CHEMICALLY-MODIFIED NI TIO2 NANOCOMPOSITE FILMS - CHARGE-TRANSFER FROM PHOTOEXCITED TIO2 PARTICLES TO HEXACYANOFERRATE REDOX CENTERS WITHINTHE FILM AND UNUSUAL PHOTOELECTROCHEMICAL BEHAVIOR/

The photoelectrochemical behavior of chemically modified Ni/TiO2 nanoc omposite films in aqueous 0.1 M NaNO3 electrolyte is described. Two ty pes of such films were prepared, either starting with a Ni/TiO2 nanoco mposite 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 wi th 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 po tentials into the NHF II --> III redox regime were accessed (>ca. 0.40 V). Thereafter, the anodic photocurrents were significantly higher th an the parent Ni/TiO2 counterpart. Type II electrodes exhibited ''bipo lar'' photoactivity, the switch from cathodic photo- to anodic photobe havior again occurring at potentials close to the NHF II --> III redox location. These observations on type I and type II nanocomposite film s stand in marked contrast to the usual photocurrent-voltage behavior of n-type semiconductor electrodes. The usual photoeffects are interpr eted within the framework of a model including charge transfer from th e photoexcited TiO2 particles to the NHF redox sites within the nanoco mposite film.