Gradient doping of phosphorus in Fe₂O₃ nanoarray photoanodes for enhanced charge separation

Hematite (α-Fe₂O₃) is a promising candidate for solar-to-hydrogen energy conversion. However, the low carrier mobility and extremely high charge recombination rate limit the practical application of hematite in solar water splitting. This paper describes the fabrication of a Fe₂O₃ photoanode with gradient incorporation of phosphorus (P) employing a facile dipping and annealing method to improve the charge separation for enhanced photoelectrochemical water oxidation. This gradient P incorporation increases the width of band bending over a large region in Fe₂O₃, which is crucial for promoting the charge separation efficiency in the bulk. Although both gradient and homogeneous P-incorporated Fe₂O₃ samples exhibit similar electrical conductivity, the Fe₂O₃ electrode with a gradient P concentration presents an additional charge separation effect. A photocurrent of ∼1.48 mA cm^-2 is obtained at 1.23 V vs. reversible hydrogen electrode (vs. RHE) under air mass 1.5G illumination. Additionally, the H₂O oxidation kinetics of Fe₂O₃ with gradient P incorporation was further improved upon loading cobalt phosphate as cocatalyst, reaching a photocurrent of ∼2.0 mA cm^-2 at 1.23 V vs. RHE.