Abstract:
Layered oxides offer a promising approach to creating nanostructured materials through exfoliation of parent crystals into single nanosheet layers. These nanosheets, which are only atoms thick but microns wide, present unique opportunities for microstructure control not available with conventional nanoparticles or nanotubes. Flocculation of exfoliated nanosheets provides an easy route to highly porous solids without further processing, which makes it possible to produce materials for catalysts, active electrodes, and other applications that benefit from macro- and mesopores and high surface areas.
In this work, birnessite, a layered polymorph of MnO2, was synthesized using solid-state and hydrothermal techniques. The high-temperature stability and microstructure evolution was studied in ion-exchanged and flocculated samples. Through controlled processing, it is possible to encourage the growth of a small amount of tunnel-strucutred α-MnO2 along with Mn2O3. While the thin nanosheets disappear with heat treatment, the high surface area of the starting porous solid is maintained. Through surface-tension effects, the sheets transform into nodules in an open network that mimics a spinodally-decomposed solid, where Mn2O3 forms an interconnected solid structure in air.
High-energy x-ray diffraction (HEXRD) was used to observe defects in nanosheets created during processing, and track changes with heat treatment. It was found that these defects play a role in the high-temperature phase stability. At temperatures as low as 200 °C, HEXRD revealed the presence of tunnel-structured MnO2, caused by the migration of surface Frenkel defects in the form of MnO6 octahedra on the surface of the sheets. Without a stabilizing cation, as the temperature increased to 600 °C, Mn2O3 begins to form. However, if a cation such as Na+ is introduced through ion exchange or via flocculation, α-MnO2 is stable to at least 800 °C.
The low-temperature conversion of δ-MnO2 nanosheet floccules to α-MnO2 yielding a porous network of nanosheets offers exciting opportunities in electrochemistry and colloidal chemistry with numerous applications.
Description:
Thesis completed in partial fulfillment of the requirements for the degree of Master of Science in Materials Science and Engineering at the Inamori School of Engineering, New York State College of Ceramics at Alfred University
