Linear carbon allotrope - Carbon atom wires prepared by pyrolysis of starch

Article abstract of DOI:10.1016/j.cplett.2004.01.007

A new method is reported to produce linear carbon allotrope from the pyrolysis of starch catalyzed by Fe. The pyrolytic product termed as carbon atom wires (CAW) is composed of winding lines with the diameter around 2.0 ?, indicated by magnified HRTEM images. The experimental results of UV and Raman spectra revealed a conjugated sequence of cumulated double bonds (=C=C=)_n presented in the hexane extract of CAW. Arguments about the sp hybridization bonding structure of CAW can also be supported by EELS and FT-IR measurements.

Full text of DOI:10.1016/j.cplett.2004.01.007

Chemical Physics Letters 385 (2004) 477–480
www.elsevier.com/locate/cplett
Linear carbon allotrope – carbon atom wires prepared by pyrolysis
of starch
a,
a
a
a
a
Kuan-Hong Xue *, Fei-Fei Tao , Wei Shen , Chun-Jian He , Qiao-Ling Chen ,
b
Li-Jun Wu , Yi-Mei Zhu
b
a
Chemistry Department, Nanjing Normal University, 122 NingHai Road, Nanjing 210097, China
b
Brookhaven National Laboratory, Upton, NY 11973, USA
Received 6 November 2003; in final form 7 January 2004
Published online:
Abstract
A new method is reported to produce linear carbon allotrope from the pyrolysis of starch catalyzed by Fe. The pyrolytic product
ꢀ
termed as carbon atom wires (CAW) is composed of winding lines with the diameter around 2.0 A, indicated by magnified HRTEM
images. The experimental results of UV and Raman spectra revealed a conjugated sequence of cumulated double bonds (@C@C@)n
presented in the hexane extract of CAW. Arguments about the sp hybridization bonding structure of CAW can also be supported by
EELS and FT-IR measurements.
Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction
Here, we report a new method capable of massively
producing linear carbon allotrope from pyrolysis of
starch.
Among all elements, carbon constructs the greatest
varieties of compounds in the world. It is well known
that carbon allotrope can be bonded with sp3 (diamond)
or sp2 (graphite, spheroidal fullerenes [1,2] and fullerene
nanotubes, including multi-wall [3] and single-wall na-
notubes [4,5]) hybridization. Another allotropic form of
carbon based on sp hybridization was produced from
graphite sublimation [6] in 1969 and synthesized from
the oxidative couplings (Hay techniques) of silyl-
protected alkynes [7] in 1972. Since then, intensive re-
searches have been performed on its preparation [8–10],
spectroscopy [11,12] and calculation [13]. Some reviews
[14–17] outlined the advancement on the subject. Al-
though the linear structure of sp-bonded carbon allo-
trope is fascinating and attractive, and the allotrope was
synthesized as early as in 1960s, the properties and ap-
plications of the linear carbon are not well revealed due
to difficulties of its instability and massive preparation.
The discovery happened in our efforts to prepare
carbon nanotubes from solid carbonaceous species. In
our experience, the diameter of carbon nanotubes pro-
duced depends upon the size of catalyst used during a
pyrolytic process of carbonaceous species. When we tried
our best to make the catalyst more and more finely and
homogeneously distributed in carbonaceous species and
in the meantime to prevent it from aggregation in the
preparation process, we found less and less carbon na-
notubes in the sight of transmission electron microscope
(TEM) of products with our efforts, instead, appearing
more and more cloudy pieces whose morphology could
not be established in the magnification of 5ꢀ104 times.
When the sample was examined with a high resolution
transmission electron microscope (HRTEM), it was
found that the cloudy pieces were of fine structures.
The terms of ÔcarbyneÕ, ÔpolyyneÕ, ÔalkynesÕ and Ôpol-
iynesÕ are often used to report the researches of the
carbon allotropy with sp-bonded structure in literatures.
To high-light the characteristic morphology of the linear
shape and of the diameter size on atomic scale, and to
* Corresponding author. Fax: +86-25-83598448.
E-mail address: khxue@njnu.edu.cn (K.-H. Xue).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2004.01.007

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K.-H. Xue et al. / Chemical Physics Letters 385 (2004) 477–480
avoid the argument on if the linear chain is constructed
in cumulated double bonds (@C@C@)n or in the alter-
native single and triple bonds (–CBC–)n, it is better to
term our product as carbon atom wires (abbreviated to
CAW later on in the Letter) rather than as the nomen-
clatures mentioned above in the literatures. Carbon
atom wires can exist stably in the forms capped by hy-
drogen atoms or other radicals.
To have a sample with narrow distribution of carbon
chain length, the product was mixed with hexane and
heated in reflux to extract small molecules of CAW.
Filtrate of the mixture was used to measure its UV and
Raman spectra on Perkin–Elmer LAMBDA 17 and on
OLYMPUS BX41 JOBIN YVON RAMAN DIVI-
SION, respectively. The proportion, soluble in hexane,
is about 0.35 wt% in the product purified by HCL. This
result does not mean that the pyrolytic product con-
tained so little CAW, but implies that the most part of
CAW possess long carbon chains. The sample for FTIR,
HRTEM and EELS measurements is the whole product,
not the small molecules of CAW in the hexane extractive
only. The FTIR spectrum was taken by the spectro-
graph Nexus 670 FT-IR (Nicolet) with 32 scans and the
resolution of 4 cmꢁ1. A CAW sample was ground with
100 times its bulk of pure potassium bromide and
pressed into a disk for the measurement. To free from
moisture, the sample and KBr used were dried in a
vacuum oven at 150 °C for 48 h.
The JEOL 3000F TEM was equipped with a field
emission gun and a Gatan energy filter and EELS
spectra were acquired in diffraction mode at the accel-
erating voltage of 300 kV, with the collection angle of 4
mrad and 0.2 eV per pixel. Common chemical methods
[18] were used to determine the quantities of alcoholic
hydroxyl group –OH by the acetic anhydride method
catalyzed by pyridine and the carboxylic acid group –
COOH by neutralization with aqueous NaOH solution.
2. Experimental
Ten grams potato starch as a carbonaceous species
used in a pyrolytic process to produce carbon atom
wires (CAW) was mixed homogeneously with 2.12 g
aqueous solution of 4.0 wt% Fe(NO3)3, i.e., in the mass
ratio of starch:Fe ¼ 1:2 ꢀ 10ꢁ3. The pyrolysis took place
in a tube-furnace, isolated from atmosphere, under a
stream of Ar/H2 mixture (Ar 80 mL/min, H2 40 mL/
min) at the reaction pressure of 0.2 MPa. The reaction
temperature was controlled in steps by a programmer,
first, heating in the rate of 15 °C/min up to 500 °C and
keeping the temperature for 6 h to reduce Fe(NO3)3 into
Fe, then, raising the temperature in the same heating
rate up to 800 °C, keeping it constant again for another
16 h to pyrolyze starch, and at last, cooling to the room
temperature in the furnace. In the last 6 h at 800 °C, the
gas stream contained only Ar gas, while H2 was stopped
adding into the reactor. About 1.2 g product was ob-
tained and purified with 6 M HCl at the reflux temper-
ature for 6 h to remove the catalyst Fe.
To determine the quantities of various elements such
as C, O, H and Fe, the measurements of X-ray photo-
electron spectroscopy (XPS) and elemental analysis were
performed on PHI 550 ESCA/SAM electron (energy)
spectroscopy, Perkin–Elmer 240C and an inductively
coupled plasma (ICP) atomic emission spectrometer
Jorrell-Ash Model J-A1100, respectively.
3. Results and discussion
Fig. 1 is of HRTEM photographs at different areas of
carbon atom wires prepared from the pyrolysis of po-
tato starch, in which the winding lines appear to have
the same thickness, around 2 A. Considering their
thickness value being comparable to the size of a single
ꢀ
Fig. 1. HRTEM photographs at different areas of carbon atom wires prepared from the pyrolysis of potato starch.

K.-H. Xue et al. / Chemical Physics Letters 385 (2004) 477–480
479
carbon atom, we term the winding lines shown in Fig. 1
as carbon atom wires.
An UV spectrum of the hexane extract of CAW is
shown in Fig. 2. The bands around 221, 199 and 272.5
nm indicate that the carbon atom numbers of the most
wires dissolved in hexane are less than ten [7,13]. Fig. 3
is of a Raman spectrum of the extractive. The peaks of
971, 999.5, 1077.5 and 1506 cmꢁ1 correspond to the
stretching frequency of @C@C@ [17,19], but there is no
apparent peak around 2100 cmꢁ1 (outside the range
shown in the figure) corresponding to the stretching
frequency of –CBC– [17] observed. Although the linear
sp-hybridized carbon allotropy can be depicted as a
sequence of cumulated double bonds (@C@C@)n or as a
chain of conjugated triple bonds (–CBC–)n, the former
is presumably the preferred bonding configure when the
carbon atom number of a chain is less than 10 [16]. The
experimental results of UV and Raman spectra revealed
that the most portion of carbon atom wires dissolved in
hexane consisted of short chains with carbon atom
numbers less than 10.
In Fig. 4a, the peak of 4.6 eV and the should of 3.4 eV
that are characteristic of the strong p–pꢂ interband
transition and p-interband plasmon, and the peak of
Fig. 4. The low loss region (a) and carbon K-edge (b) EELS spectra
from carbon atom wires.
23.6 eV and the shoulder of 20.2 eV that arise from
plasma resonance of all valence electrons (p þ r plas-
mon) can be found and are shifted to the lower energy
values than the corresponding values of graphite, as the
case of carbynes and carbynoid samples [17,20]. EELS
spectrum of carbon K-edge shown in Fig. 4b is also
characteristic of carbyne rather than graphite or dia-
mond [17,20].
An IR absorption spectrum of CAW produced from
potato starch is shown in Fig. 5. The sample measured
by FTIR is the purified product itself, not its hexane
extract as the sample for Raman measurement, con-
taining long carbon atom wires. Thus, a broad absorp-
tion around 2010 cmꢁ1 corresponding to the stretching
vibration of CBC in the chain of conjugated triple
bonds (–CBC–)n which does not appear on the Raman
Fig. 2. An UV spectrum of the hexane extract of carbon atom wires
prepared by pyrolysis of potato starch.
Fig. 3. A Raman spectrum of hexane extract of carbon atom wires
prepared by pyrolysis of potato starch.
Fig. 5. An IR absorption spectrum of carbon atom wires produced
from potato starch.

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K.-H. Xue et al. / Chemical Physics Letters 385 (2004) 477–480
spectrum and the peaks around 1400–1570 cmꢁ1 due to
various carbon skeletons such as cumulene and polyene
structure [17,21–23] can be observed. The low intensity
of the broad absorption around 2010 cmꢁ1 implies the
highly conjugated (–CBC–)n structure in the product.
As to the other bands, we refer the absorption around
1110 cmꢁ1 to C–C and C–O stretching vibration, the
band around 3440 cmꢁ1 to O–H stretching vibration,
the band around 1720 cmꢁ1 to C@O stretching vibra-
tion, and the band around 800 cmꢁ1 to the skeletal vi-
bration of alkyl group –CH3, the deformation frequency
and Ôout-of-planeÕ vibration of the alkenyl group @CH2
and OC–OH group [24,25]. These bands indicate the
presence of alcoholic hydroxyl group –OH and the
carboxylic acid group –COOH in our product, which
were confirmed by the chemical analysis method as well.
Chemical analysis detected that there were about one
alcoholic hydroxyl group –OH per 16 carbon atoms, one
–COOH group per 230 carbon atoms in CAW.
The measurements of XPS and elemental analysis
indicated that the purified CAW contained about 93%
carbon, 2% hydrogen and 5% oxygen by weight. The
trace residence of the catalyst Fe was 126 ppm in the
product purified by 6 M HCL once and could be re-
duced to 6 ppm when the product was purified twice.
It has been realized that nonreactive terminal groups
or end caps, as well as the chain conformations and in-
teractions can stabilize the linear sp structure [8,17]. The
existence of the groups detected above and the winding
line shape with small intervals among the neighboring
wires can presumably help to explain the stability of
carbon atom wires at the room temperature in air.
In addition, carbon atom wires are of one dimension
and have unfixed intervals among the neighboring wires,
and thus it is anticipated that they are easily accessible
by intercalation of different foreign atoms, molecules
and ions to form composites with various desirable
properties.
hexane consists of short chains with carbon atom
numbers less than 10.
Acknowledgements
This work was supported by National Natural Sci-
ence Foundation of China.
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