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Enhanced electrocatalytic hydrogen evolution performance of MoS2 ultrathin nanosheets via Sn doping
Accepted Manuscript Title: Enhanced Electrocatalytic Hydrogen Evolution Performance of MoS2 Ultrathin Nanosheets via Sn Doping Authors: Cuicui Du, Hao Huang, Juan Jian, Yue Wu, Mengxiang Shang, Wenbo Song PII: DOI: Reference:
S0926-860X(17)30109-6 http://dx.doi.org/doi:10.1016/j.apcata.2017.03.010 APCATA 16169
To appear in:
Applied Catalysis A: General
Received date: Revised date: Accepted date:
16-11-2016 7-3-2017 9-3-2017
Please cite this article as: Cuicui Du, Hao Huang, Juan Jian, Yue Wu, Mengxiang Shang, Wenbo Song, Enhanced Electrocatalytic Hydrogen Evolution Performance of MoS2 Ultrathin Nanosheets via Sn Doping, Applied Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2017.03.010 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.
Enhanced Electrocatalytic Hydrogen Evolution Performance of MoS2 Ultrathin Nanosheets via Sn Doping Cuicui Du, Hao Huang, Juan Jian, Yue Wu, Mengxiang Shang and Wenbo Song*
College of Chemistry, Jilin University, Changchun 130012, P.R. China
* Corresponding author. E-mail: [email protected] Fax: +86-431-85168420
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Graphical abstract]
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Highlights i.
High-quality Sn-doped MoS2 ultrathin nanosheets are controllably synthesized via a facile sacrificial template assisted pyrolytic approach. Superior HER performance with an overpotential of 28 mV at 10 mA cm-2 and a
ii.
Tafel slope of 37.2 mV dec-1 is obtained. iii.
The current density essentially remains constant after long-term stability measurement for 10 h at the overpotential of 60 mV.
iv.
The improved catalytic activity is ascribed to the increased active edge sites, enhanced intrinsic catalytic activity for each active site and accelerated electron transfer via Sn doping.
Abstract
Two-dimensional (2D) transition-metal dichalcogenides (TMDs) have drawn much attention due to their unique physical and chemical properties. Molybdenum disulfide (MoS2) is particularly promising in hydrogen evolution reaction (HER) as a substitute for noble-metal catalysts. Although numerous attempts have been made to improve the HER activity of MoS2, engineering the MoS2-based electrocatalysts with activities similar to noble-metal catalysts remains challenging. Herein, we synthesize 3
high-quality tin doped molybdenum disulfide (Sn-MoS2) ultrathin nanosheets by a g-C3N4 sacrificial template assisted thermolytic approach, and detailly investigate the role of Sn doping on the HER activity. The porous Sn-MoS2 nanosheets displays superior HER performance, exhibiting an overpotential of 28 mV at 10 mA cm-2 and a Tafel slope of 37.2 mV dec-1 with an admirable stability. Compared to pristine MoS2 nanosheets, the significantly improved catalytic activity is ascribed to the increased active edge sites, enhanced intrinsic catalytic activity for each active site as well as accelerated electron transfer upon Sn doping. This work may provide guidelines for the design and synthesis of efficient non-precious HER catalysts.
Keywords: Sn doping; MoS2 nanosheets; template synthesis; hydrogen evolution reaction
1.
Introduction The fast-growing world population requires a huge increase in energy consumption.
It is urgent to develop a clean, renewable and affordable alternative to fossil fuels which intensify the air pollution and global warming. Among the various alternative energy strategies, hydrogen as the primary carrier to construct an energy infrastructure may enable a secure and clean energy future, and the water splitting driven by renewable resource-derived electricity is a promising pathway for sustainable 4
hydrogen production. Electrocatalytic water splitting to produce H2, has attracted tremendous attention during the past few decades. It is known that an overpotential is always required for the electrochemical reaction. An efficient electrocatalyst can decrease the overpotential drastically, leading to an increase in the efficiency of energy utilization. Pt-group metals have been proven to be the most efficient catalysts for electrocatalytic hydrogen evolution reaction (HER), and show an overpotential of near zero. Unfortunately, the price of Pt-group metals is so high that limits their practical application in electrocatalytic HER. Consequently, developing efficient non-noble metal electrocatalysts, composed of earth-abundant elements, is quite appealing with the aim of providing cost-competitive hydrogen [1-4]. In recent years, 2D transition metal dichalcogenides (TMD) with unique electronic properties different from bulk materials have received much attention owing to their excellent catalytic activity and low cost compared to noble metals [5-8]. As a protocol of layered TMD, MoS2 constitutes of a stack of hexagonal layers of molybdenum atoms sandwiched between two layers of sulfur atoms (S–Mo–S) through weak van der Waals forces [9]. Tremendous attention has been devoted to MoS2 due to great potential for applications in catalyzing HER. The study of MoS2 on electrochemical HER can be traced to 2005. Hinnemannet al. found that the computational free energy of atomic hydrogen bonding to the MoS2 edge was close to that of Pt, which raised the possibility of MoS2 as a promising HER electrocatalyst in theory [10]. It is also
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the first time that the MoS2 edge structure is considered to be the actual active site. Based on the instructional studies, enhanced HER performances have been further demonstrated by creating additional HER-active sites through designing defect-rich or edge-oriented structures [11-13]. Beyond that, the HER performance of MoS2 is also limited by the poor conductivity, which is detrimental to the electron transport between electrode and electrocatalyst for effective proton reduction [14]. The electronic conductibility engineering can be achieved by two ways: (i) coupling MoS 2 with conductive species, such as graphene and carbon nanotube [15-17] and (ii) doping suitable heteroatoms into the lattice of MoS2. However, the direct coupling of MoS2 on nonactive HER substrates would potentially block the active sites of MoS2, thus significantly decreasing the HER activity. Alternatively, doping MoS2 with conductive species is a direct and viable method to improve its electronic conductibility, and thereby enhance the HER activity. As stated above, doping of MoS2 is one of the most effective ways to engineer its structure with more active sites [18-23] and higher conductivity [17, 24-26] to enhance the electrocatalytic HER efficiency. Nevertheless, it should be noted that diverse atomic doping could have different effect. Very recently, a negative effect on the electrocatalytic activity has been reported. In which, less catalytic activity for HER and ORR in comparison to their undoped counterparts is observed at Nb- and Ta-doped bulk TMDs (MoS2 and WS2) [27].
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Diverse doping strategies for MoS2 have been developed, which is divided into two types: surface charge transfer doping (the donation of charge from physically adsorbed volatile molecules [28] or alkali metals [29], and the approaches involving covalent bonding via edge functionalization [30], etc.) and substitutional doping via covalent bonding [31]. Among those, substitutional doping with heteroatoms is particularly promising in producing stable and reliable catalysts due to the strong chemical bonding between dopant and Mo (or S). The introduction of a foreign metal or nonmetal element in the MoS2 lattice to substitute Mo or S atoms may afford the opportunity to disrupt the sp2 honeycomb structure and engineer the electronic and surface structures of the host material for expanding its applications. Although isoelectronic dopants (such as Se and W) tend to more easily be incorporated, they provide no extra electrons or holes in the lattice which is possibly ineffective on the multiplication of active edge sites or conductivity. Alternatively, these drawbacks can be suppressed by non-isoelectronic doping. So far, few works emerged concerning doping of non-isoelectronic elements into the MoS2 plane to improve the HER activity. Very recently, N-doped MoS2 nanosheets with enhanced electronic conductivity have been reported, which exhibited enhanced and stable electrocatalytic activity towards HER [32]. Although numerous attempts have been made to improve the HER activity of MoS2, engineering the MoS2-based electrocatalysts with activities similar to noble-metal catalysts remains challenging.
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Therefore, it is desired to find a rational way to obtain doping-based MoS2 materials with porous structure, high surface area and even controlled HER performance. In this study, inspired by the similar hexagonal layered structure of tin disulfide (SnS 2) [33] and MoS2, as well as the potentially improved electrical conductivity originated from the incorporation of metallic Sn element, we design and synthesize Sn-doped MoS2 ultrathin nanosheets via a facile yet versatile pyrolytic approach for significantly enhanced HER catalysis. 2. Experimental section 2.1 Materials Melamine (C3H6N6), ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O), thiourea (CN2H4S) and potassium stannate trihydrate (K2SnO3·3H2O) were purchased from Sinopharm Chemical Reagent. The Pt/C catalyst (20 wt.% Pt on Vulcan XC72R carbon) was bought from Johnson Matthey Corporation. Other chemicals were obtained from Beijing Chemical Reagent Company. 2.2 Synthesis of g-C3N4 template Briefly, the g-C3N4 template was synthesized by a pyrolytic approach. In a typical experimental procedure, melamine with proper quantity was put into a crucible. The crucible was loaded into a muffle furnace, heated from room temperature to 550 oC with a rate of 4 oC min-1, kept at this temperature for 180 min, and then cooled to
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room temperature. The as-prepared g-C3N4 was yellow in colour, which was collected and grinded into powder for further use. 2.3 Synthesis of Sn-MoS2 nanosheets Briefly, 0.4 g mixture of ammonium molybdate (90 wt.%) and potassium stannate trihydrate (10 wt.%), 4.0 g g-C3N4 sacrificial template and 2.0 g thiourea were put in a mortar and grinded into homogeneous powder. The resulting powder was transferred into a quartz tube, which was then evacuated with nitrogen and sealed. In a furnace, the quartz tube was heated to 600 oC with a rate of 3 oC min-1 under nitrogen and maintained for 240 min, followed by elevating to 800 oC with a rate of 4 oC min-1. The resulting powder was collected after cooling to room temperature and labeled as Sn0.10-MoS2. For comparative study, series of samples were similarly prepared by changing the weight percentage of potassium stannate trihydrate from 5 wt.% to 12 wt.%, which were labeled as Sn0.05-MoS2, Sn0.08-MoS2 and Sn0.12-MoS2, respectively. In addition, the synthesis of pristine MoS2 nanosheets is performed, similar to the Sn-MoS2 nanosheets without the addition of potassium stannate trihydrate. 2.4 Physical characterization Powder X-ray diffraction (XRD) patterns were recorded using the Ultima IV. Scanning electron microscopy (SEM) images were collected using a HITACHI SU8020 instrument. Transmission electron microscopy (TEM) images were collected using a TecnaiF20. The X-ray photoelectron spectroscopy (XPS) spectra were
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obtained with an ESCALAB 250 spectrometer with a mono X-Ray source Al Kα excitation (1486.6 eV). Raman spectrum was taken under ambient conditions by using a Horiba LabRAM HR Evolution spectrometer with a 532 nm laser excitation. 2.5 Electrochemical Measurements All of the electrochemical measurements were performed using a three-electrode configuration
with
an
electrochemical
workstation
(CHI760E) at
ambient
temperature. A saturated calomel electrode (SCE) was used as the reference electrode and a platinum wire was used as the counter electrode. A glass carbon electrode (GCE, 3 mm in diameter) decorated with the samples was used as the working electrode. All the measurements were performed in 0.5 M H2SO4, unless otherwise noted. A homogeneous catalyst suspension was prepared by dispersing 4 mg of catalyst into 1 mL of DMF/ultrapure water (v/v=1:1) solvent followed by ultrasonication for 20 min. Then 10 μL catalyst suspension was then pipetted using a micropipettor on the GCE surface with a catalyst loading of 566 μg cm-2. For comparison, the GCE was modified with commercial 20 wt.% Pt/C catalyst with a catalyst loading of 283 μg cm-2. The working electrode was dried at ambient temperature. Once dry, 5 μL of Nafion solution (0.5 wt.%) in ethanol was dropped onto the catalyst layer to form a thin protective film. Before collecting the experimental data, the working electrodes were activated by cyclic voltammetric scanning in 0.5 M H2SO4 solution. Linear Sweep Voltammetry (LSV) measurements were conducted in a N2-gas saturated
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H2SO4 solution with a scan rate of 10 mV/s. Electrochemical impedance spectroscopy (EIS) measurements were performed at different potentials with frequency from 0.01 Hz to 10 kHz and an amplitude of 5 mV under operating conditions in 0.5 M H2SO4 solution. The Tafel slope was derived from the LSV curves by fitting experimental data to the Tafel equation: η = a + b log j, where η is the overpotential, a is the Tafel constant, b is the Tafel slope and j is the current density. The electrochemical double layer capacitance (Cdl) was determined from the CVs measured at different scan rates in a non-Faradaic region. The total current at +0.2 V (vs. RHE) is plotted against the scan rate, which was obtained from the addition of the absolute values of the current at the cathodic and anodic. The value of Cdl is half of the slope for each curve. In all measurements, the SCE reference electrode was calibrated with respect to reversible hydrogen electrode (RHE).
3. Results and discussion The typical one-step synthetic procedure via annealing for Sn-MoS2 is illustrated in Fig. 1 (for experimental details, see the experimental section). Firstly, the layered graphic carbon nitride (g-C3N4) nanosheets is prepared via the thermal decomposition of melamine to be utilized as the key sacrificial template for synthesizing the ultrathin nanosheets. Subsequently, ammonium molybdate tetrahydrate, potassium stannate trihydrate and thiourea are uniformly coated on the surface of g-C3N4 nanosheets by grinded into homogeneous powder. Finally, the mixture is calcined at desired 11
temperature under a N2 atmosphere. With elevating temperature, under the H2S gas from thiourea decomposition, the ammonium molybdate tetrahydrate and thiourea gradually transforms into the porous MoS2 ultrathin nanosheets with the aid of layered g-C3N4 sacrificial template. Meanwhile, the heteroatom Sn from the precursor of potassium stannate trihydrate is chemically doped into the MoS2 nanosheets in situ, leading to Sn-doped MoS2 nanosheets. After this calcination process, no post process is needed to remove the g-C3N4 sacrificial template which would decompose completely, inhibiting the stack of the MoS2 nanosheets during thermolysis and generating a porous structure with high surface area. In addition, the doping of Sn into MoS2 plane will accelerate the electron transfer, produce abundant lattice defects and thus bring more accessible active edge sites. The as-prepared Sn-doped MoS2 ultrathin nanosheets can be used as a highly active noble metal-free catalyst for HER. 3.1 Characterization of the as-prepared samples The as-synthesized MoS2-based samples were examined by X-ray diffraction (XRD) measurements to investigate the crystalline structure. The XRD patterns in Fig. 2a attest the purity of the pristine MoS2 and the Sn-doped product, as evidenced by the characteristic diffraction peaks for hexagonal MoS2 phase (ICDD, reference number, 00-006-0097). Aside from those of MoS2, no diffraction peaks from Sn-based substances or other impurity phases are observed, indicating that Sn heteroatoms doping does not disturb the essential crystal structure of MoS2 significantly and the
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g-C3N4 sacrificial template decomposes completely during the annealing treatment. In addition, both peaks of the (002) plane for the Sn-doped sample and the pristine MoS2 nanosheets are relatively weaker compared to the standard MoS2 pattern, indicating that the as-prepared MoS2-based samples are presented as only a few layers. The scanning electron microscopy (SEM) measurements (Fig. 2b) indeed show that the resulting Sn-MoS2 sample consists of 2D nanosheets with fluffy, crumpled and porous structure, indicating that layered g-C3N4 template plays a very important role in the formation of special structure. This structure might provide the catalyst with a greater contact area with reactants and sufficient transportation of the reactants and products, leading to outstanding performance. Compared to the pristine MoS 2 nanosheets (Fig. S1), doping of Sn heteroatoms does not influence the high-quality morphology of the Sn-MoS2 ultrathin nanosheets. Fig. 2c shows a representative low-magnification transmission electron microscopy (TEM) image for Sn-MoS2 nanosheets, which provides additional insights into the typical lamellar structure retained after Sn doping. The high-resolution TEM (HRTEM) investigation (Fig. 2d) further corroborates that the as-prepared sample is MoS2-based ultrathin nanosheets, in which well-resolved crystal lattices with an interplanar spacing of 0.62 nm can be clearly distinguished, corresponding to the distance of (002) crystal plane of hexagonal MoS2.
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To further examine the composition of Sn-MoS2, the dark field-scanning transmission electronmicroscopy (STEM) was performed. Fig. 3 shows the STEM image and the corresponding EDX maping of Mo, Sn and S, which reveals the homogeneous distribution of elements throughout the Sn-MoS2 nanosheets and indicates that Sn heteroatoms have been successfully doped into 2D MoS2 nanosheets during thermal calcination. What is more, the incorporation of tin element and the surface chemistry of Sn-MoS2 nanosheets was examined by X-ray photoelectron spectroscopy (XPS). Fig. 4a-c present the high-resolution XPS spectra of Mo 3d, S 2p and Sn 3d for Sn-MoS2. As shown in Fig. 4a, the two peaks located at 229.8 and 232.9 eV are attributed respectively to Mo 3d5/2 and Mo 3d3/2 binding energies, indicating that the Mo(VI) is reduced to Mo(IV) via annealing. Fig. 4b shows two characteristic peaks located at 162.5 and 163.8 eV corresponding to S 2p3/2 and S 2p1/2, respectively, which are typical characteristics of S2− species. In the high-resolution Sn 3d spectrum (Fig. 4c), the peaks centered at 487.8 and 496.1 eV are related with Sn 3d5/2 and Sn 3d3/2 binding energies, which confirms the presence of Sn4+ in the product [34-36]. Meanwhile, the comparative Raman spectroscopy analysis was conducted on pristine MoS2 and Sn-MoS2 to investigate the effect of Sn doping on the MoS2 nanosheets. In Fig. 4d, two characteristic peaks (E12g and A1g, corresponding to the in-plane and out-of-plane vibrations for MoS2) were observed at ∼378 and ∼402 cm-1
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in both pristine MoS2 and Sn-MoS2 nanosheets respectively, demonstrating that the Sn-MoS2 sample retains the essential crystal structure of MoS2. It is reported that the frequency difference between E12g and A1g peaks depends on the number of layers of MoS2, which normally increases with an increase in the layers [37]. From Fig. 4d, one can see that the distance between E12g and A1g peaks in the two samples is constant, indicating the uniformity of the doping concentration [38] as well as the similar number of layers for pristine MoS2 and Sn-MoS2 nanosheets [37, 39]. However, the Raman intensity upon Sn-doping is reduced, potentially stemming from the change in lattice symmetry depending on the matrix elements and selection rules for Raman active vibrational modes [40]. A smaller E12g/A1g value (0.3) than that of pristine MoS2 nanosheets (0.4) indicates abundant edge-terminated structure, which may benefit for HER catalysis [41]. 3.2 HER electrocatalytic activity evaluation The as-prepared Sn-MoS2 ultrathin nanosheets was evaluated as an electrocatalyst for hydrogen evolution reaction (HER) in acidic conditions. Herein, the electrocatalytic HER activity of Sn-MoS2 ultrathin nanosheets was assessed using a standard three-electrode configuration in 0.5 M H2SO4 electrolyte purged with N2 as described in the Experimental Section. To explore the influence of the amount of Sn-doping, a series of samples with tunable amount of Sn (denoted as Snx-MoS2, x represents the weight percentage of tin source) were similarly prepared. Fig. S2 shows
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the polarization curves for series of Sn-doped MoS2 cathode, measured in N2-saturated 0.5 M H2SO4 with a scan rate of 10 mV s-1. By varying the amount of the doped Sn, distinctive behavior among these samples is observed. At relatively low doping level (Sn0.05-MoS2, Sn0.08-MoS2 and Sn0.10-MoS2), the HER activity is distinctly improved upon increasing the amount of Sn source. By further increasing the doping amount to 12%, the catalytic activity of Sn0.12-MoS2 deteriorates. Therefore, the Sn0.10-MoS2 exhibits optimal HER catalytic performance and is utilized for detail investigations. As a control, various commercially available carbon based electrodes (including pyrolytic graphite electrode (PGE), boron-doped diamond electrode (BDDE) and screen-printed graphite electrode (SPE)) modified with the same mass of Sn-MoS2 nanosheets as that of glassy carbon electrode (GCE) are also utilized and their polarization curves are shown in Fig. 5a. By changing the supporting electrodes, distinctive HER catalytic activity is observed. The GCE modified with Sn-MoS2 nanosheets exhibits optimal HER catalytic performance. In addition, Fig. 5b displays the dependence of HER activity on the loading amount of the Sn-MoS2 ultrathin nanosheets on GCE. Apparently, the optimal loading is 566 μg cm-2. With further increasing the loading weight, the catalytic performance deteriorates. This phenomenon is likely due to the stacking of the MoS2 ultrathin nanosheets, which decreases the accessible active sites and thus inhibits the electrocatalytic activity.
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The typical cathodic polarization curve and corresponding Tafel plot of representative Sn0.10-MoS2 are shown in Fig. 6a and b. As a control, the pristine MoS2 nanosheets and a commercial Pt/C catalyst are also provided. As expected, the pristine MoS2 untrathin nanosheets obtained by our synthetic method shows commendable HER activity. Evidently, the porous framework of MoS2 ultrathin nanosheets plays an important role in providing an abundance of exposed edge sites, greater contact area with reactants and sufficient transportation of the reactants and products. Exhilaratingly, compared with pristine MoS2, greatly enhanced HER performance of Sn-MoS2 untrathin nanosheets is achieved, as indicated by the sharp increase in the magnitude of the cathodic current density with increasing overpotential, reaching 10 mA cm-2 at overpotentials of 28 mV with a Tafel slope of 37.2 mV dec-1, which is very close to that of commercial Pt/C (an overpotential of 19 mV at 10 mA cm-2 with a Tafel slope of 28.8 mV dec-1). The remarkable HER activity of the Sn-doped MoS2 ultrathin nanosheets is superior to most MoS2-based HER electrocatalysts reported so far (Table S1).
The Tafel slope for a catalyst modified electrode on HER is an inherent property indicative of the rate-determining step, which is often utilized to elucidate the dominant mechanism in HER. As is well-known, three separate reaction steps are possible when hydrogen is evolved at the catalyst modified electrode surface in acidic
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solution [42, 43], which involved a primary discharge step of a proton to form an adsorbed hydrogen atom [Volmer reaction: H3O+ + e- → Hads + H2O] and a combination step of an adsorbed hydrogen atom with a proton and an electron to form molecular hydrogen [Heyrovsky reaction: Hads + H3O+ + e- → H2 + H2O] or the recombination step of two adsorbed hydrogen atoms to form molecular hydrogen [Tafel reaction: Hads +Hads → H2]. In general, hydrogen can be produced when Volmer reaction is combined with Heyrovsky reaction or Tafel reaction, leading to either the Volmer−Heyrovsky or Volmer−Tafel mechanism. If the Volmer step is the rate-determining step, it gives a Tafel slope of ~120 mV dec-1. If the Heyrovsky or Tafel reaction is the rate-determining step, it invokes a Tafel slope of ∼40 mV dec−1 or ∼30 mV dec−1, respectively. In this work, the Tafel slope of Sn-MoS2 is 37.2 mV dec−1, revealing that the electrochemical desorption of Hads is a rate-determining step and the HER proceeds through a Volmer–Heyrovsky mechanism. The differences in electrocatalytic activities for the pristine MoS2 and the Sn-MoS2 ultrathin nanosheets are further elucidated by the charge transfer resistance (Rct), which was analyzed via the Electrical Impedance Spectroscopy (EIS) measurements performed at selected overpotential values under operating conditions. The Rct essentially describes the charge-transfer rate across the interface of electrode and electrolyte during the reactions, which can be determined from the diameter of the semicircles [16]. Fig. 6c shows the representative Nyquist plots of Sn-MoS2 collected
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at various overpotentials and the corresponding electrical equivalent circuit diagram, in which the Rct is dependent on the overpotential, decreasing with maximising overpotential, due to accelerated electron transfer capability under increasing cathodic bias. In addition, it has been well established that the Rct is related to the electrocatalysis kinetics and a lower Rct value corresponds to a faster reaction rate, leading to superior performance for HER. Fig. 6d reveals that the Rct of Sn-MoS2 is much lower than that of the pristine MoS2 at the same overpotential. Thus consistently with the LSV results (Fig. 6a), EIS demonstrates that accelerated transfers happen on Sn-MoS2 for HER. The introduction of non-isoelectronic Sn heteroatoms in the MoS2 lattice may disrupt the sp2 honeycomb structure, which is possibly effective on the multiplication active edge sites. Generally, the amount of catalytically active sites is determined by the roughness factor, which is defined as the ratio of the electrochemically active surface area to the geometric surface area of the electrode [44]. Therefore, we further monitored the change of double layer capacitance (Cdl) after Sn doping in MoS2, which is typically proportional to the electrochemically active surface area. The capacitance of the double layer at the solid-liquid interface for Sn-MoS2 and the pristine MoS2 was determined by CV measurements in the non-Faradaic region and the results are shown in Fig. S3a. The differences of the positive and negative current density at the center of the scanning potential ranges are plotted versus the voltage
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scan rates in Fig. S3b, suggesting the remarkable proliferation of catalytically active sites for Sn-MoS2 [45]. The increased active sites result from the extending defect sites, which derives from the introduction of heteroatoms into the MoS2 lattice. Thus, the remarkably enhanced electrocatalytic performance of Sn-MoS2 could be correlated to the dramatically increased accessible reactive sites exposed for HER. The turnover frequency (TOF), which is defined as the number of H2 molecules evolved per active site per unit of time [46], is used to study the intrinsic activity for the catalyst materials. By referencing the previous literature [47], the ToF of the Sn-MoS2 nanosheets as well as the pristine MoS2 nanosheets was deduced. The roughness factor (RF) for each modified electrode surface is critical for TOF calculation [48]. In this concern, the double layer capacitance technique is used to deduce the RF value. As shown in Fig. S3b, each plot is fitted to a straight line, whose slope is equal to a value of 2Cdl. The double layer capacitance of the pristine MoS2 nanosheets and the Sn-MoS2 nanosheets are 9.8 and 15.6 mF/cm2, respectively, which are much higher than most of the MoS2-based catalysts reported previously [49]. This may be related with the unique porous framework, which could provide an abundance of exposed edge sites and lead to outstanding HER activity. Taking the current density at −0.2V (vs. RHE) and the calculated RF values, the ToF values for Sn-MoS2 and pristine MoS2 nanosheets are 0.23 and 0.20 H2 s−1 per active site, respectively. The ToF of the MoS2 nanosheets increases upon Sn doping, indicating enhanced
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electrocatalytic activity of the Sn-MoS2 nanosheets compared to its counterpart. A table of output is provided in Table 1 to compare to the literature [47], which comprehensively explored the use of 2D MoS2 nanosheets as an electrocatalyst for the HER. Finally, we have measured the durability and stability of the electrode fabricated from Sn-MoS2 electrocatalyst for practical applications. This is firstly evaluated by performing cyclic voltammetry (CV) measurements on Sn-MoS2 for 2000 cycles at an accelerated scan rate in acidic media. After 2000 cycles, the polarization curve shows negligible difference in comparison with the initial one (Fig. 7a), indicating the admirable HER stability of Sn-MoS2 in acidic system. Notice that the current steadily increases along with the increased overpotential without any fluctuation and an extremely large cathodic current density of over 500 mA cm-2 is achieved at an overpotential of 500 mV. During water electrolysis, lots of bubbles are generated on the electrode surface and some of them do not get away from the electrode immediately, which leads to the direct loss of the effective active area and thus the increase of the reaction overpotential. In the present work, the steadily increased current with the overpotential manifests that the bubbles generated on the Sn-MoS2 catalyst could promptly deviate from the electrode. The long-term stability of the catalyst was assessed by potentiostatic electrolysis experiment at fixed overpotential
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of 60 mV. After a long period of 10 h, the current density essentially remains constant in this system for long-term operation, suggesting superior stability (Fig. 7b). Based on the above analysis, the remarkable HER performance of Sn-MoS2 may be attributed to following aspects: (1) The incorporation of Sn heteroatoms into MoS2 plane produces abundant lattice defects and thus brings more accessible active edge sites, as confirmed by the Raman investigation as well as the electrochemcal measurements for double layer capacitance. (2) Sn doping leads to enhanced intrinsic catalytic activity for each active site, as evidenced by the calculated TOF value. (3) Accelerated electron transfer upon Sn doping could also play an important role, as confirmed by EIS measurements. (4) The porous framework formed by the randomly distributed ultrathin nanosheets provides an abundance of exposed edge sites, a greater contact area with reactants and sufficient transportation of the reactants and products, leading to outstanding HER performance. 4. Conclusions In summary, Sn-doped MoS2 ultrathin nanosheets have been controllably synthesized using a facile in-situ pyrolytic method, in which the doping level is adjusted by controlling the weight percentage of Sn dopants. In comparison with the pristine MoS2 nanosheets, remarkablly enhanced HER performance with much lower HER overpotential (28 mV at 10 mA cm−2) and smaller Tafel slope (37.2 mV dec-1) is achieved at the Sn-MoS2 sample with an optimal doping amount. On the basis of
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these results, the feasibility to control the HER performance of the 2D MoS 2 ultrathin nanosheets by chemical doping is demonstrated, and the other perspective applications in optics and electronics are also highlighted in this work. More importantly, this one-step synthetic method via in-situ pyrolysis can be readily extended to synthesize other layered and/or transition metal based HER materials.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 21475051).
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27
(2016) 25210-25218.
28
Fig. 1. Proposed synthetic protocol for Sn-doped MoS2 nanosheets.
Fig. 2. (a) The XRD patterns of pristine MoS2 nanosheets and Sn0.10-MoS2 nanosheets; (b) The SEM, (c) TEM and (d) HRTEM images of Sn0.10-MoS2 nanosheets.
29
Fig. 3. Dark-field STEM image and the corresponding EDX elemental mapping images of Mo, S and Sn for Sn0.10-MoS2.
30
Fig. 4. The high-resolution XPS spectra of (a) Mo 3d, (b) S 2p and (c) Sn 3d for Sn0.10-MoS2; (d) The Raman spectra of pristine MoS2 and Sn0.10-MoS2.
Fig. 5. The polarization curves (vs. SCE) for (a) different underlying electrodes with a catalyst loading of 566 μg cm-2 and (b) Sn-MoS2 nanosheets modified GCE at different loading weight.
31
Fig. 6. (a) The polarization curves and (b) Tafel plots of Sn0.10-MoS2, Pt/C and the pristine MoS2; (c) Nyquist plots of Sn0.10-MoS2 at at various overpotentials (Inset: the corresponding electrical equivalent circuit diagram); (d) Nyquist plots of Sn0.10-MoS2 and the pristine MoS2 at the overpotential of 52 mV.
32
Fig. 7. (a) The polarization data initially and after 2000 cycles in 0.5 M H2SO4 and (b) time-dependent current density at static overpotential of 60 mV for 10 h.
33
Table 1. The comparative HER performance (vs. SCE) of Sn-MoS2 nanosheets with the literature [47]. [ref]
Catalyst
Underlying
Coverage
electrode
[μg cm-2]
Onset potential [V]
Roughness
TOF
factor [H2 s−1 per active site]
[21]
2D MoS2 nanosheets
GCE
1.267
-0.48
―
―
[21]
2D MoS2 nanosheets
GCE
2.019
―
2.6
―
[21]
2D MoS2 nanosheets
SPE
2.019
―
4.8
0.46 (@ -0.75 V)
[this work]
pristine MoS2
GCE
566
-0.27
146.9
0.20 (@ -0.44 V)
[this work]
Sn-MoS2
GCE
566
-0.25
233.9
0.23 (@ -0.44 V)
[this work]
Sn-MoS2
GCE
566
-0.25
233.9
0.70 (@ -0.75 V)
34
S0926-860X(17)30109-6 http://dx.doi.org/doi:10.1016/j.apcata.2017.03.010 APCATA 16169
To appear in:
Applied Catalysis A: General
Received date: Revised date: Accepted date:
16-11-2016 7-3-2017 9-3-2017
Please cite this article as: Cuicui Du, Hao Huang, Juan Jian, Yue Wu, Mengxiang Shang, Wenbo Song, Enhanced Electrocatalytic Hydrogen Evolution Performance of MoS2 Ultrathin Nanosheets via Sn Doping, Applied Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2017.03.010 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.
Enhanced Electrocatalytic Hydrogen Evolution Performance of MoS2 Ultrathin Nanosheets via Sn Doping Cuicui Du, Hao Huang, Juan Jian, Yue Wu, Mengxiang Shang and Wenbo Song*
College of Chemistry, Jilin University, Changchun 130012, P.R. China
* Corresponding author. E-mail: [email protected] Fax: +86-431-85168420
1
Graphical abstract]
2
Highlights i.
High-quality Sn-doped MoS2 ultrathin nanosheets are controllably synthesized via a facile sacrificial template assisted pyrolytic approach. Superior HER performance with an overpotential of 28 mV at 10 mA cm-2 and a
ii.
Tafel slope of 37.2 mV dec-1 is obtained. iii.
The current density essentially remains constant after long-term stability measurement for 10 h at the overpotential of 60 mV.
iv.
The improved catalytic activity is ascribed to the increased active edge sites, enhanced intrinsic catalytic activity for each active site and accelerated electron transfer via Sn doping.
Abstract
Two-dimensional (2D) transition-metal dichalcogenides (TMDs) have drawn much attention due to their unique physical and chemical properties. Molybdenum disulfide (MoS2) is particularly promising in hydrogen evolution reaction (HER) as a substitute for noble-metal catalysts. Although numerous attempts have been made to improve the HER activity of MoS2, engineering the MoS2-based electrocatalysts with activities similar to noble-metal catalysts remains challenging. Herein, we synthesize 3
high-quality tin doped molybdenum disulfide (Sn-MoS2) ultrathin nanosheets by a g-C3N4 sacrificial template assisted thermolytic approach, and detailly investigate the role of Sn doping on the HER activity. The porous Sn-MoS2 nanosheets displays superior HER performance, exhibiting an overpotential of 28 mV at 10 mA cm-2 and a Tafel slope of 37.2 mV dec-1 with an admirable stability. Compared to pristine MoS2 nanosheets, the significantly improved catalytic activity is ascribed to the increased active edge sites, enhanced intrinsic catalytic activity for each active site as well as accelerated electron transfer upon Sn doping. This work may provide guidelines for the design and synthesis of efficient non-precious HER catalysts.
Keywords: Sn doping; MoS2 nanosheets; template synthesis; hydrogen evolution reaction
1.
Introduction The fast-growing world population requires a huge increase in energy consumption.
It is urgent to develop a clean, renewable and affordable alternative to fossil fuels which intensify the air pollution and global warming. Among the various alternative energy strategies, hydrogen as the primary carrier to construct an energy infrastructure may enable a secure and clean energy future, and the water splitting driven by renewable resource-derived electricity is a promising pathway for sustainable 4
hydrogen production. Electrocatalytic water splitting to produce H2, has attracted tremendous attention during the past few decades. It is known that an overpotential is always required for the electrochemical reaction. An efficient electrocatalyst can decrease the overpotential drastically, leading to an increase in the efficiency of energy utilization. Pt-group metals have been proven to be the most efficient catalysts for electrocatalytic hydrogen evolution reaction (HER), and show an overpotential of near zero. Unfortunately, the price of Pt-group metals is so high that limits their practical application in electrocatalytic HER. Consequently, developing efficient non-noble metal electrocatalysts, composed of earth-abundant elements, is quite appealing with the aim of providing cost-competitive hydrogen [1-4]. In recent years, 2D transition metal dichalcogenides (TMD) with unique electronic properties different from bulk materials have received much attention owing to their excellent catalytic activity and low cost compared to noble metals [5-8]. As a protocol of layered TMD, MoS2 constitutes of a stack of hexagonal layers of molybdenum atoms sandwiched between two layers of sulfur atoms (S–Mo–S) through weak van der Waals forces [9]. Tremendous attention has been devoted to MoS2 due to great potential for applications in catalyzing HER. The study of MoS2 on electrochemical HER can be traced to 2005. Hinnemannet al. found that the computational free energy of atomic hydrogen bonding to the MoS2 edge was close to that of Pt, which raised the possibility of MoS2 as a promising HER electrocatalyst in theory [10]. It is also
5
the first time that the MoS2 edge structure is considered to be the actual active site. Based on the instructional studies, enhanced HER performances have been further demonstrated by creating additional HER-active sites through designing defect-rich or edge-oriented structures [11-13]. Beyond that, the HER performance of MoS2 is also limited by the poor conductivity, which is detrimental to the electron transport between electrode and electrocatalyst for effective proton reduction [14]. The electronic conductibility engineering can be achieved by two ways: (i) coupling MoS 2 with conductive species, such as graphene and carbon nanotube [15-17] and (ii) doping suitable heteroatoms into the lattice of MoS2. However, the direct coupling of MoS2 on nonactive HER substrates would potentially block the active sites of MoS2, thus significantly decreasing the HER activity. Alternatively, doping MoS2 with conductive species is a direct and viable method to improve its electronic conductibility, and thereby enhance the HER activity. As stated above, doping of MoS2 is one of the most effective ways to engineer its structure with more active sites [18-23] and higher conductivity [17, 24-26] to enhance the electrocatalytic HER efficiency. Nevertheless, it should be noted that diverse atomic doping could have different effect. Very recently, a negative effect on the electrocatalytic activity has been reported. In which, less catalytic activity for HER and ORR in comparison to their undoped counterparts is observed at Nb- and Ta-doped bulk TMDs (MoS2 and WS2) [27].
6
Diverse doping strategies for MoS2 have been developed, which is divided into two types: surface charge transfer doping (the donation of charge from physically adsorbed volatile molecules [28] or alkali metals [29], and the approaches involving covalent bonding via edge functionalization [30], etc.) and substitutional doping via covalent bonding [31]. Among those, substitutional doping with heteroatoms is particularly promising in producing stable and reliable catalysts due to the strong chemical bonding between dopant and Mo (or S). The introduction of a foreign metal or nonmetal element in the MoS2 lattice to substitute Mo or S atoms may afford the opportunity to disrupt the sp2 honeycomb structure and engineer the electronic and surface structures of the host material for expanding its applications. Although isoelectronic dopants (such as Se and W) tend to more easily be incorporated, they provide no extra electrons or holes in the lattice which is possibly ineffective on the multiplication of active edge sites or conductivity. Alternatively, these drawbacks can be suppressed by non-isoelectronic doping. So far, few works emerged concerning doping of non-isoelectronic elements into the MoS2 plane to improve the HER activity. Very recently, N-doped MoS2 nanosheets with enhanced electronic conductivity have been reported, which exhibited enhanced and stable electrocatalytic activity towards HER [32]. Although numerous attempts have been made to improve the HER activity of MoS2, engineering the MoS2-based electrocatalysts with activities similar to noble-metal catalysts remains challenging.
7
Therefore, it is desired to find a rational way to obtain doping-based MoS2 materials with porous structure, high surface area and even controlled HER performance. In this study, inspired by the similar hexagonal layered structure of tin disulfide (SnS 2) [33] and MoS2, as well as the potentially improved electrical conductivity originated from the incorporation of metallic Sn element, we design and synthesize Sn-doped MoS2 ultrathin nanosheets via a facile yet versatile pyrolytic approach for significantly enhanced HER catalysis. 2. Experimental section 2.1 Materials Melamine (C3H6N6), ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O), thiourea (CN2H4S) and potassium stannate trihydrate (K2SnO3·3H2O) were purchased from Sinopharm Chemical Reagent. The Pt/C catalyst (20 wt.% Pt on Vulcan XC72R carbon) was bought from Johnson Matthey Corporation. Other chemicals were obtained from Beijing Chemical Reagent Company. 2.2 Synthesis of g-C3N4 template Briefly, the g-C3N4 template was synthesized by a pyrolytic approach. In a typical experimental procedure, melamine with proper quantity was put into a crucible. The crucible was loaded into a muffle furnace, heated from room temperature to 550 oC with a rate of 4 oC min-1, kept at this temperature for 180 min, and then cooled to
8
room temperature. The as-prepared g-C3N4 was yellow in colour, which was collected and grinded into powder for further use. 2.3 Synthesis of Sn-MoS2 nanosheets Briefly, 0.4 g mixture of ammonium molybdate (90 wt.%) and potassium stannate trihydrate (10 wt.%), 4.0 g g-C3N4 sacrificial template and 2.0 g thiourea were put in a mortar and grinded into homogeneous powder. The resulting powder was transferred into a quartz tube, which was then evacuated with nitrogen and sealed. In a furnace, the quartz tube was heated to 600 oC with a rate of 3 oC min-1 under nitrogen and maintained for 240 min, followed by elevating to 800 oC with a rate of 4 oC min-1. The resulting powder was collected after cooling to room temperature and labeled as Sn0.10-MoS2. For comparative study, series of samples were similarly prepared by changing the weight percentage of potassium stannate trihydrate from 5 wt.% to 12 wt.%, which were labeled as Sn0.05-MoS2, Sn0.08-MoS2 and Sn0.12-MoS2, respectively. In addition, the synthesis of pristine MoS2 nanosheets is performed, similar to the Sn-MoS2 nanosheets without the addition of potassium stannate trihydrate. 2.4 Physical characterization Powder X-ray diffraction (XRD) patterns were recorded using the Ultima IV. Scanning electron microscopy (SEM) images were collected using a HITACHI SU8020 instrument. Transmission electron microscopy (TEM) images were collected using a TecnaiF20. The X-ray photoelectron spectroscopy (XPS) spectra were
9
obtained with an ESCALAB 250 spectrometer with a mono X-Ray source Al Kα excitation (1486.6 eV). Raman spectrum was taken under ambient conditions by using a Horiba LabRAM HR Evolution spectrometer with a 532 nm laser excitation. 2.5 Electrochemical Measurements All of the electrochemical measurements were performed using a three-electrode configuration
with
an
electrochemical
workstation
(CHI760E) at
ambient
temperature. A saturated calomel electrode (SCE) was used as the reference electrode and a platinum wire was used as the counter electrode. A glass carbon electrode (GCE, 3 mm in diameter) decorated with the samples was used as the working electrode. All the measurements were performed in 0.5 M H2SO4, unless otherwise noted. A homogeneous catalyst suspension was prepared by dispersing 4 mg of catalyst into 1 mL of DMF/ultrapure water (v/v=1:1) solvent followed by ultrasonication for 20 min. Then 10 μL catalyst suspension was then pipetted using a micropipettor on the GCE surface with a catalyst loading of 566 μg cm-2. For comparison, the GCE was modified with commercial 20 wt.% Pt/C catalyst with a catalyst loading of 283 μg cm-2. The working electrode was dried at ambient temperature. Once dry, 5 μL of Nafion solution (0.5 wt.%) in ethanol was dropped onto the catalyst layer to form a thin protective film. Before collecting the experimental data, the working electrodes were activated by cyclic voltammetric scanning in 0.5 M H2SO4 solution. Linear Sweep Voltammetry (LSV) measurements were conducted in a N2-gas saturated
10
H2SO4 solution with a scan rate of 10 mV/s. Electrochemical impedance spectroscopy (EIS) measurements were performed at different potentials with frequency from 0.01 Hz to 10 kHz and an amplitude of 5 mV under operating conditions in 0.5 M H2SO4 solution. The Tafel slope was derived from the LSV curves by fitting experimental data to the Tafel equation: η = a + b log j, where η is the overpotential, a is the Tafel constant, b is the Tafel slope and j is the current density. The electrochemical double layer capacitance (Cdl) was determined from the CVs measured at different scan rates in a non-Faradaic region. The total current at +0.2 V (vs. RHE) is plotted against the scan rate, which was obtained from the addition of the absolute values of the current at the cathodic and anodic. The value of Cdl is half of the slope for each curve. In all measurements, the SCE reference electrode was calibrated with respect to reversible hydrogen electrode (RHE).
3. Results and discussion The typical one-step synthetic procedure via annealing for Sn-MoS2 is illustrated in Fig. 1 (for experimental details, see the experimental section). Firstly, the layered graphic carbon nitride (g-C3N4) nanosheets is prepared via the thermal decomposition of melamine to be utilized as the key sacrificial template for synthesizing the ultrathin nanosheets. Subsequently, ammonium molybdate tetrahydrate, potassium stannate trihydrate and thiourea are uniformly coated on the surface of g-C3N4 nanosheets by grinded into homogeneous powder. Finally, the mixture is calcined at desired 11
temperature under a N2 atmosphere. With elevating temperature, under the H2S gas from thiourea decomposition, the ammonium molybdate tetrahydrate and thiourea gradually transforms into the porous MoS2 ultrathin nanosheets with the aid of layered g-C3N4 sacrificial template. Meanwhile, the heteroatom Sn from the precursor of potassium stannate trihydrate is chemically doped into the MoS2 nanosheets in situ, leading to Sn-doped MoS2 nanosheets. After this calcination process, no post process is needed to remove the g-C3N4 sacrificial template which would decompose completely, inhibiting the stack of the MoS2 nanosheets during thermolysis and generating a porous structure with high surface area. In addition, the doping of Sn into MoS2 plane will accelerate the electron transfer, produce abundant lattice defects and thus bring more accessible active edge sites. The as-prepared Sn-doped MoS2 ultrathin nanosheets can be used as a highly active noble metal-free catalyst for HER. 3.1 Characterization of the as-prepared samples The as-synthesized MoS2-based samples were examined by X-ray diffraction (XRD) measurements to investigate the crystalline structure. The XRD patterns in Fig. 2a attest the purity of the pristine MoS2 and the Sn-doped product, as evidenced by the characteristic diffraction peaks for hexagonal MoS2 phase (ICDD, reference number, 00-006-0097). Aside from those of MoS2, no diffraction peaks from Sn-based substances or other impurity phases are observed, indicating that Sn heteroatoms doping does not disturb the essential crystal structure of MoS2 significantly and the
12
g-C3N4 sacrificial template decomposes completely during the annealing treatment. In addition, both peaks of the (002) plane for the Sn-doped sample and the pristine MoS2 nanosheets are relatively weaker compared to the standard MoS2 pattern, indicating that the as-prepared MoS2-based samples are presented as only a few layers. The scanning electron microscopy (SEM) measurements (Fig. 2b) indeed show that the resulting Sn-MoS2 sample consists of 2D nanosheets with fluffy, crumpled and porous structure, indicating that layered g-C3N4 template plays a very important role in the formation of special structure. This structure might provide the catalyst with a greater contact area with reactants and sufficient transportation of the reactants and products, leading to outstanding performance. Compared to the pristine MoS 2 nanosheets (Fig. S1), doping of Sn heteroatoms does not influence the high-quality morphology of the Sn-MoS2 ultrathin nanosheets. Fig. 2c shows a representative low-magnification transmission electron microscopy (TEM) image for Sn-MoS2 nanosheets, which provides additional insights into the typical lamellar structure retained after Sn doping. The high-resolution TEM (HRTEM) investigation (Fig. 2d) further corroborates that the as-prepared sample is MoS2-based ultrathin nanosheets, in which well-resolved crystal lattices with an interplanar spacing of 0.62 nm can be clearly distinguished, corresponding to the distance of (002) crystal plane of hexagonal MoS2.
13
To further examine the composition of Sn-MoS2, the dark field-scanning transmission electronmicroscopy (STEM) was performed. Fig. 3 shows the STEM image and the corresponding EDX maping of Mo, Sn and S, which reveals the homogeneous distribution of elements throughout the Sn-MoS2 nanosheets and indicates that Sn heteroatoms have been successfully doped into 2D MoS2 nanosheets during thermal calcination. What is more, the incorporation of tin element and the surface chemistry of Sn-MoS2 nanosheets was examined by X-ray photoelectron spectroscopy (XPS). Fig. 4a-c present the high-resolution XPS spectra of Mo 3d, S 2p and Sn 3d for Sn-MoS2. As shown in Fig. 4a, the two peaks located at 229.8 and 232.9 eV are attributed respectively to Mo 3d5/2 and Mo 3d3/2 binding energies, indicating that the Mo(VI) is reduced to Mo(IV) via annealing. Fig. 4b shows two characteristic peaks located at 162.5 and 163.8 eV corresponding to S 2p3/2 and S 2p1/2, respectively, which are typical characteristics of S2− species. In the high-resolution Sn 3d spectrum (Fig. 4c), the peaks centered at 487.8 and 496.1 eV are related with Sn 3d5/2 and Sn 3d3/2 binding energies, which confirms the presence of Sn4+ in the product [34-36]. Meanwhile, the comparative Raman spectroscopy analysis was conducted on pristine MoS2 and Sn-MoS2 to investigate the effect of Sn doping on the MoS2 nanosheets. In Fig. 4d, two characteristic peaks (E12g and A1g, corresponding to the in-plane and out-of-plane vibrations for MoS2) were observed at ∼378 and ∼402 cm-1
14
in both pristine MoS2 and Sn-MoS2 nanosheets respectively, demonstrating that the Sn-MoS2 sample retains the essential crystal structure of MoS2. It is reported that the frequency difference between E12g and A1g peaks depends on the number of layers of MoS2, which normally increases with an increase in the layers [37]. From Fig. 4d, one can see that the distance between E12g and A1g peaks in the two samples is constant, indicating the uniformity of the doping concentration [38] as well as the similar number of layers for pristine MoS2 and Sn-MoS2 nanosheets [37, 39]. However, the Raman intensity upon Sn-doping is reduced, potentially stemming from the change in lattice symmetry depending on the matrix elements and selection rules for Raman active vibrational modes [40]. A smaller E12g/A1g value (0.3) than that of pristine MoS2 nanosheets (0.4) indicates abundant edge-terminated structure, which may benefit for HER catalysis [41]. 3.2 HER electrocatalytic activity evaluation The as-prepared Sn-MoS2 ultrathin nanosheets was evaluated as an electrocatalyst for hydrogen evolution reaction (HER) in acidic conditions. Herein, the electrocatalytic HER activity of Sn-MoS2 ultrathin nanosheets was assessed using a standard three-electrode configuration in 0.5 M H2SO4 electrolyte purged with N2 as described in the Experimental Section. To explore the influence of the amount of Sn-doping, a series of samples with tunable amount of Sn (denoted as Snx-MoS2, x represents the weight percentage of tin source) were similarly prepared. Fig. S2 shows
15
the polarization curves for series of Sn-doped MoS2 cathode, measured in N2-saturated 0.5 M H2SO4 with a scan rate of 10 mV s-1. By varying the amount of the doped Sn, distinctive behavior among these samples is observed. At relatively low doping level (Sn0.05-MoS2, Sn0.08-MoS2 and Sn0.10-MoS2), the HER activity is distinctly improved upon increasing the amount of Sn source. By further increasing the doping amount to 12%, the catalytic activity of Sn0.12-MoS2 deteriorates. Therefore, the Sn0.10-MoS2 exhibits optimal HER catalytic performance and is utilized for detail investigations. As a control, various commercially available carbon based electrodes (including pyrolytic graphite electrode (PGE), boron-doped diamond electrode (BDDE) and screen-printed graphite electrode (SPE)) modified with the same mass of Sn-MoS2 nanosheets as that of glassy carbon electrode (GCE) are also utilized and their polarization curves are shown in Fig. 5a. By changing the supporting electrodes, distinctive HER catalytic activity is observed. The GCE modified with Sn-MoS2 nanosheets exhibits optimal HER catalytic performance. In addition, Fig. 5b displays the dependence of HER activity on the loading amount of the Sn-MoS2 ultrathin nanosheets on GCE. Apparently, the optimal loading is 566 μg cm-2. With further increasing the loading weight, the catalytic performance deteriorates. This phenomenon is likely due to the stacking of the MoS2 ultrathin nanosheets, which decreases the accessible active sites and thus inhibits the electrocatalytic activity.
16
The typical cathodic polarization curve and corresponding Tafel plot of representative Sn0.10-MoS2 are shown in Fig. 6a and b. As a control, the pristine MoS2 nanosheets and a commercial Pt/C catalyst are also provided. As expected, the pristine MoS2 untrathin nanosheets obtained by our synthetic method shows commendable HER activity. Evidently, the porous framework of MoS2 ultrathin nanosheets plays an important role in providing an abundance of exposed edge sites, greater contact area with reactants and sufficient transportation of the reactants and products. Exhilaratingly, compared with pristine MoS2, greatly enhanced HER performance of Sn-MoS2 untrathin nanosheets is achieved, as indicated by the sharp increase in the magnitude of the cathodic current density with increasing overpotential, reaching 10 mA cm-2 at overpotentials of 28 mV with a Tafel slope of 37.2 mV dec-1, which is very close to that of commercial Pt/C (an overpotential of 19 mV at 10 mA cm-2 with a Tafel slope of 28.8 mV dec-1). The remarkable HER activity of the Sn-doped MoS2 ultrathin nanosheets is superior to most MoS2-based HER electrocatalysts reported so far (Table S1).
The Tafel slope for a catalyst modified electrode on HER is an inherent property indicative of the rate-determining step, which is often utilized to elucidate the dominant mechanism in HER. As is well-known, three separate reaction steps are possible when hydrogen is evolved at the catalyst modified electrode surface in acidic
17
solution [42, 43], which involved a primary discharge step of a proton to form an adsorbed hydrogen atom [Volmer reaction: H3O+ + e- → Hads + H2O] and a combination step of an adsorbed hydrogen atom with a proton and an electron to form molecular hydrogen [Heyrovsky reaction: Hads + H3O+ + e- → H2 + H2O] or the recombination step of two adsorbed hydrogen atoms to form molecular hydrogen [Tafel reaction: Hads +Hads → H2]. In general, hydrogen can be produced when Volmer reaction is combined with Heyrovsky reaction or Tafel reaction, leading to either the Volmer−Heyrovsky or Volmer−Tafel mechanism. If the Volmer step is the rate-determining step, it gives a Tafel slope of ~120 mV dec-1. If the Heyrovsky or Tafel reaction is the rate-determining step, it invokes a Tafel slope of ∼40 mV dec−1 or ∼30 mV dec−1, respectively. In this work, the Tafel slope of Sn-MoS2 is 37.2 mV dec−1, revealing that the electrochemical desorption of Hads is a rate-determining step and the HER proceeds through a Volmer–Heyrovsky mechanism. The differences in electrocatalytic activities for the pristine MoS2 and the Sn-MoS2 ultrathin nanosheets are further elucidated by the charge transfer resistance (Rct), which was analyzed via the Electrical Impedance Spectroscopy (EIS) measurements performed at selected overpotential values under operating conditions. The Rct essentially describes the charge-transfer rate across the interface of electrode and electrolyte during the reactions, which can be determined from the diameter of the semicircles [16]. Fig. 6c shows the representative Nyquist plots of Sn-MoS2 collected
18
at various overpotentials and the corresponding electrical equivalent circuit diagram, in which the Rct is dependent on the overpotential, decreasing with maximising overpotential, due to accelerated electron transfer capability under increasing cathodic bias. In addition, it has been well established that the Rct is related to the electrocatalysis kinetics and a lower Rct value corresponds to a faster reaction rate, leading to superior performance for HER. Fig. 6d reveals that the Rct of Sn-MoS2 is much lower than that of the pristine MoS2 at the same overpotential. Thus consistently with the LSV results (Fig. 6a), EIS demonstrates that accelerated transfers happen on Sn-MoS2 for HER. The introduction of non-isoelectronic Sn heteroatoms in the MoS2 lattice may disrupt the sp2 honeycomb structure, which is possibly effective on the multiplication active edge sites. Generally, the amount of catalytically active sites is determined by the roughness factor, which is defined as the ratio of the electrochemically active surface area to the geometric surface area of the electrode [44]. Therefore, we further monitored the change of double layer capacitance (Cdl) after Sn doping in MoS2, which is typically proportional to the electrochemically active surface area. The capacitance of the double layer at the solid-liquid interface for Sn-MoS2 and the pristine MoS2 was determined by CV measurements in the non-Faradaic region and the results are shown in Fig. S3a. The differences of the positive and negative current density at the center of the scanning potential ranges are plotted versus the voltage
19
scan rates in Fig. S3b, suggesting the remarkable proliferation of catalytically active sites for Sn-MoS2 [45]. The increased active sites result from the extending defect sites, which derives from the introduction of heteroatoms into the MoS2 lattice. Thus, the remarkably enhanced electrocatalytic performance of Sn-MoS2 could be correlated to the dramatically increased accessible reactive sites exposed for HER. The turnover frequency (TOF), which is defined as the number of H2 molecules evolved per active site per unit of time [46], is used to study the intrinsic activity for the catalyst materials. By referencing the previous literature [47], the ToF of the Sn-MoS2 nanosheets as well as the pristine MoS2 nanosheets was deduced. The roughness factor (RF) for each modified electrode surface is critical for TOF calculation [48]. In this concern, the double layer capacitance technique is used to deduce the RF value. As shown in Fig. S3b, each plot is fitted to a straight line, whose slope is equal to a value of 2Cdl. The double layer capacitance of the pristine MoS2 nanosheets and the Sn-MoS2 nanosheets are 9.8 and 15.6 mF/cm2, respectively, which are much higher than most of the MoS2-based catalysts reported previously [49]. This may be related with the unique porous framework, which could provide an abundance of exposed edge sites and lead to outstanding HER activity. Taking the current density at −0.2V (vs. RHE) and the calculated RF values, the ToF values for Sn-MoS2 and pristine MoS2 nanosheets are 0.23 and 0.20 H2 s−1 per active site, respectively. The ToF of the MoS2 nanosheets increases upon Sn doping, indicating enhanced
20
electrocatalytic activity of the Sn-MoS2 nanosheets compared to its counterpart. A table of output is provided in Table 1 to compare to the literature [47], which comprehensively explored the use of 2D MoS2 nanosheets as an electrocatalyst for the HER. Finally, we have measured the durability and stability of the electrode fabricated from Sn-MoS2 electrocatalyst for practical applications. This is firstly evaluated by performing cyclic voltammetry (CV) measurements on Sn-MoS2 for 2000 cycles at an accelerated scan rate in acidic media. After 2000 cycles, the polarization curve shows negligible difference in comparison with the initial one (Fig. 7a), indicating the admirable HER stability of Sn-MoS2 in acidic system. Notice that the current steadily increases along with the increased overpotential without any fluctuation and an extremely large cathodic current density of over 500 mA cm-2 is achieved at an overpotential of 500 mV. During water electrolysis, lots of bubbles are generated on the electrode surface and some of them do not get away from the electrode immediately, which leads to the direct loss of the effective active area and thus the increase of the reaction overpotential. In the present work, the steadily increased current with the overpotential manifests that the bubbles generated on the Sn-MoS2 catalyst could promptly deviate from the electrode. The long-term stability of the catalyst was assessed by potentiostatic electrolysis experiment at fixed overpotential
21
of 60 mV. After a long period of 10 h, the current density essentially remains constant in this system for long-term operation, suggesting superior stability (Fig. 7b). Based on the above analysis, the remarkable HER performance of Sn-MoS2 may be attributed to following aspects: (1) The incorporation of Sn heteroatoms into MoS2 plane produces abundant lattice defects and thus brings more accessible active edge sites, as confirmed by the Raman investigation as well as the electrochemcal measurements for double layer capacitance. (2) Sn doping leads to enhanced intrinsic catalytic activity for each active site, as evidenced by the calculated TOF value. (3) Accelerated electron transfer upon Sn doping could also play an important role, as confirmed by EIS measurements. (4) The porous framework formed by the randomly distributed ultrathin nanosheets provides an abundance of exposed edge sites, a greater contact area with reactants and sufficient transportation of the reactants and products, leading to outstanding HER performance. 4. Conclusions In summary, Sn-doped MoS2 ultrathin nanosheets have been controllably synthesized using a facile in-situ pyrolytic method, in which the doping level is adjusted by controlling the weight percentage of Sn dopants. In comparison with the pristine MoS2 nanosheets, remarkablly enhanced HER performance with much lower HER overpotential (28 mV at 10 mA cm−2) and smaller Tafel slope (37.2 mV dec-1) is achieved at the Sn-MoS2 sample with an optimal doping amount. On the basis of
22
these results, the feasibility to control the HER performance of the 2D MoS 2 ultrathin nanosheets by chemical doping is demonstrated, and the other perspective applications in optics and electronics are also highlighted in this work. More importantly, this one-step synthetic method via in-situ pyrolysis can be readily extended to synthesize other layered and/or transition metal based HER materials.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 21475051).
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Fig. 1. Proposed synthetic protocol for Sn-doped MoS2 nanosheets.
Fig. 2. (a) The XRD patterns of pristine MoS2 nanosheets and Sn0.10-MoS2 nanosheets; (b) The SEM, (c) TEM and (d) HRTEM images of Sn0.10-MoS2 nanosheets.
29
Fig. 3. Dark-field STEM image and the corresponding EDX elemental mapping images of Mo, S and Sn for Sn0.10-MoS2.
30
Fig. 4. The high-resolution XPS spectra of (a) Mo 3d, (b) S 2p and (c) Sn 3d for Sn0.10-MoS2; (d) The Raman spectra of pristine MoS2 and Sn0.10-MoS2.
Fig. 5. The polarization curves (vs. SCE) for (a) different underlying electrodes with a catalyst loading of 566 μg cm-2 and (b) Sn-MoS2 nanosheets modified GCE at different loading weight.
31
Fig. 6. (a) The polarization curves and (b) Tafel plots of Sn0.10-MoS2, Pt/C and the pristine MoS2; (c) Nyquist plots of Sn0.10-MoS2 at at various overpotentials (Inset: the corresponding electrical equivalent circuit diagram); (d) Nyquist plots of Sn0.10-MoS2 and the pristine MoS2 at the overpotential of 52 mV.
32
Fig. 7. (a) The polarization data initially and after 2000 cycles in 0.5 M H2SO4 and (b) time-dependent current density at static overpotential of 60 mV for 10 h.
33
Table 1. The comparative HER performance (vs. SCE) of Sn-MoS2 nanosheets with the literature [47]. [ref]
Catalyst
Underlying
Coverage
electrode
[μg cm-2]
Onset potential [V]
Roughness
TOF
factor [H2 s−1 per active site]
[21]
2D MoS2 nanosheets
GCE
1.267
-0.48
―
―
[21]
2D MoS2 nanosheets
GCE
2.019
―
2.6
―
[21]
2D MoS2 nanosheets
SPE
2.019
―
4.8
0.46 (@ -0.75 V)
[this work]
pristine MoS2
GCE
566
-0.27
146.9
0.20 (@ -0.44 V)
[this work]
Sn-MoS2
GCE
566
-0.25
233.9
0.23 (@ -0.44 V)
[this work]
Sn-MoS2
GCE
566
-0.25
233.9
0.70 (@ -0.75 V)
34