Hydroxyl Functionalization Effects on Carbene–Graphene for Enhanced Ammonia Gas Sensing

DFT study of graphene functionalized via carbene was performed to identify the preferred –OH adsorption sites and to assess how hydroxylation affects adsorption of NH3 gas. The carbene attaches to the graphene basal plane through a [2+1] cycloaddition, producing a local cyclopropane-like motif with a C–C bond. This modification introduces localized mid-gap states and asymmetric charge redistribution that create chemically active anchoring sites for –OH groups. We systematically scanned possible –OH adsorption sites and identified site-dependent binding energies. NH3 preferentially anchors at the carbene center and is further stabilized by multidentate hydrogen bonding with neighboring –OH groups. Calculated NH3 adsorption energies range from moderate values (single –OH and some two –OH symmetric sites, Eads ≈ −0.64 to −0.75 eV) to strong interaction for selected through-plane two –OH pairs (Eads ≈ −1.78 to −1.83 eV), where synergistic hydrogen bonding amplifies the NH3 interaction. Charge density difference and Bader analyses indicate polarization-dominated binding with minimal net charge transfer, consistent with hydrogen bonding rather than covalent bond formation. Desorption time estimation shows that moderate binding motifs provide rapid recovery at room temperature. We conclude that targeted placement of paired –OH groups on carbene-functionalized graphene offers a tunable route to balance sensitivity and reusability for NH3 sensing.