Carbon 42 (2004) 591–597

www.elsevier.com/locate/carbon

Preparation of highly crystalline nanofibers on Fe andFe–Ni catalysts with a variety of graphene plane alignments

Atsushi Tanaka *, Seong-Ho Yoon, Isao Mochida *

Institute for Materials Chemistry and Engineering, Kyushu University, 6-1, Kasuga, Fukuoka 816-8580, Japan

Received 25 August 2003; accepted 17 December 2003

Abstract

The present study confirmed that highly crystalline nanofibers with controlled structure may be prepared over Fe and Fe–Ni

alloy catalysts. The degree of graphitization of various carbon nanofibers (CNFs) was analyzed by using C(0 0 2) peaks from the

XRD profiles. The C(0 0 2) peaks of CNFs over Fe catalyst shifted to higher angle and became narrower as the preparation

temperature increased from 560 to 620 �C. Tubular CNFs prepared at temperature higher than 630 �C showed lower 2h angles

compared to those of platelet fibers. CNFs prepared over Fe–Ni catalysts tended to resemble those prepared over Fe catalysts. The

degree of graphitization of platelet CNFs resembled natural graphite, while d00 2 of the tubular CNFs showed values below the 3.39�A reported as a theoretical minimum for a cylindrical alignment. Lc00 2 of platelet and tubular CNFs increased by heat treatment at

2000 and 2800 �C though d00 2 changed little. A transverse section of platelet and tubular CNFs had a hexagonal shape, not a round

shape. The hexagonal column allows AB stacking of hexagonal planes that can give perfect hexagonal alignment.

� 2003 Elsevier Ltd. All rights reserved.

Keywords: A. Carbon nanotubes; Carbon filaments; C. Scanning electron microscopy; Transmission electron microscopy; X-ray diffraction

1. Introduction

A variety of CNFs, in terms of diameter, length, and

alignment of graphene sheets have been prepared from

various hydrocarbons using metal grain catalysts [1–8].

The grain catalyst can control the dimensions and

structures of resultant CNFs in a wide range throughtemperature for reduction, conditioning with carbon

source, and fiber growth. Such a grain may be segre-

gated into finer particles depending on the reduction and

reaction temperatures as well as the reductant, defining

diameter as well as graphene alignment of the resultant

CNF. Important characteristics such as electric and

thermal properties of carbon materials have been re-

ported to govern by degree of graphitization [9–12].Hence properties of CNF such as thermal and electric

conductivities, tensile modulus and storage capacity for

Li ions can be affected by degree of graphitization. So

far the degrees of graphitization of CNFs and CNTs

that have been reported are rather low, although cata-

*Corresponding authors. Tel.: +81-925-837-797; fax: +81-925-837-

798.

E-mail addresses: tanakaa9@asem.kyushu-u.ac.jp (A. Tanaka),

mochida@cm.kyushu-u.ac.jp (I. Mochida).

0008-6223/$ - see front matter � 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.carbon.2003.12.067

lytic graphitization is assumed necessary for the for-

mation of fibrous carbons [13,14].

Fe and its Ni alloys have been reported to produce

platelet and tubular CNFs at low temperature and their

degree of graphitization appears fairly high in the TEM

image [15], although carbon nanotubes have a cylindri-

cal shape, which may restrict physically the degree ofgraphitization [16].

In the present study, the degree of graphitization of a

series of CNFs prepared from CO and H2 mixtures on Fe

and its Ni alloys were systematically measured to find the

most graphitized fiber. The CNFs prepared in the pres-

ent study were also observed under HR-SEM to clarify

their three-dimensional shapes. The catalyst particles are

also observed to find relations between the shape of thecatalyst and CNF. Based on the morphology shape and

graphene alignment of CNFs, origins for the observed

high graphitization are discussed.

2. Experimental

2.1. Synthetic apparatus

The synthesis apparatus was specially designed and

constructed to allow the introduction of a reactant

592 A. Tanaka et al. / Carbon 42 (2004) 591–597

mixture (carbon monoxide and hydrogen) to a quartz

tube (45-mm inner diameter and 500 mm long) heated

by a conventional horizontal tube furnace. The gas flow

to the reactor was accurately monitored and regulatedby mass flow controllers (COFLOC Co. Ltd., Tokyo,

Japan). Powdered Fe and Fe–Ni (6/4, (wt./wt.)%) cata-

lyst (30 mg) were put on the bottom of a quartz boat,

which was placed at the center of the quartz reactor

tube. After reduction in a 20% H2–He (40:160 cm3/min)

mixture for 2 h, the reactor was purged with helium

while the system was brought to the prescribed reaction

temperature (560–675 �C). The reactant mixture CO/H2

(1/4–4/1, (vol/vol)%, total flow rate: 200 cm3/min) was

introduced into the reactor. After a reaction period of

1 h, the system was once again purged with helium and

then allowed to cool down to room temperature as re-

ported in a previous paper [15].

2.2. Characterization

The characterizations of the carbon deposits obtainedwere performed using a combination of techniques

including, high resolution scanning electron microscope

(HR-SEM, JEOL JSM 6320F, Tokyo, Japan), Field

emission transmission electron microscope (FE-TEM,

JEOL JEM-2010F, Tokyo, Japan) and X-ray diffraction

(Rigaku GeigerflexII, CuKa target, Rigaku, Tokyo,

Japan). The crystallographic parameters of d00 2 and

Lc00 2 with X-ray diffraction were calculated accordingto the Gakushin (JSPS) method [17].

2.3. Materials

The gases used in this study, CO, H2 and He, were all

99.9999% purity and were purchased from Asahi Sanso

Co. in Japan and used without further purification.

Powdered Fe and Fe–Ni (6/4) catalyst precursors were

prepared by precipitation [18].

3. Results

3.1. HR-SEM and FE-TEM images of CNFs produced

over Fe and Fe–Ni alloy catalysts at 560–675 �C

Fig. 1A shows HR-SEM and FE-TEM photographs

of (a) platelet and (b) tubular CNFs produced from CO/

H2 (1/4) over Fe catalyst. The platelet CNF(a) was

produced at 580 �C and showed a diameter range of100–200 nm. In the platelet CNF, the hexagonal planes

were stacked perpendicular to the fiber axis [19]. CNFs

prepared at 560–620 �C showed basically the same

structures. Tubular CNF(b) was produced at 645 �C and

showed a diameter range of 20–40 nm. In the tubular

CNF, the hexagonal planes were stacked parallel to the

fiber axis [19]. The CNFs prepared at 630–675 �C

showed basically the same structures. Some carbon

particles were found among tubular CNFs, however, no

amorphous carbon was found on the surfaces of either

the platelet or tubular CNFs.Fig. 1B shows HR-SEM and FE-TEM photographs

of (c) platelet and (d) tubular CNFs produced over the

Fe–Ni (6/4) catalyst. The CNF(c) produced at 580 �Chad a diameter range of 100–200 nm and the hexagonal

planes stacked perpendicular to the fiber axis in the

same way as observed in the platelet CNF produced

over the Fe catalyst. Platelet CNF produced over Fe–Ni

catalysts was rather vermicular in comparison with CNFover Fe catalyst. The CNF(d) produced at 630 �C had a

diameter range of 20–40 nm and the hexagonal planes

were stacked parallel to the fiber axis in the same way as

that of tubular CNF produced over Fe catalyst. No

carbon particles were found among these CNFs. Higher

selectivity for synthesis of both platelet and tubular

CNFs was noted over Fe–Ni catalyst. There is no

amorphous carbon on the surface of the platelet andtubular CNFs produced over Fe–Ni (6/4) catalyst.

Fig. 2 shows HR-TEM photographs of platelet and

tubular CNFs prepared over Fe and Fe–Ni catalysts,

respectively. Very high magnification showed a number

of misalignments and defects in the platelet and tubular

stacking of the rather straight hexagonal planes in the

present CNFs.

3.2. XRD analyses of CNFs

Fig. 3 shows XRD profiles of CNFs produced from

CO/H2 (4/1) over Fe catalysts at 560–675 �C. A series ofgraphite diffraction peaks shifted to higher angle and

became narrower with increasing preparation tempera-

ture, indicating progressive graphitization of the CNFs.

CNF prepared at 620 �C showed the highest diffraction

angle. The tubular CNFs prepared at temperatures

higher than 630 �C showed a 2h angle lower their

platelets.

The graphitization parameter d00 2 of CNFs producedfrom CO/H2 (4/1 and 1/4) over (a) Fe and (b) Fe–Ni (6/

4) catalysts, respectively, are illustrated in Fig. 4. Degree

of graphitization increased with reaction temperature up

to 620 �C and then decreased at higher temperature

regardless of catalyst and gas compositions. Fe catalysts

gave a d00 2 value of 3.363 �A for the particular CNF

derived from CO/H2 (4/1) at 600 �C. It must be noted

that the degree of graphitization is very close to thevalue 3.354 �A of natural graphite. Platelet CNFs pro-

duced from CO/H2 (4/1) over Fe and Fe–Ni (6/4) cata-

lysts exhibited slightly lower d00 2 values compared to

those of platelet CNFs produced from CO/H2 (1/4). The

d00 2 of tubular CNFs produced from CO/H2 (4/1) over

Fe and Fe–Ni (6/4) catalysts increased with increasing

reaction temperature as observed with CNF grown from

Fig. 1. HR-SEM and FE-TEM photographs of CNFs. (A) HR-SEM and FE-TEM photographs of platelet and tubular CNFs produced over Fe

catalyst at (a) 580 �C and (b) 645 �C. (B) HR-SEM and FE-TEM photographs of platelet and tubular CNFs produced over Fe–Ni (6/4) catalyst at

(c) 580 �C and (d) 630 �C.

A. Tanaka et al. / Carbon 42 (2004) 591–597 593

CO/H2 (1/4). Fig. 5 shows Lc00 2 of CNFs produced

from CO/H2 over (a) Fe and (b) Fe–Ni (6/4) catalysts,

respectively. The Lc00 2 of CNFs from CO/H2 (4/1) ini-

tially increased with higher reaction temperature, then

decreased over 580–600 �C regardless of catalyst. Fecatalysts produced at 580 �C platelet CNF of 33 nm

Lc00 2 from CO/H2 (4/1). The Lc00 2 of CNFs produced

from CO/H2 (1/4) over Fe and Fe–Ni (6/4) catalysts

became larger with increasing reaction temperature up

to 600 �C and did not change when reaction temperature

was raised further.

Table 1 summarizes values of d00 2 and Lc0 0 2 of as-

prepared and graphitized CNFs. The platelet CNF

showed a value of d00 2 which stayed the same after the

graphitization at 2000 and 2800 �C, respectively, whileits Lc00 2 increased slightly from 30.3 to 33.5 and 35.4nm by the same graphitization treatment. The values of

d0 0 2 of the graphitized tubular CNFs were certainly

inferior to those of as-prepared tubular CNF, while

their Lc0 0 2 increased significantly from 9.5 to 13.7 and

16.2 nm by the graphitization at 2000 and 2800 �C,respectively.

Fig. 2. HR-TEM photographs of platelet and tubular CNFs. (a) Platelet CNF as-prepared (Fe, CO/H2 ¼ 4/1, 600 �C). (b) Tubular CNF as-prepared

(Fe–Ni (6/4), CO/H2 ¼ 1/4, 630 �C).

Fig. 3. XRD profiles of CNFs produced over Fe catalyst at 560–675 �C.

594 A. Tanaka et al. / Carbon 42 (2004) 591–597

3.3. High magnified HR-SEM photographs of Fe catalyst

on CNF

Fig. 6 shows SEM photographs of Fe catalysts on

the top of (a) platelet CNF and (b) tubular CNF. Both

catalysts were definitely hexagonal. Therefore, the trans-

verse cross-section of the CNF must reflect that of

the catalyst and should be hexagonal, not round. A hex-agonal transverse shape allows graphitic structure or AB-

type stacking.

4. Discussion

The present study examined the effect of catalyst and

preparation conditions on the degree of graphitization

of CNFs made in the temperature range 560–675 �Cfrom CO/H2 over Fe and Fe–Ni (6/4) alloy catalysts.

The highly graphitic CNFs showed well-developed

graphene alignments in both platelet and tubular mor-

phologies, depending on the preparation temperature.

Such high degree of graphitization is certainly favorable

for applications as such electric and thermal conduc-

tivity in composites, and as intercalation hosts used

without extra graphitization treatment [9–12]. Such high

degree of graphitization is germane to discussion of theshape and size as well as the catalytic roles which

encouraged high graphitization of as-prepared CNF.

The values of d00 2 for as-prepared platelet and tubular

types of CNFs were 3.363 and 3.369 �A, respectively,

which are both comparable to that of natural graphite,

3.354 �A. The interlayer distances of the platelet CNFs

are hardly changed by further graphitization. Hence the

stacking order is already established at the time ofpreparation. However, the interlayer spacing of tubular

CNF was definitely increased by further graphitization.

Fig. 4. Dependence of d00 2 of CNFs on the reaction temperatures

prepared over (a) Fe and (b) Fe–Ni (6/4) catalysts.

Fig. 5. Dependence of Lc00 2 of CNFs on the reaction temperatures

prepared over (a) Fe and (b) Fe–Ni (6/4) catalysts.

Table 1

Values of d00 2 and Lc00 2 of as-prepared and their graphitized CNFs

Platelet Tubular

d00 2

(�A)

Lc00 2(nm)

d00 2

(�A)

Lc00 2(nm)

As-prepared 3.363 30.3 3.369 9.5

Graphitic temperature

2000 �C3.366 33.5 3.387 13.7

Graphitic temperature

2800 �C3.363 35.4 3.375 16.2

A. Tanaka et al. / Carbon 42 (2004) 591–597 595

In contrast, the Lc00 2 value of 30.3 and 9.5 nm for as-

prepared platelet and tubular CNFs, respectively, were

increased by further graphitization to 35.5 (slightly) and

16.2 nm (significantly), respectively. Some improvementof stacking and removal of defects is achieved by further

heat treatment.

Platelet-type CNF showed significantly better stack-

ing. Higher and ordered stacking in the platelet-type

CNF may pose no geometrical difficultly for the

graphitization of hexagonal planes stacked in a colum-

nar manner. In contrast, tubular alignment of hexagonal

planes forces their stacking distance and helicity to in-crease due to the absence of planarity, 3.39 �A being the

reported theoretical minimum in the cylindrical align-

ment [16]. However the present values are certainly

smaller for the as-prepared tubular CNFs. Thus, perfect

cylinders in three-dimensional geometry can be ruled

out. Present SEM observations suggest a hexagonal

column of tubular CNF as shown in Fig. 7. Such an

alignment is consistent with the high crystallinity.

Graphitization may remove defects to increase the Lc0 0 2values, while d00 2 increases along its change of mor-

phology.Such shape, alignment and degree of graphitization

of graphene sheets must be governed by the natures of

the catalyst particles [19]. The catalyst particle observed

on the top of the CNFs with the highest graphitization

Fig. 6. Highly magnified HR-SEM photographs of Fe catalysts on the

top of (a) platelet CNF and (b) tubular CNF.

596 A. Tanaka et al. / Carbon 42 (2004) 591–597

looked like a hexagonal column that governs the shape

of the fiber. The size of the catalyst is a little larger than

the diameter of the fiber. Thus, the active form of the

catalyst particle is segregated from the oxide grainsaccording to the conditions of reduction and fiber

growth. Higher temperature tends to reduce the sizes of

the catalyst and hence the resulting fibers.

Fig. 7. Transverse shape model of highly crystalline CNF.

The highest graphitization is observed at a well defined

temperature range of 560–675 �C. Below that tempera-

ture, catalytic graphitization by complete rearrangement

of the catalyst–carbon complex intermediate is not sat-isfactory. Above that temperature, the catalyst particle

tends to become spherical and arranges the carbon walls

to be tubular to give a more cylindrical shape to the fiber,

limiting the graphitization geometrically.

The change in the shape of the catalyst particles must

reflect their softening temperature, which allows the

deformation of the catalyst to decrease their surface

energy. So far, we cannot completely explain how thehexagonal alignment changes from platelet to tubular

types according to the synthesis temperature. The soft-

ening temperature of the catalyst, supply rate of the

carbon source and the reactivity with deposited carbon

precursors and metal catalyst, surface migration of the

carbon through the metal particle, and the solidification

rate on the fiber must all be influential. The metal sur-

face certainly plays an important role in defining thehexagonal alignment in the CNFs. Further analyses of

the intermediates are necessary.

Acknowledgements

This work was carried out within the framework of

the CREST program. The present authors acknowledgethe financial support of Japan Science and Technology

Corporation (JST).

References

[1] Ci L, Zhu H, Wei B, Xu C, Liang J, Wu D. Graphitization

behavior of carbon nanofibers prepared by the floating catalyst

method. Mater Lett 2000;43:291–4.

[2] Ci L, Li Y, Wei B, Liang J, Xu C, Wu D. Preparation of

carbon nanofibers by the floating catalyst method. Carbon 2000;

38:1933–7.

[3] Fan Y-Y, Cheng H-M, Wei Y-L, Su G, Shen Z-H. Tailoring the

diameters of vapor-grown carbon nanofibers. Carbon 2000;38:

921–7.

[4] Fan Y-Y, Cheng H-M, Wei Y-L, Su G, Shen Z-H. The influence

of preparation parameters on the mass production of vapor-

grown carbon nanofibers. Carbon 2000;38:789–95.

[5] Rodriguez NM, Kim MS, Fortin F, Mochida I, Baker RTK.

Carbon deposition on iron–nickel alloy particles. Appl Catal, A

General 1997;148:265–82.

[6] Park C, Baker RTK. Carbon deposition on iron–nickel during

interaction with ethylene–carbon monoxide–hydrogen mixtures.

J Catal 2000;190(1):104–17.

[7] Park C, Baker RTK. Carbon deposition on iron–nickel during

interaction with ethylene–hydrogen mixtures. J Catal 1998;179(2):

361–74.

[8] Park C, Rodriguez NM, Baker RTK. Carbon deposition on iron–

nickel during Interaction with carbon monoxide–hydrogen mix-

tures. J Catal 1997;169(1):212–27.

[9] Chieu TC, Dressellhaus MS, Endo M. Phys Rev B 1982;26:5867–

77.

A. Tanaka et al. / Carbon 42 (2004) 591–597 597

[10] Hishiyama Y, Nakamura M, Nagata Y, Inagaki M. Carbon 1994;

32:645–50.

[11] Kelly BT. The thermal conductivity of graphite. In: Kelly Walker

PL, editor. Chemistry and physics of carbon, vol. 5. New York:

Dekker; 1969. p. 119–215.

[12] Yang H. Preparation of carbon anodic materials with high

graphitization degree for Li ion secondary battery. Fukuoka

Japan, Kyushu University, PhD thesis, 2003. p. 109–45.

[13] Endo M, Kim YA, Takeda T, Hong SH, Matusita T, Hayashi T,

et al. Structural characterization of carbon nanofibers obtained by

hydrocarbon pyrolysis. Carbon 2001;39:2003–10.

[14] Endo M, Kim YA, Hayashi T, Yanagisawa T, Muramatsu H,

Ezaka M, et al. Microstructural changes induced in �stacked cup’

carbon nanofibers by heat treatment. Carbon 2003;41:1941–7.

[15] Tanaka A, Yoon S-H, Mochida I, Baker RTK, Rodriguez NM,

Park C-W. Structure-controlled synthesis of carbon nanofibers

from CO/H2 mixture over Fe and Fe–Ni alloy catalysts. Carbon

2002.

[16] Charlier JC, Michenaud JP. Energetics of multilayered carbon

tubules. Phys Rev Lett 1993;70:1858–61.

[17] 117 Committee of JSPS (Japan Society for Promotion of Science).

Measurement methods for crystalline parameters and size of

synthetic graphite. TANSO 1963;36:25 [in Japanese].

[18] Best RJ, Russell WW. Nickel, copper and some of their alloys as

catalysts for ethylene hydrogenation. J Amer Soc 1954;76:838–

42.

[19] Rodriguez NM, Chambers A, Baker RTK. Catalytic engineering

of carbon nanostructures. Langmuir 1995;11:3862–6.