Bimetallic nanocrystals for applications in plasmonics and catalysis

Abstract

Bimetallic nanocrystals have received growing interest owing to their diversified properties and promising applications. Seeded growth serves as a powerful route to bimetallic nanocrystals but its capability is limited by galvanic replacement reaction when the seed is more reactive than the deposited metal. This dissertation documents two strategies for achieving galvanic replacement-free seeded growth of a second metal (M: Au, Pd, or Pt) on Ag nanocrystals. The first strategy relies on the co-titration of AgNO3 and a precursor to M for the generation of Ag and M atoms via co-reduction, followed by the co-deposition on Ag nanocubes for the generation of Ag@Ag-M core-frame nanocubes. The resultant Ag@Ag-Au core-frame nanocubes exhibit enhanced surface-enhanced Raman scattering (SERS) activity at an excitation wavelength of 785 nm. On the other hand, the Ag@Ag-Pd nanocubes can play as a dual catalyst for probing stepwise catalytic reduction and oxidation reactions by SERS. The second strategy involves the use of hydroxide to transform Ag nanocubes into Ag@Au and Ag@Pt core-shell nanocubes under an alkaline condition. Upon the removal of Ag cores, the Ag@Au core-shell nanocubes can be converted into Au nanoboxes with wall thickness less than 2 nm and well-defined openings at corners. These nanoboxes embrace strong absorption in the near-infrared region for potential biomedical applications. In comparison, the Ag@Pt nanocubes can be transformed into Pt-based nanocages. These nanocages exhibit improved catalytic activity toward the oxygen reduction reaction (ORR). Collectively, this work greatly expands the utility of seeded growth for the rational design and synthesis of bimetallic nanocrystals with well-defined structures and desired properties.