Rational Design and Interfacial Engineering of an MOF-on-MOF-Derived Fe ₃ O ₄ @NiCo ₂ S ₄ Hollow Spindle-Shaped S-Scheme Heterojunction for Excellent CO ₂ Photoreduction

The development of S-scheme heterojunctions offers a powerful approach for efficient photocatalytic CO₂ reduction, leveraging enhanced charge separation and strong redox capabilities. However, efficient interfacial charge transfer continues to pose significant challenges. In this work, we designed unique Fe₃O₄@NiCo₂S₄ S-scheme heterostructured hollow spindles by using the MIL-88A(Fe) spindle as a template for the directional growth of NiCo metal-organic framework (NiCo-MOF) nanosheets, followed by hydrothermal sulfidation. This structure not only lowers the surface energy barrier for reactions but also generates an internal electric field that facilitates charge diffusion and electron transfer. Through a combination of in situ X-ray photoelectron spectroscopy (XPS), scanning Kelvin probe (SKP), electron spin resonance (ESR), and photoelectrochemical tests, the formation of an S-scheme heterojunction within Fe₃O₄@NiCo₂S₄ was confirmed. The electric field effectively traps photogenerated holes in the valence band (VB) of Fe₃O₄, while confining electrons to the conduction band (CB) of NiCo₂S₄, greatly reducing the recombination of electron-hole pairs and enhancing the efficiency of photogenerated charge-carrier utilization. Additionally, the redox capacity of the Fe₃O₄@NiCo₂S₄ heterojunction is notably enhanced. The hollow spindle architecture, with its inherent large specific surface area, improved utilization of visible light, enhanced CO₂ adsorption, and accelerated reaction rate, translates to superior photocatalytic performance. Under visible-light irradiation, the optimized Fe₃O₄@NiCo₂S₄ hollow spindles achieved CO and CH₄ production rates of 38.53 and 4.43 μmol g^-1 h^-1 through photocatalytic reduction of CO₂. This research emphasizes the synergy of an S-scheme heterojunction and a hollow spindle architecture, offering a key strategy for developing MOF-based S-scheme systems for advanced photocatalysis.