Plasmonic control of solar-driven CO2 conversion at the metal/ZnO interfaces

Molecular-level understanding of the solar-driven CO2 conversion is of importance to design high-efficiency artificial photosynthetic systems for rebalancing the global carbon cycle. Herein, some physical insights into the surface plasmon resonance (SPR) mediated CO2 photoreduction were demonstrated with metal (Au, Ag, and Pd)/3D porous ZnO nanosheets (NSs). Such plasmonic photocatalysts were designed elaborately to expose the polar {001} facet, based on the physical prototype of field-field coupling, in order to benefit chemical polarization and activation of the inert molecule. Among these plasmonic metals, gold was found to be not only more effective for promoting the solar-driven CO2 conversion, but unique for producing the higher hydrocarbon, C2H6. A 10-fold enhanced conversion efficiency and a quantum efficiency of 1.03% were achieved on Au/ZnO NSs at ca. 80% selectivity to hydrocarbons under solar light irradiation. The characterization results indicated that the metal-semiconductor interaction enables the electron-phonon decoupling to generate more amounts of energetic electrons in the excited ZnO NSs by a proposed pathway, called the SPR energy transfer induced interband transition that promotes the semiconductor photoexcitation, kinetically accelerating the conversion. Density functional theory calculations revealed that the field-field coupling greatly intensifies the surface polarization for adsorbates, charging negatively the C atom of CO2 and making O=C=O bond bent, along with the electrophilic attack by two competitive paths, leading to the concomitance of CO and CH4. The loading of plasmonic metal nanoparticles alters the molecular paths of CO2 conversion by tuning thermodynamically the first dehydroxylation step, consequently the product selectivity. Especially Au plasmon, it enables the CO hydrogenation path, making CH4 faster.