REPOSITORY > RESULTS

Abstract

Metal oxide and sulfide nanostructures are a versatile family of materials with wide-ranging applications, and their development remains ongoing. In nanoscience, attention is being directed towards their synthesis and further advancement, including the creating of new complex structures and morphologies. In this regard, ion-exchange reactions have emerged as a cutting-edge strategy for synthesizing novel ionic nanomaterials, allowing precise control over the phase and the morphology. This field continuously evolves, with discoveries of new mechanisms and phenomena contributing to improved materials being synthesized.
This thesis was focused on the synthesis of metal oxide nanostructures, specifically CuO nanowires, and their use as a model material for investigating anion-exchange transformation as a potential mechanism for the synthesis of new materials. The selection of CuO nanowires as a model material is ideal due to their one-dimensional shape, simplifying the study of phase transformations. For that reason, ultra-thin nanowires are desired for exploring the transformation processes. The synthesis of CuO nanowires was achieved through the thermal oxidation of copper in air. The method is straightforward; however, the nanowire-growth mechanism is complex and poorly understood. Hence, the research efforts in the first part of the thesis were dedicated to unraveling the mechanism behind the thermal growth of CuO nanowires. We found that the nanowires likely originated from twinned CuO grains, where oxidation preferentially occurred at the twin-boundary defect on the grain surface. During the oxidation, the NW roots become partially buried in the CuO layer growing below the nanowires. A theoretical model we developed supported the growth of nanowires and underlying oxide layers. To study the size-dependent effects on the phase transformations and to observe how nanowires behave close to the limit of quantum confinement, we also identified the optimum oxidation temperature for obtaining ultrathin CuO nanowires with the minimum diameters, serving as an ideal template material for this purpose. This limiting temperature corresponded to slightly below 200 °C, and the correlation between the nanowire’s diameter and the temperature was explained through the modeling of nucleation processes.
Following the synthesis of CuO nanowires, we carried out thermally dependent sulfurization to induce an oxide-to-sulfide phase transformation. The resulting nanostructures are morphology and phase dependent on the availability of copper cations participating in the reaction. When sulfurized nanowires were isolated, with the amount of copper involved limited to that present in the nanowires, the resulting structures were voided CuS nanowires. However, if the nanowires were still attached to the underlying substrate during sulfurization, an unlimited supply of copper from the substrate led to continued growth of the nanowire, resulting in a bulky Cu2S structure. We also developed a theoretical model to explain this intriguing observation.
The investigation into phase transformations in CuO nanowires was extended by utilizing plasma in the post-glow region of a microwave discharge as a sulfurization environment. The plasma-induced dimensionality changes by reshaping the nanowires into two-dimensional CuS structures. The proposed mechanism behind this transformation provided valuable insights into the unique processes occurring in a plasma environment and their potential in anion-exchange reactions.
The outcomes of this thesis shed light on the growth and phase transformations in copper oxide nanowires, offering valuable insights into nanoscale processes that are dependent on molecules or radicals interacting with surfaces. The understanding of the nanowire growth mechanism will aid in the development and application of copper oxide nanostructures. Additionally, the described phenomena and mechanisms of anion-exchange processes open up new possibilities for synthesizing materials through phase-transformation reactions, while also contributing to fundamental scientific knowledge.