Transition-metal chalcogenides, band

Wertheim, G. K., F. J. DiSalvo, and D. N. E. Buchanan (1973). Valence bands of layer structure transition metal chalcogenides. Solid State Comm. 13, 1228. 

The practical use of these calculations is limited, however, because the kinetics of a reaction can play an important role. This becomes quite obvious for layer compounds such as M0S2. The kinetics may be controlled by adsorption, surface chemistry, surface structure and crystal orientation. According to Fig. 8.15, pEdecomp is close to the conduction band, i.e. M0S2 is rather easily oxidized. In the case of a flat basal surface, it has been observed with several transition metal chalcogenides that the photocurrent onset at n-electrodes occurs with high overvoltages accompanied by a shift of Gfb.(see Section 5.3). Since this is caused by an accumulation of holes at the surface the hole transfer is kinetically inhibited. 

It is believed that the catalytic activity on transition metal chalcogenides takes place through the interaction of the molecular oxygen with the transition metal d-states. The bands are filled with valence electrons of the atoms up to the Fermi level as shown for Mo4Ru2Seg cluster compounds (Fig. 14.2) [21]. 

A particular class of electrode materials is represented by the transition metal chalcogenides, such as n-WSe2, n-MoSe2, and others, which form layer crystals. As already mentioned in Section 8.1.3, the basal planar surfaces of these electrodes (perpendicular to the c-axis) are relatively stable. In consequence, holes created by light excitation, are not transferred and accumulate at the surface. This leads to a large downward shift of the energy bands, as found by Mott-Schottky measurements [53] and as illustrated in Figure 8.20b (left and middle). The pho-... 

Figure 1 - General band scheme models for Transition metal chalcogenides (a). Particular cases of ZrS2 (b), NbS2 and M0S2 (c), d-cationic levels and sp anionic band at the end of a period (d).

In this chapter, we present the physical properties of cathodes materials and verify the applicability of the rigid-band model for intercalation compounds with a layered structure namely transition-metal chalcogenides MX2 (M = Ti, Ta, Mo, W X = S, Se) and oxides LLWO2 (M = Co, Ni) as well. Electrical and optical properties are investigated. For some materials, we observe different degrees of irreversibility in the intercalation process and lattice evolution to the complete destruction of the host. Since the purpose here is the study of the materials in the framework of... 

Fig. 7.3 Electron bands for transition metal oxides and chalcogenides. M = metal, X chalcogen or oxygen. The filling of the bands is appropriate for MoSj.

The sum of the two contributions is a skew parabola with its minimum moved towards the beginning of the series. Further, the skewness and the distance of the minimum from the centre of the series increase as the strength of f-p hybridization increases, as shown in fig, 59. A model similar to that summarized in fig. 58 may also be used for actinide transition metal compounds, with d bands replacing the p bands of the chalcogenide or pnictide. The resulting state densities are then similar to those in fig. 40, modified to include f states in the bands, as in sect. 5.3 for cerium compounds. 

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