Nature of band-gap states in V-doped TiO2 revealed by resonant photoemission

Band-gap states in V-doped TiO2 have been studied by photoemission spectroscopy over a range of photon energies encompassing the Ti 3p and V 3p core thresholds. The states show resonant enhancement at photon energies significantly higher than found for Ti 3d states introduced into TiO2 by oxygen deficiency or alkalimetal adsorbates. This demonstrates that the gap states relate to electrons trapped on dopant V cations rather than host Ti cations

Electronic structure of lanthanide-doped bismuth vanadates: A systematic study by x-ray photoelectron and optical spectroscopies

Monoclinic BiVO 4 has emerged in recent years as one of the most promising materials for photocatalytic evolution of oxygen under solar irradiation. However, it is in itself unable to phototcatalyze reduction of water to hydrogen due to the placement of the conduction band edge below the potential required for H 2 O/H 2 reduction. As a consequence, BiVO 4 only finds application in a hybrid system. Very recently, tetragonal lanthanide-doped BiVO 4 powders have been shown to be able to both reduce and to oxidize water under solar irradiation, but to date there has been no comprehensive study of the electronic properties of lanthanide-doped bismuth vanadates aimed at establishing the systematic trends in the electronic structure in traversing the lanthanide series. Here, the accessible family of lanthanide-doped BiVO 4 quaternary oxides of stoichiometry Bi 0.5 Ln 0.5 VO 4 (Ln = La to Lu, excluding Pm) has been studied by X-ray powder diffraction, X-ray photoemission spectroscopy, and diffuse reflectance optical spectroscopy. The compounds all adopt the tetragonal zircon structure, and lattice parameters decrease monotonically in traversing the lanthanide series. At the same time, there is an increased peak broadening in the diffraction patterns as the mismatch in ionic radius between Bi 3+ and the Ln 3+ ions increases across the series. Valence band X-ray photoemission spectra show that the final state 4f n-1 structure associated with ionization of lanthanide 4f n states is superimposed on the valence band structure of BiVO 4 in the quaternary materials: in the case of the Ce-, Pr- and Tb-doped BiVO 4 , 4f-related states appear above the top of the main valence band of BiVO 4 and account for the small bandgap in the Ce compound. In all cases, the 4f structure is characteristic of the lanthanide element in the Ln(III) oxidation state. Vanadium 2p and lanthanide 3d or 4d core level photoelectron spectra of those compounds where the lanthanide may in principle adopt a higher (Ln = Ce, Pr, Tb) or lower (Ln = Eu, Yb) oxidation state further confirm the prevalence of the Ln(III) valence state throughout. The visible region optical properties of all samples were studied by diffuse reflectance spectroscopy, with a particular focus on the optical bandgap and the details of transitions associated with localized 4f states. Taken together, the results demonstrate the remarkable tunability of optical and electronic properties for these quaternary materials

Electronic properties of antimony-doped anatase TiO2 thin films prepared by aerosol assisted chemical vapour deposition

The electronic properties of antimony-doped anatase (TiO2) thin films deposited via aerosol assisted chemical vapour deposition were investigated by a range of spectroscopic techniques. The incorporation of Sb(V) into the TiO2 lattice was characterised by X-ray absorption spectroscopy and resulted in n-type conductivity, with a decrease in sheet resistance by four-orders of magnitude compared to that of undoped TiO2 films. The films with the best electrical properties displayed charge carrier concentrations of ca. 1 × 1020 cm−3 and a specific resistivity as low as 6 × 10−2 Ω cm. Doping also resulted in an orange colouration of the films that became progressively stronger with increasing Sb content. X-ray photoelectron spectroscopy showed that substantial segregation of Sb(III) to the surface of the film was associated with the appearance of lone pair surface states lying above the top of the main O 2p valence band. The pronounced visible region absorption in the films is attributed to transitions from the Sb(III) states at surface and grain boundary interfaces into the conduction band. The segregation of Sb leads to p-type surface layers at high doping levels and limits the mobility in this new conducting oxide

Identification of the Mechanism of Electrocatalytic Ozone Generation on Ni/Sb-SnO2

This paper reports a systematic study of the codoping of SnO2with Sb and Ni to identify the mechanism responsible for the electrocatalytic generation of ozone on Ni/Sb-SnO2. On the basis of interpretation of a combination of X-ray diffraction, BET surface area measurements (N2), and thermal analysis, the formation of ozone appears to take place on particle surfaces of composite Sb-SnO2grains and is controlled by diffusion of OH along internal crystallite surfaces within the grain. Sb-doped SnO2is inactive with respect to ozone evolution in the absence of Ni, demonstrating a synergic interaction between nickel and antimony. From X-ray photoelectron spectroscopy (XPS) investigations, Sb(V) ions substitute for Sn(IV) in the lattice with a preference for centrosymmetric coordination sites, while the Sb(III) ions occur at grain surfaces or boundaries. Ni was not detected by XPS, being located in the subsurface region at concentrations below the detection limit of the instrument. In addition to identification of a possible mechanism for ozone formation, the study resulted in the production of active nanopowders which will allow the fabrication of high-surface-area anodes with the potential to exceed the space-time yields of β-PbO2anodes, permitting the application the Ni/Sb-SnO2anodes in the treatment of real waters

Nature of the band gap of In2O3 revealed by first-principles calculations and x-ray spectroscopy

Bulk and surface sensitive x-ray spectroscopic techniques are applied in tandem to show that the valence band edge for In2O3 is found significantly closer to the bottom of the conduction band than expected on the basis of the widely quoted bulk band gap of 3.75 eV. First-principles theory shows that the upper valence bands of In2O3 exhibit a small dispersion and the conduction band minimum is positioned at Gamma. However, direct optical transitions give a minimal dipole intensity until 0.8 eV below the valence band maximum. The results set an upper limit on the fundamental band gap of 2.9 eV

Revisiting the origin of satellites in core-level photoemission of transparent conducting oxides: The case of &ITn&IT-doped SnO2

The longstanding problem of interpretation of satellite structures in core-level photoemission spectra of metallic systems with a low density of conduction electrons is addressed using the specific example of Sb-doped SnO2. Comparison of ab initio many-body calculations with experimental hard x-ray photoemission spectra of the Sn 4d states shows that strong satellites are produced by coupling of the Sn core hole to the plasma oscillations of the free electrons introduced by doping. Within the same theoretical framework, spectral changes of the valence band spectra are also related to dynamical screening effects. These results demonstrate that, for the interpretation of electron correlation features in the core-level photoelectron spectra of such narrow-band materials, going beyond the homogeneous electron gas electron-plasmon coupling model is essential

Insights into the electronic structure of OsO2 using soft and hard x-ray photoelectron spectroscopy in combination with density functional theory

Theory and experiment are combined to gain an understanding of the electronic properties of OsO2, a poorly studied metallic oxide that crystallizes in the rutile structure. Hard and soft valence-band x-ray photoemission spectra of OsO2 single crystals are in broad agreement with the results of density-functional-theory calculations, aside from a feature shifted to high binding energy of the conduction band. The energy shift corresponds to the conduction electron plasmon energy measured by reflection electron energy loss spectroscopy. The plasmon satellite is reproduced by many-body perturbation theory

Oxygen deficiency and V and Sb doping in TiO2 and SnO2: Influences on surface electronic structure

The influence of oxygen deficiency and of doping with V and Sb on the surface electronic structure of TiO2 and SnO2 will be reviewed. Despite the similarity in structure of the two materials, they differ significantly in their surface electronic properties. Oxygen deficiency in TiO2-x gives rise to a new electronic state in the top half of the bulk bandgap due to electrons self trapped on Ti(III) sites, whereas in SnO2-x new 5s-5p hybrid states appear toward the bottom of the bulk bandgap. V-doping in both TiO2 and SnO2 gives rise to localized V(IV) states, whose adiabatic ionization energy defines the Fermi energy. A shift of 0.6 eV between the vertical ionization energies in the two systems is shown to be due to the differing vibrational relaxation energies associated with very different high frequency dielectric constants for the two oxides. Finally the effects of Sb doping are considered. Sb doped SnO2 is a metallic conductor with electrons in the Sn 5s conduction band. By contrast, Sb doping in TiO2 is compensated by cation vacancies and the material is a white insulator. However for both compounds, surface Sb(III) give rise to localized 5s-5p hybrid states

The Study of Adsorption on Metal Oxides by High-Resolution Electron-Energy-Loss Spectroscopy

The application of high-resolution electron-energy-loss spectroscopy to the study of adsorbates on the surfaces of single-crystal metal oxides is reviewed. The technique provides a means of measuring vibrational spectra of adsorbates over a wide energy range although when dealing with non-metallic oxide substrates a strong background of intrinsic surface phonon excitations does interfere with the adsorbate loss spectra. In metallic oxides the conduction electrons screen the substrate losses and the technique proves to be more sensitive to the presence of adsorbates. The chemical information which can be extracted from energy loss spectra is illustrated by consideration of the adsorption of water on SrTiO 3,(100) and Na O, 7 WO 3,(100). © 1985 Elsevier Science Publishers B.V