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Carbon 45 (2007) 2217–2228

Effect of different carbon sources on the growth of single-walledcarbon nanotube from MCM-41 containing nickel

Yuan Chen a,*, Bo Wang a, Lain-Jong Li b, Yanhui Yang a, Dragos Ciuparu c,Sangyun Lim d, Gary L. Haller d, Lisa D. Pfefferle d

a School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singaporeb School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore

c Department of Petroleum Processing and Petrochemistry, ‘‘Petrol-Gaze’’ University, 20000 Ploiesti, Romaniad Department of Chemical Engineering, Yale University, New Haven, CT 06520, USA

Received 20 December 2006; accepted 19 June 2007Available online 28 June 2007

Abstract

Chemical vapor deposition growth of single-walled carbon nanotubes (SWCNTs) was studied using three representative carbonsource sources: CO, ethanol, and methane, and a catalyst of Ni ions incorporated in MCM-41. The resulting SWCNTs were comparedfor similar reaction conditions. Carbon deposits were analyzed by multi-excitation wavelength Raman, TGA, TEM and AFM. Catalyticparticles in the Ni-MCM-41 catalysts were characterized by TEM and synchrotron light source X-ray absorption spectroscopy. Undersimilar synthesis conditions, SWCNTs produced from CO had a relatively smaller diameter, while those from ethanol had a larger dia-meter. Methane could not produce SWCNTs on Ni-MCM-41 under the conditions used in this research. These results demonstrate thatthree carbon sources affect the dynamic balances between metallic cluster formation and carbon deposition/precipitation on the metalliccluster surface. Controlling SWCNT diameter relies on precisely regulating this dynamic process. Using different carbon sources we areable to shift this dynamic balance and produce SWCNTs with different mean diameters.� 2007 Elsevier Ltd. All rights reserved.

1. Introduction

The properties of single-walled carbon nanotubes(SWCNTs) are determined by their structure [1]. Applica-tions of SWCNTs as electronic devices [2], chemical sensors[3], and hydrogen storage [4] have been paralleled byattempts to control their structure. One of the major obsta-cles to applications of SWCNTs is the control of the syn-thesis processes to produce SWCNTs with desiredstructure [5]. It is crucial to accumulate knowledge regard-ing SWCNTs synthesis processes, and ultimately to be ableto produce SWCNTs with desired structure.

Among SWCNT synthesis methods, chemical vapordeposition (CVD) has the advantage of producing carbon

0008-6223/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2007.06.022

* Corresponding author. Fax: +65 6791 1761.E-mail address: chenyuan@ntu.edu.sg (Y. Chen).

deposits with various structures resulting under a varietyof synthesis conditions, which opens the possibility toadjust the structure of carbon species. Catalysts and carbonsources are the two most important variables among thesesynthesis conditions. Catalysts have been prepared in vari-ous forms and compositions. Iron, cobalt and nickel arethe three most widely used metallic components in catalystsfor SWCNT synthesis [6–10]. Although many carbonsources have been investigated in SWCNT syntheses, CO,ethanol, and methane are still the popular carbon sourceswhere successful SWCNT synthesis has been reported bymany groups [6–11]. While much of the literature sub-scribes to the mechanism of initial dissolution of carbonin the metal particle followed by precipitation of carbonand grow of SWCNTs, we are of the opinion (and thereis evidence from particle size effects on steam reformingon Ni [12], as well as molecular dynamics simulation [13])that dissolution of carbon in particles of about 1 nm or

2218 Y. Chen et al. / Carbon 45 (2007) 2217–2228

smaller could not be realized. However, the formation of asurface carbide or ‘‘surface dissolution,’’ e.g. resulting fromdissociation of CO and removal of O as CO2 is known tooccur and, in any case, would be precursors to dissolution.That is, formation of an adsorbed C species on the surfaceis very likely part of the overall mechanism and the cata-lytic bond formation between two or more carbons (thegrowing SWCNTs) on the surface of the metal particle isprobably a critical step in the overall mechanism. Thus,the Fischer–Tropsch mechanism is probably a better anal-ogy than the growth of large carbon fibers on large (10’s ofnm in diameter) Ni particles, which probably does involvebulk C dissolution and re-precipitation [14]. In SWCNTsynthesis processes, the formation of metallic clusters andthe precipitation of carbon on metallic cluster surfacesare of crucial importance to control both selectivity toSWCNTs and SWCNT structures. From the kinetics per-spective, the ultimate synthesis of structure controlledSWCNTs is a twofold problem. Both the rate of formationof metallic clusters (<1 nm in diameter) and precipitationrate of carbon atoms need to be precisely controlled, inother words, the rates of these two processes should matchin a narrow parameter window.

Researchers have studied the effects of different carbonsources on the growth of nanotubes [15–18]. Mizuno et al.[19] has investigated two carbon sources (ethylene and eth-anol) with various catalysts. The importance of choosingthe appropriate combination of catalyst and carbon sourcefor efficient SWCNT synthesis has been addressed by theauthor. However, the effect of the morphology of metallicnanoparticles on the SWCNT structure under different car-bon sources has not been studied. The detailed characteriza-tion of the formation of nanoscale metallic particlesformations is not available to our best knowledge.

We have developed a Co catalyst incorporated in thepore walls of a silica MCM-41 mesoporous molecular sieveby isomorphous substitution of silicon ions with Co ions[20]. We found that the migration and sintering of Co clus-ters that normally occurs under the harsh SWCNT synthe-sis conditions can be minimized if the clusters formed bythe reduction of Co ions atomically dispersed in MCM-41 templates. The rate of Co clusters formation can alsobe regulated. High quality SWCNTs have been grown onthis Co-MCM-41 catalyst by CO disproportionation in anarrow growth window [10,21,22]. Nickel, a neighbor ofcobalt in the periodic table of elements, in a form ofwell-dispersed nanoparticles inside MCM-41 may behavesimilarly to Co-MCM-41 with respect to chemical proper-ties. Synthesis of SWCNTs by CO disproportionation onNi-MCM-41 indicated that Ni in MCM-41 would be com-pletely reduced once the catalyst is exposed to CO underSWCNT synthesis conditions, which results in SWCNTswith a wider diameter distribution compared to the Co cat-alyst [23]. We expected that carbon sources other than COmay allow a better control of the process determining thestructure uniformity of SWCNTs produced using Ni-MCM-41 catalysts.

In this contribution, we report the CVD growth ofSWCNTs on Ni ion incorporated MCM-41 catalytic tem-plates using three representative carbon sources: CO, etha-nol, and methane. The resulting SWCNTs under similarreaction conditions were comparatively characterized bymulti-excitation wavelength Raman, thermal gravimetricanalysis (TGA), transmission electron microscope (TEM)and atomic force microscope (AFM). The Ni-MCM-41catalysts were characterized by TEM and X-ray absorptionspectroscopy (XAS). Here we show that different carbonsources affect the dynamic balance existing between the for-mation of metallic clusters and carbon precipitation.SWCNTs with different mean diameters can be producedusing different carbon sources.

2. Experimental

Ni-MCM-41 catalysts with 1 wt% nickel (as measured by inductivelycoupled plasma (ICP) at Galbraith Laboratories, Inc.) were synthesizedfollowing the method described in details elsewhere [23]. It results inNi2+ ions isomorphously substituted for Si4+ ions in the MCM-41 struc-ture. The porous structure of the resulting Ni-MCM-41 catalyst and thepore size distribution (PSD) were determined by nitrogen physisorptionmeasurements in a static volumetric instrument Autosorb-1C (QuantaChrome). The average pore size was 2.9 nm calculated by the BJH method[24] and the pore size distribution was in the order of 0.1 nm full width athalf-maximum. The reducibility of Ni ions incorporated in MCM-41 hasbeen determined by temperature-programmed reduction (TPR). The TPRmeasurements on the Ni-MCM-41 catalysts were performed using thethermal conductivity detector (TCD) of a gas chromatograph (6890 plus,Agilent). The details of the TPR experiments were given in our previouscontribution [23]. Reduction of Ni in MCM-41 under hydrogen startedat about 500 �C. The maximum reduction rate of Ni-MCM-41 is reachedat about 740 �C.

SWCNTs were synthesized using three representative carbon sources:CO, ethanol, and methane. CO disproportionation (2CO! C + CO2)was performed in the same procedure described in our previous study ofCo-MCM-41 [10]. For a typical batch, 200 mg of fresh Ni-MCM-41 wereloaded into a 10 mm internal diameter quartz reactor placed in an Omegaceramic fiber radiant heater that allows precise temperature controlthroughout the catalyst bed. Prior to exposure to the carbon source, thecatalyst was heated in a rate of 20 �C/min from room temperature tothe specified prereduction temperature under 1 atm of flowing hydrogen.In the attempt to find an optimum prereduction temperature, the catalystwas maintained under hydrogen isothermally for 30 min at several reduc-tion temperatures ranged from 300 to 600 �C, followed by purging withultra high purity argon at the same temperature, and then heated to800 �C at 20 �C/min in flowing argon. Pure CO (99.5% from Airgas)passed through a carbonyl trap to eliminate Fe pentacarbonyl originatingfrom the CO container before entering the reactor. SWCNTs were grownfor 1 h under flowing CO at 6 atm pressure after the prereduction process.

In ethanol CVD (C2H5OH! 2C + 2H2 + H2O), ethanol vapor wasgenerated following the procedure introduced by Huang et al. [25]. Itwas delivered to the reactor at the reaction temperature by bubbling argon(200 sccm) into a glass saturator containing pure ethanol (held in an icebath at 0 �C). The total pressure of reactant gas is 1 atm with an ethanolvapor pressure of 11.8 torr balanced by argon. The CVD was performedfor 1 h. Methane decomposition (CH4! C + 2H2) was carried out inthe same system as CO. We have performed the synthesis with the meth-ane pressure of 6 atm and 0.4 atm (a mixture of methane and argon at1 atm, methane concentration 40 vol%), respectively.

As-synthesized carbon deposits were characterized by multi-excitationwavelength Raman spectroscopy to identify the SWCNT diameter distri-bution and the carbon species. Spectra were collected with a Renishaw

Table 1The experimental conditions used in this study for growth of SWCNTs

Carbon sources CO Ethanol Methane

Pressure Pure,6 atm

11.8 mm Hg balance in1 atm Ar

Pure, 6 atm(S3)0.4 atm (S30)

Prereduction inH2

400 �C 400 �C 400 �C

Reactiontemperature

800 �C(S1)

800 �C (S2) 800 �C650 �C (S2 0)

514.5 nm laser

RBM

Y. Chen et al. / Carbon 45 (2007) 2217–2228 2219

Ramanscope in the backscattering configuration using 514.5 nm (2.41 eV),633 nm (1.96 eV) and 785 nm (1.58 eV) laser wavelengths.

AFM images of SWCNTs were taken with an Asylum Research MFP-3D microscope in the tapping mode in air. For SWCNT samples, as-syn-thesized Ni-MCM-41 with carbon deposits was first refluxed in 1 MNaOH for 1 h twice to remove the amorphous silica templates. About1 mg of the remaining carbon deposits were mixed with 1 mg ofd(GT)20 single stranded DNA (custom-made by Integrated DNA technol-ogies, Inc. Coralville, IA). Individual SWCNT samples wrapped by DNAwere then prepared following the method developed by Zheng et al. [26].For SWCNT on catalytic templates, Ni-MCM-41 loaded with carbonwere only sonicated for 1 min in ethanol, and then deposited on a pieceof mica for measurement.

A Setaram Setsys 1750 instrument was used for TGA of the catalystsamples loaded with carbon deposits under air flow. Samples were pre-heated at 150 �C for 1 h to remove the moisture. The weight change inthe sample was monitored over the temperature program from 150 to1050 �C at 10 �C/min for two successive ramps. The second ramp was usedas a baseline to correct the first run.

The catalytic templates after SWCNT synthesis using different carbonsources were characterized by X-ray absorption spectroscopy. These mea-surements were performed at Beamline X23A2 of the National Synchro-tron Light Source, Brookhaven National Laboratory. Approximately45 mg of sample was pressed as a rectangular wafer (about 1.5 cm · 1 cm)to form 0.5 mm thick pellets. The incident and transmitted X-ray intensi-ties were measured by ion chambers filled with pure N2 placed in front andbehind the sample, respectively. A nickel foil was placed between the sec-ond and a third ion chamber as an internal reference. Extended X-rayabsorption fine structure spectroscopy (EXAFS) in the transmission modewas recorded from 200 eV below, to 900 eV above the Ni K edge. Analysisof the X-ray adsorption spectra followed the procedures described in detailin a previous report [21]. The EXAFS spectra were calibrated to the edgeenergy of the nickel foil reference. The background removal and edge-stepnormalization were performed using the FEFFIT code [27]. The theoreti-cal EXAFS functions for different nickel species (Ni and NiO) generatedby the FEFF6 program [28] were used to fit the experimental data in orderto obtain the corresponding Ni–Ni and Ni–O first-shell coordinationnumbers.

Catalyst samples after CVD were also investigated by high resolutiontransmission electron microscopy (HR-TEM) on a Tecnai F20 200 kVmicroscope from Philips.

300 600 900 1200 1500 1800

G bandS3'

S3

S2'

Inte

nsity

, a.u

.

Raman Shift, cm-1

S1

S2

D band

Fig. 1. Raman spectra of carbon deposits on Ni-MCM-41 catalystsrecorded under 514.5 nm excitation after different reaction conditions. S1:6 atm pure CO, 800 �C growth, 1 h; S2: 11.8 mm Hg ethanol in argon at1 atm, 800 �C growth, 1 h; S2 0: 11.8 mm Hg ethanol in argon at 1 atm,650 �C growth, 1 h; S3: 6 atm pure CH4, 800 �C growth, 1 h; S3 0: 0.4 atmCH4 in argon at 1 atm, 800 �C growth, 1 h. Before CVD, catalysts were allprereduced in H2 at 400 �C for 30 min.

3. Results

Temperature-programmed reduction (TPR) results [29]indicated that Ni ions were incorporated in the amorphoussilica of MCM-41, and would not be reduced in hydrogenuntil 500 �C. In previous CO disproportionation study [23],we have found that the optimum prereduction temperatureto grow SWCNTs with CO was 300 �C. However, in thisresearch, if we used the same 300 �C prereduction temper-atures, neither ethanol nor methane CVD can produce sig-nificant amount of carbon deposits. We have exploredprereduction temperatures ranging from 300 to 500 �Cfor ethanol and methane. Prereduction in hydrogen at400 �C was found to be the optimum temperature in termsof SWCNT yield. Therefore, the prereduction for all theexperiments was adjusted to 400 �C to achieve better com-parison of the results obtained from different carbonsources: Ni-MCM-41 samples were prereduced at 400 �Cin hydrogen for 30 min, then CVD experiments werecarried out at 800 �C for 1 h. Experimental conditionsand the sample labels are listed in Table 1. The effects of

the reaction temperature and pressure are discussed inSection 4.

Raman spectra for carbon deposits on Ni-MCM-41 cat-alysts excited by 514.5 nm laser are shown in Fig. 1. Thetypical spectral features of carbon deposits including theradial breathing mode (RBM) below 300 cm�1, the D bandaround 1300–1350 cm�1 and the G band around 1550–1650 cm�1 indicating SWCNTs or graphertic carbons havebeen obtained under various synthesis conditions. Thespectrum of carbon deposits produced by CO dispropor-tionation in S1 (6 atm pure CO, 800 �C growth, 1 h) hasan intense RBM peak, suggesting that SWCNTs have beensuccessfully produced. Moreover, it shows a weak D bandpeak observed for this sample which is consistent with ourprevious results [23].

300 600 900 1200 1500 1800

S3'

S3

S2'

Inte

nsity

, a.u

.Raman Shift, cm-1

785 nm laser

S1

S2

RBM

D band

G band

Fig. 3. Raman spectra of carbon deposits on Ni-MCM-41 catalystsrecorded under 785 nm excitation after different reaction conditions. S1,S2, S2 0, S3, S3 0: the same as in Fig. 1.

2220 Y. Chen et al. / Carbon 45 (2007) 2217–2228

Raman spectrum of the deposits synthesized under S2condition (11.8 torr ethanol in argon at 1 atm, 800 �Cgrowth, 1 h) shows a weak RBM peak with the 514.5 nmlaser excitation. It is well known that the characteristicpeak intensities of SWCNTs vary with the laser excitationenergy depending on the resonance conditions [30]. Multi-excitation wavelength Raman spectroscopy is thereforenecessary to properly characterize the structure of carbondeposits. The comparable Raman experiments were alsoperformed with other two laser excitation wavelengths,633 nm and 785 nm, as shown in Figs. 2 and 3, respectively.It has been observed that S2 shows RBM peaks with all theexcitations, which confirm the production of SWCNTs at800 �C by ethanol decomposition. This result is consistentwith the study of SWCNT growth using ethanol carbonsources reported by others [31].

Taking a closer look at the RBM peaks (Fig. 4) observedusing the 785 nm and 633 nm excitation, the variation inRBM frequencies for all identified peaks in S1, S2 andS2 0 are within the Raman experimental error (±1 cm�1),indicating the tube species formed from CO disproportion-ation (S1) and ethanol (S2 and S2 0) are the same. However,the intensity profile of RBM for SWCNTs grown from COis significantly different than that for SWCNTs grown fromethanol. CO has produced SWCNTs with smaller diameter(larger Raman shift according to the model of Bachilo et al.[32]) while ethanol produced SWCNTs with relatively lar-ger diameter, as summarized in Table 2, where the sug-

300 600 900 1200 1500 1800

Inte

nsity

, a.u

.

Raman Shift, cm-1

633 nm laser

S1

S2

S2'

S3

S3'

RBM

D bandG band

Fig. 2. Raman spectra of carbon deposits on Ni-MCM-41 catalystsrecorded under 633 nm excitation after different reaction conditions. S1,S2, S2 0, S3, S3 0: the same as in Fig. 1.

gested assignment of SWCNT structures is based onBachilo’s 4-parameter tight binding fitting [33,34].1

Compared to CO and ethanol, the methane moleculehas the most stable structure. High temperature decompo-sition (CH4! C + 2H2) leads to amorphous carbon whena catalyst is not present. In the presence of a catalyst, car-bon atoms can deposit on the metal surface. Carbon wouldeither diffuse on the metal surface or through the bulk oflarger particles to precipitation sites to form graphitic lay-ers, generating carbon nanofibers, or carbon nanotubes. Nicatalyst is one of the most effective catalysts tested so far todecompose methane. The Raman spectra of S3 (6 atm pureCH4, 800 �C, grown for 1 h) produced by methane decom-position are shown in Figs. 1–3 for different excitationwavelengths. Figures show a much less pronounced RBMfeatures suggesting that few SWCNTs have been synthe-sized by methane decomposition under our conditions.The increased D bands and decreased G bands of S3 com-pared to S1 indicate that significant amount of amorphouscarbon and/or graphitic carbons have been produced.

To corroborate the RBM analysis results of SWCNTdiameter and obtain more detailed structural insight, S1sample were investigated by AFM analysis (Fig. 5).Fig. 5D show a catalyst grain of Ni-MCM-41 about400 nm in diameter. Nanotube bundles growing out of

1 The optimized parameters we used for Bachilo’s 4-parameter tightbinding fitting model: A1 = 3.59 eV, B1 = 4.0 eV, b1 = 0.9 and d1 = �1.06for E11 and A2 = 3.29 eV, B2 = 4.8 eV, b2 = 2.8 and d2 = �0.2 for E22.

100 150 200 250 300 350100 150 200 250 300 350

S1

S2'

S2

S1

785 nm

Raman Shift, cm-1

S2'

S2

633 nm

Fig. 4. Radical breathing mode peaks in Raman spectra from Figs. 1–3.

Table 2Raman spectral data and assignments for samples S1, S2 and S2 0

d a (nm) RBMb (cm�1) (n,m) c

S1 S2 S2 0

785 nm excitation0.89 262.8 263.1 262.4 (7,6)0.91 258.6 258.8 (9,4)0.96 246.4 (8,6)1.03 228.5 229.2 229.2 (10,5)1.17 203.1 204.5 (11,6)1.48 163.7 –1.65 148.1 –

633 nm excitation0.83 281.6 282.4 281.9 (8,4)0.92 256.4 257.2 257.4 (11,1)0.94 251.1 252.3 251.8 (10,3)1.09 217.6 217.2 216.2 (8,8)1.24 192.3 192.5 191.9 (12,6)

RBM peaks are excited by laser wavelengths of 785 and 633 nm, respec-tively. Also contained are the suggested assignments for n and m indicesadapted from Bachilo’s 4-parameter tight binding fitting model. [33,34].

a Calculated from the sample S1 using xRBM (cm�1) = 12.5 + 223.5/d(nm) [32].

b The frequencies with relatively higher intensity are highlighted initalics.

c Suggested (n,m) assignments and predicted Raman: based on Bachilo’s4-parameter tight binding fitting [33,34]. ‘‘–’’ represents the peak unable tobe assigned due to the overlapping of several predicted RBM frequencies.

Y. Chen et al. / Carbon 45 (2007) 2217–2228 2221

catalyst extend more than 0.5 lm. In order to characterizeSWCNT diameter in AFM, individual tubes need to be dis-persed on mica surface. Fig. 5A show individual SWCNTsfrom S1 wrapped in single stranded DNA. Fig. 5C is the3D image of Fig. 5A indicating that nanotube surface isnot smooth due to the DNA strand attached on the tubesurface. Height analysis shown in Fig. 5B confirms theRaman results. The diameter of SWCNTs from S1 is about0.9 nm.

In order to evaluate the different carbon loadings onsamples S1–S3, thermo-gravimetrical analysis (TGA)results for S1–S3 Ni-MCM-41 samples after SWCNTgrowth using different carbon sources are given in Fig. 6.Temperature-programmed oxidation is currently beingwidely used to distinguish different carbon species in sam-ples containing SWCNTs. The weight loss results due tocarbon oxidation up to different temperatures are listed inTable 3. However, the oxidation temperature of SWCNThas been observed to vary considerably for samples pre-pared under different conditions [8,35]. The rather widerange of SWCNT oxidation temperatures has been attrib-uted to the differences in the catalytic activity of metallicparticles present in SWCNT samples as residues [9]. Thefirst peak centered between 150 and 300 �C has beenassigned to the oxidation of amorphous carbon compo-nents. The positive peaks around 350 �C are contributedby the oxidation of the metallic particles, initially coveredby amorphous carbon [36]. When the amorphous carbonlayer covering the nickel particles has been removed, thereduced nickel particles become accessible to the oxygenand they will be easily oxidized. Previously, the two negativepeaks around 360 and 510 �C were assigned to the oxidationof SWCNTs [36]. The same assignment can be made here aswell for S1 and S2. As confirmed by Raman spectroscopy ofcarbon loaded Ni-MCM-41 samples previously, Ramanspectra (not shown) give evidence of RBM peaks after expo-sure to air for 30 min at 300 and 450 �C, respectively. How-ever, after exposure to air at 600 �C, the disappearance ofRBM peaks suggesting complete oxidation of SWCNTs.Although S3 from methane decomposition showed asharp single peak at similar position with SWCNTs, thesame assignment cannot be made here, because Ramanspectra of Ni-MCM-41 loaded with carbon after methanedecomposition did not show any significant RBM peaks.SWCNTs were also hard to find in TEM images. And very

800pm

400

0

-400

Hei

ght

4003002001000

nm

500

400

300

200

100

0

nm

5004003002001000

nm

3

2

1

0

-1

nm

A C

B

D

Fig. 5. Atomic force microscopy (AFM) images of SWCNTs grown by CO disproportionation: (A) individual SWCNTs wrapped in single strand DNA,(B) corresponding topographic height profile along the dark lines drawn in panel A, (C) 3D image of individual SWCNTs in panel A, (D) SWCNT bundlesgrowing from a catalyst grain of Ni-MCM-41.

2222 Y. Chen et al. / Carbon 45 (2007) 2217–2228

few multi-walled carbon nanotubes (MWCNTs) wereobserved, even though some did exist. As discussed in ourprevious study [37], the strong oxidation peak around600 �C of S3 in Fig. 6 most likely can be attributed to amor-phous carbon, small amount of MWCNTs, and carbonfibers. As shown in Fig. 9C, we observed twisted carbontube bundles, amorphous carbon grown from Ni-MCM-41 catalyst under methane decomposition. The resultsshown in Fig. 6 are consistent with our previous findings[37], that the DTA results would be misleading, withoutcomplementary spectroscopic experiments, because the car-bon oxidation temperature of the high concentration amor-phous carbon in air was high enough to be considered asthat of SWCNTs.

As discussed in our previous publications [10,22,23,36],the diameter of the SWCNTs produced by transition metalincorporated MCM-41 catalysts is believed to be related tothe size of the metallic nanoparticles. Large metallic parti-cles result in the growth of MWCNTs and carbon fibers.Therefore, it is necessary to investigate the sizes of metallicclusters have been created with different carbon sources.Metallic clusters of 1 nm scale are too small to analyze withstandard X-ray diffraction. Those metallic clusters whichlead to the growth of SWCNTs, usually are from the reduc-tion of Ni ions incorporated in the MCM-41 silica matrix.

They are difficult to be imaged by TEM. In this research,Ni-MCM-41 catalyst samples have been investigated byX-ray absorption spectroscopy (XAS).

X-ray adsorption data on a fresh Ni-MCM-41 catalyst,nickel metal foil, and Ni-MCM-41 after different CVDexperiments were collected. Two features of the spectrumare shown in Fig. 7: the white line feature resulting fromthe density of unoccupied states above the Fermi level isattributed to the oxidized nickel; the pre-edge peak is char-acteristic of metallic nickel. The results here demonstratethat nickel in the fresh Ni-MCM-41 is in the oxidized statebecause nickel is incorporated in the MCM-41 matrix andstabilized by Ni–O–Si bonds. Prereduction in hydrogenfacilitates the removal of hydroxyl, two OH� ligands willbe replaced by an O2� ligand in the coordination sphereof nickel [29]. The white line decreases and the pre-edgepeak slightly increases for hydrogen reduced sample com-pared to the fresh catalyst. After CVD, the spectra ofS1–S3 are close to the spectrum of the nickel foil indicatingthat nickel in MCM-41 has been reduced. In order to char-acterize the size of the reduced nickel clusters, k2 weightedEXAFS spectra were Fourier transformed into R-space (Rrepresents the distance between atoms), as shown in theFig. 8. Spectra of fresh Ni-MCM-41 and Ni-MCM-41 afterprereduction in H2 at 400 �C both show the strong peak

0 1000 2000 3000 4000 5000 6000

-0.010

-0.008

-0.002

0.000

0.002

0.004

150 300 450 600 750 900 1050Temperature, oC

S3'

S3

S2'

S2

DTG

, wt%

/s

Time, s

S1

Fig. 6. DTA curves for as-synthesized Ni-MCM-41. The inset shows thecarbon yield calculated from DTA curves. S1, S2, S2 0, S3, S3 0: the same asin Fig. 1.

Table 3Weight loss of carbon loaded Ni-MCM-41 samples in TGA, S1, S2, S2 0,S3, S3 0: the same as in Fig. 1

Weight loss wt% Up to 300 �C Up to 1050 �C

S1 0.38 2.2S2 0.27 2.9S2 0 0.53 2.8S3 0.03 6.5S3 0 0.06 2.7

Y. Chen et al. / Carbon 45 (2007) 2217–2228 2223

intensity associated with the Ni–O bond. On the otherhand, the Ni–Ni interaction peak is the main characteristicpeak for catalysts after CVD. Calculated first-shell Ni–Niand Ni–O coordination numbers are illustrated in Table 4.The large Ni–Ni coordination number indicates that mostof Ni ions in MCM-41 have been reduced. If all Ni ionsare tetrahedrally dispersed in MCM-41 matrix, the Ni–Ocoordination number should be 4. Fresh Ni-MCM-41 hasa Ni–O coordination number of 3.98 giving evidence thatNi ions have tetrahedral coordination with surroundingoxygen anions and are isomorphously substituted for Siions in the framework. After prereduction in H2 at400 �C, the 3.13 Ni–O coordination number indicates thatmost Ni ions are still in the MCM-41 matrix. Based on ageometric model [10], the larger Ni–Ni coordination num-

ber [7,8] observed after CVD demonstrates that the averagediameter of Ni clusters is about 1–2 nm. The key questionswe should address are: (1) What is the difference of Ni ionsreduction in MCM-41 using different carbon sources? (2)Why do the metallic clusters formed with different carbonsources produce carbon deposits of different structures?

4. Discussion

From the SWCNT synthesis perspective, the size ofmetallic clusters is a key parameter to control both selectiv-ity to SWCNTs and diameter uniformity. As proposed byother researchers, the fundamental process in carbon nano-tube CVD growth on a metallic cluster has four steps: (1)adsorption of the gas precursor molecule on the metal clus-ter surface, (2) dissociation of the precursor molecule, (3)diffusion of the growth species on (or in the case of largeparticles, into) the catalyst particle, and (4) nucleationand incorporation of carbon into the growing structure[38]. Simulation studies have considered the critical forma-tion and growth of metallic clusters during the CVD pro-cesses [39–41]. It is proposed that a dynamic balancebetween metallic cluster formation and carbon precursorprecipitation plays an important role in the growth ofSWCNTs. In our previous work [42,43], we have observedthat an intermediate Co1+ species was formed by purehydrogen prereduction. The intermediate species preservethe tetrahedral coordination inside the silica frameworkand are resistant to complete reduction in hydrogen. How-ever, the intermediate Co1+ species is more reactive in thepresence of CO. Binding CO molecules may weaken theinteraction of the Co1+ species with the surrounding oxy-gen ions in the MCM-41 silica matrix, so they can nucleateinto clusters on the pore surface of MCM-41. These clus-ters keep growing until they reach the size and electronicstate required to initiate the growth of carbon nanotubes.After the initiation of SWCNT growth, the clusters areimmobilized by carbon nanotubes, and their growth isimpeded by carbon covering on their surface. This mecha-nism suggests that regulating the rates of metallic clusterformation and carbon precipitation will likely be the keyfactor to control both the selectivity to SWCNTs and theSWCNT structure. Different carbon sources should havedifferent effects on both the formation rate of Ni metallicclusters during the CVD process and the carbon precipita-tion rate on Ni clusters, which in turn should influence thestructure of the carbon deposits formed and the diameterdistribution of SWCNTs produced as well.

In this study, all Ni-MCM-41 samples were prereducedunder H2 at the same condition, and XAS results in Figs. 7and 8 indicate that Ni ions have not been significantlyreduced. The Ni–O coordination number (3.13) of 400 �Chydrogen prereduced Ni-MCM-41 suggests that the major-ity of Ni ions are still in the MCM-41 matrix, albeit oxygendefects have been introduced in the vicinity of most Niions. As a neighbor of Co in the periodic table of elements,one might expect a similar intermediate Ni1+ species may

8300 8325 8350 8375 8400 8425 8450 8475 8500

0.0

0.5

1.0

1.5

H2 reduced

white-line peak

pre-edge peakNi foil

S3'S3

S2'

S2

Norm

. absorption coefficient

Energy, eV

fresh

S1

Fig. 7. Normalized EXAFS spectra near the Ni K edge recorded for Ni-MCM-41 loaded with carbon after CVD. Spectra for the fresh Ni-MCM-41 andNi foil are given as references. S1, S2, S2 0, S3, S3 0: the same as in Fig. 1.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00.0

0.2

0.4

0.6

0.8

S3'

S3

S2

S2'k2 χ(k)

, Å-3

R, Å

Ni foil

fresh Ni-MCM-41

H2 reduced

S1

Fig. 8. EXAFS spectra of Ni-MCM-41 samples and references in R space.

Table 4Ni–Ni and Ni–O coordination numbers of Ni-MCM-41 samples

Ni–O first-shell

NNi–Oa dR (A)b r2

Fresh 3.98 ± 0.21 0.13 ± 0.02 0.Prereduced 3.13 ± 0.26 0.13 ± 0.02 0.S1 – – –S2 – – –S2 0 0.84 ± 0.31 0.12 ± 0.02 0.S3 – – –S3 0 – – –

a NNi–O average first-shell coordination of nickel–oxygen.b dR deviation from the effective half-path-length R (R is the interatomic dic r2 (·10�2 A2) mean-square deviation in R.d NNi–Ni average first-shell coordination of nickel.e ‘‘–’’ represents the peak is unable to be fitted due to the weak intensity.

2224 Y. Chen et al. / Carbon 45 (2007) 2217–2228

be produced [43]. The main difference among the carbonsources lies on their abilities to react with the Ni intermedi-ate species, which can weaken the interaction of Ni with thesilica framework of MCM-41 and, thus, initiate formationof Ni clusters.

In addition, there are two growth modes: tip- and base-growth modes. The determining factor for the growthmode was found to be the adhesion force of the catalyticmetal particles to the substrate [44]. For Ni-MCM-41 cat-alyst, there are strong chemical bonds between metal ionsand silica templates. No tip-growth modes were observedfor both ethanol and CO in TEM analysis. SWCNTs weregrown inside the pore of MCM-41 and then extended out-side as shown in Fig. 9A. For methane, even thoughSWCNTs were not produced, Ni clusters were still attachedto the MCM-41 surface as shown in Fig. 9C indicating thestrong interaction. MCM-41 structure was not damaged

Ni–Ni first-shell

c NNi–Nid dR (A)b r2c

84 –e – –78 2.86 ± 0.74 �0.04 ± 0.02 0.84

8.87 ± 0.81 �0.02 ± 0.01 0.678.67 ± 0.88 �0.02 ± 0.01 0.71

89 4.89 ± 0.83 �0.03 ± 0.02 0.797.88 ± 0.93 �0.02 ± 0.01 0.737.61 ± 0.83 �0.03 ± 0.01 0.76

stance for single scattering paths).

Fig. 9. TEM images showing the Ni-MCM-41 and carbon deposits produced: (A) SWCNTs grown from Ni-MCM-41 in ethanol decomposition at 800 �C,(B) carbon deposits on Ni-MCM-41 after ethanol decomposition at 650 �C, (C) Ni clusters covered by carbon layers and carbon deposits on Ni-MCM-41after methane decomposition, and (D) reduced Ni clusters on Ni-MCM-41 after methane decomposition.

Y. Chen et al. / Carbon 45 (2007) 2217–2228 2225

during the grown process. These suggest that they are allbase-growth modes.

Compared to methane and ethanol, CO is more reactivebased on reduction of Ni by the reactive molecule. CO dis-sociation on the Ni metal surface starts when a certain sizeof Ni cluster is formed. At 800 �C, Ni clusters grow fastunder CO. Previous in situ XAS study has shown that Niin MCM-41 can be totally reduced by CO at 750 �C in15 min [23]. When Ni clusters grow to a certain critical size,SWCNT growth can be initiated, if the disproportionationof CO can provide enough carbon surface supersaturationon Ni clusters. On the other hand, if the rate of Ni clusterformation does not match the rate of precipitation, themetallic cluster continues to grow, and the growth of thenickel clusters will eventually cease as they become coveredby layers of carbon. A dynamic balance exists, thus,between metallic cluster growth and precipitation andsupersaturation of C atoms on the cluster surface. In Figs.1–4, we have observed several peaks in RBM of S1 relating

to SWCNTs about 0.9 nm in diameter. An AFM study ofSWCNTs wrapped in DNA as shown in Fig. 5 confirmedthat the diameter of tubes. These results suggest that theremight be a narrow window of cluster size for nickel to ini-tiate SWCNT growth; therefore, not all Ni clusters couldgrow SWCNTs. An intriguing question is whether this win-dow will change under different carbon sources.

When ethanol is used as a carbon source, several differ-ences exist compared with CO. First of all, the rate of Nicluster formation under ethanol compared to under COcould be different. Ethanol could reduce Ni ions weakeningthe interaction of Ni ions with the surrounding oxygen ionsin the MCM-41 silica matrix at different rate comparedwith CO. Second, feeding C atoms by ethanol to the Nicluster surface is different than using the CO as the carbonsource. It is likely that ethanol decomposes into �OHradicals first ðC2H5OH! C2H�5þ�OHÞ, then precipitatescarbon on Ni clusters ðC2H�5 ! C�nðn¼1�2Þ þH�Þ. Oxygen-containing species, like �OH radicals, have been suggested

2226 Y. Chen et al. / Carbon 45 (2007) 2217–2228

to facilitate the oxidation of amorphous carbon and tofavor the SWCNT growth [18,31]. Carbon deposits on Niclusters may be attacked by �OH radicals, which couldlower the carbon atom concentration on Ni cluster surface.In Table 3, TGA results indicate that ethanol produceslightly less amorphous carbon compared with CO at800 �C. Furthermore, a high concentration of H� speciesdoes not favor the formation and growth of sp2 likeSWCNTs [18]. H� produced in ethanol decompositionmay change the window in which Ni clusters can initiateSWCNT growth. This is what we have observed inFig. 4. Ethanol decomposition produced SWCNTs withslightly larger mean diameter (about 1 nm) compared withSWCNTs produced by CO (about 0.9 nm). Overall,dynamic balance between metallic cluster size growth andsupersaturation/precipitation of C atoms on the clustersurface shifts under different carbon sources.

However, apparently contradicting results obtained byEXAFS indicate that Ni clusters produced under CO(NNi�Ni = 8.87 ± 0.81) is slightly larger than Ni clustersproduced under alcohol (NNi�Ni = 8.67 ± 0.88). One possi-ble explanation is that the EXAFS result is a volume aver-age of all species in the sample including cobalt ions andmetallic cobalt clusters with different sizes which includeboth those leading to SWCNT growth and those leadingto MWCNTs and graphite. The presence of a few largeparticles may significantly increase the Ni–Ni coordinationnumber estimation. On the other hand, non-reduced nickelions can decrease the Ni–Ni coordination number. The res-olution of EXAFS data we obtained on Ni-MCM-41 stilldoes not allow us to fractionalize metallic clusters leadingto the SWCNT growth from all nickel metallic clusters.It is likely that CO is more reductive to Ni-MCM-41 com-pared with ethanol. The fast and total reduction of nickelin MCM-41 under CO has been demonstrated in our pre-vious study [23]. CO reduced more nickel in the MCM-41framework which leads to a larger Ni–Ni coordinationnumber in EXAFS.

One of the key advantages of SWCNT growth with eth-anol CVD as proposed by researchers [31], is thatSWCNTs can be grown at lower temperatures, suitablefor in situ fabrication of electronic devices at 400–600 �C.We have tested the ethanol CVD growth at 650 �C (sampleS2 0). Prereduction conditions were maintained the same.Raman, TGA and XAS results for S2 0 are shown in Figs.1–4, 6–8. TEM analysis is shown in Fig. 9B. Comparedwith S2, the major difference between carbon deposits isthat more amorphous carbon has been produced, whichhas been evidenced with stronger D band in the Ramanspectra (S2 0 of Figs. 1–3) and higher peak intensities ofthe amorphous carbon in TGA in Fig. 6 and Table 3, aswell as indicated in Fig. 9B. The changes in carbon depositsshould correlate with changes of metallic Ni cluster forma-tion. XAS analysis of Ni clusters shows that Ni ions havebeen only partially reduced. We observed both a strongwhite line peak and a small pre-edge peak for S2 0 inFig. 7. The Ni–Ni peak of S2 0 in R space shown in Fig. 8

also has the lowest intensity among samples after CVD.The Ni–Ni coordination number is 4.89 and Ni–O is0.84. The Ni–O coordination number should be 4 if allNi ions tetrahedrally dispersed in the MCM-41 matrix; thissuggests that about 80% of Ni ions have been reduced at650 �C in ethanol CVD. We propose that ethanol decom-position is slow under lower reaction temperature, whichhas a twofold effect. First, some of Ni ions still stay inthe MCM-41 matrix; less Ni ions have been reduced. Sec-ond, when Ni clusters reach the critical size to growSWCNTs, carbon atom concentration on Ni cluster sur-face is too low to initiate the SWCNT growth. Once thewindow of SWCNT growth is passed, amorphous carbon,graphite/carbon fibers will be produced. That is shown inthe TEM image of Fig. 9B, where different carbon depositswere produced on Ni-MCM-41 at 650 �C ethanoldecomposition.

The last carbon source we have studied was methane.Methane (CH4! C� + H�) does not have �OH radicals asin ethanol to remove amorphous carbon. Also, a high con-centration of H� species does not favor the formation andgrowth of sp2-like SWCNTs either [18]. We propose thatunder methane CVD condition, the feed rate of carbonatoms to Ni clusters is not fast enough compared to the for-mation of Ni clusters, and the existence of high concentra-tion H� species prevents the initiation of SWCNT growth.Ni clusters continue to grow larger, resulting in the growthof graphite/carbon fibers with diameters in the 10 nm range.The growth of Ni clusters ceases when they are covered bylayers of carbon. These nickel clusters produced in methanecomposition have been observed in TEM images shown inFig. 9C and D. In many studies of methane decompositionto hydrogen and carbon nanofibers [34,45], the Ni clusters(>20 nm) detach from the catalyst support and sit at thetip of the carbon fibers. However, we found that most ofthe Ni clusters formed in our study were still attached tothe catalyst surfaces as shown in the TEM image Fig. 9C.This could be explained by the strong interaction betweenNi and the MCM-41 matrix.

In order to compare with CO disproportionation, meth-ane pressure for S3 was set at 6 atm, while the ethanol pres-sure for S2 is only at 11.8 torr. Is it possible that the carbonconcentration in Ni clusters is too high, which inactivate Niclusters? To test this hypothesis, a separate SWCNT syn-thesis experiment was performed using identical prereduc-tion and reaction temperatures, but using 0.4 atmmethane partial pressure during the SWCNT growth.The results obtained from the sample S3 0 are shown inFigs. 1–4, 7 and 8. The Raman spectra of S3 0 in Figs. 1–3 are identical with S3. No RBM peaks can be identified.XAS spectra show that Ni ions in MCM-41 are reducedafter methane CVD. These results indicate similar CVDbehaviors under 6 atm and 0.4 atm methane partial pres-sure. Therefore, this behavior is consistent with thosereported by other researchers investigating the effect ofthe methane partial pressure effect on the synthesis ofSWCNTs [46]. When the methane partial pressure is higher

Y. Chen et al. / Carbon 45 (2007) 2217–2228 2227

than a critical value (0.4 atm at 850 �C [46]), this variablewill no longer affect the growth rate of SWCNTs.

5. Summary

We have presented the CVD growth of SWCNTs usingthree representative carbon sources: CO, ethanol andmethane, and Ni incorporated MCM-41 as the catalytictemplates. SWCNTs synthesized from these three carbonsources under comparable reaction conditions were charac-terized. CO produced SWCNTs with smaller mean diame-ters. Ethanol produced SWCNTs with relatively largermean diameters compared to CO. Methane could not pro-duce SWCNTs under the conditions tested in this research.These results demonstrate that different dynamic balancesexist between metallic cluster formation and carbon precip-itation using three different carbon sources. When Ni clus-ters reach a critical size, if the C atoms feed rate to themetallic cluster surface provided by precipitation of carbonsources is in a narrow window to saturate Ni cluster sur-face, SWCNT growth can be initiated from these Ni clus-ters. Otherwise, metallic clusters continue growing, thegrowth of the nickel clusters will finally cease as they arecovered by graphite carbon layers. Controlling SWCNTstructure relies on precisely regulating the dynamic balancebetween metallic cluster formation rate and carbon sourceprecipitation rate on metallic cluster surfaces. Using differ-ent carbon sources on the same catalyst, we are able to pro-duce SWCNTs with different mean diameters.

Acknowledgments

We are grateful for the financial support from USDoE-BES for this project, and the use of the NationalSynchrotron Light Source at Brookhaven National Labo-ratory. We also thank Sang Nyon Kim and ProfessorFotios Papadimitrakopoulos at University of Connecticutfor the access to the multi-excitation wavelength Ramaninstrument; Professor Liwei Chen at Ohio University forthe access to the AFM measurement. We also thankreviewers for useful comments. This work was also sup-ported in part by the start-up fund of Nanyang Technolog-ical University and by the PD22 Grant funded by theRomanian Ministry of Education and Research.

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