Deciphering the Atomic Scale Electrocatalytic Sites in Hierarchically Porous Gold Nanostructures: Implications for Ascorbic Acid Electrooxidation

Understanding the structure and origin of catalytic sites at the nanometer/subnanometer scale in porous nanomaterials is essential for an efficient and durable catalyst design. Herein, chemically pure nanoporous gold (NPG) having a hierarchically porous network is electrochemically synthesized, and its microstructure is investigated by electron microscopies, electrochemical methods, and ab initio simulations to unravel the identification of catalytic sites for ascorbic acid (AA) electrochemical oxidation. Experimental characterizations reveal a highly porous NPG film containing polycrystalline and interconnected dendrite fractals. The film growth follows a unique pattern such that at the beginning of the film growth, Au grains are smaller, which at the later growth stage become larger and expose low-index facets and structural defects such as dislocation of Au atoms, atomic steps, and kinks. Such structural defects and facet evolutions in the NPG film cause a remarkable electrocatalytic effect toward the electrochemical oxidation of AA, which is interpreted based on density function theory (DFT) simulation. The simulation predicts stronger binding of AA on the low-indexed Au facets and grain boundaries, leading to a larger electronic charge transfer from AA to the Au atoms. In particular, grain boundary regions provide vacancy-like sites, in which part of AA locks itself, forming a stronger bond with a Au atom of partial covalent character. The low catalytic activity of the thermodynamically stable (111) facet is assigned to its flat topography, over which AA interacts through mainly van der Waals forces, causing weaker binding.