OH ^^^^ Organic components

Chemical Reaction

Scheme 15.

In view of the growing technological potential of nano-crystalline titania, several chemical synthesis routes have been employed to prepare very fine particles [453-455]. The above-mentioned physical properties of TiO2 depend on the specific crystallographic structure (Scheme 16), as it is known to exhibit three different crystallographic forms, namely anatase, rutile, and brookite. In nanosized titania, anatase and rutile phases are inhomogeneously distributed, and postsynthesis thermal treatments are necessary to induce phase transformation and/or to improve the crystallinity. Solar energy conversion devices and chemical sensors make use of the anatase form of TiO2, which is difficult to prepare in pure form and is generally accompanied by the rutile modification that dominates at high temperatures. For this reason, control over the solid-state structure is highly desirable. Although the anatase-to-rutile phase transformation in nanocrystalline samples is not well understood, a new kinetic model proposes the combined interface nucleation at certain contact areas between two anatase particles and formation and growth of rutile nuclei to be the transformation steps. Over shorter reaction times, the net transformation rate is determined by the rate of nucleation, which is initiated from rutile-like structural elements in the contact area. The activation energy of 165.6 kJ/mol for rutile nucleation within nanocrystalline anatase particles is much lower than values previously measured for rutile nucleation in coarse anatase samples (>330 kJ/mol). Over longer reaction times, the net transformation rate is determined by both nucleation and nuclei growth. These results quantitatively explain the origin of the size dependence of phase transformation rates in nanocrystalline TiO2. However, the anatase phase can be selectively obtained when the material is formed under mild conditions; for example, Rambabu et al. have used titanium tetraisopropoxide, Ti(OPr')4, as a SSP to obtain nano-crystalline oxide (5-7 nm) at temperatures as low as 85 °C by the in-situ conversion of titania sol [456]. Interestingly, Ti(OPr')4, when used in a hydrothermal synthesis, produces rutile nanoparticles, indicating the role of high-temperature and high-pressure conditions in the structure of the resulting nanoparticles.

Table 7. Influence of precursor structure on the density of resulting

Scheme 16.

It is well known that hydrolysis and condensation rates of alkoxide precursors can be tailored through the addition of multidentate ligands. However, the influence of the precursor structure on the physical properties of the resulting nanocrystalline material has received less attention. Boyle et al. have synthesized and characterized a series of car-boxylic acid-modified titanium alkoxides [328] that display a rich structural diversity in the titanium-oxygen arrangements, depending upon the steric situation around the metal center (Scheme 17).

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