Electrophoretic Deposition and Characterization of Transparent

Jan 20, 2014 - We fabricated nanocomposite films from an aqueous suspension of red-emitting YVO4:Bi3+,Eu3+ nanoparticles (hydrodynamic size: 22 ± 6 n...
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Electrophoretic Deposition and Characterization of Transparent Nanocomposite Films of YVO4:Bi3+,Eu3+ Nanophosphor and SiliconeModified Acrylic Resin Yoshiki Iso, Satoru Takeshita,* and Tetsuhiko Isobe* Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan S Supporting Information *

ABSTRACT: We fabricated nanocomposite films from an aqueous suspension of red-emitting YVO4:Bi3+,Eu3+ nanoparticles (hydrodynamic size: 22 ± 6 nm) and siliconemodified acrylic resin nanoparticles of (60 ± 15 nm) by electrophoretic deposition under application of a constant voltage. The nanocomposite films were formed from these negatively charged nanoparticles on ITO-coated glass substrates on the anodic side at the volume ratio of nanophosphor:resin ∼ 40:60. According to transmission electron microscopy observations, the YVO4:Bi3+,Eu3+ nanoparticles are well-dispersed around the resin nanoparticles. The fabricated films are transparent to the naked eye under white light because both nanoparticles show no absorption and low light scattering in the visible region. A silicone-modified acrylic resin film without the nanophosphor exhibits no absorption in the UV region (>300.0 nm). However, the fabricated nanocomposite films show near-UV absorption owing to the interband transition between the valence band and the conduction band of the YVO4:Bi3+,Eu3+ nanoparticles. A sharp emission peak corresponding to the 5D0 → 7F2 transition of Eu3+ is observed at 619.5 nm, under 365.0 nm excitation, for each nanocomposite film. The photoluminescence intensity at 619.5 nm under 365.0 nm excitation is proportional to 1−10−OD (OD: optical density at 365.0 nm) for film thicknesses ≤6 μm. This is attributed to the low light scattering from both nanoparticles in the nanocomposite film. Conversely, the observed photoluminescence intensity for film thicknesses >6 μm is higher than the value expected from the proportional relationship. This suggests that the excitation of the nanophosphors efficiently occurs due to multiple scattering of excitation light.

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INTRODUCTION Phosphor-dispersed films have been exploited for various optical devices. Organic luminescent molecules and inorganic bulk luminescent particles are used in these films.1,2 However, organic luminescent molecules decompose readily under UV light, while inorganic bulk luminescent particles are opaque because of light scattering. In contrast, inorganic nanophosphors have the following two advantages: (i) better longterm durability compared with that of organic molecules and (ii) higher transparency because of their low light scattering intensity (which is proportional to the sixth power of the particle size, on the basis of Rayleigh’s law).3 Nanophosphordispersed films have been considered for applications in displays, LEDs, and solar cells4−9 and have been fabricated by various wet processes, e.g., dip-coating,10 spin-coating,11 inkjet printing,12 screen printing,13 soft lithography,14,15 electrospinning,16,17 layer-by-layer method,18 Langmuir−Blodgett method,19 and electrophoretic deposition (EPD). EPD enables us to produce a film on an electrode in a suspension of charged particles under the application of an electric field.20−23 Migration of particles in a solution is complexly influenced by diffusion and convection as well as © 2014 American Chemical Society

electrophoresis. The electrode on which particles deposit is determined by the sign of their electric charge. EPD films have the following advantages: (i) films can be fabricated from a variety of charged materials, (ii) films are homogeneously coated on substrates with various shapes and surface roughnesses, (iii) film thickness is controlled by applied voltage and time, and (iv) micrometer-thick films can be fabricated from a suspension of well-dispersed nanoparticles, even though their concentration is low. Adjustment of concentration and surface potential of nanoparticles in a suspension is required to stabilize their dispersion without aggregation; thus, concentrations and potentials must be optimized to be able to fabricate a thick film from a colloidal solution of well-dispersed nanoparticles at low concentration. For example, ZnO film was fabricated on the cathode by EPD of ZnO nanoparticles in isopropyl alcohol.24 Moreover, if inorganic nanoparticles and organic polymers have the same sign of electric charge, organic−inorganic nanocomposite films are easily formed by simultaneous EPD on the Received: December 7, 2013 Revised: January 17, 2014 Published: January 20, 2014 1465

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same electrode.25 Gorelikov et al. fabricated nanocomposite films on the anode through EPD of CdS or CdTe nanoparticles and poly(vinyl acetate-co-crotonic acid), since both components were negatively charged in water.26 In particular, use of aqueous suspensions for EPD has become more important to reduce environmental impact.27,28 YVO4:Bi3+,Eu3+ phosphor, in which Bi3+ and Eu3+ ions substitute for some of the Y3+ ions, emits red light under nearUV light excitation. YVO4:Bi3+,Eu3+ nanophosphor is a promising candidate for near-UV LEDs and spectral downshifters for solar cells. We have synthesized YVO4:Bi3+,Eu3+ nanoparticles by a coprecipitation method via a citrate precursor and investigated their properties.29−33 A colloidal solution and a film of the YVO4:Bi3+,Eu3+ nanoparticles are transparent to the naked eye under white light because they show no absorption and low light scattering in the visible region.34 In addition, the nanophosphor has a negatively charged surface, derived from adsorbed citrate anions, and hence is well-dispersed in neutral and basic water. We therefore could fabricate transparent nanocomposite films of YVO4:Bi3+,Eu3+ nanoparticles and hydrophilic polyurethane resin because of their good affinity. Furthermore, we investigated the feasibility of using the composites as spectral downshifters for a monocrystalline silicon solar cell module.35 However, these films were degraded by photoredox reactions between the nanophosphor and the resin.36 V5+ of the nanophosphor was photoreduced to V4+ by the polyurethane resin under UV irradiation, thus decreasing the photoluminescence (PL) intensity. At the same time, the resin was decomposed through photooxidation by the nanophosphor. We also fabricated transparent nanocomposites by a sol−gel method using tetramethylammonium silicate as a silica source, since silica exhibits good mechanical strength and chemical stability.37−39 We confirmed the nanocomposites of silica containing the nanophosphors have higher photostability compared with the nanocomposite film using polyurethane resin as a matrix. This indicates that siloxane bonds play a significant role in high photostability. However, the silica matrix lacks flexibility. Flexible nanophosphor-dispersed films are needed for application in flexible optical devices, e.g., thin film solar cells. In the present work, we investigate hybridization of the YVO4:Bi3+,Eu3+ nanoparticles to silicone-modified acrylic resin. This resin is composed of acrylic resin bridged with siloxane bonds and is expected to be relatively high photostability.40 We characterize the structural and optical properties of nanocomposite EPD films containing both YVO4:Bi3+,Eu3+ nanoparticles and silicone-modified acrylic resin nanoparticles.

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shown in Figure S1 of the Supporting Information. The YVO4:Bi3+,Eu3+ nanoparticle powder sample was obtained by drying the nanophosphor paste at 30 °C for 24 h. Its composition, as determined by X-ray fluorescence (XRF) analysis, was Y:Bi:Eu = 56.8:7.3:35.9 (atomic ratio). The nanophosphor paste (2.5 g) was diluted with 26.5 g of ultrapure water under vigorous stirring followed by ultrasonication. Then, 1.0 g of basic aqueous emulsion of silicone-modified acrylic resin (35 wt %) was added to the nanophosphor colloidal solution under vigorous stirring, followed by ultrasonication. The pH value of the prepared suspension was 9.4. The volume ratio of nanophosphor:resin in the suspension was estimated to be 40:60. An ITO-coated glass substrate (25 mm × 50 mm × 1 mmt, 50 Ω/□) and a stainless steel plate (25 mm × 50 mm × 1 mmt, SUS-304) were used as the anode and cathode, respectively. Both electrodes were cleaned in acetone under ultrasonication and then washed with ultrapure water. A nanocomposite film was deposited on a 25 × 25 mm2 area of the anode by application of a constant voltage of 3.0 V for 1−180 min, as depicted in Figure 1. The distance between both electrodes was

Figure 1. Illustration of the film deposition on an ITO-coated glass substrate connected to a direct current (D.C.) power supply. kept at 10 mm using an insulating spacer. The suspension was not stirred during deposition because the influence of gravity is negligible for EPD of well-dispersed nanoparticles.41 After deposition, the asprepared film samples were gently washed with ultrapure water to remove extra suspension, and dried at 120 °C for 15 min. A nanocomposite film of polyurethane resin and the nanophosphor of 8 μm in thickness was fabricated on a soda-lime glass substrate, as described in our previous work,35 for comparison on photostability. Characterization. Electron microscope images were captured by a TEM (Tecnai 12, FEI) and a scanning electron microscope (SEM, S4700, Hitachi and Sirion, FEI). A nanocomposite film sample for TEM observation was deposited at 3.0 V for 60 s on a copper microgrid attached to the ITO-coated glass substrate using carbon adhesive tape and dried at 120 °C for 15 min. The atomic composition of the YVO4 :Bi3+,Eu3+ nanoparticle powder was determined by the fundamental parameter method on an XRF analyzer (ZSX mini II, Rigaku). Concentrations of YVO4:Bi3+,Eu3+ nanoparticles for nanocomposite films were also determined by XRF analysis. Hydrodynamic size distributions were measured with a dynamic light scattering (DLS) instrument (HPPS, Malvern Instruments). Zeta-potential profiles were measured with a zeta-potential analyzer (Zetasizer Nano Z, Malvern Instruments). For the measurements of both colloidal properties, pH values of samples were adjusted to 9.4 by addition of 0.1 M aqueous NaOH. Fourier transform infrared (FT-IR) absorption spectra were measured in pressed KBr disks on a spectrometer (FT/IR-4200, JASCO). Film thicknesses were measured with a micrometer. Optical microscope images were captured by an optical microscope (Eclipse E600, Nikon) equipped with a digital CCD camera (Evolution MP, Media Cybernetics). The nitrogen adsorption isotherm at 77 K for the YVO4:Bi3+,Eu3+ nanoparticle powder was measured with an automatic surface area analyzer (Tristar II 3200, Micromeritics) to determine the

EXPERIMENTAL SECTION

Fabrication of Nanocomposite Film Samples. The detailed preparation procedure for an aqueous paste of YVO4:Bi3+,Eu3+ nanoparticles (37.6 wt %) is described in the Supporting Information. An aqueous solution of Y(CH3COO)3 and Eu(CH3COO)3 and an ethylene glycol solution of Bi(NO3)3 were mixed with an aqueous solution of sodium citrate. An aqueous solution of Na3VO4 at pH 12.5 was added to the prepared white suspension. The mixture was adjusted to pH 9.0 by adding aqueous NaOH and aged at 85 °C for 1 h. After cooling to room temperature, a paste was collected by centrifugation and diluted with deionized water; the diluted paste was then hydrothermally treated at 130 °C for 6 h in an autoclave. The resulting colloidal solution was washed with deionized water and centrifuged to obtain the aqueous paste of YVO4:Bi3+,Eu3+ nanoparticles. Their transmission electron microscope (TEM) image is 1466

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BET specific surface area; the nanophosphor powder was heated at 100 °C for 1 h under reduced pressure before the measurement. Transmission spectra were measured on a UV/vis/near-IR optical absorption spectrometer (V-570, JASCO). PL and its excitation (PLE) spectra were measured on a fluorescence spectrometer (FP-6500, JASCO) equipped with an integrating sphere (ISF-513, JASCO); the emitted light was detected on the substrate side, and the excitation light was vertically irradiated onto the deposited film side. The spectral response was calibrated using an ethylene glycol solution of Rhodamine B (5.5 g L−1) and a standard light source (ESC-333, JASCO). Changes in PL intensity during UV light irradiation were also measured with the same fluorescence spectrometer.

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RESULTS AND DISCUSSION Colloidal Properties of YVO4:Bi3+,Eu3+ Nanoparticles and Silicone-Modified Acrylic Resin Nanoparticles. Figure 2A shows hydrodynamic size distributions of the

Figure 3. Photographs of the 20 μm thick nanocomposite film deposited for 30 min on the ITO-coated glass substrate: under white light (top) and 302 nm UV light (bottom).

microscope. However, micrometer-sized bubbles were observed in the films at deposition times >5 min (see Figure S3). The gas that formed the bubbles was most probably generated by electrolysis of water on the anode during voltage application. Figure 4 shows SEM and TEM images of a nanocomposite film. Judging from the cross-sectional SEM image (Figure 4a), the nanocomposite film was uniformly coated onto the substrate. Particles with a mean size of 68 ± 17 nm were observed in the surface SEM image (Figure 4b). This size is close to the mean hydrodynamic size of the resin nanoparticles measured by DLS. According to the TEM observation (Figure

Figure 2. (A) Hydrodynamic size distributions and (B) zeta-potential profiles of (a) YVO4:Bi3+,Eu3+ nanoparticles and (b) silicone-modified acrylic resin nanoparticles in water at pH 9.4.

YVO4:Bi3+,Eu3+ nanoparticles and the silicone-modified acrylic resin nanoparticles dispersed in water at pH 9.4. The mean sizes were 22 ± 6 and 60 ± 15 nm, respectively. Figure 2B shows zeta-potential profiles of both particles in water at pH 9.4. Their mean zeta potentials were −20 ± 15 mV for the YVO4:Bi3+,Eu3+ nanoparticles and −62 ± 14 mV for the resin nanoparticles. The aqueous suspension containing both nanoparticles was stable against sedimentation at pH 9.4 over one year. This was because of the electrostatic repulsion derived from their negatively charged surfaces. Observation of Deposited Films. Codeposition of the YVO4:Bi3+,Eu3+ nanoparticles and the resin nanoparticles onto an ITO-coated glass substrate was confirmed by FT-IR analysis (see Figure S2 and Table S1). Absorption bands derived from the nanophosphor and the resin are observed in the FT-IR spectrum of a nanocomposite film. Fabricated nanocomposite film samples were transparent to the naked eye under white light, as shown in Figure 3. When the deposition time was ≤5 min, no bubbles were observed in the films under an optical

Figure 4. Electron microscope images. SEM images of (a) cross section and (b) surface of the nanocomposite film deposited for 5 min. (c) TEM image of a nanocomposite film. 1467

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4c), dispersions of smaller dark particles and larger bright particles were observed in the film. From the specific surface area of the YVO4:Bi3+,Eu3+ nanoparticle powder, 164 m2 g−1, the primary particle size of the nanophosphor is estimated to be 7.6 nm, assuming a spherical shape. From this result, the smaller dark particles are identified as the YVO4:Bi3+,Eu3+ nanoparticles. The size of the larger bright particles is close to the mean size of the particles observed in the SEM image. These results reveal that the nanocomposite film has a nanostructure where the nanophosphors are dispersed around the resin nanoparticles. Growth of Nanocomposite Films. Film thickness and concentration of YVO4:Bi3+,Eu3+ nanoparticles for the nanocomposite films as a function of deposition time are shown in Figure 5. The film thickness increases with increasing

Figure 6. Transmission spectra of the nanocomposite film samples with different film thicknesses. A bare ITO-coated glass substrate and a blank resin film sample without nanophosphor (3 μm) are also shown.

sample is higher than that of the bare ITO-coated glass substrate in the visible region because the resin film has a lower refractive index than the ITO, and thus the resin film coating decreases the overall reflectance. Film samples containing nanophosphor exhibit lower transmittance in the near-UV region than that of the blank sample. This is attributed to absorption of the nanophosphor. The absorption peak of YVO4:Bi3+,Eu3+ in the near-UV region has been assigned to the charge transfer (CT) transition of Bi3+ → V5+.45 On the other hand, according to the recent reports of calculated results,46−48 YVO4:Bi3+,Eu3+ has the valence band, which is mainly made up of bonding O2− 2p and Bi3+ 6s orbitals, and the conduction band, which is mainly made up of V5+ 3d, antibonding O2− 2p, and Bi3+ 6p orbitals. The absorption therefore would be caused by the interband transition between the valence and conduction bands. Moreover, this absorption peak might be overlaid with the absorption peak due to the CT transition of O2− → Eu3+ around 260 nm.49 Conversely, the 3 μm thick film sample shows high transmittance (over 80%) in the visible region because of the lack of absorption and the low light scattering intensity of the YVO4:Bi3+,Eu3+ nanoparticles and the resin nanoparticles. Increasing film thickness decreases the transmittance in both the visible and near-UV regions. This might be attributed to light scattering from micrometer-sized bubbles and relatively larger resin nanoparticles. The light scattering intensity from the resin nanoparticles (some of which are >100 nm in size, as shown by DLS and SEM analyses) probably becomes more prominent for thicker films. Photoluminescence Properties of Nanocomposite Films. Figure 7 shows PL and PLE spectra of nanocomposite film samples. YVO4:Bi3+,Eu3+ emits red light caused by the f−f transitions of the Eu3+ through energy transfer from the exciton under irradiation with near-UV light. A broad and strong excitation peak is observed in the near-UV region of the PLE spectra measured at the 619.5 nm emission corresponding to the 5D0 → 7F2 transition of Eu3+. This peak is caused by the above-mentioned interband transition of YVO4:Bi3+,Eu3+ followed by the energy transfer to Eu3+ and the CT transition of O2− → Eu3+. A weak PLE peak corresponding to the 7F0 → 5 L6 transition of Eu3+ is also observed at 396.7 nm. Sharp emission peaks corresponding to the 5D0 → 7FJ (J = 1, 2, 3, 4) transitions of Eu3+ are observed in the PL spectra measured under 365.0 nm excitation. The Eu3+ ion substitutes for the 8fold-coordinated Y3+ ion with D2d symmetry in YVO4 crystal. The non-centrosymmetric environment of Eu3+ ion causes the most intense emission peak at 619.5 nm corresponding to the 5 D0 → 7F2 transition among the 5D0 → 7FJ (J = 1, 2, 3, 4)

Figure 5. Changes in film thickness and volume fraction of nanophosphors in the nanocomposite film with deposition time.

deposition time. The gradual decrease in growth rate seen here has also been reported previously for EPD under application of constant voltage.20,42−44 Nanophosphor concentrations for the nanocomposite films were ∼40 vol %, as also shown in Figure 5; this is close to the nominal volume ratio of nanophosphor:resin, 40:60, in the prepared suspension. Two kinds of particles would be deposited on the substrate competitively. The electrophoretic mobility, μ, of the nanoparticle in the dispersant is related to the zeta potential, ζ, on the basis of the Smoluchowski equation: εε μ = r 0ζ η (1) where εr and ε0 are the relative dielectric constant of the dispersant and the vacuum permittivity, respectively, and η is the dispersant viscosity. From the mean zeta potentials of both nanoparticles and this equation, the electrophoretic mobility of the resin nanoparticle is ∼3 times larger than that of the nanophosphor. Accordingly, if particle kinetics were based only on electrophoresis, the deposition of the larger resin nanoparticle would be faster than that of the smaller nanophosphor under the same electric field. This would make the actual nanophosphor concentration in the deposited film less than 40 vol %. The fact that the nominal volume fractions of the prepared suspension are nearly equal to those found in the actual deposited films indicates that the film composition is influenced by other kinetic effects, e.g., diffusion and convection, in addition to electrophoresis. Transparency of Nanocomposite Films. Figure 6 shows transmission spectra of nanocomposite film samples. A resin film without nanophosphor has no absorption in the near-UV and visible regions. The transmittance of the blank resin film 1468

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Figure 8. Relation between net PL intensity and 1 − 10−OD for the nanocomposite films, where OD is the value at 365.0 nm. λex = 365.0 nm; λem = 619.5 nm.

Figure 7. PL and PLE spectra of the nanocomposite film samples with different film thicknesses and a bare ITO-coated glass substrate. λex = 365.0 nm; λem = 619.5 nm.

transitions.50 The differences in optical density and PL intensity between nanocomposite film samples and a bare ITO-coated glass substrate are termed the net optical density (OD) and net PL intensity, respectively. When light scattering is negligible and OD depends on absorbance only, the emission intensity F is described by the equation

Figure 9. Changes in PL intensity with irradiation time of UV light. The nanocomposite film of (a) silicone-modified acrylic resin and (b) polyurethane resin containing the nanophosphors. λex = 300.0 nm; λem = 619.5 nm.

coordinating to the surface of nanoparticles. The decrease in the PL intensity is attributed to photoreduction of V5+ to V4+ by citrate anions combined with the formation of oxygen vacancies in YVO4.31 The recovery of PL intensity probably originates from photooxidation of the nanoparticles by oxygen in air. In contrast, the PL intensity for the nanocomposite film using polyurethane resin as a matrix falls to 9.2% of the initial intensity after irradiation for 6 h, as shown in Figure 9b. From this comparison, the silicone-modified acrylic resin is more photochemically stable than the polyurethane resin.

⎛ ln 10 F = (1 − 10−OD)ΦI0 = ln 10OD⎜1 − OD 2! ⎝ +

⎞ (ln 10)2 OD2 − ...⎟ΦI0 3! ⎠

(2)

where Φ is the PL quantum efficiency and I0 is the intensity of excitation light. If OD is sufficiently low, the following proportional relationship is approximately achieved: F ∝ ln 10OD ≈ 2.303OD

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(3)

CONCLUSIONS AND OUTLOOK We fabricated nanocomposite films from negatively charged YVO4:Bi3+,Eu3+ nanoparticles and silicone-modified acrylic resin nanoparticles by electrophoretic deposition under application of a constant voltage. Film thickness increased with increasing deposition time. Good dispersion of the nanophosphors around the resin nanoparticles was confirmed by transmission electron microscopy. The fabricated nanocomposite films were transparent to the naked eye under white light because the constituent nanoparticles show no absorption and low light scattering in the visible region. An emission peak at 619.5 nm, corresponding to the 5D0 → 7F2 transition of Eu3+, was observed in the photoluminescence spectra for the nanocomposite films under 365.0 nm near-UV light excitation through the interband transition between the valence band and the conduction band of YVO4:Bi3+,Eu3+. The PL intensity at 619.5 nm under 365.0 nm excitation was proportional to 1 − 10−OD for film thicknesses ≤6 μm, where OD is the optical density at 365.0 nm, demonstrating the negligibly low light

However, this should not be applied to the results of our nanocomposite films because their OD might be too high. The net PL intensity at 619.5 nm (5D0 → 7F2) under 365.0 nm excitation is plotted as a function of 1−10−OD (Figure 8), where OD is the value at 365.0 nm, and a linear relationship is found at 1 − 10−OD ≤ ∼0.3. This is attributed to the negligibly low light scattering intensity of the nanoparticles in the nanocomposite film. At 1 − 10−OD > ∼0.3, the observed PL intensity is higher than the value expected from the linear relationship. This suggests that the excitation of the nanophosphors efficiently occurs due to multiple scattering of excitation light for film thicknesses >6 μm. We characterized the photostability of the EPD film (film thickness: 6 μm) under continuous irradiation of 300.0 nm light for 6 h, as shown in Figure 9a. Its PL intensity decreases to 27.4% of the initial intensity, followed by recovering to 50.0% after irradiation for 6 h. The observed change in PL intensity can be explained by photochemical reactions with citrate anions 1469

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Notes

scattering from both types of nanoparticles in the nanocomposite film. Conversely, the observed PL intensity for film thicknesses >6 μm is higher than the value expected from the proportional relationship. This suggests that the excitation of the nanophosphors efficiently occurs due to multiple scattering of excitation light. Furthermore, the nanocomposite film using the silicone-modified acrylic resin was more photochemically stable than the nanocomposite film using the polyurethane resin. Since various conducting substrates can be used for electrophoretic deposition, we demonstrate good flexibility of a nanocomposite EPD film coated onto an ITO-coated poly(ethylene terephthalate) (PET) sheet, as shown in Figure 10. Such a flexible wavelength-conversion film could be applied in flexible optical devices; e.g., it could be used as a spectral downshifter in thin film solar cells.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank SINLOIHI Co., Ltd., for preparation of the YVO4:Bi3+,Eu3+ nanoparticle paste. We also thank DAI NIPPON TORYO Co., Ltd., for donating the silicone-modified acrylic resin emulsion.

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Figure 10. Photographs of the 2 μm thick nanocomposite film coated on the ITO-coated PET sheet under white light (top) and 302 nm UV light irradiation (bottom). The sample was bended for this demonstration.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the preparation procedure of YVO4:Bi3+,Eu3+ nanoparticles; TEM image of YVO4:Bi3+,Eu3+ nanoparticles (Figure S1); FT-IR spectra of the nanocomposite film and blank films (Figure S2); peak assignments of the FT-IR spectra (Table S1); optical microscope images of the nanocomposite films (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; Tel +81 45 566 1531; Fax +81 45 566 1551 (S.T.). *E-mail [email protected]; Tel +81 45 566 1554; Fax +81 45 566 1551 (T.I.). 1470

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dx.doi.org/10.1021/la404707r | Langmuir 2014, 30, 1465−1471