Stabilization of small organic molecules on V2C and V2CO2 MXenes: first-principles insights into the performance of van der Waals functionals and the effect of oxygen vacancies

Abstract

The adsorption of small organic molecules on pristine V₂C MXene and its derivatives is investigated by first-principles density functional theory calculations. By employing state-of-the-art van der Waals (vdW) density functionals, the binding affinity of studied molecules, i.e., CH₄, CO₂, and H₂O on MXene adsorbents is well described by more recent vdW functionals, i.e., SCAN-rvv10. Although both CH₄ and CO₂ are nonpolar molecules, on pristine and oxygen-vacancy surfaces, they show a different range of adsorption energies, in which CH₄ is more inert and has weaker binding than CO₂. CO₂ stays intact in its molecular forms for most of the tested functionals, except for the case of the vdW-DF functional, where CO₂ exhibits a dissociation regardless of its initial adsorption geometry. For full surface terminations, the adsorption affinity of all involved species is comparable within the same range, varying from −0.10 to −0.20 eV, attributed to either weak dispersion interactions or hydrogen bonds. The binding of H₂O is much more pronounced compared to CO₂ and CH₄ in the presence of oxygen vacancies with the highest adsorption energy of −1.33 eV, vs. −0.67, and −0.20 eV obtained for H₂O, CO₂, and CH₄ respectively. H₂O can dissociate with a small activation energy barrier of 0.40 eV, much smaller than its molecular adsorption energy, to further saturate itself on the surface. At high oxygen-vacancy concentrations, stronger bindings of adsorbates are found due to a preferred attachment of adsorbates to induced undercoordinated metal sites. The findings propose a potential scheme for greenhouse gas separation based on the surface modification of novel two-dimensional structures.