Photothermal Lignin-Cross-Linked Thermo-Switchable Janus Hydrogel for Light-Adaptive Water Evaporation

Solar-driven interfacial water evaporation is a promising technology for sustainable freshwater production. In this study, a series of thermo-switchable Janus hydrogel evaporators were fabricated via a facile copolymerization of N-isopropylacrylamide and acrylamide, using lignin methacrylate as the dual-functional photothermal cross-linker. No additional light absorbers or cross-linkers were required in the preparation. Under illumination, the Janus structure, featuring a reduced-hydrophilic top surface and a highly hydrophilic bottom layer, was induced in situ by the photothermal-thermoresponsive cascade, avoiding mismatches inherent in conventional predesigned structural or compositional asymmetry. The asymmetric architecture synergistically suppressed salt crystallization, relieved heat loss, and facilitated fast water transportation, resulting in an evaporation rate of 2.46 kg·m–2·h–1 under 2 sun irradiation in deionized water. The evaporation rates exceeded 2.34 kg·m–2·h–1 in 3.5 wt % NaCl and 2.25 kg·m–2·h–1 in real seawater, slightly degraded when compared to that in freshwater. In the dark, the hydrogel recovered its full hydrophilicity, which could completely dissolve the accumulated surface salt within 3 h. The light-adaptive dynamic Janus switching and hydrophilicity alternation offered a feasible route for sustainable and diurnal solar desalination, enabling daytime evaporation and nighttime regeneration. This capability was validated by stable evaporation performance during the 7 day outdoor seawater evaporation experiment. The integration of photothermal conversion and covalent cross-linking into the single lignin-derived component ensured the stability of the hydrogel for long-term evaporation and provided a promising avenue for developing simple and robust biomass-based solar-driven evaporators.