Modulating Photothermal Properties of Carbon Dots through Nitrogen Incorporation Enables Efficient Solar Water Evaporation

As a new family in carbon nanomaterials, carbon dots (CDs) are potential candidates for solar water evaporator, owing to their cost-effectiveness, non-toxicity, high solubility, and tunable optical properties. Despite such potentials, however, CDs mainly absorb solar spectrum in the ultraviolet region while their absorption in the visible region is limited, the characteristics that hinder their functionality in generating steam from solar energy. Herein, the optical and photothermal properties of CDs, derived from urea and citric acid, can be modulated by controlling their surface stoichiometry through varying the molar ratio of the precursors. Our approach is simple, fast, and highly scalable by utilizing a microwave irradiation technique. We found that increasing the nitrogen content results in broadening of the absorption spectra into the visible region due to more functional groups introduced on the CD surface that reduce the band gap, as confirmed both by X-ray photoelectron spectroscopy and theoretical calculation. Employing the CDs as photothermal materials in the volumetric solar evaporator, we demonstrate a remarkable evaporation efficiency of up to 70% along with a volumetric evaporation rate of 1.11 kg m-2 h-1 under 1 sun illumination, superior to direct bulk water heating. Furthermore, the CDs show excellent durability and stability, as demonstrated by their stable evaporation rate for 10 days, with no significant decrease in the optical and photothermal properties. This finding provides a pathway to design and functionalize CDs with controllable optical and photothermal properties for an efficient solar evaporation system.