XPS studies on surface reduction of tungsten oxide nanowire film by Ar+ bombardment

Abstract

WO3 nanowire film was bombarded by Ar ion beam in the analysis chamber of an X-ray photoelectron spectroscopy (XPS) system to produce uniform tungsten cone arrays. The WO3 nanowire film itself served as an etching mask during the Ar+ bombardment. The changes of surface chemical states and electronic structures during bombardment were monitored by in situ XPS. The morphological evolution with different Ar+ bombardment time was observed by ex situ scanning electron microscopy (SEM). At the start of Ar+ bombardment partial W6+ in WO3 was reduced to W5+ immediately, subsequently to W4+ and then to Wx+ (intermediate chemical state between W4+ and W0), finally to W0. Multiple oxidation states of tungsten coexisted until finally only W0 left. SEM images showed that the nanowires were broken and then fused together to be divided into clusters with a certain orientation after long-time high-energy ion beam bombardment. The mechanism of the ion-induced reduction during bombardment and the reason of the orientated cone arrays formation were discussed respectively.