CuS nanohybrid for electrocatalytic hydrogen evolution

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One-step synthesis of the 3D flower-like heterostructure MoS2/CuS nanohybrid for electrocatalytic hydrogen evolution Lilan Zhang a, Yali Guo a, Anam Iqbal a,b, Bo Li a,c, Deyan Gong a, Wei Liu a, Kanwal Iqbal a, Weisheng Liu a, Wenwu Qin a,* a

Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province and State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China b University of Balochistan Quetta, Pakistan c Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining, Qinghai 810008, PR China

article info

abstract

Article history:

In this work, a facile one-step hydrothermal method was developed to fabricate three types

Received 17 July 2017

different of nanomaterials: the two-dimension (2D) of MoS2 nanosheets; 3D spherical CuS

Received in revised form

nanoparticles; and 3D flower-like heterostructure of MoS2/CuS nanohybrid, respectively.

7 September 2017

The as-synthesized MoS2, CuS and MoS2/CuS were characterized by transmission electron

Accepted 24 September 2017

microscopy (TEM), field emission scanning electron microscopy (SEM) and X-ray diffraction

Available online xxx

(XRD) etc. The morphology of the MoS2/CuS nanohybrid is different from the MoS2 nano-

Keywords:

nanosheets, CuS nanoparticles and MoS2/CuS nanohybrid, were investigated by the Linear

MoS2/CuS nanohybrids

Sweep Voltammetry (LSV) and Tafel slope. The HER activity of MoS2/CuS nanohybrid is better

sheets and CuS nanoparticles. The hydrogen evolution reaction (HER) activity of MoS2

One step

than those of MoS2 nanosheets and CuS nanoparticles, which can be attributed to the good

Hydrothermal method

electron-transport ability of CuS and the strong reduction ability of hydrogen ions by MoS2.

HER

Thus, MoS2/CuS nanohybrid exhibited excellent activity for HER with a small onset potential

High activity

of 0.15 V, a low Tafel slope of 63 mV dec
1, and relatively good stability. However, the MoS2 nanosheets and CuS nanoparticles respectively shows a bigger onset potential of 0.25 V and 0.35 V, a higher Tafel slope of 165 and 185 mV dec
1. This 3D flower-like heterostructure of MoS2/CuS nanohybrid catalyst exhibits great potential for renewable energy applications. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The rapid depletion of traditional fuels and the increased demand for renewable energy sources has motivated researchers to explore options for clean, green and low-cost

energy storage devices [1,2]. The main source of fuel cell will be H2 or other renewable energy in the future, this will be the choice among the available fuel cells as it emits water as the combustion product. Water electrolysis proceeds via the following two half-cell reactions: reduction of hydrogen ions

* Corresponding author. E-mail address: [email protected] (W. Qin). https://doi.org/10.1016/j.ijhydene.2017.09.184 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang L, et al., One-step synthesis of the 3D flower-like heterostructure MoS2/CuS nanohybrid for electrocatalytic hydrogen evolution, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.184

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at the cathode (2Hþ(aq) þ 2e
/ H2(g)), i.e., the hydrogen evolution reaction (HER) [3e5], and oxidation of water (2H2O(l) / O2(g) þ 4Hþ(aq) þ 4e
), i.e., the oxygen evolution reaction (OER) [6e11]. For catalyzing HER, the best-known catalysts are noble-metals such as Pt, which is of high cost and insufficient and thereby seriously limited their practical applications, therefore it is continuously increasing interest in seeking alternative catalysts for applications in the HER. Among all the potential alternative transition metal compounds, such as MoS2 [12,13], CoS2 [14], and NiS2 [15], have demonstrated considerable electrocatalytic activity and high stability. In particular, the transition metal chalcogenide (TMC) has been widely studied as a catalyst or carrier for HER due to its low cost, high chemical stability and high electrocatalytic performance [16,17]. As a typical TMC, MoS2 possesses outstanding thermal stability and superior electrocatalytic activity, which enables its wide applications such as biosensors, electrocatalysts, supercapacitors, and energy storage devices. As compared from graphene, MoS2 is a semiconductor, so there is a transition from the indirect bandgap of the bulk to the direct gap of the monolayer: 1.2 eV of the indirect gap increases to a single layer of 1.8 eV direct band gap [18e20]. The composite of MoS2 nanosheets with metal sulfides is an ideal approach to overcome the weaknesses and optimizes the performances of MoS2 for catalysis [21]. Hence, MoS2 have an important role as an auxiliary material for improving the main catalysts, for HER. Besides, CuS as a semiconductor shows omnifarious applications including photocatalytic hydrogen production, anode material for lithium ion battery, counter electrode for quantum dots sensitized solar cells and supercapacitor electrode materials [22e25]. CuS possesses a layered crystal structure with weak Vander Waals interactions between individual plane Cu2S2 double layers [26]. Because of its aeolotropic structure, CuS can provide penetrable channels for ion adsorption and transport [27]. Meanwhile, the sulfur possesses empty 3p-orbitals and large amounts of electrons in the structure, CuS possesses a stronger tendency to capture electrons and promotes electrons with electron donor molecules at lower initial reduction potential and higher current transfer reaction [28]. These properties make CuS suitable for HER catalysis, and their catalytic activity towards the HER have been rarely investigated. The excellent catalytic activity of metallic MoS2 for the HER has contributed to substantial efforts towards increasing demand for renewable energy sources. However, the common MoS2 is less active for the HER because it has low conduction and possesses less efficient charge transfer kinetics. The CuS nanoparticles possess a better conducting and can transfer more electrons. Therefore the common MoS2 nanosheets were modified by the CuS nanoparticles. In this work, we reported a one-step hydrothermal synthesis of the 2D of MoS2 nanosheets; 3D spherical CuS nanoparticles; and 3D flowerlike heterostructure of MoS2/CuS nanohybrid, respectively. The experimental conditions are simple and controllable, and the related reagents are obtained easily. The morphology and structure of the hybrid material were studied by various techniques. In addition, the MoS2/CuS nanohybrid was firstly employed for HER applications. The HER performance of MoS2, CuS and MoS2/CuS were also investigated by the electrochemical methods.

Experimental methods Catalyst preparation (MoS2/CuS nanohybrids) In a typical synthesis, 0.23 g of Na2MoO4$2H2O, 0.3 g of thiourea, 0.38 g of CuCl2 and 0.09 g ascorbic acid sodium were added in 25 mL of deionized water, and the mixed solution was drastically magnetized stirrer for 2 h. The obtained homogeneous solution was transferred into a 25 mL Teflon-lined stainless-steel autoclave. The sealed vessel was then heated at 200  C for a desired time 24 h before it was cooled to room temperature. The products were precipitated by water and ethanol (1:1), collected via centrifugation at 8000 rpm for 5 min, and further washed four times. The product was dried on vacuum for 60  C. The synthesis of MoS2 nanosheets and CuS nanoparticles and additional data see Supporting Information.

Electrochemical tests All electrochemical measurements were conducted on the CHI660D electrochemical workstation (CH Instruments, Shanghai, China) in a three-electrode cell at room temperature. Graphite electrode and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. The working electrode was prepared on a glass carbon. Typically, a mixture containing 5.0 mg of catalyst, 2.5 mL of ethanol and 0.5 mL of Nafion solution (0.05 wt%, Gashub) was sonicated for 15 min to obtain a well dispersed ink. Then, 50 ml of the catalyst ink (containing 33.3 g of the catalyst) was loaded onto a glassy carbon electrode having a diameter of 3 mm (load ~ 0.471 mg cm
2). 0.5 M H2SO4 aqueous solution was used as the electrolyte, which was deaerated with highpurity argon prior to and throughout all the measurements. The linear sweep voltammetry (LSV) was carried out under scan rate of 2 mV s
1. Electrochemical impedance spectroscopy (EIS) experiments were performed with frequencies ranging from 100 kHz to 0.01 Hz.

Instrument The morphology and microstructure of samples were analyzed by transmission electron microscopy and (TEM) on a JEOL JEM-2100F electron microscope operating at 200 kV. X-ray diffraction (XRD) data were collected on a Model D/max-r C Xray diffract meter, operating at 40 kV and 100 mA, using CuKa radiation source (1.5406  A). Field emission scanning electron microscopy (FE-SEM) was carried out on a Hitachi S-4800. Xray photoelectron spectroscopy (XPS) was conducted on a Thermo VG Scientific ESCALAB 250 spectrometer with an Al Ka radiator. Energy dispersive X-ray spectroscopy (EDX) was collected on a JEM-2100 instrument.

Results and discussion Structure characterization The morphology and features of the as prepared MoS2 nanosheets and CuS nanoparticles were investigated through TEM and SEM characterization methods at different

Please cite this article in press as: Zhang L, et al., One-step synthesis of the 3D flower-like heterostructure MoS2/CuS nanohybrid for electrocatalytic hydrogen evolution, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.184

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magnifications. From SEM images of Fig. 1a, it can be observed that the as-prepared MoS2 is composed of uniform nanosheets with lengths of 2e3 mm; the morphology of MoS2 is similar to the 2D graphene layer. In Fig. 1b, the surface of MoS2 is smoothly; moreover, the plate structure is grown in a high density way with 10e20 layers and has an interlayer separation of 0.65 nm. The microscopic structure and morphology of the pure CuS nanoparticles were demonstrated by typical SEM and highresolution TEM (HR-TEM) in Fig. 1c and d. Fig. 1c shows the as prepared CuS nanoparticles consist of a 3D sphere. The microscopic structure of CuS nanoparticles shows two types of specific crystalline structure in Fig. 1d. The HRTEM image of a selected area of the CuS nanoparticles shows that the interplanar spacing at about 0.191 nm and 0.280 nm correspond to the (110) and (103) lattice planes of the CuS nanoparticles (0.20 nm and 0.275 nm) [29], respectively. The morphology of the as prepared MoS2/CuS nanohybrid is different with bare MoS2 nanosheets and CuS nanoparticles. Fig. 2aec shows typical SEM images of 3D MoS2/CuS nanohybrid at different magnifications, in which most of the MoS2 nanosheets were uniformly covered by CuS nanoparticles. As exhibited in Fig. 2a and b, each MoS2 nanosheets shows an average diameter of 1e2 mm. A large number of CuS nanoparticles were directly grown on the MoS2 nanosheets, forming the 3D flower-like heterostructure (Fig. 2c). In addition, the MoS2/CuS was further studied by TEM analysis, which interconnected (Fig. 2d and e) the MoS2 nanoflakes and CuS nanoparticles. Fig. 2d shows the TEM of images of MoS2/CuS. First, the MoS2 nanosheets transformed into nanoflowers and the layer number of nanoflowers is about 3e7. Plus, the CuS nanoparticles of the micromorphologies are changed into uniform nanoparticles. Finally, the MoS2 and CuS impact on each other to form a homogeneous system. Compared with

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the morphology of pure MoS2 nanosheets and CuS nanoparticles, the MoS2/CuS nanohybrid changes obviously. The MoS2/CuS nanohybrids are further analyzed by highresolution TEM (Fig. 2e). The lattice fringes with spacing of 0.196, 0.280 and 0.62 nm in the interconnected regions can be attributed to (110), (103) of face-centered cubic (fcc) CuS and (002) of MoS2, respectively. The selected-area electron diffraction patterns (SAED) (Fig. 2f) indicated two-component crystalline characteristics. Six set of spots indicating the high crystallinity and undoubtedly attributes to MoS2, and other set of rings were attributed to the CuS nanoparticles, arising from the stacking of MoS2 nanosheets with different crystallographic orientations. The elemental composition of the MoS2/CuS nanohybrid is characterized by elemental mapping and presented in Fig. 3def thought Fig. 3a. The (EDS) element mapping clearly shows that the elements of C, N, Mo, Cu and S are evenly distributed in the MoS2/CuS nanohybrid. It can be seen from the (EDS) element mapping results that the signals of all three elements (Cu, Mo, and S) are detected clearly. Fig. 3g shows the XRD patterns of MoS2, MoS2/CuS and CuS. The black curve of MoS2 can well point to the hexagonal phase of MoS2 (JCPDS No. 37-1492) with lattice constant a ¼ b ¼ 3.16, c ¼ 12.28. The observed diffraction peaks near 15.8 , 33 and 57.8 can be well assigned to the (002), (100) and (110) planes of the MoS2 hexagonal phase, respectively. No other impurity peaks were found in the spectra, indicating that the original MoS2 had high purity and crystal structure. MoS2/CuS heterozygous hybridization patterns show that in the red curve, two different lattice planes correspond to MoS2 and CuS, indicating that the composite structure is composed of MoS2 and CuS. However, it can be seen that the peak offsets near 14 , 37 and 56.7 of MoS2 are shifted 1e2 . The reason of synthesis MoS2/

Fig. 1 e Morphological and elemental characterizations: (a, b) SEM and TEM images of pristine MoS2 nanoplates. (c, d) SEM and HR-TEM images of CuS nanoparticles. Please cite this article in press as: Zhang L, et al., One-step synthesis of the 3D flower-like heterostructure MoS2/CuS nanohybrid for electrocatalytic hydrogen evolution, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.184

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Fig. 2 e (a), (b) and (c) are low- and high-magnification SEM images of MoS2/CuS nanohybrid, respectively. (d) and (e) TEM pattern of a MoS2/CuS nanohybrid. (f) Low-magnification TEM image of the pristine MoS2/CuS.

CuS is that CuS changed the crystal of MoS2 via the electrostatic attraction [30]. The blue curve of CuS can well point to the hexagonal phase of CuS (JCPDS No. 06-0464). The peak before 25 is the impurity peak for the CuS nanoparticles. X-ray photoelectron spectroscopy (XPS) measurements were used to study the oxidation states of the MoS2/CuS nanohybrids. It was observed that two typical peaks arising from 228.0 to 232.2 eV are attributed to the Mo 3d5/2 and 3d3/2 binding energies for a Mo (IV) oxidation state (Fig. 4a). The corresponding peaks for the S 2p3/2 and 2p1/2 orbitals of bivalent sulfide ions (S2
) are observed at 161.1 and 164.2 eV, respectively (Fig. 4b) [31]. For the Cu spectra, it shows that the peaks corresponding to Cu 2p are identified, suggesting that uniform bonding states are established. The binding energies of Cu 2p3/2 and Cu 2p1/2 peaks are 932.1 and 952.2 eV (Fig. 2c), respectively. In addition to this, the symmetrical shapes of the two Cu 2p XPS peaks and the “shakeup” satellite peaks in the higher binding-energy ranges imply the presence of pure CuS [32]. The XPS spectra of MoS2/CuS nanohybrid are presented in Fig. 4d. The characteristic peaks of C 1s and O 1s showed the bands were located at 284.6 and 533.2 eV, respectively. The peaks of Mo 3d, S 2p, Cu 2p are accorded with Fig. 4aec. Meanwhile, the FTIR spectra were characterized to have an insight into the structural of the MoS2, CuS, MoS2/CuS (see Supporting Information S1). For MoS2, CuS in the curve of blue and red line, the peak located at 3460 and 3458 cm
1 can be ascribed to the characteristic absorption bands of the eNH2, the peaks at 1622 and 1652 cm
1 can be attributed to symmetric stretching vibrations of carbonyl (C]O), respectively

[33,34]. For the MoS2 the peak located at 1420 cm
1 can be ascribed to the characteristic absorption bands of the C]S. Relative to the spectrum for the MoS2, the MoS2/CuS peak of 1631 cm
1 (the curve of black line) has changed slightly, and new peaks appeared at 2038 and 2005 cm
1 for MoS2/CuS. Those new peaks are attributed to stretching vibration of linear complexes in the MoS2/CuS structure. Furthermore, all the characteristic peaks of MoS2 and CuS are observed in the MoS2/CuS hybrid materials, indicating that MoS2 is successfully combined with CuS NPs. It is obvious that the catalytic performance of catalyst highly relates with its surface area. The active site is closely related to the catalytic properties of the catalyst. The catalyst with large surface area has more active sites, so the catalyst with good surface area is better. The surface areas of the title chalcogels were measured using the Brunauer
Emmett
Teller (BET) [35] model and N2 physisorption at 323 K. The pristine MoS2 showed an II-type isotherm with a small BET surface area of 2.35 m2 g
1, which is characteristic of a nonporous material (Fig. 5a). By comparison, the CuS nanoparticles showed an IV-type isotherm indicative of smallporosity with a small BET surface area of 6.66 m2 g
1 (Fig. 5b). The MoS2/CuS shows an IV-type isotherm similar to that of CuS nanoparticles (Fig. 5c). Additionally, the BET surface area of MoS2/CuS nanohybrid is 53.41 m2 g
1, which is bigger than that of MoS2 nanosheets (2.35 m2 g
1) and CuS nanoparticles (6.66 m2 g
1). This reveals that MoS2 nanosheets and CuS nanoparticles are interacted to increase the surface area of the nanocomposite [36]. The MoS2/CuS (53.41 m2 g
1)

Please cite this article in press as: Zhang L, et al., One-step synthesis of the 3D flower-like heterostructure MoS2/CuS nanohybrid for electrocatalytic hydrogen evolution, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.184

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Fig. 3 e (bef) EDS element mapping data of carbon, nitrogen, molybdenum, copper and sulfur elements throughout the MoS2/CuS nanohybrid (a). (g) XRD pattern of MoS2 nanoplates, MoS2/CuS nanohybrids, and CuS nanoparticles.

nanohybrid possess a bigger surface area than MoS2 (2.35 m2 g
1) nanosheets, and CuS (6.66 m2 g
1) nanoparticles, therefore, the MoS2/CuS nanohybrid possesses a big catalytic surface area. The reason is that the CuS nanoparticles are covered the MoS2 nanosheets of surface and their structure are influenced by each other.

Electrochemical properties Cyclic voltammetry (CV) experiments are carried out for the as-prepared nanomaterials and compared with Pt/C, which is the most efficient catalyst known for HER. All experiments are performed under identical conditions, using 0.5 M H2SO4 as the electrolyte. Fig. 5a shows the cyclic voltammetry (CV) curves of the MoS2, CuS and MoS2/CuS. The CV of MoS2/CuS nanohybrid shows a suppressed onset of hydrogen

adsorption and early onset of surface oxidation compared with that of MoS2 nanosheets and CuS nanoparticles. However, desorption peaks show an opposite trend where MoS2/ CuS shows higher desorption energy, as compared to CuS and MoS2. The HER activity might be uncertainty, because of the lack of use of H2 atmosphere. However, the results should still be in-house comparable, especially at considerable current densities, since all of the as-synthesized catalysts were tested under the same conditions. The cathode current density (j) is considered a critical evaluation criterion for HER activity [37]. The HER electrocatalytic performance of the MoS2, CuS and MoS2/CuS were evaluated using the LSV at a scan rate of between
0.75 and 0 V vs. RHE 2 mVs
1 (RHE ¼ E þ 0.224 V þ 0.059lgpH). From the before and after IRcorrected polarization curves (Fig. 5b), the CuS nanoparticles are inactive in H2SO4 solution. The CuS nanoparticles

Please cite this article in press as: Zhang L, et al., One-step synthesis of the 3D flower-like heterostructure MoS2/CuS nanohybrid for electrocatalytic hydrogen evolution, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.184

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a

3d5/2

220 222 224 226 228 230 232 234 236 238 240 242

shakeup

4+

952.2ev Cu 2p1/2

Intensity/a.u

Intensity/a.u

Mo

MoS2

932.2ev Cu 2p3/2

b

3d3/2

925

930

935

940

950

156 158 160 162 164 166 168 170 172 174 B.E.(eV)

960

S 2p1 Mo O2

O 1s

C 1s

d

S p 1/2

955

Cu 2O Cu 2p1

S 2p 3/2

Intensity/a.u

Intensity/a.u

c

945

B.E.(eV)

B.E.(eV)

0

200

400

600

800

1000

B.E.(eV)

Fig. 4 e (aec) High-resolution XPS spectra of Mo 3d, Cu 2p and S 2p in MoS2/CuS nanohybrids. (d) The XPS full spectrum of MoS2/CuS. exhibited a big overpotential (0.35 V), and the current density is less than 7.5 mA/cm2. The MoS2 nanosheets show inferior HER electrocatalytic activity, with onset potentials of 0.25 V, and catalytic currents densities are less than 10 mA/cm2 at


0.25 V vs RHE. In contrast, the MoS2/CuS nanohybrid shows a better current density (over 35 mA/cm2) than that of MoS2 (9.6 mA/cm2) and CuS (7.3 mA/cm2). The MoS2/CuS nanohybrid exhibits a small overpotential (0.15 V) compared to the

Fig. 5 e Nitrogen adsorption isotherms of the (a) MoS2/CuS, (b) MoS2 and (c) CuS samples, (insert); pore width distribution curves of the samples. Please cite this article in press as: Zhang L, et al., One-step synthesis of the 3D flower-like heterostructure MoS2/CuS nanohybrid for electrocatalytic hydrogen evolution, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.184

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6

MoS2

4

4

CuS

0

2

0 2.

0 .3

0 .4

-2 Current density (j/mA.cm )

MoS2/CuS

Adso ption

Current Density (j/ mA cm )

-2

b

8

0 .5

2 0 0.8

-2

0.6 0.4

Desoption

a

0.2 0.0

-4

-0.2 -0.4 -0.6

-6 -8

-0.8 0 1. 0

-0.4

-0.2

0.0

0.2

0.1 1

0.12

0.4

0 .13

0.1 4

0.15

0.6

-5 -10

-20

CuS iR before CuS iR after Pt/C(20%) iR before Pt/C(20%) iR after MoS2 iR before

-25

MoS2/CuS iR before

-15

MoS2 iR after MoS2/CuS iR after

-30

0.8

-0.7

Potential (V vs RHE)

c 0.35

-0.5 -0.4 -0.3 Potential (V vs.RHE)

-0.2

-0.1

0.0

350

185 mV/dec

300

0.25

MoS 2/CuS

250

165 mV/dec

0.20

Z /(Ω)

Overpotential (V vs RHE)

-0.6

d

0.30

0.15

MoS 2

200

Pt/C( 20%) CuS

150

0.10 0.05 0.00 -0.6

-0.1V

-0.29 V

-0.4

63 mV/dec

100

30 mV/dec

50

-0.2

0.0

0.2

0.4

0

0.6

0

100

200

e

4.5

500

60 0

MoS2

3.5 -2

40 0

MoS2/CuS

4.0

j=j2-j1 (j/mA.cm )

300 Z'/(Ω Ζ

-2

Log(j /Am.cm )

CuS

3.0 2.5 2.0 1.5 1.0 0.5 0

50

100

150

200

250

-1

Scan Rate (mV.s ) Fig. 6 e HER electrocatalytic performances of MoS2 nanosheets, CuS nanoparticles and MoS2/CuS nanohybrid were calculated in 0.5 M H2SO4 aqueous solution. Pt/C (20%) was used for comparison. (a) Cyclic voltammetry (CV) curves with a scan rate of 50 mV/s (insert); (b) linear sweep voltammetry (LSV) before and after iR-corrected curves; (c) Tafel plots; (d) electrochemical impedance spectroscopy (EIS). (e) Plots showing the double-layer capacitance (Cdl) for MoS2/CuS, MoS2 and CuS electrodes.

MoS2 (0.25 V) and CuS (0.35 V). The current density of commercial Pt/C catalyst is over 30 mA/cm2 under a small overpotential (0.05 V). It is very important that the activity of HER is usually evaluated by overpotential at 10 mA/cm
2. The overpotential of MoS2/CuS nanohybrid (
0.29 V) is approximate to the commercial Pt/C (
0.1 V) for the current density at 10 mA/cm2. Furthermore, the MoS2/CuS is low cost and can

be prepared by simple one-step synthesis. The Tafel slope is an inherent property of electrocatalyst, and a small Tafel slope leads to a strongly enhanced HER rate at a soft increase of overpotential [38]. To gain further insight into the MoS2/ CuS electrode, we investigated the Tafel plots for MoS2, CuS, MoS2/CuS, and Pt/C (Fig. 6c). The resulting Tafel slope of MoS2/CuS is 63 mV/dec, which is much lower than that of

Please cite this article in press as: Zhang L, et al., One-step synthesis of the 3D flower-like heterostructure MoS2/CuS nanohybrid for electrocatalytic hydrogen evolution, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.184

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Scheme 1 e Schematic representation of the HER on the MoS2/CuS nanohybrid catalyst.

MoS2 (165 mV/dec), CuS (185 mV/dec) and close to the value of Pt/C (30 mV/dec). The electrochemical impedances of the CuS nanoparticles, MoS2 nanosheets and MoS2/CuS nanohybrid were measured by the EIS measurement at a small voltage of 200 mV to provide further insight into the electrode kinetic under HER process. Fig. 6d shows that a small semicircle in EIS plot observed on the MoS2/CuS electrode, indicating the charge transfer resistance of the MoS2/CuS electrode is much smaller than that of Pt/C, MoS2 and CuS electrode. This can be attributed to the higher conductivity of MoS2/CuS than that MoS2 [39].

Cureent density (j/mA/cm )

0

The EIS data are always fitted using an equivalent circuit to gain more details on the HER process. The equivalent circuit contains Rct in series with Warburg impedance (ZD) and parallel with constant phase element (CPE). In this equivalent circuit, Re is the solution resistance, Rct is the resistance of deposited layer, CPE is the constant phase element of the electric double layer and Rct is the charge transfer resistance for the HER of the GC interface. The results illustrate that MoS2 or CuS has more Rct of about 200 or 530 W. In case of the composite of MoS2/CuS, the Rct is decreased, the Rct has been decreased from 200 W to 45 W. This is because of the increment in the number of catalytic sites of MoS2/CuS and better transportation of electrons from CuS to MoS2. Furthermore, the electrochemical active surface areas of the composites were measured by the electrochemical double-layer capacitance (Cdl). The CVs were taken out at different scan rates (0.02, 0.04, 0.06, 0.08, 0.14, 0.18 and 0.22 mV/s) in the area of faradaic potentials (0e0.3 V vs. RHE) (see Fig. S2), and the capacitive currents (DJ ¼ J2eJ1) were plotted against the scan rates where the slope was Cdl [40]. Fig. 6e shows the calculated Cdl values is 14.4 mF cm
2, 6.91 mF cm
2 and 0.437 mF cm
2 for MoS2/CuS, MoS2, and CuS, respectively. The bigger Cdl values

a initial curve after 1000 cycles

-5 -10 -15 -20 -25 -30 -35

-0.4

-0.3

-0.2

-0.1

0.0

Potential(V vs RHE)

2 Currtent density (j mA/cm )

-2

c

-4 -6 -8 -10 -12 -14 0

50

100

150

200

250

300

350

400

Time min

Fig. 7 e (a) Durability test for MoS2/CuS in 0.5 mol L¡1 H2SO4 for 1000 cycles. (b) The SEM image of MoS2/CuS after 1000 cycles. (c) Time-dependent current density curve for the MoS2/CuS nanohybrids at a static overpotential of 290 mV for 400 min. Please cite this article in press as: Zhang L, et al., One-step synthesis of the 3D flower-like heterostructure MoS2/CuS nanohybrid for electrocatalytic hydrogen evolution, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.184

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0

indicate that the MoS2/CuS samples have a bigger exposure of active edge sites and thus improved the HER activity.

Potential mechanism for the catalytic reaction The common MoS2 is less active for the HER because it has poor conduction and possesses less efficient charge transfer kinetics [41]. CuS possesses a better conducting and can transfer electrons more effectively [36]. Hence, we suspected a possible pathway for the HER catalyzed by MoS2/CuS, which is shown in Scheme 1. Firstly, the electrons have priority to approach CuS. Because of CuS aeolotropic structure, CuS can provide permeable channels for ion adsorption and transport [27]. At the same time, CuS has stronger tendency to capture the trend of electrons due to the presence of abundant electron holes in their empty 3p orbital in structures for sulfur, and MoS2 has a low initial reduction potential and high current electron donor molecules to promote the electron transfer reaction [28]. Then, the electrons were transferred to the MoS2 via the CuS and used for the HER. It is known that both Mo and S possess the capacity to accept electrons and protons [42]. Lastly, the hydrogen ions were reduced by the MoS2. According to the classical theory on the mechanism of hydrogen evolution [43], hydrogen production occurs via a fast discharge reaction (1), then a rate-determining ion þ atom reaction (2) and combination reaction (3).

9

The reusability and recyclability of MoS2/CuS nanohybrid The reusability and recyclability are crucial issues for a catalyst to be used for practical applications; therefore, the reusability of MoS2/CuS nanohybrids is investigated. Fig. 7a shows that the overpotential has no change after 1000 cycles, besides, the current density only reduced by 5 mA/cm2. The SEM morphology of the electrode is preserved after 1000 cyclic voltammograms (Fig. 7b), and it is similar to Fig. 2b. Fig. 7c shows that the MoS2/CuS nanohybrids is stable at a current density of 10 mA/cm2 and retained 89% of current density after 400 min.

Conclusion We have synthesized the MoS2 nanosheets, CuS nanoparticles and MoS2/CuS nanohybrid via a facile one-pot solvothermal approach and low-cost material. With high active sites and superior electrochemical activity, the MoS2/CuS nanohybrid catalyst exhibited better HER activity with a smaller overpotential of ~0.15 V, larger cathodic currents, and smaller Tafel slope of 63 mV/dec than that of MoS2 nanosheets and CuS nanoparticles. The BET surface area of MoS2/CuS nanohybrid was 53.41 m2/g, which is bigger than that of MoS2 and CuS. The MoS2/CuS hybrid catalyst possesses a bigger catalytic surface area than the MoS2 and CuS. Thus, the approach of CuS synthesis on MoS2 nanosheets has led to an advanced MoS2 electrocatalyst with better catalytic performance relative to common MoS2 HER electrocatalytic materials.

discharge reaction: H3Oþ þ e þ MoS2 4 MoS2
H þ H2O

(1)

ion þ atom reaction: H3Oþ þ e þ MoS2
H 4 MoS2 þ H2 þ H2O

(2)

Acknowledgments

(3)

Funding: This work was financially supported by the Natural Science Foundation of China (No. 21771092).

combination reaction: MoS2
H þ MoS2
H 4 2MoS2 þ H2

A Tafel slope of 63 mV per decade indicates a larger surface coverage of adsorbed hydrogen and a rate-determining recombination (Eq. (3)) or ion þ atom reaction. A Tafel slope of 165 mV or 185 mV could arise from various reaction pathways depending on the surface coverage [44]. For MoS2 nanosheets and CuS nanoparticles, the active sites are proposed to be on the edges which have coordinatively unsaturated Mo, S or Cu atoms [45]. The MoS2/CuS nanohybrids described may have more such unsaturated sites thanks to their amorphous nature. We suspect that this is the reason why the 3D nanohybrids are more active than the single crystals and nanoparticles. More work is required to shed light on the mechanism of H2 evolution at the nanomaterial. Given that both Mo and S are capable of accepting electrons and protons, a ‘bifunctional’, bimetallic-sulfide cooperative mode of catalysis is probable [42]. The good electron-transport properties of the CuS nanoparticles cooperated well with the MoS2 nanosheets for the HER. A large number of CuS nanoparticles were directly grown on the MoS2 nanosheets, forming the 3D flower-like heterostructure. The 3D flower-like heterostructure possesses a big catalytic surface area. Hence the HER activity of MoS2/CuS nanohybrid is better than the MoS2 nanosheets and CuS nanoparticles.

Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2017.09.184.

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Please cite this article in press as: Zhang L, et al., One-step synthesis of the 3D flower-like heterostructure MoS2/CuS nanohybrid for electrocatalytic hydrogen evolution, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.184