Agglomerates of amorphous carbon nanoparticles synthesized by a solution-phase method

Agglomerates of amorphous carbon nanoparticles synthesized by a solution-phase methodBoyang Liu a ,⁎,Haipeng Huang b ,Fuhua Zhang a ,Yun Zhou a ,Wenge Li a ,Jingwei Zhang ca Institute of Marine Materials Science and Engineering,Shanghai Maritime University,Shanghai 201306,Chinab Xiamen Products Quality Supervision &Inspection Institute,Xiamen 361004,China cLogistics Engineering College,Shanghai Maritime University,Shanghai 201306,Chinaa b s t r a c ta r t i c l e i n f o Article history:Received 23March 2011Accepted 31August 2011Available online xxxx Keywords:Amorphous materials Carbon materials Colloidal processing OxidationA novel solution-phase method is developed for preparation of agglomerates of amorphous carbon nanopar-ticles under ambient atmosphere by the reaction of ferrocene and ammonium chloride in diglycol at 200°C.Samples are characterized by X-ray diffraction,field-emission scanning electron microscopy,transmission electron microscopy and N 2adsorption –desorption isotherms.It is found that the nanoparticles are complete amorphous and agglomerate together due to the strong surface tension.The agglomerates of amorphous car-bon nanoparticles with a diameter of 20–50nm have a wide size distribution of mesopores with a Brunauer –Emmett –Teller surface area of 75.2m 2g −1.It is proposed that the dissolved reactants uniformly dispersing in the solutions could react at a molecular level to form uniform carbon nanoparticles.©2011Elsevier B.V.All rights reserved.1.IntroductionOver the past few decades,porous nanocarbons with speci fic mor-phology have attracted much attention due to their distinctive prop-erties,like sorption ability,chemical stability,low density,suitability for large scale production and wide variety of structural forms.These advantages bring a series of opportunities for the potential technological applications of the porous nanocarbons in hydrogen storage,electric double-layer capacitor,catalysis and drug delivery [1–4].The morphology and structure of porous nanocarbons are very sensitive to the synthesis method,processing,carbonization and activation conditions [5].Therefore,various synthesis methods,such as vapor-phase growth,carbonization of organic aerogels,template-assisted and solvothermal method,have been carried out for direct and controlled synthesis of porous nanocarbons with-out further activation.Vapor-phase synthesis,such as chemical vapor deposition (CVD)and flame synthesis,is a powerful technique for large scale nanocar-bon preparation with shape controlling.However these methods not only require complicated and expensive equipment,but also have dif ficulty in producing uniform samples.Carbon aerogel,con-sisting of interconnected uniform nanoparticles,is usually prepared by polycondensation of resorcinol with formaldehyde and carboniza-tion of the resulting organic gel [6],which involves a time-consuming and costly supercritical drying step.The template-assisted method can be adopted to fabricate ordered porous carbon materials [7].However,the complicated process makes it unsuitable for large-scale production for industrial applications.On the other hand,the synthesis temperature is still relatively high in these methods mentioned above.The solvothermal method can evidently lower the synthesis temperature,but the preparation should be completed in an autoclave with high pressure resistance and the reaction time is long [8].All the methods cannot be carried out in ambient atmo-sphere because oxygen should be avoided during preparation in order to prevent oxidation of carbon.Considering the disadvantages in these methods aforementioned and the close relationship between the wide application and the ef ficient synthesis,a new strategy for porous nanocarbons is still worthy of investigation.In the present paper,a novel method is developed to synthesize amorphous carbon nanoparticles in solution under ambient atmosphere.2.Experimental detailsIn a typical synthesis process,20g ammonium chloride was first dissolved in 500ml of diglycol in a four-necked round-bottomed flask equipped with an air condenser,magnetic stir bar,dropping funnel,thermometer,gas inlet tube,heating and stirring mantle.Then the solution was heated to 200°C,and 10g ferrocene dissolved by 500ml of ethanol was added drop by drop under strong magnetic stirring for 2h with a 50sccm air flow.Fig.1is the scheme of the ex-perimental setup.After the resulting black homogeneous suspension was cooled down to room temperature,2.14g of amorphous carbon nanoparticles were collected by filtration,washed with HCl solution and deionized water for several sequential times,and finally dried in an oven at 100°C for 12h.Phase characterization was performed with an X'Pert PRO X-ray diffractometer (XRD)with Cu K αradiationMaterials Letters 66(2012)199–202⁎Corresponding author.Tel.:+862138284803;fax:+862138284800.E-mail address:byliu@ (B.Liu).0167-577X/$–see front matter ©2011Elsevier B.V.All rights reserved.doi:10.1016/j.matlet.2011.08.111Contents lists available at SciVerse ScienceDirectMaterials Lettersj o u r n a l h om e p a g e :ww w.e l s e v i e r.c o m /l o c a t e /m a t l e t(λ=0.15418nm).The microstructure analysis was conducted with a JEOL JSM 7500F field emission scanning electron microscope (SEM)and a JEOL JEM 2010transmission electron microscope (TEM),oper-ating at 5and 200kV,respectively.The N 2adsorption and desorption isotherms were determined on a JW-BK-112speci fic surface area and pore size distribution analyzer (Beijing JWGB Sci.&Tech.Co.,Ltd.)at 77.4K with a sample outgassed for 5h at 200°C.Surface areas were calculated using the Brunauer –Emmett –Teller (BET)equation and pore size distributions were obtained using the Barrett –Joyner –Halenda (BJH)method using the adsorption branch of the isotherms.3.Results and discussionFig.2gives the XRD pro file of the carbon nanoparticles in the as-prepared state.The broad peak around 2θ=26°can be assigned to the amorphous phase of carbon,representing the amorphous carbon nanoparticles prepared at low synthesis temperature by the present method.For SEM observation,the as-synthesized powder was directly transferred to double sided adhesive conductive carbon tape without further coating.SEM micrographs are shown in Fig.3.Fig.3a at relative low magni fication exhibits large amount of cotton-like agglomerates less than 200nm in diameter.At high magni fication in Fig.3b,it is obvious that the agglomerates consist of nanoparticleswith dimensions ranging from 20to 50nm.In fact,the as-prepared powder suffers greatly from shrinkage during drying due to strong surface tension,resulting in the formation of agglomerated carbon nanoparticles.Typical TEM bright field morphology of the amorphous carbon nanoparticles is shown in Fig.4.It is clear that the agglomerate consists of nanoparticles as shown in Fig.4a,which is consistent with SEM micrographs.The shape of the nanoparticles is not regular and surface is not smooth either.The high resolution TEM image further proves the nanoparticles are completely amorphous,as shown in Fig.4b.Fig.5shows the nitrogen adsorption –desorption isotherm of the sample,which lies between a type II and type IV isotherm shape,sug-gesting the presence of both macro-and mesoporosity.The large hys-teresis loop at a relative pressure between 0.4and 1.0is the characteristic of a wide distribution of slit-shaped mesopores [9],which matches very well the calculated pore size distribution by the BJH method in the insert of Fig.5.The average pore diameter and BET surface area of the amorphous carbon nanoparticles is 9.8nm and 75.2m 2g −1,respectively.The formation of mesopores should be mainly attributed to the spaces between the agglomerated nano-particles,as well as the pores in the nanoparticles created by the gen-erated gases druing the synthesis.In our previous paper,we synthesized amorphous carbon nano-tubes at 200°C in air by the reaction of ferrocene and ammonium chloride [10].The chemical reaction equations are also proposed as follows:4ðC 5H 5Þ2Fe þO 2þ4NH 4Cl →4ðC 5H 5Þ2FeCl þ4NH 3þ2H 2Oð1ÞFig.1.Scheme of the experimentalsetup.Fig.2.XRD pro file of the as-prepared amorphous carbonnanoparticles.Fig. 3.SEM micrographs of the amorphous carbon nanoparticles at different magni fications.200 B.Liu et al./Materials Letters 66(2012)199–202ðC 5H 5Þ2FeCl þ3O 2þ2NH 4Cl →10C þFeCl 3þ2NH 3þ6H 2Oð2ÞAccording to the experiment,amorphous carbon can be prepared at low temperature without inert gas protection because oxygen is a reactant in the process.Therefore,the solubility of ferrocene and am-monium chloride in stable high boiling point (above 200°C)organic solvents will promise a solution-phase synthesis of amorphous car-bon under ambient atmosphere,which is very convenient as a result of no need of special and complicated equipment.In present paper,ethanol and diglycol are used to dissolve ferrocene and ammonium chloride respectively.And ammonium chloride can completely dis-solve and uniformly disperse in diglycol at 200°C.When ferrocene dissolved in ethanol was added and mixed with diglycol,ferrocene would dissolve in the diglycol and react with the ammonium chloride at a molecular level,preferring to form nanoparticles according to Eqs.(1)and (2).At the same time,the ethanol and a certain amount of unreacted ferrocene were evaporating and blowed off by the air flow because of their low melting and sublimation point,respectively.Additionally,ammonia and water vapor were generated during the formation of carbon,resulting in the unsmooth surface and meso-pores of the nanoparticles.On the basis of the above results,relatively uniform carbon nano-particles can be obtained through the reaction of ferrocene and am-monium chloride with solvent and supply of oxygen.In comparison with the previous synthesis without solvent,the solubility of the re-actants in diglycol makes it easy to control reaction rate and unifor-mity for nanoparticles.It is further found that 6.04g and 3.98g of carbon nanoparticles can be synthesized in triglycol and polyethylene glycol 400with identical conditions,respectively,as shown in Fig.S1(Supplementary materials),indicating that the morphologies are sim-ilar with that prepared in diglycol.Consequently,the properties of solvents should play an important role in the formation of carbon nanoparticles due to the interactions between the reactants and the solvent.The speci fic mechanism of this reaction warrants some fur-ther discussion in order to control the morphology,pore structure and size distribution of the nanoparticles.4.ConclusionIn conclusion,agglomerates of amorphous carbon nanoparticles with diameters of 20to 50nm are prepared by the reaction of ferro-cene and ammonium chloride in diglycol under ambient atmosphere.The present synthesis method is competitive for fabrication of porous carbon nanomaterials because of its evident advantages,including the inexpensive starting materials and instruments,low synthesis temperature and ambient atmosphere,environment friendly and ef fi-cient process,ease in procedural control.It will be much more attrac-tive for diverse applications in the future.AcknowledgmentsThis work is supported by “Chen Guang ”project of Shanghai Municipal Education Commission and Shanghai Education Develop-ment Foundation Science &Technology (No.09CG53),Natural Science Foundation of Shanghai (No.10ZR1413400),National Natural Science Foundation of China (No.51002094),Innovation Program of Shanghai Municipal Education Commission (No.10YZ112)and the Scientists-Company Cooperation Project of the Ministry of Science and Technology of China (No.2009GJC00003).Appendix A.Supplementary dataSupplementary data to this article can be found online at doi:10.1016/j.matlet.2011.08.111.Fig. 4.TEM micrographs of the amorphous carbon nanoparticles at different magni fications.Fig.5.Nitrogen adsorption –desorption isotherm and pore size distribution 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