Electrochemical and electrochromic investigation of poly-bithiophene films on a mesoporous TiO2 surface

The electrochemical behaviour and the electrochromic properties of a film of poly-bithiophene (PBT) deposited onto a mesoporous TiO2 surface layered on an optically transparent electrode (OTE) were investigated. The polymeric film was prepared by anodic polymerisation of 2,2′-bithiophene (BT) and, interestingly, a cyclic voltammetric (CV) behaviour very similar to that observed on platinum or other conducting substrates was obtained. Furthermore, both the CV pattern and the absorption properties of the film grown on TiO2 indicated that a significantly larger amount of PBT could be deposited onto the mesoporous TiO2 layer with respect to Pt or conducting SnO2 substrates having the same geometric area. This suggested that PBT growth, besides at the exposed SnO2 surface within the pores of the mesoporous network, might also occur onto the (very large) surface of the TiO2 nanocrystals. Finally, the PBT/TiO2 heterojunction was investigated for its electrochromic properties. Because of the very large specific surface concentrations of PBT and the high volume density of surface-related defects in the mesoporous TiO2 film, the hetrojunction was shown to undergo sharp optical transition upon either oxidation of the PBT film or intercalation of Li+ ions in the reduced TiO2 film.