Thermal-Oxidative Growth of Substoichiometric WO_3–x Nanowires at Mild Conditions

Herein, the growth of substoichiometric tungsten oxide (WO_3–x) nanowires (NWs) via thermal oxidation of W films (a few 10 to a few 100 nm thick) deposited by Ar-plasma sputtering on fluorine-doped tin oxide (FTO) substrates is reported. A thermal treatment at relatively low temperature (525–550 °C) in Ar at atmospheric pressure leads to the conversion of the W films into W suboxide (WO_3–x) NW arrays. Such NWs have a length of ≈500 nm and their diameter ranges from 10 to 40 nm depending on the duration of the annealing process. These growth conditions for the formation of WO_3–x NWs are significantly milder and more straightforward than those reported in previous literature. This avoids high temperatures (e.g., above 600 °C), vacuum conditions, or the addition of catalysts. A thermal-oxidative approach of these sputtered films is suitable to directly form WO_3–x NWs on various surfaces, including conductive substrates to fabricate photoelectrodes or large-scale supports, e.g., for smart windows.