Selective superantiwetting/superwetting fluorine-free nanostructured ZnO/CuO mesh membrane for efficient separation of oil/water mixture: Oxygen vacancy-dependent wetting stability studies

Selective superantiwetting/superwetting nanostructured ZnO/CuO mesh membrane was constructed using a facile method. The ZnO/CuO coated mesh exhibited switchable wettability from superhydrophobic to superhydrophilic properties using annealing treatment at high temperatures. The field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analyses were applied for characterization. The wettability was determined by water/oil static contact angle (W/OCA) and contact angle hysteresis (CAH). The coated mesh showed superhydrophobic properties without any post-treatment with a water contact angle of 161.2° ± 1.5° and contact angle hysteresis of 4.2° ± 1°. The wettability of the surface was altered to superhydrophilicity in air and underwater superoleophobicity (OCA of 162° ± 2°) after annealing at high temperatures. Evaluation of wetting stability confirmed that the ZnO/CuO surfaces exhibited long-term stability of superwetting properties towards the water after exposure to ambient air due to creating high oxygen vacancies amounts after annealing treatment compared to the ZnO coating. The prepared nanostructured surface was considered to isolate the oil/water mixtures. The results proved that the coated meshes could be applied effectively to separate oil/water mixtures with excellent separation efficiency (> 99.9%) and good flux (above 2000 L m−2 h−1) regardless of oil density, even in harsh conditions.