Fast and reversible photo-responsive wettability on TiO2 based hybrid surfaces

A hybrid surface exhibiting a fast and reversible switch between hydrophobic and hydrophilic states was prepared by spin-coating a porous TiO2 layer by a mixture of TiO2 nanoparticles with 11-(4-phenylazo) phenoxy undecanoic acid (denoted as the AzoC11 acid). The nanocomposite film corresponding to the 10% AzoC11 acid/TiO2 mass fraction exhibited both a very important and fast variation of the contact angle (by more than 120 degrees), along with an excellent reversibility. The wettability switching is explained by the trans -> cis -> trans isomerization of the AzoC11 acid under light irradiation or heating. Upon trans -> cis isomerization, optical absorption bands were found to exhibit drastic evolution, the origin of which is discussed in detail by means of a time-dependent density functional theory (TDDFT) study. Subsequent to these isomerization processes, various mechanisms explaining the wettability variations were investigated by comparing the results obtained by DFT calculations, wetting measurements for different surface ratios of the AzoC11 acid under UV irradiation, and by analyzing the molecular structure and molecular surfaces of trans and cis-isomers of the AzoC11 acid.