Can X-ray Photoelectron Spectroscopy Characterize Photocatalytically Relevant Defects in Graphitic Carbon Nitride? Re-examining Peak Assignment and Quantification Using Model Compounds

Heptazine-based compounds, often classified as graphitic carbon nitride or the g-C3N4 family of materials, have attracted a significant amount of attention since the seminal report on their photocatalytic reactivity. To achieve further progress, a better understanding of their structure–property–function relationship is necessary, which requires the development of analytical methodologies for characterizing their structure. In this work, we assessed the suitability of X-ray photoelectron spectroscopy (XPS) for the analysis of g-C3N4 materials, especially their crystallographic defects that have been implicated in the photocatalytic mechanism. Using molecular model compounds and following the latest recommendation of sample mounting, we first determined the binding energies of key functional groups, which were then used to analyze the XPS spectra of melon, the archetype g-C3N4 material. Extensive signal overlaps prevented the resolution of −NH–, −NH2, and “defect” moieties such as guanidine and nitrile/cyanamide, though urea-like defects may be discernible in the O 1s spectrum. Nevertheless, given that the quantitative uncertainty was estimated to range from 14% to 65%, structural information that can be obtained via XPS was therefore limited, necessitating additional analytical techniques for a more comprehensive characterization of g-C3N4 materials, especially for identifying the “defect” groups that are crucial to their applications.