2. Materials and methods

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

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

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 pow-

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

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

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

dioxide can have infinite solubility in anatase titanium dioxide phase.

phase, which is not observed in the literature.

that auto-neutralizing phenomenon [10, 31].

der samples [29].

68 Titanium Dioxide

phases.

In order to provide a consistent discussion about the influence of the zirconium-silicon simultaneous doping, the non-modified titanium dioxide samples were prepared in the same experimental conditions used to obtaining the doped samples. It was used the true sol-gel method involving only analytical grade reagents for the preparation of the powder samples. First, titanium (IV) isopropoxide 97% (Sigma-Aldrich) was added drop wise in glacial acetic acid 99.7% (F. Maia) under stirring. The molar ratio between the acetic acid and total metallic cations was adjusted to 4 for both samples. Due to the exothermic reaction, the homogenous mixture was cooled to room temperature by maintaining stirring for 30 min before the next step. For silicon-zirconium (Si-Zr)-doped titanium dioxide sample, tetraethyl orthosilicate 98% (Sigma-Aldrich) and zirconium (IV) propoxide 70% (Sigma-Aldrich) are added in order to obtain 5 mol % of Si-Zr doping. Then, for both samples, the isopropyl alcohol R.G. (Qhemis) was added in order to adjust the metal concentration to 0.1 molar.

After a homogenization stage through the stirring for 30 min, acidified water was added in order to promote the acid hydrolysis. For that proposes, nitric acid solution with pH 3.5 was previously prepared in order to represent a molar ratio between water and metallic cations close to 5. Thus, both composition sols were stirred for 1 hour, capped and allowed to stand at ambient conditions for 24 hours in order to complete the jellification. Both xerogels showed transparent characteristics before the drying process carried out in drying oven at 100C overnight and ground process in porcelain mortar. Finally, both dry gel samples were divided into several aliquots in order to perform the calcination step in wide range of temperature for 2 hours in a muffle type oven under static atmosphere. The expected compositions are TiO2 and Si0.25Zr0.25Ti0.95O2, for pure and Si-Zr-doped titanium dioxide-calcined powder samples, respectively.

Both dry gel compositions were characterized by simultaneous thermogravimetric and differential thermal analysis (TG/DTA) in order to verify basic differences between non-doped and Silicon-zirconium-doped gel sample along the temperature of thermal treatment. For that characterization, aliquots close to 100 mg were placed in alumina crucibles and compared with the same amount of alpha-alumina powder as standard material. Sample and reference material crucibles were submitted to heating rate of 10C min<sup>1</sup> from room temperature up to 900C under synthetic air flux of 10 mL min<sup>1</sup> by using a Netszch—Thermische Analyse equipped with TASC 414/2 controller and Pt 10 thermocouples.

The dry gel at 100C and all of the calcined powder samples in a wide temperature set were characterized by X-ray diffractometry by using D5005 Siemens equipment operating with Kalpha nickel-filtered Cu radiation from 20 to 80 (2-theta) in step scan mode in order to collect the diffraction signal in intervals of 0.02 (2-theta) during 1 s. The obtained diffraction patterns were phase identified using the JCPDS data bank [54], and the anatase and rutile structural models were taken from ICSD data bank [55]. The raw files were refined starting from the chosen anatase and rutile structural models by rietveld method [56] performed with the last upgraded DBWS software [57, 58] in order to include the size-strain calculation.
