Photocurrent of BiVO4 is limited by surface recombination, not surface catalysis.

Bismuth vanadate is one of the most promising photoanode materials for photoelectrochemical water splitting. In order to achieve high photocurrents the surface of BiVO4 always has to be modified with water oxidation catalysts, such as cobalt phosphate (CoPi), FeOOH, or NiFeO x . While this has generally been attributed to the poor intrinsic catalytic activity of BiVO4, detailed insight into the fate of the photogenerated charge carriers at the surface is still lacking. We used intensity modulated photocurrent spectroscopy (IMPS) to investigate the surface carrier dynamics of bare and CoPi-modified spray-deposited BiVO4 films. Using a model developed by Peter et al., it was possible to distinguish the reaction rate constants for surface recombination and charge transfer to the electrolyte. We found that modification with CoPi reduced the surface recombination of BiVO4 with a factor of 10-20, without significantly influencing the charge transfer kinetics. Control experiments with RuO x , one of the best known OER electrocatalysts, did not affect surface recombination and led to an actual decrease of the photocurrent. These results show that the main role of the CoPi is to passivate the surface of BiVO4 and that, contrary to earlier assumptions, the photocurrent of BiVO4 is limited by surface recombination instead of charge transfer. The importance of surface recombination is well recognized for conventional semiconductors in the field of photovoltaics; these findings show that it may also play a crucial role in oxide-based semiconductors for photoelectrochemical energy conversion.

Ruthenium sulphide thin layers as catalysts for the electrooxidation of water.

Crystalline RuS(2) layers were prepared on titanium sheets by reactive magnetron sputtering using a metallic ruthenium target and a H(2)S-Ar mixture as process gas. The ability of these layers for the electrooxidation of water (OER) was investigated by differential electrochemical mass spectrometry (DEMS) in 0.5 M H(2)SO(4) electrolyte. It was observed that the activity for water oxidation is increased with increasing temperature of the titanium substrate during the sputter deposition process whereas a competitive corrosion process is diminished. The reason for this effect seems to be a better crystallinity of these layers at higher substrate temperatures as it is proved by XRD analysis. In contrast to RuS(2) single crystals no photo effect could be observed on the sputtered layers under illumination with a tungsten lamp. Time resolved microwave conductivity analysis indicates the presence of mobile charge carriers after illumination but apparently these cannot participate in the electrooxidation of water.