Structural evolution and Raman study of nanocarbons from diamond nanoparticles

Article abstract of DOI:10.1016/j.cplett.2006.08.067

Nanocarbons with a variety of morphologies were found after diamond nanoparticles (ND) were annealed at a low temperature. The conversion of ND to nanocarbons is a process of structural evolution that causes increase of sp² bonding structure and decrease of sp³ bonding structure in nanostructure. While being annealed, ND are converted to carbon onions and undergo phase transition in a series of ND → bucky diamond → carbon onions → graphitic ribbons. The structural evolution of nanocarbons reflects the higher stability of planar graphitic ribbons over carbon onions.

Full text of DOI:10.1016/j.cplett.2006.08.067

Chemical Physics Letters 429 (2006) 479–482
www.elsevier.com/locate/cplett
Structural evolution and Raman study of nanocarbons from
diamond nanoparticles
a
a
a,b,
a
c
Zhijun Qiao , Jiajun Li , Naiqin Zhao
*, Chunsheng Shi , Philip Nash
a
School of Materials Science and Engineering, Tianjin University, Tianjin 300072, PR China
Key Laboratory of Advanced Ceramics and Machining Technology, Tianjin University, Tianjin 300072, PR China
Department of Mechanical, Materials and Aerospace Engineering, Illinois Institute of Technology, Chicago, IL 60616, USA
b
c
Received 6 May 2006; in final form 29 July 2006
Available online 18 August 2006
Abstract
Nanocarbons with a variety of morphologies were found after diamond nanoparticles (ND) were annealed at a low temperature. The
conversion of ND to nanocarbons is a process of structural evolution that causes increase of sp2 bonding structure and decrease of sp3
bonding structure in nanostructure. While being annealed, ND are converted to carbon onions and undergo phase transition in a series
of ND ! bucky diamond ! carbon onions ! graphitic ribbons. The structural evolution of nanocarbons reflects the higher stability of
planar graphitic ribbons over carbon onions.
Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction
ND at a high temperature in a closed chamber and
explained it as a result of carbon redistribution process
via carbon transport reactions [7].
It is well known that diamond with sp3 hybridization
and graphite with sp2 hybridization are traditional forms
of crystalline carbon. Since the discovery of carbon onions,
much interest has been concentrated on the structure and
morphology of the family of nanosized carbons including
diamond nanoparticles (ND), fullerenes and nanographites
[1–3]. As a new kind of carbon allotrope, fullerenic nano-
carbons (fullerenes, carbon onions and related structures)
are characteristically constructed by curved graphene,
whereas nanographites by planar graphene.
In our recent experiments, various nanocarbons includ-
ing bucky diamond, carbon onions and graphitic ribbons
were found after ND were annealed at a low temperature.
To elucidate the structural evolution, we carried out sys-
tematic experiments on ND annealed at 1400 °C. These
experiments provide us with much information that can
deepen the understanding on the structural evolution of
nanocarbon allotropes.
For nanosized carbon system, structural evolution and
relative stability between carbon allotropes are of great
importance due to their unusual properties and wide poten-
tial applications in the field of engineering. The conversion
of ND to carbon onions has been experimentally observed
and this has promoted a lot of studies in the investigation
of the structure of nanocarbons [4–6]. Kuznetsov et al.
have reported the formation of ribbon-like graphite from
2. Experimental
The average diameter of ND is about 5 nm. Annealing
of ND was carried out in a tube furnace under flowing
argon atmosphere (99.99%) at 1400 °C for 20 min, 60 min
and 120 min. Details of annealing process have been previ-
ously described [8]. The HRTEM images were obtained
using a PHILIPS TECNAI G2F20 electron microscope
operating at 200 kV. Raman spectra were determined by
Renishaw In via spectroscopy system. The laser was argon
ion laser with 514.5 nm excitation source with power out-
put of 20 mW.
*
Corresponding author. Fax: +86 22 8740 1601.
E-mail addresses: zjqtju@yahoo.com.cn (Z. Qiao), nqzhao@tju.edu.cn
(N. Zhao).
0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2006.08.067

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Z. Qiao et al. / Chemical Physics Letters 429 (2006) 479–482
3. Results and discussion
3.1. Raman spectroscopy analyses
exactly (shown in Fig. 1d). The relative intensity of ID/IG
gradually decreases with the increase of annealing time.
This means that the phase transition of nanocarbons from
ND is a gradual graphitization process, which leads to an
increase of sp2 bonding and decrease of sp3 bonding in
nanostructure. In the course of conversion there exist many
disordered graphite fragments (as seen in Fig. 2d and
Fig. 3); therefore weak shoulder of 1620 cmꢀ1 at G peak
is still present in Fig. 1c and d.
Fig. 1 shows Raman spectra of ND annealed at 1400 °C
for 20 min, 60 min and 120 min, respectively. It is known
that the C–C stretching mode of vibration can be detected
at 1332 cmꢀ1 for sp3 bonded materials of diamond. As
shown in Fig. 1a, a peak of ND Raman spectra at
1332 cmꢀ1 is observed indicating sp3 bonding structure.
Another broad peak at 1620 cmꢀ1 can be assigned to sp2
bond stretching vibrations of C@C groups [9–15]. The exis-
tence of the Raman peak at 1620 cmꢀ1 is in agreement with
HRTEM observation of the original ND covered with a
small number of amorphous carbon fragments.
As seen in Fig. 1b–d, Raman spectra of annealed ND
display two broaden bands at ꢁ1585 cmꢀ1 and
ꢁ1350 cmꢀ1 corresponding to the G-band and D-band of
carbon onions, respectively. The G-band indicates that
graphite structure develops, while the D-band points out
many structural disorders in the nanocarbons [16]. In
Fig. 1b and c, the Raman peak in the region 1332–
1350 cmꢀ1 is attributed to the overlap of sp3-bonded ND
and disordered sp2-bonded carbon, which indicates the
existence of the sp3:sp2 mixed-phase systems-bucky dia-
mond. After the annealing of ND at 1400 °C for 120 min,
the peak of sp3-bonded ND is absent and the D peak cor-
responding to disordered carbon centers at 1350 cmꢀ1
3.2. HRTEM observation
Fig. 2 presents the HRTEM images of ND annealed at
1400 °C for 20 min, 60 min and 120 min, respectively.
Fig. 2a shows an HRTEM image of original ND with
the characteristic diamond (111) d spacing of 0.206 nm.
The original ND are covered with a small number of amor-
phous graphite fragments. As seen in Fig. 2c, ND annealed
at 1400 °C for 60 min are mostly transformed into carbon
onions with a variety of morphologies and a single-wall
carbon nanohorn is also found (marked by arrow in
Fig. 2c). It can be seen that the carbon onions are irregular
and less than 10 nm. The phase transition of ND to carbon
onions is ascribed to the elimination of dangling bonds and
the closure of graphite sheets [8]. Between ND and carbon
onions, there exists an intermediary phase–bucky diamond
with a diamond core encased in fullerene-like shells (shown
in Fig. 2b).
c
a
1000 1100 1200 1300 1400 1500 1600 1700 1800
1000 1100 1200 1300 1400 1500 1600 1700 1800
d
b
1000 1100 1200 1300 1400 1500 1600 1700 1800
1000 1100 1200 1300 1400 1500 1600 1700 1800
Raman shift / cm-1
Ramanshift / cm-1
Fig. 1. Raman spectra of (a) original ND; ND annealed at 1400 °C for (b) 20 min, (c) 60 min and (d) 120 min.

Z. Qiao et al. / Chemical Physics Letters 429 (2006) 479–482
481
Fig. 2. HRTEM images of (a) original ND; ND annealed at 1400 °C for (b) 20 min, (c) 60 min and (d) 120 min.
Besides carbon onions, lots of graphitic ribbons can be
observed in the HRTEM image of ND annealed at
1400 °C for 120 min, as shown in Fig. 2d. The graphitic rib-
bons are composed of planar graphitic layers with the
length varying from 10 to 20 nm, and have interlayer spac-
ings of about 0.34 nm.While being annealed, the outer lay-
ers of assembled nanoparticles tend to link up to form
ribbon-like graphite. Fig. 3 is the HRTEM image of the
ribbon-like graphite on the surface of assembled onions.
Enoki et al. and Kuznetsov et al. have demonstrated that
impurities (oxygen and hydrogen containing gases) of flow-
ing argon atmosphere in annealing process lead to carbon
redistribution in nanocarbons with lower surface energy
[7,15,17–19]. The oxygen and hydrogen containing gases
participate in the carbon redistribution processes via
carbon transport reactions to accelerate the ND graphitiza-
tion process and decrease the temperature of graphitization
onset. The intersection of two onions forms the ignition
point and the ribbons are formed via a tangential relaxa-
tion to relieve the strain induced by the onion curvature.
When the annealing time is increased, all the curved layers
of assembled onions are changed into planar graphitic rib-
bons. It is reasonable that the conversion of planar graph-
ite from curved layers results in the drop of the surface
tension induced by the onion curvature, which is the driv-
ing force to form the graphitic ribbons. The surface tension
induced by the onion curvature makes carbon onions meta-
stable; therefore carbon onions incline to become a more
stable phase-graphitic ribbon. This change shows the
higher stability of graphitic ribbons with the length of
10–20 nm over carbon onions. It can be deduced that the
Fig. 3. HRTEM image of ribbon-like graphite by the linkup of assembled
nanoparticles.

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Z. Qiao et al. / Chemical Physics Letters 429 (2006) 479–482
graphitization of ND to nanocarbons is a process of struc-
tural evolution that causes an increase of sp2 bonding and
decrease of sp3 bonding.
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Acknowledgements
The authors are grateful for the supports from Tianjin
Natural Science Foundation (No. 05YFJZJC01900) and
Doctor Fund from the Educational Ministry of China
(No. 20050056062).
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