Titanium disulfide is an inorganic compound with the formula TiS2. A golden yellow solid with high electrical conductivity, it belongs to a group of compounds called transition metal dichalcogenides, which consist of the stoichiometry ME2. TiS2 has been employed as a cathode material in rechargeable batteries.
Structure
With a layered structure, TiS2 adopts a hexagonal close packed (hcp) structure, analogous to cadmium iodide (CdI2). In this motif, half of the octahedral holes are filled with a "cation", in this case Ti4+. Each Ti centre is surrounded by six sulfide ligands in an octahedral structure. Each sulfide is connected to three Ti centres, the geometry at S being pyramidal. Several metal dichalcogenides adopt similar structures, but some, notably MoS2, do not. The layers of TiS2 consist of covalent Ti-S bonds. The individual layers of TiS2 are bound together by van der Waals forces, which are relatively weak intermolecular forces. It crystallises in the space group Pm1. The Ti-S bond lengths are 2.423 Å.
Intercalation
The single most useful and most studied property of TiS2 is its ability to undergo intercalation upon treatment with electropositive elements. The process is a redox reaction, illustrated in the case of lithium: TiS2 + Li → LiTiS2 LiTiS2 is generally described as Li+[TiS2−]. During the intercalation and deintercalation, a range of stoichimetries are produced with the general formul LixTiS2 (x < 1). During intercalation, the interlayer spacing expands (the lattice "swells") and the electrical conductivity of the material increases. Intercalation is facilitated because of the weakness of the interlayer forces as well as the susceptibility of the Ti(IV) centers toward reduction. Intercalation can be conducted by combining a suspension of the disulfide material and a solution of the alkali metal in anhydrous ammonia. Alternatively solid TiS2 reacts with the alkali metal upon heating.
The Rigid-Band Model (RBM), which assumes that electronic band structure does not change with intercalation, describes changes in the electronic properties upon intercalation.
Deintercalation is the opposite of intercalation; the cations diffuse out from between the layers. This process is associated with recharging a Li/TiS2 battery. Intercalation and deintercalation can be monitored by cyclic voltammetry. The microstructure of the titanium disulfide greatly affects the intercalation and deintercalation kinetics. Titanium disulfide nanotubes have a higher uptake and discharge capacity than the polycrystalline structure. The higher surface area of the nanotubes is postulated to provide more binding sites for the anode ions than the polycrystalline structure.
Material properties
Formally containing the d0 ion Ti4+ and closed shell dianion S2−, TiS2 is essentially diamagnetic. Its magnetic susceptibility is 9 x 10−6 emu/mol, the value being sensitive to stoichiometry. Titanium disulfide is a semimetal, meaning there is small overlap of the conduction band and valence band.
High pressure properties
The properties of titanium disulfide powder have been studied by high pressure synchrotron x-ray diffraction (XRD) at room temperature. At ambient pressure, TiS2 behaves as semiconductor while at high pressures of 8 GPa the material behaves as a semimetal. At 15 GPa, the transport properties change. There is no significant change in the density of states at the Fermi level up to 20 GPa and phase change does not occur until 20.7 GPa. A change in the structure of TiS2 was observed at a pressure of 26.3 GPa, although the new structure of the high pressure phase has not been determined.
The unit cell of titanium disulfide is 3.407 by 5.695 angstroms. The size of the unit cell decreased at 17.8 GPa. The decrease in unit cell size was greater than was observed for MoS2 and WS2, indicating that titanium disulfide is softer and more compressible. The compression behavior of titanium disulfide is anisotropic. The axis parallel to S-Ti-S layers (c-axis) is more compressible than the axis perpendicular to S-Ti-S layers (a-axis) because of weak van der waals forces keeping S and Ti atoms together. At 17.8 GPa, the c-axis is compressed by 9.5% and the a-axis is compressed by 4%. The longitudinal sound velocity is 5284 m/s in the plane parallel to S-Ti-S layers. The longitudinal sound velocity perpendicular to the layers is 4383 m/s.
Synthesis
Titanium disulfide is prepared by the reaction of the elements around 500 °C. Ti + 2 S → TiS2