Synergy of Ultrathin CoO_x Overlayer and Nickel Single Atoms on Hematite Nanorods for Efficient Photo-Electrochemical Water Splitting

To solve surface carrier recombination and sluggish water oxidation kinetics of hematite (α-Fe₂O₃) photoanodes, herein, an attractive surface modification strategy is developed to successively deposit ultrathin CoO_x overlayer and Ni single atoms on titanium (Ti)-doped α-Fe₂O₃ (Ti:Fe₂O₃) nanorods through a two-step atomic layer deposition (ALD) and photodeposition process. The collaborative decoration of ultrathin CoO_x overlayer and Ni single atoms can trigger a big boost in photo-electrochemical (PEC) performance for water splitting over the obtained Ti:Fe₂O₃/CoO_x/Ni photoanode, with the photocurrent density reaching 1.05 mA cm⁻² at 1.23 V vs. reversible hydrogen electrode (RHE), more than three times that of Ti:Fe₂O₃ (0.326 mA cm⁻²). Electrochemical and electronic investigations reveal that the surface passivation effect of ultrathin CoO_x overlayer can reduce surface carrier recombination, while the catalysis effect of Ni single atoms can accelerate water oxidation kinetics. Moreover, theoretical calculations evidence that the synergy of ultrathin CoO_x overlayer and Ni single atoms can lower the adsorption free energy of OH* intermediates and relieve the potential-determining step (PDS) for oxygen evolution reaction (OER). This work provides an exemplary modification through rational engineering of surface electrochemical and electronic properties for the improved PEC performances, which can be applied in other metal oxide semiconductors as well.