Efficient Hydrogen Evolution on Cu Nanodots-Decorated Ni3S2

(b) The calculated adsorption free energy changes of H2O on Cu/Ni3S2, Ni3S2 and Cu. (c) Raman spectra of various catalysts before and after HER at ove...
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Article Cite This: J. Am. Chem. Soc. 2018, 140, 610−617

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Efficient Hydrogen Evolution on Cu Nanodots-Decorated Ni3S2 Nanotubes by Optimizing Atomic Hydrogen Adsorption and Desorption Jin-Xian Feng, Jin-Qi Wu, Ye-Xiang Tong, and Gao-Ren Li* MOE Laboratory of Bioinorganic and Synthetic Chemistry, The Key Lab of Low-Carbon Chemistry and Energy Conservation of Guangdong Province, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, China S Supporting Information *

ABSTRACT: Low-cost transition-metal dichalcogenides (MS2) have attracted great interest as alternative catalysts for hydrogen evolution. However, a significant challenge is the formation of sulfur−hydrogen bonds on MS2 (S−Hads), which will severely suppress hydrogen evolution reaction (HER). Here we report Cu nanodots (NDs)-decorated Ni3S2 nanotubes (NTs) supported on carbon fibers (CFs) (Cu NDs/Ni3S2 NTs-CFs) as efficient electrocatalysts for HER in alkaline media. The electronic interactions between Cu and Ni3S2 result in Cu NDs that are positively charged and can promote water adsorption and activation. Meanwhile, Ni3S2 NTs are negatively charged and can weaken S−Hads bonds formed on catalyst surfaces. Therefore, the Cu/Ni3S2 hybrids can optimize H adsorption and desorption on electrocatalysts and can promote both Volmer and Heyrovsky steps of HER. The strong interactions between Cu and Ni3S2 cause the Cu NDs/Ni3S2 NTs-CFs electrocatalysts to exhibit the outstanding HER catalytic performance with low onset potential, high catalytic activity, and excellent stability.

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INTRODUCTION Hydrogen gas (H2) is a promising energy carrier and key agent for many industrial chemical processes.1−4 Steam reforming, partial oxidation, and coal gasification suffer from serious carbon dioxide emission and are not carbon-neutral hydrogenmaking techniques. So far water electrolysis in alkaline media is an efficient method to produce high-quality H2.5−8 Pt is generally considered as one of the best catalysts toward hydrogen evolution reaction (HER), however, the high-cost of Pt limits its widespread use.9−13 Therefore, non-Pt metal-based materials should be developed as efficient HER electrocatalysts, although most of them still suffer from high overpotential and/ or poor durability.14−18 It has been reported that the typical process of HER in alkaline media on the electrocatalysts can be summarized as two radical steps, namely, eqs 1 and (2) (Volmer−Heyrovsky pathway):19,20 M + H 2O + e → M−Hads + OH−

(Volmer step)

where M refers to catalysts. In alkaline media, H2O is the reacting species for HER. The Volmer step, namely the cleavage of O−H bonds of H2O to form H atoms adsorbed on the surface of catalysts (Hads), is crucial.21,22 In addition, as the H2 is the target product, the Heyrovsky step, namely the Hads combining H from H2O into H2 gas, is also important for HER.22,23 So an ideal catalyst for HER in alkaline media should satisfy the following two requirements:24,25 (i) the catalyst should own the strong functions to activate and cleave O−H bonds of water molecules to form H atoms, and the H atoms should be easily adsorbed on the surface of catalysts (Hads); (ii) the Hads cannot be securely immobilized on the surfaces of catalysts since H2 gas should be readily released and the efficient HER is hoped to be realized. Therefore, optimizing atomic hydrogen adsorption and desorption on the electrocatalysts is very important for efficient hydrogen evolution. Metal sulfides, such as MoS2 and Ni3S2, as HER catalysts own high electrocatalytic activity and low cost, showing great potential for industrial applications.26−28 For metal sulfides, S− Hads bonds are easily generated on the surfaces of catalysts during HER, and the formation of S−Hads bonds will be

(1)

H 2O + M−Hads + e → H 2 + M + OH− (Heyrovsky step)

Received: August 10, 2017 Published: October 23, 2017

(2) © 2017 American Chemical Society

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DOI: 10.1021/jacs.7b08521 J. Am. Chem. Soc. 2018, 140, 610−617

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Figure 1. (a) Schematic illustration of the microstructure of Cu NDs/Ni3S2 NTs-CFs. (b) XRD pattern of Cu NDs/Ni3S2 NTs-CFs. (c−f) SEM images of the surface morphology of Cu NDs/Ni3S2 NTs-CFs. (g) TEM image of Cu NDs/Ni3S2 NTs. (h) TEM image with higher magnification of the wall of Cu NDs/Ni3S2 NTs. (i−k) Elemental mappings of Ni, Cu and S in Cu NDs/Ni3S2 NTs. (l) SAED pattern of Cu NDs/Ni3S2 NTs. (m) HRTEM image of Cu/Ni3S2 border.

beneficial for H adsorption.29 However, the S−Hads bonds on the surface of metal sulfides are usually so strong that making the Hads transfer to H2 is difficult.28,29 In addition, with the evolution of HER, the coordination state of metal sulfides will be coordinate saturated so that water molecules adsorption will become difficult.26,29,30 The above factors will obviously inhibit HER catalytic activity of metal sulfides. As we all know, the electronic interaction between metal and metal sulfide can make electron transfer from metal to metal sulfide,25 and this will promote water adsorption and activation and optimize H adsorption and desorption by changing electron density distributions of electrocatalysts. Accordingly, HER performance of the electrocatalysts will be obviously improved.31 Based on the above considerations, the metal/metal sulfide hybrids are studied as high-performance electrocatalysts for HER in alkaline media. Here the Cu nanodots (NDs)-decorated Ni3S2 nanotubes (NTs) supported on carbon fibers (CFs) (Cu NDs/Ni3S2 NTsCFs) were rationally designed to achieve a state-of-art nonprecious metal electrocatalyst for HER in alkaline media. Cu NDs/Ni3S2 NTs-CFs as electrocatalysts show the following advantages: (i) the hierarchical structure of Cu NDs/Ni3S2 NTs-CFs will favor mass transportation/diffusion during HER electrocatalysis; (ii) Hads can be easily realized on the surface of Ni3S2 via the formation of S−Hads bonds; (iii) the electronic interactions between Cu and Ni3S2 will make Cu positively charged and Ni3S2 negatively charged. The positively charged Cu can effectively adsorb and activate water molecules and will

benefit H−O cleavage, and the negatively charged Ni3S2 can weaken S−Hads bonds and thus will optimize H adsorption and desorption. Because of the above advantages, Cu NDs/Ni3S2 NTs-CFs hybrid catalysts show high electrocatalytic activity and excellent stability toward HER in alkaline media.

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EXPERIMENTAL SECTION

Synthesis of Electrocatalysts. In the typical procedures of the synthesis of Cu NDs/Ni3S2 NTs-CFs, all the electrochemistry-related experiments were carried out in a simple two-electrode cell by galvanostatic electrolysis. The graphite electrode was used as a counter electrode (spectral grade, 1.8 cm2). The CFs (Phychemi Company, Hong Kong) were used as a working electrode (0.5 cm × 2 cm). ZnO nanorods (NRs) were fabricated on CFs by cathodic electrodeposition at a constant current of 1.0 mA cm−2 in the solution of 0.01 M Zn(NO3)2 + 0.05 M NH4NO3 (10 mL) at 70 °C for 90 min. Ni3S2 layers were then coated on the surfaces of ZnO NRs to form ZnO@ Ni3S2 NRs-CFs by cathodic electrodeposition in the solution of 0.01 M NiCl2 + 0.1 M Na2S2O3 (10 mL) at 1.0 mA cm−2 for 8 min at 70 °C. Ni3S2 nanotubes (NTs)-CFs were fabricated by etching ZnO from ZnO@Ni3S2 NRs-CFs in 2.5 M NaOH solution (15 mL) for 2 h. Finally, Cu NDs were then decorated on the surfaces of Ni3S2 NTs to fabricate Cu NDs/Ni3S2 NTs-CFs by using chemical reduction method in the solution of 5 mM CuAc2 + 1 wt % polyvinylpyrrolidone (PVP) + 0.1 g NaBH4 (10 mL) for 10 min at 30 °C. The samples were dried in nitrogen flow at 120 °C to remove excess moisture. Cu NDs/ Ni3S2 NTs-CFs with different mass ratios of Cu/Ni3S2 can be achieved by controling deposition conditions of Cu NDs and Ni3S2 NTs. Structural and Electrochemical Characterizations. 611

DOI: 10.1021/jacs.7b08521 J. Am. Chem. Soc. 2018, 140, 610−617

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(1) The powder X-ray diffraction (XRD) patterns of samples were recorded on a Rigaku D/Max 2550 X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). (2) The X-ray photoelectron spectroscopy (XPS) spectra of samples were performed on an XPS, ESCA Lab250 X-ray photoelectron spectrometer. All XPS spectra were corrected using C 1s line at 284.6 eV, and curve fitting and background subtraction were accomplished. (3) The transmission electron microscope (TEM) images of samples were measured with a TEM, JEM-2010HR and highresolution TEM (HRTEM, 200 kV or 300 kV). (4) The scanning electron microscope (SEM) images of samples were measured with Zeiss Sigma field emission SEM (FE-SEM, JSM-6330F). (5) The Raman tests of samples were carried out on a laser microRaman spectrometer (Renishaw inVia) equipped with a He− Ne laser (wavelength = 514.5 nm) and a long working distance 50× objective lens. (6) All the electrochemical experiments were implemented in a three-electrode system with an electrochemical station (CHI 760D). The working electrode was a glassy carbon electrode. The graphite rod and the saturated calomel electrode (SCE) served as counter and reference electrodes, respectively. For HER experiments, the linear sweep voltammograms (LSVs) and cyclic voltammograms (CVs) were measured at a scan rate of 5 mV s−1 in 1.0 M KOH solution. In order to get closer to the actual situation, here the potential value of each electrode was not corrected by compensating iR drop. TOF Calculations. TOF values were calculated using eq 3: TOF(s−1) = (j × A)/(2 × F × n)

Article

RESULTS AND DISCUSSION

Cu NDs/Ni3S2 NTs-CFs are designed as high-performance and low-cost electrocatalysts for HER in alkaline solution as illustrated in Figure 1a, and the fabrication process is shown in Scheme S1. SEM image of CFs is shown in Figure S1, which shows the fiber diameters are about 7 μm. ZnO nanorods (NRs) were electrodeposited on CFs, and SEM image of ZnO NRs is shown in Figure S2. Ni3S2 layers are uniformly coated on the surface of ZnO NRs to form ZnO@Ni3S2 NRs, and Ni3S2 NTs were fabricated by removing ZnO with NaOH. SEM image, TEM image, and XRD pattern of Ni3S2 NTs-CFs are shown in Figure S3, which shows Ni3S2 NTs were successfully fabricated. Then Cu NDs were uniformly deposited on Ni3S2 NTs by chemical reduction, and Cu NDs/Ni3S2 NTs-CFs were successfully fabricated. XRD pattern of Cu NDs/Ni3S2 NTsCFs is shown in Figure 1b, which clearly shows Ni3S2 and Cu phases in Cu NDs/Ni3S2 NTs-CFs. SEM images of Cu NDs/ Ni3S2 NTs-CFs with different magnifications are shown in Figure 1c−f, which clearly shows nanotube array structure of Cu NDs/Ni 3 S2 NTs. The diameters of Cu NDs/Ni3S 2 nanotubes are ∼450 nm. To further demonstrate the hierarchical nanotube structure of Cu NDs/Ni3S2 NTs, TEM images of Cu NDs/Ni3S2 NTs are shown in Figure 1g,h, which shows the wall thickness of nanotube is 40 nm, and the Cu NDs are uniformly decorated on the walls of Ni3S2 nanotubes. The wall thicknesses of Cu NDs/Ni3S2 NTs are ∼30 nm, and the sizes of Cu NDs are ∼10 nm. In order to identify the distributions of Ni3S2 and Cu, energy disperse spectrum (EDS) element mappings were measured, and they are shown in Figure 1i−k, which further demonstrates that the Cu NDs are decorated on the surfaces of Ni3S2 NTs. The atomic ratio of Ni and S in the nanotube area is ∼1.57:1, indicating Ni and S serve as Ni3S2. The atomic ratio of Cu and O in the ND area is ∼27:1, indicating the existence of high pure metal Cu. The selected area electron diffraction (SAED) of Cu NDs/Ni3S2 NTs is shown in Figure 1l, where some of the characteristic diffraction points such as Ni3S2 (204) and (110) and Cu (101) can be clearly seen. High-resolution TEM (HRTEM) image of a typical boundary area of Cu NDs and Ni3S2 NTs is shown in Figure 1m, which shows that the Ni3S2 has interplanar spacings of 0.287, 0.204, and 0.183 nm, which are identical with those of Ni3S2(110), (202), and (113), respectively. Cu ND has an interplanar spacing of 0.287 nm, which is coincident with that of Cu(111). The above HRTEM results are well coincident with those of XRD of Cu NDs/Ni3S2 NTs. The XPS spectra of Ni 2p, S 2p, and Cu 2p of Cu NDs/Ni3S2 NTs-CFs are shown in Figure S4a,b. The deconvoluted Ni 2p3/2 profiles clearly show the peaks at 856.71 and 859.09 eV correspond to Ni0 and Ni2+ in Ni3S2 [(Ni2+)2(Ni0)(S2−)2], respectively,34 and Ni 2p1/2 profiles show the peaks at 875.63 and 876.96 eV correspond to Ni0 and Ni2+ in Ni3S2, respectively.34 XPS spectrum of S 2p of Cu NDs/Ni3S2 NTsCFs shows the peak at 162.5 eV corresponds to S 2p3/2 of S2− and the peak at 163.9 eV corresponds to S 2p1/2 of S2−.35 XPS spectrum of Cu 2p of Cu NDs/Ni3S2 NTs-CFs is shown in Figure S4c. The peaks at 932.64 and 952.54 eV can be attributed to Cu 2p3/2 and 2p1/2 of metal Cu, respectively, and they well prove the existence of metal Cu.36 Such results prove the co-existences of Ni3S2 and Cu in the Cu NDs/Ni3S2 NTsCFs. The electrocatalytic performance of Cu NDs/Ni3S2 NTs-CFs was studied in alkaline solution (1.0 M KOH, pH = 14). The

(3)

−2

where j (mA cmgeo ) is the measured current density at a definite overpotential, A (cmgeo2) is the surface area of electrocatalyst, the number 2 means 2 electrons to generate 1 mole of H2, F is Faraday’s constant (96485.3 C/mol), and n is the mole of electrochemical materials on electrode calculated from the mass and the molecular weight of the catalysts. ESCA Measurements and Calculations. To acquire ECSA of the working electrode, their roughness factor (Rf) should be obtained first according to the equation: ECSA = RfS, where S was generally equal to the geometric area of electrode (in this work, S = 1.0 cm−2). The Rf was determined by the relation Rf = Cdl/20 μF cm−2 based on the double-layer capacitance (Cdl) of a smooth metal surface (20 μF cm−2),32 where Cdl could be acquired by cyclic voltammetry measurement under the potential windows of −1.0 to −0.8 V vs SCE (1.0 M KOH solution). The scan rates were 2, 5, 10, 20, 50, and 100 mV s−1. The Cdl was estimated by plotting ja−jc at −0.9 V (where jc and ja are the cathodic and anodic current densities, respectively) vs SCE against the scan rate, where the slope was twice that of Cdl. Density Functional Theory Calculations. The computational modeling of the reactants, intermediates, and production and the reaction process involved in HER on Cu, Ni3S2 and Cu/Ni3S2 hybrid system was performed by density functional theory (DFT) method with the Gaussian 09W program,33 B3LYP method, and LanL2DZ basis set. The crystal plane of Ni3S2 for DFT calculation is (110) facet and the crystal plane of Cu for DFT calculation is (111) facet (JCPDS no. 44-1418 for Ni3S2 and JCPDS no. 65-9026 for Cu). All of the structures were fully optimized and relaxed to the ground state, and spin polarization was considered in all calculations. The convergence of energy was set to 1 × 10−8. The Gibbs free energy change (ΔG) of each reaction step is calculated as ΔG = Etot(b) − Etot(a) + ΔEZPE − T ΔS where Etot(b) is the energy of given unit cell with intermediate of latter state, Etot(a) is the energy of intermediates of previous state, ΔEZPE is the difference corresponding to zero point energy change between the intermediates of previous state and latter state, and ΔS is the entropy change between the intermediates of previous state and latter state. 612

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Figure 2. (a) Polarization curves of Cu NDs/Ni3S2 NTs-CFs, Ni3S2 NTs-CFs, Cu NDs-CFs, and CFs. (b) Tafel plots of Cu NDs/Ni3S2 NTs-CFs, Ni3S2 NTs-CFs, Cu NDs-CFs, and CFs. (c) Nyquist plots of Cu NDs/Ni3S2 NTs-CFs, Ni3S2 NTs-CFs, and Cu NDs-CFs. (d) Chronopotentiometric measurements of Cu NDs/Ni3S2 NTs-CFs at various overpotentials.

loading mass of Cu NDs/Ni3S2 NTs was 0.52 mg cm−2. Ni3S2 NTs-CFs and Cu NDs-CFs with the same loadings of Ni3S2 and Cu, respectively, were also fabricated for comparative studies (the relevant SEM images and XRD patterns are shown in Figures S3 and S5). The mass of Cu NDs can be controlled via adjusting the concentration of CuAc2. The relative results are shown in Figure S6. The polarization curves of Cu NDs/ Ni3S2 NTs-CFs with different mass ratios of Cu/Ni3S2 are shown in Figure S7. When the mass ratio of Cu/Ni3S2 is 0.18, Cu NDs/Ni3S2 NTs-CFs exhibit the highest catalytic activity. Cu NDs/Ni3S2 NTs loading can be tuned by changing electrodeposition time for Ni3S2 and the concentration of CuAc2 for Cu. In Figure S8, HER electrochemical performance with various loadings of Cu NDs/Ni3S2 NTs suggests 0.52 mg cm−2 is the optimal loading. The mass ratio of Cu/Ni3S2 in Cu NDs/Ni3S2 NTs-CFs was kept at 0.18 in all of the following experiments. The electrocatalytic activities of Cu NDs/Ni3S2 NTs-CFs, Ni3S2 NTs-CFs, and Cu NDs-CFs were also compared as shown in Figure 2a. For Cu NDs/Ni3S2 NTsCFs, the onset overpotential is only ∼60 mV, and the overpotential at the current density of 10 mA cm−2 is only 128 mV, which is much smaller than those of Ni3S2 NTs-CFs (189 mV), Cu NDs-CFs (>300 mV), and CFs (>300 mV). Cu NDs/Ni3S2 NTs-CFs also give much higher current densities than Ni3S2 NTs-CFs, Cu NDs, and CFs at the same overpotentials (e.g., at −0.15, −0.20, and −0.25 V) as shown in Figure 2a, indicating Cu NDs/Ni3S2 NTs-CFs own much higher catalytic activity than Ni3S2 NTs-CFs, Cu NDs-CFs, and CFs. The Tafel slope of Cu NDs/Ni3S2 NTs-CFs at the overpotential interval between 0.01 and 0.20 V is only ∼76.2 mV/dec, which is much smaller than those of Ni3S2 NTs-CFs (125.7 mV/dec), Cu NDs-CFs (166.7 mV/dec), and CFs (168.4 mV/dec) as shown in Figure 2b.

To investigate the catalytic kinetics under the HER process, the EIS measurements were carried out from 100 MHz to 0.01 Hz at an overpotential of 200 mV. Nyquist plots of Cu NDs/ Ni3S2 NTs-CFs, Ni3S2 NTs-CFs, and Cu NDs-CFs are shown in Figure 2c, which shows that the charge-transfer resistance of Cu NDs/Ni3S2 NTs-CFs is smaller than those of Ni3S2 NTsCFs and Cu NDs-CFs, indicating more favorable catalytic kinetics for Cu NDs/Ni3S2 NTs-CFs. The turnover frequency (TOF) values of Cu NDs/Ni3S2 NTs-CFs at different overpotentials were also calculated as shown in Figure S9, which shows that Cu NDs/Ni3S2 NTs-CFs own much larger TOF values than those of Ni3S2 NTs-CFs, Cu NDs-CFs, and CFs at different overpotentials. The electrochemically active surface areas (ECSAs) of Cu NDs/Ni3S2 NTs-CFs, Ni3S2 NTsCFs, and Cu NDs-CFs were determined by measuring the double-layer capacitance,37,38 and Figure S10 shows that the ECSA of Cu NDs/Ni3S2 NTs-CFs is much larger than those of Ni3S2 NTs-CFs and Cu NDs-CFs. So the enhanced electrocatalytic activity of Cu NDs/Ni3S2 NTs-CFs compared with those of Ni3S2 NTs-CFs, Cu NDs-CFs, and CFs can be attributed to a smaller charge-transfer resistance, larger TOF value, and larger ECSAs. Most importantly, the HER electrocatalytic activity of Cu NDs/Ni3S2 NTs-CFs is also much higher than many other recently developed electrocatalysts as shown in Table S1. To further evaluate the long-term activity retention of electrocatalysts, the chronoamperometry experiments of Cu NDs/Ni3S2 NTs-CFs were further carried out at the different overpotentials of 100, 150, 200, and 250 mV for 30 h as shown in Figure 2d, which shows the current densities almost remain unchangeable at the different overpotentials, further indicating high stability of catalytic activity of Cu NDs/Ni3S2 NTs-CFs. The polarization curves of Cu NDs/Ni3S2 NTs-CFs before and 613

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Figure 3. (a) Raman spectra of Cu NDs/Ni3S2 NTs-CFs, Ni3S2 NTs-CFs, and Cu NDs-CFs at 180−380 cm−1. XPS spectra of (b) Ni 2p, (c) S 2p of Cu NDs/Ni3S2 NTs-CFs and Ni3S2 NTs-CFs, and (d) Cu 2p of Cu NDs/Ni3S2 NTs-CFs and Cu NDs-CFs.

Figure 4. (a) The charge density distributions on Cu/Ni3S2, Ni3S2, and Cu. (b) The calculated adsorption free energy changes of H2O on Cu/Ni3S2, Ni3S2 and Cu. (c) Raman spectra of various catalysts before and after HER at overpotential of 250 mV for 30 h (dash lines: before HER; solid lines: after HER). (d) FT-IR ATR spectra of water adsorbed on various catalysts (dash lines: before HER; solid lines: after HER).

SEM/TEM images, and XPS spectra of Cu NDs/Ni3S2 NTsCFs before and after the constant HER at overpotential of 250 mV for 30 h are shown in Figures S12−S14, respectively, which show that the structure, surface morphology, and chemical state

after HER reaction of 30 h are shown in Figure S11, which shows almost no change for these polarization curves, indicating that Cu NDs/Ni3S2 NTs-CFs own excellent longterm stability of catalytic activity. In addition, XRD patterns, 614

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Figure 5. (a) H adsorption free energy profiles of Cu NDs/Ni3S2 hybrid, Ni3S2, and Cu. (b) Raman spectra of catalysts at 50 mV (dash lines) and −250 mV (solid lines) at 2000−3200 cm−1. Schematic illustration of water adsorption, water activation, and hydrogen generation processes: (c) Ni3S2 and (d) Cu/Ni3S2.

Ni3S2 NTs-CFs show positive shifts of 0.47 and 0.50 eV, respectively, compared with those of Cu NDs-CFs as shown in Figure 3d. Therefore, the strong electronic interactions happened between Cu and Ni3S2 in Cu NDs/Ni3S2 NTs-CFs. The electronic interactions between Ni3S2 and Cu will first promote water adsorption that not only will improve the Volmer step to generate Hads (eq 1) but also will improve the Heyrovsky step to transfer S−Hads into H2 (eq 2).41 The DFT calculations were carried out to determine the charge redistribution of Cu/Ni3S2 hybrid as shown in Figure 4a (the optimized structures of Cu, Ni3S2, and Cu/Ni3S2 hybrid are shown in Figure S15−S17, respectively). In the Cu/Ni3S2 hybrid, the positive charge on Cu increases, the positive charge on Ni of Ni3S2 decreases, and the negative charge on S of Ni3S2 increases compared with the individual Cu, Ni, and S, respectively, indicating the electron density on Cu decreases and the electron density on Ni3S2 increases. Considering the water is a typical polar molecule consisting of two H atoms with positive charge and an oxygen atom with negative charge (as inserted in Figure 4b), the positively charged Cu in Cu NDs/ Ni3S2 NTs-CFs will adsorb and activate water molecules by capturing O atoms of water. Here water adsorption free energies of Cu, Ni3S2, and Cu/Ni3S2 were studied by DFT calculations as shown in Figure 4b, which shows that Cu/Ni3S2 hybird owns the lowest water adsorption energy, suggesting that the water adsorption on Cu NDs/Ni3S2 NTs-CFs is easier than those on Cu NDs-CFs and Cu NDs/Ni3S2 NTs-CFs. The Cu NDs/Ni3S2 NTs-CFs was further proved to promote water adsorption and activation for HER by experimental means. As we all know, Raman shifts at 400−650 cm−1 can detect water adsorption on Cu catalysts.42 Figure 4c shows Raman spectra of Cu NDs/Ni3S2 NTs-CFs, Ni3S2 NTs-CFs, and Cu NDs-CFs before and after HER at the overpotential of

of Cu NDs/Ni3S2 NTs-CFs are hardly changed after the constant HER at overpotential of 250 mV for 30 h. Therefore, the high stability of HER catalytic activity can be attributed to high stability of surface morphology, structure, and chemical state of Cu NDs/Ni3S2 NTs-CFs. In addition, Cu NDs/Ni3S2 NTs-CFs also showed the enhanced stability compared with many other electrocatalysts in alkaline media as shown in Table S2. Here the electronic interaction between Cu and Ni3S2 plays an important role on the enhanced HER performance of Cu NDs/Ni3S2 NTs-CFs. To prove the electronic interactions of Cu and Ni3S2, Raman spectra of Cu NDs/Ni3S2 NTs-CFs, Ni3S2 NTs-CFs, and Cu NDs-CFs are measured as shown in Figure 3a. For Cu NDs-CFs, almost no characteristic Raman band is seen. For Cu NDs/Ni3S2 NTs-CFs, the characteristic Raman bands at 196.7, 218.4, 298.1, 319.8, and 345.8 cm−1 are relative to Ni−S bonds of Ni3S2.39,40 Compared with those of Ni3S2 NTs-CFs, the peaks of Cu NDs/Ni3S2 NTs-CFs all show red-shifts as shown in Figure 3a, indicating that the state of Ni− S bonds changes due to electronic interactions between Ni3S2 and Cu. Additionally, XPS spectra of Cu NDs/Ni3S2 NTs-CFs were explored to study electronic interactions between Ni3S2 and Cu in depth. Figure 3b−c shows the comparisons of Ni 2p and S 2p XPS peaks between Cu NDs/Ni3S2 NTs-CFs and Ni3S2 NTs-CFs, and Figure 3d shows the comparison of Cu 2p XPS peaks between Cu NDs/Ni3S2 NTs-CFs and Cu NDsCFs. The peaks of Ni 2p3/2 and Ni 2p1/2 of Cu NDs/Ni3S2 NTs-CFs show negative shifts of ∼0.62 and 0.98 eV, respectively, compared with those of Ni3S2 NTs-CFs as shown in Figure 3b. The peaks of S 2p3/2 and S 2p1/2 of Cu NDs/Ni3S2 NTs-CFs show negative shifts of 0.21 and 0.24 eV, respectively, compared with those of Ni3S2 NTs-CFs as shown in Figure 3c. The peaks of Cu 2p3/2 and Cu 2p1/2 of Cu NDs/ 615

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CONCLUSIONS In summary, we realized efficient HER electrocatalysis in alkaline media by using Cu NDs/Ni3S2 NTs-CFs as catalysts. The electronic interactions between Cu and Ni3S2 make Cu positively charged and will benefit water adsorption and activation, while Ni3S2 is negatively charged and will weaken S−Hads bonds formed on the surfaces of catalysts. The above electronic interactions can well optimize H adsorption and desorption on the surfaces of metal sulfide-based electrocatalysts and can efficiently promote Volmer and Heyrovsky steps of HER and realize efficient HER in alkaline media. Cu NDs/Ni3S2 NTs-CFs catalysts exhibit significantly improved electrocatalytic activity and durability for HER, such as a low onset overpotential of ∼60 mV, a low overpotential of 128 mV at 10 mA cm−2, and excellent durability with current density increase of only ∼3% for HER 30 h at the overpotential of 250 mV. Our work not only provides a new strategy to optimize H adsorption and desorption on the surface of catalysts for efficient hydrogen evolution but also develops a kind of highperformance metal sulfide-based electrocatalysts for HER in alkaline media.

250 mV for 30 h. Before HER, the weak peaks of CuO and Cu2O at 523 and 606 cm−1 are seen for Cu NDs-CFs because of the partial oxidation of Cu. However, for Cu NDs/Ni3S2 NTs-CFs, no peak is seen, indicating the electronic interactions between Ni3S2 and Cu can efficiently prevent from the oxiation of Cu. After HER 30 h, Cu NDs/Ni3S2 NTs-CFs shows a peak at 468 cm−1, suggesting the existence of Cu−OH.42 However, for Cu NDs-CFs and Ni3S2 NTs-CFs, no peak is observed at 400−650 cm−1 after HER (the disappearence of CuO and Cu2O peaks for Cu NDs-CFs can be attributed to the reduction of H2). The above results indicate that the hybridized Cu in Cu NDs/Ni3S2 NTs-CFs can promote water adsorption for HER.40 Fourier transform infrared spectroscopy with the attenuated total reflection technique (FT-IR ATR) spectra was used to further study the bonding state of the adsorb water molecules, and the relative results are shown in Figure 4d. The peak located at ∼1640 cm−1 can be assigned to the bending vibrations of O−H bonds of adsorbed water on the surfaces of catalysts. The peak of the adsorbed water on Cu NDs/Ni3S2 NTs-CFs at 1636 cm−1 shows a red-shift of ∼3 cm−1 and ∼7 cm−1 compared with those of Ni3S2 NTs-CFs and Cu NDsCFs, respectively. The above results indicate that the O−H bonds of adsorbed water molecules on Cu NDs/Ni3S2 NTsCFs become longer than those on Ni3S2 NTs-CFs and Cu NDs-CFs, suggesting that the O−H bonds of adsorbed water on Cu NDs/Ni3S2 NTs-CFs are weaker than those on Ni3S2 NTs-CFs and Cu NDs-CFs.43 Based on the above results, Cu NDs/Ni3S2 NTs-CFs as electrocatalysts can efficiently promote water adsorption and activation and accordingly will promote the Volmer step of HER to optimize H adsorption. The electronic interactions between Cu and Ni3S2 of Cu NDs/Ni3S2 NTs-CFs can also weaken S−Hads bonds formed on catalyst surfaces and will optimize H desorption to promote Heyrovsky step of HER. Using Ni3S2 as catalysts for HER, S− Hads is relatively stable and is easy to generate, but the too stable S−Hads makes H2 evolution difficult.44 In DFT calculations, the adsorption of H on Ni3S2 was found to be too strong, whereas it was too weak on Cu, resulting in low HER electrocatalytic activities on Ni3S2 and Cu, respectively. For Cu/Ni3S2 hybrid, it possesses appropriate adsorption of H as shown in Figure 5a, leading to high HER electrocatalytic activity.45,46 Raman spectra were employed to study the bonding states of S−Hads intermediate on Cu NDs/Ni3S2 NTs-CFs as shown in Figure 5b. No peak can be detected on the catalysts at the potential of 50 mV vs RHE, which is more positive than HER thermodynamic potential. After HER reaction at the potential of −250 mV vs RHE for 10 h, a peak at ∼2563 cm−1 is detected for Ni3S2 NTs-CFs, and a peak at ∼2542 cm−1 is seen for Cu NDs/Ni3S2 NTs-CFs. The above peaks correspond to S−Hads bonds formed on the catalysts during HER.47 It is worth noting that the Raman shift of S− Hads for Cu NDs/Ni3S2 NTs-CFs (2542 cm−1) is ∼21 cm−1 red shifted compared with that of Ni3S2 NTs-CFs (2563 cm−1), indicating that the S−Hads bonds on Cu NDs/Ni3S2 NTs-CFs are weaker than those on Ni3S2 NTs-CFs. Therefore, the above theoretical and experimental results both demonstrate that the Cu NDs/Ni3S2 NTs-CFs can weaken S−Hads bonds formed on the catalyst surfaces and optimize H desorption. The schematic diagram in Figure 5c,d illustrates the promotion role of Cu/ Ni3S2 hybird on the improvments of H2O adsorption and activation and the optimization of H adsorption and desorption for efficient HER compared with the individual Ni3S2.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b08521. Details of the characterizations and electrochemical data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Ye-Xiang Tong: 0000-0003-4344-443X Gao-Ren Li: 0000-0002-9222-7274 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by National Basic Research Program of China (2015CB932304 and 2016YFA0202603), NSFC (91645104), Science and Technology Program of Guangzhou (201704030019), Natural Science Foundation of Guangdong Province (2016A010104004 and 2017A010103007), and Fundamental Research Fund for the Central Universities (16lgjc67).

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