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The fact that they lie above the hull by only 0.4 eV led us to made a
choice of the threshold energy of 0.4 eV for the stability (or the feasibility
of existence) of the monolayer with the respect to the hull. The choice
of the threshold energy helps to narrow down the candidates even more,
for example compounds like OsS2, ReO2, OsSe2, ScO2, RuO2 in the 2H
class of candidates and OsO2 in the 1T class of candidates lying above
the hull by more than 0.4 eV can be safely discarded. The names of the
discarded compounds are italicized in Table 4.3 and 4.4.
• On comparing the list of the candidates with the list of the predicted 2D
materials by Lebègue et. al 74, the compounds common in both the
lists are selected. Given the fact that the heurestic approach of Lebègue
et. al which is based on the feasibility of cleaving a bulk structure along
a certain direction due to the weak interlayer interaction gives a clue that
the compounds common in both the list are potentially synthesizable,
therefore, potential candidates for the HER. The potential candidates
are marked in bold in the Table 4.3 and 4.4.
In conclusion, this work systematically explores the materials space in the
two-dimensional 2H and 1T structure for the hydrogen evolution reaction using
the free energy of hydrogen adsorption as a computational descriptor. The
requirement of the activity on the basal plane ensures the presence of large
number of active sites as compared to previously explored 2H structure of
the MoS2 which only has the activity on the edges. A fairly large window
chosen for value of the descriptor provides a flexibility to tune the adsorption
energy of the hydrogen by different means, for example, strain, environment,
doping etc. Additionally, the robust stability analysis of the candidates found
suitable for the HER provides a list of candidates which do not have very high
degree of metastability with respect to the bulk compounds thus potentially
synthesizable. The adopted approach also predicts already known MoS2 and
WS2 in 1T structure as candidates for HER thus supporting our approach.
Finally, the most probable list of candidates is proposed based on work by
Lebègue et. al. The calculations therefore invite for further investigation of
some of the best candidates suggested here like PdS2, NbS2, TiS2, TaS2, ZrS2,
PdSe2, HfS2 in the 2H structure and CrS2, TaTe2, VTe2, NbS2, CrSe2 in the
1T structure in addition to MoS2 and WS2 which are already known.