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Formation of Nanocrystalline Titanium Dioxide in Perfluorinated Ionomer Membrane Ping Liu,†,§ Jayasundera Bandara,† Yi Lin,† Derek Elgin,† Lawrence F. Allard,‡ and Ya-Ping Sun*,† Department of Chemistry and Center for Advanced Engineering Fibers and Films, Howard L. Hunter Chemistry Laboratory, Clemson University, Clemson, South Carolina 29634-0973, and High-Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6062 Received May 17, 2002. In Final Form: October 1, 2002 The formation of crystalline titanium dioxide (TiO2) nanoparticles in Nafion membrane structure was investigated. The X-ray powder diffraction and high-resolution transmission electron microscopy (TEM) results show that these nanoparticles are pure anatase TiO2. The TEM results also show that the crystalline TiO2 nanoparticles are ellipse in shape, with an estimated average aspect ratio (long axis/short axis) of 1.4. The average size of the TiO2 nanoparticles, which is independent of the loading of titanium, is similar to the estimated dimensions of the hydrophilic cavities in Nafion membrane in terms of the ion cluster model. However, there is no evidence for any TiO2 nanoparticles in the proposed channels that connect the hydrophilic cavities. The fact that the TiO2 nanoparticles are distributed homogeneously throughout the Nafion membrane structure suggests that the nanoparticles-embedded membrane still maintains a certain degree of porosity. The properties and potential applications of the Nafion-supported crystalline TiO2 nanoparticles are discussed.
Introduction Nanoscale titanium dioxide (TiO2) is among the most widely studied quantum semiconductors.1,2 A large number of methods have been developed for the preparation of TiO2 nanoparticles. Since crystalline TiO2 is required for most applications, the crystallization of TiO2 nanoparticles has also been an active area of investigation.3 For photocatalytic applications, a perfluorinated ionomer membrane is often used as support for TiO2 nanoparticles, taking advantage of the superior chemical stability and high optical quality of the membrane.4-7 Nafion is the most widely studied perfluorinated ionomer membrane.8 Thin films of Nafion membrane have been used as templates for the synthesis of nanoscale * To whom correspondence should be addressed. Telephone: 864656-5026. Fax: 864-656-5007. E-mail:
[email protected]. † Clemson University. ‡ Oak Ridge National Laboratory. § On leave from Fuzhou University, Fuzhou, China. (1) (a) Henglein, A. Chem. Rev. 1989, 89, 1861. (b) Steigerwald, N. L.; Brus, L. E. Acc. Chem. Res. 1990, 23, 183. (c) Weller, H. Adv. Mater. 1993, 5, 88. (d) Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 34. (e) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (f) Linsebigler, A. L.; Lu, G.; Yates, J. T. Chem. Rev. 1995, 95, 735. (2) (a) Schiavello, M., Ed. Photocatalysis and Environment; Kluwer Academic Publishers: Dordrecht, 1988. (b) Ollis, D. F., Al-Ekabi, H., Eds. Photocatalytic Purification of Water and Air; Elsevier: Amsterdam, 1993. (3) (a) Yin, H. B.; Wada, Y.; Kitamura, T.; Kambe, S.; Murasawa, S.; Mori, H.; Sakata, T.; Yanagida, S. J. Mater. Chem. 2001, 11, 1694. (b) Yin, H, B.; Wada, Y.; Kitamura, T.; Sumida, T.; Hasegawa, Y.; Yanagida, S. J. Mater. Chem. 2002, 12, 378. (c) Hirakawa, T.; Kominami, H.; Ohtani, B.; Nosaka, Y. J. Phys. Chem. B 2001, 105, 6993. (d) Ovenstone, J. J. Mater. Sci. 2001, 36, 1325. (4) Fan, F.-R. F.; Liu, H.-Y.; Bard, A. J. J. Phys. Chem. 1985, 89, 4418. (5) (a) Blatt, E.; Furlong, D. N.; Mau, A. W.-H.; Sasse, W. H. F.; Wells, D. Aust. J. Chem. 1989, 42, 1351. (b) Lund, E.-A.; Blatt, E.; Furlong, D. N.; Mau, A. W.-H.; Sasse, W. H. F. Aust. J. Chem. 1989, 42, 1367. (6) Miyoshi, H.; Nippa, S.; Uchida, H.; Mori, H.; Yoneyama, H. Bull. Chem. Soc. Jpn. 1990, 63, 3380. (7) Rabani, J.; Ushida, K.; Yamashita, K.; Stark, J.; Gershuni, S.; Kira, A. J. Phys. Chem. 1997, 101, 3136.
semiconductor and metal particles, including TiO2 nanoparticles.9-12 In fact, the study of the nanoparticle formation in Nafion membrane carries dual purposes. On one hand, a combination of the superior material properties of Nafion membrane with the high catalytic activities of nanocrystalline TiO2 provides a unique platform for the development of supported photocatalysts.4-7 On the other hand, crystalline TiO2 nanoparticles may serve as nanoscale probes in an understanding of the Nafion membrane structure as related to its templating effects on the formation of nanoparticles.12 For example, in the context of the ion cluster model for the Nafion membrane structure,13-15 issues such as the size, shape, and morphology of the ion clusters and the general organization of hydrophilic and hydrophobic structural domains in the (8) (a) Appleby, A. J.; Foulkes, F. R. Fuel Cell Handbook; Van Nostrand Reinhold: New York, 1989; Chapter 10. (b) Carla, H. W. J. Membr. Sci. 1996, 120, 1. (9) (a) Krishnan, M.; White, J. R.; Fox, M. A.; Bard, A. J. J. Am. Chem. Soc. 1983, 105, 7002. (b) Mau, A. W. H.; Huang, C.-B.; Kakuta, N.; Bard, A. J.; Campion, A.; Fox, M. A.; White, J. M.; Webber, S. E. J. Am. Chem. Soc. 1984, 106, 6537. (c) Kakuta, N.; White, J. M.; Campion, A.; Bard, A. J.; Fox, M. A.; Webber, S. E. J. Phys. Chem. 1985, 89, 48. (d) Kakuta, N.; Park, K. H.; Finlayson, M. F.; Ueno, A.; Bard, A. J.; Campion, A.; Fox, M. A.; Webber, S. E.; White, J. M. J. Phys. Chem. 1985, 89, 732. (e) Smotkin, E. S.; Brown, R. M.; Radenburg, L. K.; Salomon, K.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T. E.; Webber, S. E.; White, J. M. J. Phys. Chem. 1990, 94, 7543. (f) Zen, J.-M.; Chen, G. C.; Fan, F.-R. F.; Bard, A. J. Chem. Phys. Lett. 1990, 169, 23. (10) (a) Wang, Y.; Mahler, W. Opt. Commun. 1987, 61, 233. (b) Hilinski, E. F.; Lucas, P. A.; Wang, Y. J. Chem. Phys. 1988, 89, 3435. (c) Wang, Y.; Suna, A.; McHugh, J.; Hilinski, E. F.; Lucas, P. A.; Johnson, R. D. J. Chem. Phys. 1990, 92, 6927. (11) (a) Albu-Yaron, A.; Arcan, l. Thin Solid Films 1990, 185, 181. (b) Inoue, H.; Urquhart, R. S.; Nagamura, T.; Grieser, F.; Sakaguchi, H.; Furlong, D. N. Colloids Surf., A 1997, 126, 197. (12) (a) Rollins, H. W.; Whiteside, T.; Shafer, G. J.; Ma, J. J.; Tu, M. H.; Liu, J. T.; DesMarteau, D. D.; Sun, Y.-P. J. Mater. Chem. 2000, 10, 2081. (b) Rollins, H. W.; Lin, F.; Johnson, J.; Ma, J. J.; Liu, J. T.; Tu, M. H.; DesMarteau, D. D.; Sun, Y.-P. Langmuir 2000, 16, 8031. (13) Yeo, S. C.; Eisenberg, A. J. Appl. Polym. Sci. 1977, 21, 875. (14) Gierke, T. D.; Munn, G. E.; Wilson, F. C. J. Polym. Sci. 1981, 19, 1687. (15) (a) Hsu, W. Y.; Gierke, T. D. J. Membr. Sci. 1983, 13, 307. (b) Heitner-Wirguin, C. J. Membr. Sci. 1996, 120, 1.
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Crystalline TiO2 Nanoparticles in a Nafion Membrane
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membrane remain to be addressed in terms of more quantitative experimental results.16-18 Here we report an investigation on the formation of crystalline TiO2 nanoparticles in Nafion membrane under various loading conditions. The results, including those from the characterization using high-resolution transmission electron microscopy (TEM), provide new insight into the relationship between the membrane structure and the nanoparticle properties (shape, size, and location in the membrane), which may help address the issues discussed above and resolve some of the ongoing debates. Experimental Section Materials. Titanium isopropoxide (Ti(OC3H7)4) and titanium butoxide (Ti(OC4H9)4) were purchased from Aldrich. All organic solvents were of spectrophotometry grade and used as received. Water was deionized and purified by being passed through a Labconco WaterPros water purification system. Nafion ionomer membrane with an equivalent weight of 1100 was provided by Du Pont Co. The membrane films were purified to remove colored impurities using a uniform treatment procedure. In the purification, the films were immersed in concentrated nitric acid while stirring at 60 °C for 24 h. The acid was then decanted, and the films were placed sequentially in aqueous solutions of 60%, 40%, and 20% nitric acid, each for 1 h with stirring, followed by washing thoroughly with clean water until neutral. The treated Nafion membrane films were clear and optically transparent down to 200 nm. The films were kept in clean water before use. Formation of TiO2 Nanoparticles. In a typical experiment, a piece of purified and dried Nafion membrane film was soaked in a solution of Ti(OC3H7)4 in isopropanol (0.5 M) for 48 h. The film was then thoroughly washed with isopropanol to clean the film surface, followed by rinsing with acetone several times. The formation of TiO2 nanoparticles via hydrolysis in the Nafion membrane structure was accomplished by immersing the Ti(OC3H7)4)-loaded film in boiling water. Similarly, Ti(OC4H9)4 was also used as a precursor for TiO2 nanoparticles. The results obtained with the two different precursors are essentially the same. Measurements. UV/vis absorption spectra were recorded on a Shimadzu UV-3100 spectrophotometer. X-ray powder diffraction measurements were carried out on a Scintag XDS-2000 powder diffraction system. TEM images were obtained on a Hitachi HF-2000 TEM system. The specimens for TEM analyses were prepared using an Ultracut-E microtomy equipped with a diamond knife.
Results and Discussion Clean Nafion membrane films are transparent in the visible and UV down to 200 nm. Thus, the absorption spectrum of the Ti(OC3H7)4-loaded Nafion film (Figure 1) reflects the presence of titanium salt in the membrane structure. While the hydrolysis and condensation of titanium alkoxides are rapid in air, they are apparently slow in cavities of the Nafion membrane. Dipping the Ti(OC3H7)4-loaded film into ambient water resulted in little changes in the absorption spectrum. The X-ray powder diffraction analysis of the film treated with ambient water yielded no diffraction peaks of crystalline TiO2. A similarly prepared film in ambient water was also irradiated with UV light (1000-W xenon arc lamp), still producing no peaks due to crystalline TiO2 in the subsequent X-ray powder diffraction analysis. However, after the treatment in boiling water, the Ti(OC3H7)4-loaded Nafion film underwent significant changes, including more pronounced (16) Yeager, H. L.; Steck, A. J. Electrochem. Soc. 1981, 128, 1880. (17) Litt, M. H. Polym. Prepr. 1997, 38, 80. (18) (a) Bunker, C. E.; Ma, B.; Simmons, K. J.; Rollins, H. W.; Liu, J.-T.; Ma, J.-J.; Martin, C. W.; DesMarteau, D. D.; Sun, Y.-P. J. Electroanal. Chem. 1998, 459, 15. (b) Bunker, C. E.; Rollins, H. W.; Simons, K.; Liu, J.-T.; Ma, J.-J.; Martin, C. W.; DesMarteau, D. D.; Sun, Y.-P. J. Photochem. Photobiol. A: Chem. 1999, 126, 71.
Figure 1. UV/vis absorption spectra of Nafion membrane film loaded with Ti(OC3H7)4 before (- ‚ -) and after (s) treatment in boiling water. The spectrum of blank Nafion film (- - -) is also shown for comparison.
Figure 2. Powder X-ray diffraction pattern of the TiO2 nanoparticles in Nafion membrane film compared with that of the anatase TiO2 in JCPDS.
absorption in the near UV region (Figure 1). As shown in Figure 2, the X-ray powder diffraction pattern of the film treated in boiling water exhibits the characteristic peaks of crystalline TiO2 particles (corresponding to pure anatase TiO2). The nanoscopic nature of the crystalline TiO2 particles is responsible for the broadness of the X-ray powder diffraction peaks. The peak broadening is used to estimate the average TiO2 crystal grain size in terms of the DebeyeScherer equation.19
D ) Kλ/(β cos θ)
(1)
where D is the average nanocrystal diameter in angstroms, β is the corrected band broadening (fwhm), K is a constant related to the crystallite shape and way in which D and β are defined, λ is the X-ray wavelength, and θ is the diffraction angle. The average diameter thus estimated for the TiO2 nanoparticles formed in Nafion membrane cavities is 3.8 nm. The crystalline TiO2 nanoparticles were imaged directly by TEM at high resolution. For the TEM analysis, the (19) Klug, H. P.; Alexander, L. E. X-ray Diffraction Procedures; John Wiley and Sons: New York, 1959.
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Figure 4. Statistical analysis of the nanoparticle sizes obtained from the high-resolution TEM images.
Figure 3. High-resolution TEM image providing a crosssectional view of the TiO2 nanoparticles in Nafion membrane film.
TiO2 nanoparticles-embedded Nafion membrane film was microtomed to obtain ultrathin (70-90 nm) cross-sectional pieces. Shown in Figure 3 is a typical TEM image of the ultrathin specimen on a carbon-coated copper grid, which offers a cross-sectional view of TiO2 nanoparticles in the Nafion membrane film. There are clear lattice fringes on the particles, further confirming that the TiO2 nanoparticles formed in the Nafion membrane cavities are indeed crystalline. The lattice spacings of 0.352 and 0.238 nm correspond to the (101) and (004) planes of anatase TiO2, respectively. The high-resolution TEM imaging has also revealed something quite interesting. The crystalline TiO2 nanoparticles are actually ellipse in shape, with an estimated average aspect ratio (long axis/short axis) of 1.4. To the best of our knowledge, this is the first observation of ellipseshaped TiO2 nanoparticles in the polyelectrolyte membrane structure. Mechanistic details behind the formation of unsymmetric TiO2 nanoparticles are not clear. One possibility is that the ellipse shape is a result of the competition between the growth of a nanoparticle and the restriction to such growth imposed by the surrounding membrane structure. The same competition is probably responsible for the observation that the average size of the crystalline TiO2 nanoparticles is independent of the titanium salt loading in the membrane, as discussed below. The clear images of crystalline TiO2 nanoparticles enable a more accurate determination of the particle sizes. However, because of the ellipse shape, the long and short axes are averaged in the determination of an individual particle size. A statistical analysis of 123 particles (Figure 4) yields an average particle size of 3.7 nm and a size distribution standard deviation of 0.48 nm. Obviously, the average particle size values estimated in terms of the X-ray powder diffraction peak broadening and obtained from the TEM imaging are in excellent agreement. The microscopic structures of Nafion and other polyelectrolyte membranes are often described in terms of a reverse micelle-like ion cluster model.13-15 The model assumes the presence of essentially three distinctive structural regions: the perfluorinated polymer network,
Figure 5. Cartoon illustration of the ion cluster model for perfluorinated ionomer membranes.13-15
water cores, and the interfacial domain between the two regions, where the water cores in neighboring clusters are presumably interconnected through channels (Figure 5). Experimentally, the structural model is supported by results from a small-angle X-ray investigation of Nafion membrane.13 The presence of hydrophilic cavities in the ionomer membrane structure is also supported by the successful preparation of nanoparticles using the membrane as template, as reported here and in the literature.4,9-12 The average size of the crystalline TiO2 nanoparticles obtained in this work is remarkably close to the estimated size of an average cavity in hydrated Nafion membrane.8,13,20 However, there are two interesting points to be made on the basis of the results described above. One concerns the role of the proposed channels that link the ion clusters in the formation of nanoparticles. Since the channel sizes are generally expected to be smaller than those of the ion clusters, the formation of two types of nanoparticles, with the larger ones in the ion cluster cavities and smaller ones in channels, has been suggested in the literature.11a The results presented here show that at least for the formation of crystalline TiO2 nanoparticles in Nafion membrane structure there is no evidence for the presence of any smaller nanoparticles. The other point to be made is related to the fact that according to the cross-sectional view the crystalline TiO2 nanoparticles are well dispersed throughout the Nafion (20) Haubold, H. G.; Jungbluth, T. V. H.; Hiller, P. Electrochim. Acta 2001, 46, 1559.
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Figure 6. Cartoon diagram for a cross-sectional view of the Nafion membrane film embedded with TiO2 nanoparticles.
membrane film. It is understandable that the loading of titanium alkoxide in solution results in a homogeneous distribution of the salt throughout the membrane structure. However, the subsequent hydrolysis of the titanium salt and the condensation of TiO2 nanoparticles are not necessarily homogeneous processes with respect to the entire membrane structure. The formation of TiO2 nanoparticles at sites that are close to the surfaces of the Nafion membrane film might prevent the penetration of boiling water into the sites deep inside the film for the formation of TiO2 nanoparticles via the same hydrolysis-condensation processes. Thus, the fact that the crystalline TiO2 nanoparticles are well dispersed throughout the Nafion membrane structure suggests that the nanoparticlesembedded Nafion membrane film still maintains a certain degree of porosity, allowing the diffusion of solvent molecules into the inner structural domain of the film (Figure 6). This is very important to the use of Nafion membrane as host for nanoscale catalysts. The content of titanium salt in Nafion membrane can be controlled in the loading process by changing the titanium alkoxide solution concentration. In addition to the Ti(OC3H7)4 solution of 0.5 M discussed above, more dilute solutions of 0.3, 0.2, and 0.1 M concentrations were used to load the titanium salt into Nafion membrane films. When other experimental conditions were kept the same, a decrease in the titanium alkoxide solution concentration resulted in less efficient loading of the titanium salt, as confirmed by the absorption spectra of the membrane films thus obtained. The decrease in loading could possibly have two different effects on the formation of TiO2 nanoparticles: similar in number but on average smaller nanoparticles versus similar in size but smaller number of nanoparticles. The results shown in Figure 7 and Table 1 apparently support the latter, namely that the average sizes of crystalline TiO2 nanoparticles are largely unchanged with decreases in the loading solution concentration. Similar behavior has been reported for the formation of other nanoparticles in Nafion and related perfluorinated ionomer membranes.12 For example, lead sulfide (PbS) and silver (Ag0) nanoparticles formed in these membranes maintain average particle sizes of 4-5 nm and 12-14 nm, respectively, regardless of the loading conditions.12,21 Thus, in the relationship between the Nafion membrane structure and the embedded nanoparticles, it seems that the particle sizes are determined by the material properties (21) Sun, Y.-P.; Bandara, J.; Atorngitjawat, P.; Elgin, D.; Zhang, M. Manuscript in preparation.
Figure 7. Powder X-ray diffraction patterns of the Nafion membrane film-embedded TiO2 nanoparticle samples obtained with different Ti(OC3H7)4 solution concentrations under otherwise the same experimental conditions. From the top to bottom: 0.5, 0.3, 0.2, and 0.1 M. Table 1. Average Sizes of the TiO2 Nanoparticles Obtained under Different Loading Conditions Ti(OC3H7)4 solution conca (M)
avg particle size (nm)
0.5 0.3 0.2 0.1
3.8 3.9 3.8 3.9
a The time for loading and the time for treatment in boiling water are kept the same.
of the particles more than the characteristics of the Nafion membrane structure. In the nanoparticle formation, however, there is probably a competition between the need for the nanoparticle to grow and the need for the membrane to maintain its structure to restrict the nanoparticle growth. For crystalline TiO2 nanoparticles in Nafion membrane, the competition reaches a balance at an average particle size of close to 4 nm in average diameter. The formation of a smaller number of nanoparticles at a lower titanium alkoxide concentration in the Nafion membrane may have also contributed to the homogeneous dispersion of the nanoparticles throughout the membrane structure. The porosity provided by vacant ion clusters in the nanoparticles-embedded Nafion membrane films should be beneficial to the catalytic applications of these composite thin films. Acknowledgment. Financial support from DOE (DEFG02-00ER45859 and, in part, DE-FG02-91ER75666) and the Center for Advanced Engineering Fibers and Films (NSF-ERC at Clemson University) is gratefully acknowledged. D.E. was a participant in the Summer Undergraduate Research Program sponsored jointly by NSF and DOE. We also acknowledge the sponsorship by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Transportation Technologies, as part of the High Temperature Materials Laboratory User Program, Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the Department of Energy (DE-AC05-00OR22725). LA020462L
