Formation of silver nanoparticles in poly(perfluorosulfonic) acid membrane

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)+ <--> 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.