Photochemistry on metal nanoparticles

Thermal-effect dominated plasmonic catalysis on silver nanoislands

Plasmonic metal nanostructures with the intrinsic property of localized surface plasmon resonance can effectively promote energy conversion in many applications such as photocatalysis, photothermal therapy, seawater desalinization, etc. It is known that not only are plasmonically excited hot electrons generated from metal nanostructures under light irradiation, which can effectively trigger chemical reactions, but also plasmonically induced heating simultaneously occurs. Although plasmonic catalysis has been widely explored in recent years, the underlying mechanisms for distinguishing the contribution of hot electrons from thermal effects are not fully understood. Here, a simple and efficient self-assembly system using silver nanoislands as plasmonic substrates is designed to investigate the photo-induced azo coupling reaction of nitro- and amino-groups at various temperatures. In the experiments, surface-enhanced Raman spectroscopy is employed to monitor the time and temperature dependence of plasmon-induced catalytic reactions. It was found that a combination of hot electrons and thermal effects contribute to the reactivity. The thermal effects play the dominant role in the plasmon-induced azo coupling reaction of nitro-groups, which suggests that the localized temperature must be considered in the development of photonic applications based on plasmonic nanomaterials.

Light switching for product selectivity control in photocatalysis

Artificial switchable catalysis is a new, rapidly expanding field that offers great potential advantages for both homogeneous and heterogeneous catalytic systems. Light irradiation is widely accepted as the best stimulus to artificial switchable chemical systems. In recent years, tremendous progress has been made in the synthesis and application of photo-switchable catalysts that can control when and where bond formation and dissociation take place in reactant molecules. Photo-switchable catalysis is a niche area in current catalysis, on which systematic analysis and reviews are still lacking in the scientific literature, yet it offers many intriguing and versatile applications, particularly in organic synthesis. This review aims to highlight the recent advances in photo-switchable catalyst systems that can result in two different chemical product outcomes and thus achieve a degree of control over organic synthetic reactions. Furthermore, this review evaluates different approaches that have been employed to achieve dynamic control over both the catalytic function and the selectivity of several different types of synthesis reactions, along with the remaining challenges and potential opportunities. Owing to the great diversity of the types of reactions and conditions adopted, a quantitative comparison of efficiencies between considered systems is not the focus of this review, instead the review showcases how insights from successful adopted strategies can help better harness and channel the power of photoswitchability in this new and promising area of catalysis research.

Plasmon induced hot carrier distribution in Ag20 -CO composite

The interaction between plasmons and the molecules leads to the transfer of plasmon-induced hot carriers, presenting innovative opportunities for controlling chemical reactions on sub-femtosecond timescales. Through real-time time-dependent density functional theory simulations, we have investigated the enhancement of the electric field due to plasmon excitation and the subsequent generation and transfer of plasmon-induced hot carriers in a linear atomic chain of Ag20 and an Ag20 -CO composite system. By applying a Gaussian laser pulse tuned to align with the plasmon frequency, we observe a plasmon-induced transfer of hot electrons from the occupied states of Ag to the unoccupied molecular orbitals of CO. Remarkably, there is a pronounced accumulation of hot electrons and hot holes on the C and O atoms. This phenomenon arises from the electron migration from the inter-nuclear regions of the C-O bond towards the individual C and O atoms. The insights garnered from our study hold the potential to drive advancements in the development of more efficient systems for catalytic processes empowered by plasmonic interactions.

Au@C/Pt core@shell/satellite supra-nanostructures: plasmonic antenna-reactor hybrid nanocatalysts

Integration of plasmonic nanoantennas with catalytically active reactors in deliberately designed hybrid supra-nanostructures creates a dual-functional materials platform, based upon which precise modulation of catalytic reaction kinetics becomes accomplishable through optical excitations of plasmon resonances. Here, we have developed a multistep synthetic approach that enables us to assemble colloidal Au@C/Pt core@shell/satellite supra-nanostructures, in which the Au core functions as a light-harvesting plasmonic nanoantenna, the Pt satellites act as catalytically active reactors, and the C shell serves as a nanoscale dielectric spacer separating the reactors from the antenna, respectively. By adjusting several synthetic parameters, the size of the Au core, the thickness of the C shell, and the surface coverage of Pt satellites can all be tuned independently. Choosing Pt-catalyzed cascade oxidation of 3,3',5,5'-tetramethylbenzidine in an aerobic aqueous environment as a model reaction, we have systematically studied the detailed kinetic features of the catalytic reactions both in the dark and under visible light illumination over a broad range of reaction conditions, which sheds light on the interplay between plasmonic and catalytic effects in these antenna-reactor nanohybrids. The plasmonic antenna effect can be effectively harnessed to kinetically modulate multiple crucial steps during the cascade reactions, benefiting from plasmon-enhanced interband electronic transitions in the Pt satellites and plasmon-enhanced intramolecular electronic excitations in chromogenic intermediate species. In addition to the plasmonic antenna effect, photothermal transduction derived from plasmonic excitations can also provide significant contributions to the kinetic enhancements under visible light illumination. The knowledge gained from this work serves as important guiding principles for rational design and structural optimization of plasmonic antenna-reactor hybrid nanomaterials, endowing us with enhanced capabilities to kinetically modulate targeted catalytic/photocatalytic molecule-transforming processes through light illumination.

Nanocomposites Based on Spin-Crossover Nanoparticles and Silica-Coated Gold Nanorods: A Nonlinear Optical Study

A nanocomposite based on silica-coated AuNRs with the aminated silica-covered spin-crossover nanoparticles (SCO NPs) of the 1D iron(II) coordination polymer with the formula [Fe(Htrz)₂(trz)](BF₄) is presented. For the synthesis of the SCO NPs, the reverse micelle method was used, while the gold nanorods (AuNRs) were prepared with the aspect ratio AR = 6.0 using the seeded-growth method and a binary surfactant mixture composed of cetyltrimethylammonium bromide (CTAB) and sodium oleate (NaOL). The final nanocomposite was prepared using the heteroaggregation method of combining different amounts of SCO NPs with the AuNRs. The nonlinear optical (NLO) properties of the hybrid AuNRs coated with different amounts of SCO NPs were studied in detail by means of the Z-scan technique, revealing that the third-order NLO properties of the AuNRs@SCO are dependent on the amount of SCO NPs grafted onto them. However, due to the resonant nature of the excitation, SCO-induced NLO switching was not observed.

Revisit of the plasmon-mediated chemical transformation of para-aminothiophenol

Fingerprint Raman features of para-aminothiophenol (pATP) in surface-enhanced Raman scattering (SERS) spectra have been widely used to measure plasmon-driven catalytic activities because the appearance of characteristic spectral features is purported to be due to plasmon-induced chemical transformation of pATP to trans-p,p'-dimercaptoazobenzene (trans-DMAB). Here, we present a thorough comparison of SERS spectra for pATP and trans-DMAB in the extended range of frequencies covering group vibrations, skeletal vibrations, and external vibrations under various conditions. Although the fingerprint vibration modes of pATP could be almost mistaken with those of trans-DMAB, the low-frequency vibrations revealed distinct differences between pATP and DMAB. Photo-induced spectral changes of pATP in the fingerprint region were explained well by photo-thermal variation of the Au-S bond configuration, which affects the degree of the metal-to-molecule charge transfer resonance. This finding indicates that a large number of reports in the field of plasmon-mediated photochemistry must be reconsidered.

Insights into Plasmon-Induced Dimerization of Rhodanine-A Surface-Enhanced Raman Scattering Study

Plasmon-mediated chemical reactions (PMCRs) have attracted considerable interest in recent times. The PMCR initiated by hot carriers is known to be influenced by the type of metals and the excitation wavelength. Herein, we have carried out the surface-enhanced Raman scattering (SERS) investigation of rhodanine (Rd), an important pharmacologically active heterocyclic compound, adsorbed on silver and gold nanoparticles (AgNP and AuNP) using 514.5 and 632.8 nm lasers. The prominent Raman band at 1566 cm^-1 observed in the SERS spectra is attributed to the characteristic ν(C═C) stretching vibration of the Rd dimer and not of Rd tautomers. The chemical transformation of Rd to Rd dimer on metal surfaces is plausibly triggered by the indirect transfer of energetic hot electrons generated during the non-radiative decay of plasmon. The mechanism involved in the dimerization of Rd via the indirect transfer of hot electrons is also presented. The effect of wavelength on the dimerization of Rd is also observed on the AgNP surface, which indicates that the dimerization occurs more efficiently on the AgNP surface with excitation at 514.5 nm wavelength.

Effect of light at different wavelengths on polyol synthesis of silver nanocubes

Despite the presence of light-sensitive species in the polyol synthesis of silver nanocubes, the influence of light on it has yet to be investigated. Herein, we demonstrated that light radiation, by generating plasmon-based hot electrons and subsequently increasing the reduction rate of Ag⁺ in the system, in addition to enhancing the growth rate of nanocubes, causes twinned seeds, which these seeds are then converted into nanorods and right bipyramids. With shorter, higher energy wavelengths, Ag⁺ reduction progresses more quickly, resulting in structures with more twin planes. The overlap of the excitation wavelength and the band gap of Ag₂S clusters formed in the early stages of synthesis accelerates the rate of reaction at low-energy excitation. According to our findings, the surfactant polyvinylpyrrolidone acts as a photochemical relay to drive the growth of silver nanoparticles. Overall, this work emphasizes the impact of excitation light on polyol synthesis as a technique for generating Ag nanocubes of various sizes.

Mid-Infrared Response from Cr/n-Si Schottky Junction with an Ultra-Thin Cr Metal

Infrared detection technology has been widely applied in many areas. Unlike internal photoemission and the photoelectric mechanism, which are limited by the interface barrier height and material bandgap, the research of the hot carrier effect from nanometer thickness of metal could surpass the capability of silicon-based Schottky devices to detect mid-infrared and even far-infrared. In this work, we investigate the effects of physical characteristics of Cr nanometal surfaces and metal/silicon interfaces on hot carrier optical detection. Based on the results of scanning electron microscopy, atomic force microscopy, and X-ray diffraction analysis, the hot carrier effect and the variation of optical response intensity are found to depend highly on the physical properties of metal surfaces, such as surface coverage, metal thickness, and internal stress. Since the contact layer formed by Cr and Si is the main role of infrared light detection in the experiment, the higher the metal coverage, the higher the optical response. Additionally, a thicker metal surface makes the hot carriers take a longer time to convert into current signals after generation, leading to signal degradation due to the short lifetime of the hot carriers. Furthermore, the film with the best hot carrier effect induced in the Cr/Si structure is able to detect an infrared signal up to 4.2 μm. Additionally, it has a 229 times improvement in the signal-to-noise ratio (SNR) for a single band compared with ones with less favorable conditions.

Novel Clarification of Surface Plasmon Coupling Reactions of Aromatic Alkynamine and Nitro Compounds

This work presents a theoretical and experimental approach for the coupling of 4-ethynylaniline (4-APA) and 4-ethynylnitrobenzene (4-NPA) in the theoretical application of density functional theory (DFT) and experimental monitoring of surface-enhanced Raman spectroscopy (SERS). The results support electromagnetic enhancement to drive the conversion of aromatic alkynamine and nitro compounds and regulation by the catalytic coupling reaction conditions. In addition, this work investigates the adsorption site effect of surface plasmon coupling reactions of 4-APA and 4-NPA molecules into alkynyl azo compounds. This study presents theoretical and experimental images used to analyze the plasmon-driven surface catalytic reaction system.

Plasmon-assisted facile selective gaseous isopropanol dehydrogenation over Ag nanocubes

The present research showed that the non-heating effect of plasmonic absorption caused a great increase in the acetone dehydrogenation over Ag nanocubes in high selectivity at low temperatures.

On the Roles of Electron Transfer in Catalysis by Nanoclusters and Nanoparticles

Electron transfer plays a major role in chemical reactions and processes, and this is particularly true of catalysis by nanomaterials. The advent of metal nanoparticle (NP) catalysts, recently including atomically precise nanoclusters (NCs) as parts of nanocatalyst devices has brought increased control of the relationship between NP and NC structures and their catalytic functions. Consequently, the molecular definition of these new nanocatalysts has allowed a better understanding and management of various kinds of electron transfer involved in the catalytic processes. This Minireview brings a chemist's view of several major aspects of electron-transfer functions concerning NPs and NCs in catalytic processes. Particular focus concerns the role of NPs and NCs as electron reservoirs and light-induced antenna in catalytic processes from H₂ generation to more complex reactions and sustainable energy production.

Monitoring the Thiol/Thiophenol Molecule-Modulated Plasmon-Mediated Silver Oxidation with Dark-Field Optical Microscopy

Surface plasmon can trigger or accelerate many photochemical reactions, especially useful in energy and environmental industries. Recently, molecular adsorption has proven effective in modulating plasmon-mediated photochemistry, however the realized chemical reactions are limited and the underlying mechanism is still unclear. Herein, by using in situ dark-field optical microscopy, the plasmon-mediated oxidative etching of silver nanoparticles (Ag NPs), a typical hot-hole-driven reaction, is monitored continuously and quantitatively. The presence of thiol or thiophenol molecules is found essential in the silver oxidation. In addition, the rate of silver oxidation is modulated by the choice of different thiol or thiophenol molecules. Compared with the molecules having electron donating groups, the ones having electron accepting groups accelerate the silver oxidation dramatically. The thiol/thiophenol modulation is attributed to the modulation of the charge separation between the Ag NPs and the adsorbed thiol or thiophenol molecules. This work demonstrates the great potential of molecular adsorption in modulating the plasmon-mediated photochemistry, which will pave a new way for developing highly efficient plasmonic photocatalysts.

Optical Activity of Metal Nanoclusters Deposited on Regular and Doped Oxide Supports from First-Principles Simulations

We report a computational study and analysis of the optical absorption processes of Ag20 and Au20 clusters deposited on the magnesium oxide (100) facet, both regular and including point defects. Ag20 and Au20 are taken as models of metal nanoparticles and their plasmonic response, MgO as a model of a simple oxide support. We consider oxide defects both on the oxygen anion framework (i.e., a neutral oxygen vacancy) and in the magnesium cation framework (i.e., replacing Mg++ with a transition metal: Cu++ or Co++). We relax the clusters' geometries via Density-Functional Theory (DFT) and calculate the photo-absorption spectra via Time-Dependent DFT (TDDFT) simulations on the relaxed geometries. We find that the substrate/cluster interaction induces a broadening and a red-shift of the excited states of the clusters, phenomena that are enhanced by the presence of an oxygen vacancy and its localized excitations. The presence of a transition-metal dopant does not qualitatively affect the spectral profile. However, when it lies next to an oxygen vacancy for Ag20, it can strongly enhance the component of the cluster excitations perpendicular to the surface, thus favoring charge injection.

Control of Chemical Reaction Pathways by Light-Matter Coupling

Because plasmonic metal nanostructures combine strong light absorption with catalytically active surfaces, they have become platforms for the light-assisted catalysis of chemical reactions. The enhancement of reaction rates by plasmonic excitation has been extensively discussed. This review focuses on a less discussed aspect: the induction of new reaction pathways by light excitation. Through commentary on seminal reports, we describe the principles behind the optical modulation of chemical reactivity and selectivity on plasmonic metal nanostructures. Central to these phenomena are excited charge carriers generated by plasmonic excitation, which modify the energy landscape available to surface reactive species and unlock pathways not conventionally available in thermal catalysis. Photogenerated carriers can trigger bond dissociation or desorption in an adsorbate-selective manner, drive charge transfer and multielectron redox reactions, and generate radical intermediates. Through one or more of these mechanisms, a specific pathway becomes favored under light. By improved control over these mechanisms, light-assisted catalysis can be transformational for chemical synthesis and energy conversion.

First-Principles Insights into Plasmon-Induced Catalysis

The size- and shape-controlled enhanced optical response of metal nanoparticles (NPs) is referred to as a localized surface plasmon resonance (LSPR). LSPRs result in amplified surface and interparticle electric fields, which then enhance light absorption of the molecules or other materials coupled to the metallic NPs and/or generate hot carriers within the NPs themselves. When mediated by metallic NPs, photocatalysis can take advantage of this unique optical phenomenon. This review highlights the contributions of quantum mechanical modeling in understanding and guiding current attempts to incorporate plasmonic excitations to improve the kinetics of heterogeneously catalyzed reactions. A range of first-principles quantum mechanics techniques has offered insights, from ground-state density functional theory (DFT) to excited-state theories such as multireference correlated wavefunction methods. Here we discuss the advantages and limitations of these methods in the context of accurately capturing plasmonic effects, with accompanying examples.

Advancements on Basic Working Principles of Photo-Driven Oxidative Degradation of Organic Substrates over Pristine and Noble Metal-Modified TiO2. Model Case of Phenol Photo Oxidation

The specific roles played by both support and noble metals in light absorption, charge separation, and the formation of ·OH and O2− (ROS) are analyzed for light-triggered oxidation of phenol (Ph) over pristine and over noble metal (Ag, Au, Pt) -loaded TiO2. Experiments show that the supported noble metals act as a light visible absorber, assist the separation of photo-charges and reduction of O2 to O2−. The O2− oxidizes mildly Ph to oxygenated products (hydroquinone, benzoquinone, and 1,2-dihydroxibenzene). In a parallel process, ·OH radicals, yielded by TiO2, mineralize Ph to CO2 by fast reaction sequences. Radical quenching and photo electrochemical measurements (surface photovoltage) confirm independently that the production of ·OH and O2− scale with oxidative conversion of Ph. The selectivity to CO2 and mild oxidation products is the result of the interplay between catalyst activity for ·OH and for O2− production.

State-of-the-art developments in carbon-based metal nanocomposites as a catalyst: photocatalysis

The rapid progress of state-of-the-art carbon-based metals as a catalyst is playing a central role in the research area of chemical and materials engineering for effective visible-light-induced catalytic applications. Numerous admirable catalysts have been fabricated, but significant challenges persist to lower the cost and increase the action of catalysts. The development of carbon-based nanostructured materials (i.e., activated carbon, carbon nitride, graphite, fullerenes, carbon nanotubes, diamond, graphene, etc.) represents an admirable substitute to out-of-date catalysts. Significant efforts have been made by researchers toward the improvement of various carbon-based metal nanostructures as catalysts. Moreover, incredible development has been achieved in several fields of catalysis, such as visible-light-induced catalysis, electrochemical performance, energy storage, and conversion, etc. This review gives an overview of the up-to-date developments in the strategy of design and fabrication of carbon-based metal nanostructures as photo-catalysts by means of several methods within the green approach, including chemical synthesis, in situ growth, solution mixing, and hydrothermal approaches. Moreover, the photocatalytic effects of the resulting carbon-based nanostructure classifications are similarly deliberated relative to their eco-friendly applications, such as photocatalytic degradation of organic dye pollutants.

A General Approach for Monolayer Adsorption of High Weight Loadings of Uniform Nanocrystals on Oxide Supports

Monodispersed metal and semiconductor nanocrystals have attracted great attention in fundamental and applied research due to their tunable size, morphology, and well-defined chemical composition. Utilizing these nanocrystals in a controllable way is highly desirable especially when using them as building blocks for the preparation of nanostructured materials. Their deposition onto oxide materials provide them with wide applicability in many areas, including catalysis. However, so far deposition methods are limited and do not provide control to achieve high particle loadings. This study demonstrates a general approach for the deposition of hydrophobic ligand-stabilized nanocrystals on hydrophilic oxide supports without ligand-exchange. Surface functionalization of the supports with primary amine groups either using an organosilane ((3-aminopropyl)trimethoxysilane) or bonding with aminoalcohols (3-amino-1,2-propanediol) were found to significantly improve the interaction between nanocrystals and supports achieving high loadings (>10 wt. %). The bonding method with aminoalcohols guarantees the opportunity to remove the binding molecules thus allowing clean metal/oxide materials to be obtained, which is of great importance in the preparation of supported nanocrystals for heterogeneous catalysis.

Extinction Effect of Gold Nanocatalysts on Photocatalytic Activities under Plasmonic Excitation

Plasmonic nanoparticles (NPs), particularly Au NPs, are potential candidates for photocatalysts because of their unique optical properties. The size of Au NPs plays a crucial role in effective light absorption, which is an important factor in photocatalytic reactions. Although Au NP-based photocatalysts have garnered significant researched interest, the size effect of Au NPs on a photocatalytic reaction has not been sufficiently studied. We characterized the effect of size on the photocatalytic activity of Au NPs of different sizes. We found that the absorption cross-section of the Au NPs gradually increased as the size of the Au NPs increased. However, the reaction rate for each size of NP was inversely proportional to the absorption cross-section. Based on the simulation results, we found that larger Au NPs have a higher scattering factor than that of smaller Au NPs. Consequently, the scattering effect of Au NPs hinders effective light absorption, resulting in slower reaction kinetics. These findings can contribute to the rational design of high-efficiency plasmonic photocatalysts.

An enhanced plasmonic photothermal effect for crystal transformation by a heat-trapping structure

Photothermal utilization is an important approach for sustaining global ecological balance. Due to the enhancement of light absorption through surface plasmon resonance, silver or gold nanostructures can be used as efficient photothermal heat sources in visible and near-infrared regions. Herein, a heat-trapping system of self-assembled gold nanoislands with a thin Al₂O₃ layer is designed to significantly enhance the photothermal effect, which can contribute to a fast crystal transformation. Compared with pure gold nanoislands, an approximately 10-fold enhancement of the photothermal conversion efficiency is observed by using the heat-trapping layer, which results from enhanced light absorption and efficient heat utilization. With the heat-trapping layer, a relatively high and stable photothermal conversion efficiency is realized even at low temperature, and the thermal stability of the plasmonic nanostructure is also observed to improve, especially for silver nanoislands used in air. These results provide a strong additional support for the further development of photothermal applications and offer an efficient pathway for the thermal manipulation of plasmons at the nanoscale.

Spatial distribution of active sites for plasmon-induced chemical reactions triggered by well-defined plasmon modes

Plasmon-induced chemical reactions triggered by near-infrared light illumination might enable efficient photo energy conversion. Here, electrochemical oxidative polymerization of a conductive polymer was conducted on plasmonic photoconversion electrodes. The absolute electrochemical potential of the generated holes was estimated from the redox potentials of the monomers. In addition, well-defined plasmonic structures were examined to better understand the relationship between the excited plasmon mode and spatial distribution of reaction active sites. Rod structures with various lengths had distinct spatial distributions of reaction active sites that depended on the higher plasmon modes, as visualized by Raman measurements.

Sustainable existence of solid mercury (Hg) nanoparticles at room temperature and their applications

Although liquid mercury (Hg) has been known since antiquity, the formation of stable solid nano forms of Hg at room temperature has not been reported so far. Here, for the first time, we report a simple sonochemical route to obtain solid mercury nanoparticles, stabilized by reduced graphene oxide at ambient conditions. The as-formed solid Hg nanoparticles were found to exhibit remarkable rhombohedral morphology and crystallinity at room temperature. Extensive characterization using various physicochemical techniques revealed the unique properties of the solid nanoparticles of Hg compared to its bulk liquid metal phase. Furthermore, the solid nature of the Hg nanoparticles was studied electrochemically, revealing distinctive properties. We believe that solid Hg nanoparticles have the potential for important applications in the fields of electroanalytical chemistry and electrocatalysis.

Fundamentals and applications of photo-thermal catalysis

Photo-thermal catalysis has recently emerged as an alternative route to drive chemical reactions using light as an energy source.

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.

Nano-substructured plasmonic pore arrays: a robust, low cost route to reproducible hierarchical structures extended across macroscopic dimensions

Plasmonic nanostructures are important across diverse applications from sensing to renewable energy. Periodic porous array structures are particularly attractive because such topography offers a means to encapsulate or capture solution phase species and combines both propagating and localised plasmonic modes offering versatile addressability. However, in analytical spectroscopic applications, periodic pore arrays have typically reported weaker plasmonic signal enhancement compared to particulate structures. This may be addressed by introducing additional nano-structuring into the array to promote plasmonic coupling that promotes electric field-enhancement, whilst retaining pore structure. Introducing nanoparticle structures into the pores is a useful means to promote such coupling. However, current approaches rely on either expensive top-down methods or on bottom-up methods that yield random particle placement and distribution. This report describes a low cost, top-down technique for preparation of nano-sub-structured plasmonic pore arrays in a highly reproducible manner that can be applied to build arrays extending over macroscopic areas of mm² to cm². The method exploits oxygen plasma etching, under controlled conditions, of the cavity encapsulated templating polystyrene (PS) spheres used to create the periodic array. Subsequent metal deposition leads to reproducible nano-structuring within the wells of the pore array, coined in-cavity nanoparticles (icNPs). This approach was demonstrated across periodic arrays with pore/sphere diameters ranging from 500 nm to 3 μm and reliably improved the plasmonic properties of the substrate across all array dimensions compared to analogous periodic arrays without the nano-structuring. The enhancement factors achieved for metal enhanced emission and surface enhanced Raman spectroscopy depended on the substrate dimensions, with the best performance achieved for nanostructured 2 μm diameter pore arrays, where a more than 10⁴ improvement over Surface Enhanced Raman Spectroscopy (SERS) and 200-fold improvement over Metal Enhanced Fluorescence (MEF) were observed for these substrates compared with analogous unmodified pore arrays. The experiments were supported by Finite-Difference Time-Domain (FDTD) calculations used to simulate the electric field distribution as a function of pore nano-structuring.

A P/N type silicon semiconductor loaded with silver nanoparticles used as a SERS substrate to selectively drive the coupling reaction induced by surface plasmons

Semiconductor materials are favoured in the field of photocatalysis due to their unique optoelectronic properties. When a semiconductor is excited by external energy, electrons will transition through the band gap, providing electrons or holes for the reaction. This is similar to the chemical enhancement mode of a catalytic reaction initiated by the rough noble metal on the surface excited by plasmon resonance. In this study, different types of semiconductor silicon loaded with silver nanoparticles were used as SERS substrates. SERS detection of p-aminothiophenol (PATP) and p-nitrothiophenol (PNTP) probe molecules was performed using typical surface plasmon-driven coupling reactions, and the mechanism of optical drive charge transfer in semiconductor-metal-molecular systems was investigated. Scanning electron microscopy and plasmon luminescence spectroscopy were used to characterize the silver deposited on the substrate surface. Mapping technology and electrochemistry were used to characterize the photocatalytic reaction of the probe molecules. This study proposed a mechanism for the coupling reaction of "hot electrons" and "hot holes" on the surface of plasmon-driven molecules and provides a method for preparing a stable SERS substrate.

Enhanced Pollutant Adsorption and Regeneration of Layered Double Hydroxide-Based Photoregenerable Adsorbent

Efforts to combine photocatalysts with organic and inorganic adsorbents in engineered composite materials have been pursued extensively to harness sunlight for a green, sustainable regeneration of exhausted adsorbent. Recent advances combining benchmark photocatalyst, titanium dioxide (TiO₂), with an inorganic adsorbent, layered double hydroxides (LDHs), have shown potential for an inorganic adsorbent-photocatalyst system but faced critical limitations in realizing practical applications: low adsorption capacity and slow, inefficient photocatalytic regeneration. This study presents an enhanced TiO₂/LDH based material that demonstrates a dramatically increased efficiency for both decontamination through adsorption and subsequent solar, photocatalytic regeneration. The combination of delamination and high temperature treatment of LDH is utilized to drastically enhance the adsorption capacity toward model contaminant Methyl Orange to 1450-1459 mg/g, which is even higher than most commercial and lab-synthesized carbon-based adsorbents. Light-active plasmonic nanoparticles are employed to increase the photocatalytic regeneration performance, and experimental results show that the synthesized composite material regains above 97% of its adsorption capacity for 5 cycles of regeneration and readsorption. Overall, the results of this study demonstrate potential for the development of inorganic multifunctional adsorbents that can harness a variety of chemical reactions without the loss of adsorptivity over long-term use.

Au-Nanoplasmonics-Mediated Surface Plasmon-Enhanced GaN Nanostructured UV Photodetectors

The nanoplasmonic impact of chemically synthesized Au nanoparticles (Au NPs) on the performance of GaN nanostructure-based ultraviolet (UV) photodetectors is analyzed. The devices with uniformly distributed Au NPs on GaN nanostructures (nanoislands and nanoflowers) prominently respond toward UV illumination (325 nm) in both self-powered as well as photoconductive modes of operation and have shown fast and stable time-correlated response with significant enhancement in the performance parameters. A comprehensive analysis of the device design, laser power, and bias-dependent responsivity and response time is presented. The fabricated Au NP/GaN nanoflower-based device yields the highest photoresponsivity of ∼ 380 mA/W, detectivity of ∼ 10¹⁰ jones, reduced noise equivalent power of ∼ 5.5 × 10^-13 W Hz^-1/2, quantum efficiency of ∼ 145%, and fast response/recovery time of ∼40 ms. The report illustrates the mechanism where light interacts with the chemically synthesized nanoparticles guided by the surface plasmon to effectively enhance the device performance. It is observed that the Au NP-stimulated local surface plasmon resonance effect and reduced channel resistance contribute to the augmented performance of the devices. Further, the decoration of low-dimensional Au NPs on GaN nanostructures acts as a detection enhancer with a fast recovery time and paves the way toward the realization of energy-efficient optoelectronic device applications.

Critical Coupling of Visible Light Extends Hot-Electron Lifetimes for H2O2 Synthesis

Devices driven by above-equilibrium "hot" electrons are appealing for photocatalytic technologies, such as in situ H₂O₂ synthesis, but currently suffer from low (<1%) overall quantum efficiencies. Gold nanostructures excited by visible light generate hot electrons that can inject into a neighboring semiconductor to drive electrochemical reactions. Here, we designed and studied a metal-insulator-metal (MIM) structure of Au nanoparticles on a ZnO/TiO₂/Al film stack, deposited through room-temperature, lithography-free methods. Light absorption, electron injection efficiency, and photocatalytic yield in this device are superior in comparison to the same stack without Al. Our device absorbs >60% of light at the Au localized surface plasmon resonance (LSPR) peak near 530 nm-a 5-fold enhancement in Au absorption due to critical coupling to an Al film. Furthermore, we show through ultrafast pump-probe spectroscopy that the Al-coupled samples exhibit a nearly 5-fold improvement in hot-electron injection efficiency as compared to a non-Al device, with the hot-electron lifetimes extending to >2 ps in devices photoexcited with fluence of 0.1 mJ cm^-2. The use of an Al film also enhances the photocatalytic yield of H₂O₂ more than 3-fold in a visible-light-driven reactor. Altogether, we show that the critical coupling of Al films to Au nanoparticles is a low-cost, lithography-free method for improving visible-light capture, extending hot-carrier lifetimes, and ultimately increasing the rate of in situ H₂O₂ generation.

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.

Optical trapping reveals differences in dielectric and optical properties of copper nanoparticles compared to their oxides and ferrites

In a nanoplasmonic context, copper (Cu) is a potential and interesting surrogate to less accessible metals such as gold, silver or platinum. We demonstrate optical trapping of individual Cu nanoparticles with diameters between 25 and 70 nm and of two ionic Cu nanoparticle species, CuFe₂O₄ and CuZnFe₂O₄, with diameters of 90 nm using a near infrared laser and quantify their interaction with the electromagnetic field experimentally and theoretically. We find that, despite the similarity in size, the trapping stiffness and polarizability of the ferrites are significantly lower than those of Cu nanoparticles, thus inferring a different light-particle interaction. One challenge with using Cu nanoparticles in practice is that upon exposure to the normal atmosphere, Cu is spontaneously passivated by an oxide layer, thus altering its physicochemical properties. We theoretically investigate how the presence of an oxide layer influences the optical properties of Cu nanoparticles. Comparisons to experimental observations infer that oxidation of CuNPs is minimal during optical trapping. By finite element modelling we map out the expected temperature increase of the plasmonic Cu nanoparticles during optical trapping and retrieve temperature increases high enough to change the catalytic properties of the particles.

Visible-light-induced photoxidation-Povarov cascade reaction: synthesis of 2-arylquinoline through alcohol and N-benzylanilines under mild conditions via Ag/g-C3N4 nanometric semiconductor catalyst

Ag/g-C₃N₄ nanometric semiconductor catalyzed cascade reaction for the synthesis of 2-arylquinoline through alcohol and N-benzylanilines under visible light irradiation.

AgNPs encapsulated by an amine-functionalized polymer nanocatalyst for CO2fixation as a carboxylic acid and the oxidation of cyclohexane under ambient conditions

A novel catalyst comprising Ag NPs grafted to a porous polystyrene material was synthesized for the production of valuable propiolic acid derivativesviaCO₂(1 atm) incorporation, and the oxidation of cyclohexane under ambient reaction conditions.

Plasmon-Induced Dimerization of Thiazolidine-2,4-dione on Silver Nanoparticles: Revealed by Surface-Enhanced Raman Scattering Study

Surface-enhanced Raman scattering (SERS) study carried on thiazolidine-2,4-dione (TZD), a pharmacologically active heterocyclic compound, points to the presence of TZD dimer formed by plasmon-induced dimerization reaction of TZD on the surface of silver nanoparticles (Ag NP) at TZD concentrations of 10^-3 M and above. The evidence for the presence of dimer was obtained from the appearance of a prominent band at 1566 cm^-1 corresponding to the ν(C═C) band (a characteristic vibrational band observed for the Knoevenagel condensation reaction products) which is absent in the normal Raman scattering (NRS) spectra of TZD solid/solution. The observed spectrum compares well with the calculated spectrum of dimer obtained using density functional theory (DFT) calculations. The dimerization reaction is plausibly induced by the transfer of hot electrons generated by the nonradiative plasmon decay of Ag NP, and the proposed reaction mechanism is discussed. However, at lower concentrations (10^-4-10^-6 M), the characteristic dimer peak (1566 cm^-1) is absent and the SERS spectra resemble more the NRS spectrum of TZD with a few changes. The spectral analysis supported by DFT calculations showed that TZD molecules undergo deprotonation and get adsorbed on the Ag NP surface as enolate forms. The proximity of TZD molecules on the surface of Ag NP is a necessary factor for the dimerization to occur. At lower concentrations, most molecules lie apart and reactions between molecules become less feasible, and they remain as monomers on the surface, while at higher concentrations the molecules are closer to each other on the Ag NP surface favoring the dimerization reaction to take place, leading to the formation of the dimer.

Scanning probe microscopy for real-space observations of local chemical reactions induced by a localized surface plasmon

Localised surface plasmon (LSP) resonance has attracted considerable attention in recent years as an efficient driving force for chemical reactions. The chemical reactions induced by LSP are classified into two types, namely, redox reactions based on plasmon-induced charge separation (PICS) and chemical reactions induced by the direct interaction between LSP and molecules (plasmon-induced chemical reactions). Although both types of reactions have been extensively studied, the mechanisms of PICS and plasmon-induced chemical reactions remain unexplained and controversial because conventional macroscopic methods can hardly grasp the local chemical reactions induced by LSP. In order to obtain mechanistic insights, nanoscale observations and investigations are necessary. Scanning probe microscopy (SPM) is a powerful experimental tool to investigate not only the surface morphology but also the physical and chemical properties of samples at a high spatial resolution. In this perspective review, we first explain SPM combined with optical excitation, and then, review the recent studies using SPM techniques for real-space observations of the chemical reactions induced by LSP.

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.

Plasmon-Driven Rapid In Situ Formation of Luminescence Single Crystal Nanoparticle

Single crystal nanomaterials are very important for the fundamental investigation and application of luminescence. However, a very critical growth condition or high temperature treatment is always required for their preparation. Here, an easy and rapid in situ achievement of a single crystal luminescent material is realized by taking advantage of plasmon-induced thermal and catalysis effects. With the assistance of localized surface plasmon resonance of Au nanoparticles, polycrystalline NaYF₄ transforms to single crystal Y₂ O₃ in tens of milliseconds, resulting in remarkable improvement of luminescence emission. It is important to point out that the single crystal transformation is also achieved even at a very low temperature, which is impossible with conventional approaches. Such a convenient and efficient plasmon assisted scheme provides a new technology for the rapid achievement of single crystal materials and extends the application of surface plasmon to a much broader field.