Co-Doped MoS2 Nanosheets with the Dominant CoMoS Phase

Subscriber access provided by UNIV OF NEBRASKA - LINCOLN

Article

Co–doped MoS2 Nanosheets with Dominant CoMoS Phase Coated on Carbon as an Excellent Electrocatalyst for Hydrogen Evolution Xiaoping Dai, Kangli Du, Zhanzhao Li, Mengzhao Liu, Yangde Ma, Hui Sun, Xin Zhang, and Ying Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08420 • Publication Date (Web): 24 Nov 2015 Downloaded from http://pubs.acs.org on November 29, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 41

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

ACS Applied Materials & Interfaces

Co–doped MoS2 Nanosheets with Dominant CoMoS Phase Coated on Carbon as an Excellent Electrocatalyst for Hydrogen Evolution Xiaoping Daia*#, Kangli Dua,b*, Zhanzhao Lia, Mengzhao Liua, Yangde Maa, Hui Suna, Xin Zhanga #, and Ying Yanga a

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249,

China b

Sichuan Tianyi Science & Technology Co. Ltd., Chengdu 610225, China

*

These authors contributed equally to this work.

#CORRESPONDING AUTHOR. Prof X. P. Dai: State Key Laboratory of Heavy Oil Processing China University of Petroleum, Beijing 102249, PR China Tel.: +86 10 89734979; Fax: +86 10 89734979. E–mail address: [email protected] Prof X. Zhang: State Key Laboratory of Heavy Oil Processing China University of Petroleum, Beijing 102249, PR China Tel.: +86 10 89734979; Fax: +86 10 89734979. E–mail address: [email protected]

ACS Paragon Plus Environment


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

Abstract: Highly active and low–cost catalysts for hydrogen evolution reaction (HER) are crucial for the development of efficient water splitting. Molybdenum disulfide (MoS2) nanosheets possess unique physical and chemical properties, which make it as promising candidate for HER. Herein, we reported a facile, effective and scalable strategy by deposition–precipitation method to fabricate metal–doped (Fe, Co, Ni) molybdenum sulfide with few layers on carbon black as noble metal–free electrocatalysts for HER. The CoMoS phase after thermal annealing in Co–doped MoS2 plays a crucial role for the enhanced HER. The optimized Co–doped MoS2 catalyst shows superior HER performance with high exchange current density of 0.03 mA·cm–2, low onset potential of 90 mV and small Tafel slope of 50 mV·dec–1, which also exhibits excellent stability of 10000 cycles with negligible loss of the cathodic current. The superior HER activity originates from the synergistically structural and electronic modulations between MoS2 and Co ions, abundant defects in the active edge sites as well as the good balance between active sites and electronic conductivity. Thanks to their ease of synthesis, low cost and high activity, the Co–doped MoS2 catalysts appear to be promising HER catalysts for electrochemical water splitting.

Keywords: MoS2; Co–doping; deposition–precipitation method; CoMoS phase; hydrogen evolution reaction


Page 2 of 41

Page 3 of 41

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


1. INTRODUCTION Hydrogen, as high gravimetric energy carrier, can be currently generated by steam reformation or partial oxidation of hydrocarbon. However, the increasingly serious energy crisis and the environmental pollution have triggered an aggressive search for renewable and environmentally friendly alternative energy recourses.1,2 In this respect, water splitting is one of promising and appealing pathways by either light or electricity.1–7 Efficient cathode catalysts for hydrogen evolution reaction (HER) are the key to achieve optimal performance in water splitting. Up to now, the most effective HER electrocatalysts such as platinum have been limited to widespread application due to their scarcity and high costs, and it still remains a great challenge to develop highly effective catalysts based on earth–abundant elements. Several non–noble metal materials, such as transition–metal chalcogenides,2,8–15 carbides,16,17 phosphides,18 phosphosulfide,19 as well as nitrides20,21 have been reported as electrocatalysts for HER. Among these materials, molybdenum chalcogenides (MoS2), a widely used as hydrodesulfurization (HDS) catalysts in petroleum industry, has received special attention due to the close free energy of adsorbed atomic hydrogen with that of Pt–group metals (i.e., ∆GH*≈0), which make it as promising alternative for HER.22–24 However, low conductivity, limited surface area and the inert two–dimensional MoS2 surface make it challengeable to obtain high HER performance. To address the above issues, active site engineering and electronic conductivity engineering are the general strategies to conduct the rational designs on nanoscale MoS2 catalysts.8,9,24–30 Although striking achievements have been made in recent works, many practical challenges still remain to improve the activity and stability of MoS2–based catalysts for large–scale fabrication. It is well known that the HDS performance on supported MoS2 catalysts strongly depends on the promotion degree of MoS2 by the adjacent Co and Ni sulfides yielding CoMoS or NiMoS phases.31,32 The small, stable and high dispersion of nanoclusters MoS2 with dominant CoMoS ACS Paragon Plus Environment


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

(NiMoS) phases are of greatest importance in HDS reaction. For HER, the catalytic performance exhibited a strong dependence on the hydrogen adsorption energy and kinetic energy barrier of hydrogen evolution pathway. The introduction of a foreign metal element in the MoS2 lattice affords the opportunity to engineer the electronic and/or surface structures for improving the HER performances. For example, metal elements including Fe, Co, and Ni have been successfully doped into MoS2 crystal structure, and led to an increase in HER activity, which may relate to the morphology changes and/or chemical changes by creating novel bimetallic active sites with sulfur bridging.33–36 Merki et al. investigated the promotional the effects of transition metal ions on the HER activity over amorphous MoS3 films, and showed that Fe, Co and Ni ions can promote the growth of the MoS3 films to exhibit a high surface area, higher catalyst loading and significantly higher HER activity.34 The morphological and composition changes as well as the interaction between metal ions and unsaturated atoms in MoS3 play an positive role for the enhancement in HER performance. Zhang et al. reported a synthetic method of 2H–MoS2 by means of one–pot cobalt acetate/graphene oxide (GO) co–assisted hydrothermal reaction.35 The excellent HER performance could be ascribed to synergetic effects by high interconnectivity and conductivity, which accelerated the electron transfer and formed abundant defects. Tran et al. fabricated ternary sulfides of Co–W–S and Ni–W–S on a conducting electrode surface by electrodeposition process.37 The CoWSx catalysts possessed WS2–like layered structures containing Co–S–W clusters with 3 to 5 layers, and exhibited significantly improvement in HER performance. Wang et al. doped a small amount of Ni atoms into the vertically aligned MoSe2 molecular layers by thermally evaporation and rapid selenization process.38 The Ni atoms were homogeneously incorporated into the MoSe2 matrix, which led to superior HER performance. Lv et al. fabricated a series of Ni–promoted MoS2 microspheres with high active surface area and density of electrochemically active sites in HER by facile hydrothermal synthesis.39 The similar result has also been observed by Xu’s et al. on the Fe, ACS Paragon Plus Environment

Page 4 of 41

Page 5 of 41

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


Co, Ni–promoted MoS2 films, whose active surface areas were 1.4−3.3 times of that of the unpromoted MoS2+x film.24 Such direct deposition/synthesis techniques could conceivably allow the large–scale fabrication of MoS2 electrodes for energy applications. Nonetheless, many techniques employed in literature offer poor scalability. Thus, a facile, reliable and scalable method to achieve large–scale MoS2–based HER catalysts is essential for widespread application. Herein, we proposed a scalable strategy to fabricate different metal–doped (Fe, Co, Ni) molybdenum sulfides by deposition–precipitation method in an industrially compatible process, and investigated the effects of the doping of metal (Fe, Co, Ni) on the HER performance of molybdenum disulfide. The optimized Co–doping MoS2 catalyst significantly increases the HER performance with high exchange current density, low onset potential and Tafel slope. The superior HER performance are attributed to the formation of Co–Mo–S phase, which effectively reduce the charge–transfer impedance, synergistically increase structural and electronic modulations between MoS2 and Co, as well as high active surface area, indicating a promising cathode catalyst candidate. 2. EXPERIMENTAL SECTION 2.1. Materials. Ammonium tetrathiomolybdate ((NH4)2MoS4, 99.95%) and commercial Pt/C (20 wt. % Pt on Vulcan carbon black) were purchased form Alfa Aesar. Carbon black, cobalt acetate tetrahydrate (Co(CH3COO)2·4H2O), nickel acetate tetrahydrate (Ni(CH3COO)2·4H2O), iron nitrate nonahydrate (Fe(NO3)3·9H2O) and acetic acid were purchased from Sinopharm Chemical Reagent Co. Ltd. All reagents are analysis reagent (A.R.) and used as received without further purification. Deionized water (Milli–Q) was used for the synthesis of catalysts. 2.2 Catalyst Preparation. First, 0.2 g carbon black was dispersed into the mixture of 10 mL acetic acid and 20 mL deionized water under stirring at 338 K. Second, a precursor CoMoS was prepared by dropwise addition of 20 mL (0.0384 M) aqueous (NH4)2MoS4 and 10 mL (0.0154 M) aqueous Co(CH3COO)2·4H2O (Co/Mo=0.2, molar ratio) into the above solution under stirring for 2 h. Third, ACS Paragon Plus Environment


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

the resulted suspension was ﬁltered, washed with deionized water, and dried in a vacuum oven at 80 °C overnight, which was denoted as CoMoS–2. Finally, the CoMoS–2 was ﬁnally annealed at 400 o

C, 500 oC and 600 oC for 4 h under nitrogen atmosphere to obtain CoMoS–2–C400, CoMoS–2–C

and CoMoS–2–C600, respectively. The CoMoS samples with various molar ratio of Co to Mo (0, 0.1, 0.3 and 0.4) were prepared by changing the molar of Co(CH3COO)2·4H2O with similar process with CoMoS–2, denoted as Mo–S, CoMoS–1, CoMoS–3 and CoMoS–4, respectively. After thermal annealing, they are denoted as Mo–S–C, CoMoS–1–C, CoMoS–3–C and CoMoS–4–C, respectively. As comparison, the NiMoS–2–C and FeMoS–2–C were also prepared with same process with CoMoS–2–C. 2.3 Catalyst Characterization. Powder X–ray diffraction (XRD) was performed on a Brüker D8 Advance diffractometer at 40 kV and 40 mA for CuKα (λ=0.15406 nm). Raman spectra were recorded with a Renishaw Micro–Raman System 2000 spectrometer at 532 nm of a He/Cd laser. Scanning electron microscopy (SEM) images were recorded on an FEI XL30 Sirion SEM. Transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) images were obtained with FEI Tecnai G2 F20 electron microscope equipped with a field emission source at an accelerating voltage of 200 kV. X–ray photoelectron spectrum (XPS) analysis was performed on a PHI 5000 Versaprobe system using monochromatic Al Kα radiation (1486.6 eV). All binding energies were referenced to the C 1s peak at 284.6 eV. 2.4 Electrochemical Measurements. All of the electrochemical measurements were carried out at room temperature on a CHI660E electrochemical workstation in a three–electrode system with the glassy carbon electrode (GCE, 0.07065 cm2, geometric area) as the working electrode, a saturated calomel electrode (SCE) as reference electrode and a Pt slice as counter electrode. Typically, the mixture containing 4 mg catalyst, 1 mL water–ethanol (4:1, v/v) and 80 µL 0.5 wt% Nafion (Alfa


Page 6 of 41

Page 7 of 41

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


Aesar) was ultrasonicated to form ink solution. 5 µL ink solution was dropped onto the GCE (loading 0.285 mg· cm−2). The polarization curves were obtained by linear voltammetry sweeping with scan rate of 5 mV·s−1 in 0.5 M H2SO4. AC impedance measurements were carried out at η = 0.15 V from 106–0.01 Hz. The stability was carried out by continuous cyclic voltanmmetry (CV) from –0.2 to 0.3 V (vs. SCE) at a sweep rate of 100 mV·s–1. The estimation of the effective active surface area of the samples was carried out according to literature by cyclic voltammograms with various scan rates (20, 40, 60 mV/s, etc.) in the 0.1–0.2 V vs. RHE region.35,40 During the experiments, a flow of nitrogen was maintained over the electrolyte to eliminate dissolved oxygen. All results were calibrated with respect to reversible hydrogen electrode (RHE) by E(RHE) = E(SCE) + 0.273 V. Number of active sites and turnover frequency (TOF, s−1) of Co‒doped MoS2 catalysts were calculated by electrochemical approach through cyclic voltammetry measurements in pH=7 phosphate buffer at a scan rate of 50 mV·s −1 according to the previous methods.41,42 n=

Q 1 1 i ⋅ t 1 1 i ⋅ V / u 1 1 10 ⋅ S ⋅ ⋅ = ⋅ ⋅ = ⋅ ⋅ = F 2 m F 2 m F 2 m F⋅m

(1)

The per–site turnover frequencies (TOFs, s–1) can be calculated with the following equations: Q 1 i ⋅ t 1 i ⋅ V / u 1 10 ⋅ S ⋅ = ⋅ = ⋅ = F 2 F 2 F 2 F j 1 TOF = ⋅ F⋅ N 2 N=

(2) (3)

Where n is the number of active sites (mol·g–1 catalyst). Q is the integrated charge from cyclic voltammograms in pH = 7 phosphate buffer, F is the Faraday constant (96485 C·mol–1), i, V, t, u, S, m, j and N are the current (A), potential (V), sweep time (s), sweep rate (V·s–1), integrated effective area in cyclic voltammograms recorded in pH=7 phosphate buffer after deduction of the blank value for GCE, the mass of active component (denoted as CoyMoSx, the composition can be calculated from XPS survey spectra) in the catalyst, the current (A) during the linear sweep measurement in 0.5 M H2SO4, and the total number of active sites (mol), respectively. ACS Paragon Plus Environment


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

3. Results and discussion 3.1 Material Synthesis and Characterization. The Fe, Co and Ni–doped MoS2 nanosheets (MMoS) on carbon black were prepared via a facile deposition–precipitation method using (NH4)2MoS4 and Co(CH3COO)2·4H2O/Ni(CH3COO)2·4H2O)/Fe(NO3)3·9H2O as precursors in acetic acid solution (Scheme 1). The MoS42− ion with lowlying unoccupied orbitals shows versatile coordination behavior with Ni2+ or Co2+ forming complexes like [M(MoS4)2]n− or MMoS4, which have strong 3d–π interactions in the ligand orbitals (i.e. M → MoS4 electron delocalization), accompanied by reduction in the coordination symmetry and perturbation of the electronic structure.34,43–45 The introduction of Fe, Co or Ni, and heating treatment are utilized together to optimize the amorphous structure in an attempt to enhance the HER activity. Figure. 1 and Figure S1 show the XRD patterns of the as–prepared catalysts. The unannealed CoMoS–2 has the amorphous structure due to the lack of XRD peaks. As the annealing temperature increasing in N2 atmosphere, the MoS3 is reduced to MoS2 with gradually improved crystallinity and well–defined hexagonal structure (molybdenite, JCPDS card No. 37–1492) (Figure S1A). The asymmetric shapes on the low angle side of the reflections at 33, 40 and 58

o

are typical features of layered materials with a partial turbostratic

disorder.46 The sustained growth of the (002) planes at about 14 o with the increasing ratio indicates the formation of multi–layer MoS2 structure. Generally, cobalt may be present in three distinctly phase, viz., cobalt located in carbon lattice, cobalt in Co9S8 and cobalt located in CoMoS phase.47,48 The resulted catalysts with various atom ratios of Co to Mo have not shown characteristic peaks of Co species in XRD patterns (such as Co, Co9S8), implying that quite large amounts of cobalt can be accommodate in the so called “CoMoS” phase. The CoMoS phase consists of patches of single S–Mo–S slaps with cobalt atoms most probably located at Mo sites.49 We also investigate the effects of Fe and Ni, which suggest the similar results with CoMoS–2–C by the formation of FeMoS and NiMoS phase (Figure S1B).50,51 ACS Paragon Plus Environment

Page 8 of 41

Page 9 of 41

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


Raman spectroscopy was employed to further investigate the MoS2 structure for unannealed CoMoS–2, annealed MoS–C and CoMoS–2–C in Figure 2. The weak signals of distinctive A1g and E12g vibrational peaks suggests that the quantities of crystalline MoS2 is not significant in the CoMoS–2,52,53 agreeing well with the XRD characterization. The two dominant peaks at 379.6 and 401.8 cm–1 in the MoS–C correspond to the E12g and A1g modes of hexagonal MoS2 crystal, respectively. The vibration direction of A1g mode is c–axis and in–layer displacements of Mo and S atoms, which preferentially excites for the edge–terminated structure and results in a low E12g to A1g peak ratio.54 The E12g mode vibration direction is within basal plane, and preferentially excites for the terrace–terminated structure obtained, with the integrated intensity of E12g mode close to that of A1g.54,55 The relative integrated intensity ratio of the E12g to A1g peaks in Figure 2A provides rich texture information, which suggests edge–terminated structure with a small E12g peak about 42% of A1g peak. After Co–doping, the frequency of E12g peak (378.3 cm–1) decreases while that of the A1g peak (403.1 cm–1) increases for CoMoS–2–C, which are related to the layer number of MoS2.56,57 The van der Waals force in MoS2 sheets results in a higher force constant for atomic vibration with the number of layers increasing, which gives rise to blue shift of E12g peak. On the other hand, stacking–induced structure change plays a dominant role, which leads to a redshift in A1g peak. The CoMoS–2–C spectrum has the same relative intensity ratio of the E12g to A1g peaks with the MoS–C (~2.29), suggesting that it favours the vibration of A1g mode and thus the best edge–terminated structure.54,58 All of the MoS–C, CoMoS–2 and CoMoS–2–C exhibit two distinct bands at 1341 and 1590 cm–1 with similar ID/IG value (1.1), matching well with the D, G band of carbon, respectively. The morphology and structure of the as–prepared composites were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The nanoparticles with a mean diameter of about 70 nm for CoMoS–2–C is observed from the SEM image in Figure 3A. The TEM images in Figure 3B and Figure S2A–C indicate that some nanoparticles are highly dispersed, ACS Paragon Plus Environment


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

Page 10 of 41

which can be attributed to the large surface area of the carbon support and a weak metal–support interaction. The weak interaction between the carbon black and the MoS2 layers are similar to van der Waals forces.59 The HRTEM images of Figure 3C, Figure S2B, E and F suggest that MoS2 nanosheets form a thin layer on the carbon black surface. The MoS–C catalyst is composed of nearly two or three layers with a layer distance of 0.62 nm, corresponding to the (002) plane of MoS2. The HRTEM and FFT results indicate the semi–crystalline nature of the MoS3 layers over the CoMoS–2 (Figure S2D and the inset), while the CoMoS–2–C demonstrates a polycrystalline structure with continuous diffraction rings from SAED (Figure 3D).60 The element mapping and corresponding energy dispersive X–ray (EDX) images shows almost similar distribution and highly dispersion of Co and Mo atoms (Figure 3E). Notably, after thermal annealing, the crystal fringes along the curled edge are clearly observed with apparent discontinuity over the CoMoS–2–C, which can be attributed to the existence of abundant defects.35 The average slab length ( L ) and stacking layer number ( N ) of MoS2 are obtained by the following equation through statistical analyses based on 10–20 micrographs including about 100–200 slabs obtained from different parts of the MoS–C and CoMoS–2–C.61 n

∑xM i

L( N ) =

i

i =1

(4)

n

∑x

i

i =1

Where Mi is the slab length or stacking layer number of a stacked MoS2 unit, and xi is the number of slabs or stacking layer number. Table 1 gives the slab length distributions and stacking layer number distributions of the two catalysts. The MoS2 nanoslabs on the CoMoS–2–C have a smaller average length (5.4 vs. 6.9 nm) and much higher stacking (4.6 vs. 2.8 layers), indicating that the MoS2 nanoslabs on the CoMoS–2–C catalyst are well dispersed with a much narrower size distribution than that of the MoS–C. ACS Paragon Plus Environment

Page 11 of 41

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


XPS was used to identify the chemical state of the CoMoS–2, annealed MoS–C and CoMoS–2. According to the results of XPS, the S/Mo atomic ratios estimated from the integrated peak area of XPS survey spectra are 3.07, 1.98 and 2.25 for CoMoS–2, CoMoS–2–C and MoS–C, respectively, suggesting that the structure of CoMoS–2 is close to MoS3 while the structure of CoMoS–2–C and MoS–C are close to MoS2 (Figure 4A). The different atomic ratios of S/Mo between CoMoS–2–C and MoS–C may originate from the partial replacement of the molybdenum sites by cobalt atoms at the MoS2 edges.50,62 From the XPS survey spectra, the mass content of active component (CoyMoSx) can be calculated as 41.8, 39.8 and 34.2 wt. % over MoS–C, CoMoS–2–C and CoMoS–2, respectively. Four peaks are observed in the high–resolution XPS of Mo 3d–S 2s over MoS–C and CoMoS–2–C, where the one at 226.5 eV is corresponded to S 2s of MoS2. The two main peaks at 229.3 and 232.5 eV can be assigned to the Mo4+ ions while the high binding energy of Mo 3d (236.3 eV) corresponded to MoO3.52 The Mo 3d3/2 and 3d5/2 at 232.9 and 229.8 eV with separation energies close to 3.1 eV can be attributed to the presence of Mo5+ ions, implying the dominant presence of Mo5+ in the CoMoS–2.61 It is worthwhile pointing out that the peak at 227.7 eV shifts to high binging energy about 1.2 eV, which can be attributed to bridging disulfides S22–.33 The large number of bridging disulfides S22– has further been confirmed by the S 2p spectra in Figure 4C. The single doublet at 162.2 and 163.4 eV, which should attributed to the apical S2– for MoS2. Notably, another doublet at 163.9 eV and 165.0 eV is assigned to bridging disulfides S22–.63 The thermal annealing of the complexes like [M(MoS4)2]n− or MMoS4 is accompanied by molybdenum–sulfur redox processes, which include the oxidation of S2– ligands of the MoS42– anion and the reduction of molybdenum metal from MoVI to MoIV, and various thermal decomposition intermediate may exist.64 As a result, the high binding–energy doublets for bridging disulfides S22– ligand decrease or completely disappear, indicating that the bridging disulfides S22– ligand will be reduced to apical S2– ligand to form highly crystallized MoS2 layers.39,65 The Co 2p spectra are fitted with four doublets (Figure 4D), ACS Paragon Plus Environment


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

the first one at 779.2–779.6 eV from the CoMoS phase, the second one at 794.1–794.5 eV from the Co2+, and the third and fourth peaks, from a broadened peak satellite signal.66–68 The binding energy of the first peak significantly shifts to a value ~0.9 eV higher than that of Co9S8 phase, which was reported at 778.5 eV for Co9S8 phase,69,70 indicating negligible amount of Co9S8 phase in the CoMoS–2 and CoMoS–2–C. The formation of CoMoS phase also is supported by the difference of binding energy between Co 2p3/2 and Mo 3d5/2 with 549.9 eV for CoMoS–2 and CoMoS–2–C, which is relatively close to the value of 550.0 eV reported in the literature with the cobalt atoms located in a MoS2 phase.71,72 Based on these facts, it can be assumed that CoMoS phase are dominant and well–dispersed onto the surface of carbon black. 3.2. HER activity and stability. The HER performance of as–prepared samples were shown in Figure 5, and commercial Pt/C catalyst was also measured for comparison. The commercial Pt/C exhibits the highest HER catalytic performance (with a near zero overpotential), while the inferior HER activity is observed in the MoS–C. After the addition of Fe, Co and Ni, the catalytic activities towards HER are significantly promoted (Figure S3). Especially, the CoMoS–2–C exhibits the best performance with lower onset potential of nearly 90 mV and higher current density of 60.9 mA cm–2 at η=200 mV among the as–prepared FeMoS–2–C, NiMoS–2–C and annealed CoMoS catalysts with various Co/Mo ratio and calcination temperature (Figure 5A and Figure S3–S4). The trend of HER activity in these doped catalysts is CoMoS–2–C>NiMoS–2–C>FeMoS–2–C>MoS2–C, which is in good agreement with the DFT calculations.55,73 Deng et al. investigated the HER activity of MoS2 doped with Fe, Co and Ni atoms, and found the strong adsorption of H* over Ni–doped MoS2 and weak binding ability with H over Fe–doped MoS2, which could cause distinctly low HER activity.73 Coincidently, Co–doped MoS2 can provides medium adsorption free energy of H atoms (∆GH), and exhibits high HER activity of the Co–MoS2 catalyst.55,73 More importantly, the CoMoS–2–C in acidic medium shows much higher current density (61.9 mA·cm–2) at the overpotential of 200 mV ACS Paragon Plus Environment

Page 12 of 41

Page 13 of 41

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


than most of Mo–based HER electrocatalysts reported up to date (Table S1), such as oxygen–incorporated MoS2 nanosheet,7 exfoliated MoS2 nanosheets,40 amorphous MoSx,52 defect–rich MoS2,25 MoS2/rGO,8,29 MoS3/CNT,74 Co0.6Mo1.4N2,20 NiMoNx/C,21 Au–MoS2 film,22 Cu2MoS4,75 Ni–promoted MoS2,39 Co–MoS3 film,34 and Co9S8@MoS2/CNFs.76 Notably, although the CoMoS–2 has a large amount of bridging S22– ligand, which are generally highly active for HER, the HER performance is still inferior to that of CoMoS–2–C. In order to explain the reasons, CoMoS samples with various Co/Mo ratios before thermal annealing are investigated in HER (Figure S5). Very interesting, the catalytic performance remains to be similar for all the samples, implying the negligible promotion effects without CoMoS phase. The results suggest that the CoMoS phase may plays more crucial role, which can significantly decrease the hydrogen binding energy (∆GH) at the S–edge, and weaken H* adsorption.33,34,36 The effective active surface area (Aeff) was investigated by cyclic voltammetry at various scan rates (20, 40, 60 mV·s–1, etc.) in 0.1‒0.2 V vs. RHE region in Figure 5B and Figure S6. From the cyclic voltammograms, the Aeff can be calculated by plotting the △J at 0.15 V vs. RHE in CV against the scan rate (Figure 5C), where the slope is twice the Aeff.34 The calculated values of the Aeff are 4.7, 8.6 and 6.7 for MoS–C, CoMoS–2 and CoMoS–2–C, respectively. The results indicate that Co doping into the S–edge structures of MoS2 could create more active site for electrochemical activity than that prior to promotion, which was also observed in the Fe, Co and Ni–promoted MoS2+x films for HER by Xu’s group.24 However, the larger electrochemical surface areas in the CoMoS–2 than that of CoMoS–2–C are observed, which should ascribe to the abundant S22− in the CoMoS–2.77 The stability of Co–doped MoS2 towards HER were assessed by the continuous long–term cycling test (10000 cycles) in an acidic environment, which indicates the negligible decay of the cathodic currents for CoMoS–2–C (Figure 5D), while CoMoS–2 exhibits inferior stability with greater loss of the cathodic current (~18 mV at 60 mA·cm–2) (Figure S7). The engineering in Mo–S ACS Paragon Plus Environment


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

Page 14 of 41

edge increases structural stability, and further regulate the electronic structure of MoS2 semiconductors and then tune the conductivity of hybrid materials.78 The substantial long–term stability of the CoMoS–2–C suggests the great promise to fabricate cost–effective and efficient H2 evolution electrode in water electrolysis systems. 3.3 The kinetics and HER mechanism in HER. The electrode kinetics and interface reactions undergone by as–prepared samples during the HER process can be demonstrated by electrochemical impedance spectroscopy (EIS). Only one semicircle at η=150 mV is observed in each EIS nyquist plots, indicating that the equivalent circuit for the electrocatalysis is characterized by one time constant (Figure 6A and Figure S8). Generally, EIS nyquist can be fitted to an equivalent circuit (inset of Figure 6A and Figure S8), where a constant phase element (CPE) represents the deviation from the ideal capacitance behavior corresponding to a frequency–dependent phase–angle. This capacitance dispersion of solid electrodes depends strongly on the state of the electrode’s surface, e.g. its roughness and ionic adsorption. The double–layer capacitance (Cdl) of the cathode can be estimated using the following equation.79 -1

-1

C = [T ⋅ (R s + R ct ) - (1- α ) ]1/α

(5)

Where T represents the capacitance parameter obtained as the result of fitting (F s−1 cm−2), Rct is the charge–transfer resistance (Ω·cm2) and α is independent exponent (0

Co-Doped MoS2 Nanosheets with the Dominant CoMoS Phase

Recommend Documents