In Situ TEM Studies of Multicomponent Alloy Nanoparticles

This thesis is focused on the study of novel multicomponent alloy nanoparticles through advanced transmission electron microscopy (TEM) with the utilization of both ex-situ and in-situ TEM techniques. Unlike most of the traditional multicomponent nanoalloy research studies, the alloy nanoparticles investigated here are largely composed of five or more principal elements with single phase solid solution structures (high entropy alloys (HEAs)). In this work, a unique carbothermal shock (CTS) method was developed to synthesize HEA nanoparticles. The successful synthesis of multicomponent and HEA nanoparticles were verified through scanning transmission electron microscope (STEM) Z-contrast imaging and X-ray energy dispersive spectroscopy (EDS) analysis down to atomic level. Such atomic-level mixing of multiple elements was ascribed to the ultrafast heating/cooling kinetics that occurred during CTS process. The novel HEA nanoparticles were studied as catalysts and demonstrated in NH3 oxidation and NH3 decomposition applications. In both cases, the successfully synthesized HEA nanoparticles exhibited excellent performance. In addition, by tuning the elements’ content in the HEA system, the performance can be largely improved as well. The CTS process was later investigated through in situ electrical biasing TEM holder to understand the mechanisms underlying nanoparticles’ formation and stabilization on carbon substrate. It was found that CTS generated extremely high temperatures leading to CNF crystallization and salt decomposition, followed by the anchoring of metal nanoparticles onto defective graphite edge planes to form ultra-thermally stable structures. Since the combinations of five or more elements are tremendously large, computational-aided screening was carried out to guide the synthesis of complex multicomponent alloys. The synthesized nanoparticles followed the element selection guidelines and were examined through STEM, STEM-EDS for the crystal structure and mixing status information. Since the multicomponent nanoparticles always need to be operated at extreme environments, in situ heating TEM studies were carried out to give profound understanding of high temperature behaviors of HEA nanoparticles as well. This work is dedicated on the TEM studies of HEA nanoparticles and is expected to pave the roadmap for further HEA studies.