Mechanisms of Charge Transfer at the Semiconductor‐Electrolyte Interface: I . Kinetics of Electroreduction at Dark of and in Aqueous Solution on a Sintered Nb‐doped Electrode: Influence of pH
The dark cathodic reduction of and in aqueous solution on a sintered Nb‐doped electrode, at acidic and basic pH, has been studied as a function of the potential and concentration of the oxidized species. With , two different regions are observed. At low band bending (high current‐density region), the current saturates with increasing concentration of oxidant species, and a pH‐independent Tafel slope of ∼130 mV is found. By contrast, at higher band bending (low current‐density region), the current is proportional to the electrolyte concentration, and the Tafel slope increases dramatically at basic pH with respect to that at acidic pH. Ullman's model of isoenergetic electron transfer limited by transport resistance in the semiconductor depletion layer cannot account for this behavior. However, our results are compatible with the two‐step kinetic model of charge transfer, involving bandgap surface states, proposed by Vandermolenet al. (16). At least two sets of surface states with different energies within the bandgap actively mediate charge transfer through the semiconductor electrolyte interface. Experiments with show that these surface states could result from the interaction of the semiconductor surface with H+, OH−, and solvent species.