ZnO-Nanowires-Coated Smart Surface Mesh with Reversible Wettability for Efficient On-Demand Oil/Water Separation

The rapid industrial growth has led to the large production of oily wastewater. Treatment of oily wastewater is an inevitable challenge to manage the greater demand of clean water for the rapidly growing population and economy. In the present work, we have developed a smart surface mesh with reversible wetting properties via a simple, ecofriendly, and scalable approach for on-demand oil-water separation. ZnO nanowires (NWs) obtained from the chemical vapor deposition method were coated on a stainless steel (SS) mesh. The as-synthesized ZnO-NWs-coated mesh shows superhydrophilic/underwater superoleophobic behavior. This mesh works in "water-removing" mode, where the superhydrophilic as well as underwater superoleophobic nature allows the water to permeate easily through the mesh while preventing oil. The wetting property of ZnO-NWs-coated mesh can be switched easily from superhydrophilic to superhydrophobic state and vice versa by simply annealing it at 300 °C alternatively under hydrogen and oxygen environment. The separation is solely driven by gravity. Thus, the reversible wettability of ZnO NWs provides a smart surface mesh which can be switched between "oil-removing" and "water-removing" modes. It was found that for more than 10 cycles of mesh reutilization in both modes alternatively, the separation efficiency of 99.9% stayed relatively invariant, indicating a prolonged antifouling property and excellent recyclability. This work provides a simple, fast, cost-effective, and on-demand solution for oily wastewater treatment and opens up new perspectives in the field of controllable oil-water separation.