A Self-Assembled Two-Layer Structured WO3/TiO2-x Mixed Film with Improved Electrochromic Capacities

Electrochromics are a promising avenue for energy savings in tropical climates where heating of buildings is largely by transmission of infrared light through windows, causing 40% of their electrical demand to be used on air-conditioning. One technique to improve the electrochromic properties of WO3 films is to dope with a second transition metal oxide. In the present work a mixed WO3/TiO2-x film was prepared by a single step hydrothermal method. XRD investigations suggested that the doping was achieved by replacement of W6+ by Ti4+ ions in the WO3 lattice structure, rather than discrete phases of WO3 and TiO2. The composite film presented a large light modulation ability of 67% at 632.8 nm, twice that of an equivalent pure WO3 film, was well adhered to the substrate and showed good electrochromic reversibility switching rates of less than 26 s. In the colored state the transmittance in the visible light range of the mixed transition metal oxide film was below 5%, whilst in the bleach state this exceeded 70%. Electrochemical impedance measurements suggested that the improvement in electrochromic properties was due to the TiO2 doping reducing both the film's electrical resistance and its resistance to diffusion of ions within the film. (C) 2013 The Electrochemical Society. All rights reserved.