Diameter-controlled synthesis of carbon nanotubes

Article abstract of DOI:10.1021/jp0142278

Nearly monodisperse iron nanoclusters have been used to define the diameters of carbon nanotubes grown by chemical vapor deposition (CVD). Iron nanoparticles with average diameters of 3, 9, and 13 nm were used to grow carbon nanotubes with average diameters of 3, 7, and 12 nm, respectively. Transmission electron microscopy studies of the nanotubes show that the as-grown nanotubes are single-walled carbon nanotubes (SWNTs) or thin multiwalled carbon nanotubes (MWNTs) with 2 or 3 layers. Investigations of the growth conditions also demonstrate that the supply Q f carbon reactant is critical for enabling the growth of large diameter nanotubes from large iron nanoclusters, and that the growth temperature is especially important for achieving high-quality large diameter nanotubes. The implications of these results and possible applications of the nanotubes are discussed.

Full text of DOI:10.1021/jp0142278

J. Phys. Chem. B 2002, 106, 2429-2433
2429
Diameter-Controlled Synthesis of Carbon Nanotubes
,†
,‡
Chin Li Cheung, Andrea Kurtz, Hongkun Park,* and Charles M. Lieber*
Department of Chemistry and Chemical Biology, HarVard UniVersity, 12 Oxford Street,
Cambridge, Massachusetts 02138
ReceiVed: NoVember 15, 2001; In Final Form: January 18, 2002
Nearly monodisperse iron nanoclusters have been used to define the diameters of carbon nanotubes grown by
chemical vapor deposition (CVD). Iron nanoparticles with average diameters of 3, 9, and 13 nm were used
to grow carbon nanotubes with average diameters of 3, 7, and 12 nm, respectively. Transmission electron
microscopy studies of the nanotubes show that the as-grown nanotubes are single-walled carbon nanotubes
(SWNTs) or thin multiwalled carbon nanotubes (MWNTs) with 2 or 3 layers. Investigations of the growth
conditions also demonstrate that the supply of carbon reactant is critical for enabling the growth of large
diameter nanotubes from large iron nanoclusters, and that the growth temperature is especially important for
achieving high-quality large diameter nanotubes. The implications of these results and possible applications
of the nanotubes are discussed.
Introduction
Carbon nanotubes are currently the focus of intense research
due to their unique properties and potential to impact broad areas
1
of science and technology. The distinctive characteristics of
carbon nanotubes arise from the atomic structure and size of
these materials. For example, nanotubes can be either metals
or semiconductors depending on helicity and diameter. The
2
Figure 1. Schematic depicting the use of different diameter iron
nanocluster catalysts for the controlled diameter synthesis of carbon
nanotubes.
diameter of a nanotube also affects significantly its mechanical
properties, and thus can impact applications ranging from probe
3
4
microscopy tips, to electromechanical devices, and structural
large diameter nanotubes. The implications and possible ap-
plications of these nanotubes are discussed.
5
composites. Consequently, the development of methods that
control precisely the structural properties of nanotubes offers
the possibility of impacting many important areas.
Experimental Section
Control of nanotube diameter represents one of the most basic
issues in developing nanotube growth methods. Previously, it
has been suggested that the size of the growth catalyst used in
metal catalyzed chemical vapor deposition (CVD) can define
Iron nanoclusters were synthesized by thermal decomposition
of iron pentacarbonyl (Fe(CO)5) (Aldrich, 99.999%) using a
9
procedure based on a previous report. Briefly, 2 mmol of
6
Fe(CO)5 was mixed with either 1-5 mmol of oleic acid
the diameter of as-grown carbon nanotubes. This hypothesis
(
(
Aldrich, 99+%), lauric acid (Aldrich, 99.5+%) or octanoic acid
Aldrich, 99.5+%) in 10 mL of dioctyl ether (Aldrich, 99%).
has been supported by the observation that catalytic particles
at the ends of CVD-grown nanotubes have sizes commensurate
with the nanotubes diameters.3
c,6,7,8
The solutions were refluxed at 286 °C under nitrogen for 1-3
h to yield iron nanocluster solutions with distinct and nearly
monodisperse diameters.
However, direct growth of
different diameter carbon nanotubes from monodisperse catalyst
such as FeOx,3 Fe/Mo, and Co nanoclusters, which could
c
9
10
CVD growth of carbon nanotubes catalyzed by the different
size iron nanoclusters was carried out using ethylene or methane
as the carbon reactant. The catalyst was deposited on oxidized
silicon surfaces from toluene solution and then rinsed with
hexane. The substrate-supported catalyst was annealed at 800
provide much stronger support for this idea, have not yet been
reported.
Here, we demonstrate clearly the concept that different size
nanocluster catalysts can be used to control the diameters and
structures of CVD-grown nanotubes (Figure 1). We have
prepared nearly monodisperse iron nanoclusters having three
distinct average diameters, and have used these nanoclusters to
grow carbon nanotubes with similar average diameters. In
addition, investigations of the CVD growth demonstrate that
the supply of carbon reactant is critical for the preparation of
large diameter nanotubes from large iron nanoclusters, and that
growth temperature is also important for achieving high-quality
°
C for 5 min in a flow of 600 standard cubic centimeters per
minute (sccm) of argon and 400 sccm of hydrogen, and then
CVD growth was carried out by the addition of 2-200 sccm
of ethylene or 1000 sccm of methane at 800-1000 °C for 10
min.
The diameters of the iron nanoclusters and carbon nanotubes
prepared using these catalysts were determined by transmission
electron microscopy (TEM) (Philips EM 420, 100 kV, FEI,
Hillsboro, OR), and atomic force microscopy (AFM) (Nano-
scope IIIa, Digital Instruments, Santa Barbara, CA). The
reported uncertainties correspond to (1 standard deviation.
*
To whom correspondence should be addressed.
E-mail: hpark@chemistry.harvard.edu.
E-mail: cml@cmliris.harvard.edu.
†
‡
1
0.1021/jp0142278 CCC: $22.00 © 2002 American Chemical Society
Published on Web 02/16/2002

2430 J. Phys. Chem. B, Vol. 106, No. 10, 2002
Letters
Figure 2. TEM images of iron nanoclusters produced by solution-phase synthesis using (a) oleic acid, (b) lauric acid, and (c) octanoic acid. The
scale bars are 30 nm. Corresponding histograms of the nanocluster diameters are plotted in (d), (e), and (f), respectively. The solid lines correspond
to Gaussian fits.
Results and Discussion
Carbon nanotubes were grown by CVD at 800 °C with
ethylene reactant using the 3, 9 and 13 nm average diameter
iron nanoclusters as catalysts. Representative TEM images
demonstrate clearly that larger nanocluster catalyst diameters
can lead to the reproducible growth of larger diameter carbon
nanotubes (Figure 3, parts a, b, and c). The key role of the
catalyst in defining the nanotube diameters produced by CVD
is evident from the analysis of the diameter distributions.
Specifically, histograms of the carbon nanotube diameters
obtained from TEM and AFM analysis of samples prepared
using iron nanoclusters demonstrate a close correlation between
diameters of the nanocluster catalysts and nanotubes (Figure 3,
parts d, e, and f). The carbon nanotubes obtained from the 3, 9,
and 13 nm average diameter iron nanoclusters had diameters
of 2.6 ( 0.8, 7.3 ( 2.2, and 11.7 ( 3.2 nm, respectively. The
high degree of diameter control and uniformity can also be seen
clearly in AFM images of the as grown nanotubes as shown in
Figure 4. In addition to the close correlation between average
diameters of the catalysts and nanotubes, we find that diameter
distribution widths are also similar. For the largest diameter
nanotubes, there is some broadening relative to the nanocluster
catalyst distribution, and we attribute this to the greater difficulty
in achieving uniform nucleation at the largest diameters (see
below). Nevertheless, these results show quite clearly that well-
Critical to our studies of controlled diameter growth of carbon
nanotubes is the preparation of iron nanoclusters with narrow
average diameter distributions. To achieve this goal we decom-
posed Fe(CO)5 in the presence of an excess of oleic acid (C18),
lauric acid (C12) or octanoic acid (C8) acid, which function as
capping ligands for the nanoclusters as they form. Typical TEM
images of the iron nanoclusters produced using oleic, lauric,
and octanoic acids, parts a, b, and c of Figure 2, respectively,
qualitatively show our ability to control the magnitude and
uniformity of the nanocluster diameters. In general, we find that
the growth of smaller (larger) diameter nanoclusters is favored
in the presence of longer (shorter) chain-length capping ligands.
To quantify these trends and the dispersity in nanocluster
diameters we determined diameter distributions from the TEM
data. Histograms of the nanocluster diameters obtained using
oleic acid (Figure 2, part d), lauric acid (Figure 2, part e), and
octanoic acid (Figure 2, part f) capping ligands demonstrate
relatively narrow distributions with average diameters (1
standard deviation of 3.2 ( 0.8, 9.0 ( 0.9, and 12.6 ( 1.7 nm,
respectively. Significantly, these solutions of well-defined and
distinct diameter iron nanoclusters provide us with the catalyst
material needed to test clearly the possibility of diameter
controlled nanotube growth.

Letters
J. Phys. Chem. B, Vol. 106, No. 10, 2002 2431
Figure 3. TEM images of carbon nanotubes grown using (a) 3, (b) 9, and (c) 13 nm average diameters iron nanoclusters. Scale bars are 20 nm
for (a), (b), and (c). Corresponding histograms of the nanotube diameters are plotted in (d), (e), and (f), respectively. The solid lines correspond to
Gaussian fits.
defined nanocluster catalysts can be used for diameter-controlled
CVD synthesis of nanotubes.
walled MWNTs with typical wall thicknesses of 2-4 graphene
sheets (Figure 3, part c). The structural morphology of these
tubes is similar to the MWNTs grown from the medium-size-
range catalyst. Although a large fraction of SWNTs have not
been produced from the largest catalyst nanoclusters, we note
that the observed MWNTs have very thin walls and large open
internal volumes.
In addition, the partial pressure of the reactant gas was found
to be important to controlled diameter carbon nanotube growth.
As the size of nanoclusters increases from 3 to 13 nm, increased
ethylene percentages were required to produce carbon nanotubes
with diameter distributions similar to those of the corresponding
catalysts: for the 3, 9, and 13 nm diameter catalysts, 0.5, 9,
and 18, respectively, percent ethylene was used. In contrast,
The structural characteristics of these different diameter
carbon nanotubes have also been determined from TEM images.
First, nanotubes produced from small (3 nm average diameter)
iron nanoclusters consist primarily of SWNTs with ca. 30%
double-walled carbon nanotubes (DWNTs) (Figure 3, part a).
AFM images also showed that these as-grown SWNTs are
straight over lengths of hundreds of nanometers, suggesting a
relatively low density of defects. Second, the 9 nm average
diameter catalyst nanoclusters produce a mixture of SWNTs
and MWNTs (Figure 3, part b). This medium-size-range catalyst
produces SWNTs and thin-walled MWNTs with typical wall
thicknesses of 2-4 graphene layers. The well-defined lattice
fringes in TEM images of the MWNTs indicate that the as-
grown nanotubes have a high degree of local crystalline order.
Larger scale images also show that the straight, ordered
segments are often mixed with kinked and bent segments, which
arise from structural defects. Third, the large (13 nm average
diameter) diameter nanoclusters catalyze the growth of thin-
0
.5% ethylene reactant produces small diameter SWNTs ir-
respective of the diameters of iron nanoclusters used as the
catalysts. We believe that these results indicate that the initial
nucleation is critical to determining diameters of larger (>5 nm)
nanotubes, and that to achieve large size nucleation event
requires a larger flux of carbon reactant to the nanocluster
catalyst. This interpretation is consistent with results observed

2432 J. Phys. Chem. B, Vol. 106, No. 10, 2002
Letters
Figure 4. (a) AFM image of SWNTs grown from 3 nm average
diameter catalyst on an oxidized silicon surface. The image is 2 µm ×
2
µm. (b) Height cross-section taken along the white dashed line in
(a). The diameters of nanotubes in the cross-section are (from the left)
2
.86, 2.55, 2.56, and 3.11 nm, respectively.
previously using alumina-supported Fe/Mo catalyst,11 although
the catalyst diameters were not known in this earlier work.
Last, we have also carried out experiments exploring how
the growth temperature and carbon reactant species affect the
nanotubes produced from the different diameter nanocluster
catalysts. First, growth at 900 °C using the 9-nm diameter iron
nanoclusters produces large diameter, thin-walled MWNTs with
typically only 2 or 3 walls (Figure 5, part a). In addition, these
large diameter carbon nanotubes exhibit substantially fewer
defects (e.g., sharp kinks and bends) compared to similar
diameter nanotubes grown at 800 °C. However, high-temper-
ature growth from ethylene can produce substantial amorphous
carbon on nanotubes due to homogeneous decomposition of
ethylene in the gas phase. To explore the possibility of obtaining
high-quality large diameter nanotubes that are also free from
amorphous carbon coatings, we have investigated growth using
methane, which has higher homogeneous decomposition tem-
perature.12 Significantly, methane-based growth from 9 nm
average diameter nanoclusters yields relatively clean SWNTs
and DWNTs with 7 nm average diameters (Figure 5, part b).
These results thus suggest that it should be possible through
further optimization of growth conditions to produce clean and
crystalline large-diameter SWNTs.
Figure 5. (a) TEM image of ca. 7 nm diameter carbon nanotubes
grown at 900 °C using ethylene. (b) TEM image of an ca. 7 nm diameter
SWNT synthesized with methane at 850 °C. Scale bars are 40 nm.
nanotubes will facilitate the study of fundamental properties and
the exploration of new applications. For example, large-diameter
SWNTs are ideal systems for examining theoretical predictions
of reversible changes in the electronic properties of radially
13
deformed nanotubes, and moreover, it might be possible to
14
exploit such large nanotubes as electromechanical devices and
nanoreactors. Carbon nanotube samples with narrow diameter
distributions also could be used as templates for the elaboration
15
of other nanomaterials such as metal carbide nanowires. In
addition, the availability of carbon nanotubes with well-defined
and predictable diameters should have a substantial impact on
3
probe microscopy imaging. Last, we believe that the approach
demonstrated in this paper can be extended to other types of
16
pyrolytically grown nanotubes, such as BxCyNz, and thus could
open up exciting opportunities for fundamental studies and
technological applications of a wide-range of nanoscale materials
in the future.
Conclusions
We have shown that nearly monodisperse iron nanoclusters
can be used to define the diameters of carbon nanotubes grown
by metal-catalyzed CVD. Investigations of growth conditions
demonstrate that the supply of carbon reactant is important to
growth of large diameter nanotubes from large iron nanoclusters,
and that growth temperature is especially important for achieving
high-quality large diameter nanotubes. We believe that the
ability to exert a high degree of control over the diameters of
Acknowledgment. We thank Yuan Lu for assistance with
TEM measurements. C.M.L. acknowledges generous support
of this work by the AFOSR and DARPA. H.P. acknowledges
generous support of this work by the NSF MRSEC, Dreyfus
Foundation, and the Research Corporation.

Letters
J. Phys. Chem. B, Vol. 106, No. 10, 2002 2433
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