A facile one-step approach to superhydrophilic silica film with hierarchical structure using fluoroalkylsilane

Abstract

In fabricating the silica films, it's worth noted that adding fluoroalkylsilane (heptadecafluoro-1,1,2,2-tetrahydrodecyl)-trimethoxysilane (17FTMS) after or before the Stöber process will turn out opposite results. Added after the Stöber process, fluoroalkylsilane works as a modification reagent and render highly hydrophobic silica film, with a contact angle of 142.7°. Added beforehand, fluoroalkylsilane serves as the silicon precursor and will help generate superhydrophilic silica film, with a contact angle of 8.2°. The water-repellent property of highly hydrophobic silica film is provided by the fluorocarbon chains which are chemically bonded to the silica spheres. For the superhydrophilic silica film, the phenomenon is quite different in that nanoparticles of 20nm and 250nm, though with huge difference in size, coexist with one another. And fluorine is mainly found in the larger 250nm particles. In this case, fluoroalkylsilane indirectly helps to form a hierarchical structure in line with the Wenzel model which exhibits superhydrophilicity. In this paper, microscopic morphology, structure of particles, and distribution of fluorine are studied. Mechanism of how different procedures of adding the fluoroalkylsilane(17FTMS) can lead to opposite phenomenon will also be discussed based on experimental observations. In conclusion, the paper puts forth the one-step method to fabricate superhydrophilic silica film with fluoroalkylsilane via sol-gel process and it may provide a new way in preparation of surfaces with particular wetting property.