Cobalt Sulfide Nanotubes (Co9S8) Decorated with Amorphous MoSx

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Cobalt Sulphide Nanotubes (CoS) Decorated with Amorphous MoS as Highly Efficient Hydrogen Evolution Electrocatalyst x

Liqian Wu, Kaiyu Zhang, Tingting Wang, Xiaobing Xu, Yuqi Zhao, Yuan Sun, Wei Zhong, and Youwei Du ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00271 • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 7, 2018

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ACS Applied Nano Materials

Cobalt Sulphide Nanotubes (Co9S8) Decorated with Amorphous MoSx as Highly Efficient Hydrogen Evolution Electrocatalyst Liqian Wua, Kaiyu Zhanga, Tingting Wanga, Xiaobing Xua, b, Yuqi Zhaoa, Yuan Suna, Wei Zhonga* and Youwei Dua a) Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructures and Jiangsu Provincial Laboratory for Nanotechnology, Nanjing University, Nanjing, 210093, China. b) College of Electronic Engineering, Nanjing Xiaozhuang University, Nanjing, 210017, China.

In this paper, we rationally designed a novel hybrid material consisting of Co9S8 nanotubes decorated with molybdenum sulphide lamellae (Co9S8 / MoSx), which was synthesized by consecutive solvothermal treatments. The extensive characterizations of the material using X-ray diffraction (XRD), field emission scanning and transmission electron microscopy (FE-SEM and TEM), and high-resolution TEM (HRTEM) analyses certified that the decoration was amorphous MoSx. The analysis of X-ray photoelectron spectra (XPS) suggested the existence of a strong interface interaction between the two phases (Co9S8 and MoSx). Subsequently, investigated as an electrocatalyst for the hydrogen evolution reaction (HER) in acid media, the Co9S8 / MoSx hybrid exhibited a better activity than pristine MoSx and Co9S8, requiring an overpotential of -161 mV to drive a current density of -10 mA cm-2, as well as higher stability of the performance than Co9S8. Through experimental analysis, it was established that the improved HER activity and stability of the hybrid mainly resulted from the electrocatalytic synergistic effects between Co9S8 and MoSx. Keywords：Hydrogen evolution reaction, Cobalt sulfide, Molybdenum disulﬁde, Synergistic effect, Nanotubes

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1. INTRODUCTION With global warming and shortage of fossil-fuel, exploitation and utilization of renewable energy sources become an urgent need, and hydrogen as a clean energy carrier has received increasing attention.[1-3] Among various strategies to produce hydrogen, the hydrogen evolution reaction (HER) as one half reaction in electrochemical water splitting offers a promising and sustainable solution.[4-5] Thus, a high-efficiency electrocatalyst for HER to reduce overpotential and accelerate hydrogen production in the electrochemical process is urgently required. By now, Pt-group metals and their alloys as HER electrocatalysts have been found to be most effective. Nevertheless, in the light of their high cost and scarcity, large-scale applications of Pt-group metals are hindered in industrial hydrogen production. It is therefore essential to develop efficient and cheap earth-abundant alternatives as HER electrocatalysts.[6-8] As a result, more alternatives have been widely explored, including transition metal carbides, sulfides, nitrides, phosphides, selenides, etc.[9-17] Thereinto, molybdenum disulfide (MoS2) at low cost has been recognized as a promising hydrogen evolution catalyst, due to the Gibbs free energy of hydrogen adsorption for MoS2 close to that of Pt-group metals (i.e., ∆GH*≈ 0).[18-19] However, low conductivity, limited surface area, and the inert basal plane for MoS2 surface make it challenging to obtain high HER performance. To address the above issues, electronic conductivity engineering and active site engineering have been developed to synthesize various nanostructured MoS2-based HER catalysts.[20-21] One of these engineering strategies is introducing transition element, especially Co, into MoS2-based electrocatalysts, which can greatly improve the HER activity. For example, Bonde et al. found that Co coupling with S-edges in MoS2 can lower their ∆GH* from 0.18 to 0.10 eV, and thus the corresponding active sites ascribing from S-edges would increase, which promotes the HER activity of MoS2.[22] Zhang et al. fabricated metallic CoS2 nanocubes coated by amorphous Co-doped MoS2 nanosheets, and those exhibited ultrahigh HER activity, which was mainly attributed to their novel hierarchical structures and the synergistic effects induced by Co doping.[23] Likewise, Dai et al. prepared Co-doped


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MoS2 nanosheets coated on carbon, and those exhibited superior HER performance and excellent stability in acidic electrolyte. They ascribed the superior HER performance to the interface effect arising from Co doping (i.e. Co-Mo-S phase), which can not only effectively accelerate charge transfer, but also synergistically increase structural and electronic modulations between MoS2 and Co.[24] But not only that, the Co-Mo-S phase of transition metal sulphides (TMS) is also regarded as an active phase in catalytic process for hydrodesulfurization (HDS) in the petroleum industry.[25-26] Recently, it has been reported that Co9S8 / MoS2 composite catalyst can effectively improve HDS activity, owing to the synergetic effect originating from the interfacial areas between Co9S8 and MoS2 phases.[27] It is well known that a good HDS catalyst can be deemed as logical candidate for HER electrocatalyst.[13] Hence, it is valuable to design a novel Co9S8 / MoS2 hybrid and explore whether Co9S8 / MoS2 hybrid including Co-Mo-S phase can be used as an efficient electrocatalyst. In this study, we rationally designed and successfully prepared one novel hybrid consisting of Co9S8 nanotubes decorated with amorphous MoSx by solvothermal approach for the first time. On the basis of experimental results, the novel hybrid with Co-Mo-S phase as hydrogen evolution electrocatalyst indeed exhibits an excellent electrochemical activity and good stability in acid electrolyte.

2. EXPERIMENTAL SECTION Materials. All chemicals were of reagent grade and used without further purification. Cobalt chloride hexahydrate (CoCl2.6H2O) and urea (CO(NH2)2) were supplied by Sinopharm chemical reagent Co., Ltd., China. Sodium sulfide nonahydrate (Na2S·9H2O), N, N-Dimethylformamide (DMF) and hydrazine hydrate (N2H4·H2O) were provided by Nanjing Chemical Reagent Co., Ltd., China. Ammonium tetrathiomolybdate ((NH4)2MoS4) was purchased from J&K China Chemical Ltd. High purity distilled water was used in all experiments.

Synthesis of Co9S8 nanotubes. Co9S8 nanotubes were prepared using a modified hydrothermal method.[28] Firstly, Co(CO3)0.35Cl0.20(OH)1.10 nanorod bunches as ACS Paragon Plus Environment


sacrificial hard templates of Co9S8 nanotubes were prepared. In brief: CoCl2·6H2O (0.005 mol) and CO(NH2)2 (0.005 mol) were dissolved in 40 mL distilled water in a Teflon liner of 50 mL capacity, and then the liner was sealed in a stainless steel autoclave for hydrothermal reaction at 120 °C for 10 h. Secondly, the freshly Co(CO3)0.35Cl0.20(OH)1.10 (1 mmol) and Na2S solution (5 mL of a 0.50 mol L-1) were added into 40 mL distilled water in a Teflon liner of 50 mL capacity, and then the liner was sealed in a stainless steel autoclave for hydrothermal reaction at 160 °C for 8 h. In short, through two-step hydrothermal procedure, the resulting Co9S8 nanotubes were collected for next use.

Synthesis of Co9S8 / MoSx hybrid. To prepare the Co9S8 / MoSx hybrid, 30 mg as-prepared Co9S8 nanotubes and different amounts of (NH4)2MoS4, with different Co : Mo molar ratios (3:1, 2:1, 1:1, and 1:2), were mixed in 30 mL DMF, and sonicated for 30 min under ambient conditions. Afterwards, 1 mL N2H4·H2O was loaded into the suspension. After sonicated for another 30 min to disperse evenly, the mixed solution was transferred into a 50 mL Teflon liner, which was sealed in a stainless steel autoclave and maintained at 200 °C for 10 h. After cooling naturally, the resulting black products were collected by centrifugation, and then washed by distilled water and absolute ethanol several times. At last, the Co9S8 / MoSx hybrids were obtained after drying under vacuum at 80 °C for 12 h. Furthermore, in order to be convenient for discussion in the context, Co9S8 / MoSx hybrids were labelled as Co9S8 / MoSx-3:1, Co9S8 / MoSx-2:1, Co9S8 / MoSx-1:1, and Co9S8 / MoSx-1:2, respectively, according to the Co : Mo nominal molar ratios (3:1, 2:1, 1:1, and 1:2). If none of special version, Co9S8 / MoSx hybrid in the context refers to Co9S8 / MoSx-2:1. In addition, the pristine MoSx, as a comparison, was synthesized through the same experiment process lack of Co9S8 nanotubes in the solvothermal solution. Moreover, the as-prepared pure Co9S8 and pure MoSx were milled and physically mixed to form the physical mixture of Co9S8 and MoSx (Co9S8-MoSx) with the mass ratio (Co9S8 : MoSx) -2:1.


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Characterizations. The phase identification were tested using an XRD (TD-3500, China) with Cu Kα (λ = 1.5418 Å) radiation. The morphology was evaluated with FE-SEM (FEI Helios600i, USA) and TEM (Model JEOL-2010, Japan). Elemental content and elemental mapping were collected by an energy dispersive spectrometer (EDS) (Oxford X-MAX 50, UK). XPS was investigated by a PHI 5000 VersaProbe (UIVAC-PHi, Japan) using Al Kα radiation (1486.6 eV).

Electrochemical measurements. An electrochemical workstation (CHI660D) was applied to carry out the electrochemical measurements in a standard three-electrode system at room temperature. A Pt wire electrode (Figure S1) and a Ag/AgCl (in 2.5 M KCl solution) electrode were as counter electrode and reference electrode, respectively. For the preparation of working electrode, the procedure was as follows: Firstly, 5 mg of catalyst and 40 µL of Nafion solution (5 wt%) were dispersed in 1 mL of N, N-dimethylformamide (DMF), and then this mixture was sonicated for 1h to form a homogeneous ink. Subsequently, 5 µL of the dispersion was dropped onto the glassy carbon electrode (GCE, 3 mm in diameter) till the suspension was dried at room temperature. The catalyst loading on GCE is 0.35 mg cm-2. The HER activity of catalysts was evaluated by linear sweep voltamperometry (LSV) conducted at a scan rate of 5 mV s-1. The electrochemical double-layer capacitance (Cdl) was investigated through cyclic voltammetry (CV) performed with different scan rates (20-140 mV s-1) in the potential range from 0.1-0.2 V (vs. the reversible hydrogen electrode (RHE)). Electrochemical impedance spectroscopy analysis (EIS) was conducted from 106 to 10-1 Hz at η=-100, -150, and -200 mV (vs. RHE) with an amplitude of 5 mV. Besides, all tests were carried out in N2-saturated 0.5 M H2SO4 solution. All the potentials measured in this work were calibrated to reversible hydrogen electrode by the equation ((ERHE = EAg/AgCl + 0.209 + 0.059pH) V). All data were reported without iR compensation.

3. RESULTS AND DISCUSSION Characterization. Figure 1a displays the FE-SEM image of the precursor ACS Paragon Plus Environment


Co(CO3)0.35Cl0.20(OH)1.10, which presents many bunches composing of needle-like nanorods with 150-300 nm in diameter, and several micrometres in length. Figure 1b displays the XRD pattern of as-prepared Co9S8 sample and it shows peaks at 29.9°, 47.4°, and 52.0° corresponding to the Co9S8 (311), (511), and (440) planes, respectively, which are consistent with the fcc phase of Co9S8 (JCPDS 19-0364) (inset in Figure 1b). The FE-SEM image (Figure 1c) of the Co9S8 sample shows bunches assembled from many nanotubes, and the corresponding EDS elemental mapping (Figures 1d-f) validates the existence of Co and S elements have a homogeneous distribution in the sample. To further observation for the morphologies and detailed crystal structures of the Co9S8 nanotubes, TEM and HRTEM analyses were performed. Figures 1g and h show the low-magnification TEM images for the Co9S8 nanotubes, which clearly reveal their hollow structures. In the corresponding HRTEM image (Figure 1i), two interplanar distances 0.29 and 0.30 nm, which well correspond to the (222) and (311) crystal planes of Co9S8, respectively, can be clearly observed.

Figure 1. (a) FE-SEM image of the Co(CO3)0.35Cl0.20(OH)1.10 nanorods that served as the precursor. XRD pattern (b), FE-SEM image (c) and corresponding SEM-EDS elemental mapping (d-f) of as-prepared Co9S8 nanotubes. TEM (g, h) and corresponding HRTEM (i) images of the Co9S8 nanotubes.


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Figure 2a displays the XRD pattern of the pristine MoSx. No obvious crystalline peak of 2H-MoS2 (JCPDS 37-1492) (inset in Figure 2a) can be observed in the pristine MoSx, consistent with the previous report adopting the same solvothermal conditions.[29] Subsequently, the atomic ratio of Mo/S in the MoSx is analysed by XPS (Figure S2), and approximately equals 2/3, indeed deviating the normal component (namely, atomic ratio of Mo/S is 1/2) of MoS2. Furthermore, on the basis of the FE-SEM image (Figure 2b), it can be seen that the pristine MoSx embodies serious agglomeration and stacking. The corresponding EDS elemental mapping (Figures 2c-e) validates the existence of Mo and S elements have a homogeneous distribution in this sample.

Figure 2. XRD pattern (a), FE-SEM image (b) and corresponding SEM-EDS elemental mapping (c-e) of the pristine MoSx.

Next, the characterizations of the product synthesized by solvothermal treatments where the precursors are Co9S8 nanotubes and (NH4)2MoS4 were investigated. Figure



3a displays the XRD pattern of the hybrid. This hybrid displays peaks at 29.9°, 47.4°, and 52.0° corresponding to the Co9S8 (311), (511), and (440) planes, respectively. However, no obvious crystalline peak of MoS2 is observed from the XRD pattern of the hybrid. As for the morphology of this hybrid, the SEM image (Figure 3b) also presents bunches assembled from many nanotubes, similar to that of the Co9S8 sample (Figure 1c). Nonetheless, the surfaces of the hybrid are rougher, compared with Co9S8 nanotubes. In addition, the corresponding EDS elemental mapping (Figures 3c-e) indicates that apart from the existence of Co, S elements, Mo element also have a homogeneous distribution in this hybrid. Through the observation for TEM and HRTEM images, the morphologies and detailed crystal structures of the hybrid were further investigated. Figures 3f and g display the TEM images of the hybrid. Compared with Figures 1g and h, it can be evidently seen that a few nanosheets anchor on the surface of Co9S8 nanotubes, agreeing with the contrast of the SEM images (Figures 1c and 3b) for Co9S8 nanotube and the hybrid, respectively. In the HRTEM image of this hybrid (Figure 3h), the clear lattice fringes with interplanar distances of 0.30 nm can be observed, corresponding to the (311) crystal planes of Co9S8. Meanwhile, compared with pure Co9S8 (Figure 1i), several lamellar structures are observed over the periphery, and some detected interplanar distance is 0.62 nm, coincident with the (002) crystal plane of MoS2, which confirmedly prove that the lamella anchored on the surface of Co9S8 nanotube were quasi-amorphous MoSx.[29] Therefore, these experimental results above indicate Co9S8 nanotubes should be support for mediating the growth of MoSx and assembling the novel hybrid: Co9S8 nanotubes decorated with amorphous MoSx hybrid (Co9S8 / MoSx). In addition, the effect of excessive Mo precursor during synthesis on morphology of Co9S8 / MoSx hybrids were also investigated with FE-SEM. From the Figure S3, it can be seen that a little agglomeration (may ascribe from MoSx) stacks around the Co9S8 nanotubes in Co9S8 / MoSx -1:2. Therefore, the moderate amount of Mo precursor can make the generated MoSx nanosheet coated on Co9S8. However, excess precursor of Mo may make a little generated MoSx agglomerate on the surface of Co9S8.


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Figure 3. XRD pattern (a), FE-SEM image (b) and corresponding SEM-EDS elemental mapping (c-e) of the Co9S8 / MoSx hybrid. TEM (f, g) and corresponding HRTEM (h) images of the Co9S8 / MoSx hybrid.

The surface electronic state and composition of Co9S8 / MoSx hybrid were further investigated by XPS. The survey spectrum (Figure 4a) can identify the existence of C, Co, Mo, S, and O elements. The selection of the position of each peak for fitting is based on the reference[30] and the binding energies in XPS are calibrated by the C1s binding energy (284.8 eV) of carbon contamination. As shown in Figure 4b, the XP spectrum of Co 2p for Co9S8 / MoSx hybrid is deconvoluted into two spin-orbit doublets and two shakeup satellites (identified as “Sat.”). The peaks at 779.1 and 794.1 eV are assigned to the 2p3/2 and 2p1/2 of Co3+, whereas those at 781.4 and 797.1 eV are attributed to the 2p3/2 and 2p1/2 of Co2+, respectively, which agree with previous report[31] and those manifest the formation of Co9S8 nanocrystals. However, compared with the XP spectra of pure Co9S8 nanotube (778.6 and 793.8 eV, shown in Figure 4e), the binding energies of Co3+ 2p up-shift to 779.1 and 794.1 eV, respectively, suggesting a strong interface interaction between the decorated-Co9S8 and the coated-MoSx. Also, the Co3+ 2p3/2 binding energy (779.1 eV) of Co9S8 / MoSx hybrid is very close to that in a Co-Mo-S phase.[24, 32] Figure 4c depicts that the XP



spectra of Mo 3d and S 2s for Co9S8 / MoSx hybrid, which is deconvoluted into four peaks. The peak at 226.1 eV is assigned to S 2s of MoSx. The two main peaks are attributed to Mo 3d5/2 (228.9 eV) and 3d3/2 (232.2 eV) of Mo4+ of MoSx,[33] respectively. Another peak at 235.1 eV corresponds to Mo6+, which might be ascribed from the slight oxidation because of the exposure of MoSx in air.[34] Compared to the pure MoSx (229.4 and 232.7 eV, Figure 4f), the Mo4+ 3d peaks exhibit down-shifts, which further provides evidence of the strong interface interaction between Co9S8 and MoSx. In coclusion, the Co3+ 2p peaks of Co9S8 / MoSx hybrid up-shift compared to Co9S8, together with the Mo4+ 3d peaks of Co9S8 / MoSx hybrid down-shift compared to the pure MoSx, suggest the existence of strong interface interaction between the two phases, and indicate the electron transfer phenomenon from Co9S8 to MoSx by intermediate sulfur atoms, which provide the evidence of Co-Mo-S phase forming, consistent with the XPS results of Zhu et. al and Zhou et. al. [31, 33] For S 2p region (Figure 4d), the two S 2p peaks at 161.7 and 163.3 eV should be corresponding to S 2p3/2 and S 2p1/2 of Co-S bondings, respectively, in good agreement with the previous reports about Co9S8 crystals.[31, 35] The other S 2p peaks at 162.7 and 164.2 eV should be assigned to the bridging S22- or apical S2- in MoSx.[9, 36]

Meanwhile, the S 2p peaks at 162.7 and 164.2 eV may be associated with the

abundant unsaturated sulfur atoms (S22-) resulting from Co-Mo-S phase formed in the interface between Co9S8 and MoSx, and the unsaturated sulfur atoms(S22-) would be activated.[23-24] Moreover, the peak at 168.9 eV can be ascribed to the small amount of SO42- residue. (a) Co 2s C 1s

(b) Co 2p

3+

Co

2p3/2

O 1s

2+

3+

Co LMM 2+

Co

Co 2p

Co 2p3/2

Co 2p1/2

2p1/2

Sat.

Mo 3p Mo 3s

Intensity (a.u.)

Mo 3d

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Co LMM

S 2p S 2s Co 3s Mo 4s

Sat.

Co 3p Mo 4p

0

200

400 600 Binding Energy (eV)

800

1000

775

780


785 790 795 800 Binding Energy (eV)

805

810

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(c) Mo 3d-2s

(d) S 2p

Mo 3d3/2

Mo-S 2p3/2

Mo-O S 2s

220

225

Co-S 2p3/2

Intensity (a.u.)

Intensity (a.u.)

Mo 3d5/2


Co-S 2p1/2 Mo-S 2p1/2

159

240

(e) Co 2p

162

Sat.

165 168 171 Binding Energy (eV)

174

(f) Mo 3d

Intensity (a.u.)

Co9S8

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Co9S8 / MoSx

770

780


810

MoSx

Co9S8 / MoSx

220

225


240

Figure 4. (a) XPS survey spectrum, XP spectrum of (b) Co 2p, (c) Mo 3d-S 2s, (d) S 2p for the Co9S8 / MoSx hybrid. Obvious binding energy shifts are observed in the XP spectra of (e) Co 2p for the Co9S8 and Co9S8 / MoSx hybrid samples, and those of (f) Mo 3d for the MoSx and Co9S8 / MoSx hybrid samples.

Synthesis process of Co9S8 / MoSx. Co9S8 / MoSx hybrids was synthesized by consecutive solvothermal treatments, and the entire fabrication process can be illustrated in Scheme 1. Firstly, the precursor Co(CO3)0.35Cl0.20(OH)1.10 nanorod bunches was synthesized by hydrothermal process. Thereinto, when heating in aqueous solution, CO(NH2)2 is hydrolyzed to generate CO2 and NH3. Afterwards, the productions hydrolyzed are ionized to produce CO32- and OH-, and these ions react with

Co2+

ions

and

Cl-

ions

to

form

Co(CO3)0.35Cl0.20(OH)1.10.

Then,

Co(CO3)0.35Cl0.20(OH)1.10 nanorods were transformed into Co9S8 nanotubes through the nanoscale Kirkendall effect. In this case, the Co(CO3)0.35Cl0.20(OH)1.10 nanorods are reacting with Na2S solution, where Co9S8 shell is formed through the outward transport of Co2+ ions and the inward transport of S2- ions. However, the driving force



for the outward diffusion of the core material is greater, due to the relatively large concentration change in the core material between the core and the solution.[28] Therefore, the diffusion rate of the outward transport for Co2+ ions would be faster than that of the inward transport for S2- ions, which results in that with the reaction going on, the Co9S8 shell increases and the Co(CO3)0.35Cl0.20(OH)1.10 core decreases gradually. Thus, the tubular structure is formed. Finally, the Co9S8 / MoSx hybrids were synthesized by a facile solvothermal process, in which (NH4)2MoS4 is employed as a precursor reduced to MoSx around Co9S8 nanotubes substrates, and N2H4·H2O used as reducing agent.[29]

Scheme 1. The fabrication process of Co9S8 / MoSx hybrids.

Electrocatalytic activity toward HER. The HER activity of serial samples (MoSx, Co9S8, Co9S8-MoSx and Co9S8 / MoSx) was estimated by the polarization curves, which is shown in Figure 5a. Table 1 displays the onset potential (Eon), overpotential at current density of -10 mA cm-2 (η10), and cathodic current density recorded at η= -200 mV (j200) obtained from polarization curves of serial samples. Among them, Co9S8 / MoSx hybrid catalyst exhibits highest catalytic activity with the onset potential of -65 mV and the overpotential at -10 mA cm-2 of -161 mV, which is less than those of the pristine MoSx, Co9S8-MoSx and Co9S8. In addition, cathodic current


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density (-25.90 mA cm-2) at -200 mV of Co9S8 / MoSx hybrid catalyst is far larger than those of other catalysts (Co9S8 (-2.85 mA cm-2), Co9S8-MoSx (-2.43 mA cm-2), and MoSx (-2.05 mA cm-2)). The much better HER performance of the Co9S8 / MoSx hybrid with respect to the cathodes composed of individual components (pristine MoSx and pure Co9S8) or physical mixture (Co9S8-MoSx) clearly indicates the existence of the synergistic effect between MoSx and Co9S8 in the Co9S8 / MoSx hybrid. The corresponding Tafel slopes of these catalysts are determined by the linear portion of the Tafel plots, which are deduced from the polarization curves (Figure 5a), and fitting well with the Tafel equation (η=b log (j) + a, where j is the current density and b is the Tafel slope) in different overpotential ranges.[20] As shown in the Figure 5b, the obtained Tafel slope for Pt/C, Co9S8 / MoSx, Co9S8, Co9S8-MoSx and MoSx catalysts are -41, -78 -89, -98 and -126 mV dec-1, respectively. Since a small Tafel slope signifies faster increment of cathodic current density with overpotential increasing, the Co9S8 / MoSx hybrid possesses more favorable HER kinetics than the pristine MoSx, Co9S8-MoSx and Co9S8 catalysts.[37] Also, the Tafel slope is an inherent character of the catalyst, which can determine the rate-limiting step in the HER.[20, 34] Generally, there are two possible routes suggested for the HER in acid aqueous. [38] The first route is the primary discharge step (Volmer reaction: H3O++e-→Hads+H2O). The following route is the electrochemical desorption step (Heyrovsky reaction: Hads +H++e-→H2), or the recombination step (Tafel reaction: Hads+ Hads→H2). When the Volmer reaction is difficult to implement, a large Tafel slope of -120 mV dec-1 would be obtained. Inversely, if the Volmer reaction is fast and followed by a slow Heyrovsky reaction or a Tafel reaction, the corresponding Tafel slope is -40 or -30 mV dec-1, respectively. [39] According to the analysis above, the Tafel slope of Co9S8 / MoSx hybrid (-78 mV dec-1) suggests that the primary discharge, i.e. the electrochemical adsorption step, may be the rate-limiting step.[40] In addition, the exchange current density (j0) can be derived from the intercept obtained by extrapolating the linear portion of the η vs.log |j| plot to η = 0,[22] as shown in Table 1. It can be seen that the exchange current density of Co9S8 / MoSx hybrid catalyst is the ACS Paragon Plus Environment


largest value (91.2 µA cm-2). Considering that Tafel slope and exchange current density suggest total electrode activity metrics,[37] it can be sure that Co9S8 / MoSx hybrid is indeed the most active catalyst compared with the others in this work. Additionally, the excellent electrochemical performance of this hybrid catalyst is also comparable to that of other similar cobalt-based sulfide composite catalysts (see Table 2). [31, 33, 41-45] (a)

(b)0.05

0

0.00

-10

-0.05

-20

η (V)

Current (mAcm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-30

Pt/C Bare GC MoSx

-40

Co9S8-MoSx

-0.10 -0.15

-41 mV dec

Pt/C MoSx Co9S8 - MoSx Co9S8 Co9S8 / MoSx

-89 mV dec

-0.25

-0.3 -0.2 -0.1 Potential ( V vs.RHE)

0.0

-1

-1 -1 -98 mV dec

-1 -126 mV dec

-0.30

Co9S8 / MoSx

-0.4

-78 mV dec

-0.20

Co9S8

-50 -0.5

-1

-0.2

0.1

0.0

0.2

0.4 0.6 0.8 lg(-j(mA cm-2))

1.0

1.2

1.4

Figure 5. (a) Polarization curves, (b) corresponding Tafel plots of the pristine MoSx, Co9S8-MoSx, Co9S8, and Co9S8 / MoSx hybrid. Table 1. Electrochemical characteristics for the pristine MoSx, Co9S8-MoSx, Co9S8, and Co9S8 / MoSx hybrid. Eon

η10

j200

Tafel slope

j0

Cdl

(mV)

(mV)

(mA cm-2)

(mV dec-1)

(10-5A cm-2)

(mF cm-2)

MoSx

-162

-292

-2.05

-126

2.39

1.42

Co9S8 -MoSx

-150

-261

-2.43

-98

2.09

0.43

Co9S8

-131

-247

-2.85

-89

1.91

0.08

Co9S8 / MoSx

-65

-161

-25.90

-78

9.12

24.20

Pt/C

0

-29

-142.80

-41

96.47

-

sample


Page 15 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Comparison of the Co9S8 / MoSx nanotube hybrid catalyst with other catalysts. Sample

Eon (mV)

η10 (mV)

Tafel slope (mV dec-1)

Catalyst loading (mg cm-2)

Reference

Co9S8@MoS2/CNFs

-64

-190

-110

0.21

[31]

Co9S8-MoSx

-

-98

-65

5.80

[33]

Co9S8@C

-

-240

-

0.30

[41]

Co9S8@NOSC

-

-232

-69

-

[42]

CoS2/MoS2/RGO

-80

-160

-56

0.35

[43]

Co9S8/NSG-UCNTs

-

-65

-84

0.30

[44]

Co9S8/NC@MoS2

-38

-117

-69

0.28

[45]

Co9S8 / MoSx

-65

-161

-78

0.35

This work

To verify the effect of different atomic ratios (Co / Mo) for Co9S8 / MoSx on the activity of HER, electrochemical measurements of Co9S8 / MoSx synthesized with different concentrations of (NH4)2MoS4 (Co : Mo molar ratios in reactants) were carried out in a 0.5 M H2SO4 solution with the same mass loading 0.35 mg cm-2. As shown in the polarization curves and Tafel plots (Figures 6a and b), these catalysts show excellent HER activity, which is much higher than that of other catalysts (i.e. Co9S8, Co9S8-MoSx and MoSx). The vast improvement of catalytic property for these hybrids ascribes from the synergistic effect from interface structure between Co9S8 and MoSx. Among these hybrids, Co9S8 / MoSx-2:1 shows the lowest overpotential (η10 = -161 mV) and the smallest Tafel slope (-78 mV dec-1). Co9S8 / MoSx -3:1 and Co9S8 / MoSx -1:1 show overpotential (η10) of -180 mV and -173 mV, respectively. Meanwhile, the corresponding Tafel slope for them are -79 or -83 mV dec-1, respectively. Upon experimental data above, it can be clearly observed that the HER activity of Co9S8 / MoSx -3:1 and Co9S8 / MoSx -1:1 are close to that of Co9S8 / MoSx -2:1, which performs the best HER activity. Therefore, the 2:1 (Co : Mo molar ratio in reactants during the synthesis process of Co9S8 / MoSx hybrids) is the optimal ratio, and the synergistic effect between Co9S8 and MoSx of the resultant (Co9S8 / MoSx -2:1) is most pronounced. Nevertheless, the HER activity of Co9S8/ MoSx -1:2, which shows the highest overpotential (η10) of -205 mV and the largest Tafel slope (-90 mV



dec-1), is modest, compared with other Co9S8 / MoSx hybrids. We assume that, when the content of MoSx in the hybrids is excessive, the conductivity of Co9S8 / MoSx may become poor, which would weaken electrocatalytic activity of the hybrids. Through electrical measurements (as shown in Figure S4 and Table S1), resistivity ρ value of Co9S8 / MoSx -1:2 exceeds that of Co9S8 / MoSx -2:1 by one order of magnitude, demonstrating that excessive content of MoSx in the hybrids results in poorer conductivity. In addition, the effect of different atomic ratios (Co / Mo) on the activity of HER is also reported in other similar works (in Co9S8@MoSx/CC or CoS2-MoS2/CNTs).[33, 46] (a)

(b) -0.12

0

Co9S8 / MoSx-3:1 Co9S8 / MoSx-2:1

-0.14

-10


-0.16

-20

η (V)

Current (mA cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 28

-78 mV dec

-0.18

-30 Co9S8 / MoSx-3:1 Co9S8 / MoSx-2:1

-40 -50 -0.5

-79 mV dec

-0.20


-0.4


0.0

0.1

-1

-1

-1 -83 mV dec

-1 -90 mV dec

-0.22

0.6

0.8 1.0 lg(-j(mA cm-2))

1.2

1.4

Figure 6. (a) Polarization curves, (b) corresponding Tafel plots of the tubulous Co9S8 / MoSx hybrids of different molar ratios (Co / Mo).

To gain a better understanding of the improvement mechanism, we analyzed the electrochemical surface area (ECSA). Generally, the ECSA is expected to be linearly proportional to the electrochemical double-layer capacitance (Cdl).[47] We obtained the Cdl value of serial samples through plotting the difference between anodic and cathodic currents (△j=ja-jc) at a given potential (0.15 V vs. RHE) against the CV scan rates (v), (Cdl=△j/2v). The Cdl derived from Figures 7a and S5, which are the corresponding cyclic voltammograms, are shown in Figure 7b and Table 1. It can be seen that the Cdl for the Co9S8 / MoSx hybrid catalyst is 24.20 mF cm-2, which is far more than those of the pristine MoSx (1.42 mF cm-2), Co9S8-MoSx (0.43 mF cm-2) and Co9S8 (0.08 mF cm-2). Because, as the referential sample, Co9S8 morphology is similar to that of the Co9S8 / MoSx hybrid (i.e. tubulous), the increased value of Cdl for


Page 17 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the Co9S8 / MoSx hybrid is mainly attributed to the lamella textures anchored on the surface of the hybrid, and the synergistic effect between Co9S8 and MoSx.[39] Therefore, the larger ECSA for Co9S8 / MoSx hybrid contributes to the very high exposure of effective active edge sites from interface between two phases (Co9S8 and MoSx), and improve greatly catalytic performance of Co9S8 / MoSx. In order to further understand why the hybrid exhibits better electrochemical performance when compared with other catalysts, EIS measurements were also performed. Figure 7c depicts the Nyquist plots of the Co9S8 / MoSx hybrid at different applied potentials (from -100 to -200 mV vs RHE), where there are two semicircle for all cases, a small one in the high-frequency region and another large one in the low-frequency region, representing two different time constants, respectively. Besides, two obvious extremum in the phase angle curves in the Bode plots (Figure 7d) also manifest the presence of two characteristic processes relevant to two different time constants.[9] The inset of Figure 7c is the corresponding two time constant equivalent circuit model, including a solution resistance (Rs), a resistance related to surface porosity (Rp), and a charge transfer resistance (Rct) for the electrochemical reaction.[48, 49]

Because sharply decreasing with the increasing overpotentials, the semicircles at

low frequency region reflect the charge transfer resistances.[50, 51] For comparison, Nyquist plots of the other catalysts (MoSx and Co9S8) and calculated Rct values at low frequencies are shown in Figure S6 and Table S2, respectively. It can be noticed that Co9S8 / MoSx hybrid presents the smallest Rct values, clearly suggesting faster charge transport during the electrochemical HER process. The smaller Rct values of Co9S8 / MoSx hybrid compared to MoSx and Co9S8, further vertify that the electron transfers from Co9S8 to MoSx by intermediate sulfur in the hybrid,[24, 31] namely, the formation of Co-Mo-S phase. Thus, the better electrocatalytic activity of the Co9S8 / MoSx is closely relative with the synergistic effect between Co9S8 and MoSx.



(b)

Co9S8 / MoSx 140mV/s

MoSx

6

Co9S8-MoSx

5

20mV/s

0 -3

Co9S8 / MoSx

4

0.6 0.5

0.1

-2

3

Co9S8 (mA cm )

∆ j0.15V(mA cm-2)

6

3

0.4 0.3

0.15V

9

∆j

(a)

j (mA cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 28

2

0.2 0.0 20

1

-6 -9 0.10

40

60

80 100 120 140

Scan Rate(mV/s)

0 0.12

0.14 0.16 Potential (V vs RHE)

0.18

0.20

20

40

60

80

100

120

140

Scan Rate(mV/s)

Figure 7. (a) Cyclic voltammograms of Co9S8 / MoSx hybrid, (b) the estimated Cdl, (c) Nyquist plots (Inset: Two-time-constant model equivalent circuit used for data fitting of EIS spectra), and (d) Bode plots of Co9S8 / MoSx hybrid as a function of the applied overpotential.

Combining with above analysis, the main reason of the high HER electrocatalytic performance should be summarized as follows: when the new structure of Co9S8 / MoSx interfaces is formed, strong interaction ascribed from the Co-Mo-S phase between Co9S8 and MoSx, which can be certified by the XPS results, would make the electron transfer from Co to Mo by intermediate sulfur atoms occur, facilitating the carrier flow from the nanotubes to MoSx nanosheets, while activate abundant unsaturated sulfur atoms generated due to Co-Mo-S phase. Consequently, the interface interaction synergistically promote the HER electrochemical property of Co9S8 / MoSx hybrid. Finally, as an important parameter to assess electrocatalyst, the stability of catalysts was measured. The condition of the following stability tests is using a graphite rod as counter electrode. [52-54] As shown in Figure 8a, the stability of Co9S8 /


Page 19 of 28

MoSx catalyst was first tested by conducting CV from -0.4 to -0.1 V (vs. RHE) with a scan rate of 100 mV s-1. After 1000 cycles of continuous CV, there is a slight decrease in the cathodic current density. The long-term durability of the Co9S8 / MoSx for the HER was evaluated by electrolysis at an overpotential of 161 mV, which is shown in Figure 8b. Apparently, the decay in the cathodic current density is less than 10% after 30 h electrolysis. Furthermore, we also investigated the chronopotentiometric stability of the Co9S8 / MoSx at a current density of 10 mA cm-2 for more than 8 h (Figure S7). The chronopotentiometric curve remains basically constant during the whole test period and there is only about a 20 mV increase in overpotential after 8 h of testing. They all indicate that the Co9S8 / MoSx composite has stable HER activity. In addition, as a comparison, the stabilities of the other catalysts were also investigated by cycling the potential, which are shown in Figures S8a-f. Thereinto, the Co9S8 exhibits the most unstability, and its catalytic activity falls into a considerable decline (Figure S8c) after completion of 1000 cycles. However, when the Co9S8 / MoSx hybrids was formed,

the

corresponding

catalytic

stability

improved

greatly.

Just

as

Staszak-Jirkovský et al. research results,[55] by combining CoSx (higher activity but short lifetime) with MoSx (higher stability) to form a compact and robust CoMoSx sulfide structure, a considerably stable and efficient HER electrocatalyst towards both alkaline and acidic environments was successfully designed. Therefore, the stable HER activity of Co9S8 / MoSx catalyst should be owing to synergetic effect of Co-Mo-S phase from the interfacial areas between Co9S8 and MoSx. (a)

0

(b)

initial after 1000 cycles

Current density mA cm-2

-20 -30 -40 -50 -0.5

-0.4

0 Co9S8 / MoSx

-10 Current (mA cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



0.0

0.1

-5

-10

-15

-20

0

5

10

15 Times h

20

25

30

Figure 8. (a) Polarization curves of Co9S8 / MoSx hybrid before and after 1000 cycles at 100 mV/s.



(b) The current density-time (I-t) curve of the Co9S8 / MoSx at an overpotential of 161 mV for 30 h.

4. CONCLUSIONS In conclusion, hybrids being composed of Co9S8 nanotubes decorated with amorphous MoSx (Co9S8 / MoSx hybrids) were synthesized for the first time. Due to the electrocatalytic synergetic effect of Co-Mo-S phase from the interfacial areas between Co9S8 and MoSx, the novel electrocatalyst exhibits an excellent HER activity and high electrochemical stability in acid electrolyte. Meanwhile, this novel Co-Mo-S hybrid material presented herein offers a reference for the design of effective HER catalysts, for example, amorphous MoSx decorating other cobalt-based sulfide materials with different morphologies.

AUTHOR INFORMATION Corresponding Authors * Email: [email protected]

Notes The authors declare no competing financial interest.

SUPPORTING INFORMATION The Supporting Information is available: • The effect of different counter electrodes on polarization curves of the Co9S8 / MoSx hybrid. • XPS survey spectrum, XP spectrum of Mo 3d-S 2s, and S 2p for the pristine MoSx sample. • FE-SEM figure of Co9S8 / MoSx-1:2 hybrid. • Current-voltage characteristic of Co9S8 / MoSx -2:1 and Co9S8 / MoSx -1:2 at room temperature. ACS Paragon Plus Environment

Page 20 of 28

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• Comparison of resistance R (Ω), parameters measured, resistivity ρ values for the Co9S8 / MoSx -2:1 and Co9S8 / MoSx -1:2. • Cyclic voltammograms of the pristine MoSx, physical mixture (Co9S8-MoSx), and Co9S8 nanotubes. • Nyquist plots of the pristine MoSx, and Co9S8 nanotubes • Comparison of charge transfer resistance (Rct) values for the pristine MoSx, Co9S8 nanotubes, and Co9S8 / MoSx hybrid. • Time dependent potential of Co9S8 / MoSx under a current density of 10 mA cm-2. • The durability test for the pristine MoSx, Co9S8-MoSx, Co9S8 nanotubes, Co9S8 / MoSx-3:1, Co9S8 / MoSx-1:1, and Co9S8 / MoSx-1:2.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos.11474151 and 11774156), the National Key Project for Basic Research (Grant No. 2012CB932304), and PAPD, People’s Republic of China.



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Cobalt Sulfide Nanotubes (Co9S8) Decorated with Amorphous MoSx

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