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

► The WO3 nanowire film was bombarded by Ar ion beam to produce uniform tungsten cone arrays. ► We monitored the whole ion-induced reduction process of WO3 nanowires by in situ XPS measurement to study the change of W chemical states. ► Ion-induced reduction of oxides should be associated with the inherent properties of constituent elements in oxide compounds except for the preferential sputtering of oxygen. ► The formation of the directional cone arrays was due to the long-time gross surface heating of ion bombardment with the directional high-energy Ar ion beam.