Transition-Metal Decorated Aluminum Nanocrystals

Mie Resonant Metal Oxide Nanospheres for Broadband Photocatalytic Enhancements

Metal oxides are widely used in heterogeneous catalysis as supports to disperse catalytically active nanoparticles, isolated atomic sites, or even as catalysts themselves. Herein, we present a method to produce optically active metal oxide supports that exhibit size-dependent Mie resonances based on TiO2 nanospheres with tunable size, crystalline phase composition, and optical properties. Mie resonant TiO2 nanospheres were used as supports to disperse Au, Pt, and Pd nanoparticles. We have found up to a 50-fold enhancement of the electric field at the metal oxide/metal interface corresponding to wavelength-dependent multipolar resonances in the TiO2 structure. Using Au/TiO2 as a prototypical photocatalyst, we demonstrate broadband rate enhancements between 400 and 800 nm during CO oxidation, with a noticeable increase below 500 nm. This increased reactivity at higher photon energies is due to improved photon utilization and interband absorption in the gold that results in greater secondary electron generation through electron-electron scattering processes, thus leading to higher rates in conjunction with the Mie scattering TiO2 support. This study not only highlights the potential of Mie resonant TiO2 in broadband photocatalytic enhancements but also for developing various Mie resonant metal oxide supports, such as ZnO or Cu2O, which can improve photocatalytic performance for a number of critical reactions. As the chemical and energy industries move toward conversion technologies driven by renewable energy sources, the strategy of designing optical resonances into oxide supports that are already widely used could enable a straightforward adaptation of photochemical processing based on traditional heterogeneous catalysts.

Magnesium Nanoparticles for Surface-Enhanced Raman Scattering and Plasmon-Driven Catalysis

Nanostructures of some metals can sustain localized surface plasmon resonances, collective oscillations of free electrons excited by incident light. This effect results in wavelength-dependent absorption and scattering, enhancement of the incident electric field at the metal surface, and generation of hot carriers as a decay product. The enhanced electric field can be utilized to amplify the spectroscopic signal in surface-enhanced Raman scattering (SERS), while hot carriers can be exploited for catalytic applications. In recent years, cheaper and more earth abundant alternatives to traditional plasmonic Au and Ag have gained growing attention. Here, we demonstrate the ability of plasmonic Mg nanoparticles to enhance Raman scattering and drive chemical transformations upon laser irradiation. The plasmonic properties of Mg nanoparticles are characterized at the bulk and single particle level by optical spectroscopy and scanning transmission electron microscopy coupled with electron energy-loss spectroscopy and supported by numerical simulations. SERS enhancement factors of ∼102 at 532 and 633 nm are obtained using 4-mercaptobenzoic acid and 4-nitrobenzenethiol. Furthermore, the reductive coupling of 4-nitrobenzenethiol to 4,4'-dimercaptoazobenzene is observed on the surface of Mg nanoparticles under 532 nm excitation in the absence of reducing agents, indicating a plasmon-driven catalytic process. Once decorated with Pd, Mg nanostructures display an enhancement factor of 103 along with an increase in the rate of catalytic coupling. The results of this study demonstrate the successful application of plasmonic Mg nanoparticles in sensing and plasmon-enhanced catalysis.

Dynamic Behavior of Platinum Atoms and Clusters in the Native Oxide Layer of Aluminum Nanocrystals

Strong metal-support interactions (SMSIs) are well-known in the field of heterogeneous catalysis to induce the encapsulation of platinum (Pt) group metals by oxide supports through high temperature H2 reduction. However, demonstrations of SMSI overlayers have largely been limited to reducible oxides, such as TiO2 and Nb2O5. Here, we show that the amorphous native surface oxide of plasmonic aluminum nanocrystals (AlNCs) exhibits SMSI-induced encapsulation of Pt following reduction in H2 in a Pt structure dependent manner. Reductive treatment in H2 at 300 °C induces the formation of an AlOx SMSI overlayer on Pt clusters, leaving Pt single-atom sites (Ptiso) exposed available for catalysis. The remaining exposed Ptiso species possess a more uniform local coordination environment than has been observed on other forms of Al2O3, suggesting that the AlOx native oxide of AlNCs presents well-defined anchoring sites for individual Pt atoms. This observation extends our understanding of SMSIs by providing evidence that H2-induced encapsulation can occur for a wider variety of materials and should stimulate expanded studies of this effect to include nonreducible oxides with oxygen defects and the presence of disorder. It also suggests that the single-atom sites created in this manner, when combined with the plasmonic properties of the Al nanocrystal core, may allow for site-specific single-atom plasmonic photocatalysis, providing dynamic control over the light-driven reactivity in these systems.

Bimetallic copper palladium nanorods: plasmonic properties and palladium content effects

Cu is an inexpensive alternative plasmonic metal with optical behaviour comparable to Au but with much poorer environmental stability. Alloying with a more stable metal can improve stability and add functionality, with potential effects on the plasmonic properties. Here we investigate the plasmonic behaviour of Cu nanorods and Cu-CuPd nanorods containing up to 46 mass percent Pd. Monochromated scanning transmission electron microscopy electron energy-loss spectroscopy first reveals the strong length dependence of multiple plasmonic modes in Cu nanorods, where the plasmon peaks redshift and narrow with increasing length. Next, we observe an increased damping (and increased linewidth) with increasing Pd content, accompanied by minimal frequency shift. These results are corroborated by and expanded upon with numerical simulations using the electron-driven discrete dipole approximation. This study indicates that adding Pd to nanostructures of Cu is a promising method to expand the scope of their plasmonic applications.

A Pd-based plasmonic photocatalyst for nitrogen fixation through an antenna-reactor mechanism

Plasmonic metal nanocrystals (e.g., Au, Ag, and Cu) hold great promise for driving photocatalytic reactions, but little is known about the plasmonic properties of Pd nanocrystals. Herein, we constructed a plasmonic Pd/Ru antenna-reactor photocatalyst through the controllable growth of a Ru nanoarray 'reactor' on a Pd nano-octahedron 'antenna' and demonstrated a plasmonic Pd-driven N2 photofixation process. The plasmonic properties of Pd nano-octahedrons were verified using finite-difference time-domain (FDTD) simulations and refractive index sensitivity tests in water-glycerol mixtures. Notably, the constructed plasmonic antenna-reactor nanostructures exhibited superior photocatalytic activities during N2 photofixation, with a maximum ammonia production rate of 117.5 ± 15.0 μmol g-1 h-1 under visible and near-infrared (NIR) light illumination. The mechanism can be attributed to the ability of the plasmonic Pd nanoantennas to harvest light to generate abundant hot electrons and the Ru nanoreactors to provide active sites for adsorption and activation of N2. This work paves the way for the development of Pd-based plasmonic photocatalysts for efficient N2 photofixation and sheds new light on the optimal design and construction of antenna-reactor nanostructures.

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.

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

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

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.

Pulsed Photothermal Heterogeneous Catalysis

Anthropogenic climate change urgently calls for the greening and intensification of the chemical industry. Most chemical reactors make use of catalysts to increase their conversion yields, but their operation at steady-state temperatures limits their rate, selectivity, and energy efficiency. Here, we show how to break such a steady-state paradigm using ultrashort light pulses and photothermal nanoparticle arrays to modulate the temperature of catalytic sites at timescales typical of chemical processes. Using heat dissipation and time-dependent microkinetic modeling for a number of catalytic landscapes, we numerically demonstrate that pulsed photothermal catalysis can result in a favorable, dynamic mode of operation with higher energy efficiency, higher catalyst activity than for any steady-state temperature, reactor operation at room temperature, resilience against catalyst poisons, and access to adsorbed reagent distributions that are normally out of reach. Our work identifies the key experimental parameters controlling reaction rates in pulsed heterogeneous catalysis and provides specific recommendations to explore its potential in real experiments, paving the way to a more energy-efficient and process-intensive operation of catalytic reactors.

Opportunities and Challenges for Alternative Nanoplasmonic Metals: Magnesium and Beyond

Materials that sustain localized surface plasmon resonances have a broad technology potential as attractive platforms for surface-enhanced spectroscopies, chemical and biological sensing, light-driven catalysis, hyperthermal cancer therapy, waveguides, and so on. Most plasmonic nanoparticles studied to date are composed of either Ag or Au, for which a vast array of synthetic approaches are available, leading to controllable size and shape. However, recently, alternative materials capable of generating plasmonically enhanced light-matter interactions have gained prominence, notably Cu, Al, In, and Mg. In this Perspective, we give an overview of the attributes of plasmonic nanostructures that lead to their potential use and how their performance is dictated by the choice of plasmonic material, emphasizing the similarities and differences between traditional and emerging plasmonic compositions. First, we discuss the materials limitation encapsulated by the dielectric function. Then, we evaluate how size and shape maneuver localized surface plasmon resonance (LSPR) energy and field distribution and address how this impacts applications. Next, biocompatibility, reactivity, and cost, all key differences underlying the potential of non-noble metals, are highlighted. We find that metals beyond Ag and Au are of competitive plasmonic quality. We argue that by thinking outside of the box, i.e., by looking at nonconventional materials such as Mg, one can broaden the frequency range and, more importantly, combine the plasmonic response with other properties essential for the implementation of plasmonic technologies.

Thermoplasmonics in Solar Energy Conversion: Materials, Nanostructured Designs, and Applications

The indispensable requirement for sustainable development of human society has forced almost all countries to seek highly efficient and cost-effective ways to harvest and convert solar energy. Though continuous progress has advanced, it remains a daunting challenge to achieve full-spectrum solar absorption and maximize the conversion efficiency of sunlight. Recently, thermoplasmonics has emerged as a promising solution, which involves several beneficial effects including enhanced light absorption and scattering, generation and relaxation of hot carriers, as well as localized/collective heating, offering tremendous opportunities for optimized energy conversion. Besides, all these functionalities can be tailored via elaborated designs of materials and nanostructures. Here, first the fundamental physics governing thermoplasmonics is presented and then the strategies for both material selection and nanostructured designs toward more efficient energy conversion are summarized. Based on this, recent progress in thermoplasmonic applications including solar evaporation, photothermal chemistry, and thermophotovoltaic is reviewed. Finally, the corresponding challenges and prospects are discussed.

Al@TiO2 Core-Shell Nanoparticles for Plasmonic Photocatalysis

Plasmon-induced photocatalysis is a topic of rapidly increasing interest, due to its potential for substantially lowering reaction barriers and temperatures and for increasing the selectivity of chemical reactions. Of particular interest for plasmonic photocatalysis are antenna-reactor nanoparticles and nanostructures, which combine the strong light-coupling of plasmonic nanostructures with reactors that enhance chemical specificity. Here, we introduce Al@TiO₂ core-shell nanoparticles, combining earth-abundant Al nanocrystalline cores with TiO₂ layers of tunable thickness. We show that these nanoparticles are active photocatalysts for the hot electron-mediated H₂ dissociation reaction as well as for hot hole-mediated methanol dehydration. The wavelength dependence of the reaction rates suggests that the photocatalytic mechanism is plasmonic hot carrier generation with subsequent transfer of the hot carriers into the TiO₂ layer. The Al@TiO₂ antenna-reactor provides an earth-abundant solution for the future design of visible-light-driven plasmonic photocatalysts.

Control of nanoparticles synthesized via vacuum sputter deposition onto liquids: a review

Sputter deposition onto a low volatile liquid matrix is a recently developed green synthesis method for metal/metal oxide nanoparticles (NPs). In this review, we introduce the synthesis method and highlight its unique features emerging from the combination of the sputter deposition and the ability of the liquid matrix to regulate particle growth. Then, manipulating the synthesis parameters to control the particle size, composition, morphology, and crystal structure of NPs is presented. Subsequently, we evaluate the key experimental factors governing the particle characteristics and the formation of monometallic and alloy NPs to provide overall directions and insights into the preparation of NPs with desired properties. Following that, the current understanding of the growth and formation mechanism of sputtered particles in liquid media, in particular, ionic liquids and liquid polymers, during and after sputtering is emphasized. Finally, we discuss the challenges that remain and share our perspectives on the future prospects of the synthesis method and the obtained NPs.

Rh/Al Nanoantenna Photothermal Catalyst for Wide-Spectrum Solar-Driven CO2 Methanation with Nearly 100% Selectivity

Solar-powered CO₂ conversion represents a promising green and sustainable approach for achieving a carbon-neutral economy. However, the rational design of a wide-spectrum sunlight-driven catalysis system for effective CO₂ reduction is an ongoing challenge. Herein, we report the preparation of a rhodium/aluminum (Rh/Al) nanoantenna photothermal catalyst that can utilize a broad range of sunlight (from ultraviolet to the near-infrared region) for highly efficient CO₂ methanation, achieving a high CH₄ selectivity of nearly 100% and an unprecedented CH₄ productivity of 550 mmol·g^-1·h^-1 under concentrated simulated solar irradiation (11.3 W·cm^-2). Detailed control experiment results verified that the CO₂ methanation process was facilitated by the localized surface plasmonic resonance and nanoantenna effects of the Rh/Al nanostructure under light irradiation. In operando temperature-programmed Fourier transform infrared spectroscopy confirmed that CO₂ methanation on the Rh/Al nanoantenna catalyst was a multistep reaction with CO as a key intermediate. The design of a wide-spectrum solar-driven photothermal catalyst provides a feasible strategy for boosting CO₂-to-fuel conversion.

Tunable electron and hole injection channels at plasmonic Al-TiO2 interfaces

Metallic nanostructures can strongly absorb light through their plasmon excitations, whose nonradiative decay generates hot electron-hole pairs. When the metallic nanostructure is interfaced with a semiconductor, the spatial separation of hot carriers plays the central and decisive roles in photovoltaic and photocatalytic applications. In recent years, free-electron metals like Al have attracted tremendous attentions due to the much higher plasmon frequencies that could extend to the ultraviolet regime. Here, the plasmon excitations and charge separations at the Al-TiO₂ interfaces have been investigated using quantum-mechanical calculations, where the atomic structures and electronic dynamics are all treated from first-principles. It is found that the high-frequency plasmon of Al produces abundant and broad-band hot-carrier distributions, where the electron-hole symmetry is broken by the presence of the semiconductor band gap. Such an asymmetric hot-carrier distribution provides two competing channels, which can be controlled either by tuning the laser frequency, or by harnessing the plasmon frequency through the geometry and shape of the metallic nanostructure. Our study suggests that the Al plasmon offers a versatile and tunable pathway for the charge transfer and separation, and has general implications in plasmon-assisted photovoltaics and photocatalysis.

Light-Induced Charge Transfer from Transition-Metal-Doped Aluminum Clusters to Carbon Dioxide

Charge transfer between molecules and catalysts plays a critical role in determining the efficiency and yield of photochemical catalytic processes. In this paper, we study light-induced electron transfer between transition-metal-doped aluminum clusters and CO₂ molecules using first-principles time-dependent density-functional theory. Specifically, we carry out calculations for a range of dopants (Zr, Mn, Fe, Ru, Co, Ni, and Cu) and find that the resulting systems fall into two categories: Cu- and Fe-doped clusters exhibit no ground-state charge transfer, weak CO₂ adsorption, and light-induced electron transfer into the CO₂. In all other systems, we observe ground-state electron transfer into the CO₂ resulting in strong adsorption and predominantly light-induced electron back-transfer from the CO₂ into the cluster. These findings pave the way toward a rational design of atomically precise aluminum photocatalysts.

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.

Recent Advances in Plasmonic Nanostructures for Enhanced Photocatalysis and Electrocatalysis

Plasmonic nanomaterials coupled with catalytically active surfaces can provide unique opportunities for various catalysis applications, where surface plasmons produced upon proper light excitation can be adopted to drive and/or facilitate various chemical reactions. A brief introduction to the localized surface plasmon resonance and recent design and fabrication of highly efficient plasmonic nanostructures, including plasmonic metal nanostructures and metal/semiconductor heterostructures is given. Taking advantage of these plasmonic nanostructures, the following highlights summarize recent advances in plasmon-driven photochemical reactions (coupling reactions, O₂ dissociation and oxidation reactions, H₂ dissociation and hydrogenation reactions, N₂ fixation and NH₃ decomposition, and CO₂ reduction) and plasmon-enhanced electrocatalytic reactions (hydrogen evolution reaction, oxygen reduction reaction, oxygen evolution reaction, alcohol oxidation reaction, and CO₂ reduction). Theoretical and experimental approaches for understanding the underlying mechanism of surface plasmon are discussed. A proper discussion and perspective of the remaining challenges and future opportunities for plasmonic nanomaterials and plasmon-related chemistry in the field of energy conversion and storage is given in conclusion.

Lanthanide-Based Nanosensors: Refining Nanoparticle Responsiveness for Single Particle Imaging of Stimuli

Lanthanide nanoparticles (LNPs) are promising sensors of chemical, mechanical, and temperature changes; they combine the narrow-spectral emission and long-lived excited states of individual lanthanide ions with the high spatial resolution and controlled energy transfer of nanocrystalline architectures. Despite considerable progress in optimizing LNP brightness and responsiveness for dynamic sensing, detection of stimuli with a spatial resolution approaching that of individual nanoparticles remains an outstanding challenge. Here, we highlight the existing capabilities and outstanding challenges of LNP sensors, en-route to nanometer-scale, single particle sensor resolution. First, we summarize LNP sensor read-outs, including changes in emission wavelength, lifetime, intensity, and spectral ratiometric values that arise from modified energy transfer networks within nanoparticles. Then, we describe the origins of LNP sensor imprecision, including sensitivity to competing conditions, interparticle heterogeneities, such as the concentration and distribution of dopant ions, and measurement noise. Motivated by these sources of signal variance, we describe synthesis characterization feedback loops to inform and improve sensor precision, and introduce noise-equivalent sensitivity as a figure of merit of LNP sensors. Finally, we project the magnitudes of chemical and pressure stimulus resolution achievable with single LNPs at nanoscale resolution. Our perspective provides a roadmap for translating ensemble LNP sensing capabilities to the single particle level, enabling nanometer-scale sensing in biology, medicine, and sustainability.

Surface chemistry of quantum-sized metal nanoparticles under light illumination

Size reduction of metal nanoparticles increases the exposure of metal surfaces significantly, favoring heterogeneous chemistry at the surface of the nanoparticles. The optical properties of metal nanoparticles, such as light absorption, also exhibit a strong dependence on their size. It is expected that there will be strong coupling of light absorption and surface chemistry when the metal nanoparticles are small enough. For instance, metal nanoparticles with sizes in the range of 2–10 nm exhibit both surface plasmon resonances, which can efficiently produce high-energy hot electrons near the surface of the nanoparticles under light illumination, and the Coulomb blockade effect, which favors electron transfer from the metal nanoparticles to the surface adsorbates. The synergy of efficient hot electron generation and electron transfer on the surface of small metal nanoparticles leads to double-faced effects: (i) surface (adsorption) chemistry influences optical absorption in the metal nanoparticles, and (ii) optical absorption in the metal nanoparticles promotes (or inhibits) surface adsorption and heterogeneous chemistry. This review article focuses on the discussion of typical quantum phenomena in metal nanoparticles of 2–10 nm in size, which are referred to as “quantum-sized metal nanoparticles”. Both theoretical and experimental examples and results are summarized to highlight the strong correlations between the optical absorption and surface chemistry for quantum-sized metal nanoparticles of various compositions. A comprehensive understanding of these correlations may shed light on achieving high-efficiency photocatalysis and photonics.

Size reduction of metal nanoparticles increases the exposure of metal surfaces significantly, favoring heterogeneous photochemistry at the surface of the nanoparticles.

Hybrid theoretical models for molecular nanoplasmonics

The multidisciplinary nature of the research in molecular nanoplasmonics, i.e., the use of plasmonic nanostructures to enhance, control, or suppress properties of molecules interacting with light, led to contributions from different theory communities over the years, with the aim of understanding, interpreting, and predicting the physical and chemical phenomena occurring at molecular- and nano-scale in the presence of light. Multiscale hybrid techniques, using a different level of description for the molecule and the plasmonic nanosystems, permit a reliable representation of the atomistic details and of collective features, such as plasmons, in such complex systems. Here, we focus on a selected set of topics of current interest in molecular plasmonics (control of electronic excitations in light-harvesting systems, polaritonic chemistry, hot-carrier generation, and plasmon-enhanced catalysis). We discuss how their description may benefit from a hybrid modeling approach and what are the main challenges for the application of such models. In doing so, we also provide an introduction to such models and to the selected topics, as well as general discussions on their theoretical descriptions.

Morphology-Dependent Reactivity of a Plasmonic Photocatalyst

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

Shining Light on Aluminum Nanoparticle Synthesis

ConspectusAluminum in its nanostructured form is generating increasing interest because of its light-harvesting properties, achieved by excitation of its localized surface plasmon resonance. Compared to traditional plasmonic materials, the coinage metals Au and Ag, Al is far more earth-abundant and, therefore, more suitable for large-area applications or where cost may be an important factor. Its optical properties are far more flexible than either Au or Ag, supporting plasmon resonances that range from UV wavelengths, through the visible regime, and into the infrared region of the spectrum. However, the chemical synthesis of Al nanocrystals (NCs) of controlled size and shape has historically lagged far behind that of Au and Ag. This is partially due to the high reactivity of Al precursors, which react readily with O₂, H₂O, and many reagents used in traditional NC syntheses. The first chemical synthesis of Al NCs was demonstrated by Haber and Buhro in 1998, decomposing AlH₃ using titanium isopropoxide (TIP), with a number of subsequent reports refining this protocol. The role of a catalyst in Al NC synthesis is, we believe, unique to this synthetic approach. In 2015, the first synthesis of size controlled Al NCs was published by our group. Since then, we have significantly advanced Al NC synthesis, postsynthetic modifications, and applications of Al nanoparticles (NPs)-NCs with additional surface modifications-in chemical sensing and photocatalysis. Colloidal Al synthesis has its unique challenges, differing markedly from the far more familiar Au and Ag syntheses, which currently appears to present a de facto barrier to broader research activity in this field.The goal of this Account is to highlight developments in controlled synthesis of Al NCs and applications of Al NPs over the last five years. We outline techniques for successful Al NC synthesis and address some of the problems that may be encountered in this synthesis. A mechanistic understanding of AlH₃ decomposition using TIP has been developed, while new directions have been discovered for synthetic control. Facet-binding ligands, alternate Al precursors, new titanium-based reduction catalysts, even solvent composition have all been shown to control reaction products while also opening doors to future developments. A variety of postsynthetic modifications to the Al NC native oxide surface, including polymer, MOF, and transition metal island coatings have been demonstrated for applications in molecular sensing and photocatalysis. In this Account, we hope to convey that Al synthesis is more accessible than generally perceived and to encourage new synthetic development based on underlying mechanisms controlling size and shape. High selectivity in particle faceting and twinning, implementation of seeded growth principles for monodisperse samples, and the demonstration of new, practical applications of Al nanoparticles remain primary challenges in the field. As Al nanoparticle synthesis is refined and new applications emerge, colloidal Al will become an accessible and low-cost plasmonic nanomaterial complementary to Au and Ag.

Aluminum Nanocrystals Grow into Distinct Branched Aluminum Nanowire Morphologies

Plasmonic nanowires (NWs) have generated great interest in their applications in nanophotonics and nanotechnology. Here we report the synthesis of Al nanocrystals (NCs) with controlled morphologies that range from nanospheres to branched NW and NW bundles. This is accomplished by catalyzing the pyrolysis of triisobutyl aluminum (TIBA) with Tebbe's reagent, a titanium(III) catalyst with two cyclopentadienyl ligands. The ratio of TIBA to Tebbe's reagent is critical in determining the morphology of the resulting Al NC. The branched Al NWs grow in their ⟨100⟩ directions and are formed by oriented attachment of isotropic Al NCs on their {100} facets. Branched NWs are strongly absorptive from the UV to the mid-IR, with longitudinal dipolar, higher-order, and transverse plasmons, all contributing to their broadband response. This rapid Al NW synthesis enables the expanded use of Al for plasmonic and nanophotonic applications in the ultraviolet, visible, and infrared regions of the spectrum.

Quantum Leap from Gold and Silver to Aluminum Nanoplasmonics for Enhanced Biomedical Applications

Nanotechnology has been used in many biosensing and medical applications, in the form of noble metal (gold and silver) nanoparticles and nanostructured substrates. However, the translational clinical and industrial applications still need improvements of the efficiency, selectivity, cost, toxicity, reproducibility, and morphological control at the nanoscale level. In this review, we highlight the recent progress that has been made in the replacement of expensive gold and silver metals with the less expensive aluminum. In addition to low cost, other advantages of the aluminum plasmonic nanostructures include a broad spectral range from deep UV to near IR, providing additional signal enhancement and treatment mechanisms. New synergistic treatments of bacterial infections, cancer, and coronaviruses are envisioned. Coupling with gain media and quantum optical effects improve the performance of the aluminum nanostructures beyond gold and silver.

Site-Selective Nanoreactor Deposition on Photocatalytic Al Nanocubes

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

The Pimpled Gold Nanosphere: A Superior Candidate for Plasmonic Photothermal Therapy

BACKGROUND

The development of highly efficient nanoparticles to convert light to heat for anti-cancer applications is quite a challenging field of research.

METHODS

In this study, we synthesized unique pimpled gold nanospheres (PGNSs) for plasmonic photothermal therapy (PPTT). The light-to-heat conversion capability of PGNSs and PPTT damage at the cellular level were investigated using a tissue phantom model. The ability of PGNSs to induce robust cellular damage was studied during cytotoxicity tests on colorectal adenocarcinoma (DLD-1) and fibroblast cell lines. Further, a numerical model of plasmonic (COMSOL Multiphysics) properties was used with the PPTT experimental assays.

RESULTS

A low cytotoxic effect of thiolated polyethylene glycol (SH-PEG400-SH-) was observed which improved the biocompatibility of PGNSs to maintain 89.4% cell viability during cytometry assays (in terms of fibroblast cells for 24 hrs at a concentration of 300 µg/mL). The heat generated from the nanoparticle-mediated phantom models resulted in ΔT=30°C, ΔT=23.1°C and ΔT=21°C for the PGNSs, AuNRs, and AuNPs, respectively (at a 300 µg/mL concentration and for 325 sec). For the in vitro assays of PPTT on cancer cells, the PGNS group induced a 68.78% lethality (apoptosis) on DLD-1 cells. Fluorescence microscopy results showed the destruction of cell membranes and nuclei for the PPTT group. Experiments further revealed a penetration depth of sufficient PPTT damage in a physical tumor model after hematoxylin and eosin (H&E) staining through pathological studies (at depths of 2, 3 and 4 cm). Severe structural damages were observed in the tissue model through an 808-nm laser exposed to the PGNSs.

CONCLUSION

Collectively, such results show much promise for the use of the present PGNSs and photothermal therapy for numerous anti-cancer applications.

Study on the Adsorption and Activation Behaviours of Carbon Dioxide over Copper Cluster (Cu4) and Alumina-Supported Copper Catalyst (Cu4/Al2O3) by means of Density Functional Theory

The adsorption and activation of carbon dioxide over copper cluster (Cu4) and copper doped on the alumina support (Cu4/Al2O3) catalytic systems have been investigated by using density functional theory and climbing image nudged elastic band. The adsorption energies, geometrical configurations, and electronic properties are analysed. The results show the strong chemical interaction between the copper cluster and the alumina support. Both the Cu4 cluster and Cu4/Al2O3 systems have a high adsorption ability for CO2, and the adsorption process is of chemical nature. The role of the alumina support in the adsorption and activation of CO2 has been addressed. The calculated results show that the “synergistic effect” between Al2O3 and Cu4 is the key factor in the activation of CO2.

A nanofabricated plasmonic core-shell-nanoparticle library

Three-layer core-shell-nanoparticle nanoarchitectures exhibit properties not achievable by single-element nanostructures alone and have great potential to enable rationally designed functionality. However, nanofabrication strategies for crafting core-shell-nanoparticle structure arrays on surfaces are widely lacking, despite the potential of basically unlimited material combinations. Here we present a nanofabrication approach that overcomes this limitation. Using it, we produce a library of nanoarchitectures composed of a metal core and an oxide/nitride shell that is decorated with few-nanometer-sized particles with widely different material combinations. This is enabled by resolving a long-standing challenge in this field, namely the ability to grow a shell layer around a nanofabricated core without prior removal of the lithographically patterned mask, and the possibility to subsequently grow smaller metal nanoparticles locally on the shell only in close proximity of the core. Focusing on the application of such nanoarchitectures in plasmonics, we show experimentally and by Finite-Difference Time-Domain (FDTD) simulations that these structures exhibit significant optical absorption enhancement in small metal nanoparticles grown on the few nanometer thin dielectric shell layer around a plasmonic core, and derive design rules to maximize the effect by the tailored combination of the core and shell materials. We predict that these structures will find application in plasmon-mediated catalysis and nanoplasmonic sensing and spectroscopy.

The UV Plasmonic Behavior of Rhodium Tetrahedrons—A Numerical Analysis

Rhodium (Rh) nanoparticles have attracted a lot of attention due to their strong and ambient-stable UV plasmonic response. Very recently, the synthesis of Rh tetrahedra with and without concave defect-rich surfaces serving in plasmon assisted photocatalytic energy conversion has been reported. In this work, we perform a systematic numerical study on plasmonic behavior and surface charge distribution in order to optimize the use of Rh tetrahedra in surface-enhanced spectroscopies and photocatalysis. We analyze the effect of the edges and corners reshaping, a deformation already reported to appear in Rh nanocubes which have been repeatedly re-used in photocatalytic processes. It is demonstrated that rounding the edges and corners weakens both the near-field enhancement and surface charge densities in these locations, which in turn are the more reactive regions due to the presence of uncoordinated sites. In addition, we study how the near-field and charge density is redistributed on the surface of the tetrahedra when concavities of different sizes and depths are introduced. Through this study, we show that, in order to simultaneously maximize the near-field enhancement and surface charge densities in the concavity and at external edges and corners, medium size deep concavities are needed.

Aluminum Nanocubes Have Sharp Corners

Of the many plasmonic nanoparticle geometries that have been synthesized, nanocubes have been of particular interest for creating nanocavities, facilitating plasmon coupling, and enhancing phenomena dependent upon local electromagnetic fields. Here we report the straightforward colloidal synthesis of single-crystalline {100} terminated Al nanocubes by decomposing AlH₃ with Tebbe's reagent in tetrahydrofuran. The size and shape of the Al nanocubes is controlled by the reaction time and the ratio of AlH₃ to Tebbe's reagent, which, together with reaction temperature, establish kinetic control over Al nanocube growth. Al nanocubes possess strong localized field enhancements at their sharp corners and resonances highly amenable to coupling with metallic substrates. Their native oxide surface renders them extremely air stable. Chemically synthesized Al nanocubes provide an earth-abundant alternative to noble metal nanocubes for plasmonics and nanophotonics applications.

Plasmonic Photocatalysis of Nitrous Oxide into N2 and O2 Using Aluminum-Iridium Antenna-Reactor Nanoparticles

Photocatalysis with optically active "plasmonic" nanoparticles is a growing field in heterogeneous catalysis, with the potential for substantially increasing efficiencies and selectivities of chemical reactions. Here, the decomposition of nitrous oxide (N₂O), a potent anthropogenic greenhouse gas, on illuminated aluminum-iridium (Al-Ir) antenna-reactor plasmonic photocatalysts is reported. Under resonant illumination conditions, N₂ and O₂ are the only observable decomposition products, avoiding the problematic generation of NO_x species observed using other approaches. Because no appreciable change to the apparent activation energy was observed under illumination, the primary reaction enhancement mechanism for Al-Ir is likely due to photothermal heating rather than plasmon-induced hot-carrier contributions. This light-based approach can induce autocatalysis for rapid N₂O conversion, a process with highly promising potential for applications in N₂O abatement technologies, satellite propulsion, or emergency life-support systems in space stations and submarines.

Slanted Ag-Al alloy nanorods arrays for highly active and stable surface-enhanced Raman scattering substrates

Aluminum has been established as an earth-abundant and low-cost alternative to gold and silver for plasmonic applications. Particularly, aluminum largely tends to combines with oxygen compared with silver. Here, a simple glancing angle deposition technique is presented to prepare Ag-Al alloy nanorods (NRs) with a small amount of aluminum. The effect of aluminum is to combine oxygen or corroded substances under certain conditions, such as in the air and in etchants. Beside this, owing to the large diffusion coefficient of aluminum in a Si wafer, the aluminum diffuses easily into a Si wafer, so the bonding force between the Ag-Al alloy NRs and Si wafer can be improved accordingly. In this work, 3.5 at% Al alloy NRs are optimal to exhibit high surface-enhanced Raman scattering (SERS) sensitivity, long-time stability as well as strong bonding force with a Si wafer. Ag-Al alloy NRs make a metal-metal alloy a promising material platform to develop pretty sensitive as well as stable SERS substrates.

Progress in the Utilization Efficiency Improvement of Hot Carriers in Plasmon-Mediated Heterostructure Photocatalysis

The effect of plasmon-induced hot carriers (HCs) enables the possibility of applying semiconductors with wide band gaps to visible light catalysis, which becomes an emerging research field in environmental protections. Continued efforts have been made for an efficient heterostructure photocatalytic process with controllable behaviors of HCs. Recently, it has been discovered that the improvement of the utilization of HCs by band engineering is a promising strategy for an enhanced catalytic process, and relevant works have emerged for such a purpose. In this review, we give an overview of the recent progress relating to optimized methods for designing efficient photocatalysts by considering the intrinsic essence of HCs. First, the basic mechanism of the heterostructure photocatalytic process is discussed, including the formation of the Schokkty barrier and the process of photocatalysis. Then, the latest studies for improving the utilization efficiency of HCs in two aspects, the generation and extraction of HCs, are introduced. Based on this, the applications of such heterostructure photocatalysts, such as water/air treatments and organic transformations, are briefly illustrated. Finally, we conclude by discussing the remaining bottlenecks and future directions in this field.

Design of plasmonic nanomaterials for diagnostic spectrometry

Molecular diagnostics relies on the efficient extraction of biomarker information from the given bio-systems. Plasmonic nanomaterials with tailored structural parameters are promising for the development of biomarker assays due to enrichment effect and signal enhancement. Herein, we overview the recent progress on the development of plasmonic nanomaterials for diagnostic spectrometry, encompassing the interface, mechanism, and application of these materials. For interface, we summarized the types of plasmonic nanomaterials used as interfaces between different materials and light. For mechanism, we descirbe the key parameters (e.g., hot carriers and heat) that characterize the plasmonic effect of materials. For application, we highlighted recent advances in matrix assisted laser desorption/ionization mass spectrometry (MALDI MS) and surface enhanced Raman spectroscopy (SERS) toward precision in in vitro and in vivo diagnostics. We foresee the upcoming era of precision diagnostics by nano-assisted spectrometry methods in both academy and industry, which will require the interest and effort of scientists with diverse backgrounds.

Metal-organic frameworks tailor the properties of aluminum nanocrystals

Metal-organic frameworks (MOFs) and metal nanoparticles are two classes of materials that have received considerable recent attention, each for controlling chemical reactivities, albeit in very different ways. Here, we report the growth of MOF shell layers surrounding aluminum nanocrystals (Al NCs), an Earth-abundant metal with energetic, plasmonic, and photocatalytic properties. The MOF shell growth proceeds by means of dissolution-and-growth chemistry that uses the intrinsic surface oxide of the NC to obtain the Al3+ ions accommodated into the MOF nodes. Changes in the Al NC plasmon resonance provide an intrinsic optical probe of its dissolution and growth kinetics. This same chemistry enables a highly controlled oxidation of the Al NCs, providing a precise method for reducing NC size in a shape-preserving manner. The MOF shell encapsulation of the Al NCs results in increased efficiencies for plasmon-enhanced photocatalysis, which is observed for the hydrogen-deuterium exchange and reverse water-gas shift reactions.

Plasmons of hollow nanobar oligomers

Assembling metal nano-objects into well-defined configurations is an effective way to create hybrid plasmonic structures with unusual functionalities.

Optical properties of aluminum intercalated magnesium nanoparticle square array: a computational study

Magnesium nanostructures have recently emerged as a vivid and amazing plasmonic material.

Highly stable and active SERS substrates with Ag-Ti alloy nanorods

Silver (Ag) nanostructures have been intensively studied as one of the most promising surface-enhanced Raman scattering (SERS) substrates; however, their practical applications have been limited by the chemical instability with regard to oxidation, sulfuration and etching of Ag. Therefore, designing and fabricating highly active Ag nanostructures with high SERS stability has been recognized as an important research area. Herein, Ag-Ti alloy nanorods (Ag-Ti alloy NRs) are designed and fabricated by the oblique angle deposition (OAD) method to protect Ag. Taking advantage of the higher chemical activity of Ti compared with Ag, Ti can be sacrificed against oxidation and corrosion, protecting Ag in harsh environments, further ensuring long-term stability of the SERS substrates. It is demonstrated that a 2% Ti (in atoms) substrate possesses extremely high SERS sensitivity, and is stable both in air for more than 1 month and in 10 mM HNO3 solution for 1 hour. The alloy nanostructure provides a new opportunity to achieve highly sensitive and highly stable SERS substrates.

Quantum-Sized Metal Catalysts for Hot-Electron-Driven Chemical Transformation

Hot-electron-driven chemical transformation (HEDCT) represents an emerging research area in utilizing photoresponsive nanoparticles to enable efficient solar-to-chemical conversion. The unique properties of quantum-sized metal nanoparticles (QSMNPs) make them a class of photocatalysts that can generate hot electrons to drive surface chemical reactions with high quantum efficiency. Compared to the conventional thermal-driven chemical reactions, HEDCT offers the advantages of accelerating reaction rate, improving reaction selectivity, and possibly enabling the occurrence of thermodynamically endergonic reactions. Despite its embryonic stage of development, using QSMNPs for HEDCT shows great promise. Herein, a timely overview on the research progress is provided with a focus on the fundamental quantum processes involved in the photoexcitation of hot electrons and the following HEDCT on the surface of QSMNPs. The last section discusses the challenges, which also represent the opportunities for the materials research community, in designing robust QSMNP photocatalysts and understanding the fundamental quantum phenomena in HEDCT.

Effective Optical Properties of Inhomogeneously Distributed Nanoobjects in Strong Field Gradients of Nanoplasmonic Sensors

Accurate and efficient modeling of discontinuous, randomly distributed entities is a computationally challenging task, especially in the presence of large and inhomogeneous electric near-fields of plasmons. Simultaneously, the anisotropy of sensed entities and their overlap with inhomogeneous fields means that typical effective medium approaches may fail at describing their optical properties. Here, we extend the Maxwell Garnett mixing formula to overcome this limitation by introducing a gradient within the effective medium description of inhomogeneous nanoparticle layers. The effective medium layer is divided into slices with a varying volume fraction of the inclusions and, consequently, a spatially varying effective permittivity. This preserves the interplay between an anisotropic particle distribution and an inhomogeneous electric field and enables more accurate predictions than with a single effective layer. We demonstrate the usefulness of the gradient effective medium in FDTD modeling of indirect plasmonic sensing of nanoparticle sintering. First of all, it yields accurate results significantly faster than with explicitly modeled nanoparticles. Moreover, by employing the gradient effective medium approach, we prove that the detected signal is proportional to not only the nanoparticle size but also its size dispersion and potentially shape. This implies that the simple volume fraction parameter is insufficient to properly homogenize these types of nanoparticle layers and that in order to quantify optically the state of the layer more than one independent measurement should be carried out. These findings extend beyond nanoparticle sintering and could be useful in analysis of average signals in both plasmonic and dielectric systems to unveil dynamic changes in exosomes or polymer brushes, phase changes of nanoparticles, or quantifying light absorption in plasmon assisted catalysis.

Aluminum Nanorods

Al nanocrystals can be synthesized by high-temperature decomposition of triisobutyl aluminum, creating a mixture of nanoparticle geometries with a significant fraction (∼15%) being single-crystalline Al nanorods. The Al nanorods are elongated along their ⟨110⟩ direction, and generally exhibit hexagonal cross sections consisting of two adjacent {111} facets separated by {100} facets on opposite sides. Dark-field scattering spectroscopy of individual Al nanorods reveals that rods of varying aspect ratios all possess transverse quadrupolar and octupolar modes in the visible (2-3 eV) and ultraviolet (3-5 eV) regimes. Theoretical modeling indicates that the longitudinal resonances of these nanorods span the near- and mid-infrared regions of the spectrum. This work introduces a new class of anisotropic metal nanocrystals composed of single-crystalline Al, opening the door to highly modifiable plasmonic nanorods from Earth-abundant metals.

Plasmonics in the Ultraviolet with Aluminum, Gallium, Magnesium and Rhodium

Ultraviolet plasmonics (UV) has become an active topic of research due to the new challenges arising in fields such as biosensing, chemistry or spectroscopy. Recent studies have pointed out aluminum, gallium, magnesium and rhodium as promising candidates for plasmonics in the UV range. Aluminum and magnesium present a high oxidation tendency that has a critical effect in their plasmonic performance. Nevertheless, gallium and rhodium have drawn a lot of attention because of their low tendency of oxidation and, at the same time, good plasmonic response in the UV and excellent photocatalytic properties. Here, we present a short overview of the current state of UV plasmonics with the latest findings in the plasmonic response and applications of aluminum, gallium, magnesium and rhodium nanoparticles.

PtAl truncated octahedron nanocrystals for improved formic acid electrooxidation

We prepared well-defined PtAl truncated octahedron alloy nanocrystals with a large density of surface defects, which exhibit enhanced electroactivity in FOR.