Photoelectrochemical kinetics: Hydrogen evolution on p-type semiconductors

The theory of intensity-modulated photocurrent spectroscopy (IMPS) is extended to consider the two-electron two-proton hydrogen evolution reaction at p-type semiconductor photocathodes. A generalized reaction scheme is described, and two limiting cases are analyzed. The first involves photoelectrochemical desorption of hydrogen by electron/proton transfer and the second chemical desorption by reaction of adsorbed hydrogen atoms. Expressions are derived for the IMPS responses in the two cases as well as for the modulated surface electron concentration, which is related to the quasi Fermi level at the interface. The surface electron concentration expressions can be used to interpret electrical, optical, or microwave experiments that detect the excess electron concentration. The IMPS responses calculated for the two limiting cases are fitted to a simplified IMPS model that gives rate constants kt nd kr for charge transfer and recombination respectively. kt and kr are then related to the rate constants describing the two limiting schemes for the light-driven hydrogen evolution reaction.