The effect of water on the electrochromic properties of WO3 films prepared by vacuum and chemical methods

We present a comparative study on the effect of absorbed water on the properties of tungsten oxide films prepared by two different methods (e-gun evaporation, and an aqueous sol-gel technique). Scanning electron microscopy, Fourier transform infrared spectroscopy and electrochemical techniques have been used to assess the film properties. It has been found that the preparation method of the films greatly affects their water content and thus, electron gun evaporated films have less water incorporated into their structure than their sol-gel counterparts. The former are closely packed and transparent with most of their water content adsorbed on their surface, while the latter have a porous structure, being opaque, highly hydroxylated and hydrated to a lesser extent. Both types of films exhibit reversible elect rochromism, with the evaporated films being stable up to 5000 coloration-bleaching cycles and the sol-gel ones gradually degrading after 1000 cycles. Irreversible Li+ trapping related to the presence of water and hydroxyl radicals has been envisaged as the cause of the inferior cycling stability of the sol-gel films. (C) 2004 Elsevier B.V. All rights reserved.