9.6: Bonding in TiS₂, MoS₂, and Pyrite Structures
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Many layered dichalcogenides, such as TiS 2 and ZrS 2 , have the CdI 2 structure. In these compounds, as we have noted above, the metal ions are octahedrally coordinated by S. Interestingly, the structures of MoS 2 and WS 2 , while they are also layered, are different. In these cases, the metal is surrounded by a trigonal prism of sulfur atoms. NbS 2 , TaS 2 , MoSe 2 , MoTe 2 , and WSe 2 also have the trigonal prismatic molybdenite structure, which is shown below alongside a platy crystal of MoS 2 .
The coordination of the metal ions by a trigonal prism of chalcogenide ions is sterically unfavorable relative to octahedral coordination. There are close contacts between the chalcogenide ions, which are eclipsed in the stacking sequence AbA/BaB/AbA/BaB ... (where "/" indicates the van der Waals gap between layers). What stabilizes this structure?
The molybdenite structure occurs most commonly in MX 2 compounds with a d 1 or d 2 electron count . The figure below compares the splitting of d-orbital energies in the octahedral and trigonal prismatic coordination environments:
The trigonal prismatic structure is stabilized in MoS 2 by filling the lowest energy band, the d z2 . The d z2 orbital which points vertically through the triangular top and bottom faces of the trigonal prism, has the least interaction with the sulfide ligands and therefore the lowest energy. The d xz and d yz orbitals, which point at the ligands, have the highest energy. The d z2 orbital is lower in energy in this structure than the t 2g orbitals are in the octahedral structure of TiS 2 .
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d-orbital splittings and energy bands in TiS 2 and MoS 2 . MoS 2 is a semiconductor with a 1.3 eV gap between its filled and empty bands. |
Because it has an unfilled t 2g band, TiS 2 is relatively easy to reduce by intercalation with Li. For this reason, LiTiS 2 was one of the first intercalation compounds studied by Stanley Whittingham, who developed the concept of the non-aqueous lithium ion battery in the early 1970's. [3] Because it has a filled d z2 band, MoS 2 is harder to reduce, but it can be intercalated by reaction with the powerful reducing agent n-butyllithium to make Li x MoS 2 (x < 1). Atoms in the van der Waals planes of these compounds are relatively unreactive, which gives MoS 2 its good oxidative stability and enables its application as a high temperature lubricant. Atoms at the edges of the crystals are however more reactive and in fact are catalytic. High surface area MoS 2 , which has a high density of exposed edge planes, is used as a hydrodesulfurization catalyst and is also of increasing interest as an electrocatalyst for the reduction of water to hydrogen.
Layered metal dichalcogenides, including MoS 2 , WS 2 , and SnS 2 , can form closed nanostructures that take the shape of multiwalled onions and multiwalled tubes. These materials were discovered by the group of Reshef Tenne in 1992, shortly after the discovery of carbon nanotubes. Since then nanotubes have been synthesized from many other materials, including vanadium and manganese oxides.
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The pyrite (FeS 2 ) crystal structure. The structure is related to NaCl, with Fe 2 + and S 2 2 - ions occupying the cation and anion sites. |
Although early (TiS 2 ) and late (PtS 2 ) transition metal disulfides have layered structures, a number of MS 2 compounds in the middle of the transition series, such as MnS 2 , FeS 2 and RuS 2 , have three-dimensionally bonded structures. For example, FeS 2 has the pyrite structure , which is related to the NaCl structure. The reason is that FeS 2 is not Fe 4 + (S 2- ) 2 , but is actually Fe 2 + (S 2 2 - ), where S 2 2 - is the disulfide anion (which contains a single bond like the peroxide anion O 2 2 - ). S 2- is too strong a reducing agent to exist in the same compound with Fe 4 + , which is a strong oxidizing agent. Because FeS 2 is actually Fe 2 + (S 2 2 - ), it is a 1:1 compound and adopts a 1:1 structure.