Highly efficient solar-driven water evaporation through a cotton fabric evaporator with wettability gradient

Harvesting light energy and converting it to heat as terminal energy by black photothermal sheets as evaporator is a novel strategy to attain fresh water. Although immense progress has been achieved in terms of the improvement of the evaporation rate and energy efficiency, it is still challenging to limit excessive water from entering the evaporator to eliminate the energy loss. Herein, a novel cotton fabric evaporator with a superhydrophobic-hydrophobic-hydrophilic-superhydrophilic wettability gradient (CFE-WG) was constructed by the layer-by-layer self-assembly of carbon nanotubes (CNTs), followed by separate modification with polyvinyl alcohol (PVA) and polydimethylsiloxane (PDMS). The wettability gradient in the thickness direction of CFE-WG permitted the slow transfer of water from the superhydrophilic side to the hydrophilic area and ultimately trapped the water at the hydrophilic/hydrophobic (water/air) evaporation interface, which avoiding the entry of excessive saline water inside the evaporator and maximally limiting thermal leakage. Under 1 sun irradiation, this CFE-WG exhibited a high vapor generation rate up to 1.54 kg m-2h-1, corresponding to an energy conversion efficiency of 93.7%. Notably, it could be used for desalination and wastewater treatment to collect fresh water that meets drinking requirements set by the WHO. This study provides a facile strategy to prepare a scalable evaporator for portable solar water purification, industrial solar-powered water treatment, and other advanced solar thermal applications.