LETTER pubs.acs.org/Langmuir
Antireflection and Self-Cleaning Properties of a Moth-Eye-Like Surface Coated with TiO2 Particles Kazuya Nakata,*,†,‡,§ Munetoshi Sakai,†,‡ Tsuyoshi Ochiai,†,§ Taketoshi Murakami,† Katsuhiko Takagi,‡ and Akira Fujishima*,†,‡,§ †
Photocatalyst Group, KSP Building East 412, and ‡Organic Solar Cell Assessment Project, KSP Building East 308, Kanagawa Academy of Science and Technology, 3-2-1 Sakado, Takatsu-ku, Kawasaki, Kanagawa 213-0012, Japan § Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan ABSTRACT:
Poly(ethylene terephthalate) (PET) films with a moth-eye-like surface are coated with TiO2 particles to form self-cleaning antireflective films. The use of a TiO2 suspension of high concentration to coat the PET surface produces a thicker TiO2 layer with smaller pores, whereas a low concentration of a TiO2 suspension gives a thinner layer of TiO2 with larger pores. The PET films coated with TiO2 particles exhibit a high transmittance of 7695% and almost no absorption in the range of 400800 nm. The PET films coated with a TiO2 suspension with a concentration of g2 vol % exhibit superhydrophilicity after irradiation with UV light. After irradiation, the superhydrophilic nature is retained for at least 18 days. The TiO2-coated PET films showed the ability to decompose methylene blue under UV irradiation.
1. INTRODUCTION Antireflection (AR) films play an important role in a wide variety of optical technologies by reducing the reflectivity at interfaces.13 In many applications, such as flat panel displays, AR films are employed to eliminate ghost images and flickering caused by reflections from optical surfaces. AR coatings have also been widely used in energy-related applications including solar thermal and photovoltaic systems.46 Reflection losses at surface interfaces are particularly disadvantageous in technologies such as solar cells, which negatively affects their energy conversion efficiency. It is therefore necessary to reduce the intensity of reflected light to improve the overall performance and efficiency of such systems by increasing their transmission. However, a drawback limiting the use of AR films is the adhesion of grime, which reduces transmittance. TiO2 is able to remove organic contaminants adhered to its surface under illumination with UV light through self-cleaning.7 The self-cleaning properties originate from a combination of the photocatalytic oxidative decomposition of organic contaminants and superhydrophilicity, which causes water droplets to spread on the TiO2 surface, aiding the cleaning process.811 TiO2-based coatings can be easily applied to transparent substrates such as glass and plastics to provide a self-cleaning function. However, the coatings developed thus far always increase the surface reflection of the transparent substrate because of the large refractive index of TiO2 (2.52 for anatase; 2.76 for rutile). The reflection at an r 2011 American Chemical Society
airglass interface is about 4% for normal incident light. However, at an airTiO2 interface, the reflection of normal incident light can be as high as 20%.12 Therefore, self-cleaning coatings that possess low surface reflection need to be developed because they are required in many applications. A way to reduce reflections is to prepare multilayer films consisting of TiO2 and SiO2 that have low refractive indexes so as to reduce the refractive index of the multilayer film.13 Another idea is to introduce 3D porous structures. Bravo et al. demonstrated that a multilayer film consisting of SiO2 nanoparticles and polymers prepared by a layer-by-layer process showed AR properties originating from its porous structure.14 However, these films require multiple preparation procedures, which should be a major drawback for practical applications. In this work, poly(ethylene terephthalate) (PET) films possessing a moth-eye-like surface coated with TiO2 particles were fabricated. The films were characterized by field-emission scanning electron microscopy (FE-SEM) and UVvis spectroscopy, allowing their structure, transmittance, and absorbance to be determined. The self-cleaning properties of the films were evaluated by changes in the contact angle of water and their ability to decompose methylene blue under UV irradiation. Received: February 2, 2011 Revised: March 5, 2011 Published: March 10, 2011 3275
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Figure 2. (a) Transmittance and (b) absorption spectra of PET films coated with TiO2 particles. The blank represents an uncoated PET film. Figure 1. FE-SEM images of (a) a moth-eye-like PET surface coated with TiO2 particles from suspension containing TiO2 volume ratios of (b) 0.5, (c) 1, (d) 2, (e) 5, and (f) 10 vol %.
2. EXPERIMENTAL SECTION PET films with a moth-eye-like surface (Mitsubishi Rayon)15 were coated with suspensions of TiO2 particles (Tayca, TKD-701) in 2-propanol with various volume ratios (0.5, 1.0, 2.0, 5.0, and 10 vol %) by dip coating at a rate of 2.5 mm/s and are denoted as 0.5, 1.0, 2.0, 5.0, and 10 vol %, respectively. The obtained films were dried at room temperature overnight. To evaluate their photocatalytic properties, the PET films covered with TiO2 particles were coated with a saturated solution of methylene blue in 2-propanol by dip coating at a rate of 30 mm/s and then irradiated with UV light (365 nm, 2 mW/cm2).
3. RESULTS AND DISCUSSION Figure 1 shows FE-SEM images of the PET films with motheye-like surfaces before and after coating with TiO2 particles. The initial PET film before the application of a TiO2 coating has an ordered arrayed of pillars with an average diameter of 179 nm over the surface (Figure 1a). After being coated with TiO2, the tops of the pillars are fully covered with TiO2 particles. The TiO2 layer has a porous structure because of the random packing and aggregation of TiO2 particles. As the concentration of the TiO2 suspension used to prepare the film was increased, the number of TiO2 particles on the PET film increases, the porosity of the TiO2 layer decreases, and the average size of the TiO2 aggregates increases. The transmittance of the PET films coated with TiO2 particles is shown in Figure 2a. The transmittance through the blank PET film is in the 9396% range for light with a wavelength of 400800 nm whose transmittance is higher than that without the moth-eye-like surface (almost 9195%).15 After the films were coating with a 0.5 vol % TiO2 suspension, their transmittance decreased to 8595%. The coated film maintained its high transmittance in the 8392% range, even though it was tilted 45°
Figure 3. Changes in the contact angles of (a) PET films coated with TiO2 particles under UV irradiation and (b) a PET film coated with a 1 vol % suspension of TiO2 in the dark after UV irradiation. The blank represents an uncoated PET film.
with respect to the light source. As the concentration of the TiO2 suspension was increased, the transmittance decreased slightly to 7693% for the film coated with a 10 vol % TiO2 suspension but remained very transparent. The high transmittance of the coated films could be caused by the presence of pores between the TiO2 3276
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superhydrophilicity for a few days.18 A major factor causing such prolonged superhydrophilicity is the porous structure of the film, which could generate a capillary force that attracts water into the pores in the TiO2 particles and leads to a low contact angle.19 To achieve self-cleaning, a film not only must be superhydrophilic but also must be able to induce photocatalytic oxidative decomposition. To evaluate the photocatalytic oxidative decomposition performance of the TiO2-coated PET films, a conventional dye adsorptiondecomposition experiment was performed. In this experiment, PET films coated with TiO2 are modified with methylene blue. Figure 4a shows a UVvis absorption spectrum of the PET film coated with a 10 vol % TiO2 suspension after modification with methylene blue. The spectrum shows a maximum at 661 nm from methylene blue. The intensity of this peak decreases under UV irradiation, which is attributed to the decomposition of methylene blue molecules through photocatalytic reaction with TiO2.20,21 The rate of photodegradation of methylene blue increases as the thickness of the TiO2 layer on the PET film increases, as shown in Figure 4b. For the PET film with a coating made from a 10 vol % TiO2 suspension, methylene blue is rapidly decomposed under UV irradiation. Methylene blue is also decomposed by the PET film coated with a 0.5 vol % suspension of TiO2, albeit more slowly (Figure 4b). Figure 4. (a) Changes in the absorption spectra of a PET film coated with a suspension containing 1 vol % TiO2 modified with methylene blue under UV irradiation and (b) changes in the absorption at 661 nm under the UV irradiation of PET films coated with TiO2 particles. The blank represents an uncoated PET film.
particles and interfaces between the TiO2 layer and PET film because pores suppress the scattering and reflection of light.12 All of the PET films coated with TiO2 particles show almost no absorbance in the visible light range (400800 nm) and exhibit a band characteristic of TiO2 at around 308 nm (Figure 2b). The intensity of this absorbance increased linearly with the concentration of the TiO2 suspension, indicating the regular assembly of a layer of TiO2 particles on the PET film. The combination of high transmittance and low absorption in the visible light range (400800 nm) is a great advantage for self-cleaning glass. To evaluate the self-cleaning properties of the PET films coated with TiO2 particles, contact angle measurements were performed under UV irradiation. As shown in Figure 3a, the contact angle for each sample decreases upon irradiation with UV light except for the uncoated PET film, which is due to the photoinduced superhydrophilicity of the TiO2.16 This trend is enhanced as the concentration of the TiO2 suspension used to coat the PET film was increased. Although the contact angles of the coatings made from suspensions containing e1 vol % TiO2 do not show superhydrophilicity after exposure to UV irradiation for 90 min, the contact angles of the coatings made from g2 vol % TiO2 suspensions are less than 5°, so these films are superhydrophilic. The contact angles of the films should depend on the balance of surface wettability between the hydrophobicity of the PET film and the superhydrophilicity of the TiO2 particles. The wetting of the PET films before irradiation should be Wenzel mode because the PET films show a large contact angle hysteresis and no air trapping.17 After irradiation of the PET film coated with a 1.0 vol % TiO2 suspension with UV light, the superhydrophilicity is maintained for at least 18 days (Figure 3b). This result is superior to a typical TiO2-coated surface, which exhibits
4. CONCLUSIONS PET films with a moth-eye-like surface were coated with TiO2 particles to achieve films with both AR and self-cleaning properties. As the concentration of the suspension of TiO2 was increased, the layer of TiO2 particles covering the PET surface shows smaller pores and larger aggregates. UVvis spectra revealed that the PET films coated with TiO2 particles exhibit high transmittance and almost no absorption in the visible light range (400800 nm), which leads to AR properties. The same films show superhydrophilicity for samples prepared with a TiO2 suspension concentration of 2 vol % or greater. Methylene blue molecules decomposed on the PET films coated with TiO2 particles under UV irradiation, demonstrating their self-cleaning potential. Such PET films coated with TiO2 particles exhibiting both AR and self-cleaning properties will be important to improving the performance of solar cells and flat panel displays. ’ AUTHOR INFORMATION Corresponding Author
*Tel: þ81-44-819-2020. Fax: þ81-44-819-2038. E-mail:
[email protected] (K.N.),
[email protected] (A.F.).
’ ACKNOWLEDGMENT This research was partially supported by the Japan Society for the Promotion of Science (JSPS) through its “Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program)” and Tokyo University of Science, Research Institute for Science and Technology, Energy and Environment Photocatalyst Research Division. ’ REFERENCES (1) Hiller, J.; Mendelsohn, J. D.; Rubner, M. F. Nat. Mater. 2002, 1, 59–63. 3277
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