Reduction of vanadium-oxide monolayer structures

The reduction of vanadium oxide monolayer structures on Rh(111) has been investigated by variable-temperature scanning tunneling microscopy, low energy electron diffraction, photoelectron spectroscopy of the core levels, and the valence band, and by probing the phonon spectra of the oxide structures in high-resolution electron energy loss spectroscopy. A sequence of oxide phases has been observed following the reduction from the highly oxidized (7×7) R19.1°V-oxide monolayer: (5×5), (5×33) rect, (9×9), and "wagon-wheel" oxide structures are formed with decreasing chemical potential of oxygen ?O. The structures have been simulated by ab initio density functional theory, and structure models are presented. The various V-oxide structures are interrelated by common V O coordination units, and the reduction progresses mainly via the removal of V O vanadyl groups. All oxide structures are stable at the appropriate ?O only in the two-dimensional V-oxide/Rh(111) phase diagram and are thus stabilized by the metal-oxide interface. The results demonstrate that oxides in ultrathin layer form display modified physical and chemical properties as compared to the bulk oxides. Œ 2005 The American Physical Society.