The surface science of titanium dioxide - Niser

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64 U. Diebold / Surface Science Reports 48 (2003) 53±229 Table 1 (Continued ) Refractive index, n, of rutile single crystal for ordinary(1) and extraordinary(2) rays in visible and IR regions of spectrum, at 298 K 0.20 3.0 1223 0.30 4.0 1223 0.32 5.0 1223 0.50 6.0 1223 0.67 7.0 1223 0.76 8.0 1223 0.80 9.0 1223 0.84 10.0 1223 0.85 11.0 1223 0.86 12.0 1223 0.87 13.0 1223 0.88 14.0 1223 0.89 15.0 1223 a Data from [65] unless noted otherwise. A more detailed compendium of bulk properties of rutile is given in [49]. on two complementaryelectrodes (TiO 2 and WO 3 in the case of [57]), which change their color upon reduction/oxidation cycles induced by an electrical current. Polycrystalline ZnO, TiO 2 and SnO 2 , exhibit a high non-linearitybetween the current densityand the electric ®eld and are thus suitable as `varistors' for the suppression of high transient voltages [58]. Doped TiO 2 ceramics have useful varistor properties with non-linearitycoef®cient (a) values in the range a ˆ 3 12, a being de®ned bythe relationship I ˆ KV a , where I is the current, V the voltage, and K the proportionalityconstant. The presence of this potential barrier is due to the creation of defects formed during sintering of TiO 2 systems. A potential barrier associated with a double space charge distribution can originate at these defects. This phenomenon establishes variable resistance as a function of the applied electric ®eld to the solid. Metallic implants in the human bodyhave a signi®cant economic and clinical impact in the biomaterials ®eld [59]. `Commerciallypure' (CP) titanium (ASTM F67) and `extra-low interstitial' (ELI) Ti±6Al±4V alloy(ASTM F136) are the two most common implant biomaterials. There is an increasing interest in the chemical and physical nature of the oxide layer on the surface of both materials [60]. The oxide provides corrosion resistance and mayalso contribute to the biological performance of Ti at molecular and tissue levels, as suggested in the literature on osseointegrated oral and maxillofacial implants byBranemark, Kasemo and co-workers [61] in Sweden. 1.3. Outline of this review The geometric structure of various TiO 2 surfaces is discussed in Section 2. A detailed knowledge of the surface structure is the crucial ®rst step in obtaining a detailed knowledge of reaction mechanisms
U. Diebold / Surface Science Reports 48 (2003) 53±229 65 on the molecular scale. Metal oxide surfaces are prime examples of the close relationship between structure and reactivity [6], as local non-stochiometries or geometric defects directlyaffect the electronic structure. Well-tested models are available for both, `perfect' surfaces as well as surface defects on TiO 2 . Titanium dioxide crystallizes in three crystallographic phases, and the surfaces of the rutile phase have been investigated extensively. Surface science research on the technologically quite important anatase phase is just starting. The structure and stabilityof metal oxide surfaces can be predicted using the concept of autocompensation [5] or non-polarity [62]. Bulk-truncated models of various rutile and anatase TiO 2 surfaces are derived using this concept, and are compared with ab initio calculations and experimental results on surface geometrical models and relaxations. Recent scanning probe microscopyresults have given enormous insight into defect structures at TiO 2 surfaces, and have provided some surprises as well. Section 3 gives a brief summaryof the electronic structure of TiO 2 . Most of the basic understanding of the electronic structure of TiO 2 surfaces has been discussed in previous reviews [1], hence this section is kept short. Surface defects that are related to oxygen de®ciencies can be identi®ed with most electron spectroscopies, some of which are discussed in this section. The growth of metal and metal oxide overlayers on TiO 2 substrates is reviewed in Section 4. This is a veryactive and exciting area of research, and almost all metals across the periodic table have been investigated on TiO 2 . Most of the current literature on metal/TiO 2 growth has been summarized in Table 6. It is comforting to see that the basic trends for the propensityof interfacial reactions, growth morphology, geometric structure, and thermal stability that have been identi®ed early on [63] are in agreement with the more recent results. The surface chemistryof TiO 2 is reviewed in Section 5. The adsorption of inorganic molecules is discussed ®rst, and the results for each group of molecules is summarized in tables. Results on small organic molecules is then reviewed. This section closes with a brief summaryof photoinduced reactions on TiO 2 surfaces. A summaryand outlook is given at the end. 2. The structure of TiO 2 surfaces Unraveling the relationship between atomic surface structure and other physical and chemical properties is probablyone of the most important achievements of surface science. Because of the mixed ionic and covalent bonding in metal oxide systems, the surface structure has an even stronger in¯uence on local surface chemistryas compared to metals or elemental semiconductors [6]. A great amount of work has been performed on TiO 2 over the years, and has lead to an understanding that is unprecedented for a metal oxide surface. This section starts with a brief description of the bulk structure of titanium dioxide crystals, and their stable crystal planes. Because bulk non-stoichiometries in¯uence the surface properties of TiO 2 in a varietyof ways, a short discussion of bulk defects is included as well. A substantial part of the section is devoted to the rutile (1 1 0) surface. The (bulk-truncated) (1 1) surface is known with a veryhigh accuracyfrom experimental as well as theoretical studies. Nevertheless, there are some puzzling disagreements between theoryand experiment in some aspects [64]. Surface defects are categorized in step edges, oxygen vacancies, line defects (closely related to the (1 2) reconstruction), common impurities, and the manifestation of crystallographic shear planes (CSPs) at surfaces. The long-standing argument of the structure of the (1 2) phase seems to be settled, as discussed in Section 2.2.2. STM
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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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The `other' TiO 2 structure, anatas
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epitaxial growth of superconductors
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