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2007, 111, 510-513 Published on Web 12/19/2006
Use of Amorphous Carbon Nanotube Brushes as Templates to Fabricate GaN Nanotube Brushes and Related Materials Jagadeesan Dinesh, M. Eswaramoorthy, and C. N. R. Rao* Chemistry and Physics of Materials Unit, DST Unit on Nanosciene and CSIR centre of Excellence in Chemistry, Jawaharlal Nehru Centre for AdVanced Scientific Research, Jakkur P. O., Bangalore 560064, India ReceiVed: NoVember 10, 2006; In Final Form: December 3, 2006
Amorphous carbon nanotube brushes were prepared by a simple method using glucose as the carbon precursor. The functional surfaces of these nanotubes were covered with gallium ions and then calcined to get gallium oxide nanotube brushes. The gallium oxide nanotube brushes were successfully converted to crystalline GaN nanotube brushes by treatment of ammonia at 800 °C. The method is applicable to make other nanobrushes as well.
Since the discovery of the carbon nanotubes (CNTs), there have been several efforts to prepare nanotube aggregates in various forms in view of their interesting properties as well as potential applications. Thus, aligned CNTs have been prepared by several workers,1-3 the pyrolysis of ferrocene with or without an additional hydrocarbon directly yielding aligned bundles of CNTs.4 A noteworthy contribution in this direction is the preparation of CNT brushes with useful properties by Ajayan et al.5 These brushes, obtained by the chemical vapor deposition of organic precursors on partially masked SiC fibers are crystalline. This group of workers has also prepared synthetic gecko foot hairs from multiwalled CNTs on polymer surfaces.6 Though known for its versatility, the inert graphene surface of CNTs limits its use in certain applications and needs functionalization through harsh chemical and physical treatment.7 Hightemperature synthesis often results in closed-end nanotubes, and their utility as capsules for drugs, enzymes, and metal nanoparticles requires acid treatment which opens the tips of the nanotubes and in addition, produces carboxyl and hydroxyl groups on the surface, which would favor the dispersion of the nanotubes in solvents.8 It is desirable to have open-ended nanotubes with larger pore diameter to accommodate biomacromolecules. Aligning these nanotubes in the form of membranes is envisaged to have tremendous applications in batteries and fuel cells.9 They can also be used as capacitors, chemical filters and anodes for lithium-ion batteries. Aligned amorphous carbon tubes have been made hitherto by the pyrolysis of precursors such as acetylene or ethylene at high temperatures by chemical vapor deposition (CVD) using porous alumina membranes as the templates. Thus, Martin et al.10 have prepared poorly graphitic aligned carbon nanotubes by the pyrolysis of ethylene or pyrene at high temperatures using a porous anodic alumina disc. Kyotoni et al.11 produced amorphous carbon test tubes by the chemical vapor deposition of acetylene over an anodic alumina membrane. Nanopillar arrays of glassy carbon have been prepared by filling the pores of anodic alumina membranes with furfuryl alcohol and zinc chloride, followed * Corresponding author. E-mail
[email protected]. Fax: +91-8022082760.
10.1021/jp0674423 CCC: $37.00
by pyrolysis at higher temperatures.12 In all of the cases, hightemperature carbonization resulted in poorly functionalized surfaces. In the present study, we employ a novel lowtemperature chemical route to synthesize aligned, amorphouscarbon nanotube brushes with a rich functional surface by using a polycarbonate membrane and glucose as the carbon precursor. More importantly, to demonstrate their utility, we have employed the amorphous carbon nanotube brushes as templates to fabricate brushes of GaN. The GaN brushes are likely to be potentially useful in device applications. It may be noted that GaN nanotube brushes or arrays have not been reported hitherto, except for the special case where GaN arrays were obtained by epitaxial casting of ZnO nanorods.13 Although templating through porous membranes was successful to get an array of nanotubes of different organic and inorganic structures, attempts to make aligned GaN nanotubes by CVD in those membranes have failed to give organized structures. Neither the anodized alumina nor the polycarbonate membrane has been effective in obtaining the GaN nanotube arrays by chemical treatment, the acidic nature of gallium chloride being detrimental to the alumina membrane. Use of carbon nanotubes as templates results in the formation of GaN nanowires rather than the nanotubes.14,16 The nanowires are also not aligned. A simpler chemical approach to obtain such morphology has remained elusive. We have made use of amorphous carbon nanotubes as templates with good surface functionality to obtain brushes of gallium oxide and gallium nitride nanotubes. We also demonstrate that this method can be extended to fabricate nanotube brushes of other inorganic materials also. The method employed by us to prepare amorphous carbon nanotube brushes involves a simple procedure. This was done by the thermal decomposition of glucose inside the pores of a polycarbonate membrane under hydrothermal condition. In a typical procedure, polycarbonate membranes of 220 nm average diameter, 7-22 µm in thickness, pore density 109 pores per cm2 (Millipore-GTTP 04700) were taken in a 25 mL Teflon lined and sealed autoclave containing 22 mL of 0.5 M D (+) glucose (Aldrich) solution in water. The temperature of the autoclave was maintained at 180 °C for 6 h after which it was © 2007 American Chemical Society
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Figure 3. (a) Low-magnification SEM image of a Ga2O3 nanotube brush. (b) High-magnification image of a Ga2O3 nanotube brush. Inset shows the TEM image of a Ga2O3 nanotube.
Figure 1. (a) SEM image of amorphous carbon nanotube brushes. Inset shows openings of the nanotubes. (b) SEM image showing a coverage of the nanotube brushes by a thin carbon film.
Figure 4. SEM image of a GaN nanotube brush.
Figure 2. (a) TEM image of a amorphous carbon nanotubes. (b) TEM image showing cleavage along the length of the nanotube of diameter 340 nm. (c) HREM image showing occasional graphitic layers.
allowed to cool to room temperature. The brownish liquid rich in carbon spheres was discarded. The membranes that had turned brown were washed with deionized water and ethanol several times. They were later oven dried at 40 °C for 1 h. In Figure 1a, we show a SEM image of the aligned carbon nanotubes after dissolving the polycarbonate membrane using dichloromethane. The tubules are 7-10 µm long. They are broken in a few places because of the discontinuities along the pores in the polycarbonate membrane. The openings of the tubules can be seen in the SEM image in Figure 1a (see inset). A thin layer of broken carbon film, a few nanometers thick, deposited on the surface of the membrane loosely holds the tubules together (Figure 1b). In Figure 2, we show a TEM image of an amorphous carbon nanotube. The nanotubes have an inner diameter of around 170 nm and a wall thickness of ∼45 nm. Where the diameters of the tubes exceed 340 nm, we observe a cleavage along the length of the nanotubes (Figure 2b). Since the polycarbonate membrane has a 20% variation in porosity, a diameter greater than 260 nm can result from the expansion of the pore walls due to the outburst reaction accompanying carbonization of glucose within the pores of the membrane during the hydrothermal treatment. High-resolution electron microscope (HREM) images reveal the presence of graphitic layers occasionally along the walls showing thereby that the nanotubes in the brush are by and large amorphous (Figure 2c). Accordingly, the Raman spectrum shows two peaks of equal
Figure 5. (a-c) TEM images of GaN nanotubes. (d) HREM image of the wall of a GaN nanotube.
intensity at 1581 and 1370 cm-1 due to graphitic and disordered bands, respectively (Supporting Information). In order to prepare GaN brushes, we first prepared Ga2O3 brushes by making use of the amorphous CNT brushes as templates. In a typical procedure, to prepare Ga2O3 nanotube brushes, the as-prepared amorphous carbon nanotube-polycarbonate composite was soaked in 0.5 M GaCl3 solution for 5 days. The membranes were later filtered and washed with deionized water before calcination. The calcinations step involved heating of the GaCl3containing enriched composite in an alumina boat at 450 °C for 5 h in air followed by a further heating at 680 °C for 1 h in air. The rate of heating was maintained at 0.5 °C/min throughout. A white sponge like powder was obtained after cooling. The SEM image showing the Ga2O3 nanobrushes is given in Figure 3. The bristles in the brushes have diameters in the range of 90-200 nm with lengths of around 6-7 µm. Tubes of diameters greater than 250 nm also occur due to the variation in the diameters of the carbon tubes. The TEM image shown in the inset of Figure 3b reveals a tube of 200 nm diameter, with a
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Figure 6. Schematic showing the formation of nanotube brushes of carbon, Ga2O3 and GaN.
wall thickness of around 10 nm. The Ga2O3 nanotubes are poorly crystalline showing broad reflections in the X-ray diffraction pattern. The Ga2O3 nanobrushes were kept in a quartz tubular furnace, and a constant flow of 100 sccm of ammonia (99.95%) was maintained throughout the reaction. Heating was carried out at a rate of 3 °C/min till 800 °C where it was kept for 3 h. Argon was then passed into the quartz tube at the rate of 200 sccm at 800 °C. The sample obtained was light yellow in color and was allowed to cool to room temperature. Conversion of Ga2O3 nanostructures to GaN nanostructures on heating in NH3 is known.15 A SEM image of the GaN nanotube brushes is shown in Figure 4. The image is truly remarkable in that it reveals the close packing of one-dimensional GaN nanotubes. The diameter of the nanobristles varies from 100 to 230 nm. The tubes are open at one end and their surface appears to be smooth. The XRD pattern of the GaN nanotube brushes was characteristic of wurtzite structure (JCPDF 02-1078). The TEM image in Figure 5a shows a nanotube with an outer diameter of 200 nm. The wall thickness is in the range of 10-15 nm (Figure 5b). A HREM image of the wall of a GaN nanotube shows the lattice spacing of 0.244 nm corresponding to the d-spacing of (101) planes of the wurtzite phase (Figure 5c). The Raman spectrum shows broad peaks at 145, 250, 416, 553, and 720 cm-1. The bands at 145 and 720 cm-1 can be assigned to E2 (low) and A1 (LO) modes of first-order Raman phonon modes of wurtzite GaN. The bands corresponding to A1 (TO), E2 (high), and E1 (TO) merge to give the broadband centered at 550 cm-1. The band at 250 cm-1 is due to zone boundary phonons. The band edge emission peak of GaN in photoluminescence starts from 360 nm with a peak maximum at 380 nm. The lower wavelength region is associated with quantum confinement caused by the nanotubes of less than 10 nm wall thickness. The broadband edge emission could result from the wide variation in the wall thickness. The less intense peak at 420 nm is attributed to defects. In conclusion, we have successfully synthesized amorphous carbon nanotube brushes by a simple procedure and made use of these nanotube brushes to obtain crystalline GaN nanotube brushes. This process is different from that in the preparation of GaN nanostructures by heating CNTs coated with oxide (Ga2O3) precursors.16,17 The formation of GaN nanotube brushes by the ammonialysis of Ga2O3 nanotube brushes is facilitated
Figure 7. (a) Al2O3 and (b) ZnO nanotube brushes
by the poor crystallinity of the latter. What is more important is that the GaN nanotubes in the brushes are crystalline. In Figure 6, we show the process involved in the formation of the GaN nanotube brushes. Using this method, it should be possible to make porous amorphous carbon nanotube membranes (Supporting information) and nanotube brushes of inorganic materials such as ZnO, Al2O3, GaP, AlN, and BN. Preliminary studies have already shown the general applicability of the method as illustrated by the formation of ZnO and Al2O3 brushes by employing zinc acetate and aluminum nitrate as precursors (Figure 7). Supporting Information Available: The information includes a Raman spectrum of amorphous carbon nanotube brushes and SEM image of carbon nanotubes synthesized using an alumina disk as template. This material is available free of charge via Internet at http://www.pubs.acs.org References and Notes (1) Li, W. Z.; Xie, S. S.; Qian, B. L. X.; Chang, H.; Zou, B. S.; Zhou, W. Y.; Zhao, R. A.; Whang, G. Science 1996, 274, 1701. (2) Ajayan, P. M.; Stephan, O.; Colliex, C.; Trauth, D. Science 1994, 265, 1212. (3) Rao, C. N. R.; Govindaraj A. Nanotubes and Nanowires; RSC Publishing: Cambridge, 2005. (4) Rao, C. N. R.; Sen, R.; Sathishkumar, B. C.; Govindaraj, A. J. Chem. Soc. Chem. Commun. 1998, 1525. (5) Cao; Veedu, V. P.; Li, X.; Yao, Z.; Ghasemi-Nehjad, M. N.; Ajayan, P. M. Nat. Mater. 2005, 4, 540. (6) Yurdumakan, B.; Raravikar, N. R.; Ajayan, P. M.; Dhinojwala, A. Chem. Commun. 2005, 3799. (7) Tsang, S. C.; Chen, Y. K.; Harris, P. J. F.; Green, M. L. H. Nature 1994, 372, 159.
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J. Phys. Chem. C, Vol. 111, No. 2, 2007 513 (13) Goldberger, J.; Ite, R.; Zhang, Y.; Lee, S. H.; Lee, J. E.; Yana, H.; Choi, H. J.; Yang, P. Nature 2003, 422, 599. (14) Han, W.; Fan, S.; Li, Q.; Hu, Y. Science 1997, 277, 1287. (15) Hu, J.; Bando, Y.; Golberg, D.; Liu, Q. Angew. Chem., Int. Ed. 2003, 42, 3493. (16) Deepak, F. L.; Govindaraj, A.; Rao, C. N. R. J. Nanosci. Nanotechnol. 2001, 1, 303. (17) Duan, X. F.; Lieber, C. M. J. Am. Chem. Soc. 2000, 122, 188.
