The surface science of titanium dioxide - Niser

88 U. Diebold / Surface Science Reports 48 (2003) 53±229
2.2.2. Reconstructions
2.2.2.1. Reconstruction under reducing conditions: the structure(s) of the (1 2) phase. The most
commonlyobserved reconstruction on TiO 2 (1 1 0) surfaces has a (1 2) symmetry with a doubling of
the periodicityalong the ‰1 10Š direction. Various models have been suggested for this reconstruction and
the most popular ones are depicted in Fig. 18 [113].
A(1 2) LEED pattern was originallyobserved after high-temperature annealing of a reduced
TiO 2 (1 1 0) sample in ultrahigh vacuum (UHV). Based on Ti:O AES ratios it was interpreted as
alternate rows of bridging oxygen missing from the regular (1 1) surface (``missing-row model''
[111], Fig. 18a). One of the ®rst atomicallyresolved STM results on this surface was also interpreted as
missing bridging oxygen rows [114,158]; however, the Ti atoms underneath the missing oxygen atoms
had to be shifted byhalf a unit cell in [0 0 1] direction to account for the observed image contrast. A
structure with a (3 2) symmetry was reported in [159]. A model for this reconstruction was discussed
where this symmetry is achieved by removing 1/3 or 2/3 of the oxygen in the bridging oxygen rows.
However, such a reconstruction has not been reported byother groups.
The simple missing row model for the (1 2) structure in Fig. 18a has been abandoned on the basis
of more recent results. In STM the (1 2) reconstruction is commonlyobserved as a series of bright
strings along the [0 0 1] direction [113,114,123,158,160±162], see Fig. 16. At low coverage, the strings
grow preferentiallyout of the upper terrace onto the lower one (Fig. 16a) [123]. At ®rst, theyare
scattered across the terraces with a minimum distance of 13 AÊ along the [0 0 1] direction. Theyconsist
of bright double strings (although the double-ridge structure is often not resolved), with a bright dot at
the end. Antiphase boundaries are observed in high-resolution images of a fullydeveloped (1 2)-
reconstructed surface [113]. Higher periodicities, i.e., a local (1 3) reconstruction, have also been
observed [161,163,164]. In STM images the (1 2) strands generallyhave an apparent height smaller
than a regular TiO 2 step edge of 3.2 AÊ , and are in registrywith the bright rows of the (1 1) substrate.
Because most researchers report empty-state images and because these are dominated by the tunneling
into mostlyTi3d-derived states (see Section 2.2.1.3), bright strands in line with bright substrate rows
implythat the (1 2) strands are at the position of ®vefold coordinated Ti atoms and not at the bridging
oxygen atoms. STM images of a simple missing row structure are expected to show a bright feature
above the missing bridging oxygen row (provided the STM tip is a reasonable distance from the surface
[112]), inconsistent with the registryobserved experimentally. The rows can be removed bytunneling
under `extreme conditions' (V s ˆ‡1:5 V,I ˆ 10 nA [113]). First-principles total-energycalculations
show that the added Ti 2 O 3 model (discussed next) is energeticallyfavored [165] and that the missing
row structure is energeticallyequivalent to a (2 1) structure (where everyother bridging oxygen is
removed) [99]. For all these reasons, the missing-row reconstruction is no longer considered a viable
model.
Earlyon, Onishi et al. [122,123] suggested a quite different model. It consists of double rows of Ti
cations in a distorted tetrahedral con®guration (Fig. 18b). The structure has Ti 2 O 3 stoichiometry, and
the model is often called `added Ti 2 O 3 rows'. However, it needs to be emphasized that the structure
does not resemble the one found in the corundum Ti 2 O 3 structure. Rather, the Ti cations reside in
positions similar to interstitial sites in the rutile lattice [91]. Self-consistent total-energyand electronic
structure calculation found that this added `Ti 2 O 3 ' row structure has a lower surface free energythan
the missing row structure and that it is consistent with the contrast in STM [165]. Recent VASP
calculations show that such strands can be added at low energycost [91], but also that manyother