XPS studies on tungsten oxide nano-wire film

WO₃ nano-wire film was bombarded by Ar ion beam in the analysis chamber of an X-ray photo electron spectroscopy (XPS) system to produce uniform tungsten cone arrays. The WO₃ nano-wire 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 W^x+(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 nano-wires 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.

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