Ethylenediamine Assisted Synthesis of <i>o</i>‐Phenylenediamine‐Based Red Emissive Carbon Quantum Dots: A Strategy to Improve the Fluorescence Quantum Yield

AbstractO‐phenylenediamine (OPD) is commonly used as a precursor in the preparation of red emissive carbon quantum dots (R‐CQDs) due to the sp2 hybridized structure. However, the low fluorescence quantum yield (QY) of the OPD‐based R‐CQDs limits its application. Although some efforts have been made, the improvement of QY is still limited. In this paper, a strategy is proposed to improve the QY of OPD‐based R‐CQDs by introducing ethylenediamine (EDA), which plays a key role as a nitrogen (N) dopant due to its high N content. The molar ratio of OPD to EDA (MOPD/MEDA), the reaction time (t) and temperature (T), and the amount of concentrated H2SO4 (VH2SO4), are optimized. The R‐CQDs with QY as high as 32.65% and full width at half maximum (FWHM) emission as narrow as 25 nm are obtained via a hydrothermal procedure under the optimal experimental conditions i.e., MOPD/MEDA = 1/3, t = 6 h, T = 180 °C and VH2SO4 = 4 mL. Such a QY is higher than most of the reported OPD‐based R‐CQDs. Besides, it is found that concentrated H2SO4 acts as the catalyst in addition to protonation. The enhancement of QY is attributed to the increase of the aromatic N‐containing heterocyclic structures (C═N) after the introduction of EDA and catalysis by H2SO4.