Stages of the synthesis of carbon dots from citric acid and ethylenediamine: IR spectroscopy

Carbon dots synthesized by the hydrothermal method from citric acid and ethylenediamine have a luminescence quantum yield that significantly depends on the synthesis conditions and, under certain synthesis parameters, reaches more than 90%. To clarify the reasons for the dependence of the optical properties of carbon dots on the synthesis parameters, it is necessary to understand how their structure changes. For this purpose, in this paper we studied the relationship between structural changes and optical properties of carbon dots at different stages of their hydrothermal synthesis from a mixture of citric acid and ethylenediamine in a ratio of 1 : 1, achieved in 3 h of synthesis at a temperature varying from 80 to 200$^\circ$C with a step of 20$^\circ$C. As a result of the studies, the temperature boundaries of the stages of synthesis of carbon dots and changes in their structural and optical properties were determined: the beginning of the synthesis of luminescent structures; stage of active dehydration and carbonization of carbon dots with a concomitant increase in the number of both luminescent and non-luminescent chromophore structures; continuation of carbonization leading to the gradual destruction of luminescent structures. The change in chemical bonds in the reaction products during the synthesis process as the temperature increases is analyzed.