摘要:
Self-organization of triple-stranded carbon nanoropes C.-J. Su,a D. W. Hwang,b S.-H. Lin,a B.-Y. Jinb and L.-P. Hwanga,b aInstitute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan, ROC. E-mail: Hwanglp@yahoo.com.tw; Fax: z886-2-23620200; Tel: z886-2-23668287 bDepartment of Chemistry, National Taiwan University, Taipei, Taiwan, ROC. gas flow was stopped, and the reactor was cooled down to room temperature in flowing nitrogen (100 ml min21) for 14 h. The reaction product was finally collected as fine black powder from the ceramic boat. The microstructure of carbon nanoropes was studied with the aid of TEM (Hitachi H-7100, 120 kV and JEOL JEM-2010, 200 kV, equipped with ISIS 300 Energy Dispersive X-ray analyzer) and SEM (Hitachi S5000, 20 kV) after sonicating the samples in ethanol for 1 h and dispersing a drop of solution over holey carbon grids.The product prepared by this method was found to contain large numbers of irregularly curved nanofilaments with various complicated shapes and forms (Fig. 1). Among these nanofilaments, the most striking finding was that the nanoropes, as shown in area A and B of Fig. 1, appeared to consist of three helices entwining with each other. A rough estimation from these nanoropes we have found that the width of each strand is Fig. 1 A typical TEM figure showing three types of carbon nanofilaments grown by the lanthanide oxide-catalyzed decomposition of acetylene on the AlPO4-5 support. Type I and II carbon nanoropes are marked as A and B, respectively.Carbon filaments in the area C mainly consist of irregularly curved single-stranded carbon nanotubes with an average width about 10 nm. White area in the figure is a mixture of carbon black and AlPO4-5 zeolite. E-mail: byjin@www.ch.ntu.edu.tw Received 7th November 2001, Accepted 10th January 2002 Published on the Web 18th January 2002 Novel self-organized carbon nanoropes consisting of three helically coiled multi-wall nanotubes with a remarkable constant pitch over several microns were grown by the lanthanide oxide-catalyzed decomposition of gaseous acetylene on aluminophosphate (AlPO4-5) support. Direct characterization by the stereo transmission electron microscope and scanning electron microscope has convincingly shown that these three strands entwine with each other helically, which are presumably kept together by both the spontaneous curvature and van der Waals (vdW) attraction. Recently, there has been intensive interest in the structure and morphology of carbon nanofibers prepared by the catalytic decomposition of gaseous hydrocarbon using various transition metals such as iron, cobalt, nickel, and some of their alloys.1–5 These nanofibers were generally whisker-like tubular structures with diameters controlled by the size of the small metal particles. In addition to the straight tubes, several intricate shapes and structures including helices, cones, tori and rings have been identified.6–10 At higher structural level, strong inter-tube van der Waals (vdW) attraction can generate at least two kinds of self-organization among nanotubes: firstly, nanotubes can bunch together and leads to the rope formation;10 secondly, upon the ultrasonic irradiation, the nanotubes may be self-folded into ring configurations11 and stabilized by vdW attractions.However, to our knowledge, no observation of the self-organization among helically coiled nanotubes has been reported yet. In this work, we report for the first time the preparation and characterization of the novel triple-stranded helically coiled carbon nanoropes by the high-temperature decomposition of gaseous acetylene via the microporous aluminophosphate (AlPO4-5) support12,13 and rare earth metal oxide (Pr6O11) as the catalyst. The microstructure and composition of the nanoropes were characterized by transmission electron microscope (TEM), scanning electron microscope (SEM), energy dispersive X-ray (EDX), and selective area diffraction (SAD).The triple-stranded carbon nanoropes were synthesized using the AlPO4-5 supported Pr6O11 catalyst, which was prepared by the impregnation method. 0.1 g Pr6O11 fine powder was dissolved in 10 ml distilled water and stirred for 10 min at 343 K. One gram of microporous AlPO4-5 with the template was then slowly added into the solution and stirred for 1 h. The mixture was ground to fine grey powder (100 mesh) after drying in an oven at 393 K for 8 h, and the resulting catalyst was used immediately. A ceramic boat containing 100 mg of catalyst was placed inside a quartz tube located in the central part of a tubular electric furnace.The catalyst was heated at a rate of 30 K min21 from room temperature to 473 K in air and retained at 473 K for 3 h. The gas mixture (N2~100 ml min21, C2H2 ~ 10 ml min21) was introduced for about 3 min before the sample was brought to the desired reaction temperature (973 K) at a rate of 30 K min21, and the reaction was allowed to proceed at 973 K for another 2 h. Afterward, the acetylene DOI: 10.1039/b110151j PhysChemComm , 2002, 5(5), 34-36 34 This journal is # The Royal Society of Chemistry 2002 PaperFig. 2 Pitch versus diameter of triple-stranded carbon helices. The dots in the figure stand for the measurements of pitch as a function of diameter of typical nanoropes observed in the current work.The solid line corresponds to the optimal ratio of pitch and diameter (p/2r ~ p, or Q ~ 45u) for single-stranded helices in the absence of vdW interaction predicted theoretically by Ou-Yang et al.14 The computer graphs in the figure are three representative model structures for type I, type II and optimal triple-stranded nanoropes with the same arc-length for the central lines, respectively.15 Most of the triple helices observed belong to type I nanorope with low pitch angle, indicating that it is easier to grow low pitch triple-stranded helices. about 10 to 30 nm. The pitches and diameters for the triple helices vary significantly. An analysis showing the distribution of pitches and diameters of many TEM images is summarized in Fig.2 where the solid line depicts the theoretical prediction of the optimal ratio of pitch (p) and diameter (2r) for a singlestranded helix.14 Therefore, it is convenient to distinguish triple-stranded nanoropes according to the ratio of their pitch and diameter, or equivalently, the pitch angle of the helix, Q ~ arctan (p/2pr). The computer graphs in Fig. 2 are three representative model structures for type I (Q v 45u), optimal (Q ~45u) and type II (Qw45u) triple-stranded nanoropes with the same arc-length for the central lines, respectively. The pitch and diameter of a typical type I nanorope as shown in Fig. 3 are found to be 140 and 80 nm, respectively, which leads to p/2r ~ 1.8 (Q ~30u).Type II nanorope such as the representative one in Fig. 4 has a high pitch angle with a larger ratio, p/2r ~ 4 (Q ~ 52u). The pitches and diameters of the triple helices of both types are relatively uniform throughout the total length of the nanoropes ranging from 1 to 10 m. The number of repeated units in a triple helix can be as high as about 40 units, despite the fact that the detailed conformation for each pitch (Fig. 3) still shows some minor fluctuations. These small fluctuations in Fig. 3 The TEM micrograph for typical type I nanoropes. The pitch and diameter for the nanorope in the figure are about 140 and 80 nm, respectively, which leads to a low p/2r ratio of about 1.8 (Q ~ 30u). 35 PhysChemComm , 2002, 5(5), 34-36 Fig.4 The TEM image for the representative type II nanorope with the pitch and the diameter about 480 and 120 nm, respectively, which leads to a large ratio (p/2r ~ 4) and a high pitch angle (Q ~ 52u). triple-stranded carbon nanoropes might originate from the intrinsic curvature associated with the growth. The intrinsic curvature, however, has a stronger influence on the morphology of the single-stranded filaments, e.g. carbon filaments in area C of Fig. 1. Hence, the entropic effect dominates in the growth of single-stranded carbon filaments and results in conformation disorder with many complicated shapes. Unlike the single-stranded filaments, the existence of strong non-directional vdW interaction between tubes in the triplestranded nanoropes may have important implications for their geometry, thermodynamic stability, and formation mechanism.As can be seen in Fig. 2, most of the triple helices belong to type I nanoropes with low pitch angles. Since the low pitch triple helices appear to have larger vdW adhesive energy due to larger contact area among different strands, it is proposed that the vdW adhesive energy is the major factor for the stability of triple-stranded nanorope. However, additional work will be necessary for a detailed understanding of the underlying physics. It should be noted that the synthesis of triple helices is quite sensitive to the reaction temperature. Only within a narrow temperature range from 968 to 978 K have the triple-stranded coils been observed by the current method of preparation.In addition to the temperature effect, the existence of the template (TEA, triethylamine) in the zeolite is also critical for the growth of triple helices. No triple helix has been found when the template in the zeolite support is removed. This suggests that the thermal decomposition of the template in the zeolite may play a subtle role in the formation of the nanofilaments with size large enough to form the triple helices. However, within the detection limit of approximately 1%, the EDX analysis in TEM indicated that the nanoropes contain neither trace of rare earth metal Pr nor elemental nitrogen. It is also confirmed from the SAD data that there is no inclusive Pr crystal in the nanoropes. The contrast in TEM depends on the number of electrons scattered: the more atoms are in a region, the higher is the contrast.The images obtained from TEM are close to the 2D projections of the viewed objects, while there are also contributions from out of focus regions. Another possible structure, which may give rise to a similar TEM image as in Fig. 3, is a triple-stranded braid, since both the triple-stranded helix and the triple-stranded braid exhibit a similar 2D projection. Therefore, the SEM and stereo TEM were chosen to distinguish these two possible 3D structures. Fig. 5 shows the SEM micrograph of an individual type I nanorope with a pitch angle about 25u. The SEM micrograph was taken with an electron microscope operated at 20 kV. The greater depth of field of SEM provides much more information about the three-dimensional structure of the nanoropes.In fact, the micrograph in Fig. 5 indicates that the nanorope is a left-handed triple-stranded helix. This 3DFig. 5 The SEM micrograph of an individual type I nanorope with the pitch and the diameter about 150 and 110 nm, respectively, which leads to a low pitch ratio about 1.36. The SEM micrograph was taken with an electron microscope operated at 120 kV. structure is also confirmed by the stereo TEM techniques, where two TEM stereograms with a tilted angle 7u apart were taken for the nanorope in Fig. 3. The 3D appearance of the sample object can be observed by superimposing the stereo pair, and thus visualize the relative depths of three strands in the nanorope.In conclusion, both types of techniques have shown that the entwisting nanoropes are indeed triple-stranded helices instead of braids, which is consistent with the fact that we did not observe the existence of side view of a braid in all TEM images. However, the possibility of forming carbon nanobraids cannot be completely excluded. Using the stereo TEM and SEM, we were also able to identify some right-handed triple helices in addition to the lefthanded ones. This indicates that the nanoropes are a racemic mixture of two chiral forms. The authors would like to thank C.-Y. Tang for help with the stereo TEM experiment and Y.-O. Chang for the preparation of AlPO4-5. C.-J. Su acknowledges the financial support of CTCI Foundations.Funding for this work has been supported by the National Science Council, ROC. References 1 M. S. Dresselhaus, G. Dresselhaus, K. Sugihara, I. L. Spain and H. A. Goldberg, Graphite Fibers and Filaments, Springer Series in Materials Science 5, Springer-Verlag, New York, 1988. 2 M. S. Dresselhaus, G. Dresselhaus and P. C. Eklund, Science and Technology of Fullerenes and Carbon Nanotubes, Academic Press, San Diego, 1996. 3 N. M. Rodriguez, J. Mater. Res., 1993, 8, 3233. 4 R. T. Baker, Carbon, 1989, 27, 315. 5 M. Endo, K. Takeuchi, S. Igarashi, K. Kobori, M. Shiraishi and H. W. Kroto, J. Phys. 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Lett., 1997, 78, 4055. 15 The three central lines of a model structure for an ideal theoretical triple helix can be described parametrically by the 3D coordinate: {ai(t) ~ [rcos(vt z gi), rsin(vt z gi), pvt/2p], i ~ 1, 2, 3}, where 0 v t v l is the arc-length parameter, the radius of helix r, the pitch of helix p, and v21~(r2zp2)1/2. The phases of three helices are chosen to be gi ~ 0, 2p/3, 4p/3, respectively. Tube surfaces around the central lines are given byYi(t,h)~ai(t)zr0[N(t) coshz B(t) sinh], where 0 v h v 2p, N(t) is the normal vector, B(t) is the binormal vector along the central line of the tube surface, and 0 is the width of the tube. r PhysChemComm , 2002, 5(5), 34-36 36
ISSN:1460-2733
DOI:10.1039/b110151j
出版商:RSC
年代:2002
数据来源: RSC