Absolute band-edge energies are over-emphasized in the design of photoelectrochemical materials

Abstract

The absolute band-edge potentials of semiconductors and their positions relative to solution redox potentials are often invoked as design principles for photoelectrochemical devices and particulate photocatalysts. Here we show that these criteria are not necessary and limit the exploration of materials that may advance the fields of photoelectrochemistry, photochemistry and photocatalysis. We discuss how band-edge energies are not singular parameters and instead shift with pH, electrolyte type and surface chemistry. The free energies of electrons and holes, rather than those of solution redox couples, dictate overall reaction spontaneity and thus reactivity. Favourable charge-transfer kinetics can occur even when the relevant electrolyte redox potential(s) appear outside the bandgap, enabled by the inversion or accumulation of electronic charge at the semiconductor surface. This discussion informs design principles for photocatalytic systems engineering for both one-electron and multi-electron redox reactions (for example, H2 evolution, H2O oxidation and CO2 reduction). The absolute position of band edges is widely considered an indispensable design principle for selection of appropriate semiconductors for a given photo(electro)catalytic reaction. In this Perspective, the authors re-examine this idea from a viewpoint of semiconductor physics and make the case that alignment of band edges with chemical redox potentials is of limited importance.