Fabrication of bio-inspired metal-based superhydrophilic and underwater superoleophobic porous materials by hydrothermal treatment and magnetron sputtering

Abstract

Oil-water separation using porous superhydrophilic materials is a promising method to circumvent the issue of oil-polluted water by separating water from oil-water mixtures. However, fabricating metal-based porous superhydrophilic materials with stable superhydrophilicity that can recover their strong hydrophilicity and have acceptable oil-water separation efficiency without complex external stimuli is still a challenge. Inspired by the anti-wetting behavior of broccoli buds, this study successfully fabricated metal-based superhydrophilic and underwater superoleophobic porous materials by hydrothermal treatment of stainless steel meshes (SSMs) combined with magnetron sputtering of metallic Ti and W. The process was then followed with annealing at 300 °C for 4 hours. The effects of coating materials, annealing temperature, and surface structure on the wetting behavior of the prepared meshes were studied and analyzed. The modified meshes exhibited unique broccoli-like microstructures coated with thin TiO_2-x N _x /WO₃ films and showed superhydrophilicity with a 0° water contact angle (WCA) and underwater superoleophobicity with underwater oil contact angles (UOCAs) higher than 155°. They also maintained strong hydrophilicity for more than three weeks with WCAs of less than 13°. Besides, they could recover their initial superhydrophilicity with a 0° WCA after post-annealing at 80 °C for 30 minutes. Notably, the broccoli-like structures and the strong hydrophilic coatings contributed to a significant water flow rate (Q) of 3650 L m^-2 h^-1 and satisfactory oil-water separation efficiency of 98% for more than 15 separation cycles toward various oil-water mixtures. We believe that the presented method and fabricated material are promising and can be applied to induce hydrophilicity of various metallic materials for practical applications of oil-water separation, anti-fouling, microfluidic transport, and water harvesting.

Fig. 1. Schematic diagram of the fabrication procedure of the proposed surfaces.

Fig. 2. Wetting mechanism of (a) broccoli flower head, (b) hydrophobic surfaces, (c) underwater oleophobic surfaces.

Fig. 3. WCA measurements on TiN and TiN300. (a) WCA on TiN on day one, (b) WCA on TiN300 on day one, and (c) change of WCAs on TiN and TiN300 from day one to day 60 under the effect of dirt contamination, post-annealing, and UV light illumination.

Fig. 4. XPS and SEM data for TiN and TiN300 films. (a) XPS survey, (b–e) the corresponding high-resolution of (b) Ti (2p), (c) O (1s), (d) N (1s), and (e) W (4f), and (f and g) SEM data for TiN and TiN300 films.

Fig. 5. Schematic illustration of the photo-induced superhydrophilicity of the thin TiN300 film.

Fig. 6. SEM and WCA measurements of the SSMs (a) SEM of the hydrothermally treated for 12 hours SSM with TiN300 film, (b) SEM of the hydrothermally treated for 8 hours SSM with TiN300 film, (c) a water droplet on the unprocessed SSM, (d and e) time sequence of a water droplet spreading on the modified SSM.

Fig. 7. Oil contamination tests (a) paraffin liquid on the as-received SSM, (b) paraffin liquid on the modified SSMs (c) UOCA on the unprocessed SSM, (d) UOCA on the modified SSMs (e) paraffin liquid adhesion test on the modified SSMs.

Fig. 8. Schematic illustration of the wetting states (a) flat surface with water in air, (b) rough surface with water in air, (c) rough surface with oil in water.

Fig. 9. Oil–water separation mechanism and efficiency (a and b) oil–water separation mechanism using a superhydrophilic mesh, (c) oil–water separation process using the modified SSMs, and (d) oil–water separation efficiency.