Nanocrystal Surface Engineering and Catalysis in Plasmonically Coupled Nanoreactors

Abstract

Advanced catalysts capable of harnessing light energy for chemical transformations are crucial for reducing energy demand and promoting sustainable development. Plasmonic nanostructures, with their unique optoelectronic properties, have been extensively researched in recent years. However, their full potential remains largely untapped, particularly in single-component homogeneous structures. Integrating new components to form plasmonic hybrid nanostructures shows promise for revealing unforeseen enhancements or novel properties. These hybrids, combining different metals or organic components, exhibit synergistic and novel plasmonic characteristics. Such properties emerge from the integration of distinct attributes of each component, including plasmonic behavior, dielectric constants, or through the formation of heterointerfaces. However, the intricate surface and interface structures pose challenges in controlling the efficiency or selectivity of catalytic processes due to poor interfacial synergy and the highly complex molecular dynamics on metal active sites. Well-designed nanostructures serve as model catalytic platforms, enabling precise engineering of heterointerfaces, designing metal active sites, and facilitating energy transfer, thus addressing these challenges effectively. This thesis comprises the development of various plasmonic interfaces and modification using plasmonically coupled nanoreactors as model nanocatalyst platforms. Typically, metal nanocrystals (NCs) inherit specific sets of ligands and surfactants directly from the synthetic protocol. As catalysis is a surface-dependent phenomenon, these organic modifiers significantly influence catalytic surfaces and, in turn, unpredictably affect reaction kinetics and molecular dynamics. Therefore, we delve into the background of our research and outline our strategy to synthesize isolated ligand-free plasmonic metal (Au or Ag) NCs inside hollow silica (h-SiO2) shells, referred to as nanoreactors. We precisely engineer several heterointerfaces, such as plasmonic-catalytic and plasmonic-organic interfaces, for on-surface photochemical applications. In Chapter-1, we introduces "confine and shine" approach to uniformly coat the varied surface shapes of plasmonic nanocrystals (NCs), such as Au-rods, spheres, cubes, trigonal bipyramids, and dodecahedrons, with ultrathin atomically conformal layers of various catalytic noble metals, including Pt, Pd, Ru, and Rh using a light- induced self-limited epitaxial growth process. Traditional thick nonplasmonic noble metal shells over plasmonic core nanoparticles often shield and distort Localized Surface Plasmon Resonance (LSPR). Thus, by precisely modifying AuNCs surfaces with ultrathin layers of catalytic noble metals via a method termed "metal- lamination," we achieve enhanced atomic utilization efficiency and synergy between plasmonic and catalytic properties. These metal-laminated Au nanorods (NRs) demonstrate higher hot charge carrier generation and longer lifetime constants, facilitating efficient channelling of plasmonic energy to catalytic site for various organic reactions. These reactions include [Pt]-catalyzed deprotection reaction of N- pentynoyl-protected amines and reduction of nitroarenes, as well as the [Pd]- catalyzed Suzuki–Miyaura C–C cross-coupling, and Heck reaction. These hollow silica-encapsulated NRs offer promise for intracellular reactions. In Chapter-2, we explore plasmonic metal-organic interfaces to control molecular adsorption/desorption on catalyst surfaces, enhancing catalytic activity and selectivity while breaking linear scaling relationships utilizing the plasmonically integrated nanoreactor platform. Conventional flexible ligands and rigid thick metal- organic/inorganic shells often uncontrollably occupy the catalytic surface and complicate the catalytic mechanisms. Therefore, a skin-like porous covalent organic overlayer (pCOL) was selectively synthesized on NCs surfaces, preventing excessive off-surface growth. This ultrathin pCOL jacket optimizes steric and electronic microenvironments over catalytic sites, enhancing reaction selectivity without compromising rates. In a proof-of-concept application, this approach effectively mitigated over-reduction side reactions in semi-hydrogenation of alkynes. The highly ordered molecular units of pCOL with surface metal atoms ensure consistent high reaction rates and selectivity even after multiple recycling cycles. Mechanistic insights from in situ surface-enhanced Raman scattering (SERS) and density functional theory (DFT) calculations elucidate the favorable electronic coupling and steric role of the pCOL microenvironment. This work advances the effective control and harnessing of metal-organic interfaces in sustainable catalytic synthesis beyond existing literature on metal-metal/metal oxide hetero-interfaces. In Chapter-3, our research introduces a technique for finely tuning the surface of inherently chiral Au nanoparticles by applying an ultra-thin layer of catalytic metals in a precise manner. Expanding upon our prior work with hollow silica-based nanoreactors, we tackled the significant challenge of maintaining the nanoparticles' chiral properties and active sites while preventing uncontrolled growth of the catalytic metals. Through a process involving laser irradiation, we facilitated the growth of catalytic metals while ensuring a thin, atomic-layer that did not disturb the atomic chiral sites on the surface. By encapsulating the chiral Au nanoparticles within these nanoreactors, we safeguarded their sensitive intrinsic chiral surface in ligand-free environment. This method allowed for the asymmetric hydrogenation of methyl 2- phenylacrylate with exceptional enantiomeric purity. We are currently investigating the impact of the chiral microenvironment and the specific interactions between the chiral metal active sites and the organic substrate, aiming to understand how geometric structure and chiral surfaces influence this catalytic process. Further research is underway to elucidate the exact mechanisms behind chiral induction.