The kinetics of metal oxide photoanodes from charge generation to catalysis

Abstract

Generating charge carriers with lifetimes long enough to drive catalysis is a critical aspect for photoelectrochemical and photocatalytic systems, and a key determinant of their efficiency. Metal oxides are widely explored as photoanodes for photoelectrochemical water oxidation. However, their application is limited by the disparity between the picosecond–nanosecond lifetimes of electrons and holes photoexcited in bulk metal oxides versus the millisecond–second timescale of water oxidation catalysis. This Review addresses the charge-carrier dynamics underlying the performance of metal oxide photoanodes and their ability to drive photoelectrochemical water oxidation, alongside comparison with metal oxide function in photocatalytic and electrocatalytic systems. We assess the dominant kinetic processes determining photoanode performance, namely, charge generation, polaron formation and charge trapping, bulk and surface recombination, charge separation and extraction, and, finally, the kinetics of water oxidation catalysis. We examine approaches to enhance performance, including material selection, doping, nanostructuring, junction formation and/or co-catalyst deposition. Crucially, we examine how such performance enhancements can be understood from analyses of carrier dynamics and propose design guidelines for further material or device optimization. Metal oxides are widely used in photoelectrochemical and photocatalytic systems for fuel synthesis and environmental remediation. In this Review, we examine the kinetic challenges, from charge generation to water oxidation catalysis, that determine the performance of metal oxide photo(electro)catalysts.