Hot Hole Collection and Photoelectrochemical CO2 Reduction with Plasmonic Au/p-GaN Photocathodes

Oxidizing Role of Cu Cocatalysts in Unassisted Photocatalytic CO2 Reduction Using p-GaN/Al2O3/Au/Cu Heterostructures

Photocatalytic CO2 reduction to CO under unassisted (unbiased) conditions was recently demonstrated using heterostructure catalysts that combine p-type GaN with plasmonic Au nanoparticles and Cu nanoparticles as cocatalysts (p-GaN/Al2O3/Au/Cu). Here, we investigate the mechanistic role of Cu in p-GaN/Al2O3/Au/Cu under unassisted photocatalytic operating conditions using Cu K-edge X-ray absorption spectroscopy and first-principles calculations. Upon exposure to gas-phase CO2 and H2O vapor reaction conditions, the composition of the Cu nanoparticles is identified as a mixture of CuI and CuII oxide, hydroxide, and carbonate compounds without metallic Cu. These composition changes, indicating oxidative conditions, are rationalized by bulk Pourbaix thermodynamics. Under photocatalytic operating conditions with visible light excitation of the plasmonic Au nanoparticles, further oxidation of CuI to CuII is observed, indicating light-driven hole transfer from Au-to-Cu. This observation is supported by the calculated band alignments of the oxidized Cu compositions with plasmonic Au particles, where light-driven hole transfer from Au-to-Cu is found to be thermodynamically favored. These findings demonstrate that under unassisted (unbiased) gas-phase reaction conditions, Cu is found in carbonate-rich oxidized compositions rather than metallic Cu. These species then act as the active cocatalyst and play an oxidative rather than a reductive role in catalysis when coupled with plasmonic Au particles for light absorption, possibly opening an additional channel for water oxidation in this system.

Solar-Driven Sustainability: III-V Semiconductor for Green Energy Production Technologies

Long-term societal prosperity depends on addressing the world's energy and environmental problems, and photocatalysis has emerged as a viable remedy. Improving the efficiency of photocatalytic processes is fundamentally achieved by optimizing the effective utilization of solar energy and enhancing the efficient separation of photogenerated charges. It has been demonstrated that the fabrication of III-V semiconductor-based photocatalysts is effective in increasing solar light absorption, long-term stability, large-scale production and promoting charge transfer. This focused review explores on the current developments in III-V semiconductor materials for solar-powered photocatalytic systems. The review explores on various subjects, including the advancement of III-V semiconductors, photocatalytic mechanisms, and their uses in H2 conversion, CO2 reduction, environmental remediation, and photocatalytic oxidation and reduction reactions. In order to design heterostructures, the review delves into basic concepts including solar light absorption and effective charge separation. It also highlights significant advancements in green energy systems for water splitting, emphasizing the significance of establishing eco-friendly systems for CO2 reduction and hydrogen production. The main purpose is to produce hydrogen through sustainable and ecologically friendly energy conversion. The review intends to foster the development of greener and more sustainable energy source by encouraging researchers and developers to focus on practical applications and advancements in solar-powered photocatalysis.

Potential of Molecular Catalysts with Electron-Rich Transition Metal Centers for Addressing Long-Standing Chemistry Enigmas

Molecular complexes with electron-rich metal centers are highlighted as potential catalysts for the following five important chemical transformations: selective conversion of methane to methanol, capture and utilization of carbon dioxide, fixation of molecular nitrogen, water splitting, and recycling of perfluorochemicals. Our initial focus lies on negatively charged metal centers and ligands that can stabilize anionic metal atoms. Catalysts with electron-rich metal atoms (CERMAs) can sustain catalytic cycles with a "ping-pong" mechanism, where one or more electrons are transferred from the metal center to the substrate and back. The donated electrons can activate the chemical bonds of the substrate by populating its antibonding orbitals. At the last step of the catalytic cycle, the electrons return to the metal and the product interacts only weakly with the formed anion, which enables the solvent molecules to remove the product fast from the catalytic cycle and prevent subsequent unfavorable reactions. This process resembles electrocatalysis, but the metal serves as both an anode and a cathode (molecular electrocatalysis). We also analyze the usage of CERMAs as the base of Frustrated Lewis pairs proposing a new type of bimetallic catalysts. This Featured Article aspires to initiate systematic experimental and theoretical studies on CERMAs and their reactivity, the potential of which has probably been underestimated in the literature.

Catalyst-On-Hotspot Nanoarchitecture: Plasmonic Focusing of Light onto Co-Photocatalyst for Efficient Light-To-Chemical Transformation

Plasmon-mediated catalysis utilizing hybrid photocatalytic ensembles promises effective light-to-chemical transformation, but current approaches suffer from weak electromagnetic field enhancements from polycrystalline and isotropic plasmonic nanoparticles as well as poor utilization of precious co-catalyst. Here, efficient plasmon-mediated catalysis is achieved by introducing a unique catalyst-on-hotspot nanoarchitecture obtained through the strategic positioning of co-photocatalyst onto plasmonic hotspots to concentrate light energy directly at the point-of-reaction. Using environmental remediation as a proof-of-concept application, the catalyst-on-hotspot design (edge-AgOcta@Cu2O) enhances photocatalytic advanced oxidation processes to achieve superior organic-pollutant degradation at ≈81% albeit having lesser Cu2O co-photocatalyst than the fully deposited design (full-AgOcta@Cu2O). Mass-normalized rate constants of edge-AgOcta@Cu2O reveal up to 20-fold and 3-fold more efficient utilization of Cu2O and Ag nanoparticles, respectively, compared to full-AgOcta@Cu2O and standalone catalysts. Moreover, this design also exhibits catalytic performance >4-fold better than emerging hybrid photocatalytic platforms. Mechanistic studies unveil that the light-concentrating effect facilitated by the dense electromagnetic hotspots is crucial to promote the generation and utilization of energetic photocarriers for enhanced catalysis. By enabling the plasmonic focusing of light onto co-photocatalyst at the single-particle level, the unprecedented design offers valuable insights in enhancing light-driven chemical reactions and realizing efficient energy/catalyst utilizations for diverse chemical, environmental, and energy applications.

Raman Monitoring of the Electro-Optical Synergy-Induced Enhancements in Carbon-Bromine Bond Cleavage, Reaction Rate, and Product Selectivity of p-Bromothiophenol

Electro-optical synergy has recently been targeted to improve the separation of hot carriers and thereby further improve the efficiency of plasmon-mediated chemical reactions (PMCRs). However, the electro-optical synergy in PMCRs needs to be more deeply understood, and its contribution to bond dissociation and product selectivity needs to be clarified. Herein, the electro-optical synergy in plasmon-mediated reduction of p-bromothiophenol (PBTP) was studied on a plasmonic nanostructured silver electrode using in situ Raman spectroscopy and theoretical calculations. It was found that the electro-optical synergy-induced enhancements in the cleavage of carbon-bromine bonds, reaction rate, and product selectivity (4,4'-biphenyl dithiol vs thiophenol) were largely affected by the applied bias, laser wavelength, and laser power. The theoretical simulation further clarified that the strong electro-optical synergy is attributed to the matching of energy band diagrams of the plasmonic silver with those of the adsorbed PBTP molecules. A deep understanding of the electro-optical synergy in PBTP reduction and the clarification of the mechanism will be highly beneficial for the development of other highly efficient PMCRs.

Ab initio investigation of hot electron transfer in CO2 plasmonic photocatalysis in the presence of hydroxyl adsorbate

Photoreduction of carbon dioxide (CO2) on plasmonic structures is of great interest in photocatalysis to aid selectivity. While species commonly found in reaction environments and associated intermediates can steer the reaction down different pathways by altering the potential energy landscape of the system, they are often not addressed when designing efficient plasmonic catalysts. Here, we perform an atomistic study of the effect of the hydroxyl group (OH) on CO2 activation and hot electron generation and transfer using first-principles calculations. We show that the presence of OH is essential in breaking the linear symmetry of CO2, which leads to a charge redistribution and a decrease in the OCO angle to 134°, thereby activating CO2. Analysis of the partial density of states (pDOS) demonstrates that the OH group mediates the orbital hybridization between Au and CO2 resulting in more accessible states, thus facilitating charge transfer. By employing time-dependent density functional theory (TDDFT), we quantify the fraction of hot electrons directly generated into hybridized molecular states at resonance, demonstrating a broader energy distribution and an 11% increase in charge-transfer in the presence of OH groups. We further show that the spectral overlap between excitation energy and plasmon resonance plays a critical role in efficiently modulating electron transfer processes. These findings contribute to the mechanistic understanding of plasmon-mediated reactions and demonstrate the importance of co-adsorbed species in tailoring the electron transfer processes, opening new avenues for enhancing selectivity.

Hot carrier creation in a nanoparticle dimer-molecule composite

Light-matter interactions have garnered considerable interest owing to their burgeoning applications in quantum optics and plasmonics. Utilizing first principles calculations, this work explores the hot carrier (HC) generation and distribution within a composite system made up of a plasmonic nanoparticle dimer and linear polycyclic aromatic hydrocarbon (PAH) molecules. We examine the spatial and energetic distributions of HCs by initiating photoexcitation and allowing localized surface plasmon dephasing. By positioning PAH molecules within the plasmonic nanodimer's gap region, our investigation uncovers HC tuning. Notably, depending on the size of the PAH molecules, there are significant alterations in the HC distribution. Hot electrons (HEs) are distributed across both the nanodimer and the PAH molecule, while hot holes (HHs) become entirely localized on the PAH as the PAH grows larger. These findings improve our understanding of plasmon-molecule coupled states and provide guidance on how to customize HC distributions through the creation of hybrid plasmonic materials.

The impact of a dopant atom on the distribution of hot electrons and holes in Au-doped Ag nano-clusters

The generation of hot carriers (HCs) through the excitation of localized surface plasmon resonance (LSPR) in metal nanostructures is a fascinating phenomenon that fuels both fundamental and applied research. In this study, we employ first principles real-time time-dependent density-functional theory (rt-TDDFT) calculations to elucidate the creation and distribution of HCs within Au-doped Ag nanoclusters: Ag11Cl3P7H21, Ag10AucoreCl3P7H21, and Ag10AusurfCl3P7H21 nanoclusters. Our findings indicate that adjustments in HC distribution are achievable through the Au dopant atom, and precise control of HC distribution is possible by manipulating the location of the Au dopant atom. When employing a Gaussian laser pulse tailored to match the LSPR frequency, a substantial accumulation of HCs in the Ag-P bond is observed. This finding suggests a weakening of the Ag-P bonds and, consequently, the initiation of bond stretching. We propose that these findings open up possibilities for tuning HCs in Au-doped chemically functionalized Ag nanoclusters.

Au-decorated Sb2Se3 photocathodes for solar-driven CO2 reduction

Photoelectrodes with FTO/Au/Sb2Se3/TiO2/Au architecture were studied in photoelectrochemical CO2 reduction reaction (PEC CO2RR). The preparation is based on a simple spin coating technique, where nanorod-like structures were obtained for Sb2Se3, as confirmed by SEM images. A thin conformal layer of TiO2 was coated on the Sb2Se3 nanorods via ALD, which acted as both an electron transfer layer and a protective coating. Au nanoparticles were deposited as co-catalysts via photo-assisted electrodeposition at different applied potentials to control their growth and morphology. The use of such architectures has not been explored in CO2RR yet. The photoelectrochemical performance for CO2RR was investigated with different Au catalyst loadings. A photocurrent density of ∼7.5 mA cm-2 at -0.57 V vs. RHE for syngas generation was achieved, with an average Faradaic efficiency of 25 ± 6% for CO and 63 ± 12% for H2. The presented results point toward the use of Sb2Se3-based photoelectrodes in solar CO2 conversion applications.

Tailoring Hot-Carrier Distributions of Plasmonic Nanostructures through Surface Alloying

Alloyed metal nanoparticles are a promising platform for plasmonically enabled hot-carrier generation, which can be used to drive photochemical reactions. Although the non-plasmonic component in these systems has been investigated for its potential to enhance catalytic activity, its capacity to affect the photochemical process favorably has been underexplored by comparison. Here, we study the impact of surface alloy species and concentration on hot-carrier generation in Ag nanoparticles. By first-principles simulations, we photoexcite the localized surface plasmon, allow it to dephase, and calculate spatially and energetically resolved hot-carrier distributions. We show that the presence of non-noble species in the topmost surface layer drastically enhances hot-hole generation at the surface at the expense of hot-hole generation in the bulk, due to the additional d-type states that are introduced to the surface. The energy of the generated holes can be tuned by choice of the alloyant, with systematic trends across the d-band block. Already low surface alloy concentrations have a large impact, with a saturation of the enhancement effect typically close to 75% of a monolayer. Hot-electron generation at the surface is hindered slightly by alloying, but here a judicious choice of the alloy composition allows one to strike a balance between hot electrons and holes. Our work underscores the promise of utilizing multicomponent nanoparticles to achieve enhanced control over plasmonic catalysis and provides guidelines for how hot-carrier distributions can be tailored by designing the electronic structure of the surface through alloying.

Quantitative study on the photoemission of AlGaN nanoarrays based on the three-dimensional transportation within a four-step process

Photocathodes play a crucial role in photoelectronic imaging and vacuum electronic devices. The quantum efficiency of photocathodes, which determines their performance, can be enhanced through materials engineering. However, the quantum efficiency of conventional planar photocathodes remains consistently low, at around 25%. In this paper, we propose what we believe is a novel structure of AlGaN nanowire array to address this issue. We investigate the photoemission characteristics of the nanowire array using the "four-step" process, which takes into account optical absorption, electron transportation, electron emission, and electron collection. We compare the quantum efficiency of nanowire arrays with different structure sizes and Al components. After studying the effect of incident light at various angles on the nanowire array photocathode, we identify the optimal dimensional parameters: a height of 400∼500 nm and a wire width of 200∼300 nm. Furthermore, we improved the collection efficiency of the photocathode by introducing a built-in/external electric field, and obtained a 104.4% enhancement of the collection current with the built-in electric field, meanwhile the photocurrent was increased by 87% compared to the case without the external electric field. These findings demonstrate the potential of optimizing photocathode performance through the development of a novel model and adjustment of parameters, offering a promising approach for photocathode applications.

Molecular Additives Improve the Selectivity of CO2 Photoelectrochemical Reduction over Gold Nanoparticles on Gallium Nitride

Photoelectrochemical CO2 reduction (CO2R) is an appealing solution for converting carbon dioxide into higher-value products. However, CO2R in aqueous electrolytes suffers from poor selectivity due to the competitive hydrogen evolution reaction that is dominant on semiconductor surfaces in aqueous electrolytes. We demonstrate that functionalizing gold/p-type gallium nitride devices with a film derived from diphenyliodonium triflate suppresses hydrogen generation from 90% to 18%. As a result, we observe increases in the Faradaic efficiency and partial current density for carbon monoxide of 50% and 3-fold, respectively. Furthermore, we demonstrate through optical absorption measurements that the molecular film employed herein, regardless of thickness, does not affect the photocathode's light absorption. Altogether, this study provides a rigorous platform for elucidating the catalytic structure-property relationships to enable engineering of active, stable, and selective materials for photoelectrochemical CO2R.

Ultrafast hot-carrier dynamics in ultrathin monocrystalline gold

Applications in photodetection, photochemistry, and active metamaterials and metasurfaces require fundamental understanding of ultrafast nonthermal and thermal electron processes in metallic nanosystems. Significant progress has been recently achieved in synthesis and investigation of low-loss monocrystalline gold, opening up opportunities for its use in ultrathin nanophotonic architectures. Here, we reveal fundamental differences in hot-electron thermalisation dynamics between monocrystalline and polycrystalline ultrathin (down to 10 nm thickness) gold films. Comparison of weak and strong excitation regimes showcases a counterintuitive unique interplay between thermalised and non-thermalised electron dynamics in mesoscopic gold with the important influence of the X-point interband transitions on the intraband electron relaxation. We also experimentally demonstrate the effect of hot-electron transfer into a substrate and the substrate thermal properties on electron-electron and electron-phonon scattering in ultrathin films. The hot-electron injection efficiency from monocrystalline gold into TiO2, approaching 9% is measured, close to the theoretical limit. These experimental and modelling results reveal the important role of crystallinity and interfaces on the microscopic electronic processes important in numerous applications.

Boosting the Plasmon-Mediated Electrochemical Oxidation of p-Aminothiophenol with p-Hydroxythiophenol as Molecular Cocatalyst

Plasmon-mediated electrochemistry is an emerging area of interest in which the electrochemical reactions are enhanced by employing metal nanostructures possessing localized surface plasmon resonance (LSPR). However, the reaction efficacy is still far below its theoretical limit due to the ultrafast relaxation of LSPR-generated hot carriers. Herein, we introduce p-hydroxythiophenol (PHTP) as a molecular cocatalyst to significantly improve the reaction efficacy in plasmon-mediated electrochemical oxidation of p-aminothiophenol (PATP) on gold nanoparticles. Using electrochemical techniques, in situ Raman spectroscopy, and theoretical calculations, we elucidate that the presence of PHTP improves the hot hole-mediated electrochemical oxidation of PATP by 2-fold through the trapping of plasmon-mediated hot electrons. In addition, the selectivity of PATP oxidation could also be modulated by the introduction of PHTP cocatalyst. This tactic of employing molecular cocatalyst can be drawn out to endorse various plasmonic electrochemical reactions because of its simple protocol, high efficiency, and high selectivity.

Dual-Plasmon Resonance Coupling Promoting Directional Photosynthesis of Nitrate from Air

Photocatalysis, particularly plasmon-mediated photocatalysis, offers a green and sustainable approach for direct nitrogen oxidation into nitrate under ambient conditions. However, the unsatisfactory photocatalytic efficiency caused by the limited localized electromagnetic field enhancement and short hot carrier lifetime of traditional plasmonic catalysts is a stumbling block to the large-scale application of plasmon photocatalytic technology. Herein, we design and demonstrate the dual-plasmonic heterojunction (Bi/Csx WO3 ) achieves efficient and selective photocatalytic N2 oxidation. The yield of NO3 - over Bi/Csx WO3 (694.32 μg g-1  h-1 ) are 2.4 times that over Csx WO3 (292.12 μg g-1  h-1 ) under full-spectrum irradiation. The surface dual-plasmon resonance coupling effect generates a surge of localized electromagnetic field intensity to boost the formation efficiency and delay the self-thermalization of energetic hot carriers. Ultimately, electrons participate in the formation of ⋅O2 - , while holes involve in the generation of ⋅OH and the activation of N2 . The synergistic effect of multiple reactive oxygen species drives the direct photosynthesis of NO3 - , which achieves the overall-utilization of photoexcited electrons and holes in photocatalytic reaction. The concept that the dual-plasmon resonance coupling effect facilitates the directional overall-utilization of photoexcited carriers will pave a new way for the rational design of efficient photocatalytic systems.

Origin of Cycle Performance in the Ag2O/TiO2 Heterostructure Photocatalyst

In recent years, many studies on photocatalysis focused on improving efficiency. However, the cycle performance is also an important index for industrialization. Here, an Ag2O/TiO2 heterostructure photocatalyst is prepared for continuous photodegradation of methylene blue (MB) under visible light, and the samples after the first and fifth round reactions are recycled to study the microstructure evolution of the photocatalyst. The results show that the performance is obviously improved in the second round and remains stable in the following reaction round. Due to the charge transfer, Ag2O/TiO2 gradually changes to Ag2O@Ag-TiO2-x during the photocatalytic reaction. The resulting localized surface plasmon resonance effect and the change of the interface structure greatly increase the number of carriers and prolong the lifetime of carriers. Such variations of microstructures and photoelectric properties of the samples due to the charge transfer and redox reaction on the surface of the photocatalyst dominate the cycle performance.

Transport and Interfacial Injection of d-Band Hot Holes Control Plasmonic Chemistry

Harnessing nonequilibrium hot carriers from plasmonic metal nanostructures constitutes a vibrant research field with the potential to control photochemical reactions, particularly for solar fuel generation. However, a comprehensive understanding of the interplay of plasmonic hot-carrier-driven processes in metal/semiconducting heterostructures has remained elusive. In this work, we reveal the complex interdependence among plasmon excitation, hot-carrier generation, transport, and interfacial collection in plasmonic photocatalytic devices, uniquely determining the charge injection efficiency at the solid/liquid interface. Measuring the internal quantum efficiency of ultrathin (14-33 nm) single-crystalline plasmonic gold (Au) nanoantenna arrays on titanium dioxide substrates, we find that the performance of the device is limited by hot hole collection at the metal/electrolyte interface. Our solid- and liquid-state experimental approach, combined with ab initio simulations, demonstrates more efficient collection of high-energy d-band holes traveling in the [111] orientation, enhancing oxidation reactions on {111} surfaces. These findings establish new guidelines for optimizing plasmonic photocatalytic systems and optoelectronic devices.

Exceptional Spatial Variation of Charge Injection Energies on Plasmonic Surfaces

Charge injection into a molecule on a metallic interface is a key step in many photoactivated reactions. We employ the many-body perturbation theory and compute the hole and electron injection energies for CO2 molecule on an Au nanoparticle with ∼3,000 electrons and compare it to results for idealized infinite surfaces. We demonstrate a surprisingly large variation of the injection energy barrier depending on the precise molecular position on the surface. Multiple "hot spots," characterized by low energy barriers, arise due to the competition between the plasmonic coupling and the degree of hybridization between the molecule and the substrate. The charge injection barrier to the adsorbate on the nanoparticle surface decreases from the facet edge to the facet center. This finding contrasts with the typical picture in which the electric field enhancement on the nanoparticle edges is considered the most critical factor.

Nonclassical Mechanism of Metal-Enhanced Photoluminescence of Quantum Dots

Metal-enhanced photoluminescence is able to provide a robust signal even from a single emitter and is promising in applications in biosensors and optoelectronic devices. However, its realization with semiconductor nanocrystals (e.g., quantum dots, QDs) is not always straightforward due to the hidden and not fully described interactions between plasmonic nanoparticles and an emitter. Here, we demonstrate nonclassical enhancement (i.e., not a conventional electromagnetic mechanism) of the QD photoluminescence at nonplasmonic conditions and correlate it with the charge exchange processes in the system, particularly with high efficiency of the hot-hole generation in gold nanoparticles and the possibility of their transfer to QDs. The hole injection returns a QD from a charged nonemitting state caused by hole trapping by surface and/or interfacial traps into an uncharged emitting state, which leads to an increased photoluminescence intensity. These results open new insights into metal-enhanced photoluminescence, showing the importance of the QD surface states in this process.

Hot-Electron Dynamics Mediated Medical Diagnosis and Therapy

Surface plasmon resonance excitation significantly enhances the absorption of light and increases the generation of "hot" electrons, i.e., conducting electrons that are raised from their steady states to excited states. These excited electrons rapidly decay and equilibrate via radiative and nonradiative damping over several hundred femtoseconds. During the hot-electron dynamics, from their generation to the ultimate nonradiative decay, the electromagnetic field enhancement, hot electron density increase, and local heating effect are sequentially induced. Over the past decade, these physical phenomena have attracted considerable attention in the biomedical field, e.g., the rapid and accurate identification of biomolecules, precise synthesis and release of drugs, and elimination of tumors. This review highlights the recent developments in the application of hot-electron dynamics in medical diagnosis and therapy, particularly fully integrated device techniques with good application prospects. In addition, we discuss the latest experimental and theoretical studies of underlying mechanisms. From a practical standpoint, the pioneering modeling analyses and quantitative measurements in the extreme near field are summarized to illustrate the quantification of hot-electron dynamics. Finally, the prospects and remaining challenges associated with biomedical engineering based on hot-electron dynamics are presented.

Promoting plasmonic photocatalysis with ligand-induced charge separation under interband excitation

Plasmonic nanoparticles have been demonstrated to enhance photocatalysis due to their strong photon absorption and efficient hot-carrier generation. However, plasmonic photocatalysts suffer from a short lifetime of plasmon-generated hot carriers that decay through internal relaxation pathways before being harnessed for chemical reactions. Here, we demonstrate the enhanced photocatalytic reduction of gold ions on gold nanorods functionalized with polyvinylpyrrolidone. The catalytic activities of the reaction are quantified by in situ monitoring of the spectral evolution of single nanorods using a dark-field scattering microscope. We observe a 13-fold increase in the reduction rate with the excitation of d-sp interband transition compared to dark conditions, and a negligible increase in the reduction rate when excited with intraband transition. The hole scavenger only plays a minor role in the photocatalytic reduction reaction. We attribute the enhanced photocatalysis to an efficient charge separation at the gold-polyvinylpyrrolidone interface, where photogenerated d-band holes at gold transfer to the HOMO of polyvinylpyrrolidone, leading to the prolonged lifetime of the electrons that subsequently reduce gold ions to gold atoms. These results provide new insight into the design of plasmonic photocatalysts with capping ligands.

Stability of Photocathodes: A Review on Principles, Design, and Strategies

Photoelectrochemical devices based on semiconductor photoelectrode can directly convert and store solar energy into chemical fuels. Although the efficient photoelectrodes with commercially valuable solar-to-fuel energy conversion efficiency have been reported over past decades, one of the most enormous challenges is the stability of the photoelectrode due to corrosion during operation. Thus, it is of paramount importance for developing a stable photoelectrode to deploy solar-fuel production. This Review commences with a fundamental understanding of thermodynamics for photoelectrochemical reactions and the fundamentals of photocathodes. Then, the commercial application of photoelectrochemical technology is prospected. We specifically focus on recent strategies for designing photocathodes with long-term stability, including energy band alignment, hole transport/storage/blocking layer, spatial decoupling, grafting molecular catalysts, protective/passivation layer, surface element reconstruction, and solvent effects. Based on the insights gained from these effective strategies, we propose an outlook of key aspects that address the challenges for development of stable photoelectrodes in future work.

d-Band Hole Dynamics in Gold Nanoparticles Measured with Time-Resolved Emission Upconversion Microscopy

The performance of photocatalysts and photovoltaic devices can be enhanced by energetic charge carriers produced from plasmon decay, and the lifetime of these energetic carriers greatly affects overall efficiencies. Although hot electron lifetimes in plasmonic gold nanoparticles have been investigated, hot hole lifetimes have not been as thoroughly studied in plasmonic systems. Here, we demonstrate time-resolved emission upconversion microscopy and use it to resolve the lifetime and energy-dependent cooling of d-band holes formed in gold nanoparticles by plasmon excitation and by following plasmon decay into interband and then intraband electron-hole pairs.

Elucidating the Origin of Plasmon-Generated Hot Holes in Water Oxidation

Plasmon-generated hot electrons in metal/oxide heterostructures have been used extensively for driving photochemistry. However, little is known about the origin of plasmon-generated hot holes in promoting photochemical reactions. Herein, we discover that, during the nonradiative plasmon decay, the interband excitation rather than the intraband excitation generates energetic hot holes that enable to drive the water oxidation at the Au/TiO₂ interface. Distinct from lukewarm holes via the intraband excitation that only remain on Au, hot holes from the interband excitation are found to be transferred from Au into TiO₂ and stabilized by surface oxygen atoms on TiO₂, making them available to oxidize adsorbed water molecules. Taken together, our studies provide spectroscopic evidence to clarify the photophysical process for exciting plasmon-generated hot holes, unravel their atomic-level accumulation sites to maintain the strong oxidizing power in metal/oxide heterostructures, and affirm their crucial functions in governing photocatalytic oxidation reactions.

Engineering the Au-Cu2 O Crystalline Interfaces for Structural and Catalytic Integration

Precise structural control has attracted tremendous interest in pursuit of the tailoring of physical properties. Here, this work shows that through strong ligand-mediated interfacial energy control, Au-Cu₂ O dumbbell structures where both the Au nanorod (AuNR) and the partially encapsulating Cu₂ O domains are highly crystalline. The synthetic advance allows physical separation of the Au and Cu₂ O domains, in addition to the use of long nanorods with tunable absorption wavelength, and the crystalline Cu₂ O domain with well-defined facets. The interplay of plasmon and Schottky effects boosts the photocatalytic performance in the model photodegradation of methyl orange, showing superior catalytic efficiency than the AuNR@Cu₂ O core-shell structures. In addition, compared to the typical core-shell structures, the AuNR-Cu₂ O dumbbells can effectively electrochemically catalyze the CO₂ to C₂₊ products (ethanol and ethylene) via a cascade reaction pathway. The excellent dual function of both photo- and electrocatalysis can be attributed to the fine physical separation of the crystalline Au and Cu₂ O domains.

Active Site Engineering on Plasmonic Nanostructures for Efficient Photocatalysis

Plasmonic nanostructures have shown immense potential in photocatalysis because of their distinct photochemical properties associated with tunable photoresponses and strong light-matter interactions. The introduction of highly active sites is essential to fully exploit the potential of plasmonic nanostructures in photocatalysis, considering the inferior intrinsic activities of typical plasmonic metals. This review focuses on active site-engineered plasmonic nanostructures with enhanced photocatalytic performance, wherein the active sites are classified into four types (i.e., metallic sites, defect sites, ligand-grafted sites, and interface sites). The synergy between active sites and plasmonic nanostructures in photocatalysis is discussed in detail after briefly introducing the material synthesis and characterization methods. Active sites can promote the coupling of solar energy harvested by plasmonic metal to catalytic reactions in the form of local electromagnetic fields, hot carriers, and photothermal heating. Moreover, efficient energy coupling potentially regulates the reaction pathway by facilitating the excited state formation of reactants, changing the status of active sites, and creating additional active sites using photoexcited plasmonic metals. Afterward, the application of active site-engineered plasmonic nanostructures in emerging photocatalytic reactions is summarized. Finally, a summary and perspective of the existing challenges and future opportunities are presented. This review aims to deliver some insights into plasmonic photocatalysis from the perspective of active sites, expediting the discovery of high-performance plasmonic photocatalysts.

Design and synthesis of metal-free ethene-based covalent organic framework photocatalysts for efficient, selective, and long-term stable CO2 conversion into methane

The efficient and selective photocatalytic CO₂ conversion into higher-valued hydrocarbon products (e.g., methane and ethane) over covalent organic frameworks (COFs) remains a challenge, with all previously reported attempts producing carbon monoxide as the dominant product. Herein, we report a new ethene-based COF, through polycondensation of electron-rich (E)-1,2‑diphenylethene and 1,3,6,8‑tetraphenylpyrene units. The synthesized ethene-based COF functioned as an efficient metal-free photocatalyst for the conversion of CO₂ into methane under visible light irradiation, with a selectivity of 100 %, a production rate of 14.7 µmol g^-1h^-1, and an apparent quantum yield of c.a. 0.99 % at 489.5 nm, which are the most promising values reported for CO₂ conversion by a metal-free COF photocatalyst, without any support from a co-catalyst. The carbon origin of CH₄ product is confirmed by isotope tracer ¹³CO₂ experiment. Moreover, the photocatalytic system consistently produces methane for > 14 h with recyclability.

Simultaneous Harvesting of Multiple Hot Holes via Visible-Light Excitation of Plasmonic Gold Nanospheres for Selective Oxidative Bond Scission of Olefins to Carbonyls

Using visible photoexcitation of gold nanospheres we successfully demonstrate the simultaneous harvesting of plasmon-induced multiple hot holes in the complete oxidative scission of the C=C bond in styrene at room temperature to selectively form benzaldehyde and formaldehyde, which is a reaction that requires activation of multiple substrates. Our results reveal that, while extraction of hot holes becomes efficient for interband excitation, harvesting of multiple hot holes from the excited Au nanospheres becomes prevalent only beyond a threshold light intensity. We show that the alkene oxidation proceeded via a sequence of two consecutive elementary steps; namely, a binding step and a cyclic oxometallate transition state as the rate-determining step. This demonstration of plasmon-excitation-mediated harvesting of multiple hot holes without the use of an extra hole transport media opens exciting possibilities, notably for difficult catalytic transformations involving multielectron oxidation processes.

Schottky barrier effect on plasmon-induced charge transfer

Plasmon-induced charge transfer causes electron-hole spatial separation at the metal-semiconductor interface, which plays a key role in photocatalytic and photovoltaic applications. The Schottky barrier formed at the metal-semiconductor interface can modify the hot carrier dynamics. Taking the Ag-TiO₂ system as an example, we have investigated plasmon-induced charge transfer at the Schottky junction using quantum mechanical simulations. We find that the Schottky barrier induced by n-type doping enhances the electron transfer and that induced by p-type doping enhances the hole transfer, which is attributed to the shift of the Fermi energy and the band bending of the Schottky junction at the interface. The Schottky barrier also modifies the layer distribution of hot carriers. In particular, for the system with a large band bending, there exists electron-hole spatial separation inside the TiO₂ substrate. Our results reveal the mechanism and dynamics of charge transfer at the Schottky junction, and pave the way for manipulating plasmon-assisted photocatalytic and photovoltaic applications.

Bias-Free Solar-Driven Syngas Production: A Fe2 O3 Photoanode Featuring Single-Atom Cobalt Integrated with a Silver-Palladium Cathode

Photoelectrochemical syngas production from aqueous CO₂ is a promising technique for carbon capture and utilization. Herein, we demonstrate the efficient and tunable syngas production by integrating a single-atom cobalt-catalyst-decorated α-Fe₂ O₃ photoanode with a bimetallic Ag/Pd alloy cathode. A record syngas production activity of 81.9 μmol cm^-2  h^-1 (CO/H₂ ratio: ≈1 : 1) was achieved under artificial sunlight (AM 1.5 G) with an excellent durability. Systematic studies reveal that the Co single atoms effectively extract the holes from Fe₂ O₃ photoanodes and serve as active sites for promoting oxygen evolution. Simultaneously, the Pd and Ag atoms in bimetallic cathodes selectively adsorb CO₂ and protons for facilitating CO production. Further incorporation with a photovoltaic, to allow solar light (>600 nm) to be utilized, yields a bias-free CO₂ reduction device with solar-to-CO and solar-to-H₂ conversion efficiencies up to 1.33 and 1.36 %, respectively.

Understanding Wavelength-Dependent Synergies between Morphology and Photonic Design in TiO2-Based Solar Powered Redox Cells

Solar powered redox cells (SPRCs) are promising for large-scale and long-term storage of solar-energy, particularly when coupled with redox flow batteries (RFBs). While efforts have primarily focused on heterostructure engineering, the potential of synergistic morphology and photonic design has not been carefully studied. Here, we investigate the wavelength-dependent effects of light-absorption and charge transfer characteristics on the performance of gold decorated TiO₂-based SPRC photoanodes operating with RFB-compatible redox couples. Through an in-depth optical and photoelectrochemical characterization of three complementary TiO₂ microstructures, namely nanotubes, honeycombs, and nanoparticles, we elucidate the combined effects of nanometer-scale semiconductor morphology and plasmonic design across the visible spectrum. In particular, thin-walled TiO₂ nanotubes exhibit a ∼ 50% increase in solar-to-chemical efficiency (STC) compared to thick-walled TiO₂ honeycombs thanks to improved charge transfer. Au nanoparticles both increase generation and interfacial charge transfer (above bandgap) and promote hot carrier injection (below bandgap) leading to a further 25% increase in STC. Overall, Au/TiO₂ nanotubes achieve a high photocurrent at 0.098 mA/cm² and an excellent STC of 0.06%, among the highest with respect to the theoretical limit. The incident photon to current efficiency and internal quantum efficiency are also superior to those of bare TiO₂ showing maximum values of 54.7% and 67%, respectively. Overall, nanophotonic engineering that synergistically combines morphology optimization and plasmonic sensitization schemes offer new avenues for improving rechargeable solar-energy technologies such as solar redox flow batteries.

Molecular-level Manipulation of Interface Charge Transfer on Plasmonic Metal/MOF Heterostructures

Plasmon-excited hot carriers have drawn great attention for driving various chemical reactions, but the short lifetimes of hot carriers seriously restrict the performance of plasmonic photocatalysis. Constructing plasmonic metal/metal-organic framework (MOF) heterostructures has been proved as an effective strategy to extend the lifetimes of hot carriers. Due to the high molecular tunability of MOFs, the MOF substrate in plasmonic metal/MOF heterostructures is able to capture hot electrons on the conduction band of MOF and hot holes on its valence band, and thus offers an ideal platform to separately study the detailed mechanism of hot electron and hole transfer processes. This review focuses on a molecular-level understanding of both hot-electron and hot-hole transfer at plasmonic metal/MOF interfaces. The enhanced stability and photocatalytic performance by introducing MOF substrates are discussed for plasmonic metal/MOF heterostructures. Additionally, typical characterization technologies are also proposed as powerful tools for tracking hot carrier transfer process.

Cathodoluminescence and optical absorption spectroscopy of plasmonic modes in chromium micro-rods

Manipulating light at the sub-wavelength level is a crucial feature of surface plasmon resonance (SPR) properties for a wide range of nanostructures. Noble metals like Au and Ag are most commonly used as SPR materials. Significant attention is being devoted to identify and develop non-noble metal plasmonic materials whose optical properties can be reconfigured for plasmonic response by structural phase changes. Chromium (Cr) which supports plasmon resonance, is a transition metal with shiny finished, highly non-corrosive, and bio-compatible alloys, making it an alternative plasmonic material. We have synthesized Cr micro-rods from a bi-layer of Cr/Au thin films, which evolves from face centered cubic to hexagonal close packed (HCP) phase by thermal activation in a forming gas ambient. We employed optical absorption spectroscopy and cathodoluminescence (CL) imaging spectroscopy to observe the plasmonic modes from the Cr micro-rod. The origin of three emission bands that spread over the UV-Vis-IR energy range is established theoretically by considering the critical points of the second-order derivative of the macroscopic dielectric function obtained from density functional theory (DFT) matches with interband/intraband transition of electrons observed in density of states versus energy graph. The experimentally observed CL emission peaks closely match thes-dandd-dband transition obtained from DFT calculations. Our findings on plasmonic modes in Cr(HCP) phase can expand the range of plasmonic material beyond noble metal with tunable plasmonic emissions for plasmonic-based optical technology.

Core-satellite-satellite hierarchical nanostructures: assembly, plasmon coupling, and gap-selective surface-enhanced Raman scattering

The plasmonic properties of gold nanoparticles (AuNPs), such as color tunability, electric field generation, hot carrier generation, and localized heating, are significantly enhanced in the nanogaps between AuNPs. Therefore, the creation and control of nanogaps are key to developing advanced plasmonic nanomaterials. Most AuNP nanoassemblies, including dimers, trimers, and core-satellites, have a single type of nanogap within the assembly. In this study, we construct core-satellite-satellite (CSS) hierarchical, fractal-like nanostructures featuring two types of nanogaps, namely first generation nanogaps (Gap1) between the core and first satellite (Sat1) AuNPs and second generation nanogaps (Gap2) between Sat1 and second satellite (Sat2) AuNPs. The sequential and alternating immersion of glass slides in different-sized AuNPs and linkers forms CSS with perfect yield. The UV-vis spectroscopy, combined with charge density distribution calculations, reveals the nature of the plasmon coupling between the AuNPs that constitute CSS nanoassemblies. The plasmon coupling can be tuned by independently varying Gap1 and Gap2. Furthermore, we explore the electric field amplification in Gap1 and Gap2 by comparing the surface-enhanced Raman scattering signal intensity selectively from each nanogap. This new type of nanostructure provides a great flexibility to control and enhance the plasmonic properties of noble metal nanoparticles.

Hot-Carrier Transfer across a Nanoparticle-Molecule Junction: The Importance of Orbital Hybridization and Level Alignment

While direct hot-carrier transfer can increase photocatalytic activity, it is difficult to discern experimentally and competes with several other mechanisms. To shed light on these aspects, here, we model from first-principles hot-carrier generation across the interface between plasmonic nanoparticles and a CO molecule. The hot-electron transfer probability depends nonmonotonically on the nanoparticle-molecule distance and can be effective at long distances, even before a strong chemical bond can form; hot-hole transfer on the other hand is limited to shorter distances. These observations can be explained by the energetic alignment between molecular and nanoparticle states as well as the excitation frequency. The hybridization of the molecular orbitals is the key predictor for hot-carrier transfer in these systems, emphasizing the necessity of ground state hybridization for accurate predictions. Finally, we show a nontrivial dependence of the hot-carrier distribution on the excitation energy, which could be exploited when optimizing photocatalytic systems.

Hot Electron Phenomena at Solid-Liquid Interfaces

Understanding the role of energy dissipation and charge transfer under exothermic chemical reactions on metal catalyst surfaces is important for elucidating the fundamental phenomena at solid-gas and solid-liquid interfaces. Recently, many surface chemistry studies have been conducted on the solid-liquid interface, so correlating electronic excitation in the liquid-phase with the reaction mechanism plays a crucial role in heterogeneous catalysis. In this review, we introduce the detection principle of electron transfer at the solid-liquid interface by developing cutting-edge technologies with metal-semiconductor Schottky nanodiodes. The kinetics of hot electron excitation are well correlated with the reaction rates, demonstrating that the operando method for understanding nonadiabatic interactions is helpful in studying the reaction mechanism of surface molecular processes. In addition to the detection of hot electrons excited by a catalytic reaction, we highlight recent results on how the transfer of the hot electrons influences surface chemical and photoelectrochemical reactions.

Construction of Spatially Separated Gold Nanocrystal/Cuprous Oxide Architecture for Plasmon-Driven CO2 Reduction

Plasmonic hot electrons have shown great potential in photocatalysis, but little is known about the hot hole-driven chemical reactions due to the lack of desired plasmonic metal/p-type semiconductor architectures. Herein, we describe a general and robust strategy for the site-selective growth of a p-type semiconductor, Cu₂O on Au nanocrystals (NCs), to produce diverse spatially separated Au/Cu₂O heterostructures. The preferential growth of Cu₂O on the tips/ends/edges of Au NCs is directed by the sparse coverage of the surfactant molecules at the high-curvature sites of Au NCs. The obtained dumbbell-shaped nanostructures serve as the ideal platforms for probing the hot-hole-mediated CO₂ reduction reaction. Benefiting from the hot-hole injection, a new reaction pathway is unlocked, and the C₂ product activity and selectivity are significantly improved. This study demonstrates the genuine superiority of the dumbbell-shaped nanostructures in photocatalysis, offering a new unique avenue to explore the underlying mechanism of hot-hole-mediated chemical reactions.

Plasmonic hot carrier-driven photoelectrochemical water splitting on antenna–reactor Pt/Ag/TiO2 Schottky nanodiodes

Plasmonic photoelectrochemical (PEC) water splitting has excited immense interest, as it can overcome the intrinsic limitations of semiconductors, in terms of light absorption, by the localized-surface plasmon resonances effect. Here, to get insight into the role of plasmonic hot carriers in plasmonic water splitting, a rational design of an antenna–reactor type Pt/Ag/TiO2 metal–semiconductor Schottky nanodiode was fabricated and used as a photoanode. Using the designed PEC cell system combined with the Pt/Ag/TiO2 nanodiode, we show that the plasmonic hot carriers excited from Ag were utilized for the oxygen (O2) evolution reaction and, consequently, had a decisive role in the enhancement of the photocatalytic efficiency. These results were supported by finite-difference time-domain simulations, and the faradaic efficiency was measured by the amount of actual gas produced. Therefore, this study provides a deep understanding of the dynamics and mechanisms of plasmonic hot carriers in plasmonic-assisted PEC water splitting.

Plasmonic Ag-decorated Cu2O nanowires for boosting photoelectrochemical CO2 reduction to multi-carbon products

The generation of multi-carbon products on the Cu₂O photocathode remains a great challenge. Herein, effective charge separation and surface catalytic reaction are achieved for photoelectrochemical CO₂ reduction through plasmon metal (Ag) decoration on Cu₂O nanowires. The Cu₂O/Ag composite photocathode achieves a 47.7% faradaic efficiency for CH₃COOH and the generation rate is 212.7 μmol cm^-2 h^-1 under illumination, which is about five times that in dark (44.4 μmol cm^-2 h^-1) at -0.7 V vs. RHE.

1D α-Fe2O3/ZnO Junction Arrays Modified by Bi as Photocathode: High Efficiency in Photoelectrochemical Reduction of CO2 to HCOOH

Photoelectrocatalytic (PEC) CO₂ reduction to value-added chemicals is a promising solution to address the energy and environmental issues we face currently. Herein, a unique photocathode Bi@ZFO NTs (Bi and α-Fe₂O₃ co-modified ZnO nanorod arrays) with high utilization of visible light and sharp-tips effect are successfully prepared using a facile method. Impressively, the performance of Bi@ZFO NTs for PEC CO₂ reduction to HCOOH included small onset potential (-0.53 V vs RHE), Tafel slope (101.2 mV dec^-1), and a high faraday efficiency of 61.2% at -0.65 V vs RHE as well as favorable stability over 4 h in an aqueous system under visible light illumination. Also, a series of experiments were performed to investigate the origin of its high activity, indicating that the metallic Bi and α-Fe₂O₃/ZnO nanojunction should be responsible for the favorable CO₂ adsorption/activation and charge transition/carrier separation, respectively. Density functional theory calculations reveal that the Bi@ZFO NTs could lower the intermediates' energy barrier of HCOO* and HCOOH* to form HCOOH due to the strong interaction of Bi and α-Fe₂O₃/ZnO.

Steering the Pathway of Plasmon-Enhanced Photoelectrochemical CO2 Reduction by Bridging Si and Au Nanoparticles through a TiO2 Interlayer

Photoelectrochemical (PEC) conversion of CO₂ in an aqueous medium into high-energy fuels is a creative strategy for storing solar energy and closing the anthropogenic carbon cycle. However, the rational design of catalytic architectures to selectively and efficiently produce a target product such as CO has remained a grand challenge. Herein, an efficient and selective Si photocathode for CO production is reported by utilizing a TiO₂ interlayer to bridge the Au nanoparticles and n⁺ p-Si. The TiO₂ interlayer can not only effectively protect and passivate Si surface, but can also exhibit outstanding synergies with Au nanoparticles to greatly promote CO₂ reduction kinetics for CO production through stabilizing the key reaction intermediates. Specifically, the TiO₂ layer and Au nanoparticles work concertedly to enhance the separation of localized surface plasmon resonance generated hot carriers, contributing to the improved activity and selectivity for CO production by utilizing the hot electrons generated in Au nanoparticles during PEC CO₂ reduction. The optimized Au/TiO₂ /n⁺ p-Si photocathode exhibits a Faradaic efficiency of 86% and a partial current density of -5.52 mA cm^-2 at -0.8 V_RHE for CO production, which represent state-of-the-art performance in this field. Such a plasmon-enhanced strategy may pave the way for the development of high-performance PEC photocathodes for energy-efficient CO₂ utilization.

Boosting Hydrogen Evolution at Visible Light Wavelengths by Using a Photocathode with Modal Strong Coupling between Plasmons and a Fabry-Pérot Nanocavity

Hot‐hole injection from plasmonic metal nanoparticles to the valence band of p‐type semiconductors and reduction by hot electrons should be improved for efficient and tuneable reduction to obtain beneficial chemical compounds. We employed the concept of modal strong coupling between plasmons and a Fabry‐Pérot (FP) nanocavity to enhance the hot‐hole injection efficiency. We fabricated a photocathode composed of gold nanoparticles (Au−NPs), p‐type nickel oxide (NiO), and a platinum film (Pt film) (ANP). The ANP structure absorbs visible light over a broad wavelength range from 500 nm to 850 nm via hybrid modes based on the modal strong coupling between the plasmons of Au−NPs and the FP nanocavity of NiO on a Pt film. All wavelength regions of the hybrid modes of the modal strong coupling system promoted hot‐hole injection from the Au−NPs to NiO and proton/water reduction by hot electrons. The incident photon‐to‐current efficiency based on H₂ evolution through water/proton reduction by hot electrons reached 0.2 % at 650 nm and 0.04 % at 800 nm.

Construction of Heterostructured Sn/TiO2 /Si Photocathode for Efficient Photoelectrochemical CO2 Reduction

Using renewable energy to convert CO₂ into liquid products, as a sustainable way to produce fuels and chemicals, has attracted intense attention. Herein, a novel heterostructured photocathode composed of Si wafer, TiO₂ layer, and Sn metal particles has been successfully fabricated by combining of a facile hydrothermal and electrodeposition method. The obtained Sn/TiO₂ /Si photocathode shows enhanced light absorption performance by the surface plasmon resonance effect of Sn metal. Especially, the Sn/TiO₂ /Si photocathode together with rich oxygen vacancy defects jointly promote photoelectrochemical CO₂ reduction, harvesting a high faradaic efficiency of HCOOH and a desirable average current density (-4.72 mA cm^-2 ) at -1.0 V vs. reversible hydrogen electrode. Significantly, the photocathode Sn/TiO₂ /Si also shows good stability due to the design of protecting layer TiO₂ . This study provides a facile strategy of constructing an efficient photocathode to improve the light absorption performance and the electron transfer efficiency, exhibiting great potential in the CO₂ reduction.

Progress of GaN-Based Optoelectronic Devices Integrated with Optical Resonances

Being direct wide bandgap, III-nitride (III-N) semiconductors have many applications in optoelectronics, including light-emitting diodes, lasers, detectors, photocatalysis, etc. Incorporation of III-N semiconductors with high-efficiency optical resonances including surface plasmons, distributed Bragg reflectors and micro cavities, has attracted considerable interests for upgrading their performance, which can not only reveal the new coupling mechanisms between optical resonances and quasiparticles, but also unveil the shield of novel optoelectronic devices with superior performances. In this review, the content covers the recent progress of GaN-based optoelectronic devices integrated with plasmonics and/or micro resonators, including the LEDs, photodetectors, solar cells, and light photocatalysis. The authors aim to provide an inspiring insight of recent remarkable progress and breakthroughs, as well as a promising prospect for the future highly-integrated, high speed, and efficient GaN-based optoelectronic devices.

Experimental characterization techniques for plasmon-assisted chemistry

Plasmon-assisted chemistry is the result of a complex interplay between electromagnetic near fields, heat and charge transfer on the nanoscale. The disentanglement of their roles is non-trivial. Therefore, a thorough knowledge of the chemical, structural and spectral properties of the plasmonic/molecular system being used is required. Specific techniques are needed to fully characterize optical near fields, temperature and hot carriers with spatial, energetic and/or temporal resolution. The timescales for all relevant physical and chemical processes can range from a few femtoseconds to milliseconds, which necessitates the use of time-resolved techniques for monitoring the underlying dynamics. In this Review, we focus on experimental techniques to tackle these challenges. We further outline the difficulties when going from the ensemble level to single-particle measurements. Finally, a thorough understanding of plasmon-assisted chemistry also requires a substantial joint experimental and theoretical effort.

Unique Electronic Excitations at Highly Localized Plasmonic Field

ConspectusUnder visible light illuminations, noble metal nanostructures can condense photon energy into the nanoscale region. By precisely tuning the metal nanostructures, the ultimate confinement of photoenergy at the molecular scale can be obtained. At such a confined photon energy field, various unique photoresponses of molecules, such as efficient visible light energy conversion processes or efficient multielectron transfer reactions, can be observed. Light-matter interactions also increase with the condensation of photons with nanoscale regions, leading to efficient light energy utilizations. Moreover, the strong field confinement can often modulate electronic excitations beyond normal selection rules. Such unique electronic excitations could realize innovative photoenergy conversion systems. On the other hand, such interactions lead to changes in the optical absorption property of the system via the formation of hybridized electronic energy states. This hybridized state is expected to have the potential to modulate the chemical reaction pathways. Taking these facts into consideration, a probe for the molecular absorption process with high sensitivity allows us to find novel ways for further precise tuning of light-matter interactions. In this Account, we review phenomena of unique electronic excitations from the perspective of our previous investigations using surface-enhanced Raman scattering (SERS) spectroscopy at electrified interfaces. Because the enhancement mechanism of Raman scattering at interfaces is deeply correlated with the photon absorption process accompanied by the electronic excitations between molecules and electrode surfaces, the detailed SERS investigations of the well-defined system can provide information on the electronic excitation processes. Through SERS observations of single-molecule junctions at electrodes or well-defined low-dimensional carbon materials, we have observed the characteristic Raman bands containing additional polarization tensors, indicating the occurrence of electronic polarization induced by electronic excitations based on a distinct selection rule. The origins for the observed facts were attributed to the highly condensed electric field producing the huge intensity gradient at the nano scale. The electrochemical potential control of the system would be valuable for the control of the excitation process. Additionally, from Raman spectra of dye molecules coupled to the plasmonic field, the changes in the Raman scattering intensity depending on the strength of interactions suggested the modulation of the absorption characteristics of the system. In addition, we have proved that the electrochemical potential control method can be a powerful tool for the active tuning of the light-matter interaction, leading to the change in the light absorption property. The molecular behaviors of dyes in the strong-coupling regime were reversibly tuned to show intense SERS. The current descriptions provide novel insights for these unique electronic excitations, realized by the plasmon excitation, that lead to advanced photoenergy conversions beyond the limits of present systems.