Stepped-Current-Controlled Evolution of Cu(OH)2/CuO Tandem Nanostructure for Efficient Photothermal Conversion.

Photothermal conversion is an important compensation to the current energy system, which is capable of converting the sunlight into thermal energy. Copper serves as an excellent heat conductor but exhibits limited absorption over the solar spectrum, even with oxide coatings upon annealing. Herein, vertically aligned copper oxides are tailored on the copper surfaces by electrochemical anodization. The current transient renders a unique stepped profile, which corresponds to the structure evolution from a double-layered stack, i.e., the Cu(OH)₂ nanowires sitting atop the CuO nanosheets, to a single-layered CuO nanosheets. The as-anodized CuO nanosheets impart strong light absorption in the range of 200-1200 nm. Under one sun illumination, the water rises up to 102.6 °C in 20 min inside copper tubes with the CuO coatings, as compared to the 60.6 °C without the coatings. Under frozen conditions, the CuO nanosheets also result in a rapid de-icing process in just 700 s, in obvious contrast to the 1200 s for pristine copper. This is attributed to the high photothermal conversion efficiency of 73.6% for the CuO coatings, being more than doubled with respect to the copper. The photothermal coatings may find important applications in seawater desalination, evaporation-induced electricity generation, hydrogen evolution reaction, etc.