Le Mans Universite

Karolina Ordon

Czestochowa Jd

Thèse

Ahmad Mehdi

Jacek Kasperczyk Professeur

Jaroslaw Mysliwiec Professeur

Abdel Hadi Kassiba

THESE DE DOCTORAT DE

Functionalized semiconducting oxides based on bismuth vanadate with anchored organic dye molecules for photoactive applications

Karolina Ordon

To cite this version:

Karolina Ordon. Functionalized semiconducting oxides based on bismuth vanadate with anchored organic dye molecules for photoactive

CZESTOCHOWA JD University

Titre Oxydes Semi-conducteurs à base de bismuth vanadates fonctionnalisés par des colorants organiques pour des applications en photocatalyse Mots clés : BiVO4 Mésoporeux, Photocatalyse, Matériaux hybrides La recherche de nouveaux matériaux en tant que photocatalyseurs en lumière visible pour la dépollution de l'environnement (eaux, atmosphères) est un domaine de recherche très actif et suscite l'intérêt d'une large communauté scientifique en Physique, Chimie et Sciences des matériaux. Des recherches exhaustives sont actuellement menées pour améliorer l'efficacité photocatalytique de certaines classes de matériaux photoactifs connus, et pour développer la synthèse de nouveaux matériaux fonctionnels. Dans ce contexte, les semiconducteurs oxydes photoactifs à base de vanadates de bismuth (BiVO4) possédant une bande électronique au milieu du spectre visible, offrent une sérieuse alternative aux photocatalyseurs classiques efficaces (TiO2, ZnO) dont la photo-excitation requiert uniquement la fraction UV du spectre solaire. Le travail effectué dans le cadre de cette thèse est donc dédié aux matériaux à base BiVO4 sous forme d'architectures mésoporeuses ou d'assemblages hybrides associant des groupes organiques à transfert de charges.

Deux contributions majeures ont été développées dont la première portant sur la réalisation expérimentale d'architectures mésoporeuses inédites, fonctionnalisées par des groupes organiques sensibilisateurs et l'étude de leurs propriétés électroniques et optiques en vue d'optimiser leurs efficacités photocatalytiques. La deuxième partie porte sur des simulations numériques de nanostructures hybrides par des approches exploitant la méthode DFT, ab-initio ou des modèles de chimie quantique. Des systèmes modèles ont été construits associant des nanoclusters (NC) et des groupes organiques (GO). Les propriétés électroniques et optiques ainsi que les caractéristiques structurelles et vibrationnelles des systèmes (NC-GO) ont été déterminées et confrontées aux données expérimentales. Les phénomènes de transfert de charges impliqués entre les groupes organiques et la structure inorganique ont été caractérisés ainsi que leur rôle dans l'efficacité des réponses photo-catalytiques des systèmes hybrides Title Functionalized semiconducting oxides based on bismuth vanadate with anchored organic dye molecules for photoactive applications Keywords : Mesoporous BiVO4 , Photocatalysis, Hybrid Materials

The search for new materials as photocatalysts in visible light for the depollution of the environment (waters, atmospheres) is a very active field of research and attracts the interest of a large scientific community in Physics, Chemistry and Materials Science. Recent research developpements are conducted to improve the photocatalytic efficiency of certain classes of known photoactive materials, and to develop the synthesis of new functional materials. In this context, photoactive oxide semiconductors based on bismuth vanadate (BiVO4) having an electronic band in the middle of the visible spectrum, offer a serious alternative to efficient conventional photocatalysts (TiO2, ZnO) whose photo-excitation requires only the UV fraction of the solar spectrum. The work done in this thesis is therefore dedicated to BiVO4-based materials in the form of mesoporous architectures or hybrid assemblies associating organic groups with charge transfer processes.

Two major contributions have been developed, one of which is the experimental realization of novel mesoporous architectures, functionalized by sensitizing organic groups and the study of their electronic and optical properties in order to optimize their photocatalytic efficiencies. The second part deals with numerical simulations of hybrid nanostructures using approaches as the DFT method, ab-initio or quantum chemistry codes. Model systems have been constructed associating BiVO4 nanoclusters (NC) and organic groups (GO). The electronic and optical properties as well as the structural and vibrational characteristics of the systems (NC-GO) were determined and compared with the experimental data. The charge transfer phenomena involved between the organic groups and the inorganic structure were characterized as well as their role in the efficiency of photo-catalytic responses of hybrid systems investigations are carried out to improve the properties of some families of known photocatalytic materials, as well as to develop the synthesis of new functional materials that would replace classic photocatalysts. Among numerous methods, the photocatalytic properties of semiconducting oxides can be improved by increasing their active surfaces, a narrowing of their band gap as well as the use of photoactive organic dyes to sensitize the inorganic semiconducting surfaces.

The aim of the presented work is to realize mesoporous architectures based on bismuth vanadate and to investigate an influence of selected organic sensitizers on the electronic, optical, vibrational and photocatalytic properties of the monoclinic BiVO4. The increase of the active surface area was proposed as the first element improving the photocatalytic properties of the crystalline BiVO4. For this purpose, the BiVO4 was synthesized in a mesoporous form applying an original approach based on the P123 surfactant. Then, the structural, electronic and optical properties of the obtained materials were exhaustively investigated and optimized. It was found that the mesoporous BiVO4 is a good starting material to obtain an efficient photocatalyst. In addition, it was determined that the synthesis of hybrid materials by anchoring organic dye molecules on a surface of the mesoporous BiVO4 will allow a more efficient transfer of electric charge between the organic molecule and inorganic oxide positively affecting its photocatalytic properties.

Therefore, commercial D149 and two new dyes based on imino-pyridine moieties were selected as the organic sensitizers and their performances compared in the process of photoinduced charge transfer .

In order to determine the mechanisms of physical phenomena responsible for photocatalytic effects occurring in hybrid materials, a series of computer simulations and quantum-chemical calculations were performed. Applying the cluster methodology the electronic and optical parameters as well as structural and vibrational features of nanostructured BiVO4 were calculated. The considered simulations were also performed for organic dyes and hybrid materials using a semi-empirical method with the PM6 parameterization. The obtained results from computer simulations were compared to the experimental data in order to validate the relevance of the developed theoretical models.

Thus, based on computer simulations through quantum-chemical methods and the carried out experimental works, the involved charge transfer phenomena were characterized in the aim to understand the transfer of electric charges in the hybrid material and the effect of the photocatalytic responses. The role of dyes in enhancing the photocatalytic effects for some organic groups or a quenching of such effects by using other molecules was also explained. It was shown that the combination of experimental and theoretical research methods leads to quantitative understanding of the physical phenomena governing the photocatalytic properties of the photoactive semiconducting structures as BiVO4.

The performed work develops a relevant methodology applied on a promising class of photoactive materials for visible-light driven photocatalysis. The numerical approaches offer the support of experimental investigations for exhaustive understanding of the phenomena involved in the photoinduced charge transfer efficiency in the hybrid

Résumé

Les procédés photocatalytiques sont mises en oeuvre dans plusieurs phénomènes physico-chimiques et servent de base à des applications technologiques dans différents domaines tels que l'écoconstruction, l'énergie, l'environnement, la médecine, l'optoélectronique, la cosmétologie. L'ensemble de ces procédés fait intervenir des réactions catalytiques impliquant différentes classes de matériaux semi-conducteurs voire métalliques. Dans ce cadre, les matériaux photoactifs sont utilisés sous rayonnements dans des gammes spectrales adaptées à la structure électronique des matériaux. Ceci contribue notamment à des applications en thérapie photodynamique, dans la production de verres autonettoyants, la production d'hydrogène pour de nouvelles sources d'énergie et à la purification des eaux polluées. D'après le travail pionnier de Fujishima et al. (Nature 1979, 277, 637), les semi-conducteurs les plus étudiés pour la photocatalyse sont le dioxyde de titane (TiO2). Une alternative au TiO2 a été recherchée sur la base de matériaux ZnO (Chem Rev. 1995, 95, 69). Cependant, en raison des valeurs élevées de leur bande interdite, ils ne peuvent pas absorber les rayonnements de la lumière visible, ce qui limite leurs performances pour exploiter le rayonnement solaire. La recherche de nouveaux matériaux en tant que photocatalyseurs en lumière visible a attiré une large communauté de chercheurs en sciences des matériaux. Parmi les systèmes prometteurs, les vanadates de bismuth de structure monoclinique sheelite (BiVO4) possédant une bande électronique égale à Chapter 1

Introduction

Recently researchers gave the hope that photocatalysis is the phenomenon which can be used to purify water and air from pollutants. It is an ambition that the photoactivation can be achieved for non-toxic and eco-friendly compounds as be some semiconducting oxides. These materials can be activated by light resulting in decomposition of organic pollutants to small, non-toxic molecules like water and carbon dioxide. Photocatalytic phenomena have their origin in a photochemistry, heterogeneous catalysis, molecular spectroscopy, solid state physics, materials science and surface science of semiconductors. The interpenetration of the physics fields on which photocatalysis is based is presented in Schema 1.1

First time the word "photocatalysis" was used in 1910 by Plotnikow [1]. Today, the photocatalysis describes the processes activated by ultraviolet, visible or infrared radiation on the surface of the substance called "photocatalyst" (PC).

Schema 1.1. The system of dependencies of physics fields on which photocatalysis is based

Photocatalytic phenomena and applications

According to IUPAC definition, photocatalysis describes a change of the speed of chemical reactions or its initiation under influence of a radiation from UV to IR range.

The mentioned reaction occurs on a substrate called photocatalyst, which is able to absorb incident radiation. Photocatalyst promotes the reaction under an influence of light and it is not degraded during photocatalysis [2].

The heterogeneous photocatalysis deals with the interaction between the photocatalyst in solid state and liquid or gas phase of reagents and substrates. The first step of the photocatalytic reaction is an absorption of the incident radiation by the photocatalyst.

When the photon of radiation energy is equal or slightly higher than the band gap of the photocatalyst, the electron-hole pairs are created with the electrons in the conduction band and the holes in the valance band. Equation 1.1.1. describes the relationship between the band gap and the wavelength necessary to excite electrons in the semiconducting material.

(unit) .

(1.1.1.)

The process can be described by following expression:

. (1.1.2.)

The electron-hole pairs generation took place in all PC volume reached by the light with the required energy. The reaction with the environment occurs when the electrons and hole are at the surface of photocatalyst. Therefore, when the migration time of the electrons and holes to the surface of photocatalyst will be shorter than the time of the recombination, the efficiency of the photocatalytic process will be higher.

Thus, the pairs lifetime plays a key role in the PC efficiency. Schema 1.1.1.

illustrates the process that take place at the semiconducting PC activated by radiation with energy adapted for its energy band gap [3].

The organic or biological molecules adsorbed on the PC surface can be oxidized when their redox potential is located above the valence band of the semiconductor. The reduction process occurs when the redox potential of the substrate is below the conduction band of photocatalyst.

The photocatalytic reactions are used to create hydrogen from water splitting as new energy source or alternatively to decompose waste organic molecules on a window panels. In medicine, photoactive compounds are used in photodynamic therapy applied for cancer treatment.

The photoactive materials are also used in dye sensitized solar cell (DSSC) systems known as the 3 rd generation photovoltaic cells. Purification properties of the photocatalysts allow safe and friendly degradation of toxic substances through their mineralisation as nontoxic inorganic molecules including water and carbon dioxide.

During the photoactivation process the catalytic material creates active radicals able to decompose organic compounds in reduction and oxidation processes [7,8].

Photocatalytic decomposition process may be also used in production of the hydrogen from water, biomass and monohydroxyl or polihydroxy alcohols. The hydrogen generated in this process serves as substrate in chemical reaction: production hydrogen chloride, methanol, ammonia or synthetic gasoline as well as fuel for a new type of hydrogen powered cars.

Self-cleaning surfaces are built with hydrophilic-photocatalytic coating. Photoinduced reactions occurring in the coating benefit from their activation by solar radiation. The process leads to decomposition of surface organic pollutants and prevents their adhesion at the coating layer. Glasses with such photoactive surfaces are installed on the skyscrapers and high rise buildings for practical reasons [9]. One example worthy of interest concerns the facade of a Mexican hospital in a polluted environment. The entire building was covered by the film of the semiconducting titanium dioxide as photocatalyst. The photocatalytic decomposition of pollutants from air turned out to be good solution for smog in the centre of Mexico City [10].

Photoactive materials contribute also to therapeutic solutions as illustrated in the photodynamic therapy (PDT). This form of medical treatment uses photosensitive materials acting on cancer cells after their exposure to radiation. The PDT process also has the ability to kill microorganisms, including bacteria, fungi and viruses. The method has found application in treatment of many diseases including malignant tumours.

It was considered as low invasive and minimal toxic medical treatment strategy [11][12][13][14].

Photovoltaic devices are also a key illustration of photoactive materials for solar to electrical energy conversion.

Dye sensitized solar cells (DSSC) combine semiconducting materials with organic dye molecules in the way to possess adequate charge transfer between the two components in the aim to obtain the highest conversion efficiency compared to actual reached values about 15% [15][16][17].

Photoactive semiconducting oxides

In photocatalytic processes, semiconducting oxides are widely exploited because of their suitable physical peculiarities. Even limited to oxides, the number of relevant composition is wide. In this case a limitation of the purpose to the most investigated materials with respect to their efficiency and their promising applications in visible light driven photocatalysts will be applied. Beyond the photoactivity, the nontoxic nature, chemical stability and environmental friendliness are major requirement from photocatalysts to be used in applications according to the environmental preservation. Zinc oxide shows two structural variants as hexagonal wurtzite and cubic zinc blende lattice. The second structure is most stable at ambient conditions and thus the most commonly used. Its space group is the P63mc and the point group is the .

In this form, ZnO possesses a direct band gap equal to 3,37 eV and it can absorb radiation with wavelength about 375 nm [20] corresponding to the maximum absorption in the near ultraviolet.

The shift of absorption maximum to higher wavelengths can be made by doping or sensitizing process [21]. The doping operations use metallic elements as Mn [22],

Ag [23], La [24], Cu [25], Mg [26], Co [27] and non-metallic ones as C [28] or N [29].

Among the improvement, it is worth noting that the absorption spectra of the ZnO doped by the Cu2O coincide with the solar spectrum (Fig. 1.2.1.1.) [30]. It was also shown that the mentioned doping increases the photovoltaic efficiency of the ZnO.

A similar impact was induced by carbon nanofibers [31] as sensitizing agent for the ZnO based photocatalyst (Fig. 1.2.1.2.).  preserve the position of the conduction band of the semiconducting oxides or location of this band so that maintain its photoreductive activity [32].

In last years, various attempts have been carried out by introduction of non-metals in the structure. Thus, carbon [33,34], nitrogen [35,36], sulphur [37], fluorine [38], iodine [39] and boron [40] were considered and successfully cause enhanced photocatalytic activity. Among these elements, nitrogen was thought as the most effective substitution doping non-metal due to a similar size to oxygen atom as well as low ionization energy.

Consequently, the nitrogen can be easily introduced into semiconducting oxides lattice and substitute the oxygen leading to higher photocatalytic activity.

The TiO2 has been doped also by metal ions as Ag [41,42], Pt [43], Fe [44], W [45],

La [46,47], Ln [48], Au [49], Mo [50], Cu [51] as well as non-metals F [52], B/N [53] and Co [54,55] or inorganic compounds like Fe2O3 [56], SnO2 [57].

Good improvements were demonstrated by dopants which contribute to separate the photogenerated electron-hole pairs resulting in increased recombination times and then an enhancement of photocatalytic activity for both, oxidation and reduction.

Role of the material morphology and nano-structuration in the photocatalytic efficiency

Suitable photocatalytic responses of the semiconducting materials depend on their surface available for the reactions, their morphology, size and crystalline structure.

During a photocatalytic process, the suitable radiation induces the formation of electron-hole pairs with the electrons being transferred from the valence to the conduction band. The charges migrate to the surface of the compound and interact with the surrounding media (water, organic groups, small inorganic particles) to create reactive radicals as superoxide (O2 -and OH -). In such case the material with the highest interface with surrounding media is required. A promising strategy to contribute to high contact surface is based on the synthesis of meso-or nanoporous structures as well as nanostructured surfaces. A simple geometry model can be used roughly to underline the large difference between samples prepared as thin layers with an area of 0.10.1 μm 2 presenting nanostructured surface and the samples with smooth surface without any unevenness or convexity. For the nanostructured sample, its surface was treated as composed by hemispheric nanoislands with a radius about 5 nm distributed on the film. In the case the 100 pieces are distributed uniformly on the substrate (Fig. This simple model illustrates the difference of contact area between smooth and porous surface, which can be activated during a photocatalytic process. The calculations show that the nanostructured surface is more than 1,7 times larger than the smooth area.

Similar calculations can be made on nanoporous films which were experimentally realized by Guisheng Li, et all. for bismuth vanadate samples [58]. The synthesis of the nanostructured BiVO4 network is based on a hydrothermal method using a template from mesoporous silica (KIT-6). Based on the experimental investigations, the photoefficiency of the mesoporous material increased more than by 80 % compared to conventional bulk BiVO4 [58]. Beyond the above aspects, the shape of the particles has shown a significant impact on their photocatalytic activity. This was demonstrated by using nanoparticles synthesized following different chemical procedures. Thus, several shapes were obtained as flowers [59], stars [60], butterfly wings [61] or peanuts [62] and many others. The carried out experiments have shown a strong correlation between the nanoparticle shape with a high contact surface and the efficiency of the photocatalytic process.

Semiconducting nanostuctures as 0D, 1D, 2D and 3D-dimensional objects were currently synthesised with stable morphologies and sizes [63][64][65][START_REF] Liu | Self-assembling TiO2 nanorods on large graphene oxide sheets at a two-phase interface and their antirecombination in photocatalytic applications[END_REF][START_REF] Pacholski | Self-Assembly of ZnO: From Nanodots to Nanorods[END_REF]. For the particular case of the 2D systems formed as thin films, they can be deposited on several substrates as single or multilayers structures with the thickness from nano-to micrometer scale [START_REF] Shim | Effect of the variation of film thickness on the structural and optical properties of ZnO thin films deposited on sapphire substrate using PLD[END_REF][START_REF] Cheng | ZnO nanoparticulate thin film: preparation, characterization and gas-sensing property[END_REF][START_REF] Nobuaki | Preparation of Transparent TiO2 Thin Film Photocatalyst and Its Photocatalytic Activity[END_REF][START_REF] Yu | The Effect of Calcination Temperature on the Surface Microstructure and Photocatalytic Activity of TiO2 Thin Films Prepared by Liquid Phase Deposition[END_REF]. The thickness and the nature of the substrates play crucial role in the efficiency of the photocatalytic processes. From one hand, this is understood from the light penetration in the sample and then from photogeneration more or less density of electron-hole pairs in the film. On the other hand, the nature of the substrate favour the formation of interfaces with more or less defects or dislocations which may overcame the transport of the photogenerated charge carriers at the surface of the film.

When the synthesis of the film can be realized by chemical routes as sol-gel method, the possibility to increase the contact surface of the film can be realized thanks to surfactants. The obtained films exhibit mesoporous organisation with the pores surfaces controlled by the synthesis conditions and the features of the surfactants [START_REF] Choi | Thermally Stable Nanocrystalline TiO2 Photocatalysts Synthesized via Sol-Gel Methods Modified with Ionic Liquid and Surfactant Molecules[END_REF][START_REF] Chen | Long TiO2 Hollow Fibers with Mesoporous Walls: Sol-Gel Combined Electrospun Fabrication and Photocatalytic Propertiesm[END_REF].

The contact surfaces of such structures are enough high to favour enhanced efficiencies of the photocatalytic process.

It is worth noting that the nanostructures peculiarities show drastic changes on their physical properties due to the size quantum confinement effects. This occurs when at least one dimension of the system became lower than a defined critical length lc leading to modification of the electronic, optical and vibrational properties compared to those in the bulk system. The lc depends on the physical properties of the material and differs from a semiconducting, metallic or magnetic structure. In semiconductors, the Bohr radius of the bounded state of electron-hole pair defines the lc and below such length, the band gap is affected by the size quantum confinement effect [START_REF] Raty | Quantum Confinement and Fullerenelike Surface Reconstructions in Nanodiamonds[END_REF] leading to a gradual increase with decreasing the size as it is illustrated for the TiO2 (Fig.

Physical features of bismuth vanadate (BiVO4)

Bismuth vanadate is a yellow solid compound with regard to its band gap in the visible light possessing the features to be non-toxic and chemically stable in several solvents including water. After being investigated for its ferroelastic phase transition three decade earlier, the interest was reactivated in a recent period as visible light driven photocatalyst offering good alternative to the UV-photoactive TiO2. The BiVO4 exhibits three crystalline polymorphs such as tetragonal-scheelite (t-s), monoclinic scheelite (m-s), and zircon-type (z-t) structure (Fig. 1.2.3.1.) [START_REF] Park | Progress in bismuth vanadate photoanodes for use in solar water oxidation[END_REF]. However, the most commonly used structure for photocatalytic applications consist in the m-s phase [START_REF] Suarez | BiVO4 as photocatalyst for solar fuels production through water splitting: A short review[END_REF]. Therefore, the oxygen ions can be divided into two groups O1 and O2. The O1 represents oxygen ions participating to the coordination of one Bi and one V ion. The O2 represents the oxygen ions involved in the coordination with two Bi ions and one V ion. The electronic properties of the BiVO4 were studied also theoretically using the DFT methodology [START_REF] Ding | Density functional theory study on the electronic and optical properties of three crystalline phases of BiVO4[END_REF]. By this approach, Zhao and co-authors confirmed the hybridization between the Bi 6s and the O 2p orbitals at the top of the VB and proved that they are responsible for the band gap of the monoclinic BiVO4, while the CB is composed mainly by the V 3d orbitals with almost negligible influence of the O 2p and the Bi 6p orbitals. These authors point out the indirect band gap nature of the BiVO4, with an energy gap equal to 2.17 eV (Fig. [START_REF] Zhao | Electronic structure and optical properties of monoclinic clinobisvanite BiVO4[END_REF][START_REF] Walsh | Band Edge Electronic Structure of BiVO4: Elucidating the Role of the Bi s and V d Orbitals[END_REF]. This can mean that the mobility of electrons and holes in BiVO4 is expected to be similar for both charge carriers and it may have good contribution to the photocatalytic properties of the mentioned material.

As mentioned earlier, the monoclinic BiVO4 has been considered as one of the most active visible-light photocatalyst regarding its low band gap. For the electronic properties, the hybridization of the Bi 6s and O 2p orbitals causes an increase of the top of VB and then a reduction of the band gap of the monoclinic BiVO4. For semiconducting photocatalysts different attempts have been made in order to improve their electronic and optical properties and particularly to enhance their light absorption in the visible spectral range.

Kudo et all. demonstrated that by mixing of parental reactants followed by calcination process it is possible to obtain material with band gap equal to 2,4 eV contrary to the BiVO4 in tetragonal phase where the energy gap is equal to 2.9 eV [START_REF] Kudo | A Novel Aqueous Process for Preparation of Crystal Form-Controlled and Highly Crystalline BiVO4 Powder from Layered Vanadates at Room Temperature and Its Photocatalytic and Photophysical Properties[END_REF].

One of the most common estimation method to determinate the value of the band gap is the Kubelka-Munk (K-M) function F(R) which use the reflectance R value of a sample in order to approximate the optical absorbance of a semiconductor:

.

(1.2.3.1.)

After this manipulation a Tauc Plot can be constructed with:

, (1.2.3.2.)
where α is the absorption coefficient of the material, the is the energy of light, n = 1/2 is used to determine the energy gap for a direct band gap materials and n=2 for indirect semiconductors [START_REF] López | Band-gap energy estimation from diffuse reflectance measurements on sol-gel and commercial TiO2: a comparative study[END_REF]. For the thin films of the BiVO4 annealed with different temperatures, the extrapolation of the curve to the h axis gives the semiconductor band gap (Fig.

Synthesis methods of the BiVO4 and related samples morphologies

Depending on the considered method, the morphology and structural properties of the synthesized material can be modulated in large extent. The objective of this chapter section is to highlight the main features of the synthesis methods to obtain BiVO4 with defined structural organization.

Based on the synthesis methods, it has been shown the close correlation between the morphology, the structural order of the samples and the modulation of their electronic behavior. In heterogeneous photocatalysis, the reaction rate constants are highly dependent on the contact surfaces. Therefore, in the final network, the relevant synthesis methods must enhance the accessible surfaces as mesoporous organization, nanoparticles, nanoislands involved in the most external part of the thin films.

Beyond the numerous synthesis methods [START_REF] Liu | Preparation of bamboo-shaped BiVO4 nanofibers by electrospinning method and the enhanced visible-light photocatalytic activity[END_REF][START_REF] Gotić | Synthesis and characterisation of bismuth(III) vanadate[END_REF], only few experiments lead to the required features of the BiVO4 showing satisfactory photocatalytic properties. Based on experimental investigations of Fan and co-authors [START_REF] Fan | Effect of BiVO4 Crystalline Phases on the Photoinduced Carriers Behavior and Photocatalytic Activity[END_REF] was exhibited that the best photocatalytic efficiencies were confirmed for pure tetragonal or monoclinic phase of the BiVO4 (70%) compared to the non-pure polymorphs. According to the theoretical predictions [START_REF] Fan | Effect of BiVO4 Crystalline Phases on the Photoinduced Carriers Behavior and Photocatalytic Activity[END_REF] related to nanosized media, the main difference between the phases concerns the hybridization of Bi 6s and O 2p orbital in the bottom of VB of the monoclinic form which cause the photocatalytic properties of the BiVO4. For monoclinic phase, the hybridization narrows the value of the energy gap and thus stimulates more efficient photogeneration of the pairs. Consequently, more active radicals are formed in heterogeneous photocatalysis and then efficient decomposition occurs for organic molecules in water solution [START_REF] Fan | Effect of BiVO4 Crystalline Phases on the Photoinduced Carriers Behavior and Photocatalytic Activity[END_REF].

In the report of Zhang [START_REF] Zhanga | Monoclinic BiVO4 micro-/nanostructures: Microwave and ultrasonic wave combined synthesis and their visible-light photocatalytic activities[END_REF] related to the synthesis methods, the pH effect on structural, optical and photocatalytic properties was investigated. It has been shown that as the concentration of hydrogen ions in the solution increases, the synthesized particle size also increases. Therefore, the diethylene glycol was used as a solvent to maintain the appropriate pH of the final solution. With the increase in pH the size of the BiVO4 particles increased, and at pH = 8 the stoichiometry of the compound was changed. As a consequence, the photoactivity of the samples has also changed. It has been confirmed

that the photocatalytic activity decreases with the increase in particle size what can be caused by the increase in the solution pH at the final stage of the synthesis process [START_REF] Zhanga | Monoclinic BiVO4 micro-/nanostructures: Microwave and ultrasonic wave combined synthesis and their visible-light photocatalytic activities[END_REF].

Chen and co-authors have demonstrated that a higher concentration of the CH3COOH results in smaller BiVO4 particles with better photocatalytic properties than the bulk material [START_REF] Chen | Effects of citric acid and urea on the structural and morphological characteristics of BiVO4 synthesized by the sol-gel combustion method[END_REF]. Smaller particle sizes can reduce the length of migration of the photogenerated charge carriers and further reduce the rate of electrons-holes pairs recombination. The rough and relatively high specific surface provide more active sites, which is advantageous for catalytic reactions.

Sol-gel and hydrothermal synthesis are the most commonly used methods to produce nano-to micrometer sized materials with different particle morphologies. illustrates the diversity of the BiVO4 particle forms synthesized by the sol-gel method.

Irrespective to the used solvents such as 2-propanol + ethanol or water + nitric acid, The best photocatalytic efficiency is obtained for nano-or mesoporous samples as it was confirmed experimentally [START_REF] Hernandez | Considerations on Oxygen Bubble Formation and Evolution on BiVO4 Porous Anodes Used in Water Splitting Photoelectrochemical Cells[END_REF][100][START_REF] Ge | Sunlight-driven degradation of Rhodamine B by peanut-shaped porous BiVO4 nanostructures in the H2O2-containing system[END_REF]. To achieve the intended structure, a synthesis approach using a organic precursor or porous silica was used. For this purpose, the structures such as surfactants Poly(ethylene glycol)-block-poly(propylene glycol)block-poly(ethylene glycol) (P123) [START_REF] Jiang | High-performance porous spherical or octapod-like single-crystalline BiVO4 photocatalysts for the removal of phenol and methylene blue under visiblelight illumination[END_REF], porous silica as SBA-15, KIT-5 [START_REF] Tsoncheva | Cobalt oxide species supported on SBA-15, KIT-5 and KIT-6 mesoporous silicas for ethyl acetate total oxidation[END_REF], KIT-6 [58] or polymers [START_REF] Martínez-De La Cruz | Characterization of the visible-light-driven BiVO4 photocatalyst synthesized via a polymer-assisted hydrothermal method[END_REF][105][106][START_REF] Zhou | Ordered Macroporous BiVO4 Architectures with Controllable Dual, Porosity for Efficient Solar Water Splitting[END_REF] were used to obtain BiVO4 batches with high surface area (Fig. 1.2.4.3.). In this case more organic dye molecules can be adsorbed on the surface of the photocatalyst. f.) [START_REF] Zhou | Ordered Macroporous BiVO4 Architectures with Controllable Dual, Porosity for Efficient Solar Water Splitting[END_REF][START_REF] Dong | Structuring porous ''sponge-like'' BiVO4 film for efficient photocatalysis under visible light illumination[END_REF].

Photocatalytic properties of the bulk and thin film BiVO4 systems

The photoactivity of semiconducting oxides as bismuth vanadate was investigated intensively in the last decade. Their vast majority focused on the phase composition of the BiVO4. Frequently, the photocatalytic properties of the BiVO4 were defined in relation to the TiO2 in anatase crystal forms because the second one is very popular semiconducting oxide used in photocatalysis and photovoltaics processes. Its main drawback, it has a significant limitation in absorption of radiation in the visible range (Eg = 3,2 eV). The monoclinic BiVO4 is capable to absorb of visible light due to its lower energy gap (Eg = 2,4 eV). Kohtani and co-authors performed a comparison of photoactive properties between the TiO2 in anatase form to the monoclinic BiVO4 [START_REF] Kohtani | Photocatalytic Degradation of 4-n-Nonylphenol under Irradiation from Solar Simulator: Comparison between BiVO4 and TiO2 Photocatalysts[END_REF].

The impact of the shape and size of particles was also considered as a key factor in the efficiency of a photocatalytic responses. In the same context, commercial TiO2 The influence of structure at the material properties is also of great importance.

The Table 1.2.5.1. underlines the impact of shapes and size of the BiVO4 on photocatalytic properties expressed by their efficiency. In addition, morphologies with increased active surface such as dendrites, microtubes or mesoporous structures have satisfactory photocatalytic property greater than 80 % irrespective to the dye used in the photocatalytic test. Achieving high photocatalytic yields for the bulk BiVO4 material has opened the way for improving the photocatalytic processes through the increase in their characteristic parameter which is the reaction constant rates.

The development of thin films and nanostructures attracts a great interest to the BiVO4 based materials. These systems were mainly analyzed for their electrochemical properties to create the most efficient photoactivity [START_REF] Abdi | Efficient Water-Splitting Device Based on a Bismuth Vanadate Photoanode and Thin-Film Silicon Solar Cells[END_REF][116][START_REF] Qiu | Efficient solar-driven water splitting by nanocone BiVO4-perovskite tandem cells[END_REF]. Nevertheless, the O2 photoevolution analysis has confirmed satisfying photocatalytic properties of the BiVO4 in thin film form with nanoparticle size equal to 200-400 nm as well as to 20-40 nm [START_REF] Liu | Promoted Photo-oxidation Reactivity of Particulate BiVO4 Photocatalyst Prepared by a Photoassisted Sol-gel Method[END_REF][START_REF] Ke | Photocatalytic water splitting for O2 production under visible-light irradiation on BiVO4 nanoparticles in different sacrificial reagent solutions[END_REF]. Zhou and co-workers obtained similar results in the photodegradation process of 2,4-dichlorophenol by the BiVO4 thin films under visible radiations (λ > 420 nm) [START_REF] Zhou | Fabrication and photoelectrocatalytic properties of nanocrystalline monoclinic BiVO4 thin-film electrode[END_REF].

Peculiarities of hybrid systems in photoactive processes

Hybrid systems based on semiconductor materials are divided into two groups:

1. The combination of two inorganic materials forming photocatalytic tandem [START_REF] Zhang | Fabrication of SnO2-ZnO nanocomposite sensor for selective sensing of trimethylamine and the freshness of fishes[END_REF][START_REF] Yan | A selective NH3 gas sensor based on Fe2O3-ZnO nanocomposites at room temperature[END_REF];

2. The combination of inorganic material with graphene [START_REF] Lu | Microwave-assisted synthesis of graphene-ZnO nanocomposite for electrochemical supercapacitors[END_REF]129], nanotubes [START_REF] Jiang | Fabrication and characterization of ZnO-coated multi-walled carbon nanotubes with enhanced photocatalytic activity[END_REF] or an organic component such as organic dyes [START_REF] Josel | Conversion efficiency versus sensitizer for electrospun TiO2 nanorod electrodes in dye-sensitized solar cells[END_REF][START_REF] Yin | High-Performance Plastic Dye-sensitized Solar Cells Based on Low-Cost Commercial P25 TiO2 and Organic Dye[END_REF][START_REF] Miyoshi | Effect of Thin TiO2 Buffer Layer on the Performance of Plastic-based Dye-sensitized Solar Cells Using Indoline Dye[END_REF] and polymers [START_REF] Liufu | Thermal analysis and degradation mechanism of polyacrylate/ZnO nanocomposites[END_REF] creating a composite system.

For hybrid systems, analyses of the charge transfer of donor--acceptor (D--A) dyes/photocatalyst systems were carried out [START_REF] Ooyama | Dye-Sensitized Solar Cells Based On Donor-Acceptor p-Conjugated Fluorescent Dyes with a Pyridine Ring as an Electron-Withdrawing Anchoring Group[END_REF]136]. In this case the main part of the hybrid system is composed by porous oxide formed as nanometric particles deposited on semiconducting and semitransparent substrates such as fluorine doped thin oxide (FTO). It is commonly used material as a substrate [START_REF] Haid | Significant Improvement of Dye-Sensitized Solar Cell Performance by Small Structural Modification in π-Conjugated Donor-Acceptor Dyes[END_REF]. The used semiconductor oxides are titanium dioxide, zinc, vanadium or niobium oxides. On the surfaces of the semiconducting thin films organic dyes can be anchored. Their electronic features are chosen in such way that an effective absorption of visible solar radiation occurs and causes charge transfer from organic dye to the inorganic photoactive film. In the elaboration process of inorganic film sensitized by dyes, the organic molecules are  Excitation energy of the dye higher than the edge of the oxide conduction band allowing an efficient electron transfer process between the dye excited state and the CB of photocatalyst;

 Resistance to photodegradation as well as thermal and electrochemical stability.

Sensitizing photoactive materials with organic dyes were commonly synthesized using the D102 [140], D149 [START_REF] Miyoshi | Effect of Thin TiO2 Buffer Layer on the Performance of Plastic-based Dye-sensitized Solar Cells Using Indoline Dye[END_REF], N719 [START_REF] Saito | Large photocurrent generation in dye-sensitized ZnO solar cells[END_REF], Z907 [START_REF] Fattori | Adsorption and redox chemistry of cis-RuLL'(SCN)2 with L=4,4′-dicarboxylic acid-2,2′bipyridineand L'=4,4′-dinonyl-2,2′-bipyridine (Z907) at FTO and TiO2 electrode surfaces[END_REF] or black dye [START_REF] Mishra | Metal-Free Organic Dyes for Dye-Sensitized Solar Cells: From Structure: Property Relationships to Design Rules[END_REF].

Creating the hybrid systems, the properties of the organic components must be correctly chosen with respect to those of the inorganic photoactive film. In particular, the HOMO The absorption characteristic of organic dye should be situated within the visible light range. In order to minimize the charge recombination between the injected electrons and the oxidized dye, the charge should be located around the dye donor part away from the photocatalyst surface. In addition, the donor group of the dye should be hydrophobic to avoid direct contact with any aqueous medium. This criterion contributes also to increase the long-term stability of the hybrid photoactive network [START_REF] Kuang | Organic Dye-Sensitized Ionic Liquid Based Solar Cells: Remarkable Enhancement in Performance through Molecular Design of Indoline Sensitizers[END_REF].

The approach to anchor organic dye molecules to the surface of the TiO2 was developed. This enables the absorption of the visible light by the organic groups and contributes to efficient photoinduced charge transfer to the inorganic surfaces improving the photoactivity of the whole hybrid composites. The organic molecules as dyes D149 [START_REF] Liang | New Triphenylamine-Based Organic Dyes for Efficient Dye-Sensitized Solar Cells[END_REF], family of the merocyanine dyes [147] and carbon nanotube [148] were considered for titanium dioxide with the aim of increasing its photocatalytic efficiency. At the present time studies on the sensitization of the BiVO4 by organic dyes for photocatalytic applications are not widely discussed. It will be the aim of the presented work.

Conclusions

Present chapter discusses the nature of the photocatalytic process and the applications of semiconducting materials, which are able to generate active electron-hole pairs under the radiation with different wavelengths. For this purpose, the properties of the two most commonly used photocatalysts: zinc oxide and titanium dioxide were presented.

The electronic, structural, optical and photocatalytic properties of bismuth vanadate in bulk and thin film form were also discussed. At the same time an analysis was made concerning the key factors affecting the photocatalytic properties of the semiconducting oxides. On the basis of literature data, it can be concluded that:

 crystalline monoclinic BiVO4 is the most commonly used for photoactivated processes,

 synthesis way allows to achieve the intended surface affecting the photocatalytic properties of material,  annealing temperature and pH of the solution from which the BiVO4 crystallize also affects the photocatalytic properties of the material.

The selection of reaction parameters allows to control the morphology of the tested material and thus to affect its properties. Additionally, by doping or sensitizing the photocatalyst, the absorption limits can be significantly increased by controlling the range of radiation in which the material will be active. These actions are aimed at obtaining a materials which can be activated by a visible radiation and act in the photocatalytic reactions. (2.2.)

The n E is the energy of the state n. The Hamilton operator takes a form:

(2. . This means that the motion of the nuclei relative to electron motion is practically negligible. One may say also that observing the movement of "heavy" nuclei the rapidly moving electrons will be practically indescribable to the observer.

Assuming that the motion of the nuclei is insignificant in relation to electron motion the motion of nuclei may be suppressed. Separation of the electron system from the nucleus in a given atom is called adiabatic approximation. In consequence of the mentioned approximation the total wave function may be written as:

         R f R r n n n ) , (  .
(2.4)

The ) , ( R r n 
is an electron part of wave function depending on coordinates of the electrons r and parameterised on nucleus position. The nucleus part of the wave function

       R f n
shows dependence not only on the value of the parameter R, but also on the direction of its vector [2]. The resulting assumption allows toseparate electron Hamiltonian from nuclear part. As a result of this operation is received:

the electron part       , , ( , )
,

ee n n n H r R r R E r R r R    .
(2.5.)

with the electron Hamiltonian:

. (2.6.)

and the nuclear part

      NN n n n H R f R E f R   .
(2.7.)

with the nuclear Hamiltonian:

.

(2.8.)

where

( ) * J ad e E R T d      
is an average value of kinetic energy operator of nucleus motion [2]. When the nuclear kinetic energy is neglected one may subtract the kinetic energy operator  T from the Hamiltonian. In this way the last part of the Equation 2.8 is omitted. This procedure is called the Born-Oppenheimer's approximation.

Hartree-Fock methodology

The Hartree-Fock method is widely used in quantum chemistry, atomic physics, and condensed matter physics, where it allows an approximate solution of the Schrödinger equation for a multiple particle system. It is the basis of the ab initio computational method. The Hartree-Fock method is based on the variational principle which specifies that the grand state energy of the given system calculated as an expected value from proposed wave function is always greater than, or equal to, the energy that is the exact solution of the Schrödinger equation. . (2.9.)

The spinorbitals dependent on coordinates of the individual electrons. It is constructed as the product of a spin function dependent on the spin variable of an electron and an orbital function dependent on the particle's spatial variations [3].

The Hartree-Fock method is an iterative minimization of the energy of the system calculated by changing of the form of spinorbitals. This procedure is reduced to the solution of the Hartree-Fock equation similar in character to the time-independent Schrödinger equation:

    i i i i i F        . ( 2
    * 1 2 2 2 1 21 1 ( ) ( ) j i j j i
Jd rr

             . ( 2
 to j  .     * 1 2 2 2 1 21 1 ( ) ( ) j i j i j Kd rr              . (2.13.)
Total electrons energy in the Hartree-Fock method is defined as: 11 ()

nn i ij ij i i j E I J K        .
(2.14.) .

(2.11.)

where the n is a number of electrons. The Ii is one-electron integral, which can be written as:

* ( ) ( ) ( )

i i i k I k h k k d       . (2.15.)
The Jij and the Kij are two-electron integrals, coulomb and exchange, respectively defined as:

2 ** ( ) ( ) ( ) ( ) ij i j i j i j ij e J i j i j d r         . (2.16.) and 2 ** ( ) ( ) ( ) ( ) ij i j i j i j ij e K i j j i d r         . (2.17.)
The exchange integral Kij is different from zero only for parallel spin-directed electrons.

As a consequence, the triplet state always has less energy than the singlet state with the same electron configuration. Another important element is the introduction of the concept of the functional, which determines the average value of the ground state energy described by the formula: The method to find the functional was proposed by Kohn and Sham. It involves the projection of a real electron system into an auxiliary, hypothetical system of noninteracting electrons, moving in an effective potential describing the electrostatic effects of the nuclei and other electrons as well as the correlation-exchange potential. The prerequisite for this simplification is the constant density of electrons in both, the original and the newly generated system. The functional takes the form: The effective potential in the KS theory describing the non-interacting electron system takes the form:

Density functional theory (DFT)

        ext E F V r r dr      . ( 2
        00XC F T W E        . ( 2
        2 2 2 12 || KS ext XC r V r V r e dr r rr        .
(2.22.)

The correlation-exchange potential

  XC r
 is defined as an correlation-exchange energy derivative by electron density and is described by the formula:

      XC XC E r r       . (2.23.)
Knowledge of the correlation-exchange potential allows to determine the value of the total energy of a given system. The mentioned energy may be defined by an equation [4]:

            2 /2 12 12 1 12 2 2 N N XC XC N rr e E
dr dr E r r dr rr

              .
(2.24.)

Generalized gradient approximation (GGA)

DFT reduces the quantum mechanical ground state many-electron problem to selfconsistent one-electron form, through the Kohn-Sham equation. However, in practical calculations, the exchange-correlation energy as a functional of the density must be approximated. The simplest approximation is the local density approximation (LDA). It says that the depends only on the (local) density at a given point. It defined the exchange and correlation energy of homogeneous electron gas. In this approximation, the exchange-correlation energy at each point of the real space is assumed to be equal to that energy of a uniform electron gas with the same electronic density.

(2.25.)

where is the exchange-correlation energy per electron in a homogeneous electron gas with a density (r).

An improvement in the accuracy provided by the LDA can be obtained by generalised gradient approximation (GGA) functional. In the GGA approximation the functional depends on local density and its gradient. The basic idea of the GGAs is to express the exchange correlation energy in the following form:

(2.26.)

where the exchange part can be written as .

(2.27.)

The function F is a reduced density gradient for spin σ and is a local inhomogeneity parameter:

.

(2.28.)

Function F based on GGA exchange functional was developed for example by Becke (B88) and Perdew (P), by Lacks and Gordon (LG) or by Perdew, Bourke and Ernzerhof (PBE). These functionals base on exact properties without using empirical parameters.

The most popular correlation functional was implemented by Lee, Yang and Parr (LYP).

The exchange-correlation functional is usually written as the sum of an "exchange" part and a "correlation" part, where the first one contains only same-spin interactions, while the second one contains both same-spin and opposite-spin interactions.

Perdew-Burke-Ernzerhof (PBE) functional

The GGA functionals one may divide into two groups for example, the functionals that contain empirical parameters whose values have been fitted to reproduce experiment or more accurate calculations and the functionals with no empirically determined parameters. Different functionals might give different results for any particular configuration of atoms, it is important to select the functional that best suits the characteristics of the system. An example of a non-empirical GGA functional is the Perdew-Burke-Ernzerhof (PBE) functional.

The enhancement factor of the exchange functional mentioned in (2.27.) applied into PBE methodology takes the form:

(2.29.)

where 𝜅 = 0.804 is set to the maximum value allowed by the local Lieb-Oxford bound on EXC and µ = 0.21951 represents the set to recover the linear response of the uniform gas that the effective gradient coefficient for exchange cancels that for correlation [5].

Plane-wave method

In previous chapter (2.2) the theory of the density functional was discussed. There the multi-element systems of interacting electrons and ions were reduced to the problem of individual particles moving in the effective potential. Despite the above described simplification, problematic becomes the analysis of the bulk, infinite systems. The crystal is described by a cell with a finite number of atoms repeating in all three directions in space. The cell can be described as either a primitive cell or a supercell which consists of a group of atoms forming the studied crystal by a repetition. Where .

A solution of the above equation is obtained by diagonalizing the Hamiltonian matrix with matrix elements . The size of the matrix depends on the choice of cutoff energy and it becomes very large for systems containing both valence electrons and core electrons. This is a significant problem, but it can be solved after applying the approximation of pseudopotentials [6].

Most physical and chemical properties of crystals depend to a very good approximation only on the distribution of the valence electrons. The core electrons do not participate in the chemical bond. They are strongly localized around the nucleus, and their wave functions overlap only very little with the core electron wave functions from neighboring atoms. Therefore, the distribution of the core electrons basically does not change when the atoms are placed in a different chemical environment. It is thus justified to assume the core electrons to be "frozen" and to keep the core electron distribution of the isolated atom in the crystal environment.

Born-von Karman boundary conditions

The Because the , where m is an integer, in the reduced zone must be . In this way the integer m goes from to + .

The number of allowed wave vectors in the Brillouin zone equals the number of unit cells in the crystal.

Semi-empirical methods in quantum chemistry

The semi-empirical methods of quantum chemistry are based on Hartree-Fock formalism (Chapter 2.1.) with its simplification and approximation of certain parameters obtained from experiments. They are widely used in quantum chemistry for large particle systems because of considerably less computational cost than the ab initio method. In addition, the use of empirical parameters allows for some effects of electron correlation to be taken into account. The development of semi-empirical methods assumes that the Hartree-Fock formalism can be approximated or completely neglects at the level of the two-electron integrals Jij and Kij (Equation 2.16. and 2.17.). Instead to calculate these integrals the parameters best describing an experimental data or ab initio calculations are introduced. The obtained results may be wrong when the calculated molecule differs from the system for which the parameterization was done. Also the chosen properties can be calculated only by the semi-empirical methods developed to calculate the given properties. The semi-empirical methods are most commonly used for the organic chemistry, where the molecules are composed of several types of atoms, resulting in relatively easy parameterization.

The oldest semi-empirical method completely neglecting the two-electron integrals is called Hückel method and the extended Hückel method. The Hückel method treats quantum mechanically only π electrons and is intended for calculations of aromatic conjugated molecules. Extended Hückel method takes into account all the valence electrons. Among the other semi-empirical methods we can distinguish two basic groups:

 Methods in which only π electrons are quantum mechanically treated. The most important of these ones is the PPP (Pariser-Parr-Pople) method, which mainly serves to calculate excited states of the π-electrons [8,9].

 Methods in which all valence electrons are quantum mechanically treated. They include: CNDO, INDO, NDDO methods introduced by John Pople. Parameters are not based on experimental results, but on ab initio calculations with minimal basis set. They are currently rarely used, but they make a significant contribution to the development of methodology [10].

 Methods as MINDO, MNDO, AM1, PM3, PM6, RM1 and SAM1 are parameterized using experimental enthalpy of formation, dipole moments, ionization potential and geometric parameters [11]. There are also methods developed for the calculation of excited states (thus predicting electron spectra), such as ZINDO and ZINDO/S [12].

PM6 parameterization method

Over the past 40 years, the NDDO and MNDO-type methods been refined [13,14].

Numerous changes were made in the MNDO methodology. The most important of them was an addition of d orbitals to the main group elements and an introduction of diatomic parameters. In consequence the PM3 method was created [15]. By its improving the next step was a development of the PM6 parametrized method. It includes a correction in the core-core interactions defined as follows:

(2.35.)

and resulting in a convergence increasing by the addition of small perturbation. The PM6 method contains atomic and diatomic parameters for 70 elements, including transition metal groups, not previously introduced in any semi-empirical computational methods [16].

Scanning electron microscopy (SEM)

An electron microscope uses an electron beam which can be deflected and focused by electromagnetic lenses. It gives a higher resolution than the optical microscope due to the de Broglie wavelength corresponding to electrons, shorter by several orders of magnitude than the wavelength of light. Electron microscope resolution is however limited by diffraction effects.

The characteristics and quality of the scanning electron microscope (SEM) depends on the structure of the microscope and on the type of signal used to produce the image.

The resolution capacity for optical systems depends on the wavelength of light λ and the The SEM is used in material and metallurgical engineering, including fractal studies, tribiological wear, corrosion damage diagnosis, surface analysis of materials, chemical composition analysis and crystallographic orientation. In biology and medicine it is used for determining intracellular structures and processes, finding and recognizing chemicals, zoological research, quality control of medical procedures or defect characteristics [5,6].

In present work the JEOL, JSM 6510 LV scanning electron microscopy was used. In order to proper focus on the samples, the accelerating voltage and the variable slit size were adapted. Astigmatism and beam shape has been manually adjusted.

Atomic force microscopy (AFM)

Atomic force microscope (AFM) was constructed in 1986 at IBM Lab [7] and represents today an important microscopic technique with a particular interest in nanoscience. Beyond the material morphology scanning, an environmental AFM can measure electrical or magnetic properties at the nanoscale.

Contact operation mode

In contact mode, the blade is in direct contact with the sample surface and the distance between the atoms of the blade and the sample is less than 1 Å. In this mode, due to the presence of repulsive forces, the beam arm is bent in the opposite direction to the sample (Schema 3.3.1.b). The source of repulsive forces is the interaction between electron orbital atoms on the test surface and on the surface of the blade.

In this layout, it is possible to obtain very accurate surface images, but in the case of soft materials such as polymers, the impact force should be limited to prevent the destruction of the sample. Additional pressure on the sample is caused by capillary forces (water molecules adsorbed on the surface of the sample) or electrostatic forces occurring between the charges located on the surface of the sample and the measuring edge. Therefore, in some cases, to eliminate capillary and electrostatic forces the testing is carried out in liquids. AFM microscope in contact mode can simultaneously record:

1) Topographic images (constant height) when constant blade pressure is maintained on the sample while the feedback loop is on. From the images you can determine the height of the surface elements, the roughness (in nm or μm units).

2) Image of the variable force (deflection, internal sensor) from the current measurement (beam swing) when the feedback loop is off. Images are characterized by a great resolution (detail, contrast) but cannot be measured, for example, roughness.

Non-contact operation mode

In the non-contact mode, the AFM operates in the long-range attraction and the distance of the measuring blade is equal to 1 -10 nm. Due to the presence of gravity, the beam arm is bent towards the sample. The attractive forces are those of van der Waals between atoms at the test surface and on the surface of the blade. This method is designed to test very delicate and deformable materials, but the contaminant layer on the surface of the sample (e.g. adsorbed water) can introduce artifacts (Schema 3.

3.1.c).

This method is not suitable for fluid testing. In the non-contact method, the beam with a blade oscillates at a frequency close to the resonant frequency (100 -400 kHz) with a few tens of nanometres. As the blade moves closer to the sample, the frequency and period of oscillation of the probe change. It serves to control the feedback loop and to produce a topographic image of the surface.

Tapping mode

In the tapping mode, the microscope operates within the distance range shown in Schema 3.3.1.a. During a short-term contact, the kinetic energy of the probe is dissipated and it reduces the amplitude of the vibration. These changes are recorded in the feedback system where the vibration frequency is automatically selected so that the impact force between the measuring blade and the sample is kept as low as possible.

Tapping tests help to avoid damage of the sample surface by eliminating friction and adhesion forces and are especially recommended for testing easily deformable and delicate materials, both in fluids and in the air [8][9][10].

Kelvin probe force microscopy (KFM)

The Kelvin probe force microscopy (KFM), also called surface potential microscopy proposed by Nonnermacher in the 1991, is nanometric and atomic resolution technique.

KFM represents a variant of atomic force microscopy where a metalized cantilever is capable to measure the potential difference between the nozzle contained therein and the sample surface. Through this measurement, one can obtain the characteristic work function of the sample surface which gives the exact composition at the atomic level.

Through the knowledge of the work function of the sample surface, various applications are possible.

They can be correlated with the surface reconstruction, defects, or corrosion due to the working function of the atoms resulting from such phenomenon. Semiconductor, dielectrics, biological systems are able to be investigated with a molecular resolution when the topography and the surface potential can be observed.

Every atom in a solids possesses the Fermi energy defined as the energy of the electrons of the outermost atom layer, i.e. it refers to the electrons of the last occupied level.

Another important concept is the work function which consists of the energy required to remove an electron from the last energetic layer of an atom leading it to the "vacuum" where it will be free of any attraction to the atom of origin. In vacuum the electron acquires an energy different from that in the material, given the absence of interactions with the solid material.

The operation of KFM is similar to atomic force microscopy where a conductive probe A more sophisticated method involves the application of simultaneous continuous and alternating voltages in order to obtain not only the work function of the sample but also the topography of the sample.

The applications of the KFM method allowed a great evolution in the study of nanostructures and sub-nanostructures as well as surfaces of semiconductors and ionic crystals investigations [11,12].

In the presented work measurements were carried out by Atomic force microscopy (AFM) and Kelvin force microscopy (KFM) with a Multimode Nanoscope 3 from Veeco.

UV-visible Spectrophotometry

Spectrophotometry in the ultraviolet (UV) and visible (Vis) range is one of the oldest instrumental methods in chemical analysis. The phenomenon of electromagnetic radiation absorption by matter has found a wide application both in particle structure studies as well as in qualitative and quantitative chemical analysis. Absorption of visible light and ultraviolet radiation depends on the structure of the molecule and that of the compound. The absorption of defined radiation causes the transition of electrons from ground states to higher energy levels -excited states.

The rotation, vibration, and electron energies are quantized and the required energies for transitions are formulated as follows:

 For the absorption of radiation there must be such two quantum states of molecules ψm and ψn with energy difference corresponding to the energy of incident radiation hν: The measure of the intensity of the absorption band is the molar absorption coefficient εmax, at the wavelength of the absorption maximum λmax. The value of εmax is a measure of the transition probability which in turns is more higher for allowed than for forbidden transitions.

. ( 3
In a practical application, when electromagnetic radiation crosses a matter, it can be absorbed, reflected or dispersed. The intensity of the incident beam is given by: , (3.5.6.)

where Ia is intensity of radiation absorbed by the material, the It and Ir are related respectively to the transmitted and reflected as well as diffused light.

Since radiation absorption measurements are usually made in relation to the reference solution which composition should be similar to that of the sample and they should be in identical cuvettes, the reflected and diffused radiation Ir in both cases is identical and may be omitted. One may assume that the reference solution under the measurement conditions does not absorb radiation and the intensity of the radiation beam passing through the reference solution is equal to the intensity of the incident beam on the sample.

The ratio of the intensity of the radiation passing through the sample It to the intensity of radiation incident on the sample Ia (equal to the intensity of the radiation passing through the reference), is called transmittance or permeability and is denoted as: or when the transmittance is expressed in percentages: . (3.5.12.)

. ( 3
The polarization possesses a tensor representation and can be presented as an ellipsoid of polarization. When the electrical field interaction is uniform in all directions of the sample its polarization is isotropic. When the polarizability  is represented by an ellipsoid depending on the space orientation the polarization is anisotropic.

An intensity of the Raman spectra are related to the polarization of the network and can be described by the function:  Rayleigh bands: generated by the incident radiation at frequencies ν0, which does not match the energy levels of the molecule.

As the molecule after radiation exposure returns to the same energy level, this phenomenon is reduced to the classical Rayleigh diffusion.

 Stokes bands: when the molecule after exposure to radiation undergoes a transition to a higher vibration level and then to relax a photon is emitted losing its energy. When the emitted photon has less energy than the absorbed photon, this energy difference is the Stokes shift.

 Anti-Stroke bands -before being exposed to radiation the molecule was at a higher vibration level, the return to fundamental vibration level is accompanied by energy release. The energy of the diffused photon is greater than the incident beam. The anti-Stokes band appears in the Raman spectra at the opposite side compared to the Stokes band.

Photocatalytic test

Photocatalytic reactions based on the radiation inducing decomposition of organic dyes are model systems to evaluate the photocatalytic properties of semiconducting oxides.

In heterogeneous photocatalysis, the semiconductor is surrounded by an environment such as solution with dyes. The best mechanism for describing this phenomenon is the Langmuir-Hinshelwood model. Its mechanism assumes that the absorption of radiation by the photocatalyst leads to the generation of free electrons and holes, known under the common name of the excitons. The life time of the excitons is extremely short (about 30 ns in TiO2 [20]) and can lead easily to recombination phenomena which cancel the electron-hole pairs. With suitable lifetime, electrons and holes may have enough time to migrate to the surface of the photocatalyst where they may generate radicals at the interface with the surrounding media.

In the case that substrate level is small, e.g. when reacted in dilute solutions or gas phase containing small amounts of substrate, Kc value << 1. Therefore Equation 3.7.7.

can be written as: where in general, the value of kr refers to the weight of the catalyst and this parameter is used to express the activity of the photocatalyst [24,25].

, ( 3
Wu reports [26] that the concentration of the organic dye in the solution may have a significant effect on the efficiency of the photoactive process. It has been proven that an increase in the concentration of the organic compound can cause radiation shielding effect and in consequence less photons my reach the surface of the photocatalyst [26].

One of the most commonly used organic dyes in photocatalysis is methylene blue. Studies on the effect of the addition of oxidants in the dye degradation process confirm that and ions also act as electron acceptors in this case, slowing the electron-hole recombination process. In addition, to prevent electron-hole recombination by electron capture from the conductivity band, these substances participate in the formation of additional hydroxyl radicals and other reactive chemical species such as sulfate radicals [28,29].

Chapter 4

Physico-chemistry of BiVO 4

Bismuth vanadate can be synthesised using different methods such as sol-gel, hydrothermal method, RF-sputtering, ball milling, etc. Depending on the considered method, the morphology and structural properties of the obtained material can be modulated in large extent. The objective of the present chapter is to highlight the main features of the synthesis methods to obtain BiVO4 with defined microstructure organization. It will focus on an experimental approaches developed to obtain pristine BiVO4 with mesoporous morphology as well as to synthesize the hybrid BiVO4 sensitized by selected organic dyes. The samples which production is described in the present chapter were used to perform all experimental investigations presented and analysed in Chapter 5.

Synthesis of the mesoporous BiVO4

The goal of the present work is a synthesis of the mesoporous material based on BiVO4 exhibiting large available contact surfaces with external environment. In this case two Solutions A and B were prepared and mixed together under the stirring. To prepare the Solution A, first of all, the 0,5 mmol (0.058 g) of the NH4VO3 with 9 ml of 1-propanol were used. Under continuous stirring, the 0,5 ml of HNO3 (65%) drop by drop during 3 minutes was added. As a result, the solution turned to a yellow color. Then, the mixture was kept during 30 min at room temperature under continuous stirring. Next, ultrasonic bath was used during 5 minutes followed by adding 0,5 mmol (0.096g) of citric acid with stirring until dissolution. To prepare Solution B the 0.5 mmol (0.2425g) of Bi(NO3)3x5H2O was dissolved with 0.5 ml of acetic acid.

In a final stage, Solution A was mixed with Solution B, stirred 30 minutes and then the surfactant was added. Depending on the surfactant nature, two solutions were prepared.

One called the solution I containing 0.09 mmol (0.55 g) of the P123 and the second labeled solution II made by using 0.024 mmol (0.3 g) of the F127.

Hybrid composites based on mesoporous BiVO4 with anchored dyes

The All other chemicals as the tert-buthanol or acetonitrile used with BiVO4 samples were purchased from commercial company (Merck, Sigma-Aldrich) and used without further purification.

Preparation of organic dye solution:

A 3.34 mg of the D149, 4.8 mg of the LIGAND or 1mg of the COMPLEX were placed in a dark container. Next, the dyes were added to 7.5 ml of the acetonitrile and 7.5 ml of the tert-butanol. The solution was submitted to ultrasonification in water for 10 sec. and was heated at 66°C for 30 min to completely dissolve the organic dye powders. After completing the dissolution process, the solutions were cooled to RT. To prevent the problem of solution aging, all solutions were used directly, or up to 3h after being prepared.

Preparation of hybrid photoactive systems based on the BiVO4 thin film sensitized by organic dye:

The annealed thin films (as is presented in Fig. 4.1.1.) were immersed in ( of organic dye) dye solutions. This immersion is made at room temperature during 4h. Its aim is to allow efficient adsorption of dyes on the available inorganic surface of the BiVO4. In consequence the dyes coated the BiVO4 media creating the hybrid system used for the physico-chemical analysis described in Chapter 5. Finally, the hybrid composite films were dried at RT for 12h on air.

Exhaustive characterizations were carried out on hybrid systems in order to analyze their photocatalytic properties and in consequence to define their electronic charge transfer properties and the optical features of the inorganic/organic films. The electronic properties of the monoclinic scheelite BiVO4 single crystal were calculated using the CASTEP program [3,4] with implemented total energy plane-wave pseudopotential method. The electronic exchange-correlation energy was treated within the framework of the generalized gradient approximation (GGA) using Perdew-Burke-Ernzerhof (PBE) potential [5]. The Monkhorst-Pack k-points grid sampling was set at 5×5×7 points for the Brillouin zone [6]. The plane-wave basis set cut-off was chosen to be equal to 350 eV. The total energy was considered to be converged when the selfconsistent field (SCF) tolerance was equal to 10 -6 eV/atom. The ultra-soft pseudopotential was used to perform calculations for Bi 6s 2 6p 3 , V 3s 2 3p 6 3d 3 4s 2 and O 2s 2 2p 4 electrons [7].

Reference

The standard procedure before electron properties calculations is the geometry optimization of the molecular structure. In the presented work the mentioned standard geometry optimization procedure was performed as well as the electronic properties of the BiVO4 were calculated for the frozen structure with parameters obtained from the X-ray diffraction experiments. In this case, the work presents two approaches. One is taking into essential calculations the BiVO4 single crystal with geometry optimized structure (relaxed structure) and the second one without optimization (frozen structure).

During geometry optimization procedure the lattice parameters of the BiVO4 structure and its atomic coordinates were relaxed by minimizing the total energy using the DFT/PBE potential. The Broyden-Fletcher-Goldfarb-Shanno (BFGS) schema was used for the minimization algorithm [8]. The convergence parameters were chosen for the energy as low as 210 -5 eV per atom, force on the atom was less than 0.01 eVÅ -1 , stress on the atom was less than 0.02 GPa and atomic displacement was below 510 -4 Å. Table 5.1.1.1. reports the parameters of the unit cell and the length of the bonds for the relaxed and frozen structure. As a consequence of the structure relaxation, the local environments around Bi and V atoms undergo a more symmetrical organization.

The changes in the interatomic distances obtained by the geometry optimization, affect the electronic properties of the BiVO4 structure. In order to prove this hypothesis, the electron energy dispersion diagram in the Brillouin zone was calculated. The energy dispersion curves of occupied and unoccupied states exhibit a similar behavior for the optimized and frozen structures (Fig. 5.1.1.1). It means that shape of the energy levels evaluation is similar for the both optimized and for frozen structure as well.

Nevertheless, the energy gap for the optimized structure is less by 0.24 eV compared to that of the frozen structure. The obtained results for the optimized structure are consistent with the data published by Zhao et al. [10], where the band gap of the BiVO4 was evaluated theoretically using the GGA/PBE functional and was obtained to be equal to 2.17 eV. For the frozen structure, its energy gap was evaluated to 2.41 eV and is in agreement with the experimental reports [10,11]. As it was mentioned before the shapes of the energy dispersion curves in both structures are similar. In this case, the optimized and frozen structures possess an indirect band gap (Fig. The minimum of the conduction band obtained for this structure is dominated by V 3d states with small contributions from the Bi 6p and O 2p orbitals. In contrast to the above mentioned data the main contributions to the valence band states of the frozen BiVO4 crystal structure comes from the O 2p orbitals slightly hybridized with Bi 6s states.

The bottom of the conduction band calculated for the BiVO4 crystal structure is formed by the V 3d states. Usually, the valence band in metal oxides is derived from the O 2p orbitals and the conduction band is built by metal s or d orbitals. However, in monoclinic BiVO4, the valence band contains hybrid O 2p and Bi 6s orbitals, which causes an upward dispersion of the electronic structure at the Brillouin zone boundary [16]. This indicates that the frozen structure allows sound analysis of the electronic features of the BiVO4 crystal. The distortions of Bi-O and V-O bond lengths during geometry optimization enhance the impact of the V 3p electronic states in the valence band and lower the contribution of the V 3d states in the conduction band. It contributes to a more significant dispersion in the valence band of the BiVO4 crystal. These results are in agreement with the data measured by using the XPS method [17]. The obtained results suggest that, from a computational point of view, both structuresoptimized and frozenlead to coherent outcomes. The obtained data make it possible to infer that modeled nanosized clusters should be constructed with protection of the crystal internal structure (interior) and with optimized external atomic shells (exterior), according to the surface effects and environmental interactions. Analyzing the PDOS one may conclude that the transmission of the photogenerated electrons occurs between the hybridized O 2p and Bi 6s orbitals and the V 3d orbitals for the frozen structure whereas for the relaxed structure the electron excitation is between hybridized O 2p and V 3p electronic states and the V 3d electronic state. The transition between the different orbitals of the same atom is not a typical phenomenon for semiconductors. In this case the frozen structure makes it possible to more accurately reflect the actual electronic relationships in BiVO4. Therefore, later in the work, the frozen structure will serve as a benchmark for analyzing the properties of nanocrystals.

According to the literature data, the size of the BiVO4 crystallites affects the photocatalytic activity. It has been shown that nanostructures exhibit better photocatalytic properties than the bulk materials [18] however this phenomenon is not fully explained. Quantum chemical calculations may be helpful in this case.

Consequently, theoretical studies on the electronic, optical and vibrational properties of bismuth vanadate nanocrystals have been carried out. The general advantage of nanostructured systems is related to their high surface areas, which increase the efficiency of photocatalytic reactions.

Nanoclusters with a spherical shape composed by n units of the (BiVO4) and referred below as the (BiVO4)n were built using Materials Studio software. The Hessian evaluation was performed to exclude those structures giving rise to negative modes. The third family of the investigated clusters included structures with partial reconstruction occurring on the cluster surface.

In this case, the geometry optimization procedure was performed applying the PCM solvent model, where the environment was modeled as BiVO4 nanoclusters with dielectric parameters measured at room temperature [28]. Physical properties for both frozen and relaxed structures were calculated according to the procedure described earlier. The DFT/PBE and DFT/B3LYP functionals implemented in the D3MOL program were used to calculate an electronic property of the clusters. Unfortunately, the applied methodologies proved to be satisfactory only for small nanocrystals exhibiting size quantum confinement effect. In case of large systems where the value of the energy gap is going to be saturated, the DFT method becomes insufficient. The use of correct DFT functionals evaluate the changes in the energy band gap value versus the cluster size exhibiting the size inducing quantum confinement effect. Because the obtained results are underestimated comparing to the experimental data, it was decided to use the semi-empirical PM6 parameterized method implemented in Gaussian program [29] for future calculations. The energy difference between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) defined as the EHOMO-LUMO energy gap splitting calculated for the frozen and The computed data, represented by triangles, fluctuate with increasing the cluster size, but they have a tendency to be aligned to an asymptotic value close to 2.4 eV. The same value of energy gap was obtained for the bulk frozen crystal structure of the BiVO4 as well as for non-optimized nanocrystals with a size greater than 1.0 nm (nanocrystals with n > 10). The obtained results are in good agreement with the data presented by Sun et al. [32]. Calculations performed for partially reconstructed clusters show that for nanoparticles smaller than 1.1 nm (n < 20), the physical peculiarities are generally dominated by surface effects. For larger particles (sizes above 1.8 nm, n > 40), the surface influence is less pronounced compared to core atoms keeping the same coordinates as involved in the bulk monoclinic crystal structure.

The HOMO and the LUMO orbitals distribution for the nanocrystals of different sizes is demonstrated in the Table 5.1.1.2.. In the mentioned figures, the bismuth atoms are shown in yellow, oxygen in violet, while vanadium is grey. The reported data underline that the HOMO and LUMO orbitals are located in the reconstructed surface part of the cluster. Analysis of the presented orbital distribution one may observe significant similarities to the PDOS calculated for the bulk BiVO4 material. The HOMO orbitals can be compared to a valence band from which photogenerated electrons flow, while LUMO can be approximated to the conduction band. The obtained results confirm that the HOMO orbital is built by the O 2p states and the LUMO is created by the V 3d orbitals. Compared to the bulk material, where the VBM occurs from O 2p orbitals hybridised by V 3p and Bi 6s for the optimized and frozen structure, respectively, one could conclude that the size reduction of the investigated cluster does not drastically change its electronic nature. Analysing the data collected in the Table 5.1.1.2 one may see that the HOMO orbitals are located at the opposite side of cluster compared to the LUMO ones. It allows to conclude that in the photoactivated processes of electron transitions from HOMO to LUMO orbital, the surface area plays a key role. Increasing the active surface of nanoparticles by synthesis of the meso-or nanoporous material, will also increase photocatalytic capacity as it was confirmed experimentally by Zhang and coauthors [33]. The PDOS computed for the partially reconstructed (BiVO4)18 cluster indicates that the highest occupied energy levels are associated mainly with the O 2p states hybridized with the Bi 6p orbitals (Fig. 5.1.1.6). Similar properties were demonstrated for the bulk monoclinic BiVO4 with frozen atomic coordinates. The main difference between the DOS of the bulk system and the nanocluster seems to be related to the contribution from Bi 6s and Bi 6p states, respectively. Moreover, it is worth noting that for nanocrystalline (BiVO4)18 a strong correlation is involved between Bi and O orbitals. This confirms that the electronic properties of the simulated cluster are mainly driven by the core (internal)

atoms. The unoccupied orbitals with the lowest energy are composed mainly by V 3d states, with small contributions from Bi 6p and O 2p states, as was also observed for the bulk system. respectively as ( 110), ( 040), ( 002), ( 211), ( 051), ( 042), ( 202) and (161). It was also decided to investigate how the used substrates affect the crystalline structure of the deposit BiVO4 thin films. For this purpose, silicon and borofloat substrates were used to deposit the thin films by dip coating method. As discussed in Chapter 4. the silicon and borofloat substrates were initially cleaned before deposition of the films.

After deposition, the films were annealed and their structures analyzed by SEM. Fig. 5.1.1.9 reports the SEM images of the BiVO4 thin films deposited on silicon and borofloat substrates. It seems that the substrate play a key role on the morphology of the films as revealed from the shape and sizes of the deposited BiVO4 particles. The average size of the BiVO4 particles on the silicon substrate is about 130 nm compared to 46 nm for borofloat deposited film. The morphology of the obtained surfaces shows drastic differences between the two used substrates and SEM images reveal nanoisland features. Such surface shapes induce high contact area which improves the efficiency of heterogeneous photocatalysis as planned for the application of the considered mesoporous films. The realization for nanostructured morphologies constitutes a real advance compared to already reported synthesis of the BiVO4 such as powders [34,35] but the specific surfaces remains quite low compared to the surface contact of our textured films. The used synthesis method combined with the substrate effect gives better film organization compared to deposition with the CVD method leading to particle size as high as 300 nm [36]. Compared to the well-known photocatalyst as the TiO2 in a non-doped form with a spectral absorption in UV range [40,41], mesoporous thin films of the BiVO4 are more promising to harvest the visible range of the solar radiation.

The band gap of the BiVO4 thin films was calculated according to the Kubelka-Munk formula [42]. The obtained value of the Eg is equal to 2.55 eV (Fig. 5.1.2.2, bottom panel) and is in agreement with theoretical predictions (2.60 eV) performed for partially reconstructed (BiVO4)18 cluster having a size of 1.2 nm, using semi-empirical PM6 parameterized calculations. The UV-vis absorption spectrum of the frozen monoclinic BiVO4 single crystal was also calculated. These spectra related to both configurations are shown in Fig. 5.1.2.2 (top panel). The absorption edge calculated for the nano-sized cluster is shifted to higher energy compared to that of the bulk crystal.

A BF Absorbance (a.u.) Such a difference is characteristic of the size inducing quantum confinement effect and is in agreement with the work of Karunakaranet al. [43]. Moreover, for the bulk infinite structure, the optical spectrum is more extended with two main features reflecting the indirect band gap system. The UV absorption observed in monoclinic BiVO4 is associated with a band transition from O 2p to V 3d, whereas visible light absorption is due to the transition from a valence band formed by Bi 6s or a hybrid orbital of Bi 6s

and O 2p to a conduction band of V 3d. This is in agreement with the data presented in the work of Ng et al. [44].

The presented results confirm the existence of a quantum confinement size effect for both the simulated and the experimental system. As the crystallite size increases, the influence of the surface on the interaction with the environment became dominant [45,46]. It is worth noting also that at the nanoscale, the decrease in confining dimension makes the energy level discrete and widens the band gap leading the blue shift of the absorption spectra. Additionally, the surface of the partially reconstructed nanoparticles was modified according to the environmental effect of the interacting bismuth vanadate. In this case, the implemented cluster model of partially modified structures is a good model for a mesoporous material to predict its electronic properties so crucial for photocatalytic applications.

To sum up, the experimental optical features obtained on a mesoporous material are consistent with the theoretical calculations carried out for partially reconstructed nanostructures. This close correlation makes us confident in the developed numerical approach to analyze the electronic and optical effects of this class of semiconducting oxides. The size quantum confinement effect was well demonstrated theoretically on the optical spectrum of a nano-sized cluster. The photoinduced charge transfer mechanism under visible light radiation is of primary importance for the photocatalytic responses of semiconductors. In the same context, nanostructured systems composed of nanoparticles possess high specific surfaces area, which plays a crucial role in heterogeneous photocatalysis. This was also confirmed by the fact that the active HOMO and LUMO orbitals were located at the nanoparticle surface. However, as was shown from calculations, the spectral range of absorbed sunlight observed for nanoparticles was shifted to higher energies, which limits the absorption quantum efficiency. Thus, a balance must be achieved between the size reduction and the induced widening of the band gap. This is a hot topic in major photoinduced phenomena since the modulation of the electronic structure and density of states may be ensured by the size and shape design.

Analysis of the active vibrations using IR and Raman spectroscopy

The vibrational properties of the investigated materials were characterized by Raman spectrometry. These data were obtained for the synthesized BiVO4 thin films as well as on theoretically simulated clusters using quantum chemical calculations. First of all the IR absorption spectra were calculated using the PM6 methodology for both frozen and optimized clusters with different size. The calculated IR spectra for the frozen structure of nanoparticles show a broad band around 800 cm which broaden the bands leading to vibrational density of states features [47]. It gives the broad peak with the centre localized at more than 900 cm -1 .

Very similar behaviour was observed in Ruiz Preciado and co-authors work illustrating vibration properties of the NiTiO3 nanoparticles [48]. The calculations are devoted to predict the electronic peculiarities of these organic molecules in order to implement them in the photocatalytic applications. The geometry structures of the organic dyes molecules were initially optimized using quantum chemical calculations according to the total energy minimization procedures.

The calculations were performed applying semi-empirical PM6 parametrised method as it is implemented in Gaussian program [29,50]. The procedure was performed for isolated molecule in vacuum. The initial geometry was built up using the ACD/ChemSketch, an integrated software package from Advanced Chemistry Development, Inc. The minimum of the potential energy surface was calculated for neutral molecules using restricted Hartree-Fock (RHF) method with the SCF convergence criterion equal to 10 -5 hartree. No symmetry rules were imposed during the electronic structure analysis. The geometry optimization was performed in Cartesian atomic coordinates. The gradient convergence tolerance was equal to 10 -6 hartree/Bohr using the quadratic approximation (QA) method [51] updating the Hessian matrix during the optimization.

To predict electronic and vibrational properties of the investigated molecules the quantum chemical calculations were performed using their structures with optimized geometry. These properties were calculated for the isolated molecules at the RHF SCF level with energy convergence criterion chosen to be 10 -12 hartree. The geometries of the hybrid systems were optimized according to the total energy minimization using the methodology and parameters the same as it was described previously for the optimization procedure applied for the isolated dye molecules. In this case, linking vanadium atom and three neighbouring oxygen atoms located on the BiVO4 cluster surface serving as grafting sites for the dye molecules, were relaxed. The remaining part of the BiVO4 cluster was frozen. Also the atomic position of the dye molecules were allowed to be relaxed. Concluding, one may say that in the generated systems, the optimization of the atomic positions concerns the organic dye molecules

HD HL

The described charge transfer model between the organic molecule and the semiconducting nanoparicles is based on the treatment of the hybrid system as the tandem of the two components, as described in the work of Hara et al. [52,53] The nature of the connection between the photocatalyst and the organic dye has been investigated for photovoltaic systems and was described as persistent chemical bonding [54,55]. In the BiVO4/D149 hybrid systems a donor-acceptor interface is produced by a chemical bond occurring between semiconductor and the organic molecule [59,60]. In this architecture design, the organic dye assumes the role of a donor, and the semiconductor material acts as an electron acceptor.

Optical properties of the BiVO4/dye thin films

An optical properties of the BiVO4 thin films sensitized by organic dyes were investigated and the results are summarized in the Fig. 5.2.2.1. The extension of the optical spectra is enhanced in the spectral visible range up to 700 nm by using the dyes.

This effect improves the applicability of the system for visible light driven photoactivity. The broad UV-vis absorption band measured for the BiVO4/D149 system (HD) is located between 480 nm and 630 nm. The optical spectrum is composed by the absorption bands emanating from the inorganic thin film and the one related to the used organic dye (see Fig. 5.2.2.1). The appearance of the mentioned absorption band illustrates the influence of the dye on the sensitized network and traduces the involvement of charge transfer band as it was described by Hori et al. [65]. Similarly, the mentioned phenomenon occurs in the BiVO4/LIGAND system (HL), but it can be seen that the effect of the dye in this case is not so significant as it is noticed for the HD hybrid system. Unfortunately, the COMPLEX attached to the BiVO4 only changed the absorption spectrum of the system to a lesser extent. The change has not reached the limit of absorption, which is entirely in line with the absorption of the bare film of the BiVO4 and no additional band that implies a charge transfer band is observed.

127

This allows to assume that the COMPLEX did not create a coherent system with semiconductor substrate and thus only settled on the surface by intermolecular interaction rather than chemical bonding. This may mean week interactions between components of the system which, under the influence of external factors, can be separated into individual components.

Impact of anchored organic dye molecules on the Raman activity

The structural properties of the investigated hybrids were studied by Raman method and the experimental data were compared to the theoretical calculations. The experimental Raman spectra (see Fig. Asymmetric and symmetric deformation of the group occurs at 305 cm -1 and 678 cm -1 , respectively. On the other hand, the analysis of the BiVO4/COMPLEX system shows a negligible presence of the symmetrical and asymmetric deformation. The Raman spectra analysis confirms that the D149 molecule is chemically bonded to the BiVO4 nanostructure. The study of vibration properties confirms the above analysis which proved that the BiVO4/D149 system has the best compatibility with the BiVO4 mesoporous material to be used as photocatalyst. The obtained values suggest that the photocatalytic processes observed in the investigated hybrids are less efficient than using the pure mesoporous BiVO4 material.

This may be due to the fact that most of the active surface of the photocatalyst has been stuck by attaching sensitizers to it. This means a smaller surface able to generate the active oxygen and hydroxyl radicals which cause the decomposition of organic molecules in solution. The absence of changes on MB absorption spectra in solution during the experiment indicates that the dyes did not penetrate the test solution and were adsorbed on the surface throughout the process, which means that the effect of increasing the concentration of dye in solution can be ruled out.

Conclusion

On the basis of a combined strategy of computational and experimental investigations, the transfer of electric charge between the dye molecules and the nanocrystals has been demonstrated. Only the BiVO4 sensitized by the D149 molecules exhibits compatible electronic band alignment and it can be proposed to be used for the photoactiveted processes and applications. In the case of materials sensitized by LIGAND and COMPLEX molecules, no significant changes in electronic or optical properties have been observed in relation to the bare BiVO4 thin film.

The analysis using AFM instrumentation in connection with the investigations of the surface potential distribution (KFM) of mesoporous BiVO4 sensitized with organic dye molecules confirmed the previously obtained results of the theoretical analysis of the electron density distribution on the HOMO-LUMO orbital. The tests were carried out for the BiVO4 thin film samples with anchored D149, LIGAND and COMPLEX molecules before and during additional sample irradiation. For the HD and HL samples, the surface potential under the additional irradiation is blurred, while for the HC sample the reverse phenomenon was observed. In this case the potential focused on the particles of the material which was confirmed comparing the image with the surface representation.

Analyzing the optical properties of the investigated hybrid systems, one may conclude that for the BiVO4/D149 additional absorption band appears between 480 nm and 630 nm. The mentioned band corresponds to the absorption spectra of the D149. In this case the charge transfer band is observed indicating the influence of the sensitized organic molecule on the absorption capacity of the hybrid system. Anchored LIGAND and COMPLEX molecules do not create any additional absorption band. Probably it is caused by unstable connection between the organic and inorganic parts of the system.

Synthesized hybrid systems were tested as photocatalytic active materials.

Their photocatalytic capacity was estimated to be equal to 25% for the BiVO4/D149 and 30% for the BiVO4/LIGAND as well as for the BiVO4/COMPLEX. The obtained values can prove effective blocking of the active surface of the photocatalyst by anchoring organic molecules on the surface of the BiVO4. This may mean that the D149 connection process has been running with maximum efficiency.

Chapter 6

General Conclusions

The thesis work concerns the investigations of mesoporous materials based on BiVO4 sensitized by selected organic dyes to form hybrid functional materials. The performed investigations start from a synthesis of mesoporous samples as powders or thin films and the experimental investigations of their electronic, optical and structural properties.

As relevant support for experiments, computer simulations and quantum-chemical calculations were used to shed light on the photoinduced charge transfer phenomena.

Quantum-chemical calculations illustrate exhaustively the effect of individual components of the hybrid systems on their electronic and optical properties. They contribute to quantitative interpretation of the mechanism of photoinduced electric charge transfer involved in the investigated hybrid materials. Thus, it was shown that the combination of experimental and theoretical approaches draws precise roadmap of the physical phenomena that govern the photocatalytic properties of the BiVO4.

The quantum-chemical calculations performed for the bulk BiVO4 single crystal gives the following results:

1. The calculated electronic properties of the bulk BiVO4 depend on the computational procedure implemented in quantum-chemical codes. The significant influence on the electronic properties of the investigated material lies in Bi-O and V-O bond lengths and their tendency to be symmetrical following the optimization procedure. The possible reason for such isotropic bond lengths arrangement in the case of the Bi-O is due to the strong correlation between the Bi 6s and the O 2p orbitals.

2. The calculations showed that the features of the evaluated energy band are similar for both the optimized and frozen BiVO4 structure as well. Nevertheless, the energy band gap for the optimized structure is lower by 0.24 eV compared to that of the frozen structure. literatures. These results suggest that the considered nanosized clusters must have an amorphous fraction at their surface and a crystalline order in their core part. Such nano-objects reflect the nature of the BiVO4 nanostructures. The performed calculations for partially reconstructed cluster surfaces show that for nanoparticles smaller than 1.1 nm their physical peculiarities are generally dominated by surface effects. For larger particles (sizes above 1.8 nm), the surface influence is less pronounced compared to core atoms, being located at the same coordinates as those involved in bulk crystal structure.

5. The quantum-chemical calculations point out that the HOMO orbital is built by the O 2p states and the LUMO is created by V 3d orbitals. Compared to the bulk material one could conclude that the size reduction of the BiVO4 cluster does not drastically change its electronic properties.

6. The HOMO and LUMO orbitals spread at the surface of the cluster and confirm the key role of the surface in the photoactivated processes of electron transitions from HOMO to LUMO orbitals. Increasing the contact surface of nanoparticles with surrounding environment by synthesising meso-or nanoporous materials, will also improve the photocatalytic performances.

(BiVO4)18/COMPLEX, the HOMO orbital is located at the nanoparticle but the LUMO spreads on the COMPLEX molecule. This suggests that, a photoinduced electrons flow from the inorganic nanocrystal to the dye molecule limiting then the redox reactions which can manifest at the level of the inorganic network. Thus, the obtained numerical predictions point out the relevance of the (BiVO4)18/D149 hybrid system for photocatalytic activity. 9. The system based on the BiVO4 sensitized by D149 molecules exhibits compatible electronic band alignment suitable for the photoactivated processes.

In the case of materials sensitized by LIGAND and COMPLEX molecules, no significant changes in electronic or optical properties have been observed compared to the bare BiVO4 thin film. 12. The investigated hybrid systems were experimentally tested as photocatalytic active materials. Their photocatalytic activity was estimated from the degradation ratio of methylene blue (MB) in solutions. This ratio varies from 25 % for BiVO4/D149 to 30% for BiVO4/LIGAND similarly to BiVO4/COMPLEX. These measurements were realized in the configuration where the dyes cover the inorganic surfaces in contact with the (MB) solutions.

It would be relevant to proceed by using sensitized powders or alternatively free standing films sensitized by D149. This is a part of the perspectives of future developments based on the already carried out work.

To sum up, mesoporous BiVO4 materials are promising systems for photocatalysis due to the design of large contact surface required in heterogeneous catalysis with a high For the near future, the investigations can be devoted to the photoefficiency of the new functionalized hybrid materials. It is also worth noting that selected transition metal elements may also be considered to modulate the electronic and optical features of the BiVO4 materials. The goal will be to find the metal ions doping compatible with BiVO4 mesoporous architectures giving the best photocatalytic efficiencies. This approach will open potential applications of these systems in the environment preservation as for water depollution or for new sources of energy through photocatalytic production of hydrogen.

  ri, rj -Position of the i and j electrons ai -Primitive vectors of the lattice F -Reduced density gradient -Space vector -Spinorbital n  -Wave function -Wave vector Rk, Rl -Vectors of the k and l nuclei positions can harvest large fraction of the solar visible light radiations. Nowadays, exhaustive

  systems. The works associate complementary expertise contributions of two research teams responsible for quantum-chemical calculations carried out at the Jan Dlugosz University in Czestochowa (Poland) and the experimental developments of relevant materials features carried out at Le Mans University (France). The results of the present work were disseminated by publishing in high impact journals and by communications during international conferences. Streszczenie Procesy fotoaktywowane stanowią ważną grupę zjawisk wykorzystywanych w wielu zastosowaniach technologicznych w eko-budownictwie, elektronice, inżynierii materiałowej i środowiska, medycynie i kosmetologii. Materiały fotoaktywne działające w zakresie światła widzialnego oraz w nadfiolecie stosowane są między innymi w terapii fotodynamicznej, jako nowoczesne źródła energii, do produkcji szkieł samoczyszczących jak również podczas przyjaznego dla środowiska oczyszczania wody ze szkodliwych związków organicznych. Wśród całej gamy procesów fotoaktywowanych, kluczową część stanowi fotokataliza. Dotychczas, jednym z najczęściej wykorzystywanych w fotokatalizie półprzewodników jest dwutlenek tytanu (TiO2), którego własności zostały opisane w pionierskiej pracy Fujishima i współpracowników (Nature 1979, 277, 637-638). Prowadzone były również badania nad poszukiwaniem materiałów alternatywnych na bazie tlenek cynku (ZnO) (Chem. Rev. 1995, 95, 69-96). Niestety, ze względu na duże wartości wzbronionej przerwy energetycznej wymienione materiały nie mogą absorbować promieniowania w zakresie światła widzialnego, co znacząco ogranicza ich stosowalności. Alternatywą dla ZnO i TiO2 może okazać się jednoskośna odmiana wanadanu bizmutu (BiVO4), który posiada wzbronioną przerwę energetyczną równą 2,4 eV dla postaci objętościowej, co pozwala wykorzystać światło widzialne jako źródło fotoaktywacji. Obecnie prowadzone są badania mające na celu poprawienie własności znanych materiałów fotokatalitycznych, jak również syntezę nowych materiałów funkcjonalnych, które zastąpiłyby klasyczne fotokatalizatory. Wśród licznych sposobów poprawy własności fotokatalitycznych półprzewodników tlenkowych, na uwagę zasługuje zwiększenie ich powierzchni czynnej, zmniejszenie wzbronionej przerwy energetycznej oraz aktywowanie ich barwnikami organicznymi. Celem niniejszej pracy jest zbadanie wpływu wybranych sensybilizatorów organicznych na własności elektronowe, optyczne, wibracyjne oraz fotokatalityczne jednoskośnego BiVO4 o strukturze mezoporowatej w odniesieniu do materiału objętościowego. Jako pierwszy element poprawiający własności fotokatalityczne krystalicznego BiVO4 zastosowano zwiększenie jego powierzchni czynnej. W tym celu zsyntezowano wymieniony materiał w postaci mezoporowatej stosując pionierską metodę wykorzystującą surfaktant P123. Następnie przeprowadzono analizę własności strukturalnych, elektronowych i optycznych otrzymanych materiałów. Stwierdzono, że badany BiVO4 jest dobrą bazą do stworzenia wydajnego fotokatalizatora. Ponadto ustalono, że synteza materiałów hybrydowych poprzez zakotwiczanie cząsteczek barwnika organicznego na powierzchni BiVO4 pozwoli na bardziej wydajny transfer ładunku elektrycznego pomiędzy molekułą organiczną a nieorganicznym tlenkiem, co również wpłynie pozytywnie na jego własności fotokatalityczne. W związku z tym jako barwniki organiczne wybrano komercyjny D149 oraz dwa barwniki na bazie iminopirydyny. W celu określenia mechanizmów zjawisk fizycznych, zachodzących w materiałach hybrydowych, odpowiadających za efekty fotokatalityczne przeprowadzono szereg symulacji komputerowych i obliczeń kwantowo-chemicznych. Obliczono parametry elektronowe i optyczne jak również strukturalne i wibracyjne nanostruktur BiVO4 w podejściu klasterowym. Wymienione symulacje prowadzono także dla barwników organicznych i badanych materiałów hybrydowych wykorzystując metodę półempiryczną z parametryzację PM6. Jako weryfikator poprawności stosowanych modeli teoretycznych korzystano z porównań wyników badań teoretycznych z danymi eksperymentalnymi. Na drodze symulacji komputerowych oraz prac doświadczalnych stwierdzono, że zwiększenie powierzchni czynnej BiVO4 oraz sensybilizowanie go barwnikami organicznymi polepszyło transfer ładunków elektrycznych w materiale hybrydowym, co pozytywnie wpłynęło na poprawienie własności fotokatalitycznych badanego materiału. Wyjaśniono również, dlaczego niektóre barwniki wpływają pozytywnie na efekty fotokatalityczne zachodzące w materiałach hybrydowych a inne wręcz je gaszą i mogą służyć jako inhibitory powierzchniowych reakcji chemicznych. Obliczenia kwantowochemiczne obrazujące wpływ poszczególnych komponentów układu hybrydowego na jego własności elektronowe dały możliwość wyjaśnienia mechanizmu fotoindukowanego transferu ładunku elektrycznego zachodzącego w materiale barwnik/nanostruktura. Pokazano, że dzięki połączeniu eksperymentalnych i teoretycznych metod badawczych można otrzymać pełny obraz zjawisk fizycznych rządzących właściwościami fotokatalitycznymi BiVO4. Prace badawcze wykonane w ramach niniejszej rozprawy pozwoliły rozwinąć metodologię badawczą oraz analizę zjawisk fizycznych zachodzących w materiałach fotokatalitycznych na bazie BiVO4 pracujących w zakresie światła widzialnego. Wykonane prace numeryczne były wsparciem dla badań eksperymentalnych pozwalającym wyjaśnić fotoindukowane zjawiska fizyczne zachodzące w półprzewodniku, wpływające na zwiększenie efektywności transferu ładunku w systemach hybrydowych. Niniejsza prace jest wynikiem współpracy dwóch zespołów badawczych, z których jeden był odpowiedzialny za obliczenia kwantowo-chemiczne prowadzone na Akademii im. Jana Długosza w Częstochowie (Polska) oraz drugi z Uniwersytetu Le Mans (Francja), w którym prowadzono prace eksperymentalne oraz syntezę badanych materiałów. Wyniki naukowe referowanych badań opublikowano w znaczących czasopismach naukowych oraz przedstawiono na międzynarodowych konferencjach.

1. 2 . 1 . 1 )Schema 1 . 2 . 1 . 1

 2111211 Oxide materials for photocatalysisSo far, relevant and representative semiconducting oxides for photoactive applications are zinc oxides (ZnO) and titanium dioxides (TiO2). Their valence band (VB) is mainly composed from 2p orbitals of oxygen atoms, whereas metal d orbitals contribute to the conduction band (CB) (Schema 1.2.1.Band structure of the zinc oxide and titanium dioxide[18,19]

Figure 1 . 2 . 1 . 1 .Figure 1 . 2 . 1 . 2 .

 12111212 Figure 1.2.1.1. Photocatalytic efficiency of ZnO/Cu2O based photocatalysts in the photodegradation ofmethyl orange[30]

Figure 1 . 2 . 3 . 1 .

 1231 Figure 1.2.3.1. Local coordination of V and Bi ions in (a) tetragonal scheelite (t-s), (b) monoclinic scheelite (m-s), (c) and zircon-type (z-t) of BiVO4 structure. The bond lengths are demonstrated in Å(red: V, purple: Bi, and gray: O atoms)[START_REF] Park | Progress in bismuth vanadate photoanodes for use in solar water oxidation[END_REF]

Figure 1 . 2 . 3 . 2 .

 1232 Figure 1.2.3.2. Representation of the of BiVO4 crystal structure (blue: Bi, red: O, green: V). The base centered monoclinic primitive cell is indicated by the dashed black lines [79]

Figure 1 . 2 . 3 . 3 .

 1233 Figure 1.2.3.3. Partial density of states (PDOS) for the monoclinic BiVO4 crystal form[START_REF] Liu | The nature of photogenerated charge separation among different crystal facets of BiVO4 studied by a density functional theory[END_REF]

Figure 1 . 2 . 3 . 5 .

 1235 Figure 1.2.3.5. UV-Vis absorption spectra of the BiVO4 thin films annealed at 400, 500 and 600°C with a ramping rate of 3°C/min (left) and the calculated band gap extrapolation according to UV-Vis absorption spectra and Kubelka-Munk function (right) [85]

Fig. 1 . 2

 12

Fig. 1 .

 1 Fig. 1.2.4.1.a. and Fig. 1.2.4.1.c., the used synthesis sol-gel pathway lead to the same shape of the BiVO4 particles.

Figure 1 . 2 . 4 . 1 .Fig. 1 .

 12411 Figure 1.2.4.1. SEM images of BiVO4 prepared by the sol-gel method calcinated at a) 400°C [93] b) 450°C [95] c) 400°C [94]

Figure 1 . 2 . 4 . 2 .

 1242 Figure 1.2.4.2. SEM images of the BiVO4 prepared by the hydrothermal method.Autoclave was heated at a) 120°C, 6h[START_REF] Sun | Efficient Methylene Blue Removal over hydrothermally Synthesized Starlike BiVO4[END_REF] b) 120°C, 12h [97] c) 160°C, 24h[START_REF] Hojamberdiev | Morphology-controlled growth of BiVO4 crystals by hydrothermal method assisted with ethylene glycol and ethylenediamine and their photocatalytic activity[END_REF]

Figure 1 . 2 . 4 . 3 .The nanostructures exhibited in Fig. 1 . 2 . 4 . 3 .

 12431243 Figure 1.2.4.3. SEM images of porous BiVO4 synthesized with surfactant a) DMMP [108] b) P123 [102]porous silica template c) silica fiber immobilized-CNFs [109] polymer d) colloidal PS spheres[START_REF] Zhou | Ordered Macroporous BiVO4 Architectures with Controllable Dual, Porosity for Efficient Solar Water Splitting[END_REF] e) PVA[START_REF] Sarkar | Visible light photocatalysis and electron emission from porous hollow spherical BiVO4 nanostructures synthesized by a novel route[END_REF] f) PS balls[START_REF] Dong | Structuring porous ''sponge-like'' BiVO4 film for efficient photocatalysis under visible light illumination[END_REF]

(

  photodegradation of the (n-4-nonylphenol) compound. Despite significant differences in the size of the active area, the photodegradation of nanostructures in the solution after 60 minutes was practically identical for both samples[140]. At the next stage of the study, investigations were devoted to the photoactivity of monoclinic scheelite bismuth vanadate (m-BiVO4) and tetragonal (t-BiVO4) structures. The authors [140] carried out a study on photocatalytic properties through the evolution of the O2 from the AgNO3 solution using the monoclinic and tetragonal form of the BiVO4. The study was conducted for two different electromagnetic radiation wavelength, in which the samples were exposed.

Figure 1 . 2 . 5 . 1 .

 1251 Figure 1.2.5.1. Photocatalytic evolution of the O2 from aqueous AgNO3 solutions (0.05 mol/L, 320 mL)under visible light irradiation (λ > 420 nm) (left) and under ultraviolet light irradiation (300 < λ < 380 nm) (right) on (a) tetragonal BiVO4 and (b) monoclinic BiVO4 structure[START_REF] Kudo | Selective Preparation of Monoclinic and Tetragonal BiVO4 with Scheelite Structure and Their Photocatalytic Properties[END_REF]

Table 1 . 2 . 5 . 1 .

 1251 Photocatalytic efficiency [%]



  permanently attached to the surface of the oxide layer by chemical bonding between the surface of the semiconducting oxide and dye through the carboxyl group[138,[START_REF] Tian | Phenothiazine derivatives for efficient organic dye-sensitized solar cells[END_REF]. The chemical compound exhibits color if it absorbs selectively visible electromagnetic radiation. The absorption of radiation from the visible part of the spectrum is generally due to the presence in the dye the chromophore group possessing conjugated π electrons. The presence of chromophore is essential, but the color of the compound becomes more intense only after the introduction into the molecule of electron donor group (auxochrome). Thus, suitable dyes for photoactive processes must be formed by couples of chromophore and auxochrome. In view of the chemical structure, dyes are divided into a number of classes, usually from the chromophore architecture. In this frame, azo, triarylmethane, anthraquinone, indigo, xanthene, phthalocyanine define classes of dyes.The D--A structures used in such molecules should have the following requirement: Wide range of absorption of solar radiation; Presence of moieties such as -COOH, -H2PO3, -SO3H allowing an effective bonding to the inorganic photoactive film surface;

  photocatalyst-ligand charge transfer (Fig. 1.3.1.) [144]. Such requirements imply that the dye oxidation injects electrons into the conduction band of photocatalyst.

Figure 1 . 3 . 1 .

 131 Figure 1.3.1. Schematic diagram for the charge transfer under radiation for the hybrid organic/inorganic system

H

  equation is the basic differential equation for quantum chemistry, dependent on time and on all of the spatial coordinates necessary to describe the system at hand. The equation was formulated at the end of 1925 by Erwin Schrödinger. It describes changes in the quantum state of electronic systems over time. The system status is described by the wave function  also called the quantum state of the system under discussion. In the standard interpretation of the quantum mechanics, the wave function gives a complete description of a physical system. Description of the change in wave function over time gives a possibility to call the Schrödinger equation time depending. The solution of differential wave equation allows to find the wave function that carries information about the distribution of electrons and nuclei in a space defining a molecule. The time form of the Schrodinger equation is defined as: is Hamilton operator corresponding to the total energy of the system; n  is a wave function describing the quantum state n. When the Hamilton operator is independent of time the stationary Schrödinger equation looks as follows:

  3.) where: Rk, Rl -are vectors of the k and l nuclei positions; ri, rjdefine a position of the i and j electrons; M -mass of nuclei; memass of electron and . The part of the above presented equation corresponding to the and the represents the kinetic energy of electrons and nucleus, respectively. energy of an electrostatic interactions: electron-electron, electron-nucleus and nucleus-nucleus, respectively.Electrons are much lighter than nucleons located in the atomic nucleus. The rest mass of the electron is equal to kg rest mass of the proton is equal to kg

  The studied system of N electrons can be described by the wave function HF  composed by the N spinorbitals in the form of the Slater determinant:

  .10.) The can be interpreted as an energy of electron described by spinorbital in the field of other electrons. The F  is called Fock operator and it has the form: where is the operator of the kinetic energy of the electron i, of an electrostatic interaction of the electron i with the nuclei. The i h  is an one electron Hamiltonian. The j J  is the Coulomb operator of an electrostatic interaction occurring between the electron i and the electron j. Its action on the wave function i  may be defined by integral:

  electrons with the outer potential of atomic nuclei. Characters of the functional cannot be fully characterized.

  .20.) where the is the correlation-exchange energy. The mentioned energy is a consequence of the disruption of the electron density distribution. This phenomena may arise from the interactions of external or intermolecular fields. The equivalent of the Hartree-Fock equation in the DFT theory is the Kohn-Sham equation (KS): . (2.21.)

  In a crystal, regular arrangement, the potential in the Kohn-Sham equation is a periodic function of lattice vectors. In this way a wave function should satisfied the Bloch theorem:.(2.30.)where n is energy band index, is wave vector, is reciprocal space vector. The summation takes place after vectors for which . The desribes the maximum of the kinetic energy of the plane-wave. The Kohna Sham equation in the Bloch function notation can be presented as follows:(2.31.)where is the effective potential.In the representation of plane-waves, Kohn-Sham equations take on a very simple form.Substituting the trench (2.30) to equ. (2.29), and then multiplying by and by integrating after r we obtain the equation for our own values (2.32.)

  periodic boundary conditions or Born-von Karman boundary conditions impose restrictions that the wave function must be periodic in the certain Bravais lattice. It is used to describe the physical properties of the systems based on the model of ideal crystal in solid state physics. The condition can be stated as: . (2.33.) where i runs over the dimensions of the Bravais lattice, the ai are the primitive vectors of the lattice, and the Ni are integers assuming the lattice has N cells where N=N1N2N3 [7].The Born-von Karman boundary conditions provide a mathematical device to get around the physical effects of boundaries. In one dimension, the device forms the lattice into a circle of cells. To insure that there is no discontinuity of the wave function, it is

Schema 3 . 1 . 1 .

 311 based technique such as diffraction is well-known and widely used research tool in the field of solid state physics, crystallography and material sciences. For many years, it was a technique used mainly for scientific research related to determine the structure of crystallographic networks of various substances, the determination of their unite cells and lattice parameters. The X-ray based techniques has shown also an evolution to cover innovative areas such as tomography or non-destructive testing. Such applications include, for example, the determination of a phase composition of materials, the measurement of stresses and the determination of a material texture. An important factor in increasing the use of X-ray based techniques in material laboratories was the progress in the construction of X-ray diffractometers (construction of portable devices) and the creation of specialized databases and computer programs to facilitate the relatively complex analysis of diffraction data.Based on the diffraction method one may address the wide number of the compounds encountered in nature exhibiting a crystalline structure that is composed of atoms arranged in a regular and strictly structured manner determined by the crystallographic phase. The size of individual crystals or the crystalline domains within a given network can be very different. Thus, single monocrystals or polycrystalline materials can be in the size of centimetres as well as is micrometres. These features can be characteristic of materials as metals or ceramics which show polycrystalline compositions consisting of a large number of crystallites randomly arranged in the volume of material.The techniques used in X-ray diffraction are dependent on whether the object under examination is a single monocrystalline or a polycrystalline sample, but the main idea of the diffraction phenomena remains unchanged regardless of the organization of matter. The Schema 3.1.1. shows a diagram of the atoms arrangement in a simple crystallographic network with a highlight on the reticular crystallographic planes.Atoms of the network can be joined by different parallel groups of crystallographic planes. Some groups of planes as the (010) are densely arranged with atoms and others, such as (310), contain less atoms. It can be addressed that a family of plane plays the role of a grid which diffracts the X-ray beam according to defined conditions depending on the X-ray wavelength, an incidence angles and the lattice orientation (Schema 3.1.2). An exemplary arrangement of atoms in a crystalline lattice with the designed different groups (families) of mutually parallel crystallographic planes Schema 3.1.2. Diagram of X-ray reflection from two parallel atomic planes Schema 3.1.3. a) X-ray Diffraction from a single crystal, b) Diffraction reflections from a polycrystalline (powder) sample consisting of a large number of fine, randomly oriented crystallites Among the large number of randomly oriented crystallites, there will always be those whose the orientation permits to fulfil the condition for certain groups of inter-reticular planes. Thus diffraction lines from different crystallites must always lie on one cone defined by X-rays diffracted at 2θ relative to the incident X-ray beam (Schema 3.1.3.b). Diffraction lines from different families of inter-reticular planes differing by their distances d, will form separate cones of different 2θ angles (Schema 3.1.3.b). In usual X-ray diffraction studies, angular positions of the different inter-reticular planes compose the diffraction pattern [2,3]. For the investigations carried out in present work the Empyrean diffractometer from PANalytical with Cu Kα radiation (λ = 0.15418 nm) under an accelerating voltage of 45 kV and an emission current of 30 mA was used. The scale of 2θ angle was fixed between 15 -60°.

Schema 3 . 2 . 1 .

 321 Electrons incident on the sample interact with electrons of the sample and cause several emissions as secondary electrons, back scattered electrons, Auger electrons, x-rays, cathodeluminescence (Schema 3.2.1.).Each type of radiation is emitted from different depth and volume of the sample. With increasing acceleration voltage the depth and volume of the emission area is increasing (Schema 3.2.2.). The diameter of this area is also growing with applied voltage and can reach at least several times the diameter of the beam. Additionally, the depth of electron penetration affects the atomic number of the sample components. In the SEM with tungsten thermo-emitter cathode, the electron beam diameter is approximately 2 -5 nm and the limit of 4 -10 nm (at secondary electrons). Much better results are achieved with a field emission beam because in this case the scanning electron microscope resolution can reach 1 nm. Microscopes of this type are very expensive technologies, due to the high vacuum requirements to operate with field emission (p <10 -7 Pa). Effect of an electron beam on a sample surface Schema 3.2.2. Range and spatial distribution of emission of different types of electrons in a sample[4,5] The essence of SEM scanning microscopy is to scan the sample surface with an electron beam in nanometre scale. Samples are scanned by an electron beam deflected by scanning coils. The deviation of the beam forming the image on the monitor is synchronized with the deviation of the sample scanning beam. Signal from the surface of the sample, usually in the form of secondary or retrograde electrons, reaches the detector. The output from the detector controls the brightness of the image displayed on the monitor. Scanning microscope magnification is due to the ratio of the size of the scanned area of the sample. Some of the primary electrons that are scattered backwards (changing the direction of motion by more than 90 degrees due to the elastic scattering through the sample atoms) creates also backscattered electrons. The amount of emitted electrons retrograde is dependent on the atomic number Z which is the source of information on the chemical diversity of the sample. The remainder of the primary electrons, absorbed by the sample, is dispersed unevenly by the surface layers of the atoms and gradually loses energy. As a result of this interaction, the secondary electrons signal is most important in SEM. Each electron emitted by the sample possessing energy less than 50 eV is called a secondary electron. Due to low energy, only secondary electrons emitted near the surface have the chance to leave the sample and reach the detector. The amount of emitted secondary electrons gives information about the topography of the sample surface. Many of the secondary electrons leave the protuberance of the sample, while few recesses produce a topography.

1 .

 1 Among the relevant interactions that are a source of the force acting on the AFM cantilevers, the dominant are the van der Waals attractive traction and electrostatic repulsion. The interaction between the atoms of the tip and the sample is expressed by the so-called Lennard-Jones potential: , (3.3.1.) with the two parts related to the repulsion (~ σ 12 ) and the attraction (~ σ 6 ). The ε describes the depth of the potential well and σ represents the finite distance at which the intermolecular potential is zero while r is the distance between the atoms of the tip and the sample. The dependence of the Lennard-Jones potential versus the distance from the sample surface is illustrated in Schema 3.3.The AFM probes the cantilever deflection of the lever (tipped beam) with the impact forces between the tip and the atoms forming the test surface. The bending beam is converted by a detector (photodiode) into a current signal, which then serves to generate a sample image. Schema 3.3.1. (a) Force curve versus interatomic distance in AFM (b) contact mode (c) non-contact mode. scanning tunnelling microscope (STM) the AFM method can be used both in conductive and non-conductive surfaces. In addition, tests can be conducted in vacuum, in air, in gases and in liquids. The theoretical resolution in z-axis is up to 0.01 nm and in x-y axis up to 0.1 nm. In fact, the resolution is generally smaller and depends on the shape of the blade, the type of material, its roughness, and the measurement conditions. Depending on the AFM mode, the attractive or repulsive forces between the measuring blade and the sample are investigated (Schema 3.3.1.a). The attractive forces are long-range ones which can be measured in non-contact mode (Schema 3.3.1.b) in contrast to repulsive forces being measured in contact mode (Schema 3.3.1.c).

(

  cantilever) with known work function is used. The formula that will allow to obtain the work function of the sample is defined as: , (3.4.1.) where VCPD is the contact potential difference, ϕpoint is the work function of the characteristic material of the tip, ϕsample is the characteristic work function of the sample and the term (-e) is the charge of the electron. Only the work function of the tip characteristic material is known, and it remains VCPD and sample variables to be defined according to the procedure presented below. Initially the tip and the sample are without contact and differ by their different work functions and Fermi energies. The procedure consists of placing them very close one to the other to provoke the quantum tunnelling phenomenon consequently leading to Fermi energies. In this process, both the tip (probe) and the surface become charged because of depletion region at the interface [11]. Since the Fermi energy of the tip is smaller than that of the sample, the latter will decrease in the Fermi energy of the surface. The crucial point is to know that the working function of the electrons belonging to the sample surface does not change, since it is an inherent characteristic of the atom and thus the interval ϕ shows the difference between the energy of the electron at Fermi level and the energy of the electron in the vacuum. With equal Fermi levels, the difference in the potential between the tip and the sample will be due to the gap between the electrons in the vacuum. This is what was determined by the VCPD. The next step is to measure the VCPD, where the continuous voltage source VDC is applied between the nozzle and the sample. This leads to the return of the negative charges to the sample increasing its Fermi level at its initial value. The applied voltage measured and computed by the apparatus will be exactly equal to the voltage of VCPD. Thus, considering the formula (3.4.1) one can obtain the work function of the atom analysed in the sample as follows: , (3.4.2.) By rearranging the formula (3.4.2), one can get the work function of a specific point in the sample: . (3.4.3.) However, the previously mentioned methodology using a DC current is able to determine the work function of the sample only when that of the nozzle is known previously.



  photon absorption.The transition moment corresponding to the electron excitation determines the probability of absorption of matched photon according to the condition: .(3.5.5.)

  .5.7.) Transmittance is most often expressed in percentages as: . (3.5.8.) The transmittance can range from 0% to 100%. The intensity of absorbed radiation depends on the concentration of the solution c and on the thickness of the absorbing layer l (see equation 3.5.9) and is described by the Lambert-Beer law, which takes the logarithmic form: , (3.5.9.) where k is a proportionality constant (radiation absorption coefficient, often called the absorption coefficient). The decimal logarithm of the intensity ratio of the radiation beam incident on the sample I0 to the intensity of the radiation beam passing through the sample It is called absorbance. It takes values from 0 to infinity. When the concentration of the solution c is expressed in mol/dm 3 and the thickness of the layer l is expressed in cm, the proportionality coefficient k is called the molar absorption coefficient ε. In consequence the Formula 3.5.9. takes the form: . (3.5.10.) This is the basic law of absorption spectrophotometry. The relationship between absorbance and transmittance is expressed by the relation: , (3.5.11.)

( 3 .

 3 6.2.) where I (intensity of diffused radiation) is proportional to the fourth power of a radiation frequency ν. In the Raman spectra, the intensity of the radiation I exiting the sample is a linear function of the molar concentration and thickness of the layer, whereas in the IR spectrum this relation is exponential. Types of bands observed in the Raman spectra are presented in Schema 3.6.1. and they are as follows:

Schema 3 . 6 . 1 .

 361 Diagram of energy presenting different types of scatteringIn order to obtain appropriate Raman spectra, the test substance should be optically homogeneous, opaque, colourless, non-fluorinated. Quantities of substances needed for testing are of the order of one milligram[17][18][19]. In present work the vibrational properties are investigated by using a confocal micro-Raman spectrometer. The excitation was realized with a 514.5 nm laser radiation with 150 mW power. With regard to the large spectral range on which the spectra are recorded an appropriate calibration of the CCD detection system was realized.

  .7.8.) where, kr is the apparent rate of pseudo first order velocity. By integrating Equation 3.7.8. in terms of the change of concentrations from c0 to c and t0=0 one can obtain the following relation [23

  Houas and co-workers have been confirmed that methylene blue on the surface of TiO2 decomposed to the , , or [22]. The OH radicals involved in the reaction are likely to attack the groups present in one of the resonant structures of the methylene blue molecule that interact with the cobalt forces with the TiO2 surface. The next step is cleavage of groups that initiates the opening of the central aromatic ring with the heteroatoms S and N. The next two attacks of the hydroxyl radical lead to the cleavage of the two benzene rings and the formation of the sulfonic acid . The subsequent attack of the radical results in and the radical , which may react with another hydroxyl radical to give the phenolic product. Similarly, bonds occur in the methylene blue structure, and and ,27].

Figure 4 . 1 . 2 .Fig. 4 . 1 . 2 .

 412412 Figure 4.1.2. Diagram of production of thin layers of BiVO4 deposited by dip coating on borofloat substrate

  5-[[4-[4-(2,2-Diphenylethenyl)phenyl]-1,2,3-3a,4,8b-hexahydrocyclopent[b]indol-7-yl]methylene]-2-(3-ethyl-4-oxo-2-thioxo-5-thiazolidinylidene)-4-oxo-3thiazolidineacetic acid (D149) was purchased from Sigma-Aldrich and used as received to sensitize the BiVO4 mesoporous structures. The other two molecules as the N, Ndimethyl-4-((E)-(pyridin-2ylmethylene) amino)phenyl) diazenyl) aniline (LIGAND) and the N,N-dimethyl-4-((E)-(pyridin-2ylmethylene) amino)phenyl) diazenyl) aniline Zinc Chloride (COMPLEX) [1] synthesized in the Laboratoire MOLTECH-Anjou, University in Angers (France) where used as a dyes anchored at a surface of the BiVO4.

5 Results and discussion 5 . 1 .

 551 , A. Ayadi, K. Ordon, K. Iliopoulos, D.G. Branzea, A. Migalska-Zalas, M. Makowska-Janusik, A. ElGhayoury, B. Sahraoui, Zinc Induced a Dramatic Enhancement of the Non Linear Optical Properties of an Azo-based Iminopyridine Ligand, J. Phys. Chem. C 2014, 118, 14, 7545-7553 Chapter Quantum-chemical calculations and experimental investigations of the BiVO4 in bulk and nanosized form 5.1.1. Electronic and structural properties Theoretical investigations Using the Materials Studio program package [1], the monoclinic scheelite BiVO4 single crystal structure was built. The mentioned structure was constructed according to the data given by Zhao [2]. The base centered monoclinic crystal architecture belongs to space group C2/C and point group . The unit cell parameters are as follow a = 7.247 Å, b = 11.697 Å, c = 5.090 Å and =134.226 o with fractional atomic coordinates of Bi (0.000, 0.134, 0.750), V (0.000, 0.370, 0.250), O1 (0.261, 0.051, 0.380) and O2 (0.354, 0.208, 0.861).

a = 7

 7 . 330 Å b = 11.649 Å c = 5.1932 Å = 135.446 o After the geometry optimization procedure, the shortest lattice parameters (a and c) of the BiVO4 single crystal increase and the longest one (b) decreases. Also the interatomic distances were modified. The geometry optimization procedure revealed that the Bi-O bond lengths have a tendency to be symmetrical. The same situation is observed for V-O bonds. The possible reason for such isotropic bond lengths arrangement in the case of Bi-O connection is due to the strong correlation between the Bi 6s and the O 2p orbitals [9].

Figure 5 . 1 . 1 . 1 .InFig. 5 . 1 . 1 . 2 .

 51115112 Figure 5.1.1.1. Electron energy structure calculated by DFT/PBE methodology of monoclinic frozen BiVO4 structure

Figure 5 . 1 . 1 . 2 .

 5112 Figure 5.1.1.2. The PDOS calculated by DFT/PBE method for relaxed (left panel) and frozen BiVO4 structure

Fig. 5 . 1 . 1 . 3

 5113 reports an example of the constructed (BiVO4)n nanocrystal. The clusters were built with the monoclinic BiVO4 crystal structure reducing the external atoms to obtain spherical cluster with defined size and total number of the (BiVO4)n units. The number n of the structural units was changed from n = 1up to n = 56, leading to a cluster diameter up to 2.0 nm.

Figure 5 . 1 . 1 . 3

 5113 Figure 5.1.1.3 Structure of the (BiVO4)26 nanocrystal with size 1.3 nm projected along [010] crystallographic direction

Figure 5 . 1 . 1 . 4 .

 5114 Figure 5.1.1.4. The EHOMO-LUMO energy gap splitting vs size calculated by PM6 semi-empirical method for frozen and optimised BiVO4 cluster represented by the number of (BiVO4)n units

5 . 1 . 1 . 4 .

 5114 optimized structures versus their size is presented in Fig. One may see that in both cases i.e. for crystalline and amorphous nanoparticles with increasing the size of nanoclusters, their energy gap decreases in agreement with the size induced quantum confinement effect.The changes of the EHOMO-LUMO calculated for the investigated nanoparticles are consistent with the data presented by Chattopadhyayet al.[30,31]. The authors showed experimentally that the BiVO4 nanocrystals possess an indirect band gap in the range of 3.51-3.67 eV or 2.75-2.82 eV, depending on the sample preparation and the particle size. The reported results for crystalline clusters are lower than those for amorphous structures. These results suggest that the measured clusters must have an amorphous fraction at their surface and a better crystalline order in their core part (interior).

Figure 5 . 1 . 1 . 5 .

 5115 Figure 5.1.1.5. The energy gap splitting (∆EHOMO-LUMO) versus size of the system calculated by PM6 method for (BiVO4)n for surface and bulk representations

Figure 5 . 1 . 1 . 6 .

 5116 Figure 5.1.1.6. The partial density of states calculated by PM6 semi-empirical method for the partially reconstructed cluster with size 1.2 nm

Figure 5 . 1 . 1 . 8 .

 5118 Figure 5.1.1.8. XRD pattern of mesoporous BiVO4 thin film prepared by using P123 surfactant (A) and the standard JCPDS-14-0688 pattern assigning the BiVO4 crystalline structure to monoclinic scheelite phase

Figure 5 . 1 . 1 . 9 .

 5119 Figure 5.1.1.9. SEM images of the thin films of monoclinic BiVO4 deposited on the silicon (a and b) and on borofloat substrates (c and d) using the dip coating method

Figure 5 . 1 . 2 . 1 .

 5121 Figure 5.1.2.1. The UV-vis absorption spectra measured for the borofloat (BF) and the thin film of the monoclinic BiVO4 deposited on the borofloat substrate

Figure 5 . 1 . 2 . 2 .

 5122 Figure 5.1.2.2. (Top panel) Theoretical UV-vis absorption spectra calculated for bulk (solid line) and nanosized (dashed line) BiVO4 structures by using DFT/PBE and semi-empirical PM6 parameterized method, respectively. (Bottom panel) Tauc plot of [F(R)hν] 2 versus hν for the of BiVO4 monoclinic thin film deposited on the borofloat substrate

Figure 5 . 1 . 3 . 1 .

 5131 Figure 5.1.3.1. IR spectra calculated by PM6 methodology for frozen (top panel) and optimised (bottom panel) (BiVO4)n clusters of different sizes.

Fig. 5 . 1 . 3 . 3 .

 5133 The comparison is made between the Raman spectra calculated by PM6 methodology for frozen and optimized (BiVO4)18 clusters and the experimental spectrum performed with the laser radiation at 568 nm on the mesoporous BiVO4 thin film deposited on the borofloat substrate. For bismuth vanadate films deposited on the borofloat substrate, the highest peak intensity located at the 822 cm -1 corresponds to the symmetrical stretching of the V-O mode (Fig. 5.1.3.3. band A). Frozen (BiVO4)18 cluster containing the symmetrical stretching of the V-O is consistent with the experimental spectrum. The same statement is relevant for the vibrations corresponding to the asymmetric stretching V-O are at 631 cm -1 and 716 cm -1 . Asymmetric δas(VO4 3-) and symmetric δs(VO4 3-) deformation of the group occur at 323 cm -1 and 361 cm -1 , respectively [39]. In the case of mesoporous thin film structure both of these Raman bands are resolved with high intensity but the second peak is more pronounced (Fig. 5.1.3.3., band B'). From computational results on the frozen (BiVO4)18 cluster, similar behaviour is observed (Fig. 5.1.3.3., band B). The small discrepancy on the band positions may be explained by the fact that anharmonic vibration terms were not considered in the calculation model. The bands labelled as C are seen for the geometry optimized (BiVO4)40 cluster and occurs probably from the same vibrational features as those leading to the bands at 366 and 324 cm -1 . The relaxed structure do not exhibits peak corresponding to the V-O distances. The V-O bonds length relaxed according to the total energy minimization procedure modify the Raman band positions and the amorphization of the cluster create wavenumber distribution

Figure 5 . 1 . 3 . 3 . 5 . 1 . 4 . 5 . 1 . 4 . 2 .Figure 5 . 1 . 4 . 1 .

 513351451425141 Figure 5.1.3.3. Raman spectra calculated by PM6 methodology for frozen (black solid line) and optimised (black dashed line) (BiVO4)n clusters and the experimental one (red line)

Figure 5 . 1 . 4 . 2 . 1 .

 51421 Figure 5.1.4.2. Kinetics of the degradation of the MB dyes obtained from the time evolution of the intensity of the absorption band related to MB dyes content in a solution

Figure 5 . 2 . 1 . 1 .

 5211 Figure 5.2.1.1. Structure of the organic dye molecules: D149, LIGAND and COMPLEX

The Fig. 5 . 2 . 1 . 2 .

 5212 illustrates the distribution of the HOMO (left side) and LUMO (right) orbitals of the D149, LIGAND and COMPLEX calculated using the semi-empirical PM6 parameterized method. The HOMO orbital of the D149 molecule is delocalized over the π-electron system with the highest electron density located on the central nitrogen atom. It is noticed that the LUMO orbital have larger influence of the terminal carboxyl group compared to the HOMO orbital. Therefore, the HOMO-LUMO excitation induced by light irradiation could shift the electron distribution from the middle of the molecule to the anchoring moieties, thus favoring electron injection from dye to BiVO4. Whereas, for LIGAND and COMPLEX molecules was established that the HOMO orbitals are located on the dimetylamino group possessing donor character while the iminopyridine group play the role of acceptor and the LUMO orbitals are predominantly located on its atoms.Analyzing of the contribution of atomic orbitals to the formation of the LUMO orbitals the -COOH or -N were chosen in the D149 and LIGAND/COMPLEX molecules, respectively, as the groups involved in the sensitizing process of the bismuth vanadate nanocrystals. It is due to the fact that these groups are located in position where is the greatest electron concentration forming the LUMO orbitals. Hybrid systems were built using the ACD/ChemSketch program by combining an organic molecules via selected linking groups with the (BiVO4)18 nanostructure. An anchoring of the D149 is realized by removing hydrogen atom from the carboxyl group and combining the oxygen atom with the vanadium atom located on the surface of the nanocluster. In the case of systems with LIGAND and COMPLEX molecules, the connection occurs through the nitrogen atom located in the aromatic ring on which the highest density of LUMO orbitals was located.

Figure 5 . 2 . 1 . 2 .

 5212 Figure 5.2.1.2. The HOMO and LUMO orbitals of the D149, LIGAND and COMPLEX organic dyes molecules calculated by the semi-empirical PM6 parameterized method

Figure 5 . 2 . 1 . 3 .

 5213 Figure 5.2.1.3. Representation of the HOMO and LUMO orbitals calculated for the (BiVO4)18 cluster sensibilized by D149 (HD), LIGAND (HL) and COMPLEX (HC) molecules

5 . 2 . 1 . 5 . 5 . 2 . 1 . 6 . 5 . 2 . 1 . 6 .

 521552165216 The representative histograms illustrating the charge density distribution versus the surface electrical potential for the systems with anchored D149 molecules were performed. The KFM images presented in Fig. demonstrate the occurrence of drastic changes on the electrical activity of the BiVO4 based systems under light radiation. The maximum electrical potential undergoes a net change from 33/25 mV under dark to 35/55 mV with light exposure for the bare/sensibilized thin films of the BiVO4. The potential distribution changes critically between the bare BiVO4 film surface and that with organic dyes. The difference of surface potential between the bare and sensitized samples without radiation exposure was estimated to be equal to 8 mV. The justification was possible to be made taking into account the characteristic electronic phenomena such as the band bending, the Fermi levels pinning, the change on the work function expected at the organic/semiconducting heterojunction. The performed analysis is based on the donoracceptor interfaces model using the electronic band alignment theory to account for the charge transfer mechanism. Theoretical energy band diagram of the pure monoclinic BiVO4 and the one with anchored organic dye is presented in Fig. On the left side the mentioned figure shows the electronic band structure of the bare bismuth vanadate. The valence band and conduction band are placed at -7.29 eV and -4.79 eV, respectively, while the Fermi level is located at -5.29 eV (Fig. left side) [56]. As reported by Chang et al. the BiVO4 belongs to the n-type class of semiconductors, because its Fermi level is located closer to the conduction band (0.5 eV) than to the valence band (2.0 eV) [57]. The diagram on the left side illustrates also the work function Φ defined by the Fermi level and the vacuum level. The fact that the surface potential of sensibilized systems may decrease is extremely important for describing the mechanism of the electric charge transfer process. The schema presented in Fig. 5.2.1.6. on the right panel shows the BiVO4/organic dye heterojunction. The system presents a typical model of a hybrid system with the charge transfer between components.124The process proceeds by excitation of an electrons from the HOMO (-5.07 eV) to LUMO (-2.36 eV) level of the dye molecule and then transfer them to the semiconductor conduction band (-4.79 eV)[58].

Figure 5 . 2 . 1 . 5 .

 5215 Figure 5.2.1.5. Topography (A, A') and surface potential distribution of the BiVO4 thin films (B, C) and BiVO4 with anchored D149 molecules (B', C') without illumination (B and B') and under illumination (C and C'). The histograms BP, CP, B'P, C'P represent the population of the charge distribution of the samples B, C, B', C', respectively

Figure 5 . 2 . 1 . 6 .

 5216 Figure 5.2.1.6. Theoretical energy band diagram and band alignment at the interface: case of virgin thin film of the BiVO4 (left side) and sensitized donor-acceptor interface created by the D149 (right side)

Figure 5 . 2 . 2 . 1 .

 5221 Figure 5.2.2.1. UV-vis absorption spectra measured for the BiVO4 thin film (A), organic dyes D149 (D), LIGAND (L) and COMPLEX (C), and their hybrid systems HD, HL and HC solved in the acetonitrile/tertbuthanol (1/1)

5 . 2 . 3 . 1 )

 5231 show significant differences between the vibrational bands positions and their intensities expected for the individual bonds or groups of atoms. The highest deviations were observed for the BiVO4/LIGAND hybrid system. In the case of sensitized films by anchored dyes as D149 and COMPLEX, the main intense Raman band is assigned to symmetrical stretching of the V-O mode and is located approximately at the 850 cm -1 . From the theoretical Raman spectra calculations (Fig.5.2.3.1, bottom panel) one may see that the main band is located at the 817 cm -1 and corresponds to the symmetrical stretching of the V-O mode. While, the vibrations corresponding to the asymmetric stretching V-O are around 668 cm -1 and 728 cm -1 .

Figure 5 . 2 . 3 . 1 . 5 . 2 . 4 .Fig. 5 . 2 . 4 . 1 .

 52315245241 Figure 5.2.3.1. The experimental (top panel) and theoretical (bottom panel) Raman spectra obtained for the BiVO4 based hybrid systems with anchored D149 (HD), LIGAND (HL) and COMPLEX (HC) dye molecules

Figure 5 . 2 . 4 . 1 .

 5241 Figure 5.2.4.1. UV-vis absorption spectra measured for the HD, HC and HL hybrids in methylene blue solution

Figure 5 . 2 . 4 . 2 .

 5242 Figure 5.2.4.2. The MB degradation in time evaluated for the HD, HL and HC hybrid systems

140 3 .

 3 photocatalyst inducing then the redox formation of radicals.The performed work uses the appropriate methodology for the investigation of the electronic properties of the nanostructured BiVO4 as nanoparticles or mesoporous-like BiVO4. The relevant results are hereafter outlined: 4. The changes of the EHOMO-LUMO calculated for the BiVO4 nanoparticles following the size changes exhibit size quantum confinement effects. The obtained data are in agreement with the results presented in already reported

10 .

 10 AFM and KFM measurements on the hybrid systems confirmed the theoretically predicted results and the model of the photoinduced charge transfer from dye to nanostructure was validated. 11. Analysing the UV-vis absorption spectra for the (BiVO4)18/D149 hybrid system shows that the charge transfer band traduces the influence of the sensitized organic molecule on the absorption capacity of the hybrid systems. Anchored LIGAND and COMPLEX molecules do not create any additional absorption band.

  efficiency. The methodology of sensitizing the inorganic surfaces by charge transfer dyes was implemented and investigated theoretically by relevant numerical methods along with experimental realization of the considered hybrid systems. This work paves the way for future development to realize efficient photocatalysts with competitive 143 performances compared to the traditional wide band gaps semiconducting oxides as TiO2 or ZnO which harvest only UV raditation.

Figure 1 . 2 . 4 . 3 .Figure 1 . 2 . 5 . 1 .Figure 1 . 3 . 1 .

 12431251131 Figure 1.2.4.3. SEM images of porous BiVO4 synthesized with surfactant a) DMMP b) P123 porous silica template c) silica fiber immobilized-CNFs polymer d) colloidal PS spheres e) PVA f) PS balls………………………………………………………

Figure 4 . 1 . 1 .

 411 Figure 4.1.1.Annealing process applied during the thin film preparation of the BiVO4……………………………………………………..

Figure 4 . 1 . 2 .

 412 Figure 4.1.2.Diagram of production of thin layers of BiVO4 deposited by dip coating on borofloat substrate……………………………..

Figure 5 . 1 . 1 . 1 .Figure 5 . 1 . 1 . 2 .

 51115112 Figure 5.1.1.1. Electron energy structure calculated by DFT/PBE methodology of monoclinic frozen BiVO4 structure…………

Figure 5 . 1 . 1 . 3 .

 5113 Figure 5.1.1.3. Structure of (BiVO4)26 nanocrystal with size 1.3 nm projected along [010] crystallographic direction………………………...

Figure 5 . 1 . 1 . 4 .

 5114 Figure 5.1.1.4. The EHOMO-LUMO energy gap splitting vs size calculated by PM6 semi-empirical method for frozen and optimised BiVO4 cluster represented by the number of (BiVO4)n units………….

Figure 5 . 1 . 1 . 5 .

 5115 Figure 5.1.1.5. The energy gap splitting (∆EHOMO-LUMO) versus size of the system calculated by PM6 method for (BiVO4)n for surface and bulk representations……………………………………….

Figure 5 . 1 . 1 . 6 .

 5116 Figure 5.1.1.6. The partial density of states calculated by PM6 semi-empirical method for the partially reconstructed cluster with size 1.2 nm

Figure 5 . 1 . 1 . 7 .

 5117 Figure 5.1.1.7. XRD analysis of the BiVO4 powder synthesized by using P123 (A) and F127 (B) surfactant……………………………..

Figure 5 . 1 . 1 . 8 .

 5118 Figure 5.1.1.8. XRD pattern of mesoporous BiVO4 thin film prepared by using P123 as surfactant (A). Also shown the standard pattern JCPDS-14-0688 which assign the crystalline structure of BiVO4 to monoclinic scheelite phase………………………….

Figure 5 . 1 . 1 . 9 .

 5119 Figure 5.1.1.9. SEM images of the thin films of monoclinic BiVO4 deposited on the silicon (a and b) and on borofloat substrates (c and d) using the dip coating method………………………………….

Figure 5 . 1 . 1 . 10 .

 51110 Figure 5.1.1.10. AFM image of the thin film of BiVO4 deposited on a borofloat substrate. The analyzed sample size is about size 1m x 1m…………………………………………………….

Figure 5 . 1 . 2 . 1 .Figure5. 1 . 2 . 2 .

 5121122 Figure 5.1.2.1. The UV-vis absorption spectra measured for the borofloat (BF) and the thin film of the monoclinic BiVO4 deposited on the borofloat substrate…………………………………………

Figure 5 . 1 . 3 . 1 .

 5131 Figure 5.1.3.1. IR spectra calculated by PM6 methodology for frozen (top panel) and optimised (bottom panel) (BiVO4)n clusters of different sizes........................................................…………….

Figure 5 . 1 . 3 . 2 .

 5132 Figure 5.1.3.2. Raman spectra calculated by PM6 methodology for frozen (top panel) and optimized (bottom panel) (BiVO4)n clusters….

Figure 5 . 1 . 3 . 3 .

 5133 Figure 5.1.3.3. Raman spectra calculated by PM6 methodology for frozen (black solid line) and optimised (black dashed line) (BiVO4)n clusters and the experimental one (red line)…………………...

Figure 5 . 1 . 4 . 1 .Figure5. 1 . 4 . 2 .Figure 5 . 2 . 1 . 1 .

 51411425211 Figure 5.1.4.1. UV-vis spectra of degradation methylene blue dyes in solution by thin film of monoclinic BiVO4 under irradiation…………..

Figure 5 . 2 . 1 . 2 .Figure 5 . 2 . 1 . 3 .

 52125213 Figure 5.2.1.2. The HOMO and LUMO orbitals of the D149, LIGAND and COMPLEX organic dyes molecules calculated by the semiempirical PM6 parameterized method……………………

Figure 5 . 2 . 1 . 4 .Figure 5 . 2 . 1 . 5 .Figure 5 . 2 . 1 . 6 .

 521452155216 Figure 5.2.1.4. KFM measurements performed for the BiVO4 thin films sensitized by organic dye molecules D149 (HD), LIGAND (HL) and COMPLEX (HC)……………………………………

Figure 5 . 2 . 2 . 1 .

 5221 Figure 5.2.2.1. UV-vis absorption spectra measured for the BiVO4 thin film (A), organic dyes D149 (D), LIGAND (L) and COMPLEX (C), and their hybrid systems HD, HL and HC solved in the acetonitrile/tertbuthanol (1/1)………………………………….

Figure 5 . 2 . 3 . 1 .Figure 5 . 2 . 4 . 1 .Figure 5 . 2 . 4 . 2 .

 523152415242 Figure 5.2.3.1. The experimental (top panel) and theoretical (bottom panel) Raman spectra obtained for the BiVO4 based hybrid systems with anchored D149 (HD), LIGAND (HL) and COMPLEX (HC) dye molecules……………………………………………

1.2.2.1.). The estimation of available surface can be as follows: , , , . Figure 1.2.2.1. Smooth and porous surface

1.2.2.2.). Figure 1.2.2.2. The HOMO-LUMO energy gap splitting vs size calculated by the density functional theory methods as GGA and TDLDA for unpassivated (TiO2)n anatase structures [74]

1.2.3.4.). Figure 1.2.3.4. The

calculated energy band structure (left) and PDOS (right) of monoclinic BiVO4

[START_REF] Zhao | Electronic structure and optical properties of monoclinic clinobisvanite BiVO4[END_REF]

1.2.3.5.). The

	value of the band gap decreases from 2.53
	to 2.47 eV as annealing temperature increases from 400°C to 600°C, respectively [86].
	Similar dependency was examined by Zhang et al. for mixture of the t-s with m-s phase
	of the BiVO4 in the powder form. Attention was focused on annealing temperatures as
	140, 160, 180 and 200°C which gave rise to band gap equal to 2.52, 2.51, 2.41 and
	2.34 eV, respectively [87]. Experimental investigations proved that value of the energy
	gap depends on the processing temperature which control the involved crystalline
	phases.

Table 5 .

 5

	Parameters	Frozen structure	Optimised structure
		2.352 Å	2.495 Å
	Bi-O bond length [Å]	2.372 Å	2.455 Å
		2.518 Å	2.437 Å
	V-O bond length [Å]	1.762 Å 1.688 Å	1.740 Å 1.744 Å
		a = 7.247 Å
	Lattice parameters	b = 11.697 Å c = 5.090 Å
		= 134.226 o

1.1.1. Lengths of the Bi-O and V-O bonds as well as lattice parameters for the frozen and relaxed structure of BiVO4 single crystal

Table 5 .1.1.2.

 5 The HOMO and LUMO orbitals representation of (BiVO4)n frozen clusters with n = 12, 18, 24, 30, 36 calculated by the PM6 semi-empirical parametrised method

	HOMO Orbitals	LUMO Orbitals
	(BiVO4)12	diameter = 0.75 nm
	(BiVO4)18	diameter =1.2 nm
	(BiVO4)24 diameter =1.3 nm
	(BiVO4)30	diameter =1.4 nm
	(BiVO4)36	diameter =1.5 nm

) 44 (BiVO 4 ) 40 (BiVO 4 ) 36 (BiVO 4 ) 30 (BiVO 4 ) 28 (BiVO 4 ) 26 (BiVO 4 ) 20 (BiVO 4 ) 16 (BiVO 4 ) 7 (BiVO 4 ) 6 (BiVO 4 ) 5 (BiVO 4 ) 4 (BiVO 4 ) 3

  The vibrational properties were analyzed theoretically for (BiVO4)n clusters with frozen and optimized atomic coordinates. The frozen clusters possess the same atomic arrangement as involved in monoclinic bismuth vanadate. The optimized structures have a tendency to create the amorphous configurations according to the requirement of the total energy minimum.

	IR absorption [arb. units]	0 (BiVO 4 0 200 400 600 800 1000 1200 1400 (BiVO 4 ) 2  -1 [cm -1 ] 200 400 600 800 1000 1200 1400 (

BiVO 4 ) 44 (BiVO 4 ) 40 (BiVO 4 ) 36 (BiVO 4 ) 30 (BiVO 4 ) 28 (BiVO 4 ) 26 (BiVO 4 ) 20 (BiVO 4 ) 16 (BiVO 4 ) 7 (BiVO 4 ) 6 (BiVO 4 ) 5 (BiVO 4 ) 4 (BiVO 4 ) 3

	IR absorption [arb. units]
			(BiVO 4 ) 2
		-1 [cm	-1 ]

BiVO 4 ) 30 (BiVO 4 ) 24 (BiVO 4 ) 18 (BiVO 4 ) 12 Intensity(a.u.) Raman shift(cm -1 ) (BiVO 4 ) 36

  -1 , which developsThe results show that the values for both smaller and larger clusters are close to the experimental values. The biggest discrepancies are for nanocrystals with 24 and 30 units of the BiVO4, especially for structures after geometry optimization. The maximum Raman intensity for the optimized structures is shifted to longer wavelengths compared to the frozen clusters. Positions of the Raman shift for individual frozen clusters are reduced and vary up to 50 cm -1 from the experimental value and reaches 140 cm -1 for

	geometry optimized clusters.
								(BiVO 4 ) 36
								(BiVO 4 ) 30
	Intensity(a.u.)				 s (V-O)		(BiVO 4 ) 18 (BiVO 4 ) 24
								(BiVO 4 ) 12
	300	400	500	600	700	800	900	1000 1100
			Raman shift(cm -1 )
		( s (V-O)
	300	400	500	600	700	800	900	1000 1100
	Figure 5.1.3.2. Raman spectra calculated by PM6 methodology for frozen (top panel) and optimized
			(bottom panel) (BiVO4)n clusters

Table 5 .1.3.1.

 5 Maximum of Raman intensity -location of symmetrical stretching of V-O mode for the structure after and without optimization process

	Cluster	Optimized	Raman shift (cm -1 )	Frozen
	(BiVO4)12	872.65		870.00
	(BiVO4)18	964.30		824.54
	(BiVO4)24	790.17		782.24
	(BiVO4)30	748.23		708.75
	(BiVO4)36	871.70		802.67
	Comparative results between theoretical and experimental investigations are carried out
	as illustrated in

Acknowledgement

First and foremost I want to thank my supervisor Prof. Malgorzata Makowska-

AFM -Atomic Force Microscope CVD -Chemical Vapor Deposition IR -Infrared KFM -Kelvin Probe Force Microscopy PC -Photocatalyst PDT -Photodynamic Therapy SEM -Scanning Electron Eicroscope STM -Scanning Tunnelling Microscope UV Hybrid system (BiVO4)18/LIGAND HC -Hybrid system (BiVO4)18/COMPLEX ν0 -Incident radiation at frequencies LUMO -Lowest Unoccupied Molecular Orbital -Work function donor -Work function of the tip -Work function of the sample VB -Valence Band VBM -Valence band maximum B3LYP -Becke, three-parameter, Lee-Yang-Parr BFGS -Broyden-Fletcher-Goldfarb-Shanno algorithm Ecut-off -Energy Cut-off GGA -Generalized Gradient Approximation KS -Kohn-Sham equation LDA -Local Lensity Lpproximation PBE -Perdew-Burke-Ernzerhof functional PCM -Polarizable Continuum Model PDOS -

The condition that must be fulfilled to obtain constructive X-ray diffraction was reported by Bragg in 1913 and is called Bragg's Law [1]:

where n is a positive integer and λ is the wavelength of the incident radiation. One of the important conclusions of this equation is the general requirement for the wavelength of X-rays that can be used in diffractometric studies. As sin(θ) is always ≤ 1 and n ≥ 1, any diffraction lines from the crystal must lead to the condition:

with dmax the largest inter-reticular distance involved in the crystalline structure. Since the dimensions of the crystal cell are in order of several angstroms (1 = 10 -10 m) the X-ray wavelengths of the same order must be used. The relationship between the wavelength of the X-ray and its energy expressed in keV is given by the formula:

. (3.1.3.)

Typical X-ray energies used in XRD studies are between 10 and 50 keV and are therefore significantly lower than those used in classical industrial radiography.

The Bragg condition leads to the strong diffraction lines which are observed only for certain strictly defined monocrystalline orientations. In order to observe the characteristic XRD lines of a defined crystalline structure, the crystal should be rotated while simultaneously recording the overall XRD pattern. This technique is known as the rotating crystal which is used in solid state physics to determine the crystallographic structure of monocrystals. In technical applications one may dealing with polycrystalline materials consisting of a large number of small, accidentally oriented crystals for which a powder method is applied as outlined in Schema 3.1.3.

optical efficiency of the aperture-conditioned optical system which is defined by the The method can be used to determine the trace content and to purify the main ingredient [13][14][15][16].

In present work the thin film of monoclinic BiVO4 and hybrids systems were investigated by using Varian Cary 500 UV-Vis-NIR spectrometer in a wavelength range from 250 nm to 1000 nm.

Raman spectroscopy

Raman spectroscopy is based on the measures of radiation scattered from vibrational The carbonyl group react with the holes from PC decomposing organic molecules into volatile, inorganic CO2 as follows [22]:

. (3.7.6.)

The expression describing the rate of organic compounds degradation is as follows:

where k is reaction rate constant, θ shows degree of coverage and K is adsorption constant, c is concentration of the organic substance in solution.

In present work the photocatalytic properties of the BiVO4 thin films were investigated trough the degradation of organic dye molecules under irradiation coming from 60 W Xe lamp. The container consist in a glass beaker of 100 ml cylindrical reactor. The aqueous solution was charged with dyes (10 -5 M) and containing BiVO4 film set at 30 cm from the radiation source was located. This leads to a radiation power density on the photoactive film about 4 W/cm 2 . The process of methylene blue degradation was conducted at the room temperature. The kinetics was recorded each 30 min through the optical absorbance Ct of the solution charged with dyes. The photocatalytic efficiency of the thin film of monoclinic bismuth vanadate was calculated from the change of the Ct/C0 ratio in time.

Thesis and Hypothesis

The main goal of the Thesis is a synthesis and physic-chemical analysis of the sensitized mesoporous materials based on bismuth vanadate (BiVO4) as well as confirmation of their photocatalytic properties. In this case have been specified hypothesis as following:

1. Mesoporous structure of the BiVO4 increases its photocatalytic efficiency.

2. The organic dyes attached to the surface of the thin film of mesoporous BiVO4 increase the photocatalytic properties of bismuth vanadate based hybrid systems.

Preparation of substrate

Borofloat glasses were cut in pieces with size 20×20 mm 2 and cleaned by using ethanol under ultrasonification during 10 min. Subsequently, additional surface treatment was carried out by immersing borofloat substrates in 0.01mol/dm 3 solution of the HCl for 3 minutes to remove eventual impurities being unaltered by ethanol. The next step was the immersion of the substrates in 0.01 mol/dm 3 solution of the NaOH for 3 min in order to realize HCl neutralization. Finally, the substrates were removed from the NaOH solution and washed vigorously with distilled water. Thus, all residues and impurities were correctly removed from the substrates. All substrates were dried using clean compressed air and were used immediately, or up to 3h after being cleaned.

Preparation of thin films

A single layer of the synthesized solution I or solution II, respectively was deposited on the borofloat substrate using dip coating method. The deposition parameters were selected in preliminary test measurements. The speed of substrate displacement was chosen to be equal to 0,05 cm/s and room temperature was kept for all realized films.

The samples were first dried in the furnace at 80°C during 12 hours. In the last stage, the films were annealed from room temperature (RT) up to 300°C with rate 2°C/min.

Then the film was annealed with higher rate equal to 5°C /min up to 450°C. After that the sample was kept 2h at the temperature 450°C and then cooled up to 40°C with rate 5°C /min. The procedure is ilustrated in the schema The calculations of electronic properties performed for nanocrystals, require the use of two methodologies based on a semi-empirical approach and DFT methodology. The two mentioned methods were used, because it is necessary to ensure the stability of the performed simulations. The semi-empirical approach with the parameterized PM6 method [19] allows a pertinent evaluation of the physical features of clusters with large sizes and with the involvement of heavy atoms. Within such a framework, the numerical simulations were performed using the Gaussian quantum-chemical program package [20]. All the calculations were made using restricted Hartree-Fock (RHF) regime with the SCF convergence criterion set at 10 -5 hartree. The second approach was based on DFT calculations with Gaussian and DMol3 [21,22] programs. The calculations were performed using the PBE [23] and B3LYP [24] functionals with applying the DND and CEP-4G basis sets. No symmetry rules were imposed during the electronic structure analysis. The SCF density convergence criterion was equal to 10 -8 hartree. In order to reduce the required computational time, all the core electrons were represented by the effective core pseudopotential [25]. With regard to the size of the investigated clusters, the single point calculations were performed applying the RHF method.

The electronic properties of the generated nanocrystals were calculated applying the cluster approach without any saturation at their surface [26]. According to the work of Makowska-Janusik et al. [26], performed for other classes of nanocrystalline oxide systems, dangling bonds were not considered. The physical properties of the investigated (BiVO4)n nanoparticles were calculated for three different clusters families.

The first one deals with (BiVO4)n clusters possessing a monoclinic arrangement cut out of the crystal structure without any reconstruction. The second family of clusters considered the same input structures as specified previously but their geometries were fully optimized according to the total energy minimization. This means that the distances between atoms change with taking into account the influence of surface limitation. The performed procedure simulates the amorphization effect of the crystal structure. The geometry optimization was performed in Cartesian atomic coordinates with symmetry defined as C1. The gradient convergence tolerance was equal to 10 -6 hartree/Bohr using the quadratic approximation (QA) method [27] updating the Hessian matrix during the optimization.

Experimental Investigations

The performed experimental work focused on the synthesis of mesoporous thin films deposited on borofloat substrate. The synthesis procedure was described in Chapter 4.

The mesoporous BiVO4 was synthesized using two different surfactants such as P123 and F127. The resulting samples were annealed at 450°C in order to ensure the crystalline structure. XRD measurement were then performed to verify the crystalline order in the mesoporous films. The XRD analysis showed that the films synthesized by using the F127 route do not correspond to pure monoclinic phase.

Additional diffraction peaks are involved in XRD pattern between 26° and 28° irrelevant to BiVO4 polymorphs. Therefore, the synthesis with the surfactant P123 was performed in the aim to compare its effect on the crystalline BiVO4 form. This synthesis method gives satisfactory crystalline form for the synthesized samples as mesoporous powder (Fig. 5.1.1.7.).

UV-vis absorption spectra of mesoporous BiVO4 films

The UV-vis absorption spectra of the mesoporous BiVO4 films were experimentally measured and compared to theoretical predictions performed on nanocrystalline clusters. Based on the work of Sun and coauthors [37] it has been shown that the first absorption peak is shifted to lower wavelength with the increase of nanocrystalline size as also confirmed by other experimental reports [38,39]. By controlling the size, shape and porosity of the films it is possible to fine-tune the optical properties with a particular emphasis on the visible light absorption. The effect is a key requirement for the efficiency of visible light driven photocatalytic processes. The UV-vis absorption spectra obtained for films deposited on borofloat substrate are located in the range 350 -470 nm, i.e. a fraction of visible spectral range (Fig.

5.1.2.1).

for large-sized clusters (see Fig.

top panel). Experimental data obtained by

Zhang et al. [39] showed that the BiVO4 nanoparticles are characterised by a broad and strong IR band near 728 cm -1 with shoulders at 892, 822, 684 and 632 cm -1 . Compared the theoretically obtained data to the ones of Zhang et al. one may conclude that the significant influence of core atoms of the nanoparticle properties is noticed, particularly when the cluster size increases.

For small clusters, more important influence on the vibrational properties occurs from surface atoms. This is illustrated in the case of clusters with small sizes, where the IR signature is quite different for the optimised and frozen configurations (see Fig. 5.1.3.1.). This contrasts with large-sized clusters, such as (BiVO4)44, where the same shape of the IR spectra was qualitatively obtained irrespective of the atomic arrangement being optimised or frozen. The main reason for such a behaviour lies in the optimisation procedure inducing reconstruction and relaxation at the outermost cluster surfaces while preserving the core atoms. As the small clusters are highly sensitive to surface atoms, in contrast to large-sized clusters, drastic changes occur on their IR spectra, as demonstrated from the performed calculations.

Taking into account the effect of electron-phonon interaction, evaluation of the Raman spectra related to the selected clusters was performed by using the parametrized PM6 semi-empirical method. These spectra were calculated using a standard procedure implemented in the Gaussian program package. Fig. 5.1.3.2. illustrates the impact of the size on the Raman spectra of the BiVO4 clusters. Therefore, similar conclusions can be defined for Raman spectra as it was stated for calculated IR spectra. In particular, the main contribution occurs from core atoms of the clusters. For small clusters, where the ratio of the surface atoms to the internal ones is high, the main band located at 800 cm -1 is shifted and the spectrum part in the low wavenumber range is less detailed. The Raman spectrum performed for the nanosized BiVO4 is dominated by an intense Raman band at 827 cm -1 , assigned to s(V-O), and with a weak shoulder at about 709 cm -1 , assigned to as(V-O) [39]. In the Raman spectrum calculated for (BiVO4)40 one may notice the non-symmetric band at a position of 800 cm -1 with the same shoulder depicted in the low wavenumber part.

The highest intensity peak seen in the This can possibly means that the attached dye does not have significant effect on the charge transfer across the system. In this situation the dye molecule is not photoactive and the built system is not appropriate for the photocatalytic applications. Significant differences are also expressed in the (BiVO4)18/COMPLEX system (HC), where the flow of charge occurs in the opposite direction than the appropriate one. In this case the HOMO orbital is located at the nanoparticle but the LUMO is distributed at the COMPLEX molecule. It gives the possibility to conclude that, a photoinduced electrons flow from the inorganic nanocrystal to the dye molecule can occurs. In this way the photoactive charge current is blocked. Analyzing the data presented in Fig. 5.2.1.3 it can be concluded that the most appropriate for the photocatalytic application is the (BiVO4)18/D149 hybrid system (HD).

Experimental Investigations

The AFM measurements were performed on both the morphology characterizations and the electrical activity KFM probed technique. Thus, tapping modes™ and KFM images of a square surface were recorded. They reveal both the topology and the electrical potential distributions on different BiVO4 film surfaces. In consequence, experimental investigations of the BiVO4 thin film surface morphology with anchored organic molecules showed that these nanoparticles are bigger in size than the pure bare samples.

The drastic increase in particle size up to more than 70 nm illustrates significant importance of the binding agent in mesoporous thin film morphology. In addition, the analysis of the KFM measurements showed a net effect from the dye molecules on the surface potential distribution (Fig. The highest potential value is equal to 60 mV. A characteristic phenomenon occurring here consists in blurring the surface potential, especially under visible radiation exposure. For the non-irradiated samples it is noted that the greatest potential is focused on the individual particles and corresponds closely to the surface repetition of the system (Fig. 5.2.1.4, middle panel). On the other hand, in the illuminated systems the potential is blurred and does not directly correlate with the surface morphology (Fig.

5.2.1.4, right panel -HD and HL).

The resulting differences could occur due to photoactivation of organic dyes anchored on the BiVO4 surface. Electrons excited from the HOMO to LUMO levels of the organic molecules can be then transferred to the CB of the semiconductor. The increased amount of charge curriers on the surface could results in a free flow of surface charges. In the case of the (BiVO4)18/COMPLEX hybrid (HC) the situation is different than it was observed for the HD and HL hybrids. The effect of the irradiation causes the reverse charge transfer reaction exhibiting the potential located at the individual BiVO4 nanoparticles. These experimental data are in good agreement with the expected behavior from theoretical considerations. This may also illustrate the weak photoactive compatibility between the BiVO4 thin film and the COMPLEX molecule.

Surface potential under illumination

The above presented theoretical analysis of the electronic properties of the BiVO4 nanostructures underlines the relevance of mesoporous bismuth vanadate to enhance the photocatalytic activity. In this case the experimental synthesis of mesoporous samples was realized by using the surfactant P123. This synthesis method stabilizes the crystalline monoclinic sheelite structure in the mesoporous powder. For the thin films synthesis, the role of the substrate is also a matter of interest due to its effect on the morphology of the films as revealed from the shape and sizes of the experimental work for the deposition of BiVO4 particles. The UV-vis absorption spectra obtained for films deposited on borofloat substrate are located in the range 350 -470 nm, i.e. a fraction of visible spectral range can be harvested. The experimental band gap for BiVO4 thin films estimated from the Kubelka-Munk model was find equal to 2.55 eV in agreement with theoretical prediction (2.60 eV) obtained by using the semi-empirical PM6 parameterized method performed on partially reconstructed (BiVO4)18 cluster with a size of 1.2 nm. The surface of the partially reconstructed nanoparticle was modified according to the environmental effect of the interacting bismuth vanadate. In this case, the implemented cluster model of partially modified structures is a good model for a mesoporous material to predict its electronic and optical properties. 8. The location of the HOMO and LUMO orbitals calculated for the hybrid structures was analysed. In the case of (BiVO4)18/D149, it was observed that the localization of the HOMO and LUMO orbitals leads to the photoactivation of the dye and in consequence to the photoinduced charge transfer from the D149 molecule to BiVO4 nanoparticle. In the case of (BiVO4)18/LIGAND, the HOMO and LUMO orbitals are located in the BiVO4 network. This suggests that the anchored dye does not have significant effect on the charge transfer across the system. In this situation the dye molecule is not photoactive and the built system is not appropriate for the photocatalytic applications. In the case of the List of Tables Table 1.2.5.1. Summary of several monoclinic BiVO4 samples synthesized by different routes relative to the photocatalytic efficienciency………………………………………………...

List of Figures