The HER activities of these four specimens were subsequently investigated using linear sweep voltammetry (LSV) in 0.1 M aqueous HClO4. During these trials, the current on the negative side was attributed to water reduction. Both NCs decreased the onset potential and current density (Figure ), demonstrating that these materials promoted the HER. Furthermore, 3 and 4 increased the current density at a lower overpotential compared with [Au24Pt(C6)18] 0 and 2. Specifically, the onset overpotentials in the presence of [Au24Pt(C6)18] 0 and 2-4 were 392, 396, 321, and 300 mV, respectively. Similarly, at the fixed overpotential of -0.35 V vs. RHE, the mass-based activities of 3 (8.15 A mgAuPt -1 ) and 4 (11.37 A mgAuPt -1 ) were, respectively, 3.5 and 4.9 times those of [Au24Pt(C6)18] 0 (2.35 A mg AuPt -1 ) and 2 (2.32 A mg AuPt -1 ) (Figure ). These data unambiguously established that 3 and 4 were more efficient HER catalysts than [Au24Pt(C6)18] 0 and 2. LSV data obtained with these materials after five scans are presented in Figure . These LSV curves almost overlapped at all scan numbers, indicating that [Au24Pt(C6)18] 0 and 2-4 promoted the HER in a stable manner (Figure ). Our previous work showed that the alkyl chain length has a significant impact on HER activity. Hence, it was surprising to discover that 3 and 4 exhibited higher activities than [Au24Pt(C6)18] 0 , 26,44 1, and 2, despite their similar thiolate alkyl/aryl chain lengths. To understand the origin of the high HER activity of 3 and 4, Tafel plots (Figure ) were generated from the corresponding LSV curves for each of the NCs (Figure ). Although these materials provided considerably different onset potentials (Figure ), [Au24Pt(C6)18] 0 and 2-4 provided similar Tafel slopes (119, 125, 126, and 129 mV/dec, respectively) (Figure ). These values were close to the theoretical Tafel slope (120 mV/dec) of the Volmer step in the two-step HER (Figure ). Therefore, in the presence of all materials, the HER likely proceeds via a similar mechanism in which H + adsorption is the rate determining step (RDS). In the case of [Au24Pt(SR)18] 0 , theoretical analyses have indicated that H + is adsorbed on the Au12Pt core. Hence, the site of initial H + adsorption on [Au24Pt(SR)12(dithiolate)3] 0 is likely the Au12Pt core. In the differential pulse voltammetry curves, the position of the 0/+1 redox peak of 2 shifted to the negative side compared with those of 3 and 4 (Figure ). This indicates that the electronic structure of the Au12Pt core of 2 is more susceptible to oxidation (accept H + ) 45 than those of 3 and 4. However, the HER activities of 3 and 4 were higher than that of 2. These results indicated that the difference in HER activities could not be explained by the difference in the electronic structures of 2-4. Here, it is helpful to consider the manner in which H + diffuses through the electrolyte to the Au24Pt surface because the Au12Pt core is the site of the HER. According to the proton jumping theory (namely the Grotthuss mechanism), H + diffuses through water by repeatedly forming and dissociating bonds between adjacent hydronium ions (H3O + ) via a hydrogen-bond network. Therefore, water molecules (H3O + ) must penetrate the hydration structures (e.g., H5O2 + , H9O4 + , and H13O6 + ) 49,50 by avoiding the hydrophobic R groups of the -SR ligands to reach the Au12Pt surface, where H + adsorption occurs. Therefore, reducing the distance between the hydrated structure and the Au12Pt surface increases the rate of H + adsorption on the Au12Pt surface. As shown in Figure , incorporating three -SCH2-(S or CH2)-CH2S-ligands into 3 and 4 exposed the Au atoms of the metal core to a greater extent than was possible in 2. As a result, the distance between the hydrated structure and the Au12Pt surface was significantly reduced, thereby facilitating proton transfer to the Au12Pt surface. Increasing the rate of H + diffusion promotes H + adsorption and thus accelerates the HER. Indeed, introducing hydrophilic functional groups at the ends of the SR ligands of Au24Pt(SR)18 enhances the HER activity by promoting proton migration to the ligand surface. Therefore, the high HER activities of 3 and 4 likely originate in their geometric structures, which promotes the adsorption of H + on the Au12Pt surface. We expect that the origin of the slight difference in the HER activities of 3 and 4 would be revealed by investigating the diffusion of water molecules toward 3 and 4 through the hydration structures using molecular dynamics and TD-DFT calculations in future studies.