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In-Situ Hydrothermal Synthesis MoS2 / Guar Gum Carbon Nano#owers as Advanced Electrocatalysts for Electrocatalytic Hydrogen Evolution Yu Cheng, Kanglei Pang, Xiao Wu, Zhiguo Zhang, Xiaohui Xu, Junkai Ren, Wei Huang, and Rui Song ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00994 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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ACS Sustainable Chemistry & Engineering

In-Situ Hydrothermal Synthesis MoS2 / Guar Gum Carbon Nanoflowers as Advanced Electrocatalysts for Electrocatalytic Hydrogen Evolution Yu Cheng,a Kanglei Pang,ac Xiao Wu,ab Zhiguo Zhang,a Xiaohui Xu,a Junkai Ren,a Wei Huang,*ab Rui Song*a a

School of Chemical Sciences, University of Chinese Academy of Sciences, 19

Yuquan Road, Shijingshan District, Beijing, 100049, PR China b

Institute of Chemistry, Chinese Academy of Sciences (CAS), 2 Zhongguancun North

Road, Haidian District, Beijing, 100190, PR China c

Sino-Danish College (SDC), University of Chinese Academy of Sciences, 19 Yuquan

Road, Shijingshan District, Beijing, 100049, PR China *Email: [email protected]; [email protected]

ABSTRACT Herein, we report a simple in-situ hydrothermal synthetic method for the preparation of a novel three-dimensional (3D) nanoflowers forming with few-layered and expanded interlayer spacing MoS2 nanoflakes via restricting the polymerization of guar gum and the growth of MoS2. In this process, hexaammonium molybdate ((NH4)6Mo7O24·4H2O) and thiourea (CH4N2S) acted as the precursor of molybdenum and sulfur respectively, while guar gum functioned as both the template of chemical reaction and carbon source. The obtained MoS2/guar gum carbon hybrid nanoflowers (MoS2/CF) in a well-assembled 3D nanoflowers architecture provides copious active sites and thus prevents inherent stacking among MoS2 layers. Thanks to all these advantages, the electrochemical evaluation demonstrates that MoS2/CF-750 shows extraordinary HER electrocatalytic performances, possessing extremely low onset potential approximate 20 mV, the low overpotential of ~125 mV at 10 mA cm-2 and an 1

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extraordinary small Tafel slope of 34 mV dec-1, which is extremely identical to that of bulk platinum (Pt), “the gold benchmark” for hydrogen production. Moreover, the strong interactions between MoS2 nanoflakes and guar gum enable MoS2/CF-750 excellent long-term stability and microstructural integrity, presenting nearly 100% activity retention after 2000 cycles and ~95% after 16 h of chronoamperometry assessment (0.15 V). The preparation strategy is simple, inexpensive, and readily scalable, and could be extended to diverse 3D non-noble metal electrocatalysts.

Keywords: hydrogen evolution reaction (HER), molybdenum disulfide, guar gum, three-dimsensional (3D) hierarchical structure, in-situ hydrothermal

INTRODUCTION In order to meet the quickly increasing energy and environmental needs, variety of methods have been explored to seek renewable and green energy.1-5 Among the alternatives, hydrogen (H2) is a plentiful and renewable green energy, which can be reserved and distributed flexibly, and it is considered as a potential replacement for the classical fossil fuels.6 Hydrogen evolution reaction (HER) provides an efficient method for H2 production from water.7-9 Although platinum (Pt) and other noble metals materials are recognized as extremely efficient electrocatalysts for HER, large-scale preparation and application of noble metals electrocatalysts is gravely limited by their relative scarcity and high-cost.10-12 Therefore, it is urgent to exploit a high-performance, low-cost, and earth-plentiful HER electrocatalysts for achieving the ‘hydrogen economy’ .12-14 As

of

now,

non-noble-metal

electrocatalysts

including

transition

metal

chalcogenides,15-17 phosphides,18-20 nitrides,21-22 and carbides16,23 have served as significant candidates for hydrogen evolution reaction. Among them, transition metal dichalcogenides (TMDs), for their low-cost and edge active sites structure, exhibiting excellent HER electrocatalysts performances.24-25 TMDs is a series of chemical

2


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materials with the constitutional formula of MX2, in which M represents the transition metal (Mo, Ti or W), while X represents the chalcogen (S or Se) .26 As a typical layered TMDs, molybdenum disulfide (MoS2) has received tremendous attention, which shows several S-Mo-S with layer packing relying on weak van der Waals.27-28 And many researches have demonstrated that MoS2 assumes the good HER performance on account of its highly active sites exposed at the molecular structure edges.15,

29-30

In recent years, various approaches for synthesizing MoS2-based

electrocatalysts materials have been reported, such as physical mixing, solvent thermal, and chemical vapor deposition (CVD) (Table S1). However, the HER activity of MoS2 is still inferior to the platinum-based catalysts, especially the onset potential. Specifically, there are two main reasons bringing out this problem, one is the inherent layers accumulating within MoS2 that severely restricts the number of exposed active sites and robust stability, and the other is the weak electrical conductivity and poor electron transfer property which originated inherently from the large bandgap of MoS2.30-31 As recently reported the MoS2 HER performance can be efficiently improved by regulating interlayer spacing and layers of MoS2.32 Gao et al. synthesized MoS2 nanosheets using a microwave assisted strategy, and the resultant MoS2 possessing the overly enlarged interlayer spacing of 9.4 Å, mighty electrocatalytic activity with the onset potential only 103 mV.33 Therefore, in order to improve the amount of exposed active sites, it is of great importance to enlarge interlayer spacing, prevent the stacking and reduce the layer of MoS2 by bring in the extra molecules or regulating chemical reaction conditions. Meanwhile, MoS2 coupled with other carbon-based materials has been reported to be an ideal method because carbon-based materials possess distinct confinement effect and outstanding electric conductivity.31, 34 Hence, introducing the carbon substrates into MoS2 could enhance interlayer spacing and accelerate the electron transfer simultaneously. Tang et al. prepared an ultra-thin MoS2 nitrogen doped reduced graphene oxide (MoS2/N-RGO-180) electrocatalysts with expanded 3



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interlayer spacing about 9.5 Å, and the excellent electrocatalytic activity was also ascribed the synergistic effects of MoS2 nanosheets and nitrogen doped RGO.35 Zhang et al. prepared a molybdenum disulfide (MoS2) /carbon aerogel (CA) hybrids with few-layered, possessing low onset potential of 140 mV, large current density as well as outstanding stability.36 Notwithstanding the above achievements, there are still some vital problems remain to be addressed, for instance, simplifying preparation process as well as seeking sustainable and environment-friendly carbon precursor, which are quite significant for realizing the scalable preparation and application of electrocatalysts. In this work, we employ a simple and facile in-situ hydrothermal synthesis approach to prepare a novel three-dimensional (3D) MoS2/guar gum carbon nanoflowers (marked as MoS2/CF, hereinafter) with enlarged interlayer spacing of 9.5 Å by using guar gum both as reaction template and carbon precursor. Normally obtained from endosperm of guar seeds, guar gum is a water soluble, environment-friendly

branched

chain

neutral

polysaccharide

consisting

of

β-D-mannose backbone with side chains of α-linked galactose residues, which is widely applied as a thickener or stabilizing agents in practical industries, such as the pharmaceutical, cosmetic, textile printing, paper, food and oil recovery.37-38 As far as we know from the available literature, it is the first time to prepare MoS2/carbon hybrid using guar gum as carbon precursor, and more importantly, an enlarged interlayer spacing of MoS2 is successfully realized by the confined insertion of guar gum moiety. In addition, the method adopt in this case involves sequential “hydrothermal-washing-calcining” steps, which is very facile and suitable for scalable production. Based on structural characterizations, the resultant few-layered and enlarged interlayer spacing of MoS2 nanoflakes were homogeneously inlaid in the guar gum carbon substrate. The merits of this architecture are that the few-layered and enlarged interlayer spacing of MoS2 endow extraordinary high active surface area and abundant exposed active sites in HER process; additionally, the guar gum carbon 4


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substrate highly accelerate electron transport and proton (H+) trapping, and meanwhile the strong interaction between MoS2 and guar gum carbon substrate ensure robust microstructure stability in HER process. Surprisingly, the synergistic and novel 3D MoS2/CF-750 architecture results in an excellent electrocatalytic performance, exhibiting the more positive onset potential among most reported MoS2-based electrode materials when applying as HER electrocatalyst in acidic electrolyte (Table S1).

EXPERIMENTAL SECTION Materials. All chemicals were used without any further treatment. Guar gum was purchased from

Anhui

biotechnology

Factory.

Ammonium

molybdate

tetrahydrate

(NH4)6Mo7O24·4H2O, 99.8% metals basis, Sigma-Aldrich. Thiourea (CH4N2S, 99.8% metals basis), Aladdin Reagent Ltd. Sulfuric acid (H2SO4, 98.0%), Guangzhou Chemical Reagent Factory. Deionized water (18.2 MΩ) were used in solution. Preparation of MoS2/CF nanoflowers. The preparation of MoS2/CF nanoflowers is schematically shown in Figure1. In a typical in-situ hydrothermal method, 1.64 g of (NH4)6Mo7O24·4H2O and 4.23 g of CH4N2S was first dissolved in 100 mL of H2O. Then 0.35 g guar gum was added slowly to the above solution, and reacted at 35 °C for 2 h under continuous magnetic stirring. In order to make guar gum fully swelling in the solution, the above solution was remained statically at room temperature for 3 h. Subsequently, the obtained solution was moved into a 50 mL Teflon stainless-steel autoclave and then reacted at 180 °C about 24 h. The precipitates were collected by centrifugation at 12000 rpm for 10 min, and then precipitates were washed using deionized water and anhydrous ethanol for several times. The products were finally dried under vacuum oven at 60 °C for 24 h, and the obtained samples were denoted as MoS2/CF. Then the MoS2/CF was calcined at 750 °C for 2 h with heating rate of 5 °C min-1 under N2 5



atmosphere, and the resultant was denoted as MoS2/CF-750, hereinafter. For comparison, pure MoS2, guar gum, and corresponding 750 °C treated samples (denoted as MoS2, MoS2-750, guar gum and guar gum-750, respectively), were prepared under the same conditions and were subjected to the related measurements. Besides, in order to clear the influence of guar gum content and calcined temperature on HER performance, MoS2/CF-750 samples with 0.50 and 0.17 g guar gum were prepared, denoted as MoS2/CF(0.50)-750 and MoS2/CF(0.17)-750, respectively (see Table S2). Meanwhile, MoS2/CF samples with fixed guar gum content (0.35g) were calcined at 650 °C and 700 °C, denoted as MoS2/CF-650 and MoS2/CF-700, respectively (see Table S3). Characterizations. The morphology and structure were detected through transmission electron microscopy (TEM, Tecnai F20, FEI, USA), field-emission scanning electron microscopy (SEM, SU8200, Hitachi, Japan), as well as an energy-dispersive X-ray spectroscopy (EDS) affiliated to the TEM and field-emission scanning electron microscopy. The crystallinities of as-prepared electrocatalysts were characterized by X-ray diffraction (XRD) on a Rigaku Smartlab diffractometer using Cu Kα1 (λ = 1.544 Å) radiation. Raman spectra were collected with a Renishaw Invia Raman spectrometer with a 532 nm laser source. Thermogravimetric analysis (TGA) was executed on a SDT-Q600 instrument (TA Instruments, USA) in N2 atmosphere from 25 °C to 700 °C with the heating rate of 10 °C min-1. X-ray photoelectron spectra (XPS) were conducted by Thermo Scientific ESCA Lab 250Xi X-ray photoelectron spectrometer with an Al Kα X-ray radiation. Nitrogen absorption/desorption isotherms were detected at 77 K on a Micromeritics ASAP2460 instrument. The specific surface areas were computed by Brunauer-Emmett-Teller (BET) approach. As for the pore size distribution, density functional theory (DFT) was used from the absorption branch.

6


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Electrochemical measurements. The catalytic electrochemical tests were performed on the standard three-electrode setup using a CHI 660D electrochemical analyzer (Chenhua Instruments, Shanghai). The catalyst were loaded onto a glassy carbon electrode and served as a work electrode, while a platinum foil and a saturated silver chloride electrode acted as the counter and reference electrodes, respectively. To successfully modify the work electrode, 80 µL of 5 wt% Nafion solution and 4 mg of the catalyst were mixed in 920 µL of isopropyl alcohol by 30 min sonication. Then the homogeneous suspension was dispersed onto the conventional glassy carbon electrode with the mass loading about ~0.28 mg cm-2. For comparison, 20 wt % Pt/C was conducted to the electrochemical test (Figure S12). Linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and chronoamperometry measurements as well as chronopotentiometry measurements were then performed, and the LSV was measured at scan rate of 1.0 mV s-1 after cyclic voltammetry became stable. In this study, the onset potential was defined as the potential at a current density of 0.1 mA cm-2. The long-term stability was tested by chronoamperometry measurements and chronopotentiometry measurements for 16 h and 30 h, respectively. Electrochemical impedance spectroscopy (EIS) was carried out on a potential of 0.15 V at the frequency range of 100.0 kHz ~ 0.1 Hz. The electrochemical doublelayered capacitance (Cdl) was obtained by cyclic voltammetry (CV) sweeping in the range of 0 ~ 0.3 V with a series of scan rates (20 ~ 250 mV s-1). All of the electrochemical tests were conducted in 0.50 M H2SO4 with continuous purging of N2 (>99.99% purity). All the overpotential mentioned in this work were calibrated with a reversible hydrogen electrode (RHE) according to this equation ERHE = ESCE + 0.197 V.

RESULTS AND DISCUSSION Morphological and Structural Characterization. The main reasons selecting the guar gum as carbon precursor are as follows. First, 7



guar gum, served as a green and environmentally sustainable biomolecular material, can be naturally polymerized under suitable physical chemical conditions.38 Second, it involves a large number of hydroxyl groups, which endows the ability to chelate or adsorb all kinds of metal ions in the process of self-polymerization.38-39 In this case, MoS2/CF was prepared by in-situ hydrothermal method (see the experiment section). In the step one, guar gum was polymerized with ammonium molybdate tetrahydrate and thiourea in water. Then, derived MoS2/guar gum carbon nanoflowers (denoted as MoS2/CF) were subsequently carbonized at 750 °C for 2 h to form MoS2/CF-750 (Figure 1).

Figure 1. Schematic illustrations for synthesis of MoS2/CF-750. (a) Guar gum was polymerized together with ammonium molybdate tetrahydrate and thiourea using water as solvent. (b) The formation of MoS2/CF. (c) Final carbonization of MoS2/CF, the inserted guar gum were decomposed partially, leading to enlarged interlayer spacing as well as reduced layers of MoS2, all contributed to the formation of the MoS2/CF-750.

The crystalline structure and chemical constitution of products were detected by XRD. Figure 2a shows the XRD curves of MoS2/CF-750, MoS2/CF, MoS2 and MoS2-750. The characteristic diffraction peaks can be accordingly ascribed to (100), (102) and (110) facets about the hexagonal phase MoS2 (JCPDS card No. 75–1539). While the diffraction peak of MoS2/CF located in ~14.5° is manifest of the (002) facets with MoS2 interlayer spacing about ~6.2 Å,35, 40 and this peak also appears in 8


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MoS2 and MoS2-750. However, when the MoS2/CF was calcined at 750 °C to form the MoS2/CF-750, a couple of emerging characteristic peaks were clearly revealed at ~9.4° and ~18.6°, manifesting the interlayer spacing of ~9.5 Å and ~4.7 Å, respectively.35, 40 The double-enhanced d spacing demonstrates the forming of few layered structure with expanded interlayer spacing of ~9.5 Å, making the increasing of active sites more profoundly. It is noteworthy that the peak of pure MoS2 centered at ~57.5° is the (110) diffraction peak, while it is shifted to ~59.2° in MoS2-750 sample, suggesting reduced interlayer spacing when MoS2 is calcined at 750 °C subsequently. Moreover, the (103) peak weakly revealed at 2θ = 69.8° suggests that MoS2 has been anchored on the surface of guar gum in MoS2/CF-750,17, 36, 41 which is in conformity to the observation by TEM (Figure 2c-h). More insights into the chemical bond environment and microstructure of samples are securable through multiple spectroscopic characterizations. As shown, the Raman peaks at ~376 and ~405 cm-1 (Figure 2b1) are originated from E12g and A1g vibration modes of Mo-S, respectively.42 In addition, the intensity of A1g is stronger than that E12g, reflecting the MoS2/CF-750 has ample edge chemistry structures as well as the generating of few layers S-Mo-S.33, 43 Moreover, three additional peaks at ~283, ~336 and ~450 cm-1 for the MoS2/CF-750 are stemmed from E1g, 1T phase MoS2 and 2H phase MoS2. Hence, the above observation clearly manifest that 1T phase MoS2 was inserted into the 2H phase MoS2 substrate,44-45 which is in well consistent with TEM results (Figure 2h and Figure S1). More attractively, the characteristic peaks of D band (~1340 cm-1) and G band (~1585 cm-1) are observed in MoS2/CF and MoS2/CF-750 (Figure 2b2). As generally accepted, the D band is derived from the sp3-hybridized and defects, and G band is attributed to the sp2-hybridized in carbon materials.46 Furthermore, the intensity ratios of D band and G band (ID/IG) (insets are partial enlarged drawing, Figure 2b2) are calculated to be ~1.09 and ~0.89 of MoS2/CF-750 and MoS2/CF, respectively, demonstrating that the decent graphitic degree of guar gum in the MoS2/CF-750 is higher than that in MoS2/CF.47 9



Furthermore, Mo2C representative peaks located at ~987, ~660, and ~812 cm-1 are detected, which are strong evidences of the combination of MoS2 nanoflakes with guar gum. 48 However, D band, G band and Mo2C peaks are absent in MoS2 and MoS2-750 samples.

Figure 2. (a) XRD curves of MoS2, MoS2-750, MoS2/CF and MoS2/CF-750. (b) Raman patterns of MoS2, MoS2-750, MoS2/CF and MoS2/CF-750, the inset is the corresponding partially enlarged drawing. (c, f) TEM images of MoS2/CF and MoS2/CF-750, the inset is the SAED map. (d, e, g, h) HRTEM for MoS2/CF and MoS2/CF-750. (i) Structural patterns of MoS2 with the interlayer spacing of 6.2 and 9.5 Å, respectively.

The successful formation of the hybrid nanoflowers (MoS2/CF and MoS2/CF-750 samples) possessing different interlayer spacings and layers are confirmed through comprehensive FESEM and TEM analysis. The SAED pattern (inset of Figure 2c and Figure 2f) shows rings of diffraction spots assignable to a typical crystalline MoS2 in harmony with the XRD results. Figure 2e demonstrates the lattice space (d) of 0.23 nm and 0.27 nm representing the (101) and (100) plane of MoS2, respectively.35 Furthermore, Figure 2d shows multi-layered MoS2 nanoflakes, and the interlayer 10


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spacing is ~6.2 Å of MoS2/CF. However, MoS2/CF-750 (Figure 2g) manifests few-layered (~3 layers) MoS2 nanoflakes possessing the enhanced interlayer spacing about ~9.5 Å and abundant defects (Figure 2i and Figure S1), which is attributed to the partially decomposition of guar gum at high temperature resulting in enlarged interlayer spacing of MoS2. As a comparison, we found that the pure MoS2 without guar gum possesses more layers and smaller interlayer spacing, which is more remarkable after calcining at 750 °C (Figure S2), stemmed from the high temperature (>700 °C) causing the overly assembling, stacking and collapsing in the interlayers. Therefore, it is reasonable to believe that conventional interlayer spacing (~6.2 Å) exposes very little active site while the expanded interlayer spacing (~9.5 Å) is helpful to increase the amount of active sites significantly and permeable channels for ion adsorption and transport tremendiously.48-49 In addition, two distinct lattice matrices are identified by the HRTEM images (Figure 2e and 2h), indicating the MoS2/CF bears a mixed phase of 1T phase and 2H phase, which is consistent with a common honeycomb lattice.50 While the MoS2/CF-750 dominantly reveals the 1T phase (Figure S1), implying the district confinement of guar gum. The SEM images reveal that MoS2/CF-750 is consist of nanoflowers with diameter of 500 nm approximately, and these nanoflowers are constituted by plenty of papery nanoflakes with thickness about ~5 nm, whole core of radiation as well as loosely stacked together (Figure 3a and 3b).51 Furthermore, it can be seen obviously MoS2/CF-750 sample has fewer stacked nanoflakes than MoS2/CF (Figure S3). EDS analysis under high angle annular dark field (HAADF) indicates the three major component elements assume an uniform distribution. (Figure 3c and Figure S5).

11



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Figure 3. (a, b) SEM graphs of MoS2/CF-750. (c) HAADF images and Elements mapping of MoS2/CF-750, reflecting the uniform distributing of C, Mo, and S elements.

X-ray photoelectron spectroscopy (XPS) is regulated to analyze the surface chemistry of MoS2/CF-750 and MoS2/CF nanoflowers (Figure 4). The survey spectra confirm the presence of C, N, O, S and Mo elements (Figure 4a1). Figure 4b1 shows the C1s XPS spectrum, one high intensity peak for C=C/C-C (~284.6 eV) and two comparatively weak peaks for C-O (~285.9 eV) and C-N (~285.1 eV) are observed, which demonstrate the heteroatom (N or O atom) doping from guar gum and thiourea. Mo3d spectrum (Figure 4c1) exists two strong peaks of Mo3d3/2 (~232.9 eV) and Mo3d5/2 (~229.7 eV) as well as a extremely weak peak for S2s (~226.8 eV), suggesting the Mo (IV) of MoS2 oxidation state structure

52, 53

. In addition, the

relatively weak peaks located at ~236.3 eV represent for Mo3d3/2 of Mo (VI) oxidation state. Usually, the peaks of Mo (IV) 3d5/2 and 3d3/2 are assigned to the 1T phase and 2H phase, respectively; and according to the peak area, the atomic percent (at.%) of Mo (IV) 3d5/2 (~44.33) is more than 3d3/2 (~32.14), suggesting the dominance of 1T phase. Whereas, for the MoS2/CF sample the main phase is 2H phase (Figure 4c2), which is in line with the observation by HRTEM (Figure 2e and 2h). The S2p of MoS2/CF-750 (Figure 4d1) reveals two intense peaks at ~163.7 and 12


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~162.6 eV, attributing to S2p1/2 and S2p3/2 binding energies, respectively; while the peak of C-SO2 at ~168.4 eV that revealed in MoS2/CF sample (Figure 4d2) is not detected, implying the decomposition of C-SO2 in high temperature, which also helps expanding the interlayers spacing of MoS2.35,

49

As shown in Figure 4e1, N1s

spectrum suggests the existence forms of N atom, the pyridinic N (pyri-N, ~395.2 eV), graphic N (grap-N, ~399.1 eV), Mo-N coupling phase (~394.6 eV) and pyrrolic N (pyrr-N, ~396.0 eV)54. The pyri-N (~30.21 at.%) can boost the electrical conductivity as well as catalyst surface wettability, in that the presence of lone pair electrons in the carbon materials plane framework can contribute electrons to adsorb hydrogen atom, and the grap-N (~8.74 at.%) is prospected to enhance the diffusion-limited current density.48 Whereas, pyrr-N is widely accepted to have little effect on catalytic activity.48 Figure 4f1 shows the spectrum of O1s, the peaks located at ~532.4, ~530.9, ~533.6, and ~531.4 eV dispatching to Mo-O, O=C, C-OH , and C-O/O-C-N bonds, respectively. In addition, on the basis of XPS results, the atomic percent (at.%) of C and O are ~44.31, ~18.26 and ~27.13, ~7.57 of MoS2/CF and MoS2/CF-750, respectively, and the decrease of C and O is due to the decomposition of guar gum upon calcining at 750 °C (Table S4), which is verified via following TGA analysis (Figure S7). Meanwhile, it is noteworthy that Mo-C, Mo-N, Mo-S and Mo-O have synergistic effects on the improvement of electrocatalysts activity (Figure S6)35.

13



Figure 4. (a1, a2) XPS survey spectra, high resolution XPS spectrum of (b1, b2) C1s, (c1, c2) Mo3d, (d1, d2) S2p, (e1, e2) N1s, and (f1, f2) O1s, and number 1 and 2 represents MoS2/CF-750 and MoS2/CF, respectively.

To further verify the role of guar gum, TGA test is performed via a temperature procedure as that executed in the tube furnace. In the whole temperature range no apparent weight-loss behavior appear in MoS2-750 and MoS2/CF-750 samples (Figure S7), suggesting excellent thermal stability; and comparatively, MoS2/CF-750 is superior relative to MoS2-750, which manifests the strong interactions between guar gum and MoS2 in the sample of MoS2/CF-750.41 Conversely, MoS2 shows a noticeable weight-loss as well as two endothermic peaks in temperature zone of 100-350 °C ascribing to the partial decomposition of adsorbed water and excessive thiourea (free thiourea decomposed ranging 175-245 °C).41, 51 While, the weight-loss peak of MoS2/CF moves to a low temperature range (100-300 °C) which is consistent with the guar gum weight-loss behavior, suggesting the inserted guar gum is partially decomposed. Therefore, it is reasonable to believe the inserted guar gum would providing a contribution on the formation of few layers MoS2 nanoflakes. Besides, specific surface area (SSA) and pore structure are important parameters for 14


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HER

electrocatalytic

performance.

As

characterized

by

Nitrogen

adsorption-desorption isotherms, the SSA are decreased in the order, MoS2/CF-750 (~27.35 m2 g-1) > MoS2/CF (~21.25 m2 g-1) > MoS2 (~17.28 m2 g-1) > MoS2-750 (~14.01 m2 g-1) (Figure S8). Obviously, the MoS2/CF-750 assumes the highest SSA value, which is ascribed to the spherical structure of guar gum particles being beneficial to donate abundant active sites for the MoS2 nanoflakes growth. Additionally, guar gum is conducive to prevent the agglomeration of MoS2 nanoflakes, and this is particularly true upon 750 °C calcining, in that the guar gum would decompose partially at high temperature for MoS2/CF-750, which will generate more mesopores as observed from pore size distribution. Based on above observation and analysis, the feasible formation mechanism of nanoflowers structure with enlarged interlayer spacing and few layers is as follows (Figure 1). Owing to the interactions between MoO42- and guar gum, the adsorption of MoO42- on guar gum changes the guar gum and MoS2 growth behavior.37, 39, 51 When MoO42- is absent, guar gum grows isotropically and tend towards forming a spherical shape (Figure S4). When adding MoO42-, the strong interactions between MoO42- and guar gum restrain the guar gum and MoS2 growth dimensions and accordingly, MoS2-guar gum develops into nanoflakes and ultimately reforms into petals of the nanoflowers. In other words, guar gum is inserted into the interlayer of MoS2 in in-situ hydrothermal process, and then decomposed partly during the subsequent 750 °C treatment, thus, enlarging interlayer spacing as well as reducing layers of MoS2. Electrochemical Performance and Stability toward HER. The advance of constructed MoS2/CF hybrids is demonstrated in the electrocatalytic properties. As depicts in the HER polarization curves, the MoS2/CF-750 shows the best HER activity with the onset potential of only 20 mV versus RHE, and HER activity varies with the different guar gum content and calcination temperature (Figure 5a, Figure S11, Figure S12, Figure S14, Table S2 15



Page 16 of 28

and Table S3), while pure MoS2 performs relatively inferior HER activity because of the weak conductivity and finitely exposed active sites of MoS2 stacking nanoflakes. Besides, the overpotential at 10 mA cm-2 (η10) is still a direct index to compare the HER activities with varieties of electrocatalysts. MoS2/CF-750 electrocatalyst only demands the overpotential of 125 mV versus RHE to reach 10 mA cm-2, while η10 of MoS2/CF, MoS2 and MoS2-750 are 230, 255, and 465 mV, respectively (Table 1). Moreover, the electrocatalytic activity of MoS2/CF-750 is superior to many researched MoS2-based electrocatalysts, notably for onset potential and η10 (Table S1). In addition, the Faradic efficiencies of MoS2/CF-750 were calculated55,56 (see calculation details in the Supporting Information, equation S3). As seen from Figure S18, all theoretical values are highly consistent to the detected values, and all derived Faraday's

efficiencies

(96~99%)

are

quite

close

100%,

suggesting

the

outstanding HER activity. The excellent HER activity of MoS2/CF-750 can be attributed to its 3D heterostructure at two unique levels orderly. 1) at nanoscale, the few layers and large interlayer spacing afford a rich density dispersing of HER active sites on the catalyst surface; partial guar gum carbon substrate afford significant electrical conductivity required for electrocatalysis. 2) at mesoscale, whole MoS2 nanoflakes are radiate from the center, and the separation among nanoflakes provide a favorable channel for the infiltration of electrolyte. Besides, the individual nanoflower is mechanical robustly to hold its configuration integrity. Collectively, structural order at these two levels guarantee the most fraction of MoS2/CF-750 is electrochemically accessible.

16


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Figure 5. (a) LSV curves of MoS2, MoS2-750, MoS2/CF and MoS2/CF-750 as HER electrocatalysts. The inset shows the in-situ experimental observation in LSV test, where a lot of bubbles are observed. (b) Matching Tafel plots for MoS2, MoS2-750, MoS2/CF and MoS2 /CF-750. (c) CV curves of MoS2/CF-750 at varying scan rates (20-250 mV s-1). The illustration is the plot of the corresponding capacitive current at 0.35 V against the scan rate. (d) Polarization curves of samples at the original cycle and after 2000 cycles of CV scanning (0~0.3 V, 0.50 M H2SO4). (e) Chronoamperometry (j-t) tests of the long-term stability for MoS2/CF and MoS2/CF-750. (f) Nyquist curves of samples (100.0 kHz ~ 0.1Hz, potential is 0.15V).

For HER process, Tafel slopes are currently used to evaluate the type of mechanism, Volmer-Tafel or Volmer-Heyrovsky.51 Supposing only one rate-determining step, the Tafel slope of ~30, ~40 and ~120 mV dec-1 can be determined for Tafel, Volmer and Heyrovsky step, respectively. To explore the underlying mechanism of MoS2/CF, Tafel slopes deduced from the LSV curves are obtained (Figure 5b). The linear segments of the plots were fitted to the Tafel equation (η = b log(j) + a, where b is the Tafel slope and j is the current density). MoS2/CF-750 presents smallest Tafel slope approximate 34 mV dec-1, which is closing the value of merchant Pt/C electrocatalysts with around 30 mV dec-1.11 This indicates the potential of Volmer-Heyrovsky type, which signifies the rate-limiting step is hydrogen desorption. Based on the above analysis, the especially small Tafel slope of MoS2/CF-750 electrocatalyst in accordance with the adequately reformed reaction kinetics can reduce the powerful 17



Page 18 of 28

chemical or electronic coupling on the surface of catalyst, suggesting that the weakened H absorption energy upon this hetero-nanostructure is easier to remove the H2.50 In this case, the coupling interactions involved afford a more dispersive growth of MoS2 nanoflakes on guar gum surface as well as few layers and large interlayer spacing of MoS2/CF-750 in reverse endow abundant efficient active sites are major reasons for low Tafel slope of MoS2/CF-750. In addition, exchange current density (j0) is a further important evidence to assess the electrocatalytic performance, which is generally derived by extrapolation of the Tafel plots to an potential of 0 V (Table 1). It can be seen that MoS2/CF-750 presents the highest j0 (ca. 0.25 mA cm-2) compared to MoS2/CF, MoS2 and MoS2-750, indicating the fastest reaction rate per surface area.35 Furthermore, the electrochemical capacitance surface area (ECSA) is used to value the intrinsic active amount of the electrocatalyst (Figure 5c). Specifically, the double-layer capacitance (Cdl) of the MoS2/CF-750 is calculated by CV curves at a series of scan rates. The Cdl value can be obtained by plotting the ∆J at 350 mV versus RHE against the scan rate, and the slope corresponding the Cdl.51 Currently, MoS2/CF-750 presents the Cdl of ~14.23 mF cm-2 as well as the derived ECSA of ~42.35 m2 g-1 (see the calculation details in equation S1, equation S2), both higher than that of MoS2/CF (~5.69 mF cm-2, ~16.92 m2 g-1) and other samples (Figure S9 and Figure S13).

Table 1 Electrochemical parameters of samples onset potential (mV)

η10 (mV)

Tafel slope (mV dec-1)

J0 (mA cm-2)

MoS2/CF-750

20

125

34

0.25

MoS2/CF

150

230

53

0.04

MoS2

160

255

80

0.03

MoS2-750

202

465

170

0.02

18


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Meanwhile, the catalytic stability was performed via CV sweeps varying from 0 to 0.3 V at the scan rate of 50 mV s-1. After 2000 CV sweeps, a negligible degeneration of cathodic current is detected in MoS2/CF-750 (Figure 5d). Furthermore, the structure stability is determined from the SEM images, which shows no visible structure distortion after 2000 CV sweeps (Figure S10). Meanwhile, the chronoamperometry (Figure 5e, Figure S15, Figure S16) and chronopotentiometry (Figure S17) curves concretely demonstrates the long-term stability of MoS2/CF-750. Even after a long time of 30 h, there is no significant current degradation, suggesting that the MoS2/CF-750 is superiorly stable under acidic HER conditions57, 58. In addition, the charge transmission from the rich active sites to the electrodes was comparatively conducted by electrochemical impedance spectroscopy, the potential is fixed on 0.15 V to research the electrochemistry kinetics in HER system. The Nyquist curves of the catalyst modified electrodes presents a straight line in the low frequency range（Figure 5f）, and the higher slope signifies the faster charge transfer at low frequency range.36 It is distinct that MoS2/CF-750 has the highest slope, which is conducive to more charge transport. In addition, the MoS2/CF-750 possesses smaller charge transfer resistance (Rct) ~16.7 Ω than that of pure MoS2 (~55.2 Ω) and MoS2/CF (~30.5 Ω) in high frequency region. Thus, the exceedingly decreased Rct affords observably faster HER efficiency for the MoS2/CF-750 hybrid electrocatalyst.

CONCLUSIONS In conclusion, we have synthesized MoS2/CF nanoflowers using a facile hydrothermal method by introducing guar gum serve as both reaction template and carbon precursor, and the subsequent high temperature treatment. The strong interaction between MoO42- and guar gum would compel the guar gum being inserted into the interlayer of MoS2, and consequently, this effect will be helpful to prevent the inherent stacking among MoS2 layers, and lead to enlarged interlayer spacing as well as reduced layers of MoS2 nanoflakes. Impressively, the resultant MoS2/CF nanoflowers structure is conducive to expose more active sites and boost electrical 19



conductivity, and then dramatically enhancing HER activity. Hence, the obtained MoS2/CF-750 exhibits impressive HER electrocatalysis performance in acidic solution, with extremely low onset potential of 20 mV versus RHE, small Tafel slope of 34 mV dec-1 as well as long-term stability. This approach create a simple and flexible pathway to synthesize 3D non-noble metal nanocomposites using environment friendly guar gum and is beneficial to the preparation of other inexpensive and high active electrocatalysis on a large scale.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: The calculation equation of electrochemistry; TEM images; SEM images; Element mapping images; Structural models of nitrogen atom; TGA/DSC curves; Nitrogen adsorption-desorption isotherms; CV curves; LSV curves; Chronoamperometry measurements; Chronopotentiometry measurements; The amount of H2 theoretically calculated and experimentally detected; Electrochemical parameters; Atoms content. (PDF)

ACKNOWLEDGEMENTS This work is financially supported by the National Science Foundation of China (21072221, 21172252).

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A novel three-dimensional (3D) nanoflowers consisting of few-layered and enlarged interlayer spacing MoS2 nanoflakes are designed and fabricated, which exhibits extraordinary HER electrocatalytic performance, especially for onset potential and Tafel slope.


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Guar Gum ... - ACS Publications

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