Electrochromism in electrodeposited nanocrystalline WO3 films I. Electrochemical and optical properties

New hexagonal tungsten trioxide films with perpendicularly oriented nanocrystallites were synthesized using the electrodeposition technique. Electrochemical and optical responses of these films to potential scanning and potential steps were investigated for films deposited under varying conditions and with different intercalating cations. We have found that both the electrochemical kinetics and the optical switching are very fast, and in fact faster than for WO3 films deposited by any other technique. We attribute this rapid kinetics and fast optical switching ability of our WO3 films to their morphology and hexagonal structure. Especially striking feature of these films is the preferential orientation of the {0001} atomic plane of each individual nanocrystallite parallel to the film surface. Big one-dimensional hexagonal channels are thus preferentially formed normal to electrode surface and this arrangement is very favorable for the cation intercalation process. Kinetic investigations indicate that the solid state diffusion is controlling the intercalation process of H+ and D+ cations.