Atomic-level growth study of vanadium oxide nanostructures on Rh(111)

The growth and structure of ultrathin vanadium oxide films on Rh(111) has been studied by scanning tunneling microscopy, low-energy electron diffraction, high-resolution x-ray photoelectron spectroscopy, high-resolution electron energy-loss spectroscopy, and ab initio density-functional-theory calculations. For submonolayer coverages $[\ensuremath{\Theta}<0.6\mathrm{MLE}$ (monolayer equivalents)], depending on the oxide preparation route (reactive evaporation vs postoxidation), two well-ordered V-oxide phases with $(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})R19.1\ifmmode^\circ\else\textdegree\fi{}$ and $(\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13})R13.8\ifmmode^\circ\else\textdegree\fi{}$ structures and similar electronic and vibrational signatures have been observed. The $\sqrt{7}$ and $\sqrt{13}$ phases are interface stabilized and exhibit high formal oxidation states $(\ensuremath{\sim}{5}^{+}).$ In the oxide coverage range $0.6<\ensuremath{\Theta}<1.2\mathrm{MLE},$ i.e., after the completion of the first oxide layer, the $\sqrt{7}$ and $\sqrt{13}$ structures are replaced by several coexisting V-oxide phases, where the oxidation state of the V atoms progressively decreases from ${4}^{+}$ to ${2}^{+}$ with increasing oxide coverage. For coverages exceeding 2 MLE a bulk-type ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ phase with corundum structure grows epitaxially on the Rh(111) surface. The observed growth mode is examined by assessing kinetic and energetic effects in the ultrathin oxide film growth. The importance of the oxide-free areas of the metal support for the formation of highly oxidized V-oxide layers at the initial stages of growth is discussed.