Oxygen loss and recovering induced by ultrahigh vacuum and oxygen annealing on WO3 thin film surfaces: Influences on the gas response properties

Abstract

In this work the surface reversibility towards the incorporation and lack of oxygen of thin WO3 films has been studied by means of high resolution x-ray photoemission spectroscopy (XPS). The thin films (150 nm), have been thermally evaporated onto Si substrates, and either left as prepared, or pretreated in oxygen at atmospheric pressure by a 24 h long annealing at 300 °C. We used conventional monochromatized XPS measurements to follow the W4f, O1s peaks and the valence band of such samples, as introduced in ultrahigh vacuum (UHV), or after cycles of annealing in UHV and reannealing in oxygen at atmospheric pressure. We observed that annealing in UHV gives rise to a lack of oxygen in the as deposited and 300 °C annealed samples strongly modifying the W4f peak and producing the increase of metallic states at the Fermi edge. For these samples, the thermal annealing in oxygen produces recovering of the WO3 surface electronic structure and, again, a successive heating in UHV causes a lack of oxygen. This shows a substantial reversibility of the effects produced by the annealing cycles. The WO3 films submitted to the UHV thermal treatments have been also investigated as resistive gas sensors towards NO2. We observed an evident lowering of the base resistance and a decrease of their response if compared with that one of the samples not treated in vacuum. A gradual shift of the base line towards that of not UHV treated sensor has been also observed, after many working hours in air at 200 °C. This effect has been attributed to the oxygen reincorporation on the surface.