Zhenyuan Ji •Jili Wu •Xiaoping Shen •

Hu Zhou •Haitao Xi

Received:20June 2010/Accepted:31August 2010/Published online:11September 2010ÓSpringer Science+Business Media,LLC 2010

Abstract Graphene-based nanocomposites are emerging as a new class of materials that hold promise for many applications.In this article,we present a facile approach for the preparation of graphene/NiO nanocomposites using graphite oxide and nickel chloride as starting materials.The as-synthesized composites were characterized using X-ray diffraction,Fourier transform-Infrared spectroscopy,transmission electron microscopy,ultraviolet–visible spectroscopy,thermogravimetry,and differential scanning calorimetry analyses.It was shown that graphene sheets were decorated by the in situ-formed NiO nanoparticles to form a film-like composite structure and as a result,the restacking of the as-reduced graphene sheets was effec-tively prevented.The NiO-coated graphene nanocompos-ites can be expected to remarkably improve the electrochemical properties of NiO and would be the promising candidates for a variety of applications in future nanotechnology.

Introduction

Graphene,a single 2D carbon sheet with the same structure as the individual layers in graphite,has become a sparkling rising star on the horizon of materials science due to its extraordinary electrical,thermal,and mechanical proper-ties [1,2].One possible route to harnessing these excellent properties of graphene for applications would be to incor-porate graphene sheets in a composite material [3–5].It has been demonstrated that graphene-based polymer compos-ites exhibit extraordinarily low electrical percolation threshold (0.1vol%)due to large conductivity and aspect ratio of the graphene sheets (atomic thickness and micrometer-sized lateral dimensions)[3,6].Recently,graphene-based inorganic composites have been attracting more and more attention since the attachment of inorganic nanoparticles instead of polymer onto the graphene sheets may not only prevent the restacking of these sheets during the chemical reduction process,but also lead to the for-mation of a new class of graphene-based materials [7].Some graphene/inorganic nanoparticles composites have shown excellent properties,which can be applied in field emission displays,sensors,supercapacitors,batteries,catalysis,and so on [8–15].

Nickel oxide (NiO)is an antiferromagnetic semicon-ductor with a wide band gap of *3.6eV [16]and is being used in various fields such as catalysis [17],electrochromic films [18,19],fuel cell electrodes [20],and active optical fibers [21].Recently,considerable efforts have been focused on the synthesis of nanosized NiO and its com-posites [22–24]due to their potential applications in sec-ondary batteries and electrochemical capacitors [25].Especially,NiO/carbon nanotubes (CNTs)composites have been widely studied,which have shown improved capaci-tance owing to their enhanced electronic conductivity of

Z.Ji ÁJ.Wu ÁX.Shen (&)

School of Chemistry and Chemical Engineering,Jiangsu University,Zhenjiang 212013,People’s Republic of China e-mail:xiaopingshen@163.com

H.Zhou

School of Material Science and Engineering,Jiangsu University of Science and Technology,Zhenjiang 212003,People’s Republic of China

H.Xi

Key Laboratory of Fine Petrochemical Engineering of Jiangsu Province,Changzhou University,Changzhou 2131,People’s Republic of China

J Mater Sci (2011)46:1190–1195DOI 10.1007/s10853-010-42-7

Experimental

Materials and measurements

All the chemicals used in our experiments were reagent grade and used without further purification.The morphol-ogy and structure of the products were determined by transmission electron microscopy(TEM,JEM—2100)and X-ray diffraction(XRD,D/MAX2500,Rigaka)with Cu K a radiation.Samples for TEM were prepared by dropping the products on a carbon-coated copper grid after ultrasonic dispersing in absolute ethanol.Fourier transform-Infrared (FT-IR)spectra were recorded on a Nicolet FT-170SX spectrometer with KBr pellets in the4000–400cm-1 region.Ultraviolet–Visible(UV–Vis)spectroscopy mea-surements were performed on a UV-2450UV–Vis spec-trophotometer in water dispersion.The thermogravimetry (TG)and differential scanning calorimetry(DSC)mea-surements were carried out with a NETZSCH STA449C thermal analyzer,which allows simultaneous TG and DSC measurements.

Synthesis of graphite oxide

Graphite oxide was synthesized from naturalflake graphite powder by a modified Hummers method[32].In a typical synthesis, 2.0g of graphite powder was put into cold (0°C)concentrated H2SO4(100mL).Then,8.0g of KMnO4was added gradually under stirring,and the tem-perature of the mixture was kept to be below10°C by cooling.The reaction mixture was continued for2h at the temperature below10°C.Successively,the mixture was stirred at35°C for1h,and then diluted with100mL of deionized(DI)water.Because the addition of water in concentrated sulfuric acid medium released a large amount of heat,the addition of water was performed in an ice bath to keep the temperature below100°C.After adding all of the100mL of DI water,the mixture was stirred for1h, and was then further diluted to approximately300mL with DI water.Then,20mL of30%H2O2was added to the mixture to reduce the residual KMnO4.The mixture released a large amount of bubbles,and the color of the mixture changed into brilliant yellow.Finally,the mixture wasfiltered and washed with5%HCl aqueous solution (800mL)to remove metal ions followed by1.0L of DI water to remove the acid.The resulting solid was dried at 60°C for24h.For further purification,the as-obtained graphite oxide was re-dispersed in DI water and then was dialyzed for1week to remove residual salts and acids. Synthesis of graphene/NiO nanocomposites

In a typical synthesis of graphene/NiO nanocomposites, 40mg of graphite oxide was dispersed in80mL of DI water by ultrasonication.Ammonia(28wt%in water)was dropped to the graphite oxide dispersion to adjust the pH to around10.Subsequently,20mL of nickel chloride solution (6mM)and25l L of hydrazine hydrate(85%)were added to the dispersion with stirring.The mixture was transferred into a250-mL round-bottomedflask and refluxed at100°C for5h.The products were isolated by centrifugation, washed three times with both water and ethanol,andfinally dried in a vacuum oven at45°C for24h.The products obtained,which were graphene/Ni(OH)2nanocomposites as will be shown later,were annealed in nitrogen atmo-sphere at500°C for5h in a tube furnace.Graphene/NiO nanocomposites were obtained after cooling.

Results and discussion

X-ray diffraction(XRD)measurements were employed to investigate the phase and structure of the synthesized samples.As shown in Fig.1,the XRD pattern of the as-synthesized graphite oxide(Fig.1a)shows a sharp peak

at2h=10.8°,corresponding to the(001)reflection of graphite oxide[33].From Fig.1b and c,all the diffraction peaks of the graphene/Ni(OH)2and graphene/NiO nano-composites can be indexed to hexagonal Ni(OH)2(JCPDS 14-0117)and monoclinic NiO(JCPDS65-6920),respec-tively,and the broadening of the diffraction peaks suggests a very small size of the Ni(OH)2and NiO nanoparticles.In the two samples,the sharp peak at2h=10.8°disappeared, and no characteristic peak of graphite was observed,sug-gesting that the graphene oxide was well reduced,and the restacking of the as-reduced graphene sheets was effec-tively prevented[34].

The FT-IR spectra of the products are shown in Fig.2. The oxygen-containing functional groups of graphite oxide were revealed by the bands at1076,1232,1402,and 1731cm-1(Fig.2a),which correspond to C–O stretching vibrations,C–OH stretching peak,carboxyl C–O,and C=O groups,respectively.The peak at1618cm-1can be assigned to the vibrations of the adsorbed water molecules and also the contributions from the skeletal vibrations of unoxidized graphitic domains[35].Figure2b shows the FT-IR spectra of pure Ni(OH)2,which was synthesized in the same way as the graphene/Ni(OH)2nanocomposites in the absence of graphite oxide and hydrazine hydrate.The narrow peak at 32cm-1is assigned to the stretching vibrational mode of non-hydrogenbound hydroxyl groups in the brucite-like sheets,while the broad band at about3450cm-1to the stretching mode of hydrogenbound hydroxyl groups in the same layered structure.All other absorption peaks arising from the Ni(OH)2sample are also consistent with those reported in the literature[36,37].However,in the FT-IR spectra(Fig.2c,d)of both the graphene/Ni(OH)2and the graphene/NiO nanocomposites,except for the characteristic peaks of Ni(OH)2and NiO,all those bands related with the oxygen-containing functional groups almost vanished, revealing that these oxygen-containing functional groups were almost removed in the process of reduction with hydrazine hydrate,and thus the graphene oxide was trans-formed into graphene in the syntheses.In Fig.2c and d,the peaks at about1575and4cm-1can be attributed to the skeletal vibration of the graphene sheets and stretching vibration of Ni–O,respectively.

The TEM analyses were performed on the as-prepared nanocomposites to determine their features in nanometer domain.Figure3a and b shows TEM images of the as-synthesized graphene/Ni(OH)2.It can be clearly seen that the graphene nanosheets were well decorated by Ni(OH)2nanoparticles,which were densely and evenly deposited on both sides of these sheets to form a composite. Moreover,almost no Ni(OH)2nanoparticle was found outside of the graphene nanosheets,indicating a good combination between the graphene and Ni(OH)2.The selected area electron diffraction(SAED)pattern(Fig.3c) clearly shows the ring pattern arising from the hexagonal Ni(OH)2,revealing the polycrystalline nature of the Ni(OH)2nanoparticles.Figure3d and e shows TEM ima-ges of the as-synthesized graphene/NiO nanocomposites. We can see that graphene sheets were densely and evenly decorated by NiO nanoparticles,and graphene sheets did not show significant changes before and after annealing, indicating that the annealing process did not destroy the morphology and microstructure of the nanocomposites. The SAED pattern(Fig.3f)shows the ring pattern arising from the monoclinic NiO,further confirming that NiO nanoparticles were formed after the annealing of Ni(OH)2 in nitrogen atmosphere.

Figure4shows the UV–Vis absorption spectra of the as-synthesized nanocomposites,together with pure graph-ene for comparison.The pure graphene was synthesized in the same way as the graphene/Ni(OH)2nanocomposites in the absence of nickel chloride.It can be seen that the graphene(Fig.4a)shows a strong absorption peak at 266nm,which is generally regarded as the excitation of p-plasmon of graphitic structure[38].After being deco-rated by Ni(OH)2,the absorption peak at266nm almost disappears(Fig.4b).This can be reasonably explained by the microstructure of the graphene/Ni(OH)2nanocompos-ites.As shown above,the graphene sheets were completely covered by Ni(OH)2nanoparticles,which would largely weaken the absorption from graphene.Similarly,this phenomenon also happened in the case of graphene/NiO nanocomposites.However,because of the less covering on graphene sheets after Ni(OH)2was transformed into NiO,a stronger absorption from graphene can be observed in the graphene/NiO nanocomposites(Fig.4c).

The thermal properties and the composition of graphene/Ni(OH)2and graphene/NiO nanocomposite were investi-gated by TG–DSC analysis,which was performed with a heating rate of 10°C min -1in nitrogen and air atmosphere,respectively.As shown in Fig.5a,with increasing tem-perature,the graphene/Ni(OH)2nanocomposites show an obvious weight loss until ca.170°C,which can be attributed to the loss of the residual (or absorbed)solvent.Then,there is a slight weight loss in the temperature range of 170–340°C due to the decomposition of residual organic functional groups on the graphene.This is con-sistent with the thermal behavior of pure graphene sheets in N 2atmosphere as reported in the literature [39,40].Sub-sequently,an abrupt weight loss occurs at 439°C,which can be assigned to the decomposition of Ni(OH)2.Corre-spondingly,the DSC curve shows a strong exothermal peak centered at 439°C.This temperature is higher than

those

Fig.3a ,b TEM and c SAED patterns of the graphene/Ni(OH)2nanocomposites;

d ,

e TEM and

f SAED patterns of the graphene/NiO nanocomposites

In conclusion,the graphene/NiO nanocomposites have been successfully synthesized by a one-pot solution method,followed by annealing in nitrogen atmosphere.It was shown that graphene sheets were well decorated by the NiO nanoparticles to form afilm-like composite structure and as a result,the restacking of the as-reduced graphene sheets was effectively prevented.Moreover,annealing treatment in nitrogen atmosphere had no significant effect on the integrity of graphene sheets.It can be expected that the facile method presented here can be extended to the synthesis of other graphene/metal oxide nanocomposites with various applications.

Acknowledgements The authors are grateful for thefinancial sup-port from the Natural Science Foundation of Jiangsu Province(No. BK2009196),and the Key Laboratory Foundation of Fine Petro-chemical Engineering of Jiangsu Province(KF0905).References

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