Mesoporous Hollow Cu-Ni Alloy Nanocage from Core-Shell Cu@Ni

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Mesoporous Hollow Cu-Ni Alloy Nanocage from Core-Shell Cu@Ni Nanocube for Efficient Hydrogen Evolution Reaction Zhenxing Li, Chengcheng Yu, Yangyang Wen, Yang Gao, Xiaofei Xing, Zhiting Wei, Hui Sun, Ya-Wen Zhang, and Weiyu Song ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04814 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on March 28, 2019

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Mesoporous Hollow Cu-Ni Alloy Nanocage from Core-Shell Cu@Ni Nanocube for Efficient Hydrogen Evolution Reaction Zhenxing Li,*,† Chengcheng Yu,† Yangyang Wen,† Yang Gao,‡ Xiaofei Xing,† Zhiting Wei,† Hui Sun,† Ya-Wen Zhang,§ and Weiyu Song*,‡

†State

Key Laboratory of Heavy Oil Processing, College of New Energy and Materials,

China University of Petroleum (Beijing), Beijing 102249, China

‡College

§Beijing

of Science, China University of Petroleum (Beijing), Beijing, 102249, China

National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth

Materials Chemistry and Applications, PKU-HKU Joint Laboratory in Rare Earth Materials and Bioinorganic Chemistry, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China

ACS Paragon Plus Environment

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ABSTRACT ： We have created a facial self-templated method to synthesize three distinct nanostructures, including the unique edge-cut Cu@Ni nanocubes, edge-notch Cu@Ni nanocubes and mesoporous Cu-Ni nanocages by selective wet chemical etching method. Moreover, in the synthesis process, the corners of edge-cut Cu@Ni nanocubes and mesoporous Cu-Ni nanocages can be etched to produce the high catalytical active (111) facets. Impressively, compared to edge-notch Cu@Ni nanocubes and edge-cut Cu@Ni nanocubes, the Cu-Ni nanocages exhibit higher electrocatalytic activity in the hydrogen evolution reaction (HER) under alkaline conditions. When obtained overpotential is 140 mV, the current density can reach 10 mA cm-2, meantime, the corresponding Tafel slopes is 79 mV dec-1. Moreover, from the calculation results of density functional theory (DFT), it can be found that the reason why the activity of pure Ni is lower than Cu-Ni alloy is that the adsorption energy of the intermediate state (adsorbed H*) is too strong. Meanwhile the Gibbs free-energy


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(|ΔGH*|) of (111) facets is smaller than (100) facets, which brings more active sites or adsorb more hydrogen.

KEYWORDS ：self-templated, hollow structure, core-shell, mesoporous, hydrogen evolution reaction

1. INTRODUCTION

In recent years, shape-controlled metal nanoparticles were widely concerned due to application in many fields, including catalysis,1-3 electronics,4 medicine5-7 and plasmonics.8,9 Hollow nanostructures become popular in electrochemical applications with its distinctive type of architectures.10-14 Unlike the bulk structure, hollow structure has a larger specific surface area exhibits higher surface exposed active site density in catalytical reaction. Due to the atoms with high availability and the facets with controllability, the nanocages has a terrific effect to catalysis.15 Significantly, the hollow nanostructures of diverse shapes have been prepared propitiously with seed-mediated methods including core-shell structures or core-frame structures.16-19 Xia et al.20 has


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reported the cores of nanocrystals can be removed selectively by wet etching and generate the nanocages and nanoframes with open structures. The significant breakthroughs have been achieved on the noble metal synthesis into various morphologies and compositions; however, it is still a challenge to find a concise and valid way which can synthetize a well-defined hollow and core-shell nanocrystals without seed-mediated methods. Hydrogen fuels have shown their potential to replace fossil energy due to high energy density and environmental-friendly properties.21-23 Currently, the traditional Ptbased compounds are adopted as the highest active hydrogen evolution reaction electrocatalysts in acid condition.24 However, the precious metals (Pt, Pd) are rare on the planet and costly, more and more research focus has turned to non-precious metal catalysts. Developing an efficient, stable and low-costing catalysts has always been a frontier topic in the field of electrolysis water.25,26 Compared to the platinum (Pt),27 palladium (Pd),28 gold (Au)29 and other precious metals,30-32 nickel (Ni) has a higher element in the Earth, which makes Ni attractive as a low-costing catalyst for HER.33,34 The surface of nanocatalysts will affect their selectivity and reactivity, and these planes


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can be controlled by the morphology of nanocrystals. A great number of methods are used to synthesize the precious metal nanocrystals of various shapes, different types of facets on the surface and diversified structures (e.g., core-frame vs core-shell or solid vs hollow).35-37 However, the strategies which can control the shape of Ni are not mature enough, so the research of Ni morphology is still few. More recently, bimetallic synergistic catalyst, possessing a well electronic conductivity, chemical structural stability and low cost, has become a promising catalyst for a new generation of HER electrocatalysis.38-41 The facile self-templated strategy does not need to remove the inactive inner cores, which is becoming increasingly attractive approach for the preparation of hollow nanostructures. Herein, we demonstrate a facile self-templated method to synthesize three distinct nanostructures, including the unique edge-cut Cu-Ni core-shell (Cu@Ni) nanocubes, edge-notch Cu@Ni nanocubes and Cu-Ni nanocages by selective wet chemical etching method. Moreover, it should be the first time the mesoporous Cu-Ni nanocage have been prepared without the use of as-prepared seed precursors. In addition, compared to many previous efforts to control systematically Cu@Ni and Cu-Ni


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nanocrystals relying mainly on seed-mediated methods, the Cu@Ni nanocubes and CuNi nanocages are directly achieved by one pot strategy. In the synthesis process, the corners of edge-cut Cu@Ni nanocubes and mesoporous Cu-Ni nanocages can be etched to produce the high catalytical active (111) facets. Cu-Ni nanocages exhibits robust stability and excellent HER performance with overpotentials of 140 mV at 10 mA cm-2 in alkaline conditions, meantime, the corresponding Tafel slopes is 79 mV dec-1, which are superior to most reported non-precious metal electrocatalysts. The atoms on both the outer and inner sides of the Cu-Ni nanocage can be utilized, owing to the ultrathin walls and mesoporous in the Cu-Ni nanocages. The density functional theory (DFT) calculations results remains the intermediate state (adsorbed H*) on pure Ni is stronger than Cu-Ni alloy, which causes the lower HER activity. Moreover, compared with the (100) facets, the Gibbs free-energy (|ΔGH*|) on (111) facets is much smaller, which supplied more active sites to adsorb hydrogen. 2. EXPERIMENTAL SECTION


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2.1 Preparation of Cu nanocubes: For synthetizing the copper nanocubes, copper(II) acetylacetonate (10 mg, Cu(acac)2), Iron(III) chloride hexahydrate (10 mg, FeCl3·6H2O), Ascorbic acid (52.8 mg, C6H8O6), 5 mL of Oleylamine (OAm) were added into 25 ml glass tube. After the glass tube cover is covered, the mixture was ultrasonicated for around 1 h in order to forming a uniform solution. The uniform solution was heated to 180 ºC in 0.5 h, and kept at 180 ºC for 3 h in an oil bath, the glass tube after the reaction was cooled to room temperature. The samples were collected by centrifugation and washed with a hexane/ethanol mixture. 2.2 Preparation of edge-cut Cu@Ni nanocubes: In the synthesis process, nickel(II) acetylacetonate (9.8 mg, Ni(acac)2), copper(II) acetylacetonate (10 mg, Cu(acac)2), Iron(III) chloride hexahydrate (10 mg, FeCl3·6H2O), Ascorbic acid (52.8 mg, C6H8O6) and 5 mL of Oleylamine (OAm) were added into 25ml glass tube. After the glass tube cover is covered, the mixture was ultrasonicated for around 1 h in order to forming a uniform solution. The uniform solution was heated to 180 ºC in 0.5 h, and kept at 180 ºC for 3 h in an oil bath, the glass tube after the reaction was cooled to room temperature.


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The samples were collected by centrifugation and washed with a hexane/ethanol mixture. 2.3 Preparation of edge-notch Cu@Ni nanocubes: In the synthesis process, nickel(II) acetylacetonate (9.8 mg, Ni(acac)2), copper(II) acetylacetonate (10 mg, Cu(acac)2), Iron(III) chloride hexahydrate (10 mg, FeCl3·6H2O), Ascorbic acid (52.8 mg, C6H8O6), 5 mL of Oleylamine (OAm) were added into 25ml glass tube. After the glass tube cover is covered, the mixture was ultrasonicated for around 1 h in order to forming a uniform solution. The uniform solution was heated to 175 ºC in 0.5 h, and kept at 175 ºC for 3 h in an oil bath, the glass tube after the reaction was cooled to room temperature. The samples were collected by centrifugation and washed with a hexane/ethanol mixture. 2.4 Preparation of Cu-Ni nanocages: In the synthesis process, nickel(II) acetylacetonate (9.8 mg, Ni(acac)2), copper(II) acetylacetonate (10 mg, Cu(acac)2), Iron(III) chloride hexahydrate (6 mg, FeCl3·6H2O), Ascorbic acid (52.8 mg, C6H8O6), 5 mL of Oleylamine (OAm) were added into 25 ml glass tube. After the glass tube cover is covered, the mixture was ultrasonicated for around 1 h in order to forming a uniform


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solution. The uniform solution was heated to 180 ºC in 0.5 h, and kept at 180 ºC for 12 h in an oil bath, the glass tube after the reaction was cooled to room temperature. The samples were collected by centrifugation and washed with a hexane/ethanol mixture. 2.5 Preparation of Cu-Ni nanoparticle: In the synthesis process, nickel(II) acetylacetonate (514 mg, Ni(acac)2), copper(II) acetylacetonate (523 mg, Cu(acac)2), 5 mL of Oleylamine (OAm) were added into the 25 ml glass tube. After the uniform solution was bubbled with N2 and heated to 80 ºC for 20 min, and then add 0.2 ml TOP into the solution. The mixture was heated to 180 ºC for 12 h in an oil bath. The samples were collected by centrifugation and washed with a hexane/ethanol mixture. 2.6 Electrochemical Catalysts Preparation: 1 mg carbon black and 4 mg catalyst were put into the 5 ml ethanol, then the mixture was sonicated for 1 h in order to form a homogeneous solution. Next, the solution was stirred for another 8 h at 70 ℃ to eliminate the Oleylamine (OAm) after 4ml acetic acid were added to the mixture. The HER measurements performed on glassy carbon (GC) electrode were also under the identical conditions. 40 µL 5 wt% Nafion and 4 mg catalyst were placed into 960 μL ethanol solution, then the aboved mixed solution was sonicated for 0.5 h to form uniform


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solution. 5 μL uniform solution were dripped GC electrode (3 mm diameter). The HER reaction at room temperature was conducted by using a three-electrode system, with the GC electrode loaded by catalysts, Hg/HgO (1M KOH) and graphite rod served as working electrode, reference electrode and counter electrode, respectively. HER experiment was carried out with the N2-saturated 1 M KOH electrolyte. Linear sweep voltammetry (CHI760E, Shanghai Chenhua Instrument Factory, China) was carried out from -1.5 to 0 V versus RHE with a scan rate of 50 mV s-1. Cyclic voltammetric (CV) curve was measured from 0.21 to 0.31 V vs. RHE with a scan rate from 10 mV s-1 to 60 mV s-1. The long-term stability was recorded by the extended electrolysis at -22 mV for 8 hours in the N2-saturated electrolyte. The HER stabilities were also tested in CV for 2000 cycles at potentials between -0.5 and 0 V (versus RHE) with 100 mV s-1 scan rate in the N2-saturated 1.0 M KOH electrolytes. The Nyquist plot was tested at frequencies from 100 kHz to 0.01 Hz with the 200 mV overpotential. In the electrochemical measurements, the potential with respect to RHE in our measurements can be calculated as follows: E(RHE) = 0.926 V + E(Hg/HgO).


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2.7 Characterization: Scanning electron microscopy (SEM) was measured by the Hitachi SU8010 scanning electron microscope at 200 kV. The high-resolution transmission electron microscope (HRTEM) and transmission electron microscopy (TEM) were measured by the Tecnai F20 at 200 kV. The wide-angle X-ray diffraction (XRD) pattern was conducted by the Burker D8-advance X-ray diffractometer (Operating current: 40 mA, Operating Voltage: 40 KV) with Cu-Kα radiation (λ = 0.15406 nm). X-ray photoelectron spectrometer (XPS) was measured by using the Mg KR radiation (BE) of 1253.6 eV. N2 adsorption-desorption isotherm was measured with the Micromeritics ASAP 2460 analyzer (USA) at liquid nitrogen temperature (77 K). The sample was firstly degassed in a vacuum at 80 °C for 6 h prior to measurement. Pore size distribution was obtained with the Barret-Joyner-Halenda (BJH) and non-local density functional theory (NLDFT) model. The surface area was obtained using Brunauer-Emmett-Teller (BET) method. UV-visible absorption spectrum was recorded on the Hitachi U-3010 spectrometer. 2.8 Computational details: To understand the effect of Cu-doped Ni on hydrogen evolution reaction, the first principles calculations were carried out, based on the density


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functional theory (DFT) employing the Perdew-Burke-Ernzerhof (PBE)42 approximation for an exchange-correlation potential in Vienna Ab Initio Simulation Package (VASP).43,44 To model the Ni (111) and (100) surfaces, four layer p (3 × 3) slabs with 15 Å vacuum gap thickness were used. The k-points meshes of 9×9×1 were used for Brillouim zone integration, and the plane-wave basis set within the kinetic cut-off energy of 400 eV. The atomic positions were relaxed until the force on each atom was less than 0.05 eV/Å and the convergence tolerance of the energy was set to be 10-5 eV. To simulate Cu doping in Ni catalyst, we substitute a Cu atom for one surface Ni in the lattice and denote as Cu1-Ni. Furthermore, the catalyst in which two surface Ni replaced by Cu atoms was denote as Cu2-Ni. The adsorption energy of H (ΔEH) was calculated using: ΔEH =Esurf + H - Esurf - 1/2 EH2 in which Esurf + H is the surface adsorbed energy with H, Esurf is the clean surface energy, and EH2 is the gas-phase hydrogen gas energy. The H adsorption Gibbs free energy was calculated by: ΔGH =ΔEH + ΔEZPE -TΔSH where ΔSH is the entropy and ΔEZPE is the zero-point energy. ΔEZPE -TΔSH = 0.28 eV with the standard condition of T = 300K.45


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The ΔGH was obtained by: ΔGH = ΔEH + 0.28 eV 3. RESULTS AND DISCUSSION

The well-shaped Cu@Ni and Cu-Ni nanocrystals are made in a simple and facial organic solution by using metal precursors Ni(acac)2 and Cu(acac)2. The oleyl amine (OAm) is both reducing agent and solvent, in addition, the ascorbic acid (AA) is the coreducing agent, and iron(III) chloride hexahydrate (FeCl3·6H2O) is a capping agent and etchant. The ultrathin Cu-Ni nanocages, edge-cut Cu@Ni nanocubes, and edge-notch Cu@Ni nanocubes can be obtained via well-controlling the temperature, amount of FeCl3·6H2O and reaction time (Scheme 1).

Scheme 1. Schematic illustration of well-defined nanocrystals with controlled manner.


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The representative scanning electron microscopy (SEM) indicate the nanocrystals are edge-cut nanocubes. Interestingly, the edges of each nanocube are cut into two faces and the (110) and (111) faces are clearly observed in Figure 1a. The average edge length of edge-cut Cu@Ni nanocubes are 78 nm (Figure S1). The corresponding transmission electron microscopy (TEM) images (Figure 1b) shows that these nanocubes are neatly arranged on a plane. The high-resolution transmission electron microscope (HRTEM) shows that the lattice spacing of edge-cut Cu@Ni nanocubes is 0.176 nm and 0.204 nm (Figure 1c), which correspond to the (100) and (111) facets of Ni nanocrystals, confirming the formation of the Ni shell. Selected-area electron diffraction (SAED) pattern (inset of Figure 1c) shows the diffraction rings are constituted by many bright dot, means that edge-cut Cu@Ni nanocubes are constituted of polycrystalline. In order to investigate the distribute of Cu and Ni on nanocrystals, use scanning transmission electron microscopy (STEM) to measure the single edge-cut Cu@Ni nanocubes. A high-angle annular dark field (HAADF)-STEM image and energydispersive X-ray (EDX) element mapping (Figure 1e) clearly show that the edge-cut Cu@Ni nanocubes is the core-shell structure, further confirmed that the core is


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dominated by Cu while the shell is essentially made by Ni. The line scans analysis can confirm deeply that the core-shell structure of the edge-cut Cu@Ni nanocubes (Figure 1d), and Ni is distributed on the shell, while Cu is distributed within the core. The EDX analysis indicates the ratio of Cu/Ni is 53:47 (Figure S2 and Table S1).


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Figure 1. a) SEM image of edge-cut Cu@Ni nanocubes. b) TEM image of edge-cut Cu@Ni nanocubes. c) HRTEM image of edge-cut Cu@Ni nanocubes. d) Line scan analysis of edge-cut Cu@Ni nanocubes. e) EDX elemental mappings of edge-cut Cu@Ni nanocubes. f) Schematic of edge-cut Cu@Ni nanocubes. In the process of synthesizing the edge-cut Cu@Ni nanocubes, the FeCl3·6H2O plays a vital part in the edge-cut nanocube morphology. As depicted in the Figure S3, the reaction is fail to form the edge-cut nanocubes in the absence of FeCl3·6H2O. When the amount of FeCl3·6H2O is added to 13 mg, the morphology of the sample becomes nanowires, implying that the FeCl3·6H2O is a crucial shape-directing agent during the formation of edge-cut Cu@Ni nanocubes. Significantly, other synthesis factors are still identical to the edge-notch Cu@Ni nanocubes in spite of the reaction temperature decreased to 175 ℃, the unique edgenotch Cu@Ni nanocubes are obtained. Interestingly, the edges of each cube become grooves, this remarkable feature is clearly visible in the SEM (Figure 2a) and the TEM image (Figure 2b and S4). The size of as-obtained edge-notch Cu@Ni nanocubes increases and the average edge length is 94 nm (Figure S5). The HRTEM image shown that the lattice spacing of the shell of edge-notch Cu@Ni nanocubes is 0.176 nm, and


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which corresponding to (100) facets of Ni (Figure 2c). The regular square array is demonstrated by the SAED image (inset in Figure 2c), uncovering edge-notch Cu@Ni nanocubes have a high degree of crystallinity. Line scans and HAADF-STEM-EDX elemental mapping (Figure 2d, e) exhibit that Ni is distributed on the shell while Cu distributed on the core, which confirms the formation of the edge-notch Cu@Ni nanocubes. The EDX of edge-notch Cu@Ni nanocubes shows the ratio of Cu/Ni is 90:10 (Figure S6 and Table S1).


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Figure 2. a) SEM image of edge-notch Cu@Ni nanocubes. b) TEM image of edge-notch Cu@Ni nanocubes. c) HRTEM image of edge-notch Cu@Ni nanocubes. d) Line scan analysis of edge-notch Cu@Ni nanocubes. e) EDX elemental mappings of edge-notch Cu@Ni nanocubes. f) Schematic of edge-notch Cu@Ni nanocubes. When the usage amount of FeCl3·6H2O is shrinked to 6 mg and the reaction time is extended to 12 hours in synthesis process, the morphology of the Cu-Ni nanocrystals is


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changed to Cu-Ni nanocages. The chemical stability of Ni and Cu is different. Specifically, Cu is easier to etching than Ni in a solution containing Cl- and Fe3+. By using a suitable chemical etchant, the Cu-Ni nanocages are obtained by selectively etching. The representative SEM images (Figure 3a) shows that the nanocubes with many surface vacancies, whose average edge length is 62 nm (Figure S7). Interestingly, the corners of each nanocatalyst are etched to form (111) facets. As the exceptional TEM images of the prepared Cu-Ni nanocages indicated in Figure 3b, the dark contrast and intense light on the surface of Cu-Ni nanocages confirms that there are small holes in the surfaces of nanocubes (Figure S8). The HRTEM image shows that the Cu-Ni nanocage with the lattice spacing of 0.18 nm and 0.209 nm, which corresponds to the (100) and (111) facets of Cu-Ni alloy (Figure 3c). The SAED (inset in Figure 3c) cycles represent the (111) and (100) planes of the Cu-Ni nanocages, suggesting that the obtained Cu-Ni nanocage is polycrystalline. HAADF-STEM image (Figure 3e) and TEM image obviously display a distinct luminance contrast between the center and border of nanocages, strongly proving the generation of Cu-Ni nanocages with hollow structure.46


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The HAADF-STEM-EDX elemental mapping image exhibit the uniform distribution of Cu and Ni elements clearly (Figure 3e), which confirmd that the nanocrystals are hollow structure with Cu-Ni alloy. The line scans analysis further to confirms Cu and Ni are distributed in the border of nanocages uniformly (Figure 3d). The EDX of Cu-Ni nanocages shows the ratio of Cu/Ni is 19:81 (Figure S9 and Table S1).


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Figure 3. a) SEM image of Cu-Ni nanocages. b) TEM image of Cu-Ni nanocages, c) HRTEM image of Cu-Ni nanocages. d) Line scan analysis of Cu-Ni nanocages. e) EDX elemental mappings of Cu-Ni nanocages. f) Schematic of Cu-Ni nanocages. The fine structure of Cu-Ni nanocages is further investigated by TEM and HRTEM. The shell thickness of Cu-Ni nanocages is ～ 5.6 nm (Figure 4a). The pore-size distribution of Cu-Ni nanocages show the distinct peak at 3 nm, which indicates the existence of a great many of mesopores (Figure 4c). The peak located at 10 nm can be ascribed to the gap between nanocubes. The HRTEM image (Figure 4b) also clearly shows the aperture of 2.3 nm on the edge of Cu-Ni nanocages, which is agreed with the consequence of the pore-size distribution curves. Both the pore size distribution curves and HRTEM image affirm that the open channels with nanometer scale can make for improving the mass transport of the reactants.47 The N2 adsorption–desorption isotherm of Cu-Ni nanocages occurs a hysteresis loop (Figure 4d), this behavior is corresponding to the classical mesoporous materials. The Brunauer-Emmett-Teller (BET) surface area of edge-cut Cu@Ni nanocubes, edge-notch Cu@Ni nanocubes and Cu-Ni nanocages is 9.91 m2 g-1, 9.27 m2 g-1 and 21.55 m2 g-1 respectively (Figure S10). Owing to their


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prominent hollow structural, The BET surface areas of the Cu-Ni nanocages is more twice than that of edge-cut Cu@Ni nanocubes and edge-notch Cu@Ni nanocubes.

Figure 4. a) TEM image of Cu-Ni nanocages. b) HRTEM image of Cu-Ni nanocages. c) pore size distribution diagram of Cu-Ni nanocages. d) N2 adsorption-desorption isotherm of Cu-Ni nanocages. The Ultraviolet-visible spectrum (UV-vis) of the as-prepared pure Cu nanocubes (Figure S11), edge-cut Cu@Ni nanocubes, edge-notch Cu@Ni nanocubes and Cu-Ni nanocages are showed in Figure S12. Based on the Mie scattering theory,48 the absorption band of Cu nanocube in the ultraviolet-visible range is due to surface


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plasmon resonance (SPR). The Cu nanocubes exhibit a representative absorption peak at about 600 nm. The UV-vis spectra of the edge-cut Cu@Ni nanocubes, edge-notch Cu@Ni nanocubes and Cu-Ni nanocages are different from the Cu nanocubes. The SPR band below 600 nm, which results from the damping effects of Ni.49 Through the UV-vis spectrum, we can further prove the structure of as-prepared nanocrystals. The synthesized nanocrystals further to be characterized by the X-ray diffraction (XRD). The XRD of edge-cut Cu@Ni nanocubes, edge-notch Cu@Ni nanocubes and Cu-Ni nanocages are showed in Figure 5, which show that the three nanocrystals consist of Cu, Ni and Cu-Ni phase without oxides and Fe phase. The peaks of the (111) planes of Cu@Ni nanocage are observed at 43.3° for Cu (JCPDS No. 04-0836) and 44.2° for Ni (JCPDS No. 04-0850), respectively. The two weak peaks at 50.4° and 74.3° are belong to the (200) and (220) planes, respectively. The XRD pattern of Cu-Ni nanocage displays the dominant peak at 43.8°, which is correspond to the (111), and two weaker peaks at 50.9° and 74.6°, belong to the reflects of (200) and (220) faces, respectively. All the peaks of the Cu-Ni nanocage appear among the position of the peak of pure Cu and Ni, confirming the formation of Cu-Ni alloy.


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Figure 5. XRD pattern of edge-cut Cu@Ni nanocubes, edge-notch Cu@Ni nanocubes and Cu-Ni nanocages. The above experimental results were further characterized, the X-ray photoelectron spectroscopy (XPS) is used to detect the chemical elements and valence state of the products. The elemental ratio of edge-cut Cu@Ni nanocube, edge-notch Cu@Ni nanocube and Cu-Ni nanocage for Cu:Ni measured by XPS amounts about 58.5:41.5, 84.3:15.7 and 19.8:80.2, respectively (Figure S13), which match well the elemental ratio of EDX. Figure 6a shows that the Cu 2p region of the Cu-Ni nanocages can be divided into two pairs of peaks. Two Cu 2p peaks are located at 951.9 eV and 932.2 eV, which corresponding to the Cu 2p1/2 and Cu 2p3/2 states of metallic Cu. The two weaker peaks at 954.2 eV and 934.1 eV can be assigned to the CuO in the surface of the sample, its


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satellite peak around 942 eV, was assigned to the CuO species. The Figure 6b shows XPS of the two Ni 2p1/2 and Ni 2p3/2 and at 869.7 eV and 852.3 eV. The other two peaks at 873.1 eV and 855.3 eV can be assigned to the Nix+ in the surface of the sample. However, in the Ni 2p XPS spectra of the Cu-Ni nanocages, the peak of Nix+ is higher than the peak of Ni, indicating that the Ni on the surface of Cu-Ni nanocage is oxidized. The edge-cut Cu@Ni nanocubes and edge-notch Cu@Ni nanocubes are analyzed in a similar way (Figure S14).

Figure 6. a) XPS spectra of Cu 2p and b) Ni 2p of Cu-Ni nanocages. In the synthesis process, the Cu2+ is firstly reduced to Cu, due to the Cu2+ is more reduction than Ni2+. The reason that Cu2+ is reduced to form edge-cut Cu nanocubes were caused by the capping effect of Cl-.50-52 Then the Ni2+ is reduced to cover the


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surface of the Cu nanocube. During the Ni2+ reduction, some copper atoms are doped into the Ni layer through co-reduction. When the reducing agent is completely consumed, the outer Cu is etched to form surface vacancies due to the presence of Cland Fe3+ in the solution. The Cl- and Fe3+ in the solution continually etch these vacancies and eventually create a channel. The Cu can be etched following reaction:

Time-dependent experiments were implemented to study growth mechanism of CuNi nanocages. The structure evolution process for Cu-Ni nanocages were summarized in Figure S15. The products collected after3 h of reaction, the edge-cut nanocubes with an edge length of 70 nm were obtained. Noticeably, the surface of edge-cut nanocubes collected after 3 h were slightly etched, and the edges to form surface vacancies (Figure S15a). As the reaction continuously proceeded, a part of the edge-cut nanocubes were etched to form hollow structure with edge length of about 62 nm (Figure S15b). After 9 h of reaction, the size of edge-cut nanocubes did not change, but the amounts of the


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hollow structure has increased (Figure S15c). After 12 h of reaction, all the nanoparticles have become hollow structures (Figure S15d). The as-prepared Cu-Ni nanocages with two basic facets and alloy effect exhibited excellent HER activity. The three-electrode system was used to measure the HER performances of catalysts in 1 M KOH electrolyte. The edge-cut Cu@Ni nanocubes, edge-notch Cu@Ni nanocubes and Cu-Ni nanocubes were tested by loading the catalyst on glassy carbon (GC) electrode. Figure 7a exhibits the polarization curves of as-prepared catalyst, indicating that the catalytical active of edge-cut Cu@Ni nanocube was higher than that of the edge-notch Cu@Ni nanocube and that the Cu-Ni nanocages showed the highest HER catalytical activity. In order to controllable experiment, the same referential measurements were conducted using the 20% Pt/C catalyst (Figure S16). The overpotential of Cu-Ni nanocages was only 140 mV versus RHE at 10 mA cm-2, and which was much smaller than the 183 mV for the edge-cut Cu@Ni nanocubes and 155 mV for the edge-notch Cu@Ni nanocubes. Apparently, the overpotential of CuNi nanocage is the lowest among the three morphologies of catalysts, indicates that CuNi nanocage has excellent HER activity. To further explain the high electrocatalytic


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performance of Cu-Ni nanocage, the Cu nanocube (Figure S11) and Cu-Ni nanoparticle (Figure S17a) were synthesized to measure HER performance. The XRD (Figure S17b) of Cu-Ni nanoparticle strongly confirmed the successful obtain of Cu-Ni alloy nanoparticles. Figure S18 shows that the overpotential of Cu nanocube and Cu-Ni nanoparticle were much higher than Cu-Ni nanocage, indicating that the excellent catalytic performance of the Cu-Ni nanocages can be assigned to the bimetallic synergistic catalysis and the unique hollow nanostructure. Our Cu-Ni nanocages has better performance than current non-precious metal catalysts (Table S2). The Tafel slope is characteristic of electrocatalysts, which is defined by the rate-limiting step of HER.53 The Tafel slope of edge-cut Cu@Ni nanocubes and edge-notch Cu@Ni nanocubes in Figure 7b is 172 mV dec-1 and 182 mV dec-1, whereas the Cu-Ni nanocages exhibited the smaller value of 79 mV dec-1. These data can be concluded that the Cu-Ni nanocages shows higher catalytical activity than other two samples. Compared to the edge-notch Cu@Ni nanocubes, both the edge-cut Cu@Ni nanocubes and the Cu-Ni nanocages were etched into (111) faces with the high catalytical active, and thence the catalytical active of edge-cut Cu@Ni nanocube was higher than that of


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the edge-notch Cu@Ni nanocube. Meanwhile, the Cu-Ni nanocages with the unique hollow nanostructure and perfect lattice could render the catalyst much larger surface areas and active sites. The fine structure of Cu-Ni nanocages (Figure 4a, b) is beneficial to improve the mass transport and gas diffusion. Therefore, the Cu-Ni nanocages exhibit the best HER performance.

Figure 7. a) Polarization curves of edge-cut Cu@Ni nanocubes, edge-notch Cu@Ni nanocubes and Cu-Ni nanocages. b) Tafel plots of edge-cut Cu@Ni nanocubes, edge-


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notch Cu@Ni nanocubes and Cu-Ni nanocages. c) Durability test of Cu-Ni nanocages in 1 M KOH. d) Current-time curves of Cu-Ni nanocages and Pt/C (inset of d) in 1 M KOH. To investigate the stability of the Cu-Ni nanocages, the long-term cycling tests were performed at potentials between -0.5 and 0 V (versus RHE) at a scan rate of 100 mV s1.

As shown in Figure 7c, the resulting polarization curve of Cu-Ni nanocages shows a

slight increase in the overpotential after 2000 cycles. However, a significant shift was observed in the polarization curve of commercial Pt/C catalyst under the same experimental conditions (Figure S19). Due to the hollow structure, Cu-Ni nanocages shows the excellent cycling performance, while the Pt/C catalyst can be easily agglomerated during the electrocatalytic reactions. The long-term stability of Cu-Ni nanocages is examined at -22 mV for 8 h to further study the stabilities. Figure 7d display the current densities of Cu-Ni nanocage stabilized at around 19 mA cm-2, and there was no significant attenuation throughout the process. In contrast, the current density was reduced to 5 mA cm-2 for Pt/C catalyst after 10000 s (inset of Figure 7d). The electrochemically active surface area (ECSA) of the edge-cut Cu@Ni nanocubes, edge-notch Cu@Ni nanocubes and Cu-Ni nanocages samples were determined by a


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double-layer capacitance (Cdl). Figure S20a shows that the CV curves of Cu-Ni nanocages at 10, 20, 30, 40, 50 and 60 mV s-1 in the 0.21-0.31 V region. The Cdl is determined from CV curves at the potential of 0.25 V, which was calculated from the slope of linear fitting for the capacitive, and the values of the Cu-Ni nanocages, edgecut Cu@Ni nanocubes and edge-notch Cu@Ni nanocubes was 1.63 mF cm-2, 0.58 mF cm-2 and 0.48 mF cm-2, respectively (Figure S20b). The Cdl of Cu-Ni nanocages was 2.8 and 3.4 times that of edge-cut Cu@Ni nanocubes and edge-notch Cu@Ni nanocubes. The Cdl is proportional to the active surface area of Cu-Ni nanocages, which corresponds to the excellent HER activity of Cu-Ni nanocages. To further understand the HER activity of the catalysts, electrochemical impedance spectroscopy (EIS) was performed as shown in Figure S21. The Nyquist semicircle of the Cu-Ni nanocages was much lower than edge-cut Cu@Ni nanocubes and edge-notch Cu@Ni nanocubes, indicating the better electron conductivity of Cu-Ni nanocages than the other catalysts. For the purpose of understanding the high catalytic activity of the Cu-Ni nanocages toward HER, the density functional theory (DFT) calculations are used to calculate the


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H* absorption free energy (ΔGH*) of different models’ pure Ni and Cu-doped Ni and to understand the enhancement mechanism of the HER. The edge-cut Cu@Ni nanocubes with core-shell structure exposed (100) and (111) facets (Figure 1c), which corresponding to the lattice spacing of Ni. In addition, the Cu-Ni nanocages exposed (100) and (111) facets (Figure 3c), which corresponding to the lattice spacing of Cu-Ni alloy. Figure 8 showed that the three models are constructed to represent the Cu2-Ni, Cu1-Ni and pure Ni, respectively. The entire HER pathway consists of an initial state (H++e-), an intermediate state (adsorbed H*) and a final state (1/2H2), and the intermediate state of Gibbs free energy (ΔGH*) represents the catalytic activity for HER. For the hydrogen generation reaction, the |ΔGH*| of the catalyst should be close to 0 should be optimal. On (100) facets (Figure 8a), H atoms were adsorbed on the hollow site of metal, and the |ΔGH*| continuously decrease with the increase of doped Cu. Specifically, the ΔGH* is -0.29 eV, -0.31 eV and -0.34 eV for Cu2-Ni, Cu1-Ni and pure Ni catalysts indicate that the activity of pure Ni is lower than Cu-Ni alloy is that the adsorption energy of the intermediate state (adsorbed H*) is too strong. Therefore, it is


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corresponding to the Cu-Ni nanocages catalysts exhibited a higher HER activity than edge-cut Cu@Ni nanocubes and edge-notch Cu@Ni nanocubes.

Figure 8. The calculated free-energy diagram of the HER at a) (100) facets and b) (111) facets for pure Ni, Cu1-Ni and Cu2-Ni. Same process has been explored on (111) facets (Figure 8b). Indeed, to understand the enhancement mechanism of H2 generation on Cu-Ni (111) surface, electron Fermi level is calculated. The ΔGH* of the Cu2-Ni, Cu1-Ni and pure Ni is 0.13eV, -0.28 eV and -0.39 eV, respectively. The |ΔGH*| of Cu2-Ni is 0.13 eV, which is closer to zero than pure Ni, thus causing mediated adsorption-desorption behavior of the Cu-Ni nanocages to more facilitate the HER performance than edge-cut Cu@Ni nanocubes and edge-notch Cu@Ni nanocubes. Furthermore, the ΔGH* is -0.13 eV and 0.29 eV for Cu2-Ni on (111) facets and (100) facets respectively, the ΔGH* of (111)


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facets is closer to zero than (100) facets, indicating the HER performance of Cu-Ni (111) facets is higher than Cu-Ni (100) facets. In short, we have calculated Cu-Ni alloy effect for HER activity on (100) facets and (111) facets, and the results show that the (111) facets are smaller |ΔGH*| than the (100) facets, this suggests that (111) facets can provide more active sites to adsorb more hydrogen. 4. CONCLUSIONS

In summary, we have developed a novel and facial self-templated strategy to synthesize a series of nanostructures, including the unique edge-cut Cu@Ni nanocubes, edge-notch Cu@Ni nanocubes and Cu-Ni nanocages by selective wet chemical etching method. By steady controlling the amount of FeCl3·6H2O and reaction time, the mesoporous Cu-Ni nanocages are obtained. In the synthesis process, the corners of edge-cut Cu@Ni nanocubes and Cu-Ni nanocages are etched to produce the high catalytical active (111) facets. Impressively, the Cu-Ni nanocages exhibit higher electrocatalytic activity toward HER under alkaline conditions than edge-notch Cu@Ni nanocubes and edge-cut Cu@Ni nanocubes, is attributted to the mesoporous hollow


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structure, bimetallic synergistic catalysis, and higher catalytic active (111) facets. Moreover, from the calculation results of density functional theory (DFT), it can be found that the reason why the activity of pure Ni is lower than Cu-Ni alloy is that the adsorption energy of the intermediate state (adsorbed H*) is too strong. Meanwhile the Gibbs freeenergy (|ΔGH*|) of (111) facets is smaller than (100) facets, which brings more active sites or adsorb more hydrogen. This work bring a new strategy to synthesize core-shell and hollow structures, which can improve their application prospects in catalytic reactions.

ASSOCIATED CONTENT

Supporting Information. Further SEM/XPS/UV-vis characterization of the as-prepared edge-cut Cu@Ni nanocubes, edge-notch Cu@Ni nanocubes and Cu-Ni nanocages. TEM and XRD characterization of Cu-Ni nanoparticle. Polarization curves of Cu nanocube, Cu-Ni nanoparticle and Cu-Ni nanocage. The CV, Cdl and Nyquist plots of


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the as-prepared edge-cut Cu@Ni nanocubes, edge-notch Cu@Ni nanocubes and Cu-Ni nanocages. The LSV and Cycling stability of commercial Pt/C catalyst.

AUTHOR INFORMATION

Corresponding Author *Email for Z.L.: [email protected]

* Email for W.S.: [email protected]

ACKNOWLEDGMENT We gratefully acknowledge the financial support from the Beijing Natural Science Foundation (Grant Nos. 2182061).

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ACS Catalysis

TOC

The mesoporous Cu-Ni nanocages prepared by a self-templated strategy with the selective wet chemical etching method exhibit high electrocatalytic activity toward hydrogen evolution reaction, which is attributed to the mesoporous hollow structure, bimetallic synergistic catalysis, and high catalytic active (111) facets.


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Mesoporous Hollow Cu-Ni Alloy Nanocage from Core-Shell Cu@Ni

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