Anatase lattice

The structure of titanium complexes affects the formation of hydrated titanium dioxide structure, since rutile and anatase lattices are composed of TiO octahedrons connected in definite manner. The formation of anatase structure occurs when two octahedral complexes form a common vertex. When two octahedrons are united via their edges, rutile structure is formed. Based on this assumption, it is considered that if titanium (IV) complexes with one reactive centre are formed during hydrolysis, anatase structure is formed if there are two reactive centres, then rutile structure is formed. 

Electrons and holes that are generated in particulate semiconductors are localized at different defect sites on the surface and in the lattice of the particles. Electron paramagnetic resonance (EPR) results have shown that electrons are trapped as two reduced metal centers—Ti(III) sites—eoordinated either [38, 39] 1) with anatase lattice oxygen atoms only, or 2) with OH or H2O the holes are trapped as oxygen-centered radicals covalently linked to surface titanium atoms [40] (Figure 7). This is summarized by Eqs. (7)-(9). 

Figure 4.10 Proposed conceptual model of high-activity-mixed phase catalysts. Illumination promotes an electron into the rutile conduction band. This electron is then located sequentially in trapping sites in (1) the rutile lattice, (2) the anatase lattice, and (3) interfacial and surface sites. Reproduced with permission from Elsevier [38].

In order to improve the photocatalytic activity of anatase-type Ti02, substituting Ti cations or O anions in the anatase lattice has been done successfully via various chemical routes. 

A process to prepare carbon-modified Ti02, also active under visible-light irradiation, was developed heating of Ti02 powder either in a flow of carbon-containing gas (e.g., -hexane or ethanol) at 400 to 500°C or in ethanol vapor at 120°C in an antoclave). A portion of deposited carbon is thought to substitute oxygen in the anatase lattice on the particle surface. A small extent of carbon deposition, about 0.5 mass%, was found to result in only a small decrease in photoactivity, but was sufficient to make turbidity low and, as a consequence, practical water purification efficiency was markedly improved. 

High-resolution images obtained across the PEO layer are shown in Figure 5. Features of amorphous TiOa can be found in area A, The transition from amorphous titania into crystalline rutile is clearly seen in area B, while anatase lattice in area C around the pore periphery is identified. These verify the findings in the SAD analysis. 

When the host is a single oxide, incorporation is best achieved during a high temperature phase transition of the host lattice such as when Ti02 goes from anatase to mtile, or during formation from carbonates or other salts. 

This paper presents the results of ab initio calculation investigating the pressure dependence of properties of rutile, anatase and brookite, as well as of columbite and hypothetical fluorite phases. The main emphasis is on lattice properties since it was possible to locate transitions and investigate transformation precursors by using constant-pressure optimization algorithm. 

NakayamaS however, has suggested that, for rutile, which is tetragonal in structure, the strong bond between metal and oxide results from the favourable spacing between titanium ions in the rutile lattice and those in the metal structure. This explanation, however, does not account for the fact that other oxides of titanium, such as brookite, which is orthorhombic, and anatase, which is tetragonal, are also protective . 

Fio. 4. Structures of (001) crystal faces of anatase (o) clean, (6) hydrated (c) hydroxyl-ated [cut through (100) face]. The broken circles indicate how the lattice would continue. 

Compounds of stoichiometry AX2 with six-coordinated A require (according to eqn (11.1)) that X be three coordinate. Since none of the close packed lattices have cage points with three coordination, these structures are less simple. The rutile (202240) and anatase (202242) forms of Ti02 are based on FICP and FCC lattices of Ti respectively, but fitting the ions into positions of three coordination results in distortions that lower the symmetry. An alternative derivation of these structures is described in Section 11.2.2.4 below. 

Physical Properties. Of the three modifications of TiOz, rutile is the most thermodynamically stable. Nevertheless, the lattice energies of the other phases are similar and hence are stable over long periods. Above 700 °C, the monotropic conversion of anatase to rutile takes place rapidly. Brookite is difficult to produce, and therefore has no value in the TiOz pigment industry. 

Absorption of light of short wavelength (anatase < 385 nm, rutile <415 nm) occurs, producing an electron and an electron defect or hole (exciton) in the crystal lattice which migrate to the surface of the pigment. 

The difference in the catalytic activities of the anatase and rutile form is due to differences in lattice structure. It has been reported that the reducing properties of conduction-band electrons are dependent on lattice structures (Stafford et al., 1996). Anatase has the highest energy for the lowest unoccupied molecular orbital (LUMO), making it the least reactive of the three forms of Ti02 (Gratzel and Rotzinger, 1985). 

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