Spin Engineering of Ni-Based High-Entropy Oxide Cocatalyst: Synchronously Optimizing Intermediate Adsorption and Charge Carrier Separation for Photocatalytic Hydrogen Evolution.

Abstract

High-entropy materials (HEMs), as an emerging multicomponent system, exhibit significant potential in green solar energy conversion owing to their continuously tunable electronic structures. Nevertheless, the complex local chemical environments in HEMs induce significant differences in intermediate adsorption/desorption at metal active sites and complicate photoexcited electron transfer. Herein, we propose a strategy of incorporating heterogeneous transition metal Zn into a high-entropy oxide (HEO) lattice to precisely tune the spin state of Ni²⁺. Since the spin-state transition of Ni²⁺ does not involve electron transfer from t_2g to e_g orbitals, but rather only involves electron redistribution within e_g manifold, its electronic structure is particularly sensitive to lattice distortion-induced variations in crystal-field strength, thereby enabling selective regulation of Ni²⁺ from low-spin to high-spin. Field-dependent magnetization measurements and x-ray absorption spectroscopy confirm that Zn incorporation effectively modulates the spin state of Ni²⁺. Density functional theory calculations and femtosecond transient absorption spectroscopy reveal that high-spin Ni²⁺ exhibits an upshifted d-band center and pronounced spin polarization, which synergistically optimize both *H adsorption and photogenerated charge carrier separation efficiency. Consequently, 0.75%FeCoNi_0.3MnZn-HEO/Cd₃(C₃N₃S₃)₂ (0.75%Ni_0.3Zn-HEO/CdTMT) exhibits a prominent PHE rate of 47.53 mmol g^-1 h^-1. This work opens new paradigms for the rational design of spin-directed multi-component HEMs.