Highly Crystalline Pd13Cu3S7 Nanoplates Prepared via Partial Cation

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Highly Crystalline Pd CuS Nanoplates Prepared via Partial Cation Exchange of Cu S Templates as an Efficient Electrocatalyst for the Hydrogen Evolution Reaction 1.81

Jongsik Park, Haneul Jin, Jaeyoung Lee, Aram Oh, Byeongyoon Kim, Ju Hee Kim, Hionsuck Baik, Sang Hoon Joo, and Kwangyeol Lee Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03178 • Publication Date (Web): 12 Sep 2018 Downloaded from http://pubs.acs.org on September 14, 2018

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Chemistry of Materials

Highly Crystalline Pd13Cu3S7 Nanoplates Prepared via Partial Cation Exchange of Cu1.81S Templates as an Efficient Electrocatalyst for the Hydrogen Evolution Reaction Jongsik Park,†,# Haneul Jin,†,# Jaeyoung Lee,† Aram Oh,‡ Byeongyoon Kim,† Ju Hee Kim,† Hionsuck Baik,‡ Sang Hoon Joo,⊥,* and Kwangyeol Lee†,* † ‡

Department of Chemistry, Korea University, Seoul 02841, Republic of Korea Korea Basic Science Institute (KBSI), Seoul 02841, Republic of Korea

⊥

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea # These authors contributed equally. ABSTRACT: Chemical transformations via post-synthetic modification of colloidal nanocrystals have received great attention as a rational synthetic route to unprecedented nanostructures. In particular, cation exchange reaction is considered as an effective method to alter the composition of the starting nanostructures while maintaining the initial structural characteristics. Herein, we report the synthesis of highly crystalline Pd13Cu3S7 nanoplates (NPs) via partial cation exchange of the Cu1.81S phase by Pd cations, with Cu1.94S NPs and Pd13Cu3S7/Cu2-xS janus heterostructure as the intermediate phases. The highly crystalline Pd13Cu3S7 ternary NPs exhibit excellent electrocatalytic performance toward the hydrogen evolution reaction (HER) in acidic condition. The HER activity of Pd13Cu3S7 NPs with its overpotential as low as 64 mV at –10 mA cm-2 is superior to those of amorphous PdCuS and commercial Pd/C catalysts, demonstrating the importance of nanocrystal crystallinity in boosting the HER activity. They also exhibit excellent stability as compared to commercial Pt/C and Pd/C under strongly acidic conditions.

■ INTRODUCTION Post-synthetic treatments of colloidal nanocrystals have led to discovery of novel nanostructures with great compositional, structural, and morphological diversity.1-4 Cation exchange, in particular, is regarded as one of the most powerful post-synthetic modification techniques to change the composition of nanostructures while maintaining the initial morphologies.5-8 Successful cation exchange reaction requires anion frameworks possessing similar unit cell parameters between original and final nanostructures.9-11 Therefore, rational synthesis of complex heterostructures via cation exchange could be achieved only through a thorough consideration of the crystal structures of the starting nanostructure and the desired final product. Palladium-based clusters or sulfides such as Pd4S and PdS have shown good electrocatalytic performances toward the hydrogen evolution reaction (HER) or oxygen reduction reaction (ORR).12-15 Also, amorphous PdCuS nanoparticles synthesized via dealloying process show good catalytic activity and stability for the HER.16 Crystallinity of nanoparticles has been demonstrated as a critical factor to affect the electrocatalytic performance.17-20 For instance, highly crystalline MoS2 nanoparticles show better electrocatalytic activity and stability for the HER compared to amorphous MoS2 nanoparticles, because the catalytic instability is caused by the oxidation of poorly crystalline catalyst under the ambient oxygen-rich condition.17 Therefore, it is highly intriguing to synthesize highly crystalline PdCuS nanocrystals and to investigate the crystallinity effect on catalytic activity and stability of the PdCuS phase. However, thus

far, the preparation of highly crystalline PdCuS nanoparticle has not yet been reported. To synthesize mixed metal sulfides such as Pd13Cu3S7 phase, S–Pd–S bonds should be formed, as evidenced by the structure of Pd16S7 phase, which has same coordination chemistry with Pd13Cu3S7 phase. Given the formation of S–S bond is energetically more favorable than that of the Cu-S bond,21 it is necessary to employ cation exchange template that does not necessitate the breakage of S–S bond during cation exchange. Among various Cu/S phases, Cu2-xS nanoplate system appears to be a promising template to form highly crystalline Pd–Cu–S nanostructures via cation exchange. As one of Cu2-xS family, the Cu1.94S phase contains a total of 62 Cu atoms in the unit cell; 52 Cu atoms show three-fold, triangular coordination with S, 9 Cu atoms tetrahedral coordination, and 1 Cu atom linear coordination.22 Importantly, the Cu1.94S has no S–S bonds in its crystal structure. The preserved S anion framework, due to the nonbreakage nature of S–S bonds, during cation exchange would be favorable to the formation of highly crystalline binary metal sulfide nanostructures. Herein, we report a facile synthesis of highly crystalline Pd13Cu3S7 nanoplates (NPs) via partial cation exchange of Cu1.81S NPs combined with concomitant reduction of Pd species, with Cu1.94S NPs and Pd13Cu3S7/Cu2-xS janus heterostructure as the intermediate phases. The Cu1.94S phase could be in situ formed from Cu1.81S phase, because these two phases with similar coordination chemistry can be easily interconverted depending on the relative thermodynamic stabilities under given

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environment.23-25 The in situ generated Cu1.94S phase was subsequently used as the template for the generation of Pd13Cu3S7/Cu2-xS heterostructure and Pd13Cu3S7 NPs. The highly crystalline Pd13Cu3S7 NPs show excellent electrocatalytic activity toward the HER, evidenced by its overpotential as low as 64 mV to drive a current density of –10 mA cm-2, which is superior to those of amorphous PdCuS and Pd/C catalysts. Furthermore, they exhibit very high stability under strongly acidic conditions, demonstrating the importance of crystalline mixed phase in boosting the HER activity and stability.

■ EXPERIMENTAL SECTION Reagents. CuSCN (copper(I) thiocyanate, 99%), Pd(acac)2 (palladium(II) acetylacetonate, 99%), CTAC (cetyltrimethylammonium chloride), and oleylamine (98%) were purchased from Sigma-Aldrich. All reagents were used as received without further purification. Material characterization. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) studies were carried out in a TECNAI G2 F30ST microscope and Tecnai G2 20 S-twin microscope. Aberration-corrected imaging and high spatial resolution energy dispersive X-ray spectroscopy (EDS) were performed at FEI Nanoport in Eindhoven using a Titan Probe Cs TEM 300kV with Chemi-STEM technology. EDS elemental mapping data were collected using a higher efficiency detection system (Super-X detector with XFEG); it integrates 4 FEI-designed Silicon Drift Detectors (SDDs) very close to the sample area. Compared to conventional EDX detector with Schottky FEG systems, Chemi-STEM produces up to 5 times the X-ray generation with the X-FEG, and up to 10 times the Xray collection with the Super-X detector. All scanning transmission electron microscopy (STEM) images and compositional maps were acquired with the use of high-angle annular dark field (HAADF)-STEM. Powder X-ray diffraction (PXRD) patterns were collected to understand the crystal structures of Pd13Cu3S7 nanocrystals with a Rigaku Ultima III diffractometer system using a graphite monochromatized Cu-Kα radiation at 40 kV and 30 mA. Metal contents in Pd13Cu3S7 NPs/C catalysts were determined by Inductively coupled plasma atomic emission spectroscopy (ICP-AES) analyzer (700 ES, Varian). Preparation of Pd13Cu3S7 NPs. The Cu1.81S hexagonal NPs were prepared by a previously reported method using CuSCN as precursor.26 72 mg of Cu1.81S NPs powder was dissolved in 15 mL oleylamine. A slurry of Pd(acac)2 (0.04 mmol, Aldrich, 99%), cetyltrimethylammonium chloride (CTAC) (0.04 mmol, Alfa Aesar), oleylamine (45 mmol, Aldrich 98%) and the 1 mL of Cu1.81S NPs solution were placed in a 100 mL Schlenk tube. After heating at 60 oC under vacuum for 5 min, the reaction mixture was charged with 1 atm Ar. The Schlenk tube was then immerged in an oil bath, which was preheated to 160 oC. After heating at 160 oC for 30 min, the reaction mixture was cooled to room temperature, washed several times with toluene and ethanol, and separated by centrifugation. Preparation of the carbon-supported catalyst. A suspension of 20 mg of the Pd13Cu3S7 NPs and 80 mg of carbon black (Vulcan XC 72) was dispersed in 20 mL of chloroform, and the mixture was then magnetically stirred and ultrasonicated for 5 min. After centrifugation, the resulting catalyst was re-dispersed in 20 mL of acetic acid and 20 ml of chloroform then heated at 60 oC for 2 h to clean the residual surfactants. The Pd13Cu3S7 NPs/C catalyst was washed with ethanol for three

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times and dried under vacuum. Pd loading of the Pd 13Cu3S7 NPs/C was obtained by ICP-AES. Preparation of working electrode. In order to prepare ink, 3.5 mg of the carbon-supported catalyst was measured and mixed with 480 μL of ethanol (anhydrous, Sigma-Aldrich), 500 μL of freshly made D.I. water, and 20 μL of Nafion (5 wt%, Sigma-Aldrich). The mixture solution was under ultrasonication at least 30 min in an ice bath. Rotating disk electrode (RDE) was polished with 1 μm size of diamond suspension and 0.05 μm size of alumina suspension. 2 μL of the ink was dropped on the polished RDE (0.071 cm2) and spun at 800 rpm for 10 min. After that, the electrode was put in an oven at 60 oC for 5 min. Electrochemical characterization. Electrochemical characterization was performed using a CHI 7007E (CH Instruments) electrochemical analyzer at room temperature in 0.5 M H2SO4 (96%, Suprapur grade, Merck). A three-electrode system was built, and a graphite rod and Ag/AgCl (filled with saturated KCl) were used as the counter electrode and reference electrode, respectively. All data were presented after the conversion to reversible hydrogen electrode (RHE) scale by measuring the open circuit potential of Ag/AgCl with homemade RHE (H+|H2 equilibrium on Pt electrode).27 Before electrochemical measurements, 0.5 M H2SO4 was purged with highly pure N2 gas (99.9999%) for 15 min to remove impurities. Cyclic voltammetry (CV) was performed in the potential range of 0.05 - 1.1 V at a scan rate of 200 mV/s for 20 cycles prior to the evaluation of HER activity. Electrochemical impedance spectroscopy (EIS) was conducted in a frequency range from 10 kHz to 1 Hz at -70 mV (vs. RHE) with the potential amplitude of 5 mV s-1 and x-intercept of the Nyquist plot at the high-frequency region was selected for iR-compensation. After then, 10 CV cycles were measured for stable hydrogen evolution performance. Finally, electrochemical activity was assessed by linear sweep voltammetry (LSV) with an electrode rotation (1600 rpm) at a scan rate of 5 mV s-1. In order to prepare ink, 3.5 mg of the carbon supported catalyst was measured and mixed with 480 μl of ethanol (anhydrous, Sigma Aldrich), 500 μL of freshly made D.I. water, and 20 μL of Nafion (5 wt%, Sigma Aldrich). The mixture solution was under ultrasonication at least 30 min in an ice bath. Electrochemically active surface area (ECSA) was assessed by measuring double layer capacitance in a potential range of 0.3-0.5 V (vs. RHE) with different scan rates (ν; 0.01, 0.02, 0.04, 0.08, 0.1, 0.14, 0.18, and 0.2 V s-1) in order to avoid hydrogen adsorption region. Centered currents were taken to evaluate the charging current (ic). The ECSA was derived from the following equations. ic = νCdl (1) ECSA = Cdl / C (2) In this work, 34 μF cm-2 was used for Cs to calculate ECSAs.28 Faradaic efficiency was evaluated by measuring and comparing a total quantity of evolved H2 gas of Pd13Cu3S7 NPs/C, Pt/C, and theoretical value.29, 30 To measure the quantity of evolved H2, inverted mass cylinder was placed above the carbon paper electrode (1 cm2) loaded 0.1 mg cm-2 of the catalyst and a constant current of 10 mA cm-2 was introduced for 7000 s, which resulted in the total passed electric charge of 70 C. Theoretically calculated total H2 production for 7000 s was 8.13 mL.


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■ RESULTS AND DISCUSSION Representative TEM images of Cu1.81S NPs and Pd13Cu3S7 NPs are shown in Figure S1a and Figure 1a, S1b. The Pd13Cu3S7 NPs have a hexagonal plate morphology, which originates from the Cu1.81S template. The Pd13Cu3S7 NPs are monodisperse with a diameter of 92.0 ± 5.4 nm, measured from diagonal distance, and a thickness 14.1 ± 0.5 nm, as shown in Figure S1c. The formation of Pd13Cu3S7 phase during the reaction leads to increase in the diameter and thickness of Pd13Cu3S7 NPs from those of Cu1.81S seed. The measured lattice distances in yellow rectangular dotted lines in the HRTEM (Figure 1b) are 0.459 nm, 0.369 nm, and 0.263 nm, which could be indexed to {200}, {211}, and {222} facets of face-centered-cubic (fcc) Pd13Cu3S7 phase, respectively. The fast Fourier transformation (FFT) pattern of the HRTEM image with a zone axis [110] is shown in the inset of Figure 1b. The diffraction pattern of {211}facet of fcc Pd13Cu3S7 phase is clearly visible, although the {211}facet of fcc cubic crystal structure could not be usually observed at a zone axis of [110] because of its relatively weak diffraction intensity. The side view HRTEM image in Figure S2 also indicates the [110] direction, which is perpendicular to the [1-1-3] zone axis. In order to understand the detailed atomic structure of Pd13Cu3S7 NPs, we studied the HAADF-STEM image and the schematic atomic models of the NP with [110] zone axis. The unique atom stacking sequences within the enlarged HAADF-STEM image, indicated by the dotted rectangle in Figure 1c, are consistent with the atom packing patterns. The line profile analysis and elemental mapping analysis of NPs (top view in Figure 1d, e; side view in Figure 1f, g) indicate that the hexagonal NPs are composed of crystalline bimetallic chalcogenide phase. The atomic composition of Pd13Cu3S7 NPs analyzed by EDS was found to be 59.0% Pd, 12.5% Cu, and 28.5% S (Pd4.7Cu1S2.3) (Figure S3), which is similar to the theoretically calculated atomic ratio (Pd4.3Cu1S2.3).

In order to elucidate the formation mechanism of Pd13Cu3S7 NPs, we analyzed the TEM images and PXRD patterns of reaction intermediates as shown in Figure 2. TEM image of the 5 min intermediate (Figure 2a) shows that the formation of Pd13Cu3S7 ternary phase is initiated at the corner or edge site of the Cu1.81S seed. The HAADF-STEM image, corresponding elemental mapping, and line profile analysis of the 5 min intermediate indicate that the initially formed dark region is composed of Pd13Cu3S7 phase (Figure S4). The small amount of Pd13Cu3S7 phase makes it difficult to be detected through PXRD analysis. The PXRD pattern of the 5 min intermediate indicates the presence of both Cu1.81S phase and non-marginal amount of Cu1.94S phase (Figure 2d). It is known that Cu1.81S phase could be transformed to more thermodynamically stable Cu 1.94S phase.9 However, the minimization of strain energy renders the reverse phase transition from Cu1.94S to Cu1.81S more facile.8, 2325 The finding of the transition from Cu1.81S to Cu1.94S in our study might be explained by the fact that the formation of Pd13Cu3S7 phase requires the cleavage of S–S bonding in order to incorporate Pd atoms; the average S–S bonding length of Pd13Cu3S7 (0.459 nm) is closer to Cu1.94S phase (0.401 nm) than that of Cu1.81S (0.387 nm). Hence, Cu1.94S phase is expected to show more facile kinetics than Cu1.81S phase toward the transformation into the ternary phase of Pd 13Cu3S7 because it would require lower lattice transformation energy, and this might explain the observed unusual transition from Cu 1.81S to Cu1.94S. After 10 min of reaction, the dark contrast region of Pd 13Cu3S7 ternary phase gradually increased, forming heterostructures with grain boundaries between copper sulfide phase and Pd13Cu3S7 phase (Figure 2b). The transformation of copper sulfide phase into Pd13Cu3S7 could be also detected through the HRTEM image (Figure S5) and PXRD pattern (Figure 2d); among three different diffraction patterns detected, one is from the copper sulfide phase and the others are from the newly formed Pd13Cu3S7. At 20 min of reaction, most of copper sulfide regions are transformed to Pd13Cu3S7 phase (Figure 2c).

Figure 1 Characterization of Pd13Cu3S7 NPs. a) TEM image and b) HRTEM image of Pd13Cu3S7 NPs (inset: corresponding its FFT pattern). The zone axis is [110] direction. c) STEM image and unit cell model of Pd13Cu3S7 NPs. d, f) HAADF-STEM image, line profile analysis and e, g) corresponding elemental mapping analysis of Pd13Cu3S7 NPs from d, e) top view and f, g) side view, respectively.



Figure 2 TEM images of intermediates with controlling the reaction time. a) 5 min, b) 10 min, and c) 20 min. PXRD analysis depending on reaction time from 0 min (seed) to 40 min.

The dark contrast region in Figure 2c is due to the formation of Pd13Cu3S7 phase, which could be confirmed through the HRTEM analysis (Figure 3a-d). The predominant products are heterostructures composed of two domains that could be identified as Pd13Cu3S7 and Cu1.94S phases, respectively. The FFT patterns of the 20 min intermediate (Figure 3c, d) also demonstrate the existence of two different material phases, which both have well-defined single crystalline structural features. A similar anisotropic diffusion of palladium into copper sulfide was previously documented through one-pot synthesis method, but the resulting nanostructure was amorphous. 31 Therefore, the presence of well-defined copper sulfide template is critical to the preparation of the highly crystalline ternary alloy phase. The atomic arrangements of Cu1.94S and CuS are similar as shown in Figure S6, therefore, we expected that the coordination chemistry of Cu and S might induce the different crystallinity of PdCuS phase when Pd cations are inserted into the templates. CuS (Covellite) phase has 6 CuS units in the unit cell; 4 Cu atoms have tetrahedral coordination, 2 Cu atoms have triangular coordination, whereas 4 of the S atoms form disulfide bonds and 2 are single sulfide ions.32 As already described in introduction part, the formation of highly crystalline Pd 13Cu3S7 phase only occur when the Cu2-xS template is used. After 40 min of reaction, only Pd13Cu3S7 phase is observed, indicating the completion of the binary metal sulfide phase formation within 40 min. The evolution of PXRD patterns during the synthesis result suggests that the phase conversion from Cu 1.81S to Pd13Cu3S7 is very fast and the appearance of Cu1.94S intermediate phase might be indispensable in order to minimize the metal diffusion energy barrier for the transformation between two different crystal systems.

Figure 3 a) HRTEM image of reaction intermediates at 20 min. b) Enlarged HRTEM of white dotted square section in panel (a) (inset: corresponding FFT pattern). Enlarged HRTEM and FFT patterns of c) Pd13Cu3S7 phase and d) Cu1.94S phase, respectively. Schematic illustration of Pd13Cu3S7 unit cell and simulated FFT pattern of zone axis at [110] and Cu1.94S unit cell and simulated FFT pattern of zone axis at [110].

In overall, we found that Pd(acac)2 reacts with triclinic Cu1.81S hexagonal NPs at the temperature range in which the Pd precursor undergoes both cation exchange reaction and thermal decomposition with copper cation, to form highly crystalline fcc Pd13Cu3S7 NPs. At the early reaction stage, the initial triclinic Cu1.81S phase is transformed into monoclinic Cu1.94S, in which the S atoms favor the hexagonal-close-packing (hcp) arrangement and the Cu atoms are situated in the interstices. The similar S–S binding lengths in the anion frameworks of Cu 1.94S and Pd13Cu3S7 might facilitate the formation of fcc Pd13Cu3S7 phase. The overall synthetic pathway is schematically shown in Scheme 1. We further examined the changes in the chemical states of reaction intermediates through X-ray photoelectron spectroscopy (XPS) analysis (Figure 4). The Pd 3d XPS peaks are gradually shifted to lower binding energy with progress of the reaction, indicating the gradual evolution of metallic Pd species. Therefore, it appears that Pd(II) replaces Cu ions via cation ex-

Scheme 1 Schematic illustration of synthetic pathway of Pd13Cu3S7 NPs from Cu1.81S template.


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Figure 4 a) Pd 3d, b) Cu 2p, and c) S 2p XPS spectra for Pd13Cu3S7 NPs intermediates from seed to 40 min.

change initially but some of them can be further reduced to metallic Pd(0). Deposition of Pd(0) species under reducing reaction condition and infiltration into the metal sulfide matrix might be also responsible for the shift of Pd 3d peaks. In contrast to the trend of Pd 3d XPS spectra, the Cu 2p XPS peaks are gradually shifted to higher binding energy as compared to initial states. This shift might have resulted from the fact that small amounts of Cu(I) species in the structure are changed to Cu(II) species in order to provide driving force for reducing Pd(II) ions. On the other hand, the Cu(II) in the solution, liberated from nanoparticle following the cation exchange under reducing condition, can be converted back to Cu(I), which can again react with the remaining copper sulfide phase in the nanoparticle; the transformation of Cu1.81S to Cu1.94S observed in PXRD patterns might be explained by this. It was previously reported that sulfur rich Cu1.1S NPs are changed to Cu2S via reaction of Cu(I) with Cu1.1S.33 In S 2p XPS spectra, the peaks are shifted to lower binding energy upon the addition of Pd precursor. The S 2p satellite peak from near the 167 eV indicates the sulphone (SO42-) complex, which can be easily formed under air during the sample preparation. The two separate peaks originating from both Cu2S (161.92 eV) and CuS (162.1 eV) are clearly shown at 5 min and 10 min. Finally, the two peaks merge into a broad peak, reflecting the complex sulfur oxidation state change during the reaction. This process should include making new Pd‒S bonding and breaking the existing Cu‒S bonding. We also examined the effect of amount of Pd precursor on the product morphology and composition. When 0 equiv of Pd precursor was used, there was no phase change (Figure 5a and S7). Therefore, the low temperature annealing of Cu 1.81S seed in the presence of CTAC could not generate the phase of Cu1.94S. On the other hand, when 0.5 equiv of Pd precursor was used, we could observe Janus-like heterostructures, which has the same structural feature of reaction intermediates at 10 min under original reaction condition. With 2 equiv of Pd precursor,

Figure 5 a-c) The typical TEM images of Pd13Cu3S7 NPs synthesized by controlling the amount of Pd precursor. d) PXRD patterns of samples from Cu1.94S to Pd13Cu3S7 NPs using a variety of different amount of Pd precursor. Standards for Cu1.94S (JCPDS card no. 00-023- 0958, green line), Pd13Cu3S7 (JCPDS card no. 01-0752229, blue line), and Pd (JCPDS card no. 01-088-2335, orange line) are displayed.

Pd metal nanoparticles are formed as side products in addition to Pd13Cu3S7 NPs; the broad peaks at 40o, 46o and 68o in PXRD are assigned to pure Pd phase. We then examined the product patterns at different reaction temperatures. TEM images of reaction temperature dependent products are shown in Figure S8. At 80 oC, the decomposition of Pd precursor was not feasible due to the low thermal energy, and therefore, no significant changes were observed in the Cu1.81S seed (Figure S8a). At 100 oC, the growth of small Pd islands occurred on the corner sites of Cu 1.81S seed (Figure S8b). Additional growth of second metal was previously observed to take place on active sites of substrate nanoparticles such as corners or edges.34-37 At 120 oC, nonspecific, random growths of irregular islands were observed on the Cu1.81S seed (Figure S8c). At 140 oC, Janus-like Pd13Cu3S7/Cu2-xS heterostructures were observed (Figure S8d). Interestingly, the thickness of Pd-infiltrated part appears to be uniform and there is a totally unchanged copper sulfide part. The entire exposed Cu 1.94S surface was converted to Pd13Cu3S7 at 160 oC and above (Figure S8e). However, the structural deformation at corner sites was observed above 160 oC, although the CTAC was used as surfacestabilization agent (Figure S8f). As a result, the ability of CTAC capping agent for metal sulfide shape control disappears at temperatures higher than 160 oC. On the other hand, we observed a complete lack of crystallinity in the mixed metal sulfide nanoparticles when they were directly synthesized via co-reduction of Pd(acac)2, CuSCN, and CTAC at 240 oC. The broad PXRD feature in the range of 30-



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Figure 6 a) Polarization curves of Pd13Cu3S7 NPs/C (red), A-PdCuS/C (green), Cu2-xS/C (blue), commercial Pt/C (black), and commercial Pd/C (orange). b) Comparison of mass activities (jm) and specific activities (js) of Pd13Cu3S7 NPs/C, A-PdCuS/C and commercial Pd/C at 100 mV of overpotential. c) Tafel plots for HER in 0.5 M H2SO4 with a scan rate of 5 mV/s. d) Potential-time curves of Pd13Cu3S7 NPs/C, Pt/C, and Pd/C for 24 h at a constant current density of 10 mA cm-2.

45o was driven from the amorphous PdCuS (A-PdCuS) nanostructure.38 The broad PXRD peaks and ring patterns of FFT in HRTEM analysis (Figure S9) confirm the presence of amorphous phase. Minor sharp PXRD peaks and FFT spot patterns in HRTEM analysis are plausibly from the separately formed minor Cu1.94S phase, although they were not observed in TEM image. In addition, the line profile analysis and elemental mapping analysis in Figure S10 indicate the existence of PdCuS ternary composition over the entire nanoparticle. Therefore, we speculate that the PdCuS ternary phase with a high crystallinity can be obtained only through the controlled Pd atom deposition and diffusion to pre-synthesized crystalline Cu1.94S template, which would involve breakage of Cu–S and S–S bonds in order to make Pd–Cu–S bonds. We assessed electrochemical performances of Pd13Cu3S7 NPs for the HER in 0.5 M H2SO4 (Merck, Suprapur grade). For this purpose, Pd13Cu3S7 NPs, A-PdCuS, and Cu2-xS were supported on carbon black (Vulcan XC 72) to prepare Pd 13Cu3S7 NPs/C, A-PdCuS/C, and Cu2-xS/C catalysts. The electrocatalytic activities of commercial Pd/C (20% on Vulcan XC 72, Premetek) and Pt/C (HiSpec 2000, Johnson Matthey Fuel Cells) catalysts were also measured for benchmark purpose. ICP-AES analysis reveals that Pd13Cu3S7 NPs/C and A-PdCuS/C catalysts contain 15.5 wt% and 11.3 wt% of Pd, respectively (Table S1). Threeelectrode set-up was used with Ag/AgCl (filled with saturated KCl) and graphite rod as a reference and a counter electrode,

respectively. As shown in Figure 6a, polarization curves present an excellent HER performance of Pd13Cu3S7 NPs/C, which required overpotential of only 64.0 mV to reach 10 mA cm-2, while Pd/C needed more overpotential with the value of 96.6 mV. A-PdCuS/C required more overpotential than Pd 13Cu3S7 NPs/C, with the value of 107.6 mV at 10 mA cm-2. On the other hand, Cu2-xS/C shows a very poor hydrogen evolution activity, indicating the crucial role of Pd in driving HER. Mass and specific activities of Pd13Cu3S7 NPs/C, A-PdCuS/C, and Pd/C are shown in Figure 6b (left for mass activity, jm and right for a specific activity, js). Pd13Cu3S7 NPs/C shows 3.5 times and 5 times higher mass activity per mass of Pd (2.03 A mg-1Pd) than A-PdCuS/C (0.58 A mg-1Pd) and Pd/C (0.31 A mg1 Pd) at 100 mV of overpotential. ECSAs were calculated by measuring double layer capacitance (Figure S11). ECSAs of Pd13Cu3S7 NPs/C, A-PdCuS/C and Pd/C were 8.31, 9.90, and 12.22 cm2, respectively. Pd13Cu3S7 NPs/C also exhibits 5.7 and 7.5 times higher specific activity (265.58 μA cm-2) than APdCuS/C (46.91 μA cm-2) and Pd/C (35.47 μA cm-2). The superior mass and specific activities of Pd13Cu3S7 NPs/C indicate that the crystalline surface structure of Pd 13Cu3S7 NPs/C appears to promote the hydrogen evolution. Tafel plots of the catalysts also suggest a great intrinsic activity of Pd13Cu3S7 NPs/C as shown in Figure 6c. Pd13Cu3S7 NPs/C, A-PdCuS/C, Cu2-xS/C, commercial Pd/C, and Pt/C have Tafel slopes of 49.6 mV dec–1, 52.5 mV dec–1, 128.0 mV dec–1,


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Chemistry of Materials 63.9 mV dec–1, and 26.4 mV dec–1, respectively. According to the values, Pd13Cu3S7 NPs/C and A-PdCuS/C follow similar HER mechanisms while Cu2-xS/C has slower reaction kinetics. Overall, to the best of our knowledge, the Pd13Cu3S7 NPs/C ranks as a highly active catalyst among the best sulfide-based catalysts for the HER regarding both overpotential and Tafel slope (Table S2). HER proceeds through a series of three elementary reaction steps in acid.39-41 The first step is [1] Volmer reaction, which is the dissociative adsorption of H2 on the catalyst surface. The second step is [2] Heyrovsky or [3] Tafel reaction. H3O+ + e- → Had + H2O

Volmer reaction

[1]

Had + H+ + e- → H2

Heyrovsky reaction

[2]

Had + Had → H2

Tafel reaction

[3]

Volmer reaction involves a dynamic contact between catalyst and reactant, which is considered as a rate-determining step (RDS) because of its slow kinetics. Pd is well-known for its strong hydrogen affinity and is expected to increase the kinetics for Volmer reaction. Björketun et al. studied a correlation between surface coverage of Pd on a substrate metal (Au) and HER activity and showed that the Au surface covered with 60% of Pd exhibited the best exchange current density. 42 However, Pd alone is seldom considered as a great catalyst for HER.43, 44 In our study, according to the surface structure of Pd 13Cu3S7 NPs, a surface atomic portion of Pd in the nanostructure is about 65%, which is comparable to the optimum Pd coverage for the HER demonstrated in the previous work.42 Tafel slope analysis implies that Pd13Cu3S7 NPs/C follows Volmer-Heyrovsky pathway,45 which is fast hydrogen adsorption (Volmer reaction) and slow discharge reaction by the combination between adsorbed hydrogen and proton (Heyrovsky reaction). Pd/C has a higher Tafel slope than Pd13Cu3S7 NPs/C, plausibly due to too strong binding of hydrogen on the Pd surface. On the other hand, Cu 2xS/C showed the highest Tafel slope among the compared catalysts, suggesting that Volmer step limits its HER. Therefore, the high activity of Pd13Cu3S7 NPs/C toward HER could be explained by the optimal surface coverage of Pd and the crystalline surface. As shown in Figure S12, the Faradaic yields of Pd13Cu3S7 NPs/C and Pt/C were evaluated by measuring evolved H2 gas at a cathodic current of 10 mA cm-2 for 7000 s (70 C). Under an electric charge of 70 C, Pd13Cu3S7 NPs/C and Pt/C produced 8.0 mL and 8.1 mL of H2 gas, respectively, which are similar to the theoretical value of 8.1 mL, indicating that Pd13Cu3S7 NPs/C shows near 100% Faradaic efficiency for the HER. Stability tests of Pd13Cu3S7 NPs/C, Pd/C, and Pt/C were evaluated by chronopotentiometry (CP) at 10 mA cm-2 for 24 h in 0.5 M H2SO4 (Figure 5d). The overpotentials of commercial Pt/C and Pd/C after the stability test increased dramatically within 8 h due to the aggregation or the detachment of Pt catalysts from supporting carbon.46-48 In contrast, Pd13Cu3S7 NPs/C underwent much smaller increase in overpotential. These control experiments clearly demonstrate that Pd13Cu3S7 NPs has better stability than commercial noble metal catalysts. Although, the activity of Pd13Cu3S7 NPs/C decreased, it surpassed Pt/C within 4 hours, indicating stable long-term hydrogen evolution performance in an acidic electrolyte. TEM images revealed that

the morphology of Pd13Cu3S7 NPs remains intact after the stability test (Figure S13). However, due to the aggregation of NPs, the catalytic performance has been compromised to a certain degree. Moreover, the PXRD pattern of Pd 13Cu3S7 NPs/C after the stability test (Figure S14) shows that Pd13Cu3S7 NPs/C preserved its crystal phase of Pd13Cu3S7, demonstrating the robustness of the catalyst under the acidic condition. XPS analysis after stability test might explain the changes in electrocatalytic performance during operation (Figure S15). As shown in Figure S15a, Pd 3d peaks after the stability test was slightly shifted to lower binding energy region, which means an increase of the Pd(0) character. The S 2p satellite peak from near the 167 eV indicates the presence of sulphone (SO42-) complexes, which can be formed during the stability test (Figure S15b). However, it is difficult to detect the existence of the Cu phase by Cu 2p peak (Figure S15c). To identify the change of electronic states of Pd and S species, we performed deconvolution of the XPS results as shown in Figure S16. The peaks at 335.6 and 340.9 eV are attributed to Pd(0), and the peaks located at 336.4 and 341.7 eV are matched to Pd(II) states. As shown in Figure S16a, a significant amount of Pd 3d of the Pd13Cu3S7 NPs reflects zero oxidation state. It means abundant d valence electrons exist at Pd sites, which promotes M–H binding.49 The XPS results support that the electron transfer from Pd to Cu atoms at the original Pd13Cu3S7 might affect the binding strength of hydrogen and exposed active Pd sites, which leads a great HER performance of Pd13Cu3S7 NPs. In Figure S16b, the binding energy of 163.2, 162.0, and 161.4 eV are attributed to bridging S22-, S2-, and S species, respectively. The existence of S22- and/or S2- species has been linked to sites enhanced electrocatalytic performance.50

■ CONCLUSION We have demonstrated that highly crystalline Pd13Cu3S7 NPs can be synthesized by reacting Cu1.81S template with Pd(acac)2 under reducing solvent of oleylamine. Upon cation exchange reaction with the Pd precursor, Cu 1.81S NPs are sequentially transformed to Cu1.94S NPs, Pd13Cu3S7/Cu2-xS Janus heterostructure, and Pd13Cu3S7 NPs with well-developed facets. Highly crystalline Pd13Cu3S7 NPs show overpotential of only 64.0 mV at –10 mA cm-2 and Tafel slope of 49.6 mV dec-1 in 0.5 M H2SO4, thus ranking among the best sulfide-based catalysts for the HER. The HER activity of Pd 13Cu3S7 NPs is superior to amorphous PdCuS and Pd/C catalysts, highlighting the importance of crystalline facets as well as bimetallic synergy. Furthermore, Pd13Cu3S7 NPs show excellent electrochemical robustness in acidic condition; little deterioration in catalytic activity was observed during 24 h of stability test. This implies a wide opportunity in developing facet-controlled mixed metal sulfides as HER or oxygen evolution reaction (OER) catalysts. Currently we are further exploring the synthesis of various mixed metal sulfide nanocrystals for achieving enhanced and durable electrocatalytic performance.

■ ASSOCIATED CONTENT Supporting Information. This supporting information is available free of charge on the ACS Publications websites via the Internet at http://pubs.acs.org. Figures S1- S16 and tables S1 – S2 give more details on characterization of our synthesized materials and their electrocatalytic performance data. For example, additional TEM, line profile analysis,



HRTEM, XRD, elemental mapping analysis, EDS, ICP-AES, and electrocatalytic performance data are given.

■ AUTHOR INFORMATION Corresponding Author * (S. H. J.) E-mail: [email protected] * (K. Y. L.) E-mail: [email protected]

Author Contributions # J.

Park and H. Jin contributed equally to this work.

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENT This work was supported by NRF-2017R1A2B3005682, KBSI project E37300, Korea University Future Research Grant, and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1A6A3A01008861, 2018R1A6A3A01013426). S.H.J. was supported by the National Research Foundation of Korea (NRF2017R1A2B2008464). H. Jin acknowledges the Global Ph.D. Fellowship (NRF-2015H1A2A1033447). The authors thank Korea Basic Science Institute (KBSI) for the usage of their HRTEM instrument.

■ REFERENCES (1) Fenton, J. L.; Steimle, B. C.; Schaak, R. E. Tunable Intraparticle Frameworks for Creating Complex Heterostructured Nanoparticle Libraries. Science 2018, 360, 513-517. (2) Fayette, M.; Robinson, R. D. Chemical Transformations of Nanomaterials for Energy Applications. J. Mater. Chem. A 2014, 2, 59655978. (3) Moon, G. D.; Ko, S.; Min, Y.; Zeng, J.; Xia, Y.; Jeong, U. Chemical Transformations of Nanostructured Materials. Nanotoday 2011, 6, 186-203. (4) Buck, M. R.; Schaak, R. E. Emerging Strategies for the Total Synthesis of Inorganic Nanostructures. Angew. Chem. Int. Ed. 2013, 52, 6154-6178. (5) Trizio, L. D.; Manna, L. Forging Colloidal Nanostructures via Cation Exchange Reactions. Chem. Rev. 2016, 116, 10852-10887. (6) Park, J.; Park, J.; Lee, J.; Oh, A.; Baik, H.; Lee, K. Janus Nanoparticle Structural Motif Control via Asymmetric Cation Exchange in Edge-Protected Cu1.81S@IrxSy Hexagonal Nanoplates. ACS Nano 2018, 12, 7996-8005. (7) Gariano, G.; Lesnyak, V.; Brescia, R.; Bertoni, G.; Dang, Z.; Gaspari, R.; Trizio, L. D.; Manna, L. Role of the Crystal Structure in Cation Exchange Reactions Involving Colloidal Cu2Se Nanocrystals. J. Am. Chem. Soc. 2017, 139, 9583-9590. (8) Powell, A. E.; Hodges, J. M.; Schaak, R. E. Preserving Both Anion and Cation Sublattice Features during a Nanocrystal Cation-Exchange Reaction: Synthesis of Metastable Wurtzite-Type CoS and MnS. J. Am. Chem. Soc. 2016, 138, 471-474. (9) Ha, D. –H.; Caldwell, A. H.; Ward, M. J.; Honrao, S.; Mathew, K.; Hovden, R.; Koker, M. K. A.; Muller, D. A.; Hennig, R. G.; Robinson, R. D. Solid-Solid Phase Transformations Induced through Cation Exchange and Strain in 2D Heterostructured Copper Sulfide Nanocrystals. Nano Lett. 2014, 14, 7090-7099. (10) Li, H.; Brescia, R.; Povia, M.; Prato, M.; Bertoni, G.; Manna, L. Moreels, I. Synthesis of Uniform Disk-Shaped Copper Telluride Nanocrystals and Cation Exchange to Cadmium Telluride Quantum Disks with Stable Red Emission. J. Am. Chem. Soc. 2013, 135, 1227012278. (11) van der Stam, W.; Berends, A. C.; Rabouw, F. T.; Willhammar, T.; Ke, X.; Meeldijk, J. D.; Bals, S.; de Mello Donega, C. Luminescent

CuInS2 Quantum Dots by Partial Cation Exchange in Cu2-xS Nanocrystals. Chem. Mater. 2015, 27, 621-628. (12) Du, C.; Li, P.; Yang, F.; Cheng, G.; Chen, S.; Luo, W. Monodisperse Palladium Sulfide as Efficient Electrocatalyst for Oxygen Reduction Reaction. ACS Appl. Mater. Interfaces 2018, 10, 753-761. (13) Barawi, M.; Ferrer, I. J.; Ares, J. R.; Sánchez, C. Hydrogen Evolution Using Palladium Sulfide (PdS) Nanocorals as Photoanodes in Aqueous Solution. ACS Appl. Mater. Interfaces 2014, 6, 2054420549. (14) Gao, X.; Chen, W. Highly Stable and Efficient Pd6(SR)12 Cluster Catalysts for the Hydrogen and Oxygen Evolution Reactions. Chem. Commun. 2017, 53, 9733-9736. (15) Du, C.; He, S.; Liu, M.; Gao, X.; Zhang, R.; Chen, W. Novel Pd13Cu3S7 Nanotubes with High Electrocatalytic Activity towards both Oxygen Reduction and Ethanol Oxidation Reactions. CrystEngComm 2016, 18, 6055-6061. (16) Xu, W.; Zhu, S.; Liang, Y.; Cui, Z.; Yang, Z.; Inoue, A.; Wang, H. A Highly Efficient Electrocatalyst Based on Amorphous Pd-Cu-S Material for Hydrogen Evolution Reaction. J. Mater. Chem. A 2017, 5, 18793-18800. (17) Li, Y.; Yu, Y.; Huang, Y.; Nielsen, R. A.; Goddard III, W. A.; Li, Y.; Cao, L. Engineering the Composition and Crystallinity of Molybednum Sulfide for High-Performance Electrocatalytic Hydrogen Evolution. ACS Catal. 2015, 5, 448-455. (18) Li, Y.;Zhang, L. A.; Qin, Y.; Chu, F.; Kong, Y.; Tao, Y.; Li, Y.; Bu, Y.; Ding, D.; Liu, M. Crystallinity Dependence of Ruthenium Nanocatalys toward Hydrogen Evolution Reaction. ACS Catal. 2018, 8, 5714-5720. (19) Hai, X.; Zhou, W.; Chang, K.; Pang, H.; Liu, H.; Shi, L.; Ichihara, F.; Ye, J. Engineering the Crystallinity of MoS2 Monolayers for Highly Efficient Solar Hydrogen Production. J. Mater. Chem. A 2017, 5, 8591-8598. (20) Chua, C. S.; Ansovini, D.; Lee, C. J. J.; Teng, Y. T.; Ong, L. T.; Chi, D.; Andy Hor, T. S.; Raja, R.; Lim, Y. –F. The Effect of Crystallinity on Photocatalytic Performance of Co3O4 Water-Splitting Cocatalysts. Phys. Chem. Chem. Phys. 2016, 18, 5172-5178. (21) Gotsis, H. J.; Barnes, A. C.; Strange, P. Experimental and Theoretical Investigation of the Crystal Structure of CuS. J. Phys.: Condens. Matter. 1992, 4, 10461-10468. (22) Evans, H. T. Djurleite (Cu1.94S) and Low Chalcocite (Cu2S): New Crystal Structure Studies. Science 1979, 203, 356-358. (23) Zhang, H.; Zhang, Y.; Yu, J.; Yang, D. Phase-Selective Synthesis and Self-Assembly of Monodisperse Copper Sulfide Nanocrystals. J. Phys. Chem. C 2008, 112, 13390-13394. (24) Lim, W. P.; Wong, C. T.; Ang, S. L.; Low, H. Y.; Chin, W. S. Phase-Selective Synthesis of Copper Sulfide Nanocrystals. Chem. Mater. 2006, 18, 6170-6177. (25) Zhu, D.; Tang, A.; Peng, L.; Liu, Z.; Yang, C.; Teng, F. Tuning the Plasmonic Resonance of Cu2-xS Nanocrystals: Effects of the Crystal Phase, Morphology and Surface Ligands. J. Mater. Chem. C 2016, 4, 4880-4888. (26) Yoon, D.; Jin, H.; Ryu, S.; Park, S.; Baik, H.; Oh, S. J.; Haam, S.; Joo, C.; Lee, K. Scalable Synthesis of Djurleite Copper Sulphide (Cu1.94S) Hexagonal Nanoplates from a Single Precursor Copper Thiocyanate and Their Photothermal Properties. CrystEngComm 2015, 17, 4627-4631. (27) Sa, Y. J.; Seo, D. –J.; Woo, J.; Lim, J. T.; Cheon, J. Y.; Yang, S. Y.; Lee, J. M.; Kang, D.; Shin, T. J.; Shin, H. S.; Jeong, H. Y.; Kim, C. S.; Kim, M. G.; Kim, T. –Y.; Joo, S. H. A General Approach to Preferential Formation of Active Fe-Nx Sites in Fe-N/C Electrocatalysts for Efficient Oxygen Reduction Reaction. J. Am. Chem. Soc. 2016, 138, 15046-15056. (28) Łukaszewski, M.; Czerwiński, A. Electrochemical Preparation and Characterization of Thin Deposits of Pd-noble Metal Alloys. Thin Solid Films 2010, 518, 3680-3689. (29) Popczun, E.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 9267-9270.


Page 8 of 9

Page 9 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials (30) Kibsgaard, J.; Jaramillo, T. F. Molybdenum Phosphosulfide: An Active, Acid-Stable, Earth-Abundant Catalyst for the Hydrogen Evolution Reaction. Angew. Chem. Int. Ed. 2014, 53, 14433-14437. (31) Wang, Y.; Chen, Z.; Shen, R.; Cao, X.; Chen, Y.; Chen, C.; Wang, D.; Peng, Q.; Li, Y. Pd-dispersed CuS Hetero-Nanoplates for Selective Hydrogenation of Phenylacetylene. Nano Res. 2016, 9, 12091219. (32) Evans, H. T.; Konnert, J. A. Crystal Structure Refinement of Covellite. Am. Mineral. 1976, 61, 996-1000. (33) Xie, Y.; Riedinger, A.; Prato, M.; Casu, A.; Genovese, A.; Guardia, P.; Sottini, S.; Sangregorio, C.; Miszta, K.; Ghosh, S.; Pellegrino, T.; Manna, L. Copper Sulfide Nanocrystals with Tunable Composition by Reduction of Covellite Nanocrystals with Cu+ Ions. J. Am. Chem. Soc. 2013, 135, 17630-17637. (34) Wu, Y.; Wang, D.; Zhou, G.; Yu, R.; Chen, C.; Li, Y. Sophisticated Construction of Au Islands on Pt-Ni: An Ideal Trimetallic Nanoframe Catalyst. J. Am. Chem. Soc. 2014, 136, 11594-11597. (35) Jin, H.; Lee, K. W.; Khi, N. T.; An, H.; Park, J.; Baik, H.; Kim, J.; Yang, H.; Lee, K. Rational Synthesis of Heterostructured M/Pt (M=Ru or Rh) Octahedral Nanoboxes and Octapods and Their Structure-Dependent Electrochemical Activity Toward the Oxygen Evolution Reaction. Small 2015, 11, 4462-4468. (36) Kim, H.; Khi, N. T.; Yoon, J.; Yang, H.; Chae, Y.; Baik, H.; Lee, H.; Sohn, J. –H.; Lee, K. Fabrication of Hierarchical Rh Nanostructures by Understanding the Growth Kinetics of Facet-Controlled Rh Nanocrystals. Chem. Commun. 2013, 49, 2225-2227. (37) Khi, N. T.; Park, J.; Baik, H.; Lee, H.; Sohn, J. –H.; Lee, K. Facet-Controlled {100}Rh-Pt and {100}Pt-Pt Dendritic Nanostructures by Transferring the {100}facet Nature of the Core Nanocube to the Branch Nanocubes. Nanoscale 2015, 7, 3941-3946. (38) Xie, Y.; Chen, W.; Bertoni, G.; Kriegel, I. Xiong, M.; Li, N.; Prato, M.; Riedinger, A.; Sathya, A.; Manna, L. Tuning and Locking the Localized Surface Plasmon Resonances of CuS (Covellite) Nanocrystals by an Amorphous CuPdxS Shell. Chem. Mater. 2017, 29, 17161723. (39) Sheng, W.; Myint, M.; Chen, J. G.; Yan, Y. Correleating the Hydrogen Evolution Reaction Activity in Alkaline Electrolytes with the Hydrogen Binding Energy on Monometallic Surfaces. Energy Environ. Sci. 2013, 6, 1509-1512. (40) Greeley J.; Nørskov, J. K.; Kibler, L. A.; El-Aziz, A. M.; Kolb, D. M. Hydrogen Evolution over Bimetallic Systems: Understanding the Trends. ChemPhysChem 2006, 7, 1032-1035.

(41) Zhang, L.; Chang, Q.; Chen, H.; Shao, M. Recent Advances in Palladium-based Electrocatalysts for Fuel Cell Reactions and Hydrogen Evolution Reaction. Nano Energy 2016, 29, 198-219. (42) Björketun, M. E.; Karlberg, G. S.; Rossmeisl, J.; Chorkendorff, I.; Wolfschmidt, H.; Stimming, U.; Nørskov, J. K. Hydrogen Evolution on Au(111) Covered with Submonolayers of Pd. Phys. Rev. B 2011, 84, 045407. (43) Bhowmik, T.; Kundu, M. K.; Barman, S. Palladium Nanoparticle-Graphitic Carbon Nitride Porous Synergistic Catalyst for Hydrogen Evolution/Oxidation Reactions over a Broad Range of pH and Correlation of Its Catalytic Activity with Measured Hydrogen Binding Energy. ACS Catal. 2016, 6, 1929-1941. (44) Huang, B.; Chen, L.; Wang, Y.; Ouyang, L.; Ye, J. Paragenesis of Palladium-Cobalt Nanoparticle in Nitrogen-Rich Carbon Nanotubes as a Bifunctional Electrocatalyst for Hydrogen-Evolution Reaction and Oxygen-Reduction Reaction. Chem. Eur. J. 2017, 23, 7710-7718. (45) Merki, D.; Vrubel, H.; Rovelli, L.; Fierro, S.; Hu, X. Fe, Co, and Ni Ions Promote the Catalytic Activity of Amorphous Molybdenum Sulfide Films for Hydrogen Evolution. Chem. Sci. 2012, 3, 2515-2525. (46) Merki, D.; Hu, X. Recent Developments of Molybdenum and Tungsten Sulfides as Hydrogen Evolution Catalysts. Energy Environ. Sci. 2011, 4, 3878-3888. (47) Pu, Z.; Amiinu, I. S.; Kou, Z.; Li, W.; Mu, S. RuP2-Based Catalysts with Platinum-like Activity and Higher Durability for the Hydrogen Evolution Reaction at All pH Values. Angew. Chem. Int. Ed. 2017, 56, 11559-11564. (48) Liu, Y.; Mustain, W. E. Evaluation of Tungsten Carbide as the Electrocatalyst Support for Platinum Hydrogen Evolution/Oxidation Catalysts. Int. J. Hydrogen Energy 2012, 37, 8929-8938. (49) Xu, W.; Zhu, S.; Liang, Y.; Cui, Z.; Yang, Z.; Inoue, A.; Wang, H. A Highly Efficient Eelectrocatalyst Based on Amorphous Pd-Cu-S Material for Hydrogen Evolution Reaction. J. Mater. Chem. A 2017, 5, 18793-18800. (50) Chang, Y. –H.; Lin, C. –T.; Chen, T. –Y.; Hsu, C. –L.; Lee, Y. –H.; Zhang, W.; Wei, K. –H.; Li, L. –J. Highly Efficient Electrocatalytic Hydrogen Production by MoSx Grown on Graphene-Protected 3D Ni Foams. Adv. Mater. 2013, 25, 756-760.

Unprecedented Pd13Cu3S7 binary metal sulfide exhibits excellent electrocatalytic activity and durability toward hydrogen evolution reaction in acidic conditions.


Highly Crystalline Pd13Cu3S7 Nanoplates Prepared via Partial Cation

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