Carrier density and interfacial kinetics of mesoporous TiO2 in aqueous electrolyte determined by impedance spectroscopy

Abstract

Water splitting at a semiconductor/solution interface with the only input of sunlight to generate hydrogen is one of the most attractive strategies to produce and store chemical energy. In the present study we have investigated carrier dynamics and interfacial kinetics of mesoporous TiO2 in an aqueous solution. The applicability of the transmission line model for mesoporous semiconductors has been validated to identify chemical capacitance, transport resistance and charge transfer resistance in this system by testing samples of different thicknesses in the dark and under illumination. We found that both transport resistance and chemical capacitance scale well with sample thickness, while charge transfer resistance scales with thickness when the FTO substrate is not exposed to the solution. Otherwise, there is a competition between charge transfer through TiO2 and through the FTO substrate. Under illumination, the electron density is dominated by photogenerated carriers at biases below the open circuit potential, whereas at higher bias, the applied potential determines the electron density. Evidence of charge transfer via surface states has been experimentally observed and corroborated with a physical model, which explicitly includes charge transfer through a monoenergetic trap for electron and holes. This study may lay the basis for understanding more complex processes at anodic potentials on the TiO2/solution interface where water splitting reactions take place.