The surface science of titanium dioxide - Niser

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80 U. Diebold / Surface Science Reports 48 (2003) 53±229 Fig. 13. Ball-and-stick model of two terraces of TiO 2 (1 1 0). Small black balls represent Ti atoms and large white balls represent oxygen atoms. The step edge FG runs parallel to h1 11i-type directions. The smooth and rugged step edges along h0 01i in Fig. 12 are attributed to step edges AB and HI, respectively. From [116]. that contains several step edges [116]. As outlined above (Section 2.2.1.1) the same number of O ! Ti bonds and Ti ! O bonds need to be broken when cutting a TiO 2 crystal, for example, by forming a step edge byremoving part of the upper terrace. A h1 11i step edge (parallel to the diagonal of the surface unit cell) runs between the corners labeled F and G. For clarity, the bonds along this step edge are shaded with graycolor. The orientation of the step plane is …1 15†. The O atoms along the h1 11i step edge in Fig. 13 are alternatelythreefold (as in the bulk) and twofold coordinated. Formation of the step edge creates fourfold coordinated Ti atoms (terminating the Ti rows of the upper terrace) and ®vefold coordinated Ti atoms (terminating the bridging O rows). It should be pointed out that either these fourfold coordinated Ti atoms have a formal oxidation state of ‡4, or their concentration is verylow, because photoemission results of `unreduced' surfaces show no evidence of Ti 3‡ . A change in step orientation from the h1 11i to the h1 1 1i direction occurs always at the bridging oxygen rows (K in Fig. 12). The local environment of the atoms at such a kink site is not different from the rest of the step edge. Similarlyconstructed models of step edges oriented along ‰1 12Š and ‰1 15Š directions are given in [124]. There are several possibilities to form step edges parallel to the h0 01i direction. One can either cut next to the Ti atoms underneath the bridging oxygens (parallel to the arrow labeled 1 on the upper terrace in Fig. 13) or between the in-plane oxygen and titanium atoms (parallel to arrow 2). If one cuts at position 1, an autocompensated step edge is formed: for each Ti ! O bond that is broken in the upper plane, one O ! Ti bond is broken between the newlyformed bridging oxygens of the lower plane and the ®vefold coordinated titaniums of the upper plane. Note that there are two different terminations for such a step edge; the terrace maybe terminated either bya row containing in-plane oxygen atoms (step edge DC in Fig. 13) or bya row of bridging oxygens (step edge AB). Because of the observed contrast at step edges (smooth step edges terminate with a dark row, Fig. 12), a termination with bridging oxygen atoms (step AB in Fig. 13) is favored. (Fischer et al. [117] have presented a model for a h0 01i step terminating in in-plane oxygen rows. However, these authors adapted a different interpretation of the bright rows in STM as being caused bybridging oxygen rows.) If one cuts a terrace parallel to the h001i direction at position 2, the step edge that is formed is not autocompensated: onlyoxygen ! titanium bonds are broken on both the upper and on the lower
U. Diebold / Surface Science Reports 48 (2003) 53±229 81 terrace. Hence, a step edge that terminates in a row of in-plane Ti atoms is not stable and may reconstruct. This is consistent with the observation of reconstructed step edges (R in Fig. 13) appearing whenever a terrace would terminate in a bright row. A `reconstruction' was proposed [116] byremoving three of four Ti atoms as well as the neighboring in-plane O atoms (step edge HI in Fig. 13). Such a structure is consistent with the bright bumps separated by12 AÊ that are observed in STM. It also ful®lls the criterion of autocompensation. As has been point out before [116], step edges that run parallel to h1 10i type directions (step edge BC in Fig. 13) are generallynot observed. This also ®ts well into the concept of autocompensation. Such a step edge would not be not autocompensated and therefore is not expected to be stable. It should be noted that step edges playan important role in the …1 1† !…1 2† phase transformation [125]. It is interesting that the two-step h0 01i terminations with verydifferent geometries are seen. So far no temperature-dependent STM studies have been performed to ®nd out how annealing temperature affects the relative contribution of these two step edge terminations. In principle, the presence of verysmall amounts of trace impurities can also not be ruled out. In this sense, the model in Fig. 13 for the reconstructed step edge must remain speculative until supported bymore theoretical or experimental evidence. This detailed insight into step geometries and coordination number of atoms at step edges and kink sites is important, because a decrease in coordination number of surface atoms often correlates with an enhancement in chemical reactivity. Microscopic or nanoscopic particles naturally exhibit a much higher step/kink concentration than ¯at single crystals used for surface-science studies, so this issue is even more important in applications that use such materials, see Section 1.2. A few systematic studies of the effect of step edges on surface chemistryunder UHV conditions have been reported. For example, pyridine molecules have been found to be more strongly adsorbed at fourfold coordinated Ti atoms at step sites than at the ®vefold coordinated Ti atoms on the terraces [124]. On the other hand, the opposite effect has been observed byIwasawa et al. [126] for adsorption of formic acid on TiO 2 (1 1 0). Adsorption at 400±450 K resulted in particles with a strongly suppressed presence in the vicinityof step edges. Possibly, electrostatics plays a role. To this author's knowledge, virtually no theoretical work has been done to determine step geometrywith the same detail as ¯at surfaces. Because step edges break the symmetry, ®rst-principles calculations would require huge unit cells. With the advent of ever faster computers and more powerful programs such calculations maysoon become viable. 2.2.1.4.2. Oxygen vacancies created by annealing. There is overwhelming spectroscopic and chemical evidence for the presence of point defects on samples sputtered and annealed in UHV. These are attributed to vacancies in the bridging oxygen rows. Their concentration is typically reported as several percent [127,128]. These defects are of high importance for the surface properties of TiO 2 (1 1 0). No systematic study on the correlation between defect concentration and bulk properties has been performed on single crystals. EPR studies on a polycrystalline TiO 2 powder [129], reduced at temperatures between 723 and 923 K in vacuo, showed that the ratio of surface-to-bulk Ti 3‡ cations decreases as the reduction temperature is increased. Thermallyinduced point defects are visible as distributed black points on the bright oxygen rows in the AFM image in Fig. 11. As alreadypointed out, STM results are harder to interpret because of strong electronic effects. STM images of titanium dioxide surfaces that have been annealed in UHV often exhibit two kinds of atomic-scale features (Fig. 14). These appear as short bright features centered on dark rows (labeled A) and are connecting neighboring bright rows, and as dark spots on the bright rows (labeled B). The features labeled A have a densityof 7 3% per surface unit cell, consistent with O
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Page 9 and 10: Table 1 Bulk properties of titanium
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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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