Stabilization of small organic molecules on VC and VCO 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 VC 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, , CH, CO, and HO on MXene adsorbents is well described by more recent vdW functionals, , SCAN-rvv10. Although both CH and CO are nonpolar molecules, on pristine and oxygen-vacancy surfaces, they show a different range of adsorption energies, in which CH is more inert and has weaker binding than CO. CO stays intact in its molecular forms for most of the tested functionals, except for the case of the vdW-DF functional, where CO 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 HO is much more pronounced compared to CO and CH in the presence of oxygen vacancies with the highest adsorption energy of -1.33 eV, -0.67, and -0.20 eV obtained for HO, CO, and CH respectively. HO 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.