Issue 5, 2013
From the journal:

Physical Chemistry Chemical Physics

Possible presence of hydrophilic SO3H nanoclusters on the surface of dry ultrathin Nafion® films: a positron annihilation study

Hamdy F. M. Mohamed,†*a   S. Kuroda,a   Y. Kobayashi,b   N. Oshima,c   R. Suzukic  and  A. Ohiraad  

* Corresponding authors

a Fuel Cell Cutting-Edge Center (FC-Cubic), Technology Research Association, 2-3-26 Aomi, Koto-ku, Tokyo 135-0064, Japan
E-mail: hamdyfm@gmail.com

b NMIJ, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan

c RIIF, AIST, Tsukuba, Ibaraki 305-8565, Japan

d RIUED, AIST, Midorigaoka, Ikeda, Osaka 563-8577, Japan

Abstract

Solutions of Nafion® with an ion exchange capacity (IEC) of 0.91 meq g−1, which are on the verge of the formation of SO3H nanoclusters, were spin coated on silicon (Si), glassy carbon (GC) and platinum/silicon (Pt/Si) substrates to form films of up to 256 nm thickness. Nanostructure of the films was studied using Doppler broadening of annihilation radiation (DBAR), positron annihilation lifetime (PAL), X-ray photoelectron spectroscopy (XPS), an atomic force microscope (AFM) and contact angle measurements. Contact angles as low as 10 degrees indicate that the surface of dry ultrathin Nafion® films on Si is highly hydrophilic. XPS data of 10 nm thick, ultrathin film on Si show that oxygen concentration is enhanced and the SO3H group concentration, in other words, IEC on the surface is much higher than other films. The S parameter measured by DBAR of an ultrathin Nafion® film on Si is much higher than that of the films on the other substrates. We consider that a large number of hydrophilic, reversed micelle like SO3H groups are on the surface of the ultrathin Nafion® film on Si but not on the surface of other films. Positrons implanted into the film are trapped by the SO3H clusters, annihilating with the electrons of oxygen and exhibit the high S parameter. The SO3H concentration on the surface of thin Nafion® films on GC and Pt/Si substrates may not be so high as the threshold for the formation of a large number of SO3H clusters. Positrons implanted into the films annihilate mostly with fluorine atoms, resulting in a low S parameter. The film–substrate interaction plays an essential role in nanostructuring of Nafion® thin films, which may also be the case for Nafion® on the catalysts of polymer electrolyte fuel cells.

Graphical abstract: Possible presence of hydrophilic SO3H nanoclusters on the surface of dry ultrathin Nafion® films: a positron annihilation study

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Article information

DOI
https://doi.org/10.1039/C2CP43727A
Article type
Paper
Submitted
23 Oct 2012
Accepted
21 Nov 2012
First published
22 Nov 2012

Phys. Chem. Chem. Phys., 2013,15, 1518-1525

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Possible presence of hydrophilic SO3H nanoclusters on the surface of dry ultrathin Nafion® films: a positron annihilation study

H. F. M. Mohamed, S. Kuroda, Y. Kobayashi, N. Oshima, R. Suzuki and A. Ohira, Phys. Chem. Chem. Phys., 2013, 15, 1518 DOI: 10.1039/C2CP43727A

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