Fabrication of a hybrid graphenelayered double hydroxide material

Fabrication of a hybrid graphene/layered double hydroxide materialHongjuan Li,Gang Zhu,Zong-Huai Liu *,Zupei Yang,Zenglin WangKey Laboratory of Applied Surface and Colloid Chemistry (Shaanxi Normal University),Ministry of Education,Xi’an 710062,PR China School of Chemistry and Materials Science,Shaanxi Normal University,Xi’an 710062,PR ChinaA R T I C L E I N F O Article history:Received 7May 2010Accepted 29July 2010Available online 5August 2010A B S T R A C TA hybrid graphene/Ni 2+–Fe 3+layered double hydroxide material has been fabricated by the hydrothermal treatment of a mixed suspension of the exfoliated graphite oxide,Ni(NO 3)26H 2O,Fe(NO 3)3Æ9H 2O,urea and trisodium citrate.The Ni 2+–Fe 3+layered double hydroxide platelets are first homogeneously grown on the surface of GO nanosheets which are then reduced to graphene under a mild hydrothermal treatment.In the hybrid graphene/Ni 2+–Fe 3+layered double hydroxide material,the restacking of graphene nanosheets is effec-tively prevented by the formation of Ni 2+–Fe 3+layered double hydroxide platelets,and the graphene nanosheets exist in a complete exfoliation state.Ó2010Elsevier Ltd.All rights reserved.1.IntroductionSince the isolation of single-layer graphene in 2004,the prep-aration,structure and application of graphene have been widely researched because of its good electrical and thermal conductivity [1–3].In company with the recent achievement of large-area,free-standing graphenes,graphenes show pro-spective application in many fields,particularly in electronic and optoelectronic systems [4,5].However,some disadvan-tages are also found in the intrinsic property of graphene [6],which limit its applicability.Graphene-based hybrid mate-rials are quite intriguing from the perspectives of both funda-mental science and technology because the hybrid can enable versatile and tailor-made properties with performances far beyond those of the individual materials.Up to now,many graphene-based hybrid materials from graphene and certain functional particles such as polymers,metal oxides,metal particles,and organic molecules have been prepared.They present unique features and can be used in potential elec-trode materials,catalysts,biosensors,and methanol fuel cells,and so on [7–13].Ni and Fe are important transition metals due to their no-vel electronic,magnetic and optical properties.The compounds that they consist of such as layered double hydroxide (LDH)show a wide application in electro-and photoactive materi-als,catalysts,flame retardants,acid absorbents,anion exchangers,and so forth [14–16].Research results show that the composition and degree of crystallinity,the crystallite size and the size distribution affect the potential applications of LDH materials [17,18].Thus the synthesis of the transition metal layered composites such as Ni 2+–Fe 3+(or Ni 2+–Co 2+–Fe 3+)LDH with high crystallinity and well-defined hexagonal shape is of interest to probe LDH materials with novel elec-tronic,magnetic and optical properties.We have prepared Ni 2+–Fe 3+LDH material with high crystallinity and well-de-fined hexagonal shape by using urea as hydrolysis agent and trisodium citrate as complexing agent [19].In this work,we reported a simple synthesis of a hybrid graphene/Ni 2+–Fe 3+LDH material under mild hydrothermal condition.In the synthesis procedure,the exfoliated graphite oxide (GO)is simultaneously reduced to graphene in company with the homogeneous precipitation of Ni 2+–Fe 3+LDH.0008-6223/$-see front matter Ó2010Elsevier Ltd.All rights reserved.doi:10.1016/j.carbon.2010.07.053*Corresponding author at:Key Laboratory of Applied Surface and Colloid Chemistry (Shaanxi Normal University),Ministry of Education,Xi’an 710062,PR China.Tel.:+862985308442;fax:+862985307774.E-mail address:zhliu@ (Z.-H.Liu).2.Experimental2.1.MaterialsCrudeflake graphite(carbon content:99.9%)was purchased from Qingdao Aoke Co.,China.Tetramethylammonium hydroxide(TMAOH)(25wt.%)was purchased from Alfa Aesar Co.,H2O2(30%),H2SO4(98%),KMnO4,NaNO3,Ni(NO3)2Æ6H2O, Fe(NO3)3Æ9H2O,urea and trisodium citrate(TSC)were analyti-cal grade and used without further purification.Deionized water was used throughout the experiments.2.2.Synthesis and delamination of graphite oxideThe starting material,GO,was prepared from natural graphite by the Hummers method[20].GO sample(0.5g)was soaked in an aqueous solution of TMAOH(100mL,0.22mol LÀ1)for 7days at room temperature.After soaking,the colloidal sus-pension was separated by centrifugation and washing with deionized water several times until the pH was7,and the del-aminated GO slurry was obtained.2.3.Synthesis of the hybrid graphene/Ni2+–Fe3+LDH materialT ypically,Ni(NO3)2Æ6H2O,Fe(NO3)3Æ9H2O,urea and TSC were mixed together and dissolved in80mL of deionized water to give thefinal concentrations of16,respectively.The mixture wasminated GO slurry with vigorousinto Teflon-lined stainless steelmally treated at150°C for2days.Thewas centrifuged and washed within air,which was abbreviated asTo compare with the aboveexfoliated GO suspension was150°C for2days at the sameresulting product wasfiltered,washedand dried in air,which was2.4.CharacterizationX-ray diffraction(XRD)analysis was3c X-ray diffractometer with Cu K a(operation voltage and current of40tively.Transmission electroncollected using a JEOL-JEM-3010120kV.Specimens for observationing the samples into alcohol bydropped on carbon–copper grids.Theken at room temperature in theusing an ALMEGA–TM Ramanspectra were recorded using a532nmmogravimetry(TG)and differential(DSC)measurements were obtained(Q1000DSC+LNCS+FACS Q600SDT)10°C min–1.Ni and Fe contents weretively coupled plasma(ICP)atomic(IRIS Advantage)after dissolving awith an aqueous HCl solution.XPS formed with an Axis Ultra,kratos(UK)spectrometer using Al K a radiation(1486eV)at a power of150W(accelerating voltage15kV)in a vacuum of10À9Tom.The pH titration mea-surements were performed with an Orion960automatic titra-tor,and volumetric standard NaOH(0.01mol LÀ1)was used as the titrant.Before the titration experiment,GO sample(about 20mg)was dispersed in0.1mol LÀ1aqueous solution of NaCl (50mL)and then stirred for3h.3.Results and discussionThe XRD patterns of the precursor GO,the delaminated GO slurry,and the hybrid RGO–Ni–Fe LDH material are shown in Fig.1.The precursor,GO,has a layered structure with a basal spacing of0.82nm,showing the complete oxidation of graph-ite into the graphite oxide[21].After GO is treated with TMAOH solution at a molar ratio of the TMA l/GO s=5(added TMA+ion amount in the solution to the ion exchange capacity of GO)for7days,the obtained suspensions are centrifuged, and the colloidal sediments are subjected to XRD measure-ment in the wet state.The XRD pattern of the sediments ob-tained gives no clear peaks but only abroad diffraction halo in a2h range of20°–40°,in striking contrast to that of starting GO.The broad halo is most likely related to scattering from dispersed single sheets of GO and water as a solvent,which is similar to the case of layered titanic acid and manganese oxide[22,23].The broad halo indicates that the stacked sheetsFig.1–XRD patterns of the precursor GO,the delaminated GO slurry,and the hybrid RGO–Ni–Fe LDH material, respectively.4392C A R B O N48(2010)4391–4396GO is mainly reduced to graphene in the hybrid RGO–Ni–Fe LDH material.The result is also supported by the C1s XPSspectra of GO and the hybrid RGO–Ni–Fe LDH material (Fig.2).The C1s XPS spectrum of GO clearly indicates a con-siderable degree of oxidation,with three different compo-nents corresponding to carbon atoms in different functional groups:the non-oxygenated ring C(C–C),the C in C–O bonds (C–O),and the carboxylate carbon(O–C@O)[30,31].The areas of the three C1s components of graphite oxide show that the non-oxygenated ring C is about41%(284.5eV),while that of ponent is dissolved and the residual sediment is used to investigate the reduction behavior of GO to graphene.The residual sediment is washed with distilled water and dried for1day at room temperature.The XRD pattern of the ob-tained sample is shown in Fig.3.A broad peak with a basal spacing of0.36nm at2h=24.8°is observed,and it can be as-cribed to the characteristic peak of graphene[32].This result suggests that GO nanosheets are completely reduced into graphene in the hybrid RGO–Ni–Fe LDH material.In contrast, only the colloidal dispersion of GO is hydrothermally treated at150°C for2days,and the XRD pattern of the obtained sam-ple is shown in Fig.3.A clear change of the XRD patterns can be observed,and the basal spacing is reduced from0.82nm of the pristine GO to0.34nm.Meanwhile,the broad peak is very similar to that of the obtained sample after the hybrid RGO–Ni–Fe LDH material is treated with HCl solution.This result also directly proves the reduction of the GO to graphene and the formation of the RGO–Ni–Fe LDH hybrid material.The Raman spectra of the pristine GO,RGO,and the hybrid RGO–Ni–Fe LDH material in the range from400to3000cm–1 are shown in Fig.4.The G line around1597cm–1and the D line around1344cm–1are observed in the Raman spectrum of the pristine GO.In general,the G line is assigned to the E2g phonon of C sp2atoms,while the D line is a breathing mode of r-point phonons of A1g symmetry[33,34].Meanwhile, the second-order band(2D band)around2895cm–1is also ob-served,which results from a double-resonance process and links the phonon wave vectors to the electronic band struc-ture[35].On the other hand,two obvious change of Raman spectrum are found for the samples of RGO and the hybrid RGO–Ni–Fe LDH material.The second-order band around 2895cm–1disappears and the relative peak intensity of the D line to G line(I D/I G intensity)changes.In general,the I D/I G intensity ratio is a measure of disorder degree and average size of the sp2domains in graphite materials[36].The exper-imental results show that the I D/I G intensity ratio is higher than that of the pristine GO for the samples of RGO and the hybrid RGO–Ni–Fe LDH material.We think that two reasons cause the changes.One is the significant decrease of the sizeFig.2–The C1s XPS spectra of GO and the hybrid RGO–Ni–Fe LDH material.Fig.3–XRD patterns of the residual sediment obtained from the HCl leaching of the hybrid RGO–Ni–Fe LDH material and RGO.the in-plane sp 2domains due to the reduction of the exfo-liated GO [37],and the other is the remove of the oxygen func-tional groups in graphite oxide sheets and the restablishment the conjugated graphene network (sp 2carbon)[32].In addi-tion,the weak band around 563cm –1for the lattice vibration the hydroxyl layers is observed in the hybrid RGO–Ni–Fe LDH material.TEM image of the hybrid RGO–Ni–Fe LDH material shows that both irregular and thin hexagonal platelets with a mean lateral size of 350nm of Ni 2+–Fe 3+LDH nanosheets and the corrugation and scrolling image of graphene nanosheets can be observed (Fig.5).The result indicates that the platelets of Ni 2+–Fe 3+LDH are decorated in the graphene nanosheets to form the hybrid RGO–Ni–Fe LDH material.Since Ni 2+–Fe 3+LDH nanosheets are distributed on the surfaces of graphene nanosheets,the restacking of graphene nanosheets will be effectively prevented and the XRD pattern characteristic peak of graphene is not observed.For the reduced graphene,the similar crumpled silk-like image of graphene nanosheets is also observed in sample RGO in the control experiment.The result also supports the conclusion that the GO nanosheets in hybrid RGO–Ni–Fe LDH material are reduced to graphene.From the HRTEM image of the hybrid RGO–Ni–Fe LDH mate-rial,two kinds of contrast fringes can be observed.The lattice fringe with an interplanar distance of 0.23nm is ascribed to the (015)plane of the hexagonal Ni 2+–Fe 3+LDH,while the lat-tice fringe with a relative wide d -spacing of 0.52nm is caused by the ordered graphite lattices embed in the Ni 2+–Fe 3+LDH lattice fringes.The relative wide d-spacing between graphene nanosheets (0.52nm)is larger than that of graphite (0.34nm),indicating an interlayer distance increase of graphene nano-sheets after the reassembling reaction.The thermal behaviors of the pristine GO,RGO,and the hy-brid RGO–Ni–Fe LDH material are investigated by thermograv-metric analysis (TGA)in dry air.The pristine GO exhibits two steps of significant drops in mass at 229and 637°C (Fig.6),which are attributed to the removal of oxygen–functional groups and the combustion of the carbon skeleton of GO,respectively [6,24].As shown in Fig.6,RGO prepared in theFig.4–The Raman spectra of the hybrid RGO–Ni–Fe LDH material,RGO and GO.Fig.5–TEM images of the hybrid RGO–Ni–Fe LDH material,RGO and HRTEM image of the hybrid RGO–Ni–Fe LDH material,respectively.6–TGA curves of the precursor GO,RGO and the hybrid RGO–Ni–Fe LDH material,respectively.control experiment exhibits only one step of mass loss at 558°C,which corresponds to the combustion of the carbon skeleton[25].The result suggests that most of the oxygen-containing groups of the pristine GO have been removed dur-ing the hydrothermal reduction process,and it is probably an-other prove of the reduction of the pristine GO to graphene. On the other hand,some obvious changes can be observed in the DSC curve of the hybrid RGO–Ni–Fe LDH material. The two new endothermic peaks at around167and304°C are ascribed to the release of the interlayer water and the dehydroxylation of Ni2+–Fe3+LDH nanolayers in the hybrid RGO–Ni–Fe LDH material,respectively[38].Meanwhile,the exothermic peak at about398°C is attributed to the combus-tion of the carbon skeleton,which shifts to a relative lower temperature in compare with that of the pristine GO.The re-sult suggests that both the oxygenated functional groups and the thermal stability are decreased in compare with the bulk graphite powder.Similar results are observed in the litera-tures[25,39],and the reason may be attributed to the interac-tion between graphene nanosheets and Ni2+–Fe3+LDH nanosheets.The weight loss between270and520°C is about 25.3%,which corresponds to the combustion of the reduced graphene.The value is similar to the experiment one(about 26.6%)from the residual sediment obtained after the acid treatment of the hybrid RGO–Ni–Fe LDH material.According to the above results,a schematic representation of the formation process of the hybrid RGO–Ni–Fe LDH mate-rial is given in Fig.7.The pristine,GO,has a layered structure with a basal spacing of0.82nm,containing one molecular layer of water between the graphite oxide sheets.The TMA+ ions are intercalated into the interlayers of GO and followed by water washing,which results in an exfoliation of GO nano-sheets.When the slurry of exfoliated GO is soaked in a mixed solution of Ni(NO3)2Æ6H2O,Fe(NO3)3Æ9H2O,urea and TSC,metal salts can be adsorbed on the surface of GO nanosheets,and Ni2+–Fe3+LDH platelets are grown on the surface of graphene nanosheets in company with a reduction process of the pris-tine GO to graphene under a hydrothermal treatment.The Ni2+–Fe3+LDH platelets are intercalated into the nanosheets of graphene,which effectively prevents the restacking of the as-reduced graphene nanosheets[25]and the hybrid RGO–Ni–Fe LDH material isfinally obtained.4.ConclusionsA hybrid graphene/Ni2+–Fe3+LDH material has been fabri-cated by the hydrothermal treatment of a mixed suspension of the exfoliated GO,Ni(NO3)2Æ6H2O,Fe(NO3)3Æ9H2O,urea and trisodium citrate.In company with the homogeneously pre-cipitation of Ni2+–Fe3+LDH platelets on the surface of GO nanosheets,GO nanosheets are simultaneously reduced into graphene.The as-reduced graphene nanosheets exist in a complete exfoliation state in the hybrid RGO–Ni–Fe LDH material due to the formation of Ni2+–Fe3+LDH platelets, which are deposited randomly on the graphene nanosheets and prevent the restacking of the as-reduced graphene nanosheets.AcknowledgmentsWe thank National High Technology Research and Develop-ment Program of China(2007AA03Z248)and National Natural Science Foundation of China(20971082)forfinancial support this research.R E F E R E N C E S[1]Rao CNR,Biswas K,Subrahmanyama KS,Govindaraj A.Graphene,the new nanocarbon.J Mater Chem2009;19(17):2457–69.[2]Geim AK.Graphene:status and prospects.Science2009;324(5934):1530–4.[3]Rao CNR,Sood AK,Subrahmanyam KS,Govindaraj A.Graphene:the new two-dimensional nanomaterial.Angew Chem Int Ed2009;48(42):7752–78.[4]Mattevi C,Eda G,Agnoli S,Miller S,Mkhoyan KA,Celik O,et al.Evolution of electrical,chemical,and structuralproperties of transparent and conducting chemically derived graphene thinfilms.Adv Funct Mater2009;19(16):2577–83. 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