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

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 W-0), finally to W-0. Multiple oxidation states of tungsten coexisted until finally only W-0 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. (C) 2012 Elsevier B.V. All rights reserved.