Synthesis of carbon nanofibers by ethanol catalytic combustion technique

J Cent. South Univ. TechnoL. (2008)15: 15-19DOI:10.1007/s1l771-008000402 SpringerSynthesis of carbon nanofibers by ethanol catalytic combustion techniqueLI Fei(李飞, ZoU Xiao-ping(邹小平, CHENG Jin(程进, ZHANG Hong-dan(张红丹)REN Peng"fei(任鹏飞), WANG Mao-fa(王茂发), ZHU Guang(朱光)( Research Center for Sensor Technology, Beijing Information Technology Institute, Beijing 100101, China)Abstract: a general, simple and economic synthetic method for synthesizing carbon nanofibers was presented. In the method,ethanol was employed as carbon source; metal salts such as nickel nitrate, ferric nitrate and ferric chloride were used as catalystrecursor respectively; copper plate was employed as the support material. A lot of products were obtained by catalytic combustiodeposition of ethanol vapor. Then the as-prepared carbon nanofibers were characterized by field-emission scanning electronicroscopy, transmission electron microscopy, Raman spectroscopy, energy dispersion X-ray spectroscopy and selected-area electrondiffractometry. By analyzing the results of characterization, the conclusions are as follows: 1)the large catalyst particles tend to formlarge-diameter CNFs, small catalyst particles are inclinable to form small-diameter CNFs; 2)the morphology of the catalyst canaffect the final morphology of the CNFs. Moreover, the possible growth mechanisms were proposed and the degree of graphitizationof samples was estimated by Raman spectroscopy characterizationKey words: synthesis; ethanol catalytic combustion; carbon nanofibers; growth mechanism; degree of graphitizationI Introduction2 Experimentalrecent years, carbon nanofibers(CNFs)haveCNFs were formed by ECC over catalyst derivedbecome a promising research direction. It is considered from the catalyst precursors(ferric nitrate or nickelto have widely potential applications for their remarkable nitrate or ferric chloride supported on substrate.Theproperties that are similar to those of carbon nanotubes, remarkable feature of the synthetic method is the use ofsuch as high elastic modulus, strong strain, and highthe very common laboratory ethanol burnersconductivity. Syntheses of CNFs have been investigatedThe experimentsperformed in air underfor a wide range of applications today because CNFs of atmospheric pressure. The first step was to prepare thedifferent morphologies are required. So many synthetic catalyst particles that approximately determined the finalapproaches have been developed for synthesizing carbon diameter of the CNFs. Here, the wet chemical methodnanofibers, for instance, electric arc discharge, plasma was used, that is, a liquid solution containing catalystenhanced chemical vapor deposition (PECVD) and precursor in the form of metal salts such as ferric nitratechemical vapor deposition(CVD)2). However, current or nickel nitrate or ferric chloride was dropped to thesubstrate. Some quantities of metal salt were dissolved insynthetic methods suffer from high cost, complex the pure ethanol. And then the prepared solution wassonicated for tens of minutes to form a suspension ofCNFs(also known as carbon filaments) can be catalyst precursor, which provided small catalystgrown from the catalytic decomposition of certain precursor particles. One drop of the saturated catalysthydrocarbons over small catalyst particles such as iron, precursor solution was dropped with a dip-pen to thecobalt, nickel, some metal alloys and metal oxide. The copper support material, which was then placed in andiameter of the nanostructure is generally controlled by inner flame for several minutes without introducing anyhe size of the catalyst particleother gases for the CNFs growth. After a desired time,In this work, an altermative way was developed to wool-like products accumulated on the copper platesynthesize carbon nanofibres by ethanol catalytice carbon deposits were collected from thecombustion(ECC) technique by using pure ethanolsubstrate. And then the samples were examined by athe fuel, metal salt as the catalyst precursor and copper as JEOL 6500F scanning electron microscope(SEM),the substrate materialhich was used to observe the morphology of the CNfsFoundation item: Project(66167044)supported by the Academic Human Resources Deyg under the Jurisdictionof Beijing, China; Project(66062021)supported by the Science andH中国煤化工ωFellow Abroad.CNMHGCorresponding author: ZOU Xiao-ping, PhD; Tel: +86-10-64884673-816; Fax: +86-10-64879486: E-mail: xpzou2005@gmail. ccJ. Cent. South Univ. Technol. (2008)15: 15-19For transmission electron microscopy (TEM) agglomerations. These agglomerations may be the resultsbservations, the samples were prepared by sonicating in as follows: firstly, the strong cohesive forces of thepure ethanol for several minutes, followed by deposition catalyst particles can result in this case; secondly, theof a few drops of the resulting suspension on the tEm random flowing gas of air can result in the flamecopper grid. TEM with a JEOL 2010 microscope was perturbations that drive the catalyst particles to move onloyed to characterize the structure of the CNFs. the substrate, and then the metal catalyst particlesRaman spectroscopy was used to characterize the degree metal oxide catalyst particles are driven to coalesce intoof graphitization of the CNFslarge particles. Due to the high movement and reactivityof the catalyst particles, the catalyst nanoparticles are3 Results and discussionoften in the shape of catalyst agglomerations that canform carbon nanofiber clusters(as shown in Fig. 2(b)).3.1 Catalytic effectsThe morphology of the catalyst nanoparticleIt is well-known that the size of the CNF formed is approximately determines the final mordirectly related to that of the catalyst particles Fig 1 CNFs. Fig. 2(c)shows a TEM image of a pear-shapedshows TEM images of CNFs synthesized by using nickel substance inside CNF formed by using ferric chloride asnitrate, ferric nitrate and feric chloride as the catalyst catalyst precursor. From Fig. 2(c)it can be seen that thereprecursors,respectively. By careful observation of the is a pear-shaped substance containing catalystimages, it is found that the size of the catalyencapsulated in the CNe, which promotes the growth ofnanoparticles approximately determines the finCNF. The diameter of the particles at position where thediameter of the carbon nanofibers. The large catalyst black arrow points at is more than 70 nm. It can beparticles tend to form CNFs with large diameter and the inferred that this growth mode is a catalytic processsmall catalyst particles usually result in the CNFs with involving the surface diffusion of carbon atom catalystsmall diameter. This is in good agreement with KIM et particles. The carbon atoms diffuse over the catalystal's observation 4. Therefore, it is necessary to controlthis parameter. The dried catalyst powders are not surface to form a pear-like structure that emanates fromsuitable for ECC technique because the dried catalystthe circumference of the catalyst. This providesprecursor particles can easily move and easily fall from synthetic route to produce magnetically functionalizedthe support during growth. For the above reasons, the CNFs. The filled CNFs may find a lot of applicationswet chemical method was adopted, a few drop of the ranging from the implementation of individual filledcatalyst precursor solution was dipped onto the substratefibers in sensors for magnetic scanning probewith a dip pen In the initial process of the synthesis, the microscopy to the assembly of aligned high densitycatalyst precursor is decomposed into metal oxide by magnetic nanocores for future magnetic data storagecombustion. The metal oxide is very easy to be reduced devices. Moreover, the carbon shell provides an effectiveinto metal at high temperature in the reducing protection against oxidation. The exceptional mechanicalenvironment, even carbide These metals, metal oxides or properties and light mass of CNFs make them potentialcarbide play an important catalytic role in the formation filling materials in polymer composites. CNFs canof CNFs, which can catalyze the subsequent growth of improve the strength and stiffness of a polymer, as wellFig 2(a)shows the morph sogr or the wool-like to polymer based composite systems ical conductivity)CNFsas add multifunctionality(such as elect中国煤化工125nCNMHGFig. I TEM images of samples produced by using nickel nitrate(a), ferric nitrate(b)and ferric chloride(c) as catalyst precursorJ Cent. South Univ Technol. (2008)15: 15-19the catalyst particles and precipitates at the end to formthe body of the carbon fiber. Due to the exothermicdecomposition of hydrocarbons, it is believed that a可年temperature gradient exists across the catalyst particleSince the solubility of carbon in a catalyst particle istemperature dependent, precipitation of excess carbonwill occur in the colder zone behind the particle, thusallowing CNF to grow with approximate diameter as thewidth of the catalyst particle. Instead of the diffusion ofgrowth mode is a catalytica polstructure that emanates from the circumference of theFig 3(b) exhibits a CNF with a catalyst particlef Fig3(b) isobviously different from the above growth mechanism ofgrows or200nm250nmof the catalyst at the same time, which is referred as theFig 2 Microstructures of CNFs: (a)SEM image of largeig.3(c)shows a CNF with a catalyst particlewool-like agglomerations using ferric chloride as catalyst attached to one end. Observation from the TEM revealsprecursor;(b)TEM image of carbon nanofiber cluster using that certain faces of iron-containing catalyst possess thetemic nitrate as catalyst precursor, (e) TEM image of ability to precipitate dissolved carbon in the form ofcatalyst precursorgraphite platelets. The phenomenon is possiblyillustrated by the tip growth mechanism and the stack3.2 Growth mechanismmechanismof graphene layers In Fig 3(c), there isFig3()shows a CNF with a catalyst particle single face of the particle involved in the interaction withencapsulated in a CNF. There are two possible growth the hydrocarbon reactant molecule. The graphite plateletsnodes that may explain this phenomenon. The most only form from the crystallographic faces of the catalystcommonly accepted mechanism is the catalytic particle, and the catalyst stays at the growing end of thedecomposition of the carbon feedstock and bulk CNFs, which has been explained by tip growthdiffusion of carbon (. According to this growth mode, mechanism. The growth process of the CNFs may bethe hydrocarbon gas decomposes on the front-exposed explained by the stack mechanism, that is, graphenesurfaces of catalyst particles to release hydrogen and layers repeatedly deposit on certain crystallographic facecarbon, which dissolve on the particles. The dissolved of the catalyst particle. This process involves thecarbon diffuses through the catalyst particles and through feedstock diffusion on or through the catalyst particle.V凵中国煤化工125nm50nmCNMHGFig-3 TEM images of CNFs usirg ferric chloride(a), nickel nitrate(b)and ferric nitrate(c) as catalyst precursorJ. Cent. South Univ. Technol. (2008)15: 15-19Even though carbon diffusion through the catalystSelected area electron diffractometry (SAED)is anparticle has been shown to be the rate-determining step4, excellent technique to study the microstructure of thea further aspect should be taken into consideration in the CNFs ol. The SAED graph(Fig. 6)exhibits a ring patten,manner by which the hydrocarbons are bond and which indicates that the CNF is polycrystalline materialultimately react with the catalyst surface. Prior toiffusion, a crucial event should be realized to make thegrowth process continue, namely, the adsorption anddecomposition of the reactant gas The catalyst particleadsorbs hydrocarbon on certain face, then thehydrocarbon is decomposed on the catalyst particksurface, and subsequent newly arriving carbon continuesto overcoat the old one, and these events are repeateduntil the catalyst is no longer able to catalyze the growthof CNFs on account of the formation of graphiticoverlayers, which are interfered with the further growth3.3 Nanostructure and microstructureThe diameter of the most visual CNFs synthesizedis basically consistent from the base to the tip. thgrowth of a CNF is influenced by many factors,Fig 5 TEM image of product carbon nanofiber with coarseincluding the catalytic decomposition of a carbon source, surfacethe activity of the catalyst and the conditions of thegrowth. So, the CNFs can adopt various shastraight, curved and helix. Fig 4(a)shows a photographof a segment of a straight CNF. The long straight CNFsynthesized by Fe-catalyzed decomposition of ethanol.The length from arrow I to arrow 2 is more than 20 um.The CNF structure will likely be straight as long as theprecipitation of carbon on the surface of nanoparticle isperformed with constant rate. Any perturbation in growthbehavior will give rise to abnormalities in the formationof the carbon fibers and will result in the generation ofother structure forms, such as coiled configuration(Fig 4(b). It seems that the same catalyst precursor canform different structural CNFsFig 6 Selected area electron diffraction pattem of CNFs usingferric nitrate as catalyst precursor3. 4 Raman spectrum analysisRaman spectroscopy is a simple and good tool foranalyzing the structure of the CNFs.Raman spectra ofCNFs were excited by a laser with wavelength of514.5 nm at room temperature. Tow peaks(1 346.9 and400nm 1 577.6 cmin Fig. 7(a); 1 346.9 and 1 592.0 cm 'inFig 4 Microstructures of straight and helix-shaped CNFsFig. 7(b); 1 345.5 and 1 587. 7 cm in Fig. 7(c) can be(a)SEM image of straight CNF; (b)TEM image of helix- observed in the range of 1 200-1 700shaped CNRaman spectrum. It is believed that 1 250-1 450 cm"isFig 5 shows a segment of a CNF consisting ofthe disorder-induced phonon mode(D-band), whicharises frordisordered components[12). 1 550-1600cylindrical graphene sheets with a coarse surface. It isspeculated that the CNF with large surface area has中国煤化工 which is producedfropotential applications for gas storage, absorbents andmatCNMHGoder of carbonhtify well-orderedsupercapacitors )and it is assumed that the substance CNFS In Figs.7(a)and(b), peaks d are all higher thanmarked by a black arrow is amorphous carbonpeaks G, while peak D is lower than peak G in Fig. 7(c),J Cent. South Univ. Technol. (2008)15: 15-19which indicates that the sample using ferric chloride as temperature and simple setup.the catalyst precursor has relatively larger size graphite2)CNFs growth by the ECC technique requiresclusters(4) within the CNF among the three samples. catalyst particles such as Fe, Co, Ni or their oxides, aAccording to the results in Ref [15], the amount of carbon feedstock and heat. The diameter of the fibersdisordered carbon can be estimated using a fractional produced is closely related to the physical dimension ofvalue(lp(p+lG). The values off by using nickel catalyst particle And through Raman characterization onnitrate, ferric nitrate and ferric chloride as the catalyst samples, the sample using ferric chloride as the catalystprecursor are 52.46%, 53. 75%and 47.08%, respectively. precursor has higher degree of graphitization than thatIt can be seen that the amount of amorphous carbon is using other catalyst precursorsthe smallest for the sample synthesized by the interactionThe formation of CNfs is related to aof ethanol with ferric chloride precursor.combination of factors including the morphology ofcatalyst, catalyst activity, and the diffusion of carbon1346.9through/on the catalyst particles. The synthesis of CNFsis a widely debated issue, and further research will beperformed in this area, such as the treatment of substrate,the controllability of the CNF growth and the preparationReferences[1] TEO K B K, SINGH C, CHHOWALLA M, MILNE W L CatalyticAmerican Scientific Publishers, 2003: 1-22.12001800[2] CHAMBERS A, RODRIGUEZ N M, BAKER R T K Influence ofRaman shif/cm-lcopper on the structural characteristics of carbon nanofibersroduced from the cobalt-catalyzed decomposition of ethylene[].J13469er Res,l996,11(2430-4315920[3] RODRIGUEZ N M, CHAMBERS A, BAKER R T K Catalytices[J]. Langmuir, 1995, 11(10)3862-3866.[4] KIM M S, RODRIGUEZ N M, BAKER R T. The interaction ofcarbon filament[]. Journal of Catalysts, 1991, 131(1 ) 60-73.[5] BAKER R T K. Catalytic growth of carbon filaments[]. Carbon,1989,27(3):315323highly Y-branched nanotubes [J]. Chem PhysLet2005,4091):43-47.1000120016001800[7 AAYAN PM. How does a nanofiber grow[]. Nature, 2004, 427(29):Raman shift/cm[8] RODRIGUEZ N M. 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Advances in Physics, 2000, 49(6)1200Raman shift/cm"I[13] JORIO A, SOUZA FILHO A G DRESSELHAUS GDRESSELHAUS M S, SWAN A K, UNLU MS, GOLDBERG BBFig 7 Raman spectra of typical sample using different metalPIMENTA M A, HAFNER J H, LIEBER C M, SAITO R G-bandsalts ferric chloride as catalyst precursors:(a)Nickel nitrate;resonant Raman study of 62 isolated single-wall carbon nanotubes[](b)Ferric nitrate; (c) Ferric chloride[14]BOSKOVIC B O, STOLOJAN V, ZEZE D A, FORREST R D,SILvA SRP Branched carbon nanofiberssynthesis at roor4 Conclusions中国煤化工microwave plasmas][5]1)Liquid ethanol can be used as an altermative kindCNMHGembrane derived frocarbon source for CNFs preparation. The potentialnode resun]. LarDon, ZUU3, 41(15): 2961-2972advantages of this technique are low synthetic