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.