Ni2P nanocrystals modification on Ta:α-Fe2O3 photoanode for efficient photoelectrochemical water splitting: In situ formation and synergistic catalysis of Ni2P@NiOOH cocatalyst

Abstract

The application potential of hematite (α-Fe2O3) photoanode for photoelectrochemical (PEC) water splitting is restricted by its poor conductivity and severe carrier recombination. Herein, a coupling modification strategy of tantalum (Ta) doping and Ni2P nanocrystals modification is developed to realize the simultaneous optimization of photocurrent density and onset potential of α-Fe2O3 photoanode. The resulting Ni2P/Ta:α-Fe2O3 photoanode exhibits a photocurrent density of 2.98 mA/cm2 at 1.23 V vs. RHE, which is 2.76-fold higher than that of pristine α-Fe2O3. Characterization results show that Ta-doping improves the conductivity and carrier density, while Ni2P nanocrystals optimizes the hole injection efficiency and water oxidation kinetics of photoanode. Notably, Ni2P nanocrystals form a core–shell structure (Ni2P@NiOOH) in situ during the PEC reaction. Density functional theory calculations indicate that the adsorption sites for Ni2P@NiOOH cocatalyst are mainly Ni atoms at the interface between NiOOH and Ni2P, with adjacent Ni or P atoms from Ni2P core also participating in the reaction, and that the synergistic catalysis of Ni2P and NiOOH lowers the energy barrier for the key *OOH intermediates formation. Finite element simulations of the current density distribution show that Ni2P core with high conductivity exhibit a significant current-collector effect, accelerating carrier migration in Ni2P@NiOOH. This work contributes to the understanding of the catalysis of Ni2P-derived composite oxygen evolution reaction catalysts and provides a reference for the rational design of photoanode for efficient PEC water splitting.