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•TiO2/C3N4 forms 2D/2D core–shell van der Waals heterojunction.•0D Ti3C2 MXene quantum dots deposited on van der Waals heterojunction.•TiO2/C3N4/Ti3C2 composite photocatalyst with enhanced CO2 reduction activity.•S-scheme charge transfer mechanism observed between TiO2 and C3N4.•0D Ti3C2 quantum dots extract and trap electrons from C3N4.
Herein, two-dimensional (2D) TiO2 mesoporous nanosheets with three to four C3N4 layers grown in situ are employed to design a core–shell 2D/2D van der Waals heterojunction (TiO2/C3N4). Edge-terminated zero-dimensional (0D) Ti3C2 MXene quantum dots (TCQD) are subsequently integrated in the C3N4 surface via electrostatic interactions. The constructed 2D/2D/0D TiO2/C3N4/Ti3C2 composite heterojunction photocatalyst exhibits enhanced CO2 reduction activity compared to TiO2, C3N4, TiO2/C3N4, C3N4/Ti3C2 for CO and CH4 production. A step-scheme (S-scheme) charge transfer mechanism operates for the prepared samples during CO2 reduction, as authenticated by in situ X-ray photoelectron spectroscopy and electron paramagnetic resonance analysis. This study provides a paradigm of a rational structural design for regulating the number and type of heterointerfaces and further insights into the mechanism of multijunction photocatalysts.