NANO LETTERS
Silica Coated Single Walled Carbon Nanotubes
2003 Vol. 3, No. 6 775-778
Elizabeth A. Whitsitt and Andrew R. Barron* Department of Chemistry and Center for Nanoscale Science and Technology, Rice UniVersity, Houston, Texas 77005 Received March 28, 2003; Revised Manuscript Received April 24, 2003
ABSTRACT Single walled carbon nanotubes (SWNTs) have been coated with silica by the addition of a silica/H2SiF6 solution to a surfactant-stabilized solution of SWNTs. The thickness of the coating is controlled by reaction time, while the coating of individual SWNTs versus small ropes is controlled by the choice of surfactant. Individual nanotubes encased by a silica coating show retention of the characteristic Raman fluorescence. Selective etching of the silica coating allows for the exposure of either one end of the tubes or the central section. Exposure of the ends results in the formation of spontaneous interconnects between isolated SWNTs.
Single walled carbon nanotubes (SWNTs) are elongated members of the fullerene family. Since their discovery in 1993,1,2 they have come under intense multidisciplinary study because of their unique physical and chemical properties and their possible applications.3 SWNTs can be either metallic or semiconducting, depending on their helicity and diameter.4 More importantly it has been shown that these properties are sensitive to the surrounding environments. For example, the presence of O2, NH3, and many other molecules can either donate or accept electrons and alter the overall conductivity of the SWNTs.5,6 Such properties make SWNTs ideal for nanoscale sensing materials, and nanotube FETs have already been demonstrated as gas sensors.5,7 However, to introduce selectivity to nanotube sensors, certain functional groups that can selectively bind to specific target molecules need to be anchored on the nanotube surface. Unfortunately, functionalization changes the electronic properties from semiconductor or conductor to insulating and, at present, chemical functionalization is not regiospecific.8 A further major obstacle to such efforts has been diversity of tube diameters, chiral angles, and aggregation states of the tubes. Aggregation is particularly problematic because the highly polarizable, smooth-sided SWNTs readily form bundles or ropes with van der Waals binding energy of ca. 500 eV per micrometer of tube contact.9,10 This bundling perturbs the electronic structure of the tubes and precludes the separation of SWNTs by size or type. Smalley and co-workers have recently reported that individual SWNTs may be obtained encased in a cylindrical micelle by ultrasonically agitating an aqueous dispersion of raw SWNTs in a suitable surfactant.11 However, upon drying * Corresponding author. E-mail:
[email protected]; url: www.rice.edu/ barron. 10.1021/nl034186m CCC: $25.00 Published on Web 05/14/2003
© 2003 American Chemical Society
of the micellular solution, bundles re-form. Although SWNTs have been encased in a wide range of organic materials,12-15 it would be desirable to fabricate individually coated SWNTs where the coating is retained in solution and in the solid state. Of particular interest are dielectric materials such as silica, which may also be compatible with composite matrix materials. Thick coating of SiO2 on MWNTs has been reported,16,17 while thin layers have been reported on SWNTs, but these require isolation of the tubes on a surface prior to reaction.18 We have recently become interested in the seeded growth of silica films from aqueous solution under ambient conditions.19 In this regard we report herein the fabrication of silica-coated SWNTs either as small ropes or as individual tubes. Selective etching of the silica coating provides a route to site selective chemical functionalization as well as the spontaneous generation of tube-to-tube interconnects. A general synthesis of silica coated SWNTs (SiO2SWNT) is as follows: three grams of fumed silica (Aldrich, 99.8%) is added to 50 mL of 3.20 M Fluorosilicic acid (H2SiF6: Riedel de Haen, 34% pure) and allowed to stir overnight. This solution is then filtered, by vacuum, through a 0.22 µm Millipore filter. The filtrate is diluted to 1.0 M with Millipore water. One mL of this solution is added to 5 mL of an aqueous 1% sodium dodecyl sulfate (SDS, Aldrich, g99%) or 1% dodecyl trimethylammonium bromide (DTAB, Aldrich, 99%) solution containing a dispersion of raw SWNTs (30-50 mg/L) prepared by the high-pressure carbon dioxide (HiPCO) process.20 The tubes are allowed to react, with stirring and low heat (30 °C), for the allotted time in a plastic centrifuge tube. The reaction is then centrifuged. The supernatant liquid is disposed of and the solid is redispersed in ethanol. This centrifugation/redispersion process is re-
peated a minimum of six times. The coated SWNTs are then characterized in ethanol suspension or as a dried powder. It is important to ensure that a high concentration of the surfactant-SWNT solution is employed. That is, best results are seen with higher concentrations of tubes in the surfactant-SWNT solutions. If the solution is too dilute, then addition of the silica solution causes sufficient dilution to the surfactant-SWNT solution such that it is re-formed ropes that get coated, rather than individual tubes. Alternatively, if an excess of surfactant is used, to maintain SWNT dispersion, silica spheres are formed in addition to the coated tubes. In a separate experiment, we have shown that addition of (1%) SDS or DTAB to the silica solution results in the formation of silica spheres due to micelle formation. The choice of surfactant is important in determining whether individual SWNTs or small ropes are coated. In the present study, the use of an anionic surfactant (e.g., SDS) results in the formation of coated ropes, while a cationic surfactant (e.g., DTAB) results in a significant fraction of the SWNTs being individually coated. We propose that this effect is a consequence of the pH stability of the surfactantSWNT interaction. Acidification of a SDS-SWNT solution results in the immediate formation of SWNT ropes, while the DTAB-SWNT interaction is far less susceptible to changes in pH. Confirmation of rope formation is provided by Raman fluorescence measurements (see below). TEM measurements suggest that for the use of SDS each SiO2SWNT composite consists of at least two or more tubes. The thickness of the coating is dependent on the reaction mixture concentration and the reaction time. As with our prior studies19 with C60 an initial growth rate of approximately 1.3 nm.min-1 is observed, while coatings between 5 and 25 nm thick have been formed. We have previously reported that for spherical growth, etching of the silica occurs over extended reaction times, due to the formation of HF in the reaction solution (i.e., eq 1).19 H2SiF6 + 2H2O f SiO2 + 6HF
(1)
Samples for SEM analysis were sputtered with chromium to prevent charging. The use of gold resulted in a rough surface due to aggregation of metal particles on the surface of the silica. Figure 1 shows an SEM image of coated tubes formed from DTAB solution. Each SiO2 coating is of the same thickness, thus any SiO2-SWNT samples with anomalously large diameters are undoubtedly due to coating of small ropes of a few SWNTs. One such coated rope may be seen in Figure 1, as the SiO2-SWNT sample with a diameter of approximately 60 nm, as compared to the individually coated SWNTs with a diameter of approximately 41 nm. However, Raman measurements indicate the maintenance of individual tubes for the majority of semiconducting tubes. It has been previously reported that individual semiconducting SWNTs exhibit characteristic fluorescence bands in the Raman spectrum.4 Upon the formation of ropes these bands are completely quenched through the interaction with metallic tubes. Based upon the Raman fluorescence, the sample of HiPCO tubes used in the present study contains 776
Figure 1. SEM image of SiO2-SWNTs formed from DTAB solution.
Figure 2. Raman spectra of SWNTs before and after addition of the SiO2 growth solution using (a) SDS-SWNT and (b) DTABSWNT solutions.
the following n,m nanotube structures (with characteristic fluorescence band): 6,4 (1419 cm-1); 9,1 (1858 cm-1); 8,3 (2366 cm-1); 6,5 (2586 cm-1); and 7,5 (3104 cm-1), see Figure 2. During coating of SWNTs from SDS solution, rapid quenching of the fluorescence bands is observed (Figure 2a), suggesting that small ropes are coated rather than individual Nano Lett., Vol. 3, No. 6, 2003
Figure 3. SEM image of SiO2-SWNT woven fabric.
SWNTs. This confirms the TEM images of SiO2-SWNTs formed from SDS solution. A similar quenching of the fluorescence bands occurs if a SDS-SWNT solution is acidified, suggesting that the aggregation of the SWNTs into bundles with the addition of the acidic silica solution is due to the instability of SDS-SWNT interaction in acidic solution. Figure 2a shows the quenching of the Raman fluorescence after only 5 min in the SiO2 solution. In contrast, the Raman spectrum of a DTAB-SWNT solution shows only slow decrease in the fluorescence intensity upon addition of the silica growth solution. Figure 2b shows the DTAB-SWNT solution prior to mixing with the silica growth solution, and in reaction intervals of five minutes. After 15 min reaction time (consistent with ca. 18 nm coating), the fluorescence intensity is reduced to 88% of the initial intensity. With increasing coating times there is a further decrease in intensity; however, fluorescence is maintained even after reaction for >80 min. The slow decrease of intensity cannot be due to rope formation since all the SWNTs are already coated at reaction times of 5 min. We propose this decrease is due to possible scattering. It is important to note that the fluorescence shows no change in peak position, suggesting the coating does not alter the electrical properties (band gap) of the semiconducting SWNTs. If a solution/suspension of the SiO2-SWNTs is allowed to dry on a flat surface, a mat having the appearance of a woven fabric is formed, see Figure 3. In appearance this is similar to bucky-paper21 but where each tube is coated with SiO2. Any residual silica particles may be removed by gentle etching with HF solution (0.8% HF, 10 s). Extended etches result in removal of the SiO2 coating and exposure of the SWNTs. The use of SiO2 coatings allows for ready etching of the coating. Removal may be either total (i.e., to obtain the uncoated tubes) or selective (i.e., to partially expose the tubes). We have demonstrated that the SiO2-SWNT mat may be masked and etched to expose portions of the tubes. For example, HF etching the central region of the SiO2Nano Lett., Vol. 3, No. 6, 2003
Figure 4. SEM image showing the etched (a) center and (b) ends of SiO2-SWNTs.
SWNTs creates an array of exposed tubes that may be described as nano-piano wires (see Figure 4a). As may be seen from Figure 4b, the ends of the coatedSWNTs may be readily etched away using HF solution. Once this is accomplished the individual or small bundles of SWNTs return to their rope conformation due to van der Waals attraction. The spontaneous interconnection of tubes may be readily observed where the tube ends are exposed (Figure 4b). It has been recently reported that the van der Waals interaction of two tubes is sufficient to provide an ideal electrical connection. In conclusion, we have demonstrated that individual SWNTs may be coated in solution and isolated as a solid mat. The semiconducting SWNTs retain their fluorescence properties, demonstrating the isolation of individual tubes. Either the end or the center of the SiO2-SWNT may be etched and exposed. We propose that the formation of a “spontaneous interconnect”, once the tube ends are exposed, provides a route for the creation of addressable SWNT devices, while the ability to expose the central section of SWNTs has potential for sensor and device structures. Acknowledgment. Financial support for this work is provided by the Robert A. Welch Foundation. Richard E. Smalley and Valerie C. Moore are acknowledged for the supply of HiPCo SWNTs and assistance with the Kaiser Raman measurements. 777
References (1) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (2) Bethune, D. S.; Kiang, C. H.; de Vries, M. S.; Gorman, G.; Savoy, R.; Vazquez, J.; Beyers, R. Nature 1993, 363, 605. (3) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. Science of Fullerenes and Carbon Nanotubes; Academic Press: San Diego, 1996. (4) Bachilo, S. M.; Strano, M. S.; Kittrell, C.; Hauge, R. H.; Smalley, R. E.; Weisman, R. B. Science 2002, 298, 2361. (5) Kong, J.; Franklin, N. R.; Zhou, C. W.; Chapline, M. G.; Peng, S.; Cho, K. J.; Dai, H. J. Science 2000, 287, 622. (6) Collins, P. G.; Bradley, K.; Ishigami, M.; Zettl, A. Science 2000, 287, 1801. (7) Kong, J.; Chapline, M. G.; Dai, H. J. AdV. Mater. 2001, 13, 1384. (8) Hirsch, A. Angew. Chem., Int. Ed. 2002, 41, 1853. (9) Thess, A.; Lee, R.; Nikolaev, P.; Dai, H.; Petit, P.; Robert, J.; Xu C.; Lee, Y. H.; Kim, S. G.; Rinzler, A. G.; Colbert, D. T.; Scuseria, G. E.; Tomanek, D.; Fischer, J. E.; Smalley, R. E. Science 1998, 273, 483. (10) Girifalco, L. A.; Hodak, M.; Lee, R. S. Phys. ReV. B 2000, 62, 13104. (11) O’Connell, M. J.; Bachilo, S. M.; Huffman, C. B.; Moore, V. C.; Strano, M. S.; Haroz, E. H.; Rialon, K. L.; Boul, P. J.; Noon, W. H.; Kittrell, C.; Ma, J.; Hauge, R. H.; Weisman, R. B.; Smalley, R. E. Science 2002, 297, 593.
778
(12) Tsang, S. C.; Gua, Z. J.; Chen, Y. K.; Green, M. L. H.; Hill, H. A. O.; Hambley, T. W.; Sadler, P. J. Angew. Chem., Int. Ed. Engl. 1997, 36, 2198. (13) Balavoine, F.; Schultz, P.; Richard, C.; Mallouh, V.; Ebbesen, T. W.; Mioskowski, C. Angew Chem., Int. Ed. Engl. 1999, 38, 1912. (14) Chen, R. J.; Zhan, Y. G.; Wang, D. W.; Dai, H. J. J. Am. Chem. Soc. 2001, 123, 3838. (15) O’Connell, M. J.; Boul, P. J.; Ericson, L. M.; Huffman, C. B.; Wang, Y. H.; Haroz, E. H.; Kuper, C.; Tour, J.; Ausman, K. D.; Smalley, R. E. Chem. Phys. Lett. 2001, 342, 265. (16) Seeger, T.; Ko¨hler, T.; Frauenheim, T.; Grobert, N.; Ru¨hle, M.; Terrones, M.; Seifert, G. Chem. Commun. 2002, 1, 34. (17) Seeger, T.; Redlich, P.; Grobert, N.; Terrones, M.; Walton, D. R. M.; Kroto, H. W.; Ru¨hle, M. Chem. Phys. Lett. 2001, 339, 41. (18) Fu, Q.; Lu, C.; Liu, J. Nano Lett. 2002, 2, 329. (19) Whitsitt, E. A.; Barron, A. R. Chem. Commun. 2003, in press. (20) Bronikowski, M. J.; Willis, P. A.; Colbert, D. T.; Smith, K. A.; Smalley, R. E. J. Vac. Sci. Technol. A 2001, 19, 1800. (21) Rinzler, A. G.; Liu, J.; Dai, H.; Nikolaev, P.; Huffman, C. B.; Rodriguez-Macias, F. J.; Boul, P. J.; Lu, A. H.; Colbert, D. T.; Lee, R. S.; Fischer, J. E.; Rao, A. M.; Eklund, P. C.; Smalley, R. E. Appl. Phys. A 1998, 67, 29.
NL034186M
Nano Lett., Vol. 3, No. 6, 2003
