Nanoporous black silver film with high porosity for efficient solar steam generation

Given the challenges brought by the shortage of freshwater resources, solar water evaporation has been regarded as one of the most promising technologies for harnessing abundant sunlight to harvest clean water from the sea. Nanostructured metals have attracted extensive attention in solar water evaporation due to their localized surface plasmon resonance effect, but highly porous metallic films with high evaporation efficiency are challenging. Herein, a self-supporting black nanoporous silver (NP-Ag) film was fabricated by dealloying of an extremely dilute Al₉₉Ag₁ alloy. The choice of the dilute precursor guarantees the formation of the NP-Ag film with high porosity (96.5%) and low density (0.3703 g·cm^–3, even smaller than the lightest metal lithium). The three-dimensional ligament-channel network structure and the nanoscale (14.6 nm) of ligaments enable the NP-Ag film to exhibit good hydrophilicity and broadband absorption over 200‒2,500 nm. More importantly, the solar evaporator based on the NP-Ag film shows efficient solar steam generation, including the efficiency of 92.6%, the evaporation rate of 1.42 kg·m^–2·h^–1 and good cycling stability under one sun irradiation. Moreover, the NP-Ag film exhibits acceptable seawater desalination property with the ion rejection for Mg²⁺, Ca²⁺, K⁺ and Na⁺ more than 99.3%. Our findings could provide a new idea and inspiration for the design and fabrication of metal-based photothermal films in real solar evaporation applications.