Fundamentals of chemical functionalities at oxide interfaces

In the present study, we investigated the effect of cerium doping in zinc oxide matrix, used as photoactive material. Cerium ions into the matrix of ZnO can act like a 'trap' for the electrons, lowering the e⁻/h⁺ recombination rate and so increasing the photocatalytic efficiency of the ZnO.

We synthesised doped samples using a simple precipitation route. The amount of dopant used was, 1 and 10% molar. The samples have been studied via x-ray diffraction measurements for the structural characterisation; UV–visible diffuse reflectance was used for the optical analysis; Branauer–Emmett–Teller (BET) model for the measurement of the surface area. Finally, the samples have been analysed via electron paramagnetic resonance (EPR) spectroscopy for the electronic characterisation and for testing their photoactivity. The spin trapping technique was also use to measure the amount of stable radical adducts formed via reaction of OH^• radicals with molecules of the DMPO (5,5-dimethyl-1-pyrroline-N-oxide) spin probe.

Recently, a well-ordered silicatene/silicon-carbide hybrid thin-film supported on Ru(0 0 0 1) has been reported (2015 Surf. Sci. 632 9–13). The thin-film consist of a monolayer of corner sharing (SiO₄)-tetrahedra on top of a (Si₂C₃) monolayer supported on the Ru(0 0 0 1) surface. This silicatene/silicon-carbide hybrid system may exhibit interesting properties for nano-technological applications and represents another example of a 2D material. We explore the physical and chemical properties of the silicatene/silicon-carbide thin-film using DFT and compare the vibrational spectra with existing experimental data. The characteristics of the silicatene/silicon-carbide hybrid system are compared with those of the bilayer-silicatene (pure SiO₂ film). We found large differences in the adsorption modes of the two thin-films on the Ru(0 0 0 1) support. Whereas the bilayer-silicatene physisorbs on the Ru(0 0 0 1) surface, the silicatene/silicon-carbide layer binds via chemisorption. The chemical properties of the two thin-films were probed by adsorption of H atoms at various positions, as well as by Al-doping and the formation of hydroxyl groups (Al–OH). These results show that despite the similar structure of the top layer and the identical metal support (Ru), the mixed silicatene/silicon-carbide system behaves quite differently from the pure silica two-layer counterpart.

Vanadium dioxide (VO₂) has a phase transition from insulator to metal at 340 K, and this transition can be strongly modified by hydrogenation. In this work, two dimensional (2D) VO₂ sheets have been grown on Si(1 1 1) surfaces through chemical vapor deposition, and metal (Au, Pt) thin films were deposited on VO₂ surfaces by sputtering. The hydrogenation and dehydrogenation of VO₂ and metal-decorated VO₂ structures in H₂ and in air were in situ studied by Raman. We found that hydrogenation and dehydrogenation temperatures have been significantly decreased with the VO₂ surface decorated by Au and Pt. The enhanced hydrogenation and dehydrogenation reactions can be attributed to catalytic dissociation of H₂ and O₂ molecules on metal surfaces and subsequent spillover of dissociated H and O atoms to the oxide surfaces.

The structure and properties of ternary oxide materials at the nanoscale are poorly explored both on experimental and theoretical levels. With this work we demonstrate the successful on-surface synthesis of two-dimensional (2D) ternary oxide, MnWO_x and FeWO_x, nanolayers on a Pd(1 0 0) surface and the understanding of their new structure and phase behaviour with the help of state-of-art surface structure and spectroscopy techniques. We find that the 2D MnWO_x and FeWO_x phases, prepared under identical thermodynamic conditions, exhibit similar structural properties, reflecting the similarity of the bulk MnWO₄ and FeWO₄ phases with the wolframite structure. Structure models of prototypical 2D ternary oxide phases are proposed and discussed in the light of new structure architecture concepts which have no analogues in the bulk.

Thin Cr₂O₃(0 0 0 1) layers are formed by oxidation of a Cr(1 1 0) single crystal. This surface is further modified by growing an epitaxial ultrathin V₂O₃(0 0 0 1) film by reactive vapor deposition. Synchrotron based soft-x-ray photoemission spectroscopy and x-ray photoelectron diffraction are used to characterize the surface layers of these two corundum-structured oxides. By comparison of experimental XPD patterns with simulated electron multiple scattering calculations, two distinctively different surface terminations are extracted for the two oxides. While for V₂O₃ we confirm the previously proposed vanadyl-terminated surface structure, we propose a new surface structure for Cr₂O₃ that consists of excess chromium atoms occupying interstitial sub-surface sites.