Article pubs.acs.org/Langmuir
Proton Conductivities of Lamellae-Forming Bioinspired Block Copolymer Thin Films Containing Silver Nanoparticles Hiroshi Yabu,*,† Jun Matsui,‡ Mitsuo Hara,§ Shusaku Nagano,§,∥ Yasutaka Matsuo,⊥ and Yuki Nagao# †
WPI-Advanced Institute for Materials Research (AIMR), Tohoku University, 2-1-1 Katahira, Aoba-Ku, Sendai 980-8577, Japan Department of Material and Biological Chemistry, Faculty of Science, Yamagata University, 1-4-12 Koshirakawa, Yamagata 990-8560, Japan § Graduate School of Engineering, Nagoya University, Furocho, Chikusa-Ku, Nagoya 464-8603, Japan ∥ The Nagoya University Venture Business Laboratory, Nagoya University, Furocho, Chikusa-Ku, Nagoya 464-8603, Japan ⊥ Research Institute for Electronic Science (RIES), Hokkaido University, N21W10, Sapporo 001-0021, Japan # School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan ‡
S Supporting Information *
ABSTRACT: Size-controlled metal nanoparticles (NPs) were spontaneously formed when the amphiphilic diblock copolymers consisting of poly(vinyl catechol) and polystyrene (PVCa-b-PSt) were used as reductants and templates for NPs. In the present study, the proton conductivity of well-aligned lamellae structured PVCa-b-PSt films with Ag NPs was evaluated. We found that the proton conductivity of PVCab-PSt film was increased 10-fold by the addition of Ag NPs into the proton conduction channels filled with catechol moieties. In addition, the effect of humidity and the origin of proton conductivity enhancement was investigated.
1. INTRODUCTION Proton conductive polymer electrolytes are essential materials for polymer electrolyte fuel cells (PEFCs). Commercialized polyelectrolytes, including Nafion membrane, have nanochannels to transport protons with a high content of sulfonic acid groups as proton carriers.1,2 Extensive research on proton conductive materials composed of a wide variety of materials3−8 has been reported, which has led to greater focus on how to create nanochannel structures with proton conductive acidic groups. Nanochannel structures induced by self-assembly of molecules are one of the key materials to create efficient proton and ion conductive membranes. Kato, Ohno, and coworkers have reported highly conductive liquid crystalline proton and ion conductors, which form unique self-assembled nanochannel structures.9−11 Miyatake and co-workers also employed aromatic ionomers for thermally stable ionic conductors.12,13 Amphiphilic block copolymers, which form various phase-separated structures in their bulk state, are promising materials for the realization of high proton conductivity because they spontaneously self-assemble into nanoscale channel structures.14 The proton conductivity is strongly dependent on the size of the proton conduction nanochannels due to confinement of the proton conduction paths.15,16 Lodge and co-workers have synthesized a proton© XXXX American Chemical Society
exchange membrane by polymerization-induced phase separation of block copolymers composed of poly(ethylene oxide) (PEO) and polystyrene (PSt) swollen with ionic liquids.17 Amphiphilic comb-shaped block copolymers composed of fluorinated and sulfonated moieties have also exhibited high proton conductivity.18 Kim et al. have reported that lamellaeforming amphiphilic block copolymers swollen with ionic liquids exhibit high proton conductivity under water-free conditions.19 Elabd and Hickner reviewed many types of amphiphilic block copolymers that were proposed as proton conductive materials, and some of them had high proton conductivity.20 Peckham and Holdcroft have also reported recent progress on proton conductive aromatic block copolymers synthesized by polycondensation.21 Some recent studies show that the proton conductivities of polyelectrolytes with confined 2D nanochannels formed by layered structures of amphiphilic polymers have been improved with respect to the bulk system, even where weak acidic groups such as carboxylic acid have been employed as proton carriers.22,23 We have reported on the synthesis of amphiphilic diblock copolymers consisting of poly(vinyl catechol) (PVCa), which is Received: July 7, 2016 Revised: August 18, 2016
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Scheme 1. Schematic Illustration of PVCa-b-PSt Synthesis and Preparation of Ag NP-Loaded PVCa-b-PSt Nanocomposite Film
then dried at room temperature for 1 h. The synthesized PVCa-b-PSt was dissolved in tetrahydrofuran (THF) or CHCl3/MeOH (9/1 w/w) to prepare a 10 wt % solution, which was cast onto a Si wafer (1 000 rpm, 60 s) with a PVA sacrificial layer. After drying at room temperature, the prepared sample was immersed in Millipore membrane-filtered (Milli-Q) water to dissolve the PVA sacrificial layer and float a PVCa-b-PSt film onto the surface of the water. The floated film was scooped up with a 17 × 10 mm2 quartz substrate (Sendai Sekiei, Sendai) and dried in vacuo for 2 h. 2.3. Formation of Ag NP Arrays. Ag NPs arrays were formed by immersion of the PVCa-b-PSt film in 200 mM AgNO3(aq) for 4 h. After immersion, the film was washed three times with Milli-Q water and then dried at room temperature. Formation of silver NP arrays was confirmed by UV−vis spectroscopy (V-670, Jasco, Tokyo) and cross-sectional transmission electron microscopy (TEM; H-7650, Hitachi Hi-Technologies Corp., Tokyo) observations. Specimens for cross-sectional TEM observations were prepared as follows. A PVCab-PSt film or Ag NP-loaded PVCa-b-PSt film floated on a water surface was scooped up with an epoxy resin sheet (EPOC, Okenshoji Co.) and then dried at room temperature. The dried epoxy resin with sample film was embedded into another epoxy resin and cured at 70 °C for 6 h. A thinly sliced specimen was prepared using an ultramicrotome (UC-6, Lica, Germany) and placed on a Cu grid covered with a collodion membrane. The sample specimen was stained with OsO4 vapor for 2 h to stain PVCa moieties. Because OsO4 is a strong acid, it reacts with a variety of chemical compounds. It is wellknown that OsO4 reacts with phenol and catechol groups, which results in formation of complex and cross-linking structure.26 The acceleration voltage used for TEM observations was 80 kV. An energydispersive X-ray spectroscopy (EDS) spectrum was obtained by using EDAX Genesis, Ametek Co., Ltd. 2.4. Proton Conductivity Measurement. Impedance measurements of the thin films were conducted at 40−95% relative humidity (RH) using an impedance/gain-phase analyzer (SI1260, Solartron Analytical) and a dielectric interface system (1296, Solartron Analytical). The RH and temperature were controlled using a humidity- and temperature-controlled chamber (SH-221, Espec Corp.). Porous gold paint (SILBEST no. 8560, Tokuriki Chemical Research) was used to form electrodes for impedance measurement of the films. The electrode configuration was selected to obtain measurements of current flow in the plane parallel to the substrate surface. 2.5. Surface-Enhanced Raman Scattering Spectroscopy. SERS spectra of the Ag NP-loaded PVCa-b-PSt films prepared on Si substrates were measured using a Raman microscope (inVia Reflex, Renishaw, Gloucestershire, U.K.). Spin-coated film of Ag NPincorporated film on a Si substrate was used as a sample specimen. SERS measurements were conducted using a 532 nm laser. In situ SERS measurements during impedance measurements were also
known as a mussel-inspired adhesive moiety, and PSt by the reversible addition−fragmentation transfer (RAFT) polymerization of dimethoxy styrene and styrene followed by deprotection of the methoxy groups.24 These bioinspired diblock copolymers form a variety of microphase-separated structures, including spheres, cylinders, bicontinuous phases, and lamellae, depending on the copolymerization ratio of PVCa and PSt.25 Furthermore, due to the reductive properties of the two phenolic groups in catechol, we found that size-controlled metal nanoparticles (NPs) were spontaneously formed when using the diblock copolymers as reductants and templates for NPs. This effect was exploited to produce silver NP arrays in films of PVCa-b-PSt with various phase-separated structures by simply immersing the film into aqueous solutions of metal ions. These results imply that ions can be diffused into PVCa phases that are rich in acidic phenolic groups. However, the catechol moiety is a weakly acidic functional group; therefore, the proton conductivity of nanostructured films with catechol moieties has not been investigated. In the present study, the proton conductivity of PVCa-b-PSt with incorporated Ag NPs was evaluated. In addition, the effect of humidity and the origin of proton conductivity enhancement was investigated using grazing-incidence small-angle X-ray scattering (GI-SAXS), water uptake, and in situ surfaceenhanced Raman scattering (SERS) measurements.
2. EXPERIMENTAL SECTION 2.1. Synthesis of PVCa-b-PSt. PVCa-b-PSt was synthesized according to a method previously reported in the literature (Scheme 1).22 Typically, PVCa-b-PSt was synthesized by deprotection of the methoxy groups of poly(dimethoxy styrene-block-styrene) (PDMSt-bPSt) treated with BBr3 and HCl(aq). PDMSt-b-PSt was synthesized by reversible-addition−fragmentation transfer (RAFT) polymerization of dimethoxy styrene and styrene (see Figure S1, Supporting Information). The number-average molecular weight (Mn) and the weight-average molecular weight (Mw) of the precursor PDMSt-b-PSt were 9.9 and 13.4 kg/mol, respectively, and the polydispersity index (Mw/Mn) was 1.16. The PDMSt/PSt ratio was 0.31. 2.2. Film Formation. Before preparation of the PVCa-b-PSt film, a sacrificial layer of poly(vinyl alcohol) (PVA; Wako Chemical Industries Co., Ltd., Tokyo) was prepared on a 2.5 in. Si wafer (Nilaco Corp., Tokyo). PVCa moieties have high adhesivity to various inorganic substrates; therefore, the film can be transferred from an atomically flat Si substrate to a quartz substrate by dissolving the PVA layer as a sacrificial layer (see Figure S2, Supporting Information). PVA(aq) (5 wt %) was spin-coated onto the Si wafer at 1 000 rpm and B
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Figure 1. UV−vis absorption spectra for (a) PVCa-b-PSt film before and after Ag NP formation, cross-sectional TEM images of PVCa-b-PSt films (b) before and (c) after Ag NP formation, and (d) EDS spectra obtained from the film shown in (c).
the film were attributed to the Ag NPs. The Ag NPs were aligned along with the phase-separated structure of the PVCa phase. An energy-dispersive X-ray spectroscopy (EDS) spectrum (Figure 1d) of the film clearly shows peaks attributed to the L and K shells of solid Ag. These results indicate the successful formation of Ag NPs inside of the PVCa phase. SERS measurement revealed signals from catechol moieties attributed to C−H, hydrogen-bonded phenol groups, and C−O stretching, in addition to a small signal attributed to the CO stretching of quinone, which was formed by the oxidation of catechol (Figure 2). These results indicate that small numbers of catechol moieties are converted to quinone and hydrogenbonded phenolic groups are still present after formation of the Ag NPs. GI-SAXS results for the as-prepared PVCa-b-PSt film prepared on a PVA sacrificial layer at 40% RH are shown in Figure 3a. To obtain enough scattering intensity, we prepared a thicker film (ca. 1 μm) and employed it for GI-SAXS
performed (NanofinderFlex, Tokyo Instruments, Inc.; see Figure S4, Supporting Information). 2.6. GI-SAXS Measurements. GI-SAXS measurements for structural analysis of the microphase separated structures were performed with a NANO-Viewer X-ray diffractometer (Rigaku) using Cu Ka radiation (λ = 0.154 nm) as an X-ray source and an imaging plate (Fujifilm) for detection. The GI sample stage was set using a goniometer and vertical stage. 2D GI images were recorded with X-ray incidence angles adjusted between 0.16° and 0.18°, which is between the critical angles of the films. An image was obtained by exposure for 8−24 h. The diameter of the pinhole slit-collimated X-ray beam was in the range of 0.3−0.6 mm. The camera length was set at 955 mm. 2.7. Fourier-Transform Infrared Spectroscopy. Absorption spectra of PVCa-b-PSt films were obtained by Fourier-transform infrared spectroscopy attenuated total reflectance (FT-IR ATR) spectroscopy (FT/6100, Jasco, Tokyo, equipped with an ATR attachment). 2.8. Quartz Crystal Microbalance Measurement. The adsorption of water under various relative humidity conditions was evaluated using a quartz crystal microbalance (QCM; THQ-100P-SW, Tama Device, Tokyo). One drop of a solution of PVCa-b-PSt dissolved in THF was spin-coated onto a QCM chip, and the film weight was then measured under dry and humid air flow. To form Ag NPs on the PVCa-b-PSt film, the film was immersed in 200 mM AgNO3(aq) solution for 30 min.
3. RESULTS AND DISCUSSION Figure 1a shows UV−vis spectra of PVCa-b-PSt films before and after immersion in AgNO3(aq). Before immersion, the film had high optical transparency in the visible light region, and absorption of aromatic rings at lower than λ = 300 nm was observed. Immersion of the PVCa-b-PSt film in AgNO3(aq) resulted in a brown-colored film, of which the absorption peak was observed at λ = 430 nm. This result indicates the formation of Ag NPs, which have unique plasmonic absorption in the visible light region. Parts b and c of Figure 1 show crosssectional TEM images of the PVCa-b-PSt film before and after immersion in AgNO3(aq), respectively. In Figure 1b, a lamellae structure consisting of dark PVCa layers stained with OsO4 and nonstained PSt layers was stacked perpendicular to the substrate surface. The same multiple layered structures were also observed after immersion in AgNO3(aq) (Figure 1c), and black dots observed inside of the dark regions (PVCa layers) of
Figure 2. SERS spectra for the Ag NP-loaded PVCa-b-PSt film. C
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The phase-separated structures of PVCa-b-PSt are strongly dependent on the film-preparation conditions. We have reported that the same PVCa-b-PSt directly cast on a silicon substrate forms a bicontinuous phase. Furthermore, spherical phases were observed when a CHCl3/MeOH solution of PVCa-b-PSt was cast on a PVA sacrificial layer (see Figure S3, Supporting Information). When the CHCl3/MeOH were used as solvents, affinities of respective polymer segments to CHCl3 and MeOH are much different; micelles whose core has PVCa phase swelled with MeOH were formed. As a result, the spherelike phase-separated structure is selectively formed. On the other hand, when THF was used, THF is a good solvent for each polymer segment and interaction between each polymer segment and casting substrate dominated the phase-separated structure. Because Si, quartz, and PVA sacrificial layers have hydrophilic nature, the PVCa moiety selectively phaseseparated on the surface of these substrates and formed a perpendicular-stacking lamellae phase. These results demonstrate how the interior phase-separated structure of PVCa-b-PSt is very sensitive to the preparation conditions, including the substrates and solvents employed. Furthermore, the nanochannels for proton transport should be aligned from one electrode to another; therefore, the lamellae phase perpendicularly stacked to the substrate surface is a suitable phase for proton conduction. The proton conductivities of PVCa-b-PSt and Ag NP-loaded PVCa-b-PSt thin films were measured by impedance measurement under various RH conditions. Figure 4 shows the dependence of conductivity on RH for the PVCa-b-PSt and Ag NP-loaded PVCa-b-PSt thin films at 25 °C. The films were highly resistive under dry conditions. The proton conductivity of the PVCa-b-PSt thin film increased over 70% RH and reached 10−5 S/cm at 95% RH. It is noteworthy that the wellaligned lamellae phase is essential to realize such proton
Figure 3. GI-SAXS images of PVCa-b-PSt at (a) 40% RH and (b) 90% RH and (c) circular ring average of each GI-SAXS pattern, where (i) and (ii) indicate peaks at q = 0.17 nm−1 (2q = 0.23°) and q = 0.35 nm−1 (2q = 0.48°), which are attributed to d = 18.4 nm and d = 36.8 nm, respectively.
measurements (see Figure S5, Supporting Information). The 2D scattering image shows multiple ring scatterings, which implies periodic domains are inhomogeneously distributed in the film. The circular q-intensity profile in Figure 3c shows (i) first- and (ii) second-order scattering peaks, which indicate dspacings of 18.4 and 36.8 nm, respectively. The q value of the second-order peak is twice that of the first-order peak, which indicates the interior periodic structure is a lamellae phase. The average d-spacing among the lamellae layers is calculated as 19.6 nm from the TEM image, which is consistent with the GISAXS results. Furthermore, thicknesses of gray and bright regions were 7.5 and 12.1 nm, respectively, which is identical with the copolymerization ratio of VCa and St (PVCa/PSt = 1:2).
Figure 4. RH dependence of proton conductivity for PVCa-b-PSt (blue squares) and Ag NP-loaded PVCa-b-PSt (red circles) thin films. D
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Figure 5. FT-IR ATR spectra for (a, b) PVCa-b-PSt and (c, d) Ag NP-loaded PVCa-b-PSt films (a, c) before and (b, d) after immersion in water. Peak assignments (Roman numerals) are summarized in Table 1.
conduction. If the lamellae structure was distorted (such as the film shown in Figure S5, Supporting Information), the conductivity of the film was ca. 20 times lower than that of the aligned film. A conductivity hysteresis was observed with an increase and decrease of the RH, which indicates that water molecules are strongly trapped in the nanochannels and protons move through these narrow channels. The proton conductivity of the Ag NP-loaded PVCa-b-PSt thin film had higher conductivity than the PVCa-b-PSt thin film for the entire range of RHs examined. The proton conductivity of the Ag NPloaded PVCa-b-PSt thin film increased over 70% RH and reached 10−4 S/cm at 95% RH, and hysteresis in the conductivity was also observed for this film. The proton conductivity of the Ag NP-loaded PVCa-b-PSt thin film was 1 order of magnitude higher than that of the PVCa-b-PSt thin film. It should be noted that there was no obvious size change of the nanochannels before and after water uptake, as observed by GI-SAXS (parts b and c of Figure 3). The scattering position and d-spacing between the lamellae layers did not change at 90% RH. This result implies that water was incorporated into the nanochannels of PVCa phase and densely packed water layers were formed. To investigate the chemistry inside of the nanochannels, Fourier transform-infrared attenuated total reflectance (FT-IR ATR) spectra were measured for the PVCa-b-PSt and Ag NPloaded PVCa-b-PSt thin films before and after immersion in water (Figure 5a−d). Table 1 summarizes the peak assignments for the PVCa-b-PSt film. Under dry conditions, absorption attributed to the aromatic rings of styrene and catechol (δ C−H(styrene) 700 cm −1 , δ C−H(catechol) 780−960 cm −1 , δC−H(aromatic) 1 110 cm−1, νC−C 1 400−1 600 cm−1), the C−O vibration of catechol (νC−O 1 280 cm−1), and free (νO−H 3 480−3 600 cm−1) and hydrogen-bonded O−H groups (δO−H 642 cm−1 and νO−H 3 125−3 480 cm−1) are clearly observed (Figure 5a and c). However, after water uptake, a strong peak attributed to catechol C−H (δC−H(catechol) 780−960
Table 1. Assignment of FT-IR ATR Spectral Bands for PVCa-b-PSt Film notation
wavenumber (cm−1)
assignment
(i) (ii) (iii) (iv) (v) (vi) (vii) (viii) (ix) (x) (xi)
642 700 780−960 1110 1280 1360 1400−1600 2800−3000 3015−3120 3125−3480 3480−3600
δ(O−H)out‑of‑plane(hydrogen bonding) δ(C−H)styrene δ(C−H)catechol δ(C−H)out‑of‑plane ν(C−O) δ(O−H)in‑plane ν(C−C)ring ν(C−H)main chain ν(C−H)aromatic ν(O−H)intermol. hydrogen bonding ν(O−H)free
cm−1) and phenolic C−O vibration at 1 280 cm−1 were observed (Figure 5b and d). Furthermore, broad absorption attributed to free O−H groups (νO−H 3 480−3 600 cm−1) was significantly decreased. These results indicate that water molecules are adsorbed into the catechol-rich nanochannels of PVCa-b-PSt, and hydrogen bonds are formed among the water molecules. Efficient proton conduction requires hydrogen bonding among water molecules; therefore, hydrogen-bond formation during water uptake promotes proton conduction. The change in the proton conductivity of the Ag NP-loaded PVCa-b-PSt film was similar to that for the PVCa-b-PSt film (Figure 4). However, the actual proton conductivity of the Ag NP-loaded PVCa-b-PSt film was 10 times higher than that of PVCa-b-PSt film for the entire range of RH examined at 25 °C. Because the catechol group is a weak acid, this value is high enough when compared with the proton conductivity of conventional phenol groups. Figure 6 shows the weight changes of the PVCa-b-PSt and Ag NP-loaded PVCa-b-PSt films due to water uptake with a change in the external RH as measured with the QCM. The water uptake into the Ag NP-loaded E
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Figure 6. Water uptake of (a) PVCa-b-PSt and (b) Ag NP-loaded PVCa-b-PSt films with change in the RH, as measured with the QCM.
PVCa-b-PSt film increased; the weight change reached 6.0% at 95% RH (Figure 6b), which was ∼20% higher than that of the PVCa-b-PSt film (Figure 6a). Moreover, the hysteresis of water uptake was greater in the Ag NP-loaded PVCa-b-PSt film than in the PVCa-b-PSt film. From the FT-IR ATR spectra for the PVCa-b-PSt film, peaks attributed to δ(C−H)styrene and δ(C− H)catechol are clearly observed at 700 and 780−960 cm−1, respectively (parts b and d of Figure 5). Furthermore, broad absorption attributed to free and hydrogen-bonded O−H groups were observed ranging from 3 125 to 3 600 cm−1 at dry conditions (parts a and c of Figure 5); however, only a broad peak attributed to hydrogen-bonded O−H groups was observed for the Ag NP-loaded PVCa-b-PSt film after water uptake (parts b and d of Figure 5). These results indicate that the incorporation of Ag NPs induced the stronger trapping of water molecules than the original PVCa-b-PSt film. In addition, densely packed hydrogen bonding among the water molecules and hydroxyl groups of catechols enhances the proton conductivity. Benavides et al. suggested that the incorporation of NPs enhances the water uptake of films and the change in water content may increase the proton conductivity.27 This is one reason for the higher proton conductivity of the Ag NPloaded PVCa-b-PSt film than the PVCa-b-PSt film. Note that, even at 70% RH, the proton conductivity of the Ag NP-loaded PVCa-b-PSt film was higher than that of the PVCa-b-PSt film. Yaroslavtsev and co-workers reported that the proton conductivity of perfluorinated polymers with sulfonate groups (MF-4SC) containing a high content of silver or copper NPs increased with the amount of metal NPs.28 They suggested the increase in proton conductivity may be due to a decrease in channel size due to excluded volume effect of the NPs.29 However, from our TEM observations and GI-SAXS results, no obvious change in size of the channels of the PVCa-b-PSt film was observed after incorporation of the Ag NPs (Figure 1b and c). Another possible reason for the enhancement of proton conductivity is the catalytic properties of Ag NPs. Ag NPs act as oxidation or reduction catalysts.30−32 For example, Kundu et al. reported that SERS-active Ag NPs exhibit efficient catalytic activity for the reduction of 4-nitrophenol.33 The catalyst reduces the activation energies for proton exchange; therefore, the proton conductivity may also be increased. We also measured SERS spectra of the Ag NP-loaded PVCa-b-PSt film during the proton conductivity measurement, and the results
strongly suggest that the catalytic activity of the Ag NPs enhances the proton conductivity (see Figure S4, Supporting Information). The catalytic process was not dependent on the amount of water; therefore, it was concluded that the catalytic properties of the Ag NPs may play a significant role in the enhancement of proton conductivity. Note that, because the number of Ag NPs introduced into PVCa phase was not so high (up to 10% in volume) and these Ag NPs did not aggregate after measurement of proton conductivities, an electrical short did not occur. We also simulated the effect of electrical conduction of Ag NPs on impedance (see Supporting Information, Figure S6). A simple parallel equivalent circuit with one resistance and one capacitance successfully fit the experimental data of the Ag NPloaded PVCa-b-PSt thin film. The obtained resistance was found to be 10 times lower than that of original PVCa-b-PSt film. For the possibility of the electronic conduction, because the resistivity of Ag NPs is usually 8-fold lower than the estimated value,34 the resistance of the Ag NP-loaded PVCa-b-PSt thin film should be much lower if electronic conduction of Ag NPs contributes to the impedance data. The response frequency region by the electronic conduction is quite different from that by the proton conduction. Therefore, electrical conduction does not affect strongly the impedance data. For the possibility of other ionic carriers except for protons, the obtained capacitance can be regarded as one simple component. This component depends on the RH. Therefore, the observed impedance can be derived from the proton conduction, which is supported by the water uptake. Moreover, glass transition temperature is relatively high and lamellae structure is structurally stable in the whole RH regions. Thus, a segmental motion by the polymer backbone is not so effective for the enhancement of the other conduction except for protons. The response frequency region by the other ionic carriers except for protons might be different from that by the proton carrier. For these reasons, we concluded that formation of Ag NPs surely contributes to enhancement of proton conductivity. Detailed experimentation to clarify the mechanism for proton conductivity enhancement is still in progress. F
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4. CONCLUSION The proton conductivity of PVCa-b-PSt film was increased 10fold by the addition of Ag NPs into the proton conduction channels filled with catechol moieties. GI-SAXS and in situ SERS spectra were measured during the proton conduction experiment, and no obvious increase in the proton conduction channels was observed, but this conductivity enhancement may originate from the catalytic properties of the Ag NPs. There are a few reports relating the incorporation of inorganic NPs into the nanochannels of ionic conductive materials to the ionic conductivity,35 but the reports in the literature used these NPs as aligners for the nanochannels; therefore, our concept is quite new compared with the previous research. Although there are more detailed experiments, including the use of different metal sources, proton donor moieties, and phase-separated structures, the present results provide a guiding principle on how to create a new class of effective proton conductive films by embedding catalytic NPs into the proton conductive channels.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b02521. Synthesis of PVCa-b-PSt, preparation of its film, different phase-separated structures by casting chloroform/methanol solution, and in situ SERS spectra of a Agembedded PVCa-b-PSt film (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
J.M. measured in situ SERS spectra. M.H. and S.N. measured GI-SAXS and QCM. Y.M. measured SERS spectra. Y.N. measured proton conductivities. H.Y. prepared the samples, wrote the manuscript, and directed the entire project. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work has been partially supported by Grant-in-Aid for Young Researchers (A) (no. 26708025) and Grant-in-Aid for Challenging Exploratory Research (no. 16K14071), JSPS KAKENHI, Japan. J.M. thanks the support by JSPS KAKENHI Grant nos. JP26286010 and JP15H00720. H.Y. thank Minori Suzuki, AIMR, Tohoku University, for helping with TEM observation. H.Y. thanks PRESTO, JST for partially supporting this work.
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REFERENCES
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