Copolymer-Controlled Homogeneous Precipitation for the Synthesis of

Nov 23, 2006 - precipitation method under hydrothermal conditions. Scanning electron microscopy, X-ray diffraction, solid-state magic-angle spinning n...
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Langmuir 2007, 23, 4599-4605

4599

Copolymer-Controlled Homogeneous Precipitation for the Synthesis of Porous Microfibers of Alumina Peng Bai,†,‡ Fabing Su,† Pingping Wu,‡ Likui Wang,† Fang Yin Lee,† Lu Lv,† Zi-feng Yan,*,‡ and X. S. Zhao*,†,§ Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, 4 Engineering DriVe 4, Singapore 117576, State Key Laboratory for HeaVy Oil Processing, PetroChina Key Laboratory of Catalysis, China UniVersity of Petroleum, Dongying 257061, People’s Republic of China, and Nanoscience and Nanotechnology InitiatiVe, National UniVersity of Singapore, Singapore 117576 ReceiVed NoVember 23, 2006. In Final Form: January 22, 2007 Mesoporous aluminas with a uniform fibrous morphology were synthesized using a copolymer-controlled homogeneous precipitation method under hydrothermal conditions. Scanning electron microscopy, X-ray diffraction, solid-state magic-angle spinning nuclear magnetic resonance, transmission electron microscopy, thermogravimetric analysis, nitrogen adsorption, Fourier transform infrared spectrometry, and elemental analysis techniques were used to characterize the samples. The effect of various synthesis conditions on the morphology and mesoporous structure of the alumina fibers was investigated. Such porous alumina microfibers may find applications in nanotechnology and catalysis. They can also be used as advanced high-temperature composite materials and templates for fabrication of fibrous materials of various compositions, such as carbon, transition-metal oxides, and polymers.

1. Introduction Aluminas are widely used as catalyst supports, adsorbents, ceramics, and abrasives, etc.1-3 Being a catalyst support, traditional aluminas possess only textural porosity featured by a low surface area (less than 250 m2/g) and a broad pore size distribution, which limit their catalytic applications.4 Thus, a great deal of recent effort has been placed on the synthesis of mesoporous aluminas with a high specific surface area and large pore volume using various templates, including surfactants,5-8 carboxylic acids,9 and single organic molecules (e.g., glucose,10 tetraethyl glycol,11-12 and tartaric acid13). The potential application of an aluminum oxide depends not only on its porosity, but also on its morphology. For example, one-dimensional (1-D) alumina nanostructures are highly desirable in areas such as advanced high-temperature composite * To whom correspondence should be addressed. (X.S.Z.) Phone: 6565164727. Fax: 65-67791936. E-mail: [email protected]. (Z-f.Y.) Phone: 86-546-8391527. Fax: 86-546-8391974. E-mail: zfyancat@ hdpu.edu.cn. † Department of Chemical and Biomolecular Engineering, National University of Singapore. ‡ China University of Petroleum. § Nanoscience and Nanotechnology Initiative, National University of Singapore. (1) Misra, C. Industrial Alumina Chemicals; ACS Monograph 184; American Chemical Society: Washington, DC, 1986. (2) Tournier, G.; Lecroix-Repellin, M.; Pajonk, G. M. Stud. Surf. Sci. Catal. 1987, 31, 333. (3) Topsoe, H.; Clausen, B. S.; Massoth, F. E. Hydrotreating Catalysis; Springer: Berlin, 1996; p 310. (4) Bagshaw, S. A.; Pinnavaia, T. J. Angew. Chem., Int. Ed. 1996, 35, 1102. (5) Cabrera, S.; Haskouri, J. E.; Alamo, J.; Beltra´n, A.; Beltra´n, D.; Mendioroz, S.; Marcos, M. D.; Amoros, P. AdV. Mater. 1999, 11, 379. (6) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schu¨th, F.; Stucky, G. Nature 1994, 368, 317. (7) Bai, P.; Xing, W.; Zhang, Z.; Yan, Z. Mater. Lett. 2005, 59, 3128. (8) Zhang, Z.; Hicks, R.W.; Pauly, T. R.; Pinnavaia, T. J. J. Am. Chem. Soc. 2002, 124, 1592. (9) Vaudry, F.; Khodabandeh, S.; Davis, M. E. Chem. Mater. 1996, 8, 1451. (10) Xu, B.; Xiao, T.; Yan, Z.; Sun, X.; Sloan, J.; Gonza´lez-Corte´s, S. L.; Alshahrani, F.; Green, M. L. H. Microporous Mesoporous Mater. 2006, 91, 293. (11) Shan, Z.; Jansen, J. C.; Zhou, W.; Maschmeyer, Th. Appl. Catal., A 2003, 254, 339. (12) Zhang, Z.; Bai, P.; Xu, B.; Yan, Z. J. Porous Mater. 2006, 13, 255. (13) Liu, X.; Wei, Y.; Jin, D.; Shih, W. Mater. Lett. 2000, 42, 143.

materials and nanodevices because of their high dielectric constant, good thermal and chemical stability, and high mechanical modulus.14,15 Several methods have been demonstrated to prepare 1-D alumina nanostructures, including catalyst-assisted vapor-liquid-solid deposition,16 catalyst-free vapor-solid deposition,17 chemical etching,18 mechanical cleavage of porous aluminum oxide membranes,19 and electrochemical anodizing.20,21 Llusar et al.22 described a template-synthesis method for the growth of interwound alumina fibers by using an anthracenic organogelator template. Zhu and co-workers23 synthesized γ-alumina nanofibers in the presence of poly(ethylene oxide) surfactant as a template. Zhang et al.24 designed a threestep synthesis route to the preparation of lathlike and rod-shaped mesoporous alumina nanoparticles. Lee and co-workers25 reported the preparation of a unidirectional alumina nanostructure using a surfactant-templating strategy. In this work, 1-D mesoporous alumina microstructures were synthesized using a copolymer-controlled homogeneous precipitation method under hydrothermal conditions. Triblock copolymer (PEO)20-(PPO)70-(PEO)20, namely, P123, which has been shown to be a templating agent to the formation of mesoporous silicas,26 was used as a template. The experimental results showed that P123 is a highly effective agent for controlling the nucleation and growth of alumina fibers. (14) Zou, J.; Pu, L.; Bao, X.; Feng, D. Appl. Phys. Lett. 2002, 80, 1079. (15) Fang, X.; Ye, C.; Zhang, L.; Xie, T. AdV. Mater. 2005, 17, 1661. (16) Valcarcel, V.; Souto, A.; Guitian, F. AdV. Mater. 1998, 10, 138. (17) Proost, J.; Van Boxe, l. S. J. Mater. Chem. 2004, 14, 3058. (18) Xiao, Z. L.; Han, C. Y.; Welp, U.; Wang, H. H.; Kwok, W. K.; Willing, G. A.; Hiller, J. M.; Cook, R. E.; Miller, D. J. G.; Crabtree, W. Nano Lett. 2002, 2, 1293. (19) Pu, L.; Bao, X.; Zou, J.; Feng, D. Angew. Chem., Int. Ed. 2001, 40, 1490. (20) Zhang, L.; Cheng, B.; Shi, W.; Samulski, E. T. J. Mater. Chem. 2005, 15, 4889. (21) Pang, Y.; Meng, G.; Zhang, L.; Shan, W.; Zhang, C.; Gao, X.; Zhao, A.; Mao, Y. J. Solid State Electrochem. 2003, 7, 344. (22) Llusar, M.; Pidol, L.; Roux, C.; Pozzo, J. L.; Sanchez, C. Chem. Mater. 2002, 14, 5124. (23) Zhu, H. Y.; Riches, J. D.; Barry, J. C. Chem. Mater. 2002, 14, 2086. (24) Zhang, Z.; Pinnavaia, T. J. J. Am. Chem. Soc. 2002, 124, 12294. (25) Lee, H.; Kim, H.; Chung, S.; Lee, K.; Lee, H.; Lee, J. J. Am. Chem. Soc. 2003, 125, 2882. (26) Zhao, D.; Sun, J.; Li, Q.; Stucky, G. D. Chem. Mater. 2000, 12, 275.

10.1021/la0634185 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/03/2007

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Table 1. Molar Ratios of the Synthesis Precursors for Preparing Different Alumina Fiber Samplesa

a

sample

A

P

U

H

P-0 P-1 P-2 P-3 P-4 P-5 U-1 U-2 U-3 U-4 U-5 A-1 A-2 A-3 A-4 A-5 H-1 H-2 H-3 H-4 H-5

1 1 1 1 1 1 1 1 1 1 1 0.20 0.40 0.60 0.80 1.2 1 1 1 1 1

0 0.0070 0.010 0.015 0.020 0.025 0.020 0.020 0.020 0.020 0.020 0.020 0.020 0.020 0.020 0.020 0.020 0.020 0.020 0.020 0.020

9 9 9 9 9 9 3 6 10 12 20 9 9 9 9 9 9 9 9 9 9

90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 83 140 220 280 330

A:P:U:H represents the molar ratio Al(NO3)3:P123:urea:H2O.

2. Experimental Section 2.1. Preparation of Samples. In a typical synthesis, 4.64 g of P123 (Aldrich, typical Mn ) 5800) was dissolved in 65.0 mL of deionized water to form a clear solution, to which 15.0 g of Al(NO3)3‚9H2O (Merck, 98.5%) was added. After the aluminum salt was totally dissolved, 21.6 g of urea (ACS reagent, Sigma-Aldrich, 99.0-100.5%) was added. The final mixture was transferred to a Teflon-lined stainless-steel autoclave and placed in an oven at 100 °C. After 24 h, the solid was filtered off, washed with deionized water, and dried at 80 °C in a vacuum oven for 24 h. The polymer (template) was removed by calcination in air at 500 °C for 2 h with a heating rate of 1 °C/min. To study phase transition and thermal stability, calcination was also conducted at 500, 600, 700, and 800 °C. The molar ratio of the synthesis precursors was examined and optimized by changing the amount of one composition matter while maintaining the rest constant as detailed in Table 1. 2.2. Characterization. The microscopic features of the assynthesized and calcined aluminas were characterized with a JEOL JSM-5600LV scanning electron microscope operated at 15 kV, a field-emission scanning electron microscope (JSM- 6700F, JEOL Japan) operated at 5 kV, a transmission electron microscope (JEM 2010 from JEOL) operated at 200 kV, and a field-emission transmission electron microscope (JEM 2010F, JEOL, Japan) operated at 200 kV. Nitrogen adsorption/desorption isotherms of the samples were measured on a Micromeritics TRISTAR 3000 analyzer. The samples were degassed at 573 K for 4 h prior to analysis. The specific surface areas of the samples were calculated using the Brunauer-Emmett-Teller (BET) method in the relative pressure range of 0.05-0.25. The pore size distribution (PSD) curves were derived from the adsorption branches of the isotherms using the Barrett-Joyner-Halenda (BJH) method. The pore sizes were estimated from the peak positions of the BJH PSD curves. The total pore volumes were calculated from the adsorption quantity at a relative pressure of P/Po ) 0.99. The crystalline phases of the alumina samples were characterized using the X-ray diffraction (XRD-6000, Shimadzu, Japan) technique with Cu KR radiation of wavelength λ ) 0.15418 nm. Thermogravimetric and differential thermal analyses (TGA-DTA) were conducted on a thermogravimetric analyzer, TGA 2050 (DTG-60AH SHIMADZU, Japan), with an air flow rate of 50 mL/min at a heating rate of 10 °C/min. The local environments of Al were analyzed using27 Al solid-state magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy on a Bruker (27) Ramanathan, S.; Roy. S. K.; Bhat, R.; Upadhyaya, D. D.; Biswas, A. R. Ceram. Int. 1997, 23, 45.

Figure 1. SEM images of a typical as-synthesized sample at different magnifications (a-c) and a cross-section view of a fiber (d). DRX400 FT-NMR spectrometer operated at a resonance frequency of 104.3 MHz. Elemental analysis was conducted on a Perkin-Elmer 2400 series II CHNS/O analyzer. Fourier transform infrared (FTIR) spectra were collected on a Bio-Rad FTS 135 with a resolution of 4 cm-1 by using the KBr method.

3. Results and Discussion 3.1. Morphology Control. Figure 1 shows the scanning electron microscopy (SEM) images of as-synthesized sample P-4 prepared from a hydrothermal system of A:P:U:H ) 1:0.02: 9:90 (see Table 1). It is seen that the sample is made of uniformly sized microfibers. These fibers were not tangled or interwound. The length of the fibers is about 8-10 µm. The magnified SEM image depicted in Figure 1c shows that the fibers have a very smooth surface, apparently formed from small laths through a layer-by-layer packing manner. Figure 1d reveals that the crosssection of the fibers has an elliptic shape with a width of about 300 nm and a thickness of about 200 nm. The effect of the molar ratio of P123 to Al(NO3)3 on the morphology of the alumina fibers was studied, and the results are presented in Figure S1 (see the Supporting Information (SI)). It is seen that P-0, the sample prepared in the absence of P123, exhibited an irregular particle morphology with some rodlike particles. Sample P-1, which was prepared in the presence of low-concentration P123, displayed a morphology similar to that of sample P-0. However, samples P-2, P-3, P-4, and P-5 all exhibited a regular fibrous morphology. These results indicate that P123 indeed played a role in determining the particle morphology by directing the growth of inorganic aluminum species along a specific crystallographic direction.23 It may be inferred from the experimental results shown in Figure S1 that only when the molar ratio of P123 to Al(NO3)3 is above 0.01 could such a role of P123 be operative under our experimental conditions. The effect of the molar ratio of urea to Al(NO3)3 on the morphology of the precipitates was also investigated and is shown in Figure S2 (see the SI). In our experiments, it was observed that a transformation from a transparent sol to a white gel occurred when the molar ratio of urea to Al(NO3)3 was increased from 3 (sample U-1) to 6 (sample U-2). With further increasing the molar ratio to above 9, the samples were a white precipitate with a well-defined fibrous morphology as can be seen from Figure S2. The above results indicate the importance of urea in controlling the precipitation and morphology of the resultant solids. In the

Synthesis of Porous Microfibers of Alumina

homogeneous precipitation method, urea hydrolyzes slowly to release NH3 molecules, thus acting as a OH- donator. As a result, the amount of urea in the hydrothermal system determined the system pH, thus controlling the nucleation and condensation rates of aluminum oxide species. The inorganic aluminum salt concentration also has a prominent effect on the morphology of the as-synthesized samples. It was found that when the Al(NO3)3 concentration was below 0.2 M, only a transparent gel was obtained. With increasing salt concentration, a white precipitate was obtained. The gel-like product (sample A-1) is ill-defined in terms of morphology, while all precipitates display a fibrous shape (see Figure S3 in the SI). The width of the fibers increased with the salt concentration. For sample A-2 prepared with an Al(NO3)3 concentration of 0.2 M, a large portion of the fibers have a width of less than 100 nm with the smallest one at 20 nm (see Figure S3b). The thicker fibers appear to consist of bundles of smaller ones of widths in the range of 20-30 nm. For sample A-5, the fibers are much larger with a typical width of about 1 µm. It thus can be concluded that, simply by changing the salt concentration, the width of the fibers can be tuned from tens of nanometers to 1 µm. Such an effect of the salt concentration on the product morphology may be related to the ion strength of the synthesis system. A higher salt concentration, and thus a higher solution ion strength, leads to the compression of the electrical double layer of the charged primary particles, which results in their rapid coagulation to large particles. Otherwise, the charged primary nanoparticles tend to remain stable in the solution, which gives birth to thinner fibers. By changing the molar ratio of H2O to Al, a series of samples with different morphologies were observed. The H-1 sample prepared with a H2O:Al molar ratio of 83 appears to be a mixture of polymer and inorganic aluminum species aggregates (see Figure S4a in the SI). This is understood as such low water content leads P123 and aluminum species to form separate phases during the hydrothermal treatment. With increasing water content, samples H-2 and H-3 showed a regular fibrous morphology. Further increasing the water concentration to a H2O:Al molar ratio of 280 produced sample H-4 with an irregular morphology. Interestingly, the H-5 sample prepared with a H2O:Al molar ratio of 330 is made up of bundles of short flakes, which are about 1-2 µm long. Some flakes are even packed into a flowerlike shape (see Figure S4f). A simple explanation of the water effect is that the water concentration determines the aluminum salt concentration, which in turn determines the morphology of the product by influencing the solution ion strength. 3.2. Phase Identification and Chemical Composition. Previously it was reported that reaction of an aluminum inorganic salt with urea under prolonged refluxing yielded a boehmite phase.27 The XRD patterns of all as-synthesized samples can be indexed to crystalline ammonium aluminum carbonate hydroxide (AACH) with a composition of NH4[Al(OOH)HCO3] (JCPDS card no. 42-0250),28-32 which is conventionally synthesized by reaction of aluminum sulfate with ammonium hydrogen carbonate in the liquid phase.28 To the best of our knowledge, no report has been available on the synthesis of AACH using the homogeneous precipitation route. Figure 2a shows the XRD (28) Kato, S.; Iga, T.; Hatano, S.; Minowa, M.; Isawa, Y. U.S. Patent 4,053,579. (29) Ma, C.; Zhou, X.; Xu, X.; Zhu, T. Mater. Chem. Phys. 2001, 72, 374. (30) Sun, X.; Li, J.; Zhang, F.; Qin, X.; Xiu, Z.; Ru, H. J. Am. Ceram. Soc. 2003, 86, 1321. (31) Li, Z.; Feng, X.; Yao, H.; Guo, X. J. Mater. Sci. 2004, 39, 2267. (32) Morinaga, K.; Torikai, T.; Nakagawa, K.; Fujino, S. Acta Mater. 2000, 48, 4735.

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Figure 2. XRD patterns of (a) as-synthesized samples P-0 and P-4 and (b) sample P-4 calcined at different temperatures. [ denotes η-Al2O3.

patterns of as-synthesized samples P-0 and P-4, and Figure 2b shows the XRD patterns of sample P-4 calcined at different temperatures. The high intensities of the XRD peaks of the assynthesized samples indicate that the AACH phase synthesized in this work is highly crystalline. After calcination at 500 and 600 °C, both alumina samples essentially became amorphous. By increasing the calcination temperature to 700 °C, the η-Al2O3 phase was observed. The low intensity of the XRD peaks indicates the alumina sample calcined at this temperature has a low crystallinity. Further increasing the calcination temperature to 800 °C, the typical XRD pattern of the η-Al2O3 phase (JCPDS card no. 04-0875) was obtained, while the broad peaks demonstrated that the alumina fibers calcined at 800 °C are nanocrystalline, consistent with the previous results.29 Figure 3a shows the transmission electron microscopy (TEM) image of the AACH microfibers. The SAED pattern (the inset of Figure 3a) recorded from the face of the crystal having the longest axis (for example, the circled area in the TEM image) always exhibits the same single-crystalline pattern, consistent with the [001] zone of AACH crystals. The elongated microfiber shape with the exposed (001) faces demonstrated that the AACH crystal preferably grew along the crystallographic c axis under the synthesis conditions. Figure 3b shows the high-resolution TEM (HRTEM) image of the alumina microfibers calcined at 800 °C for 2 h. The estimated crystallite size is about 6 nm, and the lattice spacing is about 0.25 nm. Figure 4 shows the 27Al MAS NMR spectra of sample P-4 before and after calcination at 500 °C for 2 h. It can be seen that the as-synthesized sample exhibited a single NMR signal at about 3 ppm, showing the presence of only octahedrally coordinated Al species.33 After calcination, three well-defined signals at 65, 36, and 5 pm can be seen, demonstrating the existence of four-, five-, and six-coordinated aluminum species, respectively.34 The peak centered at 36 ppm indicated that there is a great amount of amorphous alumina domains in the calcined sample, in good agreement with the XRD results. Generally, the five-coordinated (33) Mitsui, I. Nippon Denshi Zairyo Gijutsu Kyokai Shuki Koen Gaiyoshu 1977, 14, 5. (34) Akitt, J. W. Elders, J. M. J. Chem. Soc., Dalton Trans. 1988, 1347.

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Figure 5. TGA-DTA curves of P-0 (a), P-1 (b), and P-4 (c). Table 2. Elemental Analysis Data

Figure 3. (a) TEM image and a typical SAED pattern (inset) of a single-crystalline AACH fiber. The SAED pattern is confirmed to be the [001] zone of the AACH crystal (reflections: A, (020); B, (200)) and indicates the direction of AACH microfiber growth is perpendicular to the [001] crystallographic axis. (b) HRTEM image of an alumina fiber calcined at 800 °C.

Figure 4. Solid 27Al NMR spectra of sample P-4 before and after calcination at 500 °C for 2 h.

Al atoms are considered as the Lewis acid center present in amorphous domains.4 3.3. Thermal Stability and Template Removal. Figure 5 shows the TGA curves of three representative samples. All samples have two major weight loss events. The first weight loss below 175 °C is due to the desorption of physically adsorbed water. With increasing P123 content, the weight loss of this step decreases (see Table 1 in the SI), indicating P123 has a higher affinity toward AACH crystals than water and can squeeze out the water molecules adsorbed on the AACH crystal surface.23

sample

[C], %

[H], %

[N], %

P-0 P-1 P-2 P-4

6.78 7.26 8.12 8.56

3.90 4.02 3.94 3.80

7.95 7.40 6.62 6.57

The second weight loss event in the temperature range of 175245 °C is associated with the decomposition of P123, together with the decomposition of AACH, releasing CO2, NH3, and H2O and forming alumina particles.29 With increasing P123 content, the weight loss in the second step increases, indicating more P123 had been occluded in the sample. To clarify this, samples P-0 to P-4 were measured by elemental analysis, and the results are reported in Table 2. With increasing concentration of P123 in the synthesis mixture, the C content in the samples was increased while the N content was decreased. Thus, the increase of the weight loss in the second step should be attributed to the enhanced occlusion of P123 in the as-synthesized samples. Upon removal of P123 by calcination, the morphology of the alumina samples was not markedly altered as can be seen from Figure 6. The morphologies of the alumina fibers calcined at 500 and 700 °C are the same as those of their counterparts before calcination. When the calcination temperature was increased to 800 °C, some fibers became curved, possibly because of sintering. However, the essential fiber morphology still remains. A closer observation of the alumina fibers revealed that, with increasing calcination temperature, the surface of the fibers became more and more corrugated and the layer-by-layer packing structure became more obvious. As is known, AACH is made up of Al-O octahedron chains linked by covalently sharing oxygen atoms. Between the chains are located CO32- and NH4+ ions.33 When AACH thermally decomposes, the Al-O bonds remain unchanged while the weaker bonds between CO32- and NH4+ ions and the Al-O octahedron chains are broken, releasing NH3, CO2, and H2O. This special structure of AACH endows the alumina microfibers with a high thermal stability. 3.4. Mesoporous Structure. Figure S5 in the SI shows the N2 sorption isotherms and BJH pore size distribution curves of the samples prepared with different amounts of P123 and urea, respectively. All isotherms are of typical type IV with a hysteresis loop, indicating the alumina fibers are mesoporous materials. All samples synthesized in the presence of P123 displayed steep capillary condensation and evaporation steps within the pressure range of P/Po ) 0.4-0.6, implying they have a narrow mesopore size distribution. By contrast, sample P-0 prepared without P123 exhibited a flat step over a wide pressure range (P/Po ) 0.4-0.8) in the adsorption branch, indicating it has a broad pore size distribution. It is also seen from Figure S5 that, with increasing P123 content, the primary mesopore sizes of samples P-1 to P-5 were enlarged. It was calculated that all samples synthesized in the presence of P123 had a BET surface area above 360 m2/g. However, sample P-0, which was prepared without P123, had

Synthesis of Porous Microfibers of Alumina

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Figure 7. Typical TEM images of mesoporous alumina fibers calcined at 500 °C.

Figure 6. SEM images of the P-4 sample calcined at 500 °C (a, b), 700 °C (c, d), and 800 °C (e, f).

a BET surface area of about 235 m2/g. The above data suggest that P123 triblock copolymer indeed played a role in the formation of the mesoporous aluminas. The increased pore size with increasing P123 concentration is ascribed to the increase in the aggregation number of the micelles.35 By changing the P123 concentration, the mesopore sizes can be tuned in the range of 2.7-3.2 nm, which are smaller compared with those of SBA-15 mesoporous silica. However, it is reasonable considering the high temperature and saline solution adopted in the present study, which could decrease the micelle sizes of P123.36 Further evidence of the P123 templating role can be obtained by comparing the pore sizes of samples U-3, U-4, and U-5, which were prepared with the same P123 content and showed the same pore size, 2.7 nm. Besides, the TEM images shown in Figure 7 revealed that there are two kinds of mesopores in the calcined alumina fibers. One is the channel-like pores as can be seen from Figure 7b, and the other is wormlike pores as can be seen from Figure 7c. It is believed that different mesopores may result from the removal of P123 micelles with different conformations, that is, wormlike and rodlike P123 micelles. However, the contribution from the interpartilce porosity produced by the decomposition of AACH crystals shall not be ignored either. Furthermore, the mesopore size of the alumina fibers estimated from Figure 7d is about 3 nm, which is consistent with the BET results. 3.5. Formation Mechanism of the Mesoporous Alumina Microfibers. For PEO-PPO-PEO triblock copolymers, as the temperature increases, the PPO moiety becomes more and more hydrophobic and the hydrophilicity of the PEO moiety decreases in the meantime. In a P123 solution, the cloud point of P123 at a concentration of 1% is about 90 °C.37 In the Al(NO3)3-urea(35) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145. (36) Booth, C.; Attwood, D. Macromol. Rapid Commun. 2000, 21, 501.

P123-water system, the micellization and phase transformation behaviors of P123 became more complicated due to the different effects of the aluminum salt and urea on the critical micelle concentration (cmc) of P123. The inorganic salt has an effect on the cmc similar to that of the temperature. A lower cmc is commonly found in a saline solution. However, a reverse tendency in the cmc of copolymers in the presence of urea is usually observed, which is understood as urea enhances the solubility of the PPO and PEO moieties of the copolymer.38 To understand the phase behavior of P123 and the fiber formation mechanism in the current synthesis system, transparent glass bottles were used to allow us to observe phase changes. It was observed that, at the initial stage, the synthesis system remained transparent and no phase separation occurred, implying the urea effect dominated over the salt and temperature effects and P123 can be dispersed homogeneously in the reaction system during the synthesis process. As the hydrothermal treatment went on, bubbles were seen, indicating the decomposition of urea and the onset of the following reactions:

CO(NH2)2 + H2O f CO2v + 2NH3

(1)

Al(NO3)3 + 4NH3 + CO2 + 3H2O f NH4[Al(OOH)HCO3]V + 3NH4NO3 (2) CO(NH2)2 + 3H2O + CO2 f 2NH4HCO3

(3)

Combining the above reactions yields an overall reaction:

Al(NO3)3 + 3CO(NH2)2 + 8H2O f NH4[Al(OOH)HCO3]V + 3NH4NO3 + 2NH4HCO3 The morphologies of the products separated from the hydrothermal synthesis system after different times were analyzed using SEM and are shown in Figure S6 (see the SI). After 8 h (37) http://www.basf.com/businesses/chemicals/performance/pdfs/Pluronic_ P123.pdf. (38) Desai, P. R.; Jain, N. J.; Sharma, R. K.; Bahadur, P. Colloids Surf., A 2001, 178, 57.

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Scheme 1. (a) Graphic Illustration of the Microfiber Growth Process and (b) Detailed Presentation of the Layer-by-Layer Self-Assembly Mechanism Occurring in the Formation of AACH/P123 Microfibers

of reaction, the product, which was a transparent gel, did not have a well-defined morphology as can be seen from Figure S6a. The products obtained after 18 and 20 h were white gel products and displayed a fibrous morphology. However, it is obvious that the fibers are adhered together and the length of most of the fibers is shorter than that of those shown in Figure 1. The solids separated after hydrothermal synthesis for 30 and 72 h were white precipitates with a uniform fibrous morphology, and the microfibers are clearly separated without sticking. On the basis of the above observations, here we propose a microfiber growth process as schematically illustrated in Scheme 1a to interpret the formation of the alumina fibers. Initially, numerous AACH nanocrystals were formed through the above proposed reactions because of the homogeneous increase in pH in the system with urea decomposition.39 These AACH nanocrystals adsorbed on the P123 micelle via hydrogen bonding of the oxide groups of P123 with the OH groups of AACH,23 which was confirmed by the FT-IR data shown in Figure S7 (SI). The strong bands at 3400 and 980 cm-1 are attributed to the stretching (39) Nagai. H.; Oshima, Y.; Hirano, K.; Kato, A. Br. Ceram. Trans. 1993, 92, 114.

and bending vibrations of the hydroxyl groups on the AACH crystal surface, respectively.28 The hydrogen bonding reduces the free energy of the AACH crystallites, which allows the AACH crystallites to grow into AACH/P123 nanofibers along the direction of the axis of the rod-shaped copolymer micelles.35,40,41 Zhu and co-workers have interpreted the formation of the alumina nanofibers using a surfactant-induced fiber formation (SIFF) mechanism,23 which can also be applied to explain the formation of the AACH/P123 nanofibers in our case. Also the occlusion of P123 in AACH/P123 nanofibers was confirmed earlier by TGA and elemental analysis data. The resulting AACH/P123 nanofibers then aggregate into AACH/P123 microfibers through a layer-by-layer self-assembly mechanism. Further growth of the microfibers may proceed through Ostwald ripening,42 during which larger AACH/P123 microfibers become larger while smaller AACH/P123 microfibers and AACH nanocrystals disappear. (40) Jørgensen, E. B.; Hvidt, S.; Brown, B.; Schille´n, K. Macromolecules 1997, 30, 2355. (41) Ganguly, R.; Aswal, V. K.; Hassan, P. A.; Gopalakrishnan, I. K.; Yakhmi, J. V. J. Phys. Chem. B 2005, 109, 5653. (42) Kabalnov, A. J. Dispersion Sci. Technol. 2001, 22 (1), 1.

Synthesis of Porous Microfibers of Alumina

Scheme 1b further elaborates the layer-by-layer self-assembly mechanism occurring in the formation of AACH/P123 microfibers. First, the AACH nanocrystals that adsorbed on P123 micelles underwent an SIFF mechanism to produce the AACH/P123 nanofibers. Then the resulting AACH/P123 nanofibers preferably aligned along the crystallography c axis to form AACH/P123 microfibers with the exposed (001) faces stabilized by selective adsorption of P123 monomers.43-46 The selective adsorption of copolymer monomers onto the AACH (001) faces suppresses crystal growth along the [001] zone axis. As a consequence, the AACH crystal develops into an elongated fibrous morphology with exposed (001) faces. The irregular particle morphology observed from the sample synthesized with low concentrations of P123 suggests the (001) face has a high surface free energy, which must be stabilized by polymers of high concentrations.

4. Conclusions One-dimensional mesoporous alumina microfibers can be synthesized by using a surfactant-templated homogeneous (43) Yu, S.; Co¨lfen, H.; Tauer, K.; Antonietti, M. Nat. Mater. 2005, 4, 51. (44) Liu, D.; Yates, M. Z. Langmuir 2006, 22, 5566. (45) Thachepan, S.; Li, M.; Davis, S. A.; Mann, S. Chem. Mater. 2006, 18 (15), 3557. (46) Yu, S.; Co¨lfen, H. J. Mater. Chem. 2004, 14, 2124.

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precipitation route. The typical width of the fibers is in the submicrometer range, and the average length is about 8-10 µm. By simply changing the aluminum salt concentration, the diameter of alumina fibers can be varied from tens of nanometers to about 1 µm. Calcination of the alumina fibers can enable the materials to have high surface areas up to 480 m2/g and a narrow pore size distribution. By varying the concentration of surfactant P123, the mesopore sizes can be tuned from 2.7 to 3.2 nm. Such fibrous porous aluminas may find applications in catalysis, tissue engineering, nanotechnology, etc. The fibrous morphology can be used as a hard template to fabricate other fibrous materials as has been demonstrated by carbon microfibers. Acknowledgment. We thank the National University of Singapore (NUS) for financial support. P.B. thanks the China University of Petroleum (CUP) for offering a scholarship. Supporting Information Available: SEM images, TGA data, and FT-IR spectra. This material is available free of charge via the Internet at http://pubs.acs.org. LA0634185