The surface science of titanium dioxide - Niser

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94 U. Diebold / Surface Science Reports 48 (2003) 53±229 Fig. 23. (A) Geometryof the unreconstructed TiO 2 (1 0 0)-(1 1) surface. This surface results when the same number of Ti ! OasO! Ti bonds are broken in a bulk crystal, see dashed line in (B). The formation of the …1 3† microfaceted surface, originallyproposed byZschak et al. [186] is displayed in (B). Removal of the volume labeled g produces a stoichiometric surface with {1 1 0} facets. In order to reconcile the reduced character of the …1 3† surface (observed in photoemission), the outermost bridging oxygen atoms (g 0 ) are thought to be missing as well [187]. The transition from the (1 1) surface to the …1 3† microfaceted surface was proposed to occur via breaking the bonds a which leads to the indicated relaxation and creation of an intermediate …1 3† phase [176]. (C) A surface X-raystructure analysis byZajonz et al. [192] implied a modi®ed, heavilyrelaxed model. (D) The glancing angle X-raydiffraction data that had lead to the original microfacet model was re-evaluated byLandree et al. [193] and a new structure has been proposed. The octahedra model schematic representations in (D) show the microfacet model (top) and the new model (bottom) (from Landree et al. [193]). straightforward prediction of the stable surface termination (Fig. 23A). Again, the same number of Ti ! OasO! Ti need to be broken, as indicated bythe line in Fig. 23B. This results in a strongly corrugated surface, with rows of bridging oxygen atoms at the outermost, (1 0 0)-oriented ridges (Fig. 23A). Indeed a (1 1)-terminated LEED pattern is observed on this surface after sputtering and annealing, and STM and non-contact AFM images are consistent with this model [176,177]. Several theoretical calculations have determined likelyrelaxations of the (1 1) surface [68,101,178,179]. In [178] different theoretical approaches and basis sets were tested. All these calculations agree in the general motions of the atoms, although the amount of relaxations differ somewhat. As expected from symmetry, no relaxations occur along [0 0 1]. In the [1 0 0] direction only the ®vefold coordinated Ti atoms show appreciable (downwards) relaxations [68]. Substantial relaxations occur along the [0 1 0] direction with the twofold coordinated and the threefold coordinated oxygen atoms moving in opposite direction of the ®vefold and sixfold coordinated Ti atoms. In Fig. 23A, O atoms would move to the right and Ti atoms to the left. The net effect of these displacements is to increase the effective coordination of the ®vefold coordinated Ti atoms [178].
No experimental data on relaxations of the TiO 2 (1 0 0)-(1 1) surface exist. X-rayphotoelectron and Auger electron diffraction were performed but are insensitive to the details of the surface structure [180]. 2.3.2. Reconstructions U. Diebold / Surface Science Reports 48 (2003) 53±229 95 2.3.2.1. The microfacet model of the rutile TiO 2 (1 0 0)-(1 3) surface. In addition to the (1 1)- terminated surface, a (1 3)-reconstructed surface forms relativelyeasilyupon annealing to high temperatures in UHV. It is clearlyevident from photoemission experiments that this surface is partially reduced [27,181±183]. Initially, the observed reconstruction was interpreted as removal of every third row of the outermost, `bridging' oxygen atoms [183]. Such a proposal was verymuch in line with the initiallyproposed [184] (and now largelyabandoned) `missing row' model for the (1 2) structure of the TiO 2 (1 1 0) surface (see Fig. 18a). The step structure on this surface maylead to a misinterpretation as additional (1 5) and (1 7) reconstructions as pointed out byMuryn [183]. The first STM work on a reconstructed TiO 2 (1 0 0) surface was reported byClark and Kesmodel [185]. A glancing angle X-ray diffraction and low energyelectron diffraction study[186] suggested a `microfaceted' model, as shown in Fig. 23B. Removal of the volume assigned with g in Fig. 23B creates facets of the lowest energy(1 1 0) planes. Note that, again, the same number of O ! Ti as Ti ! O bonds are broken. This results in a stoichiometric surface, with twofold coordinated bridging oxygen atoms at the outermost ridges, as is the case for the TiO 2 (1 1 0) surface. The unit cell in [0 1 0] direction is three times wider. Atomicallyresolved STM images showed bright ridges with a geometryconsistent with the microfaceted model [187,188]. The apparent height between top and bottom of the reconstruction was measured as 3 AÊ with STM, instead of the expected 5 AÊ . This has been associated with tip effects [187,188]. Scanning tunneling spectroscopy(STS) measurements showed considerable difference between dI/dV spectra taken on and in between the bright ridges, respectively [187,189]. This was interpreted as a consequence of missing oxygen atoms on the outermost ridges (i.e., removal of the oxygen atoms labeled g 0 in Fig. 23C). This results in a surface termination with threefold coordinated Ti atoms in the outermost plane, consistent with the observation of a reduced surface in photoemission [183]. A photoelectron diffraction studyof the Ti3p level was performed byHardman et al. [182]. The PED curves of the (curve ®tted) Ti 3‡ feature were evaluated. These Ti 3‡ features are supposed to come from Ti atoms located next to oxygen vacancies, and diffraction effects should give information about their surface geometry. Three different positions for vacancies on the microfaceted (1 3) surface were tested. The most likelycon®guration has the missing oxygen atoms at the outermost ridge of the (1 1 0) facets [182], consistent with the STS measurements. In this context, it is also interesting to note that the reduction state of the substrate plays a role in the formation of the (1 3) reconstruction. Almost stoichiometric, Nb-doped TiO 2 (1 0 0) ®lms are thermallymuch more stable than reduced TiO 2 substrates and do not reconstruct at temperatures where reduced TiO 2 substrates alreadyshow a clear (1 3) structure [190]. A model involving `discrete bond breaking' was proposed for the formation of the (1 3) surface [176,177]. Surfaces were prepared that exhibited both, the (1 1) termination and the ridges typical for the (1 3) surface. STM and non-contact AFM images showed an intermediate phase, which had (1 3) symmetry, but did not possess the characteristics of the microfaceted structure. Raza et al. [176] suggested that the bonds labeled a in Fig. 23B are broken, which allows the Ti atoms to relax towards the other rows of bridging oxygen atoms.
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Page 9 and 10: Table 1 Bulk properties of titanium
Page 19 and 20: The second concept, autocompensatio
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Page 47 and 48: organic molecules, see Section 5.2.
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Page 65 and 66: Table 6 Summary of growth results o
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change of Au from a noble metal in
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Table 8 (Continued ) Substrate Tech
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Table 16 Surveyof the adsorption an
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The `other' TiO 2 structure, anatas
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epitaxial growth of superconductors
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6.5. Going beyond single crystal an
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