Size-controlled synthesis of Au nanorings on Pd ultrathin nanoplates as efficient catalysts for hydrogenation

Au nanorings are of particular interest in catalysis owing to their fascinating properties, however, it still remains a tremendous challenge to generate such hollow nanostructures with small size. Here, we report the synthesis of Au nanorings with an outer diameter of less than 20 nm by a seed-mediated approach with Pd nanoplates as seeds in the presence of Ag+ ions. The incorporation of Ag+ ions substantially slows down the reduction rate of the Au precursor through an underpotential deposition (UPD) process, leading to the formation of small Au nanoparticles. These small Au nanoparticles then coalesced into nanorings on small Pd nanoplates. The outer diameter of the Au nanorings is tuned by varying the size of the Pd nanoplates. The smallest Au nanorings are evaluated as catalysts towards the hydrogenation of 4-nitrophenol, showing substantially enhanced catalytic activity relative to the Au nanoparticles with the same size. This enhancement in the catalytic activity can be attributed to the unique structural features including the large specific surface areas, convenient accessibility of both interior and exterior surfaces for the reactants, and existence of highly active sites in the interior part.