Wetting behavior of silicon nanowires array fabricated by Metal-assisted chemical etching

Abstract

A silicon (Si) substrate exhibits a hydrophobic surface (contact angle, CA ∼ 90.2⁰), and upon anodization, the porous Si (PSi) also possesses a hydrophobic surface (CA ∼ 142.4⁰). Silicon nanowires (SiNWs) array, also known as black silicon, exhibits lotus effect, i.e., water droplet role-over the surface without penetrating inside the surface. An array of SiNWs, fabricated by metal-assisted chemical etching (MACE) of Si, act as a superhydrophobic surface (i.e., CA > 150⁰) without using any functionalization material. The MACE fabricates etched-Si, using PSi substrate that also possesses a superhydrophobic surface. Trapped air, present among the conical tips of the densely formed SiNWs array, reduces the solid–liquid contact area and improves the de-wetting behavior. The trapped air increases on increasing the length of SiNWs, which further enhances the de-wetting behavior. The wetting behavior changes to hydrophobic/hydrophilic when kept for a longer duration due to the aging effect. Oxidation causes aging and changes the conical/dendrite structure of the SiNWs/etched-Si surface. The Fourier transform infrared spectroscopy confirms the hydrogen passivation on the surfaces of SiNWs because of HF etching and highly oxidized surfaces due to the effect of aging.