Ordering and interaction of molecules encapsulated ...

Ordering and interaction of molecules encapsulated in carbon nanotubes A. N. Khlobystov, K. Porfyrakis, D. A. Britz, M. Kanai, R. Scipioni, S. G. Lyapin, J. G. Wiltshire, A. Ardavan, D. Nguyen-Manh, R. J. Nicholas, D. G. Pettifor, T. J. S. Dennis and G. A. D. Briggs A clear understanding of the interactions between the building blocks of self-assembled molecular materials is essential for rational design of functional nanostructures. Intermolecular interactions have been investigated for three different classes of fullerenes in single-walled carbon nanotubes (SWNTs); van der Waals molecule – molecule and molecule – SWNT interactions control the geometry of the molecular arrays inside nanotubes; electrostatic intermolecular forces influence the alignment of polar endohedral fullerenes M@C82; and hydrogen bonding between functionalised fullerenes has a significant effect on the selectivity of insertion of functionalised fullerenes into SWNTs. MST/6104

Published by Maney Publishing (c) IOM Communications Ltd

Keywords: Carbon nanotubes, Self-assembly, Fullerenes, van der Waals interactions, Hydrogen bonding Dr Khlobystov ([email protected]), Mr Porfyrakis, Mr Britz, Dr Scipioni, Dr Nguyen-Manh, Professor Pettifor and Professor Briggs ([email protected]) are in the Department of Materials, Oxford University, Parks Road, Oxford OX1 3PH, UK. Dr Lyapin, Mr Wiltshire, Dr Ardavan and Professor Nicholas are in the Department of Physics, Oxford University, Parks Road, Oxford OX1 3PU, UK. Ms Kanai and Dr Dennis are in the Centre for Materials Research, Queen Mary University of London, Mile End Road, London E1 4NS, UK. Revised version of a presentation at ‘Nanomaterials and nanomanufacturing’, held in London on 15 – 16 December 2003; accepted 16 June 2004. # 2004 IoM Communications Ltd. Published by Maney for the Institute of Materials, Minerals and Mining.

Introduction

van der Waals interactions

It is well known that molecules can be used as building blocks for construction of complex supramolecular architectures. Molecular assembly offers cheap and easy methods for creating materials of high complexity that can spontaneously self-assemble from molecular building blocks that interact via non-covalent intermolecular forces. The nature of intermolecular interactions can be electrostatic, induction, van der Waals (vdW), charge transfer or any combination of those components. However, regardless of its nature, any intermolecular interaction is significantly weaker than covalent bonding. The low energy of the interactions provides reversible connections between constituents of a molecular array, thus allowing the array to correct any structural faults during the self-assembly process.1

Fullerenes are nanoscopic carbon cages that exhibit unique physical and chemical properties suitable for a variety of applications. Fullerenes have been proposed as promising materials for quantum computing nanodevices.2 The Buckminster fullerene C60, the simplest representative of the family of nanoscopic carbon capsules, spontaneously forms fcc-type crystals with a lattice constant3 of 1.42 nm. Each fullerene is surrounded by 12 neighbours with 0.3 nm separations between neighbouring spheres (Fig. 1). The cohesive energy per C60 is about 2 eV (190 kJ mol21), indicating a significant intermolecular interaction within the 3D crystal that can be described by the Lennard – Jones potential.4 The same type of vdW interaction takes place when fullerenes form 2D monolayers adsorbed on a graphitic substrate. The energy of the

b

a

1 a close packed arrangement of C60 in the crystal; b macroscopic crystals of C60 where molecules are perfectly ordered in all three dimensions (photograph courtesy of Dr Adam O‘Neil)

DOI 10.1179/026708304225022061

Materials Science and Technology

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970 Khlobystov et al. Molecules encapsulated in carbon nanotubes

2 Atomic structural model of a (10,10) SWNT and b same nanotube encapsulating C60 molecules


3 HRTEM micrographs showing a (Ce@C82)@SWNT and b C60@SWNT peapod structures

4 HRTEM micrographs and schematic illustrations of a transverse and b longitudinal orientations of C70 in SWNTs with diameters of 1.49 and 1.36 nm respectively

interaction in a monolayer is lower (1.52 eV per C60) as compared to the 3D crystal.5 One of the approaches for arranging C60 in 1D periodic structures utilises vdW interactions between the molecules and single-walled carbon nanotubes (SWNTs) (Fig. 2a), which are widely used as containers for ionic and molecular materials.6 SWNTs with diameters allowing for graphitic separation (y0.3 nm) between the fullerene surface and SWNT inner sidewalls can absorb fullerenes spontaneously forming ‘peapod’ structures (Fig. 2b). The energy of the fullerene encapsulation has been calculated to be about 3 eV per C60 (y290 kJ mol21) for nanotubes with diameters matching the vdW diameter of the fullerene.5,7 Fullerenes appear to be regularly spaced inside SWNTs, forming a molecular array with a repeating period of 1 nm (Figs. 2b and 3). The vdW forces acting between the molecules, and between the molecules and the nanotube, are responsible for the precise positioning of fullerenes inside SWNTs. Orientations of ellipsoidal fullerene molecules C70 inside SWNTs have been studied. Nanotubes with diameters 1.36 and 1.49 nm (as demonstrated by Raman spectroscopy) have been prepared and filled with the fullerene C70 in the gas phase. The C70@SWNT structures have been characterised in detail by high resolution transmission electron microscopy (HRTEM, JEOL4000EX, LaB6) revealing longitudinal and transverse orientations for the ellipsoidal molecule C70 inside SWNTs (Fig. 4). Extensive HRTEM imaging showed that only one type of orientation exists within each single nanotube. Physical models developed for the C70@SWNT system8 demonstrate the mechanism for the molecular alignment in nanotubes, showing that the C70 orientation is strongly dependent on the nanotube diameter. Density-functional theory calculations predict a transverse orientation for C70 in (11,11)-nanotubes (d~1.49 nm) Materials Science and Technology

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and a longitudinal orientation for C70 in (10,10)-nanotubes (d~1.36 nm),8 confirming the experimental observations. The calculations show that the fullerene orientation is primarily determined by the nanotube – C70 interactions. This indicates that the vdW forces effectively control the orientation of molecules as well as their positions in peapod structures of non-polar fullerenes.

Dipolar interactions A metal atom encapsulated in a fullerene cage transfers some or all of its valence electrons to the molecular orbitals of the fullerene cage.9 This creates an asymmetric charge distribution in M@C82 molecules (Fig. 5) resulting in high dipolar moments of 3 – 4 D.10 The electrostatic dipole interactions between the molecules influence the orientation of the fullerene cages (Fig. 6). Structures of (Ce@C82)@ SWNT peapods were investigated by HRTEM. Each single

5 Structural model of an endohedral metallofullerene M@C82

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6 Schematic representation of orientations for pairs of Ce@C82 molecules: directions of dipole moments are indicated by arrows assuming that Ce-atoms in fullerene cages are located within cross-section plane

atom of Ce can be seen in HRTEM as a dark spot within the fullerene cage (Fig. 7). The position of the Ce3z ion indicates the positive end of the molecular dipole. By examining the relative orientations between the positively and negatively charged parts of neighbouring Ce@C82 molecules, it is possible to deduce whether the electrostatic interaction has attractive or repulsive character. For example, orientations (a) and (b) shown in Fig. 6 correspond to aligning interactions, orientations (c) – (e) correspond to anti-aligning interactions and orientation (f) corresponds to little or no electrostatic interaction between two molecules, on average. HRTEM imaging of the Ce atom positions in (Ce@C82)@SWNTs can be used to determine the 2D projection of molecular dipole moments on the plane perpendicular to the electron beam. The orientation of the Ce@C82 molecule is most conveniently expressed in spherical coordinates (Fig. 8) where the vector describing position points to the Ce atom. In the absence of external electromagnetic fields, the Ce atom in an isolated Ce@C82 will be found with equal probability at any point of the surface of the sphere of constant diameter r. The plane of the HRTEM photograph coincides with the x – y plane where the projection of a Ce atom appears as a dark spot at a distance r from the centre of the fullerene cage. The direction of the dipole moment is defined by two angles, h and w. The angle h can be measured directly from TEM images; the angle w can be calculated as w~arcsin(r/h) so that angles w and 180u2w are indistinguishable. This precludes the direct determination of the dipole moment orientation by HRTEM.

7 HRTEM micrograph and schematic (Ce@C82)@SWNT peapod structure

diagram

of

8 Spherical system of coordinates applied for Ce@C82

In order to reveal the mechanism of intermolecular interactions in nanotubes, Suenaga et al.11 have measured the 2D projection of the distance between metal atoms of two neighbouring molecules (Fig. 9) as a single geometrical parameter describing dipolar interactions in (La@C82)@ SWNTs systems. An advantage of using projection of dM – M (M~Ce or La) for statistical analysis is the fact that this parameter can be obtained easily from HRTEM micrographs by measuring the distance between two dark spots corresponding to metal atoms in neighbouring fullerenes. A similar approach used for (Ce@C82)@SWNTs shows a distribution pattern similar to (La@C82)@SWNT with a maximum at dCe – Ce~1.3 nm (Fig. 9).11 Suenaga et al. interpret a non-uniform distribution of the kind shown in Fig. 9 as evidence for electrostatic interactions controlling the fullerene orientations. This method gives the distribution of the 2D projection of dCe – Ce rather than dCe – Ce itself. Spherical aberrations of the microscope and any deviation of the interfullerene separations from the vdW distance (i.e. 0.3 nm) cause a significant uncertainty in projection dCe – Ce measurements, which makes this measurement less sensitive to the intermolecular interactions than angular parameter measurements (Fig. 8).

9 Schematic representation of two Ce@C82 molecules (dCe – Ce is distance between Ce-ions) and distribution of projections of Ce – Ce distances for pairs of neighbouring molecules in (Ce@C82)@SWNTs


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12 a schematic representation of M@Cn molecules lined up in nanotube; b schematic angular dependence of dipole potential V(h) for points separated by constant distance from centre of dipole (r constant, h varying); dipole is oriented along x-axis

10 a schematic representation of M@Cn inside nanotube: nanotube direction is shown by dashed arrow, dipole direction by solid arrow, h is angle between these directions; b experimental distribution of h in (Ce@ C82)@SWNTs; c theoretical distribution of h in (Ce@ C82)@SWNTs for random orientations (no interactions)

The fullerene dipole orientation h is the only parameter that can be measured directly from HRTEM images. In the absence of interactions, the probability distribution function for h would be constant and independent of the angle at any point (Fig. 10c), owing to the spherical geometry of the system of coordinates (Fig. 8). This leads to equal probability for any value of h in isolated Ce@C82 or neighbouring Ce@C82s without any interaction, and thus h is a more appropriate parameter for statistical analysis than the projection of dCe – Ce. Angles h with respect to the nanotube axis for Ce@C82 inside nanotubes have been measured as shown in Fig. 10a. The observed distribution for h (Fig. 10b) indicates a preferential orientation with the dipole moment parallel to the nanotube axis (small angle h) for Ce@C82. Suenaga et al.11 have performed analogous statistical analysis for (La@C82)@SWNTs systems showing a similar trend.

11 Experimental distribution of |h12h2| for pairs of neighbouring fullerenes in (Ce@C82)@SWNTs


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Comparison of angles h1 and h2 for pairs of neighbouring fullerenes shows a further modest correlation between the orientations (Fig. 11). The observation of a peak in the distribution of h indicates that interactions of some kind (i.e. either between neighbouring fullerenes or between a particular fullerene and the tube) play a role in determining the fullerene orientation. The observation that |h1 – h2| peaks at 0u and is not bimodal (i.e. peaked at both 0u and 180u) confirms that interfullerene interactions are important. A candidate mechanism for interfullerene interactions is the electrostatic dipolar interaction. Each molecule in (M@Cn)@SWNTs experiences the local electrostatic field determined by two nearest neighbours constrained by the nanotube to form a chain (Fig. 12). The electrostatic interaction between Ce@C82 molecules is strongest when the dipoles are lined up within the chain (Fig. 12a). Therefore, the preferential molecular orientation observed for Ce@C82 can be explained by the presence of electrostatic interactions in (M@Cn)@SWNTs systems.

Hydrogen bonding In order to investigate the effect of chemical groups grafted on the fullerene cage (Fig. 13a) cyclopropafullerene-C60dicarboxylic acid diethyl ester [C61(COOEt)2, R~COOEt, Fig. 13], and the cyclopropafullerene-C60-dicarboxilic acid [C61(COOH)2, R~COOH, Fig. 13] have been synthesised using previously described methods.12 A technique has been developed for inserting thermally unstable molecules into SWNTs at temperatures of 30 – 50uC using supercritical carbon dioxide.13 Under these conditions the C61(COOEt)2@SWNT and C60@SWNT peapod structures were formed in a yield up to 70%. The functionalised fullerene cages appear to be more sensitive to the electron beam than non-functionalised fullerenes (Fig. 13). The molecules of C61(COOEt)2 become elongated under HRTEM imaging conditions in a few seconds, which is followed by aggregation into polymeric chains. Although the exact mechanism of the polymerisation remains unknown, we postulate that the polymerisation is most likely to occure via cyclopropane ring opening induced by the electron beam in HRTEM. Fourier transform inrared (FTIR) spectra of C61(COOEt)2@SWNTs confirmed the

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13 a structural diagram of functionalised fullerene; HRTEM micrographs of b C60@SWNT and c C61(COOEt)2@SWNT structures showing relative stability of functionalised fullerenes under electron beam: exposure time is 1 s for both images

presence of the functional groups inside SWNTs [ester C~O (1745 cm21) and ethyl C – H (2800 – 3000 cm21) groups], as HRTEM imaging showed that there was virtually no fullerene present outside the nanotube after repeated washing with CS2. The FTIR measurement confirms that the fullerene functional groups do not degrade during the filling procedure. After the same filling procedure as for C60 and C61(COOEt)2, C61(COOH)2 was found to have preferentially formed a monolayer of fullerene on the outside of the SWNTs (Fig. 14). This effect is ascribed to the ability to form hydrogen bonds by carboxylic acid groups attached to the fullerene cages in C61(COOH)2. Carboxylic groups in non-polar solvents (i.e. CO2, CCl4) readily form hydrogen bonds with 2COOH-groups attached to other molecules, and cause aggregation of C61(COOH)2 molecules. The effective size of the fullerene C61(COOH)2 functionalised with polar groups is significantly greater than the size of C60 and C61(COOEt)2, due to the intermolecular aggregation. As a result the encapsulation of C61(COOH)2 in SWNTs is sterically prohibited. HRTEM demonstrates that dimeric structures C61(COOH)2 – (HOOC)2C61 formed via hydrogen bonding are abundant on SWNTs surface (Fig. 14, inset). The molecules of C61(COOH)2 were found to be noncovalently bonded to the SWNTs, more likely interacting by vdW forces as the monolayer coating was removed by extensive washing of the sample with MeOH. After washing, the interior of the SWNTs were visible, showing encapsulation of C61(COOH)2 in less than 1% of the

SWNTs. This experiment demonstrates that the selectivity of the host – guest interaction in the peapod systems can be controlled by the ability of the functional groups to form specific intermolecular interactions. Fullerenes with relatively inert ester groups can easily enter SWNTs, whereas fullerenes with more reactive carboxylic groups preferentially coordinate to the SWNTs exterior due to the intermolecular hydrogen bonding. This type of bonding is one of the strongest and most directional non-covalent interactions.1 As a result, C61(COOH)2 molecules form stable aggregates with an effective size too large to enter the nanotubes at room temperature. Such selectivity can be used, for example, in catalysis or for molecular separation.

Conclusions The intermolecular interactions for three different classes of fullerenes in peapod systems have been investigated. Van der Waals forces are dominating interactions for non-polar fullerenes such as C60 and C70, defining intermolecular separations and molecular orientations inside SWNTs. It has been demonstrated that the orientations of ellipsoidal molecules C70 are defined by the nanotube internal diameter which can be used as a mechanism for aligning the molecules in the peapod structures.Electrostatic interactions are important for endohedral metallofullerenes M@C82 possessing high molecular dipolar moments. It has been shown that these

14 a HRTEM micrograph showing two SWNTs covered with layer of C61(COOH)2: inset shows magnified area; b schematic diagram illustating dimeric aggregate of C61(COOH)2


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molecules line up due to the dipole – dipole interactions between neighbouring fullerenes in (M@C82)@SWNTs structures. Since the encapsulated fullerenes remain mobile in SWNTs, the orientations of M@C82 may be tuned by external electrostatic fields.The molecule – nanotube interactions of fullerenes bearing functional groups can be affected by noncovalent intermolecular forces such as hydrogen bonding. C61(COOH)2 molecules aggregate due to the intermolecular interactions and coordinate preferentially to the nanotube exterior, whereas the molecules present in a discrete form can be inserted in SWNTs even at low temperature due to effective van der Waals interactions between the fullerene cage and the interior of the nanotube and a small barrier for encapsulation.The understanding of the intermolecular interactions in carbon nanomaterials is an important step towards the rational design of functional nanoscopic architectures. Precise control of the geometry of the self-assembled structures will allow fine tuning of the functional properties of these materials. Understanding these interactions is important in particular for building functional nanoelectronics from molecular selfassembled materials.

Acknowledgements The work described here is supported through the Foresight LINK Award Nanoelectronics at the Quantum Edge, funded by DTI, EPSRC (GR/R66029/01) and Hitachi Europe Ltd. The authors wish to thank the Royal Society for a Joint Research Project (Japan) and the EPSRC for grant GR/R55313/01 (TJSD), and the Overseas Research Student Scheme (DAB) for funding. Professor Shinohara is thanked for assistance in preparation of Ce@C82. AA is supported by the Royal Society.


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