Self-cleaning silica coatings on glass by single step sol–gel route

Considerable efforts have been made over the last few decades for developing self-cleaning surfaces exhibiting high water contact angles. We prepared TEOS based hydrophobic, self-cleaning silica coatings on a glass substrate by single step sol–gel process with methyltrimethoxysilane (MTMS) as a co-precursor. The coating sol was prepared by keeping the molar ratio of tetraethoxysilane (TEOS), methanol (MeOH) and water (H2O) constant at 1:33.15:6.06 respectively, throughout the experiments with NH4F (0.1 M) as a catalyst. The molar ratio (M) of the MTMS/TEOS was varied from 0 to 7.84 and its influence on the hydrophobicity was studied by static contact angle and sliding angle measurements. It was observed that, with increase in the M value from 0 to 1.57, the contact angle increased from 30° to 135°, respectively and with further increase in M value, the contact angle reduced and remained nearly constant thereafter. For coatings prepared with M values 0.78, 1.18 and 1.57, petal effect (water sticking) was observed. The sliding angle for the coatings was reduced with further increase in M values, leading to self-cleaning surface. The hydrophobicity of the silica coatings was sustained up to a temperature of 270 °C and the optical transmission observed was above 80%. The silica coatings were characterized by surface roughness measurement, field emission scanning electron microscopy (FESEM), Fourier transform infrared (FTIR) spectroscopy, percentage of optical transmission, self-cleaning test and static contact angle and sliding angle measurements.