Time-dependent electrochromism of nanocrystalline TiO2 films in propylene carbonate solution of LiClO4

Electrochromism of nanocrystalline TiO2 films under cathodic bias in propylene carbonate containing LiClO4 was studied by time-dependent absorption and fluorescence spectroscopies. The absorption spectra revealed two absorption bands near 700 and 420 nm. The absorption band near 700 nm showed a blue shift and an increase in absorbance with time. The blue shift seemed to be due to the progressive involvement of the trapped electrons in the lower energy levels within the shallow surface states. The newly observed band near 420 nm appeared to have originated from the deep surface states which, lying 1.3-1.8 eV below the conduction band, were gradually filled with electrons in the shallow surface states.