Direct synthesis of ordered mesoporous polymer/carbon nanofilaments with controlled mesostructures

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1 J Porous Mater (9) 16: DOI 10.7/s Direct synthesis of ordered mesoporous polymer/carbon nanofilaments with controlled mesostructures Rong Kou Æ Qingyuan Hu Æ Donghai Wang Æ Vijay T. John Æ Zhenzhong Yang Æ Yunfeng Lu Published online: 11 April 8 Ó Springer Science+Business Media, LLC 8 Abstract One-dimensional mesoporous polymer/carbon nanofilaments with controlled mesostructures have been prepared by an infiltration process of phenolic oligomers/ surfactant into anodized alumina membranes followed by carbonization. Transmission electron microscopy (TEM), nitrogen sorption and X-ray diffraction (XRD) investigations show that as-prepared polymer nanofilaments possess ordered mesoporous structure tunable from circular hexagonal to cubic and concentric lamellar mesostructures. After carbonization, carbon nanofilaments with corresponding circular hexagonal, cubic and concentric lamellar mesoporous structure are obtained. Keywords One-dimensional Mesoporous Polymer Carbon Nanofilaments 1 Introduction One-dimensional carbon nanostructures with high surface area may potentially enhance the device performance in energy storage and catalysis due to its low dimension and R. Kou Q. Hu D. Wang V. T. John Department of Chemical & Biomolecular Engineering, Tulane University, New Orleans, LA 70118, USA Z. Yang (&) State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Science, Beijing 080, P.R. China yangzz@iccas.ac.cn Y. Lu (&) Department of Chemical & Biomolecular Engineering, UCLA, Los Angeles, CA 90095, USA luucla@ucla.edu high surface area [1, 2]. Templated synthesis using anodized alumina membranes (AAM) as templates is widely used to synthesize one-dimensional nanostructures due to its simplicity and wide applicability of a large variety of materials [3, 4], which has been used for fabrication of mesoporous carbon nanofilaments [5, 6]. Previous reports on fabrication of carbon nanofilaments are based on replication process from mesoporous silica nanofilaments which involves infiltration of carbon precursor, carbonization, and removal of silica template [5, 6]. Recently two groups reported a simple approach to synthesize one-dimensional carbon nanostructures by confined assembly of carbon precursor and structural directing agents within AAM [7, 8]. This approach avoids tedious replication processes and silica template removal steps [5, 6, 9, 10]. However, the reported synthesis approach has not shown flexibility to control mesostructure and therefore controlled mesostructure in mesoporous carbon filaments has not been reported. Here we report a direct synthesis of one-dimensional polymer/carbon nanofilaments containing tunable ordered mesoporous structure by a facile infiltration process of phenolic oligomer/surfactant gel solution into AAM followed by direct carbonization. The resultant mesostructure in polymer/carbon nanofilaments is dependent on that of the feeding gel used for infiltration. The mesostructure of polymer/carbon nanofilaments were tunable from concentric hexagonal to cubic and concentric lamellar mesostructures. 2 Experimental The experimental procedure used to synthesize the oligomers is similar to that reported previously [11, 12]. Briefly, 0.30 g phenol, 0.06 g 20% NaOH, and 0.53 g formalin (37%) were reacted at 70 C for 1 h. After neutralization to

2 316 J Porous Mater (9) 16: ph 7 using HCl solution, water in the solution was removed under vacuum. The obtained oligomers were mixed with designed amount of Pluronic surfactant in ethanol. Surfactant used included P (EO 20 PO 70 EO 20 ) and F127 (EO 107 PO 70 EO 107 ). Viscous oligomer/surfactant composite gels obtained after removing most of the ethanol were infiltrated into AAO membranes with an average pore diameter nm (Whatman International Ltd.) at 60 C. The infiltrated AAO membranes were then heated at C in air for 24 h to allow further polymerization. Surfactant was removed by heating the infiltrated AAO membranes at 350 C in nitrogen for 2.5 h with a heating rate of 1 C/min. The carbonization process was conducted by heating the infiltrated AAO membranes in nitrogen at 900 C for 5 h with a heating rate of 1 C/min. Products of nanofilaments were obtained after removal of the AAO membranes using 5 M sodium hydroxide solution. The structure of the nanofilaments was characterized using transmission electron microscope (TEM, JEOL 2011 FasTEM, kv), nitrogen sorption (Micromeritics ASAP 2010 at 77 K), and X-ray diffraction (XRD, Simens D500, Cu-Ka, 40 kv) techniques. N 2 sorption isotherms of polymer nanofilaments were measured by using polymer/alumina composite. The sample weight used in the calculation included alumina membrane which contributes more than 50% to the whole weight. Since the difficulty to accurately calculate the exact mass of the polymer nanofilaments, the nitrogen adsorption isotherms and the corresponding pore size distributions for the polymer nanofilaments should only be used to provide qualitative information. 3 Results and discussion As synthesized polymer nanofilaments show lengths up to tens of micrometers and an average diameter of nm (Fig. 1), which is consistent with the pore structure of the AAO membranes. Mesostructure of the polymer nanofilaments can be readily tuned by adjusting surfactant concentration or by using Pluronic surfactants with different EO-block lengths. Figure 2 shows XRD patterns of polymer nanofilaments before and after surfactant removal. Before surfactant removal, the polymer nanofilaments prepared from P, phenol, formaldehyde at a molar ratio of :1:2.05 show an intense reflection peak at the d-spacing of 9.1 nm accompanied by the second peak at 4.7 nm (Fig. 2a). After surfactant removal, XRD shows a reflection peak with a decreased d-spacing of 7.8 nm (Fig. 2b). A higher P concentration (P:phenol:formaldehyde = :1:2.05) results in polymer nanofilaments with similar XRD reflections at 9.2 and 5.4 nm (Fig. 2e). After surfactant removal, broad XRD peaks centered at 6.9 and 4.5 nm were obtained (Fig. 2f). The use of F127 (containing longer EO blocks than Intensity / a.u. f e d c b a Theta / degree Fig. 1 SEM image of polymer nanofilaments prepared using AAM with pore diameter of nm Fig. 2 XRD patterns of as-synthesized polymer nanofilaments using P surfactant at low (a) and high concentration (e) and using F127 (c) as the structural directing agent, and of mesoporous polymer nanofilaments (b, d, f) prepared by removing surfactant from (a), (c) and (e) samples, respectively

3 J Porous Mater (9) 16: P) at a molar ratio of F127:phenol:formaldehyde = :1:2.05 results in polymer nanofilaments with a broad peak at 14.4 nm (Fig. 2c). After surfactant removal, the d-spacing was slightly decreased to 13.1 nm (Fig. 2d). In order to understand the mesostructure of the polymer nanofilaments, we synthesized mesoporous polymer films on glass following a similar synthesis and casting procedure. XRD patterns of these mesoporous polymers (Fig. 3) indicate the formation of 2D hexagonal (p6m with a unit cell parameter 9.8 nm), lamellar (interlayer distance of 13 nm before surfactant removal), and body-centered cubic mesostructure (Im3m with a unit cell parameter of 13 nm), which is consistent with mesostructure reported previously [11, 12]. We believe that mesostructure of the polymer nanofilaments are similar to those of the films. The less defined XRD patterns observed for the polymer nanofilaments are due to their much smaller ordered domains that diffract much less X-ray. TEM investigations further confirm the 2D hexagonal, lamellar and the cubic mesostructure within polymer nanofibers. Figure 4 shows TEM images of the mesoporous polymer nanofilaments, revealing the formation of novel ordered mesostructure in compliance with geometric constraint of the cylindrical pore. Consistent with the XRD of the mesoporous polymer nanofilaments prepared using the low P concentration (Fig. 2b), TEM shows a unique circular hexagonal mesostructure (Fig. 4a). The hexagonally arranged mesopores are clearly observed at the edges of the nanofilaments. The formation of such a circular hexagonal mesostructure is due to the bending of hexagonal liquid crystalline tubes in adapting to the curvature of AAO pore surface, which has been observed previously when silicate and surfactant were assembled within AAO membranes [13, 14]. Consistent with the XRD studies (Fig. 2e, f), TEM of the mesoporous polymer nanofilaments prepared using the high d 300 c Intensity / a.u b 210 a Theta / degree Fig. 3 XRD pattern of bulk mesoporous polymer prepared by casting precursor sol on the substrate with (a) 2D hexagonal (p6m), (b) cubic (Im3m) and lamellar mesostructure before (c) and after (d) calcination Fig. 4 Representative TEM images of mesoporous polymer nanofilaments with (a) circular hexagonal, (b) concentric lamellar, and (c) cubic mesoporous structures. Inset of (b) showing concentric lamellar polymer nanofilaments with close ends. Inset of (c) showing a cubic mesoporous polymer nanofilament at high magnification. Scale bar in the inset is 50 nm

4 318 J Porous Mater (9) 16: Fig. 5 (a) Nitrogen adsorption/ desorption isotherms of mesoporous polymer nanofilaments with circular hexagonal, cubic and concentric lamellar mesostructure; (b) BJH pore size distribution of the polymer nanofilaments confined within alumina pore channels (a) Volume Adsorbed (cm3/g STP) Hexagonal structure Cubic structure Lamellar structure Relative Pressure (b) dv/dd (cm3/g-nm) Pore Size (nm) Hexagonal structure Cubic structure Lamellar structure P concentration show a concentric lamellar mesostructure with inter-layer distances ranging from 13 to 16 nm (Fig. 4b). The formation of such concentric lamellar mesostructure is due to compliance of a lamellar mesophase with cylindrical pores that eliminates energetically unfavorable edge effects [13, 14]. Distinct from thin films containing lamellar mesostructure that collapses upon removing surfactant, the concentric lamellar structure is preserved after surfactant removal, which is consistent with the XRD study (Fig. 2f). Note that most of the concentric lamellar nanofilaments contain open ends; however, polymer nanofilaments with closed ends were also observed occasionally (see the inset of Fig. 4b), which may be due to the templating effect from the closed-end cylindrical alumina pores. Figure 4c show TEM images of mesoporous polymer nanofilaments prepared using F127 surfactant, revealing a highly ordered cubic mesostructure. Ordered domains viewed along [111] and [] directions can be clearly observed at the edge and center, respectively. The cell parameter estimated from the TEM images is approximately 12.6 nm, which is consistent with the value (13 nm) determined from XRD data. Previous research indicated that Im3m cubic mesostructured silica thin films templated by F127 surfactant tend to align their () planes parallel to vapor/liquid or liquid/solid interface [15]. The geometric confinement imposed by the cylindrical pores directs the orientation of () plane preferentially parallel to the pore axis. The inset in Fig. 4c shows a high-magnification TEM image of a cubic mesoporous polymer nanofilament, further revealing the two distinctive mesostructure orientations. While it took a large effort to synthesize enough amount of polymer nanofilaments for nitrogen sorption studies, nitrogen adsorption desorption isotherms (Fig. 5a) of the mesoporous polymer nanofilaments before removing AAO membranes show type-iv isotherms with significant adsorption desorption hysteresis. The pore diameter of the hexagonal, lamellar, and cubic polymer nanofilaments is around 9, 5, and 11 nm, respectively, according to the BJH model (Fig. 5b). The lamellar polymer nanofilaments show a relatively low surface area and pore volume, which may be due to partial structure collapse upon the surfactant removal. Note the significant nitrogen uptake at a high relative pressure ([0.7) indicates the presence of large pores, which are the spaces between the polymer nanofilaments and the AAO pore walls created due to the shrinkage of the polymer nanofilaments. Fig. 6 TEM images mesoporous carbon nanofilaments with (a) circular hexagonal, (b) cubic, and (c) concentric lamellar mesostructure

5 J Porous Mater (9) 16: Carbonization of the polymer nanofilaments converts them into mesoporous carbon nanofilaments. Figure 6 shows TEM images of mesostructured carbon nanofilaments with (a) circular hexagonal, (b) cubic and (c) concentric lamellar mesostructure, indicating that the mesostructure can be preserved through carbonization process. The average length of the mesostructured carbon nanofilaments is around several hundred nanometers, which is shorter than those of polymer nanofilaments attributable to its fragile mechanical property. The mesostructures shown in Fig. 6 are similar to those shown in Fig. 4. The cell parameter estimated from the TEM image for circular hexagonal and cubic carbon nanofilaments are slightly decreased compared with those of the polymer nanofilaments due to framework shrinkage upon carbonization. The carbon nanofilaments show less ordered concentric lamellar structure probably due to a higher degree of structural collapse upon carbonization process. 4 Conclusion In summary, we have demonstrated synthesis of onedimensional mesoporous polymer and carbon nanofilaments with controlled mesostructure via infiltration of phenolic oligomer/surfactant gel solution into cylindrical pores of AAMs. The mesostructure of polymer and carbon nanofilaments is consistent with that of feeding gels (e.g. hexagonal, cubic and lamellar) correspondingly. After carbonization, the mesoporous structure within the novel carbon nanofilaments are maintained and tunable from circular hexagonal to cubic and concentric lamellar mesostructures. Acknowledgements The work was partially funded by NASA (Grant No. NAG and NCC-3-946), Office of Naval Research, Louisiana Board of Regents (Grant No. LEQSF(1-04)- RD-B-09), National Science Foundation (Grant No. NSF-DMR and CAREER Award), and National Science Foundation of China (Grant No and ). References 1. M. Terrones, Annu. Rev. Mater. Res. 33, 419 (3) 2. A. Soffer, J. Electroanal. Chem. 38, 25 (1972) 3. C.R. Martin, Science (Washington, D.C.) 266, 1961 (1994) 4. Z. Yang, Z. Niu, X. Cao, Z. Yang, Y. Lu, Z. Hu, C.C. Han, Angew. Chem. Int. Ed. 42, 4201 (3) 5. W.S. Chae, M.J. An, S.W. Lee, M.S. Son, K.H. Yoo, Y.R. Kim, J. Phys. Chem. B, 6447 (6) 6. D.J. Cott, N. Petkov, M.A. Morris, B. Platschek, T. Bein, J.D. Holmes, J. Am. Chem. Soc. 128, 3920 (6) 7. M.B. Zheng, J.M. Cao, X.F. Ke, G.B. Ji, Y.P. Chen, K. Shen, J. Tao, Carbon 45, 1111 (7) 8. K. Wang, W. Zhang, R. Phelan, M.A. Morris, J.D. Holmes, J. Am. Chem. Soc. 129, (7) 9. D.H. Wang, H.M. Luo, R. Kou, M.P. Gil, S.G. Xiao, V.O. Golub, Z.Z. Yang, C.J. Brinker, Y.F. Lu, Angew. Chem. Int. Ed. 43, 6169 (4) 10. D.H. Wang, W.L. Zhou, B.F. McCaughy, J.E. Hampsey, X.L. Ji, Y.B. Jiang, H.F. Xu, J.K. Tang, R.H. Schmehl, C. O Connor, C.J. Brinker, Y.F. Lu, Adv. Mater. 15, 130 (3) 11. S. Tanaka, N. Nishiyama, Y. Egashira, K. Ueyama, Chem. Commun (5) 12. Y. Meng, D. Gu, F.Q. Zhang, Y.F. Shi, H.F. Yang, Z. Li, C.Z. Yu, B. Tu, D.Y. Zhao, Angew. Chem. Int. Ed. 44, 7053 (5) 13. D. Wang, R. Kou, Z. Yang, J. He, Z. Yang, Y. Lu, Chem. Commun. (Cambridge, United Kingdom) 166 (5) 14. Y. Wu, G. Cheng, K. Katsov, S.W. Sides, J. Wang, J. Tang, G.H. Fredrickson, M. Moskovits, G.D. Stucky, Nat. Mater. 3, 816 (4) 15. D.Y. Zhao, P.D. Yang, N. Melosh, Y.L. Feng, B.F. Chmelka, G. Stucky, Adv. Mater. (Weinheim, Germany) 10, 1380 (1998)