Insertion of H +, Li +, Na + and K + into thin films prepared from silicotungstic acid—a photoelectron spectroscopy study

Abstract

Electrochemical insertion by a set of different ions (H +, Li +, Na + and K +) into a tungsten oxide thin film was studied by photoelectron spectroscopy. The tungsten oxide thin film incorporating Si atoms was produced from a silicotungstic acid (SiWA) solution. The insertion compounds were measured by core level photoelectron spectroscopy (W 4 f) and the contributions from ions of different oxidation states could be monitored simultaneously. SiWA films having a W 6+/W tot ratio of 0.7 could be prepared for all cations investigated. At this ratio the W 4 f core level electronic structure for H + inserted SiWA films was found to be very similar to that of H + inserted into crystalline monoclinic WO 3 in that both films show the presence of W 4+, W 5+ and W 6+. The measurements on Li + inserted SiWA films indicate an electronic structure very similar to that of the smaller (H +) ion. The K + inserted film displays a similar behaviour although the existence of W 4+ was difficult to ascertain. Interestingly, a different behaviour was observed for the Na + inserted compound. In this case, the binding energy shift of the W 4 f peak upon reduction is clearly different from that obtained for the other insertion materials.