Dual-Defective Two-Dimensional/Two-Dimensional Z-Scheme Heterojunctions for CO₂ Reduction

The target of photocatalytic CO₂ reduction is to achieve high selectivity, efficiency, and stability for a single chemical/fuel production. The construction of conventional Z-scheme heterojunctions is beneficial to improve the interfacial charge separation and redox capacities. However, the random dimensions of junction component(s) undermine the charge-to-surface transport for catalytic reactions, and the limited chemical structures of catalysts restrict surface activity/selectivity tailoring. In this work, we successfully overcome these issues by stacking/constructing an ultrathin dual-defective two-dimensional (2D)/2D Z-scheme heterojunction with growing functional anionic vacancies onto both reductive and oxidative components of the Z-scheme. The O-vacancy-rich BiOCl/N-vacancy-rich g-C₃N₄-based 2D Z-scheme exhibits excellent photoactivity in CO₂ reduction. The rate of CO₂ photoreduction to CO is around 45.33 μmol g^-1 h^-1, which is 11.7- and 12.2-fold those of untreated bulk g-C₃N₄ and pristine BiOCl, respectively. Among them, N-vacancy-rich g-C₃N₄ exhibits active and selective photoreduction ability, accompanied with oxidation reactions from O-vacancy-rich BiOCl. Such ultrathin defective Z-schemes not only retain their original features, i.e., enhanced charge separation and redox capacities, but also extend to lower energy photon absorption and ameliorate charge-to-surface transport in two redox components. Besides, density functional theory calculations unveiled the thermodynamically favored CO₂-to-CO reduction path and energy barrier’s stepwise reduction at the COOH-to-CO rate-limiting step from defective g-C₃N₄ to the single redox component defective junction and further to the defective junction with both redox components. This work provides an effective adaptable dual-defect engineering on 2D/2D heterojunctions to enhance CO₂ photoreduction.