Metal Identity as a Design Switch: Engineering Salt Adaptive Photothermal Evaporation in Conjugated Metal-organic Frameworks

Photothermal interfacial evaporation represents a promising route to address global water scarcity, yet achieving simultaneously high efficiency and robust salt tolerance remains a critical challenge. The decisive role of the metal center in governing the hierarchical architecture and evaporation dynamics of metal–organic frameworks (MOFs), especially in saline environments, remains underexplored, despite their versatile, tunable structures for interfacial design. Herein, using a scalable series of two-dimensional conjugated M–HITP frameworks, we demonstrate that metal identity acts as a central design switch to direct coordination geometry, long-range order, and ultimately, photothermal performance. While all M–HITP variants exhibit exceptional solar evaporation rates that surpass conventional systems, only Ni–HITP displays a distinctive salt-adaptive enhancement, with its evaporation rate rising from 3.28 kg·m–2·h–1 in pure water to 4.80 kg·m–2·h–1 in 3.5 wt% NaCl. This unique behavior originates from the strong metal–ligand coupling and highly ordered layered structure, which promotes selective salt-ion enrichment at the evaporation interface. Such interfacial ion accumulation reconstructs the hydrogen-bond network of water, significantly reducing the effective evaporation enthalpy from 667.43 J·g–1 to 424.04 J·g–1. This work not only deciphers the metal center structure performance interplay in conjugated MOFs but also establishes a novel salt-utilization strategy, paving a rational design pathway toward adaptive, high-performance photothermal evaporators for real-world saline water treatment.