In-Situ Transmission Electron Microscopy Investigations of Manganese Oxide and Gadolinium Oxide

Abstract

Nanomaterials have been widely studied owing to their interesting properties and potential applications. In this thesis, two novel nanomaterials were studied using in-situ transmission electron microscopy (TEM): (1) MnO nanostructures and (2) Gd2O3 nanofoam. In-situ TEM is used because it enables heating inside the microscope, which allows for real-time viewing of the heating induced changes to the material. The aim of the first part of this thesis was to observe the predicted heating induced square to trigonal lattice transformation in MnO nanosheets. Manganese oxide nanostructures were synthesized using a salt-templating method. Depending on the precursor, this resulted in the formation of either nanoparticles or nanosheets, both of which were heated inside the TEM. The MnO nanoparticles were embedded in amorphous carbon and spontaneously oxidized to Mn3O4 under ambient conditions depending on the thickness of the carbon layer. When heated, the nanoparticles reduced back to MnO, indicating a reversible transformation between MnO and Mn3O4. The synthesized MnO nanosheets transformed from a square lattice to a trigonal lattice at room temperature as soon as the salt template was removed, thereby confirming the predicted lattice transformation. In addition to manganese oxide nanostructures, gadolinium oxide was synthesized via gel combustion, resulting in highly porous and crystalline nanofoams. The aim of this project was to characterize the porosity and study the formation of the material during combustion synthesis. The material was characterized by physisorption, indicating a large specific surface area of 67 m2/g, and a 3D model of the structure made using electron tomography visualized the interconnected pore structure of the bulk synthesized Gd2O3. The Gd gel was applied to heating chips and heated in-situ and ex-situ to further analyze the synthesis mechanism. The pore structure was highly dependent on the experimental conditions, indicating that the structure could be tuned by varying the temperature and atmosphere during synthesis. Porous Gd2O3 can be used to treat contaminated water and in (photo)catalysis applications, and gel-combustion synthesis is a low-cost and energy-efficient method to synthesize large amounts of this highly porous Gd2O3.