Letter pubs.acs.org/Langmuir
Simple Method for Preparing Superhydrophobic Paper: SprayDeposited Hydrophobic Silica Nanoparticle Coatings Exhibit High Water-Repellency and Transparency Hitoshi Ogihara,* Jing Xie, Jun Okagaki, and Tetsuo Saji Department of Chemistry & Materials Science, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo 152-8552, Japan S Supporting Information *
ABSTRACT: Superhydrophobic and transparent coatings are deposited onto paper by spraying alcohol suspensions of SiO2 nanoparticles. Superhydrophobicity depends on the aggregation states of nanoparticles, which are determined by the type of alcohol used in the suspensions. The superhydrophobicity of the paper is maintained after touching the paper with a bare finger.
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INTRODUCTION Paper is a common and essential material in daily life. Paper is made from wood, and its main compound is fibrous cellulose. Because cellulose is a structural polysaccharide that has many hydroxyl groups, paper shows hydrophilicity. If paper has water repellency, the waterproof paper would be useful in various fields. Recently, to impart high water repellency to materials, superhydrophobic coatings have been studied. A superhydrophobic surface is defined as a surface with both a water contact angle (CA) > 150° and a water sliding angle (SA) < 10°, which means that a water droplet that settles on the superhydrophobic surface is almost a sphere and easily rolls off.1−6 According to previous research, superhydrophobic surfaces have to possess a low surface energy and high surface roughness. A water droplet that settles on a rough surface contacts both the constituent materials of the surface and the air trapped within the rough structure. Air is an absolutely hydrophobic material (its CA is 180°); therefore, surface roughness contributes to an increase in surface hydrophobicity. Although superhydrophobic coatings have been vigorously researched, studies on superhydrophobic paper are limited because most methods for preparing superhydrophobic coatings cannot be simply applied for paper. Paper has different properties from typical substrates, such as glass and metals; cellulose, which is the main component of paper, is easily decomposed and damaged by chemical, physical, and thermal treatments. So far, plasma treatments7 and the rapid expansion of supercritical solutions technique8 have been proposed for preparing superhydrophobic paper, but these methods require specialized and costly instrumentation. Chemical modification of paper in the solution phase to prepare superhydrophobic paper has also been reported.9,10 The wet-chemical methods are simpler processes, but treatment in the solution phase would © 2012 American Chemical Society
lead to shrinkage of the paper and the dissolution of the print on the paper. Other approaches to impart superhydrophobicity to paper, such as a multilayer deposition of polydiallyldimethylammonium chloride and SiO2 particles, followed by a fluorination surface treatment with perfluorooctyltriethoxysilane,11 coating of fluoro-containing SiO2 nanoparticles prepared by the cohydrolysis of TEOS/fluorinated alkyl silane with NH3·H2O,12 and coating of organic nanoparaticles13,14 have been reported. These previously reported methods to make paper substrates superhydrophobic are sophisticated; nevertheless, the transparency of the coatings was not discussed in these previous studies. Transparency is an important property in superhydrophobic coatings, especially in superhydrophobic paper, because the visibility of the print on the paper should be maintained after applying the superhydrophobic coating. However, it is not easy to achieve both superhydrophobicity and transparency. Surface roughness, which is an indispensable requirement for exhibiting superhydrophobicity, often causes light scattering, which would make the surface opaque. Thus, precise control of surface roughness is necessary to achieve both superhydrophobicity and transparency. To date, superhydrophobic and transparent coatings have been mainly achieved on glass substrates,1,2,4 but these techniques cannot simply be applied for paper substrates because the chemical/ physical properties of paper are quite different from those of glass. Very recently, Zhang et al. reported the application of transparent and superhydrophobic polymer coatings on paper.15 In this excellent study, nanoporous polydivinylbenzene Received: November 14, 2011 Revised: February 14, 2012 Published: February 27, 2012 4605
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prepared using solvothermal synthesis was painted on paper. Herein, we demonstrate that special nanomaterials and costly instruments are not necessary for preparing superhydrophobic and transparent films on paper. By simply spraying alcohol suspensions containing hydrophobic SiO2 nanoparticles, superhydrophobic and transparent SiO2 nanoparticle coatings were formed on paper.
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EXPERIMENTAL DETAILS
Pretreatment of SiO2 Nanoparticles. Two grams of SiO2 nanoparticles (C.I. Kasei Co., Ltd.; the mean particle size estimated by the specific surface area is ca. 25 nm) and 40 mL of dehydrated toluene were placed into a Schlenk flask. After adding 1 mL of dodecyltrichlorosilane, they were refluxed for 3 h. The procedure imparts hydrophobicity to the SiO2 nanoparticles by immobilizing dodecyltrichloro groups on their surface. After filtration, the obtained SiO2 particles were dried at 353 K and ground to a fine powder using a mortar. Spray-Coating. The spray-coating procedures are similar to those in our previous study.16 SiO2 particles (0.1 g) were added to 5 mL of alcohol (ethanol, 1-propanol, or 1-butanol). After stirring (30 min) and sonicating (20 min), the suspension was manually sprayed over standard printer-grade paper (JP-MLT2; Sanwa Supply) using a glass vaporizer (Tokyo Glass Kikai Co., Ltd.; Supporting Information Figure S1). The resulting samples were placed horizontally and dried at room temperature overnight. Characterization. The CAs of water droplets (3 μL) on the samples were measured using a contact angle meter (Kyowa Interfacial Science Japan, CA-D). Scanning electron microscopy (SEM) images were obtained using VE-8800 (Keyence) and S-4700 (Hitachi) instruments. Prior to SEM measurements, a thin Au layer (ca. 5 nm) was deposited on the specimens by magnetron sputtering.
Figure 2. Contact and sliding angles (CA and SA) of paper substrates spray-coated with different amount of SiO2 nanoparticles (25 nm).
Table 1. Contact and Sliding Angles (CA and SA) of SiO2 Nanoparticle Coatings on Paper Substrates alcohol
particle size/nm
contact angle/deg
sliding angle/deg
ethanol 1-propanol 1-butanol ethanol ethanol
25 25 25 250 500
155 154 149 148 126
7 16 50 −a −a
a
Sliding angles of these coatings could not be measured because a water droplet did not roll off at any angle (i.e., a water droplet was pinned).
aggregates is faster. As discussed below, the aggregation state of SiO2 nanoparticles in suspensions has a significant influence on the hydrophobicity of SiO2 nanoparticle coatings. Figure 2 shows CA and SA of paper substrates spray-coated with different amount of SiO2 nanoparticles. The amount of SiO2 nanoparticle coatings was adjusted by the number of spray-coatings. As increasing the amount of SiO2 nanoparticle coatings, hydrophobicity of the paper became higher, and SiO2 nanoparticle-coated paper showed superhydrophobicity when the amount of SiO2 nanoparticle coatings was higher than ca. 0.4 mg silica /cm 2 paper . However, a large amount of SiO 2 nanoparticles is unfavorable for transparent coatings; therefore, in subsequent experiments, we repeated spray-coating (20−30 times) to adjust the amount of deposited SiO2 nanoparticles to be ca. 0.4−0.5 mgsilica/cm2paper. Table 1 summarizes CAs and SAs of SiO2 nanoparticle coatings on paper. At first, we will focus on the effect of the type of alcohol on the hydrophobicity of SiO2 nanoparticle coatings. The CAs and SAs depended on the kind of alcohol; the hydrophobicity of the coatings decreased as the length of the hydrocarbon chains in the alcohols increased. As a result, the preparation of superhydrophobic paper (CA > 150° and SA < 10°) was achieved only with ethanol. Note that the hydrophobicity of alcohols does not directly affect the hydrophobicity of the resulting coatings because the alcohols are removed through evaporation. As described above, the surface hydrophobicity depends on the surface energy and the surface roughness. In this study, the surface energy of the SiO2 nanoparticles was the same; therefore, the surface roughness, which is composed of deposited SiO2 nanoparticles, would determine the surface hydrophobicity. To discuss surface morphology, SEM images of SiO2 nanoparticles coatings prepared using ethanol and 1-butanol were measured (Figure 3). Low magnification SEM images shows that SiO2 nano-
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RESULTS AND DISCUSSION Figure 1 shows photographs of SiO2 nanoparticle suspensions 10 min after sonication. Obviously, the sedimentation rate of SiO2 nanoparticles was faster in a lower alcohol, which indicates that the aggregation states of SiO2 nanoparticles are different in these suspensions. The particle size distribution of SiO2 nanoparticles in the alcohols (Supporting Information Figure S2) suggests that the SiO2 nanoparticles tend to form larger aggregates in lower alcohols. The average sizes of the SiO2 nanoparticle aggregates were approximately 470 nm in ethanol, 410 nm in 1-propanol, and 350 nm in 1-butanol. It is reasonable that hydrophobic SiO2 nanoparticles with dodecyl groups tend to aggregate in lower alcohols (i.e., less hydrophobic alcohols) and the sedimentation rate of larger
Figure 1. Photographs of SiO2 nanoparticle suspensions in (a) ethanol, (b) 1-propanol, and (c) 1-butanol. 4606
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Figure 3. SEM images of SiO2 nanoparticle coatings prepared using ethanol (a−c) and 1-butanol (d−f).
particles uniformly cover the paper substrates, and the morphology was similar in the two coatings (Figure 3a and d; a SEM image of a pristine paper substrate is shown in the Supporting Information, Figure S3). However, in highermagnification images, we can see morphological differences in the two coatings. Comparing Figure 3b and e, it seems that surface structures between the two coatings are different; the coating from ethanol suspension of nanoparticle had rougher surface. Figure 3c shows the SiO2 coating prepared from ethanol suspension had micrometer-sized roughness, while relatively flat surface was observed in the coating from 1butanol suspension (Figure 3f). In addition to the micrometersized roughness, nanometer-sized roughness was also present in the coatings because they consist of nanometer-sized SiO2 particles (inset in Figure 3c). Generally, superhydrophobicity tends to be exhibited when hydrophobic surfaces have hierarchical roughness (both nanometer- and micrometersized roughness).17,18 Based on these SEM images, it is reasonable that superhydrophobicity was exhibited only when the ethanol suspension was used, because a hierarchically rougher surface was formed in the case of the ethanol suspension. We assume that the morphological difference would be attributed to the difference in the aggregation states of the SiO2 nanoparticles in the alcohols. As shown in Figure 1 and Supporting Information Figure S2, the SiO2 nanoparticles are dispersed finely in 1-butanol, and it is likely that a flat coating is formed when spraying the finely dispersed SiO2 nanoparticles. In contrast, the SiO2 nanoparticles in ethanol are relatively aggregated, causing the surface of the resulting coatings to be rougher, which is why superhydrophobic paper was formed only from the ethanol suspension. In addition to the type of alcohols, particle size of SiO2 affected hydrophobicity of SiO2 particle coatings; larger SiO2 particles showed less hydrophobicity (Table 1). Similar
Figure 4. Photographs of (a) paper, (b) photograph, and (c) cotton on some part of which were spray-coated with SiO2 nanoparticles (25 nm) using ethanol. Red dashed square frame indicates spray-coated area.
tendency is observed in SiO2 particle coatings prepared using electrophoretic deposition.19 The previous research suggests that roughness of particle coatings depends on the particle size because void spaces in the particles, whose size is determined by particle size, form the roughness. In this study, nanometersized void spaces were observed (inset in Figure 3c), and the void spaces grew with increasing particle size (Supporting Information Figure S4). Therefore, smaller SiO2 particles are suitable to form hierarchical roughness on paper substrates. Figure 4 shows the appearance of printed paper, photograph, and cotton, some of which were spray-coated with SiO2 nanoparticles. In all samples, water droplets on the superhydrophobic area resemble a sphere. Moreover, transparency of the coatings was confirmed; the visibility of the samples was not changed after spray-coating the SiO2 nanoparticles. The water repellency test for a bent piece of paper showed that the superhydrophobic paper was flexible and repelled running water continuously (see the video in the Supporting Information). Finally, we examined the strength of the superhydrophobic coatings by pressing the superhydrophobic paper with a bare finger (Supporting Information Figure S5). The values of CA and SA of the superhydrophobic paper after being pressed were 153° and 9.5° (the CA and SA of the paper before pressing were 155° and 7.2°), respectively, which indicates that the superhydrophobic coatings persist after being touched with bare hands. In addition to touching with hands, superhydrophobicity was maintained after folding it. These results indicate that adhesive force between SiO2 nanoparticles and cellulose fibers is relatively high. Dodecyl groups on SiO2 4607
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(7) Balu, B.; Breedveld, V.; Hess, D. W. Fabrication of “Roll-off” and “Sticky” Superhydrophobic Cellulose Surfaces via Plasma Processing. Langmuir 2008, 24, 4785−4790. (8) Quan, C.; Werner, O.; Wågberg, L.; Turner, C. Generation of Superhydrophobic Paper Surfaces by a Rapidly Expanding Supercritical Carbon Dioxide-Alkyl Ketene Dimer Solution. J. Supercrit. Fluids 2009, 49, 117−124. (9) Li, S.; Zhang, S.; Wang, X. Fabrication of Superhydrophobic Cellulose-Based Materials through a Solution-Immersion Process. Langmuir 2008, 24, 5585−5590. (10) Nyström, D.; Lindqvist, J.; Ö stmark, E.; Hult, A.; Malmström, E. Superhydrophobic Bio-Fibre Surfaces via Tailored Grafting Architecture. Chem. Commun. 2006, 3594−3596. (11) Yang, H.; Deng, Y. Preparation and Physical Properties of Superhydrophobic Papers. J. Colloid Interface Sci. 2008, 325, 588−593. (12) Wang, H.; Fang, J.; Cheng, T.; Ding, J.; Qu, L.; Dai, L.; Wang, X.; Lin, T. One-Step Coating of Fluoro-Containing Silica Nanoparticles for Universal Generation of Surface Superhydrophobicity. Chem. Commun. 2008, 877−879. (13) Samyn, P.; Schoukens, G.; Vonck, L.; Stanssens, D.; Van den Abbeele, H. How Thermal Curing of an Organic Paper Coating Changes Topography, Chemistry, and Wettability. Langmuir 2011, 27, 8509−8521. (14) Stanssens, D.; Van den Abbeele, H.; Vonck, L.; Schoukens, G.; Deconinck, M.; Samyn, P. Creating Water-Repellent and SuperHydrophobic Cellulose Substrates by Deposition of Organic Nanoparticles. Mater. Lett. 2011, 65, 1781−1784. (15) Zhang, Y.-L.; Wang, J.-N.; He, Y.; He, Y.; Xu, B.-B.; Wei, S.; Xiao, F.-S. Solvothermal Synthesis of Nanoporous Polymer Chalk for Painting Superhydrophobic Surfaces. Langmuir 2011, 27, 12585− 12590. (16) Ogihara, H.; Okagaki, J.; Saji, T. Facile Fabrication of Colored Superhydrophobic Coatings by Spraying a Pigment Nanoparticle Suspension. Langmuir 2011, 27, 9069−9072. (17) Bhushan, B.; Jung, Y. C.; Niemietz, A.; Koch, K. Lotus-Like Biomimetic Hierarchical Structures Developed by the Self-Assembly of Tubular Plant Waxes. Langmuir 2009, 25, 1659−1666. (18) Lee, Y.; Park, S.-H.; Kim, K.-B.; Lee, J.-K. Fabrication of Hierarchical Structures on a Polymer Surface to Mimic Natural Superhydrophobic Surfaces. Adv. Mater. 2007, 19, 2330−2335. (19) Ogihara, H.; Katayama, T.; Saji, T. One-step Electrophoretic Deposition for the Preparation of Superhydrophobic Silica Particle/ Trimethylsiloxysilicate Composite Coatings. J. Colloid Interface Sci. 2011, 362, 560−565.
nanoparticles and hydroxyl groups on cellulose could not have chemical and electrostatic interaction; alternatively, van der Waals force would be present between them. From the viewpoint of practical applications, the mechanical strength of the superhydrophobic paper is an important property.
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CONCLUSION Superhydrophobic paper could be prepared by spraying an ethanol suspension containing commercially available SiO2 nanoparticles at room temperature and under atmospheric pressure. The hydrophobicity of the coatings could be controlled by two factors: the aggregation state of the SiO2 nanoparticles, which depends on the type of alcohol in the suspension, and the SiO2 particle size. The visibility of characters on the paper was maintained after coating with SiO2 nanoparticles. The advantage of our method is the simplicity of the process; this study clearly indicates that costly instrumentation, extreme reaction conditions, and specialized nanomaterials are not necessary to prepare superhydrophobic and transparent films on paper.
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ASSOCIATED CONTENT
S Supporting Information *
Photographs of apparatus, particle size distribution of SiO2 nanoparticles, SEM images of paper substrate and SiO2 coatings, and results of the strength test for the superhydrophobic paper. This material is available free of charge via the Internet at http://pubs.acs.org.
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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 This work is supported by the Mizuho Foundation for the Promotion of Sciences and the Tokyo Tech Young Investigator Engineering Award. We thank Prof. Y. Wada and Dr. D. Mochizuki (Department of Applied Chemistry, Tokyo Institute of Technology) for measuring the particle size distribution of nanoparticles and Mr. J. Koki (Center for Advanced Materials Analysis, Tokyo Institute of Technology) for measuring FESEM images.
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REFERENCES
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