The abnormal photophysical properties of metal nanoparticles and their applicability as a single nanoprobe to observe a variety of heterogeneous catalytic reactions have actively reported. These studies mainly use scattered light enhanced by localized surface plasmon resonance (LSPR) in the visible region. Since plasmon characteristics are very sensitive to the electronic structure, shape, and surrounding environment of the particles, it is also possible to modulate them and expand their applicabilities. Especially, Au, Ag, and Cu have a high scattering efficiency due to the behavior of free electron density near the Fermi level, and the combination of these materials enables to form new plasmonic materials with diverse physicochemical properties. In this study, I have investigated the chemical and electrochemical synthesis of bimetallic nanoparticles, and their formation mechanism based on their plasmonic properties.
In chapter 2, intermetallic Au1-xCux nanoparticles with a specific atomic arrangement were synthesized according to the composition, and their plasmonic natures were elucidated using single-particle spectroscopy and time-dependent density functional theory. In this experiment, a hypsochromic shift by increasing a Cu composition was explained by considering a bound electron effect on energy dissipation, as well as free-electron effect on the external electric field. Furthermore, we emphasized the importance of interpreting electronic structures for plasmonic characteristics.
In chapter 3, bimetallic nanoparticles were manipulated by electrochemical methods, and their formation mechanism was monitored using single-particle plasmon voltammetry (spPV). We fabricated the spectroelectrochemical cell for real-time and single-particle monitoring of electrochemical reactions and applied to the formation of Cu-on-Ag bimetallic nanoparticles. Using linear sweep voltammetry (LSV), we induced the preferential Cu deposition on individual Ag nanocubes with distinct morphologies, such as atop tetrapod, dendritic sphere, and multiple cube clusters. Besides, single-particle plasmon tracking revealed that selective bulk deposition and abnormal underpotential deposition of Cu determined their mechanistic features.
In chapter 4, we investigated the structural complexity induced by cysteine blocking in electrochemical Cu deposition on Ag nanocubes. In these experiments, differences in the relative growth rate among crystallographic planes by cysteine adsorption gave rise to morphological diversity, including face-overgrown, four-leaf clover, and octapod-like. A combination of electron microscopic images and spPV provided insights on the structural changes and their mechanism. The synthesis and plasmonic characteristics of bimetallic nanoparticles, and precise engineering of composition and morphology make it possible to design for desired photophysical and catalytic applications towards perfect control of heterogeneous reactions.