Identifying the active sites of Cu nanoparticles that convert CO2 to multi-carbon (C2+) materials has remained elusive. It is caused by the reconstruction of Cu nanoparticles during electrochemical CO2 reduction and the unrevealed effect of capping ligands covering the Cu surface. We show that the C2+ selectivity and morphological evolution of Cu nanoparticles largely depend on the density of the capping ligand, tetradecylphosphonate (TDP). Ultraviolet-ozone pre-treatment of Cu nanoparticles reduced the density of TDP. Desorption of the remaining ligands was expedited within 3 h of the CO2 reduction process, and the concurrent aggregation of NPs formed bare Cu clusters. The increased C2+ selectivity of >50% faradaic efficiency revealed that the existing grain boundaries of Cu clusters promoted CO dimerization to produce C2+ materials. Despite similar cluster formation, Cu nanoparticles without ultraviolet-ozone treatment showed poor C2+ selectivity, suggesting that the remaining TDP, half of the original concentration, passivated the grain boundaries. Further the morphological transformation of Cu did not increase C2+ selectivity even after 20 h of the reaction.