Nucleation Of Nanocrystals Research Articles

Continuous Production of Water‐Soluble Nanocrystals through Anti‐Solvent Precipitation in a Fluidic Device

AbstractThis paper describes a simple and robust method for the continuous production of water‐soluble nanocrystals using anti‐solvent precipitation under diffusion control in a fluidic device. We use sodium chloride (NaCl) as an example to demonstrate the concept. In a typical process, aqueous NaCl and ethanol (the anti‐solvent) serve as the focused and focusing phases, respectively, for the generation of a coaxial‐flow system. Upon contact with each other, the rapid diffusion between water and ethanol leads to the formation of NaCl nanocrystals at the interface while a gradient in NaCl concentration is created along the flow direction. The nucleation and growth of NaCl nanocrystals can be readily tuned by varying the hydrodynamic parameters such as the ratio between the flow rates of the two phases and the total volumetric rate. By optimizing these parameters, we are able to produce NaCl nanocubes and nanospheres as small as 20 nm and 6 nm, respectively, while attaining a narrow distribution in size. We have also successfully generated KCl nanocrystals with similar controls, demonstrating the generality of this method for the production of water‐soluble nanocrystals.

Amorphous Steel Coatings Deposited by Cold-Gas Spraying

Cold-gas spray (CGS) deposition of amorphous steel coatings starting from a commercial feedstock powder containing boron, tungsten, and silicon was investigated. Microstructural characterization, carried out by X-ray diffraction (XRD), transmission electron microscopy, and backscattered electron diffraction (EBSD) analysis, confirmed the amorphous nature of deposited coatings. The amorphization phenomenon is related to high-strain/strain-rate deformation with shear instability caused by very high particle kinetic energy, with a mechanism that resembles the severe plastic deformation process. The CGS coatings were heat-treated at temperatures ranging from 650 to 850 °C to induce partial recrystallization. The effect of nanocrystal nucleation and growth on the hardness of the coatings was investigated, and the hardness of heat-treated samples was found to increase with respect to as-sprayed coatings, outperforming conventional high-velocity oxy-fuel (HVOF) deposits. Hardness was found to decrease after prolonged (<90 min) or higher temperature (>750 °C) exposures.

Surface molecular kinetics on the outermost layer characterized by nucleation of Mg-vapor atoms

The state of organic surfaces/interfaces greatly affects the performance of organic electronic devices. Various methods have been reported to investigate the surface glass transition temperature, which is based on the dynamics of molecules or polymer chains existing at the depth of several to several-tens nanometers from surface. However, no report has published about the characterization of molecular kinetics on the outermost surface. We report that the surface nucleation of Mg-vapor atoms reflects the molecular kinetics on the outermost surface. The nucleation rate of vaporized atoms/molecules is generally accelerated on steps existed on crystalline surface, to the contrary we found suppressed nucleation of Mg nanocrystals around the steps on a diarylethene crystalline surface during Mg-vapor deposition. On the other hand, Pb, Bi, and Te nanocrystals tended to accumulate the steps. The difference between acceleration and suppression of metal nucleation on the steps was attributed to molecular kinetics on the outermost surface and intensity of van der Waals interaction between metal atoms and the molecules. Mg having the weak interaction with organic molecules is suitable for characterizing molecular kinetics on the outermost surface. The observation of Mg nucleation can play a role in measuring molecular kinetics on the outermost surfaces. The state of organic surfaces/interfaces greatly affects the performance of organic electronic devices. Various methods have been reported to investigate the surface glass transition temperature, which is based on the dynamics of molecules or polymer chains existing at the depth of several to several-tens nanometers from surface. However, no report has published about the characterization of molecular kinetics on the outermost surface. We report that the surface nucleation of Mg-vapor atoms reflects the molecular kinetics on the outermost surface. The nucleation rate of vaporized atoms/molecules is generally accelerated on steps existed on crystalline surface, to the contrary we found suppressed nucleation of Mg nanocrystals around the steps on a diarylethene crystalline surface during Mg-vapor deposition. On the other hand, Pb, Bi, and Te nanocrystals tended to accumulate the steps. The difference between acceleration and suppression of metal nucleation on the steps was attributed to molecular kinetics on the outermost surface and intensity of van der Waals interaction between metal atoms and the molecules. Mg having the weak interaction with organic molecules is suitable for characterizing molecular kinetics on the outermost surface. The observation of Mg nucleation can play a role in measuring molecular kinetics on the outermost surfaces. • The surface nucleation of Mg-vapor atoms reflects the molecular kinetics on the outermost surface. • We found suppressed nucleation of Mg nanocrystals around the steps on a diarylethene crystalline surface. • The suppressed nucleation was attributed to surface molecular kinetics and weak van der Waals interaction.

Facile and scalable fabrication of self-assembled Cu architecture with superior antioxidative properties and improved sinterability as a conductive ink for flexible electronics

The inherent susceptibility to oxidation and poor sinterability significantly limit the practical application of Cu-based conductive inks. Most methodologies employed for the inks like organic polymer coatings and inorganic metal deposition are generally ineffective. Herein, we report the design of a novel hierarchical Cu architecture to simultaneously improve the antioxidative and sinterability via a self-passivation mechanism and loose interior structures. The hierarchical Cu architecture was prepared using copper hydroxide, L-ascorbic acid, and polyvinylpyrrolidone in aqueous solution; 40 g Cu were prepared in a scale-up experiment. A possible growth mechanism is proposed, involving the Cu2O-templated and mediated nucleation and growth of Cu nanocrystals, followed by the PVP-directed electrostatic self-assembly of Cu nanocrystals. The synthesized Cu shows high oxidation resistance after stored in ambient environment for 90 d by self-passivation, wherein the dense oxidized external layer prevented further oxidation of Cu, unlike other antioxidative strategies. In addition, the structure became 2D flake after a simple ball-milling for 10 min of 2000r, thus forming a good conductive network at the temperature of 180 °C. Importantly, no obvious decline in the electrical performance after severe surface oxidation. Although the structure cannot offer excellent conductive performance, but it proposes a new solution for the balance of antioxidative capabilities and good sinterability in Cu nanomaterials, thus facilitating greater utilization of Cu-based conductive inks for emerging flexible electronic applications.

Ordered nanocrystal formation in Cu50Zr45Ti5 metallic glass

The paper reports formation of ordered nanocrystals in amorphous Cu50Zr45Ti5 metallic glass (MG) when exposed to ions in focused ion beam (FIB) instrument. The specimen was subjected to 30 keV Ga ion beam during trenching, lift out of a lamella, and thinning to a thickness of about 100 nm. Transmission electron microscopy confirms formation of ordered nanocrystals 2–5 nm in size, aligned along lines, which have ∼10 nm separation distance. It is hypothesized that solid flow induced by ion bombardment leads to ripple formation. The diffusible short-range order quasicrystals agglomerate in the crest regions under stress and the agglomeration is followed by local nanocrystal nucleation. The FIB process is unable to induce such an ordered nanocrystal matrix in MG Ti40Cu31Pd14Zr10Sn2Si3. The difference in radiation resistance is explained by the level of complexity in the correlated diffusion needed for crystal nucleation, which is reflected by the width of the supercooled liquid region.

A Peptide-Based Method for the Fabrication of 1D Rail-Like Nanoparticle Chains and 2D Nanoparticle Membranes: Higher-Order Self-Assembly.

Functionalized histidine-rich peptide sequences were designed for the site-directed assembly of nanoparticles. TEM and AFM images shown that the peptides self-assembled into well-ordered nanofibrils at pH 7.2. The nanofibrils could lie parallel to one another and form membranes when the solution was acidic (pH 3.8) resulting from the hierarchical assembly of the nanofibrils in the direction of the peptide backbone. These peptide structures served as a template for nucleation and growth of Au nanocrystals. Further characterization showed that the Au nanocrystals grew on both sides of the nanofibrils, and a 1D system with a rail-like structure and a 2D membrane were synthesized after reduction with hydrazine hydrate at neutral and acidic pH values, respectively. The size and packing density of the Au nanocrystals were positively correlated with the incubation time of the Au ions. This approach can be extended further to the controlled synthesis of 1D and 2D architectures formed from metals, metal sulfides, and metal oxides in a low-cost and simple manner. Finally, the nanostructures could catalyze the reduction of p-nitrophenol with rate constants of 0.83±0.14 and 0.69±0.09 min-1 for the 1D and 2D structures, respectively.

Organic Templates for Inorganic Nanocrystal Growth

Nanocrystals provide a variety of size and shape‐dependent properties with applications in a wide range of areas, gaining much attention in the past few years. However, due to the nature of the kinetic nanocrystal growth, the procedures often require strict experimental conditions and the shape and size of nanocrystals are difficult to control. In such context, organic templates, which are artificially modified or synthesized, can direct inorganic nanocrystal nucleation and growth to achieve desired shape, size and ultimately properties. In this review article, two general categories of organic templates used for making inorganic nanomaterials are discussed: biotemplates (e.g., peptide, lipid, DNA, and capsid) and block copolymer templates (e.g., block copolymer micelles, star‐like block copolymer unimicelles). The goal of this review is to bridge these gaps and help foster a greater awareness of the range and applicability of different organic templating techniques within the field of nanotechnology.

Cross-sectional transmission electron microscopy studies of boron ion implantation in hexagonal boron nitride

We have reported on the implantation of boron ion into hexagonal boron nitride (h-BN) material to aid the nucleation of cubic boron nitride nanocrystals (nc-BN).Single crystal h-BN was implanted with boron ion at 150 keV at fluences of the order of 1015 and 1017 ions/cm2 at room temperature. High Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM) mapping showed a variation in image contrast in samples irradiated to a fluence of 1 × 1015 ions/cm2. As predicted by Stopping Range and Ions in Materials (SRIM) calculations, the implanted region with the highest damage density appeared to have a bright contrast in HAADF-STEM which represented the high density c-BN symmetry. High Resolution Transmission Electron Microscopy (HRTEM) and electron diffraction measurements showed regions with nc-BN for samples implanted with low fluences and amorphous BN after implantation with high fluences indicating a fluence-related phase transition in BN. Raman spectroscopy showed the emergence of longitudinal optical frequency mode associated with c-BN after implantation.

Green synthesis of biocompatible trypsin-conjugated Ag nanocomposite with antibacterial activity

In this work, water-soluble Ag nanoparticles were prepared in aqueous solution by using trypsin as reducing and capping agent. The protein-assisted synthetic strategy eliminates the need of intermediate protecting and linking agents compared with organometallic approach, which is simple, effect, less energy consuming, and closer to the requirements of green chemistry. The morphology, size and antibacterial activity properties could be controlled by varying experimental conditions. The results of FT-IR and SDS-PAGE analysis indicated that trypsin molecules could control the nucleation and growth of nanocrystals through chemical interaction between Ag and functional groups of trypsin. The binding of trypsin on the surface of Ag nanoparticles significantly reduced nano-toxicity through capping effect. The trypsin-conjugated Ag nanoparticles exhibited strong antibacterial activity toward both Gram-positive and Gram-negative bacteria due to small size and specific morphologies. Compared with traditional antibacterial materials, the water-solubility and biocompatibility make the products more suitable for the application in biological and medical science.

In situ growth of Co3O4 on nitrogen-doped hollow carbon nanospheres as air electrode for lithium-air batteries

Design and synthesis of efficient bifunctional electrocatalysts for both oxygen reduction reaction and oxygen evolution reactions are of great significant for metal-air batteries. In this work, bifunctional catalysts consisting of Co3O4 nanocrystals and nitrogen-doped hollow carbon nanospheres are synthesized through in situ growth of Co3O4 nanocrystals on the surface of nitrogen-doped hollow carbon nanospheres. The observed CoN bond formation is an indication of the nucleation of Co3O4 nanocrystals starting from N-sites in nitrogen-doped hollow carbon nanospheres. The resulted hybrids exhibit improved activity towards oxygen reduction reaction compared to pristine nitrogen-doped hollow carbon nanospheres in terms of the 42 mV positive shift of half-wave potential and comparable activity towards oxygen evolution reactions with commercial RuO2 and IrO2 catalysts. The thus-assembled Li-O2 battery delivers an initial discharge capacity of 3325 mAhg−1 at 100 mAg−1 using mixed gas of O2 and Ar (20% of O2 in volume). The battery fails after 27 discharge/charge cycles due to the accumulation of discharge products on electrode.

Millisecond CdS nanocrystal nucleation and growth studied by microfluidics with in situ spectroscopy

Microfluidic methods allowing fast mixing allow studying fast chemical reactions on the millisecond time scale. Here the kinetics of nucleation and growth of thioglycerol- and l-cysteine-capped CdS quantum dots is investigated using a continuous-flow microfluidic device in combination with in situ microscopy and spectroscopic techniques. A microfluidic device is developed to three-dimensionally focus a central aqueous Cd2+-stream, which is completely enclosed by a water buffer layer, into a stream of aqueous S2−. Diffusion-limited nucleation and growth of CdS nanoparticles occurs in the buffer layer downstream the reaction channel which can be followed by in situ confocal laser scanning microscopy and UV-Vis absorption spectroscopy on the time scale of the first 1–100 ms, giving direct and unique insights into early colloidal crystal nucleation and growth processes.

Spatially Localized Synthesis and Structural Characterization of Platinum Nanocrystals Obtained Using UV Light.

Platinum nanocrystals with a fine control of the crystal domain size in the range 1.0–2.2 nm are produced by tuning the NaOH concentration during the UV-induced reduction of H2PtCl6 in surfactant-free alkaline ethylene glycol. The colloidal solutions obtained are characterized by transmission electron microscopy and pair distribution function analysis, allowing analysis of both atomic and nanoscale structures. The obtained nanoparticles exhibit a face-centered cubic crystal structure even for the smallest nanoparticles, and the cubic unit cell parameter is significantly reduced with decreasing crystallite size. It is further demonstrated how the “UV-approach” can be used to achieve spatial control of the nucleation and growth of the platinum nanocrystals, which is not possible by thermal reduction.

Features of copper bromide nanocrystals formation in fluorophosphate glass

Formation of CuBr nanocrystals in fluorophosphate glass is investigated for the first time. The features of the nanocrystals nucleation and growth during isothermal treatment of the glass are analyzed based on spectral studies results. The mean size of the CuBr nanocrystals formed via thermo-induced crystallization varies within 7–10 nm. The nanocrystals size of 9 nm is revealed to be critical, below which the CuBr nanocrystals possess cubic structure, and above which the structure of the nanocrystals becomes hexagonal. While during spontaneous nanocrystals nucleation from the glass melt, without additional thermal inducing, the CuBr crystal structure remains cubic regardless of the size. A preliminary explanation of the effects revealed is proposed, assuming the principal role of internal pressure in drops of a copper halide liquid in a glass matrix.

Nucleation and growth of hydroxyapatite nanocrystals by hydrothermal method

Hydroxyapatite (HA) was synthesized by using a hydrothermal method with Ca(NO3)2·4H2O and H3PO4. We use x-ray diffraction and field-emission scanning electron microscopy to investigate how pH, reaction temperature, hydrothermal-reaction time, and calcium-ion concentration affects the microstructure and the growth of HA crystals. In addition, we discuss the growth mechanism. The results show that the crystals grow more completely and that the aspect ratio tends to increase with increasing hydrothermal-reaction time, reaction temperature, and calcium-ion concentration. The pH of the system strongly impacts the growth of HA crystals. With increasing pH, the HA crystal grain size and aspect ratio decrease significantly. By using 1 mol/L calcium-ion concentration, pH = 10, and a hydrothermal reaction at 200 °C for 8 h, we obtain high crystallinity and crystal clear of the growth polarity with hexagonal 60–100-nm-long columnar HA, 30–40 nm in diameter. The mechanisms producing this growth may be the effect of growth conditions on ion concentration, thereby changing the HA crystal growth rate along the different crystal axes.

In Situ Atomic-Level Tracking of Heterogeneous Nucleation in Nanocrystal Growth with an Isocyanide Molecular Probe.

We report the use of 2,6-dimethylphenyl isocyanide (2,6-DMPI) as a spectroscopic probe to study the heterogeneous nucleation and deposition of Pd on Ag nanocubes under different conditions by surface-enhanced Raman scattering. As a major advantage, the spectroscopic analysis can be performed in situ and in real time with the nanoparticles still suspended in the reaction solution. The success of this method relies on the distinctive stretching frequencies (νNC) of the isocyanide group in 2,6-DMPI when it binds to Ag and Pd atoms through σ donation and π-back-donation, respectively. Significantly, we discovered that νNC was sensitive to the arrangement of Pd adatoms on the Ag surface. For example, when the isocyanide group bound to one, two, and three Pd atoms, we would observe the atop, bridge, and hollow configurations, respectively, at different νNC frequencies. As such, the νNC band could serve as a characteristic reporter for the Pd adatoms being deposited onto different types of facets on Ag nanocubes with atomic-level sensitivity. When 2,6-DMPI molecules were introduced into the reaction solution, we further demonstrated in situ tracking of heterogeneous nucleation and early stage deposition of Pd on Ag nanocubes by monitoring the evolution of νNC bands for both Ag and Pd surface atoms as a function of reaction time. This in situ technique opens up the opportunity to investigate the roles played by reaction temperature and the type of Pd(II) precursor in influencing the heterogeneous nucleation and growth of bimetallic nanocrystals. The sensitivity of isocyanide group to Pd atoms helps elucidate some of the details on the reduction, deposition, and diffusion processes involved in heterogeneous nucleation.

Stabilization of Ag-Au Bimetallic Nanocrystals in Aquatic Environments Mediated by Dissolved Organic Matter: A Mechanistic Perspective.

Gold and silver nanoparticles can be stabilized endogenously within aquatic environments from dissolved ionic species as a result of mineralization induced by dissolved organic matter. However, the ability of fulvic and humic acids to stabilize bimetallic nanoparticles is entirely unexplored. Elucidating the formation of such particles is imperative given their potential ecological toxicity. Herein, we demonstrate the nucleation, growth, and stabilization of bimetallic Ag-Au nanocrystals from the interactions of Ag+ and Au3+ with Suwannee River fulvic and humic acids. The mechanisms underpinning the stabilization of Ag-Au alloy NPs at different pH (6.0-9.0) values are studied by UV-vis spectrophotometry, X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). Complexation of free Ag+ and Au3+ ions with the Lewis basic groups (carbonyls, carboxyls, and thiols) of FA and HA, followed by electron-transfer from redox-active moieties present in dissolved organic matter initiates the nucleation of the NPs. Alloy formation and interdiffusion of Au and Ag atoms are further facilitated by a galvanic replacement reaction between AuCl4- and Ag. Charge-transfer from Au to Ag stabilizes the formed bimetallic NPs. A more pronounced agglomeration of the Ag-Au NPs is observed when HA is used compared to FA as the reducing agent. The bimetallic NPs are stable for greater than four months, which suggests the possible persistence and dispersion of these materials in aquatic environments. The mechanistic ideas have broad generalizability to reductive mineralization processes mediated by dissolved organic matter.

Facile synthesis of Pd concave nanocubes: From kinetics to mechanistic understanding and rationally designed protocol

We report a rationally designed one-pot method for the facile synthesis of Pd concave nanocubes in an aqueous solution at room temperature by manipulating the reduction kinetics through the selection of a proper combination of a salt precursor (PdBr42–) and reductant (sodium ascorbate). Our kinetic analysis demonstrates that, through this selection, the nucleation and growth of Pd nanocrystals could be effectively separated into two kinetic regimes involving distinctive reduction pathways: i) solution reduction for the initial formation of single-crystal seeds and ii) surface reduction for the formation of concave nanocrystals via autocatalytic growth from the single-crystal seeds. The suppressed surface diffusion at room temperature, when coupled with the capping effect of bromide ions, ultimately leads to the formation of concave nanocubes with an asymmetric shape and high-index facets, whose synthesis would otherwise require multiple steps and the use of elevated temperatures.

Nucleation and Growth Behavior of CdSe Nanocrystals Synthesized in the Presence of Oleylamine Coordinating Ligand.

We established the dual role played by oleylamine (OAm) during the synthesis of CdSe nanocrystals (NCs) by the hot injection method. Earlier reports suggest its role either as nucleating or as passivating agent in controlling the growth of CdSe NCs. Remarkably, by exploring four different synthesis routes, in which the reactant addition, timing, and concentration were varied, we found that both these two phenomena coexist and control synthesis. While examining if there is any effect of concentration of OAm on this synthesis, we found that at lower contents of OAm (<0.5 mmol) it prominently acts as an agent acting on nucleation, increasing the number of nuclei, reducing the nuclei (initial monomer units) size, and thereby increasing the NCs concentration resulting in a small NCs size, down to 2.7 nm. Whereas at higher contents (>1.0 mmol), it served more as a passivating agent by deterring both nucleation and growth processes, so generating fewer NCs with larger size up to 3.6 nm. Thus, adjusting the influence between nucleation and passivation, we can better control the final NCs size and so tune their size-dependent optical properties.

In-situ reaction-growth of PtNiX nanocrystals on supports for enhanced electrochemical catalytic oxidation of ethanol via continuous flow microfluidic process

An in-situ composite methodology was first proposed to prepare carbon supported PtNix (X = 1, 3, 1/3) via online reaction, nucleation and growth of nanocrystals (NCs) on carbon support along the microfluidic channel. Uniform dispersed ultra-small PtNiX NCs (1 nm–3 nm) with high surface exposed Pt atoms and enhanced nanocrystal-carbon interface interaction can be constructed. The annealed nanocomposites with a Pt/Ni ratio of 1/3 exhibit a mass catalytic activity of 7.6 times higher than the commercial Pt/C nanocatalysts in the ethanol oxidation reaction. The chronoamperometry measurement indicates that their mass activity still retains 3.6 times of the commercial Pt/C nanocatalysts at 4000 s. Moreover, PtNi3/C nanocatalysts have the highest tolerance to CO poisoning according to CO stripping measurements.

Phase Transformation of GeO2 Glass to Nanocrystals under Ambient Conditions.

Theoretically, the accomplishment of phase transformation requires sufficient energy to overcome the barriers of structure rearrangements. The transition of an amorphous structure to a crystalline structure is implemented traditionally by heating at high temperatures. However, phase transformation under ambient condition without involving external energy has not been reported. Here, we demonstrate that the phase transformation of GeO2 glass to nanocrystals can be triggered at ambient conditions when subjected to aqueous environments. In this case, continuous chemical reactions between amorphous GeO2 and water are responsible for the amorphous-to-crystalline transition. The dynamic evolution process is monitored by using in situ liquid-cell transmission electron microscopy, clearly revealing this phase transformation. It is the hydrolysis of amorphous GeO2 that leads to the formation of clusters with a size of ∼0.4 nm, followed by the development of dense liquid clusters, which subsequently aggregate to facilitate the nucleation and growth of GeO2 nanocrystals. Our finding breaks the traditional understanding of phase transformation and will bring about a significant revolution and contribution to the classical glass-crystallization theories.