First-Principles Characterization of Potassium Intercalation in Hexagonal 2H-MoS2

Periodic density functional theory calculations were performed to study the structural and electronic properties of potassium intercalated into hexagonal MoS2 (2H-MoS2). Metallic potassium (K) atoms are incrementally loaded in the hexagonal sites of the interstitial spaces between MoS2 layers of the 2H-MoS2 bulk structure generating KxMoS2 (0.125 <= x <= 1.0) structures. To accommodate the potassium atoms, the interstitial spacing c parameter in the 2H-MoS2 bulk expands to 15.871 angstrom in K0.125MoS2. The second lowest potassium loading concentration (K0.25MoS2) results in the largest interstitial spacing expansion (to c = 16.617 angstrom). Our calculations show that there is a small gradual contraction of the interstitial spacing as the potassium loading increases with c = 14.785 angstrom for KMoS2. This interstitial contraction is correlated with an in-plane expansion of the MoS2 layers, which is in good agreement with experimental X-ray diffraction (XRD) measurements. The electronic analysis shows that potassium readily donates its 4s electron to the conduction band of the MoS2, and is largely ionic in character. As a result of the electron donation, the KxMoS2 system changes from a semiconductor to a more metallic system with increasing potassium intercalation. For loadings 0.25 <= x <= 0.625, triangular Mo-Mo-Mo moieties are prominent and tend to form interlayer rhombitrihexagonal tessellated patterns. Intercalation of H2O molecules that solvate the K cations is likely to occur in catalytic conditions. The inclusion of two H2O molecules per K atom in the K0.25MoS2 structure shows good agreement with XRD measurements.