Strong light-absorbing properties allow plasmonic metal nanoparticles to serve as antennas for other catalysts to function as photocatalysts. To achieve plasmonic photocatalysis, the hot charge carriers created when light is absorbed must be harnessed before they decay through internal relaxation pathways. We demonstrate the role of photogenerated hot holes in the oxidative dissolution of individual gold nanorods with millisecond time resolution while tuning charge-carrier density and photon energy using snapshot hyperspectral imaging. We show that light-induced hot charge carriers enhance the rate of gold oxidation and subsequent electrodissolution. Importantly, we distinguish how hot holes generated from interband transitions versus hot holes around the Fermi level contribute to photooxidative dissolution. The results provide new insights into hot-hole-driven processes with relevance to photocatalysis while emphasizing the need for statistical descriptions of nonequilibrium processes on innately heterogeneous nanoparticle supports.

Metallic nanoparticles supporting a localized surface plasmon resonance have emerged as promising platforms for nanoscopic labels, sensors, and (photo-) catalysts. To use nanoparticles in these capacities, and to gain mechanistic insight into the reactivity of inherently heterogeneous nanoparticles, single-particle characterization approaches are needed. Single-particle scattering spectroscopy has become an important, highly sensitive tool for localizing single plasmonic nanoparticles and studying their optical properties, local environment, and reactivity. In this review, we discuss approaches taken for collecting the scattered light from single particles, their advantages and disadvantages, and present some recent applications. We introduce techniques for the excitation and detection of single-particle scattering such as high-angle dark-field excitation, total internal reflection dark-field excitation, scanning near-field microscopy, and interferometric scattering. We also describe methods to achieve polarization-resolved excitation and detection. We then discuss different approaches for scanning, ratiometric, snapshot, and interferometric hyperspectral imaging techniques used to extract spectral information. Finally, we provide a brief overview of specialized setups for in situ measurements of nanoparticles in liquid systems and setups coupled to scanning tip microscopes.

Resolving the dynamics of photoexcited d-holes in metallic nanostructures is one step in the rational design of plasmonic photocatalysis. Here, we track the creation and relaxation of charge carriers in catalytically important plasmonic copper nanocubes using single-particle spectroscopy. We show that interband transitions dominate the absorption and emission of 60−160 nm copper nanocubes, and in contrast to gold nanorods, the LSPR is less important to the emission. We uncover the plasmonic enhancement of emission hidden in ensemble measurements by resolving the size-dependent line shape and quantum yield. Quantum yield analysis reveals the energy dependence of d-hole lifetimes. We demonstrate that the effect of excitation energy on quantum yield is stronger in copper nanocubes than in gold nanocubes of equivalent size due to differences in the band structure and the probabilities of hole scattering. Finally, we establish that emission spectroscopy is useful to understand the physical chemistry of d-holes in plasmonic photocatalysts.

Gold−silver alloy nanoparticles are interesting for multiple applications, including heterogeneous catalysis, optical sensing, and antimicrobial properties. The inert element gold acts as a stabilizer for silver to prevent particle corrosion, or conversely, to control the release kinetics of antimicrobial silver ions for long-term efficiency at minimum cytotoxicity. However, little is known about the kinetics of silver ion leaching from bimetallic nanoparticles and how it is correlated with silver content, especially not on a single-particle level. To characterize the kinetics of silver ion release from gold−silver alloy nanoparticles, we employed a combination of electron microscopy and single-particle hyperspectral imaging with an acquisition speed fast enough to capture the irreversible silver ion leaching. Single-particle leaching profiles revealed a reduction in silver ion leaching rate due to the alloying with gold as well as two leaching stages, with a large heterogeneity in rate constants. We modeled the initial leaching stage as a shrinking-particle with a rate constant that exponentially depends on the silver content. The second, slower leaching stage is controlled by the electrochemical oxidation potential of the alloy being steadily increased by the change in relative gold content and diffusion of silver atoms through the lattice. Interestingly, individual nanoparticles with similar sizes and compositions exhibited completely different silver ion leaching yields. Most nanoparticles released silver completely, but 25% of them appeared to arrest leaching. Additionally, nanoparticles became slightly porous. Alloy nanoparticles, produced by scalable laser ablation in liquid, together with kinetic studies of silver ion leaching, provide an approach to design the durability or bioactivity of alloy nanoparticles.

Reactive hot spots on plasmonic nanoparticles have attracted attention for photocatalysis as they allow for efficient catalyst design. While sharp tips have been identified as optimal features for field enhancement and hot electron generation, the locations of catalytically promising d-band holes are less clear. Here we exploit d-band hole-enhanced dissolution of gold nanorods as a model reaction to locate reactive hot spots produced from direct interband transitions, while the role of the plasmon is to follow the reaction optically in real time. Using a combination of single-particle electrochemistry and single-particle spectroscopy, we determine that d-band holes increase the rate of gold nanorod electrodissolution at their tips. While nanorods dissolve isotropically in the dark, the same nanoparticles switch to tip-enhanced dissolution upon illimitation with 488 nm light. Electron microscopy confirms that dissolution enhancement is exclusively at the tips of the nanorods, consistent with previous theoretical work that predicts the location of d-band holes. We, therefore, conclude that d-band holes drive reactions selectively at the nanorod tips.