Investigation of the oxidation of NaxWO3 surfaces

Abstract

The oxidation of single-crystal (100) surfaces of sodium tungsten bronzes Na_xWO₃ has been investigated at temperatures between 180 and 600 °C. At ambient pressures and a temperature of 500 °C, a non-conducting overlayer grows coherently on the oxide bronze surface. The thickness may be monitored by the development of interference fringes in infrared reflection spectroscopy. X-Ray photoelectron spectroscopy shows that the W ions in the overlayer are in the maximal oxidation state and that the characteristic plasmon found in high-resolution electron energy loss spectroscopy of the bronzes vanishes completely on oxidation. The overlayer has a complex but characteristic phonon spectrum seen in infrared reflection spectroscopy, different from that of WO₃ or Na₂WO₄. These phases are also absent in X-ray diffraction patterns from oxidised layers.

The rate of growth of the oxidised layer at ambient pressures is too rapid on the XPS depth scale to follow the initial stages of oxidation, but at lower pressures of the order of 100 mbar progressive changes in core-level XPS may be used to chart the oxidation of W to the W^VI state.