© Copyright 2006 American Chemical Society
NOVEMBER 7, 2006 VOLUME 22, NUMBER 23
Letters Synthesis of Ordered Mesoporous Zirconium Phosphate Films by Spin Coating and Vapor Treatments Yuko Nishiyama,† Shunsuke Tanaka,†,‡ Hugh W. Hillhouse,‡ Norikazu Nishiyama,*,† Yasuyuki Egashira,† and Korekazu Ueyama† DiVision of Chemical Engineering, Graduate School of Engineering Science, Osaka UniVersity, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan, and School of Chemical Engineering, Purdue UniVersity, 480 Stadium Mall DriVe, West Lafayette, Indiana 47907 ReceiVed May 25, 2006. In Final Form: October 1, 2006 Ordered mesoporous zirconium phosphate films were prepared on a silicon substrate by spin coating using a mixture of zirconium isopropoxide, triethyl phosphate, Pluronic P123 triblock copolymer, nitric acid, ethanol, and water. The spin-on film was consecutively treated with vapors of phosphoric acid and ammonia. The post-vapor treatments effectively enhanced the thermal stability of an ordered mesostructure when heated to 500 °C. XRD and TEM analyses show that the calcined zirconium phosphate film has a hexagonal structure with straight channels parallel to the film surface. The zirconium phosphate film exhibited high proton conductivity of 0.02 S/cm parallel to the film surface at 80% RH and 25 °C.
Introduction The preparation of inorganic mesoporous films has attracted considerable attention because of their possible use in membrane separations, chemical sensors, optical devices, and electronic devices such as low-k dielectric films and electrolytes. Mesoporous silica materials1,2 and mesoporous silica films3-14 have conventionally been fabricated by deposition of surfactant* To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +81-6-6850-6256. Fax: +816-6850-6256. † Osaka University. ‡ Purdue University. (1) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988. (2) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature (London) 1992, 359, 710. (3) Yang, H.; Coombs, N.; Sokolov, I.; Ozin, G. A. Nature (London) 1996, 381, 589. (4) Ryoo, R.; Ko, C. H.; Cho, S. J.; Kim, J. M. J. Phys. Chem. B 1997, 101, 10610. (5) Schacht, S.; Huo, Q.; Voigt-Martin, I. G.; Stuky, G. D.; Schuth, F. Science 1996, 273, 768. (6) Aksay, I. A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberger, P. M.; Gruner, S. M. Science 1996, 273, 892.
silicate composites from a liquid phase under acidic or basic conditions. This surfactant-assisted method has been extended to the synthesis of various multicomponent mesostructures such as metal oxides and phosphates.15-17 Ordered mesoporous zirconium phosphate is an attractive material for use in proton(7) Yang, H.; Kuperman, A.; Coombs, N.; Mamich-Afara, S.; Ozin, G. A. Nature (London) 1996, 379, 703. (8) Ogawa, M. J. Chem. Soc., Chem. Commun. 1996, 1149. (9) Ogawa, M.; Masukawa, N. Microporous Mesoporous Mater. 2000, 38, 35. (10) Miyata, H.; Kuroda, K. Chem. Mater. 1999, 11, 1609. (11) Lu, Y.; Ganguli, R.; Drewien, C. A.; Anderson, M. T.; Brinker, C. J.; Gong, W.; Guo, Y.; Soyez, H.; Dunn, B.; Huang, M. H.; Zink, J. I. Nature (London) 1997, 389, 364. (12) Sellinger, A.; Weiss, P. R.; Nguyen, A.; Lu, Y.; Assink, R. A.; Gong, W.; Brinker, C. J. Nature (London) 1998, 394, 256. (13) Nishiyama, N.; Tanaka, S.; Egashira, Y.; Oku, Y.; Ueyama, K. Chem. Mater. 2002, 14, 4229. (14) Park, D.-H.; Nishiyama, N.; Egashira, Y.; Ueyama, K. Ind. Eng. Chem. Res. 2001, 40, 6105. (15) Tian, B.; Liu, X.; Tu, B.; Yu, C.; Fan, J.; Wang, L.; Xie, S.; Stucky, G. D.; Zhao, D. Nat. Mater. 2003, 2, 159. (16) Rodoriguez-Castellon, E.; Jimenez-Jimenez, J.; Jimenez-Lomenez, A.; Maireles-Torres, P.; Ramos-Barrado, J. R.; Jones, D. J.; Roziere, J. Solid State Ionics 1999, 125, 407. (17) Kleitz, F.; Thomson, J. S.; Liu, Z.; Terasaki, O.; Schuth, F. Chem. Mater. 2002, 14, 4134.
10.1021/la061493+ CCC: $33.50 © 2006 American Chemical Society Published on Web 10/17/2006
9470 Langmuir, Vol. 22, No. 23, 2006
Letters
conducting devices and catalysts.16 Kleitz et al.17 have synthesized mesoporous zirconium phosphate in powder form using alkyltrimethylammonium bromide. However, ordered mesoporous zirconium phosphate films have not been reported because the mesoporous films prepared by dip coating or spin coating at room temperature have a serious problem with respect to thermal stability. A deterioration of the periodic order of these mesostructures has been inevitable under calcination because the formation of inorganic networks in pore walls is insufficient compared to that in silicate. In the present study, we have developed a new method of synthesizing ordered mesoporous zirconium phosphate films with high thermal stability. The structure of the zirconium phosphate films and their proton conductivity are shown here. Experimental Section Preparation. A precursor solution was prepared using zirconium isopropoxide (∼75 wt % Zr(OPr)n solution in free propanol), triethyl phosphate (PO(OEt)3), Pluronic P123 triblock copolymer, 10 wt % nitric acid solution (HNO3), ethanol (EtOH), and water. A precipitation formed instantaneously after a mixture of a Zr(OPr)n solution and PO(OEt)3 was added to a diluted HNO3 solution. The mixture was stirred until the precipitate disappeared. Then, Pluronic P123 dissolved in EtOH was added to the mixture. The molar ratio of the mixture was 2.0:1.0:0.1:2.4:110:40:3.6 Zr(OPr)n/PO(EtO)3/Pluronic P123/HNO3/H2O/EtOH/PrOH. After being stirred for 1.25 h, the solution was deposited on a silicon substrate by spin coating (50 rpm for 10 s followed by 4000 rpm for 60 s). The deposited film was exposed to a vapor of phosphoric acid (H3PO4) at autogenous pressure as follows: The film was arranged to lie vertically in a closed vessel (50 cm3). A small amount of H3PO4 was placed in the bottom of the vessel apart from the substrate. The vessel was placed in an oven at 90 °C for 72 h and at 150 °C for 24 h. Then, the film was exposed to ammonia (NH3) vapor at 60 °C for 7 h in the same manner. Calcination was performed at 300 °C for 5 h with a heating rate of 1 °C/min to remove the P123 triblock copolymer template. Characterization. X-ray diffraction (XRD) patterns of mesostructured films were recorded on a Rigaku Mini-Flex using Cu KR radiation with λ ) 1.5418 Å in θ-2θ scan mode. Fourier transform infrared (FTIR) spectra were recorded on an FTIR8200PC spectrometer (Shimadzu Co.). Field emission scanning electron microscope (FE-SEM) images were recorded on a Hitachi S-5000L microscope at an acceleration voltage of 18 kV. Transmission electron microscope (TEM) images of a calcined zirconium phosphate film peeled from the substrate were recorded on a Hitachi H800 electron microscope at an acceleration voltage of 200 kV. The chemical composition was measured by an energy disperse X-ray analyzer (EDX: Perkin-Elmer 2400II). Impedance spectroscopy was used to determine the proton conductivity of the film in the direction of the substrate surface. Platinum electrodes were deposited on the zirconium phosphate films with a Quick Coater VPS-20. (Ulvac Kiko, Inc.) The proton conductivity of the films was measured with an impedance analyzer (Solatron SI 1260) in the frequency range of 10-1-106 Hz.
Results and Discussion Figure 1 shows XRD patterns of an as-coated film, films treated with H3PO4 and NH3, and a calcined film. The XRD pattern of the as-coated film has three reflection peaks that correspond to (100), (200), and (300) reflections. The d spacing of the peak at the lowest angle was 5.3 nm. A nanocomposite of P123zirconium phosphate was thought to arrange with lamellar symmetry. Upon treatment with H3PO4 vapor, the (200) and (300) reflection peaks disappeared, indicating that the zirconium phosphate composite rearranged to a hexagonal phase under a penetration of H3PO4 molecules. The absence of the (110) reflection indicates that the family of planes of the hexagonal unit cell is oriented parallel to the surface of the silicon substrate.
Figure 1. XRD patterns of the zirconium phosphate films: (a) as-coated, (b) H3PO4-treated, (c) NH3-treated, and (d) calcined.
Figure 2. FTIR spectra of the zirconium phosphate films: (a) ascoated, (b) H3PO4-treated, (c) NH3-treated, and (d) calcined.
The ordered hexagonal structure was retained after template removal although the d(100) spacing decreased to 3.3 nm. The 2D-GISAXS pattern of the film suggests that the calcined film possesses c2mm structure oriented with the plane parallel to the substrate (Supporting Information, Figure S1). The periodic structure of the zirconium phosphate film without post-treatment using H3PO4 and NH3 vapors was not retained under calcination. Apparently, post-treatment with the vapors was very effective at enhancing the thermal stability of the ordered mesostructure. Figure 2 shows the FTIR spectra of the zirconium phosphate films. The absence of the bands at ∼2854 and ∼2925 cm-1 for the calcined film, which are ascribed to symmetric and asymmetric vibrations of the C-H groups, indicates the complete removal of P123 molecules after calcination. A new broad band, which appeared around 1036 cm-1 after calcination, is attributed to the P-O stretching and bending vibration.18 The broad band around 3270 cm-1 can be attributed to OH groups and physically adsorbed water,19 suggesting that the calcined zirconium phosphate film is hydrophilic in nature. Molar ratios of Zr/P in zirconium phosphate films were determined by EDX measurements. The Zr/P ratio of the film is decreased from 1.5 to 1.2 after the vapor treatment of H3PO4, indicating that H3PO4 penetrated into the spin-on zirconium phosphate film. This result suggests that a small amount of H3PO4 incorporated into the zirconium phosphate walls effectively enhanced the thermal stability. It seemed that a decrease in the molar ratio of Zr/P in the film induced a transformation from the lamellar to hexagonal phase. However, spin coating using solutions with lower Zr/P molar ratios did not produce a hexagonal (18) Wu, P.; Liu, Y.; He, M.; Iwamoto, M. Chem. Mater. 2005, 17, 3921. (19) Alberti, G.; Casciola, M.; Donnadio, A.; Piaggio, P.; Pica, M.; Sisani, M. Solid State Ionics 2005, 176, 2893.
Letters
Langmuir, Vol. 22, No. 23, 2006 9471
Figure 3. FE-SEM images of the cross section of the mesoporous zirconium phosphate films.
Figure 4. TEM micrograph of the mesoporous zirconium phosphate thin films.
structure. This result suggests that reaction conditions for the vapor treatment such as temperature and vapor pressure are important in inducing the phase transformation. Furthermore, thermal stability was improved by NH3 treatment. For reference, XRD patterns of mesoporous zirconium phosphate films without NH3 treatment are shown in the Supporting Information (Figure S2). After calcination of the H3PO4-treated film but without NH3 treatment, the d(100) spacing was reduced to 2.00 nm. The hydrolysis of Zr(OPr)n and PO(OEt)3 and their condensation to Zr-O-P must be promoted in the presence of basic NH4OH and acidic H3PO4, which act as catalysts for the hydrolysis and condensation reactions. An enhancement of the formation of a zirconia phosphate framework resulted in improved thermal stability of the ordered periodic structure. The molar ratio of Zr/P in the calcined film was 2.1. Unreacted PO(OEt)3 in the film was supposed to evaporate under a heating process of calcination. Figure 3 shows FE-SEM images of the cross section of the mesoporous zirconium phosphate films. The pores run parallel to the film surface. The film thickness was about 275 nm. A TEM image of the calcined mesoporous zirconium phosphate film peeled from the Si substrate is presented in Figure 4. The TEM image must be observed in the direction perpendicular to the film surface. The parts showing light and dark contrasts correspond to pores and walls, respectively. From FE-SEM and TEM observations and XRD results, ordered channels run parallel to the film surface. The distance from layer to layer is about 3 nm, which is consistent with the result of XRD measurements (d ) 3.3 nm). XRD patterns of the calcined zirconium phosphate films heated to different temperatures are shown in Figure 5. The periodic
Figure 5. XRD patterns of mesoporous zirconium phosphate films heated to different temperatures.
Figure 6. Proton conductivities of zirconium phosphate films heated to different temperatures.
mesostructure of the zirconium phosphate was retained after heating although a structural contraction was observed. The d(100) spacings of zirconium phosphate were 3.29, 2.90, and 2.69 nm, respectively, at 300, 400, and 500 °C. There are no peaks at higher than 8°, indicating that the formed zirconium phosphate possesses an amorphous structure and that the crystallization of zirconia or zirconium phosphate did not occur up to 500 °C. Proton conductivity along the film surface was measured at 25 °C and 80% relative humidity after depositing platinum electrodes on the films. The proton conductivity was calculated from a semicircular arc in a Cole-Cole plot. Figure 6 shows the proton conductivities of the zirconium phosphate films heated to different temperatures. The proton conductivity for zirconium phosphate calcined at 300 °C was 2.2 × 10-2 S/cm and higher than for the other two samples. An increase in the heating
9472 Langmuir, Vol. 22, No. 23, 2006
Figure 7. Plausible structure of the zirconium phosphate film.
temperature resulted in a decrease in the proton conductivity. From the FTIR spectra of zirconium phosphate films heated to different temperatures (Supporting Information, Figure S3), the concentrations of P-OH and physically adsorbed H2O on the pore surface seem to decreased at elevated temperature. The P-OH groups and physically adsorbed H2O on the pore surface must contribute to the proton conductivity of the zirconium phosphate film. The proton conductivity of the ordered mesoporous zirconium phosphate film in this study is much higher than the reported data for disordered mesoporous zirconium phosphate.20 The high proton conductivity can be explained by the high surface concentration of P-OH on the uniform pore surface. The FTIR spectra in Figure 2 indicate that this material possesses a large number of OH groups on the inner surface even after calcination. In addition to that, the proton conductivity must be strongly (20) Rodorı´guez-Castello´n, E.; Jime´nez- Jime´nez. J.; Jime´nez-Lo´pez, A.; Maireles-Torres, P.; Ramos-Barrado, J. R.; Jones, D. J.; Rozie´re, J. Solid State Ionics 1999, 125, 407.
Letters
related to the orientation of the pore channels. We assume that the mesoporous zirconium phosphate with domain structure was randomly oriented with respect to the directions of the film surface as shown in Figure 7. The proton conductivity along the film surface must be much higher than that perpendicular to the surface. In conclusion, a mesoporous film with a zirconium phosphate framework has been synthesized by spin coating using a triblock copolymer to assemble an ordered mesostructure. To stabilize the framework of zirconium phosphate, vapor treatments of H3PO4 and NH3 were consecutively performed before calcination. The post-vapor treatments effectively enhanced the thermal stability of the periodic structure up to 500 °C. The zirconium phosphate film exhibited a high proton conductivity of 0.02 S/cm at 80% RH and 25 °C parallel to the film surface. Acknowledgment. We thank the GHAS laboratory at Osaka University for the EDX and FE-SEM measurements. We also thank Professor H. Mori and Dr. T. Sakata (the Research Center for Ultrahigh Voltage Electron Microscopy at Osaka University) for the TEM measurements. S.T. acknowledges the Japan Society for the Promotion of Science (JSPS) research fellowships. Y.N. acknowledges the center of excellence (21COE) program “Creation of Integrated EcoChemistry” of Osaka University. This study was partially supported by a grant-in-aid from the Ministry of Education, Science, Sports and Culture of Japan (no. 18360374). Supporting Information Available: GISAX pattern of the calcined zirconium phosphate film. XRD patterns of mesoporous zirconium phosphate films without NH3 treatment. FTIR spectra of zirconium phosphates heated to different temperatures. This material is available free of charge via the Internet at http://pubs.acs.org. LA061493+
