Journal of Membrane Science 245 (2004) 211–217 Nanoscale cavities in ionomer membrane for the formation of nanoparticles Ya-Ping Sun ∗ , Pornpen Atorngitjawat, Yi Lin, Ping Liu, Pankaj Pathak, Jayasundera Bandara, Derek Elgin, Mingzhen Zhang Department of Chemistry, 233 Howard L. Hunter Chemistry Laboratory, Clemson University, Clemson, SC 29634-0973, USA Received 26 March 2004; received in revised form 28 July 2004; accepted 30 July 2004 Available online 1 October 2004 Abstract Silver nanoparticles of a relatively narrow size distribution were prepared in the structural cavities of the commercial ionomer membrane thin film Nafion. UV–vis absorption, X-ray powder diffraction, and transmission electron microscopy methods were used in the characterization of the nanoparticle–Nafion composite thin films. The population of silver nanoparticles in the membrane structure could be varied by changing the preparation conditions, but the sizes of the nanoparticles were found to remain generally similar. The results were explained in terms of the hosting of the silver nanoparticles in homogeneously distributed structural domains in the membrane structure, where each domain is probably a group of interconnected reverse micelle-like ion clusters according to the ion cluster model for ionomer membranes already in the literature. © 2004 Elsevier B.V. All rights reserved. Keywords: Ion cluster model; Ion-exchange membranes; Membrane reactors; Microporous and porous membranes; Nanoparticles 1. Introduction The microscopic structure in perfluorinated ionomer membranes as represented by commercial Nafion film is understood in terms of a reverse micelle-like ion cluster model [1–8]. According to the model, there are essentially three distinctive structural regions: the perfluorinated poly- mer network, water cores, and the interfacial domain be- tween the two regions, where the water cores in neighbor- ing clusters are presumably interconnected through chan- nels [3,5]. The presence of reverse micelle-like hydrophilic cavities in Nafion and related ionomer membranes is sup- ported by their successful uses in the preparation of embed- ded nanoparticles [9–13]. In fact, the preparation and char- acterization of the nanoscale particles incorporated in the ionomer membrane structure have served dual purposes. On one hand, the hydrophilic cavities are excellent templates for ∗ Corresponding author. Tel.: +1 864 656 5026; fax: +1 864 656 5007. E-mail address: syaping@clemson.edu (Y.-P. Sun). the formation of nanoparticles. When properly controlled, nanoparticle–membrane composites of high optical quality can be obtained. Because of the excellent materials prop- erties of Nafion membrane, these nanocomposite thin films have unique applications, such as being used as catalytic (es- pecially photocatalytic) and optical materials [9,10a,11,14]. For example, nanoscale semiconductors and related compos- ites were prepared in Nafion membrane for the photocatalytic production of hydrogen and photocatalytic reduction of car- bon dioxide [9,14]. On the other hand, the nanoparticles are staining agents or nanoscale probes for an understanding of the structural details in the ionomer membranes. Direct mi- croscopy imaging of the Nafion and related ionomer mem- branes embedded with nanoscale semiconductors and metals have provided new insight into the membrane structural do- mains and properties [12a,13]. Here, we report an investigation on the formation of silver nanoparticles in the hydrophilic cavities in Nafion membrane under various experimental conditions, resulting in nanocom- posite thin films of different silver loadings. Interestingly, the 0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.07.026