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.
Using a multiscale
computational scheme, we study the trends in
distribution and composition of the surface functional groups −O,
−OH, and −F on two-dimensional (2D) transition metal
carbides and nitrides (MXenes). We consider Ti
2
N, Ti
4
N
3
, Nb
2
C, Nb
4
C
3
, Ti
2
C, and Ti
3
C
2
to explore MXenes
with different chemistry and different number of atomic layers. Using
a combination of cluster expansion, Monte Carlo, and density functional
theory methods, we study the distribution and composition of functional
groups at experimentally relevant conditions. We show that mixtures
of functional groups are favorable on all studied MXene surfaces.
The distribution of functional groups appears to be largely independent
of the type of metal, carbon, or nitrogen species and/or number of
atomic layers in the MXene. We further show that some properties (e.g.,
the work function) strongly depend on the surface composition, while
others, for example, the electric conductivity, exhibit only a weak
dependence.
Vertical van der Waals (vdW) heterostructures of 2D crystals with defined interlayer twist are of interest for band-structure engineering via twist moiré superlattice potentials. To date, twist-heterostructures have been realized by micromechanical stacking. Direct synthesis is hindered by the tendency toward equilibrium stacking without interlayer twist. Here, we demonstrate that growing a 2D crystal with fixed azimuthal alignment to the substrate followed by transformation of this intermediate enables a potentially scalable synthesis of twisted heterostructures. Microscopy during growth of ultrathin orthorhombic SnS on trigonal SnS2 shows that vdW epitaxy yields azimuthal order even for non-isotypic 2D crystals. Excess sulfur drives a spontaneous transformation of the few-layer SnS to SnS2, whose orientation – rotated 30° against the underlying SnS2 crystal – is defined by the SnS intermediate rather than the substrate. Preferential nucleation of additional SnS on such twisted domains repeats the process, promising the realization of complex twisted stacks by bottom-up synthesis.
Two-dimensional MXenes
have recently received increased attention
due to their facile synthesis process and extraordinary properties
suitable for many different applications. During the wet etching synthesis
of MXenes, native defects, such as metal and carbon or nitrogen vacancies,
are produced, but the underlying defect formation processes are poorly
understood. Here, we employ first-principles calculations to evaluate
formation energies of Ti, C, and N vacancies in Ti3C2 and Ti2N MXenes under etching conditions. We carefully
account for the mixed functionalization of the surfaces as well as
the chemical environment in the solution (pH and electrode potential).
We observe that the formation energies of the metal vacancies differ
significantly for different types of surface functionalization as
well as for different local and global environments. We attribute
these differences to electrostatic interactions between vacancies
and the surrounding functional groups. We predict that Ti vacancies
will be prevalent on bare or OH-functionalized surfaces but not on
O-functionalized ones. In contrast, C and N vacancies are more prevalent
in O-functionalized surfaces. In addition, our results suggest that
the pH value of the etching solution and the electrode potential strongly
affect vacancy formation. In particular, the predicted conditions
at which abundant vacancy formation is expected are compared to experiments
and found to coincide with conditions at which MXenes oxidize readily.
This suggests that Ti vacancy formation is a crucial step in initiating
the oxidation process.
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