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C A R B O N 4 6 ( 2 0 0 8 ) 1 7 9 2 – 1 8 2 8

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Effect of hole size on the incorporation of C60 moleculesinside single-wall carbon nanohorns and their release

Jing Fana, Ryota Yugeb, Alan Maignea,c, Jin Miyawakia, Sumio Iijimaa,b,d,Masako Yudasakaa,b,*

aSORST/JST, NEC Corporation, 34 Miyukigaoka, Tsukuba 305-8501, JapanbNEC Corporation, 34 Miyukigaoka, Tsukuba 305-8501, JapancUniversite Paris-Sud, CNRS, UMR 8502, 91405 Orsay, FrancedMeijo University, 1-501 Shiogamaguchi, Tenpaku-ku, Nagoya 468-8502, Japan

A R T I C L E I N F O

Article history:

Received 20 September 2007

Accepted 18 June 2008

Available online 10 July 2008

0008-6223/$ - see front matter � 2008 Eldoi:10.1016/j.carbon.2008.06.056

* Corresponding author: Address: NEC CoE-mail address: yudasaka@frl.cl.nec.c

A B S T R A C T

We created holes in single-wall carbon nanohorns (SWCNH) by oxidation (SWCNHox) and

investigated how the hole size affected the incorporation of C60 in SWCNHox and their

release from it. The incorporation of C60 inside SWCNHox first occurred when the holes

were opened by oxidation at 475 �C. It was followed by a steep increase in the incorporation

quantity with increasing oxidation temperature. The release rate of C60 from inside

SWCNHox was slower than that of C60 crystallites located outside, and did not depend

on the oxidation temperature (475–550 �C), indicating that the release rates were not influ-

enced by the hole sizes.

� 2008 Elsevier Ltd. All rights reserved.

Incorporation of materials, such as C60 [1], into carbon nano-

tubes have attracted the attention of researchers due to their

potential application, and the incorporation methods in gas

[2] and liquid phase [3,4] were developed. Not only the incor-

poration but also the release of C60 is possible, suggesting po-

tential application of the carbon nanotubes to various fields,

such as drug delivery. Irrespective of the progress of study

on the incorporation and release, however, how the incorpo-

ration and release of molecules are influenced by the hole

sizes has not been well studied. To address this issue, we used

single-wall carbon nanohorns (SWCNH) [5–7] to investigate

the influence of hole size on the incorporation and release

of C60 molecules. The SWCNH were oxidized in flowing air

with a temperature rise rate of 1 �C/min from room tempera-

ture to a target temperature (Tox) of 400, 450, 475, 500, 525,

550, or 575 �C [8]. As a result, holes were created in the walls

of SWCNH, which is noted as SWCNHox hereafter. In loading

sevier Ltd. All rights reserve

rporation, 34 Miyukigaoka,o.jp (M. Yudasaka).

C60 into SWCNH or SWCNHox by the nanoprecipitation meth-

od [6], we first mixed 6 mg of C60, 30 mg of SWCNH or

SWCNHox and 40 mL of toluene in a beaker. Then, toluene

was evaporated in a N2 gas flow. The quantity ratio of C60

loaded on SWCNH (C60/SWCNH) or SWCNHox (C60@

SWCNHox) was estimated through thermogravimetric (TG)

measurement performed at a heating rate of 10 �C/min in

an atmosphere of 100% O2 gas [6]. Fig. 1 shows that the

C60 quantity increased steeply at Tox above 475 �C.

XRD results for C60/SWCNH, C60@SWCNHox450, and

C60@SWCNHox575 (Fig. 2a, b, and g) exhibited peaks dif-

fracted from C60 crystals with sizes of about 30 nm, as esti-

mated from the full widths at half maximum of the peak at

17.8� using Scherrer’s formula. The crystals were too large to

exist inside SWCNHox (diameters: 2–5 nm), so they had to

be outside the SWCNHox. In the case of C60@SWCNHox475–

550, the XRD results did not exhibit any diffraction peaks of

d.

Tsukuba 305-8501, Japan. Fax: +81 29 850 1366.

20 40 400 450 500 5500.00

0.05

0.10

0.15

0.20C

60 /

SWC

NH

ox (

g/g)

Tox (oC)

As-

grow

n

Fig. 1 – Quantities of C60 loaded in SWCNHox.

C60

(a) as-grown(b) Tox=450

(c) 475

(d) 500

(e) 525

(f) 550

(g) 575

18 20 22

Inte

nsity

2 Theta

Fig. 2 – XRD patterns of C60 crystals and C60/SWCNH (a),

C60@SWCNHox450 (b), C60@SWCNHox475 (c),

C60@SWCNHox500 (d), C60@SWCNHox525 (e),

C60@SWCNHox550 (f), and C60@SWCNHox575 (g).

0 10 15 20 250.0

0.5

1.0

Con

cent

ratio

n of

C60

(µM

)

Immersion period (h)

450

as-grown

525

Tox = 575oC

0.0 0.5 1.040

60

80

100

0 20 25

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Rel

ease

rat

e (%

/h)

Immersion period (h)

as-grown

Tox=400oC

450

475

500

525

550

575

0

20

40

60

80

100

Nor

mal

ized

con

cent

ratio

n of

C60

Immersion period (h)

as-grown

Tox=450oC

525

575

5

0 10 15 20 255

1 2 3 4

1.5

a

b

c

Fig. 3 – Quantities of C60 released from C60/SWCNH (square),

C60@SWCNHox with Tox of 450 (triangle), 525 (lateral

triangle), and 575 �C (circle) in a time series estimated from

the maximum absorbance of UV/vis absorption spectra at

336 nm (a). Normalized release quantities of C60 (b). Release

rates calculated from the normalized release-curves (c).

C A R B O N 4 6 ( 2 0 0 8 ) 1 7 9 2 – 1 8 2 8 1793

C60 crystals (Fig. 2c–f), indicating there were few C60 crystals

outside the SWCNHox. The filling factor of C60 inside

SWCNHox550 was about 30% where the density of the C60

crystal (1.68 g /mL) and the pore volume of the inside space

of the SWCNHox (0.36 mL/g) [9] were applied. When Tox

was 450 �C or below, the hole size was too small for C60 to en-

ter inside SWCNHox.

TGA (Fig. 1) and XRD (Fig. 2) results indicated that the C60

incorporation in SWCNHox had a distinct threshold at Tox

of 475 �C. According to previous reports, small molecules,

such as N2 and xylene, exhibited a gradual increase in ad-

sorbed quantities with increasing Tox (300–500 �C), reaching

the maximum at Tox of 500 �C; that is, there was no thresh-

1794 C A R B O N 4 6 ( 2 0 0 8 ) 1 7 9 2 – 1 8 2 8

old-like Tox [10]. The kinetic diameter of C60 is 0.92 nm, while

those for N2 and xylene are 0.36 and 0.64 nm, respectively,

meaning that the number of the holes with sizes larger than

0.36 or 0.64 nm increased gradually with Tox, while the holes

larger than 0.92 nm opened only at and above Tox of 475 �C.

Thus, we consider that the molecular sieve effect of

SWCNHox proposed by Murata et al. [10] may be more effec-

tive for large molecules.

To study the release of C60 from C60@SWCNHox, 2 mg of

C60/SWCNH or C60@SWCNHox was immersed in 300 mL of

toluene/ethanol (volume ratio = 4:1) solution for time periods

ranging from 2 min to 1 day. The UV/vis absorption spectrum

of the supernatant, that is, the solution of the released C60

and toluene/ethanol, was measured. From the absorption

intensity at 336 nm, we estimated the quantity of released

C60 [6]. Typical dissolution profiles of C60 loaded in SWCNH

and SWCNHox are shown in Fig. 3. The raw data in Fig. 3a

are normalized by the almost saturated C60 quantities mea-

sured at 24 h (Fig. 3b). The normalized C60 quantities versus

immersion period curves were differentiated with respect to

the immersion period, and then the release rates were ob-

tained (Fig. 3c). It is clear that the primary release rates in

Fig. 3c were fast when there were the C60 crystallites outside

the SWCNHox. The primary release rates of C60 located inside

SWCNHox (Tox: 475–550 �C) were slow, though their varia-

tions with Tox were not systematic. We think that the slow re-

lease rates of C60 from inside SWCNHox was caused by the

confinement and stabilization of C60 inside SWCNHox and

that the influence of the hole size and existence of XRD-unde-

tectable C60 outside the SWCNHox caused their variations in

the primary release rates.

In conclusion, we found that C60 incorporation inside

SWCNHox exhibited the distinct threshold at Tox of

475 �C, when the hole sizes would correspond to the kinetic

diameter of C60, 0.92 nm. The release rates of the C60 from

inside SWCNHox did not depend on the Tox (475–550 �C),

indicating that the release rates were not influenced by

the hole sizes. This suggests that no attractive interaction

was active between the hole edges and C60. We consider

that the C60 release rate would be influenced by the hole

sizes, if the hole edges had certain molecules that attract

the C60 molecules.

R E F E R E N C E S

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[3] Yudasaka M, Ajima K, Suenaga K, Ichihashi T, Hashimoto A,Iijima S. Nano-extraction and nano-condensation for C60

incorporation into single-wall carbon nanotubes in liquidphases. Chem Phys Lett 2003;380:42–6.

[4] Simon F, Kuzmany H, Rauf H, Pichler T, Bernardi J, Peterlik H,et al. Low temperature fullerene encapsulation in single wallcarbon nanotubes; synthesis of N@C60@SWCNT. Chem PhysLett 2004;383:362–7.

[5] Iijima S, Yudaska M, Yamada R, Bandow S, Suenaga K, KokaiF, et al. Nano-aggregates of single-walled graphitic carbonnano-horns. Chem Phys Lett 1999;309:165–70.

[6] Yuge R, Yudasaka M, Miyawaki J, Kubo Y, Ichihashi T, Imai H,et al. Controlling the incorporation and release of C60 innanometer-scale hollow spaces inside single-wall carbonnanohorns. J Phys Chem B 2005;109:17861–7.

[7] Fan J, Yudasaka M, Yuge R, Futaba DM, Hata K, Iijima S.Efficiency of C60 incorporation in and release from single-wallcarbon nanotubes depending on their diameters. J PhysChem B 2007;45:722–6.

[8] Fan J, Yudasaka M, Miyawaki J, Ajima K, Murata K, Iijima S.Control of hole opening in single-wall carbon nanotubes andsingle-wall carbon nanohorns using oxygen. J Phys Chem B2006;110:1587–91.

[9] Murata K, Kaneko, Steele WA, Kokai F, Takahashi K, Kasuya D,et al. Molecular potential structure of heat-treated single-wall carbon nanohorn assemblies. J Phys Chem B2001;105:10210–6.

[10] Murata K, Hirahara K, Yudasaka M, Iijima S. Nanowindow-induced molecular sieving effect in a single-wall carbonnanohorn. J Phys Chem B 2002;106:12668–9.