Interfacial Control of Reaction Kinetics in Oxides

Revealing Temperature-Dependent Oxidation Dynamics of Ni Nanoparticles via Ambient Pressure Transmission Electron Microscopy

Understanding the oxidation mechanism of metal nanoparticles under ambient pressure is extremely important to make the best use of them in a variety of applications. Through ambient pressure transmission electron microscopy, we in situ investigated the dynamic oxidation processes of Ni nanoparticles at different temperatures under atmospheric pressure, and a temperature-dependent oxidation behavior was revealed. At a relatively low temperature (e.g., 600 °C), the oxidation of Ni nanoparticles underwent a classic Kirkendall process, accompanied by the formation of oxide shells. In contrast, at a higher temperature (e.g., 800 °C), the oxidation began with a single crystal nucleus at the metal surface and then proceeded along the metal/oxide interface without voids formed during the whole process. Through our experiments and density functional theory calculations, a temperature-dependent oxidation mechanism based on Ni nanoparticles was proposed, which was derived from the discrepancy of gas adsorption and diffusion rates under different temperatures.

Yttrium Manganese Oxide Phase Stability and Selectivity Using Lithium Carbonate Assisted Metathesis Reactions

In solid-state chemistry, stable phases are often missed if their synthesis is impractical, such as when decomposition or a polymorphic transition occurs at relatively low temperature. In the preparation of complex oxides, reaction temperatures commonly exceed 1000 °C with little to no control of the reaction pathway. Thus, a prerequisite for exploring the synthesis of complex oxides is to identify reactions with intermediates that are kinetically competent at low temperatures, as provided by assisted metathesis reactions. Here, we study the assisted metathesis reaction Mn₂O₃ + 2.2YCl₃·6H₂O + 3Li₂CO₃ → 2YMnO₃ + 5.8LiCl + 0.2LiYCl₄ + 3CO₂ using in situ synchrotron X-ray diffraction. By changing the atmosphere, oxygen vs inert gas, the reaction product changes from the overoxidized perovskite YMnO_3+δ to the hexagonal YMnO₃ polymorph at the reaction temperature of 850 °C, respectively. Analysis of the reaction pathways reveals two parallel reaction pathways in forming YMnO₃ phases: (1) the slow reaction of metal oxides in a LiCl flux (Y₂O₃ + Mn₂O₃ [Formula: see text] 2YMnO₃) and (2) the fast reaction from ternary intermediates (YOCl + LiMnO₂ → LiCl + YMnO₃). Control reactions reveal that both proposed pathways in isolation result in product formation, but the direct preparation of ternary intermediates (YOCl + LiMnO₂ → LiCl + YMnO₃) occurs at lower temperatures (500 °C) and shorter times (<24 h) and forms nominally stoichiometric orthorhombic YMnO₃. These ternary intermediates react at a faster rate than the slow stepwise oxygenation of yttrium chloride to Y₂O₃ (YCl₃ → YOCl → Y₃O₄Cl → Y₂O₃), which is relatively inert. These results support a kinetically controlled reaction pathway to form YMnO₃ phases in assisted metathesis reactions with phase selectivity achievable through changes to reaction atmosphere.

On the Influence of Applied Fields on Spinel Formation

Interfaces play an important role in determining the effect of electric fields on the mechanism of the formation of spinel by solid-state reaction. The reaction occurs by the movement of phase boundaries but the rate of this movement can be affected by grain boundaries in the reactants or in the reaction product. Only by understanding these relationships will it be possible to engineer their behavior. As a particular example of such a study, Mgln2O4 can be formed by the reaction between single-crystal MgO substrate and a thin film of In2O3with or without an applied electric field. High-resolution backscattered electron (BSE) imaging and electron backscattered diffraction (EBSD) in a scanning electron microscope (SEM) has been used to obtain complementary chemical and crystallographic information.