Transition metal atom adsorption on the titanium carbide MXene: Trends across the periodic table for the bare and O-terminated surfaces

Abstract

MXenes are a family of two-dimensional materials of great interest due to their unique properties, e.g., adjustability based on changes in their composition, structure, and surface functionality, which grant MXenes a variety of applications. One way of changing the catalytic effect of MXenes consists of adsorbing isolated metallic elements, such as transition metals (TMs), onto their surface, leading to the formation of single-atom catalysts. Herewith, the adsorption behavior of 31 TMs on the surface of two titanium carbide MXenes, viz. ${\mathrm{Ti}}_{2}\mathrm{C}$ and ${\mathrm{Ti}}_{2}{\mathrm{CO}}_{2}$, is analyzed by means of density-functional theory (DFT) calculations. We find that the oxygen surface termination causes most of the TM atoms to adsorb on a hollow site above a carbon atom, whereas on bare ${\mathrm{Ti}}_{2}\mathrm{C}$, the adsorption preference follows a pattern related to groups of the periodic table. The interaction between the TM atoms and the surface of both ${\mathrm{Ti}}_{2}\mathrm{C}$ and ${\mathrm{Ti}}_{2}{\mathrm{CO}}_{2}$ is strong, as demonstrated by the calculated adsorption energies, which range between about \ensuremath{-}1 and \ensuremath{-}9 eV on either surface. Upon adsorption on ${\mathrm{Ti}}_{2}{\mathrm{CO}}_{2}$, electrons are transferred from the adatom to the MXene surface, whereas on ${\mathrm{Ti}}_{2}\mathrm{C}$, the only TM atoms for which this happens are the ones in group 3 of the periodic table. All the other transition metal atoms become negatively charged after adsorption on ${\mathrm{Ti}}_{2}\mathrm{C}$. On the oxygen-covered MXene, stronger adsorptions are accompanied by higher charge transfers. The energy barriers for TM adatom diffusion on ${\mathrm{Ti}}_{2}\mathrm{C}$ are very small, meaning that the adatoms can move rather freely along it. On ${\mathrm{Ti}}_{2}{\mathrm{CO}}_{2}$, however, higher diffusion barriers were found, many being above 1 eV, which suggests that the oxygen termination layer blocks the diffusion. On both surfaces, the highest diffusion barriers were found to correspond to the TM elements which adsorb most strongly.