Zinc-Reduced CQDs with Highly Improved Stability, Enhanced Fluorescence, and Refined Solid-State Applications

Carbon quantum dots (CQDs) have attracted considerable interest because of their advantages of low cost, nontoxicity, easy preparation, and diverse optical properties. Photophysical properties and stability are two key issues that affect the further use of CQDs, especially for solid-state applications. Here, we report a facile and effective method to enhance the fluorescence and moisture-resistance of CQDs. CQDs were synthesized by the thermolysis of citric acid on a multigram scale and were readily used to react with zinc powder in an aqueous solution to produce zinc-reduced CQDs (Zn-CQDs). The process was operated under mild conditions and completed in 10 min. The fluorescence intensity of the CQDs increased more than three times after reacting with zinc. In contrast to the pristine CQD solids, which easily absorbed moisture and turned into viscous liquids, the as-prepared Zn-CQD blocks possessed an ordered porous structure and were immune to moisture in the air. They remained stable under ambient conditions for months. Comprehensive studies were carried out to explore the reaction mechanism and material properties. It was revealed that zinc likely served as a reductant in the process. On the basis of the satisfactory properties of the Zn-CQD blocks, their solid-state applications were studied. A Zn-CQD-based fluorescent film for temperature monitoring was fabricated with favorable linear response and reversibility. Moreover, the as-obtained Zn-CQD blocks showed remarkable adsorption of particulate matter (PM). Therefore, it is expected that they may find practical use in the purification of PM-polluted air.