CARBON
4 6 ( 2 0 0 8 ) 1 5 7 9 –1 5 8 7
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
The effect of graphitization catalyst on the structure
and porosity of SiC derived carbons
Maike Käärika,b,*, Mati Aruleppb, Mati Karelsona, Jaan Leisa,b
a
Institute of Chemistry, University of Tartu, 2 Jakobi Street, 51014 Tartu, Estonia
Tartu Technologies Ltd., 185 Riia Street, 51014 Tartu, Estonia
b
A R T I C L E I N F O
A B S T R A C T
Article history:
A number of carbide-derived carbon (CDC) samples were synthesized through the reaction
Received 28 April 2008
between a-SiC and gaseous chlorine at temperatures 900, 1000 and 1100 °C and by varying
Accepted 1 July 2008
the amount of catalyst. The chlorides of Co(II), Ni(II) and Fe(III) were used as catalytic addi-
Available online 10 July 2008
tives in a range of concentration of 0.1–5 wt%. The structural differences of the obtained
carbons were studied by low-temperature nitrogen adsorption, X-ray diffraction and
Raman spectroscopy. Results showed that porosity, specific surface area and graphitization
degree of the CDC materials is a function of chlorination temperature and catalyst concentration, which agrees with previous results. It was shown that the catalytic graphitization
only weakly influences the La value of the crystallites, which according to the Raman scattering is 4–5 nm in both the highly disordered SiC derived carbons and in fully graphitic
carbons made from SiC containing 15 wt% of surface-contacted Co–Ni–Fe catalyst. The surface area of the CDC materials can be controlled in the range of 300–1350 m2 g1, depending
on the amount of catalysts used.
Ó 2008 Elsevier Ltd. All rights reserved.
1.
Introduction
In last decade the carbide-derived carbon materials (CDC)
have been under extensive research. It has been demonstrated that it is possible to synthesize almost all known carbon structures including amorphous [1] and nanocrystalline
carbon [2], carbon onions [3], nano-barrels [4], nano-diamond
[5], graphite ribbons and ordered graphite [6]. There are various applications suggested for CDC based materials, most of
them being adsorption based: molecular sieves, adsorbents
for gas chromatography, electrodes for supercapacitors and
Li-ion batteries, hydrogen storage, and many more [7]. Furthermore, the very recent studies have shown their potential
suitability also for the electronic applications, such as fieldemission displays [8]. The electron emission ability of CDC
is strongly influenced by its nanostructure and the ordering
of graphene sheets in carbon. Therefore it is important to
learn more about the ways to vary the graphitic order in
CDC materials and precisely to control the parameters affecting the CDC nanostructure.
CDC has been produced from many different carbides [9]
such as Al4C3 [4,10], SiC [11,12], TiC [13,14], ZrC [15], NbC
[16], B4C [17], Fe3C [6], and VC [18]. Previous studies show that
carbon particles ‘‘remember’’ the shape and size of origin carbide and are greatly influenced by origin carbides chemical
and structural compositions. Hence, it is possible to synthesize carbon materials with desired macro- and microstructure
by varying the carbide type and synthesis parameters. It is
well known that increasing synthesis temperature produces
a more ordered structure [4]. The graphitization degree and
nanostructure formation can be influenced by catalysts. It is
possible to synthesize graphite-structured carbon at substantially lower temperatures by using chlorides of d-metals as
catalysts [10,13].
* Corresponding author: Address: Institute of Chemistry, University of Tartu, 2 Jakobi Street, 51014 Tartu, Estonia. Fax: +372 7428467.
E-mail address: [email protected] (M. Käärik).
0008-6223/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carbon.2008.07.003
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The present paper deals with the catalyst assisted carbonization of a-SiC, which behaves noticeably differently compared to the most of metal carbides, because of worse
chemical reactivity. This paper supplements the series of articles about the catalytic carbonization of carbides. The previous studies on the catalyst assisted chlorination of TiC [13]
and Al4C3 [10] did not consider the fact that catalytic effect
of the d-metal chlorides is more or less local. However, it inevitably gives a raise to the heterogeneous carbon structure, but
can also be used for developing the structurally layered CDC
particles.
This is also the first time when the structural aspects of
catalytically born CDC are systematically considered by
means of Raman and XRD analysis. Through the adsorption
and structural study, we show the effect of synthesis temperature and amount of the Co–Ni–Fe catalyst on the structural
order and porosity of wide range of a-SiC derived carbons.
We show that it is possible to assure a good control over the
relative structural ordering, which for instance, is an important issue regarding the electronic and field-emission properties of CDC.
2.
Experimental
All CDC samples of this study were made by the following
general method. SiC (Sika Tech, FCP13C, 0.8 lm) placed in
quartz boat was thoroughly mixed with the different amounts
of graphitization catalyst, which was composed from the
equal quantities of cobalt(II), nickel(II) and iron(III) chlorides
dissolved in ethanol. The amount of catalyst in different
experiments was varied between 0 and 150 mg/g of the carbide. The ethanol was evaporated by heating the reaction vessel at temperature 70–100 °C. The dry SiC/catalyst composite
was thereafter placed in the quartz boat and was reacted with
a flow of chlorine gas (99.999%) in a horizontal quartz tube at
a fixed temperature. The experiments were made at 900 °C,
1000 °C and 1100 °C. The by-product, SiCl4, was removed by
the stream of excess chlorine. During the heating and cooling
the reactor was flushed with a slow stream of argon. After
chlorination the product was additionally treated with hydro-
gen at 800 °C to deeply dechlorinate the sample. The final
yields of the carbon were between 80% and 93% from theoretical. The general reaction equation for SiC derived carbons is
given below
SiC þ 2Cl2 ! C þ SiCl4
Sorption analysis of carbon samples was done at the boiling
temperature of nitrogen using a Gemini Sorptometer 2337
(Micromeritics). The raw data were collected and treated with
the ‘‘Stardriver’’ software. The samples were degassed overnight under vacuum at 300 °C and backfilled with argon gas
before the measurement. The specific surface area (SA) of carbon was calculated according to BET theory [19] up to the
nitrogen relative pressure of P/P0 = 0.2. The total volume of
pores (Vp) was measured at near to a saturation pressure (P/
P0 = 0.97). The volume of micropores (Vl) was estimated from
t-plot method by using Harkins-Jura statistical thickness.
The vibration spectra of CDC samples were recorded by
micro-Raman spectrometer using Nd:YAG laser (k = 532 nm)
with a scanning range of 700–4000 cm1. The Raman spectra
of carbon powders of this study show two basic peaks at
wavelength 1350 cm1 (D-band) and 1580 cm1 (G-band)
that is in agreement with the general observations on amorphous carbons [20]. The ORIGIN software was used for the
analyses of raw spectra. In this work two Lorentzian curves
fittings were done to the G- and D-bands, respectively.
The wide-angle X-ray diffraction measurements were performed by Siemens powder diffractometer using Cu Ka radiation (k = 1.54 Å). The diffraction patterns were recorded at
25 °C and treated by the ORIGIN software.
3.
Results and discussion
A series of carbon materials (samples 1–15) were synthesized
from SiC without additives and from SiC mixed with varied
amount of Ni, Co and Fe chlorides. A comparison between
specific surface areas (SA) and pore volumes (Vp, Vl) of these
carbon materials is presented in Table 1. A general observation of this study is that with larger amount of catalyst and
higher synthesis temperature larger pores and smaller sur-
Table 1 – Porosity characteristics of SiC-derived carbons evaluated from the N2 adsorption measurements
Tchlor [°C]
Amount of catalyst [mg/g]
SA [m2 g1]
Vp [cm3 g1]
Vl [cm3 g1]
1
2
3
4
5
900
0
3
15
30
150
1359
1177
918
897
543
0.71
0.63
0.61
0.59
0.35
0.59
0.51
0.37
0.36
0.22
6
7
8
9
10
1000
0
3
15
30
150
1336
1235
956
714
368
0.71
0.70
0.52
0.52
0.41
0.58
0.53
0.40
0.28
0.11
11
12
13
14
15
1100
0
3
15
30
150
1327
1110
810
551
296
0.69
0.59
0.55
0.41
0.29
0.57
0.48
0.33
0.22
0.10
Carbon #
CARBON
4 6 ( 20 0 8 ) 1 5 7 9–15 8 7
face area were obtained. It was also expected as both the high
chlorination temperature and the graphitization catalyst increase the structural order. This gives raise to the graphitic
nano-clusters and multilayered ribbons, which significantly
reduce the specific surface, but may increase the overall
porosity due to the voids between the graphitic clusters and
nano-particles [13]. For example, the CDC sample 1, made
without catalyst at 900 °C, has the highest specific surface
area (SA) 1359 m2 g1, while the SA of sample 5, made with
150 mg/g catalysts at 900 °C, is only 543 m2 g1. Similar tendencies were revealed at all considered temperatures
(900 °C, 1000 °C and 1100 °C). Generally, the specific surface
areas decrease while the chlorination temperature increases,
however, the exception in this trend is the ‘‘low-temperature’’
samples 1–3, which have the smallest micropores among the
samples of this study. The temperature influence on the poresize of CDC materials has been previously discussed by Gogotsi et al. [21]. It has also been reported that the peak pore-size
of SiC-derived carbons is 0.65 nm [21]. Obviously, the diffusion
of adsorbate into the smallest micropores of samples 1–3 is
restricted that leads to the lower surface area compared to
the respective samples 6–8 made at 1000 °C. This suggestion
is in agreement with the small hysteresis observed in desorption curves of samples 1–3 as the possible result of molecular
sieving. The adsorption isotherms of CDC materials of this
study are presented in Fig. 1, which demonstrate that increasing of the concentration of catalysts in reaction medium decreases the relative amount of micropores and increases the
amount of mesopores. Without catalyst and with small
amount of catalyst the adsorption isotherms of samples belong to type I by the Brunauer classification [22], which is a
characteristic of microporous materials by IUPAC [23]. The
isotherms at increased catalyst concentration become more
similar to type II isotherms thus confirming the wider poresize distribution. The H4 type hysteresis loop in respective
1581
adsorption isotherms is seen between a relative pressure of
0.4 and 0.5. This type of hysteresis is caused by the capillary
condensation of adsorbate in the slit-shaped mesopores,
which is typical for partially graphitic carbon materials. The
hysteresis loop widened for the all samples synthesized at
1100 °C as represented in Fig. 1. This confirms that the higher
the chlorination temperature, the more graphitised the carbon is. In conclusion, the adsorption isotherms of a-SiC derived carbons are in good agreement with the previous
studies about TiC and Al4C3 derived carbons [10,13], which
all demonstrate that the graphitization increases by increasing amount of catalytic transition metals and likewise with
the reaction temperature. However, it has to be noted that
the effect of catalysts on CDC formation from a-SiC appears
at significantly higher temperature and is weaker than in
the case of above-mentioned titanium and aluminium
carbides.
The Raman spectra of carbon samples derived from a-SiC
at 1100 °C at various amounts of catalyst are shown in
Fig. 2. The first-order Raman spectrum features two peaks:
disorder-induced peak (D-band) at wavelength 1350 cm1
and the graphite peak (G-band) at 1580 cm1. The spectrum
also shows a second-order peak of the D-band (2D) at
2700 cm1, which is sometimes named as the G 0 -band as it
appears in more graphitic carbons. The increase of secondorder D-peak is related to the ordering of the graphitic structure [20,24]. In this study, the Lorentzian fitting to the
recorded Raman scattering spectra was used to extract the
G- and D-bands, whose shape and relative intensities carry
the important information about the catalyst behaviour on
the structural order and crystallinity of the CDC studied.
The Lorentzian curves for G- and D-bands for sample 15 are
presented in Fig. 3. Note that the fitting with two peaks is
not always justified, especially in the case of highly amorphous carbon structures. In such cases, the fitting by three
Fig. 1 – Low-temperature nitrogen adsorption isotherms for carbon materials made at 900–1100 °C at various amount of
catalyst, noted in figure.
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Fig. 2 – Raman spectra of carbon materials made at 1100 °C at various amount of catalyst, noted in figure.
Fig. 3 – Two Lorentzian curves fitting to the first-order Raman spectrum of sample 15.
or four peaks is advisable [25]. Nevertheless, in this work we
have used two-peak fitting, which produced reasonably good
matching to the experimental curve. The square of the correlation coefficient of the fitted curves was R2 0.99 for all 15
samples. Fig. 4 shows the dependence of the ratio of the
heights of D- and G-peaks (ID/IG) on the amount of catalyst
and on the chlorination temperature. It is apparent that the
samples made at 1000 °C and 1100 °C have a similar tendency
of graphitization according to ID/IG ratios, but are rather different from those made at 900 °C. A noticeable difference between the shapes of D- and G-bands was observed (cf. Fig. 5).
Very low intensity of diffusive Raman signals reveals the
highly amorphous structure of ‘‘low-temperature’’ SiC derived
carbons compared to the samples made at 1000 °C and
CARBON
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1583
Fig. 4 – Dependency of the ratio of the intensities of D- and G-peaks (ID/IG) on the amount of catalyst at various chlorination
temperatures.
Fig. 5 – Comparison of D- and G-bands of carbon materials made at 900 °C and 1000 °C at various amount of catalyst, noted in
figure.
1100 °C. One of the reasons is that at lower synthesis temperature (T = 900 °C) the graphitization and ordering of graphene
layers is affected mainly by the amount of surface-contacted
catalyst while at higher temperature the influence of the
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chlorination temperature becomes more important. The general conclusion is that the increase of the reaction temperature and catalysts concentration increases the intensity of
the G-band and decreases the intensity of D-band. The shape
of these peaks particularly depends on the crystallite size (La)
that often is calculated from the empirical formula proposed
by Tuinstra and Koening (T–K) [26]:
CðkL Þ=La ¼ ID =IG
The wavelength dependent prefactor C(kL) is derived as
C(kL) = 12.6 + 0.033kL, where kL = 532 nm. The minimum La
for which T–K formula has been directly verified is 2 nm
[20,25,27]. The ID/IG ratio and the La values according to T–K
for the samples of this study are given in Table 2. However,
it has to be considered that the T–K equation is originally derived for the polycrystalline graphitic carbons constituted in
100% of sp2 hybrid C–C bonds. The recent studies have revealed that CDC materials may include up to 10% of sp3 carbon depending on the precursor carbide and the conditions
of chlorination [28]. For the carbons including small amount
of sp3 bonds, Ferrari and Robertson (F–R) have proposed the
following equation [25]:
C0 ðkL Þ=L2a ¼ ID =IG
where the wavelength dependent prefactor is C 0 (kL) C/8. The
La values for the samples of this study according to F–R are in
the range of 1.1–1.4 nm, which coincide the in-plane correlation length for the different CDC type carbons made without
the catalytic additives [29].
Certainly, it is not a straightforward task to analyse the
structural order and crystallinity of this kind catalytically
graphitised carbons, since the catalyst in a solid-phase conversion of carbide into carbon has more or less a local effect.
Therefore, the surface of carbon particles formed with the
assisting catalyst support is heterogeneous regarding to the
core of particles, where the structure formation has been
mainly controlled by the reaction temperature. However,
according to the adsorption analysis discussed above, it is
also evident that at higher catalyst concentrations, the catalytic influence on the graphitization goes deeper in particles.
Only diffusion of catalysts in the particles during chlorination
can explain the surface areas as low as 300 m2 g1, e.g. for the
sample 15. Considering all this complexity, the average La values based on the ID/IG most probably lay in between of those
estimated by T–K and F–R equations.
It is still interesting to note that the La values seem to be
rather independent on the graphitization, which indicates
that even in highly graphitic samples the stacks of parallel
graphene layers must be noticeably curved.
The further problem with the Raman spectra of disordered
carbons is that the ID/IG ratio is dispersive because the D-peak
is dispersive, especially for highly amorphous carbon [24,30].
Therefore, it is being advisable to use direct XRD methods to
evaluate La [27,31]. Unfortunately, the problems arising due
to the heterogeneous structure of catalytically made CDCs,
are also met in the diffraction analysis.
The X-ray diffraction patterns of carbon samples synthesized at different temperatures and various amounts of catalyst are shown in Fig. 6. It was observed that the samples
made without catalyst produce only one broad diffraction
peak at 2H 43°, which is characteristic to the amorphous
carbon without a long range structure. The adding of only
minor amount of Ni–Co–Fe catalyst to the carbide creates
some structural order most probably on surface of the carbon
particles that is reflected by the Bragg 002 diffraction at 2H
26°. The large angle between diffraction pattern and the
baseline under the 002 peak, however, reveals the prevalence
of disordered amorphous structure of these carbons. Increasing of the catalyst amount gives raise to the four diffraction
peaks. The 002 and 004 Bragg diffraction peaks at 2H 26°
and 54° correspond to parallel graphene layers. The 10 and
11 diffraction peaks at 2H 43° and 78° characterize the 2D
in-plane symmetry along the graphene layers. This kind diffraction patterns are usual for turbostratic graphite. Turbostratic nature of the carbons of this study is well supported by
the d002 values, which range between 0.341 and 0.344 nm thus
Table 2 – Structural characteristics of SiC-derived carbons evaluated from the Raman D- and G-bands and X-ray 002, 100
and 10 diffraction patterns
Carbon #
ID/IG
La [nm]
(T–K)
d002 [nm]
I002/I10
(F–R)
La [nm]
Lc [nm]
(1 0 0)
(1 0)
1
2
3
4
5
1.12
1.09
1.09
1.11
1.04
4.5
4.6
4.6
4.5
4.8
1.3
1.3
1.3
1.3
1.3
–
–
0.343
0.343
0.343
–
–
1.98
3.40
5.08
–
–
–
10.1
15.1
2.2
2.8
4.2
5.6
7.0
–
–
5.9
6.2
7.8
6
7
8
9
10
1.1
1.09
1.03
0.88
0.84
4.6
4.6
4.9
5.7
6.0
1.3
1.3
1.3
1.2
1.2
–
0.343
0.344
0.342
0.342
–
0.58
2.64
3.65
5.53
–
–
–
17.0
20.4
2.8
3.6
5.3
4.4
6.7
–
6.4
6.6
7.0
8.5
11
12
13
14
15
1.14
1.01
0.91
0.93
0.81
4.4
5.0
5.5
5.4
6.2
1.4
1.3
1.2
1.2
1.1
–
0.342
0.342
0.343
0.341
–
0.58
3.56
5.56
6.20
–
–
10.4
16.7
18.0
2.6
2.5
5.8
4.4
5.9
–
8.5
9.4
9.3
9.5
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1585
Fig. 6 – Comparison of X-ray diffraction patterns of carbons synthesized at different chlorination temperatures by using the
various amount of catalyst, noted in figure.
confirming the significantly lower van der Waals interactions
between graphene layers compared to the perfect graphite
configuration. As the graphitization increases, the 10 peak is
supposed to split into 100 and 101 peaks. The effect was seen
for the carbons made by using of large amounts of the catalyst (P30 mg/g). However, the clear separation of 100 and
101 peaks was not observed even for highly graphitic samples
that confirms their turbostratic structure. It must be noticed
that catalytically made TiC derived carbons also possess turbostratic, curved lamellar structure rather than that of crystalline graphite [13]. The changes in structural order in
carbon samples can be seen by comparing the relative intensities of 002 and 10 reflections. The respective I002/I10 values
are presented in Table 2. In several former studies the ratio
of 002 and 10 peaks has also been used to quantify the parallelly stacked graphene layers [32].
The crystallite sizes La and Lc for the in-plane and crosssection of the multilayered carbon structures were calculated
using Scherrer’s formula:
La; c ¼ Kk=b cos H;
where k is the wavelength of the X-ray (0.154 nm), H is the position of the peak and b is the half-height width of the peak in
2H (rad) units. K is a constant depending on the reflection
plane (0.89 for the 002 peak in Lc calculations and 1.84 for
10 and 100 peaks in La calculations). The calculated crystallite
sizes are collected in Table 2. It was observed that the increase
of the Lc values is proportional with the increase of both, the
amount of catalyst and the temperature of chlorination. Since
the 002 peak occurs only in diffraction patterns of the CDC
samples made with the catalyst, it is obvious that the Lc value
specifically describes the catalytically generated multilayered
carbon clusters. It is interesting to note that the temperature
up to 1100 °C without the presence of catalyst in reaction
medium is not sufficient to initiate the graphitization, whereby in the presence of catalyst the temperature effect is noticeable. The earlier studies of the catalytic chlorination of TiC
and Al4C3 also indicated that the Co–Ni–Fe catalyst is activated at certain temperature, above which it significantly affects the graphitization [10,13]. The temperature, at which
the catalytic effect is established, is different for different carbides, evidently due to the different chemical activity of the
by-produced chloride in the reaction of chlorine with the
carbide.
Generally, it is recommended to use the graphite 100 peak
for the La calculation. Unfortunately, not always is this peak
well defined that may induce some discrepancies in the La
calculations [31]. In this study, the 100 and 101 peaks were
possible to separate only for the CDC samples made with larger concentration of catalysts. The separation of peaks was
done by Lorentzian curve fitting. The La values calculated
from the graphite 100 peak do not describe the average inplane correlation length of the catalytically made CDC samples, but obviously characterize only the more ordered surface of CDC particles, which was graphitised due to the
surface-contacted catalysts. Therefore, the XRD derived La
values are several times bigger than those from the Raman
spectra. The La values were also calculated from the 10 peak
of turbostratic carbon (see in Table 2), which is supposed to
give the more adequate characteristic for the structurally heterogeneous carbon samples of this study. Indeed, the estimated values, which vary from 2.2 nm to 7.0 nm, are much
closer to those from Raman spectra – 4.4 nm to 6.2 nm according to T–K equation.
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Conclusions
A carbon material was synthesized by chlorination of silicon
carbide (a-SiC) at three different temperatures and with different amounts of catalyst, composed from the equal quantities of Co(II), Ni(II) and Fe(III) chlorides. A range of catalyst
concentration in carbide was 0–15 wt%. It was shown that
the structure of the carbon depends on both the catalyst concentration and the synthesis temperature. The material with
the largest specific surface area was synthesized without catalyst at 900 °C and the material with the smallest specific surface area was made at 1100 °C by using 15 wt% of the catalyst.
The Raman and XRD studies show that the size of the crystallite increases with the increase of both catalyst concentration
and the temperature. Yet the changes in La values were rather
small according to the ID/IG from Raman spectra. According to
the Raman spectra, the bulk structure of a-SiC derived carbons made at 900 °C is amorphous even in the catalytically
made samples, which produce the well observed Bragg 002
diffraction peak in XRD pattern. It was also revealed that
the Ni–Co–Fe catalyst significantly increases the structural order in the SiC derived carbon at 1000–1100 °C. However, eventhough the angle of baseline under the 002 signal of carbon,
made from SiC with high concentration of catalysts, closes
to 0°, which indicates on the absence of amorphous phase
in carbon, the XRD and Raman analyses both show that the
disorder has not completely disappeared.
Finally, it may be concluded that the structural order,
porosity and the relative amount of graphitic phase of a-SiC
derived carbon can be well tuned by varying of chlorination
temperature and amount of Ni–Co–Fe catalyst surface-contacted to silicon carbide that is beneficial for developing the
CDC based materials for electronic applications.
Acknowledgements
This work was partly supported by Tartu Tehnoloogiad OÜ.
The authors wish to thank colleagues from Tartu Tehnoloogiad for assisting in the sample preparation and investigation.
Dr. Ahti Niilisk and Mr. Martti Pärs are thanked for the help
with the Raman study of carbon samples.
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