One-Step Hydrothermal Fabrication of Three-dimensional MoS2 Nanoflower using Polypyrrole as Template for Efficient Hydrogen Evolution Reaction

Herein, a facile and cost-effective strategy for hydrothermal synthesis of three-dimensional (3D) MoS2 with adequate active edge sites and advanced hydrogen evolution reaction (HER) performance using polypyrrole (PPy) as template is reported. The MoS2 is first thermally nucleated using hexaammonium heptamolybdate tetrahydrate (NH4)6Mo7O24·4H2O and thiourea as precursor in the presence of PPy, and then they are further annealed to remove PPy at higher temperature to generate 3D MoS2-P. Morphology and composition characterizations reveal that the 3D MoS2-P exhibits a nanoflower morphology. It presents larger stretched “thin folding leaves” and higher mesoporous volume of 0.608 cm3 g−1 than the MoS2 without PPy as template. Importantly, the 3D MoS2-P shows enhanced HER catalytic activity (onset potential at −100 mV) than previously reports that MoS2-based HER catalysts. The large “thin folding leaves” possessing efficient edge active sites and defects are responsible to for the enhanced HER performance, while the high mesoporous volume facilitates the transfer of reaction substrate. Our study provides a facile and cost-effective method for synthesis of 3D MoS2 with advanced HER performances, which has great potential for larger-scale production and practical industrial applications.

modification including chemical doping 15 , adjustment of the metastable 1T-phase 16,17 and strain treating 18 . Furthermore, fabrication of efficient 3D MoS 2 such as porous nanosheets 19 , vertical nanoflakes 20 , core-shell MoO 3 -MoS 2 nanowires 21,22 and double-gyroid morphology 23 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 MoS 2 naonosheets rooting into polyaniline nanofibers, It showed good catalytic properties and stability 21 . Kibsgaard et al. engineered the surface structure of MoS 2 to preferentially expose edge sites for improved catalytic activity by successfully synthesizing contiguous large-area thin films of a highly ordered double-gyroid MoS 2 with nanopores 23 . Our group designed a 3D nitrogen-doped graphene supported MoS 2 as an advanced HER catalyst 24 . Among them, various 3D MoS 2 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 MoS 2 -based catalysts with high HER activity.
Herein, a one-step hydrothermal synthesis route for 3D MoS 2 flower with advanced HER performance using PPy (MoS 2 -P) was developed. The PPy shows excellent conductivity. Specially, it displays good stability 24 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 MoS 2 nanosheets on the surface of the PPy instead of forming independent nanoflowers 25 . Comprehensive characterizations revealed a nanoflower morphology of the resulting MoS 2 -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 MoS 2 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 MoS 2 with advanced HER performances and practical industrial application.

Results
The one-step hydrothermal synthesis method is schematically illustrated in Fig. 1. (NH 4 ) 6 Mo 7 O 24 ·4H 2 O and thiourea were chosen as the precursor for large-quantity of MoS 2 preparation in the presence of PPy, The PPy was employed as the template to generate a MoS 2 structure with large surface area and adequate active edges. The (NH 4 ) 6 Mo 7 O 24 ·4H 2 O, thiourea and PPy were simultaneously transferred into the Teflon-lined stainless-steel autoclave for hydrothermal synthesis to generate MoS 2 /PPy composite. The resulting composite was further annealed at higher temperature to remove the PPy 24,26 to produce 3D MoS 2 -P. The PPy played critical roles in the formation of unique morphology and electrochemical performance of the MoS 2 -P, which will be addressed in the following section.
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 MoS 2 without PPy as template displaying a compact nanoflower structure (Fig. 2b), the MoS 2 -P presented a nanoflower morphology with larger stretched "thin folding leaves" (Fig. 2a). PPy acts as a soft template for MoS 2 in the experiments, providing a substrate to support the growth of MoS 2 . Thus, the morphology of the PPy will influence of the morphology of MoS 2 -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 MoS 2 -P (Fig. 2b). As shown in Fig. S2, the annealing process could effectively remove the PPy 24 . The annealing process also induce tortuous and cleaved lattice in the MoS 2 -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 MoS 2 -P ( Fig. 2c) compared to the closed nanoflower of MoS 2 without PPy as template Scientific RepoRts | 7:42309 | DOI: 10.1038/srep42309 (Fig. 2d). These results revealed that the PPy template effectively inhibited the aggregation of MoS 2 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 MoS 2 that with lots of active edges hided inside the nanoflowers. The high-resoflution TEM (HRTEM) image analysis of MoS 2 -P displayed a clean lattice structure with an interplanar spacing of 0.65 nm (Fig. 2e) ascribed to the (002) planes of MoS 2 11 , similar to the MoS 2 ( Fig. 2f), which suggested that the MoS 2 was successfully synthesized. Importantly, numerous tortuous and cleaved lattice structures were observed in the MoS 2 -P (circle in Fig. 2e), indicating the formation of numerous defects during the PPy-assisted MoS 2 hydrothermal synthesis.
As shown in Fig. 3a, both of the nitrogen (N 2 ) adsorption-desorption isotherms of the MoS 2 and MoS 2 -P presented typical IV isothermals hysteresis loop associated with large size mesoporous. The H3 hysteresis loop of MoS 2 -P and MoS 2 indicated the presence of slit nanopore 27 . Brunauer-Emmett-Teller (BET) calculation revealed that MoS 2 -P displayed six times larger surface area of 431.2 m 2 g −1 than that of MoS 2 (60.3 m 2 g −1 ). Although MoS 2 -P and MoS 2 displayed similar pore size of 19 nm, MoS 2 -P showed a four-fold higher pore volume of 0.608 cm 3 g −1 than that of MoS 2 with a pore volume of 0.156 cm 3 g −1 (Fig. 3b). Thus, the PPy and annealing process rendered MoS 2 -P with large surface area and adequate nanopores.  (Fig. 4a). The slight shift of (100) and (110) between MoS 2 and MoS 2 -P was resulted from the crystal lattice tortuosity (λ = 2dsinθ ) (Fig. 4a, inset). The recovery of characteristic peaks in MoS 2 -P compared to PPy encapsulated MoS 2 (PPy-MoS 2 ) (Fig. S4) as well as the thermogravimetric analysis curve of MoS 2 -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 E 2g , A 1g respectively. It suggested the resulted MoS 2 -P (Fig. 4b, blue curve) and MoS 2 (Fig. 4b,  black curve) were mainly 2H-MoS 2, and the slight red shift of A 1g in MoS 2 -P compared to MoS 2 was caused by the crystal lattice tortuosity 28 .
The components of the MoS 2 samples were investigated by X-ray photoelectron spectroscopy (XPS). The MoS 2 and MoS 2 -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 MoS 2 -P (Fig. S6b) compared to MoS 2 (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 performance 24 . As shown in the Mo 3d spectrum of MoS 2 -P (Fig. 4c, blue curve), the peak located at 229.8 and 232.9 eV is assigned to Mo 3d 5/2 and Mo 3d 3/2, respectively 29,30 , which further suggested the resulting MoS 2 -P was mainly consisted of 2H semiconducting structure 16 (Fig. 4c, black curve). Similarly, the characteristic peaks of S 2p spectrum in MoS 2 -P (Fig. 4d, blue curve) at 162.4 and 163.5 eV attributed to S 2p 3/2 and S 2p 1/2 were presented (black curve), indicating the domain oxidation state of S 2− 31 . The XPS analysis revealed that the MoS 2 -P and MoS 2 have little difference in elemental composition and bonding configuration. Similar to MoS 2 , the MoS 2 -P displayed a strong absorption 671 nm, and the band gap was calculated to be 1.52 eV (Fig. S6), agreeing with previous report 32 .
The HER electrochemical performance of the MoS 2 -P was further investigated. Figure 5a shows representative linear sweep voltammetry (LSV) response for the bulk MoS 2 , commercial Pt-C, MoS 2 and MoS 2 -P. The MoS 2 exhibited superior HER performance (black curve) than the raw bulk MoS 2 (purple curve) 11 . The MoS 2 -P (blue curve) displayed enhanced HER catalytic activity with negative onset overpotential of 100 mV than that of MoS 2 with an onset overpotential of 170 mV. The MoS 2 -P presented superior an overpotential of − 251 mV for 10 mA cm −2 to the MoS 2 of 350 mV and bulk MoS 2 of 578mV (Fig. 5a, inset). The MoS 2 -P displayed a smaller Tafel slope of 80.5 mV/dec (Fig. 5b, blue curve) than MoS 2 of 95.9 mV/dec (Fig. 5b, black curve) and bulk MoS 2 of 143.3 mV/ dec (Fig. 5b, purple curve). These results confirmed the enhanced HER performance of MoS 2 -P than MoS 2 and bulk MoS 2 .
To obtain more information about the intrinsic catalytic activity, the turnover frequency (TOF) for the active sites of different MoS 2 catalysts was calculated using the roughness factor method according to the following equations 33 . where D c and D s were the density of active sites for catalyst (Sites/cm 2 ) and standard sample (Sites/cm 2 ), the C dls and C dlc were the double layer capacitor (C dl ) for standard MoS 2 (60 μ F 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 MoS 2 -P was 3.35 × 10 17 sites/cm 2 , 1.2 times and 27 times higher than that of MoS 2 (2.85 × 10 17 sites/cm 2 ) and that of bulk MoS 2 (1.24 × 10 16 sites/cm 2 ), respectively. The MoS 2 -P presented enhanced TOF of 0.85s −1 to MoS 2 (0.67s −1 ) and bulk MoS 2 (0.54s −1 ), further indicating advanced HER catalytic activity of MoS 2 -P. Electrochemical impedance spectroscopy (EIS) was used to characterize the interfacial reaction and electron-transfer kinetics in HER. As shown in Fig. 5c, MoS 2 -P displayed the lowest faradaic impedance and smallest charge transfer resistance (R ct ) among these MoS 2 catalysts. For the long-time durability, the MoS 2 -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 MoS 2 edge active site are favorable to hydrogen adsorption, leading to the efficient hydrogen evolution 7 (Fig. S8). We further compared the HER performance of the MoS 2 -P to previous reports of MoS 2 -based HER catalysts. As shown in Table S1, the MoS 2 -P showed more competitive performance than most of the previous MoS 2 -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/cm 2 )) for MoS 2 -P is 8.2 times higher than that of MoS 2 and 378 times higher than that of bulk MoS 2 (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 MoS 2 -P facilitates the efficient mass transfer.

Discussion
In this study, we have developed a facile and cost-effective strategy for large-scale synthesis of 3D MoS 2 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 MoS 2 -P. Electrochemical characterizations demonstrated that the prepared MoS 2 -P displayed advanced HER performance. It presenting superior onset overpotential, Tafel plot and lower faradaic impedance than MoS 2 without using PPy as template, which was competitive to most of the reported analogous MoS 2 -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 MoS 2 with advanced HER performances holds great promising in practical industrial application. % 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 (H 2 SO 4 , 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.  (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 MoS 2 was prepared in a similar procedure except using PPy as template.

Materials Preparation of MoS 2 and MoS
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 H 2 SO 4 aqueous solution. A saturated calomel electrode (Hg/HgCl 2 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 μ L of Nafion solution and ultrasonicated for 15 min. A 20 μ L 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 H 2 SO 4 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 10 6 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 H 2 -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 H 2 SO 4 , E (RHE) = E (SCE) + 0.26 V. All the potentials reported in our manuscript are against RHE.