Selective growth of SWNTs on partially reduced monometallic cobalt catalyst

Article abstract of DOI:10.1039/c0cc02751k

SiO₂ supported cobalt (Co) catalyst could be partially reduced and anchored by unreduced Co ions during a carbon monoxide (CO) chemical vapor deposition (CVD) process. This resulted in the formation of sub-nanometre metallic Co clusters catalyzing the growth of single-walled carbon nanotubes (SWNTs) with a narrow diameter distribution.

Full text of DOI:10.1039/c0cc02751k

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Selective growth of SWNTs on partially reduced monometallic
cobalt catalyst
a
b
b
b
Maoshuai He,* Alexander I. Chernov, Pavel V. Fedotov, Elena D. Obraztsova,
a
c
d
c
c
Emma Rikkinen, Zhen Zhu, Jani Sainio, Hua Jiang, Albert G. Nasibulin,
ce
a
a
Esko I. Kauppinen,* Marita Niemela and A. Outi I. Krause
¨
Received 23rd July 2010, Accepted 4th November 2010
DOI: 10.1039/c0cc02751k
SiO
and anchored by unreduced Co ions during a carbon monoxide
CO) chemical vapor deposition (CVD) process. This resulted in
2
supported cobalt (Co) catalyst could be partially reduced
size of the Co nanoparticles was well controlled by the
3
,5
occlusion of the MCM-41 framework.
Nevertheless, the
(
use of an expensive MCM-41 mesoporous material as a
template limits the scale-up potential of SWNT production.
To develop an economically viable method, it is desirable to
develop a monometallic catalyst on an ordinary support for
synthesizing high quality SWNTs with only a few species.
Atomic layer deposition (ALD) allows the production of a
the formation of sub-nanometre metallic Co clusters catalyzing
the growth of single-walled carbon nanotubes (SWNTs) with a
narrow diameter distribution.
SWNTs are very attractive one-dimensional materials with a
variety of potential applications, such as electron-field
emission, electrochemical probes, membranes for microfluidic
7,8
catalyst on
a
support with better dispersion
than
9
impregnation or co-precipitation techniques. The reason for
the high dispersion is that the catalyst precursor is attached to
the support via a chemical reaction with the surface functional
groups. For example, Haller et al. have synthesized Co grafted
SBA-15 through the ALD technique for growing high quality
1
devices, and nanoelectronic devices. The performance of
SWNT-based materials depends greatly on the diameter and
chirality of the nanotube. The ultimate goal in carbon
nanotube production is to find a catalytic system that would
2
7
allow for the growth of SWNTs of a single helicity. Significant
SWNTs at high temperatures. It was also discovered that the
work has been conducted to achieve this goal in the area of
3
Co ions were substituted for silicon and formed a tetrahedral
7
–6
9
selective growth,
which is aimed at lowering the reaction
2 4
structure, which could be the surface cobalt silicate (Co SiO ).
temperature of a specific catalyst to obtain a narrow chirality
distribution of SWNTs. Co-based catalysts were usually
selected due to the possibility of forming cobalt carbonyl-like
This enhanced the metal–support interaction and provided a
good yield in the SWNT synthesis even on a catalyst prepared
9
by the impregnation technique. Nevertheless, due to the
3
–5
species which act as precursors for growing SWNTs in a CO
CVD process. To grow SWNTs, it is first necessary to fully
necessarily high catalyst pre-reduction and lack of anchor site,
the use of these catalysts did not lead to the selective growth of
7
,9
reduce the catalyst in a hydrogen (H
2
) atmosphere. However,
SWNTs.
In this work, we demonstrate the low temperature growth of
SWNTs on Co grafted SiO (Grace 432 silica, 0.5–1.0 mm
during the reduction, the Co species are highly mobile and tend
to form large particles which result in the growth of multi-
walled carbon nanotubes (MWNTs). To constrain the mobility
2
2
À1
particle size, surface area 320 m g ) catalyst deposited by the
8
ALD technique. The precursor used for the deposition was
of the Co species on the support, either a MoO
process) or a MCM-41 mesoporous support has to be
x
(CoMoCAT
3
cobalt(III) acetylacetonate (Co(acac) , Aldrich, 98%). The
3
–5
introduced.
entire ALD process was carried out at a reduced pressure
(6–10 kPa). Prior to deposition, the silica support was
preheated in the ALD reactor at 400 1C for 5 h in a nitrogen
However, the introduced MoOx will finally result in the
formation of an Mo C impurity which is difficult to remove
2
4
from the SWNTs produced. Instead, Co incorporated in
3
atmosphere. The Co(acac) precursor was then evaporated at
MCM-41 has recently proven to be a good template for the
growth of SWNTs with a narrow diameter distribution: the
190 1C and passed through the silica bed. After deposition for
6 h and flushing with nitrogen, the catalyst was annealed at
4
50 1C with air for 4 h to remove the acac-ligands. The catalyst
6
thus produced was thereafter introduced into a CVD set-up
for growing SWNTs. The catalyst was heated to the desired
temperature (500 1C to 900 1C) under an argon (Ar) flow
a
Department of Biotechnology and Chemical Technology, School of
Science and Technology, Aalto University, PO Box 16100, FI-00076
Aalto, Finland. E-mail: maoshuai.he@hut.fi; Fax: +358 947022622;
Tel: +358 947022874
A.M. Prokhorov General Physics Institute RAS, 38 Vavilov Street,
119991 Moscow, Russia
NanoMaterials Group, Department of Applied Physics and Center for
New Materials, School of Science and Technology, Aalto University,
PO Box 15100, FI-00076 Aalto, Finland.
3
À1
(
50 cm min ) at atmospheric pressure. After reaching the
3
b
À1
desired temperature, CO (50 cm min ) replaced Ar and the
growth process lasted for 10 min.
c
2
The Co concentration on the SiO support is 8.0 atomic
percent, which was determined by X-ray photoelectron
spectroscopy (XPS, ESCA SSX-100). As demonstrated in the
10
E-mail: esko.kauppinen@hut.fi; Fax: +358 947023517;
Tel: +358 405098064
Department of Applied Physics, School of Science and Technology,
Aalto University, PO Box 11100, FI-00076 Aalto, Finland
VTT Biotechnology, PO Box 1000, FI-02044, Espoo, Finland
d
e
previous studies, X-ray diffraction patterns of the catalyst
showed very weak reflections which suggests the possible
presence of small particles. The extent of reduction is
This journal is ꢀc The Royal Society of Chemistry 2011
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Fig. 3 (a) Raman spectra (632.8 nm) of the as-synthesized SWNTs
grown at a temperature of 500 1C, 550 1C, 600 1C, 650 1C and 700 1C.
All the spectra were normalized with G intensity. (b) TEM image of
SWNTs grown at 600 1C.
Fig. 1
H
2
2
-TPR pattern of Co–SiO synthesized by ALD from
Co(acac)
3
as a precursor.
temperature (6, 22 and 23 for carbon nanotubes grown at
500 1C, 550 1C and 700 1C, respectively). Carbon nanotube
samples grown at 600 1C were finely ground using an agate
mortar and collected on a carbonized copper grid for
transmission electron microscope (TEM, Philips CM-200FEG)
characterization, besides SWNTs and small diameter catalyst
particles, no other form of carbon was observed (Fig. 3b).
The diameter of SWNTs grown at different temperatures was
evaluated by the radial breathing mode (RBM) frequencies
in the Raman spectra. For SWNTs grown at 700 1C,
relatively low at a temperature below 700 1C, as estimated by a
temperature programmed reduction (TPR) profile (Fig. 1)
2
which was obtained from monitored H consumption by a
Hiden mass spectrometer (AMI-200R catalyst characterization
system). The low extent of reduction obtained is in agreement
8
with our previous studies, wherein the degree of reduction of
2 2
Co/SiO catalyst reduced at 550 1C in H was ranging from 4%
to 28% depending on the content of Co. Meanwhile, the TPR
pattern shows that the reduction started at around 500 1C and
continued to well above 800 1C implying a possible formation of
À1
the relative intensity of the RBM centered at 190 cm
12
9
,10
Co SiO during the catalyst preparation process.
2
Based on
(1.26 nm, o = 223.5/d + 12.5) is relatively high. When the
reaction temperature is lowered, the intensity of the RBM peak
4
11a
the binary CoO and SiO phase diagram, the formation of
2
À1
À1
Co SiO can only start to occur at around 900 1C. However,
at 190 cm decreases gradually, and the RBM peaks centered
À1
2
4
1
1b
considering the small size of Co and the presence of H O
2
at 280 cm ((7,5) tube) and 290 cm ((8,3) tube) become much
which results from the decomposition of the Co(acac)
formation of surface Co SiO is highly plausible during the
catalyst preparation process.
3
, the
more dominant. Meanwhile, the intensities of intermediate
3
frequency modes, which correlate with small diameter SWNTs,
2
4
become significant in the Raman spectra of samples grown at low
temperatures.
It was found that carbon nanotubes can grow on the Co
catalyst in a wide temperature window (500 1C–900 1C). Fig. 2
displays scanning electron microscope (SEM, Leo Gemini 982)
images of the carbon nanotubes grown at temperatures
ranging from 550 1C to 700 1C. Carbon nanotubes, with high
density, can be observed on samples grown in this temperature
range (Fig. 2). Carbon nanotubes can even grow at 500 1C, but
the density is rather low. This observation is in agreement with
the Raman spectroscopy (He–Ne laser, 632.8 nm, JY LabRam
UV-vis-NIR absorption spectroscopy (Perkin Elmer
Lambda 950) was also used to characterize the suspensions
of SWNTs grown at different temperatures. To disperse
SWNTs, as-synthesized SWNTs were mixed with sodium
cholate aqueous solution (2 wt%) and sonicated at 80 W (tip
sonicator) for 60 min, the dispersions were centrifuged at
2
50 000g for 60 min to remove the SiO support and metallic
particles (the centrifugation does not affect the SWNT
diameter distribution). Only a few SWNT species, such as
(6,4), (6,5), (7,5), (8,3), (8,4) and (7,6) were observed in the
suspension of SWNTs grown at 600 1C (Fig. 4a). This is well in
agreement with the photoluminescence (PL) excitation
3
00) characterization result (Fig. 3a), which shows a high D
mode intensity for carbon nanotubes grown at 500 1C. The
intensity ratio of G mode to D mode, which is related to the
quality of carbon nanotubes, increases with increasing growth
Fig. 4 (a) UV-vis-NIR absorption spectra of sodium cholate dispersed
SWNTs grown at a temperature of 600 1C and 700 1C. M11, S22 and S11
represent the lowest metallic, second lowest semiconducting and lowest
semiconducting absorption, respectively. (b) The contour plot of
normalized photoluminescence emission intensities under the various
excitation energies for SWNTs grown at 600 1C.
2
Fig. 2 SEM images of SWNTs grown on Co–SiO catalyst at different
temperatures: (a) 550 1C, (b) 600 1C, (c) 650 1C and (d) 700 1C. The
scale bars are 500 nm in all images.
1
220 | Chem. Commun., 2011, 47, 1219–1221
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In summary, we have demonstrated the selective growth of
SWNTs on partially reduced monometallic Co catalyst with
low temperature CVD. Because of the modest reduction
temperature of the catalyst, the catalyst could be partially
reduced by CO and anchored on the surface by unreduced
Fig. 5 Schematic illustration of SWNT growth on a partially reduced
Co–SiO catalyst.
2
Co ions without a high temperature H pre-reduction process.
2
As-formed sub-nanometre Co clusters can thus catalyze the
growth of small diameter SWNTs with a narrow diameter
distribution. Although ALD is not an efficient technique for
the preparation of catalyst in large quantities, similar catalyst
could be prepared using impregnation or co-precipitation
techniques by selecting proper precursors and a subsequent
calcination process.
mapping (Horiba Jobin-Yvon NanoLog system with InGaAs
CCD detection) result (Fig. 4b). The most bright PL peak
corresponds to the (6,5) nanotubes, and its intensity is two
times higher than that of the second brightest peak
corresponding to (7,5) tubes. Compared to the SWNTs
1
3
grown on CoMoCAT at ambient pressure,
the
monometallic Co catalyzed SWNTs consist of much smaller
diameter species. It is noteworthy that although predominant
small diameter (6,5) tubes were produced in the Co–MCM-41
This work was supported by TEKES, Academy of Finland
(
Project Nos 128445 and 128495), CNB-E project in Aalto
University through the Multidisciplinary Institute of
Digitalization and Energy (MIDE) program, European FP6
Project 033350, Project RFBR-09-02-91076 and Project
ROSNAUKA 02.523.12.3020. We thank Dr Sanna
Airasinen, MSc Kati Vilonen and MSc Sonja Kouva for
their help in preparing this manuscript and experimental
support.
3
SWNTs and commercial grade CoMoCAT SWNTs, it was
1
indeed the applied high pressure during the growth processes
4
that induced the growth of small diameter SWNTs.
3
,5,6
As reported in the previous work,
the SWNT diameter
shifts towards a large value and the diameter distribution gets
broad with increasing growth temperature. From the
comparison of absorption spectra between SWNTs grown at
6
00 1C and those grown at 700 1C (Fig. 4a), it can be clearly
Notes and references
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absorption spectra of SWNTs grown at 700 1C.
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1
6
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1
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Chem. Commun., 2011, 47, 1219–1221 | 1221