Supplementary Material
CoFe2O4 nanoparticles as a catalyst: Synthesis of a forest of vertically
aligned CNTs of uniform diameters by plasma enhanced CVD
Ankur Baliyan, Takahiro Fukuda, Yasuhiro Hayasaki, Takashi Uchida, Yoshikata
Nakajima, Tatsuro Hanajiri and Toru Maekawa
Contents
1. TEM images of nanoparticles (magnified version)
2. Distributions of the diameters of nanoparticles
3. XRD analysis of CoFe2O4 particles
4. EDS spectra of nanoparticles
5. FTIR spectra of CoFe2O4 nanoparticles
6. TEM images of CNTs (magnified version)
7. Distribution of the internal diameters of CNTs
8. Diameter of nanoparticles after complete reduction (theoretical calculation)
9. XPS analysis of nanoparticles
10. TEM images of CNTs (open end and a nanoparticle trapped at the tip)
11. Distributions of the number of walls of CNTs
12. Crystal characteristics in the austenite and ferrite phases
13. Distributions of nanoparticles on silicon substrates
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1. TEM images of nanoparticles (magnified version)
Fig. S1 TEM images of catalytic nanoparticles. (A) The diameter is 4.8 nm. (B) 6.9 nm.
2. Distributions of the diameters of nanoparticles
Fig. S2 Distributions of the diameters of catalytic nanoparticles. (A) The diameter is
4.8 ± 0.34 nm. (B) 6.9 ± 0.31 nm.
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3. XRD analysis of CoFe2O4 particles
X-ray diffraction patterns of as-synthesised nanoparticles are shown in Fig. S3. The
positions and intensities of all of the peaks are the same as those corresponding to
JCPDS card number 22-1086, which indicates that the nanoparticles have crystalline
structures. The same behaviour was observed in the SEAD patterns (see Fig. 1 in the
main text).
Fig. S3 XRD patterns of catalytic nanoparticles. XRD diffractograms indicated by
blue and red curves, respectively, correspond to particles of 4.8 and 6.9 nm diameters.
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4. EDS spectra of nanoparticles
Fig. S4-1 EDS spectra of catalytic nanoparticles. (A) EDS spectrum of catalytic
nanoparticles of 4.80 nm diameter. The stoichiometric ratio of Fe to Co was 2:1. (B)
EDS spectrum of catalytic nanoparticles of 6.90 nm diameter. The stoichiometric ratio
of Fe to Co was 2.5:1.
Fig. S4-2 Point localised EDS spectra. (A) EDS spectrum of a nanoparticle before
CVD. (B) EDS spectrum of a nanoparticle trapped at the tip of a CNT after CVD.
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5. FTIR spectra of CoFe2O4 nanoparticles
The FTIR spectra of the ligand attached to the CoFe2O4 nanoparticles of 4.8 and 6.9 nm
diameters are shown below. The peaks at 2854 and 2925 cm-1 are due to the symmetric
and asymmetric CH2 stretching modes, whereas the peak at 3005 cm-1 due to the C-H
bond adjacent to the C=C bond. The peaks at 1528 and 1426 cm-1 are associated with
the asymmetric and symmetric COO- stretching. The spectra show that the surface of
nanoparticles of both 4.8 and 6.9 nm diameters was coated with oleic acid.
Fig. S5 FTIR spectra of CoFe2O4 nanoparticles coated with oleic acid. The red
curve represents the spectrum of CoFe2O4 particles of 4.8 nm diameter, whereas
the blue curve CoFe2O4 particles of 6.9 nm.
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6. TEM images of CNTs (magnified version)
Fig. S6 TEM images of CNTs. (A) The internal diameter is 4.4 nm. (B) 6.2 nm.
7. Distribution of the internal diameters of CNTs
Fig. S7 Distributions of the internal diameters of CNTs. (A) CNTs grown using
nanoparticles of 4.8 nm diameter. The internal diameter is 4.4 ± 0.21 nm. (B) CNTs
grown using nanoparticles of 6.9 nm diameter. The internal diameter is 6.2 ± 0.41 nm.
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8. Diameter of nanoparticles after complete reduction (theoretical calculation)
The diameter of nanoparticles after complete reduction is theoretically calculated as
follows. The mass density of CoFe2O4 is 5.3 g ml-1 and the volumes of particles of 4.8
and 6.9 nm diameters are, respectively, 5.79 × 10-20 and 17.2 × 10-20 ml. The volume of
each particle was converted into the mass; i.e., 3.07 × 10-19 and 9.12 × 10-20 g. The mass
fraction of Co plus Fe to CoFe2O4 is 0.73 and therefore the mass of Co and Fe in a
CoFe2O4 particle of 4.8 and 6.9 nm diameters are, respectively, 2.24 × 10-20 and 6.66 ×
10-19 g. The mass of Co and Fe is converted into the volume using the density of iron;
i.e., 7.8 g ml-1, and finally the diameters of the nanoparticles after complete reduction
are estimated to be 3.78 and 5.46 nm, respectively. A small correction is necessary to
obtain the correlation between the above theoretical diameters of nanoparticles and the
average internal diameters of CNTs; that is, 0.33 nm, which is the distance between
adjacent graphitic walls, should be added to the diameters of the nanoparticles. The final
internal diameters of CNTs would be 4.11 and 5.80 nm, respectively, which are very
close to those obtained in the present experiment.
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9. XPS analysis of nanoparticles
We investigated the reduction of CoFe2O4 particles to Fe-Co alloy ones by XPS analysis
of nanoparticles before and after the application of an H2 flow at 760 °C. The XPS
peaks of Fe 2p3/2, 2p1/2 and Co 2p3/2, 2p1/2 for the CoFe2O4 nanoparticles used as an
initial catalyst supported on a silicon substrate are shown in Fig. S6. Blue and red
curves represent, respectively, the spectra before and after the application of H2 flow.
The binding energies for Fe 2p3/2, Fe 2p1/2 and Co 2p3/2, Co 2p1/2 before the application
of H2 flow are, respectively, 711.0, 724.6 and 780.0, 796.5 eV. The differences in the
binding energies of Fe and Co in oxidised state corresponding to 2p3/2 and 2p1/2 are 13.6
and 15.5 eV, which are very close to the binding energy differences of CoFe2O4. After
the application of H2 flow, the peaks corresponding to Fe 2p1/2 and Co 2p1/2 at 724 and
796.5 eV have completely disappeared and those corresponding to Fe 2p3/2 and Co 2p3/2
have decreased. Two new peaks corresponding to Fe 2p3/2, Fe 2p1/2 and Co 2p3/2, Co
2p1/2 appeared at 706, 720 and 777.9, 793.5 eV, respectively. The sharp narrow shapes
and positions of the new peaks exactly coincide with pure Fe and Co (Wagner et al.
1979). Furthermore, the differences in the binding energies between Fe 2p3/2 and Fe
2p1/2 and between Co 2p3/2 and Co 2p1/2 are, respectively, 13.2 and 15.1 eV, which are
very close to the binding energy differences of metallic Fe and Co (Wagner et al. 1979).
The above result indicates that iron oxide and cobalt oxide have been reduced to the
metallic form of iron and cobalt. It can therefore be concluded that the shrinkage in the
size of the nanoparticles was caused by the phase change; i.e., the reduction of
nanoparticles from ferrite to metallic constituent of Fe-Co alloy.
Wagner CD, Riggs WM, Davis LE, Moulder JF, Mullenberg GE (1979) Handbook of X-ray
Photoelectron Spectroscopy, Eden Prairie, MN, USA: Perkin-Elmer Corp., pp.76-78.
Fig. S9 XPS analysis of catalytic nanoparticles before and after the application of H2 flow. (A)
Reduction from oxidised Fe to metallic Fe. (B) Reduction from oxidised Co to metallic Co.
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10. TEM images of CNTs (open end and a nanoparticle trapped at the tip)
HRTEM images of an open end of a CNT and a nanoparticle trapped at the tip of a CNT
are shown in Fig. S9. It is clearly shown that there is no metal/metal-oxide interface, no
segregated core-shell structure, but it is an alloy of CoFe2 since the crystalline lattice
belongs to the atomic plane (110) of CoFe2 and the interatomic distance was 2.02 Å,
which coincides with that reported before (Jurca et al. 2004).
Jurca IS. Meny C, Viart N, Ulhaq-Bouillet C, Panissod P, Pourroy G (2004) Growth, structure
and morphology of CoFe2/CoFe2O4 multilayers. Thin Solid Films 444:58-63
Fig. S10 TEM images of carbon nanotubes. (A) Open-end of a CNT. (B) Nanoparticle
trapped at the tip of a CNT.
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11. Distributions of the number of walls of CNTs
Fig. S11 Number of CNT walls. (A) CNTs grown using nanoparticles of 4.8 nm diameter.
(B) CNTs grown using nanoparticles of 6.9 nm diameter.
12. Crystal characteristics in the austenite and ferrite phases
Crystal
structure
FCC
BCC
Interstitial site
Octahedral
Tetrahedral
Octahedral
Tetrahedral
Number of voids
per unit cell
4
8
6
12
Number of voids
per unit atoms
1
2
3
6
Radius of a void
of iron, nm
0.052
0.028
0.019
0.036
13. Distributions of nanoparticles on silicon substrates
The ligand started to be stripped off from the surface of nanoparticles at 760 °C.
However, no coagulation was observed among the nanoparticles during the temperature
rise, noting that it took approximately 4 minutes the temperature to reach 760 °C,
whereas in the presence of hydrogen plasma, nanoparticles coagulated to form larger
particles when the diameter of particles was greater than 3.8 nm and the concentration
was 0.04 mg ml-1 (see Figs. S12-1 and S12-2). Agglomeration was mainly dominated
by plasma, temperature and the size of catalytic nanoparticles. CNTs of large internal
diameters can be grown by increasing the interparticle distances. In the present case, a
dense forest of vertically aligned CNTs of uniform diameters and lengths was
successfully grown thanks to the reduction in the concentration of nanoparticles to 0.03
mg ml-1, and the shrinkage of the size of the nanoparticles caused by the phase
transition.
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Fig. S13-1 HRSEM images of catalytic nanoparticles of 4.8 nm diameter deposited on a silicon
substrate. (A) Initial state. The concentration is 0.04 mg ml-1. (B) Distribution of nanoparticles
on the silicon substrate at 760 °C in the absence of hydrogen plasma. (C) Distribution of
nanoparticles on the silicon substrate at 760 °C in the presence of hydrogen plasma. There was
no agglomeration of nanoparticles on the silicon substrates at 760 °C in the absence of hydrogen
plasma thanks to the oleic acid ligand attached to the nanoparticles even when the concentration
was as high as 0.04 mg ml-1, whereas in the presence of hydrogen plasma, nanoparticles
coagulated to form larger particles.
Fig. S13-2 HRSEM images of catalytic nanoparticles of 6.9 nm diameter deposited on a silicon
substrate. (A) Initial state. The concentration is 0.04 mg ml-1. (B) Distribution of nanoparticles
on the silicon substrate at 760 °C in the absence of hydrogen plasma. (C) Distribution of
nanoparticles on the silicon substrate at 760 °C in the presence of hydrogen plasma. There was
no agglomeration of nanoparticles on the silicon substrates at 760 °C in the absence of hydrogen
plasma even when the concentration was as high as 0.04 mg ml-1, whereas in the presence of
hydrogen plasma, nanoparticles coagulated to form larger particles when the diameter of
particles was greater than 3.8 nm and the concentration was 0.04 mg ml-1. It is therefore
supposed that a dense forest of VA-CNTs of uniform diameters and lengths was successfully
grown thanks to the reduction in the concentration of nanoparticles to 0.03 mg ml-1, and the
shrinkage of the size of the nanoparticles caused by the phase transition.
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