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Langmuir 2008, 24, 368-370
Using Collagen Fiber as a Template to Synthesize Hierarchical Mesoporous Alumina Fiber Dehui Deng,† Rui Tang,‡ Xuepin Liao,*,‡ and Bi Shi*,† Key Laboratory of Leather Chemistry and Engineering of Ministry of Education, and Department of Biomass Chemistry and Leather Engineering, Sichuan UniVersity, Chengdu 610065, P. R. China ReceiVed September 12, 2007. In Final Form: NoVember 7, 2007 Hierarchical mesoporous alumina fiber was synthesized by using collagen fiber as the template, and characterized by means of scanning electron microscopy, transmission electron microscopy, N2 adsorption techniques, X-ray photoelectron spectroscopy, and X-ray diffraction. The alumina fiber obtained is approximately 1-4 µm in outer diameter and 0.5-1 mm in length. The pore size distribution of the alumina fiber is narrow (2-20 nm), and its pore size is controllable by varying preparation methods. This study indicates that collagen fiber, which has hierarchical supermolecular structure, could be used as an ideal template to prepare well-defined porous metal oxide fibers.
Recently, the structural design of inorganic materials by using organic matrices as templates has received increasing attention.1 In particular, the organic matrices with fibrous or tubular structures, such as organogelalors,2 DNA,3 cellulose,4 and tobacco mosaic virus,5 are most often used to synthesize fibrous or tubular inorganic materials with unique pore or surface structures, such as silica, carbon, and metal oxide. However, the finding of novel organic matrices which can be used as structure-directing agents for synthesizing inorganic fibrous materials is still an attractive challenge. Herein, the template synthesis of ordered mesoporous alumina fiber by using collagen fiber as a structure-directing agent is reported. This kind of material is promising for applications in the fields of adsorption,6 catalyst, and catalyst support.7 Collagen fiber, one of the most abundant biomass in the natural world, mainly comes from the skin of animals and is traditionally used as a raw material in leather manufacturing.8 The collagen molecule, having a rodlike shape with 1.5 nm in diameter and 300 nm in length, is composed of three polypeptide chains with a helical structure (a right-handed triple helix). The triple helices are staggered (on the fourth level of order) along their heliced long axis with a “gap” region corresponding to a length of 67 nm,9 as shown in Scheme 1a. The triple-helical molecules * Corresponding author. E-mail:
[email protected] and E-mail:
[email protected]. † Key Laboratory of Leather Chemistry and Engineering of Ministry of Education. ‡ Department of Biomass Chemistry and Leather Engineering. (1) (a) Cha, J. N.; Stucky, G. D.; Morse, D. E.; Deming, T. J. Nature (London) 2000, 403, 289. (b) Trewyn, B. G.; Whitman, C. M.; Lin, V. S. Y. Nano Lett. 2004, 4, 2139. (c) van Bommel, K. J. C.; Friggeri, A.; Shinkal, S. Angew. Chem., Int. Ed. 2003, 42, 980. (d) Zhou, Y.; Schattka, J. H.; Antonietti, M. Nano Lett. 2004, 4, 477. (2) (a) Jung, J. H.; Ono, Y.; Hanabusa, K.; Shinkai, S. J. Am. Chem. Soc. 2000, 122, 5008. (b) Jung, J. H.; Ono, Y.; Shinkai, S. Langmuir 2000, 16, 1643. (3) Braun, E.; Eichen, Y.; Sivan, U.; Ben-Yoseph, G. Nature (London) 1998, 391, 775. (4) Polarz, S.; Smarsly, B.; Schattka, J. H. Chem. Mater. 2002, 14, 2940. (5) (a) Dujardin, E.; Peet, C.; Stubbs, G.; Culver, J. N.; Mann, S. Nano Lett. 2003, 3, 413. (b) Fowler, C. E.; Shenton, W.; Stubbs, G.; Mann, S. AdV. Mater. 2001, 13, 1266. (6) (a) Ng, L. M.; Lyth, E.; Zeller, M. V.; Boyd, D. L. Langmuir 1995, 11, 127. (b) Li, P.; Ng, L. M.; Liang, J. Surf. Sci. 1997, 380, 530. (c) Wu, X. Y.; Cong, P. H.; Mori, S. Appl. Surf. Sci. 2002, 201, 115. (7) (a) Reichertz, P. P.; Yost, W. J. J. Chem. Phys. 1946, 14, 495. (b) Gates, B. C.; Katzer, R. J.; Schuit, G. C. A. Chemistry of Catalytic Processes; McGrawHill: New York, 1979. (c) Bond, C. G. J. Catal. 1997, 116, 192. (d) Burch, R. Catal. Lett. 1997, 43, 19. (8) Tang, R.; Liao, X. P.; Liu, X.; Shi, B. Chem. Commun. 2005, 47, 5882. (9) Smith, J. W. Nature (London) 1968, 219, 157.
Scheme 1. Schematic Illustration Showing the Preparation Process of Alumina Fiber by Using Collagen Fiber as a Templatea
a (a) Longitudinal view of collagen molecules in staggered array. (b) Al(III) chelateded with collagen fiber via black wattle tannin. (c) Black wattle tannin.
aggregate through fibrillogenesis into microfibrils consisting of four to eight collagen molecules and further into fibrils. These collagen fibrils organize into fibers, which can form even larger fiber bundles.10 Therefore, it can be inferred that porous inorganic fibrous materials with ordered morphology might be obtained by using collagen fiber as a template. According to the principles of leather processing, collagen fiber which contains abundant functional groups, like -OH, -COOH, and -NH2, is ready to react with some metal ions, such as Cr(III), Al(III), Zr(IV), Ti(IV), and so forth.11 Hence, the porous alumina fiber is possibly obtained via the reaction between Al(III) and collagen fiber, followed by removing collagen fiber template by heat treatment. On the basis of this idea, we have tried to synthesize the alumina fiber by direct reaction between Al(III) and collagen fiber. However, the fiber structure could hardly be obtained after removing the collagen fiber template by heat treatment. The reason should be that Al(III) mainly reacts with carboxy groups of collagen through (10) Friess, W. Eur. J. Pharm. Biopharm. 1998, 45, 113. (11) (a) Evans, N. A.; Milligan, B.; Montgomery, K. C. J. Am. Leather Chem. Assoc. 1987, 82, 86. (b) Liao, X. P.; Shi, B. EnViron. Sci. Technol. 2005, 39, 4628.
10.1021/la702831m CCC: $40.75 © 2008 American Chemical Society Published on Web 12/19/2007
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Langmuir, Vol. 24, No. 2, 2008 369 Table 1. Structural Parameters of Samples Calcined at 800 °C
Figure 1. SEM images, showing alumina fiber with hierachical morphology. (a) Ordered alumina fiber. (b) Smaller fibers longitudinally aligning and assembling into alumina fiber. The sample was obtained via calcining at 800 °C.
Figure 2. TEM micrographs of alumina fiber at different magnifications, obtained at 800 °C.
electrovalent bond rather than through covalent complexation,12a and as a result, the supporting function of collagen fibers is relatively weak. Therefore, the fibrous structure of the alumina is easily destroyed during the process of removing the template. To solve the problem, the chemical modification of collagen fiber should be undertaken to stabilize its fibrous structure and enhance its fixing capacity to Al(III). Vegetable tannins, which are rich in orthohydroxyls and thus have a high affinity for both metal ions and collagen fiber,13,14 are excellent chemicals able to provide these functions. It has been confirmed that, in the reaction system of collagen-tannin-Al(III), tannins react with collagen through multiple hydrogen bonds while the orthophenolic hydroxys of tannins chelate with Al(III). The formula of the reaction can be described as follows: Collagen-Tannin-Al(III) -Tannin-Collagen12b In fact, the synergistic reaction of Al(III) and vegetable tannins with collagen fiber has been used as an effective tanning method in leather manufacturing,12a which can increase the shrinkage temperature of collagen fiber from 50-60 °C to 100-120 °C.13 On the other hand, the anchor effect of tannins can prevent the aggregation and growth of alumina crystallites during the process of heat treatment. Furthermore, the tannins interacted with collagen fiber can act as porogen, and the porous alumina can probably be obtained after removing the collagen fiber template. According to the analysis above, the alumina fiber was synthesized in three steps, as depicted in Scheme 1. First, vegetable (12) (a) Covington, A. D. Chem. Soc. ReV. 1997, 26, 111. (b) Hernandez, J. H.; Kallenberger, W. E. J. Am. Leather Chem. Assoc. 1984, 79, 182. (13) Shi, B.; Di, Y. Plant Polyphenol; Science Press: Beijing, 2000. (14) Liao, X. P.; Lu, Z. B.; Du, X.; liu, X.; Shi, B. EnViron. Sci. Technol. 2004, 38, 324.
samplea
BET surface area (m2 g-1)
BJH surface area A (m2 g-1)
BJH pore volume V (cm3 g-1)
pore sizeb (nm)
Al10 Col10 Al20 Col10 Al30 Col10
226 198 198
255 225 242
0.328 0.440 0.479
5.1 7.8 7.9
a AlXColY: X, Y (unit: g) represents the dosage of Al (SO ) ‚18H O 2 4 3 2 and collagen fiber, respectively. b The pore size (4V/A) was calculated by BJH pore volume (V) and BJH surface area (A).
tannin (black wattle tannin) is immobilized onto collagen fiber, mainly through hydrogen bonds between vegetable tannins and collagen fiber. Second, Al(III) is chelated with phenolic hydroxy groups of vegetable tannin. Finally, the collagen fiber template is removed by heat treatment in air. The alumina fiber obtained exhibits hierarchical morphology with several levels of structural organization. As shown in the scanning electron microscopy (SEM) images presented in Figure 1, the alumina fibers are of highly ordered organization with approximately 1-4 µm in outer diameter and 0.5-1 mm in length (Figure 1a), which is assigned with the diameter and length of the bundle of natural collagen fiber (Supporting Information Figure S1a). Furthermore, the alumina fiber consists of smaller ordered fibers with an average diameter of 60 nm (Figure 1b), which is in good agreement with the diameter of natural collagen fiber (Supporting Information Figure S1b). Therefore, it can be concluded that the well-defined morphology of natural collagen fiber is mostly preserved. The mesoporous morphology in the alumina fiber was investigated by transmission electron microscopy (TEM), as shown in Figure 2. It can be observed that the mesopore size is about 8 nm, and the mesopores are wellaligned with a high degree of order. The aligned feature also corresponds to the smaller fibrous structure of the alumina fiber, which has been observed in Figure 1b. These results indicate that the alumina fiber synthesized can preserve even the very fine structure of collagen fiber. The mesoporous alumina fiber is further analyzed by means of N2 adsorption techniques. The Barrett-Joyner-Halenda (BJH) distribution curves were calculated according to the setup program based on the theory proposed by Barrett et al.15a It is observed that the pore size is in the range 2-20 nm, showing a narrow distribution. As presented in Figure 3a, the hysteresis loop is clear and the adsorption-desorption isotherms are the typical type IV according to the classification of Brunauer, which is associated with the characteristic of mesoporous materials.15b In addition, the surface area of BJH is higher than that of Brunauer-Emmett-Teller (BET), suggesting that there are almost no micropores existing in alumina fiber. Furthermore, the pore size is tunable depending on the mass ratio of Al(III) and collagen fiber and the preparation conditions. As shown in Figure 3b and Table 1, the pore size and the pore volume are both increased with the increase of Al(III), from 5.1 to 7.9 nm and 0.328 to 0.479 cm3 g-1, respectively, though the surface area is almost unchanged. On the other hand, the calcining temperature also has a remarkable effect on the pore volume and pore distribution (Supporting Information Figure S2 and Table S1). When the calcining temperature is increased from 600 to 800 °C, the pore volume is increased from 0.357 to 0.479 cm3 g-1. At 600 °C, a bimodal porosity with micro (12%) and meso (88%) (15) (a) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373. (b) Kruk, M.; Jaroniec, M. Chem. Mater. 2001, 13, 3169.
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Figure 3. N2 adsorption-desorption isotherms and BJH distribution curves (the mass ratios of Al2(SO4)3‚18H2O and collagen fiber were 9, 30:10; O, 20:10; and 2, 10:10, respectively). (a) N2 adsorption-desorption isotherms of the samples. (b) BJH distribution curves of the samples. All samples were obtained at 800 °C.
Figure 4. XPS of Al2O3 fiber obtained at 800 °C: (a) Al 2p spectrum; (b) O 1s spectrum.
is observed. However, the micropore has almost entirely disappeared when the calcining temperature is increased to 700 °C. Elemental analysis indicates that the content of carbon in all alumina fiber samples is below 1%, and no other heteroelements, such as N and S, are detected, suggesting that there are no unreacted precursors or byproducts in the materials obtained. X-ray photoelectron spectroscopy (XPS) analysis (Figure 4) reveals that the Al (2p) and O (1p) peaks lie at 74.2 and 531.2 eV, respectively, which is in agreement with literature data of Al3+ and O2- species in Al2O3.16 Furthermore, the X-ray diffraction (XRD) analysis was carried out to investigate the crystal structure of alumina fiber (Supporting Information Figure S3). It was observed that, when the sample was obtained below the calcining temperature of 700 °C, no obvious diffraction peak emerged, which indicates that alumina exists mainly in the amorphous phase. However, when the sample was calcined above (16) (a) Moffitt, C. E.; Chen, B.; Wieliczka, D. M.; Kruger, M. B. Solid State Commun. 2000, 116, 631. (b) NIST Standard Reference Database 20, version 3.0, June, 2000.
800 °C, some broad peaks at 2θ ) 37.6°, 45.8°, and 66.8° were presented, which are assigned to (311), (400), and (440) reflections of γ-Al2O3, a cubic unit cell with space group Fd3m.17 In summary, hierarchical alumina fiber with ordered mesopore distribution has been successfully prepared by using collagen fiber as a template, and the pore size is controllable via varying preparation methods or conditions. The alumina fiber obtained is believed to have great potential to be used as catalyst or catalyst support with high activity and selectivity, owing to its high surface area and shape-selective properties. Furthermore, hierarchical mesoporous ZrO2 fiber and TiO2 fiber have also been successfully synthesized by using collagen fiber as a template in this study (Supporting Information Figure S4 and Figure S5). Therefore, it can be concluded that collagen fiber could be used as a general template to synthesize porous metal oxide fibers. Acknowledgment. The continued partial supports by the Key Program of National Science Fund of China (20536030), the National Natural Science Foundation of China (20776090) and the National Science Fund for Distinguished Young Scholars (20325619) are gratefully acknowledged. Supporting Information Available: Experimental procedures; SEM images of collagen fiber, ZrO2 fiber, and TiO2 fiber; N2 isotherms and BJH distribution curves of different samples; tables of structural parameters for different samples; XRD patterns of alumina fiber. This material is available free of charge via the Internet at http://pubs.acs.org. LA702831M (17) (a) Wang, J. A.; Bokhimi, X.; Morales, A.; Novaro, O. J. Phys. Chem. B 1999, 103, 299. (b) Hada, K.; Nagai, M.; Omi, S. J. Phys. Chem. B 2000, 104, 2090.
