4746
Langmuir 2007, 23, 4746-4748
Vapor-Phase Synthesis of Mesoporous SiO2-P2O5 Thin Films Norikazu Nishiyama,* Junji Kaihara, Yuko 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 ReceiVed January 11, 2007. In Final Form: February 13, 2007 Mesoporous SiO2-P2O5 films were synthesized from the vapor phase onto a silicon substrate. First, a precursor solution of cetyltrimethylammonium bromide (C16TAB), H3PO4, ethanol, and water was deposited on a silicon substrate by a spin-coating method. Then, the C16TAB-H3PO4 composite film was treated with tetraethoxysilane (TEOS) vapor at 90-180 °C for 2.5 h. The H3PO4-C16TAB composite formed a hexagonal structure on the silicon substrate before vapor treatment. The TEOS molecules penetrated into the film without a phase transition. The periodic mesostructure of the SiO2-P2O5 films was retained after calcination. The calcined films showed a high proton conductivity of about 0.55 S/cm at room temperature. The molar ratio of P/Si in the SiO2-P2O5 film was as high as 0.43, a level that was not attained by a premixing sol-gel method. The high phosphate group content and the ordered periodic mesostructure contributed to the high proton conductivity.
Introduction Recently, direct methanol fuel cells (DMFCs) with proton conducting electrolyte membranes have been of great interest for use in portable power sources and electric vehicles. A protonbased energy system is clean and produces high energy density without the emission of harmful pollutants. Porous inorganic materials with phosphate groups1-4 and sulfonic acid groups5 are expected to exhibit high proton conductivity. The hydrated water adsorbed in micropores and mesopores is considered to act as a proton donor. Mesoporous materials prepared by a surfactant-assisted method have a large surface area (>700 m2/ g) for water adsorption. In addition, the periodic order of their pore structure may improve their proton conductivity.6 Recently, we have reported that ordered mesoporous zirconium phosphate films prepared by spin coating exhibited a high proton conductivity of 0.02 S/cm parallel to the film surface at 80% RH and 25 °C.7 In the present study, we have prepared ordered mesoporous SiO2P2O5 films by a vapor-phase method that was developed for silica films.8,9 Experimental Section Preparation. Surfactant molecules were deposited onto a silicon substrate by a spin-coating method. The precursor solution was prepared using cetyltrimethylammonium bromide (C16TAB), H3PO4, EtOH, and deionized water in a mole ratio of 1.5H3PO4/0.75C16TAB/ 50EtOH/100H2O. The surfactant-solvent mixture was dropped onto * To whom correspondence should be addressed. E-mail: nisiyama@ cheng.es.osaka-u.ac.jp. Phone: +81-6-6850-6256. Fax: +81-6-6850-6256. (1) Alberti, G.; Casciola, M.; Cavalaglio, S.; Vivani, R. Solid State Ionics 1999, 125, 91. (2) Rodriguez-Castellon, E.; Jimenez-Jimenez, J.; Jimenez-Lopez, A.; MairelesTorres, P.; Ramos-Barrado, J. R.; Jones, D. J.; Roziere, J. Solid State Ionics 1999, 125, 407. (3) Nagami, M.; Daiko, Y.; Akai, T.; Kasuga, T. J. Phys. Chem. B 2001, 105, 4653. (4) Daiko, Y.; Kasuga, T.; Nogami, M. Chem. Mater. 2002, 14, 4624. (5) Diaz, I.; Mohino, F.; Perez-Pariente, J.; Sastre, E. Appl. Catal., A 2001, 205, 19. (6) Matsuda, A.; Nono, Y.; Kanzaki, T.; Tadanaga, K.; Tatsumisago, M.; Minami, T. Solid State Ionics 2001, 145, 13. (7) Nishiyama, Y.; Tanaka, S.; Hillhouse, H. W.; Nishiyama, N.; Egashira, Y.; Ueyama, K. Langmuir 2006, 22, 9469. (8) Nishiyama, N.; Tanaka, S.; Egashira, Y.; Oku, Y.; Ueyama, K. Chem. Mater. 2003, 15, 1006. (9) Tanaka, S.; Nishiyama, N.; Oku, Y.; Egashira, Y.; Ueyama, K. J. Am. Chem. Soc. 2004, 126, 4854.
the silicon substrate while it was spinning at 500 rpm, and then the substrate was spun up to 4000 rpm for 60 s. The H3PO4-C16TAB composite film was arranged to lie vertically in a closed vessel (50 cm3). A small amount of tetraethoxysilane (TEOS) was placed in the bottom of the vessel apart from the substrate. The vessel was placed in an oven at 90-180 °C for 2.5 h. Calcination was performed at 400 °C in air for 5 h with a heating rate of 1 °C/min. Characterization. The X-ray diffraction patterns (XRD) of mesostructured films were recorded on a Rigaku Mini-Flex with Cu KR radiation using λ ) 1.5418 Å in a θ-2θ scanning mode. Fourier transform infrared (FTIR) spectra of mesostructured films were recorded on an FTIR-8200PC spectrometer (Shimadzu Co.) at 4 cm-1 resolutions. Field-emission scanning electron microscope (FESEM) images were recorded on a Hitachi S-5000L microscope at an acceleration voltage of 18 kV. The porous structures were observed with an scanning transmission electron microscope (STEM, Hitachi HD2000). The proton conductivity was measured by the ac impedance method with an impedance analyzer (Solatron SI 1260). Platinum electrodes were deposited on the SiO2-P2O5 films with a Quick Coater VPS20 (ULVAC KIKO, Inc.) The film was placed in a humiditycontrolled chamber at 20 °C. The impedance was obtained from a semicircular arc in a Cole-Cole plot over a frequency range of 10-1-105 Hz. The proton conductivity was calculated from the impedance at a frequency of 0.1-10 Hz and from the film thickness and distance between the electrodes (2 mm).
Results and Discussion The XRD patterns of the resultant mesostructured SiO2-P2O5 films are shown in Figure 1. The spin cast film of H3PO4C16TAB already has a hexagonal structure. The absence of the (110) reflection indicates that the (100) family of planes of the hexagonal unit cell is oriented parallel to the surface of the silicon substrate.10 After the TEOS-vapor treatment, the (100) reflection peak was shifted to lower angles. This result indicates that partially hydrolyzed TEOS molecules penetrate into a hydrophilic H3PO4H2O region around the C16TAB molecules, resulting in an expansion of the periodic distance. We previously reported the vapor-phase synthesis of mesoporous silica films.8 In that case, C16TAB surfactant molecules were arranged with lamellar symmetry on the silicon substrate. In addition, when penetrated (10) Hillhouse, H. W.; van Egmond, J. W.; Tspatis, M.; Hanson, J. C.; Larese, J. Z. Microporous Mesoporous Mater. 2001, 44-45, 639.
10.1021/la070081d CCC: $37.00 © 2007 American Chemical Society Published on Web 03/21/2007
Letters
Figure 1. XRD patterns of SiO2-P2O5 films prepared by the vaporphase method: (a) H3PO4-C16TAB, (b) TEOS vapor-treated, and (c) calcined.
Figure 2. Proposed model of the formation of the mesostructured SiO2-P2O5 film.
Langmuir, Vol. 23, No. 9, 2007 4747
Figure 4. FTIR spectra of the SiO2-P2O5 films prepared by the vapor-phase method: (a) H3PO4-deposited, (b) TEOS vapor-treated (90 °C), and (c) calcined (400 °C).
Figure 5. FE-SEM image of the SiO2-P2O5 film prepared by the vapor-phase method. The treatment temperature is 90 °C.
by the TEOS vapor, the TEOS-C16TAB composite rearranged to a hexagonal phase. However, in this study the H3PO4-C16TAB composite forms a hexagonal structure on the silicon substrate before vapor treatment. A similar phase transition was reported for a H2SO4-C16TAB composite.11 It seems that large amounts of H3O+ and PO43- could adsorb onto the hydrophilic heads of C16TAB molecules. The bulky heads of the surfactants caused
the arrangement of the H3PO4-C16TAB composite into hexagonal symmetry. A proposed model for the formation of the mesostructured SiO2-P2O5 film is shown in Figure 2. The TEOS molecules penetrate into the film without a phase transition. The periodic mesostructure of the SiO2-P2O5 films was retained after calcination. The effect of the treatment temperature on the proton conductivity of the SiO2-P2O5 films is presented in Figure 3. At 0.55 S cm-1, the proton conductivity of the film synthesized at 90 °C was much higher than the reported values for Nafion (5 × 10-2 S cm-1). An increase in temperature during TEOS vapor treatment resulted in a decrease in the proton conductivity. Silica particles were deposited on the surface of the films prepared above 150 °C. The reaction rate of the TEOS deposition onto the film is increased at high temperatures compared to the penetration rate of TEOS into the film. The existence of a nonporous silica layer reduces the overall proton conductivity. The proton conductivity of mesoporous pure-silica films synthesized by the vapor-phase method was 10-4 S/cm. This result suggests that Si-O-P-OH groups are present in the SiO2P2O5 films and contribute to the high proton conductivity. The formation of Si-O-P bonds in phosphosilicate gels derived from TEOS-H3PO4 mixtures heated to 150 °C has been reported by Matsuda et al.12 The proton conductivity at a relative humidity of 5% is similar to that at a relative humidity of 80%. The humidity for the capillary condensation of water was estimated to be 42% when the pore size was assumed to be about 2.4 nm. These results suggest that capillary-condensed water is not required for high proton
(11) Tanaka, S.; Maruo, T.; Nishiyama, N.; Egashira, Y.; Ueyama, K. Chem. Lett. 2005, 34, 1148.
(12) Masuda, A.; Kanzaki, T.; Tadanaga, K.; Tatsumisago, M.; Minami, T.; Solid State Ionics 2002, 154-155, 687.
Figure 3. Proton conductivity of SiO2-P2O5 films prepared by the vapor-phase method. The relative humidity is 80%.
4748 Langmuir, Vol. 23, No. 9, 2007
Letters
that it adsorbs water even at low humidity. This result suggests that adsorbed (not condensed) water on the pore surface plays an important role in proton conductivity. FE-SEM and TEM images of a cross-section of the calcined SiO2-P2O5 film are provided in Figures 5 and 6, respectively. The film thickness was about 400 nm. The ordered pore channels with a hexagonal arrangement were observed in the TEM image, a finding that is consistent with the XRD results. We peeled the film off of the substrate and measured the P/Si molar ratio by EDX analysis. The measured P/Si ratio was 0.43. Such a high P content was not attained by the premixing sol-gel method because of the collapse of the ordered structure. The proton conductivity of the disordered SiO2-P2O5 films (0.01 S/cm) was much smaller than that of the ordered SiO2-P2O5 films (0.55 S/cm); evidently, the proton conductivity is affected not only by the content of P-OH but also by the porous structure. The high proton conductivity can be explained by a high surface concentration of P-OH on the uniform pore surface. The FTIR spectra in Figure 4 support the contention 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 related to the orientation of the pore channels. The TEM image reinforces the finding that the mesoporous, domain-structured SiO2-P2O5 was randomly oriented with respect to the direction of the film surface. The horizontal domain size of mesoporous silica was less than 100 nm. The proton conductivity along the film surface was 102 times higher than that perpendicular to the surface. Both the high content of phosphate groups in and on the pore wall and the highly ordered periodic structure must contribute to the high proton conductivity of the SiO2-P2O5 film synthesized by the vapor-phase method.
Figure 6. TEM images of the SiO2-P2O5 film prepared by the vapor-phase method. The treatment temperature is 90 °C.
Acknowledgment. This study was supported by Grant-inAid from the Ministry of Education, Science, Sports and Culture of Japan (no. 18360374). We acknowledge the GHAS laboratory and Mr. M. Kawashima at Osaka University for the FE-SEM measurements. We also acknowledge Rohm Co., Ltd. for the TEM measurements. Y.N. acknowledges the Center of excellence (21COE) program “Creation of Integrated EcoChemistry” of Osaka University.
conductivity. As shown in the FTIR spectra in Figure 4, the hydrophilic nature of the pore surface of Si-O-P-OH ensures
LA070081D
