Soft Chemistry Routes
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Abstract
Topochemical Reactions
A solid‐state reaction is said to be topochemically controlled when the reactivity is controlled by the crystal structure rather than by the chemical nature of the constituents. The products obtained in many solid‐state decompositions are determined by topochemical factors, especially when the reaction occurs within the solid without the separation of a new phase. In topotactic solid‐state reactions, the atomic arrangement in the reactant crystal remains largely unaffected during the course of the reaction, except for changes in dimension in one or more directions. Orientational relations between the parent and the product phases are generally found. Many developments in solid‐state chemistry owe much to the investigations carried out on MoO3 and WO3 (for example, crystallographic shear planes). Topochemical dehydration has been used for sometime to prepare new metastable solids (for example, the synthesis of Ti2Nb2O9 from HTiNbO5). This strategy has been extended to perovskites.
Intercalation Chemistry
Intercalation reactions of solids involve the insertion of a guest species (ion or molecule) into a solid host lattice without any major rearrangement of the solid structure. A variety of layered structures act as hosts. The general feature of these structures is that the interlayer interactions are weak while the intralayer bonding is strong. Alkali metal intercalation in dichalcogenides is achieved by direct reaction of the elements around 1070 K in sealed tubes. Alkali metal intercalation compounds with dichalcogenides form hydrated phases. Metal phosphorus trisulfides undergo redox intercalation reactions just as the dichalcogenides and also ion exchange reactions. Pillaring is another intercalation reaction that enables synthesis of metastable oxide material. Pillaring refers to intercalation of robust, thermally stable, molecular species that prop the layers apart and convert the two‐dimensional interlayer space into micropores of molecular dimensions, similar to the pores in zeolites.
Ion Exchange Reactions
Ion exchange in fast‐ion conductors such as β‐alumina is well known. It can be carried out in aqueous as well as molten salt media conditions. Ion exchange in inorganic solids is a general phenomenon, not restricted to fast‐ion conductors alone. Kinetic and thermodynamic aspects of ion exchange in inorganic solids were examined by England. Their results reveal that ion exchange is a phenomenon that occurs even when the diffusion coefficients are as small as ˜10‐12cm2/s, at temperatures far below the sintering temperatures of solids. Ion exchange occurs at a considerable rate in stoichiometric solids as well. A variety of inorganic solids have been exchanged with protons to give new phases, some of which exhibit high protonic conduction. Ion exchange chemistry of layered metal chalcogenides is not explored much compared to that of metal oxides. These are by and large limited to alkali ion‐containing transition metal dichalcogenides.
Use of Fluxes
Use of molten salts as reactive fluxes is a non‐topochemical route that enables the synthesis of metastable phases, especially at intermediate temperatures (150 to 500 degrees Celsius) between those employed in the hydrothermal route and the conventional ceramic route. Strong alkaline media, either in the form of solid fluxes or molten (or aqueous) solutions, enable the synthesis of novel oxides. The alkali flux stabilizes higher oxidation states of metals by providing an oxidizing atmosphere. Alkali carbonate fluxes have been traditionally used to prepare transition metal oxides such as LaNiO3 with Ni in the +3 state. A good example of an oxide synthesized in a strongly alkaline medium is the pyrochlore, Pb2(Ru2‐xPbx)O7‐y, where Pb is in the +4 state. This oxide is a bifunctional electrocatalyst. The procedure for preparation involves bubbling oxygen through a solution of Pb and Ru salts in strong KOH at 320 K.
Sol–Gel Synthesis
The sol‐gel method has provided a very important means of preparing inorganic oxides. It is a wet chemical method and a multistep process involving both chemical and physical processes such as hydrolysis, polymerization, drying and densification. Important features of the sol‐gel method are better homogeneity compared to the traditional ceramic method, high purity, lower processing temperature, more uniform phase distribution in multicomponent systems, better size and morphological control, the possibility of preparing new crystalline and non‐crystalline materials and, lastly, easy preparation of thin films and coatings. The six important steps in sol‐gel synthesis include: hydrolysis, polymerization, gelation, drying, dehydration and densification. The sol‐gel technique has been used to prepare sub‐micrometer metal oxide powders with a narrow particle size distribution and unique particle shapes.
Electrochemical Methods
Electrochemical methods have been employed to advantage for the synthesis of many solid materials. Typical materials prepared in this manner are metal borides, carbides, suicides, oxides and sulfides. Vanadate spinels of the formula MV2O4 as well as tungsten bronzes A5WO3 have been prepared by the electrochemical route. Electrochemical oxidation has been employed to prepare oxygen‐excess La2CuO4 and other related materials. Thin films of BaTiO3 and lead zirconate titanate have been prepared by cathodic reduction. Intercalation of alkali metals in host solids is readily accomplished electrochemically.
Hydrothermal, Solvothermal and Ionothermal Synthesis
In recent years, hydrothermal synthesis has been employed to prepare various inorganic materials such as metal oxides, chalcogenides, metal‐organic frameworks, porous materials and nanomaterials. Hydrothermal high‐pressure synthesis under closed system conditions has been employed for the preparation of higher‐valence metal oxides. Solvothermal synthesis is similar to hydrothermal synthesis but uses organic solvents such as toluene, decalin and octadecene instead of water. Solvothermal reactions have been extensively employed to prepare inorganic nanocrystals. In solvothermal synthesis, the size and shape of nanocrystals are controlled by the concentration of precursors and the reaction temperature. Ionothermal synthesis involves the use of an ionic liquid as the solvent in the synthesis of novel inorganic compounds.
Identifiers
book ISBN : | 9781118832547 |
book e-ISBN : | 9781118892671 |
DOI | 10.1002/9781118892671.ch10 |
Keywords
reactant crystal solid‐state reaction topochemical dehydration topochemical factors alkali metal intercalation dichalcogenides host lattice intercalation reactions pillaring β‐alumina inorganic solids ion exchange layered metal chalcogenides metal oxide alkali carbonate fluxes bifunctional electrocatalyst metal oxide metastable phases molten salts non‐topochemical route reactive fluxes metal oxide powders polymerization sol‐gel synthesis BaTiO3 cathodic reduction electrochemical methods MV2O4 hydrothermal synthesis ionothermal synthesis nanocrystals solvothermal synthesis
reactant crystal solid‐state reaction topochemical dehydration topochemical factors alkali metal intercalation dichalcogenides host lattice intercalation reactions pillaring β‐alumina inorganic solids ion exchange layered metal chalcogenides metal oxide alkali carbonate fluxes bifunctional electrocatalyst metal oxide metastable phases molten salts non‐topochemical route reactive fluxes metal oxide powders polymerization sol‐gel synthesis BaTiO3 cathodic reduction electrochemical methods MV2O4 hydrothermal synthesis ionothermal synthesis nanocrystals solvothermal synthesis