The surface science of titanium dioxide - Niser

90 U. Diebold / Surface Science Reports 48 (2003) 53±229
Fig. 19. STM images …500 Ð 500 І of a TiO 2 (1 1 0) surface taken at room temperature. 18 O 2 (1 10 6 mbar) was dosed
at (a) 500 K, (b) 520 K, (c) 550 K, (d) 660 K for 10 min, (e) 710 K for 15 min, and (f) 830 K for 20 min. Before each gas
exposure, the sample was sputtered and annealed in UHV at 880 K for 30 min which renders ¯at, (1 1)-terminated surfaces.
From Li et al. [75]. # 1999 Elsevier.
`age' or color) of the crystal. These parameters have been investigated in detail by Li et al.
[75,144,156,170,171]. For example, Fig. 19 shows the effect of annealing in 1 10 6 mbar O 2 at
various temperatures [75]. Before each gas exposure, a ¯at (1 1)-terminated surface was prepared by
sputtering and annealing in UHV. The surface morphologyof the oxygen-exposed sample is very
temperature dependent. For medium temperatures, surfaces are relativelyrough with manysmall-scale
features. Isotopicallylabeled 18 O 2 gas was used for the annealing excursions. In SSIMS and low-energy
He ‡ ion scattering measurements, the signal from 18 O atoms can clearlybe separated from the
(naturallymuch more common) 16 O isotope in the crystal. All the surfaces in Fig. 19 showed an
enrichment with 18 O, with a maximum of 18 O surface concentration around 660 K. The structures that
form for intermediate annealing temperatures (520±660 K) are better seen in the small-scale image in
Fig. 20. Theyconsist of small, (1 1)-terminated islands and irregular networks of connected
`rosettes', i.e., six bright spots in a pseudohexagonal arrangement, as well as small strands. A model for
the rosette network is shown in Fig. 21. It consists of atoms in bulk-like positions with some atoms
missing from the regular (1 1) structure. LDA-based ®rst-principles calculations [75] showed that
such a rosette structure is stable. The same calculations also predict sizable relaxations.