9576
J. Phys. Chem. B 2000, 104, 9576-9579
ARTICLES Bolaamphiphile Nanotube-Templated Metallized Wires Hiroshi Matsui,* Su Pan, Bogdan Gologan, and Seth H. Jonas UniVersity of Central Florida, Department of Chemistry, Orlando, Florida 32816 ReceiVed: February 29, 2000
This paper introduces a new method to produce metal-coated nanowires using self-assembled bolaamphiphile nanotubes as templates. Electroless Ni and Cu coatings on the bolaamphiphile nanotubes were achieved in Ni and Cu baths with reducing agents. Raman microscopic analysis indicates that non-hydrogen-bonded amide groups of the bolaamphiphile nanotubes capture metal ions and form stable metal-amide complexes. These complexes seem to become efficient nucleation sites and produce metallic coatings around the bolaamphiphile nanotubes. This method may therefore be applied to construct nano-electric circuits since the metal-coated bolaamphiphile nanotubes can grow on Au electrodes via self-assembled carboxylic acid thiol monolayers.
Nanometer-sized architectures of metals and semiconductors have recently been recognized to have great potential for nanoelectronic devices, nano-optical devices, and chemical sensors.1,2 Self-assembling has been successful in the construction of complex functional nanosized devices.3,4 Biological assemblies and organic assemblies have been used as templates and coated by metals and inorganic molecules for miniaturization of electronics and communication technologies.5-7 For example, DNA was used as a template to construct a silver nanowire connecting two electrodes.8 Bacterium assemblies were also templated to produce nano-ferromagnet fibers.9 Bolaamphiphiles have been studied as models of functionalized membranes,10,11 while the interesting self-assembling natures of the bolaamphiphiles were recently reported.12-16 Assembled structures of bis(N-R-amido-glycylglycine)-1,7heptane dicarboxylate (Figure 1), one of the bolaamphiphiles, display a sensitivity to pH.16 In an acidic solution, the heptane bolaamphiphile grows to a crystalline tubule, while a helical ribbon structure is formed in a basic solution. The degree of carboxylic acid protonation was used to control the final assembled structures since the structures are determined by the strengths of the amide-amide and carboxylic acid dimer hydrogen bonds. The tubules have an average size of 700 nm in diameter and 10 µm in length. The assembled structure of the heptane dicarboxylate nanotube was previously determined using X-ray and Raman studies.16 An interesting feature of this assembled structure is that a pair of amide groups bind the neighboring dicarboxylate’s amide bonds via hydrogen bonds while the other pair of amide groups are free from the hydrogen bonds in Figure 2. These amide groups point toward either the z direction or the 1/2y + 1/ z direction as shown in Figure 2a. These non-hydrogen2 bonded amide groups may capture metal ions such as Pt, Pd, Cu, and Ni (Figure 2b) since this group of metals forms a stable four-coordinate planar structure as zwitterions with amino acid dimers.17 The amide-metal complexes in the nanotubes may * Corresponding author. E-mail:
[email protected].
Figure 1. Chemical structure of monomeric bolaamphiphile, bis(NR-amido-glycylglycine)-1,7-heptane dicarboxylate.
serve as nucleation sites and produce stable coatings around the heptane dicarboxylate nanotubes. The difficulty in organic nanotube metallization is to create organic-inorganic junctions on the nanotube surfaces. But the heptane dicarboxylate nanotubes may overcome this difficulty through the intercalating of non-hydrogen-bonded amide sites. In other words, the metal-intercalating amide groups may serve as organic-inorganic junctions. Therefore, the heptane dicarboxylate nanotubes have the potential to be excellent templates for inorganic and metallic nanowires. To examine this hypothesis, the heptane dicarboxylate nanotubes were coated by Ni and Cu, respectively. First, the heptane dicarboxylate nanotubes were assembled in a pH 6 citric acid/ sodium citrate buffer solution. In this solution, the tubule structures of the bolaamphiphiles appeared after two weeks at room temperature. Further details of the tube assembly method are described elsewhere.15,16 Then electroless coatings were developed by loading the nanotubes in a Ni bath (0.08 M NiCl2‚ 6H2O) or in a Cu bath (0.35 M CuCl2‚2H2O) for 2 h. After metal ions were reduced by hypophosphite (0.1 M) for Ni coating and dimethylaminoborane (0.2 M) for Cu coating, the nanotubes were rinsed with water several times. Cu and Ni coat the nanotube with and without the use of a Pd catalyst. Figure 3a shows a scanning electron micrograph of Ni-coated tubes with Pd. An average diameter of the Cu-coated nanotubes is approximately 700 nm and the Ni-coated tubes have an average diameter of 1 µm. Before coating, the nanotube was observed to have a tubular structure.15,16 A hole was observed at the end of the tubule structure after the Ni coating in Figure 3a. The hollow structure is also confirmed in a transmission electron micrograph of the Ni-coated tube (Figure 3b). These results
10.1021/jp000762g CCC: $19.00 © 2000 American Chemical Society Published on Web 09/20/2000
Bolaamphiphile Nanotube-Templated Metallized Wires
J. Phys. Chem. B, Vol. 104, No. 41, 2000 9577
Figure 2. (a) Assembled structures of the heptane bolaamphiphile in the tubule. A pair of heptane bolaamphiphiles are connected by hydrogen bonds between two COOH groups via acid-acid dimer interactions in the x direction. An intermolecular amide-amide hydrogen bond is formed along the z direction and along the 1/2y + 1/2z directions, respectively. (b) Assembled structures of the heptane bolaamphiphile viewed in the xz plane. Possible occupied sites of metal ions are shown in arrows.
show that the metallized tube preserves a hollow structure. It should be noted that the scanning electron micrographs of the nanotubes were unobtainable when the nanotubes lacked metal coatings, since the noncoated nanotube was easily charged up under the 10 kV of the electron accelerate voltage of the SEM due to lack of conductivity. The metal coatings were also confirmed by elemental analysis of the nanotube surfaces using an energy-dispersive X-ray spectrometer (EDAX) with 10 kV of electron accelerate voltage, while uniformity of the metal coverage on the nanotube surfaces was not yet evaluated. To identify the binding sites of metal ions in the heptane dicarboxylate nanotube, vibrational modes of the metal ionheptane dicarboxylate nanotube complexes before reduction were probed by a Raman microscope (Jobin Yvon/Horiba,
LabRam) with 632.8 nm excitation. Figure 4 shows Raman spectra of the noncoated nanotube, Ni nanotube, and NiCl2 solution. The Ni-O asymmetric stretch mode and the Ni-N symmetric stretch mode appear at 284 and 442 cm-1, respectively, depicted with dotted lines in Figure 4a. These assignments are consistent with the vibrational frequencies of Ni-N and Ni-O stretching modes in bis(glycino) Ni complex, 289 and 441 cm-1.18 This result indicates that the Ni ions are captured by the non-hydrogen-bonded amide groups of the nanotubes. It should be noted that nickel-nitrogen vibrations appear in the Raman spectra only when Ni ions bind nitrogen atoms of the amide groups in the nanotubes. These sites seem to serve as nucleation sites for further metallization. Peaks at 545 and 595 cm-1 are assigned as the vibrational modes of the heptane
9578 J. Phys. Chem. B, Vol. 104, No. 41, 2000
Matsui et al.
Figure 4. Raman spectra of (a) noncoated nanotube, (b) Ni-doped nanotube, (c) Ni solution, and (d) Ni-doped nanotube after the addition of EDTA. Dotted lines represent peak positions of the Ni-O asymmetric stretch mode (284 cm-1) and the Ni-N symmetric stretch mode (442 cm-1).
Figure 3. (a) Scanning electron micrograph of Ni-coated bolaamphiphile nanotubes. EDAX showed that the intensity ratio of Ni KR and C KR peaks is 1:7. (b) Transmission electron micrograph of Nicoated bolaamphiphile nanotube.
dicarboxylate from the comparison between spectra (a) and (b) in Figure 4. There are no matching peaks between the Nibolaamphiphile nanotube spectrum (b) and NiCl2 solution spectrum (c), which shows that the salts were removed from the Ni-doped nanotubes after rinsing with water. Raman spectra of the noncoated nanotube, Cu nanotube, and CuCl2 solution are shown in Figure 5. In Figure 5b, the Cu-O asymmetric stretching and the Cu-N symmetric stretching modes appear at 332 and 450 cm-1, respectively, marked by dotted lines. The Cu-N and the Cu-O stretching frequencies of the bis(glycino) Cu complex, 337 and 462 cm-1, agree with the above assignments.18 Copper-nitrogen vibrations appear in the Raman spectra only when the Cu ions bind the nitrogen atoms of the amide groups in the nanotubes. Raman spectra (b) and (c) in Figure 5 also show that the Cu salts were removed completely from the Cu-doped nanotubes after rinsing. To demonstrate the validity of this scheme, whether the bound metal ions are required as templating species, we studied helical assembly of the bolaamphiphile molecules using the Raman microscope. The helical assembly of the bolaamphiphile molecules was assembled in a basic solution of NaOH (50 mM). This helical assembly has the same molecular arrangement as the bolaamphiphile nanotube, as shown in Figure 2, except all
Figure 5. Raman spectra of (a) noncoated nanotube, (b) Cu-doped nanotube, (c) Cu solution, and (d) Cu-doped nanotube after the addition of EDTA. Dotted lines represent peak positions of the Cu-O asymmetric stretch mode (332 cm-1) and the Cu-N symmetric stretch mode (450 cm-1).
of the amide groups bind amide groups of the adjacent bolaamphiphile molecules via hydrogen bonds.16 In other words, the helical assembly does not have non-hydrogen-bonded sites of the amide groups for the metal-ion intercalation. Therefore, examination of the helical assembly serves as a model study to verify the proposed metallization scheme. We applied the same coating procedure to the helical assembly and studied their Raman spectra. Figure 6a is the spectrum of the helical assembly. Figure 6b,c are the spectra of helical assemblies after they were loaded into the Ni bath and the Cu bath, respectively.
Bolaamphiphile Nanotube-Templated Metallized Wires
J. Phys. Chem. B, Vol. 104, No. 41, 2000 9579 agents. Metal ions are observed to coordinate between nonhydrogen-bonded amide groups of neighboring bolaamphiphile molecules. The bolaamphiphile nanotubes may be used as templates for nanowires connecting nano-electrodes to build nano-electric circuits. Attachment of the metallic nanowires onto the nano-electrodes can be achieved by self-assembling the bolaamphiphile nanotubes onto the electrodes via carboxylic acid-thiol self-assembly monolayers.19 Acknowledgment. This work was supported by Office of the Vice President for Research and Graduate Studies and Advanced Materials Processing and Analysis Center (AMPAC) at University of Central Florida. H.M. acknowledges Mr. Zia Ur Rahman at University of Central Florida for assistance in the SEM and TEM studies. References and Notes
Figure 6. Raman spectra of (a) helical assembly of the bolaamphiphile, (b) helical assembly of the bolaamphiphile after loading in a Ni bath (0.08 M NiCl2‚6H2O) for a day, and (c) helical assembly of the bolaamphiphile after loading in a Cu bath (0.35 M CuCl2‚2H2O) for a day.
All of the peak positions coincide with Figure 6a-c, and no additional peaks corresponding to metal-oxygen and metalnitrogen vibrations were observed in Figure 6b,c. This spectral comparison indicates that metal ions were not intercalated by the helical assembly and this should occur through the lack of free amide group sites to accept metal ions in the helical assembly. This control experiment supports the proposed metallization mechanism for the bolaamphiphile nanotubes. Reversibility of the metallization process was also examined by adding EDTA (50 mM in NaOH 1.0 M) into the metal ionnanotube solutions. Spectral changes of the Cu and Ni nanotubes before and after the addition of EDTA were not detected within 3 days (Figures 4d, 5d). It seems that binding between metal ions and amide groups of the nanotubes is strong and the ions could not be removed by the EDTA. In summary, bolaamphiphile nanotube-templated Ni and Cu nanowires can be produced by electroless metallization of the bolaamphiphile nanotubes in the Ni and Cu baths with reducing
(1) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2098. (2) Tans, S. J.; Devoret, M. H.; Dai, H.; Thess, A.; Smalley, R. E.; Geerligs, L. J.; Dekker, C. Nature 1997, 386, 474. (3) Li, M.; Schnnablegger, H.; Mann, S. Nature 1999, 402, 393. (4) Archibald, D. D.; Mann, S. Nature 1993, 364, 430. (5) Alivisatos, A. P.; Johnsson, K. P.; Peng, X.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P., Jr.; Schultz, P. G. Nature 1996, 382, 609. (6) Loweth, C. J.; Caldwell, W. B.; Peng, X.; Alivisatos, A. P.; Schultz, P. G. Angew. Chem., Int. Ed. Engl. 1999, 38, 1808. (7) Wang, Z. L.; Harfenist, S. A.; Whetten, R. L.; Bentley, J.; Evans, N. D. J. Phys. Chem. B 1998, 102, 3068. (8) Braun, E.; Eichen, Y.; Sivan, U.; Ben-Yoseph, G. Nature 1998, 391, 775. (9) Field, M.; Smith, C. J.; Awshalom, D. D.; Mayes, E. L.; Davis, S. A.; Mann, S. Appl. Phys. Lett. 1998, 73, 1739. (10) Thompson, D. H.; Wong, K. F.; Humphry-Baker, R.; Wheeler, J. J.; Kim, J.-M.; Rananavare, B. J. Am. Chem. Soc. 1992, 114, 9035. (11) Meglio, C. D.; Rananavare, S. B.; Svenson, S.; Thompson, D. H. Langmuir 2000, 16, 128. (12) Bergeron, R. J.; Phanstiel, O., IV; Yao, G. W.; Milstein, S.; Weimar, W. R. J. Am. Chem. Soc. 1994, 116, 8479. (13) Bergeron, R. J.; Yao, G. W.; Erdos, G. W.; Milstein, S.; Gao, F.; Weimar, W. R.; Phanstiel, O., IV. J. Am. Chem. Soc. 1995, 117, 6658. (14) Matsui, H.; Gologan, B.; Schaffer, H.; Adar, F.; Seconi, D.; Phanstiel, O., IV. Langmuir 2000, 16, 3148. (15) Kogiso, M.; Ohnishi, S.; Yase, K.; Masuda, M.; Shimizu, T. Langmuir 1998, 14, 4978. (16) Matsui, H.; Gologan, B. J. Phys. Chem. B 2000, 104, 3383. (17) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; John Wiley & Sons: New York, 1997; Vol. Part B. (18) Kincard, J. R.; Nakamoto, K. Spectrochim. Acta 1976, 32A, 277. (19) Matsui, H.; Gologan, B.; Pan, S. Chem. Commun. 2000, submitted.
