Thiomolybdate [Mo3S13]2â Nanoclusters Anchored on Reduced

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Thiomolybdate [Mo3S13]2- nanoclusters anchored on reduced graphene oxide-carbon nanotubes aerogels for efficient electrocatalytic hydrogen evolution Yanan Shang, Xing Xu, Baoyu Gao, and Zhongfei Ren ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01713 • Publication Date (Web): 17 Aug 2017 Downloaded from http://pubs.acs.org on August 18, 2017

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ACS Sustainable Chemistry & Engineering

Thiomolybdate [Mo3S13]2- nanoclusters anchored on reduced graphene oxide-carbon nanotubes aerogels for efficient electrocatalytic hydrogen evolution Yanan Shang, Xing Xu*, Baoyu Gao*, Zhongfei Ren Key Laboratory of Water Pollution Control and Recycling (Shandong), School of Environmental Science and Engineering, Shandong University, Jinan 250100, PR China

*To whom correspondence should be addressed E-mail: [email protected] (Xing Xu) E-mail: [email protected] (Baoyu Gao)

Abstract: Thiomolybdate [Mo3S13]2- nanoclusters anchored on the reduced graphene oxide-carbon nanotubes (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 the [Mo3S13]2nanoclusters 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 S 2p region of Mo3S13@rGO-CNTs, indicating that the [Mo3S13]2- clusters were bond onto the rGO-CNTs aerogels through S-O binding. The strong support of rGO-CNTs aerogel suppressed the aggregation of [Mo3S13]2- nanoclusters, exposing more active surface and electrons diffusions on surface of Mo3S13@rGO-CNTs aerogel. Mo3S13@rGO-CNTs aerogel laden with 20 mg of [Mo3S13]2- exhibited the close HER performance as compared with that of [Mo3S13-120]@rGO-CNTs aerogel 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, the Mo3S13@rGO-CNTs aerogel enabled remarkable electrochemical performances showing a low overpotential (0.179 V at 10 mA cm-2) with small Tafel slope, reduced transfer resistance, and excellent stability. 1 ACS Paragon Plus Environment


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Keywords: Hydrogen evolution reaction; Reduced graphene oxide; Carbon nanotubes; Aerogels; [Mo3S13]2- clusters

Introduction Hydrogen, a vital chemical ingredient, is widely used in the process of petroleum 1-5

refining, composition of nitrogenous fertilizer and metal reduction ect

. Besides,

hydrogen is also considered as energy sources in the future. For the increasing population and shrink of fossil energy all the world, the demand of hydrogen may be rising up 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 green-house effect because the CO2 was produced. If H2 can be generated from water using a renewable energy, then H2 also becomes a renewable fuel; this will relieve the stress of energy meanwhile 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. The HER is half of the water-splitting reaction which needs active catalysts to improve the reaction efficiency through minimizing the over potential 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 over potential 19-21. Nonetheless, Pt is costly and rare on the earth, which restricts its extensive use in the industry. Transition metals with cheap and ample on the 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 the similar structures to that of Pt

20-22

. As a result,

they have been considered as the substitutes to replace Pt in HER process. For example, the character of the edge sulfur atoms of MoS2 was similar to that of the active enzymatic centers 23. And also, MoS2 nanoparticles exerted favorably catalytic activity of HER

11, 24-27

. However, sulfur atoms in the MoS2 nanoparticles showed

lower HER activity because only the edge S atoms were activity 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 the [Mo3S13]2− 2 ACS Paragon Plus Environment

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nanoclusters intrinsically exposed numerous active sulfur complexes on the molecular edge

31

. The high catalytic activity of [Mo3S13]2− clusters complex was partially

ascribed to their three diverse kinds of sulfur ligands, which was located at the edge of the molecule. This distinctive formation in [Mo3S13]2− makes it possible to be comparable with the expensive noble-metal catalysts

31

, such as Pt. In addition to

this, [Mo3S13]2− has been reported with extremely stability when served as co-catalysts and electro-catalysts 32-33. Recent results demonstrated that catalyst activity for HER can be boosted by assembling them on some highly conductive nano-carbon composites, e.g. reduced graphene oxide (rGO), carbon nanotubes (CNTs), and other MoSx-carbon electrode materials

11, 24, 34-39

. These nano-carbons would be beneficial to the distribution of

catalysts and aggrandize the conductivity of the whole catalyst. In addition, recent studies reported that three dementional (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.

Experimental Materials. Graphite powder (GP, 99.95 wt%), (NH4)6Mo7O24·4H2O, ammonium polysulfide solution (25 wt%), and ascorbic acid (99.7 wt%) was purchased from Sinopharm Chemical Reagent Co., Ltd. The CNTs was purchased from Chinese Academy of Sciences Chengdu Organic Chemical Co., Ltd. Graphene oxide (GO) was synthesized through a modified Hummer’s method and its detailed procedure was given in Supporting Information Appendix S1. All aqueous solutions were prepared using ultra-pure water (18.2 MΩ cm−1, Millipore). Preparation

of

Mo3S13@rGO-CNTS.

Pure

[Mo3S13]2-

nanoclusters

[(NH4)2Mo3S13·nH2O] was prepared according to the method of Müller et al 3 ACS Paragon Plus Environment

44

. In


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details, 4.0 g of (NH4)6Mo7O24·4H2O was dissolved in 20 ml of ultra-pure water in a 250 ml of 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 ultra-pure water, and then 200 mL of absolute ethyl alcohol was mixed with the unpurified [Mo3S13]2nanoclusters for a continuous stir (10 h). Thereafter, the sample was soaked in the carbon disulfide for ultrasonic cleaning (10 min). It was then followed by centrifugation at 10000 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 aerogel comprised of three steps

11, 45-46

. (i)

Well-distributed GO solution (100 mL) and CNTs solution (100 mL) were prepared under ultrasound (1 h) in a 250 mL of flask. The mixture was further ultrasonicated for 2 h and followed by a 3 h of 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 the rGO-CNTs aerogel was prepared (Figure S1 and Figure S2, which revealed the anchored CNTs on rGO sheets). Preparation of [Mo3S13]@rGO-CNTs aerogels was presented as follows. A certain amount of (NH4)2Mo3S13·nH2O powder (5, 20, 40, 80, 120, 200 mg) and 40 mg of rGO/CNTs aerogel were dispersed into 10 ml of N, N dimethylformamide (DMF) in a 20 mL Teflon autoclave. Then the hybrid was sonicated at 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 centrifugalized at 10000 rpm for 10 min to remove the DMF when the autoclave was cooled down. Thereafter, a sequence of centrifugation of the sample was executed with ultra-pure 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) 4 ACS Paragon Plus Environment

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were

prepared

and

they

[Mo3S13-20]@rGO-CNTs,

were

named

as

[Mo3S13-40]@rGO-CNTs,

[Mo3S13-5]@rGO-CNTs, [Mo3S13-80]@rGO-CNTs,

[Mo3S13-120]@rGO-CNTs, and [Mo3S13-200]@rGO-CNTs aerogels. The detailed preparation procedures were presented in Figure 1.

Figure 1 Scheme for the preparation procedures of Mo3S13@rGO-CNTs

Electrochemical

Electrochemical

measurements.

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 Pt electrode as counter electrode and Ag/AgCl electrode as 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 1mL DMF and sonicated for 30 min. Then 5 µL of mixture was drop upon the glazed surface of GCE. The working electrode was then prepared after being dried at room temperature. All the potentials mentioned in the work were 5 ACS Paragon Plus Environment


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calibrated to 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 electrochemical 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 20000 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 the 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 microscopy 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 the micro-electrophoresis apparatus (JS94H, Zhongchen Digital Technical Apparatus Co., Ltd). BET surface area and pore size distribution was carried out using Micro Meritics ASAP-2010C Instrument (Norcross, GA). Fourier Transform Infrared Spectroscopy (FT-IR) of GO, rGO-CNTs aerogels, and Mo3S13@rGO-CNTs aerogels were determined by the spectroscopy (Perkin–Elmer spectrometer).

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Results and discussions Morphological and chemical characteristics of (NH4)2Mo3S13·nH2O. Crystal morphologies as well as structural properties of pure (NH4)2Mo3S13·nH2O were presented in Figure 2. The TEM results indicated that the particle sizes of [Mo3S13]2nanoclusters 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 was shown in Figure 2b. 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]2clusters could be evaluated by the XPS measurement and the relevant XPS spectra were given in Figure 2c-d and Figure S3. The Mo 3d region was evaluated by deconvolution according to the peak fitting and the results in Figure 2c revealed that the 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 eV, 164.6 eV), corresponding to the bridging S22- ligands and the apical S2- ligand of the [Mo3S13]2- clusters, and (iii) the last doublet at higher binding energies (164.4 eV, 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 was 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 7 ACS Paragon Plus Environment


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(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]2nanoclusters shown in Figure S4b also revealed four characteristic peaks, corresponding to the v(Mo-Mo), v(Mo-S), v(Mo3-Sa), and v(S-S) 48 .

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; (d) S 2p spectrum of [Mo3S13]2- nanoclusters

Morphological and chemical nature of Mo3S13@rGO-CNTs aerogels. TEM, SEM and elemental distribution mapping of typical Mo3S13@rGO-CNTs aerogel (Mo3S13-20@rGO-CNTs aerogel) were presented in Figure 3. TEM micrographs shown in Figure 3 a-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 8 ACS Paragon Plus Environment

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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 meso-porous 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 the pure [Mo3S13]2- nanoclusters. The morphology of Mo3S13@rGO-CNTs aerogels was also investigated by SEM observation. The results in Figure 3d-e exhibited a well-defined 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).



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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, MAG×15000, (f) elemental distribution mapping, C, O, S, and Mo

The vibrational modes of chemical bonds in Mo3S13@rGO-CNTs aerogels can be evaluated by the 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 aerogel, and typical Mo3S13@rGO-CNTs aerogel using 532 nm excitation were presented in Figure 4(a) and (b). The Raman peaks of rGO-CNTS aerogel were located at 1347, 1576 and 2686 cm-1

50, 53-54


. [Mo3S13]2- nanoclusters

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showed four strong features at 180-230 cm−1, 350−410 cm−1, 450−465 cm−1 and 520-560 cm-1, which were assigned to the v(Mo-Mo), v(Mo-S), 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 Mo3S13@rGO-CNTs aerogel. This indicated that the morphologies or chemical compositions of [Mo3S13]2- nanoclusters were still stable as the [Mo3S13]2nanoclusters were introduced on the rGO-CNTs aerogel at 200 oC.

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

FTIR spectra of GO, [Mo3S13] nanoclusters, rGO-CNTS aerogel, and Mo3S13@rGO-CNTs aerogels were displayed in Figure 5a and Figure S4a. For the GO and GO-CNTS aerogel, 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 vibration of O-H, C=C, and C=O. 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 11 ACS Paragon Plus Environment


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indicated that more [Mo3S13]2- nanoclusters were anchored onto the rGO-CNTS aerogels. XRD pattern of GO, [Mo3S13]2- clusters, GO-CNTs aerogel, rGO-CNTs aerogel, and Mo3S13@rGO-CNTs aerogel were presented in Figure 5b. A broad peak appeared at 26.6o which accorded with the characteristic peak of CNTs structure in rGO-CNTs aerogel. The diffraction peak located at 10.0o representing the oxygen-containing functional groups, which existed in both patterns of GO and rGO-CNTs aerogel

49,

56-57

. However, it disappeared after reduction process in rGO-CNTs aerogel. And this

also contributed to high electrical conductivity of the catalytic nanocomposites. Interestingly, distinct (110) peak of (NH4)2Mo3S13·nH2O at 10.1o was vague in the XRD patterns of the Mo3S13@rGO-CNTs aerogel, which implied that the low degree of crystallinity of [Mo3S13]2- nanoclusters after anchoring onto the rGO-CNTs aerogel 32,

58

.

The

[Mo3S13]2-

nanoclusters

with

defect-rich

structure

in

the

Mo3S13@rGO-CNTs aerogel can be conducive to its 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.


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

XPS spectra of GO-CNTs aerogel, rGO-CNTs aerogel, and multiple Mo3S13@rGO-CNTS aerogel samples were presented in Figure 5c-e and Figure S7-8. 13 ACS Paragon Plus Environment


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The C 1s region of GO-CNTs aerogel exhibited two broad features which were de-convoluted 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 aerogel and Mo3S13@rGO-CNTs

aerogels revealed the same features that have been assigned for GO-CNTs aerogel, 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 aerogel. The deconvolution of Mo 3d region in all Mo3S13@rGO-CNTs aerogels were consistent with

the

pure

(NH4)2Mo3S13·nH2O,

indicating

the

existed

Mo[IV]

in

Mo3S13@rGO-CNTS aerogels (Figure 5d). The increased vibrations of Mo 3d peaks in Mo3S13-200@rGO-CNTs aerogel can be interpreted in terms of the more [Mo3S13]2nanoclusters bond on the rGO-CNTs aerogel. The S 2p region of all Mo3S13@rGO-CNTs aerogels samples can be deconvoluted into two doublets (2p1/2, 2p3/2). The doublet at (162.1 eV, 163.3 eV) represented the terminal S22- ligands in [Mo3S13]2- clusters and the doublet at (163.5 eV, 164.6 eV) was assigned to the apical S2- ligands and bridging S22- ligand (Figure 5e). The new peak at (168.8 eV) corresponded to the characteristic S-O binding in S 2p region, and the vibration of S-O peak in Mo3S13-200@rGO-CNTs aerogel was relatively higher than those of [Mo3S13-20, 80]@rGO-CNTs aerogels. This indicated that the [Mo3S13]2- nanoclusters were anchored onto the rGO-CNTs aerogels through S-O binding. Zeta potential of rGO-CNTs aerogel, [Mo3S13]2- nanoclusters as well as all Mo3S13@rGO-CNTs aerogel samples was detected and the results were presented in Figure S9-10. Results indicated that both the rGO-CNTs aerogel and [Mo3S13]2nanoclusters were negatively charged. As a result, [Mo3S13]2- nanoclusters were combined with the rGO-CNTs aerogel through the strong chemical bonding rather than the electrostatic attraction 63. HER activity of Mo3S13@rGO-CNTs aerogels in 0.5 M H2SO4. HER activity of

bare

rGO-CNTs aerogels,

pure

Mo3S13

clusters as well as

various

Mo3S13@rGO-CNTS aerogels (drop-casting 5 mg of samples on GCE) were presented in Figure 6. Based on the HER polarization curves of all samples (Figure 6a 14 ACS Paragon Plus Environment

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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 aerogel gradually increased as the laden [Mo3S13] increased from 5 mg to 120 mg. The onset potential of all Mo3S13@rGO-CNTs aerogels was in the range of -110 ~ -135 mV. The [Mo3S13-120]@rGO-CNTs aerogel 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 aerogel laden with 20 mg of [Mo3S13]2- exhibited the close HER performance as compared with that of [Mo3S13-120]@rGO-CNTs aerogel. This indicated the extremely high HER performance of [Mo3S13]2- even at low mass. However, as [Mo3S13]2- mass loading further increased from 120 mg to 200 mg, the overpotential (10 mA cm-2) of Mo3S13@rGO-CNTs aerogel gradually increased from 0.179 V to 0.210 V, indicating the decreased HER activity. The observed superior HER activity of rGO-CNTs aerogel laden with lower mass (20-120 mg) of [Mo3S13] over samples with higher [Mo3S13] laden (120-200 mg) can be attributed to the following two aspects. On the one hand, the highly conductive rGO-CNTs aerogel contributed to the high electrical conductivity of the catalyst and facilitated a fast electron-transfer process by improving the electrical contact between the electrode and the active sites [Mo3S13]2nanoclusters. However, excessive loading of [Mo3S13]2- nanoclusters in the structure of rGO-CNTs aerogel 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 weak synergetic effect of the hybrid Mo3S13@rGO-CNTs aerogel catalyst

55, 64-65

.

The substantial agglomeration of excessive [Mo3S13]2- on rGO-CNTs aerogel was observed in Figure S12. This weak synergetic effect due to the excessive catalysts were in accordance with previous research results 66-67. To further investigate the HER activity of multiple Mo3S13@rGO-CNTs aerogels, 15 ACS Paragon Plus Environment


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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 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]@rGO-CNTs 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 of 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 surface of Mo3S13@rGO-CNTs aerogels. To ensure a complete characterization, the 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 were presented in Figure 6c, and Figure S14. Nyquist plots of bare [Mo3S13]2- nanoclusters and bare rGO-CNTs aerogel were given in Figure S15. The significantly reduced charge-transfer resistance (Rct) was realized for all Mo3S13@rGO-CNTs aerogels in contrast to the bare [Mo3S13]2- nanoclusters and bare rGO-CNTs aerogel, 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 was also presented in Figure S16, in which R1 (R2), R3, CPE, and W represent the solution resistance, charge transfer resistance, the constant phase element, and the dispersion resistance 39, 68-69

.

All these EIS parameters were presented in Table S2. Results indicated that all 16 ACS Paragon Plus Environment

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[Mo3S13]@rGO-CNTs aerogels achieved the 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 aerogels 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 were given in Figure 6d and Figure S17. For rGO-CNTs aerogels loading with 20, 40, 80 and 120 mg of [Mo3S13]2nanoclusters, no appreciable activity change was observed after 1000 cycles; this indicated the excellent stability of these samples during HER. This observation further suggested that the [Mo3S13-20, 40, 80, 120]@rGO-CNTs aerogels remain intact with the [Mo3S13]2- nanoclusters firmly immobilized in rGO-CNTs aerogels. In contrast, an obvious activity loss was observed in the [Mo3S13-200]@rGO-CNTs aerogel. This further proved that the substantial agglomeration yielded from excess [Mo3S13]2nanoclusters would reduce the HER stability of Mo3S13@rGO-CNTs aerogel. To evaluate the durability of Mo3S13@rGO-CNTs aerogel for continuous HER at static overpotential, the current-time plot was determined at the potential of 0.200 V (vs.RHE) for 20000 s and the results were shown in Figure S18. The current density exhibited only a slight decrease after a long period of 20000 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 was also presented in Figure S11, S13, S14, and S17. For all Mo3S13@rGO-CNTs aerogel samples, the increased deposition of [Mo3S13]2nanoclusters from 1 mg to 5 mg onto the GCE greatly improved the HER performance. This was partially due to the spontaneous increase of both rGO-CNTs aerogels and [Mo3S13]2- nanoclusters contents, maintaining the synergetic effect of the 17 ACS Paragon Plus Environment


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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.

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; (d) Polarization curves of all samples initially and after 1000 cycles in 0.5 M H2SO4

The TEM of Mo3S13@rGO-CNTs aerogel after 1000 cycles of HER indicated that the Mo3S13@rGO-CNTs aerogel still retained its structural integrity with morphology of the catalyst almost kept unchanged (Figure 7a, b). All these results indicated that the Mo3S13@rGO-CNTs aerogel was a highly efficient electrocatalyst for HER. The 18 ACS Paragon Plus Environment

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significant number of active edge sites were inherently created by the [Mo3S13]2nanoclusters anchored in Mo3S13@rGO-CNTs aerogel. And also, the strong support of rGO-CNTs aerogel exposed more active sites and electrons diffusions on surface of Mo3S13@rGO-CNTs aerogel (Figure 7c). As a result, rGO-CNTs aerogel laden with a relatively small amount (20-120 mg) of [Mo3S13]2- nanoclusters exhibited low over potential, small Tafel slope, high electro-conductibility and excellent durability for HER in 0.5 M H2SO4 solution, which could be comparable with most relevant electrocatalysts (Table S3).

Figure 7 TEM of Mo3S13@rGO-CNTs aerogel after 1000 cycles of HER: (a) 100 nm; (b) 20 nm (inserted figure: 5 nm); (c) Mechanism of HER on surface of Mo3S13@rGO-CNTs aerogels

Conclusions The [Mo3S13]2- nanoclusters based catalyst, Mo3S13@rGO-CNTs 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@rGO-CNTs 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 pattern indicated the [Mo3S13]2- nanoclusters have poor crystallinity, and this defect-rich property of [Mo3S13]2- nanoclusters in Mo3S13@rGO-CNTs aerogel can be 19 ACS Paragon Plus Environment


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beneficial in terms of electrocatalytic activity. And also, the strong support of rGO-CNTs aerogel suppressed the aggregation of [Mo3S13]2- nanoclusters, exposing more active sites and electrons diffusions on surface of Mo3S13@rGO-CNTs aerogel. The rGO-CNTs aerogel loaded with 20-120 mg of [Mo3S13]2- nanoclusters exhibited better HER performance as compared with that of [Mo3S13-200]@rGO-CNTs aerogel. Excessive loading of [Mo3S13]2- nanoclusters in the structure of rGO-CNTs aerogel resulted in high internal resistance and substantial agglomeration, thus leading to worse HER performance.

ASSOCIATED CONTENT Supporting Information Additional

characteristics

(AFM,

XPS,

EDS,

FTIR,

XRD,

N2

adsorption/desorption isotherm, Zeta potential) of samples, polarization curves (initially and after 1000 cycles), Tafel slopes, Nyquist plots of different Mo3S13@rGO-CNTs aerogel samples AUTHOR INFORMATION Corresponding Author *Tel: +86-531-88361912, E-mail: [email protected] (Xing Xu); [email protected] (Baoyu Gao)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The research was supported by the National Natural Science Foundation of China (51178252, 51508307, 51508308), China Postdoctoral Science Foundation funded project (2014M560556, 2015T80721). This work was also supported by grants from Tai Shan Scholar Foundation (No. ts201511003).

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For Table of Contents Use Only.

Brief Summary Thiomolybdate [Mo3S13]2- nanoclusters anchored on the reduced graphene oxide-carbon nanotubes aerogels were used as a catalyst converting water to sustinable hydrogen energy.



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

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