Synthetic Control of Crystal Structure and the Resulting Chemical and Physical Properties of Early Transition Metal Oxides and Carbides.

摘要

Scaling materials to nanometer-sized dimensions provides an opportunity to stabilize polymorphs at room temperature that are otherwise inaccessible. The relatively facile accessibility of metastable crystal structures is a consequence of both thermodynamic and kinetic effects. The high surface-to-volume ratio of nanostructures implies that surface energy terms can play a significant role (comparable to the bulk energy) in determining the stable polymorph at room temperature.1 From a kinetic perspective, the absence of extended defects in nanostructures can preclude phase transitions to stable polymorphs, thereby trapping the system within the metastable phase, which can possibly represent a local thermodynamic minimum. The focus of this dissertation is primarily on exploring the influence of finite size and solid-solution formation on the phase diagrams of early transition metal oxides. Elucidation of the nanoscale phase diagram necessitates the development of appropriate synthetic methods. Much of this dissertation will thus focus on explorations of reactions yielding metal oxide nanocrystals at relatively low temperatures. Some synthetic explorations of early transition metal carbides are also illustrated.