Fourier Transform Infrared study of tungsten oxide thin films and their coloration by lithium

摘要

The technique of Fourier Transform Infrared (FTIR) Spectroscopy has been used to study tungsten oxide (WO$-3$/) thin films. The effect of coloration with lithium has been investigated and leads to the conclusion that the coloration mechanism for lithium in WO$-3$/ requires a structural reorganization induced by the co-insertion of the lithium ion and an electron. This can produce a valence change in one of the tungsten atoms, necessary for absorption of radiation by the inter-valence charge transfer mechanism. There is a clear redistribution of the infra-red active modes when the material is colored with lithium or sodium, and this can be used to characterize the film to some extent. Furthermore, the FTIR spectral changes in anomalously coloring WO$-3$/ as reported previously, have been investigated. No change is seen in the FTIR spectra until the onset of the electrochromic coloration, indicating the lithium ion initially occupies a different site or performs a different role to that seen in normal coloration. This evidence, when combined with other information, leads to the conclusion that the lithium may react with excess oxygen in the WO$-3$/ forming a second chemical phase, perhaps of lithium oxide.