Metal Nanocrystals Research Articles

Size dependence of the phase transition temperature of metal nanocrystals

Melting, freezing, and evaporation are common phase transition processes in metallic nanocrystals. Predicting these phase transitions and understanding their underlying mechanisms are crucial for the design, preparation, and application of nanomaterials. In this work, we establish the thermodynamic models to describe the size dependence of cohesive energy Ec, melting temperature Tm, freezing temperature Tf, and evaporation temperature Te. By comparing the predictions with the available experimental results, a good agreement is obtained. In addition, we find that at the same diameter D, the Tm(D)/Tm, Tf(D)/Tf, and Te(D)/Te of nanocrystals increase with the improvement of the ratio of solid-liquid interface energy γsl to interfacial stress f, indicating that γsl/f determines the thermal stability of the surfaces.

Shape Transformation via Etching and Regrowth: A Systematic Study of Pd Nanocrystals with Different Shapes and Twin Structures.

This article describes a systematic study of the oxidative etching and regrowth behaviors of Pd nanocrystals, including single-crystal cubes bounded by {100} facets, single-crystal octahedra and tetrahedra enclosed by {111} facets; and multiple-twinned icosahedra covered by {111} facets and twin boundaries. During etching, Pd atoms are preferentially oxidized and removed from the corners regardless of the type of nanocrystal, and the resultant Pd2+ ions are then reduced back to elemental Pd. For cubes and icosahedra, the newly formed Pd atoms are deposited on the {100} facets and twin boundaries, respectively, due to their relatively higher energies. For octahedra and tetrahedra, the Pd atoms self-nucleate in the solution phase, followed by their growth into small particles. We can control the regrowth rate relative to etching rate by varying the concentration of HCl in the reaction solution. As the concentration of HCl is increased, 18-nm Pd cubes are transformed into octahedra of 23, 18, and 13 nm, respectively, in edge length. Due to the absence of regrowth, however, Pd octahedra are transformed into truncated octahedra, cuboctahedra, and spheres with decreasing sizes whereas Pd tetrahedra evolve into truncated tetrahedra and spheres. In contrast, Pd icosahedra with twin boundaries on the surface are converted to asymmetric icosahedra, flower-like icosahedra, and spheres. This work not only advances the understanding of etching and growth behaviors of metal nanocrystals with various shapes and twin structures but also offers an alternative method for controlling their shape and size.

Nanoscale goldbeating: Solid-state transformation of 0D and 1D gold nanoparticles to anisotropic 2D morphologies.

Goldbeating is the ancient craft of thinning bulk gold (Au) into gossamer leaves. Pioneered by ancient Egyptian craftsmen, modern mechanized iterations of this technique can fabricate sheets as thin as ∼100 nm. We take inspiration from this millennia-old craft and adapt it to the nanoscale regime, using colloidally synthesized 0D/1D Au nanoparticles (AuNPs) as highly ductile and malleable nanoscopic Au ingots and subjecting them to solid-state, uniaxial compression. The applied stress induces anisotropic morphological transformation of AuNPs into 2D leaf form and elucidates insights into metal nanocrystal deformation at the extreme length scales. The induced 2D morphology is found to be dependent on the precursor 0D/1D NP morphology, size (0D nanosphere diameter and 1D nanorod diameter and length), and their on-substrate arrangement (e.g., interparticle separation and packing order) prior to compression. Overall, this versatile and generalizable solid-state compression technique enables new pathways to synthesize and investigate the anisotropic morphological transformation of arbitrary NPs and their resultant emergent phenomena.

Enhanced bifunctional electrocatalytic performance of NiFe-layered double hydroxide activated by ultrasonic-assisted loading of Pd nanoclusters

In this study, metallic Pd nanocrystals loaded on NiFe-layered double hydroxide (Pd/NiFe-LDH-u) electrocatalyst for water splitting is synthesized via a novel ultrasonic-assisted ion exchange and in situ reduction method. The ultrasonic-assisted loaded Pd clusters (with an average size 2.84 nm) are in high crystallinity. The optimized Pd/NiFe-LDH-u displays remarkable oxygen evolution reaction (OER) catalytic activity with a low overpotential of 267 mV at 20 mA cm−2. The hydrogen evolution reaction (HER) mass activity of Pd/NiFe-LDH-u is up to 6.37 A mg−1 at an overpotential of 300 mV, which is nearly 10 times higher than that of bare Pd and outperforming most reported Pd-based catalysts. Both the experimental results and density functional theory (DFT) calculations prove that the ultrasonic-assisted loading effectively strengthens the interaction between Pd and NiFe-layered double hydroxide (NiFe-LDH), resulting in improved dispersion uniformity, catalytic stability, electrical conductivity and bifunctional catalytic activities.

Surface-Enhanced Raman Spectroscopy of Benzylpenicillin Using Silver Nanocrystals Modified with Moroccan Plant Extracts

Green chemical routes for the synthesis of colloidal metal nanocrystals have been of great interest, namely in the context of nanosciences associated with biological applications. Among these methods, the synthesis of metal colloids using medicinal plant extracts originates nanocrystals having surfaces modified with chemical compounds of biological origin, which can be further explored in association with conventional pharmaceutics. In this context, the development of spectroscopic methods that seeks for understanding the potential benefits of using formulations that contain natural compounds and metal nanoparticles with therapeutic properties is of relevance. This research describes the chemical synthesis of silver colloids via the reduction of Ag(I) in the presence of distinct aqueous plant extracts. The selected extracts were obtained from Moroccan plants that have been used in traditional therapeutic practices over the centuries. The method led to stable colloids comprising polydispersed Ag nanocrystals that show surface-enhanced Raman scattering (SERS) activity. As an illustrative scenario, these colloids have been applied to the SERS detection of the natural β-lactam antibiotic benzylpenicillin, also known as penicillin G (PG). Our results indicate that all the Ag colloids tested with the different plant extracts are SERS-active for PG without showing detrimental interference from chemical adsorbates originated from the extracts. Therefore, this spectroscopic method can be further explored for monitoring nanoformulations of pharmaceuticals and metal colloids obtained using biological synthesis.

NIR enhancement induced by refractive index differences between noble metal particles and nanocrystals in glass-ceramics

The use of optically active ions doped into photonic media containing noble metals is an intriguing area of photoluminescence (PL) research. In this paper, we present a new approach using nickel ions doped into photonic glass ceramics (GCs) containing Au metal NCs. We exploit the unique PL properties of Ni2+ embedded in ZnAl2O4 nanocrystals (NCs). Under the same conditions, the Au-ZAO duplex samples exhibit 2–3 times higher near-infrared (NIR) emission intensity and higher absorption intensity compared to the samples without Au metal. We provide direct evidence of energy harvesting around NCs through numerical simulations. The significant NIR PL properties of Ni2+ in Au-ZAO GCs can be explained by the significant dielectric coefficient difference between the Au metal, the surrounding medium, and the ZAO NCs. Furthermore, the intense and broad emission spectra of our samples cover the entire second biological NIR region, which makes them particularly useful for biomedical applications.

One-Pot Biosynthesis of Carbon-Coated Silver Nanoparticles Using Palm Leaves as a Reductant and a Carbon Source.

In this study, carbon-coated silver nanoparticles (Ag@C NPs) were synthesized with a one-pot hydrothermal method using palm leaves as a reductant and a carbon source. SEM, TEM, XRD, Raman, and UV-vis analyses were employed to characterize the as-prepared Ag@C NPs. Results showed that the diameter of silver nanoparticles (Ag NPs) and the coating thickness could be controlled by changing the amount of biomass and the reaction temperature. The diameter ranged from 68.33 to 143.15 nm, while the coating thickness ranged from 1.74 to 4.70 nm. As the biomass amount and the reaction temperature increased, the diameter of Ag NPs and the coating thickness became larger. Thus, this work provided a green, simple, and feasible method for the preparation of metal nanocrystals.

Surface Engineering of Metal and Semiconductor Nanocrystal Assemblies and Their Optical and Electronic Devices.

ConspectusColloidal nanocrystals (NCs) are composed of inorganic cores and organic or inorganic ligand shells and serve as building blocks of NC assemblies. Metal and semiconductor NCs are well known for the size-dependent physical properties of their cores. The large NC surface-to-volume ratio and the space between NCs in assemblies places significant importance on the composition of the NC surface and ligand shell. Nonaqueous colloidal NC syntheses use relatively long organic ligands to control NC size and uniformity during growth and to prepare stable NC dispersions. However, these ligands create large interparticle distances that dilute the metal and semiconductor NC properties of their assemblies. In this Account, we describe postsynthesis chemical treatments to engineer the NC surface and design the optical and electronic properties of NC assemblies. In metal NC assemblies, compact ligand exchange reduces the interparticle distance and drives an insulator-to-metal transition tuning the dc resistivity over a 1010 range and the real part of the optical dielectric function from positive to negative across the visible-to-IR region. Juxtaposing NC and bulk metal thin films in bilayers allows the differential chemical and thermal addressability of the NC surface to be exploited in device fabrication. Ligand exchange and thermal annealing densifies the NC layer, creating interfacial misfit strain that triggers folding of the bilayers and is used to fabricate, with only one lithography step, large-area 3D chiral metamaterials. In semiconductor NC assemblies, chemical treatments such as ligand exchange, doping, and cation exchange control the interparticle distance and composition to add impurities, tailor stoichiometry, or make entirely new compounds. These treatments are employed in longer studied II-VI and IV-VI materials and are being developed as interest in III-V and I-III-VI2 NC materials grows. NC surface engineering is used to design NC assemblies with tailored carrier energy, type, concentration, mobility, and lifetime. Compact ligand exchange increases the coupling between NCs but can introduce intragap states that scatter and reduce the lifetime of carriers. Hybrid ligand exchange with two different chemistries can enhance the mobility-lifetime product. Doping increases carrier concentration, shifts the Fermi energy, and increases carrier mobility, creating n- and p-type building blocks for optoelectronic and electronic devices and circuits. Surface engineering of semiconductor NC assemblies is also important to modify device interfaces to allow the stacking and patterning of NC layers and to realize excellent device performance. It is used to construct NC-integrated circuits, exploiting the library of metal, semiconductor, and insulator NCs, to achieve all-NC, solution-fabricated transistors.

Carbon Aerogels Loaded with Noble Metal Nanocrystal Electrocatalysts for Efficient Full Water Splitting

It is always a challenge to improve the stability of the electrocatalyst and reduce noble metal loading in the process of hydrogen production from electrolytic water. Herein, we have designed a general carbon aerogel (CG) loaded with noble metal ultrafine nanocrystalline electrocatalysts for efficient full water splitting. The precursor polyethyleneimine/polydopamine co-modified cellulose nanocrystal (CNC)-loaded noble metal by the hydrothermal method and carbonized at high temperature to produce the CG-loaded noble metal ultrafine nanocatalysts (size < 2.5 nm). Through the control of components, different nanocatalysts (defined as CG-M-X, M as Pt and Ir and X as the quantity score) exhibit extraordinary electrocatalytic activity. To achieve a catalytic current of 10 mA cm–2 in 1 M KOH electrolyte, the overpotential required for hydrogen evolution was 23.6 mV for CG-Pt-1.19% and 242 mV for oxygen evolution in CG-Ir-1.62%. When assembled into the full water splitting system, CG-Pt-1.19%||CG-Ir-1.62% delivers a current density of 10 mA cm–2 at a low voltage of 1.43 V, which can steadily drive the complete decomposition of water at nearly 100% Faraday efficiency for up to 50 h without any significant performance degradation. Furthermore, the Pt38 cluster model is constructed, the electrolytic water activity on the Pt38 cluster is higher than the Pt(111) surface, and the N doping on CG regulates the local electric field of Pt38 cluster, which is more conducive to the electrocatalytic hydrogen evolution reaction.

Nonlinear Electromagnetic Properties of Thinfilm Nanocomposites (CoFeZr)x(MgF2)100−x

The aim of this work is a comprehensive study of the effect of variable atomic composition and structural-phase state of (CoFeZr)x(MgF2)100−x nanocomposites (NCs) on their nonlinear electronic and magnetic/magneto-optical properties. Micrometer-thick nanocomposite layers on the glass substrates were obtained by ion-beam sputtering of a composite target in the argon atmosphere in a wide range of compositions x = 9–51 at·%. The value of the resistive percolation threshold, xper = 34 at·%, determined from the concentration dependencies of the electrical resistance of NCs, coincides with the beginning of nucleation of metallic nanocrystals CoFeZr in MgF2 dielectric matrix. The absolute value of maximum magnetoresistance of NCs is 2.4% in a magnetic field of 5.5 kG at x = 25 at·%, up to the percolation threshold. Two maxima appear in the concentration dependencies of magneto-optical transversal Kerr effect, one of which, at x = 34 at·%, corresponds to the formation of CoFeZr alloy nanocrystals of a hexagonal structure, and the second one at x = 45 at·% corresponds to the phase transition of nanocrystals from a hexagonal to a cubic body-centered structure. The magnetic percolation threshold in (CoFeZr)x(MgF2)100−x system at xfm = 34 at·%, with the appearance of a hysteresis loop and a coercive force of Hc ≈ 8 Oe, coincides with the resistive percolation threshold xper = 34 at·%. Concentration dependence of the coercive force showed that at low contents of metallic alloy x &lt; 30 at·%, NCs are superparamagnetic (Hc = 0). With an increase of the alloy content, in the region of magnetic and resistive percolation thresholds, NCs exhibit a magnetically soft ferromagnetic character and do not change it far beyond the percolation threshold, with the maximum value of the coercive force Hc &lt; 30 Oe.

Low Overpotential Electroreduction of CO2 on Porous SnO2/ZnO Catalysts

Abstract SnO2-based materials are promising catalysts for CO2 electrochemical reduction due to their attractive selectivity for C1 products (formate and carbon monoxide) but they tend to suffer high overpotential and poor stability. Here, a porous SnO2/ZnO catalyst is synthesized via hydroxides coprecipitation, hydrothermal treatment, and carbon black template calcination. SnO2 nanocrystals are produced by calcination of tin hydroxides while the growth of ZnO nanocrystals is associated with carbon black template. The porous SnO2/ZnO catalyst presents a stable Faradaic efficiency of &amp;gt;90% for CO2 reduction at an applied voltage of −0.7 V versus reversible hydrogen electrode and a C1 current density of 9.53 mA/cm2 over a testing period of 100 h. The improved performance is originated from abundant hetero-junctions and lattice defects of SnO2 and ZnO nanocrystals, large specific surface area, and grain boundary. This study provides a facile method to fabricate porous and nanocrystal metal oxides electrocatalysts for electrochemical processes.

Sandwich-Kernelled AgCu Nanoclusters with Golden Ratio Geometry and Promising Photothermal Efficiency.

Metal nanoclusters have recently attracted extensive interest from the scientific community. However, unlike carbon-based materials and metal nanocrystals, they rarely exhibit a sheet kernel structure, probably owing to the instability caused by the high exposure of metal atoms (particularly in the relatively less noble Ag or Cu nanoclusters) in such a structure. Herein, we synthesized a novel AgCu nanocluster with a sandwich-like kernel (diameter≈0.9 nm and length≈0.25 nm) by introducing the furfuryl mercaptan ligand (FUR) and the alloying strategy. Interestingly, the kernel consists of a centered silver atom and two planar Ag10 pentacle units with completely mirrored symmetry after a rotation of 36 degrees. The two Ag10 pentacles and some extended structures show an unreported golden ratio geometry, and the two inner five-membered rings and the centered Ag atom form an unanticipated full-metal ferrocene-like structure. The featured kernel structure causes the dominant radial direction transition of excitation electrons, as determined via time-dependent density functional theory calculations, which affords the protruding absorption at 612 nm and contributes to the promising photothermal conversion efficiency of 67.6 % of the as-obtained nanocluster, having important implications for structure-property correlation and the development of nanocluser-based photothermal materials.

Sandwich‐Kernelled AgCu Nanoclusters with Golden Ratio Geometry and Promising Photothermal Efficiency

AbstractMetal nanoclusters have recently attracted extensive interest from the scientific community. However, unlike carbon‐based materials and metal nanocrystals, they rarely exhibit a sheet kernel structure, probably owing to the instability caused by the high exposure of metal atoms (particularly in the relatively less noble Ag or Cu nanoclusters) in such a structure. Herein, we synthesized a novel AgCu nanocluster with a sandwich‐like kernel (diameter≈0.9 nm and length≈0.25 nm) by introducing the furfuryl mercaptan ligand (FUR) and the alloying strategy. Interestingly, the kernel consists of a centered silver atom and two planar Ag10 pentacle units with completely mirrored symmetry after a rotation of 36 degrees. The two Ag10 pentacles and some extended structures show an unreported golden ratio geometry, and the two inner five‐membered rings and the centered Ag atom form an unanticipated full‐metal ferrocene‐like structure. The featured kernel structure causes the dominant radial direction transition of excitation electrons, as determined via time‐dependent density functional theory calculations, which affords the protruding absorption at 612 nm and contributes to the promising photothermal conversion efficiency of 67.6 % of the as‐obtained nanocluster, having important implications for structure‐property correlation and the development of nanocluser‐based photothermal materials.

Digitizing protocols into single reactors for the one-pot synthesis of nanomaterials

Digitizing protocols into single reactors for the one-pot synthesis of nanomaterials

Colloidal Synthesis of Multinary Alkali-Metal Chalcogenides Containing Bi and Sb: An Emerging Class of I-V-VI2 Nanocrystals with Tunable Composition and Interesting Properties.

The growth mechanism and synthetic controls for colloidal multinary metal chalcogenide nanocrystals (NCs) involving alkali metals and the pnictogen metals Sb and Bi are unknown. Sb and Bi are prone to form metallic nanocrystals that stay as impurities in the final product. Herein, we synthesize colloidal NaBi1-xSbxSe2-ySy NCs using amine-thiol-Se chemistry. We find that ternary NaBiSe2 NCs initiate with Bi0 nuclei and an amorphous intermediate nanoparticle formation that gradually transforms into NaBiSe2 upon Se addition. Furthermore, we extend our methods to substitute Sb in place of Bi and S in place of Se. Our findings show the initial quasi-cubic morphology transforms into a spherical shape upon increased Sb substitution, and the S incorporation promotes elongation along the <111> direction. We further investigate the thermoelectric transport properties of the Sb-substituted material displaying very low thermal conductivity and n-type transport behavior. Notably, the NaBi0.75Sb0.25Se2 material exhibits an ultralow thermal conductivity of 0.25 W·m-1·K-1 at 596 K with an average thermal conductivity of 0.35 W·m-1·K-1 between 358 and 596 K and a ZTmax of 0.24.

ZnCo-ZIF-based nanoarchitectonics of honeycomblike N-doped carbon for oxygen reduction reaction and supercapacitor

Nitrogen-doped carbon (NC) materials combined with transition metal nanocrystals are potential multifunctional materials for energy conversion and storage. Herein, zinc and cobalt doped zeolite imidazole frameworks (ZnCo-ZIF) were used as precursors for the synthesis of honeycomb-like NC. Due to the NaCl-assisted pyrolysis and selenylation, as-prepared honeycomb-like NC was well-distributed with ZnSe and Co nanocrystals. This unique morphology with abundant porous channels and wrinkles not only improves the dispersion and exposure of ZnSe and Co composites, but also facilitates mass and electron transfer. For oxygen reduction reaction, it exhibited a 0.976 V onset potential and a 0.923 V half-wave potential; and for supercapacitors, its specific capacitance was 915.9 F·g−1 at 1 A·g−1 in 2 M KOH. Thus, NaCl-assisted pyrolysis and selenylation can provide a feasible strategy for preparing high performance multifunctional materials.

Surface engineering of metallic nanocrystals via atomic structure and composition control for boosting electrocatalysis

Since the clean energy industry emerged, developing efficient nanocrystal catalysts has attracted ever-increasing attention. Recently, the utilization of metal nanocrystals as catalysts for electrochemical reactions is entering a new era with the development of theories and techniques that help incorporate surface chemistry into nanoscale materials. Current approaches in the field of nanocrystal catalysts include detailed analyses and modifications of the surface atoms of nanocrystals, with which optimal structures and compositions for target electrochemical reactions could be realized. This review presents two major strategies to engineer the surface structure of nanocrystals: control over the atomic arrangement and composition of nanocrystal surfaces. The first section mainly covers the modification of surface atom arrangements with various methods, including the induction of various facets, strains, and defects. The generation of anomalous crystal structures of nanocrystals is also discussed. The second section encompasses recent advances in controlling the composition of nanocrystal surfaces by bringing high entropy or periodicity to the metal elements in nanocrystals to attain high electrocatalytic activity and stability.

Toward a Quantitative Understanding of Crystal-Phase Engineering of Ru Nanocrystals

The crystal phase with a specific stacking sequence of atoms largely affects the catalytic performance of metal nanocrystals. Since the control of the phase at the same composition is extremely difficult, the phase-dependent performance of metal nanocrystals is studied rarely. Here, we show the synthesis of Ru nanocrystals with different percentages of face-centered cubic (FCC) and hexagonal close-packed (HCP) phases via kinetic control, further revealing a quantitative correlation between the phase percentage of Ru nanocrystals and the initial reduction rate of Ru(III) precursors. Specifically, we manipulate the single parameter-initial reduction rate by controlling the Ru(III) injection rate into the dropwise synthesis at a fixed reaction temperature and correlate the kinetic data with the Ru phase percentage analyzed by atomic-resolution electron microscopy and synchrotron X-ray scattering. Based on the quantitative analysis, the ranges of initial reduction rates of Ru precursors can be determined for synthesizing Ru nanocrystals with the percentages of unusual FCC phase from 9.0 to 55.1%. We demonstrate that a low initial reduction rate corresponds to the crystallization of the Ru HCP phase, while a high initial reduction rate favors the crystallization of the FCC lattice. Furthermore, we also systematically examine the catalytic performance of Ru nanocrystals with different phases.

Passively Q-switched ytterbium doped fiber laser based on a weak-photothermal saturable absorber

Recently, plasmonic nanocrystals (NCs) have attracted much attention due to their wide applications in bioimaging, photothermal therapy and photonics devices. Compared to other metal NCs, copper NCs show a weak photo-thermal effect, which shows that the copper NCs are a good candidate for nonlinear optical materials. In this paper, we demonstrated a Q-switched fiber laser at 1 μm wavelength by using Cu1.8S NCs as a saturable absorber (SA). By incorporating the Cu1.8S SA into an ytterbium doped fiber laser cavity pumped by a 980 nm laser diode, stable passive Q-switching at 1038 nm was obtained for a threshold pump power of 63 mW. On gradually increasing the pump power from 64 mW to 160 mW, the repetition rate of a Q-switched laser increases from 25.7 kHz to 54.5 kHz and the pulse duration decreases from 6.7 μs to 2.33 μs. Our results showed that Cu1.8S NCs is an effective SA for constructing pulsed lasers with a broadband operating wavelength.

Surface strain-regulated twin-rich Au-Pt bimetallic nanowire for highly efficient alcohol oxidation reaction

Metallic nanocrystals with abundant metastable twin boundaries have demonstrated great potential in prompting the improvement of electrocatalytic reactivity, such as alcohol oxidation reaction (AOR) in fuel cell applications. However, modulating the lattice and electronic structures of such twin-rich nanocatalysts to optimize the AOR activity remains a huge challenge. Herein, jagged Au-Pt bimetallic nanowires with abundant twin boundaries along the 〈111〉 direction have been constructed via a seeded synthesis method. Importantly, the lattice strain and electronic structures of Pt in the twin-rich nanowires have been feasibly tuned, resulting in the regulation of AOR performances. Impressively, the jagged AuPt3 nanowires, with an atomic ratio of ∼1:3, exhibit the highest mass activity toward ethanol oxidation reaction (EOR) and methanol oxidation reaction (MOR), outperforming AuPt9, AuPt, Pt nanowires and commercial Pt/C catalysts. The structure–activity correlation for the AOR follows a volcano-shaped curve with the increase of Pt ratios. Theoretical results suggest the weakened bonding of Pt-CO and strengthened Pt-OH on the AuPt3 (111)-twin site. Our work develops a simple yet effective method to synthesize twin-rich bimetallic nanostructures with controlled lattice strain and illustrates their structure-dependent properties, opening the way to develop high-performance electrocatalysts for promising applications.