Russell G. Egdell

Stereochemistry of post-transition metal oxides: revision of the classical lone pair model

The chemistry of post transition metals is dominated by the group oxidation state N and a lower N-2 oxidation state, which is associated with occupation of a metal s(2) lone pair, as found in compounds of Tl(I), Pb(II) and Bi(III). The preference of these cations for non-centrosymmetric coordination environments has previously been rationalised in terms of direct hybridisation of metal s and p valence orbitals, thus lowering the internal electronic energy of the N-2 ion. This explanation in terms of an on-site second-order Jahn-Teller effect remains the contemporary textbook explanation. In this tutorial review, we review recent progress in this area, based on quantum chemical calculations and X-ray spectroscopic measurements. This recent work has led to a revised model, which highlights the important role of covalent interaction with oxygen in mediating lone pair formation for metal oxides. The role of the anion p atomic orbital in chemical bonding is key to explaining why chalcogenides display a weaker preference for structural distortions in comparison to oxides and halides. The underlying chemical interactions are responsible for the unique physicochemical properties of oxides containing lone pairs and, in particular, to their application as photocatalysts (BiVO(4)), ferroelectrics (PbTiO(3)), multi-ferroics (BiFeO(3)) and p-type semiconductors (SnO). The exploration of lone pair systems remains a viable a venue for the design of functional multi-component oxide compounds.

Surface properties of antimony doped tin(IV) oxide: A study by electron spectroscopy

Abstract Pure and antimony doped tin(IV) oxide (SnO 2 ) have been studied by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) and high resolution electron energy loss spectroscopy (HREELS). Pronounced surface enrichment by antimony in the doped material is evident from XPS, although both UPS and HREELS indicate that no increase in the free-carrier concentration close to the surface is associated with this segregation. Ultraviolet photoemission from undoped SnO 2 is dominated by the filled O 2p valence band, the overall bandwidth being in good agreement with KKR calculations. However, a localized 5s-5p hybrid state associated with surface tin(II) ions gives rise to a feature in photoemission between the bulk conduction and valence bands. Carriers introduced by doping 3% antimony into the SnO 2 lattice occupy a free-electron-like conduction band of predominantly metal 5s atomic character and may be observed directly in UPS. The Drude-like response of the conduction electrons produces a surface plasmon loss in low energy electron scattering spectra whose energy (0.55 eV) can be calculated from a simple free-electron model assuming an effective mass ratio close to unity. The localized electrons associated with surface antimony atoms are more tightly bound than electrons occupying the tin surface states; we speculate that this provides the thermodynamic driving force for surface enrichment in antimony.

High resolution x-ray photoemission study of plasma oxidation of indium–tin–oxide thin film surfaces

The influence of plasma oxidation and other surface pretreatments on the electronic structure of indium–tin–oxide (ITO) thin films has been studied by high resolution x-ray photoemission spectroscopy. Plasma oxidation compensates n-type doping in the near surface region and leads to a reduction in the energy of plasmon satellite structure observed in In 3d core level spectra. In parallel, the Fermi level moves down within the conduction band, leading to a shift to low binding energy for both core and valence band photoemission features; and the work function increases by a value that corresponds roughly to the core and valence band binding energy shifts. These observations suggest that the conduction band of ITO is fixed relative to the vacuum level and that changes of work function are dominated by shifts of the Fermi level within the conduction band.

The metal-to-semiconductor transition in ternary ruthenium (IV) oxides: a study by electron spectroscopy

The pyrochlore ruthenium (IV) oxides Pb2Ru2O7-y, Bi2Ru2O7, Y2Ru2O7 and the perovskites SrRuO3, CaRuO3 have been studied by ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS) and by high-resolution electron-energy-loss spectroscopy (HREELS). The presence of a Fermi edge in UPS confirms that, with the exception of Y2Ru2O7, these materials are metallic; the density of states at the Fermi energy g( epsilon F) decreases in the sequence Pb2Ru2O7-y>Bi2Ru2O7>SrRuO3>CaRuO3. The probability of reaching final states in XPS where a Ru:3d core hole is screened by conduction electrons decreases in the same sequence, and for Y2Ru2O7 only unscreened final states are observed. In parallel with these trends, the conduction-electron plasmon loss found in HREELS moves to lower energy with decreasing g( epsilon F); plasmon excitation is not evident in the energy-loss spectrum of semiconducting Y2Ru2O7. The results are consistent with a progressive decrease in the transfer-energy (resonance) integral that determines the Ru:4d bandwidth in the series Pb2Ru2O7-y>Bi2Ru2O7>SrRuO3>CaRuO3>Y2Ru2O7, and with a transition to non-metallic behaviour due to correlation-induced electron localisation in Y2Ru2O7. A technique for the calculation of the width of photoemission lines in UPS from HREELS data is presented.

Electronic states at oxygen deficient WO3(001) surfaces: a study by resonant photoemission

Abstract Electronic states associated with oxygen deficiency at WO 3 (001) surfaces have been studied by photoemission with the photon energy tuned to a resonance maximum in the W 5d ionization cross-section profile. Mild ion bombardment followed by UHV annealing leads to a reduced surface where the predominant W 5d states lie “deep” within the bulk bandgap. After more energetic and prolonged bombardment and annealing, a quasi metallic surface is obtained with a significant density of states at the Fermi energy. The valence level photoemission measurements are compared with results from scanning tunnelling microscopy and shallow W 4f core level photoemission. The “deep” states are associated with pairs of W ions σ-bonded together in defect troughs that can be imaged in real space by STM. By contrast the metallic states are assigned to reduced (1 × 1) terrace areas where all the W ions lose on-top oxygen.

Antimony-doped tin(IV) oxide: Surface composition and electronic structure

Abstract Antimony-doped tin(IV) oxide Sn1−xSbxO2 prepared by a high-temperature (1300 K) solid-state synthetic procedure has been studied over the composition range0

N-type doping in CdO ceramics: a study by EELS and photoemission spectroscopy

Abstract Ceramic samples of yttrium- and indium-doped CdO ( Cd 1 − x Y x O with 0 ≤ x ≤ 0.035 and Cd 1 − x In x O with 0 ≤ x ≤ 0.023) have been studied by electron energy loss spectroscopy (EELS) and ultraviolet and X-ray photoemission spectroscopy (UPS and XPS). Both In and Y act as efficient n-type dopants, although the carrier concentration is higher in In-doped material. The maximum surface plasmon loss energy in EELS is 0.66 eV in Y-doped samples and 0.72 eV for in-doped samples. Effective masses ( m ∗) increase with increasing carrier concentration N and obey an approximately linear variation of the form m∗ = m ∗ o + cN , where c is a constant. UPS shows a well-defined conduction band feature for doped samples. A shift of the valence band edge towards high binding energy due to chemical doping was observed in He(I) UPS, which is confirmed by shifts of O 2p valence band features and Cd 4d core level peaks in high resolution XPS. However, the shifts are less than expected from the increased widths of the occupied part of the conduction band. This is due to shrinkage of the bandgap with doping. XPS demonstrates that there is pronounced surface segregation of dopant atoms. However, the segregated dopant atoms do not act as donor centres.

Growth of In2O3(100) on Y-stabilized ZrO2(100) by O-plasma assisted molecular beam epitaxy

Thin films of In2O3 have been grown on Y-stabilized ZrO2(100) by oxygen plasma assisted molecular beam epitaxy with a substrate temperature of 650°C. Ordered epitaxial growth was confirmed by high resolution transmission electron microscopy. The position of the valence band onset in the x-ray photoemission spectra of the epitaxial films is found to be inconsistent with the widely quoted value of 3.75eV for the fundamental bandgap of In2O3 and suggests a revised value of 2.67eV.

An STM study of surface structures on WO3(001)

Structure on the (001) surface of monoclinic WO3 has been studied by scanning tunnelling microscopy. Electrostatic instability at the polar surface of WO3(001) is avoided by termination of the outer WO2 layer with half a monolayer of oxygen ions to produce a (2 × 2)R45° arrangement of on-top oxygen. An additional periodicity is introduced in some terrace areas by tilting of WO6 octahedra to produce alternately long and short on-top OO separations along the [110] direction. This produces a (2 × 2) superstructure approximating to the space group c2mm. Prominent defect troughs on oxygen annealed surfaces are attributed to missing oxygen ions. Topographically resolved I-V curves show that significant filled state electron density resides in the trough bottoms. Argon ion bombardment and subsequent annealing leads to the appearance of defects in which on-top oxygen is removed from adjacent W ions to generate linear features three unit cells wide running along the [100] direction. These new defects aggregate with the troughs to produce characteristic branched structures. Prolonged bombardment and annealing leads eventually to large terrace areas displaying a new (2 × 2) superstructure in which only a 14 the W ions carry on-top oxygen.

Surface Energies Control the Self-Organization of Oriented In2O3 Nanostructures on Cubic Zirconia

Highly aligned one-dimensional (1D) nanorods of the transparent conducting oxide In(2)O(3) have been grown on (110)-oriented Y-stabilized ZrO(2) substrates, whereas growth on (100) and (111) substrates leads respectively to blocklike 3D islands and continuous films. It is shown that the striking influence of substrate orientation on the growth morphology is controlled by differences in energies between the low index surfaces of In(2)O(3) and that spontaneous self-organization is driven by minimization of surface energies.