Pt Interface

Article pubs.acs.org/JPCC

Molecular Structure of Buried Perfluorosulfonated Ionomer/Pt Interface Probed by Vibrational Sum Frequency Generation Spectroscopy Ichizo Yagi,*,†,‡ Kiyoshi Inokuma,‡ Ken’ichi Kimijima,‡ and Hideo Notsu‡ †

Faculty of Environmental Earth Science, Hokkaido University, N10W5, Kita-ku, Sapporo 060-0810, Japan Fuel Cell Cutting-Edge Center Technology Research Association (FC-Cubic TRA), 2-3-26 Aomi, Koto-ku, Tokyo 135-0064, Japan

‡

ABSTRACT: Perfluorosulfonated ionomer (PFSI), such as Nafion, in polymer electrolyte fuel cells (PEFCs) has been recognized as an important component to shuttle protons during the electrocatalytic reactions, especially the oxygen reduction reaction (ORR) at the cathode. However, a molecular structure of PFSIs inside catalyst layers in PEFCs has been unclear, since the polymers surrounding the gasdiffusion electrode with meso-to-macroporous structures have been considered to be much more complicated to resolve. Recent progress in the environmental electron microscopic technology clarified the real thickness of PFSI films to be only several nanometers, which can be analyzed by spectroscopic techniques and simulation modelings with molecular insights. Infrared and Raman spectroscopies were promptly applied to obtain the molecular arrangement of Nafion on the surface of Pt catalysts, but the thicknesses of the Nafion films seemed to be much thicker than the real thicknesses of PFSI in catalyst layers. In the present study, the preparation method of an ultrathin film of Nafion on Pt surface with 18.2 MΩ cm from a water-purification system (Milli-Q Gradient A10, Millipore). Nafion dispersion (5%) (DE520, Wako Pure Chemical Industries), 10% Aquivion dispersion (D83-10E, Solvay Solexis), ethanol (Wako Pure Chemical Industries), and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP, Wako Pure Chemical Industries) were used without further purification. The molecular structures of Nafion and Aquivion are shown in Scheme 1. Sample Preparation. Pt thin films of 20 nm thickness were sputter-deposited by Turbo Sputter Corter (Emitech, K575X) on 10 × 10 mm Si(100) wafers with adhesive layers of 3.5 nm Ti. Because the deposited Pt film surfaces were slightly contaminated by naturally adsorbed hydrocarbon impurities, the Pt surfaces were treated in a UV/ozone cleaner (UV253HS, Filgen) for 5 min before preparation of PFSI thin films. For the thickness-dependent measurements of PFSI films, Nafion thin films were mainly prepared. Various volumes of diluted Nafion dispersion solution were cast on the cleaned Pt surfaces to control the film thickness. Nafion solution (0.05 wt %) diluted with HFIP was typically used to obtain PFSI film with homogeneous and controlled film thicknesses more than 5 nm, and further dilution to 0.01 wt % was required to reduce the 26183

dx.doi.org/10.1021/jp5083592 | J. Phys. Chem. C 2014, 118, 26182−26190

The Journal of Physical Chemistry C

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membranes, the spectra became constant since the penetration depth of IR from Ge IRE was shorter than the thicknesses of membranes. Raman Scattering Spectroscopy. Raman scattering spectra were obtained with a commercial confocal Raman microscope (SENTERRA, Bruker Optics). Light from a He− Ne laser (λ = 632.8 nm) with a power of 20 mW was focused on the surface of the samples at the microscope stage through 20× objective lens with a NA of 0.4 (LMPlanFL, Olympus). Each spectrum was collected with a spectral integration time of 30 s and a spectral resolution better than 15 cm−1. Vibrational Sum Frequency Spectroscopy. The VSFG spectra were recorded in the C−F stretching and S−O stretching region (900−1650 cm−1) by a broadband femtosecond VSFG spectrometer consisting of a femtosecond Ti:sapphire oscillator/regenerative amplifier laser system (Integra-c, Quantronix), 100 fs, 2.35 mJ, 804 nm, 1 kHz. About 60% of the laser output was used to pump an optical parametric amplifier (OPA/OPG) system (TOPAS-c, Light Conversion, Inc.) to generate a broadband tunable IR beam with a bandwidth of ca. 200 cm−1 through an AgGaS2 crystal for differential frequency generation (DFG). The remaining 40% of the broadband visible output at 804 nm was converted to a picosecond pulse tunable from the UV to near-IR region (220− 1000 nm) by another OPA/OPG system (TOPAS-white-NB, Light Conversion, Inc.). Typically, visible wavelength of 600 nm was selected for narrow-band visible beam in the present study. The visible and infrared beams overlapped in a copropagating geometry on the substrate with incident angles of 65° and 50°, respectively. The SFG signal was dispersed through a monochromator (MS3501i, Solar-TII) and collected by a charge-coupled device (CCD) camera (DU420A-BU2, Andor Technology). The VSFG spectra were recorded in ppp (in the order of SFG, Vis, IR) polarization combination and were accumulated for 3 min. The intensities of the SFG spectra were normalized by VSFG spectra obtained at a clean gold substrate with an acquisition time of 1 min. To cover the abovementioned frequency region, broadband VSFG spectra collected at the 4 or 5 different center wavelengths of IR were required and the properly normalized spectra can be connected without any further corrections.

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Figure 1. ATR-IR spectra of (a−f) Nafion thin films on Pt and (g) Nafion 117 membrane. The thicknesses of Nafion films are (a) 1, (b) 2, (c) 6, (d) 9, (e) 26, and (f) 33 nm.

RESULTS AND DISCUSSION Figure 1 shows ATR-IR spectra obtained at Nafion thin films on Pt substrates with different designed thicknesses, t = (a) 1, (b) 2, (c) 5, (d) 10, (e) 25, and (f) 50 nm, which are calculated from the following equation:13 t=

VdIC N dNA × 100

Table 1. Thicknesses of Prepared Nafion Thin Films on Pt Surfaces

(1)

where V, dI, dN, CN, and A are a volume of Nafion solution, the density of Nafion solution, the density of dry ionomer, an ionomer concentration in the volume %, and a surface area, respectively. In Figure 1g, the ATR-IR spectrum measured at a Nafion 117 membrane is shown as a reference. Spectroellipsometry measurements have determined their real thicknesses to be summarized in Table 1. It is noteworthy that a much thicker spot with a diameter of 2 mm was locally formed during the drying process at the Nafion film on Pt with a designed thickness of 50 nm, resulting in the thinner Nafion thickness of ca. 33 nm at the homogeneous area. Thus, the real thicknesses of t = ca. 1, 2, 5, 9, 26, and 33 nm are used in the text below.

a

designed thickness/nm

real thicknessa/nm

1.0 2.0 5.0 10.0 25.0 50.0

0.99 2.18 5.73 9.25 25.7 32.8

Obtained by Cauchy fit of spectroellipsometric data.

The shape of each ATR-IR spectrum is similar to one in the previous paper, which was measured by the polarizationmodulated IR reflection absorption spectroscopy (PM-IRRAS) at Nafion thin film spin-coated on a Pt substrate.6 When the film thickness of Nafion decreases, the intensities of observed bands clearly decrease, as shown in Figure 1. The penetration 26184



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depth, dp, of IR from Ge IRE can be calculated from the following equation,14 dp =

λ 2πn1

1 2

sin θ −

2

( ) n2 n1

(2)

where θ, λ, n1, and n2, are the incident angle, an IR wavelength, and the refractive indices of ATR prism and sample, respectively. The calculated dp is dependent on the IR wavelength to be evaluated in the range 0.8−1.2 μm for the corresponding frequency region. Because it is much larger than the thicknesses of Nafion thin films in the present study, all the molecular vibrations inside Nafion thin films can be detected. Thus, the decrease in the absorbance of the vibrational bands with the reduction in the Nafion film thickness can be attributed to the decrease in the corresponding chemical species in the Nafion thin film. Figure 2 shows Raman spectra of (a) a Nafion thin film at a Pt substrate with a designed thickness of 33 nm and (b) a

Figure 3. VSFG spectra of Nafion thin films on Pt with thicknesses of (a) 1, (b) 2, (c) 6, (d) 9, (e) 26, and (f) 33 nm.

shapes and the intensities of several bands do not change in the range of the Nafion film thickness from 5 to 33 nm. Since the VSFG spectroscopy can detect only the vibrational modes with both the IR- and Raman-activity, the shape of the VSFG spectra seems to resemble the Raman spectrum in Figure 2. However, the sensitivity of the VSFG is quite higher than that of Raman spectroscopy without surface-enhancement effect, resulting in the detection of vibrational modes in the Nafion thin film in the submonolayer thickness. When the thickness of Nafion film decreases less than 5 nm, the band intensities seem to reduce depending on the film thickness. The intensity of VSFG signal (ISFG(ω)) is proportional to the square of the absolute value of the effective second-order susceptibility tensor of the interface (χ(2) eff ) as the following equation:

Figure 2. Raman spectra of (a) Nafion thin film with 33 nm thickness on Pt and (b) Nafion membrane.

Nafion 117 membrane. Although Raman measurements were tried at all the samples with three different excitation wavelengths, distinct spectral features in the corresponding frequency region were not obtained at the Nafion films with thicknesses less than 33 nm. The Raman spectra in Figure 2 are significantly similar to those in the previous reports,7,15 which were obtained at both Nafion 117 membrane and Nafion gel in water. Figure 3 shows VSFG spectra obtained at Nafion thin films on Pt substrates with various thicknesses, t = (a) 1, (b) 2, (c) 5, (d) 9, (e) 26, and (f) 33 nm. It can be easily noticed that the 26185


The Journal of Physical Chemistry C (2) 2 ISFG(ω) ∝ |χeff | =

∑ q

Aq ω − ωq + i Γq

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literature6,7,15−19 are summarized in Table 2. Note that the assignments for IR spectra are very complicated and some different assignments are put together in Table 2. As has been well-known, VSFG spectroscopy can detect vibrational modes of both IR- and Raman-active. Thus, the vibrational bands observed in VSFG spectra should be included in both ATR-IR and Raman spectra. In ATR-IR spectra, there is very large and broad absorption by perfluoroalkyl chains, which consist of both backbone and side-chains of Nafion, in the frequency region between 1100 and 1400 cm−1. In addition, the spectral assignment for the fluorocarbons in the frequency region between 1250 and 1350 cm−1, where the relatively large vibrational feature is observed in the VSFG of Nafion thin film on Pt, is currently a matter of considerable debate as an “axial” symmetric CF2 stretching mode20−25 is favored by some authors, while other authors suggest an assignment to CF3 asymmetric26−30 or CF3 symmetric stretching30−32 vibrations even for the simple systems, such as perfluorinated selfassembled monolayers (FSAMs). In addition, C−C stretching vibration also can be assignable to the higher frequency bands from Table 2, but the C−C stretching band is known as IRinactive;33 then this mode can be excluded from the assignment for the VSFG band between 1250 and 1350 cm−1, because both IR- and Raman-active modes can be VSFG-active. Although precise assignments of the CF related bands are desired to determine the molecular arrangement of Nafion in the ultrathin film, the complicated molecular structure of polymer is not proper to discuss at the present stage.34 Thus, it can be useful to compare the thickness-dependent behavior of a vibrational band, which coincides in all the three kinds of vibrational spectra and can be assignable to the chemical species. In Table 2, two vibrational bands observed at around 970 and 1060 cm−1 coincide their frequencies in the three kinds of vibrational spectra. The assignment of the former band is not agreed even in the recent papers to be various vibrations, such as νs(C−O− C), δ(C−O−C), and coupled modes of νs(SO3−) + νas(C−O− C(A)) + ρr(C−O−C(B)), where νs, νas, δ, and ρr denote symmetric stretching, asymmetric stretching, bending, and rocking, respectively, and C−O−C(A) and (B) represent ether groups in proximity to the backbone and the sulfonate group, respectively (see Scheme 1). On the other hand, the vibrational band at ∼1060 cm−1 is agreed to be assigned as νs(SO3−), while the mechanical coupling with νas(C−O−C(A)) is described as a result of DFT calculation.6 Therefore, the vibrational band at 1060 cm−1 is assigned to νs(SO3−) mode in the present article, and the thickness-dependent behavior of the band is analyzed. Figure 5 shows the thickness dependences of integrated area of the IR absorption band and fitted amplitude, A1060, of the VSFG band at ∼1060 cm−1. Because the thickness-dependent series of Raman spectra was not obtained, it cannot appear in the following discussion. A linear correlation between the integrated IR absorbance and the thickness of Nafion thin film is obtained, while A1060 seems to saturate at the film thickness of 5 nm, as has been expected from thickness dependence of ATR-IR (Figure 1) and VSFG (Figure 3) spectra. The difference between the film thickness-dependent behaviors of the IR and VSFG bands at the same frequency of 1060 cm−1 should be attributed to the inherent interfacial selectivity of VSFG spectroscopy. The linear correlation between the IR absorption and film thickness seems reasonable by assuming that the ATR-IR can detect all the vibrations in the Nafion thin films, as mentioned above. Here we should discuss about the surface selectivity of IR spectroscopy described in the previous

2 (2) exp(iϕq)+χNR

(3)

χ(2) NR

where arises from the nonresonant background contribution and Aq, ωq, and Γq are the oscillator amplitude, resonant frequency, and damping coefficient of the qth vibrational mode, respectively. Each peak on the VSFG spectra was fitted by eq 3 using Aq, ωq, and Γq as fitting parameters. Fitting by eq 3 gives five vibrational bands for all the VSFG spectra in Figure 3. The above-mentioned results obtained by three different vibrational spectroscopies are compared in the following sections. Although the shapes of the VSFG spectra in Figure 3 seem significantly different from the ATR-IR spectra of Nafion thin films at Pt substrates, the several vibrational bands coincide each other. Besides, the Raman spectrum in Figure 2 seems similar to the VSFG spectra, but the frequencies of the several bands in the higher-frequency region are slightly different, maybe due to the overlapping of several vibrational bands. ATR-IR, Raman, and VSFG spectra of the Nafion thin film with a thickness of 35 nm on the Pt substrate are compared in Figure 4, and the frequencies of the corresponding vibrational bands in these spectra and typical assignments in the

Figure 4. (a) ATR-IR, (b) VSFG, and (c) Raman spectra of Nafion thin film with 33 nm thickness. 26186



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Table 2. Assignment of Vibrational Modes Observed in Raman and IR in the Previous Studies for Nafion Membranes and Thin Filmsa Raman7,15

IR6,16−19

membrane

thin film/Pt

assign.

membrane

thin film/Pt

972

971

δ(COC)

971 984

1001 1065

νs(SO3−) νs(SO3−)

νs(SO3−) + νas(COCA) + ρr(COCB)6,17 ω(CF2) + δs(COCB)6,17

1060

1061

1150

νs(CF2)

1178 1233 1270 1302

νas(SO3−) νs(CF3) νas(CF3) νs(CC)

969 982 993sh18 1056, 1063 1134 1139, 1143 1155 1192, 1199 1217, 1224 1271, 1280 1299, 1301 1320, 1325 1350, 1357

νs(SO3−) + νas(COCA),6,17 νs(SO3−),16,18 νs(SO3−) + νs(CF2)18 νas(COC)18 νs(CF2)18 νas(SO3−)16 δs(CF2) + ρr(CF2) + ω(COCA),6 νas(CF2)16,18 νas(SO3−) + νas(CF2) + νas(CF3),19 νas(CF2) + νas(CF3)18 νas(CF3) + δs(COCA) + δs(COCB),6 νas(SO3−)18 νs(CF), νs(CC) + νas(BBStr)19 δs(CF2),6 νas(CF3),16,18 νs(BBStr,op)19 νas(SO2),18 νs(BBStr,ip) + ρr(COCB)19

1296

1382

1379

1164 1260 1322

assign.

νs(CC)

νs, symmetric stretching; νas, asymmetric stretching; δs, scissoring; ρr, rocking; ω, wagging; BBStr, backbone stretching; ip, in-phase; op, out-ofphase; sh, shoulder; COCA and COCB, see Scheme 1. a

functional groups ordered by the Pt surface.6 It seems not to be a precise description for thicker film more than monolayer on metal surfaces, because the PM-IRRAS can detect ordered vibrational modes perpendicular to the metal surface. For example, PM-IRRAS at Langmuir−Blodgett (LB) multilayers and polyelectrolyte multilayers on metal surfaces shows a linear correlation between the film thickness and absorbance of vibrational bands,35−37 indicating that the PM-IRRAS is not selective to the chemical species directly adsorbing at the metal surfaces. The authors measured PM-IRRAS spectra at much thicker Nafion films on Pt than those of the present study, and detailed thickness dependences were not examined. In addition, the vibrational modes perpendicular to the surface can be enhanced even in the ATR-IR spectrum at metal surfaces, because the electric field distribution of the evanescent wave at the substrate/sample interface is modified so only the ppolarized light gives rise to a surface-selection rule similar to that more usually associated in IRRAS. In fact, ATR-IR spectra of the Nafion thin films on Pt (Figure 1a−f) and Nafion membrane (Figure 1g) in the present study are similar to the PM-IRRAS at the Nafion film on Pt and the ATR-IR spectrum of the Nafion 117 membrane in the literature,6 respectively. Therefore, the difference between ATR-IR spectra (and PMIRRAS) of the Nafion membrane and the thin film on Pt can be attributed to the difference of molecular arrangement (ordering) of Nafion, rather than the selectivity of PM-IRRAS toward molecular vibrations adsorbing at the Pt surfaces.

Figure 5. Thickness dependences of integrated area of IR absorption band and fitted amplitude, A1060, of VSFG band at ∼1060 cm−1. At least three ATR-IR and VSFG spectra at the different points are measured, and the values are in the plot.

paper.6 The authors compared ATR-IR spectrum of a Nafion membrane and PM-IRRAS of Nafion thin film on Pt substrate to be different from each other, and they mentioned that PMIRRAS enhances (relative to the ATR) vibrational modes of

Figure 6. Possible molecular arrangements of sulfonate terminals adsorbing at (a) Nafion/Pt interface, (b) air/Nafion interface, and (c) both Nafion/Pt and air/Nafion interfaces. 26187



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Figure 7. One of the possible molecular arrangements of PFSI molecules in Nafion film formed on Pt surface.

capacity of the Nafion used in the present study is 1.03−1.12 meq g−1, and the number of sulfonate terminals contained in the Nafion thin film of 5 nm thickness can be calculated to be 2.14−2.35 × 10−10 mol cm−2. If the most stable Pt(111) surface is dominant at the Pt sputtered film, the surface coverage of sulfonate simply corresponds to 0.20−0.23 ML of a Pt atomic layer. Electrochemical measurements at Nafion-cast Pt(111) electrode by Subbaraman et al. indicated that the pseudocapacitive peaks appear in cyclic voltammogram (CV) as a result of adsorption/desorption of terminal sulfonate and the coverage of adsorbed sulfonate is 5 nm means the saturation of interfacial ordering of SO3− terminals toward the corresponding interface. The polarization combination of ppp for the VSFG measurements at metal surfaces causes the predominant contribution of (2) χ(2) zzz, one of the tensor elements of χeff in eq 3. Thus, the − observation of νs(SO3 ) can be associated with the ordering of transition dipoles rather perpendicular to the interface, meaning adsorption at the interface. However, there are two interfaces, air/Nafion and Nafion/Pt, at Nafion thin film/Pt substrate samples. If the SO3− terminals perpendicularly ordered at the air/Nafion or Nafion/Pt interface are assumed, each interface can contribute to the VSFG signal. In Figure 6, rough images of the molecular arrangements of Nafion adsorbing at the (a) Nafion/Pt, (b) air/Nafion, and (c) both interfaces are shown. In the case of (c), the directions of νs(SO3−) modes become opposite, resulting in the disappearance of VSFG signal due to the establishment of inversion symmetry. The contact angle of water at the topmost surface of Nafion thin film on Pt was ca. 101.2 ± 1.4°, similar to that at plasma-polymerized perfluorocarbon film38 rather than the sputtered polytetrafluoroethylene (PTFE) and massive PTFE.38−40 Such a hydrophobic surface cannot be expected from the molecular arrangement shown in cases (b) and (c) of Figure 6. Thus, we conclude that the terminal sulfonates of the side-chains in Nafion adsorb on the Pt surfaces, as has been illustrated in previous reports.6,41 It is noteworthy that the VSFG spectra cannot provide any information about molecular arrangement of Nafion in the bulky region more than 5 nm, because VSFG is canceled out if the inversion symmetry is established. For example, random molecular arrangement of sulfonate terminals may cause the formation of macroscopic centrosymmetry,42 and multilamella structure keeping water channels results in the formation of a pair of sulfonates with opposite orientation,42,43 which raised the microscopic inversion symmetry. One of the possible molecular arrangements is shown in Figure 7. The saturation of A1060 at the Nafion film thickness around 5 nm in Figure 5 is quantitatively estimated. The total acid 26188



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twice that compared to the value obtained by electrochemical charge density for pseudocapacitive current peak attributed to the sulfonate terminal adsorption/desorption.4 Because the Pt substrates are polycrystalline film prepared by sputtering in the present study, the systematic experiments using Pt(hkl) singlecrystalline surfaces are required to obtain the atomic and molecular picture at the Nafion/Pt interfaces from the quantitative viewpoint.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +81-11-706-4526. Fax: +81-11-706-4529. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The present study was financially supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan and a Grant-in-Aid for Scientific Research (C) (No. 24550002) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan.

Figure 8. VSFG spectra of (a) Nafion and (b) Aquivion thin films on Pt substrates with thicknesses of ca. 50 nm.

on Pt substrates with thicknesses of ca. 50 nm. The thin film of Aquivion was also prepared by drop-casting with the same condition as that of Nafion. Almost the same features are observed in the lower-frequency region, while a broad and doublet-like feature at ∼1300 cm−1 is observed only at the Nafion thin film on Pt surface. As is shown in Scheme 1, the differences between the molecular structures of Nafion and Aquivion are the length of side-chain and the existence of CF3 group in the side-chain of Nafion. The present results indicate that the corresponding VSFG feature only observed at Nafion thin film can be assigned to the molecular vibration of CF3 functional group in the side-chain. However, the overlap of broad and relatively small contributions probably due to the terminal CF3 or “axial” CF2 of perfluoroalkyl backbones in the corresponding frequency region makes it difficult to fit or deconvolute the VSFG spectra precisely. Heterodyne VSFG spectroscopy, which can extract imaginary spectrum of χ(2) and/or precise DFT calculation for both Nafion and Aquivion thin films, can help the assignment of the corresponding bands, resulting in detailed understanding of interfacial molecular arrangements of PFSIs.

(1) More, K.; Borup, R.; Reeves, K. Identifying Contributing Degradation Phenomena in PEM Fuel Cell Membrane Electride Assemblies Via Electron Microscopy. ECS Trans. 2006, 3, 717−733. (2) Gómez-Marín, A. M.; Berná, A.; Feliu, J. M. Spectroelectrochemical Studies of the Pt(111)/Nafion Interface Cast Electrode. J. Phys. Chem. C 2010, 114, 20130−20140. (3) Subbaraman, R.; Strmcnik, D.; Paulikas, A. P.; Stamenkovic, V. R.; Markovic, N. M. Oxygen Reduction Reaction at Three-Phase Interfaces. ChemPhysChem 2010, 11, 2825−2833. (4) Subbaraman, R.; Strmcnik, D.; Stamenkovic, V.; Markovic, N. M. Three Phase Interfaces at Electrified Metal−Solid Electrolyte Systems. 1. Study of the Pt(hkl)−Nafion Interface. J. Phys. Chem. C 2010, 114, 8414−8422. (5) Ahmed, M.; Morgan, D.; Attard, G. A.; Wright, E.; Thompsett, D.; Sharman, J. Unprecedented Structural Sensitivity toward Average Terrace Width: Nafion Adsorption at Pt{hkl} Electrodes. J. Phys. Chem. C 2011, 115, 17020−17027. (6) Kendrick, I.; Kumari, D.; Yakaboski, A.; Dimakis, N.; Smotkin, E. S. Elucidating the Ionomer-Electrified Metal Interface. J. Am. Chem. Soc. 2010, 132, 17611−17616. (7) Zeng, J.; Jean, D.-i.; Ji, C.; Zou, S. In Situ Surface-Enhanced Raman Spectroscopic Studies of Nafion Adsorption on Au and Pt Electrodes. Langmuir 2012, 28, 957−964. (8) Noguchi, H.; Taneda, K.; Naohara, H.; Uosaki, K. Humidity dependent structure of water at the interfaces between perfluorosulfonated ionomer thin film and Pt and HOPG studied by sum frequency generation spectroscopy. Electrochem. Commun. 2013, 27, 5−8. (9) Iqbal, S.; Podgaynyy, N.; Ahmed, M.; Attard, G. A.; Baltruschat, H. Surface morphological studies of Nafion/Pt(100) interface. Electrochim. Acta 2014, 144, 141−146. (10) Webber, M.; Dimakis, N.; Kumari, D.; Fuccillo, M.; Smotkin, E. S. Mechanically Coupled Internal Coordinates of Ionomer Vibrational Modes. Macromolecules 2010, 43, 5500−5502. (11) Zaera, F. Probing liquid/solid interfaces at the molecular level. Chem. Rev. 2012, 112, 2920−2986. (12) Ekgasit, S.; Pattayakorn, N.; Tongsakul, D.; Thammacharoen, C.; Kongyou, T. A Novel ATR FT-IR Microspectroscopy Technique for Surface Contamination Analysis without Interference of the Substrate. Anal. Sci. 2007, 23, 863−868. (13) Higuchi, E.; Uchida, H.; Watanabe, M. Effect of loading level in platinum-dispersed carbon black electrocatalysts on oxygen reduction

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CONCLUSION Inherently surface-selective VSFG spectroscopy was applied to PFSIs/Pt interfaces and the adsorption of sulfonate terminals toward the Pt surface was nearly confirmed from the thicknessdependent band intensities of symmetric SO3− stretching in both VSFG and ATR-IR spectra. Although the present results support the picture reported in the previous paper, the rigorousness of the VSFG measurements seems to exceed that obtained by PM-IRRAS, because the VSFG spectroscopy can extract the vibrations in interfacial molecular layers with submonolayer coverage. The saturation of VSFG amplitude at 1060 cm−1, which can be attributed to the SO3− symmetric stretching, at the Nafion thickness of ca. 5 nm corresponded to the terminal sulfonate coverage of 0.2 at Pt(111) surface and the sulfonate terminals in the bulky phase, which was expected to appear in Nafion thin films thicker than 5 nm, formed in a (macroscopically centrosymmetric) random orientation42 or (locally inversion symmetric) dimer configuration.42,43 The surface coverage of 0.2 for sulfonate terminals at Pt(111) was 26189



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