Composition-Dependent Catalytic Activities of Noble-Metal-Free NiS

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Composition-Dependent Catalytic Activities of NobleMetal-Free NiS/NiS for Hydrogen Evolution Reaction 3

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Zhixiao Qin, Yubin Chen, Zhenxiong Huang, Jinzhan Su, Zhidan Diao, and Liejin Guo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05230 • Publication Date (Web): 27 Jun 2016 Downloaded from http://pubs.acs.org on June 28, 2016

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The Journal of Physical Chemistry

Composition-Dependent Catalytic Activities of Noble-MetalFree NiS/Ni3S4 for Hydrogen Evolution Reaction Zhixiao Qin, Yubin Chen*, Zhenxiong Huang, Jinzhan Su, Zhidan Diao, and Liejin Guo* International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Shaanxi 710049, P. R. China. ABSTRACT: The development of efficient noble-metal-free hydrogen evolution catalysts is quite appealing with the aim of providing cost-competitive hydrogen. Herein, nickel sulfides (NiSx) with tunable NiS/Ni3S4 molar ratios were synthesized via a simple hydrothermal method. Detailed electrochemical studies under neutral conditions indicated that the electrocatalytic property of NiSx catalysts was determined by the composition. Notably, the NiSx sample with the NiS/Ni3S4 molar ratio of 1.0 exhibited the lowest overpotential and charge-transfer resistance. As analyzed from the Tafel plots, the rate determining step of NiSx catalysts for hydrogen generation was the Volmer step, in which the proton adsorption played a key role. Theoretical calculation revealed that NiS and Ni3S4 exhibited the metallic behaviors with different work functions. Consequently, the NiSx sample with the NiS/Ni3S4 molar ratio of 1.0 owned the most adsorbed protons, which led to the highest electrocatalytic property. Meanwhile, NiSx was demonstrated to be efficient cocatalysts to promote photocatalytic hydrogen generation. NiSx/CdS with the

ACS Paragon Plus Environment


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NiS/Ni3S4 molar ratio of 1.0 showed the best photocatalytic activity with the apparent quantum efficiency of 56.5% at 420 nm. This result was in good agreement with the electrocatalytic activities of NiSx samples, indicating the intrinsic property for efficient hydrogen generation. INTRODUCTION With the increasing concern over the energy demand and global warming caused by the use of conventional fossil fuels, hydrogen has received great attentions as a clean and renewable energy carrier. Among various hydrogen fabrication routes, it is of particular interest to produce hydrogen from water, an abundant and renewable hydrogen source. Electrocatalytic and photocatalytic hydrogen generation from water are promising schemes for sustainable hydrogen generation.1-6 The key issue of these techniques is to develop highly efficient and low-cost hydrogen evolution reaction (HER) catalysts. Noble metals (e.g., Pt) are commonly examined HER catalysts at present.7-9 However, developing efficient noble-metal-free HER catalysts, preferably based on earth-abundant elements, is quite appealing with the aim of providing cost-competitive hydrogen.10-16 Inspired by the composition and structure of [NiFe] hydrogenases,17-19 researchers have been exploring efficient nickel-based HER catalysts for energy conversion processes. In particular, low-cost nickel sulfides (NiSx) have been widely investigated for electrocatalytic hydrogen generation, since they can be prepared by facile routes, and high catalytic properties can be simultaneously achieved.20-27 For instance, NiS2 was investigated as an active non-precious hydrogen evolution catalyst, which


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exhibited excellent catalytic activity in an acidic electrolyte.28 Similarly, Ni3S2 was found to be a competent HER catalyst with robust activities not only in neutral buffer but also in natural water.29 Meanwhile, nickel sulfides were demonstrated to be excellent cocatalysts for photocatalytic hydrogen production. NiS/CdS and NiS/ZnxCd1-xS/reduced graphene oxide nanocomposites were proved to be highly active photocatalysts for H2 production.30,

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NiS2 nanoparticles deposited on the

CdLa2S4 photocatalyst could remarkably increase the H2 evolution rate of CdLa2S4.32 Recently, our group reported that nickel sulfides cocatalysts could be intergrown with host photocatalysts, which led to the apparently improved photocatalytic hydrogen generation.33 On the basis of the above results, it can be summarized that NiSx could act as efficient HER catalysts for both electrocatalytic and photocatalytic hydrogen evolution. It should be noted that nickel sulfides always exhibit complicated compositions and structures,34 which can have a great influence on the HER activity. For example, Sun et al. reported that NiS, NiS2, and Ni3S2 compounds exhibited varied HER activities due to their different physicochemical properties.35 Sung and coworkers found that NiS and Ni3S2 showed an apparent difference in electrocatalytic activity owing to their varied atomic configurations and crystalline structures.23 Recently, Fu et al. found that NiS/Ni3S2 nanorod composite array exhibited higher electrocatalytic activity than Ni3S2 and Pt for dye-sensitized solar cells.36 However, most of present studies were focused on the comparison of single component, the detailed study of NiSx-based composite catalysts hopefully for improved hydrogen production were



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still limited. In addition, although numerous catalysts for electrocatalytic hydrogen production have been demonstrated to be efficient cocatalysts for photocatalytic hydrogen production, few studies about the catalysts for simultaneously facilitating the HER both electrocatalytically and photocatalytically have been reported.37 Therefore, the comprehensive understanding of the catalytic properties for NiSx-based composites could promote the development of highly efficient NiSx catalysts for electrocatalytic and photocatalytic hydrogen generation. In the present study, NiS/Ni3S4 composites with different compositions were synthesized via a simple hydrothermal method. It was observed that the NiS/Ni3S4 molar ratio was determined by the initial S2-/Ni2+ molar ratio. When the NiS/Ni3S4 molar ratio was 1.0, the NiSx sample exhibited the highest properties both for electrocatalytic and photocatalytic hydrogen generation. Theoretical calculations and experimental studies were conducted to reveal the underlying mechanism. EXPERIMENTAL SECTION Material Synthesis Nickel acetate tetrahydrate (Ni(OAc)2·4H2O, 98%), thioacetamide (TAA, 99%), cadmium nitrate tetrahydrate (Cd(NO3)2·4H2O, 99%), sodium hydroxide (NaOH, 96%), sodium sulfate (Na2SO4, 99%), ethanol (C2H6O, 99.7%), and lactic acid (C3H6O3, 85%) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used without further purification. NiSx samples were synthesized via a simple hydrothermal method. In a typical


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procedure, 2 mmol of Ni(OAc)2·4H2O was added into 60 mL of ethanol. After the suspension was stirred uniformly, varied amount of thioacetamide as a sulfur source was added. After 20 min of stirring, the suspension was then transferred into a 110 mL Teflon-lined stainless steel autoclave and heated at 150 °C for 2 h. After the autoclave cooled naturally in the air, the resulting precipitate was thoroughly washed with ethanol and deionized water and finally dried at 80 °C under vacuum conditions for 8 h. The obtained product was denoted as NiSx-m, where m represented the initial S2-/Ni2+ molar ratio for the hydrothermal treatment. NiSx/CdS photocatalysts were synthesized via a two-step route. CdS photocatalysts were firstly synthesized by a hydrothermal route. 0.02 mol of Cd(NO3)2·4H2O were dissolved into 40 mL of deionized water, and 40 mL of NaOH solution (OH-/Cd2+ molar ratio was 6) was then added. The suspension was stirred continuously for 20 min, and an excess amount of thioacetamide (S2-/Cd2+ molar ratio was 4) was added, which was then hydrothermally treated at 150 °C for 20 h to obtain the CdS products. In the second step, 5 mmol of as-prepared CdS powders were dispersed into 60 mL of ethanol under stirring, and an appropriate amount of Ni(OAc)2·4H2O was added (Ni2+/Cd2+ molar ratio was 0.1). After the suspension was stirred uniformly, varied amount of thioacetamide was added. Then, the hydrothermal treatment was applied for the synthesis of NiSx/CdS samples at 150 °C for 2 h. The obtained product was denoted as NiSx/CdS-n, where n represented the initial S2-/Ni2+ molar ratio for the second hydrothermal treatment. Characterization



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X-ray powder diffraction (XRD) patterns were obtained in a PANalytical X’pert MPD Pro X-ray diffractometer (CuKα irradiation at 40 kV and 40 mA). The UV-visible (UV-vis) absorption spectra were measured on a HITACHI U-4100 spectrophotometer. Transmission electron microscope (TEM) images and STEM-EDS mapping were determined using a FEI Tecnai G2 F30 S-Twin microscope with an OXFORD MAX-80 energy dispersive X-ray spectrometer. Scanning electron microscope (SEM) images were recorded by a JEOL JSM-7800F microscope. The specific surface area was conducted at 77 K in the Beckman Coulter SA3100 plus instrument using the Brunauer-Emmett-Teller (BET) method. The elemental composition could be determined by an ICPE-9000 inductively coupled plasma atomic emission spectrometer (ICP-AES). The X-ray photoelectron spectroscopy (XPS) measurements were conducted on an Axis Ultra, Kratos (UK) multifunctional X-ray spectrometer using monochromatic Al Kα radiation. Binding energies were calibrated relative to the C 1s peak (284.8 eV) from adventitious carbon adsorbed on the surface of the samples. Electrochemical and Photocatalytic Measurements Electrochemical measurements were carried out using a CHI 760D electrochemical workstation (CH Instruments, Inc., USA) in a three-electrode system. The working electrode was fabricated by a drop casting approach (Supporting Information). The reference electrode was an Ag/AgCl electrode, and the counter electrode was a platinum foil. Linear sweep voltammogram was conducted in a N2-saturated 0.5 M Na2SO4 electrolyte at a scan rate of 100 mV/s. The measured potentials vs Ag/AgCl


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were converted to a reversible hydrogen electrode (RHE) scale via the Nernst equation. Electrochemical impedance spectroscopy (EIS) was measured in deaerated 0.5 M Na2SO4 electrolyte when the working electrode was biased at a constant potential of -0.2 V vs RHE. The frequency was from 100 kHz to 1 Hz with a 5 mV AC dither. Photocatalytic hydrogen generation was evaluated in a side irradiation Pyrex cell with a magnetic stirring. 0.1 g of as-prepared photocatalysts was added into 200 mL of aqueous solution containing 30 vol% lactic acid as the sacrificial reagents. Before irradiation, nitrogen was purged through the reaction cell for 30 min to remove air in the dark. The reaction temperature was kept at 35 °C. A 300 W Xe-lamp equipped with a 420 nm cutoff filter was employed to provide the visible light irradiation. The amount of generated H2 was determined with a TCD gas chromatograph. The average hydrogen production rates were calculated based on hydrogen generation amount in the first 5 h reaction. The apparent quantum efficiency (AQE) could be calculated according to Equation (1).

AQE(%) =

The number of evolved hydrogen molecules × 2 ×100% The number of incident photons

(1)

RESULTS AND DISCUSSION Morphology, Structure and Composition Figure 1 shows SEM and TEM images of various NiSx-m samples, where m represented the initial S2-/Ni2+ molar ratio for the hydrothermal process. As observed in Figure 1a-e, the hydrothermal prepared NiSx-m samples all exhibited hollow spheres with the diameter of ca. 200 nm. It was found that the hollow sphere was



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made up of NiSx nanoparticles, indicating the polycrystalline character. TEM image (Figure 1f) reveals that the bright area in the center of the sphere could be observed for NiSx-3 sample, which was a typical feature of hollow spheres. The hollow structure is considered highly beneficial for the surface reaction, owing to the large surface area and closely packed interpenetrating networks.38, 39 It should be noted that the similar morphologies of different NiSx-m samples excluded the influence of the morphology on their HER properties. Furthermore, the BET specific surface area of each catalyst was investigated. As shown in Table S1, all NiSx-m samples exhibited similar specific surface area, which revealed that the specific surface area should have no influence on the varied activities.

Figure 1 SEM images of (a) NiSx-1, (b) NiSx-2, (c) NiSx-3, (d) NiSx-4, (e) NiSx-5. TEM image of (f) NiSx-3. X-ray diffraction (XRD) spectroscopy was employed to investigate the crystal phases of NiSx-m samples. As shown in Figure 2, the diffraction peaks of


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hydrothermally synthesized NiSx-m could be well assigned to hexagonal NiS (JCPDS no. 75-0613) and cubic Ni3S4 (JCPDS no. 43-1469). When the initial S2-/Ni2+ molar ratio was increased, it was noted that the intensities of diffraction peaks corresponding to Ni3S4 were gradually increased. It was thus inferred that NiSx-1 sample had massive hexagonal NiS and less cubic Ni3S4, while NiSx-5 sample had massive cubic Ni3S4 and less hexagonal NiS. However, pure NiS or Ni3S4 could not be synthesized by this hydrothermal method. As shown in Figure S1, when the initial S2-/Ni2+ molar ratio was further tuned to 0.6 and 7, the prepared NiSx sample was still a composite of hexagonal NiS and cubic Ni3S4.

Figure 2 XRD patterns of NiSx-m samples with different initial S2-/Ni2+ molar ratios. XPS measurements were conducted to investigate the surface chemical states and compositions of NiSx-m samples. As shown in Figure 3, the binding energies of Ni 2p3/2 and 2p1/2 for all NiSx-m samples were located at around 853.0 and 872.0 eV, and the peaks at around 858.0 and 877.0 eV corresponded to their “shake-up” peaks. The close examination revealed that the peaks of Ni 2p3/2 could be fitted with two separate peaks respectively located at ca. 852.8 and 854.4 eV, which indicated that both NiS



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and Ni3S4 were achieved.40 Furthermore, when the initial S2-/Ni2+ molar ratio was increased, it was noted that the Ni 2p3/2 peak area of Ni3S4 was gradually increased. The fitted results of Ni 2p3/2 XPS spectra were shown in Table 1. The NiS/Ni3S4 molar ratio of the NiSx-m samples could be calculated from the area ratio of the Ni 2p3/2 peaks.41, 42 As shown in Table 1, the NiS/Ni3S4 peak area ratios of NiSx-1 sample was 1.7 and NiSx-5 sample was 0.5, indicating that NiSx-1 sample had massive NiS, while NiSx-5 sample had massive Ni3S4. This phenomenon was consistent with the XRD result. In particular, the NiS/Ni3S4 molar ratio of NiSx-3 sample was exactly 1.0. To further confirm the Ni/S molar ratio, ICP-AES elemental analysis of all NiSx-m samples were carried out. As shown in Table S2, the Ni/S molar ratios determined by ICP-AES analysis were in good agreement with the values by XPS measurement.

Figure 3 Ni 2p XPS spectra of (a) NiSx-1, (b) NiSx-2, (c) NiSx-3, (d) NiSx-4, (e) NiSx-5. The fitted NiS (red) and Ni3S4 (blue) peaks were displayed in each spectrum.


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Table 1. The binding energies and area ratios of Ni 2p3/2 peaks for NiS/Ni3S4.

Sample

Binding energy of Ni

Binding energy of Ni

Area ratio of

2p3/2 for NiS (eV)

2p3/2 for Ni3S4 (eV)

Ni 2p3/2 peaks (NiS/Ni3S4)

NiSx-1

852.9

854.6

1.7

NiSx-2

852.8

854.2

1.3

NiSx-3

852.9

854.4

1.0

NiSx-4

852.8

854.2

0.7

NiSx-5

852.9

854.2

0.5

Electrocatalytic Properties for Hydrogen Generation Linear sweep voltammetry (LSV) was performed to examine the electrocatalytic activities of varied NiSx-m samples loaded on the glassy carbon electrodes in a N2-saturated neutral-pH (0.5 M Na2SO4) aqueous solution. As shown in Figure 4, all five samples enabled electrocatalytic H2 evolution under negative bias, but there was a marked difference in performance. A more relevant metric by which to compare catalysts is the potential required to reach an operating current density of interest. If we measured the cathodic overpotential (η) required for the different electrocatalysts to drive the HER at 10 mA/cm2, it was noticed that with the increased S2-/Ni2+ molar ratio, the overpotential steady decreased and then underwent an increase. It was apparent that NiSx-3 sample exhibited the prominent activity for the HER with the lowest overpotential of 221 mV to drive 10 mA/cm2. This value compares favourably to the results of the reported Ni-S electrodes in neutral solution.24, 29, 43 To further understand the varied HER performance, the electrochemical active surface area (ECSA) of each catalyst was studied, which was known to distinctly impact the



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electrocatalytic performance. Since it is defiant to directly measure the absolute ECSA, a widely adopted method is to derive the relative ECSA based on the measurement of double-layer capacitance in the non-Faradaic potential region.44 It is generally accepted that the double-layer capacitance is linearly proportional to ECSA.44 The double-layer capacitance of a film can be conveniently deduced from cyclic voltammetry measurements at various scan rates. Figure S2a shows the capacitive currents of the NiSx-3 sample with different scan rates. The capacitive currents at 0.65 V vs RHE under different scan rates were plotted in Figure S2b, with the slope corresponding to the double-layer capacitance. Concerning the similar double-layer capacitance values for all NiSx-m samples, the ECSA was not the critical factor accounting for the varied HER activities.

Figure 4 Linear sweep voltammograms for different NiSx-m samples in 0.5 M Na2SO4 solution. Electrochemical impedance spectroscopy (EIS) was measured to elucidate the charge-transfer process of NiSx-m electrodes. As shown in Figure 5, the Nyquist impedance plots for these electrodes could be fitted to an equivalent circuit consisting


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of Rs as the series resistance, the constant phase element (CPE), and the charge-transfer resistance from the electrode to the electrolyte (Rct).45 The fitted parameters were summarized in Table 2. It was observed that the Rs values for all samples were basically the same, which were much lower than the Rct values. This phenomenon indicated that Rct was the major resistance for the electrocatalytic reaction. As shown in Table 2, the charge-transfer resistance Rct for NiSx-3 sample was 66.01 Ω/cm2, which was dramatically lower compared to those of other electrodes, indicating its favourable reaction kinetics.46 The lowest charge-transfer resistance of NiSx-3 sample indicated the superior electrocatalytic activity.

Figure 5 EIS Nyquist plots of NiSx-m electrodes at -0.2 V vs RHE in N2-saturated 0.5 M Na2SO4; The inset shows the equivalent circuit.



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Table 2. Resistance values of Rs and Rct as obtained from the EIS measurements for various NiSx electrodes. Electrode

Rs (Ω/cm2)

Rct (Ω/cm2)

NiSx-1

13.75

140.84

NiSx-2

9.16

119.42

NiSx-3

10.93

66.01

NiSx-4

15.49

147.84

NiSx-5

7.84

219.38

To understand the underlying mechanism of the varied HER activities, the corresponding Tafel plots based on LSV curves were acquired. As shown in Figure 6, Tafel slopes of 116, 109, 101, 125 and 148 mV/dec were respectively observed for NiSx-1, NiSx-2, NiSx-3, NiSx-4, and NiSx-5. It is well known that the Tafel slope is an inherent property indicative of the rate determining step for HER.47 Three possible reaction steps have been suggested for HER: First is a primary discharge step (Volmer step, 120 mV/dec) followed by either an electrochemical desorption step (Heyrovsky step, 40 mV/dec) or a recombination step (Tafel step, 30 mV/dec).48 In our case, the Tafel slopes of all NiSx-m samples (101-148 mV/dec) implied that the rate determining step was the Volmer step, in which the proton adsorption played a key role.23 In the case of NiSx-3 sample, the Tafel slope was the lowest, indicating that the proton adsorption was much easier than other NiSx samples. Therefore, it was inferred that the superior HER activity of NiSx-3 sample could originate from the much easier proton adsorption.


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Figure 6 Tafel plots of (a) NiSx-1, (b) NiSx-2, (c) NiSx-3, (d) NiSx-4, and (e) NiSx-5 derived from the corresponding linear sweep voltammograms in 0.5 M Na2SO4 solution. The dashed lines are the linear fittings. Theoretical computation using density functional theory (DFT) calculation was subsequently conducted to examine the electronic behaviors of NiSx-m samples. Herein, DFT computations were performed to examine the total density of states (DOS) of hexagonal NiS and cubic Ni3S4. The detailed theoretical computation method was listed in the Supporting Information. As shown in Figure 7a and b, NiS and Ni3S4 had very similar DOS distributions and their Fermi levels crossed the nickel d band, which indicated the metallic behavior.49-52



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Figure 7 The calculated total (per cell) density of states for (a) NiS and (b) Ni3S4. The Fermi level is denoted by the dashed lines at the energy of 0 eV. The calculated slab modules of the energy change along the c-direction of the slab cell for (c) NiS and (d) Ni3S4. Fermi level is of paramount importance for the control of charge carrier transport. In a sense, the work function represents the energy barrier that prevents an electron on the Fermi level from escaping the solid.53 In order to obtain the work functions of NiS and Ni3S4, we created two slab modules for the calculation. Both of the slabs consisted of a 20 Å vacuum layer and the surface cleaved for bulk material. NiS (001) surface and Ni3S4 (111) surface were chosen, because they were close-packing plane. Figure 7c and d showed the energy changed along the c-direction of the slab cell. Horizontal platforms in both graphs represented the energy levels of vacuum layers (Evac) and Fermi levels (Ef), calculated via the DFT method. Consequently, the work functions of NiS and Ni3S4 were determined to be 5.11 and 4.97 eV, respectively.


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As shown in Figure 8a, due to the different work functions of Ni3S4 and NiS, electrons would transfer from Ni3S4 to NiS to equilibrate the Fermi levels when they got a contact.54 In this case, the accumulated electrons in NiS would lead to the absorption of cations (e.g, H+, Na+) from the electrolyte. On the contrary, anions (e.g, OH-, SO42-) could be absorbed on the surface of Ni3S4. When electrons migrated into the NiSx electrode, they preferred to move to NiS for the hydrogen evolution reaction. As analyzed from the Tafel plots, the rate determining step of NiSx-m samples for electrocatalytic hydrogen generation should be the Volmer step, in which the proton adsorption played a key role. In the present case, the number of protons adsorbed by NiS was directly proportional to the amount of accumulated electrons in NiS, which was related to the Fermi level shift. When NiS was interacted with Ni3S4, the Fermi level shift was influenced by the NiS/Ni3S4 molar ratio.54 According to our calculation (Supporting Information), the amount of accumulated electrons in NiS was first increased and then decreased with the reduced NiS/Ni3S4 molar ratio. As a consequence, the number of adsorbed protons exhibited the same trend (Figure 8b). Particularly, as the NiS/Ni3S4 molar ratio was 1.0, the amount of accumulated electrons was the largest, leading to the most adsorbed protons. As shown in Figure 8c, the amount of accumulated electrons had the same trend with the current density of NiSx samples with different NiS/Ni3S4 molar ratios. Accordingly, it could be revealed that the most adsorbed protons from the largest amount of accumulated electrons in NiS promoted the Volmer reaction, which resulted in the lowest overpotential and charge-transfer resistance for the NiSx-3 electrode. It is worth noting that Au-Pt



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bimetallic with Au/Pt molar ratio of 1:1 also exhibited the prominent electrocatalytic activity.55, 56

Figure 8 (a) Schematic illustration for hydrogen production mechanism of the NiSx electrode. (b) The different number of ions adsorbed by NiSx electrodes with the varied NiS/Ni3S4 molar ratios. (c) The amount of accumulated electrons in NiS (blue curve based on Equation (4) in the Supporting Information) and the current density (red points corresponding to the current density at -0.2 V vs RHE based on LSV curves) with the varied NiS/Ni3S4 molar ratios. NiSx as cocatalysts for photocatalytic hydrogen generation In addition to serving as an efficient HER electrocatalyst, NiSx samples were examined as cocatalysts to promote photocatalytic hydrogen generation. NiSx nanoparticles were loaded onto CdS using a two-step route to generated NiSx/CdS-n, where n represented the initial S2-/Ni2+ molar ratio for the second-step hydrothermal treatment. As shown in Figure S3, the diffraction peaks of hydrothermally synthesized CdS and NiSx/CdS-n samples could be well assigned to hexagonal CdS (JCPDS no. 77-2306). No obvious NiSx peaks were observed, possibly owing to the low amount of NiSx. UV-vis absorption spectra and the plots of (αhν)2 versus hν (Figure S4)


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showed that all samples have consistent absorption edges, indicating that nickel ions were not incorporated into CdS lattices. The morphologies of CdS and various NiSx/CdS-n samples were examined by SEM. As shown in Figure S5, NiSx nanoparticles were formed on the surface of CdS polyhedral nanocrystals. Figure 9 shows a TEM image of the NiSx/CdS-3 sample, along with the STEM-EDS element mapping. As shown in Figure 9a, NiSx nanoparticles were successfully deposited on the surface of CdS nanocrystal. HRTEM images (Figure 9b and c) indicated that NiSx-3 nanoparticles on the surface of NiSx/CdS-3 sample included NiS and Ni3S4. The STEM-EDS element mapping of NiSx/CdS-3 sample (Figure 9d-g) further demonstrated that NiSx-3 nanoparticles were successfully deposited on the surface of CdS nanocrystals. Besides, the NiS/Ni3S4 molar ratio of NiSx/CdS-3 sample was measured by Ni 2p XPS spectra. As shown in Figure S6, the NiS/Ni3S4 molar ratio of NiSx/CdS-3 sample was about 1.0. This result was consistent with the pure NiSx-3 sample, indicating that the composition of NiSx nanoparticles was not changed when they were deposited on the surface of CdS nanocrystals.



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Figure 9 (a) TEM image of NiSx/CdS-3, (b, c) HRTEM images of NiSx/CdS-3, and (d-g) STEM-EDS element mapping of NiSx/CdS-3 nanocomposite. Photocatalytic hydrogen generation was performed under visible light with lactic acid as a sacrificial electron donor. Recent studies have shown that lactic acid is an efficient sacrificial electron donor for NiSx/CdS photocatalyst, which can consume photogenerated holes by oxidizing lactic acid to pyruvic acid.30, 33 Figure 10a shows the photocatalytic activities of NiSx/CdS-n samples. With the increased S2-/Ni2+ molar ratio, the hydrogen production rate initially increased and then underwent a decrease. It could be revealed that the property of NiSx as cocatalysts for hydrogen production was firstly increased and then reduced with the gradual increase of the NiS/Ni3S4 molar ratio. NiSx/CdS-3 sample exhibited the best photocatalytic activity with the NiS/Ni3S4 molar ratio of 1.0. The trend was in good agreement with the electrocatalytic result of NiSx-m samples, indicating the intrinsic property of NiSx-m for efficient hydrogen generation. The apparent quantum efficiency of NiSx/CdS-3 sample was calculated to be 56.5% at 420 nm, and the long-time photocatalytic test was carried out to investigate the stability of NiSx/CdS-3 sample. As displayed in


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Figure 10b, over the 20 hours’ reaction, NiSx/CdS-3 photocatalyst didn’t show apparent decrease in the photocatalytic activity. To further confirm the durability of the catalysts, XRD and XPS data of NiSx/CdS-3 catalyst before and after the photocatalytic reaction were measured. As shown in Figure S7, the crystalline structure and elemental composition were not changed after the 20 hours’ reaction, indicating the good durability.

Figure 10 (a) Photocatalytic hydrogen production over NiSx/CdS-n photocatalysts. (b) Long-time photocatalytic test of NiSx/CdS-3 sample for hydrogen production. Reaction conditions: 0.1 g of photocatalysts; 200 mL of aqueous solution containing 30 vol % lactic acid; 300 W Xe lamp equipped with a cutoff filter (λ ≥ 420 nm). CONCLUSIONS



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In summary, we have synthesized nickel sulfides with tunable compositions via a simple hydrothermal method, and confirmed their electrocatalytic activities for efficient hydrogen generation. It was found that the NiSx sample with the NiS/Ni3S4 molar ratio of 1.0 exhibited the lowest overpotential and charge-transfer resistance. As deduced from the Tafel plots and theoretical calculation, the rate determining step of NiSx samples for hydrogen generation was the Volmer step, in which the proton adsorption played a key role. NiS and Ni3S4 exhibited the metallic behaviors with different work functions, and the NiSx sample with the NiS/Ni3S4 molar ratio of 1.0 owned the most adsorbed photons, which led to the highest electrocatalytic property. At the same time, NiSx was proved to be efficient cocatalysts to promote photocatalytic hydrogen generation. The NiSx/CdS sample with the NiS/Ni3S4 molar ratio of 1.0 showed the best photocatalytic activity with the apparent quantum efficiency of 56.5% at 420 nm, which was in good agreement with the electrocatalytic result of NiSx samples. It is expected that the comprehensive examination of present NiSx-based composites could promote the development of highly efficient noble-metal-free catalysts for hydrogen generation. ACKNOWLEDGMENT The authors thank the financial support from the National Natural Science Foundation of China (Nos. 51236007 and 51323011), the grant support from the China Postdoctoral Science Foundation (No. 2014M560768), and the China Fundamental Research Funds for the Central Universities (xjj2015041).


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ASSOCIATED CONTENT Supporting Information Electrode fabrication; Theoretical computation method; Calculation of the amount of accumulated electrons; XRD patterns of NiSx prepared with different initial S2-/Ni2+ molar ratios; The BET specific surface area of different NiSx-m samples; The Ni/S molar ratios determined by XPS and ICP-AES analysis; Cyclic voltammograms showing the capacitive currents for the NiSx-3 sample at different scan rates; The linear relationship between the capacitive current and the scan rate for the NiSx-m samples; XRD patterns of CdS and NiSx/CdS-n prepared with different initial S2-/Ni2+ molar ratios; UV-vis absorption spectra and the plots of (αhν)2 versus hν for CdS and NiSx/CdS-n prepared with different initial S2-/Ni2+ molar ratios; SEM images of CdS, NiSx/CdS-1, NiSx/CdS-2, NiSx/CdS-3, NiSx/CdS-4, NiSx/CdS-5; Ni 2p XPS spectra of NiSx/CdS-3 sample; XRD patterns and Ni 2p XPS spectra of NiSx/CdS-3 sample before and after the photocatalytic reaction. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Y. Chen). Tel.: +86-29-82668296.

Fax:

+86-29-82669033. *E-mail:

[email protected]

(L.

Guo).

Tel.:

+86-29-82669033.


+86-29-82663895.

Fax:


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Table of Contents (TOC)


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Composition-Dependent Catalytic Activities of Noble-Metal-Free NiS

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