NANO LETTERS
Noncovalent Self-Assembly of Carbon Nanotubes for Construction of “Cages”
2002 Vol. 2, No. 5 531-533
Masahito Sano,*,† Ayumi Kamino, Junko Okamura, and Seiji Shinkai‡ Chemotransfiguration Project - JST, 2432 Kurume, Fukuoka 839-0861, Japan Received February 13, 2002; Revised Manuscript Received March 15, 2002
ABSTRACT A hollow spherical cage made of nested single-walled carbon nanotubes is constructed using self-assembly techniques. Nanotubes are first adsorbed onto amine-terminated silica gels in solution. Then, the drying and readsorption cycle grow the nanotube nest layer-by-layer. Finally, silica gels are etched away to give hollow cages. The initial amine−nanotube interactions determine the nested network, and nanotube−nanotube interactions thicken each arm of the nest.
The construction of super-structures made of single-walled carbon nanotubes (SWNTs) is one of the tasks in which nanotube chemistry can play a central role. So far, ringshaped and star-shaped SWNT super-structures have been synthesized by applying organic reactions in solution.1,2 The main difficulty in these reactions is the strong tendency of SWNTs to coagulate, resulting in poor solubility. If spontaneous aggregation can be regulated, SWNTs may be shown as a useful building component of supramolecular chemistry. In this report, we show that properly controlled kinetics and stabilization processes can re-direct spontaneous aggregation to form super-structures defined by a template object. To make SWNTs disperse in common solvents, various techniques, such as shortening by ultrasonication,3 derivatizing with alkyl chains,4,5 and mixing with surfactants or surface active polymers,6 have been utilized. Because the total van der Waals attraction between SWNTs along their fiber axes is enormous, self-aggregation is thermodynamically favored. Thus, long SWNTs tend to bundle spontaneously to form ropes even in these dispersions. If the affinity of bundles toward solvents is low, then the dispersion becomes only kinetically stable, which is the case for shortened SWNTs without derivatization or surfactants.7 This kinetically stable colloidal state of SWNTs is used for selfassembly processes. In particular, we employ layer-bylayer growth of adsorbed films, which has been applied extensively in supramolecular chemistry.8 As an example, we describe a procedure to make hollow spherical cages made only of SWNTs. Nanotube spherical cages are selfsupporting and may find applications in the area of microwave radiation. * To whom correspondence should be addressed. E-mail: mass@ yz.yamagata-u.ac.jp. † Present address: Department of Polymer Science and Engineering, Yamagata University 992-8510, Japan. E-mail:
[email protected]. ‡ Present address: Department of Chemistry and Biochemistry, Kyushu University 812-8581, Japan. E-mail:
[email protected]. 10.1021/nl025525z CCC: $22.00 Published on Web 04/10/2002
© 2002 American Chemical Society
The process consists of the layer-by-layer adsorption of SWNTs onto template silica gels (6 µm in average diameter) and stabilization of adsorbed tubes (Scheme 1). To initiate the adsorption process, amine-SWNT interactions are employed.3b,6c,9 Pristine SWNTs (diameter 1.2 nm) are cut by ultrasonication in strong acids.3 Cut-SWNTs become hydrophilic and can be dispersed in water. Very short tube materials have been removed to avoid complications arising from adsorption onto small pores in silica gels. The dispersion is centrifuged at 3500g, and only the supernatant solution is retained, 1. Immediately afterward, amine-functionalized silica gels10 (amine spheres) are added to adsorb SWNTs, 2. After a typical immersion time of 30 min, SWNT-covered amine spheres (SWNT spheres) are forced to sediment by centrifuging at 100g, and the supernatant solution is decanted, 3. SWNT spheres are collected on a Teflon filter and dried in oven, 4. Once dried, SWNT spheres are extremely stable (Figure 1b). SWNTs do not desorb even when the dried SWNT spheres are re-dispersed in solution. For additional cycles of coating, dried SWNT spheres are added to the freshly prepared SWNT dispersion and processes 2 through 4 are repeated. Silica at any cycle of coating can be etched away by HF to give spherical cages made of only SWNTs, 5 (Figure 2). In solution, cut-SWNTs aggregate spontaneously.7 It is, however, a rather slow process with a time constant of 470 h.11 Thus, coagulation of SWNTs in solution is negligible within an adsorption time of 30 min. In fact, no materials were seen to sediment at 3 in the absence of amine spheres. A controlled experiment has shown that no adsorption occurs on silica gels without amine-functionalization (Figure 1a). This indicates that, during adsorption at 2, SWNTs are adsorbed either onto amine surfaces or on those SWNTs that have been already adsorbed. To see the relative contribution of each process, the morphology of SWNT spheres and the
Scheme 1.
Self-Assembling Processes
thickness of SWNT layers were followed as a function of numbers of layering cycles. Thickness was estimated as follows. Slow evaporation of water from a cast film of cages often flattened the cages into circular disks. Atomic force microscopy was employed to construct 3D images of a disk. Thickness was calculated as the volume of a flattened circular disk divided by the area of the disk. After the first cycle, a network of SWNTs covers the amine sphere partially (Figure 1b). There are large portions of amine surfaces still uncovered. These uncovered spaces make the average thickness the low value of 1 nm and the rms roughness (of flattened disk surfaces) to near zero. SEM reveals that, as the number of layering cycles increases, the same network feature persists with no significant filling of the uncovered spaces. The thickness increases to 11.7 ( 2.8 nm and the roughness to 4 nm after 3 cycles, and then to 12.3 ( 6.0 and 7 nm after 5 cycles. The large standard deviation and increasing roughness indicate that overlayering does not smooth surfaces at all. Rather, it enhances roughing. These results suggest the following model. SWNTs are first adsorbed onto the amine surface. The SWNT-amine adsorption continues until the surface coverage reaches a point where a part of the incoming SWNT necessarily comes in near contact with the already adsorbed SWNTs. Then, van der Waals attractions by the adsorbed SWNTs capture the incoming SWNT and prevent it from reaching the amine surface. The first adsorbed SWNTs determine the network structure, and subsequent adsorption simply thickens each arm of the network. This model is supported by recent studies estimating the binding energies between amine and SWNTs to 23 kcal/mol8 and between two (10, 10) SWNTs to 22 kcal/ mol/nm.12 Only a few nm of the tube-tube contact length is sufficient to favor the SWNT-SWNT adsorption. Our choice of the layer-by-layer process with a short adsorption time is to isolate SWNT spheres before SWNTs 532
aggregate to significant sizes in solution. At the same time, it minimizes “cross-linking” of neighboring SWNT spheres
Figure 1. SEM image taken after the first cycle adsorption of SWNTs using (a) original silica spheres and (b) amine-functionalized silica spheres. In (a), a drop of the adsorption solution 2 was used to show that, despite the presence of SWNTs in solution, no adsorption occurs on silica gels without amine-functionalization. The SWNT spheres in (b) are after the drying process. PtPd coated. The bar is 3 µm in length. Nano Lett., Vol. 2, No. 5, 2002
Figure 2. SEM image of SWNT cages after 3 cycles. Some cages were crashed and deformed as water evaporated during the sample preparation. The bar is 15 mm in length.
first adsorbed onto amine spheres. It is clear that there is a larger fraction of shorter tubes in the adsorbed case. Multicurve analyses indicate that the distribution of adsorbed SWNTs (3b) can be separated into the distribution of tubes in the original solution (3a) and a Gaussian centered at 530 nm. We speculate that the excess distribution at 530 nm is the preferred adsorption length determined by the persistence length and the radius of sphere. Although a formal theory is required to verify this, the result signifies that tube lengths must be considered carefully when objects with different sizes or shapes are used as templates. The present paper describes a methodology to fabricate supramolecular structures made of SWNTs. It utilizes the self-aggregation tendency of SWNT itself and, thus, is energy efficient. It can produce a large amount of super-structures with simple procedures. Because SWNTs can be metallic or semiconducting, cages of micron sizes may find applications in electromagnetic radiation phenomena. References
Figure 3. Length distributions of SWNTs that are (a) in the original solution and (b) adsorbed onto amine spheres. The distribution in (b) can be decomposed to the distribution curve of (a) and a Gaussian peaked at 530 nm.
caused by SWNTs that bridge over many spheres. Because the adsorption is interrupted well before the system reaches equilibrium, the adsorbed SWNTs at 3 are not stable. If they are brought into contact with water, some SWNTs are desorbed. Stabilization is achieved by the drying process, 4. During drying, evaporating the solvent helps adjacent tubes to come close together. More tubes come into direct van der Waals contact and contribute to the cohesive energy of the 3D network. Once dried, we could not desorb SWNTs without destroying SWNT spheres. One of the characteristic features of SWNTs is stiffness, which is expected to play an important role in the adsorption onto curved surfaces. Figure 3a shows a length distribution of original SWNTs in 1 used for the self-assembly. A recent study gives the persistence length of SWNTs to be 800 nm,1 implying that most of tubes are considered to be stiff and straight. Figure 3b displays a length distribution of SWNTs
Nano Lett., Vol. 2, No. 5, 2002
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