Activating MoS2 with Super-High Nitrogen-Doping Concentration as

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

Activating MoS with Super-High Nitrogen-Doping Concentration as Efficient Catalyst for Hydrogen Evolution Reaction Qian Yang, Zegao Wang, Lichun Dong, Wenbin Zhao, Yan Jin, Liang Fang, Baoshan Hu, and Mingdong Dong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00059 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 12, 2019

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The Journal of Physical Chemistry

Activating MoS2 with Super-High Nitrogen-Doping Concentration as Efficient Catalyst for Hydrogen Evolution Reaction Qian Yang,†,‡ Zegao Wang,‡,# Lichun Dong,†, §,∥ Wenbin Zhao,† Yan Jin,† Liang Fang,⊥ Baoshan Hu,*,† Mingdong Dong*,‡

†School

of Chemistry and Chemical Engineering, Chongqing University, Chongqing

401331, China

‡Interdisciplinary

Nanoscience Center (iNANO), Aarhus University, DK-8000 Aarhus C,

Denmark

#School

of Materials Science and Engineering, Sichuan University, Chengdu 610065, PR

China

ACS Paragon Plus Environment

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The Journal of Physical Chemistry 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

§School

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of Chemistry and Chemical Engineering, Yangtze Normal University, Fuling

408100, Chongqing, China

∥Key

Laboratory of Low-grade Energy Utilization Technologies & Systems of the

Ministry of Education, Chongqing University, Chongqing 40004, China

⊥College

of Physics, Chongqing University, Chongqing 401331, China

ABSTRACT

The development of nonprecious electrocatalysts with high hydrogen evolution reaction (HER) activity for water splitting is highly desirable but remains a significant challenge. Molybdenum disulfide (MoS2) has been demonstrated as a good candidate, however, the insufficient active sites along with the poor conductivity significantly hinder the overall efficiency of MoS2. In this work, we present a method to activate commercial MoS2 by high concentration nitrogen doping via a facile high-temperature treatment routine. The dominant N-doping mechanism is demonstrated to be an appropriate one-to-one substitution of sulfur atoms, which is confirmed by the approximate constancy between


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the atomic ratio of Mo/(S+N) and stoichiometric number of original MoS2. By controlling the activation time and temperature, the concentration of the doped nitrogen atoms can be tuned up to 41 at.%. The HER activity of the as-prepared materials was evaluated as electrode materials, showing that the catalytic activity is strongly correlated with the doped nitrogen concentration, and the catalytic current density of N-doped MoS2 can reach 15 times higher than that of the pristine MoS2. The prominent improvement of HER performance for N-MoS2 can be attributed to rich active sites, higher electron concentration around active sites and ameliorative conductivity induced by N incorporation. The facile and controllable approach to activate MoS2 for achieving highlevel N-doping developed in this study can shed significant light on the preparation of heteroatoms doped electrocatalytic materials.


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INTRODUCTION

Compared with the conventional routines for hydrogen production from fossil fuels by steam reforming or partial oxidation of methane and coal gasification,1 water splitting provides a more sustainable pathway to generate hydrogen with abundant water as raw materials. Among the various techniques of water splitting including thermolysis,2 electrolysis,3-4 photolysis,5 photoelectrolysis,6 etc., electrolysis has received the most attention over the past decade due to the potential to meet primary energy requirements in the near future.7 Pt-based catalysts have been demonstrated to be the most effective electrocatalyst of hydrogen evolution reaction (HER) for large cathodic current densities at

low

overpotentials.8-9

However,

the

commercial

applications

of

Pt-based

electrocatalysts were constrained by the limited abundance and high cost.8, 10 Thus, the development of nonprecious electrocatalysts with high HER activity for water splitting is highly desirable but remains a significant challenge.


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In the past decade, MoS2 has attracted tremendous attentions as economical HER electrocatalytic catalysts due to its relatively high activity originating from the moderate Gibbs free energy of hydrogen adsorption (ΔGH*) at the Mo-edge sites. However, the basal planes and S-edge of MoS2 are catalytically inert, and the electro-conductivity of common 2H-phase MoS2 is poor.11-12 These two factors severely hinder the overall efficiency of MoS2 in catalyzing the hydrogen evolution reactions.13 The widely reported deliberate strategies, such as morphology design (e.g. small-size fragments14 and threedimensional MoS2 nanoflower15), vacancies introduction (S-vacancies),16 crystal structure/phase transformation (metallic-phase MoS2),2,

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and composite structure

(MoO3@MoS2 nanowires)13 can enhance the electrocatalytic activity of MoS2 by increasing active sites and improving conductivity, but suffer from the disadvantages of complex process and high cost. Heteroatom doping has been well recognized as an effective way to tune the chemical, electronic and optical properties of materials.17-18 Metal elements (e.g. Pt, Co, Ni, Fe, Zn, Cu, W, etc.) can modify the S-edges of MoS2 to supply additional active sites,19-21 while the incorporation of non-metal elements, including O,22-23 B,24 Cl25 and N26, has also been


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demonstrated as an efficient approach to activate MoS2. In previous works, several methods have been employed to dope N atoms into MoS2 for HER, such as plasma treatment, sol-gel method and solvothermal method.26-28 Xiao et al.27 synthesized Ndoped MoS2 nanosheets as an efficient HER catalyst by a sol-gel routine, and revealed the dual-function of nitrogen dopants to activate the HER activity of S-edges and enhance the conductivity of MoS2. Azcatl et al.26 proposed an realizable strategy for doping nitrogen into MoS2 through a remote N2 plasma surface treatment, and demonstrated that the doping mechanism of nitrogen atoms was to substitute for sulfur atoms. Li et al.28 fabricated N-doped MoS2 as an effective HER catalyst by using a solvothermal method, and found that the superior activity of the catalyst may originate from the more exposed active Mo-sites. However, these methods suffer from relatively low concentration and/or undesirable side effects. In this paper, we report a facile method to prepare N-doped MoS2 materials for highefficient HER catalysts by incorporating high concentration of nitrogen atoms into the commercial MoS2 under the atmosphere of ammonia. By using the developed method, the N-doping concentration can reach up to 41 at.%, which is several times higher than


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that achieved by the previous approaches including the plasma treatment, hydrothermal / solvothermal, and sol-gel methods. The doping mechanism of nitrogen was verified to be an appropriate one-to-one substitution of sulfur atoms. Electrochemical evaluation demonstrated that the N-doped MoS2 (N-MoS2) materials exhibit much better HER performance than the pristine MoS2 due to the increased active sites, higher electron concentration and ameliorated conductivity. The facile and controllable approach to activate MoS2 for achieving high-level N-doping can pave a practical train of thought for the preparation of heteroatoms doped electrocatalytic materials.

EXPERIMENTAL

Synthesis of N-doped MoS2. N-MoS2 samples were synthesized by thermally treating commercial MoS2 (< 2μm, Aldrich) up to 900 °C under the atmosphere of ammonia. In a quartz tube pre-purged with ammonia gas, the MoS2 powder was annealed for different times at different temperatures with a heating rate of 10 °C/min under 100 sccm ammonia. After cooling the materials down to room temperature, N-MoS2 samples were obtained.


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The as-synthesized N-incorporated MoS2 samples were denoted as N-MoS2-a-b, where a represents the activation temperature (°C) and b represents the activation time (h). Materials Characterizations. X-ray photoelectron spectroscopy (XPS) was measured on a Kratos Axis Ultra DLD spectrometer under a vacuum of 1×10-9 torr. X-ray diffraction (XRD) was performed on a STOE diffractometer with a 2θ span of 10-80°. Transmission electron microscopy (TEM) images and energy-dispersive X-ray spectroscopy (EDS) mapping images were taken on a FEI Talos™ F200X transmission electron microscope. Raman spectra were recorded by using a Renishaw Raman Microscope/Spectrometer at an excitation wavelength of 514 nm. The N2 adsorption isotherms and pore diameters were obtained by Brunauer-Emmett-Teller (BET) measurements using an ASAP 2020 surface area analyzer. Electrochemical Measurements. Electrochemical measurements were performed by using an electrochemical workstation (CHI660E) with a standard three-electrode system. Typically, 4 mg of catalyst sample was dispersed in 1 mL of DI water, and sonicated for 1 h to obtain a homogeneous suspension. Then 5 μL of suspension was dropped onto a glassy carbon electrode (3 mm in diameter) with a loading of 0.283 mg cm-2. Subsquently,


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Nafion solution was pipetted on the electrode after it was dried. The glassy carbon covered with the as-synthesized N-MoS2 catalyst was used as the working electrode. Liner sweep voltammetry (LSV) measurements were conducted in 0.5 M of H2SO4 electrolyte at a scan rate of 5mV s-1 using Pt and Ag/AgCl as the counter electrode and reference electrode, respectively. Before each measurement, the electrolyte was degassed with blowing N2. All the reported potentials were referenced to a revisable hydrogen electrode (RHE) by the equation E(RHE) = E(Ag/AgCl) + 0.197 V.

RESULTS AND DISCUSSION

Theoretically, the Mo-N bond is more stable than Mo-S, indicated by its shorter bond length (2.03 Å vs 2.43 Å),29-30 thus it has a great potential for nitrogen atoms to replace sulfur atoms in MoS2. In the previous studies for preparing N-MoS2,27-28 the N-doping concentration is mostly lower than 8 at.% since solid materials, e.g. thiourea or cyanoguanidine, were usually used as the nitrogen sources, whose capacity to supply nitrogen elements via their thermal decomposition to ammonia is limited. In the method developed in this study, the continuous supply of gas ammonia as the N source offer the


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opportunity for high-level N-doping,

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attaining the successful preparation of N-MoS2

materials with the doping concentration up to 41 at.%. In the XPS survey of MoS2 and N-MoS2-900-2 (Figure 1a), the signals corresponding to the elements of Mo, S and C can be clearly identified. The successful doping of Nelements in N-MoS2 can be confirmed by the high-resolution N1s and Mo 3p profiles (Figure 1b and c). In Figure 1b, the N 1s peak located at 398.1 eV is attributed to the Mo-N bonds, while the peak at 395.5 eV is corresponding to Mo 3p3/2.31 For the high resolution Mo 3d (Figure 1c), the two major peaks at 232.7 and 229.5 eV can be assigned to Mo4+ 3d3/2 and Mo4+ 3d5/2, respectively;31-32 while the peak at 235.8 eV is attributed to Mo6+,33 suggesting the existence of metallic Mo-N configuration, which is able to improve the conductivity of N-MoS2.34 The high-resolution S 2p spectrum is dominated by a doublet of S 2p1/2 at 163.7 eV and S 2p3/2 at 162.5 eV, revealing the S2- state of S elements in MoS2 (Figure 1d). More importantly, the disappearance of the peak at 164.8 eV suggests that the intensity of S-N bonds is below the detection limit, implying that the nitrogen atoms are mainly bonded with Mo rather than S in N-MoS2.26 In addition, the


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slight red shifts of Mo 3d and S 2p along with the broadened full width at half-maximum confirm the electrons interaction between N, Mo and S after N doping. For exploring the effect of activation temperature, XPS spectra of the as-prepared NMoS2 materials prepared at different temperatures are compared in Figure S1. It can be seen that the N-doping concentration continuously increases with the elevation of activation temperatures, verified by the enhanced intensity of N1s peaks as well as the Mo6+ peak at the higher activation temperatures. In contrast, Figure S2 shows that the intensity of N1s peaks firstly increases with an extension in activation temperature (0.5-2 h), reaching the strongest at 2 h, and then drops significantly with a further extension in reaction time (2-4 h). The variation of the Mo6+ peak also demonstrated that the N-doping concentration reaches the maximum at the activation time of 2 h. The accurate quantitative analysis of every element is difficult due to shallow analysis depth of XPS, but high resolution XPS spectra still can provide quantitative information to a certain extent.26-28 In Figure 1e, in parallel with the increase of nitrogen concentration, the variation of the sulfur concentration in N-MoS2 shows a downward trend, indicating the substitution of S atoms by N atoms during the first two hours of activation. The slight


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deviation of the x value at 1 h in a nominal expression ‘MoSxNy’ is likely to derive from the surface sensitive effect of XPS. Furthermore, the atomic ratio of Mo/(S+N) in all the assynthesized N-MoS2 samples irrespective of the activation time is close to the stoichiometric number of pristine MoS2 (Table 1). The concurrent variation and constant summation of S and N atoms unravel logically that the incorporation mechanism of N atoms into MoS2 is the substitution of S atoms by N atoms, forming new Mo-N bonds. The N concentration in the N-MoS2 sample obtained at different activation times is displayed in Figure 1f. The highest N content can reach up to about 41 at.%, which is much higher than the doping level of around 8 at.% reported in previous studies.27


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Figure 1. (a) XPS survey spectra of pristine MoS2 and N-MoS2-900-2. (b-d) High resolution XPS spectra of N 1s, Mo 3d, and S 2p of pristine MoS2 and N-MoS2-900-2. Stoichiometry (e) and N atom concentration (at.%) (f) in the N-MoS2 (MoSxNy) as a function of activation time.

Table 1. Atoms ratio and N concentration of N-MoS2 prepared with different activation times at 900 °C.

Mo:(S+N)

N atomic Conc.

(at%)

(N/(Mo+S+N), at%)

Activation time (h)

Mo:S:N

0

1 : 1.77 : 0

1:1.77

0

0.5

1 : 1.42 : 0.47

1:1.89

16.3

1.0

1: 1.54: 0.53

1:2.07

17.3

2.0

1 : 0.76 : 1.23

1:1.99

41.1

4.0

1 : 1.14 : 0.66

1:1.80

23.6

The high resolution TEM (HR-TEM) image in Figure S3 depicts the periodic atoms arrangement of pristine MoS2 and the interplanar spacing of 0.27 nm matches well with that of (100) facets.35 After N-doping, the atomic arrangement on the surface of N-MoS2


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becomes disordered, as shown in Figure 2a, which can possibly facilitate the HER activity of the catalysts by providing additional active sites.36 The EDS spectrum in Figure 2b confirms that the atomic ratio of Mo/(S+N) in N-MoS2-900-2 is close to that in the pristine MoS2, consistent with the XPS results, further verifying that the dominant N-doping mechanism is to substitute for sulfur atoms. Moreover, the EDS mapping images of NMoS2-900-2 in Figure 2c-g indicate that nitrogen atoms are successfully introduced into MoS2 and uniformly distributed over the sample.

Figure 2. (a) TEM image, (b) EDS spectrum and (c) STEM image of N-MoS2. (d-g) The corresponding EDS mapping images of Mo, N, S and overlap elements.


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The as-synthesized N-MoS2 materials were further characterized via XRD to verify the effect of the activation temperature and time. In Figure 3a and b, the peaks at around 14.4°, 32.6°, 33.6°, 36.0°, 39.7°, 50.0°, 58.5°, 60.2° can be attributed to the (002), (100), (101), (102), (103), (105), (110) and (008) planes of MoS2, respectively.37 As the sample treated at 900 °C, the new diffraction peaks located at 31.9°, 49.0°, 65.0°, 74.3°, 76.9° and 78.3° are corresponding to the (002), (202), (220), (222), (400) and (204) phases of hexagonal molybdenum nitride (JCPDS no. 25-1367), respectively, agreeing well with the EDS and XPS analyses. On the contrary, no obvious molybdenum nitride peaks can be observed in the XRD spectra of the N-MoS2 samples synthesized at low reaction temperatures of 400 and 500 °C because of the low N-doping concentration. Figure 3b displays the XRD spectra of N-MoS2 samples activated at 900 °C with different reaction times. For the samples activated for 0.5 h and 1 h, the XRD patterns are similar as that of pristine MoS2; while when the activation time reached 2 h, the peaks of molybdenum nitride can be obviously observed. Furthermore, when the activation time was extended to 4 h, the characteristic peaks of molybdenum nitride become unexpectedly weaker. Under the high-temperature treatment, a variety of free radicals, i.e. –NH2, –NH–, –N and


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–H, can be generated by the decomposition of ammonia. Therefore, it is reasonable to hypothesize that the doping sites in MoS2 can be rapidly saturated by nitrogen atoms after suitable activation time, while overlong activation time would result in the escape of nitrogen atoms from N-MoS2. The possible mechanism is that Mo6+ are reduced to lowvalence ions due to the existence of hydrogen (in accordance with the XPS data), resulting in the deviation of the redundant nitrogen from N-MoS2.38 In addition, the overlong thermal treatment can possibly induce hydrogen etching, causing the structure collapse of MoS2 to a certain degree.39 Consequently, the activation time for the preparation of N-MoS2 materials should be controlled to manipulate the N-doping concentration. The Raman spectra of all samples in Figure 3c and d exhibit two characteristic bands of MoS2, corresponding to the in-plane vibration (E2g) of Mo and S atoms and out-ofplane vibration (A1g) of S atoms.40 Compared with those of the pristine MoS2, both A1g and E2g bands in the Raman spectra of N-MoS2 show gradual red shifts (up to ~2.3 cm-1) with the increase of activation temperature, revealing the enhancement of electron density around active sites at the doping sites due to the electron-donating effect of


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nitrogen atoms, which can promote the catalytic HER activity of the materials.41-42 In the rate-limiting step, the increased electron concentration around active sites can accelerate the charge transfer process.43 And the high electron density around active sites can also weaken the adsorption of H atoms, facilitating the following H combination and desorption reactions.44 The Raman characteristic peaks of N-MoS2 also exhibit clear response to the activation time. Compared with that of the pristine MoS2, a minor red shift of A1g and E2g (~1 cm-1) occurs in the spectrum of N-MoS2-900-0.5, however, when the activation time was prolonged to 1, 2 or 4 h, the red shift keep invariant (~2 cm-1). In addition, the absence of J1, J2, J3 Raman peaks at 153.2 cm-1, 226.4 cm-1, 336.7 cm-1 (Figure S4) as well as no peaks at around 9.81°, 19.62° in XRD certify that N-MoS2 still exist as 2H phase, rather than 1T phase.45-46


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Figure 3. XRD and Raman spectra of pristine MoS2 and N-MoS2 treated at different activation temperatures (a, c) and for different activation times (b, d). The peaks of molybdenum nitride are marked by black stars.

The textural property of the as-synthesized N-MoS2 materials was also investigated by N2 adsorption-desorption analysis. According to the adsorption-desorption isotherms (Figure S5), the BET specific surface area of N-MoS2-900-2 is determined to be 7.1 m2


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g-1. The Barrett-Joyner-Halenda (BJH) pore size distribution curve (Figure S5) reveals that most of pores in N-MoS2-900-2 are micro- and meso-pores mainly ranging from 0.8 nm to 50 nm, while main pores in pristine MoS2 are macropore. Three factors could be assigned to generate micro- and meso-pores in N-MoS2: (i) the increased roughness and crack deflections at the grain boundary of MoS2 caused by the structural strain;47-48 (ii) the weaker vdW force between MoS2 layers induced by NH3 intercalation and gas volume expansion;49 (iii) the collapse of macropores caused by thermal annealing.50 Anyway, the texture of N-MoS2 is still approximate to the structural characteristics of bulk MoS2, which is determined by the nature of raw materials. The electrochemical performance of the as-prepared N-MoS2 samples was characterized in 0.5 M of H2SO4 by using a three-electrode setup. Figure 4a shows the linear sweep voltammetry (LSV) polarization curves of the samples prepared at different activation temperatures, exhibiting that the overpotential for N-MoS2-900-2 is much lower than that for the pristine MoS2, N-MoS2-400-2 and N-MoS2-500-2. Moreover, N-MoS2900-2 displays a current density of about 14 mA cm-2 at a potential of 0.50 V, which is about 15, 3.6 and 2.6 times higher than that of the pristine MoS2, N-MoS2-400-2 and N-


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MoS2-500-2, respectively. These observations verified that N-doping can enhance the HER activity of MoS2, and the N-MoS2-900-2 sample turns to exhibit the best electrochemical performance. To obtain further insight into the intrinsic HER kinetics of N-MoS2, Tafel plots of all the as-synthesized samples are explored (Figure 4b). The resulted Tafel slope for the pristine MoS2 is 107 mV/decade, whereas N-MoS2 samples prepared at different activation temperatures exhibit much lower Tafel slope of 95, 88, 77 mV/decade, respectively. Specially, the Tafel slope for N-MoS2-900-2 is even lower than that for the Ni-doped MoS251 and pure MoS2 nanosheets52 reported in the literatures. For HER in acidic media, three principle steps for conversion of H+ to H2 have been well known as follows. Volmer discharge reaction: H 3O

+

+ e →Hads + H2O (1)

Heyrovsky desorption reaction: Hads + H3O

+

+ e →H2 + H2O (2)

Tafel combination reaction: Hads + Hads→H2 (3) ACS Paragon Plus Environment

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In the Volmer discharge reaction, a hydrated proton combines with an electron, generating an adsorbed H atom, as shown in equation (1). Two possible routes can achieve the formation of H2 molecule from the adsorbed H atoms: i) the adsorbed H atom reacts with a hydrated proton and an electron (Heyrovsky desorption reaction); ii) two adsorbed H atoms combine directly (Tafel combination reactions). When the Volmer reaction is the rate-limiting step, the catalyst owns a high Tafel slope of 120 mV/decade, while the Tafel slope for a rate-limiting Heyrovsky desorption reaction or Tafel combination reaction can be reduced to 40 mV/decade or 30 mV/decade, respectively. In this study, the Tafel slopes locate in a range of 77-107 mV/decade, suggesting that the HER reaction on N-MoS2 follows the Volmer-Heyrovsky mechanism. From the perspective of Tafel plots53 and ΔGH*27, 54, N-MoS2 is more active than pure molybdenum nitride for HER.


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Figure 4. (a) LSV polarization curves and (b) the corresponding Tafel plots of pristine MoS2 and N-MoS2 activated at 400 °C, 500 °C and 900 °C. (c) Linear fitting of the capacitive currents of the catalysts vs scan rates.


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To further clarify the effect of N-doping to enhance the HER activity of MoS2, we further study the double layer capacitance (Cdl) of the as-synthesized materials to evaluate the amount of active HER sites. The CV curves in a potential of 0.1-0.2 V vs RHE with different scan rates are shown in Figure S6. The cycle curves of the pristine MoS2, NMoS2-400-2 and N-MoS2-500-2 are all sharp while that of N-MoS2-900-2 exhibit a rectangle-like shape, demonstrating its larger Cdl than the other samples.9 According to the difference between anodic and cathodic currents (ΔJ = Ja-Jc) at 0.15 V (vs RHE) as a function of scan rate in Figure 4c, the Cdl value of the materials can be calculated by the slope of fitted line following an equation of Cdl=ΔJ/2ν.55 Compared with the pristine MoS2, N-MoS2 exhibits much higher Cdl values. Specially, the Cdl of N-MoS2-900-2 is 400μF, which is 7-fold higher than that of pristine MoS2, suggesting that N-doping can significantly increase the active sites of MoS2 to give higher HER performance. Nevertheless, compared with some reported works, the absolute value of Cdl is still low, which is determined by the nature of raw materials (bulk MoS2). Affirmatively, a much higher Cdl and activity will be achieved by applying the strategy of our work into electrode materials with high specific surface area. Apart from the increased active sites, the N incorporation


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also can enhance the conductivity of MoS2. Electrochemical impedance spectroscopy (EIS) measurements were performed to explore the charge transfer kinetics in HER, as shown in Figure S7. After N doping, the charge transfer resistance (Rct) decrease from 20.1 Ω to 11.5 Ω, reflecting enhanced conductivity after N incorporation in accordance with the XPS and XRD data. The HER performance of the as-synthesized N-MoS2 materials at 900 °C was evaluated, and the polarization curves of N-MoS2 prepared with different activation times are shown in Figure 5a. The results show that the HER activity of N-MoS2 samples firstly increases with an increase in activation time due to a higher N-doping concentration, reaching the maximum when the activation time is 2 h, and then decreases with a further increase in activation time. The N-MoS2-900-2 exhibits the best performance with a LSV current density of 14 mA cm-2 at the potential of 0.50 V, which is 15, 1, 0.4 times higher than that of pristine MoS2, N-MoS2-900-0.5, N-MoS2-900-1, and N-MoS2-900-4, respectively. Moreover, the corresponding Tafel slopes of N-MoS2 with different activation times reveal same variation tendency, as shown in Figure 5b. Derived from the CV curves for treated


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MoS2 at 900 °C in Figure 5c, d and Figure S8, the Cdl of N-MoS2-900-2 is highest (Figure 5e), which is in accordance with its HER performance.


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Figure 5. (a) LSV polarization curves and (b) the corresponding Tafel plots of pristine MoS2 and N-MoS2 prepared with different activation time at 900 °C. (c-d) Electrochemical cyclic voltammogram of pristine MoS2 and N-MoS2 at different scan rates (4-50 mV). (e) Linear fitting of the capacitive currents of the catalysts vs scan rates. (f) Scatter comparison of Tafel slope vs Cdl for N-MoS2-900-2 in this work and MoS2 materials in other literatures.

Since the geometrical area can not reflect the actual catalytic area of HER electrocatalysts, electrochemical active surface area (ECSA) values are always employed to evaluate the intrinsic property of the catalysts, which can be calculated from Cdl (detailed calculation is shown in the Supporting Information). Accordingly, the ECSA values of pristine MoS2 and N-MoS2-900-2 are calculated to be 1.27 and 9.98 cm2, respectively, indicating that the active surface area of MoS2 can be remarkably increased by N-doping. As shown in Table S1, the value of ECSAmass/SBET (Electrochemical active surface area on a unit physical surface area, distribution density of active sites) of NMoS2-900-2 reaches up to 7.03, which is commensurate with that of the previously


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reported ultrathin MoS2/graphene, Mo-N/C@MoS2 and hierarchical MoSe2-CoSe2 nanotubes;56-58 therefore, it is confirmed that the electrochemical active sites of N-MoS2900-2 are densely distributed. In a methodological sense, this work provides an effective approach to effectively increase the density of electrochemical active sites of materials, which can be applied into electrode materials with high specific surface area to achieve ultra-high HER activity. Recently, a variety of strategies for enhancing the HER activity of MoS2 have been reported, i.e. morphology optimization (flower15 and film59), incorporation of O atoms (OMoS2),36 defect creation by plasma treatment (Ar-MoS2 and O2-MoS2),23 and composite preparation (MoS2/rGO)9. Figure 5f compares the Tafel slopes as a function of Cdl for the N-MoS2-900-2 sample prepared in this work and the representative MoS2 materials in the literatures, reflecting the relationship between the catalytic activity and active HER sites. The results show that, although not superior to the optimized MoS2 film, the N-MoS2-9002 sample exhibits much better HER performance for per active site, roughly comparative with MoS2 flower, O-MoS2, O2-MoS2 and Ar-MoS2 samples.15,

23, 36

In order to further

understand the intrinsic catalytic property of N-MoS2, the parameter of per-site turnover


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frequency (TOF) was calculated to accurately evaluate the average activity of each site according to the approach reported by Benck et al.,60 which has been employed in many previous publications61-62 (detailed calculation shown in the Supporting Information). A TOF value of 5.6 H2/s per site was obtained for N-MoS2-900-2 at a current density of 14 mA cm-2 (at 0.5 V), which is comparable with that of the other MoS2 catalysts reported in the literatures.63 As summarized in Table S2, the TOF of N-MoS2-900-2 is 15 times higher than that of the pristine MoS2 at an overpotential of 0.5 V, in accordance with the performance of HER. The larger TOF value of N-MoS2-900-2 is ascribed to the high electron density around the doping sites in MoS2 induced by the N-doping. In summary, the prepared N-MoS2-900-2 sample exhibited excellent HER performance, which can be attributed to three factors induced by the N-doping: i) the disordered structures can offer abundant active sites of HER; ii) N-doping can enhance the electron concentration around the doping sites in MoS2, resulting in its excellent catalytic ability of per active site; iii) the generation of molybdenum nitride can enhance the intrinsic conductivity of the materials.


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CONCLUSIONS

N-doped MoS2 materials with efficient HER activity were synthesized by activating commercial MoS2 via a facile high-temperature treatment method. By legitimately regulating the activation temperature and time, the N-doping concentration up can reach up to 41 at.%. The dominant doping mechanism are determined as a one-to-one substitution of sulfur atoms by N atoms, proved by the approximate constancy between the atomic ratio of Mo/(S+N) and stoichiometric number of original MoS2. XPS and XRD analyses verified the generation of molybdenum nitride in N-MoS2, which can enhance the electrical conductivity of the materials. HR-TEM images demonstrated that N-doping can induce the disordered atomic arrangement on N-MoS2 surfaces, facilitating the HER activity of the catalysts by providing additional active sites. Moreover, the enhanced electron density around the doping sites induced by N doping can alter the whole process of HER. Consequently, the N-MoS2 catalyst exhibits much higher HER performance than the pristine MoS2 with a LSV current density of 14 mA cm-2 and a TOF of 5.6 H2/s per site at the potential of 0.50 V. The facile and controllable approach to activate MoS2 for


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achieving high-level N-doping developed in this study provides a simply routine for the activation procedure of HER electrocatalytic materials.

ASSOCIATED CONTENT

Supporting Information The following files are available free of charge. XPS data; HR-TEM image; Raman spectra; BET data; Electrochemical cyclic voltammogram; Electrochemical impedance spectra; detail calculation and data of ECSA and TOF (PDF).

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (B.H.). *E-mail: [email protected] (M.D.).

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT

This work was financially supported by the National Natural Science Foundation of China (Grant No. 51502026, 21606026 and 21776025), the Natural Science Foundation of Chongqing (Grant No. cstc2016jcyjA0230), the Fundamental Research Funds for the Central

Universities

(Grant

No.

YJ201893,

106112017CDJXY220004,

106112017CDJPT220001 and 2018CDXYHG0028), the National High Technology Research and Development Program of China (Grant No. 2015AA034801), the Danish National Research Foundation and AUFF NOVA-project from Aarhus Universitets Forskningsfond.

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TOC graphic


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Activating MoS2 with Super-High Nitrogen-Doping Concentration as

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