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4.3 Stability with respect to the standard states
In the last chapter, the standard heat of formation of the compounds was discussed.
It has to be negative for a compound if the compound has to be stable
with respect to the standard states of the constituent elements. Therefore, as
a first step the calculation of the heat of formation of the compounds has been
performed for all the 2D materials explored here. The heatmap in the Figure
4.2 shows the heats of formation of the compounds in the 2H and 1T structure
and the difference in energy of the two structures. The figure shows that a
significant fraction of the compounds have positive heats of formation thus
unstable 62. Figure 4.2 (c) shows the difference in energies of the 2H and 1T
structure. The figure clearly shows that in most of the cases the 2H and 1T
structures are energetically very close. One of the important implication of the
two structures having similar energy is that the HER active phase can be synthesized
and stabilized under normal condition with suitable synthetic routes
and the same fact has been realized in the case of MoS2 and WS2 50, 48.
However, an ideal situation would be that the HER active phase is the most
stable phase. But, if that is not the case then a small degree of metastability
would make it feasible to synthesize the HER active phase. As a side note,
since the standard heat of formation by definition is the stability with respect
to standard states, the stability with respect to other competing phases might
also be important, however, stability with respect to the other phases has only
been considered for the compounds meeting the criteria for the HER activity.
However, Figure 4.2 only shows the heats of formation of the perfectly
symmetrical 2H and 1T structures. But, as discussed in the last section the
possible distortions have also been explored for all the compounds hence it
is crucial to assess the energy difference of the perfectly symmetrical and the
distorted phase of the compounds. Figure 4.3 shows the relative energy of
the distorted phase with respect to the symmetric phase. The white squares
corresponds to the compounds manifesting massive distortions leading to the
structures not belonging to either of the 2H or 1T class, therefore, they are
ignored. As can be seen from the figure, a large fraction of compounds do not
show any distortions.
4.4 Adsorption of hydrogen on the basal planes
One of the widely accepted mechanism for the HER is the Volmer-Heyrovsky
mechanism which is a two step process; the first step is the adsorption of H
on the active site and the second step is the bond formation between the two