Infrared ATR Spectroscopic Studies on Hydrated Tungsten Oxide Thin Films

The water absorption behavior of tungsten oxide films has been studied by infrared attenuated total reflection (ATR) spectroscopy. Our results indicate that most of the water in the film is present in the initial stage of deposition and hardly any water is added at greater thicknesses. It is also seen that the water content in WO ~ decreases as the temperature ofdep-osition increases. The spectra of thin WO3 films hydrated under controlled conditions have been studied, and new bands in the mid-infrared region have been found. Thin Ag over- and underlayers have been used in order to enhance the IR ab-sorption. The spectra of hydrated tungsten bronze and hydrated tungsten oxide in the presence of silver have been found to be similar. Tungsten oxide (WO3) has been found to be a very versa-tile material for electrochromic (EC) devices. The composi-tion of the EC phase, as determined by thermogravimetric and thermal differential analysis, has been found (1) to be a fractionally hydrated WO3- xH20, where x depends on the temperature ofdeposition. The coloration mechanism con-sists of a reversible lectron transfer between W + 6 and W ~, with the formation of tungsten bronze in the presence of cations such as H*, Li § and Ag § according to the follow-ing reaction WO3 + xH ~ + xe- ~ H.~.WO~ Tungsten bronzes are deep blue compounds exhibiting fairly high electrical conductivity, and can be switched back to the original colorless state. The EC properties (electrical conductivity and rate of coloration) have been shown (2, 3) to be strongly influenced by the porosity and the content of water in the film. Although many investi-gators have recognized the importance of water content, only Tecently has the relationship between the structure and the EC properties of WO3 " xH20 been studied. Yoshiike and Kondo (3) pointed out that the water ab-sorbed in the material reacts gradually with tungsten oxide, giving a hydroxylated film with a weak EC coloration as per the following reaction WO3 9 mH20 + xH § + xe- ~ WO3-~. (OH)- mH2