In-situ observation of plasmon-controlled photocatalytic dehydrogenation of individual palladium nanoparticles

Plasmon-Switched Kinetics for Formic Acid Dehydrogenation: Selective Adsorption Driven by Local Field and Hot Carriers

Understanding illumination-mediated kinetics is essential for catalyst design in plasmon catalysis. Here we prepare Pd-based plasmonic catalysts with tunable electronic structures to reveal the underlying illumination-enhanced kinetic mechanisms for formic acid (HCOOH) dehydrogenation. We demonstrate a kinetic switch from a competitive Langmuir-Hinshelwood adsorption mode in dark to a non-competitive type under irradiation triggered by local field and hot carriers. Specifically, the electromagnetic field induces a spatial-temporal separation of dehydrogenation-favorable configurations of reactant molecule HCOOH and HCOO- due to their natural different polarities. Meanwhile, the generated energetic carriers can serve as active sites for selective molecular adsorption. The hot electrons act as adsorption sites for HCOOH, while holes prefer to adsorb HCOO-. Such unique non-competitive adsorption kinetics induced by plasmon effects serves as another typical characteristic of plasmonic catalysis that remarkably differs from thermocatalysis. This work unravels unique adsorption transformations and a kinetic switching driven by plasmon nonthermal effects.

Solar light driven atomic and electronic transformations in a plasmonic Ni@NiO/NiCO3 photocatalyst revealed by ambient pressure X-ray photoelectron spectroscopy

This work employs ambient pressure X-ray photoelectron spectroscopy (APXPS) to delve into the atomic and electronic transformations of a core-shell Ni@NiO/NiCO3 photocatalyst - a model system for visible light active plasmonic photocatalysts used in water splitting for hydrogen production. This catalyst exhibits reversible structural and electronic changes in response to water vapor and solar simulator light. In this study, APXPS spectra were obtained under a 1 millibar water vapor pressure, employing a solar simulator with an AM 1.5 filter to measure spectral data under visible light illumination. The in situ APXPS spectra indicate that the metallic Ni core absorbs the light, exciting plasmons, and creates hot electrons that are subsequently utilized through hot electron injection in the hydrogen evolution reaction (HER) by NiCO3. Additionally, the data show that NiO undergoes reversible oxidation to NiOOH in the presence of water vapor and light. The present work also investigates the contribution of carbonate and its involvement in the photocatalytic reaction mechanism, shedding light on this seldom-explored aspect of photocatalysis. The APXPS results highlight the photochemical reduction of carbonates into -COOH, contributing to the deactivation of the photocatalyst. This work demonstrates the APXPS efficacy in examining photochemical reactions, charge transfer dynamics and intermediates in potential photocatalysts under near realistic conditions.

Enhanced Light-Matter Interaction in Metallic Nanoparticles: A Generic Strategy of Smart Void Filling

The intrinsic properties of materials play a substantial role in light-matter interactions, impacting both bulk metals and nanostructures. While plasmonic nanostructures exhibit strong interactions with photons via plasmon resonances, achieving efficient light absorption/scattering in other transition metals remains a challenge, impeding various applications related to optoelectronics, chemistry, and energy harvesting. Here, we propose a universal strategy to enhance light-matter interaction, through introducing voids onto the surface of metallic nanoparticles. This strategy spans nine metals including those traditionally considered optically inactive. The absorption cross section of void-filled nanoparticles surpasses the value of plasmonic (Ag/Au) counterparts with tunable resonance peaks across a broad spectral range. Notably, this enhancement is achieved under arbitrary polarizations and varied particle sizes and in the presence of geometric disorder, highlighting the universal adaptability. Our strategy holds promise for inspiring emerging devices in photocatalysis, bioimaging, optical sensing, and beyond, particularly when metals other than gold or silver are preferred.

Exploiting hot electrons from a plasmon nanohybrid system for the photoelectroreduction of CO2

Plasmonic materials convert light into hot carriers and heat to mediate catalytic transformation. The participation of hot carriers (photocatalysis) remains a subject of vigorous debate, often argued on the basis that carriers have ultrashort lifetime incompatible with drive photochemical processes. This study utilises plasmon hot electrons directly in the photoelectrocatalytic reduction of CO2 to CO via a Ppasmonic nanohybrid. Through the deliberate construction of a plasmonic nanohybrid system comprising NiO/Au/ReI(phen-NH2)(CO)3Cl (phen-NH2 = 1,10-Phenanthrolin-5-amine) that is unstable above 580 K; it was possible to demonstrate hot electrons are the main culprit in CO2 reduction. The engagement of hot electrons in the catalytic process is derived from many approaches that cover the processes in real-time, from ultrafast charge generation and separation to catalysis occurring on the minute scale. Unbiased in situ FTIR spectroscopy confirmed the stepwise reduction of the catalytic system. This, coupled with the low thermal stability of the ReI(phen-NH2)(CO)3Cl complex, explicitly establishes plasmonic hot carriers as the primary contributors to the process. Therefore, mediating catalytic reactions by plasmon hot carriers is feasible and holds promise for further exploration. Plasmonic nanohybrid systems can leverage plasmon's unique photophysics and capabilities because they expedite the carrier's lifetime.

Electrogenerated chemiluminescence imaging of plasmon-induced electrochemical reactions at single nanocatalysts

This study explores plasmon-induced electrochemical reactions on single nanoparticles using electrogenerated chemiluminescence microscopy (ECLM). Under laser irradiation, real-time screening showed lower plasmon-induced reaction efficiency for bimetallic Au@Pt nanoparticles compared to monometallic Au nanoparticles. ECLM offers a high-throughput imaging and precise quantitative approach for analyzing photo-electrochemical conversion at single nanoparticle level, valuable for both theoretical exploration and optimization of plasmonic nanocatalysts.

Hydrogen evolution with hot electrons on a plasmonic-molecular catalyst hybrid system

Plasmonic systems convert light into electrical charges and heat, mediating catalytic transformations. However, there is ongoing controversy regarding the involvement of hot carriers in the catalytic process. In this study, we demonstrate the direct utilisation of plasmon hot electrons in the hydrogen evolution reaction with visible light. We intentionally assemble a plasmonic nanohybrid system comprising NiO/Au/[Co(1,10-Phenanthrolin-5-amine)2(H2O)2], which is unstable at water thermolysis temperatures. This assembly limits the plasmon thermal contribution while ensuring that hot carriers are the primary contributors to the catalytic process. By combining photoelectrocatalysis with advanced in situ spectroscopies, we can substantiate a reaction mechanism in which plasmon-induced hot electrons play a crucial role. These plasmonic hot electrons are directed into phenanthroline ligands, facilitating the rapid, concerted proton-electron transfer steps essential for hydrogen generation. The catalytic response to light modulation aligns with the distinctive profile of a hot carrier-mediated process, featuring a positive, though non-essential, heat contribution.

Advancements of Intense Terahertz Field Focusing on Metallic Nanoarchitectures for Monitoring Hidden Interatomic Gas-Matter Interactions

With the advancements of nanotechnology, innovative photonic designs coupled with functional materials provide a unique way to acquire, share, and respond effectively to information. It is found that the simple deposition of a 30 nm-thick palladium nanofilm on a terahertz (THz) metasurface chip with a 14 nm-wide effective nanogap of asymmetric materials and geometries allows the tracking of both interatomic and interfacial gas-matter interactions, including gas adsorption, hydrogenation (or dehydrogenation), metal phase changes, and unique water-forming reactions. Combinatorial analyses by simulation and experimental measurements demonstrate the distinct nanostructures, which leads to significant light-matter interactions and corresponding THz absorption in a real-time, highly repeatable, and reliable manner. The complex lattice dynamics and intrinsic properties of metals influenced by hydrogen gas exposure are also thoroughly examined using systematically controlled ternary gas mixture devices that mimic normal temperature and pressure. Furthermore, the novel degrees of freedom are utilized to analyze various physical phenomena, and thus, analytical methods that enable the tracking of unknown hidden stages of water-forming reactions resulting in water growth are introduced. A single exposure of the wave spectrum emphasizes the robustness of the proposed THz nanoscopic probe, bridging the gap between fundamental laboratory research and industry.

In Situ TEM Characterization and Modulation for Phase Engineering of Nanomaterials

Solid-state phase transformation is an intriguing phenomenon in crystalline or noncrystalline solids due to the distinct physical and chemical properties that can be obtained and modified by phase engineering. Compared to bulk solids, nanomaterials exhibit enhanced capability for phase engineering due to their small sizes and high surface-to-volume ratios, facilitating various emerging applications. To establish a comprehensive atomistic understanding of phase engineering, in situ transmission electron microscopy (TEM) techniques have emerged as powerful tools, providing unprecedented atomic-resolution imaging, multiple characterization and stimulation mechanisms, and real-time integrations with various external fields. In this Review, we present a comprehensive overview of recent advances in in situ TEM studies to characterize and modulate nanomaterials for phase transformations under different stimuli, including mechanical, thermal, electrical, environmental, optical, and magnetic factors. We briefly introduce crystalline structures and polymorphism and then summarize phase stability and phase transformation models. The advanced experimental setups of in situ techniques are outlined and the advantages of in situ TEM phase engineering are highlighted, as demonstrated via several representative examples. Besides, the distinctive properties that can be obtained from in situ phase engineering are presented. Finally, current challenges and future research opportunities, along with their potential applications, are suggested.

Photo-enhanced dehydrogenation of formic acid on Pd-based hybrid plasmonic nanostructures

Coupling visible light with Pd-based hybrid plasmonic nanostructures has effectively enhanced formic acid (FA) dehydrogenation at room temperature. Unlike conventional heating to achieve higher product yield, the plasmonic effect supplies a unique surface environment through the local electromagnetic field and hot charge carriers, avoiding unfavorable energy consumption and attenuated selectivity. In this minireview, we summarized the latest advances in plasmon-enhanced FA dehydrogenation, including geometry/size-dependent dehydrogenation activities, and further catalytic enhancement by coupling local surface plasmon resonance (LSPR) with Fermi level engineering or alloying effect. Furthermore, some representative cases were taken to interpret the mechanisms of hot charge carriers and the local electromagnetic field on molecular adsorption/activation. Finally, a summary of current limitations and future directions was outlined from the perspectives of mechanism and materials design for the field of plasmon-enhanced FA decomposition.

New insights into the influence of plasmonic and non-plasmonic nanostructures on the photocatalytic activity of titanium dioxide

The results of this work cover the influence of plasmonic (gold) and non-plasmonic (palladium) nanostructures on the photocatalytic activity and redox properties of titanium dioxide. Materials decorated with gold, palladium and both materials were examined using photoelectrochemical and spectroelectrochemical methods to establish the changes introduced by the modifications and the possibility of the influence of the plasmonic effect from gold on their activity. Additionally, the photocatalytic tests of hydroxyl radical generation and hydrogen evolution were performed to confirm the activity of modified materials in oxidation and reduction reactions. It turned out that in the observed system the catalytic properties of palladium determine mostly the activity of modified materials, and the surface plasmon resonance of gold does not affect the activity. Moreover, the influence of the nanostructures on the activity, besides the catalytic performance, is the same for plasmonic and non-plasmonic ones and results in a change in the redox properties of the semiconductor.

Plasmonic Cavity-Catalysis by Standing Hot Carrier Waves

Manipulating active sites of catalysts is crucial but challenging in catalysis science and engineering. Beyond the design of the composition and structure of catalysts, the confined electromagnetic field in optical cavities has recently become a promising method for catalyzing chemical reactions via strong light-matter interactions. Another form of confined electromagnetic field, the charge density wave in plasmonic cavities, however, still needs to be explored for catalysis. Here, we present an unprecedented catalytic mode based on plasmonic cavities, called plasmonic cavity-catalysis. We achieve direct control of catalytic sites in plasmonic cavities through standing hot carrier waves. Periodic catalytic hotspots are formed because of localized energy and carrier distribution and can be well tuned by cavity geometry, charge density, and excitation angle. We also found that the catalytic activity of the cavity mode increases several orders of magnitude compared with conventional plasmonic catalysis. We ultimately demonstrate that the locally concentrated long-lived hot carriers in the standing wave mode underlie the formation of the catalytic hotspots. Plasmonic cavity-catalysis provides a new approach to manipulate the catalytic sites and rates and may expand the frontier of heterogeneous catalysis.

Promises of Plasmonic Antenna-Reactor Systems in Gas-Phase CO2 Photocatalysis

Sunlight-driven photocatalytic CO2 reduction provides intriguing opportunities for addressing the energy and environmental crises faced by humans. The rational combination of plasmonic antennas and active transition metal-based catalysts, known as "antenna-reactor" (AR) nanostructures, allows the simultaneous optimization of optical and catalytic performances of photocatalysts, and thus holds great promise for CO2 photocatalysis. Such design combines the favorable absorption, radiative, and photochemical properties of the plasmonic components with the great catalytic potentials and conductivities of the reactor components. In this review, recent developments of photocatalysts based on plasmonic AR systems for various gas-phase CO2 reduction reactions with emphasis on the electronic structure of plasmonic and catalytic metals, plasmon-driven catalytic pathways, and the role of AR complex in photocatalytic processes are summarized. Perspectives in terms of challenges and future research in this area are also highlighted.

In Situ Electron Microscopy of Transformations of Copper Nanoparticles under Plasmonic Excitation

Metal nanoparticles are attracting interest for their light-absorption properties, but such materials are known to dynamically evolve under the action of chemical and physical perturbations, resulting in changes in their structure and composition. Using a transmission electron microscope equipped for optical excitation of the specimen, the structural evolution of Cu-based nanoparticles under simultaneous electron beam irradiation and plasmonic excitation was investigated with high spatiotemporal resolution. These nanoparticles initially have a Cu core-Cu₂O oxide shell structure, but over the course of imaging, they undergo hollowing via the nanoscale Kirkendall effect. We captured the nucleation of a void within the core, which then rapidly grows along specific crystallographic directions until the core is hollowed out. Hollowing is triggered by electron-beam irradiation; plasmonic excitation enhances the kinetics of the transformation likely by the effect of photothermal heating.

Nano-biosensor for SARS-CoV-2/COVID-19 detection: methods, mechanism and interface design

The epidemic of coronavirus disease 2019 (COVID-19) was a huge disaster to human society. The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which led to COVID-19, has resulted in a large number of deaths. Even though the reverse transcription-polymerase chain reaction (RT-PCR) is the most efficient method for the detection of SARS-CoV-2, the disadvantages (such as long detection time, professional operators, expensive instruments, and laboratory equipment) limit its application. In this review, the different kinds of nano-biosensors based on surface-enhanced Raman scattering (SERS), surface plasmon resonance (SPR), field-effect transistor (FET), fluorescence methods, and electrochemical methods are summarized, starting with a concise description of their sensing mechanism. The different bioprobes (such as ACE2, S protein-antibody, IgG antibody, IgM antibody, and SARS-CoV-2 DNA probes) with different bio-principles are introduced. The key structural components of the biosensors are briefly introduced to give readers an understanding of the principles behind the testing methods. In particular, SARS-CoV-2-related RNA mutation detection and its challenges are also briefly described. We hope that this review will encourage readers with different research backgrounds to design SARS-CoV-2 nano-biosensors with high selectivity and sensitivity.

Time-resolved transmission electron microscopy for nanoscale chemical dynamics

The ability of transmission electron microscopy (TEM) to image a structure ranging from millimetres to Ångströms has made it an indispensable component of the toolkit of modern chemists. TEM has enabled unprecedented understanding of the atomic structures of materials and how structure relates to properties and functions. Recent developments in TEM have advanced the technique beyond static material characterization to probing structural evolution on the nanoscale in real time. Accompanying advances in data collection have pushed the temporal resolution into the microsecond regime with the use of direct-electron detectors and down to the femtosecond regime with pump-probe microscopy. Consequently, studies have deftly applied TEM for understanding nanoscale dynamics, often in operando. In this Review, time-resolved in situ TEM techniques and their applications for probing chemical and physical processes are discussed, along with emerging directions in the TEM field.

Light-Induced In Situ Transmission Electron Microscopy─Development, Challenges, and Perspectives

Transmission electron microscopy is a basic technique used for examining matter at the highest magnification scale available. One of its most challenging branches is in situ microscopy, in which dynamic processes are observed in real time. Among the various stimuli, like strain, temperature, and magnetic or electric fields, the light-matter interaction is rarely observed. However, in recent years, a significant increase in the interest in this technique has been observed. Therefore, I present a summary and critical review of all the in situ experiments performed with light, various technical possibilities for bringing radiation inside the transmission electron microscope, and the most important differences between the effects of light and electrons on the studied matter. Finally, I summarize the most promising directions for further research using light excitation.

Combination of Plasmon-Mediated Photochemistry and Seed-Mediated Methods for Synthesis of Bicomponent Nanocrystals

Plasmon resonances of metal nanocrystals resulted from free electrons oscillating around nanocrystals, leading to a strong electromagnetic field around them. Because these oscillating electrons possess higher energy than the original ones, also known as hot electrons, these were widely used as photocatalysts for various reactions. Also, the strength and distribution of the electromagnetic field around the nanocrystals strongly depended on their morphology and excited irradiation, which led to the reaction environment around nanocrystals being controllable. Here, we integrated the seed-mediated and plasmon-mediated photochemistry methods for fabricating bimetallic and semiconductor-metal nanocrystals with controllable morphologies and compositions of the nanocrystals, resulting from the highly anisotropic reaction environment around the nanocrystals. The highly anisotropic reaction environment around the template nanocrystal was caused by the distribution of electromagnetic fields around it and its exposure area in the reaction solution. This new synthesis method should enable the fabrication of various multicomponent nanocrystals with desirable functions for potential applications, such as photocatalysts, chemical sensors, biosensors, biomedicines, etc.

Plasmonically enhanced electrochemistry boosted by nonaqueous solvent

Plasmon excitation of metal electrodes is known to enhance important energy related electrochemical transformations in aqueous media. However, the low solubility of nonpolar gases and molecular reagents involved in many energy conversion reactions limits the number of products formed per unit time in aqueous media. In this Communication, we use linear sweep voltammetry to measure how electrochemical H2O reduction in a nonaqueous solvent, acetonitrile, is enhanced by excitation of a plasmonic electrode. Plasmonically excited electrochemically roughened Au electrodes are found to produce photopotentials as large as 175 mV, which can be harnessed to lower the applied electrical bias required to drive the formation of H2. As the solvent polarity increases, by an increase in the concentration of H2O, the measured photopotential rapidly drops off to ∼50 mV. We propose a mechanism by which an increase in the H2O concentration increasingly stabilizes the photocharged plasmonic electrode, lowering the photopotential available to assist in the electrochemical reaction. Our study demonstrates that solvent polarity is an essential experimental parameter to optimize plasmonic enhancement in electrochemistry.

Correlating structure, morphology and properties of metal nanostructures by combining single-particle optical spectroscopy and electron microscopy

The nanoscale morphology of metal nanostructures directly defines their optical, catalytic and electronic properties and even small morphological changes can cause significant property variations. On the one hand, this dependence allows for precisely tuning and exploring properties by shape engineering; next to advanced synthesis protocols, post-synthesis modification through tailored laser modification has become an emerging tool to do so. On the other hand, with this interconnection also comes the quest for detailed structure-property correlation and understanding of laser-induced reshaping processes on the individual nanostructure level beyond ensemble averages. With the development of single-particle (ultrafast) optical spectroscopy techniques and advanced electron microscopy such understanding can in principle be gained at the femtosecond temporal and atomic spatial scale, respectively. However, accessing both on the same individual nanostructure is far from straightforward as it requires the combination of optical spectroscopy and electron microscopy. In this Minireview, we highlight key studies from recent years that performed such correlative measurements on the same individual metal nanostructure either in a consecutive ex situ manner or in situ inside the electron microscope. We demonstrate that such a detailed correlation is critical for revealing the full picture of the structure-property relationship and the physics behind light-induced nanostructure modifications. We put emphasis on the advantages and disadvantages of each methodology as well as on the unique information that one can gain only by correlative studies performed on the same individual nanostructure and end with an outlook on possible further development of this field in the near future.

Thermo-photo catalysis: a whole greater than the sum of its parts

Thermo-photo catalysis, which is the catalysis with the participation of both thermal and photo energies, not only reduces the large energy consumption of thermal catalysis but also addresses the low efficiency of photocatalysis. As a whole greater than the sum of its parts, thermo-photo catalysis has been proven as an effective and promising technology to drive chemical reactions. In this review, we first clarify the definition (beyond photo-thermal catalysis and plasmonic catalysis), classification, and principles of thermo-photo catalysis and then reveal its superiority over individual thermal catalysis and photocatalysis. After elucidating the design principles and strategies toward highly efficient thermo-photo catalytic systems, an ample discussion on the synergetic effects of thermal and photo energies is provided from two perspectives, namely, the promotion of photocatalysis by thermal energy and the promotion of thermal catalysis by photo energy. Subsequently, state-of-the-art techniques applied to explore thermo-photo catalytic mechanisms are reviewed, followed by a summary on the broad applications of thermo-photo catalysis and its energy management toward industrialization. In the end, current challenges and potential research directions related to thermo-photo catalysis are outlined.

Ultrafast Plasmonics Beyond the Perturbative Regime: Breaking the Electronic-Optical Dynamics Correspondence

The transient optical response of plasmonic nanostructures has recently been the focus of extensive research. Accurate prediction of the ultrafast dynamics following excitation of hot electrons by ultrashort laser pulses is of major relevance in a variety of contexts from the study of light harvesting and photocatalytic processes to nonlinear nanophotonics and the all-optical modulation of light. So far, all studies have assumed the correspondence between the temporal evolution of the dynamic optical signal, retrieved by transient absorption spectroscopy, and that of the photoexcited hot electrons, described in terms of their temperature. Here, we show both theoretically and experimentally that this correspondence does not hold under a nonperturbative excitation regime. Our results indicate that the main mechanism responsible for the breaking of the correspondence between electronic and optical dynamics is universal in plasmonics, being dominated by the nonlinear smearing of the Fermi-Dirac occupation probability at high hot-electron temperatures.

Exploiting Plasmonic Hot Spots in Au-Based Nanostructures for Sensing and Photocatalysis

ConspectusLocalized surface plasmon resonance is a unique property appearing in certain metal nanostructures, which can generate hot carriers (electrons and holes) and bring about an intense electromagnetic field localized near the surface of nanostructures. Specific locations, such as the rough surfaces and gaps in nanostructures, where a strong electromagnetic field is formed are referred to as hot spots. Hot-spot-containing plasmonic nanostructures have shown great promise in molecular sensing and plasmon-induced catalytic applications by exploiting the unique optical properties of hot spots. In this Account, we will review our recent developments in the synthesis of Au nanostructures consisting of multiple hot spots and Au-based heteronanostructures combining secondary active metals or semiconductors with Au nanostructures as promising plasmonic platforms for hot-spot-induced sensing and photocatalysis. We first provide a brief introduction to Au nanocrystals and Au nanoparticle assemblies with multiple hot spots. High-index-faceted hexoctahedral Au nanocrystals having multiple high-curvature vertices and edges are beneficial for the generation of an intense and reproducible electromagnetic field, which can enhance the performance of surface-enhanced Raman scattering-based molecular sensing. In addition, the engineering of interparticle gaps in Au nanoparticle assemblies to have a controlled size and a certain number of gaps can lead to the enhancement of plasmonic properties due to the significant amplification of the electromagnetic field at interparticle gaps. We then discuss hot-spot-containing Au-based heteronanostructures prepared by growing secondary components on the aforementioned Au nanostructures. With a combination of merit from strong plasmon energy formed by hot spots and catalytically active secondary materials, Au-based heteronanostructures have emerged as an attractive and versatile catalyst platform for various photocatalytic reactions. Through the control of key factors governing the photocatalysis of Au-based heteronanostructures, such as the coupling manner, shell thickness of secondary materials, and intimacy of contact, the plasmon energy formation of heteronanostructures and its transfer to catalytically active materials can be optimized, leading to the promotion of photocatalysis, such as photocatalytic hydrogen evolution. The rational design of Au nanostructures and Au-based heteronanostructures with multiple hot spots not only could realize enhanced sensing and photocatalysis but also could enable the understanding of the geometry-performance relationship. It is envisioned that the developed strategies can offer new opportunities for the design of various high-efficiency catalytic platforms.

Insight into ultrasensitive and high-stability flocculation-enhanced Raman spectroscopy for the in situ noninvasive probing of cupping effect substances

The combination of in situ TEM and theoretical analysis is applied to further analyze the behavior of nanoparticles in flocculation; particularly, for the in situ noninvasive probing of effect substances after cupping.

Optical Pulling Forces Enabled by Hyperbolic Metamaterials

We propose a novel approach to generating optical pulling forces on a gold nanowire, which are placed inside or above a hyperbolic metamaterial and subjected to plane wave illumination. Two mechanisms are found to induce the optical pulling force, including the concave isofrequency contour of the hyperbolic metamaterial and the excitation of directional surface plasmon polaritons. We systematically study the optical forces under various conditions, including the wavelength, the angle of incidence of light, and the nanowire radius. It is shown that the optical pulling force enabled by hyperbolic metamaterials is broadband and insensitive to the angle of incidence. The mechanisms and results reported here open a new avenue to manipulating nanoscale objects.

Energy-resolved plasmonic chemistry in individual nanoreactors

Plasmonic resonances can concentrate light into exceptionally small volumes, which approach the molecular scale. The extreme light confinement provides an advantageous pathway to probe molecules at the surface of plasmonic nanostructures with highly sensitive spectroscopies, such as surface-enhanced Raman scattering. Unavoidable energy losses associated with metals, which are usually seen as a nuisance, carry invaluable information on energy transfer to the adsorbed molecules through the resonance linewidth. We measured a thousand single nanocavities with sharp gap plasmon resonances spanning the red to near-infrared spectral range and used changes in their linewidth, peak energy and surface-enhanced Raman scattering spectra to monitor energy transfer and plasmon-driven chemical reactions at their surface. Using methylene blue as a model system, we measured shifts in the absorption spectrum of molecules following surface adsorption and revealed a rich plasmon-driven reactivity landscape that consists of distinct reaction pathways that occur in separate resonance energy windows.

Plasmonic Coupling Architectures for Enhanced Photocatalysis

Plasmonic photocatalysis is a promising approach for solar energy transformation. Comparing with isolated metal nanoparticles, the plasmonic coupling architectures can provide further strengthened local electromagnetic field and boosted light-harvesting capability through optimal control over the composition, spacing, and orientation of individual nanocomponents. As such, when integrated with semiconductor photocatalysts, the coupled metal nanostructures can dramatically promote exciton generation and separation through plasmonic-coupling-driven charge/energy transfer toward superior photocatalytic efficiencies. Herein, the principles of the plasmonic coupling effect are presented and recent progress on the construction of plasmonic coupling architectures and their integration with semiconductors for enhanced photocatalytic reactions is summarized. In addition, the remaining challenges as to the rational design and utilization of plasmon coupling structures are elaborated, and some prospects to inspire new opportunities on the future development of plasmonic coupling structures for efficient and sustainable light-driven reactions are raised.

Advancing Plasmon-Induced Selectivity in Chemical Transformations with Optically Coupled Transmission Electron Microscopy

Nanoparticle photocatalysts are essential to processes ranging from chemical production and water purification to air filtration and surgical instrument sterilization. Photochemical reactions are generally mediated by the illumination of metallic and/or semiconducting nanomaterials, which provide the necessary optical absorption, electronic band structure, and surface faceting to drive molecular reactions. However, with reaction efficiency and selectivity dictated by atomic and molecular interactions, imaging and controlling photochemistry at the atomic scale are necessary to both understand reaction mechanisms and to improve nanomaterials for next-generation catalysts. Here, we describe how advances in plasmonics, combined with advances in electron microscopy, particularly optically coupled transmission electron microscopy (OTEM), can be used to image and control light-induced chemical transformations at the nanoscale. We focus on our group's research investigating the interaction between hydrogen gas and Pd nanoparticles, which presents an important model system for understanding both hydrogenation catalysis and hydrogen storage. The studies described in this Account primarily rely on an environmental transmission electron microscope, a tool capable of circumventing traditional TEM's high-vacuum requirements, outfitted with optical sources and detectors to couple light into and out of the microscope. First, we describe the H₂ loading kinetics of individual Pd nanoparticles. When confined to sizes of less than ∼100 nm, single-crystalline Pd nanoparticles exhibit coherent phase transformations between the hydrogen-poor α-phase and hydrogen-rich β-phase, as revealed through monitoring the bulk plasmon resonance with electron energy loss spectroscopy. Next, we describe how contrast imaging techniques, such as phase contrast STEM and displaced-aperture dark field, can be employed as real-time techniques to image phase transformations with 100 ms temporal resolution. Studies of multiply twinned Pd nanoparticles and high aspect ratio Pd nanorods demonstrate that internal strain and grain boundaries can lead to partial hydrogenation within individual nanoparticles. Finally, we describe how OTEM can be used to locally probe nanoparticle dynamics under optical excitation and in reactive chemical environments. Under illumination, multicomponent plasmonic photocatalysts consisting of a gold nanoparticle "antenna" and a Pd "reactor" show clear α-phase nucleation in regions close to electromagnetic "hot spots" when near plasmonic antennas. Importantly, these hot spots need not correspond to the traditionally active, energetically preferred sites of catalytic nanoparticles. Nonthermal effects imparted by plasmonic nanoparticles, including electromagnetic field enhancement and plasmon-derived hot carriers, are crucial to explaining the site selectivity observed in PdH_x phase transformations under illumination. This Account demonstrates how light can contribute to selective chemical phenomena in plasmonic heterostructures, en route to sustainable, solar-driven chemical production.

Fiber-optic sample illuminator design for the observation of light induced phenomena with transmission electron microscopy in situ: Antimicrobial photodynamic therapy

Antibacterial photodynamic therapy is a promising treatment for problematic infections caused by bacteria and fungi. Despite its undoubted effectiveness, the ultrastructural mechanism of microbial death remains not fully described and distinct organisms respond to the treatment with different efficacy. For this reason, it was decided to try imaging the process using the in situ transmission electron microscopy method. To conduct an observational experiment, the microscope was significantly modified. Liquid cell methods were used, electron doses and their influence on the sample were estimated, and a fiber-optic sample illuminator was designed and built. The modifications allowed for the light-induced characterization of photosensitizer-bacteria interaction. Microscope modification is a promising platform for further studies of light-induced phenomena in both life and material science.

In-situ nanospectroscopic imaging of plasmon-induced two-dimensional [4+4]-cycloaddition polymerization on Au(111)

Plasmon-induced chemical reactions (PICRs) have recently become promising approaches for highly efficient light-chemical energy conversion. However, an in-depth understanding of their mechanisms at the nanoscale still remains challenging. Here, we present an in-situ investigation by tip-enhanced Raman spectroscopy (TERS) imaging of the plasmon-induced [4+4]-cycloaddition polymerization within anthracene-based monomer monolayers physisorbed on Au(111), and complement the experimental results with density functional theory (DFT) calculations. This two-dimensional (2D) polymerization can be flexibly triggered and manipulated by the hot carriers, and be monitored simultaneously by TERS in real time and space. TERS imaging provides direct evidence for covalent bond formation with ca. 3.7 nm spatial resolution under ambient conditions. Combined with DFT calculations, the TERS results demonstrate that the lateral polymerization on Au(111) occurs by a hot electron tunneling mechanism, and crosslinks form via a self-stimulating growth mechanism. We show that TERS is promising to be plasmon-induced nanolithography for organic 2D materials.

Gallium Plasmonic Nanoantennas Unveiling Multiple Kinetics of Hydrogen Sensing, Storage, and Spillover

Hydrogen is the key element to accomplish a carbon-free based economy. Here, the first evidence of plasmonic gallium (Ga) nanoantennas is provided as nanoreactors supported on sapphire (α-Al₂ O₃ ) acting as direct plasmon-enhanced photocatalyst for hydrogen sensing, storage, and spillover. The role of plasmon-catalyzed electron transfer between hydrogen and plasmonic Ga nanoparticle in the activation of those processes is highlighted, as opposed to conventional refractive index-change-based sensing. This study reveals that, while temperature selectively operates those various processes, longitudinal (LO-LSPR) and transverse (TO-LSPR) localized surface plasmon resonances of supported Ga nanoparticles open selectivity of localized reaction pathways at specific sites corresponding to the electromagnetic hot-spots. Specifically, the TO-LSPR couples light into the surface dissociative adsorption of hydrogen and formation of hydrides, whereas the LO-LSPR activates heterogeneous reactions at the interface with the support, that is, hydrogen spillover into α-Al₂ O₃ and reverse-oxygen spillover from α-Al₂ O_3. This Ga-based plasmon-catalytic platform expands the application of supported plasmon-catalysis to hydrogen technologies, including reversible fast hydrogen sensing in a timescale of a few seconds with a limit of detection as low as 5 ppm and in a broad temperature range from room-temperature up to 600 °C while remaining stable and reusable over an extended period of time.

Plasmonic metal nanostructures: concepts, challenges and opportunities in photo-mediated chemical transformations

Plasmonic metal nanostructures (PMNs) are characterized by the plasmon oscillation of conduction band electron in response to external radiation, enabling strong light absorption and scattering capacities and near-field amplification. Owing to these enhanced light-matter interactions, PMNs have garnered extensive research interest in the past decades. Notably, a growingly large number of reports show that the energetics and kinetics of chemical transformations on PMNs can be modified upon photoexcitation of their plasmons, giving rise to a new paradigm of manipulating the reaction rate and selectivity of chemical reactions. On the other hand, there is urgent need to achieve clear understanding of the mechanism underlying the photo-mediated chemical transformations on PMNs for unleashing their full potential in converting solar energy to chemicals. In this perspective, we review current fundamental concepts of photo-mediated chemical transformations executed at PMNs. Three pivotal mechanistic questions, i.e., thermal and nonthermal effects, direct and indirect charge transfer processes, and the specific impacts of plasmon-induced potentials, are explored based on recent studies. We highlight the critical aspects in which major advancements should be made to facilitate the rational design and optimization of photo-mediated chemical transformations on PMNs in the future.

Light Driven Mechanism of Carbon Dioxide Reduction Reaction to Carbon Monoxide on Gold Nanoparticles: A Theoretical Prediction

Insightful understanding of the light driven CO₂ reduction reaction (CO₂RR) mechanism on gold nanoparticles is one of the important issues in the plasmon mediated photocatalytic study. Herein, time-dependent density functional theory and reduced two-state model are adopted to investigate the photoinduced charge transfer in interfaces. According to the excitation energy and orbital coupling, the light driven mechanism of CO₂RR on gold nanoparticles can be described as follows: the light induces electron excitation and then transfers to the physisorbed CO₂, and CO₂ can relax to a bent structure adsorbed on gold nanoparticles, and the adsorbed C-O bonds are dissociated finally. Moreover, our calculated results demonstrate that the s, p, and d electron excitations of gold nanoparticles are the major contribution for the CO₂ adsorption and the C-O dissociation process, respectively. This work would promote the understanding of the light driven electron transfer and photocatalytic CO₂RR on the noble metal.

Driving energetically unfavorable dehydrogenation dynamics with plasmonics

Nanoparticle surface structure and geometry generally dictate where chemical transformations occur, with higher chemical activity at sites with lower activation energies. Here, we show how optical excitation of plasmons enables spatially modified phase transformations, activating otherwise energetically unfavorable sites. We have designed a crossed-bar Au-PdH _x antenna-reactor system that localizes electromagnetic enhancement away from the innately reactive PdH _x nanorod tips. Using optically coupled in situ environmental transmission electron microscopy, we track the dehydrogenation of individual antenna-reactor pairs with varying optical illumination intensity, wavelength, and hydrogen pressure. Our in situ experiments show that plasmons enable new catalytic sites, including dehydrogenation at the nanorod faces. Molecular dynamics simulations confirm that these new nucleation sites are energetically unfavorable in equilibrium and only accessible through tailored plasmonic excitation.

Selectivity control of organic chemical synthesis over plasmonic metal-based photocatalysts

The factors, issues, and design of plasmonic metal-based photocatalysts for selective photosynthesis of organic chemicals have been discussed.

In Situ Tracking of Colloidally Stable and Ordered Assemblies of Gold Nanorods

Solution-phase self-assembly of anisotropic nanoparticles into complex 2D and 3D assemblies is one of the most promising strategies toward obtaining nanoparticle-based materials and devices with unique optical properties at the macroscale. However, controlling this process with single-particle precision is highly demanding, mostly due to insufficient understanding of the self-assembly process at the nanoscale. We report the use of in situ environmental scanning transmission electron microscopy (WetSTEM), combined with UV/vis spectroscopy, small-angle X-ray diffraction (SAXRD) and multiscale modeling, to draw a detailed picture of the dynamics of vertically aligned assemblies of gold nanorods. Detailed understanding of the self-assembly/disassembly mechanisms is obtained from real-time observations, which provide direct evidence of the colloidal stability of side-to-side nanorod clusters. Structural details and the forces governing the disassembly process are revealed with single particle resolution as well as in bulk samples, by combined experimental and theoretical modeling. In particular, this study provides unique information on the evolution of the orientational order of nanorods within side-to-side 2D assemblies and shows that both electrostatic (at the nanoscale) and thermal (in bulk) stimuli can be used to drive the process. These results not only give insight into the interactions between nanorods and the stability of their assemblies, thereby assisting the design of ordered, anisotropic nanomaterials but also broaden the available toolbox for in situ tracking of nanoparticle behavior at the single-particle level.

Fabrication and characterization of ternary sepiolite/g-C3N4/Pd composites for improvement of photocatalytic degradation of ciprofloxacin under visible light irradiation

The development of high-quality photocatalytic materials for the degradation of organic pollutants under visible light irradiation is a vital field of research. In the present study, a composite of natural sepiolite clay and synthetic graphitic carbon nitride (CN) mixed with dispersed palladium nanoparticles was developed for the efficient photocatalytic degradation of ciprofloxacin (CIP) under visible light irradiation. The sepiolite, CN, and composite materials were characterized by several techniques. The sepiolite/CN composite (SC30%) displayed superior activity than pristine sepiolite and CN, resulted from the generation of new electron trap states in the interfacial contract between sepiolite and CN to suppress the charge recombination of CN. Furthermore, the well-dispersed of 1 wt% Pd-nanoparticles in the SC30% composite collectively enhanced CIP degradation by avoiding the recombination of photogenerated electrons and holes. Additionally, the electron trap states on the surface of all samples were studied using novel reversed double-beam photoacoustic spectroscopy to understand electron transfer in the composites related to the photocatalytic degradation mechanism of CIP. The developed sepiolite/CN/Pd(0) composite can act as a potential catalyst for the degradation of organic pollutants in wastewater under visible light irradiation.

Morphology-Dependent Reactivity of a Plasmonic Photocatalyst

The shape of a plasmonic nanoparticle strongly controls its light-matter interaction, which in turn affects how specific morphologies may be used in applications such as sensing, photodetection, and active pixel displays. Here, we show that particle shape also controls plasmonic photocatalytic activity. Three different Al nanocrystal morphologies, octopods, nanocubes, and nanocrystals, all with very similar plasmon resonance frequencies, were used as photocatalysts for the H₂ dissociation reaction. We observe widely varying reaction rates for the three different morphologies. Octopods show a 10 times higher reaction rate than nanocrystals and a 5 times higher rate than nanocubes, with lower apparent activation energies than either nanocubes or nanocrystals by 45% and 49%, respectively. A theoretical model of hot electron direct transfer from photoexcited Al nanoparticles to H₂ molecules is consistent with this observed morphological dependence. This research strongly suggests that nanoparticle geometry, in addition to plasmon resonance energy, is a critical factor in plasmonic photocatalyst design.

Site-Selective Nanoreactor Deposition on Photocatalytic Al Nanocubes

Photoactivation of catalytic materials through plasmon-coupled energy transfer has created new possibilities for expanding the scope of light-driven heterogeneous catalysis. Here we present a nanoengineered plasmonic photocatalyst consisting of catalytic Pd islands preferentially grown on vertices of Al nanocubes. The regioselective Pd deposition on Al nanocubes does not rely on complex surface ligands, in contrast to site-specific transition-metal deposition on gold nanoparticles. We show that the strong local field enhancement on the sharp nanocube vertices provides a mechanism for efficient coupling of the plasmonic Al antenna to adjacent Pd nanoparticles. A substantial increase in photocatalytic H₂ dissociation on Pd-bound Al nanocubes relative to pristine Al nanocubes can be observed, incentivizing further engineering of heterometallic antenna-reactor photocatalysts. Controlled growth of catalytic materials on plasmonic hot spots can result in more efficient use of the localized surface plasmon energy for photocatalysis, while minimizing the amount and cost of precious transition-metal catalysts.

Thermal effects - an alternative mechanism for plasmon-assisted photocatalysis

Recent experiments claimed that the catalysis of reaction rates in numerous bond-dissociation reactions occurs via the decrease of activation barriers driven by non-equilibrium ("hot") electrons in illuminated plasmonic metal nanoparticles. Thus, these experiments identify plasmon-assisted photocatalysis as a promising path for enhancing the efficiency of various chemical reactions. Here, we argue that what appears to be photocatalysis is much more likely thermo-catalysis, driven by the well-known plasmon-enhanced ability of illuminated metallic nanoparticles to serve as heat sources. Specifically, we point to some of the most important papers in the field, and show that a simple theory of illumination-induced heating can explain the extracted experimental data to remarkable agreement, with minimal to no fit parameters. We further show that any small temperature difference between the photocatalysis experiment and a control experiment performed under external heating is effectively amplified by the exponential sensitivity of the reaction, and is very likely to be interpreted incorrectly as "hot" electron effects.

Hot Electrons Generated in Chiral Plasmonic Nanocrystals as a Mechanism for Surface Photochemistry and Chiral Growth

The realization of chiral photochemical reactions at the molecular level has proven to be a challenging task, with invariably low efficiencies originating from very small optical circular dichroism signals. On the contrary, colloidal nanocrystals offer a very large differential response to circularly polarized light when designed with chiral geometries. We propose taking advantage of this capability, introducing a novel mechanism driving surface photochemistry in a chiral nanocrystal. Plasmonic nanocrystals exhibit anomalously large asymmetry factors in optical circular dichroism (CD), and the related hot-electron generation shows in turn a very strong asymmetry, serving as a mechanism for chiral growth. Through theoretical modeling, we show that chiral plasmonic nanocrystals can enable chiral surface growth based on the generation of energetic (hot) electrons. Using simple and realistic phenomenological models, we illustrate how this kind of surface photochemistry can be observed experimentally. The proposed mechanism is efficient if it operates on an already strongly chiral nanocrystal, whereas our proposed mechanism does not show chiral growth for initially nonchiral structures in a solution. The asymmetry factors for the chiral effects, driven by hot electrons, exceed the values observed in chiral molecular photophysics at least 10-fold. The proposed chiral-growth mechanism for the transformation of plasmonic colloids is fundamentally different to the traditional schemes of chiral photochemistry at the molecular level.

Arrays of Plasmonic Nanoparticle Dimers with Defined Nanogap Spacers

Plasmonic molecules are building blocks of metallic nanostructures that give rise to intriguing optical phenomena with similarities to those seen in molecular systems. The ability to design plasmonic hybrid structures and molecules with nanometric resolution would enable applications in optical metamaterials and sensing that presently cannot be demonstrated, because of a lack of suitable fabrication methods allowing the structural control of the plasmonic atoms on a large scale. Here we demonstrate a wafer-scale "lithography-free" parallel fabrication scheme to realize nanogap plasmonic meta-molecules with precise control over their size, shape, material, and orientation. We demonstrate how we can tune the corresponding coupled resonances through the entire visible spectrum. Our fabrication method, based on glancing angle physical vapor deposition with gradient shadowing, permits critical parameters to be varied across the wafer and thus is ideally suited to screen potential structures. We obtain billions of aligned dimer structures with controlled variation of the spectral properties across the wafer. We spectroscopically map the plasmonic resonances of gold dimer structures and show that they not only are in good agreement with numerically modeled spectra, but also remain functional, at least for a year, in ambient conditions.

Plasmon Enhanced Fluorescence and Raman Scattering by [Au-Ag Alloy NP Cluster]@SiO2 Core-Shell Nanostructure

Silica-shell coated noble metal nanoparticles have shown a good performance in surface enhanced fluorescence and Raman scattering. However, silica-shell coated single noble nanoparticle cannot effectively enhance the optical signal due to the relative weak near-field enhancement. In this paper, [Au-Ag alloy NP cluster]@SiO₂ core-shell nanostructure is employed to achieve the effective electric field enhancement. With the specific structure, simultaneous Raman scattering and fluorescence emission enhancement is obtained, and the enhancement comparison of fluorescence emission with Raman scattering in different type agglomeration of metal NPs is investigated in-situ. With different thickness of SiO₂ shell, the optimized Raman and fluorescence enhancement systems are obtained, respectively, and corresponding study of power dependence are investigated in detail. The selectively enhanced Raman and fluorescence can be realized via controlling the shell thickness and laser power. Our work provides a non-polarization dependent [metal NP cluster]@SiO₂ system, which may have a promising application in portable chemical and biochemistry detecting.

Hierarchical Control of Plasmonic Nanochemistry in Microreactor

A microreactor that can confine chemical reactions exclusively in tiny vessels with the volume of ∼0.015 μm³ is introduced. Aluminum inversed hollow nanocone arrays (IHNAs) are fabricated by a simple and efficient colloidal lithography method. Ag and Au nanoparticles (NPs), as well as polypyrrole, grow exclusively in the conic cavities under light illumination. The photocatalytic effect arising from the plasmonic enhanced electric fields (E-fields) of IHNAs boosts the reactions and is in charge of the submicrometer site-selectivity. By partially inhibiting light to IHNAs, various hierarchical patterns at the macro-, micro-, and sub-microscale are obtained, inspiring a facile patterning technique by varying the light source. In addition, the Al IHNA films are transferred to flexible and curved substrates with unchanged performances, showing high flexibility for wide applications. Microreactors based on the IHNAs will contribute to the control of chemical reactions at different dimensions and offer great potentials in developing novel nanofabrication techniques.

Plasmon-Driven Catalysis on Molecules and Nanomaterials

As a new class of photocatalysts, plasmonic noble metal nanoparticles with the unique ability to harvest solar energy across the entire visible spectrum and produce effective energy conversion have been explored as a promising pathway for the energy crisis. The resonant excitation of surface plasmon resonance allows the nanoparticles to collect the energy of photons to form a highly enhanced electromagnetic field, and the energy stored in the plasmonic field can induce hot carriers in the metal. The hot electron-hole pairs ultimately dissipate by coupling to phonon modes of the metal nanoparticles, resulting in a higher lattice temperature. The plasmonic electromagnetic field, hot electrons, and heat can catalyze chemical reactions of reactants near the surface of the plasmonic metal nanoparticles. This Account summarizes recent theoretical and experimental advances on the excitation mechanisms and energy transfer pathways in the plasmonic catalysis on molecules. Especially, current advances on plasmon-driven crystal growth and transformation of nanomaterials are introduced. The efficiency of the chemical reaction can be dramatically increased by the plasmonic electromagnetic field because of its higher density of photons. Similar to traditional photocatalysis, energy overlap between the plasmonic field and the HOMO-LUMO gap of the reactant is needed to realize resonant energy transfer. For hot-carrier-driven catalysis, hot electrons generated by plasmon decay can be transferred to the reactant through the indirect electron transfer or direct electron excitation process. For this mechanism, the energy of hot electrons needs to overlap with the unoccupied orbitals of the reactant, and the particular chemical channel can be selectively enhanced by controlling the energy distribution of hot electrons. In addition, the local thermal effect following plasmon decay offers an opportunity to facilitate chemical reactions at room temperature. Importantly, surface plasmons can not only catalyze chemical reactions of molecules but also induce crystal growth and transformation of nanomaterials. As a new development in plasmonic catalysis, plasmon-driven crystal transformation reveals a more powerful aspect of the catalysis effect, which opens the new field of plasmonic catalysis. We believe that this Account will promote clear understanding of plasmonic catalysis on both molecules and materials and contribute to the design of highly tunable catalytic systems to realize crystal transformations that are essential to achieve efficient solar-to-chemical energy conversion.