Chalcogenide powders

In an extension of the spray-drying technique called spray roasting , evaporative decomposition of solutions (EDS) , spray pyrolysis , or aerosol pyrolysis , the temperature of the heated chamber is high enough to decompose the dried salts after the solvent has evaporated. Nitrate salts are used because of their low decomposition temperatures. The technique eliminates the problems of handling dried nitrate powders, which can be hydroscopic. These methods are used to prepare chalcogenide powders" and barium titanate . 

Just as, in Group VB, niobium, so, in this Group, molybdenum provides most of the examples of the chalcogenide halides. The occurrence and preparation of such compounds are described in numerous publications. In most cases, they have been obtained as powders, with the composition based on chemical analyses only. The presence of defined, homogeneous phases is, therefore, in many cases doubtful. In addition, some published results are contradictory. A decision is possible where a complete structure analysis has been made. As will be shown later, the formation of metal-metal bonds (so-called clusters), as in the case of niobium, is the most characteristic building-principle. Such clusters... 

The structures of the rhenium chalcogenide halides have not been studied. X-Ray powder data were collected, in order to prove the homogeneity of the compounds 140, 263, 264, 353). 

Hydrothermal synthesis of ZnSe and CdSe, using powdered Se with either Zn or Cd heated to 180°C in a water filled autoclave, yields 70-100 nm particles without the need of any stabilizer or capping agent. While most hydrothermal synthesis routes of chalcogenides involve the preparation of binary systems, more complicated ternary compounds can also be prepared. For example, Znln2S4, a ternary chalogenide photocatalyst has been hydrothermally prepared and found to photocatalytically reduce water under visible... 

As an example, Fig. 5.6 depicts a typical diffraction spectrum. It is evident that long range order does not exist in our chalcogenide samples. However, the broad difffactrogram peak centered at 20 = 42.5° has the characteristic of a nanodivided ruthenium metal [22]. This points out that the active center in this chalcogenide materials is essentially of metallic nature. The material, either in powder or colloidal form, was analyzed by the EXAFS technique [11]. The local range order of this technique allowed for some structural determination of our samples. Thus, for example, the co-ordination distances for ruthenium-selenium and ruthenium-ruthenium are R(RU-se) = 2.43 A y R(ru.rU) = 2.64 A, respectively. The metal-metal co-ordination distance is of the same order of magnitude as that of well known cluster based materials such as the Chevrel phase [35, 37], cf. Fig. 5.2b. This testifies that the used chemical route leads to the formation of cluster-like materials. 

The novel cluster-like chalcogenide material RuxSey deposited in thin [5, 26, 31, 36] and ultra-thin layers [9, 11] or in powder form embedded in a polymer matrix [30] was found to be an efficient catalyst for the molecular oxygen reduction in acid medium. Fig. 5.10 summarizes the current-potential (j-E) characteristics as a function of the substrate s nature. First of all, one can appreciate that similar activities are obtained from materials synthesized in powder or in colloidal form when deposited onto GC (Fig. 5.10, compare curves (1) and (2)). For the sake of comparison, the j-E characteristic generated on the naked GC substrate for the electrochemical process is contrasted in curve (5). 

The most likely application of the chalcogenide nanotubes is as solid lubricants. Mo and W chalcogenides are widely used as solid lubricants. It has been observed that the hollow nanoparticles of WS2 show better tribological properties and act as a better lubricant compared to the bulk phase in every respect (friction, wear and life-time of the lubricant).142 Tribological properties of 2H-MoS2 and WS2 powder can be attributed to the weak van der Waals forces between the layers which allow easy shear of the films with respect to each other. The mechanism in the WS2 nanostructures is somewhat different and the better tribological properties may arise from the rolling friction allowed by the round shape of the nanostructures. 

The other compounds that have been examined are compiled in Table 14. These materials have been studied both as photoanodes (Entries 2 4) and in powder form (Entries 1, 5 10). As with their oxide counterparts, the chalcogenide family is also rich in solid solution chemistries and Entries 6, 7, 9 and 10 in Table 14 exemplify this trend. These materials are further discussed in Section 12 of this Chapter. 

Alloys Borates Solid-state Chemistry Carbides Transition Metal Solid-state Chemistry Chalcogenides Solid-state Chemistry Diffraction Methods in Inorganic Chemistry Electronic Structure of Solids Fluorides Solid-state Chemistry Halides Solid-state Chemistry Intercalation Chemistry Ionic Conductors Magnetic Oxides Magnetism of Extended Arrays in Inorganic Solids Nitrides Transition Metal Solid-state Chemistry Noncrystalline Solids Oxide Catalysts in Solid-state Chemistry Oxides Solid-state Chemistry Quasicrystals Semiconductor Interfaces Solids Characterization by Powder Diffraction Solids Computer Modeling Superconductivity Surfaces. 

Apart from the oxides and sulphides, other groups of simple compounds are the halides, chalcogenides and molybdates, and there are a number of well characterised organic compounds. Simple halides can be prepared by direct reaction between molybdenum powder and chlorine, fluorine or bromine, and the pentachloride MoClg has been used as a chlorination catalyst. 

Two different types of reactors are used depending on the product synthesized. The first type can maintain pressures up to 150 atm, and is widely used for production of powders in gasless and gas-solid systems. Carbides, borides, silicides, intermetallics, chalcogenides, phosphides, and nitrides are usually produced in this type of reactor. The second type, a high-pressure reactor (up to 2000 atm), is used for the production of nitride-based articles and materials, since higher initial sample densities require elevated reactant gas pressures for full conversion. For example, well-sintered pure BN ceramic with a porosity of about 20-35% was synthesized at 100 to 5000-atm nitrogen pressure (Merzhanov, 1992). Additional examples are discussed in Section III. 

---