Abstract
Materials synthesis in the liquid phase, or wet-chemical synthesis, utilizes a solution medium in which the target materials are generated from a series of chemical and physical transformations. Although this route is central in organic chemistry, for materials synthesis the low operational temperature range of the solvent (usually below 200 °C, in extreme 350 °C) is a serious restriction. Here, salt melt synthesis (SMS) which employs a molten inorganic salt as the medium emerges as an important complementary route to conventional liquid phase synthesis. Depending on the nature of the salt, the operational temperature ranges from near 100 °C to over 1000 °C, thus allowing the access to a broad range of inorganic crystalline materials and carbons. The recent progress in SMS of inorganic materials, including oxide ceramic powders, semiconductors and carbon nanostructures, is reviewed here. We will introduce in general the range of accessible materials by SMS from oxides to non-oxides, and discuss in detail based on selected examples the mechanisms of structural evolution and the influence of synthetic conditions for certain materials. In the later sections we also present the recent developments in SMS for the synthesis of organic solids: covalent frameworks and polymeric semiconductors. Throughout this review, special emphasis is placed on materials with nanostructures generated by SMS, and the possible modulation of materials structures at the nanoscale in the salt melt. The review is finalized with the summary of the current achievements and problems, and suggestions for potential future directions in SMS.
Highlights
In the past decades, many discoveries in physics and chemistry have been triggered by the breakthrough in fabrication procedures that allowed unprecedented easy access to the desired materials and material structures
From a chemistry point of view, material synthesis is the result of chemical transformations which can be performed in a gaseous, solid state or solution environment
They can be divided into two groups according to the covalent bonding strength: (i) compounds like borides, carbides and silicides, which are built up with strong metal–nonmetal covalent bonds which usually require a very high crystallization temperature; and (ii) chalcogenides, which are much weaker in bonding energy and can be in general accessed in mild temperatures
Summary
Many discoveries in physics and chemistry have been triggered by the breakthrough in fabrication procedures that allowed unprecedented easy access to the desired materials and material structures. The use of different types of ‘‘fluxes’’, including low melting metals and salts, has been extensively explored for the synthesis of metallic and non-metallic materials in the form of either single crystals or polycrystalline powders.[10,11] Compared to solid state reactions for which the rates are usually seriously limited by the slow diffusion of the reactants, the flux method lowers the reaction temperature as it allows faster mass transfer transport in the liquid phase by means of convection and diffusion For those solvent-based synthetic routes solvation is a crucial step, but molecular solvents hardly solvate many inorganics like metals and oxides. The review will be finalized by giving some potential future directions for material synthesis by SMS
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