Applied Surface Science 257 (2011) 8640–8646 Contents lists available at ScienceDirect Applied Surface Science jou rn al h om epa g e: www.elsevier.com/locate/apsusc Surface modification on a glass surface with a combination technique of sol–gel and air brushing processes Meng-Yu Tsai a , Chin-Chi Hsu a , Ping-Hei Chen a,∗ , Chao-Sung Lin b , Alexander Chen c a Department of Mechanical Engineering, National Taiwan University, 10617, Taiwan b Department of Materials Science and Engineering, National Taiwan University, 10617, Taiwan c Department of Chemistry Engineering, University of Minnesota, United States a r t i c l e i n f o Article history: Received 6 January 2011 Received in revised form 14 April 2011 Accepted 9 May 2011 Available online 14 May 2011 Keywords: Superhydrophobic Transparent Glass Sol–gel Silica particle Air brushing a b s t r a c t This study fabricated the large area and optically transparent superhydrophobic silica based films on glass surface with optimized hardness. A silane coupling agent, tetraethoxysilane (TEOS), effectively bonds silica particles onto the glass substrate. Desired surface roughness was obtained by adjusting nano silica particles concentration of the precursors prepared by the sol–gel process. Silica suspension was coated onto the glass substrate by the air brushing methods. This method can deposit a uniform, transparent coating on the glass substrate efficiently. Diluting the precursor by adding ethanol or a mixture of D.I. water and ethanol further improved the transmittance and superhydrophobicity efficiency. The results showed that as the silica particle concentration and the thickness of the coating were increased, the surface roughness was enhanced. Rougher surface displayed a higher superhydrophobicity and lower transmittance. Therefore, the concentration of silica particle, volume of coatings, and the ratio of ethanol and D.I. water are of great importance to deposit a transparent, superhydrophobic coating on glass. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Natural hydrophobic performance has recently received much attention and inspired mimetic researches. Many plants, including the lotus leaves, show specific wetting characteristics of superhy- drophobicity and self-cleaning properties. Surfaces with a water contact angle larger than 150 ◦ and a tilt angle less than 10 ◦ is generally called superhydrophobicity, showing almost no contact between liquid droplets and the surface. Superhydrophobic glass surface is one of the most important materials for sustainable energy technology, such as solar cells and solar buildings. If a surface has a characteristic of superhy- drophobicity, water droplets can easily carry dusts away while the droplets moving on the surface. Therefore, such modified sur- face can achieve a function of self-cleaning [1]. On the other hand, the other properties of modified glasses, such as transmittance, surface and hardness should be taken into consideration if such a superhydrophobic surface is applied to an outdoor system. For example, solar energy panels, the modified surfaces must have enough hardness to sustain the impact by dusts in the atmo- ∗ Corresponding author at: Mechanical Engineering Department, National Taiwan University, No. 1, Roosevelt Road Sec. 4, Taipei 106, Taiwan. Tel.: +886 2 33662689; fax: +886 2 23670781/23631755. E-mail address: phchen@ntu.edu.tw (P.-H. Chen). spheres. If the modified glass is applied to a solar cell panel, the surface must have enough transparency to allow sufficient sunlight through the surface to maintain the efficiency of solar cell. More- over, coating technique is also an important issue for fabricating the superhydrophobic glass with an applicable size. The lotus- effect is typically considered to be induced by the combination of hydrophobic material and hierarchical micro/nano structures on a leaf surface [2]. The methods to fabricate lotus-leaf like surface [3–6] or other biomimic structure with specific chemical composition have been investigated in recent years. Up to now, the combination of sol–gel and dip-coating techniques are widely adopted for fabricating superhydrophobic and transparent glass surface [7–10]. Further- more, electrochemical reaction [11], chemical etching [12,13], lithography [14], thermal imprinting [15] and vapor deposition [16] have also been used to fabricate superhydrophobic surface. The superhydrophobic surface can typically be obtained by com- bining the intrinsic hydrophobic property of materials and the surface roughness in sol–gel method. The hydrophobic effect can be increased either using fluorinated silanes [17,18] or alkylated silanes [19,20] because all of the functional groups exposed are highly hydrophobic. Furthermore, one way is to construct a rough glass surface by coating particles on the glass substrate with the help of silanes as a coupling agent. The particles used include SiO 2 [21], TiO 2 [21], ZrO 2 [21] and ZnO [22]. 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.05.041