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Component-Controlled Synthesis of Necklace-like Hollow NiRu Nanoalloys as Electrocatalysts for Hydrogen Evolution Reaction Caihua Zhang, Ying Liu, Yingxue Chang, Yanan Lu, Shulin Zhao, Dongdong Xu, Zhihui Dai, Min Han, and Jianchun Bao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 08 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

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ACS Applied Materials & Interfaces

Component-Controlled Synthesis of Necklace-like Hollow NixRuy Nanoalloys as Electrocatalysts for Hydrogen Evolution Reaction Caihua Zhang,†,‡ Ying Liu,† Yingxue Chang,† Yanan Lu, † Shulin Zhao,† Dongdong Xu,† Zhihui Dai, † Min Han,†,┴,*, and Jianchun Bao†,* †

Jiangsu Key Laboratory of Biofunctional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, P. R. China ‡

┴

College of Science, Nanjing Forestry University, Nanjing 210037, P. R. China

State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China

ABSTRACT: Developing high-efficient and long-durable nanoalloy electrocatalysts toward hydrogen evolution reaction (HER) are highly desirable for implementation of water splitting technique to prepare clean fuels. Though great progress that has been achieved, controllable synthesis of hollow NixRuy nanoalloys with a wide component ratio range remains a challenge and their applications for HER have not been explored. Here, a series of necklace-like hollow NixRuy nanoalloys (Ni72Ru28, Ni63Ru37, Ni43Ru57, and Ni29Ru71) are prepared using the galvanic replacement reaction between the Ni nanochains and RuCl3·3H2O, and the hollowing process based on Kirkendall effect. Electrochemical tests reveal that those NixRuy nanoalloys can efficiently catalyze HER in acidic media. Among them, the Ni43Ru57 nanoalloy exhibits the highest catalytic activity with an overpotential of 41 mV to attain a current density of -10 mA

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cm-2, outperforming to other NixRuy nanoalloys and close to commercial Pt/C. Additionally, its current density will exceed Pt/C catalyst as the overpotential surpasses 102 mV. Moreover, such Ni43Ru57 nanoalloy also shows an exceptional durability that can continuously work for 8 hours only with a little loss of activity. Deduced from some featured spectroscopic and electrochemical analysis, the excellent catalytic performance of Ni43Ru57 nanoalloy is attributed to the proper component ratio and effective electronic coupling of Ni and Ru, causing the faster interfacial electron transfer kinetics and more available active sites on it compared with other NixRuy nanoalloy ones. KEYWORDS: Nanoalloys, hollow nanostructures, Kirkendall effect, Ni-Ru, component control, electrocatalysis, hydrogen evolution reaction

1. INTRODUCTION Hydrogen is the most promising energy carriers that can be used to replace fossil fuels.1,2 Electrolysis of water is believed to be an ideal avenue for producing high-purity H2 at largescale.3 For this purpose, the key lies on finding high-efficient and long-durable electrocatalysts to improve the sluggish kinetics of oxygen evolution reaction and reduce the overpotential of hydrogen evolution reaction (HER).4 Though Pt and Pt-based nanoalloys are the most efficient catalysts for HER in acidic media, the scarcity and high cost of Pt limit its large-scale commercial application, which urgently needs to develop cheap and robust non-Pt electrocatalysts for HER.5 Recently, several kinds of non-Pt nanostructured electrocatalysts toward HER have been developed, such as metal chalcogenides (e.g. MoS2,6 NiSe2,7 iron-nickel sulfide8) and related composites (e.g. MoS2/Au,9 CoSe2/CP,10 Fe1−xCoxS2/CNT11), metal phosphides (e.g.


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Ni2P,12 Ni12P5,13 MoP14) and related hybrids (e.g. CoP/CNT,15 MoP/S16, CoP/S17), metal carbides (e.g. Mo2C,18 Ni3C-GNRs19), metal nitrides (e.g. Co0.6Mo1.4N2,20 P-WN/rGO21), metal oxides (e.g. MoO2/rGO,22 WO2-CMNs23), and bimetallic or multi-metallic nanoalloys (e.g. NiAu/Au,24 CoNi@NC25). Among those non-Pt HER catalysts, nanoalloys have garnered much attention due to their good electrical conductivity and tunable surface electronic structure by engineering component, shape, defects or strains, facilitating to mediate the electrocatalytic activity and stability for HER. As one of the important platinum-group metals, Ru is cheaper than that of Pt, Pd, and Ir,26 and widely used in many industrially important reactions such as hydrogenation, ammonia synthesis, activation of C-H bond, and CO methanation.27-31 Recently, some unsupported or supported Ru nanostructures with well-defined size, morphology and phase structures have been synthesized, and their applications for dehydrogenation of ammonia borane, Fischer-Tropsch synthesis, lithium-oxygen batteries, and HER have been explored.32-37 However, as for HER, the activity of Ru nanostructures is still lower than that of commercial Pt/C catalyst,38,39 which needs to be further improved for practical application in electrolysis of water to produce H2. Previous studies have proven that combination of noble metals and other cheap transition metals to form nanoalloy is a promising avenue to develop advanced catalysts due to the synergistic effect and efficient electronic coupling of the two constituent metals, which may lead to the combined changes in the average energy of the surface d-band and in the width of the d-band.40 Among cheap transition metals, Ni is often used to catalyze HER in alkaline media,41,42 and easy to corrosion in acid environment. While Ru possesses better acid-resistance ability than Ni.43 Thus, combination of Ni and Ru to form Ni-Ru nanoalloy with variable component ratio is expected to create high-performance HER electrocatalyst in acidic media by fostering their strength and


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circumventing their weakness. Nonetheless, such two metal elements are difficult to form alloy at arbitrary atomic ratio because their crystal phase structure and atomic radius have a large difference. As for Ru, its hexagonal phase is thermodynamically stable, and cubic phase is metastable. While for Ni, its thermodynamically stable phase is the cubic phase, whose hexagonal phase is metastable. Moreover, the atomic radius of Ru is larger than that of Ni. To obtain desired Ni-Ru nanoalloy, the selection of proper synthetic method and control of component ratio are very vital. We note that some solid Ni-Ru nanoalloys with limited Ni/Ru molar ratio have been reported, but most of their applications focus on magnetism and hydrogenation of unsaturated organic compounds.43-46 To the best of our knowledge, there is nearly no report on hollow Ni-Ru nanoalloys, albeit hollow bimetallic nanostructures have unique structural merits and show greatly enhanced catalytic performance compared with their solid counterparts.47 Moreover, their electrocatalytic applications for HER are not fully explored. So, it is highly desirable but imperative to fabricate hollow Ni-Ru nanoalloys with tunable component ratios and examine their electrocatalytic properties toward HER. Here, we report the synthesis of a series of necklace-like hollow NixRuy nanoalloys (Ni72Ru28, Ni63Ru37, Ni43Ru57, and Ni29Ru71) and their electrocatalytic performance for HER. The typical synthesis is based on the galvanic replacement reaction between the Ni nanochains and RuCl3·3H2O. By fixing the amount of Ni nanochains and only adjusting the concentration of Ru precursors, the hollow NixRuy nanoalloys with variable Ni and Ru molar ratio can be obtained due to the Kirkendall effect. Compared with the related single-metal nanostructures, those NixRuy nanoalloys show greatly enhanced electrocatalytic performance toward HER in acid media. Among them, the Ni43Ru57 nanoalloy exhibits the highest HER activity with the overpotential of 41 mV to attain a current density of -10 mA cm-2, which is superior to other


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NixRuy nanoalloys and close to commercial Pt/C catalyst. As the overpotential reaches 102 mV, its current densities will surpass Pt/C catalyst. Moreover, the Ni43Ru57 nanoalloy can reserve 95.4% of its initial current density after continuously cycling for 8 h at a constant potential of 0.1 V (vs. RHE), showing an exceptional durability yet. As revealed by a series of spectroscopic and electrochemical analysis, the excellent catalytic performance of the Ni43Ru57 nanoalloy results from the proper component ratio and the effective electronic coupling of Ni and Ru, which lead to the faster interfacial electron transfer dynamics and more available active sites on it in relative to other NixRuy nanoalloy ones.

2. EXPERIMENTAL SECTION 2.1. Chemicals and materials. Ni(COOH)2·2H2O (99.9%, Alfa Aesar), RuCl3·3H2O (98%, Energy Chemical), terpilenol (AR, Shanghai Lingfeng Chemical Reagent Co. Ltd.), dodecylamine (> 98%, Sigma-Aldrich), 1-octadecene (90%, Sigma-Aldrich), heptane (≥ 97%, Sinopharm Chemical Reagent Co. Ltd.), ethylene glycol (≥ 99%, Sinopharm Chemical Reagent Co. Ltd.), and absolute alcohol (AR, Sinopharm Chemical Reagent Co. Ltd.) were used as received, unless otherwise stated. The commercial Pt/C (20%) catalyst was bought from Alfa Aesar. 2.2. Synthesis of Ni nanochains. The Ni nanochains were synthesized through the development and modification of our previous method. 48 For each synthesis, 800 mg of Ni(COOH)2·2H2O, 4 mL of terpilenol, 5 mL of dodecylamine, and 8 mL of 1-octodecene were added into a 250 mL three-neck flask in turn. Then, the above mixtures were heated to 120 °C with a heating rate of 3 o

C min-1, and kept at 120 oC for 1 h. Subsequently, the mixtures were further heated to 160 °C

and kept at 160 °C for 30 min. After naturally cooled down to 25 oC, the black precipitates were


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separated by centrifugation, and washed with heptane and absolute alcohol for several times. Finally, the black products were dried in vacuum at 40 oC for 2 h, and used as the sacrificial template for preparing NixRuy nanoalloys. 2.3. Synthesis of necklace-like hollow NixRuy nanoalloys and Ru nanostructures. The necklace-like hollow Ni43Ru57 nanoalloy was synthesized according to the following procedure: In a typical synthesis, 0.666 mmol of pre-synthesized Ni nanochains were dispersed in 10 mL of ethylene glycol (EG) to form a uniform suspension by heating from room temperature to 110 °C at a rate of 7 °C min-1 and kept at 110 °C for 10 min. Then, 10 mL of EG solution that dissolving 0.333 mmol of RuCl3·3H2O, was rapidly injected into the above suspension and let it react at 110 °C for 3 h. Finally, the reaction system was naturally cooled down to 25 oC. After separated by centrifugation and washed with absolute alcohol for several times, the black product was collected and dried in vacuum at 40 oC for 2 h. Keeping other conditions constant and only changing the amount of RuCl3·3H2O to let the Ni/Ru precursor molar ratio to be 4:1, 3:1, 1.8:1, and 1:2 respectively, the other NixRuy nanoalloys including Ni72Ru28, Ni63Ru37 and Ni29Ru71, and Ru nanostructures, could also be obtained. 2.4. Materials characterization. The X-ray energy dispersive spectra (EDS) were performed on a JSM-5610LV-Vantage energy spectrometer. The powder X-ray diffraction (XRD) patterns were recorded on a Smart Lab (9 kW) X-Ray diffractometer (Rigaku Corporation) equipped with the rotating anode Cu target (Kα radiation, λ= 1.54060 Å) and D/tex Ultra 250 high speed detector. And the related working voltage and current were 45 kV and 200 mA, respectively. The MDI Jade 5.0 software was employed to analyze the acquired diffraction data. The Induction Couple Plasma (ICP) tests were performed on Jarrel-Ash 1100+2000 Quantometer. The transmission


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electron microscopy (TEM) images and high-resolution transmission electron microscopy (HRTEM) images were acquired on JEOL-2100F apparatus. Further high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) images and elemental mapping analysis were carried out on FEI, Talos F200X apparatus. For TEM, HRTEM and elemental mapping analysis, the accelerating voltages that used for each test were 200 kV. The X-ray photoelectron spectra (XPS) were taken on a scanning X-ray microprobe (PHI 5000 Versa, ULACPHI, Inc.) using Al Kα radiation. The electrochemical impedance spectra (EIS) tests were executed on CHI 660E electrochemical workstation in 0.5 M H2SO4 solution at 298 K by applying a 10 mV potential modulation at the overpotential of 170 mV for HER. The EIS data were recorded within the frequency range of 10-1-106 Hz. The fitting of the measured data were carried out using the Zsimpwin software. 2.5. Electrocatalytic tests. The electrocatalytic experiments were performed on CHI 660E electrochemical workstation using 0.5 M H2SO4 solution as the electrolyte. A standard threeelectrode electrochemical cell was employed for all the tests. The graphite rod electrode, saturated calomel electrode (SCE) and the NixRuy nanoalloys or other control catalysts (initial Ni nanochains, hollow Ru nanostructures and Pt/C) modified glassy carbon electrodes (GCE) were used as the counter electrode, reference electrode and working electrode, respectively. For comparison, the platinum gauze electrode was also employed as the counter electrode to carry out the catalytic test. The working electrode was fabricated as follows: 2 mg of the NixRuy nanoalloys or other control catalysts were dispersed into 2 mL of water/ethanol solution (3:1 v/v) under ultrasonication to form a homogeneous catalyst ink. Then, 20 µL of each catalyst ink, 5 uL of 1 wt % Nafion solution were dropped onto the surface of pre-cleaned GCE (3 mm in diameter) and dried at ambient condition. The loading amount of all the catalysts on GCE surface was 0.28


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mg cm-2. Before the electrocatalytic experiments, continuous cyclic voltammetry sweeps were operated to remove trace of EG from the surface of those NixRuy nanoalloy catalysts. After that, the HER performances of those NixRuy nanoalloy catalysts were evaluated using linear sweep voltammetry (LSV) with a scan rate of 5 mV s-1. It should be mentioned that all the polarization curves were corrected for iR contribution within the cell using the iR automatic compensation function of the electrochemical workstation. All potentials reported in this study, were calibrated to reversible hydrogen electrode (RHE). The chronoamperometric measurement was carried out in 0.5 M H2SO4 solution by applying a potential pulse at -0.1 V (vs. RHE) during 8 h at 298 K. To prevent the formation of bubbles, the magnetic stirring by a stir bar with the rotation rate of 1000 rpm was employed for the LSV and chronoamperometric experiments. For all the tests, the electrolyte solution was purged with high purity N2 gas.

3. RESULTS and DISCUSSION 3.1 Component and microstructure characterization To describe briefly, the Ni43Ru57 nanoalloy will be taken as a typical example to introduce the detailed analysis and characterization process. Its component, crystallinity and microstructure are characterized by EDS, ICP, XRD, TEM, HRTEM, HAADF-STEM and elemental mapping analysis. The EDS pattern (Figure 1A) only shows the presence of Ni and Ru elements except for a small C peak and O peak that result from the adsorbed organic capping reagent. Integral calculation indicates that the atomic ratio of Ni to Ru is close to 50:50. The more accurate molar ratio of Ni to Ru in the final product is found to be 43:57 based on the ICP test (Table 1), which is basically consistent with the EDS analysis. Thus, this sample is denoted as Ni43Ru57. The related XRD pattern is shown in Figure 1B. Besides the three small diffraction


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peaks (purple dashed lines indicated) that assigned to the face-centered cubic (fcc) phase Ni (JCPDS-70-1849) or Ni-rich fcc alloy, the other diffraction peaks marked by the orange dashed lines are located between that of hexagonal close packed (hcp) Ru (JCPDS-65-1863) and hcp Ni (JCPDS-45-1027), implying that the formation of Ni-Ru nanoalloy with hcp structure. To observe more clearly, the XRD pattern has been magnified and given in the supporting information (SI), Figure S1. The XRD analysis confirms that the Ni43Ru57 sample is the hcp NiRu nanoalloy along with small residual fcc Ni or Ni-rich fcc alloy. The average crystal size of the nanoalloy in Ni43Ru57 sample is calculated to be approximately 5 nm according to the Scherer’s empirical equation.49 The related alloying degree is estimated to be 33 using the method developed by Antolini and co-authors.50 The morphology and microstructure of the Ni43Ru57 nanoalloy were further examined by TEM and HRTEM. From the low- and high-magnification TEM images (Figure 1C-D), it can be seen that the shape of the Ni43Ru57 nanoalloy is alike to the necklace, which is spontaneously assembled by the quasi-spherical nano-building blocks. The average diameter of those nano-building blocks is ca. 70-90 nm, similar to that of initial Ni nanochains template (SI, Figure S2). That is to say, the Ni43Ru57 nanoalloy almost inherits the shape of the initial Ni nanochains. Nonetheless, in contrast to the initial Ni nanochains, the surfaces of the Ni43Ru57 nanoalloy become very rough. Moreover, there is a difference on contrast between the outer shell layer and the inner core of each nano-building block, revealing the formation of hollow structure in the obtained Ni43Ru57 nanoalloy. The corresponding highresolution TEM (HRTEM) image for the outer shell layer of an individual hollow sphere building block in the necklace-like Ni43Ru57 nanoalloy is shown in Figure 1E, from which we can see that the shell layer is made up of many small nanocrystals. The average crystal size of these small nanocrystals is measured to be approximately 5 ± 1 nm, which is in accordance with


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the calculated value from Scherrer’s equation. The grain boundaries or voids between those small nanocrystals can be observed. Moreover, the clear lattice fringes for those small nanocrystals are also seen, and the lattice spacing is measured to be about 0.204 nm, which is between the interplanar separation of (101) plane of hcp Ru (JCPDS-65-1863) and (011) plane of hcp Ni (JCPDS-45-1027). This result confirms that the formation of Ni-Ru nanoalloy with the hcp structure. The related fast Fourier transform (FFT) pattern (inset of Figure 1E) for all the small nanocrystals in the shell layer of an individual hollow sphere building block shows the hexagonal symmetry structure, further proving the obtained Ni43Ru57 nanoalloy is the hcp phase. Related element line-scanning profile (Figure 1F) for an individual hollow sphere building block (as the yellow arrow indicated direction) shows fewer elements in the core compared with that of the shell, proving the existence of hollow structure. Relatively speaking, the content of Ni that close to core region is slightly higher than that of Ru (Figure 1F), implying that there may be trace amount of Ni residual or Ni-rich alloy phase. Further HAADF-STEM image (Figure 1G) and elemental area-scanning mapping analysis (Figure 1H-I) for one necklace-like hollow Ni43Ru57 nanoalloy (for clearly, only shows three how spheres in series) confirm that the Ni and Ru elements are almost uniformly distributed along the whole necklace-like hollow nanoalloy. To further clarify the distribution of Ni and Ru elements on the surface of the nanoalloy, the shell layer region of an individual hollow sphere building block (yellow rectangular frame marked in Figure 1G) is chosen to make the HAADF-STEM and elemental mapping analysis yet. As illustrated in Figure 1J-L, we can see that the Ni and Ru elements are also nearly homogeneous distributed on the surface of an individual hollow sphere building block.


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Figure 1. (A-B) EDS and XRD patterns of the as-synthesized necklace-like hollow Ni43Ru57 nanoalloy. (C-D) Low- and high-magnification TEM images of the necklace-like hollow


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Ni43Ru57 nanoalloy. (E) HRTEM image for the outer shell layer of an individual hollow sphere building block in one necklace-like Ni43Ru57 nanoalloy. The inset shows the FFT pattern of an individual nanocrystal in the shell layer. (F) Element line-scanning profile for an individual hollow sphere building block along the yellow line indicated direction. (G-I) HAADF-STEM image and elemental mapping analysis for the necklace-like hollow Ni43Ru57 nanoalloy. (J-L) HAADF-STEM image and elemental mapping analysis for the outer shell layer of an individual hollow sphere building block as the yellow rectangular frame indicated area in (G).

Moreover, using the similar synthetic strategy, other NixRuy (Ni72Ru28, Ni63Ru37, and Ni29Ru71) nanoalloys also can be obtained. The corresponding EDS patterns are given in SI, Figure S3. Their accurate component ratios are identified by ICP, and the related data are summarized in Table 1. Figure 2 shows the XRD patterns of Ni72Ru28, Ni63Ru37, and Ni29Ru71 nanoalloys, from which we can see that their diffraction peaks (orange dashed lines) are gradually shifted, and close to that of pure hollow Ru nanostructures with the increase of the molar percentage of Ru. As a whole, their diffraction peaks are located between that of hcp-structured Ru (JCPDS-657645) and hcp-structured Ni (JCPDS-45-1027), indicating the formation of hcp-structured nanoalloys. It should be mentioned that the hcp phase has two unit cell parameters, ao and co. So, the detailed shifting direction of the diffraction peaks for those NixRuy nanoalloys may has two cases in relative to that of hcp-structured Ru and hcp-structured Ni. Additionally, with the increase of the content of Ru, the intensity of the diffraction peaks (purple dashed line) that assigned of residual Ni or Ni-rich fcc-structured alloy are gradually reduced. The average sizes of the crystallites in Ni72Ru28, Ni63Ru37, and Ni29Ru71 nanoalloys are calculated to be 6 ± 1, 6 ± 1, and 3 ± 1 nm, respectively, according to the Scherer’s equation. The related TEM images for those NixRuy nanoalloys as well as Ru nanostructures are given in Figure 3, from which the


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necklace-like hollow nanostructures can be clearly observed. With the increase of the content of Ru, their shell thicknesses are found to be gradually reduced and the well-defined hollow shapes will appear.

Table 1. Summary of Metal Contents of the reaction products measured from ICP and the corresponding denominations Ni/Ru precursors molar ratio

Ni/Ru molar ratio in the obtained product

Denomination

1:2

0: 100

Ru

1.8:1

29:71

Ni29Ru71

2:1

43:57

Ni43Ru57

3:1

63:37

Ni63Ru37

4:1

72:28

Ni72Ru28


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Figure 2. XRD patterns of the necklace-like hollow Ru nanostructures and other NixRuy (Ni72Ru28, Ni63Ru37, Ni29Ru71) nanoalloys.

Figure 3. TEM images of the as-prepared other necklace-like hollow NixRuy nanoalloys and Ru nanostructures: (A) Ni72Ru28, (B) Ni63Ru37, (C) Ni29Ru71 and (D) Ru.

3.2 Growth process discussion Based on the above experimental results, the formation processes for those necklace-like hollow NixRuy nanoalloys are suggested. From the thermodynamic data, we know that the


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standard electrode potential of Ru3+/Ru couple is 0.400 V (vs. SHE (standard hydrogen electrode)), which is much higher than that of Ni2+/Ni couple (-0.257 V vs. SHE). Once RuCl3·3H2O solution is added into the Ni nanochains suspension, the redox reaction will occur immediately due to the spontaneous formation of the galvanic cell. The related reaction can be described as follows: 3Ni + 2Ru3+ → 3Ni2+ + 2Ru. Here, the initial Ni nanochains are served as the sacrificial template or negative electrode and Ni resources, which are oxidized to form Ni2+ ions and outer-diffuse into the solution that leave voids in the interiors of the Ni cores. Since the Ru3+ and Ni2+ ions have their characteristic colors, the process for the outer-diffusion and release of Ni2+ ions has been monitored by recording the color change during the reaction with a digital camera. From the related photo images (SI, Figure S4), we can see that the supernatant of the reaction system is changed from colorless to light green after the reaction, which is the characteristic color for Ni2+ ions. Such experimental fact confirms the release of Ni2+ ions into the reaction solution. That is to say, with the proceeding of the reaction, the hollow Ni cores will be generated due to the Kirkendall effect.,51, 52 As we know, there is no external circuit and the Ru salt precursors are directly connected with the in-situ formed hollow Ni cores. Thus, the reduced Ru (0) atoms will spontaneously deposit on the surface of the hollow Ni cores. The large lattice mismatch between Ni and Ru as well as their differences on bond dissociation energies (Ni-Ni, 204 KJ. mol-1; Ru-Ru, 193 KJ mol-1)53 or cohesive energies will provide the driving force for the diffusion or incorporation of Ru atoms into Ni lattice to form the necklace-like hollow NixRuy nanoalloy. As for the interesting phase transformation of the formed NixRuy nanoalloy, the possible reason may be as follows: the atomic radius of Ru is larger than that of Ni, so the incorporation of Ru into Ni may induce the distortion of the original Ni lattice (fcc) and let it transform from fcc to hcp phase structure, which can match the thermodynamic stable Ru


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structure (hcp) and eventually form the newly Ni-Ru solid solution or alloy with the hcp phase. To fully understand the phase transformation mechanism, it needs some advanced in-situ characterization techniques to provide more useful information.

3.3 Electrochemical analysis The electrocatalytic properties of the necklace-like hollow NixRuy nanoalloys were evaluated in 0.5 M H2SO4 solution by using HER as a probe reaction. For comparison, the initial Ni nanochains, the necklace-like hollow Ru nanostructures and commercial Pt/C catalyst are used as the control catalysts. Figure 4A shows the HER polarization curves of those seven catalysts using the graphite rod as the counter electrode with the iR drop correction, which are identical to or overlapped with that obtained using platinum gauze electrode as the counter electrode (SI, Figure S5). Compared with that of initial Ni nanochains, the necklace-like hollow Ru nanostructures and previously reported Ru nanoparticles,37 the obtained NixRuy nanoalloys show greatly enhanced HER activity, implying that the incorporation of Ni into Ru lattice to form nanoalloy can efficiently modulate the electronic structure of Ru and improve its catalytic performance. The onset overpotential (ηo) of the Ni43Ru57 nanoalloy for HER is nearly 0 mV, which is comparable to commercial Pt/C catalyst, and much lower than that of Ni29Ru71 (ηo ≈ 15 mV), Ni63Ru37 (ηo ≈ 12 mV) and Ni72Ru28 (ηo ≈ 20 mV). As for the overpotential that needed to reach the current density of -10 mA cm-2 (η10), the similar trend can be found (Figure 4B). That is to say, among those NixRuy nanoalloys, the Ni43Ru57 possesses the highest catalytic activity toward HER, which is superior to other reported non-Pt HER catalysts (e.g. NiSe2,7 MoS2/Au,9 Fe1−xCoxS2/CNT,11 Ni2P,12 Ni12P5,13 Mo2C,18 P-WN/rGO,21 CoNi@NC,25 Ni-Sn@C,54 and IrNiN /C, 55 etc.), and close to commercial Pt/C catalyst. It should be mentioned that the current density of the Ni43Ru57 nanoalloy will surpass Pt/C catalyst when the overpotential is higher than 102


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mV. Moreover, based on the value of η10, the catalytic activity of the Ni43Ru57 nanoalloy even outperform recently reported some Pt-based nanohybrids, such as Pt-TiS2 and Pt-TaS2.56 The detailed data for comparison of the Ni43Ru57 nanoalloy with other reported non-Pt HER electrocatalysts are summarized in SI, Table S1.

Figure 4. (A) HER polarization curves for the necklace-like hollow Ni29Ru71, Ni43Ru57, Ni63Ru37, and Ni72Ru28 nanoalloys as well as Ru nanostructures, initial Ni nanochains and Pt/C catalyst. (B) Comparing the overpotentials of NixRuy nanoalloys, Ru nanostructures and Pt/C catalyst that needed to attain the current density of -10 mA cm-2. (C) Tafel plots for NixRuy nanoalloys, Ru nanostructures and Pt/C catalyst. (D) Durability test for the Ni43Ru57 nanoalloy.


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To further understand the catalytic behaviors of those NixRuy nanoalloys catalysts, the related Tafel plots are obtained according to the Tafel equation (η = a + b log j, where η, a, b and j are the overpotential, the intercept, Tafel slope and the current density, respectively) by choosing the linear portions of their HER polarization curves, shown in Figure 4C. Usually, there are three possible reaction steps suggested for HER in acidic media.57 The first step is a primary discharge process (Volmer reaction)58: (1) H3O+ + e- → Hads + H2O, b ≈ 120 mV dec-1. This step is followed by either an electrochemical desorption step (Heyrovsky reaction): (2) Hads+ H3O+ + e→H2 +H2O, b ≈ 40 mV dec-1; or a recombination step (Tafel reaction): (3) Hads + Hads →H2, b ≈ 30 mV dec-1. The rate-limiting step determines the HER mechanism, which can be deduced from the calculated Tafel slope. For instance, the HER on a single crystalline Pt surface abides by the Volmer-Tafel mechanism, and the recombination step is the rate-limiting step at low overpotentials, as attested by the measured Tafel slope of 40 mV dec-1.12, 57 In our work, the measured Tafel slope values for Ni29Ru71, Ni43Ru57, Ni63Ru37, and Ni72Ru28 nanoalloys as well as commercial Pt/C catalyst are 39, 31, 32, 41 and 30 mV dec-1, respectively indicating that their HER mechanisms are similar to that of single crystalline Pt catalyst. While for Ru nanostructures, the Tafel slope is measured to be 54 mV dec-1, revealing that the HER on their surface complies with the Volmer-Heyrovsky mechanism, and the electrochemical desorption step is the rate-limiting step. Moreover, from the Tafel curves using an extrapolation method, the exchanged current density (jo) of the Ni29Ru71, Ni43Ru57, Ni63Ru37, and Ni72Ru28 nanoalloys as well as Ru nanostructures and Pt/C catalyst are calculated to be 3.12 x10-4, 6.17 x 10-4, 3.53 x10-4,

2.99 x 10-4, 2.49 x10-4, 4.2 x10-4 A cm-2, respectively. Usually, the higher the jo value, the higher the activity of a specific catalyst. In contrast to pure Ru nanostructures, the higher jo values of those NixRuy nanoalloys imply that their catalytic activities are greatly enhanced by alloying Ru


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with Ni. Among those NixRuy nanoalloys, the highest jo value observed on Ni43Ru57 reveals that it possesses the highest catalytic activity for HER, albeit its HER mechanism is similar to other NixRuy nanoalloy ones. Besides the catalytic activity and mechanism, the long-term stability is another important parameter for determining the application of a specific electrocatalyst. The durability test for Ni43Ru57 nanoalloy is further performed using the chronoamperometric method at a constant potential of -0.1 V (vs RHE). The corresponding stability curve is shown in Figure 4D. After continuously cycling for 8 hours, 95.4% of its initial current density can be reserved, implying that the Ni43Ru57 nanoalloy also possesses very high stability in acidic environment. To get some insight on the origin of the excellent catalytic performance of the Ni43Ru57 nanoalloy, XPS was firstly used to characterize its chemical state and surface electronic structures. Owing to the overlapping of Ru 3d peak with that of C1s signal from the adventitious carbon,59 the Ru 3p region is collected to discuss the electronic structure change of Ru by alloying with Ni. To show the evolution trends of the electronic structures for Ni and Ru, the XPS spectra for Ni29Ru71, Ni63Ru39 and Ni72Ru28 nanoalloys are also measured, and placed together with that of Ni43Ru47 nanoalloy for comparison. As shown in Figure 5A, the Ni 2p core level spectra reveal the presence of the metallic Ni (0) with the binding energy (BE) centered at about 852.8-853.1 eV, and the oxidized Ni in those NixRuy nanoalloys.44 With the decrease of Ru molar percentage, the Ni 2P spectra of those NixRuy nanoalloys are slightly shifted to high BE direction, indicating that the interaction between Ni and Ru leads to the slight variation of Ni’s surface electronic structures in the nanoalloys. While for the Ru 3p core level spectra (for comparison only shows Ru 3p3/2 peaks in Figure 5B, and the full spectra are shown in SI, Figure S6) of those NixRuy nanoalloys, they show the existence of the metallic Ru (0) with the BE


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centered at 461.2-461.7 eV and the oxidized Ru species centered at 463.9-463.2 eV.44 As a whole, compared with that of pure Ru nanostructures, the Ru 3p3/2 peaks of those NixRuy nanoalloys are shifted to lower BE direction with the increase of Ni molar percentage, implying that there are efficient electronic coupling between Ni and Ru, and the electrons may transfer from Ni to Ru that leads the Ru to bear negative charge in form. That is to say, the interatomic charge polarization47 occurs in those NixRuy nanoalloys. Among them, the biggest negative shift (0.5 eV in relative to Ru nanostructures) of Ru 3P3/2 peak is observed in the Ni43Ru57 nanoalloy, revealing that there is the strongest electronic coupling or interatomic charge polarization in it. The detailed deconvolution of Ni 2p and Ru 3p3/2 peaks for those NixRuy nanoalloys are given in SI, Figure S7-8. And the related data are summarized in SI, Table S2. In a word, the XPS results confirm that the electronic coupling of Ni and Ru in the Ni43Ru57 sample is much stronger than that in other NixRuy nanoalloy ones.


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Figure 5. (A) Comparing the Ni 2P core level XPS spectrum of the Ni43Ru57 nanoalloy with that of other NixRuy nanoalloys. (B) Comparing the Ru 3p3/2 core level XPS spectrum of the Ni43Ru57 nanoalloy with that of other NixRuy nanoalloys and pure Ru nanostructures.

Moreover, the electrochemical impedance spectra (EIS) were employed to study the interfacial electrons transfer dynamics of those NixRuy nanoalloys. The related Nyquist plots and equivalent circuit are shown in Figure 6. The equivalent circuit consists of five components, including Rs, Rp, Rct and two constant phase elements (CPEs). Among them, Rs represents the resistance of the electrolyte and intrinsic resistance of the active materials that modified on the electrode; Rp refers to the resistance of an electrode porosity response; Rct stands for the electron or charge transfer resistance, which determines the interfacial electron transfer kinetics of each catalyst; CPE1 refers to the capacitance components of an electrode porosity response, and CPE2 corresponds to the HER charge-transfer double-layer capacitance components.60 For comparison, various physical parameters obtained by fitting the Nyquist plots of the examined catalysts are listed in SI, Table S3. The Rct of Ni43Ru57, Ni63Ru37, Ni29Ru71, and Ni72Ru28 nanoalloys are identified to be 9, 19, 31, and 43 Ω, respectively, which are much lower than that of pure Ru nanostructures (65 Ω) and initial Ni nanochains (169 Ω). This result reveals that alloying Ni with Ru can enhance electrical conductivity and improve interfacial electron transfer kinetics due to the efficient electronic coupling between Ni and Ru. Based on the Rct value of those four NixRuy nanoalloys, we can identify that their interfacial electron transfer kinetics follows the order: Ni43Ru57 > Ni63Ru37 > Ni29Ru71 > Ni72Ru28, which is consistent with their HER activity exactly. Among them, the Ni43Ru57 nanoalloy exhibits the fastest interfacial electronic transfer kinetics, so it shows the best electrocatalytic activity toward HER.


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Figure 6. (A) Nyquist curves for the necklace-like hollow Ru nanostructures (a), Ni29Ru71 (b), Ni43Ru57 (c), Ni63Ru37 (d), and Ni72Ru28 (e) nanoalloys obtained in 0.5 M H2SO4 solution. (B) The corresponding Nyquist curves for the initial Ni nanochains. The insets are the related equivalent circuits that used for fitting the Nyquist curves of those catalysts.

In addition, to tentatively compare the number of the available catalytic active sites, the capacitance of the catalyst-solution interface and the roughness factors of those NixRuy nanoalloys and Ru nanostructures were further examined. As shown in Figure 7A-E, the current densities are measured from 1 mV s-1 to 9 mV s-1 at the potential window of 0.185-0.235 V (vs. RHE) and can be plotted as a function of the scan rate (Figure 7F) to study the effective surface area of those catalysts by evaluating the electrochemical double-layer capacitance (Cdl) 61, 62 and the roughness factor (Rf). The Cdl for those catalysts can be calculated through plotting the ∆j at E = 0.21 V (vs. RHE) against the scan rates. And the slope of the fitted line is the Cdl value. As illustrated in Figure 7F, the Cdl values of the Ni43Ru57, Ni63Ru37, Ni29Ru71, Ni72Ru28 nanoalloys and Ru nanostructures are 7.6, 6.8, 5.9, 5.5, and 4.6 mF cm-2, respectively, implying that their


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electrochemical active surface area follows the order: Ni43Ru57 > Ni63Ru37 > Ni29Ru71 > Ni72Ru28 > Ru. To better understand the intrinsic activity of those NixRuy nanoalloys, the current densities in their HER polarization plots are normalized according to the ratio of their electrochemical active surface area. The apparent current density of Ru nanostructures is chosen as a standard or reference. As shown in SI, Figure S9, we can see that the current densities of Ni43Ru57, Ni63Ru37, Ni29Ru71 and Ni72Ru28 nanoalloys are actually decreased after normalized by the ratio of their electrochemical active surface areas, but the trends of their catalytic activity are unchanged. Moreover, the Rf of the Ni43Ru57, Ni63Ru37, Ni29Ru71, Ni72Ru28 nanoalloys and Ru nanostructures are calculated to be 380, 340, 295, 275, and 230, respectively, which follows the same order as that for Cdl. The sequences of Cdl and Rf for those NixRuy nanoalloy catalysts are in accordance with their HER activity exactly. Compared with other NixRuy nanoalloy ones, the Ni43Ru57 nanoalloy exhibits the highest Cdl and Rf values, revealing that there are more available active sites on it. So, the Ni43Ru57 nanoalloy shows the best catalytic performance. That is to say, except for the fastest interfacial electron transfer kinetics, the highest Cdl and Rf values will make the Ni43Ru57 nanoalloy possess more available active sites, which also offer the positive contribution for enhancing its catalytic performance.


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Figure 7. (A-E) CV curves for the necklace-like hollow Ru nanostructures, Ni29Ru71, Ni43Ru57, Ni63Ru37 and Ni72Ru28 nanoalloys measured at 0.185-0.235V (vs. RHE) with the scan rate ranging from 1 to 9 mV s-1. (F) Current density as a function of the scan rate for those Ni29Ru71, Ni43Ru57, Ni63Ru37 and Ni72Ru28 nanoalloys and Ru nanostructures.

4. CONCLUSIONS


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In summary, the novel necklace-like hollow NixRuy nanoalloys with variable component ratios have been successfully synthesized through a facile galvanic replacement strategy, i.e., adding RuCl3·3H2O precursors into pre-synthesized Ni nanochains solution. By simply adjusting the amount of Ru salt precursors and taking advantage of the Kirkendall effect, the component ratio of the obtained NixRuy nanoalloys can be well-controlled and the hollow structures can be carved out in the final obtained nanoalloys. Due to strong synergistic effect or efficient electronic coupling, those NixRuy nanoalloys show greatly enhanced electrocatalytic HER activity and stability in acidic media compared with that of pure Ni and pure Ru nanostructures. Among them, the Ni43Ru57 nanoalloy exhibits the highest HER activity with a Tafel slope of ∼31 mV dec-1 and the overpotential of 41 mV to afford a current density of -10 mA cm-2, which is close to commercial Pt/C, and competitive with the-state-of-art non-Pt HER catalysts. Except for the high activity, the Ni43Ru57 nanoalloy also possesses an excellent stability. The excellent catalytic performance of such Ni43Ru57 nanoalloy originates from the proper component ratio and effective electronic coupling of Ni and Ru, which endow the faster interfacial electron transfer kinetics and more available active sites on it that evidenced by spectroscopic and electrochemical analysis. This work not only offers a facile avenue for component-controlled synthesis of hollow bimetallic nanoalloys but also screens out a highly active and robust nanoalloy electrocatalyst for splitting water to produce clean H2 fuel.

AUTHOR INFORMATION Corresponding Author * Phone: 86-25-85891051. Fax: +86-25-85891051 E-mail: [email protected] (Prof. Dr. M. Han);


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[email protected] (Prof. Dr. J. C. Bao). Author Contributions All authors have given approval to the final version of the manuscript. † C. Zhang and Y. Liu contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China for the projects (Nos. 21271105, 21471081, 21671106, and 21501095), research fund from the Priority Academic Program Development of Jiangsu Higher Education Institutions, the Program for Outstanding Innovation Research Team of Universities in Jiangsu Province, the Graduate Student Innovation Fund in Jiangsu Province (KYLX16_1267), and the opening research foundations of State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Solid State Microstructures, Nanjing University. Supporting Information. This information contains the magnified XRD pattern for Ni43Ru57 nanoalloy (Figure S1), characterization of initial Ni nanochains (Figure S2), the EDS patterns for other NixRuy nanoalloys (Figure S3), photo images for monitoring the color change before and after the reaction (Figure S4), comparison of HER plots using two different counter electrodes (Figure S5), comparison of HER performance of Ni43Ru57 nanoalloy with other reported catalysts (Table S1), the XPS spectra (Figure S6-8 and Table S2) and the fitted various physical parameters from the Nyquist plots of NixRuy nanoalloys and single metal catalysts (Table S3), and the HER polarization plots with the normalized current densities (Figure S9). This material is available free of charge via the Internet at http://pubs.acs.org.


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