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J. Phys. Chem. C 2008, 112, 926-931
Preparation and Characterization of Porous Carbon Nanofibers from Thermal Decomposition of Poly(ethylene glycol) Chao-Wei Huang, Sheng-Cheng Chiu, Wang-Hua Lin, and Yuan-Yao Li* Department of Chemical Engineering, National Chung Cheng UniVersity, Chia-Yi 62102, Taiwan, Republic of China ReceiVed: October 7, 2007
Porous carbon nanofibers (CNFs) were synthesized on wafer uniformly by thermal decomposition of poly(ethylene glycol) with the presence of nickel catalyst at 600 °C under nitrogen atmosphere. High purity porous CNFs with diameters of 40-60 nm and a few micrometers in length demonstrated a unique character of graphitic structure and open-edge structure of mesopores. In comparison with other methods for the formation of porous CNFs such as activation of carbon fibers or template method, our method provides a one-step process in which the pores can be created while the fibers grow. Characterizations of porous CNFs found that the disordered graphene layers were generated from polycrystalline Ni catalysts, and the stacking of disordered graphene layers created 3-6 nm mesopores with open edges. Porous CNFs with open edges, which is a different morphology from that of conventional activated CNFs, were suggested to be a good medium for mass transport, while graphene layers may serve as a good electrical conductive medium for the applications of electrodes, catalyst supports, and adsorption.
Introduction Porous carbon nanomaterials such as ordered mesoporous carbon,1-8 porous carbon nanofibers (CNFs),9 and activated carbon nanofibers (ACNFs)10-19 have attracted much attention because of their unique structure, high surface area, and physical and chemical properties. These properties lead to a wide range of applications such as adsorbents,1,19 catalyst supports,4 gas storage,5,6 sensors,20,21 and electrode materials.7-13,22,23 In general, porous carbon nanomaterials are fabricated using templates1-8 and physical or chemical activation of carbon materials,10-19 while most mesoporous carbon materials are formed using a template synthetic process. The procedure of the template method is generally as follows: (a) incorporation of carbon precursor into the pores of the mesostructure material (template), (b) carbonization, and (c) removal of the mesostructure template using hydrofluoric acid or NaOH solution, leaving a porous solid having pores that are similar in size and shape to the original template. However, the template synthetic method is complex and time-consuming because the template is required to be removed by difficult processes. Porous carbon nanomaterials with one-dimensional structure are usually fabricated by physical or chemical activation of CNFs. Physical activation commonly uses steam or CO2 as the activating agent for CNFs to create pore texture.10,11,16-19 Chemical activation with alkali compounds such as KOH and NaOH is a well-known technique to activate carbon materials with porous texture.12-15 The ACNFs formed by activation usually create a massive amount of micropores and a relatively small amount of mesopores. For the adsorption applications on macromolecules (large molecules), it generally requires mesoporous materials as adsorbents. For the electrode application, a higher graphitization degree of carbon materials is required.24 As a result, one-dimensional carbon nanomaterials with meso* Corresponding author. Phone: +886-5-2720411, ext. 33403. Fax: +886-5-2721206. E-mail:
[email protected].
porous and graphitic structure are required for the above purposes so that the materials will be a promising candidate for electrode materials.25 Therefore, the aim of this study was to design and fabricate porous CNF materials using a one-step synthesis method without the assistance of activation or template. Thermal decomposition of a mixture containing poly(ethylene glycol) (PEG) as the carbon source and nickel chloride (NiCl2) as the catalytic precursor was carried out to study the feasibility for the synthesis of porous CNFs. It is expected that the porous CNFs possess a characteristic of mesoporous and graphitic structure. The effect of catalyst and temperature on the stacking of the graphene layer was investigated. A nitrogen adsorption method was used to examine the surface texture and pore size distribution. The synthesized materials were characterized by high-resolution transmission electron microscopy (HR-TEM), field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and Raman spectra. Experimental Methods PEG (MW: 8000, Sigma) and NiCl2 (purity 97%, RDH) were employed as the carbon source and the catalytic precursor, respectively, for the formation of porous CNFs. The PEG was dissolved in aqueous catalytic solution (NiCl2, 0.05 M) and then well-mixed by a magnetic stirrer in a 60 °C water bath for 1 h. The mixture was then applied on a P-type silicon wafer and dried at room temperature until the mixture reached a constant weight. The catalyst-containing polymeric mixture was placed in a quartz boat and loaded into a tubular furnace. Thermal treatment of the mixture was carried out with a ramping rate of 15 °C/min to 400 °C under a nitrogen atmosphere and kept at this temperature for 1 h for complete decomposition of the mixture. The temperature was then elevated to 600 °C and kept for 1 h for the synthesis of porous CNFs.
10.1021/jp709795d CCC: $40.75 © 2008 American Chemical Society Published on Web 01/05/2008
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Figure 1. (a) SEM and (b) HR-SEM images of porous CNFs prepared by thermal decomposition of the PEG/NiCl2 mixture.
The morphology of porous CNFs was examined by FE-SEM (Hitachi S-4800), while HR-TEM (Philips Tecnai-20 G-2, Philips Tecnai F20) was employed to understand the crystal structure and graphene layer arrangement of porous CNFs. The collected CNF powder from silicon wafers was dispersed in alcohol by sonication. A drop of well-dispersed CNF solution was applied on a TEM copper grid, and the solvent was allowed to vaporize before being loaded into the TEM chamber. Nitrogen adsorption was performed by an ASAP 2020 (Micromeritics) system at 77 K. The sample was degassed in a vacuum at 200 °C for 4 h prior to the measurements. Pore size and size distribution were calculated by the density functional theory (DFT) method.25 The Raman spectrum analysis was carried out at room temperature using a 3D Nanometer Scale Raman PL Microspectrometer (Tokyo Instruments, Inc., a 632.8 nm HeNe laser). The crystallographic data collected by XRD (Shimadzu, XRD-6000) used Cu KR radiation from the rotating anode X-ray source. TGA (Perkin-Elmer, Diamond TG/DTA) was used to investigate the thermal stability of porous CNFs. Approximate 10 mg of the material was heated in an open platinum pan at a ramping rate of 10 °C/min to 800 °C under air atmosphere. Results and Discussion The thermal decomposition analysis of the mixture (PEG/ NiCl2) was conducted by thermogravimetry-differential scanning calorimetry-mass spectrometry/derivative thermogravim-
Figure 2. (a) TEM and (b) HR-TEM images of porous CNFs.
etry and reported in our previous study26 to form platelet graphite nanofibers (PGNFs). In general, five primary gases (CO, CO2, H2O, CH4 and H2) were formed from thermal decomposition of the mixture. The produced gases reacted with the catalyst to form carbon nanomaterials during the thermal decomposition process. As can be seen in Figure 1a, high-purity porous CNFs 40-60 nm in diameter and several micrometers in length can be fabricated at 600 °C. The HR-SEM image in Figure 1b reveals that the porous CNFs have rough surfaces and high porous structures, which are different from the ACNFs made by the oxidation process.10-19 The TEM image in Figure 2a shows obviously that there was a large amount of pores in the fiber, and the pore sizes were about 3-6 nm. The HR-TEM image in Figure 2b demonstrates that, in the porous CNFs, the orientation of the graphene layers were disordered, which led to the formation of a large portion of voids (spaces/pores) in the fiber. The pore structure is different from that of conventional ACNFs or ordered mesoporous carbon. The pore size of porous CNFs is mainly in the range of mesopores but not in a wide-range distribution from micropores to macropores as ACNFs. In addition, the graphitization degree of the fiber is high, which is beneficial for the use of electrodes or electrically conductive applications.24 Figure 3a shows that the porous CNFs were grown on catalysts from multiple directions. This indicates
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Figure 3. (a) SEM image of porous CNFs (four-directional growth from the catalyst). (b) HR-TEM image of the interface between the catalyst and the porous CNF. (c) EDS of the catalyst particle in panel d. (d) SAED from the interface between catalyst particle and the porous CNF.
that carbon species deposited simultaneously on the different facets of the catalyst. Figure 3b shows the interface of graphene layers and the catalyst. The rough surface of Ni-nanocatalyst was suggested to be the evidence of the poor crystallinity of catalyst, which causes the growth of disordered graphene layers and forms the porous CNFs. Kim et al.27 reported that there is a subtle relationship between the degree of ordering in the deposited CNFs and the ability of the metal catalyst particle to undergo a strong interaction with graphite. Rodriguez28 also found that the geometry of a metal catalyst interacting with a graphite support is governed to a large degree by the strength of interaction between the metal atoms and graphite support. The metal catalyst, which has good wetting characteristics with respect to graphite, forms highly graphitic carbon nanostructures, whereas a metal catalyst that exhibits a weaker interaction with graphite fabricates nanofiber structures with many defects. We think that, because of insufficient thermal energy at 600 °C, the catalyst was generated in the form of polycrystalline grains with rough texture. This reduced or disabled the wetting ability of the carbon species and formed the disordered graphene layers with porous structure. Therefore, the polycrystalline catalyst is unable to form highly ordered graphitic carbon nanostructures.
The energy-dispersive X-ray spectroscopy (EDS) data in Figure 3c and the selected-area electron diffraction (SAED) pattern in Figure 3d show that the final form of catalyst was confirmed as nickel nanoparticles with a small amount of doped carbon (NiCx, x ) 0.015). In Figure 3d, intense arc fragments correspond to graphite planes (002), and the catalyst particle has a twin boundary at the (111) plane, which is typical for face-centered-cubic (fcc) structure. The twin structure is usually formed by a lower energy or temperature synthesis process. The growth direction of the nanofibers coincides with the 〈111〉 direction of the FCC structure of NiCx (x ) 0.015), which belongs to the twin planes. In other words, the porous CNF was grown from the (111) plane of NiCx (x ) 0.015). Figure 4 shows the Raman spectrum of the as-produced porous CNF. The two characteristic peaks that lie at about 1330 cm-1 and 1590 cm-1 correspond to the D-band and the G-band, respectively. The D-band is disorder-induced because of defects, dislocations, and lattice distortions in carbon structures.29,30 The G-band is explained as the stretching vibration mode of graphite crystals.31 The relatively peak-integrated intensity of the D-band to the G-band is about 1.6, indicating the faulty long-range ordered crystalline and the existence of disordered carbon
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Figure 4. Raman spectrum of the porous CNFs.
Figure 7. (a) Nitrogen adsorption-desorption isotherms for the porous CNFs. (b) Corresponding pore size distribution of porous CNFs calculated by the DFT method. Figure 5. XRD pattern of the porous CNFs.
Figure 6. TGA of (1) porous CNFs, (2) PGNFs, and (3) MWCNTs.
structure. Figure 5 shows the XRD patterns of the as-produced porous CNFs. All the diffraction peaks have been identified and indexed on the basis of a graphitic crystalline structure and a NiCx (x ) 0.015) fcc structure. The analysis of the above results (Figures 4 and 5) indicated that the as-produced porous CNFs were of graphitic and disordered structure, which is consistent with the result of TEM. The thermal stability of the as-produced porous CNFs was studied by TGA. Figure 6 shows the TGA results of (1) the porous CNFs, (2) PGNFs26 synthesized by the same method at 750 °C and (3) multiwalled carbon nanotubes (MWCNTs) synthesized by the chemical vapor deposition method. As can be seen, the thermal oxidation of
the porous CNFs occurred dramatically at a temperature of about 500 °C, which is lower than the oxidation temperature of PGNFs and MWCNTs. The result is expected because the disordered graphitic structure and edge plane openings of porous CNFs were less thermally stable than the basal plane of MWCNTs and PGNFs. Porous CNFs were burnt off at 600 °C, and only a small amount of catalyst (about 5 wt %) remained. Figure 7a shows the nitrogen adsorption isotherm of the porous CNFs. The result reveals that the isotherm is a Type IV isotherm with an H3 hysteresis loop, which is the characteristic of flat particle aggregates or slit-shaped pores.32 In Figure 7b, the pore size distribution of porous CNFs calculated using DFT analysis shows that the pores centered at 3 nm. The pore size distribution is mainly in 3-6 nm. It is consistent with the pore observation by SEM and TEM images. Surface area studied by the BET equation found that the surface area of the porous CNFs was 302 m2/g. The total pore volume by single point calculation was 0.46 cm3/g, estimated at a relative pressure of about 0.98. Temperature effect on the growth of porous CNFs was investigated. Figure 8 shows the thermal decomposition of the mixture (PEG/NiCl2) at 500 °C, 550 °C, 600 °C, 650 °C, 700 °C, and 750 °C. As can be seen, irregular shape particles with diameters of 50-150 nm were formed at 500 °C (Figure 8a). The particles were identified as the catalyst-encapsulated carbon particles by EDS and TEM. No fiber was found in the sample. We believe that the decomposition temperature of 500 °C does not provide sufficient energy for the growth of carbon fibers. Figure 8b shows as-produced materials at 550 °C. It is clear that a small amount of the curly fibers with rough surfaces were found on the substrate, and the fiber diameters were in the range of 20-100 nm. In addition to the
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Figure 8. Temperature effect on the growth of CNFs at (a) 500 °C, (b) 550 °C, (c) 600 °C, (d) 650 °C, (e) 700 °C, and (f) 750 °C.
fibers, catalyst-encapsulated carbon particles still can be observed. According to the experiments, we think that the formation of catalyst may not be completed at 550 °C for the growth of porous CNFs, so that the produced materials were the combination of catalyst-encapsulated carbon particles and carbon fibers. Figure 8c shows high-purity porous CNFs with a narrow diameter distribution (40-60 nm) synthesized at 600 °C. This is the optimal temperature found for the growth of porous CNFs. In the current study, the porous CNFs grown on a 4 cm × 4 cm grid uniformly can be achieved. If the temperature increased to 650 °C, as shown in Figure 8d, in the case of some fibers, the CNF morphology changed slightly from rough and porous surface to platelet-like surface. This is clearly seen while the temperature increased to 700 °C, as shown in Figure 8e. More PGNFs can be observed on the substrate instead of porous CNFs. An optimal temperature of 750 °C was found for the formation of high-purity PGNFs26 shown in Figure 8f. We found that temperature played an important role on the morphology of catalyst, which then significantly affects the growth of porous CNFs or PGNFs. The transformation of the catalyst shape was from the irregular nanoparticles for the synthesis of porous CNFs at 600 °C to the cubic-like nanoparticles for the synthesis of PGNFs at 750 °C.26 Conclusion We demonstrated the formation of porous CNFs using thermal decomposition of a mixture containing PEG and NiCl2. Without activation or template processing, high-purity porous CNFs with high porosity and uniform mesopores (3-6 nm) can be synthesized by a mild thermal process at 600 °C under a nitrogen atmosphere. Characterizations reveal that the porous CNFs consisted of disordered graphene layers forming a large portion of mesopores with open edges. Porous CNFs with open edges were suggested to be a good medium for mass transport while graphene layers may serve as a good electrical conductive medium for the applications of electrodes, catalyst supports, and adsorption.
Acknowledgment. The financial support of this work by the National Science Council of the Republic of China under Contract No. NSC 96-2120-M-006-006 is gratefully acknowledged. References and Notes (1) Ryoo, R.; Joo, S. H.; Kruk, M.; Jaroniec, M. AdV. Mater. 2001, 13, 677. (2) Kim, T. W.; Park, I. S.; Ryoo, R. Angew. Chem., Int. Ed. 2003, 42, 4375. (3) Li, Z.; Zhang, J.; Li, Y.; Guan, Y.; Feng, Z.; Li, C. J. Mater. Chem. 2006, 16, 1350. (4) Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Nature 2001, 412, 169. (5) Zhou, H.; Zhu, S.; Honma, I.; Seki, K. Chem. Phys. Lett. 2004, 396, 252. (6) Vinu, A.; Ariga, K.; Mori, T.; Nakanishi, T.; Hishita, S.; Golberg, D.; Bando, Y. AdV. Mater. 2005, 17, 1648. (7) Lee, J.; Kim, J.; Lee, Y.; Yoon, S.; Oh, S. M.; Hyeon, T. Chem. Mater. 2004, 16, 3323. (8) Yang, H.; Yan, Y.; Liu, Y.; Zhang, F.; Zhang, R.; Meng, Y.; Li, M.; Xie, S.; Tu, B.; Zhao, D. J. Phys. Chem. B 2004, 108, 17320. (9) Tao, X. Y.; Zhang, X. B.; Zhang, L.; Cheng, J. P.; Liu, F.; Luo, J. H.; Luo, Z. Q.; Geise, H. J. Carbon 2006, 44, 1425. (10) Yang, K. S.; Kim, C.; Park, S. H.; Kim, J. H.; Lee, W. J. J. Biomed. Nanotechnol. 2006, 2, 103. (11) Kim, C. J. Power Sources 2005, 142, 382. (12) Merino, C.; Soto, P.; Vilaplana-Ortego, E.; Gomez de Salazar, J. M.; Pico, F.; Rojo, J. M. Carbon 2005, 43, 551. (13) Kim, S. U.; Lee, K. H. Chem. Phys. Lett. 2004, 400, 253. (14) Yoon, S. H.; Lim, S.; Song, Y.; Ota, Y.; Qiao, W.; Tanaka, A.; Mochida, I. Carbon 2004, 42, 1723. (15) Luxembourg, D.; Py, X.; Didion, A.; Gadiou, R.; Vix-Guterl, C.; Flamant, G. Microporous Mesoporous Mater. 2007, 98, 123. (16) Lim, S.; Hong, S. H.; Qiao, W.; Whitehurst, D.; Yoon, S. H.; Mochida, I.; An, B.; Yokogawa, K. Carbon 2007, 45, 173. (17) Li, P.; Zhao, T. J.; Zhou, J. H.; Sui, Z. J.; Dai, Y. C.; Yuan, W. K. Carbon 2005, 43, 2701. (18) Suzuki, M. Carbon 1994, 32, 577. (19) Li, Y. Y.; Mochidzuki, K.; Sakoda, A.; Suzuki, M. Carbon 2001, 39, 2143. (20) Gavalas, V. G.; Chaniotakis, N. A.; Gibson, T. D. Biosens. Bioelectron. 1998, 13, 1205. (21) Sotiropoulou, S.; Gavalas, V.; Vamvakaki, V.; Chaniotakis, N. A. Biosens. Bioelectron. 2003, 18, 211. (22) Yoon, S.; Lee, J.; Hyeon, T.; Oh, S. M. J. Electrochem. Soc. 2000, 147, 2507.
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