Photo-Enhanced Catalysis of Nano-Plasmonic Array at the Single-Molecule Level


Plasmonic catalysts have emerged as a promising material to promote photo-enhanced chemical reactions because of their high photo-absorption efficiency and tunable absorption. Gold, one of the best plasmonic metals, has been widely applied to the field of organic transformations under visible-light irradiation via localized surface plasmon resonance (LSPR). To fully utilize the photons within the solar radiation spectrum, significant efforts have been put forth to the development of nanogold disk arrays to maximize the catalysis surface area and resonance wavelength to the near-infrared region. However, where and how the catalytic reaction happened and how the LSPR illumination enhanced catalytic efficiency are not fully understood. Here we examined LSPR catalytic reactions resazurin to resorufin reduced by NH2OH in nanoporous gold disk (NPGD) array using single-molecule super-resolution microscopy. We observed the main activity happens in the catalytic hot spots and the inter-catalyst area. Interestingly, we found the local hot spots showed all different kinds of trends (increase, decrease, or oscillating) other than the overall linear increase. By analyzing the location of LSPR decreased catalytic hot spots with the 561 nm excitation profile, we suggested that the LSPR induced catalysis was competing with interband excitation induced catalysis, and the LSPR one had much higher efficiency. We further investigated the reason why the activity mainly happened in the catalyst-free inter-disk area and was mainly confined within the small area. Correlating the adsorption and the catalytic results of the NPGD array, we found the NH2OH was accumulating on the periphery and the inter-catalyst region and could guide the intermediate chemical species. By studying the adsorption properties of two adsorbates on NPGD with opposite charges, we found LSPR effect may modulate the surface potential of NPGD to enhance or suppress the adsorption in a small area. This work showed the LSPR catalysis is a net result of the interplay between LSPR enhanced adsorption and other well-known effects (such as thermal or hot electron/hole). These findings provide the knowledge base to design a higher efficient LSPR catalyst by manipulating the spatial arrangement of activity sites of reactions or adsorption sites of reactants.