USING CONVENTIONAL AND \u3cem\u3eIN SITU\u3c/em\u3e TRANSMISSION ELECTRON MICROSCOPY TECHNIQUES TO UNDERSTAND NANOSCALE CRYSTALLOGRAPHY

Transmission electron microscopy (TEM) is a powerful tool for studying solidstate crystalline systems. With the advances in aberration correction, monochromation, and in situ capabilities, these microscopes are now more useful for addressing fundamental materials chemistry problems than ever before. This dissertation will illustrate the ways in which I have been using high-resolution imaging and in situ heating in the TEM during my Ph.D. research to investigate unique solid state chemistry questions. This dissertation will focus on four unique crystal systems: thermoelectric skutterudite crystals, vapor-liquid-solid (VLS) grown nanowires, and hafnium dioxide nanorods. Although these systems are very different from one another, high resolution and/or in situ heating in TEM is an integral part of each study. Through these techniques, we gain insight and knowledge of these systems that may have gone unknown through different analysis techniques. The experiments I will describe in some cases provide surprising and unexpected results that arise from the nanoscale nature of the materials and would be difficult to observe through bulk analytical methods. The work presented here helps to demonstrate the strength and versatility of TEM to address solid state chemistry questions

Real-time Atomistic Observation of Structural Phase Transformations in Individual Hafnia Nanorods

High-temperature phases of hafnium dioxide have exceptionally high dielectric constants and large bandgaps, but quenching them to room temperature remains a challenge. Scaling the bulk form to nanocrystals, while successful in stabilizing the tetragonal phase of isomorphous ZrO2, has produced nanorods with a twinned version of the room temperature monoclinic phase in HfO2. Here we use in situ heating in a scanning transmission electron microscope to observe the transformation of an HfO2 nanorod from monoclinic to tetragonal, with a transformation temperature suppressed by over 1000°C from bulk. When the nanorod is annealed, we observe with atomic-scale resolution the transformation from twinned-monoclinic to tetragonal, starting at a twin boundary and propagating via coherent transformation dislocation; the nanorod is reduced to hafnium on cooling. Unlike the bulk displacive transition, nanoscale size-confinement enables us to manipulate the transformation mechanism, and we observe discrete nucleation events and sigmoidal nucleation and growth kinetics

Effect of pH on the Selectivity of γ‐MnO2 Electrocatalysts towards Oxygen Evolution Reaction in the Presence of Chloride Ions in Alkaline Environment

Abstract In this manuscript, we explore the effect of pH on the selectivity of a hydrothermally synthesized nanostructured γ‐MnO2 electrocatalyst in the alkaline environment. Selectivity of electrodeposited γ‐MnO2 toward oxygen evolution reaction (OER) in 0.5 M NaCl at pH 12 has been demonstrated by Fujimura et al (Mat. Sci. Eng., A267 (1999) 254–259). Herein, we extend the pH region from pH 8.2 to pH 13 and demonstrate by thin‐film rotating disk electrode (RDE) method that the catalyst is selective toward the OER at current densities up to ca 15 mA/cm2 in the entire range of pH, if buffer is utilized to mitigate the effect of local pH. In synthetic seawater, at pH 8.2, the catalyst is not selective toward the OER. The analysis of the OER Tafel slopes at low current densities (<3 mA/cm2) shows that the slopes were not affected by pH in 0.5 M NaCl solutions, which suggests the same OER mechanism. At the same time, reaction rate decreased with decrease in pH. Nafion ionomer in the catalyst layer may adversely affect the catalyst's performance at pH ≤12 by limiting diffusion of OH− ions through the Nafion film. Catalyst layers need to be carefully designed to avoid negative effects of Nafion

Real-Time Observation of the Solid–Liquid–Vapor Dissolution of Individual Tin(IV) Oxide Nanowires

The well-known vapor–liquid–solid (VLS) mechanism results in high-purity, single-crystalline wires with few defects and controllable diameters, and is the method of choice for the growth of nanowires for a vast array of nanoelectronic devices. It is of utmost importance, therefore, to understand how such wires interact with metallic interconnects–an understanding which relies on comprehensive knowledge of the initial growth process, in which a crystalline wire is ejected from a metallic particle. Though ubiquitous, even in the case of single elemental nanowires the VLS mechanism is complicated by competing processes at multiple heterogeneous interfaces, and despite decades of study, there are still aspects of the mechanism which are not well understood. Recent breakthroughs in studying the mechanism and kinetics of VLS growth have been strongly aided by the use of <i>in situ</i> techniques, and would have been impossible through other means. As well as several systematic studies of nanowire growth, reports which focus on the role and the nature of the catalyst tip reveal that the stability of the droplet is a crucial factor in determining nanowire morphology and crystallinity. Additionally, a reverse of the VLS process dubbed solid–liquid–vapor (SLV) has been found to result in the formation of cavities, or “negative nanowires”. Here, we present a series of heating studies conducted <i>in situ</i> in the transmission electron microscope (TEM), in which we observe the complete dissolution of metal oxide nanowires into the metal catalyst particles at their tips. We are able to consistently explain our observations using a solid–liquid–vapor (SLV) type mechanism in which both evaporation at the liquid–vapor interface and adhesion of the catalyst droplet to the substrate surface contribute to the overall rate