A simple method for the continuous production of carbon nanotubes

Article abstract of DOI:10.1016/S0009-2614(00)00151-2

A simplified arc-method is presented which allows for the continuous synthesis of multi-walled carbon nanotubes. In its basic form, the method requires only a dc power supply, a graphite electrode, and a container of liquid nitrogen with no need for pumps, seals, water-cooled vacuum chambers, or purge-gas handling systems. High-quality nanotubes are produced in high yield. The system is easily adaptable to produce large quantities of nanotubes continuously without interruptions for electrode replacement, chamber cleaning, or gas purging.

Full text of DOI:10.1016/S0009-2614(00)00151-2

24 March 2000
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Chemical Physics Letters 319 2000 457–459
www.elsevier.nlrlocatercplett
A simple method for the continuous production
of carbon nanotubes
a,b
a,b
c
M. Ishigami , John Cumings , A. Zettl a,b,), S. Chen
a
Department of Physics, UniÕersity of California at Berkeley, Berkeley, CA 94720, USA
b
Materials Sciences DiÕision, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
c
National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
Received 18 November 1999
Abstract
A simplified arc-method is presented which allows for the continuous synthesis of multi-walled carbon nanotubes. In its
basic form, the method requires only a dc power supply, a graphite electrode, and a container of liquid nitrogen with no need
for pumps, seals, water-cooled vacuum chambers, or purge-gas handling systems. High-quality nanotubes are produced in
high yield. The system is easily adaptable to produce large quantities of nanotubes continuously without interruptions for
electrode replacement, chamber cleaning, or gas purging. q 2000 Elsevier Science B.V. All rights reserved.
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Carbon nanotubes 1 have unique mechanical and
that of arc-plasma growth, which necessitates a liq-
uid-cooled sealed chamber with special sliding or
bellows seals for continuously-cooled electrodes,
electrical properties that make them attractive sys-
tems for fundamental scientific studies as well as for
a wide range of applications including electron field
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complex gas handling equipment 12,19 , and time-
consuming purge cycles. After each limited synthesis
run, the chamber must be opened in order to extract
the nanotube product. Laser vaporization methods
suffer similar constraints. Even CVD methods, which
hold some promise for scalability, necessitate cum-
bersome gas handling systems, high-temperature fur-
naces, and carefully prepared consumable substrate
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emission sources 2–4 , nanoscale electronic devices
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5–8 , chemical filters and storage systems 9 , and
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mechanical reinforcements 10,11 . Various synthesis
methods have been developed for the production of
nanotubes, including carbon arc plasma growth 12–
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14 , laser vaporization 15 , and chemical vapor de-
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position CVD 16–18 . While these methods can
produce high-quality nanotubes in yields suitable for
limited research use, no method has been developed
for the production of high-quality tubes which is
continuous and easily adaptable to industrial produc-
tion levels. The most common synthesis method is
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catalysts 18 .
Here we describe a simplified carbon-arc nan-
otube synthesis method that eliminates nearly all of
the complex and expensive machinery associated
with conventional nanotube growth techniques. In-
deed, all that is required is an insulated bucket full of
liquid nitrogen, graphite electrodes, and a dc power
supply. The reaction can run in a continuous fashion
and can be scaled up for industrial production levels.
)
Corresponding author. Fax: q1-510-643-8497; e-mail:
azettl@physics.berkeley.edu
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
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PII: S0009-2614 00 00151-2

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M. Ishigami et al.rChemical Physics Letters 319 2000 457–459
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provides a protective oxygen-free environment as
well as being the source of the buffer gas during the
arcing process. The liquid nitrogen further maintains
the necessary temperature gradient within the plasma.
All complex vacuum seals are eliminated, including
the typically troublesome sliding seals around the
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electrodes. Finally, the nanotube products which
have been shown to be extremely air-sensitive after
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synthesis 20,21 are easily maintained and trans-
ported in an inert environment following production.
During operation of our prototype system using a
1r4" graphite anode and 1r2" graphite cathode,
multi-walled nanotubes were produced using a dc
current of 60 A at 20–25 V; the arc was extremely
stable. The arc reaction consumed ;0.2 l of liquid
nitrogen per minute of operation. The total yield of
nanotube-rich material was 44 mg per minute per
cm2 of anode surface area, a comparable per unit
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anode area yield to that of an optimized conven-
tional but unscalable arc reaction 12–14 .
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To characterize the nanotubes produced by our
synthesis method, the raw nanotube-containing prod-
uct was crushed and sprinkled onto transmission
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electron microscope TEM grids and imaged with a
high-resolution TEM operating at 200 keV. Multi-
walled nanotubes were omnipresent on the grids with
high relative yield. Fig. 2 shows a representative
Fig. 1. Schematic of continuous production liquid-nitrogen reac-
tion chamber for nanotube synthesis.
High-quality multi-walled carbon nanotubes are pro-
duced at high production rates.
Fig. 1 shows a schematic of the synthesis appara-
tus. To grow nanotubes, a graphite anode is dipped
into an open container of liquid nitrogen containing a
short non-consumable copper or graphite cathode.
The electrodes are momentarily brought into contact
and an arc is struck. Nanotubes formed in the arc-
plasma region drop and collect on the bottom of the
bucket. For continuous operation, the bottom of the
bucket is funnel-shaped and sealed with a valve
which opens automatically and periodically to flush
nanotubes from the vessel. In this configuration, the
graphite anode rods are threaded one into another
and continuously fed through a simple sliding elec-
trical contact. The level of liquid nitrogen in the
insulated vessel is automatically maintained using a
level sensor and fill pipe. This production scheme
allows for nanotube synthesis at a 100% duty cycle.
The advantages of this simplified design are many.
No gas purging is required, as the liquid nitrogen
Fig. 2. High-resolution transmission electron microscope image of
a multi-walled carbon nanotube produced by the liquid-nitrogen
arc reaction method.

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M. Ishigami et al.rChemical Physics Letters 319 2000 457–459
459
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nanotubes were of high quality with 4–8 layers and
with long and straight parallel walls with only occa-
sional surface contamination. In fact, the tubes grown
using our liquid-nitrogen method appear to have
consistently cleaner surfaces than tubes grown using
other methods. We further characterized the stoi-
chiometry of our nanotubes using electron energy
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This work was supported in part by the National
Science Foundation under Grant Nos. DMR-9501156
and DMR-9801738 and by the Director, Office of
Energy Research, Office of Basic Energy Sciences,
Material Sciences Division of the US Department of
Energy, under Contract Number DE-AC-03-
76SF00098. M.I. acknowledges the support of the
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