Reversible wettability control of silicon nanowire surfaces: From superhydrophilicity to superhydrophobicity

Bio-inspired superhydrophobic surfaces have attracted considerable attention due to their potential applications. Although various techniques to fabricate artificial superhydrophobic surfaces have been demonstrated, most of the methods lack water adherence or controllable wetting properties of the surfaces, which hinders their practical usage. In this paper, we present a simple approach to fabricate water-adhesive superhydrophobic silicon nanowire (Si NW) surfaces by applying a thermal annealing treatment in oxygen ambient. The Si NW arrays were fabricated using a metal assisted chemical etching method. After the cycled rapid thermal annealing (RTA) process at 1000 C under oxygen ambient, the water contact angle of the Si NW surface changed dramatically from 0 to 154.3 with high water-adhesive properties. This drastic change of the wettability could be attributed to the formed siloxane groups (- Si-O-Si-) on the thermally-treated Si NW surfaces; H₂O is released from two adjacent silanol groups (-Si-O-H) to form siloxane groups during the RTA process. When the annealed Si NW was exposed in air, the wettability of the superhydrophobic Si NW was reconverted due to the re-formation of silanol groups (-Si-O-H). The wettability conversion of Si NW between superhydrophilic and superhydrophobic was repeated with good reversibility.