Anatase, crystal phases

On heating to higher temperatures, no crystalline phases are observed until anatase crystallizes at 1000 °C. At 1400 °C, anatase, rutile and crystobalite are the only products. No single-phase material is obtained. The lack of correspondence between the TGA ceramic yield and the theoretical ceramic yield calculated for TiSi04 presages this problem. The exact reasons for the formation of a mixed-oxide phase are unknown at the moment, but they clearly contrast with the behavior of the Zr and Hf analogs. 

Fig.2 shows the XRD patterns of the initial powder and sintered Ti02 sheets, the initial phase of the powder is anatase, after sintering at 1100°C for Ih, the crystal phase of the Ti02 sheets has totally transformed to rutile, and this result can be confirmed by the main peaks at 25.3° and 27.4°, which are corresponding to the characteristie peak of anatase and rutile respectively. High temperature sintering favorably induces the phase transformation from anatase to rutile. 

XRD studies show that synthesized composites do not contain any crystal phase, just an amorphous phase. Optical absorption measurements prove that synthesized nanocomposites are containing Ti02 and Ti phases. For comparative analysis the pure Ti containing thin film was deposited onto the cold substrate (77 K) and onto the substrate at room temperature. The same result was obtained XRD analysis shows that the synthesized films only contain the amorphous phase. Kinetics of the electrical resistance increase with the air exposure of Ti/PPX nanocomposites (after synthesis under vacuum) is similar to that of the Al/PPX ones. For a metal content below the percolation threshold the metal particles became insulator within several seconds, whereas for the samples beyond the threshold the observed resistance increase is per cents within several hours. DTA analysis revealed that the heating of amorphous Ti02 nanoparticles up to a temperature of 480°C leads to a phase transformation to anatase, whereas heating up to 580°C results in the anatase transformation to the mtile structure. 

Depending on the voliune filHng factor of the matrix material, substantial shrinkage of the porous network accompanied by crack formation may occur for low filHng fractions during template removal. Furthermore, annealing the repHca material at elevated temperatures may lead to a transformation from one modification or crystal phase to another that is thermodynamically more stable at these temperatures, as shown for the thermal conversion of amorphous titania or its anatase phase to the rutile structure [79]. 

Wang, J., Chen, C., Liu, Y, et al. 2008a. Potential neurological lesion after nasal instillation of Ti02 nanoparticles in the anatase and rutile crystal phases. Toxicology Letters 183(1-3), 72-80. 

For many years, it has been known that the different crystalline phases of titanium dioxide give rise to unique spectra [24,25]. In Fig. 11, the Raman spectra of anatase and rutile titanium dioxide, two possible titanium dioxide crystal phases, are shown. The different crystal phases have different mechanical and optical properties, and thus it is important to control the crystal phase for the final application of the material. 

Gallardo Amores, J.M., Sanchez Escribano, V., and Busca, G. Anatase crystal growth and phase transformation to rntUe in high-area Ti02, Mo03-Ti02 and related catalytic materials. J. Mater. Chem. 1995, 5, 1245-1249. 

The physical properties of the solids obtained by thermohydrolysis of TiCU in aqueous solutions are strongly influenced by the synthetic variables. In particular, acidity, presence (and nature) of anions, and titanivun concentration govern the composition and the photoreactivity of the Ti02 photocatalysts (Cheng et al., 1995 Koelsch et al., 2004). Depending on the e>q)erimental conditions, ratile or anatase, binary mixtures of anatase and rutile or anatase and brookite, or temaiy mixtures of anatase, brookite and ratile, can be obtained. Table 1 shows the crystal phase composition of some selected samples prepared under different experimental conditions. 

A facile way to prepare active Ti02 photocalalysts has been developed. The crystal phase eomposition of the samples can be easity tailored by simply varying the type of aqueous solution. The most efficient samples consisted of a ternary mixture of anatase, brookite and ratile. The presence of junctions among different polymorphic Ti02 phases favours the separation of the photogenerated electron-hole pairs, enhancing the catalyst activity. 

Figure C2.17.8. Powder x-ray diffraction (PXRD) from amoriDhous and nanocry stalline Ti02 nanocrystals. Powder x-ray diffraction is an important test for nanocrystal quality. In the top panel, nanoparticles of titania provide no crystalline reflections. These samples, while showing some evidence of crystallinity in TEM, have a major amoriDhous component. A similar reaction, perfonned with a crystallizing agent at high temperature, provides well defined reflections which allow the anatase phase to be clearly identified.

Unlike melting and the solid-solid phase transitions discussed in the next section, these phase changes are not reversible processes they occur because the crystal stmcture of the nanocrystal is metastable. For example, titania made in the nanophase always adopts the anatase stmcture. At higher temperatures the material spontaneously transfonns to the mtile bulk stable phase [211, 212 and 213]. The role of grain size in these metastable-stable transitions is not well established the issue is complicated by the fact that the transition is accompanied by grain growth which clouds the inteiyDretation of size-dependent data [214, 215 and 216]. In situ TEM studies, however, indicate that the surface chemistry of the nanocrystals play a cmcial role in the transition temperatures [217, 218]. 

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