Please do not adjust margins Please do not adjust margins Thus, this study successfully elucidated the electronic structure and the origin of the optical absorption spectrum of [Pt17(CO)12(PPh3)8] z using both experimental and theoretical approaches. These findings align with previous research on ligand-protected Au-, Ag-, and Cu NCs. Specifically, several key findings regarding the electronic structure have been observed: 1) a distinct peak structure appears in the optical absorption spectrum when the metal core is refined to ~1 nm; 2) the peak structure in the visible region is mainly caused by the absorption of the metal core; 31 3) differences in the structure of the functional groups at the end of the ligands have negligible effect on optical absorption. Electrocatalytic activity of Pt17/CB for the HER Having established the geometrical and electronic structures of the Pt NCs, we next aimed to create highly active catalysts by increasing the number of active sites through miniaturization. Although many highly active Pt NCs have been reported, most require special equipment and reagents. Accordingly, those methods are not suitable for the preparation of the practical Pt NC catalyst. Although simple methods have also been reported for the preparation of Pt NCs, Pt NCs synthesized by those methods typically includes the variation in the number of constituent atoms. Accordingly, they do not necessarily show high activity specialized in the NC-size region. Therefore, in this work, we attempted to prepare the Pt NC catalysts using [Pt17(CO)12(PPh3)8] z , which can be obtained with atomic precision by simple synthesis in air, as a precursor to establish a method for preparing the practical NC catalyst with high HER activity. It can be expected that using such an atomically precise Pt NC as catalyst also help to understand the mechanisms of the HER which occurs over NC catalysts. In the experiment, following a previously established method with a slight modification, Pt17 NCs were supported on CB (Fig. ) with a Pt loading of 1.0 wt% through chemisorption, and the ligands were removed by calcination at 200 °C. Transmission electron microscopy (TEM) images of the calcined Pt17 NC-supported CB catalyst (Pt17/CB(1.0 wt% Pt)) showed that the particle size of Pt17 NC remains largely unchanged during the adsorption and calcination processes (Figs. and). X-ray photoelectron spectroscopy (XPS) indicated that the Pt in Pt17/CB(1.0 wt% Pt) is in a largely metallic electronic state with a slight positive charge, similar to the previously reported one (Figs. and). To evaluate their HER activities, a catalyst slurry was prepared using Pt17/CB(1.0 wt% Pt) and then deposited onto a glassy carbon electrode (GCE) to create the working electrode. After electrochemical cleaning to remove residual organic substances, the HER activity was measured and compared with that of a catalyst made by diluting commercial Pt NPs/CB(46.9 wt% Pt; TEC10E50E) with CB until 1.0 wt% Pt to prepare Pt NPs/CB(1.0 wt% Pt). Fig. . shows the linear sweep voltammetry (LSV) curves for Pt17/CB(1.0 wt% Pt) and Pt NPs/CB(1.0 wt% Pt). The results show that Pt17/CB(1.0 wt% Pt) generates a larger reduction current than Pt NPs/CB(1.0 wt% Pt) starting around 0.0 V vs. reversible hydrogen electrode (RHE), indicating that Pt17/CB(1.0 wt% Pt) has superior HER activity than Pt NPs/CB(1.0 wt% Pt). Comparison of current values at -0.055 V vs. RHE with the appropriate Tafel slope in the HER (Fig. ) revealed that Pt17/CB(1.0 wt% Pt) has 1.26-times the HER activity (7.54 vs.