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

In this work the surface reversibility towards the incorporation and lack o f oxygen of thin WO3 films has been studied by means of high resolution x-r ay photoemission spectroscopy (XPS). The thin films (150 nm), have been the rmally evaporated onto Si substrates, and either left as prepared, or pretr eated in oxygen at atmospheric pressure by a 24 h long annealing at 300 deg reesC. We used conventional monochromatized XPS measurements to follow the W-4f, O-3, peaks and the valence band of such samples, as introduced in ult rahigh vacuum (UHV), or after cycles of annealing in UHV and reannealing in oxygen at atmospheric pressure. We observed that annealing in UHV gives ri se to a lack of oxygen in the as deposited and 300 degreesC annealed sample s strongly modifying the W4f peak and producing the increase of metallic st ates at the Fermi edge. For these samples, the thermal annealing in oxygen produces recovering of the W03 surface electronic structure and, again, a s uccessive heating in UHV causes a lack of oxygen. This shows a substantial reversibility of the effects produced by the annealing cycles. The WO3 film s submitted to the UHV thermal treatments have been also investigated as re sistive gas sensors towards NO2. We observed an evident lowering of the bas e 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 degreesC. This effect has been attributed to the oxygen reincorporation on the surface. (C) 2001 American Vacuum Society.