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.