Carbon nanotube synthesis in a flame using

DOI:10.1007/s00339-003-2196-3Appl.Phys.A 77,885–889(2003)Materials Science &ProcessingApplied Physics Ar.l.vander wal 1,u g.m.berger 1t.m.ticich 2Carbon nanotube synthesis in a flame using laser ablation for in situ catalyst generation1NCMR,c /o NASA–Glenn Research Center,M.S.110–321000,Brookpark Rd.,Cleveland,OH 44135,USA2CentenaryCollege of Louisiana,Dept.of Chemistry,2911Centenary Blvd.,Shreveport,LA 71134,USAReceived:4November 2002/Accepted:2April 2003Published online:8July 2003•©Springer-Verlag 2003ABSTRACTLaser ablation of either Ni or Fe is used to createnanoparticles within a reactive flame environment for catalysis of carbon nanotubes (CNTs).Ablation of Fe in a CO-enriched flame produces single-walled nanotubes,whereas,ablation of Ni in an acetylene-enriched flame produces carbon nanofibers.These results illustrate that the materials for catalyst particle formation and CNT,SWNT or nanofiber,inception and growth in the aerosol phase can be supplied from separate sources;a metal-carbon mixture produced by condensation is not neces-sary.Both particle formation and CNT inception can begin from molecular species in a laser-ablation approach within the com-plex chemical environment of a flame.Moreover,SWNTs and nanofibers can be synthesized within very short timescales,of the order of tens of milliseconds.Finally,high-intensity pulsed laser light can destroy CNTs through either vaporization or co-alescence induced by melting.PACS 42.62Fi;81.05.Tp;82.80.Ch;81.15Fg1IntroductionAlthough single-walled nanotubes (SWNTs)wereinitially discovered within an arc discharge [1],researchers rapidly turned to laser ablation of metal–graphite compos-ite targets within high-temperature tube furnaces for their synthesis [2].In the laser-ablation–furnace approach,a laser pulse creates a superheated plasma consisting of carbon and metal species from the composite target.The high-temperature furnace slows the resulting condensation pro-cesses;as the plasma cools,carbon clusters are allowed to form.This is followed some milliseconds later by the forma-tion of metal clusters [3–5].The interaction of these species then generates carbon nanotubes (CNTs).Therein,the condi-tions of laser power [6],furnace temperature [7,8]and cata-lyst composition [9]are critical in the laser-ablation–furnace method for determining the residence time and identity of the species forming in the high-temperature environment.In add-ition to the inert gas identity [10]and flow rate [9,11],the effects of pressure [12,13],target placement relative to the central high-temperature zone [4,5,11,14]and the expan-sion dynamics along with cooling rate of the laser-generatedu Fax:+1-216/433-3793,E-mail:randy@plume [3–5,8,11,12,14–19]have been well documented.Despite these studies,however,many questions remain re-garding the details of the mechanism by which the condensing plume of metal atoms and carbon species interact to produce SWNTs.We have examined laser ablation in the flame environ-ment,as a point of comparison with the high-temperature furnace experiments for aerosol synthesis of CNTs.In a flame,the carbon feedstock is readily provided by the flame gases so that the ablation target need only supply the metal catalyst.Ablation and atomization of this target create a supersatu-rated metal vapor within the carbon-rich flame gases.Upon condensation and coalescence of metal nanoparticles,carbon nanotube catalysis can begin.In general,we are pursuing flame synthesis as a scalable method for CNT synthesis.The flame environment is appeal-ing because such systems have demonstrated scalability.They can be energy efficient since a portion of the fuel is burned to create the elevated temperature while the remainder of the fuel and its incomplete combustion by-products can serve as the reactive carbon source for nanotube synthesis.In addition,the chemical and thermal environments of a flame can be eas-ily varied by adjusting the fuel–air equivalence ratio and by the addition or substitution of ser ablation is used here only as a tool for pure catalyst introduction to provide a direct comparison with ablation–furnace synthesis and to gain fundamental insight into CNT inception and growth and the observed preferential reactivities of the different catalyst metals with CO and C 2H 2apart from solvent /solute evapora-tive and decomposition effects.A flame presents a radical departure from both the chem-ical and flow environments used in the laser-ablation–furnace approach.A flame environment is chemically complex with a temperature-dependent composition [20].The chemistry of the flame is determined by the choice of fuel(s)and overall fuel–air equivalence ratio [21].This contrasts with the high-purity,inert-environment characteristic of the laser–ablation furnace method,which consists only of vaporized metal,car-bon and inert carrier gas.A flame also presents a very different flow field rela-tive to the quasi-static flow established within the flow tube of a high-temperature furnace.In the laser-ablation–furnace approach,the slow gas flow and near-isothermal conditions allow the laser ablation plume to evolve into roll-up vor-886Applied Physics A–Materials Science&Processingtices,as revealed by imaging diagnostics[4,5,8,11,15,17]. It has been postulated that these are the regions where SWNT inception and growth occur due to the confinement of reac-tive species[14].The slow gasflow through theflow tube allows extended residence times for these vortices within the high-temperature zone of the furnace[1,4,5,11,12, 14,18].These extended residence times also allow sub-sequent laser pulses to interact with previously generated plumes[15].The complexities engendered by these con-ditions for SWNT inception and growth are not yet fully understood.Due to the density difference between the post-flame gases at elevated temperature and the ambient air,theflowfield of a laminarflame within a normal gravity environment is dom-inated by buoyancy-induced convection[22].Though fre-quently not appreciated,buoyant acceleration can result in flow velocities exceeding1m/s within a few cm of theflame front[23].Thus,within theflowfield of normal laboratory-scaleflames,the timescale for nanotube inception and growth will generally be limited to less than100ms.Therein,nano-tube synthesis under these conditions represents a radical de-parture from the more commonly used furnace conditions and could shed light on the CNT inception and growth processes in a variety of systems.Ablation is used here only as a tool to provide insights into the metal-catalyzed CNTs in the aerosol phase where the metal catalyst is independent from interactions with a support material.Many other methods could be used in a practical large-scale synthesis.By using ablation to cre-ate the pure-metal catalyst particles,rather than aerosol nebulization,catalyst particle build-up occurs from atomic and molecular species.This eliminates the complexity of solvent evaporation,solvent precipitation and interactions of decomposition products with the nucleating or growing metal nanoparticle[24].Similarly Ding et al.have sepa-rated the catalyst and fuel sources by using laser ablation of a Ni target within a low-pressure Ar:C2H2gas mix-ture in a high-temperature furnace whereby MWNTs were synthesized[25].This environment imparts some of the afore-mentioned complications,thus leaving key questions unad-dressed.Creation of the metal catalyst particle apart from con-densing carbon,as in the ablation–furnace or arc-discharge approaches,affords an opportunity to address the particle for-mation and SWNT nucleation processes separately.Therein, this approach is expected to provide answers to the ques-tions posed below.Such knowledge is expected to guide the direction/approach for scalingflame synthesis of these desir-able nanoproducts.1.Are the interactions between the condensing metal speciesand condensed elemental carbon particles necessary for SWNT inception and/or growth?2.Can active catalyst particle inception and growth occurwithin the complex chemical environment of theflame,in particular,aflame containing a high concentration of CO and perhaps other oxygenated species?3.Can SWNT,MWNT or nanofiber inception and growthoccur within the timescale of a buoyantly drivenflame?4.Are interactions with subsequent laser pulses necessary/advantageous for SWNT or nanofiber growth?2ExperimentalLaser ablation experiments were performed in the post-combustion exhaust gases of a rich,premixed acetylene–airflame.A water-cooled McKenna burner with a central tube for gas addition to the post-flame gases supported aflame in which the acetylene–air equivalence ratio was1.5.Two dif-ferent mixtures supplemented thisflame in our experiments, one of CO,H2and He each with0.25SLPMflows(“CO-enhanced”conditions)and another of C2H2,H2and CO with flows of0.100,0.500,and0.25SLPM,respectively(“C2H2-enhanced”conditions).The dynamic expansion of the abla-tion plume thoroughly mixed these added gases within the post-flame exhaust.Metal targets of either Fe or Ni were placed immediately adjacent to theflame.In either case,Nd:YAG laser light at 532nm was focused on the target by a250-mm-focal-length, plano-convex lens.The laser energy incident upon the target was300mJ/pulse.The focal point was placed approximately 1cm beyond the target to minimize laser-induced breakdown in the transient ablation plume emerging from the target. The placement of the target was just outside of a centrally placed chimney,a stainless steel tube with an outer diam-eter of5cm and a length of7.5cm that served to both sta-bilize theflame and confine the ablation plume.The laser beam traversed theflame1cm above the burner and just be-low the chimney,thus allowing ablation products from the target to be readily entrained by theflame gases into the chimney.To study the interaction of pulsed laser light with grown CNTs,a Nd:YAG laser at30Hz provided pulsed laser light at 1064nm with energies between130–200mJ/pulse.Dichroic mirrors directed the beam across the chimney,where the beam diameter was roughly8mm.Material from theflame was thermophoretically sampled, roughly1.5cm above the chimney.In these experiments,the dwell time of the probe within theflame was maintained at 0.5s to ensure capture of laser-altered material,whereas,nor-mally,it was300ms.In these experiments,nebulization of the appropriate metal salt solution was used to supply the cata-lysts,as reported previously[26–28].For all samples,TEM was performed at200keV,with an instrumental resolution of 0.14nm,upon the directly sampled material with no subse-quent processing or purification.3Results and discussionFigure1shows TEM images of material sampled directly on a TEM grid from the post-flame reactiveflow dur-ing Fe ablation in a CO-enhancedflame.As the overview image shows,SWNTs over a micron in length were read-ily found.The higher magnification image reveals that the SWNTs were frequently found together as ropes,consisting of several nanotubes.The mean observed diameter of1.4nm (based on12samples)is consistent with the range observed in laser-ablation–furnace syntheses[9,11,12,14].It is also commensurate with the size of the Fe catalyst nanoparticles. Some carbonaceous material was observed along the SWNTs and bundles,as is also commonly observed in laser-ablation–furnace synthesis.VANDER WAL et al.Carbon nanotube synthesis in a flame using laser ablation for in situ catalyst generation887FIGURE 1TEM images of SWNTs as directly sampled from the flameenvironment.The catalyst is Fe in the CO-enriched flame mixtureThese images,however,illustrate an important feature of our flame environment in that other types of carbonaceous debris,e.g.carbon particles,onions,fingers,etc.that are produced using metal–carbon composite targets are not co-generated.We attribute this to both the molecular species sup-plying the carbon feedstock and to the other species present in the flame environment.Species such as H 2and H 2O can etch amorphous carbon from catalyst particles and CNT sur-faces [27,29–33].Although the catalyst generation process and the carbon supply are not completely decoupled in our ex-periment,these results demonstrate a potential advantage of separating them.Results using Ni as the catalyst in an acetylene-enriched flame are shown in the images in Fig.2.In contrast to Fe -catalyzed SWNTs obtained within the CO -enriched flame mixtures,nanofibers are produced by the Ni catalyst under acetylene-enriched conditions.We distinguish nanofibers from multi-walled carbon nanotubes (MWNTs)based on their structural differences.MWNTs consist of nearly concen-tric graphitic cylinders,whereas carbon nanofibers (CNFs)are comprised of relatively short graphene segments which are oriented at some angle relative to the nanofiber axis.As Fig.2shows,nanofibers with lengths ofseveral-hundred FIGURE 2TEM images of nanofibers as directly sampled from the flameenvironment.The catalyst is Ni in the C 2H 2-enriched flame mixturenanometers were found in association with the Ni particles.The larger size and corresponding structure of the Ni nanopar-ticles is reflected in the diameter and morphology of the nanofibers they catalyze.These results are consistent with earlier observations from flame synthesis using nebulized so-lutions of Fe or Ni salts as precursor reagents to prepare the metal nanoparticle catalysts [26–28].In these studies,Fe was also observed to catalyze SWNTs while Ni was observed to catalyze CNFs.It is significant to note that Ni particles comparable in size to active Fe particles do not catalyze SWNTs using a flame-gas mixture rich in acetylene.Only larger Ni nanoparticles,a few nanometers in size,appear to be active in that environ-ment.In fact,smaller Ni nanoparticles appear to be absent from the post-combustion gases sampled.Conversely,the Fe nanoparticles have a mean size significantly smaller than that of the Ni nanoparticles and appear not to grow beyond a size of about 1.5nm .The implications of these differences for de-scribing SWNT and CNF growth in terms of the yarmulke and CSDP mechanisms,respectively,have been discussed else-where [28].Using the measured pyrolysis flame temperature along the axial streamline,the integrated residence time along888Applied Physics A–Materials Science&Processingthis streamline based on buoyant acceleration of theflame gases is calculated to be approximately100ms[23,34]. While the in situ ablation does introduce some pertur-bation to the gasflow within theflame,the steady-state temperature profile and resulting acceleration and particle residence times are taken to be indicative of the synthe-sis conditions.The above estimate is conservative as the growth undoubtedly occurs within a limited region of the flame that corresponds to only a fraction of the total resi-dence time.Even so,this timescale estimate is ten-fold less than that based onflow rates through process tubes within furnaces[2,4,5,8,9,11,14,18]used in the laser-ablation approach.The interaction(s)of pulsed laser light with CNTs has been a subject of speculation.It has been suggested that in-teractions of subsequent laser light pulses could serve to an-neal the developing CNTs[15].Alternately,based on spec-troscopic investigations of the laser generated plasma plume, Scott et al.[15]and Arepalli et al.[19]suggest thattheFIGURE3TEM image of CNTs subjected to pulsed laser light.The Ni cat-alyzed nanofibers were irradiated by laser light directed across the chimney top and the“annealed”products were thermophoretically sampled directly from the post-flame gases.The images are representative of those obtained using laserfluences within the range of130–200mJ/cm2.Notably,the laser fluences were significantly lower than typically used in the laser-ablation–furnace approach laser light may produce C2and other carbon species by ablation of carbon clusters thereby providing growth mate-rial to the CNTs.In our characterization of laser-induced incandescence(LII)applied to in situ detection of CNTs and nanofibers,we observed that the interaction of the laser light with the CNTs altered their optical emission signature upon subsequent pulsed laser heating[35].Similar results have been observed with LII applied to soot within com-bustion systems[36–38].Given the initial goal of observ-ing morphological changes in CNTs upon interaction with pulsed laser light,initial experiments were performed upon nanofibers.The TEM images of Fig.3show thermophoretically sam-pled material from theflame upon in-situ laser irradiation of Ni catalyzed nanofibers.They reveal greatly enlarged catalyst particles within a carbonaceous matrix.The higher magni-fication image reveals that the residual products bear little structural similarity to the original nanofibers when compared to Fig.2.Catalyst particles appear to have sintered or an-nealed together while at the elevated temperature induced by the laser light.The carbon from the original nanofibers appears to have reformed and graphitized around the metal particles.Remaining portions of the nanofibers may have been vaporized.We note that any laser-induced vaporization would create C n species that could then serve as growth media for the nanofibers within the laser-ablation–furnace synthe-sis approach.These altered structures are consistent with the changes in the optical emission signatures observed in our LII experiments mentioned above.4ConclusionsIn summary,our results provide answers to the questions raised in the introduction:1.Interaction between condensing metal and carbon clustersis not necessary for aerosol synthesis of SWNTs.SWNT inception and growth can occur apart from condensing carbon produced by ablation.2.Even within the presence of oxygen-containing species,creation/nucleation of metal catalyst particles that are vi-able for nanotube catalysis can occur within the complex chemical environment of theflame.3.Within the limited timescale of a buoyantly drivenflame,<100ms,metal catalyst particle nucleation plus growth to reach a catalytically active size and SWNT,MWNT and CNF inception and growth can occur through molecular build-up.4.Pulsed laser light is deleterious to ser-inducedsintering,coalescence and vaporization processes can occur.Most significantly,these studies confirm earlier results re-garding preferential reactivity of specifically sized catalyst particles towards particular reactive gases.ACKNOWLEDGEMENTS This work was supported by a NASA NRA97-HEDs-01combustion award(RVW)and through a NASA–Glenn Director’s Strategic Research Fund administered through NASA Cooperative Agreement No.NAC3–544with The National Center for Micro-gravity Research on Fluids and Combustion(NCMR)at The NASA–Glenn Research Center.The authors gratefully acknowledge Dr.Y.L.Chen and D.R. Hull for the TEM imaging.VANDER WAL et al.Carbon nanotube synthesis in aflame using laser ablation for in situ catalyst generation889REFERENCES1S.Iijima,T.Ichihasahi:Nature363,603(1993)2T.Guo,P.Nikolaev,A.Thess,D.T.Colbert,R.E.Smalley:Chem.Phys.Lett.243,49(1995)3G.De Boer,S.Arepalli,W.Holmes,P.Nikolaev,C.Range,C.D.Scott: J.Appl.Phys.89,5760(2001)4A.A.Puretzky,D.B.Geohegan,X.Fan,S.J.Pennycook:Appl.Phys.A 70,153(2000)5A.A.Puretzky,D.B.Geohegan,X.Fan,S.J.Pennycook:Appl.Phys.Lett.76,182(2000)6A.C.Dillon,P.A.Parialla,J.L.Alleman,J.D.Perkins,M.J.Heben: Chem.Phys.Lett.316,13(2000)7M.Yudasaka,T.Ichihashi,S.Iijima:J.Phys.Chem.B102,10201 (1998)8F.Kokai,K.Takahashi,M.Yudasaka,R.Yamada,T.Ichihashi,S.Iijima: J.Phys.Chem.B103,4346(1999)9H.Katura,Y.Kumazawa,Y.Maniwa,Y.Ohtsuka,R.Sen,S.Suzuki, Y.Achiba:Carbon38,1691(2000)10Y.Zhang,H.Gu,S.Iijima:Appl.Phys.Lett.73,3827(1998)11R.Sen,Y.Ohtsuka,T.Ishigaki,D.Kasuya,S.Suzuki,H.Kataura, Y.Achiba:Chem.Phys.Lett.332,467(2000)12F.Kokai,K.Takahashi,D.Kasuya,M.Yudasaka,S.Iijima:Appl.Phys.A73,401(2001)13M.Yudasaka,T.Komatsu,T.Ichihashi,Y.Achiba,S.Iijima:J.Phys.Chem.B102,4892(1998)14S.Arepalli,P.Nikolaev,W.Holmes,B.S.Files:Appl.Phys.Lett.78, 1610(2001)15C.D.Scott,S.Arepalli,P.Nikolaev,R.E.Smalley:Appl.Phys.A72,573 (2001)16M.Yudasaka,F.Kokai,K.Takahashi,R.Yamada,N.Sensui,T.Ichi-hashi,S.Iijima:J.Phys.Chem.B103,3576(1999)17F.Kokai,K.Takahashi,M.Yudasaka,S.Iijima:J.Phys.Chem.B103, 8686(1999)18F.Kokai,K.Takahashi,M.Yudasaka,S.Iijima:J.Phys.Chem.B104, 6777(2000)19S.Arepalli,P.Nikolaev,W.Holmes,C.D.Scott:Appl.Phys.A70,125 (2000)20S.R.Turns:An Introduction to Combustion:Concepts and Applications (McGraw Hill Inc.,New York1996)21R.J.Kee,F.M.Ripley,ler:Sandia National Laboratories Report No.SAND89–8009B(1989)w,G.M.Faeth:Prog.Energy Combust.Sci.20,64(1994)23F.G.Roper:Comb.and Flame29,219(1977)24A.Gurav,T.Kodas,T.Pluym,Y.Xiong:Aerosol Sci.Technol.19,411 (1992)25H.Kanzow,A.Schmalz,A.Ding:Chem.Phys.Lett.295,525(1998) 26R.L.Vander Wal:Carbon40,2101(2002)27R.L.Vander Wal:Comb.and Flame130,37(2002)28R.L.Vander Wal,L.J.Hall:Chem.Phys.Lett.349,178(2001)29P.E.Nolan,D.C.Lynch,A.H.Cutler:Carbon32,477(1994)30K.L.Yang,R.T.Yang:Carbon24,687(1986)31G.A.Somorjai,C.M.Kim,C.Knight:In Surface Science of Catalysis: In Situ Probes and Reaction Kinetics,ed.by D.J.Dwyer,F.M.Hoffman (The American Chemical Society,Washington DC1992)32C.Park,N.M.Rodriguez,R.T.K.Baker:J.Catalysis169,212(1997) 33P.McAllister,E.E.Wolf:J.Catal.138,129(1992)34R.L.Vander Wal,T.M.Ticich,V.E.Curtis:Chem.Phys.Lett.323,217 (2000)35R.L.Vander Wal,G.M.Berger,P.D.Patel,T.M.Ticich:Appl.Opt.41, 5678(2002)36R.L.Vander Wal,T.M.Ticich,A.B.Stephens:Appl.Phys.B67,115 (1998)37R.L.Vander Wal,K.A.Jensen:Appl.Opt.37,1607(1998)38R.L.Vander Wal,M.Y.Choi:Carbon37,231(1999)。