On the Thermal Stability of Aryl Groups Chemisorbed on Graphite

The covalent functionalization of carbon-based materials through aryldiazonium chemistry has emerged as a powerful tool for physicochemical property modification. However, the characterization techniques traditionally used to assess the stability of any modification, such as thermogravimetric analysis (TGA), do not allow covalent detachment to be discriminated from the loss of physisorbed material. Here, we present a general method to differentiate these two processes by combining scanning probe microscopy with Raman spectroscopy. Using covalently modified highly oriented pyrolytic graphite (HOPG), we show that the thermally induced covalent bond breaking between the aryl groups and the HOPG surface can be directly monitored through this combined approach. Moreover, temperature-dependent Raman spectroscopy allows the kinetics of this bond breaking to be studied. Desorption activation energies can thus be initially estimated for aryl groups bearing different substituents. Finally, we highlight that the pristine HOPG structure can be restored at a relatively low temperature, opening the way for reversible covalent modification. Our results alight on a general methodology for the full characterization of covalently modified carbon materials.