Anomalous Salt-Enhanced Evaporation in Conjugated Metal-Organic Frameworks: The Critical Role of Metal Centers in Tailoring Interfacial Water Dynamics.

Abstract

Photothermal interfacial evaporation is a promising route to alleviate global water scarcity, but simultaneously achieving high efficiency and robust salt tolerance remains a critical challenge. Here, we construct a series of scalable two-dimensional conjugated M-HITP (M = Ni, Co, Pt) frameworks with an invariant ligand backbone. The resulting composite evaporators all exhibit high evaporation rates in pure water, with Pt-HITP reaching up to 4.55 kg m-2 h-1. The metal center plays a decisive role in governing the hierarchical architecture and evaporation dynamics of metal-organic frameworks (MOFs), enabling reliable light absorption and efficient energy conversion. Only Ni-HITP displays a salt-adaptive enhancement, and its evaporation rate increases from 3.28 kg m-2 h-1 in pure water to 3.80 kg m-2 h-1 in 3.5 wt % NaCl solution. This anomalous behavior arises from the synergistic effect between the dispersed salt-retention microenvironments formed by the relatively continuous wrinkled nanosheet coating of Ni-HITP and the Na+-induced reorganization of the hydrogen-bonding network of interfacial water, which reduces the effective evaporation enthalpy of interfacial water in Ni-HITP. This work not only deciphers the interplay between metal-center identity and the structural/evaporative performance of conjugated MOFs, but also establishes a salt-utilization strategy, providing a rational design pathway toward adaptive, high-performance photothermal evaporators for saline water treatment.