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Title

Proton Conductivity and Oriented Structure of
Nafion Thin Films on the Au‑Deposited Surface and
MgO Substrate

Author(s)Nagao, Yuki; Enta, Aoi; Suwansoontorn, Athchaya;Ono, Yutaro

Citation

ECS Transactions, 88(1): 249‑258

Issue Date

2018‑12‑13

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/16671

RightsCopyright (C)2018 ECS. Yuki Nagao, Aoi Enta,
Athchaya Suwansoontorn and Yutaro Ono, ECS
Transactions, 88(1), 2018, pp.249‑258. This is
the Accepted Manuscript version of an article
accepted for publication in ECS Transactions.
The Electrochemical Society and IOP Publishing
Ltd are not responsible for any errors oromissions in this version of the manuscript or
any version derived from it. The Version ofRecord is available online athttps://doi.org/10.1149/08801.0249ecst.

Description

Japan Advanced Institute of Science and Technology

Proton Conductivity and Oriented Structure of Nafion Thin Filmson the Au-deposited Surface and MgO Substrate
Yuki Nagao*, Aoi Enta, Athchaya Suwansoontorn, Yutaro Ono
School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1
Asahidai, Nomi, Ishikawa 923-1292, JAPAN*ynagao@jaist.ac.jp
In-plane proton conductivity and interfacial structure of Nafion thinfilms on the Au-deposited surface and MgO(100) substrate were
investigated. Proton conductivity on both Au-deposited and MgO
surfaces clearly decreased with decreasing the thickness except theconditions of 80% and 95% relative humidity on the Au-deposited
surface. Proton conductivity drop on the MgO substrate was muchheavier than that on the Au-deposited surface. Highly oriented
structure was observed in both Nafion thin films on the Audeposited surface and MgO surface by a p-polarized multiple angle
incidence resolution spectrometry. While thickness dependent
structural change was observed on the Au-deposited surface like theprevious reported Nafion thin films on the Pt-deposited surface,
thickness dependence of the structural change was not observed
markedly on the MgO substrate.
Introduction
Highly proton-conductive polymers have attracted the attention of researchers foruse in energy conversion, sensors, catalysts, and other applications. One fundamental
approach to create highly proton-conductive polymers is based on phase segregatedstructures composed of hydrophobic backbones and hydrophilic parts with strong acidgroups (1-3). Nafion is the most investigated proton conductive membrane (4-6).
Since Siroma and co-workers reported on the proton conductivity drop withdecreasing the thickness of the Nafion thin films (7), confined Nafion thin films are
attracting attention of researchers because of different phase segregated structure andproton transport property compared to the commercial Nafion membranes. Karan and coworkers summarized early works for Nafion thin films (8). Kusoglu and Weber reviewed
studies of both Nafion thick membranes and thin films reported by the end of 2016 (6).
Many researches on the interfacial structure (8-39) and proton transport property (7, 19-21,
32, 33, 37, 40-48) have been dedicated. Nafion thin films can be prepared from acommercially available dispersion using spin casting, drop casting, Langmuir–Blodgett,Langmuir–Schaefer, spray deposition, and self-assembled adsorption on various substrates.
One pioneer study for the interfacial structure of the hydrated Nafion thin film examinedan alternating water-rich and polymer-rich lamellar structure on the SiO2 substrateusing neutron reflectometry (13). On the other hand, a single hydrated layer appeared atmetal interfaces such as Pt and Au surfaces (12, 13). Results suggest that the interfacial
structure of the Nafion thin films depends on the substrate surface.
In the previous study, our group demonstrated the in-plane proton conductivity dropof Nafion thin films depends on the substrate surfaces. (19, 20, 33). An inkjet process toprepare three phase boundary was also demonstrated (49). Surface morphology of Nafion
ultrathin film prepared by the inkjet process can be changed to the ultra-smooth surface bya solvent annealing method (47). Compared to the commercially available thick Nafion
membrane, all Nafion thin films decreases in-plane conductivity. However, degree ofsuppressed conductivity is different with the substrate surfaces in the thickness range ofseveral hundreds nano from a few nanometers. To the best of our knowledge, there is noreport on the thickness dependence of in-plane proton conductivity of Nafion thin films on
the Au and MgO(100) surfaces. The report on the interfacial structure on these surfaces
has not also been sufficiently discussed. In this study, thickness dependence of proton
conductivity and interfacial structure on the Au-deposited and MgO(100) surfaces were
investigated.
Experimental
MgO(100) (Furuuchi Chemical Industries Corp., Japan) and Au-deposited Si wafer
were used as substrates. The Si wafer was used as received without removing native oxide
layer. The substrate size is 15 × 15 × 0.5 mm. Substrate was washed by 2-propanol andcleaned by plasma treatment (Cute-MP; Femto Science, Korea) (50). Au-deposited Si
wafer was prepared using radio frequency (RF) magnetron sputtering system (KXS-110;
Kenix Co. Ltd.). Deposition was done in 30 s with 15 W at room temperature with an Ar
atmosphere. The thickness of the Au-deposited layer was ca. 10–15 nm determined byatomic force microscopy (AFM) (VN-8000; Keyence Co.) equipped with DFM/SS modecantilever (OP-75041; Keyence Co.). The Au-deposited surface was characterized using
X-ray photoelectron spectroscopy (XPS) (Kratos AXIS-ULTRA DLD; Shimadzu Corp.).
Binding energy was calibrated to C 1s electrons (284.5 eV). Nafion (DE521 CS type; Wako
Pure Chemical Industries Ltd.) thin films were prepared with different dispersion
concentration by spin-coating. Thickness was determined by AFM measurements andwhite light interferometric microscope (BW-S506; Nikon Corp.). Proton conductivity ofthe thin film was examined through impedance measurements using a frequency response
analyzer and a high-frequency dielectric interface (SI1260 and SI1296; Solartron
Anlytical). Relative humidity (RH) and temperature were controlled using a computercontrolled chamber (SH-221; Espec Corp.). For thin film conductivity measurement, twoprobe method was applied to obtain in-plane proton conductivity parallel to the thin film.
Gold contacts were used as electrodes with a porous gold paste (SILBEST No. 8560;
Tokuriki Chemical Research Co. Ltd.). Impedance data were collected for frequencies of
1 Hz and 10 MHz, with an applied alternating potential of 50 mV. Thin-film conductivity(σ) was calculated as,
𝑑𝑑

σ = 𝑅𝑅𝑅𝑅𝑅𝑅 , (1)where d signifies the distance between the gold electrodes, R denotes the resistance value
obtained from the impedance, l and t respectively stand the contact electrode length andthe thickness of the film. p-Polarized multiple angle incidence resolution spectrometry(pMAIRS) technique (51, 52) was used to investigate the molecular orientation of the thinfilm on the substrates. The p-MAIRS measurements were taken using an Fourier transform
infrared (FT-IR) spectrometer (Nicolet 6700; Thermo Fisher Scientific Inc.) equipped witha mercury–cadmium–telluride (MCT) detector and ZnSe polarizer. Single-beam spectra
were collected from 38° to 8° in 6° steps between the angle of incidence. The humidity ofthe sample compartment was less than 5% under room temperature. The p-MAIRS analysis

from the collected spectra was conducted automatically using p-MAIRS analyzer software(Thermo Fisher Scientific Inc.).
Results and Discussion
Nafion thin films on Au-deposited surface
Thickness dependence of the proton conductivity on the SiO2 substrates has beenreported in several literature (7, 21, 33, 43). Siroma and co-workers demonstrated
depression of proton conductivity in recast Nafion films (7). The high proton conductivity
is derived from a phase segregation between hydrophobic backbone and hydrophilic
sulfonic acid groups at the side chain (1, 4, 5). Modestino and co-workers reported that
degree of phase segregation decreases with decreasing thickness of the Nafion thin films(21). Ono and Nagao show that the proton conductivity decreases with decreasing the
thickness of Nafion thin films on the thin Pt-deposited surface (33). However, the protontransport on the Au-deposited surface has not been sufficiently discussed. For the in-plane
proton conductivity measurements on the Au-deposited surface, thickness of the Audeposited surface should be thin because of suppressing the electronic conductivity by the
Au layer (33). Fig. 1 shows the AFM images of the top and cross section views for the Audeposited surface on SiO2 substrate. Thickness of Au layer was estimated as 10 nm thick.
The resistance of the Au layer was higher than that of the Nafion thin films in the all RH
ranges. The Si 2p XPS spectrum suggested that deposited Au formed island structures on
the SiO2 surface. Therefore Au deposited layer is not so conductive compared to the Nafion
thin film layer.

Figure 1. AFM images of the top view and cross section views for the Au-deposited surface
on SiO2 substrate.
Figure 2 shows thickness dependence of proton conductivity of the Nafion thin films
on the Au-deposited surface. Proton conductivity of 20, 90, and 200 nm thick films showed
1.5 × 10-2, 1.7 × 10-2, and 1.6 × 10-2 S cm-1 at the 95% RH, respectively. Compared to thethick Nafion membrane, the proton conductivity of Nafion thin films was lower in the
thickness range of 20 – 200 nm. In the previous report used by SiO2 and Pt-deposited
surfaces, proton conductivity decreased with decreasing thickness (33). In the case of Audeposited surface, proton conductivity of Nafion thin films showed almost the same value
at the 80% and 95% RH. In the lower RH condition, proton conductivity dropped withdecreasing the thickness. Compared to the Nafion thin films on the SiO2 substrates, thedecrease of the proton conductivity on the Pt- and Au-deposited metal surfaces wassuppressed in the thinner region. Shim and co-workers reported that the water uptake of
Nafion thin films with ca. 80 nm thick on SiO2, Pt-deposited, and Au-deposited surfaces
exhibit almost similar value in the RH range of 0 – 80% (46). Kusoglu and co-workers
suggest the substrate/film interactions influence the structural order (28). Therefore we
speculated that the difference of the proton conductivity between the metal and Si wafer
surfaces might be derived from the different thin film nano-structures.
-1

Log (σ / S cm-1)

-2
-3
-4
-5
40% RH
60% RH

-6
0

50

298K

80% RH
95% RH100 150 200 250

Thickness / nm

Figure 2. Thickness dependence of the proton conductivity for the Nafion thin films on the
Au-deposited surface at various RH conditions.
pMAIRS requires transparent substrates (51, 52). To the best of our knowledge,
pMAIRS study has not been established using Au-deposited surfaces. Therefore Audeposition on the Si wafer was optimized. Thickness of Au-deposited surfaces should bethin as possible to apply pMAIRS (33). Fig. 3 shows the AFM images of the top and crosssection views for the Au-deposited surface on Si wafer. Thickness of Au was estimated as
10 nm thick. This transmittance of the IR single beam was 95% compared to the nondeposited Si wafer.

Figure 3. AFM images of the top view and cross section views for the Au-deposited surface
on Si wafer.
Figure 4 shows Au 4f XPS spectrum for the Au-deposited surface on the Si wafer.
Binding energies of Au 4f7/2, 4f5/2 electrons at 84.0 eV and 87.7 eV, respectively, suggest

Au0. Since signal in the Si 2p XPS spectrum decreased but was still observed after the Audeposition, Au was deposited as island structures on the Si wafer.
5

5x10

Intensity / cps

5

4x10

Au-deposited surface on Si
only Si

Au 4f

5

3x10

5

2x10

5

1x10

0
95

90

85

Binding energy / eV

80

Figure 4. Au 4f XPS spectra of bare Si wafer and Au-deposited surface on Si wafer.
Figure 5 shows incident angle dependence of the IR transmission spectra of Nafion
thin film on the Au-deposited surface. At the wavenumber around 1250 cm-1, absorption
band strongly depends on the incident angle. This result suggests that the thin film on the
Au-deposited surface exhibited an oriented structure. The highly oriented Nafion thin filmon the Pt-deposited surface has been reported in our previous literature (20).

Absorbance

0.10
0.08
0.06
0.04

38 deg32 deg26 deg20 deg14 deg
8 deg

0.02
0
1400 1350 1300 1250 1200 1150 1100
-1

Wavenumber / cm
Figure 5. Incident angle dependence of the IR transmission spectra of the 200 nm thick
Nafion film on the Au-deposited surface.

Figure 6 shows pMAIR spectra of Nafion thin films with different thickness on the
Au-deposited surface. The bands at 1150, 1210 – 1240, and 1300 cm-1 are assigned to νas(CF2), [νas (CF2) and νas (SO3-)], and ν (C-C) (53). The IP and OP spectra did not show thesimilar spectral shape. Results suggest that the Nafion thin films on the Au-deposited
surface have oriented structure. This orientation has been reported in our previous literatureusing Si and Pt-deposited surfaces (33). IP spectrum showed the well-known spectrum of
the thick Nafion membrane (53). The characteristic band at 1260 cm-1 was observed onlyin the OP spectrum. This attribution of the absorption band is unsolved yet (54, 55). Some
attributions were reported as νas(CF3) + δs(COC) (15), νas(CF2) (35), ν (CF2) (56), νas(CF3)(57), and –SO3- (58, 59) vibration modes. Ozhukil Kollath and co-workers discussed theorigin of thickness dependent peak from 1223 to 1259 cm-1 by attenuated total reflection(ATR)-FTIR spectra of Nafion films (60). However, our results showed that the peak

position was thickness independent in the thickness range of 210 – 20 nm. Therefore ourobserved peak at 1260 cm-1 can be considered as the different origin with the reported
thickness dependent peak (35, 60). In our results, each absorbance in the IP and OP spectra
decreased with decreasing thickness. The ratio of absorption bands at 1260 cm-1 in the OP
spectrum and at 1215 cm-1 in the IP spectrum gradually changed along the thickness change.
This large change of the absorption ratio can be observed on the Pt-deposited surface, but
not on the Si wafer surface. Results indicate that interfacial structure of Nafion thin films
is different between the metal and Si wafer surfaces.

Figure 6. IR pMAIR spectra of Nafion thin films on the Au-deposited surfaces. The
thickness of Nafion thin films is (a)210 nm, (b)150 nm, (c)90 nm, and (d)20 nm thick.
Nafion thin films on MgO(100) surface
In the previous section, the decrease of the proton conductivity on the Au-deposited
surface was suppressed compared to that on the SiO2 substrates. In our previous study,
proton conductivity of Nafion thin films on the MgO(100) substrate strongly decreased
compared to that of the commercial thick Nafion membranes (19). To investigate thethickness dependence of in-plane proton conductivity on the MgO(100) substrate, RHcontrolled impedance measurements were carried out. Fig. 7 shows thickness dependenceof proton conductivity for Nafion thin films on the MgO(100) substrates. The proton
conductivity was lower than that on the Au-deposited surface. The conductivity on the
MgO(100) substrate decreased by one order of magnitude with decreasing the thickness.
Fig. 8 shows pMAIR spectra of Nafion thin films with different thickness on the
MgO(100) substrates. In common with the results on the Au-deposited surface, all thinfilms showed the highly oriented structure on MgO substrates. The difference between the
Au-deposited surface and MgO surface can be seen in the absorption ratio of the bands
between 1260 cm-1 in the OP spectrum and 1215 cm-1 in the IP spectrum. The ratio wasalmost thickness independent in the case of MgO substrates. Both Au-deposited and Ptdeposited surfaces show the similar trend of the thickness dependent absorption ratio.
Though the peak assignment at 1260 cm-1 in the OP spectrum is still unsolved, results
demonstrate that the interfacial oriented structure on metal-deposited surfaces graduallychanges with decreasing the thickness of Nafion thin films.
-3-1

Log (σ / S cm )

-4
-5
-6

298K

-7
-8
0

40% RH
60% RH
80% RH50

100 150 200 250

Thickness / nm

Figure 7. Thickness dependence of the proton conductivity for the Nafion thin films on the
MgO(100) substrate at various RH conditions.

Figure 8. IR pMAIR spectra of Nafion thin films on the MgO(100) substrates. The
thickness of Nafion thin films is (a)240 nm, (b)150 nm, (c)30 nm, and (d)15 nm thick.

In this study, results suggest that the metal-deposited surface can suppress thedecrease of the in-plane proton conductivity in the thin film forms of Nafion compared to
the SiO2 and MgO(100) substrates. The metal-deposited surface also causes the change ofthe interfacial oriented structure. Some parameters should be discussed to unveil the origin
for the surface-selective structural change: surface charge, surface flatness, surfacehydrophilicity, conductivity (derived from electrons and holes), etc. Some parameters canbe discussed using self-assembled monolayer techniques in future.
Conclusions
Nafion thin films are attracting attention of researchers because of different phasesegregated structure and proton transport property compared to the commercial thick
Nafion membranes. In the previous study, proton conductivity drop was reported withdecreasing thickness of Nafion thin films on SiO2 substrates. In this study, pMAIRS study
using the Au-deposited surface was newly established. Then in-plane proton conductivity
and interfacial structure of Nafion thin films on the Au-deposited surface and MgO(100)substrate were investigated. Apparent drop of proton conductivity was observed on both
Au-deposited and MgO surfaces compared to the commercial Nafion membrane. A larger
proton conductivity drop on the MgO substrate was found than that on the Au-deposited
surface. For the interfacial structure by pMAIRS study, highly oriented structure was
elucidated in both Nafion thin films on the Au-deposited surface and MgO surface. Nafion
thin films on the Au-deposited surface exhibited the thickness dependent structural change.
This trend can be seen in the case on the Pt-deposited surface. Results indicate that metaldeposited surfaces gradually changes the interfacial structure with decreasing the thicknessof Nafion thin films. In the case of MgO substrates, no apparent structural change wasobserved. We infer that the metal-deposited surfaces suppress the drop of in-plane proton
conductivity of the Nafion thin films compared to the SiO2 and MgO(100) surfaces.
Acknowledgments
Y.N. thanks Dr. Vinayaraj Ozhukil Kollath and Prof. Carol Korzeniewski for their time
for discussion. This work was partially supported by JSPS KAKENHI Grant Number
JP18K05257.
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