JOURNAL OF APPLIED PHYSICSVOLUME 88, NUMBER 9

1 NOVEMBER 2000

Formation of Co filled carbon nanocapsules by metal-template graphitizationof diamond nanoparticles
Satoshi Tomita and Masahiro Hikita
Graduate School of Science and Technology, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan

Minoru Fujii and Shinji Hayashia)
Department of Electrical and Electronics Engineering, Faculty of Engineering, Kobe University, Rokkodai,
Nada, Kobe 657-8501, Japan

Kensuke Akamatsu and Shigehito Deki
Department of Chemical Science and Engineering, Faculty of Engineering, Kobe University, Rokkodai,
Nada, Kobe 657-8501, Japan

Hidehiro Yasudab)
Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, Mihogaoka, Ibaraki,
Osaka 567-0047, Japan

共Received 20 June 2000; accepted for publication 17 August 2000兲
Co filled carbon nanocapsules, which are formed by a heat treatment of the mixture of Co anddiamond nanoparticles, have been studied by in situ transmission electron microscopy 共TEM兲, x-ray
diffraction, and Raman spectroscopy. Raman studies show that the heat treatment reduces thesurface native oxide 共Co3 O4 ) of Co nanoparticles. The reduction is accompanied by graphitizationof diamond nanoparticles, indicating that diamond nanoparticles being in contact with the metallic
Co are transformed into graphitic coating. The in situ TEM studies show that the graphitic coating
is formed in the heating process, not in the cooling process. Furthermore, once the coating iscompleted, the number of the graphitic layers is almost constant on further heating and cooling.
These results allow us to conclude that metallic Co particles simply act as templates for graphitic
coating. © 2000 American Institute of Physics. 关S0021-8979共00兲07522-8兴

I. INTRODUCTION

modified arc discharge, the thickness of the coating increases
by postannealing. They claimed that the addition of graphitic
layers results from a precipitation of carbon previously dissolved in the metal, suggesting the dissolution-precipitation
mechanism in these systems.
Very recently, we succeeded in preparing magneticmetal filled carbon nanocapsules via solid-state reaction.14
The method used is the heat treatment of the mixture ofmagnetic-metal nanoparticles about 30 nm in diameter anddiamond nanoparticles about 5 nm in diameter. After theheat treatment at 1700 °C, all the magnetic-metal nanoparticles were encapsulated with uniform thin graphitic layers.
The thickness of the coating was nearly the same as thediameter of the initial diamond nanoparticles. These results
imply that diamond nanoparticles being in contact with themetals are transformed into the thin coating. However, there
was no clearcut explanation for the detailed formation
mechanism of the graphitic coating, although it was obviousthat the formation is assisted by the magnetic-metals.
In this article, we intend to clarify the formation mechanism of Co filled carbon nanocapsules by in situ transmission
electron microscopy 共TEM兲, x-ray diffraction 共XRD兲, andRaman spectroscopy. In the initial mixture, surface oxidized
␤ -Co nanoparticles about 30 nm in diameter are surroundedby diamond nanoparticles about 5 nm in diameter. Raman
study indicates that the surface native oxide 共Co3 O4 ) is reduced to metallic Co as the heat treatment temperature increases. The reduction is accompanied by the drastic

Metal nanoparticles wrapped in concentric layers of encapsulating graphitic carbon, called carbon nanocapsules,
have been intensively studied since LaC2 was successfully
encapsulated.1,2 Particularly for magnetic-metal filled carbon
nanocapsules, the interesting magnetic behavior of magneticmetal nanoparticles and the oxidation resistance of the carbon coating have led to the consideration of these particles
for applications in areas such as magnetic data storage, magnetic tonor for xerography, fellofluids, and contrast agents inmagnetic resonance imaging.3 Graphitic coating increases
the effective distance of neighboring magnetic particles.4
The increase reduces particle–particles exchange coupling,which is an important issue in ongoing attempt to synthesize
extremely high-density magnetic recording media.5–7
Magnetic-metal filled carbon nanocapsules have been
produced by various methods via vapor phase reaction, e.g.,standard arc methods,8,9 modified arc methods,4,10 and sputtering methods.7,11,12 The nanocapsules are thought to beformed by so-called ‘‘dissolution-precipitation mechanism,’’in which carbon is dissolved into the metal, and graphite
precipitates as coating layers on quenching. Jiao andSeraphin13 showed that, for the nanocapsules prepared by thea兲Author to whom correspondence should be addressed; electronic mail:hayashi@eedept.kobe-u.ac.jpb兲Present address: National Research Institute for Metals, Sakura, Tsukuba,
Ibaraki 305-0003, Japan.
0021-8979/2000/88(9)/5452/5/$17.00

5452© 2000 American Institute of Physics

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Tomita et al.

J. Appl. Phys., Vol. 88, No. 9, 1 November 2000

5453

FIG. 2. XRD profiles of initial Co nanoparticles before heat treatment 共a兲
and the mixture of Co and diamond nanoparticles after the heat treatment at
1700 °C 共b兲.

FIG. 1. A TEM image 共a兲 and a SAED pattern 共b兲 of the initial mixture of
Co and diamond nanoparticles. Co nanoparticles about 30 nm in diameter
共indicated by arrows兲 are surrounded by diamond nanoparticles about 5 nm
in diameter.

progress of graphitization of diamond nanoparticles. The result supports our hypothesis that diamond nanoparticles being in contact with metallic Co are transformed into graphitic
coating. In situ TEM experiments show that the graphitic
coating is formed not on the cooling but on the heating process. Once the coating is completely formed, the number ofthe graphitic layers is almost constant on further heating andcooling. These results brought by in situ TEM studies cannot
be explained within the framework of the dissolutionprecipitation mechanism. We thus propose a metal-templategraphitization mechanism, in which metallic Co particles
simply act as templates for the coating.

II. EXPERIMENT

Co filled carbon nanocapsules were prepared by the heat
treatment of the mixture of Co and diamond nanoparticles.14
Diamond nanoparticles about 5 nm in diameter were mixed
with Co nanoparticles about 30 nm in diameter using ultrasonic dispersion. Both of the nanoparticles were commercially available. The mixture was put in a hole 共2.5 mm indiameter兲 which was drilled through a graphite rod 共5 mm indiameter and 7 mm in length兲. The graphite rod was thenheat treated at the temperatures ranging from 300 to 800 °C

in vacuum (⬍2⫻10⫺3 Pa兲 in a conventional electric furnace, and from 800 to 1700 °C by an electron beam irradiation in vacuum (⬍2.5⫻10⫺3 Pa兲.
TEM studies were performed with JEOL JEM-2010 and
Hitachi HF-2000 transmission electron microscopes at an acceleration voltage of 200 kV. For the in situ observation, thesample was placed on a W filament, which was attached tothe sample holder, and resistively heated. The temperature of
the sample was roughly estimated from the current, which issupplied to a W filament, by using a calibration curve obtained from standard samples 共Au兲. An XRD measurement
was performed with an x-ray diffractometer 关RINT-2100
共RIGAKU兲兴 with the Cu K ␣ radiation. Raman spectroscopy
was carried out with a SPEX 1877 Triplemate equipped witha charge coupled device. Spectra were recorded in a 90°
scattering geometry. The excitation source was the 514.5 nm
line of an Ar-ion laser. The excitation power was kept lessthan 10 mW in order to avoid local heating.
III. RESULTS AND DISCUSSION

Figures 1共a兲 and 1共b兲, respectively, show a TEM image
and a selected-area electron diffraction 共SAED兲 pattern ofthe mixture of Co and diamond nanoparticles before the heat
treatment. Figure 1共b兲 also exhibits diffraction patterns calculated from the American Society for Testing and Materials
data15 for ␤ -Co with a fcc structure, Co3 O4 and diamond; thelength of the bars represents diffraction intensity. The SAED
pattern indicates that the initial mixture consists of ␤ -Co,
Co3 O4 , and diamond. Co3 O4 is a native oxide formed on thesurface of Co nanoparticles.16 The presence of Co3 O4 as wellas ␤ -Co in the initial Co nanoparticles is also confirmed by
an XRD study as shown in Fig. 2共a兲. Therefore, dark patches
in Fig. 1共a兲 共indicated by arrows兲 correspond to ␤ -Co nano-

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5454

J. Appl. Phys., Vol. 88, No. 9, 1 November 2000

Tomita et al.

FIG. 4. A typical TEM image of a graphitic-carbon encapsulated Co nanoparticles formed after heat treatment at 1700 °C.
FIG. 3. Raman spectra obtained from initial mixture 共curve a兲 and samples
with different heat treatment temperatures 共curves b–f兲. The top curve g
represents a Raman spectrum of the diamond nanoparticles heat treated at
800 °C without Co nanoparticles.

particles with surface oxides, and the surroundings are diamond nanoparticles about 5 nm in diameter. The fcc structure is the high temperature phase for Co and normallypresents only above 420 °C. The presence of the high temperature phase at room temperature is probably due to thesmall particle nature of the material and/or ‘‘quenching’’
effects.17
Figure 3 shows the heat treatment temperature dependence of Raman spectra. Before heat treatment 共curve a兲, thespectrum exhibits a weak Raman band at about 1325 cm⫺1
which originates from diamond nanoparticles 共indicated byan arrow兲. The band is slightly shifted toward a lower frequency compared with that for the bulk diamond 共1332
cm⫺1 ). This shift is explained by the phonon confinement
effects.18–20 In addition to the diamond band, a broad band
can be seen at a higher wave number region. The broad bandis thought to originate from the surface amorphous carbon
layers of the diamond nanoparticles.20,21 The spectrum a also
shows three Raman bands at about 483, 523, and 691 cm⫺1 .
These bands are attributed to Co3 O4 . 22
The carbon-related features above 1200 cm⫺1 do notchange up to the heat treatment temperature of 500 °C
共curves b and c兲. In contrast, the Co3 O4 bands become
weaker, suggesting that Co3 O4 is reduced to metallic Co
with increasing the heat treatment temperature. The Co3 O4bands almost disappear at 600 °C 共curve d兲. Since Co3 O4 isnormally stable up to 600 °C, diamond nanoparticles may act
as a reducing agent. We confirmed the reduction of Co3 O4

also by differential scanning calorimetry, which shows a endothermic peak around 600 °C.
The disappearance of Co3 O4 bands at 600 °C is accompanied by the drastic change in carbon-related features; theband due to the graphitic structure (G band兲 at about 1580cm⫺1 and disorder-induced band 共called D band兲 at about
1350 cm⫺1 emerge. This implies that when the reduction of
Co3 O4 is completed, the graphitization of diamond nanoparticles starts. The reduction of surface Co3 O4 layers allows
diamond nanoparticles to directly contact with metallic Co
particles. The direct contact enables diamond nanoparticles
to be transformed into graphitic coating owing to the catalytic effect of metallic Co for graphitization.23,24 The top
curve g in Fig. 3 represents a spectrum of the diamond nanoparticles annealed at 800 °C without mixing with Co particles. The carbon-related features are smeared and very
similar to those in curves b and c, suggesting that, without
Co nanoparticles, graphitization does not proceed even at
800 °C.
Figure 4 shows a TEM image for the sample heat treated
at 1700 °C. A nanocapsule about 65 nm in diameter with thincoating can be seen. The spacing of the lattice fringes of thecoating is around 0.34 nm, which is close to that of thegraphite 共002兲 planes. The thickness of the coating is nearly
the same as the average diameter of diamond nanoparticles.
XRD profile 关Fig. 2共b兲兴 shows the presence of graphitic carbon and ␤ -Co. Neither carbide nor oxide phase is found in
the profile. Although not shown here, the TEM observation
indicated that the graphitic coating starts at about 600 °C.
The results of TEM, SAED, XRD, and Raman studies
revealed that diamond nanoparticles being in contact withmetallic Co are transformed into graphitic coating, resulting
in the formation of Co filled carbon nanocapsules. Neverthe-

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Tomita et al.

J. Appl. Phys., Vol. 88, No. 9, 1 November 2000

FIG. 5. The in situ TEM image of the coating on heating process. The
coating consists of 6 or 7 graphitic layers. The micrograph also shows defects as indicated by arrows a, b, c, and d.

less, the formation mechanism of the graphitic coating, particularly the role of metallic Co, is still unclear. In order togain more detailed information about the formation mechanism, we performed in situ TEM experiments.
The initial mixture was put into the microscope and thesample temperature was gradually increased by increasing
the current supplied to a W filament 共heating process兲. The
lattice fringes due to Co oxide disappeared above 600 °C,
and then the graphitic coating of the nanocapsules is formed.
Figure 5 shows an in situ TEM image of a part of the coating
during the heating process. The temperature of the sample
was estimated to be about 1100 °C. Clear lattice fringes corresponding to 共002兲 planes of graphite can be seen. The coating consists of 6 or 7 graphitic layers and is about 2 nm inthickness. Once the coating was completely formed, thenumber of graphitic layers was found to be almost constanton further heating. Figure 5 also shows that the coating has
defects as indicated by arrows a, b, c, and d.
After heating up to 1100 °C, we gradually decreased thesample temperature by decreasing the current 共cooling process兲. Upon cooling, Fig. 6 was obtained at the same place as

5455

in Fig. 5; the point a ⬘ is the same place as the point a in Fig.
5. Note here that the number of the layers on cooling isalmost the same as that on heating. However, the crystallinity of the graphitic coating is much different. The coating
becomes disordered and is similar to turbostratic carbon thatis made up of small segments of graphitic sheets stacked
roughly parallel to the particle surface. The disorder isthought to be induced by the different heat-expansion coefficient between Co and graphite.
From these results of in situ TEM experiments, we will
discuss here the formation mechanism of the graphitic coating and the role of metallic Co. It is obvious that the transformation of diamond nanoparticles into graphitic coating is
assisted by the Co nanoparticles. Catalytic effects by themetals in graphitization have been investigated for somedecades.23–25 Sinclair and co-workers26–28 showed that amorphous carbon in contact with Co fully crystallizes between
500 and 600 °C. This graphitization temperature is close to
the present result; present Raman study indicates that diamond nanoparticles are graphitized at about 600 °C. Theysuggested a model of metal-mediated graphitization, in
which carbon is dissolved into the metal, and upon supersaturation, graphite precipitates outwardly at the interface,
just similar to the dissolution-precipitation mechanism. However, as depicted in a binary alloy phase diagram,29 the equilibrium solubility of carbon in Co at 600 °C is extremely low
共about 0.2 at. %兲; the carbon does not dissolve into Co.
Therefore, there has been ambiguity whether or not graphitization at 600 °C is really mediated by the dissolution ofcarbon atoms into the interstitial and/or substitutional sites inmetallic Co.
For the nanocapsules prepared by modified arc discharge
method,13 the thickness of the coating increased by postannealing, suggesting that the addition of graphitic layers results from a precipitation of carbon previously dissolved in
the metal. This is thought to be an evidence of dissolutionprecipitation mechanism. On the contrary, present TEM
studies show that, once the coating is formed, the number ofthe graphitic layers does not increase on further heating andcooling. The present formation mechanism is thus different
from the dissolution-precipitation mechanism. In otherwords, diamond nanoparticles are not dissolved into Co
nanoparticles but directly transformed into the graphitic coating on the surface of Co. It is likely that the surface of
metallic Co provides a fast path to release the excess freeenergy of diamond nanoparticles30 and a medium for the
diffusion of mobile carbon atoms,31 thus facilitating crystalline growth of graphitic layers. As a conclusion, we suggestthat the present formation mechanism of Co filled carbon
nanocapsules is a metal-template graphitization, in which
metallic Co simply acts as templates for the coating.
IV. CONCLUSION

FIG. 6. The in situ TEM image on cooling process at the same place as inFig. 5. Note that the number of the layers on cooling is almost the same asthat on heating although the crystallinity is much different.

The formation of Co filled carbon nanocapsules has beenstudied by in situ TEM, XRD and Raman spectroscopy. In
the initial mixture, surface-oxidized ␤ -Co nanoparticles
about 30 nm in diameter were surrounded by diamond nanoparticles about 5 nm in diameter. Raman studies showed that

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5456

the reduction of Co3 O4 to metallic Co at about 600 °C isaccompanied by the drastic progress of graphitization of diamond nanoparticles, suggesting that diamond nanoparticles
being in contact with the metallic Co are transformed to thegraphitic coating layers. In situ TEM experiments showedthat the graphitic coating is formed not on cooling but onheating process. Moreover, once the coating was formed, thenumber of the graphitic layers was almost constant on further
heating and cooling. These results allow us to conclude thatmetallic Co simply acts as templates for the formation of thegraphitic coating 共a metal-template graphitization mechanism兲. Metallic Co particles provide the place to release theexcess free energy of diamond nanoparticles and diamondnanoparticles are recrystallized as graphitic coating layers on
the surface of Co.
ACKNOWLEDGMENTS

The authors are grateful to Kouichi Sawada for his valuable assistance in this work. S. T. wishes to acknowledge thefinancial support from the Japan Society for the Promotion of
Science 共JSPS兲. This work is supported by a Grant-in-Aid
for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan, and a Grant for Research for
the Future Program from the JSPS 共JSPS-RFTF 96P-00305
and JSPS-RFTF-98P-01203兲.
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