MoS2 Thin Sheet Growing on Nitrogen Self-Doped ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

MoS2 Thin Sheet Growing on Nitrogen Self-Doped Mesoporous Graphic Carbon Derived from ZIF‑8 with Highly Electrocatalytic Performance on Hydrogen Evolution Reaction Jinhui Tong,*,† Qing Li,† Wenyan Li,† Wenhui Wang,† Wenmei Ma,† Bitao Su,† and Lili Bo*,‡ †

Key Laboratory of Polymer Materials of Gansu Province, Key Laboratory of Eco-Environment-Related Polymer Materials Ministry of Education, College of Chemistry and Chemical Engineering, Northwest Normal University, 967 Anning East Road, Lanzhou 730070, Gansu, People’s Republic of China ‡ College of Science, Gansu Agricultural University, No. 1 Yingmen village, Anning District, Lanzhou 730070, Gansu, People’s Republic of China S Supporting Information *

ABSTRACT: Several composite catalysts were simply prepared by growing a MoS2 sheet on N-doped mesoporous graphic carbon derived from ZIF-8. The as-prepared catalysts were well characterized by transmission electron microscopy (TEM), Raman spectrometry, X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and N2 adsorption−desorption analysis. The catalysts have shown greatly improved electrocatalytic performance in the hydrogen evolution reaction (HER) in 0.5 M H2SO4 compared to the corresponding MoS2, and as low as 185 mV of overpotential at 10 mA/cm2 and 57.0 mV/decade of Tafel slope were obtained. The catalyst also exhibits great stability, and only an 18 mV increase in overpotential was observed after 1000 cycles. KEYWORDS: Molybdenum disulfide, N-doped mesoporous carbon, Metal−organic framework, Hydrogen evolution reaction

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small special surface areas.13,14 Except for fabrication of metal doped hollow M−MoSx (M = Co, Ni; x = 2, 3),15,16 a most effective strategy to address the above issues is supporting MoS2 on some proper support. In this respect, various carbon materials have been used as supports;17 carbon nanotubes (CNTs)18 and reduced graphene oxide (rGO)19 are most popular. However, these star materials are limited to large scale use by their rigid preparation conditions and high costs. As a kind of alternative, graphitic carbons, especially nitrogen-doped porous carbon materials, have attracted more and more attention in recent years.20,21 Numerous technologies have been developed to synthesize carbon materials using various precursors.22,23 Among the precursors, metal−organic frameworks (MOFs), as a kind of native porous crystalline material, have become both promising precursors and templates for the synthesis of heteroatom doped porous carbon materials due to flexible tunability in compositions, structures, and properties of MOFs.24,25 Zeolitic imidazolate framework 8 (ZIF-8), a subclass of nitrogen-containing MOFs constructed by 2methylimidazole and Zn2+, has high thermal and chemical

INTRODUCTION With the growing global demand for sustainable development and environmental protection, the development of clean energy has attracted more and more interest. Hydrogen, as the representative of clean fuel, is considered as a promising alternative to traditional fossil fuels in the future.1 More and more efforts have been put into developing efficient technologies for hydrogen production. Compared with alkane reforming, the presently dominant method for hydrogen production, the electrocatalytic hydrogen evolution reaction (HER), by splitting of water, is a cleaner and sustainable alternative. However, catalysts are needed to overcome the energy barrier of proton reduction to improve the reaction rate. Up to now, noble metals, especially Pt-based catalysts, were proved to be the most effective catalysts for HER. However, limited reserves and high prices severely prevent large scale application of noble-based catalysts.2−4 Thus, cost-effective catalysts with high catalytic activities are urgently desirable. Recently, transition metal sulfides, such as CoS2, MoS2, VS2, and WS2, have already been developed as effective electrocatalysts for HER.5−8 Especially, MoS2 is regarded as a preferable one due to its low free energy for adsorption of protons9 as well as its low cost and high stability in acid.10−12 Unfortunately, the HER activity of MoS2 is limited by intrinsic low conductivity, rare exposure active sites, as well as relatively © XXXX American Chemical Society

Received: July 6, 2017 Revised: September 14, 2017

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DOI: 10.1021/acssuschemeng.7b02244 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

the electrolyte was degassed by bubbling highly pure nitrogen for at least 30 min to ensure the H2O/N2 equilibrium at 0 V vs RHE. Linear sweep voltammetry (LSV) was conducted in 0.5 M H2SO4 with a scan rate of 50 mV/s. For a Tafel plot, the linear portion is fitted to the Tafel equation. Cyclic voltammetry (CV) measurements for long-term stability were performed at room temperature, and the overpotential was scanned from −1 to 0 V (vs RHE) at a sweep rate of 50 mV/s. All data have been corrected for a small ohmic drop based on impedance spectroscopy. All the potentials reported in our manuscript were calibrated to a reversible hydrogen electrode (RHE).

stability and is a preferable precursor in preparation of nitrogendoped porous materials.26,27 In this work, a MoS2 thin sheet was grown on N-doped mesoporous carbon derived from pyrolysis of ZIF-8 accompanied by activation using ZnCl2 at 800 °C. The composite materials exhibit remarkable HER activities in 0.5 M H2SO4, and a low overpotential of 185 mV and a Tafel slope of 55 mV/ dec have been obtained.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION Catalyst Characterization. Figure 1 shows the XRD patterns of the catalysts. The diffraction peaks around 2θ =

Reagents and Equipment. All reagents were of analytic grade and used without any further purification. X-ray diffraction (XRD) characterization was performed on a Shimadzu XD-3A instrument using filtered Cu Kα radiation (40 kV, 30 mA). X-ray photoelectron spectrum (XPS) analysis was performed on a PHI 5000 Versaprobe system using monochromatic Al Kα radiation (1486.6 eV). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterizations were performed on a Carl Zeiss Ultra Plus and HITACHI H-8100 instrument, respectively. The specific surface area and pore size distribution of the samples were determined on a Micromeritics ASAP 2010 instrument at liquid nitrogen temperature. The contents of carbon and nitrogen were determined on a flash 2000 organic elemental analyzer (Thermo Scientific). Preparation of ZIF-8. ZIF-8 was prepared according to a reported method28 as follows: 4.390 g of zinc acetate (Zn(CH3COO)2·2H2O) dissolved in 26 mL of deionized water formed solution A. Next, 3.284 g of 2-methylimidazole (Hmim, C4H6N2) dissolved in 45.5 mL of ammonium hydroxide solution (NH3·H2O, 25 wt %) formed solution B. Then, solution A was added to solution B, and the mixture was stirred for 24 h at room temperature. The solid product was collected by filtration, washed with deionized water to neutral pH, and finally dried at 60 °C under a vacuum overnight, obtaining the product ZIF-8. Preparation of N-Doped Carbon. N-doped carbon support was prepared by pyrolysis of ZIF-8 using ZnCl2 as an activating agent. Typically, 1.0 g of as-prepared ZIF-8 and 4.0 g of ZnCl2 were dispersed in 5 mL of ethanol. The mixture was evaporated until dry under magnetic stirring. Then, the dried precursor was pyrolyzed at 800 °C for 5 h in a tube furnace under a N2 atmosphere with a heating rate of 5 °C/min. The obtained sample was then treated with 1 mol/L of HCl to remove the zinc species. The residual was completely washed with distilled water and dried under 120 °C overnight, obtaining the final N-doped carbon, which was designated as NCNH-800. Synthesis of the Composite Catalysts. MoS2 was grown on the as-prepared N-doped carbon by a hydrothermal method as reported previously.29 Typically, 50 mg of NCNH-800 was ultrasonically dispersed into 70 mL of distilled water to form a homogeneous suspension. Then, 2.48 mmol of Na2MoO4·2H2O and 9.85 mmol of thiourea were dissolved in the suspension under vigorous stirring. The resulting mixture was transferred to a 100 mL stainless steel oven and allowed to react for 24 h at 180 °C and then cooled to room temperature. The resulting solid was collected by centrifugation and washed several times with distilled water and absolute ethanol and, then, dried under a vacuum at 60 °C for 12 h to obtain the final catalyst denoted as MoS2/NCNH-800-50 (50 represent the quantity of the support used). The other two samples, MoS2/NCNH-800-25 and MoS2/NCNH-800-75, were also prepared using the same method. For comparison purposes, MoS2 was also prepared under the same procedures in the absence of NCNH-800. Electrochemical Measurements. Electrochemical measurements were performed on a CHI 760E electrochemical analyzer using a three electrode system. A platinum wire and Ag/AgCl (in 3 M KCl) electrode were used as the auxiliary electrode and reference electrode, respectively. The working electrode was prepared as follows: 2.5 mg of catalyst was dispersed in 1.0 mL of a water/ethanol mixture solvent (0.98 mL ethanol and 20 μL water). The suspension was ultrasonicated for 30 min to form a homogeneous ink. Then, 3 μL of the ink was loaded onto a 3 mm glassy carbon electrode and dried under room temperature to be tested. The catalyst loaded on the electrode is 0.105 mg/cm2. Before the electrochemical measurement,

Figure 1. XRD patterns of the as-prepared support and catalysts.

14.4°, 29.1°, 49.5°, and 58.9° can be well indexed to (002), (004), (105), and (110) diffraction of the MoS2 crystal (JCPDS NO. 65-1951), respectively. The broad diffraction peak at 2θ = 24.4° should be ascribed to (002) diffractions of graphitic carbon. This confirms that the as-prepared supports comprise graphitic carbon. Figure 2 presents typical SEM and TEM photographs of the support NCNH-800 and the sample MoS2/NCNH-800-50. Different from previous reports in which the obtained carbon inherited the original morphology of ZIF-8 crystals and comprised mostly micropores,30 the SEM image of the support NCNH-800 (Figure 2a) shows irregular porous carbon due to the activation by ZnCl2 in the process of our pyrolysis process. The TEM image of NCNH-800 evidenced the existence of fruitful mesopores in the carbon matrix (Figure 2b). In the sample MoS2/NCNH-800-50, the support was covered by MoS2 and formed sphere-like aggregates (Figure 2c). The TEM image of the sample MoS2/NCNH-800-50 further confirmed that the support was covered by a MoS2 thin sheet (Figure 2d). Elemental mapping analysis based on the SEM image of the sample MoS2/NCNH-800-50 confirms the presence and even distribution of S, Mo, C, and N in the sample (Figure 2e,f). As shown in the HRTEM image of MoS2 nanosheets in the sample MoS2/NCNH-800-50 (Figure 3), the fringes with a lattice spacing of 0.6 nm correspond to the (002) plane of MoS2 with the layered structures.31 XPS characterization of the sample MoS2/NCNH-800-50 was shown in Figure 4. The content of carbon and nitrogen determined by elemental analysis and percentage of various C B


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Figure 2. SEM images of (a) NCNH-800 and (c) MoS2/NCNH-800-50. TEM images of (b) NCNH-800 and (d) MoS2/NCNH-800-50. Elemental mapping (e) and EDS analysis (f) of the sample MoS2/NCNH-800-50.

port.12,32−34 The high binding energy of Mo 3d (236.3 eV) corresponds to trace MoO3 existing in the sample. The S 2p electrons exhibited various bonding energies for S2− (161.68 eV, 163.4 and 168.8 eV). Bridging apical S2− could result from the unsaturated S atoms and is known as an active site for HER, as shown in Figure 4b. Thus, better HER performance can be expected from the freshly prepared S-rich MoS2 sample. The C 1s spectrum of MoS2/NCNH-800-50 was fitted into three peaks assigned to the graphitic carbon (284.7 eV), carbon in C−N (285.6 eV), and carbon in C−O (288.6 eV), respectively (Figure 4c). The atomic content of graphitic carbon is as high as 50.97%, indicating the catalyst with high content of graphitization and great conductivity. Figure 4d shows the fit peaks of N 1s and Mo 2p3/2 spectra. The three deconvoluted peaks can be assigned to the Mo 2p3/2 (395.7 eV), pyridinic N (398.48 eV), and pyrrolic N (399.8), respectively. The element analysis results show that the nitrogen content in the support is as high as 11.07%, and the atomic content of pyrrolic N is as high as 62.32% determined by XPS (Table 1); this may account

Figure 3. HRTEM image of the MoS2 nanosheets in the sample MoS2/NCNH-800-50.

and N based on relative integrated intensities (atom %) of XPS spectra were listed in Table 1. The high resolution XPS of Mo 3d and S 2s exhibited five peaks (Figure 4a). The lowest one at 226.7 eV corresponded to S 2s peak of MoS2. The two main peaks at 229.6 and 233.0 eV were the Mo 3d5/2 and Mo 3d3/2 peaks of MoS2 growing on a NCNH-800 carbon supC


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Figure 4. XPS survey of MoS2−NCNH-800-50.

Electrocatalytic Activity toward HER. The activities of the as-prepared catalysts on electrocatalytic HER were investigated in 0.5 M H2SO4 by linear sweep voltammetry (LSV), and the results were shown in Figure 6. Compared with the support NCNH-800 and MoS2, all the composite catalysts exhibit greatly improved catalytic activities except for the sample NCNH-800-75, the catalytic performance of which is even much lower than MoS2. It can be seen clearly that the sample MoS2/NCNH-800-50 has shown the best HER activity, and the lowest overpotential of 185 mV was obtained at 10 mA/cm2, which was much lower than 206 mV, 603 mV, 474 mV, and 493 mV for MoS2/NCNH-800-25, MoS2/NCNH800-75, pristine MoS2, and NCNH-800, respectively (as shown in Figure 6a). The Tafel plots of the as-prepared samples were shown in Figure 6b. The Tafel slope was an inherent property of the catalyst that was determined by the rate-limiting step of the HER. The Tafel slopes for NCNH-800, MoS2, MoS2/NCNH800-25, MoS2/NCNH-800-50, and MoS2/NCNH-800-75 were 219.0, 144.5, 71.1, 57.0, and 220.5 mV/dec, respectively, indicating that the HER process is likely to occur by a Volmer− Heyrovsky mechanism in which a fast discharge step (Volmer reaction, H3O+ + e− → Hads + H2O) is followed by an electrochemical desorption step (Heyrovsky reaction, Hads + H3O+ + e− → H2 + H2O) and the desorption of hydrogen is the rate limiting step.36,37 The Tafel slope for the sample MoS2/NCNH-800-50 was 57.0 mV/dec, which is only 27.0 mV/dec higher than that for Pt/C (20%). Additionally, electrochemical impedance spectroscopy (EIS) was used to investigate the kinetics on the catalysts under the catalytic HER operating conditions (Figure 6c). The chargetransfer impedance (RCT) for the samples based on Figure 6c

Table 1. Total Content and Atomic Percentage of Carbon and Nitrogen in the Sample MoS2−NCNH-800-50 carbon content (%)

nitrogen content (%)

graphitic C (atom %)

67.30 C in C−N (atom %)

C in C−O (atom %)

50.97

25.97

23.06

11.07 pyridinic-N pyrrolic-N (atom %) (atom %) 37.68

62.32

for the high catalytic performance of the sample MoS2− NCNH-800-50. The N2 adsorption−desorption isotherms and pore size distribution plots for the support and the catalysts were shown in Figure 5. The corresponding morphological parameters of the as-prepared samples were listed in Table 2. As can be seen, the support NCNH-800 exhibits a typical type-I isotherm with high BET surface areas of 1304.0 m2/g and a high pore volume of 0.682 cm3/g. The sample also shows narrow pore size distribution with a fraction of micropores around 1.3 nm and major mesopores around 3.5 nm. The mesoporous BET surface area and pore volume of NCNH-800 are 1040.0 m2/g and 0.531 cm3/g, respectively, which is near 80% of the total. As for the composite catalyst, they all exhibit a typical type-II isotherm with a sharp capillary condensation step in the relative pressure (P/P0) range from 0.6 to 1.0 as well as a H3-type hysteresis loop. This indicates the presence of plenty of hierarchical pore structures in the samples as evidenced by the pore size distribution plots.35 Compared with the support NCNH-800, the BET surface areas and pore volumes of the composite catalysts dropped sharply due to being covered by the MoS2 sheet. However, the composite catalysts are with a broad distribution of pore size and mainly with mesopores around 20 nm. D


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Figure 5. N2 adsorption−desorption isotherms and the corresponding pore size-distribution profiles (inset) of (a) NCNH-800, (b) MoS2/NCNH800-25, (c) MoS2/NCNH-800-50, and (d) MoS2/NCNH-800-75.

scanning potential ranges are plotted versus the voltage scan rates in Figure 7, in which the slopes are the EDLCs.39 The Cdl of the samples increased in the order MoS2/NCNH800-75 < NCNH-800 < MoS2 < MoS2/NCNH-800-25 < MoS2/NCNH-800-50. This order is consistent with the increasing order of the catalytic performances of the samples in respect to overpotentials. In particular, the Cdl for MoS2/ NCNH-800-50 is 16.38 mF·cm−2, which is 2.3 times that of MoS2/NCNH-800-25 (7.21 mF·cm−2), 4.5 times that of the pristine MoS2 (3.65 mF·cm−2) and NCNH-800 (3.61 mF· cm−2), and 4.8 times that of MoS2/NCNH-800-75 (3.39 mF· cm−2). This result reveals that the catalyst MoS2/NCNH-80050 has a much higher effective active surface area than the pristine MoS2, NCNH-800, and the other two composite catalysts. Besides activity, durability is another important requirement for good HER electrocatalysts to guarantee sustainable H2 generation. To evaluate the long-term stability of the MoS2/ NCNH-800-50, continuous cyclic voltammetry was performed in the range of 0.2 to −0.4 V (vs RHE) with a scan rate of 50 mV/s in 0.5 M H2SO4 for 1000 cycles. Figure 6d shows the LSV curves recorded before and after 1000 cycles. The results evidence that the catalyst exhibits great stability, and only a 18 mV increase in overpotential was observed after 1000 cycles. The durability of the catalyst MoS2/NCNH-800-50 was further evaluated with a chronoamperometry test procedure at 285 mV overpotential, and the Pt/C (20 wt %) was also evaluated under

Table 2. Morphological Parameters of the As-Prepared Samples sample SBET (m2/g) pore volume (cm3/g)

total microporous mesoporous total microporous mesoporous

NCNH800 1304.0 264.0 1040.0 0.682 0.151 0.531

MoS2/ NCNH800-25

MoS2/ NCNH800-50

MoS2/ NCNH800-75

282.0 86.0 196.0 0.030

12.4 0.4 12.0 0.047

0.030

0.047

5.8 4.6 1.2 0.034 0.002 0.032

is in the order MoS2/NCNH-800-25 < MoS2 < MoS2/NCNH800-50 < NCNH-800 < MoS2/NCNH-800-75. The MoS2/ NCNH-800-50 material exhibited a medium RCT of 110 Ω. The low RCT could mean much faster HER kinetics over the catalyst, which may due to high conductivity resulting from high nitrogen content and mesoporous structure of the support.38 To estimate the effective surface areas, we employed the CV method to measure the electrochemical double-layer capacitances (EDLCs), Cdl, as shown in Figure 7. The potential range where no faradic current occurred was selected for the catalysts (CV curves of the samples can be seen in Supporting Information Figures S1−S5). The halves of the positive and negative current density differences at the center of the E


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Figure 6. HER catalytic activities of the as-prepared samples. (a) LSV curves. (b) Tafel plots. (c) EIS Nyquist plots. (d) Durability test for the sample MoS2/NCNH-800-50.

Figure 8. Current−time chronoamperometric curves of Pt/C (20%) and MoS2/NCNH-800-50 at −0.285 V (vs RHE) in 0.5 M H2SO4.

Figure 7. Relations of half difference between anodic and cathodic current at 0.35 V with various scan rates from 5 mV/s to 25 mV/s.

the same conditions for comparison purposes. As described in Figure 8, after a 35 000 s test, almost no decrease was observed in the current density on Pt/C (20 wt %). As for the sample MoS2/NCNH-800-50, the current density decreased from 0.130 to 0.128, that is, only a 1.5% drop was observed. This result indicates that the as-prepared MoS2/NCNH-800-50 has excellent durability under acid conditions for HER.

catalysts were prepared by hydrothermally growing a MoS2 thin sheet on the as-prepared carbon support. The obtained catalysts exhibit great enhanced electrocatalytic activity toward HER compared to the corresponding MoS2, and as low as a 185 mV overpotential and 57 mV/dec Tafel slope can be obtained. Great improvement in the activity of the catalyst may mainly be due to great conductivity and mesoporous structure of the carbon matrix. This work is more vivid evidence that the pursuit of proper carbon supports is crucial for the development of new efficient non-noble metal eletrocatalysts for the hydrogen evolution reaction.

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CONCLUSIONS The nitrogen self-doped graphic carbon was prepared simply by pyrolysis of ZIF-8 with activation using ZnCl2. The obtained carbon contained a high level of nitrogen and comprised plenty of mesopores and thus the great conductivity. The composite F


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available ACS Publications website at DOI: meng.7b02244. Cyclic voltammetry curves of the ments of double-layer capacitance

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free of charge on the 10.1021/acssuschesamples for measure(PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jinhui Tong: 0000-0002-9919-3708 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China (21363021, 51302222), Natural Science Foundation of Gansu Province (1308RJYA017), and Program for Changjiang Scholars and Innovative Research Team in University (IRT15R56) for financial support.

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REFERENCES

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MoS2 Thin Sheet Growing on Nitrogen Self-Doped ... - ACS Publications

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