Preparation of a platelike carbon nanomaterial using MgO as a template

Article abstract of DOI:10.1134/S0020168512020070

A carbon nanomaterial in the form of hollow hexagonal platelets with shells of disordered graphene layers has been synthesized through CH₄ pyrolysis on pseudomorphic hexagonal MgO platelets 1-2 μm in average size, followed by dissolution of the magnesium oxide. The material has a specific surface area above 1300 m²/g, specific pore volume of 3.23 cm ³/g, and resistivity of 0.08 Ω cm. Pleiades Publishing, Ltd., 2012.

Full text of DOI:10.1134/S0020168512020070

ISSN 0020ꢀ1685, Inorganic Materials, 2012, Vol. 48, No. 3, pp. 244–248. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © S.Yu. Davydov, A.Yu. Kryukov, V.O. Gerya, I.M. Izvol’skii, E.G. Rakov, 2012, published in Neorganicheskie Materialy, 2012, Vol. 48, No. 3, pp. 297–301.
Preparation of a Platelike Carbon Nanomaterial
Using MgO As a Template
S. Yu. Davydova, A. Yu. Kryukovb, V. O. Geryab, I. M. Izvol’skiia, and E. G. Rakovb
a OOO Global SO, Moscow, Russia
b Mendeleev University of Chemical Technology, Miusskaya pl. 9, Moscow, 125047 Russia
eꢀmail: davydoff_s@mail.ru
Received May 23, 2011
Abstract—A carbon nanomaterial in the form of hollow hexagonal platelets with shells of disordered
graphene layers has been synthesized through CH4 pyrolysis on pseudomorphic hexagonal MgO platelets
1–2 m in average size, followed by dissolution of the magnesium oxide. The material has a specific surface
μ
area above 1300 m2/g, specific pore volume of 3.23 cm3/g, and resistivity of 0.08
Ω
cm.
DOI: 10.1134/S0020168512020070
INTRODUCTION
The objectives of this work were to prepare flat
MgO particles, select conditions for graphene shell
synthesis on such particles, and investigate some propꢀ
erties of the shells.
Magnesium oxide has surface defect sites with
strongly basic (alkaline) properties [1] and can be used
as a specific catalyst for many reactions [2], including
hydrocarbon pyrolysis [3–7].
Graphene shells forming on MgO nanoparticles
during pyrolysis under relatively mild conditions have
the form of nanoislands one to three graphene layers in
thickness [4]. According to Rümmeli et al. [7], the
number of graphene layers grown on MgO through
С6Н12 pyrolysis is usually three to eight, independent
of the process duration. MgO particles retain their
shape and can be removed by dissolving them away in an
acid, which attests to good permeability of the graphene
shells. Rümmeli et al. [7] presented micrographs of
electronꢀtransparent shells in the form of square plateꢀ
lets one to three graphene layers in thickness.
EXPERIMENTAL
The starting material used was commercially availꢀ
able Mg(OH)2. The thermal decomposition of the
Mg(OH)2 was carried out in air with allowance for earꢀ
lier results on the kinetics of the process [12] and our
thermal analysis data. A weighed amount of Mg(OH)2
was heated by raising the temperature at a rate of
20°C/min to 800°С, followed by slow cooling.
According to Chizallet et al. [13], it is at 800°С that
OHꢀfree MgO surfaces can be prepared.
Graphene layers were grown through the pyrolysis
of natural gas (>95% СН4) in the range 600–800 for
a short time by rapidly heating a weighed amount of
the gas to a predetermined temperature and then rapꢀ
idly cooling it to room temperature.
°C
MgO particles prepared through Mg(OCH3)2
hydrolysis in a mixture of СН3ОН and С6Н5СН3, folꢀ
lowed by drying under supercritical conditions, and
having a specific surface area
S
= 380 m2/g were
up to
reported to have graphene shells ranging in
S
Electronꢀmicroscopic examination was performed
on a scanning (JEOL JSMꢀ6490LV) and a transmisꢀ
sion (JEOL JEMꢀ2100FꢀCs) electron microscope.
The specific surface area was determined by
Brunauer–Emmett–Teller measurements using a
Quantachrome Autosorbꢀ1C/MS/TPR analyzer.
Porosity was evaluated from the N2 volume adsorbed,
using a SorbiꢀMS instrument. The pore size distribuꢀ
tion was calculated in the Barrett–Joyner–Halenda
model. Electrical conductivity was measured by the
fourꢀpoint probe technique using an MHꢀ10 microꢀ
ohmmeter.
2000–2500 m2/g [6]. It is reasonable to expect that
such shells consist predominantly of one or two
graphene layers.
The results of the aboveꢀmentioned studies demꢀ
onstrate that, under certain conditions, catalytic
hydrocarbon pyrolysis is a selfꢀextinguishing process
which stops when the oxide surface becomes covered
with a small number of graphene layers.
Мg(OH)2, often used as a MgO precursor in variꢀ
ous studies [4, 8, 9], is known to crystallize in hexagoꢀ
nal symmetry (sp. gr. С3m) and to have a tendency to
form flat hexagonal prisms with a base side of 50–
60 nm [10, 11].
Thermoanalytical (DTA + DTG) curves were
obtained on a Navas TGA 1000 analyzer. The meaꢀ
244

PREPARATION OF A PLATELIKE CARBON NANOMATERIAL
245
The thermal decomposition of Mg(OH)2 to MgO
had little effect on the shape and size of the particles
(Fig. 2), causing some of the agglomerates to break
down. The platelets ranged in thickness from 86 to
173 nm (with an average of 140 nm), and the hexagon
side length was 550 to 910 nm (with an average of
720 nm).
The
= 65.07 m2/g, was an order of magnitude greater
than the of the parent Mg(OH)2. This attested to
S obtained after the thermal decomposition,
S
S
cracking and disintegration of the parent particles,
with no changes in shape (pseudomorphic conversion
of Mg(OH)2 into MgO). The resultant MgO contained
pores with an average diameter of 25.4 nm and specific
volume of 0.41 cm3/g.
1 μm
Fig. 1. SEM image of Mg(OH) precursor particles.
2
The present results demonstrate that MgO can be
prepared in the form of comparatively large platelike
polycrystals similar in shape to the parent magnesium
hydroxide particles, which is important for subsequent
deposition of graphene layers on MgO. The MgO
platelets were slightly larger than the Mg(OH)2 plateꢀ
lets (by ~15–20% in thickness and 3–5% in lateral
size).
surements were made in air at a heating rate of
10 C/min.
°
RESULTS AND DISCUSSION
Thermal decomposition of magnesium hydroxide.
Figure 1 is a scanning electron microscopy (SEM)
image of Mg(OH)2 precursor particles. The particles
have the form of microcrystals in the shape of hexagoꢀ
nal platelets with rounded corners. The shape of the
crystals is due to the hexagonal crystal lattice of
Mg(OH)2. The platelets ranged in thickness from 67 to
170 nm (with an average of 115 nm as derived from
40 measurements), and the hexagon side length was
490 to 974 nm (with an average of 690 nm as derived
from eight measurements); that is, the particles were
substantially larger than those described in previous
Methane pyrolysis on MgO platelets. Figure 3 is an
SEM micrograph of 800°C methane pyrolysis prodꢀ
ucts. The pyrolysis products were similar in shape to
the Mg(OH)2 and MgO particles, with the only differꢀ
ence that, after MgO dissolution, they had the form of
hollow shells. The shells ranged in thickness from 65 to
140 nm (with an average of 95 nm as derived from
26 measurements), and the base side length of the flat
hexagonal prisms ranged from 520 to 850 nm (with an
average of 670 nm as derived from 33 measurements).
The thickness of the hollow carbon particles was less
than that of the Mg(OН)2 and MgO particles by 6–10
reports [10, 11]. The
S of the Mg(OH)2 particles was
6 m2/g. The particles were arranged at random and and 30–35%, respectively, and the hexagon sides were
formed agglomerates consisting of up to 20 particles. shorter by 2–5 and 5–10%, which can be accounted
1 μm
1 μm
Fig. 2. SEM image of MgO particles produced by the therꢀ
Fig. 3. SEM image of the product of methane pyrolysis on
MgO.
mal decomposition of Mg(OH) .
2
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246
DAVYDOV et al.
and a pyrolysis time of 10, 15, and 30 min, the yield
was 3.9, 4.8, and 8.0 wt % relative to MgO.
The highꢀresolution transmission electron microsꢀ
copy (HRTEM) image in Fig. 4 demonstrates that the
graphene shells are composed of one to four irregular
graphene layers. The material consists of blocks and
has a certain texture due to preferential orientation of
the MgO platelets. The blocks are almost rectangular
in shape and 10–15 nm in size (Fig. 5). Clearly, the
graphene shells obtained in our experiments are aggreꢀ
gates.
Because of their small thickness, the shells are
transparent to electrons, as illustrated by images of
particles normal to the beam direction (Fig. 6). It is
worth noting rather even edges of the particles, which
have the form of hexagons with a vertex angle slightly
above 120
°.
The specific surface area of the synthesized
graphene material depends on pyrolysis conditions:
S
10 nm
= 1585, 1335, and 1297 m2/g at pyrolysis times of 10,
Fig. 4. Graphene layers in the product of methane pyrolyꢀ
sis on MgO (HRTEM image).
15, and 30 min, respectively. These
S values correꢀ
spond to bilayer graphene particles (the values calcuꢀ
lated for monoꢀ and bilayer graphene are 2630 and
1315 m2/g, respectively). The material is porous, with
a net specific pore volume of 3.23 cm3/g and average
pore diameter of 9.98 nm. The pore size distribution is
bimodal: the pores 3.7 nm in diameter account for the
largest volume. The second maximum, at 11.5 nm,
coincides with the average pore size in the magnesium
oxide.
for by the slight contraction of the carbon shells during
dissolution and subsequent drying.
Like in previous studies [3–7], MgO was used as a
template, but the size of the graphene shells in our
experiments considerably exceeded that of the hollow
carbon particles described earlier (1.4–10 nm). The
graphene shell yield was rather moderate: at 800
°C
5 nm
20 nm
Fig. 5. Texture of the product of methane pyrolysis on
MgO (HRTEM image).
Fig. 6. Layer of transparent carbon shells (HRTEM
image).
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PREPARATION OF A PLATELIKE CARBON NANOMATERIAL
247
Comparison with the porosity of other carbon enabled the first synthesis of flat carbon shells similar
materials (e.g., with data in Beyaz et al. [14]) demonꢀ in shape to the Mg(OH)2 microcrystals. The shells
strates that the specific micropore volume in the
have a large specific surface area and high porosity,
graphene shells (1.22 cm3/g) considerably exceeds levꢀ are good conductors, and consist of blocks containꢀ
els typical of activated carbon, carbon nanotubes, and ing a small number of graphene layers with irregular
carbon nanofibers. Moreover, the fraction of structure.
micropores in the graphene shells is also very large and
Such carbon shells have considerable potential for
roughly corresponds to their fraction in carbon nanoꢀ
practical application and can probably be produced by
tubes after oxidative activation [15]. At the same time,
pyrolysis of other hydrocarbons.
the size of the pores that make the largest contribution
to the total pore volume in the graphene shells is conꢀ
ACKNOWLEDGMENTS
siderably greater than that in singleꢀwalled (1.20 nm
[16]) and multiwalled (0.6 nm [17]) carbon nanotubes.
We are grateful to S.V. Savilov for his assistance in
the electronꢀmicroscopic work.
It is reasonable to assume that grapheneꢀcoated
MgO particles and the graphene shells are of interest
as unique adsorbents and electrodes for lithium ion
batteries and electrochemical capacitors [18]. The use
of graphene shells as electrodes is also prompted by the
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