Chemically modified Ni/TiO2 nanocomposite films: Charge transfer from photoexcited TiO2 particles to hexacyanoferrate redox centers within the film and unusual photoelectrochemical behavior

The photoelectrochemical behavior of chemically modified Ni/TiO2 nanocomposite films in aqueous 0.1 M NaNO3 electrolyte is described. Two types of such films were prepared, either starting with a Ni/TiO2 nanocomposite photoelectrode that was subsequently cycled in 0.01 M K3Fe(CN)6 + 0.1 M NaNO3 (type I) or with a Ni electrode derivatized in situ with nickel hexacyanoferrate (NHF) and TiO2 (type II). The photoactivity of both types of films was compared with the parent Ni/TiO2 film in 0.1 M NaNO3. Type I electrodes exhibited minimal photoactivity until potentials into the NHF II --> III redox regime were accessed (>ca. 0.40 V). Thereafter, the anodic photocurrents were significantly higher than the parent Ni/TiO2 counterpart. Type II electrodes exhibited ''bipolar'' photoactivity, the switch from cathodic photo- to anodic photobehavior again occurring at potentials close to the NHF II --> III redox location. These observations on type I and type II nanocomposite films stand in marked contrast to the usual photocurrent-voltage behavior of n-type semiconductor electrodes. The usual photoeffects are interpreted within the framework of a model including charge transfer from the photoexcited TiO2 particles to the NHF redox sites within the nanocomposite film.