Chronopotentiometry of Titania Film Electrodes in Aqueous Media

Temporal changes in the potential of TiO 2 films (supported on Au substrates) were monitored under ultraviolet irradiation in aqueous media. Prior to these experiments, mimicing heterogeneous photocatalysis reaction conditions, the open-circuit potentials ( E oc ) of the prepared TiO 2 films were monitored as a function of medium pH in the 3−11 range. A slope of −26 mV/pH was measured; this value is compared with previous studies of the pH dependence of E oc and the TiO 2 flat-band potential, E fb . Perhaps the most remarkable aspect of the presented ( E ph − t ) profiles concerns the very slow (minute regime) changes in the TiO 2 film potential when the UV light is turned on. The extent of negative potential excursion (when the UV light is initially switched on) and the plateau potential attained (at times greater than ∼5 min) are dependent on the nature of the metal ion “depolarizer”, the incident photon flux, and the presence of any co-additive in the 0.05 M Na 2 SO 4 supporting electrolyte. When the light is turned off, the TiO 2 film potential returns to its rest value (in the “dark”) in exponential fashion. A simple model, based on initial photocharging of the TiO 2 film/solution interface and subsequent carrier trapping/detrapping events, is presented to account for the observed trends in the measured ( E ph − t ) profiles. Simulations based on this model afford chronopotentiometric profiles in good agreement with their experimental counterparts.