Aluminum Nanocrystals Research Articles

Clustering characteristic diffraction vectors in 4-D STEM data sets from overlapping structures in nanocrystalline and amorphous materials

We describe a method for identifying and clustering diffraction vectors in four-dimensional (4-D) scanning transmission electron microscopy data to determine characteristic diffraction patterns from overlapping structures in projection. First, the data is convolved with a 4-D kernel, then diffraction vectors are identified and clustered using both density-based clustering and a metric that emphasizes rotational symmetries. The method works well for both crystalline and amorphous samples and in high- and low-dose experiments. A simulated dataset of overlapping aluminum nanocrystals provides performance metrics as a function of Poisson noise and the number of overlapping structures. Experimental data from an aluminum nanocrystal sample shows similar performance. For an amorphous Pd77.5Cu6Si16.5 thin film, experiments measuring glassy structure show strong evidence of 4- and 6-fold symmetry structures. A significant background arises from the diffraction of overlapping structures. Quantifying this background helps to separate contributions from single, rotationally symmetric structures vs. apparent symmetries arising from overlapping structures in projection.

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.

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.

Capacitively coupled nonthermal plasma synthesis of aluminum nanocrystals for enhanced yield and size control

Uniform-size, non-native oxide-passivated metallic aluminum nanoparticles (Al NPs) have desirable properties for fuel applications, battery components, plasmonics, and hydrogen catalysis. Nonthermal plasma-assisted synthesis of Al NPs was previously achieved with an inductively coupled plasma (ICP) reactor, but the low production rate and limited tunability of particle size were key barriers to the applications of this material. This work focuses on the application of capacitively coupled plasma (CCP) to achieve improved control over Al NP size and a ten-fold increase in yield. In contrast with many other materials, where NP size is controlled via the gas residence time in the reactor, the Al NP size appeared to depend on the power input to the CCP system. The results indicate that the CCP reactor assembly, with a hydrogen-rich argon/hydrogen plasma, was able to produce Al NPs with diameters that were tunable between 8 and 21 nm at a rate up ∼ 100 mg h−1. X-ray diffraction indicates that a hydrogen-rich environment results in crystalline metal Al particles. The improved synthesis control of the CCP system compared to the ICP system is interpreted in terms of the CCP’s lower plasma density, as determined by double Langmuir probe measurements, leading to reduced NP heating in the CCP that is more amenable to NP nucleation and growth.

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.

Branched Aluminum Nanocrystals with Internal Hot Spots: Synthesis and Single-Particle Surface-Enhanced Raman Scattering.

Owing to their unique and sustainable surface plasmonic properties, Al nanocrystals have attracted increasing attention for plasmonic-enhanced applications, including single-particle surface-enhanced Raman scattering (SERS). However, whether Al nanocrystals can achieve single-particle SERS is still unknown, mainly due to the synthetic difficulty of Al nanocrystals with internal gaps. Herein, we report a regrowth method for the synthesis of Al nanohexapods with tunable and uniform internal gaps for single-particle SERS with an enhancement factor of up to 1.79 × 108. The uniform branches of the Al nanohexapods can be systematically tuned regarding their dimensions, terminated facets, and internal gaps. The Al nanohexapods generate hot spots concentrated in the internal gaps due to the strong plasmonic coupling between the branches. A single-particle SERS measurement of Al nanohexapods shows strong Raman signals with maximum enhancement factors comparable to that of Au counterparts. The large enhancement factor indicates that Al nanohexapods are good candidates for single-particle SERS.

Nucleation kinetics model for primary crystallization in Al-Y-Fe metallic glass.

The high density of aluminum nanocrystals (>1021m-3) that develop during the primary crystallization in Al-based metallic glasses indicates a high nucleation rate (∼1018m-3s-1). Several studies have been advanced to account for the primary crystallization behavior, but none have been developed to completely describe the reaction kinetics. Recently, structural analysis by fluctuation electron microscopy has demonstrated the presence of the Al-like medium range order (MRO) regions as a spatial heterogeneity in as-spun Al88Y7Fe5 metallic glass that is representative for the class of Al-based amorphous alloys that develop Al nanocrystals during primary crystallization. From the structural characterization, an MRO seeded nucleation configuration is established, whereby the Al nanocrystals are catalyzed by the MRO core to decrease the nucleation barrier. The MRO seeded nucleation model and the kinetic data from the delay time (τ) measurement provide a full accounting of the evolution of the Al nanocrystal density (Nv) during the primary crystallization under isothermal annealing treatments. Moreover, the calculated values of the steady state nucleation rates (Jss) predicted by the nucleation model agree with the experimental results. Moreover, the model satisfies constraints on the structural, thermodynamic, and kinetic parameters, such as the critical nucleus size, the interface energy, and the volume-free energy driving force that are essential for a fully self-consistent nucleation kinetics analysis. The nucleation kinetics model can be applied more broadly to materials that are characterized by the presence of spatial heterogeneities.

On modification of silumins

It has been shown that the elementary crystal cells of the basic products of modification of silumins, aluminum and silicon do not correspond to the principle of structural and dimensional correspondence of Dankov – Konobeevsky. Modifying elements B, Sr, P significantly increase the surface energy of liquid silumins, and Zr and Ti practically do not affect it. Modification of silumins is a nanostructured process. Ti, Zr, B form intermetallides that refine aluminum nanocrystals from demodifying adsorbed hydrogen atoms. Phosphorus, atomically dissolving in silumin, refiners silicon nanocrystals from adsorbed oxygen. Na and Sr form an emulsion and colloid in liquid silumin, which protect eutectic microcrystals of α‑and βSi‑phases from demodifying molecular hydrogen. All these processes increase the concentration of crystallization centers of dendritic microcrystals α‑and βSi‑phase, make them more branched and compact.

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.

Dissolution of hydrogen in metals and casting alloys

Thermodynamic calculations have shown that hydrogen from atmospheric water vapors can penetrate aluminum, iron and copper, especially into their melts. Hydrogen atoms in these metals are in free and adsorbed states. Strong and dense oxide films on the surfaces of aluminum, iron and copper significantly reduce the solubility of hydrogen in these metals. Copper and iron nanocrystals more actively adsorb atomic hydrogen than aluminum nanocrystals. This is one of the main reasons for the weak solubility of hydrogen in aluminum and the large desorption of hydrogen atoms in the crystallization of aluminum compared to iron and copper.

Facile one-pot synthesis of functional hydrochar catalyst for biomass valorization

Corn stover derived hydrochar (HC) doped with aluminum nanoparticles was synthesized through the facile one-pot hydrothermal carbonization for glucose to fructose isomerization. Amorphous aluminum nanoparticles with crystalline size less than 0.5 nm were uniformly loaded on hydrochar. The phase of the Al-oxide nanoparticles was tailored through aerobic/anaerobic calcination. The results showed that the amorphous aluminum nanocrystals with six coordinated Al-oxide structure were formed under oxygen calcination, which facilitated the high catalytic activity and selectivity of glucose isomerization. It was found that catalysts prepared at hydrothermal temperature of 180℃ and further calcinated at 300℃ under aerobic condition (Al/HC180-300/O) achieved a fructose yield of 42.6% and a selectivity of 83.6% at 180 °C for 5 min. The kinetics, catalytic mechanism, and the deactivation mechanism of the glucose to fructose were further studied using Al/HC180-300/O catalyst. This study provides a high energy efficiency and low metal loading carbon-based catalysts synthetic approach applicable for biorefinery.

Aluminum Nanocrystals: Sub‐3 nm Aluminum Nanocrystals Exhibiting Cluster‐Like Optical Properties (Small 27/2021)

Ultrasmall aluminum nanocrystals (<3 nm) with cluster-like optical properties are prepared with a simple and robust synthetic chemical method in article number 2002524 by Xiaoyu Cheng, Sailing He, and co-workers. Phosphonic acids are used to surface-modify and stabilize aluminum nanocrystals in solution. Ultrasmall aluminum nanocrystals exhibit bright photoluminescence in the ultraviolet range, and their lifetime is ultrashort and wavelength-dependent.

Numerical Simulation of Fracture of Titanium and Aluminum Nanocrystals by the Molecular Dynamics Method

Numerical Simulation of Fracture of Titanium and Aluminum Nanocrystals by the Molecular Dynamics Method

Aluminum nanocrystals evolving from cluster to metallic state: Size tunability and spectral evidence

Sub-3 nm aluminum (Al) nanocrystal is an emerging class of nanomaterial with properties distinct to noble metal nanoclusters. The complete solution synthesis of aluminum nanoclusters was recently reported, and their photoluminescence (PL) observed for the first time. At the moment, there exists no method to tune the size of ultrasmall aluminum nanocrystals in solution thus no knowledge on the boundary state between aluminum nanoclusters to plasmonic nanoparticles. In this work, it is demonstrated a study of size-controlled solution synthesis of ultrasmall aluminum nanocrystals with size controlled between ∼ 2.2 to ∼ 3.8 nm. Increasing the size results in three sets of spectral responses: (1) absorption due to nascent plasmons generated at ∼ 340 nm for larger particles, confirmed by Mie theory calculations; (2) significant decreased quantum yield of PL from ∼ 7.8% to ∼ 2.4%, indicating reduced quantum confinement effects and increased metallicity; (3) drop of fluorescence lifetime was observed, especially when the diameter of aluminum nanoparticles was changed from ∼ 3.0 to ∼ 3.8 nm. This study provides experimental evidence and insights to the transitional state between aluminum nanoclusters to plasmonic nanoparticles, which seems to occur at size larger than gold nanoclusters.

Numerical Simulation of Fracture of Titanium and Aluminum Nanocrystals by the Molecular Dynamics Method

Numerical Simulation of Fracture of Titanium and Aluminum Nanocrystals by the Molecular Dynamics Method

Sub-3nm Aluminum Nanocrystals Exhibiting Cluster-Like Optical Properties.

Metal nanoclusters with distinct photophysical and photochemical properties have drawn intense research interests for their applications in optoelectronics, catalysis, and biomedicine. Herein, strong evidence is provided that light metal is capable of generating comparable optical responses of noble metal nanoclusters, but at much shorter wavelength. Air-stable, size-uniform, sub-3nm aluminum nanocrystals are prepared with simple solution based synthetic procedures, with photoluminescence located in the ultraviolet range and short exciton lifetime. Partial modulation of the photoluminescence is achieved, indicating the key role of surface oxides. This work is envisioned to inspire new frontiers of nanocluster research with light metals.

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.

Correction: General criteria for evaluating suitable polymer ligands for the synthesis of aluminum nanocrystals.

Correction for 'General criteria for evaluating suitable polymer ligands for the synthesis of aluminum nanocrystals' by Hua Yu et al., Chem. Commun., 2020, 56, 217-220, DOI: .

General criteria for evaluating suitable polymer ligands for the synthesis of aluminum nanocrystals.

Herein, we evaluate the suitablity of polystyrenes with thiol, dithioester and trithiocarbonate end-groups for the synthesis of Al nanocrystals. Both the end-groups and molecular weight of the polymer ligands play important roles for controlling the size and shape of the Al nanocrystals. A general criterion for evaluating polymers as the ligand for the synthesis of Al nanocrystals is proposed.