Synthesis of High-Entropy Alloy Nanoparticles and Compounds Using Germanium-Based Nanostructures

Abstract

High-entropy alloy nanoparticles and compounds exhibit significant potential in various emerging energy storage and conversion fields due to their compositional flexibility for precise tuning of desired functionalities. The research described in this thesis aims to explore germanium-based nanostructures for the synthesis of high-entropy alloy nanoparticles and compounds. In Chapter 1, the principles of high-entropy alloys, nanoparticles, and compounds are introduced. The most common synthetic methods that provide high-entropy alloy nanoparticles are discussed, highlighting the importance of rapid heating and cooling. Additionally, a compilation of high-entropy compounds is presented. The chapter concludes with a detailed discussion of germanium nanostructures, including nanoparticles and nanosheets, and the synthetic methodologies used to prepare them. Chapter 2 outlines a method for introducing a suite of metal nanoparticles (i.e., Au, Ag, Cu, Pd, Pt) onto the surfaces of germanium nanosheets (GeNSs). Characterization confirming the integrity of the GeNS structure and the formation of metal nanoparticles is then discussed. The resulting materials are investigated as photocatalysts for the selective oxidation of benzyl alcohol, demonstrating higher efficiency compared to freestanding metal nanoparticles and GeNSs. The direct linking of metal nanoparticles to GeNSs showcases a synergistic effect, and thin films of the decorated GeNSs offer convenient catalyst recovery and recyclability. Chapter 3 details the deposition of high-entropy alloy nanoparticles onto various germanium surfaces through surface-mediated reduction. Techniques including XRD, FTIR, XPS, and TEM confirm the structure, composition, and morphology of the resulting nanoparticles. Freestanding high-entropy alloy nanoparticles can be liberated from GeNSs via UV light irradiation while maintaining their single-phase solid solution structure. This methodology is extended to different germanium substrates. Chapter 4 introduces a method for preparing high-entropy germanides through rapid thermal annealing, presenting two types of high-entropy germanides (i.e., AuAgCuPdPtGe and FeCoNiCrVGe). XRD, XPS, and TEM analyses confirm the synthesis and composition of the high-entropy germanides. The formation and growth mechanisms are studied using in situ heating experiments, revealing a multi-stage process involving the initial decomposition of germanane, followed by gradual grain growth and rapid grain growth at elevated temperatures. A summary of the experimental chapters is provided in Chapter 5, along with future research directions.