A robust bilayer nanofilm fabricated on copper foam for oil-water separation with improved performances

In this work, a facile approach of covalent layer-by-layer assembly was applied to fabricate a bilayer nanofilm which renders roughened copper foam superhydrophobic with robust performances. Before constructing the bilayer nanofilm, the surface of the foam was roughened by using KOH-K2S2O8 as the oxidation system, giving rise to hierarchical structures of flower-like protrusions and petals. A multifunctional polymeric nanofilm, capable of resisting copper corrosion and serving as an activated interface simultaneously, was first introduced onto the copper surface by heating self-assembled monolayers (SAMs) of one triazinedithiolsilane compound (designated as TESPA). Then, perfluorodecyltrichlorosilane (abbreviated as PFDTCS) was anchored onto the TESPA-modified copper surface. Consequently, a bilayer nanofilm of PFDTCS-TESPA is generated on the hierarchical foam that possesses outstanding superhydrophobic capability. The as-prepared foam exhibits excellent reusability and separation efficiency. After fifty recycles, the separation efficiency exceeds 98% while the surface still retains remarkable superhydrophobicity. The improved efficiency and recycle number are outstanding compared to those of similar systems reported previously. X-ray photoelectron spectroscopy (XPS) was conducted to reveal the mechanism of the robust performances of the bilayer-coated foam. The results show that TESPA can react with copper oxide (CuO) through SH groups yielding TESPA SAM. Upon heating, the newly formed disulfide units (-SS-) and siloxane networks (SiOSi) of the TESPA polymeric nanofilm protect the copper; the outward silanol groups (SiOH) endow the surface with activating ability. PFDTCS can covalently bond to the TESPA polymeric nanofilm via these SiOH groups. The specific arrangement of PFDTCS-TESPA and the chemical bonds of Cu(I) S, as well as the three-dimensional (3D) cross-linked textures of -SS-units and SiOSi networks, cooperatively enhance the chemical durability of the bilayer. The design strategy of preparing the bilayer nanofilm via an interlayer (e.g., TESPA as a molecular adhesive displayed in this work) can also be applied in other superhydrophobic systems for improved long-term utilization.