Mechanistic investigation of water oxidation on hematite photoanodes using intensity-modulated photocurrent spectroscopy

Exploring the nature of photogenerated intermediates during the photoelectrochemical (PEC) oxidation of water is a crucial step to enhance the performance of PEC devices. In this work, the PEC water oxidation on different hematite photoanodes, either bare or modified with cobalt-based oxygen evolution co-catalyst, has been investigated using intensity-modulated photocurrent spectroscopy (IMPS) . The phenomenological rate constants for charge transfer and recombination have been determined at different bias potentials. Their potential dependence trends have been analyzed using a mechanistic model that combines the IMPS theory and the bulk characteristic of the semiconductor. Based on this model, the nature of the formed intermediates during water oxidation is explored. It is found that water oxidation takes place through the coupling of two mobile surface intermediates and two adjacent metal sites are involved in the process. The rate constants for the trapping of the photogenerated holes and the coupling of the mobile surface intermediates have been determined. All investigated photoanodes exhibit comparable rate constants for the trapping as well as for the coupling steps. The surface modification does not significantly influence the trapping and coupling rate constants but remarkably reduces the rate constant of recombination. These results indicate that the efficiency of hematite photoanode is limited by its intrinsic properties and the surface modification has a restricted impact on the overall performance.