Ionic Liquid-Assisted Synthesis of Nanoscale (MoS2) - ACS Publications

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Ionic Liquid-assisted Synthesis of Nanoscale (MoS2)x(SnO2)1–x on Reduced Graphene Oxide for the Electrocatalytic Hydrogen Evolution Reaction Sudhir Ravula, Chi Zhang, Jeremy B Essner, John David Robertson, Jian Lin, and Gary A. Baker ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13578 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 10, 2017

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ACS Applied Materials & Interfaces

Ionic Liquid-Assisted Synthesis of Nanoscale (MoS2)x(SnO2)1– x on Reduced Graphene Oxide for the Electrocatalytic Hydrogen Evolution Reaction Sudhir Ravula,a Chi Zhang,b Jeremy B. Essner,a J. David Robertson,a,c Jian Lin,b and Gary A. Bakera* a. Department of Chemistry, University of Missouri, Columbia, MO, 65211, USA. b. Department of Mechanical & Aerospace Engineering, University of Missouri, Columbia, MO, 65211, USA. c. University of Missouri Research Reactor, University of Missouri, Columbia, MO, 65211, USA.

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ABSTRACT Layered transition metal dichalcogenides (TMDs) have attracted increased attention due to their enhanced hydrogen evolution reaction (HER) performance. More specifically, ternary TMD nanohybrids, such as MoS2(1–x)Se2x or bimetallic sulfides, have arisen as promising electrocatalysts compared to MoS2 and MoSe2 due to their electronic, morphologic, and size tunabilities. Herein, we report the successful synthesis of fewlayered MoS2/rGO, SnS2/rGO, and (MoS2)x(SnO2)1–x/rGO nanohybrids anchored on reduced graphene oxide (rGO) through a facile hydrothermal reaction in the presence of ionic liquids as stabilizing, de-layering agents. Spectroscopic and microscopic techniques (electron microscopy, X-ray diffraction, Raman spectroscopy, neutron activation analysis, UV-Vis spectrophotometry) are used to validate the hierarchical properties, phase identity, and the smooth compositional tunability of the (MoS2)x(SnO2)1–x/rGO nanohybrids. Linear sweep voltammetry measurements reveal that incorporation of Sn into the ternary nanohybrids (as a discrete SnO2 phase) greatly reduces the overpotential by 90–130 mV relative to the MoS2 electrocatalyst. Significantly, the (MoS2)0.6(SnO2)0.4/rGO nanohybrid displays superior catalytic performance over MoS2 alone, exhibiting a low overpotential (η10) of 263 ± 5 mV and a small Tafel slope of 50.8 mV dec–1. The hybrid catalyst shows high stability for the HER in acidic solutions, with negligible activity loss after 1000 cycles. The hierarchical structures and large surface areas possessing exposed, active edge sites make few-layered (MoS2)x(SnO2)1–x/rGO nanohybrids promising non-precious metal electrocatalysts for the HER.

KEYWORDS Ionic liquids, transition metal dichalcogenides, reduced graphene oxide, hydrogen evolution reaction, electrocatalysis

CORRESPONDING AUTHOR *Department of Chemistry, University [email protected], Tel 573-882-1811

of

Missouri,

2

Columbia,


MO,

65211,

USA.

Email:

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1. INTRODUCTION Currently, hydrogen is being enthusiastically investigated as a principal energy carrier in the paradigm shift from the existing petroleum-based economy.1-2 Typically, hydrogen (H2) is produced through alkane reforming which relies heavily on non-renewable petroleum feedstocks and releases harmful pollutants (e.g., CO, CO2) in the reformation process.1-4 The sustainable production of H2 gas through the electrochemical splitting of water has attracted growing attention since it offers a cleaner alternative, yielding only H2 and O2 as products from an essentially limitless source.2-5 Unfortunately, the most effective electrocatalysts for the hydrogen half-reaction, referred to as the hydrogen evolution reaction (HER, 2H+ + 2e– → H2), rely on expensive and scarce noble metals, particularly platinum (Pt).2-5 Hence, the development of low-cost and efficient HER catalysts from earth-abundant sources is a priority in clean energy initiatives. Recently, two-dimensional (2-D) layered transition metal dichalcogenides (TMDs, examples of which include MX2 for M = Mo, W; X = S, Se) were reported to exhibit strong activity in HER electrocatalysis.2-10 More specifically, MoS2 has been demonstrated as a promising alternative to Pt in the HER since its activity is only 57 times lower than Pt while being 1000-fold cheaper.2,

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However, computational and

experimental evidence indicates that the active sites of MoS2 are limited to the edges of the MoS2 plates which, unfortunately, constitute only a small proportion of the total surface area.2-5 One promising approach to enhancing the overall electrocatalytic activity of MoS2 in the HER involves maximizing the number of active edges by reducing the MoS2 sheets to nanoscale dimensions.2,

5

However, generating nanoscale MoS2 which possesses

abundant active edges remains challenging due to thermodynamic issues, such as

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unfavorable energetics and structural instability.11 As one potential path forward, ionic liquids (ILs) have recently been demonstrated to stabilize the active surfaces of nanocrystalline catalysts, although improvement in the intrinsic activity of heterogeneous catalysts has seen limited study.12-13 Due to their designer nature, task-specific ILs can, in principle, be deliberately made to thermodynamically passivate unstable nanocrystalline surfaces, therefore playing a dual-role of solvent and stabilizer. Notably, Maschmeyer and co-workers demonstrated the synthesis of MoS2 catalysts using a series of ILs which yielded a de-layered morphology with improved crystallinity and a greater exposure of the active sites, resulting in enhanced HER electrocatalytic activity.7, 12 Unfortunately, the resistance to electron transfer along the lateral surface of the lamellar crystals and the strong intersheet van der Waals interactions results in poor conductivity and aggregation phenomena, respectively, both of which hinder the catalytic performance of MoS2 nanosheets. These issues can potentially be mitigated by forming heterojunctions between the metals and conductive carbon-based materials, such as graphene oxide (GO),8, 14-18 carbon nanofibers (CNFs),19-20 carbon nanotube (CNT),21-23 or mesoporous carbon. For example, Li et al. solvothermally synthesized a highly active HER composite of MoS2 nanoparticles on rGO which possessed a low overpotential (~0.1 V) and a small Tafel slope (an inherent property determined by the rate-limiting step of the HER and a key figure of merit for characterizing HER electrocatalysts) of 41 mV dec–1 due to the highly exposed edges sites and improved conductivity.8 Another approach to improving the intrinsic activity of the MoS2 active edges is to incorporate catalytic promoters such as Ni, Co, and Fe6,

24-27

to arrive at a hybrid

nanoscale catalyst that possesses a lower overpotential and a smaller Tafel slope. Along these lines, Lin and Ni recently reported that a second metal within the MoS2 lattice could

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induce alterations in the energy states due to the change in the occupation and energy of the anti-bonding defect levels in the band gap.26 Similarly, a variety of strategies have been employed to tune the d-band gap of the molybdenum and to optimize the hydrogen adsorption free energy, which can also lead to improvements in the intrinsic activity of the catalysts. For instance, Hu and co-workers incorporated Fe, Co, and Ni ions into MoS3 films, resulting in higher HER activity under both acidic and neutral conditions.24 It was proposed that the introduction of these metals greatly promoted the growth of the amorphous MoS3 films, resulting in high surface area composites with high catalytic loading. Similarly, Faber et al. prepared thin films of earth-abundant metal pyrites on graphite or glass substrates through thermal sulfidation of the metal precursors, employing the thin films for HER and polysulfide reactions.27 In addition, several other ternary catalysts such as MoS2(1–x)Se2x,28-30 MoxW(1–x)S2,30-32 and WS2(1–x)Se2x,33-34 have been reported to outperform their binary counterparts in the HER. While all of these nanohybrids were efficient catalysts for H2 generation, they generally required complex and timely synthetic procedures, highlighting the need for simpler and more expedient methods toward the heteroatom doping of TMD films. One potential elemental dopant that is both cheap and abundant, but has seen little study for the HER, is tin (Sn). Of particular interest, SnS nanostructures coupled with N-doped graphene were recently reported by Lee and co-workers to function as HER catalysts in the electrochemical splitting of water.35 Moreover, tin sulfide derivatives have moved into the limelight because of their diverse structural morphologies, many of which exhibit high electrical conductivity in lithium batteries, supercapacitors, solar cells, and catalysis.36-39 Therefore, the incorporation of Sn into MoS2 nanostructures to achieve HER electrocatalysts appears ripe for exploration.

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Herein, we report a facile hydrothermal approach that employed a range of ILs as dual exfoliating and stabilizing agents to synthesize stable, few-layered (MoS2)x(SnO2)1–x nanoparticles anchored onto reduced GO (rGO) as a conductive support. This represents the first example of an MoS2/SnO2 nanohybrid explored for the HER. The unique structures of the (MoS2)x(SnO2)1–x/rGO nanocomposites afforded catalysts with excellent HER activity. Significantly, the (MoS2)x(SnO2)1–x nanohybrids showed a marked improvement in the overpotential and Tafel slope over their MoS2/rGO counterpart (the x=0.6 hybrid gave the highest performance), consistent with the notion that the electronic interaction of multiple metal species can synergistically enhance the intrinsic activity of the electrocatalyst. Notably, the Sn is incorporated into the nanohybrid as a discrete oxide nanophase (as opposed to doping of the MoS2 lattice), suggesting that nanoscale heterojunctions may play an important role in the significantly enhanced HER performance achieved. This finding opens up new pathways and strategies for other TMD-derived HER electrocatalysts designed to incorporate nanoscale heterojunctions. 2. EXPERIMENTAL SECTION 2.1. Materials and reagents. All experiments were carried out using Ultrapure Millipore water (18.2 MΩ·cm). Sodium molybdate dihydrate powder (331058, ≥99%), tin(II) chloride (208256, 98%), thiourea (T7875, ≥99%), potassium permanganate (399124, ≥99%), sulfuric acid (339741, 99.99%), hydrogen peroxide (216763, 30 wt% in water), sodium nitrate (71752, ≥99.999% metals basis), tris(2-hydroxyethyl)methylammonium methylsulfate, [(HOEt)3MeN][MeSO4] (91198), and 1-butyl-3-methylimidazolium 2-(2methoxyethoxy)ethyl sulfate, [Bmim][2-(2-methoxyethoxy)EtSO4] (67421, ≥95.0%) were purchased from Sigma-Aldrich (St. Louis, MO). 1-Butyl-3-methylimidazolium tetrafluoroborate, [Bmim][BF4] (IL-0012-HP-100, 99%) and graphite powder (SP-1, 200

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mesh, ≤74 µm, Lot no. 011705) were acquired from IoLiTec Ionic Liquids Technologies Inc. (Tuscsaloosa, AL) and Bay Carbon, Inc. (Bay City, MI), respectively. A low-power bath sonicator (Branson 3510) was employed for the exfoliation. 1-Octyl-3methylimidazolium tetrafluoroborate [Omim][BF4], 1-butyl-3-methylimidazolium butyl sulfonate

[Bmim][BuSO3],

and

N-butyl-N-methylpyrrolidinum

trifluoromethane

sulfonate [C4mPy][TfO] were synthesized in-house.40 2.2. Characterization techniques. Transmission electron microscopy (TEM) studies were conducted on carbon-coated copper grids (Ted Pella, Inc. 01822-F, support films, ultrathin carbon type-A, 400 mesh copper grid) or Pacific Grid-Tech 400 mesh copper grids coated with Quantifoil film (EDX measurements), using an FEI Tecnai (F30 G2, Twin) microscope operated at a 200 keV accelerating electron voltage. Raman spectra were collected on a Renishaw RM1000 Raman spectrometer that employed a 785 nm laser. Absorbance spectra of the exfoliated (MoS2)x(SnO2)1–x/rGO aqueous ethanolic solutions (diluted 9-fold) were measured in 1-cm pathlength disposable PMMA cuvettes using a UV-Vis spectrophotometer (Hitachi U-3000). X-ray diffraction patterns of (MoS2)x(SnO2)1–x/rGO nanohybrid samples were recorded at 25 °C using a Bruker Prospector CCD system using CuKα radiation (λ = 1.5418 Å) from an Incoatec IMuS microfocus tube and were measured with the spectral range of 2θ from 10° to 80°. The mole fractions of Sn and Mo within the nanohybrids were determined by instrumental neutron activation analysis (NAA) performed at the University of Missouri Research Reactor (MURR). To perform NAA, a 1–3 mg amount of each sample was carefully massed and sealed within a 0.5 mL high-density polyethylene vial followed by irradiation for 15 s in the pneumatic tube facility at MURR. The thermal flux in the irradiation position was approximately 6 × 1013 n cm–2 s–1. The samples were allowed to

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decay for 3 min and then the isotope decays of interest were monitored in real-time for 1 min at a sample-to-detector distance of 6.0 cm. The spectrometer consisted of a 25% high-purity germanium detector with a full-width-half-maximum (FWHM) resolution of 1.9 keV at 1331 keV, a loss-free counting module (Canberra 599), and a digital signal processor (Canberra 9660). Dead times were below 1% for every analysis. The mass of Sn was quantified by measuring the 332 keV gamma-ray from the β– decay of 125mSn (t1/2 = 9.5 min) and the mass of Mo was quantified by measuring the 590 keV gamma-ray from the β– decay of

101

Mo (t1/2 = 14.6 min). The areas of the gamma-ray peaks were

determined automatically with the Genie ESP spectroscopy package. Comparator standards were prepared gravimetrically from 1000 ppm certified standard solutions of Mo and Sn (High-Purity Standards, Charleston, SC) and were subjected to the same treatment as the samples to generate calibration curves. The average response function for the three Mo standards was 1907 ± 52 counts per microgram and the average response function for the three Sn standards was 12381 ± 88 counts per microgram. 2.3. Synthesis of graphene oxide (GO). GO was prepared according to the modified Hummer’s method.41 In a typical synthesis, 1.0 g of graphite powder and 0.5 g of NaNO3 were suspended in concentrated H2SO4 in a 250 mL round bottom flask and the suspension cooled to 0 °C in an ice bath. Next, 3.0 g of KMnO4 was added portion-wise to the mixture under magnetic stirring (250 rpm), maintaining a temperature below 20 °C during the addition. After the KMnO4 addition, the ice bath was removed and the solution was allowed to stir at 35 °C for an additional 1 h. The mixture gradually became a thick, brownish-grey slurry. The oxidation reaction was quenched by the addition of cold water (46 mL) under magnetic stirring [Caution: addition of cold water to the mixture causes violent effervescence and a drastic rise in temperature to >90 °C]. The resultant brown

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suspension was allowed to stir for another 20 min at room temperature, followed by a 180 mL addition of cold water. Finally, 10 mL of 30 wt% H2O2 was added drop-wise to reduce any unreacted permanganate and the solution was stirred for a final 30 min. During this entire process, the color of the solution changed from brown to yellow. The yellow solution was centrifuged at 8,000 rpm for 10 min and the resulting sediment washed several times with water until the pH became neutral. The obtained GO was exfoliated in water by bath sonication for 2 h. The dispersed solution was centrifuged at 3,000 rpm for 30 min to remove any un-exfoliated GO and the top 80% of the supernatant was collected. The supernatant was lyophilized for 72 h to obtain GO as a fluffy beige powder which was stored under 4 °C refrigeration until needed. 2.4. Synthesis of MoS2/rGO, SnS2/rGO, and (MoS2)x(SnO2)1–x/rGO. The MoS2/rGO, SnS2/rGO and (MoS2)x(SnO2)1–x/r GO nanohybrids were prepared by a facile hydrothermal method. In a typical synthesis, 20 mg of the previously synthesized GO (Section 2.3) was dispersed in 10 mL of water and sonicated for 30 min to form a homogeneous suspension. Next, 500 mg (2.07 mmol) of Na2MoO4·2H2O or 391 mg (2.1 mmol) of SnCl2 was dissolved in the exfoliated GO suspension and stirred for 30 min. Then, 200 mg of [Bmim][BF4] and 315 mg (4.14 mmol) of thiourea were separately added to the above suspension and stirred for 10 min after each addition. The mixture (total water volume ~15 mL) was transferred to a Teflon-lined stainless steel autoclave and hydrothermally treated at 240 °C for 20 h in a programmable oven. After cooling, the black precipitate was collected via centrifugation at 8,000 rpm for 10 min and sequentially washed with water and then ethanol. The washing steps were repeated a minimum of six times to ensure complete removal of excess ionic liquid. The final products were collected by one last centrifugation step and were then dried at 80 °C

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overnight to obtain the MoS2/rGO and SnS2/rGO nanocomposites. The (MoS2)x(SnO2)1– x/rGO

nanohybrids (where, x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9) were

synthesized in a similar fashion except the molybdenum and tin precursors were mixed in the desired stoichiometric proportions relative to thiourea. The IL and GO amounts remained unchanged. 2.5. Electrochemical Measurements. Linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy were conducted in a three-electrode cell configuration using a CHI 708E electrochemical station (CH Instruments, USA) using 0.5 M H2SO4. A silver/silver chloride (Ag/AgCl) and a graphite rod were used as the reference electrode and counter electrode, respectively. 4 mg of the nanohybrid catalyst and 30 µL of 5 wt% Nafion solution were dispersed in 1 mL of a water:ethanol mixture (0.75:0.25 v/v) via sonication until a homogeneous ink was formed. An aliquot of the catalyst ink (5 µL) was then loaded onto a 3-mm diameter glassy carbon electrode working electrode. Before measurement, the electrolyte was bubbled with H2 for about 15 min and H2 purging was continued during the measurement. The reference electrode was calibrated with respect to a reversible hydrogen electrode (RHE) using Pt wires as both the working and counter electrodes at a scan rate of 0.1 mV s–1. The average of the two potentials of each CV curve where the current crossed zero was viewed as the thermodynamic potential. All the electrochemical measurements were iR-corrected. 2.6. Exfoliation of (MoS2)x(SnO2)1–x/rGO nanohybrids. Briefly, 25 mg of the bulk (MoS2)x(SnO2)1–x/rGO crystals were dispersed in 10 mL of a water-ethanol mixture (2:3, v/v) and bath sonicated for 6 h.42 The dispersed solutions were centrifuged at 4000 rpm for 20 min to remove any un-exfoliated material, resulting in stable (MoS2)x(SnO2)1–

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x/rGO

nanohybrid colloidal dispersions. The supernatant solution was diluted 9-fold using

water-ethanol (2:3, v/v) before conducting UV-Vis absorption measurements.

3. RESULTS AND DISCUSSION (MoS2)x(SnO2)1–x/rGO nanohybrids (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0) were synthesized in the presence of various ILs using a facile hydrothermal synthetic approach. The specific details can be found in the Experimental Section (Sections 2.3 and 2.4). Briefly, GO) was synthesized based on a modified Hummer’s method41 and was dispersed in water via bath sonication for 30 min to obtain a uniform solution. The successful formation of GO was confirmed with X-ray diffraction (Figure S1). Next, the metal precursors (Na2MoO4 and/or SnCl2) and thiourea were added to the GO suspension and mixed by mechanical stirring at room temperature. The surface functional groups (e.g., hydroxyl, carboxyl, and epoxy groups) of the hydrophilic GO play an important role in establishing robust electrical contact (heterointerface) between the nanoparticles generated and the reduced graphene oxide (rGO) substrate since these functionalities act as both nucleation sites8 and potential oxidizing agents43 for nanoparticle formation. Compounds such as L-cysteine and thiourea have been used as reducing agents during the hydrothermal synthesis of MoS2/rGO nanocomposites, because they release H2S in situ through hydrolysis,14, 16 resulting in simultaneous reduction of GO to rGO and reduction of GO-adsorbed MoO42– to yield nanoscale MoS2 anchored onto an rGO platform.15 For ternary nanohybrid synthesis, when the SnCl2 precursor was mixed with a solution containing MoO42– and GO, a dark green precipitate was observed, possibly metathesis with formation of Sn[MoO4], although reduction of Mo(VI) by Sn(II) is also likely. Thus,

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prior to hydrothermal treatment, the reaction mixture likely contains Sn[MoO4] as well as Mo(IV) and Sn(IV) species. Based on evidence we present later, we propose that, because Sn is more oxophilic, Mo competes for the sulfur (sulfide) from thiourea decomposition to give rise to discrete MoS2 and SnO2 (and not SnS2) phases, accounting for the production of (MoS2)x(SnO2)1–x/rGO nanohybrids. That is, although tin sulfides will putatively form initially under the hydrothermal conditions employed, the Mo species scavenge the sulfide and the oxide to generate SnO2 is provided by GO, MoO42–, and air/solvent. As we will show, the Mo:Sn ratio in the final nanohybrid can be smoothly controlled by altering the initial stoichiometry between the precursor metal salts Na2MoO4 and SnCl2. Importantly, across all stoichiometries, the metal ratio in the final product reflects the precursor Mo:Sn metal ratio and this ratio profoundly influences the electrocatalytic activity for the HER. Finally, IL was added as an exfoliating and stabilizing agent in order to improve nanocrystalline stability and to suppress aggregation/restacking, to achieve a more delayered morphology. It has been well reported that cationic surfactants or solvents, such as imidazolium-derived ILs, are adsorbed to the negatively-charged surface of GO through electrostatic and π-π interactions, which not only prevent the GO from restacking after reduction but also aid in screening the GO’s negative charge, allowing otherwise electrostatically-repelled MoO42– to sorb to the GO surface during synthesis.15, 44-45 This mixture was transferred to an autoclave and subjected to a hydrothermal treatment for 20 h at 240 °C to generate (MoS2)x(SnO2)1–x/rGO nanohybrids as prospective HER electrocatalysts.

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The sizes and morphologies of the (MoS2)x(SnO2)1–x/rGO nanohybrids (synthesized in the presence of [Bmim][BF4]) were examined by TEM imaging (Figure 1 and Figures S2–4). Panels A–C in Figure 1 provide representative images of the MoS2/rGO nanohybrids which are comprised of thin MoS2 ribbons connected side-byside to form layered stacks uniformly decorated onto the underlying rGO sheets. These MoS2 stacks lie flat to the graphene surface (Figure 1B) and appear well connected at the heterointerface, supporting rapid electron transfer. Most of the MoS2 particles are bent and corrugated which minimizes the surface energy of the material and results in an abundance of exposed edge sites. Figure 1C highlights these exposed edge sites in which the MoS2 particles comprise of multi-layered (5–10 layers) nanostructures. More specifically, the parallel banding pattern shown in the inset of Figure 1C corresponds to a 5-layers MoS2 ribbon with a spacing of 0.65 nm relative to the (002) plane of MoS2. Figure 1G–I displays representative TEM images of the SnS2/rGO nanohybrid, distinctly revealing a variety of irregular nanoscale morphologies that are abundantly dispersed on the rGO surface. The average particle size of the SnS2 is 9 ± 2 nm, but large nanoplates up to hundreds of nanometers in length can also be observed. For nanohybrid materials containing both metals, a remarkable difference in morphology was observed (Figure 1D–F) with a fairly uniform size distribution of relatively quasi-spherical nanoparticles having average diameters of 7 ± 2 nm being present. The inset images in panels F and I of Figure 1 display the lattice fringes of the (MoS2)0.5(SnO2)0.5, SnS, and SnS2 nanoparticles with lattice spacings of 0.316, 0.305, and 0.277 nm which correspond well with reported literature values for the (001), (101), and (101) planes of SnO2,46 SnS,47-48 and SnS2,49-50 respectively. Additional TEM images of these samples are provided in Figures S2–S4. TEM was also conducted on (MoS2)0.6(SnO2)0.4/rGO nanohybrids

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prepared with the ILs [(HOEt)3MeN][MeSO4] and [C4mPy][TfO], micrographs of which are provided in Figures S5 and S6, respectively. The resultant nanoparticles are similar to those obtained using [Bmim][BF4], indicating the diversity of ILs that can be employed using this synthetic approach. Careful image analysis reveals that the (MoS2)x(SnO2)1–x nanoparticles exhibit larger surface areas than their mono-metallic counterparts, suggesting a greater abundance of exposed active edges. These structural and morphological features are highly advantageous for catalysts in various applications such as supercapacitors, batteries, and HER.27-28, 31, 33-34 To accurately determine the elemental composition of the prepared nanohybrids, comparator standard neutron activation analysis (NAA) was conducted. NAA can readily provide multi-element analysis of major, minor, and trace elements of milligram-quantity samples. Figure 2A and Table S1 summarize the relative amounts of Mo and Sn present within the nanohybrids. For illustration, molybdenum-to-tin (Mo:Sn) molar ratios of 0.78:0.22, 0.56:0.44, 0.51:0.49, 0.38:0.62, and 0.18:0.82 were measured for (MoS2)x(SnO2)1–x/rGO nanohybrids for x = 0.8, 0.6, 0.5, 0.4 and 0.2, respectively. As shown in Figure 2A, the actual content of Mo (%Mo measured) present within the nanohybrids generally lies in excellent agreement (linear correlation, r2 = 0.995) with the initial stoichiometry (%Mo theoretical) of Mo:Sn used during the hydrothermal synthesis, suggesting that the present methodology is highly effective for controllably tuning the nanohybrid metal composition. We note that (MoS2)0.1(SnO2)0.9/rGO is a statisticallysignificant outlier in the sense that the final Mo content is underrepresented in the final nanohybrid compared to its feed ratio in the synthesis. Other than this one exception, energy-dispersive X-ray (EDX) microanalysis (Figures S2–S6) results are consistent with

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NAA data, further corroborating the elemental composition being essentially the same as the mixing ratio. Figure 2B depicts UV-Vis absorption spectra of few-layered (MoS2)x(SnO2)1– x/rGO

nanohybrid dispersions for varying x, as indicated. In the MoS2/rGO nanohybrid,

two characteristic absorption bands appear at 667 and 616 nm, corresponding to the A1 and B1 peaks, respectively. These peaks arise from the direct excitonic electronic transition at the K point of the first Brillouin zone, while the broad absorption band between 350 and 500 nm (λmax ≈ 425 nm), referred to as the C and D peaks, arises from the direct excitonic electronic transition at the M point. These optical features are consistent with previously reported literature on MoS2.42,

51-52

Since no characteristic

peaks are observed for SnS2/rGO in the visible region, as the Sn content in the (MoS2)x(SnO2)1–x/rGO nanohybrid increases, the overall absorbance naturally decreases. Interestingly, however, peaks associated with MoS2 shift to slightly longer wavelengths and, as x decreases below 0.8, peaks become gradually dampened, providing evidence for the formation of a ternary hybrid wherein the SnO2 component influences the optical features of the MoS2 nanophase, possibly at heterojunctions present within the (MoS2)x(SnO2)1–x/rGO nanohybrid. The photograph provided in Figure 2C demonstrates the Tyndall effect for laser light scattering from (MoS2)x(SnO2)1–x/rGO suspensions. The exfoliated MoS2/rGO and SnS2/rGO samples appear dark green and pale green, respectively, and remain well dispersed for at least a month, indicating a high degree of colloidal stability. Although the exfoliated (MoS2)x(SnO2)1–x/rGO dispersions sediment more rapidly with increasing Sn content, they are easily re-suspended by bath sonication (3 min). In fact, seven-month-old (MoS2)x(SnO2)1–x/rGO sediments redispersed in this

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manner exhibit UV-Vis absorption spectra identical to those of the original freshly-made material, as shown in Figure S7. The XRD patterns of the ternary (MoS2)x(SnO2)1–x/rGO nanohybrids prepared using [Bmim][BF4] for varying x are shown in Figure 3A. For the MoS2/rGO hybrid, the diffraction peaks at 2θ values of 14.04, 33.44, 39.08, 59.12, 60.72, and 69.22°, correspond to the (002), (100), (103), (110), (008) and (203) planes of MoS2, respectively. These peaks match well with the literature values for 2H-MoS2 crystals.51 Similarly, the diffractions at 15.14, 30.76, 32.22, 42.02, 50.14, 52.64, 54.52, 60.88, and 67.32° observed for SnS2/rGO correspond to the (001), (002), (011), (012) (110), (111), (103) (201), and (202) planes of SnS2, respectively. These peaks are in good agreement with the literature values for hexagonal SnS2.36-38 In addition, diffraction peaks (marked by * in Figure 3A) were observed at 26.08, 27.54, 32.06, 39.30, 42.74, 44.84, 45.66, 48.78, 56.7, and 64.42° (2θ) for the SnS2/rGO hybrid which correspond to the (120), (021), (111), (131), (210), (141), (002), (211), (042), and (251) planes of orthorhombic SnS nanocrystals.35,

38, 53

This suggests that the SnS2/rGO nanohybrids consist of both

hexagonal SnS2 and orthorhombic SnS crystals which is consistent with observation of both nanoparticles and nanoplates in TEM. Unexpectedly, as the proportion of Sn increases in the ternary nanohybrid, sharp and intense peaks emerge arising from SnO2 (marked # in Figures 3A and S7), which is further supported by EDX analysis (Figures S3–S6). The formation of SnO2 was repeatably observed in the ternary nanohybrids from several different individual batches synthesized using the same experimental parameters. Notably, however, the XRD pattern of Mo-free SnS2/rGO shows no evidence for SnO2 formation. Thus, although GO has been previously shown to act as an oxidant in the conversion of Sn(II) to Sn(IV),43 we postulate that the introduction of MoO42– into the

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hydrothermal reaction mixture results in competition between Mo and Sn for thioureaderived sulfide to produce discrete MoS2 and SnO2 phases, yielding (MoS2)x(SnO2)1– x/rGO

nanohybrids. It is likely that these phases are thermodynamic sinks with high

lattice energies such that the intermetallic MoxSn1–xS2 is not easily accessible. The XRD patterns of the (MoS2)x(SnO2)1–x nanohybrids synthesized using different ILs (Figure S8) were consistent with the results mentioned above, further highlighting the generality of this approach using a range of ILs. In addition, the absence of the (002) diffraction peak near 24° for rGO in the XRD patterns of the (MoS2)x(SnO2)1–x nanohybrids highlights the outstanding and wide-ranging ability of ILs to stabilize rGO nanosheets, preventing them from restacking. We note that the growth of MoS2 and SnSx/SnO2 nanocrystals directly on the rGO nanosheet surface can additionally aid in the goal of impeding rGO layer restacking.37 Raman spectra further confirm the formation of MoS2 and the reduction of GO during the hydrothermal treatment. Figure 3B clearly reveals characteristic bands for MoS2 and rGO for the (MoS2)x(SnO2)1–x/rGO nanohybrids even after incorporation of Sn. Typically, bulk MoS2 displays two prominent peaks at approximately 382 cm–1 (E12g mode) and 409 cm–1 (A1g mode). The peak intensities and widths of these two Raman modes have been used analytically to determine the number of MoS2 layers previously.51, 54

The measured peak widths for bulk MoS2, MoS2/rGO, (MoS2)0.5(SnO2)0.5/rGO, and

(MoS2)0.6(SnO2)0.4/rGO nanohybrids are 26.70, 16.13, 13.98, and 13.98 cm–1, respectively. These values indicate that the MoS2 within the nanohybrids consists of only a few layers, although the interaction between Mo and Sn atoms might soften the Mo-S modes, decreasing their frequency widths.28 The SnS2/rGO nanohybrid displays peaks at 317, 1334, and 1616 cm–1 (Figure S9) which correspond to the A1g mode of SnS2 and the

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D and G modes of rGO, respectively, consistent with successful formation of the SnS2/rGO nanohybrid. The D (disorder) and G (sp2-graphitic) bands of GO15 were used to determine the ID/IG ratio of the (r)GO, which was found to be 1.12, 1.18, 1.23, 1.29, and 1.26 for GO and the (MoS2)x(SnO2)1–x/rGO nanohybrids (x = 1.0, 0.6, 0.5, 0.0), respectively. The ID/IG values for the (MoS2)x(SnO2)1–x/rGO nanohybrids were close to the value for GO, suggesting that the nanoparticles were adsorbed onto the rGO surface and not doped into the graphene layers.36 These results further corroborate that the obtained (MoS2)x(SnO2)1–x/rGO nanohybrids are comprised of rGO layers and crystalline MoS2 and SnS2/SnO2 nanoparticles. To assess the performance of (MoS2)x(SnO2)1–x nanoparticles decorated onto rGO for HER electrocatalysis, linear sweep voltammetry (LSV) was performed in 0.5 M H2SO4 solution using a typical three-electrode system (see Section 2.5 for experimental details). Glassy carbon was used as the working electrode since it is an ideal electrode substrate for electrocatalyst deposition due to its negligible HER activity within the voltage range of interest (–0.5 to 0.1 V). Figure 4A shows polarization curves measured for MoS2/rGO, SnS2/rGO, and (MoS2)x(SnO2)1–x/rGO nanohybrids. A polarization curve measured under identical conditions using a Pt/C (20 wt% Pt) catalyst in provided as a benchmark. One of the key parameters normally used to evaluate HER performance is the overpotential (η10) defined as the potential at a cathodic current density of 10 mA cm–2. The Pt/C reference catalyst gave an exceptionally high catalytic activity with an η10 value of 67 mV, in accord with literature values.8 As shown in Figure 4A, MoS2/rGO displayed vastly better HER performance over SnS2/rGO, although the multi-layered MoS2 morphology suggests further scope for improvement. Despite the very low electrocatalytic activity of SnS2, incorporation of Sn alongside the MoS2 in the form of

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(MoS2)x(SnO2)1–x/rGO prominently reduced the overpotential by 90–130 mV compared to MoS2/rGO. More specifically, the η10 values of the (MoS2)x(SnO2)1–x/rGO nanohybrids for x = 0.8, 0.6, 0.5, 0.4 and 0.2 were 359 ± 25, 263 ± 5, 274 ± 4, 297 ± 7 and 315 ± 14 mV, respectively, with the (MoS2)0.6(SnO2)0.4/rGO nanohybrid displaying the highest HER activity. Figure 4B displays how dramatically the overpotential for (MoS2)x(SnO2)1– x/rGO

electrocatalysts varies with Mo:Sn composition. Importantly, the overpotentials for

the best performing ternary (MoS2)x(SnO2)1–x/rGO nanohybrids are comparable to or better than many recently reported values (Table S2).28, 31, 33-34, 42 In spite of XRD data which point toward Sn incorporation in the form of a distinct SnO2 phase, this dramatic improvement, amounting to hundreds of mV decrease in η10, is assuredly significant. Given the fact that an intermetallic (e.g., MoxSn1–xS2) involving intimate mixing or alloying of Mo and Sn is not accessible by this route, the improved electrocatalytic performance displayed by (MoS2)x(SnO2)1–x/rGO is probably a result of synergistic interactions between the exposed edges of the MoS2 and SnO2 phases and/or more efficient electronic coupling to rGO with improved electron transfer efficiency and electrode kinetics. Due to the low electrocatalytic activity of the SnS2/rGO nanohybrid and the presence of SnO2 within the ternary hybrids, one plausible explanation for the catalytic boost is that the SnO2 is functioning as secondary scaffolding onto which minuscule MoS2 nanoislands exist, although TEM imaging failed to evidence such heterostructures. Although this topic deserves further scrutiny, given the fact that MoS2 and SnO2 clearly form discrete phases, one likely must evoke the existence of heterojunctions or heterointerfaces to account for the increased electrocatalytic activity of the ternary (MoS2)x(SnO2)1–x/rGO catalysts. In addition to excellent performance for the HER, the (MoS2)0.6(SnO2)0.4/rGO nanohybrid electrode only showed a marginal decrease

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in the cathodic current and catalytic activity after 1000 continuous operation cycles (Figure S10), demonstrating the robustness of the nanohybrid catalysts. Figure 5 displays Tafel plots derived from the corresponding polarization curves in Figure 4A. Tafel plots can be used to predict the electrocatalytic reaction mechanism via the Tafel slope (bf). The linear portions of the Tafel plots were fit to the Tafel equation (η = bf log( j ) + a), yielding bf of 31.5, 105.5, 133.2, 50.8, 58.0, 58.4, 58.9, and 196.1 mV per decade (mV dec–1) for Pt/C and (MoS2)x(SnO2)1–x nanohybrids for x = 1.0, 0.8, 0.6, 0.5, 0.4, 0.2 and 0.0, respectively. Using Pt/C as a benchmark, the activity of the nanohybrid electrocatalysts towards the HER could be judged by comparing their respective bf values. Notably, the much lower bf value of (MoS2)0.6(SnO2)0.4/rGO (bf = 50.8 mV dec–1) compared to MoS2/rGO (bf = 105.5 mV dec–1) confirms that the catalytic activity is substantially improved by incorporation of Sn. This drastic decrease in the Tafel slope is attributed to an increase in the quantity of active edge sites and/or higher conductivity within the (MoS2)0.6(SnO2)0.4/rGO hybrid. Although bf can be used to qualitatively assess the electrocatalytic reaction mechanism, unfortunately, the exact HER mechanism and reaction pathway that predominates still remains elusive.8 The three reactions that have been suggested for HER in an acidic medium are a primary discharge step (Volmer reaction), an electrochemical desorption step (Heyrovsky reaction), and a recombination step (Tafel reaction). The theoretical bf values of Volmer, Heyrovsky, and Tafel reactions are 120, 40, and 30 mV dec–1, respectively. In view of bf values between 50 and 60 mV dec–1 for the (MoS2)0.6(SnO2)0.4/rGO hybrid, a combination of the Volmer reaction involving the electrochemical step of adsorbing hydrogen atoms in the active sites and the Heyrovsky reaction involving the formation of surface-bound H2 should dominate

the

kinetics.

Thus,

the

smaller

20

Tafel


slope

observed

for

the

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(MoS2)0.6(SnO2)0.4/rGO nanohybrid confirms the generation of accessible and catalytically active edges toward low-cost and efficient HER catalysis. To better understand the HER interfacial reactions and electrode charge transport mechanisms of the electrocatalysts, electrochemical impedance spectroscopy (EIS) measurements were conducted. The diameter of the small semicircles in the EIS plot (Figure S11) indicates the interfacial charge-transfer resistance (Rct) values for the electrocatalysts. The MoS2/rGO, SnS2/rGO, and (MoS2)0.6(SnO2)0.4/rGO nanohybrids show Rct values of 1.2 kΩ, >16 kΩ, and 150 Ω, respectively. The significantly lower Rct for (MoS2)0.6(SnO2)0.4/rGO indicates better electron transport within the nanohybrid which agrees well with the faster reaction rates observed in the LSV experiments, further confirming the synergistic benefit of Sn incorporation. Finally, Figure S12 presents polarization curves and corresponding Tafel plots for (MoS2)0.6(SnO2)0.4/rGO nanohybrids prepared using six different ILs in addition to [Bmim][BF4]. These nanohybrids exhibit η10 values in the range of 265–301 mV and showed bf values similar to those prepared with [Bmim][BF4] (Table S3). Interestingly, the use of non-aromatic ILs—such as the quaternary ammonium examples [(HOEt)3MeN][MeSO4] and the pyrrolidinium-based [C4mPy]—results in similarly-low Tafel slopes as for [Bmim][BF4], suggesting that π-π interactions may not be the primary factor leading to a de-layered rGO morphology. Certainly, these results suggest that a multitude of ILs can serve as competent stabilizing agents in this capacity while pointing to the crucial role and promise of Sn incorporation for enhancing the HER activity of MoS2.

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4. CONCLUSIONS In summary, we have successfully demonstrated a facile hydrothermal method to generate (MoS2)x(SnO2)1–x/rGO nanohybrids as efficient HER electrocatalysts using ionic liquids as agents for achieving a more de-layered morphology by suppression of graphene plane aggregation/restacking. X-ray diffraction results reveal that Mo outcompetes Sn for sulfur to result in discrete MoS2 and SnO2 nanophases supported on rGO. Conveniently, the Mo:Sn ratio within the final nanohybrid is controlled by the starting stoichiometry of the metal precursors. The Sn content was found to profoundly influence the electrocatalytic activity of the catalyst for the HER. The (MoS2)0.6(SnO2) 0.4/rGO hybrid showed the best overall performance, yielding an overpotential, η10, of 263 mV (i.e., 131 mV lower than that for the MoS2/rGO analog) and a small Tafel slope (50.8 mV dec–1), possibly due to heterointerfacial interactions between exposed active sites on MoS2 and SnO2 nanoscale phases and/or more efficient charge transport and electronic coupling. Overall, this one-pot methodology offers a promising strategy for realizing designer electrocatalysts for the HER based on tuning the compositions of non-precious metal nanoscale hybrids. Although the exact origin and impact of the SnO2 nanophase remains under investigation, the fact that its integration with MoS2 greatly improves the HER activity also merits further study.

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FIGURES AND FIGURE CAPTIONS

Figure 1. Representative TEM images of the (A–C) MoS2/rGO, (D–F) (MoS2)0.5(SnO2)0.5/rGO, and (G–I) SnS2/rGO nanohybrids. The inset of (C) shows the spacing between the MoS2 layers while (F) and (I) highlight the lattice fringes of the indicated nanoparticles within their respective nanohybrid.

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Figure 2. (A) Plot showing the correlation between the actual Mo fractional content measured by neutron activation analysis for (MoS2)x(SnO2)1–x/rGO nanohybrids prepared using [Bmim][BF4] and the content predicted on the basis of the initial Mo:Sn stoichiometry provided during synthesis (r2 = 0.995). The dashed line represents the case where the (MoS2)x(SnO2)1–x composition exactly reflects the reagent metallic ratio. (B) UV-Vis absorption spectra of (MoS2)x(SnO2)1–x/rGO nanohybrid dispersions in water-ethanol (2:3, v/v) for varying molar ratios (x), which highlight the loss of the characteristic MoS2 absorption bands as the Sn content increases. This attenuation in absorbance arises due to both decreased overall MoS2 content and suppression of the Mo excitonic peaks by Sn. (C) Digital photograph of diluted (MoS2)x(SnO2)1–x/rGO nanohybrid dispersions illustrating the Tyndall light scattering effect under green (λ = 532 nm) laser pointer illumination.

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Figure 3. (A) XRD pattern and (B) Raman spectra of (MoS2)x(SnO2)1–x/rGO nanohybrids prepared using [Bmim][BF4] for varying x, as indicated. The crystal planes of SnS and SnO2 nanoparticles are denoted with * and #, respectively. The spectra confirm the formation of nanoscale (MoS2)x(SnO2)1–x on the rGO surface.

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Figure 4. (A) Linear sweep voltammetry curves measured for (MoS2)x(SnO2)1–x/rGO nanohybrids prepared using [Bmim][BF4] for different values of x. The ternary nanohybrids exhibit significantly improved performance in comparison to both MoS2 and SnS2. (B) Extracted overpotentials of (MoS2)x(SnO2)1–x/rGO nanohybrids with respect to %Mo content. The error bars denote standard deviations based on five measurements. The open red circle marked with an asterisk indicates that the η10 value for the SnS2/rGO nanohybrid is above 500 mV. All electrochemical measurements were iR-corrected.

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Figure 5. Tafel plots for (MoS2)x(SnO2)1–x/rGO nanohybrids prepared using [Bmim][BF4]. These results highlight the substantial improvement in HER activity that arises when intermediate Mo:Sn compositions are present in the hybrid catalyst. The (MoS2)0.6(SnO2)0.4/rGO nanohybrid showed the best performance, with a Tafel slope of 50.8 mV/dec.

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ACKNOWLEDGEMENTS G.A.B. thanks the donors of the Petroleum Research Fund (grant no. 51865-DNI10), administered by the American Chemical Society, for support of this research. We also thank Dr. Charles L. Barnes for his kind assistance with powder X-ray diffraction measurements. SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website at DOI: XRD patterns of GO and (MoS2)0.6(SnO2)0.4/rGO nanohybrids using different ionic liquids, additional TEM images, EDX spectra, tabulated NAA results, Raman spectra of GO and SnS2/rGO nanohybrid, additional polarization and Tafel plots, and tabulated HER electrocatalyst comparisons (PDF) ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Gary Baker: 0000-0002-3052-7730 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. REFERENCES (1) Turner, J. A., Sustainable Hydrogen Production. Science 2004, 305, 972–974. (2) Laursen, A. B.; Kegnaes, S.; Dahl, S.; Chorkendorff, I., Molybdenum Sulfides– Efficient and Viable Materials for Electro- and Photoelectrocatalytic Hydrogen Evolution. Energ. Environ. Sci. 2012, 5, 5577-5591. (3) Yan, Y.; Xia, B.; Xu, Z.; Wang, X., Recent Development of Molybdenum Sulfides as Advanced Electrocatalysts for Hydrogen Evolution Reaction. ACS Catal. 2014, 4, 1693– 1705. (4) Merki, D.; Hu, X., Recent Developments of Molybdenum and Tungsten Sulfides as Hydrogen Evolution Catalysts. Energ. Environ. Sci. 2011, 4, 3878-3888. (5) Jaramillo, T. F.; Jørgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I., Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts. Science 2007, 317, 100-102. (6) Wu, Z.; Fang, B.; Wang, Z.; Wang, C.; Liu, Z.; Liu, F.; Wang, W.; Alfantazi, A.; Wang, D.; Wilkinson, D. P., MoS2 Nanosheets: A Designed Structure with High Active Site Density for the Hydrogen Evolution Reaction. ACS Catal. 2013, 3, 2101-2107. (7) Lau, V. W.-h.; Masters, A. F.; Bond, A. M.; Maschmeyer, T., Ionic-Liquid-Mediated Active-Site Control of MoS2 for the Electrocatalytic Hydrogen Evolution Reaction. Chem.-Eur. J. 2012, 18, 8230-8239.

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Ionic Liquid-Assisted Synthesis of Nanoscale (MoS2) - ACS Publications

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