Thesis title in French (Titre de thèse en français):

Composites ZnO(coeur)/TiO 2 (coquille): influence de la microstructure de TiO 2 , du dopage par azote et de la décoration avec des nanoparticules d'or sur l'activité photocatalytique et photoélectrochimique

Thesis title in Polish (Tytuł rozprawy po polsku):

Układy hybrydowe -ZnO(rdzeń)/TiO 2 (powłoka): wpływ mikrostruktury TiO 2 , domieszkowania azotem i obecności nanocząstek Au na aktywność fotokatalityczną i fotoelektrochemiczną układu

Abstract

Efficient use of renewable energies is one of the most difficult technological challenges facing humanity. Among all renewable energy sources, the sunlight is considered as the most abundant and accessible one. To convert it into usable and controllable form, the modern technology relies on generation of electron-hole pairs in semiconductors upon light absorption. The obtained separated charge carriers possess extra energy brought by the converted sunlight which can be further utilized in various ways. Currently, the most common approach consists in its direct transformation into electricity in p-n junctions. Alternatively, the electrons and holes can be used to perform chemical reactions. The electrons can be transferred to reduce various organic compounds or inorganic species, while simultaneously the holes can play the role of oxidizer by subtracting the electrons from the other substances. Through these reactions it would be possible to accumulate the solar energy in chemical species allowing thus to alleviate the intermittence of the sunlight. Unfortunately, the existing materials do not easily cross the laboratory level to realize this approach on industrial scale. That is why the development of new semiconductor photocatalysts, which harvest and convert efficiently the visible part of solar spectrum, is of paramount importance.

For many years the most often studied photocatalytic materials have been ZnO and TiO 2 . However, it has been recently shown that composites based on ZnO and TiO 2 possess even more promising properties than many other semiconductors in various photocatalytic applications. Despite many works reporting high photoactivity of these composites in different applications, the detailed information about the structureproperties correlation is lacking. In order to fill this gap we decided to focus our attention to study the influence of microstructure of ZnO/TiO 2 composites on their properties in photocatalytic degradation of organic pollutants, and in application in half-reaction of ‗solar fuel' generation, namely photoassisted water electro-oxidation. To realize such study the composites should satisfy the requirements of a high surface area and good electric conductivity. We chose therefore the design based on ZnO nanorods supported on ITO (Indium Tin Oxide)-coated glass electrode. The ZnO nanorods (NRs) were then covered with a layer of TiO 2 under different deposition conditions. The composition and microstructure of the obtained ZnO(core)/TiO 2 (shell) composites were modified in the aim to elucidate how these parameters influence their photocatalytic activity.

Consequently, the efforts were made to impart visible light activity to the elaborated ZnO/TiO 2 composites by modifying titanium dioxide layers with nitrogen and decoration with Au nanoparticles.

The thesis consists of three parts: Bibliography, Experimental and Results and Discussion.

First part of the present PhD thesis, Bibliography, is dedicated to the analysis of literature concerning fundamental properties of semiconductor materials, solid/electrolyte interface as well as principles of photocatalysis and photoelectrochemical water oxidation. Also, the photocatalytic properties of ZnO and TiO 2 , and those of their composites are reviewed in this section. Furthermore, methods for improvement visible-light absorption are also described, i.e. N-doping and surface plasmonic effects due to the noble metal nanoparticles (Au NPs) deposited on semiconductors.

The second part, Experimental, covers the preparation procedures and LSV (Linear Sweep Voltammetry), CV (Cyclic Voltammetry), chronoamperometry, chronopotentiometry, as well as the set-ups elaborated by the author for the purpose of photocatalytic and photoelectrochemical measurements.

The main results of the PhD thesis are presented in the third part, Results and Discussion, consisting of four chapters.

In the first chapter (Chapter 4.1), the results of the studies on electrochemical seeding of ITO-electrode in Zn(CH 3 COO) 2 solution are presented. The length and width of ZnO nanorods grown by hydrothermal method from Zn(NO 3 ) 2 aqueous solution on Zn/ZnO-seeded ITO substrate were shown to depend strongly on initial Zn 2+ concentration and the synthesis duration. The arrays of well-separated ZnO ‗obelisk-like' nanorods of width varied from 100 nm at tips to ~ 300 nm at bottom and average length of 1.9 µm were prepared under optimized conditions, and used as starting point for further fabrication of (core)ZnO/TiO 2 (shell) composites.

In the second chapter (Chapter 4.2), a simple and low-cost sol-gel method was developed in order to form TiO 2 thin layers on ZnO nanorods by hydrolysis of titanium(IV) butoxide. The results of studies lead to elaboration of two most distinctive variants of sol-gel procedure that allow to deposit TiO 2 layers of controlled thicknesses and different morphology (rugged or compact). The rugged TiO 2 layers were obtained after 6 hours of one step sol-gel synthesis followed by calcination of the sample at 450 o C, ensuring formation of anatase-TiO 2 , whereas the uniform coating of 25 nm -40 nm thickness was obtained via three successive 30 min-synthesis with the intermediate calcination of the sample after each deposition cycle. The composite containing the rugged TiO 2 layer was shown to possess significantly higher activity in model pollutant (methylene blue, MB) degradation and in photoassisted H 2 O electrooxidation under 400 nm monochromatic light irradiation. This improved photoactivity was correlated with the composite microstructure and attributed to a higher porosity and better accessibility of ZnO/TiO 2 interface region through the rugged TiO 2 layer by the reagents. The TiO 2 (shell) layers of similar morphology were also prepared by atomic layer (ALD) and chemical vapor deposition (CVD) techniques and it was shown that the composites fabricated by us with the use of simple sol-gel procedure yield comparable (or even higher) photoactivity. Finally, it was confirmed by total organic carbon (TOC)

analysis that the ZnO/TiO 2 composites elaborated in this work are also active in decomposition of the pollutants in a dumb hill leachate solution (waste water) under 400 nm monochromatic irradiation.

In Chapter 4.3 it is shown that the ZnO/TiO 2 interface plays a key role in enhancement of photodecomposition of MB under 400 nm illumination. The increase of photocatalytic activity was attributed to the shift of absorption edge of ZnO/TiO 2 towards visible light in comparison to that of the ZnO(core)-etched TiO 2 . Further enhancement of photocatalytic activity of ZnO/TiO 2 was achieved through its additional calcination at 450 °C for 3 h. This simple treatment brings 40% increase in the rate of MB decomposition and a two-fold rise of the photocurrent in H 2 O oxidation.

Measurements of open circuit potential (V OC ) showed that the improved properties of additionally calcined ZnO/TiO 2 composites stem from the decrease of electron-hole recombination rate. STEM (Scanning Transmission Electron Microscopy) studies showed that the additional calcination resulted in formation of voids at the ZnO/TiO 2 interface. EDX (Energy Dispersive X-ray) analysis and XPS (X-ray Photoelectron Spectroscopy) results proved that formation of voids is accompanied by the outward diffusion of Zn ions into TiO 2 layer and allowed to conclude about the existence of the Kirkendall effect at ZnO/TiO 2 interface. Occurrence of this effect observed for the first time at unusually moderate temperature (450 °C) was shown and attributed to a highly defective nature of the surface layer of the ZnO nanorods.

In the last chapter (Chapter 4.4), the composites consisting of ITO-supported ZnO nanorods covered with nitrogen-doped titanium dioxide, TiO 2 (N), shell were decorated with gold nanoparticles (Au NPs) in order to improve their photocatalytic activity under visible light. The photocatalytic properties of ZnO/TiO 2 /Au and ZnO/TiO 2 (N)/Au ternary composites were studied under illumination with Xe lamp equipped with a 400 nm cut-off filter. It was found that low Au NPs loading (0.37% at.) resulted in 60% enhancement of photocatalytic decolorization of MB under visible light with respect to the Au-free sample owing to plasmonic effects. Also, a simultaneous N-doping and Au

NPs-decoration allows to multiply by three the photocurrent in photoelectrochemical water oxidation at the potential of 0.8 V vs. Ag/AgCl. It was also demonstrated that the Au-decorated composites possess a strong electrocatalytic activity in reduction O 2 to active oxygen species (via formation of O 2 ⦁ -radicals) under a small negative bias (-0.25 V vs. Ag/AgCl) in dark. Illumination of the polarized sample with visible light was shown to enhance this process resulting in rapid decomposition a model pollutant (MB) even in the presence of Na 2 SO 4 . This approach allows to completely overcome a problem of inhibition of the photocatalytic process by dissolved inorganic salts on non-polarized catalysts, thus meeting the aim of promising material for photoelectrocatalytic remediation of waste water, often containing a significant amount of inorganic ions.

Streszczenie (abstract in Polish)

Opracowanie technologii umożliwiających efektywne wykorzystanie odnawialnych źródeł energii należy do największych wyzwań stojących przed ludzkością. Energia słoneczna jest najławiej dostępnym i niewyczerpywalnym źródłem energii. Jej przetworzenie w formę przemysłowo użyteczną odbywa się poprzez wytworzenie par elektron-dziura w oświetlonym półprzewodniku, a nastepnie ich wykorzystanie w odpowiedni sposób. Najlepiej znane i najczęściej praktykowane jest bezpośrednie przekształcenie tych nośników w energię elektryczną dzięki zastosowaniu pólprzewodnikowego złącza p-n. Alternatywnym sposobem jest wykorzystanie wygenerowanych par elektron-dziura w fotokatalitycznych reakcjach chemicznych.

Elektron może wziąć udział w reakcji redukcji, natomiast dziura w utlenieniu różnych zwiazków organicznych lub nieorganicznych na granicy faz półprzewodnik/roztwór.

Dzięki tym reakcjom można gromadzić energię słoneczną w postaci związków chemicznych i w ten sposób łagodzić problem okresowej jej dostępności. Niestety, stosowane obecnie półprzewodniki ze względu na pewne ograniczenia nie dają jeszcze możliwości zastosowania tej technologii na skalę przemysłową i dlatego istnieje potrzeba opracowania nowych materiałów lub układów hybrydowych.

Przez wiele lat materiałami najczęściej stosowanymi w fotokatalizie był tlenek cynku (ZnO) i dwutlenek tytanu (TiO 2 ), jednakże oba te materiały charakteryzuja się stosunkowo dużą szybkością rekombinacji fotowygenerowanych nośników ładunku i dużą energią pasma zabronionego, co w konsekwencji przekłada się na małą wydajność przetwarzania energii słonecznej docierającej do powierzchni Ziemi.

Résumé de la thèse (abstract in French)

L'utilisation efficace des énergies renouvelables est l'un des défis technologiques les plus difficiles auxquels l'humanité est confrontée. Parmi toutes les sources d'énergie renouvelables, la lumière du soleil est considérée comme la plus abondante et accessible.

Pour la convertir en une forme utilisable la technologie moderne repose sur la génération de paires électron-trou dans les semi-conducteurs lors de l'absorption de la lumière. Les porteurs de charge séparés obtenus possèdent une énergie supplémentaire apportée par la lumière du soleil qui peut être utilisée de différentes façons. À l'heure actuelle, l'approche la plus utilisée consiste en sa transformation directe en électricité dans les jonctions p-n. La première partie de la présente thèse, Bibliographie, est consacrée à l'analyse de la littérature sur les propriétés fondamentales des matériaux semi-conducteurs, l'interface solide-électrolyte ainsi que les principes de la photocatalyse et de l'oxydation photoélectrochimique de l'eau. En outre, les propriétés photocatalytiques de ZnO et TiO 2 , et celles de leurs composites sont examinées dans cette section. Aussi, des procédés pour améliorer l'absorption de la lumière visible sont également décrits, c'est-à-dire les effets de dopage par azote et de plasmon de surface dû à la présence de nanoparticules de métaux nobles (Au NPs) déposées sur la surface des semi-conducteurs. Dans le Chapitre 4.3, nous montrons que l'interface ZnO/TiO 2 joue un rôle clé dans l'amélioration de la photodégradation de MB sous illumination de 400 nm.

La partie

L'augmentation de l'activité photocatalytique a été attribuée à l'extension de l'absorption optique de ZnO/TiO 2 vers le domaine de la lumière visible ce qui n'est pas le cas pour

List of abbreviations and acronyms

The symbols listed below follow the recommendations of the

Introduction

Among many fascinating properties of semiconductors one of the most important is their capacity to absorb UV-vis light with formation of electron-hole pairs if the light energy exceeds the band gap of the semiconductor (Figure 1 The electrons and holes produced upon light absorption can also be used for performing chemical reactions. The electrons arriving at the surface of semiconductor particles can be transferred to various organic or inorganic species resulting in their reduction. The holes can play the role of oxidizer by subtracting the electrons from the species localized on the particle surface. The electrochemical potential of the produced charge carriers is determined by the position of the valence band (VB) and conduction band (CB) and may thus be adjusted by choosing an appropriate material. This flexibility permits to perform various transformations using the electrons and holes produced in semiconductor after the light absorption. In the present manuscript we will be concerned with two types of reactions.

First of them is the photoassisted water splitting in which the electrons are used to reduce protons to molecular hydrogen while the holes oxidize water molecules producing oxygen (Figure 1). This reaction, frequently referred to as ‗artificial photosynthesis', is of paramount importance since it allows to produce a valuable fuel (H 2 ) from abundant resource (H 2 O) using free energy (solar light). This process can also help to alleviate the main drawback of solar energyits intermittent character.

In the second type of reactions the photogenerated electrons and holes are involved in formation of very reactive radicals O 2 ⦁ -and ⦁ OH, respectively. These radicals are powerful in decomposition of various organic compounds, known as toxic pollutants, with one or many double bonds. Photocatalytic water purification is one of the most important challenges in the environmental protection because, in contrast to many conventional methods, may lead to a complete mineralization of polluting molecules.

To accomplish the reaction of water splitting without any bias (so called This approach allows to match better the properties of the materials with the conditions needed for each reaction. Indeed, while the proton reduction proceeds relatively rapidly on the surface of noble metals, water oxidation is a sluggish four-electron reaction which limits the overall process [2]. That is why a significant effort has been devoted to the search of semiconducting materials capable to realize water oxidation which allowed to identify some promising materials: TiO 2 , Fe 2 O 3 , WO 3 , BiVO 4 [3].

In the case of pollution abatement the reduction and oxidation reactions may occur on the surface of the same band gap material. According to thermodynamics, the photogenerated electrons and holes are powerful in photodegradation process if the bottom of conduction band is located above the reduction potential of O 2 to O 2 ⦁ - (-0.28 V vs. NHE at pH 7) while the top of the valence band is below the oxidation potentials of OH -or H 2 O to ⦁ OH radical (2.27 V vs. NHE at pH 7 [5]). These thermodynamic requirements are fulfilled by semiconductors with large band gap energy. Additionally, the material for photocatalytic water purification must be stable in a wide pH range, non-toxic, easily processable and of low cost. Given all these requirements TiO 2 has appeared as the most popular material for this application.

Despite the attractiveness of titanium dioxide both as anode for photoassisted water splitting and water purification, it possesses two major drawbacks. As mentioned above its band gap allows to use only a small fraction of the sunlight. The second serious shortcoming of TiO 2 is a high rate of recombination of photogenerated charge carriers leading to low photonic efficiency. The onset of TiO 2 absorption may be to some extent shifted toward the visible light through non-metal doping [6] or deposition of plasmonic metal nanoparticles [START_REF] Hua | High-performance 1D type-II TiO 2 @ZnO core-shell nanorods arrays photoanodes for photoelectrochemical solar fuel production[END_REF]. The composites based on ZnO and TiO 2 have been shown to possess promising properties in various photocatalytic applications despite the similarity of the band gap structures of these two oxides (see the references in the Bibliography part). A high hole mobility in ZnO (205 cm 2 V -1 s -1 , [13]) or formation of mid-gap states [14] have been proposed as the origin of this enhancement. However, the details of the involved mechanism are still unknown.

In order to obtain the material applicable both in organics degradation and water splitting a high surface area and good electric conductivity are needed. We chose therefore the design based on ZnO nanorods supported on ITO-covered glass support.

The ZnO nanorods were then covered by a layer of titanium dioxide under different conditions. The composition and microstructure of the obtained composites were varied in the aim to elucidate how these parameters influence their catalytic activity.

Additionally, we tried to impart visible light activity to the composites using methods known for pure titanium dioxide: the TiO 2 layer was doped with nitrogen and decorated with gold nanoparticles.

Based on a complementary experience of Warsaw and Dijon teams in synthesis and characterization of nanostructured materials we decided to prepare and study in detail the influence of preparation parameters on the photocatalytic properties of ZnO/TiO 2 nanostructured composites.

First part of the dissertation, Bibliography, is dedicated to the analysis of literature concerning fundamentals of semiconductor materials, solid/electrolyte interface as well as principles of photocatalysis and photoelectrochemical water oxidation. Also, properties of ZnO, TiO 2 , and their composites as ZnO/TiO 2 in the field applied photocatalysis are reviewed and covered in this section. Furthermore, methods for improvement toward visible light absorption are also described, i.e. N-doping and surface plasmonic effects due to the noble metal nanoparticles deposited on semiconductors (Au NPs).

The second part, Experimental, covers details of electrochemical seeding of ITOcatalyst support, hydrothermal growth of ZnO nanorods, sol-gel TiO 2 (and N-doped TiO 2 ) preparation and deposition onto ZnO, and photodeposition methods of Au NPs.

Also, the methods and techniques used in this work are described, as well as the set-ups elaborated for the purpose of this work, i.e. irradiation LED devices.

In the third part, the main results, discussion, and conclusions are given. semiconductor or insulator depending on value of its band gap (E g ). Accumulation of atoms in solid mater results in formation of ‗energy bands': electron-occupied valence (VB) and electron-free conduction (CB). In metals electron-unoccupied CB coincides with electron-occupied VB (Figure 3A). The charge carriers can in this case move freely from VB to CB without energy activation, providing therefore a good electric conductivity. In case of insulators the forbidden zone (energy gap) is large, E g > 6 eV, and so the electric conductivity is completely suppressed. In case of semiconductors (E g ~ 1 -6 eV) an external thermal energy activation or absorption of light (hv) is necessary to excite and transfer electrons from VB to CB (Figure 3B). Also, contrary to metals, the electric conductivity of SCs increases at elevated temperatures [15].

Figure 3 Band structures of metals and insulators (A), electron-hole photogeneration in intrinsic (undoped and defect free) semiconductor (B), and band structure of ntype and p-type semiconductors (C).

As consequence of electron (e -) transition from VB to CB, in the VB a hole (h + ) is left.

However, a real metal oxide semiconductor crystal possesses structural defects and/or impurities, which result in appearance of donor or acceptor energy levels, close to the edges of CB or VB, respectively (Figure 3C). In general, electron band-to-band transitions are much stronger than these involving additional energy levels due to their low population. A semiconductor (SC) that is of an n-type holds additional energy levels occupied by electrons, whereas the one with extra empty energy levels located closer to VB, which can be occupied by the electrons from VB is of a p-type. Such situations lead to the shift of Fermi level (E F ) energy from the middle band gap position typical for intrinsic semiconductor to CB for n-type SC or to VB in p-type SC. Fermi level can be either described as energy level at which probability of an electronic state being occupied is 0.5 or in terms of thermodynamics it corresponds to the electrochemical potential of electron in solids. In this context it is worth to mention that TiO 2 , similarly to ZnO, is an n-type SC [16]. In general, the fate of a pair of separated charge carriers can be dualistic, either they migrate to the surface of semiconductor and take part in redox reactions (e r -, h r + : reactive electron and hole), or recombine to release energy in the form of light emission (hv 1 ) or thermal dissipation (Figure 4A). Also, two different types of electron transitions characterize semiconductors (Figure 4B).

In a simplified energy description, electron transition can proceed when the minimum of CB is located at the same value of wave vector (k) as the maximum of VB and such transition is called ‗direct'. On the other hand, when these positions maxima are displaced, the ‗indirect' transition occurs. While the quantum momentum is conserved in direct transition and only photon energy is needed for such, the indirect transition requires additionally absorbed phonon to obey the laws of momentum transition to larger k value. According to the literature, the probability of electron transition in a case of indirect SC is lower by almost two orders of magnitude than in direct SC [START_REF] Schiavello | Photoelectrochemistry, photocatalysis and photoreactors[END_REF]. An example of SC of direct transition occurrence is ZnO, whereas TiO 2 exhibits indirect behavior.

A B

Rapid recombination of photogenerated electrons and holes is a key issue in many semiconductors, such as TiO 2 , hampering efficient utilization of the charges. The charge carrier generation lasts 10 -6 ns [START_REF] Hoffmann | Environmental Applications of Semiconductor Photocatalysis[END_REF], and the trapped charge carriers lifetimes are enlisted in the Figure 5.

Figure 5 Time scale of different electron-involved processes occurring in semiconductor/electrolyte system [START_REF] Schneider | Understanding TiO 2 Photocatalysis: Mechanisms and Materials[END_REF].

Nanomaterialsthe advantage over the bulk

A material with at least one linear dimension lower than 100 nm can be defined as a

nanomaterial. An enormous attention is attracted mostly due to their dramatically different properties in a comparison to bulk-scale counterparts. Among many unique features of nanomaterials, one in particular should be mentioned here. Nanomaterials are characterized with a large surface area and in this sense high surface energy ( ) is a consequence of as such, expressed by:

(

where is a material density, M is molar weight, d is particle diameter, whereas is specific surface energy. According to the eq. 1, surface energy increases inversely proportional to the size of particle. As consequence, the thermodynamic driving force to minimize the surface energy is strong and has many consequences, such as determination of a shape of growing particle (long rods or flat plates). With a simple assumption of a tetragonal particle with sides ‗a' and ‗b' have related surface energies ‗ a ' and ‗ b ', so the surface energy is given by:

(2)

Assuming constant volume, and giving minimum surface energy ( ), one arrives:

(3) then if a > b , nanoparticles grow and/or agglomerate in the form of rods, whereas for the condition a < b a plate-shape dominates. Surface energy, and so the particle shape and size, strongly influence a phase transformation temperature i.e. anatase/rutile TiO 2 . It is worth to mention that different planes possess different surface energy, which can contribute to overall photoactivity of the material [START_REF] Ohno | Photocatalytic activities of pure rutile particles isolated from TiO 2 powder by dissolving the anatase component in HF solution[END_REF], i.e. (001) planes of anatase TiO 2 exhibits low photocatalytic activity [START_REF] Selçuk | Surface Structure and Reactivity of Anatase TiO 2 Crystals with Dominant {001} Facets[END_REF], whereas the same (001) planes of rutile TiO 2 happens to be highly reactive [START_REF] Sosnowchik | Titanium dioxide nanoswords with highly reactive, photocatalytic facets[END_REF]. Majima et al. [START_REF] Tachikawa | Design of a highly sensitive fluorescent probe for interfacial electron transfer on a TiO 2 surface[END_REF] reported that the energetically stable {101} crystal face ( = 0.44 J m -2 ) showed a higher photocatalytic activity than the {001} face with a higher surface energy ( = 0.90 J m -2 ). Also the group of Manivannan et al. [START_REF] Wu | Shape-enhanced photocatalytic activity of single-crystalline anatase TiO 2 (101) nanobelts[END_REF] found that the TiO 2 {101} face is highly reactive surface for the reduction of molecular O 2 molecules to O 2 •-. Preparation methods, such as sol-gel, result in formation of uniformly shaped particles ( a ≈ b ), often with sharp edges after calcination. Since, in sharp crystals the {101} faces dominate, the photoelectron charge transfer to acceptor-electrolyte is promoted, thus reducing the electron-hole recombination [START_REF] Li | Particle size, shape and activity for photocatalysis on titania anatase nanoparticles in aqueous surroundings[END_REF]. Similar conclusion was drawn from studies of low-energy {101} and high-energy {001} TiO 2 crystal facets by Roy et al. [START_REF] Roy | Synergy of Low-Energy {101} and High-Energy {001} TiO 2 Crystal Facets for Enhanced Photocatalysis[END_REF], who reported enhanced photocatalytic degradation on methyl orange (MO) by synthesized material with appropriate ratio of these facets. Therefore, it is also important to design carefully and prepare a nanomaterial, taking into account shape, size, faces, and polycrystallinity.

Moreover, it should be mentioned that excess of the surface energy, caused by surface deformations is responsible for formation of electrical potential barrier at the solid/electrolyte interface [START_REF] Zhao | Sol-gel preparation of Ti 1-x V x O 2 solid solution film electrodes with conspicuous photoresponse in the visible region[END_REF], which is also important for electrochemical properties.

Relation of semiconductor E g with the size of nanoparticles can be presented in the simplified form as follows [START_REF] Vollath | Nanoparticles -Nanocomposites -Nanomaterials, An Introduciton for Beginners[END_REF]:

where is the band gap energy of bulk material, d is diameter of a nanoparticle, and α is quantity proportional factor. This relation shows that the band gap of SC nanomaterials decreases with increased particle size. Also, it should be mentioned that in the nanosized materials the effective diffusion length of photogenerated charge carriers to the surface is shortened providing therefore their more efficient utilization.

Among all 1D nanostructures, nanorods and nanotubes are the most attractive for electrode preparation since the effective working surface area increases significantly.

Columnar nanostructure arrays show better electron drift mobility than non-hierarchical, random networks of porous film (i.e. for TiO 2 films: array (A): 5.6 •10 -6 cm 2 V -1 s -1 , porous network (B): 6.6 •10 -7 cm 2 V -1 s -1 , Figure 6). In this sense, higher electron drift mobility yields better electrochemical responses [START_REF] Aduda | Effect of morphology on electron drift mobility in porous TiO 2[END_REF].

Figure 6 SEM cross-section images of array (A), and porous network TiO 2 (B) [START_REF] Aduda | Effect of morphology on electron drift mobility in porous TiO 2[END_REF].

Optical properties

The ‗optical band gap' of a semiconductor may be determined from UV-vis absorption spectrum with the use of Lambert-Beer law:

[ ] (5)
where I 0 is light intensity before, whereas I tr after passing through a thin film of thickness d, and α is absorption coefficient of the solid material. Here, it should be noted that the equation 5 can be used in the case of flat material surface without light intensity loses because of scattering and reflection. Also, α depends on the energy of light, and its value should be specified for the given wavelength. The intensity of light decreases after passing throughout the material and the distance at which 63% of initial light intensity

1 𝜇𝑚 A B
remains, is called absorption length or optical penetration depth, expressed explicitly as α -1 , and it depends on irradiation light wavelength [START_REF] Hitchman | Studies of TiO 2 thin films prepared by chemical vapour deposition for photocatalytic and photoelectrocatalytic degradation of 4-chlorophenol[END_REF]. For rutile TiO 2 the value of α -1 changes from 10 nm at 300 nm via 1 µm for 380 nm [START_REF] Eagles | Polar modes of lattice vibration and polaron coupling constants in rutile[END_REF], and to about ~ 40 µm for light irradiation of 400 nm wavelength [START_REF] Möllers | On the Origin of the Photocatalytic Deposition of Noble Metals on TiO 2[END_REF]. The absorption coefficient is correlated with the optical band gap energy according to the Tauc equation [START_REF] Tauc | Absorption edge and internal electric fields in amorphous semiconductors[END_REF][START_REF] Tauc | Optical properties and electronic structure of amorphous germanium[END_REF][START_REF] Pankove | Optical Processes in Semiconductors[END_REF]:

( ) ( ) (6)
where is material constant (10 5 -10 6 cm -1 eV -1 ), n is a type of electron transition given for indirect (n = 2) and direct (n = ½) semiconductor, recalling that TiO 2 is indirect, and ZnO is direct SC. Plotting ( ) vs. energy of incident light ( ) and extrapolating straight line to zero value, one can determine the optical band gap value (Figure 7). The absorption coefficient ( ), according to the Urbach rule [38], depends on temperature and photon energy, accordingly to the equation:

( ) * ( ) + (7)
where is optical energy coordinate of the convergence point and is steepness parameter of absorption edge defined as [39]:

( ) ( ) (8)
where is a temperature independent constant which represents the phonon energy associated with the so-called ‗Urbach absorption tail'. According to equation 7, the absorption coefficient increases exponentially in the range of absorption edge. While approaching to it, the density of states of VB and CB, related to population of their surface states, manifests in stepwise increase of , which explains the Urbach absorption ‗tail'. This means that more pronounced absorption should be observed when approaching the band gap value of SC.

As it was discussed above, the materials possess structural defects manifesting in step decreasing values of (αhv) 1/n . Such weak absorption shoulders are proportional to the density of defect states in the material. In this sense, a less defective crystal has more perpendicular inclination of absorbance (Figure 7). Also, due to low concentration of defect states the intensity of such transition is much weaker than the band-to-band transition. Consequently, absorption of the material at lower energies, i.e. towards visible light can be improved by introducing more defect states. This can be done by doping a material or increasing concentration of oxygen vacancies in metal oxides. The other possibility is to use plasmonic metal nanoparticles.

Semiconductor/solution interface and related phenomena

Theory of the charge transfer at the semiconductor/solution interface has been described by the 41,42].

There are two principle reasons of differences in the charge transfer process at the semiconductor/solution and metal/solution interface. One is much lower concentration of the mobile charge carriers in semiconductor leading to different structure of a double layer (Figure 8A,B) and in consequence different kinetic limitations, and the second one is involvement of two kinds of the charge carriers (electron and holes) in the interfacial reactions at semiconductor/solution interface [43]. After immersion of n-type semiconductor in the solution containing a redox couple (in dark), the electrons flow from one phase to another to reach the equilibration of Fermi energy levels of the both phases (E F and E F,redox , respectively). This leads to redistribution of charge carriers within the near-interface region in the semiconductor of 10 nm -1000 nm thickness (called as space-charge layer) and eventually to the ‗band bending' and potential drop  SC across the space-charge layer (Figure 8B andD). When the E F of n-type semiconductor before equilibration lies above E F,redox , the electrons from donor levels of semiconductor flow towards the electrolyte, leading to development of positively charged space charge layer (depletion layer) in the semiconductor and causing upward band bending. At the same time, the counterbalance negative charges are accumulated from electrolyte solution side and the potential drop  H is formed in the Helmholtz layer of thickness 0.4 nm -0.6 nm (Figure 8B). However, because of lower charge density in the space charge layer of semiconductor than that in the Helmholtz layer, the potential drop ( SC ) in the semiconductor is the main factor determining the total capacitance of the interface. In contrast the Helmholtz layer cannot be neglected at the metal/solution interface because of very high electron density in metal with respect to the charge density (ions) in the solution (Figure 8A). In case when there is no excess charge on the semiconductor side of the interface, the Fermi level energy is called as flat band energy, E fb = E F .

Band bending in a semiconductor can be also induced by applying an external bias potential (V a ) to SC. Application of anodic polarization resembles n-type SC band bending; however when V a is equal to E fb (vs. NHE), then the band bending is not observed and the space-charge layer is not developed.

Under illumination the generated charge carriers are separated between CB and VB, so their energies are different from the dark equilibrated state, and can be described by equation:

( ) (9)
where E F is the energy of Fermi level in dark, ∆q s denotes concentration difference of charge carriers under steady-state illumination period and at the dark thermal equilibrium (q 0 ) [44]. Thus, under illumination, E F splits into two quasi-Fermi levels, one for electrons ( ) and second for holes ( ) (Figure 9). For the reaction to happen the standard redox potentials of the solution species should be located within the SC band gap. However, the oxidation and reduction potentials of donor/acceptor species may differ from the standard redox potential even by 0.5 eV due to contribution of reorganization energy caused by a solvent [47]. Since the exact A B

energies of reorganization and quasi-Fermi levels cannot be precisely estimated, in practice the equilibrium redox potential values and CB/VB positions are used for thermodynamic predictions of process occurrence.

Application of external potential separates and makes drift the charge carriers, i.e. the electrons are moved via external circuit to the counter electrode and the holes are transferred to the semiconductor surface. Recombination thus is hindered.

Thermodynamically it is favorable for electrons to lower their energy and holes move in opposite direction (upwards) in energy scale.

The maximum photovoltage can be calculated accordingly to:

V ph = E F,redox - (10)
Under anodic polarization and in dark the weak ‗thermal' current, however negligible, can be recorded (Figure 10A The obtained photocurrent density (j ph ) is a difference between the current density under illumination (j ill ) and in dark (j dark ):

j ph = j ill -j dark (11)
The photocurrent is a sum of contributions from the charge carriers created in the space charge region and transported to the surface by electric field and those produced out of depletion layer and transported by diffusion field. In the simplest case, without recombination, the total photocurrent for monochromatic light derived from Gärtner model [51] is given by equation:

*1 ( ) + ( 12
)
where I o is incident light intensity at the surface, α is the absorption coefficient, W sc is a depletion layer thickness and L is the minority carrier diffusion length (Figure 10C).

The value of L is a function of the life time of minority charge carriers () and diffusion coefficient (D):

√ (13)
The dependence of the photocurrent on potential may be derived using the relationship between W sc and potential difference (V a -V fb ) [51]:

( ( ) ) (14)
where N D is the donor density,  is the dielectric constant of the semiconductor.

When the absorption coefficient or the thickness of the space charge is very large (αW sc » 1), then j ph = eI o , i.e. all created charge carriers contribute to the photocurrent and the photocurrent reaches the saturation value (Figure 10A,line c).

Ideally, point of zero photocurrent under illumination refers to situation when working electrode potential V a is equal to V fb (potential of a flat band), in other words onset of photocurrent corresponds to V fb (Figure 10B). It is also worth to note that the value of V fb determined from photocurrent measurement is dependent on solution pH, and in case of TiO 2 it fulfills the equation [52]:

(vs. NHE) (15)

However, due to presence of defects such as surface states, a zero point photocurrent can vary. Such current-voltage relationship can be observed for crystallites of 3 nm -30 nm size, despite the fact that space-charge layer is not formed [53,54,55]. This can be observed only when such particles form aggregates in the form of thick porous films or networks.

Thus, the photocurrent value shows the effectiveness of photogeneration of charge carriers. Also, when j ph value is greater than zero it means that the electrode plays the role of photoanode, whereas if j ph < 0 the electrode is photocathode.

As presented above, photocurrent can be measured under constant illumination with linear change of applied bias (i.e. linear sweep voltammetry, LSV), but also under constant bias with dark and illuminated periods (transients, Figure 11). In the initial seconds of light on/off transitions the characteristic spikes followed by exponential current decays within a few seconds to the steady-state values are observed. They The intensity of the spikes depends on the rate of charge carrier recombination. In order to estimate the ratio of the rate constants for charge transfer across the semiconductor/solution interface (k tr ) and for e -/h + recombination (k rec ), the photocurrent transients j(t) can be analyzed according to the model proposed by Peter [57]. As reported, the decay time constant () is determined by (k tr + k rec ) -1 :

( ) (16)
while the value of  can be derived from the normalized photocurrent decay :

( ) ( ) ( ) ( ) (17)
where j(0) and j(∞) are the current densities at t = 0 and at steady-state (∞), respectively.

The values of decay time constants can be obtained by fitting the exponential function to the experimental data. Then, the ratio of k tr /k rec determined from the equation for steadystate current:

( ) ( ) (18)
allows to conclude whether the charge transfer dominates over the recombination ((k tr /k rec ) > 1) or situation is opposite ((k tr /k rec ) < 1).

Photopotential of the electrode can be determined when no bias is applied to the photoelectrode according to the eq. [59]:

V ph = |V OC,ill -V OC,0 | (19)
where, V OC,ill and V OC,0 are the open-circuit potentials (OCP) under illumination and in dark, respectively. In practice, V OC,ill for n-type semiconductors is more negative under illumination than in dark (V OC,ill < V OC,0 ). Similarly to photocurrent transient measurements, OCP is recorded in light on/off periods. In common OCP behavior (chronopotentiometry) for n-type electrode (i.e. TiO 2 ) one can distinguish several regions (A -E in Figure 12A). Once the potential of the electrode in dark is stabilized in terms of half-reaction kinetics (Region A) the light can be turned on and immediate drop of photopotential is observed due to photogeneration and separation of the charge carriers (Region B). When charge generation and consumption are equilibrated the photopotential stabilizes (Region C). At this point light can be turned off and the photopotential decays (V OC ) by slow relaxation (Region D) to initial value, reaching finally steady state in dark (Region E). For semiconductor electrodes such as ZnO, and TiO 2 the results of V OC decay can be used to estimate electron lifetime ( ),

[62] accordingly to the eq.:[63]

( ) (20)
where is the Boltzmann constant, is the elementary charge and is the temperature expressed in Kelvins, and t is decay time. Such approach allows to estimate the overall lifetime of trapped and free electrons upon their photogeneration in semiconductors (Figure 12B), and as visible the lifetime may be strongly influenced by its composition.

Photoelectrochemical water oxidation

Photo-assisted water oxidation is a half-reaction of water splitting. Such process however, cannot proceeds on neither ZnO nor TiO 2 without application of external bias potential because H + /H 2 redox potential is above the Fermi level energy of the semiconductor (Figure 13).

A B

Therefore, the three electrode system is needed equipped with a photoanode (i.e. n-type SC), Pt cathode and the reference electrode (for example Ag/AgCl). Water is oxidized by holes, whereas the electrons photogenerated in photoanode flow to Pt cathode where hydrogen is then generated, according to reaction scheme:

2h + + H 2 O → ½ O 2 + 2H + : at photoanode (21)
2H + + 2e -→ H 2 : at cathode [START_REF] Selçuk | Surface Structure and Reactivity of Anatase TiO 2 Crystals with Dominant {001} Facets[END_REF] Photoanode composed of TiO 2 exhibits maximal photovoltage of 0.7 V -0.9 V [64],

which is too small for water splitting without bias. The overall water splitting reaction:

H 2 O → H 2 + ½ O 2 (23)
is an endothermic reaction with standard free enthalpy of 236.8 kJ mol -1 . Half-reactions of oxidation and reduction are:

reduction: 2H 2 O + 2e -→ H 2 + 2OH -E 0 red = -0.41 V vs. NHE at pH 7 (24)
oxidation: O 2 + 4e -+ 4H + → 2H 2 O E 0 ox = 0.82 V vs. NHE at pH 7 [START_REF] Wu | Shape-enhanced photocatalytic activity of single-crystalline anatase TiO 2 (101) nanobelts[END_REF] and the summary energetic process can be correlated with standard free energy accordingly:

E = (E 0 red -E 0 ox )e = ∆G 0 H2O / 2N A = hv = 1.23 eV (26)
Contrary to photocatalytic degradation of organic pollutants where the light is ‗utilized' to accelerate spontaneous processes, the water oxidation might be considered as light energy ‗storage' since the process is not spontaneous under ambient conditions. In other words, water splitting has ∆G 0 > 0, which is so-called uphill electron transfer process, while photocatalysis ∆G 0 < 0 is a downhill electron transfer process.

To carry the water splitting reaction on TiO 2 photoanode the overpotential is required.

The O 2 /H 2 O reaction typically requires an additional 0.275 V (U a , Figure 13) anodic overpotential, while H + /H 2 half-reaction requires an additional 0.05 V (U c , Figure 13) value, the excess of energy is needed to overcome kinetic limitations of this reaction, and so the E g should be therefore ≥ 2 eV [65]. To conclude, some general demands for effective semiconductor working as photo-anode can be specified:

cathodic
(1) band gap should be of approx. 1.9 eV (it is a compromise of small band gap which promotes absorption of wide range of light spectrum, however not too small to overcome kinetic limitations),

(2) CB should be more negative than reduction potential of water (E CB < E red H2O ),

(3) VB should be more positive than oxidation potential of water (E VB > E ox H2O ), (4) electron transfer via SC/electrolyte interface should be fast,

(5) photocorrosion should be eliminated.

Either water oxidation or water reduction can be conducted or these reactions are sacrificial oxidation and sacrificial reduction, since reductant and oxidant are consumed in the processes. Despite recent results on determination of the mechanism of water oxidation, still discussion in literature is continued. Thus ones consider that surface states at semiconductor/solution interface serve as intermediate place for O 2 evolution, the others postulate that surface • OH radicals in (TiOH) + may serve as an initial stage of the process involving h + . Contrary to that, it was also reported that O 2 evolution may lead to formation of surface OH radicals. Moreover, the others showed that h + can be trapped in the lattice of O atoms without formation of OH radicals [20, and references therein].

Heterogeneous photocatalysis

Heterogeneous photocatalysis is currently one of the most attractive among other advanced oxidation methods for remediation of the environment, especially air and water purification (Figure 14A). Its advantage arises from a unique combination of opportunities that biological or chemical methods solely cannot offer nor achieve expected effect of degradation of pollutants. The method was enlisted as one of the advanced oxidation processes (AOP), with advantages of relatively cheap photocatalytic materials which do not require additional chemical substances to work, free of chemical wastes and the processes proceeds in ambient temperature. Oxidative degradation of organic pollutants initially was being achieved with ‗old' methods that are based on formation of reactive ozone (O 3 ). AOP are the methods that generate other oxidizing agents such as hydroxyl radicals ( • OH), which are more reactive than ‗reference' O 3 , by far of a few or several orders of magnitude (Figure 14B). Therefore, the heterogeneous photocatalysis attracts a great attention since it still requires further improvement and understanding as ‗relatively' recent and need-to-develop method and technique. There are few models to describe quantitatively the effectiveness of heterogeneous photocatalytic process. One of the classical kinetic models for heterogeneous photocatalytic degradation process is the Langmuir-Hinshelwood model, which is based on classical equilibrium coverage on uniform surface, represented by equation: [START_REF] Roy | Synergy of Low-Energy {101} and High-Energy {001} TiO 2 Crystal Facets for Enhanced Photocatalysis[END_REF] where is adsorption equilibrium constant, and is reactant concentration, and the rate of degradation can be expressed as: [START_REF] Zhao | Sol-gel preparation of Ti 1-x V x O 2 solid solution film electrodes with conspicuous photoresponse in the visible region[END_REF] where, is time of irradiation, is reaction constant. Assumptions to the model are:

(1) at equilibrium state the population of adsorption sites is constant,

(2) only one reactant occupies one adsorption site,

(3) reactant molecules do not interact between themselves, (4) the adsorption is reversible,

(5) rate of adsorption is higher than any subsequent reactions, (6) energy of adsorption is the same at each adsorption site.

For initial concentrations c 0 lower that 10 -3 M, the equation is simplified by the assumption K a c << 1:

A B ( ) (29)
where, k app is apparent first-order rate constant (Figure 16A). Such model can be applied in MB decomposition and quantitative comparison of photocatalytic activity of catalysts can be based on the determined k app values. Holes:

TiO 2 + hν → TiO 2 (e -+ h + ) (30) TiO 2 (h + ) + RX ad → TiO 2 + RX ad •+ (31) TiO 2 (h + ) + H 2 O ad → TiO 2 + • OH ad + H + (32)
TiO 2 (h + ) + -OH ad → TiO 2 + • OH ad [START_REF] Möllers | On the Origin of the Photocatalytic Deposition of Noble Metals on TiO 2[END_REF] Electrons:

(ii) (iv) (iv) (iii) (iii) (ii) (i) A B TiO 2 (e -) + O 2 → TiO 2 + O 2 •- (34) O 2 •-+ H + → HO 2 • (35) O 2 •-+ HO 2 • → • OH + O 2 + H 2 O 2 (36) 2HO 2 • → O 2 + H 2 O 2 (37) TiO 2 (e -) + H 2 O 2 → TiO 2 + OH -+ • OH (38) • OH + • OH → H 2 O 2 (39) H 2 O 2 + • OH → H 2 O + HO 2 • ( 40
)
There is also possibility for reaction to occur:

H 2 O 2 +TiO 2 (e -) + H + → • OH + H 2 O (41) O 2 •-+ h + → 1 O 2 (42)
Reactions ( 32) and ( 33) generate highly reactive hydroxyl radicals ( • OH ad ), which are mainly responsible for degradation of adsorbed organic pollutants at SC surface. Other highly active species O 2 •-and ‗free' • OH are also involved in this process.

Photocatalytic anti-bacterial properties of TiO 2 have been also reported [72], however for bacteria disinfection are mainly responsible ‗free' • OH radicals generated in electron- [88,[START_REF] Mills | Overview of the current ISO tests for photocatalytic materials[END_REF].

Model pollutant photocatalytic degradation

Some dyes like methylene blue (MB), methyl orange (MO), or Rhodamine B (RhB) are widely used as probe molecules for determination of photocatalytic activity of catalysts and in the present work MB was chosen for this purpose. Decolorization of MB follows the apparent first-order kinetics which is convenient for determination of decolorization rate constants for catalyst and their possible quantitative comparison. MB UV-vis spectrum covers a region of negligible absorption within 350 nm -500 nm, whereas two peak located around 614 nm and 664 nm can be seen (Figure 18A). The former can be ascribed to dimer form of MB, whereas the latter peak is ascribed to electron transition in the monomer (Figure 18B). According to the literature, in 10 µM aqueous solution of MB only 7% is in dimer form [START_REF] Mills | Photobleaching of methylene blue sensitized by TiO 2 : an ambiguous system?[END_REF].

Figure 18 UV-vis absorption spectrum of methylene blue (MB) in aqueous solution (A) [START_REF] Melgoza | Comparative efficiencies of the decolourisation of Methylene Blue using Fenton's and photo-Fenton's reactions[END_REF], and chemical structure of MB monomer and dimer (B) [START_REF] Patil | Self-aggregation of Methylene Blue in aqueous medium and aqueous solutions of Bu4NBr and urea[END_REF].

Therefore, determination of photocatalytic activity of a catalyst in MB decomposition under irradiation with light of 350 nm -500 nm is not influenced by the dye selfdecolorization. However, under visible light MB photobleaches reversibly in the presence of catalyst and sacrificial electron donor (SED) in deaereated solutions. The product of such reaction, colorless leuco-MB (LMB), is unstable in time, and finally returns to initial MB form.

However, in case when molecular oxygen is present in MB solution, the dye undergoes irreversible process, known as photosensitization-autoxidation. In such case, the electrons from dye excited state (MB*) are transferred to CB of semiconductor ( 1), and then to O 2 leading to formation of O 2 •-, leaving the oxidized form of dye (MB •+ ). The electrons injected to SC may lead to formation of hydroxyl radicals ( • OH) and those to degradation the dye.

Under visible light the semiconductor of the wide band gap does not play a role of photocatalyst, since it is not a generation source of e -/h + , but only a medium for injection of electrons to its surface reactive centers.

Considering MB photodegradation by photoactive material upon its illumination with the light of sufficient energy, the photogenerated electrons are transferred from CB of semiconductor to dissolved oxygen (O 2 ), to form superoxide radicals (O 2 •-). On the other hand, the holes participate in a formation of another radical species to oxidation of organic compounds [START_REF] Gouvêa | Semiconductorassisted photocatalytic degradation of reactive dyes in aqueous solution[END_REF]. However, because of a rather small difference between the reduction potential of oxygen to superoxide radical (O 2 •-; -0.28 V vs. NHE)

and the potentials of the lower edges of conduction bands of TiO 2 (about -0.5 V vs. NHE at pH 7) [11,[START_REF] Park | Surface modification of TiO 2 photocatalyst for environmental applications[END_REF] and ZnO (about -0.63 V vs. NHE) [START_REF] Jacobsson | Photoelectrochemical Determination of the Absolute Band Edge Positions as a Function of Particle Size for ZnO Quantum Dots[END_REF], the reducing power of conduction band electrons of these semiconductors is rather low and decolorization of MB more likely occurs via oxidation by holes from the valence band. Methylene blue dissolved in DI water exists in cationic form. Thus, its adsorption on semiconductors depends on pH of the solution and the isoelectric point (IEP) of TiO 2 [START_REF] Zhou | Degradation Kinetics of Photoelectrocatalysis on Landfill Leachate Using Codoped TiO 2 /Ti Photoelectrodes[END_REF]: Figure 19 Photodegradation mechanism pathway of methylene blue (MB) [START_REF] Houas | Photocatalytic degradation pathway of methylene blue in water[END_REF].

pH

Zinc Oxide: properties

ZnO crystals exist in three forms: cubic rocksalt, zinc blende, or wurzite type (Figure 20A). Under normal conditions the thermodynamically stable form of ZnO is wurzite of hexagonal structure with lattice parameters a = 3.25 Å and c = 5.2 Å, and can be obtained in the form of large crystals [START_REF] Reynolds | Optically pumped ultraviolet lasing from ZnO[END_REF].

Due to the wide band gap of ~ 3.37 eV (at room temperature) [START_REF] Mang | Band gaps, crystal-field splitting, spin-orbit coupling, and exciton binding energies in ZnO under hydrostatic pressure[END_REF] and high exciton binding energy of 60 meV, ZnO possess efficient exciton emission [100,101]. Also in ZnO, a high electron mobility of 205 -400 cm 2 V -1 s -1 is observed [102], which is higher than that in TiO 2 : 10 -5 cm 2 V -1 s -1 [103,104]. One of the major drawbacks of ZnO is its chemical dissolution in acidic and strong basic aqueous solutions (pH > 13).

ZnO nanostructures can be prepared by various methods including: ( 1 . Moreover, because of this deep green emission (585 nm) the material found its successful application in LED [START_REF] Fan | ZnO-Based Light-Emitting Diodes Optoelectronics -Advanced Materials and Devices[END_REF] and laser [125] devices.

Titanium Dioxide: properties

TiO 2 exists in nature in three polymorph forms including brookite, anatase, and rutile (Figure 22). Among all forms of TiO 2 , the most attractive one is metastable anatase, as it generally exhibits higher photocatalytic activity than the others [126]. Brookite can be described as oxygen-deficient form of TiO 2 and similarly to anatase it is less thermodynamically stable than rutile in normal conditions. Those metastable forms of TiO 2 can be transformed into rutile at high temperatures (600 o C -800 o C) [127,128].

However, the exact transformation temperature strongly depends on many parameters i.e.

size of particles, preparation conditions, or presence of structural defects [129]. Despite the fact that the mobility of charge carriers is the highest in rutile due to better crystallinity, anatase exhibits more effective use of charge carriers in the reactions since it possesses more defect sites such as oxygen vacancies, which serves as active reaction centers. TiO 2 anatase is an example of n-type semiconductor with possession of dominant native defects (often oxygen vacancies), which can be formed its preparation Because of the high refractive index (anatase: 2.45, rutile: 2.70 [START_REF] Yuwono | Controlling the crystallinity and nonlinear optical properties of transparent TiO 2 -PMMA nanohybrids[END_REF]) and nontoxicity, TiO 2 is used as an outdoor paint component, which also serves as a photocatalytic self-cleaning surface [136,137]. Other popular application fields of TiO 2 include gas sensors [START_REF] Lin | Gas-sensing properties of nanocrystalline TiO 2[END_REF], optics [START_REF] Paul Van Hal | Photoinduced Electron Transfer and Photovoltaic Response of a MDMO-PPV:TiO 2 Bulk-Heterojunction[END_REF], or ceramics [140]. TiO 2 is chemically stable in a wide range of pH: 1 -14, and therefore may be used in harsh conditions, and so the most promising field of its application is photocatalytic degradation of organic pollutants

[141]. However, TiO 2 suffers from fast recombination of charge carriers [142], and light absorption limited to UV range (E g ~ 3.0 -3.2 eV).

(cm 2 V s -1 ) 5 -180 2 •10 -3 E g ~ 3.3 [145] ~ 3.1 E CB -0.63 -0.5 E VB ~ 2.5 ~ 2.7 A B

Doping of TiO 2

In order to extend the range of light absorption by TiO 2 to visible, a doping by either metals or non-metals was widely used. This may result in appearance of new midgap energy states and in some cases like N-doping in band gap decrease (Figure 23).

Incorporation of different cations or anions into crystallographic structure of metal oxide, in most cases, also increases its photocatalytic activity [146,147,148,149,150]. N [160,161], S [162], I [START_REF] Lisowska-Oleksiak | Preparation and characterisation of visible light responsive iodine doped TiO 2 electrodes[END_REF]164], F [165], P [START_REF] Sotelo-Vazquez | Multifunctional P-Doped TiO 2 Films: A New Approach to Self-Cleaning, Transparent Conducting Oxide Materials[END_REF], or B [167,168]. Among all anionic dopants, nitrogen seems to be the most popular, since the absorption of visible light is higher for N-doped TiO 2 than for other dopants i.e. S-doped TiO 2 , and finally leads to better photocatalytic activity of the modified semiconductor [169]. Co-doping with two different anions is also possible and the doped material exhibits higher activity than each single anion-doped SC, i.e. N-,S-doped TiO 2 [170]. Mixed simultaneous co-doping (cationic and anionic) also promotes higher activity, i.e. Ce-,N-doped TiO 2 [171], Ni-,Ndoped TiO 2 [START_REF] Selcuk | Hydrogen production by photocatalytic water-splitting using nitrogen and metal co-doped TiO 2 powder photocatalyst[END_REF], or Nb-,N-doped TiO 2 [173]. The crucial factor that influences photoactivity is the level of doping; either too low or too high is undesirable. Doping level of TiO 2 is usually around or less than 1%, and in case of N-doping it was found that approx. 0.25% is the optimal value to achieve the highest catalytic activity, TiO 1.9925 N 0.0075 [174].

Though N-doping of TiO 2 always results in the increase of absorption towards visible light but it does not lead in parallel to systematic enhancement of photocatalytic activity [175,176]. In case of photo-electrolysis of water on N-doped TiO 2 electrodes a key issue is the photocurrent stability [177]. Combination of experimental (XPS) and theoretical (density of states (DOS) calculations) studies allows to understand the nature, role of N-doping and shed some new light on importance of doping methods [178].

N-doping can be achieved by different methods and techniques, most popular are:

(1) liquid phase method (sol-gel) [179,180]. In such approach, nitrogen is incorporated into TiO 2 lattice during its precipitation. Prior to this, TiO 2 precursor, such as All methods incorporate nitrogen in substitutional and interstitial sites (Figure 24A, and 24B). According to the literature, for N1s core binding energy within the range of 396 eV -398 eV corresponds to nitrogen incorporated into metal oxide structure as N-Ti-N, therefore substituting oxygen. Also, the nitrogen state was defined as either anion [191] or atom [192]. Binding energies of about 400 eV are assigned to Ti-O-N or Ti-N-O linkage, characteristic of interstitial N atoms [193,[START_REF] Bhirud | In-situ preparation of N-TiO 2 /grapheme nanocomposite and its enhanced photocatalytic hydrogen production by H 2 S splitting under solar light[END_REF]. Also, N1s peaks of about 402 eV were ascribed to chemisorbed N 2 or adsorbed organic species [195]. Substitutional nitrogen introduces N2p energy levels little above VB of TiO 2 (0.14 eV), while interstitial energy levels are located higher (0.73 eV above TiO 2 VB) [START_REF] Valentin | Theory of carbon doping of titanium dioxide[END_REF][START_REF] Valentin | Characterization of paramagnetic species in N-doped TiO 2 powders by EPR spectroscopy and DFT calculations[END_REF]198].

Substitutional is a negative oxidation state, interstitial is positive.

Studies show that the increased absorption of visible light is indeed due to appearance of new ‗nitrogen-related' energy levels but also due to additional oxygen vacancies energy levels formed in a SC [199], according to the reaction scheme:

TiO 2-2x N 2x → TiO 2-3x N 2x + xV O + ½xO 2 (45)
As postulated, oxygen vacancies increase the light absorption in visible range and the role of nitrogen incorporation is also prevention of reoxidation of formed oxygen vacancies, and this may lead to the increase of material photoactivity [200]. However, N- ) and diamagnetic (N b -). The reactions with oxygen at each state are different and can be presented as follows:

N b -+ O 2 → N b • ‖ + O 2 •- (46) N b • + O 2 → N b + + O 2 •- (47

Mechanism of action of plasmonic NP: 1) Hot electron injection

Regarding aspect of band positions in SC and energy levels of metal NPs, the Fermi level for noble metals is located approx. at 0 V vs. NHE, in case of Au it is slightly more positive (E F,Au = 0.45 V). The energy of plasmonic resonance varies from 1.0 eV to 4.0 eV; therefore, electron transfer from noble nanoparticles to CB of SC is possible.

Association of a SC and Me NPs causing Fermi level energies of Me NPs and SC to equalize and formation of Schottky (φ B ) energy barrier at SC/Me interface (Figure 25B).

Once formed LSPR exists for about 30 fs. After this time the plasmon dephases i.e. the electron oscillations lose their collective nature. In small particles (< 15 nm) this process results in formation of so-called hot electrons having excess of energy corresponding to the plasmon frequency (560 nm or 2.2 eV in the case of Au NPs). Given that the difference between E F,Au and is about 1 eV, the hot electrons are able to overcome the Schottky barrier and can be injected into the conducting band of TiO 2 (Figure 26).

These electrons can then be involved in the photocatalytic reactions.

For example, a degradation of organic pollutants can be presented as follows [207]:

Ag NPs + hv → Ag NPs * (48) Ag NPs * + TiO 2 →Ag NPs •+ + TiO 2 (e -) (49) TiO 2 (e -) + O 2 → TiO 2 + O 2 •- ( 50
) Ag NPs •+ + Cl -→ Ag NPs + Cl 0 (51)
Apart from formation of previously described photocatalytic reactivity of O 2 •-, also Cl 0 can cause degradation of organic pollutant.

Figure 26 Scheme depicting hot electron transfer from plasmonic nanoparticle with excited electron above Fermi level of metal (E F,M ). Only these electrons whose energy can overcome Schottky barrier (φ SB = φ M -χ S )

, where φ M is work function of a metal, χ S is electron affinity of the semiconductor, can be transferred to semiconductor [208].

2) Plasmon induced resonance energy transfer (PIRET)

Due to LSPR the electric field around the nanostructure is strongly amplified (up to 10 6 times) creating so-called hot spots. The energy of this field can be transferred to SC particle through a resonant non-radiative transfer which results in increase of the rate of electron-hole generation. This phenomenon occurs only if the light absorption of SC overlaps with the plasmonic band of the metal. For this reason, for Au NPs (plasmon resonance at ~ 560 nm) the PIRET is observed only when they are associated with Ndoped TiO 2 but not with pure TiO 2 which does not absorb light in visible range (see below). Another important condition for optimal PIRET is the distance between the metal NP and SC. Even if the effect is observed for the metal particles in direct contact with the surface of SC, the enhancement is stronger when two counterparts are separated by 5 nm -7 nm of a dielectric layer (Figure 27).

Preparation of Au nanoparticles on semiconductor surface

Noble metal nanoparticles can be prepared and deposited on semiconductor surface by the following methods: It was found that the energy of Au surface plasmon resonance (SPR) was not high enough to form e -/h + pairs by radiative energy transfer. In case of Ag (inset Figure 29A)

the improved formation of e -/h + in N-TiO 2 was due to strong light scattering on Ag nanocubes. However, it was found that extinction efficiency depends on the size and shape of the particle (Figure 29C). Also local electromagnetic field enhancement was observed on Ag resulting in higher formation of e -/h + in close-contact N-TiO 2 , ultimately resulting in improving water splitting efficiency (Figure 29B). [START_REF] Ingram | Water Splitting on Composite Plasmonic-Metal/Semiconductor Photoelectrodes: Evidence for Selective Plasmon-Induced Formation of Charge Carriers near the Semiconductor Surface[END_REF].

A B Rosseler et al. [224] reported hydrogen production from photoelectrochemical water splitting on Pt-or Au-TiO 2 (anatase/rutile) composites performed in pure water as a hydrogen source, and methanol as a sacrificial additive agent under solar light irradiation. Prepared TiO 2 with 1:1 anatase to rutile ratio was controlled by addition of polyoxyethelene(10)cetyl ether (Brij56) to be also compared with P25 commercial catalyst. It was shown that non-decorated TiO 2 exhibits negligible water splitting efficiency, while in contrast a small decoration of TiO 2 with Pt (wt. 2%) or Au (wt. 2%)

significantly enhanced H 2 production from water splitting, showing also higher efficiency with Au than with Pt (Figure 31). The higher efficiency for Au/TiO 2 than for Pt/TiO 2 was ascribed to almost much higher reversible reaction of water splitting on Pt than on Au, and nature and concentration noble metal NPs on SC and the contact between them. Photocurrent under UV : Pt-Au/TiO 2 : ~ 70 µA cm -2 ; Pt/TiO 2 : ~ 38 µA cm -2 Au/TiO 2 : ~ 20 µA cm -2 ; TiO 2 : ~ 13 µA cm -2 Photocurrent under AM1.5 : Pt-Au/TiO 2 : ~ 2.2 µA cm -2 ; Pt/TiO 2 : ~ 0.3 µA cm -2 Au/TiO 2 : ~ 1.5 µA cm -2 ; TiO 2 : none [START_REF] Melvin | M-Au/TiO 2 (M = Ag, Pd, and Pt) nanophotocatalyst for overall solar water splitting: role of interfaces[END_REF] Au/TiO 2 TiO 2 nanotubes (NT) prepared by anodization of Ti foil, Au by pulse electrodeposition PC: 0.1 M Na 2 SO 4 under 0.6 V vs. SCE, and also PEC: degradation of MO 300 W Xe lamp with ultraviolet filter (λ > 420 nm)

Photocurrent: Au/TiO 2 : 7 µA cm -2 ; TiO 2 : 1 µA cm -2 , MO photoelectrodegradation: Au/TiO 2 : 0.156 h -1 ; TiO 2 : 0.008 h -1

[235]

Properties of semiconductor-based composites

Shape engineering

The advantages of composite material over its bare components might be the increased light absorption in the visible range, and also more effective charge separation due to the presence of the interface. Among many morphological combinations of components, some general types can be distinguished, such as simple blends, one material decorated with another, or core/shell structure (Figure 32). In the first type randomly shaped and sized particles are freely mixed together without well-defined interface; however the simplicity of preparation of such composites is their strong advantage. The second type, also a powder, the composite loaded particles is more sophisticated than the previous example and requires more effort to prepare. In such case, one material is deposited on another one with specific intention of architecture or morphology. The core/shell materials possess developed interfacial contact between two components, which may result in maximizing enhancement effects. Therefore, for the purposes of desired application one can attempt to optimize composition morphology and architecture to achieve the required properties.

Band gap alignment

In general, one can consider three types of heterojunctions in the composites consisted of two different semiconductors (SC x /SC y ), taking into account arrangement of their valence and conduction bands. In the first type (I), E CB of SC 1 is located below E CB of SC 2 and therefore, the photoexcited electrons may be transferred from CB of SC 2 to CB of SC 1 , towards less negative potential energy, while the holes are injected in opposite direction from VB of SC 1 to VB of SC 2 , (Figure 33A). For heterojunction of type II, where CB of SC 3 is below CB of SC 1 , the electrons are moved from SC 1 to SC 3 , and holes in opposite direction (Figure 33B). Type III heterojunction consists of SC 1 /SC 4 ,

A C B
where CB and VB of SC 4 are located inside the band gap of SC 1 , and owing to this, the electron and hole are both transferred from SC 1 to SC 4 (Figure 33C). According to the literature, ZnO/TiO 2 composite can be ascribed to the type I, if (i.e. anatase/rutile TiO 2 is about 0.2 eV) but high enough to improve the electron transfer from anatase to rutile (which is well crystallizedless defective), or the electrons migrate from well crystallized (less defective) rutile to the surface states in more defective anatase. Due to these effects, higher photocatalytic activities are observed for P25 than for each separated material [244,245], and for different weight ratios of anatase/rutile. Similarly to P25, the difference between the band energy alignment of ZnO and TiO 2 -anatase is not significant. However, association of these two oxides (ZnO, TiO 2 ) has been shown to possess very promising photocatalytic properties in various applications, as in the case of widely used commercial P25. nitrogen and/or decorated with Au NPs will be described.

Non-supported ZnO/TiO 2 composites

The examples of the works describing the photocatalytic properties of various unsupported ZnO/TiO 2 composites are given in Table 5. Because of unsupported nature of these materials the photoelectrochemical properties were not studied and only their photocatalytic activities were measured. Analysis of this data shows that despite a widely varying preparation conditions almost in all cases the combination of ZnO and TiO 2 allows to obtain a gain in catalytic activity in degradation of organic species of different nature under UV light. Despite the fact that in some cases the increase is not very significant, these studies show that ZnO/TiO 2 combination is definitely worth being studied in detail.

To better understand the factors influencing the performance of such materials and to gain insight in the mechanism of this enhancement one needs to work with the composites having well-defined microstructure. Growing these materials on conducting support like ITO or FTO allows also to study them by electrochemical methods. That is why many attempts to synthesize such supported composites with well-defined microstructure and to characterize their properties have been done. that the electron transfer was enhanced by formation of branched hierarchical structures.

Also, it was highlighted that the presence of ZnO/TiO 2 heterojunction interface plays an important role in effective exciton dissociation. Hernández et al. [275] have prepared 1D (core)ZnO/TiO 2 (shell) composites on FTO glass electrode. ZnO was grown hydrothermally on FTO-seeded substrate, while TiO 2 was prepared by a sol-gel method. The composite was used in photoelectrochemical water splitting under solar light irradiation and the effectiveness was two and four times higher than those of ZnO and TiO 2 counterparts (Figure 34). To the best of our knowledge, there are only a few reports on ternary ZnO/TiO 2 /Au composites. Recently, Kumar et al. [287] have reported preparation of ZnO/TiO 2 /Au (ZTA) for photocatalytic degradation of organic pollutants. The ZnO nanorods were covered with TiO 2 by sol-gel method and decorated with Au nanoparticles synthesized by citrate-caped method (Figure 37A). The photocatalytic not photoactivated, and finally both effects were taken into account under solar light (UV-vis). For water photo-oxidation at zero bias voltage the highest photocurrents were recorded under visible light for ZnO/TiO 2 /Au due to plasmonic effect (Figure 37D).

However, the transient currents were less stable than those for ZnO/TiO 2 . Shao et al. [288] reported preparation of ZnO/TiO 2 nanofibers decorated with Au nanoparticles for photocatalytic degradation of methyl orange (MO) and 4-nitrophenol TiO 2 /Au, respectively, and correspondingly 60, 68, 79, and 81 under UV irradiation.

Degradation under UV was also confirmed by HPLC measurements. The observed differences between UV and sun light degradation efficiencies were ascribed to higher intensity of UV light that excites directly semiconductors, the presence of Au co-catalyst with active reaction centers for electrons, improved charge separation at Au/SC interface and formation of active oxygen. More effective degradation (by order of magnitude)

under sun light in case of Au-containing in a comparison to their Au-free counterparts was ascribed to the plasmonic effects of Au.

Conclusions and the aim of thesis

Enhancement of photocatalytic properties of ZnO/TiO 2 composites with respect to photoactivity of non-coupled materials is well documented in the literature. In general, this improvement has been ascribed to several reasons, such as more efficient electronhole separation, lower recombination rate of photogenerated charge carriers as well as higher surface area due to porosity of TiO 2 shell. However, these explanations have been often provided without strong support by the kinetic parameters of the interfacial charge transfer processes because most of composites were used in the form suspensions in the solution, excluding application of electrochemical techniques. There are only a few reports on ZnO/TiO 2 composites deposited on the conducting substrates but investigations of a dynamics of electron-hole recombination process in these systems are still missing. Little attention is also devoted to the influence of morphology of the coreshell composite and microstructural properties of the ZnO/TiO 2 interface on photocatalytic performance of the system.

Therefore, the aim of this PhD project was to provide the systematic studies of the ZnO/TiO 2 composites in the form of core/shell nanostructures deposited on the conducting substrate to establish the clear correlation between synthesis conditions of the system, morphology and photocatalytic activity in photodecolorization of methylene blue and water photoelectrochemical oxidation. A particular interest will be focused on the role of the boundary between ZnO and TiO 2 in efficient separation of the photogenerated charge carriers. The dynamics of the electron-hole recombination process and charge transfer at the composite/solution interface will be studied by means of electrochemical methods.

The studies will be also addressed to improvement of the photocatalytic performance of the composites in the visible light by modification of TiO 2 shell by doping with nitrogen and deposition of noble metal nanoparticles on the composite surface. The synergetic effect of these two approaches, postulated for TiO 2 , has not explored in the literature as yet with respect to ZnO/TiO 2 composite. Another novelty will be application of small cathodic bias to utilize the active oxygen species formed during oxygen electroreduction on Au-decorated composites in photocatalytic decolorization of MB. Aqueous solution were prepared with the use of deionized water (DI) (Millipore, 18 MΩ cm) was used. Indium tin oxide (ITO) conducting glass support of a sheet resistance 20 Ω □ -1 was purchased from Prazisions Glass & Optik GmbH (Germany).

Experimental

Synthesis

The ZnO/TiO 2 core/shell composites were prepared in a sequence of consecutive stages described below.

ITO Pre-treatment and electrochemical seeding (preparation stage 1)

In all experiments the glass plates covered with indium tin oxide (ITO) were used as substrates for all ZnO/TiO 2 composites and their further modifications (N-doping, and The ITO substrates of the size approx. 1.5 cm 2 × 3 cm 2 (the surface covered with catalysts was about 1 cm 2 -1.5 cm 2 ) were ultrasonically cleaned and degreased with acetone and then immersed for 10 s in 3 M NaOH, rinsed with distilled H 2 O, etched for 15 s in concentrated H 2 SO 4 and finally rinsed again thoroughly with distilled (DI) water.

According to the literature [292], etching of ITO in acid peels off an ‗initial' layer and exposes a ‗fresh' surface which is more active due to higher surface energy. Moreover, the etching increases the density of hydroxyl groups on the ITO surface and in consequence density of the nucleation centers for nucleation-growth process.

In order to deposit a uniform layer of ZnO nanorods on ITO, the substrate should be

( ) (53)
(54)

( ) → (55)
In such pH conditions the growth is promoted by high concentration of Zn(NH) 4 2+ as presented in Figure 39B. Taking into account the fact that water added to isopropanol to hydrolyze titanium(IV)

butoxide is also involved in hydration of NH 4 + and Cl -ions, the volume of water injected to the reactor mixture was tripled with respect to that added in the ‗standard' procedure described above. Moreover, the synthesis time was shortened to 30 min. Next, the samples were calcined at 450 o C for 1 h as in the case of pristine (non-doped) ZnO/TiO 2 composites. This modified sol-gel procedure was repeated three times and therefore, the obtained core/shell systems with N-doped TiO 2 layer; ZnO/TiO 2 (N), are denoted as ‗3c(N)'.

TiO 2 prepared by ALD and CVD techniques

In

Characterization methods

Electron microscopies (SEM, TEM) and elemental analysis of the samples (EDX)

Scanning electron microscopy (SEM) was used to examine the surface morphology of the samples. In general, electrons generated in a emitter impacts are accelerated by electromagnetic field and impacts the sample emitting various types of electrons depending on the penetration depth (R), which can be expressed by equation [297,298]:

( ) (58)
where, A is mass number (g mole -1 ), E 0 is energy of accelerated electrons that impacts the sample (keV), is density of the material (g cm -3 ), and Z is atomic number. After impacting the surface of material, the secondary electrons (SE) are formed depending on the penetration depth (see scheme Transmission electron microscopy (TEM) allows to observe internal structure of the samples, also with a high resolution mode (HR-TEM). Resolution of TEM images (δ) depends on electron acceleration beam voltage (V), accordingly to the equation:

√ (59)
where (n sin α) is numerical aperture, m 0 is the rest mass of electron, and e is the electron charge. Also, to observe thicker sample higher electron energy (eV) is necessary.

Samples absorb and scatter the electrons and in general elastically scattered electrons are used for imaging.

Often TEM can operate in two modes: bright-field (BF) and dark-field (DF)

imaging. The sample is placed in the line of optical axis of electron beam and detector. If electrons that passed through a sample and reached detector placed in line of initial beam, the image that is created is in BF mode. In case, when scattered electrons were Observations under high resolution regime allow to use angular detector, which collects inelastically scattered electrons. The observed difference in image contrast is due to different electron scattering, and this depends on atomic number Z, so-called Z-contrast.

Most efficient contrast differences for elements are observed when transmitted electrons that left the specimen are collected at high angles with respect to the optic axis (highangle annular dark field detection (HAADF). Such mode is one of possibilities for signal collection in STEM. Others includes: (1) bright field detection (STEM-BF) that allow to collect transmitted electrons leaving the specimen at low angles with respect to the optic axis, (2) EELSelectron energy loss spectroscopy, where the spectrum is plotted as function of number electrons that left the specimen with a loss of energy due to interaction vs. that lost energy. Also the (3) X-ray energy dispersion spectroscopy (EDX) is enabled during STEM observation that include mapping of elements, line scan and point analysis.

EDX was used to provide information of elemental composition, atomic distribution (mapping), and semi-quantitative analysis. In general, atoms emit characteristic X-rays, which can be related to Moseley's law:

( ) (60)
where, v is the frequency of emitted X-ray line, α and β are line constants, and Z is atomic number of an element. Electrons emit lines accordingly to their donation of filling the vacancy in shell orbitals around the atom nucleus, i.e. Kα, Kβ, Lα. In EDS energy dispersion spectra these lines correspond to the peak energy positions usually within the range of 1 keV to 20 keV. Area under the peak can give information about amount of the element in a sample.

In this work, internal microstructures were studied by means of high resolution (HR-TEM), while high-angle annular dark-field scanning electron microscopy (HAADF-STEM) was performed on JEOL JEM-2100F working at 200 kV. The samples prepared for TEM characterization were detached from the ITO support by scratching with prior dropping of ethanol on the surface. Then a drop (ethanol and removed samples) was placed at copper mesh grid.

Optical properties

In order to investigate optical absorption properties of the samples within the range of 350 -700 nm, the Shimadzu UV-3600 spectrometer equipped with an integration sphere was used. The UV-vis spectra were used to determine the optical band gap energy, E g , by means of the Tauc equation:

[304] ( ) (61)
where is the photon energy, is the frequency-independent constant, and exponent depends on the nature of transition in the semiconductor, i.e., = ½ for allowed direct and = 2 for allowed indirect transition. Regardless of some controversy, it has been found that ZnO is an n-type direct band gap semiconductor, [305] while anatase-TiO 2 is a material with the indirect transition [306,307,308]. Optical band gap energies were determined from the plot of ( ) , from intersection of the tangent line with the horizontal axis.

Such spectral methodology approach has some limitations in a regard to the samples.

The equation is applicable to layered films. In case of non-flat layered samples the scattered and reflected light should be also taken into considerations. Such problem is solved in spectrophotometers equipped with integration sphere attachment that allow to collect the reflected flux. Integration sphere attachment is coated from the inside usually with polytetrafluoroethylene (PTFE) or barium sulfate (BaSO 4 ) which diffuse reflectance is close to 1 [309], and geometry set-up configurations with sample being illuminated with diffused light ‗illuminated geometry' (Figure 41A) or with light in-axis to the sample in angle < 10 o ‗view geometry' (Figure 41B). In both cases baffles are placed between the sample and ‗illuminated' or ‗view' geometry to avoid superposing the reflected light.

Figure 41 Geometric set-up configurations (‗illuminated' and ‗viewed', A and B, respectively) of integration sphere attachment used in optical band gap determination from UV-vis spectra [309].

Tauc theory is sufficient for optical band gap determination, and examples are found for bare materials [310], and also for composites [311]. In the shape of (αhv) 1/n vs. hv one can observe inclination of the curve which can be correlated with the band-to-band transitions between CB and VB in the SC.

XRD

Crystallinity of the samples was studied using X-ray diffraction (XRD). The X-ray beam arrives at the sample's surface under an θ angle and are scattered to leave the sample at the same angle. By analysis of the peak width in XRD pattern, the crystal size can be determined using the Scherrer relationship:

(62

)
where L is an average particle size, K is the Sherrer constant (0.9) and β is peak width at half height. For thin layers deposited on the solid substrate the grazing-angle XRD (GAXRD) technique was used in order to obtain the information about the surface layer of the composites. GAXRD measurements were performed with diffractometer Bruker D8-A25 Discover equipped with a LYNXEYE XE detector.

XPS

X-ray Photoelectron spectroscopy (XPS) was applied in order to determinate atomic states and surface composition of samples by usage of apparatus PHI 5000Versaprobe

A B

with mono-chromatic Al Kα1 X-Rays. In short, the analysis is based on energy exchange between photons from the beam with electrons in atoms of specimen. Then excited electron may escape from the atom and these emitted electrons possess certain kinetic energy accordingly to the equation [312,313]:

hv = E f -E i + E' k (63)
Where, hv is energy of incident electron, E f is the total energy of the final state of the ion, E i is the initial state of the atom, and E' k is photoelectron kinetic energy. All elements within a certain chemical compound, except hydrogen, emit characteristic core photoelectrons. For a certain element (atom) in a chemical compound the binding energies of atom's core level is different because of other surrounding atoms, which is a base for XPS analysis of the compound composition. Therefore qualitative analysis is based on peak position at certain binding energy of photoelectrons from these atoms (elements), while (relative) quantitative analysis is based on the calculation of the area under this peak. The analysis takes place under low pressure conditions, usually lower then 10 -7 torr. The 95% of the signal collected is from less than 10 nm thick layer, making XPS analysis strongly surface adapted.

Photoluminescence

Once the material absorb the light enters an electronic excitation state. Then, a process of spontaneous emission of photon may occur in order to release absorbed energy by the material, called photoluminescence (PL). In such method a sample is illuminated with excitation wavelength and the emitted light is analyzed. The information provided by PL of solid surfaces concerns its nature and modification states [314], as well as the electronic energy levels present in a sample are the information that one can obtain by analyzing PL emission spectra. The photoluminescence (PL) emission spectra of the samples were recorded using Fluorolog-3 spectrofluorometer (Horiba Scientific) with a 400 nm excitation wavelength at room temperature. analysis by infrared detector. Area under the peak of detected CO 2 is correlated with the amount of carbon in the analyzed solution.

The measurements were performed in co-operation with Mr. M. Sulwiński from the Faculty of Biology, University of Warsaw.

Linear sweep voltammetry, cyclic voltammetry, chronoamperometry

(Photo)electrochemical measurements were performed with the use of potentiostat Autolab (Metrohm B. V., The Netherlands) in three-electrode cell, with a composite sample as the working electrode (WE) (approx. 1 cm 2 geometrical working surface area), Ag/AgCl/Cl -(3 M KCl) as a reference electrode (RE), and Pt wire as a counter electrode (CE). To provide electrical conductivity in the solution an electrolyte should be present, i.e. Na 2 SO 4 (in this studies 5 mM).

However, when redox reactions occur under certain potential, the Faradaic current is observed. In cyclic voltammetry (CV) and linear sweep voltammetry (LSV) an external potential applied to the WE is changed linearly in time with the scan rate , usually from the range 20 mV s -1 -100 mV s -1 . In CV after reaching the final potential E f , the polarization direction is reversed to arrive back to the initial value E i , as illustrated in If needed, the solutions were deareated by purging with argon for 30 min.

Chronoamperometry is a technique where constant potential is applied to the working electrode and current flow is observed over time. Integration of surface below the I(t) curve corresponds to the charge passed involved in the process. In such method the solution is not stirred so the transport of electroactive species to the electrode occurs only by diffusion. Therefore, the current is a function of time according to Cottrell equation:

(64)
where, n is number of electrons involved in the process, F is Faraday's constant, A is geometrical working area of electrode, D is diffusion coefficient of electroactive species, c 0 is nominal concentration of electroactive substance in the bulk of solution, and t is time of applied bias. Also, worth to mention is that charging of double layer takes place within milliseconds.

Photocurrent and open circuit potential

The fundamentals of photocurrent and open circuit potential measurements were described in detail in previous paragraph 2. 1.4. 450W arc xenon lamp (Oriel)) equipped with a 400 nm cut-off filter. In order to perform tests with monochromatic light, an originally designed light emitting diode-based device was assembled (see the scheme and a photo in Figure 4A, and 43C). The core of the device relies on a high power emitter (400 nm) with a narrow relative spectral power distribution. LEDs were provided by LED Engin (USA). In case, when tests were performed using visible light the distance was adjusted to keep the same intensity. The optical light irradiance of the LEDs was measured with the use of an IL1700

International Light (USA) radiometer. The LED illumination device allowed to maintain a constant temperature of 24.6 o C in aqueous solutions in the cuvette at a distance of 1 cm from the emitters (Figure 43B). The samples were placed on the rear wall of the cuvette at a distance of 2 cm from the LED light source. MB negligibly absorb light between 350 nm and 450 nm, and molar extinction coefficient of ε(664 nm) = 7402.8 m 2 mol -1 in aqueous solution of 10 µM, which corresponds to absorption A' λ,664nm = 0.74, while for 20 µM is A‖ λ,664nm = 1.48

[317]. According to the ISO directive a tested sample geometrical area should be between (100 ± 1) mm 2 and (1500 ± 15) mm 2 . The absorption spectra of MB solution were recorded in interval periods no longer than 20 min (usually 10 min or 15 min). For this, the quartz cuvette (with all transparent walls) was placed directly in spectrometer chamber.

Prior to PC tests, the samples were conditioned in dark in 20 µM MB aqueous solutions for 24 h in order to avoid a concentration drop due to adsorption of MB during the main PC test. UV-vis spectra of MB solutions were collected using Shimadzu UV-3600 spectrometer and the change of MB concentration was determined from decrease of Thus, the kinetic data obtained under illumination were analyzed by plotting the natural logarithm of concentration ratio (c/c 0 ) as a function of time, and an apparent first-order rate constants (k 400 ) were determined from the slope of this linear plot. An additional source of OH -ions is needed for the precipitation of Zn(OH) 2 and may be provided by electrochemical reduction of molecular oxygen present in the solution, according to the scheme: Ag/AgCl, which can be explained by above mentioned reduction of dissolved oxygen (reaction scheme 67). In contrast, in the deareated solution (red curve -2) no current was observed until -1.1 V. In the range of potentials more negative than -1.1 V, reduction of Zn 2+ and deposition of metallic Zn occurs ( = -0.76 V, Zn 2+ /Zn (~ -1.0 V vs.

Results and Discussion

Growth of ZnO nanorods on ITO-covered plates

O 2 + 2H 2 O + 4 e -→ 4 OH -E 0 = 0.4 V vs. NHE (
Ag/AgCl). During the reverse scan the metallic Zn is reoxidized to Zn 2+ ions giving rise to the anodic oxidation peak at about -0.9 V. The charge balance between the reduction and oxidation charges (curve 2, inset, Figure 44A) obtained in deaerated solution indicates that Zn deposition/dissolution process is the only process occurring on the electrode. In contrast, when the deposition was performed in non-dearated solution the charge involved in reoxidation was only 55% of the reduction charge (curve 1, inset, Since the pH of solution used for the electrodeposition was 6.2 the reduction of oxygen at the ITO electrode probably resulted in a local increase of pH above this threshold value, leading to precipitation of native amorphous zinc oxide. In order to keep the preparation procedure simple, the potentiostatic ITO seeding was limited to nondearated solution conditions. Therefore, the Zn nanosheets can occur less probably at the close neighboring to the well-developed crystals than in a further distance from them. So the crystals are likely to be formed from Zn smaller structures. In order to form the well-developed Zn-crystal seed layer it was found that at least the sufficient charge density (≥ 70 mC cm -2 ) should pass. On the other hand, high charge density (≥ 280 mC cm -2 ) results in formation of overgrown Zn crystals eventually leading to their fusion into big chain-crystal structures (Figure 46D). Therefore, the current density of about 140 mC cm -2 seems to be the most reported elsewhere [START_REF] Gao | Metal/Semiconductor Core/Shell Nanodisks and Nanotubes[END_REF][START_REF] Liu | A low-temperature synthesis of ultraviolet-light-emitting ZnO nanotubes and tubular whiskers[END_REF].

In order to confirm the formation of ZnO on the surface of Zn, the samples before and after calcination were examined by XRD. In XRD pattern obtained for non-calcined sample (Figure 47A) one can observe only the signals corresponding to reflections from (002), ( 100), ( 101) and ( 102) planes of hexagonal Zn, respectively at 2θ : 36.4 47B). This peak was not observed in the initial sample due to dominant amount of Zn over ZnO superficial native layers formed in effect of Zn-passivation during sample exposure to air.

Hydrothermal growth of ZnO nanorods

ZnO nanostructures were fabricated via hydrothermal synthesis at 80 o C from aqueous solution containing Zn(NO 3 ) 2 with ammonia added to fix pH at 10.6. The synthesis products were ‗star-like' ZnO structures formed in the bulk of solution (Figure 48A) and epitaxially grown ‗obelisk-like' ZnO nanostructures on Zn/ZnO seeds (Figure 48B). It is worth to note that the seeding stage is essential for the immobilization of the ZnO nanostructures on the surface, since a simple rinsing with a stream of water removes all bulk-synthetized forms of ZnO structures from the plates.

Figure 48 SEM images of ZnO ‗star-like' structures formed in the bulk of solution (A), and SEM image of the edge between ZnO nanorods grown on Zn/ZnO seeded (1) and non-seeded (2) parts of the ITO plate (B).

The next task was to determine how the synthesis conditions, such as concentration of Zn 2+ ions and reaction time influence on diameter and lengths of nanorods grown on

A B A B 1 2
the seeded substrate. This is the key issue because the ZnO nanorods should be further covered with layers of TiO 2 and finally decorated with Au nanoparticles. Thus, the distances between the ZnO nanorods should be large enough for these further modifications. The exemplary results are compared in Figure 49. As shown in Figure 49A, when initial concentration of Zn 2+ was 0.01 M, the nanorods were very thin and their tips had tendency to coalesce likely due to high surface energy. In contrast, the space between nanorods obtained at Zn 2+ concentration 0.08 M is not sufficient for further modification (Figure 49B). It was found that the optimal results were obtained after 2.5 h of hydrothermal synthesis from the solution of 0.04 M Zn(NO 3 ) 2 . The average length of the nanorods after 2.5 h of synthesis was 1.9 µm, whereas the width varied from about 100 nm (at the top excluding sharp tip ending) to about 300 nm at the bottom of nanorods as they resembles ‗obelisklike' structures. The hexagonal ZnO nanorods with smooth sides (inset in Figure 50A)

grown on the Zn/ZnO nanosheets structures were arranged in a uniform fashion (Figure 50C), whereas those grown on the Zn/ZnO crystals formed ‗urchin-like' structures as illustrated in Figures 50B and 50D

Figure 50 SEM images of ZnO nanorods obtained by hydrothermal method: top view of a large surface area (A) with a high-resolution micrograph of tip of a single ZnO nanorod (inset), urchin-like Zn/ZnO structure (B), and cross section views (B, C) seen in lower secondary and low-angle backscattered electron projection (LABE). XRD pattern of ZnO nanorods grown on Zn seeded ITO plate (E).

Since the urchin-like structures are formed randomly having also different size, the thickness of the resultant catalyst is not uniform. However, such urchin-like ZnO nanostructures may be advantageous for catalytic or sensor applications where large area of ZnO surface and easy access of the reagents to the nanorods are required. To our knowledge, only Cheng et al. [334] obtained similar electrode composed of urchin-like

Zn/ZnO structures deposited on ITO by using complicated thermal vaporization techniques. It has been reported that urchin-like ITO/Zn/ZnO electrodes shown almost three times higher photocurrent than the electrodes of ZnO nanorods array in electro- Figure 51a shows the simplified scheme of the elaborated procedure, whereas the SEM images presented in Figure 51b depicts the results obtained in the consecutive preparation stages of ITO/Zn/ZnO(nanorods) composites.

Influence of the microstructure of TiO2 layer on the properties of

ZnO/TiO 2 composites

Developing and characterization of core/shell ZnO/TiO 2 composites

As it was demonstrated in Chapter 4.1, the optimized synthesis conditions resulted in preparation of obelisk-like ZnO nanostructures with sharp tips and smooth side walls.

These ZnO nanostructures served as a core material for coverage with TiO 2 shell. At first, it was intended to develop a simple, relatively rapid and reproducible procedure that can result in well-defined core/shell ZnO/TiO 2 composites with separated nanorods array. Therefore, in order to cover the ZnO nanorods with a thin layer of TiO 2 , a sol-gel method was chosen. The attempts were undertaken to determine the influence of some procedure parameters on morphology of the resultant TiO 2 layer i.e. deposition time, molar and volume ratios of all synthesis precursors or even injection rate of water necessary to initiate the hydrolysis of titanium precursor. Titanium butoxide was chosen as an inexpensive TiO 2 precursor, while isopropanol was selected as a water-free reaction medium. The SEM images presented in (Figure 52) show morphology of samples prepared by sol-gel method when synthesis conditions were not well Further investigations were carried out in order to optimize deposition procedure to cover the ZnO nanorods with thin layer of TiO 2 . Ultimately, the optimization resulted in preparation of two different, most distinctive morphologies of TiO 2 layers. In the first case, precipitation/deposition of TiO 2 was carried out for 6 h in a one deposition cycle, and so the ZnO/TiO 2 composite was denoted as ‗1c'. The second type of ZnO/TiO 2 sample was prepared in three consecutive cycles, however, each the one lasting 30 min (‗3c'); for more details of the preparation procedure, see section 'TiO 2 Sol-Gel Deposition' in the Experimental part.

Since, as-deposited TiO 2 via sol-gel method is amorphous and in this form should have negligible photocatalytic properties [START_REF] Ohtani | Photocatalytic Activity of Amorphous-Anatase Mixture of Titanium(IV) Oxide Particles Suspended in Aqueous Solutions[END_REF], it was necessary to convert it into anatase by calcination. On the other hand, the temperature of calcination should be chosen with caution because it is an important factor influencing material properties. Taking into account complexity of designed ITO/ZnO/TiO 2 material one shall bear in mind the limitations of its components, i.e. 600 o C and above, can strongly reduce conductivity of indium tin oxide in ITO [336,337], which may finally result in lower electroactivity when the composite is used as an electrode. The materials prepared by sol-gel method are known to contain organic residues and the amorphous-to-anatase phase transformation temperature strongly depends on the size of TiO 2 nanoparticles and synthesis conditions. For this reason, TGA-DSC was used to determine the temperature at which these residues are completely eliminated and on the other hand, TiO 2 is transformed into anatase.

First, pure TiO 2 formed in bulk of the solution during Ti(OC 4 H 9 ) 4 hydrolysis was characterized (Figure 53A). Analysis of the weight changes (red curve) and corresponding thermal effects (black curve) allowed to distinguish the following stages.

The weight loss observed below 100 °C, accompanied by an endothermic broad peak, can be attributed to the dehydroxylation of the formed titanium (oxo)hydroxide. Further increase of the temperature to 200 o C -300 °C results in an additional weight loss coinciding with a significant exothermic effect. This combination indicates clearly a burning out of the organic species occluded in the sample. The last process occurring at 400 °C results in a sharp heat release without any noticeable weight variation which allows to assign it to crystallization of anatase. The amount of the TiO 2 deposited on ZnO nanorods in a single sample was insufficient for correct TGA-DSC measurements. Therefore, the analysis was done on the material gathered from several identically prepared 3c samples so the weight of collected material was sufficient (ca. 10 mg). The TGA-DSC profiles were also recorded for bare ZnO nanorods (Figure 53B, dotted curves) and this material was used as the reference one for the ZnO/TiO 2 system (Figure 53B, solid lines).The result obtained for bare ZnO is in accordance with general behavior of materials without phase transition. Although the peaks for ZnO/TiO 2 sample are less intense and broader than those for pure TiO 2 (Figure 53A), the same trends can be distinguished in the obtained TGA-DSC profiles. Below 100 °C the weight loss is endothermic and thus it can be attributed to the dehydroxylation of the amorphous titanium hydroxide. At higher temperatures the exothermic peaks are observed which are due to burning out of organic residues and crystallization of anatase phase. The broadening of the exothermic peaks in the case of ZnO/TiO 2 can be explained by its specific preparation procedure. Indeed, three layers produced in different steps can have slightly different thermal behavior stemming possibly from their varying arrangement relatively to ZnO nanorods. Such heterogeneity should naturally result in broader peaks than those for the non-supported solid prepared in a single step. The TGA-DSC data show that the calcination temperature of 450 °C is sufficient for burning out the organic residues. It follows also that at this temperature the deposited amorphous layer is transformed into anatase with a high rate as suggested by the sharpness of the corresponding exothermic peak. This finding confirms that the chosen calcination time (1 h) allows a complete conversion of the amorphous titanium (oxo)hydroxide into a crystalline anatase. Also it should be noted that the initial layers formed on ZnO nanorods consisted of hydrated amorphous titanium dioxide in the form of titanium hydroxides, TiO(OH) 2 or Ti(OH) 4 . After calcination of the samples at 450 °C for 1 h the surface of the nanorods became rough due to shrinking of deposit in effect of removal of water. For these reasons SEM images of both 1c and 3c composites after calcination were collected.

As it is shown in SEM image (Figure 54A) the ZnO surface at low magnification is not much changed in both ZnO/TiO 2 composites when compared to initial, raw ZnO samples (Chapter 4.1, Figure 50). However, at higher magnifications one can observe that the surface of ZnO nanorods has been significantly modified (Figures 54B, and54C). From comparison of the pictures taken at higher magnification it is evident that the surface of ZnO nanorods is covered with the layer of deposit. It is worth noting that in both cases the surface is free of TiO 2 particle aggregates or clusters.

Another fact should be also noted hereat higher magnifications it is visible that the surface of 1c sample is more corrugated and rough (Figure 54A) than that of 3c composite (Figure 54C). The SEM pictures presented in Although the layer is composed of three thinner layers formed in separate deposition cycles, one cannot distinguish the frontiers between them. This means that nanoparticles formed in subsequent deposition cycles filled the gaps formed after calcination of the previous layer. It is also interesting to note that at high magnification one can see that the boundary between ZnO and TiO 2 is not smooth but some brighter pits are located in ZnO nanorods. These empty spaces could be formed according to the mechanism analogous to void formation in the Kirkendall effect [338], consisted in partial diffusion of elements during calcination. The detailed discussion of this phenomenon will be presented in Chapter 4.3. As to the procedure itself, we can conclude that it matched the prerequisite of a good reproducibility. The summary of the schemes of sol-gel deposition of TiO 2 on ZnO nanorods is presented in Figure 56. The spectroscopic tests (Figure 57A) proved that the decrease of methylene blue (MB) concentration in aqueous solutions, due to dye adsorption on the composites, was greater for 1c sample than for 3c one, whereas adsorption on ZnO was negligible. The mass of ZnO nanostructures measured with the sensitive microbalance was 1.15 mg per 1 cm 2 of seeded ITO substrate (geometrical size of the sample). The results of EDX analysis (Ti/Zn ratio) allow to estimate that the amount of TiO 2 deposited on ZnO nanorods in 1c procedure was about 0.11 mg per 1 cm 2 , while for 3c was almost three times higher, 0.28 mg cm -2 .

The isoelectric point of TiO 2 may vary [339,340] (101), ( 004), ( 200), ( 105), ( 211), ( 204), ( 116), ( 220), ( 215 The results of the quantitative analysis of relative amount of Ti and Zn in different samples are presented in Table 9 along with EDX-SEM results. It follows from the EDX results that the increased number of sol-gel cycles (from one to three) allows to deposit higher amount of TiO 2 , despite shortening of each cycle 6 h to 30 min. The total Ti/Zn ratio increased from 0.10 in 1c samples to 0.27 in 3c composites. In contrast, the XPS measurements deliver information about surface Ti/Zn ratio and allow to estimate the efficiency of the coverage of ZnO with TiO 2 layer. The data in Table 9 show that the trend is similar to that observed for the total amount of TiO 2 . In this regard, the multiplication of cycle numbers brings a considerable increase of Ti/Zn ratio in the surface layer. where, is the intensity of the beam reaching detector, expresses the intensity of initial beam, and Δθ is the deviation of the angle from 45 o . The basic scheme of application of the equation is illustrated as inset in Figure 60.

The estimation for the flat ZnO surface covered with a continuous layer of crystalline TiO 2 -anatase, showed that Zn atoms should not be seen in XPS if the thickness of TiO 2 layer exceeds 4 nm (Figure 60). The thickness of the shell formed in the 3c sample (ca. 40 nm, see Figure 55) is however much larger than the calculated limit. Assuming that this TiO 2 shell is continuous, the presence of Zn signal in XPS spectra can be explained only if one admits that TiO 2 layer is porous. In this case, the effective density of the layer will be much lower than that of crystalline TiO 2 anatase and the photoelectrons emitted by Zn atoms will be able to reach the external surface of the particles. The presence of an intense signal of Zn in the XPS spectra of ZnO/TiO 2 is thus a clear indication that the produced TiO 2 anatase shell is porous.

In order, to characterize optical properties of the ZnO/TiO 2 composites, UV-vis absorbance spectra of the 3c and 1c samples are compared in Figure 61 with the results obtained for bare ZnO nanorods and TiO 2 powder obtained via sol-gel method. As visible, the absorption edges for single oxides are at about 400 nm for ZnO or even slightly lower ca. 390 nm for TiO 2 . In contrast, the composite ZnO/TiO 2 samples demonstrated noticeably red-shifted absorption onset (to ~ 450 nm). The spectra of the composites reveal also a pronounced low-energy absorption tail (‗Urbach tail') increasing from about 550 nm, which may be a result of appearance of new interface energy states in the band gaps of ZnO and TiO 2 . Since, these new interface levels can be localized closely to the conduction bands, excitation to them may be probable [START_REF] Hoffmann | Environmental Applications of Semiconductor Photocatalysis[END_REF].

Figure 61 Absorbance spectra of 3c (blue curve, 1) and 1c ZnO/TiO 2 systems (red curve, 2); the orange curve (3) to ZnO sample, whereas the black curve ( 4) corresponds to the spectrum of TiO 2 powder obtained by sol-gel method and calcined at 450 o C (A). Inset A represents the plots of ( ) as a function of photon energy ( ) for direct (n = ½) and indirect (n = 2) transitions in semiconductors. The ( ) plots were calculated from the absorbance spectra. ( ) as a function of photon energy (hv) for determination of optical energy band gaps of pristine ZnO and TiO 2 powder (B).

Since the TiO 2 is an external layer thus, it was assumed in calculations and plotting that the exponent in the component materials is = 2. However, concerning morphology and thickness of the TiO 2 layers one should compare both extremes (n = ½, n = 2) as it has been reasonably presented elsewhere for ZnFe 2 O 4 /TiO 2 composite systems [350]. As depicted in the inset in Figure 61A, the lowest value of optical energy band gap was found for 3c ZnO/TiO 2 composite (~ 2.78 eV) and a little higher (~ 2.86 eV) was obtained for the 1c ZnO/TiO 2 (Table 9). And yet, each system showed lower value of optical energy band gap than any particular separate bare material (ZnO (nanorods): 3.05 eV and TiO 2 (powder): 2.95 eV, Figure 59B). At this point it is also worth noting that zinc titanates of various Ti-to-Zn atomic ratios had their absorption edges in deep UV range (< 380 nm) and optical energy band gap values varied from 3.32 to 3.70 eV [351,352]. Thus, the optical behavior of the ZnO/TiO 2 systems can be ascribed to appearance at ZnO/TiO 2 interface of new states whose energy levels are localized inside the band gap. While formation of such states was previously postulated in ZnO/TiO 2 systems, their physical nature has not been elucidated up to date. In our case, the high concentration of the vacancies is evidenced by the formation of voids (see Figure 55C) which are in fact the clusters of such vacancies [353]. The energy levels corresponding to the vacancies in ZnO are localized inside the band gap [354] giving rise to sub-band gap optical transitions in the visible range. Also in such energy states the electrons' lifetime can be effectively prolonged, hindering the recombination [START_REF] Linsebigler | Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results[END_REF][START_REF] Navarro | Photocatalytic hydrogen evolution from CdS-ZnO-CdO systems under visible light irradiation: Effect of thermal treatment and presence of Pt and Ru cocatalysts[END_REF]. It would be therefore interesting to characterize the influence of these new interface energy states on photocatalytic efficiency, to do that the monochromatic irradiation (400 nm) emitted by LED was chosen.

Photocatalytic tests

In order to evaluate photocatalytic activity of the ZnO/TiO 2 composites, photodecolorization of a model pollutant (methylene blue, MB) was investigated according to the International Standard Organization procedure [87]. By choosing the monochromatic light of 400 nm wavelength photosensitization of MB was eliminated [START_REF] Mills | Photobleaching of methylene blue sensitised by TiO 2 : an ambiguous system?[END_REF]. The mechanism of MB photodegradation by semiconductor materials in aqueous solutions can be explained by formation of oxidizing agents and proceeds through the initial stage which is decolorization (see Bibliography paragraph 2.1.19 ‗MB photocatalytic degradation'). The decolorization rate of 10 µM MB aqueous solution under illumination with the LED of the wavelength 400 nm was determined from decrease of absorbance maximum intensity in UV-vis spectrum of the solution at the wavelength of 664 nm (Figures 62A and62B). In order to avoid the decrease of MB concentration in the test solution due to adsorption of the dye on TiO 2 , all samples prior to exact photocatalytic measurements were held for 24 h in the 20 µM MB solution to attain an adsorption-desorption equilibrium. Together with the results obtained for illuminated samples, the data for ‗dark' samples were collected as a reference. Also, to prove a complete mineralization of MB the total organic carbon (TOC) content was measured in the MB solutions before and after 120 min of illumination for 3c and 1c

samples and the results were compared with decolorization percentage (Figure 62D).

According to the band gap values listed in Table 9, the photon energy corresponding to the wavelength 400 nm (3.1 eV) is high enough to excite the electrons from valence to conduction band of ZnO nanorods and ZnO/TiO 2 systems. However, the photocatalytic experiments shown that the performance of ZnO/TiO 2 systems is better than that of bare ZnO nanorods (Figure 62B). Moreover, the apparent first-order constant rate obtained for 1c sample (k 400 = 1.2

•10 -2 min -1 ) is almost two times higher than that of 3c sample (k 400 = 0.7 •10 -2 min -1 ) and even six times higher than that of the bare ZnO (k 400 = 0.2 •10 -2 min -1 ). A small value of the rate constant for ZnO is in accordance with very low absorbance at 400 nm observed in UV-vis spectrum of ZnO (Figure 61). Also, the activity of 3c used without any calcinations at 450 o C was k 400 = 0.4 •10 -2 min -1 (not included in Figure 62), which supports the importance of amorphous-to-anatase TiO 2 phase transformation. Alongside MB decolorization tests, the total organic carbon (TOC) analyses were done for 1c and 3c samples (Figure 62D) in order to confirm degradation of the organic pollutant. According to the literature [START_REF] Houas | Photocatalytic degradation pathway of methylene blue in water[END_REF], MB photodegradation proceeds through chain reaction stages to final simple inorganic species such as NH 4 + , NO 3 -, SO 4 2-, CO 2 . The TOC measurements of MB solutions before and after 120 min irradiation under 400 nm in the presence of the ZnO/TiO 2 composites showed that indeed MB is mineralized and organic carbon removal was greater for 1c (45% removal) than for 3c (31% removal). It is worth noting that in both cases the percentage of MB decolorization was higher than the TOC removal. Since the decolorization process is the first stage of a dye degradation, the differences are justified and consistent with the data reported also for other dye decomposition e.g. methyl orange (MO) [START_REF] Baiocchi | Characterization of methyl orange and its photocatalytic degradation products by HPLC/UV-VIS diode array and atmospheric pressure ionization quadrupole ion trap mass spectrometry[END_REF].

The increase of absorbance in visible range after deposition of TiO 2 on ZnO may be a result of appearance of new interface energy levels as it was discussed in previous section. On the other hand, according to the literature, a lower edge of conduction band (CB) in ZnO lies a little above the CB of TiO 2 and therefore, the photogenerated electrons are transferred from CB of ZnO to CB of TiO 2 , whereas the holes from VB of TiO 2 to VB of ZnO. Thus, the formation of ZnO/TiO 2 interface may lead not only to lowering excitation energy but also to better separation of charge carriers preventing fast charge carriers recombination.

In order to prove a significance of the hole-dependent dye decolorization mechanism rather than that via photoexcited electrons, the similar catalytic tests were performed with addition of methanol, known as an efficient scavenger of the photogenerated holes [START_REF] Prairie | An investigation of titanium dioxide photocatalysis for the treatment of water contaminated with metals and organic chemicals[END_REF][START_REF] Valentin | Hole Scavenging by Organic Adsorbates on the TiO2 Surface: A DFT Model Study[END_REF]. The results showed that photocatalytic activities of all samples decreased substantially (see comparison of the results in Figure 62C). The most notable drop of the rate constant value by factor 4 (from 1.2 •10 -2 min -1 without to 0.30

•10 -2 min -1 in the presence of methanol) was recorded for 1c sample. The rate constants of the process performed with two other samples decreased twice, namely to k MetOH = 0.36 •10 -2 min -1 for the 3c sample and to k MetOH = 0.08 •10 -2 min -1 for ZnO. These results support the assumption that MB decomposition mechanism can be associated mainly with the photogenerated holes and formation of • OH radicals rather than with photoexcited electrons and O 2 •-.

As it was presented in SEM and TEM micrographs (Figures 54 and55) the TiO 2 layers in two composite samples differ significantly in morphology. The TiO 2 layer in 1c

sample is rugged and porous, while that in 3c is smooth and compact. Therefore, the interface between ZnO and TiO 2 in 1c is more easily accessible for reacting species (H 2 O, O 2 and MB) in comparison to situation in 3c. Thus, it explains higher photocatalytic activity of 1c sample. At this point; it is worth to recall that MB adsorption (during conditioning) was 3.5 times higher on 1c than on 3c, which may also contribute to higher efficiency during photocatalytic test.

Photoelectrochemical tests

Photoelectrochemical properties of the composites were firstly studied in aqueous solutions, namely, in the reaction of water photo-oxidation expressed by the following equation:

2H 2 O -4e → O 2 + 4H + (69)
The mechanism of the process is still a subject of intensive studies and discussions in the literature [361]. Nonetheless, the anodic behavior of the semiconductor electrode in aqueous solution under illumination is often used as a test of photoelectrocatalytic properties of semiconductor materials. Therefore, 1c and 3c samples were studied at a constant positive potential of 0.8 V vs. Ag/AgCl in the solution of pH 6.8 containing 5 mM Na 2 SO 4 in dark and under illumination with light of the wavelength 400 nm. As visible in Figure 63, in both cases the steady state photocurrent was achieved just after switching the light on but its value was almost six-times higher for the sample 1c than that for 3c electrode (0.75 mA cm -2 and 0.12 mA cm -2 , respectively). This difference may be ascribed to more extended surface area of TiO 2 deposited in 1c procedure and probably higher amount of surface bound hydroxyl groups (Ti-OH) or/and surface bound water molecules (Ti-OH 2 ) which may be oxidized by the photogenerated holes. In order to gain more insight into electrochemical behavior of the materials studied, ZnO nanorods and ZnO/TiO 2 systems deposited on ITO were tested by means of cyclic voltammetry in the solution of 0.1 M LiClO 4 containing ferrocene (Fc) as a redox probe.

The measurements were performed in dark and under illumination. The cyclic voltammograms obtained in all systems studied are typical of n-type semiconductor in the redox electrolyte [362,363]. Namely, the dark oxidation currents at E > 0 V vs.

Ag/AgCl are very small because of the very low population of the minority carriers (holes) at the semiconductor/solution interface. On the other hand, the reduction process with participation of the majority charge carriers (electrons) can proceed and cathodic current is dependent on the potential and concentration of electroactive species at the electrode. As visible in Figure 64, the cathodic and anodic dark currents obtained for ITO/ZnO/TiO 2 electrodes are noticeably higher than those recorded for bare ITO/ZnO.

This may be ascribed to increase of the electrode/solution interfacial area after deposition of TiO 2 . A slightly higher current density obtained for 1c than that for 3c ZnO/TiO 2 system (c.f. lines 1 in Figures 64B and 64C Comparison of the plots obtained for the three systems studied under illumination with xenon lamp shows that the oxidation current density is practically the same for 1c and 3c ITO/ZnO/TiO 2 electrodes (curves in Figures 65B and65C) and two times higher than that for bare ITO/ZnO (curve 3 in Figure 65A). This difference may be ascribed to larger surface area of TiO 2 with respect to ZnO or/and formation of the interface states at the ZnO/TiO 2 boundary, which may be involved in the charge transfer from the metal oxide to ferrocene. On the other hand, the presence of these surface states may also result in sub-band gap transition leading to photoelectrochemical activity of the system in visible range, which was confirmed after application of UV cut-off filter the wavelengths below 400 nm (curve 2 in Figure 65). The photoelectrochemical results obtained in anodic potential range are in conformity with photocatalytic data. It is also worth noting that in all cases the current density of the oxidation wave was practically independent on the scan rate (see inset in Figure 65D) probably due to relatively slow transport of the minority charge carriers (holes) to the semiconductor surface and/or slow rate of the charge transfer process at the electrode/solution interface. Thus, the sample morphology and accessibility of the reagent to the interface have a small influence on the value of the anodic current.

In contrast, in the range of negative potentials the reduction peak is well developed and its location was dependent on the system studied, i.e. -0.1 V in the case of ITO/ZnO and ITO/ZnO/TiO 2 (1c) and more negative than -0.2 V for ITO/ZnO/TiO 2 (3c) electrode. This is probably a consequence of different resistance of the systems. One may expect that the layer of relatively compact layer of TiO 2 in 3c sample may be a source of additional ohmic drop at the electrode/solution interface in comparison to that of 1c electrode.

CP CD AD  1/2 (V 1/2 s -1/2 ) i (A) AP A B C D
It is also important to know that the intensity of the cathodic peak current is linearly dependent on the square root of the scan rate as for diffusion controlled process. This means that the reaction of ferrocenium ion reduction by photoexcited electrons (majority charge carriers) is relatively fast.To conclude, the photoelectrochemical tests showed that deposition of TiO 2 thin layers on ITO/ZnO nanostructured electrodes results in the increase of the photocurrent density for oxidation/reduction of ferrocene in acetonitrile, with respect to that on the bare ITO/ZnO, both in anodic and cathodic potential ranges.

However, the value of the photocurrent is nearly the same for 1c and 3c samples. In contrast, a strong influence of the photocurrent density on the film morphology was observed for H 2 O or OH -oxidation in aqueous solution of Na 2 SO 4 . This is probably a result of improved adsorption of H 2 O and OH -groups on more developed surface area of 1c sample than on the surface of more compact 3c TiO 2 layer.

Photocatalytic tests with landfill leachate

The catalysts were tested so far in photocatalytic decolorization of a model dye pollutant.

However, remediation of real waste water is a big challenge. Many difficulties and limitations for a photocatalyst to work efficiently in the solutions containing a mixture of organic and inorganic pollutants appear. Besides unknown composition of dissolved matter, there are more harsh conditions that may completely deactivate the catalyst, such as, irreversible adsorption of species which cannot be degraded (blocking effect), high or low value of pH, which can cause dissolution or passivation of a catalyst, or even high concentration of inorganic salts that can strongly inhibit the activity [366,367]. Two complementary methods that allow to track remediation of waste water from organic pollutants are spectroscopic observations of absorption at 254 nm, which in general is ascribed to π-π* transitions in aromatic (benzene-based) compounds, and the second is total organic carbon analysis.

The waste water leachate samples, as-received, were very concentrated and therefore they had to be diluted 25-times with DI water before the decolorization tests to avoid saturation of the UV-vis spectrometer's detector. The pH of diluted solution was 9. Prior to the tests, the catalysts were conditioned for 24 hours in diluted waste water solution to reach adsorption-desorption equilibrium. The typical result of a decolorization of landfill leachate solution under 400 nm irradiation is presented on Figure 66A.

The investigations were performed for 1c, 3c and ZnO/TiO 2 composite with N-doped TiO 2 layer (3c(N)), prepared accordingly to the procedure described in the Chapter 4.4 (paragraph 4.1). The UV-vis spectrum of leachate solution is complex with broad shoulder around 270 nm, as presented on Figure 66A, showing its photocatalytic decolorization with usage of 3c(N). Upon irradiation of the samples, a continuous decrease of absorption was observed and after 600 min the absorption above 350 nm practically decreased to zero while the shoulder remains at almost the same wavelength.

The absorption in the UV can be attributed to π→π* transitions in organic compounds with double C=C bonds, but it is impossible to identify the exact components of the leachate solution. Nonetheless, the photodecolorization tests were done with usage of other elaborated samples as well as for commercial catalyst P25 deposited on ITO. The latter one of the same mass (estimated by sensitive microbalance) as that of ZnO/TiO 2 catalyst was deposited by dispersing P25 powder in ethanol and dropwise placed on ITO surface, followed by calcination at 300 o C for 15 min. The relative changes of absorbance at the wavelength of 254 nm in a function of irradiation time with monochromatic light (400 nm) are presented in Figure 66B. It is important to know that the decolorization rate of the leachate solution with the use of all ZnO/TiO 2 samples was markedly higher than that with commercial P25 catalysts and the catalysts' activity increased in the order: ZnO < P25 < 3c < 1c < 3c(N). The results of TOC analysis performed for all samples (Figure 66C) confirmed the same trend of activities. However as it is generally observed, the TOC removal was smaller than absorption decrease at 254 nm.

In order to determine the long term stability of the most efficient 3c(N) catalyst, this sample was tested in three subsequent photocatalytic runs. However, for these tests waste water solution was not diluted with DI water before photocatalytic tests to provide ‗real life' conditions. None diluted leachate solution saturates the UV-vis spectrometer detector, therefore in order to measure the absorption spectra of the solution, we followed the standard procedure of taking out a small portion (of about 0.1 ml from 2.5 ml total sample) of solution from the entire volume and was diluted 25-times. The results are presented in Figure 66D, and show almost identical decrease of absorption after the same time irradiation time intervals (2, 6, and 11 h). This proves that the 3c(N) sample is stable in harsh, non-diluted, as-received the landfill leachate and can still remediate the solution with the same activity in consecutive photocatalytic runs. The deposition parameters in CVD and ALD were adjusted to achieve the thickness of TiO 2 layer similar to that obtained by of sol-gel synthesis. In such case, the comparison between all samples can be more reliable, even if some differences are inevitable. In both techniques TiO 2 layers were formed from the same starting precursor, titanium(IV) isopropoxide; The SEM images of the ZnO/TiO 2 ‗CVD' and ‗ALD' samples are compared in Figure 67. As visible, in both cases the TiO 2 layer deposited on ZnO is relatively thin because the nanorods remained well separated, as it was in the case of the sol-gel prepared ZnO/TiO 2 composites. However, at close inspection one can notice that the ends of the nanorods covered with TiO 2 with the use of ‗CVD' are thicker and the layer is more rough (resembling a stock of ‗fish scales') the than that deposited with ‗ALD' (Figures 67B and 67D,respectively). It is also interesting to note that the roughness of deposited HAADF-STEM images of CVD-deposited TiO 2 show that the shell thickness is not uniform along ZnO nanorods, as it was initially suggested by SEM, and the whole composite nanorods resembles a shape of ‗matches'. Moreover, TiO 2 shell consists of an array of TiO 2 needles on already existed ZnO nanorods array (array-on-array). The length of a single TiO 2 nanorod obviously varies along ZnO surface; however it does not exceed 150 nm on top of ZnO, whereas the diameter seems to be about 30 nm. Similar specific alignment of the TiO 2 nanorods deposited on also hydrothermally grown ZnO was reported by Gao et al. [370], however by different techniquemagnetron sputtering.

They also admitted that ZnO/TiO 2 interface is of a great importance for the special phase structure formation, and as such could relevantly impact photocatalytic properties of the material. As shown on the HR-TEM image (Figure 68B) the TiO 2 nanorods are wellcrystallized and the interplanar distance was measured to be 3.5 Å, which corresponds to As it is evidenced, the images the TiO 2 shell formed is uniform along the whole ZnO nanorod surface and appear to be composed of extremely small particles (smaller than 5 nm), whereas the thickness of the shell was about 30 nm. Such small particles that are forming continuous layer are a challenge for HR-TEM determination of interplanar Optical properties of the ‗CVD' and ‗ALD' samples were measured within the range of 350 nm -650 nm wavelengths and compared with bare ZnO and ‗sol-gel' ZnO/TiO 2 composites (Figure 70A). The results showed that both ‗ALD' and ‗CVD' samples have almost identical absorption profile as the 1c sample: the absorption ‗Urbach tail' and onset edge. These results suggest similar contact between ZnO surface and TiO 2 particles for 1c, ‗CVD', and ‗ALD', whereas 3c seems to possess higher amount ZnO and TiO 2 bounded as the absorption especially with ‗Urbach tail' range is higher.

XRD patterns for ‗CVD' and ‗ALD' samples were also recorded in order to confirm the (k 400 ) are listed in the Table 11. As presented, the most active sample was 1c ‗sol-gel' (k 400 = 1.2 •10 -2 min -1 ). However, slightly lower, but still comparative performance was recorded for ZnO/TiO 2 composite with TiO 2 layer prepared by ALD (k 400 = 1.1 •10 -2 min -1 ). On the other hand, both samples evidently decolorize MB better than the 3c ‗solgel' (k 400 = 0.7 •10 -2 min -1 ), and ‗CVD' samples (k 400 = 0.8 •10 -2 min -1 ). The latter two samples showed also similar photocatalytic activities in MB decolorization. According to HAADF-STEM observations, 1c and ‗CVD' samples should have better accessibility to the ZnO/TiO 2 interface than the 3c and ‗ALD' samples. A difference between 1c and ‗CVD' is seen in continuity of the TiO 2 layer along ZnO nanorods.

While the former one is covered with a uniform, rugged TiO 2 layer, the latter has only deposit on the tips of ZnO. Therefore, the 1c yields higher photoactivity than the ‗CVD' sample, because of better accessibility of ZnO/TiO 2 interface. On the other hand, the ‗ALD' sample showed also high photoactivity, despite the fact of continuity of the TiO 2 layer. This may suggest high porosity of TiO 2 layer in ‗ALD' sample.

Conclusions

In summary, the hybrid composite ZnO/TiO 2 systems were prepared by deposition of TiO 2 via simple sol-gel method on ZnO nanorods hydrothermally grown on ITO surface.

Synthesis parameters such as deposition time or repetition of single sol-gel cycle, allowed to prepare either rugged TiO 2 layers or compact and continuous TiO 2 coatings.

The rugged TiO 2 layers were obtained after 6 hours of one step sol-gel synthesis followed by calcination of the sample (1c), whereas the uniform coating of thickness of The photoelectrochemical studies with the presence of ferrocene showed that higher current densities were observed for ITO/ZnO/TiO 2 composite electrodes than for ITO/ZnO sample in a broad potential range. These results confirm and emphasize the significance of the TiO 2 layer morphology on photoelectrochemical properties of ZnO/TiO 2 core/shell systems.

The photocatalytic properties of ZnO/TiO 2 composites prepared by sol-gel method were compared with those of the materials produced by ALD and CVD. It was evidenced the developed sol-gel procedure of TiO 2 deposition on ZnO nanorods resulted in preparation of the composite catalyst of higher performance than the catalyst with TiO 2 layer prepared by CVD technique and comparable performance with ALD technique.

Taking into account possibilities for preparation of the catalysts is seems more facile to use the sol-gel method, which can be easily implemented, whereas the use of ALD and CVD requires the specific apparatus, and often training to operate. The 3c composite was chosen over other obtained ZnO/TiO 2 samples, as this material exhibits continuous and well-distinguished boundary between ZnO and TiO 2 .

Also, EDX-SEM analysis across 100 µm 2 area of samples after 20 h, 40 h, and 50 h of treatment in HCl (aq.) showed that the Ti:Zn ratio was almost the same in all cases (3. 1, 3.3, and 3.3, respectively), confirming that 20 h of treatment was sufficiently long to The results presented above suggest that the additional calcination significantly modifies the interfacial ZnO/TiO 2 region, and thus influences the photocatalytic activity of the composites. Improvement of the photoactivity by a simple additional calcination is an interesting result and we attempted to understand the origin of this enhancement.

Explanation of improved properties of additionally calcined ZnO/TiO 2

First, we supposed that the enhancement stems from the decrease of the optical band gap and therefore, better absorption in visible range after additional calcination. As visible, in Tauc plot (inset, Figure 74A) indicate that the additional calcination leads to a small increase, from 2.8 eV for 3c to 2.9 eV for 3c-Cal. However, the absorbance at 400 nm (at which the photocatalytic tests were performed) of 3c-Cal sample is very similar to that of 3c. Thus, the change of the optical properties can hardly explain the observed increase of photocatalytic activity. The similarity of the optical properties of 3c and 3c-Cal samples at 400 nm means that the number of generated electron-holes pairs is similar in both cases. Also, the XRD patterns are very similar for both samples (Figure 74B). All peak positions and intensities of ZnO, ITO, and TiO 2 -anatase in the XRD patterns, which were previously found in 3c, were not changed after the additional calcination (3c-Cal), and neither rutile phase of TiO 2 was formed during this process.

In these conditions the higher photocatalytic activity of 3c-Cal might be related to a was observed in the course of V oc transients during light-off relaxation from the illuminated state to the dark equilibrium.

In the case of the 3c sample, an initial dark value of V oc was reached within 120 s, whereas the relaxation time was much longer (~ 700 s) for the 3c-Cal. These results confirm our hypothesis that photogenerated electron-hole pairs in the samples differ in recombination lifetimes. For ZnO and TiO 2 the results of V oc decay can be used to estimate electron lifetime, [374] accordingly to the eq. ( 20).

Inset in Figure 75 shows that indeed in the 3c-Cal sample the electron-hole pair recombination process is slower than that in the initial 3c sample. These results allow to ascribe the improved photocatalytic activity of the additionally calcined sample to the prolonged photoelectron and hole lifetime. Thus, the photogenerated charge carriers can be more efficiently utilized in the decomposition of MB.

Figure 75 Open-circuit potential (V oc ) decay after switching off the light for the 3c [START_REF] Koch | The influence of film crystallinity on I the coupling efficiency of ZnO optical modulator waveguides[END_REF] and 3c-Cal (2) samples. The V oc measurements were recorded in deareated aqueous solution of 0.2 M KCl in phosphate buffer of pH 6.8. Inset: the plots of lifetime ( e ) of the photogenerated charge carriers, derived from equation [START_REF] Schneider | Understanding TiO 2 Photocatalysis: Mechanisms and Materials[END_REF] as a function of V oc for 3c and 3c-Cal.

In order to examine how the hindered electron-hole recombination in the 3c-Cal composite influences the photoelectrochemical properties of the system, the investigation The same conclusions may be derived from the photocurrent transients recorded at constant potential of 0.8 V vs. Ag/AgCl, presented in Figure 76B, where the photocurrent is almost two times higher for the additionally calcined sample. It is also important to note that after illumination of the electrode the photocurrent stabilizes within several seconds and the response is reproducible in the successive dark and illumination cycles. Such strong positive effect of calcination on the photocatalytic and photoelectrochemical properties of the ZnO/TiO 2 system can be a result of morphological changes, especially at the interface. In order to verify this point we characterized the structural changes occurring during calcination. To this end, XPS and STEM with EDX elemental mapping and line scans were used. The obtained results are described in the next section. To confirm the results of EDX analyses, XPS spectra were measured for the 3c, 3c-HCl, 3c-Cal, and 3c-Cal-HCl samples (Figure 80). In the XPS spectra of all samples, the position of main Ti (2p 1/2 ) signal corresponds to TiO 2 , the Zn (2p 3/2 ) signal to ZnO, and the O1s signals corresponds to O 2-in the lattice of metal oxides and hydroxyl groups [381,382]. The Zn:Ti atomic ratios calculated from the XPS spectra are given in Table 12. The Zn amount in the external TiO 2 layer increases significantly by factor of 2.7 when the 3c sample is additionally calcined (3c-Cal). The same tendency is observed when comparing 3c-HCl and 3c-Cal-HCl (factor of increase: 1.8). These results thus confirm that the formation of voids is accompanied by Zn 2+ diffusion into the TiO 2 layer.

Also, STEM images and XPS results ensure that the zinc signal was detected from Znmodified most external parts TiO 2 layer and not from ZnO-core, due to porosity of TiO 2 layer. The data listed in Table 12 also show that the increase of the Zn/Ti ratio in the outer layer is well correlated with the increase of photocatalytic activity of the samples. Table 12 The Zn:Ti ratios determined from XPS spectra of 3c composite, 3c composite after additional calcination (3c-Cal), and after .

The values of apparent first-order rate constant (k 400 ) of photocatalytic MB decolorization on the samples are also given in the latter column. the oxygen vacancies impact the photocatalytic properties of TiO 2 . These parameters appear to be favorable in our case for decreasing the electron-hole recombination rate and consequently for improving photocatalytic and photoelectrochemical properties of the composite.

Our findings about modification of TiO 2 layer in the calcined composite allow to explain the stronger variation of the catalytic activity in MB decolorization observed in the etched samples (0.12 •10 -3 min -1 for 3c-HCl vs. 0.34 •10 -3 min -1 for 3c-Cal-HCl)

than for the composite ones (7 •10 -3 min -1 for 3c vs. 10 -2 min -1 for 3c-Cal).

As it was demonstrated in the Chapter 4.2. paragraph 4.2.2, that high photocatalytic activity of ZnO/TiO 2 composites under 400 nm illumination was achieved due to appearance of two effects: i) formation of additional energy levels within the band gap (allowing the sub-band optical transitions) and ii) enhancement of the separation efficiency of photogenerated e -/h + pairs. The additional calcination of the ZnO/TiO 2 composite brings up the third effectthe increase of charge carriers lifetime. The first and the second effects were hence dominantly acting in the pristine 3c sample due to the presence of ZnO/TiO 2 interface. The increase of the lifetime of the charge carriers appearing in 3c-Cal sample seems to be of a lower importance than these two above mentioned effects and therefore, it does not lead to a dramatic increase of its photocatalytic activity in comparison with that of 3c sample. In contrast, in the etched samples (3c-HCl and 3c-Cal-HCl) the ZnO/TiO 2 interface was eliminated and for that reason, the related interfacial effects are no longer valid. The only difference between these two samples is a slower electron-hole recombination in 3c-Cal-HCl due to the presence of oxygen vacancies formed upon incorporation of Zn cations into TiO 2 . This effect, although ‗masked' in the composites, becomes an essential one in the etched samples. This explains why the relative increase of the rate constant upon additional calcination is much larger in case of the HCl-etched samples than for the non-treated ones.

Migration of Zn ions into TiO 2 layer proceeds in our composite at 450 o C which is an unusually low temperature for such phenomenon to occur. Thus, in a similar nanostructured (core)ZnO/TiO 2 (shell) system the reaction proceeded at 700 °C [387].

The higher reactivity of ZnO in our case is supposed to be related with some particular properties of the surface layer of ZnO nanorods which is in direct contact with the TiO 2 shell. To clarify this point the surface microstructure of as-grown and calcined ZnO nanorods was characterized by HAADF-STEM. 82C). Also, it should be noted that prolongation of calcination time from 1 h to 3 h (Figure 82D), or 6 h (Figure 82E) have no further influence on ZnO surface morphology. We speculate that these defects may be formed during crystallization of external amorphous layer of zinc (oxo)hydroxide obtained in the hydrothermal synthesis which can be seen as blurred external layer in

Conclusions

In this Chapter it was directly proven that presence of both ZnO(core) and TiO 2 (shell) is needed for obtaining photoactive composites, which efficiently decolorize MB under 400 nm light irradiation. The origin of this effectiveness was shown to be related to the improved absorption in the visible range due to formation of new electronic mid gap states at the ZnO/TiO 2 interface.

We found that an additional calcination of the ZnO/TiO 2 composite at 450 °C leads to the increase of photocatalytic activity in MB decolorization and remarkable improvement of the photocurrent response of the composite in photoassisted water oxidation.

The measurements of open circuit potential transients showed that this effect stems from a lower electron-hole recombination rate in the additionally calcined composite. The observed improvement of the photocatalytic properties was found to be accompanied by a pronounced modification of the microstructure of the ZnO/TiO 2 composite. Despite a moderate value of the used temperature (450 °C), voids appear at ZnO/TiO 2 interface and Zn ions migrate into TiO 2 shell. We suggest that formation of the Zn-enriched TiO 2 shell is responsible for the observed enhancement of the photocatalytic and photoelectrochemical properties. This phenomenon was ascribed to the Kirkendall effect which was observed for the first time at such low temperature due to defective surface structure of ZnO nanorods. Given the importance of ZnO and TiO 2 in photocatalysis these findings may be useful in designing more efficient photoactive materials, as well as in application of ZnO/TiO 2 composites in degradation of other pollutants.

gap just above the upper edge of the valence band [START_REF] Irie | Nitrogen-Concentration Dependence on Photocatalytic Activity of TiO 2-x N x Powders[END_REF][START_REF] Valentin | Origin of the different photoactivity of N-doped anatase and rutile TiO 2[END_REF]390]. Consequently, it makes possible to transfer the electrons from the nitrogen midgap state to the conduction band of TiO 2 . It is also worth noting that the ‗red' shift of the spectrum of the sample 3c(N) is not accompanied with formation of the additional absorption shoulder, thus suggesting a homogeneous distribution of a nitrogen dopant in TiO 2 . Following the Tauc equation, the energy of optical band gap (E g ) of semiconductor (or composite) can be determined Photodeposition of Au nanoparticles on both samples results in appearance of strong and broad absorption bands with maxima located at approx. 556 nm for 3c(N)-Au and 545 nm for 1c-Au (curves 5 and 6 in Figure 85A, respectively). Both peaks may be ascribed to the plasmonic bands of metallic gold nanoparticles [392]. A direct interaction between Au and TiO 2 is responsible for a red shift of the plasmon absorption band compared to that of nanoparticles suspended in the solution (530 nm) and for extended absorption in infrared region [START_REF] Subramanian | Semiconductor-Metal Composite Nanostructures. To What Extent Do Metal Nanoparticles Improve the Photocatalytic Activity of TiO 2 Films?[END_REF]. The different position of the plasmonic peaks in 1c and 3c(N) samples is possibly due to the well-known sensitivity of this parameter to the composition of the species being in contact with the plasmonic nanoparticles [394].

Chemical composition

The differences observed in optical absorption spectra of the samples are reflected in their different compositions (Table 13). The chemical composition of the samples was The XPS spectrum of Au is complex because of its strong overlap with the peaks of Zn (Figure 87B). However, in the spectra of both 1c-Au and 3c(N)-Au samples, four peaks can be clearly distinguished. Zn peaks of 3p This aspect was also taken into account in selection of the sample for the studies on the influence of nitrogen doping and decoration of the ZnO/TiO 2 surface with Au nanoparticles on photocatalytic and photo-electrochemical properties of the composites.

Au-nanoparticle decorated composites

The gold nanoparticles are easily detected as bright spots in SEM images obtained using the low-angle backscattered electron detection which signal intensity is proportional to atomic number of the analyzed element (Figure 89). Therefore, Au nanoparticles can be easily seen in 1c and 3c(N) nanostructures as spots of much brighter contrast than Zn or Ti (in ZnO or TiO 2 ) (Au nanoparticles are indicated by red circles in the SEM images). The spatial resolution of the SEM is however insufficient to determine precisely the size and characterize in detail the morphology of Au nanoparticles. This information was obtained using HR-TEM and HAADF-STEM techniques (Figure 90). Analysis of the images shows that despite the presence of large particles (> 30 nm), the size of the most particles lies within the range 7 nm -25 nm which is in agreement with the average value determined from XRD (Table 13). Such a broad dispersion of the particles size is not surprising since a simple photodeposition was used without addition of any growthlimiting agents. The similarity of the size distribution of gold particles for 1c-Au and 3c(N)-Au shows that the presence of nitrogen in the latter sample does not influence the process of photodeposition. Another important feature of the obtained samples is a high crystallinity of the obtained nanoparticles which is attested by the high resolution images (Figures 90E and90F). The EDX analysis with a high spatial resolution present in HAADF-STEM setup was used also to answer a question concerning the chemical form of gold present in our samples. Given the preparation procedure and short time of photodeposition (20 s) one can suppose that the reduction of gold is incomplete and some fraction may be present in form of ionic species dispersed on the surface of TiO 2 . To check this possibility we analyzed by EDX the chemical composition of two different areas: one contained gold nanoparticles (pt 1) and another one was free of them (pt 2) (Figure 91). While the gold is clearly detected in the first area, it is completely absent in the second one. If gold were present in any ionic form it would have been distributed homogeneously and observed everywhere. Its absence in the second area thus strongly supports that gold is present only in the form of nanoparticles. Addition of the Au nanoparticles results in a 36% increase of the rate of MB decolorization for nitrogen-incorporated samples (k vis = 0.41 •10 -2 min -1 for 3c(N), and k vis = 0.51 •10 -2 min -1 for 3c(N)-Au). On the other hand, the photocatalytic activity of the nitrogen-free composite was more prone to the influence of Au nanoparticles since it resulted in almost 60% enhancement (k vis = 0.39 •10 -2 min -1 for 1c, and k vis = 0.64 •10 -2 min -1 for 1c-Au). It is important to notice that the measurements of TOC showed that MB is not only decolorized but also partially mineralized under used conditions (Figure 93). The slightly lower degree of mineralization than that of decolorization is due to slower kinetics of the former process [START_REF] Houas | Photocatalytic degradation pathway of methylene blue in water[END_REF]. The enhancement of the photocatalytic activity of gold-containing samples suggests that the plasmonic excitation in Au nanoparticles enhances the visible light response of ZnO/TiO 2 core/shell composites. One can suppose that this increase of activity should be even greater under bias conditions when the generated e -/h + pairs are separated more efficiently. That is why in the following sections the photoelectrochemical properties of the composites in different reactions were characterized in detail. It is interesting to note, that deposition of the Au nanoparticles on ZnO/TiO 2 composites gives rise to significant increase of the photocurrent in anodic potential range, while the reduction current is strongly enhanced even without illumination (lines 4 -7 in Figure 94A). The possible reason of the latter effect is catalytic electroreduction of dissolved oxygen on Au, which was confirmed by the result of a control experiment As visible, all current responses are very reproducible in the subsequent darkillumination cycles and it was found that the photocurrent was stable even after 50 on/off cycles (Figure 96). The steady-state current is similar to that in is greater for the process occurring on Au-loaded electrodes, suggesting higher recombination rate. In order to estimate a ratio of the rate constants for charge transfer across the semiconductor/solution interface (k tr ) and for e -/h + recombination (k rec ), the photocurrent transients j(t) were analyzed according to the protocol proposed by Peter's relation (see Bibliography section for more details):

( ) ( ) (73)
As one can see from the plots presented in Figure 95 and data listed in Table 14, the doping of TiO 2 with N leads to two opposite effects. On one hand, it results in higher steady-state photocurrents (due to higher amount of electrons and holes photogenerated by visible light) but on the other, the appearance of new N-doping levels in TiO 2 may also act as the trapping states promoting the e -/h + recombination [422]. The latter effect is probably responsible for relatively low performance of N-doped sample in photodecolorization of MB with respect to that of non-doped composites. The data listed in Table 14 indicate also enhancement of recombination after decoration of both types of the samples (1c and 3c(N)) with Au nanoparticles. This may be rationalized taking into account participation in this process of the electrons appeared in the conduction band of TiO 2 owing to plasmonic effect.

On the other hand, one can expect that these extra electrons may be involved in reduction of dissolved oxygen under cathodic polarization of the electrode (reaction According to the literature, photodegradation of organic dyes, such as methylene blue (MB) or methyl orange (MO) at TiO 2 surface occurs mainly via highly active hydroxyl radicals formed in oxidation of H 2 O or surface hydroxyl groups by photogenerated holes [423]. Moreover, a positive potential applied to the working electrode induces the transport of photo-excited electrons to the counter electrode via an external circuit, preventing the e -/h + recombination [424,[START_REF] Egerton | Does photoelectrocatalysis by TiO 2 work?[END_REF]. Therefore, the photoelectrochemical degradation of these dyes is usually investigated in the positive potential range [426,427]. It has been also postulated that the electrons transferred to the counter electrode may be consumed in oxygen reduction to superoxide and then to hydroxyl radical which is further active in the degradation of dye at the metal electrode but not on semiconductor. 15. However, LMB is stable only in anaerobic conditions, while in the presence of oxygen it is regenerated back to MB. The results presented in Figure 98B and in Table 15 indicate that the illumination of 1c sample accelerated MB decolorization, resulting in the increase of the rate constant from 0.16 •10 -2 min -1 to 0.38 •10 -2 min -1 . Much stronger changes were found when the same experiments were carried out with the use of the Audecorated ITO/ZnO/TiO 2 electrode. The rate constant of MB decolorization raised from 0.38 •10 -2 min -1 for 1c to 0.81 •10 -2 min -1 for 1c-Au sample i.e. to the value higher than that obtained in the photocatalytic tests without polarization (see Figure 98).

In order to confirm the crucial role of oxygen reduction in photoelectrochemical degradation of MB, the control experiment was performed with the use of 1c-Au under the same conditions but in deoxygenated solution. As visible in Figure 98C and in Table 15, the potential-assisted decolorization rate constant considerably decreased from 0.81 •10 -2 min -1 in the presence of oxygen to 0.22 •10 -2 min -1 in deaerated solution.

Thus, it is evident that the active oxygen species formed in process of oxygen electroreduction on Au-decorated composites participate in photocatalytic decolorization of MB.

The presence of supporting electrolyte (Na 2 SO 4 ), added to provide a sufficient conductivity of the solution allowed us to observe an unexpected effect of cathodic polarization on the rate of MB decomposition. Indeed, according to the literature, inorganic anions such as SO 4 2-, NO 3 -, H 2 PO 4 -inhibit the photocatalytic activity of TiO 2 in MB degradation [366,367]. Our experiments show that this negative effect can be strongly attenuated by polarizing 1c-Au composite at -0.25 V (Figure 98C and Table 15). We notice that the apparent first-order rate constant of MB decolorization in the presence of Na 2 SO 4 increases dramatically when polarization is applied (from 0.11 •10 -2 min -1 to 0.81 •10 -2 min -1 ). Cathodic polarization of 1c-Au allows thus to suppress the inhibition effect of Na 2 SO 4 on the photocatalytic activity of the composite.

It is worth noting that under used conditions MB is not only decolorized but also mineralized, as follows from TOC analysis after illumination for 60 min. The increase of the activity of 1c-Au in the presence on Na 2 SO 4 through application of negative bias is probably due to a higher concentration of active species produced from oxygen electrocatalytic reduction on Au 0 nanoparticles. It is not clear if such enhancement can be observed for other dyes and other inorganic salts. The future study involving different substrates would allow to answer this question.

Conclusions

In this chapter it was found that the visible light response of ZnO(core)/TiO 2 (shell) composites can be considerably improved through N-doping of TiO 2 shell and its decoration with Au 0 nanoparticles. Both promoters were introduced through simple procedures under mild conditions. N-doping was achieved by precipitation of amorphous titanium precursor in the presence of NH 4 Cl. The gold nanoparticles of the size 7 nm -25 nm were grown by photodeposition for 20 s from [AuCl 4-x (OH) x ] -precursor solution (pH ~ 6.7). Thus obtained nitrogen-and gold-promoted composites showed a significantly higher photocatalytic activity in comparison with that of the non-promoted materials in different test reactions realized under Xe lamp illumination with a 400 nm cut-off filter. It was found that plasmonic effect in Au-decorated ZnO/TiO 2 composite increased the rate of MB decolorization by 60% in comparison with the non-promoted sample. Moreover, due to the presence of Au NPs, a three-fold increase of photocurrent in photoassisted water oxidation was achieved under potential of 0.8 V vs. Ag/AgCl.

We also found that the presence of gold in the composite provokes appearance of cathodic current which was shown to originate from the reduction of dissolved O 2 . The active species produced by this reaction allow to gain a significant increase of the rate of MB decolorization in the presence of dissolved inorganic salt (Na 2 SO 4 ). The rate at the Au 0 -promoted ZnO/TiO 2 core/shell composite polarized at -0.25 V vs. Ag/AgCl was found to be seven times higher than that on the same composite but without applied potential. This finding might be important for practical application of the elaborated composites for purification of waste water which usually contain considerable amounts of inorganic salts.

show that the accessibility of the interface and its microstructure are of crucial importance for the performance of composites and these parameters should therefore be taken into account when developing new materials.

Extension of light absorption into visible range. A serious drawback of ZnO and TiO 2 is the large band gap allowing utilization of the light only with wavelength below 400 nm. One of the major results of our work is the observation of optical absorption at higher wavelength attributed to formation of midgap states at the ZnO/TiO 2 interface. It is worth noting that this unexpected effect has not been systematically observed in ZnO/TiO 2 composites described in literature indicating that its origin must depend on the used preparation method. Even if the exact nature of the midgap states is not known, some observations allowed us to correlate their appearance with the presence of structural defects. In order to further increase the visible light response of our composites, the N-doping of TiO 2 layer was combined with decoration by Au nanoparticles. This approach, well documented for pure TiO 2 , has been shown for the first time to be efficient as well for ZnO/TiO 2 composites bringing a significant increase of activity in photoassisted water oxidation under visible light.

Application of external potential. The use of Au-decorated composites permitted us to observe another important effect: small cathodic polarization of material considerably increased the rate of MB degradation in the presence of mineral salts. This effect was shown to be due to increase of production of active species by reduction of dissolved oxygen. Given that waste water subjected to photocatalytic depollution always contains mineral salts, the observed enhancement might be very helpful in development of novel advanced water purification techniques.

The perspectives of the present work are related to the aspects described above.

Regarding the preparation method, it would be interesting to see if the composites prepared by sol-gel method but having a simple structure (i.e. ZnO/TiO 2 bilayer) possess the high activity found in our materials. This question is important in the context of water purification in which conducting composites are not needed and simpler configuration may be applied. The appearance of optical absorption above 400 nm in our composites is another point worth further investigation. Better understanding of the nature of the formed mid gap states at ZnO/TiO 2 interface would be highly desirable in order to further increase their concentration. The investigation of N-doped and Au-

  Photoelectron Spectroscopy), EDS ‗or EDX' (Energy Dispersive Spectroscopy) analyses connected with electron microscopy techniques, UV-vis Spectroscopy and DRS (Diffuse Reflectance UV-vis Spectroscopy, TGA-DSC (Thermogravimetry Differential Scanning Calorimetry Analysis), TOC (Total Organic Carbon) analysis, RT-PL (Room Temperature Photoluminescence Spectroscopy), electrochemical techniques including:

  Alternativement, les électrons et les trous peuvent être utilisés pour effectuer des réactions chimiques. Les électrons peuvent être transférés pour réduire divers composés organiques ou des espèces inorganiques, alors que les trous peuvent jouer le rôle d'oxydant en soustrayant les électrons des autres composés. Grâce à ces réactions, il serait possible d'accumuler l'énergie solaire dans des espèces chimiques permettant ainsi d'atténuer les difficultés liées à l'intermittence de la lumière du soleil. Malheureusement, les matériaux existants ne franchissent pas facilement le niveau de laboratoire pour réaliser cette approche à l'échelle industrielle. C'est pourquoi le développement de nouveaux photocatalyseurs à base de semi-conducteurs, qui récoltent et convertissent efficacement la lumière solaire, est d'une importance primordiale. Pendant de nombreuses années, les matériaux photocatalytiques les plus souvent étudiés ont été ZnO et TiO 2 . Cependant, on a récemment montré que les composites basés sur ZnO et TiO 2 possèdent des propriétés encore plus prometteuses que de nombreux autres semi-conducteurs dans diverses applications photocatalytiques. Malgré de nombreux travaux décrivant une grande activité photocatalytique de ces composites dans différentes applications, les informations détaillées sur la corrélation structurepropriétés font défaut. Afin de combler cette lacune, nous avons décidé de concentrer notre attention sur l'étude de l'influence de la microstructure des composites ZnO/TiO 2 sur leurs propriétés dans la dégradation photocatalytique des polluants organiques, et dans la demi-réaction de la génération de «combustible solaire», à savoir oxydation électrochimique photoassistée de l'eau. Pour réaliser une telle étude, les composites doivent satisfaire aux exigences d'une surface élevée et d'une bonne conductivité électrique. Nous choisissons donc la conception basée sur les nanobâtonnets de ZnO supportés sur l'électrode en verre revêtue d'ITO (Indium Tin Oxide). Les nanobâtonnets de ZnO (NR) ont ensuite été recouverts d'une couche de TiO 2 dans différentes conditions. La composition et la microstructure des composites ZnO(coeur)/ TiO 2 (coquille) obtenus ont été modifiées dans le but d'élucider comment ces paramètres influencent leur activité photocatalytique. Par conséquent, les efforts ont été fournis pour conférer une activité dans le domaine de visible aux composites ZnO/TiO 2 élaborés en modifiant les couches de dioxyde de titane avec dopage par de l'azote et la décoration avec des nanoparticules Au. La thèse comprend trois parties: la Bibliographie, Expérimentale et Résultats et Discussion.

  photodéposition des nanoparticules d'or. Deuxièmement, les techniques de caractérisation sont présentées : MEB (microscopie électronique à balayage), TEM (microscopie électronique en transmission), STEM-HAADF (microscopie électronique en transmission par balayage avec un détecteur angulaire de grand angle), EDS (spectroscopie de dispersion de rayons X), XPS (spectroscopie photoélectronique par rayons X), DRX (diffraction des rayons X), ATG-DSC (analyse thermogravimétrique et calorimétrie différentielle), TOC (analyse du carbone total), spectroscopie UV-visible (en solution ou en phase solide), spectroscopie de photoluminescence, LSV (voltampérométrie par balayage), CV (voltampérométrie cyclique). Dans cette partie nous décrivons aussi des dispositifs pour les mesures de l'activité photocatalytique construits dans le cadre de cette thèse.

  ). Thus obtained separated charge carriers possess extra energy brought by light which can be used in different ways. Currently the most important use of this energy is the direct production of electricity in p-n junctions created in photovoltaic (PV) materials. This technology represents the main use of the solar light with 177 GW installed global capacity (in 2014)with 90% of it being produced with planar structures based on silicon in mono-or polycrystalline forms[START_REF] Koch | The influence of film crystallinity on I the coupling efficiency of ZnO optical modulator waveguides[END_REF].

Figure 1

 1 Figure 1 Scheme of photocatalytic action on a metal oxide semiconductor, such as TiO 2 .

  photoelectrochemical cell (PEC, Figure2).

Figure 2

 2 Figure 2 Scheme of photoelectrochemical cell (PEC) for water splitting application.

  Acceptor and donor levels may serve as trapping levels for electrons and holes. Trapped charge carriers (e tr -, h tr + ) may be deactivated therein and not participate in any surface reaction, ultimately recombining and releasing energy by locally heating crystal structure or by emission of light of a lower energy than the energy of light absorbed (hv 2 ).

Figure 4 (

 4 Figure 4 (Photo)electron-hole pair possible utilization in a reaction at solid/liquid interface or its radiative (hv 1 , hv 2 ) recombination (A) [17], direct and indirect electron transitions in a semiconductor (B).

Figure 7

 7 Figure 7 Determination of optical band gap values of pure SnO 2 (black circles), and Nidoped SnO 2 (red stars) counterpart from their absorption spectra (inset) [37].

Figure 8

 8 Figure 8 Comparison of the charge and potential distribution at the metal/solution (A) and n-type semiconductor/solution (B) interface. N-type semiconductor before (C) and after (D) contact with a redox couple in solution in dark.

Figure 9 N

 9 Figure 9 N-type semiconductor/electrolyte interface in dark (A) and under illumination (B) [45].

  photocurrent (Figure 10A, line c). With an increase of negative applied potential over the value of V fb , the electrons from semiconductor Fermi level are transferred to the electron acceptor in the solution (A) and cathodic current increases. In this range the photocurrent is close to dark current because illumination does not change noticeably the population of the majority charge carriers (Figure 10A, lines b and d).

Figure 10

 10 Figure 10 Ideal current-potential characteristic for n-type semiconductor in dark (a, b) and under illumination (c, d) (A) [48]. Linear sweep voltammogram (LSV) for Ti/TiO 2 in 0.1 mol dm -3 Na 2 SO 4 , in the dark and under illumination. Sweep rate 10 mV s -1 , pH 5.6 [49]. Model of the n-type semiconductor/electrolyte boundary under illumination with indication of characteristic regions defined as depletion layer width (W sc ), minority carriers diffusion length (L) and light penetration depth (1/) [50] (C).

  originate from electron recombination with free or surface-trapped holes [56]. After the spike during the light ‗on' period the current value stabilizes (j(∞)) if the rate of arrival of holes is balanced by recombination and the charge transfer at the semiconductor/solution interface [57]. When the light is switched off, the production of holes is interrupted and the electrons move back towards the surface to recombine with the remaining holes resulting in the spike in opposite direction.

Figure 11

 11 Figure 11 Chronoamperometric response of n-type semiconductor electrode during light ‗on' and ‗off' periods, adapted from [58].

Figure 12

 12 Figure 12 Model OCP behavior of n-type semiconductor (A) [60], and dependence of average carrier lifetime vs. OCP for thin TiO 2 and PbS modified TiO 2 films (B) [61].

Figure 13

 13 Figure 13 Scheme of n-type semiconductor photoanode and metal cathode in photoelectrochemical cell under light illumination and anodic bias [65].

Figure 14 Figure 15

 1415 Figure 14 Methods and techniques used for water remediation (A) [67], comparison of the rate constants of decomposition reactions with the use of reactive ozone (O 3 ) and hydroxyl radicals ( • OH) (B) [68].Photogenerated hydroxyl radicals can degrade organic pollutant compound in either one or two ways: by abstraction of hydrogen atom to form water (i.e. from alkenes or alcohols, Figure15A) and/or can be added to aromatic compounds initiating its decomposition chain reactions (Figure15B)[70].

Figure 16

 16 Figure 16 Determination of apparent first-order kinetic rate constant based on Langmuir-Hinshelwood model (A), and photocatalytic stages of the process associated with energy level scheme of semiconductor (B) [69].

  photogenerated holes they are primary species in degradation of the organic pollutants.

Figure 17

 17 Figure 17 Redox potential dependence on pH (A) [75], and energy positions for ZnO and TiO 2 anatase calculated for pH 7 (B). As seen on the schemes, CB position of ZnO and TiO 2 are only slightly more negative than O 2 /O 2 •-redox couple potential allowing photogenerated electrons to be transferred from near-CB and CB energy levels to O 2 . At the same time, the photogenerated holes are supplied by an electron donor i.e. H 2 O whose redox potential are more negative than near-VB and VB energy level positions. Not all photogenerated electron-hole pairs are participating in the reactions, even if energetically it is favorable. This is due to the size of particles and the charge carrier diffusion distance to the surface of SC/reaction center before they recombine, and in general it is a fewseveral nanometers [76].

  solution < IEP, then: TiOH + H + ↔ TiOH 2 + (43) pH solution > IEP, then: TiOH + OH -↔ TiO -+ H 2 O (44) MB adsorption/desorption on TiO 2 surface should reach equilibrium within 1 h [97]. However according to ISO procedure [87] one should maintain the catalyst in a contact with MB solution for 24 h before photocatalytic tests. Photodegradation mechanism of MB proceeds via first initial stage of N=C bond breaking, which is responsible for observed decolorization, and then via sequence of reactions to final simple inorganic compounds as presented in Figure 19.

  photocatalytic properties of ZnO are consequence of surface and native defects (i.e. vacancies, interstitials, and antisites)[110,111]. The defect ionization energies vary from ~ 0.05 to 2.8 eV[112].

Figure 20 (Figure 21

 2021 Figure 20 (A) ZnO crystal structure of rocksalt (a), zinc blende (b), and wurzite (c) [113]. A scheme depicting stages of hydrothermal growth mechanism of ZnO nanorods (B) [115].

[

  130]. Band gap energy (E g ) in TiO 2 anatase, brookite, and rutile are ~ 3.2 eV, ~ 3.1 eV, and ~ 3.0 eV, respectively[START_REF] Di Paola | Brookite, the least known TiO 2 photocatalyst[END_REF]132]. While the VB energy level is the same for all forms of TiO 2 (E VB = 2.64 V at pH 7) [133], the position of CB is different for each form.Optical absorption is affected by different electron transition process; which in case of anatase is indirect, while direct in rutile.

Figure 22

 22 Figure 22 Ball-stick representation of periodic structures of rutile (A) and anatase (B) TiO 2 [134], blue represents Ti, whereas red stands for O ions. Rutile cell distances are a = b = 4.49 Å, c = 3.01 Å, for anatase TiO 2 : a =b = 3.77 Å, c = 9.56 Å.

Figure 23

 23 Figure 23 Band structure of TiO 2 undoped TiO 2 and influence of N-doping on band gap energy value (E g ) [151]. TiO 2 can be doped with metal cations such as Cr 3+ [152,153], Cu 2+ [154], Ni 3+ [155], Fe 3+ [156], V 4+/5+ [157], Nd 3+ [158], Co 2+ [159], and/or with non-metal anions, i.e.

  titanium(IV) propoxide is dissolved in alcohol medium in the presence of a nitrogen precursor. Examples of nitrogen precursor are inorganic salts NH 4 Cl [181], urea [182], or NH 3(aq.) [183]. (2) Annealing of TiO 2 at elevated temperatures (500 o C -700 o C) in a flow of inert gas containing nitrogen precursor, i.e. ammonia [184], guanidine hydrochloride [185]. Other methods include: (3) electrochemical N-doping under constant potential in a solution containing nitrogen precursor, such as triethylamine, urea [186,187], (4) hydrothermal synthesis methods [188,189], (5) nitrogen incorporation in a plasma conditions at 400 o C [190].

  doping may not always results in increase of photoactivity despite the fact of increased light absorption. An explanation of lowering photoactivity was postulated, based of EPR results[201], taking into account formation of two kinds of nitrogen-related states above VB of TiO 2 : paramagnetic (N b •

  energy state. Holes can be only localized at N b • or N b + centers which explain their low reactivity due to strongly suppressed mobility.

Figure 24 1 .

 241 Figure 24 Titanium and nitrogen bonding in substiutional (A), and interstitial (B) positions [202]. UV-vis absorbance spectra (C), and XPS spectra (D) of bare and N-doped TiO 2 [203].

( 1 )

 1 used to enhance the light conversion by a semiconductor by associating it with a metal nanostructure. Three different mechanisms have been proposed to explain the increase of photocatalytic activity [204]: Hot carrier injection (2) Plasmon-induced resonance energy transfer (PIRET) (3) Light rapping enhancement Below we present in more detail these three mechanisms along with the conditions in which they are realized.

Figure 25

 25 Figure 25 Scheme depicting plasmon oscillation of electron ‗cloud' in a metal sphere particle (A) [205]. Noble metal (Me) nanoparticle (red) and n-type SC (green) energy diagram level showing formation of Schottky barrier φ B at Me/SC interface and plasmonic electron transfer from Me to CB of SC [206].

Figure 27

 27 Figure 27 Local field intensity enhancement of two proximate particles of Au/TiO 2 (A) and the enhancement dependence of light utilization on Me NPs and semiconductor distance (B) [209].

Figure 28

 28 Figure 28 Light scattering on large (> 30 nm) plasmonic nanoparticlesabsorption enhancement through increasing light penetration path [211].

  Linic and Ingram [222] demonstrated preparation of N-TiO 2 , Ag/N-TiO 2 , and Au/N-TiO 2 photoanodes for water splitting under visible light irradiation. Ag cube-like particles had edge length of 118 ± 25 nm, while spherical Au particles were 24.5 ± 4.5 nm in diameter and both were prepared by polyol synthesis procedure. It was shown that Au-, and Ag-loaded catalysts exhibited plasmonic peaks approx. at 530 nm and 650 nm for Au, and 410 nm and 590 nm for Ag (Figure29A).

Figure 29

 29 Figure 29 Optical properties of TiO 2 , N-TiO 2 , Au/N-TiO 2 , and Ag/N-TiO 2 composites (A), extinction efficiency as function of Ag-cube edge length or Au-sphere radius within the 400 nm -500 nm wavelength range (B), photocurrent response of photoelectrodes under 400 -900 nm irradiation (C) [219].

  photocatalytic water splitting under 663 nm visible) monochromatic light irradiations. It was found that photocurrent observed for Au/TiO 2 (0.3 µA) was strongly enhanced (by factor of 66) in a comparison with bare TiO 2 (4.5 nA, Figure30A). Also photocurrent enhancement ratio was shown to be the highest in visible range with the maximum of about 7 located at around 650 nm excitation wavelength (Figure30B).The observed photocatalytic improvement was assigned to the presence of plasmonic Au NPs which exhibited strong local electromagnetic field enhancement and short minority carrier diffusion length in TiO 2 .

Figure 30 (

 30 Figure 30 (A) Photocurrent transient obtained on TiO 2 nanotube (NT) electrode (black solid line), and for the same electrode after deposition of 5 nm Au layer (red dashed line). (B) Photocurrent (PC) enhancement ratio spectrum obtained from comparison of results on Au/TiO 2 (NT) and bare TiO 2 (NT) [222].

Figure 31

 31 Figure 31 Hydrogen evolution on Au/TiO 2 and Pt/TiO 2 photocatalysts in photoelectrochemical water splitting with different metal/TiO 2 weight ratios [223].

Figure 32

 32 Figure 32 Possible combinations of two materials: a simple mixture (A), loading (B), core/shell (C) [236].

Figure 33

 33 Figure 33 Schemes depicting possible heterojunction configurations in SC 1 /SC 2 type I (A), SC 1 /SC 3 type II (B), and SC 1 /SC 4 type III (C),adapted from [237].

  TiO 2 deposition was done at different duration time, 5 and 10 min. Deposition of TiO 2 on ZnO resulted in a slight band gap narrowing with respect to the latter material, i.e. from 3.31 eV for ZnO to about 3.24 eV for ZnO/TiO 2 . This was attributed to high scattering effects caused by high crystallinity in absence of oxygen vacancies. According to the authors the higher photocurrents observed for ZnO/TiO 2 composites were due to more efficient charge transfer at SC/electrolyte interface, and faster electron transport from active layer to FTO. However, the authors presented different reasons of origin of the effects. They postulated that TiO 2 particles effectively passivate recombination surface sites in ZnO, hindering recombination of the charge carriers. They also pointed out faster electron transfer in ZnO than in TiO 2 due to formation of heterojunction. Moreover, the improved properties were explained by higher active surface area due to the TiO 2 deposition.

Figure 34 SEM

 34 Figure 34 SEM images of bare ZnO (A), and core/shell ZnO/TiO 2 composites with TiO 2 layer prepared after 5 min (B), and 10 min (C) of impregnation. Photocurrent transients of the respective photoanodes under solar light (450 W Xe lamp) irradiation at 0.2 V vs. Ag/AgCl (D) [274].

Figure 35

 35 Figure 35 Transient current under dark and illuminated periods for corresponding ZnO and ZnO/TiO 2 composites [275].

2. 2 . 4 N

 24 photocatalytic decomposition of NO x gas under visible and/or UV light. ZnO nanorods deposited on a glass substrate by hydrothermal method were etched with NH 3 and cetyltrimethyl ammonium bromide solution to form ZnO nanotubes. Then, the nanotubes were spin-coated with N-doped TiO 2 prepared by solvothermal reaction from TiCl 3 and hexamethylenetetramine (HMT) as nitrogen doping precursor. A red-shift of absorption edge of ZnO/TiO 2-x N y samples with respect to that of bare ZnO (Figure36A) was ascribed to the contribution of heterojunction. Photocatalytic activity of the samples were studies in conversion NO x gas under irradiation of the wavelength λ > 510 nm, λ > 400 nm, and λ > 290 nm by using 450 W high-pressure mercury lamp with appropriate light filters (Figure36B). The increased photoactivity of ZnO(nanotube)/TiO 2-x N y composite under λ > 400 nm and λ > 290 nm was ascribed to unique surface features of the wellaligned nanotube structure and promoted charge separation at the ZnO/TiO 2 interface.

Figure 36

 36 Figure 36 Optical properties of ZnO, and ZnO/TiO 2-x N y composites (A), and photocatalytic degradation of NO x under λ > 510 nm, λ > 400 nm, and λ > 290 nm light irradiations on: (a) TiO 2 (P25) powder, (b) ZnO nanorod arrays on the glass substrate, (c) ZnO nanotube arrays on the glass substrate, (d) TiO 2-x N y film on the glass substrate, (e) ZnO nanorod arrays/TiO 2-x N y composite on the glass substrate, and (f) ZnO nanotube arrays/TiO 2-x N y composite on the glass substrate [285].

  in photodegradation of 2.5 • 10 -5 M MB (methylene blue), CR (Congo red), and MG (malachite green) dyes mixture of 1:1:1 molar ratio (Figure37Band 37C) under irradiation with light of the wavelength 365 nm, visible > 420 nm, and UV-vis with tungsten halogen lamp. The photocatalyst, in the form of powder (10 mg) was loaded in 25 ml of pollutant mixture solution. The photocatalytic decolorization of the solution was ascribed to formation of super oxide radicals from dissolved molecular oxygen.

Figure 37

 37 Figure 37 TEM image of ZnO/TiO 2 /Au ternary composite (A). Photocatalytic decolorization of methylene blue (MB), and Congo red (CR) under solar (UVvis) light irradiation for ZnO, TiO 2 , ZnO/TiO 2 , (ZT), and ZnO/TiO 2 /Au (ZTA) catalysts (B). Photodecolorization of dye pollutants under UV, visible, and solar light in the presence of ZnO/TiO 2 /Au ternary composite (C). Water photooxidation under zero bias voltage (vs. Ag/AgCl) for ZTA and ZT under visible light irradiation [287].

( 4 -

 4 photodegradation of respectively). This enhancement was ascribed to several effects: efficient separation of charge carriers at ZnO/Au and TiO 2 /Au interface due to formation of Schottky barrier, as well as to inhibition of recombination at the ZnO/TiO 2 interface. This was confirmed by photoluminescence (PL) emission spectra.

Figure 38

 38 Figure 38 Optical properties of ZnO, TiO 2 , ZnO/TiO 2 nanofibers, and ZnO/TiO 2 /Au (A). Inset A: plasmonic peak position of Au in the ternary composite. Percentage of methyl orange (B) and 4-nitrophenol (C) degradation after 30 min irradiation with UV light with the use of mono-, binary and ternary photocatalysts [288].

  Au-decoration). It is composed of Sn doped In 2 O 3 , as In 2-x Sn x O 3-2x (or In 2 O 3 /SnO 2 , usually 9:1 by weight). All details, has been recently summarized in an interesting review concerning preparation, physical and chemical properties of transparent conductive oxides electrodes [291].

  ‗seeded' with a thin layer of ZnO, playing the role of nucleation centers in the synthesis stage. Electrochemical deposition of the seed layers was performed in a three-electrode, single compartment electrochemical cell with ITO working electrode, a Pt counter electrode and aqueous Ag/AgCl/Cl -(3 M KCl) reference electrode (denoted further in the text as Ag/AgCl). The process was carried out in an aqueous solutions of 0.1 M Zn(CH 3 COO) 2 by potentiostatic method at polarization potential of -1.2 V vs. Ag/AgCl using AUTOLAB PGSTAT 30 (Ecochemie, The Netherlands). In effect the ITO was covered with Zn nanoplates and nanoblocks (more details and SEM images of the obtained seed layers are presented Chapter 4.1). Then, the plates were annealed in air at 300 °C for 1 h to transform the superficial layer of Zn into ZnO.3.2.2. ZnO hydrothermal synthesis (preparation stage 2)ZnO nanorods were prepared on seeded ITO surface via hydrothermal synthesis from optimized conditions: 0.04 M Zn(NO 3 ) 2 aqueous solution at fixed pH 10.6 (adjusted by addition of NH 3(aq.) ), in tightly closed Teflon® reactor (presented in Figure39A) at 80 o C [293]. After 2.5 h, the reactor was quickly cooled down to ambient temperature under cold water stream. Such quenching treatment should results in improved crystallinity of ZnO [294]. The samples were washed thoroughly with deionized water and calcined at 300 o C for 1 h (hereafter, ZnO-plates). The hydrothermal synthesis of ZnO in the presence of NH 3(aq.) can be summarized in equations 53 -55:

Figure 39 4 →

 394 Figure 39 Photo of hydrothermal reactors used in synthesis (A) and zinc species fractions in the hydrothermal synthesis solution as function of pH (B) [295].

  case of CVD technique, TiO 2 layers are deposited from the top target source placed above ITO/ZnO sample, whereas in ALD the sample is placed in sprayed atmosphere of the precursor. Deposition in case of CVD is continuous one stage and was carried out from the solution of titanium(IV) isopropoxide for 400 s at 450 o C, while in ALD deposition at ~ 280 o C using multiple cycles. Since, the temperature could not provoke formation of well-crystallized TiO 2 , contrary to CVD; in this case sample was further calcined at 450 o C for 1 h. Each deposition cycle was initialed by injection of sprayed water into the chamber where a sample and sprayed titanium precursor were placed.The synthesis was performed by Dr. Ludovic Avril from the SIOM team of ICB laboratory from Université de Bourgogne Franche-Comté.Decoration of ZnO/TiO 2 composites of the similar morphology of the shell layer (1c and 3c(N)) with Au nanoparticles was achieved by a photodeposition method. Since the amphoteric ZnO dissolves in acids, a simple procedure reported in the literature[296] with the use of HAuCl 4 as gold precursor was modified to reach, non-corrosive conditions. Thus, the pH of aqueous solution obtained by dissolution of 0.083 g HAuCl 4 • xH 2 O in 50 ml DI water, was adjusted to ~ 6.7 with 1 M NaOH, so that [Au(Cl) x (OH) 4-x ] -complex ions were formed. Then, 1c and 3c(N) samples were immersed into the solution and irradiated for 20 s each with 365 nm monochromatic light from LED illumination device (140 mW cm -2 ). Under UV irradiation, the photoexcited electrons generated in ZnO/TiO 2 composite reduce gold ionic precursor and Au nanoparticles are deposited on the semiconductor surface. Finally, the samples were washed with DI water and calcined at 400 o C for 2 h. The obtained Au-promoted ZnO/TiO 2 and ZnO/TiO 2 (N) composites, are hereafter named as ‗1c-Au' and ‗3c(N)-Au', respectively.

  Figure 40A: SE1, SE2, SE3). The electrons are also reflected from the sample by elastic scattering and these back-scattered electrons (BSE) vary in their amount and direction due to the composition and topography of the surface.All these electrons are collected by detectors, and since they possess different energies the signal of different resolution and contrast is recorded. The BSE electrons have energies greater than 50 eV, whereas SE poses energy lower than 50 eV. Brightness and contrast of SE is related to low energy that is electron flux and dose of beam. Small energies may result in accumulation of electrons (negative charging effect) if the sample is a poor electric conductor. Therefore, samples must be electrically conductive, and if not they undergo metallization by sputtering thin layers of Au (a few nm) or a layer of carbon (~ 13 nm). In case of BSE their emitted number is proportional to atomic number of the element in a sample. BSE that are emitted at low angle from a sample can be collected by a LABE detector, which is more sensitive to surface information (Figures40B and 40C). LABE and SE resolutions are comparable[299].

Figure 40

 40 Figure 40 Scheme of electron beam interaction with a material, and subsequent generation of various electron types (A) [300]. SEM image of ZnO (Z Zn = 30) nanorods decorated with Au (Z Au = 79) nanoparticles in SE1 detection mode (B) and LABE mode (C), own results.

  creation the mode is called DF. The image contrast is a result of different absorption (BF) or scattering of incident electrons (DF) by different material nature (density) and thickness. The later one comes from incoherent elastic scattering, while the former one from coherent elastic scattering at Bragg angles [302]. In DF mode properly oriented electron beam towards a sample containing well crystallized particles results in bright contrast. Opposite to that BF imaging results a brighter contrast for material thinner and amorphous. Useful and interesting possibilities are provided further by Scanning Transmission Electron Microscopy (STEM) that allow to scan the probe across the sample, due to additional scan coils to TEM, thus generating various scattered electron signals that after detection are plotted as a function of probe position [303].

  ) together with differential scanning calorimetry(DSC) analyses allow to measure the changes of material weight (TGA) and the simultaneous energy release or absorption in the form of heat (DSC), respectively, while the sample is being heated up, cooled down or kept in isothermal conditions[315]. With the usage of these combine techniques one can i.e. determine the temperature of solid material phase transformation, which usually is connected by noticeable energy change with no weight mass change. The analyses in this work were performed with the use of a SENSYS Evo TGA-DSC, Setaram Instruments (France). Ramping of temperature was set at 5 o C min -1 . The technique provides information of thermal behavior of the sample. The mass of the sample was approx. 10 mg, placed inside platinum crucible under air or nitrogen flow with a flow rate of 20 cm 3 min -1 . (TOC) content in solution was determined by means of Analytik Jena AG TOC analyzer (multi N/C 3100). In such measurement, the solution were pretreated by adding 25 µl of4 M HCl (aq.) in order to remove carbonates. Next, solution was purged with a flow of oxygen. Then samples are burned out at the Pt catalyst at 800 o C, while the water from the solution is soaked by Peltier cooler. Detection is based on CO 2

Figure 42A .

 42A Figure 42A. When a redox reversible couple is present in the solution, in the cyclic voltammogram one can observe the formation of anodic oxidation peak at the potential E pa and cathodic reduction peak at E pc (Figure42B). For ideally reversible redox couple the difference between the peaks potentials ∆E p = E pa -E pc should be 59/n mV.

Figure 42

 42 Figure 42 Potential change in time and a model reversible cyclic voltammogram for reversible redox couple [316].

Figure 43

 43 Figure 43 Scheme (A) and of originally designed LED device used in photocatalytic and photoelectrochemical tests. Temperature change over time of MB aqueous solution in a cuvette placed at fixed distance of 1 cm from LED emitter (B), and photo of the LED device (C) and the UV-vis spectrometer chamber (D); arrows the between figures C and D, symbolize the measurement approach.

  the wavelength of 664 nm. The MB decolorization on TiO 2 obeys first-order kinetics and therefore, apparent first-order rate constants (k vis ) were determined from the linear slope of logarithm of concentration ratio (c/c o ) as a function of time. It is assumed that decolorization of MB on TiO 2 obeys a first-

  Figure 44A, show that in the non-deareated solution the reduction current (black

FigureFigure 44

 44 Figure 44A), i.e. a concurrent irreversible reaction to Zn 2+ /Zn reduction occurred on the electrode in the cathodic range. There are two reasons of this imbalance: reduction of molecular oxygen dissolved in the solution and passivation of deposited Zn, which could be a result of formation of native amorphous zinc hydroxides. According to the literature, the critical value of pH at which an aqueous solution becomes supersaturated with Zn(OH) 2 at 25 o C should be 6.7 [320].

  Figure44Bpresents a chronoamperometric curve obtained in this solution at polarization potential of -1.2 V. The overall shape of current transient is typical of the nucleation and growth process[321]. Namely, the current density after a short minimum, located at about 0.03 s, rises to the plateau and then increases very slowly but continuously in the longer time scale, as for kinetically controlled process.The morphology of the samples obtained after the passage of charge density of about 140 mC cm -2 is illustrated in Figure45A. The Zn-nanosheet structures show dominantly vertical arrangements to the ITO substrate. The formation of plate-like Zn nanostructures by electrochemical methods or thermal metal-vapor depositions have been already reported in the literature[322,323] and the zinc plates obtained by these methods were much thicker (above 100 nm). In contrast, in this work the Zn nanosheets were only a few nm in possibly the thickest part (the darkest edges, Figure45B), and had tendency to grow in multi-layered patterns. The formation of Zn nanosheet structures has been also reported for electrochemical deposition of Zn/Zn 5 (OH) 8 Cl 2 • H 2 O [324] and for Zn(OH) 2 synthesized chemically in liquid-liquid biphasic (organic-aqueous) system containing hydrophobic long-chain carboxylate ions (as heptanoate, decanoate) in organic (xylene) phase [325].

Figure 45

 45 Figure 45 SEM image of Zn seeded ITO surface (A), and TEM image of representative Zn nanosheets deposited and physically removed from ITO surface (B).

Figure 46

 46 Figure 46 SEM images of Zn seed layers potentiostatically deposited on ITO electrode at constant potential of -1.2 V in the non-deaerated solution of 0.1 M Zn(CH 3 COO) 2 at different progression stages of Zn crystal formation.

Figure 47

 47 Figure 47 XRD patterns of the seed layer deposited in aqueous solution of 0.1 M Zn(CH 3 COO) 2 at constant potential of -1.2 V before (A) and after calcination (B). Peaks labeled with asterisks correspond to ITO.

Figure 49

 49 Figure 49 SEM images of ZnO nanorods grown from 0.01 M (A) and 0.08 M (B) initial zinc nitrate concentration after 2.5 h [333], ZnO nanorods grown from 0.02 M zinc nitrate solution after 1 h (C).

  . The XRD pattern of the sample shows dominant contribution of (002) ZnO plane at 2θ = 34.4 o which confirms high preferential A B C orientation and alignment of the ZnO structures. However, other weaker signals from ZnO can be noticed at 2θ: 36.3 o of (101), 47.5 o of (102), 56.6 o of (110), 62.9 o of (103), 68.0 o of (112) planes. The presence of these signals in the XRD pattern can be assigned to different orientation of ZnO nanorods which grown on the side walls of the Zn/ZnO seeds. Other signals than ZnO, i.e. originating from Zn or ITO are negligible suggesting compact coverage of the support surface.

  at 0.8 V (vs. SCE reference electrode) under xenon lamp light irradiation. The results presented above show that similar urchin-like Zn/ZnO nanorods can be obtained on Zn-seeded ITO by means of simple electrochemical/chemical methods, which allow for easy control of nanorods parameters. ZnO seed layers of different morphology (nanosheets and crystals) were electrochemically deposited (at the charge density ~ 140 mC cm -2 ) at the potential of -1.2 V (vs. Ag/AgCl) on ITO electrodes from aqueous non-dearated solution of 0.1 M Zn(CH 3 COO) 2 . The obtained substrate was used for hydrothermal synthesis of ‗obelisk-like' ZnO nanorods. The length and width of obtained ZnO nanorods depend strongly on Zn 2+ concentration and synthesis duration. The arrays of well-separated ZnO nanorods of width varied from 100 nm at tips to ~ 300 nm at bottom and average length of 1.9 µm were prepared in optimized conditions: 2.5 h of growth in 0.04 M Zn(NO 3 ) 2 at pH 10.6 (fixed with ammonia) at 80°C. ZnO nanorods were fabricated in the form of the uniform array on the regions of ITO layer seeded with Zn/ZnO nanosheets, whereas urchin-like ZnO structures were obtained on elemental Zn crystals. The optimization of synthesis conditions allowed to obtain Zn/ZnO(nanorods) well attached to ITO with a good electric contact with the substrate and highly developed surface. Such ITO/Zn/ZnO electrode may be used as the base for further modifications, to prepare core/shell ZnO/TiO 2 composites.

  established. The ZnO nanorods visible in Figure 52A remained almost unmodified, while these in Figure 52B are covered with a thick TiO 2 layer, filling also the space between the nanorods. The jointed ZnO nanorod array and the TiO 2 layer resemble one dense integument that possesses characteristic cracks, visible in the SEM image, formed during calcination.

Figure 52

 52 Figure 52 SEM images of ZnO nanorods with insufficient (A) and excessive (B) amountof TiO 2 deposited by sol-gel method. Inset B shows side view of the ZnO/TiO 2 array detached from the ITO-support.

Figure 53

 53 Figure53Thermogravimetric(TGA, red curves,[START_REF] Koch | The influence of film crystallinity on I the coupling efficiency of ZnO optical modulator waveguides[END_REF] 3, 5) and differential scanning calorimetry (DSC, black curves 2, 4, 6) results for TiO 2 powder precipitated in the bulk of the deposition solution (solid lines, 53A) and for ZnO/TiO 2 composite detached from the support (solid lines, 53B). The dotted lines in Figure 53B correspond to the results obtained for bare ZnO nanorods. All measurements were performed under air flow.

  Figure 54C indicate that ZnO nanorods after three deposition cycles of TiO 2 are covered with a uniform, continuous and smooth coating. Application of low-angle backscattered electron detector (LABE) allowed distinguishing inner ZnO core (less transparent, brighter contrast) from the external TiO 2 shell especially closer to the edge of nanorod (more transparent, darker contrast) due to difference in atomic numbers of Zn and Ti (Figure 54D). Such detection was not possible for 1c sample probably because of a thinner TiO 2 layer deposited on the nanorods.

Figure 54

 54 Figure 54 SEM images of 3c (A, C, D) and 1c (B) ZnO/TiO 2 samples. Single nanorods observed at higher magnification using secondary electron imaging (SEI, C), and low-angle backscattering electron detection -LABE (D), yellow dashed line indicates the boundary between ZnO and TiO 2 . More detailed study of the samples morphology was performed by TEM. A rough TiO 2 layer in 1c sample is well visible in TEM images presented in Figures 55A and 55B. It is also worth noting that in this case the TiO 2 layer is composed of nanoparticles of various shape and size (diameter in the range 3 nm -8 nm, based on analysis of several TEM images of different 1c nanorods). The micrographs presented in Figures 55C and 55D

Figure 55

 55 Figure 55 TEM images of 1c (A, B) and 3c (C, D) ZnO/TiO 2 composites.

Figure 56

 56 Figure 56 Schemes of the two approaches in preparation TiO 2 thin layers on ZnO nanorods: 1c (App. 1) and 3c (App. 2).

  ),(224). No other peaks were observed in TiO 2 powder suggesting formation of pure anatase phase. As to the XRD results of ITO supported samples, all recorded peaks are characteristic of ZnO Standard Pattern (ICDD Card. No. 00-036-1451). Among all ZnO-matched peaks in the samples, the two are particularly noticeable, due to their strong intensity, at 34.4 o and 62.9 o , suggesting strong preferential orientation of ZnO along crystallographic directions[002] and[103], respectively.

  By the reason of such strong alignment of ZnO nanorods and very small amount of deposited TiO 2 , the XRD patterns of ZnO/TiO 2 samples are mostly dominated by these two ZnO signals. Nevertheless, the most intense TiO 2 peak (101) can be still distinguished in the 3c XRD pattern in which the TiO 2 deposited amount was expected to be the highest. The XRD results along with the TGA-DSC data confirmed thus the presence of TiO 2 -anatase after calcination at 450 o C.

1 Figure 58 Figure 59 .

 15859 Figure 58 XRD patterns of: TiO 2 powder obtained in the bulk of the solution by sol-gel method and calcined at 450 o C (red, line 1), 1c (black, line 3) and 3c (grey, line 2) ITO/ZnO/TiO 2 samples. Blue pattern (line 5) corresponds to the reference TiO 2 -anatase, whereas green (line 4) to ZnO. Inset in the right top corner depicts anatase-TiO 2 peak position of (101) planes found in a 3c sample (A). HR-TEM image of TiO 2 nanoparticles with indicated interplanar distance of 3.5 Å corresponding to the anatase-TiO 2 plane (101). Surface chemical composition of ZnO nanorods and ZnO/TiO 2 core/shell systems was determined by analysis of Ti2p, Zn2p and O1s peaks in XPS spectra presented in Figure 59. The positions of peaks of Zn 2p 1/2 and 2p 3/2 at 1021.4 eV and 1044 eV in the spectrum of ZnO nanorods are typical for Zn with oxygen surroundings [341,342]. The same conclusion can be drawn for Ti atoms as Ti 2p 3/2 and Ti 2p 1/2 peaks observed respectively at 458.5 eV and 464 eV, characteristic of Ti-O binding energies in TiO 2 nanoparticles [343,344]. The O1s peak in bare and TiO 2 -covered ZnO nanorods has a complex structure with a pronounced shoulder. The position of the main component (530.2 eV) is typical for oxygen atoms in oxides [345,346]. The shoulder with the maximum at 532.8 eV can be attributed to the surface OH groups [347]. The OH groups detected on the surface of both samples after calcination (300 °C for bare ZnO nanorods or 450 °C for ZnO/TiO 2 ) cannot be due to incomplete dehydroxylation since according to

Figure 59

 59 Figure 59 XPS spectra of pristine ZnO (top row), and ZnO/TiO 2 composites: 1c (middle row) and 3c (bottom row).

Figure 60

 60 Figure 60 Theoretical Zn / Ti atomic ratio as a function of TiO 2 layer thickness deposited on ZnO, determined by calculations by means of QUASES-IMFP-TPP2M software. Inset: scheme on the right side of the curves illustrates the basis of the analysis concept (B).

  Figure 62 (A) Evolution of absorption spectra of aqueous solution of MB during photocatalytic process on 1c ZnO/TiO 2 composite under 400 nm irradiation. (B) Relative concentration changes (c/c o ) of MB determined from decrease of absorption peak at 664 nm during photocatalytic process with the use of ZnO (black triangles, 1), and ZnO/TiO 2 composites: 3c (blue squares, 2) and 1c (red circles, 3). (C) The linear plots of -ln(c/c o ) vs. Time (R 2 > 0.99) used for determination of the apparent first-order rate constants of MB decolorization (k 400 ) from data in figure 62B (lines 1 -3. The reference tests carried out in ‗dark' conditions -ZnO (half-filled triangles), 3c (half-filled squares), and 1c (half-filled circles). (D) Comparison of total organic carbon (TOC) content in the solutions and MB photodecolorization efficiency after 120 min on 3c and 1c samples under 400 nm light irradiation.

Figure 63

 63 Figure 63 Chronoamperometric curves obtained on 1c ZnO/TiO 2 (curve 1, red), and 3c ZnO/TiO 2 (curve 2, blue) composites in dark and under illumination, at constant potential of 0.8 V vs. Ag/AgCl in aqueous solution of 5 mM Na 2 SO 4 .

Figure 64

 64 Figure 64 Cyclic voltammograms obtained on 3c ITO/ZnO/TiO 2 in dark (dashed lines), and under illumination (solid lines) in acetonitrile solution of 0.1 M LiClO 4 (curves 1 and 1') and the same solution containing 10 -3 M Fc (curves 2 and 2').

Figure 65

 65 Figure 65 Comparison of the cyclic voltammograms obtained for ITO/ZnO (A), 3c ITO/ZnO/TiO 2 (B) and 1c ITO/ZnO/TiO 2 (C) in dark (curves 1) and under illumination with xenon lamp with cut-off ≤ 400 nm filter (curves 2) and without filter (curves 3) in the solution of 1 mM Fc in 0.1 M LiClO 4 /AN. Curve 4 in the plot (A) corresponds to the cyclic voltammogram obtained in the same solution on small Pt disc electrode. Figure 63D presents the dependences of the cathodic peak currents (▲dark ‗CD', ▲photo ‗CP') and anodic currents (•dark ‗AD', •photo ‗AP') at the potential of 1 V on the root of the scan rate for 1c sample.

  Figure 66 UV-vis absorption of waste water leachate under irradiation of 400 nm monochromatic light and in presence of ZnO/TiO 2 (N) composite -3c(N) photocatalyst (A), comparison of different photocatalyst activity in remediation of waste water solution (B), TOC removal from initial sample after 360 min of 400 nm irradiation in presence of the photocatalysts (C), evolution of absorption spectra of non-diluted waste water solution under illumination and presence of 3c(N). Inset D: absorption decrease at 254 nm with respect to initial value after 1 st (pruple), 2 nd (green), and 3 rd (grey) photocatalytic run on 3c(N) under 400 nm irradiation after 2, 6, and 11 hours.

Figure 67

 67 Figure 67 SEM images of ZnO/TiO 2 composites with prepared TiO 2 shells by CVD (A, B) and ALD (C, D) techniques.

  TiO 2 surface layer seems to increase from bottom to top, Figure 67B. Such observation A B D Cconfirms initial predictions of the more top-targeted deposition technique. In case of ALD-prepared TiO 2 shell the composite nanorods ZnO/TiO 2 seems to possess strong surface uniform roughness consisted of tiny deposited nanoparticle (Figure67D). More details about morphology were revealed on HAADF-STEM and HR-TEM images of ZnO/TiO 2 composited with TiO 2 layers prepared by CVD (Figure68) and ALD (Figure69).

Figure 68

 68 Figure 68 HAADF-STEM (A) and HR-TEM (B) images of ZnO/TiO 2 nanorods with TiO 2 layers prepared by CVD. Specific alignment of the TiO 2 deposited on ZnO is indicated with yellow arrow in red circle.

(

  Figures 69 HAADF-STEM (A), TEM (B), HR-TEM (C) images of ZnO/TiO 2 nanorodswith TiO 2 layers prepared by ALD technique.

  the overlapped particles. However, it was possible to notice in the images presented in Figures 69C some interplanar distances of 3.5 Å of well-crystallized TiO 2 anatase. The ALD preparation resulted, therefore, in a composite which appear like 3c ‗sol-gel' sample in two aspects: i) the continuity of TiO 2 shell and ii) the overall thickness of the layer. To conclude, one can observe that sol-gel samples: 1c and 3c have their similar counterparts prepared by CVD and ALD, respectively.

  presence of anatase TiO 2 in the samples). As visible in Figure 70B, all peaks corresponding to ITO and ZnO are also present in both ‗ALD' and ‗CVD' spectra, as it was observed in the ‗sol-gel' samples. The signals of TiO 2 anatase can be also detected, but only at 2θ = 25.3 o , (101) plane, due to the low amount of deposited material.

Figure 70

 70 Figure 70 UV-vis spectra of ZnO/TiO 2 composites with TiO 2 shells prepared by CVD, and ALD with a comparison of bare ZnO, and the ‗sol-gel 'ZnO/TiO 2 composites (A), XRD patterns of the ‗CVD' and ‗ALD' composite samples.

  photocatalytic activity in comparison with ITO/ZnO under irradiation with 400 nm light.Higher photocatalytic activity was observed for 1c sample than for 3c both in MB degradation and in photoassisted water oxidation. This effect was attributed to a more open structure of TiO 2 shell in 1c sample which allows better access of the reacting species from solution to the ZnO/TiO 2 interface.

1 .

 1 Evidence for a key role of ZnO/TiO 2 interface in photocatalysis In order to prove the importance of ZnO/TiO 2 interface in the photocatalytic decolorization of MB, comparative studies were performed on the 3c sample and on the same sample after dissolution of ZnO core in HCl (aq.) at pH 3 for 20 h (hereafter designated as '3c-HCl'), Figure 71.

Figure 71

 71 Figure 71 SEM (A, B) and TEM (C) images of hollow nanostructures obtained afterremoval of ZnO-core in HCl (aq.) from initial (core)ZnO/TiO 2 (shell) composites.

  The results presented in Figure72Aindicate that decolorization rate of MB significantly decreased when the 3c sample was treated with HCl (aq.)(20 h) and the apparent rate constant of the process (k 400 ) decreased by a factor 6.4 with respect to that obtained for the non-treated 3c sample. These results show that the presence of ZnO/TiO 2 interface plays indeed a crucial role in the photocatalytic process. In order to understand the origin of this effect the optical absorbance spectra of the samples were recorded (Figure72B).

Figure 72

 72 Figure 72 Photodecolorization of MB with use of the ZnO/TiO 2 composites before (blue scatter 1, 3c) and after (red scatter 2, 3c-HCl) dissolution of ZnO-cores in HCl (aq.) (A). Inset A: values of apparent first-order rate constants (k 400 ) calculated for 3c (blue) and 3c-HCl (red). Absorption spectra of 3c (blue curve 1), 3c-HCl (red curve 2), whereas the black curve 3 corresponds to the spectrum obtained for TiO 2 (B). The UV-vis absorption spectrum of the 3c sample (curve 1 in Figure 72B) presents two characteristic features: formation of a pronounced ‗Urbach tail' in the wavelengths range from 600 nm to 420 nm, absent in the spectrum of TiO 2 powder, and the shift of the absorption threshold towards visible range. Consequently, these two effects may be ascribed to the formation of interfacial states at the ZnO/TiO 2 boundary, with the energy levels localized in the band gap [371]. Hence, the sub-band gap optical transitions become possible as it was discussed more in detail in Chapter 4.2, paragraph 4.2.1.However, the treatment with HCl (aq.) leads to the shift of the sample absorption spectrum back towards UV range and to a significant diminution of the absorbance tail. This behavior may be explained by disappearance of the interfacial states due to dissolution of ZnO in HCl (aq.) . Since the results presented above supported the key role of ZnO/TiO 2 interface in photocatalytic activity of the composite, an attempt to modify the interfacial

Figure 73

 73 Figure 73 Photodecolorization of MB on the catalysts: 3c and 3c-Cal (A), 3c-HCl and 3c-Cal-HCl (B).

Figure 74A ,

 74A Figure74A, the spectrum of 3c-Cal slightly differs from that of 3c in the visible range, between 420 nm and 600 nm. The calcination leads to diminishing of the ‗Urbach tail' and decrease of the absorbance in this range. The optical band gaps determined from the

  lower electron-hole recombination rate in this sample. To check this possibility the composites were characterized by measurements of open-circuit potential (V oc ) decay.

Figure 74 (

 74 Figure 74 (A) Optical properties of the ZnO/TiO 2 composites: initial 3c sample (1, blue curves), and after additional calcination 3c-Cal (2, black curves). Inset (A): the Tauc plots used for determination of optical band gaps (E g ) of the samples. (B) XRD patterns of the initial 3c sample and after an additional calcination for 3 h at 450 o C (3c-Cal).

  photocurrent was obtained for the 3c-Cal electrode than that for the 3c sample, in a broad range of applied potentials (from 0.4 V to 1.0 V) under illumination at 400 nm (solid

Figure 76 (

 76 Figure76(A) Linear sweep voltammograms for the 3c (blue curves, 1 and 3) and 3c-Cal(black curves, 2 and 4) electrodes in dark (dash-dotted curves) and under illumination (solid curves). LSV taken at a sweep rate of 10 mV s -1 in 5 mM Na 2 SO 4 aqueous solution, while the photocurrent transients (B) of 3c (blue, 1) and 3c-Cal (black, 2) were measured at constant potential of 0.8 V vs. E Ag/AgCl .At the same time the electrons from CB are transported to the ITO substrate and then to the external circuit. The significant increase of photocurrent upon the additional calcination can be explained, as in the case of V oc by prolonged lifetime of the photogenerated charge carriers and increased flux of the electrons to the external circuit.

  was used to examine thermal modifications of the ZnO/TiO 2 composites. By means of HAADF-STEM technique it is possible to acquire images in which the signal is proportional not only to the thickness but also to atomic number of the elements (Z-contrast). In HAADF-STEM images presented in Figure77, the external TiO 2 shell, the ZnO core, and so the internal structure of composites can be clearly distinguished. Figures 77A and 77B present, respectively, the results for ZnO nanorods covered with as-deposited amorphous TiO 2 layers (prepared in three subsequent deposition cycles without any calcination after each one) and the same sample after calcination for only 1 h at 450 o C (the calcination was done after the third deposition cycle).

Figure 77

 77 Figure 77 HAADF-STEM images of ZnO nanorods covered with amorphous TiO 2 layer after three deposition cycles without any calcination (A), and the same sample after 1 h of calcination at 450 o C (B). Yellow arrow on the Figure B depicts initial spot of void enlargement. Images of ZnO/TiO 2 composites: 3c (C) and 3c-Cal (D, E). Inset D, in red frame, shows a hollow channel through the 3c-Cal nanorod, whereas SEM image of the same sample, intentionally broken, reveals internal structure seen from the top view.

Figure 78

 78 Figure 78 STEM-EDX elemental maps of 3c (1), and 3c-Cal (2). White scale bars length =100 nm. EDX-STEM atomic mapping and line scan profiles of the ZnO/TiO 2 systems were acquired to confirm the presence of zinc in the TiO 2 external layer. The maps of elements distribution (Figure 78) indicate that zinc is indeed present in the TiO 2 layers of the 3c and 3c-Cal samples, before and after the additional calcination.

  line scan analyses were done along the red lines as indicated in the HAADF-STEM images in Figure 79. As visible, the amount of zinc increases in the outer part of TiO 2 layer after additional calcination of 3c. These results showed that the process of zinc ions diffusion into the TiO 2 layer took place already in the 3c sample and becomes more pronounced after additional calcination (3c-Cal).

Figure 79

 79 Figure 79 Zinc count profiles obtained from linear EDX-STEM analysis across the ZnO/TiO 2 nanorods: 3c and 3c-Cal. Insets: HAADF-STEM images of the 3c (bottom) and 3c-Cal (top) samples with red arrows indicating the line scan direction.

Figure 80

 80 Figure 80 XPS spectra of ZnO/TiO 2 composite systems: 3c, 3c-Cal, 3c-HCl, and 3c-Cal-HCl.

From

  the conclusion driven from the enrichment of TiO 2 with zinc a question regarding the nature of the thus formed phase arises. It should be noted that a gradient of Zn concentration exists across the TiO 2 layer and consequently different phases might be formed depending on the distance from ZnO surface. Thus, zinc-rich titanates (Zn 2 TiO 4 or ZnTiO 3 ) may exist in the close proximity to ZnO. In contrast, the bulk of TiO 2 layer can accommodate Zn ions in the form of a zinc-poor solid solution. The absence of any peak of zinc titanates in the diffractograms of 3c-Cal, Figure74B) does not however allow to exclude their formation but indicates that the main phase is a zinc-containing TiO 2 -anatase. This fact points to a possible reason of the increase of the photoexcited electron lifetime observed in the 3c-Cal material. We attribute it to the presence of oxygen vacancies whose formation allows to compensate the charge of Zn 2+ cations incorporated into the TiO 2 -anatase structure. Higher concentration of the oxygen vacancies in the Zn-enriched TiO 2 is confirmed by photoluminescence (PL) spectra of 3c-HCl and 3c-Cal-HCl (Figure81). The intensity of the peak at 510 nm, whose appearance is correlated with formation of oxygen vacancies[383,384,385], increases after additional calcination.

Figure 81

 81 Figure81Room-temperature photoluminescence spectra of 3c-HCl[START_REF] Koch | The influence of film crystallinity on I the coupling efficiency of ZnO optical modulator waveguides[END_REF], 3c-Cal-HCl (2), and ITO-support as reference (3), excited at 400 nm.

Figure 82A .

 82A Figure 82A. Crystallization of this layer during calcination at 300 o C results in its shrinking and formation of a ‗dented' surface layer. Such rough surface should be enriched with steps and/or kinks which contain an important number of undercoordinated Zn 2+ possessing high reactivity. During sol-gel deposition the amorphous Ti-containing species fill up the ‗dents' at the ZnO surface and thus come in a close contact with the highly reactive zinc species. This particular configuration would allow for diffusion of Zn 2+ into TiO 2 already at 450 o C. We consider that both this specific surface roughness of ZnO nanorods and the used sol-gel deposition method make possible Zn ion diffusion at such moderate temperature.

Figure 82 Figure 83

 8283 Figure 82 HAADF-STEM images of hydrothermally as-synthesized ZnO nanorods (A), and after their calcination at 300 o C for 1 h (B). Samples B after subsequent their calcination at 450 o C for: 1 h, 3 h, and 6 h are presented on images, C, D, and E, respectively.

[

  391], as it is shown in Figure 85B. The 3c(N) composite proved to have lower value of E g (~ 2.78 eV) in a comparison to that of the 1c sample (E g ~ 2.90 eV).

Figure 85

 85 Figure 85 (A) Comparison of UV-vis absorption spectra of ZnO NRs (curve 1) TiO 2 (curve 2) and composites: ZnO/TiO 2 (1c, curve 3), ZnO/TiO 2 (N) (3c(N), curve 4), ZnO/TiO 2 /Au (1c-Au, curve 5) and ZnO/TiO 2 (N)/Au (3c(N)-Au, curve 6). (B) Determination of optical band-gap energies from the absorption spectra of 1c (curve 1) and 3c(N) (curve 2) samples.

1 Figure 86

 186 Figure 86 XRD patterns used for the determination of the average sizes of Au NPs from the broadening of 111 peak of metallic gold (located at 2θ = 38.2).

15

  These findings allow to conclude that the used sol-gel procedure resulted in successful doping of TiO 2 layer with nitrogen. Concerning the spectra of Ti and Zn, their positions are 458.2 eV and 464.0 eV for Ti2p, and 1021.6 eV and 1044.6 eV for Zn2p, corresponding to typical binding energies in metal oxides[402,403] (Figure87C, 87D, 87E). In case of the O1s XPS spectrum, the maximum at 529.8 eV assigned to oxygen in metal oxides [404] is followed by a shoulder corresponding to oxygen of the surface adsorbed hydroxyl groups[405].

  Figure 87 XPS spectra of N1s (A) in nitrogen doped TiO 2 deposited on ZnO, and Zn3p and Au4f in Au-decorated composites (B). XPS spectra of Ti2p (C), Zn2p (D), O1s (E) in nitrogen doped TiO 2 deposited on ZnO.

4. 4 . 3 .

 43 Microstructure of the composites N-modified TiO 2 in ZnO/TiO 2 composite -3c(N) The microstructure of 1c sample was characterized in detail in our previous work. The SEM and HAADF-STEM images of 3c(N) sample are presented in Figure 88. They reveal the presence of a continuous and visibly porous layer of TiO 2 anatase of ca. 20 nm in thickness (Figure 88B). Similarity of microstructure of TiO 2 layer in 1c and 3c(N) Chapter 4.2, Figure 55) justifies our choice of 1c as the nitrogen free composite for comparison with 3c(N).

Figure 88

 88 Figure 88 SEM (A) and HAADF-STEM (B) images of 3c(N) sample.

Figure 89 SEM

 89 Figure 89 SEM images of 3c(N)-Au (A) and 1c-Au (B) samples in low-angle backscattered electron (LABE) detection mode.

Figure 90

 90 Figure 90 TEM and HAADF-STEM images of 3c(N)-Au (A, B), 1c-Au (C, D) samples and HR-TEM images of Au NPs deposited on 3c(N)-Au (E), and 1c-Au (F), respectively.

1 AuFigure 91

 191 Figure 91 EDX chemical analysis in HAADF-STEM mode on selected areas: with (pt 1)and without Au nanoparticles (pt 2).

Figure 92

 92 Figure 92 Decolorization of MB under visible light irradiation (Xe lamp with 400 nm cut-off filter) without any catalyst (line 0) and in the presence of composites: 1c (line 1), 3c(N) (line 2), 3c(N)-Au (line 3), and 1c-Au (line 4) (A) and determination of k vis -values of the catalysts (B).

Figure 93

 93 PhotoelectrocatalysisMB decolorization

Figure 94 Figure 94 (

 9494 Figure 94 presents a comparison of the linear sweep voltammograms obtained for the non-modified ZnO/TiO 2 composite (sample 1c), modified by N-doping (sample 3c(N)), and the two other samples additionally decorated with Au nanoparticles (samples 1c-Au and 3c(N)-Au, respectively). The voltammograms were recorded in dark and under illumination with visible light (λ > 400 nm).

  photoelectrochemical activity of the samples was studied at constant potential. The current transients recorded for all studied electrodes at the potential of 0.8 V vs. Ag/AgCl in dark and under illumination are presented inFigure 95.

Figure 95

 95 Figure 95 Comparison of the current transient in dark and under illumination with visible light at constant potential of 0.8 V vs. Ag/AgCl for the samples: (A): 1c-Au (plot 1) and 1c (plot 2); (B): 3c(N)-Au (plot 3) and 3c(N) (plot 4).

Figure 95 ,Figure 96

 9596 Figure 96 Long-term chronoamperometric tests for the sample 3c(N) in dark andunderillumination at constant potential of 0.8 V vs. Ag/AgCl.

  current spikes just after the switching light on, followed by exponential decays within 1 -3 seconds to the steady-state values. The current overshoot in opposite direction and decay back to zero was observed after switching light off. These characteristic spikes in the onset regions of photo-and dark currents, are an evidence for e -/h + surface recombination, competing with the light-driven electrode reactions at the semiconductor/solution interface[57]. It is interesting to note, that intensity of the spikes

(

  Figure 97C).

Figure 97

 97 Figure 97 Current transients recorded during polarization of ZnO/TiO 2 /Au electrode (1c-Au) (A) and ZnO/TiO 2 (1c) (B) in dark and under illumination with visible light in aqueous solution of 5 mM Na 2 SO 4 at constant potential of -0.25 V vs. Ag/AgCl. Current transients recorded during polarization of ZnO/TiO 2 (N)/Au electrode (3c(N)-Au) (C) and ZnO/TiO 2 (N) (3c(N)) (D) in dark and under illumination with visible light in aqueous the solution containing 5 mM Na 2 SO 4 at constant potential of -0.25 V vs. Ag/AgCl.

  presented above indicate that the electrochemical reduction of oxygen at Au-decorated electrodes (1c-Au, 3c(N)-Au) is enhanced by visible light, we decided to verify if products of O 2 electroreduction may improve photoelectrochemical degradation of methylene blue in the range of negative potentials. At first it was necessary to examine the electrochemical behavior of methylene blue on ZnO/TiO 2 electrodes (1c) without Au nanoparticles. The cyclic voltammograms were recorded in the potential range from 0.8 V to -0.4 V vs. Ag/AgCl in dark and under visible light illumination in aqueous solution of 10 µM MB, containing 5 mM Na 2 SO 4 as the supporting electrolyte. As visible in Figure98A, a quasi-reversible redox behavior with cathodic peak at -0.27 V and its anodic counterpart at -0.18 V is observed in dark, which may be ascribed to reduction of MB to semi-reduced radical MB•-[428] and its reoxidation back to MB. The intensity of the reduction peak increased only slightly under illumination with visible light. In the case of voltammograms performed on the Au-decorated electrode (1c-Au), the cathodic current related to catalytic oxygen reduction dominates and increases significantly under illumination of the sample with visible light. Thus, if this process leads to production of superoxide radicals and then in the consecutive reactions (71) and (72) to formation of • OH, the photoelectrocatalytic degradation of MB at the negative polarization potential should be improved. In order to verify this reasoning, the UV-vis spectra of the MB solution were recorded after every 15 min of electrolysis to determine the change in MB concentration. The measurements were carried out both for pristine (1c) and Au-decorated (1c-Au) electrodes (Figures98B-D. The corresponding apparent rate constants of MB decolorization are summarized in Table

Figure 98

 98 Figure 98 Cyclic voltammograms (A) and relative changes of MB concentration at constant potential of -0.25 V vs. Ag/AgCl (B) in dark and under visible light illumination for samples 1c (curves 1-dark and 2-vis) and 1c-Au (curves 3-dark and 4-vis) in aqueous solution of 5 mM Na 2 SO 4 containing 10 M MB. (C) Influence of oxygen in the solution and application of external cathodic potential (-0.25 V vs. Ag/AgCl) on photoelectrocatalytic decolorization of MB with the use of ZnO/TiO 2 /Au composite (1c-Au), and determination of k vis -values of MB decolorization (D).

  Figure 98B, polarization of the Au-free sample (1c) in dark at the potential of -0.25 V vs. Ag/AgCl corresponding to the reduction peak of MB to MB •-, results in a slight decolorization of MB. The relative decrease of MB concentration in the solution after 60 min of electrolysis was about 9%. According to the literature, the semi reduced MB •-radical may undergo to disproportionation to MB and colorless leuco-methylene blue (LMB).

  Nanodruty zostały następnie pokryte cienką warstwą TiO 2 , stosując różne warunki jego osadzania. Skład i mikrostruktura uzyskanych kopozytów ZnO(rdzeń)/ TiO 2 (powłoka) były modyfikowane w celu określenia wpływu tych parametrów na właściwości fotokatalityczne układu. TiO 2 /Au i ZnO/TiO 2 (N)/Au były badane po ich oświetleniu lampą ksenonową z filtrem odcinającym wszystkie długości fali światła poniżej 400 nm. Stwierdzono, że po osadzeniu nawet niewielkiej ilości nanoczastek Au (0.37 % at.) na ZnO/TiO 2 , dzięki efektowi plazmonowemu uzyskuje się 60 % wzrost szybkości fotodekoloryzacji MB w

	zawartości dostepnością granicy faz ZnO/TiO 2 dla reagentów. Dla porównania, powłoki TiO 2 o całkowitego węgla organicznego), RT-PL (spektroskopia
	fotoluminescencyjna w warunkach temperatury pokojowej), techniki elektrochemiczne: podobnej morfologii zostały również osadzone na nanodrutach ZnO metodami ALD
	m.in. LSV (woltamperometria z liniową zmianą potencjału), CV (cykliczna (osadzania wartw atomowych) i CVD (chemicznego osadzania z fazy gazowej) i okazało
	woltamperomeria), chronoamperometria, chronopotencjometria, oraz autorski zestaw się, że kompozyty przygotowane przy użyciu opisanej powyżej prostej metody zol-żel
	Następnie, zastosowane zostały metody uaktywnienia kompozytu w świetle widzialnym, składający się z diody (LED) wysokiej mocy, emitującej monochromatyczne światło o wykazują porównywalną (a nawet wyższą) fotoaktywność. Ponadto wykazano i świetle widzialnym, natomiast zastosowanie kompozytu ZnO/TiO 2 (N)/Au pozwoliło na
	poprzez zdomieszkowanie TiO 2 azotem oraz osadzenie na powierzchni katalizatora długości fali 365 nm lub 400 nm, wykorzystany do oświetlania układów w badaniach potwierdzono metodą pomiaru całkowitego węgla organicznego (TOC), że opracowane trzykrotny wzrost fotoprądu w procesie utlenienia wody przy potencjale 0.8 V względem
	nanocząstek Au. fotokatalitycznych i fotoelektrokatalitycznych kompozyty ZnO/TiO 2 są aktywne w fotokatalitycznym rozkładzie zanieczyszczeń elektrody Ag/AgCl. Wykazano również, że kompozyty z osadzonym złotem posiadaja
	Niniejsza rozprawa składa się z trzech częsci: Bibliography, Experimental i Results and Discussion. organicznych zawartych w próbach wody pochodzacej z terenów z miejskich wysypisk silne właściwości elektrokatalityczne w reakcji redukcji tlenu do aktywnych form tlenu Najważniejsze wyniki uzyskane w trakcie realizacji pracy zostały przedstawione i przedyskutowane w trzeciej części pracy, zatytułowanej Results and Discussion, śmieci. (poprzez tworzenie rodników nadtlenowych O 2 ⦁ -), przy stosunkowo niewielkim
	składającej się z trzech rozdziałów. potencjale ujemnej polaryzacji (-0.25 V). Oświetlenie spolaryzowanej próbki światłem W rozdziale trzecim (Chapter 4.3) wykazano, że kluczową rolę w zwiększeniu Pierwsza część, Bibliography, stanowi przegląd literatury dotyczącej tematu rozprawy, najważniejszych właściwości półprzewodników, granicy aktywności fotokatalitycznej opracowanych kompozytów w widzialnym spowodowało wzmocnienie tego procesu, w wyniku czego nastąpił wzrost świetle faz szybkości fotorozkładu MB, nawet w obecnosci Na 2 SO
	półprzewodnik/roztwór oraz zjawisk i procesów związanych z fotokatalizą i
	Ostatnio wykazano,
	że kompozyty ZnO/TiO 2 posiadają korzystniejsze właściwości fotokatalityczne niż	2
	każdy z tych materiałów osobno. Pomimo wielu doniesień litaraturowych metodą zol-żel i jego domieszkowania azotem oraz procedury fotoosadzania
	potwierdzających wysoką fotoaktywność tych układów w różnych zastosowaniach, nanocząstek Au. Ponadto krótko opisane zostały metody pomiarowe stosowane w trakcie spektroskopii fotoelektronów (XPS) dowiodła, że procesowi temu towarzyszy dyfuzja
	wciąż jednak brakuje danych dotyczacych zależności między ich strukturą i realizacji badań: SEM (skanningowa mikroskopia elektronowa), HAADF-STEM Pokrycie ZnO zwartą warstwą TiO 2 o grubości 25 nm -40 nm uzyskiwano w wyniku jonów Zn do warstwy TiO 2 . Wyniki te pozwoliły na sformułowanie wniosku o
	właściwościami. Aby wypełnić tę lukę, w ramach niniejszego projektu doktorskiego (skanningowa transmisyjna mikroskopia elektronowa z obrazowaniem za pomocą trzech nastepujących po sobie 30-minutowych syntez, z wygrzewaniem próbki między wystąpieniu efektu Kirkendalla na granicy faz ZnO/TiO 2 . Zaobserwowanie tego efektu
	podjęto systematyczne badania nad wpływem mikrostruktury kompozytów ZnO/TiO 2 elektronów rozproszonych pod dużymi kątami), XRD (rentgenowska analiza dyfrakcyjna kolejnymi cyklami osadzania. Pierwszy z tych kompozytów wykazywał znacząco po raz pierwszy w tak niskiej temperaturze (450 o C) było możliwe dzięki dużej ilości
	typu rdzeń-powłoka na ich aktywność w procesach fotodegradacji modelowych promieniowania X), XPS (spektroskopia fotoelektronów w zakresie promieniowania X), wyższą aktywność zarówno w procesie fotodegradacji modelowego zanieczyszczenia defektów strukturalnych na powierzchni ZnO.
	EDS ‗lub EDX' (rentgenowska spektroskopia energodyspersyjna) analiza połączona z organicznego (błękitu metylwnowego, w skrócie MB), jak i fotoelektrochemicznego zanieczyszczeń organicznych i fotoelektrochemicznego utleniania wody. Stosowane do technikami mikroskopii elektronowej, spektroskopia absorpcyjno-emisyjna UV-vis, DRS utleniania wody, przy naświetlaniu próbki światłem monochromatycznym o długości fali Ostatni rozdział (Chapter 4.4) dotyczy kompozytów ITO/ZnO/TiO 2 dodatkowo
	(spektroskopia odbicia dyfuzyjnego w zakresie UV-vis), TGA-DSC (analiza 400 nm. Wspomniana zwiększona fotoaktywność została skorelowana z mikrostrukturą
	termograwimetryczna połączona z skanningową kalorymetrią różnicową), TOC (analiza kompozytu i wyjaśniona większą porowatością warstwy TiO 2 , a w konsekwencji lepszą

tego typu badań kompozyty powinny posiadać dużą powierzchnię właściwą i wykazywać dobre przewodnictwo elektryczne. Dlatego w pracy skoncentrowano się na układach bazujących na nanodrutach ZnO wytworzonych na elektrodzie ITO (szkle pokrytym cienką warstwą tlenku indowo-cynowego). fotoelektrochemicznym utlenianiem wody. W części tej opisane zostały również fizykochemiczne i fotokatalityczne właściwości ZnO i TiO 2 oraz układów kompozytowych. Przedstawione zostały również metody zwiększenia absorpcji promieniowania w zakresie światła widzialnego, takie jak domieszkowanie TiO 2 azotem oraz wykorzystanie efektu plazmonowego nanocząstek metali szlachetnych (np. Au) osadzonych na powierzchni półprzewodników. W drugiej części, zatytułowanej Experimental, opisane zostały procedury opracowane do syntezy układów ZnO/TiO 2 oraz metody stosowane w pracy do ich charakterystyki. Podane zostały szczegółowe informacje dotyczące elektrochemicznego zasiewania podłoża ITO, hydrotermalnego wzrostu nanodrutów ZnO, syntezy TiO W pierwszym rozdziale (Chapter 4.1) przedstawione zostały wyniki badań dotyczących elektrochemicznego zasiewania elektrody ITO cienką warstwą Zn/ZnO, w roztworze Zn(CH 3 COO) 2 . Stanowiła ona podłoże dla nanodrutów ZnO, których długość i średnica kontrolowane były poprzez zmianę stężenia jonów Zn 2+ i czas trwania syntezy hydrotermalnej. Po optymalizacji parametrów syntezy, na zasianym podłożu ITO uzyskiwane były w sposób powtarzalny warstwy nanodrutów, dobrze od siebie odseparowanych, w kształcie "obelisków‖ o średnicy ok. 100 nm u wierzchołka i 300 nm u podstawy oraz długości ok. 1.9 μm. Drugi rozdział (Chapter 4.2) dotyczy metody pokrywania powierzchni nanodrutów ZnO cienką warstwą TiO 2 poprzez hydrolizę butanolanu tytanu (IV). W rezultacie zaproponowano dwa warianty syntezy zol-żel, prowadzące do uzyskania warstw o różnej morfologii, chropowatej i zwartej. Warstwa chropowata była uzyskiwana w trakcie jednoetapowej, 6-godzinnej syntezy, po zakończeniu której próbka była wygrzewana w temperaturze 450 o C w celu przekształcenia bezpostaciowego TiO 2 w formę anatazu. monochromatycznym (= 400 nm), odgrywa granica faz ZnO/TiO 2 . Efekt ten został przypisany przesunięciu granicy absorpcji światła przez ten układ w kierunku fal dłuższych w stosunku do zakresu promieniowania absorbowanego przez warstwę TiO 2 pozostałą po wytrawieniu rdzenia ZnO. Dalszy wzrost aktywności fotoktalitycznej ZnO/TiO 2 osiagnięto po dodatkowym wygrzaniu kompozytu przez 3 h w temperaturze 450 o C. Dzięki tej prostej procedurze uzyskano 40% wzrost szybkości fotorozkładu MB i dwukrotny wzrost fotopradów w reakcji utleniania wody. Wyniki pomiaru zmian potencjału obwodu otwartego (V oc ) dowiodły, że przyczyną poprawy właściwości fotokatalitycznych układu ZnO/TiO 2 po dodatkowej kalcynacji jest zmiejszenie szybkości rekombinacji fotowygenerowanych nośników ładunku. Badania z zastosowaniem skaningowego transmisyjnego mikroskopu elektronowego (STEM) wykazały, że dodatkowa kalcynacja układu prowadzi do wytworzenia "luk‖ na granicy faz ZnO i TiO 2 . Analiza składu pierwiastkowego metodą rentgenowską (EDX) i metodą zmodyfikowanych poprzez zdomieszkowanie warstwy TiO 2 azotem (TiO 2 (N)) i osadzenie na niej nanoczastek Au w celu zwiekszenia ich fotoaktywności tych układów w zakresie światła widzialnego. Właściwości fotokatalityczne kompozytów ZnO/4 . To rozwiazanie pozwoliło przezwyciężyć problem inhibicji procesu fotokatalitycznego przez obecność w roztworze anionów soli nieorganicznych. Dzieki temu opracowane w niniejszej rozprawie układy stanowią bardzo atrakcyjny materiał fotokatalityczny, który może zostać zastosowany do oczyszczania ścieków zawierajacych często znaczne ilości jonów soli nieorganicznych.

  TiO 2 non associé avec le coeur de ZnO. Une amélioration supplémentaire de l'activité photocatalytique de ZnO/TiO 2 a été obtenue grâce à sa calcination supplémentaire à 450 °C pendant 3 h. Ce traitement simple augmente de 40% la vitesse de décomposition du MB et multiplie par deux le photocourant dans l'oxydation de H 2 O. Les mesures du 0.25 V contre Ag/AgCl). L'illumination de l'échantillon polarisé avec de la lumière visible a permis encore d'améliorer ce processus, ce qui a provoqué une décomposition rapide du MB même en présence de Na 2 SO 4 . Cette approche permet donc de surmonter le problème d'inhibition du processus photocatalytique sur des catalyseurs non polarisés par des sels inorganiques dissous. Elle présente ainsi une voie prometteuse vers une nouvelle technologie de purification photoélectrocatalytique des eaux usées, contenant souvent des quantités importantes de sels inorganiques.

potentiel de circuit ouvert ont montré que les propriétés améliorées des composites ZnO/TiO 2 calcinés proviennent de la diminution de la vitesse de recombinaison électrontrou. HAADF-STEM a montré que la calcination supplémentaire a entraîné la formation de cavités à l'interface ZnO/TiO 2 . Les analyses EDX et XPS ont révélé que la formation des cavités s'accompagne de la diffusion externe des ions Zn dans la couche de TiO 2 et a permis ainsi d'établir l'existence de l'effet Kirkendall à l'interface ZnO/TiO 2 .

L'apparition de cet effet observé pour la première fois à une température modérée (450 °C) a été attribuée à une concentration importante de défauts dans la couche superficielle des nanobâtonnets de ZnO. Dans le dernier chapitre (Chapitre 4.4), les composites constitués de nanobâtonnets de ZnO supportés sur ITO et recouverts de dioxyde de titane dopé à l'azote, TiO 2 (N), ont été décorés de nanoparticules d'or (Au NP) afin d'améliorer leur activité photocatalytique sous lumière visible. Les propriétés photocatalytiques des composites ternaires ZnO/TiO 2 /Au et ZnO/TiO 2 (N)/Au ont été étudiées sous illumination de lampe Xe équipé d'un filtre pour le rayonnement en dessous de 400 nm. Nous avons constaté que même une faible teneur en or (0,37% en poids) entraîne une amélioration de la décoloration photocatalytique de MB sous lumière visible de 60% par rapport à l'échantillon exempt d'or en raison de l'effet plasmonique. En outre, une utilisation simultanée de dopage par l'azote et de la décoration par Au NP permet de multiplier par trois le photocourant de l'oxydation photoélectrochimique de l'eau à 0,8 V (Ag/AgCl). Il a également été démontré que les composites décorés de nanoparticules d'or possèdent en absence d'illumination une forte activité électrocatalytique dans la réduction O 2 en des espèces d'oxygène actives en photocatalyse (via la formation de radicaux) sous un petit biais négatif (-

. General properties of semiconductors 2.1.1 Electronic structure

  Chapter 4.2 concerns preparation of ZnO/TiO 2 core/shell composites by sol-gel method, their detailed characterization, and photocatalytic and photoelectrochemical properties. Also, comparison to other deposition techniques (ALD, CVD) is given. The Chapter 4.3 reveals the origin of enhancement nature of the ZnO/TiO 2 interface on photocatalytic and photoelectrochemical properties.Electronic band structure theory of solid-state mater classifies a material as metal,

	2. Bibliography
	2.1
	Final Chapter 4.4 describes preparation, characterization of N-doped TiO 2 and
	ZnO/TiO 2 /Au ternary composites for improved photoactivity under visible light.
	The results from this work allow to correlate ZnO/TiO 2 composites architecture,
	i.e. shape, size, and morphology, as well as N-doping TiO 2 and Au-decoration, with
	optical properties, photocatalytic, and photoelectrochemical activity under
	monochromatic (400 nm) and/or visible light irradiation.
	Electrochimica Acta 246 (2017) 213 -225

This part consists of four chapters: Chapter 4.1 deals with preparation of ITO-catalyst supports and optimization of hydrothermal growth of ZnO, The obtained results have been published in the following papers: (1) K. Zarębska, M. Kwiatkowski, M. Gniadek, M. Skompska, Electrodeposition of Zn(OH) 2 , ZnO thin filmsand nanosheet-like Zn seed layers and influence of their morphology on the growth of ZnO nanorods, Electrochimica Acta 98 (2013) 255 -262 (2) M. Kwiatkowski, I. Bezverkhyy, M. Skompska, ZnO nanorods covered with a TiO 2 layer: simple sol-gel preparation, and optical, photocatalytic and photoelectrochemical properties, Journal of Materials Chemistry A 3 (2015) 12748 (3) M. Kwiatkowski, R. Chassagnon, O. Heintz, N. Geoffroy, M. Skompska, I. Bezverkhyy, Improvement of photocatalytic and photoelectrochemical activity of ZnO/TiO 2 core/shell system through additional calcination: Insight into the mechanism, Applied Catalysis B: Environmental 204 (2017) 200 -208 (4) M. Kwiatkowski, R. Chassagnon, N. Geoffroy, F. Herbst, O. Heintz, I. Bezverkhyy, M. Skompska, Enhancement of visible light photoelectrocatalytic activity of ZnO (core)/TiO 2 (shell) composite by N-doping and decorating with Au 0 Nanoparticles,

Table 1

 1 Basic crystal properties of ZnO and TiO 2 .

		ZnO wurzite	TiO 2 anatase
	crystal system	Hexagonal [143]	Tetragonal
	electron Hall mobility at 300 K (cm 2 V s -1 )	200	4 -20 [144]
	hole Hall mobility at 300 K

13.4. Examples of plasmonic metal nanoparticles/TiO 2 composites

  , after some time taken out and dried in air. Then, in hydrogen atmosphere at 200 -400 o C are annealed to form metallic gold Au 0[212,213].

	2) Photoreduction method; in such approach semiconductor (i.e. TiO 2 , ZnO, α-Fe 2 O 3 ,
	WO 3 , etc.) is irradiated by UV light in noble metal precursor solution. Electrons
	photogenerated in SC take part in reduction of i.e. Au-precursor (i.e. Au(OH) x Cl 4-x ] -, pH
	adjusted from AuCl 4	-) and simultaneously are deposited on the surface [214,215,216].
	3) Other methods including chemical reduction of gold precursor i.e. HAuCl 4 aqueous
	solution with addition of NaBH 4 in a presence of SC support material [217], physical
	vapor deposition [218], or ion exchange [219] i.e. as following in reaction scheme:
		BiOBr + Ag + → AgBr + BiO +	(52)
	2.1.

1) Precipitation-deposition; i.e. Au aqueous precursor solution with Au 3+ (AuCl 4 -) with adjusted pH by i.e. NaOH, NH 3 ·H 2 O. Then, TiO 2 is suspended or immersed in the A B solutionNoble metal nanoparticles can work specifically better in degradation of selected compounds, i.e. methanol, or nitrobenzene [220], whereas for dichloropropionic acid (DCA) cause the decrease of degradation rate [221]. Photocatalytic performances regarding utilization of SC/Me NPs in degradation of organic pollutants are summarized in Table 2, while examples in photoelectrochemical application, such as water oxidation, are presented in Table 3. Also the most interesting examples in photoelectrochemical applications are described in more detail below.

Table 2

 2 Photocatalytic degradation of organic pollutants on plasmonic Me NPs/TiO 2 composites.

	Type of	Preparation method of noble NPs	Photocatalytic test	Irradiation Type	Ref.
	composition
	Ag/TiO 2	Ag on TiO 2 was heated in N 2 at 800 °C to	MB decomposition increased by a factor of 7 with comparison to TiO 2	10 W black-fluorescent lamp (peak	[225]
		form Ag NPs		emission 380 nm)
	Au/TiO 2 /SiO 2	Turkevich method	MB solution after 2.5 h complete decolorization on	Xe lamp with 420 nm cut-off filter,	[226]
			SiO 2 /TiO 2 /Au, 20% decolorize on SiO 2 /TiO 2	450 W (visible light)
	Ag/TiO 2	Ag NPs were synthesized via the citrate	MB decolorization on Ag/TiO 2 enhancement factors over bare TiO 2 with:	100 W light source with λ of	[227]
		reduction method	a) Ag-nanowires by 2.1; b) Ag-spheres by 2.4; c) Ag-nanocubes by 3.8	365 nm (fwhm of 4 nm)
	Ag/TiO 2	Ag-ion-loaded TiO 2 , and then reduced with NaBH 4	TiO 2 k = 0.007 min -1 ; Ag/TiO 2 k = 0.029 min -1 Ag/TiO 2 : k = 0.0723 min -1 ; and under visible light: MB decolorization under UV light: TiO 2 : k = 0.0186 min -1 ;	and 36 W UV lamp UV cut-off filter 150 W xenon lamp with 400 nm	[228]
	Au/TiO 2 Au/TiO 2	Au NPs photodeposition from HAuCl 4 Ti 3+ -ion-assisted	2-propanol oxidation (5 vol %) Au/TiO 2 activity 0.5 µmol h -1 ; TiO 2 0.04 µmol h -1 Benzene oxidation, for 180 min, yields:	visible light irradiation λ > 450 nm 300 W Xe lamp with UV cut-off	[229] [230]
	Ag/TiO 2	UV-reduced TiO 2 , additions:	Au/TiO 2 ~ 60%; Pt/TiO 2 ~ 30%; Ag/TiO 2 : negligible	filter, visible light (λ > 400 nm)
	Pt/TiO 2	HAuCl 4 , AgNO 3 , or H 2 PtCl 6
	Au/TiO 2	Sol-gel: titanium(IV) butoxide and		110 W high-pressure sodium lamp,	[231]
		tetrachloroauric acid -HAuCl 4	052	visible light of 400 -800 nm
	Au/TiO 2 Pd/TiO 2	treatment of TiF 4 with HAuCl 4 , or H 2 PtCl 6 , Me@TiO 2 core/shell, hydrothermal	apparent first-order constant (k, h -1 ): Rhodamine B (RhB) decolorization,	for visible light, λ > 400 nm 300 W xenon lamp cut off filter	[232]
	Pt/TiO 2	or PdCl 2	Au/TiO 2 : 0.83; Pd/TiO 2 : 2.29; Pt/TiO 2 : 0.96 ; P25 : 0.72

MB decolorization (165 mL aqueous, 12 mg /L): pseudo-first-order kinetic constant (k, min -1 ): pure TiO 2 : 0.0144 ; 0.5% wt. Au/TiO 2 : 0.

Table 3

 3 Photoelectrochemical activity of Me NPs/TiO 2 composite electrodes.

	Type of composition	Preparation method	Photoelectrocatalytic test	Irradiation Type	Photoelectrochemical activity	Ref.
	Au/TiO 2	with TiO 2 (2.5 cm 2 ) ~ 10 µm thick layer Optical transparent electrodes coated	vs. SCE 0.05 M NaOH	UV irradiation, 150 W xenon lamp,	+0.75 V : Au/TiO 2 : ~ 0.42 mA; TiO 2 : ~ 0.09 mA Photocurrent under constant potential :	[233]
		immersed in toluene 9 mM Au colloid		λ > 300 nm	-0.95 V : Au/TiO 2 : ~ 0.37 mA; TiO 2 : ~ 0.04 mA
	Au/TiO 2 Pt-Au/TiO 2	One-pot photodeposition synthesis under λ = 365 nm and 310 nm on P25 TiO 2	0.5 M KOH 0 V vs. Ag/AgCl	UV (400 W Hg vapor lamp, 240 mW cm -2 ),
		from HAuCl 4 , H 2 PtCl 6
				solar simulator
				300 W with AM1.5
				(100 mW cm -2 )

Table 4

 4 Examples of different types of SC x /SC y with different VB and CB positioning used in photocatalytic and photoelectrocatalytic applications.In this chapter the properties of ZnO/TiO 2 composites will be described. Due to a large number of publications on this subject our description cannot be exhaustive. Instead, we tried to analyze the most representative works and formulate the general trends

	Composite	Type	Ref.	Composite	Type	Ref.
		(when SC 1 is TiO 2 )			(when SC 1 is ZnO)
	TiO 2 /CdS	I	[246, 247]	ZnO/CdS	I	[248]
	TiO 2 /ZnO	I	[249,250]	ZnO/SnO 2	II	[251]
	TiO 2 /CeO 2	I	[252]	ZnO/CuO	III	[253]
	TiO 2 /Bi 2 O 3	I	[254]	ZnO/CeO 2	II	[255]
	TiO 2 /SnO 2	II	[256]	ZnO/ZrO x	Reverse III	[257]
	TiO 2 /WO 3	III	[258,259,260]	ZnO/ZnS	II	[261]
	TiO 2 /ZrO 2	Reverse III	[262,263]
	TiO 2 /CuO	III	[264]

the materials based on supported ZnO nanorods decorated with TiO 2 nanoparticles whose structure is similar to that of materials studied in the present thesis. Finally, in section 2.2.3.4 several examples of ZnO(core)/TiO 2 (shell) composites doped with

Table 5

 5 Photocatalytic properties of ZnO/TiO 2 non-supported composites

	Type of ZnO/TiO 2 composition	Preparation method	Photocatalytic test	Irradiation type	Decolorization, degradation, or absorbance decrease:	Ref.
	Powder:	homogenous hydrolysis	MO decolorization,	125 W high pressure	MO decolorization after 20 min:	[265]
	spherical shape	from Zn(CH 3 COO) 2 and	catalyst loading 2.5 g dm -3	Hg lamp λ = 365 nm	ZnO/TiO 2 : ~ 88%;	ZnO: ~ 50%; TiO 2 : ~ 75%
		Ti{O(C 4 H 9 )} 4
	Powder:	solid-state dispersion	4-chlorophenol	16 W UV-B lamp	4-chlorophenol degradation degree after 75 min):	[266]
	highest activity for		(25 mg dm -3 ), catalyst loading	λ: 303 nm -578 nm	ZnO/TiO 2 : ~ 100%;	ZnO ~ 75%; TiO 2 ~ 84%
	wt. ratio 1:4; ZnO:TiO 2		100 mg /50 ml
	Nanofibers:	electrospinning	Probe compounds: 8 ppm	12 W (RhB decolor.),	Degree of RhB decolorization after 24 min:	[267]
	best results for ZnO/TiO 2 (wt.	with Zn(CH 3 COO) 2 and	RhB, and 20 ppm phenol	24 W (phenol degrad.)	ZnO/TiO 2 : ~100%;	TiO 2 : ~ 65%
	%) 84.2/15.8	Ti{OCH(CH 3 ) 2 } 4		UV lamp, λ = 365 nm	Degree of phenol degradation after 20 h:
					ZnO/TiO 2 : ~ 75%;	TiO 2 : ~ 60%
	Powder:	ZnO hydrothermally	MB (10 ppm 25 ml), 20 mg	UV irradiation	MB decolorization after 180 min:	[268]
	ZnO/TiO 2 flowers/deposit Powder:	prepared mixed with P25 coating of functionalized	catalyst loading Rhodamine 6G: 10 -5 M,	λ = 365 nm High pressure	ZnO/TiO 2 : ~ 70%; ZnO: ~ 48%; TiO 2 (P25): ~ 65% Rhodamine 6G complete decolorization after:	[269]
	Nanostructured hollow mixed	polystyrene beads with	70 ml, catalyst loading: 60 mg	Hg (Xe) lamp,	20 min (ZnO/TiO 2 ); 75 min (ZnO); 100 min (TiO 2 )
	spheres Powder:	ZnO and TiO 2 phase selective growth of	MO (10 mg dm -3 , 50 mL),	UV-vis irradiation 300 W Xe lamp	MO decolorization after 30 min:	[270]
	ZnO/TiO 2 in the form of frame	ZnO (5 mg) on TiO 2	process carried out in 10 o C,	λ: 220 nm -420 nm	ZnO/TiO 2 (frame network): ~85%
	network	anatase (5 mg) faces	catalyst loading 10 mg		ZnO/TiO 2 (mixed particles): ~ 53%
	Powder: TiO 2 (anatase)/ZnO from 10% molar Zn 2+ concentration Powder:	TiO 2 prepared by hydrolysis of TiCl 3 , ZnO formed by impregnation method from Zn(NO 3 ) 2 hydrothermal ZnO mixed	4-nitrophenol (20 mg/L), 0.5 L, catalyst loading 1 g/L MB (20 ppm, 200 ml),	25 W Hg lamp, photon flux 13.5 mW cm -2 500 W high pressure	zero-order rate constant: ZnO/TiO 2 : 56 •10 6 M h -1 TiO 2 : 52 •10 6 M h -1 ZnO: 18 •10 6 M h -1 Pseudo-first order constant values (min -1 ):	[271] [272]
	ZnO/TiO 2 (non-supported NRs)/NPs)	with sodium titanate and kept for 24 h at 180 o C	catalyst loading 80 mg	Hg lamp λ = 368 nm	ZnO/TiO 2 : k = 0.138;	ZnO NRs: k = 0.029

2 Nanostructured supported ZnO/TiO 2 composites

  The photocatalytic and photoelectrochemical properties of supported composites which do not contain ZnO nanorods are presented in Table6. Mostly these types of composites consist of TiO 2 nanotubes or nanorods grown on conducting support and decorated with ZnO nanoparticles. As mentioned before, such arrangement allows to measure the photocurrent in photoassisted water oxidation. The corresponding values are also given in Table6.A well-defined structure of these supported composites presented in this Tableallowedto apply the electrochemical methods to characterize some features of the mechanism allowing higher performance of the composites in comparison with pure ZnO and TiO 2 .

	Ozer et al. [274] fabricated the branched hierarchical ZnO/TiO 2 nanostructures by
	combined hydrothermal (ZnO) and electrospinning (TiO 2 ) methods. Different
	preparation parameters resulted in tuning of ZnO/TiO 2 surface area and morphology
	roughness. It has been demonstrated by electrochemical impedance spectroscopy (EIS)

2.2.3.3 Supported ZnO nanorod (core)/TiO 2 (shell) composites

Table

7

summarizes the properties of the composites consisting of supported ZnO nanorods covered with TiO 2 nanoparticles.

Wang et al. [273]

prepared film of all-nanoparticle ZnO/TiO 2 composite by layerby-layer deposited on FTO. It was shown that the improved photocurrent transient response of ZnO/TiO 2 (~ 5 µA cm -2 ) with respect to that of TiO 2 counterpart (~ 0.75 µA cm -2 ) was due to the presence and good connection ZnO/TiO 2 heterojunction. It was also observed by electrochemical impedance spectroscopy (EIS) that the composite possessed lower charge transfer resistance than the bare TiO 2 . It was explained and concluded that better charge separation in ZnO/TiO 2 was because of the presence of ZnO in ZnO/TiO 2 , which (according to cited literature) should show lower recombination rate and faster electron transport in ZnO than in TiO 2 .

Table 6

 6 Photoactivity of supported ZnO/TiO 2 composites (all architectures except ZnO(nanorods)/TiO 2 ).

	Type of ZnO/TiO 2 composite	Preparation method	Photocatalytic (PC) test or	Irradiation type	Photocatalytic or	Ref.
			photoelectrocatalytic (PEC) test		photoelectrochemical activity
	TiO 2 nanotubes (NT) with	TiO 2 (NT): anodization of Ti foil,	PC: MB decolorization (5 mg/L)	12 W UV light	MB decolorization after 150 min:	[277]
	ZnO	ZnO electrochemical deposition		λ = 365 nm	TiO 2 /ZnO: ~ 80%;	TiO 2 : ~ 57%
	TiO 2 /ZnO (nanorods)/ (shell)	FTO/TiO 2 hydrothermal synth., ZnO by atomic later deposition (ALD)	solution, also cyclic voltammetry (CV) 50 mV s -1 PEC: 0.1 M NaOH, N 2 purged	1.5 filter) λ > 300 nm 100 mW cm -2 300 W Xe lamp (AM	TiO 2 : negligible; TiO 2 /ZnO: ~130 µA cm -2 Water splitting: photocurrent at 0.4 V:	[278]
	TiO 2 /ZnO (nanorods)/ (shell)	covered with ZnO shell FTO/TiO 2 hydrothermal synth.,	1 M Na 2 SO 4 PEC: Water splitting	solar light, 100 mW cm -2 500 W Xe lamp,	TiO 2 : 0.15 mA cm -2 0.7 V (vs. RHE): TiO 2 /ZnO : 0.65 mA cm -2 ; LSV and transient photocurrent under constant	[279]
	ZnO/TiO 2 (NPs/NPs)	Layer-by-layer deposition on FTO	1 mM NaNO 3 , applied potential: +0.4 V vs. Ag/AgCl	4 W UV lamp 1.2 mW cm -2 , λ: 365 nm	TiO 2 counterpart (~ 0.75 µA cm -2 ZnO/TiO 2 (~ 5 µA cm -2 )	[280]
	TiO 2 /ZnO	TiO 2 by anodization Ti foil,	PC: degradation of RhB	300 W Xe lamp	PC: RhB degradation: TiO 2 /ZnO ~ 78%;	[281]
	(nanotubes)/(shell)	ZnO by impregnation	PEC: water oxidation in 0.1 M Na 2 SO 4	λ = 365 ± 15 nm	TiO 2 : ~ 63%; ZnO film ~ 16% PEC: water oxidation: ZnO/TiO 2 ~ 2.5 •10 -4 A cm -2 ; TiO 2 ~ 1.5 •10 -4 A cm -2

Table 7

 7 Photoactivity of supported ZnO(nanorods)/TiO 2 composites.

	Type of	Preparation method	Photocatalytic (PC) test or	Irradiation type	Photocatalytic or	Ref.
	ZnO/TiO 2 composition		Photoelectrocatalytic (PEC) test		Photoelectrochemical activity
	ZnO/TiO 2	ZnO hydrothermal growth, TiO 2	PC: 20 ppm or 10 ppm Orange II	for UV 18 W BLB T8	Orange II decolorization (after 210 min) under	[282]
		prepared from ethanol suspension	solution, volume of 150 ml	lamp, λ: 315 -400 nm,	UV:
				for visible, linear lamp	ZnO/TiO 2 : ~ 53%; ZnO: ~ 28%
				500 W, 9700 Lm	under visible: ZnO/TiO 2 : ~ 72%; ZnO: ~ 35%
	ZnO/TiO 2	film was prepared on quartz	PC: MO or MB (20 mg/L)	500 W high-pressure	Decolorization without catalyst (after 10 h):	[283]
		substrate by pulse laser		mercury lamp for	MO: ~ 6.8%; MB: ~ 5.9%
		deposition		visible light	with: ZnO: MO: ~ 19%;	MB: ~ 8.7%
					TiO 2 : MO: ~ 25.5%; MB: ~ 12.3%
	ZnO/TiO 2 nanorods/nanobelts	ZnO via solution dipping technique TiO 2 via hydrothermal synth.,	oxidation, 0.5 M Na 2 SO 4 , under 0.5 V vs. Ag/AgCl PEC: photoassisted water	intensity ~ 120 mW cm -2 Xe lamp, irradiance	ZnO/TiO 2 : MO: ~ 32.9%, MB: ~18.6% TiO 2 /ZnO: ~ 1.80 mA cm -2 Photocurrent: ZnO (NRs): ~ 0.25 mA cm -2 ;	[284]
	ZnO/TIO 2	ZnO via hydrothermally, TiO 2	PC: Bromo-Pyrogallol red	25 W mercury tube	Decolorization of Br-PGR after 210 min:	[285]
	NRs/'nano-brush'	magnetron sputtering	(Br-PGR) 0.02 mg/L	lamp UV λ: 254 nm	ZnO/TiO 2 : 81%; TiO 2 film: 32%

3.1. Chemical reagents and materials

  Chemicals (provided by Sigma-Aldrich and ROTH) and substrates used in this work for synthesis of composites, their modifications and photocatalytic tests:

	gold(III) chloride hydrate (~52% Au basis),	HAuCl 4 • ~ 2H 2 O
	zinc nitrate hexahydrate (≥ 99%),	Zn(NO 3 ) 2 • 6H 2 O
	titanium(IV) butoxide (97%),	Ti(OCH 2 CH 2 CH 2 CH 3 ) 4
	2-propanol (isopropanol) (≥ 99.9%),	C 3 H 8 O
	zinc acetate dihydrate (≥ 99.5%),	Zn(CH 3 COO) 2 • 2H 2 O
	hydrochloric acid 36%,	HCl (aq.)
	sodium hydroxide,	NaOH
	sodium sulfate,	Na 2 SO 4
	potassium chloride,	KCl
	ammonium hydroxide solution (≥ 23%) ,	NH 4 OH (aq.)
	ammonium chloride	NH 4 Cl
	methylene blue (≥ 95%),	C 16 H 18 N 3 SCl

Table 8

 8 Acronyms of the composite systems elaborated and studied in this work.

	Acronym	Composition	Description of composite
	of sample
	1c	ZnO/TiO 2	ZnO NRs covered with TiO 2 (sol-gel) 1 deposition cycle for 6 h, followed by calcination at 450 o C for 1 h
	3c	ZnO/TiO 2	ZnO NRs covered with TiO 2 (sol-gel) 3 deposition cycles, each one for 30 min and followed by calcination at 450 o C for 1 h
	‗ALD'	ZnO/TiO 2	ZnO NRs covered with TiO 2 prepared by Atomic Layer Deposition
			technique
	‗CVD'	ZnO/TiO 2	ZnO NRs covered with TiO 2 prepared by Chemical Vapor Deposition
			technique
	3c-Cal	ZnO/Zn:TiO 2	3c sample treated by additional calcination at 450 o C
			for 3 h
	3c-HCl 3c-Cal-HCl	remained * (shell) TiO 2 remained Zn:TiO 2 *	3c sample treated by HCl (aq.) for 20 h pH 3 (ZnO-cores etched) 3c-Cal sample treated by HCl (aq.) for 20 h pH 3
		(shell)	(ZnO-cores etched)
	3c(N)	ZnO/TiO 2 (N) N-doped TiO 2	TiO 2 (sol-gel) 3 deposition cycles, each one for 30 min with a presence of NH 4 Cl and followed by calcination at 450 o C for 1 h
	1c-Au	ZnO/TiO 2 /Au	1c sample with deposited Au NPs by photodeposition for 20 s under 365 nm irradiation and calcined at 400 o C for 2 h
	3c(N)-Au	ZnO/TiO 2 (N)/Au	3c(N) sample with deposited Au NPs by photodeposition for 20 s under 365 nm irradiation and calcined at 400 o C for 2 h

  photocurrent measurements were done with the use of Autolab, in three electrode cell, with elaborated ITO-based photocatalysts (i.e. ITO/ZnO, ITO/ZnO/TiO 2 , ITO/ZnO/TiO 2 /Au, etc.) as working electrode, a Pt wire as counter electrode and aqueous Ag/AgCl as a reference electrode, in 5 mM Na 2 SO 4 aqueous solution. The working electrode surface area was approx. 1 cm 2 . The V oc experiments were carried out

	in deareated aqueous solution of 0.2 M KCl in phosphate buffer of pH 6.8.
	3.3.10 Set-up for photocatalytic and photoelectrochemical measurements
	Photocatalytic (PC) activity of the samples was tested by measurement of
	photodecolorization rate of model pollutant, methylene blue (MB), according to the
	International Standard Procedure [87]. The PC tests were performed in quartz cell
	containing 2.5 ml (all experiments described in Chapters 4.2 and 4.3) and 5 ml (all tests
	in Chapter 4.4) of 10 µM MB aqueous solutions under either LED 400 nm irradiation or
	visible light irradiation of 160 mW cm -2 intensity generated by an arc xenon lamp (

in Bibliography part. In this work the

A B

4.1.1. Preparation of ITO-plates seeded with nanocrystals

	All ZnO/TiO 2 -based catalysts for photocatalytic and (photo)electrochemical
	measurements were fabricated on ITO conductive glass plates. It is essential to provide
	good attachment of the catalyst to the substrate to avoid any weight losses during
	catalytic process and ensure good electrical contact between the catalyst and electrode
	surface to achieve satisfactory electrochemical properties.	In this context,
	electrochemical seeding methods surpass commonly used physical adsorption techniques
	such as the ‗doctor blade' or sputtering methods. Taking into account that the catalysts
	elaborated in this work are based on ZnO nanorods (subsequently covered with TiO 2 and
	decorated with Au nanoparticles), potentiostatic reduction of Zn 2+ to metallic Zn-seed
	crystals, and their further superficial transformation into ZnO, satisfies the needs of
	proper surface immobilization and allow keeping good electric contact between Zn/ZnO
	seeds and ZnO nanorods grown epitaxially in the following stage by a hydrothermal
	method.
	Zinc acetate is a precursor more often used for chemical seeding [318] or chemical bath
	deposition [319] of ZnO nanostructures than for an electrochemical seeding of the
	substrate. In aqueous solutions, Zn(CH 3 COO) 2 undergoes partial hydrolysis according to
	the scheme:
	Zn(CH 3 COO) 2 + H 2 O → Zn(OH) + + 2CH 3 COO -+ H +	(66)

  therefore it was reasonable to determine it for TiO 2 nanoparticles obtained by sol-gel method used in this work for fabrication of ZnO/TiO 2 composite. Thus, the powder synthesized in the bulk of the solution was calcined at 450 o C for 1 h and suspended in aqueous solutions of different pH (adjusted with HCl or NaOH). As visible in Figure57B, the isoelectric point determined from pH-dependence of zeta potential is about 4.3, i.e. at neutral pH the surface of TiO 2 nanoparticles is negatively charged. Since methylene blue is a cationic dye, one can expect its preferential adsorption on the surface of TiO 2 -modified ZnO nanorods. This has been confirmed by UV-vis measurements of MB solution being in contact with ZnO/TiO 2 composites.

	Figure 57 Absorption spectra of initial aqueous solution of 20 µM MB (curve 1) and
	after adsorption of MB for 24 h on ITO/ZnO (curve 2), 3c (curve, 3) and 1c
	(curve, 4) (A) and zeta potential (ζ) of TiO 2 sol-gel prepared nanoparticles in
	a function of pH (B).
	XRD characterization of the two discussed ITO/ZnO/TiO 2 composites and TiO 2
	powder after calcination at 450 o C was performed to confirm a complete amorphous-to-
	anatase transformation upon calcination. The XRD patterns recorded for these materials
	are presented in Figure 58A together with ZnO and TiO 2 -anatase reference patterns. The
	XRD pattern of TiO 2 powder calcined at 450 °C for 1 h fits to that of anatase-TiO 2
	(ICDD Card No. 03-065-5714). The peaks observed at 2θ angle: 25.3°, 37.8°, 48.0°,
	54.0°, 55.1°, 62.5°, 68.7°, 70.2°, 75.1°, 82.7°, correspond respectively to the planes:

The amount of MB molecules adsorbed on the ZnO/TiO 2 composites, estimated from absorption spectra, is 9 •10 -8 mol per 1 mg of TiO 2 for 1c sample and 3.5 times lower (2.6 •10 -8 mol per 1 mg of TiO 2 ) for 3c sample. These results clearly demonstrate that TiO 2 layer obtained within 1c synthesis procedure possess more developed surface than that formed via 3c procedure which is consistent with TEM images presented above.

Table 9

 9 Titanium-to-zinc ratios obtained from XPS, EDX-SEM measurements for composite systems and optical energy band gap values determined from UV-vis spectra.If not stated, the calcination was performed at 450 o C for 1 h.

	Comparison of the XPS results with TEM observations allows to obtain additional
	information about textural properties of the TiO 2 layers. It is well known that a surface
	layer analyzed by XPS in inorganic materials is about several nanometers. Using
	QUASES-IMFP-TPP2M	software	(available	free	at

http://www.quases.com/products/quases-imfp-tpp2m/) it is possible to estimate the thickness of coating-layer material that prevents from XPS detection of under-layer second material. The thickness of the coating material (J) can be estimated using the equation [349]:

(68)

Table 10

 10 Results of photocatalytic decolorization of MB (performed under irradiation 400 nm -LED, and xenon arc lamp with 400 nm cut-off filter)

	Sample	Irradiation: LED	Irradiation: xenon lamp	Irradiation: LED
		k 400 (10 -2 min -1 )	with filter < 400 nm k xenon (•10 -2 min -1 )	addition of methanol 3% v/v k 400 (•10 -2 min -1 )
	ZnO	0.2	0.3	0.09
	1c (ZnO/TiO 2 )	1.2	0.8	0.30
	3c (ZnO/TiO 2 )	0.7	0.6	0.36

Table 11

 11 Photocatalytic properties of ZnO/TiO 2 core/shell composites, with TiO 2 shell prepared by the sol-gel method (1c and 3c),‗CVD', and ‗ALD' techniques.

	ZnO/TiO 2	Decolorization of MB	EDX
	Sample	under 400 nm k 400 (•10 -2 min -1 )	Ti / Zn ratio
	‗CVD'	0.8	0.27
	‗ALD'	1.1	0.06
	3c	0.7	0.27
	1c	1.2	0.10

4.3.4. Origin of the modification of the ZnO/TiO 2 interface at moderate temperature

  Results of HAADF-STEM imaging of as-synthetized ZnO nanorods, the same nanorods after calcination at 300 o C, and ZnO nanorods calcined at 450 o C are presented in Figures82A -82E. It can be seen that ZnO nanorods after calcination at 300 o C contain numerous small dark spots on its surface. These extended surface defects designated as ‗dents' can be clearly observed on the HAADF-STEM image of ZnO nanorods calcined at elevated temperature at 450 o C for 1 h (Figure

  SEM analysis. It can be noticed that amount of TiO 2 deposited in the presence of NH 4 Cl is smaller than that obtained in salt-free conditions. Indeed, according to our previous results, the Ti/Zn ratio in the sample prepared within three cycles of deposition without NH 4 Cl (3c) was 0.27, while it was only 0.05 in the 3c(N)

	determined by EDX-sample. Such a strong decrease of the amount of deposited TiO 2 despite the same
	reaction time can be attributed to the presence of chloride anions which were shown to
	slow down the hydrolysis of titanium alkoxides [395]. The amount of Au deposited on
	both samples (1.1 -1.8 wt. %) is similar to that reported for other Au-loaded
	photocatalysts [396,397,398]. It is worth noting that the Ti/Au atomic ratios are similar
	A for both gold-containing composites (11.9 and 11.3 for 1c-Au and 3c(N)-Au, B
	respectively) which is important for correct comparison of their photocatalytic
	properties. The same observation holds also for the average sizes of the gold particles
	estimated from the broadening of 111 peak of metallic gold (located at 2θ = 38.2 o ) in the
	XRD patterns of the samples (Figure 86).

Table 13

 13 Chemical composition of initial ZnO/TiO 2 as well as N-doped and Au-deposited composites.

	Sample		Composition, at. %	crystallite size of Au, nm
			(EDX-SEM analysis)	(XRD)
		Zn	Ti	Au
	1c	46 4.6	-	-
	1c-Au	45 4.4	0.37	17
				(1.8 wt. %)
	3c(N)	47 2.3	-	-
	3c(N)-Au 46 2.6	0.23
				(1.1 wt. %)

Table 14 .

 14 Ratio of charge transfer and recombination rate constants determined from photocurrent transients.

	Sample	k tr /k rec
	1c	4.06
	1c-Au	1.05
	3c(N)	2.45
	3c(N)-Au	0.75

Table 15

 15 The apparent first-order rate constants for MB decolorization at -0.25 V in the presence of 5 mM Na 2 SO 4 .

	Sample	k vis (•10 -2 min -1 )
	1c (dark)	0.16
	1c (visible)	0.38
	1c-Au (dark)	0.30
	1c-Au (visible)	0.81
	1c-Au (visible) deaerated	0.22
	1c-Au (visible) non-polarized	0.11

Acknowledgements

I would like to deliver my deepest gratitude to my supervisors, Prof Dr. Magdalena Skompska and Dr. Igor Bezverkhyy, for their dedication, encouragement and outstanding

Imparting visible light activity to the ZnO/TiO 2 composites 4.4.1 Optimization of N-doping of TiO 2

In order to obtain nitrogen-modified TiO 2 powder the same sol-gel approach was used.

Firstly, the weight of TiO 2 powder obtained in a single 30 min cycle was estimated and calculated into molar yield. Addition of nitrogen precursor salt (NH 4 Cl) during precipitation was adjusted to this TiO 2 molar yield in a mole ratio of TiO 2 :N-atom varying from 1:1 to 1:20. The optical properties of obtained N-doped TiO 2 powders are presented in Figure 84. As seen the highest absorption, and improvement of absorption towards visible light was observed for a powder prepared in middle-way conditions, that is with initial molar ratio TiO 2 :N 1:10 and. The preparation conditions of such powder were then chosen for our next stage of N-doped TiO 2 covering of ZnO nanostructures.

General conclusions and perspectives

The particulars results obtained in the present thesis were summarized in the corresponding chapters of the Results and Discussion part. The aim of this section is to put our particular findings for ZnO/TiO 2 core/shell composites in a more general context in order to see if they can be useful for other types of nanostructured composites used in the field of heterogeneous photocatalysis.

High potential of wet chemical synthetic methods. Our study showed that simple and easily scalable chemical approaches can be successfully employed to construct rather complex nanostructured systems. Indeed the ZnO nanorods of variable morphology were successfully grown on the seed layer prepared by electrochemical deposition from aqueous solution. The sol-gel method, used to deposit TiO 2 layer, is well established and scalable technique. Despite its simplicity it allowed to obtain the layers of very different microstructure by varying the duration of deposition and number of cycles. Moreover, the same approach made possible N-doping of TiO 2 under similar reaction conditions by adding ammonium salt in the reaction solution. Another important fact to underline is the similarity of the photocatalytic activities of the composites obtained by ALD and CVD and those of the materials prepared by sol-gel approach. This fact shows that wet chemical methods can successfully compete with the complex physical approaches in production of highly active photocatalysts.

Key role of the interface.

The results obtained in this thesis clearly show that the interface between the components of composite plays a crucial role in their photocatalytic behavior. In general the performance of composites in photocatalysis is analyzed in terms of electron and hole transfer between two (or more) components having different band structures. Our results show that this analysis should be completed by taking into account the microstructure of the components and of the interface between them. Indeed using the same oxides (ZnO and TiO 2 ) we were able to construct the composites with considerably different photocatalytic activities by varying two parameters. First, we increased the porosity of the anatase layer and by this we improved the accessibility of the interface. Second, we changed the interface structure through the outward Zn 2+ diffusion into TiO 2 layer upon additional calcination (the Kirkendall effect). Application of these approaches allowed to improve significantly the activity of the composites in MB degradation and in photoassisted water oxidation. These findings decorated composites deserves also further development. First of all, other methods of N-doping and Au deposition should be evaluated. Second, composite configurations, e.g.

ZnO/Au/N-TiO 2 , can be prepared and characterized. Third, the generality of the observed photoelectrocatalytic activity of these samples worth being studied by substrates other than MB, for example phenol and its derivatives.

The results presented in this dissertation could be considered also in a broader context.

The knowledge of correlation between synthesis-morphology-properties observed for our ZnO/TiO 2 composites may transferable to other metal oxide composites, may be useful in elaboration of novel visible light-driven photocatalysts.