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Title

Highly Oriented Sulfonic Acid Groups in a Nafion
Thin Film on Si Substrate

Author(s)Nagao, Yuki

Citation

Journal of Physical Chemistry C, 117(7): 3294‑
3297

Issue Date

2013‑02‑07

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/12062

RightsYuki Nagao, Journal of Physical Chemistry C,
2013, 117(7), pp.3294‑3297. This document is theunedited author's version of a Submitted Workthat was subsequently accepted for publication in
Langmuir, copyright (c) American Chemical Society
after peer review. To access the final edited andpublished work, see[http://pubs.acs.org/articlesonrequest/AOR‑
AVVpyuGrFBbkVIQADUFp].

Description

Japan Advanced Institute of Science and Technology

Highly Oriented Sulfonic Acid Groups in a Nafion
Thin Film on Si Substrate
Yuki Nagao*
School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai,
Nomi, Ishikawa 923-1292 Japan
Keywords: Proton conductivity, Multiple-angle incidence resolution spectrometry, Interface,
Molecular oriented structure, Substrate
Abstract

Solid state ionics is a research field attracting much current attentions because of the ideal powersources for use with portable electronic devices having high power-to-weight ratios. One of themost urgent subjects in this field is to understand proton transport properties at the interface
between inorganic materials and polymer electrolyte from the viewpoint of developing much
more powerful energy. In this study, a 150-nm-thick Nafion thin film was prepared byspincoating on a silicon (Si) substrate to investigate the proton transport property at the interface.
The infrared (IR) p-polarized multiple-angle incidence resolution spectrometry (p-MAIRS)
technique was applied to investigate the in-plane (IP) and out-of-plane (OP) spectra to thesurface. The IP spectrum showed a well-known spectrum, but the OP spectrum was quite
different from the IP spectrum. An anomalous IR peak was observed in the OP spectrum at 1260

1

cm-1. From density functional theory (DFT) calculations, this peak was attributed to the –SO3H
vibration modes between two sulfonic acid groups with hydrogen bonds. These results
demonstrate that the Nafion thin film on Si substrate had a highly oriented structure with thesulfonic acid groups at the side chain. Impedance measurements of Nafion thin film wereconducted to investigate the proton transport property of the Nafion thin film on SiO2 substrate.
The proton conductivity of the thin film exhibited a lower value than that of the commercial
Nafion membrane. The low proton conductivity of the Nafion thin film was related with thesehighly oriented structures.

1. Introduction
Polymer electrolyte membrane fuel cells such as direct methanol fuel cells (DMFCs) are ideal
power sources for portable electronic devices having high power-to-weight ratios. The energy
density of DMFC increases along with the methanol concentration. An issue hinderingdevelopment of these fuel cells is methanol crossover through an electrolyte membrane, which
decreases cell voltage. This crossover phenomenon mainly occurs by the expansion of electrolyte
membrane because of water or methanol absorption.1 To solve this crossover problem, Munakata
and Kanamura et al. sought to use the concept of three-dimensionally ordered macroporous(3DOM) silica composite membrane including polymer electrolytes.2-6 The methanol
permeability of their composite membranes was reduced and the proton conductivity was
improved.
To obtain effective and high-power energy sources of the fuel cell, understanding the interface
between a proton conductive material and another material is necessary. Many results have beenreported from studies using the interface between inorganic materials and proton conductive2

polymers.7-9 Some reports described that the proton transport properties were improved byadding fillers, which presents advantages for the suppression of swelling and shrinking of the
proton conductive membrane surrounded by water and methanol. The origin was inferred as
related to their structural modification. Using the interface between the polymer thin films andinorganic substrates, anomalous proton transport properties were also reported.10,11 However, themechanism and structural information at the interface are insufficiently revealed.
Nafion, a commercially available ionomer produced by Du Pont, is the most widely studied
polymer electrolyte membrane because it exhibits high conductivity and good mechanicalstability.12-14 The Nafion chemical structure consists of a perfluorocarbon backbone and side
chains terminated with sulfonic acid groups as shown in Figure 1. Nafion is also used as asolution state with various solvents for preparing membrane-electrode assembly. Catalytic
reactions occur mainly at the triple phase boundary, which consists of the catalyst, proton
conducting electrolyte, and gaseous reactant. Proton-conductive ionomers such as Nafion
solution are used to fabricate suitable triple phase boundaries. A liquid electrolyte can be a usefulionomer not only to fabricate the membrane but also to function as binder to form a triple phaseboundary because of their excellent properties of proton conductivity and penetration to thecatalytic layer in the electrode.
In an earlier report, the proton conductivity of the Nafion thin film on the MgO(100) substrate
was described as quite lower than that of the commercial Nafion membrane.15 The infrared (IR)p-polarized multiple-angle incidence resolution spectrometry (p-MAIRS) technique developed
by Hasegawa16-19 revealed that the Nafion thin film had a highly oriented molecular structure.
However, the attribution was not fully analyzed at that time. As described herein, an IR pMAIRS technique and impedance measurements were conducted to investigate the proton3transport property of the Nafion thin film on a Silicon (Si) substrate. Infrared p-MAIRS is
increasingly regarded as a powerful spectroscopic tool for revealing molecular orientation in thinfilms. The p-MAIRS analysis showed that in-plane (IP) and out-of-plane (OP) transition dipoles
are distinguishable in an identical infrared transparent substrate. These results are applicable tofurther development of much more powerful polymer electrolyte membrane fuel cells.

Fig. 1 Nafion structure.
2. Experimental
The Nafion thin film was prepared using an Active ACT-200 Spincoater on a high-resistivity
and mirror-polished Si substrate. To improve the hydrophilicity on the surface of the substrate
before spincoat, plasma treatment was performed using a vacuum plasma system (Cute-MP;
Femto Science, Korea). Nafion solution was obtained from Wako Pure Chemical Industries Ltd.
The obtained thin film on the Si substrate was dried in a desiccator for 12 hr. Then its thickness
was measured using a surface profiler (Dektak 8; Veeco Instruments). The thin film thickness
was ca. 150 nm.
To investigate IP and OP molecular vibrations in an identical 150-nm-thick film of Nafion on
the Si substrate, the infrared p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) technique was performed. The p-MAIRS measurements were taken using an FT-IR
spectrometer (Nicolet 6700; Thermo-Fisher Scientific Inc.) equipped with a mercury-cadmium-4

telluride (MCT) detector. Single-beam spectra were collected from 38° through 8° by 6° steps
between the angle of incidence. The optical configuration is shown in Figure 2. The aperture was
fully opened (size of 150); a metal plate with small pores was placed in the light path of theincidental beam to prevent saturation. The p-MAIRS analysis of the collected spectra wasconducted automatically using p-MAIRS analyzer software (Thermo Fisher Scientific Inc.).
Density functional theory (DFT) calculations were performed using the DMol3 package inMaterials Studio v6.0.0 (Accelrys Software Inc.). The Perdew–Burke–Ernzerhof (PBE) function
was chosen. The convergence threshold for the maximum force and maximum displacement fornormal geometry optimization were set respectively to 0.004 Ha Å-1 and 0.005 Å.
To investigate the proton transport properties of the thin film on SiO2 substrate, impedance
measurements of the thin films were conducted in the relative humidity (RH) range of 40–90%
with an Impedance/gain-phase analyzer (Solartron 1260; Solartron Analytical) and a dielectricinterface system (1296; Solartron Analytical). The RH and temperature were controlled with ahumidity-controlled and temperature-controlled chamber (SH-221; Espec Corp). In the
impedance measurements of the thin films, the electrode configuration was selected to obtainmeasurements of the current flow in the plane parallel to the substrate surface. The electrodeusing Au paste is located at the edge of the thin film. The electrode configuration is parallel. The
distance between electrodes is ca. 3 mm. The porous Au electrode is also covered at the side of
the thin film. The conductivity of the SiO2 substrate is negligible because its resistance is muchhigher than that of the thin film.

5Fig. 2. Configuration of the polarized incident beam path and incident angle.
3. Results and discussions
Figure 3 shows the incident angle dependence of the transmission IR spectra. The shape of thespectra depends strongly on the incident angle. The absorbance around 1260 cm-1 increased withthe angle of incidence. This result suggests that the thin film had a highly oriented structure. For
a low incident angle, the peak was not observed around 1260 cm-1. Therefore no IP transition
dipole moment exists around that wavenumber. However, the peak gradually appeared with theincident angle, meaning that OP transition dipole moments were excited by polarized light.

Fig. 3. Incident angle dependence of the transmission IR spectra.

6

To discuss the details of the molecular orientation, p-MAIR spectra after the collection of
single-beam spectra measured from 38° through 8° by 6° steps are shown in Figure 4. A Nafion
membrane consists of a perfluorocarbon backbone and side chains terminated with sulfonic acidgroups. The IP spectrum shows the well-known spectrum of Nafion membrane.20 However, theOP spectrum was quite different from the IP spectrum. The greatly differing shapes of thespectra indicate clearly that the 150-nm-thick film of Nafion has an anisotropic moleculararrangement. The nanostructure of commercial Nafion membranes at different states of hydrationby IR and Raman spectroscopy was investigated by Gruger et al.20 The CF2 stretching vibration
bands ((CF2) at 1155, 1210, and 1235 cm-1) and C–C stretching vibration band ((C–C) at 1300
cm-1) were observed in the IP spectrum. The CF2 stretching vibration bands ((CF2) at 1210 cm1) were overlapped on the SO3- stretching vibration band. The characteristic absorption band at

1260 cm-1 was observed in the OP spectrum. Many investigations of IR spectra were conductedusing commercial Nafion films with several hundred or ten micrometer order thickness.
However, no absorption band at 1260 cm-1 has been reported.20-25 Since this band is also
recognized in Figure 3, the band at 1260 cm-1 is not an artifact. A similar result, obtained using
p-MAIRS method, was described in an earlier report of a study using a MgO(100) substrate.15
The peak at 1260 cm-1 had not been attributed at that time. A possible reason was inferred: theMgO surface was modified by the reaction with Nafion because of the strong acidity andsulfonic acid magnesium salt was created in the interface. Then Mg2+ ion may migrate to the film.
However, that presumption is contradicted by the present result obtained using Si substratebecause Si substrate does not include any metal ions.

7Fig. 4. Infrared p-MAIR spectra after the collection of single-beam spectra measured from 38°
through 8° by 6° steps.
Another explanation for the anomalous peak at 1260 cm-1 is the coupling vibrational modes via
hydrogen bonds between two or more sulfonic acid groups. To analyze the vibration modes
around 1260 cm-1, a DFT calculation was conducted. Three molecular structures were produced
as Nafion-like models, as shown in Figure 5. The geometry of their molecular structure was
optimized and vibrational analysis was applied. The result shows that the structures of Figures
5(a) and 5(b) did not exhibit any vibration mode around 1260 cm-1. However, the dimer structureof Figure 5(c) exhibited some vibration modes around 1260 cm-1. Every mode of 1238, 1249,
1271, and 1276 cm-1 resulted from the OH vibrational mode of the sulfonic acid groupsinteracted with another sulfonic acid groups via hydrogen bonds. These modes did not appear in
the monomer structure shown in Figures 5(a) and 5(b). Since MAIRS spectra reveals both IP and
OP spectra from an identical sample, all the transition moments with any directions appear in the
spectra. Therefore these results suggest that the sulfonic acid groups are strongly coupled rather
than separated. Considering the result of p-MAIR spectra in this study, the 150-nm-thick thin

8

film of Nafion had a highly oriented structure, which was built by hydrogen bonds of sulfonicacid groups at the side chain.

(a)

(b)

(c)

Fig. 5. Nafion-like models for DFT calculation: C, gray; F, light blue; H, white; O, red; S, yellow.
Light-blue dotted lines show hydrogen bonds.
Impedance measurements were conducted to check the proton transport property of the thinfilm on Si substrate. The Si substrate is conductive compared to the Nafion thin film. Therefore,
the electrical conductivities between the Si substrate and Nafion thin film were not separated.
Normally, the surface of the Si substrate was oxidized and became SiO2. Therefore, the proton
conductivity was measured using the SiO2 substrate. Figure 6 shows the RH dependence of the
proton conductivity of the Nafion thin film. The thin film of the proton conductivity depended on9the RH. The proton conductivity of the thin film on SiO2 substrate was 10-2 S cm-1 under RH of
90% and 298K. However, its proton conductivity was a lower value than that of the commercial
bulk Nafion membrane. Presumably, the origin for the lower proton conductivity in the Nafion
film results from the highly oriented structure with sulfonic acid groups.

Fig. 6. Relative humidity dependence of proton conductivity for the Nafion thin film on SiO2
substrate.
Proton transport properties at the interface between inorganic materials and polymer
electrolyte have been investigated widely. Some reports described that the proton transport
properties were improved by the filler-impregnated composite membranes,6 and the origin wasinterpreted regarding their structural modification. The surface structure modified by theinterface can contribute to these improvements of the proton transport properties. Tamura and
Kawakami found that the composite bulk membrane containing uniaxially aligned sulfonatedpolyimide nanofibers exhibited a high proton conductivity.26 This result shows a possibility for
improving the proton transport property using the molecular orientation. For this Nafion thin filmon the MgO substrate, it is presumed that the change of proton transport property is derived from10

this highly oriented structure at the interface. The highly oriented sulfonic acid groups used inthis study can contribute to a discussion about proton transport property at the interface.

4. Conclusions
A highly oriented structure in the Nafion thin film on Si substrate was revealed using the IR pMAIRS technique. Based on DFT calculations, an anomalous IR peak at 1260 cm-1 was
attributed to –SO3H vibration modes between two sulfonic acid groups with hydrogen bonds.
The low proton conductivity of the Nafion thin film on SiO2 substrate was related with highlyoriented sulfonic acid groups. The results are expected to be useful for investigations of Nafion
thin film at the interface.
This work was supported by the Japan Society for the Promotion of Science (JSPS) throughthe Funding Program for Next Generation World-Leading Researchers (NEXT Program),initiated by the Council for Science and Technology Policy (CSTP).

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ASSOCIATED CONTENT
Supporting Information
The measurement of p-MAIRS. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author*ynagao@jaist.ac.jp Phone: +81(Japan)-761-51-1541.
Funding Sources
This work was supported by the Japan Society for the Promotion of Science (JSPS) through theFunding Program for Next Generation World-Leading Researchers (NEXT Program), initiatedby the Council for Science and Technology Policy (CSTP).

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