The lattice constant (a) of Ti 2 CSO MXene is 3.106 Å and that of Ti 2 CSSe MXene is 3.214 Å. The thickness (t) of the MXenes in out-of-plane direction were calculated as the distance between S and O/Se surfaces. The thicknesses of Ti 2 CSO and Ti 2 CSSe MXene are 5.000 and 5.684 Å, respectively. It is clear that Ti 2 CS 2 has a and t values which are almost equal to the respective values of Ti 2 CSSe MXene (see Table ). As a consequence of having the shortest lattice constant and thickness, Ti 2 CO 2 has the lowest volume of the functionalized Ti 2 C materials in Table . This could be because of lower atomic radius of O relative to S and Se and higher attraction between Ti and O in Ti 2 CO 2 as proved by the short Ti-O bond lengths and high charge transfer of Ti and O.