Fabrication of transparent superhydrophobic porous silica coating for self-cleaning and anti-fogging
Introduction
Self-cleaning has been one of the most desirable properties for coating research [1], [2], [3], [4]. The typical self-cleaning phenomenon in nature is observed on lotus leaves [5]. Water droplet that settles on lotus leaf keeps a nearly perfect spherical shape and can easily roll off taking away dust particles from the surface. Mimicking the surface morphology of lotus leaf let to the development of various artificial superhydrophobic surfaces. It has been proved that the coexistence of surface roughness and low surface energy material is the prerequisite for achieving superhydrophobicity [6], [7], [8]. By definition, superhydrophobic surfaces are characterized by water contact angle values larger than 150° and sliding angle values less than 10° [9]. Because of their unique self-cleaning characteristic, superhydrophobic surfaces have many potential applications in interdisciplinary technological fields, including microelectronics, biosensors, smart structural coating materials, and microfluids [10], [11], [12], [13].
Various technologies have been developed for fabricating superhydrophobic surfaces, such as plasma treatment [14], chemical etching [15], chemical vapor deposition (CVD) [16], sol–gel process [17], [18], [19], spin-coating [20], colloidal assembly [21], [22], electrospinning [23], [24], layer-by-layer(LBL) assembly [25], [26], [27], dip-coating [28] and so forth. Among them, LBL assembly is a good choice to acquire superhydrophobic surfaces because of its great simplicity, scalability, speed, and compatibility with a variety of substrates.
In the past few years, there have been intensive studies on the fabrication of highly optical transparent superhydrophobic coating for expending application fields to various glass-based products, such as self-cleaning solar panels, antifogging glasses, and raindrop-repellent windows [29], [30], [31], [32]. However, it is a challenge to achieve both superhydrophobicity and high transparency on the same surface. The surface must be sufficiently rough to obtain high contact angle and low sliding angle, but the high dimensions of the roughness features will decrease transmittance of light due to light scattering. Light scattering is a function of the surface feature sizes and the refractive index of the materials. Therefore, precise turning of the surface roughness and control the refractive index of the coating materials are important to prevent light scattering and to fabricate both high transparency and superhydrophobic surfaces.
Hollow nanoparticles with lower refractive index than solid nanoparticles are suitable to prepare transparent coatings [33], [34]. On the other hand, raspberry-like nanoparticles are promising candidates for the fabrication of superhydrophobic surfaces due to their hierarchical structure [35]. Therefore, the fabrication of rationally designed surface structures using raspberry-like hollow silica particles as building blocks is expected to produce surfaces of both transparent and superhydrophobic properties on glass. It is in fact the case. The fabrication of transparent superhydrophilic porous silica surfaces on glass substrates was realized by LBL assembly of raspberry-like polystyrene@silica (PS@SiO2) composite nanoparticles, followed by thermal annealing. After modification with low surface energy material, the porous silica coating showed excellent supherhydrophobic property. Furthermore, the anti-fogging property of superhydrophilic and supherhydrophobic porous silica coatings were studied to explore their potential anti-fogging application.
Section snippets
Materials
Polyvinylpyrrolidone (PVP, Mw=58,000), α,α′-Azodiisobutyramidine dihydrochloride (AIBA) and poly(diallyldimethylammonium chloride) (PDDA, Mw=200,000−350,000, 20 wt%) were supplied by Aladdin. Sodium poly (4-styrenesulfonate) (PSS, Mw=70,000) were obtained from Alfa Aesar. Styrene, cetyltrimethylammonium bromide (CTAB), tetraethoxysilane (TEOS), ammonia solution (NH4OH, 25 wt%), sulfuric acid (H2SO4, 98%), hydrogen peroxide (H2O2, 30%) and absolute ethanol (99.5%) were purchased from Shanghai
Surface morphology and wettability of porous silica coatings
It has been well known that the template-assisted method is a type procedure for the preparation of hollow silica structure because of the easily controllable size and morphology. PS microspheres with average diameter approx. 300 nm were synthesized via soap free emulsion polymerization using PVP as stabilizer (Supporting Information, Fig. 1S) and taken as core for the preparation of raspberry-like PS@SiO2 microspheres. The surface structure of raspberry-like PS@SiO2 microspheres and hollow SiO2
Conclusions
In summary, we report the fabrication of transparent superhydrophobic porous silica coatings through the deposition of raspberry-like PS@SiO2 microspheres using the LBL assembly and calcination. The porous silica coating of two assembly cycles showed a high contact angle of 159°±2°and a sliding angle of 7°±1.5°, indicating the excellent self-cleaning property. Different water contact angles and the transparency could be obtained by changing the number of assembly cycles. In addition, the
Acknowledgments
The work was supported by the Natural Science Foundation of Jiangsu Province of China (Grants BK20150072).
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