NANO LETTERS
Hydrophobic Anchoring of Monolayer-Protected Gold Nanoclusters to Carbon Nanotubes
2003 Vol. 3, No. 3 279-282
A. V. Ellis,† K. Vijayamohanan,†,§ R. Goswami,† N. Chakrapani,† L. S. Ramanathan,‡ P. M. Ajayan,† and G. Ramanath*,† Materials Science and Engineering Department, and Chemistry Department, Rensselaer Polytechnic Institute, Troy, New York 12180 Received October 1, 2002; Revised Manuscript Received January 2, 2003
ABSTRACT Creating hybrid nanostructures consisting of disparate nanoscale blocks is of interest for exploring new types of quantum device architectures. Here, we demonstrate the novel anchoring of monolayer-protected gold nanoclusters of 1−3 nm diameter to sidewalls of carbon nanotubes (CNTs) via hydrophobic interactions between octanethiols capping the nanoclusters and acetone-activated CNT surfaces. Such molecularly interlinked hybrid nanoblocks are attractive for building biocompatible nanodevices.
Carbon nanotubes have remarkable electronic properties owing to their unique molecular structure and are attractive building blocks for creating next generation electronic devices and networks.1 Examples of nanotube-based devices include single-electron transistors,2,3 molecular diodes,4-6 memory elements,7 and logic gates.8,9 There is a great deal of interest in devising strategies to individually address each molecular unit, and interconnect them, without adversely affecting the local electronic structure.10,11 One approach to enable this is to attach metal- or semiconductor nanoclusters to nanotubes. This approach is also attractive for creating molecular-level hybrid units (e.g., in this case, a quantum dot attached to a molecular wire) and will allow the exploration of new properties and effects that arise from electronic-structure-level interactions between the constituent molecular units and applications such as active nanodevices and heterogeneous nanocatalysts. Since carbon nanotubes are chemically inert, activating their surfaces is an essential prerequisite for linking nanoclusters to them. Chemical treatments such as wet oxidation in HNO312,13 can functionalize nanotube surfaces with anchor groups such as hydroxyl (-OH), carboxyl (-COOH), and carbonyl (>CdO)14 that are necessary to tether metal ions to the tube. Recent studies have utilized covalent interactions in these groups to attach nanoclusters to single-walled nanotubes (SWNTs). For example, Lordi et al.12 used in situ reduction of K2PtCl4 and ion exchange to tether Pt clusters on nanotubes via carboxyl groups. More recently, Azamian * Corresponding author. E-mail:
[email protected]. † Materials Science and Engineering Department. ‡ Chemistry Department. § On sabbatical from National Chemical Laboratories, Pune, India. 10.1021/nl025824o CCC: $25.00 Published on Web 02/05/2003
© 2003 American Chemical Society
et al.13 attached Au nanoclusters to SWNTs via thioamides obtained by converting carboxyl groups on SWNT surfaces. Other strategies including electrostatic deposition and physisorption of nanoparticles of Au, Pt, and Ag onto acid-treated multiwalled carbon nanotubes (MWNTs) have also been reported.14-16 These nanoclusters, however, tend to rapidly coalesce on the nanotube templates resulting in wire-like structures consisting of large polycrystals.17 Attachment of compound semiconductor nanoclusters (e.g., CdSe)18,19 via covalent linkages, and anchoring of metal complexes20,21 to oxidized nanotube surfaces have also been reported. Here we report a unique approach to connect monolayerprotected gold nanoclusters to non-oxidized carbon nanotubes by means of relatively weak (e.g., ∼0.1 eV) hydrophobic, rather than covalent, interactions. The anchorage is provided by interdigitation of alkyl chains of self-assembled molecular layers (SAMs) capping the nanoclusters and molecular moieties adsorbed on nanotubes. Since the structure and chemistry of SAMs are similar to many biomolecules such as proteins, cluster attachment through SAMs will enable the design and creation of new bio-inspired hybrid nanodevices. For example, creating metal-SAM-nanotube units will open up possibilities for replicating quantum effects such as single-electron hopping and coulomb blockade in nanotubes and nanoclusters functionalized with proteins. Moreover, it is conceivable to modulate such effects by adjusting SAM chain length and structure, enabling molecular-level design of nanodevices for switching, sensing, and information storage. MWNTs synthesized by the arc-discharge method were treated with acetone in an ultrasonic bath for 1 h and dried
Figure 1. UV-visible absorption spectra of OT-capped Au nanoclusters in toluene before (squares) and after (circles) attachment onto acetone-treated MWNTs. Spectra from acetone-treated MWNTs (triangles) and pristine MWNTs (diamonds)-identical before and after Au-OT treatment-are shown for reference.
in air for 15 h to remove the free acetone. These activated tubes were dispersed in toluene by agitation in an ultrasonic bath, prior to being mixed with OT-capped gold nanoclusters. The OT-capped nanoclusters were prepared following the Brust method,22,23 but without the use of phase transfer agents. A 2 mM auric chloride solution in deionized water was mixed with 5 mM OT in toluene in a 1:2 ratio and stirred vigorously for 30 min. This two-phase mixture was reduced by slowly adding a 0.1 M aqueous NaBH4. Stirring was continued until all the metallic ions were completely converted to OT-capped clusters, which migrate to the toluene layer. The two layers were then isolated in a separating funnel, and the resulting toluene layer was washed five times with deionized water and dried in a N2 atmosphere. To attach gold nanoclusters to MWNT surfaces, the dried OT-capped nanoclusters were added to acetone-treated MWNTs dissolved in toluene. The results reported here, unless mentioned otherwise, were those obtained after removing unattached nanoclusters by a 5-min agitation in an ultrasonic bath, followed by decanting the supernatant liquid, and subsequent washing six times with toluene. The excess toluene was removed by drying in air for 12 h. MWNTs-with and without Au attachment-dispersed in a toluene medium were transferred to a carbon-coated Cu grid for high-resolution transmission electron microscopy (HRTEM) measurements in a JEOL-2010 TEM operated at 200 kV. Transmission-mode Fourier transform infrared (FTIR) spectroscopy was carried out in a Bio-Rad Excalibur series FTS 3000MX spectrophotometer. The samples for FTIR consisted of 150 mg KBr disks containing 0.1 mg of each dried sample. The results reported here were obtained from 200 scans at a 4 cm-1 resolution, and were verified at least four times with different nanocluster samples prepared and stored under identical conditions. Figure 1 shows UV-visible absorption spectra of OTcapped gold nanoclusters before and after attachment onto 280
acetone-treated carbon nanotubes. The Au-OT clusters exhibit a strong surface plasmon peak at ∼400 nm, in contrast with the well-known 520 nm peak characteristic of >3 nm nanoclusters.24-26 We attribute this result to surface plasmon excitation from ∼1-3 nm-diameter gold nanoclusters consisting of AuCl3 complexes and/or 3-13 Au atoms, consistent with previous reports.27,28 The small peak at ∼310 nm corresponds to the ligand-to-metal charge-transfer absorption band of AuCl4-.29 Both these peaks remain, albeit at a decreased intensity, in acetone-treated MWNTs mixed with OT-capped Au nanoclusters, even after repeated (6 times) washing and vigorous ultrasonication. This result indicates that the gold nanoclusters are attached to the nanotubes. The retention of the optical absorption signatures of the nanoclusters suggests that cluster attachment does not have an adverse influence on the optical properties of the nanoclusters due to phenomena such as coalescence. We note that Au-OT nanoclusters do not attach to MWNTs that are not treated with acetone; the nanoclusters get removed during repeated washing in toluene. This is indicated by the absence of surface plasmon peaks in UV-visible spectra obtained from mixtures of Au-OT and pristine MWNTs. Thus, acetone treatment is essential to enable the connection of nanoclusters to nanotubes in our experiments. Figure 2b is a representative TEM micrograph of a MWNT with 1-3 nm nanoclusters, indicated by spheres of dark contrast, attached to it. A micrograph of a nanotube prior to cluster attachment is also shown in Figure 2a for comparison. Many nanoclusters exhibit Moire´ contrast arising from the superimposition of the gold lattice planes with that of MWNT shells, indicating that nanoclusters are either on, or close to, the nanotube. An example of such contrast is seen in the nanocluster marked by the arrow in Figure 2c. The gold nanoclusters have an fcc structure, indicated by the spacing of {111} lattice planes measured from HRTEM micrographs (e.g., see Figure 2d), and that of (111) and (200) reflections seen in electron diffraction patterns (see Figure 2e). In many instances, we also observe assemblies of nanoclusters attached to the nanotubes, as shown in Figure 2f. The intercluster spacings, measured by constructing Voronoi30 cells around each nanocluster, have discrete values with a high frequency between ∼1.1-1.5 nm (see Figure 2g). This spacing range correlates well with 1.5 to 2 times the length of OT (∼0.7 nm), suggesting interdigitation between OT molecules that cap adjacent clusters. Note that the lowest intercluster spacing of ∼0.8 nm corresponds to the highest degree of interdigitation between OT chains. Spacings higher than 1.5 nm most likely arise from clusters belonging to adjacent subassemblies that are not linked to each other. To determine the mechanism by which the Au clusters were attached to the nanotubes, we carried out transmissionmode FTIR measurements. Our results, shown in Figure 3 and Table 1 and described below, indicate that the nanoclusters are attached to nanotubes via hydrophobic interactions27 between CH3 groups in acetone (on MWNTs) and the alkyl chains in OT molecules (on the nanoclusters). Nano Lett., Vol. 3, No. 3, 2003
Figure 3. FTIR spectra of MWNTs (a) before and (b) after acetone activation; and (c) OT-capped gold nanoclusters attached to acetonetreated MWNTs. (d) Reference spectrum from OT-capped Au clusters before attachment on to the MWNTs. The dashed lines show the positions of the asymmetric and symmetric modes of CH3 and CH2.
Figure 2. Representative HRTEM micrographs showing (a) lowmagnification of a pristine nanotube, (b) low-magnification of Au nanoclusters attached on a nanotube, and (c) high-magnification views of a 10-nm-diameter MWNT with Au nanoclusters attached to it. (d) HRTEM image of a gold nanocluster showing {111} lattice fringes. (e) Diffraction pattern showing (111) and (200) reflections from fcc Au and (0002) reflections from MWNTs. (f) Example HRTEM micrograph showing nanocluster assemblies attached to a MWNT. Arrows indicate the tube walls. The lines drawn in (f) illustrate the scheme used to measure intercluster spacings between nearest neighbor nanoclusters, for two example nanoclusters. (g) Histogram of spacings between nearest-neighbor nanoclusters.
Both pristine and acetone-activated MWNTs, prior to cluster attachment, show no observable absorption signatures corresponding to carbonyl groups >CdO (e.g., at 1750 cm-1). However, C-O stretch signatures expected at ∼1150 cm-1 could not be delineated due to peaks from CH wagging vibrations in the 1350-1150 cm-1 range. Acetone treatment increases the asymmetric CH3 stretching peak intensity a 2956 cm-1, indicating adsorption. This is also seen in Table 1, which lists the relative changes in CH3/CH2 intensity ratio for MWNTs before and after acetone treatment, and for Au clusters before and after attachment. Spectra from the nanoclusters prior to attachment (see spectrum d in Figure 3) show absorption peaks at 2871 and 2956 cm-1 corresponding to unhindered symmetric and asymmetric stretching, respectively, of terminal CH3 groups in the OT molecular caps. This indicates a high degree of flexibility in the OT molecules that cap the nanoclusters, as expected, and is corroborated by a series of discrete equidistant twisting-rocking and wagging progression CH2 modes between 1150 and 1305 cm-1 (not shown). Nano Lett., Vol. 3, No. 3, 2003
Table 1. Ratios of Asymmetric CH3 (2956 cm-1) to Asymmetric CH2 (2926 cm-1) Stretching Vibrations for MWNTs and OT-Capped Au Nanoclusters before and after Various Treatments or Attachment sample pristine MWNTs acetone-treated MWNTs Au-OT attached to acetone-activated MWNTs pristine OT-capped Au nanoclusters
CH3/CH2 asymmetric stretching peak ratio 0.5 0.63 0.30 0.87
Upon attaching nanoclusters to MWNTs both symmetric and asymmetric CH3 stretching modes diminish by more than 25% (compare c and d in Figure 3, and see Table 1). Furthermore, the CH3 to CH2 ratio for the asymmetric stretch decreases by ∼52 and 66%, when compared with that of acetone-treated tubes and unattached OT-capped nanoclusters, respectively (compare c and b in Figure 3, and see Table 1). We attribute both these results to the decreased flexibility of OT molecules on the nanoclusters, and acetone molecules on MWNTs, due to hydrophobic interchain interactions between CH3 groups of acetone and alkyl chains of OT. This constrained environment that enables nanocluster attachment is consistent with the increased gauche defect concentration indicated by the broadening, and decreased intensity of the CH2 twisting rocking mode peaks, described earlier. Based upon the above, we propose that OT-capped gold nanoclusters are anchored to acetone-activated MWNTs by attractive interactions resulting from interdigitation of the alkyl chains of OT and the methyl termini of the acetone. Figure 4 is a schematic sketch that illustrates the proposed mode of attachment of OT-capped Au nanoclusters on 281
Figure 4. Schematic sketch (not to scale) illustrating the proposed mechanism of attachment of OT-capped Au nanoclusters to acetoneactivated MWNTs. Interdigitation between OT molecules of adjacent nanoclusters is also shown. Dashed lines in the acetone molecules illustrate that acetone is not attached to nanotubes by CdO type bonds.
acetone-activated MWNTs. The absence of CdO bonds in acetone-treated nanotubes suggests that acetone is adsorbed to the nanotube walls via C-O-C type bonds or resonance structures, in configurations where the terminal methyl groups point away from the nanotube surfaces. This view is supported by ab initio calculations based on density functional theory showing that acetone attaches preferentially at defects such as vacancies on nanotube sidewalls, via ∼0.903.3 eV interactions resulting from C(acetone)-O-C(nanotube) bonds, rather than CdO bonds, on nanotube surfaces.31 Preferred adsorption of acetone and cluster attachment at defects are consistent with previous works32 which suggest that ultrasonication creates defects on nanotube surfaces, thereby facilitating functionalization. In summary, we have shown that SAM-protected gold clusters can be attached to acetone-activated carbon nanotubes through hydrophobic interactions. Such molecular interactions are compatible with many biological systems and will be important for exploring and creating a rich variety of molecular nanostructures for device applications. Acknowledgment. We gratefully acknowledge funding from a seed grant from NSF- Nanoscale Science and Engineering Center at RPI (G.R. and AE), a gift grant from Philip Morris, USA (G.R., K.V. and PA), and G.R.’s NSFCAREER award DMR 994478. References (1) Carbon Nanotubes: Synthesis, Structure Properties and Applications; Dresselhaus, M. S.; Dresselhaus, G.; Avouris, Ph.; Springer-Verlag: Berlin, 2001.
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