BiVO₄ charge transfer control by a water-insoluble iron complex for solar water oxidation

Photoelectrochemical water splitting can become efficient by grafting co-catalysts on semiconductors that improve the interfacial oxygen evolution reaction. We applied a simple non-noble metal pre-catalyst, [Fe^II(PBI)₃]²⁺ (PBI is 2-(2′-pyridyl)benzimidazole ligand) for this purpose on a nanopyramidal BiVO₄ semiconductor that was morphologically optimal for efficient light harvesting, but its performance suffered from V-poor surface recombination sites. The [Fe^II(PBI)₃]²⁺ in situ transformed to α-Fe₂O₃ nanoparticles on V-vacant areas of BiVO₄ mending their photocurrent-limiting effect. Photoelectrochemistry at pH 8.2 confirmed that the α-Fe₂O₃ co-catalyst improved the charge transfer efficiency by an order of magnitude, suppressed the recombination in the bulk and reduced the charge transfer resistance. Overall, the α-Fe₂O₃ suppressed the recombination on the V-poor surface, while at high potentials it provided high-valent centers for the oxygen evolution. The resulting photocurrent density far exceeding that of BiVO₄ or samples modified by FeCl₃ or Fe(NO₃)₃ underlines the metallochaperone-like effect of the PBI ligand.