Direct imaging of intra-cage structure in titanium-carbide
endohedral metallofullerene
Yuta Sato,1 Takashi Yumura,1,2 Kazu Suenaga,1 Hiroe Moribe,3 Daisuke Nishide,3
Masashi Ishida,3 Hisanori Shinohara,3 and Sumio Iijima1,2
1
Research Center for Advanced Carbon Materials, National Institute of Advanced
Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan
2
Department of Materials Science and Engineering, Meijo University, Nagoya
468-8502, Japan
3
Department of Chemistry and Institute for Advanced Research, Nagoya University,
Nagoya 464-8602, Japan
Supporting Information
S1: Experimental procedures
S2: EELS observation
S3: Optimized structures of (Ti2C2)@C78 isomers
References
1
S1. Experimental procedures
Preparation of Ti2C80 from Ti/C composite rods by direct current (DC) arc
discharge and the isolation using high-performance liquid chromatography (HPLC)
were performed in the same manner as described elsewhere.8* Isolated Ti2C80
molecules were incorporated into open-ended single-walled carbon nanotubes
(SWNTs) by vaporization in vacuum.23 The SWNTs filled with Ti2C80 were dispersed
in n-hexane, and then fixed on a copper TEM grid coated with holey carbon.
A JEOL 2010F microscope equipped with a Gatan detector (model 794) based
on charge-coupled devices (CCD) was used for HR-TEM observation at an
accelerating voltage of 120 kV. This observation condition enables one to visualize the
moiré pattern coming from the zigzag chains on the rolled-up graphene sheet of an
SWNT.24 In the present study, however, the contrast of the moiré pattern was
minimized by careful adjustment of defocus value (f) not to disturb the molecular
images of incorporated Ti2C80 (see Fig. S1-1).
–––––––––––––––
* In the previous report,8 it was supposed that the obtained product is a 3:1 mixture of
the two Ti2@C80 isomers based on the D5h- and Ih-C80 cages, respectively, and that they
are inseparable from each other by HPLC. The structures of Ti2C80, however, are
actually ascribed to the (Ti2C2)@C78 types, as shown in the present study and in Ref.
[9–12,17].
2
FIG. S1-1. (a) Phase contrast transfer functions for the present HR-TEM observation at
f of –320 (i), –380 (ii) and –470 Å (iii; Scherzer focus). The red line indicates the
spatial frequency of 0.469 Å -1 which corresponds to the distance between the adjacent
zigzag chains on the rolled-up graphene sheet of an SWNT (2.13 Å). This figure
indicates that the contrast of the moiré pattern coming from these zigzag chains is
maximized and minimized at f of –320 and –380 Å, respectively. (b) HR-TEM
images of the SWNT with the chiral index of (16,3) experimentally obtained (left) and
simulated (right) using f of –320 (top), –380 (middle) and –470 Å (bottom).
3
S2. EELS observation
The effect of the incorporation of Ti2C80 into SWNTs on the charge state of
the encaged Ti atoms was examined by in situ EELS observation using a Gatan
ENFINA spectrometer attached to the electron microscope. The EELS spectrum
obtained for a bundle consisting of about ten SWNTs filled with Ti 2C80 shows two
peaks ascribed to the Ti L2 and L3 edges at around 466.0 and 460.5 eV, respectively
(Fig. S2-1). These EELS edges are almost identical to those previously observed for
the original crystalline Ti2C80, in which the positive charge of each Ti atom is found to
be two or less.8
The average intermolecular spacing of Ti 2C80 incorporated in SWNTs is
determined to be 10.3 ± 0.4 Å based on HR-TEM images, which is almost identical to
that found for the empty C70 (10.2 ± 0.4 Å25) but smaller than those for Gd@C82 and
Sc2@C84 metallofullerenes (11.0 ± 0.3 Å23,25). The average diameter of SWNTs filled
with Ti2C80 is estimated to be 14.3 ± 0.4 Å.
4
FIG. S2-1. The EELS spectrum obtained for the Ti 2C80 molecules incorporated into
SWNTs. (Inset) The HR-TEM image of the bundle of SWNTs filled with Ti2C80 for
which the EELS spectrum was obtained.
5
S3. Optimized structures of (Ti2C2)@C78 isomers
The original D3h symmetry of the C78 cage in the end-on type (Ti2C2)@C78
isomer is not disturbed by the linear arrangement of the Ti 2C2 cluster on the C3 rotation
axis (Figure 3a). The calculated 13C NMR chemical shifts of this cage are classified
into eight groups in the range of 129–145 ppm.10 On the other hand, the one-sided
arrangement of Ti2C2 in the side-on type (Ti2C2)@C78 isomer (Figure 3b) degrades the
D3h cage symmetry to as low as Cs, and causes the significant split of the NMR peaks
ranging from 125 to 162 ppm.10 Relative intensities of these split peaks, therefore,
should be lower than those of the end-on type, even if the yields of the two isomers are
comparable with each other. The side-on type isomer is expected to be energetically
less favored than the end-on type by 1.66 eV,9 and to exist as a minor component in the
Ti2C80 specimen. Thus the side-on type isomer does not give the NMR peaks with
detectable intensities in the previous experiment, 8 but is detected by the present
HR-TEM study. Each of the eight NMR peaks at 130–145 ppm observed for Ti2C808 is
reproduced by the end-on type (Ti2C2)@C78 model with the accuracy of –1.3 to +0.6
ppm,10 while the intensity of the peak coming from the encaged carbon dimer (which
should be one sixth of the full-intensity peaks from the cage) has been below the
detection limit.
The detailed configurations of the Ti2C2 clusters in the optimized (Ti2C2)@C78
models are shown in Figure S3-1. The Ti–C distances are 1.995 and 2.021–2.053 Å in
the end-on and side-on type Ti2C2 clusters, respectively, and they are shorter than the
Sc–C distances in the Sc2C2 cluster (which is essentially the side-on type) encapsulated
in the (Sc2C2)@C84 molecule (2.263 Å).4 This implies that the carbon dimers in
(Ti2C2)@C78 isomers, especially the one in the end-on type, should be more strongly
fixed to the Ti atoms than that to the Sc atoms in (Sc 2C2)@C84. The activation energy
estimated for the transition of Ti2C2 from the end-on type into the side-on type is as
high as 2.11 eV,9 suggesting that the rotational motion of the carbon dimer in the
fullerene cage is less likely to occur in (Ti 2C2)@C78 than in (Sc2C2)@C84.5 Even when
Ti2C2 takes the side-on type configuration, the Ti atoms favor the one-sided
arrangement inside the C78 cage unlike the Sc atoms in (Sc2C2)@C84, hindering the C2
rotation.
6
Figure S3-1. Ti–C bond lengths in the optimized structures of the (Ti 2C2)@C78 isomers
having the end-on (a) and side-on (b) type Ti2C2 clusters. Red and yellow balls denote
the encaged Ti and C atoms, respectively. Azure-colored hexagonal and pentagonal
rings correspond to those in Fig. 3.
7
References
23
K. Hirahara, K. Suenaga, S. Bandow, H. Kato, T. Okazaki, H. Shinohara, S. Iijima,
Phys. Rev. Lett. 85, 5384 (2000).
24
A. Hashimoto, K. Suenaga, A. Gloter, K. Urita, S. Iijima, Nature (London) 430, 870
(2004).
25
K. Hirahara, S. Bandow, K. Suenaga, H. Kato, T. Okazaki, H. Shinohara, S. Iijima,
Phys. Rev. B 64, 115420 (2001).
8