Introduction

Hydrogen is a green and sustainable energy, acting as attractive alternative of traditional fossil fuels to alleviate the energy crisis and environmental pollution1,2. Electrolytic water is an efficient and clean technology for hydrogen evolution3, and Pt-based noble metal electrocatalysts are commonly employed in hydrogen evolution reaction (HER) to improve the reaction efficiency. However, the high cost and rare reserve are still hinder its practical application4. Efficient alternatives are under urgent need. Recently, two dimensional layered molybdenum disulfide (MoS2) based materials are emerging as the promising HER. It was supported by both theoretical calculations and experimental studies5,6. Density function theory (DFT) calculation revealed that the thermodynamic free energy of H adsorption on the unsaturated sulfur atoms at MoS2 edge sites was fit for HER application7. Additionally, its relatively low cost, abundant reserve and good stability make MoS2 a promising alternative candidate of Pt in HER8,9. However, the relatively lower catalytic activity of bulk MoS2 compared to Pt trigger intense research that improve the catalytic capability10.

Generally, three strategies are employed to improve the catalytic properties of MoS2. First, much effort has devoted to generation of defects to obtain more active edges per unit area, and thus more active sites. Advanced hydrothermal synthesis methods11, including plasma-engineered12,13 or rough substrate chemical vapor deposition (CVD)14 are designed to generate defects for enhanced HER. Xie et al. developed a hydrothermal routine and paved the way of engineering defects into MoS2 using high concentration of the precursors and different amounts of thiourea11. The second strategy strives to strengthen the electrochemical properties by atomic-scale modification including chemical doping15, adjustment of the metastable 1T-phase16,17 and strain treating18. Furthermore, fabrication of efficient 3D MoS2 such as porous nanosheets19, vertical nanoflakes20, core-shell MoO3-MoS2 nanowires21,22 and double-gyroid morphology23 etc. are explored to increase specific surface area and active sites, leading to high catalytic activity. Zhang et al. designed an edge-rich and highly ordered MoS2 naonosheets rooting into polyaniline nanofibers, It showed good catalytic properties and stability21. Kibsgaard et al. engineered the surface structure of MoS2 to preferentially expose edge sites for improved catalytic activity by successfully synthesizing contiguous large-area thin films of a highly ordered double-gyroid MoS2 with nanopores23. Our group designed a 3D nitrogen-doped graphene supported MoS2 as an advanced HER catalyst24. Among them, various 3D MoS2 structures with high-surface and exposed active sites present great prospect in large-scale and practical applications.

Although reasonable progresses have been achieved in fabrication of 3D structure, it still suffers from some deficiencies such as complicated operation with multiple steps and inefficient catalytic activity. It is highly desirable to develop a facile and straightforward approach to fabricate cost-effective MoS2-based catalysts with high HER activity.

Herein, a one-step hydrothermal synthesis route for 3D MoS2 flower with advanced HER performance using PPy (MoS2-P) was developed. The PPy shows excellent conductivity. Specially, it displays good stability24 to keep its morphology during the hydrothermal synthesis, which is good candidate for soft template. Moreover, it has been demonstrated that the molybdenum sulfide anions prefer to attach and dope into PPy, inducing formation of MoS2 nanosheets on the surface of the PPy instead of forming independent nanoflowers25. Comprehensive characterizations revealed a nanoflower morphology of the resulting MoS2-P. It presented larger area “thin folding leaves” and much more mesoporous pore than the counterpart without PPy as template. Enhanced HER performance was demonstrated. The “thin folding leaves” possessed adequate edge active sites, enabling the enhanced HER performance. The higher mesoporous volume facilitates efficient transfer of reaction substrate. The PPy template could generate nanoflower morphology with larger stretched “thin folding leaves” and higher mesoporous volume as well as much more tortuous and cleaved lattice structures than the MoS2 without PPy as template. By adjusting the concentration of the PPy, both the morphology and the defects can be controllably engineered. The proposed method is a facile and cost-effective, providing great potential for larger-scale production of 3D MoS2 with advanced HER performances and practical industrial application.

Results

The one-step hydrothermal synthesis method is schematically illustrated in Fig. 1. (NH4)6Mo7O24·4H2O and thiourea were chosen as the precursor for large-quantity of MoS2 preparation in the presence of PPy, The PPy was employed as the template to generate a MoS2 structure with large surface area and adequate active edges. The (NH4)6Mo7O24·4H2O, thiourea and PPy were simultaneously transferred into the Teflon-lined stainless-steel autoclave for hydrothermal synthesis to generate MoS2/PPy composite. The resulting composite was further annealed at higher temperature to remove the PPy24,26 to produce 3D MoS2-P. The PPy played critical roles in the formation of unique morphology and electrochemical performance of the MoS2-P, which will be addressed in the following section.

Figure 1
figure 1

Schematic illustration of the morphological evolution process of the MoS2-P.

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Scanning electron microscope (SEM) and transmission electron microscope (TEM) were employed to characterize the morphology of the synthesized composites. As shown in Fig. 2, in comparison with the MoS2 without PPy as template displaying a compact nanoflower structure (Fig. 2b), the MoS2-P presented a nanoflower morphology with larger stretched “thin folding leaves” (Fig. 2a). PPy acts as a soft template for MoS2 in the experiments, providing a substrate to support the growth of MoS2. Thus, the morphology of the PPy will influence of the morphology of MoS2-P. The SEM image (Fig. S1) shows that the morphology of PPy is an irregular sphere with rough surface, which provide a template for the formation of nanoflower of MoS2-P (Fig. 2b). As shown in Fig. S2, the annealing process could effectively remove the PPy24. The annealing process also induce tortuous and cleaved lattice in the MoS2-P. The large “thin folding leaves” of the nanoflower structure possessing efficient edge active sites and defects are responsible to the enhanced HER performance. The decrease of carbon component during the annealing process further confirmed the effective removal of PPy (Fig. S3). TEM image confirms the larger stretched thin films of MoS2-P (Fig. 2c) compared to the closed nanoflower of MoS2 without PPy as template (Fig. 2d). These results revealed that the PPy template effectively inhibited the aggregation of MoS2 during hydrothermal synthesis process. The formation of stretched “thin folding leaves” is vital for HER, because the adequate exposure of active sites facilitate the efficient substrate accessibility compared to the MoS2 that with lots of active edges hided inside the nanoflowers. The high-resoflution TEM (HRTEM) image analysis of MoS2-P displayed a clean lattice structure with an interplanar spacing of 0.65 nm (Fig. 2e) ascribed to the (002) planes of MoS211, similar to the MoS2 (Fig. 2f), which suggested that the MoS2 was successfully synthesized. Importantly, numerous tortuous and cleaved lattice structures were observed in the MoS2-P (circle in Fig. 2e), indicating the formation of numerous defects during the PPy-assisted MoS2 hydrothermal synthesis.

Figure 2
figure 2

(a) SEM image of MoS2-P; (b) SEM image of MoS2; (c) TEM image of MoS2-P; (d) TEM image of MoS2; (e) HRTEM image of MoS2-P; (f) HRTEM image of MoS2.

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As shown in Fig. 3a, both of the nitrogen (N2) adsorption-desorption isotherms of the MoS2 and MoS2-P presented typical IV isothermals hysteresis loop associated with large size mesoporous. The H3 hysteresis loop of MoS2-P and MoS2 indicated the presence of slit nanopore27. Brunauer–Emmett–Teller (BET) calculation revealed that MoS2-P displayed six times larger surface area of 431.2 m2 g−1 than that of MoS2 (60.3 m2 g−1). Although MoS2-P and MoS2 displayed similar pore size of 19 nm, MoS2-P showed a four-fold higher pore volume of 0.608 cm3 g−1 than that of MoS2 with a pore volume of 0.156 cm3 g−1 (Fig. 3b). Thus, the PPy and annealing process rendered MoS2-P with large surface area and adequate nanopores.

Figure 3
figure 3

(a) Nitrogen adsorption-desorption characterizations of MoS2-P (blue curve) and MoS2 (black curve); (b) the pore size distribution of MoS2-P (blue curve) and MoS2 (black curve).

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As shown in Fig. 4a, in agreement with MoS2 (black curve), the characteristic peaks of MoS2-P at 2θ = 14.3, 33.6, 29.7, 59.0 corresponding to the (002), (100), (103), (110) planes (blue curve). It confirmed the well defined and hexagonally symmetric structured of MoS2 (JCPDS card no. 77–1716). It indicated less layers of MoS2-P than MoS2 according to Scherrer analysis of the half maximum (FWHM) value in the (002) diffraction peak17 (Fig. 4a). The slight shift of (100) and (110) between MoS2 and MoS2-P was resulted from the crystal lattice tortuosity (λ = 2dsinθ) (Fig. 4a, inset). The recovery of characteristic peaks in MoS2-P compared to PPy encapsulated MoS2 (PPy-MoS2) (Fig. S4) as well as the thermogravimetric analysis curve of MoS2-P (Fig. S5) proved the annealing process removed effectively the PPy. The two strong characteristic peaks located at 376 and 402 cm−1 were ascribed to E2g, A1g respectively. It suggested the resulted MoS2-P (Fig. 4b, blue curve) and MoS2 (Fig. 4b, black curve) were mainly 2H-MoS2, and the slight red shift of A1g in MoS2-P compared to MoS2 was caused by the crystal lattice tortuosity28.

Figure 4
figure 4

Characterization of MoS2 and MoS2-P by (a) XRD; (b) Raman spectroscopy and High-resolution (c) Mo 3d and (d) S 2p XPS spectra. Inset a: the magnified image of the (100) and (110) peak of the MoS2 and MoS2-P.

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The components of the MoS2 samples were investigated by X-ray photoelectron spectroscopy (XPS). The MoS2 and MoS2-P showed the characteristic peaks of the Mo 3d, S 2p and O 1s (Fig. S6). The appearance of weak N 1s peak located at 396 eV and slight increase of C1s at 285.2eV in MoS2-P (Fig. S6b) compared to MoS2 (Fig. S6a) was resulted from carbonization remnants of PPy during the annealing process. It suggested that PPy might carbonized partially rather than decomposed completely, and it might also generated active nitrogen hybrid species for enhanced HER performance24. As shown in the Mo 3d spectrum of MoS2-P (Fig. 4c, blue curve), the peak located at 229.8 and 232.9 eV is assigned to Mo 3d5/2 and Mo 3d3/2, respectively29,30, which further suggested the resulting MoS2-P was mainly consisted of 2H semiconducting structure16 (Fig. 4c, black curve). Similarly, the characteristic peaks of S 2p spectrum in MoS2-P (Fig. 4d, blue curve) at 162.4 and 163.5 eV attributed to S 2p3/2 and S 2p1/2 were presented (black curve), indicating the domain oxidation state of S2− 31. The XPS analysis revealed that the MoS2-P and MoS2 have little difference in elemental composition and bonding configuration. Similar to MoS2, the MoS2-P displayed a strong absorption 671 nm, and the band gap was calculated to be 1.52 eV (Fig. S6), agreeing with previous report32.

The HER electrochemical performance of the MoS2-P was further investigated. Figure 5a shows representative linear sweep voltammetry (LSV) response for the bulk MoS2, commercial Pt-C, MoS2 and MoS2-P. The MoS2 exhibited superior HER performance (black curve) than the raw bulk MoS2 (purple curve)11. The MoS2-P (blue curve) displayed enhanced HER catalytic activity with negative onset overpotential of 100 mV than that of MoS2 with an onset overpotential of 170 mV. The MoS2-P presented superior an overpotential of −251 mV for 10 mA cm−2 to the MoS2 of 350 mV and bulk MoS2 of 578mV (Fig. 5a, inset). The MoS2-P displayed a smaller Tafel slope of 80.5 mV/dec (Fig. 5b, blue curve) than MoS2 of 95.9 mV/dec (Fig. 5b, black curve) and bulk MoS2 of 143.3 mV/dec (Fig. 5b, purple curve). These results confirmed the enhanced HER performance of MoS2-P than MoS2 and bulk MoS2.

Figure 5
figure 5

(a) Polarization curves obtained of catalysts as indicated; inset: the overpotential of catalysts when the current is 10 mA cm−2 (b) Corresponding Tafel plots recorded on glassy carbon electrodes with a catalyst loading of 0.28 mg cm−2. (c) Nyquist plots of the MoS2 catalyst recorded. Inset c: equivalent circuit of the EIS spectroscopy. (d) Durability test showing negligible current loss even after 2000 cycles.

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To obtain more information about the intrinsic catalytic activity, the turnover frequency (TOF) for the active sites of different MoS2 catalysts was calculated using the roughness factor method according to the following equations33.

where Dc and Ds were the density of active sites for catalyst (Sites/cm2) and standard sample (Sites/cm2), the Cdlsand Cdlc were the double layer capacitor (Cdl) for standard MoS2 (60 μF cm−1) and catalysts calculated by the cyclic voltammetry (CV) experiment at different scan rates (Fig. S7), the j (A cm−2) was the current density of LSV at −400 mV and q was the elementary charge (1.6 × 10–19 C). The active sites density of MoS2-P was 3.35 × 1017 sites/cm2, 1.2 times and 27 times higher than that of MoS2 (2.85 × 1017 sites/cm2) and that of bulk MoS2 (1.24 × 1016 sites/cm2), respectively. The MoS2-P presented enhanced TOF of 0.85s−1 to MoS2 (0.67s−1) and bulk MoS2 (0.54s−1), further indicating advanced HER catalytic activity of MoS2-P. Electrochemical impedance spectroscopy (EIS) was used to characterize the interfacial reaction and electron-transfer kinetics in HER. As shown in Fig. 5c, MoS2-P displayed the lowest faradaic impedance and smallest charge transfer resistance (Rct) among these MoS2 catalysts. For the long-time durability, the MoS2-P showed a negligible decrease in the current density after a long period of 2000 potential-cycling between 0 and −0.5 V, indicating the outstanding electrochemical stability and its promising potential for the practical application (Fig. 5d).

Briefly, the most important step involved in the HER process is the hydrogen adsorption, which require appropriate Gibbs free energy of the catalyst. Increasing theoretical and experimental reports confirmed the Gibbs free energy of H adsorption on the unsaturated atoms at MoS2 edge active site are favorable to hydrogen adsorption, leading to the efficient hydrogen evolution7 (Fig. S8). We further compared the HER performance of the MoS2-P to previous reports of MoS2-based HER catalysts. As shown in Table S1, the MoS2-P showed more competitive performance than most of the previous MoS2-based catalyst. The enhanced HER performance could be explained as follows: First, the unique nanoflower morphology of large stretched “thin folding leaves” allowed considerable active site exposure for HER (Fig. 2a and c). The quantity of total active sites (calculated by the surface area multiply the density of active sites for catalyst (Sites/cm2)) for MoS2-P is 8.2 times higher than that of MoS2 and 378 times higher than that of bulk MoS2 (Fig. 6). Secondly, the numerous defects resulted from the tortuous and cleaved lattice formed during the annealing process also contribute to enhance HER performance (Fig. 2e). Additionally, the high mesoporous volume of MoS2-P facilitates the efficient mass transfer.

Figure 6
figure 6

Ratios of surface area, active sites per surface area and total active sites of MoS2 and MoS2-P versus the bulk MoS2.

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Discussion

In this study, we have developed a facile and cost-effective strategy for large-scale synthesis of 3D MoS2 nanoflower with large stretched “thin folding leaves” and considerable nanopores by using a PPy-assisted one-step hydrothermal routine. Microscopic and spectroscopic tools including SEM, TEM, HRTEM, BET, XRD, XPS and Raman spectroscopy was employed to comprehensively characterize the morphology and component of the MoS2-P. Electrochemical characterizations demonstrated that the prepared MoS2-P displayed advanced HER performance. It presenting superior onset overpotential, Tafel plot and lower faradaic impedance than MoS2 without using PPy as template, which was competitive to most of the reported analogous MoS2-based catalyst. It also displayed long outstanding electrochemical stability for the practical application. It was demonstrated that the high quantity of exposed active sites on the large surface and the defects formed during the hydrothermal synthesis synergistically contribute to the advanced HER catalytic activity, while unique mesoporous structure facilitates the accessibility of the reacted substrate. The facile and cost-effective method for larger-scale synthesis 3D MoS2 with advanced HER performances holds great promising in practical industrial application.

Methods

Materials and Reagents

(NH4)6 Mo7O24 · 4H2O, Polypyrrole (PPy, un-doped, extent of labeling: ~20 wt. % loading, composite with carbon black and the CAS number is 30604-81-0) and Pt/C (10% Pt) were obtained from Sigma-Aldrich. Thiourea and KOH were from Sinopharm Chemical Reagent Co., Ltd. Sulfuric acid (H2SO4, 95–98%) and ethanol (99.9%) was purchased from Beijing Chemical Works. All aqueous solutions were prepared with ultrapure water obtained from a Millipore water purification system (≥18 MΩ, Milli-Q, Millipore).

X-ray diffraction (XRD) was performed by a Rigaku X-ray diffractometer with Cu KR target. The porosity was measured with a nitrogen adsorption-desorption isotherm using a surface area analyzer (QuadraSorb SI 2000–08, Quantachrome Instruments). The morphologies of products were observed under a field-emission scanning electron microscope (SEM; HITACHI S-4800) and a transmission electron microscope (TEM; JEM-2010, 200 kV). X-ray photoelectron spectroscopy (XPS) analysis was performed using an AXIS ULTRADLD instrument equipped with an Al Kα X-ray source. Raman spectra were recorded on an InVia-Reflex Raman microscope with a laser excitation wavelength of 532 nm.

Materials Preparation of MoS2 and MoS2-P

MoS2-P were synthesized by a hydrothermal synthesis; typically, 44.8 mg polypyrrole (PPy), 123.625 mg hexaammonium heptamolybdate tetrahydrate (0.1 mmol, (NH4)6 Mo7O24·4H2O, i.e. 0.7 mmol Mo) and 228.375 mg thiourea (3 mmol) were dissolved in ultrapure water (20 mL) under vigorous stirring to form a homogeneous solution. Then, the solution was transferred into a 25 mL Teflon-lined stainless steel autoclave and maintained at 180 °C for 24 h, and the reaction system was then cooled to room temperature. The final product was washed thoroughly with water and ethanol to remove any possible ions, and the as-prepared hydrogel was directly dehydrated via a freeze-drying process to maintain the 3D monolithic architecture, and then annealed at 600 °C for 3 h under argon. As a control, the MoS2 was prepared in a similar procedure except using PPy as template.

Electrochemical Characterization

Electrochemical measurements were performed using a CHI 852C electrochemical workstation (Shanghai Chenhua Instrument Co., China) with a standard three-electrode setup in 0.5 M H2SO4 aqueous solution. A saturated calomel electrode (Hg/HgCl2 in saturated KCl) and a graphite rod were used as the reference electrode and the counter electrode, respectively. A glass carbon rotating ring-disk electrodes (RRDE) loading the catalyst was used as the working electrode. Experimentally, 1 mg of the respective catalyst powder was dispersed in 1 mL of ethanol with 50 μL of Nafion solution and ultrasonicated for 15 min. A 20 μL of the resulting solution was dropped onto the glassy-carbon disk (diameter of 3 mm) using a microliter syringe and dried at room temperature. The catalyst loadings were all 0.28 mg cm−2. Liner sweep voltammetry (LSV) was performed in nitrogen-statured 0.5 M H2SO4 at a scan rate of 10 mV s−1 at 1400 rpm. Electrochemical impedance spectroscopy (EIS) was measured in the same configuration from 10−2 to 106 Hz with modulation amplitude of 5 mV. SCE was calibrated to reversible hydrogen electrode (RHE). A Pt wire was used as the working electrode and the counter electrode, respectively, and the calibration was carried out in a high purity H2-saturated electrolyte at a scan rate of 0.1 mV s−1. The potential at which the current crossed zero was taken to be the thermodynamic potential for the hydrogen electrode reactions. In 0.5 M H2SO4, E (RHE) = E (SCE) + 0.26 V. All the potentials reported in our manuscript are against RHE.

Additional Information

How to cite this article: Lu, X. et al. One-Step Hydrothermal Fabrication of Three-dimensional MoS2 Nanoflower using Polypyrrole as Template for Efficient Hydrogen Evolution Reaction. Sci. Rep. 7, 42309; doi: 10.1038/srep42309 (2017).

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