G. Thornton

Scanning tunnelling microscopy studies of α-Fe2O3(0001)

Abstract Scanning tunnelling microscopy (STM) images of two different reconstructions of an α-Fe2O3(0001) crystal are presented. Annealing the sample to 1000 K creates a selvedge stabilised by a thin film of Fe3O4, with its (111) plane parallel to the basal plane of the underlying substrate. The STM images confirm that this surface is structurally equivalent to that previously reported for the surface of Fe3O4(111) single crystals, in that two coexisting terminations, denoted A and B, are present separated by alternate steps. Termination A has been identified with 1 4 ML of O atoms capping 3 4 ML of Fe atoms, while termination B consists of 1 2 ML of Fe atoms overlaying a close-packed O layer. Some regions of the sample are disordered but contain small triangular islands of termination A. This structure is attributed to Ar ion induced sputter damage. A different termination, created by annealing the sample at 1100 K in 1 × 10−6 mbar O2, has a distinctive hexagonal LEED pattern, with all the main beams floreted, being surrounded by a hexagon of smaller spots. The STM results show that this surface is stabilized by coexisting α-Fe2O3(0001) and Fe1−xO(111) phases, with each phase existing in atomically well ordered islands of mesoscopic dimensions. The islands themselves are arranged to form a superlattice. The formation of this superlattice can be explained in terms of the lattice mismatch between the two types of oxygen sub-lattices.

Imaging the polar and non-polar surfaces of ZnO with STM

Scanning tunneling microscopy has been used to image the non-polar (1010) and polar (0001)-O surfaces of zinc oxide. Atomic resolution of one sublattice is achieved in the case of ZnO(1010). Images of the (0001) surface evidence a high density of (1010) step edges that locally have the same structure. They occur in four forms, two being as triangular pits and protrusions on flat terraces. They also appear as regular step edges, which alternate with sawtooth step edges. Together these form a double-step, which provides an explanation for earlier LEED results.

HREELS study of the interaction of formic acid with ZnO(101̄0) and ZnO(0001̄)-O

HREELS has been used to characterise the adsorption of formic acid on ZnO(1010) and ZnO(0001)O. The vibrational structure is consistent with that of surface formate bonded to Zn2+ sites. On the basis of previous assignments, the frequency separations of the vs(OCO) and va(OCO) modes indicate that formate adopts a bidentate and monodentate-type configuration on the (1010) and (0001)O surfaces, respectively.

Step and point defect effects on TiO2(100) reactivity

Abstract The influence of steps and point defects on the reactivity of TiO 2 (100) to H 2 O has been examined using photoemission spectroscopy. A vicinal surface was prepared by polishing a sample 2.6 ± 0.1° off the (100) plane towards [001]. Point defects in the form of oxygen vacancies were introduced onto the stepped and planar 1 × 1 stoichiometric (100) surfaces by thermal annealing, a 1 × 3 ordered defect array being formed. The interaction of H 2 O with TiO 2 (100) at 130 K and 293 K is found to be independent of O vacancy and step density. H 2 O adsorbs molecularly at 130 K and dissociates to form OH at 293 K.

Reactions at oxide surfaces

Describes some recent studies of adsorbate interactions at oxide surfaces using photo-emission, surface EXAFS and NEXAFS which illustrate the potential of the techniques to provide structural and mechanistic information. The examples chosen are a SEXAFS study of the NiO(100)-H2S reaction, which results in reduction of the substrate to form a Ni(100)c(2*2)S raft; a NEXAFS study of the thermally activated reaction of SO2 with TiO2(110) resulting in surface sulphate formation; and photo-emission work which evidences the dissociation of H2O by SrTiO3(100) catalytic step sites.

SrTiO3(100) step sites as catalytic centers for H2O dissociation

Abstract Features arising from surface Sr(Ti) atoms on the SrO(TiO2) terraces of SrTiO3 (100) have been resolved in core-level photoemission spectra, which provides new insights into the surface electronic structure and reactivity of transition metal oxides. The surface Sr 3d (Ti3s) features lie to lower (higher) initial energy of the bulk-derived peaks by ca. 1.0 eV (1.7 eV), being consistent with the expected enhancement of covalent bonding in the TiO2-terrace surface. Step-sites, which connect the two types of terrace, are found to act as catalytic centers for H2O dissociation.

Magnetic properties of stoichiometric and nonstoichiometric ultrathin Fe3O4(111) films on Al2O3(0001)

A detailed characterization of magnetic oxide films is essential to enable their use in magnetoresistive devices since their properties depend critically on stoichiometry and structural order. Here, the composition and magnetic properties of ultrathin iron oxide films grown epitaxially on Al2O3(0001) have been characterized using x-ray magnetic circular dichroism XMCD and magnetoresistance (MR) measurements. The XMCD data show by comparison with theoretical calculations that we have successfully found growth conditions for well ordered epitaxial films with Fe3O4 stoichiometry. Nonstoichiometric films exhibit, in addition to a relative reduction in Fe2+ ions, a net transfer of Fe3+ from tetrahedral to octahedral sites. The in-plane MR for both these films is found to be 1% at room temperature in a field of 1 T even though the electrical conductivity differs by a factor of 5.

Probing molecular orientation in corrosion inhibition via a NEXAFS study of benzotriazole and related molecules on Cu(100)

Abstract The orientation of benzotriazole on Cu(100) has been studied using near-edge X-ray absorption fine structure (NEXAFS) at the C and N K-edges. At submonolayer coverage, benzotriazole (C6H5N3) is found to adsorb with the molecular plane close to perpendicular to the Cu(100) surface, as indicated by the polarization dependence of the π∗ features. The orientation of benzimidazole (C7H6N2) and 1-methyl benzotriazole (C7H7N3) on Cu(100) have also been studied, and are found not to be oriented at submonolayer coverage. This, along with the orientation at multilayer coverage suggests a specific first-bonding-layer origin for the corrosion inhibitor properties of benzotriazole.

Interaction of O2 with SnO2(110)1×1 and 4×1

Abstract TPD, photoemission, work function and band-bending measurements have explored the interaction of O2 with two clean surface reconstructions of SnO2(110). While the photoemission spectra are little modified by adsorption, the TPD and work function results suggest the formation of superoxo, O2--type species in both cases.

Growth of copper and palladium on α-Al2O3(0001)

Non-contact atomic force microscopy (NC-AFM) has been used to image the room-temperature growth of copper and palladium on the (1 x 1) and (root 31 x root 31) R +/- 9 degrees terminations of alpha-Al2O3(0001). Three-dimensional (3D) clusters of palladium are observed on both the (1 x 1) and the (root 31 x root 31) R +/- 9 degrees terminations, with 3D clusters of copper observed on the reconstructed surface. There is evidence of step-edge-dominated growth of palladium on the (root 31 x root 31) R +/- 9 degrees termination