Gasochromic Coloration Research Articles

Gasochromic mechanism in a-WO3 thin films based on Raman spectroscopic studies

The gasochromic mechanism in amorphous tungsten oxide films has been investigated using Raman scattering measurements. The Raman spectra of as-deposited films show two strong peaks at 770 and 950 cm−1 due to vibrations of the O–W6+–O and W6+=O bonds, respectively. When the Pd/a-WO3 films are gasochromically colored by exposure to diluted hydrogen gas, extra Raman peaks due to O–W5+–O and W5+=O bonds appear at 330 and 450 cm−1, respectively. The Raman spectra of a-WO3 films bleached with isotopic heavy oxygen (18O) show that the gasochromic coloration in a-WO3 films is directly related to the double injection of hydrogen ions and electrons.

Mechanism of the gasochromic coloration of porous WO 3 films

Films of WO 3 were coated with platinum and coloured by exposure to diluted hydrogen gas. Bleaching was achieved with diluted oxygen. Because of its simple layer structure, this so-called gasochromic device is suitable for large-area window applications. By variation of several parameters, such as film thickness and gas concentration, and by considering the bleaching in vacuum and argon, we found a model for the mechanism of the reaction. It is strikingly different to the ideas of the double injection model, and there are indications that the gasochromic and electrochromic reactions are similar for porous films. According to our model, H 2 is dissociated on the platinum, transferred into a pore or grain boundary of the WO 3 and creates water and an oxygen vacancy. The O vacancy diffuses into the interior of the grain and the water slowly leaves the film. The bleaching in argon or vacuum is assumed to be the reverse reaction. The bleaching by oxygen is suggested to occur by by the dissociation of the O 2 on the platinum, transfer into a pore and recombination with the oxygen vacancy.