Stoichiometric water splitting using a p-type Fe2O3 based photocathode with the aid of a multi-heterojunction

Abstract

Fe₂O₃-based photocathodes are one of the least expensive options for hydrogen generation by water splitting. Although p-type N,Zn-doped Fe₂O₃ (N,Zn–Fe₂O₃) has been reported to possess a negative conduction band minimum position sufficient for photocathodic hydrogen generation, the efficiency and stability of the resulting H₂ production is low and the reaction is sacrificial. In the present work, analysis by hard X-ray photoelectron spectroscopy (HAXPES) showed that these negative characteristics result from the self-redox reaction of p-type Fe₂O₃. Based on this result, a TiO₂ layer was introduced onto the surface of p-type N,Zn–Fe₂O₃ to passivate surface defects. In addition, to ensure efficient electron transfer, a thin Cr₂O₃ layer was also inserted between N,Zn–Fe₂O₃ and a bottom side conductive oxide layer to generate a favorable band alignment for hole transfer. The resulting Pt/TiO₂/N,Zn–Fe₂O₃/Cr₂O₃ electrode exhibits a highly stable, significantly enhanced cathodic photocurrent during H₂ production under AM 1.5 irradiation. The mechanism providing this improvement was investigated by combining electrochemical impedance spectroscopy, open-circuit voltage decay analysis and scanning tunneling electron microscopy-energy dispersive X-ray spectroscopy. Stoichiometric water splitting without an external electrical bias was also demonstrated by connecting the Fe₂O₃-based photocathode to an n-type SrTiO_3−x photoanode, representing the first-ever example of stoichiometric overall water splitting using an Fe-based photocathode.