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Active basal plane catalytic activity and conductivity in Zn doped MoS2 nanosheets for efficient hydrogen evolution
Accepted Manuscript Active basal plane catalytic activity and conductivity in Zn doped MoS2 nanosheets for efficient hydrogen evolution Peitao Liu, Jingyi Zhu, Jingyan Zhang, Kun Tao, Daqiang Gao, Pinxian Xi PII:
S0013-4686(17)32444-1
DOI:
10.1016/j.electacta.2017.11.080
Reference:
EA 30670
To appear in:
Electrochimica Acta
Received Date: 5 June 2017 Revised Date:
16 October 2017
Accepted Date: 12 November 2017
Please cite this article as: P. Liu, J. Zhu, J. Zhang, K. Tao, D. Gao, P. Xi, Active basal plane catalytic activity and conductivity in Zn doped MoS2 nanosheets for efficient hydrogen evolution, Electrochimica Acta (2017), doi: 10.1016/j.electacta.2017.11.080. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Active basal plane catalytic activity and conductivity in Zn doped MoS2 nanosheets for efficient hydrogen evolution Peitao Liua , Jingyi Zhu b , Jingyan Zhanga , Kun Taoa, Daqiang Gaoa*, Pinxian
a
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Xic*. Key Laboratory for Magnetism and Magnetic Materials of MOE, Key Laboratory of Special Function Materials and Structure Design of MOE, Lanzhou University, Lanzhou 730000, P. R. China.
Department of Physics and Astronomy, Clemson Nanomaterials Center and
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b
COMSET, Clemson University, Clemson, SC 29634, USA Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu
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c
Province and The Research Center of Biomedical Nanotechnology, Lanzhou University, Lanzhou, 730000, P. R. China The two authors have equal contribution to this work
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* Corresponding author: [email protected], [email protected]. Abstract
Transition metal dichalcogenides based electrocatalysts have been proposed as substitutes for noble metals to generate hydrogen in electrolysis of water, but these
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alternatives usually suffer from their inferior performance of intrinsic semiconducting property and inert basal plane. Here
Zn dopants are proved to can intrigue the
catalytic activity of MoS2 nanosheets with the low overpotential of 140 mV at 100
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mA/cm2 and Tafel slope of 35 mV/dec. Density functional theory calculations are performed to unravel the underlying mechanism, revealing that the inert basal plane is activated by Zn dopants with the H adsorption free energy (△GH*) of -0.07. Further, the charge distribution spreading over the basal plane induced by strong hybridizations of metal d-states and sulphurous p-states play a key role for promoting H atom adsorption and desorption kinetics simultaneously. This study opens new opportunities for the designing and understanding of MoS2-based catalysts for water splitting or other applications. 1. Introduction With the increasing attestations on excessive consumption fossil fuel and their
ACCEPTED MANUSCRIPT homologous environmental pollution problems, numerous efforts have been devoted to search for sustainable alternative energy. Hydrogen (H2), with a high gravimetric energy density, is considered to be the excellent candidate of the mobile energy carrier.[1, 2] Importantly, H2 can be made from the electrolysis of water without any CO2 emission, where the electricity could be produced from intermittent solar and
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wind energy resources. To improve the energy efficiency, efficient and earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) are needed. Although platinum (Pt) and its alloys have been proved to be the benchmark catalysts for HER, the prohibitive cost and scarcity hamper their widespread commercial use. Therefore,
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searching for earth-abundant materials based catalysts is critical for the future hydrogen economy.[3, 4]
During the past years, MoS2-based materials have been proved to be the
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economical and ecofriendly HER catalysts owing to their two dimensional (2D) structure, electrocatalytic performance, and cost-effectiveness.[5-7] There is a consensus that the catalysts' HER performance is highly depending on the synergistic effect of the active sites and the conductivity of the materials.[8, 9] Therefore, for optimizing the HER efficiency of the MoS2-based catalysts, researches have been mainly focused on two main strategies: 1) to enhance the amounts of active site and 2)
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to increase the conductivity.[10] Monolayer MoS2 belongs to direct semiconductor with the band gap of 1.8 V, so the conductivity of MoS2 catalysts can generally be enhanced via utilizing conductive supports from metals or carbon materials.[11, 12] For the active sites of the MoS2, both density function theory (DFT) calculation and
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subsequent experimental results reveal that their edge sites are active for HER while their basal plane is catalytically inert.[13] Therefore, extensive efforts have been
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devoted to develope nano- or defect-riched MoS2 structures to maximize their active edge sites.[14, 15] However, besides increasing the amount of exposed edge sites, little attention has been paid to explore the strategies of stimulating the catalytic activity of inert in-plane atoms of 2D MoS2 to enhance the HER kinetics.[16, 17] Recently, several attempts have been made to activate the basal plane of
2H-MoS2. For example, Deng et.al demonstrated that the catalytic activity of in-plane S atoms of MoS2 can be significantly aroused via single-atom Pt metal doping in HER.[18] Ye et. al reported the improved HER activity of MoS2 and MoP via formation of a MoS2(1– x)Px solid solution and their calculation results demonstrated that P-doping can activate MoS2 basal plane toward HER activity by reducing the
ACCEPTED MANUSCRIPT adsorption free energy (△GH*).[19] Huang et.al further revealed that P-doping could dramatically reduce Mo valence charge and can active the inert MoS2 basal plane or S-edge with the assistant of O elements co-doping.[20] These reports have stimulated further development of MoS2 systems to bring high HER performance by arousing their activity of basal plane with dopants. However, there remains several inevitable formation of MoP and oxide phases.
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issues in these methods, such as the use of precious metal Pt or the spontaneous Recently, as predicted by Deng et.al in the volcano plot for varied single-atom metal doped MoS2, the non-precious metal dopants of Zn could also be good
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candidate to tune the catalytic activity of MoS2.[18] Efficient all-pH value electrochemical and photocatalytic hydrogen evolution was also observed in hollow Co-based bimetallic sulfide polyhedral by Zn doping.[21] These results indicate that
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Zn dopants are the good candidate to activate the active sites of other catalysts. Therefore, we here report the HER performance of Zn doped MoS2 catalysts with systematic experimental and first-principles study. A simple hydrothermal method has been used to fabricate Zn doped MoS2 nanosheets, where the best HER catalytic activity of the Zn doped MoS2 catalyst possesss an onset overpotential of 40 mV, an overpotential of 140 mV at 100 mA/cm2 and a Tafel slope of 35 mV/dec. Through the
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measurements of double-layer capacitance, turnover frequency and electrochemical impedance of Zn doped MoS2 catalysts, we draw the conclusion that both the amount of active HER sites and the conductivity of MoS2 nanosheets increase as results of Zn doping. Further first-principle calculation results confirm our experimental results that
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Zn dopants could activate the HER activity of MoS2 basal plane, and concurrently enhance electronic conductivity of the basal plane to promote fast charge transfer in
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HER process. The finding opens a new door to realize non-noble-metal-doped MoS2 for activating their inert basal plane. 2. Experimental 2.1 Synthesis
Pure and Zn doped MoS2 nanosheets are fabricated by the simple hydorthemal
method. In brief, 1.5 g sodium molybdate dihydrate [Na2MoO4·2H2O], 2.2 g thioacetamide [CH3CSNH2] and different amounts of zinc nitrate hexahydrate ( ZnNO3·6H2O) (0 g, 0.1 g, 0.2 g, 0.4 g, 0.6 g) were dissolver in 80 ml deionized water under magnetic stirring. The mixed solution was stirred for 1 h and then transferred into a 100 ml Teflon-lined stainless steel autoclave, heated at 200 °C for
ACCEPTED MANUSCRIPT 24 h. Then the black products were separated by a string machine, heating 60 °C for further use. In order to succedent study, the typical as-prepared samples were named as MoS2 (0 g), Zn-MoS2 (0.2 g) and ZnS/MoS2 (0.6 g). 2.2 Materials Characterizations X-ray diffractometer (XRD; X’pert Pro Philips with Cu K radiation) was used to
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study the crystal structure. Scanning electron microscopy (SEM; Hitachi S-4800) and transmission electron microscopy (TEM; Tecnai G2 F30, FEI) were employed to image the microstructure and atomic structure of the samples. Energy-dispersive X-ray spectroscopy (EDX) attached in the TEM was used to obtain elemental
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mappings. X-ray photoelectron spectroscopy (XPS; Kratos AXIS Ultra) and Raman spectrometer (Jobin-Yvon LabRam HR80) were utilized to analyze the chemical states
and
bonding
characteristics
of
the
N-doped
MoS2
samples.
The
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Brunauer-Emmett-Teller (BET) surface area and pore width were measured by using a Micrometrics ASAP 2020 V403 measurement. 2.3 Electrochemical Measurements
Catalyst inks was typically made from dispersing 10 mg of catalyst and 10 mg of carbon black (Vulcan XC72) in 50 ml petroleum ether. After drying, 3 mg of this catalyst, 30 µL Nafion-117 solution and 1470 L N,N dimethylformamide (DMF) were
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added into 5 mL container and ultrasonication for 30 min. Afterward, an aliquot of 3 µL was pipetted onto the glassy carbon electrode to obtain the catalyst loading of 0.32 mg cm-2. A CHI 660E electrochemical workstation was used to perform all electrochemical tests with a standard three-electrode cell. Ag/AgCl and graphite rod
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were used as reference and counter electrodes, respectively. Linear sweep voltammetry (LSV) measurements were conducted at a scan rate of 2 mV/s at 23 ºC in
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0.5 M H2SO4 electrolyte. The electrolyte was desecrated by purging N2 for 30 min prior to the measurements. The stability of the catalysts was evaluated by carried out cyclic voltammetry measurement at a scan rate of 100 mV/s for 10000 cycles followed by LSV measurement at a scan rate of 2 mV/s. All polarization curves were iR-corrected unless noted. All the potentials were calibrated with respect to a reversible hydrogen electrode (RHE) by the equation E(RHE) = E(Ag/AgCl) + 197 mV. The Nyquist plots were measured with frequencies ranging from 100 kHz to 0.1 Hz at potential of 260 mV vs. RHE. A simplified Randles circuit is used to fit the impedance data to extract the solution and charge-transfer resistances (RS and RCT). 2.4 First-principles Calculations and Modeling
ACCEPTED MANUSCRIPT All calculations were performed within the framework of of density functional theory (DFT) as implemented in the Vienna Ab initio Simulation Package (VASP).[22] The exchange–correlation interactions were treated by generalized gradient approximation (GGA) parameterized by Perdew, Burke, and Ernzerhof (PBE). The interaction between ions and electrons was described using the projector augmented
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wave (PAW). For the plane-wave basis restriction, the cut-off energy of 400 eV was set in all calculations. For the Brillouin-zone integration, K-points were sampled under Monkhorst-Pack.[23] The atomic structures for all models were fully relaxed with self-consistency accuracy of 10-5 eV reached for electronic loops. For the MoS2
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monolayer, vacuum slab of 15 Å was inserted in z direction for surface isolation to prevent interaction between two neighboring surfaces. A 4×4×1 supercell was adopted to assess the electronic property and hydrogen evolution activity with the calculated 3. Results and discussions
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lattice constants of a = b = 3.24 Å after fully relaxation.
X-ray diffractometer (XRD) and Raman results indicate that as the increasing of the Zn dopants, Zn atoms are firstly doped into MoS2 matrix and then form the second impurity phase of ZnS with the increasing of the Zn dopants (Figure S1). Here, three representative samples of pure MoS2 (MoS2), Zn doped MoS2 with the Zn
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concentration of 6.1 at. % (Zn-MoS2) and the MoS2 with ZnS impurity phase (Zn/MoS2) are employed to study the effect of Zn dopants on the microstructure and HER activity of MoS2. As shown in Figure 1a, samples MoS2 and Zn-MoS2 possess six distinct diffraction peaks of (002), (100), (103), (105), (110) and (108),
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corresponds to pristine MoS2 [JCPDS#73-1508]. Nevertheless, sample ZnS/MoS2 shows the new diffraction peaks which correspond to the (111) and (311) peaks of
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cubic sphalerite ZnS [JCPDS#77-2100]. Raman results shown in Figure 1b further verify the structure of these three samples. For samples MoS2 and Zn-MoS2, only two vibration modes of Eଶ and Aଵ for MoS2 are observed, while the vibration peaks for ZnS can be clearly seen in the sample ZnS/MoS2,[21] consisting with the XRD results. More importantly, the Brunauer-Emmett-Teller (BET) results shown in Figure 1c reveal that the sample Zn-MoS2 exhibits a lager BET area (68.658 m2/g) and pore volume (0.148 cm3/g) than that of the pristine MoS2 (7.831 m2/g, 0.098 cm3/g), which could provide more contact areas for the catalyst and the electrolyte in further HER testing.[24, 25] In addition, it can be seen that the sample Zn-MoS2 shows the flower-like shape and nanosheets structure as usual, which are confirmed by the
ACCEPTED MANUSCRIPT Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images shown in Figure 1d-1f. Further, the EDS mapping results in Figure 1e confirm the existence of Mo, S and Zn elements in the sample. Clearly, the Zn elements are uniformly distributed in the nanosheets, indicating that Zn is triumphantly introduced
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in the prepared Zn-MoS2.
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Figure 1. (a)- (b) The XRD and Raman results of sample MoS2, Zn-MoS2 and ZnS/MoS2. BET results of sample MoS2 and Zn-MoS2. (d)SEM, (e) TEM, (f) HRTEM and EDS mapping of sample Zn-MoS2.
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X-ray photoelectron spectroscopy (XPS) was employed to further investigate the chemical constituents state in these three samples. As displayed in Figure 2a, the two
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distinct peaks around 229.1 eV and 232.2 eV of Zn-MoS2 can attribute to Mo 3d5/2 and Mo 3d3/2, respectively, which corresponds with pure MoS2, indicating that the nature structure of MoS2 is largely retained even after Zn doping.[26] Comparing with them, the ZnS/MoS2 (228.9 eV, 231.9 eV) shows a little shift of the Mo 3d peaks by vast Zn doping, suggesting that suitable amount of Zn source plays a key role for compound ZnS/MoS2. To further investigate the impact of Zn dopants in MoS2 atomic structure, S 2p was employed to probe the inner structure of Zn-MoS2 and ZnS-MoS2, as shown in Figure 2b. Similarly, comparing with MoS2 and Zn-MoS2 (S 2p3/2 161.8 eV, S 2p1/2 163.1eV) the ZnS/MoS2 (161.6 eV, 163.2 eV) shows a little shift of the S 2p peaks. Hence, the S 2p spectra can be divided into four peaks, which conform to Zn-S bonds (161.7 eV, 162.9 eV) and Mo-S bonds (162 eV. 163.1 eV).[27, 28] The Zn
ACCEPTED MANUSCRIPT 2p spectrum (Figure 2c) illustrates the peaks of Zn 2p3/2 and Zn 2p1/2 with binding energies at 1020.8 eV and 1043.8 eV, indicating Zn has been successfully doped into
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MoS2 nanosheets, the results are in outstanding agreement with EDS mapping.[29]
Figure 2. XPS results. (a) Mo 3d XPS spectra, (b) S 2p XPS spectra and (c) Zn 2p XPS spectra of
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MoS2, Zn-MoS2 and ZnS/MoS2.
Further, we evaluated the effect of Zn dopants on the HER performance of MoS2, where three electrocatalysts of MoS2, Zn-MoS2 and ZnS/MoS2 compounds are tested in a 3-electrode cell containing 0.5 M H2SO4 electrolyte (see Methods in the
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Supporting Information).[30] Potential are measured versus saturated calomel electrode (SCE), and are presented vs. RHE. As can be seen from the linear scan voltammetry (LSV) curves (seen from Figure 3a), sample Zn-MoS2 exhibits the
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smallest onset overpotential of 40 mV comparing with that of 115 mV for MoS2 nanosheets and 94 mV for ZnS/MoS2 compounds. The results suggest that the catalytic activity of the MoS2 nanosheets can be enhanced by Zn dopants. To get a
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HER current of 100 mA/cm2, overpotential values of 97, 140, 276, and 297 mV are needed for the Pt, Zn-MoS2, ZnS/MoS2, and MoS2 catalysts, respectively. Further, the HER kinetics of the above electrocatalysis is analyzed by their related Tafel plots. It can be seen from Figure 3b that MoS2 and ZnS/MoS2 compounds show the Tafel slope of 85 and 76 mV/dec, concurring with other previous reported values. The Zn-MoS2 nanosheets exhibit a Tafel slope of 35 mV/dec, much closer to the value of Pt. Such Tafel slope suggests that the HER occurs on Zn-MoS2 catalyst via a Volmer-Heyrovsky mechanism
and
that
the
electrochemical
desorption
is
rate-limiting.[31] Thus, the lowest overpotential and smallest Tafel slope for Zn doped
ACCEPTED MANUSCRIPT MoS2 nanosheets imply its superior HER activity. What's more, we fitted the exchange current density by the Butler-Volmer equation (shown in supporting information S4). [32] The results indicate that the exchange current density of Zn-MoS2 is 0.145 mA/cm2, larger than that of the MoS2 (0.035 mA/cm2) and
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ZnS/MoS2 (0.085 mA/cm2), revealing that the catalyst of Zn-MoS2 has more active electrocatalytic surface area toward the HER than the others. Further, we tested the stability of Zn-MoS2 electrode by cyclic voltammetry scan between 0.20 and −0.20 V vs RHE at an fast scan rate of 100 mV/s.[33] As can be seen from Figure 3c, the
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polarization curve exhibits little shift with the initial one after 10000 cycles. The durability of the catalyst conducted in a constant current density of −18 mA/cm2
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electrolysis shows that the potential can be maintained at around −100 mV with almost no noticeable degradation for 20 h (shown in Figure 3d). [34] These results suggest that the Zn-doped MoS2 electrode has excellent long-term stability for HER. In addition, we tested the XRD, HRTEM and XPS results of Zn doped MoS2 catalysts after its long time stability test (supporting information S5) . As can be seen, there are
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no changes on morphology, phase structure and binding energies for the catalyst of Zn/MoS2 after its long time electrochemical testing, indicating its good HER structure stability.
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Here, to elucidate the functions of Zn dopants for enhancing the HER activity of MoS2 nanosheets, we further investigate their conductivity and active HER catalytic sites, which are known as the key factors for the HER performance of MoS2 system.
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Firstly, we further investigate the electrode kinetics of the MoS2, Zn-MoS2 and ZnS/MoS2 through the electrochemical impedance spectroscopy (EIS) measurement in HER process from 100 kHz to 0.1 Hz (shown in Figure 3e). The charge-transfer resistance (RCT) and solution resistance (RS) could be obtained via fitting the plots using a simplified Randles circuit model shown in inset. It is proposed that a low RCT value means rapid charge transfer over the catalytic electrode and hence a rapid reaction rate in electrocatalytic kinetics.[35] The calculated RCT values of catalysts Zn-MoS2, ZnS/MoS2 and MoS2 are 12 Ω, 35 Ω and 55 Ω, respectively, implying higher conductivity and faster electron transfer process for Zn doped MoS2
ACCEPTED MANUSCRIPT catalyst.[36] Secondly, we measured the electrochemical double layer capacitances (Cdl) of the catalysts, which are expected to be proportional to their electrochemical surface areas, by a simple cyclic voltammetry method. It can be seen from Figure 3f that the Cdl value of Zn-MoS2 (32 mF/cm2) is higher than that of MoS2 (11 mF/cm2)
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and ZnS/MoS2 compounds (27 mF/cm2), demonstrating that doping Zn elements into MoS2 matrix could lead to an increase in active sites, which is beneficial to enhance the HER activity. Furthermore, to evaluate the intrinsic catalytic activity of active sites, the turnover frequency was putted out for calculating the active sites through an
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electrochemical approach in the phosphate solution with 0.2 M PBS. As shown in Figure S6, it can be seen that the catalysts have the TOF of 0.03, 0.06 and 0.67 H2/s at
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new active sites in MoS2. [37]
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potential of 100 mV, respectively, indicating that proper Zn dopants indeed introduce
Figure 3. (a) Polarization data and (b) Tafel plots of MoS2, Zn-MoS2, ZnS/MoS2, and Pt/C electrodes in 0.5 M H2SO4. (c) The comparison of polarization curves of Zn-MoS2 after continuous sweeps 10000 cycles at 100 mV/s in 0.5 M H2SO4. (d) The chronoamperometry measurement of Zn-MoS2 under a fixed overpotential of 100 mV (vs RHE) over 20 h. (e) Electrochemical impedance spectroscopy (EIS) Nyquist plots at −0.15 V (vs RHE) for MoS2,
ACCEPTED MANUSCRIPT Zn-MoS2 and ZnS/MoS2 electrodes. The data were fitted using the modifed Randles circuits. (f) Charging current density differences plotted against scan rate of MoS2, Zn-MoS2 and ZnS/MoS2, Cdl is equivalent to the slope of the fitted line.
Furthermore, we extensively compared HER activity and kinetics of our prepared
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Zn doped MoS2 catalysts with previously reported MoS2-based HER catalysts. It can be seen from Figure 4 and Table S1 (Supporting Information), the HER performance of Zn doped MoS2 nanosheets is among the best reported for MoS2-based catalysts. Thus, all these results clearly reveal that the Zn doped MoS2 nanosheets has excellent
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performance for HER and show potential for practical applications in future water
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splitting.
Figure 4. HER activity comparison graph showing the Tafel slope (mV/ dec) with overpotential
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(mV vs. RHE) to reach a current density of 10 mA/cm2.
As we know, the hydrogen evolution activity is strongly correlated with the free
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energy of hydrogen adsorption to the catalyst surface, where a good electrocatalyst is thought to possess a moderate H adsorption free energy (△GH*) and the △GH* value close to zero indicates the superior HER activity with the optimal balance between the absorption and the removal of hydrogen atoms on the active sites.[38] Here, density functional theory (DFT) analysis is carried out to explore the origin of superior catalytic process upon Zn doped MoS2 nanosheets in our case. As shown in Figure 5, calculation results indicate thatΔGH* of the pristine 2H-MoS2 in basal plane is about 2.1 eV, corresponds to previous reports, revealing its inert basal plane.[39] For Zn doped MoS2 (one Mo atom replaced by one Zn atom in 4×4×1 supercell), all possible active sites of Mo, Zn, and S sites in the basal plane are investigated. Results
ACCEPTED MANUSCRIPT indicate that hydrogen atom adsorbed at Zn and Mo sites are unstable, but adsorbed at S site is stable, with the △GH* value of −0.07 eV for the S sites neighboring to Zn dopant. These results suggest that the in-plane S atoms could be activated by the presence of Zn dopants, and possess an HER activity comparable or even higher to the Mo-edge related S atoms (△GH* ~0.07). Our calculation results have perfectly
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explained the experimental observation of outstanding HER performance for Zn doped MoS2 nanosheets.
Further, as discussed before, the Nyquist plots show a decreasing charge-transfer resistance upon Zn elements doping, which is closely associated with improved
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electrical conductivity of the catalysts.[40] Therefore, the nature of the Zn dopants' activity is further revealed through investigating its electronic structure. Band structures and the density of states (DOS) plots of monolayer pristine MoS2 and Zn
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doped MoS2 are shown in Figure 6a and 6b, respectively. It can be seen that some new bands appear near the Fermi level when Zn dopants are introduced, which is also confirmed by the DOS results where most of the densities of states in Zn doped MoS2 come from the hybridization of metal d-states and sulphurous p-states. These new states are responsible for the improvement of the conductivity in Zn doped MoS2 to promote rapid charge transfer over the basal plane in HER progress.[41] Further, the
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partial charge density of the Zn doped MoS2 monolayer for the bands near Fermi level is shown in Figure 6d to visualize the distribution of conducting charges over the basal plane. It can be seen that the conducting charges uniformly spread over the basal plane, which can effectively facilitate charge transfer over the catalytic electrode and
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reduce HER overpotential.[42],[43] Thus, as discussed above, the outstanding catalytic activity of Zn-MoS2 comparing with that of pristine MoS2 can be attributed
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to following factors: i) the larger surface area of Zn-MoS2 facilitates the contact between the catalysts and electrolyte, as well as, exposing more active sites; ii) Zn dopants could induce more distortion in MoS2 nanosheets, where it has been proved that the S-vacancies are new catalytic sites in the basal plane and the gap states around the Fermi level allow hydrogen to bind directly to exposed Mo atoms;[44, 45] iii) the introduction of Zn dopants not only optimizes the catalytic activity of MoS2 basal planes by lowering the △GH* value of the in-plane S atom, but also enhances the conducting charges over the basal plane to faster the charge transfer over the catalytic electrode, guaranteeing its excellent catalytic activity.
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Figure 5. (a) Free energy versus the reaction coordinate of HER for the pristine and Zn doped MoS2 monolayer. (b) Optimized atoms structure of the Zn-doped MoS2 basal plane with H
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adatom.
Figure 6. Electronic band structure and density of states (DOS) plots of monolayer (a) pristine MoS2 and (b) Zn doped MoS2. (c) The projected density of states (PDOS) plots of Mo 3d (purple line), S 2p (blue line) and Zn 3d (orange line) orbitals for Zn doped MoS2. (d) Partial charge density of the Zn doped MoS2 monolayer for the bands within 1 eV below Fermi level. The isosurface value for the plot is set at 0.005e/bohr3.
4. Conclusions In summary, our systematic experimental and theoretical results demonstrate that
ACCEPTED MANUSCRIPT the catalytic activity of in-plane S atoms, as well as the basal plane conductivity of MoS2 can be aroused via Zn doping in HER. Electrocatalysis testing results reveal a significantly enhanced HER activity of Zn doped MoS2 nanosheets compared with pure MoS2, originating from the reduced adsorption behavior of H atoms on the in-plane S sites neighboring to doped Zn atoms. Our observation is further proved by
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density functional theory calculation. Besides the activated basal plane active sites, Zn dopants can also improve the intrinsic electronic conductivity of MoS2 by strong hybridizations of metal d-states and sulphurous p-states, leading to the rapid charge transfer in HER. This study provides a new strategy to enhance the HER performance
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of other MoS2-based catalysts by doping. Supporting Information.
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The Supporting Information associated with this article can be found in the online version. The Supporting information includes the following: Experimental Section, Calculation details, extra SEM pictures, XRD and TEM results, Polarization curves of all the Zn doped MoS2 samples. Acknowledgments
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This work is supported by the National Natural Science Foundation of China (Grant No. 11474137 and 21571089), the Fundamental Research Funds for the Central Universities
(GrantNo.lzujbky-2014-27,
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[18] J. Deng, H. Li, J. Xiao, Y. Tu, D. Deng, H. Yang, H. Tian, J. Li, P. Ren, X. Bao, Energy Environ. Sci., 8 [19] R. Ye, P. del Angel-Vicente, Y. Liu, M.J. Arellano-Jimenez, Z. Peng, T. Wang, Y. Li, B.I. Yakobson, S.H. Wei, M.J. Yacaman, J.M. Tour, Adv. Mater., 28 (2016) 1427-1432.
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[23] P. Wisesa, K.A. McGill, T. Mueller, Phys Rev B, 93 (2016). [24] M. Li, X. Liu, Y. Xiong, X. Bo, Y. Zhang, C. Han, L. Guo, J. Mater. Chem. A, 3 (2015) 4255-4265. [25] C. Sun, J. Zhang, J. Ma, P. Liu, D. Gao, K. Tao, D. Xue, J. Mater. Chem. A, 4 (2016) 11234-11238. [26] L. Zheng, S. Han, H. Liu, P. Yu, X. Fang, Small, 12 (2016) 1527-1536.
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Liu, N. De Marco, Y. Yang, Nat. Nanotechnol., 11 (2016) 75-81. [30] D. Akyüz, B. Keskin, U. Şahintürk, A. Koca, Appl. Catal. B-Environ., 188 (2016) 217-226. [31] L. Liao, S. Wang, J. Xiao, X. Bian, Y. Zhang, M.D. Scanlon, X. Hu, Y. Tang, B. Liu, H.H. Girault, Energy Environ. Sci., 7 (2014) 387-392. [32] A. Oh, Y.J. Sa, H. Hwang, H. Baik, J. Kim, B. Kim, S.H. Joo, K. Lee, Nanoscale, 8 (2016) 16379-16386. [33] Y. Wang, L. Chen, X. Yu, Y. Wang, G. Zheng, Adv. Energy Mater., 7 (2017) 1601390. [34] C. Xia, H. Liang, J. Zhu, U. Schwingenschlögl, H.N. Alshareef, Adv. Energy Mater., (2017) 1602089. [35] J. J. Zhang, X. L. Sui, G. S. Huang, D. M. Gu, Z. B. Wang, J. Mater. Chem. A, 5 (2017) 4067-4074. [36] W. Xiao, P. Liu, J. Zhang, W. Song, Y.P. Feng, D. Gao, J. Ding, Adv. Energy Mater., (2016) 1602086. [37] B. Seo, D.S. Baek, Y.J. Sa, S.H. Joo, CrystEngComm, 18 (2016) 6083-6089. [38] Y. Zhao, S. Wang, C. Li, X. Yu, C. Zhu, X. Zhang, Y. Chen, RSC Adv., 6 (2016) 7370-7377. [39] J. Hu, C. X. Zhang, X. Y. Meng, H. Lin, C. Hu, X. Long, S. H. Yang, J. Mater. Chem. A, 1 (2017). [40] J.D. Benck, T.R. Hellstern, J. Kibsgaard, P. Chakthranont, T.F. Jaramillo, ACS Catalysis, 4 (2014)
ACCEPTED MANUSCRIPT 3957-3971. [41] J. Greeley, T.F. Jaramillo, J. Bonde, I.B. Chorkendorff, J.K. Norskov, Nat. Mater., 5 (2006) 909-913. [42] D. Voiry, R. Fullon, J. Yang, E.S.C. de Carvalho Castro, R. Kappera, I. Bozkurt, D. Kaplan, M.J. Lagos, P.E. Batson, G. Gupta, A.D. Mohite, L. Dong, D. Er, V.B. Shenoy, T. Asefa, M. Chhowalla, Nat. Mater., 15 (2016) 1003-1009. [43] T. S. Li, G. l. Galli, J. Phys. Chem. C, 111 (2007) 16192-16196. [44] H. Li, C. Tsai, A. L. Koh, L. Cai, A. W. Contryman, A. H. Fragapane, J. Zhao, H. S. Han, H. C.
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(2016) 16524-16530.
S0013-4686(17)32444-1
DOI:
10.1016/j.electacta.2017.11.080
Reference:
EA 30670
To appear in:
Electrochimica Acta
Received Date: 5 June 2017 Revised Date:
16 October 2017
Accepted Date: 12 November 2017
Please cite this article as: P. Liu, J. Zhu, J. Zhang, K. Tao, D. Gao, P. Xi, Active basal plane catalytic activity and conductivity in Zn doped MoS2 nanosheets for efficient hydrogen evolution, Electrochimica Acta (2017), doi: 10.1016/j.electacta.2017.11.080. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Active basal plane catalytic activity and conductivity in Zn doped MoS2 nanosheets for efficient hydrogen evolution Peitao Liua , Jingyi Zhu b , Jingyan Zhanga , Kun Taoa, Daqiang Gaoa*, Pinxian
a
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Xic*. Key Laboratory for Magnetism and Magnetic Materials of MOE, Key Laboratory of Special Function Materials and Structure Design of MOE, Lanzhou University, Lanzhou 730000, P. R. China.
Department of Physics and Astronomy, Clemson Nanomaterials Center and
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b
COMSET, Clemson University, Clemson, SC 29634, USA Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu
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c
Province and The Research Center of Biomedical Nanotechnology, Lanzhou University, Lanzhou, 730000, P. R. China The two authors have equal contribution to this work
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* Corresponding author: [email protected], [email protected]. Abstract
Transition metal dichalcogenides based electrocatalysts have been proposed as substitutes for noble metals to generate hydrogen in electrolysis of water, but these
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alternatives usually suffer from their inferior performance of intrinsic semiconducting property and inert basal plane. Here
Zn dopants are proved to can intrigue the
catalytic activity of MoS2 nanosheets with the low overpotential of 140 mV at 100
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mA/cm2 and Tafel slope of 35 mV/dec. Density functional theory calculations are performed to unravel the underlying mechanism, revealing that the inert basal plane is activated by Zn dopants with the H adsorption free energy (△GH*) of -0.07. Further, the charge distribution spreading over the basal plane induced by strong hybridizations of metal d-states and sulphurous p-states play a key role for promoting H atom adsorption and desorption kinetics simultaneously. This study opens new opportunities for the designing and understanding of MoS2-based catalysts for water splitting or other applications. 1. Introduction With the increasing attestations on excessive consumption fossil fuel and their
ACCEPTED MANUSCRIPT homologous environmental pollution problems, numerous efforts have been devoted to search for sustainable alternative energy. Hydrogen (H2), with a high gravimetric energy density, is considered to be the excellent candidate of the mobile energy carrier.[1, 2] Importantly, H2 can be made from the electrolysis of water without any CO2 emission, where the electricity could be produced from intermittent solar and
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wind energy resources. To improve the energy efficiency, efficient and earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) are needed. Although platinum (Pt) and its alloys have been proved to be the benchmark catalysts for HER, the prohibitive cost and scarcity hamper their widespread commercial use. Therefore,
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searching for earth-abundant materials based catalysts is critical for the future hydrogen economy.[3, 4]
During the past years, MoS2-based materials have been proved to be the
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economical and ecofriendly HER catalysts owing to their two dimensional (2D) structure, electrocatalytic performance, and cost-effectiveness.[5-7] There is a consensus that the catalysts' HER performance is highly depending on the synergistic effect of the active sites and the conductivity of the materials.[8, 9] Therefore, for optimizing the HER efficiency of the MoS2-based catalysts, researches have been mainly focused on two main strategies: 1) to enhance the amounts of active site and 2)
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to increase the conductivity.[10] Monolayer MoS2 belongs to direct semiconductor with the band gap of 1.8 V, so the conductivity of MoS2 catalysts can generally be enhanced via utilizing conductive supports from metals or carbon materials.[11, 12] For the active sites of the MoS2, both density function theory (DFT) calculation and
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subsequent experimental results reveal that their edge sites are active for HER while their basal plane is catalytically inert.[13] Therefore, extensive efforts have been
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devoted to develope nano- or defect-riched MoS2 structures to maximize their active edge sites.[14, 15] However, besides increasing the amount of exposed edge sites, little attention has been paid to explore the strategies of stimulating the catalytic activity of inert in-plane atoms of 2D MoS2 to enhance the HER kinetics.[16, 17] Recently, several attempts have been made to activate the basal plane of
2H-MoS2. For example, Deng et.al demonstrated that the catalytic activity of in-plane S atoms of MoS2 can be significantly aroused via single-atom Pt metal doping in HER.[18] Ye et. al reported the improved HER activity of MoS2 and MoP via formation of a MoS2(1– x)Px solid solution and their calculation results demonstrated that P-doping can activate MoS2 basal plane toward HER activity by reducing the
ACCEPTED MANUSCRIPT adsorption free energy (△GH*).[19] Huang et.al further revealed that P-doping could dramatically reduce Mo valence charge and can active the inert MoS2 basal plane or S-edge with the assistant of O elements co-doping.[20] These reports have stimulated further development of MoS2 systems to bring high HER performance by arousing their activity of basal plane with dopants. However, there remains several inevitable formation of MoP and oxide phases.
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issues in these methods, such as the use of precious metal Pt or the spontaneous Recently, as predicted by Deng et.al in the volcano plot for varied single-atom metal doped MoS2, the non-precious metal dopants of Zn could also be good
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candidate to tune the catalytic activity of MoS2.[18] Efficient all-pH value electrochemical and photocatalytic hydrogen evolution was also observed in hollow Co-based bimetallic sulfide polyhedral by Zn doping.[21] These results indicate that
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Zn dopants are the good candidate to activate the active sites of other catalysts. Therefore, we here report the HER performance of Zn doped MoS2 catalysts with systematic experimental and first-principles study. A simple hydrothermal method has been used to fabricate Zn doped MoS2 nanosheets, where the best HER catalytic activity of the Zn doped MoS2 catalyst possesss an onset overpotential of 40 mV, an overpotential of 140 mV at 100 mA/cm2 and a Tafel slope of 35 mV/dec. Through the
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measurements of double-layer capacitance, turnover frequency and electrochemical impedance of Zn doped MoS2 catalysts, we draw the conclusion that both the amount of active HER sites and the conductivity of MoS2 nanosheets increase as results of Zn doping. Further first-principle calculation results confirm our experimental results that
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Zn dopants could activate the HER activity of MoS2 basal plane, and concurrently enhance electronic conductivity of the basal plane to promote fast charge transfer in
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HER process. The finding opens a new door to realize non-noble-metal-doped MoS2 for activating their inert basal plane. 2. Experimental 2.1 Synthesis
Pure and Zn doped MoS2 nanosheets are fabricated by the simple hydorthemal
method. In brief, 1.5 g sodium molybdate dihydrate [Na2MoO4·2H2O], 2.2 g thioacetamide [CH3CSNH2] and different amounts of zinc nitrate hexahydrate ( ZnNO3·6H2O) (0 g, 0.1 g, 0.2 g, 0.4 g, 0.6 g) were dissolver in 80 ml deionized water under magnetic stirring. The mixed solution was stirred for 1 h and then transferred into a 100 ml Teflon-lined stainless steel autoclave, heated at 200 °C for
ACCEPTED MANUSCRIPT 24 h. Then the black products were separated by a string machine, heating 60 °C for further use. In order to succedent study, the typical as-prepared samples were named as MoS2 (0 g), Zn-MoS2 (0.2 g) and ZnS/MoS2 (0.6 g). 2.2 Materials Characterizations X-ray diffractometer (XRD; X’pert Pro Philips with Cu K radiation) was used to
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study the crystal structure. Scanning electron microscopy (SEM; Hitachi S-4800) and transmission electron microscopy (TEM; Tecnai G2 F30, FEI) were employed to image the microstructure and atomic structure of the samples. Energy-dispersive X-ray spectroscopy (EDX) attached in the TEM was used to obtain elemental
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mappings. X-ray photoelectron spectroscopy (XPS; Kratos AXIS Ultra) and Raman spectrometer (Jobin-Yvon LabRam HR80) were utilized to analyze the chemical states
and
bonding
characteristics
of
the
N-doped
MoS2
samples.
The
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Brunauer-Emmett-Teller (BET) surface area and pore width were measured by using a Micrometrics ASAP 2020 V403 measurement. 2.3 Electrochemical Measurements
Catalyst inks was typically made from dispersing 10 mg of catalyst and 10 mg of carbon black (Vulcan XC72) in 50 ml petroleum ether. After drying, 3 mg of this catalyst, 30 µL Nafion-117 solution and 1470 L N,N dimethylformamide (DMF) were
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added into 5 mL container and ultrasonication for 30 min. Afterward, an aliquot of 3 µL was pipetted onto the glassy carbon electrode to obtain the catalyst loading of 0.32 mg cm-2. A CHI 660E electrochemical workstation was used to perform all electrochemical tests with a standard three-electrode cell. Ag/AgCl and graphite rod
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were used as reference and counter electrodes, respectively. Linear sweep voltammetry (LSV) measurements were conducted at a scan rate of 2 mV/s at 23 ºC in
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0.5 M H2SO4 electrolyte. The electrolyte was desecrated by purging N2 for 30 min prior to the measurements. The stability of the catalysts was evaluated by carried out cyclic voltammetry measurement at a scan rate of 100 mV/s for 10000 cycles followed by LSV measurement at a scan rate of 2 mV/s. All polarization curves were iR-corrected unless noted. All the potentials were calibrated with respect to a reversible hydrogen electrode (RHE) by the equation E(RHE) = E(Ag/AgCl) + 197 mV. The Nyquist plots were measured with frequencies ranging from 100 kHz to 0.1 Hz at potential of 260 mV vs. RHE. A simplified Randles circuit is used to fit the impedance data to extract the solution and charge-transfer resistances (RS and RCT). 2.4 First-principles Calculations and Modeling
ACCEPTED MANUSCRIPT All calculations were performed within the framework of of density functional theory (DFT) as implemented in the Vienna Ab initio Simulation Package (VASP).[22] The exchange–correlation interactions were treated by generalized gradient approximation (GGA) parameterized by Perdew, Burke, and Ernzerhof (PBE). The interaction between ions and electrons was described using the projector augmented
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wave (PAW). For the plane-wave basis restriction, the cut-off energy of 400 eV was set in all calculations. For the Brillouin-zone integration, K-points were sampled under Monkhorst-Pack.[23] The atomic structures for all models were fully relaxed with self-consistency accuracy of 10-5 eV reached for electronic loops. For the MoS2
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monolayer, vacuum slab of 15 Å was inserted in z direction for surface isolation to prevent interaction between two neighboring surfaces. A 4×4×1 supercell was adopted to assess the electronic property and hydrogen evolution activity with the calculated 3. Results and discussions
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lattice constants of a = b = 3.24 Å after fully relaxation.
X-ray diffractometer (XRD) and Raman results indicate that as the increasing of the Zn dopants, Zn atoms are firstly doped into MoS2 matrix and then form the second impurity phase of ZnS with the increasing of the Zn dopants (Figure S1). Here, three representative samples of pure MoS2 (MoS2), Zn doped MoS2 with the Zn
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concentration of 6.1 at. % (Zn-MoS2) and the MoS2 with ZnS impurity phase (Zn/MoS2) are employed to study the effect of Zn dopants on the microstructure and HER activity of MoS2. As shown in Figure 1a, samples MoS2 and Zn-MoS2 possess six distinct diffraction peaks of (002), (100), (103), (105), (110) and (108),
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corresponds to pristine MoS2 [JCPDS#73-1508]. Nevertheless, sample ZnS/MoS2 shows the new diffraction peaks which correspond to the (111) and (311) peaks of
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cubic sphalerite ZnS [JCPDS#77-2100]. Raman results shown in Figure 1b further verify the structure of these three samples. For samples MoS2 and Zn-MoS2, only two vibration modes of Eଶ and Aଵ for MoS2 are observed, while the vibration peaks for ZnS can be clearly seen in the sample ZnS/MoS2,[21] consisting with the XRD results. More importantly, the Brunauer-Emmett-Teller (BET) results shown in Figure 1c reveal that the sample Zn-MoS2 exhibits a lager BET area (68.658 m2/g) and pore volume (0.148 cm3/g) than that of the pristine MoS2 (7.831 m2/g, 0.098 cm3/g), which could provide more contact areas for the catalyst and the electrolyte in further HER testing.[24, 25] In addition, it can be seen that the sample Zn-MoS2 shows the flower-like shape and nanosheets structure as usual, which are confirmed by the
ACCEPTED MANUSCRIPT Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images shown in Figure 1d-1f. Further, the EDS mapping results in Figure 1e confirm the existence of Mo, S and Zn elements in the sample. Clearly, the Zn elements are uniformly distributed in the nanosheets, indicating that Zn is triumphantly introduced
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in the prepared Zn-MoS2.
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Figure 1. (a)- (b) The XRD and Raman results of sample MoS2, Zn-MoS2 and ZnS/MoS2. BET results of sample MoS2 and Zn-MoS2. (d)SEM, (e) TEM, (f) HRTEM and EDS mapping of sample Zn-MoS2.
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X-ray photoelectron spectroscopy (XPS) was employed to further investigate the chemical constituents state in these three samples. As displayed in Figure 2a, the two
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distinct peaks around 229.1 eV and 232.2 eV of Zn-MoS2 can attribute to Mo 3d5/2 and Mo 3d3/2, respectively, which corresponds with pure MoS2, indicating that the nature structure of MoS2 is largely retained even after Zn doping.[26] Comparing with them, the ZnS/MoS2 (228.9 eV, 231.9 eV) shows a little shift of the Mo 3d peaks by vast Zn doping, suggesting that suitable amount of Zn source plays a key role for compound ZnS/MoS2. To further investigate the impact of Zn dopants in MoS2 atomic structure, S 2p was employed to probe the inner structure of Zn-MoS2 and ZnS-MoS2, as shown in Figure 2b. Similarly, comparing with MoS2 and Zn-MoS2 (S 2p3/2 161.8 eV, S 2p1/2 163.1eV) the ZnS/MoS2 (161.6 eV, 163.2 eV) shows a little shift of the S 2p peaks. Hence, the S 2p spectra can be divided into four peaks, which conform to Zn-S bonds (161.7 eV, 162.9 eV) and Mo-S bonds (162 eV. 163.1 eV).[27, 28] The Zn
ACCEPTED MANUSCRIPT 2p spectrum (Figure 2c) illustrates the peaks of Zn 2p3/2 and Zn 2p1/2 with binding energies at 1020.8 eV and 1043.8 eV, indicating Zn has been successfully doped into
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MoS2 nanosheets, the results are in outstanding agreement with EDS mapping.[29]
Figure 2. XPS results. (a) Mo 3d XPS spectra, (b) S 2p XPS spectra and (c) Zn 2p XPS spectra of
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MoS2, Zn-MoS2 and ZnS/MoS2.
Further, we evaluated the effect of Zn dopants on the HER performance of MoS2, where three electrocatalysts of MoS2, Zn-MoS2 and ZnS/MoS2 compounds are tested in a 3-electrode cell containing 0.5 M H2SO4 electrolyte (see Methods in the
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Supporting Information).[30] Potential are measured versus saturated calomel electrode (SCE), and are presented vs. RHE. As can be seen from the linear scan voltammetry (LSV) curves (seen from Figure 3a), sample Zn-MoS2 exhibits the
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smallest onset overpotential of 40 mV comparing with that of 115 mV for MoS2 nanosheets and 94 mV for ZnS/MoS2 compounds. The results suggest that the catalytic activity of the MoS2 nanosheets can be enhanced by Zn dopants. To get a
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HER current of 100 mA/cm2, overpotential values of 97, 140, 276, and 297 mV are needed for the Pt, Zn-MoS2, ZnS/MoS2, and MoS2 catalysts, respectively. Further, the HER kinetics of the above electrocatalysis is analyzed by their related Tafel plots. It can be seen from Figure 3b that MoS2 and ZnS/MoS2 compounds show the Tafel slope of 85 and 76 mV/dec, concurring with other previous reported values. The Zn-MoS2 nanosheets exhibit a Tafel slope of 35 mV/dec, much closer to the value of Pt. Such Tafel slope suggests that the HER occurs on Zn-MoS2 catalyst via a Volmer-Heyrovsky mechanism
and
that
the
electrochemical
desorption
is
rate-limiting.[31] Thus, the lowest overpotential and smallest Tafel slope for Zn doped
ACCEPTED MANUSCRIPT MoS2 nanosheets imply its superior HER activity. What's more, we fitted the exchange current density by the Butler-Volmer equation (shown in supporting information S4). [32] The results indicate that the exchange current density of Zn-MoS2 is 0.145 mA/cm2, larger than that of the MoS2 (0.035 mA/cm2) and
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ZnS/MoS2 (0.085 mA/cm2), revealing that the catalyst of Zn-MoS2 has more active electrocatalytic surface area toward the HER than the others. Further, we tested the stability of Zn-MoS2 electrode by cyclic voltammetry scan between 0.20 and −0.20 V vs RHE at an fast scan rate of 100 mV/s.[33] As can be seen from Figure 3c, the
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polarization curve exhibits little shift with the initial one after 10000 cycles. The durability of the catalyst conducted in a constant current density of −18 mA/cm2
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electrolysis shows that the potential can be maintained at around −100 mV with almost no noticeable degradation for 20 h (shown in Figure 3d). [34] These results suggest that the Zn-doped MoS2 electrode has excellent long-term stability for HER. In addition, we tested the XRD, HRTEM and XPS results of Zn doped MoS2 catalysts after its long time stability test (supporting information S5) . As can be seen, there are
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no changes on morphology, phase structure and binding energies for the catalyst of Zn/MoS2 after its long time electrochemical testing, indicating its good HER structure stability.
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Here, to elucidate the functions of Zn dopants for enhancing the HER activity of MoS2 nanosheets, we further investigate their conductivity and active HER catalytic sites, which are known as the key factors for the HER performance of MoS2 system.
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Firstly, we further investigate the electrode kinetics of the MoS2, Zn-MoS2 and ZnS/MoS2 through the electrochemical impedance spectroscopy (EIS) measurement in HER process from 100 kHz to 0.1 Hz (shown in Figure 3e). The charge-transfer resistance (RCT) and solution resistance (RS) could be obtained via fitting the plots using a simplified Randles circuit model shown in inset. It is proposed that a low RCT value means rapid charge transfer over the catalytic electrode and hence a rapid reaction rate in electrocatalytic kinetics.[35] The calculated RCT values of catalysts Zn-MoS2, ZnS/MoS2 and MoS2 are 12 Ω, 35 Ω and 55 Ω, respectively, implying higher conductivity and faster electron transfer process for Zn doped MoS2
ACCEPTED MANUSCRIPT catalyst.[36] Secondly, we measured the electrochemical double layer capacitances (Cdl) of the catalysts, which are expected to be proportional to their electrochemical surface areas, by a simple cyclic voltammetry method. It can be seen from Figure 3f that the Cdl value of Zn-MoS2 (32 mF/cm2) is higher than that of MoS2 (11 mF/cm2)
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and ZnS/MoS2 compounds (27 mF/cm2), demonstrating that doping Zn elements into MoS2 matrix could lead to an increase in active sites, which is beneficial to enhance the HER activity. Furthermore, to evaluate the intrinsic catalytic activity of active sites, the turnover frequency was putted out for calculating the active sites through an
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electrochemical approach in the phosphate solution with 0.2 M PBS. As shown in Figure S6, it can be seen that the catalysts have the TOF of 0.03, 0.06 and 0.67 H2/s at
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new active sites in MoS2. [37]
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potential of 100 mV, respectively, indicating that proper Zn dopants indeed introduce
Figure 3. (a) Polarization data and (b) Tafel plots of MoS2, Zn-MoS2, ZnS/MoS2, and Pt/C electrodes in 0.5 M H2SO4. (c) The comparison of polarization curves of Zn-MoS2 after continuous sweeps 10000 cycles at 100 mV/s in 0.5 M H2SO4. (d) The chronoamperometry measurement of Zn-MoS2 under a fixed overpotential of 100 mV (vs RHE) over 20 h. (e) Electrochemical impedance spectroscopy (EIS) Nyquist plots at −0.15 V (vs RHE) for MoS2,
ACCEPTED MANUSCRIPT Zn-MoS2 and ZnS/MoS2 electrodes. The data were fitted using the modifed Randles circuits. (f) Charging current density differences plotted against scan rate of MoS2, Zn-MoS2 and ZnS/MoS2, Cdl is equivalent to the slope of the fitted line.
Furthermore, we extensively compared HER activity and kinetics of our prepared
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Zn doped MoS2 catalysts with previously reported MoS2-based HER catalysts. It can be seen from Figure 4 and Table S1 (Supporting Information), the HER performance of Zn doped MoS2 nanosheets is among the best reported for MoS2-based catalysts. Thus, all these results clearly reveal that the Zn doped MoS2 nanosheets has excellent
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performance for HER and show potential for practical applications in future water
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splitting.
Figure 4. HER activity comparison graph showing the Tafel slope (mV/ dec) with overpotential
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(mV vs. RHE) to reach a current density of 10 mA/cm2.
As we know, the hydrogen evolution activity is strongly correlated with the free
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energy of hydrogen adsorption to the catalyst surface, where a good electrocatalyst is thought to possess a moderate H adsorption free energy (△GH*) and the △GH* value close to zero indicates the superior HER activity with the optimal balance between the absorption and the removal of hydrogen atoms on the active sites.[38] Here, density functional theory (DFT) analysis is carried out to explore the origin of superior catalytic process upon Zn doped MoS2 nanosheets in our case. As shown in Figure 5, calculation results indicate thatΔGH* of the pristine 2H-MoS2 in basal plane is about 2.1 eV, corresponds to previous reports, revealing its inert basal plane.[39] For Zn doped MoS2 (one Mo atom replaced by one Zn atom in 4×4×1 supercell), all possible active sites of Mo, Zn, and S sites in the basal plane are investigated. Results
ACCEPTED MANUSCRIPT indicate that hydrogen atom adsorbed at Zn and Mo sites are unstable, but adsorbed at S site is stable, with the △GH* value of −0.07 eV for the S sites neighboring to Zn dopant. These results suggest that the in-plane S atoms could be activated by the presence of Zn dopants, and possess an HER activity comparable or even higher to the Mo-edge related S atoms (△GH* ~0.07). Our calculation results have perfectly
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explained the experimental observation of outstanding HER performance for Zn doped MoS2 nanosheets.
Further, as discussed before, the Nyquist plots show a decreasing charge-transfer resistance upon Zn elements doping, which is closely associated with improved
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electrical conductivity of the catalysts.[40] Therefore, the nature of the Zn dopants' activity is further revealed through investigating its electronic structure. Band structures and the density of states (DOS) plots of monolayer pristine MoS2 and Zn
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doped MoS2 are shown in Figure 6a and 6b, respectively. It can be seen that some new bands appear near the Fermi level when Zn dopants are introduced, which is also confirmed by the DOS results where most of the densities of states in Zn doped MoS2 come from the hybridization of metal d-states and sulphurous p-states. These new states are responsible for the improvement of the conductivity in Zn doped MoS2 to promote rapid charge transfer over the basal plane in HER progress.[41] Further, the
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partial charge density of the Zn doped MoS2 monolayer for the bands near Fermi level is shown in Figure 6d to visualize the distribution of conducting charges over the basal plane. It can be seen that the conducting charges uniformly spread over the basal plane, which can effectively facilitate charge transfer over the catalytic electrode and
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reduce HER overpotential.[42],[43] Thus, as discussed above, the outstanding catalytic activity of Zn-MoS2 comparing with that of pristine MoS2 can be attributed
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to following factors: i) the larger surface area of Zn-MoS2 facilitates the contact between the catalysts and electrolyte, as well as, exposing more active sites; ii) Zn dopants could induce more distortion in MoS2 nanosheets, where it has been proved that the S-vacancies are new catalytic sites in the basal plane and the gap states around the Fermi level allow hydrogen to bind directly to exposed Mo atoms;[44, 45] iii) the introduction of Zn dopants not only optimizes the catalytic activity of MoS2 basal planes by lowering the △GH* value of the in-plane S atom, but also enhances the conducting charges over the basal plane to faster the charge transfer over the catalytic electrode, guaranteeing its excellent catalytic activity.
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Figure 5. (a) Free energy versus the reaction coordinate of HER for the pristine and Zn doped MoS2 monolayer. (b) Optimized atoms structure of the Zn-doped MoS2 basal plane with H
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adatom.
Figure 6. Electronic band structure and density of states (DOS) plots of monolayer (a) pristine MoS2 and (b) Zn doped MoS2. (c) The projected density of states (PDOS) plots of Mo 3d (purple line), S 2p (blue line) and Zn 3d (orange line) orbitals for Zn doped MoS2. (d) Partial charge density of the Zn doped MoS2 monolayer for the bands within 1 eV below Fermi level. The isosurface value for the plot is set at 0.005e/bohr3.
4. Conclusions In summary, our systematic experimental and theoretical results demonstrate that
ACCEPTED MANUSCRIPT the catalytic activity of in-plane S atoms, as well as the basal plane conductivity of MoS2 can be aroused via Zn doping in HER. Electrocatalysis testing results reveal a significantly enhanced HER activity of Zn doped MoS2 nanosheets compared with pure MoS2, originating from the reduced adsorption behavior of H atoms on the in-plane S sites neighboring to doped Zn atoms. Our observation is further proved by
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density functional theory calculation. Besides the activated basal plane active sites, Zn dopants can also improve the intrinsic electronic conductivity of MoS2 by strong hybridizations of metal d-states and sulphurous p-states, leading to the rapid charge transfer in HER. This study provides a new strategy to enhance the HER performance
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of other MoS2-based catalysts by doping. Supporting Information.
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The Supporting Information associated with this article can be found in the online version. The Supporting information includes the following: Experimental Section, Calculation details, extra SEM pictures, XRD and TEM results, Polarization curves of all the Zn doped MoS2 samples. Acknowledgments
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This work is supported by the National Natural Science Foundation of China (Grant No. 11474137 and 21571089), the Fundamental Research Funds for the Central Universities
(GrantNo.lzujbky-2014-27,
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