Calculation on surface energy and electronic properties of CoS.

Density functional theory was employed to investigate the (111), (200), (210), (211) and (220) surfaces of CoS. The surface energies were calculated with a sulfur environment using first-principle-based thermodynamics. It is founded that surfaces with metal atoms at their outermost layer have higher energy. The stoichiometric (220) surface terminated by two layer of sulfur atoms is most stable under the sulfur-rich condition, while the non-stoichiometric (211) surface terminated by a layer of Co atoms has the lower energy under the sulfur-poor environment. The electric structure results show that the front valence electrons of (200) surface are active, indicating that there may be some active sites on this face. There is an energy gap between the stoichiometric (220) and (211), which has low Fermi energy, indicating that their electronic structures are dynamically stable. Spin-polarized bands are calculated on the stoichiometric surfaces, and these two (200) and (210) surfaces are predicted to be noticeably spin-polarized. The Bravais-Friedel-Donnay-Harker (BFDH) method is adopted to predict crystal growth habit. The results show that the most important crystal planes for the CoS crystal growth are (111) and (200) planes, and the macroscopic morphology of CoS crystal may be spherical, cubic, octahedral, prismatic or plate-shaped, which have been verified by experiments.