Surface defects play an important role in catalysis and photocatalysis. They consist of surface reaction sites with highly unsaturated valences that promote reactant adsorption. Defects strongly influence the surface electronic structure and reactivity of catalytic materials; in photocatalytic materials, they affect the efficiency of charge separation and photocatalyst performance.
Layered materials, such as ultrathin nanosheets with thicknesses approaching atomic dimensions, possess abundant coordinatively unsaturated metal defect sites, which can serve as active sites for catalytic transformations. The large specific surface areas of ultrathin nanosheets also maximize the active sites’ availability to reactants bypassing traditional diffusion barriers that limit the performance of conventional 3-D catalysts and photocatalysts.
Zhang and his colleagues synthesized ultrathin zinc-containing layered double hydroxide (LDH) nanosheets with coordinatively unsaturated zinc defects (yellow circles in Figure 1). Under UV–vis irradiation, the high surface area of ultrathin ZnAl-LDH nanosheets showed excellent activity for the photoreduction of CO2 to CO in the presence of water vapor. The catalytic activity of ZnAl-LDH was ≈20 times greater than that of a commercial ZnO nanoparticle reference photocatalyst.
The researchers introduced coordinatively unsaturated zinc ions by increasing the density of oxygen-vacancy defects (Vo) surrounding the cations. To achieve the high Vo concentrations, they decreased the lateral dimension of ZnAl-LDH sheets from 5 μm to 40 nm and then further reduced the nanoplatelet thickness to ≈2 structural layers. Advanced characterization measurements provided clear evidence for the formation of Zn+-Vo complexes in these ultrathin ZnAl-LDH nanosheets. Density functional theory (DFT) calculations showed that the coordinatively unsaturated zinc centers serve as trapping sites to efficiently promote the adsorption of CO2 on the surface of LDH nanosheets. The sites also facilitate electron transfer to the adsorbate, thereby enhancing the rates of photocatalytic CO2 reduction to CO in the presence of water vapor.
This work provides a solid platform for development of defect-containing catalysts with high specific surface areas for efficient CO2 photoreduction and other applications. (Adv. Mater. DOI: 10.1002/adma.201503730; Adv. Energy Mater. DOI:10.1002/aenm201501974)