Reconstructing Oxygen Vacancies in the Bulk and Nickel Oxyhydroxide Overlayer to Promote the Hematite Photoanode for Photoelectrochemical Water Oxidation

Surface engineering, as an efficient strategy, can improve the photoelectrochemical water splitting (PEC-WS) performance for converting inexhaustible sunlight into clean hydrogen fuel. Oxyhydroxides and p–n heterojunctions have been demonstrated as efficient catalysts for the water oxidation reaction. In this work, to address the drawbacks of poor conductivity and sluggish oxidation kinetics of hematite, we introduce a p-type NiOOH overlayer as a surface catalyst onto n-type Sn-doping hematite (Sn@α-Fe2O3) photoanode. The oxygen vacancies (Ov) are reconstructed both in the bulk of Sn@α-Fe2O3 and the surface decoration layer of NiOOH via Ar plasma treatment, effectively reducing unavoidable defects introduced by the NiOOH overlayer. Compared with the original Sn@α-Fe2O3 photoanode, the Sn@α-Fe2O3/NiOOH–Ar photoanode exhibits a significant increase in photocurrent density (at 1.23 VRHE) of ∼3 times and a decrease in the onset potential of ∼200 mV. The performance improvement can be ascribed to the synergistic effect of the p–n junctions formed by NiOOH decoration and improved conductivity through oxygen vacancy reconstruction, which remarkably improves carrier separation in the bulk of α-Fe2O3 and suppresses carrier recombination on the photoanode surface. Moreover, the density functional theory (DFT) calculation proves that the real active sites are farther from (rather than near) the oxygen vacancies.