The effects of chemical and thermal exfoliation on the physico-chemical and optical properties of carbon nitrides

Carbon-based nanomaterials, such as polymeric graphitic carbon nitrides, have garnered attention due to their metal-free structure, exceptional thermal and mechanical stability and unique extended aromatic system, imparting them with semiconductor properties. Furthermore, their efficient electron storage and fast charge transfer have opened new research possibilities in a plethora of applications. However, bulk polymeric carbon nitrides present limited surface areas and scarce functional groups, limiting their application. Exfoliation of these bulk systems, to nanosheets, is an effective way to overcome these drawbacks. However, there is a lack of understanding as to how these exfoliation processes affect their properties. Herein, we present a study of the effects of thermal and acid exfoliation processes on melamine, dicyandiamide and urea-based carbon nitrides' morphology and optical properties. We investigate how both exfoliation methods affect their microstructure, surface area and thermal stability. The thermally treated samples lead to single sheets and more organized structures without significantly impacting the surface chemistry. Contrarily, the acid treatment increases the samples' thickness while reducing their length and imparts them with oxygenated moieties. Both treatments affect the bandgap energies and electronic transitions, influencing their fluorescence profiles, lifetimes and fluorescence quantum yields. Our results provide a comprehensive understanding of how the exfoliation processes can be used to modulate the properties of polymeric graphitic carbon nitrides for the development of efficient materials in applications spanning wastewater treatment to biodiesel production.