Spatial-separation of photoinduced charge carriers in semiconductor nanostructures for efficient photocatalysis

Abstract

Advances in nanotechnology have enabled precise design of catalytic sites for $CO_2$ photoreduction, pushing product selectivity to near-unity. However, activity of most nanostructured photocatalysts remains underwhelming, due to fast recombination of photogenerated electron-hole pairs and sluggish hole transfer. To address these issues, we construct colloidal CdS nanosheets with the large basal planes terminated by $S^{2-}$-atomic layers as intrinsic photocatalysts ($CdS-S^{2-}$ NSs). Experimental investigation reveals that the $S^{2-}$-termination endows ultrathin CdS-$S^{2-}$ NSs with facet-resolved redox-catalytic sites: oxidation occurs on $S^{2-}$-terminated large basal facets and reduction happens on side facets. Such allocation of redox sites not only promotes spatial separation of photoinduced electrons and holes, but also facilitates balanced extraction of holes and electrons by shortening the hole diffusion distance along (001) direction of the ultrathin NSs. Consequently, the CdS-$S^{2-}$ NSs exhibit superb performance for photocatalytic $CO_2$-to-CO conversion, which was verified by the isotope-labeled experiments, at a record-breaking performance: CO selectivity of 99%, CO formation rate of 2.13 mol $g^{-1} h^{-1}$, and effective apparent quantum efficiency (denoted by AQE*) of 42.1% under the irradiation (340 nm to 450 nm) of a solar simulator (AM 1.5G). The breakthrough performance achieved in this work provides novel insights on precise design of nanostructures for selective and efficient $CO_2$ photoreduction. Furthermore, our investigation of planar surfaces in zinc blende (ZB) and wurtzite (WZ) CdS nanosheets revealed the surface-dependent passivation effect in 2D nanosheets. The intrinsic atomic structure of these nanosheets plays a critical role in determining their suitability for surface engineering and their potential for photocatalytic reactions. ZB nanosheets with polar basal planes exhibited a strong passivation effect, and their photocatalytic activity depended heavily on the passivation layer. In contrast, WZ nanosheets possessed inherently stable nonpolar basal planes, resulting in a weaker passivation effect. Consequently, additional $S^{2-}$ and MPA-passivation on WZ nanosheets proved to be unstable and susceptible to degradation during photocatalysis. Expanding upon this understanding, we demonstrated the potential of CdS-based nanosheets for $CO_2$ photoreduction in aqueous solutions and dual-functional photocatalysis. These findings contribute to the advancement of photocatalytic technologies and open up new opportunities for the development of highly efficient and sustainable photocatalysts.