A Ligand-Exchange Route to Nobel Metal Nanocrystals with a Clean Surface for Enhanced Optical and Catalytic Properties

Noble metal nanocrystals that are free of capping ligands promise significantly enhanced activities in surface-enhanced Raman scattering (SERS) and catalytic applications. Conventional physical and chemical processes to remove the capping ligands are usually too harsh to retain the morphology and surface structure of the noble metal nanocrystals. In this work, a mild, effective, and robust strategy is presented to remove the capping ligands from the surface of noble metal nanocrystals. Polyvinylpyrrolidone and oleylamine, which are generally known to adsorb strongly on the metal surface, have been successfully removed from the Au and Pt nanocrystals, respectively, by a convenient ligand-exchange process with diethylamine. The resulting surface-clean Au nanospheres and nanoflowers show significantly enhanced activity in SERS with great potential in single-particle SERS applications. The surface-clean Pt nanocrystals show highly improved electrocatalytic activity, compared with those cleaned by conventional methods. It is believed that this novel ligand-exchange strategy opens up new opportunities in eliminating the effect of the capping ligands for optimal activities of colloidal noble metal nanocrystals in a variety of applications.