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 $(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})\mathrm{R}19.1\ifmmode^\circ\else\textdegree\fi{}$ V-oxide monolayer: $(5\ifmmode\times\else\texttimes\fi{}5)$, $(5\ifmmode\times\else\texttimes\fi{}3\sqrt{3})\mathrm{rect}$, $(9\ifmmode\times\else\texttimes\fi{}9)$, and ``wagon-wheel'' oxide structures are formed with decreasing chemical potential of oxygen ${\ensuremath{\mu}}_{\mathrm{O}}$. 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 $\mathrm{V}\mathrm{O}$ coordination units, and the reduction progresses mainly via the removal of $\mathrm{V}\mathrm{O}$ vanadyl groups. All oxide structures are stable at the appropriate ${\ensuremath{\mu}}_{\mathrm{O}}$ 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.