Fluorescent carbon dots synthesized in solid phase and air for application in LEDs

Carbon dots (CDs) have been widely adopted as optical materials because of their excellent luminescent properties. However, most of the reported synthetic methods are conducted in solvents, especially hydrothermal/solvothermal reactions, leading to intractable problems such as toxic and flammable solvents, complex and inseparable by-products, and dangerously high pressures and temperatures. Solid-phase synthesis of CDs in air is an effective solution to overcome the above issues, but solid reactions always result in uncontrolled growth and agglomeration of nanoparticles. In this study, some inorganic salts are selected as catalysts for synthesizing CDs in solid states and air, which also play as dispersants to hinder CDs aggregation. In the meantime, some aromatic derivatives containing hydroxyl and amino groups are chosen as carbon sources, ground with the optimized catalyst, and then heated together in air. The production yields are affected by the reaction time and reactant ratio, while the graphitization degrees of the CDs are determined by the reaction temperature. The I_G/I_D value of their Raman spectra increases from 0.59 to 0.85, and the particle size decreases from 2.5 to 1.4 nm when the synthesis temperature is increased from 200 to 280 °C. The as-prepared CDs show emission peaks ranging from 366 to 606 nm, with the photoluminescence (PL) quantum yield up to 53%. Their emission color variation mainly results from different carbon sources, which can be ascribed to the differences in the element composition, functional groups, and graphitic nitrogen content of these CDs. By dispersing CDs of different concentrations into polyvinyl alcohol (PVA) and combining them with blue LEDs, cold, standard, and warm white light emitting devices (WLEDs) are prepared, with a color rendering index (CRI) up to 84. Since the as-prepared CDs have antioxidant ability at high temperature, the as-prepared WLEDs have long lifespans, remaining the effective white luminescence after 72 h continuous work.