Oxygen Species Role in the Nitrogen Doping of N-rGO from GO

Graphene is a nanostructure of great interest due to its extraordinary properties such as high surface area, high thermal conductivity, high mobility of charge carriers and excellent mechanical properties. However, the chemical stability of Graphene causes it to be a very inert material in chemical interactions with its environment and therefore of little catalytic activity. However, Nitrogen-doped Graphene has emerged as a great alternative to metal catalysts due to its catalytic properties. The charge distribution of the Carbon atoms is influenced by the neighboring-Nitrogen atoms doping the material, which induces active regions on the Nitrogen-doped Graphene (N-rGO) surface. These regions or active sites, together with its property as a good electrical conductor, allow it to participate in electrocatalytic reactions of great interest such as: the oxygen reduction reaction (ORR) <b>[1]</b>, the iodide reduction reaction (IRR) <b>[2]</b> or charge storage; which are essential for fuel cells, dye-sensitized solar cells (DSSC), supercapacitors or Metal-Air batteries.<br/>These types of applications require considerable amounts of material, so it is vital to implement and understand the mechanisms involved in scalable treatments to achieve N-doping. In this sense, thermal treatments as a post-synthesis method correspond to a simple and easily scalable technique. Furthermore, it is well known that Nitrogen atoms can be incorporated into the sp² Carbon lattice in different configurations: pyridine nitrogen, graphitic nitrogen, pyrrolic nitrogen or even as oxidized nitrogen species <b>[1-2]</b>. However, each of these configurations or Nitrogen species may have different roles in the reactions of interest <b>[1]</b>; therefore, being able to modify or modulate the proportion between the Nitrogen species present in the N-rGO would be very useful.<br/>The present work analyzes in detail the doping of Graphene with Nitrogen through post-synthesis thermal treatments in an inert atmosphere, using Graphene Oxide (GO) as a precursor, seeking to understand at atomic level and propose a reaction mechanism. Special emphasis has been placed on the role of oxygen species present in GO as precursor and its importance in the incorporation of Nitrogen atoms into the N-rGO network. This analysis has been carried out using different sets of samples with a systematic monitoring of their physico-chemical and structural properties using X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), Raman spectroscopy, X-ray diffraction (XRD), optical microscopy and atomic force microscopy (AFM). Even more, it is shown how the temperature in a second thermal treatment on the N-rGO obtained, plays a key role to modify or modulate the proportion of the different Nitrogen species present in the final N-rGO.<br/><br/><b>[1] </b>Fernandez-Escamilla H.N. et al. <u><b>Advanced Energy Materials</b></u> <b>11</b> (3): 2002459 <b><i>(2021)</i></b>.<br/><b>[2] </b>Ruiz-Marizcal J.M. et al. <u><b>Carbon</b></u> <b>167</b>: 209-218 <b><i>(2020)</i></b>.<br/><br/><b>Acknowledgements:</b> The authors thank CONACyT for financial support through project A1-S-17539 and UNAM for DGAPA-PAPIIT project IN111223 and DGTIC-UNAM projects LANCAD-UNAM-DGTIC-382. J.M.R.-M. thanks CONACyT for his Ph.D. scholarship. The authors thank Francisco Ruiz, Jaime Mendoza, Eduardo Murillo, Israel Gradilla, Eloisa Aparicio and Jesus A. Diaz for technical assistance.