DFT study of synergistic gas sensing using electrically conductive mixed ligand 2D-MOF

Abstract

Two-dimensional conductive metal-organic frameworks (2D-cMOFs) have been adopted in electrochemical sensing applications owing to their superior electrical conductivity and large surface area. Here, we performed a density functional theory (DFT) analysis to study the synergistic impact of introducing a secondary organic ligand to the 2D-cMOF system. In this study, cobalt-hexaiminobenzene (Co-HIB) and cobalt-2,3,6,7,10,11-hexaiminotriphenylene (Co-HITP) were combined to form a mixed ligand MOF named, Co-HIB-HITP. A DFT-level comparative study was designed to access stability, selectivity, and gas adsorption mechanism, important factors in sensing material development. A potential energy surface calculation predicted the structural stability of Co-HIB-HITP at larger interlayer displacement around 3.6 ~ 4.2 Å regions along the ab-plane than its unmixed states, Co-HIB and Co-HITP, indicating the tunability of the stacking mode using the mixed ligand system. Furthermore, the adsorption capabilities of NH3, H2S, NO, and NO2 exhibited the superiority of Co-HIB-HITP over unmixed 2D-cMOFs by showing a synergy effect of 158% and 170% improvement in adsorption energy on H2S and NH3, respectively. Finally, an electron charge density analysis revealed Co-HIB-HITP’s unique stacking mode and Co-metal density as contributing factors to its gas-selective synergy effect. The AB stacked layers and an intermediate metal density (5.25 %) significantly improved the electrostatic interactions with H2S and NH3 by inducing a change in the chemical environment of the gas binding sites. This work proposes the dual-ligand 2D-cMOF as the promising design strategy for the next-generation sensing material.