A Porous Interfacial Photothermal Layer of Fused Core-Shell Carbon-Polymer Nanospheres for Directional Salt Crystallization and Zero-Liquid-Discharge.

Abstract

Interfacial solar evaporation offers a sustainable route for freshwater production; however, effective zero-liquid-discharge (ZLD) operation under hypersaline conditions remains fundamentally challenging because high salinity suppresses water activity and vapor pressure while intensifying coupled heat, mass, and salt management. Herein, we report a hierarchical hydrogel evaporator featuring a thin, porous interfacial photothermal layer composed of fused core-shell carbon-polymer nanospheres, fabricated via interfacial precipitation polymerization. The surface-confined photothermal layer ensures effective thermal confinement and abundant liquid-vapor interfaces, while the underlying hydrogel matrix sustains continuous water replenishment, delivering an exceptional evaporation rate of 6.2 kg m^-2 h^-1 under one-sun irradiation without requiring bulky 3D architectures. During ZLD operation, direction-dependent transport within the thin porous layer drives lateral advection of concentrated brine away from the active evaporation interface while suppressing through-thickness salt accumulation, thereby enabling localized crystallization in the distal region. As a result, near-complete spatial separation of water evaporation and salt crystallization is achieved from 20 wt.% brine. Outdoor demonstration further delivers 26.6 L m^-2 day^-1 of freshwater and 7.7 kg m^-2 day^-1 of recovered salt, underscoring the potential of this platform for practical ZLD desalination and mineral recovery.