Study on the Influence Mechanism of Cobalt and Iron Doping on the Adsorption of H₂S, NO, NO₂ and SO₂ Gas Molecules by Two-Dimensional C₉N₄ Materials

Abstract

This study systematically investigates the influence mechanisms of cobalt (Co) and iron (Fe) doping 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 (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 are calculated. Combined with the thermodynamic analysis of recovery time, the regulatory laws of transition metal d-electron configurations on adsorption selectivity are 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). Among them, 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 shows that the orbital hybridization energy gaps between Co³⁺ and gas molecules (such as -3.5 eV and -2 eV in the H₂S system) are lower than those in the Fe³⁺ system (such as -1 eV in the H₂S system), confirming that the Co-doped system has stronger bonding stability. Differential charge density calculations show that the charge transfer amount 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.