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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Low-Temperature Synthesis of Cuboid Silver Tetrathiotungstate (Ag2WS4) as Electrocatalyst for Hydrogen Evolution Reaction Fengping Zhan,† Qinghua Wang,† Yibing Li,‡ Xin Bo,‡ Qingxiang Wang,*,†,‡ Fei Gao,† and Chuan Zhao*,‡ †

College of Chemistry and Environment, Fujian Province Key Laboratory of Modern Analytical Science and Separation Technology, Minnan Normal University, Zhangzhou 363000, P. R. China ‡ School of Chemistry, The University of New South Wales, Sydney 2052, Australia W Web-Enhanced Feature * S Supporting Information *

ABSTRACT: Ternary bimetallic sulfide silver tetrathiotungstate (Ag2WS4) was prepared by a simple low-temperature precipitation method. The structure of Ag 2 WS 4 was determined using the data from powder X-ray diffraction, which is in space group I4̅2m (I-phase) with a tetragonal unit cell of a = b = 5.950 71 Å and c = 9.562 65 Å, containing layers of edge-sharing AgS4 and WS4 tetrahedra. The Ag2WS4 was also used as an electrocatalyst for the hydrogen evolution reaction (HER) in acid electrolyte. The results demonstrated that, attributed to its unique composition, large electrochemical active surface area, and high electric conductivity, the Ag2WS4 exhibited superior electrocatalytic activity for HER, in comparison with the ternary counterparts of WS2 and Ag2S with a low onset potential and small Tafel slope. The Ag2WS4 also presented superior stability and maintained the electrocatalytic activity of HER for at least 24 h in 0.5 M H2SO4.

1. INTRODUCTION With rapid growth of global energy demand and increasing problem of environmental pollution problem caused by the overuse of the fossil fuel, much effort has been paid to develop novel sustainable and clean energy. Molecular hydrogen (H2) is proposed as a zero emission and renewable energy carrier to replace exhaustible fossil fuels.1 Electrochemical water splitting is one of the most promising solutions to obtain clean H2 fuels due to its advantages of simplicity, high H2 purity, availability, renewable, and environmentally friendly energy conversion strategy.2,3 However, the H2 production from electrochemcial water splitting suffered from the disadvantage of high overpotential of hydrogen evolution reaction (HER), which limited its large-scale industrial application. The platinum (Pt)based materials are state-of-art electrocatalysts to be capable of lowering overpotential of HER and decrease the energy consumption, but unfortunately, their scarcity, high price, and inferior durability inhibited the commercial utilization. Recently, more and more scientists make their efforts in seeking new electrocatalysts to achieve high efficiency for HER.4−6 Among these electrocatalysts, the transition-metal sulfide received considerable attention due to outstanding electronic structure, tunable composition, and abundance in earth.7−14 The tetrathiometalates MS42− (M = V, Mo, W, and Re)based ternary bimetallic sulfides (TBS) are a kind of metal-rich compounds.15−17 The characterization studies show that the TBS usually consist of square packed layers with metal atoms © XXXX American Chemical Society

sandwiched between S atoms in adjacent layers. The transition metal or M predominately binds with S by ionization with some covalent features, and then the adjacent layers interact with each other via van der Waals forces.17 On the basis of their unique constitution and structure, the TBS has been utilized as electrode materials in the energy field.16,18,19 For example, Hu et al.16 have also prepared ultrathin hydrogenated-Cu2WS4 nanosheets and used them as an electrode material to fabricate a flexible all-solid-state supercapacitor. The result showed that the Cu2WS4 varied from semiconductive to metallic through hydrogen incorporation, leading to outstanding charge− discharge performance and high stability (3000 cycles). Jing et al.18 have obtained a novel Cu2WS4, being of unique decahedral structure with energy gap of 2.1 eV, and utilized it as a novel photocatalyst for hydrogen generation. The result showed that the photocatalyst presented the highest activity of 135 μmol/h for H2 evolution with the apparent quantum yield of 11% at 425 nm. Tran et al.19 reported that the Cu2MoS4 catalysts with a well-defined crystal structure can be applied to HER in aqueous solution in a wide pH range, representing an attractive alternative to platinum. Both the experimental and theoretical results suggest that the transition-metal ions in the TBS can obviously improve the HER activity by enhancing its intrinsic catalytic activity and effective electrochemical surface area.15,20 However, to the best of our knowledge, the transition Received: January 12, 2018

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DOI: 10.1021/acs.inorgchem.8b00108 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a) Rietveld refinement of Ag2WS4 showing observed (black), calculated (red), and difference (green) plots; the structure of body-centered Ag2WS4 viewed along the 010 direction (b), 001 direction (c), and 111 direction (d). WS4 tetrahedra are shown in cyan, AgS4 tetrahedra are shown in purple, and the sulfur atoms are shown in yellow. The morphologies of the synthesized materials were obtained by scanning electron microscopy (SEM) with a Hitachi S-4800 scanning electron microscope. Energy-dispersive X-ray spectra (EDS) and elemental mapping data were recorded on JSM-60−10LA. PXRD was characterized by a Rigaku, Ultima IV diffractometer with Cu Kα radiation (λ = 1.5418 Å). X-ray photoelectron spectroscopy (XPS) was determined on Kratos Axis Ultra using Al Kα (1486.6 eV) radiation. Transmission electron microscopy (TEM) was completed on Tecnai F20, 200 kV from FEI Company. Inductively coupled plasma mass spectrometry (ICP-MS) analysis was performed on Agilent7500CE. The pH of H2SO4 was directly measured by a digital pH 211 microprocessor. 2.2. Preparation of Ag2WS4. Prior to the synthesis of Ag2WS4, the precursor of (NH4)2WS4 was synthesized according to a previously reported procedure.21 In brief, 20 mmol (5.0 g) of H2WO4 was dissolved in 40 mL of concentrated NH3·H2O to form the dispersion, which was followed by aerating with H2S at room temperature until reaching saturation. Then the mixture was stirred at 60 °C for 20 min. After filtration of the mixture, the solution was heated at 60 °C again and continuously bubbled with H2S for 8 h. The reaction solution was then cooled to 15 °C while maintaining the H2S flow, during which the orange solid product of (NH4)2WS4 was formed. The product was then separated by filtration, washed with isopropanol and ethyl ether, and dried in vacuum at last. The Ag2WS4 was prepared via a facile ion-exchange reaction: first, 0.25 mmol (0.0846 g) of (NH4)2WS4 was dissolved in 2 mL of H2O to obtain an orange solution. Then the solution was one-time added into 1.5 mL of H2O containing 0.50 mmol (0.0887 g) of AgNO3 under constant magnetic stirring. After it reacted for 5 h, the solution was gradually changed to dark brown. Thereafter, the reaction mixture was kept stirring for 48 h, and the solution was retained at room temperature for 2 d. Thus, the precipitate was formed, and the product was collected through centrifuging with ultrapure water and absolute ethanol several times and finally dried at 40 °C overnight. For comparison, the Ag2S was also prepared by a simple precipitation reaction of AgNO3 and Na2S with 2:1 molar ratio at room temperature. To test HER performance in an acidic medium, the chemical stability of the Ag2WS4 was investigated. In brief, the Ag2WS4 powder was immersed in excess amount of 0.5 M H2SO4. The product was

metals used for synthesizing TBS are mainly focused on copper, nickel, and cobalt,15,16,20 and the silver ion (Ag+)-based TBS has not been reported. Further, the reported methods for the synthesis of the copper-, nickel-, and cobalt-based TBS usually required the toxic-organic solvents and high reaction temperature, posing significant risks to health and environments and concerns for energy consumption. In the present article, we show a low-temperature route with low energy consumption and environmental-friendly aqueous solvent to synthesize a new ternary sulfide, silver tetrathiotungstate (Ag2WS4). In brief, the Ag2WS4 material was directly generated by adding (NH4)2WS4 into AgNO3 aqueous solution through a facile chemical precipitation method. The structure of Ag2WS4 was solved using the data from powder X-ray diffraction (PXRD) techniques. It was obtained that the Ag2WS4 phase crystallizes in the tetragonal system I4̅2m containing a square planar arrangement that constituted by interlinked silver and tungsten atoms tetrahedrally coordinated to the bridging sulfur atoms. When the Ag2WS4 served as an electrocatalyst for the acidic HER, excellent electrocatalytic activity as indicated by low onset potential and Tafel slope was achieved. Also, extraordinary stability was obtained during durability test, which shows that the facilely synthesized Ag2WS4 can be used as a new electrocatalyst for energy conversion.

2. EXPERIMENTAL SECTION 2.1. Reagents and Apparatus. Silver nitrate (AgNO3) was purchased from Sinopharm Chemical Reagent Co. Ltd. Nafion 117 (5 wt %) was obtained from Shanghai New-energy Technology Co. Ltd. Pt/C (10%, Pt on Vulcan XC-72R carbon support), tungstic acid (H2WO4), and tungsten sulfide (WS2) were provided by Aladdin Reagent Co. Ltd. Concentrated ammonia solution (NH3·H2O, 25− 28%) was purchased from Xilong Co. Ltd. All the other chemicals were of analytical reagent grade and used without further purification. The ultrapure water (18 MΩ cm) used throughout all experiments was prepared from a Milli-Q system (Millipore). B


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Inorganic Chemistry collected and dried for 24 h; after that the vacuum was applied. The PXRD patterns of both the untreated and acid-soaked Ag2WS4 were taken. 2.3. Fabrication of the Modified Electrodes. Before fabrication, the coating ink was prepared by dispersing 5 mg of Ag2WS4 in a mixture containing 200 μL of ethanol, 800 μL of ultrapure water, and 30 μL of 5% Nafion through ultrasonication for 30 min. Then, 10 μL of the prepared ink was cast onto a freshly cleaned glassy carbon electrode (GCE) surface. After the catalyst ink was dried in air, the Ag2WS4−Nafion modified GCE (Ag2WS4−Nafion/GCE) was obtained. The loading amount of Ag2WS4 was calculated to be 0.68 mg cm−2. For comparison, the control electrodes of Nafion/GCE, Ag2SNafion/GCE, WS2−Nafion/GCE, and Pt/C-Nafion/GCE were fabricated in the same way. 2.4. Electrochemical Measurement. Electrochemical measurements were performed on a CHI 660E electrochemical workstation. A standard three-electrode system is composed of Ag/AgCl electrode (saturated KCl) as reference electrode, a bare or modified GCE (diameter = 3 mm) as working electrode, and a graphite rod as counter electrode, respectively. Before electrochemical measurement, the electrolyte of 0.5 M sulfuric acid (H2SO4, pH = 0.3, which is directly measured by a digital pH meter) was bubbled with N2 for 20 min, and then linear-sweep voltammetry (LSV) was performed at a scan rate of 5 mV s−1 for the polarization curve tests. The test potential was then converted to the reversible hydrogen electrode (RHE) potential according to the formula: E(RHE) = E(Ag/AgCl) + 0.0592 pH + 0.197 V,22 where E(Ag/AgCl) is a potential versus Ag/AgCl (saturated KCl) electrode, pH is the pH value of electrolyte, and E(RHE) is a potential versus RHE. Multistep chronopotentiometry with current being increased from 50 to 500 mA cm−2 with an improvement step of 50 mA cm−2 per 500 s, chronoamperometry (i−t curve) conducted at a static potential of −385 mV (vs RHE) and cyclic voltammetry (CV) performed between −180 and −430 mV (vs RHE) at a sweep rate of 100 mV s−1 were applied for long-term stability study of the synthesized material for HER. 2.5. Structure Determination from Powder X-ray Diffraction Data. The PXRD patterns were recorded in a 2θ range from 15 to 140° with a 0.0144° step size and a measurement time of 7.5 s per step. The pattern was indexed by an N-TREOR09 program embedded in the EXPO2014 package.23 The space group was assigned using the FINDSPACE module of EXPO2014. The cell and diffraction pattern profile parameters were refined according to the Le Bail algorithm.24 The background was modeled by a 20th-order polynomial function of the Chebyschev type; peak profiles were described by the Pearson VII function. The structure was solved by direct method and refined by the Rietveld method using EXPO2014 program. 2.6. Density Functional Theory Calculations. Density functional theory (DFT) calculations using the Gaussian 09 package were performed to estimate the possible catalytic active site in Ag2WS4, which was optimized by the B3LYP method. LANL2DZ polar basis set was adopted for Ag and W atoms; the 6-311G (d) basis set was used for S and H atoms.

Table 1. Crystal Data and Structure Refinement Parameters compound Fw (g mol−1) system space group a (Å) b (Å) c (Å) α = β = γ (deg) V (Å3) Z Dx (g cm−3) Rp Rwp χ2 RBragg RF

Ag2WS4 527.86 tetragonal I4̅2m 5.9507(1) 5.9507(1) 9.5626(5) 90 338.62(3) 2 5.177 0.0937 0.1294 1.280 0.1114 0.0960

of a = b = 5.9507 Å and c = 9.5626 Å. The fractional atomic coordinates and isotropic thermal factors (Uiso values fall in the range from 0.0245 to 0.0349) for Ag2WS4 are listed in Table 2, showing three symmetric independent atoms in each unit cell. Table 2. Fractional Atomic Coordinates and Isotropic Thermal Factors atom

x

y

z

Uiso, Å2

W Ag S

1.0000 1.0000 0.7901(13)

1.0000 0.5000 0.7901(13)

0.5000 0.5000 0.6435(11)

0.0295 0.0349 0.0245

Table 3 lists the bond lengths and angles of Ag2WS4. The distortions of the WS4 units from tetrahedral geometry are Table 3. Bond Lengths and Bond Angles for Ag2WS4

a

bond lengths, Å

bond angles, deg

W−S 2.236(7) Ag−S 2.534(5) Ag−W 2.975(6) S···S 3.637(2)a

W−S−Ag 76.9(1) S−W−S 112.1(1) S−W−S 104.3(2) S−Ag−S 94.1(3) S−Ag−S 120.9(4)

The van der Waals contact between S atoms in adjacent layers.

manifested by the angles of S−W−S and S−Ag−S, which are in the ranges from 104.3 to 112.1° and from 94.1 to 120.9°, respectively. It shows that WS4 tetrahedron deviated slightly from ideal tetrahedral geometry, while AgS4 tetrahedron is much more distorted.25 According to previous studies,18 such crystal distortion will benefit the charge separation as well as hydrogen generation. The W−S bond length of 2.236 Å is longer than that observed in previously isolated WS42− ions with terminal S atoms, such as in (NH4)2WS4 (2.17 Å),26 indicating the presence of weaker interactions between W and S atoms in the synthesized Ag2WS4.27 In addition, the Ag−W bond length of 2.975 Å is shorter than those in (PPh3)Ag2WS4 (3.056 Å, PPh3 = triphenylphosphine)28 and [Ag4W2S8(PPh3)4] (2.997 Å),29 suggesting stronger metal−metal interactions between Ag and W atoms in the synthesized product. The Ag−S bond length of 2.534 Å is very close to those observed in other [AgWS4] units, such as 2.52 Å in [WS4Ag·NH3C(CH2OH)3·2DMF]n (DMF = dimethylforma-

3. RESULTS AND DISCUSSION 3.1. X-ray Crystallographic Study of Ag2WS4. Figure 1a shows the collected PXRD data for the synthesized Ag2WS4 powder, fitted results, and their difference. Since the single crystal of Ag2WS4 is hard for us to obtain in the experiment, the structure was solved by analysis of PXRD data. As seen, the final Rietveld refinement showed a good accordance between the fitting and experimental results. The agreement factors of the pattern R factor (Rp) = 0.0937, the weighted pattern R factor (Rwp) = 0.1294, the Bragg intensities R factor (RBragg) = 0.1114, and the structure amplitudes R factor (RF) = 0.0960 were obtained, confirming the formation of Ag2WS4. In addition, from the summary of the structure refinement listed in Table 1, it can be obtained that the synthesized sample phase crystallizes in the tetragonal system I42̅ m, with cell parameters C


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Inorganic Chemistry

Figure 2. Low-magnification (a) and high-magnification (b) SEM images of Ag2WS4. (c) SEM and the corresponding elemental mapping image of Ag2WS4. (d) EDX of Ag2WS4 with the elemental compositions. (e) TEM image of an Ag2WS4 cuboid with granular particles on the surface and (f) HRTEM image showing the layer spacing.

Figure 3. (a) XPS survey spectra of Ag2WS4; (b) Ag 3d, (c) W 4f, and (d) S 2p spectra of Ag2WS4 catalyst.

mide)30 and 2.54 Å in [WS4Ag2(Hmimt)2]n (Hmimt = 1methylimidazoline-2(3H)-thione).31 The shortest S···S distance

is only 3.637 Å for the sulfur atoms in adjacent layers, which is close to the 3.699 Å as observed in Cu2WS4,25 revealing that D


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Inorganic Chemistry

Figure 4. (a) LSV curves for different electrodes in 0.5 M H2SO4 with a scan rate of 5 mV s−1, and (b) the corresponding Tafel plots. (c) Multicurrent chronopotentiometric process of Ag2WS4−Nafion/GCE. The current density started at 50 mA cm−2 and ended at 500 mA cm−2, with an increment of 50 mA cm−2 per 500 s; (d) LSV curves of Ag2WS4−Nafion/GCE before and after 1000 and 2000 CV cycles. (inset) Time dependence of the constant current density at −385 mV (vs RHE).

granular particles attached to the surface. To probe the structure of the granular particles, the high-resolution (HR) TEM images were taken, and the results showed that clear lattice fringes with an interplanar distance of 0.509 nm were observed (Figure 2f), which can be indexed to the (101) crystal plane of the tetragonal Ag2WS4 phase. This result confirmed that the granular particles are also composed of Ag2WS4. The survey XPS spectrum of Ag2WS4 (Figure 3a) manifests that the product is composed of Ag, W, and S elements. The high-resolution scans of the Ag, W, and S electrons are depicted in Figure 3b−d, respectively. The Ag 3d5/2 peak at 367.8 eV and the Ag 3d3/2 peak at 373.8 eV imply that the oxidation state of the silver species corresponds to Ag+.32 The W 4f7/2, 4f5/2, and 5p3/2 levels are located at 33.1, 35.3, and 37.7 eV, respectively, which suggests that tungsten is in its W6+ oxidation state as it is in the WS42− ions.33,34 The S 2p3/2 peaks at 161.3 eV and the S 2p1/2 peaks at 162.5 eV can be ascribed to sulfur species in S2− state.32,35 3.3. HER Performance of Ag2WS4. The electrocatalytic performance of Ag2WS4 for HER was tested in 0.5 M H2SO4 using a conventional three-electrode system, and the result was compared with those from Ag2S and commercial WS2 and Pt/C electrodes. Figure 4a displays the polarization curves using different electrodes. As expected, the Pt/C exhibits high HER performance with a very low overpotential, while the Nafion/ GCE, WS2−Nafion/GCE, and Ag2S-Nafion/GCE showed lower HER activity with a negligible catalytic current density. However, it is interesting that the Ag2WS4−Nafion/GCE exhibits significant electrocatalytic activity for HER with an onset potential of −212 mV (vs RHE) and an overpotential of

the van der Waals interaction also exists between the adjacent layers of the synthesized Ag2WS4. From the PXRD data, the crystal model of Ag2WS4 was further simulated. Figure 1b−d shows the simulated crystal structure of Ag2WS4 viewed along the 010 (b), 001 (c), and 111 (d) directions. As seen, the Ag, W, and S atoms within the layers are covalently bonded. Four WS4 units coordinated to four Ag atoms in a square array, and two-dimensional (2D) layers containing WS4 and AgS4 distorted tetrahedral were formed via edge sharing of the squares. Furthermore, through the S···S van der Waals interactions between the adjacent layers, a quasi three-dimensional (3D) structure was constructed. 3.2. SEM, TEM, and XPS Characterization of Ag2WS4. To gain detailed insights into the morphology, the prepared Ag2WS4 was examined by SEM. As shown in low-resolution SEM image (Figure 2a), the obtained products present cuboid shape. The magnified SEM of a single cuboid showed that there are some tiny granular particles covering the surface of the surface (Figure 2b), which was expected to be beneficial for increasing the surface area of the sample. The elemental mapping images of energy-dispersive X-ray (EDX) as shown in Figure 2c reveals that all the Ag, W, and S elements are uniformly distributed on the cube. The atomic ratio of Ag, W, and S was determined to be 2.1:1.1:3.9 (Figure 2d), which is in good consistency with the atomic ratio of Ag2WS4 as demonstrated by the PXRD refinement analysis, confirming the successful synthesis of Ag2WS4. The morphology and composition of the synthesized Ag2WS4 were also characterized by TEM. As seen in Figure 2e, the single cuboid has the cuboid shape with numerous E


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Inorganic Chemistry 329 mV for current density of 10 mA cm−2. This result showed that the synthesized Ag2WS4 has markedly enhanced catalytic performance for HER in comparison with WS2 and Ag2S due to its unique ternary elemental composition. It is worth noting that the lower electrocatalytic activity of the WS2−Nafion/GCE in HER, which appears to be contradictory to previously reported results of some 2D WS2 thin sheets with rich edge,36,37 is probably owing to the large grain size and highly compact layered structure (i.e., without exfoliation), as shown in the SEM images in Figure S1. This results in lower specific surface area, fewer active sites, and reduced electrochemical activity of commercial WS2. However, although the Ag2WS4 reported here showed better performance than the commercially purchased WS2 and some previously reported WS2 in HER,38,39 it is still not as good as the vertically oriented WS2 nanosheets (onset potential of −110 mV, overpotential of 225 mV at 10 mA cm−2) directly grown on carbon cloth.40 Through comparison, we think that such an excellent performance of the WS2 nanosheets is due to the unique vertical layer orientation of directly grown WS2 nanosheets with larger exposed active edge site, and the relatively poor electrocatalytic performance of our Ag2WS4 is related to its highly packed cuboid shape. Furthermore, the Tafel slopes of different electrodes were determined from the LSV tests and compared. Figure 4b shows the plots of the relationship of overpotential versus log j obtained from the linear portions at low overpotential of LSV curves. Through fitting the linear relationship of overpotential versus log j to the following Tafel equation: η = b log j + a,41 where η is the overpotential, j is the current density, and a is the Tafel intercept, the value of the Tafel slope (b) for Ag2WS4 electrode was determined to be 62 mV dec−1, which is larger than that of Pt/C but significantly lower than control electrodes including Nafion/GCE, WS2−Nafion/GCE, and Ag2S-Nafion/ GCE. This value is also significantly lower than the reported Wor Ag-based HER catalysts in acid medium (WS2(1−x)Se2x, 105 mV dec−1;42 WO3−x, 89 mV dec−1;43 Ag2S/CuS, 75 mV dec−1;44 Ag2S/Ag, 102 mV dec−1;45 WC, 84 mV dec−1),46 suggesting that the synthesized Ag2WS4 is a highly efficient electrocatalyst for HER. In addition, from the value of the Tafel slope (62 mV dec−1), the electrocatalytic mechanism of Ag2WS4 for HER can be ascribed to a sequential process of Volmer (H+ + e− → H*) step and Heyrovsky (H+ + e− + H* → H2) step.47 To investigate the intrinsic catalytic active site of the synthesized Ag2WS4 for HER, the binding energies of different elements in Ag2WS4 with hydrogen (H) were calculated by DFT. The results showed that the binding energies of H−Ag, H−W, and H−S were −1.42, −1.22, and −2.92 eV, respectively, suggesting that H atom preferentially bound to S site rather than Ag and W sites, and so the intrinsic active site for HER in Ag2WS4 should also be S2−. This result is also in good consistency with that for the other sulfide-based HER electrocatalysts.14,48 The stability of Ag2WS4−Nafion/GCE for HER was further investigated by multistep chronopotentiometric technology. Figure 4c displays a multistep chronopotentiometric curve for Ag2WS4 in 0.5 M H2SO4, with the current being increased from 50 to 500 mA cm−2 with an improvement step of 50 mA cm−2 per 500 s. At the initial current value of 50 mA cm−2, the potential immediately leveled off at −402 mV (vs RHE) and remained constant for the remaining 500 s. The overpotentials also remained stable at each current density in the test range,

confirming high catalytic activity as well as excellent stability of Ag2WS4 in a current density range from 50 to 500 mA cm−2. Continuous CV sweeping and the time dependence of the constant current density were also applied to evaluate the stability of material during long-term HER (Figure 4d). CV sweeps between −0.18 and −0.43 V (vs RHE) were performed at a scan rate of 100 mV s−1. After 2000 cycles, the LSV curve obtained is almost identical to the initial LSV, indicating superior cycling performance and no degradation of the Ag2WS4 for HER electrocatalysis. The long-term stability of Ag2WS4 was further performed at a constant overpotential of 385 mV (inset of Figure 4d). As observed, the catalytic current showed a slight decrease in the first 1.5 h likely due to the coverage of generated H2 bubbles on the electrode, and then the current kept constant for the rest of the electrolysis test (24 h). Several spikes that appeared in the chronoamperometric response could be ascribed to the detachment of large H2 bubbles from the electrode surface,49 which was testified by direct observation of vigorous effervescence and rapid dissipation of gas bubbles into solution during the electrolysis process (Supplementary Movie). All these results confirmed that the Ag2WS4 exhibited excellent electrocatalytic stability in accompaniment with effective detaching kinetic of bubbles for HER catalysis. In this work, the chemical stability of synthesized Ag2WS4 was investigated by comparing the XRD patterns and TEM images of the material before and after soaking in 0.5 M H2SO4 for 24 h. As shown in Figure S2-a, compared with the pristine Ag2WS4, there is no remarkable change in the PXRD pattern for the H2SO4-soaked Ag2WS4, suggesting good chemical stability of this material in acidic media. The PXRD patterns before and after the electrocatalysis were also taken and compared (Figure S2-b). The result showed that all the characteristic diffraction peaks remained after 24 h of HER test, further confirming that the reduction condition during the HER process had no influence on the structure and composition of Ag2WS4. TEM characterization results showed that Ag2WS4 still exhibited the cuboid shape with numerous granular particles attached to the surface after soaking in H2SO4 for 24 h (Figure S3-a), and the interplanar distance (0.503 nm) of the lattice fringe is also well-consistent with that without H2SO4 soaking, showing that H2SO4 has no influence on the morphology and microstructure of Ag2WS4. To further probe whether the metal ion (Ag+) can be leached from the Ag2WS4 by H2SO4, the soaked solution was detected by ICP-MS analysis, and the result showed that no Ag+ was detected. This result proved that the metal leaching is also negligible for Ag2WS4 in H2SO4. It is noticeable that the Ag+ is easily reduced under a lowpotential region due to the high standard potential (+0.8 V) for the Ag+/Ag couple, but from above characterization, we can know that the Ag+ in Ag2WS4 had not been changed to Ag0 under the reductive HER condition. We think that this phenomenon was related to the formation of insoluble Ag2WS4, from which the standard potential of Ag2WS4/Ag couple reduced greatly. Something similar also happened for the other Ag+-based insoluble compound. For example, the standard potential for the Ag+/Ag couple in insoluble Ag2S was decreased to −0.69 V50 and also presented high stability for HER in acid media.44,45 The electronic conductivity of synthesized Ag2WS4 was studied by electrochemical impedance spectroscopy (EIS) and compared with other control materials. The obtained Nyquist F


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Inorganic Chemistry

Figure 5. (a) Nyquist plots of different electrodes in 0.5 M H2SO4 at an applied potential of −249 mV (vs RHE). (b) Nyquist plots of Ag2WS4− Nafion/GCE as a function of applied potential from −249 mV (vs RHE) to −372 mV (vs RHE). (c) Plot of overpotential vs log(1/Rct).

Figure 6. (a) CV obtained with Ag2WS4−Nafion/GCE with scan rates changing from 2 to 400 mV s−1. (b) The corresponding cathodic and anodic current plot (i) vs scan rate (v).

plots (Figure 5a) reveal the Rct value of 2.65 kΩ in 0.5 M H2SO4 for Ag2WS4−Nafion/GCE, which is far smaller than that of Nafion/GCE (54.2 kΩ), WS2−Nafion/GCE (25.9 kΩ), and Ag2S-Nafion/GCE (5.3 kΩ), indicating a faster electrontransfer kinetics of Ag2WS4−Nafion in comparison with the other control samples. The smaller Rct of Ag2WS4 modified electrode is probably attributed to introducing Ag ions for the significant enhancement in the intrinsic electrocatalytic capacity of the material. In addition, the Nyquist plots of Ag2WS4−Nafion/GCE under different operating potentials were recorded. As shown in Figure 5b, the Rct values of Ag2WS4 modified GCE decreased significantly upon increase of overpotentials, from 2.650 kΩ at −249 mV (vs RHE) to only 664.1 Ω at −285 mV (vs RHE), demonstrating that the electron transfer and HER kinetics were significantly accelerated upon overpotential increasing at the solid/liquid interface. Furthermore, the Tafel slope during the electrochemical test was also determined from the EIS results according to the method proposed by Murthy et al.51 Figure 5c shows the plot of log(1/Rct) values versus the corresponding overpotentials, from which a Tafel slope of 65 mV dec−1 that is very close to 62 mV dec−1 of Ag2WS4 from the LSV

measurement was achieved, suggesting that the obtained Tafel slope value is reliable. The electrochemically active surface area (ECSA) of Ag2WS4 and the control materials were further calculated by determining the double layer capacitance (Cdl) from recording anodic−cathodic currents (i) in non-Faradaic potential region52 (see the Supporting Information for detailed calculation). Figure 6a shows the CVs at different scan rate ranging from 2 to 400 mV s−1 and the corresponding plots of i versus ν (Figure 6b). Then the Cdl value of Ag2WS4−Nafion/GCE was determined to be 0.102 mF. Thus, the Cdl-derived ECSA for Ag2WS4−Nafion/GCE was calculated to be 2.91 cm2. For comparison, the CVs and the corresponding plots of i versus ν of Nafion/GCE (Figure S4-a,b), WS2−Nafion/GCE (Figure S4-c,d), and Ag2S-Nafion/GCE (Figure S4-e,f) at different scan rates were also measured, and the ECSA values of Nafion/ GCE, WS2−Nafion/GCE, and Ag2S-Nafion/GCE were calculated to be 0.46, 0.17, and 0.34 cm2, respectively. The ECSA of Ag2WS4 is 6−17-fold larger than the control electrodes, indicating that more surface active sites are available for electrochemical reactions in Ag2WS4−Nafion/GCE. To compare the electrocatalytic performance of the synthesized Ag2WS4 with other material more accurately, the G


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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (21275127), Australian Research Council (DP160103107 and FT170100224), Oversea Visiting Program of China Scholarship Council ([2016]3035), and Scientific Research Foundation for Young Researcher of Minnan Normal University.

turnover frequency (TOF), namely, the number of hydrogen molecules evolved per second per active site of Ag2WS4, was also determined (see the Supporting Information for detailed calculation). On the basis of the density of Ag2WS4 from crystal structure and above-obtained ECSA value, the TOF was calculated to be 0.083 s−1 at η = 150 mV. These values compared favorably to the other state-of-the-art ternary HER catalysts such as MoSSe (0.0808 s−1, at η = 150 mV)53 and WSSe (0.055 s−1, at η = 160 mV)54 suggesting high intrinsic activity of each Ag2WS4 active site. Collectively, the above results suggest that the high catalytic activity of Ag2WS4 is originated from the fast electron transfer kinetic, large catalytic active area, fast mass transport, as well as high intrinsic activity of Ag2WS4, which all enable highly efficient HER at large current densities.

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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/acs.inorgchem.8b00108. Measurement of electrochemically active surface area, calculation of turnover frequency, SEM images of WS2, PXRD patterns, TEM image of Ag2WS4, CV data, additional refs (PDF) W Web-Enhanced Feature *

Movie showing vigorous effervescence and rapid dissipation of gas bubbles into solution during electrolysis.

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REFERENCES

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4. CONCLUSION In summary, a new member of ternary metal sulfide family, Ag2WS4, was prepared under a low-temperature condition, and its structure was well-resolved by PXRD. Electrochemical tests showed that the Ag2WS4 presented a lower onset potential and a smaller Tafel slope than the binary counterparts such as Ag2S and commercial WS2 in acidic solution for HER. Also, on the one hand, the synthesized Ag2WS4 presented no change in chemical structure after HER test for 24 h, suggesting that the synthesized Ag2WS4 can be used as a highly stable electrocatalyst for practical application for hydrogen production. On the other hand, the bulky layer-structured Ag2WS4 has the potential to be exfoliated into mono- or few-layered material through appropriate method (like graphite being exfoliated into graphene), which will greatly enhance the specific area and the electrocatalytic site, making the material to be more promising in electrocatalysis of HER.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86-596-2591445. Fax: +86-596-2520035. (Q.-X.W.) *E-mail: [email protected]. (C.Z.) ORCID

Yibing Li: 0000-0002-1729-5963 Qingxiang Wang: 0000-0002-8952-9925 Chuan Zhao: 0000-0001-7007-5946 Notes

The authors declare no competing financial interest. H


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