Phase engineering of multimetallic nanostructures for electrocatalytic applications

• Main Author: Cui Xiaoya
• Other Authors: Zhang Qichun
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Materials::Nanostructured materials; Engineering::Materials::Energy materials
• Online Access: https://hdl.handle.net/10356/146078
• Institution: Nanyang Technological University

Noble metallic nanomaterials have abundant potential applications in the field of energy conversion. Synthesizing novel crystal-structured nanomaterials such as multicomponent nanostructures provides a facile way to reach novel functionalities owing to the modification of atomic arrangements, electronic structures, and exceptional functionalities originating from their synergistic components, which is also a promising strategy for novel nanocatalyst development. To date, various effective approaches have been fostered to discover the novel nanocatalysts with fine-tuned size, morphology, structure and composition, which can indeed optimize the electrocatalytic performance. Nonetheless, it is indeed fully challengeable to prepare multimetallic hybrid nanostructures with desired chemical composition, surface structure, and crystal phase for specific applications, which are critical in fundamental studies, as well as practical applications. In this thesis, I will present my research works on structure engineering (i.e., phase engineering and defects engineering) of multimetallic nanostructures via wet chemical synthesis and electron beam irradiation under high-resolution TEM for electrocatalytic applications. Structure engineering of noble metal nanomaterials are effective strategy to obtain novel nanostructures with desired morphology, composition, defects, and crystal phase. A series of Au nanostructures with unconventional phases such as fcc, hcp (i.e., 2H), 4H, 4H/fcc, and 2H/fcc heterophase have been synthesized via wet chemical synthesis. In the first project, the effects on the synthesis of multimetallic nanostructures of various Au templates such as fcc Au nanowires, 4H/fcc Au nanorods and 4H Au nanoribbons are investigated systematically. In addition, the crystal structure effects on the as-synthesized structures and properties are studied. In this project, the noble metallic hybrid heterostructures with various morphologies and crystalline structures were well characterized. Moreover, the crystal phase selective growth and etching strategies will be further studied. As known, defects such as twins, stacking faults, atomic steps and lattice strain in metallic nanostructures can modulate their atomic arrangement in atomic scale, electronic structures and surface activities, and thus improve their electrocatalytic performance. However, defect engineering in multimetallic nanostructures with desired composition, morphology and surface structure, which can further optimize their stability and activity, still remains a great challenge. Therefore, the rational design and synthesis of multimetallic nanostructures with rich defects are highly desirable, which is important not only in fundamental studies but also in practical applications. Phase transformation of the aforementioned Au based nanostructures with unconventional phases can generate various novel nanostructures with new phase, which can be induced by ligand exchange, metal coating, high pressure and high temperature. Nevertheless, direct observation of phase transformation pathway is still of great challenge. In the second project, the size-dependent phase transformation between various Au based nanostructures with novel phases are investigated under electron beam irradiation via in-situ TEM observation. Particularly, the phase transformation is dependent on the ratio of the size of the monocrystalline nanoparticle (NP) to the diameter of 4H nanodomains in 4H/fcc Au nanorod (4H AuND). Furthermore, molecular dynamics simulation and theoretical modeling are used to explain the experimental results, giving a size-dependent phase transformation diagram which provides a general guidance to predict the phase transformation pathway between fcc and 4H Au nanomaterials.