g-C3N4/CoN4 heterojunction as a sensor for detecting volatile organic compounds: a density functional study.

Utilizing density functional theory (DFT) calculations, we investigated the adsorption behavior of key exhaled breath biomarkers - benzene, toluene, aniline, and o-toluidine - on both pristine g-C3N4 and g-C3N4/CoN4 composite surfaces. By analyzing the key parameters such as adsorption energy, electronic density of states (DOS), band structures, charge density difference, conductivity and work function we gained critical insight into the interaction mechanisms between the gas molecules and the sensor surfaces. Our results reveal that the incorporation of CoN4 significantly enhances the chemical reactivity and stability of the g-C3N4 substrate, further improving upon gas adsorption. Notably, benzene, toluene, and aniline exhibit reversible adsorption behavior on g-C3N4/CoN4, highlighting their suitability for reusable gas sensor applications. In contrast, o-toluidine shows irreversible binding, potentially limiting its reusability. Aniline exhibits the lowest band gap and highest conductivity, highest sensitivity and strongest orbital hybridization, confirming that the g-C3N4/CoN4 substrate acts as the best sensor towards aniline gas molecules, among the investigated molecules. Band structure analysis of aniline adsorbed on the g-C3N4/CoN4 heterostructure further confirms that the composite exhibits improved electrical conductivity, even at room temperature, reinforcing its potential for biomarker sensing in human breath analysis. These theoretical findings establish g-C3N4/CoN4 as a promising candidate material and provide predictive guidelines for future experimental investigations into selective and sensitive detection of benzene-related VOCs.