Two-dimensional transition metal chalcogenides with hexagonal and orthorhombic structures: candidates for auxetics and photocatalysts

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.