Mechanism for plasmon-generated solvated electrons

Plasma-Generated Free Electrons Induced Perfluorooctanoic Acid Efficient Degradation at the Gas-Liquid Interface

Low-temperature plasma, generating both reductive electrons and diverse oxidative species, has demonstrated considerable potential for the degradation of perfluorooctanoic acid (PFOA). However, limited understanding of electron propagation mechanisms during discharge has led previous research to focus on hydrated electrons (eaq-) while neglecting free electrons (e-). In this study, a consistent and modeled dielectric barrier discharge (DBD) plasma was employed to degrade PFOA. Contribution analysis indicated that reactions driven by e- were dominant, with substantial contributions from hydroxyl radical (•OH)-mediated oxidation. By integrating a kinetic model with a streamer solver, a basic discharge unit model was developed. Simulation of e- streamer propagation identified a high-intensity response electric field formed by the e- memory effect, with a peak strength of 1.816 × 106 V/m. This electric field facilitated a secondary acceleration of e-, allowing e- to penetrate the surface water layer and directly attack PFOA via chain-shortening mechanisms. The delocalized state of e- restricted degradation primarily to the gas-liquid interface, minimizing interference from the surrounding medium. This study highlights the previously overlooked role of e- and provides essential theoretical insights for the plasma-based treatment of PFOA-contaminated water.

Alchemically-glazed plasmonic nanocavities using atomic layer metals: controllably synergizing catalysis and plasmonics

Plasmonic nanocavities offer exceptional confinement of light, making them effective for energy conversion applications. However, limitations with stability, materials, and chemical activity have impeded their practical implementation. Here we integrate ultrathin palladium (Pd) metal films from sub- to few- atomic monolayers inside plasmonic nanocavities using underpotential deposition. Despite the poor plasmonic properties of bulk Pd in the visible region, minimal loss in optical field enhancement is delivered along with Pd chemical enhancement, as confirmed by ab initio calculations. Such synergistic effects significantly enhance photocatalytic activity of the plasmonic nanocavities as well as photostability by suppressing surface atom migration. We show the atomic alchemical-glazing approach is general for a range of catalytic metals that bridge plasmonic and chemical catalysis, yielding broad applications in photocatalysis for optimal chemical transformation.

Advances in plasma-driven solution electrochemistry

Energetic species produced by gas-phase plasmas that impinge on a liquid surface can initiate physicochemical processes at the gas/liquid interface and in the liquid phase. The interaction of these energetic species with the liquid phase can initiate chemical reaction pathways referred to as plasma-driven solution electrochemistry (PDSE). There are several processing opportunities and challenges presented by PDSE. These include the potential use of PDSE to activate chemical pathways that are difficult to activate with other approaches as well as the use of renewable electricity to generate plasmas that could make these liquid-phase chemical conversion processes more sustainable and environmentally friendly. In this review, we focus on PDSE as an approach for controlled and selective chemical conversion including the synthesis of nanoparticles and polymers with desired but currently uncontrollable or unattainable properties as the next step in the use of PDSE. The underpinning redox chemistry and transport processes of PDSE are reviewed as many PDSE-driven processes are transport-limited due to the many short-lived highly reactive species involved.

Photoexcited Hot Electron Catalysis in Plasmon-Resonant Grating Structures with Platinum, Nickel, and Ruthenium Coatings

We report the electrochemical potential dependence of photocatalysis produced by hot electrons in plasmon-resonant grating structures. Here, corrugated metal surfaces with a period of 520 nm are illuminated with 785 nm wavelength laser light swept as a function of incident angle. At incident angles corresponding to plasmon-resonant excitation, we observe sharp peaks in the electrochemical photocurrent and dips in the photoreflectance consistent with the conditions under which there is wavevector matching between the incident light and the spacing between the lines in the grating. In addition to the bare plasmonic metal surface (i.e., Au), which is catalytically inert, we have measured grating structures with a thin layer of Pt, Ru, and Ni catalyst coatings. For the bare Au grating, we observe that the plasmon-resonant photocurrent remains relatively featureless over the applied potential range from -0.8 to +1.2 V vs NHE. For the Pt-coated grating, we observe a sharp peak around -0.3 V vs NHE, three times larger than the bare Au grating, and near complete suppression of the oxidation half-reaction, reflecting the reducing nature of Pt as a good hydrogen evolution reaction catalyst. The photocurrent associated with the Pt-coated grating is less noisy and produces higher photocurrents than the bare Au grating due to the faster kinetics (i.e., charge transfer) associated with the Pt-coated surface. The plasmon-resonant grating structures enable us to compare plasmon-resonant excitation with that of bulk metal interband absorption simply by rotating the polarization of the light while leaving all other parameters of the experiment fixed (i.e., wavelength, potential, electrochemical solution, sample surface, etc.). A 64X plasmon-resonant enhancement (i.e., p-to-s polarized photocurrent ratio) is observed for the Pt-coated grating compared to 28X for the bare grating. The nickel-coated grating shows an increase in the hot-electron photocurrent enhancement in both oxidation and reduction half-reactions. Similarly, Ru-coated gratings show an increase in hot-electron photocurrents in the oxidation half-reaction compared to the bare Au grating. Plasmon-resonant enhancement factors of 36X and 15X are observed in the p-to-s polarized photocurrent ratio for the Ni and Ru gratings, respectively.

Photoemission Enhancement of Plasmonic Hot Electrons by Au Antenna-Sensitizer Complexes

The photoemission of surface plasmon decay-produced hot electrons is usually of very low efficiencies, hindering the practical utilization of such nonequilibrium charge carriers in harvesting photons with less energy than the semiconductor band gap for more efficient solar energy collection and photodetection. However, it has been demonstrated that the photoemission efficiency of small metal clusters increases as the particle size decreases. Recent studies have also shown that the photoemission efficiency of surface plasmon-yielded hot carriers can be intrinsically improved through proper material construction. In this paper, we report that the photoemission efficiency of hot electrons on the Au nanodisk-cluster complex/TiO2 interface can be dramatically enhanced under optical nanoantenna-sensitizer design. Such an enhancement is dominantly attributed to three factors. First, the large plasmonic nanodisk antennas provide a significantly enhanced optical near field, which largely increases light absorption in the small Au clusters that are acting as hot electron injection sensitizers. Second, the sub-3 nm size of the Au clusters facilitates the collection of delocalized spreading charges by the semiconductor. Third, the hybrid interface and molecule-like energy level of the Au cluster result in a much longer lifetime of excited electrons. Our results provide a promising approach for the effective harvesting of solar energy with plasmonic antenna-sensitizer complexes.

A Quasi-Bound States in the Continuum Dielectric Metasurface-Based Antenna-Reactor Photocatalyst

Metasurfaces are a class of two-dimensional artificial resonators, creating new opportunities for strong light-matter interactions. One type of nonradiative optical metasurface that enables substantial light concentration is based on quasi-Bound States in the Continuum (quasi-BIC). Here we report the design and fabrication of a quasi-BIC dielectric metasurface that serves as an optical frequency antenna for photocatalysis. By depositing Ni nanoparticle reactors onto the metasurface, we create an antenna-reactor photocatalyst, where the virtually lossless metasurface funnels light to drive a chemical reaction. This quasi-BIC-Ni antenna-reactor drives H2 dissociation under resonant illumination, showing strong polarization, wavelength, and optical power dependencies. Both E-field-induced electronic and photothermal heating effects drive the reaction, supported by load-dependent reactivity studies and our theoretical model. This study unlocks new opportunities for photocatalysis that employ dielectric metasurfaces for light harvesting in an antenna-reactor format.

Sustainable chemistry with plasmonic photocatalysts

There is a pressing global need to increase the use of renewable energy sources and limit greenhouse gas emissions. Towards this goal, highly efficient and molecularly selective chemical processes that operate under mild conditions are critical. Plasmonic photocatalysis uses optically-resonant metallic nanoparticles and their resulting plasmonic, electronic, and phononic light-matter interactions to drive chemical reactions. The promise of simultaneous high-efficiency and product-selective reactions with plasmon photocatalysis provides a compelling opportunity to rethink how chemistry is achieved. Plasmonic nanoparticles serve as nanoscale 'antennas' that enable strong light-matter interactions, surpassing the light-harvesting capabilities one would expect purely from their size. Complex composite structures, combining engineered light harvesters with more chemically active components, are a focal point of current research endeavors. In this review, we provide an overview of recent advances in plasmonic catalysis. We start with a discussion of the relevant mechanisms in photochemical transformations and explain hot-carrier generation and distributions from several ubiquitous plasmonic antennae. Then we highlight three important types of catalytic processes for sustainable chemistry: ammonia synthesis, hydrogen production and CO2 reduction. To help elucidate the reaction mechanism, both state-of-art electromagnetic calculations and quantum mechanistic calculations are discussed. This review provides insights to better understand the mechanism of plasmonic photocatalysis with a variety of metallic and composite nanostructures toward designing and controlling improved platforms for green chemistry in the future.

Plasma Electrochemistry for Carbon-Carbon Bond Formation via Pinacol Coupling

The formation of carbon-carbon bonds by pinacol coupling of aldehydes and ketones requires a large negative reduction potential, often realized with a stoichiometric reducing reagent. Here, we use solvated electrons generated via a plasma-liquid process. Parametric studies with methyl-4-formylbenzoate reveal that selectivity over the competing reduction to the alcohol requires careful control over mass transport. The generality is demonstrated with benzaldehydes, benzyl ketones, and furfural. A reaction-diffusion model explains the observed kinetics, and ab initio calculations provide insight into the mechanism. This study opens the possibility of a metal-free, electrically-powered, sustainable method for reductive organic reactions.