pH-Dependent Distribution of Functional Groups on Titanium-Based MXenes

MXenes
are a new rapidly developing class of two-dimensional materials
with suitable properties for a broad range of applications. It has
been shown that during synthesis of these materials the surfaces are
usually functionalized by O, OH, and F and further suggested that
controlling the surface allows controlling the material properties.
However, a proper understanding of the surface structure is still
missing, with a significant discrepancy between computational and
experimental studies. Experiments consistently show formation of surfaces
with mixed terminations, whereas computational studies point toward
pure terminated surfaces. Here, we explain the formation of mixed
functionalization on the surface of titanium-based two-dimensional
carbides, Ti2C and Ti3C2, using a
multiscale modeling scheme. Our scheme is based on calculating Gibbs
free energy of formation by a combination of electronic structure
calculations with cluster expansion and Monte Carlo simulations. Our
calculations show formation of mixtures of O, OH, and F on the surface
with the composition depending on pH, temperature, and the work function.
On the other hand, our results also suggest a limited stable range
of compositions, which challenges the paradigm of a high tunability
of MXene properties.