Journal of Science: Advanced Materials and Devices (2018) 359e365 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article g-MnS nanoparticles anchored reduced graphene oxide: Electrode materials for high performance supercapacitors S Ranganatha*, N Munichandraiah Department of Inorganic & Physical Chemistry, Indian Institute of Science, C V Raman Avenue, Bengaluru 560012, India a r t i c l e i n f o a b s t r a c t Article history: Received June 2018 Received in revised form 27 June 2018 Accepted July 2018 Available online July 2018 g-MnS/reduced graphene oxide composites (g-MnS/rGO) were successfully synthesized by a simple one Keywords: Supercapacitors Reduced graphene oxide g-MnS Composite rGO pot solvothermal route Their structure, morphology and electrochemical properties were studied with respect to applications as a supercapacitor electrode material The specific capacity of g-MnS/rGO is 1009 C/g at A/g and retains 82% of initial capacity over 2000 cycles at A/g whereas pristine g-MnS delivers only 480 C/g at A/g with a capacity retention of 64% Thus, g-MnS/rGO proves to be a promising electrode material, which exhibits high the specific capacity and stable long cycle life © 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Introduction In the current scenario, more and more efforts are focussed on suitable and environmentally friendly energy converting and energy storage materials Electrochemical supercapacitors are one such class of materials, which offer high energy density, fast charge/ discharge rate, long cycle life, etc They are mechanically classified as electrochemical double layer capacitors and pseudocapacitors [1e5] Some of the recent literature reports on transition metal oxides and sulphides have described their significance as potential candidates for supercapacitor electrode materials [6e10] Manganese sulphide being a wide gap semiconductor, has found potential applications in short wavelength opto electronics, luminescents and magnetic semiconductors technological fields These materials are used for semiconductor spin-based electronics or spintronics due to their magnetic and magneto-optical properties which arise from spin-exchange interactions between the dopant ions and the semiconductor charge carriers [1,3,4] Recent reports clarify that metal sulphides, in particular MnS, are promising materials for supercapacitors MnS is known to exhibit strong redox peaks in the cyclic voltammogram which is attributed to the non-linear * Corresponding author E-mail address: kamath.ranganath@gmail.com (S Ranganatha) Peer review under responsibility of Vietnam National University, Hanoi dependence of charge storage vs potential advocating it's faradaic or battery type behaviour [11] MnS is known to crystallize in three different polymorphic forms, namely, a-MnS with rock salt structure, b-MnS with zinc-blends structure and g-MnS with wurzite structure g-MnS stands superior in electrochemical performances due to its laminar nanostructure facilitating easy penetration of electrolytes and intercalation of ions affecting the capacitive behaviour positively [12e17] MnS is less focussed for supercapacitive applications due to its poor cycling ability and low electronic conductivity [18,19] Carbon based materials like activated carbon, carbon nanotubes, graphenes etc are very potential electrode candidates for supercapacitors and batteries which offer high power density and long cycle life Unfortunately the charge storage mechanism limits its energy density Recently, scientists targeting bridging the gap of this power density and energy density by combining the contributions of both pseudocapacitive materials like metal sulphides/oxides with conducting materials Popularly, the conducting polymer polyaniline is being used to wrap pseudocapacitive materials, thus to enhance the performance In recent past, studies dealing with anchoring the metal sulphides/oxide nanoparticles to graphene sheets are gaining importance because of their high conductivity and very high specific surface area [20] Using graphene as a matrix for MnS will be a good idea to facilitate large electrode/electrolyte interfaces for charge/discharge reactions and to enhance the conductivity [21e26] https://doi.org/10.1016/j.jsamd.2018.07.001 2468-2179/© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) 360 S Ranganatha, N Munichandraiah / Journal of Science: Advanced Materials and Devices (2018) 359e365 In this work, the MnS anchored reduced graphene oxide (rGO) composite has been successfully designed by the facile solvothermal method Its supercapacitive performance has also been evaluated showing that g-MnS/rGO composite possesses better capacities compared to the pristine g-MnS Samples were characterized by various techniques, such as X-ray diffraction (XRD), Tunneling electron microscopy (TEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) Experimental To synthesize the graphene oxide, 10 g of graphite powders and g NaNO3 were mixed and added to 220 mL conc H2SO4 which was kept in an ice bath 30 g of KMnO4 was slowly added with constant stirring After 30 min, the mixture was further stirred at 35 C for h 460 mL of water and 80 mL of H2O2 were then added slowly to the solution After cooling, the mixture was filtered and washed with 10% HCl and deionized water until the sulfate ions In brief, rGO was synthesized using the oxidation of graphite by KMnO4 and H2O2 and NaNO2, and by the subsequent hydrothermal reduction with an ammonia solution [26] g-MnS/rGO was prepared using a solvothermal procedure based on rGO dispersed glycerol, MnCl2 4H2O and thioacetamide at 190 C for h [21] 2.1 Preparation of rGO Fig (a) XRD patterns of GO & rGO, (b) g-MnS, g-MnS/rGO, (c) TEM image and SAED pattern of g-MnS, (d) TEM image of g-MnS/rGO, rGO sheets as inset, (e) HRTEM of g-MnS, (f) TEM and SAED pattern of g-MnS/rGO S Ranganatha, N Munichandraiah / Journal of Science: Advanced Materials and Devices (2018) 359e365 361 Fig (a) XPS spectrum of g-MnS, (b) High resolution spectrum of Mn, (c) High resolution spectrum of S, (d) Raman spectra of GO, rGO and g-MnS/rGO were removed As-prepared GO was reduced by the solvothermal method using NH4OH Nearly 50 mg GO was dispersed in 60 mL ethanol and sonicated for h 10 mL of NH4OH was added and reduced hydrothermally at 180 C for 10 h [27] 2.2 Preparation of g-MnS/rGO To prepare the g-MnS/rGO, 60 mg of rGO was dispersed in 60 mL of glycerol by sonication for h 0.01 mmol of MnCl2$4H2O and 0.01 mmol Thioacetamide were dissolved in 10 mL distilled water individually Both of these solutions were added to 60 mL of glycerol, well mixed and stirred then transferred into a Teflon lined autoclave of 100 mL capacity The autoclave was sealed and maintained at 190 C for h Precipitates were washed and dried The same procedure was followed to synthesize the g-MnS, except the addition of rGO 2.3 Characterization Powder X-ray diffraction (XRD) patterns were recorded using a PANylatical diffractometer with Cu Ka (Wavelength ¼ 1.5438 Å) incident radiation as the source The surface area and the pore size distribution of the samples were measured using the micromeritics surface area analyzer of the model ASAP 2020 The X-ray photoelectron spectra (XPS) were collected on an AXIS ULTRA X-ray photoelectron spectrometer Microscopy images of the samples were recorded using the FEI Tecnai T-20 e 200 kV transmission electron microscope (TEM) and FEI Co equipped with an EDAX system at an accelerating voltage of 10 kV The Raman spectra were measured by a Horiba Jobin Yvon LabRam HR spectrometer having an 0.2 mW power laser of 514.5 nm wavelength illustrating the sample surface 2.4 Preparation of electrodes and electrochemical experiments For the fabrication of the electrodes, the active material (70 wt.%), conductive carbon (Ketjen black, 15 wt.%) and polyvinylidine fluoride (15 wt.%) were mixed in a mortar A few drops of N-methyl pyrrolidone were added to form a slurry This slurry was coated on a carbon paper with a geometrical area of cm2 and then dried at 100 C under reduced pressure The coating and drying steps were repeated to get the mass of the active material 362 S Ranganatha, N Munichandraiah / Journal of Science: Advanced Materials and Devices (2018) 359e365 0.8e1 mg/cm2 The electrodes were finally dried for 12 h An electrochemical cell was assembled using the material coated carbon paper, Pt and a saturated calomel electrode (SCE) as the working, counter and reference electrodes, respectively, in a glass container All potential values are reported against SCE reference The cyclic voltammetry (CV) and the galvanostatic charge/discharge cycling were measured by the Biologic SA multichannel potentiostat/galvanostat of the model VMP3, in a 6M KOH solution The electrochemical impedance spectroscopic measurements (EIS) were done using the Electrochemical Analyzer model CHI608C in the range 0.01 Hze100 kHz with an alternating voltage perturbation of mV The galvanostatic charge/discharge cycling tests were performed and the discharge specific capacity (C) was calculated using the relation C ¼ It/m, where I is the current, t the discharge time, DE the potential window and m the mass of the active material on the working electrode Results and discussion XRD patterns for graphite oxide and reduced graphene oxide are shown in Fig 1(a) A peak at 10.6 in GO indicates the oxidation of graphite This characteristic peak vanishes as rGO forms, indicating the periodic layered structure of rGO sheets The peak emerged at 24 advocates the formation of graphene and its amorphous structure Fig 1(b) refers to the XRD pattern of the gMnS (JCPDS file # 40-1289) with the characteristic diffraction peaks at 26 , 28 , 37.8 , 46 , 50.2 , 54.5 , 61.5 , 70 and 78.5 corresponding to (100), (002), (102), (110), (103), (112), (202), (203) and (105), respectively [21,23] Fig 1(c) shows the TEM image and the SAED pattern of the g-MnS indicating its well dispersed nanoparticles and polycrystalline nature with distinct diffraction rings Fig 1(d) and (f) show the TEM images of g-MnS/ rGO wherein the rGO sheets are shown with the arrow marks demonstrating the anchoring of g-MnS on to the rGO sheets Also, an image of the individual rGO sheets is provided as an inset in Fig 1(d) The SAED pattern of g-MnS/rGO with no distinct diffraction rings suggests the amorphous nature of this material The HRTEM image of the composite shown in Fig 1(e) clearly pronounces the characteristic lattice fringes with a lattice plane space of 0.32 nm, which corresponds to the (002) plane of g-MnS in agreement with the XRD results Fig 2(a) shows a broad XPS survey spectrum of g-MnS indicating the presence of the n, S, C and O elements The binding energies at about 642.1 and 655.1 eV (Fig 2(b)) can be assigned to Mn 2p3/2 and Mn 2p1/2, respectively The peaks at 162 and 164.5 eV (Fig 2(c)) are attributed to the binding energies of S 2p3/2 and S 2p1/2, respectively These values are matched with corresponding literature values and conrmed that Mn2ỵ and S2 are present in the sample The peak at around 169 eV, suggests that a part of S2À on the MnS surface in the as-synthesized material has been oxidized [21,22] In Raman spectra of GO, rGO and g-MnS/rGO (Fig 2(d)), D and G bands of the graphene are exhibited by the curves at 1349 and 1586 cmÀ1, the representing poorly crystallized graphite and crystal graphite's stretching mode, respectively ID/IG ratio convey the quality of graphene and it gets improved from 0.98 to 1.36 for GO to rGO and it is 1.38 for that of g-MnS/rGO which is ascribed to the new and smaller sp2 domains formed during the reduction of GO The composite shows a characteristic peak at 645 cmÀ1 confirming the presence of g-MnS [21e24] The specific surface area was calculated using the BrunauereEmmetteTeller (BET) method from the adsorption branch of isotherms in p/p0 range of 0.1e0.2 (Fig 3) The inset of 3(a) depicts the isotherms of the as-prepared g-MnS The adsorption and the desorption branches (Fig 3(a)) exhibit a loop at the high relative pressure indicating a porous nature of the compound In the case of g-MnS/rGO, there were 28 cm3/g of N2 adsorbed at p/p0 ¼ 0.99 and the sample possesses a specific surface area of 6.8 m2/g whereas for g-MnS the corresponding values are 2.5 cm3/g and 1.2 m2/g, respectively According to Fig 3(b), the BJH curves of the composite depict a pore size distribution with a prominent maximum at around 20 nm Fig 4(a) and (b) depict the CV diagrams of the g-MnS and g-MnS/rGO electrodes, respectively Broad voltammograms with current peaks corresponding to redox reactions are observed at all scan rates suggesting a faradaic nature of g-MnS This behavior is contrary to the electric double layer capacitor behaviour, where rectangular CV is the signature The experimental result showing the incremental current responses with the increasing scan rates signify the diffusion limited redox behavior Furthermore, the redox peaks move with the increasing scan rates can be attributed as being responsible for the limitation of the diffusion rate to satisfy the electronic neutralization [28,29] The variable oxidation states of Mn (Mn2ỵ/Mn3ỵ/Mn4ỵ) in MnS contribute to the faradaic Fig (a) Adsorption e desorption isotherms from the BET experiment (The inset: isotherms of MnS enlarged), (b) pore size distribution for the g-MnS and g-MnS/rGO S Ranganatha, N Munichandraiah / Journal of Science: Advanced Materials and Devices (2018) 359e365 363 Fig (aeb) CV diagrams of g-MnS and g-MnS/rGO, (ced) Galvanostatic charge/discharge profiles of g-MnS and g-MnS/rGO, (e) Variation of the specific capacity with the specific current, (f) Stability of the electrodes upon cycling, (g) Nyquist plots for samples 364 S Ranganatha, N Munichandraiah / Journal of Science: Advanced Materials and Devices (2018) 359e365 capacity The CV diagrams reflect a good reversibility of the corresponding electrode processes; and also the large integrated area assures a consequential remarkable capacity The following redox reactions can be proposed [21e26] MnS ỵ OH 4MnSOH ỵ e MnSOH ỵ OH 4MnSO ỵ H2 O ỵ e The charge-discharge voltage profiles registered at different specific currents are shown in Fig 4(ced) The symmetric characteristics of the chargeedischarge curves suggests a satisfactory reversibility w.r.t faradaic reactions The pristine g-MnS shows a specific capacity of 480 C/g at A/g and 47 C/g at 15 A/g, whereas, g-MnS/rGO offers a high capacity 1009 C/g at A/g and 90 C/g at high specific current 15 A/g (Fig 4(e)) Also, to test the cyclic stability of the materials, 2000 cycles were run at A/g (Fig 4(f)) The composite retains 82% of the initial capacity whereas the pristine g-MnS retains only 64% In Nyquist plots (Fig 4(g)) the intersection of the semi-circle at high frequencies on the real axis reflects the solution resistance RS, whereas, the diameter of the semi-circle is equated to the charge-transfer resistance Rct of the interface electrode/electrolyte The Rct values for g-MnS and g-MnS/rGO are U and 0.3 U, respectively, suggesting a lower intrinsic resistance and a better capacitive behavior of g-MnS/rGO The straight line or a spike seen in the low frequency region in the case of g-MnS/rGO represents the resistance to the diffusion of the electrolyte ions to the electrode interior The angle of the straight line with respect to the horizontal axis closely to 90 suggests the fast electrolyte diffusion and adsorption to the electrode surface attesting the ideal capacitor characteristics [28e30] A quick review on the previous reports on MnS as a supercapacitor material manifests the superiority and novelty of the present work Quan et al., fabricated the a-MnS/rGO solvothermally and studied its electrochemical properties It exhibits 513 C/g at A/g of specific current [31] Recently, MnS/rGO was fabricated by Xu et al also using the solvothermal method The group could obtain a high capacity up to 540 C/g at A/g [32] a-MnS nanosheets were generated adopting the hydrothermal route by Li et al The maximum capacity of that synthesized material was 137.6 C/g at 0.5 A/g [33] In a recent study by Hou et al., aimed to synthesize a g-MnS/CNT hybrid by the two steps hydrothermal method They obtained a capacity up to 353 C/g at A/g from the g-MnS anchored CNT hybrid material [30] In an attempt to synthesise MnS nanocrystals, Tang et al., obtained MnS nanospheres with appreciable electrochemical properties including a specific capacity of 490 C/g [34] In comparison with these literature reports, our g-MnS/rGO hybrid material, wherein g-MnS is anchored on the surface of the highly conducting rGO, exhibits superior electrochemical capacitive characteristics making it a reliable candidate for high performance supercapacitors This superiority can be attributed to the few advantages of the composite Anchoring is possible due to the direct covalent bonding and Van der Waal's attraction at oxygen, containing the functional groups on rGO g-MnS anchored on rGO sheets can interact well and favor the electron transportation The diffusion paths for the electrolyte ions are significantly shortened due to the particle anchoring to rGO sheets by spacing effect, thereby increasing the effective surface contact to the electrolyte Lastly, the outstanding electron transportation from the particles to the underlying rGO sheets speeds up the faradaic reactions, even at the high specific currents So, this unique structural feature of the composite material provides a condition wherein the electrolyte utilizes both g-MnS and rGO to the maximum Conclusion We successfully synthesized the g-MnS anchored rGO composite for an electrochemical supercapacitor via the solvothermal method The material showed an appreciably high capacity of 1009 C/g at A/g and a 82% capacity retention over 2000 cycles at A/g This material with high effective contact surface and electronic conductivity facilitates effective faradaic reactions at the interface of g-MnS and the electrolyte As a consequence, g-MnS/rGO exhibits a superior specific capacity and an exceptional cycling stability Acknowledgments S R acknowledges the financial support from the University Grant Commission (UGC), Government of India, under Dr D.S Kothari postdoctoral fellowship program [Ref No F.4-2/2006(BSR)/ CH/14-15/0133] References [1] B.E Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer Academic/Plenum Publishers, New York, 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