Growth of carbon nanotubes over nonmetallic based catalysts

Growth of carbon nanotubes over nonmetallic based catalysts. Phát triển các ống nano carbon trên chất xúc tác phi kim loại. Growth of carbon nanotubes over nonmetallic based catalysts. Phát triển các ống nano carbon trên chất xúc tác phi kim loại.Growth of carbon nanotubes over nonmetallic based catalysts. Phát triển các ống nano carbon trên chất xúc tác phi kim loại.

Trang 1

Pleasecitethisarticleinpressas:L.-L.Tan,etal.,Growthofcarbonnanotubesovernon-metallicbasedcatalysts:Areviewontherecent developments,Catal.Today(2012),http://dx.doi.org/10.1016/j.cattod.2012.10.023

ARTICLE IN PRESS

G Model

CATTOD-8288; No of Pages 12

Catalysis Today xxx (2012) xxx– xxx

ContentslistsavailableatSciVerseScienceDirect

Catalysis Today

j o ur na l ho me p a g e :w w w e l s e v i e r c o m / l o c a t e / c a t t o d

Review

Lling-Lling Tana, Wee-Jun Onga, Siang-Piao Chaia,∗, Abdul Rahman Mohamedb

a Chemical Engineering Discipline, School of Engineering, Monash University, Jalan Lagoon Selatan, Bandar Sunway, 46150 Selangor Darul Ehsan, Malaysia

b School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Pulau Pinang, Malaysia

a r t i c l e i n f o

Article history:

Received 16 July 2012

Received in revised form 1 October 2012

Accepted 17 October 2012

Available online xxx

Keywords:

Metal-free-catalyst

Carbon nanotubes

Nanoparticles

Chemical vapor deposition

Methane decomposition

Catalyst preparation

a b s t r a c t

Theconventionalmethodtosynthesizecarbonnanotubes(CNTs)requirestheuseofmetalliccatalysts However,themetallicparticleimpuritiesusuallycontaminatetheCNTs,andaredifﬁculttoremove with-outintroducingdefectsandcontaminations.AnalternativeandmoredesirableapproachistogrowCNTs directlyonasubstratewithoutfurthertreatmentbeingrequired.Thereviewpresentsanddiscussesthe currentdevelopmentofthecontrolledsynthesisofSWCNTsand/orMWCNTsusingnon-metallic cata-lysts,whichcomprisesofvariousceramicsandsemiconductorsascatalyticparticles.All-carbonsystems forCNTgrowthwithoutemployinganycatalystsarealsodiscussed.Despitetheenormousstridesinthe synthesisofCNTs,thepreciseatomisticmechanismexplainingtheirnucleationandgrowthstillremains unclear.Therefore,thedifferentgrowthsystemsproposedbyseveralauthorsarealsoexaminedinthis article.Thereviewconcludeswithasummaryandanoutlookonthechallengesandfuturedirectionsin themetal-free-catalystgrowthofCNTs

Contents

1 Introduction 00

2 Generalbackgroundonthemetal-freecatalystgrowthofCNTs 00

3 Non-metalliccatalysts 00

3.1 Semiconductor–catalystsystems 00

3.2 Nanoparticulateoxidecatalysts 00

3.3 Othercatalystsystems 00

4 Controllingthelength,diameterandchiralityofCNTsovernon-metalliccatalysts 00

5 Growthmechanismdiscussion 00

6 Summaryandoutlook 00

Acknowledgement 00

References 00

1 Introduction

Tubularformofcarbonproductsknownascarbonﬁlamentswas

ﬁrstobservedusingelectronmicroscopesaround1950[1].Since

theobservationanddetailedstructuralstudyofcarbonnanotubes

(CNTs)byIijimaoftheNECCorporationin1991[2],CNTshave

stimulatedextensiveresearchactivitiesinmostareasofscience

andengineeringdue totheirextraordinaryphysicaland

chemi-calproperties,includinghighmechanicalstrength,highelectron

∗ Corresponding author Tel.: +60 355146234.

E-mail address: chai.siang.piao@monash.edu (S.-P Chai).

conductivityandsuperiorsurfaceproperty.In1991,Iijimainitially observedonlymulti-walledcarbonnanotubes(MWCNTs)grown

in an arc discharge process, and it wasnot until 2 years later whensingle-walledcarbonnanotubes(SWCNTs)withdiameters between1.1and1.3nmweresynthesizedusinglaserablation[3,4]

In1996,Smalley’sgroupsuccessfullyproducedbundlesofSWCNTs fortheﬁrsttime[5]

Currently, thevast majority of researchis beingcarried out

onSWCNTs,astheyareknowntopossessremarkableelectronic and mechanical properties Theyrepresent theultimatecarbon ﬁber,withthehighestthermalconductivityandthehighest ten-silestrengthofanymaterial[6,7].Hugeeffortshavebeenspent

bytheinternationalscientiﬁccommunityinordertostudytheir

http://dx.doi.org/10.1016/j.cattod.2012.10.023

Trang 2

G Model

applicationinﬁeldsincludingmedicine[8–10],catalystsupport

for electrode of fuel cells [11,12], high-performance adsorbent

[13–15],ﬁeldemitterforﬁeld-emissiondisplay[16,17],electrode

materialforenergystoragedevice[18,19]andsoon

ThethreemajortechniquesforthefabricationofCNTsarearc

discharge[3],laserablation[20]andchemical vapordeposition

(CVD)[21,22].Amongthesetechniques,CVDshowsobvious

advan-tagesforthegrowthofCNTsintermsoflowgrowthtemperature,

goodcontrollabilityandeasinessofscalingup,atarelativelylow

cost.ItispossibletocontrolthegrowthandthestructureofCNTsby

adjustingreactionparameterssuchasgrowthtemperature,carbon

source,andcatalystconcentration[23].Thegrowthprocess

typ-icallyrequirestheassistanceofmetalliccatalysts,predominantly

3dvalencetransitionmetalnanoparticles(Fe, Coand Ni)inthe

decompositionprocessofhydrocarbonsduetohighsolubilityand

diffusionrateofcarbonathightemperatures.Generally,for

sup-portedmetalcatalysts,therearetwo proposedmechanismsfor

thegrowthofCNTs,namelytip-growthandbase-growthmodel,

bothofwhicharebasedonthevapor–liquid–solid(VLS)theory

describedbyBakerandco-workers[24,25].Initially,itwasbelieved

thattheapplicationofthesecarbide-formingmetalsis

indispens-ableforthegrowthofCNTs.However,recentexperimental[26,27]

andtheoretical[28]studiesdemonstratedthatthechiralitiesof

theSWCNTsproducedcouldbecontrolledtosomeextentbythe

selectionofcatalysts.Sincethen,manyothermetallicspeciessuch

asAu,Ag,Cu,Pd, andRh have beenreportedtoyieldSWCNTs

[29,30].Despitetheseadvancements,themetalspeciesremaining

in theCNTsamples would resultin major drawbacksfor their

intrinsic property characterization.Due to thechemical, redox,

andmagneticpropertiesofthemetalnanoparticles,interference

withthecorrespondingtubepropertiescannotbeavoided[31,32]

TheperformanceofCNT-basedmaterialsascatalystsupportand

itsapplicationinmolecularelectronics,biology,andmedicineare

alsoobscuredbythepresenceofmetalcatalystparticles[33,34].In

semiconductorelectronicsfabrication(CMOS)processes,the

adap-tationofCNTsontoelectronicsubstrateswillresultindetrimental

incompatibilityissuesduetothepresenceofmetalparticle

impu-rities.Thetoxicityand effectsofmetalnanoparticlesonhuman

healthhavehinderedtheuseofCNTsinbiologyanddrug

deliv-eryapplications.Furthermore,inmanycases,thecatalystparticles

arecoveredbyacarbonshell,whichpresentsadditionalproblems

forthenon-destructivepuriﬁcationoftheCNTse.g.bytreatment

withnon-oxidizingacids[35].Ithasbeenuntilnowanintractable

problemincompletelyeliminatingmetalcatalystsfromCNT

sam-pleswithoutintroducingdefectsandcontaminations.Overthepast

5years,many non-metallicspecies havebeenreportedtohave

theabilitytocatalyzeCNTgrowthunderspeciﬁcconditions.The

listencompassesvariousceramics(e.g.Al2O3 andZrO2)[36,37]

and semiconductors(e.g.Si, SiC and Ge)[38–40].The fact that

CNTscanbegrownovernon-metalliccatalystsisamajor

break-throughinnanotechnology.ThegrowthofCNTsovernon-metallic

catalystswillplayveryimportantrolesinfacilitatingthe

applica-tionsofCNTmaterialsintheﬁeldofnanoelectronics,photonics,

biomedicine,membranetechnologyand soforth In addition,it

enablessimplerpuriﬁcationtechniquesandmitigatestoxicity

con-cerns.Membrane-andCNT-nanoﬂuidics-basedresearchcanalso

beneﬁtfromthenon-metallicCNTsinbypassingoneormore

man-ufacturingsteps,thusleadingtolowercosts

Comprehensivereviewsontheproductionandgrowth

mech-anismofCNTshavebeenpublished[41–49].Thegrowthprocess

andmechanismformetalliccatalystshavebeenadequately

dis-cussed[42],whereasaclearmechanismisstillbeingsoughtforthe

properunderstandingofCNTgrowthovernon-metalliccatalysts

Asopposetometalliccatalysts,thesynthesisofCNTsoverthe

non-metalliccatalystsstillsuffersfromlowyield,makingitunattractive

formassproductionatthemoment.Nevertheless,sinceanumber

ofimportantﬁndingshavebeenreportedonthegrowthofCNTs overnon-metalliccatalysts,webelievethatareviewonthis sub-jectistimelytopromotelatestdevelopmentsinthisinteresting areaofresearchtoshedadifferentlightonpreviousexperiments Thereviewbeginswithanoverviewofthenon-metalliccatalyst growthofCNTs,followedbyanin-depthanalysisofCNTgrowth basedonsemiconductors,ceramics,all-carbonsystems,andother newuncommoncatalysts.Thedifferentgrowthmechanisms, par-ticularlyonthegraphitizationofsilica,arealsoexaminedinthis article.Finally,wewillpresentasummaryofchallengesandfuture directionsforinvestigation

2 General background on the metal-free catalyst growth of CNTs

Usingmetallic catalystshaslongbeenconsidered indispens-ableforthenucleationand growthofCNTs.Onlyveryrecently, severalgroupshave demonstratedthepossibilitytogrowCNTs fromceramic[36,37],semiconducting[38–40,50–53],and nano-sizeddiamondparticles[54],allofwhichwereconsideredinactive

inthegrowthofCNTsinthepast.Moreinterestingly,denseCNTs werealsodemonstratedtogrowonporouscarbonsubstrates with-outemployinganycatalysts[55,56].Theseﬁndingsclearlyshow thatthedecompositionofhydrocarbonsandCNTproductionare notboundtothefunctionsofmetallicparticles.Instead,Takagietal [38]proposedthattheessentialroleofcatalystsistoprovidea tem-plateforcapformation.ItshouldbenotedthatthegrowthofCNTs overthesenewlyfoundspeciesdoesnotalwayshappen.The tem-plateforCNTnucleationrequiresthepresenceofporousstructures

ornanoparticleswiththeappropriatediameterandtheright cur-vature.Itissaidthatthenano-scalecurvaturesprovideaplatform

onwhichcarbonatomscanformahemisphericalcap,whereCNTs aregrowninaself-assembledfunction[36,38,57]

TherearefourfactorsforthegrowthofCNTs:(1)thecatalystsize [29],(2)thecatalyst/substratepretreatment[38],(3)thegrowth temperature[58]and(4)theroleofwater[59].Thesolubilityof carbonincatalystparticlesandtheprecipitationratesofcarbon fromcatalystparticlesbothshowgreatdependenceonthe cata-lystsize.Whenthesizeofparticlesisbelow10nm,thequantum sizeeffectsgreatlyinﬂuencethepropertiesofthecatalyst parti-cles[60,61].ThesynthesisofSWCNTsgenerallyrequirescatalyst particleswithseveralnanometersindiameters,preferably3nm

orless.Withanincreaseinparticlesize,thenumberofwallsand diametersofthenanotubeswouldincrease.Interestingly,SWCNTs havebeenreportedtogrowonlargeAl2O3particles,rangingfrom severaltensofnanometertohundredsofnanometers[36] Scan-ningelectronmicroscopy(SEM)imagesrevealedthattheSWCNTs weregrownonnano-sizedprotrusions,onceagainindicatingthe importanceofnano-scalecurvaturesinthenucleationand subse-quentgrowthofCNTs.ThesecondfactorforthegrowthofCNTs

isthepretreatmentofthecatalyst/substrate.Itwasreportedthat preheatingtheparticlesinairat950–1000◦Cnotonlyincreases theyieldofSWCNTs,butalsoimprovestheactivityofthecatalyst particlesandremoveshydrocarboncontaminantsontheparticles [58,62].Ontheotherhand,itisnotedthattheannealingduration (from1to60min)inairhadnosigniﬁcanteffectonthegrowthof CNTs.ThisdirectlyimpliedthatpretreatmentofSiO2substratesin airwasmerelyaimedtocleanthesubstratesurface[58]

Thegrowthtemperature isalsoconsideredtobeone ofthe essential factors which affects the growth of CNTs Liu et al [58] investigatedthe effectof growthtemperature from800◦C

to 900◦C on the yield of SWCNTs grown on a SiO2 substrate Theyreportedthata hightemperatureof850◦C wasimportant

toinducethepyrolysisofethanol.However,whenthe tempera-tureincreasedabove900◦C,ahigherthermaldecompositionrate

Trang 3

G Model

ofcarbonsourcewouldtakeplace,leadingtothecoverageof

sub-stratesbyathickencapsulatedcarbonlayerinashortresidence

time,resultinginnoCNTgrowth.Asimilartrendofresultswas

reportedinthestudybyXuetal.[59].Theystudiedthetemperature

dependenceoftheCNTgrowthrangingfrom750◦Cto950◦Con

silicananoparticlessynthesizedfromthethermaldecomposition

ofPSS-(2-(trans-3,4-Cyclohexanediol)ethyl)-Heptaisobutyl

substi-tuted(POSS) Apparently,noCNTscouldbeproducedwhenthe

growthtemperaturewastoolow(at750◦C)ortoohigh(at950◦C)

Theyclaimedthatwhenthetemperaturewasincreasedfrom850◦C

to900◦C,theCNTcontentdecreaseddrasticallyfrom47%to20.3%

duetomuchmoreaggregationofSiO2particles.Veryrecently,Xu

etal.[59]demonstratedtheeffectofintroducingwaterinthe

syn-thesisofCNTsonsilicananoparticles.Theyfoundthatwaterplays

acleansingrolebyremovingamorphouscarbonandalsoplaysan

indirectroleinpromotingCNTgrowth.Thisobservationissimilar

tothatreportedinthepreviousstudiesonthegrowthofCNTsover

metalcatalystparticles[63,64].Theystudiedtheinﬂuenceofwater

contentfrom0to10vol%.Theresultsshowedthatintheabsenceof

water,CNTswithsomeamorphouscarbonwereobservedwhereas

muchlessamorphouscarbonwith2%water.However,by

increas-ingthewatercontentfrom2%to5%and10%,respectively,much

lessCNTswereobtained

Mostrecently,Liuetal.[65]reportedthatthechemical

composi-tionofthecatalystparticlesisanothercrucialfactorforthegrowth

ofSWCNTs,inadditiontothewell-knowncatalystsizeeffect.In

thepaper,theauthorsstudiedtheCVDgrowthofSWCNTsfrom

SiOxandSinanoparticles.Atomicforcemicroscopy(AFM)andSEM

imagesshowedthegrowthofdenseSWCNTsonSiOxnanoparticles

whilenoCNTswereobservedonSinanoparticles.Density

func-tionaltheory(DFT) calculationshavealsoindicatedthat oxygen

atomscanimprovethecaptureofcarbon-bearingmolecules.This

indicatesthatoxygenmayplayanimportantroleinpromotingthe

formationofgraphiticcarbonstructuresandfacilitatingthegrowth

ofSWCNTsonoxygen-containingSiOxnanoparticles[65]

3 Non-metallic catalysts

3.1 Semiconductor–catalystsystems

Thenon-metallicsynthesisofCNTswasdemonstratedforthe

ﬁrsttimein1997,inwhichaﬁlmofwell-orientedCNTswere

pro-ducedbythesublimationdecompositionofSiCnanoparticlesat

hightemperatures(>1700◦C)[66].However,thesuccessof

tran-sitionmetal-basedcatalystsandthelargenumberofresearchers

workingonCVDatthattimeleftthisworklargelyignored.With

therecentdevelopmentsinnovelcatalysts,interestintheuseof

non-metalliccatalystsforCNTgrowthisgraduallyrising.Oneofthe

mostwidelyusedcatalystsinthesynthesisofCNTsisSiC.Growth

ofCNTsusingthiscatalystcanbeachievedbyannealingSiC

parti-cles[38,53,66],amorphousSiCﬁlms[52],orhexagonalSiC(6H-SiC)

[67]invacuum.Kusunokietal.[68]demonstratedthatunderlow

vacuumconditions,SiCoxidizesaccordingtothefollowingprocess:

TheoxidationofSiCresultsintheformationofinitialnucleation

caps,whichthenenablethesubsequentconstructionofCNTs

How-ever,themechanismbehindtheirformationstillremainsunclear

Severalgroupsclaimed that theformationof thenanocaps

fol-lowedthetransformation processof graphenelayers[69,70]or

amorphouscarbonclusters[71],whileothersarguedthattheir

for-mationisaresultofconvexstructuresontheSiCsurface[72–75]

Mostrecently,Wanget al.[51] suggested theformationof SiO

nanoclustersattheC/SiCinterface.Theauthorsclaimedthatthe

possible roles of these nanoclusters may be twofold.First, the

curvatureofthemoltenSiOnanoclustersprovidesaplatformfor graphene lifting, thus leading to the formation of hemispheri-cal carbon nanocaps and subsequent CNT growth Second, the coordination-unsaturatedSi speciesin thenanoclusterspresent highactivityfortheincorporationofcarbonatoms,andfacilitate theattachmentofcarbonatomstothetubeedges

InadditiontoSiC,theuseofGecatalystsinthesynthesisofCNTs hasalsobeenextensivelyresearched[40,76].Thepioneeringwork

byUchinoetal.[76]employedcarbon-dopedSiGeislandsonSi substratestosynthesizeCNTsinCH4-CVD.Theauthorssuggested thatthegrowthofCNTsoccurredfromGeclusters,which were producedfollowingthechemicaloxidation andannealing treat-mentoftheSiGelayers.SinceSihasthegreaterthermodynamic tendencytobeoxidizedascomparedtoGe,theoxidation treat-mentresultsintheformationofSiO2and thesegregationofGe clusters.Theseclustersthenserveascatalystsforthegrowthof CNTs.OthergrowthtechniquesassociatedwiththeuseofGe cata-lystsincludethosewhicharebasedonGeStranksi–Krastanowdots,

Genanoparticlesformedbyionimplantation[77]andcolloidalGe nanoparticles[78,79]

Takagietal.[38]alsodemonstratedtheproductionofCNTsover semiconductornanoparticlesSi,GeandSiCinethanol-CVD Single-walledordouble-walledCNTs,withdiameterslessthan5nmwere produced.TheCNTyieldfromGenanoparticleswasfoundtobe higherthanthoseofSiandSiC.SincethemeltingpointofGeis knowntobelowercomparedtoSiandSiC,itisplausiblethattheGe nanoparticlesareinmoltenstateduringthegrowthprocess,thus contributingtothehigherCNTyield.Despitetheadvancements madeinthedevelopmentofthesenon-metalliccatalysts,theyield wasstillmuchlowercomparedtothe3dvalencetransitionmetals

ofFe,NiandCo SinceSi,Ge andSiCshouldhave littleactivity

inthisprocess,ahighergrowthtemperaturewouldbenecessary

toinducethepyrolysisofthecarbonsource.Anotherfactorthat mightcontributetothedeviationinCNTyieldisthephaseofthe nanoparticles(solidorliquid)becauseitrelatestotheprecipitation mechanismofcarbonatomsonitssurface.Othersemiconductor catalyststhathavebeenreportedinliteratureincludeZnO[80], TiO2[81]andTe[82].Theseﬁndingsprovidemoreinsightsinto theactualroleofcatalystsandarehelpfulforunderstandingthe mechanismbehindthegrowthofCNTs

3.2 Nanoparticulateoxidecatalysts

In catalyst-supported CVD,pureoxides such asSiO2,Al2O3, TiO2,ZrO2 andMgOarebasicallyemployedasphysicalsupports forthecatalysts.However,recentexperimentalstudiesrevealed thatoxideshavetheabilitytogrowgraphiticsheetsundertypical CVDconditionsforCNTgrowth[83,84].Rümmelietal.[83] demon-stratedthatdifﬁcult-to-reducenanoparticleoxidesareextremely effectiveinpromotingorderedcarbon(graphene)growth.In con-trast,nocarbonformationwasreportedonbulk/ﬁlmsamples.The authorsattributedthisdifferencetothepresenceofsurfacedefect sitesonthenanoparticleoxides,andproposedthattheinterface betweenthecatalystparticleandthesubstratebehavesasa cir-culardefectsite wherenanotubegrowthcantakeplace.Huang

etal.[81]reportedthatmanyoxidenanoparticlesincludingSiO2,

Al2O3,Er2O3andalllanthanideoxidesexceptpromethiumoxide areactiveforthegrowthofSWCNTs

Amongthenanoparticleoxidesreportedinliterature,theuseof SiO2asacatalystforCNTandgrapheneproductionisofparticular interestduetotheirpotentialapplicationsinsilicon-based technol-ogy[85].Todate,differentapproachestoSiO2catalystpreparation havebeenreported.In2009,Liuetal.[86]reportedthata 30-nm-thickSiO2ﬁlmdepositedontoaSi/SiO2wafercoulddirectlyserve

asasubstrateforSWCNTgrowthinaCH4-CVDprocessat900◦C

A dense and large-area of uniform SWCNTswere reproducibly

Trang 4

G Model

Fig 1.SEM images of the SWCNTs grown on Si/SiO 2 wafer: (a) overview of the cross pattern, (b and c) enlarged images of the blue square area in image (a) Reprinted with permission from [86] (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of the article.)

Copyright (2009) American Chemical Society.

obtainedonthesubstratesurface,indicatingtheeffectivenessof

thesynthesisapproach.Thesamegroupofresearchersalso

devel-oped a facile “scratching” methodfor the patterned growth of

SWCNTs,inwhichaSi/SiO2waferwasscratchedusinganotherone

withasharptiptoobtainadesiredpattern,beforebeingsubjected

toCH4-CVD.FromSEMobservations(Fig.1 CNTscanbefound

onscratchedareasbutnotonnanoscopicallysmoothsurfaces.It

issaidthatthe“scratching”betweenthetwowaferswillcrackthe

thermallygrownSiO2layerandconsequentlygeneratesomeactive

sitesforthegrowthofSWCNTs.Alongthesamelines,Huangetal

[81]scratchedacleanSi/SiO2waferwithadiamondbladewithout

puttinganycatalystonthesurface.SWCNTswithanarrow

diam-eterdistributionrangingfrom0.8to1.4nmwereobserved.This

indicatesthatonlySiO2 withanappropriatecatalystsizeofless

than2nmisactiveforSWCNTgrowth.Thepresenceofoxidized

SiwasconﬁrmedbytheXPSanalysisattherichregionof

SWC-NTs.The“scratching”approachisbothsimpleandcheap,without

requiringtheneedforcomplexpatterningprocesstogrowSWCNTs

atapredeﬁnedpositionfordevicefabrication.However,sincethe

structureofthesenanoparticlesﬂuctuate,itisdifﬁculttoprecisely

controlthestructureoftheas-grownCNTs

Inadditiontothe“scratching”approach,awetchemical

etch-ingprocesshasalsobeenreportedtogenerateSiO2nanoparticles

[81,87].Huangetal.[81]etchedaSi/SiO2waferwithanaqueous

solutionofHF,followedbythermallyannealingitinairat1000◦C

for1h.AcirculartracewasformedontheSiwaferafterdryingas

showninFig.2a.ItwasproposedthatHFdissolvestheSiO2layer

whichishydrophilicandtheaqueoussolutionshrinksontothe

bot-tomSilayerwhichishydrophobic.PartsoftheSiO2aresaidtobe

dissolved,leavingsmallSiO2particlesinthewaterdropontheSi

wafer.Theseparticlesthenserveasnucleationsitesforthegrowth

ofCNTsinthesubsequentCVDprocess.BasedonSEMobservations,

highlydenserandomSWCNTscanbeobservedaroundthecircular

trace(Fig.2 andc)whilecarbonﬁlamentorMWCNTsareobserved

insidethecircle(Fig.2d)

Thereare otherreportsontheuseofSiO2 catalystsforCNT

growth.Mostrecently,Xuetal.[59]demonstratedtheﬁrst

exam-pleoftheuseofSiO2nanoparticlesforthecontinuoussynthesis

ofMWCNTsbyalcohol-CVD.Inanotherpaper,Liuetal.[58]

suc-cessfullysynthesizeddenseSWCNTsbysimplyannealingtheSiO2

substratesinH2athightemperatures(950–1000◦C)beforeCVD

Theauthorsproposedthattheannealingtreatmentathigh

tem-peraturelocallyevaporatesSiO2substratesurfacesresultinginthe

formationofdefectsonthesurfaces.Thesedefectsprovide

nuclea-tionsitesfortheproductionofcarbonnanoparticles,andassistthe

formationofcarbonnanocaps,thusresultinginthegrowthof

SWC-NTs.ThegrowthmechanismofCNTsoverSiO2nanoparticleshas

yettobeclariﬁed.SomearguethatSiO2undergoescarbothermal

reductiontoSiC,whileothersclaimthatitremainsstable

through-outthegrowthprocess.Anindepthanalysisofthegraphitization

mechanismofSiO2catalystsispresentedinSection5

Inthepast,Al2O3 ceramicnanoparticleswereregardedasan inactive catalyst inthe growthof CNTs andwere only usedas

abufferlayertodispersemetalliccatalystparticlesandenhance theircatalyticpropertiesinCNTgrowth[88].However,inarecent studybyLiuetal.[36],denseSWCNTlayerscatalyzedbyAl2O3 nanoparticlesweresuccessfullygrownusinganalcohol-CVD.The morphologiesofAl2O3 particleswerefoundtobedifferent com-paredtometalliccatalystparticlessuchasFe,CoandNi,whichare typicallysphericalorsemi-sphericalwithasmoothsurface.The authorsindicatethattheAl2O3particlesarelikelytobeinsolid stateduringtheCNTgrowthprocess.Thissigniﬁesthatagrowth modelotherthanthetraditionalVLSmechanismmustbeinvolved forthesesolidcatalysts.Theresearchalsoindicatesthe possibil-ity of growinglarge area“catalyst-free” SWCNTs onﬂat Al2O3 substratesby simply manipulating the nanostructureson their surfaces

Steineretal.[37]successfullysynthesizedbothSWCNTsand MWCNTsonZrO2nanoparticlesbythermalCVD.ZrO2offers sev-eraladvantagesasacatalystforCNTgrowthduetoitsnonmagnetic nature,makingit aninterestingpossiblecatalyst forelectronics application.TheauthorsdemonstratedthatsolidZrO2 nanoparti-clesareactivecatalysts,andneitherreducetoZrnorcarbonizeinto ZrCduringthegrowthprocess.Thisobservationissubstantiatedby theunderstandingofZrO2chemistryathightemperaturesin liter-ature.ZrO2isknowntonotbereducedbyH2,evenattemperatures

of1500◦Candhigher[89].Sincethistemperatureismuchhigher thanthegrowthtemperatureusedinthestudy,reductionofZrO2

toZrorZrCishighlyunlikely

BothAl2O3andZrO2possesshighmeltingpoints(>2000◦C)in theirbulkform.Hence,itisunlikelythattheywouldbeinmolten stateduringtheentiregrowthprocess,evenafterfactoringin par-ticlesizeeffects.Moreover,giventhelowcarbondiffusivityinbulk ZrO2andAl2O3,thesuccessfulgrowthofCNTsissuggestivetooccur viaasurface-bornemechanismwhichwillbediscussedindetailin Section5

3.3 Othercatalystsystems

In2009,Takagietal.[54]demonstratedthatnanosizeddiamond particles(4–5nm)actedasCNTgrowthnucleieffectivelyinCVD Interestingly,thediamondnanoparticlesdidnot fusewitheach other,evenwhenagglomeratedparticles wereusedin theCVD process.Particledensitywasenhancedduetothenon-fusion char-acteristicofnanodiamondparticles.Theﬁndingssuggestthatthe growthofCNTsoccurredfromsolidcarbonnanoparticles Further-more,sincebulkdiffusionofcarbonintonanodiamondparticlesis unlikely,thesurfaceofthediamondmustplayanimportantrole

inthesynthesisofCNTs,inwhichitprovidesatemplateforthe formationofCNTcaps

Inrecentyears,abroadarrayofgrowthroutesusingpure car-bonsystemswithoutanyadditionofcatalystparticleshasbeen

Trang 5

G Model

Fig 2. SEM images of CNTs grown on HF-treated Si/SiO 2 wafer: (a) overview of the circular trace, (b) indicated area B in image (a), (c) high magniﬁcation of image (b) and (d) inner area of the circle.

Reprinted with permission from [81] Copyright (2009) American Chemical Society.

developed.ThegrowthofCNTsongraphiticsurfaceshasbeen

suc-cessfullydemonstratedinvariousworksbyLinetal.[55,56].The

authorsemployedcarbonblack,ﬂakegraphitepowderandhighly

orientedpyrolyticgraphiteas substratestogrowCNTsby CVD

Previousstudiesindicatethatdanglingstructuresofcarbonshave

thecatalyticability todecomposehydrocarbons[90,91].Hence,

the defective sites present on the substrateswill result in the

decompositionofthecarbonfeedstockandleadtothesubsequent

formationofamorphousnanobumps,asshowninFig.3.Oncethese

nanobumpshaveformed,whetherfromalift-offprocessor

self-assemblymechanism,thegrowthofCNTscanoccur.However,two

majordisadvantagesstillneedtobeovercomebeforecatalyst-free

CNTgrowthcanbeemployedinmacroscopicapplications:the

sig-niﬁcantlylow synthesisyieldand thehighgrowthtemperature

required.Twoof thesefactorsareunattractiveforscalingupto

massproduction

Recentdevelopmentsinnanomaterialsynthesisand

character-izationhavebroughtupmanynewcatalystsfortheproduction

ofCNTs.Apartfromthenon-metalliccatalystsmentionedabove,

mostrecently,calciumsilicate,CaSiO3wasshowntohavethe

abil-itytocatalyzeCNTgrowthonapyrolyticgraphitepapertape[92]

Theauthorsproposedthattheroleofsilicateissimilartothatofa

transitionmetalandisbasedonthesolubilityofthecarboninthe

silicate.Theelectronenergylossspectra(EELS)alsorevealedthat

thecarbonsurroundingthecatalystismoreamorphousthanthat

inthetube,whichsupportsthesolidstatetransformationof

car-bon[92].RecentreportsonthegrowthofCNTsovernon-metallic

catalystsaresummarizedinTable1

4 Controlling the length, diameter and chirality of CNTs

over non-metallic catalysts

AlmostallcurrentlyavailabletechnologiesforCNTfabrication

canonlyproducemixturesofCNTswitharangeof(n,m)indices

[48].ThisposesahugelimitationfortheapplicationofCNTs.Hence,

thegrowthofCNTswithwell-controlledstructuresishighly

desir-ableforbothfundamentalresearchandpracticalapplications[44]

Liuetal.[95]reportedthedirectlength-sortedgrowthof SWC-NTsusingSiO2nanoparticles.TheauthorsfoundthatSiO2catalyzed grownSWCNTshaveanextremelylowgrowthvelocityof8.3nm/s, whichisabout300timesslowerthanthatofthecommonlyusedCo catalystforSWCNTgrowthatthesamereactioncondition.Theslow growth velocity allowsdirect length-sortedgrowth of SWCNTs withaveragelengthsof149,342and483nmbysimplyadjusting thegrowthdurationscorrespondingly[95].Owingtotheirﬁnite lengtheffect,theshortSWCNTsarebelieved todisplay intrigu-ing physicsand are attractivefor various practical applications includingscanningprobes[96–98],catalystsupports[99], biologi-calimaging[100,101],molecularsensing[100],electronicdevices [100,102]andsoon.Furthermore,comparativestudiesrevealed thatSiO2catalystexhibitsconsiderablylongercatalyticactivetime comparedtothecommonlyusedCocatalyst,whichlosesits cat-alyticactivityafterashortperiodoftime

Recently, the growth of SWCNTs with controlled diameters usingSiO2nanoparticlesweredemonstratedbyChenandZhang [103].VarioussizesofSiO2nanoparticlesweregeneratedbythe thermal oxidation of3-aminopropyltriethoxysilane(APTES) lay-erswithdifferentthicknesses.ItwasshownthatthesizeofSiO2 nanoparticles increased with the number of assembled APTES layers Thesenanoparticlesservedas nucleationcenters,where SWCNTswithdiametersrangingfrom0.90to1.82nmweregrown

in ethanol-CVD The ﬁndings clearly indicated a direct correla-tionbetweenSWCNTdiameterandSiO2nanoparticlesize.Fig.4 presentstheschematicofthepreparationproceduresfromAPTES layerstoSiO2nanoparticleswithcontrolledsizes,followedbythe growthofSWCNTsinCVD

Chirality-selective synthesis ofSWCNTs isessential for their application in nanoelectronic devices [104] because electronic structuresaredeﬁnedbythechiralindex(n,m).Itiswellknown thatthestructuresofSWCNTsaredeterminedbytheinitialcarbon structure asthegrowthcommences.Thestructureof this“cap”

is determinedduringthenucleationstage.Hence,it is possible

tocontrolthechiralityofSWCNTsbycontrollingtheprocessof capformation.Yuetal.[105]demonstratedarationalapproachto

Fig 3.SEM image of (a) carbon nanobumps initially formed on the surface of the acid-treated ﬂake graphite, TEM images of (b) a typical nanobumps and (c) an as-grown MWCNT with the close-cap feature.

Trang 6

Table 1

Recent reports on the growth of CNTs over non-metallic catalysts.

Si or Ge nanoparticles on

SiC(0 0 0 1) substrate

SiC substrate cleaned using H 2 SO 4 /H 2 O 2 (4:1) oxidation and HF-etching Annealing and Si deposition on SiC substrate at 1000 ◦ C in UHV Deposition of Ge or Si at room temperature

Carbon source: EtOH Carrier gas: Ar/H 2

Temperature: 850 ◦ C Reaction time: 10–30 min

SWCNTs and DWCNTs, diameters

<5 nm

SiC nanoparticles on

Si(1 1 1) substrate

Si substrate cleaned using H 2 SO 4 /H 2 O 2 (4:1) oxidation and HF-etching, followed by soaking in EtOH solution SiC nanoparticles formed by heating substrate at 1000 ◦ C in UHV

SWCNTs and DWCNTs, diameters

<5 nm

Si(1 0 0) wafer scratched by

another Si wafer or

diamond scriber

Si wafer cleaned in HF solution for 2 min Si wafer or diamond scriber used to produce growth patterns on wafer

by mechanical scribing

Carbon source: CH 4

Carrier gas: N 2

Temperature: 850◦C Reaction time: 3–5 min

nanotubes/␮m 2

[50]

Carbon-doped SiGe islands

on Si(0 0 1) substrate

SiGe (30% Ge) deposited by CVD on Si wafers, followed by carbon doping using ion implantation Substrate cleaned in

HF solution and chemical oxidation using 30% H 2 O 2 at room temperature

Carbon source: CH 4

Carrier gas: Ar/H 2

Temperature: 850◦C Reaction time: 10 min

SWCNTs, diameters ranging from 1.2 to 1.6 nm

Ge Stranksi–Krastanow

dots on Si substrate

Ge dots formed by CVD deposition of Ge atop a thin Si buffer layer Substrate cleaned in HF solution and chemical oxidation using 30% H 2 O 2 at room temperature.

Carbon source: CH 4

Carrier gas: Ar/H 2

Temperature: 850 ◦ C Reaction time: 10 min

Ge nanocrystals on Si/SiO 2

wafer

Ge nanocrystals formed by implanting Ge into SiO 2 layer and annealing in N 2 , followed by HF-etching

Carbon source: CH 4

Carrier gas: H 2

Temperature: 850–1000 ◦ C Reaction time: 20 min

4.1 ± 1.2 in length/␮m 2

[40]

Ge nanoparticles on

Si(0 0 1) patterned by

nanoindentation

Si sample patterned by nanoindentation subjected to a cleaning stage by cyclical ultrasonic baths in ethylic alcohol and deionized water, followed by annealing in UHV at 600 ◦ C for 30 min

Carbon source: C 2 H 2

Carrier gas: H 2

MWCNTs, originating from Ge nanoparticles < 50 nm in diameter

Te nanoparticles on Si/SiO 2

wafer

Either single crystal TDEC or TeCl 4 was employed as catalyst precursor in CVD

SWCNTs, high percentage (92.2%)

of superlong semiconducting SWCNTs

8–10 SWCNTs/100 ␮m

[82]

ZnO nanoparticles on

Si/SiO 2 wafer

EtOH solution of ZnCl 2 and PVP was dropped onto the wafer, followed by calcination in air at 700 ◦ C for 5 min

Carbon source: EtOH Carrier gas: H 2

SWCNTs, average diameter of 1.2 nm

1–2 SWCNTs/10 ␮m

[80]

TiO 2 nanoparticles on

Si/SiO 2 wafer

TiO 2 sol synthesized by the reaction of Ti(OC 4 H 9 ) 4 with ethanol in inorganic acid under stirring Sol was dispersed

on substrate followed by sintering at 900 ◦ C for 1 h

Carbon source: CH 4 or EtOH Carrier gas: Ar/H 2

SiO 2 ﬁlm on Si/SiO 2 wafer Substrate ﬁrst cleaned by sonication and then sputtering

deposited with a 30-nm-thick SiO 2 layer

Carbon source: CH 4

Carrier gas: Ar/H 2

Trang 7

Table 1 (Continued)

Si/SiO 2 wafer scratched by

diamond blade

Substrate ﬁrst cleaned by piranha solution, followed by washing with deionized water and acetone under sonication

SWCNTs, diameters in the range of 0.8–1.4 nm

Si/SiO 2 wafer etched with

HF

Substrate ﬁrst cleaned by piranha solution, followed by washing with deionized water and acetone under sonication, then thermally annealed in air at 1000 ◦ C for

1 h.

SiO 2 nanoparticles on Si

substrate

Catalyst precursor POSS was dissolved in ethanol and then nebulized using a 1.7 MHz ultrasonic beam

MWCNTs, diameters in the range

of 13–16 nm

SiO 2 nanoparticles on

Si/SiO 2 substrate

Substrate was water-plasma etched at 30 W, 250 kHz and 0.62 Torr for 20 min

Carbon source: CH 4

Carrier gas: Ar/H 2

SWCNTs, diameters in the range of 1.29–1.65 nm

Al 2 O 3 nanoparticles on

Si/SiO 2 wafer

Aluminum acetate powder was dispersed in ethanol solution, and then loaded onto the wafer by dip-coating.

Substrate then calcined in air at 950◦C for 30 min

Carbon source: CH 4

Carrier gas: Ar/H 2

Nanodiamond particles on

graphite plates

Nanodiamond particles were produced by the detonation method, dispersed in ethanol and enclosed in amorphous carbon or graphite, followed by annealing in air at 600–700 ◦ C for 1–15 min

SWCNTs, diameters ranging from 1

to 2 nm

Porous carbon black

CB-BP2000

CB treated in HCl, washed in pure water and then dried in oven at 120◦C

Carrier gas: Ar Temperature: 800 ◦ C Reaction time: 30 min

MWCNTs, diameters ranging from

20 to 80 nm

Flake graphite

powder/highly oriented

pyrolytic graphite

Graphite samples subjected to O 2 oxidation, acid treatment in HNO 3 , and laser ablation to generate defects and oxygenated functional groups

Carrier gas: He Temperature: 850 ◦ C Reaction time: 30 min

MWCNTs, diameters ranging from

20 to 90 nm

8.38 × 10−2g CNT/g graphite · h

[56]

Amorphous carbon (a-C)

layer on glass substrates

Glass substrates cleaned in trichloroethylene, acetone, methanol and distilled water a-C layers deposited on glass

by a RF magnetron sputtering method

Carbon source: CH 4

Carrier gas: Ar/H 2

CaSiO 3 on pyrolytic

graphite paper

CaSiO 3 sol produced by mixing CaCl 2 , Si(OC 2 H 5 ) 4 , ethanol and NaOH Graphite dipped in sol and dried in air at room temperature

Carbon source: EtOH Carrier gas: Ar Temperature: 1200–1400 ◦ C Reaction time: 60 min

MWCNTs, ﬁlled with amorphous CaSiO 3

Trang 8

G Model

Fig 4. Schematic of diameter controlled growth of SWCNTs from SiO 2 nanoparticles: (a) APTES with different layers were assembled on the Si substrate, (b) discrete SiO 2

and (c) growth of SWCNTs.

engineerfullerenecapsforgrowingSWCNTswithcontrolled

struc-turesviaCVD.Inthepaper,fullerenecapswereobtainedfroma

precursorcalledfullerendionederived frompristineC60.Several

pretreatmentprocedures were employed to acquire the

hemi-sphericalfullerene,asshowninFig.5a.Resultsshowedthatthe

temperatureusedforthermaloxidationstronglyaffectedthesize

andstructureofthecapandhence,thediameterdistributionofthe

as-grownSWCNTs.Strongeroxidationtreatments(450◦C

oxida-tioninair)promotedtheproductionofsmalldiameterSWCNTs,

whileweakeroxidation treatments(350◦Coxidation inair)led

tolargediameterSWCNTs.Interestingly,theas-grownSWCNTs

fromthermally openedC60showedstep-likediameter

distribu-tionscomparedtoSWCNTscatalyzedbyFenanoparticles[105]

Inanotherpaper,Yaoetal.[106]appliedtheconceptof“cloning”

andpresentedanapproachtogrowSWCNTswithcontrolled

chi-ralityusinganopen-endgrowthmechanism.Theschematicforthis

approachisdepictedinFig.5b.SWCNTswithapredetermined

chi-ralityandopenendswereemployedas“seeds/catalysts”.Duplicate

SWCNTscouldbecontinuouslygrownfromtheparentSWCNTs

segmentsbythedirectadditionofCX(mainlyC2and/orC3)

radi-calstotheopen-endseeds.Itwasreportedthatmorethan600short

seedsegmentsweremeasuredandtheyieldofcloningwas

rela-tivelylow(around9%).Theyieldcanbegreatlyimprovedupto40%

bygrowingSWCNTsonquartzsubstrate.BasedonAFMandRaman

spectroscopycharacterizations,theduplicatenanotubewasshown

toexhibitsimilarstructuretoitsparentnanotube[106]

5 Growth mechanism discussion

DespiteenormousstridesinthesynthesisofCNTs,the mecha-nismsregardingtheirnucleationandgrowthstillremainahighly debatedissue Themostwidely acceptedCNTgrowthmodel is theVLStheory[107,108].Fig.6showsthethreesuccessivesteps involvedintheVLSmechanism.Themodelassumesthata car-boncontaininggasprecursoradsorbsontothecatalystparticleto formelementarycarbonatoms.Next,thecatalyticallydecomposed carbonatomsdissolveinthebulkofthenanoparticletoforma liquidmetastablecarbideanddiffusewithintheparticle.Finally, uponreachingsupersaturatedstate,solidcarbonprecipitatesout

inatubular,crystallineform[42].However,resultspresentedin thisworksuggestthattherearesomeobservational inconsisten-cieswhichdo not supportthis mechanism forCVDproduction DuetothesheernumberofinformationonCNTsynthesisroutes, theprimarygoalhereistofocusonmajorelementsinthecurrent understandingofCNTgrowth,highlightpointsofcontroversyand presentnewﬁndings

It is often argued that the termination or prevention of CNTgrowthis attributed tocatalyst poisoning [109].How this

Fig 5.(a) CNT growth from opened C 60 Reprinted with permission from [105] Copyright (2010) American Chemical Society (b) An opened CNT Reprinted with permission from [106] Copyright (2009) American Chemical Society.

Trang 9

G Model

Fig 6. The three steps involved in the VLS mechanism: (a) decomposition of the carbon-containing precursor on the catalyst surface, (b) diffusion of carbon atoms through the particle and (c) precipitation of carbon at the catalyst–support interface and formation of a nanotube.

poisoningactuallyoccurshasnotbeendemonstratedandhasbeen

questionedbyReillyandWhitten[110].Theypointedoutthe

con-tradictionintheargumentthatanamorphouscarboncoatingon

thecatalystparticlehaltsgrowth,yetwhentheparticleiscoated

withgraphiticcarbon(CNTgrowth),itisnotconsideredpoisoned,

viz.theyareapparentlystillabletodecomposehydrocarbons

Fur-thermore,thefactthatCNTscanbegrownfromsemiconducting

catalystse.g.Si,SiCandGe[38,76]furtherweakenstheaccepted

notionthatmetalliccatalystparticlesareessentialforthe

decom-positionof thehydrocarbon.Reillyand Whitten[110]proposed

thatamorelikelyscenarioisthatafreeradicalcondensate(FRC)

formsduringhydrocarbonpyrolysis.Theprocessbeginswiththe

breakingofacarbon–hydrogenorcarbon–carbonbondwitheach

fragmentkeepingoneelectrontoformtworadicals.Thepresence

oftheradicalthusenablesrapidarrangementofcarbonbonds.In

theFRCmodel,thecatalystroleistosimplyprovideaninterface

fortheformationofhemisphericalcapsatnucleationbecausethis

reducesthehightotalsurfaceenergyoftheparticlecausedbyits

highcurvature

ItiswellestablishedthatmetalcatalystssuchasAu,AgandCu

havelowsolubilityofcarbon[111–113].Unlikeirongroupmetals

Fe,Niand Co,thesemetal catalystsdonot possessd-vacancies

[114,115].Noactivesites arepresenttodissolvecarbon;hence

neithersaturationnorprecipitationispossible.Nevertheless,these

elementshavebeenreportedtoyieldSWCNTs[29].Furthermore,

inthecaseofstableoxidessuchasAl2O3andZrO2,theVLStheory

isclearlyunrealisticascarbondissolutionisunlikelybutprobably

occursthroughasurface-bornemechanism.Similarargumentscan

beusedtoexplainthenucleationofCNTsfromnanosizeddiamond

particles.Takagietal.[54]proposedanewmodel,the“vapor–solid

surface–solid”(VSSS)toexplainthegrowthofSWCNTsondiamond

surfaces.Theauthorssuggestedtheformationofagrapheneisland

withaﬁve-memberedringonthesp2-relaxeddiamondsurface

Thecurvedgraphiteislandthenliftsofftheparticlesurfaceexcept

foritsedges,thusformingaCNTcap.Theedgeoftheas-formed

capissaidtobechemicallyactiveandservesasincorporationsites

forcarbonatoms,whereCNTgrowthisinduced.Theschematicfor

thegrowthofSWCNTsonansp2-solid-carbonsurfaceisdepictedin

Fig.7,wherethecorecanbediamond,Si,SiC,orAl2O3[62].Inother

caseswhereoxidescanbereducedtoformcarbides,bulkcarbon

dissolutionandprecipitationinamannersimilartotheVLStheory

maybevalid

Ontheotherhand,thefactthatCNTscanbegrownonpure

carbonsystems withoutanyadditionofcatalystsindicatesthat

catalystparticles,eithermetallicornon-metallic,arenot a

pre-requisitefor thegrowth of CNTs.This doesnot imply thatthe

catalyst’sstructuringroleislost.Instead,anoxidesupportorsimply

unsaturatedbondsattheedgesofgraphiticlayerscanfulﬁllthe

roleofprovidinganinterfacefororderedcarbonformation.This

meansthat thesubstrateis thecatalyst forgraphiticformation

Experimentalevidencefromdifferentstudiesshowedtheaddition

Fig 7.Schematic of SWCNT growth on an sp 2 -solid-carbon surface Reprinted with permission from [62] Copyright (2009) Tsinghua University Press and Springer-Verlag.

ofcarbontotheedgesoffreestandinggraphiticedges[116–118].In thisscenario,carbonspeciesdiffusealongthesurfaceofgraphitic speciesandsubsequentlyadsorbedtotheedges.Thismechanism can beused to explain thegrowth of SWCNTs nucleated from openedfullerenes[105]andtheformationofMWCNTsongraphitic surfaces[55,56]

TherecentsuccessbyLiuetal.[86]andHuangetal.[81]in syn-thesizingSWCNTsfromSiO2 nanoparticlessupportedonSi/SiO2 substratesclearlyhighlightsthecatalyticgraphitizationpotential

of SiO2 nanoparticles.Asthere appear tobe severalconﬂicting results in explaining the growth of CNTs over SiO2 nanoparti-cles,wewillfocusourdiscussionheretowardthegraphitization mechanismofthisparticularcatalyst.Akeyquestionregardingthe useofSiO2asagraphitizationcatalystiswhethercarbidephases formin thereaction, orwhetherit remains stable.Bachmatiuk

etal.[85,119]investigatedSiO2 nanoparticlesaftera CVD reac-tion.Fig.8showsa schematicoverviewillustratingthestepsin theformationofCNTs,asproposedbytheauthors.Inthemodel, SiO2nanoparticlesﬁrstreducetoSiCviaacarbothermalreaction TheSiCparticlesthencoalesce,leadingtotheformationofCNTs consistingofstackedgraphitic“yarmulke-like”caps.Transmission electronmicroscopy(TEM),infrared(IR)andRamanspectroscopy conﬁrmedtheparticlesattherootoftheCNTstobeSiC.Theirdata pointstowardthecarbothermalreductionofSiO2accordingtothe overallreactionbelow

ItiswellknownthatSiCisproducedthroughtheformationof intermediateSiO.Therefore,theoverallreaction(2)canbedivided intotwoelementaryprocesses:

TheformationofthestackedcupCNTsandtheparticleshape suggestthatthelikelymechanismisthedissolutionofcarbonfrom thevaporphaseintotheparticlesfollowedbyprecipitationfrom

aliquid orliquid-likeparticle, viz.theVLSmechanismasfound withmetalcatalystsisprobablyoccurringinthiscaseaswell[85]

Trang 10

G Model

Fig 8. Schematic of the carbothermal reduction of SiO 2 to SiC and carbon nanostructure formation: (a) SiO 2 is reduced to SiC by carbothermal reduction, (b) SiC nanoparticles coalesce and (c) carbon caps form on the surface of SiC particles through precipitation and/or SiC decomposition.

Fig 9.General pathway for the growth of SWCNT on SiO 2 nanoparticles.

ThisﬁndingisincontrastwiththeXPSconductedbyHuangetal

[81]whichdidnotshowanycarbideformationandhencethey

arguedthatthegrowthofCNTsoccurredfromSiO2nanoparticles

Huangetal.[81]proposedthatnanosizedSiO2(<2nm)isinmolten

stateatgrowthtemperatureandthehighﬂuctuationofthe

liquid-likestructureallowsSiandOatomstomovearoundquickly,thus

creatingaspaceholeordislocation,whichhavethecapabilityto

decomposehydrocarbonmoleculesandgrowCNTs.However,it

shouldbenotedthatBachmatiuketal.[85]alsofoundnotraceof

carbideformationwhenusingXPSdespiteothertechniquesclearly

demonstratingthepresenceofcarbides.Therefore,Rummelietal

[120]pointedoutthatXPS,whichisasurfacesensitivetechnique,

maynotbewellsuitedtodetermineifoxidesusedascatalystsfor

CNTgrowthreducetocarbidesornotduringthesynthesisprocess

Mostrecently,analternativetheorybehindthenucleationof

CNTsonSiO2nanoparticleshasbeenproposedbyPageetal.[121]

Theauthorsindicatedthatavapor–solid–solid(VSS)mechanism,

rather than a VLS mechanism is responsiblefor thegrowth of

CNTsonSiO2nanoparticles.Quantum-chemicalmolecular

dynam-ics(QM/MD)simulations wereusedtostudythe CH4-CVD and

CNTnucleationprocess.Uponsupplyofthecarbonfeedstockto

thesurfaceofamodelSiO2nanoparticle,COwasproducedasthe

mainchemicalproductoftheCVDprocess.TheproductionofCO

occurredsimultaneouslywiththecarbothermalreductionofthe

SiO2nanoparticles.However,twoprimarydifferencesfrom

exper-imentalobservationsbyBachmatiuketal.[85]areevident.First,

theauthorsfoundthatthecarbidestructureformedfromthe

carbo-thermalreductionofSiO2isclearlyanamorphousone,i.e.nobulk

SiCstructurewasformedasaresultofthereductionprocess

Sec-ond,thecarbothermalreductionofSiO2byCH4waslimitedtothe

surface/subsurfacelayersoftheSiO2nanoparticle,withthecore

oftheSiO2 nanoparticleremainingoxygen-rich.Basedon

simu-lations,theﬁrststageoftheCNTnucleationprocessfeaturedthe

coalescenceofcarbonatomsontheSiO2surface,resultinginthe

formationofextendedpolyynechains.Athigherconcentrations

ofcarbon,theisomerizationofthesepolyynechainsresultedin

theformationofisolatedsp2-carbonnetworksontheSiO2surface, andtheformationofCNTcapstructure.Fig.9showsthegeneral pathwayforthegrowthofSWCNTonSiO2 nanoparticles.These simulationsindicatethatcarbonsaturationoftheSiO2surfaceisa prerequisiteforCNTnucleation.Hence,Pageetal.[121]concluded thatelementsofSiO2-catalyzedCNTnucleationareirreconcilable withthoseofaVLS-typemechanismbutproceedsaccordingtoa VSSmechanism.FundamentaldifferencesbetweenCNTnucleation

onnon-traditionalandtraditionalcatalystsarethereforeobserved

6 Summary and outlook

Inthisreview,therecentdevelopmentsinthemetal-free cata-lystgrowthofCNTshavebeenstudied.ThefabricationofSWCNTs wasshowntobepossiblefromsemiconductingandceramic cat-alystsystems.Thisrevealsthatthecatalyticdecompositionofthe carbonfeedstockandthegraphitizationabilityarenotessentialina catalyst.Therecentdevelopmentoftheﬁeldisevenmoreexciting ThegrowthofCNTsispossibleonpurecarbonsystemswithoutany additionofcatalystparticles.Thisﬁndinghighlightsthatparticles, eithermetallicornon-metallic,arenotnecessaryforthegrowthof CNTs.Thisleadstoarecentdiscoveryoftheroleofthecatalystfor thegrowthofCNTswhereonlyananoscalecurvatureisrequired However,amoredetailedinsightintothisnewareaisnecessaryto providefundamentalunderstandingonthecatalyticgrowth mech-anismandprocessofCNTs.Despitethetremendousadvancesthat havebeenmade,thereremainsa fairamountofcontroversyin explainingthegrowthmechanismofCNTs.Thereasonforthisis duetothesheernumberofpossiblesynthesisroutesandthefact thatthereisnosingleuniversalgrowthmode

TheintegrationofCNTsintosuccessfulapplicationsand large-scaleproductionprocesses requiretheunderstandingof several fundamentalissues,whichareyettobeaddressed.Forexample,

anintriguingquestionconcerningnanotubegrowthfromthenewly developedcatalystsiswhetherallsubstanceswithasuitable par-ticlesize (≤5nm)arecapableofgrowingSWCNTsregardlessof

Định dạng
Số trang	12
Dung lượng	0,93 MB