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 properties 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 possible 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 responses 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 voltammetric 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.