Rapid fabrication of angle-independent structurally colored films with a superhydrophobic property
Introduction
Chemical pigments used in industries produced from organic or inorganic chemicals are easy to fade over time or upon exposure to light [1], [2], [3], [4]. Recent years, structural colors that originate from interference, diffraction, or scattering of visible light, with the anti-photo bleaching character have attracted great attentions in energy-saving reflective displaying [5], [6], [7], [8], [9], [10], [11], [12], [13]. Commonly observed structural colors could be divided into two classes: one is the iridescent colors from periodic nano- and microstructures, as illustrated in butterfly wings and beetle scales [14], [15], [16]; and another one is the non-iridescent color from amorphous photonic structures (APSs), as illustrated in feathers of many birds which are produced by quasi-random arrays of air vacuoles in the medullary keratin [17], [18].
Non-iridescent structural color is originated from amorphous photonic structures (APSs), in which the interference condition does not vary with orientation, and the color is independent of the viewing angles., non-iridescent structural colors have attracted great attentions in potential applications due to the requirement of broad viewing angle, such as building skins, textiles, display boards, print media, cosmetics, colorimetric sensors, and optical devices [6], [19], [20], [21], [22]. Active research has been conducted on creating angle-independent structural color by packing colloidal crystals into APSs through a variety of approaches such as spin coating, drop-casting, and spray coating [12], [13], [14], [15], [16], [17], [18], [19]. Among them, spray coating offers the benefits of rapid patterning and mass production over a large area on both planar and curved surfaces. Ge et al. prepared a composite film consisting of a thin layer of quasi-amorphous array of silica nanoparticles embedded in bulk elastomeric PDMS [23]. However, the colors emitted from colloidal APSs are very pale since incoherent light scattering across the entire visible region is very strong. To reduce the contribution of multiple-scattered light to the overall scattering spectrum, black component with the character of a high absorption across the entire visible region has been mixed into the spray coating solution [24], [25], [26].
However, the commonly used SiO2 are typically hydrophilic and negatively charged, and brilliant structural colors will disappear once the films are wet due to the decreased refractive index contrast within the films [27]. This disadvantage limits their applications only in a dried environment. Superhydrophobic surfaces with contact angle greater than 150° and slide angle lower than 10° have received great interests within the scientific community as well as the industrial world over the last two decades [28], [29], [30], [31]. Inspired by nature, such as the wings of the Morpho butterfly, a peacock feather and beetle shells, which still maintain brilliant colors even under wet conditions [32]. Thus, it is useful to apply a thin layer of transparent low-surface energy coating on the generated rough surface to prepare superhydrophobic surfaces. [33], [34], [35], Method such as post-treat the colored films with fluoroalkyl silane or alkyl silane were utilized to make superhydrophobic films previously [36], [37], [38], [39]. However, most of them were achieved by vapor deposition under vacuum or by solution casting, in which the post-treatment step is time consuming and may not be desirable for applications in consumer [12]. Tang et al. reported a simple fabrication of colloidal crystal structural color films with good mechanical stability and high hydrophobicity [40]. They also prepared a heat-resistant photonic crystal structural color films fabricated by assembling microspheres of methyl methacrylate (MMA) and methacrylic acid (MAA) through a vertical-lifting process [41]. But the key to superhydrophobic surfaces lies within the combination of surface chemical compositions and topographic structures [28].
Recently, Ge et al. reported superhydrophobic and angle-independent colored films prepared by spray coating of fluorosilane functionalized SiO2 [27]. However, it has been recognized that some fluorochemicals have potential risks to human health and environment [42]. PDMS is a typical elastomeric material with low surface energy of about 20 mN/m has many excellent properties, such as hydrophobicity, low toxicity, long-term endurance, and transparency, make it an attractive material for outdoor applications [43], [44], [45], [46], [47].
There are two general strategies to create superhydrophobic surfaces: (1) introduction of surface roughness or porosity on a low surface energy material, and (2) creation of roughness on surface, followed by deposition of a low surface energy material on top of it. The second approach is simple, low-cost, and versatile.
In this article, we report the structurally colored films with excellent superhydrophobic property by spray coating of monodisperse SiO2 nanospheres and PDMS solution. The fabricating process is very simple and suitable for mass production over a large area on both planar and curved surfaces. Moreover, the stop-bands could be easily tuned by adjusting the size of SiO2 nanospheres. This structurally colored film exhibited a high contact angle (∼165°) and a low roll-off angle (<2°).
Section snippets
Materials
Tetraethoxysilane (TEOS), ethanol, and ammonia (28%) were purchased from Sinopharm Chemical Reagent Co., Ltd of China. Polydimethylsiloxane (PDMS, Sylgard 184 Silicone Elastomer Kit with components of PDMS base and curing agent) was purchased from Dow Corning. Tetrahydrofuran (THF), ethanol (EtOH) was commercially obtained without further purification. Deionized water (18.2 MΩ cm resistivity) was used in all experiments.
Synthesis of SiO2 nanospheres
Monodispersed silica nanoparticles (NPs) were synthesized according to the
Morphology of the structurally colored films
SiO2 is the primary component of soil and found in abundant supply in nature. Which is commonly used to build structurally colored films due to the simplicity in synthesizes and chemically stable. Moreover, in vivo, toxicity of silica particles that are greater than 300 nm in diameter has not been detected [48]. Therefore, submicrometer-sized silica particle become one of the best candidates for fabricating environmental friendly materials. Here, we fabricated the monodisperse silica NPs via a
Conclusions
In summary, superhydrophobic and angle-independent structurally colored films have been prepared by spray-coating of hydrophobic PDMS on SiO2 films. The angle-independent colors could be tuned from blue, green to red by varying the size of the monodispersed silica NPs. And this coating with high water contact angles (167°) and low roll-off angles (<2°) of could effectively prevent the color from fading away when in contact with water. The spray coating method with simple and fast characters
Acknowledgments
This work was supported by the National Natural Science Foundation of China (51472153, 51232008).
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