Synthesis and characterization of transparent hydrophobic silica thin films by single step sol–gel process and dip coating

The work presented here describes the room temperature synthesis of highly transparent and hydrophobic silica coatings using tetramethoxysilane (TMOS) as a precursor and trimethylchlorosilane (TMCS) as a surface modifying agent via a simple dip coating technique. By varying the TMCS/TMOS molar ratio, the hydrophobic surface properties have been tailored, covering the entire range from wetting to non-wetting. The contact angle of a water droplet for TMCS/TMOS molar ratios between 0.6 and 1.4 was around 120°; below 0.6 these values continuously decreased to reach approximately 12° for a pure TMOS film. The resulting silica-based coatings have shown an optical transmission over the visible range up to 85% (in reference to 100% transmission defined by a plain glass substrate). The as-prepared hydrophobic silica films retained their hydrophobicity up to a temperature of 230 °C. At higher temperatures, a gradual loss of the hydrophobic properties was observed. There is a tradeoff between optical transmittance and non-wetting behavior. The best coating performance in terms of both, high optical transmission and water contact angle were obtained for a TMOS:MeOH:TMCS:H2O molar ratio of 1:14.8:1.4:3.3, respectively. In addition to contact angle and optical transparency measurements, the films were also characterized by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and thermal stability measurements.