Pyrolysis in the mesophase: A chemist's approach toward preparing carbon nano- and microparticles

Article abstract of DOI:10.1021/ja020363u

A mild pyrolytic method is proposed for the generation of different carbon micro- and nanoparticles that are either unprecedented or have never been reported under the present experimental conditions. A hexa-alkyl-substituted hexa-peri-hexabenzocoronene serves as a graphite-like starting compound that melts into a discotic liquid crystalline phase prior to heat-induced cross-linking and dehydrogenation. An essential feature of the process is that the liquid crystalline order persists even above 400 °C, i.e., during alkyl chain cleavage. The present approach bears a resemblance to carbomesophase formation during graphitization starting from pitch. The pyrolysis products are characterized by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry, optical microscopy, selected area electron diffraction (SAED), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM).

Full text of DOI:10.1021/ja020363u

Published on Web 10/09/2002
Pyrolysis in the Mesophase: A Chemist’s Approach toward
Preparing Carbon Nano- and Microparticles
†
‡
†
†
Lileta Gherghel, Christian K ¨u bel, G ¨u nter Lieser, Hans-Joachim R ¨a der, and
,
†
Klaus M ¨u llen*
Contribution from the Max Planck Institute for Polymer Research, Ackermannweg 10,
5128 Mainz, Germany, and FEI Company, Achtseweg Noord 5,
5
5
600 MD EindhoVen, The Netherlands
Received March 11, 2002
Abstract: A mild pyrolytic method is proposed for the generation of different carbon micro- and nanoparticles
that are either unprecedented or have never been reported under the present experimental conditions. A
hexa-alkyl-substituted hexa-peri-hexabenzocoronene serves as a graphite-like starting compound that melts
into a discotic liquid crystalline phase prior to heat-induced cross-linking and dehydrogenation. An essential
feature of the process is that the liquid crystalline order persists even above 400 °C, i.e., during alkyl chain
cleavage. The present approach bears a resemblance to carbomesophase formation during graphitization
starting from pitch. The pyrolysis products are characterized by matrix-assisted laser desorption/ionization
time-of-flight (MALDI-TOF) mass spectrometry, optical microscopy, selected area electron diffraction (SAED),
scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM).
Introduction
methods, such as vacuum pyrolysis, catalytic pyrolysis of
8
b,10
organic compounds,
and catalytic chemical vapor deposi-
The chemistry of carbon in the past decade has been
dominated by the detection of fullerenes and carbon nanopar-
ticles as new carbon allotropes.2 The latter are of interest not
1
1
tion, can also be utilized to generate carbon nanoparticles.
In contrast to the common pyrolytic syntheses of carbon
nanomaterials, which usually occur catalytically or implicate
drastic conditions, we propose here a simple and effective new
route to carbon nano- and microparticles, by pyrolysis of well-
defined polycyclic aromatic hydrocarbons (PAHs) under mild
conditions. This approach is completely different from gas-phase
and solid-state pyrolysis but has similarity to the classical liquid-
phase carbonization of organic precursors such as small aromatic
1
,3
only for fundamental studies of mechanical and electric proper-
4
5,6
ties but also for a range of practical applications. Preparation
of carbon nanotubes, carbon onions, and other classes of carbon
6
particles typically requires high temperature or high pressure.
In many examples, carbon materials are synthesized by electric-
arc discharge techniques using graphite electrodes or laser
3,7
8
ablation of graphite targets. Chemical routes have also been
1
2-15
molecules, petroleum, or coal-tar pitches.
Because of its
9
attempted. It is well-known that in these cases the reactions
molecular structure (a two-dimensional disklike graphite subunit)
are chemically complicated and difficult to control. Pyrolytic
(
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(b) Guo, T.; Nikolaev, P.; Thess, A.; Colbert, D. T.; Smalley, R. E. Chem.
Phys. Lett. 1995, 243, 49-54.
*
Corresponding author: e-mail muellen@mpip-mainz.mpg.de.
Max Planck Institute for Polymer Research.
FEI Company.
†
‡
(
1) (a) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E.
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Fostiropoulos, K.; Huffman, D. R. Nature (London) 1990, 347, 354-358.
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H. W. J. Phys. Chem. Solids 1993, 54, 1841-1848. (b) Diaz, G.; Benaissa,
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(
(
(
13130
9
J. AM. CHEM. SOC. 2002, 124, 13130-13138
10.1021/ja020363u CCC: $22.00 © 2002 American Chemical Society

Preparing Carbon Nano- and Microparticles
A R T I C L E S
Chart 1. Molecules 1a and 1b and the Concept of the Pyrolytic
Approach.
Figure 1. MALDI-TOF mass spectra of pyrolytic products measured with
TCNQ as matrix. (a) Starting compound 1b. (b) Product distribution after
6
4
h of heating at 400 °C. (c) Product distribution after 72 h of heating at
00 °C; (inset) expanded view of mass range >2000 Da.
Obtained after the First Step of Pyrolysis. We have recently
shown that PAHs such as hexa-peri-hexabenzocoronene (HBC)
1
a (Chart 1) and higher homologues can be readily synthesized
and characterized as well-defined “graphite” molecules. The
synthetic approach, which involves intramolecular cyclodehy-
drogenation of oligophenylene precursors, can be modified to
attach alkyl substituents (1b). As mentioned above, alkyl-
substituted HBC 1b shows an extremely stable discotic me-
sophase and a remarkably wide thermal LC phase window (TkfD
)
107 °C and max Ti ) 417 °C) in which the molecules are
organized in an ordered columnar arrangement with a hexagonal
1
6
and its exceptional mesomorphic properties, alkyl-substituted
hexa-peri-hexabenzocoronene (HBC) 1b (Chart 1) is an excel-
lent candidate for this study.16 The flexible alkyl chains
dramatically lower the melting points and induce the formation
of extremely stable discotic mesophases with a columnar
arrangement of the disk molecules. This is utilized during
thermolysis at moderate temperatures to build up even larger
structures that preserve the liquid crystalline (LC) order. The
concept rests upon thermal oligomerization of 1b subunits
following the cleavage of alkyl side chains under preservation
of the initial mesomorphic order. This enables the lowering of
the classical temperature of graphitization and it is expected
that, upon further increase of the temperature up to 800 °C, the
extended PAH structures would lead to the formation of
graphitic structures.
superstructure. As previously mentioned, the aim of the first
step of the pyrolysis is to obtain a network of PAHs in which
the supramolecular order of the liquid crystalline phase is
preserved and to stabilize this established order. To elucidate
the initial reaction mechanisms and the progress of our “soft-
pyrolysis” experiment, the intermediate products were investi-
gated by matrix-assisted laser desorption/ionization time-of-
flight mass spectrometry (MALDI-TOF MS) at different time
lengths of thermal treatment. Therefore, two pyrolysis experi-
ments were performed with 1b under identical conditions in
evacuated and sealed quartz ampules at 400 °C (for details see
Experimental Section). The first ampule was opened after 6 h
of thermal treatment and the second after 72 h. The MALDI-
TOF measurements were carried out with a sample preparation
especially developed for the analysis of commonly insoluble
large PAHs. It is known from previous investigations1
7,18
that
On the basis of this concept, we chose to perform the
pyrolysis in two steps: (1) heating to 400 °C to form large
intermediate structures and prolonged heat treatment at this
temperature to provide a high orientation of these structural
units, as a consequence of their mobility in the melt; and (2)
further heating to the final temperature at max 800 °C, a
PAHs have to be measured with very soft desorption and
ionization conditions because of their tendency to form frag-
ments and coalescence products under laser irradiation. Details
of the sample preparation for MALDI-TOF analysis are
described in the Experimental Section.
temperature remarkably lower than the normally used graphi-
The spectrum in Figure 1a was measured before thermal
treatment and shows exclusively the molecular ion of 1b at
tization temperatures (2000-3000 °C),1
2-14
with development
of graphite-related structures.
(
(
(
16) Watson, M. D.; Fechtenk o¨ tter, A.; M u¨ llen, K. Chem. ReV. 2001, 101, 1267-
1300.
Results and Discussion
17) Yoshimura, K.; Przybilla, L.; Ito, S.; Brand, J. D.; Wehmeier, M.; R a¨ der,
H. J.; M u¨ llen K. Macromol. Chem. Phys. 2001, 202, 215-222.
18) Przybilla, L.; Brand, J. D.; Yoshimura, K.; R a¨ der, H. J.; M u¨ llen, K. Anal.
Chem. 2000, 72, 4591-4597.
A. Formation of Large Mesoscopic Structures. 1. MALDI-
TOF Mass Spectrometric Characterization of Products
J. AM. CHEM. SOC.
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A R T I C L E S
531.3 Da as a radical cation produced by photoionization. The
Gherghel et al.
1
small signals in the low molecular weight region are due to the
tetracyanoquinodimethane (TCNQ) matrix. Figure 1b reveals
the spectrum of the same sample after heat treatment for 6 h at
4
00 °C.19 At this early state of thermal decomposition, numerous
new compounds are already visible, which form two main
groups of mass distributions around approximately 1200 and
2
400 Da. In the lower mass range, near the molecular ion of
the intact PAH at 1531.3 Da, a series of intense signals appears
with masses of 1377.1, 1222.9, 1068.7, and 914.6 Da. The mass
differences are multiples of 154.2 Da and originate from a
preferential loss of several C11H22 moieties.
Obviously the thermal fragmentation of the complete dodecyl
side chains seems to be unfavorable because of the special
stabilization of the benzyl-like structures that can be formed
when a residual methylene group is left at the HBC core. Mass
differences of 14 Da, appearing among the most intense thermal
fragments, can be assigned to different numbers of residual
methylene units at the HBC core and indicate that the cleavage
of the alkyl side groups of 1b also takes place at any position
of the saturated hydrocarbon chain. The second distribution of
signals in a mass range between 2000 and 2800 Da in Figure
Figure 2. (a) Optical micrograph between crossed polarizers of 1b after
annealing at 400 °C for 72 h; scale bar 10 µm. (b) Surface of the same
sample imaged with differential interference contrast in reflected light; scale
bar 50 µm. (c) Optical micrograph of 1b between crossed polarizers after
short annealing at 400 °C; scale bar 100 µm. (d) Optical micrograph of 1b
between crossed polarizers after prolonged annealing for 72 h at 400 °C;
scale bar 20 µm. The micrographs were recorded at room temperature.
1b shows, on average, twice the molecular weight of the thermal
fragments described before. The interpretation of the exact
isotopically resolved masses suggests that this mass distribution
is due to dimeric molecules with two intact HBC cores, linked
by an alkyl spacer. The close relationship between the low
molecular weight distribution around 1200 Da and the high
molecular weight distribution around 2400 Da proves that the
dimeric molecules are created by a statistical recombination of
reactive thermal fragments of 1b during pyrolysis. A magnified
view of the mass region above 3000 Da shows a third
distribution of signals around 3600 Da, indicating that even
trimeric molecules are formed at this early stage of the process.
The mass spectrum obtained after 72 h of heat treatment of
increased number of double bonds and rearrangement reactions
in the alkyl chains. In addition to the molecular weight
distributions of monomers and dimers, higher aggregates (up
to heptamers) are observed also at this extended time of
2
0a
pyrolysis.
The results of MALDI-TOF mass spectrometry reveal that
the oligomerization of 1b occurs in two steps: (1) preferential
cleavage of the CR-Câ alkyl bonds and formation of free
radicals, followed by (2) their intermolecular cross-linking.2
The aromatic core of HBC remains intact during heat treatment
at 400 °C. With increasing time of pyrolysis, a general tendency
toward products with increasing molecular weight and increasing
carbon content is observed. From the alteration of the product
distribution during the reaction time, it can be concluded that a
prolonged pyrolytic decomposition at 400 °C proceeds until the
saturated alkyl chains are completely removed or form unsatu-
rated or even aromatic structures themselves by rearrangement
or cross-linking reactions.
0b
1b at 400 °C (Figure 1c) still shows a significant signal of the
intact starting compound at 1532 Da, while the structured
molecular weight distribution of Figure 1b, which originates
from a preferential cleavage of C11H22 hydrocarbon chains, has
disappeared completely. The most intense signals between 550
and 700 Da are equally spaced by 14 Da and are caused by
intact HBC cores bearing different numbers of CH2 units. The
lowest mass of this signal series at 564 Da can be assigned to
an intact HBC core substituted with three methyl groups. This
demonstrates that the cleavage of alkyl chains now also takes
place directly at the aromatic system within this period of
extended thermal treatment. The second molecular weight
distribution around 1250 Da is due to dimeric HBC molecules
as already observed in the spectrum of Figure 1b, however, with
significantly lower molecular weights due to the increased loss
of alkyl chains. The most intense signal of this series at 1238
Da can be assigned exactly to a chemically bound dimer of a
thermal HBC fragment with seven CH2 units at 620 Da, which
is the most probable structure in the low mass region. Whereas
the signals of the low mass end of the HBC dimer distribution
are equally spaced by 14 Da, this signal structure is lost in the
high mass end where a continuum of signals can be observed
at each nominal mass value. This can be explained by an
2
. Investigation by Thermogravimetric Analysis and
Characterization of Pyrolytic Products (Large Mesoscopic
Structures) by Optical Microscopy. Investigation of 1b by
thermogravimetric analysis (TGA), performed under a nitrogen
-
1
atmosphere at a heating rate of 10 K min from room
temperature up to 900 °C, shows that the substance is stable up
to 300 °C. Above this temperature the decomposition proceeds
in two steps. The first step occurs in a temperature range
between 300 and 500 °C, showing a weight decrease of 55-
5
8%, and this process can be explained by the gradual loss of
alkyl side chains. Above 500 °C the thermal degradation
includes the loss of aromatic units. The thermogravimetric
analysis of the pyrolytic products of 1b, obtained by heat
treatment for 72 h at 400 °C, was different than that of 1b.
Now the weight loss occurs constantly and completely, in one
step, in the temperature range between 450 and 840 °C. Possible
(
19) The pyrolytic products obtained by heat treatment for 6 h at 400 °C are
soluble in organic solvents and permit the use of conventional techniques
for structure elucidation.
(20) (a) Ida, T.; Akada, K.; Okuda, T.; Miyake, M.; Nomura, M. Carbon 1995,
33 (5), 625-631. (b) Lewis, I. C.; Kovac, C. A. Carbon 1978, 16, 425-
429.
13132 J. AM. CHEM. SOC.
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Preparing Carbon Nano- and Microparticles
A R T I C L E S
Table 1. Overview of the Micro- and Nanoparticles Formed by Mild Pyrolysis
type
description
dimensions
temp (°C)
substrate
A
B
microspheres (Figure 3a)
microfibers with “bamboo-shaped”
structure (Figure 3b,c)
diameter a few hundred nanometers to 2 µm
up to 10 µm in cross section and 1-2 mm in length; linear chains
of “compartments” of length 10-20 µm
diameter about 20 nm; lengths from a few hundred nanometers to
a few micrometers
800
800
quartz
quartz
C
nanowires (Figures 3d, 4, and 5)
800
quartz
a
D
E
F
G
H
I
cylindrical nanorods
outer diameters in the 100 nm range; lengths up to a few micrometers
diameters in the 100 nm range; lengths up to a few micrometers
edge lengths a few hundred nanometers to 1 µm
square cross sections of about 500 nm, lengths several micrometers
dimensions less than 200 nm
100-200 nm diameter and micrometer lengths
stems and branches up to 500 nm
1-10 mm cross section and micrometer lengths
800
800
800
800
650
650
650
650
quartz
quartz
quartz
quartz
quartz
mica
mica
mica
a
nanorods with 90° kink (Figure 6a)
nanoobjects with quadratic geometry
nanocolumns (Figure 6b)
doughnutlike nanoparticles (Figure 7)
nanosticks (Figure 8a)
Y-branched nanosticks (Figure 8a)
microobjects with hexagonal symmetry and
zigzag shape (Figure 8c,d)
3
1
a
a
J
K
3
5
L
rootlike microobjects (Figure 9)
cross sections 400 nm-1 µm; lengths 30 µm and more
650
mica
a
Precursor compound 1b was coated between two quartz plates and preheated at 220 °C.
Figure 3. SEM images of micro- and nanoparticles formed by pyrolysis at 800 °C and deposited on quartz. (a) Microobjects with radial spherical texture
(
type A); scale bar 5 µm. (b, c) Carbon microfibers with bamboo-shaped structure (type B); scale bars (b) 100 µm, (c) 500 µm. (d) Coalescence of the
nanowires in the bundle (type C); scale bar 200 nm.
reasons for the finally occurring 100% mass loss of this sample
can be either the complete thermal decomposition into volatile
fragments or the total oxidation by traces of oxygen.
disappear and domains with fanlike textures emerge, indicating
the existence of a columnar discotic phase. These domains grow
during heat treatment up to 400 °C. Figure 2c shows the
dominant liquid crystalline fanlike texture obtained upon short
heating of 1b at 400 °C. Annealing the sample for 1 h at 400
°C under nitrogen atmosphere does not change the thermal
behavior of the sample significantly. Further increase of the
temperature leads to the transition into an optically isotropic
phase at 420 °C.
2
1
For a further characterization of the pyrolytic products
obtained after heating for 72 h at 400 °C, investigations by
optical microscopy between crossed polarizers were conducted.
Figure 2a shows well-developed birefringent domains with
fanlike textures, which are visible also by differential interfer-
ence contrast in reflected light (Figure 2b). These textures,
typical for a columnar mesophase, suggest that the liquid
crystalline order persists even during alkyl chain cleavage and
intermolecular cross-linking reactions to form oligomers.
An investigation of the liquid crystalline textures reveals that,
above 230 °C, residual areas of the crystalline morphology
Comparatively, the prolonged pyrolysis of the sample for 72
h at 400 °C stabilizes the liquid crystalline texture, and
subsequent heating to 600 °C, the maximum temperature
attainable with the hot stage of the optical microscope, does
not destroy this texture. Figure 2d shows the dominant liquid
crystalline fanlike texture upon prolonged annealing of the
sample at 400 °C.
(
21) For thermogravimetric analysis of 1b and of the pyrolytic products formed
at 400 °C, see Supporting Information.
J. AM. CHEM. SOC.
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A R T I C L E S
Gherghel et al.
Figure 4. HRTEM images showing isolated graphitic particles; scale bars: (a) 10 nm; (b) 5 nm.
Figure 5. Morphology of the coalescence of the nanowires in the bundle (type C). (a) HRTEM image showing the multiwall stacking of graphite sheets
d(002) ) 3.48 Å]. (b) HRTEM image showing a hollow circular structure with an unusual bright central core; scale bars: (a, b) 10 nm.
[
A similar experiment, carried out under prolonged heating
100 nm), black film, possessing a metallic luster. Scanning
electron microscopy (SEM) examination shows that, in addition,
a large quantity of discrete nano- and microobjects were formed
(Table 1). The preponderant part of them consisted of free-
standing objects in the ampule and the rest of the particles
growing on the top of the film, which were unproblematically
isolated by mechanical separation. The yield of such structures,
estimated by gravimetric determination and by semiquantitative
evaluation by optical and electron microscopy, was on the order
of 60%. Elemental analysis of the pyrolytic products indicates
only a small residual hydrogen content of 0.37-0.42 wt %
(original 10.64 wt %).
for 72 h at only 355 °C, shows that the resulting textures are
similar to those in Figure 2d. In contrast to the textures obtained
by prolonged heating at 400 °C, they did not resist further
heating and transformed at about 425 °C into an isotropic melt.
These experiments demonstrate that prolonged heat treatment
of 1b at 400 °C is the best condition for the formation of
extremely stable discotic mesophases. Accordingly, 1b can be
pyrolyzed in the mesophase, leading to a network structure in
which the mesomorphic order of the pyrolytic products remains
preserved. Therefore some preorganization is created before the
final pyrolytic step for 24 h at 800 °C.
B. Formation of Graphitic Structures. 1. Effects of
Temperature. In the following experiments, the two stages of
pyrolysis of 1b (Chart 1) were carried out successively, without
opening of the ampules between the thermal pretreatment at
The structure of the films was analyzed by selected area
electron diffraction (SAED), which reveals the characteristic
lattice spacing for the 100, 101, 112, and 002 reflections
occurring in graphite. The distance between the graphite layers
was determined to be 0.348 nm, from the 002 reflection. The
diffraction pattern exhibits broad uniform rings indicating
graphite grains orientated randomly within the carbon film.
However, the 002 reflection forms a sharper diffraction spot
with a misorientation of only 35° over an area of 10 µm
diameter. Thus the graphite layers oriented perpendicular to the
film possess a high long-range order. These results together with
the small residual hydrogen content clearly suggest that the films
400 °C and the final pyrolytic step at 800 °C (for details see
Experimental Section). Quartz plates were inserted into the
ampule as substrate for deposition of pyrolytic products. After
completion of the heat treatment, the evacuated and sealed quartz
ampules were opened, and the pyrolytic products were collected
from the surface of the substrate, as well as from free-standing
material in the ampules, and analyzed. The inside walls of the
ampules and the quartz plates were coated with a thin (80-
13134 J. AM. CHEM. SOC.
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Preparing Carbon Nano- and Microparticles
A R T I C L E S
Figure 6. SEM images of polyhedral forms of pyrolytic carbon. (a) Nanorod with kink (type E); scale bar 1 µm. (b) Example of the hollow columns of type
G with square cross sections; scale bar 500 nm.
result from the elimination of hydrogen and fusion to larger
graphitic structures.
Among the discrete particles were carbon spheres (type A),
ranging in diameter from a few hundred nanometers to 2 µm,
which at some spots appeared to just emerge from the deposited
1
2-15,22
carbon surface.
The radial spherical textures (Figure 3a)
consisted of highly regular single spheres well separated from
each other, aggregated spheres, and spherulitic nodules included
in the deposited carbon surface. The carbon spheres are
comparable to the “mesophase spheres” produced during
carbonization of a variety of carbonaceous precursors and
reported by Oberlin.1 Also present were long carbon microfi-
bers, up to 10 µm in cross section, with a peculiar bamboolike
shape (type B). They are composed of linear chains of
2a
“
compartments”, ranging in length from 10 to 20 µm (Figure
3
b,c), and resemble CCVD (catalytic chemical vapor deposition)
23
carbon filaments reported by Dresselhaus et al.
The third type of carbon particles detected (type C) were
bundles of nanowires, about 20 nm in diameter and ranging in
length from a few hundred nanometers to a few micrometers.
The SEM image of these coiled nanowires, similar to the
filamentous carbon reported by Oberlin14 upon heat treatment
of CO in the presence of metal-based catalysts, is shown in
Figure 3d. SAED of type C nanoobjects exhibits the charac-
teristic lattice spacings of graphite powder. A more detailed
analysis by high-resolution transmission electron microscopy
Figure 7. SEM images of round-shaped structures and carbon nanoparticles
of type H; scale bars: (a) 500 nm; (b) 200 nm.
(
HRTEM) shows micrographs that are dominated by graphite
are relatively thick in the beam direction (Figure 5). One further
notable aspect, clearly illustrated in Figure 5b, is that the central
core often appears unusually bright. We therefore suppose that
these particles are short tubes. High-resolution images indicate
that the tubes have a concentric rather than a spiraling structure
and that the central cores of many of these particles are irregular
in shape, different from the circular central cores of the classical
nanotubes. Another example of a graphitic particle with a
remarkable rounded profile, comparable to the spheroidal
structure of the type originally reported by Iijima,25 is shown
in Figure 4b. The high-resolution image of this particle exhibits
a “multiwall” circular shape with about 25 layers and a regular,
uniform contrast (note the disappearance of the empty space in
the center).
layers with a spacing of about 0.348 nm and oriented perpen-
dicular to the electron beam. These graphite layers typically
form “multiwall” structures with 5-50 graphite layers and
create complex morphologies, consisting of a combination of
straight domains, constantly bent walls, closed graphitic band
form, and particles with rounded or faceted outer shapes (Figures
2
4
4
and 5). The wall thickness and the ratio of straight domains
to bent walls vary, giving rise to structural motifs on different
length scales.
Remarkably enough, the high-resolution images reveal that
many objects consist of aggregates of presumably hollow tubular
structures, which exhibit strong contrast, suggesting that they
(
22) Hishiyama, Y.; Yoshida, A.; Inagaki, M. Carbon 1982, 20 (1), 79-84.
23) Dressselhaus, M. S.; Dresselhaus, G.; Sugihara, K.; Spain, I. L.; Goldberg,
H. A. Graphite Fibers and Filaments; M u¨ ller, K. A., Gonser, U., Panish,
M. B., Mooradian, A., Sakaki, H., Eds.; Springer-Verlag: Berlin and
Heidelberg, 1988.
(
To study the influence of the temperature profile and
simultaneously the importance of the liquid crystalline properties
(
24) Ugarte, D. Carbon 1995, 33 (7), 989-993.
(25) Iijima, S. J. Cryst. Growth 1980, 50, 675-683.
J. AM. CHEM. SOC.
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A R T I C L E S
Gherghel et al.
Figure 8. SEM images of carbon pyrolytic structures formed on mica. (a) Carbon nanosticks (type I) and Y-branched carbon fibers (type J); scale bar 1 µm.
(
b) Fusion of type I and J nanoparticles; scale bar 500 nm. (c) Type K microparticle with hexagonal symmetry and zigzag shape; scale bar 5 µm. (d)
Overview of a “giant” (millimeter length) pyrolytic object; scale bar 20 µm.
of 1b on the formation of pyrolytic products, we applied a
second technique for sample preparation: 1b was coated
between two quartz plates and preheated for 10 min to 220 °C,
a temperature at which the anisotropic melt was formed but
without bond cleavage. After this pretreatment, the quartz plates
were inserted into the ampule, which was then evacuated and
sealed. The pyrolysis was again conducted in two steps: first
for 72 h at 400 °C and then finally for 24 h at 800 °C. SEM
studies of the discrete particles formed reveal straight and
twisted cylindrical nanorods (type D) as well as nanorods
possessing kinks with 90° angles between the arms (type E)
grow on top of the carbon film perpendicular to its surface.30
Columns broken off from the base give rise to quadratic craters
on the fractured surface. These observations together with the
SEM images of the well-defined cages of the broken cubes lead
to the conclusion that these objects are hollow. The results of
these experiments support the importance of the temperature
profile and the final temperature for product formation.
31
To further examine the influence of the temperature in the
final stage of the pyrolysis, several experiments were performed
by a similar procedure, with the same temperature profile during
the first stage of the pyrolysis (for 72 h at 400 °C) but with
differing maximum temperatures during the final stage (see
Experimental Section). Thus, when the samples were heated
for 24 h at 650 °C, in addition to the bamboo-shaped microfibers
(Figure 6a). The columns of type D are round, with thin external
walls and large hollow cores, as well as with thick external walls
and small hollow cores. The outer diameters lie in the 100 nm
range and the lengths are up to a few micrometers. In addition
to the particles of type D and E, nanoobjects with a quadratic
(type B), round-shaped, well-separated nanoobjects with small
aspect ratio were produced. The latter show often a doughnutlike
geometry, like hollow cubes and rectangular parallelepipeds
structure, with a defined channel in the center (type H) (Figure
6-29
(
type F),2
with edges ranging in length from a few hundred
32
7
).
nanometers to 1 mm, and micrometer-long columns with square
cross sections of about 500 nm (type G) were found (Figure
Alternatively, another experiment was carried out by skipping
the thermal pretreatment at 400 °C and heating precursor 1b
directly to 800 °C. As a result, only an irregular film without
6
b). The latter are new unusual forms of pyrolytic carbon that
(
(
(
(
26) Gogotsi, Y. G.; Libera, J. A.; Kalashnikov, N.; Yoshimura, M. Science
000, 29, 317-320.
27) Gogotsi, Y. G.; Yaroshenko, V. P.; Porz, F. J. Mater. Sci. Lett. 1992, 11,
08-310.
28) Suzuki, T.; Utsumi, Y.; Sasaki, K.; Shibuki, K. J. Appl. Phys. 1995, 77
7), 3450-3452.
29) Saito, Y., Matsumoto, T. Nature 1998, 392, 237.
2
(30) Ajayan, P. M.; Nugent, J. M.; Siegel, R. W.; Wei, B.; Kohler-Redlich, P.
Nature 2000, 404, 243.
3
(31) For SEM images of various forms of pyrolytic carbon formed during the
pyrolysis at 800 °C (nanostructures with square cross section), see
Supporting Information.
(
(32) Ihara, S.; Itoh, S. Carbon 1995, 33 (7), 931-939.
13136 J. AM. CHEM. SOC.
9
VOL. 124, NO. 44, 2002

Preparing Carbon Nano- and Microparticles
A R T I C L E S
Figure 9. SEM images of microobjects of type L formed by pyrolysis at 650 °C and deposited on mica. (a) Zigzag shape of the branches; scale bar 5 µm.
(
b) Multiple branched microobject; scale bar 2 µm. (c, inset d) Representative hexagonal cross section of the branches; scale bars (c) 5 µm; (d) 1 µm. (e)
Round shape of the top of the branches; scale bar 2 µm.
the presence of micro- and nanoobjects was observed. Since
the TGA measurement of sample 1b, pretreated for 72 h at 400
fusion of many nanosticks, at first into structured ensembles,
and further to bigger, parallel-oriented structures with hexagonal
symmetry and a zigzag shape (type K).
°C, finally shows a complete decomposition, we cannot exclude
completely that the formation of nanoparticles may also take
place in the gas phase, possibly by evaporation of reactive
fragments. However, the absence of nanoparticles after direct
heating to 800 ˚C unambiguously confirms our concept and
supports the essential role of the mesophase during the first step
of pyrolysis for particle formation.
This unusual formation, apparently the preferred structure on
mica, was also observed for microobjects of type L (Table 1).
3
5
These rootlike microobjects, 30 µm and more in length, are
filled, multiple-branched, and taper outward to the surface. The
branches of the microroots have a hexagonal geometry and cross
sections between 400 nm and 1 µm (Figure 9). Only the tops
of the branches possess a round shape and in some cases kinks
or helical-coiled structures. It must be stressed that these
pyrolytic products, with hexagonal symmetry and zigzag shapes,
are new and do not correspond to any of the hitherto known
2. Effects of Substrate. The same procedure and temperature
profile during the pyrolysis (heat treatment for 72 h at 400 °C
and further heating for 24 h at 650 °C) were utilized for the
studies of the role of the substrate on the formation of pyrolytic
products. Using mica instead of quartz substrates gave rise to
the formation both of films coated on the surface and of discrete
particles (Table 1). However, the shape of the latter is signifi-
cantly different from the ones characteristic for the pyrolytic
products deposited on quartz, and the semiquantitative evaluation
by optical and electron microscopy of different sites and samples
suggests even higher yields of micro- and nanoparticles formed
on mica than on quartz. SEM analysis shows that, during the
pyrolysis, carbon deposits on mica as a surface with spherulitic
morphology and several round nodules emerge from the surface
and form nanosticks (type I), with 100-200 nm diameter and
micrometer length. Many of them show a three-point junction
3
6, 37
types of carbon micro- or nanoparticles.
The growth mechanism of the polyhedral structures of type
K and L is clearly different from that observed for the spheres
and fibers. We suppose that 1b or the subsequent pyrolytic
products interact with the metal oxides in the mica. This process,
which occurs in the interstices between the substrate and the
carbon film, supports the formation of nuclei for nanoparticles
of type I and J as well as the growth of branched structures and
ordered ensembles of the carbon microobjects with hexagonal
geometry. It follows that the role of the substrate is decisive
for the formation and growth of the pyrolytic products.
3. Role of Alkyl Substitution. The requirement of the liquid
(
type J), similar to the Y-junctions observed by pyrolysis of
crystalline properties of the precursor for our pyrolytic approach
is also supported by a further experiment. Unsubstituted HBC
33,34
organometallic precursors or of hydrocarbons with catalysts.
The SEM image (Figure 8a) indicates Y-branched carbon fibers,
consisting of stems and branches up to 500 nm long, and on
the top of type I nanoparticles, the initial stage of the formation
of Y-junctions. The SEM images in Figure 8b-d suggest the
1
a, unable to form an ordered mesophase, was also subjected
(35) For SEM image (overview) of a rootlike microobject (type L) formed by
pyrolysis at 650 °C and deposited on mica, see Supporting Information.
(
36) Endo, M.; Saito, R.; Dresselhaus, M. S.; Dresselhaus, G. Carbon Nanotubes:
Preparation and Properties; Ebbesen, T. W., Ed.; CRC Press: Boca Raton,
FL, 1997; pp 35-110.
(33) Satishkumar, B. C.; Thomas, P. J.; Govindaraj, A.; Rao, C. N. R. Appl.
Phys. Lett. 2000, 77, 2530-2532.
(37) Ugarte, D. Carbon Nanotubes; Endo, M., Iijima, S., Dresselhaus, M. S.,
Eds.; Pergamon: Oxford, 1996; pp 163-167.
(34) Li, J.; Papadopoulos, C.; Xu, J. Nature 1999, 402, 253-254.
J. AM. CHEM. SOC.
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A R T I C L E S
Gherghel et al.
to pyrolysis, yielding black products completely different from
those of 1b. SEM reveals disordered and spongy structures and
the absence of discrete micro- and nanoparticles. These results
are fully consistent with literature describing the pyrolysis of
unsubstituted PAHs.3 It is obvious that a properly controlled
alkyl chain cleavage of 1b and a “directed” cross-coupling of
radical intermediates in an ordered mesophase are key prereq-
uisites for the formation of large mesoscopic structures which
further lead to the formation of graphitic structures.
Typically, the precursor compounds 1a and 1b were placed into the
ampule together with quartz plates as substrate for deposition of the
pyrolytic products and heated at 2 °C/min to 400 °C. This temperature
was maintained for 72 h before it was raised for another 24 h to 800
°
C or to 650 °C. For the second technique of sample preparation, 1b
was coated between two quartz plates and preheated for 10 min to 220
C. After this pretreatment, the quartz plates were inserted into the
8,39
°
ampule, which was then evacuated and sealed. The samples were then
further heated at 400 °C for 72 h and to 800 °C for another 24 h. The
pyrolysis products were well reproducible under analogous preparation
conditions.
Conclusions and Outlook
Characterization. For the matrix-assisted laser desorption/ionization
time-of-flight mass spectrometry (MALDI-TOF MS) measurements, a
sample preparation method was applied that was especially developed
We present here a versatile method for producing different
forms of carbon micro- and nanoparticles by pyrolysis under
soft conditions, in the absence of metal-based catalysts. The
extremely stable discotic mesophase of the precursor 1b provides
a preorganization during the first stage of the pyrolysis, which
is essential for the pyrolytic “graphitization” in the anisotropic
melt. The experiments prove that prolonged heat treatment at
18
for MALDI-TOF analysis of large PAHs. This sample preparation
avoids the laser radiation-induced creation of new compounds with
lower as well as higher molecular weights than the original molecules,
which are only detectable in the mass spectrometer and are not present
in the original sample. To make sure that the mass spectra only show
the signals of compounds produced by thermal treatment, we applied
the matrix 7,7,8,8-tetracyanoquinodimethane (TCNQ) and used only
low desorption laser power. Additionally, the samples were prepared
by mechanical mixing with the matrix without the use of solvents, which
4
00 °C leads to the formation of large mesomorphic structures
by intermolecular coupling reactions and to a stabilization of
the mesophase. Further increase of the temperature leads to
graphitization processes and the formation of various shapes
of carbon nano- and microparticles. Many discrete carbon
objects are completely new (types G-I, K, and L) or have never
enables also the detection of insoluble compounds possibly produced
18,44
during thermal treatment.
The mass spectra were recorded on a
Bruker Reflex I MALDI-TOF mass spectrometer equipped with a N
2
been reported on the present length scale or under the conditions
laser (λ ) 337 nm) operating at a pulse rate of 3 Hz.
utilized here (types A-F and J).4
0-42
Experimental control is
A Zeiss Axiophot optical microscope equipped with polarizers and
a nitrogen-flushed Linkam THM 600 hot stage was used to characterize
the polarization microscopy textures and to estimate clearing temper-
possible, e.g., by the substrate and the temperature profile. Some
of the nano- and microparticles obtained by our pyrolysis
method show many similarities to various flying particles
-
1
atures. Heating and cooling rates varied between 5 and 20 °C min
.
Elemental analysis were performed at the Institut f u¨ r Organische
Chemie, Johannes Gutenberg-Universit a¨ t Mainz (Germany).
SAED and SEM investigations were performed on a Leo 912 Omega
energy filter transmission electron microscope and LEO 1530 field
emission scanning electron microscope, respectively. High-resolution
TEM observations were conducted on a CM 20 (200 kV) electron
microscope.
(
aerosols) in the atmosphere, which have a significant influence
on the regional and global climate and on the formation of
4
3
clouds. These observations open new perspectives for these
pyrolytic products as model systems for certain aerosols. Further
studies of the mechanism of selective particle formation are
under way, involving especially new PAH starting materials.
Our efforts are directed to PAH molecules with ever-increasing
π-systems, which are able to form stable discotic mesophases
with a stacking of the π-layers. This could lead to designer-
made carbon objects for nanoelectronics. An important objective
of our further work is also the investigation of the mechanical
and electrical properties of these new carbon objects. Micro-
and nanomanipulators constitute convenient tools not only for
selection of certain particles but also for elucidation of the
possibility of reshaping and stretching these particles and for
the determination of their electrical conductivity.
Acknowledgment. We thank G. Glasser for the SEM
measurements. Financial support by the European Commission
(TMR Project SISITOMAS, Project DISCEL and MAC-MES),
the European Science Foundation (Project Smarton), and the
Bundesministerium f u¨ r Bildung und Forschung (Multifunctional
Materials and Miniaturized Devices Center) is gratefully
acknowledged.
Supporting Information Available: Figures showing ther-
mogravimetric analysis of 1b and of the pyrolytic products
formed at 400 °C and SEM images of micro- and nanoparticles
of type F and L. This material is available free of charge via
the Internet at http://pubs.acs.org
Experimental Section
Sample Preparation. All pyrolytic experiments were carried out
-5
in evacuated (10 mbar) and sealed quartz ampules in an electric oven.
JA020363U
(
38) Boenigk, W.; Haenel, M. W.; Zander, M. Fuel 1995, 74, 305-306.
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