A facile bioinspired strategy for accelerating water collection enabled by passive radiative cooling and wettability engineering

Vapor condensation and condensate collection play a crucial role in solar water desalination, which is a promising solution to the shortage of freshwater. However, the currently used condensers based on thermal conduction and/or convection cooling are limited in cooling power for vapor condensation, whilst their surface wettability requires appropriate modulation to facilitate condensate collection. In this work, we achieve efficient water collection via using a wettability and spectral selectivity engineered coating comprising hydrophobic SiO2 and hydrophilic TiO2 nanospheres at an optimized weight ratio. The tuning of surface wettability promotes a dropwise collection mode, leading to maximized droplet harvesting performance of 1047 mg·cm−2·h−1. Further, the coating can spontaneously cool itself by reflecting ~90% of the incident sunlight whilst emitting >95% of thermal infrared rays, leads to a daytime sub-ambient temperature drop of 7 °C. Hence, under an ultra-humid condition that mimics a solar water-purification system, the coating that integrates daytime radiative cooling with dropwise collection demonstrates an excellent water production rate, which is superior to those of the hydrophilic surface and the surface with restricted radiative cooling access to the sky. Moreover, the coating comprising TiO2 photocatalyst can recover its wettability through the photocatalytic degradation of adsorbed oily dirt pollutants.