Ligand removal of Au 25 nanoclusters by thermal and electrochemical treatments for selective CO 2 electroreduction to CO.

Undercoordinated metal nanoclusters have shown great promise for various catalytic applications. However, their activity is often limited by the covalently bonded ligands, which could block the active surface sites. Here, we investigate the ligand removal process for Au ₂₅ nanoclusters using both thermal and electrochemical treatments, as well as its impact on the electroreduction of CO ₂ to CO. The Au ₂₅ nanoclusters are synthesized with 2-phenylethanethiol as the capping agent and anchored on sulfur-doped graphene. The thiolate ligands can be readily removed under either thermal annealing at ≥180°C or electrochemical biasing at ≤-0.5 V vs reversible hydrogen electrode, as evidenced by the Cu underpotential deposition surface area measurement, x-ray photoelectron spectroscopy, and extended x-ray absorption fine structure spectroscopy. However, these ligand-removing treatments also trigger the structural evolution of Au ₂₅ nanoclusters concomitantly. The thermally and electrochemically treated Au ₂₅ nanoclusters show enhanced activity and selectivity for the electrochemical CO ₂ -to-CO conversion than their pristine counterpart, which is attributed to the exposure of undercoordinated Au sites on the surface after ligand removal. This work provides facile strategies to strip away the staple ligands from metal nanoclusters and highlights its importance in promoting the catalytic performances.