Raman spectroscopic studies of Ni–W oxide thin films

We report on a Raman spectroscopic study of sputtered nickel–tungsten oxide films. Their crystallographic and chemical identification with electrochemical lithium insertion and extraction are obtained by Raman spectroscopy. The Raman spectra of as-deposited Ni–W oxide films show a strong peak at 525 cm−1 due to vibrations of the NiO bonds and two weaker peaks at 875 and 950 cm−1, which we attribute to the vibrations of the WO bonds. The Raman spectrum of the as-deposited Ni–W oxide film shows a blueshift of the NiO stretching mode compared to that of a NiOx film. When lithium ions and electrons are inserted, the overall Raman intensity increases due to electrochromic bleaching and the relative intensity of the peak at 875 cm−1 also increases. By comparing these results with results from the same measurements using sodium ions, we conclude that this reversible peak at 875 cm−1 is not directly related to the Li or Na ions.