Growth of carbon nanowires and nanotubes via ultrarapid heating of ethanol vapor

Article abstract of DOI:10.1023/A:1023271531066

A method is proposed for producing carbon nanomaterials by ultrarapid heating of vapors of organic compounds to high temperatures, and an experimental setup for implementing this method is described. The starting reagents used are ethanol and mixtures of ethanol with water, glycerol, and ferrocene. At heater temperatures of 1500-2000°C and substrate temperatures of 600-1000°C, carbon nanowires and nanotubes are obtained. The nanowires attain 100 μm in length and range in thickness from 30 to 150 nm. The nanotubes have bamboo or fish-bone structures, with a thickness from 20 to 50 nm. The deposition of nanotubes on supported catalysts (iron, nickel, gold, and others) is also examined. It is shown that, under certain conditions, selective deposition of carbon nanotubes (nanowires) onto catalyst-coated parts of the substrate is possible.

Full text of DOI:10.1023/A:1023271531066

Inorganic Materials, Vol. 39, No. 4, 2003, pp. 353–356. Translated from Neorganicheskie Materialy, Vol. 39, No. 4, 2003, pp. 433–437.
Original Russian Text Copyright © 2003 by Red’kin, Malyarevich.
Growth of Carbon Nanowires and Nanotubes
via Ultrarapid Heating of Ethanol Vapor
A. N. Red’kin and L. V. Malyarevich
Institute of Microelectronics Technology and High-Purity Materials, Russian Academy of Sciences,
Chernogolovka, Moscow oblast, 142432 Russia
e-mail: arcadii@ipmt-hpm.ac.ru
Received July 4, 2002; in final form, October 28, 2002
Abstract—A method is proposed for producing carbon nanomaterials by ultrarapid heating of vapors of
organic compounds to high temperatures, and an experimental setup for implementing this method is described.
The starting reagents used are ethanol and mixtures of ethanol with water, glycerol, and ferrocene. At heater
temperatures of 1500–2000°C and substrate temperatures of 600–1000°C, carbon nanowires and nanotubes are
obtained. The nanowires attain 100 µm in length and range in thickness from 30 to 150 nm. The nanotubes have
“bamboo” or “fish-bone” structures, with a thickness from 20 to 50 nm. The deposition of nanotubes on sup-
ported catalysts (iron, nickel, gold, and others) is also examined. It is shown that, under certain conditions,
selective deposition of carbon nanotubes (nanowires) onto catalyst-coated parts of the substrate is possible.
INTRODUCTION
work is to examine the potentialities of the proposed
method with application to the pyrolysis of ethanol
vapor and ethanol-based mixtures.
After the discovery of carbon nanotubes in 1991,
considerable effort has been concentrated on the devel-
opment of new approaches to producing nanotubes and
optimization of the known methods. One of the most
common and effective methods for producing nano-
tubes is arc vaporization [1]. The main drawback to this
method is the formation of various carbon structures
and carbonized catalyst particles, with a content of nan-
otubes no higher than a few tens percent. The separa-
tion of nanotubes from such mixtures and their purifi-
cation requires much effort and time. Moreover, this
method is unsuitable for growing uniform layers of car-
bon nanotubes on substrate surfaces.
1
5
2
4
3
In recent years, many studies have been devoted to
nanotube growth via high-temperature (as a rule, cata-
lyzed) decomposition of carbon compounds—acety-
lene [2], methane [3], carbon monoxide [3, 4], and oth-
ers [5]. Typical catalysts are transition metals (often,
iron-group metals) and their compounds [2–6]. How-
ever, classical pyrolysis with the use of a flow reactor
and external heater (furnace) is of limited application
because the process cannot be run continuously, the
dimensions of the substrate are limited by those of the
reactor, and the deposition temperature is, as a rule, no
higher than 1000–1100°C. Moreover, since the reactor
is heated throughout its length, the reaction mixture
cannot be heated or cooled rapidly. At the same time,
the factors in question are known to be of key impor-
tance in the formation of carbon nanomaterials [7].
Here, we describe a pyrolytic method which allows the
above limitations to be overcome. The purpose of this
6
Fig. 1. Experimental setup for the preparation of carbon
nanomaterials: (1) quartz reactor, (2) graphite heater,
(3) source of the starting reagent, (4) current leads,
(5) glassy carbon bell jar, (6) current source.
0020-1685/03/3904-0353$25.00 © 2003 MAIK “Nauka/Interperiodica”

354
RED’KIN, MALYAREVICH
EXPERIMENTAL
The experimental setup is schematized in Fig. 1.
The heater temperature was varied from 1000 to
2000°C. As the starting reagent, we used ethanol and
mixtures of ethanol with water, glycerol, and ferrocene.
The reaction mixture is heated by a graphite rod 50 mm The reaction products were characterized by differen-
tial thermal analysis (DTA), x-ray diffraction (XRD),
and scanning and transmission electron microscopy. In
DTA, we used an RIF-5 precision temperature control-
ler. The heating rate was 2°C/min. Analyses were car-
ried out in flowing air (50 ml/min), which was delivered
through a thin tube. The morphology of the deposits
was examined on a JEOL JSM-840A scanning electron
microscope (SEM). Their structure was analyzed on a
JEOL JEM-2000FX transmission electron microscope
(TEM).
in length and 5 mm in diameter, which is introduced
into the reactor. This allows the thermal-activation zone
to be separated from the deposition zone and the pro-
cess temperature to be raised substantially (as com-
pared to conventional pyrolytic methods), without rais-
ing the substrate temperature, which may even be
reduced. Thus, pyrolysis occurs in a high-temperature
zone near the heater. The reaction products either con-
dense on the substrate, situated near the heater, or leave
the reaction zone, together with the gas evolved.
Another distinguishing feature of the method is
ultrarapid heating of the reagent vapor, which is
ensured by minimizing the separation between the
cooled source of the starting reagent and the heater.
Carbon nanomaterials were produced with or with-
out catalysts. Some of the reaction mixtures studied
contained ferrocene as a catalytic additive. As sup-
ported catalysts, we used iron, nickel, cobalt, gold, or
Fe0.7Cr0.2Ni0.1 films 100–200 Å in thickness. The films
2 µm
1 µm
(a)
(b)
(c)
Fig. 2. SEM images of carbon nanomaterials produced by ultrarapid heating of ethanol vapor.
INORGANIC MATERIALS Vol. 39 No. 4 2003

GROWTH OF CARBON NANOWIRES AND NANOTUBES VIA ULTRARAPID HEATING
RESULTS AND DISCUSSION
355
High-temperature reactions typically yield a mix-
ture of different carbon structures. The composition of
such mixtures can be determined by DTA because the
oxidation temperature in oxygen depends on the carbon
structure [8, 9]. Since the process is accompanied by
heat release, the oxidation temperature is easy to deter-
mine by DTA and can be used to identify the carbon
structure.
1
To ensure proper interpretation of DTA results, we
first analyzed fullerene samples, graphite powder, fine-
particle diamond powder, their mixtures, and noncrys-
talline carbon (activated carbon and fine-particle glassy
carbon). Under the conditions of this study, the oxida-
tion temperature (maximum peak temperature) was
520–530°C for fullerene, 730–760°C for graphite, and
920–930°C for diamond. Noncrystalline carbon was
found to oxidize in the range 400–600°C.
Analysis of the bulk products obtained from ethanol
vapor and a 1 : 1 mixture of water and ethanol at heater
temperatures above 1200°C indicates the presence of
noncrystalline carbon and a phase close in oxidation
temperature to graphite. The DTA curves of the mate-
rial deposited on a graphite substrate near the heater
show only one peak corresponding to an oxidation tem-
perature of 740–770°C. XRD examination reveals the
2
20 nm
Fig. 3. TEM image of carbon nanotubes with (1) bamboo
and (2) fish-bone structures produced by ultrarapid heating
of ethanol vapor.
were deposited by thermal evaporation in a VUP-5 vac-
uum unit. Some of the samples were oxidized in air at
550°C for 5 h. The substrates were made of graphite or
silicon.
1 µm
Fig. 4. Selective deposition of carbon nanotubes on a silicon substrate; the process was catalyzed by a 100-Å-thick Fe Cr Ni
0.7 0.2 0.1
film preoxidized in air at 550°C.
INORGANIC MATERIALS Vol. 39 No. 4 2003

356
RED’KIN, MALYAREVICH
presence of graphene planes. Therefore, it seems likely ducing carbon nanomaterials. Further, more in-depth
that the deposit consists of multiwalled carbon nano- studies are planned to assess the effects of the tempera-
tubes and nanowires, the closest analogues of graphite. ture, reaction medium, catalyst, and other factors on the
effectiveness of the process and the phase composition
and structure of the deposit.
SEM examination demonstrates that the deposits on
graphite substrates do have a fibrous structure (Fig. 2a).
The nanowire length may attain several tens of microns,
depending on the process duration. The growth rate of
nanowires is 1–3 µm/min. Occasionally, we observe
secondary nanowire growth, leading to the formation of
garlandlike structures (Fig. 2b). The introduction of fer-
rocene vapor into the reaction mixture notably changes
the deposit structure: the fibrous material becomes
denser, and some nanowires are terminated with nano-
particles consisting of a material different from the
deposit material (judging from the electron-micro-
scopic contrast). The nanoparticles are close in diame-
ter to the nanowires and seem to consist of iron metal or
iron carbide (Fig. 2c).
The structure of the nanomaterials was studied in
greater detail by TEM. The samples were found to con-
sist of both nanowires and nanotubes. The nanowire
thickness ranges from 30 to 150 nm. The nanotubes
have “bamboo” or “fish-bone” structures (Fig. 3) with
an outer diameter of 20–50 nm. Note that, according to
electron-microscopic results, the samples contained
insignificant amounts of nonfibrous material.
We also examined the potential of the proposed
method for depositing a layer of nanotubes on sup-
ported catalysts. The process was found to be catalyzed
by thin films of iron, nickel, cobalt, and their alloys.
The catalytic effect was far more pronounced if the cat-
alyst film was thermally oxidized in air. Gold was
found to exhibit the highest catalytic activity. This find-
ing is of considerable interest because the catalytic
properties of the iron-group metals are well known,
whereas little or no data have been reported on the cat-
alytic activity of gold in the growth of carbon nanoma-
terials. Copper and aluminum films showed very low or
no catalytic activity.
The catalytic effect of metals is well illustrated by
the selective deposition of nanotubes on a silicon sub-
strate coated with a patterned metal film: no nanostruc-
tures are formed on the uncoated parts of the substrate.
An example of such a structure is shown in Fig. 4. The
inset displays the fibrous structure of the deposit at an
increased magnification.
ACKNOWLEDGMENTS
We are grateful to I.I. Khodos and S.V. Dubonos for
performing the electron-microscopic examination of
the samples.
This work was supported by the Russian Academy
of Sciences through the Nanomaterials and Supramo-
lecular Systems Integrated Program.
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The present results provide convincing evidence
that the proposed method has great potential for pro-
INORGANIC MATERIALS Vol. 39 No. 4 2003