Thermal Decomposition of Silver Acetate: Physico-Geometrical Kinetic Features and Formation of Silver Nanoparticles
The thermal decomposition of silver acetate (CH3COOAg) was investigated to reveal the factors controlling the formation of Ag nanoparticles (NPs). The overall kinetic behavior was interpreted as partially overlapping two reaction steps using systematic kinetic and morphological analyses. Although the apparent activation energies were comparable (approximately 75 kJ mol–1), the initial reaction step was regulated by the first order law because of the consumption of reactive sites on the end surfaces of columnar crystals, whereas the subsequent reaction step advanced by shrinkage of the side surfaces of the crystals with an accelerating linear shrinkage rate, resulting in slimming of the crystals. A large surface area of the reactant crystals was exposed to the reaction atmosphere during the course of the reaction by the self-induced migration of the Ag product to the surfaces of the Ag-NP aggregates formed at certain parts of the reactant surfaces. As a result, the atmospheric water vapor affected the kinetic behavior by significantly lowering the reaction temperature. As a possible explanation for these phenomena, a physical mechanism involving evaporation of the reactant and simultaneous condensation of the product is proposed herein.
The thermal decomposition of solids is largely regulated by the reaction geometry and the chemical and physical processes at the reaction interface, including destruction of the reactant crystal lattice, nucleation and crystal growth of the solid product, diffusional removal of gaseous product, and heat transfer. The actual reaction occurring at the reaction interface is thus largely influenced by the self-generated reaction conditions caused by the interactions of the component chemical and physical processes and the related mass and heat-transfer phenomena. In addition, various physicochemical and physico-geometrical events occur, which regulate the overall kinetics of the thermal decomposition.
The practical reaction conditions are generally selected by considering the empirical relationships among the reactant, reaction conditions, and product, as well as the overall rate behavior, as revealed using thermoanalytical measurements. In turn, such complex reaction behaviors and special events can sometimes afford NPs and structural materials comprised thereof. One example is the formation of Au-NPs by the rapid and violent fragmentation of reactant particles during the thermal decomposition of gold acetate as reported by Bakrania et al. The other is the generation of a Ag2CO3–Ag2O core–shell structure during the thermal decomposition of silver carbonate, which exhibits high visible light efficiency in the photocatalytic degradation of pollutants. To identify the major factors that control the formation kinetics of nanosized and structural materials and to determine the most appropriate reaction conditions of the heat treatment in more sophisticated manner, it is necessary to reveal the kinetic characteristics of the thermal decomposition from the viewpoints of physico-chemistry and physico-geometry.
Because of their utilities for various purposes, syntheses of Ag-NPs and their structural and composite materials via thermal decomposition of solid precursors have been extensively studied using different Ag compounds as precursor materials. Among others, the thermal decomposition of silver acetate has been studied for preparing nanowires, nanorods, and catalysts, and these products have been used in electric devices. Logvinenko and Siffiqui determined the apparent kinetic parameters for the thermal decomposition under linearly increasing temperatures in flowing inert gas and in air. Further extension of the kinetic characterization of the thermal decomposition process from the viewpoints of physico-chemistry and physico-geometry would provide direct information useful for controlling the reaction process and the properties of the Ag-NP product.
Along this line, the present study focused on the characteristics of the physicochemical and physico-geometrical mechanisms of the thermal decomposition of silver acetate to form Ag-NPs and the overall kinetics of the process. The kinetic analyses were carried out through systematic measurements of kinetic rate data and logically coordinated kinetic calculations involving kinetic deconvolution analysis. The results were interpreted with the aid of microscopic observations of morphological changes of the sample during the reaction. The investigation was further extended to evaluate the impact of atmospheric water vapor on the kinetics of the overall reaction and on each reaction step. Through these investigations, the rarely observed special features of the thermal decomposition were revealed, and the possible causes were examined to gain information essential for the sophisticated design of the controlled synthesis of Ag-NPs and structural materials comprised thereof via thermal decomposition of silver acetate.
