Photothermal Porous Material with Gradient Hydrophobicity for Fast and Highly Selective Oil/Water Separation and Crude Oil Recovery.

Oil spills and oily wastewater discharges have posed severe threats to the ecosystem and human health, yet efficient cleanup and recovery remain huge challenges. The absorption of crude oil is especially difficult due to its high viscosity. In this study, we propose a strategy for the fast and highly selective absorption of crude oil as well as other oils and organic solvents with variable viscosity by combining the desert beetle's back-inspired gradient hydrophobicity with the photothermal effect to enhance the absorption rate. The oil-absorbent material was prepared through the alkylsilane-based gradient chemical modification of MXene-polyurethane sponges. The hydrophobic gradient across the composite sponge offers an extra driving force for the selective oil wetting in the sponge. Owing to the synergistic effect between gradient wettability and photothermal heating, a faster absorption rate, in addition to the high separation rate, was achieved for a variety of oils, including thick crude oil, thin crude oil, and light diesel oil, compared to that without gradient wettability. The as-prepared material is robust with good repeatability for the oil absorption. The surface silane modification was also demonstrated to help prevent the oxidation of MXene, facilitating the long-term stability of the material. This study will enlighten the development of fast and highly selective liquid absorbents.