Shining Light on Aluminum Nanoparticle Synthesis

Sustainable Fabrication and Transfer of High-Precision Nanoparticle Arrays Using Recyclable Chemical Pattern Templates

Nanoparticle (NP) arrays, particularly those with plasmonic properties, have diverse applications in electronics, photonics, catalysis, and biosensing, but their precise and scalable fabrication remains challenging. In this work, a facile chemical-based strategy is presented for the fabrication of precise NP patterns using a combination of soft thermal nanoimprinting and template-directed assembly. The approach enables the creation of well-defined NP arrays with single-particle resolution and yields over 99%, covering a diverse range of NP sizes from 30 to 150 nm. These patterns can be transferred onto various substrates including semiconductors, insulators, 2D materials, and flexible polymers, maintaining high uniformity and repeatability for over 60 cycles with minimal degradation. Moreover, the method enables the fabrication of extensive NP arrays up to 1 cm2 with a positional accuracy of ±11 nm for 30 nm NPs. As a result, the obtained silver NP arrays exhibit ultranarrow surface lattice resonances with a linewidth of 4 nm and a quality factor (Q) of 216. The method offers new avenues for the creation of plasmonic NP arrays for flexible and wearable devices.

Deciphering the Photocatalysis Mechanism of Semimetallic Bismuth Nanoparticles

Metallic nanoparticle photocatalysts have been developed in various catalytic systems over the past few decades, including diverse noble and non-noble metals with plasmonic properties. The hot-carrier-induced mechanism is one of the most appealing pathways as it can provide energetic electrons or holes for driving thermodynamically unfavorable reactions or increasing the reaction rate. In this work, we evaluate the photocatalytic performance of semimetallic bismuth nanoparticles and offer detailed mechanistic interpretations in terms of hot carriers and interband transitions. The photocatalyzed nitrophenol reduction with sodium borohydride serves as a model reaction, and a wavelength-dependent study reveals the contribution of hot carriers. It is demonstrated that light irradiation under shorter wavelengths could produce deeper hot holes in bismuth nanoparticles, which can be quenched more effectively by hole scavengers, thus facilitating the electron-transfer process and resulting in larger apparent reaction rate constants. The observed photocatalysis enhancement accounts for the unique band structure with an extremely small band gap and exclusive interband absorption in the visible region. This proof-of-concept work offers a different perspective on the photocatalysis mechanism of bismuth nanoparticles and could help us better understand the role of hot carriers involved in photocatalysis, especially with interband transitions.

Plasmonics Meets Perovskite Photovoltaics: Innovations and Challenges in Boosting Efficiency

Perovskite solar cells (PSCs) have garnered immense attention in recent years due to their outstanding optoelectronic properties and cost-effective fabrication methods, establishing them as promising candidates for next-generation photovoltaic technologies. Among the diverse strategies aimed at enhancing the power conversion efficiency (PCE) of PSCs, the incorporation of plasmonic nanoparticles has emerged as a pioneering approach. This review summarizes the latest research advancements in the utilization of plasmonic nanoparticles to enhance the performance of PSCs. We delve into the fundamental principles of plasmonic resonance and its interaction with perovskite materials, highlighting how localized surface plasmons can effectively broaden light absorption, facilitate hot-electron transfer (HET), and optimize charge separation dynamics. Recent strategies, including the design of tailored metal nanoparticles (MNPs), gratings, and hybrid plasmonic-photonic architectures, are critically evaluated for their efficacy in enhancing light trapping, increasing photocurrent, and mitigating charge recombination. Additionally, this review addresses the challenges associated with the integration of plasmonic elements into PSCs, including issues of scalability, compatibility, and cost-effectiveness. Finally, the review provides insights into future research directions aimed at advancing the field, thereby paving the way for next-generation, high-performance perovskite-based photovoltaic technologies.

Deciphering the Underlying Mechanism of the Fourth Entity in Medium-Entropy NiCoFeMP toward Boosting Oxygen Evolution Electrocatalysis

High-/medium-entropy materials have been explored as promising electrocatalysts for water splitting due to their unique physical and chemical properties. Unfortunately, state-of-the-art materials face the dilemma of explaining the enhancement mechanism, which is now limited to theoretical models or an unclear cocktail effect. Herein, a medium-entropy NiCoFeMnP with an advanced hierarchical particle-nanosheet-tumbleweed nanostructure has been synthesized via simple precursor preparation and subsequent phosphorization. Evaluated as the electrocatalyst for oxygen evolution reaction (OER), the medium-entropy NiCoFeMnP displays a lower overpotential of 272 mV at a current density of 10 mA cm-2, and more favorable kinetics than the binary NiFeP, ternary NiCoFeP, quaternary NiCoFeCuP and NiCoFeCrP counterparts, and other reported high-/medium-entropy electrocatalysts. Careful experimental analyses reveal that the incorporation of Mn can significantly regulate the electronic structure of Ni, Co, and Fe sites. More importantly, the Mn introduction and entropy stabilization effect in the reconstructed metal (oxy)hydroxide simultaneously promote the lattice oxygen mechanism, improving the activity. This work sheds new light on the design of high-/medium-entropy materials from an in-depth understanding of the underlying mechanism for improving energy conversion efficiency.

Plasmon-enhanced fluorescence for biophotonics and bio-analytical applications

Fluorescence spectroscopy serves as an ultrasensitive sophisticated tool where background noises which serve as a major impediment to the detection of the desired signals can be safely avoided for detections down to the single-molecule levels. One such way of bypassing background noise is plasmon-enhanced fluorescence (PEF), where the interactions of fluorophores at the surface of metals or plasmonic nanoparticles are probed. The underlying condition is a significant spectral overlap between the localized surface plasmon resonance (LSPR) of the nanoparticle and the absorption or emission spectra of the fluorophore. The rationale being the coupling of the excited state of the fluorophore with the localized surface plasmon leads to an augmented emission, owing to local field enhancement. It is manifested in enhanced quantum yields concurrent with a decrease in fluorescence lifetimes, owing to an increase in radiative rate constants. This improvement in detection provided by PEF allows a significant scope of expansion in the domain of weakly emitting fluorophores which otherwise would have remained unperceivable. The concept of coupling of weak emitters with plasmons can bypass the problems of photobleaching, opening up avenues of imaging with significantly higher sensitivity and improved resolution. Furthermore, amplification of the emission signal by the coupling of free electrons of the metal nanoparticles with the electrons of the fluorophore provides ample opportunities for achieving lower detection limits that are involved in biological imaging and molecular sensing. One avenue that has attracted significant attraction in the last few years is the fast, label-free detection of bio-analytes under physiological conditions using plasmonic nanoparticles for point-of-care analysis. This review focusses on the applications of plasmonic nanomaterials in the field of biosensing, imaging with a brief introduction on the different aspects of LSPR and fabrication techniques.

Reduced-Dimensionality Al Nanocrystals: Nanowires, Nanobars, and Nanomoustaches

Aluminum nanocrystals created by catalyst-driven colloidal synthesis support excellent plasmonic properties, due to their high level of elemental purity, monocrystallinity, and controlled size and shape. Reduction in the rate of nanocrystal growth enables the synthesis of highly anisotropic Al nanowires, nanobars, and singly twinned "nanomoustaches". Electron energy loss spectroscopy was used to study the plasmonic properties of these nanocrystals, spanning the broad energy range needed to map their plasmonic modes. The coupling between these nanocrystals and other plasmonic metal nanostructures, specifically Ag nanocubes and Au films of controlled nanoscale thickness, was investigated. Al nanocrystals show excellent long-term stability under atmospheric conditions, providing a practical alternative to coinage metal-based nanowires in assembled nanoscale devices.

Tailoring the aluminum nanocrystal surface oxide for all-aluminum-based antenna-reactor plasmonic photocatalysts

Aluminum nanocrystals (AlNCs) are of increasing interest as sustainable, earth-abundant nanoparticles for visible wavelength plasmonics and as versatile nanoantennas for energy-efficient plasmonic photocatalysis. Here, we show that annealing AlNCs under various gases and thermal conditions induces substantial, systematic changes in their surface oxide, modifying crystalline phase, surface morphology, density, and defect type and concentration. Tailoring the surface oxide properties enables AlNCs to function as all-aluminum-based antenna-reactor plasmonic photocatalysts, with the modified surface oxides providing varying reactivities and selectivities for several chemical reactions.

Sensitive Aluminum SPR Sensors Prepared by Thermal Evaporation Deposition

We used straightforward thermal evaporation deposition to form thin Al films on fused silica slides as surface plasmon resonance (SPR) sensors in the blue visible region. Compared to other studies, we achieved high-quality Al SPR sensors with a low vacuum level at 7 × 10-4 Pa and a low deposition rate between 1.47 and 3.41 nm/s. These Al films have an atomic-level surface roughness. With our recipe, the requirements for deposition conditions are relaxed, and the operation time is reduced remarkably. The experimental sensitivity of the bulk refractive index measurements using 405 nm probing light is as high as 149.9°/RIU. Compared with other studies, our blue visible Al SPR completes the Al SPR working frequency range from deep UV to near-infrared which is much broader than the working range of Au SPR sensors. The cost of Al material is cheap, and the deposition instrument is also economic and operation easy. Considering the compatibility with most of the nanofabrication procedures and stability from the native oxide layer, Al SPR sensors have a huge potential to replace Au SPR sensors as the new golden standard of SPR sensing technology.

Controlling the Surface of Aluminum Nanocrystals: From Aluminum Oxide to Aluminum Fluoride

Aluminum nanocrystals are emerging as a promising alternative to silver and gold for various applications ranging from plasmonic functionalities to photocatalysis and as energetic materials. Such nanocrystals often exhibit an inherent surface oxidation layer, as aluminum is highly reactive. Its controlled removal is challenging but required, as it can hinder the properties of the encaged metal. Herein, two wet-chemical colloidal approaches toward the surface coating of Al nanocrystals, which afford control over the surface chemistry of the nanocrystals and the oxide thickness, are presented. The first approach utilizes oleic acid as a surface ligand by its addition toward the end of the Al nanocrystals synthesis, and the second approach is the post-synthesis treatment of Al nanocrystals with NOBF4 , in a "wet" colloidal-based approach, which is found to etch and fluorinate the surface oxides. As surface chemistry is an important handle for controlling materials' properties, this research paves a path for manipulating Al nanocrystals while promoting their utilization in diverse applications.

Photoactivated plasmonic nanohybrid fibers with prolonged trapping of excited charge carriers for SERS analysis of biomolecules

The quest to enhance Raman spectroscopic signals through the rational design of plasmonic substrates has enabled the detection and characterization of pharmaceutically important molecules with low scattering cross-sections, such as amino acids and proteins, and is helping in making forays into the diverse field of biomedical sciences. This work presents a simple strategy for synthesizing silver nanoparticles-incorporated alumina nanofibers (Ag-AlNFs) utilizing controlled microwave synthesis for enhancing the surface-enhanced Raman chemical enhancement factor through photo-induced charge accumulation at the plasmonic-dielectric interface. The plasmonic-dielectric fibers serve as excellent charge carrier trappers, as evident from the ultrafast transient absorption spectroscopy studies. Apart from chemical enhancement, the increase in electronic surface charge also enables the protein disulfide bonds to capture these electrons and form a transient disulfide electron adduct radical, which converts to free thiol radical on dissociation. This allows protein molecules to bind to the nanoparticle's surface with the favorable silver thiol bond leading to greater surface affinity and larger SERS enhancement. The proposed Ag-AlNFs represent a cost-effective material that can be potentially used to probe biological systems in a label-free manner by photoactivating the SERS substrate for obtaining higher enhancement factors.

Chemical and Physical Properties of Photonic Noble-Metal Nanomaterials

Colloidal noble metal nanoparticles (NPs) are composed of metal cores and organic or inorganic ligand shells. These NPs support size- and shape-dependent plasmonic resonances. They can be assembled from dispersions into artificial metamolecules which have collective plasmonic resonances originating from coupled bright and dark optical electric and magnetic modes that form depending on the size and shape of the constituent NPs and their number, arrangement, and interparticle distance. NPs can also be assembled into extended 2D and 3D metamaterials that are glassy thin films or ordered thin films or crystals, also known as superlattices and supercrystals. The metamaterials have tunable optical properties that depend on the size, shape, and composition of the NPs, and on the number of NP layers and their interparticle distance. Interestingly, strong light-matter interactions in superlattices form plasmon polaritons. Tunable interparticle distances allow designer materials with dielectric functions tailorable from that characteristic of an insulator to that of a metal, and serve as strong optical absorbers or scatterers, respectively. In combination with lithography techniques, these extended assemblies can be patterned to create subwavelength NP superstructures and form large-area 2D and 3D metamaterials that manipulate the amplitude, phase, and polarization of transmitted or reflected light.

Plate-Like Colloidal Metal Nanoparticles

The pseudo-two-dimensional (2D) morphology of plate-like metal nanoparticles makes them one of the most anisotropic, mechanistically understood, and tunable structures available. Although well-known for their superior plasmonic properties, recent progress in the 2D growth of various other materials has led to an increasingly diverse family of plate-like metal nanoparticles, giving rise to numerous appealing properties and applications. In this review, we summarize recent progress on the solution-phase growth of colloidal plate-like metal nanoparticles, including plasmonic and other metals, with an emphasis on mechanistic insights for different synthetic strategies, the crystallographic habits of different metals, and the use of nanoplates as scaffolds for the synthesis of other derivative structures. We additionally highlight representative self-assembly techniques and provide a brief overview on the attractive properties and unique versatility benefiting from the 2D morphology. Finally, we share our opinions on the existing challenges and future perspectives for plate-like metal nanomaterials.

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.

Facet Tunability of Aluminum Nanocrystals

Aluminum nanocrystals (Al NCs) with a well-defined size and shape combine unique plasmonic properties with high earth abundance, potentially ideal for applications where sustainability and cost are important factors. It has recently been shown that single-crystal Al {100} nanocubes can be synthesized by the decomposition of AlH₃ with Tebbe's reagent, a titanium(IV) catalyst with two cyclopentadienyl ligands. By systematically modifying the catalyst molecular structure, control of the NC growth morphology is observed spectroscopically, as the catalyst stabilizes the {100} NC facets. By varying the catalyst concentration, Al NC faceted growth is tunable from {100} faceted nanocubes to {111} faceted octahedra. This study provides direct insight into the role of catalyst molecular structure in controlling Al NC morphology.

Plasmonic Hybrid Nanostructures in Photocatalysis: Structures, Mechanisms, and Applications

(Sun)Light is an abundantly available sustainable source of energy that has been used in catalyzing chemical reactions for several decades now. In particular, studies related to the interaction of light with plasmonic nanostructures have been receiving increased attention. These structures display the unique property of localized surface plasmon resonance, which converts light of a specific wavelength range into hot charge carriers, along with strong local electromagnetic fields, and/or heat, which may all enhance the reaction efficiency in their own way. These unique properties of plasmonic nanoparticles can be conveniently tuned by varying the metal type, size, shape, and dielectric environment, thus prompting a research focus on rationally designed plasmonic hybrid nanostructures. In this review, the term "hybrid" implies nanomaterials that consist of multiple plasmonic or non-plasmonic materials, forming complex configurations in the geometry and/or at the atomic level. We discuss the synthetic techniques and evolution of such hybrid plasmonic nanostructures giving rise to a wide variety of material and geometric configurations. Bimetallic alloys, which result in a new set of opto-physical parameters, are compared with core-shell configurations. For the latter, the use of metal, semiconductor, and polymer shells is reviewed. Also, more complex structures such as Janus and antenna reactor composites are discussed. This review further summarizes the studies exploiting plasmonic hybrids to elucidate the plasmonic-photocatalytic mechanism. Finally, we review the implementation of these plasmonic hybrids in different photocatalytic application domains such as H₂ generation, CO₂ reduction, water purification, air purification, and disinfection.

A Dual Catalyst Strategy for Controlling Aluminum Nanocrystal Growth

The synthesis of Al nanocrystals (Al NCs) is a rapidly expanding field, but there are few strategies for size and morphology control. Here we introduce a dual catalyst approach for the synthesis of Al NCs to control both NC size and shape. By using one catalyst that nucleates growth more rapidly than a second catalyst whose ligands affect NC morphology during growth, one can obtain both size and shape control of the resulting Al NCs. The combination of the two catalysts (1) titanium isopropoxide (TIP), for rapid nucleation, and (2) Tebbe's reagent, for specific facet-promoting growth, yields {100}-faceted Al NCs with tunable diameters between 35 and 65 nm. This dual-catalyst strategy could dramatically expand the possible outcomes for Al NC growth, opening the door to new controlled morphologies and a deeper understanding of earth-abundant plasmonic nanocrystal synthesis.

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.

Room-temperature liquid-phase synthesis of aluminium nanoparticles

Aluminium nanoparticles, Al(0), are obtained via liquid-phase synthesis at 25 °C. Accordingly, AlBr₃ is reduced by lithium naphthalenide ([LiNaph]) in toluene in the presence of N,N,N',N'-tetramethylethylenediamine (TMEDA). The Al(0) nanoparticles are small (5.6 ± 1.5 nm) and highly crystalline. A light yellow colour and absorption at 250-350 nm are related to the plasmon-resonance absorption. Due to TMEDA functionalization, the Al(0) nanoparticles are colloidally and chemically stable, but show high reactivity after TMEDA removal.

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.

Revealing the effects of molecular orientations on the azo-coupling reaction of nitro compounds driven by surface plasmonic resonances

A recent report on the azo coupling of 4-nitrobenzo-15-crown-ether (4NB15C) and 4-nitrothiophenol (4NTP) indicated that the reaction barrier could be reduced greatly with surface plasmonic effects on silver dendritic nanostructures in aqueous solution. Accordingly, an azo coupling reaction mechanism was proposed based on one or two SERS peaks. Toward a profound understanding of this azo coupling reaction mechanism, it is crucial to scrutinize the origin of the full SERS spectrum. Here, we construct a molecular model consisting of 4NTP and 4NB15C on an Ag₇ cluster that simulates a silver dendritic nanostructure, and investigate the SERS spectra of the azo coupling of these two molecules. We propose five different adsorption sites and 13 different orientations of 4NTP on the Ag₇ cluster and optimize the geometries of the five configurations. With each optimized configuration of 4NTP adsorbed on Ag₇, we further consider the azo coupling product with a 4NB15C molecule and simulate the corresponding Raman spectra. Comparing the measured Raman spectra and model analysis, we conclude that the azo coupling reaction depends decisively on a parallel molecular orientation of the adsorbed 4NTP relative to the facets of Ag₇, the orientation of which further directs the subsequent reaction for the product of 4NB15C-4NTP.

Tip-Enhanced Raman Analysis of Plasmonic and Photocatalytic Properties of Copper Nanomaterials

Theoretical predictions suggest that, in addition to gold (Au) and silver (Ag), several other metals such as copper (Cu) and aluminum (Al) can be used as plasmonic materials. However, their plasmonic and photocatalytic properties remain poorly understood. In this contribution, we employed tip-enhanced Raman spectroscopy to examine photocatalytic properties of Cu nanowires and nanocubes (CuNWs and CuNCs). Our results show that both CuNWs and CuNCs demonstrate a far more efficient photocatalytic dimerization of 4-nitrobenzenethiol to 4,4'-dimercaptoazobenzene than Au nano and microplates. We also found that CuNWs and CuNCs can neither reduce 4-mercaptobenzoic acid (4-MBA) to the corresponding aromatic alcohol nor dearboxylate it forming benzenethiol. We infer that this is due to a unique coordination of 4-MBA on Cu surfaces that was only rarely observed on Au and Ag nanomaterials. Finally, we found that Cu nanostructures can oxidize 4-mercapto-phenyl-methanol to 4-MBA, which was previously only observed on gold-platinum nanoplates.