Langmuir 2000, 16, 8031-8036
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Nanoscale Cavities for Nanoparticles in Perfluorinated Ionomer Membranes Harry W. Rollins, Feng Lin, Jermaine Johnson, Jing-Ji Ma, Jin-Tao Liu, Ming-Hu Tu, Darryl D. DesMarteau, and Ya-Ping Sun* Department of Chemistry, Howard L. Hunter Chemistry Laboratory, Clemson University, Clemson, South Carolina 29634-0973 Received December 7, 1999. In Final Form: July 19, 2000 The preparation and characterization of nanoscale silver sulfide and silver particles in Nafion and perfluorinated sulfonimide ionomer membranes are reported. The results show that the nanoparticles are hosted in the membrane structure in an isolated fashion, with no indication of channels-like domains as proposed in the ion cluster model for the ionomer membranes. These randomly dispersed nanoparticles are also significantly larger than the average size of the reverse micelle-like hydrophilic cavities estimated in the literature for Nafion membrane. The properties of the silver sulfide and silver nanoparticles are presented, and their implications to the understanding of membrane nanoscopic structural details are discussed.
Introduction Perfluorinated ionomer membranes, represented by the commercial thin film Nafion, have received much attention for several important existing and potential applications.1-4 The structure and properties of perfluorinated ionomer membranes are currently understood in terms of a reverse micelle-like ion cluster model for Nafion.5-12 The model assumes the presence of essentially three distinctive structural regions: the perfluorinated polymer network, water cores, and the interfacial domain between the two regions, where the water cores in neighboring clusters are presumably interconnected through channels (Figure 1).7,9 Although the microstructural model was supported by results from a small-angle X-ray study of Nafion membrane,5 issues such as the shape and morphology of the ion clusters, the significance and dimension of the interfacial region, and the general organization of hydrophilic and hydrophobic structural domains in Nafion and related ionomer membranes are still being debated.8,11,12 The presence of hydrophilic cavities in the ionomer membrane structure was also supported by experiments in which nanoparticles were prepared using the cavities in Nafion membrane as templates.13-23 For example, nanoscale semiconductors CdS and TiO2 and nanocom(1) Cutler, S. G. In Ions in Polymers; Eisenberg, A., Ed.; American Chemical Society: Washington, DC, 1980; Chapter 9. (2) Perfluorinated Ionomers Membranes; Eisenberg, A., Yeager, H. L., Eds.; American Chemical Society: Washington, DC, 1982. (3) La Conti, A. B.; Fragala, A. R.; Boyack, J. R. In Electrode Materials and Processes for Energy Conversion and Storage; McIntyre, J. D., Srinivasan, S., Will, F. G., Eds.; The Electrochemical Society: Princeton, NJ, 1977; p 354. (4) Appleby, A. J.; Foulkes, F. R. In Fuel Cell Handbook; Van Nostrand Reinhold: New York, 1989; Chapter 10. (5) Yeo, S. C.; Eisenberg, A. J. Appl. Polym. Sci. 1977, 21, 875. (6) Falk, M. Can. J. Chem. 1980, 58, 1495. (7) Gierke, T. D.; Munn, G. E.; Wilson, F. C. J. Polym. Sci. 1981, 19, 1687. (8) Yeager, H. L.; Steck, A. J. Electrochem. Soc. 1981, 128, 1880. (9) Hsu, W. Y.; Gierke, T. D. J. Membr. Sci. 1983, 13. (10) Heitner-Wirguin, C. J. Membr. Sci. 1996, 120, 1. (11) Litt, M. H. Polm. Prepr. 1997, 38, 80. (12) 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.
Figure 1. The proposed structure of ion clusters.7,9
posites such as CdS/ZnS and CdS/Pt were prepared in Nafion membrane for the photocatalytic production of hydrogen. These investigations also served another purpose, such that the incorporation of nanoparticles into the membrane allowed a direct imaging of the hosting cavities in the membrane structure, complementary to the electron microscopy studies of stained Nafion membrane films.24,25 In most reports of nanoparticles in Nafion (13) Krishnan, M.; White, J. R.; Fox, M. A.; Bard, A. J. J. Am. Chem. Soc. 1983, 105, 7002. (14) 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. (15) Kakuta, N.; White, J. M.; Campion, A.; Bard, A. J.; Fox, M. A.; Webber, S. E. J. Phys. Chem. 1985, 89, 48. (16) 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. (17) Wang, Y.; Mahler, W. Opt. Commun. 1987, 61, 233. (18) Hilinski, E. F.; Lucas, P. A.; Wang, Y. J. Chem. Phys. 1988, 89, 3435. (19) Wang, Y.; Suna, A.; McHugh, J.; Hilinski, E. F.; Lucas, P. A.; Johnson, R. D. J. Chem. Phys. 1990, 92, 6927. (20) 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. (21) Zen, J.-M.; Chen, G.; Fan, F.-R. F.; Bard, A. J. Chem. Phys. Lett. 1990, 169, 23. (22) Albu-Yaron, A.; Arcan, l. Thin Solid Films 1990, 185, 181. (23) Inoue, H.; Urquhart, R. S.; Nagamura, T.; Grieser, F.; Sakaguchi, H.; Furlong, D. N. Colloids Surf. A 1997, 126, 197. (24) Ceynowa, J. Polymer 1978, 19, 73.
10.1021/la991593y CCC: $19.00 © 2000 American Chemical Society Published on Web 09/19/2000
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membrane films, however, the primary characterization techniques were optical absorption and powder X-ray diffraction, from which parameters such as average particle sizes were estimated. There were only a few investigations that provided clear images of nanoparticles embedded in the structural cavities of Nafion membrane films. The preparation and characterization of nanoscale semiconductors and metals in the ionomer membrane structure apparently carry dual purposes. The structural cavities in ionomer membranes serve as excellent templates for the formation of nanoparticles. Despite the obvious disadvantage that the nanomaterials thus prepared are difficult to separate from the templates, these nanoparticle-membrane composites may still find unique applications, such as being used as catalytic and optical materials.13-23 On the other hand, the semiconductor and metal particles embedded in the membrane films serve as staining agents or probes for an understanding of nanoscopic details of the ionomer membrane structure. Here we report a study of perfluorinated ionomer membranes, including Nafion (I) and sulfonimide (II), via the preparation and characterization of silver sulfide and silver nanoparticles in the membrane structure. The
results show that the ionomer membranes contain similar hosts for the nanoparticle formation and that the hosting cavities are randomly distributed throughout the membrane structure. The properties of the nanoscale particles in the ionomer membranes are presented, and their implications to the understanding of membrane nanoscopic structural details are discussed. Experimental Section Materials. Silver nitrate (AgNO3), sodium sulfide (Na2S), and sodium borohydride (NaBH4) were purchased from Aldrich. Spectroscopy grade organic solvents were used as received. Water was deionized and purified by being passed through a Labconco WaterPros water purification system. Nafion ionomer (I) membrane with an equivalent weight of 1100 was provided by Du Pont Company. The sulfonimide ionomer (II) was prepared in house via the copolymerization of the sulfonimide monomer with tetrafluoroethene.26,27 Details on the synthesis will be reported separately. The membrane was obtained by wet casting from a solution of the sulfonimide ionomer in dimethylformimide, followed by careful annealing at a high temperature. The equivalent weight of the sulfonimide ionomer membrane was estimated to be ∼1200 in terms of titration. The ionomer 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 (25) Porat, Z.; Fryer, J. R.; Huxham, M.; Rubinstein, I. J. Phys. Chem. 1995, 99, 4667. (26) DesMarteau, D. D. J. Fluorine Chem. 1995, 72, 203. (27) DesMarteau, D. D. U.S. Patent No. US 5,463,005.
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Figure 2. The powder X-ray diffraction pattern of the Ag2S nanoparticles embedded in structural cavities of Nafion membrane is compared with the reference in the JCPDS database. 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. The treated ionomer membrane films were clear and optically transparent down to 200 nm. Unless specified otherwise, the membrane films were investigated in the sodium form. Converting the membranes to the sodium form was achieved by soaking the treated films in a 0.1 M aqueous solution of sodium hydroxide with stirring for 24 h, followed by washing thoroughly with clean water until neutral. The membrane films in the sodium form were again clear and optically transparent down to 200 nm. The purified membranes were kept fully hydrated before and during the preparation of nanoparticles. Measurements. Powder X-ray diffraction measurements were carried out on a Scintag XDS-2000 powder diffraction system. Transmission electron microscopy (TEM) images were obtained on either a Hitachi 600AB transmission electron microscope or a Hitachi 7000 transmission electron microscope.
Results and Discussion Silver Sulfide in Nafion. Nanoscale Ag2S particles were prepared in the structural cavities of Nafion membrane. The preparation procedure was as follows. A piece of clean Nafion film was soaked in an aqueous solution of AgNO3 (0.01 M) for 10 min, followed by rinsing with water to clean the film surface. After the surface was blotted dry, the film containing Ag+ was immersed in an aqueous solution of Na2S (0.1 M) for 2.5 h for the formation of Ag2S particles under fully hydrated condition. The particles were identified by their X-ray diffraction pattern, which matches well with the Ag2S reference in the JCPDS database (Figure 2). The broadness in observed diffraction peaks is characteristic of the Ag2S particles being nanoscopic; and the diffraction peak broadening can be used to estimate the average particle size in terms of the Debeye-Scherer equation28
D ) Kλ/(β cos θ)
(1)
where D is the average particle diameter in angstrom, β is the corrected band broadening (fwhm), K is a constant related to the crystallite shape and the way in which D and β are defined, λ is the X-ray wavelength, and θ is the diffraction angle. For the diffraction pattern shown in Figure 2, the average particle size in the corresponding (28) Klug, H. P.; Alexander, L. E. X-ray Diffraction Procedures; John Wiley and Sons: New York, 1959.
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Figure 3. TEM image for a cross-sectional view of the Ag2S nanoparticles embedded in structural cavities of Nafion membrane film.
Ag2S-Nafion sample was estimated to be 24 nm in diameter. However, the accuracy of such an estimate may be questionable. For the Ag2S-Nafion samples prepared repeatedly under the same conditions, while their X-ray diffraction patterns are rather similar qualitatively, the degree of peak broadening varies quantitatively from sample to sample, resulting in different average particle size values (as large as 50 nm for some samples). A likely reason for the large difference in estimating the average particle size on the basis of the Debeye-Scherer equation is that the samples may have somewhat different crystallinities, where the crystallinity of nanoparticles also affects the peak broadening in observed diffraction patterns. Such an assessment is consistent with the results from the TEM analysis of the Ag2S-Nafion samples. For TEM measurements, ultrathin cross-sectional slices of the Nafion film embedded with Ag2S nanoparticles were prepared. In the preparation, the film was placed in an epoxy resin and allowed to harden, and then was cut using an ultra-microtome with a diamond knife. The TEM image of such a slice corresponds to a cross-sectional view of the Nafion membrane film loaded with Ag2S nanoparticles. As shown in Figure 3, the TEM image consists of welldispersed dark spots due to Ag2S particles. A statistical analysis of the image yielded an average particle size of 10.5 nm in diameter, with the size distribution standard deviation of 2.2 nm (Figure 4). These clearly identifiable Ag2S particles, formed under the condition of fully hydrated Nafion membrane, are likely hosted in the nanoscale cavities in the Nafion membrane structure. However, the relatively high contrast for the particles in the image makes it difficult to look at the other structural elements of the membrane. On the other hand, the results seem to suggest that the hosting cavities for the nanoparticles are isolated, with essentially no Ag2S in the proposed connecting channels in terms of the ion cluster model.7,9 However, for the film sample prepared with loading at an intermediate AgNO3 concentration, it contained a lower population of Ag2S nanoparticles; however, the observed particles were similar in size to those listed in Table 1. The loading of Ag2S particles in the ionomer membrane could be varied via the use of different preparation conditions. Intuitively, it appeared that the loading was most sensitive to the concentration of the aqueous AgNO3 solution. For example, when a piece of Nafion film was soaked in a 0.2 mM aqueous solution of AgNO3, followed by the same treatments as those discussed above, the loading of Ag2S in the membrane became too low for the
Figure 4. Histogram from the TEM image for Ag2S nanoparticles. The average particle size is 10.5 nm, with the size distribution standard deviation of 2.2 nm. Table 1. Preparations and Properties of Silver Nanoparticles in Ionomer Membranes AgNO3 solution
NaBH4 solution
membrane
conc. (M)
time (min)
conc. (M)
time
av particle size (nm)a
Nafion Nafion Nafion Nafion sulfonimide
0.01 0.01 0.11 0.11 0.01
10 10 10 10 10
0.66 0.66 0.66 0.66 0.66
2.5 h 2.5 h 10 min 10 min 2.5 h
13.5 15 12.5 13 13
a Calculated from the X-ray peak broadening using the DebeyeScherer equation.
Figure 5. The powder X-ray diffraction pattern of the silver nanoparticles embedded in structural cavities of Nafion membrane is compared with the reference in the JCPDS database.
X-ray diffraction measurement, and the treated film appeared unchanged from the fresh Nafion sample. Silver in Nafion. Nanoscale silver metal particles were prepared in the structural cavities of Nafion membrane. In the preparation, a piece of Nafion film was soaked in an aqueous solution of AgNO3 (0.01 M) for 10 min. Then, after its surface was washed with water and blotted dry, the Nafion film containing Ag+ was immersed in a freshly prepared aqueous solution of NaBH4 (0.66 M) for chemical
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Figure 6. TEM images for cross-sectional views of the silver nanoparticles embedded in structural cavities of Nafion (top) and the sulfonimide ionomer (bottom) membrane films.
reduction under fully hydrated film condition. The formation of nanoscale silver metal was confirmed by the X-ray diffraction pattern of the film sample, which consists of broad peaks but matches well with the silver reference in the JCPDS database (Figure 5). The average silver particle size in the silver-Nafion sample was estimated from the X-ray diffraction peak broadening in terms of the DebeyeScherer equation (eq 1), 13.5 nm in diameter. Unlike those of the Ag2S-Nafion samples discussed above, X-ray diffraction patterns of the silver-Nafion samples obtained from repeated preparations agree well, resulting in similar particle size values (Table 1). These results are also comparable to the average silver particle size determined from the TEM analysis. Ultrathin cross-sectional slices of the silver-Nafion sample were prepared for TEM measurements. The TEM image of such a slice in Figure 6a shows clearly silver particles that are close to spheric in shape. A statistical analysis of the particles yielded an average size of 13 nm in diameter, with the size distribution standard deviation of 3.4 nm (Figure 7a). Here, the average particle sizes
determined from the X-ray diffraction, and TEM results of the same silver-Nafion sample are in excellent agreement. This indicates that the nanoscale silver particles hosted in the structural cavities of Nafion membrane have high crystallinity, which is critical to the accuracy in the calculation of the Debeye-Scherer equation (eq 1).28 These results also support, from an opposite side, the assessment discussed above that the Ag2S nanoparticles prepared in Nafion membrane are probably not highly crystalline materials. The TEM image of the silver-Nafion sample also shows high contrast between the silver particles and the Nafion membrane background, more so than in the image of the Ag2S-Nafion sample (Figure 6a vs Figure 3), probably due to the high electron density of silver metal. Thus, it becomes more conclusive for the silver-Nafion sample that the nanoscale silver particles are isolated in welldispersed structural cavities of the membrane, with essentially no silver in the proposed channels-like structural domains, which presumably connect the ion cluster cavities.7,9
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Figure 7. Histograms from the TEM images for silver nanoparticles in Nafion (top) and the sulfonimide ionomer (bottom) membranes. The average particle sizes are 13 and 12.5 nm, with the size distribution standard deviations of 3.4 and 3.3 nm, for the silver-Nafion and silver-sulfonimide samples, respectively.
Silver particles hosted in Nafion membrane could also be prepared under somewhat different experimental conditions. As compared in Table 1, the average particle sizes in the silver-Nafion samples obtained with the same procedure but different experimental parameters are quite similar. Silver in Sulfonimide Ionomer Membrane. Nanoscale silver particles were also prepared in structural cavities of the perfluorinated sulfonimide ionomer (II) membrane, which has the same physical appearance as Nafion film. The same preparation procedure as that for the silver-Nafion sample was used, and the experimental conditions are listed in Table 1. The sulfonimide ionomer membrane film loaded with silver particles were also cut into ultrathin slices using an ultra-microtome with a diamond knife for TEM measurements. Shown in Figure 6b is the TEM image of such a slice, which again consists of randomly dispersed well-defined silver nanoparticles. A statistical analysis of the silver particles yielded an average size of 12.5 nm in diameter, with the size distribution standard deviation of 3.3 nm (Figure 7b). The contrast between the silver particles and the membrane background in this TEM image is even sharper than that in the image of the silver-Nafion sample (Figure 6). Thus, an unambiguous conclusion from the TEM image is that the nanoscale silver particles are isolated only in the proposed hydrophilic cavities of the ionomer membrane structure. Mechanistic Implications. According to the ion cluster model,5-12 the perfluorinated ionomer membranes may be characterized by the presence of randomly distributed reverse micelle-like cavities in the membrane structure, and the hydrophilic cavities are presumably connected via channels to facilitate ion transportation. As reported in the literature, the size of ion clusters in fully hydrated Nafion membrane was estimated to be on
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the order of 4 nm in diameter (Figure 1).5 Thus, in principle, one might expect the formation of Ag2S and silver particles in the same dimension as that of the ion clusters, which serve as hosts, in the Nafion membrane structure under the same fully hydrated condition; one might also expect the observation of nanoscale particles in the connecting channels. Contrary to such expectations, however, the observed Ag2S and silver nanoparticles are hosted in the ionomer membrane structure in an isolated fashion, without any indication of particles in the channels-like domains in the TEM images. In addition, these isolated nanoparticles are on average significantly larger than the estimated size of the ion clusters in Nafion membrane. A possible explanation for the results is that the ion clustersbased structure of ionomer membranes is somewhat flexible, allowing the extra growth of particles in the hydrophilic cavities via squeezing the connecting channels. Here, the formation of a smaller number of larger particles rather than a larger number of smaller particles in ionomer membranes is probably comparable to the agglomeration of nanoparticles in a suspension. Interestingly, however, the presumably enlarged cavities in the membrane structure produce nanoparticles of similar sizes, as reflected in the relatively narrow particle size distributions (Figures 4 and 7). The results seem to suggest that the ion clusters and their associated channels-like domains are reasonably uniform in both the total cavity volume and the distribution throughout the ionomer membrane structure. The preparations of Ag2S and silver in Nafion membrane involve different reactions, but the Ag2S and silver nanoparticles thus obtained are comparable in sizes. The observed Ag2S and silver particle sizes, 10-15 nm in diameter, probably represent the typical volumes of individual ion clusters combined with their associated channels that are squeezed in the formation of the nanoparticles. In addition, the average size of the silver nanoparticles in the sulfonimide ionomer membrane also falls into this range, which may be considered as evidence for similarities in nanoscopic structures between these different ionomer membranes. This is consistent with the results from comparative luminescence spectroscopic investigations, which also indicate great similarities in structure and properties between the sulfonimide ionomer and Nafion membranes.12,29 Additional experimental investigations are required for a detailed mechanistic understanding. For both Nafion and the sulfonimide ionomer membranes, the hydrophilic structural cavities serve as templates for conveniently producing metal sulfide and metal nanoparticles of reasonably narrow size distributions. The method is applicable to potentially a variety of materials.30-32 These nanoparticle-ionomer membrane composite films may find unique applications in areas such as the search for new catalysts and the development of novel magnetic materials. Acknowledgment. We thank JoAn Hudson for experimental assistance. Financial support from the De(29) Bunker, C. E.; Rollins, H. W.; Ma, B.; Simmons, K. J.; Liu, J.-T.; Ma, J.-J.; Martin, C. W.; DesMarteau, D. D.; Sun, Y.-P. J. Photochem. Photobiol. 1999, 126, 71. (30) (a) Ng Cheong Chan, Y.; Schrock, R. R.; Cohen, R. E. J. Am. Chem. Soc. 1992, 114, 7295. (b) Chan, Y. N. C.; Schrock, R. R.; Cohen, R. E. Chem. Mater. 1992, 4, 24. (c) Kane, R. S.; Cohen, R. E.; Silbey, R. Chem. Mater. 1996, 8, 1919. (31) Olshavsky, M. A.; Allcock, H. R. Chem. Mater. 1997, 9, 1367. (32) (a) Moffitt, M.; McMahon, L.; Pessel, V.; Eisenberg, A. Chem. Mater. 1995, 7, 1185. (b) Moffitt, M.; Eisenberg, A. Chem. Mater. 1995, 7, 1178. (c) Moffitt, M.; Vali, H.; Eisenberg, A. Chem. Mater. 1998, 10, 1021.
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partment of Energy (Cooperative Agreement DE-FG0291ER75666) and, in part, the National Science Foundation (CHE-9727506 and EPS-9986522) (Y.-P.S.) and ARPA/ URI (N00014-92-J-1848) (D.D.D.) is gratefully acknowledged. J.J. was a participant in the Summer Under-
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graduate Research Program sponsored jointly by the National Science Foundation (CHE-9100387 and CHE9619573) and by Clemson University. LA991593Y
