Nano-sized double helices and braids: Interesting carbon nanostructures

Article abstract of DOI:10.1016/S0025-5408(02)01035-8

Two interesting carbon nanostructures, the double helices and the nanobraids, have been synthesized by pyrolysis of acetone at 715°C, using iron nanoparticles as catalysts. The double helix was formed by two nanofibers, strictly coiled together with a strikingly constant pitch. Such a rope-like structure usually extended hundreds of microns in length, and provided excellent strength, stability and flexibility. The nanobraids appeared to be partially rolled up from carbon layers. Electron microscopy was applied to characterize these carbon forms.

Full text of DOI:10.1016/S0025-5408(02)01035-8

Materials Research Bulletin 38 (2003) 261±267
Nano-sized double helices and braids: interesting
carbon nanostructures
Jun Liu, X. Zhang*, Yingjiu Zhang, Xihua Chen, Jing Zhu
Electron Microscopy Laboratory, School of Materials Science and Engineering,
Tsinghua University, Beijing 100084, PR China
Received 3April 2002; received in revised form 21 August 2002; accepted 6 November 2002
Abstract
Two interesting carbon nanostructures, the double helices and the nanobraids, have been synthesized by
pyrolysis of acetone at 715 8C, using iron nanoparticles as catalysts. The double helix was formed by two
nano®bers, strictly coiled together with a strikingly constant pitch. Such a rope-like structure usually extended
hundreds of microns in length, and provided excellent strength, stability and ¯exibility. The nanobraids appeared
to be partially rolled up from carbon layers. Electron microscopy was applied to characterize these carbon forms.
# 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Nanostructures; Vapor deposition; Electron microscopy
1. Introduction
Carbon is probably the most versatile element because it can form a number of structures besides
diamond and graphite. The discovery of fullerenes [1], carbon nanotubes [2], and carbon onions [3], has
excited numerous speculations on the potential applications of these carbons [4±11], and indicated the
diversity and variety of nonplanar graphitic structures. Recently, carbon micro-coils [12±14], carbon
catabashes [15], onion-like marbles and bats [16], etc. have been fabricated by various methods, such as
arc discharge [17] and catalytic decomposition of hydrocarbon [12±14,18]. They all consist of curved
graphite planes and possess somewhat twisted morphology. Among them, the coiled carbon ®laments
or tubes are of particular interest, because they have peculiar spiral structures, thus considered as ideal
candidates for electromagnetic wave adsorbents, tunable micro-devices, bioactivators, Li-battery
electrodes and hydrogen containers [13,15].
*
Corresponding author. Tel.: ‡86-10-62773999; fax: ‡86-10-62772507.
E-mail address: xzzhang@tsinghua.edu.cn (X. Zhang).
0025-5408/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0025-5408(02)01035-8

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Coiled carbon ®bers have been synthesized by Ni catalyzed pyrolysis of acetylene [12±14]. They
were usually coiled regularly with a remarkably constant pitch, and coiled in the right and left
clockwise in the same amount. Unfortunately, the processes are generally very accidental with poor
reproducibility, and their ®ber diameters and coil diameters are basically on the micrometer scale [12±14].
As many researchers are aiming to obtain nano-scale coiled structures, coiled carbon nano®bers have also
been reported [19], however, the regularity of their spiral structure is not at all satisfactory. As far as we
know, nano-sized double helical carbon structure has not been reported yet.
Here in this letter we report, for the ®rst time, the synthesis of two carbon nanostructures: nano
double helices and braids, with fairly good reproducibility by catalytic pyrolysis of acetone at 715 8C.
These nano-sized carbon forms are coiled with strikingly constant pitch, or impressively ordered
morphology. Their unique morphology and structures are studied, and preliminary discussion on the
growth mechanism is presented.
2. Experimental
In the present work, iron nanoparticles embedded in mesoporous silica, were used as the catalyst for
pyrolysis of acetone to obtain carbon products. The reaction system is shown as Fig. 1.
The catalyst was prepared by a sol±gel process as the following: 50 ml tetraethoxysilane (TEOS),
52 ml dry ethynol, and 57 ml iron nitrate aqueous solution (0.1 mol/l) were mixed together and
magneto-stirred for 30 min. Then 2 ml concentrated hydrogen ¯uoride was added in, and the mixture
were stirred for 15 more minutes. After gelation, the mixture was dried at 60 8C for 1 week, and then
calcined at 450 8C under 10À2 torr for 10 h. As a result, a silica network embedded with uniformly
dispersed iron oxide nanoparticles were obtained [20].
Before used as catalysts, these iron oxide particles were reduced by H2 (500 sccm) at 550 8C for 5 h.
Then the furnace was heated to 715 8C in 10 min, and the H2 (500 sccm) was forced to ¯ow through
acetone to carry the acetone vapor into the reaction system. Typical reaction time was 30 min.
The product was then characterized by transmission electron microscopy (TEM, JEM-200CX, JEOL,
Japan), high-resolution transmission electron microscopy (HRTEM, JEM-2010F, JEOL, Japan), energy
Fig. 1. Schematic layout of the reaction system.

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dispersive spectrum (EDS, Link ISIS, Oxford, UK), and electron energy loss spectrum (EELS, GIF,
Gatan, US).
3. Results
After the reaction, a black wool-like layer was formed on the inner wall of the quartz tube. In this
product, two interesting carbon forms ( Figs. 2 and 3, respectively), as well as some carbon nanotubes,
were found.
As in Fig. 2, two nano®bers were strictly coiled together to form a double helix, which is very much
like the twisted pairs extensively used in electronics and telecommunications. It can be found from
Fig. 2 that the outer diameter of the twisted pair is ꢀ50 nm and each single ®ber is about 21 nm in
diameter. With strikingly constant pitch, this rope-like structure shows great ¯exibility and usually
extends up to several microns long. The component nanon®bers remain closely twisted together all the
length, even toward the end. And partially unfastened twisted pair has not been found. Because of its
¯exibility, stability and possible electromagnetic properties, this double helical structure is promising as
the connection wires for the Micro/Nano Electronic and Mechanical System (MEMS/NEMS) or as the
reinforcement in the nanocomposites.
In the EDS of the double helices, with a detection limit of approximately 1%, only carbon was
detected. It is thus believed that these double helices are composed of pure carbon, although existence
of a small amount of hydrogen is still arguable. It should be noted that, metal catalysts could often be
found on the end of carbon nanotubes/nano®bers in other reports [21,22], which is usually explained
with a VLS growth mechanism. Nevertheless, in our work, metal catalyst was not observed on the
end, or any other part, of the double helices, suggesting a different growth mechanism. EELS of
an individual twisted pair also con®rms its composition (Fig. 4a). In the near-edge ®ne structure, it
exhibits the onset of the leading peak at DE ˆ 284 eV, corresponding to C 1s ! pà transitions, and the
following peak up from 290 eV, mainly due to transitions from C 1s ! pà states. This near-edge ®ne
Fig. 2. Morphological image of the double helix.

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Fig. 3. Morphological image of the nanobraids (together with a carbon nano®ber).
Fig. 4. EELS of (a) the double helices; (b) the nanobraids.

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265
Fig. 5. HRTEM image of the double helix.
structure, which is very similar to that of carbon onions [16], con®rms that carbon is in sp2
hybridization state, and consequently a graphitic short-range ordering.
The high-resolution TEM images (Fig. 5) reveals the detailed arrangement of these graphitic layers in
the double helices. The uneven contrast and defocus of the adjacent part of the nano®bers re¯ects the
un¯atness of the entire structure. And the abrupt contrast change around the nodes distinctively reveals
the three-dimension spiral nature of the double helix structure. The short-range ordered graphitic layers
could be clearly observed, aligned to the shape of twisting nano®bers. These layers are incontinuously
curved, due to the twist and strain of the carbon nano®bers.
The other interesting carbon structures found in the product looks really like a braid, as shown in
Fig. 3. It seems to be partially rolled up from a layer, but what is intriguing is its periodic and regular
morphology. These braids range from 10 nm to hundreds of nanometer in width, and usually extend as
long as tens of micrometer. They are also made of pure carbon, in sp2 hybridization state, which is
con®rmed by EDS and EELS (Fig. 4b).
The HRTEM image (Fig. 6) reveals the detailed structure of the braided carbon, which is closer to
graphite than the double helix. The graphitic layers stack more orderly, and the (0 0 2) spacing is
calculated to be 0.34 nm, in good consistence with that of standard graphite.

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Fig. 6. HRTEM image of the carbon nanobraids.
4. Discussion
As mentioned before, the double helices are very stable, although some strain and stress might have
been introduced from the twisted structure. They can maintain this structure after 3030 min' ultrasonic
oscillation, and remain unchanged for at least 6 months in air. As the high activity of carbon nanotubes
or nano®bers has generally been explained with the large surface areas and huge number of dangling
bonds [23,24]. Accordingly, the stability of the double helices might have resulted from pairing of these
dangling bonds, which leads to a strong interaction between the two nano®bers and greatly lowered the
energy of the system. Thus, by coupling the two nano®bers, the twisted structure was stabilized.
As in other work, the micro-sized catalysts produced only micro-coiled carbon [12,25], thus the key
to the synthesis of nano-sized coiled ®bers/tubes could be the nano-sized catalyst particles used to
decompose acetone. The VLS mechanism is not likely working here, as mentioned before, because
catalyst particles were not found on the ends of the double helices. The growth might be in two
processes: ®rst, H2 and acetone vapor were adsorbed on the catalyst surface, then the acetone was
reduced, and carbon left on the catalyst. The iron particles simply supplied surfaces for reaction, and no
catalyst liquid was formed, just as in the VLS mechanism. Therefore, the interaction between double
helices and catalysts was very weak, and they were detached during ultrasonic oscillation. And the
newly reduced iron catalysts kept the regular crystal shape, thus when two carbon nano®bers were
growing from different faces, such as (1 0 0) and (0 0 1), they got twisted with each other to reduce

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dangling bonds. Otherwise, if one predominating nano®ber rose from a catalyst particle, it would just
grow into carbon nanotubes. However, the way to a controlled production has not been found, although
the double helices could be obtained with fairly good reproduction.
On the other hand the braided carbon with regular graphite structure, seems to be partially rolled up
from a carbon layer. It is believed to be the mid-products of the growth of carbon nanotubes, although
the growth mechanism and its relationship with carbon nanotubes are yet to be studied.
In summary, two interesting carbon nanostructures, the double helices and braids, have been
synthesized by catalytic pyrolysis of acetone and characterized with TEM, EDS and EELS, etc. They
were all made of pure carbon of sp2 hybridization state. Preliminary discussion on the growth
mechanism was presented. These carbon structures were fabricated in the same reaction system where
carbon nanotubes were produced, further studies on their structure, property and growth processes
could reveal the growth mechanism of carbon nanotubes. The carbon double helices are promising for
applications in MEMS/NEMS, or nanocoposites.
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