growth of novel carbon phases by methane inﬁltration of ... · gen upon inﬁltration using the...

www.elsevier.com/locate/carbon

Carbon 45 (2007) 1085–1096

Growth of novel carbon phases by methane infiltrationof free-standing single-walled carbon nanotube films

Artur Böttcher a,*, Frank Hennrich b, Harald Rösner b, Sharali Malik b,Manfred M. Kappes a,b,*, Sven Lichtenberg c, Günter Schoch c, Olaf Deutschmann c

a Institut für Physikalische Chemie, Universität Karlsruhe, Fritz-Habel-Weg 4, D-76128 Karlsruhe, Germanyb Institut für Nanotechnologie, Forschungszentrum Karlsruhe, D-76021 Karlsruhe, Germany

c Institut für Technische Chemie und Polymerchemie, Universität Karlsruhe, 76131 Karlsruhe, Germany

Received 21 June 2006; accepted 8 December 2006Available online 30 January 2007

Abstract

High-temperature methane infiltration of thin, free-standing films of acid-treated single-walled carbon nanotubes (SWCNT) has beenstudied by means of scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. In the early stages ofinfiltration, carbon nuclei form predominantly at SWCNT bundle intersections. Further growth proceeds via the formation of graphitenanosheets – without further influence of the nanotube support. Both sheet edges and their structural imperfections act as reaction cen-ters for subsequent deposition, likely giving rise to autocatalytic deposition kinetics. In contrast, infiltration with a H2:CH4 (24:1) mixtureleads to the reductive activation of residual Ni/Co impurities embedded in the precursor SWCNT-felt. This is associated with a differentpredominant carbon deposition mode in which multiwalled carbon nanotubes grow out from the substrate.� 2006 Elsevier Ltd. All rights reserved.

1. Introduction

Thin films of partially oriented single-walled carbonnanotube (SWNT) bundles, may be thought of as a newclass of essentially monodispersed all-carbon materials[1]. They exhibit high-temperature stability [2] as well assurprisingly high tensile strength [3]. The correspondingYoung’s modulus can range up to at least 24 MPa, makingthese films and particularly composites derived from them,potentially interesting for applications [4]. We haveshown that thin, free-standing films of SWCNT bundles(=SWCNTF) can be readily generated from aqueousSWCNT suspensions by filtration through nanopore filters[5]. Careful removal of the deposited material from the fil-

0008-6223/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2006.12.008

* Corresponding authors. Address: Institut für Physikalische Chemie,Universität Karlsruhe, Fritz-Habel-Weg 4, D-76128 Karlsruhe, Germany.Tel.: +49 7216083254; fax: +49 7216087232.

E-mail addresses: [email protected] (A.Böttcher), [email protected] (M.M. Kappes).

ter results in large area (10 cm2), free-standing films whichare homogeneous on length scales greater than 200 nm.This was demonstrated both for as-prepared and for nitricacid-treated (reduced metal content) SWNT materials [5].Film thickness can be controlled over the range from200 nm up to several lm by varying the suspension vol-ume/concentration. In free-standing SWCNTF samples,bundles lie preferentially in the plane of the film but areotherwise aligned randomly. Mass densities are approxi-mately 1 g/cm3 [4]. Varying the rate of solvent removal dur-ing filtration, results in some vertical layering which isassociated with slight density differences between layers.

The mechanical properties of more conventional porouscarbon materials can often be significantly strengthened bymethane gas infiltration/pyrolitic carbon deposition at highsubstrate temperatures [6]. This motivated us to performanalogous experiments with methane infiltration ofSWCNTF. Additionally, we were interested in the associ-ated elementary reaction steps and how these differ fromreactions occurring at graphite single crystal surfaces under

mailto:[email protected]:[email protected]

1086 A. Böttcher et al. / Carbon 45 (2007) 1085–1096

otherwise identical reaction conditions [7]. Below we reportthe growth modes and the morphology of the carbonphases generated when exposing acid-treated SWCNTFto (i) pure methane or (ii) to methane in excess hydrogenmixtures (1:24) at reaction temperatures of 1373 K. Resul-tant materials were studied using a combination of SEM,TEM and Raman spectroscopy. Using pure methane asthe gaseous feed, we observed nucleation of nanograph-ene-like sheets at SWNT junctions followed by autocata-lytic growth along the edges of these nascent graphiticsheets. Transition metal impurities (deriving from the cata-lyst for SWCNT preparation) contained within theSWNTF material become reductively activated by hydro-gen upon infiltration using the methane hydrogen mixture.This leads to the (metal catalyzed) formation of multi-walled carbon nanotubes (MWCNTs).

Metal catalyzed CVD growth of SWCNTs andMWCNTs is experimentally well established and has beenthe subject of numerous studies using a variety of gaseouscarbon feeds. More recently, there have also been reportsof the ‘‘second generation’’ growth of nanotubes fromnanotube substrates following the renewed deposition oftransition metal catalysts [8]. This is presently of particularinterest in the context of ‘‘cloning’’ SWCNTs, i.e. extend-ing preexisting SWCNTs by reinitiating their growth – ide-ally while retaining chiral index [9]. In the work reportedbelow we show that acid-treated SWCNTF substrates withtrace amounts of nickel and cobalt impurities providenucleation sites for autocatalytic nanographene depositionwhen exposed to pure methane at high temperatures. Nei-ther continued SWCNT growth nor new nanotube forma-tion is observed under these conditions. In contrast, thetransition metal impurities (metal oxides or carbon coatedmetal particles) can be reductively activated with an excessof hydrogen. (New) MWCNT growth then ensues in addi-tion to deposition of nanographene.

Fig. 1. Schematic of the hot-wall reactor used to infiltrate SWCNTF(cross-section along the axis of the graphite block). The SWCNTF-sampleis mounted between two ceramics plates with 10 mm large holes forinfiltration of both sites of the felt. The main methane flow is marked bylarge arrows. Methane molecules reach the sample via diffusion throughthe narrow cylindrical slit between inner and outer graphitic blocks asindicated by short backwards directed arrows.

2. Experimental

The SWCNTs used in this study were fabricated by laser ablation/vaporization of a composite carbon target containing Ni/Co catalyst aspreviously described. Ablation was carried out under 500 mbar flowingAr within a furnace held at a temperature of typically 1447 K similar towhat was already described in Ref. [10]. SWCNTs exhibit a rather narrowdiameter distribution 1.2 ± 0.2 nm and a broad length distribution. Theattractive van der Waals interaction between individual nanotubes resultsin the formation of densely packed SWCNT bundles (with diameters from20 nm to 120 nm). These are harvested from the flowing argon after exit-ing the oven by filtering in extraction thimbles (Schleicher & Schuell). Theresultant low density ‘‘as-prepared’’ SWCNT felts contain significantamounts of metal catalyst and amorphous carbon impurities. In orderto reduce these impurities, as-prepared material was refluxed in 3 M nitricacid to yield ‘‘acid-treated SWCNTs’’ (residual metal contents of 1–2 wt%[1]). Large area, free-standing thin films (SWCNTF) of acid-treatedSWCNT bundles were generated as previously described in [5]. Thisinvolves peeling filtered SWCNT material from a nanopore filter (cellulosenitrate 0.2 lm, Millipore). Correspondingly, the two faces of eachSWCNTF sample differ slightly in their microscopic morphology. The sidefacing the nanopore filter exhibits some SWCNT bundle pullout, whereasthe side facing the SWNT suspension is flatter. For infiltration studies,

SWNTF samples were formed as discs with a diameter of 16 mm and amean thickness of about 50 ± 5 lm. Prior to gas infiltration, they wereclamped between two ceramic plates (PBN, pyrolitic bore nitride, Schunk)with 12 mm large holes and inserted into a hot-wall reactor.

Infiltration of SWCNTF samples was performed in a hot-wall reactorwhich had previously been used for chemical vapor infiltration of ‘‘con-ventional’’ carbon/carbon composites (Fig. 1, [11–13]). The reactor com-prises a hollow graphite block through which the reaction gas flows.The ceramic holder containing the SWCNTF sample is inserted into thegraphite block such that the flatter side of the nanotube film faces towardsthe diffusing reagent gas flow. The reactor is resistively heated by an outerheating jacket. The temperature of the graphite walls was measuredusing a PtRh-thermocouple and stabilized to within 5 K by a controller.The SWCNTF sample was located in the center of a 12 cm long zone ofconstant working temperature of typically 1373 K. Prior to reachingthe SWCNTF sample, methane molecules must transit a hot inlet chan-nel equilibrating them to the reaction temperature and leading to signifi-cant vibronic excitation, some dissociation and associated secondaryreactions.

Before starting the infiltration procedure, the SWCNTF-loaded reac-tor was rinsed with argon for 30 min at the working temperature. Aftercompleting the infiltration, the reaction gas flow was turned off and thereactor was cooled down with argon to near room temperature (at20 kPa base pressure) over typically 5-6 h before extracting samples.Experiments were performed under pressure and temperature conditions

A. Böttcher et al. / Carbon 45 (2007) 1085–1096 1087

which have previously been applied to the infiltration of oriented feltsbased on polyacrylonitrile microfibers (PAN, Conrati, Surface Trans-form), i.e. at a methane partial pressure of 20 kPa, at temperatures near1373 K, and under stabilized gas flow of 2.4 l/h corresponding to a resi-dence time of 0.5 s in the hot zone of the furnace [12,13]. The experimentswith H2/CH4 mixtures were also performed at a total gas pressure of20 kPa using a partial pressure ratio of p(H2)/p(CH4) = 24:1.

The reactor set-up allows only ex situ characterization of the infiltratedsamples. SEM investigations were performed with a LEO1530 scanningelectron microscope which provides a lateral resolution better than10 nm. The SEM was equipped with an X-ray (EDX) detector (EDAXCompany). TEM investigations were performed with a Tecnai F20ST elec-tron microscope (FEI Company) operating at 200 kV. The TEM wasequipped with a field emission gun, a Super-Twin objective lens and a darkfield (HAADF) detector for scanning transmission electron microscopy(STEM). It is capable of reaching a resolution of 0.235 nm and an infor-mation depth below 0.14 nm.

Raman spectra were acquired with a confocal Raman microscope(WiTec) with a spectral resolution of 3.75 cm�1 using backscattering con-figuration and a 100 fold objective (�1 lm spot size) excited with an Arion laser at 514.5 nm excitation with a power density of c � 1 · 105 W/cm2.

3. Results

3.1. CH4 infiltration

Fig. 2 contains SEM images which illustrate three repre-sentative stages of the methane infiltration process. Allimages presented were taken from the central region ofthe flat SWNTF side (see above). Fig. 2A shows a0.5 · 0.5 lm area before infiltration. Note the interwovenSWNT bundles as well as residual amorphous carbonand metal particulate impurities (deriving from the NiCocatalyst and evidenced in trace amounts by EDX).

The SEM image (1 · 1 lm2) shown in Fig. 2B illustratesthe surface topography created by exposing the SWNTF tomethane for 6 min at 1373 K – corresponding to the earlyinfiltration stage. Relative to Fig. 2A, we observe the for-mation of carbon nuclei predominantly at bundle intersec-tions. Exposed lower-lying bundles are seen to have a lowerdensity of (smaller) nuclei than the topmost bundles. Alto-gether we observe roughly 80 (circular) carbon nuclei per1 lm2 SWCNTF surface area with a diameter distributionpeaked at 12 nm. Assuming hemispherical deposits with amean mass density of 2.2 g/cm3 (similar to graphite) thetotal mass deposited during the initial infiltration stagecan be estimated as 8 · 10�17 g. When relating this massto the total number of impinging molecules (5.4 · 1026

CH4/cm�2 simply assuming all impinging molecules are

CH4) we infer an initial deposition probability in the rangeof 10�9.

The preferential location of carbon nuclei at bundlecrossings may be rationalized by considering the meandistance between two adjacent bundle crossings (meanmesh size d < 80 nm) versus the mean diffusion lengthfor carbon carriers moving along the bundles. The lattercan be derived from the analogous system, C/HOPG.For substrate temperature of 1373 K the diffusion lengthof carbon carriers on a perfect basal plane can reach avalue of 500 nm [14,15]. Thus, the relation d� LDiff,

implies that elevated equilibrium concentrations ofmigrating carbon carriers can be expected at bundle cross-ings correspondingly raising the yield for carbon deposi-tion there.

The preferential location of carbon nuclei at bundleintersections on the topmost layers of the bundle networkresults from the fact that the mean mesh size d (70 nmdiameter bundles with a mean separation of 80 nm) is atleast three orders of magnitude smaller than the mean freepath Lfree of gaseous carbon carriers (Lfree = 6.8 lm atp = 200 mbar, T = 1373 K). The ratio d/Lfree implies amean penetration depth comparable to the diameter of arepresentative bundle [16]. Furthermore diffusion into theSWCNTF is expected to be slow due to the high tortuosityof the material. Thus, SWCNTF infiltration is restricted tothe topmost bundle layers and proceeds predominantly vianucleation and subsequent growth of carbon deposits atbundle crossings. We cannot rule out that bundle crossingsexhibit modified electronic properties compared to freeparts of the bundles. This might raise the sticking coeffi-cients for CH4 or CH3 at such sites [11].

Fig. 2C (surface area 1.5 · 1.5 lm) shows a typicalimage of the second infiltration stage, which begins whenthe network of SWCNT bundles becomes buried belowgrowing carbon phases. Correspondingly, the SWCNTFsubstrate has no further apparent influence on the deposi-tion. SEM images reveal that the carbon phases grown dur-ing this stage comprise randomly stacked and orientedgraphene flakes (mean size of 40–60 nm) completely cover-ing the SWNTF material. Additionally, this phase is alsodecorated by bright sphere-like carbon deposits whichappear to exhibit a bimodal size distribution (centered inFig. 2C at 35 nm and 450 nm) with the smaller islandsdominant.

In order to prove the low effective reagent penetrationdepth already inferred from SEM images of the first growthstage, SEM images were taken of the deposit/SWCNTFinterface following second stage growth. Fig. 2C*, whichis an oblique angle view illustrates the typical interfacetopography. The SWCNTF region appears to be termi-nated by a thin layer of variously shaped and oriented car-bon nanosheets. The ca. 100 nm width of this intermixingregion provides a rough measure of the penetration lengthand confirms that the infiltration-induced densification ofSWCNTF samples is restricted to the topmost bundle lay-ers only.

Fig. 2D illustrates the final stage of the infiltration whichis reached upon exposing the SWCNTF to CH4 for120 min (imaged surface area 10 · 10 lm). The resultingsurface topography is dominated by densely packedspheres with a mean diameter of 800 nm. We estimate thecorresponding deposition probability to be in the rangeof 10�6 – again assuming a carbon density of 2.2 g/cm3

(this corresponds to a mean deposition rate of 6 ·10�14 g/s onto cross-sectional area of 1 cm2, consistent witha value of less than 2 · 10�13 g/s as determined from thedeposited layer thickness in Fig. 2C*).

Fig. 2. SEM images illustrating three representative stages of methane infiltration of SWCNTF at 1373 K and at a partial pressure of 20 kPa. Image (A)shows a 0.5 · 0.5 lm area of SWCNTF prior to exposure. Images B–D have been taken after infiltrating for 6, 30 and 120 min, respectively (correspondingimage sizes: B(1 · 1 lm), C(1.5 · 1.5 lm), D(10 · 10 lm)). C* shows an oblique angle SEM image of a fracture through the deposit/SWNT interface.


The fact that the mean deposition probability increasesfrom 10�9 to 10�6 with progressing reaction can be under-stood when assuming that the deposits grow via formationof defected graphene sheets. The only reaction centers, atwhich the incorporation of carbon can take place are latticedefects and sheet edges. In this way the corresponding reac-

tion rate scales approximately with the mean area of thegraphene layers created during the foregoing infiltrationstage, a · a. The averaged reaction rate increases with pro-gressing deposition and saturates when the mean free pathof the migrating molecules becomes lower than the sheetsize, LDiff < a � 500 nm. At the same point, nanosheet


stacking begins. We rationalize the rough surface texture ofthe large carbon hemispheres observed in the finalinfiltration stage as comprising randomly oriented graph-ene sheets and stacks (1.6 · 1.6 lm image D* in Fig. 2).Analogous phenomena, i.e. defect nucleated graphene layerstacking, has also been observed during carbon depositionon intentionally defected HOPG surfaces upon exposingthem to methyl radicals [17].

The infiltration of SWCNTF has been found to bestrongly dependent on reactor temperature. No significantamounts of deposited carbon could be found by SEM orTEM for infiltration times up to 2 h at temperatures lowerthan 1023 K. Fig. 3 shows SEM images corresponding to2 h infiltration at 1173 (upper) and 1373 K (lower), respec-tively. Whereas the sample infiltrated at 1173 K still exhib-its recognizable traces of the SWCNTF bundle networkcovered by a thin carbon layer (inset 1 · 1 lm, upperpanel), the 1373 K treated sample manifests densely packed

Fig. 3. Two SEM images illustrating thermal activation of the methane dtopography as observed after 2 h of methane infiltration at a temperature of 1SWCNT network still visible underneath. SEM images shown in the lower panNote the prevalence of densely packed hemispherical carbon deposits. The lat

carbon hemispheres without any traces of the SWCNTsubstrate. These ca. 1 lm diameter hemispheres grown at1373 K indicate extensive carbon deposition. Togetherthese results imply thermally activated growth with theeffective onset of extensive deposition occurring between1173 and 1273 K.

Several scenarios have been proposed to rationalizemethane infiltration – including the deposition of largerhydrocarbons/aromatics produced from the methane feedin a combination of homogeneous and heterogeneousupstream reactions. In one comparatively simple and there-fore commonly envisaged deposition scenario [18], the(nominally) first step corresponds to the dissociation ofimpinging CH4 molecule to create radicals such as CH3at the surface. The corresponding activation energy isaround 4.23 eV [19]. In spite of considerable vibronic exci-tation of incident methane molecules, the weak interactionbetween CH4 and the basal plain of HOPG or with SWNT

eposition process. The upper panel (including inset) shows the surface173 K. The 1 · 1 lm inset indicates a thin deposit layer with traces of theel were taken after performing the same infiltration procedure at 1373 K.

ter appear to be terminated by small carbon nanosheets.


sidewalls makes activation at the majority ‘‘surface’’improbable. Note that the corresponding methane bindingenergies have been calculated to range from 72 meV for(17,0) SWNT to 190 meV for (10,0) SWNT [20] (experi-mental values as derived from low-coverage adsorption iso-therms measured on SWNT bundles are only slightlyhigher (221.8 meV, [20,21])). Thus, estimated residencetimes are between 10�12 and 10�13 s [22] for infiltrationtemperatures of 1373 K. This in turn implies that (further)vibronic excitation of migrating molecules due to energytransfer from surface phonons will be inefficient. We there-fore speculate that CH4 dissociation only takes place atdefects of SWCNTs (or graphene edges/defects). The resi-dence time of the dissociation products, CH3, is consider-ably higher than that of methane molecules. The nearlyten times higher binding energies suggest residence timeson the order of 10�7 s at substrate temperature of 1373 K[10]. Thus, an elevated equilibrium concentration of activecarbon carriers as well as other derived recombinationproducts (C2H2, C2H4, C6H6, etc.) might be expected inthe vicinity of defect sites.

At room temperature, defect sites in graphene sheets(and likely SWCNTs) are typically terminated, e.g. by–C–OH or –C–H end groups [23]. Such end groups mustbe removed in order for methane to be activated. In thecase of HOPG, this can be done by heating to 1073–1173 K, leading to the formation of unsaturated arm-chairand zig-zag sites. While DFT calculations suggest prohibi-tive barriers for simple dissociation of terminating bonds[24], the concerted abstraction of adjacent hydrogen atomsterminating armchair edges appears to be the energeticallyfavored channel for edge activation (Eact = 2.96 eV[25,26]). Thus, we speculate that the activation of carbondeposition observed here between 1173 and 1273 K reflectsthe activation of edge and defects sites of graphene sheets,in turn enabling the decomposition of methane.

It is conceivable that defect activation and methanedissociation may also proceed in a concerted fashion, e.g.Hterminating + (CH4)migrating! H2 + CH3, where hydrogenremoval from graphene edges is accompanied by dissocia-tion of the CH4 molecule and H2 abstraction followed byedge-mediated inclusion of the radical dissociation prod-uct. This surface reaction is thought to exhibit quite highrates at 1000 K in the range of 10�12 cm3/s [24]. Formally,the graphene growth based on carbon inclusion at stepedges corresponds to a replacement of hydrogen termina-tions –C–H with –C–C–H groups. This step has to beaccompanied by second H2 abstraction event, –C–HþCH�3 ! –C–C–H2 þH2. However, due to the geometryof armchair edges the latter step appears to be facili-tated when the inclusion event proceeds in a concertedway, where simultaneously two CH�3 become temporar-ily adopted along the scheme: ð–C–HÞ2 þ 2CH

�3 !

ð–C–C–HÞ2 þ 3H2, i.e. the armchair sites become bridgedby intermediate C2H2 precursor molecules. Such associa-tive formation of molecular precursors from two hot CH4or CH3 is well established among gas-phase reactions [25].

3.2. CH4/H2 based infiltration

Next we explored how an excess of molecular hydrogenin the reaction mixture (CH4/H2 = 1/24) influences thetopography of the carbon phase deposited (keeping allother experimental parameters constant). Fig. 4 showsthe sample surface upon exposing to CH4/H2 mixturesfor 5 and 90 h, A/A* and B/B*, respectively. Fig. 4A indi-cates that two entirely different growth modes occur in par-allel. The prevailing large-area phase corresponds torandomly oriented nanographene sheets. Interspersedamong these are regular tubular structures generally grow-ing out from the surface – often slightly tilted with respectto the surface normal. The mean lateral extent of the sheetsis 40 nm, whereas the tubes exhibit a narrow diameter dis-tribution centered at 170 nm. After 5 h of growth the meanlength of these tubes has reached a value of around 1.3 lm– with respect to the underlying nanographene sheet sur-face. The morphology of the nanosheets is very similar tothat exhibited by a carbon phase fabricated via rfplasma-enhanced chemical vapor deposition on varioussubstrates at 953 K in CH4/H2 at 12 Pa [27]. We note thatin the latter study, the corresponding carbon phase wasgenerated without any catalyst and was also assigned tofree-standing subnanometer sheets.

The observation of tubes is plausible when consideringknown CVD growth methods for generating carbon nano-tubes [28]. All are based on metal particle catalyzed growth(Fe, Ni and Co usually). We note that the acid treatedSWCNTF samples applied here still contain traces of theoriginal Co/Ni catalyst (as evidenced by EDX and previouschemical analysis). These apparently become active in theinfiltration process. HRTEM image A* in Fig. 4 shows asection of a typical multiwalled nanotube. The image indi-cates that the nanotube comprises graphene layers orientedalong the tube axis with an interlayer space of 0.361 nm –slightly higher than in graphite (0.335 nm). The diameterof the central hollow region ranges from 40 to 60 nm. Wecannot distinguish whether the tubes are concentric shellmultiwalled nanotubes (MWNTs) or whether they corre-spond to graphene sheets wrapped up spirally around thetube axis.

Tubular structures were not observed upon pure meth-ane infiltration. Moreover, pristine SWCNTFs, not puri-fied by acid treatment and infiltrated by CH4/H2 mixtureunder standard conditions, exhibit much higher densitiesof worm- and tube-like carbon structures than was foundfor purified felts. Thus this growth mode can be attributedto the activating role of molecular hydrogen. The nature ofthe metal residue in SWCNTF after acid treatment isunclear. It is probably present mainly in the form of (acidimpervious) carbon coated catalyst particles plus someadditional smaller metal oxide particulates [5]. We specu-late that the residual metal remains inactive in the meth-ane-based infiltration. It starts to contribute to thegrowth of carbon nanotubes only when the carbon coverbecomes etched by hydrogen and the metal particles

Fig. 4. SEM images of a surface created by exposing SWCNTF at 1373 K to a CH4/H2 (1:24) mixture for 5 (A) and 90 h (B), respectively. HRTEM imageA* shows a typical multi-walled nanotube (MWNT) such as those seen in A to be growing out from the substrate (which comprises primarilynanographene sheets). SEM image B* is a detail of a typical branch as shown in B. B* shows clearly that free-standing carbon nanosheets as decorating thebranches comprise the majority growth mode.


become accessible for impinging methane molecules. Wenote that high-temperature hydrogen etching is an estab-lished method to remove amorphous carbon from variouscarbon phases [29].

The two growth modes described above also dominatethe intermediate infiltration stages. However, free-standingtubes appear shorter than in the initial deposition stage.Furthermore tube ends now have become enlarged relativeto their ‘‘stems’’. This can be explained by reduced accessof impinging molecular carriers to the catalyst/nanotubeinterface as the nanosheet layer becomes thicker. Conse-quently, the continued growth of nanotubes out from themetal catalyst base slows down considerably. Instead fur-ther growth is increasingly governed by methane moleculesimpinging on and migrating to the top of the tubes wherethey are apparently incorporated into the edges of graph-ene layers terminating the MWCNTs.

Fig. 4B shows a ‘‘final stage’’ SEM image of theSWCNTF surface following methane/hydrogen infiltrationfor 90 h. We note that the regular cylindrical structures(MWNTs) observed previously are no longer found.Instead, we observe an irregular network of 0.8–1.1 lm

thick tubular ‘‘branch’’-like objects which are covered bycarbon nanosheets. These ‘‘branches’’ form a porousmaterial with an apparent mean distance between near-est-neighbour branches D of about 1.5–2 lm. D is compara-ble to the mean free path of methane molecules underour infiltration pressure and consequently determines theeffective penetration depth. The SEM image shown inFig. 4B* provides a higher resolution view of the terminat-ing nanosheets. Note that the topography of this phase isvery close to that obtained by Wang et al. [27,30,31] uponplasma-enhanced chemical vapor deposition of CH4/H2mixture onto numerous substrates at 980 K (gas pressure:12 Pa, substrates: Si, W, Mo, Zr, Ti, Hf, Nb, Ta, Cr,SiO2, Al2O3). The transition from metal catalyzedMWCNT growth to the branch-like growth mode proba-bly occurs when the metal/nanotube interface becomesinaccessible to migrating molecular carriers due to the dif-fusion barrier provided by the rapidly growing nanosheetlayer. It is not clear how the branches form. Conceivably,they originate from MWCNT ends and reflect a change-over from more in-plane growth into a vertically drivenmode.


The above arguments invoke nanographene depositionon MWCNT sidewalls. A related experiment performedon aligned SWCNT bundles suggests that this is plausible.For this we achieved SWNT bundle alignment across anSWCNTF crack as previously described [3]. Fig. 5 showsSEM images of such a disrupted area after exposing thesample to CH4/H2 mixture kept at 1373 K for 6 h. Carbonnanosheets start to grow, apparently by pinning at the sideof the bundles and continue their lateral growth in severalmodes as determined by the orientation of the bundle back-bone. Some nanographene sheets appear to partially wraparound the bundle – either nearly radially or tangentiallywith respect to the bundle axis. The majority of the nano-graphene sheets appear to be folded and perpendicularlyoriented with respect to the bundle axis. We note that thereis significantly more nanographene growth at the bundlebase than in the suspended regions. This observation canbe understood when considering that only the sheet edges

Fig. 5. SEM images of a crack in a SWNTF after exposing the substrate to apanel). The higher resolution image shown in the lower panel provides an exa

comprise the reaction centers (after a nucleation stage).Consequently, the size of an individual sheet governs itsreaction rate (as long as the sheet size is lower than the dif-fusion length). The number of molecular carriers collectedby a large sheet area is considerably higher than that for asmall feature on a thin suspended- or free-standing bundle.

The concept of nanographene sheets extending out fromand sometimes partially wrapping SWCNTs as evidencedby Fig. 5 seems to be in contradiction with the apparentlymore planar growth of free-standing nanosheets during theinitial infiltration stage of SWCNTF with CH4/H2(Fig. 4A). A plausible explanation can be found in the dif-ferent topographies of the substrates, e.g. the mean dis-tance between SWCNT bundles is about 40 nm in theformer case and in the range of 1 lm in the latter.

In a related experiment, carbon nanotubes have recentlybeen exposed to Ar/CH4 microwave plasma [32]. This leadsto the formation of covalently bound ‘‘graphitic platelet

CH4/H2 mixture (1:24) at an oven temperature of 1373 K for 6 h (uppermple of spiral growth of nanosheets around a separated bundle.


wings’’ much like the nanographene sheets observed in thepresent study. The authors have rationalized their plateletsin terms CNT sidewall damage due to radical attack – lead-ing to more reactive edges and spiral growth.

3.3. Raman spectroscopy

The carbon phases created by pure CH4 and CH4/H2infiltration were compared by Raman spectroscopy. Spec-trum (a) in Fig. 6 has been taken from a purified SWCNTFsample. It exhibits all features described in the literature ascharacteristic for these felts (see e.g. [33]): the radial-breathing mode (RBM) in the region 100–300 cm�1 andthe Raman-allowed tangential G-mode in the range1530–1610 cm�1. The latter is in fact a sum of severalhigh-energy tangential modes which originate from break-ing the vibration symmetry when the graphene sheetbecomes wrapped into a cylinder. The G mode effectivelycomprises two intense superimposed peaks labeled G+,for atomic displacements along the tube axis, and G�,for displacements along the circumferential direction. Inaddition, the SWNTF Raman spectra exhibit two features

2 h

30 h

90 h

1000 2000 3000

Raman Shift Δν /cm-1

G’

oTO

G

RBM D

CH4/H2SWNTF

a

b

c

d

Fig. 6. Raman spectra taken after exposing SWCNTF to a CH4/H2mixture for 0, 2, 30 and 90 h at 1373 K (spectra a–d, respectively). Allspectra were taken ex situ in air at room temperature.

known as D and G 0 bands. The feature around 1350 cm�1

is called the disorder-induced D mode. It corresponds tothe iTO phonon branch in graphite [34] and is frequentlyused as a simple measure of the number density of defectsin carbon materials. The G 0 band around 2700 cm�1 repre-sents the second harmonic of the D band [35]. A weak fea-ture (at the right side of G) higher in energy than theintense G-mode centered at 1750 cm�1 has recently beenattributed to an overtone of an infrared-active out-of-plainmode (oTO) [36].

Fig. 6b–d shows Raman spectra of samples prepared fol-lowing 2, 30 and 90 h of CH4/H2-infiltration at 1373 K,respectively. Spectrum (b) in Fig. 6 demonstrates that thecarbon deposited in 2 h of infiltration does not significantlymodify the Raman features characteristic for SWCNTF(Fig. 6a), i.e. the lateral concentration of carbon nuclei pin-ned at bundles is below the detection limit. Spectrum (c) inFig. 6, taken after 30 h, no longer shows any RBM signal.Instead it exhibits considerably broadened and very intenseD and G 0 mode features. In particular the D mode corre-sponding to the carbon phase deposited, appears nearlyfour times more intense than for the starting material.The oTO mode which is the signature of an out-of-plaindisplacement of carbon atoms in SWCNTs disappearsentirely. Taken together these spectral features confirmthat the SWNTF substrate is fully covered by carbon enti-ties after 30 h of exposure. Spectrum (d) shows the Ramanfeatures of carbon deposits obtained after a 90 h infiltra-tion. This spectrum corresponds to the situation illustratedby SEM images in Fig. 4B. No significant changes betweenspectrum (c) and (d) could be found implying that the mor-phology of the carbon phase does not significantly changebetween 30 and 90 h. Further growth proceeds mainly viathe formation of new graphite sheets as evidenced byFig. 4. In the resulting carbon phase the intensity ratio,g = I(D)/I(G) saturates at a level of 0.83. This ratio hasbeen used as a simple indicator of the lateral dimension kof the growing carbon nanosheets [37,38]. The 30–50 nmnanosheets covering the branches resolved by SEM inFig. 4B entirely support this assignment. We note further,that studies of well-defined carbon materials have shownthat a single strong G 0 mode around 2700 cm�1 is a signof total lack of c-axis order [34,39], consistent with thenearly random orientation of nanosheets shown in Fig. 4B.

Fig. 7 compares three Raman spectra taken: (a) at thefinal stage of pure methane infiltration – correspondingto surface topography shown in Fig. 2D, (b) at the finalstage of CH4/H2 infiltration (see Fig. 4B), (c) for a cleanHOPG surface. We denote the long-infiltration carbonphases obtained for pure methane and methane/hydrogenreagents as MI and MHI, respectively. A striking attributeof the MI phase (Fig. 7a) is the counter correlated behaviorof the D and G 0 modes: strong D mode intensity is accom-panied by correspondingly reduced G 0 mode signal. TheD–G doublet is atypically broad and the correspond-ing intensity ratio g has an uncharacteristically high valueof 1.3. Both the strong D band and weak G 0 mode, are

1000 2000 3000

1 5 8 5

CH4

CH4/H2

HOPG

Raman Shift Δν /cm-1

DG

G’

a

b

c

Fig. 7. Raman spectra of carbon deposits formed on SWCNTF during thefinal stages of methane infiltration (MI phase, spectrum a) and methane/hydrogen infiltration (MHI phase, spectrum b). Spectrum (a) has beentaken from the MI phase shown in Fig. 2D, and spectrum (b) representsthe MHI phase illustrated in Fig. 4B. For comparison, the Ramanspectrum of a clean HOPG surface is shown in panel c.


opposite to the observation for the MHI phase (Fig. 7b).Consequently, based on the assignment/quantification ofthese features as first proposed by Wilhelm et al. [38], theMI deposits comprise much smaller nanosheets than isthe case for the MHI phase, i.e. k(CH4)� k(CH4/H2).Moreover, the weak G 0 band observed for the MI phaseindicates significant stacking order along the c-axis, i.e.the small nanosheets are predominantly stacked parallelto the substrate. Thus, we infer that the large hemispheresobserved on the mesoscopic scale (Fig. 2D and D*) are infact comprised of stacked nanographene sheets. For com-parison, Fig. 7c shows a Raman spectrum of highly ori-ented pyrolytic graphite, HOPG, which exhibits only twonarrow peaks, G and G 0 (in accordance with [38]). BothMI and MHI phases are clearly distinguishable from the‘‘ideal’’ case of a HOPG sample by their disorder and highdensity of defects as manifested by an intense D mode.

3.4. Comparison between MI and MHI

Two striking aspects become evident when comparingthe MI and MHI experiments: (1) the admixture of hydro-gen leads to the activation of multiwalled carbon nanotube

growth, (2) whereas the MI phase is composed of smalldefected nanosheets aligned in stacked layers, the MHIphase is dominated by randomly distributed, free-standingnanosheets which are less defected and on average largerthan those observed in the MI phase. The first point israther well established in the literature [40], hydrogen acti-vates metal catalysts and stabilizes the growth of carbonnanotubes. The second point, different morphologies ofthe nanosheets, is more complex and can only be tenta-tively rationalized. Both phases, MI and MHI, exhibit car-bon nanosheets as the common building blocks. Thissimilarity suggests that the main carbon inclusion mecha-nism is not significantly modified by the admixture ofmolecular hydrogen.

We rationalize this as follows. Molecular hydrogen onlyinteracts weakly with graphite terraces (0.051 eV [41]). H2dissociation on graphite terraces ending with two H atomscovalently bound to lattice carbons is thought to be endo-thermic by about 3.4 eV [42]. Similarly unlikely is the inter-calation of a gas-phase H2 into the space between twooutermost graphene layers [43]. Recent calculations basedon spin-polarized and gradient-corrected density functionaltheory (DFT) suggest that the barrier to molecular hydro-gen dissociation followed by H-addition at the (free) edgesof a graphene sheet is very low (about 0.2 eV). The wholeprocess is exothermic by �5.3 eV [37]. Thus, at surface tem-perature of 1400 K (kT � 0.12 eV) molecular hydrogen dis-sociation at the edges of a growing carbon nanosheet can beconsidered to be an efficient reaction. However, because ofits weak interaction energy with the graphite basal plane,the adsorption-conditioned residence time of a H2 moleculeis about 100 times shorter than is the case for a migratingCH�3 (5.7 nm diffusion length at 1373 K). Thus, despite theunfavourable partial pressure ratio (24:1), the flux of car-bon carriers towards the sheet edge reaction centers canbe expected to be significantly larger than the correspondingH2 flux. As a corollary, hydrogen termination of the edgescan only hinder carbon inclusion by terminating the activeedge sites but it cannot block them entirely. We speculatethat in MHI deposition the role of molecular hydrogen isessentially to remove and redistribute the reaction heatwhich is first localized at the edges of the growing nano-sheets. Such nanosheet-hydrogen collisional energy transfercould be the reason for the observed larger size of the indi-vidual flakes as well as the reason why there is little stackingof sheets in MHI deposition. We note that Louchev et al.[44] have invoked such a process as important for the con-trol of length and quality during growth of nanotubes.

4. Conclusions

The experimental findings presented here can be summa-rized as follows:

1. The initial infiltration stage results in the formationof carbon nuclei predominantly pinned at bundlecrossings. During this nucleation stage, the probability


for carbon deposition is roughly 10�9 per methanecollision.

2. In both MI and MHI deposition, only several layers ofthe SWCNTF become infiltrated. The carbon depositscreated in the topmost regions block further access ofimpinging methane molecules to the deeper lying SWNTlayers. This is a consequence of the unfavorable ratiobetween the SWCNTF mesh size and the mean free pathin gas phase. As a result methane CVD does not appearto present a viable method for mechanically strengthen-ing SWCNTF with thicknesses greater than the diffusiondepth.

3. Further growth of carbon phases proceeds autocatalyti-cally via formation of carbon nanosheets pinned at ran-domly distributed sites of the carbonaceous surface. Thetopography of the final MI phase is dominated by den-sely packed carbon hemispheres which are composed ofstacked graphite nanosheets. At this later depositionstage, the probability for carbon incorporation levelsoff at 10�6.

4. MHI deposits also grow via the formation and lateralexpansion of carbon nanosheets, but at intermediatetimes, the resulting carbonaceous surface is deco-rated with MWCNTs which emanate from Ni/Cocatalyst particles lying below the surface plane. Hydro-gen admixture activates the buried metal catalyst inSWCNTF and initializes the growth of MWCNTs. Thisgrowth mode slows down considerably when diffusiveaccess to the metal–nanotube interface becomes hin-dered. Subsequently the growth changes into upwardsdriven growth of lm-sized carbon branches covered byfree-standing carbon nanosheets.

Acknowledgments

We thank the Deutsche Forschungsgemeinschaft forsupport of this project under Sonderforschungsbereich 551‘‘Kohlenstoff aus der Gasphase: Elementarreaktionen,Strukturen und Werkstoffe’’. We also thank ReginaFischer and Carrisa Stairs for preparation of SWNTF.AB thanks Dr. Boris Reznik for many illuminating discus-sions on the basics of chemical vapour infiltration.

References

[1] Hennrich F, Wellmann R, Malik S, Lebedkin S, Kappes MM.Reversible modification of the absorption properties of single-walledcarbon nanotube thin films via nitric acid exposure. Phys Chem ChemPhys 2003;5:178–83.

[2] Wei J, Zhu H, Wu D, Wei B. Carbon nanotube filaments inhousehold light bulbs. Appl Phys Lett 2004;84:4869–71.

[3] Malik S, Rösner H, Hennrich F, Böttcher A, Kappes MM, Beck T,et al. Failure mechanism of single-walled carbon nanotubes thin filmsunder tensile loads. Phys Chem Chem Phys 2004;6:3540–4.

[4] Mamedov AA, Kotov NA, Prato M, Guldi DM, Wicksted JP, HirschA, et al. Molecular design of strong single-wall carbon nano-tube/polyelectrolyte multilayer composites. Nature Mater 2002;1:190–4.

[5] Hennrich F, Lebedkin S, Malik S, Tracy J, Barczewski M, Rösner H,et al. Preparation, characterization and applications of free-standingsingle walled carbon nanotube thin films. Phys Chem Chem Phys2002;4:2273–7.

[6] Delhaès P, Trinquecoste M, Lines J-F, Cosculluela A,Goyhénèche J-M, Couzi M, et al. Fast densification processes andmatrix characterisations. Carbon 2005;43:681–91.

[7] Hüttinger KJ. Fundamentals of chemical vapor deposition in hot wallreactors. In: Delhaes P, editor. World of carbon. Fibers andcomposites, 2. London: Taylor and Francis; 2003.

[8] Klinke C, Delvigne E, Barth JV, Kern K. Enhanced field emissionfrom multiwall carbon nanotube. J Phys Chem B 2005;109:21677–80.

[9] Wang Y, Kim MJ, Shan H, Kittrell C, Fan H, Ericson LM, et al.Continued growth of single-walled carbon nanotubes. Nano Letters2005;5:997–1002.

[10] Lebedkin S, Schweiss P, Renker B, Malik S, Hennrich F, NeumaierM, et al. Single wall carbon nanotubes with diameter approching6 nm obtained by laser vaporization. Carbon 2002;40:417–23.

[11] Benzinger W, Hüttinger KJ. Chemistry and kinetics of chemicalvapor infiltration of pyrocarbon. VI. Mechanical and structuralproperties of infiltrated carbon fiber felt. Carbon 1999;37:1311–6.

[12] Reznik B, Gerthsen D, Hüttinger KJ. Micro- and nanostructure ofcarbon matrix of infiltrated carbon fiber felts. Carbon 2001;39:215–29.

[13] Vitali L, Burghard M, Liu L, Wu SY, Jayanths CS, Kern K, et al.Phonon spectromicroscopy of carbon nanostructures with atomicresolution. Phys Rev Lett 2004;93:1361031–5.

[14] Louchev OA. Formation mechanism of pentagonal defects andbamboo-like structures in carbon nanotube growth mediated bysurface diffusion. Phys Status Solidi 2002;193:585–9.

[15] Hsu WL. Gas-phase kinetics during mcrowave plasma-assisteddiamond deposition: is the hydrocarbon product distribution dictatedby neutral–neutral interactions? J Appl Phys 1992;72:3102–9.

[16] Yoon S-H, Lim S, Hong SH, Qiao W, Whitehurst DD, Mochida I,et al. A conceptual model for the structure of catalytically growncarbon nano-fibers. Carbon 2005;43:1828–38.

[17] Wellmann R, Böttcher A, Kappes MM, Kohl U, Niehus H. Growthof graphene layers on HOPG via exposure to methyl radicals. Surf Sci2003;542:81–93.

[18] Muradow N, Smith F, Raissi AT. Catalytic activity of carbons formethane decomposition reaction. Catal Today 2005;102:225–33.

[19] Laidler KJ, Casey EJ. Heats of dissociation of carbon–hydrogenbonds in methane and its radicals. J Chem Phys 1949;17:1087–91.

[20] Zhao J, Buldum A, Han J, Lu JP. Gas molecule adsorption incarbon nanotubes and nanotube bundles. Nanotechnology 2002;13:195–200.

[21] Weber SE, Talapatra S, Journet C, Zambano A, Migone AD.Determination of the binding energy of methane on single-walledcarbon nanotube bundles. Phys Rev B 2000;61:13150–4.

[22] Lee YH, Kim SG, Tomanek D. Catalytic growth of single-wall carbon nanotubes: an ab initio study. Phys Rev Lett 1997;78:2393–6.

[23] Kwon S, Vidic R, Borguet E. The effect of surface chemical functionalgroups on the adsorption and desorption of a polar molecule,acetone, from a model carbonaceous surface, graphite. Surf Sci2003;522:17–26.

[24] May K, Dapprich S, Furche F, Unterreiner BV, Ahlrichs R.Structures C–H and C–CH3 bond energies at borders of polycyclicaromatic hydrocarbons. Phys Chem Chem Phys 2000;2:5084–8.

[25] Hsu WL. Mole fractions of H, CH3, and other species duringfilament-assisted diamond growth. Appl Phys Lett 1991;59:1427–9.

[26] Ashfold MNR, May PW, Petherbridge JR, Rosser KN, Smith JA,Mankelevich YA, et al. Unravelling aspects of the gas phasechemistry involved in diamond chemical vapor deposition. PhysChem Chem Phys 2001;3:3471–85.

[27] Wang JJ, Zhu MY, Outlaw RA, Zhao X, Manos DM, Holloway BC,et al. Free standing subnanometer graphite sheets. Appl Phys Lett2004;85:1265–7.


[28] Takehira K, Ohi T, Shishido T, Kawabata T, Takaki K. Catalyticgrowth of carbon fibers from methane and ethylene on carbon-supported Ni catalysts. Appl Catal A 2005;283:137–45.

[29] Klusek Z, Datta PK, Kozlowski W. Nanoscale studies of theoxidation and hydrogenation of graphite surface. Corros Sci2003;45:1383–93.

[30] Wang JJ, Zhu M, Outlaw RA, Zhao X, Manos DM, Holloway BC,et al. Synthesis of carbon nanosheets by inductively coupled radio-frequency plasma enhanced chemical vapor deposition. Carbon2004;42:2867–72.

[31] French BL, Wang JJ, Zhu MY, Holloway BC. Structural character-ization of carbon nanosheets via X-ray scattering. J Appl Phys2005;97:114317–24.

[32] Trasobares S, Ewels C, Birrell J, Stephan O, Wei B, Carlisle J, et al.Carbon nanotubes with graphitic wings. Adv Mater 2004;16:610.

[33] Brown SDM, Corio P, Marucci A, Pimenta MA, Dresselhaus MS,Dresselhaus G, et al. Second-order resonant Raman spectra ofsingle-walled carbon nanotubes. Phys Rev B 2000;61:7734–42.

[34] Thomsen C, Reich S. Double resonant Raman scattering in graphite.Phys Rev Lett 2000;85:5214–7.

[35] Dresselhaus MS, Dresselhaus G, Saito R, Jorio A. Raman Spectros-copy of carbon nanotubes. Phys Rep 2005;409:47–99.

[36] Brar VW, Samsonidze GG, Dresselhaus MS, Dresselhaus G, Saito R,Swan AK, et al. Second-order harmonic and combination modes in

graphite, single-wall carbon nanotube bundles, and isolated single-wall carbon nanotubes. Phys Rev B 2002;66:155418–28.

[37] Tuinstra F, Koenig JL. Raman spectrum of graphite. J Chem Phys1970;53:1126–30.

[38] Wilhelm H, Lelaurain M, McRae E, Humbert B. Raman spectro-scopic studies on well-defined carbonaceous materials of strong two-dimensional character. J Appl Phys 1998;84:6552–8.

[39] Reich S, Thomsen C. Raman spectroscopy of graphite. Philos TransR Soc Lond A 2004;362:227–88.

[40] Ivanov V, Nagy JB, Lambin P, Lucas A, Zhang XB, Zhang XF, et al.The study of carbon nanotubules produced by catalytic method.Chem Phys Lett 1994;223:329–35.

[41] Sha X, Jackson B. The location of adsorbed hydrogen in graphitenanostructures. J Am Chem Soc 2004;126:13095–9.

[42] Sha XW, Jackson B. First principle study of structural and energeticproperties of H atoms on graphite (0001) surface. Surf Sci 2002;496:318–30.

[43] Miura Y, Kasai H, Diño W, Nakanishi H, Sugimoto T. Firstprinciples studies for the dissociative adsorption of H2 on graphene.J Appl Phys 2003;93:3395–400.

[44] Louchev OA, Kanda H, Rosén A, Bolton K. Thermal physicsin carbon nanotube growth kinetics. J Chem Phys 2004;121:446–56.

Growth of novel carbon phases by methane infiltration of free-standing single-walled carbon nanotube filmsIntroductionExperimentalResultsCH4 infiltrationCH4/H2 based infiltrationRaman spectroscopyComparison between MI and MHI

ConclusionsAcknowledgmentsReferences

growth of novel carbon phases by methane inﬁltration of ... · gen upon inﬁltration using the...

Documents