Studies on the influence of cobalt- and iron-doping mechanisms on the adsorption of H2S, NO, NO2 and SO2 gas molecules by two-dimensional C9N4 materials

Abstract

We have systematically investigated the influence of cobalt (Co)- and iron (Fe)-doping mechanisms on the adsorption of H₂S, NO, NO₂, and SO₂ gas molecules by two-dimensional C₉N₄ materials using density functional theory (DFT) combined with the generalized gradient approximation with the Hubbard U correction (GGA+U) method. By constructing a 4 × 4 × 1 supercell model, the adsorption structures, thermodynamic parameters, electronic structures, and charge transfer characteristics of different doping systems were determined. By analyzing the recovery time at different temperatures, the regulation patterns of adsorption selectivity by the d-electron configuration of transition metals were revealed. The structure optimization results show that gas molecules are mainly adsorbed in an end-on coordination mode on the surface of Co-/Fe-doped C₉N₄. The adsorption energies of the Co-doped system for H₂S, NO, and NO₂ (−2.553 to −1.919 eV) are significantly higher than those of the Fe-doped system (−2.228 to −0.393 eV). The adsorption energy of Co-C₉N₄ for NO (−2.553 eV) is 89% higher than that of Fe-C₉N₄ (−1.347 eV), which is attributed to the superior d-orbital hybridization ability of Co²⁺ (3d⁷). Density of states (DOS) analysis showed that the orbital hybridization energy gaps between Co³⁺ and gas molecules (−3.5 eV and −2 eV in the H₂S system) are lower than those in the Fe³⁺ system (−1 eV in the H₂S system), confirming that the Co-doped system has stronger bonding stability. Differential charge density calculations showed that the charge-transfer value of Fe-C₉N₄ for NO (0.43 e) is 48% higher than that of Co-C₉N₄ (0.29 e), which originates from the electron donation–acceptance flexibility of the half-filled configuration of Fe³⁺ (3d⁵). SO₂ in Fe-C₉N₄ has recovery times of 9.46 × 10⁻⁹–4.4 × 10⁻⁶ s at 298–498 K, balancing stability and desorption, providing a basis for designing selective 2D gas-sensing materials.