A salt-resistant Janus evaporator assembled from ultralong hydroxyapatite nanowires and nickel oxide for efficient and recyclable solar desalination

Solar energy-driven interfacial water evaporation is a promising energy utilization technology in the field of seawater desalination and water purification. However, the accumulation of salt on the heating surface severely impairs the water evaporation performance and long-time stability. Herein, we demonstrate a new kind of photothermal paper comprising a high-temperature-resistant paper made from ultralong hydroxyapatite nanowires and glass fibers and black nickel oxide (NiO) nanoparticles for solar energy-driven desalination. Owing to the high photothermal conversion ability, fast water transportation in the air-laid paper, and good heat insulation, the hydrophilic HN/NiO photothermal paper can achieve efficient, stable and recyclable water evaporation performance. In addition, a Janus HN/NiO photothermal paper based on hydrophobic sodium oleate-modified ultralong hydroxyapatite nanowires has been developed, and it has a high water evaporation efficiency of 83.5% under 1 kW m-2 irradiation. In particular, with the bottom hydrophobic ultralong hydroxyapatite nanowire layer and water-transporting channels in the air-laid paper to facilitate salt exchange, the as-prepared Janus evaporator exhibits no salt accumulation on the surface, high performance and long-time stable desalination using simulated seawater (3.5 wt% NaCl). Furthermore, the Janus evaporator with the hydrophobic ultralong hydroxyapatite nanowire substrate can be extended to support other photothermal materials such as black titanium oxide (Ti2O3) and Ketjen black carbon. The as-prepared Janus HN/Ti2O3 and Janus HN/KB photothermal paper also exhibit salt-resistant desalination function. The as-prepared Janus salt-resistant photothermal paper with efficient, stable and recyclable merits has great potential in solar energy-driven desalination and water purification.