Langmuir 2006, 22, 1817-1821
1817
Assembly of Metal Nanoparticle-Carbon Nanotube Composite Materials at the Liquid/Liquid Interface Kang Yeol Lee,† Minjung Kim,† Joeoong Hahn,‡ Jung Sang Suh,‡ Inhyung Lee,‡ Kwan Kim,‡ and Sang Woo Han*,† Nanomaterials Laboratory, Department of Chemistry and Research Institute of Natural Sciences, Gyeongsang National UniVersity, Jinju 660-701, Korea, and School of Chemistry, Seoul National UniVersity, Seoul 151-742, Korea ReceiVed September 7, 2005. In Final Form: NoVember 22, 2005 Carbon nanotubes (CNTs)-mediated self-assembly of metal (Au and Ag) nanoparticles at the liquid/liquid interface in the form of a stable nanocomposite film is reported. The metallic luster results from the electronic coupling of nanoparticles, suggesting the formation of closely packed nanoparticle thin films. The interfacial film could be transferred to mica substrates and carbon-coated transmission electron microscopy (TEM) grids. The transferred films were very stable for a prolonged time. The samples were characterized by UV-vis spectroscopy, scanning electron microscopy (SEM), TEM, and X-ray photoelectron spectroscopy (XPS). SEM and TEM results show that the films formed at the liquid/liquid interface are indeed composite materials consisting of CNTs and nanoparticles. XPS measurements further indicate the presence of the interaction between nanoparticles and CNTs.
Introduction The intense research activity in the field of nanoparticles is motivated by the search for new materials in order to further miniaturize electronic devices, as well as by the fundamental question of how molecular electronic properties evolve with increasing size in this intermediate region between molecular and solid-state physics.1 In this respect, molecularly bridged nanoparticle aggregates have been attracting growing interest as a result of their collective electronic, optical, and magnetic properties being distinctly different from a corresponding collection of individual nanoparticles or the extended solid.2-5 The properties of two-dimensional assemblies of metal nanoparticles are controlled by the composition, geometry, and spatial arrangement of the nanoparticle building blocks. Such structures have been used for a variety of important applications in catalysis, photonics, electronics, and biological sensing.6 The 2D/3D control over the spatial arrangement of nanoparticles is primarily based on the thiolamphilic nature of metal nanoparticles,2 hydrogenbonding interactions,3 the highly specific recognition interaction of antigens/antibodies,4 and specific base-pairing interactions between DNA and its complementary strand.5 The liquid/liquid interface has also served as a fertile medium for nanoparticle assembly.7 Recently, Kumar et al. have observed that aromatic molecules such as benzene and anthracene present in the organic phase bind strongly with aqueous gold nanoparticles * To whom correspondence should be addressed. E-mail: swhan@ gsnu.ac.kr. † Gyeongsang National University. ‡ Seoul National University. (1) Sugimoto, T. Fine Particles: Synthesis, Characterization, and Mechanisms of Growth; Marcel Dekker: New York, 2000. (2) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690. (3) Boal, A. K.; Ilhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746. (4) Mann, S.; Shenton, W.; Li, M.; Connolly, S.; Fitzmaurice, D. AdV. Mater. 2000, 12, 147. (5) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (6) (a) Maier, S. A.; Brongersma, M. L.; Kik, P. G.; Meltzer, S.; Requicha, A. A. G.; Atwater, H. A. AdV. Mater. 2001, 13, 1501. (b) Maxwell, D. J.; Taylor, J. R.; Nie, S. M. J. Am. Chem. Soc. 2002, 124, 9606. (c) Pellegrino, T.; Kudera, S.; Liedl, T.; MuCoz Javier, A.; Manna, L.; Parak, W. J. Small 2005, 1, 48. (7) Yogev, D.; Efrima, S. J. Phys. Chem. 1988, 92, 5754.
(AuNPs).8 This process leads to the immobilization of the AuNPs in the form of a highly localized film at the interface. Most recently, Reincke and co-workers reported that the introduction of ethanol can pull hydrophilic citrate-stabilized AuNPs into the water/heptane interface, leading to a closely packed monolayer.9 Meanwhile, Duan et al. directed the assembly of hydrophobic and hydrophilic nanoparticles at water/oil interfaces by capping the nanoparticles with appropriate ligands.10 These previous reports demonstrate a promising way to create a 2D or 3D arrangement of hydrophobic or hydrophilic nanoparticles at water/ oil interfaces. Herein, we present our noticeable finding that carbon nanotubes (CNTs) can mediate self-assembly of metal nanoparticles at the liquid/liquid interface in the form of a stable nanocomposite film. CNTs are of great interest due to their unique electronic, chemical, and mechanical properties for creating new-generation electronic devices and networks.11 Their potential applications include electron field emitters,12 quantum wires,13 molecular filters,14 artificial muscles,15 etc. From the point of application aspect, it is very requisite to develop synthetic methodologies for making CNTs-based nanocomposites because physicochemical properties can be tuned by spatial arrangement of their components. Experimental Section The growth of nanotubes was carried out in a horizontal quartz tube furnace at 1000 °C. The catalysts for growth of single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (8) Kumar, A.; Mandal, S.; Mathew, S. P.; Selvakannan, P. R.; Mandale, A. B.; Chaudhari, R. V.; Sastry, M. Langmuir 2002, 18, 6478. (9) Reincke, F.; Hickey, S. G.; Kegel, W. K.; Vanmaekelbergh, D. Angew. Chem., Int. Ed. 2004, 43, 458. (10) Duan, H.; Wang, D.; Kurth, D. G.; Mo¨hwald, H. Angew. Chem., Int. Ed. 2004, 43, 5639. (11) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787. (12) Fan, S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.; Cassell, A. M.; Dai, H. Science 1999, 283, 512. (13) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. Science of Fullerenes and Carbon Nanotubes; Academic Press: San Diego, CA, 1996. (14) Che, G.; Lakshmi, B. B.; Fisher, E. R.; Martin, C. R. Nature 1998, 393, 346. (15) Baughman, R. H.; Cui, C.; Zakhidov, A. A.; Iqbal, Z.; Barisci, J. N.; Spinks, G. M.; Wallace, G. G.; Mazzoldi, A.; de Rossi, D.; Rinzler, A. G.; Jaschinski, O.; Roth, S.; Kertesz, M. Science 1999, 284, 1340.
10.1021/la052435b CCC: $33.50 © 2006 American Chemical Society Published on Web 01/21/2006
1818 Langmuir, Vol. 22, No. 4, 2006 (MWCNTs) were prepared by a sol-gel method.16 The methane decomposition reaction was conducted at the ambient pressure. A mixture of H2/CH4 ) 4 v/v at a flow of 400 sccm was introduced for the growth of SWCNTs, whereas a flow of 400 sccm (H2/CH4 ) 1 v/v) was inserted for the growth of MWCNTs. The reaction was maintained for 30 min, after which the furnace was allowed to cool to room temperature under a N2 purging. Hydrochloric acid treatment at room temperature and oxidation process in air at 450 °C have been performed in order to purify the CNTs by removing the MgO substrate, metal catalyst, and any amorphous carbon. The diameters of the SWCNTs and MWCNTs are about 1 nm and 10-20 nm, respectively. The gold sol was prepared by following the literature with a difference only in the molar ratio of HAuCl4 to sodium citrate.17 Namely, 40 mg of HAuCl4 (Aldrich) was initially dissolved in 90 mL of water, and the solution was heated to boiling. A total of 10.2 mL of aqueous solution of sodium citrate (40 mM) was then added to the HAuCl4 solution under vigorous stirring, and boiling was continued for ca. 15 min. The concentration of gold nanoparticles was calculated to be about 6.3 nM by assuming an average 19.6 nm diameter for all nanoparticles. In a typical preparation of silver nanoparticles (AgNPs), 47.5 mL of highly purified water were prepared. A 1 mL aqueous solution of sodium citrate (30mM) and 25 mL aqueous solution of AgNO3 (5 mM) were added. A 1 mL aliquot of freshly prepared NaBH4 (50 mM) was quickly added, and the solution immediately turned into a light yellow color. An aqueous solution of poly(vinyl pyrrolidone) (PVP, 5 mg/mL, Mw ≈ 55 000) (1 mL) was subsequently added by dropwise addition to the solution as a particle stabilizing agent.18 Note that this solution had to be vigorously stirred with a magnetic bar (with a rotation rate of 1200 rpm) during the entire process. CNT-metal nanoparticle films were fabricated as follows. The 5.0 mL of the freshly prepared metal (Au and Ag) hydrosol was taken in a vial along with 4.0 mL of diethyl ether resulting in a biphasic mixture with the colorless organic part on top and wine-red (AuNPs) or yellow (AgNPs) colored hydrosol below. Upon addition of 1.0 mL of a solution of CNTs in N,N-dimethylformamide (DMF) to this mixture, a thin film of metal reflectance and blue transmittance is formed immediately at the water/oil interface. The UV-vis absorption spectra were recorded at room temperature using a SINCO S-3100 spectrophotometer. The scanning electron micrographs of the samples were taken with a field emission scanning electron microscope (FESEM, Phillips Model XL30 S FEG). The transmission electron microscopy (TEM) images were acquired by using a JEOL JEM-2010 transmission electron microscope operating at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a VG Scientific ESCALAB 250 spectrometer, using Al KR X-ray (1486.6 eV) as the light source. The base pressure of the chamber was ∼1 × 10-10 Torr, and the electron takeoff angle was 90°.
Results and Discussion Aqueous suspensions of AuNPs with average diameter of 19.6 ( 3.7 nm were prepared by citrate reduction method as described above. The freshly prepared Au hydrosol was taken in a vial along with diethyl ether resulting in a biphasic mixture with the colorless organic part on top and the wine-red-colored hydrosol below (inset of Figure 1, vial to the left). Upon addition of DMF solution of SWCNTs to this mixture, a thin film of golden reflectance and blue transmittance is formed immediately at the water/oil interface (Figure 1, vial at the center). This interfacial entrapment of AuNPs may be accelerated by gentle shaking. The gold hydrosol was now colorless, clearly indicating transfer of the gold nanoparticles to the interface. This metallic luster results from the electronic coupling of AuNPs, suggesting the formation of closely packed nanoparticle thin films.19 The UV-vis spectra (16) Suh, D. J.; Park, T.-J. Chem. Mater. 1997, 9, 1903. (17) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391. (18) Sun, Y.; Mayers, B.; Xia, Y. Nano Lett. 2003, 3, 675.
Lee et al.
Figure 1. UV-vis spectra recorded from the (a) MWCNT- and (b) SWCNT-AuNP films, (c) AuNPs cast film, and (d) as-prepared aqueous gold sol solution. The inset shows vials before (vial on the left) and after the addition of SWCNTs (vial at the center) and MWCNTs (vial on the right).
demonstrate that no AuNPs exist in water or are transferred into the diethyl ether phase. We have obtained the same results with MWCNTs (Figure 1, vial to the right). In a control experiment, we injected only DMF into the Au hydrosol/diethyl ether biphasic mixture. However, films with metallic luster were not fabricated at the water/oil interface. Therefore, the DMF does not influence the formation of interfacial films. This shows that the nanotube itself can lead to direct assembly of AuNPs. The interfacial films could be easily transferred to solid substrates. In fact, the biphasic mixture was poured into a Petri dish and the diethyl ether phase evaporated to leave a uniform thin film on the surface of water. This film was lifted onto mica substrates and carbon-coated TEM grids for further analysis. The transferred films were very stable for a prolonged time. The physicochemical characteristics of these films were examined by various analytical tools such as UV-vis spectroscopy, SEM, TEM, and XPS. The UV-vis spectra recorded from the films on mica are shown in Figure 1 (curves a and b for MWCNT- and SWCNTmediated interfacial films, respectively). For comparison, UVvis spectra recorded from AuNPs cast film on mica and asprepared aqueous Au sol solution are also shown in Figure 1. All of the spectra exhibit bands due to the surface plasmon.20 One can notice that the transverse plasmon components observed at 526 and 570 nm in the spectra of aqueous Au sol and AuNPs cast film, respectively, almost completely disappear and noticeably red-shifted and broad plasmon resonance peaks occur around 770 and 750 nm in the case of the SWCNT- and MWCNTmediated films, respectively. It is well-known that, as gold and silver colloidal particles aggregate into string-like structures, there is growth of a higher wavelength component that shifts to the red and increases in intensity as the aggregation proceeds.19 The long wavelength component (the longitudinal plasmon resonance) arises due to coupling of the plasma modes of the individual clusters. The observed characteristic red-shift and dampening of the Au plasmon resonance upon interfacial film (19) (a) Mulvaney, P. Langmuir 1996, 12, 788. (b) Blatchford, C. G.; Campbell, J. R.; Creighton, J. A. Surf. Sci. 1982, 120, 435. (20) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: New York, 1995.
Assembly of CNT Composite Materials
Figure 2. (a) SEM and (b) TEM images of MWCNT-AuNP films.
formation thus indicates an extended assembly of nanoparticles in the composite film.5 The morphologies and microstructures of the as-prepared interfacial films were studied by means of SEM and TEM. Figure 2a shows a typical SEM image of MWCNT-mediated interfacial film on mica. The image indicates that the substrate was covered with large domains of the materials without any apparent disruption. As shown in the figure, the films formed at the liquid/ liquid interface are indeed composite materials consist of CNTs and AuNPs. The higher magnification TEM image of the CNTsAuNPs composite film presented in Figure 2b further reveals that interfacial films should be formed by assembly of nanoparticles and CNTs. In the previous report on the assembly of AuNPs at the liquid/liquid interface, particles in the 2-D assembly are regularly in plane packed and well separated from one another although some degree of sintering of the particles appears to have occurred.8-10 Other earlier studies have also demonstrated the very regular, hexagonal arrangement of silica-coated gold nanoparticles21 as well as CdSe quantum dots.22 We speculate that the difference could be due to the fact that the AuNPs of this study are assembled at the liquid/liquid interface by nanostructured materials, i.e., CNTs as mediator. In the previous studies, small organic ligands have been used to transfer nanoparticles into the interface, which cannot hinder the spatial arrangement of the nanoparticles. On the contrary, the intrinsic structure of CNTs as well as the interaction between nanoparticles and CNTs do not lead to good two-dimensional ordering of AuNPs in the present composite materials. Our method for assembly of AuNPs and CNTs has been successfully extended to other metal or semiconductor nanoparticles. Following the aforementioned interfacial entrapment procedure, aqueous AgNPs of ca. 6 nm in size, prepared by reduction of silver nitrate by sodium borohydride,18 were assembled with CNTs at the water/oil interface. The as-made AgNP-CNT composites display interfacial behavior similar to that of AuNP-CNT composites; they prefer to reside at the water/oil interface, creating a film with metallic reflectance. The UV-vis spectra recorded from the AgNP-CNT films on mica show that the transverse plasmon component observed at 390 nm in the spectrum of aqueous Ag sol almost completely disappears and noticeably red-shifted and broad plasmon resonance peaks occur around 535 nm. This indicates an assembly of nanoparticles in the nanocomposites films. Typical TEM images of the CNT-AgNP composite films (Figure 3a) show the formation of a AgNP layer. Enlarged images (inset of Figure 3a) also indicate that the interfacial films are nanocomposites consisting of AgNPs and CNTs. On the other hand, we could make TiO2 nanoparticles (avg. size of ∼20 nm, Degussa P-25)/ CNT composite materials by the same experimental protocol (21) Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329. (22) (a) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723. (b) Brown, L. O.; Hutchinson, J. E. J. Am. Chem. Soc. 1999, 121, 882.
Langmuir, Vol. 22, No. 4, 2006 1819
Figure 3. TEM images of (a) AgNP- and (b) TiO2 nanoparticleMWCNT films. The enlarged images are shown in each inset.
Figure 4. High-resolution C 1s XPS features obtained from (a) SWCNT, (b) MWCNT, (c) SWCNT-AuNP composites, (d) MWCNT-AuNP composites, and (e) AuNPs.
used in the case of Au and Ag nanoparticles. A typical TEM image of the TiO2 nanoparticle-CNT composite films is shown in the Figure 3b. To gain insight into the interaction between nanoparticles and CNTs, we have performed XPS measurements. XPS turned out to be a powerful tool for the investigation of functionalized CNTs.23-25 High-resolution C 1s X-ray photoelectron spectra of CNTs and CNT-AuNP composite films are shown in Figure 4. The C 1s peaks of the SWCNTs (Figure 4a) and the MWCNTs (Figure 4b) consist of several components. The peak separations were carried out on the basis of the analysis of the C 1s XPS spectrum of the freshly cleaved HOPG surface.25,26 Component (23) Holzinger, M.; Abraham, J.; Whelan, P.; Graupner, R.; Ley, L.; Hennrich, F.; Kappes, M.; Hirsch, A. J. Am. Chem. Soc. 2003, 125, 8566. (24) Banerjee, S.; Wong, S. S. J. Am. Chem. Soc. 2003, 125, 10342. (25) Luong, J. H. T.; Hrapovic, S.; Liu, Y.; Yang, D.-Q.; Sacher, E.; Wang, D.; Kingston, C. T.; Enright, G. D. J. Phys. Chem. B 2005, 109, 1400. (26) (a) Yang, D.-Q.; Sacher, E. Surf. Sci. 2002, 504, 125. (b) Yang, D.-Q.; Sacher, E. Surf. Sci. 2002, 516, 43.
1820 Langmuir, Vol. 22, No. 4, 2006
Figure 5. High-resolution Au 4f XPS features obtained from (a) SWCNT-AuNP composites, (b) MWCNT-AuNP composites, and (c) AuNPs.
peaks are found at 284.6 eV (extensively delocalized sp2hybridized carbon), 285.6 eV (defect-containing sp2-hybridized carbon), and 286.5 eV (sp3 defects). The π* r π shake-up of the 284.6 eV component appears at 291.4 eV. When the CNTAuNP nanocomposite films were formed, it was found that the high binding energy C 1s peaks (291.4 eV) almost completely disappeared and the intensity of the peaks from the defectcontaining sp2-hybridized carbon increased (see Figure 4, panels c and d). These indicate the formation of composite materials by the interaction between CNT and AuNP. Since the AuNPs used in this work are stabilized by citrate molecules, it may be possible that the XPS spectral features of composites are caused by both CNTs and citrates. However, the characteristic peak from the carboxylate (-COO-) moiety of citrate molecule observed at 288.7 eV in the XPS spectrum of the AuNPs (Figure 4e)27 could be hardly identified in the spectra of composite films. From this result, we can conjecture that much of the citrate molecules are exchanged by CNTs along the formation of composites. We have also measured the Au 4f XPS spectra of the AuNPs and CNT-AuNP composite films. As shown in Figure 5c, the Au 4f XPS spectrum obtained from the AuNPs was characterized by peaks with binding energies of 87.5 eV for 4f5/2 and 83.8 eV for 4f7/2, both distinctive for Au metal. The Au 4f5/2 and Au 4f7/2 peaks for the interfacial films indicate the existence of the AuNPs (Figure 5, panels a and b). It is noticeable that the Au 4f5/2 and Au 4f7/2 peaks shift to higher binding energies upon the composites (27) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer: Eden Prairie, MN, 1992.
Lee et al.
formation; +0.6 eV for SWCNTs and +0.2 eV for MWCNTs. In fact, many references report present high binding energy values for metals, depending on the nature of the supporting substrate, the dispersion, and the particle size.28 Mason, who studied cluster-support interactions, concluded that, for interactive substrates such as carbon, the metal-support interactions occurred and the XPS peaks for the cluster shift to higher binding energies relative to that of the bulk metal.29 Therefore, the observed peak shifts indicate the presence of the interaction between AuNPs and CNTs. However, we cannot rule out the possibility of substrate effects as the cause of peak shifts because XPS line widths are changing in the three sets of data presented in Figure 5. To optimize the potential applications of CNTs, it is essential to modify the inert sidewall by chemical functionalization and/ or attach suitable nanostructures to the nanotubes.30 The size and shape of metal nanoparticles are responsible for interesting electronic properties.31 Therefore, attaching metal nanoparticles to nanotube sidewalls is of interest for obtaining nanotube/ nanoparticle hybrid materials with useful properties32 and for forming metal nanowires on nanotube templates.33 In light of this, a series of practical approaches has been suggested to modify the inert sidewalls of CNTs with nanoparticles. Previous approaches to metal nanoparticle functionalization of nanotubes include physical evaporation,32,33 attachment after oxidation of nanotubes,34 solid-state reaction with metal salts at elevated temperatures,35 and electroless deposition from salt solutions.36 In this work, we can synthesize nanotube/nanoparticle hybrid materials through a simple interfacial entrapment process. The mechanism of assembly of CNTs and metal nanoparticles to form stable interfacial films is not clear yet. However, we conjecture that charge reduction of the nanoparticles surface through the interaction between nanoparticles and CNTs is a driving force for the formation of interfacial films. As discussed in the previous report, adsorption of a charged nanocrystal at the water/oil interface must be driven by a reduction of the interfacial energy; the repulsion between the charged nanocrystals in the monolayer forms a counteracting force.9 The reduction of the charge density on the nanocrystals is a prerequisite for the formation of the nanocrystal monolayers at the water/oil interface. Upon addition of CNTs, the charge on the nanoparticle surface decreases through displacement of the adsorbed citrate ions by CNTs. Thus, a decrease in the charge density of the nanocrystals leads to the controlled formation of a monolayer of nanocrystalCNT hybrids at the water/oil interface. Actually, the removal of citrate ions upon hybrid formation was verified by XPS measurements (Figure 4). The remaining question is what the nature of interaction between nanoparticles and CNTs is. As (28) (a) Walter, E. C.; Penner, R. M. Anal. Chem. 2002, 74, 1546. (b) Fritsch, A.; Le´gare´, P. Surf. Sci. 1985, 162, 742. (c) Fleisch, T. H.; Hicks, R. F.; Bell, A. T. J. Catal. 1984, 87, 398. (d) Takasu, Y.; Unwin, R.; Tesche, B.; Bradshaw, A. M. Surf. Sci. 1978, 77, 219. (29) (a) Mason, M. G.; Gerenser, L. J.; Lee, S. T. Phys. ReV. Lett. 1977, 39, 288. (b) Mason, M. G. Phys. ReV. B 1983, 27, 748. (30) (a) Fullam, S.; Cottel, D.; Rensmo, H.; Fitzmaurice, D. AdV. Mater. 2000, 12, 1430. (b) Chen, Q.; Dai, L.; Gao, M.; Huang, S.; Mau, A. J. Phys. Chem. B 2001, 105, 618. (c) Chen, R. J.; Zhang, Y.; Wang, D.; Dai, H. J. Am. Chem. Soc. 2001, 123, 3838 (d) Jiang, K. Y.; Eitan, A.; Schadler, L. S.; Ajayan, P. M.; Siegel, R. W.; Grobert, N.; Mayne, M.; Reyes-Reyes, M.; Terrones, H.; Terrones, M. Nano Lett. 2003, 3, 275. (31) (a) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293. (b) Shipway, A. N.; Willner, I. Chem. Commun. 2001, 2035. (c) Imahori, H.; Fukuzumi, S. AdV. Mater. 2001, 13, 1197. (32) Kong, J.; Chapline, M.; Dai, H. AdV. Mater. 2001, 13, 1384. (33) Bezryadin, A.; Lau, C. N.; Tinkham, M. Nature 2000, 404, 971. (34) Azamian, B. R.; Coleman, K. S.; Davis, J. J.; Hanson, N.; Green, M. L. H. Chem. Commun. 2002, 366. (35) Xue, B.; Chen, P.; Hong, Q.; Lin, J.; Tan, K. L. J. Mater. Chem. 2001, 11, 2378. (36) Choi, H. C.; Shim, M.; Bangsaruntip, S.; Dai, H. J. Am. Chem. Soc. 2002, 124, 9058.
Assembly of CNT Composite Materials
Langmuir, Vol. 22, No. 4, 2006 1821
mentioned in Introduction, Kumar and co-workers found that benzene and anthracene molecules bind to the surface of gold nanoparticles leading to the assembly of the nanoparticles in 2-D assemblies at the liquid-liquid interface.8 In the case of the gold nanoparticle-benzene film on quartz, an absorption band was observed at 202 nm, which can be attributed to Au+ in nanoparticle surface-bound AuCl2- ions. The XPS results also indicated the presence of Cl in the Au nano-benzene film. On the basis of the fact that Au+ binds to aromatic molecules through cation-π interactions,37 they argued that this interaction is the likely mode of binding of both benzene and anthracene to the gold nanoparticles. However, we could not observe an absorption band near 200 nm. Furthermore, there is no trace of Cl in the obtained XPS spectra. The mechanism suggested by Kumar et al. can thus be ruled out for explaining our results. The interaction between metal nanoparticles and CNTs may be explained by referring to a mechanism proposed in the previous study on the dispersable CNTs/gold hybrids. Through the spectroscopic observation of the presence of strong electron coupling between nanoparticles and CNTs, Rahman and co-workers suggested an electron-transfer process as the cause of hybrid formation.38 In the study on the aggregation of AuNPs in toluene solvent mediated by fullerene molecules, Brust et al. have also suggested the involvement of an electron-transfer from the AuNPs to the adsorbed fullerene molecules in aggregation of AuNPs by C60.39 On the basis of these previous reports and also the fact that electron-accepting/donating features of Au or Ag nanoparticles and likewise the ambivalent character of CNTs (i.e., they show both reductive and oxidative charging in electrochemical experiments),40 we feel that a shift of electron density is thought
to be responsible for the observed formation of hybrid materials although the nature of the electronically coupled state remains an intriguing question. The observed XPS spectral changes upon hybrid formation (see Figures 4 and 5) may be ascribed to the results of electron-transfer between CNTs and nanoparticles. Since the electronic coupling should affect physicochemical properties of the composite materials, we are currently exploring the altered redox features in the nanoparticle-CNT hybrids to investigate the exact binding nature.
(37) Gimeno, M. C.; Jones, P. G.; Laguna, A.; Sarroca, C.; Calhorda, M. J.; Veiros, L. F. Chem. Eur. J. 1998, 4, 2308. (38) Rahman, G. M. A.; Guldi, D. M.; Zambon, E.; Pasquato, L.; Tagmatarchis, N.; Prato, M. Small 2005, 1, 527. (39) Brust, M.; Kiely, C. J.; Bethell, D.; Schiffrin, D. J. J. Am. Chem. Soc. 1998, 120, 12367.
LA052435B
Conclusions We have shown that CNTs mediate the transfer of metal nanoparticles in water solution to the diethyl ether/water interface, which results in directing the self-assembly of nanoparticles in the form of novel nanocomposite films. The interfacial film could be transferred to different solid substrates and analyzed by various spectroscopic techniques. The experimental results indicate the formation of composite films at water/oil interfaces with the nanoparticles bound to the surface of the CNTs. XPS results imply interfacial interaction between nanoparticles and CNTs. Currently, we are attempting to synthesize other composite films by using interactions between CNTs and various nanoscopic metal particles or metal precursors. The resulting nanocomposite materials would be expected to be useful for applications including high-efficiency computing, high-density data storage media, light harvesting in photovoltaic cells, high-strength textiles, and supersensitive sensors.41 Acknowledgment. This work was supported by the Korea Research Foundation Grant (KRF-2004-041-C00172).
(40) Melle-Franco, M.; Marcaccio, M.; Paolucci, D.; Paolucci, F.; Georgakilas, V.; Guldi, D. M.; Prato, M.; Zerbetto, F. J. Am. Chem. Soc. 2004, 126, 1646. (41) Avouris, P. Acc. Chem. Res. 2002, 35, 1026.
