Facile oxidation reaction to produce monolayered highly crystalline nitrogen-doped graphene quantum dots

Abstract

Nitrogen-doped graphene quantum dots (NGQDs) made by standard hydrothermal synthesis route were found to produce non-uniform nanostructures that exhibit poor crystallinity and unstable optical properties. In this study, a simple post synthesis oxidation step was shown to significantly improve the overall crystallinity and stability of the nanostructures via removal of reaction by-products and oxidation of reactive functional groups, generating oxidized nitrogen-doped graphene quantum dots (Ox-NGQDs). Through transmission electron microscopy (TEM) and atomic force microscopy (AFM) the Ox-NGQDs were found to be uniform monolayered nanostructures with lateral size of 20–25 nm. The appearance of G and D bands in the Raman spectrum of the Ox-NGQDs sample confirmed that the reaction by-products had been removed, and indicated that the Ox-NGQDs were composed of a highly ordered graphitic structure. X-ray photoelectron spectroscopy (XPS) indicated that oxidation of the surface functional groups took place. The Ox-NGQDs produced in this work gained a significant increase in visible absorbance, which is important in a wide range of potential light harvesting applications and uses.