Template synthesis of one-dimensional perovskites (ABO₃): the effects of grain size and aspect ratio on the electronic properties

Abstract

Metal oxides, that have the perovskite (ABO3) crystal structure, are versatile materials known for their dielectric (paraelectric), ferroelectric, magnetic, piezoelectric, optic, and superconductivity. The phase or symmetry of the unit cell, concentrations of the cations within solid solutions, and defects in the crystalline lattice, can influence these properties. Thus, it is imperative to understand how crystal chemistry is affected by nanoscaling. A plethora of compounds within the perovskite family exist, however; this work has focused on a few choice perovskites known for being examples of paraelectric, ferroelectric, antiferroelectric, and magnetic materials. Barium titanate (BaTiO3), strontium titanate (SrTiO3), lead titanate (PbTiO3), lead zirconate (PbZrO3), and neodymium nickelate (NdNiO3) were chosen for this study. Template synthesis was used to fabricate one-dimensional materials for the investigation on the effects of grain size, crystallite size, and aspect ratio upon the physical properties of the perovskites. This dissertation reports on the sol-gel template synthesis and characterization of the first perovskite nanotubes. Methods were further developed to include the formation of perovskite nano fibers. Chelate and Pechini sol-gel routes were employed to coat inorganic and organic membrane templates. Nanotubes were formed within 200 nm pore Whatman anodisc aluminum oxide membranes. Both nanotubes and nanofibers were prepared using 50, 100, and 200 nm Whatman track-etched polycarbonate membranes. Electron microscopy revealed that morphology depended upon the selection of templates employed, concentration of the precursor sol-gel, and crystallization conditions. A fundamental study of how grain/crystallite size and aspect ratio of the one dimensional morphologies affect the electronic phase transitions were monitored through the comparison of bulk powders to nanostructured materials using electron diffraction, X-ray diffraction, Raman spectroscopy, differential scanning calorimetry, and UV-visible optical diffuse reflectance spectroscopy. Results indicated that the perovskites synthesized through the templates used in this research did not show significant deviations in their properties from the bulk control powders except in crystallite sizes formed.