A Self-Template Strategy for the Synthesis of Mesoporous Carbon Nanofibers as Advanced...

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Wei Li, Fan Zhang, Yuqian Dou, Zhangxiong Wu, Haijing Liu, Xufang Qian, Dong Gu, Yongyao Xia, Bo Tu,* and Dongyuan Zhao*

A Self-Template Strategy for the Synthesis of Mesoporous Carbon Nanofi bers as Advanced Supercapacitor Electrodes

Over the past few decades, nanostructured carbon materials have attracted tremendous attention because of their fascinating properties and potential applications in the fi elds of electronics, adsorption, water purifi cation, catalysis, etc. [ 1–3 ] Considering the ever-increasing demands for electrical energy storage, it is both urgent and crucial to develop simple and effi cient tech-niques to create new porous carbon materials with both high surface areas and desirable architectures, since such materials are essential for electrochemical capacitors (ECs). [ 4–8 ]

So far, signifi cant advances have been achieved in the prepara-tion of porous carbon materials with tailored architectures. [ 9–15 ] However, it is diffi cult but desirable to precisely and simultane-ously manipulate the pore texture and the morphology, espe-cially with regard to construction of 1D nanostructures with a 3D interconnected mesoporous texture. It is well known that such 1D architectures not only provide shortened pathways for electron transport, but also, remarkably, facilitate the penetra-tion of electrolyte from the direction perpendicular to the lon-gitudinal axis of the fi ber, as well as having high ion-accessible surface areas. Thus, the unique structure gives rise to excel-lent performance as electrode materials for ECs. [ 4 ] Therefore, many studies have focused on exploiting state-of-the-art porous carbon nanofi bers (CNFs) to build new devices with improved functions. To this end, the electrospinning technique has been widely adopted to fabricate porous CNFs through addition of porogens or subsequent activation processes. [ 16–19 ] However, it is very diffi cult to achieve fi ne control of the properties of the resulting carbon materials in terms of surface area, pore size, and surface functionality. Recently, inspired by confi ned syn-thesis in channels of anodic alumina, extensive research efforts have been devoted to producing porous CNFs by carbonizing proper precursors within suitable templates. [ 20–24 ] However, this is an obviously laborious and industrially unfavorable method with multiple steps and time-consuming procedures. To date, only a few methods have been reported for the direct syn-thesis of porous CNFs through carbonization of 1D polymers,

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W. Li , F. Zhang , Y. Q. Dou , Z. X. Wu , H. J. Liu , X. F. Qian , D. Gu , Prof. Y. Xia , Prof. B. Tu , Prof. D. Y. Zhao Department of Chemistry Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials and Laboratory of Advanced Materials Fudan University Shanghai 200433, P. R. China Fax: + 86–21-5163–0307 E-mail: [email protected]; [email protected]

DOI: 10.1002/aenm.201000096

however, fi bers so produced possess small pore sizes and low surface area. [ 25,26 ] It is still a great challenge, but of great sig-nifi cance for electrochemical systems, to develop a facile, eco-nomical, and template-free method to conveniently synthesize mesoporous carbon nanofi bers with high surface area, large pore size, and plentiful surface functionalities.

Herein, we report a novel self-template strategy for the syn-thesis of mesoporous carbon nanofi bers through a solution-growth process, using ethylene glycol (EG) as the carbon pre-cursor and Zn(CH 3 COO) 2 as the structural constructor as well as the porogen, wherein the initially formed zinc glycolate acts as the build-in template during the subsequent carboni-zation process. As the low cost of Zn(CH 3 COO) 2 and the easy removal of ZnO, this method avoids the requirements for hard templates, [ 20 ] complicated activations, [ 16 ] aggressive chemi-cals [ 22 ] and specifi c precursors. [ 25 ] Moreover, it would be indus-trially feasible, owing to its ease and low cost of production. The obtained mesoporous carbon nanofi bers possess a well-designed 1D nanostructure and a 3D interconnected mesopo-rous texture, uniformly sized mesopore and high surface area, as well as plentiful oxygen functional groups on the surface, which give rise to excellent performances as an electrode mate-rial for ECs.

The formation process of the mesoporous carbon nanofi bers is based on a solution-growth pathway ( Scheme 1 ). Firstly, zinc ions, as the structural constructor, are reacted with EG at 150 ° C for 1 h to co-assemble into zinc glycolate fi bers (Scheme 1 a and b) with diameters of ∼ 280 nm and lengths up to 30 μ m (Figure S1, Supporting Information). The fi bers are very stable with no evident change after reaction overnight, and the initial reaction temperature can be changed from 140 to 170 ° C. A strong dif-fraction peak around 10 ° can be observed in the powder X-ray diffraction (XRD) pattern of the zinc glycolate (Figure S2, Supporting Information), indicating that it is highly crystalline, well consistent with the typical feature of metal glycolates. [ 27 , 28 ] A well-resolved bond at 66.7 ppm in the 13 C solid NMR spectrum (Figure S3, Supporting Information) reveals a more deshielded environment of CH 2 unit as compared to the standard bond of pure EG, suggesting that the acetate groups are replaced by EG units. [ 28 ] This is further supported by FTIR analysis (Figure S4, Supporting Information). The peaks at 2920 and 2860 cm − 1 corresponding to CH 2 bands, and those at 1070 and 1024 cm − 1 attributed to C-O and C-C vibration, agree well with assign-ment to EG. In addition, the new peak at 440 cm − 1 reveals the presence of Zn-O band. The thermogravimetric analysis (TGA) curve under air shows ∼ 38 wt% weight loss at 360 ° C from the organic components (Figure S5, Supporting Information), sug-gesting a molar ratio of approximately 1:1 for Zn:EG. All these

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Scheme 1 . Schematic illustration of the formation process of the mes-oporous carbon nanofi bers (CNFs) and mesoporous ZnO nanotubes.

Figure 1 . a) The SEM image of the ZnO/carbon composites (inset is the high-magnifi cation SEM image, clearly showing the high quality fi bers). b) The FESEM image of a single ZnO/carbon composite fi ber. c, d) TEM images and the corresponding SAED pattern taken from individual fi ber (inset d) of the ZnO/carbon composites. e) The HRTEM image of ZnO/carbon composites, the arrows clearly show the ZnO crystals embedded in the carbon matrix. f) The STEM image and cross-sectional composi-tional line profi les of a single ZnO/carbon composite fi ber.

results demonstrate that the zinc glycolate has a linear poly-meric molecular structure and self-assemble into ordered fi bers through van der Waals interactions. [ 27 ]

When being subjected to the thermal treatment in Ar at 600 ° C, zinc glycolate acted as the build-in template and was transformed into the ZnO/carbon composites (Scheme 1 c). Scanning electron microscopy (SEM) images show that the composites have high-quality fi brous morphology with diam-eters of ∼ 240 nm and lengths of ∼ 30 μ m ( Figure 1 a). During the thermal treatment process, zinc ions in-situ crystallized into zinc oxides, whilst the organic units were carbonized into amorphous carbon frameworks as evidenced by the XRD pat-tern (Figure S6a, Supporting Information). All diffraction peaks are well consistent with the standard values for the bulk wur-tzite-type ZnO (JCPDS card 36–1451). The presence of Zn, O and C elements in the composites was also confi rmed by the energy dispersive X-ray spectrum (EDX) (Figure S7a). The TGA curve shows that the carbon content in the composites is ∼ 9 wt% (Figure S8, Supporting Information). The carboniza-tion yield, calculated on the basis of the organic components is as high as ∼ 16.8%. The surface of the composites is decorated with a large number of particles with diameters of 10–20 nm (Figure 1 b), indicating a phase separation between ZnO and carbon at a high temperature. Transmission electron micro-scopy (TEM) image further shows that the composites retain the fi brous morphology with a high density of large nanopar-ticles (10–20 nm) dispersed on the surface (Figure 1 c and d). The corresponding selected-area electron diffraction (SAED) pattern (Figure 1 d, inset) reveals the polycrystalline feature of ZnO. The high-resolution TEM (HRTEM) image clearly reveals some of small ZnO crystals are homogeneously dispersed in the carbon matrix with particle sizes of ∼ 4 nm (Figure 1 e). The external particle sizes (10–20 nm) are much larger than that of the embedded ones ( ∼ 4 nm), which should be ascribed to the

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severe transfer and aggregation of zinc species from inward to outward during the thermal treatment process, thus forming a ZnO shell. Meanwhile, since the highly uniform distribution of zinc ions within the zinc glycolate, the initial carbonized matrix effectively restricts the growth of the crystallite, which leads to the formation of the smaller nanocrystal through the in-situ crystallization. As a result, the scanning-TEM image (Figure 1 f) of a single fi ber clearly shows different electron contrasts, with the dark and light areas due to zinc and carbon-enriched compositions, respectively. Moreover, the corresponding cross-section elemental line scanning reveals much higher intensity of zinc on the shell than that of the central part, while the oppo-site trend is observed for carbon (Figure 1 f).

Mesoporous carbon nanofi bers can be obtained after leaching ZnO crystals (Scheme 1 d). The XRD pattern (Figure S6b, Supporting Information) displays two broad diffraction peaks at 2 θ values of 25 ° and 44 ° for amorphous carbon, indicating the carbon species is present in the resulting CNFs. The complete removal of Zn was further confi rmed by the EDX (Figure S7b, Supporting Information) and X-ray photoelectron spectrum (XPS) (Figure S9, Supporting Information) analyses. The mes-oporous CNFs retain the originally fi brous morphology with diameters of ∼ 180 nm and lengths of up to 20 μ m, and are entangled together without any by-products ( Figure 2 a). The decreased diameter of the CNFs further demonstrates the core-shell structure of the ZnO/carbon composites with a ZnO shell thickness about 30 nm. Etching the ZnO shell generates a large portion of holes randomly distributed on the surface with very

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Figure 2 . a) The SEM image of the mesoporous carbon nanofi bers (CNFs) (inset is the high-magnifi cation SEM image, clearly showing the high quality fi bers). b) The FESEM image of a single mesoporous CNFs. c,d) TEM images of mesoporous CNFs. e,f) HRTEM images of the sur-face of mesoporous CNFs, the arrow clearly shows the large and small mesopores coated with very thin carbon sheets, respectively.

Figure 3 . Nitrogen sorption isotherms of the mesoporous carbon nanofi bers. The inset (a) is the corresponding pore size distribution curve and cumulative pore volume curve, (b) is the schematic cross-section representation of the mesoporous carbon nanofi bers with 3D intercon-nected mesopore texture.

thin carbon walls (Figure 2 b). The TEM image also indicates the well-defi ned CNFs and some of them are bundled together due to the bundled glycolate fi bers (Figure 2 c). Many 3D inter-connected mesopores are homogeneously located to construct the whole fi ber (Figure 2 d). The large pores on the surface have diameters of 10–20 nm (Figure 2 e), which can be effectively covered by electrolyte ions, thus facilitating their penetration. The HRTEM image (Figure 2 f) reveals that there are small mesopores ( ∼ 4 nm) connected with the large ones and the pore walls consist of amorphous carbon with disordered graphene sheets, which provide a minimized inner-pore resistance, leading to a high ion-accessible surface area.

The nitrogen sorption isotherms of the mesoporous carbon nanofi bers ( Figure 3 ) exhibit a combined type-I/IV sorption iso-therms with a BET surface area of 1725 m 2 /g, which is much higher than those of the porous CNFs previously reported (10–1140 m 2 /g). [ 16–26 ] It should be noted that only low detect-able surface area (82 m 2 /g) was observed for ZnO/carbon com-posites (Figure S10, Supporting Information), clearly indicating that most of the surface area is produced upon the removal of ZnO crystals. The total pore volume is about 2.7 cm 3 /g with the contribution of 0.5 cm 3 /g from the micropore (Figure 3 , inset a), revealing the presence of excessive micropores. The pore size distribution curve (Figure 3 , inset a) derived from the adsorp-tion branch of the isotherm using nonlocal density functional theory (NLDFT) method clearly shows bimodal-pores with detectable sizes of 1.1 and 3.4 nm. The micropores at the mean value of 1.1 nm are generated during the carbonization process, which offer a good charge accommodation. While the mes-opores of 10–20 and 3.4 nm are contributed from the removal of the ZnO crystals dispersed on the surface and embedded in the carbon matrix, respectively. A schematic cross-section rep-resentation of mesoporous CNFs with 3D interconnected mes-opore texture is presented in Figure 3 (inset b). Those results agree well with above data derived from the SEM/TEM meas-urements (Figure 1 and 2). Therefore, zinc species not only act


as the structural constructor, but also as an in-situ porogen. The chemical composition of the CNFs is 86.5% C, 9.0% O and 2.5% H from the elemental analysis, well in accordance with the XPS result (Figure S9, Supporting Information, 90.0% C, 10.0% O), showing the moderate oxygen functional groups in the fi bers.

On the other hand, the removal of organic components by combustion in air led to the formation of the mesoporous ZnO nanotubes (Scheme 1 e) with lengths of ∼ 25 μ m, and the outer and inner diameters of ∼ 190 and ∼ 90 nm, respectively (Figure S11, Supporting Information). Such unique structure further demonstrates that zinc species transfer and aggregate from inward to outward during the thermal treatment process. The diameter of the nanotubes ( ∼ 190 nm) is smaller than that of the composites ( ∼ 240 nm), indicating the presence of large structural shrinkage without the in situ carbon support. Therefore, the initial carbonized matrix can effectively restricts the transfer of zinc species and the growth of the crystallite, leading to the unique structure of the ZnO/carbon compos-ites (Figure 1 ). ZnO crystals were dispersed homogeneously, not only on the surface (10–20 nm) but also inside the carbon matrix ( ∼ 4 nm) of the composites. When the ZnO crystals were removed via using HCl aqueous solution, the mesopores were formed in the CNF matrix. Thus, mesoporous carbon nanofi bers can be successfully fabricated by this novel self-template strategy.

The electrochemical capacitive performances of the mesopo-rous carbon nanofi bers were evaluated by cyclic voltammetry (CV) and galvanostatic charge/discharge technique. The CV curve ( Figure 4 a) measured at a sweep rate of 5 mV · s − 1 displays relatively good rectangular profi le, showing a typical charac-teristic of double-layer capacitance. Compared to that of com-mercial activated carbons (ACs), the CNFs present a couple of wide and vague redox peaks at ∼ −0.4 V, which can be ascribed to the redox reactions of the surface oxygen groups. [ 29,30 ] These oxygen groups can not only provide pseudocapacitance, but


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Figure 4 . a) Cyclic voltammmograms of the mesoporous carbon nanofi bers and commercial activated carbons (ACs, a product with a pore diameter of 2.2 nm, a surface area of ∼ 2071 m 2 /g and the micropore surface area of ∼ 477 m 2 /g) at a scan rate of 5 mV · s − 1 in 6.0 M KOH solution. b) Galvanostatic charge/discharge curves of the mesoporous carbon nanofi bers at different current densities.

also enhance the surface wettability by electrolyte and ensure a complete utilization of the exposed surface for charge storage. [ 14 ] The specifi c capacitance is up to 280 F · g − 1 at a current density of 0.5 A · g − 1 (Figure 4 b), much higher than 202 F · g − 1 for the commercial ACs. Moreover, the capacitance of the mesoporous fi bers can still remain 200 F · g − 1 even at a higher current density of 5 A · g − 1 . Furthermore, cyclic tests show that 91% of its initial capacity can be maintained even after 1000 cycles (Figure S12, Supporting Information). The superior performance should be attributed to the unique architecture of the CNFs. Firstly, the 1D nanostructures provide a shortened path for electron transport and electrolyte penetration, which was confi rmed by electro-chemical impedance spectroscopy (EIS) (Figure S13, Supporting Information). The width of the semicircle impedance loop in the high frequency region refl ects resistance to mass transfer/diffusion rate of ions through the mesoporous structure of the carbon, and this resistance to electrolyte transport is observed to be smaller for mesoporous CNFs than ACs. [ 31 ] Meanwhile, the 3D interconnected pore texture combined with the large surface area offers a good charge accommodation and ability to handle high current loads. In addition, the well-defi ned oxygen functionalities not only enhance the surface wettability, but also afford high pseudocapacitance. The electrochemical perform-ance of the CNFs was also investigated using an organic elec-trolyte (Figure S14, Supporting Information), which exhibits a specifi c capacitance of ∼ 168 F · g − 1 at a sweep rate of 5 mV · s − 1 , higher than pioneered capacitors. [ 10 , 24 ] All these results demon-strate that the mesoporous carbon nanofi bers are very prom-ising as an advanced electrode for ECs.

In conclusion, we have demonstrated a facial self-template strategy for the synthesis of the mesoporous carbon nanofi bers based on a solution-growth pathway, using ethylene glycol (EG) as the carbon precursor, Zn(CH 3 COO) 2 as the structural con-structor as well as the porogen, and initially formed zinc gly-colate fi bers as the built-in template during the subsequent carbonization process. The mesoporous CNFs exhibit a high surface area of 1725 m 2 /g, a 3D interconnected pore texture, a large pore size of 3.4 nm, and fi nely developed oxygenated sur-face functionalities ( ∼ 9 wt%). The obtained fi bers show excellent supercapacitance performance in both aqueous ( ∼ 280 F · g − 1 ) and organic electrolytes ( ∼ 168 F · g − 1 ), and a good cyclic stability. This is a typically new type of synergistic effect toward the

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, WeinhAdv. Energy Mater. 2011, 1, 382–386

development of advanced carbon electrodes for high-performance electrochemical capacitors. Overall, our results may provide new insights for convenient synthesis of novel porous carbon materials as well as well-dispersed nanoparticles/carbon com-posites with desired nanostructures and architectures.

Experimental Section Preparation of mesoporous carbon nanofi bers

(CNFs): In a typical procedure, anhydrous zinc acetate (2.0 g) was added to EG (45 ml) and the mixture was stirred at 150 ° C for 90 min, which was air-proofed by a liquid seal setup. The slurry was collected by centrifugation, followed by calcination

at 600 ° C for 2 h in Ar. The heating rates were set at 5 and 2 ° C/min below and above 400 ° C, respectively. The composites were immersed into a 2.0 M aqueous HCl solution (20 ml) with stirring overnight. Mesoporous carbon nanofi bers were obtained after being washed with water and ethanol, and then dried at room temperature. To obtain mesoporous ZnO nanotubes, the slurry was collected by centrifugation, followed by calcination at 300 ° C with a heating rate of 3 ° C/min for 2 h in air. Electrochemical measurements : The electrode was prepared by mixing active materials (70 wt%), carbon black (20 wt%), and poly (tetrafl uoroethylene) (PTFE) binder (10 wt%) well. In the case of using 6 M KOH solution as the electrolyte, the above slurries were pressed onto a nickel foam. Cyclic voltammetry (CV), electrochemical impedance spectroscopy and charge-discharge tests were carried out using a three-electrode glass cell, in which platinum and Hg/HgO electrodes (0.052 V vs. a normal hydrogen electrode, NHE) were used as the counter and reference electrodes, respectively. The experiments were performed using an electrochemical analyzer, CHI 606B under ambient conditions. In both cases, the typical mass load was about 5 mg/cm 2 . In the case of using 1.0 M (C 2 H 5 ) 4 NBF 4 in propylene carbonate (PC) (Honeywell Corp.) as an organic electrolyte, the slurries were coated on metal foil (Al foil and steel foils were employed as current collectors for positive and negative, respectively). The electrochemical measurements were conducted in a CR2016 coin-type cell, and carried out on a Solartron Instrument Model 1287 electrochemical interface controlled by a computer.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported by the NSF of China (20871030, 20721063, and 20821140537), the State Key Basic Research Program of the PRC (2009CB930400), Shanghai Leading Academic Discipline Project (B108), and Science & Technology Commission of Shanghai Municipality (08DZ2270500). We greatly appreciate the fi nancial supports from from Delta Environmental & Educational Foundation (Taiwan) and Fudan Graduate Innovation Funds.

Received: December 25, 2010 Revised: February 27, 2011

Published online: April 15, 2011

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