Confined Self-Assembly Approach to Produce Ultrathin Carbon

Apr 16, 2009 - Additionally, the described strategy is extendable. By designing an appropriate surfactant, it is also possible for the fabrication of ...
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Confined Self-Assembly Approach to Produce Ultrathin Carbon Nanofibers Weixia Zhang, Jiecheng Cui, Cheng-an Tao, Changxu Lin, Yiguang Wu, and Guangtao Li* Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry Tsinghua University, 100084 Beijing, China Received February 3, 2009. Revised Manuscript Received March 2, 2009 A surfactant containing a terminal carbon source moiety was synthesized and used simultaneously as both template molecule and carbon source. On the basis of this special structure-directing agent, an efficient strategy for producing uniform carbon nanowires with diameter below 1 nm was developed using a confined self-assembly approach. Besides the capability of producing ultralong and thin carbon wires inaccessible by the previously reported approaches, the method described here presents many advantages such as the direct use of residue iron complex as catalyst for carbonization and no requirement of conventional tedious infiltration process of carbon source into small channels. Different methods including SEM, TEM, XRD, Raman spectroscopy, and conductivity measurement were employed to characterize the formed ultrathin carbon nanofibers. Additionally, the described strategy is extendable. By designing an appropriate surfactant, it is also possible for the fabrication of the finely structured carbon network and ultrathin graphitic sheets through the construction of the corresponding cubic and lamellar mesostructured templates.

Introduction Owing to their unique physicochemical properties and a broad spectrum of potential applications in catalysis, energy storage, chemical sensors, nanocomposites and nanoelectronics and so forth, the controlled fabrication of one-dimensional (1D) nanostructured carbon materials with well-defined size and morphology has been extensively exploited in recent years.1-5 The most widely used method is the catalytic decomposition of various hydrocarbon sources, in which carbon nanotubes (CNT) or nanofibers (CNF) with diameters in the range of 20 to 150 nm can be attained by varying the size of the catalysis nanoparticles.6,7 Recently, porous materials with 1D channels (e.g., porous anodic aluminum oxide and mesoporous silica) have become attractive templates for the generation of 1D carbons.8-13 Although these template syntheses allow production of CNT or CNF with several-nanometer diameters by tuning the pore size, further size reduction of 1D carbons by using the methods developed so far is difficult to achieve. Herein, we report for the first time the preparation of well-defined carbon nanofibers with diameters below 1 nm through a facile, confined self-assembly process within the pores of mesoporous silicate. *Corresponding author. Tel: +86-10-62792905. Fax: +86-10-62792905. E-mail: [email protected]. (1) Wang, K.; Zhang, W.; Phelan, R.; Morris, M. A.; Holms, J. D. J. Am. Chem. Soc. 2007, 129, 13388. (2) Peng, H.; Jain, M.; Li, Q.; Peterson, D. E.; Zhu, Y.; Jia, Q. J. Am. Chem. Soc. 2008, 130, 1130. (3) Masarapu, C.; Wei, B. Langmuir 2007, 23, 9046. (4) Chen, S.; Jiang, Y.; Wang, Z.; Zhang, X.; Dai, L.; Smet, M. Langmuir 2008, 24, 9233. (5) Qu, L.; Du, F.; Dai, L. Nano Lett. 2008, 8, 2682. (6) Kukovitsky, E. F.; Lvov, S. G.; Sainov, N. A.; Shustov, V. A. Chernozatonskii, L. A. Chem. Phys. Lett. 2002, 355, 497. (7) Gunjishima, I.; Inoue, T.; Okamoto, A. Langmuir 2008, 24, 2407. (8) Liang, C.; Li, Z.; Dai, S. Angew. Chem., Int. Ed. 2008, 47, 3696. (9) Lee, J.; Kim, J.; Hyeon, T. Adv. Mater. 2006, 18, 2073. (10) Cott, D. J.; Petkov, N.; Morris, M. A.; Platschek, B.; Bein, T.; Holmes, J. D. J. Am. Chem. Soc. 2006, 128, 3920. (11) Park, C.; Keane, M. A. Langmuir 2001, 17, 8386. (12) Moller, K.; Bein, T. Chem. Mater. 1998, 10, 2950. (13) Wang, Z.; Stein, A. Chem. Mater. 2008, 20, 1029.

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Experimental Section Chemicals. 1,12-Dibromododecane and 1-methylimidazole were received from Aldrich. Pyrrole, petroleum ether, diethyl ether, N,N-dimethylformamide, dichloromethane, methanol, toluene, sodium hydride, anhydrous magnesium sulfate, ferric chloride anhydrous, tetraethoxysilane (TEOS), ammonia solution, acetone, chloroform, and ethanol were purchased from Beijing Chemical Company. All chemicals were used without further purification. Synthesis of Polymerizable Surfactant PyC12MIM+Br-. PyC12MIM+Br- was synthesized according to the following route. First, pyrrole (15 mmol) was added dropwise at 0 °C to a dry DMF solution (150 mL) containing 1,12-dibromododecane (43 mmol) and NaH (43 mmol). The resulting solution was stirred overnight. The crude product was purified by chromatography on silica gel with petroleum ether as an eluent. Subsequently, the resulting bromide (16 mmol) was added dropwise under vigorous stirring to a 50 mL toluene solution of 1-methylimidazole (18 mmol), and the resulting mixture was refluxed for 10 h and then cooled to room temperature. The crude product was purified by chromatography on silica gel with CH2Cl2/CH3OH (10:1) as an eluent, affording the desired product as brownish oil (yield: 70%). 1H NMR (CDCl3, δ): 10.27 (s, 1H), 7.51 (d, 1H), 7.35 (d, 1H), 6.58 (dd, 2H), 6.04 (dd, 2H), 4.24 (t, 2H), 4.05 (s, 3H), 3.77 (t, 2H), 1.84 (m, 2H), 1.66 (m, 2H), 1.20 (m, 16H).

Preparation of Mesostructured Silica with PyC12MIM+Br- as Structure-Directing Agent. Hexagonal mesoporous silica (MCM-41) was prepared in basic medium by a hydrothermal procedure using tetraethoxysilane (TEOS) as silica source and PyC12MIM+Br- as structure-directing agent and carbon source. In a typical synthesis, PyC12MIM+Brwas dissolved in deionized water with NH3 3 H2O under mild magnetic stirring for 1 h to get the homogeneous mixture, and then TEOS was added dropwise at room temperature. The molar compositions of the starting mixtures were 1 TEOS/0.12 PyC12MIM+Br-/7 NH3 3 H2O/100 H2O. The resulting mixture was stirred at room temperature for 1 h and then transferred into an autoclave and left standing at 80 °C for 3 days. After hydrothermal treatment, the mixtures were filtered, and white powders

Published on Web 04/16/2009

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were washed with Millipore water and acetone. The samples were dried in the air at room temperature.

Preparation of Polypyrrole (PPy) Nanowires within Mesoporous Channels. The as-synthesized mesoporous silica were dispersed in chloroform with vigorous stirring at RT, and then the chloroform containing FeCl3 was added to convert the densely packed pyrrole moieties within the channels of the silica into conjugated PPy nanowires. After stirring for 30 min, a black solid was collected, washed thoroughly with water, and dried in the air. Then, the residue was dispersed in aqueous HF to remove the silica framework. The formed black powder was collected by filtration, rinsed thoroughly with water and acetone, and dried in the air.

Preparation of Ultrathin Carbon Fibers within Channels. To get carbon nanowires, the silica with the formed PPy within channels was pyrolyzed in a nitrogen flow at 600, 700, or 800 °C for 5 h. The silica frameworks were removed using HF, and then the residues was collected, rinsed thoroughly with water and ethanol, and dried in air for further experiments. Charaterization. XRD measurements were performed using Bruker D8 Advance X-ray powder diffractometer. TEM images were obtained using JEM 2010 high-resolution transmission electronic microscope at an acceleration voltage of 120 kV. Fourier transform infrared (FTIR) spectra were recorded on AVATAR 360 ESP FTS spectrophotometer. Ultraviolet-visible (UV-vis) spectra were collected on PerkinElmer Lambda35 spectrometer. N2 adsorption-desorption isotherms were measured with a Micromeritics ASAP2010 apparatus. Raman spectra were collected on RM 2000 microscopic confocal Raman spectrometer.

Figure 1. Schematic illustration of the developed strategy for producing carbon nanofibers with diameters below 1 nm.

Results and Discussion In previous studies, the template synthesis of carbon nanofibers involves the introduction of carbon precursor into channels of the template followed by carbonization and template dissolution.8,9 This approach affords nanofibers with diameter comparable to that of the employed channels, but this technique is only limited to the diameter down to about 2 nm by tuning the size of the channels.14 In contrast, our strategy relies on the utilization of a special structure-directing surfactant for the construction of mesoporous template. This surfactant contains a terminal carbon source moiety and can function simultaneously as both template molecule and carbon source. During the formation of the mesoporous template, carbon source moieties could be densely packed in a controlled linear fashion within channels, providing the prerequisite for producing ultrathin carbon nanofibers after carbonization. In our case, polymerizable pyrrole was used as a carbon source moiety. After the in situ polymerization of the organized pyrrole moieties in channels using FeCl3, the resulting polypyrrole (PPy) molecular wires were transferred into carbon nanofibers under N2-protected high temperature, where residual iron salts were directly employed as catalyst for the conversion. The overall fabrication procedure is presented in Figure 1. Methylimidiazolium-based surfactant bearing terminal pyrrole moiety (PyC12MIM+Br-) was used as structure-directing agent for the preparation of carbon source filled mesotructured silicate. In a typical synthesis, 2.0 g tetraethoxysilane (TEOS) was added under vigorous stirring to an aqueous solution of NH4OH containing PyC12MIM+Br-. A molar ratio of TEOS/H2O/ PyC12MIM+Br-/NH4OH (26%) = 1:100:0.12:7 was established in the reaction mixture. After 60 min of stirring at RT, the solution was left standing at 80 °C for 72 h in an autoclave. A white powder was collected by filtration and washed with (14) Tian, B.; Che, S.; Liu, Z.; Liu, X.; Fan, W.; Tatsumi, T.; Terasaki, O.; Zhao, D. Chem. Commun. 2003, 2726.

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Figure 2. XRD patterns (A) of as-synthesized mesoporous silicate (a), the silicate after the in situ polymerization of pyrrole units (b) and the silicate after the polymerization followed by carbonization (c), Inset is the wide-angle XRD patterns of the corresponding silicates (a-c) and carbon nanowires (d). TEM images of assynthesized mesoporous silicate (B), the silicate after the in situ polymerization of pyrrole units (C), and the silicate after the polymerization followed by carbonization (D).

deionized water and acetone. Figure 2A (curve a) shows the powder X-ray diffraction (XRD) pattern of the as-synthesized silica. One very intense diffraction peak and two weak peaks were detected. These peaks could be indexed as (10), (11), and (20) and are typical of the reflections of a highly ordered hexagonal mesoporous solid. The d-spacing, calculated from the position of the intense peak (10), gives a repeating distance of 3.62 nm between the pore centers. Complementary to the XRD data, transmission electron microscopy (TEM) reveals mesopores with a channel structure throughout the sample (Figure 2B). The conversion of the densely packed pyrrole moieties within the channels of the obtained silica into conjugated polypyrrole molecular wires was accomplished by using 0.2 g anhydrous FeCl3 as an oxidant in 2 mL chloroform.15 After 30 min reaction (15) Guo, R.; Li, G.; Zhang, W.; Shen, G.; Shen, D. ChemPhysChem 2005, 6, 2025.

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Figure 3. (A) UV/vis absorption spectra of the as-synthesized silica (dash dot) and the formed PPy (solid line); (B)TEM images of PPy fibers after the removal of silica framework (negatively stained by 2% phosphotungstic acid aqueous solution); (C) PPy fiber at high magnification.

time, a brown solid was collected by filtration and dried in the air. As confirmed by XRD and TEM (Figure 2A, curve b, and Figure 2C), the mesostructure of the treated silica was completely maintained. FT-IR study revealed that R-R coupling of the pyrrole units occurred and conjugated PPy was formed in the channels (Supporting Information Figure S1). UV/vis spectroscopy further confirmed the formation of conjugated PPy. The as-synthesized silica does not exhibit any absorption in the range from 350 to 700 nm. After polymerization, the characteristic π-π* absorption of conjugated PPy at 403 nm occurred (Figure 3A). Direct visualization of the encapsulated PPy molecular wires was realized after the removal of the silica framework using HF. Under the inspection of TEM, a large amount of mesofibers were observed (Figure 3B). The length of these fibers is comparable to the particle size of the silica host (Supporting Information Figure S2), suggesting that the channels and the formed PPy run the entire length of the particles. At higher magnification, the TEM image reveals that each single PPy fiber is composed of numerous discernible, highly aligned unidirectional molecular wires (Figure 3C), not disordered as expected. This result indicates that, probably due to the existence of micropores in walls separating mesoporous channels, the formed PPy nanowires were partially cross-linked during the polymerization reaction. Due to the spatial confinement of the silica framework, the PPy chains in the channels were elongated and straight, leading to increasing π-conjugation length. Advantageously, after the polymerization process a considerable amount of iron complexes remained in silica sample as proved by energy-dispersive X-ray spectroscopy (Supporting Information Figure S3), which could Langmuir 2009, 25(14), 8235–8239

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Figure 4. (A) TEM image of carbon nanowires after the removal of silica shell; (B,C) HRTEM of carbon nanowires from different detection directions; (D) TEM image of carbon nanowires obtained from the infiltration of ionic liquid ([C12mim]Br) into the silica channels followed by the removal of silica framework; (E) TEM image of the single carbon nanowire with a diameter of about 0.28 nm.

be conveniently used to enhance the formation of graphite during the carbonization process. Therefore, in our case, it is found that the PPy molecular wires in the presence of iron complexes can form graphitic structures at a low temperature (∼800 °C). Figure 2A (curve c) and Figure 2D show the XRD pattern and TEM image of the silica sample pyrolyzed at 800 °C for 5 h under N2 atmosphere, indicative of good preservation of the ordered channel structure. Extraction of the mesoporous supports with diluted HF solution (2%) indeed affords massive black carbon fibers with several-micrometer lengths (Figure 4A). In the highresolution TEM (HRTEM) image of these fibers, bundles containing numerous aligned ultrathin nanowires were clearly observed (Figure 4B). The average spacing between adjacent nanowires is about 3.7 A˚, being different from the typical distance of a well-ordered stacking of graphite layers (3.4 A˚).16 The formed carbon nanofibers hold them together and formed ordered structures (Figure 4B,C). We detected the feature peak for the ordered structure at 24.8° from XRD measurements (curve d in the inset of Figure 2A), which is consistent with the calculated value (24.6°) based on the repeating distance of 0.37 nm between the centers of adjacent carbon fibers. Top-view TEM characterization of these objects exhibits nanowire array structure (Figure 4C). Moreover, like the as-synthesized silica as well as the silica after the in situ polymerization of pyrrole units, the silica sample after carbonization displays only the typical broad diffraction of the amorphous structure of the silica at 2θ = 22°17,18 (16) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183. (17) Zhu, H.; Lee, B.; Dai, S.; Overbury, S. H. Langmuir 2003, 19, 3974. (18) Inagaki, S.; Guan, S.; Ohsuna, T.; Terasaki, O. Nature (London) 2002, 416, 304.

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Figure 5. Nitrogen adsorption isotherms of as-synthesized silica (9), carbonized silica (O), and calcined silica (2). Inset is the corresponding pore-size distribution curves. The adsorption amount of as-synthesized silica was magnified by 10 times for clarity. Table 1. Nitrogen Adsorption-Desorption Isotherm Data and the dSpacing Calculated from the (100) X-ray Diffraction Peak of the Silica Samplesa

silica sample

nitrogen adsorption-desorption isotherm

XRD pattern

SBET/m2 g-1 Vt/cm3 g-1 DBJH/nm

d(10)/nm

as-synthesized silica 11.5 0.03 3.62 carbonized silica 738.2 0.43 2.54 3.40 calcined silica 1006.2 0.67 2.4 3.37 a SBET, BET specific surface area; Vt, total pore volume; DBJH, pore diameter calculated using the BJH method.

(inset in Figure 2A), and the characteristic diffraction of the graphite structure at 2θ = 26.4°(inset in Figure 2A) was not detected.19,20 These results clearly disclose that the formed carbon materials are really uniform ultrathin nanowires rather than lamellar graphitic layers. These carbon nanowires have a diameter of about 2.8 A˚, and, to the best of our knowledge, are the thinnest reported so far. Interestingly, we noticed that the carbon bundles (Figure 4A) resemble the afore-mentioned PPy fibers in appearance (Figure 3B). It seems very likely that each PPy molecular wire could be individually converted into the corresponding carbon nanowire. Like other nanostructured carbon materials, the prepared carbon wires tend to aggregate together. The attempt to achieve isolated carbon wires by using different methods seems difficult. Finally, however, we found that, instead of direct removal of silica framework using HF, the infiltration of ionic liquid ([C12mim]Br) into the silica channels followed by the removal of silica shell can afford to some extent the dispersed carbon nanowires (Figure 4D). In addition, well-defined single carbon wires with a diameter of 0.28 nm were observed (Figure 4E). During our work, we also tried to find the carbon nanofibers in channels before the silica template was removed. Unfortunately, all attempts to see such an arrangement failed, probably due to the low contrast between the thin carbon fiber and silica framework as well as the very small thickness of the carbon fibers. In addition, a size reduction between the polypyrrole fibers and the resulting carbon bundles was observed. This phenomenon originates from the formation of more compact structures accompanied by dehydrogenation, denitrogenation, and aroma(19) Wakabayashi, K.; Pierre, C.; Dikin, D. A.; Ruoff, R. S.; Ramanathan, T.; Brinson, L. C.; Torkelson, J. M. Macromolecules 2008, 41, 1905. (20) Jeong, H.; Lee, Y.; Lahaye, R. J. W.; Park, M.; An, K. J. Am. Chem. Soc. 2008, 130, 1362.

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Figure 6. Raman spectrum (A) and I-V curve (B) of the fabricated carbon nanofibers after the removal of silica shell.

tization in carbonization process. A similar result was also observed in previous works. N2 adsorption-desorption isotherms were measured on the as-synthesized, the carbonized, and the calcined silica samples (Figure 5). Summary of the BET surface area, total pore volumes, and pore diameters of these samples is given in Table 1. The as-synthesized silica exhibits very low BET surface area (11.5 m2/g) and pore volume (0.03 cm3/g), implying a significantly high degree of pore loading of the surfactant molecules. After the carbonization, the silica displayed clear N2 adsorptiondesorption isotherms with a distinct capillary condensation step and a narrow pore size distribution (inset in Figure 5). In addition, the BET surface area and pore volume increased considerably to 738 m2/g and 0.43 cm3/g. Nevertheless, compared to the calcined silica, which exhibited the typical BET surface area (1006 m2/g) and pore volume (0.67 cm3/g) of mesoporous silica, the carbonized silica sample showed a medium BET surface area and somewhat smaller pore volume. This slightly decrease in the pore volume clearly indicates the incomplete filling with carbon nanowires inside the silica channels and that there is a lot of free space between nanowires and pore walls of silica. These results are consistent with the formation of ultrathin nanofibers within mesoporous channels (Figure 1). Raman spectrum of the obtained nanofibers reveals two distinct peaks at 1350 cm-1 and 1590 cm-1 (Figure 6A), confirming the presence of amorphous and graphitic carbon domains.21 The 1590 cm-1 band is associated with a graphitic carbon with sp2 electronic configuration. The band at 1350 cm-1 arises from polycrystalline graphite, and can be attributed to diamondlike carbon atoms with sp3 configuration. The area ratio (Asp3/Asp2) of the 1350 cm-1 band to the 1590 cm-1 band is 0.83, indicating that a higher degree of graphitization occurred in our system. (21) Jang, J.; Bae, J. Angew. Chem., Int. Ed. 2004, 43, 3803.

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In this work, we found that the area ratio of Asp3 to Asp2 decreased with increasing carbonization temperature (Supporting Information Figures S4-S5). Thus, the carbonization of the PPy molecular wires at higher temperature can lead to further graphitized structures. Nevertheless, it is still difficult for us to define the microstructure of the formed carbon nanofibers. How can the carbon atoms are arranged in a carbon nanofiber with diameter of 0.28 nm? Now, intensive investigation is ongoing in our laboratory to understand this question. We hope that in the near future a structure model can be given to illustrate the arrangement. All attempts to measure the conductivity of single carbon wire have failed, due to the difficulty isolating the single carbon wires. Thus, a nanosheet (320 μm  320 μm  40 μm) composed of the fabricated carbon nanowires was prepared, and its electrical transport properties was studied by a two-terminal current-voltage measurement system (Supporting Information Figure S6). I-V measurements revealed a good linear nature (Figure 6B), suggesting ohmic behavior. The electrical conductivity calculated from I-V curve is 1.5 S cm-1, which is much higher than the polymer-derived carbon nanowires prepared using zeolite or mesoporous templates.22,23

Conclusions We developed an efficient strategy for producing uniform carbon nanofibers with diameters below 1 nm using a confined self-assembly approach. The crucial point of this strategy is the (22) Wu, C.; Bein, T. Science 1994, 266, 1013. (23) Hudson, M. J.; Peckett, J. W.; Sibley, C. S.; Harris, P. J. F. Ind. Eng. Chem. Res. 2008, 47, 2605.

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utilization of a special surfactant for the construction of a mesoporous template, which functions both as structure-directing agent and carbon source. Besides the capability of producing ultralong and thin carbon fibers inaccessible by the previously reported approaches, the method described here has many advantages such as the direct use of residue iron complex as catalyst for carbonization and no requirement of conventional tedious infiltration process of carbon source into small channels. Additionally, preliminary experiments showed that the described strategy is extendable. By designing an appropriate surfactant, it is also possible to fabricate finely structured carbon network and ultrathin graphitic sheets through the construction of the corresponding cubic and lamellar mesostructured templates. Systematic investigation is ongoing in our laboratory. Acknowledgment. This work was supported by the NSFC (20473044, 20533050 and 50673048), 973 (2006CB806200), and Transregional Project (TRR6). We thank Prof. Wenping Hu and his co-workers at the Institute of Chemistry, Chinese Academic Science for the current-voltage measurements. Supporting Information Available: FT-IR spectra of PyC12MIM+Br- monomer and PPy fibers after the removal of silica framework (Figure S1). EDS of as-synthesized silica after polymerization process and carbon nanofibers after removal silica shell (Figure S2). Raman spectra and TEM images of the carbon nanofibers prepared at different temperatures (Figure S3-S4). Schematic array and photograph of the used current-voltage measurement system (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org.

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