Toward carbon dots from citric acid and ethylenediamine, part 1: Structure, optical properties, main luminophore at different stages of synthesis

Carbon dots (CDs) from citric acid (CA) and ethylenediamine (EDA) synthesized under certain parameters of hydrothermal synthesis are reported to demonstrate ultra-bright luminescence in the blue-violet region with a quantum yield up to similar to 100 %. However, the questions remain: is this luminescence really belong to the nanoparticles or to concomitant molecular luminophores; at what stage of the CDs' synthesis such luminophores are formed and lost; how exactly structure of the reacted precursors changes when the synthesis parameters change? In this study, to answer these questions, the array of 392 samples of ethylenediamine and citric acid aqueous solutions undergone the process of hydrothermal method of synthesis, varying EDA:CA ratio in the range of 0-20:1, temperature in 80-200 degrees C, and reaction time in 0.5-6 h. For all samples the luminescence excitation-emission matrices, optical absorption and FTIR spectra were obtained, quantum yields and luminophores' intensity of samples' luminescence at an excitation wavelength of 350 nm were calculated. Based on the obtained data, the processes of CDs' gradual synthesis - polymerization, dehydration and carbonization - were identified, the changes in the composition of the reaction products during different stages of synthesis were revealed. It was established that the formation of the main samples' luminophores starts with the polymerization of precursors, accelerates with the initial carbonization of the samples, while the stage of graphitizing carbonization - formation of CDs cores - brings their partially destruction.