Can X‑ray\nPhotoelectron Spectroscopy Characterize\nPhotocatalytically Relevant Defects in Graphitic Carbon Nitride? Re-examining\nPeak Assignment and Quantification Using Model Compounds

Heptazine-based compounds, often classified as graphitic\ncarbon\nnitride or the g-C<sub>3</sub>N<sub>4</sub> family of materials, have\nattracted a significant amount of attention since the seminal report\non their photocatalytic reactivity. To achieve further progress, a\nbetter understanding of their structure–property–function\nrelationship is necessary, which requires the development of analytical\nmethodologies for characterizing their structure. In this work, we\nassessed the suitability of X-ray photoelectron spectroscopy (XPS)\nfor the analysis of g-C<sub>3</sub>N<sub>4</sub> materials, especially\ntheir crystallographic defects that have been implicated in the photocatalytic\nmechanism. Using molecular model compounds and following the latest\nrecommendation of sample mounting, we first determined the binding\nenergies of key functional groups, which were then used to analyze\nthe XPS spectra of melon, the archetype g-C<sub>3</sub>N<sub>4</sub> material. Extensive signal overlaps prevented the resolution of\n−NH–, −NH<sub>2</sub>, and “defect”\nmoieties such as guanidine and nitrile/cyanamide, though urea-like\ndefects may be discernible in the O 1s spectrum. Nevertheless, given\nthat the quantitative uncertainty was estimated to range from 14%\nto 65%, structural information that can be obtained via XPS was therefore\nlimited, necessitating additional analytical techniques for a more\ncomprehensive characterization of g-C<sub>3</sub>N<sub>4</sub> materials,\nespecially for identifying the “defect” groups that\nare crucial to their applications.