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Nitrogen doped MoS2 nanosheets synthesized via a low-temperature process as electrocatalysts with enhanced activity for hydrogen evolution reaction
Journal of Power Sources 356 (2017) 133e139
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Nitrogen doped MoS2 nanosheets synthesized via a low-temperature process as electrocatalysts with enhanced activity for hydrogen evolution reaction Ruchun Li a, b, 1, Linjing Yang a, 1, Tanli Xiong a, Yisheng Wu a, Lindie Cao a, Dingsheng Yuan b, Weijia Zhou a, * a
New Energy Research Institute, Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, School of Environment and Energy, South China University of Technology, Guangzhou Higher Education Mega Center, Guangzhou, Guangdong, 510006, China School of Chemistry and Materials, Jinan University, Guangzhou, 510632, China
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A simple strategy was reported to fabricate N-doped MoS2 as HER catalysts. N-doped MoS2 revealed an enhanced HER performance than pure MoS2. The DFT confirmed that more active sites were produced by doped N atoms.
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Article history: Received 18 December 2016 Received in revised form 13 April 2017 Accepted 15 April 2017
Highly active and earth-abundant catalysts for hydrogen evolution reaction (HER) play a crucial in the development of efficient water splitting to produce hydrogen fuel. Here, we reported a simple, facile and effective strategy to fabricate N-doped molybdenum sulfide (N-doped MoS2) as noble metal-free catalysts for HER. Compared with pure MoS2, the obtained N-doped MoS2 catalyst revealed enhanced HER performance with low overpotential of 168 mV (10 mA cm2), small Tafel slope of 40.5 mV dec1 and excellent stability. The superior HER activity may originate from both the exposed Mo active sites due to S defects and the optimized electron density state of S atoms by N doping. More importantly, due to its simple synthesis method, earth-abundant catalysts and high catalytic activity, the N-doped MoS2 will become a promising HER catalysts for water splitting. © 2017 Elsevier B.V. All rights reserved.
Keywords: Hydrogen evolution reaction Molybdenum sulfide Nitrogen doping Theoretical calculation
1. Introduction Hydrogen (H2), as a clean and promising energy, has received tremendous interest due to the increasing environmental concerns and consumption of fossil fuels [1e4]. The hydrogen evolution
* Corresponding author. E-mail address: [email protected] (W. Zhou). 1 These authors contributed equally to the work. http://dx.doi.org/10.1016/j.jpowsour.2017.04.060 0378-7753/© 2017 Elsevier B.V. All rights reserved.
reaction (HER) from water splitting has been widely regarded as a sustainable clean pathway for hydrogen production [5e8]. During HER process, the catalyst plays an essential role in reducing overpotential, promoting the reaction kinetics and thus enhancing the HER catalytic efficiency. Pt-based catalysts have been so far the most efficient catalysts for HER, but their high cost and scarcity have hampered its widespread applications [9e13]. Therefore, it remains a hugely challenge to develop the inexpensive and earthabundant catalyst within good catalytic property and high cycling stability.
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Recently, transition metal sulfides (MoS2, WS2 et al.) [13e18] have been intensively investigated as HER catalysts. Theoretical and experimental studies have indicated that Mo and S edges of MoS2 are catalytically active sites for HER [19e21]. To improve the HER catalytic activity of MoS2, some general strategies have been attempted [22e28], including the controlled particle size/ morphology, improved electrical conductivity, composite effect and element doping, etc. Note that element doping is one of the effective methods to rationally optimize HER activity of MoS2 by changing the atomic arrangement and electron density state [29e31]. It has been reported that metal (such as Co, Ni, Fe and Cu) [30,31] or non-metal (N, P, O, Se, Cl et al.) [32e35] atoms were doped to replace Mo or S atoms, respectively, which modified their electronic properties and boost intrinsic conductivity. Sun et al. [32] have developed a flower-like N-doped WS2 which were synthesized by the solegel process and subsequent annealing treatment. N-doping could introduce more charge carriers and improve the intrinsic conductivity and thus revealed the enhanced electrochemical HER activity. Nevertheless, to the best of our knowledge, majority of N atoms doping required complicated and multi-step process, such as high-temperature calcination [36] or harsh requirement (N2 plasma) [37,38]. Therefore, it would be worthwhile to design a relatively simple, low-temperature and efficient approach for the preparation of N atoms doped MoS2 to obtain enhanced HER performance. Here, we developed a nitrogen doped MoS2 (N-doped MoS2) as an efficient catalyst for HER by simple hydrothermal reaction. By incorporation of nitrogen atom (replace S atom), more active edge sites were exposed and thus enhanced catalytic performance of MoS2. The theoretical calculations confirmed that the active sites in N-doped MoS2 were S atoms tuned by N doping and Mo atoms tuned by S defects. As expected, the obtained N-doped MoS2 presented an enhanced HER catalytic activity with low overpotential of -168 mV (-10 mA cm-2), small Tafel plot of 40.5 dec mV-1 and superior long-term stability. 2. Experimental section Chemicals: ammonium tetrathiomolybdate ((NH4)2MoS4), dicyandiamide (C2H4N4) and iso-Propyl alcohol (C3H8O) were purchased from Sinopharm Chemical Reagents Beijing Co. All of the reagents were of analytical grade and used without further treatment. Deionized (D.I.) water was purified using a Milli-Q system (Millipore, Billerica, USA). 2.1. Synthesis of N-doped MoS2 In a typical reaction, 20 mg of (NH4)2MoS4 and 10 mg C2H4N4 were dissolved in 20 mL of iso-Propyl alcohol. The solution was transferred to a Teflon-lined stainless steel autoclave and then heated in an electric oven at 200 C for 24 h. The black product, Ndoped MoS2, was harvested after centrifugation and dried at 70 C for 12 h. For comparison, pure MoS2 nanosheets without C2H4N4 and N-doped MoS2 with different C2H4N4 amount (20 and 30 mg) were synthesized under the similar conditions, named as pure MoS2, N-doped MoS2-1 and N-doped MoS2-2, respectively. 2.2. Structural characterizations The morphology of the samples was characterized using a Fieldemission scanning electron microscopic (FESEM, Model JSM7600F). Transmission electron microscopy (TEM) and highresolution TEM (HRTEM) observations were recorded on a highresolution transmission electron microscopy (HRTEM, JEOL JEM20100). Powder X-ray diffraction (XRD) patterns of the samples
were recorded with a Bruke D8 Advance powder X-ray diffractometer with Cu Ka (l ¼ 0.15406 nm) radiation. Raman spectra were recorded on a RENISHAW in Via instrument with an Ar laser source of 488 nm in a macroscopic configuration. X-ray photoelectron spectroscopic (XPS) measurements were performed using a PHI X-tool instrument (Ulvac-Phi). Nitrogen adsorptiondesorption analysis was conducted with an ASAP 2020 instrument to evaluate the BET surface areas of the samples. 2.3. Electrochemical measurements Electrochemical measurements were performed with an electrochemical workstation (CHI 760C, CH Instruments Inc.) in a 0.5 M H2SO4 aqueous solution. An Ag/AgCl electrode (saturated KCl) and carbon rod were used as the reference and counter electrodes, respectively. 4 mg of the catalyst powders was dispersed in 1 mL of 4: 1 (v/v) watereethanol mixed solvents along with 80 mL of Nafion solution, and the mixture was sonicated for 30 min. Then, 5 mL of the above solution was drop-cast onto the surface of a glassy carbon (GC) disk electrode at a catalyst loading of 0.285 mg cm2. The catalyst film was dried at room temperature. Polarization curves were acquired at a potential scan rate of 5 mV s1. Accelerated stability tests were performed in 0.5 M H2SO4 at room temperature by potential cycling between 0.3 and 0.3 V (vs. RHE) at a sweep rate of 100 mV s1 for a given number of cycles. Currentetime responses were monitored by chronoamperometric measurements for 50 h. 2.4. DFT calculations DFT calculations: the calculations were performed using the density functional theory (DFT), as implemented in the Vienna Ab Initio Simulation Package (VASP) [39,40]. The projected augmented wave method was used to describe the ionic cores [41,42], and the electronic exchangeecorrelation energy was modelled using the PerdeweBurkeeErnzerhof functional within the generalized gradient approximation [43,44]. A large supercell (5 5 x 1) was used to simulate a hydrogen atom adsorption of MoS2 with a S vaccancy defect and a N atom doped. The Brillouin zone integration was performed on a G-centered 4 4 x 1 k point mesh, and the Gaussian smearing width was 0.05 eV. The cutoff energy was 450 eV during the calculations. All the atoms were fully optimized until the maximum residual forces of each atom were less than 0.02 eV/Å. The Gibbs free energy of hydrogen absorption (DGH) was calculated with a corrected formula as follows [45]: DGH ¼ DEH þ 0.24 eV, where DEH ¼EHþMoS2 - EMoS2 - 0:5EH2 (where EHþMoS2 , EMoS2 and EH2 refer to the total energy of MoS2 with a hydrogen atom absorbed, the total energy of MoS2 and the total energy of a hydrogen molecule in the gas phase, respectively). 3. Results and discussion The N-doped MoS2 and MoS2 were synthesized by simple hydrothermal reaction at reactively low temperature. The crystalline nature of obtained materials was investigated by powder X-ray diffraction (XRD). As shown in Fig. 1a, the obtained MoS2 showed four peaks at 14.1, 33.4 40.0 and 58.9 , corresponding to the (002), (100) (103) and (110) lattice plane, agreeing well with the standard pattern of hexagonal MoS2 (JCPDS card No. 73-1508) [16]. Nevertheless, after doping nitrogen, the (002) peak of N-doped MoS2 became much broader and no other peaks in the N-doped MoS2 diffraction pattern could be detected, possibly due to the formation of plentiful defect in N-doped MoS2. In addition, nitrogen adsorptionedesorption isotherm and pore size distribution of samples were shown in Fig. 1b. The BET surface area of the N-doped
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Fig. 1. The XRD pattern (a), Nitrogen adsorption-desorption isotherm (b) and pore size distribution (inset) of the N-doped MoS2 and MoS2.
MoS2 and MoS2 was 71.7 m2 g-1 and 22.9 m2 g-1 with pore volume of 0.225 and 0.101 cm3 g-1, respectively, which indicated more catalytically active sites for N-doped MoS2. The obvious peak at 9 nm (inset in Fig. 1b) in the pore size distribution was observed in N-doped MoS2. This was possibly attributed to the destruction of the two-dimensional structure of MoS2 and formation of abundant defects during the hydrothermal reaction with the adding of dicyandiamide. The chemical composition was investigated by X-ray photoelectron spectroscopic (XPS), as shown in Fig. 2. The highresolution Mo 3d spectrum (Fig. 2b) of N-doped MoS2 can be deconvolved into two peaks of Mo 3d5/2 and Mo 3d3/2 at 228.6 and
231.8 eV. From the high-resolution S 2p spectrum (Fig. 2c), the peaks at 161.3 eV and 162.6 eV were related to S 2p3/2 and S 2p1/2 binding energies, respectively, consistent with the oxidation state of S2 in N-doped MoS2 [46]. In addition, the N 1s XPS spectrum was important for N-doped MoS2, which was depicted in Fig. 2d. One peak at 394.5 eV was Mo 3p3/2, and other obvious peak at 398.1 eV was attributed to N 1s, which were only observed in Ndoped MoS2. The N element content in N-doped MoS2 was about 5.7 at% and the adjustable doping content listed in Table S1. More importantly, compared with those of MoS2, the Mo 3d and S 2p peaks in the XPS spectrum of N-doped MoS2 negatively shifted ~0.40 eV and 0.53 eV, respectively, confirming the strong electron
Fig. 2. XPS survey spectrum of (a) full spectrum, (b) Mo 3d, (c) S 2p and (d) N 1s for N-doped MoS2 and MoS2.
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Fig. 3. The SEM (a), TEM (b) and HRTEM (c) of N-doped MoS2; (d) the corresponding EDS elemental mapping images of Mo, S and N..
interactions after N atom doped in MoS2. Fig. 3a showed the field-emission scanning electron microscopy (FESEM) image of N-doped MoS2. The sheet-like nanostructures can be clearly seen in N-doped MoS2, which were more dispersed and homogeneous than pure MoS2 (Fig. 3a and Fig. S1a). However, the excessive dicyandiamide also resulted in the aggregation of MoS2 (Figs. S1c and d). The TEM image in Fig. 3b further revealed the nanosheet with ultrathin graphene-like structure, which was consistent with the SEM results. Moreover, the HRTEM image of N-doped MoS2 in Fig. 3c obviously displayed the crystal planes where the lattice fringes with an inter-planar distance of 0.61 nm and 0.27 nm corresponded to the (002) and (100) planes of MoS2, respectively. The (002) planes could evaluate the layered structure of N-doped MoS2, which comprised only 2-6 layers as shown in Fig. 3b. The abundant defects in (002) and (100) planes caused the incomplete and twisted lattices as showed in Fig. 3c. Energy dispersive spectrometer (EDS)-based elemental mapping in Fig. 3d further confirmed the N atom has been successfully doped in MoS2. The HER catalytic activities of samples including the MoS2, Ndoped MoS2 and commercial 20 wt% Pt/C were examined in 0.5 M H2SO4 by LSV measurements. As expected, the 20 wt% Pt/C showed the most excellent catalytic property with a near-zero overpotential (vs. RHE) in Fig. 4a. Furthermore, for N-doped MoS2, one can see that the overpotential of 168 mV was required to achieve the cathodic current of 10 mA cm2, which was much smaller than that (395 mV) of MoS2 catalysts. Compared with pure MoS2, the N-doped MoS2 become more active and presented an increase in exchange current densities by at least 20-fold. The Ndoped MoS2 by adding different amount of dicyandiamide were synthesized. The according HER performance was also examined (Fig. S3), showing the worse HER activity with increasing adding amount of dicyandiamide. Compared with the SEM and XPS results, the N-doped MoS2 presents the best HER catalytic activity due to the balance of dispersity and the N amount in samples. Moreover, the enhanced HER activity was further estimated by comparing the Tafel slopes which were fitted to the Tafel equation (h ¼ blog j þ a,
where h represent overpotential, j is the current density, b is the Tafel slope and a is a constant). As shown in Fig. 4b, yielded Tafel slope of N-doped MoS2 was 40.5 mV dec1, much smaller than that of MoS2 (101.7 mV dec1), but lightly larger than that of 20 wt% Pt/C (31.5 mV dec1). A lower Tafel slope suggested a more drastic increase of HER currents with increasing applying potential and hence leaded to a better HER performance. Note that the mechanism for HER in acid media typically involves three major reactions [19,20]: H3Oþ þ e-catalyst 4 H-catalyst þ H2O (Volmer reaction, 120 mV dec1) (1) 2H-catalyst 4 H2-catalyst (Tafel reaction, 30 mV dec1)
(2)
H3Oþ þ e-catalyst þ H-catalyst 4 H2-catalyst þ catalyst þ H2O (Heyrovsky reaction, 40 mV dec1) (3) The observed Tafel slope value of 40.5 mV dec1 for N-doped MoS2 suggested that the Volmer-Heyrovsky reaction mechanism dominated during the HER process and the desorption step (Heyrovsky reaction) was the rate determining step. Electrochemical impedance spectroscopy (EIS) measurements were studied to probe the HER catalytic kinetics. As shown in Fig. 4c, the charge transfer resistance (Rct) of the N-doped MoS2 electrode at 200 mV overpotentials was 58.4 U which lower than that of MoS2 (295.5 U), implying a faster reaction rate. Moreover, the Nyquist plots of the N-doped MoS2 electrode at various overpotentials were revealed in Fig. S4, where one can be seen that the Rct from the diameter of the semicircles obviously diminished with increasing overpotential, from 243.3 U at 150 mV to 58.4 U at 200 mV, suggesting fast charge transfer and high HER kinetics. Typically, the effective electrochemical area of the N-doped MoS2 can be estimated by measuring the capacitance of double layer at the solid-liquid interface. As shown in Fig. S5 and Fig. 4d, the N-doped MoS2 electrode offered a higher double layer capacitance (14.3 mF cm2) than that (2.92 mF cm2) of MoS2, indicating
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Fig. 4. (a) Polarization curves of MoS2, N-doped MoS2, and 20 wt% Pt/C for HER in 0.5 M H2SO4; (b) Corresponding Tafel plots derived from (a); (c) Electrochemical impedance spectroscopy of N-doped MoS2 and MoS2 electrodes at overpotentials of 200 mV; (d) Linear fitting of the capacitive currents of the N-doped MoS2 vs scan rates; (e) Current-time plot of the N-doped MoS2 electrode; (f) HER polarization curves of N-doped MoS2 before and after 5000 cycles in the stability test.
a more catalytic sites. In addition, a rectangular shape in CV curves (Fig. S5) was inherited although the scan rate increased from 40 to 200 mV s1 which represented a facile ion transport and good ionic conductivity, agreeing well with the EIS results. Generally, the cycling stability is a key evaluating parameter for the catalysts, which was measured by the chronoamperometric measurements and the continuous CV cycles. As shown in Fig. 4e, the potential displayed a negligible increment which demonstrated that the N-doped MoS2 catalyst possessed excellent stability in 0.5 M H2SO4 at 200 mV for 50 h. Moreover, the superior stability was further confirmed by the continuous CV measurements in the range of 0.3e0.3 V at 100 mV s-1 for 5000 cycles. Fig. 4f presented the polarization curve at the first cycle and 5000th cycle, which only negatively shifted 3 mV at current density of 30 mA cm1, reflected its good durability. In addition, compared with the XRD and XPS results before and after cycling measurement in Fig. S6, no obvious
change can be seen which further demonstrated an outstanding stability of the N-doped MoS2. The effect of N doping into MoS2 catalyst for HER was investigated by density functional theory (DFT) calculations. The DFT calculation was focused on the free energy change of absorbed hydrogen on different active sites, which was an important intermediate state in HER. To simplify the calculations, computable model of N-doped MoS2 with two S atom defects and one N doping atom was constructed (Fig. 5a). For comparison, the model of pure MoS2 was also constructed (Fig. S7). Highly efficient HER catalysts have a jDGH*j close to zero, and this value depends on the geometric and electronic structures of the catalysts. A DGH* that is too positive or too negative is detrimental to the HER because H*cannot efficiently absorb on the catalyst or desorb from the catalyst, respectively. As showed Fig. 5b, the large DGH* value of 2.11 eV on S atoms was obtained in pure MoS2. Because that the inert S atoms
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Fig. 5. (a) Structural model and (b) Gibbs free energy of H* adsorption (DGH*) profile for various sites on N-doped MoS2.
cover the active Mo atoms to cause the poor HER activity, which was consistent with reported results. However, after removing S atoms by N atoms, the exposed Mo atoms combined with H atom to obtain smaller DGH* value of 0.63 eV. It's interesting that DGH* value for S atoms (0.71 eV) were much smaller than that of N atoms (1.43 eV), implying that the N atoms were not active sites, which only effectively change the electronic state density of S atoms in Ndoped MoS2. Therefore, the DFT results implied that the two kinds of active sites in N-doped MoS2 were S atoms tuned by N doping and more Mo atoms exposed by S defects. 4. Conclusion In summary, we have developed an effective and lowtemperature synthesis method to prepare the N-doped MoS2 as efficient electrocatalysts for HER. By introducing nitrogen atoms, 1) the N-doped MoS2 revealed more homogeneous than pure MoS2, resulting in the higher electrochemical active area; 2) more Mo active sites were exposed by abundant defects and the DGH* of S atoms were simultaneously tuned by doped N atoms. Thus, compared with pure MoS2, the N-doped MoS2 become more active for HER and revealed a low overpotential of 168 mV (10 mA cm2), small Tafel slope of 40.5 mV dec1 and excellent stability. The doping effect of N-doped MoS2 will encourage new opportunities to develop high-performance catalysts for HER. Acknowledgment This work was supported by Project of Public Interest Research and Capacity Building of Guangdong Province (2014A010106005), Guangdong Innovative and Entrepreneurial Research Team Program (2014ZT05N200) and the National Natural Science Foundation of China (51502096). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.04.060. References [1] Y. Hou, M.R. Lohe, J. Zhang, S. Liu, X. Zhuang, X. Feng, Energy Environ. Sci. 9 (2016) 478e483. [2] J. Lai, S. Li, F. Wu, M. Saqib, R. Luque, G. Xu, Energy Environ. Sci. 9 (2016) 1210e1214. [3] J. Huang, D. Hou, Y. Zhou, W. Zhou, G. Li, Z. Tang, L. Li, S. Chen, J. Mater. Chem. A 3 (2015) 22886e22891. [4] L. Yang, W. Zhou, D. Hou, K. Zhou, G. Li, Z. Tang, L. Li, S. Chen, Nanoscale 7 (2015) 5203e5208. [5] X. Zou, Y. Zhang, Chem. Soc. Rev. 44 (2015) 5148e5180. [6] X. Yu, S. Zhang, C. Li, C. Zhu, Y. Chen, P. Gao, L. Qi, X. Zhang, Nanoscale 8 (2016)
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Nitrogen doped MoS2 nanosheets synthesized via a low-temperature process as electrocatalysts with enhanced activity for hydrogen evolution reaction Ruchun Li a, b, 1, Linjing Yang a, 1, Tanli Xiong a, Yisheng Wu a, Lindie Cao a, Dingsheng Yuan b, Weijia Zhou a, * a
New Energy Research Institute, Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, School of Environment and Energy, South China University of Technology, Guangzhou Higher Education Mega Center, Guangzhou, Guangdong, 510006, China School of Chemistry and Materials, Jinan University, Guangzhou, 510632, China
b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A simple strategy was reported to fabricate N-doped MoS2 as HER catalysts. N-doped MoS2 revealed an enhanced HER performance than pure MoS2. The DFT confirmed that more active sites were produced by doped N atoms.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 December 2016 Received in revised form 13 April 2017 Accepted 15 April 2017
Highly active and earth-abundant catalysts for hydrogen evolution reaction (HER) play a crucial in the development of efficient water splitting to produce hydrogen fuel. Here, we reported a simple, facile and effective strategy to fabricate N-doped molybdenum sulfide (N-doped MoS2) as noble metal-free catalysts for HER. Compared with pure MoS2, the obtained N-doped MoS2 catalyst revealed enhanced HER performance with low overpotential of 168 mV (10 mA cm2), small Tafel slope of 40.5 mV dec1 and excellent stability. The superior HER activity may originate from both the exposed Mo active sites due to S defects and the optimized electron density state of S atoms by N doping. More importantly, due to its simple synthesis method, earth-abundant catalysts and high catalytic activity, the N-doped MoS2 will become a promising HER catalysts for water splitting. © 2017 Elsevier B.V. All rights reserved.
Keywords: Hydrogen evolution reaction Molybdenum sulfide Nitrogen doping Theoretical calculation
1. Introduction Hydrogen (H2), as a clean and promising energy, has received tremendous interest due to the increasing environmental concerns and consumption of fossil fuels [1e4]. The hydrogen evolution
* Corresponding author. E-mail address: [email protected] (W. Zhou). 1 These authors contributed equally to the work. http://dx.doi.org/10.1016/j.jpowsour.2017.04.060 0378-7753/© 2017 Elsevier B.V. All rights reserved.
reaction (HER) from water splitting has been widely regarded as a sustainable clean pathway for hydrogen production [5e8]. During HER process, the catalyst plays an essential role in reducing overpotential, promoting the reaction kinetics and thus enhancing the HER catalytic efficiency. Pt-based catalysts have been so far the most efficient catalysts for HER, but their high cost and scarcity have hampered its widespread applications [9e13]. Therefore, it remains a hugely challenge to develop the inexpensive and earthabundant catalyst within good catalytic property and high cycling stability.
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Recently, transition metal sulfides (MoS2, WS2 et al.) [13e18] have been intensively investigated as HER catalysts. Theoretical and experimental studies have indicated that Mo and S edges of MoS2 are catalytically active sites for HER [19e21]. To improve the HER catalytic activity of MoS2, some general strategies have been attempted [22e28], including the controlled particle size/ morphology, improved electrical conductivity, composite effect and element doping, etc. Note that element doping is one of the effective methods to rationally optimize HER activity of MoS2 by changing the atomic arrangement and electron density state [29e31]. It has been reported that metal (such as Co, Ni, Fe and Cu) [30,31] or non-metal (N, P, O, Se, Cl et al.) [32e35] atoms were doped to replace Mo or S atoms, respectively, which modified their electronic properties and boost intrinsic conductivity. Sun et al. [32] have developed a flower-like N-doped WS2 which were synthesized by the solegel process and subsequent annealing treatment. N-doping could introduce more charge carriers and improve the intrinsic conductivity and thus revealed the enhanced electrochemical HER activity. Nevertheless, to the best of our knowledge, majority of N atoms doping required complicated and multi-step process, such as high-temperature calcination [36] or harsh requirement (N2 plasma) [37,38]. Therefore, it would be worthwhile to design a relatively simple, low-temperature and efficient approach for the preparation of N atoms doped MoS2 to obtain enhanced HER performance. Here, we developed a nitrogen doped MoS2 (N-doped MoS2) as an efficient catalyst for HER by simple hydrothermal reaction. By incorporation of nitrogen atom (replace S atom), more active edge sites were exposed and thus enhanced catalytic performance of MoS2. The theoretical calculations confirmed that the active sites in N-doped MoS2 were S atoms tuned by N doping and Mo atoms tuned by S defects. As expected, the obtained N-doped MoS2 presented an enhanced HER catalytic activity with low overpotential of -168 mV (-10 mA cm-2), small Tafel plot of 40.5 dec mV-1 and superior long-term stability. 2. Experimental section Chemicals: ammonium tetrathiomolybdate ((NH4)2MoS4), dicyandiamide (C2H4N4) and iso-Propyl alcohol (C3H8O) were purchased from Sinopharm Chemical Reagents Beijing Co. All of the reagents were of analytical grade and used without further treatment. Deionized (D.I.) water was purified using a Milli-Q system (Millipore, Billerica, USA). 2.1. Synthesis of N-doped MoS2 In a typical reaction, 20 mg of (NH4)2MoS4 and 10 mg C2H4N4 were dissolved in 20 mL of iso-Propyl alcohol. The solution was transferred to a Teflon-lined stainless steel autoclave and then heated in an electric oven at 200 C for 24 h. The black product, Ndoped MoS2, was harvested after centrifugation and dried at 70 C for 12 h. For comparison, pure MoS2 nanosheets without C2H4N4 and N-doped MoS2 with different C2H4N4 amount (20 and 30 mg) were synthesized under the similar conditions, named as pure MoS2, N-doped MoS2-1 and N-doped MoS2-2, respectively. 2.2. Structural characterizations The morphology of the samples was characterized using a Fieldemission scanning electron microscopic (FESEM, Model JSM7600F). Transmission electron microscopy (TEM) and highresolution TEM (HRTEM) observations were recorded on a highresolution transmission electron microscopy (HRTEM, JEOL JEM20100). Powder X-ray diffraction (XRD) patterns of the samples
were recorded with a Bruke D8 Advance powder X-ray diffractometer with Cu Ka (l ¼ 0.15406 nm) radiation. Raman spectra were recorded on a RENISHAW in Via instrument with an Ar laser source of 488 nm in a macroscopic configuration. X-ray photoelectron spectroscopic (XPS) measurements were performed using a PHI X-tool instrument (Ulvac-Phi). Nitrogen adsorptiondesorption analysis was conducted with an ASAP 2020 instrument to evaluate the BET surface areas of the samples. 2.3. Electrochemical measurements Electrochemical measurements were performed with an electrochemical workstation (CHI 760C, CH Instruments Inc.) in a 0.5 M H2SO4 aqueous solution. An Ag/AgCl electrode (saturated KCl) and carbon rod were used as the reference and counter electrodes, respectively. 4 mg of the catalyst powders was dispersed in 1 mL of 4: 1 (v/v) watereethanol mixed solvents along with 80 mL of Nafion solution, and the mixture was sonicated for 30 min. Then, 5 mL of the above solution was drop-cast onto the surface of a glassy carbon (GC) disk electrode at a catalyst loading of 0.285 mg cm2. The catalyst film was dried at room temperature. Polarization curves were acquired at a potential scan rate of 5 mV s1. Accelerated stability tests were performed in 0.5 M H2SO4 at room temperature by potential cycling between 0.3 and 0.3 V (vs. RHE) at a sweep rate of 100 mV s1 for a given number of cycles. Currentetime responses were monitored by chronoamperometric measurements for 50 h. 2.4. DFT calculations DFT calculations: the calculations were performed using the density functional theory (DFT), as implemented in the Vienna Ab Initio Simulation Package (VASP) [39,40]. The projected augmented wave method was used to describe the ionic cores [41,42], and the electronic exchangeecorrelation energy was modelled using the PerdeweBurkeeErnzerhof functional within the generalized gradient approximation [43,44]. A large supercell (5 5 x 1) was used to simulate a hydrogen atom adsorption of MoS2 with a S vaccancy defect and a N atom doped. The Brillouin zone integration was performed on a G-centered 4 4 x 1 k point mesh, and the Gaussian smearing width was 0.05 eV. The cutoff energy was 450 eV during the calculations. All the atoms were fully optimized until the maximum residual forces of each atom were less than 0.02 eV/Å. The Gibbs free energy of hydrogen absorption (DGH) was calculated with a corrected formula as follows [45]: DGH ¼ DEH þ 0.24 eV, where DEH ¼EHþMoS2 - EMoS2 - 0:5EH2 (where EHþMoS2 , EMoS2 and EH2 refer to the total energy of MoS2 with a hydrogen atom absorbed, the total energy of MoS2 and the total energy of a hydrogen molecule in the gas phase, respectively). 3. Results and discussion The N-doped MoS2 and MoS2 were synthesized by simple hydrothermal reaction at reactively low temperature. The crystalline nature of obtained materials was investigated by powder X-ray diffraction (XRD). As shown in Fig. 1a, the obtained MoS2 showed four peaks at 14.1, 33.4 40.0 and 58.9 , corresponding to the (002), (100) (103) and (110) lattice plane, agreeing well with the standard pattern of hexagonal MoS2 (JCPDS card No. 73-1508) [16]. Nevertheless, after doping nitrogen, the (002) peak of N-doped MoS2 became much broader and no other peaks in the N-doped MoS2 diffraction pattern could be detected, possibly due to the formation of plentiful defect in N-doped MoS2. In addition, nitrogen adsorptionedesorption isotherm and pore size distribution of samples were shown in Fig. 1b. The BET surface area of the N-doped
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Fig. 1. The XRD pattern (a), Nitrogen adsorption-desorption isotherm (b) and pore size distribution (inset) of the N-doped MoS2 and MoS2.
MoS2 and MoS2 was 71.7 m2 g-1 and 22.9 m2 g-1 with pore volume of 0.225 and 0.101 cm3 g-1, respectively, which indicated more catalytically active sites for N-doped MoS2. The obvious peak at 9 nm (inset in Fig. 1b) in the pore size distribution was observed in N-doped MoS2. This was possibly attributed to the destruction of the two-dimensional structure of MoS2 and formation of abundant defects during the hydrothermal reaction with the adding of dicyandiamide. The chemical composition was investigated by X-ray photoelectron spectroscopic (XPS), as shown in Fig. 2. The highresolution Mo 3d spectrum (Fig. 2b) of N-doped MoS2 can be deconvolved into two peaks of Mo 3d5/2 and Mo 3d3/2 at 228.6 and
231.8 eV. From the high-resolution S 2p spectrum (Fig. 2c), the peaks at 161.3 eV and 162.6 eV were related to S 2p3/2 and S 2p1/2 binding energies, respectively, consistent with the oxidation state of S2 in N-doped MoS2 [46]. In addition, the N 1s XPS spectrum was important for N-doped MoS2, which was depicted in Fig. 2d. One peak at 394.5 eV was Mo 3p3/2, and other obvious peak at 398.1 eV was attributed to N 1s, which were only observed in Ndoped MoS2. The N element content in N-doped MoS2 was about 5.7 at% and the adjustable doping content listed in Table S1. More importantly, compared with those of MoS2, the Mo 3d and S 2p peaks in the XPS spectrum of N-doped MoS2 negatively shifted ~0.40 eV and 0.53 eV, respectively, confirming the strong electron
Fig. 2. XPS survey spectrum of (a) full spectrum, (b) Mo 3d, (c) S 2p and (d) N 1s for N-doped MoS2 and MoS2.
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Fig. 3. The SEM (a), TEM (b) and HRTEM (c) of N-doped MoS2; (d) the corresponding EDS elemental mapping images of Mo, S and N..
interactions after N atom doped in MoS2. Fig. 3a showed the field-emission scanning electron microscopy (FESEM) image of N-doped MoS2. The sheet-like nanostructures can be clearly seen in N-doped MoS2, which were more dispersed and homogeneous than pure MoS2 (Fig. 3a and Fig. S1a). However, the excessive dicyandiamide also resulted in the aggregation of MoS2 (Figs. S1c and d). The TEM image in Fig. 3b further revealed the nanosheet with ultrathin graphene-like structure, which was consistent with the SEM results. Moreover, the HRTEM image of N-doped MoS2 in Fig. 3c obviously displayed the crystal planes where the lattice fringes with an inter-planar distance of 0.61 nm and 0.27 nm corresponded to the (002) and (100) planes of MoS2, respectively. The (002) planes could evaluate the layered structure of N-doped MoS2, which comprised only 2-6 layers as shown in Fig. 3b. The abundant defects in (002) and (100) planes caused the incomplete and twisted lattices as showed in Fig. 3c. Energy dispersive spectrometer (EDS)-based elemental mapping in Fig. 3d further confirmed the N atom has been successfully doped in MoS2. The HER catalytic activities of samples including the MoS2, Ndoped MoS2 and commercial 20 wt% Pt/C were examined in 0.5 M H2SO4 by LSV measurements. As expected, the 20 wt% Pt/C showed the most excellent catalytic property with a near-zero overpotential (vs. RHE) in Fig. 4a. Furthermore, for N-doped MoS2, one can see that the overpotential of 168 mV was required to achieve the cathodic current of 10 mA cm2, which was much smaller than that (395 mV) of MoS2 catalysts. Compared with pure MoS2, the N-doped MoS2 become more active and presented an increase in exchange current densities by at least 20-fold. The Ndoped MoS2 by adding different amount of dicyandiamide were synthesized. The according HER performance was also examined (Fig. S3), showing the worse HER activity with increasing adding amount of dicyandiamide. Compared with the SEM and XPS results, the N-doped MoS2 presents the best HER catalytic activity due to the balance of dispersity and the N amount in samples. Moreover, the enhanced HER activity was further estimated by comparing the Tafel slopes which were fitted to the Tafel equation (h ¼ blog j þ a,
where h represent overpotential, j is the current density, b is the Tafel slope and a is a constant). As shown in Fig. 4b, yielded Tafel slope of N-doped MoS2 was 40.5 mV dec1, much smaller than that of MoS2 (101.7 mV dec1), but lightly larger than that of 20 wt% Pt/C (31.5 mV dec1). A lower Tafel slope suggested a more drastic increase of HER currents with increasing applying potential and hence leaded to a better HER performance. Note that the mechanism for HER in acid media typically involves three major reactions [19,20]: H3Oþ þ e-catalyst 4 H-catalyst þ H2O (Volmer reaction, 120 mV dec1) (1) 2H-catalyst 4 H2-catalyst (Tafel reaction, 30 mV dec1)
(2)
H3Oþ þ e-catalyst þ H-catalyst 4 H2-catalyst þ catalyst þ H2O (Heyrovsky reaction, 40 mV dec1) (3) The observed Tafel slope value of 40.5 mV dec1 for N-doped MoS2 suggested that the Volmer-Heyrovsky reaction mechanism dominated during the HER process and the desorption step (Heyrovsky reaction) was the rate determining step. Electrochemical impedance spectroscopy (EIS) measurements were studied to probe the HER catalytic kinetics. As shown in Fig. 4c, the charge transfer resistance (Rct) of the N-doped MoS2 electrode at 200 mV overpotentials was 58.4 U which lower than that of MoS2 (295.5 U), implying a faster reaction rate. Moreover, the Nyquist plots of the N-doped MoS2 electrode at various overpotentials were revealed in Fig. S4, where one can be seen that the Rct from the diameter of the semicircles obviously diminished with increasing overpotential, from 243.3 U at 150 mV to 58.4 U at 200 mV, suggesting fast charge transfer and high HER kinetics. Typically, the effective electrochemical area of the N-doped MoS2 can be estimated by measuring the capacitance of double layer at the solid-liquid interface. As shown in Fig. S5 and Fig. 4d, the N-doped MoS2 electrode offered a higher double layer capacitance (14.3 mF cm2) than that (2.92 mF cm2) of MoS2, indicating
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Fig. 4. (a) Polarization curves of MoS2, N-doped MoS2, and 20 wt% Pt/C for HER in 0.5 M H2SO4; (b) Corresponding Tafel plots derived from (a); (c) Electrochemical impedance spectroscopy of N-doped MoS2 and MoS2 electrodes at overpotentials of 200 mV; (d) Linear fitting of the capacitive currents of the N-doped MoS2 vs scan rates; (e) Current-time plot of the N-doped MoS2 electrode; (f) HER polarization curves of N-doped MoS2 before and after 5000 cycles in the stability test.
a more catalytic sites. In addition, a rectangular shape in CV curves (Fig. S5) was inherited although the scan rate increased from 40 to 200 mV s1 which represented a facile ion transport and good ionic conductivity, agreeing well with the EIS results. Generally, the cycling stability is a key evaluating parameter for the catalysts, which was measured by the chronoamperometric measurements and the continuous CV cycles. As shown in Fig. 4e, the potential displayed a negligible increment which demonstrated that the N-doped MoS2 catalyst possessed excellent stability in 0.5 M H2SO4 at 200 mV for 50 h. Moreover, the superior stability was further confirmed by the continuous CV measurements in the range of 0.3e0.3 V at 100 mV s-1 for 5000 cycles. Fig. 4f presented the polarization curve at the first cycle and 5000th cycle, which only negatively shifted 3 mV at current density of 30 mA cm1, reflected its good durability. In addition, compared with the XRD and XPS results before and after cycling measurement in Fig. S6, no obvious
change can be seen which further demonstrated an outstanding stability of the N-doped MoS2. The effect of N doping into MoS2 catalyst for HER was investigated by density functional theory (DFT) calculations. The DFT calculation was focused on the free energy change of absorbed hydrogen on different active sites, which was an important intermediate state in HER. To simplify the calculations, computable model of N-doped MoS2 with two S atom defects and one N doping atom was constructed (Fig. 5a). For comparison, the model of pure MoS2 was also constructed (Fig. S7). Highly efficient HER catalysts have a jDGH*j close to zero, and this value depends on the geometric and electronic structures of the catalysts. A DGH* that is too positive or too negative is detrimental to the HER because H*cannot efficiently absorb on the catalyst or desorb from the catalyst, respectively. As showed Fig. 5b, the large DGH* value of 2.11 eV on S atoms was obtained in pure MoS2. Because that the inert S atoms
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Fig. 5. (a) Structural model and (b) Gibbs free energy of H* adsorption (DGH*) profile for various sites on N-doped MoS2.
cover the active Mo atoms to cause the poor HER activity, which was consistent with reported results. However, after removing S atoms by N atoms, the exposed Mo atoms combined with H atom to obtain smaller DGH* value of 0.63 eV. It's interesting that DGH* value for S atoms (0.71 eV) were much smaller than that of N atoms (1.43 eV), implying that the N atoms were not active sites, which only effectively change the electronic state density of S atoms in Ndoped MoS2. Therefore, the DFT results implied that the two kinds of active sites in N-doped MoS2 were S atoms tuned by N doping and more Mo atoms exposed by S defects. 4. Conclusion In summary, we have developed an effective and lowtemperature synthesis method to prepare the N-doped MoS2 as efficient electrocatalysts for HER. By introducing nitrogen atoms, 1) the N-doped MoS2 revealed more homogeneous than pure MoS2, resulting in the higher electrochemical active area; 2) more Mo active sites were exposed by abundant defects and the DGH* of S atoms were simultaneously tuned by doped N atoms. Thus, compared with pure MoS2, the N-doped MoS2 become more active for HER and revealed a low overpotential of 168 mV (10 mA cm2), small Tafel slope of 40.5 mV dec1 and excellent stability. The doping effect of N-doped MoS2 will encourage new opportunities to develop high-performance catalysts for HER. Acknowledgment This work was supported by Project of Public Interest Research and Capacity Building of Guangdong Province (2014A010106005), Guangdong Innovative and Entrepreneurial Research Team Program (2014ZT05N200) and the National Natural Science Foundation of China (51502096). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.04.060. References [1] Y. Hou, M.R. Lohe, J. Zhang, S. Liu, X. Zhuang, X. Feng, Energy Environ. Sci. 9 (2016) 478e483. [2] J. Lai, S. Li, F. Wu, M. Saqib, R. Luque, G. Xu, Energy Environ. Sci. 9 (2016) 1210e1214. [3] J. Huang, D. Hou, Y. Zhou, W. Zhou, G. Li, Z. Tang, L. Li, S. Chen, J. Mater. Chem. A 3 (2015) 22886e22891. [4] L. Yang, W. Zhou, D. Hou, K. Zhou, G. Li, Z. Tang, L. Li, S. Chen, Nanoscale 7 (2015) 5203e5208. [5] X. Zou, Y. Zhang, Chem. Soc. Rev. 44 (2015) 5148e5180. [6] X. Yu, S. Zhang, C. Li, C. Zhu, Y. Chen, P. Gao, L. Qi, X. Zhang, Nanoscale 8 (2016)
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