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

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Abstract
W O 3 nanowirefilms 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 W O 3 nanowirefilms show excellent electrochromic properties, including a rapid response time ( ∼ 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 λ > 800 nm between the nanowirefilm and other traditional films is presented in the end.

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