Surface Reconstruction of Cobalt Species on Amorphous Cobalt Silicate-Coated Fluorine-Doped Hematite for Efficient Photoelectrochemical Water Oxidation

The slow kinetics of photoelectrochemical (PEC) water oxidation reaction is the bottleneck of PEC water splitting. Here, we report a comprehensive method to improve the PEC water oxidation performance of a hematite (α-Fe₂O₃) photoanode, that is, fluorine doping and an ultrathin amorphous cobalt silicate (Co-Sil) oxygen evolution reaction (OER) cocatalyst by photo-assisted electrophoretic deposition (PEPD). Detailed investigations reveal that fluorine doping can reduce the interfacial transfer resistance of charge and increase the carrier density to improve the conductivity of hematite. Also, simultaneously, the Co-Sil is used as an excellent OER cocatalyst to accelerate OER kinetics. Specifically, surface reconstruction of cobalt species occurred, and its average oxidation state increased significantly, which was more conducive to water oxidation. In addition, the presence of silicate groups could reduce the OOH* adsorption free energy. The synergistic effect of these efforts significantly reduced the onset potential and overpotential and enhanced the charge separation of the α-Fe₂O₃ photoanode, resulting in an excellent photocurrent density around 2.61 mA cm^-2 at 1.23 V vs RHE (4.75 times higher than the primitive α-Fe₂O₃). This work provides a feasible strategy for the construction and development of a potential hematite photoanode.