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Highly Ordered Lamellar Mesostructure of Nanocrystalline PbSO4 Prepared by Hydrothermal Treatment Bin Deng,†,‡ Cun-Chang Wang,‡ Qiang-Guo Li,‡ and An-Wu Xu*,† DiVision of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, and Hefei 230026, China, Department of Chemistry and Life Sciences, Xiangnan UniVersity, Chenzhou 423000, China ReceiVed: April 21, 2009; ReVised Manuscript ReceiVed: September 7, 2009
A lamellar PbSO4 mesophase with well-crystallized walls was synthesized using sodium dodecyl sulfate (SDS) as a template and bis[3-(triethoxysilyl)propyl]tetrasulfide (TESPTS) as a structure-stabilizing agent through hydrothermal treatment method. The obtained layered mesostructure was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM), energy-dispersive X-ray spectroscopy (EDS), and IR spectrum. The photoluminescence properties of the products were also investigated. The sample gives two UV emission bands centered at 340 and 360 nm and four visible light emission bands in the range of 400-500 nm using an excitation wavelength of 270 nm. The results have demonstrated that TESPTS with more hydrophobic character could be responsible for the formation of this stabilized lamellar PbSO4 mesophase. A possible formation mechanism for the lamellar PbSO4 mesostructure was also proposed. Such lamellar mesostructures could provide a useful precursor for potential applications of PbSO4. 1. Introduction It is well-known that the properties of nanocrystals depend not only on their chemical composition but also on their shape, phase, structure, size, and size distribution.1-3 In the past few years, a number of methods have been developed to control not only the morphology but also the size and structures of nanocrystals. Among these methods, the template self-assembly process has been demonstrated to be an effective route because the preorganized template can regulate the nucleation, growth, morphology, and orientation of nanocrystals.4-6 Some kinds of templates, cylindrical,7 reversed micelles,8 vesicles,9 cubic phases of surfactants,10 liquid crystalline phases,11 surfactant monolayers and Langmuir-Blodgett films,12 aqueous organosiloxane solutions,13 and block copolymers14 were used as the template to synthesize metal, metal oxide, metal sulfide, and other semiconductors. The goal is not only to optimally control the morphology, shape, size, and polydispersity of nanocrystals but also to fabricate novel organic/inorganic composite nanomaterials with interesting optical, electrical, and magnetic properties. Since the first report of M41S silica mesophase in 1992,15,16 surfactant-templated synthetic processes based on the hydrolysis and cross-linking of inorganic precursors at the surfaces of supramolecular surfactant assemblies have been used to synthesize new families of hexagonal, cubic, or lamellar mesostructures.17-20 Self-assembly of molecules into a fascinating diversity of mesostructured frameworks has attracted much attention from a wide range of scientific interests and applications.21-23 A number of layered materials such as silicas, transition metal oxides, and aluminophosphates have been obtained using surfactants such as primary amines and quaternary ammonium * To whom correspondence should be addressed. Fax: 86-551-3600724. E-mail:
[email protected]. † University of Science and Technology of China. ‡ Xiangnan University.
ions.24-26 However, most of the related lamellar mesostructures reported to date readily collapse after hydrothermal treatment.21,24-26 Lead-acid batteries play an important role in secondary batteries owing to their high ratio of performance to cost and applications in many fields, such as electromedical equipment.27 As a precursor of positive active material (PAM) of lead-acid battery, PbSO4 has many advantages.28 It is also a promising scintillator material for the use in PET detector systems due to its high photon stopping power, yield, and fast decay speed.29 What is more, PbSO4 is a well-known industrial product and has a wide range of applications in white dye, quickly driedpaint, and so on.30 Up to date, many kinds of sulfates with different morphologies have been prepared through various methods, such as platelike PbSO4 nanocrystals,30 lead sulfate films with two-layer structure,31 lead sulfate crystals,32 and single-crystal PbSO4 nanorods.33 Among the various inorganic structures with nanoscale dimensions, well-ordered mesostructures, and morphological specificity, the fabrication of multifarious mesostructured materials with a lamellar structure has received much attention because of the potential applications in membrane-based separations, selective catalysis, and sensors.34 However, most of the related lamellar mesostructures reported to date readily collapsed to amorphous oxides.35 So it is fascinating and challenging to synthesize mesoporous materials with highly ordered and thermally stable lamellar structure via a simple hydrothermal or sol-gel process. In this study, stabilized lamellar PbSO4 nanocrystals with well-crystallized walls were synthesized by hydrothermal treatment of a precursor layered Pb(DS)2-TESPTS composite. To the best of our knowledge, this is the first experimental example that highly ordered lamellar PbSO4 nanocrystals have been synthesized effectively on a large scale by this simple hydrothermal approach.
10.1021/jp9036765 CCC: $40.75 2009 American Chemical Society Published on Web 10/06/2009
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2. Experimental Section 2.1. Synthesis of Lamellar Pb(DS)2-TESPTS Mesophase Precursor. All reagents were analytical grade and used without further purification. The sodium dodecyl sulfate (SDS) was purchased from Aldrich. Lead acetate trihydrate (Pb(CH3COO)2 · 3H2O), bis[3-(triethoxysilyl)propyl]tetrasulfide ((C2H5O)3Si(CH2)3S4(CH2)3Si(OC2H5)3, named as TESPTS), and absolute ethanol were obtained from Shanghai Chemical Reagents Co. The water used in all experiments was prepared in a three-stage Millipore Milli-Q plus 185 purification system. In a typical synthesis, 0.01 mol of SDS was dissolved in 45 mL of deionized water to form a transparent solution under stirring; 0.005 mol of lead acetate and 0.0025 mol of TESPTS were dissolved in 2 mL of deionized water and 30 mL of absolute alcohol under stirring for 24 h in order to ensure TESPTS hydrolyzed completely, then slowly added in the above SDS solution. After complete addition, the pH of the mixture solution was adjusted to the range of 3.0-6.0 by addition of HCl aqueous solution. After reacting for 24 h at room temperature under stirring, the resulting solid product was filtered, washed with distilled water and absolute alcohol to remove free ions possibly remaining in the final products, and finally dried at 30 °C in air. The obtained precipitate is nominated as Pb(DS)2-TESPTS, and a yield of about 85-90% is estimated on the basis of Pb. 2.2. Preparation of Highly Ordered Lamellar Structure of Crystalline PbSO4. An amount of 0.12 g of the obtained Pb(DS)2-TESPTS composite was added in 16 mL of water under stirring for 30 min, then the mixture was poured into a Teflon-lined stainless steel autoclave (capacity, 20 mL), then sealed and maintained in an oven at 130 °C for 12 h. After the reaction was completed, the resulting solid product was filtered, washed with distilled water and absolute alcohol, and finally dried at 60 °C in air. 2.3. Characterization. The low-angle and wide-angle X-ray powder diffraction (XRD) diagrams of all samples were obtained using a Rigaku/Max-3A X-ray diffractometer using Cu KR radiation (λ ) 1.5418 Å), with the operation power maintained at 3 kW. Scanning electron microscopy (SEM) images were performed on a JEOL JEM-6330F microscope operating at a beam energy 15 kV. Transmission electron microscopic (TEM) images, high-resolution transmission electron microscopic (HRTEM) images, and the selected area electron diffraction (SAED) patterns were obtained on a JEOL-2010 microscope with an accelerating voltage of 200 kV. Energy-dispersive X-ray spectroscopy (EDS) was attached to the JEOL 2010. Sample grids were prepared by sonicating powdered samples in ethanol for 10 min and evaporating one drop of the suspension onto a carbon-coated, holey film supported on a copper grid for TEM measurements. Fourier transform infrared (FTIR) spectra were recorded using the KBr pellet method on a Bruker EQUINOX FT-IR spectrometer in the range of 400-4000 cm-1 at a resolution of 2 cm-1. The photoluminescence (PL) emission spectra were recorded with a F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source; PL spectra were recorded under an excitation wavelength of 270 nm at room temperature. 3. Results and Discussion The layered Pb(DS)2-TESPTS mesophase precursor was first synthesized at room temperature by using SDS as the template in the presence of a organosilane coupling agent TESPTS. The synthetic procedure results in the formation of a gray sheetlike product. Figure 1 shows the powder X-ray diffraction (XRD) pattern of the as-synthesized Pb(DS)2-TESPTS mesophase
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Figure 1. XRD pattern of the obtained lamellar Pb(DS)2-TESPTS mesophase. Inset: low-angle XRD pattern of the sample.
precursor. The appearance of two groups of 00L reflections for the Pb(DS)2-TESPTS precursor at low angles indicates that a layered structure with two interlayer d-spacings was formed. The interlayer distances of the precursor compounds are determined based on [001] reflections. The peak with the low intensity at 2θ ) 3.52° reflects one type of interlayer distance with d value of 2.52 nm, corresponding to the 001(I) diffraction of the bilamellar phase. The peaks at 2θ ) 7.05, 10.56, and 14.08° could be assigned to its second-, third-, and fourth-order reflections, respectively. Another interlayer distance with d value is found to be 2.35 nm, as determined by the peak at 2θ ) 3.77° ascribed to the 001(II) diffraction of the bilamellar phase. The appearance of two layer distances are not equal to the length of two all-trans dodecyl sulfate chains, which is roughly 40-42 Å,36 indicating that partial overlap of the alkyl chains and/or tilt angles in the molecule axis between the inorganic layers could be present. The difference in the layer distances may be attributed to the difference in the overlap and/or tilt angles. Moreover, a large number of other reflections that appeared at the wide-angle part of the pattern are much smaller, indicating that the lamellar structure is almost perfectly ordered to a very long range. Figure 2a shows the possible arrangements of the intercalated SDS anions that can account for the observed occurrence of two different expansions in interlayer spacings. Two panels as shown in Figure 2a are mainly made up of the SDS template and inorganic precursors by electrostatic interactions, in which two all-trans dodecyl chains sandwich between the internal surfaces of the opposing inorganic layers via partial overlap of hydrocarbon chains and/or tilted bonding. It should be noted that other possibilities for the formation of bilayers can not be excluded and deserve further study in detail. TESPTS, an organotriethoxysilane with tetrasulfide group among its molecular structure containing covalently attached hydrophobic tails, becomes amphiphilic when silanol (Si-OH) groups are formed by hydrolysis.37 The polar segment is a silanol group (-Si-OH) that can interact with the hydrophilic sulfate head of SDS and inorganic species in system through hydrogen bonding and van der Waals force, and the nonpolar parts are hydrocarbon and sulfur chains which could be incorporated into the hydrophobic segment of SDS bilayers and span between the Pb2+ sheets. Therefore, hydrolyzed TESPTS products act as a coupling agent between the SDS organic molecules and inorganic species, as shown in Figure 2a. The SEM image of layered Pb(DS)2-TESPTS mesophase precursor is shown in Figure 3. It is important to notice that the lamellar framework is a common characteristic for the sheetlike morphology; each
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Figure 2. (a) Cartoon illustrating the formation of SDS-mediated assembly of bilamellar structured Pb(DS)2-TESPTS mesophase precursor. (b) Schematic drawing of the mechanism model of hydrolyzed TESPTS for stabilizing as-made lamellar PbSO4 nanocrystals.
Figure 3. SEM image of the obtained lamellar Pb(DS)2-TESPTS mesophase at room temperature in the presence of TESPTS by using SDS as the template.
layer is accumulated regularly in a parallel way. The average length and thickness of the aggregates are estimated in the range of 3-5 and 0.5-1 µm, respectively. To obtain lamellar PbSO4 nanocrystals, we further carried out hydrothermal treatment of layered Pb(DS)2-TESPTS precursor at 130 °C for 12 h in an autoclave and found that a highly ordered lamellar structure of crystalline PbSO4 was obtained. Figure 4a shows a wide-angle XRD pattern of the samples after hydrothermal treatment. All diffraction peaks shown in Figure 4a can be indexed as a primitive orthorhombic lattice of PbSO4 (anglesite) with cell parameters a ) 6.96, b ) 8.48, and c ) 5.37 Å, which are in good agreement with those reported in the literature (JCPDS card no. 36-1461). The most intensified peak for (020) indicates that the PbSO4 crystals are highly oriented along this direction. It is worth noting that the obtained PbSO4 crystals have a highly ordered lamellar structure, as clearly shown in Figure 4b. An interlayer distance is found to be 3.96 nm, as determined by the first peak in Figure
Figure 4. Wide-angle (a) and low-angle (b) XRD patterns of the as-made lamellar PbSO4 nanocrystals grown by hydrothermal treatment at 130 °C for 12 h.
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Figure 5. TEM images (A, B, E, and F) and HRTEM image (C and D) of an ultrathin section of the PbSO4 products obtained under hydrothermal treatment at 130 °C for 12 h. The inset in panel D is the SAED pattern taken from the [-110] zone axis. The white arrows point to nanorods formed by the fusion of lamella.
4b. When hydrothermal treatment temperature was increased over than 150 °C, the low-angle XRD peaks completely disappeared (data not shown), indicating that the lamellar structure collapsed. On the other hand, in the absence of TESPTS silane, layered mesophase was also produced; however, no lamellar PbSO4 crystals was obtained under hydrothermal treatment, indicating that TESPTS plays an important role in stabilizing the layered structure of PbSO4 crystals due to its strong hydrophobic feature and the inorganic/organic coupling effect. The possible mechanism of TESPTS for stabilizing asmade lamellar PbSO4 nanocrystals and spanning between inorganic layers is shown in Figure 2b. Transmission electron microscopy and HRTEM were also employed to provide further insight into the structure and morphology of the synthesized lamellar PbSO4 nanocrystals. A typical TEM image of an ultrathin section of the obtained lamellar PbSO4 nanocrystals, as shown in Figure 5A, displays a large domain of highly ordered lamellar mesostructure,
consisting of alternating fringes of dark and bright contrast. The dark contrasts represent the inorganic walls composed of PbSO4 nanocrystals, and the bright layers are the organic layers, which is the space between them in which SDS molecules and silane TESPTS molecules are embedded. The interlayer distance, as measured from the TEM images, is 4.05 nm, in agreement with the basal spacing of 3.96 nm obtained from the low-angle XRD analysis. Moreover, the inorganic PbSO4 layers are well crystallized, as clearly observed in the HRTEM image (Figure 5C). The distance between the lattice fringes is calculated to be 4.23 Å, consistent with the face spacing of (020) of anglesite PbSO4 crystals (d020 ) 4.24 Å). It has to be mentioned that a small quantity of PbSO4 nanorods (ca. 10%) can also be observed in the final product (Figure 5B). The TEM image in Figure 5B shows that the diameter of each nanorod is about 10-15 nm, and the nanorods tend to align in a parallel fashion. The formation of nanorods is caused by fusion of the PbSO4 walls when some SDS molecules were extruded, as clearly
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Figure 6. EDS spectrum of the as-prepared lamellar PbSO4 mesophase. Cu came from the TEM grid.
shown in Figure 5, parts E and F (the white arrows point to nanorods formed by the fusion of lamella). Therefore, the lamella are likely to be metastable structures or an intermediate for the formation of nanorods. Figure 5D is a HRTEM image taken from a single nanorod (as indicated in the white frame in Figure 5B), giving resolved lattice fringes of (010) planes of PbSO4 nanocrystals (d010 ) 0.847 nm). The electron diffraction pattern (inset in Figure 5D) recorded from the [-110] zone axis confirms that each nanorod is a perfect single crystal. The nanorods grow along the [010] direction. This is agreement with the XRD pattern showing the most intensified peak of the (020) face. Previous study also shows that PbSO4 nanorods grew along the [010] direction.35 Energy-dispersive X-ray spectroscopy analysis shows that the sample contains Pb, S, and O elements (Figure 6), confirming the composition of the product is PbSO4. In the anion surfactant-inorganic layered solid intercalating systems, the anion headgroup is believed to be tethered to the internal surface of the galleries of the layered inorganic hosts by coulomb forces. The surfactant molecules can adopt a variety of structures in the galleries, such as monolayer, bilayer, and paraffin-type monolayer, depending on the grafting density and the structure of the surfactant methylene tail. In addition, the methylene chain can behave as the trans or gauche conformation, which differ from each other distinctively in their vibrational modes. FTIR spectroscopy has been used as a sensitive tool to probe the structure and organization of these samples. Figure 7, parts A and B, shows the IR spectrum of commercial bulk PbSO4 crystals and the obtained lamellar PbSO4 product, respectively. Good band resolution is achieved for vibration modes both of the organic and inorganic moieties. Bands due to the lamellar PbSO4 framework are very similar to those appearing in the IR spectrum of bulk PbSO4 crystal, both in energy and relative intensity. It is well-known that the positions of the antisymmetric stretching band (2920 cm-1) and the symmetric stretching band (2850 cm-1) of CH2 groups (abbreviated Vas (CH2) and Vs (CH2), respectively) in n-alkyl chains are sensitive to chain conformation and that these two bands shift to lower wavenumbers if disorder is introduced into the n-alkyl chains.36,38-40 Each IR spectrum of lamellar PbSO4 mesophases exhibits the Vas (CH2) band at 2920 cm-1 and the Vs (CH2) band at 2852 cm-1, indicating that the n-alkyl chains of SDS possess an ordered all-trans conformation in the interlayer space. On the other hand, The strong dodecyl sulfate S-O stretching peaks around 1200 cm-1 and CO-S stretch peak at 855 cm-1 also prove that the surfactant of SDS is an integer component of the composite material obtained.41 In addition, about the characteristics of the TESTPS molecules in the product, the FTIR spectrum shows sharp bands in the highenergy range (2956, 1620, 1467, 886, 789, 684, and 471 cm-1)
Figure 7. FTIR spectra of bulk PbSO4 crystals (A) and our obtained lamellar PbSO4 nanocrystals (B).
that can be assigned to C-H, O-H, Si-O, Si-C, and S-S vibration modes for TESTPS molecules, respectively.42 The results clearly show the existence of TESPTS in the layered PbSO4 products. The room-temperature PL properties of as-prepared lamellar PbSO4 nanocrystals were investigated. The sample gives two emission bands centered at 340 and 360 nm (UV band) using an excitation wavelength of 270 nm, as shown in Figure 8. Both emission peaks are consistent with that reported previously which was attributed to radiative dissociation of an anion exciton.35,43 In addition, the highly ordered lamellar structure of crystalline PbSO4 also gives four well-resolved emission bands in the range of 400-500 nm, which are not observed previously, and their origins need to be further investigated. The luminescence properties of the obtained layered PbSO4 crystals could be of interest for engineering of the scintillation properties and fundamental research.
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Deng et al. groups including hydrocarbon chains and tetrasulfide bonds. The silanol groups are capable of interacting with PbSO4 through hydrogen bonding, whereas the nonpolar groups inhabiting between two inorganic walls are hydrophobic, leading to a strong hydrothermal stability of the lamellar PbSO4 mesostructure (Figure 2b). Detailed studies on the stabilizing mechanism of this lamellar structure under hydrothermal treatment in the present system are now in progress. 4. Conclusions
Figure 8. Room-temperature PL spectrum of the as-prepared lamellar PbSO4 mesophase with excited wavelength of 270 nm.
In control experiments, the influence of the pH value of the solution and the molar ratio of SDS to TESPTS on the growth of layered PbSO4 nanocrystals was also investigated. It is found that ordered lamellar Pb(DS)2-TESPTS precursor was not formed under neutral or alkaline conditions. By increasing the molar ratio of SDS to TESPTS to twice or higher than that in the typical synthesis while without changing other experimental conditions, rodlike PbSO4 nanocrystals were obtained instead of a lamellar nanocrystal structure. In addition, lamellar PbSO4 nanocrystals could not be observed in the absence of TESPTS, indicating TESPTS can stabilize layered PbSO4 mesostructure under hydrothermal conditions. Our experimental results show that the crystal formation of PbSO4 with two-dimensional lamellar structure originates from layered Pb(DS)2-TESPTS mesophase precursors and is stabilized by TESPTS. We suggest that the following possible formation mechanism of the PbSO4 lamellar structure could be divided into three steps: (i) The SDS surfactant molecules condensed into aggregations with Pb2+ ions intercalated into the interspaces between the headgroups of SDS to form DS--Pb2+ ion pairs through strong electrostatic interactions; at the same time, hydrolyzed TESPTS molecules intervened in the internal of SDS bilayer by hydrogen bonding and hydrophobic interaction of long alkyl and tetrasulfide chains (Figure 2a). (ii) The condensation process continued and produced ordered lamellar assemblies of Pb(DS)2-TESPTS. (iii) Under hydrothermal treatment in an autoclave at gradually elevated temperature, these SDS molecules began to partially decompose SO42-, which reacted with Pb2+ to form lamellar PbSO4 nanocrystals (Figure 2b). The whole reaction process for the formation of PbSO4 lamellar structure can be simply expressed as
DS- + Pb2+ + TESPTS f layered Pb(DS)2 TESPTS f lamellar PbSO4 nanocrystals Here, what needs to be stressed is that TESPTS molecules are the choice of “silane coupling agent” and play a crucial role in stabilizing lamellar structure of the final PbSO4 nanocrystals. TESPTS can function as a bridge between inorganic PbSO4 nanocrystals and hydrophobic hydrocarbon chains of SDS bilayer and enhances the PbSO4-SDS interaction due to the two kinds of different chemical groups (Figure 2b). One is polar groups such as silanol groups (Si-OH), the other is nonpolar
In summary, PbSO4 mesostructured materials with a good crystallinity and highly ordered lamellar structure have been prepared for the first time. The precursor of layered Pb(DS)2-TESPTS mesophase was synthesized at room temperature by using SDS and lead acetate as reactants and TESPTS as a structure-stabilizing agent. Well-crystallized PbSO4 nanocrystals with a lamellar structure were produced by in situ reaction of sulfate ions coming from SDS with Pb2+ by hydrothermal treatment of the layered Pb(DS)2-TESPTS precursor at 130 °C for 12 h. The lamellar structure of PbSO4 nanocrystals was maintained due to the protection effect of hydrophobic TESPTS molecules. The main advantage of this novel synthetic route is that the lamellar structure of PbSO4 nanocrystals shows highly hydrothermal stability against collapse during hydrothermal crystallization process. The obtained products show interesting PL properties and could find potential applications. Acknowledgment. Support from the National Natural Science Foundation of China (20671096), the 100 Talents program of the Chinese Academy of Sciences, and Anhui Natural Science Foundation Key Project (ZD200902) is gratefully acknowledged. The authors thank the anonymous referee for critical comments on the manuscript. References and Notes (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Lieber, C. M. Solid State Commun. 1998, 107, 607. (3) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700. (4) Van Blaaderen, A.; Ruel, R.; Wiltzius, P. Nature 1997, 385, 321. (5) Sellinger, A.; Weiss, P. M.; Nguyen, A.; Lu, Y.; Assink, R. A.; Gong, W.; Brinker, C. J. Nature 1998, 394, 256. (6) Wang, T. X.; Xu, A. W.; Co¨lfen, H. Angew. Chem., Int. Ed. 2006, 45, 4451. (7) Pileni, M. P. Langmuir 1997, 13, 3266. (8) Breulmann, M.; Co¨lfen, H.; Hentze, H. P.; Antonietti, M.; Walsh, D.; Mann, S. AdV. Mater. 1998, 10, 237. (9) Kennedy, M. T.; Korgel, B. A.; Monbouquette, H. G.; Zasadzinski, J. A. Chem. Mater. 1998, 10, 2116. (10) Attard, G. S.; Bartlett, P. N.; Coleman, N. R. B.; Elliott, J. M.; Owen, J. R.; Wang, J. H. Science 1997, 278, 838. (11) Yang, J. P.; Qadri, S. B.; Ratna, B. R. J. Phys. Chem. 1996, 100, 17255. (12) Frostman, L. M.; Ward, M. D. Langmuir 1997, 13, 330. (13) Oliver, S. R.; Ozin, G. A. J. Mater. Chem. 1998, 8, 1081. (14) Kunieda, H.; Uddin, M. H.; Horii, M.; Furukawa, H.; Harashima, A. J. Phys. Chem. B 2001, 105, 5419. (15) Kresge, C. T.; Leonowick, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (16) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowick, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (17) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56. (18) (a) Antonelli, M.; Ying, J. Y. Angew. Chem., Int. Ed. 1995, 34, 2014. (b) Goltner, C. G.; Henke, S.; Weissenberger, M. C.; Antonietti, M. Angew. Chem., Int. Ed. 1998, 37, 613. (19) (a) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (b) Kim, J. M.;
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