Plasmonic modulation of spatially confined reactions on a silver nanoprism
Creators: name: Mahasivam, Sanje
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340303
subject: Nanochemistry
classificationCode: 401605
subject: Functional materials
classificationCode: 401807
subject: Nanomaterials

description: Light-induced nanoparticle photochemistry and photobiology continue to draw significant interest due to their vast potential across industrial catalyses to therapeutics applications. Plasmons are collective electron oscillations that occur when an incident electromagnetic field interacts with free electrons on a metal surface. These plasmons are prominently exhibited by noble metals such as silver (Ag), gold (Au), and copper (Cu) at nanoscale dimensions. The size, shape, and composition of the nanoparticles can radically alter plasmonic characteristics. The localised surface plasmon resonances (LSPRs) resulting from light-matter interactions lead to interesting optoelectronic effects such as optical near-field enhancement close to the nanoparticle surface, generation of hot carriers, and photo-thermal effects.  These phenomena have been of significant research interest, particularly to harness them for chemical and biology applications. 

Anisotropic nanoparticles such as silver nanoprisms (AgPRs) exhibit multiple LSPR modes, like the in-plane and out-of-plane dipolar and quadrupolar modes. These modes can be selectively photoexcited using narrow bandwidth light sources, which leads to localised hot spots within the nanoparticle depending on the mode excited. These plasmonic hotspots can also act as chemical hotspots that can be used to drive and control chemical reactions on nanoparticle surfaces. An LSPR-driven spatially controlled chemical reaction offers unique prospects in the field of single atom and single particle catalysis, as it may allow light to be used as an external trigger to promote chemical reactions at specific locations within a nanoparticle.

The aim of this thesis is to control the chemical reaction sites within a plasmonic nanoparticle through selective optical excitations of plasmon eigenmodes and while doing so, develop unique hybrid materials that have different components precisely positioned at specific sub-sites within a single nanoparticle. The thesis employed AgPRs as the representative anisotropic plasmonic nanoparticle to study the outlined aims. Further, to achieve the core objective, as a proof of concept, the plasmon-induced processes were coupled with a well-established reaction system, the galvanic replacement reaction (GRR) that involves a spontaneous redox reaction between two metallic components. The thesis initially investigated the influence of plasmons on a thermodynamically feasible GRR between AgPRs and tetrachloroaurate ([AuCl4]-) ions. Subsequently, the learnings from this study were implemented to study the influence of plasmonic chemistry in advancing a thermodynamically-unfavourable reaction between AgPRs and tetrachloroplatinate ([PtCl4]2-) ions. Thus, the thesis dived into exploring the possibility of “turning impossible into possible via plasmonics”.

The unique anisotropic feature of an AgPR allow selective excitation of its LSPR modes through optical illumination. The excitation of the in-plane and out-of-plane dipole plasmons selectively produces the optical hot spots at the tips and edges of the AgPR, respectively. Thus, the thesis investigated the possibility of selectively driving the GRR processes at these specific sub-sites in the AgPR where corresponding LSPR modes were excited. The overall outcomes revealed that metallic Au0 and Pt0 can in fact be successfully deposited on the tips and edges of an AgPR through selective excitation of the LSPR modes.

The research also saw a careful deployment of a range of analysis techniques to obtain deep mechanistic insights into the underlying plasmonic chemistry. These include high resolution transmission electron microscopy and x-ray diffraction studies to carefully study the crystal structure and defect states in the AgPR; low-loss electron energy loss spectroscopy to map the plasmons across the AgPR surface as well as to quantify the relative changes in the thickness of the nanoprism across the AgPR; chemical mapping of AgPRs via energy dispersive x-ray mapping to study the elemental distribution profiles; x-ray photoemission spectroscopy to validate the formation of reaction products as well as to study the fate of capping agents during plasmonic catalysis; atomic emission spectroscopy to quantify the degree of GRR; absorption spectroscopy to study the changes in the optical properties of AgPRs during GRR; as well as electrochemical analysis of reactions to understand why theoretically-prohibited GRR between AgPRs and tetrachloroplatinate ions takes place.

These investigations generated enriched knowledge about the underlying mechanisms of chemical reactions that can be influenced by the excitation of the specific LSPR modes of an AgPR. The role of both short-lived non-thermalised and long-lived thermalised ‘hot’ charge carriers in influencing different aspects of the GRR was established. Both photonic energy and excitation of specific LSPR modes were observed to influence the GRR under photoillumination conditions. While photonic energy was found to increase the efficiency of GRR, the plasmonic excitation allowed spatial biasing of the GRR at the tips and edges of the AgPR. While it remained challenging to completely disentangle the effects of photonic and plasmonic chemistry, the investigations suggested that the reaction thermodynamics is controlled by the photonic energy, whereas plasmons control the kinetics of the reaction. The study also revealed that often overlooked reaction parameters, such as the crystal defects, capping agents, and the stability of the reactant molecules can play a non-trivial role during plasmon-enhanced chemical reactions. In particular, citrate molecules bound onto AgPRs played a key role in completely changing the mechanism of GRR under photoillumination. This allowed overcoming the thermodynamic barrier of the reaction between Ag and tetrachloroplatinate ions, and “turning impossible into possible via plasmonics”.

Overall, this thesis is an attempt to advance our understanding of plasmonic chemistry and plasmonic catalysis. The methodologies, tools and techniques developed during the course of this thesis, the key observations made, and the future directions proposed, could benefit those seeking to further advance this important area of research.