D-band state control engineering over ZnIn₂S₄ for enhanced photoreduction of CO₂ to CH₄

Metal-based photocatalysts with d¹⁰ electronic configurations exhibit good photocatalytic performance due to strong band edge dispersion, however, the weak bonding between d¹⁰ metal sites and CO₂ through 2p-3d orbital hybridization limits their activity and selectivity for CO₂ to CH₄ conversion. Herein, a strategy for modulating the d-band center is proposed to promote the formation of CH₄ in the photocatalytic CO₂ reduction process. In a model system taking ZnIn₂S₄ (ZIS) as photocatalysts, highly thermodynamically electronegative elements (such as Bi, Cu, and Co) are doped to upshift the d-band center of ZIS, enhance the selectivity and yield of CH₄. In the absence of cocatalysts or photosensitizers, the CO₂ photoreduction products of all doped ZIS samples shifts from pure CO to a mixture of CO and CH₄, with CH₄ being the predominant product. Among all samples, Bi-doped ZnIn₂S₄ (Bi-ZIS) demonstrates the highest performance, achieving a CH₄ selectivity of 63.68 % and a high evolution rate of 21.07 µmol·g^-1·h^-1 during visible-light-driven CO₂ reduction. Density functional theory (DFT) calculations indicate that doping highly electronegative elements can modulate the electron cloud distribution of ZIS, thereby raising its d-band center. This shift reduces the energy barrier for CO₂ photoreduction to CH₄, enhancing the binding energy between active sites and intermediates (such as *OCH₂ and *OCH₃), facilitating the formation of CH₄. Consequently, this study not only validates the effectiveness of the d-band center modulation strategy but also offers a novel perspective for optimizing the activity and product selectivity of d¹⁰ metal-based photocatalysts in CO₂RR.