MoS2 Nanosheets Supported on Hollow Carbon Spheres as Efficient

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MoS2 Nanosheets Supported on Hollow Carbon Spheres as Efficient Catalysts for Electrochemical Hydrogen Evolution Reaction Wenyue Li,†,§ Zhenyu Zhang,‡,§ Wenjun Zhang,*,‡ and Shouzhong Zou*,† †

Department of Chemistry, American University, 4400 Massachusetts Avenue NW, Washington, DC 20016, United States Department of Physics and Materials Science, Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, Tat Chee Avenue, Kowloon 999077, Hong Kong, China

‡

S Supporting Information *

ABSTRACT: Hybridizing structured carbon materials with MoS2 has been demonstrated to be an effective method to increase the electrochemical hydrogen evolution reaction (HER) activity and durability of MoS2. In this study, we report the growth of MoS2 nanosheets on the surface of uniform hollow carbon spheres (HCS) to form a hydrangea-like nanocomposite. The HCS were formed through carbonization of a phenol formaldehyde template, and the MoS2 nanosheets were grown on the HCS surfaces through a hydrothermal method. The nanocomposites have the advantages of significantly improved electrical conductivity, ease of varying the MoS2 loading, and minimizing stacking of MoS2 nanosheets, which are manifested by their remarkably improved HER performance. The well-tuned carbon−MoS2 composite shows a Tafel slope of 48.9 mV dec−1, an onset potential of −0.079 V (vs reversible hydrogen electrode), and an overpotential of 126 mV at the current density of 10 mA cm−2 after 1000 potential cycles.

1. INTRODUCTION Proton exchange membrane fuel cells (PEMFCs) convert chemical energy stored in hydrogen gas directly into electricity with a high efficiency and zero emission of pollutants and are therefore promising devices for producing clean energy. Among other technical bottlenecks that hinder the large-scale applications of PEMFCs are the hydrogen production and storage.1−3 To date, nearly all of the hydrogen is produced from steam reforming of natural gas, which is energy demanding and has a large carbon footprint. The produced hydrogen often contains a trace amount of carbon monoxide, which poisons the Pt-based catalysts in PEMFCs. In contrast, electrolysis of water produces clean hydrogen. Although Pt is the most active catalyst for the hydrogen evolution reaction (HER) due to its optimal atomic hydrogen adsorption energy,4 the scarcity and high cost of Pt limit its use in practice. Seeking non-noble metal HER catalysts with high electrocatalytic activity is therefore critical for the large-scale implementation of hydrogen-related energy applications. Many materials consisting of non-noble transition metal (including Fe, Co, Ni, Mo, W, etc.) borides/carbides/nitrides/ phosphides/oxides/sulfides/selenides have been explored for hydrogen production.5−8 Among them, molybdenum disulfide (MoS2) with a lamellar hexagonal structure similar to graphite has recently emerged as a very promising nonprecious metal HER catalyst with attractive electrochemical performance in acidic electrolytes.9−13 Density functional theory (DFT) calculations predict that only the edges of S−Mo−S in the © 2017 American Chemical Society

MoS2 sheets are active for H* adsorption, whereas the basal plane is electrocatalytically inert.14,15 In addition, it has also been shown that catalysts with moderate hydrogen adsorption have a higher HER activity.4,15 Experimentally, the edge sites have been proved active sites9−11,14,15 and the catalytic activity decreases by a factor of 4.5 with each additional layer due to the high interlayer electron transport energy barrier.16 Different methods have been employed to promote the HER activity of MoS2, including: (1) boosting the electron transfer rate by changing its phase and structure or compositing MoS2 with other highly conductive materials;17−19 (2) increasing the active surface area and edge density of MoS2 by alleviating the agglomeration of MoS2 sheets or constructing nanoscale architectures;20−22 and (3) introducing heteroatoms or defects into MoS2 planes to optimize the hydrogen adsorption energy.23−26 A combination of the above strategies has often been used.27−33 For instance, Dai’s group synthesized a MoS2/reduced graphene oxide hybrid catalyst exhibiting excellent HER activity, which was attributed to the highly exposed edges of MoS2 and the excellent electrical coupling of MoS2 to the underlying graphene sheets.34 Edgeoriented MoS2 films were successfully fabricated by Tour’s group through reactions of sulfur with anodically formed sponge-like Mo oxide films on Mo substrates and showed enhanced activity and long-term cycling stability for HER.35 By means of Received: June 8, 2017 Accepted: August 14, 2017 Published: August 29, 2017 5087

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Figure 1. Schematic of the synthesis of MoS2 nanosheets supported on mesoporous hollow carbon nanospheres (RF: resorcinol−formaldehyde).

Figure 2. TEM images of (a) HCS (inset: high-magnification image) and (b, c, e) C−M2. (d) HRTEM image of C−M2 (inset: its SAED pattern); (f−h) EELS mapping images of C−M2 for C, Mo, and S, respectively.

controllable disorder engineering and oxygen incorporation in MoS2 ultrathin nanosheets, Xie and co-workers developed an optimized catalyst with a superior activity for HER.36 In this study, we report the synthesis and characterizations of a hydrangea-like carbon−MoS2 hybrid (C−M) with MoS2 nanosheets loosely covering hollow carbon nanospheres (HCS) that were prepared by a facile two-step approach. The loading of the MoS2 nanosheets can be conveniently controlled by adjusting the weight ratio of the carbon and molybdenum sources. Due to the minimization of MoS2 nanosheet stacking and the enhanced electron transport through the HCS, the optimized C−M

exhibits much better HER activity than pure MoS2 nanosheets. Furthermore, the hydrangea-like C−M hybrid exhibits superior electrochemical stability during prolonged HER measurements. It is anticipated that the hydrangea-like C−M hybrids may also find applications in lithium-ion batteries and thermal therapy because of their large surface areas and stable structures.

2. RESULTS AND DISCUSSION The synthesis processes for the C−M hybrids are illustrated in Figure 1. Uniform SiO2 nanospheres were fabricated by a sol−gel method using the Stöber reaction and then encapsulated with a 5088

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Figure 3. (a) XRD patterns and (b) Raman spectra of different C−M hybrids. XPS fine spectra of (c) Mo 3d and (d) S 2p of C−M2.

structure by changing the mass ratio between the HCS and Mo source is demonstrated in the TEM images in Figures 2 and S3. The hydrangea-like structure well maintained the hollow sphere feature of HCS after being encapsulated by MoS2 nanosheets, which is exhibited in Figure 2b,c. The MoS2 nanosheets stacked loosely on the surface of carbon spheres, which benefits electrolyte penetration and is expected to result in a highly effective catalytic layer. High-resolution TEM (HRTEM) image and selected area electron diffraction (SAED) pattern of C−M2 are shown in Figure 2d. The lattice fringes with a d-spacing of 0.65 nm can be assigned to the (002) plane of MoS2.37 The characteristic diffraction rings in the SAED pattern correspond to the (002), (100), and (110) planes of MoS2.38,39 The elemental distribution of C−M2 is disclosed by the electron energy loss spectroscopy (EELS) mapping images of C, Mo, and S (Figure 2f−h). As expected, the Mo and S atoms are distributed homogenously over the carbon hollow spheres. Figure 3a shows the X-ray diffraction (XRD) patterns of C−M hybrids with different loadings of MoS2. The diffraction peak at ∼22° can be assigned to the hollow carbon sphere,40 and the peaks at ∼33 and 58° are from the (100) and (110) lattice planes of MoS2 nanosheets, respectively.41,42 Raman spectra in Figure 3b reveal the two characteristic peaks of carbon materials, located at 1359 and 1596 cm−1 corresponding to the D and G bands.43 The intensity of G band is higher than that of D band, indicating

resorcinol−formaldehyde (RF) layer, which was transformed into a carbon shell in the subsequent thermal annealing treatment. The SiO2 core was removed by hydrofluoric acid (HF) etching, leading to the formation of HCS with a diameter of ∼300 nm, as demonstrated by the scanning electron microscopy (SEM) image in Figure S1a. MoS2 nanosheets were then grown on the surface of HCS via a hydrothermal process. As shown by the SEM image in Figure S1b, the surface of HCS became quite rough after the reaction, suggesting the formation of the MoS2 layer. By varying the mass ratio between HCS and molybdenum source, the thickness of the MoS2 layer can be tuned accordingly (Figures S1c and S1d). These hybrids are denoted as C−M1, C− M2 and C−M3. Transmission electron microscopy (TEM) was used to further characterize the structure of the HCS and C−M hybrids. A TEM image of HCS is presented in Figure 2a.The hollow structure can be clearly observed, and the thickness of the carbon layer is about 30 nm. Due to their well-defined hollow structure, the HCS have a large Brunauer−Emmett−Teller surface area of 725.8 m2 g−1 with uniform pore sizes of about 3.2 nm (Figure S2). The hollow carbon spheres were used as the substrate to support MoS2 nanosheets. The large surface area of the HCS provides more attaching sites, where MoS2 nanosheets can grow, which alleviates the agglomeration of MoS2 nanosheets. The ability to control the density of MoS2 sheets in the hydrangea-like 5089

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Figure 4. (a) Polarization curves (inset: enlarged section of the polarization curves with the dotted line indicating the current density at 10 mA cm−2) and (b) the corresponding Tafel plots of different samples obtained in N2-saturated 0.5 M H2SO4 solution with a scan rate of 5 mV s−1. (c) Polarization curves of C−M2 obtained after different potential cycles demonstrating the potential cycle stability. (d) Nyquist plots acquired at −0.200 V.

signifying the formation of MoO3 suggests that the oxidation of MoS2 is insignificant. The HER performance of different samples was evaluated in a N2-saturated 0.5 M H2SO4 solution. The onset potential of each material was first compared. Onset potential is the highest (for cathodic reactions) potential at which a reaction product is formed and is frequently used as an indicator of the HER catalytic activity. As shown in Figure 4a, the HCS sample exhibits very poor HER activity as revealed by the absence of a significant reduction current. In contrast, all of the C−M samples show higher electrocatalytic activities than those of pure MoS2, clearly demonstrating the advantages of using the HCS substrate. Among the C−M hybrids, C−M2 shows the best electrochemical activity, with an onset potential of about −0.128 V and an overpotential of 0.198 V at the current density of 10 mA cm−2, which are commonly used for evaluating the electrocatalytic activities of HER catalysts.51,52 Interestingly, the performance of C−M hybrids is not monotonically improved by increasing the amount of MoS2 in the samples. Although the C−M3 hybrid has the highest MoS2 loading, its overpotential is nearly the same as that of C−M1, which is probably due to the decrease of electrochemical surface area (ECSA) of MoS2 resulting from the stacking of MoS2 sheets. To support this assertion, double-layer capacitance (Cdl), which is proportional to ECSA and often used in HER studies,53,54 was measured. As shown in Figure S6, the Cdl

that the carbon nanospheres are well graphitized during the annealing process.44 The weak peaks at about 377 and 405 cm−1, respectively, agree well with the in-plane E12g and out-of-plane A1g phonon modes of MoS2 nanosheets.45,46 The two peaks at 820 and 995 cm−1 clearly seen on C−M1 are, respectively, from symmetric and asymmetric stretching modes of MoO3,47 which is likely formed by the oxidation of MoS2 in air induced by laser heating of the carbon support.48 The loading mass of MoS2 nanosheets in different samples can be quantitatively determined by the thermogravimetric analysis (TGA) curves of C−M samples and the corresponding oxidation of MoS2 in air (Figure S4). The MoS2 contents in C−M samples were evaluated as 42.8 wt % for C−M1, 49.4 wt % for C−M2, and 54.4 wt % for C−M3. The X-ray photoelectron spectroscopy (XPS) survey spectrum of C−M2 is shown in Figure S5, in which only C, O, Mo, and S elements were detected, confirming the purity of the samples. The high-resolution spectra of Mo 3d and S 2p of M−C2 are shown in Figure 3c,d, respectively. The peaks at 228.7 and 232.4 eV are ascribed to 3d5/2 and 3d3/2 of Mo4+, respectively. Those located at 232.5 and 235.4 eV are associated with Mo6+ resulting from the surface oxidation of MoS2 in air. The small satellite peak around 225.8 eV corresponds to 2s of S species.49 Accordingly, the S 2p region can be deconvoluted into two separate peaks arising from S 2p1/2 and S 2p3/2, which are consistent with the S2− state of sulfur.50 The absence of a strong peak at about 236 eV 5090

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Figure 5. (a) Chronoamperometric curve for C−M2 recorded at −0.250 V. (b) TEM and (c) HRTEM images of C−M2 after the stability test.

the Rct for C−M2 is about half of that of pure MoS2, which can be attributed to the less stacking of MoS2 layers arising from the hydrangea-like structure and possibly the better electrical conductivity of the HCS substrate that facilitates the electron transport between the catalytic sites and the GC electrode. To test the long-term performance of the catalyst, chronoamperometry measurement was also conducted at −0.250 V for a duration of 24 h, and the result is displayed in Figure 5a. The current increased from ca. 55 to 75 mA cm−2 in the first 17 h and then decreased to about 68 mA cm−2 at 24 h. The initial current increase, which is frequently observed in HER tests,26,59,60 might be attributed to the activation of the catalyst, for example, removal of surface contaminants and improvement of hydrophilicity of the catalyst. This observation is in harmony with the improvements of onset potential and overpotential at −10 mA cm−2 after the repetitive potential cycling (vide supra). The slight decrease of the current after 17 h of operation may be caused by the depletion of H+ in the solution, the accumulation of H2 bubbles on the electrode surface, or the detachment of the catalyst aggravated by the H2 bubbling. To examine structural stability of the catalyst, TEM and HRTEM images of C−M2 after the chronoamperometric test were acquired (Figure 5b,c). The hydrangea-like structure is well preserved, demonstrating the strong bonding between the MoS2 nanosheets and the HCS substrate. The characteristic lattice fringe of MoS2 is still clearly displayed in Figure 5c, confirming the good structure stability of C−M2. The electrochemical performances of C−M2 are superior to or at least close to those reported for MoS2-based materials in terms of the onset potential (η) at 10 mA cm−2 and Tafel slope, as shown in Table S1. The high HER activity and stability of C−M2 can be attributed to the following four aspects: (1) the good conductivity of HCS facilitates the electron transport between the catalytic sites and the electrode substrate, resulting in a decreased Rct; (2) due to the support of HCS, the agglomeration of MoS2 nanosheets are largely alleviated, which ensures a large amount of MoS2 edges exposed to the electrolyte during the HER; (3) the loose packing of MoS2 nanosheets on the HCS

values are in the order of MoS2 < C−M3 < C−M1 < C−M2, indicating that the C−M2 catalyst has indeed the largest ECSA. Tafel analysis of HER polarization curve was then applied to evaluate the HER kinetics. The linear part of the Tafel plots was fitted to the Tafel equation η = a + b log(j), where η is the overpotential, b is the Tafel slope, and j is the current density.55 As shown in Figure 4b, C−M2 shows the smallest Tafel slope of 49.3 mV dec−1 compared with 56.3, 53.9, and 58.5 mV dec−1 for C−M1, C−M3, and MoS2, respectively, further confirming its outstanding catalytic activity. A Tafel slope close to 40 mV dec−1 indicates that the HER follows the Volmer−Heyrovsky reaction mechanism, which can be summarized as the hydrogen adsorption (H3O+ + e− → Hads + H2O) and desorption (Hads + H3O+ + e− → H2 + H2O) processes.56 Potential sweeps between 0.200 and −0.800 V at a scan rate of 50 mV s−1 were further carried out to assess the durability of C−M2. The polarization curves recorded at 5 mV s−1 after a given number of potential cycles are presented in Figure 4c. The onset potential was increasing with the cycling number, and the current density was also increasing appreciably, indicating that C−M2 is activated during the potential sweep process. After 1000 potential cycles, the onset potential increased to 0.079 V, the overpotential at 10 mA cm−2 became 0.126 V, and the Tafel slope decreased to 48.9 mV dec−1. This activation can be attributed to the increasing hydrophilicity of C−M2, which facilitates the electrolyte penetrating into the active sites of MoS2 and boosts the charge-transfer kinetics of HER9 or the surface cleaning. Upon further increasing the potential cycles, the current density dropped slightly, presumably due to the detachment of the catalyst from the glassy carbon (GC) electrode (GCE) and the depletion of protons in the diffusion layer. To understand the improved HER activity of C−M samples, analysis of electrochemical impedance spectra (EIS) was carried out, and the results are shown in Figure 4d. Only one semicircle was observed for each sample in the Nyquist plot. From the radius of these semicircles, the charge-transfer resistance (Rct) of HER can be assessed.57,58 The Rct values of C−M hybrids are clearly lower than those of pure MoS2 nanosheets. In particular, 5091

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4.4. Electrochemical Measurements. A CH Instruments 660C electrochemical workstation was used to evaluate the electrochemical HER activities of various catalysts in a threeelectrode glass cell with the catalyst-modified glassy carbon electrode as the working electrode, a Pt plate as the counter electrode, and a Ag/AgCl as the reference electrode. All of the electrode potentials are reported with respect to a reversible hydrogen electrode. For the preparation of the working electrode, 5 mg of as-synthesized catalyst was dispersed in a mixture of 20 μL of Nafion solution (5.0% Nafion in ethanol) and 1 mL of ethanol. The mixture was sonicated for homogenization. A small amount (2 μL) of the suspension was pipetted out and dropped onto a glassy carbon electrode (GCE) with a diameter of 3 mm and then fully dried in air. Linear sweep voltammetry with a 5 mV s−1 scan rate was carried out from 0.200 to −0.800 V in a 0.5 M H2SO4 solution. Test for the cycling stability was performed with a scan rate of 50 mV s−1 for 2000 cycles. The time-dependent current density curve was tested at −0.25 V for about 24 h. All of the data were obtained without IR compensation. We are aware of the possibility of Pt contaminations on the working electrode from the dissolution of the Pt counter electrode after prolonged potential cycling.62 However, our previous studies following the similar potential cycling protocol did not show the presence of Pt in energydispersive X-ray spectroscopy or XPS images.9 EIS were acquired over a frequency range from 100 kHz to 10 mHz at −0.200 V.

facilitates the penetration of electrolytes into the inner space of the catalyst, taking full advantage of the MoS2 catalytic layers; and (4) the strong attachment of the MoS2 on HCS guarantees the long-term structural stability during H2 bubbling.

3. CONCLUSIONS In conclusion, MoS2 nanosheets supported on HCS with a hydrangea-like structure were synthesized by a hydrothermal method. These hybrids exhibited superior electrocatalytic activities toward HER to the pure MoS2. By varying the mass ratio of MoS2 to HCS in the synthesis, the loading of MoS2 can be controlled. From the HER studies, it is found that moderate loading of MoS2 nanosheets yields the best overall HER performance, which can be attributed to the optimal coverage that provides sufficient active edge sites yet avoids significant MoS2 nanosheets stacking. 4. EXPERIMENTAL SECTION 4.1. Synthesis of Mesoporous Hollow Carbon Spheres (HCS). Mesoporous HCS were synthesized via the Stöber reaction as reported previously.61 In a typical synthesis process, 2 mL of ammonia aqueous solution (28 wt %) was mixed with 6 mL of deionized (DI) water and 38.5 mL of methanol. Tetraethyl orthosilicate (TEOS, 1.6 mL) was added into the above wellmixed solution under stirring, followed by the addition of 0.2 g of resorcinol and 0.3 of mL formaldehyde solution (37 wt %) at an interval of 10 min. The mixture was stirred at room temperature for 24 h and then transferred into a 50 mL Teflon-lined autoclave and kept at 100 °C for another 24 h. The product was filtrated and washed with DI water and ethanol in turn for several times. After being dried in an oven, the SiO2/resorcinol product was annealed at 750 °C in an Ar/H2 atmosphere for 2 h. Finally, the SiO2 templates were removed by immersing the sample in a HF solution for 6 h and the HCS were obtained after filtration, washing, and drying processes. 4.2. Growth of MoS2 Nanosheets on HCS. HCS (0.05 g) was dispersed in DI water under the assistance of ultrasonication. Sodium molybdate (0.08 g) and L-cysteine (0.32 g) were then added into the solution under vigorous stirring. The solution was transferred into a 50 mL Teflon-lined autoclave and heated at 200 °C for 12 h. The resulting black solid was filtrated and washed with water and ethanol successively for several times and then dried in an oven at 80 °C (designated as C−M1). C−M materials with different carbon/molybdenum ratios were also synthesized via increasing the amount of HCS (0.04 and 0.03 g for C−M2 and C−M3, respectively). MoS2 nanosheets were prepared under the same conditions without using HCS. 4.3. Materials Characterization. Microstructures and morphologies of the samples were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) using Philips XL30 FEG and JEOL JEM-2100F FEG, respectively. Crystallographic structures of the materials were characterized with an X-ray powder diffractometer (D8Advance; Bruker, Germany). Raman spectroscopy was carried out using a Renishaw 200 Raman microscope with a 633 nm excitation source. Surface areas of the samples were measured with nitrogen adsorption isotherms at 77 K on a Micromeritics ASAP 2020 apparatus. Surface chemical states of the samples were analyzed by X-ray photoelectron spectroscopy (XPS; VG Scientific ESCALab220i-XL). Thermogravimetric analysis (TGA) was performed using a Mettler Toledo TGA/DSC 1 thermal analyzer system in air.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b00755. SEM and additional TEM images of the samples; nitrogen adsorption/desorption isotherms and pore distribution of HCS; TGA analysis; XPS full spectrum of C−M2; CV curves of different samples for calculating ECSA; table of comparison of HER performance with literature results (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.Z.). *E-mail: [email protected] (S.Z.). ORCID

Wenjun Zhang: 0000-0002-4497-0688 Shouzhong Zou: 0000-0003-1952-8799 Author Contributions §

W.L. and Z.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by American University through a startup fund to S.Z. REFERENCES

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