32
adsorbed hydrogen to evolve the gaseous hydrogen 63, 64. Schematically, the
energetics of the process is shown in the Figure 4.4.
Figure 4.4: Schematic of the Volmer-Heyrovsky route for the HER.
The product and the reactant are at the same level of energy in the Figure
4.4 due to the assumption that the process is at equlibrium under standard
conditions thus have zero free energy. Active site in the figure is denoted by
the *. It can be seen from the figure that the intermediate H may lie higher
or lower in energy than the product and the reactant. If the intermediate lies
higher in energy than the reactant then the first step will be uphill and if it lies
lower in the energy than the reactant then the second process will be uphill.
Therefore, based on the thermodynamic argument if the process has a zero
barrier, then the free energy for the adsorption of hydrogen has to be zero
48, 65, 42. Although the free energy for the hydrogen adsorption provides a
descriptor for the HER activity, it does not provide any information about the
kinetic barrier for the process but we do not explore the kinetic pathways in
this work.
In order to assess the reactivity of the basal plane the hydrogen adsorption
energy has been calculated for different active sites on the surface. We find that
the hydrogen prefers to adsorb on chalcogen/oxygen atoms in tilted positions
in most of the cases and does not prefers to adsorb on the metal site. In
a perfectly symmetric structure all the chalcogen atoms are equivalent thus
considering just one of them suffices. However, in the distorted structures,
the broken symmetry leads to inequivalent chalcogen sites, therefore, all the
inequivalent sites have been explored for the hydrogen adsorption and the
site with the lowest adsorption energy has been chosen for further analysis.
The coverage of 0.25 monolayer (ML) has been chosen initially and the higher
coverage (0.5 ML) is only considered for those structures which bind hydrogen
too strongly (Hads
H −0.8) for 0.25 ML. However, it has been found that at
higher coverages the structures massively distort leading to the structures not
belonging to either of the 2H or 1T class, therefore, higher coverages have not
been considered any further.