Ionic liquids (ILs) are effective CO₂ capture media and recent experimental evidence has demonstrated that the addition of two-dimensional (2D) nanomaterials into ILs can effectively improve their CO₂ capturing capability. However, an in-depth mechanism on how 2D nanomaterials enhance CO₂ absorption is poorly documented. In this study, the adsorption of CO₂ by a representative IL, namely 1-ethyl-3-methyl-imidazole-tetrafluoroborate ([EMIM][BF₄]), coated on graphene (GRA, the prototype 2D nanomaterial) and nitrogenized graphene (C₃N) was investigated by molecular dynamics simulations. The influence of the IL film thickness on the amount of CO₂ adsorption was systematically analyzed. Our data clearly indicate that at the IL-gas interface the CO₂ accumulation is significantly enhanced. In contrast, at the IL-GRA and IL-C₃N interfaces, only slight enhancement was observed for CO₂ accumulation. Quantitative calculations of the adsorption-free energy for CO₂ inside the IL film further support the simulation results. Our present results also reveal that the sub-nanometer IL film possesses a considerably high CO₂ capture efficiency because of the formation of the reduced bulk IL region. Moreover, the nanomaterial substrate surfaces can effectively accelerate the diffusion of CO₂, which is beneficial for the CO₂ mass transfer. In general, our theoretical study provides a deep microscopic understanding of the CO₂ capture by nanomaterials and IL composites. These results could benefit the design and fabrication of a high-performance CO₂ capture and storage medium through the synthetic effects of ILs and nanomaterials.