Low-Temperature Production of Genuinely Amorphous Carbon from Highly Reactive Nanoacetylide Precursors

Article abstract of DOI:10.1155/2016/7840687

Copper acetylide is a well-known explosive compound. However, when the size of it crystals is reduced to the nanoscale, its explosive nature is lost, owing to a much lower thermal conductance that inhibits explosive chain reactions. This less explosive character can be exploited for the production of new carbon materials. Generally, amorphous carbon is prepared by carbonization of organic compounds exposed to high temperature, which can induce partial crystallization in graphite. In this work, we present a new method in which the carbonization reaction can proceed at a lower annealing temperature (under 150°C) owing to the highly reactive nature of copper acetylide, thus avoiding crystallization processes and enabling the production of genuinely amorphous carbon materials.

Full text of DOI:10.1155/2016/7840687

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Journal of Chemistry
Volume 2016, Article ID 7840687, 6 pages
http://dx.doi.org/10.1155/2016/7840687
Research Article
Low-Temperature Production of Genuinely Amorphous Carbon
from Highly Reactive Nanoacetylide Precursors
Ken Judai, Naoyuki Iguchi, and Yoshikiyo Hatakeyama
College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan
Correspondence should be addressed to Ken Judai; judai@chs.nihon-u.ac.jp
Received 7 September 2015; Accepted 14 March 2016
Academic Editor: Jean-Marie Nedelec
Copyright © 2016 Ken Judai et al. is is an open access article distributed under the Creative Commons Attribution License, which
permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Copper acetylide is a well-known explosive compound. However, when the size of it crystals is reduced to the nanoscale, its explosive
nature is lost, owing to a much lower thermal conductance that inhibits explosive chain reactions. is less explosive character can
be exploited for the production of new carbon materials. Generally, amorphous carbon is prepared by carbonization of organic
compounds exposed to high temperature, which can induce partial crystallization in graphite. In this work, we present a new
method in which the carbonization reaction can proceed at a lower annealing temperature (under 150∘C) owing to the highly
reactive nature of copper acetylide, thus avoiding crystallization processes and enabling the production of genuinely amorphous
carbon materials.
1. Introduction
[7], catalysis [8], and so on [9, 10]. e explosive nature of
compounds like copper acetylide has limited investigations
on their use [11]. However, reducing the size of the crystals
to the nanoscale eliminates their explosive nature, due to
the much lower thermal conductance that inhibits explosive
chain reactions. e less explosive character of the nanosized
compounds can be exploited for the production of new noble
carbon materials.
Herein, we report a new method for preparing gen-
uinely amorphous carbon materials. Self-assembled copper
acetylide nanowires were used as precursors for amorphous
carbon [9]. As the high reactivity of copper acetylide allows
the carbonization reaction to proceed at a lower annealing
temperature, unwanted crystallization processes are avoided
and genuinely amorphous carbon materials can be prepared.
Although thin amorphous carbon films can be prepared
by physical vapor deposition at low temperature [12–14],
this work approach can produce amorphous carbon in bulk
amounts, not in only film amounts.
Amorphous carbon has been known for a long time under
various denominations such as coal, soot, and carbide-
derived carbon. is form of carbon consists of free, reactive
carbon, without any crystalline structure. However, most
carbon materials can be categorized into diamond-like-
carbon, graphite-based carbon, and carbyne, which represent
the main crystalline forms of carbon, involving sp3, sp2, and
sp carbon hybridization, respectively [1–5]. On the other
hand, no apparent progress has been made in the production
of genuinely amorphous carbon.
Research on the carbyne carbon form has been limited
because of its instability, whereas diamond-like carbon has
been studied extensively only in the last decade. Amorphous
carbon is usually associated with graphite-based materials
[1]. It is generally prepared by carbonization of organic
compounds exposed to a high temperature, which can induce
partial crystallization in graphite [6]. For the production
of commercial carbon materials, graphite-based carbon is
activated in order to increase its surface area. e activated
carbon can no longer be considered as amorphous.
In addition to the amorphous character, the new carbon
material exhibited a high surface area associated with a meso-
and microporous structure. Mesoporous carbon materials
have recently attracted interest as viable and inexpensive
materials for use in numerous applications, such as super
We have previously investigated the nanostructure of
metal acetylide molecules and their application to gas sensors

2
Journal of Chemistry
C2H2(1%) + Ar
5mL/min
the original optical setup and a 16 cm focusing spectrometer
(Andor, SR163). e Raman signals were gathered with
backscattering geometry by a microscopic objective lens
and were accumulated on a CCD detector for 30 min. is
analysis was applied not only to the amorphous carbon
prepared in this work but also to graphite and commercial
activated carbon (Darco5 G-60) samples, in order to compare
their crystalline quality. Further thermogravimetric (Rigaku,
TG8120) and nitrogen absorption-desorption (BEL Japan,
BELSORP-mini) analyses were carried out on the amor-
phous carbon sample. e exact specific surface area was
derived using the Brunauer-Emmett-Teller (BET) analysis
from absorption-desorption data.
Ar
C2Cu2
Pump
Ammonia water
100 mL (5%)
CuCl 1g
Pump
150∘C
r.t.
2h
24h
3. Results and Discussion
We have succeeded in producing carbon materials by only
employing processes performed under 150∘C. e annealing
temperature is one of the most important parameters to
design and control the chemical properties of carbon materi-
als. e main advantage of low-temperature carbonization is
the high degree of amorphousness produced. Typical carbon
materials are exposed to over 800∘C for the carbonization,
which induces crystallization of carbon to graphite [6]. In
order to assess this, we have compared the spectroscopic
properties of three samples: the carbon prepared in this work,
graphite as an example of perfect crystalline structure, and
commercial activated carbon.
e crystallinity or amorphousness can be checked by X-
ray diffraction measurements. Figure 2 shows powder XRD
patterns for the three carbon materials examined. A strong
peak was observed for graphite, denoting perfect crystallinity.
Even though the measurements were taken with the same
sample amounts and for the same duration, the peak intensity
of the activated carbon was two orders of magnitude lower
than that of graphite. is difference is due to the lower
crystal quality of the carbon sample: in other words, the
amorphousness of the activated carbon resulted in a lower
intensity of the XRD signal. e XRD peak signal for the
amorphous carbon in this work was buried in noise even for
the same measuring conditions, which highlights its superior
amorphousness compared to that of the activated carbon.
No structural information could be obtained from the XRD
measurements for the carbon material prepared in this work,
owing to its highly amorphous character.
A Raman spectroscopic analysis was performed to com-
pare in detail the three carbon materials. Figure 3 illustrates
the Raman spectra of the samples excited by a 532 nm laser.
e vertical scales of the Raman spectra were normalized in
order to compare the intensities of the different samples.
e Raman spectrum for graphite (Figure 3(a)) shows the
sharp G band peak at 1585 cm−1, which is the most prominent
vibration of the graphite lattice. e other prominent mode is
the D band around 1400 cm−1, which is associated with the
occurrence of defects or of a large number of edges in the
graphite carbon sample [1]. A large number of edges denote a
small domain size in graphite. Even though no D band peak
could be observed, a 2D band at double vibrational energy
H2SO4
Figure 1: Illustration of the method used to prepare amorphous
carbon under 150∘C. Copper acetylide precursor was prepared via
the reaction of CuCl and C H under NH water. en the precursor
2
2
3
was separated by suction filtration and annealed at 150∘C. Finally,
sulfuric acid treatment was carried out for eliminating copper.
capacitors [10], absorbents, electrodes [2], and catalytic sup-
ports [8, 15]. e unique amorphous character combined
with the high surface area of the materials introduced in this
work will significantly enhance its applicability in the above
applications.
2. Materials and Methods
A schematic procedure for the preparation of amorphous
carbon is illustrated in Figure 1. Copper (I) chloride (1 g)
was dissolved in 5% aqueous ammonia solution (100 mL),
and acetylene gas was introduced into the solution flask at a
flow rate of 0.05 mL/min for 3 h, afer which a dark brown
precipitate of copper acetylide (C Cu ) was obtained; the
2
2
chemical reaction is shown as follows:
2CuCl + C H + 2NH ꢀꢁ C Cu ↓ +2NH Cl
(1)
2
2
3
2
2
4
e extremely slow flow rate is the key to generate self-
assembled C Cu nanowires [9]. e product was sepa-
2
2
rated by suction filtration and annealed under vacuum. To
avoid explosive reactions, the sample was slowly heated up
(1∘C/min) to 150∘C. Carbonization of C Cu was achieved by
2
2
maintaining the sample at 150∘C for 24 h for the separation
reaction resulting in copper and carbon. Copper was then
eliminated by dipping the sample in concentrated sulfuric
acid for 30 min. is process allowed us to obtain genuinely
amorphous carbon at a temperature as low as 150∘C.
e amorphous carbon prepared as described above was
analyzed by powder X-ray diffraction (XRD) measurements
using Cu Kꢂ emissions (Rigaku, RINT-2000) and Raman
spectroscopy with a 532 nm laser (Elforlight, G4 60), with

Journal of Chemistry
3
×105
2
1
0
1000
0
22
23
24
25
26
2ꢀ
27
28
29
30
22
23
24
25
(b)
26
2ꢀ
27
28
29
30
(a)
500
0
22
23
24
25
26
27
28
29
30
2ꢀ
(c)
Figure 2: Powder X-ray diffraction patterns for graphite (a), commercial activated carbon (b), and amorphous carbon prepared in this work
(c).
1000
1500
2000
2500
3000 1000
1500
2000
2500
3000
Raman shif ((m−1
)
Raman shif ((m−1
)
(a)
(b)
1000
1500
2000
2500
3000
Raman shif ((m−1
)
(c)
Figure 3: Raman spectra for graphite (a), commercial activated carbon (b), and amorphous carbon prepared in this work (c). e samples
were excited by a 532 nm laser light with 30 mW power.
could be detected. e D band corresponds to an optically
forbidden Raman transition, which can only occur at defect
carbons or edges in graphite. However, the 2D band transition
can be optically allowed even in the perfect graphite lattice.
In the Raman spectrum of activated carbon (Figure 3(b)),
the D band can be clearly observed at 1357 cm−1, indicating
that the activated carbon sample contains a large amount of
defects. A comparison of the Raman spectra of graphite and

4
Journal of Chemistry
0
−20
activated carbon shows that the peak widths of the G band
are also different. e chemical environments of graphitic
carbon differ from those found in amorphous carbon, which
manifests itself as a broad peak in the Raman spectrum.
e peak width in these spectra thus represents a suitable
parameter for estimating the amorphousness of a sample.
e Raman spectrum for the amorphous carbon in
this work (Figure 3(c)) exhibits broader G and D band
peaks than those of the activated carbon. is means that
the low-temperature carbonization process can generate a
carbon material of lower crystallinity and more genuinely
amorphous than commercial activated carbon. e peak
position of the G band is 1603 cm−1, and the ratio of the D
−40
−60
−80
band intensity to that of the G band, ꢃ /ꢃ , is 0.95. e Raman
G
spectrum in this work closely resembDles that of an amorphous
carbon film fabricated by physical vapor deposition [1, 4, 12–
14]. However, this process can produce genuinely amorphous
carbon in bulk amounts and not just in amounts required
for film formation. Low-temperature carbonization is the key
point to producing amorphous carbon.
−100
0
200
400
600
800
Temperature (∘C)
Figure 4: ermogravimetric spectrum under 20% oxygen flow
with a temperature ramp rate of 10∘C/min.
It should be pointed out that the signals in the present
Raman spectra can be assigned to the graphitic G and
D bands. e copper acetylide precursor of the present
amorphous carbon contains a carbon-carbon triple bond.
Although there are some techniques that can produce carbon
materials with triply bonded carbons, the carbon triple
bond of polycarbyne shows an intense Raman peak around
2000 cm−1 [16, 17]; no such polycarbyne peak can be observed
in Figure 3(c). erefore, despite using acetylide precursors,
which contain carbon triple bonds, the resulting materials
3000
2500
2000
Desorption
1500
are based on the graphite structure. Preliminary 13C NMR
1000
Adsorption
measurements also identified the chemical shif signals of sp2
carbon (graphite), but not of sp (carbyne) or sp3 (diamond)
carbon. Whereas the Raman spectrum mainly provides
information on the carbon hybridization, basically showing
the predominance of a graphite-like carbon environment, the
broadness of the G band and the intensity of the D band
indicate the genuinely amorphous character of the sample.
ermogravimetric analysis was performed for the amor-
phous carbon prepared in this work, as shown in Figure 4.
e sample was heated up at a ramp rate of 10∘C/min
under atmospheric (20%) oxygen gas concentration. Around
10% weight loss can be observed below 100∘C; this was
owing to the desorption of water molecules from the carbon
porous surface. e combustion of carbon starts at 150∘C.
is relatively low combustion temperature denotes chemical
instability of carbon. It should be pointed out that the burning
temperatures of the sample spanned a wide range, from 150
to 550∘C. A wide range of different chemical environments
must then be present in the amorphous carbon, and a
corresponding wide range of combustion temperatures is
also expected. e wide range of the temperature regions
in the thermogravimetric spectrum confirms the genuinely
amorphous character of the carbon prepared in this work.
e total weight loss over 600∘C reaches up to 97%. e
contamination of metal ash was less than 3%. Considering
the method used for the preparation of the samples, most
of this contamination should be represented by elemental
copper.
500
0
0.0
0.2
0.4
0.6
0.8
1.0
Pressure P/P
0
Figure 5: Nitrogen absorption-desorption equilibrium curves for
the amorphous carbon prepared in this work. e sample was
preannealed at 100∘C for 6 h in vacuum and was analyzed at the
liquid nitrogen temperature of 77 K.
Nitrogen adsorption-desorption experiments were also
carried out for the amorphous carbon prepared in this work.
e specific surface area determined from these measure-
ments represents one of the most important properties of
carbon materials. Figure 5 shows the nitrogen adsorption and
desorption isothermal equilibrium curves of the amorphous
carbon. Two distinct regions are visible in the curves, a
microporous region for relative pressures below 0.1, and
a mesoporous region at a relative pressure between 0.4
and 1.0. e amorphous carbon produced in this work
contained substantial amounts of both microporous and
mesoporous surfaces. Even though the generation method
did not involve carbon activation, the microporous specific

Journal of Chemistry
5
Competing Interests
0.004
0.003
0.002
0.001
0.000
e authors declare that there is no conflict of interests
regarding the publication of this paper.
540m2/g
Acknowledgments
is work was partially supported by the N. Research Project
at Nihon University. e authors acknowledge Professor
Hashimoto and Dr. Niwa for technical support in the X-ray
diffraction and thermogravimetric and nitrogen absorption-
desorption measurements.
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We have succeeded in preparing genuinely amorphous car-
bon materials. e high reactivity of copper acetylide allows
the carbonization reaction to proceed at a lower anneal-
ing temperature (under 150∘C), and carbon materials with
superior amorphousness can be prepared. In addition to the
amorphous character, the new carbon material exhibited a
high surface area associated with a meso- and microporous
structure even though the material was produced without any
activation process. e unique amorphous character com-
bined with the high surface area will significantly enhance its
applicability.

6
Journal of Chemistry
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