Ethylenediamine Assisted Synthesis of o-Phenylenediamine-Based Red Emissive Carbon Quantum Dots: A Strategy to Improve the Fluorescence Quantum Yield

O-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 degrees 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 & boxH;N) after the introduction of EDA and catalysis by H2SO4.