1966

Russian Journal of Physical Chemistry, Vol. 78, No. 12, 2004, pp. 1966–1971. Translated from Zhurnal Fizicheskoi Khimii, Vol. 78, No. 12, 2004, pp. 2222–2227.Original Russian Text Copyright © 2004 by Rakov, Grishin, Gavrilov, Rakova, Nasibulin, Dzhian, KauppinenEnglish Translation Copyright © 2004 by MAIK “Nauka /Interperiodica” (Russia).

INTRODUCTION

Carbon nanotubes with a small number of wallshave wide potential applications [1, 2]. Nanotubes areprepared by graphite vaporization and desublimationand catalytic decomposition or pyrolysis of carbon-containing reagents [3, 4]. The pyrolytic method is con-sidered most promising for large-scale use. Pyrolysiscan be performed in apparatus simple in design, andthis method is easily scalable [4]. The most convenientsource of carbon for the synthesis of nanotubes is meth-ane, which does not undergo noncatalytic pyrolysis atlow temperatures and, therefore, does not give amor-phous carbon impurities under optimum conditions.

Quite a number of works have been concerned withcatalytic pyrolysis of

ëç

4

and the formation of nano-tubes. In many of them, the synthesis of single-wallnanotubes was performed using three-component cata-lysts containing an active metal (Fe, Co, or Ni), a pro-moter (Mo or W compounds), and a carrier that pre-vented active metal nanoparticles from aggregation(Mg, Al, Si, and other metal oxides) [5–9]. In particular,the influence of the composition of the Co–Mo/MgOcatalyst on the yield and quality of nanotubes was stud-ied in [8]. It was shown that the highest yield of single-wall nanotubes and the lowest amount of amorphouscarbon impurity were obtained at a Mo : Co : Mg =1 : 5 : 94 atomic ratio. At the same time, the morphol-ogy of the products was not characterized in [8].

The purpose of this work was a more detailed studyof the structure, morphology, and properties of nano-tubes and nanoparticles formed in the pyrolysis of

ëç

4

on a catalyst with the Mo : Co : Mg = 1 : 5 : 94 atomicratio.

EXPERIMENTAL

The catalyst was synthesized by the combustionmethod developed by Patil [10] and briefly described in[8]. The method was slightly modified: instead of citricacid, we used glycine, which allowed us to suppress theformation of nitrogen oxides. A porcelain cup (250 ml)contained a mixture of

Mg(NO

3

)

2

·

6H

2

O

[10.0 g, ch. d. a.(pure for analysis) grade],

Co(NO

3

)

2

·

6H

2

O

[0.6 g, ch.(pure) grade],

(NH

4

)

6

Mo

7

O

24

·

4H

2

O

[0.073 g, kh. ch.(chemically pure) grade],

NH

2

CH

2

COOH

(6.0 g, ch.grade), and

H

2

O

(2.5 ml). The suspension was heatedunder stirring to its complete solution, introduced intoa furnace heated to

550°ë

, and held there for 20 min.The specific surface area of the catalyst was 110–160 m

2

/g (that of the catalyst used in [8] was 176.5 m

2

/g).The pyrolysis of

ëç

4

(natural gas) was performedusing a unit that included a horizontal quartz tube reac-tor 56 mm in diameter and 900 mm long. The gasentered through a special pipe into the center of thereactor and exited from both reactor ends. A quartz boatwith the catalyst (up to 4.0 g) was placed into the centerof the reactor. The reactor was blown through with amixture of

CH

4

(150 cm

3

/min) and

H

2

(600 cm

3

/min)for 10 min, then heated to

900°C

under the same condi-tions at a rate of 30 K/min, held at the final temperaturefor 1 h, and cooled in a flow of

H

2

.The products were purified from the major amount of

the catalyst by heating in 23% HCl (treatment with con-centrated

HNO

3

as in [8] caused the oxidation of part ofsingle-wall nanotubes), washed with distilled water, anddried at

110°C

. The yield of carbonaceous products wasdetermined from the catalyst weight loss, taking into

The Morphology of Pyrolytic Carbon Nanotubeswith a Small Number of Walls

E. G. Rakov

1

, D. A. Grishin

1

, Yu. V. Gavrilov

1

, E. V. Rakova

2

, A. G. Nasibulin

3

, H. Jiang

3

, and E. I. Kauppinen

3, 4

1

Mendeleev University of Chemical Technology, Miusskaya pl. 9, Moscow, 125190 Russia

2

Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, 117333 Russia

3

Center for New Materials, Helsinki University of Technology, Helsinki, Finland

4

Technical Research Center of Finland, Helsinki, FinlandE-mail: rakov@rctu.ru

Received December 25, 2003

Abstract

—A mixture of carbon nanotubes with one to four walls and nanofibers was prepared by catalyticpyrolysis of

ëç

4

. The structure and morphology of the products were studied by usual and high-resolutiontransmission electron microscopy in combination with Raman spectroscopy, X-ray diffraction, and derivatog-raphy. The outside diameter of double-wall nanotubes changed from 1.8 to 6.7 nm, and that of triple- and qua-druple-walls nanotubes, from 2.6 to 6.2 and from 4.0 to 7.7 nm, respectively. Double-, triple-, and quadruple-layer nanotubes had conic caps. A scheme of the formation of nanotubes was suggested.

STRUCTURE OF MATTER AND QUANTUM CHEMISTRY

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY

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No. 12

2004

THE MORPHOLOGY OF PYROLYTIC CARBON NANOTUBES 1967

account that the weight of the catalyst changed by 16%in control experiments (in the absence of

CH

4

).The ash content in the products (the residual content

of the catalyst) was determined gravimetrically afterwashing the product with acid by oxidation in air at

850°C

.The phase composition and structure of the products

were studied by transmission electron microscopy on aJEM 100C instrument and Philips EM430 and PhilipsCM200 FEG high-resolution transmission electronmicroscopes and also using a Tesla BS340 scanningelectron microscope, a Jobin-Yvon U1000 Ramanspectrometer (wavelength 488 nm), and a DRON-2X-ray diffractometer. The oxidation of the products wasstudied on a Paulik-Paulik-Erdei derivatograph. Thespecific surface area was measured by the method ofnitrogen desorption.

RESULTS AND DISCUSSION

The yield of solid carbonaceous products was 1.8–2.0 g, and the specific efficiency of the catalyst was0.45–0.50 g/g (0.41 g/g according to [8]). After boilingin HCl (15 min), washing to a neutral reaction, and dry-ing, the specific surface area of the products was400 m

2

/g (703.7 m

2

/g according to [8]), and the ashcontent in them was 11–14 wt % (20% according to[8]). Further boiling (up to 1 h) did not decrease theamount of ash. Similar results were obtained after treat-ing nanotubes with glacial acetic acid and 23% perchlo-ric acid. The obtained specific surface value corre-sponded to nanotubes with a small number of walls [11]and bundled single- and double-wall nanotubes.

A larger

H

2

: CH

4

ratio and a shorter duration ofholding at the final high temperature were used in [8].According to [8], single-wall nanotubes were predomi-nantly formed under these conditions. We, however,were unable to determine the quantitative differencebetween the yields of one-layer nanotubes under theconditions used in [8] and this work. In addition, theformation of nanofibers was not observed in [8].

The electron microscopy data (Fig. 1) showed thatthe solid products obtained in this work were mixturesof comparatively rectilinear or weakly bent nanotubeswith a small amount of strongly curved nanofibers withuneven outside surfaces and internal partitions. Thisleads us to suggest that pyrolysis occurred by two well-known mechanisms under the conditions used in thiswork: tip and root [3]. During tip growth, catalyst par-ticles reside at the tip of a growing fiber (some of theseparticles are shown as dark round formations in Fig. 1),and, during root growth, they remain on the surface ofa carrier and cause growth of nanotubes. Earlier, such a“dual” mechanism was observed for the pyrolysis of

C

2

H

4

under mild conditions (

550°C

) on the catalyst

Fe–Mo/Al

2

O

3

[12].The major carbon form in the pyrolysis products

was nanotubes with one to four walls, among whichdouble-wall nanotubes predominated (Fig. 2). Single-and double-wall nanotubes were mostly present as bun-dles 8–30 nm in diameter and up to several micronslong. The radial breathing modes at 300, 220, and170 cm

–1

in the Raman spectra of the products (Fig. 3)can be assigned to single-wall nanotubes 0.9, 1.0, and1.4 nm in diameter, respectively [13]. Note that, in [8],pyrolysis largely yielded single-wall nanotubes 1.16 nm

60 nm

Fig. 1.

Electron microscopic image of carbon nanotubesand nanofibers.

10 nm

100 002

004

Fig. 2.

Photomicrography of the product at a high resolutionand diffraction pattern from a bundle of nanotubes.

1968

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY

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2004

RAKOV

et al

.

in diameter. The formation of double-wall nanotubeswas not mentioned in [8], probably because double-wallnanotubes looked like single-wall nanotubes at low reso-lution.

The diffraction pattern from a selected region withtubes having two to four walls is shown in Fig. 2. The

conclusion can be drawn that tube walls are parallel, asfollows from the presence of an equatorial line withstrong 002 and weaker 004 reflections. The arcs thatcorrespond to graphite 100 and 110 reflections are splitand elongated in the direction perpendicular to the tubeaxes. According to the concept of the reciprocal latticeof cylindrical nanotubes [14], the deformation of thesereflections is caused by the small number of walls innanotubes, and their splitting is a consequence of dif-ferent angles of twist (different chiralities) of walls.The diffraction pattern from a separate quadruple-wallnanotube shown in Fig. 4 also shows that different wallshave different chiralities.

A comparison of the Raman data with the electronmicroscopic results also allows us to suggest that thespecified lines can be related to internal double-wallnanotube walls. Indeed, according to measurementsperformed for 18 isolated double-wall nanotubes, theiroutside diameter (

D

out

) changes within 1.8–7.1 nm, andtubes with diameters of 4.5

±

0.5 nm predominate,whereas the smallest inside diameters (

D

in

) were 0.9and 1.1 nm (Table 1). Unfortunately, the accuracy ofmeasurements was no more than 0.1 nm, and we weretherefore unable to obtain the dependence of the inter-planar distance

d

on

D

out

. We can only note that the

d

=0.4 nm value coincides with the measurement resultsreported in [15, 16] and that

d

tends to increase as

D

out

grows. At the same time, it increases by a power lawwith an exponent smaller than one, and the

D

out

/

D

in

ratio therefore decreases from 2.0 (

D

out

= 1.8 nm) to1.22 (

D

out

= 7.1 nm), whereas we might expect a slowerdecrease at a constant

d

value. It is also probable thatthe specified tendency is related to the presence ofdefects in nanotube walls.

The radial breathing modes of double-wall nano-tubes of the most frequently occurring diameter shouldlie near 50 cm

–1

. In this Raman spectrum region, weindeed observed a strong signal, but we were unable todetermine if this signal was related specifically to dou-ble-wall nanotubes. The Raman spectrum region below100 cm

–1

was not reported in the other works concernedwith double-wall nanotubes.

The specified range of nanotube diameters corre-sponds to that observed for double-wall nanotubes pre-pared by the arc [15] (2.4–5.5 nm) and pyrolytic [16](1.3–6 nm, catalyst

Fe–Mo/Al

2

O

3

) methods and issomewhat broader than that reported in [17] (1.75–3.1 nm,catalyst

Fe–Mo/Al

2

O

3

), [18] (1.1–3.5 nm, catalystCo/MgO), and [19] (1–3 nm, pyrolysis with a volatileFe catalyst). The closeness of the most typical diameterto 4.5 nm is substantiated by the results obtainedin [20].

Triple- and quadruple-wall nanotubes are present inmuch smaller amounts in the products of

CH

4

pyrolysis(eight three-layer and six quadruple-wall nanotubeswere observed in 10 photomicrographies). Probably forthis reason, the spread of their geometric characteristicsis much narrower (Table 2). The single quintuple-wall

1

0

I

,

arb. units

ν

, Òm

–1

2

1200200 400 1400 1600 1800

×

5

×

5

×

1

×

1

b‡

3

4

5

6

Fig. 3.

Raman spectra of (

a

) products washed with an acidand (

b

) products annealed at

340°C

for 2 h in air and washedonce more.

10 nm

100002

Fig. 4. Photomicrography of the product at a high resolutionand diffraction pattern from a quadruple-wall nanotube.

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY Vol. 78 No. 12 2004

THE MORPHOLOGY OF PYROLYTIC CARBON NANOTUBES 1969

nanotube has a variable outside diameter (5.9–6.6 nm).The trend in d variations observed for two-layer nan-otubes was not characteristic of nanotubes with alarger number of layers.

An interesting morphological feature of double-,triple-, and quadruple-wall nanotubes obtained bypyrolysis is the formation of peculiar tips resemblingcaps or the rostrum of belemnite. Such caps are seen inFig. 2 (on two- and four-layer nanotubes) and 4 (twosuperimposed caps; the upper cap is double-wall andthe lower one, triple-wall). The cone angle of all suchcaps changes smoothly over the distance L, which,according to measurements, equals approximately(2.1−3.5)Dout. A comparatively smooth change in theangle over a small distance is not characteristic of nan-otubes of the “classic” type, because any bend of theirwalls can only occur if topological defects of the formof pentagons appear in the regular graphene shell.Smooth angle variations in tips of polylayer nanotubesare possible if defects are consistently distributed notonly in one wall but also in different walls and if thedensity of defects is comparatively high.

The angle at the cap vertex is not constant. Forstrictly conic structures of regular graphene layers, thecone angle cannot take on random values. It equals19.2° if five five-membered rings are introduced and38.9° if there are four such rings. In the products thatwe are considering, the situation is more complex,because tips smoothly transform into tubes. It followsthat five-membered rings are distributed over the wholetip surface.

Scroll-type structures are observed in several photo-micrographies. These structures have different numbersof graphene layers on the two sides of their axes. Inaddition to nanofibers, which, as is seen in Fig. 1, differslightly in diameter from the most large-sized nano-

tubes, the products contain a small amount of graphi-tized particles and onion structures. The internal cavi-ties of some of the particles are filled with the catalyst.

Metal particles of a certain size participate in theformation of nanotubes and nanofibers. If this size isexceeded, we observe the formation of multilayergraphene shells on cobalt particles. A special search forsuch particles (we observed seven of them) and mea-surements showed that their mean diameter varied from4.6 to 13.1 nm, the majority of them being 6–7 nm insize, and the number of graphene layers in the shellswas three to eight. This leads us to suggest that the for-mation of nanotubes and nanofibers is catalyzed by par-ticles of size 5 nm or less.

Attempts at determining the period d002 by X-raydiffraction were unsuccessful. The X-ray patterns didnot contain well-defined 002 reflections characteristicof graphite-like structures. At the same time, the Raman(G band at 1300 cm–1 in Fig. 3) and electron micros-copy data are evidence that the products contain disor-dered carbon on the surface of separate nanotubes ortheir bundles and probably in nanotube cavities. Some-times, it takes the form of short strongly curves“scraps” of graphene planes (Figs. 2, 4).

Annealing in Ar (1000°ë, 1 h) did not cause sub-stantial changes in the morphology of CH4 pyrolysisproducts. The fraction of removed components was7 wt %. Annealing in H2 (950°C, 2 h) resulted in a 52%weight loss. The relative contents of disordered carbonand nanotubes with small diameters then decreasedsomewhat.

Several experiments were performed under isother-mal conditions, when heating was performed in a flowof Ar and a mixture of CH4 and H2 was supplied for 1 hinto a reactor heated to 900°ë. The yield of nanotubeswas close to that obtained under nonisothermal condi-

Table 1. Dout, Din, and d values (nm) for 18 isolated double-wall nanotubes

Dout Din d Dout Din d Dout Din d

1.8 0.9 0.4 4.0 3.0 0.5 4.9 4.0 0.4

2.0 1.1 0.5 4.0 3.1 0.4 5.0 4.0 0.5

2.8 2.1 0.4 4.4 3.5 0.4 5.0 4.0 0.5

3.0 2.1 0.5 4.5 3.5 0.5 5.4 4.0 0.7

3.2 2.2 0.5 4.5 3.7 0.4 6.7 5.5 0.6

3.9 2.9 0.5 4.5 3.7 0.4 7.1 5.8 0.6

Table 2. Dout, Din, and d values (nm) for triple- and quadruple-wall nanotubes

Dout Din d Dout Din d Dout Din d Dout Din d Dout Din d

Triple-wall nanotubes Quadruple-wall nanotubes

2.6 0.6 0.5 3.0 1.4 0.4 6.0 4.4 0.4 4.0 1.3 0.5 5.9 3.1 0.5

2.8 1.0 0.5 3.2 1.4 0.4 6.2 4.6 0.4 4.8 2.4 0.4 7.7 5.0 0.4

2.9 1.2 0.4 4.5 2.8 0.4 4.9 2.4 0.4 7.7 5.2 0.4

1970

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY Vol. 78 No. 12 2004

RAKOV et al.

tions, but the quality of nanotubes was somewhatworse. The preliminary reduction of the catalyst in aflow of H2, especially at high temperatures, increasedthe relative content of nanofibers and nanotubes with alarge number of walls.

Derivatographic studies in air showed that the oxi-dation of the crudely purified product began at 360°C.A differential thermal analysis curve maximum wasobserved at 430°C. Under isothermal conditions, theproduct was stable in air at 320°C (its weight did notchange during 2 h). The weight loss at 340°C was 5%in 1 h and 11% in 2 h. Subsequent treatment of theproduct annealed for 2 h with HCl decreased the ashcontent to 3–5%.

The probable scheme of the formation of carbonnanotubes with a small number of walls shown in Fig. 5is a development of the mechanism suggested in [21].It takes into account that the catalyst consists of parti-cles that differ in size and composition. Indeed, underthe pyrolysis conditions, cobalt can be present in thecatalyst as ëÓ0, ëÓé, Co3O4, ëÓåÓé4, cobalt molyb-dates of other compositions, ëÓxMg1 – xO solid solu-tion, and more complex compounds and solid solutions

(oxides with different degrees of metal oxidation, car-bides, etc.). In addition, separate catalyst particles andseparate regions of one particle can presumably havedifferent compositions, and their composition canchange, especially at the initial pyrolysis stage.

The size of particles primarily determines the diam-eter of nanotubes, and their composition determines thenumber of nanotube walls. Both the upper and lowerlimits of the size of particles are important. The differ-ence between the upper and lower limits of the size ofcatalyst particles is very small in the formation of sin-gle-wall nanotubes, and the size itself is of ~1–3 nm. Itappears that particles of sizes ~1–4, ~2–5, and ~2–6 nmare necessary for the growth of double-, triple-, andquadruple-wall nanotubes, respectively.

The splitting off of a catalyst particle from the car-rier (probably related to a change in the composition ofparticles) results in a change in the mechanism ofgrowth and the formation of nanofibers. When the sizeof the catalyst particles exceeds a certain boundaryvalue (which varies from 4 to 6–7 nm and supposedlydepends on the composition), this causes their occlu-sion without the formation of nanotubes.

The character of the distribution of catalyst particleson a substrate, the porosity of the substrate, and the cur-vature of its surface can influence the behavior of theseparticles. The formation of bundles of nanotubesaccording to the suggested scheme occurs during pyrol-ysis and subsequent product processing.

ACKNOWLEDGMENTS

The authors thank N.N. Mel’nik for the recording ofthe Raman spectra. This work was financially sup-ported by the Russian Foundation for Basic Research(project no. 01-03-33225), the Ministry of Education ofthe Russian Federation (the program Chemical Tech-nology, project no. 06.06.017), and the Finnish Acad-emy of Sciences (grant no. 205456).

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1–2r, nm

2–3 5–153–5

SWNT

DWNTDWNT

åWNíDWNTåWNí NF NP

åWNí

1

2

3

4

Fig. 5. Scheme of formation of nanotubes: (1) catalyst par-ticles of different size r and composition on an inert sub-strate (MgO); (2) beginning of formation of tips andgraphene shells of nanoparticles (NP); (3) beginning of thegrowth of single-wall nanotubes (SWNT), the formation oftips of double- and multiwall nanotubes (DWNT andMWNT), the splitting off of a catalyst particle linked withnanofiber (NF), and the growth of graphene shells on nano-particles; and (4) the removal of single-, double-, and mul-tiwall nanotubes and nanofibers and the cessation of thegrowth of graphene shells on nanoparticles.

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