Combining the lotus leaf effect with artificial photosynthesis: regeneration of underwater superhydrophobicity of hierarchical ZnO/Si surfaces by solar water splitting

Fabrication of stable superhydrophobic surfaces in dynamic circumstances is a key issue for practical uses of non-wetting surfaces. However, superhydrophobic surfaces have finite lifetime in underwater conditions due to the diffusion of gas pockets into the water. To overcome this limited lifetime of underwater superhydrophobicity, this study introduces a novel method for regenerating a continuous air interlayer on superhydrophobic ZnO nanorod/Si micropost hierarchical structures (HRs) via the combination of two biomimetic properties of natural leaf: superhydrophobicity from the lotus leaf effect and solar water splitting from photosynthesis. The designed n/p junction in the ZnO/Si HRs allowed for highly stable gas interlayer in water and regeneration of the underwater superhydrophobicity due to the unique ability of the surface to capture and retain a stable gas layer. Furthermore, we developed a model to determine the optimum structural factors of hierarchical ZnO/Si surfaces that aid the formation of an air interlayer to completely regenerate the superhydrophobicity. We also verified that this model satisfactorily predicted the regeneration of underwater superhydrophobicity under various experimental conditions. The regenerative method developed in this work is expected to broaden the range of potential applications involving superhydrophobic surfaces and to create new opportunities for related technologies. By combining the lotus-leaf effect with artificial photosynthesis, a team in Korea has realized a self-sustaining superhydrophobic surface. Superhydrophobic surfaces that mimic lotus leaves have many potential applications including drag reduction, anti-fouling and anti-corrosion. But since their superhydrophobicity stems from a trapped gas layer, which tends to diffuse when submerged in water, they gradually lose their superhydrophobicity in water. Now, scientists at Pohang University of Science and Technology have realized a superhydrophobic surface that maintains a continuous gas layer through the generation of hydrogen gas by water splitting, thus allowing the surface to remain superhydrophobic under water. They also developed a theoretical model for determining the optimal geometry of surface structures for the formation of the gas layer and demonstrated its agreement with experimental results. Regenerative underwater superhydrophobicity was achieved in hierarchical ZnO/Si surfaces via hydrogen gas from photoelectrochemical reaction and unique surface structures for capturing and retaining a stable gas layer. Furthermore, we developed a model to determine the optimum structural factors of hierarchical ZnO/Si for complete regeneration of superhydrophobicity.