Single-walled carbon nanotube growth using [Fe3( 3-O)(-O2CR)6(L)3]n+ complexes as catalyst precursors

Article abstract of DOI:10.1039/b512885d

We present herein the VLS growth of SWNTs from oxo-hexacarboxylate-triron precursors, [Fe₃O(O₂CCH₃)₆(EtOH) ₃] (1) and [Fe₃O(O₂CCH₂OMe) ₆(H₂O)₃][FeCl₄] (2), on spin-on-glass surfaces, using C₂H₄/H₂ (750 °C) and CH₄/H₂ (800 and 900 °C) growth conditions. The SWNTs have been characterized by AFM, SEM and Raman spectroscopy. The characteristics of the SWNTs are found to be independent of the identity of the precursor complex or the solvent from which it is spin-coated. The as grown SWNTs show a low level of side-wall defects and have an average diameter of 1.2-1.4 nm with a narrow distribution of diameters. At 750 and 800 °C the SWNTs are grown with a range of lengths (300 nm-9 m), but at 900 °C only the longer SWNTs are observed (6-8 m). The yield of SWNTs per unit area of catalyst nanoparticle decreases with the growth temperature. We have demonstrated that spin coating of molecular precursors allows for the formation of catalyst nanoparticles suitable for growth of SWNTs with a high degree of uniformity in the diameter, without the formation of preformed clusters of a set diameter. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b512885d

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PAPER
www.rsc.org/dalton | Dalton Transactions
Single-walled carbon nanotube growth using [Fe3(l3-O)(l-O2CR)6(L)3]n+
complexes as catalyst precursors†‡
Douglas Ogrin,a Ramon Colorado, Jr.,a Benji Maruyama,b Mark J. Pender,b Richard E. Smalleya,c and
Andrew R. Barron*a
Received 13th September 2005, Accepted 25th October 2005
First published as an Advance Article on the web 9th November 2005
DOI: 10.1039/b512885d
We present herein the VLS growth of SWNTs from oxo-hexacarboxylate–triron precursors,
[Fe3O(O2CCH3)6(EtOH)3] (1) and [Fe3O(O2CCH2OMe)6(H2O)3][FeCl4] (2), on spin-on-glass surfaces,
using C2H4/H2 (750 ◦C) and CH4/H2 (800 and 900 ◦C) growth conditions. The SWNTs have been
characterized by AFM, SEM and Raman spectroscopy. The characteristics of the SWNTs are found to
be independent of the identity of the precursor complex or the solvent from which it is spin-coated. The
as grown SWNTs show a low level of side-wall defects and have an average diameter of 1.2–1.4 nm with
a narrow distribution of diameters. At 750 and 800 ◦C the SWNTs are grown with a range of lengths
(300 nm–9 lm), but at 900 ◦C only the longer SWNTs are observed (6–8 lm). The yield of SWNTs
per unit area of catalyst nanoparticle decreases with the growth temperature. We have demonstrated
that spin coating of molecular precursors allows for the formation of catalyst nanoparticles suitable for
growth of SWNTs with a high degree of uniformity in the diameter, without the formation of
preformed clusters of a set diameter.
(with a high yield, i.e., SWNT : catalyst ratio) are desired since
Introduction
polydispersed metal nanoparticle catalysts result in SWNTs with
The growth of carbon nanotubes from supported catalyst particles
a wide size distribution. Several approaches have been made to
occurs by vapor–liquid–solid (VLS) growth,1 that is believed
prepare catalysts with highly controlled size distribution in the
to occur via two mechanistic steps: nucleation and growth.2,3
hope of synthesizing SWNTs with narrow diameter distributions,
including, the use of supports,8 diblock copolymer micelles,9
ferritin5,10 and dendrimers.11 These supports are usually treated
with an iron salt, and then annealed to form Fe2O3 nanoparticles.
Arrays of posts and strips, patterned into a thermally evaporated
iron surface, have also shown success.12 In a recent publication Liu
and co-workers have shown the iron–molybdenum nanocluster
[HxPMo12O40⊂H4Mo72Fe30(O2CMe)15O254(H2O)98] (FeMoC) to be
a suitable catalyst precursor for the growth of SWNTs.13
We are interested in the application of small molecular precur-
sors for VLS growth of uniform SWNTs, in particular, on surfaces.
Our rationalization for the use of the small molecular precursors
was based upon two questions. First, if a simple molecular species
is used as the catalyst precursor what control over SWNT diameter
is possible? It has been postulated that preformed clusters of a
set diameter (without aggregation) are required for surface VLS
growth of SWNTs with uniform diameters. An individual molecule
as a precursor can be assumed to be too small for seeding SWNT
growth. Thus, either aggregation of the precursor on the surface
or aggregation of the metal atoms after reduction must occur
in order to generate a cluster of sufficient size. If aggregation
were random, then a wide range of catalyst cluster sizes, and
therefore distribution of SWNT diameters would be expected.
Second, can the crystal packing/intermolecular forces between
different molecular clusters be used as a route to fabrication of
catalyst particles with different sizes?
Nucleation involves both the formation of a catalyst particle and
the initial construction of the carbon framework on which the tube
is later grown. At growth temperatures the catalyst metal atoms
aggregate to form a liquid metal nanoparticle. For the growth
of single walled carbon nanotubes (SWNTs), it is proposed that
the nanoparticle needs to consist of ca. 50–200 metal atoms (1–
100 nm).4 The size of this particle has been proposed to have an
effect on the diameter and type (as defined by the SWNT’s m,n
value) of tube produced.4–6 Although evidence for this postulate
is limited, it is an attractive research goal that specific catalysts
(or catalyst precursors) can be designed to provide a specific m,n
SWNT or at least a limited range of SWNT types.
A variety of transition-metal catalysts and alloys have been
studied. The metals most commonly used for SWNT growth are
iron, cobalt, nickel, molybdenum, and various bimetallic alloys of
these metals.7 Irrespective of the metal source, catalysts that enable
narrow size and chirality distributions in SWNT production
aDepartment of Chemistry and Center for Nanoscale Science and Technol-
ogy, Rice University, Houston, Texas, 77005, USA. E-mail: arb@rice.edu
bMaterials and Manufacturing Directorate, Air Force Research Laboratory,
Wright Patterson AFB, Ohio, 45433, USA
cCarbon Nanotechnology Laboratory, Rice University, Houston, Texas,
77005, USA
† Electronic supplementary information (ESI) available: Fig. S1: TGA plot
for 2. Fig. S2: A SEM micrograph of SWNTs grown on SOG substrate
from 1 using CH4/H2 (1 : 1) at 800 ◦C. Fig. S3: Plot of percentage of active
catalysts as a function of growth temperature. See DOI: 10.1039/b512885d
‡ Dedicated to the memory of our recently departed colleague, Richard
E. Smalley. He will be sorely missed.
In choosing a suitable class of precursor the following points
were considered: high yield synthesis allowing for solubility in
a wide range of solvents through different functional groups,
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and multiple metal atoms without being of sufficient size to
form a nanocluster on their own. The oxo-hexacarboxylate–
triron compounds, [Fe3(l3-O)(l-O2CR)6(L)3]n , fulfill these needs.
These compounds contain either [Fe(III)3], [Fe(III)2Fe(II)], or
[Fe(III)Fe(II)2] cores depending on whether the complex is cationic,
neutral, or anionic, respectively.14 Derivatives may be prepared
with a range of both carboxylate substituents and neutral donor
ligands, and many of the examples have been crystallographically
characterized. The oxo-hexacarboxylate–triron compounds are
too small to act as catalyst precursors and therefore the formation
of a catalyst particle will require aggregation. The desire to produce
an EtOH or H2O soluble derivative for attachment to a function-
alized SWNT prompted the synthesis of [Fe3O(O2CMe)6(EtOH)3]
(1) and [Fe3O(O2CCH2OMe)6(H2O)3][FeCl4] (2). We have previ-
ously shown that methoxyacetic acid derived nanoparticles show
good solubility in water and alcohols.15
substrates coated with a spin-on-glass (SOG) layer (see Experi-
mental section). The SOG is used to provide a smooth, uniform
surface. The choice of 0.1 mM solutions was to allow for sufficient
precursor to be deposited without formation of micron sized crys-
tals or precursor that result in the formation of multi wall carbon
nanotubes (MWNTs). The samples were placed in the hot zone of
Mullite tube mounted in an electric tube furnace, and subjected to
a three-step growth process: thermolysis, reduction and growth.
Decomposition of the precursors was accomplished by heating
to the desired growth temperature under argon. The thermogravi-
metric analysis (TGA) of compounds 1 and 2 under argon show
thermolysis via loss of co◦ordinated solvent ligands followed by
decomposition up to 300 C. Compound 2 loses one equivalent
of Fe per formula (presumably as chloride) between 750 and
950 ◦C. Thus, nanoparticles prepared from compound 2 when
heated to 750 ◦C will contain chlorine which will allow for a
comparison of the possible effects of chlorine in the precursor.
Reduction of the iron oxide features at under a hydrogen
atmosphere (see below) was followed by the introduction of
the growth gas and cessation of the argon flow. Three growth
gas/temperature conditions were explored. Ethylene has been
found to be a feedstock for growth at lower temperatures, while
methane generally provides cleaner growth, i.e., in the absence of
amorphous carbon. Although prior experiments with molecular
precursors and CH4 have generally been run at 900 ◦C we
chose to also study lower temperature growth. This was for two
reasons. First, this temperature is the upper limit for the growth
chamber for in-situ Raman monitoring, and second, it allows
for a comparison with the C2H4 growth. A summary of growth
conditions is given in Table 1.
The growth of SWNTs is confirmed by AFM measurements of
the diameters of the SWNTs (Table 1) and more importantly by
in-situ Raman spectroscopy within a growth chamber for samples
grown from compound 1 in CH4/H2 at 800 ◦C (see Experimental
section). As may be seen from Fig. 1, the spectra of the surface
before and after a growth run indicate the appearance of the
tangential band (G-band) at 1590 cm−1 that is characteristic of
a SWNT. The lack of a disorder band (D) band) at ca. 1350 cm−1
which indicates sp3 hybdrization on the sidewalls, is evidence
that the SWNTs grown have very few defects and low sidewall
functionalization.18
Scanning electron microscopy (SEM) of the surface after growth
for samples grown at 750 and 800 ◦C clearly show the presence
of features consistent with SWNT growth (e.g., Fig. 2). Samples
grown from CH4/H2 at 900 ◦C show only sparse coverage. AFM
allows for more detailed analysis of the samples.
We present herein the VLS growth of SWNTs from [Fe3(l3-O)(l-
O2CR)6(L)3]n+ precursors on spin-on-glass surfaces. The effects of
precursor identity (n = 0, 1), carbon source (CH4 vs. C2H4), and
growth temperature (750–900 ◦C), on the uniformity and yield of
SWNTs are discussed.
Results and discussion
The EtOH soluble acetate derivative, [Fe3O(O2CMe)6(EtOH)3]
(1) was prepared by solvent exchange from the aquo-complex,
[Fe3O(O2CMe)6(H2O)3].16 Reaction of FeCl3 with methoxyacetic
acid/NaHCO3 in water allowed for the isolation of dark red
crystals of [Fe3O(O2CCH2OMe)6(H2O)3][FeCl4] (2). The structure
of 2 has been confirmed by X-ray crystallography.17 The choice
of both neutral and ionic complexes was aimed at determining
the effects of solvent (and therefore evaporation rates) during spin
coating of the catalyst precursor. In addition, the crystal structure
of compound 2 exhibits an extended structure in which the clusters
are linked by intra-trimer hydrogen bonding to form a zigzag
motif that form sheets via hydrogen bonding involving disordered
waters of hydration. The [FeCl4]− anion is intercalated between the
hydrogen-bonded sheets.17 The aggregation of compound 2 should
be dominated by hydrogen bonding and ion interactions, while
that of compound 1 is only reliant on van der Waals interactions.
Thus, the aggregation of compound 2 should be favored over the
aggregation of compound 1. Finally, the presence of chlorine in a
catalyst precursor has been thought to be detrimental to SWNT
growth unless prior thermolysis to Fe2O3 is accomplished.
Dilute solutions (0.1 mM) of compounds 1 and 2 were prepared
in EtOH and H2O, respectively, and spin-coated onto silicon wafer
Table 1 Summary of SWNT growth experiments
Precursor/solventa
Growth gas
Td/◦C
Average SWNT diametere/nm
SWNT length rangee/lm
Yield (%)f
b
1/EtOH
1/EtOH
1/EtOH
2/H2O
C2H4/H2
750
800
900
750
800
900
1.2 0.2
1.3 0.2
1.4 0.3
1.3 0.2
1.3 0.2
1.4 0.3
0.3–7
0.3–6
6–8
0.5–9
0.3–6
6–8
30–50
15–25
1–2
30–50
15–25
1–2
c
CH4/H2
CH4/H2
c
b
C2H4/H2
c
2/H2O
CH4/H2
CH4/H2
c
2/H2O
a 10−4 M. b Gas ratio of 1 : 4. c Gas ratio of 1 : 1. d All samples heated to growth temperature under Ar. All growth runs of 15 min. e Measured by AFM.
f Number of SWNTs grown per catalyst feature in a 1 lm × 1 lm square.
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Fig. 1 Raman spectra of sample (a) before and (b) after growth run with
[Fe3O(O2CCH3)6(EtOH)3] (1) using CH4/H2 (1 : 1) at 800 ◦C.
Fig. 3 Representative AFM image of SWNTs grown from (a) [Fe3O-
(O2CCH3)6(EtOH)3] (1) and (b) [Fe3O(O2CCH2OMe)6(H2O)3][FeCl4] (2)
using C2H4/H2 (1 : 4) at 750 ◦C. Images are 10 × 10 lm.
Fig. 2 A representative SEM micrograph of SWNTs grown on SOG
substrate from [Fe3O(O2CCH2OMe)6(H2O)3][FeCl4] (2) using CH4/H2 (1 :
1) at 800 ◦C.
Fig. 3 shows representative AFM images of SWNTs grown
from compounds 1 and 2 using C2H4/H2 growth conditions. It
may be clearly seen that there is a high proportion of SWNTs
per catalyst feature suggesting that the majority of catalyst
particles formed from the catalyst precursor are active for SWNT
growth. The similarity of results for compounds 1 and 2 suggest
that the presence of chloride ligands in the precursor is not a
detriment to SWNT growth. Based upon the analysis of catalyst
particles vs. SWNTs grown over a series of 1 × 1 lm squares
analyzed by AFM, there is no difference in activity between
the catalysts formed from [Fe3O(O2CMe)6(EtOH)3] in EtOH and
[Fe3O(O2CCH2OMe)6(H2O)3][FeCl4] in H2O (Table 1). Samples
grown using CH4/H2 at 800 ◦C show a slightly lower coverage
(Fig. 4) to that observed with C2H4/H2 (Fig. 3). However, it is
worth noting that the range of length of the grown SWNT remains
essentially constant (Table 1). This would suggest that despite
the higher temperatures and different growth gases the growth
rates are unchanged but the fraction of active catalysts decreases
(to 15–25% @ 800 ◦C). The fraction of active catalysts decreases
significantly for the CH4/H2 growth at 900 ◦C (see Table 1). The
number of active catalysts decreases◦exponentially with increased
temperature. Samples grown at 900 C using CH4/H2 show very
few SWNTs per unit area compared to the number of catalyst
particles (Fig.◦ 5). Although the average lengths of the SWNTs
grown at 900 C are much longer (6–8 lm) than those grown at
lower temperatures (300 nm, ∼9 lm), the SWNTs grown at 900 ◦C
appear to be identical to the longest grown at lower temperatures,
i.e., the maximum growth rates are unchanged with temperature.
We propose that increasing the growth temperature does not
significantly alter the growth rate due to the feedstock-limited
conditions (i.e., the relative concentrations of CH4 or C2H4 and
H2) under which these experiments were conducted. Nevertheless,
increasing the temperature does deactivate a significant fraction
of the catalyst particles.
The above results demonstrate that in this system the growth
of SWNTs is independent of the identity of the catalyst precursor
and the solvent it is spin-coated from. An explanation of this
observation is obtained by a consideration of the size of the
catalyst particles formeduponthe thermolysis andreductionofthe
catalyst precursor molecules. Fig. 6 shows AFM images of samples
of 1 and 2 spin-coated onto SOG substrates after reduction
with H2 at 900 ◦C. The catalyst nanoparticles formed from the
reductive decomposition of compounds 1 and 2 on SOG are 8.0
1.0 nm and 9.0
0.3 nm, respectively. Despite the differences
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Fig. 5 Representative AFM image of long SWNT grown from (a) [Fe3O-
(O2CCH3)6(EtOH)3] (1) and (b) [Fe3O(O2CCH2OMe)6(H2O)3][FeCl4] (2)
using CH4/H2 (1 : 1) at 900 ◦C. Images are 2 × 2 lm.
Fig. 4 Representative AFM image of SWNTs grown from (a) [Fe3O-
(O2CCH3)6(EtOH)3] (1) and (b) [Fe3O(O2CCH2OMe)6(H2O)3][FeCl4] (2)
using CH4/H2 (1 : 1) at 800 ◦C. Images are 5 × 5 lm.
for SWNTs grown from the 2 nm diameter molecular nanocluster
[HxPMo12O40⊂H4Mo72Fe30(O2CMe)15O254(H2O)98] (38%).13
The size distributions for SWNTs grown from [Fe3O(O2CMe)6-
(EtOH)3] and [Fe3O(O2CCH2OMe)6(H2O)3][FeCl4] using C2H4/
H2 at 750 ◦C and CH4/H2 at 800 ◦C are shown in Fig. 7.
A consideration of the average SWNT diameter (Table 1) as a
function of precursor and/or growth conditions suggests that
there is some effect of these parameters the size distribution plots.
However, from Fig. 7 there is a slight shift towards larger diameter
for the higher temperature runs, but the statistical significance of
this is questionable. We propose that the size distribution of the
SWNTs is essentially independent of the growth temperature/gas
and the precursor. It should be noted that given the paucity of
SWNTs on each wafer for the CH4/H2 @ 900 ◦C growth runs the
sample size is insufficient to allow good statistics.
As noted in the Introduction, it has been proposed that the
diameters of SWNTs are proportionally related to the sizes of the
catalytic nanoparticles used in the surface VLS growth process.4–6
It should be noted that some of the evidence for this proposal
is based upon results with MWNTs.6 A consequence of this
proposal has been the search for precursor particles constrained
to a small size and narrow size distribution.11,13,19–21 Our present
results using molecular complexes that are themselves too small
to grow SWNTs without aggregation, show that large catalyst
in the catalyst precursor compound solubilities and solid-state
aggregation (van der Waals interactions vs. hydrogen bonding),
upon reduction with H2 at 900 ◦C the resulting Fe catalyst
particles are remarkably similar. We note that these particles
are significantly larger than observed for previous constrained
precursors. Thus, irrespective of the identity of the precursor
complex or the solvent the resulting catalyst particles are of
comparable size. This suggests that for the SOG surfaces, diffusion
and surface tension effects control the catalyst size.
As noted in the Introduction the size of the catalyst particle
has been proposed to have an effect on the diameter and type
(as defined by the SWNT’s m,n value) of tube produced.4–6 In
this regard, previous researchers have postulated that in order
for SWNTs to be grown with narrow distribution of diameters
constrained sized catalysts or single molecular catalysts must be
employed. In the present work, we have shown that irrespective
of the identity of the precursor or its solid-state structure,17 or
the solvent from which it is deposited, similar and uniformly
sized catalyst particles are obtained. Furthermore, as may be
seen from Table 1 the diameter of the resulting SWNTs are
within a narrow range. It should be noted that the percentage
standard deviation (15–21%) is significantly lower than observed
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nanoparticle size for VLS growth from iron catalysts. All values
were obtained using AFM measurements. It can be seen that
(with the exception of precursors forming bundles or MWT,
see below) catalyst precursors between 1 and 9 nm result in a
moderate range of SWNT diameters (0.8–1.5 nm). Our results
suggest that controlling the size of the catalyst nanoparticle is not
necessary for the growth of SWNTs with a small diameter and
a narrow distribution and that simple unconstrained molecular
precursor offer as much control over the SWNT growth than
constrained precursors. We note that the published AFM data
for the two values in Fig. 8 that appear to show increased SWNT
diameter with increased catalyst size (formed using FeCl3/poly(2-
vinylpyridine)21 and Fe-doped apoferritin19) show evidence for the
formation of bundles or ropes. It is not clear that this effect or
the formation of MWNTs is responsible for the apparent large
diameter range. In this regard, we note that if during spin coating
of compounds 1 or 2 large micron sized crystals are formed then
MWNT are formed in place of SWNTs (Fig. 9).
Fig. 6 Representative AFM image of Fe nanoparticle catalysts formed
from the reductive decomposition of spin-coated (a) [Fe3O(O2CCH3)6-
(EtOH)3] (1) and (b) [Fe3O(O2CCH2OMe)6(H2O)3][FeCl4] (2). Images are
1 × 1 lm.
Fig. 8 Plot of catalyst diameter with deviation vs. the diameter of the
SWNTs grown on a surface by VLS from the Fe-containing catalyst
using CH4/H2 growth gas. Based upon known diameters for SWNTs it
is expected that bundles (ropes) would have a diameter >2 nm. Values for
SWNTs grown from compounds 1 and 2 indicated.
Fig.
7 Diameter distribution of the SWNTs grown from [Fe3O-
(O2CCH3)6(EtOH)3] (1) using C2H4/H2 (1:4) at 750 ◦C (black) or CH4/H2
(1 : 1) at 800 ◦C (dark grey) and [Fe3O(O2CCH2OMe)6(H2O)3][FeCl4] (2)
using C2H4/H2 (1 : 4) at 750 ◦C (grey) or CH4/H2 (1 : 1) at 800 ◦C (light
grey).
nanoparticles (8–9 nm) allow for the growth of SWNTs with
diameters comparable to prior results with smaller nanoparticles.
Fig. 8 shows a plot of SWNT diameter as a function of catalyst
Fig. 9 A representative SEM micrograph of MWNTs grown from
[Fe3O(O2CCH3)6(EtOH)3] (1) using C2H4/H2 (1 : 4) at 750 ◦C.
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Given that the rate of precursor decomposition (for a given
precursor, i.e., CH4 vs. C2H4) and C–C bond formation should be
constant for a given catalyst (Fe) the rate of SWNT growth should
be constant between individual SWNTs if initiation events occur
simultaneously. An interesting observation of a unique feature
gives insight to the “catalyst nucleation” and “growth” steps in
growth. Fig. 10 shows an AFM image of two SWNTs that have
grown from two catalyst particles (labeled) and have bisected at
the same point in space. The SWNTs are similar in diameter and
the initial catalyst particle may be seen from their relative height to
be positioned at the open ends of the “V”. The intersection of the
two SWNTs does not show any catalyst, precluding the possibility
that two SWNTs grew from a single catalyst particle. The similarity
in lengths suggest that in order for mutual termination to occur
at the same point on the surface, the nucleation and growth of
the SWNT must have occurred at the same time and rate. This
would suggest that any catalyst particle that is going to be active
is activated at the initial stages of the growth reaction. If it does
not initiate growth it will not do so at a later time. It also suggests
that given similar surface interactions (roughness, obstacles, etc.)
the SWNTs will grow at the same rate.
result in surface parallel growth (Fig. 11(a)) the ones deactivated
with increased temperatures while those resulting in surface-free
growth (Fig. 11(b)) appear to be stable to higher temperatures?
Furthermore, why are a significant fraction of catalysts inactive
even at relatively low growth temperatures? In order to investigate
these questions we are continuing our studies in the area of surface
catalyst interactions.
Fig. 11 Schematic representation of supported catalyst SWNTs growth in
which the SWNT grows parallel to the surface (a) or out from the surface
(b).
Conclusions
We have demonstrated that SWNTs can been grown using
simple molecular iron precursors, [Fe3O(O2CMe)6(EtOH)3] and
[Fe3O(O2CCH2OMe)6(H2O)3][FeCl4] spin-coated onto a SOG
substrate. The advantages of this class of precursor is the ease
of synthesis of various substituents allowing for control over
solubility, that in-turn allows for spin coating onto substrates from
a wide range of solvents.
While the relative activity of SWNT growth appears to be
independent of the precursor and possibly the growth gas, there is
clearly a dependence on the growth temperature. At 900 ◦C there
is a marked deactivation of the catalysts since inactive catalyst
particles are still seen on the surface along with the grown SWNTs.
Interestingly, the catalysts that remain active are those that grow
the longest SWNTs. It has been proposed that the formation of an
amorphous carbon coating is the likely cause of this inactivation
of the catalyst particles at high temperature.19 However, given the
complex interplay of catalyst activation, nucleation, growth, and
termination the reasons why the ‘most active’ catalysts remain
active at higher temperatures, but the catalysts with apparently
lower activity cease under the same condition are unclear. We
propose that a future goal of surface VLS growth of SWNTs is
an understanding of the apparent low overall activity of catalyst
particles.
Fig. 10 AFM image of two SWNTs mutually terminated during the
growth from [Fe3O(O2CCH2OMe)6(H2O)3][FeCl4] (2) using CH4/H2 (1 :
1) at 800 ◦C. Image is 3 × 3 lm.
Finally, observation of Figs. 3–5 shows that the shorter SWNTs
(<1 lm) are straight, while the longest SWNTs (6–8 lm) gently
curve. We propose the straight SWNTs are a result of growth
along the surface (Fig. 11(a)) while the longer SWNTs are formed
by growth out of the plane of the surface (Fig. 11(b)). The growth
rate of the former will be limited due to SWNT · · · SiO2 surface
interactions, while the latter will have unrestricted growth away
from the surface. Once the reaction run is complete (and the gas
flow is removed) the SWNTs grown out of the surface will fall over,
resulting in the images shown in Figs. 3–5. We propose that, in the
absence of additional factors, the rate of SWNT growth is reduced
by a factor of ca. 8 due to the interaction with the surface. This
proposal suggests a further question. Why are the catalysts that
We have demonstrated that spin coating of molecular precursors
allows for the formation of catalyst nanoparticles suitable for
growth of SWNTs with a high degree of uniformity in the diameter,
without the formation of preformed clusters of a set diameter.
Unfortunately, the crystal packing/intermolecular forces between
different molecular clusters do not appear to provide a route to
controlling the SWNT diameter. The result of this present work
questions the ability of particular sized precursor particles to
determine the diameter of the SWNTs grown from those particles.
While this does not offer hope for designed catalysts for SWNT
growth, it does suggest that a single catalyst precursor may be used
in the amplification of preformed SWNTs irrespective of their m,n
values. Our research in this area is continuing.
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with CH4 (225 sccm, 99.9%) and H2 (225 sccm) for 15 min. The
system was then cooled to room temperature under Ar. With these
flow rates; the formation of amorphous carbon was not a problem.
Flow rates for runs using C2H4 and H2 were 100 sccm and 400 sccm,
respectively.
Experimental
Fe3O(O2CCH3)6(H2O)3 was prepared according to a previ-
ously reported method.16 Methoxyacetic acid (Aldrich), acetic
acid (Fisher), sodium bicarbonate (Fisher), 200 proof EtOH
(Pharmco), and FeCl3·6H2O (Alfa Aesar) were ACS reagent grade
or better and used as received. Spin-on-glass was purchased from
Honeywell and used as received. Thermal gravimetric analysis
was carried out on a Seiko TG/DTA 200. Atomic force mi-
croscopy measurements were obtained using a Digital Instruments
NanoScope IIIa scanning probe microscope in tapping mode. A
RTESP type NanoprobeTM SPM Tip with a drive frequency of
300 kHZ was used. Tips were replaced frequently to ensure the
accuracy of the images and limit tip artifacts. Images were taken at
a scan frequency of 1–2 Hz and 256–512 samples line−1. Scanning
electron micrographs were obtained on a FEI XL30 without a
conductive coating.
In-situ Raman growth was performed on a TS1500 Hotstage
from Linkam Scientific Instruments Ltd. fitted to a Renishaw
InVia(tm) Raman microscope, and plumbed with UHP gases was
used for the growth runs at atmospheric pressure. Flow rates were
15 sccm for argon and 100 sccm for both methane and hydrogen.
A drop of solution of [Fe3O(O2CMe)6(EtOH)3] was spin coated
onto the wafer at 3000 rpm for 40 s. The silicon wafer with catalyst
was placed in the well of the stage’s ceramic heating element. The
stage was flus◦hed with argon as the temperature was raised to
800 ◦C at 10 C.min−1. Once at temperature, the hydrogen and
methane were turned on, followed by the argon being turned off.
The growth run lasted for 15 min, after which the argon was turned
back on, and the hydrogen and methane turned off. The stage
was allowed to cool to room temperature. With these flow rates;
the formation of amorphous carbon was not observed by visual
observation of the heating element, or from significant Raman D
peak intensity. Before, during and after growth, Raman spectra
were acquired using a 514 nm Ar Ion laser with a rated power
of 150 mW. The laser power delivered to the sample is normally
4.8 mW, however this was somewhat reduced due to transmission
losses through the sapphire window of the hotstage. Spectra taken
at the growth temperature were not useful due to excessive thermal
background.
[Fe3O(O2CMe)6(EtOH)3] (1)
[Fe3O(O2CMe)6(H2O)3] (500 mg, 0.845 mmol) was suspended in
EtOH (100 mL). The solution was heated to reflux for 2 h, at which
time the solution became a deep red, and all solids dissolved. The
solution was cooled to room temperature and the volatiles were
removed in vacuo to give a brick red solid. Yield: 475 mg, 95%.
UV-vis: 203 nm and 310 nm. IR (cm−1): 3379 (m, OH), 2966 (m),
1594 [m, mas(COO−)], 1410 [s, CO2], 1345 (w), 1260 (m), 1092 (m),
1026 (m), 800 (w) and 657 (Fe3O).
[Fe3O(O2CCH2OMe)6(H2O)3][FeCl4] (2)
Acknowledgements
Methoxyacetic acid (5.87 g, 65.2 mmol) was slowly added to an
aqueous solution of sodium bicarbonate (5.46 g, 65.2 mmol). The
solution was gently heated until the effervescence (CO2) ceased.
The solution was added dropwise to an aqueous solution of
FeCl3·6H2O (8.81 g, 32.6 mmol). The reaction was stirred for 4 h
at room temperature. The solvent was removed in vacuo to give a
dark red powder. Yield: 80%, 8.40 g. UV-vis (H2O): k = 204 (e =
21157 L mol−1 cm−1) and 282 (e = 5979 L mol−1 cm−1) nm. IR:
3338 (m, OH), 2931 (m), 2831 (m), 1596 (s, CO2), 1440 (s, CO2),
1409 (s), 1198 (s, asym. OCH3), 1116 (s, sym. OCH3), 941 (m), 919
(m), 708 (m, Fe3O) cm−1.
Financial support for this work is provided by the Robert A. Welch
Foundation and the Defense Advanced Research Projects Agency
(DARPA).
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