CO2 Reduction by Plasmonic Au Nanoparticle-Decorated TiO2 Photocatalyst with an Ultrathin Al2O3 Interlayer

A well-designed nanocomposite structure, Au/Al2O3/TiO2, that is Au-decorated TiO2 with an atomic-layer-deposited Al2O3 interlayer between Au and TiO2, was synthesized to study the role of plasmonic Au nanoparticles on photocatalytic CO2 reduction as well as the influence of the interlayer. Localized surface plasmon resonance induced hot electron injection and near-field enhancement, together with the enhanced charge separation caused by the Au NPs were found responsible for the promoted photocatalytic CO2 reduction on Au-modified TiO2. The Al2O3 interlayer inhibited electron transfer and dampened the near-field enhancement effect; however, it also served as passivation layer to suppress electron–hole recombination on the TiO2 surface. To further identify the significance of each of the mechanisms, photocatalysts with tailored thickness of Al2O3 interlayer and varied size of Au NPs were carefully synthesized, characterized, and tested for CO2 photoreduction with water. A finite difference time domain simulation was also conducted to correlate with the catalytic activity. The results show that the hot electron injection phenomenon was observable but very weak under visible light excitation. Charge separation and near-field enhancement were more dominant mechanisms under UV–vis irradiation and were closely related to the size of Au NPs. Charge separation was more significant for smaller Au NPs, whereas stronger near-field enhancement was found on larger Au NPs. At an optimal thickness of Al2O3 interlayer, the positive effects of the interlayer outperformed the negative ones, resulting in a significant improvement in CO2 photoreduction.