Hollow Rh-COF@COF S-Scheme Heterojunction for Photocatalytic Nicotinamide Cofactor Regeneration

Photocatalytic regeneration of a nicotinamide cofactor, nicotinamide adenine dinucleotide phosphate (NADPH), has emerged as an ideal cascade partner for combination with enzymatic transformations, but it still suffers from low efficiency due to the recombination of photogenerated charge carriers. Herein, a hierarchical Rh-covalent organic framework (COF)@COF core–shell hollow sphere (CSHS) with S-scheme heterojunction is proposed to enhance the charge carrier transfer and utilization in photocatalysis to regenerate the expensive NADPH. The hierarchical Rh-COFBpy@ hollow spherical COF (HCOF) CSHS was prepared by simple sequential in situ growth. The turnover frequency of Rh-COFBpy@HCOF25 for NADPH regeneration reached 2.6 mmol·gRh-COF–1·h–1, which was 3.7 times that of pure Rh-COFBpy under visible light. Density functional theory calculation and X-ray photoelectron spectroscopy analysis showed that an internal electric field directed from Rh-COFBpy to HCOF was formed in Rh-COFBpy@HCOF CSHS, which accelerated the photogenerated electron transfer from HCOF to Rh-COFBpy. In situ irradiated XPS analyses and photoirradiated Kelvin probe measurement revealed the S-scheme charge-transfer mechanism within Rh-COFBpy@HCOF. The Rh-COFBpy@HCOF photocatalytic NADPH regeneration system was further coupled with enzymatic reduction of C═C, achieving a photoenzyme cascade reaction. This work provides a protocol for the design and utilization of COF-based artificial photosynthetic systems.