Ligand Engineered MnO Catalytic Nanoparticles As an Efficient Charge Transfer Mediator for Photoelectrochemical Water Oxidation

Photoelectrochemical (PEC) water splitting is regarded as one of the most promising approaches for the generation of hydrogen as a renewable and carbon-free fuel. Bismuth vanadate (BiVO 4 ) has been recognized a highly promising photoanode material for solar water splitting. However the PEC performance of BiVO 4 remains significantly poorer than the theoretical value due to the severe recombination of photogenerated charge carrier near the solid/electrolyte interface. A novel strategy is therefore essential for the progress of highly efficient PEC water splitting. The band edge positions of semiconductors determine their functionality in photoelectrochemical water splitting. While surface modification on the catalytic surface is known to enable to facilitate modification of the band structure, its crucial role in water splitting efficiency has not yet been fully understood. Here, we firstly demonstrate the ligand engineering effect of manganese oxide Co-catalyst nanoparticles (MnO NPs) on bismuth vanadate (BiVO 4 )-based cascade anodes, and achieve a remarkably enhanced photocurrent density of 6.25 mA/cm 2 at the 1.23 V versus reversible hydrogen electrode. It is the highest value among the previously reported BiVO 4 -based anodes. We verified that improved photoactivity is closely related to the substantial shifts in band edge energies that originate from both the induced dipole at the ligand/MnO interface and intrinsic dipole of the ligand. In particular, combined spectroscopic analysis and electrochemical study revealed the clear relationship between the surface structure driven by the ligand treatment and the band edge positions of MnO NPs for photoelectrochemical water oxidation. Our systematic study provides general strategy for enabling new active semiconductor photocatalyst and is applicable to solar water splitting system.