Hydrothermal synthesis and chromic properties of hexagonal WO3 nanowires**Project supported by the Program for New Century Excellent Talents in University (Grant No. NCET-07-0278), the Hunan Provincial Natural Science Fund of China (Grant Nos. 08JJ1001 and 07JJ6009), the Major Research plan of National Natural Science Foundation of China (Grant No. 90606010) and the Program for Excellent Talents in Hunan Normal University,

Abstract

This paper reports that highly purified hexagonal WO3 nanowires are synthesized by a simple hydrothermal method. The as-synthesized WO3 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 WO3 nanowires decrease observably upon exposure to ultraviolet light or NH3 gas. It is also found that there are electrons being trapped or released in individual WO3 nanowires when scanning bias voltage in different directions upon exposure to ultraviolet and NH3 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 NH3 gas exposure, which serve as colour centres and trap electrons as polarons. The experimental results also suggest that the hexagonal WO3 nanowires will be a good candidate for sensing reduced gas such as NH3.