The photoelectrochemistry of colloidal semiconductors

Results are presented for the dark electrochemistry and the photoelectrochemistry for CdS colloidal particles studied with the optical rotating disc electrode (ORDE). The electron transfer reactions between the particles and the electrode are found to be irreversible. In the dark, electrons are transferred from a range of populated surface states, while in the light, transfer takes place from the particle conduction band. However, in agreement with theoretical predictions, the kinetics obey the Levich equation in the dark and the corresponding relationship in the light. Several hundred electrons may be transferred on each encounter between a particle and the ORDE. Transient techniques allow the calculation of a pseudo-first order rate constant for the loss of photogenerated electrons from the conduction band to inter-band trapped holes and theory is presented explaining this. Added FeEDTA- and Fe(CN)63- species and found to be good scavengers for photogenerated electrons, and second order rate constants for electron transfer between the species and particles are reported. Added Ru(NH3)63+ is found to be a good scavenger for photogenerated holes. Heptyl viologen is found to act as a recombination centre in much the same way that the inter-band trapped holes do, and the second order rate constant for electron loss to solution HV2+ is calculated. Results for the quantum efficiencies for the photogeneration of electrons in both the absence and presence of added charge carrier scavengers are reported and explained. Finally, recommendations are made as to the nature of the ideal electron scavenger to be used in ORDE experiments. (author)