Noble-metal nanocrystals with metastable phases and their applications
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Nguyen, Quynh Nhu
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Abstract
Noble-metal nanocrystals serve as an innovative platform for the rational development of next-generation catalysts, owing to their tunable physical attributes, including size, shape, composition, and defects. The ability to modify atomic configurations as materials transition to nanoscale dimensions has pivoted the focus of recent research towards manipulating the atomic packing within the internal structure, particularly the crystal structure or phase, to enhance existing properties and uncover new phenomena. Such a precise control over the polymorphism of nanocrystals opens a new avenue to boost their catalytic performance by modifying the interactions between surface atoms and reaction intermediates. This dissertation presents a series of systematic studies that delve into the thermodynamic and kinetic factors responsible for the seed-mediated growth of noble-metal nanocrystals with unconventional, metastable crystal phases, together with an evaluation of their potential applications in catalysis. With an initial focus on Ru, I first elucidated the roles of shape and size of the preformed seeds in controlling the crystal phase taken by the deposited Ru overlayers, using fcc-Pd cubic and octahedral nanocrystals with different edge lengths as seeds. The Ru shell consistently adopted a metastable face-centered cubic (fcc) phase regardless of the size of the cubic seeds, while it retained the fcc phase on small octahedral seeds before reverting to the native hexagonal close-packed (hcp) phase on larger ones. This phenomenon could be attributed to the differences in surface atomic arrangement of the {100} facets displayed on a cubic seed and the facets of hcp-Ru, forcing the deposited Ru to take the fcc phase. In contrast, the {111} facets on an octahedral seed could be symmetrically aligned with the {0001} facets of hcp-Ru, allowing the deposited Ru to take either an hcp or fcc phase. The crystal phase of the Ru shell was then determined by the interplay between the surface and bulk energies as the area proportions of edges and faces on the octahedral seed changed with size. When tested as catalysts towards ethylene glycol oxidation, the fcc-Ru outperforms hcp-Ru, while the cubic shape is advantageous over the octahedral counterpart. Further exploration into other experimental parameters affecting the outcome of a phase-selective epitaxial growth demonstrated that the reduction kinetics played a more dominant role than the templating effect from the preformed seed in dictating the crystal phase of the deposited overlayers. Specifically, the use of ethylene glycol and triethylene glycol led to the formation of Ru shell in its natural hcp and metastable fcc phases, respectively, regardless of the size and phase of the seed. Quantitative measurements and theoretical calculations suggested that this trend was a manifestation of the different reduction kinetics associated with the precursor and the chosen polyol, which in turn, affected the reduction pathway (solution versus surface) and packing sequence of the deposited Ru atoms. Building on these insights, I extended the seed-mediated growth to obtain hcp-Rh deposition on hcp-Ru seeds. Under careful control of the reaction kinetics via dropwise injection of the precursor, Rh could be deposited on hcp-Ru seeds in either a novel, metastable hcp phase, or its native fcc phase, depending on the growth mode employed to achieve a balance between the bulk and surface energies. The metastable hcp-Rh phase within the Ru@Rh core-shell nanocrystals exhibited stability up to 400 °C and enhanced catalytic activity for ethanol oxidation reaction, highlighting the potential of phase-controlled synthesis for developing efficient catalysts.
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2024-07-26
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Dissertation