Few-Layered Trigonal WS2 Nanosheet-Coated Graphite Foam as an

Aug 29, 2017 - ... Jurong Island 627833, Singapore. ⊥ Institute of Environmental Health and Pollution Control, School of Environmental Science and E...
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Few-Layered Trigonal WS2 Nanosheet-Coated Graphite Foam as an Efficient Free-Standing Electrode for a Hydrogen Evolution Reaction Xiaomeng Guo,†,∇ Junyi Ji,‡,∇ Quanguo Jiang,§ Lili Zhang,∥ Zhimin Ao,*,⊥ Xiaobin Fan,† Shaobin Wang,# Yang Li,† Fengbao Zhang,† Guoliang Zhang,† and Wenchao Peng*,† †

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China School of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, China § College of Mechanics and Materials, Hohai University, Nanjing 210098, China ∥ Institute of Chemical and Engineering Sciences, A*STAR, 1 Pesek Road, Jurong Island 627833, Singapore ⊥ Institute of Environmental Health and Pollution Control, School of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou 510006, China # Department of Chemical Engineering, Curtin University, Perth, Western Australia 6845, Australia ‡

S Supporting Information *

ABSTRACT: Few-layered tungsten disulfide (WS2) with a controlled-phase ratio (the highest trigonal-phase ratio being 67%) was exfoliated via lithium insertion. The exfoliated WS2 nanosheets were then anchored onto three-dimensional (3D) graphite foam (GF) to fabricate free-standing binder-free electrodes. The 3D GF can increase the interfacial contact between the WS2 nanosheets and the electrolyte and facilitate ion transfer. Without the nonconductive binder, an intimate contact between the WS2 and GF interface can be created, leading to the improvement of electrical conductivity. In comparison to the pure WS2 nanosheets, the overpotential for a hydrogen evolution reaction is significantly decreased from 350 mV to 190 mV at 10 mA/cm2, and no deactivation occurs after 1000 cycles. The density functional theory computations reveal that the efficient catalytic activity of the trigonal-phase WS2/GF electrode is attributed to the lower Gibbs free energy for H* adsorption and higher electrical conductivity. KEYWORDS: tungsten disulfide, mixed phase, graphite foam, hydrogen evolution, binder-free electrode

1. INTRODUCTION Hydrogen is regarded as a clean energy carrier due to its high energy density and nonexistent carbon emission during combustion.1 Electrochemical catalytic hydrogen evolution reaction (HER) is an efficient method for the production of hydrogen.2−4 Due to the presence of overpotential, a large electrolysis voltage is always needed, which leads to the waste of electric energy.5 Hence, advanced catalysts for HER with lower overpotentials and high efficiencies are the keys to largescale applications.6,7 Although Pt-group metals are the most effective HER catalysts, the application is limited by the rareness and high cost of Pt-group metals.8,9 Therefore, the development of low-cost and easy-scalable HER catalysts with high activity still remains a challenge. Transition-metal dichalcogenides (TMDs) represent an alternative group of 2D-layered materials that differ from the semimetallic character of graphene.10−12 The properties of TMDs can be tailored according to the crystalline structure, specifically, the number and stacking sequence of their nanosheets.13−15 Notably, their electrochemical activities can also be greatly improved by decreasing their number of layers to just a single layer or a few layers.16−19 Among the available techniques, lithium insertion is the most widely used and © 2017 American Chemical Society

efficient method to obtain TMD nanosheets in a large scale.20,21 Tungsten disulfide (WS2) is a TMD material widely used as an electrochemical catalyst, a photocatalytic cocatalyst, and a lithium-battery electrode.22,23 The WS2 material has two different main phases (hexagonal 2H phase and trigonal 1T phase) with distinct electronic structures.24 2H-WS2 behaves as a semiconductor, while the 1T-WS2 is a metallic material with better electrochemical catalytic activity.24 A phase transition from 2H-WS2 to 1T-WS2 can happen during the lithiuminsertion process.20,21 The presence of more 1T phase could provide more active sites and better electric conductivity.25−27 Therefore, the percentage of metallic 1T phase of WS2 will affect the catalytic activity in electrochemical reactions greatly.28,29 However, the electrical conductivity of pure WS2 nanosheets is relatively low, which will hamper the HER application, especially under a high catalytic current.30 Generally, free-standing electrodes could be designed on the basis of the Cu grid and Ni foam due to their good mechanical properties.31,32 Three-dimensional graphite foam (3D-GF) is Received: May 10, 2017 Accepted: August 29, 2017 Published: August 29, 2017 30591

DOI: 10.1021/acsami.7b06613 ACS Appl. Mater. Interfaces 2017, 9, 30591−30598

Research Article

ACS Applied Materials & Interfaces Scheme 1. Illustration of the WS2/GF Electrode Fabrication

Figure 1. XRD (a) and Raman spectra (b) for the WS2 before and after the exfoliation.

Figure 2. XPS spectra of the bulk WS2 (a,d); I-WS2 (b,e); II-WS2 (c,f).

exfoliation of bulk WS2 via lithium insertion.24 The exfoliated 1T-WS2 nanosheets were drop-casted onto the surface of the 3D-GF to obtain the WS2/GF composites. Due to the interconnected structure and relatively strong mechanical strength of 3D-GF, the composite can be directly used as a

an emerging material for the fabrication of free-standing electrodes due to its excellent electric conductivity, enhanced chemical stability, and good tailorable mechanical properties.33 In this study, few-layered WS2 nanosheets with a controlledphase ratio at the highest 1T ratio of 67% was obtained by 30592

DOI: 10.1021/acsami.7b06613 ACS Appl. Mater. Interfaces 2017, 9, 30591−30598

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ACS Applied Materials & Interfaces

Figure 3. TEM images of the bulk 2H-WS2 with different magnifications (a,b); the hexagonal prismatic structure of 2H phase WS2 (c); the exfoliated WS2 nanosheets (d); the blown-up image and corresponding FFT pattern of trigonal 1T phase (e), which is the enlargement of area 1 in Figure 3d; hexagonal prismatic 2H (f), which is the enlargement of area 2 in Figure 3d.

free-standing and binder-free electrode for HER.33,34 In comparison with the pure WS2 nanosheets, the overpotential can be remarkably decreased from −350 mV to −190 mV at 10 mA/cm2. This enhanced activity is attributed to the unique structure of the WS2/GF electrode. The intimate “sheet contact” between the WS2 and GF can improve the electrical conductivity of the electrode, while the 3D structure of GF can increase the interfacial contact between the WS2 nanosheets and the electrolyte for efficient ion transfer. The density functional theory computations further reveal that the efficient catalytic activity of the WS2/GF electrode is due to the low Gibbs free energy for H* adsorption and high electrical conductivity. Therefore, this readily scalable WS2/GF composite will be a promising free-standing binder-free electrode for practical hydrogen production.

respectively. For the bulk 2H-WS2, these two characteristic peaks are very sharp. However, they become broader for IIWS2, and new Raman shifts J1, J2, and J3 in the lower-frequency regions appear, indicating the presence of 1T-type WS2 nanosheets. X-ray photoelectron spectroscopy (XPS) is another effective tool to distinguish the 2H and 1T polymorphs of TMD materials. As shown in Figure 2a, two predominant peaks of W 4f7/2 and W 4f5/2 at 32.8 and 35.0 eV can be observed, indicating the 2H phase nature of the bulk WS2. The weak peak of W 5p3/2 at 37.9 eV is attributed to the slight oxidation of WS2. After the lithium intercalation, as shown in Figure 2b,c, the intensities of the 2H-WS2 peaks decrease, and an additional pair of 1T peaks appears, whose binding energies are ∼1.0 eV lower than those of the 2H-WS2. Similarly, for sulfur, a new pair of peaks at 163.6 and 162.4 eV are found besides the known doublet peaks of 2H-WS2 at 161.8 and 163.0 eV (Figure 2d−f). The parallel shift of the original peaks to lower binding energy suggests the appearance of the 1T phase in WS2. Interestingly, the peak for the SO bond at 169.2 eV disappears after the exfoliation, indicating the strong reductivity of the nbutylithium. The morphology of WS2 before and after exfoliation is shown in Figure 3. Before exfoliation, the bulk WS2 is observed in thick multilayer-stacked large crystals (Figure 3a−c). The hexagonal prismatic structure in Figure 3c corresponds to the 2H phase. After the exfoliation, thinner WS2 nanosheets with a few layers can be observed (Figure 3d); the thickness of the WS2 nanosheets from atomic force microscope is ∼3 nm (Figure S3), which corresponds to a layer number of 3. The exfoliated WS2 presents disordered crystal domains, indicating the coexistence of both 2H and 1T phases (Figure 3d). In the selected areas, a hexagonal lattice of 2H-WS2 can be observed in Figure 3e, while the 1T-WS2 in Figure 3f shows a trigonal structure. The corresponding FFT patterns for 1T- and 2HWS2 are also displayed as the insets of Figure 3e,f, respectively. As shown in the inset of Figure 3f, the FFT pattern of the 2HWS2 displays the typical hexagonal spots, whereas the pattern of 1T-WS2 shows an extra hexagonal spot at 30° angular spacing between the hexagonal spots (inset of Figure 3f). These patterns are in agreement with the structure models for 1T- and 2H-WS2 in Figure 3e,f, respectively. The TGA thermogram of GF (Figure S4) shows that 100% of the GF could be burned by calcination at 900 °C, indicating the successful removal of nickel substrate. Moreover, the EDS

2. RESULTS AND DISCUSSION The WS2/GF electrodes are synthesized as illustrated in Scheme 1. The bulk WS2 was first exfoliated using the lithiuminsertion method,13 and the percentage of 1T-WS2 phase could be adjusted with different amounts of n-butyllithium addition. At a molar ratio of WS2/n-butyllithium of ∼1:0.4, the obtained weight percentage of 1T-WS2 was ∼50%. Increasing the ratio to ∼1:2.3, the weight percentage of 1T-WS2 could be increased to ∼67%. This 1T percentage could not be increased further by adjusting the amount of n-butyllithium in solution. In this study, the exfoliated WS2 sample with less intercalation agent (1:0.4) was labeled as I-WS2, while the one with more intercalation agent (1:2.3) was marked as II-WS2. The 3D conductive graphite foam was synthesized as reported before.35,36 The free-standing WS2/GF electrode can be prepared by drop-casting the exfoliated WS2 nanosheets solution onto the GF struts. The XRD patterns of the WS2 before and after the exfoliation are shown in Figure 1a. The bulk 2H-WS2 sample has a good crystalline structure and shows well-resolved diffraction peaks. After the exfoliation, the intensities for all the diffraction peaks of II-WS2 (Figure S2) obviously decrease, while the absence of strong reflection peak leads to the non-negligible noise. Moreover, the in-plane (02−2) reflection at 31.8° can be distinguished, indicating the presence of 1T phase of WS2 nanosheets after the exfoliation. 24 Raman spectroscopy measurements were also performed to further confirm the varying phases (Figure 1b). The characteristic Raman peaks at 366 and 431 cm−1 for the E12g and A1g for WS2 can be observed, 30593

DOI: 10.1021/acsami.7b06613 ACS Appl. Mater. Interfaces 2017, 9, 30591−30598

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ACS Applied Materials & Interfaces

Figure 4. SEM images of GF (a); WS2/GF with different magnifications (b,c), the inset of c shows the WS2 nanosheets anchored on the surface of GF network; selected SEM image of WS2/GF and its corresponding elemental-mapping images (d−g).

Figure 5. Electrochemical performance of the obtained materials. The effect of WS2 (1T: 67%) loading on the HER activity (a); the effect of 1T weight percentage on the HER activity, I-WS2 (1T: 50%), and II-WS2 (1T: 67%) (b); stability test for the II-WS2/GF electrodes (c); EIS spectra for the obtained electrodes (d); optimized II-WS2/GF electrodes for HER test (e); the corresponding Tafel plots (f).

the contact interface.37 Figures 4d−g show the elemental distribution of W, S, and C in the WS2/GF composite. The mapping images of W and S show the significant inheritance of the GF structure, and the absence of the Ni element could also reveal the complete removal of the Ni template. In addition, the surface morphology of the WS2/GF composite is not broken even after 200 cycles of HER tests (Figure S6), indicating the tight coating of the WS2 nanosheets on the GF surface. Electrochemical hydrogen evolution of the WS2/GF electrodes was then investigated in a standard three-electrode system in 0.5 M H2SO4 solution. To optimize the fabrication of the electrodes, the effect of the loading of exfoliated WS2 (67 wt % 1T-WS2) on GF was evaluated. As shown in Figure 5a, the best activity for HER can be obtained with 60 μg of WS2 supported on GF (0.8 cm2); there may have been trade-offs between the

spectra of the WS2/GF electrode (Figure S5) possess only carbon, tungsten, and sulfur elements, and no residual nickel could be detected. The free-standing GF with a high electrical conductivity and 3D-interconnected network may act as an excellent support for WS2-based electrocatalysts (Figure 4a). The surface area of GF was determined to be ∼4 m2/g, and the diameter of the pores was ∼200 μm according to the SEM image in Figure 4a. After drop-casting, the exfoliated WS2 was anchored on the GF surface uniformly, and the structure of the GF was kept unchanged (Figure 4b). The 3D-interconnected porous structure of GF provides larger interfacial contact between the electrocatalyst and the electrolyte as compared with a plate-current collector. The WS2 nanosheets were tightly coated on the GF surface via a sheet contact (Figure 4c). This “sheet contact” can ensure strong adhesion between the WS2 nanosheets and GF, thus facilitating the electron transfer from 30594

DOI: 10.1021/acsami.7b06613 ACS Appl. Mater. Interfaces 2017, 9, 30591−30598

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ACS Applied Materials & Interfaces electrical conductivity (lower loading amount) and active-site density. The influence of the GF structure on catalyst performance was investigated, and two WS2 nanosheets, I-WS2 and II-WS2, were used for comparison. The I-WS2/GCE and II-WS2/GCE were fabricated with Nafion as the binder agent (Figure S7). As shown in Figure 5b, the electrode with a larger 1T percentage (67%) has a higher activity for both the glassy carbon electrode (GCE) and GF, which is in line with the theoretical prediction and other experimental results.38 With the presence of GF, the overpotential largely decreases from −350 mV of II-WS2/GCE to −190 mV of II-WS2/GF at the cathodic current densities of 10 mA/cm2 (i.e., η10). Furthermore, the Tafel slopes were calculated to be 237, 91, and 84 mV for the GF, II-WS2/GCE, and II-WS2/GF, respectively (Figure 5f). As the bare GF shows a weak activity (η10= −696 mV, Figure 5e), the earlier overpotenial and smaller Tafel slope of II-WS2/GF should attribute to the synergetic effect of WS2 nanosheets and GF. As shown in Table S1, we listed some representative free-standing HER electrodes for comparison. The II-WS2/GF in this study exhibits the smallest overpotential of η10 with an easy and scalable synthesis process.24,39−42 The stability of the optimized electrode was also investigated, and no deactivation can be observed even after 1000 cycles (Figure 5c). To further understand the mechanism of the enhanced activity, the electrochemical impedance spectroscopy (EIS) spectra for these electrodes were determined (Figure 5d). The charge-transfer resistance of GF at the electrode/electrolyte interface was the smallest (∼1.3 Ω) due to its good electrical conductivity. When the GF is used as the support, the chargetransfer resistance of the II-WS2/GF electrode can be highly reduced from ∼7.5 (II-WS2/GCE) to ∼2.0 Ω. The reduced charge-transfer resistance can afford faster HER kinetics with the WS2/GF, thus increasing the catalytic activity. Therefore, the enhancement in HER performance of the WS2/GF may be due to the following factors: (1) the WS2 nanosheets anchored on 3D-GF can greatly increase the interfacial contact between WS2 and electrolyte, and the large pores on GF can facilitate the electrolyte ion transfer; (2) the intimate sheet contact between the WS2 and GF can improve the electrical conductivity through the interface. To interpret the experimental results and understand the role of graphite substrate as well as the effect of different phases of WS2 on the performance of HER, WS2/graphene heterointerfaces were established and studied by density functional theory (DFT) calculations using a Dmol3 package,43 and the corresponding band structure and adsorption behaviors of the hydrogen atom on the WS2 and WS2/graphene heterointerface were considered. Several prototypes of WS2 have been reported, including 2H, 1T, and 1T′. On the basis of our DFT calculations, the 1T-WS2 is metastable and spontaneously transforms into the distorted structure (1T′-WS2) after adsorbing an H atom, which is consistent with the previous reports.11,44 Thus, the HER activity of 2H-WS2 and 1T′-WS2 were studied by the DFT method, and their atomic configurations are shown in Figure 6a,c, respectively. The band structures of 2H-WS2 and 1T′-WS2 are shown in Figure 6e,g, where 2H-WS2 has a direct band gap of 1.82 eV at Kpoint, while 1T′-WS2 has an indirect band gap of 0.05 eV. The WS2/graphene heterointerface can improve electric conductivity compared with that of bare WS2. The structural models of 2H-WS2/graphene and 1T′-WS2/graphene are shown in Figure 6b,d, respectively. The band structures of the bilayer WS2/

Figure 6. Atomic structures of 2H-WS2 (a), 2H-WS2/graphene (b), 1T′-WS2 (c), and 1T′-WS2/graphene (d). Panels (e−h) show the band structures of 2H-WS2, 2H-WS2/graphene, 1T′-WS2, and 1T′WS2/graphene, respectively. The yellow/green, blue, and gray atoms represent S, Mo, and C atoms, respectively. The dashed line denotes the Fermi level.

graphene system in both 2H and 1T′ phases were calculated and are respectively shown in Figure 6f,h, where the bilayer system has a near-zero band gap and similar conductive behavior as pristine graphene at the Dirac K-point. Therefore, to support WS2, nanosheets on GF can obtain an improved electrode with higher electrical conductivity and facilitate the HER process. HER activity of a catalyst can be well described by free energy of adsorbed hydrogen atom ΔGH. The configurations for the adsorption of H atom on WS2 systems are shown in Figure 7. For 2H-WS2 and 2H-WS2/graphene, the hydrogen atom prefers to adsorb on the top of the S atom (Figure 7a,b). For 1T′-WS2 and 1T′-WS2/graphene, the S atoms are not in the same plane, and there are two possible adsorption positions, i.e., site 1 and site 2. According to DFT calculations, the hydrogen atom prefers to adsorb on the S atom at site 1 (Figure 7c,d). On the basis of the definition of free energy for hydrogen adsorption, ΔGH values are 2.21, 2.16, 0.20, and 0.31 eV for 2H-WS2, 2H-WS2/graphene, 1T′-WS2, and 1T′-WS2/ graphene, respectively. It is commonly accepted that the best HER activity corresponds to ΔGH = 0.45 More negative ΔGH will lead to stronger bonding of adsorbed hydrogen, while more positive ΔGH will make the protons bond too weakly to the catalyst surface and, thus, both conditions will lead to slow HER kinetics. Therefore, 1T′-WS2 and 1T′-WS2/graphene show an excellent HER activity compared with the 2H-WS2 and 2H-WS2/graphene. Therefore, on the basis of the ΔGH values and band structure, it can be determined that more 1T′-WS2 and graphene substrate participate in HER processes, and the corresponding HER processes will be promoted significantly.

3. CONCLUSION In summary, few-layered WS2 nanosheets with a controllable phase ratio (1 T ratio up to 67%) were obtained via a lithiuminsertion exfoliation method. Free-standing and conductive 30595

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Fabrication of WS2/GCE Electrode. Typically, 5 mg of WS2 and 20 μL of Nafion solution were dispersed in 1 mL of water and sonicated for 0.5 h to form a homogeneous ink. A WS2 ink solution of the same concentration but without Nafion addition was also prepared. After that, 5 μL of these two inks were then loaded onto glassy carbon electrodes (GCE) that were 3 mm in diameter, separately. Fabrication of WS2/GF Electrode. The desired amount of isopropanol was added in the as-obtained WS2 aqueous solution. This mixture solution was then drop-casted onto the surface of GF, and the final electrodes can be obtained after drying at room temperature under nitrogen atmosphere. Characterizations. The samples were characterized by scanning electron microscopy (SEM) (Hitachi S4800), energy-dispersive X-ray spectroscopy (EDX; FEI NOVA NanoSEM430), transmission electron microscopy (TEM) (JEM-2100F), X-ray photoelectron spectroscopy (XPS) (PerkinElmer, PHI1600 spectrometer), X-ray diffraction (XRD) (AXS D8-Focus), and Raman spectroscopy (Renishaw in Via reflex Raman spectrometer with 532 nm laser excitation). Electrochemical Measurements. The activity tests of the synthesized catalysts for HER was carried out using a CHI 660E workstation (Chenhua Instruments, Shanghai, China). Linear sweep voltammetry with a scan rate of 10 mV s−1 was conducted in 0.5 M H2SO4 using an Ag/AgCl electrode as the reference electrode, a Pt wire as the counter electrode, and the obtained electrode as the working electrode. The electrochemical impedance spectroscopy (EIS) measurements were carried out at the same workstation with a frequency range from 0.01 to 1 M Hz. For measurements of exfoliated WS2 nanosheets, the working electrode was prepared by loading WS2 aqueous solution with 5 wt % Nafion binder on the glass carbon electrode. Density Functional Theory Calculations. The density functional theory (DFT) calculations were carried out by using a Dmol3 package.35 Exchange−correlation functions are taken as generalized gradient approximation (GGA) with Perdew−Burke−Ernzerhof (PBE).22 DFT semicore pseudopotential (DSPP) core treatment is implemented for relativistic effects and double numerical plus polarization (DNP) is employed as the basis set. The simulation cell for pristine WS2 consists of a 4 × 4 WS2 supercell with a vacuum width of 20 Å above the WS2 layer to minimize the interlayer interaction. For the WS2/graphene heterostructure, a 4 × 4 WS2 supercell and 5 × 5 graphene supercell were used, resulting in a small lattice mismatch of 3.5%. The K-point is set to 6 × 6 × 1, and all atoms are allowed to relax. ΔGH of the adsorbed hydrogen was obtained by ΔGnH = ΔEnH + ΔEZPE − TΔSH, where ΔEZPE − TΔSH is 0.28 eV46 in standard conditions at T = 300 K with ΔEZPE and ΔSH being changes in zeropoint energy and entropy between the adsorbed atomic hydrogen and hydrogen in gas phase, respectively. Thus, ΔGnH is simplified to ΔGnH = ΔEnH + 0.28. ΔEnH is the adsorption energy of the nth hydrogen and is defined as ΔEnH = EWS2+nH − EWS2+(n−1)H − 1/2EH2, where EWS2+nH and EWS2+(n−1)H represent the total energies of the WS2 system with n and n − 1 adsorbed hydrogen atoms on the surface, respectively, while EH2 represents the total energy of a gas-phase hydrogen molecule.

Figure 7. Hydrogen atom adsorbed on 2H-WS2 (a), 2H-WS2/ graphene (b), 1T′-WS2 (c), and 1T′-WS2/graphene (d), respectively. The white atom represents the H atom, and the numbers indicate different adsorption positions for the hydrogen atom.

WS2/GF composite was fabricated by simple drop-casting of WS2 nanosheets onto the 3D conductive graphite foam, and an intimate “sheet contact” between the WS2 and GF was observed. With improved electrical conductivity and iontransfer rate, the WS2/GF composite can act as an efficient binder-free HER electrode. An early overpotential of 190 mV at 10 mA/cm2 can be achieved, better than most of the present free-standing HER electrodes. This easy and scalable WS2/GF composite with high catalytic activity and long-term stability can be a promising binder-free electrode for large-scale hydrogen production.

4. EXPERIMENTAL SECTION Exfoliation of WS2. WS2 powders (2.38 or 1.25 g) were added into a 50 mL Schlenk flask. After the air in the flask was swept with argon, 6 or 18 mL of n-butylithium/hexane solution (1.6 M in hexane) was injected into it and then stirred for 48 h at 67 °C. Then the mixture was sonicated for 180 min at room temperature. Successively, the dispersion was centrifuged to obtain the solid samples, and the precipitates were washed three times with n-hexane to remove the organic residues and excess lithium. The final exfoliated WS2 nanosheet solution can be obtained after further washing with water. Synthesis of Graphite Foam. The graphite foam was prepared following the previously reported works.35,36 In brief, Ni foam was cut into slices with ∼10 cm in length and ∼2 cm in width and then placed into the center of a 1 in. hot wall furnace (TF55035A-1, Linderburg/ Blue M). The quartz tube was evacuated and then filled with Ar (100 sccm, 99.99%) until the chamber reached atmospheric pressure. The furnace was heated to 1050 °C at atmospheric pressure under a flow of Ar (40 sccm) and H2 (10 sccm, 99.99%), and then the Ni foam was annealed under H2 for 30 min. After that, the H2 was adjusted to 5 sccm, and CH4 (5 sccm, 99.99%) was introduced. After the Ni foam grew for 60 min at 1050 °C, the furnace was cooled down at a rate of 10 °C min−1 to room temperature. The graphite-coated Ni foam was cut into ∼0.8 cm by ∼2 cm strips and placed in 3 wt % HCl aqueous solution at 80 °C for 24 h to remove Ni. The graphite foam was gently bathed in deionized water several times and rinsed in ethanol before drying at 35 °C.

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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/acsami.7b06613. Optical image of aqueous solution of bulk WS2 and exfoliated WS2; XRD and Raman spectra for the I-WS2 and II-WS2; AFM image of the exfoliated WS2 nanosheets; TGA thermogram of graphite foam; EDS images of WS2/GF composite; SEM images of WS2/GF and WS 2 /Gf through 100 and 200 cycles of cyclic voltammetry; the electrochemical performance of the WS2/GCE electrodes with and without binder; compar30596

DOI: 10.1021/acsami.7b06613 ACS Appl. Mater. Interfaces 2017, 9, 30591−30598

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ison of HER activities for the WS2/GF and other HER electrocatalysts (PDF)

AUTHOR INFORMATION

Corresponding Authors

*W.P.: E-mail: [email protected] *Z.A.: E-mail: [email protected] ORCID

Xiaobin Fan: 0000-0002-9615-3866 Yang Li: 0000-0003-3003-9857 Wenchao Peng: 0000-0002-1515-8287 Author Contributions ∇

X.G. and J.J. contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by No. 21506158 and 21607029 from the National Natural Science Foundation of China (NSFC), No. 2015M571652 from the China Postdoctoral Science Foundation, the “1000 Plan” for the Young Professionals Program of China, and the “100 Talents” Program of Guangdong University of Technology.

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DOI: 10.1021/acsami.7b06613 ACS Appl. Mater. Interfaces 2017, 9, 30591−30598