  In this paper, we perform theoretical study on the physical properties of
two-dimensional transition metal chalcogenides MX$_{2}$ and M$_{2}$X$_{3}$ (M=
Ni, Pd; X= S, Se, Te). These studied materials are classified in three stable
phases according to their lattice structures: hexagonal MX$_{2}$, orthorhombic
MX$_{2}$ and orthorhombic M$_{2}$X$_{3}$. They have either isotropic or
anisotropic in-plane properties depending on their symmetries. In particular,
the orthorhombic MX$_{2}$ and M$_{2}$X$_{3}$ have low lattice symmetry and
present highly anisotropic properties. The orthorhombic MX$_{2}$ possess giant
negative in-plane Poisson's ratios, different from the other two phases.
Moreover, by joint analysis of band gap, band edge and optical absorption, the
orthorhombic MX$_{2}$ and M$_{2}$X$_{3}$ are found to be highly efficient as
water splitting photocatalysts within the visible and ultraviolet sunlight
regions.

Materials Science