Transition metals dichalcogenides

It is therefore probably true to say that the only practical source of any of the five compounds is by synthesis, with the result that they are all more expensive than molybdenum disulphide. The synthesis in each case is by heating the powdered metal with sulphur or selenium. The product in most cases is non-stoichiometric and the crystals which form initially from the walls of the reaction vessel do not have the desired hexagonal lamellar structure. The hexagonal crystal structure can usually be obtained by annealing or holding the product at 200°C or more in an inert atmosphere or vacuum. 

Their crystal structures have been mentioned briefly in connection with intercalation in Section 14.2. All five compounds can be obtained in the layered hexagonal crystal form, and most are also found in rhombohedral or trigonal form. The compounds of the Group 6 metals, molybdenum and tungsten, as well as niobium diselenide, have a hexagonal form similar to that of molybdenum disulphide, in which the metal atoms in one layer are displaced sideways from those in the layers immediately above and below. This structure results in the widest interlamellar spacing, the easiest interlamellar shear, and the lowest friction. 

However, pure niobium disulphide has a hexagonal structure in which the metal atoms in each layer are located directly above or below those in adjacent layers. Jamison showed that with this configuration niobium disulphide is not a good lubricant. He postulated that when niobium disulphide behaves as a good lubricant, additionai niobium atoms intercalated into the structure will have resulted in a change in eiectron bonding to favour the molybdenum disulphide structure. 

Test Conditions Slow Ball on flat, Flat/flat Compact Sphere on Flat on  

Subject to this complication, the friction properties of the group are generally similar to those of molybdenum disulphide. The actual coefficients of friction vary with load, speed, temperature and humidity, but some reported figures are shown in Table 14.6. The chemical properties are also similar to those of molybdenum disulphide. They are resistant to attack by water, alkalis and most acids, but are attacked by aqua regia and hot concentrated hydrochloric, nitric or sulphuric acids. The most significant differences are in their electrical conductivity and their oxidation resistance. 

Considerable attention has been devoted to the study of intercalation compounds of the dichalcogenides (Whittingham, 1978 Subba Rao Shafer, 1979). Intercalation compounds of dichalcogenides can be divided into three categories (a) compounds with Lewis base type molecules such as ammonia, n-alkylamines, pyridines etc. (b) compounds with metal cations or molecular cations, Li, Na, K, etc., or [(C5H5)2Co] and (c) compounds containing both cations and neutral polar (solvated) molecules in the van der Waals gap. 

Intercalation with organic compounds has been carried out by direct reaction of the 

The dichalcogenides of group VI do not readily form intercalates with organic molecules. But n-alkylammonium compounds can be prepared by ion-exchange reactions of the hydrated sodium intercalation compounds such as Na j(H20)o g M0S2. Alkylammonium compounds prepared by this route show lattice expansions that depend on the alkyl chain length, very similar to the MX2-n-alkylamine systems described above. This raises the fundamental question whether protonated amines are the actual intercalated species in both the cases, involving reduction of the host. In fact, this constitutes the basis of the new model for the intercalation reaction in MX2 proposed by Schollhorn (1980). 

Intercalation of both cationic and neutral (solvated) molecular species in MXj is particularly important when the reaction occurs in the presence of polar solvents, especially at low temperatures (Fig. 8.8). A simplified scheme of reactions which explains this type of intercalation has been proposed by Schollhorn (1980)  

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Jobic S, Brec R, Rouxel J (1992) Occurrence and characterization of anionic bondings in transition metal dichalcogenides. J AUoy Compd 178 253-283... 

Wilson JA, Yoffe AD (1969) The transition metal dichalcogenides. Discussion and interpretation of the observed optical, electrical and structural properties. Adv Phys 18(73) 193-335... 

Lucovsky G, White RM, Benda JA, ReveUi JE (1973) Infrared-reflectance spectra of layered Group-IV and Group-VI transition-metal dichalcogenides. Phys Rev B 7 3859-3870 Cordes H, Schmid-Fetzer R (1994) Phase equilibria in the U-Te system. J AUoy Compd 216 197-206... 

Tiibutsch H (1978) Hole reactions from d-energy bands of layer type group VI transition metal dichalcogenides New perspectives for electrochemical solar energy conversion. J... 

A critical review on the foundation and earlier results on metal intercalates of the transition metal dichalcogenides and related host materials can be found in the seminal paper of Whittingham [53]. The electrochemical and transport properties... 

Subba Rao GV, Tsang JC (1974) Electrolysis method of intercalation of layered transition metal dichalcogenides. Mater Res BuU 9 921-926... 

Ki, W. Huang, X. Li, J. Young, D. L. Zhang, Y. 2007. Highly conductive group VI transition metal dichalcogenide films by solution-processed deposition. J. Mater. Res. 22 1390-1395. 

Chen X, Fan R (2001) Low temperature hydrothermal synthesis of transition metal dichalcogenides. Chem Mater 13 802-805... 

Omloo and Jellinek7 have described the synthesis and characterization of intercalation compounds of alkali metals with the group V layered transition metal dichalcogenides. Typically, these types of intercalation complexes are sensitive to moisture and must be handled in dry argon or nitrogen atmospheres. The alkali metal atoms occupy either octahedral or trigonal prismatic holes between X-M—X slabs. There are two principal means by which these compounds may be prepared. 

IV layered transition metal dichalcogenide-alkali metal intercalation compounds. The advantage of this method is that it is carried out at room temperature and, consequently, there is less likelihood of reaction between sodium and the reaction vessel. On the other hand, this method is more difficult in that it involves the use of liquid NH3. Furthermore, undesirable side reactions may occur if the NH3 is not dried thoroughly or if the reaction vessel is not clean. For example,... 

DiSalvo et al.9 have carried out a systematic survey of intercalation compounds of 2H(a)-TaS2 with post-transition metals. In particular, the system SnxTaS2 was found to exist in two composition domains, 0 < x < /3 and x = 1. The following discussion briefly describes the techniques used by DiSalvo to synthesize the compound SnTaS2. Syntheses of other transition and post-transition metal intercalation complexes with the layered transition metal dichalcogenides are discussed in References 9 and 20-24. 

J. A. Wilson and A. D. Yoffe, Adv. Phys., 18, No. 73, 193-335 (1969). This article offers a complete discussion of the polymorphic phases found in the various layered transition metal dichalcogenides. 

Figure 1.33. Polytypes of transition-metal dichalcogenides (1120 sections) large circles, chalcogen small circles, metal.

Figure 4.17 Electrical resistivity parallel to the layers of transition-metal dichalcogenides. (After Wilson et al, 1975.)...

In summary, it has been demonstrated that surface morphology is critically important in determining the performance of solar cells with layered compound semiconductors. Steps on structured surfaces of transition metal dichalcogenides have been identified as carrier recombination sites. The region defined by the depth of the space charge layer parallel to the van der Waals planes can be considered as essentially "dead" in the sense that its photoresponse is negligible. As the "step model" predicts, marked improvement in solar cell performance is found on samples with smooth surfaces. 

The major classes of conducting host lattices are summarized in Table 2. Of these, graphite and the layered transition metal dichalcogenides see Chalcogenides Solid-state Chemistry) have been investigated in the greatest detail. The conducting host lattices have the feature that... 

Intercalation compounds of the dichalcogenides can be formed with organic guest molecules under sflictly anhydrous conditions by direct reaction at temperatures up to about 200 °C. Early studies of the transition metal dichalcogenides largely focused on this type of reaction after the initial discovery of amide intercalation in TiS2 and the subsequent... 

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