Hollow Spheres Based on Mesostructured Lead Titanate with

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Langmuir 2003, 19, 1362-1367

Hollow Spheres Based on Mesostructured Lead Titanate with Amorphous Framework Mingmei Wu,*,† Guangguo Wang,† Huifang Xu,‡ Junbiao Long,† Fanny L. Y. Shek,§ Samuel M.-F. Lo,§ Ian D. Williams,§ Shouhua Feng,| and Ruren Xu| State Laboratory of Optoelectronic Materials and Technologies, and School of Chemistry and Chemical Engineering, Sun Yat-Sen (Zhongshan) University, Guangzhou, 510275, P. R. China, Department of Earth and Planetary Science, University of New Mexico, 200 Yale Boulevard, Albuquerque, New Mexico 87131, Department of Chemistry, Hong Kong University of Science and Technology, Clear Water Bay, Clear Water Bay, Kowloon, Hong Kong, China, and Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130023, P. R. China Received February 12, 2002. In Final Form: October 21, 2002 Hollow spheres of mesostructured lead titanate, denoted as PTM-1, have been prepared via a combined oil-in-water emulsion mediated and neutral amine supermolecular templated route. The variety of reaction temperatures and KOH concentrations indicates hollow spheres can be formed under a very critical condition. The structure and composition of the as-synthesized PTM-1 have been determined by powder X-ray diffraction, high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS), CHN (carbon-hydrogen-nitrogen) elemental analysis, and thermal analysis. Chemical extraction of organic templates by a cosolvent of weak acid and alcohol has resulted in the formation of a new mesoporous material of non-silica oxide with high porosity.

1. Introduction In both fundamental and industrial studies, hollow spheres and tubes are attracting great attention. It is well-known that buckyballs and buckytubes are purely carbonaceous members in the class of hollow organic materials. The templates of solid organic polymer (polystyrene, PS) beads and carbon nanotubes have been used to produce macroporous materials, hollow spheres, and tubes of inorganic materials of either silica or non-silica oxides, such as anatase and rutile.1-7 These hollow spheres of either silica or non-silica oxides directed by PS beads are generally structured by nonporous shells. Porous shells building hollow spheres have rarely been reported, especially for non-silica oxides. Recently, hollow spheres * Author to whom correspondence should be addressed at Department of Chemistry, Sun Yat-Sen (Zhongshan) University, Guangzhou, 510275, P. R. China. Tel & Fax: 86-20-84111038, 8620-84036766. E-mail: [email protected]. † Sun Yat-Sen (Zhongshan) University. ‡ University of New Mexico. § Hong Kong University of Science and Technology. | Jilin University. (1) Soten, I.; Ozin, G. A. Curr. Opin. Colloid Interface Sci. 1999, 4, 325-337. (2) Jiang, P.; Bertone, J. F.; Colvin, V. L. Science 2001, 291, 453457. (3) (a) Stein, A. Microporous Mesoporous Mater. 2001, 44-45, 227239. (b) Holland, B. T.; Blanford, C. F.; Do, T.; Stein, A. Chem. Mater. 1999, 11, 795-805. (c) Wang, D.-Y.; Caruso, R. A.; Caruso, F. Chem. Mater. 2001, 13, 364-371. (4) Gundiah, G.; Rao, C. N. R. Solid State Sci. 2000, 2, 877-882. (5) (a) Caruso, R. A.; Susha, A.; Caruso, F. Chem. Mater. 2001, 13, 400-409. (b) Caruso, F.; Shi, X.-Y.; Caruso, R. A.; Susha, A. Adv. Mater. 2001, 13, 740-743. (c) Zhong, Z.; Yin, Y.-D.; Gates, B.; Xia, Y.-N. Adv. Mater. 2000, 12, 206-209. (d) Yin, Y.-D.; Lu, Y.; Gates, B.; Xia, Y.-N. Chem. Mater. 2001, 13, 1146-1148. (e) Li, Y.-L.; Ishigaki, T. Chem. Mater. 2001, 13, 1577-1584. (6) Fleming, M. S.; Mandal, T. K.; Walt, D. R. Chem. Mater. 2001, 13, 2210-2216. (7) Rao, C. R. N.; Satishkumar, B. C.; Govindaraj, A. Chem. Commun. 1997, 1581-1852.

of some silicate zeolites with micropores have been fabricated through a layer-by-layer technique using PS beads as templates.8 The strategy of oil-in-water emulsion chemistry has been applied to create hollow spheres of calcium carbonate9 and even mesostructured silica.10-12 Through liquid crystalline templates, mesoporous aluminosilicate, pure alumina, and pure silica have been created with hierarchical morphologies for various potential applications in chemical catalysis, separation, chromatography, microencapsulation, electronic engineering, and optical devices.10-23 Of these, mesostructured (8) (a) Wang, X.-D.; Yang, W.-L.; Tang, Y.; Wang, Y.-J.; Fu, S.-K.; Gao, Z. Chem. Commun. 2000, 2161-2162. (b) Zhu, G.-S.; Qiu, S.-L.; Terasaki, O.; Wei, Y. J. Am. Chem. Soc. 2001, 123, 7723-7724. (9) Walsh D.; Mann, S. Science 1995, 377, 320-323. (10) (a) Schacht, S.; Huo, Q.; Voigt-Martin, I. G.; Stucky, G. D.; Schu¨th, F. Science 1996, 273, 768-771. (b) Singh, P. S.; Kosuge, K. Chem. Lett. 1998, 101-102. (b) Kosuge, K.; Singh, P. S. Microporous Mesoporous Mater. 2001, 44-45, 139-145. (11) Lin, H.-P.; Cheng, Y.-R.; Mou, C.-Y. Chem. Mater. 1998, 10, 3772-3776. (12) Bruinsma, P. J.; Kim, A. Y.; Liu, J.; Baskaran, S. Chem. Mater. 1997, 9, 2507-2512. (13) (a) Kosuge, K.; Singh, P. S. Chem. Mater. 2001, 13, 2476-2482. (b) Gru¨n, M.; Lauer, I.; Unger, K. K. Adv. Mater. 1997, 9, 254-257. (c) Huo, Q.-S.; Feng, J.-L.; Schu¨th, F.; Stucky, G. D. Chem. Mater. 1997, 9, 14-17. (d) Boissie`re, C.; Lee, A. V. D.; Mansouri, A. E.; Larbot, A.; Prouzet, E. Chem. Commun. 1999, 2047-2048. (14) Lu, Y.-F.; Fan, H.-Y.; Stump, A.; Ward, T. L.; Rieker, T.; Brinker, C. J. Nature 1999, 398, 223-226. (15) For example, see: (a) Mou, C.-Y.; Lin, H.-P. Pure Appl. Chem. 2000, 72, 137-146. (b) Mokaya, R. Microporous Mesoporous Mater. 2001, 44-45, 119-127. (c) Wang, L.-Z.; Shi, J.-L.; Gao, J.-H.; Tomura, S.; Yang, D.-S. J. Non-Cryst. Solids 2000, 278, 178-186. (d) Yang, H.; Coombs, N.; Ozin, G. A. Nature 1997, 386, 692-695. (e) Ozin, G. A.; Yang, H.; Sokolov, I.; Coombs, N. Adv. Mater. 1997, 9, 662-667. (f) Yang, H.; Vovk, G.; Coombs, N.; Sokolov, I.; Ozin, G. A. J. Mater. Chem. 1998, 8, 743-750. (g) Boissie`re, C.; Larbot, A.; Lee, A. V. D.; Kooyman, P. J.; Prouzet, E. Chem. Mater. 2000, 12, 2902-2913. (h) Tanev, P. T.; Liang, Y.; Pinnavaia, T. J. J. Am. Chem. Soc. 1997, 119, 8616-8624. (16) (a) Lin, H.-P.; Mou, C.-Y.; Liu, S. B. Adv. Mater. 2000, 12, 103106. (b) Lin, H.-P.; Mou, C.-Y. Science 1996, 273, 765-768.

10.1021/la020159k CCC: $25.00 © 2003 American Chemical Society Published on Web 01/16/2003

Hollow Spheres Based on Mesostructured Pb Titanate

silicas of both hard and hollow spheres are of significantly current interest.10-14 In the mesostructured inorganic/organic composites, there are compatible interactions between the interfaces of inorganic networks and organic surfactant assemblies through covalent, electrostatic, or hydrogen bonding.20-23 The hydrogen bonding between neutral alkylamine micelles and metal alkoxide reagents forces self-assembly of mesostructure with wormhole frameworks, that is not readily accessible by general covalent or electrostatic templating.23 Thick inorganic walls with improved stability have been generated by using long-chain alkylamines as a template due to weak repulsive interaction at the organic/inorganic interphase.23 Relative to charged surfactants, neutral surfactants can be removed noncorrosively by solvent extraction.23 Recently, more and more research has been focusing on non-silica mesostructured materials.18,19,24-33 These materials include transition metal oxides,24-30 tin oxide,31 lead oxide,20 magnetic rare earth oxides,32 complex oxides,30 oxide solid solutions,24 and even sulfides.33 Neutral amine supermolecular assembly pathways have allowed successful formation of not only mesostructured silica,23 but also metal oxides,25-27 and even sulfides as well.33 Owing to wide application in chemical catalysis, electronic devices, and optoelectronic sensors, many results on synthesis, structure, porosity, and physical properties of titania and titanates have been published.1-5,27-43 Complex oxides of lead and titanium crystallizing in perovskite, pyrochlore, or tetragonal body-centered structure, and PbO-TiO2 solid solutions have been synthesized hydrothermally in alkaline aqueous mediums. The extra (17) (a) Huo, Q.-S.; Zhao, D.-Y.; Feng, J.-L.; Weston, K.; Buratto, S. K.; Stucky, G. D.; Schacht, S.; Schu¨th, F. Adv. Mater. 1997, 9, 974-978. (b) Yang, P.-D.; Zhao, D.-Y.; Chmelka, B.-F.; Stucky, G. D. Chem. Mater. 1998, 10, 2033-2036. (18) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56-77. (19) Sayari, A.; Liu, P. Microporous Mater. 1997, 12, 149-177. (20) Huo, Q.-S.; Margolese, D. I.; Ciesla, U.; Feng, P.-Y.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schu¨th, F.; Stucky, G. D. Nature 1994, 368, 317-323. (21) (a) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science 1995, 269, 1242-1244. (b) Zhao, D.-Y.; Feng, J.-L.; Huo, Q.-S.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548-552. (22) (a) Bagshaw, S. A.; Pinnavaia, T. J. Angew. Chem., Int. Ed. Engl. 1996, 35, 1102-1105. (b) Prouzet, E.; Pinnavaia, T. J. Angew. Chem., Int. Ed. Engl. 1997, 36, 516-518. (23) (a) Tanev, P. T.; Pinnavaia, T. J. Science 1995, 267, 865-867. (b) Mori, Y.; Pinnavaia, T. J. Chem. Mater. 2001, 13, 2173-2178. (c) Pauly, T. R.; Pinnavaia, T. J. Chem. Mater. 2001, 13, 987-993. (24) Mamak, M.; Coombs, N.; Ozin, G. J. Am. Chem. Soc. 2000, 122, 8932-8939. (25) Sun, T.; Ying, J. Y. Nature 1997, 389, 704-706. (26) Ulagappan, N.; Rao, C. R. N. Chem. Commun. 1996, 16851686. (27) Wang, Y.-Q.; Tang, X.-H.; Yin, L.-X.; Huang, W.-P.; Hacohen, Y. R.; Gedanken, A. Adv. Mater. 2000, 12, 1183-1186. (28) Antonelli, D. M.; Ying, J. Y. Angew. Chem., Int. Ed. Engl. 1995, 34, 2014-2017. (29) Stone, V. F.; Davis, J. R. J. Chem. Mater. 1998, 10, 1468-1474. (30) Yang, P.-D.; Zhao, D.-Y.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Nature 1998, 396, 152-155. (b) Yang, P.-D.; Zhao, D.-Y.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 1999, 11, 2813-2826. (31) Severin, K. G.; Abdel-Fattah, T. M.; Pinnavaia, T. J. Chem. Commun. 1998, 1471-1472. (32) Yada, M.; Kitamura, H.; Ichinose, A.; Machida, M.; Kijima, T. Angew. Chem., Int. Ed. Engl. 1999, 38, 3506-3510. (33) Li, J.-Q.; Nazar, L. F. Chem. Commun. 2000, 1749-1750. (34) For examples, see: (a) Vichi, F. M.; Tejedor-Tejedor, M. I.; Anderson, M. A. Chem. Mater. 2000, 12, 1762-1770. (b) Kelly, C. A.; Farzad, F.; Thompson, D. W.; Meyer, G. J. Langmuir 1999, 15, 731737. (c) Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weisso¨rtel, F.; Salbeck, J.; Spreitzer, H.; Gra¨tzel, M. Nature 1998, 395, 583-585. (d) Papageorgiou, N.; Barbe´, C.; Gra¨tzel, M. J. Phys. Chem. B 1998, 102, 4156-4164. (e) Burnside, S.; Moser, J. E.; Brooks, K.; Gra¨tzel, M.; Cahen, D. J. Phys. Chem. B 1999, 103, 9328-9332.

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impurity of lead in an as-synthesized product can be noncorrosively washed away with dilute acetic acid.38-40 The variety of reaction conditions afford the possibilities to yield various structures of products with different compositions and morphologies as well, for instance, perovskite-structured lead titanate with a hard spherical morphology.40 It has been reported that perovskite PbTiO3 particles have been prepared via use of water-in-oil emulsions (or reverse micelles) that act as microreactors to confine crystal growth.43 Conversely, can we create hollow spheres with lead titanate shells through the oilin-water route, especially, porous lead titanate shells? Much effort has been made to prepare porous electronic materials with specific morphologies to optimize the mass transport properties.34,37,42 Considering the similarity of the electronegativity of lead and titanium to that of silicon and aluminum, it will be possible to prepare porous lead titanate.4,42 In this paper, we will describe the preparation of mesoporous lead titanate, a non-silica oxide by using alkylamines as surfactants. As the formation of vesicletemplated mesolamellar aluminophosphates,44 the design and fabrication of a specified macroscopically bulky morphology are of technological and commercial importance.1 As oil-in-water emulsions could yield macroporous materials and hollow spheres of mesoporous silica, herein hollow spheres of mesostructured lead titanate have been explored via an oil-in-water emulsion-mediated route. 2. Experimental Procedure Preparation. On consideration of the disadvantage of strongly and directly electrostatic interaction between counterionic organic surfactants and inorganic framework (S+I- or S-I+ pathway) for the creation of hollow spheres of mesostructured composite, neutral amine surfactants (S°) have been used herein to direct the formation of mesostructured lead titanate. The much less reactive titanium (acetylacetone) alkoxide has been used as inorganic precursor of titanium to prepare mesostructured TiO2 by a modified ligand-stablized sol-gel method.28,29 Here, titanium butoxide is used as the titanium source and dissolved into the oil droplets of acetylacetone. Acetylacetone (2,4-pentanedione) can also act as a bidentate ligand to titanium atom to inhibit hydrolysis and condensation of the titanium alkoxide. Milder hydrolysis of the alkoxide can moderate the precipitation of dense (35) For example, see: (a) Wu, M.-M.; Long, J.-B.; Wang, G.-G.; Huang, A.-H.; Luo, Y.-J.; Feng, S.-H.; Xu, R.-R. J. Am. Ceram. Soc. 1999, 82, 3254-3256. (b) Wang, H.-Z.; Zhao, F.-L.; He, Y.-J.; Zheng, X.-G.; Huang, X.-G.; Wu, M.-M. Opt. Lett. 1998, 23, 777-779. (c) Wu, M.-M.; Long, J.-M.; Huang, A.-H.; Luo, Y.-J.; Feng, S.-H.; Xu, R.-R. Langmuir 1999, 15, 8822-8825. (36) Sharma, P. K.; Varadan, V. V.; Varadan, V. K. Chem. Mater. 2000, 12, 2590-2596. (37) For example, see: (a) Dogheche, E.; Jaber, B.; Re´miens, D. Appl. Opt. 1998, 37, 4245-4248. (b) Yamamura, M.; Tsuzuki, N.; Okado, H.; Wakatsuki, T.; Otsuka, K. Appl. Catal. A: General 1994, 115, 269283. (c) Kholkin, A.; Seifert, A.; Setter, N. Appl. Phys. Lett. 1998, 72, 3374-3376. (38) (a) Lencka, M. M.; Riman, R. E. Chem. Mater. 1993, 5, 61-70. (b) Lencka, M. M.; Riman, R. E. J. Am. Ceram. Soc. 1993, 76, 26492659. (39) For example, see: (a) Cheng, H.-M.; Ma, J.-M.; Zhou, Z.-G.; Qiang D.; Li, Y.-X.; Yao, X. J. Am. Ceram. Soc. 1992, 75, 1123-1128. (b) Cheng, H.-M.; Ma, J.-M.; Zhao, Z.-G. Chem. Mater. 1994, 6, 1033-1040. (c) Gelabert, M. C.; Gersten. B. L.; Riman, R. E. J. Cryst. Growth 2000, 211, 497-500. (40) Choi, J. Y.; Kim, C. H.; Kim, D. K. J. Am. Ceram. Soc. 1998, 81, 1353-1356. (41) Imhof, A.; Pine, D. J. Nature 1997, 389, 948-951. (42) Guo, Y.-H.; Qiu, S.-L.; Pang, W.-Q.; Ohnishi, N.; Hiraga, K. Stud. Surf. Sci. Catal. 1994, 84, 251-258. (43) (a) Fang, J.; Wang, J.; Ng, S. C.; Chew, C. H.; Gan, L. M. J. Mater. Sci. 1999, 34, 1943-1952. (b) Lu, C.-H.; Wu, Y.-P. Mater. Lett. 1996, 27, 13-16. (44) (a) Oliver, S.; Kuperman, A.; Coombs, N.; Lough, A.; Ozin, G. A. Nature 1995, 378, 47-50. (b) Oliver, S.; Coombs, N.; Ozin, G. A. Adv. Mater. 1995, 7, 931-935. (c) Ozin, G. A.; Oliver, S. Adv. Mater. 1995, 7, 943-947.

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mass and probably lead to the yield of elegant morphology of mesostructured composites. All chemical reagents used in our experiment were directly from commercially available sources without further purification. Titanium butoxide and dodecylamine were obtained from Xinhua Chemicals and Shanghai Chemical Plant III, respectively. All other regents were purchased from Guangzhou Chemical Company. In a typical synthesis, 1.7 mL of titanium butoxide was added dropwise into 2.0 mL of acetylacetone under magnetic stirring to form a clear mixture. With contined stirring, a solution of 1.0 g of dodecylamine in 2.0 mL of 1-butanol was then slowly introduced as a template for the formation of the mesostructure. Following this step was a slow addition of 16 mL of an aqueous solution of 0.33 M lead acetate into the above organic/titania composite under milder stirring. A clear sol was formed. Finally, with the addition of 16 mL of an aqueous solution of 0.5 M KOH, the sol turned into an opaque white suspension and the pH reached 9.0-9.5. This pH value was suitable for hydrothermal synthesis of lead titanate, according to the thermodynamic modeling and experiment results.38-40 The resulting mixture with a molar composition of 5.3PbO:5.0TiO2:20acetylacetone:5.5dodecylamine:22 1-butanol:4.0K2O:1780H2O was aged in a tightly closed Teflon-lined stainless steel autoclave (purchased from Yongjia) at 80 °C for 10 h. The capacity of the Teflon cup was 50 mL. The molar composition was fixed at the above ratio for synthesis in the text if not otherwise specialized. The concentration of KOH was described as a ratio of K2O/H2O for accuracy. After the autoclave was cooled to room temperature, a yellowish precipitate was separated by filtration and then washed with acetone, alcohol, and distilled water, respectively, for 2-3 times. Finally, the yellowish precipitate was dried in a desiccator at ambient temperature. Characterization. Products were characterized by powder X-ray diffraction collected on a Philips model PW1830 diffractometer using Cu KR radiation (λ ) 0.1541 nm) and a graphite monochromator operating at 40 kV and 30 mA with a scanning rate at 0.05° 2θ s-1. Sizes and shapes of particles were confirmed on a JEOL JSM-6330F field emission scanning electron microscope. Dried samples for scanning electron microscopy (SEM) were placed on glass-slice surfaces and then sputter-coated with platinum. The TEM images were obtained on a JEOL 2010 highresolution transmission electron microscope. Samples for HRTEM were prepared by dispersing powders directly on holey carboncoated copper grids. EDS data were recorded on the Link ISIS300X Oxford Instrument with a 20.0 kV accelerating voltage. Thermogravimetric analysis (TGA) of a sample was performed in a platinum crucible on Perkin-Elmer thermal analysis equipment with thermally and chemically inert Al2O3 powder as reference, operating at a heating rate of 5 °C min-1 between 20 °C and 700 °C. Chemical extraction is a promising method to remove amine molecules from voids of mesostructured composites.27,29 A cosolvent of ethanol and dilute acetic acid was used to extract organic molecules from voids. After chemical leaching at room temperature for 12 h, the powder was filtered, washed, and dried in air at 150 °C or 250 °C for 5 h. N2 adsorption-desorption isotherms were measured at 77 K on a Micromeritics ASAP 2010 system using static adsorption procedures after samples had been outgassed in a vacuum of about 10-6 Torr at 150 °C overnight. Calculations of surface area were supplied directly by commercial software accompanying the system according to the IUPAC recommendations.45 The porosity was defined as the total volume of pores inside the oxide and was calculated on the basis of the single-point total pore volume and the density of the inorganic network.

3. Results and Discussion In hydrothermal synthesis of lead titanate, the ratio of lead to titanium is usually slightly higher than one and sodium or potassium hydroxide has been used as mineralizer.38 Generally speaking, anatase will be formed if (45) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603-619.

Wu et al.

Figure 1. Powder XRD patterns of mesoporous lead titanate PTM-1 (a) before and (b) after chemical extraction and followed by physical calcinations. Inset: selected-area electron diffraction (SAED).

no alkaline mineralizer is used under hydrothermal conditions.38 In this work, the ratio of lead/titanium was fixed at 5.3:5.0 and aqueous solution of KOH was served as mineralizer to promote the generation of chemically mixed frameworks of PbO-TiO2. Various sets of experimental results show that the optimal mixture for preparation of the mesophase is at a molar composition of 5.3PbO:5.0TiO2:20acetylacetone:5.5dodecylamine:22 1butanol:1780 H2O. Without the addition of KOH solution, only the amorphous phase or poor mesostructure is produced. Well-defined mesostructures of lead titanate can be formed at molar ratios of K2O/H2O in a range from 4.0/1780 to 10.0/1780. The product formed from the typical mixture at 80 °C for 10 h, being designated as PTM-1, is identified by powder X-ray diffraction (XRD) as shown in Figure 1a. The first intense diffraction peak locates at a d-value of 3.6 nm. The lack of apparently discrete peaks at higher angles, and the broad and weak band around 30°/2θ implies the long-range disorder and amorphous inorganic framework. At K2O/H2O molar ratios of 4.0/ 1780 and 10.0/1780, both the d spacing and the intensity of the intense peak increase as the reaction temperatures increasing to 80 °C (Figure 2). This is indicative of the so-called S°I° pathway.23 It has been well and extensively documented that the diffraction at a lower angle (