Facile Synthesis Of MoS₂/Graphene Nanocomposites And Their Electrochemical Application Research

Due to its favorable properties,such as excellent electrical conductivity,high carrier mobility,large surface area and excellent material flexibility,graphene has been widely applied in fields such as nano-electronic devices,lithium ion batteries and supercapacitors.The discovery of graphene has also opened the floodgates to the research of other 2D nanomaterisl like MoS₂ and SnS₂.As a typical metal dichalcogenides,MoS₂ exhibits a layered graphite-like structure.It can be used in the lithium ions storage field and delivers a higher lithium storage specific capacity than that of graphite.On the other hand,the sulfur edges of MoS₂ layer is active for electrocatalytic hydrogen evolution recation（HER）.However,as a semiconductor material,the electronic conductivity of MoS₂ is very low and MoS₂ nanosheets tend to restack because of van der Waals.In this paper,MoS₂/graphene nanocomposites were prepared by hydrothermal method with the assistance of supramolecule,ion liquid and so on.The morphology,microstructure,as well the electrochemical lithium storage and electrocatalytic HER performances of the as-formed nanocomposites were characterized and investigated.The MoS₂/graphene composites were synthesized by a hydrothermal route in the presence of graphene oxide sheets with assistance from poly（diallyldimethylammonium chloride）（PDDA）.The results of characterizations show that the MoS₂/graphene composites exhibit a graphene-like morphology,in which single-or few-layer MoS₂ sheets are homogenously dispersed and well anchored on the graphene surface.The robust heterostructure and morphology of MoS₂/graphene composites can not only provide more sites and channels for lithium ion accommodation and diffusion,but also facilitate electrolyte access.The heterostructure also maximizes the synergistic effects between MoS₂ and graphene.Therefore,MoS₂/graphene composites exhibit significantly improved electrochemical performances for reversible lithium storage.In particular,the MoS₂/graphene composite prepared with monomer concentration of PDDA being 0.02 M exhibits a reversible capacity of 1100 mAh/g with enhanced rate capability.The reversible capacity of 856 mAh/g after 900 cycles at 500 mA/g can be remained,indicating its excellent cyclic stability.MoS₂/reduced graphene oxide composites were prepared through hydrothermal route and under the mediation of a water-souble supramolecule（pillar[5]arene）.The microstructure and morphology of the composites obtained with the mediation of different concentrations of supramolecule were characterized by XRD,SEM,TEM/HRTEM,Raman and EDX.And their lithium storage performances were also tested.The characterization results show that when the concentration of supramolecule is 0.002 M,the obtained composite display a wrinkled thin flaky appearance,in which there are a lot of irregular pores and apertures.Few-layer MoS₂ sheets are well dispersed and anchored on the rGO surface.When evaluated as a host material for lithium storage,the composite exhibits a much higher specific capacity of 1150 mAh/g and an enhanced high-rate capability of 875 mAh/g at a current density of 1000 mA/g.The improved performance can be ascribed to the robust composite structure and better synergic effects between few-layer MoS₂ and graphene.The ionic liquid（IL,[BMIM]BF₄）is employed to mediate the preparation of molybdenum disulfide/graphene composites by hydrothermal route in the presence of graphene oxide sheets.The effects of ionic liquid on the microstructure and hydrogen evolution reaction performances of molybdenum disulfide/graphene are investigated.The molybdenum disulfide/graphene composite（MoS₂/G-IL10）prepared with 1.0 mL of ionic liquid displays numerous de-layered molybdenum disulfide sheets with short slab length and discontinuous crystal fringes on the surface of graphene.Due to the rich exposed edge sites,as well as the synergetic effect of de-layered molybdenum disulfide and graphene sheets,the MoS₂/graphene composite shows excellent electrocatalytic performance for hydrogen evolution reaction with a small Tafel slope of 52 mV/dec in acidic medium.A facile one-pot hydrothermal route with the mediation of ionic liquid（IL,[BMIM]BF₄）is presented to synthesize MoS₂/nitrogen-doped graphene（MoS₂/NG-IL）composites.It is found that as-prepared MoS₂/NG-IL composites exhibit that the de-layered MoS₂ sheets with more exposed edge sites and defects are well anchored on the N-doped graphene.Due to the synergism between de-layered MoS₂ and N-doped graphene,MoS₂/NG-IL composites exhibit better electrochemical performances for hydrogen evolution reaction（HER）and reversible lithium storage in comparison with MoS₂.When 1,0 mL of IL is added in hydrothermal solution,the obtained composite displays very high electrocatalytic activity for HER with a low onset overpotential of 80 mV and a small Tafel slope of 48.0 mV/dec in 0.5 M H2SO4.As anode of lithium ion battery,it delivers a reversible capacity as high as 1169 mAh/g at 100 mA/g and enhanced rate capability of 782 mAh/g at 1000 mA/g.After 800 cycles,a reversible capacity of about 800 mAh/g at 500 mAh/g can be retained,indicating its excellent cyclic stability.Cobalt-doped MoS₂/graphene（Co-MoS₂/G）hybrids are fabricated through a one-pot hydrothermal method by reacting Na2Moo4 and graphene oxide with L-cysteine in the presence of Co（CH3COO）2-4H2o.It is found that the Co-doping not only changes the morphology and microstructure of the Co-MoS₂/G hybrids,but also improves the intrinsic electrocatalytic activity of their active sites.Especially,the Co-MoS₂/G-3 hybrid prepared with 0.15 mmol Co（CH3COO）2·4H2O displays numerous Co-MoS₂ short sheets which are well-anchored on the graphene surface,and provide more exposed active edge sites for the hydrogen evolution reaction（HER）.With such merits,the Co-MoS₂/G-3 hybrid shows a remarkable catalytic activity toward HER with a low Tafel slope of 44.3 mV/dec and excellent durability（minimal performance degradation after 1000 cycles）.