1. Introduction

The titanium dioxide TiO2 is a semiconductor ceramic material widely applied for polluted water remediation and self-cleaning surfaces. The main basis of titanium dioxide powder and thin film performance is the electron excitation from valance band to conducting one, which can be carried out by lighting the material surface with radiation energy greater than its band gap. Thus, the larger the number of electron-hole pairs, the greater the rate of degradation of organic compounds by oxy-reduction [1–5].

There are some mechanism details to be considered after the initiation of lighting process and the final mineralization of organic compounds, which can be changed as a function of the organic molecule type and its semi-decomposed by-products, organics concentration and pH of solution, distance between the radiation source and the semiconductor material, radiation intensity and losses in the optical path, as well as some characteristics of the powder or thin film morphologies. But, the characteristic of the titanium dioxide semiconductor is the most important factor to be considered, including the morphology, composition and crystalline structure [6–9].

First of all, the importance of solid morphology is an intuitive aspect due to the basis of heterogeneous catalysis, and the several papers on the literature relating to the synthesis of titanium dioxide nanoparticles provide a good idea of the importance of a large specific area for powders. Innovative semiconductor arrangements like p-n junction, titanium dioxide glass-ceramic, ion-sodium battery based on anatase titanium dioxide ceramic matrix are also investigated in order to provide new applications concerning novel structures and morphologies degrees, making the titanium dioxide much more investigated [10–12].

In sequence, at first sight, the composition aspect seems not be applicable because high purity titanium dioxide should have stoichiometry defined as TiO2. However, the fact is titanium dioxide is an intrinsic N-type semiconductor due to oxygen vacancies generated during the heat treatment, mainly for samples with high amount of anatase phase. Nevertheless, the non-doped n-type anatase titanium dioxide powder or thin films are usually applied as photocatalyst materials in advanced oxidation process for organic compounds. The justification is based on the yield of long-lived extrinsic photoholes, which is able to promote the water oxidation [13, 14].

The non-stoichiometry characteristic of titanium dioxide is very similar to the zinc oxide, so that many investigations consider the addition of extrinsic dopants in order to change semiconductor predominant behaviour. But, when the concentration of cation dopants becomes higher or when anion dopants are inserted in oxygen site, the crystalline structure starts to play an important role. In general, metallic cations with different oxidation states than tetravalent titanium are investigated and the energy bandgap and other photonic aspects are availed [15–17]. But, tetravalent titanium vacancies can be also formed through alternative procedures in order to generate some interesting properties, like high p-type conductivity and better photocatalytic performance [18].

Several works repot related the insertion of heterovalent metallic cations in titanium dioxide lead to the decreasing in the temperature of anatase-to-rutile phase transition and also to more amounts of rutile phase if the powder samples are heat treated up to 600C [19–22]. Most of the doping approaches aim to shift the absorption edge to lower frequencies than ultraviolet range in order to utilize the titanium dioxide in solar photocatalysis. The nitrogen insertion by replacing lattice oxygen in anatase phase of titanium dioxide has also this central objective. However, there are strong suspicions that the desired surface electronic enhancement was followed by a considerable lattice surface distortion [23, 24].

1. Introduction

64 Titanium Dioxide

performance [18].

organic compounds by oxy-reduction [1–5].

The titanium dioxide TiO2 is a semiconductor ceramic material widely applied for polluted water remediation and self-cleaning surfaces. The main basis of titanium dioxide powder and thin film performance is the electron excitation from valance band to conducting one, which can be carried out by lighting the material surface with radiation energy greater than its band gap. Thus, the larger the number of electron-hole pairs, the greater the rate of degradation of

There are some mechanism details to be considered after the initiation of lighting process and the final mineralization of organic compounds, which can be changed as a function of the organic molecule type and its semi-decomposed by-products, organics concentration and pH of solution, distance between the radiation source and the semiconductor material, radiation intensity and losses in the optical path, as well as some characteristics of the powder or thin film morphologies. But, the characteristic of the titanium dioxide semiconductor is the most important factor

First of all, the importance of solid morphology is an intuitive aspect due to the basis of heterogeneous catalysis, and the several papers on the literature relating to the synthesis of titanium dioxide nanoparticles provide a good idea of the importance of a large specific area for powders. Innovative semiconductor arrangements like p-n junction, titanium dioxide glass-ceramic, ion-sodium battery based on anatase titanium dioxide ceramic matrix are also investigated in order to provide new applications concerning novel structures and morphol-

In sequence, at first sight, the composition aspect seems not be applicable because high purity titanium dioxide should have stoichiometry defined as TiO2. However, the fact is titanium dioxide is an intrinsic N-type semiconductor due to oxygen vacancies generated during the heat treatment, mainly for samples with high amount of anatase phase. Nevertheless, the non-doped n-type anatase titanium dioxide powder or thin films are usually applied as photocatalyst materials in advanced oxidation process for organic compounds. The justification is based on the yield of long-lived extrinsic photoholes, which is able to promote the water oxidation [13, 14]. The non-stoichiometry characteristic of titanium dioxide is very similar to the zinc oxide, so that many investigations consider the addition of extrinsic dopants in order to change semiconductor predominant behaviour. But, when the concentration of cation dopants becomes higher or when anion dopants are inserted in oxygen site, the crystalline structure starts to play an important role. In general, metallic cations with different oxidation states than tetravalent titanium are investigated and the energy bandgap and other photonic aspects are availed [15–17]. But, tetravalent titanium vacancies can be also formed through alternative procedures in order to generate some interesting properties, like high p-type conductivity and better photocatalytic

Several works repot related the insertion of heterovalent metallic cations in titanium dioxide lead to the decreasing in the temperature of anatase-to-rutile phase transition and also to more amounts of rutile phase if the powder samples are heat treated up to 600C [19–22]. Most of the

to be considered, including the morphology, composition and crystalline structure [6–9].

ogies degrees, making the titanium dioxide much more investigated [10–12].

The most of the doped titanium dioxide structures present different effects for anatase and rutile polymorphism due to the not well-controlled sample preparation as well as overlapping of several different effects. The oxygen vacancies seem to occur in association with most dopant insertion, and their concentrations are very sensitive to heat treatments. In addition to the lattice distortions and the changes in cation gap states caused by doping process, the oxygen vacancies also act as hole-trapping sites [25–27].

Even for that high-ordered single-crystal rutile sample, the trivalent titanium acting as trap site in (110) surface was found [28]. Any of those events seems to be significantly responsible for the enhancing or damaging of the titanium dioxide photocatalyst performance. Thus, wellcharacterized structural model systems are required to understand other changes in the material bulk or surface [29].

The exclusive structural aspects deserve special attention in order to understand a little more about the interfacial charge transfer in complex surface structure of the titanium dioxide photocatalyst, because the free energy on the single crystal surface is affected by its crystallographic orientation [30]. Different crystallographic planes were investigated separately in single crystal samples, and an interesting hypothesis was proposed concerning the surface atoms alignment. A reasonable experiment set including the etching of high aligned (001) facet proved that the more aligned the oxygen and titanium atoms on the surface, the greater are the chances of recombination of the electron-hole pairs [27].

The mechanisms of chemical reaction as a function of crystalline phase are also investigated. The single crystalline rutile (110) and anatase (101) samples submitted to the oxygen and water adsorptions at low temperatures, during the reheating up to room temperature, can be very useful to understand several aspects of photocatalysis mechanisms. Both rutile (110) and anatase (101) surfaces experience the reaction between oxygen and water molecules in order to form terminal hydroxyl groups on the phase surfaces. While the hydroxyl groups formed on the rutile (110) surface are highly bridged, on the anatase (101) surface the hydroxyl groups remain isolated and stable even after annealing above 130C. The water molecules desorbs readily at room temperature from anatase (101) surfaces, whereas the oxygen ones remain undissociated and adsorbed in the vicinity of extrinsic P-type defects, and those differences have important consequences for photocatalysis application [31].

It is well understood that the anatase-to-rutile phase transition in polycrystalline titanium dioxide powder samples is strongly dependent on the anatase phase characteristics, mainly the impurity amount and the extrinsic dopant type. But, some structural surface aspects for non-doped and high purity powder samples are also indicative signals, such as the nucleation of the rutile phase by anatase (112) surface. A different intermediate metastable phase dissimilar to the prior anatase or the final rutile phases was observed, which showed to be sensitive to the compressive strain and the phase anisotropy [32, 33].

The photonic behaviour is strongly influenced by predominant phase characteristics, but some results attributed to the partial anatase-to-rutile phase transition through the controlled heat treatment leading to an enhancement in the photocatalytic performance. However, this hypothesis is controversial, because there are serious evidences that the anatase-to-rutile phase transition can start only the previous ordering in anatase phase. Thus, the relationship between well-crystallized anatase structure and photocatalytic activity can aid to understand some aspects not yet clarified about this material [34, 35].

The irreversible anatase-to-rutile phase transition occurring in temperatures above 600C is a well-known fact for any experienced researcher working with titanium dioxide samples. It is also easy to verify that increasing the rutile phase amount is proportional to the temperature of heat treatment beyond 600C. The anatase phase in titanium dioxide powder samples can be obtained even at room temperatures if no high amounts of organics are present in precursor materials, such as that provided by conventional sol-gel method. No structural differences seem to occur between the fresh hydrolysed samples and the heat treated one until 150C, because the anatase phase nuclei is a water-driven process in order to form oxy-hydroxide titanium, which is relatively stable up to 150C [36–38].

The continuous increase in temperature of heat treatment triggers several sequential decomposition stages in fresh hydrolysed gels, concerning the elimination of several residual compounds, such as the adsorbed water, hydroxyl groups and organic by-products. Unless the organic by-product types are able to form carbonaceous solid compounds, above the 250C, the anatase phase is already an impurity-free material. Nevertheless, the desidroxylation stage is followed by the formation of cross-linked nuclei, which creates high amount of structural defects, in special, the oxygen vacancies. Those defects only can be eliminated overcoming the energy barrier of titanium-oxygen bonds, which leads to the structural destroy-rebuilding processes. This mechanism is the basis of the reconstructive transformation observed in the anatase-to-rutile phase transition [24, 32] and that is why the effective anatase phase crystallization only occurs when the rutile phase starts to form.

It was demonstrated earlier that the oxygen vacancies in titanium dioxide material configure an N-type semiconductor, which seems to be a not good photocatalyst due to high electronhole combination occurring in oxygen vacancies. In addition, the formation of hydroxyl radical in aqueous media is also dependent on positive holes. Consequently, several dopants with lower oxidation states than titanium (IV) are investigated in order to make that material a P-type semiconductor [23].

In parallel, the synthesis method also plays an important role on the anatase crystallization and the sol-gel method is one of the most used to synthesize titanium dioxide powder and thin film samples. That chemical route is able to synthesize well-crystallized titanium dioxide samples with anatase single phase at very low temperature because the titanium dioxide formation is a water-driven process that occurs even at room temperature and leads invariably to anatase single phase. It is possible to use only volatile coadjutant reagents, such as the metallic alkoxide precursors and acetic acid complexing agent, so that very little amount of organic wastes gets retained when the fresh gels are dried at 100C or almost nothing if dried above 150C. There is no chance for the existence of organic solid residues, even if metallic nitrates are used as dopant precursor reagents, which makes the sol-gel method the preferential choice in order to obtain nanoparticles of titanium dioxide in anatase single phase with relative success [39].

The photonic behaviour is strongly influenced by predominant phase characteristics, but some results attributed to the partial anatase-to-rutile phase transition through the controlled heat treatment leading to an enhancement in the photocatalytic performance. However, this hypothesis is controversial, because there are serious evidences that the anatase-to-rutile phase transition can start only the previous ordering in anatase phase. Thus, the relationship between well-crystallized anatase structure and photocatalytic activity can aid to understand

The irreversible anatase-to-rutile phase transition occurring in temperatures above 600C is a well-known fact for any experienced researcher working with titanium dioxide samples. It is also easy to verify that increasing the rutile phase amount is proportional to the temperature of heat treatment beyond 600C. The anatase phase in titanium dioxide powder samples can be obtained even at room temperatures if no high amounts of organics are present in precursor materials, such as that provided by conventional sol-gel method. No structural differences seem to occur between the fresh hydrolysed samples and the heat treated one until 150C, because the anatase phase nuclei is a water-driven process in order to form oxy-hydroxide

The continuous increase in temperature of heat treatment triggers several sequential decomposition stages in fresh hydrolysed gels, concerning the elimination of several residual compounds, such as the adsorbed water, hydroxyl groups and organic by-products. Unless the organic by-product types are able to form carbonaceous solid compounds, above the 250C, the anatase phase is already an impurity-free material. Nevertheless, the desidroxylation stage is followed by the formation of cross-linked nuclei, which creates high amount of structural defects, in special, the oxygen vacancies. Those defects only can be eliminated overcoming the energy barrier of titanium-oxygen bonds, which leads to the structural destroy-rebuilding processes. This mechanism is the basis of the reconstructive transformation observed in the anatase-to-rutile phase transition [24, 32] and that is why the effective anatase phase crystalli-

It was demonstrated earlier that the oxygen vacancies in titanium dioxide material configure an N-type semiconductor, which seems to be a not good photocatalyst due to high electronhole combination occurring in oxygen vacancies. In addition, the formation of hydroxyl radical in aqueous media is also dependent on positive holes. Consequently, several dopants with lower oxidation states than titanium (IV) are investigated in order to make that material

In parallel, the synthesis method also plays an important role on the anatase crystallization and the sol-gel method is one of the most used to synthesize titanium dioxide powder and thin film samples. That chemical route is able to synthesize well-crystallized titanium dioxide samples with anatase single phase at very low temperature because the titanium dioxide formation is a water-driven process that occurs even at room temperature and leads invariably to anatase single phase. It is possible to use only volatile coadjutant reagents, such as the metallic alkoxide precursors and acetic acid complexing agent, so that very little amount of organic wastes gets retained when the fresh gels are dried at 100C or almost nothing if dried above 150C. There is no chance for the existence of organic solid residues, even if metallic

some aspects not yet clarified about this material [34, 35].

66 Titanium Dioxide

titanium, which is relatively stable up to 150C [36–38].

zation only occurs when the rutile phase starts to form.

a P-type semiconductor [23].

In addition, the amounts of alcohol solvent and water as hydrolyzing agent are also important on the particle growth stage, because the cross-linked bonds are affected directly by those coadjutant reagents during the first stages of titanium oxy-hydroxide nuclei formation. It is almost a consensus that the mean particle diameter reduces as a function of water content in the jellifying process [40], due to the more separation of nuclei from each other. Nevertheless, that means a more amount of terminal hydroxyl groups in anatase phase particle surface and, consequently, a more amount of cross-linked bonds emerging during the drying stage.

The anatase thermal stability depends on the defect concentration, including the oxygen vacancies. The oxygen vacancies concentration can be dependent on the amount of the crosslinked bonds among the nanoparticles so that the titanium dioxide nanoparticles can present a significant increase in oxygen vacancies when compared to the coarse particles. Thus, the literature shows that the thermal treatment to crystallize the titanium dioxide nanoparticles is preferentially carried out in temperatures below 500C, which is still far from the energetic barrier for anatase-to-rutile phase transition [41]. Less often, the nanoparticles can be carefully thermal treated under step-by-step process by increasing the temperature from 250 to 600C, at most, if it is desirable to obtain anatase single phase [42], which is probably due to the lower stability for anatase phase in nanoparticle form [43, 44].

On the other hand, homovalent cation doping seems to reduce also the oxygen vacancies, which can also be associated with cross-linked metal-oxygen bonds in anatase phase. The insertion of zirconium at 10 mol% in titanium dioxide powder samples avoids the full anatase-to-rutile phase transition at higher temperatures than 600C, leading to the calcined material to present high amounts of anatase phase when calcined up to 750C. By increasing the zirconium content and calcining the material at same temperature, much more amounts of anatase phase are observed in powder samples. However, an orthorhombic zirconium titanate secondary phase starts to crystallize in addition to the remaining anatase phase when the zirconium content is higher than 10 mol% [45].

By comparing the ionic radii for Ti (IV) and Zr (IV) hexacoordinate cations, it is possible to infer that the zirconium cation is bigger (72 pm) than titanium one (61 pm) [46]. As consequence, if the zirconium substitution in titanium site was successful, then a proportional anatase lattice expansion must be observed as a function of zirconium content until at certain concentration limit, at least [28, 45].

Another homovalent cation dopant investigated for anatase phase stabilization is the tetravalent silicon [47]. Thermal analysis from fresh gel has shown that the presence of silicon dopant can delay the anatase crystallization, which can be visualized by an exothermic peak, towards higher temperatures than undoped titanium dioxide. The cell volume for several calcined samples decreases continually as a function of silicon content, besides the crystallinity loss, as visualized by the X-ray diffraction patterns followed by rietveld refinement. The calcined samples in wide temperature range show an increase in temperature of anatase-to-rutile phase transition, but until a certain dopant concentration limit, signalled as 5 mol%. Different from zirconium dopant, the silicon one does not seem to generate secondary phases, but only to increase the atomic disordering in anatase phase [48]. Those results imply that the silicon dioxide can have infinite solubility in anatase titanium dioxide phase.

Some researchers have believed that the silicon cation is so smaller that its perfect accommodation in interstitial of titanium and oxygen sites of low dense anatase phase is possible. Thus, the explanation for anatase stabilization is justified, because the high dense rutile phase does not have enough interstitial space to accommodate hexacoordinate tetravalent silicon cations, avoiding the phase transition. That consideration is reasonable but is not true, because the ionic radii for hexacoordinate silicon (VI) is 40 pm [46], which means the titanium and silicon cations should experience some repulsion with each other to lead a lattice expansion in anatase phase, which is not observed in the literature.

Furthermore, an important ab-initio study taking into account the alternative silicon dopant positions in anatase and rutile phases buries the idea of interstitial silicon in titanium dioxide sample once and for all. The calculated results suggest that the interstitial tetravalent silicon cation is energetically favourable neither for anatase phase nor for rutile one. Also, the calculated results are in concordance with lattice parameter contraction; thus, it is not a good idea to continue with the consideration of interstitial tetravalent silicon cations in titanium dioxide anatase phase as the cause for anatase stabilization. At least while the X-ray diffraction data are showing the consistent anatase lattice contraction for silicon-doped titanium-doped powder samples [29].

The stabilization of anatase phase caused by homovalent dopants seems to be related to the increase in cross-linked metal-oxygen bond energies still in anatase phase. In addition, the reported enhancement in photocatalytic performance for homovalent doping cannot be related exclusively to the decrease in oxygen vacancies [28]. The effect in crystal surface can play an important role in order to reduce the recombination of electron-hole pair, and the anatase phase surface presenting oxygen and cations outer of the plane seems to be crucial to avoid that auto-neutralizing phenomenon [10, 31].

The first attempt to use simultaneously silicon and zirconium dopants was published in 2006 [49] but did not contribute to the correct understanding of the anatase phase stabilization in titanium dioxide samples. First, because the anatase phase stabilization occurring as a function temperature increasing or even significant changes in anatase-to-rutile phase transition were not demonstrated, according the X-ray diffraction patterns available on that work. Except at 700C, all of the samples presented rutile phase and the authors chose to explain the results by considering no dopant substitutions, but the effect of anatase interparticle secondary phases.

In addition, considerable confusion can occur if the dopant concentrations were referred as weight percent, due to the enormous difference in atomic weight among the metal constituents, and is not a good choice in order to permit the correct understanding of the progressive doping effects. The results provided by simultaneous and equal weight doping elements for silicon and zirconium at 5 wt% [51] correspond to 8.4 mol% for silicon and 2.6 mol% for zirconium, which means a Si/Zr molar ratio more than 3. Even so, no considerable differences were observed comparing the isolated zirconium or silicon doping under the point of view of anatase phase stabilization [47, 48, 50–52].

Other consideration about the mechanism of the anatase-to-rutile phase transition is no longer valid, specially, the affirmation that the anatase phase starts to convert into rutile one at lower temperatures through particle agglomeration in a kinetically controlled reaction. It was demonstrated already that there is a thermodynamic stability for anatase phase at standard conditions of temperature and pressure [29], which is determined probably by titanium-oxygen bond energies [24]. The kinetic component starts to play an important role only after overcoming the energetic barrier at very high temperatures, like above 900C [28].

Only in 2016, another publication reporting the silicon-zirconium-doped titanium dioxide sample was found [53]. There are several doped titanium dioxide nanoparticles calcined at 500C only, but unexpectedly, the authors discuss about the anatase-to-rutile phase transition as a function of dopant type for anatase single phase samples. In addition, a sample containing only silicon and zirconium dopants was also not prepared so that it was not possible to evaluate the effects of the simultaneous silicon and zirconium doping on titanium dioxide. Despite that the authors affirm that the sample simultaneously doped with copper, silicon and zirconium cations at 15 mol% in total concentration and equal parts showed a great improvement in photocatalytic performance for the degradation of methyl orange.

No other articles about the simultaneous silicon- and zirconium-doped titanium dioxide sample were found in the literature. That way, the present work aims to provide an accurate investigation about exclusive and simultaneous silicon and zirconium doping in titanium dioxide powder samples obtained through the sol-gel method in order to demonstrate the structural basis of anatase phase stabilization. It is very important to keep in mind that equal parts of tetravalent silicon and zirconium cations represent the average ionic radii close to the tetravalent titanium cation, considering all of those in hexacoordinate sites. It is also important to know that zircon silicate ZrSiO4 possesses also anatase phase, which can be thermally stable until very high temperatures [46, 54].
