Crystalline titania film

The thermal stability of mesoporous frameworks substantially increases with an increase in the wall thickness and pore size, which can be varied even for the same template by changing the processing conditions. Ozin et al.55 developed a way to prepare crystalline titania films with a 2D-hexagonal architecture by replacement of ethanol in the Pluronic-containing precursor solution with more hydrophobic butanol-1. The latter promotes phase separation at low surfactant-to-titania ratios, resulting in thicker pore walls, which are more compatible with the crystal growth during subsequent calcination. 

Titania films prepared by the methods described above are, however, just partially crystalline. Although WAXS patterns indicate formation of anatase crystals of ca. 10-12nm in size (Fig. 9.3a), the electron microscopy study demonstrates that the elongated crystals are actually embedded into an amorphous mesoporous matrix (Fig. 9.3c). The degree of crystallinity for such films usually does not exceed 60% attempts to increase it by calcination at higher temperatures cause uncontrolled crystal growth, which leads to collapse of mesoporos-ity and a drastic decrease in the surface area (Fig. 9.3d). 

Thermodynamic considerations suggest that such transformation phenomena are thermally controlled. In fact, amorphous phase coatings spontaneously crystallize above some critical temperature. The transformation is manifested in the Raman spectrum by dramatic band intensity increases and marked band narrowing (20). When metastable crystalline treated, recrystallization temperature phase occurs as the temperature-dependent spectra shown in Figure 13, which depicts the irreversible transformation of anatase to rutile in a thin titania film. 

Adherent, conformal LPD ceramic film either crystalline (Method 2) or amorphous (Method 1) can be obtained on polyimides. This is in contrast to silanol-bearing surfaces (good for Method 1 only) or sulfonated surfaces (good for Method 2 only). The fact that the polyimides accommodate both titania preparations may be due to the partial hydrolysis of the polymer surface under oxide deposition conditions. This provides a mix of carboxylic acid and amide sites that anchor the titania by a combination of coulombic and chelation-based effects.22 An important lesson of this work is that the interaction of the polymer surface with the deposition solution may create oxide film anchoring sites. This does not negate activating the polymer surface. It recognizes that the polymer surface can react further under the deposition conditions. 

Very high deposition rates of titania were observed with the mono- or dinuclear titaninm S-diketoesters Ti(mpd)(mdop)2 (28, 17-28 nmmin ) and [Ti(mpd)(mdop)(p,-OMe)]2 (9-24 nmmin ), 5 to 6 times higher than those of Ti(thd)2(OPr-/)2 (26b) °. The higher deposition rates can be explained by a weaker bonding of an ester moiety to titaninm compared to that from a ketone. The films obtained from the dimeric species were crystalline (anatase) and showed some carbon impnrity (<3%). Dimerization of complex 28 was observed when dilnting with methanol, as shown in equation 12. 

Since natural sunlight can only penetrate a few microns depth, the use of thin films of titania applied to ceramic or metallic supports as maintenance free decontamination catalysts for the photocatalytic oxidation of volatile organic compounds is of interest for the abatement or control of these emissions. The sol-gel technology can be readily incorporated as a washcoating step of the catalyst supports that may be subsequently heat-treated to fix the titania to the support. The surface area, porosity and crystalline phases present in these gels is important in controlling their catalytic activity. Furthermore, the thermal stability and development of porosity with heat-treatment was important if the sol-gel route is to be used as a washcoating step to produce thin films. 

The properties of the deposited Ti02 layer, such as thickness, uniformity, and crystallinity, can be tailored using different solvents such as water, ethanol, or a mixture therefrom. The addition of poly(vinyl pyrroli-done) to the solvents prevents the aggregation of the Ti02 nanoparticles. The titania layer on the parylene film is stable and cannot be removed by a simple washing procedure with water, ethanol, or acetone. 

Wong, M.S., S.W. Hsu, K.K. Rao, and C.P. Kumar, Influence of crystallinity and carbon content on visible light photocatalysis of carbon doped titania thin films. 

Tantalum- and niobium-doped (up to 10 atom%) titania nanopowders as base materials for thick film gas sensors were synthesized recently (Traversa et al., 2000). The source of titanium, Ti(0 Pr)4, was dissolved in absolute ethanol and the solution added dropwise into a 1 1 ethanol/water solution to obtain a precipitate. On calcination, the amorphous precipitates converted to the crystalline forms of titania, as also other phases, as shown in Table 7-2. 

The possibihty of obtaining nanopartides upon exposure to hard X-rays is not restricted to metallic nanopartides, in fact oxide nanopartides within a mesopo-rous ordered matrix have been also obtained. One example is ceriimi oxide nanopartides within a titania mesoporous ordered film to produce ceria NPs, the titania mesopores have been impregnated with a ceria precursor solution and then exposed to hard X-rays. Crystalline cerium oxide NPs with an average size of 4 nm have been grown within the mesopores, and different patterns with spatial control of the nanoparticle growth on the micrometer scale have been obtained [121]. 

---