Yolk-like non-stoichiometric nickel sulfide-based Janus hydrogel photothermal film for enhanced solar-driven water evaporation and multi-media purification.

Solar-driven interface water evaporation is a promising strategy for desalination and wastewater treatment. However, it remains a huge challenge to simultaneously achieve a high light-to-heat conversion efficiency (η) and multi-media evaporation applications. In this study, a highly efficient Janus hydrogel photothermal film was developed using yolk-like non-stoichiometric nickel sulfide (NiS) microspheres and agar hydrogel. The NiS immobilized in the agar hydrogel has full-spectrum absorption characteristics at 200-2500 nm, which can perform efficient light-to-heat conversion and regulate water transport channels. Additionally, the pure agar in the bottom can transport water effectively and avoid heat loss. By the pouring method, the Janus hydrogel film can be easily prepared into various shapes; hence, it can be adjusted depending on the environment in which it is used. The optimized Janus hydrogel film (Janus hydrogel-1) possessed good hydrophilicity and showed an excellent solar evaporation rate of 1.45 kg mh, and a high η of 97% under one-sun irradiation. Theoretical simulation results showed that the outstanding water evaporation performance of Janus hydrogel-1 was mainly due to its relatively free water transport channels. Janus hydrogel-1 can be used for water evaporation applications in various media, including seawater, heavy metal ion/organic wastewater, and domestic sewage. Our work highlights the great potential of Janus hydrogel-1 for realizing a highly effective solar energy-driven interface water evaporation and multi-media purification.