Ceria Hollow Nanospheres Produced by a Template-Free Microwave

May 13, 2010 - People's Republic of China, Beijing National Laboratory for Molecular Science, ... facile template-free method for the synthesis of cer...
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J. Phys. Chem. C 2010, 114, 9865–9870

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Ceria Hollow Nanospheres Produced by a Template-Free Microwave-Assisted Hydrothermal Method for Heavy Metal Ion Removal and Catalysis Chang-Yan Cao,†,‡ Zhi-Min Cui,§ Chao-Qiu Chen,‡ Wei-Guo Song,*,‡ and Wei Cai*,† School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, People’s Republic of China, Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, People’s Republic of China, and School of Chemistry and EnVironmental Engineering, Beijing UniVersity of Aeronautics and Astronautics, Beijing, 100191, People’s Republic of China ReceiVed: February 21, 2010; ReVised Manuscript ReceiVed: May 3, 2010

Ceria hollow nanospheres composed of CeO2 nanocrystals were synthesized via a template-free and microwaveassisted aqueous hydrothermal method. This is a low-cost and environmentally benign method. The chemicals used are all environmentally benign materials (cerium nitrate, urea, and water). An Ostwald ripening mechanism coupled with a self-templated, self-assembly process, in which amorphous solid spheres are converted to crystalline nanocrystals and the latter self-assemble into hollow structures, was proposed for the formation of the hollow structures. The products were characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution TEM, energy-dispersive X-ray analysis, X-ray photoelectron spectroscopy, and N2 adsorption-desorption methods. These ceria hollow nanospheres show an excellent adsorption capacity for heavy metal ions, for example, 22.4 mg g-1 for As(V) and 15.4 mg g-1 for Cr(VI). These values are significantly higher than reported data from other ceria nanostructures. These ceria hollow nanospheres are also excellent supports for gold nanoparticles, forming a Au/CeO2 composite catalyst. In CO oxidation, a 100% CO conversion was achieved at room temperature. Introduction Ceria is a very useful rare earth metal oxide in several key applications, including adsorbents, catalysts, UV blocking and shielding materials, polishing materials, fuel cells, gas sensors, and luminescence.1-6 The properties of ceria vary significantly with different morphologies, sizes, shapes, surface areas, and exposed crystal phases. Nano size effects further induce new properties for nanometer-sized ceria, such as new catalytic activity,7 blue shift in absorption spectra,8 lattice expansion,9 phase transformation,10 and photovoltaic response.11 In the past few years, ceria nanostructures with various morphologies, such as nanoparticles,12 nanorods,13 nanowires,14 nanotubes,15 nanopolyhedrons,16 three-dimensional flower-like structures,17 and hollow structures,18 have been successfully fabricated by a variety of methods. In particular, ceria hollow nanosphere structures have several advantages in adsorption and catalysis. The hollow interior space effectively enhances the spatial dispersion, which results in not only higher surface area but also facile mass transportation of molecules to the active sites. Many methods have been reported for the preparation of hollow structures. The most common methods are template-based ones, including soft template methods using organic surfactants19,20 or hard template methods using solid templates.21-23 For example, Stucky et al.18 and Caruso et al.24 prepared ceria hollow nanospheres using silica and polystyrene bead as hard templates, respectively. These methods require templates that need to be removed by calcination or strong acid/base erosion. Ostwald ripening and the * To whom correspondence should be addressed. E-mail: wsong@ iccas.ac.cn (W.-G.S.), [email protected] (W.C.). † Harbin Institute of Technology. ‡ Chinese Academy of Sciences. § Beijing University of Aeronautics and Astronautics.

Kirkendall effect based methods have also been used to prepare hollow structures.25-33 However, ceria hollow nanospheres are only reported by template methods.18,24 The development of a facile template-free method for the synthesis of ceria hollow nanospheres may promote their applications. Herein, we report a template-free, low cost, and environmentally benign route to prepare ceria hollow nanospheres. All chemicals used are environmentally benign materials, and the process is fast due to microwave assistance. A self-templated, self-assembly process coupled with an Ostwald ripening mechanism is proposed to explain the formation of the hollow structures. The ceria hollow nanospheres have a uniform size of 260 nm and are composed of CeO2 nanocrystals of about 14 nm. They have a high surface area of 72 m2 g-1. These ceria hollow nanospheres show an excellent adsorption capacity for heavy metal ions, for example, 22.4 mg g-1 for As(V) and 15.4 mg g-1 for Cr(VI). These adsorption capacities are 70 times higher than that of the commercial bulk ceria material. Ceria hollow nanospheres are also excellent supports for gold nanoparticles. The Au/CeO2 catalyst is able to remove trace CO in air completely at room temperature. Experimental Section Materials. All the materials, including cerium(III) nitrate hexahydrate (Ce(NO3)3 · 6H2O), urea, HAuCl4 · 4H2O, sodium hydroxide, ammonia, and ethanol, were used as received from Beijing Chemicals Co. (Beijing, China). Preparation of Ceria Precursor and Ceria. In a typical procedure, 1 mmol of Ce(NO3)3 · 6H2O and 1.5 mmol of urea were dissolved in 100 mL of deionized water, and then the reaction solution was poured into a Teflon-lined autoclave. The autoclave was sealed and placed in a programmable microwave oven (MDS-6, Shanghai Sino Microwave Chemistry Technology

10.1021/jp101553x  2010 American Chemical Society Published on Web 05/13/2010

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Co., Ltd.). The oven was heated to 170 °C in 2 min by microwave irradiation and then kept at that temperature for 30 min. After cooling to room temperature, a white precipitate was collected as the ceria precursor by centrifugation and washed with ethanol several times. Ceria was obtained after calcinations of the ceria precursors in air at 500 °C for 2 h. Preparation of Ceria-Supported Gold Catalyst. Au was deposited on the ceria hollow nanosphere support through the deposition-precipitation (DP) method according to the literature.7,17 In brief, 0.55 mL of HAuCl4 · 4H2O solution (0.1 M) was added to 20 mL of deionized water, and the pH value was adjusted to 10 under continually stirring by the addition of 0.2 M NaOH. The solution was then added to a slurry containing 0.2 g of ceria hollow nanospheres in deionized water (50 mL). The pH value of the mixing solution was adjusted to 10 again with 0.2 M NaOH. After the suspension was stirred overnight at room temperature, the solid was isolated by centrifugation and washed with deionized water several times and then dried at 80 °C overnight. Characterization. X-ray diffraction (XRD) patterns were obtained with a Rigaku D/max-2500 diffractometer with Cu KR radiation (λ ) 1.5418 Å) at 40 kV and 100 mA. The microscopic features of the samples were characterized by field emission scanning electron microscopy (FESEM, JEOL 6701F), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) (FEI Tecnai F20). X-ray photoelectron spectroscopy (XPS) analysis was performed in a VG ESCALab 220i-XL spectrometer with 300 W Al KR radiation. The binding energies are referenced to the C 1s line at 284.8 eV from adventitious carbon. The nitrogen adsorption and desorption isotherms were measured on a Quantachrome Autosorb AS-1 instrument. The pore size distributions were derived from the desorption branches of the isotherm with the Barrett-Joyner-Halenda (BJH) model. Water Treatment Experiments. Solutions containing different concentrations of As(V), Cr(VI), and Pb(II) were prepared using Na2HAsO4 · 7H2O, K2Cr2O7, and Pb(NO3)2 as the sources of heavy metal ions, respectively. The pH value was adjusted to 3 using HCl (2 M) for As(V). Ceria (0.02 g) was then added to 10 mL of the above solution under stirring at room temperature. After the specified time, the solid and liquid were separated immediately and inductively coupled plasma-optical emission spectroscopy (Shimazu ICPE-9000) was used to measure the concentration of metal ions in the remaining solution. CO Oxidation Test. The CO oxidation test was performed in a quartz tubular reactor (7 mm inside diameter) loaded with 50 mg of CeO2 or Au/CeO2 hollow nanospheres and 450 mg of sea sand. The catalyst was activated first in an air flow (25 mL min-1) at 300 °C for 1 h. The CO oxidation test was carried out with a 25 sccm flow of 1% CO in nitrogen and 25 sccm air. The gas composition was monitored by online gas chromatography (Scimadzu, GC-14C). Results and Discussion Unlike previously reported methods for hollow structures,19-33 neither an organic template nor an organic solvent is used in this work to produce the ceria hollow nanospheres. Microwave heating has major advantages in low cost and time saving. It reduces the reaction time for the hydrothermal process to only 30 min. In addition, microwave heating leads to uniform heating of the whole synthesis mixture, which led to the homogeneous shapes and sizes of the nanoparticles. These make this aqueous route a low-cost and environmentally friendly one. Monodis-

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Figure 1. (a) SEM image of the ceria precursor. (b) High-magnification SEM image of the ceria precursor (inset shows a broken nanosphere, scale bar ) 100 nm). (c) TEM image of the ceria precursor.

Figure 2. XRD patterns of the products obtained at different times, (a) 1, (b) 3, (c) 5, and (d) 30 min, and of (e) ceria.

persed ceria hollow nanospheres with uniform sizes are produced with a near 100% yield based on the cerium nitrate consumption. The SEM image of the typical ceria precursor (Figure 1a) shows that nearly monodispersed nanospheres with average diameter of 260 nm are obtained at 170 °C for 30 min. A closer examination by high-magnification SEM indicates that the entire structure of the sphere consists of many small nanoparticles (Figure 1b). The hollow structures can be identified by several broken ceria precursors, as shown in the inset in Figure 1b. The TEM image in Figure 1c confirms that the products have hollow structures, and the 30 nm shell consists of small nanoparticles. The powder X-ray diffraction (XRD) pattern of the typical ceria precursors is shown in Figure 2d. The main diffraction peaks can be mostly indexed to the hexagonal phase of CeCO3OH (JCPDS No. 41-0013) and the cubic phase of ceria (JCPDS No. 34-0394). XPS analysis indicates that cerium atoms mainly exist as Ce(III) in the ceria precursor, and the energydispersive X-ray (EDX) analysis confirmed that the ceria

Ceria Hollow Nanospheres

Figure 3. TEM images of ceria precursors obtained at different reaction times: (a) 1, (b) 3, (c) 5, and (d) 30 min (scale bar ) 200 nm).

SCHEME 1: Schematic Illustration of the Ostwald Ripening Coupled Self-Templated, Self-Assembly Process of the Ceria Precursor

precursor consisted solely of cerium, carbon, and oxygen (Supporting Information, Figures S1 and S2). To investigate the formation process of the hollow nanosphere structures, samples prepared at different reaction times are collected and investigated by TEM and XRD. Solid and amorphous spheres are obtained at the initial stage (1 min after microwave heating, Figures 2a and 3a). Figure 3b shows that the sample collected after 3 min has many small nanoparticles attached on the surface of the spheres. The product is mainly composed of crystals of unknown structure (Figure 2b). After 5 min, a mixture of core-shell structures and hollow spheres is obtained (Figure 3c) with a similar crystalline structure. All the spheres become hollow after 30 min (Figure 3d), and the product is mainly CeCO3OH and ceria. The diffraction peak at 18° is almost disappeared. The crystallinity evolution suggests a typical Ostwald ripening process. However, the formation of hollow structures is unusual because no surfactant or hard template is intentionally used during the process. From the above time-dependent results, we propose a new mechanism for the formation of ceria hollow nanospheres. This is an Ostwald ripening coupled self-templated, self-assembly process, as schematically illustrated in Scheme 1. At 1 min after microwave heating started, amorphous solid spheres are formed in order to minimization the total surface energy. As the reaction proceeded, small nanocrystals begin to form via an Ostwald ripening process at the interface between the solid spheres and the solution. These nanocrystals self-assemble onto the surface of the remaining solid spheres. By this, each solid sphere acts as a “template” for nanocrystals formed from the same sphere itself. With this Ostwald ripening and self-templated, selfassembly process going on, more and more nanocrystals are formed and assembled around the solid core. Once there are

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Figure 4. (a) SEM image, (b) TEM image, (c) SAED pattern, and (d) HRTEM image of the ceria hollow nanospheres.

enough nanocrystals produced and assembled, they can stack against each other, forming a stable shell. A semicore/shelllike structure is thus formed at this stage. The remaining core does not act as a template after this stage because the shell is stable enough to self-support. By Ostwald ripening, the remaining core is converted to nanocrystals, which attach to the shell to finalize the whole process for the hollow structure. Such a Ostwald ripening coupled self-templated, self-assembly mechanism explains well the effects of reaction temperature on the formation of the ceria precursor hollow nanospheres (Supporting Information, Figure S3). At the lower temperature, for example, 120 °C, only solid product is collected. This is because Ostwald ripening needs a higher temperature to proceed. When the reaction temperature is increased to 150 °C, hollow nanospheres started to form, although there are still some solid spheres. When the temperature is further increased to 170 °C, all hollow spheres are formed (Figure 1). Urea plays an essential role by providing a steady OH- ion source through gradual hydrolysis. The reaction between the cerium ions and OH- ions is thus steady and slow. When sodium hydroxide or ammonia is used in place of urea, while other conditions are kept the same, only aggregate nanoparticles are obtained (Supporting Information, Figure S4). The XRD pattern (Figure 2e) of the calcined product matches well with the cubic phase CeO2 (JCPDS No. 34-0394). The broaden diffraction peaks indicate the small size of nanocrystals. The average grain size of the nanocrystals is ca. 14 nm, as calculated from the width of diffraction peaks using the Scherrer formula. The SEM image in Figure 4a shows a very similar morphology, indicating that calcination does not change the total nanosphere morphology. The TEM image (Figure 4b) further indicates that ceria remains a hollow nanosphere structure. The selected-area electron diffraction (SAED) pattern (Figure 4c) shows that ceria hollow nanospheres are composed of high crystalline nanocrystals. The lattice fringes in the high-resolution TEM (HRTEM) image (Figure 4d) shows a spacing of 0.31 nm due to the (111) planes and a spacing of 0.27 nm from the (200) planes of cubic CeO2. The XPS spectrum of the calcined sample for Ce 3d states is typical of the Ce4+ (Supporting Information, Figure S1b).

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qe ) qmbCe /(1 + bCe)

Figure 5. Nitrogen adsorption-desorption isotherm (with the BJH pore size distribution plot in the inset) of the ceria nanospheres.

The nitrogen adsorption study shows that the BET surface area of the ceria nanospheres is 72 m2 g-1, calculated from the nitrogen adsorption and desorption isotherms of the sample (Figure 5). A sharp pore size distribution with an average pore size of 4 nm was obtained by the BJH method. These pores are very like due to the void space among the stacked hollow spheres. Ceria is widely used in many different application areas owing to its outstanding physical and chemical properties, including depolluting gaseous noxious compounds from automobiles and industrial exhaust (such as removal of SO2 and CO), removing heavy metal ions from wastewater, etc.1 In our previous work,17 flower-like hierarchical structured ceria shows a good heavy metal ion adsorption capacity for As(V) and Cr(VI). However, the special surface area of flower-like ceria (34.1 m2 g-1) is only a half of that of ceria hollow nanospheres in this work. Therefore, we expect that the ceria hollow nanospheres in this paper would show better adsorption capacity. Figure 6a shows a very fast adsorption rate of heavy metal ions at room temperature for As(V) and Cr(VI) solutions. Adsorption isothermals for these heavy metal ions are shown in Figure 6b. The Langmuir adsorption model is used to calculate the maximum adsorption capacity34

where qm (mg g-1) is the maximum sorption capacity. Table 1 lists the adsorption capacities of ceria hollow nanospheres and other comparing samples. The maximum adsorption capacity of the ceria hollow nanospheres is 22.4 mg g-1 for As(V) and 15.4 mg g-1 for Cr(VI). These values are much higher than those from flower-like ceria and R-Fe2O3 nanostructures in our previous studies17,35 and about 70 times higher than that of commercial bulk ceria. Ceria can remove not only anions, such as As(V) and Cr(VI), but also cations, such as Pb(II) (Supporting Information, Figure S5). The adsorption capacity for Pb(II) is 9.2 mg g-1 at a pH value of 7. A recycling test shows that their capacities in heavy metal ion removal can be regenerated with NaOH treatment. The mechanism for the heavy metal ion’s adsorption by metal oxide is likely the combination of static electrical attraction between oxides and heavy metal ions and ion exchange.36 The porous structure and the high surface area of the ceria hollow nanospheres are two desirable features for heavy metal ion removal. In catalysis, the selective removal of trace CO from the hydrogen source for hydrogen fuel cells is one of the very important catalytic processes. Ceria has a high oxygen storage capacity, which enables ceria to be an excellent support for Au nanoparticles.37-40 The hollow interior space of ceria hollow nanospheres effectively enhances the spatial dispersion, which facilitates the mass transportation of molecules to the active sites during CO oxidation. In this work, the ability of ceria hollow nanospheres as supports for Au nanoparticles was tested. Au nanoparticles are deposited on the ceria hollow nanospheres by a deposition-precipitation (DP) method. The total Au loading of the Au/CeO2 hollow nanospheres is 4 wt %, as determined by elemental analysis. The HRTEM image (Figure 7) of the Au/ceria composite shows that the Au nanoparticles (dark dots) with a diameter of 4 ( 0.4 nm are highly dispersed on the ceria support. The lattice fringes of Au nanocrystals and ceria supports are simultaneously observed from the same TEM

Figure 6. (a) Time-dependent concentration of As(V) (initial concentration of 13 mg L-1) and Cr(VI) (initial concentration of 26.8 mg L-1) using ceria hollow nanospheres. (b) Adsorption isothermals of As(V) and Cr(VI) using ceria hollow nanospheres.

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TABLE 1: BET Surface Area and Maximum Removal Capacity of Ceria Hollow Nanospheres, Flower-like Ceria, Flower-like r-Fe2O3, and Commercial Bulk Ceria at the Same Experimental Conditions adsorbent sample

ceria hollow nanospheres flower-like ceria (ref 17) flower-like R-Fe2O3 (ref 35) commercial bulk ceria

maximum removal maximum removal BET surface capacity of As(V) capacity of Cr(VI) area (m2 g-1) (mg g-1) (mg g-1)

72

22.4

15.4

34.1

14.4

5.9

40

7.6

5.4

2

0.3

0.37

image, providing unambigous evidence for strong interaction between Au nanoparticles and the ceria support. The spacing of 0.23 and 0.20 nm are indexed to the (111) planes and the (200) planes of Au, respectively, and a spacing of 0.31 nm is indexed to the (111) planes of CeO2. The Au/ceria composite catalyst shows excellent activity in CO oxidation. As shown in Figure 8, the CO conversion is 100% at ambient temperature. This activity is much higher than that of the Au/ceria composite catalyst in our previous work,17 where flower-like ceria was used as the support. In comparsion, the ceria hollow nanospheres alone show a similar activity as flowerlike ceria for CO oxidation. A 100% CO conversion was reached at about 370 °C. The addition of Au nanoparticles significantly increased the catalytic activity of ceria. This could be attributed to the strong synergistic interaction between Au and CeO2. Ceria supplied reactive oxygen by oxygen releasing-uptake through the redox process. During the catalytic CO oxidation process, reactive oxygen from CeO2 reacted with CO molecules adsorbed on Au nanoparticles to form CO2. To investigate the durability of the Au/CeO2 catalyst, another run was carried out after the catalyst had been continuously used for 10 h under the flow of mixture gas at room temperature. CO could still be removed completely in air, indicating that the Au/CeO2 catalyst had a high catalytic activity and high durability. Corma et al. repoted that the high

Figure 8. CO conversion as a function of temperature for the ceria hollow nanospheres and Au/CeO2.

activity of Au/CeO2 for gold-catalyzed reactions is due to the strong synergistic interaction between Au and ceria, which induced the presence of positively charged gold species at the Au/ceria interface.41-43 The XPS spectrum of Au 4f states of the Au/CeO2 nanospheres proves the existence of the positively charged gold species (Supporting Information, Figure S6). This is consistent with the synergistic effect. As for ceria itself, theoretical studies have shown that ceria with mostly the (111) phase exposed is more active than ceria with the (110) phase exposed.44 Li et al. proposed that, besides different crystal phases having different catalytic activities, oxygen vancancy is also an important factor for ceria in CO oxidation13 because oxygen transportation during the redox process involving the Ce4+/Ce3+ couple is largely controlled by the oxygen vacancies. In this work, however, the ceria hollow nanospheres are composed of ceria nanocrystals, and several phases, such as (111) and (200), are simultaneously exposed. In addition, for the Au/ceria composite, the activity from Au nanoparticles overwhelms the activity of ceria in CO oxidation, making it hard to evaluate the contributions from ceria alone. Conclusions In short summary, we produced ceria hollow nanospheres via a low-cost and environmentally benign method, which is a template-free and microwave-assisted method. A self-templated, self-assembly process coupled with Ostwald ripening is proposed for the formation of the hollow structures. The ceria hollow nanospheres have very high adsorption capacities to remove heavy metal ions, for example, As(V) and Cr(VI). The ceria hollow nanospheres are also excellent supports for gold nanoparticles; the composite catalyst can oxide CO at room temperature. Acknowledgment. We gratefully thank the financial support from the National Natural Science Foundation of China (NSFC 50725207, 20873156, and 20821003), the Ministry of Science and Technology (MOST 2007CB936400 and 2009CB930400), the Excellent Youth Foundation of Heilongjiang Province of China (No. JC200715), and the Chinese Academy of Sciences.

Figure 7. HRTEM image of Au/CeO2 hollow nanospheres (inset is the TEM image of an individual Au/CeO2 nanosphere).

Supporting Information Available: XPS analysis of the Ce 3d states of the ceria precursor and ceria hollow nanospheres, energy-dispersive X-ray (EDX) spectroscopic analysis of the typical ceria precursor, SEM and TEM images of the ceria precursors obtained at different reaction temperatures or using different alkalis, time-dependent concentration of Pb(II) and

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adsorption isothermals of Pb(II) at pH 3 and pH 7 using ceria hollow nanospheres, and XPS analysis of the Au 4f states of the Au/CeO2 hollow nanospheres. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Trovarelli, A.; Leitenburg, C.; Boaro, M.; Dolcetti, G. Catal. Today 1999, 50, 353–367. (2) Yabe, S.; Sato, T. J. Solid State Chem. 2003, 171, 7–11. (3) Park, S. D.; Vohs, J. M.; Gorte, R. J. Nature 2000, 404, 265–267. (4) Fujihara, S.; Oikawa, M. J. Appl. Phys. 2004, 95, 8002–8006. (5) Wang, L. Y.; Zhang, K. L.; Song, Z. T.; Feng, S. L. Appl. Surf. Sci. 2007, 253, 4951–4954. (6) Brosha, E. L.; Mukundan, R.; Brown, D. R.; Garzon, F. H.; Visser, J. H. Solid State Ionics 2002, 148, 61–69. (7) Carrettin, S.; Concepcion, P.; Corma, A.; Nieto, J. M. L.; Puntes, V. F. Angew. Chem., Int. Ed. 2004, 43, 2538–2540. (8) Tsunekawa, S.; Fukuda, T.; Kasuya, A. J. Appl. Phys. 2000, 87, 1318–1321. (9) Tsunekawa, S.; Ishikawa, K.; Li, Z. Q.; Kawazoe, Y.; Kasuya, A. Phys. ReV. Lett. 2000, 85, 3440–3443. (10) Wang, Z. W.; Saxena, S. K.; Pischedda, V.; Liermann, H. P.; Zha, C. S. Phys. ReV. B 2001, 64, 012102. (11) Corma, A.; Atienzar, P.; Garcia, H.; Chane-Ching, J. Y. Nat. Mater. 2004, 3, 394–397. (12) Huo, Z. Y.; Chen, C.; Liu, X. W.; Chu, D. R.; Li, H. H.; Peng, Q.; Li, Y. D. Chem. Commun. 2008, 32, 3741–3743. (13) Liu, X. W.; Zhou, K. B.; Wang, L.; Wang, B. Y.; Li, Y. D. J. Am. Chem. Soc. 2009, 131, 3140–3141. (14) Yu, T. Y.; Joo, J.; Park, Y. I.; Hyeon, T. Angew. Chem., Int. Ed. 2005, 44, 7411–7414. (15) Tang, C. C.; Bando, Y.; Liu, B. D.; Golberg, D. AdV. Mater. 2005, 17, 3005–3009. (16) Si, R.; Zhang, Y. W.; You, L. P.; Yan, C. H. Angew. Chem., Int. Ed. 2005, 44, 3256–3260. (17) Zhong, L. S.; Hu, J. S.; Cao, A. M.; Liu, Q.; Song, W. G.; Wan, L. J. Chem. Mater. 2007, 19, 1648–1655. (18) Strandwitz, N. C.; Stucky, G. D. Chem. Mater. 2009, 21, 4577– 4582. (19) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature. 1992, 359, 710–712. (20) Yang, P. D.; Zhao, D. Y.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Nature 1998, 396, 152–155.

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