3. Results and discussion

Figure 1(a) shows the TG/DTG/DTA curves for both dry gel samples, where it is possible to observe higher weight losses for Si-Zr-doped titanium dioxide dry gel in all of the weight loss stages from room temperature up to 460C. The first weight loss is an endothermic event associated to the volatilisation processes of residual alcohols and acetic acid, including probable ester and nitro compounds, which possess boiling temperatures up to 100C. In addition to the slight narrow endothermic peak observed for Si-Zr doped titanium dioxide dry gel, the volatile compounds present in that sample were 1.7%, against 1. 3% for pure one, considering the final temperature at 180C for both curves, according to the DTA peaks.

Structural Aspects of Anatase to Rutile Phase Transition in Titanium Dioxide Powders Elucidated by… http://dx.doi.org/10.5772/intechopen.68601 71

% of Si-Zr doping. Then, for both samples, the isopropyl alcohol R.G. (Qhemis) was added in

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,

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

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

Figure 1(a) shows the TG/DTG/DTA curves for both dry gel samples, where it is possible to observe higher weight losses for Si-Zr-doped titanium dioxide dry gel in all of the weight loss stages from room temperature up to 460C. The first weight loss is an endothermic event associated to the volatilisation processes of residual alcohols and acetic acid, including probable ester and nitro compounds, which possess boiling temperatures up to 100C. In addition to the slight narrow endothermic peak observed for Si-Zr doped titanium dioxide dry gel, the volatile compounds present in that sample were 1.7%, against 1. 3% for pure one, considering

upgraded DBWS software [57, 58] in order to include the size-strain calculation.

the final temperature at 180C for both curves, according to the DTA peaks.

order to adjust the metal concentration to 0.1 molar.

with TASC 414/2 controller and Pt 10 thermocouples.

3. Results and discussion

respectively.

70 Titanium Dioxide

Figure 1. Thermogravimetric and differential thermal analysis for pure and silicon-zircon-doped titanium dioxide dry gel samples: (a) entire TG/DTG/DTA curves and (b) amplification of TG curves between 390 and 900C.

Residual organic compounds and free-water are desorbed from porous matrixes between 180 and 250C, markedly a kinetic event, as can be verified by the absence of DTG or DTA peaks in that region. That desorption event represented a weight loss of 0.7% for Si-Zr-doped titanium dioxide dry gel, against 0.4% for the same event in pure titanium dioxide sample. Thus, the total weight losses observed for Si-Zr-doped and pure titanium dioxide dry gels from 30 to 250C increase for 2.4 and 1.7%, respectively.

The first exothermic event for both samples occurs between 250 and 460C, which was associated to the cross-link bonding among the hydroxyl groups in nuclei surface through the dehydroxylation process. The water losses associated to that event were close to 2.8% for Si-Zr-doped titanium dioxide sample and 2.6% for pure one. It is also possible to notice that the exothermic peaks (DTA) precede the water weight loss (DTG) in almost 20C, which prove that the water loss is slower than the dehydroxylation process.

Supposedly, this result is coherent with expected reduction of empty volume among the nuclei due to the pore constriction associated to the cross-link bonding, which harms the water desorption. It is also possible to notice that the exothermic peak for Si-Zr-doped titanium dioxide sample is considerably more intense than that observed for the pure one. Then, already during the dehydroxylation process, an important energetic difference is observed for zircon silicate titanium dioxide samples when compared to the pure one, which can justify possible changes in photocatalysis performances due to the important role the hydroxyl groups in anatase surface have in charge transference and electron-hole recombining processes.

Finally, the anatase crystallization process is observed after the last weight loss and the Si-Zrdoped titanium dioxide samples present two evidenced exothermic events. According to correlated work [59], the presence of silicon dioxide in titanium dioxide seems to favour the crystallization process as a facilitator of the oxygen vacancy elimination. In fact, it is possible to visualize that the Si-Zr-doped titanium-doped sample has both events more exothermically energetic than the pure one. But, other considerable differences are also observed concerning the temperature of maximum energy. The Si-Zr-doped titanium sample possesses the first crystallization stage starting below 400C, already during the cross-link bonding, and ends at 550C. For the pure titanium dioxide sample, there is only a shoulder at 650C in DTA curve, which is very difficult to view without the graphic software help.

Taking into account that the shoulder in pure titanium dioxide sample occurs above the typical anatase-to-rutile phase transition, it is possible to infer that some amounts of rutile phase probably is already present at that 650C. Thus, the third exothermic event for pure-doped titanium-doped sample is centred at 800C, probably associated to the complete formation of rutile phase. On the other hand, the Si-Zr-doped titanium dioxide sample presents the third exothermic event probably above 900C (not observed due the analysis ending). Nevertheless, there is no evidence that the rutile phase is already present above 900C, because the zircon silicate has an increasingly stable anatase structure with increasing calcination time and temperature. Thus, 5 mol% of Si-Zr may be sufficient to prevent the rutile phase transition in titanium dioxide samples calcined until 900C, at least.

Figure 1(b) shows the amplification of TG curves between 390 and 896C in order to show the weight gain for both samples starting to occur immediately after the final weight loss. Then, it is acceptable that the hypothesis based on oxygen vacancies elimination starts after the ending of cross-link bonding. Both titanium dioxide samples present significant weight gain due to the oxygen incorporation. However, the pure titanium dioxide sample presents higher 0.5% more oxygen incorporation than the Si-Zr-doped one. In addition, the weight gain for Si-Zr-doped sample only becomes remarkable after 550C.

One important relation between both the samples related to the anatase phase stabilization can be established by considering that the oxygen incorporation is not finished at 900C due to the kinetic component. Thus, more weight gain can continue to occur until both samples reach a similar weight gain values. If it is true, then the velocity constant for oxygen incorporation in the Si-Zr-doped sample is lower because the same weight gain is reached for pure sample at 811C, which represents 85C lower than the Si-Zr-doped sample. An accurate investigation of that dependence would be very interesting to clarify the energetic changes provided by simultaneous silicon-zirconium doping process.

In Figure 2, X-ray diffraction patterns showing the phase evolution starting from 100C for 24 hours to 900C for 2 hours are shown. The pure titanium dioxide sample presents anatase single phase up to 600C, which means the rutile phase starts to form at 650C, according to DTA analysis evidence. On one hand, a continuous increase in temperature of calcination leads to much more rutile phase in the samples until the formation of rutile single phase in pure titanium dioxide powder samples above the 800C (Figure 2(a)). On the other hand, the Si-Zr-doped titanium dioxide samples present no evidence of rutile phase even at 900C (Figure 2(b)).

The angle degrees and the relative intensities for anatase phase peaks in all of the calcination temperatures fit with those available on JCPDS card number 21-1272, whereas the rutile phase

Structural Aspects of Anatase to Rutile Phase Transition in Titanium Dioxide Powders Elucidated by… http://dx.doi.org/10.5772/intechopen.68601 73

visualize that the Si-Zr-doped titanium-doped sample has both events more exothermically energetic than the pure one. But, other considerable differences are also observed concerning the temperature of maximum energy. The Si-Zr-doped titanium sample possesses the first crystallization stage starting below 400C, already during the cross-link bonding, and ends at 550C. For the pure titanium dioxide sample, there is only a shoulder at 650C in DTA curve,

Taking into account that the shoulder in pure titanium dioxide sample occurs above the typical anatase-to-rutile phase transition, it is possible to infer that some amounts of rutile phase probably is already present at that 650C. Thus, the third exothermic event for pure-doped titanium-doped sample is centred at 800C, probably associated to the complete formation of rutile phase. On the other hand, the Si-Zr-doped titanium dioxide sample presents the third exothermic event probably above 900C (not observed due the analysis ending). Nevertheless, there is no evidence that the rutile phase is already present above 900C, because the zircon silicate has an increasingly stable anatase structure with increasing calcination time and temperature. Thus, 5 mol% of Si-Zr may be sufficient to prevent the rutile phase transition in

Figure 1(b) shows the amplification of TG curves between 390 and 896C in order to show the weight gain for both samples starting to occur immediately after the final weight loss. Then, it is acceptable that the hypothesis based on oxygen vacancies elimination starts after the ending of cross-link bonding. Both titanium dioxide samples present significant weight gain due to the oxygen incorporation. However, the pure titanium dioxide sample presents higher 0.5% more oxygen incorporation than the Si-Zr-doped one. In addition, the weight gain for Si-Zr-doped

One important relation between both the samples related to the anatase phase stabilization can be established by considering that the oxygen incorporation is not finished at 900C due to the kinetic component. Thus, more weight gain can continue to occur until both samples reach a similar weight gain values. If it is true, then the velocity constant for oxygen incorporation in the Si-Zr-doped sample is lower because the same weight gain is reached for pure sample at 811C, which represents 85C lower than the Si-Zr-doped sample. An accurate investigation of that dependence would be very interesting to clarify the energetic changes provided by

In Figure 2, X-ray diffraction patterns showing the phase evolution starting from 100C for 24 hours to 900C for 2 hours are shown. The pure titanium dioxide sample presents anatase single phase up to 600C, which means the rutile phase starts to form at 650C, according to DTA analysis evidence. On one hand, a continuous increase in temperature of calcination leads to much more rutile phase in the samples until the formation of rutile single phase in pure titanium dioxide powder samples above the 800C (Figure 2(a)). On the other hand, the Si-Zr-doped titanium dioxide samples present no evidence of rutile phase even at 900C

The angle degrees and the relative intensities for anatase phase peaks in all of the calcination temperatures fit with those available on JCPDS card number 21-1272, whereas the rutile phase

which is very difficult to view without the graphic software help.

titanium dioxide samples calcined until 900C, at least.

sample only becomes remarkable after 550C.

simultaneous silicon-zirconium doping process.

(Figure 2(b)).

72 Titanium Dioxide

Figure 2. X-ray diffraction patterns showing the phase evolution for starting from 100C for 24 hours up to 900C for 2 hours: (a) pure and (b) Si-Zr titanium dioxide samples.

was perfectly identified with the angle degrees and the relative intensities available on JCPDS card number 21-1276. Nevertheless, it is easy to observe a considerable peak enlargement for Si-Zr-doped samples for all temperatures, if compared to the pure ones. However, neither the angle degrees nor the relative intensities are changed for the anatase peaks in Si-Zr-doped titanium dioxide samples along the temperature of calcination.

In order to understand the structural effects beyond the visual observation of the X-ray diffraction patterns, a structural refinement was carried out for all of the samples. That procedure starts choosing the structural models for anatase and rutile phase available on the ICSD data bank. The best adequacy was obtained by setting the lattice parameters and atomic positions according to card numbers 82084 and 53997, for anatase and rutile phase, respectively. After more than 3000 cycles, the refinement factors reach the minimum permitted for statistically expected values provided by method. More information about that methodology is available on the specific literature [56–58, 60].

The lattice parameters of each phase are shown in Figure 3. It is possible to observe a consistent variation for all of the refined parameters, making possible an appropriated discussion. For anatase phase, both compositions, the lattice parameters present the same variation samples and, except for 100C, the vector a and c values are slightly higher for pure titanium dioxide samples, at least up to 600C, where the anatase phase for pure titanium dioxide sample becomes unstable (Figure 3(a)). By considering tetravalent silicon and zirconium hexacoordinate dopant cations, with ionic radii of 40 pm and 72 pm, respectively, the average radii are close to 56 pm. Therefore, the substituting cations constitute as smaller than hexacoordinate tetravalent titanium substituted one (61 pm). Thus, it is a coherent result that the slight lattice contraction was observed for the Si-Zr doped samples.

Maybe it is not important to explain the basis of the anatase phase stabilization, but the inverse effect that occurred at 100C is in consequence of the higher amount hydroxyl groups in Si-Zrdoped titanium dioxide dry gel, as was demonstrated in TG analysis. The importance of the Rietveld analysis is established just above the 650C and remarkable difference is observed among the samples originated from both the compositions. In temperatures close to anatase-to-rutile

Figure 3. Refined lattice parameters for anatase phase as a function of calcination temperature for: (a) vectors values and (b) calculated unit cell volume and tetragonality (c/a).

phase transition, the both vectors a and c of anatase phase for pure titanium dioxide samples start to reduce, marking the initial collapse of anatase phase, which will no longer be present at 800C. By observing the same variation for Si-Zr-doped samples, it is possible to observe that behaviour is characteristic of anatase phase crystallization, but only for pure titanium dioxide sample and is followed by the anatase to rutile phase transition.

By observing the variation of anatase cell volume in Figure 3(b) it is possible to notice an impressive variation similarity with the variation of vector a (Figure 3(a)), for both samples. It is possible to observe the tetragonality changes between the pure and the Si-Zr-doped titanium dioxide samples in the same way, but only displaced to higher temperature for Si-Zr-doped one. Thus, those results corroborate the previous DTA differences and the tetragonality of anatase single phase in the Si-Zr-doped titanium dioxide sample calcined at 900C is very close to the well-crystallized anatase single phase observed in the pure titanium dioxide sample calcined at 600C.

The reliable crystallization process was investigated through the size-strain calculation according to the methodology well-established [60] and using tungsten carbide as standard material in order to evaluate the instrumental contributions. It was noticed that the crystallite size stays in low values until the beginning of anatase-to-rutile phase formation and the peak narrowing is originated from the reduction of the lattice microstrain practically (Figure 4(a)). Thus, the crystallite coalescence occurs as a consequence of the destroying-rebuilt oxygen-metallic cation bonds process starting at 650C, regardless of anatase-to rutile phase transition. Beyond that reconstructive transformation, above 700C, the crystallite sizes considerably increase despite of

Structural Aspects of Anatase to Rutile Phase Transition in Titanium Dioxide Powders Elucidated by… http://dx.doi.org/10.5772/intechopen.68601 75

Figure 4. Calculated data through the refined lattice parameters and peak profiles: (a) crystallite size and lattice microstrain and (b) isotropic thermal parameters.

some variation on lattice microstrain. In Figure 4(b), it is possible to observe that the anatase structure starts with spacious oxygen site through the negative values for isotropic thermal parameters. As the atomic scattering is associated to a specific site volume, it is probable that the hydroxyl groups push oxygen atoms away from each other. The isotropic parameters become higher after the dehydroxylation process and that results are coherent with cell volume variation and thermal analysis results. However, for pure titanium dioxide samples, the atoms experience a new localized spacing in the temperature close to the anatase-to-rutile phase transition, different from Si-Zr-doped ones. That event indicates the reconstructive transformation occurs through the destroying-rebuilt bonds, because the variation of the isotropic thermal parameters presents noticeable inflection during that process.

### 4. Conclusions

phase transition, the both vectors a and c of anatase phase for pure titanium dioxide samples start to reduce, marking the initial collapse of anatase phase, which will no longer be present at 800C. By observing the same variation for Si-Zr-doped samples, it is possible to observe that behaviour is characteristic of anatase phase crystallization, but only for pure titanium dioxide sample and is

Figure 3. Refined lattice parameters for anatase phase as a function of calcination temperature for: (a) vectors values and

By observing the variation of anatase cell volume in Figure 3(b) it is possible to notice an impressive variation similarity with the variation of vector a (Figure 3(a)), for both samples. It is possible to observe the tetragonality changes between the pure and the Si-Zr-doped titanium dioxide samples in the same way, but only displaced to higher temperature for Si-Zr-doped one. Thus, those results corroborate the previous DTA differences and the tetragonality of anatase single phase in the Si-Zr-doped titanium dioxide sample calcined at 900C is very close to the well-crystallized anatase single phase observed in the pure titanium dioxide

The reliable crystallization process was investigated through the size-strain calculation according to the methodology well-established [60] and using tungsten carbide as standard material in order to evaluate the instrumental contributions. It was noticed that the crystallite size stays in low values until the beginning of anatase-to-rutile phase formation and the peak narrowing is originated from the reduction of the lattice microstrain practically (Figure 4(a)). Thus, the crystallite coalescence occurs as a consequence of the destroying-rebuilt oxygen-metallic cation bonds process starting at 650C, regardless of anatase-to rutile phase transition. Beyond that reconstructive transformation, above 700C, the crystallite sizes considerably increase despite of

followed by the anatase to rutile phase transition.

(b) calculated unit cell volume and tetragonality (c/a).

74 Titanium Dioxide

sample calcined at 600C.

The accurate structural characterization was carried out by X-ray diffractometry and rietveld refinement in titanium dioxide sample synthesized through sol-gel method and calcined in wide temperature range in order to evaluate the effects on the thermal stabilization of the zirconium silicate doping at 5 mol%. The discussion of structural variations in both pure and Si-Zr-doped sample compositions was supported by corresponding thermal analysis taken from dry gels and consistent correlations were found.

The silicon and zircon dopants lead to more retention of residues coming from jellying process and also the fresh anatase formed is more hydrolyzed. All of the exothermic events associated to the phase crystallization in both samples are displaced to higher temperatures if compared to the pure titanium dioxide sample. Thus, the final crystallization associated to oxygen vacancies elimination as a function of oxygen incorporation occurs above 900C.

By observing the X-ray diffraction patterns, it was possible to prove that the Si-Zr dopants at concentration of 5 mol% are able to prevent the rutile phase transition in titanium dioxide samples calcined up to 900C. But, the rietveld analysis was very important to found the structural basis for that behaviour and the known reconstructive transformation was proved for the anatase-to-rutile phase transition. It was possible to show that the isotropic thermal parameters for oxygen and titanium atoms are considerably affected just during the anatase phase collapse and rutile one rising.

The crystallite coalescence responsible for anatase crystallization does not occur in pure titanium dioxide samples, and the X-ray diffraction peaks narrowing observed for calcined samples in several publications is consequence only of the microstrain reduction. Thus, the pure titanium dioxide samples cannot be crystallized as anatase phase without the parallel rutile phase conversion, different from the Si-Zr-doped titanium dioxide sample. Consequently, well-crystallized anatase single phase at 900C can be obtained by doping the titanium dioxide samples with 5 mol% of Si-Zr. That composition can become a new semiconductor matrix for the investigation of new dopants in order to improve further the photocatalysis performance or maybe, other possible applications.
