Nitrogen-doped carbon quantum dots obtained hydrothermally from citric acid and urea: The role of the specific nitrogen centers in their electrochemical and optical responses

The knowledge of how the different types of nitrogen centers affect the electrochemical and optical prop- erties of Nitrogen-doped carbon quantum dots (N-CDs) is of paramount importance in the design and synthesis of these nanostructures. In fact, from the knowledge of the redox centers of N-CDs it is possi- ble tune their energetic levels in order to realize the suitable matches for the design of opto-electronic, photovoltaic or catalytic devices. Herein we present a study of the electrochemical and the optical re- sponses of N-CDs, obtained by hydrothermal method from citric acid and urea. The main issue is to investigate the role played by the specific nitrogen centers in their redox processes. From the voltam- metric responses we showed that the observed oxidation processes of N-CDs involve their graphitic and pyrrolic N-atoms, respectively, while the reduction process regards their protonated pyridinic N-atoms. Then, from spectro-electrochemical determinations, we explained that the oxidation of the graphitic N- atoms is a two electron process that gives an N -oxide form through the reaction with a molecule of water and the loss of two protons. Finally we discussed the observed pH-dependence of both the optical and the electrochemical responses of our N-CDs to explain the role played by their pyridinic N-atoms.