Molybdenum Disulfide Nanoflakes Covered Carbonized Catkin

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Molybdenum disulfide nanoflakes covered carbonized catkin microtube hybrids as superior catalysts for electrochemical hydrogen evolution Shanshan Tong, Shenghua Ma, Yingchun Su, Qingchuan Li, Xuejing Wang, and Xiaojun Han ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.7b04751 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 22, 2018

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ACS Sustainable Chemistry & Engineering

Molybdenum

disulfide

nanoflakes

covered

carbonized catkin microtube hybrids as superior catalysts for electrochemical hydrogen evolution Shanshan Tong,†,a, b Shenghua Ma,†,c Yingchun Su,b Qingchuan Li,b Xuejing Wang,b and Xiaojun Han*,b a

(Shaanxi Key Laboratory of Natural Products & Chemical Biology, College of Chemistry and

Pharmacy, Northwest A&F University, Yangling, Shaanxi 712100, P. R. China) b

(State Key Laboratory of Urban Water Resource and Environment, School of Chemistry and

Chemical Engineering, Harbin Institute of Technology, Harbin 150001, P. R. China) c

(Institute of Photonics & Photon Technology, Northwest University, Xi'an 710069, P. R.

China) † S.S. Tong and S.H. Ma. contributed equally to this work. * Corresponding author. E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Synergistic effect of non-noble-metal, low-cost and high-performance hybrid electrocatalysts was often used for hydrogen generation (HER). Herein, the molybdenum disulfide nanoflakes were in-suit grown on carbon microtube derived from natural tubular willow catkins as templates to construct the three-dimensional Catkins/MoS2 hybrids. Catkins/MoS2 hybrids exhibited excellent HER activity in acidic condition with a remarkable small onset overpotential of 50 mV, a Tafel slope of 62 mV dec−1, a comparatively small overpotential of 136 mV required to obtain 10 mA cm−2, and a Faraday yield of ~100%. The as-synthesized catalysts also presented impressive durability and long-term stability. The superior electrocatalytic activity of Catkins/MoS2 hybrids were resulted from the unique hybrid structure of the catkins microtube and MoS2 nanoflakes providing abundant catalytically active sites, and the annealed-catkins providing the good conductive supports. The hybrids may find potential in practical hydrogen generation.

KEYWORDS: Carbon microtube, Catkin, Molybdenum disulfide, Hydrogen evolution reaction, Electrocatalysis


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INTRODUCTION The increasing global concerns about energy consumption and environmental quality have attracted wide attention in developing new technologies for renewable energy sources.1 The widespread use of hydrogen can reduce the consumption of fossil fuels and lighten the pressure of environment pollution, because it is a clean and sustainable fuel.2 The splitting of water for H2 production has drawn great attentions.3 The hydrogen evolution reaction (HER) is a basic process in electrochemically generated hydrogen, where hydrogen ions are electroreduced into large amount of molecular hydrogen.4 However, due to the existence of a large overpotential for the HER, an efficient electrocatalyst is usually required. The most effective catalyst is well known to be Pt and Pt-based alloys, which have rather low overpotential (close to zero) and a very small value of Tafel slope as well as an extremely high exchange current density, but these catalysts are unfortunately limited by their scarcity and expensive price. Consequently, catalysts without noble metal were developed by tremendous novel materials, such as, metal oxides,5,6 alloys,7,8 carbon materials,9 metal phosphide nanoparticles,10-12 metal-organic frameworks,13 and transition metal dichalcogenides.14-16 Molybdenum disulfide (MoS2) nanocomposites have been reported to be a promising electrocatalyst in HER.17,18 Experiments and theoretical calculations suggest that MoS2 has potential to be a candidate to replace Pt in electrochemical hydrogen evolution.19,20 In general, the density of the exposed active edge sites is positively correlative with HER activity.21 So, it is an effective method increasing the amount of active sites of MoS2 catalysts to enhance activity.20,22 Nanosized MoS2 is regarded as an effective electrocatalyst to generate hydrogen, but its poor bulk conduction and anisotropic electrical transport limit its catalytic activity.23 These disadvantages could be overcome by combining single-layer MoS2-based catalysts with


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conductive and easily percolating materials.24,25 The HER performance of MoS2 was improved by the combination with carbon materials.26-28 Therefore, it is important to choose the partner material to improve the electrocatalytic performance. Carbon materials have attracted great attentions because of their tunable pore structure, high surface area, superior conductivities, and chemical stabilities. Among various precursors of carbon materials, biomass becomes an attractive candidate, because of its large specific surface area, hierarchical porous architecture (i.e., micropores and mesopores), and high coverage of surface heteroatom (such as O, N, P, etc.) content.29 Compared with the artificially synthesized carbon materials, biomass-based ones are available in a cheap, environmentally friendly manner, and show more attractive performances.30 By typical carbonizations, many biomass materials served as both template and precursor, for example silk cocoon and cotton, could be directly carbonized into carbon microfibers. The most tubular carbon materials are fabricated through template31 and chemical vapor deposition methods.32 The willow catkins as a kind of common biomass have a tubular structure and they are ideal biomass for carbon microtubes. In addition, the amino acid in willow catkins provides the nitrogen source for doping nitrogen element into carbon microtubes. Willow catkins are considered as a waste and a kind source of air pollution.33 Taken together, willow catkins are good candidates as carbon material precursors. There is no report on HER catalysts based on the combination of carbonized willow catkin and MoS2 nanoflakes. Herein, we synthesized the novel nanoflake-on-tubular microstructure containing ultrathin MoS2 nanoflakes on carbonized catkin microtubes, named as Catkins/MoS2 hybrids. As expected, excellent HER catalytic performance was realized due to the combination of


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synergistic effect from the hybrid structure and good conductive supports. The electrocatalysts may find real application for energy conversion and hydrogen production.

EXPERIMENTAL SECTION Materials. Sodium molybdate (Na2MoO4), thiourea (CH4N2S), Pt/C (10 wt%) and 5 wt% Nafion solution were purchased from Sigma-Aldrich. Sodium oleate (>97%) and oleic acid were obtained from Aladdin Co. (Shanghai, China). Sulfuric acid (H2SO4) and absolute ethanol were gained from Beijing Chemical Reagent Co. Ltd (Beijing, China). All the solutions are prepared using pure water (18.2 MΩ) which was purified through a Millipore system. Preparation of Catkins microtubes. Catkins were collected in the campus of Harbin Institute of Technology, Heilongjiang Province, China. A certain amount of catkins were washed many times with deionized water in order to remove the seed and other impurities, and then were soaked in HNO3 (2.0 mol L−1) for 24 h. The obtained materials were washed for 5 times using pure water. After drying at 50 ○C over night, the catkins were carbonized in a horizontal tube furnace at 500 ○C, 700 ○C, and 900 ○C for 4 h. The carbonized processes were performed under nitrogen atmosphere and the heating rate was 3 ○C min−1. The obtained carbonized catkins were named Catkins-500, Catkins-700, and Catkins-900 according to the heating temperature, respectively. Preparation of Catkins/MoS2 hybrids. The above as-obtained carbonized catkins (40 mg) were immersed in the mixed solution (pH = 0.8) containing Na2MoO4 (0.5 g), thiourea (0.45 g), sodium oleate (0.5 g), oleic acid (0.5 mL), pure water (7.5 mL), and ethanol (7.5 mL). Afterwards the mixture was transferred into a hydrothermal reactor at 200 ○C for 24 h. The hydrothermal reactor was Telfon-lined stainless-steel autoclave. After naturally cooling down to


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25 ○C, water and absolute ethanol were used to wash the resulting sample. Then the product was dried in a vacuum oven at 60 ○C for 4 h. Preparation of pure MoS2 nanoflakes. MoS2 nanoflakes were made by a modified method of the previous report.34 Na2MoO4 (0.5 g) and thiourea (0.45 g) were dissolved in 15 mL mixture solution of water and ethanol (volume ratio 1: 1). Sodium oleate (0.5 g) and oleic acid (0.5 mL) were added to the above mixture with 15 min of sonication to disperse the obtained solution. The pH of resulting mixture was adjusted to about 0.8 by using 37 % wt HCl aqueous solution, which was transferred into an autoclave at 200 oC for 24 h. The water and ethanol were used to wash the sample before drying under vacuum at 60 ○C oven overnight. Electrochemical measurements. Autolab electrochemical workstation (PGSTAT320N, Switzerland) was used to evaluate the performance of the samples in a standard three-electrode electrochemical cell. The electrolyte solution was 0.5 M H2SO4. A glassy carbon electrode (4 mm in diameter), a platinum wire and a saturated calomel electrode (SCE) were regarded as working electrode, counter electrode and reference electrode, respectively. Sample (4 mg) was dispersed into a mixture of ethanol and water (3:7, v/v) solution (1 mL). After sonication for 20 min, the dispersion (10 µL) was dropped onto the freshly polished glassy carbon electrode. The amount of catalyst loading was 0.318 mg cm−2. 5 µL of Nafion solution was dropped onto the glassy carbon electrode after catalyst-modification and dried in air. For comparison, measurements were performed with a commercial Pt-loaded carbon catalyst (20 wt% Pt/C). A reversible hydrogen electrode (RHE) was used as a reference to calibrate all the potentials. In the applied voltage range from 0.2 V to −0.40 V, the polarization curves were achieved from linear sweep voltammetry (LSV) measurements. The scan rate was 5 mV s−1. Cyclic


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voltammetry (CV) was conducted between −0.2 V and 0.2 V at 50 mV s−1 for 1000 cycles. The beginning overpotential of linear region in the Tafel plot was the onset overpotential. Electrochemical impedance spectrum (EIS) was also measured from 100 kHz to 0.1 Hz with amplitude of −150 mV versus RHE. Characterizations. JEOL JEM-1230 transmission electron microscope (TEM, 80 kV) and Hitachi S-4800 field emission scanning electron microscope (FESEM) was used to characterize the size and morphology of the materials. High-resolution TEM (HRTEM) and electron diffraction (ED) pattern were performed on FEI TF-20 at 200 kV. A Thermo ESCALAB 250 spectrometer was used to measure X-ray photoelectron spectroscopy (XPS) at 15 kV and 17 mA. A Bruker D8 ADVANCE X-ray powder diffractometer (XRD) was used to examine the phase structure from 10○ to 70○. A DSC-TGA thermogravimetric analyzer (Q1000DSC+LNCS+FACS Q600SDT, TA Instruments, USA) was used for the thermogravimetric analysis (TGA) from room temperature to 1000 °C. The atmosphere was N2 and the heating rate was 10 °C min‒1. The mineral elements of catkins were measured by Thermo Fisher iCAPQc ICP-MS. ASAP2020 Micromeritics was used to carry out the Brunauer Emmett Teller (BET) surface area. The quantitative analysis of the carbon and sulfur elements was carried out on the vario EL cube (Elementar Analysensysteme GmbH). Raman spectra were tested on a HORIBA LabRAM HR Evolution Raman microscope with a 532 nm excitation laser.


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N2 Annealing

Pristine catkins

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Carbonized-Catkins

MoS2coating

Catkins/MoS2

Figure 1. Schematic of the preparing process of Catkins/MoS2 hybrids.

RESULTS AND DISCUSSION Characterization of Catkins/MoS2 hybrids. We provided a two-step strategy to construct microtube hybrids, with first step to carbonize raw willow catkins as supported materials, and the second step to in-situ grow MoS2 nanoflakes on the carbonize catkins wall. The formation process of Catkins/MoS2 hybrids was schematically illustrated in Figure 1. SEM and TEM were conducted to observe the material morphology. As shown in Figure S1, pristine MoS2 of thin plate-like nanoflakes were obtained as 100−200 nm in the lateral size of the nanoflakes. Figure 2 shows the FESEM image of willow catkins, Catkins-900, and Catkins-900/MoS2 hybrids. The


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catkins show a hollow tubular structure. Their diameter and thickness were ~10 µm and ~1 µm, respectively (Figure 2a, b). The high magnification SEM image presents willow catkins with the smooth surface (Figure 2c). As expected, after calcinations, these willow catkins still maintained the open tubular structure with a rough and porous surface, and became much shorter than that of the precursors (Figures. 2d, e, f). Figures 2g and h show the low magnification SEM images of Catkins-900/MoS2 hybrids. The dense MoS2 nanoflakes arrays were covered on the entire surface of the Catkins-900 uniformly. The vertically growing nanoflakes on catkins microtube were also revealed by the high magnification SEM image (Figure 2i). FESEM imaging (Figure S2) were further used to characterize the Catkins-500/MoS2 and Catkins-700/MoS2, illustrating that their morphologies are similar to those of Catkins-900/MoS2. As shown in EDX elemental map (Figure S3), the elements of C, O, Mo, S, N, and P were found in the hybrids and uniformly distributed over the entire hybrids.


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a)

b)

50 µm

d)

c)

10 µm

e)

g)

f)

h)

50 µm

2 µm

10 µm

50 µm

Page 10 of 29

2 µm

i)

10 µm

2 µm

Figure 2. FESEM images of pristine catkins (a‒c), Catkins-900 (d‒f), and Catkins-900/MoS2 hybrids (g‒i).

Figure 3 shows the low-magnification TEM images of the Catkins-900 and Catkins900/MoS2 hybrids. In comparison to the Catkins-900 (Figure 3a−c), we can clearly see the uniform growth of MoS2 nanoflakes surrounding the catkins microtube backbone (Figure 3d−f), indicating the successful wrapping of MoS2 on the willow catkins.


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a)

b)

1 µm

d)

c)

e)

1 µm

100 nm

200 nm

f)

200 nm

100 nm

Figure 3. TEM images of Catkins-900 (a‒c) and Catkins-900/MoS2 hybrids (d−f).

Further structural information of the MoS2, Catkins-900 and Catkins-900/MoS2 hybrids was obtained by HRTEM. As shown in Figure 4a, MoS2 crystal fringes are clearly observed. A lattice spacing of 0.25 nm and an interlayer spacing of 0.62 nm were observed from HRTEM image. The observed 0.25 nm spacing was related to the (101) planes. The 0.62 nm spacing was related to the (002) planes of the natural MoS219. The ED pattern of MoS2 gave the hexagonal structure (Figure 4b). Figure 4c-d shows a tubular structure of the catkins-900 and obvious lattice fringes can be observed in Figure 4e-f, confirming the partially graphitization. Figure 4gi shows the HRTEM of the Catkins-900/MoS2 hybrids. The thickness of the MoS2 on the catkins900 is about 500 nm, further confirming the MoS2 grown on the annealing catkins.


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Figure 4. HRTEM images of MoS2 (a), Catkins-900 (c‒e) and Catkins-900/MoS2 hybrids (g−i). ED pattern of MoS2 (b), and Catkins-900 (f).


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The structural information was investigated through X-ray diffraction. In Figure 5A, for the poorly crystallization,27 three broad and weak diffraction peaks can be observed from the XRD pattern of MoS2 nanoflakes. Their peaking center are at approximately ~ 17.4°, 32.9° and 58.2°, assigned to the (002), (100) and (110) planes in the hexagonal phase MoS2, respectively. The Catkins-900 microtube displays a broad feature at 23.06° and a weak one at 43.26°, which is corresponded to the (002) and (100) planes of graphite,35 indicating that well-developed graphitic structures are dominant in the obtained Catkins-900. For Catkins-900/MoS2 hybrids, the characteristic diffraction peaks of MoS2 were identified, suggesting that MoS2 were successful grown onto the hybrids. The surface elements of the various samples and their variation in surface electron states were examined by XPS analysis. The XPS survey spectrum indicated the presence of the intrinsic nature of C, O, Mo, S, and N species in Catkins-900/MoS2 hybrids (Figure 5B). For Catkins-900, it contains C, O, N, and P species. Compared with MoS2 sample, enhanced signal of C 1s, weakened signal of Mo 3d, and weakened signal of S 2p were clearly observed in Catkins-900/MoS2 hybrids. The XPS peaks at 228.4 eV and 231.7 eV from Mo 3d5/2 and Mo 3d3/2 are the characteristic peaks. They suggested Mo(IV) are present in MoS2 and Catkins900/MoS2 hybrids (Figure 5C). The S 2p region of the Catkins-900/MoS2 hybrids exhibited 2p1/2 and 2p3/2 at 162.5 eV and 161.4 eV, indicating the existence of S2− or bridging S22− (Figure 5D). The peaks at 398 eV − 401.5 eV in the high-resolution N 1s spectrum (Figure S4) was referred to graphitic-N, pyridinic-N, pyrrolic/ pyridone-N, respectively. From the P 2p XPS spectra (Figure S5), phosphorus signal in the Catkins-900 was discovered at 133.6 eV. However, the phosphorus signal is neglectable for the Catkins-900/MoS2. This may be caused by the decrease of phosphorus concentration after coating the MoS2 nanoflakes on the surface of the


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Catkins-900. Notably, 0.7 eV negatively shifts in binding energies of S 2p as well as 0.8 eV negatively shifts in Mo 3d were observed in Catkins-900/MoS2 hybrids compared with those of MoS2 nanoflakes. In the magnified N1s XPS spectra, there is 1 eV shift to lower binding energies in the N peak of Catkins-900/MoS2 hybrids compared with that of Catkins-900. This is likely due to the charge transfer from Catkins-900 to MoS2 through the electronic interactions.36,37 Such effects suggest the combination of Catkins-900 and MoS2 is tightly so that it will be beneficial for electron transport during HER.38 TGA was used to determine the amount of MoS2 on the surface of Catkins-900. Figure 5E displays the weight losses of MoS2, Catkins-900 and Catkins-900/MoS2. It is found that the weight loss region (about 375 °C) could be attributed to the decomposition of MoS2. The significant weight reduction in the high-temperature region (about 760 °C) was due to the decomposition of Catkins-900. Thus, the amount of MoS2 was determined to be 74.5 wt%. The element analysis of three samples of C and S element were calculated quantitatively as listed in Table 1. According to the C element, there is 72.0 wt% MoS2 in Catkins-900/MoS2, which is similar to the result from TGA.

Table 1. Mass percentage (wt%) of C and S elements in the MoS2, carbonized-catkins, and Catkins/MoS2 hybrids Samples

Composition (%) C

S

MoS2

5.92

28.36

Catkins-900

76.36

0.836

Catkins-900/MoS2

25.62

23.737


14

o

MoS2 23

o

43.26

o

Catkins-900

N1s

O1s

Catkins-900

P2p

58.2

MoS 2

C1s

o

Mo3p1/2 Mo3p3/2

32.9

Intensity (a.u.)

Intensity (a.u.)

Catkins-900/MoS 2

C1s

B)

o

17.4

S 2p

A)

Catkins-900/MoS2

10

20

30

40

50

60

70

80

600

2θ (degree) 228.4 eV

Catkins-900/MoS2 Mo 3d 5/2 228.9 eV

Mo 3d3/2 232.1 eV

MoS2

240

236

232

228

224

Binding Energy (eV)

E)

3

90 80 70 Catkins-900/MoS2

50

Catkins-900 MoS2

0

200

400

600

800 o

Temperature ( C)

Catkins-900/MoS2 S 2p 1/2 163.1 eV

200

100

1000

S 2p3/2 161.4 eV

S 2p3/2 161.8 eV

MoS2

164

162

160

Binding Energy (eV)

F)

100

60

300

S 2p 1/2 162.5 eV

166

Quantity adsorbed (cm /g STP)

Intensity (a.u.)

231.7 eV

400

D)

Mo 3d5/2

Mo 3d3/2

500

Binding Energy (eV)

Intensity (a.u.)

C)

Weight (%)

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


Mo 3d

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80

Adsorption desorption

60 SBET= 45.07 m2g-1

40 20 0

0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0)

Figure 5. XRD patterns (A), XPS survey spectra of different materials (B), and high-resolution XPS spectra of Mo 3d (C), S 2p (D) for as-prepared MoS2 and Catkins-900/MoS2. (E) TGA data of MoS2, Catkins-900, Catkins-900/MoS2. (F) Nitrogen adsorption-desorption isotherms of the Catkins-900/MoS2 hybrid


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The specific surface areas of MoS2, Catkins-900 and Catkins-900/MoS2 were obtained by N2 adsorption-desorption. From Figure 5F, the specific surface area of Catkins-900/MoS2 hybrid (45.07 m2 g−1) was higher than that of MoS2 (11.42 m2 g−1) and lower than that of Catkins-900 (349 m2 g−1) (Figure S5 and Table S1, Supporting Information). The porous surface structure and large specific surface area of Catkins-900 is believed to improve the HER electrocatalytic performance. In the Raman spectroscopy (Figure S6), two strong peaks around 1350 cm−1and 1575 cm−1 were observed for the Catkins-900 corresponding to the disordered carbon and the order graphitic carbon, respectively. They become weaker after coating with MoS2. ICP analysis was performed to investigate the impurities introduced by soaking step of the Catkins-900. Pristine catkins were soaked in different concentrations of HNO3 (0 M, 2 M and 16 M) for 24 h, and then they were washed using ultrapure water until pH 7.0 is reached. A vacuum oven was used to try the resulting samples at 60 ○C. After the Catkins annealed at 900 ○C in N2 protection for 4h, the obtained products were named as 0 M-Catkins-900, 2 M-Catkins-900 and 16 M-Catkins-900. At last, equal amounts of above samples were immersed in 6 mL of the concentrated HNO3 at room temperature for 12 h under stirring. The ICP-MS data about leaching solution of these materials were shown in Table S2, revealing the elements content of three samples have little difference, which suggested the negligible effect of the HNO3 concentration on the Catkins-900, consistent with the results of Electrochemical LSV (Figure S7).


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Pt/C Catkins-500 Catkins-700 Catkins-900 MoS2

-10 -20

Catkins-500@MoS2 Catkins-700@MoS2 Catkins-900@MoS2

-30 -0.6 -0.4 -0.2 0.0

0.4

0.2

98 m

0.0 0.0

0.3

c V/de

/dec 83 mV /dec 78 mV ec 62 mV/d 29.8 mV/dec

0.1

0.6

0.9

1.2

log [|j (mA cm )|]

D)

MoS2

40

Catkins-500@MoS2 Catkins-700@MoS2

30

-Z'' (Ω)

H2 ( µmol)

0.3

Catkins-700/MoS2 Catkins-900/MoS2 Pt/C

-2

60 40

Catkins-900@MoS2

20 10

20 0

MoS2 Catkins-500/MoS2

-0.3

Theory Experiment

80

0.4

0.6

Potential (V vs. RHE)

C)

0 0

2000

4000

6000

Time (s)

0

F)

0

20

40

60

80

z' (Ω) 0 -5

-10

-2

j (mA cm )

E)

0.2

Overpotential (V)

B)

0

-2

j (mA cm )

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


-20

1st cycle 1000th cycle

-30

-10 -15 -20

-0.3

-0.2

-0.1

0.0

0.1

0

Potential (V vs. RHE)

2000

4000

6000

Time (min)

Figure 6. A) Polarization curves. B) Tafel plots. C) The theoretical (black dot) and experimentally observed (red dot) H2 yield as a function of time in 0.5 M H2SO4 solution with Catkins-900/MoS2 hybrids. D) EIS Nyquist plots of different samples at an overpotential of 150 mV vs. RHE. E) Stability test for Catkins-900/MoS2 hybrids with initial polarization curve and


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the curve after 1000 cycles. F) Current density‒time (j ‒ t) curves of Catkins-900/MoS2 hybrids (ƞ=150 mV vs. RHE) for 100 h.

Electrochemical catalytic performance of Catkins/MoS2 hybrids. The catalytic performance of Catkins/MoS2 hybrids, carbonized-Catkins, MoS2, and commercial Pt/C for HER was evaluated. The linear scan voltammetry (LSV) curves of Catkins-500, Catkins-700, Catkins900, MoS2, Catkins-500/MoS2, Catkins-700/MoS2, Catkins-900/MoS2, and commercial Pt/C were shown in Figure 6A. An enlarged LSV plots was shown in Figure S8. Both different temperature carbonized-Catkins and MoS2 nanoflakes demonstrated rather poor HER activity. While, Catkins-500/MoS2, Catkins-700/MoS2, Catkins-900/MoS2 exhibited a substantially enhanced catalytic activity for HER. Not surprisingly, the Catkins-900/MoS2 materials possessed a very small onset overpotential (50 mV) at 1 mA cm−2 and an overpotential of –136 mV at 10 mA cm−2. In contrast, MoS2, Catkins-500/MoS2, and Catkins-700/MoS2 presented onset overpotentials of 232 mV, 153 mV, and 84 mV (1 mA cm−2), and 348 mV, 230 mV, and 152 mV at 10 mA cm−2, respectively. The results show that the HER activity of Catkins-900/MoS2 is better than that of MoS2, Catkins-500/MoS2, and Catkins-700/MoS2, indicating the graphitized degree of carbon and the internal electrical conductivity were favorable for improving the HER capacity with increasing annealed temperature. Moreover, Figure S9 summarized the onset overpotential of all samples. It can be directly seen that the onset overpotential of the Catkins900/MoS2 is lower than other catalyst apart from the commercial Pt/C catalyst. It is worth noting that by choosing Catkins-900 as supporting substrate material, Catkins900/MoS2 hybrid exhibits the superior HER activity. This could be attributed to the sufficient coverage of ultrathin MoS2 nanoflakes on Catkins-900 mcirotubes and the increasing exposure of efficient active sites. In addition, Catkins microtubes afford an interconnected conducting


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network to enhance the electrical conductivity of the materials, and further improve the rate of electrocatalytic reaction. Therefore, synergistic effect of unique structure and electron conductivity improved the electrocatalytic HER performance. The literatures about MoS2-based catalysts were surveyed to compare with our Catkins900/MoS2 material for the acidic HER activity (see the statistical results in Table 2). From Table

2, the phosphorus-containing nano-materials showed the remarkable HER performance, such as MoS2−BP nanosheets23 and [email protected] Our material is better than most other MoS2-based catalysts.

Table 2. Comparison of HER performance in acidic electrolytes for Catkins/MoS2 hybrids with other recently reported MoS2-based electrocatalysts Electrocatalysta C@MoS2 nanoboxes C/MoS2@G Catkins@MoS2 CoxCeO2+x–MoS2 Co doped-MoS2 CoS2@MoS2 CoS2@MoS2/RGO Co9S8 @MoS2/CNFs Coral-shaped MoS2@GQD Cu7S4@MoS2 Few-layered MoS2-CB Metal phase MoS2 MoS2−BP MoS2 /CNFs MoS2/CNT MoS2 nanosheets MoS2 nanotubes MoS2@MoP MoS2/MoSe2– graphene aerogels MoS2/RGO MoS2/rGO/PPD/OMWCNT

ƞ at 1 mA cm−2 (mV) ~140 165 50 158 195 ~200 ~50 64 95

ƞ at 10 mA cm−2 (mV) 165 224 136 ~210 ~260 290 98 190 ~120

Tafel slope (mV dec-1) 55 46 62 70 105 85.9 37.4 110 40

Mass loading (mg cm−2) 1.0 0.283 0.318 0.2 0.2 0.285 0.285 0.212 Not given

[44] [45

~60 ~106 ~53

133 ~160 175

48 58 41

0.28 0.28 0.043

[40] [46] [47]

~48 224 100 290 220 29 70

85 342 ~200 308 ~260 108 ~190

68 110 47 90 110 76 61

0.102 0.212 0.286 0.0013 0.2 0.35 Not given

[23] [44] [26] [48] [43] [39] [49]

~150 90

163 ~130

51.3 48

0.285 0.262

[27] [50]


Reference [41] [42] This work [43] [27]

19


MoS2-PPy MoS2 /WS2 MoS2 with hydrogen annealing MoS2 after oxygen plasma treatment

Page 20 of 29

100 ~120 ~440

~256 ~240 Not given

80.5 69 147

0.28 Not given Not given

~400

630

171

Not given

[51] [52] [53]

a

BP: black phosphorus CB: Carbon black CNFs: Carbon nanofibers CNT: Carbon nanotube G: Graphene GQD: graphene quantum dot RGO: Reduction of graphene oxide PPD: pphenylenediamine PPy: Polypyrrole O-MWCNT: O-Functionalized Multiwall Carbon Nanotubes

Tafel slopes are used to probe the mechanism of hydrogen evolution. A multistep reaction route was proposed in hydrogen evolution for converting H+ to H2 in acidic media. Two different mechanisms (the adsorption of H atom and formation of H2 molecule) with three principle steps were proposed.

H3O+ (aq) + e- + ∗ → H* + H 2 O(aq) Volmer (120 mV dec-1 )

(1)

H∗ + H3O+ (aq) + e- → H 2 (g) + H 2O(aq) Heyrovsky (40 mV dec-1 )

(2)

H∗ + H∗ → H 2 (g) Tafel (30 mV dec-1 )

(3)

They are known as the Volmer, Heyrovsky, and Tafel reactions.54 According to the multistep reaction process, when the Volmer reaction, Heyrovsky or Tafel reactions is the rate-controlling step of the HER, the slope of 120 mV dec−1, 40 or 30 mV dec−1 should be obtained. For the same product of H2 molecule, Heyrovsky and Tafel steps are parallel to each other, where the fastest step controls the kinetics. Tafel plots for several catalysts are presented (Figure 6B). Tafel slopes were obtained from the linear region of Tafel equation55, which presents the values of Tafel slopes to be 98 mV dec−1, 83 mV dec−1, 78 mV dec−1, 62 mV dec−1, and 29.8 mV dec−1 for


20

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MoS2, Catkins-500/MoS2 Catkins-700/MoS2, Catkins-900/MoS2, and Pt/C, respectively. The Tafel slope of Catkins-900/MoS2 was the lowest among all catalysts except for commercial Pt/C catalyst. This suggests that the HER process occurs on Catkins-900/MoS2 in a Volmer– Heyrovsky route, and Heyrovsky reaction was the rate-determining step. The intrinsic activities were investigated by the exchange current densities (j0) of various samples. The j0 of Catkins900/MoS2 is calculated to be 74.64 µA cm−2 at an overpotential of 0 V, which is 62.4 times higher than that of MoS2. Catkins-900/MoS2 also performs better than Catkins-700/MoS2 hybrids (17.06 µA cm−2) and Catkins-500/MoS2 hybrids (5.62 µA cm−2). The faradic efficiency was evaluated by the percentage of hydrogen production, which was calculated from galvanostatic electrolysis and water-gas displacing method, respectively. The faradic efficiency of approximate 100% was obtained (Figure 6C). Electrochemical impedance spectroscopy (EIS) is a useful tool to study the electron transfer kinetics and intrinsic conductivity of the electrocatalyst.56 The charge-transfer resistance (Rct) of Catkins-900/MoS2 hybrids is the smallest, which implied that the carbonized-catkins support indeed improved the intrinsic conductivity of Catkins-900/MoS2 hybrids (Figure 6D). As a result, through the combination of synergistically structural and electronic modulation, Catkins900/MoS2 possesses both abundant active sites and high conductivity to achieve excellent electrocatalysis for HER process. The effective electrode surface area was estimated by electrochemical double-layer capacitance (Cdl). From in Figure S10, Catkins-900/MoS2 hybrids exhibit larger Cdl of 28.34 mF cm‒2 comparing with the MoS2 (13.29 mF cm‒2), which indicates that the Catkins-900/MoS2 hybrids hold a great number of exposed effective active sites to obtain excellent HER activity.27


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Catalytic stability is another important parameter for a good electrocatalyst. The LSV curves of Catkins-900/MoS2 hybrids show their high stability since the first curve is similar to the one after 1000 cycles (Figure 6E). The current density–time curve under continuous operation at 150 mV vs. RHE for 100 h (Figure 6F) showed a small decay of current density, which is probably due to the loss of hybrids from the electrode surface.

CONCLUSIONS In summary, a novel hybrid micro/nanostructure of ultrathin MoS2 flake supported on catkins carbon microtube has been successfully constructed. These Catkins/MoS2 hybrids exhibited enhanced electrocatalytic properties, which could be contributed to their unique structure and enhanced electrical conductivity. The carbon microtube acted as a conductive channel to accelerate electron transfer of MoS2 flakes. In addition, more active sites of the defect-rich MoS2 were exposed in Catkins/MoS2 hybrids, which would have undoubtedly contributed to the improvements of electrochemical performance for the HER. As a result, the modified catalyst exhibited remarkable HER activity with a rather low onset overpotential of 50 mV, a small Tafel slope of 62 mV decade−1 and outstanding durability, making Catkins/MoS2 hybrids an attractive candidate for HER electrocatalysts.


22

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. FESEM images of Catkins-500/MoS2 and Catkins-700/MoS2; SEM-EDX elemental mapping of pristine catkins , annealed catkins at 900oC, MoS2 and Catkins-900/MoS2 hybrids; N1s and P 2p XPS spectra; N2 adsorption-desorption isotherms; physical properties; Raman spectra; ICP-MS results; enlarged LSV plots; comparison of onset overpotential; and Electrochemical cyclic voltammogram.

AUTHOR INFORMATION † S.S. Tong and S.H. Ma. contributed equally to this work.

Corresponding Author *E-mail: [email protected] (X. Han)

ORCID Xiaojun Han: 0000-0001-8571-6187

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Key R & D Program of China (2016YFC0401104), HIT Environment and Ecology Innovation Special Funds (HSCJ201617), the National Natural Science Foundation of China (Grant No.21773050, 21528501, 21505106), the Harbin


23


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Distinguished Young Scholars Fund (No. 2017RAYXJ024), the Fundamental Research Funds for the Central Universities (No. 2452017177), and Young Talent Fund of University Association for Science and Technology in Shaanxi, China (20170703).

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(56) Raza, A. M. A.; Young, J. H.; Hee, L. M.; Suk, S. H.; Sung, L. J. Efficient hydrogen evolution reaction catalysis in alkaline media by all-in-one MoS2 with multifunctional active sites. Adv. Mater. 2018, 30 (20), 1707105, DOI 10.1002/adma.201707105.


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Table of Contents

Molybdenum disulfide nanoflakes were in-suit grown on carbon microtube deriving from natural tubular willow catkins to construct the three-dimensional hybrids for hydrogen evolution reaction.


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Molybdenum Disulfide Nanoflakes Covered Carbonized Catkin

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