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

WO 3 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 WO 3 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 W 6+ in WO 3 was reduced to W 5+ immediately, subsequently to W 4+ and then to W x+ (intermediate chemical state between W 4+ 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. © 2012 Elsevier B.V. All rights reserved.