The surface science of titanium dioxide - Niser

92 U. Diebold / Surface Science Reports 48 (2003) 53±229
The rosette structure can be explained simplybythe formation of (partiallyincomplete) TiO 2 layers
through a growth process where the Ti atoms come from the reduced bulk and the 18 O from the gas
phase. The kinetics of the growth determines the relative concentration of the incomplete structures (the
rosettes and strands) and the (1 1) islands on the surface. The surface is ¯at and (1 1)-terminated
when the growth is slow in comparison with surface diffusion processes. This can be achieved in
various ways, either during annealing at high temperatures, Fig. 19f, or when the ¯ux of one of the
constituents is small (in verylight samples with a small concentration of interstitials [156]) or at lower
O 2 background pressures. Conversely, on very dark crystals, or at intermediate temperatures, a
substantial part of the surface can be covered with rosette networks. Note that 60% of the Ti atoms in
the rosettes are fourfold coordinated, whereas the Ti atoms exposed on the (1 1) surface are ®vefold
coordinated. Clearlythe possibilitythat such structures can form needs to be taken into account when
preparing rutile (1 1 0) surfaces for surface chemistryexperiments (see Section 5).
The surface structure in Fig. 19e, obtained after annealing in 1 10 6 mbar 18 O 2 at 710 K, shows the
presence of (1 2) strands on otherwise ¯at, (1 1)-terminated surfaces. This is in agreement with
Onishi and Iwasawa's [169] results described above. From atomicallyresolved images of strands
connected to rosettes as well as UHV annealing experiments of restructured surfaces it was concluded
[144] that these strands also have the Ti 2 O 3 structure depicted in Fig. 18b.
The dependence of the restructuring (as well as the type of (1 2) reconstruction) on the reduction
state of the bulk was resolved byBennett et al. [76]. High-temperature STM studies were performed on
two different TiO 2 samples. On a dark blue/black crystal (that showed already evidence for CSP
formation, i.e., darker than cube 3 in Fig. 5) two different structures were observed (see Fig. 22). The
dark and bright strings in Fig. 22 were attributed to added Ti 2 O 3 rows (Fig. 18b) and added rows of a
bulk-terminated TiO 2 layer (Fig. 18c, but with bridging oxygens at the center of the strands),
respectively. The latter structure also appears cross-linked with partial `rosettes' (Fig. 21). Both the
added-row structure and the rosettes are just incomplete TiO 2 structures that form during the growth of
additional TiO 2 (1 1 0)-(1 1) layers. The authors have published impressive web-based STM `movies'
[173] (which can be viewed at http://www.njp.org/) of the growth process that show the cyclic
completion of terraces and new formation of the cross-linked added-row structures. In contrast, the dark
rows (the Ti 2 O 3 added rows) appeared relativelyunreactive for additional growth.
2.2.3. Recommendations for surface preparation
Although the TiO 2 (1 1 0)-(1 1) surface is considered the `best-characterized', prototypical metal
oxide surface, the above summaryclearlyshows that its atomic-level structure is quite complex. The
recent STM results summarized above clearlyindicate that both the oxidation conditions and the history
of the TiO 2 (1 1 0) sample have signi®cant bearing on the morphologyof the surface, the presence of
strands, rosettes, or CSPs. The variations in the surface structure with O 2 pressure, crystal temperature and
bulk defect densityare so vast that one could suspect chemistryof the TiO 2 (1 1 0) surface to be
signi®cantlyvariant for samples oxidized under different conditions. For example, the issue of whether
water is molecularlyor dissociativelyadsorbed on TiO 2 (110)[127,128,138,174,175] (see Section 5.1.2)
maybe signi®cantlyclouded in the literature because of studies in which the morphologyof the surface
was unknowinglydisordered bythe presence of the rosettes and/or strands observed recentlybySTM.
This level of ambiguitymayalso permeate manyother adsorption studies on TiO 2 (110).
Guidelines of surface preparation of TiO 2 (1 1 0) can be extracted from recent work [75,76,150,156,172].
If solely(1 1)-terminated surfaces are desired, light blue crystals (as depicted in Fig. 5) should be used.