Thiomolybdate [Mo3S13]2â Nanoclusters Anchored on Reduced

Research Article pubs.acs.org/journal/ascecg

Thiomolybdate [Mo3S13]2− Nanoclusters Anchored on Reduced Graphene Oxide-Carbon Nanotube Aerogels for Efficient Electrocatalytic Hydrogen Evolution Yanan Shang, Xing Xu,* Baoyu Gao,* and Zhongfei Ren Key Laboratory of Water Pollution Control and Recycling (Shandong), School of Environmental Science and Engineering, Shandong University, Jinan 250100, P.R. China S Supporting Information *

ABSTRACT: Thiomolybdate [Mo3S13]2− nanoclusters anchored on reduced graphene oxide-carbon nanotube (rGO-CNTs) aerogels were used as a new catalyst for efficient electrocatalytic hydrogen evolution. The elemental distribution of sulfur (S) corresponded well to the Mo distribution, and both Mo and S elements distributed evenly in the Mo3S13@rGO-CNTs aerogels. Results indicated that [Mo3S13]2− nanoclusters inherently exposed a high number of active edge sites, which greatly improved the electrocatalytic hydrogen evolution. The new peak at 168.8 eV corresponded to the characteristic S−O binding in the S 2p region of Mo3S13@rGOCNTs, indicating that the [Mo3S13]2− clusters were bond onto the rGO-CNTs aerogels through S−O binding. The strong support of rGO-CNTs aerogels suppressed the aggregation of [Mo3S13]2− nanoclusters, exposing more active surface and electrons diffusions on the surface of Mo3S13@rGO-CNTs aerogels. Mo3S13@rGO-CNTs aerogels laden with 20 mg of [Mo3S13]2− exhibited close hydrogen evolution reaction (HER) performance as compared with that of [Mo3S13-120]@rGO-CNTs aerogels laden with 120 mg of [Mo3S13]2− nanoclusters. This indicated the extremely high HER performance of [Mo3S13]2− even at low mass. As a result, Mo3S13@rGO-CNTs aerogels enabled remarkable electrochemical performances showing a low overpotential (0.179 V at 10 mA cm−2) with a small Tafel slope, reduced transfer resistance, and excellent stability. KEYWORDS: Hydrogen evolution reaction, Reduced graphene oxide, Carbon nanotubes, Aerogels, [Mo3S13]2− clusters

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INTRODUCTION Hydrogen, a vital chemical ingredient, is widely used in the process of petroleum refining, composition of nitrogenous fertilizer, metal reduction, etc.1−5 Hydrogen is also considered as an energy source for the future. For the increasing population and decreasing fossil energy supplies all over the world, the demand for hydrogen may be rising continuously.6 Nowadays, almost all hydrogen (H2) is produced based on the steam reforming of natural gas (CH4) at refineries (CH4 + 2H2O → 4H2 + CO2).7,8 However, these processes may aggravate the greenhouse effect because CO2 was produced. If H2 can be generated from water using a renewable energy, then H2 also becomes a renewable fuel; this then will relieve the stress of producing energy.9 Hence, reducing water by electrolysis to produce hydrogen efficiently raises more concerns about the hydrogen evolution reaction (HER, 2H+ + 2e− → H2).1−3,6,10−17 HER is half of the water-splitting reaction which needs active catalysts to improve the reaction efficiency through minimizing the overpotential.2,18 Among all catalysts for HER, platinum (Pt) is the best choice in that it contributes to a high reaction rate only with an extremely low overpotential.19−21 Nonethe© 2017 American Chemical Society

less, Pt is costly and rare on Earth, which restricts its extensive use in the industry. Transition metals that are cheap and ample on Earth are active centers of some enzymes, such as hydrogenase and nitrogenase. It was reported that some of their chemical compounds (e.g., carbides and sulfides) have similar structures to that of Pt.20−22 As a result, they have been considered as the substitutes to replace Pt in the HER process. For example, the character of the edge sulfur atoms of MoS2 was similar to that of the active enzymatic centers.23 Also, MoS2 nanoparticles exerted favorably catalytic activity for HER.11,24−27 However, sulfur atoms in the MoS2 nanoparticles showed lower HER activity because only the edge S atoms were active.28−30 Therefore, more S atoms in MoS2 particles could be exposed to their edge, and this will enable improving their HER activities. Kibsgaard et al. reported that [Mo3S13]2− nanoclusters intrinsically exposed numerous active sulfur complexes on the molecular edge.31 The high catalytic activity of the [Mo3S13]2− clusters complex was partially ascribed to its Received: May 30, 2017 Revised: July 9, 2017 Published: August 17, 2017 8908

DOI: 10.1021/acssuschemeng.7b01713 ACS Sustainable Chem. Eng. 2017, 5, 8908−8917

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ACS Sustainable Chemistry & Engineering

Figure 1. Scheme for preparation procedures of Mo3S13@rGO-CNTs. rpm for 10 min. This process was conducted three times. Finally, the sample was rinsed with toluene before airing for a further extract of the residual S element and organic solvent. Preparation of rGO-CNTs aerogels comprised three steps:11,45,46 (i) A well-distributed GO solution (100 mL) and CNTs solution (100 mL) were prepared under ultrasound (1 h) in a 250 mL flask. The mixture was further ultrasonicated for 2 h, followed by a 3 h stirring to form a homogeneous suspension. (ii) Thereafter, 1 g of ascorbic acid (99.7 wt %) was added, and the mixture was stirred for 24 h. (iii) The suspension was freeze-dried for 48 h, and then, rGO-CNTs aerogels were prepared (Figures S1 and S2), which revealed anchored CNTs on rGO sheets. Preparation of [Mo3S13]@rGO-CNTs aerogels is presented as follows. A certain amount of (NH4)2Mo3S13·nH2O powder (5, 20, 40, 80, 120, 200 mg) and 40 mg of rGO/CNTs aerogels were dispersed into 10 mL of N,N dimethylformamide (DMF) in a 20 mL Teflon autoclave. Then, the hybrid was sonicated at an indoor temperature for 10 min to form a homogeneous suspension. After that, the autoclave was sealed and kept at 180 °C for 10 h under spontaneous pressure. The sample was centrifuged at 10,000 rpm for 10 min to remove the DMF when the autoclave cooled. Thereafter, a sequence of centrifugation of the sample was executed with ultrapure water for a further purification. After freeze-drying for 24 h, multiple rGO-CNTs aerogels introduced with different amounts of [Mo3S13] (5, 20, 40, 80, 120, 200 mg) were prepared, and they were named [Mo3S13-5]@rGOCNTs, [Mo 3 S 13 -20]@rGO-CNTs, [Mo 3 S 13 -40]@rGO-CNTs, [Mo3S13-80]@rGO-CNTs, [Mo3S13-120]@rGO-CNTs, and [Mo3S13200]@rGO-CNTs aerogels. The detailed preparation procedures are presented in Figure 1. Electrochemical Measurements. Electrochemical measurements were performed on a CHI 760D electrochemical workstation (Shanghai Chenhua Instrument Co., China). All electrochemical tests were carried out in a typical three-electrode cell with an electrolyte of 0.5 M H2SO4 solution using a Pt electrode as the counter electrode and Ag/AgCl electrode as the reference electrode. The stability tests were examined by successively cycling the potential between −0.1 and −0.8 V at a scan rate of 50 mV/s. The working electrode was modified from a glassy carbon electrode (GCE, 3 mm) by loading with different Mo3S13@rGO-CNTs aerogel samples. In brief, 1, 3, and 5 mg of [Mo3S13-5, 20, 40, 80, 120, 200]@ rGO-CNTs aerogels and 40 μL Nafion (5%) were dispersed in 1 mL of DMF and sonicated for 30 min. Then, 5 μL of mixture was dropped upon the glazed surface of a GCE. The working electrode was then prepared after being dried at room temperature. All the potentials mentioned in the work were calibrated to a reversible hydrogen electrode (RHE). In 0.5 M H2SO4, ERHE = EAg/AgCl + 0.059 × pH + 0.1988.47 The scan rate for polarization measurements was maintained at 1 mV s−1. To ensure a complete characterization, the electro-

three diverse kinds of sulfur ligands, which are located at the edge of the molecule. This distinctive formation in [Mo3S13]2− makes it possible to be comparable with the expensive noblemetal catalysts,31 such as Pt. In addition to this, [Mo3S13]2− has been reported to be extremely stable when used as co-catalysts and electrocatalysts.32,33 Recent results demonstrated that catalyst activity for HER can be boosted by assembling them on some highly conductive nanocarbon composites, e.g., reduced graphene oxide (rGO), carbon nanotubes (CNTs), and other MoSx-carbon electrode materials.11,24,34−39 These nanocarbons would be beneficial to the distribution of catalysts and aggrandize the conductivity of the whole catalyst. In addition, recent studies reported that three-dimensional (3D) carbon aerogel materials combining CNTs and rGO can further strengthen this process in electrocatalytic applications.40−43 Herein, an electrode material was prepared by anchoring the [Mo3S13]2− nanoclusters [(NH4)2Mo3S13·nH2O] onto the rGO/CNTs aerogels. Hydrogen evolution performances of the electrode materials loaded with different amounts of [Mo3S13]2− nanoclusters were investigated. In addition, their physicochemical properties were also determined.

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EXPERIMENTAL SECTION

Materials. Graphite powder (GP, 99.95 wt %), (NH4)6Mo7O24· 4H2O, ammonium polysulfide solution (25 wt %), and ascorbic acid (99.7 wt %) were purchased from Sinopharm Chemical Reagent Co., Ltd. CNTs were purchased from the Chinese Academy of Sciences, Chengdu Organic Chemical Co., Ltd. Graphene oxide (GO) was synthesized through a modified Hummer’s method, and its detailed procedure is given in the Supporting Information, Appendix S1. All aqueous solutions were prepared using ultrapure water (18.2 MΩ cm−1, Millipore). Preparation of Mo3S13@rGO-CNTS. Pure [Mo3S13]2− nanoclusters [(NH4)2Mo3S13·nH2O] were prepared according to the method of Müller et al.44 In detail, 4.0 g of (NH4)6Mo7O24·4H2O was dissolved in 20 mL of ultrapure water in a 250 mL triangle flask, followed by adding 120 mL of ammonium polysulfide. The flask was heated on an oil bath stewing for 5 d, and then the deep red primary [(NH4)2Mo3S13·nH2O] was produced. This primary product needs to be purified in order to remove the residual S atoms. It was rinsed with ultrapure water, and then, 200 mL of absolute ethyl alcohol was mixed with the unpurified [Mo3S13]2− nanoclusters for a continuous stir (10 h). Thereafter, the sample was soaked in carbon disulfide for ultrasonic cleaning (10 min). It was then followed by centrifugation at 10,000 8909


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Figure 2. (a) TEM of (NH4)2Mo3S13·nH2O. (b) XRD pattern of (NH4)2Mo3S13·nH2O. XPS spectrum of [Mo3S13]2− nanoclusters, (c) Mo 3d spectrum of [Mo3S13]2− clusters, and (d) S 2p spectrum of [Mo3S13]2− nanoclusters.

The synthesized [Mo3S13]2− clusters have the dominant diffraction peaks at (110) and (111), which well corresponded to the powder XRD patterns of simulated (NH4)2Mo3S13·nH2O reported in the literature (JCPDS 76-2038). The characteristic binding energies of each element in synthesized [Mo3S13]2− clusters could be evaluated by XPS measurements, and relevant XPS spectra are given in Figure 2c and d and Figure S3. The Mo 3d region was evaluated by deconvolution according to the peak fitting, and the results in Figure 2c reveal that Mo 3d 5/2 and Mo 3d 3/2 were at positions of 228.9 and 223.2 eV. This indicated the oxidation state of Mo[IV] in [Mo3S13]2− clusters.32,48 In addition, a broad S 2s feature (226.8 eV) was observed near the Mo 3d5/2 peak, which indicated the multiple chemical states of sulfur. The S 2p spectrum of [Mo3S13]2− clusters could be fitted with three distinct doublets (2p1/2, 2p3/2):44,48 (i) one doublet at lower binding energies (162.1 eV, 163.3 eV), which arose from the terminal S22− ligands, (ii) one doublet at 163.5 and 164.6 eV, corresponding to the bridging S22− ligands and the apical S2− ligand of the [Mo3S13]2− clusters, respectively, and (iii) the last doublet at higher binding energies (164.4 and 165.5 eV), reflecting the residual sulfur from the polysulfide solution during the (NH4)2Mo3S13·nH2O preparation. The three different doublets (2p3/2, 2p1/2) which reflected the terminal S22− ligands (in blue), bridging S22− ligands together with the apical S2− ligand (in pink) and residual sulfur (in yellow) from the polysulfide solution are presented Figure 2d. According to results of deconvolution, the area of doublets in bridging (together with apical) ligand and terminal ligands have a ratio of 1.10, which corresponded to the S structure of [Mo3S13]2− with bridging (together with apical) ligands/terminal ligands = 1.17 (bridging ligand:apical ligand:terminal ligand = 6:1:6). FTIR spectra of [Mo3S13]2− nanoclusters exhibited the Mo− S binding at peaks of 508.0, 934.9, 1393.9, and 1635.5 cm−1 (Figure S4a). Raman spectra of [Mo3S13]2− nanoclusters shown in Figure S4b also reveal four characteristic peaks, corresponding to v(Mo-Mo), v(Mo-S), v(Mo3-Sa), and v(S-S).48

chemical impedance spectroscopy (EIS) measurements were recorded over five frequency decades, from 10 kHz to 0.1 Hz. All EIS data were fit and statistically analyzed by the Zview software. Current−time responses were monitored by chronoamperometric measurements for up to 20,000 s at the applied potential of −0.2 V (vs RHE). To assess the stability of all Mo3S13@rGO-CNTs aerogels samples, continuous cyclic voltammograms to up to 1000 cycles and galvanostatic polarization curves at a current density of 10 mA/cm2 were recorded. Characterizations. The structure and morphology of Mo3S13@ rGO-CNTs aerogels were determined by a field-emission scanning electron microscope (FE-SEM, QUANTA430), and the chemical composition was investigated by energy dispersive X-ray spectroscopy (EDX). Transmission electron microscopy (TEM) images of (NH4)2Mo3S13·nH2O and Mo3S13@rGO-CNTs aerogels were measured by a FEI Tecnai G2 F20 S-TWIN transmission electron microscope with an accelerating voltage of 200 kV. Raman spectroscopy was measured on a Raman spectrometer with a 532 nm laser (LabRAM HR800, Horiba Jobin Yvon). X-ray powder diffraction (XRD) patterns were determined by powder X-ray diffractometer (powder XRD, D8 ADVANCE, Bruker) equipped with Cu Kα radiation (40 kV and 200 mA). X-ray photoelectron spectroscopy (XPS) spectra were collected on an XPS spectrometer (K-alpha, Thermo VG Scientific). Zeta potentials of GO, rGO-CNTs aerogels, and Mo3S13@rGO-CNTs aerogels were determined by a microelectrophoresis apparatus (JS94H, Zhongchen Digital Technical Apparatus Co., Ltd.). BET surface area and pore size distribution were carried out using a Micro Meritics ASAP-2010C instrument (Norcross, GA). Fourier transform infrared spectroscopy (FT-IR) of GO, rGOCNTs aerogels, and Mo3S13@rGO-CNTs aerogels was determined by a Perkin−Elmer spectrometer.

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RESULTS AND DISCUSSIONS Morphological and Chemical Characteristics of (NH 4) 2Mo 3S13·nH 2O. Crystal morphologies as well as structural properties of pure (NH4) 2Mo 3 S 13·nH 2 O are presented in Figure 2. The TEM results indicated that the particle sizes of [Mo3S13]2− nanoclusters were in the range of 5−20 nm, and its interplanar spacing was measured to be 0.235 nm (Figure 2a). The X-ray diffraction (XRD) patterns of synthesized [Mo3S13]2− nanoclusters are shown in Figure 2b. 8910


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ACS Sustainable Chemistry & Engineering Morphological and Chemical Nature of Mo3S13@rGOCNTs Aerogels. TEM, SEM, and elemental distribution mapping of typical Mo3S13@rGO-CNTs aerogels (Mo3S1320@rGO-CNTs aerogels) are presented in Figure 3. TEM

of Mo3S13@rGO-CNTs aerogels was also investigated by SEM observation. The results in Figure 3d and e exhibited a welldefined 3D hierarchical structure, in which the CNTs were anchored on the rGO sheets. The EDS results confirmed the Mo and S in the Mo3S13@rGO-CNTs aerogels (Figure S6). The elemental distribution of S corresponded to the Mo distribution, and both the Mo and S elements distributed evenly in the Mo3S13@rGO-CNTs aerogels (Figure 3f). The vibrational modes of chemical bonds in Mo3S13@rGOCNTs aerogels can be evaluated by Raman spectroscopy, which provided a specific spectrum that can be applied to identify their chemical natures.13,52 The Raman spectra of [Mo3S13]2− nanoclusters, rGO-CNTS aerogels, and typical Mo3S13@rGOCNTs aerogels using 532 nm excitation are presented in Figure 4a and b. Raman peaks of rGO-CNTS aerogels are located at

Figure 4. Raman spectra of [Mo3S13]2− nanoclusters, rGO-CNTs aerogels, and [Mo3S13-20]@rGO-CNTs aerogels using 532 nm excitation: (a) 100−3000 cm−1 and (b) 100−800 cm−1.

1347, 1576, and 2686 cm−1.50,53,54 [Mo3S13]2− nanoclusters showed four strong features at 180−230, 350−410, 450−465, and 520−560 cm−1, which were assigned to v(Mo-Mo), v(MoS), v(Mo3-Sa), and v(S-S) in [Mo3S13]2− nanoclusters.55 After the loading of [Mo3S13]2− nanoclusters, these characteristic peaks in pure [Mo3S13]2− nanoclusters were also observed in the Raman spectrum of a Mo3S13@rGO-CNTs aerogel. This indicated that the morphologies or chemical compositions of [Mo3S13]2− nanoclusters were still stable as the [Mo3S13]2− nanoclusters were introduced on rGO-CNTs aerogels at 200 °C. FTIR spectra of GO, [Mo3S13] nanoclusters, rGO-CNTS aerogels, and Mo3S13@rGO-CNTs aerogels are displayed in Figure 5a and Figure S4a. For the GO and GO-CNTS aerogels, the adsorption bands at 1191 and 1071 cm−1 indicated the vibration of C−O. The specific peaks at 1461, 1636, and 1725 cm−1 were assigned to the vibrations of O−H, CC, and C O, respectively. The band at 3437 cm−1 corresponded to the C−OH and −OH in interlayer water. After reduction by the ascorbic acid, these oxygen-containing groups in all rGO-CNTs and Mo3S13@rGO-CNTs aerogels were greatly weakened. The new characteristics peaks of Mo3S13@rGO-CNTs aerogels located at 528.2, 943.2, 1399.6, and 1636.9 cm−1 corresponded well to the FTIR spectrum of pure [Mo3S13]2− nanoclusters (Figure S4b), and their intensities were greatly increased in [Mo3S13-20, -80, -120 and -200]@rGO-CNTs aerogels. This result indicated that more [Mo3S13]2− nanoclusters were anchored onto the rGO-CNTS aerogels. XRD patterns of GO, [Mo3S13]2− clusters, GO-CNTs aerogels, rGO-CNTs aerogels, and Mo3S13@rGO-CNTs aerogels are presented in Figure 5b. A broad peak appeared at 26.6° which accorded with the characteristic peak of CNTs structures in rGO-CNTs aerogels. The diffraction peak located at 10.0° represented the oxygen-containing functional groups,

Figure 3. TEM of Mo3S13@rGO-CNTS aerogels: (a) 1 μm, (b) 100 nm, (c) 20 nm, (d) 5 nm. SEM and element distribution mapping of typical Mo3S13@rGO-CNTS aerogels (Mo3S13-20@rGO-CNTS aerogels): (e) SEM, ×15000 and (f) elemental distribution mapping, C, O, S, and Mo.

micrographs shown in Figure 3a and b depicted that the rGO sheets were adhered with a great number of CNTs (diameter of about 20 nm). A close view revealed that the CNTs were vertically aligned to the rGO-CNTs aerogels support, leading to the formation of a large number of empty cavities.48−50 This was consistent with the specific surface area and pore volume of rGO-CNTs, which were significantly higher than those of GO (Figure S5 and Table S1). As a result, the rGO-CNTs aerogels provided more sites for [Mo3S13]2− nanoclusters to occupy. The pore size distribution of Mo3S13@rGO-CNTs was also presented in Table S1 and the average pore width of Mo3S13@ rGO-CNTs centered at 10.02 nm, which was in the mesoporous range (2−50 nm). Mesoporous materials with pore size distribution more than 1 nm (2−50 nm) are reported to have good electrochemical capability because of their magnificent electrical conductivity.34,51 TEM micrographs shown in Figure 3c exhibited the anchored [Mo3S13]2− nanoclusters on the rGO-CNTs aerogels. Simultaneously, the lattice spacing of laden [Mo3S13]2− nanoclusters on Mo3S13@ rGO-CNTs aerogels was measured to be 0.235 nm, which was consistent with pure [Mo3S13]2− nanoclusters. The morphology 8911


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Figure 5. (a) XRD pattern of GO, [Mo3S13]2− clusters, GO-CNTs aerogels, rGO-CNTs aerogels, and Mo3S13@rGO-CNTs aerogels. (b) FTIR spectra of GO, [Mo3S13] clusters, rGO-CNTS aerogels, and Mo3S13@rGO-CNTs aerogels. (c) XPS spectra of multiple Mo3S13@rGO-CNTS aerogel samples, (d) Mo 3d region, and (e) S 2p region.

which existed in both patterns of GO and rGO-CNTs aerogels.49,56,57 However, it disappeared after the reduction process in rGO-CNTs aerogels. This also contributed to high electrical conductivity of the catalytic nanocomposites. Interestingly, the distinct (110) peak of (NH4)2Mo3S13·nH2O at 10.1° was vague in the XRD patterns of the Mo3S13@rGOCNTs aerogels, which implied a low degree of crystallinity in [Mo3S13]2− nanoclusters after anchoring onto rGO-CNTs aerogels.32,58 The [Mo3S13]2− nanoclusters with defect-rich structures in the Mo3S13@rGO-CNTs aerogels can be conducive to electrocatalytic activity.59 This is because the existence of abundant defects consequently gives rise to more edge sites with lower crystallinity, benefiting its electrocatalytic hydrogen evolution.60

XPS spectra of GO-CNTs aerogel, rGO-CNTs aerogel, and multiple Mo3S13@rGO-CNTS aerogel samples are presented in Figure 5c−e and Figures S7 and S8. The C 1s region of GOCNTs aerogels exhibited two broad features which were deconvoluted into three components, corresponding to carbon atoms of CC, C−O, and HO-CO.61,62 The C 1s region of rGO-CNTs aerogels and Mo3S13@rGO-CNTs aerogels revealed the same features that have been assigned for GO-CNTs aerogels, but the peak intensities of all oxygen functional groups (C−O and HO−CO) were greatly reduced, indicative of successful reduction of GO-CNTs aerogels. The deconvolution of the Mo 3d region in all Mo3S13@rGO-CNTs aerogels was consistent with pure (NH4)2Mo3S13·nH2O, indicating the existence of Mo[IV] in Mo3S13@rGO-CNTS aerogels (Figure 5d). The increased vibrations of Mo 3d peaks in Mo3S13-200@ 8912


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Figure 6. HER performance of bare rGO-CNTs, bare [Mo3S13]2− nanoclusters, rGO laden with [Mo3S13]2−, and all Mo3S13@rGO-CNTs aerogel samples (drop-casting 5 mg on the GCE): (a) polarization curves, (b) Tafel plots, (c) Nyquist plots, and (d) polarization curves of all samples initially and after 1000 cycles in 0.5 M H2SO4.

samples (Figure 6a and Figure S11), it was obvious that the GCE loaded with bare rGO-CNTs aerogels or pure Mo3S13 clusters showed negligible HER performance. In contrast, the HER activity of Mo3S13@rGO-CNTs aerogels gradually increased as the laden [Mo3S13] increased from 5 to 120 mg. The onset potential of all Mo3S13@rGO-CNTs aerogels was in the range from −110 to −135 mV. The [Mo3S13-120]@rGOCNTs aerogels exhibited a small overpotential of −179 mV at current density of 10 mA cm−2, which was smaller than those of [Mo3S13-5, -20, -40, -80]@rGO-CNTs aerogels (10 mA cm−2 at 0.263, 0.208, 0.199, 0.193 V). The higher Mo3S13 mass loading exposed more active sites, enhancing the HER performance. In addition, the Mo3S13@rGO-CNTs aerogels laden with 20 mg of [Mo3S13]2− exhibited close HER performance compared with that of [Mo3S13-120]@rGO-CNTs aerogels. This indicated an extremely high HER performance of [Mo3S13]2− even at low mass. However, as [Mo3S13]2− mass loading further increased from 120 to 200 mg, the overpotential (10 mA cm−2) of Mo3S13@rGO-CNTs aerogels gradually increased from 0.179 to 0.210 V, indicating decreased HER activity. The observed superior HER activity of rGO-CNTs aerogels laden with lower mass (20−120 mg) of [Mo3S13] over samples laden with higher mass of [Mo3S13] (120−200 mg) can be attributed to the following two aspects. On the one hand, the highly conductive rGO-CNTs aerogels contributed to the high electrical conductivity of the catalyst and facilitated a fast electrontransfer process by improving the electrical contact between the

rGO-CNTs aerogels can be interpreted in terms of more [Mo3S13]2− nanoclusters bonded on rGO-CNTs aerogels. The S 2p region of all Mo3S13@rGO-CNTs aerogel samples can be deconvoluted into two doublets (2p1/2, 2p3/2). The doublets at 162.1 and 163.3 eV represented the terminal S22− ligands in [Mo3S13]2− clusters, and the doublets at 163.5 and 164.6 eV were assigned to the apical S2− ligand and bridging S22− ligand (Figure 5e), respectively. The new peak at 168.8 eV corresponded to the characteristic S−O binding in the S 2p region, and the vibration of the S−O peak in Mo3S13-200@ rGO-CNTs aerogel was relatively higher than those of [Mo3S1320, 80]@rGO-CNTs aerogels. This indicated that the [Mo3S13]2− nanoclusters were anchored onto the rGO-CNTs aerogels through S−O binding. The zeta potential of rGO-CNTs aerogels and [Mo3S13]2− nanoclusters as well as all Mo3S13@rGO-CNTs aerogel samples was detected, and the results are presented in Figures S9 and S10. Results indicated that both the rGO-CNTs aerogels and [Mo3S13]2− nanoclusters were negatively charged. As a result, [Mo3S13]2− nanoclusters were combined with rGO-CNTs aerogels through strong chemical bonding rather than electrostatic attraction.63 HER Activity of Mo3S13@rGO-CNTs Aerogels in 0.5 M H2SO4. HER activity of bare rGO-CNTs aerogels and pure Mo3S13 clusters as well as various Mo3S13@rGO-CNTS aerogels (drop-casting 5 mg of samples on GCE) are presented in Figure 6. Based on the HER polarization curves of all 8913


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Figure 7. TEM of Mo3S13@rGO-CNTs aerogels after 1000 cycles of HER: (a) 100 nm and (b) 20 nm (insert: 5 nm). (c) Mechanism of HER on surface of Mo3S13@rGO-CNTs aerogels.

electrode and the active sites [Mo3S13]2− nanoclusters. However, excessive loading of [Mo3S13]2− nanoclusters in the structure of rGO-CNTs aerogels would increase the internal resistance and decrease the electron/proton transfer, thus leading to worse HER performance. On the other hand, the substantial agglomeration derived from excessive [Mo3S13]2− nanoclusters also led to a weak synergetic effect of the hybrid Mo3S13@rGO-CNTs aerogel catalyst.55,64,65 The substantial agglomeration of excessive [Mo3S13]2− on rGO-CNTs aerogels is observed in Figure S12. This weak synergetic effect due to excessive catalysts was in accordance with previous research results.66,67 To further investigate the HER activity of multiple Mo3S13@ rGO-CNTs aerogels, the Tafel slopes were determined by fitting the linear portions of Tafel plots with the Tafel formula (η = a + blogj, where b is the Tafel slop, and j is the current density). The Tafel slopes described the relationship between overpotential and current density and were determined by the rate-limiting step of HER. As shown in Figure 6b and Figure S13, the Tafel slopes for bare rGO-CNTs aerogels, pure Mo3S13 clusters, and the [Mo3S13-5, -20, -40, -80, -120, 200]@rGOCNTs aerogels were measured to be 252.2, 93.3, 67.8, 60.2, 71.8, 65.3, 67.4, and 68.2 mV dec−1, respectively. The small Tafel slopes (60.2−71.8 mV dec−1) of all Mo3S13@rGO-CNTs aerogel samples indicated an improved HER catalysis rate under a constant increase in overpotential, which will be beneficial for practical applications. Electrochemical impedance spectroscopy (EIS) was applied to further investigate the electrode/electrolyte interface and corresponding electrocatalytic processes that occurred on surfaces of Mo3S13@rGO-CNTs aerogels. To ensure a complete characterization, EIS measurements were recorded over five frequency decades, from 10 kHz to 0.1 Hz. Nyquist plots for impedance data for different Mo3S13@rGO-CNTs aerogels are presented in Figure 6c and Figure S14. Nyquist plots of bare [Mo3S13]2− nanoclusters and bare rGO-CNTs aerogels are given in Figure S15. The significantly reduced charge-transfer resistance (Rct) was realized for all Mo3S13@ rGO-CNTs aerogels in contrast to the bare [Mo 3S13]2− nanoclusters and bare rGO-CNTs aerogels, which demonstrated the enhanced conductivity and higher charge transfer

rate of Mo3S13@rGO-CNTs aerogels. The charge-transfer resistance Rct at the interface of electrode/electrolyte was always evaluated based on the semicircles at high frequencies. The lower Rct value was likely related to the more porous structure and higher surface area, which substantially decreased the electron transfer resistance at the electrode/electrolyte interface. All EIS data were fit by the means of Zview software. The equivalent electrical circuit is also presented in Figure S16, in which R1 (R2), R3, CPE, and W represent the solution resistance, charge-transfer resistance, constant phase element, and dispersion resistance.39,68,69 All these EIS parameters are presented in Table S2. Results indicated that all [Mo3S13]@ rGO-CNTs aerogels achieved low charge-transfer resistances (0.069−2.34 Ω) and dispersion resistances (3.96−15.64 Ω). These low resistance results were always related to the fast HER kinetic reaction rate, which corresponded well to the HER results observed in this work. In addition to the HER efficiency, the stability during HER is another critical factor for evaluating a catalyst. To assess the stability of all Mo3S13@rGO-CNTs aerogel samples, continuous cyclic voltammograms to up to 1000 cycles and galvanostatic polarization curves at a current density of 10 mA/cm2 were recorded. The LSV curves recorded at 5 mV/s for Mo3S13-laden rGO-CNTs aerogels before and after performing 1000 continuous cyclic voltammograms are given in Figure 6d and Figure S17. For rGO-CNTs aerogels loaded with 20, 40, 80, and 120 mg of [Mo3S13]2− nanoclusters, no appreciable activity change was observed after 1000 cycles; this indicated the excellent stability of these samples during HER. This observation further suggested that [Mo3S13-20, -40, -80, -120] @rGO-CNTs aerogels remain intact with [Mo3S13]2− nanoclusters firmly immobilized in rGO-CNTs aerogels. In contrast, an obvious activity loss was observed in [Mo3S13-200]@rGOCNTs aerogels. This further proved that the substantial agglomeration yielded from excess [Mo3S13]2− nanoclusters would reduce the HER stability of Mo3S13@rGO-CNTs aerogels. To evaluate the durability of Mo3S13@rGO-CNTs aerogels for continuous HER at static overpotential, the current−time plot was determined at the potential of 0.200 V (vsRHE) for 20,000 s, and the results are shown in Figure S18. The current density exhibited only a slight decrease after a long 8914


Research Article

ACS Sustainable Chemistry & Engineering period of 20,000 s, indicating the strong durability of [Mo3S13] @rGO-CNTs for HER. HER performance of the GCE loaded with different amounts (1, 3, and 5 mg) of Mo3S13@rGO-CNTs aerogels is also presented in Figures S11, S13, S14, and S17. For all Mo3S13@ rGO-CNTs aerogel samples, the increased deposition of [Mo3S13]2− nanoclusters from 1 to 5 mg onto GCE greatly improved the HER performance. This was partially due to the spontaneous increase in both rGO-CNTs aerogels and [Mo3S13]2− nanocluster contents, maintaining the synergetic effect of the hybrid Mo3S13@rGO-CNTs aerogel catalyst during the electrocatalytic hydrogen evolution. All Tafel slopes, charge-transfer resistance (Rct), and stability data considering the deposited amounts of Mo3S13@rGO-CNTs aerogels on GCE corresponded well to the HER activity. The TEM of Mo3S13@rGO-CNTs aerogels after 1000 cycles of HER indicated that Mo3S13@rGO-CNTs aerogels still retained their structural integrity with morphology of the catalyst almost kept unchanged (Figure 7a, b). All these results indicated that Mo3S13@rGO-CNTs aerogels were highly efficient electrocatalysts for HER. The significant number of active edge sites were inherently created by the [Mo3S13]2− nanoclusters anchored in Mo3S13@rGO-CNTs aerogels. Also, the strong support of rGO-CNTs aerogels exposed more active sites and electrons diffusions on the surface of Mo3S13@rGOCNTs aerogels (Figure 7c). As a result, rGO-CNTs aerogels laden with a relatively small amount (20−120 mg) of [Mo3S13]2− nanoclusters exhibited low overpotential, small Tafel slope, high electroconductibility, and excellent durability for HER in a 0.5 M H2SO4 solution, which could be comparable with most relevant electrocatalysts (Table S3).

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +86-531-88361912. E-mail: [email protected] (Xing Xu). *Tel: +86-531-88361912. E-mail: [email protected] (Baoyu Gao). ORCID

Xing Xu: 0000-0001-7245-179X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was supported by the National Natural Science Foundation of China (51178252, 51508307, 51508308) and China Postdoctoral Science Foundation funded project (2014M560556, 2015T80721). This work was also supported by grants from the Tai Shan Scholar Foundation (ts201511003).

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REFERENCES

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CONCLUSIONS The [Mo3S13]2− nanoclusters-based catalyst, Mo3S13@rGOCNTs aerogels, were used as a new catalyst for HER. TEM micrographs exhibited the immobilized [Mo3S13]2− nanoclusters on the rGO-CNTs aerogels. Simultaneously, the lattice spacing of laden [Mo3S13]2− nanoclusters on Mo3S13@rGOCNTs aerogels was measured to be 0.235 nm. The significant number of active edge sites were inherently exposed in the [Mo3S13]2− nanoclusters, which greatly improved the HER performance. XRD patterns indicated the [Mo3S13]2− nanoclusters have poor crystallinity, and this defect-rich property of [Mo3S13]2− nanoclusters in Mo3S13@rGO-CNTs aerogels can be beneficial in terms of electrocatalytic activity. Also, the strong support of rGO-CNTs aerogels suppressed the aggregation of [Mo3S13]2− nanoclusters, exposing more active sites and electron diffusions on the surface of Mo3S13@rGOCNTs aerogels. rGO-CNTs aerogels loaded with 20−120 mg of [Mo3S13]2− nanoclusters exhibited better HER performance as compared with that of [Mo3S13-200]@rGO-CNTs aerogels. Excessive loading of [Mo3S13]2− nanoclusters in the structures of rGO-CNTs aerogels resulted in high internal resistance and substantial agglomeration, thus leading to worse HER performance.

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Additional characteristics (AFM, XPS, EDS, FTIR, XRD, N2 adsorption/desorption isotherm, zeta potential) of samples, polarization curves (initially and after 1000 cycles), Tafel slopes, and Nyquist plots of different Mo3S13@rGO-CNTs aerogel samples. (PDF)

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01713. 8915


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Thiomolybdate [Mo3S13]2â Nanoclusters Anchored on Reduced

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