Construction of two-dimensionally relative p-n heterojunction for efficient photocatalytic redox reactions under visible light

[Display omitted] •Relative p-n junction CdS/g-C3N4 with higher photocatalytic redox abilities is fabricated.•Z-scheme mechanism verified by experimental and computational analyses.•The built-in internal electric field enhances driving redox force. Precisely modulating the interfacial contact of heterojunction photocatalysts and regulating the spatial transfer of photoexcited charge carriers are the core factors to promote the photocatalytic efficiency for energy and environmental applications. Herein, novel 2D/2D relative p-n heterojunction CdS/g-C3N4 with Z-scheme (or S-scheme) was fabricated by in-situ growing ultrasmall 2D CdS nanoflakes on the surface of 2D g-C3N4 nanosheets. The optimized 2D/2D heterojunction structure with proper band configuration and suitable heterojunction interface can improve photoexcited charge carriers separation spatially and transfer to the ideal reaction sites. Therefore, 2D/2D CdS/g-C3N4 composites show high photocatalytic reduction (H2 evolution) and oxidation (selective oxidation of 5-hydroxymethylfurfural into 2,5-diformylfuran) activities, which are about 26.8 times and 3.51 times higher than individual g-C3N4 and are about 4.4 times and 2.39 times higher than individual CdS, respectively. The mechanisms including direction and inner impetus of the photoexcited charge separation and transfer in the composite photocatalyst are proposed and verified by experimental and computational analyses. This investigation highlights the promising scope to intelligently regulate the spatial separation and transfer of photoexcited charge carriers and rationally construct new relative p-n junction or Z-scheme photocatalysts for energy and environmental applications.