Hole storage overlayer of amorphous hafnium oxide for boosting hematite-based solar water splitting

Hole storage layer (HSL) has been proved to be effective for constructing highly efficient photoelectrochemical (PEC) systems owing to its timely extraction and temporary storage of the photogenerated holes. Here we demonstrate an ultrathin HSL of amorphous hafnium oxide (HfOx) by a combined strategy of hybrid microwave annealing (HMA) and in-situ Hf doping, which enhances the PEC performance collaboratively: i) in-situ doping of Hf4+ ions into hematite lattices to enhance the electrical conductivity in the bulk; and ii) amorphous HfOx overlayer on hematite surface as a HSL to promote efficient charge separation between electrons and oxidizing equivalents in water oxidation. As a result, the fabricated Hf:Fe2O3 @HfOx nanorod photoanode achieves a photocurrent density of 3.47 mA cm-2 at 1.23 VRHE under simulated sunlight (100 mW cm-2). With NiCoFe (OH)x co-catalyst modification, the photocurrent density further increases to 4.13 mA cm-2 at 1.23 VRHE, which is 3 times higher than that of bare hematite (1.24 mA cm-2) and comparable to the state-of-the-art hematite photoanode performance. This work demonstrates the high potential of the HMA-induced engineering of transition metal oxides applicable to the field of solar energy conversion.