Plasmon-enhanced Photocatalysis Research Articles

Differentiating Plasmon-Enhanced Chemical Reactions on AgPd Hollow Nanoplates through Surface-Enhanced Raman Spectroscopy.

Plasmonic photocatalysis demonstrates great potential for efficiently harnessing light energy. However, the underlying mechanisms remain enigmatic due to the transient nature of the reaction processes. Typically, plasmonic photocatalysis relies on the excitation of surface plasmon resonance (SPR) in plasmonic materials, such as metal nanoparticles, leading to the generation of high-energy or "hot electrons", albeit accompanied by photothermal heating or Joule effect. The ability of hot electrons to participate in chemical reactions is one of the key mechanisms, underlying the enhanced photocatalytic activity observed in plasmonic photocatalysis. Interestingly, surface-enhanced Raman scattering (SERS) spectroscopy allows the analysis of chemical reactions driven by hot electrons, as both SERS and hot electrons stem from the decay of SPR and occur at the hot spots. Herein, we propose a highly efficient SERS substrate based on cellulose paper loaded with either Ag nanoplates (Ag NPs) or AgPd hollow nanoplates (AgPd HNPs) for the in situ monitoring of C-C homocoupling reactions. The data analysis allowed us to disentangle the impact of hot electrons and the Joule effect on plasmon-enhanced photocatalysis. Computational simulations revealed an increase in the rate of excitation of hot carriers from single/isolated AgPd HNPs to an in-plane with a vertical stacking assembly, suggesting its promise as a photocatalyst under broadband light. In addition, the results suggest that the incorporation of Pd into an alloy with plasmonic properties may enhance its catalytic performance under light irradiation due to the collection of plasmon-excitation-induced hot electrons. This work has demonstrated the performance-oriented synthesis of hybrid nanostructures, providing a unique route to uncover the mechanism of plasmon-enhanced photocatalysis.

Site-Selective Bimetallic Heterogeneous Nanostructures for Plasmon-Enhanced Photocatalysis of Nitrobenzene Hydrogenation

Site-Selective Bimetallic Heterogeneous Nanostructures for Plasmon-Enhanced Photocatalysis of Nitrobenzene Hydrogenation

Molecular scale nanophotonics: hot carriers, strong coupling, and electrically driven plasmonic processes

Abstract Plasmonic modes confined to metallic nanostructures at the atomic and molecular scale push the boundaries of light–matter interactions. Within these extreme plasmonic structures of ultrathin nanogaps, coupled nanoparticles, and tunnelling junctions, new physical phenomena arise when plasmon resonances couple to electronic, exitonic, or vibrational excitations, as well as the efficient generation of non-radiative hot carriers. This review surveys the latest experimental and theoretical advances in the regime of extreme nano-plasmonics, with an emphasis on plasmon-induced hot carriers, strong coupling effects, and electrically driven processes at the molecular scale. We will also highlight related nanophotonic and optoelectronic applications including plasmon-enhanced molecular light sources, photocatalysis, photodetection, and strong coupling with low dimensional materials.

Direct Hot-Electron Transfer at the Au Nanoparticle/Monolayer Transition-Metal Dichalcogenide Interface Observed with Ultrahigh Spatiotemporal Resolution.

Plasmon-induced hot-electron transfer at the metallic nanoparticle/semiconductor interface is the basis of plasmon-enhanced photocatalysis and energy harvesting. However, limited by the nanoscale size of hot spots and femtosecond time scale of hot-electron transfer, direct observation is still challenging. Herein, by using spatiotemporal-resolved photoemission electron microscopy with a two-color pump-probe beamline, we directly observed such a process with a concise system, the Au nanoparticle/monolayer transition-metal dichalcogenide (TMD) interface. The ultrafast hot-electron transfer from Au nanoparticles to monolayer TMDs and the plasmon-enhanced transfer process were directly measured and verified through an in situ comparison with the Au film/TMD interface and free TMDs. The lifetime at the Au nanoparticle/MoSe2 interface decreased from 410 to 42 fs, while the photoemission intensities exhibited a 27-fold increase compared to free MoSe2. We also measured the evolution of hot electrons in the energy distributions, indicating the hot-electron injection and decay happened in an ultrafast time scale of ∼50 fs without observable electron cooling.

Advances in fundamentals and application of plasmon-assisted CO2 photoreduction

Abstract Artificial photosynthesis of hydrocarbons from carbon dioxide (CO2) has the potential to provide renewable fuels at the scale needed to meet global decarbonization targets. However, CO2 is a notoriously inert molecule and converting it to energy dense hydrocarbons is a complex, multistep process, which can proceed through several intermediates. Recently, the ability of plasmonic nanoparticles to steer the reaction down specific pathways and enhance both reaction rate and selectivity has garnered significant attention due to its potential for sustainable energy production and environmental mitigation. The plasmonic excitation of strong and confined optical near-fields, energetic hot carriers and localized heating can be harnessed to control or enhance chemical reaction pathways. However, despite many seminal contributions, the anticipated transformative impact of plasmonics in selective CO2 photocatalysis has yet to materialize in practical applications. This is due to the lack of a complete theoretical framework on the plasmonic action mechanisms, as well as the challenge of finding efficient materials with high scalability potential. In this review, we aim to provide a comprehensive and critical discussion on recent advancements in plasmon-enhanced CO2 photoreduction, highlighting emerging trends and challenges in this field. We delve into the fundamental principles of plasmonics, discussing the seminal works that led to ongoing debates on the reaction mechanism, and we introduce the most recent ab initio advances, which could help disentangle these effects. We then synthesize experimental advances and in situ measurements on plasmon CO2 photoreduction before concluding with our perspective and outlook on the field of plasmon-enhanced photocatalysis.

Streamlines of the Poynting Vector and Chirality Flux around a Plasmonic Bowtie Nanoantenna.

The streamlines of the energy flux (Poynting vectors) and chirality flux as well as the intensity of the electric field around various plasmonic nanostructures (nanocube, nanocuboid, nanotriangle, hexagonal nanoplate and bowtie nanoantenna) induced by a circularly polarized (CP) or linearly polarized (LP) light were studied theoretically. The boundary element method combined with the method of moment was used to solve a set of surface integral equations, based on the Stratton-Chu formulation, for analyzing the highly distorted electromagnetic (EM) field in the proximity of these nanostructures. We discovered that the winding behavior of these streamlines exhibits versatility for various modes of the surface plasmon resonance of different nanostructures. Recently, using plasmonic nanostructures to facilitate a photochemical reaction has gained significant attention, where the hot carriers (electrons) play important roles. Our findings reveal a connection between the flow pattern of energy flux and the morphology of the photochemical deposition around various plasmonic nanostructures irradiated by a CP light. For example, numerical results exhibit vertically helical streamlines of the Poynting vector around an Au nanocube and transversely twisted-roll streamlines around a nanocuboid. Additionally, the behaviors of the winding energy and chirality fluxes at the gap and corners of a plasmonic bowtie nanoantenna, implying a highly twisted EM field, depend on the polarization of the incident LP light. Our analysis of the streamlines of the Poynting vector and chirality flux offers an insight into the formation of plasmon-enhanced photocatalysis.

Plasmon-Enhanced Photocatalysis Based on Plasmonic Nanoparticles for Energy and Environmental Solutions: A Review

Plasmonic nanoparticles are able to enhance the photocatalytic performance of semiconductor nanoparticles using four main mechanisms of light scattering, light concentration, hot electron injections, and plasmon-induced resonance energy transfer. Pushing the semiconductor nanoparticles’ absorption range to the visible and near-infrared and boosting the electron–hole generation quantum yield are the major effects. However, most of the reviewed papers lack a systematic design for conjugation of plasmonic nanoparticles with semiconductor nanoparticles, and they mainly benefited from the general enhancement because of the generated localized surface plasmon resonances. In this Review, we have systematically showed the importance of rational optical design of plasmonic-semiconductor nanoparticle conjugation for an efficient photocatalytic activity. The application of plasmon-enhanced phenomenon for the environmental remediation and green energy production was thoroughly reviewed. The degradation of organic dyes, air pollutants, volatile organic compounds, pesticides, and pharmaceutical compounds were the major reviewed works in environmental applications, while the green energy productions were limited to the role of plasmonic nanoparticles for the enhancement of H2/O2 production, water splitting, and CO2 reduction. This review can be a useful guideline for researchers working on enhancing the photocatalytic activity of the semiconductor nanoparticles and those interested in plasmon-enhanced phenomena by emphasizing the underlying mechanisms.

Plasmon-enhanced photocatalysis using gold nanoparticles encapsulated in nanoscale molybdenum oxide shell

Using solar energy to enhance the transformation rate of organic molecules is a promising strategy to advance chemical synthesis and environmental remediation. Plasmonic nanoparticles responsive to sunlight show great promise in the catalysis of chemical reactions. In this work, we used a straightforward wet-chemistry method to synthesize plasmonic octahedral gold nanoparticles (NPs) coated with thin molybdenum oxide (MoO3−x ), Au@MoO3−x NPs, which exhibited strong surface plasmon resonance in a broad wavelength range. The synthesized Au@MoO3−x NPs were characterized by UV–vis, SEM, TEM, EDS, XPS, and the electrochemical technique of cyclic voltammetry (CV). The catalytic performance of Au@MoO3−x NPs under visible light irradiation was investigated using the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) as a model reaction. The presence of a thin capping layer of MoO3−x on our Au NPs contributed to the broadening of their range of absorption of visible light, resulting in a stronger intra-particle plasmonic resonance and the modulation of surface energy and electronic state. Accordingly, the kinetics of plasmon photocatalytic transformation of 4-NP to 4-AP was significantly accelerated (by a factor of 8.1) under visible light, compared to uncapped Au NPs in the dark. Our as-synthesized Au@MoO3−x NPs is an example that the range of plasmonic wavelengths of NPs can be effectively broadened by coating them with another plasmon-active (semiconducting) material, which substantially improves their plasmonic photocatalytic performance. Meanwhile, the synthesized Au@MoO3−x NPs can be used to accelerate the transformation of organic molecules under visible light irradiation.

Oblique-Incidence-Excited Localized Hot Spots in Plasmonic Particle-on-Film Nanocavities

Regarding plasmonic nanocavities formed by particle-on-film, the nanogap hot spots play an essential role in wide applications including plasmon-enhanced spectroscopy, ultrasensitive sensing, photovoltaics, and photocatalysis. However, hot spots at the nanogap are symmetrically distributed on both sides of the junction under normal incidence. Herein, under the condition of oblique incidence, properties of plasmonic hot spots in the Au@SiO2 particle-on-film have been investigated theoretically. We find that when the plane wave is incident obliquely, the hot spots in the gap are not only symmetrically distributed on both sides of the contact point but also circularly distributed around the junction at specific excitation wavelengths. With the adjustment of the incidence angle, the maximum surface-enhanced Raman spectroscopy enhancement factor from this circularly distributed hot spot reaches 11.3 orders in magnitude. Moreover, the resonant mode at 785 nm has been readily matched by optimizing the size of the Au core, and the optimal thickness of the silica shell with 1 nm for achieving the maximum electromagnetic (EM) field enhancement was further demonstrated. These findings offer a new strategy for manipulating locations of localized hot spots with strong near-field enhancement in plasmonic nanocavities.

(Invited) Beyond Plasmonic Effect of Metal Nanoparticles in Photocatalysis and Electrocatalysis

Metal nanoparticles such as gold, silver and copper, exhibit localized surface plasmon resonance effect. The plasmon-enhanced photocatalysis and electrocatalysis of metal nanoparticles receive increasing attention. In addition, metal nanoparticles show other significant effects beyond the plasmonic effect. This talk will discuss and separate other effects of metal nanoparticles from the plasmonic effect.

Palladium nanoparticles decorated MXene for plasmon-enhanced photocatalysis

Two-dimensional MXene nanomaterials have found a wide application range in recent years. Here, Ti3C2 MXene nanosheets exhibiting inherent photothermal activity reveal synergistic plasmon-enhanced photocatalytic properties upon rational surface decoration with palladium nanoparticles (NPs). Pd incorporation induces a 1.7-fold increase in the number of defect sites on the MXene surface. Most importantly, the introduced Pd nanoparticles favour an effective separation and collection of localized surface plasmon resonance (LSPR)-induced hot charge carriers generated at the surface of semi-metallic Ti3C2 NS, further facilitating the generation of hydrogen peroxide (H2O2). The generated H2O2 is sequentially decomposed to hydroxyl radicals via the peroxidase (POD)-like activity of Pd NPs. The Pd@MXene shows approximately 2-fold enhancement of photocatalytic activity and excellent photostability under laser irradiation. Taken together, this study highlights the promise of constructing active and stable MXene-based nanohybrids for highly effective photo-responsive nanomedicine.

In situ SERS monitoring of plasmon-driven catalytic reaction on gap-controlled Ag nanoparticle arrays under 785 nm irradiation

Plasmon-enhanced photocatalysis has attracted considerable attention due to its low energy consumption and high energy throughput. Surface-enhanced Raman scattering (SERS) is a highly sensitive and label-free nondestructive tool to investigate plasmon-driven photocatalytic reactions. Herein, we present a facile method to fabricate gap-controlled Ag nanoparticle (NP) arrays with uniform and high-density distribution of hot spots, which can be employed as both efficient plasmonic photocatalysts and stable SERS platforms. The plasmon-driven catalytic reaction of 4-nitrobenzenethiol (4NBT), which transforms it into p, p′-dimercaptoazobenzene (DMAB), is detected by using an in situ SERS technique at the excited wavelength of 785 nm. According to the temperature and laser power density dependent photocatalytic reaction rates observed on the Ag NP arrays, we quantitatively determined that the reductive coupling of 4NBT is more likely to occur as the gap decreases. The finite-difference time-domain (FDTD) simulation results demonstrate that the plasmonic hot spots are significantly enhanced with a decrease in gap, which in turn reduces activation energy. The gap-controlled Ag NP arrays are efficient for both promotion and detection of plasmon-driven catalytic reactions, and may pave a pathway for implementing efficient plasmonic photocatalytic platforms.

3D flower-like Ag/Bi5O7I embedded on an acrylate fluoroboron polymer as a multifunctional assembly film for ultrastable plasmon-enhanced photocatalysis and antibiosis

Flower-like Bi5O7I microspheres modified with Ag nanoparticles are prepared, exhibiting excellent surface plasmon resonance. Bi5O7I/Ag/acrylate fluoroboron polymer (AFBP) ternary assembly films are also fabricated using Bi5O7I/Ag composites uniformly distributed in acrylate fluoroboron polymer. The structure and photocatalytic properties of Bi5O7I/Ag are characterized. Characterization results indicate that Bi5O7I/Ag has a large specific surface area, a low band gap, and excellent visible light response. To evaluate the properties of the Bi5O7I/Ag/AFBP ternary assembly film, laboratory diatom assays and antibacterial activity are tested. The results show excellent diatom anti-settlement performance and high antibacterial rates of 99.63% for Escherichia coli and 99.92% for Staphylococcus aureus. Photocatalytic degradation of 2,4,6-trichlorophenol by visible-light irradiation for 150 min shows a removal rate of 95%, which is attributed to charge efficiency separation. Simultaneously, the photodegradation pathway and TOC of 2,4,6-trichlorophenol are analyzed. Thus, this study shows the significance of assembly film for potential marine applications in the future.

Plasmonic Photocatalytic Enhancement of L-Cysteine Self-Assembled Gold Nanoparticle Clusters for Fenton Reaction Catalysis

Plasmon-enhanced photocatalysis has the potential to reduce activation energies and decrease temperature requirements, which increases catalyst stability and lowers process operating costs. The near-field enhancement that occurs at junctions between plasmonic nanoparticle clusters (i.e., hot spots) has been well-studied for sensing applications (e.g., Raman scattering). However, experimental insight into the effect of nanoparticle cluster hot spots on plasmon-enhanced photocatalysis is lacking. We demonstrate that catalytic activity is increased when clusters of gold nanoparticles (AuNPs) are formed relative to isolated particles using the same catalyst loading. Through experimental controls, we conclude that this catalytic enhancement is most likely due to the formation of plasmonic hot spots. Clusters of AuNPs were formed by adding L-cysteine to an AuNP dispersion, and a 20 ± 12% enhancement in the photocatalytic dye degradation rate was observed using a Fenton process. While this report may be a modest enhancement relative to the spectacular near-field electromagnetic field enhancements predicted by simulation at the nanoparticle junction, this finding supports the recent work of Srimanta et al. that plasmonic hot spots contribute to catalytic rate enchantments. It is anticipated that further self-assembly strategies to optimize interparticle orientations and cluster size distributions will improve the enhancement due to the formation of hot spots, and careful control will be required. For example, excess L-cysteine addition revealed extensive aggregation and subsequent rate reductions.

Scalable and stable silica-coated silver nanoparticles, produced by electron beam evaporation and rapid thermal annealing, for plasmon-enhanced photocatalysis

This study demonstrates an easy, scalable way to fabricate stable plasmonic photocatalysts for gas-phase photocatalysis. Silver nanoparticles, coated with silica for improved stability, were fabricated via electron beam evaporation and rapid thermal annealing, modeled via finite difference time domain simulations, and characterized. Catalytic performance was evaluated by monitoring total volatile organic compound concentration from toluene degradation in a gas-phase recirculating batch reactor. Results of cycled experiments demonstrate 5-fold increase in apparent quantum yield and 18% increase in reaction rate constant over TiO2. This approach may enable plasmonic photocatalysis by offering a simple, scalable fabrication method more reliable than colloidal synthesis.

Ordered Macroporous Photonic Crystal Hot Electron Plasmonic Photocatalysts

Plasmon-enhanced photocatalysis provides opportunities for controlling chemical reaction rates, especially when the influence of the light is to enhance or alter charge transfer properties. Semiconductor photocatalysts with an inverse opal or photonic crystal structure not only provide large, open and accessible surface area, they are electrically interconnected as a porous network. This becomes a useful scaffold to dock metallic nanoparticles whose size can be tuned to absorb specific frequencies resonant with their localised surface plasmon resonance. Coupling this with the nature of the metal-semiconductor band structure (where the metal oxide is wide band gap) and exploiting certain properties of photonic crystals, more efficient photocatalysts are possible. We demonstrate semiconducting photonic crystal plasmonic photocatalysts using V2O5 and TiO2 as visible light semiconductor catalysts that showed superior performance to a conventional TiO2 support for hydrogenation of 4-nitrophenol. The approach married the photonic band gap, metal oxide semiconductor bandgap, slow photon effect and localised surface plasmon polaritons to maximum photon absorption to modulate charge transfer and electron density either in the metal nanoparticle, or the conduction band of the semiconductor to give a better photocatalyst.

Enhanced photocurrent generation from indium–tin-oxide/Fe2TiO5 hybrid nanocone arrays

Semiconductor oxide materials, including α-Fe2O3 (hematite), have been investigated as possible candidates for large-scale solar-to-chemical energy conversion. However, many of these materials suffer from inefficient charge transport leading to undesirable charge recombination. Although plasmon-enhanced photocatalysis has achieved some success in enhancing light absorption and hot carrier generation, the intrinsic ohmic losses within the metal hinder its wide applications. Here, we use large-area indium-tin-oxide (ITO) nanocone arrays that exhibit both high electrical transport, strong light trapping and enlarged surface area with reduced ohmic losses. We further adopted sputtered iron titanate (Fe2TiO5) thin films that show better photocatalytic performance than hematite. The photoanode made of all-oxide ITO/Fe2TiO5 nanocone array exhibited Mie resonance-enhanced photocurrent generation up to 31 times. These ITO nanocones can be readily combined with other oxide materials providing great opportunities for efficient photon management in enhanced photocatalysis.

Single-Molecular Catalysis Identifying Activation Energy of the Intermediate Product and Rate-Limiting Step in Plasmonic Photocatalysis.

Plasmon-mediated photocatalysis provides a novel strategy for harvesting solar energy. Identification of the rate-determining step and its activation energy in plasmon-mediated photocatalysis plays critical roles for understanding the contribution of hot carriers, which facilitates rational designation of catalysts with integrated high photochemical conversion efficiency and catalytic performance. However, it remains a challenge due to a lack of research tools with spatiotemporal resolution that are capable of capturing intermediates. In this work, we used a single-molecule fluorescence approach to investigate a localized surface plasmon resonance (LSPR)-enhanced photocatalytic reaction with subturnover resolution. By introducing variable temperature as an independent parameter in plasmonic photocatalysis, the activation energies of tandem reaction steps, including intermediate generation, product generation, and product desorption, were clearly differentiated, and intermediate generation was found to be the rate-limiting step. Remarkably, the cause of the plasmon-enhanced catalysis performance was found to be its ability of lowering the activation energy of intermediate generation. This study gives new insight into the photochemical energy conversion pathways in plasmon-enhanced photocatalysis and sheds light on designing high-performance plasmonic catalysts.

Ultrasmall Designed Plasmon Resonators by Fused Colloidal Nanopatterning.

This work presents a procedure for large-area patterning of designed plasmon resonators that are much smaller than possible with conventional lithography techniques. Fused Colloidal Nanopatterning combines directed self-assembly and controlled fusing of spherical colloidal nanoparticles. The two-step approach first patterns a surface covered with hydrogen silsesquioxane, an electron beam resist, forming traps into which the colloidal gold nanoparticles self-assemble. Second, the patterned nanoparticles are controllably fused to form plasmon resonators of any 2D designed shape. The heights and widths of the plasmon resonators are determined by the diameter of the nanoparticle building blocks, which can be well below 10 nm. By performing the fusing step with UV ozone and heat exposure, we demonstrate that the process is easily scalable to cover large areas on silicon wafers with designed gold nanostructures. The procedure neither requires adhesion layers nor a lift-off process, making it ideally suited for plasmonics, in comparison with regular electron beam lithography. We use monochromated electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy and boundary element method simulations to demonstrate that the designed plasmon resonators are directly tunable via the pattern design. We foresee future expansion of this approach for applications such as plasmon-enhanced photocatalysis and for large-scale patterning where chemical, optical, or confinement properties require sub-10 nm metal lines.

Compact Integration of TiO₂ Nanoparticles into the Cross-Points of 3D Vertically Stacked Ag Nanowires for Plasmon-Enhanced Photocatalysis.

The compact integration of semiconductor TiO2 nanoparticles (NPs) into the 3D crossed region of stacked plasmonic Ag nanowires (NWs) enhanced the photocatalytic activities through synergistic effects between the strong localized surface plasmon resonance (LSPR) excitation at the 3D cross-points of the Ag NWs and the efficient hot electron transfer at the interface between the Ag NWs and the TiO2 NPs. This paper explored new hybrid nanostructures based on the selective assembly of TiO2 NPs onto 3D cross-points of vertically stacked Ag NWs. The assembled TiO2 NPs directly contacted the 3D Ag NWs; therefore, charge separation occurred efficiently at the interface between the Ag NWs and the TiO2 NPs. The composite nanomaterials exhibited high extinction across the ultraviolet-visible range, rendering the nanomaterials high-performance photocatalysts across the full (ultraviolet-visible) and the visible spectral regions. Theoretical simulations clearly revealed that the local plasmonic field was highly enhanced at the 3D crossed regions of the vertically stacked Ag NWs. A Raman spectroscopic analysis of probe dye molecules under photodegradation conditions clearly revealed that the nanogap in the 3D crossed region was crucial for facilitating plasmon-enhanced photocatalysis and plasmon-enhanced spectroscopy.