Imaging the Structure of Water Confined in CO₂Islands with Atomic Resolution

The structure of interfacial water plays a pivotal role in CO₂ activation and conversion processes, governing the reaction pathways and efficiencies. Although the adsorption of CO₂ and H₂O on metal substrates has been extensively investigated individually, the atomic-scale structures arising from their coadsorption remain insufficiently elucidated. In this work, we use scanning tunneling microscopy (STM), noncontact atomic force microscopy (nc-AFM), and density functional theory (DFT) to directly identify the structures of coadsorbed CO₂ and H₂O mixtures on the Au(111) surface at the atomic scale. Our results reveal that CO₂ molecules self-assemble into cyclic frameworks or extended islands, and water molecules can penetrate the interior of the CO₂ frameworks. Notably, over a wide range of coverages and H₂O-to-CO₂ ratios, the water molecules were consistently surrounded by CO₂ molecules, indicating that they were confined within the two-dimensional (2D) CO₂ islands. Within the confined domains, H₂O adopts three types of configurations─isolated monomers, zigzag chains, and cyclic clusters, with cyclic clusters that are composed of fused five- and six-membered rings, constituting the predominant structural motif. Furthermore, the chirality of the CO₂ frameworks does not influence the OH orientation within cyclic water clusters, suggesting the relatively weak interaction between surface water and CO₂ molecules. This work provides atomic-scale insights into the fundamental behavior of the CO₂–H₂O assemblies on metal surfaces.