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 CO 2 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 Fe 3 O 4 @NiCo 2 S 4 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 Fe 3 O 4 @NiCo 2 S 4 was confirmed. The electric field effectively traps photogenerated holes in the valence band (VB) of Fe 3 O 4, while confining electrons to the conduction band (CB) of NiCo 2 S 4, greatly reducing the recombination of electron–hole pairs and enhancing the efficiency of photogenerated charge-carrier utilization. Additionally, the redox capacity of the Fe 3 O 4 @NiCo 2 S 4 heterojunction is notably enhanced. The hollow spindle architecture, with its inherent large specific surface area, improved utilization of visible light, enhanced CO 2 adsorption, and accelerated reaction rate, translates to superior photocatalytic performance. Under visible-light irradiation, the optimized Fe 3 O 4 @NiCo 2 S 4 hollow spindles achieved CO and CH 4 production rates of 38.53 and 4.43 μmol g –1 h –1 through photocatalytic reduction of CO 2 . 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.