Ligand-regulated ORR activity of Au nanoparticles in alkaline medium: the importance of surface coverage of ligands

Abstract

The functionalization of organic ligands on nanoparticles (NPs) to create metal–organic interfaces can tune their activity and selectivity in catalytic reactions. Compared to the well-documented electronic and steric contribution of organic ligands, the coverage of ligands is also a significant factor but has been rarely investigated. Herein, we employ two methods, potential cycling and thermal annealing, to judiciously remove surface ligands from gold nanoparticles (AuNPs) and systematically investigate the influence of surface coverage of ligands on the catalytic performance. Potential cycling enables mild removal of ligands without changing the size of AuNPs and thus provides ideal models to identify the intrinsic influence of the surface coverage of ligands on oxygen reduction reaction (ORR). The ORR activity and selectivity is quantitatively evaluated by plotting the half potential and electron transfer number versus the surface coverage of ligands. Thermal annealing at different temperatures produces differently sized AuNPs with various surface ligand coverages. Compared to the size effect, we found that the surface coverage of ligands played a dominant role in the ORR activity of AuNPs. Most importantly, capping AuNPs with oleylamine and sodium citrate and reverse poisoning clean AuNPs with thiol, butylamine and CTAB further reveal that the difference in the ORR activity/selectivity of the capped AuNPs is directly related to the surface coverage of the ligands regardless of the diverse chemical nature of these ligands. This work highlights that the surface coverage of ligands should be considered as an important factor for catalytic reactions accounting for metal–organic interfaces.