Synthesis of metal nanomaterials with unconventional crystal phases and their applications in electrocatalysis

• Main Author: Liu, Jiawei
• Other Authors: Alex Yan Qingyu
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Nanotechnology
• Online Access: https://hdl.handle.net/10356/152581
• Institution: Nanyang Technological University

As an intrinsic characteristic of crystalline materials, crystal phase has been viewed as an important parameter to tune the physiochemical properties of metallic nanomaterials for various applications including catalysis. Unconventional crystal phases, which are distinct from their thermodynamically stable bulk phases, can also be stabilized in metallic nanomaterials. This thesis aims to develop facile methods to synthesize mono- and bi-metallic nanomaterials with both conventional and unconventional crystal phases, investigate their structural features, and explore the phase-dependent electrocatalytic applications. First, phase-controlled synthesis of two-dimensional (2D) Au nanostructures, including the square-like 2H/face-centered cubic (fcc) Au nanosheets, square-like 2H Au nanosheets and triangular fcc Au nanoplates, is achieved via facile wet-chemical methods. Then, the unique structural features of the 2D 2H/fcc Au nanosheets are studied in detail. They are used as templates to systematically investigate how different phase domains in different regions of the 2H/fcc Au nanosheets affect the nucleation and growth of one-dimensional (1D) Rh nanorods. Capitalizing on different surface energies among different structural domains (i.e., 2H phase, fcc phase, fcc-defects) and those among different regions (i.e., 2H/fcc edges, fcc edges, 2H/fcc basal planes) of the 2H/fcc Au nanosheets, by precisely tuning the reaction conditions, selective epaxial growth of Rh nanorods is achieved, resulting in the formation of a series of 1D/2D Rh-Au heterostructures, in which the Rh nanorods show different levels of site-selectivity and growth density. As a comparison, the growth of Pt nanorods as the secondary metal and the usage of 2H Au nanosheets and fcc Au nanoplates as templates have also been studied. Besides, taking advantage of the 1D/2D hierarchical structure, the 1D/2D Rh-Au heterostructure shows promising electrocatalytic performance toward the hydrogen evolution reaction (HER) in acidic media. On the other hand, phase-controlled synthesis of bimetallic Pd3Sn nanorods, including the ordered L12 phase and the disordered fcc phase, is achieved via wet-chemical methods. Experimentally, the usage of different types of tin precursors and solvents is decisive for controlling their crystal phases. This method can be further extended to synthesize ternary PdCuSn nanorods with the unconventional L12 phase. These nanorods, with similar size and morphology, but different crystal phases, provide an effective platform to explore the crystal phase-dependent catalytic applications. When they are employed as electrocatalysts toward the electrochemical ethanol oxidation reaction (EOR) in alkaline media, both the mass activity and stability follow the trend that the L12 PdCuSn > L12 Pd3Sn > fcc Pd3Sn > Pd/C, demonstrating the significance of alloying effect and crystal phase in determining their catalytic performance.