Characterization of photoelectrocatalytic processes at nanoporous TiO2 film electrodes:: Photocatalytic oxidation of glucose

The effect of potential bias, light intensity, and the concentration of the photohole scavenger (glucose) on the photocurrent responses of the nanoporous TiO2 film electrodes were investigated with the focus on the overall photoelectrocatalytic oxidation process. The electron transport in TiO2 film was the rate-limiting step at low potential bias, while the interfacial reactions became rate-limiting step at high potential bias. A linear photocurrent/potential characteristic can be obtained within a wide range of applied potential bias. Within this linear range, the electrodes behave as a constant resistance rather than a variable resistance, which is unlike a photoelectrochemical process at a bulk semiconductor electrode. The resistance consists of two components: a variant component and an invariant component. The former is inversely proportional to maximum photocurrent, due to the free electron concentration change as a result of the consumption of photoholes through interfacial reaction. The hypothesis of the photoelectron and photohole separation being fulfilled through interfacial reaction was confirmed experimentally. The invariant component of the resistance is attributed to the sum of ohmic contact impedance at the ITO/TiO2 interface and crystalline boundary impedance during electron migration under an electric field. A model for the overall photoelectrocatalytic oxidation process was proposed and explained based on the experimental results.