Hongqing Shi

Bridging the temperature and pressure gaps: close-packed transition metal surfaces in an oxygen environment

An understanding of the interaction of atoms and molecules with solid surfaces on the microscopic level is of crucial importance to many, if not most, modern high-tech materials applications. Obtaining such accurate, quantitative information has traditionally been the realm of surface science experiments, carried out under ultra-high vacuum conditions. Over recent years scientists have realized the importance of obtaining such knowledge also under the high pressure and temperature conditions under which many industrial processes take place, e.g. heterogeneous catalysis, since the material under these conditions may be quite different to that under the conditions of typical surface science experiments. Theoretical studies too have been aimed at bridging the so-called pressure and temperature gaps, and great strides have been made in recent years, often in conjunction with experiment. Here we review recent progress in the understanding of the hexagonal close-packed surfaces of late transition and noble metals in an oxygen environment, which is of relevance to many heterogeneous catalytic reactions. In many cases it is found that, on exposure to high oxygen pressures and elevated temperatures, thin oxide-like structures form which may or may not be stable, and which may have little similarity to the bulk oxides, and thus possess unique chemical and physical properties.

Real Time Determination of the Electronic Structure of Unstable Reaction Intermediates during Au2O3 Reduction.

Chemical reactions are always associated with electronic structure changes of the involved chemical species. Determining the electronic configuration of an atom allows probing its chemical state and gives understanding of the reaction pathways. However, often the reactions are too complex and too fast to be measured at in situ conditions due to slow and/or insensitive experimental techniques. A short-lived Au2O compound has been detected for the first time under in situ conditions during the temperature-programmed reduction of Au2O3. A time-resolved resonant inelastic X-ray scattering experiment (RIXS) allowed the determination of changes in the Au electronic structure, enabling a better understanding of the reaction mechanism of Au(III) reduction. On the basis of time-resolved RIXS data analysis combined with genetic algorithm methodology, we determined the electronic structure of the metastable Au2O intermediate species. The data analysis showed a notably larger value for the lattice constant of the intermediate Au as compared to the theoretical predictions. With support of DFT calculations, we found that such a structure may indeed be formed and that the expanded lattice constant is due to the termination of Au2O on the Au2O3 structure.

Single P and As dopants in the Si(001) surface

Using first-principles density functional theory, we discuss doping of the Si(001) surface by a single substitutional phosphorus or arsenic atom. We show that there are two competing atomic structures for isolated Si-P and Si-As heterodimers, and that the donor electron is delocalized over the surface. We also show that the Si atom dangling bond of one of these heterodimer structures can be progressively charged by additional electrons. It is predicted that surface charge accumulation as a result of tip-induced band bending leads to structural and electronic changes of the Si-P and Si-As heterodimers which could be observed experimentally. Scanning tunneling microscopy (STM) measurements of the Si-P heterodimer on a n-type Si(001) surface reveal structural characteristics and a bias-voltage dependent appearance, consistent with these predictions. STM measurements for the As:Si(001) system are predicted to exhibit similar behavior to P:Si(001).