Electrochromic properties of WO3 nanowire films and mechanism responsible for the near infrared absorption

WO3 nanowire films were prepared by thermal evaporation. The electrochromic properties and relative near infrared absorption mechanism of the films in detail are investigated by electrochemistry, ultraviolet-visible-near infrared spectroscopy, x-ray diffraction spectroscopy, and some theoretical approaches. The studies indicate that WO3 nanowire films show excellent electrochromic properties, including a rapid response time (similar to 3 s) and a large contrast ratio between the bleached and the colored states. These improvements are due to the loose structure of the films and the small diameter of a single nanowire. It is also shown that a lithium tungsten bronze structure appears when the film is colored, causing two broad absorption bands centering around 920 and 1800 nm. A calculation including the Urbach effect, cation-polaron absorption and large polaron absorption was carried out to interpret the absorbance. A brief discussion of the discrepancy of the transmittance spectrum in the region lambda > 800 nm between the nanowire film and other traditional films is presented in the end. (c) 2007 American Institute of Physics.