Critical role of lattice defects in structural transformation and catalytic reactions in crystalline solids

Abstract

Over the past two decades, the main research direction of materials science and engineering has been to select materials with inherent properties or to control microstructures in order to change the properties of materials. However, as the materials used in the industry gradually become finer, researchers have begun to look at the atomic unit rather than analyzing the physical properties of the material through the microscopic aspect. Now, the trend of material study has reached the stage of controlling the overall properties by controlling the arrangement of ions and atoms in the crystal lattice. Therefore, besides the synthesis of nano-scale materials, chemical reaction at the atomic level and physical control have become a key field of basic research for developing new conceptual materials. For control of the atomic-level crystal lattices and defects, it is necessary to cooperate with the field of electron microscopy which can directly observe atomic arrangement in the materials. The correlation between the material structures and physicochemical properties can be accurately clarified using electron microscopy. Accordingly, a spherical aberration corrector capable of correcting the aberration of transmission electron microscopy was developed in the 2000s and contributes greatly to the observation and control of atomic defects. In recent years, research has begun to focus on the solid-chemical approach at the atomic scale. Research support in this field has been expanded in the USA, Japan and China, and many research results have been published in professional journals. Since the physical properties of each material, such as electrochemical properties, dielectric properties, and magnetism, are directly affected by the atomic arrangement within the energy material, studies related to it are likely to continue to increase. The field of energy materials benefited most from these research trends. Electrochemical properties of energy storage and energy conversion materials are closely related to the atomic arrangement in the bulk or surface. Indeed, the cause of capacity fading in cathodes of the lithium ion batteries, which is energy storage materials, has been clarified through the transmission electron microscopy, and there is a movement to control and improve characteristics by controlling the crystalline defects. In the field of energy conversion, it has been reported that the catalytic activity is changed according to the atomic arrangement of the surface of the metal alloy through the transmission electron microscopic analysis. In this study, we investigate the role of defects on the structural transition and catalytic reactions in crystalline solids using atomic-scale observation techniques. For this purpose, lithium spinel cathodes known as energy-storage materials and a gold thin film known as energy conversion materials were selected. $LiNi_{0.5}Mn_{1.5}O_4$, a high-power cathode material in the lithium secondary batteries, has two different space groups according to the chemical ordering between $Mn^{4+}$ and $Ni^{2+}$ cations located at octahedral sites. Since the two phases have different electrochemical characteristics, studies has been conducted to obtain optimum characteristics by controlling of the chemical ordering. Although it is essential to study the intermediate phase for the systematical synthesis between the two phases, the research of the intermediate phase has been insufficient. In chapter 2, we performed atomic and large-scale investigation using transmission electron microscopy and in situ X-ray powder diffractions in order to clarify the intermediate phases. In addition, we calculated energy of each transition state by theoretical ab initio calculation. Consequently, we critically reveals that the chemical ordering transition occurs through the formation of unstable Frenkel defects. Furthermore, it proved that a large amount of energy is needed during the cation ordering transition. Based on this study, in chapter 3, we conducted a follow-up study on the correlation between defect formation energy and phase transition temperature, and experimentally proved that formation energy of the Frenkel defect is closely related to phase transition temperature. Chapter 4 clarifies the relationship between catalytic properties and structure defects in gold thin films. Recently, studies have been made to form nano-sized gold catalysts in order to improve the selectivity of CO at low overvoltage in a $CO_2$ reduction reaction. We have succeeded in converting the surface of the gold thin film into a nanoporous structure through easy electrochemical treatments and observed a large amount of nanoparticles containing grain boundaries on the surface. The nanoporous structured Au thin films reduce the overpotential of $CO_2$ reduction reaction and improve the selectivity of CO. Through the comparison with the annealed gold thin films, the grain boundaries plays an important role in the improvement of CO selectivity.