Jesús Vargas Hernández

Keywords: Metal doping, TiO2, Photocatalyst, Nanopowders, Thin films, Sol-gel, Hydrothermal, Spin coating, Dip coating, Methylene blue, EPR, XRD, Raman, XPS, SEM, TEM

Résumé

Le travail de thèse porte sur les méthodes de synthèse de nanostructures de dioxyde de titane et de leurs études physicochimiques afin de préciser les corrélations entre la morphologie, le dopage métallique, les caractéristiques structurales avec l'efficacité photocatalytique. Le grand intérêt pour les nanomateriaux TiO2 réside dans la mise au point de nouvelles sources d'énergie ou la conservation de l'environnement par des processus photocatalytiques. Cependant, la limitation principale de TiO2 est du au large gap électronique (eV ~3,2) du polymorphe Anatase. Ainsi, un des objectifs importants pour l'amélioration de l'efficacité des nanomatériaux TiO2 est d'augmenter leur photoactivité en décalant la création de paires d'électron-trou de l'UV à la gamme du visible. D'ailleurs, on a montré que l'utilisation de nanostructures 1D de TiO2 (nanotubes) a amélioré la collection de charges, en favorisant leur transport dans les structures 1D, qui par conséquent réduit au minimum la recombinaison et prolonge les durées de vie des électrons. La première partie de ce travail est dédiée à la synthèse des nanopoudres TiO2 dopées par des ions métalliques (Ag, Cu, Eu) préparés par sol-gel. Même avec différents éléments de dopage qui apparemment peuvent adopter le même état de valence (2+) (Cu2+, Ag2+, Eu2+), différents comportements ont été démontrés pour l'incorporation efficace de ces ions dans la structure de TiO 2 . L'anomalie entre les rayons ioniques des différents éléments utilisés module le rapport du dopage substitutionnel. Ceci est en effet réalisé pour Cu2+ mais dans moins d'ampleur pour Ag2+ tandis que les ions d'europium forment une ségrégation de phase Eu2Ti2O7. La dégradation de colorants de bleu de méthylène (MB) a été améliorée légèrement avec les échantillons dopés Ag. La raison a été attribuée aux clusters métalliques Ag qui ont été en effet mis en évidence à travers leur bande d'absorption plasmonique. La deuxième partie porte sur des couches minces de TiO2 dopés (Cu, Ag, et Eu) qui ont été élaborés par sol-gel et spin-coating et dipcoating. Les paramètres optimaux ont été obtenus pour réaliser les films cristallins mais présentant une organisation mésoporouse qui dépend également du processus de dopage. Des études de Photocatalyse ont été également réalisées et l'efficacité des films ont été comparées en fonction des éléments dopants. La troisième partie de la thèse est liée à la modification morphologique des nanoparticules pour former des nanotubes à l'aide de la méthode hydrothermale sous pression contrôlée. Un plan d'expérience basé sur la méthode Taguchi a été utilisé pour la détermination des paramètres optimaux. Les nanotubes TiO2 augmentent la surface spécifique en comparaison avec les nanoparticles. La dégradation de bleu de méthylène par les nanotubes a montré une efficacité photocatalytique plus élevée qu'avec les nanopowders TiO2 pures et dopés Ag. Mots clés : Dopage métallique , TiO2, Photocatalyse, Nanopoudres, films minces, sol-gel, synthèse hydrothermale, spin-coating, dipcoating, mésoporeuse structures, colorants orgaiques, RPE, XRD, Raman, XPS, MEB, MET.

Abstract

The thesis work is focused on the synthesis methods of titanium dioxide nanostructures and their physico-chemical studies in order to point out the correlations between the morphology, metal doping, structural features with the photocatalytic efficiency. The great interest on TiO2 nanomaterials deals with new sources of energy or in the environment preservation through the photocatalytic properties. However, the main limitations is due to the wide band gap (~3.2 eV) of the anatase polymorph. Thus, a major objective for improvement of the performance of TiO2 nanomaterials is to increase their photoactivity by shifting the onset of the electron-hole pairs creation from UV to the visible range. Moreover, it was found that using onedimensional (1-D) TiO2 (nanotubes) improved the charge collection by 1D nanostructures which consequently minimizes the recombination and prolongate the electron lifetimes. The first part of this work is focused on the synthesis of TiO2 nanopowders doped with metallic ions (Ag, Cu, Eu) prepared by Solgel. Even with different doping elements which apparently can adopt the same valence state (2+) such as (Cu 2+ , Ag 2+ ,Eu 2+ ), different behaviors were demonstrated for the effective incorporation of these ions in the host structure of TiO2. The discrepancy between ionic radii of the different used elements modulates the ratio of the substitutional doping. This is indeed achieved for Cu 2+ but in less extent for Ag 2+ while Europium ions form segregated phase as Eu2Ti2O7. The experiments on the degradation of methylene blue (MB) dyes have shown slight improvement with Ag-doped samples. The reason was tentatively attributed to the Ag clusters which were indeed demonstrated through their plasmon optical band. The second part of the work concerns thin films of TiO2 doped (Cu, Ag, and Eu) which were elaborated by spin coating and dip coating. The optimal parameters were obtained to achieve crystalline films but presenting mesoporous organisation which also depends on the doping process. Photocatalysis investigations were also realized and the efficiency of the films compared as function of the doping elements. The third part of the thesis is related to the morphological modification from nanoparticles to nanotubes by using the hydrothermal method with controlled pressure. An experimental design based on Taguchi Method was employed for the determination of the optimal parameters. TiO2 nanotubes increase the surface area in comparison with TiO2 nanoparticles. TiO2 nanotubes were tested for the methylene blue degradation and show a higher photocatalytic efficiency than TiO2 nanopowders and TIO2 doped with Ag. 16. XPS spectra of pure TiO2. Fig. 17. Absorption spectra of methylene blu e photodegradation by pure TiO 2 powder. Fig. 18 First o rder Kinetics of methylene blue photodeg radation by pure TiO 2 powder.. Fig. 19. X-ray diffra ctograms of 3%Cu-TiO2 annealed at different tempera tures. Fig. 20. X-ray diffractogram of TiO2 doped with copper (1 , 2 and 3% mol) annealed at 500°C. Fig. 21. Raman spectra of TiO2 doped with copper (1, 2 and 3% mol) annealed at 500°C. Fig. 22. EPR spectrum of Cu 2+ ions in TiO2 with the experimental (continuous) and the simulated one (dashed). Fig. 23. Optical spectroscopy. a) Diffuse reflectance (%) spectra of TiO2 doped powders with copper (1, 2 and 3% mol) with the corresponding b) Kubelka-Munk absorption curves, c) Determination of band gap. Fig. 24. Jahn-Teller(J-T) distortion of the octahedral symmetry of TiO6 and changes in the d-d transition from octahedral to tetragonal phase. In undistorted octahedral coordination, only a single electronic transition occu rs Fig. 25. XPS spectra of 3%Cu-TiO2 annealed at 500°C. Fig. 26. Absorption spectra of methylene blu e photodegradation by TiO 2 doped with copper (1, 2 and 3% mol). Fig. 27. First order Kinetics of methylene blu e photodegradation by TiO 2 doped with copper (1, 2 and 3% mol). Fig. 28. X-ray diffractograms of 3%Ag-TiO2 annealed at different temp eratu res. Fig. 29. X-ray diffractogram of TiO2 doped with silver (1, 2 and 3% mol) annealed at 500°C. Fig. 30. Raman spectra of TiO2 doped with silver (1, 2 and 3% mol) annealed at 500°C. Fig. 31. EPR spectra of 3 at.% Ag doped TiO2 powder with th e experimental (tick line) and simulation (thin lin e). Fig. 32. Optical spectroscopy. a) Diffuse reflectance (%) spectra of TiO2 doped with silver (1, 2 and 3% mol) with the corresponding b) Kubelka-Munk absorption curves, c) Determination of band gap. Fig. 33. XPS spectra of 3%Ag-TiO2 annealed at 500°C. Fig. 34. Absorption spectra of methylene blu e photodegradation by TiO 2 doped with silver (1 , 2 and 3% mol). vii Fig. 35. First order Kinetics of methylene blu e photodegradation by TiO2 doped with silver (1, 2 and 3% mol). Fig. 36. X-ray diffractograms of 3%Eu-TiO2 annealed at different temp era tures. Fig. 37. X-ray diffractogram of TiO2 doped with eu ropium (1, 2 and 3% mol) annealed a t 800°C. Fig. 38. Raman spectra of TiO2 doped with europium (1 , 2 and 3% mol) annealed at 800°C. Fig. 39. EPR spectra of Eu-TiO2 at different concentrations 3 at.% (grey line), 2at.% (thick black line), 1 at.% (dashed line). Fig. 40. Optical spectroscopy. a) Diffuse reflectance (%) spectra of TiO2 doped with europium (1, 2 and 3% mol) with the co rresponding b) Kubelka-Munk absorption cu rves, c) Determination of band gap. Fig. 41. XPS spectra of 3%Eu-TiO2 annealed at 500°C. Fig. 42. Absorption spectra of methylene blu e photodegradation by TiO 2 doped with europium (1, 2 and 3% mol).78 Fig. 43. First order Kinetics of methylene blu e photodegradation by TiO2 doped with silver (1, 2 and 3% mol). Fig. [START_REF] Xue | Enhanced conversion efficiency of flexible dye-sensitized solar cells by optimization of the nanoparticle size with an electrophoretic deposition technique[END_REF]. Semi-log plot of σdc conductivity versus inverse temp eratu re fo r pure and metal-doped TiO2. Fig. 45 First o rder Kinetics of methylene blue photodeg radation by 3% M-TiO2 (M=Cu, Ag, and Eu).

vi

LIST OF FIGURES

Introduction.

Introduction.

TiO2 powders have been commonly used as white pigments from ancient times. They are inexpensive, chemically stable and harmless, and have no absorption in the visible region.

Therefore, they have a white color. However, the chemical stability of TiO2 holds only in the dark. Instead, it is active under UV light irradiation, inducing some chemical reactions. Such activity under sunlight was known from the flaking of paints and the degradation of fabrics incorporating TiO2. Scientific studies on such photoactivity of TiO2 have been reported since the early part of the 20 th century [1,2]. For example, there was a report on the photobleaching of dyes by TiO2 both in vacuo and in oxygen in1938. It was reported that UV absorption produces active oxygen species on the TiO2 surface, causing the photobleaching of dyes. It was also known that titania itself does not change through the photoreaction, although the ''photocatalyst'' terminology was not used for TiO2 in the report, but called a photosensitizer [3].

It is equivocal when and who started utilizing first such a photochemical power of TiO2 to induce chemical reactions actively, but at least in Japan, there were a series of reports by Mashio et al., from 1956, entitled ''Autooxidation by TiO2 as a photocatalyst'' [4]. They dispersed TiO2 powders into various organic solvents such as alcohols and hydrocarbons followed by the UV irradiation with an Hg lamp. They observed the autooxidation of solvents and the simultaneous formation of H2O2 under ambient conditions. It is interesting to note that they had already compared the photocatalytic activities of various TiO2 powders using twelve types of commercial anatase and three types of rutile, and concluded that the anatase activity of the autooxidation is much higher than that of rutile, suggesting a fairly high degree of progress of the research.

In 1972, Fujishima and Honda discovered the phenomenon of the photocatalytic splitting of water on a TiO2 electrode under ultraviolet (UV) light. Since then, enormous efforts have been devoted to the research of TiO2 material, which has led to many promising applications in areas ranging from photovoltaics and photocatalysis to photo-/electrochromics and sensors. These applications can be roughly divided into "energy" and "environmental" categories, many of which depend not only on the properties of the TiO2 material itself but also on the modifications of the TiO2 material host (e.g., with inorganic and organic dyes) and on the interactions of TiO2 materials with environment [5]. One of the most interesting aspects of TiO2 is that the types of photochemistry responsible for photocatalysis and hydrophilicity are completely different, even though both can occur simultaneously on the same surface [6].

New physical and chemical properties emerge when the size of the material becomes smaller and smaller, and down to the nanometer scale. Properties also vary as the shapes of the shrinking nanomaterials change. Among the unique properties of nanomaterials, the movement of electrons and holes in semiconductor nanomaterials is primarily governed by the well-known quantum confinement, and the transport properties related to phonons and photons are largely affected by the size and geometry of the materials. The specific surface area and surface-to-volume ratio increase dramatically as the size of material decreases. The high surface area brought about by small particle size is beneficial to many TiO2-based devices, as it facilitates reaction/interaction between the devices and the interacting media, which mainly occurs on the surface or at the interface and strongly depends on the surface area of the material. As the most promising photocatalyst, TiO2 materials are expected to play an important role in helping solve many serious environmental and pollution challenges [5].

Since the bulk of the terrestrial solar energy is in the visible spectrum, any shift in the optical response of titania, or its high-bandgap energy metal oxide counterparts, from the UV towards the visible spectrum offers the potential for a significant, positive impact on the photocatalytic and photoelectrochemical utility of the material(s) [7]. Some studies about morphological modification have shown that one-dimensional structures have a better performance for photovoltaic and photocatalytic applications [8][9][10]. The doping process is an effective approach to improve the visible light activity. Doping with cations or anions may alter the band gap or introduction of intraband gap states which may make TiO2 absorb the visible light. [11].

The present thesis is focused on the morphology and structural study of TiO2. The morphological modification from nanoparticles to nanotubes by the hydrothermal method with control of pressure was studied. The other path followed, is the study of additions of other elements to the crystal structure of TiO2 (doping) trough ions which promising increase the photochemical properties of the semiconductor, synthesized by the sol-gel method. Thin films from the same systems doping were fabricated by dip coating, and spin coating techniques through a stable sol.

CHAPTER I Fundamentals

I. Fundamentals 1.1. Titanium dioxide (TiO 2 ).
TiO2 is a chemically stable, nontoxic, biocompatible, inexpensive material with very high dielectric constant and interesting photocatalytic activities. It is a wide-gap semiconductor, and depending on its chemical composition, it shows a large range of electrical conductivity. In general, TiO2 has two stable crystalline structures: anatase and rutile. Usually, natural rutile crystals are impure, and, therefore, early investigations were limited to keramic samples only, but later (around 1950s), a colorless, large, single crystal of synthetic rutile was grown by the Boule technique [1].

Thereafter, most of the research was done on the electro-optical characterization and defect chemistry of single rutile crystals. The stoichiometry of the rutile TiO2 is highly dependent on its deposition parameters, especially on annealing conditions and atmosphere [2]. Earlier, it was assumed that the properties of anatase would be similar to those of rutile until a new solar cell concept was reported using anatase, which cannot be realized by rutile structure. [3].

1.1.1 Crystal structure and properties.

Besides the four polymorphs of TiO2 found in nature (i.e., anatase (tetragonal), brookite (orthorhombic), rutile (tetragonal), and TiO2 (B) (monoclinic)), two additional high-pressure forms have been synthesized starting from rutile: TiO2(II) [103], which has the PbO2 structure, and TiO2(H) with the hollandite structure [4,5].

Rutile and anatase structures can be described regarding chains of TiO6 octahedra, where each Ti 4+ ion is surrounded by an octahedron of six O 2-ions. The two crystal structures differ in the distortion of each octahedron and by the assembly pattern of the octahedral chains . In rutile, the octahedron shows a slight orthorhombic distortion; in anatase, the octahedron is significantly distorted so that its symmetry is lower than orthorhombic. The Ti-Ti distances in anatase are larger, whereas the Ti-O distances are shorter than those in rutile. In the rutile structure, each octahedron is in contact with ten neighbor octahedrons (two sharing edge oxygen pairs and eight sharing corner oxygen atoms), while, in the anatase structure, each octahedron is in contact with eight neighbors (four sharing an edge and four sharing a corner). These differences in lattice structures cause different mass densities and electronic band structures between the two forms of TiO2 [6]. Among these structures, rutile is the most stable phase, whereas anatase is metastable at room temperature and transforms irreversibly into rutile upon heating above a threshold temperature around 1000 °C (which is in atmospheric conditions, but the threshold temperature can vary from 400 °C to 1200 °C, depending on the grain size, ambient conditions, and impurities). Natural rutile crystals exhibit predominantly (110) surface, which is considered to be the most stable surface of stoichiometric rutile, whereas anatase is the most stable with (101) surface [2].

The application of anatase phase or rutile phase depends on its crystallinity and isomorphism.

Therefore the application can vary. For example, the anatase phase is most used in charge separation devices, while rutile is predominantly used in gas sensors and dielectric layer [7]. The rest of the t2g bands are antibonding with p states. The main peak of the t2g bands is identified to be mostly dyz and dzx states [6]. The optical properties of semiconductors, such as the absorbance and emission, result from the excitonic transitions. The influence of size on these properties occurs when the diameter is comparable to or smaller than the exciton diameter; the consequent changes in the electronic structure are called the confinement effect. Excitons are imaginary quasiparticles produced by the pairing of an electron (from the conduction band) and a hole (from the valence band) resulting from the Coulomb interaction. Such crystallite size in semiconductor nanocrystals is understandable from the work of Brus [8], and his effective mass approximation theory. According to the theory, model

𝐸 𝑔 * = 𝐸 𝑔 + 𝜋 2 ℎ 2 2𝑅 2 [ 1 𝑚 𝑒 1 𝑚 ℎ ] -[ 1.786𝑒 2 𝜀𝑅 ] Equation 1
where Eg and Eg* are the bandgap value of the bulk and confined crystallite, respectively; ε is the dielectric constant; R is the radius of the crystallite; me is the effective mass of electron; and mh is the effective mass of the hole. This theory has also been applied to determine the bandgap widening (∆Eg = Eg*-Eg) in ZnO, SnO2, and TiO2 nanocrystals. A plot between the crystallite diameter and bandgap widening is presented in Fig.

3 [9]. 1.2 Fundamentals and photocatalysis mechanism of TiO 2 .

TiO2 is characterized by the presence of photoinduced phenomena. These are depicted in Fig. 5. All these photoinduced processes originate from the semiconductor band gap. When photons have a higher energy than this band gap, they can be absorbed, and an electron is promoted to the CB, leaving a hole in the VB. This excited electron can either be used directly to create electricity in photovoltaic solar cells or drive a chemical reaction, which is called photocatalysis.

A special phenomenon was recently discovered: trapping of holes at the TiO2 surface causes a high wettability and is termed 'photoinduced super hydrophilicity' (PSH). All photoinduced phenomena involve surface bound redox reactions. [5]. The first report on the efficient hydrogen production from water and organic compounds was published in Nature at the time of the second oil crisis [14], and TiO2 photocatalysis drew the attention of many people as one of the promising methods for hydrogen production. However, even though the reaction efficiency is very high, TiO2 can absorb only the UV light contained in a solar spectrum, which is only about 3%. Therefore, from the viewpoint of H2 production technology, TiO2 photocatalysis is not very attractive. Various other semiconductors with a small band gap, such as CdS and CdSe, were investigated, but their efficiency and stability were much Instead, the research shifted to the utilization of the strong photoproduced oxidation power of TiO2 for the destruction of pollutants. The first such reports were those of Frank and Bard in 1977, in which they described the decomposition of cyanide in the presence of aqueous TiO2 suspensions.

In the 1980s, detoxications of various harmful compounds in both water and air were demonstrated using powdered TiO2 actively as potential purification methods of wastewater and polluted air. The platinization of TiO2 was not necessary in this case, and TiO2 powder itself was used under ambient condition [15] [16].

Photocatalysis is based on the excitation of a semiconductor material with irradiation of light energy at least equal or greater to that of the band gap energy of the material. An electron in an electron filled valence band (VB) is excited upon irradiation of light to a vacant conduction band (CB) and leave behind a positive hole (h + ) in the VB. These electrons and holes (e -and h + ) are mainly responsible for the generation of active species which degrade the target molecules.

These charge carriers e -and h + drive the reduction and oxidation, respectively. These charge carriers migrate to the photocatalyst surface and indicate the secondary reactions with the adsorbed components on the surface of the solid semiconductor. The photoexcited e-in CB changes oxygen to the superoxide radicals and hydroperoxide radicals while positive h + in VB can oxidize the adsorbed water or hydroxyl ions to form hydroxyl radicals Fig. 6. These reactive species (superoxide radicals or hydroperoxide radicals and hydroxyl radicals) can be involved in the degradation of surface adsorbed compounds. The overall process can be summarized as follows:

Semiconductor material

(𝑇𝑖𝑂 2 ) + ℎ𝑣 → ℎ 𝑉𝐵 + + 𝑒 𝐶𝐵 - Equation 2 𝑒 𝐶𝐵 -+ 𝑂 2 → 𝑂 • 2 (𝑅𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛) Equation 3 𝑒 𝐶𝐵 -+ 𝐻 2 𝑂 → 𝑂𝐻 • + 𝐻 + (𝑂𝑥𝑖𝑑𝑎𝑡𝑖𝑜𝑛) Equation 4 𝑂 2 + 𝐻 + → 𝐻𝑂 2 • • Equation 5 𝑒 𝐵𝐶 -+ 𝐻𝑂 2 • → 𝐻𝑂 2 - Equation 6 𝐻𝑂 2 -+ ℎ 𝑉𝐵 + → 𝐻 2 𝑂 2 Equation 7
TiO2 photocatalyst is only restricted to absorb the light in the ultraviolet region (wavelength < 390 nm) due to the wide band gap (3.2 eV for anatase and 3.0 eV for rutile). The photoexcitation of TiO2 generates the electron/hole pairs. The UV light activity and fast recombination of electron/hole pairs limit the use of TiO2 in visible light and the photocatalytic efficacy, respectively.

Because of solar light contains only about 2-3 % UV light, it is necessary to develop TiO2 based material which can harvest solar light. Modifications by various strategies have been applied to make the TiO2 as visible light active material such as doping with metal and non-metals, codoping, coupling, etc [17].

Doping of TiO2

The optical response of any material is largely determined by its underlying electronic structure.

The electronic properties of material are closely related to its chemical composition (chemical nature of the bonds between the atoms or ions), its atomic arrangement, and its physical dimension (confinement of carriers) for nanometer-sized materials. The chemical composition of TiO2 can be altered by doping. Specifically, the metal (titanium) or the nonmetal (oxygen) component can be replaced to alter the material's optical properties. It is desirable to maintain the integrity of the crystal structure of the photocatalytic host material and to produce favorable changes in electronic structure. It appears easier to substitute the Ti 4+ cation in TiO2 with other transition metals, and it is more difficult to replace the O 2-anion with other anions due to differences in charge states and ionic radii. The small size of the nanoparticle is beneficial for the modification of the chemical composition of TiO2 due to the higher tolerance of the structural distortion than that of bulk materials induced by the inherent lattice strain in nanomaterials [6].

1.3.1 Metal Doped TiO2.

Metal doping has proved to be a successful approach for obtaining photocatalysts with improved photonic efficiencies, generally using less than 10 % of doping species for modifying the semiconductor. With this strategy, it is possible to shift the optical response towards higher wavelengths, increasing the photoactivity under visible irradiation, and to hinder electron-hole recombination. This may allow using sunlight as an inexpensive and renewable energy source in photocatalytic applications. Besides optical absorption, other properties that strongly influence the photocatalytic activity, such as surface area, pore size, density of hydroxyl groups, surface acidity, and adsorption/desorption properties, can also be modulated by incorporating a dopant into the photocatalyst structure.

Impregnation, co-precipitation, ion implantation technique, and sol-gel methods can be employed to incorporate cationic dopants into TiO2 [18].

Doping by metal ions into the TiO2 structure (Cationic doping) has demonstrated the efficient method for enhancing the visible light absorption. The electrons injected into CB from VB can be easily transported to the surface of the catalyst and electrons can be easily trapped by doping with transition metals, noble metals, other metals, rare earth elements. Doping of metallic cations enhance the redox potential of the radicals generated during photocatalysis and reduce the e-/h+ recombination lifetime. [17].

Surface Dispersion versus Lattice Incorporation

The presence of dopants can alter the charge transfer properties of TiO2, modify the adsorption capacity of its surface, or produce a shift of the absorption edge of the photocatalyst to the visible range. This last modification is highly desirable for a more efficient use of sunlight. It can be associated with a decrease in the band gap, by the interaction of the cation states with the valence or conduction bands of anatase, or with the creation of intra-band-gap states, induced by the incorporation of the dopant ions.

Pronounced redshifts can be obtained using ion implantation processes, which consist of the acceleration of the dopant ions in an electric field and their impact on the semiconductor.

In 1994 [19], a systematic study of quantum-sized (3-4 nm) doped TiO2 with 21 cations was conducted by Hoffmann and co-workers, who reported on the influence of the metal dopants on the photoreactivity by acting as electron and hole traps, and by altering the electron-hole recombination rate as follows:

𝑀 𝑛+ + 𝑒 𝐶𝐵 -→ 𝑀 (𝑛-1)+ electron trap Equation 8𝑀 𝑛+ + ℎ 𝑉𝐵 + → 𝑀 (𝑛+1)+ Equation 9

with the energy level of M n+ /M (n-1)+ located below the conduction band edge and the energy level of M (n+1) +/M n+ above the valence band edge. The presence of these intra-band-gap energy levels would lead to the already mentioned red shift and the photocatalytic response under visible light.

They also observed the existence of an optimum dopant concentration, above which the photoactivity decreases. In the case of quantum-sized particles, this behavior has been attributed

to an increasing recombination rate with dopant concentration due to the concomitant decrease in the average distance between trap sites.

An effective absorption of visible light does not always guarantee an enhanced photocatalytic activity under visible irradiation. A dopant can act as an effective mediator of interfacial charge transfer, inhibiting the electron-hole recombination, or it can produce the opposite effect. Despite an extended response in the visible range, metal ions can act as recombination centers, decreasing the photocatalytic activity of the catalyst under UV irradiation Depending on the dopant, two different situations can be reached, as schematically represented in Fig. 7: (1) a p-type doping, by the introduction of cations with a lower valence than that of Ti 4+ (M n+ ,n≤4), which act as acceptors of electrons; (2) an n-type doping by incorporating electron donors, cations with higher valence than Ti 4+ (Mn+,n≥ 5).

Metal dopants can occupy substitutional or interstitial sites in the titania lattice or form segregated metal oxide clusters or islands on the surface, depending on nature (primarily electronegativity and ionic radius) and concentration of the doping metal and the synthesis procedure. The inherent lattice strain of nanomaterials facilitates the structural distortion of nanosized TiO2 produced by the modification of its chemical composition. Substitutional doping of TiO2 will tend to occur when the electronegativity and ionic radius of the doping metal cations match those of Ti 4+ , while interstitial doping will be preferred if the electronegativity of the doping metal ions approaches that of Ti 4+ , but its ionic radius is smaller. On the other hand, metal ions with larger ionic radii than Ti 4+ present lower solubility in the TiO2 lattice, which hinders their substitutional incorporation, and will be more probably found as dispersed metal oxides. In contrast to substitutional doping, a red shift in the absorption spectrum is not expected in the case of surface dispersion. To keep the charge neutrality in the structure of doped TiO2, cation or anion vacancies are created. In these cases, the presence of defects associated with the creation of oxygen vacancies may also contribute to the visible light absorption of the modified TiO2.

The solubility of the metal cations in the anatase structure is governed by parameters such as the primary particle size, the M n+ /Ti 4+ radius ratio, and the structural strain, the latter influenced in turn by factors such as the metal oxidation state. A large M n+ /Ti 4+ radius ratio favors the presence of oxygen vacancies, which efficiently release strain in the anatase structure. The solubility of M n+ will determine the tendency of the dopant element to form solid solutions, with a homogeneous chemical composition throughout the particle (Ti1-xMxO2), or to segregate at the surface of TiO2.

In any case, at concentrations above the solubility limit of each metal ion, an enrichment of the dopant at the surface is expected, finally leading to the formation of pure MOx phases.

Furthermore, surface or bulk metal doping can stabilize the anatase or the rutile phases of titania and modify the temperature of the anatase-to-rutile transformation. This effect has been ascribed to a hindered crystal growth due to the formation of Ti-O-M bonds. It has been suggested that the creation of oxygen vacancies, derived from the incorporation of ions of lower valence than Ti 4+ and relatively small ionic radius, accelerates the phase transition by reducing the strain energy that must be overcome for the rearrangement of the Ti-O octahedra and the other way around for ions of higher valence and larger ionic radius. Thus, for instance, the presence of Zr 4+ seems to inhibit this transition, stabilizing the anatase phase at higher temperatures than for bare TiO2. Similarly, doping with Al, Ni, Ga, Nb, Ta, and W has proved to retard the anatase-to-rutile transformation, while Mn, Fe, Cu, or Zn promote, generally, this transition [18].

1.3.2 System of TiO2 doped with Cu.

The metal doping is known as the second generation of photocatalysts and has shown high activity for pollutants degradation. Copper is an important dopant because of its high electronic conductivity; low-cost and is highly available in the earth crust.

Nagaveni et al., [20] doped the titania with metal ions Cu 2+ , V 5+ , Fe 3+ , Zr 4+ by the combustion method, and found that metal ions improve the performance in the 4-nitropheno degradation in comparison with the commercial TiO2 pure, Degussa P25.

Xiaoqing Qiu et al., [21], found that the combined effect of Cu in the TiO2 is beneficial for the photooxidation of the volatile organic compounds.

Wu et al., [22], synthesized a compound of TiO2 with Cu2O clusters for carbon dioxide reduction by the sol-gel method. The study showed that the presence of copper clusters, electrons are enriched owing to the alignment of Fermi levels of the metal and the semiconductor, which is the Schottky barrier. An optimal Cu-loaded titania is a highly efficient photocatalyst for CO2 reduction since copper is an effective electron trapper, able to reduce the recombination of electron-hole.

Biswakit Choudhury et al., [23], doped TiO2 with copper by the sol-gel method. The study revealed Silver nanoparticles have prospective applications including biosensing, biodiagnostics, optical fibers, and antimicrobial and photocatalytic uses. Silver ions are known to cause denaturation of proteins present in bacterial cell walls and slow down bacterial growth. The simplest photocatalytic mechanism of silver ions is that it may take part in catalytic oxidation reactions between oxygen molecules in the cell and hydrogen atoms of thiol groups, i.e., two thiol groups become covalently bonded to one another through disulfide bonds (R-S-S-R), which leads to blocking of respiration and cell death of the bacteria, [26].

Mitsunobu Sato et al., [24], examined thin films of a composite of TiO2 with silver nanoparticles by the Molecular Precursor method. The study showed that the thin films with silver loading less than 40% mol are not effective under vis-irradiation, though they can work as a photocatalyst under UV-irradiation. Contrarily, the composite thin films of Ag content larger than 50 mol% showed the vis-responsive activity, whose level was slightly lower than the decreased UVsensitivity.

Ashutosh Pandey et al., [26], synthesized TiO2 doping with silver by the sol-gel method for antibacterial purpose. The results showed that the doping systems have a shifting of the absorption edge to the visible region. The antibacterial efficiency increase for the TiO2 doping with silver in comparison with the TiO2 pure, due de decreasing of electron/hole recombination, according to the photoluminescence test.

Surface deposition of silver nanoparticles (Ag NPs) onto the 100% anatase titania (Ag/TiO 2) for the evolution of surface plasmon resonance (SPR) was achieved sustainably with the assistance of solar energy was studied by Pichiah Saravanan et al., [25],. The synthesized plasmonic photocatalyst Ag/TiO2 unveiled an impressive photocatalytic activity for EDCs degradation under artificial visible light. Beside they established the photocatalysis mechanism of TiO2 doping with silver. When Ag/TiO2 is exposed to visible light irradiation, the electrons below the Fermi level (Ef) of the Ag NPs will be excited to the surface plasmon states, leaving positive charges (h + ) below the Ef. Thus, with the contact of Ag NPs on the surface of TiO2, the energetic electrons from Ag will be excited to the CB of TiO2. As the CB of TiO2 is an electron acceptor, it readily accepts the electrons and forms superoxide anion radicals (•O 2-) and followed by the protonation that yields

•HO2 radicals. This •HO2 radicals easily combines with the trapped electrons resulting into H2O2

and finally forming a photodegradation active species •OH radicals. Moreover, the Schottky barrier was formed at the interface of Ag-TiO2 because the Fermi level of TiO2 is higher than that of Ag which will hinder the transfer of an electron from the Ag to TiO2. However, it is proven that electrons can transfer from the Ag to TiO2 due to its strong electron oscillating collectively on the SPR excitation. These lead to interband excitation giving sufficient energy to the electrons to overcome the Schottky barrier at the interface. Thus it triggers the energetic electron to be transferred to the CB of TiO2 and lead to the enriched formation of •OH radicals thus enhancing the photocatalytic oxidation.

Non-metal doping of TiO2.

Nonmetal-doped TiO2 nanomaterials have been regarded as the third generation photocatalyst.

Various nonmetal doped TiO2 nanomaterials have been widely studied for their visible light photocatalytic activities. Nonmetal-doped TiO2 nanomaterials have been demonstrated to have improved photocatalytic activities compared to those for pure TiO2 nanomaterials, especially in the visible light region. [6].

It is known that the TiO2 doped with Nitrogen, has a higher photocatalytic activity under the visible than pure TiO2. However, it showed less activity in the UV light, [27].

Titania doping with Sulphur has shown a very similar behavior, with high activity in the visible region and low activity in the UV light, however, depending on the synthesis conditions the visible light absorption will change, due to the different charge behavior of the samples, [28].

In metal-doped TiO2, the cations replace titanium atoms; meanwhile, in the non-metal-doped TiO2, the anionic dopants are expected to substitute the oxygen atoms, although cationic substitutions may take place as well with some non-metals. Even though titania with partial substitution at the oxygen sites is usually represented in the literature as TiO2-xAx, where A is the anion, this representative formula must be carefully employed and seems questionable for very low A/Ti ratios.

The aim of absorbing radiation less energetic than UV-A requires the band-gap narrowing, by the elevation of the valence band maximum, the lowering of the conduction band minimum or the introduction of localized energy levels, [18].

The main requirements for the anionic dopant to elevate the valence band maximum are the following [29]:

1. Lower electronegativity than oxygen, so that the dopant states can be involved in the formation of a new valence band at a higher energy level. The Pauling electronegativity of O is 3.4, higher than that of all non-metals, except F, with an electronegativity of 3.98.

2. Similar size to that of lattice O atoms, so that the substitutional anions do not alter the structure uniformity and, thus, the intrinsic properties of the semiconductor. N presents the most similar anionic radius.

Many successful photocatalysts have been synthesized. The main challenge remains to achieve higher photocatalytic activity under visible than under ultraviolet light. Therefore, the optimization of the dopants choice and doping strategies continues, [18].

1.3.5 Doping with rare earth.

Luminescent nanomaterials have gained considerable attention in recent years due to the breakthrough developments of technology in various areas such as electronics, photonics, displays, optical amplifications, lasers, fluorescent sensing, biomedical engineering, [9] and environmental control. The long emission lifetime and rich spectral properties of certain rare-earth

[RE] ions are highly attractive in many ways. However, RE ions alone are weakly fluorescent due to the parity forbade f-f transitions. Therefore, the use of host materials is crucial to excite the RE ions efficiently in a wide spectral range to utilize their full potential in optoelectronic devices. Oxide lattices have proved to be an excellent host material due to their good thermal, chemical, and mechanical stabilities. Among them, Y2O3 is a promising host for RE ions due to its low phonon frequencies, which make the nonradiative relaxation of the excited states inefficient. However, the high costs associated with synthesis have restricted its further use. As an alternative, TiO2, a well-known wide bandgap semiconductor, has demonstrated the possibility to be a good sensitizer to absorb light and transfer energy to RE ions. Moreover, the high refractive index and high transparency of TiO2 in the visible and infrared regions make it possible to use in optical devices, [30].

Doping TiO2 with rare earth metals increases the photoactivity of nitrite oxidation in the following order: Gd 3+ > Nd 3+ > La 3+ > Pr 3+ (Er 3+ ) > Ce 3+ > Sm 3+ . The photodegradation rate is higher than that for bare TiO2 due to the higher absorption (red shift) and an increase of interfacial electron rate. The highest enhancement in photoactivity is obtained for ca. 0.5 wt% rare-earth-ion doping (which may favor an efficient separation of the charge carriers), [31].

Furthermore, owing to its good mechanical, optical, and thermal properties, TiO2 is believed to be the ideal host system for incorporating rare-earth (RE) impurity ions as dopants. One of the major advantages offered by these materials is the ability to tailor size dependent optical properties and to achieve high luminescence response through the sensitization by the host. Several reports have demonstrated the intense RE 3+ emission of RE incorporated TiO2 nanoscale systems along with a suitable explanation of the energy transfer mechanism. On the other hand, the RE ions can promote the degradation process (photocatalysis) of many hazardous pollutants and dyes through active interaction with the organic molecules of the pollutants. The incorporation of lanthanide ions into the TiO2 host may help organic pollutants to concentrate on the semiconductor surface, [32].

Substitution of Ti 4+ by d n metallic ions in the TiO2 lattice creates allowed energy states in the band gap of TiO2, which may induce photoactive transitions in the visible light, due to an excitation of an electron from this energy level into the TiO2 conduction band [5,33].

Lanthanide ions are known for their ability to form complexes with various Lewis bases (e.g., acids, amines, aldehydes, alcohols, thiols, etc.) through the interaction of these functional groups with the f-orbitals of the lanthanides [5].

System of TiO2 doping with Eu.

Compared to other trivalent RE ions, Eu 3+ gives strong red emission due to intra 4f carrier transitions thus making the system more useful in technologically important optoelectronic and display elements. Besides that, Eu-doped TiO2 nanomaterials could exhibit better photocatalytic activity as compared to their undoped counterpart. This is because of generation of an adequate number of electron trap centers by Eu 3+ ions [32].

However, due to the large mismatch of ionic radii (Eu 3+ =0.95Å and Ti 4+ = 0.68 Å) and charge imbalance between the Ti +4 and Eu +3 ions, successful incorporation of Eu ions into TiO2 nanocrystals through a soft, wet-chemical route remains a great challenge. In most of the cases, Eu +3 ions either tend to locate on a crystal surface, causing an undesired Eu-Eu interaction, or form Eu2O3 aggregates, which act as quenching sites, resulting in a drastic decrease in the luminescent intensity, [30].

Mou Pal et al., [30], doped TiO2 with Eu by the sol-gel method with different doping concentrations, and shown that as the Europium content increase the size of the particles decrease, suggesting that Europium suppresses the growth of TiO2 nanoparticles. Besides, the induced crystallization reduces the photoluminescence intensity, suggesting amorphous TiO2 as an ideal framework for an efficient energy transfer between the titania host and incorporated Eu 3+ ions. Electrons are initially excited to the conduction band of TiO2 on irradiating UV light and then relaxed to the defect states. Since the defect states of TiO2 are located at higher energies than those of the emitting state ( 5 D0) of Eu +3 ions, energy transfer to the crystal field states ( 7 Fj) of Eu +3 occurs, resulting in efficient PL.

. Dambarudhar et al., [32], doped TiO2 by the sol-gel method, and found that systems with Europium content of 1 to 5 %mol increased the photocatalytic activity,. The reason for the increased photodegradation efficiency is ascribed to the introduction of Eu 3+ doping states, which helped to inhibit e -/h + pair recombination while enhancing interfacial charge. Eu-doped TiO2 nanoparticles is a better choice over the undoped ones, owing to the effective free radical generation via participation of new energy levels and lower rate of e -and h + recombination that would facilitate photocatalytic reactions. Typically, nanoporous film based on TiO2 nanoparticles ( ~20 nm in diameter) has shown high energy conversion efficiency due to large amounts of adsorbed dye molecules. However, when the photoexcited electrons are excited and pass through the interparticle grain boundaries or suffer surface trapping/detrapping effects, the subsequent electron accumulation can enhance the recombination probability, causing restricted performance of solar cells, [34].

Nowdays a model exists for the recombination explanation. In a solar cell when current is drawn from the cell, injected electrons move toward the anode contact either by hopping between sites (the hopping model) or by a random walk process in which the electrons spend some time immobilized in trap sites from which they are excited thermally back to the conduction band (the multiple trapping model). In either model, the electrons may be localized near the surface or in bulk, [35,36].

However it was found that using one-dimensional (1-D) TiO2 (in the form of nanotubes, nanowires, nanofibers, and nanorods) as the photoanode material for DSSCs significantly improved the charge collection, by providing a highway to transport photoelectrons, which consequently minimizes the recombination and increase the electron lifetime [34]. The Fig. 11 Recently, various one-dimensional (1D) nanostructures, including nanowires (NW), nanorods (NR), and nanotubes (NT), have attracted much interest due to their advantages for charge carrier transport and interfacial recombination. However, there are still some technical problems and challenges in these pure 1D nanostructures, such as lower dye loading and lower effective surface area, resulting in lower energy conversion efficiency compared with the 3D-TiO2

nanoparticles. The combined nanostructure (TP) consisted of 1D-nanotubes and 3Dnanoparticles can combine each advantage of 1D and 3D nanostructure, and thus is promising to exhibit higher energy conversion efficiency, [37].

In the last 30 years of research, the particulate form of titanium dioxide (TiOx; x~2) has been the most widely studied photocatalyst for a variety of light-driven reactions. Admittedly, some pressing and yet-to-be-resolved issues regarding TiO2 nanoparticles (T_NPs), nanoparticulate films, and monoliths include inefficient light absorption and e -/h + separation deficiency.

In contrast, the performance of one-dimensional (1D) nanostructures of TiOx-nanotubes or nanorods-has been reported to exhibit much better performance than nanoparticulate systems;

the much lower number of grain boundaries in the 1D nanostructures is believed to allow improved charge separation as shown in Fig. 12, [38].

(a) (b) Figura 2. La principal diferencia entre a) partículas y b) arquitectura de una película nanotubula r es la larga presencia de límites de grano entre las partículas.

2.4.1 Morfología tubular TiO2 y su aplicación en la DSSC.

Los nanotubos de TiO2 (TNT) han obtenidos considerables intereses como un fotocatalizador efectivo debido a sus diversas ventajas promovidas por su microestructura, como alta superficie específica y alto volumen de poro. Los nanotubos son capaces de adsorber una considerable cantidad de contaminantes debido a las propiedades antes mencionadas [39].

Tubular morphology for photovoltaic applications.

TiO2 nanotubes (TNTs) have recently garnered considerable interest as an effective photocatalyst due to their various advantages promoted by their microstructure, such as a large specific surface area and high pore volume. TNTs can absorb a considerable amount of pollutants because of their large specific surface areas and high pore volumes, and this adsorption plays an important role in the degradation of those pollutants, [39]. Wook Choi et al., [11], found that the efficiency of dye-sensitized solar cells [DSSCs] was enhanced by combining the use of TiO2 nanotubes [TNTs] and nanoparticles. The nanotubes were fabricated by the hydrothermal method. Besides the study showed that the morphology properties changes depending on the hydrothermal temperature conditions. The major photovoltaic performance was achieved when a mix of nanotubes and nanoparticles is used in the DSSC.

Ana Flávia Nogueira et al., [43], fabricated nanotubes by the sol-gel and hydrothermal method, and found that nanotubes have a high superficial area allowing much more dye to be chemically absorbed and creates an ample ion path for fast ionic motion from the bulk to the inner parts of the film. Solar cells assembled by combining the TiO2 nanotubes and the plasticized polymer electrolyte exhibited an overall efficiency of 4.03% at 100 mW cm -2 , higher than the devices assembled solely with TiO2 nanoparticles.

Changjian Lin at al., [37], fabricated a combined structure with a mixture of TiO2 nanotubes and particles by anodization of a dispersion of TiO2 nanoparticles were uniformly dispersed on the surface of TiO2 nanotubes with the diameter of 110-130 nm and the length of 15 μm. It was found that the energy conversion efficiency of the solar cell with composite photoanode was 5.75%, enhanced up to 25.8% compared to that of the photoanode of pure TiO2 nanotube structure in the same condition.

Yuan Lin et al., [START_REF] Xue | Enhanced conversion efficiency of flexible dye-sensitized solar cells by optimization of the nanoparticle size with an electrophoretic deposition technique[END_REF], prepared an electrophoretical deposition of nanoparticles inside the nanotubes to increase the surface area for dye loading. The dye loading increased 47.2% after depositing the nanoparticles. A conversion efficiency of 6.28% was achieved for DSSCs with a flexible photoanode.

Xia Tao et al., [45], fabricated a photoanode with bilayer-structured film with TiO2 nanocrystals as underlayer and TiO2 nanotubes as overlayer. The charge recombination behavior of cells was investigated by electrochemical impedance spectra, and the results showed that double layer-TiO2 film-based cell possessed the lowest transfer resistance and the longest electron lifetime.

CHAPTER II Metal doping of TiO2 powders

II. Metal doping of TiO2 powders. Lanthanide ions are known for their ability to form complexes with various Lewis bases (e.g., acids, amines, aldehydes, alcohols, thiols, etc.) in the interaction of these functional groups with the f-orbitals of lanthanides [4]. The incorporation of lanthanide ions into the TiO2 host may help organic pollutants to concentrate on the semiconductor surface. [5]. According to Xu et al., the TiO2 doping with lanthanides increase the photocurrent, and it implies an effective separation of electron-hole pairs [6].

Experimental design.

The TiO2 doping with metallics elements was made according to the following experimental designs:

In early studies, anatase TiO2 was believed to be a more efficient photocatalyst than rutile TiO2 due to its higher Fermi level and a higher degree of hydroxylation [7]. On the other hand, the incorporation of doping ions can modify the phase temperature transition from anatase phase to rutile. Therefore a variation in the thermal treatment temperature was made to stabalize the anatase phase according to the XRD results. Only samples of TiO2 doping with 3% mol were employed for this experimental design showed in Table 1.

Table 1. Experimental design for the TiO2 doped with metallic ions (3% mol) by the temperature variation.

Ti O2 Ti O2:3 % Cu Ti O2:3 % Ag Ti O2:3 % Eu

500° C 600° C 650° C 700° C 800° C 800° C
When the temperature of the stable Anatase phase was found, the doping with 1, 2 and 3% mol was made, according to the experimental design in Table 2. The structural, electronic and optical properties of TiO2 was studied and finally, the dye degradation was evaluated (photocatalytic test). The thermal treatment temperature was 500°C for the samples of TiO2 pure, TiO2:Cu and TiO2:Ag and 800°C for TiO2:Eu, due to a very well crystallization of anatase phase.

2.1.5 Synthesis procedure of TiO2: Metal.

Materials.

The pure TiO2 and doped were synthesized by the soft chemical method sol-gel using titanium isopropoxide (IV) as a reactant. The reactants used are listed below:

 Titanium (IV) isopropoxide (TTIP) of Strem Chemicals (98%).

 Acetic acid of Fisher Scientific.

 2-Propanol (99.5%) de Alfa Aesar GmbH.

 Anhydrous copper (II) acetate (98%) of Alfa Aesar GmbH.

 Hydrated europium (III) acetate (99.9%) of Strem Chemicals  Silver acetate(99%) of Strem Chemicals.

2.1.5.2. Synthesis of pure TiO2.

The synthesis procedure for TiO2 nanoparticles consists in adding 3.3 ml of TTIP to 8.85 ml of 2propanol used as solvent. The mixture was kept under vigorous magnetic stirring for 15 min at room temperature. Then, 3.0 ml of acetic acid was added and the stirring performed for 30 min.

The obtained product was dried at 120°C for 48 hours and then calcined in air at 500 and 800 °C.

2.1.5.3 Synthesis of TiO 2 doped with metallic ions powders.

The experimental procedure for the synthesis of metal-doped TiO2 nanoparticles (with a metal ratio of 1%, 2%, and 3 mol. %) was performed by using TTIP and ethanol as solvent under vigorous magnetic stirring for 15 min. For the doping process, appropriate amounts of copper acetate or silver acetate were added to the solution and stirred until a clear solution was formed, and then the procedure described above was followed. For the Eu-doping, the solution was hermetically closed and ultrasonicated for 2 hours, then heated at 60°C in an oven for 12 hours and ultrasonicated again for 2 hours before adding the acetic acid Then the sols were dried at 120 °C. Cu or Ag-doped TiO2 were calcined in air at 500 °C with the anatase phase stabilized at this temperature. For Eu doping, the product was calcined at 800 °C due to the stabilization of the anatase phase at a higher temperature compared to doping with the other elements. The samples calcined at 500 °C and 600 °C show only the anatase phase, (JPDCS-01-089-4921).

The first peak in 2theta=~27° appear from 700°C, and it is related to the peak in the plane (110) from the rutile phase (JPDCS 01-086-0147). At 800 °C the main peak related is more intense that the main peak of the plane (101) from the anatase. It suggests that at 800 °C the predominant phase is rutile. The temperature of the phase transformation depend on the synthesis conditions fo TiO2 nanoparticles.

Phase identification of TiO2 powders by Raman spectroscopy.

The anatase and rutile phases of TiO2 can be precisely identified from their Raman active modes related to the involved crystalline structure. The anatase phase shows major Raman bands at 144, 197, 399, 515, 519 (superimposed with the 515 cm -1 band), and 639 cm -1 . These bands can be attributed to the six Raman-active modes of anatase phase with the symmetries of Eg, Eg, B1g, A1g, B1g, and Eg, respectively. The typical Raman bands due to rutile phase appear at 143

(superimposed with the 144 cm -1 band due to anatase phase), and the ones 235, 447, and 612 cm -1 , which can be ascribed to the B1g, two-phonon scattering, Eg, and A1g modes of rutile phase, respectively. Additionally, the band at 144 cm -1 is the strongest one for the anatase phase and the band at 143 cm -1 is the weakest one for the rutile phase [8][9][10][11].

The Raman modes of the samples of pure TiO2 annealed at 500 °C and 800°C are shown in the Fig. 14. For the pure TiO2 annealed at 500°C the peaks at 144, 395, 514 and 636 cm -1 corresponds to the anatase phase, while the TiO2 annealed at 800 °C, shows peaks corresponding to the anatase and rutile phase. The peaks at 144 and 515 cm -1 correspond to the anatase phase and 235, 447 and 612 cm -1 correspond to the rutile phase. It means that both phases are presents in the sample, these results are in agreement with the X-ray results.

Optical characterization of pure TiO2 powders.

The main mechanism of light absorption in pure semiconductors is direct interband electron transitions. This absorption is particularly small in indirect semiconductors, e.g., TiO2, where the direct electron transitions between the band centers are prohibited by the crystal symmetry.

Considerable enhancement of the absorption is expected in small TiO2 nanocrystals, as well as in porous and microcrystalline semiconductors when the interface atoms is sufficiently large [12]. The diffuse reflectance spectra of pure TiO2 powders annealed at 500 °C is shown in Fig. 15.

They have a strong absorption in the UV region with a low absorption in the visible range. The absorbance spectrum is obtained using the Kubelka Munk function:

(𝑅) = (1 -𝑅) 2 2𝑅
Equation 10

The band gap was obtained using the Kubelka Munk modified function [F(R) x hv] 1/2 vs light absorption energy plot.

A band gap of 3.2 eV was measured for the TiO2 powder annealed at 500 °C. This value is in accordance with the literature. [13][14][15][16].

The absorption in the UV region is associated to the O 2-(2p)Ti 4+ (3d) charge transfer process [17]. In TiO2, the valence band (VB) is composed of O 2p states, and the conduction band (CB) is composed of Ti 3d states [13].

2.2.4 Chemical Analysis by X-ray Photoelectron Spectroscopy o f pure TiO2 powders.

The XPS technique was employed to explore the chemical states of the elements on the surface sample. In this section, TiO2 annealed at 500 °C was the only one analyzed. Subsequently, in the next sections it will be compared with the doped systems.

The XPS spectra of O 1s of TiO2 in the Fig. 16 a) are attributed to lattice oxygen (530.3 eV), surface hydroxyl oxygen (531± 0.5 eV) and adsorbed oxygen (533±1 eV) [18,19]. Two wellresolved peaks at 464.5 and 458.9 eV were observed from the Ti 2p core-level spectrum ( Fig. 16 b), which can be assigned to Ti 2p1/2 and Ti 2p3/2 spin-orbital components in TiO2, respectively [20]. XPS results indicate that TiO2 has a stoichiometric composition, no impurities that can modify the peaks position were detected. The most important parameter of this equation is (k), named as constant rate or constant of reaction with units in min -1 , Fig. 18. The higher the k constant and the more efficient is the photodegradation of methylene blue. All the photocatalysis experiments were done under the same conditions for a real effective comparison. When metal ion dopants are used, the properties of TiO2 are modified The modifications are not only dependent on the type of dopant, but also on its concentration and distribution. It has been demonstrated that the transformation rate is dependent on the conditions of doping. Indeed, the way that the dopant is incorporated, and also its oxidation state, chemical environment, and distribution, could have an effect on the phase transformation [21].

Doping of Cu increases the concentration of grain boundary defects. Oxygen vacancies generated by doping migrate to the grain boundary and thereby increases defect content in this region [22]. At elevated temperatures, the substitution of Ti 4+ by Cu 2+ increases the oxygen vacancy concentration and decreases the free electron concentration. The excess of oxygen vacancies created in the TiO2 crystal lattice is responsible for anatase to rutile phase transition.

Dopants with an oxidation state of 3+ or lower, when placed in the titania lattice points, create a charge-compensating anion vacancy and cause a transformation to the rutile phase [23]. In this section, the TiO2 powders were doped with copper in concentrations of 1%, 2% and 3% mol, by the sol-gel method. X-ray diffraction was employed for phase determination and calculation of crystallite. The samples were annealed at 500 °C to stabilize the anatase phase.

According to the Fig. 20, all the peaks correspond to the anatase phase. It must be noted that the presence of copper oxide cannot be observed even in the sample with the highest copper content. The similarity in ionic radius of Cu 2+ (0.73 Å) to that of Ti 4+ (0.64 Å) enable copper to substitutionally replaces it in the titanium lattice. Cu could be placed in the interstitial sites due to rTi < rCu producing the strain of the titania lattice, and hence a displacement of the (1 0 1) signal in the XRD pattern [2,23]. The diffractograms of TiO2 doped with copper were fitted using High Score Plus software, for determining the peaks position in 2theta corresponding to the main plane of the anatase phase (101), and for the measurement of the FWHM, showed in Table 4. The peak positions of the plane (101) was located at values very similar in 2theta. The FWHM changed due to the doping effect, this variation is related to the evolution of the crystallite size caused by the insertion of Cu 2+ . No Raman modes from copper oxide were detected by this technique. The observed ones related to anatase were located at 43, 396, 516 and 641 cm -1 , see Fig. 21. An adjustement of the Raman line was realized for the main vibration mode Eg1, using the Voigt function for changes in the Raman shift and the FWHM caused by the doping effect, see Table 5.

The Eg peak is associated with the symmetric stretching vibration of O-Ti-O in TiO2 [10,13]. No particular shift of the Raman band position was detected while a net broadening of the FWHM was observed for the doping samples. Parker and Siegel [24], reported that this broadening of the Raman peaks is due to the presence of oxygen vacancies. The formation of oxygen vacancies near of the Cu is theoretically possible in TiO2 doped with copper [25].

2.3.4 BET surface area measurements of TiO2:Cu powders.

The BET surface area, pore volume and pore size of the pure TiO2 pure and the TiO2 doped powders with copper are shown in Table 6. The doped sample has a lower surface area than the pure TiO2 powder. This behavior could be due to the agglomeration process or ripening caused by the presence of oxygen vacancies [26][27][28]. The particle size was determined by the BET surface area results. The involved interactions between electronic spin and the applied magnetic field as well as with the nuclear spin are summarized in the following spin Hamiltonian:

𝐻 ̂= 𝛽𝐵 ⃗ 𝑔 ̃𝑆 + 𝐼 𝐴 ̃𝑆 Equation 12
Where B stands for the applied magnetic field, the Landé tensor depending on the electronic structure of the Cu 2+ ion in its local environment, the hyperfine coupling tensor between electronic spin (S) and nuclear one (I). The simulation of the experimental spectrum yields the theoretical values of the EPR spectral parameters summarized in Table 7. The theoretical spectrum is compared to the experimental one (Fig. 22) based on the use of two crystalline sites for Cu 2+ ions. The only difference between spectral parameters lies in the variation in one component of g-tensor (ΔgZ=0.04). The existence of two crystalline host sites for Cu 2+ seems involved in Cu-TiO2 with rhombic symmetry which may occur from the substitution of Cu 2+ in Ti 4+ ions. The distortion of the local sites is expected due to the difference between the ionic radii of Ti (4+): 0.51-0.6 and Cu (2+): 0.65-0.73. The g-tensor components are in the same order of magnitude as those obtained in Cu-doped BiVO4. [29].

Thus, distorted oxygen environments constitute the local hosting sites for Cu 2+ ions. The doping process consists in the incorporation of Cu 2+ ions in the host crystalline sites of TiO2. It is worth noting that there is no occurrence in the host matrix of Cu-rich phase segregation or Cu clusters.

The small EPR signal located in the high magnetic field side correspond to g-factor about 1.96.

This value is well identified for Ti 3+ ions which occur after irradiation of the sample by Xe-lamp which also contain a spectral band in the UV range.

The gx, gy, gz are the components of g-tensor, Ax, Ay and Az the hyperfine components of Atensor and LW the line width of EPR lines. The main goal of doping with metallic ions is to increases the light absorption in the visible region.

The nature of TiO2 allows the absorption in the UV region. However, UV light represent only the 10 % of the solar light. With UV-Vis spectroscopy, it is possible to appreciate the optical behavior of the doping systems.

The absorption in the UV region is associated to the O 2-(2p)Ti 4+ (3d) charge transfer process.

The absorption in the visibe range increases for the sample doped with copper due to the combination of the charge transfer processes O 2-(2p)Ti 4+ (3d) and O 2-(2p)Cu 2+ (3d). The absorption band at 600-1000 nm can be attributed to copper transitions 2 Eg-2 T2g for copper ions localized in the symmetry of the distortion octahedral. [30], see Fig. 24. Indeed Cu 2+ has a d 9 electronic configuration, and in the pure octahedral coordination, the 2 D state of Cu 2+ is split into 2 Eg ground state and 2 T2g excited state with a single electronic transition.

UV

Vis Vis

However, from the XRD and Raman spectra of the samples, it was found that doping of Cu 2+

generates oxygen vacancies which lie near Cu 2+ . Therefore, Cu 2+ is no longer in the pure octahedral (Oh) symmetry, and the symmetry is slightly distorted. A Jahn-Teller (J-T) distortion is taking place, changing the symmetry from Oh to D4h. Due to the J-T effect, the ground state 2 Eg is further splitted into 2 B1g ground and 2 Ag excited states, and the 2 T2g is separated into 2 B2g ground and 2 Eg excited states. Therefore, the possible optical transitions that may give rise to the above peaks are 2 B1g → 2 Ag, 2 B1g → 2 B2g, and B → Eg. The Fig. 24 shows the octahedral distortion to tetragonal local symmetry. [13].

The band gap of the doping samples decrease in comparison with the pure TiO2 as it can be observed in Fig. 23 c). Cu 2+ forms sub-band states in the band gap of TiO2. Along with Cu 2+ , oxygen defect band states are also formed in the band gap. In pure TiO2, the electronic transition occurs directly from VB to CB. However, on Cu doping, the electrons are not directly excited to CB since the unoccupied Cu 2+ s-d states and oxygen vacancies capture the electrons. The charged oxygen defect states formed in TiO2 are F (two electrons), F + (single electron), and F ++ (devoid electrons). The oxygen vacancy states that may capture electrons are F + and F ++ , respectively. Therefore, the sub-band states of Cu 2+ and oxygen defects are responsible for the reduction of the effective band gap of TiO2 nanoparticles [13]. The XPS spectrum of Ti 4+ for the 3%Cu-TiO2 can be observed in the Fig. 25 c) and shows a lower binding energy in comparison with the pure TiO2. A decrease of Ti 2p3/2 binding energy is a direct measure of the lowering of valence state level of Ti 4+ . Formation of oxygen vacancies, as a result of doping, results in an electron density decrease and a partial reduction of Ti 4+ to Ti 3+ and to Ti 2+ , which reflects a decrease in Ti 2p3/2 binding energy. [33,34]. This was clearly shown by EPR method with the existence of Ti 3+ EPR spectrum in Cu doped TiO2. In Table 8 is summarized the peaks positions of the XPS spectra previously discussed. The photodegradation of the dye (methylene blue 2x10 -5 Mol) by pure or doped TiO2 powders annealed at 500 °C when were irradiated with light from a Xenon lamp was studied. The irradiation power was from 120 mW/cm 2 and a distance of 70 cm between the lamp and the sample. A magnetic stirring was used during the test.

During the test, the absorbance of the methylene blue was measured with a VIS spectrometer in intervals of 10 minutes. The Fig. 26 shows the methylene blue spectra for the samples of TiO2 doped with copper (1, 2 and 3 %mol). These spectra were integrated for the area under a curve measurements. When Cu 2+ replaces Ti 4+ in the crystalline structure it can act as recombination centers for the electrons and holes photoinduced trough the following process:

𝑀 𝑛+ + 𝑒 𝑐𝑏 -→ 𝑀 (𝑛-1)+ 𝐸𝑙𝑒𝑐𝑡𝑟𝑜𝑛 𝑡𝑟𝑎𝑝 Equation 13𝑀 𝑛+ + ℎ 𝑣𝑏 + → 𝑀 (𝑛+1)+ 𝐻𝑜𝑙𝑒 𝑡𝑟𝑎𝑝𝑠 Equation 14Where the energy level for M n+ /M (n-1)+ lies below the conduction band edge (Ecb) and the energy level for M n+ /M (n+1)+ above the valance band edge (Evb) [19,35,36]. TiO2 powders doped with silver were synthesized by the sol-gel technique and analyzed by X-ray diffraction for the crystalline phase determination. The annealed temperatures were varied to find a stable anatase phase.

Fig. 28 shows the diffractograms of the 3% Ag-TiO2 powder with different annealing temperatures. It can be observed that at 500°C and 600°C only exist the anatase phase. At 700 °C a very well formed rutile phase is present; while for pure TiO2 poweder at this temperature the predominant phase is anatase. This means that silver addition decreases the phase transformation temperature of the system. It is well accepted that the concentration of oxygen vacancies strongly influences the rate of the anatase to rutile transformation. Because the radius of Ag + ion (ca. 126 pm) is much larger than that of Ti 4+ ion (ca. 68 pm), the Ag + ions introduced by the sol-gel process could not enter into the lattice of anatase phase to form a stable solid solution. During the drying and calcination process with the elimination of liquids and organic substances and the crystallization of anatase phase, the uniformly dispersed Ag + ions would gradually migrate from the volume of anatase grains to the surface of the TiO2 powder under the action of heat. The density of surfaces defects at the surface of anatase grains then increases, which favors the rutile nucleation.

On the other hand, due to the high redox potential for the Ag + ion, by heat and TiO2 photocatalytic reduction, the Ag + ions spreaded on the surface anatase grains would gradually be reduced into Ag 0 .

For charge compensation, oxygen vacancies occur:

𝑂 𝑂 = 𝑉 𝑂 •• + 1 2 𝑂 2(𝑔) + 2𝑒 - Equation 15
The concentration of oxygen vacancies at the surface of the TiO2 powder then increases, which facilitates the bond rupture and ionic movement necessary for the formation of the rutile phase.

The anatase to rutile phase transformation is then accelerated [37].

2.4.2 Phase identification by XRD of TiO2:Ag powders. Effect of the concentration.

In this section, TiO2 was doped with silver in concentrations of 1%, 2% and 3% mol, by the solgel method. X-ray diffraction was employed for phase determination and estimation of crystallite size. The samples were annealed at 500 °C.

Fig. 29 shows the diffractograms of the TiO2 doped powders with silver (1, 2 and 3% mol). XRD patterns do not show any noticeable change in the number of diffraction lines due to the presence of the silver dopant. This suggests that the Ag nanoparticles were well scattered located on the surface of TiO2 grains. Due to the higher ionic radii of Ag (1.26 Å), is complicated to incorporate into the lattice structure [38]. The diffractograms of TiO2 doped with silver were refined using High Score Plus software, to determine the involved crystalline structure by the main plane of the anatase phase (101), and for the measurement of the FWHM, reported in Table 9. The peaks position of the plane (101) was located at values very similar in 2theta. The FWHM changed due to the doping effect, this variation is related to the crystallite size being modified by the incoporation of Ag in the host titania.

The presence of structural defects generate a charge decompensation favoring the agglomeration process and therefore the slight increased of particle size, see Table 10. Despite the crystallite size increased by the doping effect, the sample doped with 3% mol has a similar crystallite size than the doped with 1%. It could be due to the silver incorporated on the TiO2 surface that favor the growth of titania nanoparticle. However, the slight increase of the Ag-TiO2 crystallites is contradictory to several reports where doping the structure induce a reduction of the size of particle. Additional BET measurements as discussed below can shed light also on the particle growth due to doping. Raman bands related to oxides of silver were not detected by this technique. The Raman modes of the anatase were shown at 43, 396, 516 y 641 cm -1 , see Fig. 30. For TiO2 doped with silver we may notice additional band at 80 cm -1 which correspond to metallic Ag, [39,40]. A nonlinear fitting of the Raman spectra was realized for the main vibration mode Eg1, using the Voigt function for changes in the Raman shift and the FWHM caused by the doping effect, see Table 5. A shift and broadening of the main Raman mode of anatase Eg1 can be observed for the TiO2 doped powders with silver. Quiñones et al [39], attributed this effect to a volume contraction and radial pressure increase, which consequently changes the force constant and vibrational amplitude of the nearest neighbor bond. The Eg peak is associated with the symmetric stretching vibration of O-Ti-O in TiO2 [10,13].

The intensity of the bands relevant to TiO2 decreases with increasing Ag amount (prolonging reaction time), which implies that there is an interaction between Ag and TiO2 probably due to some plasmon effect which can alter the Raman bands. [41].

2.3.4 BET surface area measurements of TiO2:Ag poweders.

BET surface area measurements are shown in Table 12 and lower for the sample doped with TiO2. The decrease in surface area of Ag-loaded TiO2 powders might be due to a block of fine capillaries of parent TiO2 surface by metal film islands [16]. The diameter particle size was determinate using the BET surface area. Thus, an only low fraction of substitutional silver seems realized in the sample with incorporation in crystalline sites leading to the well resolved hyperfine structure as illustrated by the doublet in Fig. 31. Under irradiation by wide spectral range from Xe-lamp, we notice the development of narrow EPR line which originates from color centers induced by UV radiation in TiO2. However, in contrast to Cu 2+ doping case, there is no evidence of valence change of Titanium ions leading to Ti 3+ . It seems that, in the present case of doping, the color centers are formed preferentially to the formation of Ti 3+ as observed for copper doping. It is worth noting that Ag doping ratio didn't induce important EPR spectra. Even with the highest used ratio of 3 at.%, a low signal on noise ratio remains on the recorded EPR spectrum. This behavior suggests the possibility of Ag rich phases segregated from the host TiO2 structure.

Alternatively, if the valance state is Ag + , no EPR signal may be induced in that case. Optical absorption measurements are then helpful to point out the eventual involvement of Ag metal clusters as discussed below. 2.4.6 Optical characterization of TiO2:Ag powder.

The main goal of doping with metallic ions consistes in an increase of the light absorption in the visible region. UV-Vis spectroscopy is appropriate possible to measure the optical behavior of the doping systems.

The absorption in the UV region is associated to the O 2-(2p)Ti 4+ (3d) charge transfer process [17]. A strong absorption is observed at 600 nm for the TiO2 doped with silver Fig. 32.

This significant change is strongly attributed to localized surface plasmon resonance (LPSRs)

induced by Ag NPs. The presence of Ag NPs on the titania surface had induce charge oscillations of the metallic particles enhancing then the electromagnetic (light) fields in the vicinity of titania [3]. It is also noteworthy that the visible light absorption peaks of the various Ag doping are unchanged, except the intensities but additional plasmonic band can be seen on the optical spectra.

The peak position in the absorption spectrum depends on various factors, as the temperature of annealing, concentration, agglomeration of silver nanoparticles, silver precursors, and particle size, among others. The peak absorption formed in the visible region corresponds to a behavior generated by silver particles on the surface of TiO2 at temperatures higher than 400 °C [42]. XPS spectroscopy was employed for the determination of the chemical state of Ag and Ti in the annealed samples. The Fig. 33 shows the XPS spectrum of 3%Ag-TiO2 powder and it is compared with the pure TiO2 powder.

The strong existence of silver nanoparticles can be distinguished by the center peaks at the binding energies of 368.1 and 374.09 eV corresponding to Ag 3d5/2 and Ag 3d3/2, see Fig. 33 a).

These results confirm the existence of silver nanoparticles as evidenve by Raman spectroscopy [3,41,43]. The existence of AgO wasnt shown as the corresponding lines must appear at the binding energy of 367.7 eV for Ag 3d5/2 [START_REF] Xue | Enhanced conversion efficiency of flexible dye-sensitized solar cells by optimization of the nanoparticle size with an electrophoretic deposition technique[END_REF], besides has been reported that the bonds Ag-Ag and Ti-O are stronger than the chemical bond of Ag-O, which favors the metallic silver cluster formation [45].The XPS spectra for O 1s and Ti2p is shown in the Fig. 33 b) and c). It can be observed that the peaks for the samples of TiO2 pure and doped powders have the same binding energy. For the Ti 2p3/2 its binding energy of 458.70 eV and 464.42 eV for Ti2p1/2. For O 1s a binding energy of 530.07 is expected. In the Table 14 are summarized the binding energies corresponding to the XPS analysis. It can be observed a broadening of the peaks of 2p3/2, Ti2p1/2 and O1s of the 3%Ag-TiO2 powder in comparison with the pure TiO2 powders. According to Young Ku et al. [START_REF] Kuo | Analysis of silver particles incorporated on TiO2 coatings for the photodecomposition of o-cresol[END_REF], as the amount of silver incorporated on TiO2 is increased, the Ti 2p peaks become broader and composed by superposition of XPS at the energies 457.4 eV and 458.7 eV matching the trivalent and tetravalent states of Ti, respectively. It could be a consequence of the presence of Ti 3+ resulting from the incorportation of Ag ions inside the host titania. The photodegradation of organic dyes as (methylene blue 2x10 -5 Mol) by Ag doped TiO2 poweders annealed at 500 °C were irradiated with light from a Xenon lamp. The irradiation power was of 120 mW/cm 2 and the distance of 70 cm between the lamp and the sample. A magnetic stirring was used during the test.

The photocatalysis was characterized by, the absorbance of the methylene blue measured with a VIS spectrometer in intervals of 10 minutes. Fig. 26 shows the optical absorption of MB for the samples of TiO2 doped powders with silver (1, 2 and 3 %mol). These spectra were integrated for the area under a curve measurements. The samples of TiO2 doped with silver show a higher constant rate in comparison of pure TIO2, this is because the Fermi level of Ag is lower than that of TiO2 so that electrons may transfer to the Ag deposited on surface of TiO2, which this induce a decrease in the outer electron cloud density on Ti ion. The accumulation of electrons on silver particles results in the negative charge enrichment of the metal surface. This enhances the separation of reduction and oxidation surface site. Hence, the extent of surface recombination is lowered and availability of holes and electrons for redox. Subsequently, electrons migrate from silver-laden particles to O2 molecules. Moreover, the density of free electrons in Ag-TiO2 coatings is less than in the pure TiO2 coating and will promote the photoabsorption of O2 molecules on the surface of photocatalysts. Therefore, the rate of electron transfer to oxygen increases and the recombination rate of electron/hole pairs decreases [START_REF] Kuo | Analysis of silver particles incorporated on TiO2 coatings for the photodecomposition of o-cresol[END_REF][START_REF] Xin | Effects of Simultaneously Doped and Deposited Ag on the Photocatalytic Activity and Surface States of TiO2[END_REF][START_REF] Takai | Capture, Store, and Discharge. Shuttling Photogenerated Electrons across TiO2-Silver Interface[END_REF]. The samples doped with europium which maintains the anatase phase even at 800 °C (JPDCS-01-089-4921) and only minor fraction of rutile phase appears. This is in contrast to the samples doped with silver and copper at 800°C which show a complete rutile phase was achieved. The samples doping with europium at 1, 2 and 3% mol were annealed at 800 °C. For the sample 3%Eu-TiO2, it could be observed a peak corresponding to Eu2Ti2O7 (JCPDS 01-087-1852), Fig. 37. Yuhong et al [4], established that this remarkable shift of crystallization and the phase transformation to high temperatures is caused by the substitution of titanium ions by europium ions within the structural framework. However, no such a conclusuion can be drawn since we do not show the incorportation of Eu ions in the lattice from the EPR investigations discussed below.

Thus, the location of minor phase, rich in Eu, prevent the coalescence of TiO2 particles. The diffractograms of TiO2 doped with Europium were fitted using High Score Plus software, for determining the peaks position in 2theta corresponding to the main plane of the anatase phase (101), and for the measurement of the FWHM, showed in Table 15Table 4. The peaks position of the plane (101) was located at values very similar of 2theta. The FWHM changed is induced due to the doping effect, this variation is related to the crystallite size caused by the effect of Eu 2+ ant the high annealing temperature.

According to the particle size estimated, see Table 16, it decrease as the europium concentration increase, this due the Eu ions suppresses the growth of TiO2 nanocrystals to a great extent [START_REF] Pal | Effects of crystallization and dopant concentration on the emission behavior of TiO2:Eu nanophosphors[END_REF]. The Raman modes of the anatase are associated to the bands at 43, 396, 516 and 641 cm -1 corresponding to the doping with Europium (1, 2 and 3% mol). For pure TiO2, at 800°C a mix of phases of anatase and rutile is present. The bands corresponding to Eu2Ti2O2 were not identified by Raman spectroscopy, it could be due to the small concentration of the Europium ions As the Raman bands are well resolved and sharp, we may conclude the good crystalline quality of the host titania. The effect of Eu doping seems not to affect the Raman spectra. This is a consequence of phase segragation of Eu from the titania lattice. B 1(g) A

 Rutile      A 1(g) E g B 1(g) (a) (d) (c) (b)
 Anatase   (a) Fig. 38. Raman spectra of TiO2 doped with europium (1, 2 and 3% mol) annealed at 800°C. An adjustement of the Raman spectra was realized for the main vibration mode Eg1, using the Voigt function. The aim is to reval the effect of EU doping on the vibrational propertie of Titania structure, see Table 17. Increased oxygen vacancies in the crystal structure of TiO2 induce a shift of the wavenumber of the anatase Eg mode (144 cm -1 ). This may imply that the discrepancies in the oxygen vacancy level have not been significant enough to cause Raman band shifting. This is understood from the rather limited substitutional Eu 3+ doping in the TiO2 lattice [START_REF] Li | Phase structure and luminescence properties of Eu3+-doped TiO2 nanocrystals synthesized by Ar/O2 radio frequency thermal plasma oxidation of liquid precursor mists[END_REF]. The BET surface area, pore volume and pore size of the TiO2 pure and the TiO2 doped with europium, are shown in Table 18. The sample doping has a lower surface area than pure TiO2.

This could be due to the high annealing temperature for the anatase stabilization, favoring the particles growth and agglomeration. The particle size was determined by the BET surface area method. Hoewever, we may note that the great difference in the annealing temeprature can explain such difference in the particle size. The role of Eu doping is quite minor at this stage s ince as shown by EPR (seem below) the doping ions have tendancy at this temperature to segregate from host titania.

2.5.5 EPR evaluation of TiO 2 :Eu powders.

The Eu doping seems the more critical among the above-used elements. The features of the spectra are less resolved and we do not see the characteristic U forms which are reported for this ions in its valence state Eu 2+ . In that case, the electronic spin is S=7/2 and characteristic lines must be seen in the spectra. Instead, we only notice large signals (Fig. 39) with intensities correlated with the doping ratios (1, 2, 3 at.%). However, the absence of characteristic (S=7/2) Eu 2+ EPR lines due to the incorporation of Eu ions in the host crystalline sites can be understood by the large departure between Eu ionic radius almost twice that of TiO2. However, the existence of weak and broad EPR signal with intensity correlated with the doping rates underlines the involvement of Eu 2+ in the samples. The weak intensity reflects a small fraction of Eu ions in the valence states Eu 2+ and the large line width can be explained probably by clustering of Eu 2+ ions submitted to exchange interactions. According to XRD investigations have shown a phase segregation with rich europium ions as Eu2Ti2O7 within the titania host matrix. As the valence state Eu 3+ , no EPR signal can be obtained from such ions. Also, the above results are consistent with the Europium ions being mainly in the valance state Eu 3+ . In addition, the exposure of Eu-TiO2 to Xe-Lamp radiation as it was experienced with the other doping ions, didn't show any modification of the EPR spectra. This means that the photoactive efficiency of Eu-doped titania must be quite different compared to the other doping elements as it is discussed in forthcoming section.

EPR s ignal Fig. 39. EPR spectra of Eu-TiO2 at different concentrations 3 at.% (grey line), 2at.% (thick black line), 1 at.% (dashed line).

2.5.6 Optical characterization of TiO2:Eu powders.

The samples doped with Europium show an optical behavior similar to that of the pure TiO2. The absorption in the UV region is associated to the O 2-(2p)Ti 4+ (3d) charge transfer process [17].

The absorption peaks of Eu 3+ appear at 210 and 250 nm for the transitions 5d 4f, and at 394 and 464 nm for 7 F0 5 L6 and 7 F0 5 D2, see Fig. 40 a).

The band gap decrease when the concentration of the rare earth dopant increase [START_REF] Štengl | Preparation and photocatalytic activity of rare earth doped TiO2 nanoparticles[END_REF]. The variation in the optical band edge could occur due to the incorporation of localized impurity states owing to Eu3 + doping [5], see Fig. 40 c). XPS spectroscopy was employed for the determination of the chemical state of Eu and Ti in the annealed samples. The Fig. 41 shows the XPS spectrum of 3%Eu-TiO2 and it is compared with the TiO2 pure.

The peak at 1134 eV of the doping sample corresponds to the Eu 3d5/2. The binding energy positions were attributed to Eu(III). There were observed two peaks at 1125 and 1154 eV that could be attributed to shake satellites of Eu 3d. These peaks have also been attributed to Eu(II)

present on the surface. It is possible that Eu(III) will be reduced to Eu(II) when oxygen defects are created [START_REF] Kang | Full characterization of Eu(OH)3 and Eu2O3 nanorods[END_REF].

The The photodegradation of the dye (methylene blue 2x10 -5 Mol) by TiO2 doped annealed at 800 °C were irradiated with light from a Xenon lamp. The irradiation power was from 120 mW/cm 2 and a distance of 70 cm between the lamp and the sample. In solution also submitted to magetic stirring During the photocatalytic process test, the absorbance of the methylene blue was measured with a VIS spectrometer in intervals of 10 minutes. The Fig. 42 shows the methylene blue spectra for the samples of TiO2 doped with europium (1, 2 and 3 %mol). These spectra were integrated for the area under a curve measurements. According to some reports, doping with concentrations lower than 0.5 mol, improve the photocatalytic degradation. However when the concentration of doping ions is high, the space charge region becomes very narrow and the penetration depth of light into TiO2 greatly exceeds the space charge layer; therefore the recombination of the photogenerated electron-hole pairs in semiconductor become easier decreasing the photocatalytic activity of the TiO2 for the methylene blue degradation process [6,[START_REF] Štengl | Preparation and photocatalytic activity of rare earth doped TiO2 nanoparticles[END_REF]. Excess amounts of rare earth oxide covering the surface of TiO2 would increase the number of recombination centers and result in low photoactivity [START_REF] Khataee | Europiumdoped ZnO as a visible light responsive nanocatalyst: Sonochemical synthesis, characterization and response surface modeling of photocatalytic process[END_REF].

2.6 .Conclusions.

Comparative conductivity versus doping.

Electrical characterizations were carried out on the pure and metal-doped TiO2 in order to compare the thermal behavior of their electrical conductivity in the permanent regime. The obtained results are summarized in a semi-logarithmic plot depicted in Fig. 44, where thermally activated conductivity was reported with values up to 0.1 S.m -1 . However, irrespective to the doping element, no improvement of the conductivity was realized by doping and even the tendency is a decrease of the conductivity for doping elements such as Cu and Eu. Several reasons may be considered to account for the decrease of the conductivity. Particularly, due to the required neutrality in the substitutional sites, doping may induce the creation of oxygen vacancies which act as traps for electrons of TiO2 usually considered as an n-type semiconductor.

The presence of oxygen vacancies induces the formation of energetically favorable valence state as Ti 3+ . This is clearly demonstrated by EPR investigations in Cu-doped samples by the occurrence of EPR spectra related to paramagnetic Ti3+ involved in the lattice with a density estimated to 10 17 Ti 3+ ion/gram. Thus, EPR and conductivity measurements give complementary information on the role of Cu 2+ ions in TiO2. For doping by Ag and Eu, composite systems are formed and consist of metallic Ag clusters or Eu2Ti2O7 minor phase embedded in the TiO2 host media. Such a departure to stoichiometric materials and the heterogeneous distribution of minor phases or clusters limit the charge carrier mobility reducing the hole conductivity of the doped samples compared to pure sample. where σ is the sample conductivity (S/cm) at temperature T (Kelvin); σ0 the pre-exponential factor and represents the nominal conductivity at infinite temperature; Ea is the activation energy for the conduction process (J mol -1 ) and Rg is the Universal Gas constant 8.3141 (J mol -1 K -1 ). In the work discussed below, ln(r) is plotted against 1000/T hence multiplying the slope of the plot by Rg, obtains Ea in kJ mol -1 which is the more usual unit (to convert activation energy in kJ mol -1 to eV/ion, Ea should be divided by 96.485). The activation energy can be physically interpreted as the energy barrier that must be overcome for an ion to conduct [START_REF] Mccarter | Conductivity/activation energy relationships for cement-based materials undergoing cyclic thermal excursions[END_REF]. The activation values are presented in two temperatures range according to the plot, see Table 19 2.6.2 Comparative performance versus doping .

Comparative investigations were devoted to metal-doped TiO2 where doping process has shown versatile behavior and dependence on the nature of the used elements. Even, with identical and stable valence states such as (Cu 2+ , Ag 2+ , Eu 2+ ), different behaviors were demonstrated for the effective incorporation of these ions in the host structure of TiO2. The discrepancy between ionic radii of the different used elements modulates the ratio of the substitutional doping. This is indeed achieved for Cu 2+ but in less extent for Ag 2+ while Europium ions form segregated phase as Eu2Ti2O7. The structural features are also dependent on the doping nature. This is illustrated by an increase or a decrease of the anatase-to-rutile phase transformation temperature.

Interestingly, in the case of Eu-TiO2 nanoparticles, europium preserves the anatase phase at 800°C, while the other samples show a complete rutile phase transformation at a lower powders [1]. The present chapter focuses on the elaboration of thin films of TiO2 doped with metallic ions (Cu, Ag, and Eu) by two methods of fabrications: spin coating and dip coating.

3.1 Surfactants and nanoscience.

Surfactants are amphiphilic molecules-that is, they have one part prefers water and another part prefers oil. These dual characteristics give them a range of properties connected to two key behaviors-adsorption at interfaces and self-assembly in solution. The molecular group possesses the head group which may be charged or may be polar due to the inclusion of oxygen or similar atoms; the tail group generally comprises hydrocarbon chains [2].

It is clear that a vast range of chemical groups can fulfill these requirements, and so, a large number of chemically different surfactants are possible.

Among the well known surfactant, Pluronic F 127 was employed for the fabrication of the films.

The objective is to increase the surface area for increasing the contact between the semiconductor and the dye (methylene blue), in the photocatalysis test.

Experimental design.

The TiO2:M films elaboration was carried out following the next experimental design, see Table 20. The experimental procedure is detailed below.

Materials.

The TiO2 pure and doped were synthesized by the soft chemical method sol-gel using titanium isopropoxide (IV) as a precursor. The reactants used are listed below:

 Titanium (IV) isopropoxide (TTIP) of Strem Chemicals.

 Acetic acid of Fisher Scientific.

 2-Propanol (99.5%) de Alfa Aesar GmbH.

 Anhydrous copper (II) acetate (98%) of Alfa Aesar GmbH.

 Hydrated europium (III) acetate (99.9%) of Strem Chemicals  Silver acetate (99%) of Strem Chemicals.

 Pluronic F-127.

3.3.2. Preparation of the sol for TiO2 thin films elaboration.

The synthesis procedure for TiO2 nanoparticles consists in adding 3.3 ml of TTIP to 5.8 ml of 2propanol used as a solvent. The mixture was kept under vigorous magnetic stirring for 15 min at room temperature. Then, 3.0 ml of acetic acid was added and the stirring performed for 30 min.

In another flask, 0.52 g of F127 was mixed with 3 ml of 2-Propanol and were stirred until getting a complete dissolution. The both solutions were mixed and stirred for 30 minutes. The sol was kept close to avoid the contact with humidity of the environment.

3.3.3 Preparation of the sol for TiO2 doped with metallic ions.

The experimental procedure for the synthesis of metal-doped TiO2 films (with a metal ratio of 3 mol. %) was performed by using TTIP and ethanol as solvent under vigorous magnetic stirring for 15 min. For the doping process, appropriate amounts of copper acetate or silver acetate were added to the solution and stirred until a clear solution was formed, and then the procedure described above was followed. For the Eu-doping, the solution was hermetically closed and ultrasonicated for 2 hours, then heated at 60°C in an air oven for 12 hours and ultrasonicated again for 2 hours before adding the acetic acid. In another flask, 0.52 g of F127 was mixed with 3 ml of 2-Propanol and were stirred until getting a complete dissolution. The both solutions were mixed and stirred for 30 minutes. The sol was kept close to avoid the contact with humidity of the environment.

Cleaning of the substrates.

Two kinds of substrates were employed: borosilicate and silicon crystalline. The silicon crystalline was employed for AFM analysis and Raman spectroscopy. The substrates were immersed in ethyl alcohol and were ultrasonicated for lapses of ten minutes. The substrates dimensions were 15 mm x 15 mm. For the films of TiO2 pure and TiO2:M annealed at 450 °C the anatase phase is present ((JPDCS-01-089-4921). The film of Ag3%-TiO2 shows a better peak definition in comparison with the other samples, it could be due the radius of Ag + ion (ca. 126 pm) is much larger than that of Ti 4+ ion (ca. 68 pm), the Ag + ions introduced by the sol-gel process could not enter into the lattice of anatase phase to form a stable solid solution. During the drying and calcination process with the elimination of liquids and organic substance and the crystallization of anatase phase, the uniformly dispersed Ag + ions would gradually migrate from the volume of anatase grains to the surface and further to the surface of the TiO2 powder under the action of heat. The density of surfaces defects at the surface of anatase grains then increases, which favors the rutile nucleation. Therefore the crystallization is improved for this system in comparison with the TiO2 pure.

In the case of the film of 3%Cu-TiO2, according to the results for the same system in powder, there is a copper incorporation in the titania lattice, favoring the presence of oxygen vacancies.

The diffractogram shows a quality similar than in pure TiO2.

For the film of 3%Eu-TiO2, the peaks intensity have not a good definition; it means that the film has not a good crystallization. This result is in accordance with the already reported results for powders. Europium ions will inhibit the phase transition of anatase-to-rutile through the formation of a Ti-O-Eu bond. It is necessary to increase the annealing temperature to obtain well crystalline anatase phase. However at higher temperature can contaminate the semiconductor with impurities of the substrate. For pure and doped TiO2 Raman bands at 145, 396, 516 y 641 cm -1 were detected corresponding to the anatase Raman modes for the films elaborated by spin and dip coating. An extra signal corresponding to silicon of the substrate was detected. Because of the thickness of the samples, the beam can pass through the film and detect the substrate.

For the TiO2 doped with silver samples a peak at 80 cm -1 were detected and correspond to metallic Ag [3,4]. The films elaborated by spin coating present higher intensity in comparison with the films elaborated by dip coating. It could be due to many factors, like thickness, roughness, a surface cover area among others. However, exists another factor that can affect the signal of the spectra as the focus made with a microscope in the equipment.

The doped films show a low shift in the main peak of anatase at 144 cm -1 , in similar way as already observed in powders. The morphological analysis was done by AFM analysis. Information as thickness and morphology were obtained.

In the Fig. 48 and Fig. 50 can show the topographic images from the films elaborated with spin and dip coating respectively. It can be observed that even when it was employed the same solution the morphology obtained was different due to the experimental conditions used for each technique.

It is important to mention that each film was synthesized by superoposition of theree deposition run. This procedure allow to obtain an increasing of the thickness and roughness of the surface area. Therefore the contact surfaces increase between the film and the dye in the photocatalysis test.

Accordance to the literature, it was observed that the addition of dopant elements in the solution increase the contact angle between the solution and the substrate, this information is important for the interpretation of the obtained results. [5,6]. A low contact angle means that the strong adhesion between the solution and the substrate is predominant, on the other hand, a high contact angle means that the cohesion forces in the solution are stronger, so it is more difficult the adhesion with the substrate. The morphology obtained by spin coating is the result of the centripetal force combined with the surface tension of the solution which pushes the solution for creating films uniformly, see Fig. 48.

The horizontal design of this technique can elaborate uniform films and besides a higher thickness in comparison with the films elaborated by dip coating, according to Table 21.

It can be observed that the film of pure TiO2 has a lower thickness than the doping films for the samples elaborated by spin coating. It is due to the low contact angle for the film of pure TiO2, favoring the formation of a homogeneous flat film, see Fig. 49 a). The TiO2 doped films present higher roughness and thickness in comparison with the pure TiO2; it means that the contact angle of the solution increase forming a heterogeneous film, Fig. 49 b).

Th1 Th2

Pure TiO2 Indeed, in amorphous materials, the absorption due to the band-to-band transitions that determine the optical band gap, Eg, is given by:

(∝ ℎ𝑣) = 𝑞(ℎ𝑣 -𝐸𝑔) 𝛾 Equation 18
where Eg is the optical band gap, q is a constant, and γ = 0.5 or 2 for indirect and direct optical transitions, respectively [8].

It can be observed that for the films elaborated by dip coating the transmittance in the visible region is higher in comparison with the films elaborated by spin coating. It could be due the low thickness and low surface area which allows to the visible light pass crossing through the film.

TiO2 particles, which may cause a decrease in the number of surface active sites and an increase in the opacity and the light scattering of TiO2 particles more likely to decrease the passage of radiation through the sample [7]. 3.8 Photocatalytic evaluation of T iO2: M films.

The photodegradation of the dye (methylene blue 2x10 -5 Mol) by pure TiO2 annealed at 500 °C when were irradiated with light from a Xenon lamp was studied. The irradiation power was from 320 mW/cm 2 and a distance of 35 cm between the lamp and the sample. A magnetic stirring was used during the test.

During the test, the absorbance of the methylene blue was measured with a VIS spectrometer in intervals of 10 minutes. The Fig. 53 and Fig. 54 shows the methylene blue optical absorbance spectra for the films of pure TiO2 and TiO2:M elaborated by spin coating and dip coating respectively. These spectra were integrated for the area under a curve measurements. the volume of anatase grains to the surface and further to the surface of the TiO2 powder under the action of heat. Therefore silver could not be on the surface of the TiO2 as in the system powders, reducing the electronic charge transfer between the TiO2 and the silver nanoparticle [9].

In the case of the films doped with Europium, even when its photocatalytic efficiency is lower than pure TiO2, its efficiency is similar than the obtained by the films doped with silver. It could be due to the temperature and the thermal treatment was lower in powders than the films, avoiding the formation of secondary compounds as Eu2Ti2O7, which decrease the photocatalytic efficiency [10].

For the films elaborated by spin coating they show more photocatalytic efficiency than the films elaborated by dip coating. The possible origin is that the films elaborated by spin coating have more roughness and percentage of the cover area according to the AFM analysis. The photocatalytic efficiency is strongly influenced by the semiconductor concentration. These properties improve the contact between the semiconductor and the dyes favoring the degradation reactions.

Spin coating

Conclusions

The film morphology obtained by the two techniques were completely different. The films elaborated by spin coating show higher photocatalytic degradation, due to the higher thickness, roughness, and percentage of covered area. These properties are obtained due to the centripetal force combined with the surface tension of the solution which pushes the solution for creating films uniform.

The films elaborated by dip coating have a low percentage of covered area. It could be due to the effect of the high contact angle, which indicates strong cohesion force in solution and then difficult adhesion with the substrate. Another important factor is the concentrations of acetic acid, which for which the vapor can dilute the formed film when it is withdrawing from the solution.

CHAPTER IV TiO2 Nanotubes

IV. TiO2 nanotubes for photocatalysis applications.

Introduction.

The fabrication of titania nanotubes by hydrothermal synthesis is performed by reacting titania nanopowders with an alkaline aqueous solution. While the hydrothermal method of titania nanotube production has been comprehensively investigated in the past decade, the formation mechanisms, compositions, crystalline structures, thermal stabilities, and post-treatment functions remain areas of debate [1].

As a low-temperature technology, hydrothermal synthesis is environmentally friendly in that the reaction takes place in aqueous solutions within a closed system, using water as the reaction medium. This technique is usually carried out in an autoclave (a steel pressure vessel) under controlled temperature and pressure. The operating temperature is held above the wáter boiling point to self-generate saturated vapor pressure. The internal pressure generated in the autoclave is governed by the operating temperature and the presence of aqueous solutions in the autoclave. TiO2 nanotubes are obtained when TiO2 powders are mixed with 2.5-20 M sodium hydroxide aqueous solution maintained at 20-110 °C for 20 h in the autoclave [2].

The hydrothermal method is widely applied in titania nanotubes production because of its many advantages, such as high reactivity, low energy requirement, relatively non-polluting set-up and simple control of the aqueous solution. The reaction pathway is very sensitive to the experimental conditions, such as pH, temperature and hydrothermal treatment time, but the technique achieves a high yield of titania nanotubes cheaply and in a relatively simpler manner under optimized conditions. There is three main reactions steps in the hydrothermal method: (a) generation of the alkaline titanate nanotubes; (b) substitution of alkali ions with protons; and (c) heat dehydration reactions in air. The hydrothermal method is amenable to the preparation of TiO2 nanotubes with different crystalline phases such as the anatase, brookite, monoclinic and rutile phases. [1].

Experimental design.

Taguchi method is a very useful tool to solve the complex and confusing problems with fewest variables and fewer tests in many areas. It uses an orthogonal array to determine the effect of the variables on characteristic properties and to optimize conditions of selected variables. This statistical method was successfully utilized to optimize the preparation condition of ceramic materials which have been reported elsewhere. [4].

The Taguchi method uses orthogonal arrays to organize the parameters affecting the process and the levels at which they should be varied. It allows for the determination of factor mostly affecting a process performance characteristic with a minimum amount of experimentation [5].

Taguchi method has been employed to determine the best synthesis conditions to get TiO2 nanotubes. The conditions variables studied were pressure, time, temperature and molar ratio TiO2/NaOH. The experimental matrix is shown in Table 22 were formulated in accordant to the parameters that affect the morphology and the properties from the nanotubes during the synthesis, [6,7]. By the parameters and levels used, an L9 orthogonal array was employed and is shown in Table 23; it established 9 experiment to study the parameters effect.

Chapter The TiO2 nanotubes synthesis were made by the methodology employed for Kasuga, through a sol-gel method for TiO2 nanoparticles and hydrothermal with control of pressure for TiO2

nanotubes with an alkali treatment of NaOH at a temperature around the boiling point of the water.

 Titanium (IV) isopropoxide of Strem Chemicals.

 Acetic Acid.

 2-propanol (99.5%) of Alfa Aesar GmbH.

 Triton X-100 4.2.2Sol-Gel method.

The synthesis procedure for TiO2 nanoparticles consisted in mixing 3.3 ml of titanium tetra isopropoxide with 5 ml of 2-propanol for 15 minutes with magnetic stirring at room temperature.

Then 3.3 ml of acetic acid were added to the previous solutions and was kept it during 30 minutes for to get a very homogeneous solution.

In another recipe, 1,4 gr de Triton X-100 were mixed with 3.8 ml of 2-propanol. Both solutions were mixed and stirred until transparent solutions were obtained and then were dried at 120 °C for 12 hours and then were annealed in air at 500°C for three hours. The TiO2 powders treated by the hydrothermal method were analyzed by the X-ray diffraction for the phase analysis. The Fig. 57 shows that for the samples TG-1 to TG9 and TiO2 nanoparticles have the characteristics peaks from the planes of anatase phase (JPDCS-01-089-4921). A small fraction of Brookite is presented with a peak in 30.8 grades; this peak was generated due to the presence of the surfactant Triton X-100. There is not the presence of another phase reported by other researchers, as titanates, H2TinO2n+1, NaOH or Na2Ti3O7 among others. [6,8,9]. It could be due to the repetitive washing steps with HCl that can eliminate all the sodium in the systems. The post thermal treatment brings on the compositional change from H2TinO2n+1 to TiO2 [10]. A nonlinear fitting of the main peak of the anatase phase (Eg1), was realized using the Voigt function, to detect a shift in the Raman signal and a change in the dimensions of the Full width at half maximums (FWHM) is shown in Table 24. The peak Eg1 is related to the symmetric vibration of O-Ti-O in the TiO2. [11,12]. A shift in the Raman peaks was observed for the samples from TG1 to TG9 in comparison of the pure TiO2 and a widely of the FWHM. This behavior is due to the change in the particle size during the hydrothermal process and the presence of oxygen vacancies. The diffuse reflectance spectroscopy was used to analyze the optical properties and calculate the band gap of the TiO2 nanotubes.

The diffuse reflectance spectra of TiO2 nanotubes has a similar behavior than TiO2 nanoparticles, Fig. 59. No other peaks were observed except around 350 nm in the diffuse reflectance spectra of the TiO2 systems, associated to the O 2 (2p)  Ti 4+ (3d) charge transfer process [13].

The samples (TG1-TG9) increase their absorption in the visible region in comparison to the pure TiO2. It could be due to the presence of oxygen vacancies that can effectively extend the visible light absorption, and the band gap narrowing is closely related to the O vacancy concentrations.

The oxygen vacancies were created by the hydrothermal process [14]. When the concentration of oxygen vacancies is increased, the impurity states become more delocalized and overlap with the valance band edge, resulting to the band gap narrowing [15].

Compared with the conventional doping methods, an oxygen vacancy is a kind of self-doping without introducing any impurity elements, which is more favorable for preserving the intrinsic crystal structures of the semiconductor and leads to enhanced photocatalytic performances under visible light irradiation [12,15]. A qualitative analysis was made using the SEM images, to evaluate the percentage of nanotubes formation in the hydrothermal process.

The hydrothermal process with a controlled pressure mechanism has been barely using it; the conventional hydrothermal is the method widely used it for the obtention of TiO2 nanotubes. Due in the hydrothermal process exists many parameters to establish that is the reason for the employment of Taguchi method for to know the role of the parameter in the formation of nanotubes.

The analysis of the Taguchi method was made taking in account the formation of TiO2 nanotubes versus the parameter varied, as is shown in the Fig. 63. According to the results, the highest conditions of temperature, pressure and time promote the formation of TiO2 nanotubes. In the case of NaOH/TiO2, the intermediate condition shows the highest value of TIO2 nanotubes formation. These results are in concordance with the obtained for others authors, employing a conventional hydrothermal method [16][17][18]. Surface Area Determination of TiO2 nanotubes by the BET method (Branauer, Emmet y Teller).

The BET surface area were determined by BET technique; the results are shown in Table 25. It is evident that the samples that were treated by the hydrothermal method have a BET surface area higher than pure TiO2 synthesized by sol-gel method; even the samples with non-tubular morphology the surface area increased. The methylene blue was degraded by the TiO2 powder under irradiation from Xe lamp. The irradiation incident power on the samples was of 60 mW/cm 2 : and the distance between the lamp and the sample is 70 cm. The initial methylene blue concentration is 2x10 -5 Mol. During the test, a continuous magnetic stirring was done. The evolution of the optical absorption of methylene blue as from TiO2 nanoparticles and TiO2 nanotubes photocatalysis is illustrated in the Fig. 64 and Fig. 65 respectively. The peak integration is measured to obtain the kinetics of degradation. The photocatalytic efficiency is evaluated by the kinetics of degradation as is shown in the Fig. 66. Is evident that the morphology plays a very important role in the photocatalysis process. The samples with a nanotubular morphology (TG2 to TG7) have a higher constant rate in comparison with the pure TiO2. The samples TG8 and TG9 with a spherical morphology have a constant rate lower than pure TiO2 even when they have a surface area similar than the samples with nanotubes; this is because the recombination rate is higher in nanoparticles than nanotubes. Sun et al. reported that tubular structure facilitates the electronic mobility due the reduced grain boundaries, which is expected that improve the charge separation for photocatalytic reactions [19].

Nanotubular materials are of great interest because of their exceptional electronic and mechanical properties, high photocatalytic activity and superficial area and pore volume [20].

Compared to spherical particles, one-dimensional TiO2 nanostructures could provide a high surface area and a high interfacial charge transfer rate. The carriers are free to move throughout the length of these nanostructures, which is expected to reduce the e-/h+ recombination probability. Titania nanotubes have a relatively high surface area compared to non-porous titania, and time-resolved diffuse reflectance spectroscopy has shown that charge recombination is disfavored by the tubular morphology of the titania [21].

4.4 Conclusions.

The hydrothermal method offers an advantage as high reactivity, low energy requirements and simple control of aqua solutions in nanotubes fabrication. The Taguchi method leads to knowing the optimal parameters for nanotubes synthesis. It will be interested in consider higher intervals of parameters in a new experimental design to know the effect of the parameters in the tubular formation.

It has been shown that tubular morphology is more efficient for methylene blue degradation in comparison of TiO2 nanoparticles. The presence of oxygen vacancies and the high surface area generated during the morphological change from spherical to tubular in the hydrothermal method can improve the degradation of the dye. Beside promote the decreasing of electron-hole recombination by the charge transfer trough the length of nanotubes.

Finally it is important to mention that tubular morphology is better photocatalyst in comparison with TiO2 doped with silver.

V GENERAL CONCLUSIONS

V General conclusions.

The present work pretended to study to the TiO2 in two mainly paths 1. Doping with metallic ions 2. Morphological Modification Metallic doping was employed for the increasing of the optical absorption of TiO2 in the Visible region, while the morphological modification option was used to overcome the nanoparticles issues, as the effective charge transfer due to the high quantity of grain boundary.

For TiO2 doping with metallic ions, the main goal was to introduce the metallic ions inside the titania lattice for keep the integrity of the crystalline structure and produce favorable changes in the electronic structure, however depending on the dopant ion nature (electronegativity and ionic radio) will be its incorporation in the system. It could be substitutional (Cu doping), as metallic segregates (Ag doping) and phases segregations (Eu doping). Besides it could be observed that the presence of dopants elements influence on the phase transformation temperature of TiO 2 when it transforms from anatase to rutile. Cu and Ag decrease the phase transformation temperature, for Eu, the opposite effect was observed.

For copper doping, a substitutional incorporation was done, and it increases the visible light absorption. Cu-doped TiO2 showed a lower degradation efficiency in comparison with the pure TiO2, due copper in such concentrations act as a recombination center for photoinduced electrons and holes.

For silver doping, the visible light absorption and the photocatalytic efficiency is higher than the pure TiO2, due that silver is deposited on the titania surface, and allows for negative charge accumulation promoting the photoabsorption of the O2 molecules, improving the dye degradation.

For europium doping, due to the ion size, favor the electron-hole recombinations at this doping concentrations.

The morphological modification by the hydrothermal method with pressure control allows the formation of TiO2 nanotubes starting form nanoparticles. An experimental design 2 k-1 based on the Taguchi philosophy was employed to study the effect of the parameter in the nanotubes formation and the dye degradation efficiency. The methylene blue degradations were higher for the samples submitted at high conditions as pressure, time, temperature and concentrations, due to the fact they allow the nanotubes formation in the hydrothermal process. Results are related the efficient electronic transport of the 1D structures, the high surface area and the vacancies generation in comparison with the TiO2 nanoparticles.

Methylene blue degradation was higher for when TiO2 nanotubes were employed in comparison with the metal doped-TiO2 and pure TiO2. The morphological modification seems to be the option for increase the photocatalytic efficiency of TiO2, however, must consider the long synthesis process for its formation.

Fig. 1 .

 1 Fig. 1. Crystalline structure of TiO2 anatase (a), rutile (b). Fig. 2. Molecular-orbital bonding structure of anatase TiO2: (a) atomic levels; (b) crystal-field split levels; (c) final interaction states. The thin solid line and dashed line rep resent larg e and small contribu tions, respectively. Fig. 3. Crystalline size effect on optical properties of nanocrystals; For ZnO: me * =10, mh*=0.8, Eg=3.2 eV, and Ɛ=86; para TiO2:me * =10 mh * =0.8, Eg=3 .2 eV, and Ɛ=86; and for SnO2: me*=0.3, mh * =0.8, Eg=3.6 eV and Ɛ=12. Fig. 4. Illustration of the modification paths fo r TiO2 nanomaterials. Fig. 5. Photoinduced pro cesses on TiO2. Fig. 6. Photocatalytic process. Fig. 7. Schematic representation of the energy levels of doped semiconductors: p-type doping (left) and n-type doping (right). Fig. 8. Schematic representation of the distortion of the TiO 6 octahedra on doping Cu 2+ ion on the Ti 4+ lattice site. Fig. 9. Schematic diagram of photocatalysis mechanism Ag/TiO2 photocatalyst under artificial visible light irradiation. Fig. 10. Schematic illustration of the possible mechanism of en ergy transfer from th e TiO 2 host to Eu 3+ . Fig. 11 Models of charge recombination in a semiconductor particle; a) Surface trapping sites and b)Within particle trapping sites. Fig. 12. The key difference between the (a) particulate and (b) nanotubular film architecture is the large presence of inter-particle grain boundaries as shown in this representative schematic with TiO 2 films as an example oxide photocatalyst. Fig. 13 X-ray diffractogram of TiO2 pure at different th ermal treatmen ts. Fig. 14. Raman spectra of pure TiO2 heat treated at 500°C and 800°C. Fig. 15. Optical spectroscopy. a) Diffuse reflectance (%) spectra of pure TiO2 powders with the corresponding b) Kubelka-Munk absorption curves, c) Determination of band gap. Fig. 16. XPS spectra of pure TiO2. Fig. 17. Absorption spectra of methylene blu e photodegradation by pure TiO 2 powder. Fig. 18 First o rder Kinetics of methylene blue photodeg radation by pure TiO 2 powder.. Fig. 19. X-ray diffra ctograms of 3%Cu-TiO2 annealed at different tempera tures. Fig. 20. X-ray diffractogram of TiO2 doped with copper (1 , 2 and 3% mol) annealed at 500°C. Fig. 21. Raman spectra of TiO2 doped with copper (1, 2 and 3% mol) annealed at 500°C. Fig. 22. EPR spectrum of Cu 2+ ions in TiO2 with the experimental (continuous) and the simulated one (dashed). Fig. 23. Optical spectroscopy. a) Diffuse reflectance (%) spectra of TiO2 doped powders with copper (1, 2 and 3% mol) with the corresponding b) Kubelka-Munk absorption curves, c) Determination of band gap. Fig. 24. Jahn-Teller(J-T) distortion of the octahedral symmetry of TiO6 and changes in the d-d transition from octahedral to tetragonal phase. In undistorted octahedral coordination, only a single electronic transition occu rs Fig. 25. XPS spectra of 3%Cu-TiO2 annealed at 500°C. Fig. 26. Absorption spectra of methylene blu e photodegradation by TiO 2 doped with copper (1, 2 and 3% mol). Fig. 27. First order Kinetics of methylene blu e photodegradation by TiO 2 doped with copper (1, 2 and 3% mol). Fig. 28. X-ray diffractograms of 3%Ag-TiO2 annealed at different temp eratu res. Fig. 29. X-ray diffractogram of TiO2 doped with silver (1, 2 and 3% mol) annealed at 500°C. Fig. 30. Raman spectra of TiO2 doped with silver (1, 2 and 3% mol) annealed at 500°C. Fig. 31. EPR spectra of 3 at.% Ag doped TiO2 powder with th e experimental (tick line) and simulation (thin lin e). Fig. 32. Optical spectroscopy. a) Diffuse reflectance (%) spectra of TiO2 doped with silver (1, 2 and 3% mol) with the corresponding b) Kubelka-Munk absorption curves, c) Determination of band gap. Fig. 33. XPS spectra of 3%Ag-TiO2 annealed at 500°C. Fig. 34. Absorption spectra of methylene blu e photodegradation by TiO 2 doped with silver (1 , 2 and 3% mol).

Fig. 46 X

 46 Fig.35. First order Kinetics of methylene blu e photodegradation by TiO2 doped with silver(1, 2 and 3% mol). Fig.36. X-ray diffractograms of 3%Eu-TiO2 annealed at different temp era tures. Fig.37. X-ray diffractogram of TiO2 doped with eu ropium (1, 2 and 3% mol) annealed a t 800°C. Fig.38. Raman spectra of TiO2 doped with europium (1 , 2 and 3% mol) annealed at 800°C. Fig.39. EPR spectra of Eu-TiO2 at different concentrations 3 at.% (grey line), 2at.% (thick black line), 1 at.% (dashed line). Fig. 40. Optical spectroscopy. a) Diffuse reflectance (%) spectra of TiO2 doped with europium (1, 2 and 3% mol) with the co rresponding b) Kubelka-Munk absorption cu rves, c) Determination of band gap. Fig. 41. XPS spectra of 3%Eu-TiO2 annealed at 500°C. Fig. 42. Absorption spectra of methylene blu e photodegradation by TiO 2 doped with europium (1, 2 and 3% mol).78 Fig. 43. First order Kinetics of methylene blu e photodegradation by TiO2 doped with silver (1, 2 and 3% mol). Fig. 44. Semi-log plot of σdc conductivity versus inverse temp eratu re fo r pure and metal-doped TiO2. Fig. 45 First o rder Kinetics of methylene blue photodeg radation by 3% M-TiO2 (M=Cu, Ag, and Eu). Fig. 46 X-ray diffractograms of 3%M -TiO2 films (Cu, Ag, and Eu) annealed at 450 °C elabora ted by spin coating. Fig. 47. Raman spectra of TiO2 pure and doped (Cu, Ag, and Eu) elaborated by a) dip coating and b) spin coating, annealed at 450 °C. Fig. 48. AFM morphology of TiO2 film elaborated by spin coating, (10 μm x 10 μm): a) Pure TiO 2, b) TiO2: Cu 3% mol, c) TiO2:Ag 3% mol and d) TiO2:Eu. Fig. 49. Doping effect in the contact angle fo r films elabora ted by spin coating : a ) Pure TiO2 and b) Doped TiO2 . Fig. 50. AFM morphology of TiO2 film elaborated by dip coating (10 μm x 10 μm): a) Pure TiO 2, b) TiO2: Cu 3% mol, c) TiO2:Ag 3% mol and d) TiO2:Eu. Fig. 51. Doping effect in the contact angle fo r films elabora ted by spin coating : a) Pure TiO 2 and b) Doped TiO2. Fig. 52. Optical characterization of M-TiO2 films. Transmittance spectra and band gap determination of a) and c) respectively of M-TiO2 films elaborated by spin coating. Transmittance spectra and band gap determination of b) and d) respectively of M-TiO2 films elaborated by dip coating. Fig. 53. Absorbance spectra of the methylene blue by the photodegradation with M-TiO2 films elaborated by spin coating. Fig. 54. Absorbance spectra of the methylene blue by the photodegradation with M-TiO2 films elaborated by dip coating. Fig. 55. First order Kinetics of methylene blue photodegradation with M-TiO2 films elaborated by a) dip coating and b) spin coating. Fig. 56. Formation mechanism of TiO2 nanotubes using hydrothermal method Fig. 57. X-ray diffractogram of TiO2 nanotubes (TG1-TG9). Fig. 58. Raman spectra of TiO2 nanotubes (TG1-TG9). Fig. 59. Diffuse reflectance spectroscopy; a) Diffuse reflectance (%R) spectra, b) Kubelka -Munk absorption curves and determination of band gap of TiO2 nanotubes. Fig. 60. SEM image of TiO2 nanoparticles annealed a t 500°C. Fig. 61. SEM images of the samples treated by hydroth ermal method (TG1-TG9). Fig. 62. TEM image of the TiO2 nanotube (TG5). Fig. 63. Influence of design parameters on the formation of TiO 2 nanotubes. Fig. 64. Evolution of the optical abso rption of methylene blu e degradation by TiO2 nanoparticles. Fig. 65 Evolution of the optical absorption of methylene blue deg radation by TiO 2 nanotubes (TG1-TG9) Fig. 66.Pseudo first-order kinetics of degradation of Methylene Blue solutions. Fig. 67. Conventional autoclave type Tuttle Roy Fig. 68. Phase diagram of water. Fig. 69. Varia tion of the dielectric constant of water with temp eratu re and pressure. Fig. 70. Schematic diagram of an X-ray diffractometer. Fig. 71. PANalytical θ/θ Bragg-Brentano X´pert MDP PRO diffra cto meter fo r measurements of powd ers. Fig. 72. PANanalytical diffractometer with a design of low in cidence X-ray.

Fig. 1 .

 1 Fig. 1. Crystalline structure of TiO2 anatase (a), rutile (b).

1. 1 . 2

 12 Electronic properties of TiO 2 nanomaterials. The density of states of TiO2 is composed of Ti eg,Ti t2g (dyz,dzx, and dxy),O pσ (in the Ti3O cluster plane), and O pπ (out of the Ti3O cluster plane), as shown in Fig. 2. The upper valence bands can be decomposed into three main regions; the σ bonding in the lower energy region mainly due to O pσ bonding; the π bonding in the middle energy region; and O pπ states in the higher energy region due to O pπ nonbonding states at the top of the valence bands where the hybridization with d states is almost negligible. The contribution of the π bonding is much weaker than that of the σ bonding. The conduction bands are decomposed into Ti eg (>5 eV) and t2g bands (<5 eV). The dxy states are dominantly located at the bottom of the conduction bands (the vertical dashed line).

Fig. 2 .

 2 Fig. 2. Molecular-orbital bonding structure of anatase TiO2: (a) atomic levels; (b) crystal-field split levels; (c) final interaction states. The thin solid line and dashed line represent large and small contributions, respectively.

Fig. 3 .

 3 Fig. 3. Crystalline size effect on optical properties of nanocrystals; For ZnO: m e * =10, mh*=0.8, Eg=3.2 eV, and Ɛ=86; para TiO2:m e * =10 mh * =0.8, Eg=3.2 eV, and Ɛ=86; and for SnO2: m e*=0.3, mh * =0.8, Eg=3.6 eV and Ɛ=12.

Fig. 4 Fig. 4 .

 44 Fig.4illustrates the modification paths in improving the properties (mainly the optical properties) of TiO2 nanomaterials[10].

Fig. 5 .

 5 Fig. 5. Photoinduced processes on TiO2.

  Photovoltaics

Fig. 6 .

 6 Fig. 6. Photocatalytic process.

Fig. 7 .

 7 Fig. 7. Schematic representation of the energy levels of doped semiconductors: p-type doping (left) and n-type doping (right).

Fig. 8 .Fig. 9 .

 89 Fig. 8. Schematic representation of the distortion of the TiO6 octahedra on doping Cu 2+ ion on the Ti 4+ lattice site.

Fig. 10 .

 10 Fig. 10. Schematic illustration of the possible mechanism of energy transfer from the TiO2 host to Eu 3+ .

  shows two models of charge recombination in a semiconductor nanoparticle. In both cases, cations are immobile, while electrons can move between trap sites on the surface of the nanoparticle by thermal activation to the conduction band. The model of Nelson et al., is shown on the left and corresponds to the nearest neighbor random walk. The random flight model on the right corresponds to a completely random choice of the next trapping site for an electron within the nanoparticle after its detrapping and diffusion in the conduction band [36].

Fig. 11

 11 Fig. 11 Models of charge recombination in a semiconductor particle; a) Surface trapping sites and b)Within particle trapping sites.

  TiO2 nanotubes are expected to have advantages over TiO2 particles. First, their one-dimensional structure can reduce the transport dimensionality and recombination of the photoinjected electrons. Second, their tubular structure provides a large area for hosting dye molecules and receiving illumination. Nonetheless, many technical issues remain to be dealt with before the TiO2 nanotube-based DSSCs can finally mature,[40]. Hoda S. Hafez[41], synthesized TiO2 nanotubes by the hydrothermal method in a KOH/NaOH solution. The semiconductor was successfully tested in the degradation of the commercial

Fig. 12 .

 12 Fig. 12. The key difference between the (a) particulate and (b) nanotubular film architecture is the large presence of inter-particle grain boundaries as shown in this representative schematic with TiO2 films as an exam ple oxide photocatalyst.

Figure 3

 3 Figure 3 Dye sensitized solar cell: a) particles b) nanotubes [42].

2. 1

 1 Synthesis of TiO2 doped with metallic ions.TiO2 nanomaterials have been synthesized by the sol-gel method, a widely used method in making various ceramic materials. In a typical sol-gel process, the hydrolysis of titanium alkoxide or halide precursor and subsequent condensation finally lead to the formation of the TiO2 inorganic framework. This process normally proceeds via an acid controlled hydrolysis step of titanium (IV) alkoxide followed by condensation. The development of Ti-O-Ti chains through alkoxylation is favored for low content of water, with low hydrolysis rates and excess titanium alkoxide in the reaction mixture. Three-dimensional polymeric skeletons with close packing result from the development of Ti-O-Ti chains since a part of the Ti are coordinated with O atoms[1].2.1.1 TiO2 doping with Cu.The doping with metal ions induces effects on the photocatalytic activity of TiO2. Some authors claim that the metal doping should decrease the photo threshold energy of TiO2, producing a significant band gap narrowing, which allows lowering the energy consumption; meanwhile, other authors consider that the metal doping could work as a recombination center for electrons and holes, avoiding a fast recombination; and thus increasing the TiO2 photoreactivity[2].2.1.2 TiO2 doping with Ag.Int the recent years the plasmonic driven photocatalysts achieved with the support of noble metal(s) has become a key focus in many studies. Decorating with single or bi-noble metal(s) onto semiconductor spinal had proven to trigger the localized surface plasmon resonance (LSPRs) for enhanced visible light photocatalytic activities. A plasmon band is generated due to the collective oscillations of conduction electron induced by the electric field of the incident visible light photons and further potentially declines the recombination rate of photogenerated electronhole pairs of the semiconductor[3].2.1.3 TiO2 doping with Eu.

2. 2 Fig. 13 X

 213 Fig. 13 X-ray diffractogram of TiO2 pure at different thermal treatments.

Fig. 14 .

 14 Fig. 14. Raman spectra of pure TiO2 heat treated at 500°C and 800°C.

Fig. 15 .

 15 Fig. 15. Optical spectroscopy. a) Diffuse reflectance (%) spectra of pure TiO2 powders with the corresponding b) Kubelka-Munk absorption curves, c) Determination of band gap.

Fig. 16 .Fig. 17 .

 1617 Fig. 16. XPS spectra of pure TiO2.

1 Fig. 18 First 2 Fig. 19 .

 118219 Fig. 18 First order Kinetics of methylene blue photodegradation by pure TiO2 powder..

2. 3 . 2

 32 Phase identification by XRD of TiO2:Cu powders. Effect of the concentration.

Fig. 20 .

 20 Fig. 20. X-ray diffractogram of TiO2 doped with copper (1, 2 and 3% mol) annealed at 500°C.

Table 4 . 3

 43 Diffraction line analysis of the main peak corresponding to the plane (101) from anatase phase, for TiO2 doped powders with copper (1, 2 and 3 % mol): Sa m p l e Pe a k p o si ti on o f th e p l ane (10 1 ) / d e g re e s Fu l l w i d th a t h a l f m e d i u Phase identification by Raman spectroscopy of TiO2:Cu powders.Raman spectroscopy was employed for the characterization of the vibrational modes related to the crystalline structure of TiO2 powders and to probe the structural changes due to doping.The ionic size of Cu 2+ (0.73 Å) is larger than that of Ti 4+ (0.64 Å), and hence, doping of this ion will distort the lattice structure of TiO2. As there is charge difference between Cu 2+ and Ti 4+ , doping of Cu generates oxygen vacancies in the lattice of TiO2 to maintain the charge neutrality. If doping occurs on the substitutional position of the Ti 4+ site, the Ti-O-Ti bond will be distorted and a new Cu-O-Ti or Cu-O-Cu bond will be formed. Therefore, disturbance of the Ti-O-Ti bonds and the formation of new Cu-O bonds will affect the Raman-active modes and will result in the broadening and shifting of the Raman bands[13].

Fig. 21 .

 21 Fig. 21. Raman spectra of TiO2 doped with copper (1, 2 and 3% mol) annealed at 500°C.

2. 3 . 5

 35 EPR evaluation of TiO 2 :Cu powders.Electron paramagnetic resonance (EPR) is an important tool to determine the valence states of doping elements and their local environments in the host matrices. As the TiO2 is a photoactive materials able to react with radiations in suitable spectral range, the experiments were conducted at room temperature (RT) and at low temperature 150 K as well as in-situ under irradiation by using the light spectrum of Xe source with a nominal power of 10 W on the sample.A measurement under Xe lamp radiation contributes to the creation of an additional small signal superimposed to the EPR spectrum of Cu 2+ . To identify the main features of the EPR signal, a simulation was realized taking into account the electronic spin of Cu 2+ (S=1/2) and the nuclear spin of the two main isotopes of Cu nuclei (I=3/2).

Fig. 22 .

 22 Fig. 22. EPR spectrum of Cu 2+ ions in TiO2 with the experimental (continuous) and the simulated one (dashed).

Fig. 23 .

 23 Fig. 23. Optical spectroscopy. a) Diffuse reflectance (%) spectra of TiO2 doped powders with copper (1, 2 and 3% mol) with the corresponding b) Kubelka-Munk absorption curves, c) Determination of band gap.

Fig. 24 .Fig. 25 .

 2425 Fig. 24. Jahn-Teller(J-T) distortion of the octahedral symmetry of TiO6 and changes in the d-d transition from octahedral to tetragonal phase. In undistorted octahedral coordination, only a single electronic transition occurs

Fig. 26 . 1 Fig. 27 .

 26127 Fig. 26. Absorption spectra of methylene blue photodegradation by TiO2 doped with copper (1, 2 and 3% mol).

2. 4 . 1

 41 Phase identification by XRD of TiO2:Ag Powders. Effect of the temperature.

Fig. 28 .

 28 Fig. 28. X-ray diffractograms of 3%Ag-TiO2 annealed at different temperatures.

Fig. 29 .

 29 Fig. 29. X-ray diffractogram of TiO2 doped with silver (1, 2 and 3% mol) annealed at 500°C.

3

 3 (101) diffraction line as function of the Ag doping ratio : Sa m p l e Pe a k p o si ti on o f th e p l ane (10 1 ) / d e g re e s Fu l l w i d th a t h a l f m e d i u m . Phase identification by Raman spectroscopy of TiO2:Ag powders.

Fig. 30 .

 30 Fig. 30. Raman spectra of TiO2 doped with silver (1, 2 and 3% mol) annealed at 500°C.

2. 4 . 5

 45 EPR investigations of TiO2:Ag powders.As Cu 2+ , Ag 2+ ions possess an electronic configuration with d 9 orbitals and then the paramagnetic behavior is expected with an effective spin (S=1/2). The EPR spectrum of 3 at.% of Ag-doped TiO2 powders is shown in Fig.31, along with an additional narrow line which emanates from color centers in TiO2. The simulation of the EPR spectrum is performed based on the above spin hamiltonian with electronic spin S=1/2 and nuclear spin I=1/2 for Ag nuclei. The spectral parameters of Ag 2+ doped TiO2 are summarized in table 1. Only one crystalline site is occupied by Ag 2+ in contrast to Cu 2+ discussed above with two involved substitutional sites. The low quality of the EPR signal/noise ratio indicates a lower concentration of substitutional Ag 2+ with regard to the departure of its ionic radius (0.94) compared to titanium ion. The anisotropy of the g-tensor which shows a rhombic local symmetry around Ag 2+ ions is inferred in a similar way as shown above for Cu2+ doping.

Fig. 31 .

 31 Fig. 31. EPR spectra of 3 at.% Ag doped TiO2 powder with the experimental (tick line) and simulation (thin line).

Fig. 32 .

 32 Fig. 32. Optical spectroscopy. a) Diffuse reflectance (%) spectra of TiO2 doped with silver (1, 2 and 3% mol) with the corresponding b) Kubelka-Munk absorption curves, c) Determination of band gap.

7

 7 Chemical Analysis by XPS of TiO2:Ag powders.

Fig. 33 .

 33 Fig. 33. XPS spectra of 3%Ag-TiO2 annealed at 500°C.

Fig. 34 . 1 Fig. 35 .

 34135 Fig. 34. Absorption spectra of methylene blue photodegradation by TiO2 doped with silver (1, 2 and 3% mol).

2. 5 . 1 7 Fig. 36 .

 51736 Fig. 36. X-ray diffractograms of 3%Eu-TiO2 annealed at different temperatures.

2. 5 . 2

 52 Phase identification by XRD of TiO2:Eu powders. Effect of the concentration.

7 Fig. 37 .

 737 Fig. 37. X-ray diffractogram of TiO2 doped with europium (1, 2 and 3% mol) annealed at 800°C.

Fig. 40 .

 40 Fig. 40. Optical spectroscopy. a) Diffuse reflectance (%) spectra of TiO2 doped with europium (1, 2 and 3% mol) with the corresponding b) Kubelka-Munk absorption curves, c) Determination of band gap.

Fig. 41 .

 41 Fig. 41. XPS spectra of 3%Eu-TiO2 annealed at 500°C.

2. 5 . 8

 58 Photocatalytic evaluation of TiO2:Eu powders.

Fig. 42 . 1 Fig. 43 .

 42143 Fig. 42. Absorption spectra of methylene blue photodegradation by TiO2 doped with europium (1, 2 and 3% mol).

Fig. 44 .

 44 Fig. 44. Semi-log plot of σdc conductivity versus inverse temperature for pure and metal-doped TiO2.

  Conductivity measurements for the concrete mixes have been presented in an Arrhenius format,

  temperature (~600°C). The electronic behavior is also drastically modified and depends on the doping elements with the effective conductivity being lowered compared to the undoped sample. The microstructure changes such with low specific surfaces in doped samples correlate with the formation of metallic clusters for Ag doping or europium oxide clusters for Eu doping. The increase of interfaces and composition changes by such doping may overcome the conduction channels compared to undoped sample. Alternatively, the red shift of the band gap for all doped samples was clearly noticed with the expectation of harvesting a wide light spectrum for photocatalysis reactions. The experiments on the degradation of methylene blue dyes have shown slight improvement with Ag-doped samples. The reason was tentatively attributed to the Ag clusters which were indeed demonstrated through their plasmon optical band. The induced surface plasmon resonances enhance the intensity of light radiation at the TiO2 interfaces and improve the efficiency of photocatalytic degradation of dyes in solutions, see Fig. 45.

Fig. 45 First

 45 Fig. 45 First order Kinetics of methylene blue photodegradation by 3% M-TiO2 (M=Cu, Ag, and Eu).

3. 3 . 5 Fig. 46 X

 3546 Fig. 46 X-ray diffractograms of 3%M-TiO2 films (Cu, Ag, and Eu) annealed at 450 °C elaborated by spin coating.

3. 5

 5 Phase identification by Raman spectroscopy of pure TiO2 and TiO2:M 3% mol, elaborated by spin coating.

Fig. 47 .

 47 Fig. 47. Raman spectra of TiO2 pure and doped (Cu, Ag, and Eu) elaborated by a) dip coating and b) spin coating, annealed at 450 °C.

3. 6

 6 Morphological analysis of TiO2:M films elaborated by dip coating and spin coating.

  Sa m p l eTh i ckness (spi n co a ti n g ) / nm Esp e so r (d i p co a ti n g )

Fig. 49 .

 49 Fig. 49. Doping effect in the contact angle for films elaborated by spin coating: a) Pure TiO2 and b) Doped TiO2 .

Fig. 48 .Fig. 51 .

 4851 Fig. 48. AFM morphology of TiO2 film elaborated by spin coating, (10 μm x 10 μm): a) Pure TiO2, b) TiO2: Cu 3% mol, c) TiO2:Ag 3% mol and d) TiO2:Eu.

Fig. 50 .

 50 Fig. 50. AFM morphology of TiO2 film elaborated by dip coating (10 μm x 10 μm): a) Pure TiO2, b) TiO2: Cu 3% mol, c) TiO2:Ag 3% mol and d) TiO2:Eu.

3. 7

 7 Optical characterization of TiO2:M films elaborated by spin coating and dip coating.The optical properties of the films were evaluated by measurements of their optical transmittance.The investigated films were deposited on borosilicate glass.The band gas was determinated using the Tauc plot

Fig. 52 .

 52 Fig. 52. Optical characterization of M-TiO2 films. Transmittance spectra and band gap determination of a) and c) respectively of M-TiO2 films elaborated by spin coating. Transmittance spectra and band gap determination of b) and d) respectively of M-TiO2 films elaborated by dip coating.

Fig. 53 .

 53 Fig. 53. Absorbance spectra of the methylene blue by the photodegradation with M-TiO2 films elaborated by spin coating.

Fig. 55 .

 55 Fig. 55. First order Kinetics of methylene blue photodegradation with M-TiO2 films elaborated by a) dip coating and b) spin coating.

Fig. 56 .

 56 Fig. 56. Formation mechanism of TiO2 nanotubes using hydrothermal method

Table 23 .

 23 Taguchi L9 orthogonal array for TiO2 nanotubes synthesis.Exp e ri m e n tMo l a r R a ti o

4. 2 . 4 . 3

 243 3Hydrothermal method.TiO2 nanoparticles obtained by sol-gel were treated with a solution of NaOH in a time and temperature specified in the experimental design. The autoclave w as pressurized with nitrogen at the pressure established in the experimental design at the beginning of the test. Subsequently, the product was washed with distilled water until reach a pH of 7. Then it was washed with 1 M of HCl. Again the product was washed with distilled water until reach a pH 7. The treated powders were separated by centrifugation. The solid was dried at 90°C and then annealed at 450°C for 30 minutes. Results and discussion of TiO2 nanotubes. 4.3.1 Phase identification of TiO 2 nanotubes by X-ray Diffraction.

  Fig. 57. X-ray diffractogram of TiO2 nanotubes (TG1-TG9).

4. 3 .Fig. 58 .

 358 Fig. 58. Raman spectra of TiO2 nanotubes (TG1-TG9).

Fig. 59 .

 59 Fig. 59. Diffuse reflectance spectroscopy; a) Diffuse reflectance (%R) spectra, b) Kubelka -Munk absorption curves and determination of band gap of TiO2 nanotubes.

4. 3 . 4

 34 Morphology analysis of TiO 2 nanotubes. The morphology analysis was made by Scanning electron microscopy and the Transmission electronic microscopy. The TiO2 nanoparticles synthesized by sol gel with the surfactant are shown in the Fig. 60¡Error! No se encuentra el origen de la referencia. The surfactant was added for reducing the agglomeration of the particles. The particle size of TiO2 nanoparticles is around 20 nm.

Fig. 60 .

 60 Fig. 60. SEM image of TiO2 nanoparticles annealed at 500°C.

Fig. 61 .

 61 Fig. 61. SEM images of the samples treated by hydrothermal method (TG1-TG9).

Fig. 62 .

 62 Fig. 62. TEM image of the TiO2 nanotube (TG5).

Fig. 63 .

 63 Fig. 63. Influence of design parameters on the formation of TiO2 nanotubes.

Fig. 64 .Fig. 65

 6465 Fig. 64. Evolution of the optical absorption of methylene blue degradation by TiO2 nanoparticles.

1 Fig. 66 .

 166 Fig. 66.Pseudo first-order kinetics of degradation of Methylene Blue solutions.

Table 2 .

 2 Experimental design for TiO2 doping with metallic ions (1, 2 y 3% mol).

		Cu 2+	Ag 2+	Eu 3+
	Ti O2 p u ro .		-
	Ti O2:% M	1% mol a 500°C	1% mol a 500°C	1% mol a 800°C
	Ti O2:% M	2% mol a 500°C	2% mol a 500°C	2% mol a 800°C
	Ti O2:% M	3% mol a 500°C	3% mol a 500°C	3% mol a 800°C

Table 3 .

 3 Crystallite size for TiO2 Doped powders with copper (1, 2 and 3% mol).

		Ti O2 pure	1 % C u -Ti O2	2 % C u -Ti O2	3 % C u -Ti O2
	C rysta l l i te si ze	20 nm	29 nm	24 nm	37 nm

Table 6 .

 6 BET surface area, pore volume and pore size of TiO2:Cu.

	Sa m p l e	BET su rfa ce a re a / m 2 g -1	Po re vo l u m e / cm 3 g -1	Po ro si ze / n m	Ave rage pa rti cl e d i a m e te r / n m
	TiO2 at 500°C	31.3	0.078	9	49
	3 % Cu-TiO2 at 500°C	22.6	0.056	10	68

Table 5 .

 5 Analysis of the main Raman peak of the anatase Eg1 of TiO2:Cu.

	Sa m p l e s	Eg1 p eak position / cm -1	Fu l l w i d th a t h a l f m e d i u m .
	TiO2	142.9	18.4
	1%Cu-TiO2	142.7	19.3
	2%Cu-TiO2	143.1	19.02
	3%Cu-TiO2	143.1	19.02

Table 7 .

 7 EPR values obtained of EPR experimental data of 3%Cu-TiO2 powders.

	Metal doping gx	gy	gz	Ax (G)	Ay (G)	Az (G)	LW (G)	%
	Cu 2+ (1)	2.06 (2)	2.15 (2)	2.36 (2)	30 (3)	50 (5)	100 (5)	25	38
	Cu 2+ (2)	2.06 (2)	2.15 (2)	2.40 (2)	30 (3)	50 (5)	100 (5)	25	62

2.3.6 Optical characterization of TiO2:Cu powders.

Table 8 .

 8 XPS binding energies of 3%Cu-TiO2 powder.

	Bi n d i n g e n e rg y / e V

2.3.8 Photocatalytic evaluation of TiO 2 : Cu powders.

Table 10 .

 10 Crystallite size for TiO2. Doped powders with silver (1, 2 and 3% mol).

		Ti O2 pure	1 % Ag -Ti O2	2 % Ag -Ti O2	3 % Ag -Ti O2
	C rysta l l i te si ze	20 nm	30 nm	26 nm	32 nm

Table 9 .

 9 Position and FWHM of the

Table 12 .

 12 BET surface area, pore volume and pore size of TiO2:Ag powders.

	Sa m p l e	BET su rfa ce a re a / m 2 g -1	Po re vo l u m e / cm 3 g -1	Po re si ze / n m	Ave rage pa rti cl e d i a m e te r / n m
	TiO2 at 500°C	31.3	0.078	9.9	49
	3%Ag-TiO2 at 500°C	21.4	0.073	13.7	71

Table 11 .

 11 Analysis of the main Raman band associted to Eg1 Raman active mode of anatase of TiO2:Ag poweders.

	Sa m p l e s	Eg1 p eak position / cm -1	Fu l l w i d th a t h a l f m e d ium.
	TiO2	142.9	18.4
	1%Ag-TiO2	145.9	20.6
	2%Ag-TiO2	145.9	22.8
	3%Ag-TiO2	147.5	24.6

Table 13 .

 13 EPR values obtained of EPR experimental data of 3%Ag-TiO2 powders.

	Metalic doping	gx	gy	gz	Ax (G)	Ay (G)	Az (G)	LW (G)	%
	Ag 2+	2	2.05	2.21	20	20	30	10-20 G 100
	Colour centers (Ag-TiO2)	2.0035	2.0035 2.0035 0	0	0	3	100

Table 14 .

 14 XPS binding energies of 3%Ag-TiO2 powder.

				Bi n d i n g e n e rg y / e V
		Ti 2 p 3/2	Ti 2 p 1/2	O1 s	Ag 3 d 3/2	Ag 3 d 5/2
	Ti O2	458.7	464.38	530.07	-	-
	3 % Ag -Ti O2	458.35	464.38	530.07	374.09	368.10
	2.4.8 Photocatalytic evaluation of TiO2:Ag powders.

Table 16 .

 16 Crystallite size for TiO2. doping with europium (1, 2 and 3% mol).

		Ti O2 p u re	1 % Eu -Ti O2	2 % Eu -Ti O2	3 % Eu -Ti O2
	C rysta l l i te si ze	20 nm	44 nm	28 nm	25 nm

Table 15 .

 15 Peak analysis of the main peak corresponding to the plane (101) from anatase phase, for TiO2 doped

	with europium (1, 2 and 3 % mol):
	Sa m p l e	p l ane (10 1 ) / d e g re e s Pe a k p o si ti on o f th e	Fu l l w i d th a t h a l f m e d i u m
	TiO2	25.35	0.242
	1%Eu-TiO2	25.32	0.204
	2%Eu-TiO2	25.29	0.272
	3%Eu-TiO2	25.30	0.326

2.5.3 Phase identification by Raman spectroscopy of TiO2:Eu powders.

Table 17 .

 17 Analysis of the main Raman peak of the anatase Eg1 of TiO2:Eu.

	Sa m p l e s	Eg1 p e a k p o si ti o n / cm -1	Fu l l w i d th a t h a l f m e d i u m .
	TiO2	144.7	17.21
	1%Eu-TiO2	144.45	18.20
	2%Eu-TiO2	144.68	18.58
	3%Eu-TiO2	144.08	21.09

2.5.4 BET surface area measurements of TiO2:Eu powders.

Table 18 .

 18 BET surface area, pore volume and pore size of TiO2:Eu.

	Sa m p l e	BET su rfa ce a re a / m 2 g -1	Po re vo l u m e / cm 3 g -1	Po ro si ze / n m	Ave rage parti cl e d i a m e te r / n m
	Ti O2 a t 5 0 0 °C	31.3324	0.078245	9.989	49 nm
	3 % Eu -Ti O2 a t	10.0270	0.044292	17.66	153 nm
	8 0 0 °C

Table 19 .

 19 The activation energy of the samples 3%M-TiO2.

		Activation energy.	Activation energy.
	TiO 2	0.44 eV ( 193-313 K)	0.15 eV (333-473 K)
	3% Cu-TiO 2	0.37 eV ( 193-333 K)	0.16 eV (353-473 K)
	3% Ag-TiO 2	0.14 eV ( 193-313 K)	0.17 eV (333-473 K)
	3% Eu-TiO 2	0.59 eV ( 193-333 K)	0.10 eV (353-473 K)

Table 20 .

 20 Experimental design for TiO2 doping with metallic ions (Cu, Ag y Eu) and TiO2 pure.

		Sp i n C o a ti n g	D i p co a ti n g .
	Ti O2	450°C	450°C
	Ti O2:%Cu	3% mol at 450°C	3% mol at 450°C
	Ti O2:% Ag	3% mol at 450°C	3% mol at 450°C
	Ti O2:% Eu	3% mol at 450°C	3% mol at 450°C
	3.3 Synthesis of TiO2:Metal thin films.

Table 21 .

 21 Thickness measurement by AFM of the films elaborated by spin coating and dip coating.

Table 22 .

 22 Parameter Level of process parameters.

Table 24 .

 24 Analysis of the main Raman peak for the anatase phase, Eg1. Optical analysis of TiO 2 nanotubes.

	Sa m p l e	1 Pe ak position (Eg1) / cm -	Fu l l w i d th a t h a l f m a xi m u m .
	TiO2	150.44	20.59
	TG 1	150.91	22.99
	TG 2	150.98	21.49
	TG 3	151.41	23.20
	TG 4	150.26	21.25
	TG 5	150.02	20.52
	TG 6	149.33	21.37
	TG 7	149.7	21.43
	TG 8	150.54	22.65
	TG 9	149.95	21.87

Table 25 .

 25 BET results of TiO2 nanotubes. Photocatalytic degradation of methylene blue in w ater by TiO2 nanotubes.

	Mu e stra	BET Surface area / m 2 g -1	Po re vo l u m e / cm 3 g -1	Po re si ze / n m
	Ti O2 a 500°C	25.24	0.09	14.4
	TG 1	88.62	0.15	6.9
	TG 2	83.9	0.16	7.8
	TG 3	90.62	0.31	13.7
	TG 4	72.93	0.23	12.9
	TG 5	81.2	0.22	11.2
	TG 6	80.68	0.16	8.1
	TG 7	76.72	0.16	85.3
	TG8	79.16	0.20	10.4
	TG9	80.1	0.21	10.6

CHAPTER III

TiO2:M films elaborated by spincoating and dip coating III. TiO2:M films elaborated by spin coating and dip coating.

Introduction.

TiO2 has several naturally occurring modifications, the most common ones being rutile, anatase, and brookita polymorphs. Most research on photocatalysis is performed using anatase, while in all other aspects rutile has been much more thoroughly characterized (e.g. electronic behavior, bulk structural investigations). This is mostly due to the preparation methods: While it is easier to obtain anatase than rutile at low temperatures below 600°C, rutile is stable at temperatures above 800°C, which is the transition temperature to the thermodynamically most stable phase rutile. While TiO2 powders or films obtained at low temperatures are finely grained (nanocrystalline) because little grain growth can occur, grain growth at temperatures above 800°C can be substantial. Therefore, depending on the preparation method, the nature of the TiO2 particle or film surface, as well as the active surface area are usually very different for anatase and rutile then it is withdrawn with a constant speed according to many tests made previously (results are not shown).

The conditions for thin films elaboration are below.

Immersion speed: 100 mm/s Withdrawn speed: 5 mm / s Covered surface area: 1 cm 2 .

Number of deposits: 3

Subsequently, the films were dried in air and then annealed in air at 450 °C for 30 min with an air cooling.

TiO2 films elaborated by spin coating.

The TiO2 pure and TiO2:M films were elaborated with the sols described in sections 3.3.2 and 3.3.3, by spin technique, which consists in putting a drop of the sol in the center of the substrate submitted to it is spinning with an established speed for 30 seconds. The speed of spinning was established according to tests done previously (results are not shown).

The conditions for thin films elaboration are below.

The speed of spinning: 5000 rpm.

Number of layers: 3

Subsequently, the films were dried in air and then annealed in air at 450 °C for 30 min with an air cooling. The photocatalytic process consists in the concentration reduction (C) during a time (t), following a first order Kinect employing the equation:

The most important parameter of this equation is (k), named as constant rate or constant of reaction with units of min -1 , Fig. 55 . When this factor is higher means that the semiconductor is more efficient in the photodegradation of methylene blue.

All the photocatalysis experiments were done under the same conditions for a real comparison.

It can be observed that for the films of TiO2 doped with metallic ions (TiO2:M) have a constant rate lower than pure TiO2. It means the photodegradation of the methylene blue is lower in comparison with the pure TiO2. These results are in accordance with the obtained by the powders in Chapter 2.

In the specific case of the film of Ag-TiO2 posseses a photodegradation efficiency lower than pure TiO2. This result is not the same as the obtained by the powders in Chapter 2. It could be due the time and temperature of the thermal treatment previously mentioned during the drying and calcination process. In these conditions, the elimination of liquids and organic substance and the crystallization of anatase phase, the uniformly dispersed Ag + ions would gradually migrate from

Appendix.

VI. Appendix 6.1 Synthesis methods 6.1.1 Sol-Gel.

Sol-gel refers to solution-gelation and is a widely used method for producing ceramic materials with high purity and homogeneity. This approach offers control of the composition and structure at the molecular level. The process involves the generation of a colloidal suspension, which is subsequently converted to gel. In the process, reactive metal precursors are initially hydrolized, followed by condensation and polymerization reactions. The metal alkoxides are organometallic compounds with an organic ligand attached to a metal or metalloid atom. These alkoxides are the result of direct or indirect reactions between a metal (M) and alcohol (ROH); typical examples include methoxide (OMe; MOCH3) and ethoxide (OEt; MOC2H5). During hydrolysis, the alkoxy groups (OR) are replaced by hydroxy ligands (OH), i.e.

𝑀(𝑂𝑅

where R is the alkyl group (CnH2n+1). The mechanism of this reaction involves the addition of a negatively charged HO d-group to the positively charged metal center (M d+ ) followed by the removal of ROH. Several factors are known to affect the hydrolysis reaction: (a) the nature of the alkyl group, (b) the nature of the solvent, (c) the concentration of each species in the solvent, (d) temperature, (e) the water to alkoxide molar ratio, and (f) the presence of acid or base catalysts.

128 Subsequent condensation eliminates either water or alcohol to produce metal oxide or hydroxide linkages. In this process, the two mononuclear complexes of M, each comprising only one metal M, can react with each another to form a polynuclear complex consisting of two metal atoms.

Condensation only occurs when at least one hydroxo ligand is bonded to the cation M; this is designated as M-OH for simplicity. Condensation can proceed via olation and oxolation reactions.

Olation is a reaction in which a hydroxo or an "ol" bridge M-OH-M bond is formed between two cations whereas oxalation involves the formation of oxo bridges M-O-M between to metal cations.

The "ol" or "oxo" bridges between the two metal atoms lead to the formation of condensed oxide or hydroxide species. Under acid conditions, three-dimensional solid phase networks consisting of extended linear M-O-M chain polymers are fabricated. Inorganic polymerization is believed to occur in three stages during the acid catalyzed condensation: (a) polymerization of monomer units to form particles, (b) particle growth, and (c) particles linking to form chains then solid networks that extend throughout the liquid medium, which thickens it to a gel. The removal of the solvents and appropriate drying techniques are important steps to achieve gel densification. When a solvent evaporates from the gel under atmospheric conditions, the capillary pressure due to the interfacial tension of the solvent puts high stress on the gel network. This leads to considerable shrinkage and fracture of the gel during the drying process. The resulting hard, glassy, and porous product is called a xerogel. When the liquid within the gel is removed above its critical temperature and pressure (hypercritical) in an autoclave, the capillary pressure can be eliminated. The product obtained from this process is referred to as an aerogel. The crystalline nature of the amorphous aerogel was achieved after heat treatment. This transition exhibits crystallites with high surface areas and unique properties [1]. The term hydrothermal usually refers to any heterogeneous reaction in the presence of aqueous solvents or mineralizers under high pressure and temperature conditions to dissolve and recrystallize (recover) materials that are relatively insoluble under ordinary conditions [2].

Water as a reaction medium.

Water is one of the most important solvents present in nature in abundant amount and has remarkable properties as a reaction medium under hydrothermal conditions. Water shows different characteristics under hydrothermal conditions than that of standard conditions. One of the biggest advantages of using water is the environmental benefit and cheaper than other solvents, and it can act as a catalyst for the formation of desired materials by tuning the temperature and the pressure. It is nontoxic, nonflammable, noncarcinogenic, nonmutagenic, and Hydrothermal solvents have different properties at above 100°C and above 1 atm, especially at the critical point. To understand hydrothermal reactions, the properties of the solvent under hydrothermal conditions must be known very well.

In the Fig. 68, the critical point marks the end of liquid-vapor coexistence curve at the critical temperature, Tc, and pressure, Pc, in a phase diagram for a pure homogenous substance. A fluid is defined as being supercritical if it is maintained at conditions above its critical temperature and pressure. The properties of supercritical fluids (SCFs) vary depending on the pressure and temperature and frequently described as being intermediate between those of a gas and a liquid.

As the temperature increases, the liquid becomes less dense due to thermal expansion, and at the same time, the gas becomes denser. At the critical point, the densities of both phases become the same. The compound is neither liquid nor gas any longer above the critical point, and it becomes a supercritical fluid. After that, the phases of liquid and gas are not distinguishable, and properties of SCF will be between gas and liquid. Diffusivity and viscosity symbolize transport properties that influence rates of mass transfer.

Diffusivity is at least an order of magnitude higher, and viscosity is lower compared with a liquid Appendix 131 solvent. This means that diffusivity of reactants in SCF will occur faster than that in a liquid solvent, which means that solids can dissolve and migrate more rapidly in SCFs. High diffusivity, low viscosity and intermediate density of water increase the rate of the reaction [4].

The dielectric constant that is defined as the ability of a solvent to charge separate increases sharply with the pressure in the compressible region that refers to the area around the critical point in which compressibility is considerably greater than would be forecasted from the ideal gas law. This behavior is also parallel to a change in density, as shown in Fig. 69. Density changes sharply but continuously with pressure in the compressible region. One of the most important advantages of hydrothermal solvents is that a change in density affects the solvating power. A decrease in the density results in a significant change in solvating ability [4]. X-ray powder diffraction (XRD) is a rapid analytical technique primarily used for phase identification of a crystalline material and can provide information on unit cell dimensions. The analyzed material is finely ground, homogenized, and average bulk composition is determined.

Max von Laue, in 1912, discovered that crystalline substances act as three-dimensional diffraction gratings for X-ray wavelengths similar to the spacing of planes in a crystal lattice. X-ray diffraction is now a common technique for the study of crystal structures and atomic spacing.

X-ray diffraction is based on constructive interference of monochromatic X-rays and a crystalline sample. These X-rays are generated by a cathode ray tube, filtered to produce monochromatic radiation, collimated to concentrate, and directed toward the sample. The interaction of the incident rays with the sample produces constructive interference (and a diffracted ray) when conditions satisfy Bragg's Law (nλ=2d sin θ). This law relates the wavelength of electromagnetic radiation to the diffraction angle and the lattice spacing in a crystalline sample. These diffracted X-rays are then detected, processed and counted. By scanning the sample through a range of 2θangles, all possible diffraction directions of the lattice should be attained due to the random orientation of the powdered material. Conversion of the diffraction peaks to d-spacings allows identification of the mineral because each mineral has a set of unique d-spacings. Typically, this is achieved by comparison of d-spacings with standard reference patterns All diffraction methods are based on generation of X-rays in an X-ray tube. These X-rays are directed at the sample, and the diffracted rays are collected. A key component of all diffraction is the angle between the incident and diffracted rays. Powder and single crystal diffraction vary in instrumentation beyond this. X-ray diffractometers consist of three basic elements: an X-ray tube, a sample holder, and an Xray detector. X-rays are generated in a cathode ray tube by heating a filament to produce electrons, accelerating the electrons toward a target by applying a voltage, and bombarding the target material with electrons. When electrons have sufficient energy to dislodge inner shell electrons of the target material, characteristic X-ray spectra are produced. These spectra consist 133 of several components, the most common being Kα and Kβ. Kα consists, in part, of Kα1 and Kα2.

Kα1 has a slightly shorter wavelength and twice the intensity as Kα2. The specific wavelengths are characteristic of the target material (Cu, Fe, Mo, Cr). Filtering, by foils or crystal monochromators, is required to produce monochromatic X-rays needed for diffraction. Kα1and Kα2 are sufficiently close in wavelength such that a weighted average of the two is used. Copper is the most common target material for single-crystal diffraction, with CuKα radiation = 1.5418Å. These X-rays are collimated and directed onto the sample. As the sample and detector are rotated, the intensity of the reflected X-rays is recorded. When the geometry of the incident X-rays impinging the sample satisfies the Bragg Equation, constructive interference occurs and a peak in intensity occurs. A detector records and processes this X-ray signal and converts the signal to a count rate which is then output to a device such as a printer or computer monitor [5,6]. For the powders samples, the analysis was made at room temperature in a PANalytical θ/θ Bragg-Brentano X´pert MDP PRO diffractometer with a radiation of Cu-Kα (λ=1.5406 Å) and a X´Celerator detector. The diagrams were recorded in a range of 2 theta from 10° to 80°, see Fig.

71.

134 Thin-film measurements were done on a PANanalytical diffractometer with a design of low incidence X-ray.

Surface analysis.

Surface área helps determine such things as how solids burn, dissolve, and react with other materials. To determine the surface area, solid samples are pretreated by applying some combination of heat, vacuum, and/or flowing gas to remove adsorbed contaminants acquired (typically water and carbon dioxide) from atmospheric exposure. The solid is then cooled, under vacuum, usually to cryogenic temperature (77 k, -195°C). An adsorptive (typically nitrogen) is dosed to the solid in controlled increments. After each dose of adsorptive, the pressure is allowed to equilibrate and the quantity adsorbed is calculated. The quantity adsorbed at each pressure ( and temperature) defines and adsorption isotherm, from which the quantity of gas required to form a monolayer over the external surface of the solid is determined. With the area covered by each adsorbed gas molecule known, the surface area can be calculated. Surface area determinations involve creating the conditions required to absorb and average monolayer of molecules onto a simple. By extending this process so that the gas is allowed to condense in the pores, the sample´s fine pore structure can be evaluated. As pressure increases, the gas condenses first in the pores with the smallest dimensions. The pressure is increased until saturation is reached, at which time all pores are filled with liquid. The adsorptive gas pressure then is reduced incrementally, evaporating the condensed gas from the system. Evaluation of the adsorption and desorption branches of these isotherms and the hysteresis between them reveals information about the size, volume, and area [7].

The steps for surface area measurements, porosity, pore size and pore distributions are described in the Fig. 74 Where it assumes that all the particles are spherical with the same size [8][9][10][11].

Atomic force microscope.

An atomic force microscope is a type of high-resolution scanning probe microscope that has a resolution that you can measure in fractions of a nanometer. It was pioneered in 1986 by Nobel Prize Winner Gerd Binnig along with Calvin Quate and Christoph Gerber. [12].

One of the most important tools for imaging on the nanometer scale, Atomic Force Microscopy uses a cantilever with a sharp probe that scans the surface of the specimen. When the tip of the probe travels near to a surface, the forces between the tip and sample deflect the cantilever according to Hooke's law. Atomic force microscopy will measure a number of different forces depending on the situation and the sample that you want to measure. As well as the forces, other microscopes can include a probe that performs more specialized measurements, such as temperature. The force deflects the cantilever, and this changes the reflection of a laser beam that shines on the top surface of the cantilever onto an array of photodiodes. The variation of the laser beam is a measure of the applied forces.

There are two primary modes of operation for an atomic force microscope, namely contact mode and non-contact mode depending on whether the cantilever vibrates during the operation. In contact mode, the cantilever drags across the sample surface, and it uses the deflection of the cantilever to measure the contours of the surface. To eliminate the noise and drift that can affect a static signal, low stiffness cantilevers are used, but this allows strong attractive forces to pull the tip to the surface. To eliminate this attraction, the tip is in contact with the surface where the overall force is repulsive.

In non-contact mode, the tip vibrates slightly above its resonance frequency and does not contact the surface of the sample. Any long range forces, like van der Waals forces, decreases the resonant frequency of the cantilever. Most samples will form a layer of moisture on the surface if stored at ambient conditions, and this can make it difficult to measure the sample accurately.

If the probe tip is close enough to detect the short-range forces then it is close enough to stick to the moisture. One way around this is tapping, or dynamic contact mode.

In tapping mode, the cantilever uses a piezoelectric element mounted on the top to oscillate it at near to its resonance frequency with an amplitude of up to 200nm. The forces cause the amplitude to decrease as the tip gets close to the surface, and the height of the cantilever adjusts to keep the amplitude constant. This tapping results in less damage to the sample than contact mode and is more accurate than non-contact mode when moisture is present in a sample [13]. Diamagnetic oxide materials can also be studied using suitable paramagnetic probes, including nitroxides and transition metal ions. These observable paramagnetic centers may include surface defects, inorganic or organic radicals, metal cations or supported metal complexes and clusters.

Each of these paramagnetic species will produce a characteristic EPR signature with well defi ned spin Hamiltonian parameters. However, the magnetic properties, stability and reactivity of these centers may vary dramatically depending on the nature of the support or the measurement conditions. In some case, radicals stable on one surface may be transient on another, while variations in the EPR spectra of these radicals may be observed simply by altering the pretreatment conditions of the support. Furthermore, the spin Hamiltonian parameters for a particular paramagnetic species may vary greatly from one support to another. A number of "external" perturbations, such as the specific location of the species on a surface, the presence of other interaction species in the catalytic system, the size of the metal particles, and so on, can alter the spin Hamiltonian parameters, resulting in a significantly modified EPR profile.

Finally, the accurate analysis of the experimental spectrum can only be achieved by simulation and the resulting spin Hamiltonian parameters can then be compared to values obtained by theoretical treatments.

The electron Zeeman Interaction.

The electron is a negatively charged particle which possesses orbital angular momentum as it moves around the nucleus. The electron also possesses spin angular momentum S as it spins about its own axis. The magnitude of S is given by

Equation 22

where S= the spin quantum number and h= Planck ' s constant. By restricting the dimension to one specified direction, usually assigned the z direction, then the component of the spin angular momentum can only assume two values

The term M S can have (2S + 1) different values: +S, (S -1), (S -2) and so on. If the possible values of MS differ by one and range from -S to + S then the only two possible values for M S are +1/2 and -1 /2 for a single unpaired electron.

The most important physical consequence of the electron spin is the associated magnetic moment, μ e. This magnetic moment is directly proportional to the spin angular momentum and one may therefore write

The negative sign arises from the fact that the magnetic momentum of the electron is collinear but antiparallel to the spin itself. The factor (geμB) is referred to as the magnetogyric ratio and is composed of two important factors. The Bohr magneton, μB , is the magnetic moment for one unit of quantum mechanical angular momentum:

where e is the electron charge, me is the electron mass and ђ= h / 2π. The factor, ge, is known as the free electron g-factor with a value of 2.002319304386.

This magnetic moment interacts with the applied magnetic field. In classical terms the energy of the interaction between the magnetic moment ( μ) and the field ( B ) is described by

141

For a quantum mechanical system one must replace μ by the corresponding operator, giving the following simple spin Hamiltonian for a free electron in a magnetic field:

If the field is defined along the z-direction, then the scalar product simplifies to the following Hamiltonian:

𝐻 ̂= 𝑔 𝑒 𝜇 𝐵 𝑆 𝑍 * 𝐵 Equation 28The Sz value in the above equation can then be replaced by MS (in quantum mechanical terms the only operator in the right-hand term above is Sz), giving 𝐸 = 𝑔 𝑒 𝜇 𝐵 𝐵𝑀 𝑆 Equation 29Since MS = ± 1/2 only two energy states are available, which are degenerate in the absence of a magnetic field, but as B increases this degeneracy is lifted linearly as illustrated in . The separation of the two levels can be matched to a quantum of radiation through the Bohr frequency condition:

The existence of two Zeeman levels, and the possibility of inducing transitions from the lower energy level to the higher energy level is the very basis of EPR spectroscopy [14]. where h is the constant of Planck and v is the frequency of the electromagnetic wave. Therefore the change in the vibrational status of the molecule (∆Ef-i) can be expressed as:

If in Equation 32we substitute the definition of frequency with the number of waves in the distance light travels in one second, where c is the velocity of light and λ is the wavelenght of the radiation, we find that

and since c is constant and h is constant, the change in the difference of energy can be expressed in terms of the reciprocal of the wavelength, whose dimension is cm -1 , and it is denominated the wavenumber, this is the normal unit used both in Raman and Infrared spectroscopy [15].

The Raman experiments were carried out on XploRA (Jobin-Yvon-Horiba) at room temperature under microscope Olympus with a magnification 50X and using the laser radiation (532 nm) as the excitation source with a nominal power at the sample of about 1.24 mW, Fig. 79. According to L-H model, when initial concentration Co (ppm) is very small the following pseudofirst order rate equation is followed

where k is pseudo-first order rate constant (hr -1 ) and t is time (hr). A plot of ln (C/Co) versus time represents a straight line, the slope of which upon linear regression equals the pseudo-first order rate constant k (hr -1 ) [17].