Computational investigation of carboxyl-functionalized hydrogen-substituted graphdiyne for tracing and trapping nitrogen and its oxides

Outdoor air pollution has emerged as a critical global concern, primarily driven by the rapid increase in automobile usage, which has led to elevated emissions of nitrogen and its oxides (N₂, N₂O, NO, and NO₂). These exhaust gases pose severe health risks, including cardiovascular and respiratory diseases. Therefore, it is essential not only to detect but also to capture these harmful species, for which gas sensors play a vital role. Motivated by this urgent need, we have computationally designed carboxyl-functionalized hydrogen-substituted graphdiyne (COOH-HsGDY) and evaluated its gas adsorption capability as a potential carbon-based sensing material. Density functional theory (DFT) calculations have been performed to investigate the structural stability, electronic properties, and adsorption behavior of COOH-HsGDY toward nitrogen and its oxides. The results reveal that the NO₂ molecule exhibits the most favorable adsorption, with an adsorption energy of -0.493 eV. Electronic structure analyses, including total and partial density of states (TDOS and PDOS), indicate a reduction in the energy gap (E_g) upon gas adsorption, suggesting enhanced electrical conductivity, while PDOS analysis further evidences orbital overlap that supports strong gas-substrate interactions. Non-covalent interaction (NCI) plots and reduced density gradient (RDG) analyses confirm that the adsorption process is primarily governed by hydrogen-bonding interactions. In addition, charge density difference mapping and Löwdin charge analysis demonstrate significant charge transfer of 1.2914e^- and 1.2394e^- during the adsorption of N₂O and NO₂ molecules, respectively. Sensitivity and recovery time evaluations further highlight the sensor response and reusability of the material. Overall, these findings identify COOH-HsGDY as a highly promising candidate for the detection and capture of nitrogen and its oxides.