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We have employed elevated temperature scanning tunneling microscopy to elucidate the reactivity and surface structure of the ͑1 3 2͒ reconstructed TiO 2 ͑110͒ surface. We find two distinctly different ordered surface reconstructions depending upon the level of bulk reduction of the crystal (degree of nonstoichiometry). On the near stoichiometric surface reactivity to oxygen is low and attributed to the formation of the stable Ti 2 O 3 type termination. For heavily reduced crystals a cross-linked ͑1 3 2͒ reconstruction forms with high activity to oxygen resulting in a well-defined cyclic reaction (TiO 2 growth). [S0031-9007(99)09062-6] PACS numbers: 68.35.Bs, 82.65.Jv, 82.65.My The ability to prepare conducting TiO 2 samples in UHV has lead to a wealth of studies on its surface structure and chemistry [1][2][3][4][5][6][7][8][9][10][11][12][13]. However, there has been relatively little work exploiting the effect of varying the bulk stoichiometry on surface structure and reactivity [8]. The link between surface structure and bulk defect formation is strong in reducible d 0 metal oxides (TiO 2 , V 2 O 5 , MoO 3 , and WO 3 ) where defects can cluster and form crystallographic shear planes (CS) within a crystal [1,14,15]. These have recently been shown to terminate at the surface in a well-ordered array of half-height steps [16][17][18][19] on TiO 2 ͑110͒.There are at present two structural models considered to be in broad agreement with the wide range of techniques that have been used to probe the TiO 2 ͑110͒-͑1 3 2͒ reconstruction. The first, proposed by Onishi et al., is for an added row of stoichiometry Ti 2 O 3 which grows upon the ͑1 3 1͒ terrace in an oxygen ambient as a result of transport of Ti n1 interstitials to the surface [2,3,20]. More recently, a second added row model has been proposed by Pang et al. on the basis of atomic resolution STM data and theoretical calculations in which the added rows consist of strings of the fully reduced bulk termination ͑Ti 3 O 5 ͒ [9]. However, there are a number of reports in the literature of ͑1 3 2͒ rows that have been stabilized by cross-linking every few tens of Å [5,6,8]. Most notably these crosslinks are reported to be more prevalent after annealing a reduced surface in oxygen. Structural models proposed to explain the cross-links were based on a missing row reconstruction that has largely been rejected because of conflicting evidence by other techniques.In this Letter we propose that both models are, in essence, correct as we show that there are indeed two forms of the ͑1 3 2͒ reconstruction on the surface, the straight ͑1 3 2͒, and the cross-linked ͑1 3 2͒. With the aid of time resolved reoxidation scanning tunneling microscopy (STM) experiments on lightly and heavily reduced TiO 2 ͑110͒ crystals we show that the surface termination is dictated by bulk nonstoichiometry. These observations are also compatible with, and contribute to, recent reports of the reoxidation behavior of TiO 2 ͑110͒ [5,6,13]. The ability to image the reoxidation process in situ is ess...

We have used scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS) to investigate the thermal stability of Pd(111) islands and thin films on TiO2(110)-(1 × 2). Two new nano-structures were observed to form on the surface of the Pd by annealing to 973 K. Atomic resolution STM images show a “pinwheel” super-structure. One domain has a unit cell with respect to the Pd(111) whereas the other domain has a unit cell. Coexisting with this phase is a structure consisting of zigzag rows that run along the close-packed directions of the Pd(111) islands. This has a rectangular unit cell incommensurate with both the substrate TiO2(110) and the Pd(111) islands. STM shows these two structures merge with no noticeable domain barriers or steps, suggesting a close relationship between the two. LEED shows several distinct, overlapping patterns that can be identified with the surface structures observed; TiO2(110)-(1 × 2), Pd(111)-(1 × 1), the hexagonal pinwheel structure, and the rectangular zigzag unit cell. XPS at normal and grazing emission show the encapsulating layer to be composed of TiO x with Ti predominantly in ∼2+ or ∼3+ oxidation states. The proposed models are structurally consistent with the LEED and STM data and have stoichiometries of TiO and TiO1.4, chemically consistent with the XPS spectra. The STM images of the zigzags bear a strong similarity to structures seen for annealed Pt islands on TiO2(110)-(1 × 1) and TiO x supported on Pt(111), while the pinwheel structure is similar to annealed Cr on Pt(111). We discuss the similarities of our structures to those seen before for these related systems.

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Point defects in metal oxides such as TiO 2 are key to their applications in numerous technologies. The investigation of thermally induced nonstoichiometry in TiO 2 is complicated by the difficulties in preparing and determining a desired degree of nonstoichiometry. We study controlled self-doping of TiO 2 by adsorption of 1/8 and 1/16 monolayer Ti at the ͑110͒ surface using a combination of experimental and computational approaches to unravel the details of the adsorption process and the oxidation state of Ti. Upon adsorption of Ti, x-ray and ultraviolet photoemission spectroscopy ͑XPS and UPS͒ show formation of reduced Ti. Comparison of pure density functional theory ͑DFT͒ with experiment shows that pure DFT provides an inconsistent description of the electronic structure. To surmount this difficulty, we apply DFT corrected for on-site Coulomb interaction ͑DFT+ U͒ to describe reduced Ti ions. The optimal value of U is 3 eV, determined from comparison of the computed Ti 3d electronic density of states with the UPS data. DFT+ U and UPS show the appearance of a Ti 3d adsorbate-induced state at 1.3 eV above the valence band and 1.0 eV below the conduction band. The computations show that the adsorbed Ti atom is oxidized to Ti 2+ and a fivefold coordinated surface Ti atom is reduced to Ti 3+ , while the remaining electron is distributed among other surface Ti atoms. The UPS data are best fitted with reduced Ti 2+ and Ti 3+ ions. These results demonstrate that the complexity of doped metal oxides is best understood with a combination of experiment and appropriate computations.

The re-oxidation of slightly reduced TiO2(110) surfaces by exposure to an oxygen pressure of ~2 × 10-7 mbar in the temperature range 473-1000 K occurs by re-growth of TiO2 overlayers by diffusion of Ti n+ interstitials from the bulk. Starting with a (1 × 2) reconstructed surface, scanning tunnelling microscope images of the surface reacting under these conditions show that the (1 × 1) islands nucleate within the (1 × 2) layer and grow laterally. As the islands reach a critical size, which is temperature dependent, a new (1 × 2) layer begins to nucleate and grow. At both extremes of the temperature range nucleation of the second layer occurs before coalescence of the (1 × 1) islands, however, at temperatures between 673 and 773 K large areas of (1 × 1) surface form before growth of the second layer. The reaction is cyclic and several layers of TiO2 can be grown in this way.

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