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

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.