ABSTRACT
Monolayer vanadium oxide films grown on Pt(111) can be reversibly switched between an oxygen-poor and an oxygen-rich composition, equivalent to V2O3 and V2O5, respectively. While the overall oxygen storage capacity of the film is quantified by X-ray photoelectron spectroscopy, the atomic binding sites of the extra O species are determined by low-temperature scanning tunneling microscopy and electron diffraction. In the O-poor phase, the oxide takes the form of a honeycomb lattice that gets partially covered with vanadyl (V=O) groups at higher O exposure. Upon transition to the O-rich phase, isolated V6O12 rings emerge in the film first, which then evolves towards a disordered O-V-O trilayer on the Pt(111) surface. Our works thus unravels the microscopic nature of reversible oxygen storage in a model system for heterogeneous catalysis.
ABSTRACT
Low-temperature scanning tunnelling microscopy (STM) is employed to study electron-stimulated desorption of vanadyl groups from an ultrathin vanadium oxide film. The vanadia patches are prepared by reactive vapour deposition of V onto a Ru(0001) surface and comprise a highly ordered network of six and twelve membered V-O rings, some of them terminated by upright V[double bond, length as m-dash]O groups. The vanadyl units can be desorbed via electron injection from the STM tip in a reliable fashion. From hundreds of individual experiments, desorption rates are determined as a function of bias voltage and tunnelling current. Data analysis reveals a distinct threshold behaviour with bias onsets at +3.3 V and -2.6 V for positive and negative polarity, respectively. The desorption rate varies quadratically (cubically) with the tunnelling current at positive (negative) sample bias, indicating that V[double bond, length as m-dash]O desorption is a many-electron process. Based on our findings, a mechanism for desorption is proposed that includes resonant tunnelling into anti-bonding or out of bonding orbitals, followed by vibrational ladder climbing in the binding potential of the V[double bond, length as m-dash]O ad-system. The underlying electronic states can be identified directly in the STM conductance spectra taken on the oxide surface.
