Magnetic Nanoparticle Cluster-Based 3D Solar Steam Generator with High Recyclability

We developed a three-dimensional (3D) solar steam generator (SSG) with a hierarchical spiny structure across its entire surface, employing a magnetized iron cylinder and magnetic nanoparticle clusters (MNCs). In the two-dimensional (2D) SSG configuration, MNCs positioned on a disc-shaped permanent magnet formed spiny structures aligned with the magnetic field lines. Upon exposure to sunlight, this hierarchical spiny structure significantly enhanced light absorption via multiple reflections and promoted water evaporation by expanding the surface area. Transitioning from a 2D to a 3D structure using a cylindrical permanent magnet in the 3D SSG improves the evaporation performance by effectively harnessing environmental energy for dark evaporation. However, generating a spiny structure on the side surface of the cylindrical magnet presents challenges owing to the weaker magnetic field on the side surface. We addressed this limitation by magnetizing a hollow iron cylinder placed over a permanent magnet, which led to the creation of a uniform magnetic field and produced a hierarchical spiny structure across the entire surface. The 3D SSG achieved an evaporation rate of 2.8 kg m(-2) h(-1), which is 1.64 times higher than that of the 2D SSG, attributable to its efficient environmental energy utilization. Furthermore, we demonstrated the recyclability and self-regeneration of the 3D SSG through evaporation experiments using a 3.5 wt % NaCl solution.