Simple and High-Yield Preparation of Carbon-Black-Supported ~1-nm Platinum Nanoclusters and Their Oxygen Reduction Reactivity

1 Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, Kagurazaka, Shinjuku-ku, Tokyo 162−8601 (Japan), E-mail; negishi@rs.tus.ac.jp 2 Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University, 1–1 Minami-Osawa, Hachioji-shi, Tokyo 192−0397 (Japan) 3 Department of Chemistry, University of Adelaide, Adelaide, South Australia 5005 (Australia) ‡ These authors contributed equally

The high-angle annular dark field scanning TEM (HAADF-STEM) images were obtained by ultra-high-resolution transmission electron microscope (The FEI Titan Themis 80−200) operating at 200 kV, with a beam convergence semi angle of 25 mrad and HAADF collection angle from 56−200 mrad. Elemental maps were acquired using a super X detector and low-background sample holder.
Pt L3-edge X-ray absorption fine structure (XAFS) measurements were performed at beamline BL01B1 of the SPring-8 facility of the Japan Synchrotron Radiation Research Institute (proposal numbers 2020A0695, 2020A1410 and 2020A1219). The incident X-ray beam was monochromatized by a Si(111) double-crystal monochromator. As references, XAFS spectra of Pt foil and solid PtO2 were recorded in transmission mode using ionization chambers. The Pt L3-edge XAFS spectra of the samples were measured in fluorescence mode using a 19-element Ge solid-state detector at room temperature. The X-ray energies for the Pt L3-edges were calibrated using Pt foil. The XANES and EXAFS spectra were analyzed using xTunes 4 as follows. The χ spectra were extracted by subtracting the atomic absorption background using cubic spline interpolation and normalized to the edge height. The normalized data were used as the XANES spectra. The k 3 -weighted χ spectra in the k range 3.0-14.0 Å −1 for the Pt L3-edge were Fourier transformed into r space for structural analysis.
The Pt 4f and sulfur (S) 2p X-ray photoelectron spectroscopy (XPS) spectra were collected by using a JPS-9010MC electron spectrometer (JEOL, Tokyo, Japan) at a base pressure of ∼2 × 10 −8 Torr. X-rays from the Mg-Kα line (1253.6 eV) were used for excitation. Each NCs was deposited on an Ag plate and the spectra were calibrated with the peak energies of Ag 3d5/2 (368.22 eV).
The ultraviolet-visible (UV-vis) absorption spectra of products were acquired in dichloromethane solution at room temperature with a V-630 spectrometer (JASCO, Tokyo, Japan).
Fourier transform infrared (FT-IR) spectra of the product were obtained using the attenuated total reflectance (ATR) method in the region between 400 and 4000 cm −1 by a FT/IR-4600-ATR-PRO ONE spectrometer (JASCO, Tokyo, Japan) equipped with a DLATGS detector as the average of 50 scans at 4 cm −1 resolution.
The matrix assisted laser desorption/ionization (MALDI) mass spectra were recorded with a JMS-S3000 spiral time-of-flight mass spectrometer (JEOL, Tokyo, Japan) equipped with a semiconductor laser (λ = 349 nm). DCTB was used as the MALDI matrix. To minimize NC dissociation induced by laser irradiation, the NC-to-matrix ratio was fixed at 1:1000.
Thermogravimetric analysis (TGA) was performed with a TGA2000SA (Bruker, Massachusetts, USA) and at a heating rate of 5 °C min under a nitrogen (N2) atmosphere to 900 °C from room temperature.
TG-MS was performed with an STA 2500 Regulus (NETZSCH, Bavaria, Germany) and a JMS-Q 1500GC (JEOL, Tokyo, Japan) at a heating rate of 5 °C min under a He atmosphere over the temperature range of 40-900 °C.
ICP-MS was performed with an Agilent 7500c spectrometer (Agilent Technologies, Tokyo, Japan). Bismuth was used as the internal standard. The ICP-MS measurements were performed for the solution before and after mixing 1−3 with CB to estimate the adsorbed or loaded Pt content.

Electrochemical measurement
All electrochemical measurements were performed using a CHI 710D electrochemical work-station (ALS, Osaka, Japan) with a RRDE-3A rotating ring disk electrode apparatus (BAS, Tokyo, Japan). A rotating disk electrode (RDE, f = 5 mm) was polished with diamond paste and alumina paste and then sonicated in water before the usage. A counter electrode, Pt ring electrode, was cleaned by a sonication in 10% HNO3 before the usage. Silver-silver chloride (Ag/AgCl) electrode was used as reference electrode. In the setup, first, the catalyst slurry prepared using the method described in section 2.2.3 was sonicated in ice bath for 30 min and then 10 μL of the catalyst slurry was carefully dropped onto RDE by spin-coat method ( Figure S23a). 5 After the catalyst slurry was dried enough, each electrode was set into electrochemical measurement system containing 0.10 mol/L of HClO4 solution (pH = 1.0) as electrolyte ( Figure S23b).
In the measurements, N2 gas was first bubbled for 30 min and then cyclic voltammetry (CV) was conducted 100 times in the region from −0.016 V to 1.254 V (vs. reversible hydrogen electrode; RHE) with a scanning rate of 200 mV/s for the cleaning of the electrodes. 5 Thereafter, CV was performed 3 times in the region from −0.016 V to 1.000 V (vs. RHE) with the rate of 20 mV/s to evaluate ECSA. After CV, a linear sweep voltammetry (LSV) was performed under N2 atmosphere in the region from 0.000 V to 1.000 V (vs. RHE) with the rate of 10 mV/s. Then, LSV was performed under oxygen (O2) gas atmosphere in the region from 0.000 V to 1.000 V (vs. RHE) with the rate of 10 mV/s while rotating RDE at the rate of 400, 900, 1600, 2500 rpm ( Figure S24 and S25).
In the durability test, a cleaning of the electrodes was initially undertaken. Then, CV was performed 3 times in the region from −0.016 V to 1.000 V (vs. RHE) with scanning rate of 20 mV/s to evaluate ECSA. This value was used as ECSA of the sample before durability test. Thereafter, the catalyst was kept under the potential of 1.000 V (vs. RHE) for 30 sec and then exposed to the repeated CV sweep in the region from 1.000 to 1.500 V (vs. RHE) at the rate of 500 mV/s. This repeated CV sweep was performed until the ECSA value fell down below 50% of the initial value ( Figure S32 and S33). 6

Analysis 5.1. Estimation of Chemical Composition for 1−3
Assuming that in the MALDI-MS spectrum of 1−3 the peaks attributed to PtnSm(CO)l are caused by S−C bond dissociation ( Figure  S12 and S13), we explored the combination of n, m, and l attributed to the strongest peak in the MALDI-MS spectrum ( Figure 3a). The weight ratio of Pt, PET, and CO calculated for Ptn(PET)m(CO)l using the obtained the combination of n, m, and l was constrained so that it was consistent with the result of the TGA (Figure 3b): the weight loss in the range of 160−300 °C is equivalent to the weight of PhC2H4 caused by the S−C bond dissociation ( Figure S14), that in the range of 750−900 °C corresponds to the weight of S and CO ( Figure S14), the residual weight at 900 °C corresponds to the weight of Pt (Table S2). Considering the experimental error, we searched for combinations of n, m, and l with a width of ± 5 Da for the mass-to-charge ratio (m/z) and ± 2% for TGA curve. As a result, only the chemical compositions shown in Table S3−5 didn't contradict with the results of MALDI-MS and TGA. Although this is a rough estimation, considering the results obtained for Au25(SR) 18,7,8 Au38(SR) 24,9,10 and Au144(SR)60 11,12 (SR = alkanethiolate or PET) the number of Pt atoms calculated on this assumption seems to reflect the actual value relatively well.

Estimation of Particle Size of Pt Core for 1−3
The following equation was used in the calculation of the particle diameter (D; nm) of Pt core using the density of bulk Pt for 1−3: where n is the number of Pt atoms estimated from MALDI-MS and TG-MS results (Table S3−5), π is circular constant (3.14), ρ is the density of bulk Pt (21.45 g cm −3 ), NA is Avogadro constant (6.02 × 10 23 mol −1 ), and M is the atomic weight of Pt (195.1 g mol −1 ).

Estimation of the Number of Transferred Electrons for Each Catalyst at Diffusion-Limited Region and Mass Activity.
These values are estimated using the Koutecky-Levich equation; 13,14 where Ik is kinetic current, Id is diffusion limiting current, n is the number of transfer electrons, F is Faraday constant, A is geometric is viscosity of the electrolyte (1.0×10 −2 cm 2 /s), and ω is angular rotation speed of the electrode. Mass activity is obtained from Ik by dividing the measured current by geometric area of electrode (A) and loading weight of Pt ( Figure S26 and S27).   Figure 3b and Figure S14. In the table, the value of Pt is fixed to 1.             This MS spectrum implies that a similar S−C dissociation also occurred for 2. Similar results were also observed for 1 and 3.               (Table S7) and the mass activity (middle of Figure 6a) were calculated on the basis of these results.    (Table S7) and the mass activity (middle of Figure 6a) were calculated on the basis of this result. Figure S31. Comparison of (a) ECSA, (b) ORR mass activity at 0.6 V vs. RHE, and (c) ORR specific activity between Pt~51/CB (5.1 wt% Pt; Table S6) and

Additional Figures
PtNP/CB (6.5 wt% Pt). The PtNP/CB (6.5 wt% Pt) was prepared by adding the CB to the commercial PtNP/CB (13 wt% Pt). In these figures, the average value over 3 times experiments are described for Pt~51/CB (5.1 wt% Pt). Figure S32. Comparison of LSV curves between Pt~51/CB (5.14 wt% Pt (Table S6); cyan), Pt~66/CB (5.56 wt% Pt (Table S6); red), and commercial PtNP/CB (6.5 wt% Pt; black). These curves were obtained by the LSV at a rotating rate of 400, 900, 1600, and 2500 rpm under O2 atmosphere (the baseline obtained by LSV under N2 atmosphere are subtracted from these curves). These results indicate that the overpotential is lower for Pt~51/CB and Pt~66/CB compared with the commercial PtNP/CB. Figure S33. Protocol of the durability test used in this work. 20 Figure S34. Flow of the durability test used in this work. 20 Figure S35. Normalized ECSA of Pt~66/CB (red) and commercial PtNP/CB (black) during accelerated durability test. In this experiment, Pt~66 with 5.6 wt% Pt (Table S6) and PtNP/CB with 6.5 wt% Pt were used since the large difference in loading weigh might affect the durability of the catalysts. The values of the actual ECSA before and after the durability test are also shown in this figure.