26
tunately, relatively limited number of active sites on the edges gives very low
exchange current density for the HER. However, recent experiments on the
other polymorph of the MoS2 and WS2 known as 1T structure demonstrated
the activity of the basal plane for the HER thus giving access to relatively
larger number of active sites 49, 48, 50. Additionally, the difference in energy
of the 1T and 2H phase MoS2 or WS2 decreases as the dimensionality
of the system is reduced from three (bulk) to two (monolayer) thus making it
feasible to synthesize the HER active metastable phase in the 2D form 55.
The different activity of the 2H and 1T phase broadens the materials space
for HER which is the basis of this work. In this work, the basal planes of 100
different metal dichalcogenides and oxides have been explored in both the 2H
and 1T structure for the HER. Primarily, criterion of stability of the material
with respect to the standard reference phases and other competing phases and
the free energy of the hydrogen adsorption on the basal plane has been used
as descriptors for the screening of materials for the HER.
4.2 Details of the atomic structure
The 2H and 1T structures differ by the arrangement of the chalcogen/oxygen
atom around the metal atoms. The 2H structure has prismatic arrangement of
the chalcogen/oxygen atoms around the metal atom whereas in the 1T structure
they are octahedrally arranged. The 2H and 1T structures are shown
in the Figure 4.1 (a) and (f) respectively. The black square represents the
unit cell of the structures. Other structures shown in the 2H and 1T class are
the distorted derivatives of the 2H and 1T structures. The distorted structures
have been broadly classified based on their symmetry group which have
been identified using certain cutoff for the rotations/translations to account
for the residual forces in the structures. In order to identify the the distorted
structures, atoms are slightly displaced from their symmetric position in a
bigger unit cell in order to break the symmetry of the structure and then the
relaxation is performed.
The above procedure captures all the distortions if any in the 2×2 unit cell.
There might be other distortions in the larger unit cell but those cases have not
been considered here. Fortunately, the charge density wave (CDW) structures
in compounds like TiS2 56, 57 distorted structure of MoS2, WS2 etc. 58, 59,
exhibiting quantum spin Hall effect (QSH) and the distortions in ReS2 60 are
captured by the above procedure thus supporting our results. However, the
choice of 0.01 eV/atom for the threshold of energy to differentiate between
the symmetrical and the distorted structure categorize TiS2 as symmetrical