Cooperite (PtS) is
one of the main sources of platinum in the world
and has not been given much attention, in particular from the computational
aspect. Besides, the surface stability of cooperite is not fully understood,
in particular the preferred surface cleavage. In the current study,
we employed computer modeling methods within the plane-wave framework
of density functional theory with dispersion correction and the U parameter to correctly predict the bulk and surface properties.
We reconstructed and calculated the geometries and surface energies
of (001), (100), (101), (112), (110), (111), and (211) cooperite surfaces
of stoichiometric planes. The Pt d-orbitals with U = 4.5 eV and S p-orbitals with U = 5.5 eV were
found optimum to correctly predict a band gap of 1.408 eV for the
bulk cooperite model, which agreed with an experimental value of 1.41
eV. The PtS-, Pt-, and S-terminated surfaces were investigated. The
structural and electronic properties of the reconstructed surfaces
were discussed in detail. We observed one major mechanism of relaxation
of cooperite surface reconstructions that emerged from this study,
which was the formation of Pt–Pt bonds. It emanated that the
(110) and (111) cooperite surfaces underwent significant reconstruction
in which the Pt2+ cation relaxed into the surface, forming
new Pt–Pt (Pt2
2+) bonds. Similar behavior
was perceived for (101) and (211) surfaces, where the Pt2+ cation relaxed inward and sideways on the surface, forming new Pt–Pt
(Pt2
2+) bonds. The surface stability decreased
in the order (101) > (100) ≈ (112) > (211) > (111)
> (110)
> (001), indicating that the (101) surface was the most stable,
leading
to an octahedron cooperite crystal morphology with truncated corners
under equilibrium conditions. However, the electronic structures indicated
that the chemical reactivity stability of the surfaces would be determined
by band gaps. It was found that the (112) surface had a larger band
gap than the other surfaces and thus was a chemical stability competitor
to the (101) surface. In addition, it was established that the surfaces
had different reactivities, which largely depended on the atomic coordination
and charge state based on population atomic charges. This study has
shown that cooperite has many planes/surface cleavages as determined
by the computed crystal morphology, which is in agreement with experimental
X-ray diffraction (XRD) pattern findings and the formation of irregular
morphology shapes.