Facile Synthesis of Highly Ordered Mesoporous ZnTiO3 with

Published: October 24, 2011 r 2011 American Chemical Society ... revolution in nanoscience, engineering, and technology is being driven by our ability...
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Facile Synthesis of Highly Ordered Mesoporous ZnTiO3 with Crystalline Walls by Self-Adjusting Method Zhen-Xing Li, Fu-Bo Shi, Yi Ding, Tao Zhang, and Chun-Hua Yan* Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications & PKU-HKU Joint Laboratory in Rare Earth Materials and Bioinorganic Chemistry, Peking University, Beijing 100871, China

bS Supporting Information ABSTRACT: Highly ordered mesoporous ZnTiO3 with crystalline walls was directly prepared by a sol gel process combined with evaporation induced self-assembly in ethanol, using amphiphilic triblock copolymers as structure directing agents. The whole process is self-adjusting to organize the networkforming metal oxide species without additional acid or base. The mesoporous material is pure cubic-phase ZnTiO3 and has large surface area (up to 134 m2/g), large pore volume (0.17 cm3/g), and narrow pore size distribution (3 4.5 nm). The optic behavior was systematically studied, which is very helpful to understand the mesoporous ZnTiO3 material either in fundamental study or for potential applications in optics and catalysis. This work provides a “self-adjusting” approach to fabricate the mesoporous functional materials with diverse compositions: the diverse hydrolysis condensation kinetics of various metal oxides is homogenized to yield stable multicomponent precursors. The development of such a simple, versatile, and reproducible method is important for applications in practice.

’ INTRODUCTION With booming development on nanomaterials, the new revolution in nanoscience, engineering, and technology is being driven by our ability to manipulate matter to create “designer” structures.1 The mesoporous materials are regarded as designable nanomaterials. Since the discovery of ordered mesoporous silica materials in 1992,2 this research field has been widely studied because of the potential applications of these materials as supports for catalysts and other realms in chemistry.3 The ordered mesoporous materials provide not only large surface areas, but also highly uniform channels, narrow pore size distribution, and tunable pore sizes over a considerably wide range. Recently, great interest has been focused on the mesoporous metal oxides due to their potential application in various areas in electronic, photocatalytic, photovoltaic, and energy storage applications.4 Compared to single-metal oxides which have been widely researched,5 multimetal oxides, such as those of the fluorite, perovskite, tetragonal, and ilmenite structures, promise the construction of robust multifunctional materials.6 The compositional and structural diversity of metal oxides lead to an array of unique chemical and physical properties. With advanced properties such as selective oxidation, electron or ionic percolation, and energy transfer, many metal oxides are multicomponent systems containing two or more types of metal ions. For this purpose, it is a key to simultaneous control of the nanoscale structure and composition of such materials in a facile and inexpensive way. r 2011 American Chemical Society

The fabrication of mesoporous films, powders, and monoliths is extremely important for applications in which high interfacial surface area is required, for example, heterogeneous catalysts, photocatalysis, environmental cleaning fuel cells, and conversion of solar energy.7 Moreover, this strategy to obtain nanocrystalline multimetal oxides bridges the gap between mesoporous materials and the remarkable properties of binary, ternary, or quaternary metallic oxides.6 Although crystalline ordered mesostructured multimetallic materials have been obtained by an impregnation and inverse replica nanocasting technique using mesoporous silica or carbon as template,8 10 the direct preparation of mesoporous multimetallic oxides is complicated, because it is difficult to control the multimetallic hydrolysis behavior.11 Titania (TiO2) and zinc oxide (ZnO) are both wide bandgap semiconductors with several favorable properties and extensive applications, and have attracted much interest in either single material12 or ZnO TiO2 composites.13 There are three compounds existing in the ZnO TiO2 system: Zn2TiO4, ZnTiO3, and Zn2Ti3O8.14 From an industrial perspective, ZnTiO3 is a promising material for applications in gas sensor, microwave dielectrics, white pigment, and catalytic sorbents for desulfurization of hot coal.15 ZnTiO3 also has potential applications in nonlinear optics,16 and as luminescent material.17 To the best of Received: September 3, 2011 Revised: October 23, 2011 Published: October 24, 2011 14589

dx.doi.org/10.1021/la2034615 | Langmuir 2011, 27, 14589–14593

Langmuir our knowledge, few reports have demonstrated the fabrication of mesostructured ZnTiO3,15a and the ordered crystalline mesostructured ZnTiO3 is not reported, which is expected for optoelectronic applications. Here, we demonstrate first a direct and facile approach to the systematic synthesis of mesostructured ZnTiO3 with crystalline walls. The general synthesis strategy is based on a sol gel process combined with evaporation induced selfassembly (EISA) in ethanol using amphiphilic triblock copolymers as structure directing agents (SDAs) without additional acid or base. Zinc nitrate and titanyl acetylacetonate were chosen as cheaper precursors. Ordered mesostructured ZnTiO3 has high Brunauer Emmett Teller (BET) surface area (up to 134 m2/g), large pore volume (0.17 cm3/g), and uniform pore size (the mean value of 3.3 nm). The optic behavior of this material was clearly observed and has been systematically studied, which offers more comprehensive knowledge either in fundamental study or for potential applications in optics and catalysis.

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Scheme 1. Schematic Illustration for the Synthesis Procedure of the Highly Ordered Mesoporous ZnTiO3

’ EXPERIMENTAL SECTION Materials. Pluronic F127 (Mav = 12 600, EO106PO70EO106) was purchased from Aldrich and Sigma-Aldrich Chemical Inc. Titanyl acetylacetonate (TiO(acac)2) was purchased from Alfa Aesar Company. Znic nitrate (Zn(NO3)2 3 6H2O) was purchased from Beijing Chemical Reagent Company. All other chemicals were used as received. Synthesis of Mesoporous ZnTiO3. 0.8 g of Pluronic F127 was dissolved in 20 mL of ethanol at room temperature (RT). Then, 0.744 g (2.5 mmol) of Zn(NO3)2 3 6H2O and 0.655 g (2.5 mmol) of TiO(acac)2 were added into the above solution with vigorous stirring. The mixture was covered with polyethylene (PE) film. After stirring for at least 5 h at room temperature, the homogeneous sol was transferred to an oven and underwent solvent evaporation. After two days aging at 40 °C, gel product was dried in another oven at 100 °C for 1 day. Calcination was carried out by slowly increasing temperature from room temperature to 400 °C (1 °C/min ramping rate) and heating at 400 °C for 4 h in air. Characterization. The small- and wide-angle X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX-2000 diffractometer (Japan) using Cu Kα (λ = 1.5406 Å) radiation. The lattice parameters were calculated with the least-squares method. The average crystal domain size of the ZnTiO3 crystallite size (D) was estimated with the Scherrer equation: D = 0.90λ/β cos θ where θ is the diffraction angle of the (311) peak of the cubic phase, and β is the full width at halfmaximum (fwhm) of the (311) peak in radians, which is calibrated from high-purity silicon. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) were performed with a JEOL-2100 transmission electron microscope (Japan) operated at 200 kV. Energy-dispersive X-ray spectroscopy (EDS) were taken on a JEOL2100F transmission electron microscope (Japan) equipped with an EDS detector. The photoluminescence spectra were recorded on a Hitachi F-4500 fluorescence spectrophotometer equipped with an external tunable 2 W 980 nm laser diode. The UV vis spectra were recorded on a Hitachi U-3010 spectrometer. The nitrogen adsorption and desorption isotherms at 78.3 K were measured on an ASAP 2020 analyzer (Micromeritics Co. Ltd.). Measurements were performed after outgassing the sample at 573 K under a vacuum, down to a residual pressure less than 10 3 Torr. With the Barrett Joyner Halenda (BJH) model, the pore volumes and pore size distributions were derived from the desorption branches of the isotherms. The Zn and Ti contents of mesoporous ZnTiO3 material calcined at 400 °C were determined on an inductively coupled plasma-atomic emission spectrometer (ICP-AES) (Vista, Varian).

’ RESULTS AND DISCUSSION Highly ordered mesoporous ZnTiO3 are synthesized via the simple EISA method using amphiphilic triblock copolymers (PEO-PPO-PEO) as templates. Zinc nitrate and titanyl acetylacetonate were chosen as metal precursors, followed by a thermal process to remove organic templates (Scheme 1). In the selfassembly process, the poly(ethylene oxide) (PEO) segments are incorporated into the rigid matrix composed of zinc and titanium ions, while the poly(propylene oxide) (PPO) segments form the rigid hydrophobic core.18 High-molecular-weight copolymers with long hydrophobic segments are promising candidates to template the large-pore mesoporous materials.19 21 Zhao et al. introduced a new concept about “acid base pairs” in the synthesis of mesoporous materials, and they successfully obtained many mesoporous metal phosphates without extra acid or base.22 Evidence for the formation of mesostructures is provided by small-angle X-ray diffraction (XRD) patterns shown in Figure 1a. The sample shows a very strong diffraction peak around 0.96°, and one poorly resolved diffraction peak around 1.6° is observed, which, according to the transmission electron microscopy (TEM) observation, can be attributed to the 2D space group with p6m symmetry. The wide-angle XRD pattern (Figure 1b) of as-synthesized mesoporous ZnTiO3 is indexed to the cubic phase (ICDD Card No. 39 0190), and all the diffraction peaks can be indexed to (220), (311), (400), (511), and (440) reflections. The broadening of the reflections distinctly indicates the intrinsic nature of nanocrystals, and the average crystallite size (D) of ZnTiO3 nanocrystals is around 3 4 nm, calculated by the Sherrer formula. The TEM images and corresponding schematic diagrams illustrating the microstructure (Figure 2a and b) reveal a high degree of periodicity over large domains, viewed along the [001] and [110] orientations. On the basis of the small-angle XRD and TEM data, it is reasonable that the synthesized mesoporous ZnTiO3 possesses highly ordered mesostructure. The above TEM images are representative ones selected from large amounts of TEM images to give a clear and accurate picture of the ordered structure. More images are shown in Supporting Information (SI) to display the reproducibility as well as to give a more 14590

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Figure 2. TEM images (a) and (b) of the ordered mesoporous ZnTiO3 viewed along [110] and [001] orientation (the insets in a and b are the corresponding schematic diagrams illustrating the microstructure); HRTEM images (c) and image (d) of the ordered mesoporous ZnTiO3 viewed along [110] and [001] orientation (the inset in c is corresponding SAED pattern).

Figure 1. (a) Small- and (b) wide-angle XRD patterns of the ordered mesoporous ZnTiO3.

complete description (SI Figure S1). The high-resolution TEM (HRTEM) images (Figure 2c and d) show that the walls are highly crystalline with a lattice spacing of 0.29 nm agreeing well with the value of 0.297 nm for the (220) planes of cubic phase ZnTiO3, and composed of ZnTiO3 nanocrystals with size of ca. 3 nm. The selective area electron diffraction (SAED) pattern of ordered mesostructure domains further confirms that the mesoporous wall is crystalline (the inset in Figure 2d), which agrees well with the wide-angle XRD result above. The high-angle annular dark-field scanning TEM (HAADF STEM) image further confirms the ordered structure (Figure 3a). The elemental mappings of the mesoporous ZnTiO3 are obtained by energy-dispersive spectroscopy (EDS) for Zn and Ti. Clearly, Zn and Ti were uniformly distributed throughout the mesoporous materials (Figure 3b,c). EDS point scanning experiments at arbitrary points of the samples confirm that Zn and Ti are present with an atomic ratio of 1:1 (representative data shown in Figure 3d), which is similar to the original molar ratio of the feed. This result is also confirmed by the analysis of an inductively coupled plasma-atomic emission spectrometer (ICP-AES). The peaks of Cr and Cu elements come from the holder and copper mesh of sample. This indicates that Zn2+ and Ti4+ are distributed

Figure 3. HAADF STEM (a) image of the ordered mesoporous ZnTiO3; EDS mappings of Ti (b) and Zn (c) elements of the ordered mesostructure; EDS point spectrum (d) of the ordered mesoporous ZnTiO3.

homogeneously in the framework of the mesostructure, further confirming the observed results from elemental map. The porosity of the ZnTiO3 material was distinctly confirmed by the analysis of the nitrogen adsorption desorption isotherms. Nitrogen adsorption desorption isotherms of the material is shown in Figure 4. It exhibits type IV isotherm with a type H2 hysteresis loop according to the IUPAC classification, signifying the uniform cylindrical mesoporous structure. The presence of a pronounced hysteresis loop in the isotherm curve is indicative of the pores in a 3D intersection network.23 The Barrett Joyner Halenda (BJH) analysis shows a high BET surface area of 134 m2/g and large pore volume of 0.17 cm3/g. Narrow pore size distribution (3 4.5 nm) is obtained, indicating more uniform pore structure. 14591

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Figure 6. Photoluminescence spectra of the ordered mesoporous ZnTiO3.

Figure 4. (a) Nitrogen adsorption desorption isotherms and (b) pore size distribution curves of the ordered mesoporous ZnTiO3.

Figure 5. UV vis absorption spectra of the ordered mesoporous ZnTiO3 (the inset is the corresponding plot of (αhν)2 versus photon energy).

In order to reveal the band gap energy of crystalline mesostructured ZnTiO3, the UV vis absorption spectra were recorded (Figure 5). This material shows a strong absorption band (below

350 nm in wavelength) at the UV region. The plot of (αhν)2 vs photon energy of ZnTiO3 is shown in inset of Figure 5. For direct transitions, α near the absorption edge can be expressed in the following equation: α µ (hν Ed)1/2/hν, where α is the absorption coefficient, hν is photon energy, and Ed is the band gap energy for direct transitions.24 In the present case, the optical band gap is about 3.78 eV, which is significantly larger than that reported for the bulk material (3.5 eV).25 It has been theoretically deduced that the value of blue-shifting is inversely proportional to the square of the crystallite size due to quantum confinement effect.26 Photoluminescence (PL) in mesostructured ZnTiO3 originates from the local distortion of the Ti4+ ions within the TiO6 octahedrons, self-trapped excitons, oxygen vacancies, surface states, or charge transfers via intrinsic defects inside an oxygen octahedron.27 30 Because of its high symmetry, direct electron transitions from the valence to the conduction band are forbidden in bulk cubic ZnTiO3. Figure 6 shows the PL spectra of the crystalline mesostructured ZnTiO3 excited at the wavelength of 200 nm, in which an emission peak in the wavelength of about 468 nm can be clearly seen, indicating that the defects exist in the crystalline ordered mesostructure ZnTiO3 materials, and the emission results from defects.31,32 Therefore, these results strongly suggest that the ordered mesoporous ZnTiO3 materials have potential applications in optical and catalytic fields.

’ CONCLUSIONS In summary, we have developed a facile method to produce highly ordered mesoporous ZnTiO3 with crystalline walls, from the inexpensive and commercially available polymers in the ethanol solution, using zinc nitrate and titanyl acetylacetonate as metal precursors. All the hydrolysis, condensation, and assembly processes are self-adjusting without additional acid or base. The mesoporous material above is pure cubic-phase ZnTiO3 and has large surface area, large pore volume, and narrow pore size distribution. The optic behavior was clearly observed and has been systematically studied. It is very helpful to understand the mesoporous ZnTiO3 material either in fundamental study or for potential applications in optics and catalysis. This work provides a “self-adjusting” approach to fabricate the mesoporous functional materials with diverse compositions: the diverse hydrolysis condensation kinetics of various metal oxides 14592

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Langmuir is homogenized to yield stable multicomponent precursors. The development of such a simple, versatile, and reproducible method is important for applications in practice.

’ ASSOCIATED CONTENT

bS

Supporting Information. More TEM images of the ordered mesoporous ZnTiO3 viewed along [110] and [001] orientation (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Fax: +86-10-6275-4179; E-mail: [email protected].

’ ACKNOWLEDGMENT This work is supported by NSFC (20821091 and 20931160429). ’ REFERENCES (1) (a) Liu, H.; Xu, J.; Li, Y.; Li, Y. Acc. Chem. Res. 2010, 43, 1496. (b) Guo, Y.; Tang, Q.; Liu, H.; Zhang, Y.; Li, Y.; Hu, W.; Wang, S.; Zhu, D. J. Am. Chem. Soc. 2008, 130, 9198. (c) Liu, H.; Li, Y.; Jiang, L.; Luo, H.; Xiao, S.; Fang, H.; Li, H.; Zhu, D.; Yu, D.; Xu, J.; Xiang, B. J. Am. Chem. Soc. 2002, 124, 13370. (2) (a) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (b) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenkert, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (3) (a) Taguchi, A.; Sch€uth, F. Microporous Mesoporous Mater. 2005, 77, 1. (b) Sauer, J.; Kaskel, S.; Janicke, M.; Sch€uth, F. Stud. Surf. Sci. Catal. 2001, 135, 4740. (4) (a) Morris, C. A.; Anderson, M. L.; Stroud, R. M.; Merzbacher, C. I.; Rolison, D. R. Science 1999, 284, 622. (b) Carreon, M. A.; Guliants, V. V. Eur. J. Inorg. Chem. 2005, 1, 27. (c) Krawiec, P.; Kockrick, E.; Simon, P.; Auffermann, G.; Kaskel, S. Chem. Mater. 2006, 18, 2663. (d) Zhang, L.; Papaefthymiou, G. C.; Ying, J. Y. J. Phys. Chem. B 2001, 105, 414. (e) Molnar, E.; Konya, Z.; Kiricsi, I. J. Therm. Anal. Calorim. 2005, 79, 573. (5) (a) Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Sch€uth, F.; Stucky, G. D. Nature 1994, 368, 317. (b) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Nature 1998, 396, 152. (c) Crepaldi, E. L.; De A. A. SolerIllia, G. J.; Grosso, D.; Cagnol, F.; Ribot, F.; Sanchez, C. J. Am. Chem. Soc. 2003, 125, 9770. (d) Sinha, A. K.; Suzuki, K. Angew. Chem., Int. Ed. 2005, 44, 271. (e) Shibata, H.; Ogura, T.; Mukai, T.; Ohkubo, T.; Sakai, H.; Abe, M. J. Am. Chem. Soc. 2005, 127, 16396. (f) Wang, Y.; Yang, C. M.; Schmidt, W.; Spliethoff, B.; Bill, E.; Sch€uth, F. Adv. Mater. 2005, 17, 53. (g) Jiao, F.; Harrison, A.; Jumas, J. C.; Chadwick, A. V.; Kockelmann, W.; Bruce, P. G. J. Am. Chem. Soc. 2006, 128, 5468. (h) Lai, X.; Li, X.; Geng, W.; Tu, J.; Li, J.; Qiu, S. Angew. Chem., Int. Ed. 2007, 46, 738. (6) Grosso, D.; Boissiere, C.; Smarsly, B.; Brezesinski, T.; Pinna, N.; Albouy, P. A.; Amenitsch, H.; Antonietti, M.; Sanchez, C. Nat. Mater. 2004, 3, 787. (7) (a) Bartl, M. H.; Boettcher, S. W.; Frindell, K. L.; Stucky, G. D. Acc. Chem. Res. 2005, 38, 263. (b) Corma, A. Chem. Rev. 1997, 97, 2373. (8) Lu, A. H.; Sch€uth, F. Adv. Mater. 2006, 18, 1793. (9) Lee, J.; Kim, J.; Hyeon, T. Adv. Mater. 2006, 18, 2073. (10) (a) Jiao, F.; Shaju, K. M.; Bruce, P. G. Angew. Chem., Int. Ed. 2005, 44, 6550. (b) Luo, J. Y.; Wang, Y. G.; Xiong, H. M.; Xia, Y. Y. Chem. Mater. 2007, 19, 4791. (11) Fan, J.; Boettcher, S. W.; Stucky, G. D. Chem. Mater. 2006, 18, 6391. (12) (a) Chen, C. C.; Ma, W. H.; Zhao, J. C. Chem. Soc. Rev. 2010, 39, 4206. (b) Wang, Q.; Zhang, M.; Chen, C. C.; Ma, W. H.; Zhao, J. C.

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