Evaluation of p-type PdO as a photocathode in water photoelectrolysis

Abstract The electronic and electrochemical properties of vapour-grown single-crystal PdO are reported; this PdO is a p-type semiconductor with a bandgap of about 0.8 eV, corresponding to a strongly forbidden d-d transition. A higher-energy transition, with a threshold near 2.2eV, is assigned to 0 2p — Pd 4d charge transfer. The flat-band potential PdO appears to be 0.7 ± 0.1V (NHE) in 0.5M H 2 SO 4 , but the photoresponse of the crystals is poor owing both to unfavourable bulk properties and to feeble faradaic kinetics for hydrogen evolution. The cathodic decomposition of PdO to palladium metal is a strongly competing reaction under potential biases that invert the surface region.