Enhancement of the selectivity of MXenes (M₂C, M = Ti, V, Nb, Mo) <i>via</i> oxygen-functionalization: promising materials for gas-sensing and -separation

Abstract

Two-dimensional graphene-like materials, namely MXenes, have been proposed as potential materials for various applications. In this work, the reactivity and selectivity of four MXenes (i.e. M₂C (M = Ti, V, Nb, Mo)) and their oxygen-functionalized forms (i.e. O-MXenes or M₂CO₂) toward gas molecules were investigated by using the plane wave-based Density Functional Theory (DFT) calculations. Small gas molecules, which are commonly found in flue gas streams, are considered herein. Our results demonstrated that MXenes are very reactive. Chemisorption is a predominant process for gas adsorption on MXenes. Simultaneously dissociative adsorption can be observed in most cases. The high reactivity of their non-functionalized surface is attractive for catalytic applications. In contrast, their reactivity is reduced, but the selectivity is improved upon oxygen functionalization. Mo₂CO₂ and V₂CO₂ present good selectivity toward NO molecules, while Nb₂CO₂ and Ti₂CO₂ show good selectivity toward NH₃. The electronic charge properties explain the nature of the substrates and also interactions between them and the adsorbed gases. Our results indicated that O-MXenes are potential materials for gas-separation/capture, -storage, -sensing, etc. Furthermore, their structural stability and SO₂-tolerant nature are attractive properties for using them in a wide range of applications. Our finding provides good information to narrow down the choices of materials to be tested in future experimental work.