One-pot solvothermal synthesis of sandwich-like graphene nanosheets Fe3O4 hybrid

One-pot solvothermal synthesis of sandwich-like graphene nanosheets/Fe 3O 4hybrid material and its microwave electromagnetic propertiesYingqing Zhan,Fanbin Meng,Yajie Lei,Rui Zhao,Jiachun Zhong,Xiaobo Liu ⁎Research Branch of Functional Materials,Institute of Microelectronic and Solid State Electronic,University of Electronic Science and Technology of China,Chengdu 610054,PR Chinaa b s t r a c ta r t i c l e i n f o Article history:Received 18January 2011Accepted 3March 2011Available online 9March 2011Keywords:Graphene nanosheets Magnetite Sandwich-likeMagnetic propertiesMicrowave electromagnetic propertiesA novel sandwich-like graphene nanosheets (GNs)/Fe 3O 4hybrid material was synthesized through a facile one-pot solvothermal method using FeCl 3as iron source,ethylene glycol as the reducing agent and graphene nanosheets as templates.The Fe 3O 4nanoparticles,with the average diameters of ca.40nm,were self-assembled on the graphene nanosheets through electrostatic attraction and formed sandwich-like nanostructure.The ferromagnetic signature emerged with the saturated magnetization of ~72.3emu g −1,and the coercive force of ~196.1Oe at 300K.The magnetic loss was caused mainly by natural resonance which is in agreement with the Kittel equation.The novel electromagnetic hybrid material is believed to have potential applications in the microwave absorbing performances.©2011Elsevier B.V.All rights reserved.1.IntroductionGraphene,as a unique two-dimensional carbon material,has received much attention from both the experimental and theoretical scienti fic communities [1,2].Because of the special surface properties and layered structure,they become potential nanoscale building blocks for new hybrid materials [3,4].Recent studies have shown that deposition of inorganic nanoparticles,such as Au,Cu 2O,etc.,onto graphene or GO sheets reveals special features in new hybrids and the nanocomposites have potential applications in catalysis,fuel cells,lithium ion batteries,etc.[5–7].It is well-known that magnetite nanoparticles have long been of scienti fic and technological interest because of their unique magnetic and electric properties [8,9].In the previous researches,Jia and co-workers [10]and Zhan al.[11]have done some work in the solvothermal synthesis of the heterojunction of Fe 3O 4with CNTs,revealing that CNTs/Fe 3O 4hybrid materials can extend the applica-tions and reinforce the pristine properties of simplex Fe 3O 4and CNTs materials.In comparison with CNTs,graphene possesses similar physical properties but larger surface areas,which can be considered as an unrolled CNT [12].Furthermore,more effective absorption might be obtained due to the advantages of platelet-shaped materials than the rod-shaped and sphere-shaped ones in microwave absorp-tion applications.Therefore,graphene as a low cost alternative to CNTs in nanocomposites is highly expected.Herein,to broaden the horizon of using graphene as nanoscale building blocks for new hybrids,we reported a facile one-pot solvothermal synthesis of sandwich-like graphene nanosheets/Fe 3O 4hybrid material.The structure,magnetic and microwave electromag-netic properties of as-prepared hybrid material were investigated.2.ExperimentalGraphene nanosheets were prepared according to the reference reported previously [13].Typical synthesis of GNs/Fe 3O 4hybrid material was carried out in a solvothermal system by reduction reaction between FeCl 3and ethylene glycol in the presence of GNs.0.5g FeCl 3·6H 2O was dissolved into 200mL ethylene glycol,followed by addition of 5.0g polyethylene glycol and 0.5g GNs to form a black solution with the help of an ultrasonic bath.After that,18g NaAc was slowly added into the solution with vigorous stirring at 60°C for 2h.The mixture was sealed in a Te flon-lined stainless steel autoclave and maintained at 200°C for 15h,and cooled to room temperature.The black product was rinsed with ethanol for several times,and dried at 60°C for 8h.The synthesized product was characterized by X-ray diffraction (Rigaku RINT2400with Cu K αradiation),scanning electron microscopy (JSM,6490LV)and transmission electron microscopy (Hitach,H600).Magnetic study was performed by a vibrating sample magnetometer (VSM,Riken Denshi,BHV-525).Electromagnetic (EM)parameters were measured by a vector network analyzer (HP8722ES),in which the GNs/Fe 3O 4hybrid material was mixed with paraf fin at certain mass filling ratio (30wt.%)and compressed to standard ring shapes (outer diameter:7mm,inner diameter:3mm,and thickness:2mm).Materials Letters 65(2011)1737–1740⁎Corresponding author.Tel./fax:+862883207326.E-mail address:liuxb@ (X.Liu).0167-577X/$–see front matter ©2011Elsevier B.V.All rights reserved.doi:10.1016/j.matlet.2011.03.019Contents lists available at ScienceDirectMaterials Lettersj o u r na l ho m e p a g e :w w w.e l s ev i e r.c o m /l o c a t e /m a t l e t3.Results and discussionFig.1a shows the illustration of synthesis process of the graphene nanosheets/magnetite hybrid material.It is well-known that some carbon double bonds were oxidized after the acidic intercalation and thermal treatment,resulting in the presence of oxygen-containing functional groups such as–COOH,and–OH on the surface of the graphene nanosheets[14].Meanwhile,the amount and distribution of the carboxylic groups have great influence on the further positive-selective modification of graphene nanosheets with Fe3O4.The positive metal ions in the system would attach to and interact with the carboxylic groups via electrostatic attraction and serve as nucleation precursors.In our case,ferric ions in the solution would like to attach to some particular positions of the graphene nanosheets with high density of carboxyl and in situ reduced into Fe3O4 crystallites during following solvothermal process.According to our previous experiments,we found that Fe3O4beads were constructed with tiny nanocrystallites through oriented aggregation on the both sides of graphene nanosheets.Finally a sandwich-like nanostructure of graphene nanosheets/Fe3O4hybrid material was formed.At the same time,the weak capping agent PEG could prevent the fast growth or oxidation of crystallite and be helpful to the solvothermal process.Fig.1b shows the XRD patterns of graphene nanosheets and the product obtained from solvothermal process.The diffraction peaks of as-received graphene nanosheets at26.24°and49.32could be attributed to the graphite structure(002)and(100)planes of the graphene nanosheets.After surface decoration,six new peaks at30.1°, 35.5°,43.1°,53.4°,56.9°and62.4°were observed,while the characteristic peak of GNs remained unchanged.The positions and relative intensities of these non-GNs related new peaks match well with the(220),(311),(400),(422),(511)and(440)planes of the standards XRD data for the cubic spinal crystal structure of bulk magnetite(JCPDSfile No.19–0629).No peaks corresponding to impurities are detected.So it could be concluded that a kind of magnetite-GNs heterostructure was formed during the solvothermal process.According to the Debye–Scherrer equation D=Kλ/βcosθ,the average diameter of magnetite beads depositing on the GNs calculated from XRD data was about35nm.Fig.1.(a)Illustration of synthesis process of sandwich-like graphene nanosheets/Fe3O4hybrid material and(b)XRD patterns of GNs and GNs/Fe3O4hybridmaterial.Fig.2.(a)SEM image of GNs,(b)TEM image of GNs,(c)SEM image of GNs/Fe3O4hybrid material and(d)TEM image of GNs/Fe3O4hybrid material.1738Y.Zhan et al./Materials Letters65(2011)1737–1740The morphology of graphene nanosheets and GNs/Fe 3O 4hybrid material were investigated by scanning electron microscopy (SEM)and transmission electron microscopy (TEM).As seen from Fig.2a –b,the GNs are 10–20nm thick and 5–20μm in diameter.Fig.2c shows the SEM image of GNs/Fe 3O 4hybrid material.It can be clearly seen that the graphene nanosheets were well coated by Fe 3O 4nanopar-ticles,which densely and evenly deposited on both sides of these sheets to form a sandwich-like composite structure.Moreover,almost no Fe 3O 4nanoparticles were found outside of the graphene nanosheets.This indicates that solvothermal synthesis of graphene nanosheets/Fe 3O 4hybrid material is of high ef ficiency.TEM image (Fig.2d)indicates that magnetite particles formed on GNs are nearly monodisperse and the particle sizes are 35–40nm,matching well with that obtained from XRD data.The magnetic hysteresis curve of the sample (Fig.3)measured at 300K illustrates the strong magnetic response to a varying magnetic field.The saturation magnetization value of graphene/Fe 3O 4inorganic hybrid is 72.3emu g −1,which is lower than that of the pure Fe 3O 4.The decrease of the saturation magnetization can be mainly attributed to the existence of graphene nanosheets and smaller size than that of pure Fe 3O 4which was synthesized in the same method [8].This agrees with the known fact that the magnetization of small particles decreases as the particle size decreases.In addition,the coercive force is 196.1Oe at 300K.As synthesized graphene nanosheets/Fe 3O 4hybrid material can be easily dispersed in ethanol and remains very stable for at least 2weeks at room temperature.These "magnetic GNs"could be quickly separated from their dispersion by holding the sample close to a commercial magnet as shown in the inset of Fig.3,indicating that it is possible to manipulate these magnetic GNs by an external magnetic field.Fig.4a shows the frequency dependence of the real (ε′)and the imaginary parts (ε′′)of the relative complex permittivity of the wax-GNs/Fe 3O 4hybrid material.It can be found that the values of ε′and ε′′declined from 85.2to 52.6and from 35.9to 14.8,respectively,over the 1–18GHz frequency range.According to the free electron theory [15],ε′′=1/2ε0πρƒ,where ρis the resistivity.It can be speculated that the lower ε′′values of GNs/Fe 3O 4hybrid material at 1–18GHz indicated higher electric resistivity with respect to other microwave absorption materials,e.g.,ε′′=40–90for Fe/CNTs and ε′′=45–50for carbon-coated Fe composites [14].In general,the high electric resistivity is of great advantage for striking a balance between permeability and permittivity,thus decreasing the re flection coef ficient of the absorber compared with other magnetic materials for microwave absorbing application.Fig.4b shows the complex permeability of the GNs/Fe 3O 4hybrid material with the magnetite depositing in the frequency of 1–18GHz.It reveals that real part (μ′)exhibits an abrupt decrease from 2.7to 1.3at the 1–8GHz range and retains an approximate constant 1.3over 8–18GHz.Meanwhile,as seen from the imaginary part of perme-ability,the resonance peak is near 1.16GHz.The Kittel equation hasbeen widely used to calculate the natural resonance frequency of magnetite materials;the natural resonance of the sphere-shaped magnet is [16].f r =γHawhere γ=28GHz T −1,is the gyromagnetic ratio and Ha=4|K 1|/3μ0M s is the effective anisotropy field.The saturation magnetization μ0M s is 0.542T and the anisotropic coef ficient K 1for the fcc-type bulk magnetite is about −9×103J m −3,so the theoretical calculation of the natural resonance frequency should be γHa ≈1.2GHz,which matches well with the experimental data of 1.16GHz.This demonstrates that the as-synthesized product shows mainly sphere-like morphology and the magnetic loss is caused mainly by the natural resonance in the microwave region.4.ConclusionIn summary,we developed a facile and effective solvothermal method for preparing the GNs/Fe 3O 4hybrid material,in which the Fe 3O 4nanoparticles were self-assembled on the surface of graphene nanosheets through electrostatic attraction,forming a sandwich-like structure.Moreover,the as-prepared GNs/Fe 3O 4hybrid material possesses high saturated magnetization.The magnetic loss was caused mainly by natural resonance,which is in agreement with the Kittel equation.It is believed that such hybrid material can find 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