Hydrothermal synthesis and chromic properties of hexagonal WO ₃ nanowires

This paper reports that highly purified hexagonal WO 3 nanowires are synthesized by a simple hydrothermal method. The as-synthesized WO 3 nanowires are investigated in detail by ultraviolet—visible—near infrared spectroscopy and electrical transport measurements under different conditions. It finds that the optical band gap and the diffuse reflection coefficient in the wavelength region above 450 nm of WO 3 nanowires decrease observably upon exposure to ultraviolet light or NH 3 gas. It is also found that there are electrons being trapped or released in individual WO 3 nanowires when scanning bias voltage in different directions upon exposure to ultraviolet and NH 3 gas. The experimental results suggest that the chromic properties might be attributed to the injection/extraction of hydrogen ions induced by ultraviolet light irradiation in air or creation/annihilation of oxygen vacancies induced by NH 3 gas exposure, which serve as colour centres and trap electrons as polarons. The experimental results also suggest that the hexagonal WO 3 nanowires will be a good candidate for sensing reduced gas such as NH 3 .