In spite of the singularities of the intercalation chemistry of MoS 2 compared with those of other transition metal dichalcogenides, quantitative theoretical descriptions permitting to understand the peculiarities of its chemistry are still lacking. Intercalation dynamics is certainly also favoured by the activation process by lowering the energy barriers for lithium migration. The success of this procedure arises from the activation of the matrix by three ways: the reduction of the matrix by lithium intercalation, the stabilization of the octahedral modification, and the physical separation of the molecular layers. The colloidal suspension of MoS 2 molecular layers is then flocculated in the presence of the donor. The key for such a development is an experimental method based on the exfoliation of the pristine matrix by a rapid hydrolysis of the lithiated product ( 5). Although the direct intercalation of amines or similar electron donors into MoS 2 is not possible, a renaissance of the MoS 2 intercalation chemistry has been observed. The intercalation process is however relatively slow, so drastic reaction conditions or special procedures are necessary ( 19). Chemical lithium intercalation into MoS 2 is widely known ( 17, 18). According to a qualitative description based on the comparison with the electronic band structure determined for TaS 2, 1T-MoS 2 should exhibit metal-like behavior because of a broad, partially occupied conduction band ( 15, 16). Thus, the pristine semiconducting 2H b-MoS 2 modification, exhibiting MoS 6 units with a trigonal prismatic arrangement of sulfur atoms around the molybdenum, changes upon intercalation to a distorted 1T-MoS 2 modification with the molybdenum coordinated octahedrically ( 12, 13) which, as recently stablished by electron crystallographic studies, corresponds more properly to a WTe 2 type structure ( 14). Such a hindrance should be apparently removed by a relatively drastic phase change. In the latter, the charge transfer implies to occupy levels of relatively high energy in an originally empty conduction band. In systems with the Fermi level in the conduction band such a charge transfer is certainly thermodynamically easier than in a semiconductor like the MoS 2 with a band gap of about 1.2 eV ( 11). Normally the intercalation in transition metal dichalcogenides is associated to a redox process in which charge is transferred to the matrix ( 9, 10). There are indeed features in both the thermodynamics and kinetics of the intercalation process in MoS 2 which have retarded the progress of its chemistry. Such a late development certainly deserves some attention. Contrasting with other transition metal dichalcogenides as those of Ti, Ta or Zr, whose ability for intercalating a variety of chemical species was rather well known still in the 70Âs ( 6- 8), most molybdenum intercalation compounds have been described during the last decade. The interest on the intercalation chemistry of molybdenum disulfide has been growing notoriously during the last years ( 1-5). Key Words: Electron chemical potential, quantum chemical model, molydenum disulfide, electrochemical charge capacity, lithium intercalation. The model permits moreover to identify a sequence of octahedral and tetrahedral sites as the more favorable migration pathway for the diffusion of lithium through the interlaminar space. Contrasting with classical descriptions like the gas lattice model assuming complete lithium-MoS 2 one electron transfer, proposed model leads, agreeing with previous experimental evidence, to a system in which electron density is partially retained in the lithium atom. Experimentally observed trends of the charge capacity in the range 0
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |