Super water- and oil-repellent surfaces on intrinsically hydrophilic and oleophilic porous silicon films.

We demonstrate that porous Si films fabricated by a convenient gold-assisted electroless etching process, which possess a hierarchical porous structure consisting of micrometer-sized asperities superimposed onto a network of nanometer-sized pores, are able to induce a superhydrophobic phenomenon on an intrinsically hydrophilic hydrogen-terminated Si surface and a superoleophobic phenomenon on an intrinsically oleophilic self-assembled monolayer-coated Si surface. Through comparison with porous Si films consisting of vertically aligned straight pores, which are hydrophilic and oleophilic, we show that an overhang structure resulting from the hierarchical porous structure is essential to preventing water and oil from penetrating the texture of the films and inducing the observed macroscopic superhydrophobic and superoleophobic phenomena.