关键词: 溶胶-凝胶, 金属纳米材料, 纳米合金, 热力学计算, 金属氧化物标准电极电位

Abstract:

The sol-gel method, developed for over 150 years, is a conventional route for designing and preparing various kinds of metal oxide materials. In the sol-gel method, different chemical agents are homogenously mixed together in aqueous or organic solutions. During the evaporation of the solvents, the solution transforms to sol and gel through polycondensation or polyesterification reaction, and the dried gel is obtained after the complete evaporation of the solvent. Then, the dried precursor is often heat-treated in air at high temperature to induce the formation of oxide materials, especially the multi-component oxide materials that are difficult to prepare using other methods. Recently, new developments have been achieved in the sol-gel method. The application of the sol-gel method has been extended to the preparation of metallic nanomaterials, especially the alloy nanocrystals. For instance, the sol-gel method can be used to prepare CoPt and FePt hard magnetic alloy nanocrystals; CoCrCuNiAl high-entropy alloy nanocrystals; Ni₃Fe and Cu₃Pt alloy nanocrystals with equilibrium-ordered crystalline phases; and Ni, Cu, Bi, Sb, Te, Ag, Pt, and Pd monometallic nanocrystals. This article reviews the recent progresses in the sol-gel method for designing and preparing metallic and alloy nanocrystals, as well as the detailed experimental procedures and the different metallic nanocrystals that can be obtained by the sol-gel method. The crystalline phase formed in the final calcined products can be determined from the thermodynamic calculations of the sol-gel method. The thermodynamic model involves the calculation of the Gibbs free energy change of the reaction between the metallic oxide and reducing gases, such as hydrogen. A negative change and a positive change in the Gibbs free energy of the reaction correspond to the formation of metallic and alloy crystalline phases, or oxide crystalline phase, respectively. Based on the thermodynamic calculations and the relationship between the Gibbs free energy and standard electrodynamic potential of the chemical reaction, a new parameter, metal oxide standard electrode potential, was proposed. This electrode potential is different from the conventional standard metal electrode potential. A metallic crystalline phase is obtained if the electrode potential of the corresponding metal oxide is positive, while a metal oxide crystalline phase is obtained if the electrode potential of the metal oxide is negative. We also discuss the possible applications, including the magnetic and electrocatalytic applications, of the metallic and alloy nanocrystals that have been obtained by the sol-gel method. Finally, the future prospects of the application of the sol-gel method in designing metallic and alloy nanocrystals are discussed.

Key words: Sol-gel, Metallic nanomaterials, Nanoalloy, Thermodyanmical calculations, Metal oxide standard electrode potential