The influence of oxygen content in the sputtering gas on the self-synthesis of tungsten oxide nanowires on sputter-deposited tungsten films

The self-synthesis of tungsten oxide (W₁₈O₄₉) nanowires on sputter-deposited W films prepared under different O ₂/Ar flow rate ratios (OAFRRs) in the sputtering gas is reported. After thermally annealing at 700-850 °C in N₂ ambient for 15 min, dense and well crystalline W₁₈O₄₉ (010) nanowires or nanobelts were obtained depending on the oxygen content in the sputtering gas. Experimental results show that the annealing temperature required for the full growth of W₁₈O₄₉ nanowires reduced when the OAFRR in the sputtering gas was increased. It is found that the oxygen absorbed in the surface region is responsible for the growth of nanowires. As the OAFRR was increased to (8 sccm)/(24 sccm), which resulted in a saturated oxygen content of about 55 at.% inside the W film, large-scale nanobelts or nanosheets of W₁₈O₄₉ were grown. The possible growth mechanism which governs the evolution from nanowires to nanobelts as the OAFRR was changed is also discussed.