Control of the chemical and physical behaviour of silicon surfaces for enhancing the transition from hydrophilic to superhydrophobic surfaces

Abstract

In this work, we present a convenient and simple experimental approach based on the modification of chemical and topographical state of silicon surfaces for enhancing the transition from hydrophilic to superhydrophobic surfaces. Using the concept of self assembly technique and chemical deposition process, we have constructed two microstructured silicon substrates: hydrophilic surface (with high surface density of amine groups) and superhydrophobic surface (having long carbon chains moieties). In the first case (amine-surface -NH2), amine functionalized silica particles were chemically immobilized on cleaned surface by nucleophilic addition between epoxy (or carboxyl) and amine groups. SEM images of the resulting substrate indicate that the roughness become very important and a microstructured surface (honeycomb-like structure) has been observed when carboxyl-silanes CPS was used as coupling agent. Using Moon's method, we found a higher surface density of amino groups which reaches 25 amino groups by 100Å2 relatively higher than the aminosilylated surface (∼3 amino groups by 100Å2). Of course, this subsequent change could be explained by the fact that an increasing surface area (high roughness) enhances surface density. While, in the second case (alkyl surface—[CH2]18), silica particles have been chemically attached onto aminosilylated surface by Stober's method in situ. In this case, we attempt that the ammonia reactive catalyze the Stober reaction directly on the surface of the substrate. Deposited silica particles films were then hydrophobized by chemisorption of OTS molecules by SAMs technique. Using contact angle measurements, we found a CA value of about 170° and a strong superhydrophobic property of the substrate has been obtained. This behaviour could be explained by the contribution of two factors: chemical composition and geometric microstructure. Our result demonstrates the possibility to construct covalent structures with functionalized silica particles and promote their application in other fields like biosensors, optoelectronic, wettability control devices.