Article pubs.acs.org/Langmuir
Hierarchical Nanoporous Glass with Antireflectivity and Superhydrophilicity by One-Pot Etching Takuya Fujima,*,† Eitaro Futakuchi,† Tomohiro Tomita,† Yoshihisa Orai,‡ and Takeshi Sunaoshi‡ †
Faculty of Engineering, Tokyo City University, 1-28-1 Tamazutsumi, Setagaya, Tokyo 156-8557, Japan Electron Microscope Group, Global Application Center, Hitachi High-Technologies Corporation, Ichige 882, Hitachinaka, Ibaraki 312-0057, Japan
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ABSTRACT: We have developed a hierarchical nanoporous layer (HNL) on silicate glass by a simple one-pot etching method. The HNL has a threedimensionally continuous spongelike structure with a pore size of a few tens of nanometers on its apparent surface. The pore size gradually decreases from the apparent surface to the HNL−bulk interface. This HNL bestows significant properties to glass: low optical reflectivity that reflects 7% less visible light than nontreated glass and long-persistence superhydrophilicity that keeps its water contact angle at about 5° for more than 1 year. The superhydrophilicity also realizes antifogging and antifouling functionalities.
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any complicated processes or the use of a template. Samples with the HNL exhibited a higher transparency for visible light than untreated glass, retained their superhydrophilicity (water contact angle of approximately 5°) for more than 140 days, and exhibited antifouling and antifogging properties. The HNL were formed on glass slides by etching the slides in a heated aqueous NaHCO3 solution. Soda lime glass slides (1 mm thick) purchased from Matsunami Glass Ind., Ltd. were used after being subjected to an ultrasonic wash. The cleaned glass slides were immersed in a 0.5 mol dm−3 aqueous NaHCO3 solution and maintained at 120 °C for 20 h. The treated samples were then rinsed with water, and their properties were evaluated. The treated samples changed their thickness by less than 1 μm during the above preparation process. The structures formed on the glass slides were observed using a scanning electron microscope (SEM, SU-9000, Hitachi Hi-Technologies) without the vapor deposition of a conductive material such as a Pt−Pd alloy. Micrographs were obtained of the sample surface from the normal direction and of the crosssectional surface from the in-plane direction after cracking the samples. The former provided detailed images of the formed structure, and the latter revealed the changes in the structure with depth. The hydrophilicity of the porous layers was investigated by measuring the water contact angle of the samples. Purified water (2 × 10−6 dm3) was gently placed on the samples, and then images from the in-plane and normal directions were
uperhydrophilic surfaces generally do not retain their properties for very long because of their high surface free energy. Dust readily adheres to such high-energy surfaces, lowering their surface free energy.1 One of the most widely known superhydrophilic materials is photocatalytic TiO2, which is used as a coating on windows, car mirrors, and so forth.2 Such coatings, which are deactivated in several days, require UV light irradiation to revitalize their superhydrophilicity. Although imparting microroughness to a hydrophilic surface is a method for enhancing its hydrophilicity through the creation of a larger intrinsic surface area, as indicated by the Wenzel equation,3 this roughness does not improve the durability of the hydrophilicity. However, a porous structure can be regarded as a 3D expansion of the roughness that can persist for longer periods when exposed to dust. In fact, a surface coating containing porous silica particles reportedly exhibited superhydrophilic properties4 for several weeks.5 Hierarchical nanoporous materials, which contain pores whose sizes change with some regularity as a function of the depth of the surface, are attracting significant attention,6−8 and it is expected that metal adsorbents for the treatment of contaminated water,9 containers for drug delivery systems (DDS),10 and so forth will be realized using these materials. These functionalities arise from highly effective molecular diffusion: diffusive molecules can smoothly reach small pores with a large surface area by passing through larger pores. Silicabased materials with such a hierarchical porous structure have reportedly been fabricated by using special templates and the sol−gel method.11,12 Here we report a new hierarchical nanoporous layer (HNL) formed on a silicate glass surface via a simple etching process in an aqueous solution that is cost-effective and does not involve © XXXX American Chemical Society
Received: July 22, 2014 Revised: October 15, 2014
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It can also be seen in Figure 1b that the pore size gradually decreased from the apparent surface to the interface of the porous layer and the bulk substrate. That is, a hierarchical porous structure was spontaneously formed during the simple etching process, unlike previously reported methods that required the use of a template.11,12 The glass slides with the HNL appeared to be colorless and transparent and looked similar to untreated glass. The total reflectivity of the samples, determined using an optical spectrometer (Hitachi U-4100) with an integrating sphere for visible light, was 7% lower than for the untreated glass for the entire wavelength range evaluated, as shown in Figure 2.
obtained. The former were used for samples exhibiting rather large contact angles, and the latter were used for samples with small contact angles because of their high hydrophilicity. Such a superhydrophilic surface leads to the formation of an amoebalike indeterminate water drop rather than a spherical one. Therefore, the footprint of the water drop on such a surface was determined by analyzing the images and then calculating the contact angle equivalent to that of a spherical water drop with the same footprint area and volume. The dried samples were stored between experiments in a nonclean room where an oil-sealed rotary pump is on for typically a few tens of hours every week and a few tens of people are coming in and going out every day. Figure 1 shows SEM micrographs of the (a) HNL surface and (b) a cross-sectional view of the layer and of nontreated
Figure 2. Total reflectivity of visible light on HNL-treated and untreated glass slides. The HNL glass exhibited about 7% lower reflectance over the entire visible-light region without changing its spectral shape.
The wavelength independence is thought to be due to the gradual change in the pore size of the HNL. The change in the pore size results in a change in the effective refractive index because the spatial scale of the porous structure is much smaller than the wavelengths of visible light. Such a gradual change in refractive index suppresses reflection over a wide wavelength range and is known as the moth-eye effect.13,14 This behavior is unlike typical antireflection (AR) coatings with homogeneous refractive indices that reduce reflection over very narrow wavelength ranges. Notably, the HNL prepared via etching exhibits high and long-lasting hydrophilicity. Figure 3 shows the time variation of the water contact angle on the HNL glass, which was approximately 5° and persisted for more than 140 days, confirming the superhydrophilicity of the HNL. This durability is significantly greater than for superhydrophilic surfaces developed to date, including surfaces treated with vacuum ultraviolet (VUV) light, those coated with photocatalytic TiO2
Figure 1. SEM micrographs of the hierarchical nanoporous layer (HNL). (a) Sample surface from the normal direction and (b) crosssectional surface of the layer and of nontreated glass (inset). A 3D, continuous spongelike structure is formed with a pore size that gradually varies from tens of nanometers at the surface to a few nanometers near the bulk interface.
glass in the inset. A 3D continuous porous structure that resembles a sponge with a pore size of a few tens of nanometres is clearly observed in the figures, though such a microscopic structure does not exist on the nontreated glass. Additionally, the solid and hollow phases within the structure look bicontinuous. The thickness of the porous layer shown in Figure 1b was observed to be several hundreds of nanometers. Notably, the thickness and pore size varied as a function of the preparation conditions (temperature, concentration of the solution, and immersion time).
Figure 3. Water contact angle on the HNL glass. The strong affinity for water, indicated by a contact angle of approximately 5°, was retained for more than 140 days. B
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Figure 4. Antifogging of the (a) HNL-treated and (b) untreated glasses. The letters behind the HNL glass are decipherable, whereas the untreated glass is completely fogged.
(without UV irradiation), and those coated with porous particles. Such superhydrophilicity imparts remarkable functionalities to surfaces, including antifogging and antifouling.15,16 The former occurs because water droplets that condense on a superhydrophilic surface as dew spread out and connect to one another, forming an aqueous thin film that suppresses the diffused reflection of light (fogging). In the latter phenomenon, water penetrates between the superhydrophilic surface and the fouling substance, preventing its attachment to the surface because of the significantly higher energetic stabilization of the surface in contact with the water than that in contact with the foulant. Antifogging and/or antifouling glass is desired for the production of mirrors, window glass, and decorative panels. Figure 4 shows glass plates with and without the HNL coating that were exposed to warm, humid air on one side and cool, dry air on the other side. The antifogging effect on the HNL glass can be clearly seen under these conditions. It should be noted that the HNL glass was clearly transparent, not because it was dry but because it was homogeneously wet due to the formation of a thin water film on its surface. The nontreated glass was also wet but with numerous small water droplets, and thus fogging occurred. As expected, the HNL-treated glass also exhibited antifouling properties because of its superhydrophilicity. A drop of red chilli oil was placed on nontreated and HNL-treated glass plates (Figure 5a), which were then rinsed with water for several seconds. As can be seen in Figure 5b, all of the oil was removed from the HNL-treated glass after rinsing, but some of the oil remained adhered to the nontreated glass. According to the Wenzel equation, a hydrophilic surface with microroughness has enhanced apparent hydrophilicity that is due to an increase in the intrinsic surface area.3 The HNL layer consists of a continuous 3D porous structure with a large intrinsic surface area that extends from the apparent surface toward the bulk glass. Such a structure gives rise to capillarity within itself that enables the spread of water throughout the HNL structure beneath the apparent surface to exhibit superhydrophilicity. We speculate the large intrinsic surface area in the HNL also results in the extended persistence of the superhydrophilicity from an energy viewpoint. Because volatile hydrocarbons contained in the ambient air should be adsorbed onto the intrinsic glass surface because of its large surface free energy, the hydrophilicity of the silicate glass gradually decreases to mild hydrophilicity, which corresponds to a water contact angle of 40−50° for a flat surface. Because the apparent hydrophilicity, determined by the water contact angle, is derived by a
Figure 5. Antifouling properties of the HNL-treated glass (left side) compared to those of the untreated glass (right side). Red-colored chilli oil was (a) dropped on the samples and then (b) briefly rinsed with water. The oil was completely removed from the HNL-treated glass, but some remained on the untreated glass.
large surface free energy per unit of apparent area, structures with a large intrinsic surface per unit of apparent area can retain a large apparent surface free energy even after contamination. Therefore, the HNL with a significantly larger intrinsic surface area could retain its superhydrophilicity even after contamination that vitiates other superhydrophilic surfaces such as micropatterned indented surfaces to be mildly hydrophilic. The HNL glass maintains its superhydrophilic properties for a long time, not because of the nature of the material but because of its 3D nanostructure. The detailed mechanism of HNL formation during the simple etching process is still unclear. We found that HNL was also formed by other aqueous solutions such as KHCO3 and NH4HCO3. The weak alkalis are probably clues to the mechanism. To summarize, we have developed a hierarchical nanoporous layer (HNL) on a silicate glass surface via a simple one-pot etching process. The HNL-treated glass has remarkably higher transparency than untreated glass over the entire visible light region and retains its high superhydrophilicity for a significantly longer time than do previously reported materials. These characteristics, in combination with its antifogging and C
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antifouling properties, make the HNL-treated glass a promising material for various practical applications.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We sincerely thank Mr. R. Tamochi (Hitachi Hi-Tech. Co.) and Prof. Yoshida (Tokyo City University) for fruitful discussions.
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REFERENCES
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