The surface science of titanium dioxide - Niser

68 U. Diebold / Surface Science Reports 48 (2003) 53±229
Fig. 4. Phase diagram of the Ti±O system taken from Samsonov [65]. The region Ti 2 O 3 ±TiO 2 contains Ti 2 O 3 ,Ti 3 O 5 ,seven
discrete phases of the homologous series Ti n O 2n 1 (Magneli phases), and TiO 2 . See [65] for a more detailed description.
The two approaches that are commonlyused to predict the structure and stabilityof oxide surfaces are
exempli®ed in detail for the rutile (1 1 0) surface. For anatase, the (1 0 1) and the (1 0 0)/(0 1 0) surface
planes are found in powder materials, together with some (0 0 1). The (1 0 1) surface was calculated to
have the lowest surface energy, even lower than the rutile (1 1 0) surface [70]. First experimental results
on anatase (0 0 1) and (1 0 1) are discussed at the end of this section.
2.1.1. Bulk defects
The titanium±oxygen phase diagram is very rich with many stable phases with a variety of crystal
structures, see Fig. 4 [65]. Consequently, TiO 2 can be reduced easily. Bulk reduction and the resulting
color centers are re¯ected in a pronounced color change of TiO 2 single crystals from initially
transparent to light and, eventually, dark blue, see Fig. 5. These intrinsic defects result in n-type doping
and high conductivity, see Table 2. The high conductivitymakes TiO 2 single crystals such a convenient
oxide system for experimentalists.
As has been pointed out recently [71], bulk defects playa major role in a varietyof surface
phenomena where annealing to high temperatures is necessary, e.g. during the encapsulation of Pt
[18,20,72], in bulk-assisted reoxidation [73,74], in restructuring and reconstruction processes [75,76],
and adsorption of sulfur and other inorganic compounds [77]. The relationship between crystal color,
conductivity, bulk defects as characterized by EPR measurements, and surface structure of rutile (1 1 0)
has been investigated systematically by Li et al. [71], and the samples reproduced in Fig. 5 have been
used in this study. The electric properties in dependence on the bulk defect concentration has been
investigated in [78,79].
The bulk structure of reduced TiO 2 x crystals is quite complex with a various types of defects such as
doublycharged oxygen vacancies, Ti 3‡ and Ti 4‡ interstitials, and planar defects such as CSPs. The
defect structure varies with oxygen de®ciency which depends on temperature, gas pressure, impurities,
etc. Despite years of research, the question of which type of defect is dominant in which region of
oxygen de®ciency is still subject to debate [78,80]. It was shown that the dominant type are Ti
interstitials in the region from TiO 1.9996 to TiO 1.9999 (from 3:7 10 18 to 1:3 19 19 missing O atoms