Ulrike Diebold

Epitaxial growth and properties of ferromagnetic co-doped TiO2 anatase

We have used oxygen-plasma-assisted molecular-beam epitaxy (OPA-MBE) to grow CoxTi1−xO2 anatase on SrTiO3(001) for x=∼0.01–0.10, and have measured the structural, compositional, and magnetic properties of the resulting films. Whether epitaxial or polycrystalline, these CoxTi1−xO2 films are ferromagnetic semiconductors at and above room temperature. However, the magnetic and structural properties depend critically on the Co distribution, which varies widely with growth conditions. Co is substitutional in the anatase lattice and in the +2 formal oxidation state in ferromagnetic CoxTi1−xO2. The magnetic properties of OPA-MBE grown material are significantly better than those of analogous pulsed laser deposition-grown material.

Interaction of water, oxygen, and hydrogen with TiO2(110) surfaces having different defect densities

We have studied the stoichiometric (annealed in oxygen), the slightly oxygen‐deficient (annealed in vacuum), and the highly defective (sputtered with Ar+) TiO2(110) surfaces and their reactivities to molecular oxygen, molecular water, and 50‐eV hydrogen ions using x‐ray photoelectron spectroscopy (XPS) and low‐energy ion scattering spectroscopy (LEIS). The use of isotopically labeled 18O enables us to distinguish adsorbed oxygen from lattice oxygen, and the concentration of surface oxygen vacancies is titrated by 18O2 adsorption. LEIS (1‐keV He+) is used to analyze the chemical composition of the outermost surface layer before and after 18O2 and H218O exposure. Water adsorbs on both stoichiometric and slightly O‐deficient surfaces [with oxygen vacancies ∼0 and 0.08 monolayer (ML), respectively] at room temperature. There is little or no dependence of saturation water coverage (lower limit of ∼0.07 ML for both surfaces) on the concentration of surface oxygen vacancies. On the highly defective surfaces, the...

Reaction of O2 with Subsurface Oxygen Vacancies on TiO2 Anatase (101)

Oxide Chemistry Below the Surface Although metal oxides, such as titanium dioxide (TiO2), are used for catalytic oxidation reactions and photocatalysis, the O2 does not react directly with substrates. Vacancies in the surface region of the TiO2 rutile phase can transfer a negative charge to adsorbed O2 to create more reactive species. By contrast, in anatase—the phase associated with nanoscale TiO2 particles—subsurface vacancies form. Setvin et al. (p. 988) used a scanning tunneling microscopy tip to pull these vacancies to the surface in a niobiumdoped anatase crystal and followed the transformation of adsorbed O2− into a peroxo species and a bridging O2 dimer. Subsurface oxygen vacancies created at an anatase surface play a key role in forming a bridging oxygen (O2) dimer from adsorbed O2. Oxygen (O2) adsorbed on metal oxides is important in catalytic oxidation reactions, chemical sensing, and photocatalysis. Strong adsorption requires transfer of negative charge from oxygen vacancies (VOs) or dopants, for example. With scanning tunneling microscopy, we observed, transformed, and, in conjunction with theory, identified the nature of O2 molecules on the (101) surface of anatase (titanium oxide, TiO2) doped with niobium. VOs reside exclusively in the bulk, but we pull them to the surface with a strongly negatively charged scanning tunneling microscope tip. O2 adsorbed as superoxo (O2–) at fivefold-coordinated Ti sites was transformed to peroxo (O22–) and, via reaction with a VO, placed into an anion surface lattice site as an (O2)O species. This so-called bridging dimer also formed when O2 directly reacted with VOs at or below the surface.

Titanium and reduced titania overlayers on titanium dioxide(110)

Abstract The adsorption of titanium on titanium dioxide TiO 2 (110) has been studied by X-ray photoelectron spectroscopy (XPS) and low energy ion scattering (LEIS). The XPS data for Ti overlayers are interpreted using peak fitting based on experimental standard spectra. 4 A of Ti deposited at 150 K reacts with the substrate to produce ≈ 12 A of intermediate oxidation state Ti. Adsorption of neutral metal begins on top of this interface oxide film, but 20 A of deposited Ti are needed to cover the oxide completely. LEIS data indicate a tendency for clustering of Ti on top of the interface oxide. Ar + sputtering of stoichiometric TiO 2 leads to preferential loss of O from the near surface region. This reduction of the clean, annealed oxide surface by Ar + ion bombardment starts immediately and does not reach a steady state until 3 × 10 17 ions cm −2 , at which point the reduced overlayer is 17 A thick.

Local ordering and electronic signatures of submonolayer water on anatase TiO2(101)

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.

Hydrogen Bonding Controls the Dynamics of Catechol Adsorbed on a TiO2(110) Surface

Stop or Go on Oxide Surfaces Direct studies of surface diffusion with instruments such as the scanning tunneling microscope (STM) have often focused on species on metal surfaces, but surface diffusion can play an important role for reactions on metal oxide surfaces. Li et al. (p. 882) used STM and density functional theory calculations to study how catechol (a benzene ring bearing two −OH groups) diffuses on the surface of the rutile phase of titanium dioxide. Both mobile and immobile species were observed on the time scale of minutes while making repeated STM scans. Hydrogen atom transfers between surface OH groups and the molecule changed the interaction energy between the molecule and the surface, and hence the barrier for diffusion. The diffusion barrier for an organic molecule on a hydroxylated metal oxide surface depends on hydrogen bond formation. Direct studies of how organic molecules diffuse on metal oxide surfaces can provide insights into catalysis and molecular assembly processes. We studied individual catechol molecules, C6H4(OH)2, on a rutile TiO2(110) surface with scanning tunneling microscopy. Surface hydroxyls enhanced the diffusivity of adsorbed catecholates. The capture and release of a proton caused individual molecules to switch between mobile and immobile states within a measurement period of minutes. Density functional theory calculations showed that the transfer of hydrogen from surface hydroxyls to the molecule and its interaction with surface hydroxyls substantially lowered the activation barrier for rotational motion across the surface. Hydrogen bonding can play an essential role in the initial stages of the dynamics of molecular assembly.

Correlation between bonding geometry and band gap states at organic-inorganic interfaces: catechol on rutile TiO2(110).

Adsorbate-induced band gap states in semiconductors are of particular interest due to the potential of increased light absorption and photoreactivity. A combined theoretical and experimental (STM, photoemission) study of the molecular-scale factors involved in the formation of gap states in TiO(2) is presented. Using the organic catechol on rutile TiO(2)(110) as a model system, it is found that the bonding geometry strongly affects the molecular electronic structure. At saturation catechol forms an ordered 4 x 1 overlayer. This structure is attributed to catechol adsorbed on rows of surface Ti atoms with the molecular plane tilted from the surface normal in an alternating fashion. In the computed lowest-energy structure, one of the two terminal OH groups at each catechol dissociates and the O binds to a surface Ti atom in a monodentate configuration, whereas the other OH group forms an H-bond to the next catechol neighbor. Through proton exchange with the surface, this structure can easily transform into one where both OH groups dissociate and the catechol is bound to two surface Ti in a bidentate configuration. Only bidendate catechol introduces states in the band gap of TiO(2).

Oxygen-induced restructuring of the TiO2(110) surface: a comprehensive study

We report a comprehensive experimental and theoretical study of the eVect of oxidizing a TiO 2 (110) surface at moderate temperatures. The surfaces are investigated with scanning tunneling microscopy (STM ), low-energy He+ ion scattering (LEIS ) and static secondary ion mass spectroscopy (SSIMS ). Flat (1◊1)-terminated TiO 2 (110) surfaces are obtained by sputtering and annealing in UHV at 880 K. These surfaces are exposed to oxygen gas at elevated temperatures in the range 470‐830 K. Formation of irregular networks of pseudo-hexagonal rosettes (6.5 A ˚ ◊ 6A ˚ ) and small (11:0] oriented (1◊1) islands along with {001}-oriented strands is induced at temperatures from 470 to 660 K. After annealing above 830 K, only regular (1◊1) terraces and white strands are observed. The composition of these oxygen-induced phases is quantified using 18O 2 gas in combination with LEIS and SSIMS measurements. The dependence of the restructuring process on annealing time, annealing temperature, and sample history is systematically investigated. Exposure to H 2 18O and air in the same temperature regime fails to induce the restructuring. UHV annealing of restructured, oxygen-enriched TiO 2 (110) surface smooths the surfaces and converts the rosette networks into strands and finally into the regular (1◊1) terraces. This is reported in an accompanying paper [M. Li, W. Hebenstreit, U. Diebold, Phys. Rev. B (1999), submitted ]. The rosette model is supported by first-principles density functional calculations which show a stable structure results, accompanied by significant relaxations from bulk-truncated positions. A mechanism for the dynamic processes of the formation of rosettes and (1◊1) islands is presented and the importance of these results for the surface chemistry of TiO 2 (110) surfaces is discussed.

Direct view at excess electrons in TiO2 rutile and anatase.

A combination of scanning tunneling microscopy and spectroscopy and density functional theory is used to characterize excess electrons in TiO2 rutile and anatase, two prototypical materials with identical chemical composition but different crystal lattices. In rutile, excess electrons can localize at any lattice Ti atom, forming a small polaron, which can easily hop to neighboring sites. In contrast, electrons in anatase prefer a free-carrier state, and can only be trapped near oxygen vacancies or form shallow donor states bound to Nb dopants. The present study conclusively explains the differences between the two polymorphs and indicates that even small structural variations in the crystal lattice can lead to a very different behavior.

Ultrathin metal film growth on TiO2(110): an overview

Abstract The interface between an oxide substrate and a metal overlayer may crucially influence the macroscopic behavior of technological devices such as sensors, catalysts, ceramics, and semiconductor chips. Hence investigations of adsorption, nucleation and growth of ultrathin metal films on metal oxide surfaces have attracted increasing interest during recent years. Experimental results on the growth of metal overlayers on a model oxide, TiO 2 (110), are reviewed. The emphasis is on the very initial stages of overlayer growth and on extracting general trends on metal/metal-oxide interaction by comparing results from different metal overlayers. The electronic and geometric structure, growth mode, interface formation, and thermal stability of metal films are discussed. The strength of the oxidation/reduction reaction at the interface, the wetting ability and the tendency to form a disordered layer are all related to the reactivity of the overlayer metal towards oxygen. We propose that ‘reactive adsorption’ accounts for the observed trends.