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2008, 112, 4024-4028 Published on Web 02/19/2008
Direct Synthesis of Carbon-Rich Composite Sub-microtubes by Combination of a Solvothermal Route and a Succeeding Self-Assembly Process Yong-Jie Zhan and Shu-Hong Yu* DiVision of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, The School of Chemistry & Materials, UniVersity of Science and Technology of China, Heifei 230026, China ReceiVed: January 2, 2008; In Final Form: February 4, 2008
A new self-assembly route at room temperature to produce carbon-rich composite sub-microtubes with large aspect ratios through aging a solution containing amorphous carbon nanoparticles with functional groups produced by a solvothermal carbonization of glucose in pyridine solution has been reported. The driving force of the latter assembly phenomena can be the affinities of different solutions for the functional groups grafted on the carbonaceous nanoparticles. This method shows the possibility to produce similar tubular structures by taking advantage of moderate intermolecular forces among the chemically grafted carbon nanoparticles.
Since the discovery of carbon nanotubes (CNTs) by Iijima,1 their special structure and outstanding mechanical electrical properties2 have drawn much attention, and related research is always one focus of nanoscience and nanotechnology. Various methods have been developed to prepare CNTs, such as the original arc discharge,1 laser ablation,3 and chemical vapor deposition (CVD) methods.4 All of these methods need elevated temperatures. Solvothermal and hydrothermal treatment of organic precursors to synthesize nanotubes, nanospheres, and shell materials of nanocables can provide relatively mild ways to advanced nanomaterials. To produce multiwall carbon nanotubes (MWCNTs), hexachlorobenzene was reduced by metallic potassium in the presence of a Co/Ni catalyst at 350 °C under benzene thermal conditions,5 and this strategy has been adopted in other solvent systems also.6 Recently, a basecatalyzed process called “modified Wolff-Kishner reduction” has been introduced to synthesize multiwall carbon nanotubes at 160 and 180 °C.7 A synergistic soft-hard template method has been developed to synthesize flexible metal (Ag, Cu)@crosslinked poly(vinyl alcohol) (PVA) coaxial nanocables and Te@cross-linked PVA.8 Carbohydrates such as starch and glucose are another commonly used organic source to prepare composite structures on nanometer and submicron scales. For example, glucose was used to synthesize noble-metal (Au, Ag) core/shell nanoparticles and Te@carbon-rich composite nanocables, and starch was used to synthesize a Ag/carbon hybrid nanostructure.9 Generally, there are still organic groups of different contents in these carbon-coating layers as a result of incomplete hydrothermal carbonization, and therefore, compared with those typical CNTs, these carbon layers show different properties; for example, the carbon nanowires produced by hydrothermal approach display remarkable reactivity and the capability for in situ loading with noble-metal nanoparticles of metals such as Pd, Pt, and Au.10 Recently, short and highly * To whom correspondence should be addressed. E-mail: shyu@ ustc.edu.cn. Fax: + 86 551 3603040.
10.1021/jp800017v CCC: $40.75
ordered single-walled mixed-oxide nanotubes have been assembled from amorphous nanoparticles by Mukherjee et al.11 In this Letter, we report a simple new solution approach to synthesize a kind of carbon-rich composite sub-microtube from amorphous carbon-rich composite nanoparticles by a novel combination of typical solvothermal process and a succeeding self-assembly process. Carbon nanoparticles are prepared by glucose under pyridine thermal conditions, and the associated organic functional groups on surfaces play key roles in the subsequent self-assembly process of tubular structures. Such a self-assembly-induced structure shows flexibility in deformation and reassembly, which is not the character for common carbon nanotubes. All chemicals used were of analytical grade and used without further purification. In a typical experimental procedure, analytically pure ammonium sulfocyanate (10-3 mol) and glucose (2 × 10-3 mol) were dissolved in 30 mL of pyridine; after a stirring for 20 min, the clear solution was transferred into a 34 mL Teflon-lined stainless steel autoclave, which was sealed and maintained at 180 °C for 12 h and then air-cooled to room temperature. The obtained black solution was then diluted with 50 mL of distilled water, and the mixture was removed in a beaker sealed with a plastic membrane, on which fissures were made for slow volatilization of the solution; the beaker was placed in a ventilating cabinet for a week, and about 80% of the volume of the solution within was kept in this process. A brown or brownish-black floccule formed in the residual solution gradually. The floccule was collected and washed with distilled water several times to remove pyridine and any remnants, and then, it was dried at 60 °C. The as-prepared product was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Raman, Fourier transformation infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), energydispersive X-ray spectroscopy (EDX), elementary analysis, and © 2008 American Chemical Society
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Figure 1. SEM and TEM images of the carbon-rich composite sub-microtubes. (a) A general overview of the sub-microtubes. The inset photograph images in (a) are untreated solution through the solvothermal process (left) and the as-prepared sample redispersed in distilled water (right). (b and c) High-magnification SEM images, clearly showing the open ends of these tubes. (d-f) TEM images of the sub-microtubes with different magnification.
analyses about its specific surface area and aperture sizes. SEM and TEM images were taken on a JSM-6700F field emission scanning electron microanalyzer and Hitachi Model H-800 transmission electron microscope, respectively, HRTEM was recorded on a JEOL-2010 transmission electron microscope, the Raman spectrum was carried out on a RAMANLOG 6 laser Raman spectrometer with a resolution of 0.15 cm-1(546 nm), and the FTIR spectrum was obtained on a MAGNA-IR 750 spectrometer with a resolution of 0.1 cm-1. The XPS information was obtained on a ESCALAB MK II spectrometer, the EDX information was on a JEOL JSM 6700 scanning electron microanalyzer, the element analysis of the product was measured on an Elementar Vario EL-III, and the specific surface area and aperture sizes measurement was on a DSC-60 thermal analyzer (SHIMADZU). Figure 1 reveals that the product is composed of long tubular fibers with an external diameter of about 90-200 nm and a wall thickness of less than 50 nm, and the lengths can exceed 10 µm. The final drying process could break some submicrotubes (Figure 1b). These tubes have open ends and relatively smooth interior and external walls (Figure 1c,f). Heavily bent tubes were found in the sample with a larger external diameter about 600 nm (Figure 2a), showing the perfect flexibility of this structure. The high-resolution TEM (HRTEM) image of one open end of a typical tube shows that it is amorphous (Figure 2b). The BJH isothermal adsorptiondesorption pattern shown in Figure 2c indicates the porous character of the product. The BET surface area of the product is 7.51 m2/g, in which the micropore area is 1.88 m2/g and external surface area is 5.63 m2/g. The Raman spectrum in Figure 3 shows two bands at 1540 and 1364 cm-1, respectively, that are very similar to the bands of typical graphite powder,12 corresponding to the E2g2 vibration mode of the graphite layers (G-band) and the A1g mode for the disordered graphite structures (D-band). However, the thickness and the amorphous structure of the tube walls and the typical
inner and outer diameters of these tubes are totally different from those for carbon nanotubes, although they have similar Raman spectrum features. The FTIR spectrum in Figure 4 was also used to characterize these sub-microtubes. Compared with the spectrum of glucose, it has few but broader absorption peaks and still shows clear succession to this main carbon source; the peaks at 3406, 2919, 2851, and 1466 cm-1 correspond to those at 3411, 2924, 2854, and 1460 cm-1 for glucose, respectively. The OH bending vibration at 3406 cm-1 implies the existence of residual hydroxyl groups. The peaks at 1629, 1064, and 606 cm-1 correspond to peaks 1633, 1064, and 603 cm-1 for pyridine, revealing that pyridine is not only a solution but also takes part in solvothermal reactions. The CdC vibration at 1629 cm-1 can also be partially attributed to aromatization of glucose in solution.9 These results suggest that there are a large amount of residues including hydroxyl groups and pyridyl groups in these carbon tubes due to an incomplete carbonization process, and they play important roles in the formation process of the tubular structures. The X-ray photoelectron spectroscopy (XPS) reveals the S2p, O1s, and N1s binding energies of the sample at 163.45, 532.15, and 399.35 eV, respectively (see Supporting Information Figure S1). The asymmetry peak of S2p with a shoulder of lower energy shows the different chemical valences of element S in the sample. The standard data in NIST (National Institute of Standards and Technology, NIST X-ray Photoelectron Spectroscopy Database) shows that the S2p binding energy in ammonium thiocyanate is about 162-162.1 eV, that in thiourea is about 161.4 eV (as another possible isomeride at higher temperature), and that in elemental S is about 162.9-164.8 eV. Thus, part of the element S in the sample exists in the form of a simple substance according to the fitted value of the S2p peak; the corresponding proportion is dominant (94%), which has no infrared activity but has Raman activity. The latter is not indicated clearly in above Raman spectrum; however, it implies
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Figure 2. (a) TEM image of the tubes that shows perfect flexibility; (b) HRTEM image of one open end of a typical tube; and (c) the BJH isothermal adsorption-desorption pattern.
Figure 3. Raman spectrum of the sample, showing obvious character of amorphous carbon.
that its content (in all elements) is relative lower. The molar ratio of S/N/O in the sample is about 1:2:2. The typical EDX spectrum verifies the existence of element C, O, and S, and the data taken from eight random-selected sub-microtubes indicates that the mean molar ratios of elements C, O, and S are about 14:2:1 (see Supporting Information Figure S2). The molar ratio of element C to element H determined by the elemental analysis (with Elementar Vario EL-III) is about 1:1.2. Combing these data with the above XPS results, a semiquantitative result can be obtained, that is, the mass ratio of major elements C, N, H, S, and O in the sample is 1:0.42: 0.10:0.50:0.47, and the corresponding molar ratios are 1:0.36: 1.20:0.19:0.35. The element C is found to be about 40 wt %, holding the highest content in the sample, and the amount of residual groups that hold elements S, N, H, and O are
Figure 4. FTIR spectrum of the sub-microtubes.
considerable, Thus, the tubular structures are, in fact, carbonrich composites. Except for the presence of elements N and S, the Raman and FTIR spectra reveal an obvious similarity between the submicrotubes and the hydrothermal products reported previously.9 As-prepared tubes are formed spontaneously in a self-assembly process at room temperature, and the previous solvothermal and hydrothermal processes themselves cannot directly fabricate such a similar tubular structure. Different morphologies of the initial and intermediate products found in the evolution process of tubular structures are shown in Figure 5. The initial solution after solvothermal reaction can form carbon films containing tiny nanoparticles (Figure 5a,b). The product collected at an early stage from the aging solution diluted with water for 24 h is a mixture of nanoparticles and sub-microtubes with a rather rough surface (Figure 5c), indicating that the tubes are indeed
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Figure 5. (a) SEM image of membranes formed by air-dried pyridine solution that was not diluted with water; (b) a high-resolution SEM image showing that the membranes are composed of tiny nanoparticles; (c) a mixture of granular and tubular structures formed at an early stage of the self-assembling process after aging for 24 h in a water-diluted solution; and (d) sub-microtubes formed at a later stage after aging for 72 h.
SCHEME 1: Schematic Illustration of the Possible Formation Mechanism of Carbon-Rich Composite Sub-microtubes by the Self-Assembly of Carbon Nanoparticles Grafted with Different Proportions of Hydrophilic and “Pyridylphilic” Residual Groups in the Water-Pyridine System and the Heterogeneous Radial Distribution of Nanoparticles in the Tube Walls
self-assembled from tiny carbon nanoparticles. When the time goes on, the nanoparticles in solution gradually disappear, and the formed sub-microtubes have a smoother surface (Figure 5d). The solvothermal treatment of glucose in pyridine has not been reported previously. In general, hydrothermal carbonization of glucose still results in a complex mixture of organic compounds.9 In some cases, the LaMer model is used to
qualitatively explain many experimental phenomena;13 dehydration, aromatization, and polymerization are considered to be major processes to determine the species of the final products. The obtained carbon particles from hydrothermal processes are generally composed of carbonized cores and the hydrophilic surfaces, which are produced by incomplete carbonization. As for the present carbon particles from pyridine thermal processes,
4028 J. Phys. Chem. C, Vol. 112, No. 11, 2008 besides hydrophilic groups on their surfaces, the presence of elements N, H, and S in the final product also suggests the existence of other adhering pyridyl groups or sulfocyanic acid groups. During the succeeding intermolecular dehydration process, these groups can be interweaved onto carbon-rich surfaces and therefore affect the hydrophilic property of the product. Despite the good intersolubility of water and pyridine, these two solutions can still have different affinities for those groups, and therefore, they show different affinities for the carbon nanoparticles grafted with various functional groups, which will delicately affect the self-assembly process of these nanoparticles in the mixed solution. Considering the self-assembly of surfactant molecules to micelles in various solutions and emulsions, the formation of the sub-microtubes here could share a similar mechanism. As shown in Scheme 1, the residual groups on carbon nanoparticles can be divided into hydrophilic groups and so-called “pyridylphilic” groups. In the mixed solution of water and pyridine, the nanoparticles grafted with these groups of different proportions are driven by two kinds of affinities and adjust their mutual positions to agglomerate and form a tubular structure at last. The inner surface and exterior surface have functional groups of different affinities; if the “pyridylphilic” groups on the inner surfaces are the dominant ones, the tubes will encapsulate relative pyridine-rich microenvironments inside and vice versa. Such an assembly process is also similar to that proposed to explain the self-assembly of supramolecular macroscopic tubes by Yan et al.14 In a series of experiments, hydrothermal carbonization of glucose without the addition of pyridine could only produce microspheres9,15 (see Supporting Information in Figure S3), implying that the presence of the “pyridylphilic” groups is essential to the subsequent self-assembly of carbon nanoparticles to a more complicated tubular structures observed here. To understand the effect of element S in the tubes, the stability of the tubes after being attacked by Ag+ was examined (see Supporting Information Figure S4). TEM images of the tubes after being immersed in AgNO3 solution (0.01 M) for 1 week reveal that their former tubular structure becomes ambiguous and vulnerable to the irradiation of electrons beams, whereas the original ones are tough enough to endure such beam irradiation. The formation of Ag2S nanoparticles, their adhesion, and penetration give these tubes deeper color; at the same time, the loss of element S in the original structure seems to decrease the stability of these tubes. Once these tubes are self-assembled, they can keep their structures firmly in water, under dry air condition, and at a relative higher temperature of 300 °C. If these tubes are reimmersed in pure pyridine and ultrasonic oscillated for just several minutes, their smooth walls become coarse, and some spherical masses or nanospheres appear. The results confirmed that the tubes tend to disassemble or reassemble carbon nanoparticles into new stable structures in a changed solution environment, in which “pyridylphilic” surfaces rather than hydrophilic surfaces are more stable. All of these processes are likewise driven by the different affinities. This phenomenon provides indirect evidence for the proposed mechanism. In other experiments, when surfactants were introduced in the selfassembly process, secondary branches with a different length could also be synthesized (see Supporting Information Figure S5), showing the potential ability to prepare other carbon-rich architectures using this kind of carbon-rich composite microtubes as the precursor.
Letters In summary, a novel method to synthesize carbon-rich composite sub-microtubes with large aspect ratios has been discovered by a solvothermal carbonization of glucose in pyridine solution and a succeeding self-assembly process. The driving force of the latter assembly phenomena can be the affinities of different solutions for those groups grafted on the carbonaceous nanoparticles. This method shows the possibility to produce similar tubular structures by taking advantage of moderate intermolecular forces among the chemically grafted carbon nanoparticles. Acknowledgment. This work is supported by the special funding support from the Centurial Program of Chinese Academy of Sciences, the National Science Foundation of China (NSFC) (Grants Nos. 50732006, 20325104, 20621061, 20671085, and 50372065), the 973 project (2005CB623601), Anhui Development Fund for Talent Personnel and Anhui Education Committee (2006Z027, ZD2007004-1), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, the Specialized Research Fund for the Doctoral Program (SRFDP) of Higher Education State Education Ministry, and the PartnerGroup of the Chinese Academy of Sciences-The Max Planck Society. Supporting Information Available: XPS spectra, EDX spectrum, SEM images, TEM images of the product, and intermediate products. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Iijima, S. Nature 1991, 354, 56. (2) Dresselhaus, M. S.; Dressehaus, G.; Eklund P. C. Science of Fullenrenes and Carbon Nanotubes; Academic Press: New York, 1996. (3) Thess, A.; Lee, R.; Nikolaev, P.; Dai, H. J.; Petit, P.; Robert, J.; Xu, C. H.; Lee, Y. H.; Kim, S. G.; Rinzler, A. G.; Colbert, D. T.; Scuseria, G. E.; Tomanek, D.; Fischer, J. E.; Smalley, R. E. Science 1996, 273, 483. (4) (a) Li, W. Z.; Xie, S. S.; Chang, B. H.; Zou, B. S.; Zhou, W. Y.; Zhao, R. A.; Wang, G. Science 1996, 274, 1701. (b) Kong, J.; Soh, H. T.; Cassell, A. M.; Quate, C. F.; Dai, H. J. Nature 1998, 395, 878. (c) Cassell, A. M.; Franklin, N. R.; Tombler, T. W.; Chan, E. M.; Han, J.; Dai, H. J. J. Am. Chem. Soc. 1999, 121, 7975. (d) Jiang, K. L.; Li, Q. Q.; Fan, S. S. Nature 2002, 419, 801. (5) Jiang, Y.; Wu, Y.; Zhang, S. Y.; Xu, C. Y.; Yu, W. C.; Xie, Y.; Qian, Y. T. J. Am. Chem. Soc. 2000, 122, 12383. (6) (a) Shao, M. W.; Li, Q.; Wu, J.; Xie, B.; Zhang, S. Y.; Qian, Y. T. Carbon 2000, 40, 2961. (b) Wang, X. J.; Lu, J.; Xie, Y.; Du, G. A.; Guo, X. Q.; Zhang, S. Y. J. Phys. Chem. B 2002, 106, 933. (c) Liu, J. W.; Shao, M. W.; Chen, X. Y.; Yu, W. C.; Liu, X. M.; Qian, Y. T. J. Am. Chem. Soc. 2003, 125, 8088. (7) (a) Wang, W. Z.; Poudel, B.; Wang, D. Z.; Ren, Z. F. J. Am. Chem. Soc. 2005, 127, 18018. (b) Wang, W. Z.; Kunwar, K.; Huang, J. Y.; Wang, D. Z.; Ren, Z. F. Nanotechnology 2005, 16, 21. (8) (a) Luo, L. B.; Yu, S. H.; Qian, H. S.; Zhou, T. J. Am. Chem. Soc. 2005, 127, 2822. (b) Gong, J. Y.; Luo, L. B.; Yu, S. H.; Qian, H. S.; Fei, L. F. J. Mater. Chem. 2006, 16, 101. (c) Qian, H. S.; Luo, L. B.; Gong, J. Y.; Yu, S. H.; Li, T. W.; Fei, L. F. Cryst. Growth Des. 2006, 6, 607. (9) (a) Li, X. M.; Li, Y. D. Angew. Chem., Int. Ed. 2004, 43, 597. (b) Qian, H. S.; Yu, S. H.; Luo, L. B.; Gong, J. Y.; Fei, L. F.; Liu, X. M. Chem. Mater. 2006, 18, 2102. (c) Yu, S. H.; Cui, X. J.; Li, L. L.; Li, K.; Yu, B.; Antonietti, M.; Co¨lfen, H. AdV. Mater. 2004, 16, 1636. (10) Qian, H. S.; Yu, S. H. AdV. Funct. Mater. 2007, 17, 637. (11) Mukherjee, S.; Kim, K.; Nair S. J. Am. Chem. Soc. 2007, 129, 6820. (12) (a) Tuinstra, F.; Koenig, J. L. J. Chem. Phys. 1969, 53, 1126. (b) Lee, E. H.; Hembree, D. M.; Rao, G. R.; Mansur, L. K. Phys. ReV. B. 1993, 48, 15540. (13) LaMer, V. K. Ind. Eng. Chem. 1952, 44, 1270. (14) Yan, D. Y.; Zhou, Y. F.; Hou, J. Science 2004, 303, 65. (15) Wang, Q.; Li, H.; Chen, L. Q.; Huang, X. J. Carbon 2001, 39, 2211.
