Enhanced interfacial solar desalination using nano-engineered MoO _x photothermal evaporators

Interfacial solar desalination relies on enhanced optical absorbance, heat localization at the air/water interface, and effective water management on photothermal evaporators. However, its commercialization is hindered by marginal vaporization rates, processing challenges, and unacceptable stability. This study presents a novel substoichiometric molybdenum oxide (MoO _x ) solar absorber with a unique nanochannel-on-microchannel architecture, designed to enhance broadband absorbance, concentrate heat within thin layers of water, and promote superwicking. For the first time, a tightly focused, non-diffracting Bessel laser beam is employed to create nanochannels layered over hierarchically designed open microchannels. The nanochannels promote cluster evaporation by distributing water in very thin layers, while the hierarchical morphology and rough oxide microchannels contribute to strong broadband absorbance and generate capillary forces that enable superwicking on the surfaces at any angle. Outdoor tests demonstrated exceptional performance, with evaporation rates of 4.21 kg m^-2 h^-1 under 1 sun and 19.3 kg m^-2 h^-1 under 3 suns, outperforming existing evaporators. Comparison of these rates with indoor rates under controlled lab conditions suggests that ∼50% of the total evaporation rate was contributed by wind and ambient temperature. Moreover, the impact of water salinity on interfacial evaporation is revealed by performing experiments and comparing results from both saline and deionized water. Salt ions that are specifically adsorbed at the solution/MoO _x interface are found to inhibit direct contact between MoO _x and the secondary water, thereby enhancing evaporation by lowering the adsorption energy. A comprehensive analysis of hydrogen bonding states, the electrical double layer, temperature measurements, vaporization enthalpy, and efficiency calculations corroborates the performance improvements. Our findings demonstrate significant potential for large-scale solar desalination and provide new possibilities for advancing interfacial solar desalination.