Large-scale synthesis of carbon spheres by reduction of supercritical CO2 with metallic calcium

Article abstract of DOI:10.1016/j.cplett.2006.01.119

Carbon spheres with approximate uniform diameters of about 1-2 μm were synthesized by chemical reduction of supercritical CO₂ with metallic calcium at 550 °C. The products were characterized by techniques including XRD, scanning electron micros

Full text of DOI:10.1016/j.cplett.2006.01.119

Chemical Physics Letters 421 (2006) 584–588
www.elsevier.com/locate/cplett
Large-scale synthesis of carbon spheres by reduction
of supercritical CO2 with metallic calcium
a,
b
a
a
a
Zhengsong Lou *, Changle Chen , Dejian Zhao , Shengli Luo , Zhongchun Li
a
Laboratory of Precious Metal Processing Technology and Application, School of Applied Chemistry, Jiangsu Teachers University of Technology,
Changzhou Yuying 1, Changzhou, Jiangsu 213001, PR China
b
Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, IL 60637, USA
Received 17 October 2005; in final form 24 January 2006
Available online 28 February 2006
Abstract
Carbon spheres with approximate uniform diameters of about 1–2 lm were synthesized by chemical reduction of supercritical CO2
with metallic calcium at 550 °C. The products were characterized by techniques including XRD, scanning electron microscope, Raman,
and transmission electron microscopy. The formation mechanism was thought to be as follows: the thermal movement of carbon nano-
granules on the surface of metallic calcium leads to the formation of carbon spheres, which was confirmed by reference experiments at
different temperatures. What is more, the formation mechanism of carbon sphere in supercritical CO2 system was discussed.
Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction
chemical reduction of CO2 with alkali metals, which pro-
vides new perspectives for the synthesis of large size dia-
mond and other carbon-based materials as well as the
comprehensive utilization of CO2. In order to further
explore the reduction mechanism, we substitute alkali met-
als with metallic calcium, which is an alkali earth metal and
formulate a simple way of preparing large-scale carbon
spheres. Based on the data obtained, a possible mechanism
is suggested to illustrate the formation of carbon spheres in
supercritical CO2 system.
The discovery of fullerenes and carbon nano-tubes [1,2]
has stimulated intense interest in carbon structures, among
which carbon spheres (CPS) attracts more and more atten-
tion from both academic and industrial fields. Because of
its intrinsic properties such as high strength, high thermal
resistance, light weight, carbon spheres could be used as
high strength composites, catalyst carriers, lubricants, gas
storage media and wear-resistant materials [3]. There have
been many attempts in the synthesis of this carbon struc-
ture such as the carbon-arc technique [4], the ultrasonic
treatment [5], the high-energy electron beam irradiation
[6], the thermal treatment of carbonaceous materials [7],
the chemical vapor deposition [8], the thermal treatment
of pure carbon soot [9], the ultradispersed diamond pow-
ders (2–6 nm) [10] and the plasma torch process [11].
Recently, the reduction of carbonate or supercritical car-
bon dioxide also proves to be practical [12,13].
2. Experimental
In a typical experiment, dry ice (freshly made from high
purity CO2 gas (99.99%)) (10.0 g) and metallic Ca (1.2 g)
were put into a stainless autoclave of 15-mL capacity.
The vessel was then immediately closed tightly and heated
to 550 °C, and kept at this temperature for 24 h. If it was
not specifically mentioned, most of the reactions were car-
ried out at 550 °C. The reaction took place at an autogenic
pressure depending on the amount of carbon dioxide and
calcium added. After the cooling of the sample to room
temperature, the solid product was collected and rinsed
with ethanol three times to get rid of a small quantity of
Our group has successfully prepared diamond [14,15],
carbon nano-tubes [16] and carbon spheres [13] by the
*
Corresponding author. Fax: +86 519 6999516.
E-mail addresses: lzs@jstu.edu.cn, lzs@mail.ustc.edn.cn (Z. Lou).
0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2006.01.119

Z. Lou et al. / Chemical Physics Letters 421 (2006) 584–588
585
unreacted Ca. The main products identified were carbon
and CaCO3; CaCO3 was removed by 6.0 mol/L HCl aque-
ous solutions. The remaining solid precipitate, weighed
0.16 g, was confirmed main containing carbon spheres
and amorphous carbon. The volume fraction of the carbon
spheres was approximately 80% of carbonaceous materials,
which was evaluated from the SEM images.
The X-ray diffraction (XRD) analysis was performed
using a Rigaku (Japan) D/max-cA X-ray diffractometer
equipped with graphite monochromatized Cu Ka radiation
˚
(k = 1.54178 A). The scanning electron microscopy (SEM)
images and energy dispersive X-ray analysis (EDX) were
taken on XT30 ESEM-TMP scanning electron microscope
equipped with INCA400 energy dispersive X-ray detector.
The Raman spectrum was recorded at ambient tempera-
ture on a Spex 1403 Raman spectrometer with an argon-
ion laser at an excitation wavelength of 514.5 nm. The
transmission electron microscopy (TEM) images were
recorded on a Hitachi H-800 transmission electron
microscope.
3. Results and discussion
Fig. 2. SEM images of the products grown at 550 °C after treated by HCl
aqueous solutions (6 mol/L).
Fig. 1 shows the XRD pattern of the sample after trea-
ted by 6.0 mol/L HCl aqueous solutions. The XRD data
are typical of so-called ‘non-graphitic’ carbon (also termed
‘turbostratic’ carbon), showing diffuse interference maxima
in XRD [17]. Moreover, the broad (10) diffraction origi-
nates from a two-dimensional lattice. It can also be con-
formed by the HRTEM image of the sample (not shown).
The SEM images of the products (Fig. 2) indicate that
the final products consist of a large quantity of carbon
spheres and the carbon spheres are in perfect spherical
morphology whose surface is very smooth without cracks.
It can also be seen that the sample comprises carbon
spheres 500–1800 nm in diameter. When the temperature
is increased to 600 °C, the diameter of carbon sphere also
increases significantly. As shown in Fig. 5A, the carbon
spheres with an average diameter about 2500 nm, and the
largest ones with sizes around 5 lm found present in the
product; moreover, some carbon spheres show the elliptic
shape, which is in agreement with the result of TEM obser-
vation. The carbon spheres are with a narrow range of
diameters, which shows a potential of controlling the size
for commercial application.
A typical Raman spectrum (Fig. 3A) of the sample
shows that there are two strong peaks at 1600 and
1340 cmꢀ1. The peak at 1600 cmꢀ1 (G-band) corresponds
Fig. 3. Raman spectrum of the products after treated by HCl aqueous
solutions (6 mol/L): (a) the experiment was carried out at 550 °C and (b)
the experiment was carried out at 600 °C.
Fig. 1. XRD pattern of the products after treatment by HCl aqueous
solutions (6 mol/L).

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Z. Lou et al. / Chemical Physics Letters 421 (2006) 584–588
to an E2g mode of graphite and is related to the vibration of
sp2-bonded carbon atoms in a two-dimensional hexagonal
lattice, such as in a graphite layer [18]. The peak at
1340 cmꢀ1 (D-band) is associated with vibrations of carbon
atoms with dangling bonds in plane terminations of disor-
dered graphite. This peak is quite high indicating that in
the basal plane there exist a two-dimensional disorder,
which is quite common in mild temperature routes [19].
The value of ID/IG is indicative of the graphitization degree
of carbon material. ID/IG value of the sample in the present
study is 0.81, which suggests a decrease in the number of
the six fold ring of graphitized plane.
Fig. 4 shows the TEM images and electron diffraction
pattern of the as-prepared carbon spheres. TEM image
(Fig. 4A, D) of the samples grown at 550 °C investigations
indicate the carbon spheres in the sample appear to be of
nearly perfect spheres with diameters range from 500 to
1500 nm. Smaller carbon spheres with sizes around 500–
900 nm and carbon nano-rods are also found present in
the product (Fig. 4D). Fig. 4B, C shows the samples
obtained at 600 °C consist of carbon spheres 2.5–5 lm in
diameter, and the elliptic carbon spheres are observed. In
Fig. 4E, the produces grown at 500 °C contain carbon
nano-rods, but no carbon spheres are seen. The selected
area electron diffraction pattern (Fig. 4F) of the carbon
spheres grown at 550 °C comprises two diffraction rings
corresponding to (002) and (10) reflections of graphite,
confirming the XRD result.
To investigate the effect of reaction condition on the for-
mation of carbon spheres a series of experiments were car-
ried out by altering experimental parameters of the process.
It was found that the reaction temperature played a critical
role in the formation of these carbon spheres. The process
proceeded at the temperatures lower than 450 °C could not
initiate the reaction. When the reaction was conducted at
500 °C, the main products were nano-rods and graphite.
Carbon spheres became the major products at 550 °C, as
shown in Fig. 2A. As the temperature was further increased
to 600 °C, a large number of the larger carbon spheres
formed in the as-synthesized sample, as shown in Fig. 5.
In the above reactant conditions, it was also found that
no CNTs were formed.
Compared with my previous work that carbon spheres
consisting of amorphous carbon cores and graphene shells
were once synthesized by reduction of CO2 using Li as the
reducing agent and proposed a possible growth mecha-
nism that nano-scale rough surface of lithium formed at
the interface between supercritical carbon dioxide and
liquid lithium took the roles of both the reductant for
reduction of carbon dioxide to carbon and the template
for growth of carbon spheres [13], there are many differ-
ences in them. Firstly, the melting point of metallic Ca
is 847 °C, which means that its phase in the reaction sys-
tem is solid. In order to make sure whether the metallic
Ca is solid in the reaction system, we conducted experi-
ments exactly the same as that described above except
Fig. 4. The TEM images and electron diffraction pattern of the as-prepared carbon spheres: (A, D) TEM images of the samples obtained at 550 °C; (B, C)
TEM image of the samples grown at 600 °C; (E) the TEM image of the products grown at 500 °C and (F) the electron diffraction pattern of a carbon
sphere grown at 550 °C.

Z. Lou et al. / Chemical Physics Letters 421 (2006) 584–588
587
image of the quadrangled-boxed area in Fig. 5A clearly
shows such small carbon spheres in the initial stage of
the growth, although most of the spheres show relative uni-
form diameters. At the same time, higher temperature
means larger size and more defects and disorders in carbon
spheres, which are confirmed by TEM images (Fig. 4C) and
Raman spectrum (Fig. 3B). At 600 °C carbon spheres are
2–5 lm larger than those of 550 °C in general. At 600 °C
the thermal movement becomes much fiercer and leads to
the formation of more defects and disorders in carbon
spheres. This could be conformed by Raman analysis
shown in Fig. 3B, in which ID/IG value is 1.16, much higher
than that of the products at 550 °C; moreover, a very small
proportion of the carbon spheres show the elliptic shape
(Fig. 4B). While the temperature is at 500 °C, the carbon
nano-granules tend to stay at certain place, causing the for-
mation of carbon nano-rods (Fig. 4E). This could explain
the fact that the nano-rods are short and do not have uni-
form diameters. The TEM image (Fig. 4D) of the products
synthesized at the critical temperature: 550 °C shows the
co-presence of carbon spheres and carbon nano-rods, with
carbon spheres as dominant products, providing further
evidences to our proposed mechanism. Then considering
the fact that the main products are carbon nano-rods at
500 °C and carbon spheres at 550 °C, the conclusion could
be drawn that the temperature play an important role on
the final shape of the products, which means that we could
control the shape of the products as well as the size of
spheres by carefully designing the reaction conditions.
According to our mechanism, between the calcium atom
and carbon atom lays oxygen atom, which means that
there should be no calcium in the as-synthesized carbon
spheres. We carry out SEM–EDX analysis of the carbon
sphere (the product without any treatment) shown in the
elliptic-boxed area in Fig. 5B, and no calcium was detected
in it. As to the other elements than carbon, oxygen atom
may be due to the influence of O2 in the air, while Cr
and Fe elements may come from the steel reactor.
Fig. 5. (A) SEM image of the products grown at 600 °C without any
treatment, the quadrangled-boxed area shows carbon spheres in the initial
stage of the growth (B). EDX of carbon spheres shown in the elliptic-
boxed area in (A).
that 3.0 g Ca was used. The shape of the Ca we used was
spherical shapes with diameters of 1–2 mm. After the
experiment, the shape remained but the diameters dimin-
ished, indicating that the Ca did not melt during the
experiment. Therefore, the droplet models cannot be
adopted in the Ca–CO2 system to describe the formation
of carbon spheres. Secondly, the crystallinity of carbon
spheres and the reaction temperature decreased. Finally,
the yield and the content of carbon spheres increased sig-
nificantly, and no CNTs were grown.
We propose a possible mechanism for the formation of
carbon spheres. In our experiment, metallic Ca atom bonds
with O atom in CO2 molecule, forming CaO and CO, then
Ca atom reacts with CO and lead to the emergence of car-
bon nano-granules. At the same time, at certain reaction
temperature CO could decompose as follows:
4. Conclusions
In summary, approximate uniform diameter carbon
spheres have been successfully synthesized in high yield
by chemical reduction of dense CO2 with metallic calcium
at 550 °C. It is found that the reaction temperature has cru-
cial effects on the morphology of the final product. At the
same time, the growth mechanism of carbon spheres in
supercritical CO2 system is suggested. This experiment
may provide a new method for the large-scale preparation
of carbon spheres as well as the comprehensive treatment
of CO2.
CO ! CO2 þ C
While this reaction could not form a large quantity of
carbon products, because of the excess amount of CO2.
So the resulted C from the above reaction might account
for amorphous carbon other than carbon spheres in the
products. Then if the temperature is high (P550 °C), the
thermal movement of the carbon nano-granules will be
fierce, and the nano-granules could roll on the surface of
the metallic calcium and incorporate new carbon atoms,
which leads to the formation of carbon spheres. Because
of the statistic rule of thermal movement, most of the car-
bon spheres are perfect spherical. In the framework of our
hypothesis, the synthesized carbon spheres should follow a
process of ‘growing’ from small to large, and the SEM
Acknowledgement
This work was supported by the National Natural Sci-
ence Foundation of China (20321101, 20125103 and
90206034).

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Z. Lou et al. / Chemical Physics Letters 421 (2006) 584–588
[10] V.L. Kuznetsov, A.L. Chuvilin, Y.V. Butenko, I.Y. Mal’kov, V.M.
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