C A R B O N 4 7 ( 2 0 0 9 ) 3 8 4 – 3 9 5

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Growth kinetics of MWCNTs synthesized by acontinuous-feed CVD method

Illayathambi Kunadiana, Rodney Andrewsa,*, Dali Qiana, M. Pinar Mengucb

aCenter for Applied Energy Research, University of Kentucky, 2540 Research Park Drive, Lexington, KY 40511-8410, USAbDepartment of Mechanical Engineering, University of Kentucky, 269 Ralph G. Anderson Building, Lexington, KY 40506, USA

A R T I C L E I N F O

Article history:

Received 14 January 2008

Accepted 7 October 2008

Available online 1 November 2008

0008-6223/$ - see front matter � 2008 Elsevidoi:10.1016/j.carbon.2008.10.022

* Corresponding author: Fax: +1 859 257 0220E-mail address: andrews@caer.uky.edu (R

A B S T R A C T

Unlike two-step chemical vapor deposition (CVD) methods using pre-deposited catalyst

particles, in a continuous-feed CVD process, the liquid feed (consisting of catalytic precur-

sor and hydrocarbon source) is continuously supplied into the reactor causing catalyst par-

ticle formation, nucleation of carbon nanotubes (CNTs) and CNT growth to occur

simultaneously throughout the reaction period. In order to observe these processes, CVD

experiments were conducted for different durations (30 s to 3 h) and the product multi-

walled carbon nanotubes (MWCNTs) were characterized using scanning electron micros-

copy. It was found that the nanotubes did not grow in the vapor phase and that

substrates played an important role in the growth by providing a place for them to anchor

before growth took place. Based on transmission electron microscopy images, it has been

suggested that MWCNTs grew by root-growth mechanism from the catalyst particles that

were deposited on the substrate during the early stages. At long process times, continu-

ously supplied feed gas produced additional catalyst particles which were deposited mostly

on the growing nanotube mat. Due to weak catalyst–mat interaction, the additional nano-

tubes grew by tip growth. A comprehensive MWCNT growth model has been presented for

the continuous-feed CVD.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

The study of the growth of carbon nanotubes started with the

study of growth of carbon filaments in the 1970s [1–3]. The

growth of carbon nanotubes and the type of carbon nano-

tubes (SWCNT or MWCNT) synthesized differs significantly

depending on the synthesis method used due to different

growth environments. Consequently, several growth mecha-

nisms have been proposed [4–9]. A number of theories have

been postulated for SWCNT growth, some from vapor phase

[10–12] and others from a condensed phase [13,14], with

either tip-growth [15,16] or root growth [13,14,17,18]. The

structure of the SWCNTs produced was found to be depen-

er Ltd. All rights reserved

.

. Andrews).

dent on the growth parameters such as reaction time, furnace

temperature, catalyst particles, promoters (S, Bi, or Pb), and

reaction gas [4,19,20].

Several groups [4,5,21–28] have studied the growth of well-

aligned MWCNT mats by CVD in an attempt to optimize the

process parameters. Sinnott et al. [4] have claimed formation

and growth of MWCNTs via CVD to be an extension of the

process described by Baker [2] and suggested a tip-growth (de-

tached particle) model for the growth of MWCNTs based on

transmission electron microscopy (TEM) images. Qian [5] sug-

gested a combined root-growth and tip-growth model for the

growth of MWCNTs via CVD. One theory has claimed sequen-

tial growth of various shells of the MWCNTs [29,30] where

.

C A R B O N 4 7 ( 2 0 0 9 ) 3 8 4 – 3 9 5 385

each following nanotube shell was formed by carbon adsorp-

tion at the previous shell surface. Still, another theory has

suggested simultaneous growth of the tube shells [31]. Vari-

ous studies have shown that the diameter and length of the

nanotubes depend on the size of the initial catalyst particles

formed on the substrates [4,5,32,33]. Studies have also shown

that the electric field influences the alignment of carbon

nanotubes [34,35].

Over the years, the growth of CNTs has been studied

extensively using several approaches. In situ observation of

the nanotube growth has been made by using an environ-

mental transmission microscope [36]. These tools are imprac-

tical to use due to high equipment cost and harsh operating

temperatures that could damage any diagnostic tool. They

may require major modifications to the existing experimental

setup. Optical characterization techniques based on laser-

beam attenuation and scattering [34,37] have also been used

in situ during the synthesis of the carbon nanotubes. Geohe-

gan et al. have used Febry–Perot interference fringes and

attenuation of a reflected HeNe laser beam to study the

growth rates and kinetics of MWCNT arrays [37]. Due to opti-

cal limitations, the system is not capable of capturing the

early stages of the growth of nanotubes. Computer simula-

tions like molecular dynamics simulations [8,38–41], and

computational fluid dynamics [42–44] have been used to mod-

el the CNT growth from the catalyst particles.

The growth of the CNTs has been successfully studied

using ex situ characterization of the product CNTs via electron

microscopy [4–9,15,32,33,35,45–49]. Fan et al. have predicted

the growth of CNT array using carbon isotope labeling method

involving labeling the nanotube array with tracing chemicals

and then detecting the isotope compositions of each section

in the nanotubes by a micro-Raman after growth [46]. Com-

pared to other methods, ex situ characterization techniques

are more cost effective for monitoring CNT growth because

this method allows for conducting CVD experiments using

available equipment.

Most of the existing growth models describe the CNT

growth mechanism for the two-step CVD method. However,

the continuous-feed CVD growth mechanism is more compli-

cated than the two-step CVD method because catalyst parti-

cle formation, nucleation of CNTs and their growth all take

place simultaneously throughout the reaction period as the li-

quid feed (consisting of catalytic precursor and hydrocarbon

source) is continuously fed into the reactor. It is possible to

control the diameter of the nanotubes grown via two-step

CVD by controlling the diameter of the pre-deposited catalyst

particles. In contrast to this, in a continuous-feed CVD, it is al-

most impossible to control the diameter of the nanotubes

grown due to lack of methods to control the diameter of the

catalyst particles generated in the reactor. The growth models

available for the continuous-feed CVD method [2,4,47,50–52]

do not contain sufficient experimental evidence of the inter-

mediate stages that lead to the formation of MWCNT-mat. It

is also unclear whether the nanotubes were initially formed

in the gas phase or if the substrates played a role on the for-

mation of aligned carbon nanotubes.

This paper describes the various stages of growth of

MWCNTs starting from catalyst particle formation, then

MWCNT formation from catalyst particles, MWCNT growth

and organization to form regular arrays of aligned MWCNT-

mats on the surface of a quartz substrate. The experimental

work was accomplished by conducting CVD runs at different

reaction times and performing ex situ characterization of the

products later. Gas chromatography (GC) was performed on

the exhaust gas samples in order to study the major hydrocar-

bons produced. The results were compared with those avail-

able for the CVD method using pre-deposited catalyst particles.

2. Experimental setup and procedure

The CVD experiments were conducted using the same reactor

system previously described by Andrews et al. [25]. The feed

solution consisted of ferrocene which was used as the precur-

sor for producing Fe catalyst particles and xylene which was

used as the hydrocarbon source. Approximately 6.5 mol% of

ferrocene was dissolved in xylene to obtain feed solution with

�0.75 at% Fe/C ratio. The liquid feed was pumped into the pre-

heater zone of the two-stage reactor where it was preheated to

�240 �C prior to its entry into the furnace. The liquid was

immediately volatilized and swept into the reaction zone of

a furnace maintained at 700 �C, under a reducing atmosphere

(90% Ar/10% H2). The reactor was operated at a modest over-

pressure, 250 Pa, at all times to prevent any oxygen influx.

The experiments were conducted at different reaction times

starting from the point where the feed gas just entered

the reaction zone to about 3 h afterward. The MWCNTs were

grown on small quartz substrates (1 cm · 1 cm) which were

placed in the furnace area of the tubular reactor. The sub-

strates were removed at the end of the reaction and were later

characterized using SEM.

3. Experimental results

Fig. 1a shows the SEM images of the MWCNTs grown at differ-

ent reaction times: initial, 30 s, 1 min, 3, 5, and 15 min. The

term ‘‘initial’’ denotes the period when the feed gas just en-

tered the reaction zone of the CVD reactor. The injection of

the feed stock material was immediately stopped and the

reactor was purged with Argon. The reactor was left to cool

down while the quartz substrate on which the reaction prod-

ucts were formed was removed and characterized using SEM.

CVD experiments were then repeated at different reaction

times. The reaction times were calculated immediately after

the feed gas entered the reactor.

3.1. Initial stages of MWCNT growth

During the initial stage, small particles were observed spar-

sely deposited on the surface of the substrate. The size of

these particles ranged from 20 to 100 nm, coinciding with

the various outer diameters of the nanotubes as shown in

previous studies [4,5,25,32,33]. This variability of the sizes of

the Fe catalyst particles deposited accounts for the wide

range of CNT diameters observed. At this stage, the chemical

composition of the catalyst particles could not be evaluated

due to the extremely small sizes of the particles. Regardless,

even when the Fe catalyst particles had some carbon content,

they did not possess any filamentous structure.

Fig. 1 – (a) SEM images of MWCNT growth and (b) plot of MWCNT and catalyst particle count on substrate surface with

change in reaction time.

386 C A R B O N 4 7 ( 2 0 0 9 ) 3 8 4 – 3 9 5

At 30 s, the substrate was covered densely with free cata-

lyst particles. Some short nanotubes that could have nucle-

ated from the metal catalyst particles were also observed.

After the first minute of the process, the substrate was den-

sely covered with particles and sparsely covered with nano-

tubes of different lengths. This indicated that while catalyst

particles were being deposited on the substrate, some

MWCNTs began to nucleate from the catalyst particles and

others that had already nucleated began to grow longer. At

3 min, MWCNTs appear to have grown in random directions

on the substrate due to low carbon nanotube areal densities.

Some catalyst particles also were found on the substrate,

which indicated that particle deposition was still taking place

at that stage. At 5 min, most of the substrate area was covered

with nanotubes. As the areal number density increased,

growth in random directions ceased and the MWCNTs were

forced to grow vertically, perpendicular to the substrate due

to geometric constraints. (Einarsson et al. have reported that

vertical growth of SWCNTs was caused when the density of

the nanotube array impeded the growth in all directions par-

allel to the substrate surface [45]). At 15 min, the entire sub-

strate area was covered with MWCNTs. Some catalyst

particles deposited on top of the growing MWCNTs and

formed additional tubes. As the reaction time progressed,

the array density and the thickness of the mat continued to

increase.

Fig. 1b shows the number of carbon nanotubes and metal

catalyst particles counted in a square area on the surface of

the substrates plotted against the reaction time. The catalyst

particles illustrated here refers to the free Fe catalyst particles

which had not become part of the growing MWCNTs. The par-

ticle count was higher than the nanotube count during the

initial time indicating that the deposition of metal particles

took place before the formation of MWCNTs. The nanotube

count increased while the free particle count on the surface

of the substrate dropped steadily. This could mean that the

C A R B O N 4 7 ( 2 0 0 9 ) 3 8 4 – 3 9 5 387

metal particles that were deposited on the surface of the sub-

strate became part of the MWCNTs that nucleated from them.

The number of free catalyst particles decreased to zero once

all the particles were used up to form MWCNTs and there

was no room left on the substrate for further deposition of

metal particles. Particle deposition continued to take place

on top of the growing MWCNTs.

3.2. Role of substrate on the growth of MWCNTs

Fig. 2 shows MWCNTs grown on a quartz substrate that was

non-uniformly sputter coated with gold. It was observed that

the MWCNTs did not grow on the gold surface but grew from

quartz surfaces beneath the gold layer as if their bases were

anchored to the quartz substrate. This indicated that there

must have been some strong interaction between the

MWCNTs and the substrate, possibly when the catalyst parti-

cles were deposited during the initial stages. Once the catalyst

particles established a bond with the substrate, the carbon

nanotubes nucleated and grew from them. If the MWCNTs

were formed in the vapor phase, they would have appeared

lying on top of the gold surface. This confirmed the hypothe-

sis that carbon nanotube growth does not take place in the

vapor phase and that the substrate played an important role

in the initiation of nanotube growth [25]. Li et al. [17] have also

shown the importance of the presence of a solid substrate

interface for the growth of carbon nanotubes.

3.3. Transmission electron microscopy of MWCNTs

Transmission electron microscopy (TEM) and high resolution

transmission electron microscopy (HRTEM) were performed

on the MWCNTs (Fig. 3) in order to determine the mecha-

nisms that governed the growth process. The TEM/HRTEM

images revealed that a majority of the MWCNTs contained

Fig. 2 – SEM images of MWCNT growth on quartz su

catalyst particles at the root end. The MWCNTs with catalyst

particles at the root end likely grew from the catalyst particles

that were deposited on the substrate during the initial stages

where catalyst–substrate interaction was highly favorable.

The catalyst particles at the root end exhibited a neck like re-

gion and traces of the catalyst were found along the inner

core of the tubes (Fig. 3a). As shown in Fig. 3c, some MWCNTs

did not contain catalyst particles due to losses from scraping

the nanotubes off the substrate prior to microscopy. The

growing end of the nanotube exhibited a closed-capped struc-

ture which is known to be energetically more stable than

open-ended structures [39]. Catalyst particles were also ob-

served on the tip–end of the nanotubes. These nanotubes

with catalyst particles at the tip end likely grew from catalyst

particles that were deposited at a later stage on top of the

growing nanotube mat. The weaker catalyst interaction with

the nanotube mat likely caused the nanotubes to grow by

tip-growth. Additionally, catalyst particles were observed on

the tip and root ends of the nanotubes. Perhaps this was

caused by the splitting of catalyst particles: while the lower

part of the catalyst was firmly attached to the substrate the

upper part was lifted off along with the growing end of the

nanotube. This theory is supported by the presence of traces

of catalyst particles observed along the inner core of the

tubes. Secondary branching of the nanotubes was also ob-

served arising from the splitting of catalyst particles present

at the growing end of the nanotubes.

3.4. Growth of MWCNT mats with reaction time

Fig. 4 shows SEM images of MWCNT mats grown at different

reaction times taken at 65� angle from the surface of the sub-

strate. The thickness of the mat increased steadily with reac-

tion time. The MWCNTs were well aligned in the vertical

direction perpendicular to the substrate [5]. They appeared

bstrate non-uniformly sputter coated with gold.

Fig. 3 – TEM/HRTEM images of MWCNTs: (a) catalyst at root end with neck region, black arrows showing catalyst inclusions

within the tube core, (b) catalyst at root, (c) open-ended root-end with catalyst removed, (d) and (e) closed-capped growing tip

end, (f) catalyst particle at tip end, (g) catalyst particle at tip and root ends, (h) and (i) secondary-branching of nanotubes,

arrows show catalyst inclusions within tube core.

388 C A R B O N 4 7 ( 2 0 0 9 ) 3 8 4 – 3 9 5

to be tilted to the left in the SEM images due to displacement

where lines were marked on the substrate using a scriber. The

MWCNTs appeared to be randomly oriented on the top sur-

face of the mat which may have been due to two reasons: (i)

during the initial stages of growth, the low array density

caused the nanotubes to grow in random directions on the

substrate. As the reaction time progressed, nanotube crowd-

ing forced the nanotubes to grow in an aligned manner per-

pendicular to the substrate that carried the coiled

nanotubes above it (as described by Fan et al. [46]); (ii) the con-

tinuous supply of feed gas produced additional catalyst parti-

cles that deposit randomly on the growing nanotubes and

grew into randomly oriented nanotubes on the surface of

the nanotube mat.

Three separate growth experiments were conducted at dif-

ferent reaction times and the average thickness of the

MWCNT mat was measured and plotted against the reaction

time (Fig. 5). The maximum variability in the mat thickness

was less than 5 lm, which showed the consistency of our

CVD experiments. The mat growth rate curve exhibited a sig-

moidal shape, although different in magnitude, as previously

obtained by Baker [2]. The growth curve showed an initial

acceleration period, a constant growth period and a final

slowing-down period. A sudden burst in the mat growth dur-

ing the initial acceleration period may have been due to the

availability of optimal conditions that were conducive for

the growth of CNTs such as temperature, open areas on the

substrate, and feed gas concentration. The final slowing-

down period may have been caused either when the substrate

became saturated with CNTs or when the catalyst particles

were completely covered with carbon layers. This conclusion

was based on the assumption that the root-growth

Fig. 4 – SEM images of growth of MWCNT mat at different reaction times: 5, 10, 20, 30, 60 and 180 min.

Fig. 5 – Growth rate curve of MWCNT mat.

C A R B O N 4 7 ( 2 0 0 9 ) 3 8 4 – 3 9 5 389

mechanism dominated the growth process of MWCNTs and

the growth of nanotubes via tip-growth (from the catalyst par-

ticles that were deposited on the growing mat) was relatively

insignificant.

3.5. Deposition pattern of MWCNTs across the CVDreactor

MWCNT deposition pattern across the CVD reactor was inves-

tigated by taking SEM images in the radial direction (x-axis) of

the substrate and the number of particles and nanotubes per

square area at different locations on the substrate were plot-

ted as shown in Fig. 6. It was observed that the catalyst parti-

cle and MWCNT number density was greater at the center

than in the region close to the walls of the CVD reactor. Com-

mercially available computational fluid dynamics (CFD) soft-

ware, Fluent 6.3, was used to investigate the velocity profiles

43

2

1

5

6

7

Middle

Left Extreme Right Extreme

x

y

z

(a)

6µm 6µm

6µm

6µm

6µm

6µm

6µm

0

10

20

30

40

50

60

1 2 3 4 5 6 7

Cou

nt (

#)

Location across reactor

Catalyst particlesMWCNTs

(b)

0

0.1

0.2

0.3

0.4

0.5

1 2 3 4 5 6 7

Velo

city

(m

/s)

Location across reactor

(c)

Fig. 6 – (a) SEM images of MWCNTs in the radial direction of the reactor. Catalyst particles and MWCNTs are denser in middle

than toward the extreme edges in the x-direction of the reactor, (b) MWCNT and catalyst particle count at different location in

the radial direction of the reactor. MWCNT and particle count is maximum in the center and decreases toward the reactor

walls, (c) velocity magnitude (m/s) of feed gas generated by CFD analysis: velocity is maximum at the center and decreases

toward the walls.

390 C A R B O N 4 7 ( 2 0 0 9 ) 3 8 4 – 3 9 5

of the feed gas inside the CVD reactor. The fluid dynamics

model adopted was based on 2-D, laminar, steady-state flow

with both gas-phase and surface chemical reactions. The in-

let gas velocity (�1.5 m/s) used in the CFD calculations was

based on the flow rate of argon (675 sccm) and hydrogen

(75 sccm) and the feed rate of xylene/ferrocene mixture

(1 ml/h). The velocity contours of the feed gas inside the

CVD reactor predicted by CFD calculations (Fig. 6c) indicated

that the feed gas velocity attained a maximum value at the

center of the reactor and decreased to zero at the walls. Since

the availability of feed material acted as the driving force for

the diffusion of carbon into the catalyst particles leading to

CNT growth, higher feed gas velocity in the center of the reac-

tor could have led to the increase in the CNT formation in that

zone when compared to the region that was closer to the

walls.

3.6. Analysis of exhaust gas using gas chromatography

The exhaust gases from the CVD reactor that used both the

two-step CVD method and the one with continuous-feed each

were collected at different reaction times (5, 15, 40 and

60 min) using gas bags and were submitted for GC analysis

for measurement of CmHn concentration. Mass spectra analy-

sis of the exhaust gas from the CVD method with pre-depos-

ited catalyst particles (Fig. 7a) showed that the larger C8H10

and C7H8 accounted for more than 94% of the total hydrocar-

bon molecules in the tail gas. However, the concentrations of

smaller molecules including benzene, methane and propane

were only about 6% in the tail gas. The analysis of the gas

samples collected from the continuous-feed CVD method

(Fig. 7b) showed that the larger C8H10 and C7H8 molecules ac-

counted for about 80% of the total hydrocarbon molecules in

0

20

40

60

80

100

0

Com

poun

d (w

t.%)

Reaction Time (Minutes)

Xylene TolueneXylene MethaneBenzene Propane

(a)

0

20

40

60

80

100

Com

poun

d (w

t%)

Reaction Time (minutes)

m+p-Xylene TolueneMethane Benzeneo-Xylene EthylbenzeneButane Ethane

(b)

20 40 60

0 20 40 60

Fig. 7 – CnHm molecule concentration in the tail gas from: (a)

a two-step CVD method involving pre-deposited catalyst

particles and (b) continuous-feed CVD method measured by

a mass spectrometer at different reaction times.

C A R B O N 4 7 ( 2 0 0 9 ) 3 8 4 – 3 9 5 391

the tail gas which indicated that about 20% of the larger mol-

ecules were converted into smaller molecules. This difference

signified greater catalytic activity during the continuous CVD

process, whether on the surface, or, in the gas phase.

4. Comprehensive growth model

Based on the SEM, TEM, and HRTEM images of the MWCNTs

synthesized by the continuous-feed CVD method the follow-

ing growth mechanisms have been proposed as shown in

Fig. 8.

4.1. Root-growth mechanism

The first step in the growth of nanotubes involves the thermal

decomposition of ferrocene to produce clusters of Fe atoms.

During the initial stages, the open areas on the quartz sub-

strate favor strong catalyst–substrate interactions, and fea-

ture growth by the root-growth mechanism as shown in

Fig. 8a. The pyrolysis of xylene produces carbon atoms which

are extremely mobile on metal surfaces leading to rapid diffu-

sion over and through the metal particles. Carbon diffuses

along the concentration gradient and precipitates crystalline

graphite around and below the bisecting diameter of the cat-

alyst particles. The basal planes of the graphite crystallites

are oriented tangential to the curved surface of the catalyst

particle [4] (the details of the crystallographic structure and

orientation of the iron catalyst particles encapsulated in the

CNTs can be obtained elsewhere [44]). The large radius of cur-

vature resulting from the extremely small diameters of the

metal particles imposes a large strain on the basal planes of

the crystallites forcing them to take the continuous filamen-

tous surface form of the carbon nanotubes. Several concen-

tric cylindrical structures are formed in a nested fashion

from the catalyst particle. Since the basal planes of the graph-

ite crystallites are oriented tangential to the curved surface of

the catalyst particle, at a critical radius of curvature of the ex-

posed catalyst particle, further formation of a cylindrical

structure becomes almost impossible due to a significant in-

crease in the strain of the basal planes. This results in the for-

mation of a hollow core in the interior of the MWCNTs.

As the reaction progresses, the dissociated Fe atoms may

diffuse into the old catalyst particles that are part of the grow-

ing tubes resulting in an increase in the diameter of the cata-

lyst particles. As more carbon atoms diffuse into these

enlarged catalyst particles, additional tube walls may be

formed resulting in coarsening of the nanotube near the

root-end. While this portion of the nanotube is being ex-

truded, if the catalyst particle continues to increase in diam-

eter, the nanotube near the base will broaden a little more

with a diameter slightly greater than the region above it. As

these sequence of events are repeated tapered nanotube

structures are formed [5]. The growing end of the MWCNTs

exhibits a closed-capped structure due to the high dangling

bond energy required to keep it open (it has been theoretically

shown that the closed-capped form of nanotube growth is

energetically more favorable than the open-ended growth

[39]). As the nanotube increases in length the catalyst particle

at the root initially forms a neck which breaks off leaving

traces of catalyst particles within the inner core of the tubes

as evident in the TEM/STEM images in Fig. 3. The growth of

these nanotubes stops when the catalyst particle is com-

pletely covered with layers of carbon. The encapsulation of

the carbon nanotubes with catalyst particles has also been

shown by previously by Xiang et al. [18]. They have shown

that the bottom ends of the carbon nanotubes act as active

growth sites and the entrained catalyst particles move bot-

tom–up as the nanotubes increase in length [18].

4.2. Tip-growth mechanism

The growing nanotube mat may also act as a substrate for

nanotube growth. The tip-growth may occur from the newly

formed catalyst particles that are deposited on the growing

nanotube mat as shown in Fig. 8b. The weak interaction of

the catalyst particles with the nanotube mat causes the cata-

lyst particles to lift off with the concentric cylindrical struc-

tures formed below it. As explained earlier, at the critical

radius of curvature of the catalyst particle, further formation

of cylindrical structures is prevented and a hollow core is pro-

duced. With the base of the nanotube anchored to the nano-

tube mat, the tip carrying the catalyst particle grows toward

the region of higher feed gas concentration (this type of

growth is also referred to as the ‘‘kite mechanism’’ in which

the active end of the nanotube floats while the section close

to the original site where the catalyst was deposited forms

the van der Waals contacts with the substrate [6]). The diffu-

sion of carbon and Fe atoms takes place through the exposed

Fig. 8 – Schematic of (a) root growth: (I) Fe catalyst particle formation, (II) diffusion of carbon into catalyst, (III) formation of

nested MWCNT structure, (IV) enlargement of catalyst particle, (V) formation of neck and trapping of catalyst inside nanotube

core, (VI) catalyst covered with carbon layers and growth stoppage; (b) tip growth: (I) Deposition of Fe catalyst particle on

growing mat, (II) diffusion of carbon into catalyst, (III) lift-off of catalyst particle due to weak interaction with mat, (IV)

diffusion of Fe atoms on exposed end of catalyst, (V) catalyst covered with carbon layers and growth stoppage; and (c)

combined root-tip-growth mechanisms: (I) Fe catalyst particle formation, (II) diffusion of carbon into catalyst, (III) splitting of

catalyst particle, (IV) upper portion of catalyst particle carried away on growing tip of nanotube, (V) catalyst covered with

carbon layers and growth stoppage or (V(a) and (b)) further splitting of catalyst particle and formation of secondary branches.

392 C A R B O N 4 7 ( 2 0 0 9 ) 3 8 4 – 3 9 5

portion of the catalyst particle. As the catalyst particle moves

away from the mat, a portion of the catalyst breaks off and

gets trapped within the inner core of the tube. The tip growth

stops when the catalyst particle is covered with carbon layers

or when the supply of feed gas is cut off.

4.3. Combined root-tip-growth mechanism

Larger catalyst particles form as a result of agglomeration of

smaller catalyst particles. The combined root-tip-growth re-

sults from the splitting of these larger catalyst particles

(Fig. 8c). With the lower portion of the catalyst firmly an-

chored to the quartz substrate, the upper portion breaks away

from the original catalyst and is lifted off along with the grow-

ing end of the nanotube. This sort of growth favors elongation

of the nanotubes. The growth of the nanotube stops when the

catalyst particle is completely covered with carbon layers.

Alternatively, additional Fe atoms could diffuse into the ex-

posed catalyst particle and cause it to enlarge. The catalyst

particle eventually splits and gives rise to secondary

branches. Certain portions of the catalyst particle break off

and get trapped within the inner core of the tubes.

4.4. Sequence of growth events on the substrate

Since the feed material is continuously supplied throughout

the reaction period in a continuous-feed CVD reactor, the for-

mation of the catalyst particles followed by the formation of

MWCNTs take place throughout the reaction period making

the growth process on the surface of the substrate chaotic.

Using SEM images and previous experimental observations,

a sequence of growth events is presented in Fig. 9. As ob-

served in the SEM images, the deposition of catalyst particles

takes place first. This is followed by the nucleation of carbon

nanotubes via diffusion of carbon atoms into the catalyst par-

ticles. With a lot of empty surface area on the substrate, the

Fig. 9 – Sequence of events of the growth of MWCNTs on the

quartz substrate in a continuous-feed CVD process: (a)

random deposition of metal catalyst particles of different

diameters on the substrate, (b) nucleation of carbon

nanotubes by diffusion of carbon atoms into the deposited

catalyst particles and further deposition of catalyst particles,

(c) random growth of MWCNTs via root-growth; catalyst

particle enlargement leading to tube coarsening near root-

end; catalyst particle-splitting, (d) crowding of MWCNTs

leading to vertical alignment; deposition of new particles on

the growing mat, part of catalyst carried away on the

growing end of the tubes, branching of tubes from catalyst

particles that were carried away on growing end, (e)

MWCNTs grow by ‘‘tip-growth’’ from catalyst particles

deposited on growing nanotube mat; vertically-aligned

MWCNT-mat.

C A R B O N 4 7 ( 2 0 0 9 ) 3 8 4 – 3 9 5 393

catalyst particles are generously deposited on the substrate

and establish a strong interaction with the quartz substrate,

favoring nanotube growth via root-growth mechanism. Ini-

tially, the nanotubes are sparsely distributed and grow in ran-

dom directions on the substrate. While the nanotubes

continue to grow in the axial direction, the tube walls close

to the catalyst particles begin to coarsen via epitaxial growth

leading to the formation of taper-shaped nanotubes [5]. The

continuously supplied feed gas produces more clusters of Fe

atoms. The crowding of the nanotubes prevents these newly

formed catalyst particles from reaching the substrate. These

catalyst particles get deposited randomly on the growing

mat and grow via tip-growth due to weaker interaction with

the mat. The larger catalyst particle split and the growing

nanotube carries a portion of the catalyst on the tip end. A

portion of the catalyst splits further and gives rise to addi-

tional branches. As the nanotube number density increases,

they begin to align themselves into regular arrays in the ver-

tical direction perpendicular to the substrate carrying the

coiled nanotubes on the growing end of the mat.

5. Conclusions

The various events that take place on the substrate starting

from the formation of catalyst particles to the evolution of

MWCNT-mats have been successfully studied using ex situ

characterization methods. Based on the SEM images during

the initial stages of growth, it has been shown that the

MWCNTs do not appear to have grown in the vapor phase

and that substrate played an important role in the nucleation

of the nanotube by providing an anchorage point for the nano-

tubes during growth. During the initial stages, the open areas

in the substrate made it possible for generous deposition of

free catalyst particles on the substrate and for strong cata-

lyst–substrate interaction. This led to the majority of the

growth to take place by root-growth. As the substrate filled in

with nanotubes and the availability of open areas on the sub-

strate decreased, the newly formed catalyst particles were

deposited on the growing nanotube mat. The weak catalyst

interaction with the mat forced the catalyst particles to lift

off while the MWCNT bases remained anchored to the mat in

a tip-growth mechanism. Geometric constraints forced the

nanotubes to grow vertically, perpendicular to the substrate.

The catalyst particles that were carried away on the grow-

ing tip of the nanotubes led to tube elongation and formation

of secondary branches. As predicted by CFD, higher flows at

the center led to increased surface diffusion. This led to in-

creased MWCNT deposition in the center compared to regions

close to the walls that had been observed in the experimental

results. The comprehensive growth model developed in this

work is expected to help researchers to have a better under-

standing of the growth of MWCNTs synthesized via continu-

ous-feed CVD and also to possibly achieve better control of

the diameter, length and quality of synthesized products

which might be used in different fields of nanotechnology.

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