NiCo2S4 cocatalyst supported Si nanowire heterostructure for improved solar-driven water reduction: experimental and theoretical insights

Abstract

We report a heterostructure photocathode composed of the optimal gradient of nickel cobalt disulphide layers (NiCo₂S₄ NS) over a p-Si nanowire (Si NW) surface designed for the solar-assisted water reduction process. The optimal Si NW/NiCo₂S₄ NS heterostructure exhibits enriched solar water reduction activity with a photocurrent density of 15 mA cm⁻² at −0.8 V vs. RHE applied bias and an onset potential of 301 mV vs. RHE under simulated solar irradiation. Moreover, the fabricated heterostructure photocathode (Si NW/NiCo₂S₄ NS) at the applied bias of −0.3 V vs. RHE produces a hydrogen gas evolution rate of around 53.75 μmol cm⁻² h⁻¹. The accomplished catalytic activity of the heterostructure can be attributed to the HER electrocatalyst properties of NiCo₂S₄ as an interfacial layer, such as interfacial energies, charge transfer kinetics, and solar-driven water reduction activity at the photocathode/electrolyte interface. An electrochemical impedance spectroscopy study demonstrates a significant reduction in charge transfer resistance that results in rapid electron transfer at the interface for efficient charge separation and migration processes. The density functional theory calculations reveal that NiCo₂S₄ NS has a suitable electronic band alignment with a water redox potential advancing the catalytic efficiency owing to barrier-free electron transport with near-zero Gibbs free energy of hydrogen adsorption on hetero-interface sites (ΔG_H* = 0.02 eV at Co1 site). Consequently, the proposed heterostructure scheme is a promising strategy for designing a high-performance Si photocathode for solar water reduction.