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with respect to the standard states of the elements whereas there might be
other potentially competing phases hampering the growth of the 2D materials
for the HER. Therefore, it is crucial to have a deeper look on the stability of
materials potentially suitable for the HER. If the metastability of the candidate
material comes out to be too large as compared to the competing phase then its
unlikely that the candidate material can be synthesized and stabilized under
normal conditions. Therefore, the stability check for the candidate material
has been performed with respect to the other competing phases with same
stochiometry. Additionally, in the present work we do not explore the stability
of the compounds in aqueous medium because in some recent works a good
control over the stability of the compounds in water has been achieved by
the use of stabilizing agents 72. All the competing bulk structures are taken
from the The Open Quantum Materials Database (OQMD) 73 and then the
energy of candidate material is compared to the energy of the convex hull in
order to have an estimate of the degree of metastability. The data is shown in
the Table 4.3 and 4.4.
H in the tables denotes standard heats of formation calculated with the
FERE method and Hhull denotes the convex hull 73. The symbol * in
superscript denotes the cases where the convex hull has been calculated as the
linear combination of the energies of two compounds because no compounds
were present with 1:2 stoichiometry in the database. Hhull denotes the energy
of the monolayer with respect to the convex hull. HExpt is the experimental
heat of formation of the compound (if available) lying on the convex hull. The
initial list of the candidates for the HER is also compared to the predicted
2D materials by Lebègue et. al 74. In order to have an estimate of the
metastability of the 2H structure with respect to the 1T structure or viceversa
the difference of the two is also shown as H2H/1T (H1T/2H). Finally,
the previously discussed probability P(|G| 0.5) is also listed in the table.
All the energies mentioned in the table are in eV/atom.
Few important points worth noticing in the table are:
• In all the cases the 2H and 1T structure do not differ by more than 0.4
eV which is similar to the degree of metastability in MoS2 and WS2 in
the 2H and 1T structure and both the compounds can be synthesized in
the stable 2H phase and metastable 1T phase under normal conditions.
Similar degree of metastability in other compounds suggest that if they
can be synthesized in one structure then it is likely that they can be
synthesized in the other structure as well.
• Few of the HER materials like PdS2, PdTe2 proposed in this work have
also been predicted by Lebègue et. al 74 to exist in the monolayer form.