Anal. Chem. 2006, 78, 7169-7174
Formation of Silver Nanoparticles in Poly(perfluorosulfonic) Acid Membrane A. Sachdeva, Suparna Sodaye, A. K. Pandey,* and A. Goswami
Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400 085, India
The formation of silver nanoparticles by chemical reduction of Ag+-loaded Nafion-117 membrane with NaBH4 was studied using radioactivity tagged ions. The counterion+ exchange method (Ag+ (m) h Na(s)) was used to obtain a membrane sample with a varying proportion of Ag+ ions. The X-ray elemental mapping across the thickness of the membrane by energy-dispersive X-ray spectrometer attached to the environmental scanning electron microscope (ESEM/EDAX) indicated that Na+ and Ag+ were uniformly distributed in the membrane samples before reduction. The average size of nanoparticles formed after reduction was found to be 15 ( 3 nm, irrespective of the concentration of silver ions present in the membrane before reduction. Energy-dispersive X-ray fluorescence (EDXRF) analyses of the membrane samples, carried out before and after reduction, indicated that the Ag concentration on the membrane surface was considerably increased after reduction. EDXRF measurements of the membrane samples, obtained from reduction carried out in a dead end cell, indicated that Ag nanoparticles were formed only on the membrane surface exposed to NaBH4 solution. Reduction carried out with NaBH4 tagged with 22Na showed that the formation of Ag nanoparticles involved exchange of Ag+ ions from ion-exchange sites in the interior of the membrane with Na+ ions, followed by reduction of Ag+ ions with BH4- ions at the surface of membrane. The study of self-diffusion of water, Na+, and Cs+ ions in the membrane loaded with Ag nanoparticles indicated that formation of Ag nanoparticles did not affect the diffusional transport properties of the membrane. The ion-exchange capacity and water uptake capacity were also not affected by the formation of Ag nanoparticles in the membrane. The spatial distribution of Ag nanoparticles across the thickness of the membrane obtained by ESEM/EDAX showed that Ag nanoparticles were confined to a few-micrometer surface layer of the membrane. Based on these observations, an attempt has been made to explain the mechanism of the formation of Ag nanoparticles in the membrane. Utilization of nanoparticles often requires the construction of integrated chemical systems.1 Most popular of these are polymersupported nanoparticles. These systems are known for their * Corresponding author. E-mail:
[email protected]. Tel: +91-2225590641. FAX: +91-22-25505151. (1) Krishnan, M.; White, J. R.; Fox, M. A.; Bard, A. J. J. Am. Chem. Soc. 1983, 105, 7002-7003. 10.1021/ac060647n CCC: $33.50 Published on Web 09/15/2006
© 2006 American Chemical Society
unique electronic, optical, electrooptical, electrochemical, magnetic, and catalytic properties.2 The properties of nanoparticles in the host material are strongly related to their size, shape, and distribution in the host matrix.3 However, control over particle size, size distribution, and metal concentration in the composite is a challenging task. In this regard, poly(perfluorosulfonic) acid membrane (Nafion-117) has received much attention as a host for a variety of nanoparticles such as Pt, Ag, CdS, Ag2S, Fe2O3, Fe3O4, TiO2, SiO2, and ZrO2 due to the following: (i) its superior chemical stability, which prevents the agglomeration and corrosion of the nanoparticles, (ii) its high optical quality, (iii) its ease in loading of a variety of metal ions via ion-exchange mechanism and subsequent formation of nanoparticles in the membrane, and (iv) its ease of handling and recycling it for catalytic purposes.4-14 Nafion is extensively used in chlor-alkali industries, water electrolysis, polymer electrolyte fuel cells, sensors, and Donnan (2) (a) Henglein, A. Chem. Rev. 1989, 89, 1861-1871. (b) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69-96. (c) Moffitt, M.; McMohan, L.; Pessel, V.; Eisenberg, A. Chem. Mater. 1995, 7, 1185-1192. (d) Moffitt, M.; Vali, H.; Eisenberg, A. Chem. Mater. 1998, 10, 1021-1028. (e) Sidorov, S. N.; Bronstein, L. M.; Davankov, V. A.; Tsyurupa, M. P.; Solodovnikov, S. P.; Valetsky, P. M.; Wilder, E. A.; Spontak, R. J. Chem. Mater. 1999, 11, 3210-3215. (f) Bronstein, L. M.; Chernyshov, D. M.; Valetsky, P. M.; Wilder, E. A.; Spontak, R. J. Langmuir 2000, 16, 8221-8225. (3) (a) Barnet, R. N.; Landman, U. Nature 1997, 387, 788-791. (b) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17-30. (c) Morley, K. S.; Marr, P. C.; Webb, P. B.; Berry, A. R.; Allison, F. J.; Moldovan, G.; Brown, P. D.; Howdle, S. M. J. Mater. Chem. 2002, 12, 1898-1905.(d) Stranik, O.; McEvoy, H. M.; McDonagh, C.; MacCraith, B. D. Sens. Actuators, B 2005, 107, 148-153. (4) 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, 6537-6542. (5) Kakuta, N.; White, J. M.; Campion, A.; Bard, A. J.; Fox., M. A.; Webber, S. E. J. Phys. Chem. 1985, 89, 48-52. (6) Smotkin, E. S.; Brown, R. M., Jr.; Rabenberg, 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, 7543-7549. (7) 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, 8031-8036. (8) Liu, P.; Bandara, J.; Lin, Y.; Elgin, D.; Allard, L. F.; Sun, Y.-P. Langmuir 2002, 18, 10389-10401, and references therein. (9) Sun, Y.-P.; Atorngitjawat, P.; Lin, Y.; Liu, P.; Pathak, P.; Bandara, J.; Elgin, D.; Zhang, M. J. Membr. Sci. 2004, 245, 211-217. (10) Zhang, Y.; Kang, D.; Saquing, C.; Aindow, M.; Erkey, C. Ind. Eng. Chem. Res. 2005, 44, 4161-4164. (11) Zhang, Y.; Erkey, C. Ind. Eng. Chem. Res. 2005, 44, 5312-5317. (12) Wang, S.; Liu, P.; Wang, X.; Fu, X. Langmuir 2005, 21, 11969-11973, and references therein. (13) Mu, S.; Tang, H.; Wan, Z.; Pan, M.; Yuan, R. Electrochem. Commun. 2005, 7, 1143-1147. (14) Jalani, N. H.; Dunn, K.; Datta, R. Electrochim. Acta 2005, 51, 553-560.
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dialysis-based applications.15,16 The chemical structure of Nafion117 membrane consists of a poly(tertrafluoroethylene) (PTFE) backbone and regularly spaced long perfluorovinyl ether pendant side chain terminated by a sulfonate ionic group.15,16 The absence of chemical cross-linking between polymer chains in the Nafion is responsible for its phase segregation into hydrophilic and hydrophobic domains.17 According to the cluster-channel network model of Nafion membrane, the polymer chains form reverse micelles in which sulfonate groups are lined in the wall encapsulating water cavities.18 The water clusters are linked with narrow channels, which have been experimentally confirmed by electrolithography of the Nafion membrane.19 The presence of water clusters in Nafion-117 membrane has prompted a number of studies to use these clusters as template for different nanoparticles.4-14 On the other hand, attempts have been made to use Ag and Ag2S nanoparticles formed in the membrane as a probe for understanding the nanostructure of the Nafion-117 membrane.7,9 However, contrary to the expected diameter of 4-5 nm of water clusters in Nafion-117 membrane, the sizes of nanoparticles vary from 2 to 40 nm, depending upon the type of nanoparticle and its method of preparation.4-14 Also, there is ambiguity about the spatial distribution of the nanoparticles in the membrane.4,7,12,20 These disparities indicate that the role played by the cluster-channel structure of Nafion-117 membrane in the formation of nanoparticles has been oversimplified. In the present paper, we have studied role of permselectivity in the mechanism of formation of Ag nanoparticles in the Nafion117 membrane using BH4- anions (NaBH4) as a reducing agent. In view of many possible applications of such systems, the ion and water transport properties of the Ag nanoparticle-loaded membrane have also been examined. Radiotracers of 22Na, 137Cs, 3H, and 110mAg have been employed in these studies. The membrane samples with varying silver content were obtained by a counterion-exchange method and were subsequently reduced with NaBH4 to get Ag nanoparticle-loaded Nafion-117 membranes. UV-visible spectra and X-ray diffraction (XRD) of the membrane sample have been used to confirm the formation of Ag nanoparticles and to determine the size of the nanoparticles, respectively. Energy-dispersive X-ray fluorescence (EDXRF) of the membrane samples has been used to study the surface concentration of Ag+ ions or Ag nanoparticles. The changes in spatial distributions of Ag in the membrane samples before and after reduction with BH4have been studied by environmental scanning electron microscope. The experimental observations of these studies have been used to describe the possible mechanism of formation of Ag nanoparticles in the Nafion-117 membrane. (15) Heitner-Wirguin, C. J. Membr. Sci. 1996, 120, 1-33. (16) Mauritz, K. A.; Moore, R. B. Chem. Rev. 2004, 104, 4535-4585. (17) (a) Falk, M. Can. J. Chem. 1980, 58, 1495-1501. (b) Yeager, H. L.; Steck, A. J. Electrochem. Soc. 1981, 128, 1880-1884. (c) Pineri, M.; Duplessix, R.; Valino, F. In Perfluorinated Ionomer Membranes; Eisenberg A., Yeager H. L., Eds.; ACS Symposium Series 180; American Chemical Society: Washington, DC, 1982; pp 49-282. (d) Steck, A.; Yeager, H. L. Anal. Chem. 1980, 52, 1215-1218. (e) Okada, T.; Satou, H.; Okuno, M.; Yuasa, M. J. Phys. Chem. B 2002, 106, 1267-1278. (18) (a) Gierke, T. D.; Munn, G. E.; Wilson, F. C. J. Polym. Sci. Polym. Phys. Ed. 1981, 19, 1687. (b) Hsu, W. Y.; Gierke, T. D. J. Membr. Sci 1983, 13, 307-326. (19) Chou, J.; McFarland, E. W.; Metiu, H. J. Phys. Chem. B 2005, 109, 32523256. (20) Kim, K. J.; Shahinpoor, M. Smart Mater. Struct. 2003, 12, 65-79.
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EXPERIMENTAL SECTION Materials. AR grade chemicals (NaNO3, AgNO3, CsNO3, NaBH4) and deionized water (18 MΩ/cm, Gradient A-10 model, Milli-Q USA) were used in the present study. Nafion-117 ionexchange membrane with an equivalent weight of 1100 and thickness of 178 mm (Aldrich) was used after conditioning as described elsewhere.21-23 Radiotracers 3H (tritium-tagged water), 22Na,137Cs, and 110mAg were obtained from the Board of Radiation and Isotope Technology, Mumbai, India. The γ-activity of 22Na, 137Cs, and 110mAg in the membrane was monitored by a well-type NaI(Tl) detector connected to a multichannel analyzer. The β-radioactivity of tritiated water (HTO) was measured by spiking the sample in a vial containing 5 mL of scintillation cocktail (2,5diphenyloxazole ) 10 g, 1,4-di-2-(5-phenyloxazolyl)benzene ) 0.25 g, and naphthalene ) 100 g in 1000 mL of 1,4-dioxane solvent), and counting the samples with a Packard Liquid Scintillation Analyzer (model Tri-Carb 2100 TR). Loading of Ag+ Ions in Nafion. The membranes samples (2 × 2 cm2) in Na+ form were equilibrated with 10 mL of 0.1 mol‚L-1 solution of NaCl containing known radioactivity of 22Na for 24 h. These membrane samples containing 22Na radiotracer were equilibrated with a 0.25 mol‚L-1 solution of AgNO3, and complete loading of Ag+ was ensured from the absence of γ-activity of 22Na in the membrane samples. Finally, these membrane samples were equilibrated with 110mAg-tagged aqueous AgNO3 solution to obtain Ag+ form of the membrane labeled with 110mAg. Membrane samples with different amounts of Ag+ ions were obtained by equilibrating the membrane in Ag+ form with 25 mL of a vigorously stirred (∼52 rad/s) solution of 0.25 mol‚L-1 NaNO3 for different lengths of time. The fraction of Ag+ ions in the ion-exchange sites of membrane was obtained from the ratio of radioactivity of 110mAg (counts/s) in the membrane at a given equilibration time to initial radioactivity of 110mAg (counts/s) in the membrane. The variation of Ag+ ion content in the membrane as a function of equilibration time in NaNO3 solution is shown in Figure 1. The membrane samples with very low amount of Ag+ ions (
