Synthesis and Surface Properties of Hierarchical SiO2 Coating Layers by Forming Au Nanoparticles

Abstract

Superhydrophobic <TEX>$SiO_2$</TEX> layers with a micro-nano hierarchical surface structure were prepared. <TEX>$SiO_2$</TEX> layers deposited via an electrospray method combined with a sol-gel chemical route were rough on the microscale. Au particles were decorated on the surface of the microscale-rough <TEX>$SiO_2$</TEX> layers by use of the photo-reduction process with different intensities (<TEX>$0.11-1.9mW/cm^2$</TEX>) and illumination times (60-240 sec) of ultraviolet light. With the aid of nanoscale Au nanoparticles, this consequently resulted in a micro-nano hierarchical surface structure. Subsequent fluorination treatment with a solution containing trichloro(1H,2H,2H,2H-perfluorooctyl)silane fluorinated the hierarchical <TEX>$SiO_2$</TEX> layers. The change in surface roughness factor was in good agreement with that observed for the water contact angle, where the surface roughness factor developed as a measure needed to evaluate the degree of surface roughness. The resulting <TEX>$SiO_2$</TEX> layers revealed excellent repellency toward various liquid droplets with different surface tensions ranging from 46 to 72.3 mN/m. Especially, the micro-nano hierarchical surface created at an illumination intensity of <TEX>$0.11mW/cm^2$</TEX> and illumination time of 60 sec showed the largest water contact angle of <TEX>$170^{\circ}$</TEX>. Based on the Cassie-Baxter and Young-Dupre equations, the surface fraction and work of adhesion for the micronano hierarchical <TEX>$SiO_2$</TEX> layers were evaluated. The work of adhesion was estimated to be less than <TEX>$3{\times}10^{-3}N/m$</TEX> for all the liquid droplets. This exceptionally small work of adhesion is likely to be responsible for the strong repellency of the liquids to the micro-nano hierarchical <TEX>$SiO_2$</TEX> layers.