Fabrication of Silica Nanoparticle Monolayer Arrays Using an Anodic

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Fabrication of Silica Nanoparticle Monolayer Arrays Using an Anodic Aluminum Oxide Template Kazutoshi Sekiguchi,†,‡ Takayuki Nakanishi,*,† Hiroyo Segawa,†,§ and Atsuo Yasumori*,† †

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Department of Materials Science and Technology, Faculty of Industrial Science and Technology, Tokyo University of Science, 6-3-1 Niijuku, Katsushika-ku, Tokyo 125-8585, Japan ‡ Materials Research Laboratories, Nissan Chemical Corporation, 488-6 Suzumi-cho, Funabashi, Chiba 274-0052, Japan § National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan S Supporting Information *

ABSTRACT: Non-close-packed (NCP) silica nanoparticle monolayer arrays (SNMA) on ordered porous anodic aluminum oxide (AAO) templates were fabricated for the first time by a novel two-step spin-coating technique. The obtained NCP-SNMA-AAO was composed of silica nanoparticles (average primary particle size of 440 nm) and well-organized nanopores on the AAO substrates. NCP-SNMA-AAO with a supporting ratio of 87% silica nanoparticles showed a hydrophilic surface (water contact angle of 51.0°), while the original AAO substrate shows a hydrophobic surface (water contact angle of 107.9°). The maximum coefficient of static friction was decreased by 29% (0.327 → 0.233). The coefficient of dynamic friction was also decreased by 20% (0.281 → 0.226). We found that controlling the silica supporting ratio using the two-step spin-coating technique is an effective approach for surface modification of an AAO substrate.

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INTRODUCTION Supporting nanoparticles on various substrates have been studied to realize unique functionalities. Nanoparticle monolayer arrays utilizing self-assembly of nanosized silica (SiO2) particle dispersions have interesting features in various material fields such as optics,1,2 photonics,3 and colloidal lithography.4 There are three typical types of silica nanoparticle monolayer arrays (SNMAs): a random structure,5−9 a close-packed structure,2−4,7−10 and an ordered non-close-packed (NCP) structure with regular particle spacing.11−14 Ordered NCPSNMA is expected to induce characteristic functionalities because of the control of the particle distance. Yan et al. reported the fabrication of NCP-SNMA by swelling or stretching of a dimethylpolysiloxane (PDMS) elastomer substrate.13 Xia et al. reported the fabrication of a desirable patterned silicon substrate with silica nanoparticles by an interferometric lithography technique to control the surface modification.11 Various ordered arrangements of silica nanoparticles (SNs) on substrates are strongly required as photonic material designs such as a reflectance and a structural color.1,2,6,7 As a novel technique for fabrication of ordered NCP-SNMA, an anodic aluminum oxide (AAO) substrate has received much attention as a nanotemplate. It is well known that the arrangement of AAO-nanosized pores can be controlled by electrochemical anodization conditions.15−18 For that reason, the AAO substrate is one of the famous nanotemplates to prepare multifunctional nanomaterials such as nanoparticle arrays19,20 and nanometal pillar arrays21 for various applications. Wang and co-workers reported the fabrication of NCP © XXXX American Chemical Society

silver nanoparticle monolayer arrays on an organized AAO substrate by an electrochemical synthesis for surface-enhanced Raman scattering (SERS).19 In addition, Gaponenko and coworkers achieved the fabrication of a multilayer of disordered SNs on an AAO substrate by an impregnation technique in the dispersion of SNs.5 An advantage of a method of supporting nanoparticles on substrates is that synthesis conditions of the nanoparticles are not limited by the properties of the substrates such as the chemical resistance and heat resistance. However, the direct loading method for the fabrication of NCP-SNMAAAO using the dispersion of SNs and its properties has not been investigated in detail. In this study, we investigated the optimal conditions for the fabrication of NCP-SNMA in which SNs were arranged uniformly on the surface of an AAO substrate, and we evaluated the surface physical properties of wettability and friction. Here, we propose an AAO substrate as a wellorganized template for fabricating NCP-SNMA by the spincoating technique (Figure 1a). The spin-coating method is a suitable technique for the fabrication of SNMA on various substrates other than an AAO substrate.3,7,8,11 Two-step spin coating enables fabrication of highly filled NCP-SNMA on substrates. Application of the two-step spin-coating technique for an AAO substrate is expected to enable fabrication of highly filled NCP-SNMA. Adjustments of the particle size of SNs and the pore distance of AAO are important for Received: July 9, 2019 Accepted: August 6, 2019

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DOI: 10.1021/acsomega.9b02114 ACS Omega XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematic illustration of the method for fabrication of SNMA on AAO by the spin-coating method. Examples of the spincoating conditions of (b) the one-step technique and (c) the two-step technique.

Figure 2. (a) Digital camera image, FESEM images of (b) low magnification and (c) high magnification of the top view, and (d) pore distance distribution of the AAO substrate.

fabrication of NCP-SNMA-AAO. Therefore, controlling the AAO nanostructure is also one of the key factors for the fabrication of NCP-SNMA-AAO. Consequently, highly filled NCP-SNMA on an AAO substrate was successfully fabricated by the two-step spincoating technique. The results showed that the AAO substrate can be used as a template for fabrication of NCP-SNMA without surface modification of the particles. Furthermore, the characteristic surface physical properties were changed by the silica supporting ratio on the AAO substrate. The NCP-SNMA obtained on the AAO substrate indicates the possibility of controlling the surface wettability (hydrophilicity/hydrophobicity) and the friction coefficient to improve surface physical properties of AAO substrates.

Figure 3. (a) FESEM image, (b) particle size distribution, and (c) digital camera image of an aqueous dispersion of silica nanoparticles.

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RESULTS AND DISCUSSION Preparation of an AAO Substrate and an Aqueous Dispersion of Silica Nanoparticles. An AAO film was fabricated on a high purity aluminum plate by two-step anodization in the area of the red solid box (approximately 1 × 3 cm) as shown in Figure 2a. Nanopores with a relatively uniform pore size distribution were observed on the AAO substrate surface by an FESEM as shown in Figure 2b,c. Figure 2d shows the pore distance distribution of the AAO substrate measured with an image analyzer. The center position of the nanopores of the AAO substrate was measured manually. The AAO substrate had an average pore distance of 550±81 nm and a coefficient of variation (CV) of 14.6%. Figure 3a shows an FESEM image of SNs in the aqueous dispersion, which is shown in Figure 3c. As shown in Figure 3a, nonporous and spherical particles with a narrow particle size distribution were observed. Figure 3b shows the particle size distribution in the FESEM image of the SNs measured with the image analyzer. The SNs had an average particle size of 440±20 nm and a CV of 4.6% with a narrow distribution. Fabrication of SNMA Supported on an AAO Substrate. To obtain the SNMA, the influence of the rotation speed by the one-step spin-coating method in the supported state of SNs on the AAO substrate was investigated. The change in the loading condition of the SNs on the AAO substrate with change in the spinning rate is shown in Figure 4. For samples prepared at 200 and 500 rpm, the SNs were

Figure 4. FESEM images of silica nanoparticles on AAO substrates fabricated by the one-step spin-coating technique at (a) 200 rpm, (b) 500 rpm, (c) 1000 rpm, (d) 2000 rpm, (e) 3000 rpm, and (f) before spin-coating with a silica concentration of 10 wt %. Illustrations of (g) a multilayer of silica nanoparticles, (h) a monolayer and a double layer of silica nanoparticles, and (i) a monolayer of lowly filled silica nanoparticles on AAO substrates.

arranged as accumulated highly filled multilayers on most of the surface of the AAO substrate as shown in Figure 4a,b, respectively. This self-assembled multilayered structure of SNs was formed by the interparticle capillary force of the SNs during the drying process as shown in Figure 4g as a schematic illustration model.22 For the samples prepared at 1000 and 2000 rpm, the SNs were arranged as a monolayer or a double B

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layer on the surface of the AAO substrate as shown in Figure 4c,d, respectively. It is possible that these morphologies were formed due to the nonuniform presence of the aqueous dispersion of SNs on the AAO substrate during spin coatings as shown in Figure 4h as a schematic illustration model. For the samples prepared at 3000 rpm, the SNs were arranged as a monolayer on the nanopores of the AAO substrate as shown in Figure 4e. As the rotation speed is increased, it is possible that the monolayer of SNs was formed due to the uniform presence of the aqueous dispersion of SNs on the AAO substrate during spin coating. As a result, we succeeded in obtaining uniform SNMA-AAO at 3000 rpm as shown in Figure 4i. The supporting ratio of the obtained SNMA-AAO at 3000 rpm was shown to be 28% by using the image analyzer. The rotation speed is one of the important parameters of spin coating for fabricating SNMA-AAO. We demonstrated fabrication of SNMA-AAO with a supporting ratio of 28% at 3000 rpm for 10 s by the onestep spin-coating method. Although SNMA-AAO was obtained, a supporting ratio of 28% was insufficient. Therefore, optimal conditions using the two-step technique were investigated to obtain highly filled SNMA-AAO. Ogi and coworkers reported that highly filled close-packed SNMA could be obtained on a sapphire (monocrystal aluminum oxide) substrate with a super smooth surface by changing the rotation speed during spin coating in two steps.7 The two-step spincoating technique consists of two processes. In the first step with the low rotation speed, the uniform and optimal thickness of the dispersion on the substrate is obtained by optimizing both the removal amount of the dispersion by the centrifugal force and the evaporation rate of the dispersion medium. As a result, the uniformity of self-assembly of the nanoparticles guided by immersion capillary forces is improved. In the second step with the high rotation speed, the nanoparticles near the surface of the substrate are attached on the substrate due to increase of surface capillary forces caused by the rapid evaporation of the dispersion medium. Application of the twostep spin-coating technique for the AAO substrate having the nanostructure is expected to enable fabrication of highly filled NCP-SNMA. In the experiments, the effects of the first-step rotation time and silica concentration when using the two-step spin-coating method were investigated. Changes in the SNs on the AAO substrate with changes in the first-step rotation time and silica concentration are shown in Figure 5. With first-step rotation times of 10 and 30 s and a silica concentration of 10 wt %, SNs were arranged as a monolayer on almost all of the surface of the AAO substrate as shown in Figure 5b,c, respectively. A schematic illustration model is shown in Figure 5j. The supporting ratios were 55 and 49%. The supporting ratio of SNMA-AAO was increased by approximately two times when the two-step spin-coating technique was used. This result showed that the two-step spin-coating technique is effective for AAO substrates with nanostructures as well as sapphire substrates with a super smooth surface. With first-step rotation times of 0 and 10 s and a silica concentration of 20 wt %, SNs were arranged as a double layer on almost all of the surface of the AAO substrate as shown in Figure 5e,f, respectively. A schematic illustration model is shown in Figure 5i. The formation of a double-layered structure of SNs was promoted by increasing the silica concentration in the dispersion. With a first-step rotation time of 60 s and silica concentrations of 10 and 20 wt %, SNs were partially arranged as double layers on the surface of the AAO substrate as shown

Figure 5. FESEM images of silica nanoparticles supported on AAO substrates fabricated by the two-step spin-coating technique with firststep rotation times of (a, e) 0 s, (b, f) 10 s, (c, g) 30 s, and (d, h) 60 s. Silica concentrations were (a−d) 10 wt % and (e−h) 20 wt %. Illustrations of (i) a monolayer and a double layer of silica nanoparticles, (j) a monolayer of half-filled silica nanoparticles, and (k) a monolayer of highly filled silica nanoparticles on AAO substrates.

in Figure 5d,h, respectively. A schematic illustration model is shown in Figure 5i. As the first-step rotation time was increased, a partial double-layered structure of SNs was formed by evaporating the dispersion medium of the SNs before the second-step spin-coating process. With a first-step rotation time of 30 s and a silica concentration of 20 wt %, SNs were arranged as a highly filled monolayer on almost all of the surfaces of the AAO substrate as shown in Figure 5g. A schematic illustration model is shown in Figure 5k. The supporting ratio of SNMA-AAO was 87%. A NCP structure of the obtained SNMA-AAO with a supporting ratio of 87% was observed as shown in Figure 6. The average particle distance of

Figure 6. FESEM images of (a) a top view and (b) a 3D view. (c) Schematic illustration of SNMA-AAO substrate.

the NCP-SNMA was shown to be 120±56 nm by using the image analyzer (see Figure S1). Additionally, the appearance of the obtained NCP-SNMA-AAO substrate slightly changed to a matte tone. A highly filled NCP-SNMA-AAO was obtained with a first-step rotation time of 30 s and a silica concentration of 20 wt %. By optimizing the rotation speed, rotation time, and silica concentration when using the two-step spin-coating method, we succeeded in the fabrication of a highly filled NCP-SNMA-AAO with a supporting ratio of 87% as shown in Figure 6c as a schematic illustration model. We also showed that not only could highly filled NCP-SNMA be obtained on the AAO substrate with a nanostructure as well as on sapphire substrates with a super smooth surface but also the SNs were preferentially supported on the nanopores of the AAO substrate. Mathur and co-workers reported the polystyrene nanoparticles were preferentially arranged in the nanosized shallow valleys of the corrugated Si substrates and suggested the ordering force caused by the surface nanoroughness of the substrates.23 Schmudde and co-workers reported the SNs having the nanoroughness surface were arranged as NCPC

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that express heterogeneous and homogeneous states, respectively. Apparent contact angles of a rough surface are given by the Wenzel equation (θW) and the Cassie−Baxter equation (θCB), respectively:

SNMA on the nanostructured Au substrates due to the increase of friction forces between the nanoparticles and substrates.24 It is believed that the SNs are preferentially arranged in the AAO nanopore due to the surface nanoroughness of the AAO substrates. Characterization of the SNMA-AAO Substrate. It is generally known that wettability changes due to the lotus effect caused by nanostructures on a solid surface.25−27 For example, Lee et al. reported change in the wettability by fabricating ordered nanostructures on a polystyrene substrate.25 Thus, it was expected that wettability on the SNMA-AAO substrate would be changed by varying the nanostructure. Therefore, the water contact angles of SNMA-AAO with different supporting ratios were investigated. Figure 7 shows the contact angles and optical images of 1 μL of DI water of SNMA-AAO substrates with different

cos θ W = r cos θY

(1)

cos θCB = f1 cos θ1 + f2 cos θ2 , f1 + f2 = 1

(2)

where r is the surface roughness factor, θY is the Young’s contact angle of a smooth surface, f1 and f 2 are ratio of two different materials, and θ1 and θ2 are each Young’s contact angle of two different materials, respectively.28,29 However, the Cassie−Baxter equation cannot express the heterogeneous surface composed of three different materials such as the SNMA-AAO substrates. Therefore, the contact angle for the heterogeneous surface composed of three different materials is defined by the expanded Cassie−Baxter equation (θECB): cos θECB = f1 cos θ1 + f2 cos θ2 + f3 cos θ3 , f1 + f2 + f3 (3) =1 According to the Cassie−Baxter (eq 2) and the measurement result by using the image analyzer as shown in Figure S2, a calculated contact angle of the obtained AAO substrate was 103°. The result was close to the measured contact angle of 107.3 ± 0.7°. On the other hand, according to the expanded Cassie−Baxter (eq 3) and measurement results by using the image analyzer as shown in Figure S3 and Table S1, calculated contact angles of the SNMA-AAO substrates with supporting ratios of 28, 49, 55, and 87% were 89, 81, 76, and 64°, respectively. The calculated contact angles of the SNMA-AAO substrates showed a different value from the measured contact angles of 107.3 ± 0.7, 103.5 ± 0.7, 107.7 ± 1.5, 99.2 ± 1.2, and 51.0 ± 3.0°, respectively. Figure 8 shows a schematic illustration of the water contact angle of the SNMA-AAO substrate with each supporting ratio

Figure 7. (a) Water contact angles on AAO substrates at different supporting ratios of silica nanoparticles on AAO nanopores. Circle solid and box symbols refer to the measured water contact angles and the calculated water contact angles, respectively. An error bar means one standard deviation (SD). (b)−(f) Optical images of DI water droplets on AAO substrates at different supporting ratios of silica nanoparticles.

supporting ratios. The water contact angle is an average value of three measurement results (n = 3). Error bars indicate one standard deviation (SD). The water contact angles with supporting ratios of 0, 28, 49, 55, and 87% were 107.3 ± 0.7, 103.5 ± 0.7, 107.7 ± 1.5, 99.2 ± 1.2, and 51.0 ± 3.0°, respectively. The water contact angle of the AAO substrate (with a supporting ratio of 0%) was hydrophobic with 107.3 ± 0.7°. The pore size of the AAO substrate of about 150−380 nm shows hydrophobicity due to the lotus effect.26 The produced AAO substrate showed the same hydrophobicity as that in the literature. SNMA-AAO substrates with supporting ratios of 28, 49, and 55% had hydrophobic surfaces similar to that of the AAO substrate. On the other hand, the surface of the highly filled NCP-SNMA-AAO substrate with a supporting ratio of 87% was changed from hydrophobic to hydrophilic with a water contact angle of 51.0 ± 3.0°. The hydrophilicity/hydrophobicity of the AAO substrate surface can be controlled by the supporting ratio of SNMA. It is well known that wettability of a rough surface can be described by the Wenzel model and the Cassie−Baxter model

Figure 8. Illustrations of silica nanoparticles on AAO nanopores at supporting ratios of (a) 0%, (b) 49%, and (c) 87%.

of SNs. The AAO substrate showed hydrophobicity due to the Cassie−Baxter state as shown in Figure 8a. On the other hand, the highly filled NCP-SNMA-AAO with a supporting ratio of 87% showed hydrophilicity. It is well known that the surfaces of SNs exhibit hydrophilicity due to silanol groups.30 Hydrophilicity of the highly filled NCP-SNMA-AAO probably occurred due to the silanol groups of SNs and changed from the Cassie−Baxter state to the Wenzel state as shown in Figure 8c. However, the half-filled SNMA-AAO showed the same hydrophobicity as the AAO substrate. The result indicated the surface state of the half-filled SNMA-AAO maintained the Cassie−Baxter state as shown in Figure 8b. In addition, hydrophobicity due to the nanostructure of the AAO substrate might be stronger than hydrophilicity due to the silanol group of SNs. D

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substrate was caused by the reduction in the contact area of the measurement probe because of the convex structure by SNs on the nanopores of the AAO substrate. We showed that the highly filled NCP-SNMA on the AAO substrate have a lubricating function.

As other surface properties, it is known that a friction coefficient varies with changes in surface structures. For example, Choi et al. reported that the friction coefficient of AAO substrates was varied by changes in the pore size and porosity of AAO substrates.31 Zhang et al. reported that the friction coefficient of the AAO substrate laminated with PTFE particles by the electrophoresis method was decreased.32 Thus, it was expected that the friction coefficient on the SNMA-AAO substrate would be changed by varying the nanostructure. Therefore, the friction coefficient of the highly filled SNMAAAO substrate was investigated. Figure 9 shows the coefficients of dynamic friction and the maximum coefficients of static friction of the AAO substrate

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CONCLUSIONS The supporting conditions of SNMA on the surface of an AAO substrate by the spin-coating method were investigated. We successfully fabricated a highly filled NCP-SNMA-AAO by optimizing the spin-coating conditions without surface modification of the SNs and AAO substrate. The two-step spin-coating technique is an effective technique for fabricating the highly filled SNMA not only on flat substrates but also on nanostructured substrates. An AAO substrate is an important substrate as a template for fabricating NCP nanoparticle monolayer arrays. The surface wettability of the highly filled NCP-SNMA-AAO changed from hydrophobicity to hydrophilicity (107.9 → 51.0°). Moreover, both the maximum coefficient of static friction and the coefficient of dynamic friction were decreased by 29% (0.327 → 0.233) and 20% (0.281 → 0.226), respectively. Controlling the surface wettability (hydrophilicity/hydrophobicity) and the friction coefficient by the obtained SNMA-AAO is expected to improve the surface physical properties of an AAO substrate.

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Figure 9. Coefficients of dynamic friction of (a) the AAO substrate and (b) the highly filled NCP-SNMA-AAO substrate. Maximum coefficients of static friction of (c) the AAO substrate and (d) the highly filled NCP-SNMA-AAO substrate.

EXPERIMENTAL SECTION Chemicals and Materials. Acetone (EL grade) and 99.5% ethyl alcohol (guaranteed reagent grade) were purchased from Junsei Chemical Co., Ltd. Perchloric acid (60%) (HClO4, guaranteed reagent grade) and chromic anhydride (guaranteed reagent grade) were purchased from FUJIFILM Wako Pure Chemical Corporation. Phosphoric acid (85%) (H3PO4, guaranteed reagent grade) and 0.1 mol/L sodium hydroxide solution (for volumetric titration) were purchased from Kanto Chemical Co., Inc. A high purity aluminum plate (purity of 99.99%, 1 mm in thickness) was purchased from Showa Denko K. K. A platinum plate (purity of 99.98%, 0.10 mm in thickness) was purchased from The Nilaco Corporation. An aqueous dispersion of commercial silica nanoparticles (Snowtex MP-4540M, silica concentration of 40 wt %, pH value of 9.3) was provided by Nissan Chemical Corporation. Deionized water (DI water) with a resistivity of 18.2 MΩ·cm that was obtained from Milli Q integral5 (Merck Millipore) was used in all experiments. Preparation of an AAO Substrate. An AAO substrate was fabricated using the two-step anodization method.15,18 The anodization condition was adjusted according to the particle size of SNs. A high purity aluminum plate of 1 mm in thickness was cut into rectangular specimens of 10 × 50 mm in size. Each specimen of the high purity aluminum plate was degreased in acetone with ultrasonification for 10 min at room temperature. After drying the specimen, it was electropolished under a constant current of 0.2 A/cm2 (direct-current stabilized power supply device, CPS-3025L, CUSTOM Corporation) in a 4:1 volume mixture of ethyl alcohol and 60 wt % perchloric acid for 10 min at 1−5 °C to reduce surface roughness. The specimen was used as an anode, and a platinum plate was used as a cathode. After the electropolishing, the specimen was rinsed in DI water for more than 30 min. The electropolished specimen was first anodized under a constant voltage of 200 V (compact DC power supply,

and the highly filled NCP-SNMA-AAO substrate with a supporting ratio of 87%. The coefficient of dynamic friction and the maximum coefficient of static friction were obtained automatically from the measurement results as shown in Figure S4. Each friction coefficient is an average value of three measurement results (n = 3). Error bars indicate one standard deviation (SD). The coefficients of dynamic friction of the AAO substrate and the highly filled NCP-SNMA-AAO substrate were 0.281 ± 0.18 and 0.226 ± 0.08, respectively, and the maximum coefficients of static friction of the AAO substrate and the highly filled NCP-SNMA-AAO substrate were 0.327 ± 0.09 and 0.233 ± 0.05, respectively. The coefficient of dynamic friction and the maximum coefficient of static friction of the highly filled NCP-SNMA-AAO substrate were decreased by 20 and 29%, respectively. Additionally, the difference between the coefficient of dynamic friction and the maximum coefficient of static friction became small, from 0.046 to 0.007. Figure 10 shows a schematic illustration of the friction coefficient measurements of the AAO substrate and the highly filled NCP-SNMA-AAO substrate. It is possible that the decrease in the friction coefficient of the highly filled NCP-SNMA-AAO

Figure 10. Illustrations of (a) the AAO substrate and (b) the highly filled NCP-SNMA-AAO substrate during friction coefficient measurements. E

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Scientific Co., Ltd.) at a load of 100 g, a velocity of 1 mm/s, and a measurement range of 10 mm.

PMX350-0.2A, Kikusui Electronics Corporation) in 0.1 wt % phosphoric acid solution for 8 h at 20 °C with stirring. After the first anodization, the AAO film was rinsed several times in DI water, and the AAO film was removed by immersing the specimen in a mixture of 5 wt % phosphoric acid and 1.8 wt % chromic acid solution for 2 h at 60 °C. The specimen was rinsed again several times in DI water and reanodized under a constant voltage of 200 V in 0.1 wt % phosphoric acid solution for 2 h at 20 °C with stirring. After the second anodization, the obtained AAO substrate was rinsed several times in DI water and was dried at room temperature for 1 day under an air atmosphere. Preparation of a Dispersion of Silica Nanoparticles. An aqueous dispersion of SNs (Snowtex MP-4540M) was centrifuged at 1000g for 20 min at 20 °C. After removal of the supernatant, the same volume of DI water as the supernatant was added, and the SNs were redispersed by ultrasonification. These centrifugation−redispersion processes were repeated seven times. The silica concentration of the purified aqueous dispersion of SNs was adjusted to 10 or 20 wt %, and the pH value was adjusted to 10 with DI water and 0.1 mol/L sodium hydroxide solution. The adjusted aqueous dispersion of SNs was filtrated (1.0 μm membrane filter, Whatman Puradisc PES Syringe Filters, GE Healthcare) before spin coating. Spin-Coating of Silica Nanoparticles. The AAO substrate was placed on a spin coater (MS-B100, Mikasa Co., Ltd.). The spin-coating methods used in the experiments were the one-step technique and the two-step technique (see Figure 1b,c). The two-step technique is a supporting technique for obtaining a high-density monolayer of nanoparticles on the substrate in which the substrate is spun at a low spin speed (first step) to make the dispersion uniform on the substrate and then accelerated to a second higher spin speed (second step) to remove excess dispersion.7 For all spin-coating experiments, 0.5 mL of 10 or 20 wt % SN aqueous dispersion (pH 10) was used. The AAO substrate was spun at 200−3000 rpm for 10−60 s at room temperature and at a relative humidity of 50−60%. The layer of SNs supported on the AAO substrate was dried at room temperature for 1 day under an air atmosphere. Characterization. The fabricated AAO substrate, the adjusted aqueous dispersion of SNs, and the supported state of SNs on AAO pores were observed with a field emission scanning electron microscope (FESEM, JSM-7400F, JEOL Ltd.) at 1 kV. By analyzing the FESEM images with an image analyzer (Luzex AP, Nireco Corporation), the pore distance (D) and the ratio of the pore area (f 2) of the AAO substrate, the particle size (R) of the SNs, the particle distance (d), the ratio of the area (f 3), and the supporting ratio (S) of the SNs supported on the AAO substrate were calculated. The supporting ratio (S) was calculated only when SNMA-AAO was obtained and by the following equation: S (%) =

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b02114. Particle distance distribution, measurement results of f1−3 for the expanded Cassie−Baxter equation, water contact angles calculated by the expanded Cassie− Baxter equation, and friction coefficient measurements of AAO substrate or the NCP-SNMA-AAO substrates (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.N.). *E-mail: [email protected] (A.Y.). ORCID

Takayuki Nakanishi: 0000-0003-3412-2842 Author Contributions

K.S. designed the research, performed the experiments, and analyzed results. H.S. instructed K.S. the technique of the anodic aluminum oxide fabrication. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare the following competing financial interest(s): Kazutoshi Sekiguchi is an employee of Nissan Chemical Corporation.

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ACKNOWLEDGMENTS This work was financially supported by Nissan Chemical Corporation. REFERENCES

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number of silica nanoparticles on AAO pores number of AAO pores × 100

ASSOCIATED CONTENT

(4)

The wettability and friction coefficient of the obtained SNMA-AAO were estimated by using a contact angle meter (DM-501Hi, Kyowa Interface Science Co., Ltd.) at a DI water droplet of 1 μL and a variable normal load friction and a wear measurement system (Tribogear Type: HHS2000, Shinto F

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DOI: 10.1021/acsomega.9b02114 ACS Omega XXXX, XXX, XXX−XXX