Layered Transition Metal Chalcogenides

BF BF and the photoanodic BF BF were separated by only -150 mV, allowing the authors to assign an accurate flat band potential of +0.30 (+0.05) V vs. SCE to M0S2. 

Fan and Bard [140] employed capacitance and voltammetric measurements to investigate the photoelectrochemical behavior of n-WSe2, p-WSe2, and n-MoSe2 single-crystal electrodes in aqueous solutions containing various redox couples, namely Br /Br2, I /I, Fe(CN)6 /Fe(CN)6 , Fe /Fe, and 

MV /MV (HV is heptyl viologen and MV is methyl viologen). The specific effects of iodide on the electrochemical behavior of the layer-type compounds were compared, and the characteristics of several PEC cells were described. The interface energies for n-MoSe2 in contact with various redox couples were given as in Fig. 5.9. 

The rhenium dichalcogenides, ReS2 and ReSe2, have a distorted CdCl2 layered structure leading to triclinic symmetry, with the Re t/-states forming the top of the 

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Schbllhom R, Meyer H (1974) Cathodic reduction of layered transition metal chalcogenides. Mater Res Bull 9 1237-1245... 

Li J, Badding ME, DiSalvo FJ (1992) New layered ternary niobium teUurides Synthesis, structure, and properties of NbMTe2 (M = Fe, Co). Inorg Chem 31 1050-1054 Tremel W, Kleinke H, Derstroff V, Reisner C (1995) Transition metal chalcogenides New views on an old topic. J Alloy Compd 219 73-82... 

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... 

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 hydrothermal method has been used to prepare monodispersed ZnS (6 nm) [10] and CdS nanocrystals (16 nm) [11]. By hydrothermal polymerization and simultaneous sulfidation processes, nanocomposites CdS/poly(vinyl acetate) nanorods [12] and nanospheres [13] were synthesized. In aqueous hydrazine solutions, nonstoichiometric metal telluride nanocrystallites such as Cu2.86Te2, CuyTes, Cuy-xTe, and Ag7Te4 [14], and cubic CogSg were hydrothermally synthesized [15]. Other transition metal chalcogenides, such as single-molecular-layer M0S2 [16] and MoSey [17] were also prepared under hydrothermal conditions. 

The transition metal chalcogenides such as n-WSe2 are a particular class of electrode materials, and their photoelectrochemical behaviour is of interest from the fundamental point of view. If the basal planar surfaces (perpendicular to the c-axis) with a low density of steps are contacting the electrolyte, these layered materials are relatively stable. Since the corrosion rate is very small, the anodic photocurrent occurs at a high overvoltage with respect to the flatband potential in the dark. As discussed in Section 2.3.1 (Fig. 2.15), the flatband potential t/fb is shifted on illumination because holes accumulate at the surface. On addition of a redox couple such as [Fe(phen)3] ... 

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. 

In the oxide literature, there are several reviews specific to the different families, such as layered transition metal oxides and metal phosphonates. Synthesis of metal nitrides has been discussed by Di Salvo. Precursor synthesis of oxides, chalcogenides and other materials is being pursued with vigour. - Thus, single-molecule precursors of chalcogenide containing compound semiconductors have b n developed. Kanatzidis has reviewed the application of molten poly-chalcophosphate fluxes for the synthesis of complex metal thiophosphates and selenophosphates. 

D. Syntheses of Layered Ternary Transition Metal Chalcogenides... 

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-... 

Rouxel J (1978) Alkali metal intercalation compounds of transition metal chalcogenides TX2, TX3 and TX4 chacogenides. In Levy F (ed) Intercalated layer materials. Reidel, Dordrecht, pp 201-250... 

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... 

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