Syngas molecules as probes for defects in 2D hexagonal boron nitride: their adsorption and vibrations

Single-layer, defect-laden hexagonal boron nitride (dh-BN) is attracting a great deal of attention for its diverse applications: catalysis on the one hand, and single photon emission on the other. As possible probes for identifying some common defects in single-layer h-BN, we present results of ab initio calculations for the adsorption and vibrational characteristics of syngas molecules (H₂, CO, CO₂) on dh-BN containing one of four types of defects: nitrogen vacancy (V_N), boron vacancy (V_B), Stone-Wales defect (SW), and nitrogen substituted by boron (B_N). Through a comparative examination of adsorption features, charge transfer, electronic structure, and vibrational spectrum, we obtain a deep understanding of the interaction of these molecules with dh-BN and the role of the defect states. We find that while CO, CO₂ and atomic hydrogen chemisorb, molecular H₂ physisorbs on dh-BN with the four considered defect types. V_N and V_B show strong affinity for CO and CO₂ since the defect states induced by them lie close to the Fermi level. SW does not favor adsorption of these small molecules, as the process for each is endothermic. In the case of B_N, CO adsorbs strongly but CO₂ only weakly. Vibrational frequencies of notable modes localized at the adsorbed molecules are analyzed and suggested as measures for identification of the defect type. Through a simple comparison of adsorption characteristics of the molecules on these defects, we propose dh-BN with V_N to be a good catalyst candidate for CO₂ hydrogenation.