Anatase is a metastable polymorph of TiO2. In contrast to the more widely studied TiO2 rutile, O vacancies (V(O)'s) are not stable at the anatase (101) surface. Low-temperature STM shows that surface V(O)'s, created by electron bombardment at 105 K, start migrating to subsurface sites at temperatures ≥200 K. After an initial decrease of the V(O) density, a temperature-dependent dynamic equilibrium is established where V(O)'s move to subsurface sites and back again, as seen in time-lapse STM images. We estimate that activation energies for subsurface migration lie between 0.6 and 1.2 eV; in comparison, density functional theory calculations predict a barrier of ca. 0.75 eV. The wide scatter of the experimental values might be attributed to inhomogeneously distributed subsurface defects in the reduced sample.

A natural TiO2 anatase crystal, cut to exhibit its (010) surface, was cleaved by breaking off one of its corners. The resulting sample exhibited a small, flat area ca. 2 mm(2) in size with a (101) orientation as confirmed by LEED. The evolution of the surface morphology was monitored with UHV-STM. After one sputtering/annealing cycle the surface is characterized by periodic ridges that run parallel to the [010] direction. The ridges are ∼ 3 nm high and 10-15 nm wide and have a spacing of 30 nm. Interestingly, [Formula: see text]-oriented step edges are not observed, despite them having the lowest formation energy. The ridges flatten with repeated sputter/annealing cycles. After a total of three cycles a flat surface is achieved, which exhibits trapezoidal terraces that are typical for anatase (101). The importance of preparing such a pristine surface for understanding the surface structure and chemistry of TiO2 anatase is discussed.

We present a combined experimental and theoretical study of the adsorption of water on the rutile TiO2(011)-2x1 surface, whose "brookite (001)-like" reconstruction has been recently elucidated. By using scanning tunneling microscopy and density functional theory calculations, we provide evidence that water adsorbs weakly on the stoichiometric surface, while hydroxyls resulting from water dissociation at surface O vacancies act as nucleation centers for the growth of H-bonded water clusters that are confined in one dimension.

An extensive search for possible structural models of the (2×1)-reconstructed rutile TiO2(011) surface was carried out by means of density functional theory (DFT) calculations. A number of models were identified that have much lower surface energies than the previously-proposed ‘titanyl’ and ‘microfaceting’ models. These new structures were tested with surface X-ray diffraction (SXRD) and voltage-dependent STM measurements. The model that is (by far) energetically most stable shows also the best agreement with SXRD data. Calculated STM images agree with the experimental ones for appropriate tunneling conditions. In contrast to previously-proposed models, this structure is not of missing-row type; because of its similarity to the fully optimized brookite TiO2(001) surface, we call it the ‘brookite (001)-like’ model. The new surface structure exhibits two different types of undercoordinated oxygen and titanium atoms, and is, in its stoichiometric form, predicted to be rather inert towards the adsorption of probe molecules.

The interaction of water with metal oxide surfaces is of fundamental importance to various fields of science, ranging from geophysics to catalysis and biochemistry. In particular, the discovery that TiO(2) photocatalyses the dissociation of water has triggered broad interest and intensive studies of water adsorption on TiO(2) over decades. So far, these studies have mostly focused on the (110) surface of the most stable polymorph of TiO(2), rutile, whereas it is the metastable anatase form that is generally considered photocatalytically more efficient. The present combined experimental (scanning tunnelling microscopy) and theoretical (density functional theory and first-principles molecular dynamics) study gives atomic-scale insights into the adsorption of water on anatase (101), the most frequently exposed surface of this TiO(2) polymorph. Water adsorbs as an intact monomer with a computed binding energy of 730 meV. The charge rearrangement at the molecule-anatase interface affects the adsorption of further water molecules, resulting in short-range repulsive and attractive interactions along the [010] and directions, respectively, and a locally ordered (2x2) superstructure of molecular water.

Scanning tunneling microscopy (STM) images taken on a freshly cleaved anatase TiO2(101) sample show an almost perfect surface with very few subsurface impurities and adsorbates. Surface oxygen vacancies are not typically present but can be induced by electron bombardment. In contrast, a reduced anatase (101) crystal shows isolated as well as ordered intrinsic subsurface defects in STM, consistent with density functional theory (DFT) calculations which predict that O vacancies (V_{O}'s) at subsurface and bulk sites are significantly more stable than on the surface.

Using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED), the structure of the anatase TiO2 (514) surface, similar to 10 degrees vicinal to the lowest-energy (101) plane, has been studied. The surface was found to facet into a structure composed of ridges with a uniform width of five lattice units. On the basis of atomically resolved STM and electron counting rules, it is proposed that the sides of the ridges are parallel to (1 (1) over bar0) and (112) planes. These sides might be reconstructed to stabilize the microfaceted structure. Vapor-deposited gold shows pronounced clustering between the ridges, indicating a one-dimensional template effect of the vicinal surface, which supports denser and more uniformly sized Au clusters, as compared to the flat (101) surface.

When low-energy electrons strike a titanium dioxide surface, they may cause the desorption of surface oxygen. Oxygen vacancies that result from irradiating a TiO 2 (011)-2×1 surface with electrons with an energy of 300 electron volts were analyzed by scanning tunneling microscopy. The cross section for desorbing oxygen from the pristine surface was found to be 9 (±6) × 10 –17 square centimeters, which means that the initial electronic excitation was converted into atomic motion with a probability near unity. Once an O vacancy had formed, the desorption cross sections for its nearest and next-nearest oxygen neighbors were reduced by factors of 100 and 10, respectively. This site-specific desorption probability resulted in one-dimensional arrays of oxygen vacancies.

The a-plane Mg x Zn1−x O (0 ≤ x ≤ 0.3) films were grown on r-plane (\( 01\bar 12 \)) sapphire substrates using metal-organic chemical vapor deposition (MOCVD). Growth was done at temperatures from 450°C to 500°C, with a typical growth rate of ∼500 nm/h. Field emission scanning electron microscopy (FESEM) images show that the films are smooth and dense. X-ray diffraction (XRD) scans confirm good crystallinity of the films. The interface of Mg x Zn1−x O films with r-sapphire was found to be semicoherent as characterized by high-resolution transmission electron microscopy (HRTEM). The Mg x Zn1−x O surfaces were characterized using scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV). Low-energy electron diffraction (LEED) shows well-ordered and single-crystalline surfaces. The films have a characteristic wavelike surface morphology with needle-shaped domains running predominantly along the crystallographic c-direction. Photoluminescence (PL) measurements show a strong near-band-edge emission without observable deep level emission, indicating a low defect concentration. In-plane optical anisotropic transmission was observed by polarized transmission measurements.

The adsorption properties of Au and Pt metal nanoclusters on TiO2 anatase (101) were calculated using density functional theory. Structures and energetics of adsorbed Au and Pt monomers, dimers, and trimers at clean anatase TiO2(101) terraces and two major step edges, as well as O-vacancies, were systematically determined. The theoretical predictions were tested by vapor-depositing small coverages of Au and Pt on anatase (101) and investigating the resulting clusters with Scanning Tunneling Microscopy. On the clean surface, Au shows a strong tendency to form large clusters that nucleate on step edges. A preference for adsorption at type D-(112) steps is observed, which is probably a result of kinetic effects. For Pt, clusters as small as monomers are observed on the terraces, in agreement with the predicted large binding energy of 2.2 eV. Step edges play a less important role than in the case of Au. Oxygen vacancies, produced by electron irradiation, dramatically influence the growth of Au, while the nucleation behavior of Pt was found to be less affected.

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The titanium dioxide rutile (011) (equivalent to (101)) surface reconstructs to a stable (2 · 1) structure upon sputtering and annealing in ultrahigh vacuum. A previously proposed model (T.J. Beck, A. Klust, M. Batzill, U. Diebold, C. Di Valentin, A. Selloni, Phys. Rev. Lett. 93 (2004) 036104/1) containing onefold coordinated oxygen atoms (titanyl groups, Ti@O) is supported by Scanning Tunneling Microscopy (STM) measurements. These Ti@O sites are imaged bright in empty-states STM. A few percent of these terminal oxygen atoms are missing at vacuum-annealed surfaces of bulk-reduced samples. These O vacancies are imaged as dark spots. Their number density depends on the reduction state of the bulk. Double vacancies are the most commonly observed defect configuration; single vacancies and vacancies involving several O atoms are present as well. Formation of oxygen vacancies can be suppressed by annealing a sputtered surface first in vacuum and then in oxygen; annealing a sputtered surface in oxygen results in surface restructuring and a (3 · 1) phase. Anti-phase domain boundaries in the (2 · 1) structure are active adsorption sites. Segregation of calcium impurities from the bulk results in an ordered overlayer that exhibits domains with a centered (2 · 1) periodicity in STM. 2006 Elsevier B.V. All rights reserved.

The titanium dioxide rutile (0 1 1) (equivalent to (1 0 1)) surface reconstructs to a stable (2 × 1) structure upon sputtering and annealing in ultrahigh vacuum. A previously proposed model (T.J. Beck, A. Klust, M. Batzill, U. Diebold, C. Di Valentin, A. Selloni, Phys. Rev. Lett. 93 (2004) 036104/1) containing onefold coordinated oxygen atoms (titanyl groups, TiO) is supported by Scanning Tunneling Microscopy (STM) measurements. These TiO sites are imaged bright in empty-states STM. A few percent of these terminal oxygen atoms are missing at vacuum-annealed surfaces of bulk-reduced samples. These O vacancies are imaged as dark spots. Their number density depends on the reduction state of the bulk. Double vacancies are the most commonly observed defect configuration; single vacancies and vacancies involving several O atoms are present as well. Formation of oxygen vacancies can be suppressed by annealing a sputtered surface first in vacuum and then in oxygen; annealing a sputtered surface in oxygen results in surface restructuring and a (3 × 1) phase. Anti-phase domain boundaries in the (2 × 1) structure are active adsorption sites. Segregation of calcium impurities from the bulk results in an ordered overlayer that exhibits domains with a centered (2 × 1) periodicity in STM.

A combined scanning tunneling microscopy and density-functional theory (DFT) study shows a rich structure of water monolayers adsorbed on ZnO(1010) at room temperature. Most of the water is in a lowest-energy configuration where every second molecule is dissociated. It coexists with an energetically almost degenerate configuration consisting of a fully molecular water monolayer. Parts of the layer continuously switch back and forth between these two states. DFT calculations reveal that water molecules repeatedly associate and dissociate in this sustained dynamical process.

Copper, vapor-deposited on the polar, Zn-terminated ZnO(0001) surface is investigated in view of its suitability as model system for the technologically important Cu/ZnO catalyst. The structure and electronic properties of Cu clusters on ZnO(0001)–Zn have been studied with scanning tunneling microscopy (STM), low energy electron diffraction (LEED), ultraviolet photoelectron spectroscopy (UPS), and low-energy He+ ion scattering (LEIS). At room temperature copper grows as two-dimensional (2D) clusters only at very low coverages of 0.001–0.05 equivalent monolayers (ML). At coverages greater than 0.01 ML, 3D clusters start to develop. This is contrasted to Cu growth on the oxygen-terminated ZnO(0001bar) surface, where a strong adhesion between Cu and the ZnO substrate results in an initial wetting of the surface by Cu. On ZnO(0001)–Zn, surface roughness and sputter damage change the growth mode to more 2D-like. Annealing in UHV results in well-separated, hexagonal clusters rotationally aligned with the substrate. Annealing of 2–5 ML Cu deposits on the ZnO(0001)–Zn surface in 10−6 mbar O2 results in the formation of a (√3 × √3)R30° superstructure with respect to the ZnO lattice. This superstructure likely contains Cu+ sites. The suitability of the different surface morphologies to probe specific sites that are thought to be active for catalytic processes is discussed.

Half-dissociated: Experimental and computational findings conclude that water forms a highly ordered superstructure on defect-free surfaces of zinc oxide, in which every second water molecule is dissociated (see picture). The results are of general relevance for heterogeneous catalysis.