Stages Of Particle Growth Research Articles

Migration of zeolite-encapsulated Pt and Au under reducing environments

Simulations reveal accelerated migration of Pt@zeolite by reducing adsorbates and the importance of PtCO in early stages of particle growth.

Two Sides of the Same Coin: Light as a Tool to Control and Map Microsphere Design

Herein, we establish the effect of intensity and wavelength on the size of microparticles formed via precipitation polymerization, employing photocrosslinkable prepolymers. Simultaneous measurement of backscattered laser irradiation enabled real-time tracking of particle growth and provides the ability to vary the LED intensity (λmax = 415 nm) during various stages of particle growth. Critically, particle diameters can be controlled between 200 and 700 nm by varying the LED power from 73 to 0.63 mW, respectively. High intensities during the nucleation phase-spanning only the initial seconds-were found to dictate the particle diameter, irrespective of the energy used during the growth phase. Finally, a bathochromic shift was observed between the wavelength generating the highest rate of particle formation and the absorbance maxima of the photoactive group. We submit that these findings are broadly applicable in the continuously developing field of photoinitiated synthesis of polymer particles.

Understanding the anatase-rutile stability in flame-made TiO2

The relative stability of anatase and rutile in stagnation flame synthesis with stoichiometric mixtures is investigated experimentally. The measurements reveal a high sensitivity of anatase-rutile composition to the flame dilution. It is demonstrated that anatase formation is favoured in more dilute (colder) flames while rutile is favoured in less dilute (hotter) flames. A particle model with a detailed description of aggregate morphology and crystal phase composition is applied to investigate the anatase-rutile stability. A phase transformation model is implemented in which rutile is formed for particles larger than a “crossover” size while anatase is formed for those smaller. Two formation mechanisms/pathways are discussed and evaluated. In the first pathway, the nascent particles are assumed to be stoichiometric and the crossover size is determined solely by the surface free energy. This hypothesis captures the general trend in the measured anatase-rutile composition but fails to explain the sensitivity. In the second pathway, non-stoichiometric TiO 2 − x oxide intermediates are assumed and the crossover size is hypothesised to be composition-dependent. This shows an excellent agreement with the experimental data. However, this hypothesis is found to be strongly influenced by assumptions about the initial particle growth stages. This study demonstrates the importance of a better description of the high-temperature chemistry and initial clustering mechanism in order to understand the crystal phase formation.

Energy Modeling of Competition Between Tubular and Platy Morphologies of Chrysotile and Halloysite Layers

AbstractThe present study considered the problem of halloysite nanoscroll synthesis by energy modeling of the formation of chrysotile and halloysite particles. The main aim of the study was to reveal an energy preference between scrolled and platy morphologies of the particles. Both hydrosilicates possess the ability to scroll spontaneously but relatively facile hydrothermal synthesis of the nanoscrolls is available only to the former, whereas halloysite forms mainly plates under the same conditions. This issue was investigated by a phenomenological energy model, taking into account: (1) strain energy due to the size difference between metal oxide and silica sheets; (2) surface-energy difference on the opposite sides of the layer; and (3) adhesion energy. Calculations showed that the halloysite layer had a significant scrolling potential due to the first energy component, but the surface-energy difference acted in the opposite direction and tried to unbend the layer. In contrast, these two actions were co-directional in chrysotile layers. In both cases, the formation of multi-layered plates became more energy favorable when the specific surface energy of the edges decreased. In the range 0.5–3 J/m2 for the specific surface energy, only halloysite layers showed an energy preference for platy particles over nanoscrolls, especially at small layer sizes. Certain processes, such as hydration, could reduce the corresponding specific surface energy value and, as a result, could stabilize the platy morphology of halloysite at the earliest stages of particle growth under hydrothermal conditions.

In Situ and Real-Time Monitoring of Nanoparticle Formation in Microemulsion by Means of Dielectric Spectroscopy.

Dielectric spectroscopy was employed, for the first time, to monitor the formation process of silica nanoparticles in a nonionic surfactant-based microemulsion in situ and in real time. Two dominant relaxations were observed in the frequency range of 1 MHz-3 GHz during this process. The relaxation at the lower frequency range was confirmed to be mainly ascribed to interfacial polarization, whose relaxation parameters, together with the electrical property of the synthesis system, were used to characterize the evolution of this dynamic formation process. Four evolution stages are distinctively revealed, including an induction stage, a nucleation dominant stage, an early particle growth stage, and a late growth stage. The dynamic features at each evolution stages were discussed in terms of the dielectric characteristics of the system. It is strongly suggested that dielectric spectroscopy is an effective tool for the in situ mechanistic study of nanoparticle formation in microemulsion.

Controlled Growth of Srx Ba1-x Nb2 O6 Hopper- and Cube-Shaped Nanostructures by Hydrothermal Synthesis.

Controlling the shape and size of nanostructured materials has been a topic of interest in the field of material science for decades. In this work, the ferroelectric material SrxBa1−xNb2O6 (x=0.32–0.82, SBN) was prepared by hydrothermal synthesis, and the morphology is controllably changed from cube‐shaped to hollow‐ended structures based on a fundamental understanding of the precursor chemistry. Synchrotron X‐ray total scattering and PDF analysis was used to reveal the structure of the Nb‐acid precursor, showing Lindqvist‐like motifs. The changing growth mechanism, from layer‐by‐layer growth forming cubes to hopper‐growth giving hollow‐ended structures, is attributed to differences in supersaturation. Transmission electron microscopy revealed an inhomogeneous composition along the length of the hollow‐ended particles, which is explained by preferential formation of the high entropy composition, SBN33, at the initial stages of particle nucleation and growth.

Relative kinetics of nucleation and condensation of silane pyrolysis in a helium atmosphere provide mechanistic insight in the initial stages of particle formation and growth

Chemical nucleation for the synthesis of solid nanoparticles from gas phase precursor is a fundamental process of technological relevance, yet, not well understood. In this work, a silane pyrolysis system was built to obtain insights in particle formation and growth. Primary particle analysis was used to determine the relative condensation and nucleation rates in the aerosol process of silane pyrolysis. TEM, XRD and SMPS allow the comparison between primary particle and agglomerate dynamics. The analysis reveals that nucleation is correlated to silane concentration to the power of 2/3. This relationship and other observations are used to assess the impact of magnetic interactions in classical Brownian agglomeration in the initial stages of primary particle dynamics in silane pyrolysis. Furthermore, this mechanism is consistent with key observations reported in the literature regarding nanoparticles of various materials, suggesting the proposed mechanism could be valid for more materials and deserves further attention.

WITHDRAWN: Relative kinetics of nucleation and condensation of silane pyrolysis in a helium atmosphere provide mechanistic insight in the initial stages of particle formation and growth

WITHDRAWN: Relative kinetics of nucleation and condensation of silane pyrolysis in a helium atmosphere provide mechanistic insight in the initial stages of particle formation and growth

On the early stages of soot formation: Molecular structure elucidation by high-resolution atomic force microscopy

The early stages of soot formation, namely inception and growth, are highly debated and central to many ongoing studies in combustion research. Here, we provide new insights into these processes from studying different soot samples by atomic force microscopy (AFM). Soot has been extracted from a slightly sooting, premixed ethylene/air flame both at the onset of the nucleation process, where the particle size is of the order of 2–4 nm, and at the initial stage of particle growth, where slightly larger particles are present. Subsequently, the molecular constituents from both stages of soot formation were investigated using high-resolution AFM with CO-functionalized tips. In addition, we studied a model compound to confirm the atomic contrast and AFM-based unambiguous identification of aliphatic pentagonal rings, which were frequently observed on the periphery of the aromatic soot molecules. We show that the removal of hydrogen from such moieties could be a pathway to resonantly stabilized π-radicals, which were detected in both investigated stages of the soot formation process. Such π-radicals could be highly important in particle nucleation, as they provide a rational explanation for the binding forces among aromatic molecules.

Production and Measurement of Organic Particulate Matter in a Flow Tube Reactor

Organic particulate matter (PM) is increasingly recognized as important to the Earth's climate system as well as public health in urban regions, and the production of synthetic PM for laboratory studies have become a widespread necessity. Herein, experimental protocols demonstrate approaches to produce aerosolized organic PM by α-pinene ozonolysis in a flow tube reactor. Methods are described for measuring the size distributions and morphology of the aerosol particles. The video demonstrates basic operations of the flow tube reactor and related instrumentation. The first part of the video shows the procedure for preparing gas-phase reactants, ozonolysis, and production of organic PM. The second part of the video shows the procedures for determining the properties of the produced particle population. The particle number-diameter distributions show different stages of particle growth, namely condensation, coagulation, or a combination of both, depending on reaction conditions. The particle morphology is characterized by an aerosol particle mass analyzer (APM) and a scanning electron microscope (SEM). The results confirm the existence of non-spherical particles that have grown from coagulation for specific reaction conditions. The experimental results also indicate that the flow tube reactor can be used to study the physical and chemical properties of organic PM for relatively high concentrations and short time frames.

Collisional growth rate and correction factor for TiO2 nanoparticles at high temperatures in free molecular regime

Flame and furnace aerosol reactors are used extensively for the synthesis of nanoparticles at high temperatures (800–2000 K), however, there is a limited understanding of the early stages of particle growth by collision at these temperatures. An experimental set-up to quantify the collisional growth rate and collisional correction factor (relative to the kinetic theory of gases) for flame synthesized TiO2 nanoparticles in the free molecular regime is described. Spherical charge-neutral nanoparticles with precise control of size are selected from a flame aerosol reactor (FLAR), and then passed through a downstream furnace reactor tube maintained at a high temperature. Initial and final particle size distributions (PSD) are measured using a scanning mobility particle sizer (SMPS), and the PSD is represented by a lognormal distribution. The general dynamic equation (GDE) for simultaneous coagulation and diffusive deposition is solved numerically for a laminar pipe flow using the method of moments. By comparing the experimental data with the model prediction, a correction factor to collisional growth rate is calculated. It was found that for particles in the size range 5–8 nm, there was significant enhancement in the collisional growth rate for TiO2 nanoparticles at 400 °C. This enhancement in growth rate decreased as temperature increased (upto 800 °C). The collisional correction factor values are compared to that of three existing models in the literature which predict similar trends with temperature, however, these models under-predict the value of the collisional correction factor. Reasons for these deviations from existing models are elucidated.

Nanospectroscopy Captures Nanoscale Compositional Zonation in Barite Solid Solutions

Scientists have long suspected that compositionally zoned particles can form under far-from equilibrium precipitation conditions, but their inferences have been based on bulk solid and solution measurements. We are the first to directly observe nanoscale trace element compositional zonation in <10 µm-sized particles using X-ray fluorescence nanospectroscopy at the Hard X-ray Nanoprobe (HXN) Beamline at National Synchrotron Light Source II (NSLS-II). Through high-resolution images, compositional zonation was observed in barite (BaSO4) particles precipitated from aqueous solution, in which Sr2+ cations as well as HAsO42− anions were co-precipitated into (Ba,Sr)SO4 or Ba(SO4,HAsO4) solid solutions. Under high salinity conditions (NaCl ≥ 1.0 M), bands contained ~3.5 to ~5 times more trace element compared to the center of the particle formed in early stages of particle growth. Quantitative analysis of Sr and As fractional substitution allowed us to determine that different crystallographic growth directions incorporated trace elements to different extents. These findings provide supporting evidence that barite solid solutions have great potential for trace element incorporation; this has significant implications for environmental and engineered systems that remove hazardous substances from water.

A novel synthetic route for high-index faceted iron oxide concave nanocubes with high T2 relaxivity for in vivo MRI applications.

Iron oxide nanoparticles (IONPs) with high-index facets have shown great potential as high performance T2 contrast agents for MRI. Previous synthetic approaches focused mainly on ion-directed or oxidative etching methods. Herein, we report a new synthetic route for preparing high-index faceted iron oxide concave nanocubes using a bulky coordinating solvent. Through the systematic replacement of a non-coordinating solvent, 1-octadecene, with trioctylamine, the solvent interaction with the nanoparticle surface is modified, thereby, promoting the growth evolution of the IONPs from spherical to concave cubic morphology. The presence of the bulky trioctylamine solvent results in particle size increase and the formation of nanoparticles with enhanced shape anisotropy. A well-defined concave nanocube structure was evident from the early stages of particle growth, further confirming the important role of bulky coordinating solvents in nanoparticle structural development. The unique concave nanocube morphology has a direct influence on the magnetic properties of the IONPs, ultimately leading to an ultra-high T2 relaxivity (862.2 mM-1 s-1), and a 2-fold enhancement in T2*-weighted in vivo MRI contrast compared to spherical IONP analogs.

Cobalt-Assisted Morphology and Assembly Control of Co-Doped ZnO Nanoparticles.

The morphology of metal oxide nanostructures influences the response of the materials in a given application. In addition to changing the composition, doping can also modify the morphology of a host nanomaterial. Herein, we determine the effect of dopant concentration, reaction temperature, and reaction time on the morphology and assembly of CoxZn1−xO nanoparticles synthesized through non-aqueous sol-gel in benzyl alcohol. With the increase of the atom % of cobalt incorporated from 0 to 15, the shape of the nanoparticles changes from near spherical, to irregular, and finally to triangular. The tendency of the particles to assemble increases in the same direction, with Co0.05Zn0.95O consisting of non-assembled particles, whereas Co0.15Zn0.85O consists of triangular nanoparticles forming spherical structures. The morphology and assembly process are also sensitive to the reaction temperature. The assembly process is found to occur during the nucleation or the early stages of particle growth. The cobalt ions promote the change in the shape during the growth stage of the nanoparticles.

Evolution of Nickel, Manganese, and Cobalt Hydroxide Precursor for Li-Ion Battery Cathode Materials in Co-Precipitation Reactions

Evolution of nickel, manganese, and cobalt hydroxide precursor, Ni1/3Mn1/3Co1/3(OH)2, in typical co-precipitation reactions is investigated at different pH levels (i.e. 10.6 and 11.4). The growth of the Ni1/3Mn1/3Co1/3(OH)2 primary and secondary particle is monitored and their particle size distribution revealed different numbers of growth stages for samples prepared at pH 10.6 and pH 11.4. Experimental The experiment is carried out in a 1L CSTR with water bath jacket (60 oC) to synthesize Ni1/3Mn1/3Co1/3(OH)2. The reaction proceeds with the addition of 5.0 M NH3(aq) at 15 mL/h and 2.0 M MSO4 (M = Ni, Co, and/or Mn in desired ratios) at 20 mL/h.(1) A pH meter with feedback to the pump is used to monitor the pH value, and 4.0 M NaOH solution is automatically added to the reaction contents by the pump to maintain the desired pH. Samples are prepared with two solution pH levels of 10.6 and 11.4. An overhead rotator is used to stir the solution, and nitrogen gas continuously bubbled into the solution throughout the synthesis. Sample synthesized in the pH 10.6 solution is referred as sample A, while the one synthesized at the high pH level of 11.4 is referred as sample B. The total reaction time is 3 hrs. Ex-situ samples are drawn from the reactor with pipette during the reaction at certain timer intervals. These samples are immediately diluted in vials to avoid further co-precipitation reaction before characterizing their particle size distribution using particle size analyzer (Cilas 1190LD, Cilas Particle Size). Results and Discussion Our analysis focuses on the growth kinetics of the hydroxide primary and secondary particles. Figure 1 (black-square line) shows the change in the median particle size of sample A as a function of reaction time. The secondary particle grows from 1.5 to 16 µm in 3 hours. Three stages of particle growth could be observed: (I) growth and agglomeration of primary particles (before 45 mins); (II) mixed stage (45~100 mins); and (III) surface smoothing of secondary particles (after 100 mins). In stage I, primary particles are formed through instant nucleation, growth, and agglomeration after the addition of transition metal sources. The growth rate is as high as 11.0 µm/h, which is due to the immediate agglomeration of newly formed primary particles, decreasing their surface energy. In contrast, the growth rate of the particles dropped significantly to 0.9 µm/h in stage III. During this stage, the newly added transition metal ions form smooth layers in the void space of the secondary particles surfaces, which consists of disordered nanoplates. This could be proven by the SEM images (not shown here). In stage II, the growth rate of the secondary particles is about 3.2 µm/h, representing a mixed effect of stage I and stage III. This suggests that the growth and agglomeration of the primary particles as well as the surface smoothing of the secondary particles occur at the same time. The change in the growth mechanism is directly related to the concentration of total transition metal ions and hydroxide in the reactor. A quick calculation shows the concentration of transition metal is 0.05 M and 0.1 M at the first turning point (1 hour) and second turning point (2 hour), respectively. Also shown in Figure 1 (red-circle line) is the change in the secondary particle size as a function of reaction time for sample B synthesized at pH 11.4. In this case, the secondary particles grow from 1.5 to 8 µm in 3 hours. Unlike sample A, only two stages of particle growth can be observed: (I) growth and agglomeration of primary particle at 5.1 µm/h for reaction time smaller than 40 mins; and (II) surface smoothing of the secondary particles at 0.9 µm/h for longer reaction time. The lack of the mixed stage in sample B indicates the primary particle agglomeration is not taking place on the formed secondary particles after 40 mins. This observation is also supported by the SEM images.. The concentration of transition metal in the reactor at the turning point (i.e. 40 mins) is about 0.03 M. A. van Bommel and J. R. Dahn, Chem Mater, 21, 1500 (2009). Figure 1

Growth rates of fine aerosol particles at a site near Beijing in June 2013

Growth of fine aerosol particles is investigated during the Aerosol–CCN–Cloud Closure Experiment campaign in June 2013 at an urban site near Beijing. Analyses show a high frequency (∼ 50%) of fine aerosol particle growth events, and show that the growth rates range from 2.1 to 6.5 nm h−1 with a mean value of ∼ 5.1 nm h−1. A review of previous studies indicates that at least four mechanisms can affect the growth of fine aerosol particles: vapor condensation, intramodal coagulation, extramodal coagulation, and multi-phase chemical reaction. At the initial stage of fine aerosol particle growth, condensational growth usually plays a major role and coagulation efficiency generally increases with particle sizes. An overview of previous studies shows higher growth rates over megacity, urban and boreal forest regions than over rural and oceanic regions. This is most likely due to the higher condensational vapor, which can cause strong condensational growth of fine aerosol particles. Associated with these multiple factors of influence, there are large uncertainties for the aerosol particle growth rates, even at the same location.

Water-based binary polyol process for the controllable synthesis of silver nanoparticles inhibiting human and foodborne pathogenic bacteria.

The polyol process is a widely used strategy for producing nanoparticles from various reducible metallic precursors; however, it requires a bulk polyol liquid reaction with additional protective agents at high temperatures. Here, we report a water-based binary polyol process using low concentrations of high-molecular-weight polyethylene glycol (100 000 g mol−1, 2 wt%) and ethylene glycol (5 wt%). The entangled conformation of the polyethylene glycol in water and the increased number of reducing sites by the ethylene glycol cooperatively contributed to the stability and effectiveness of reduction reaction and particle growth, producing uniformly sized silver nanoparticles (15.8 ± 2.2 nm) with no additional protective agents at a mild temperature of 80 °C. The measurement of particle size throughout the reaction and the dependence of the optical density of a silver colloidal solution on the concentration of ethylene glycol revealed three stages of particle growth. The minimum inhibitory concentrations of the purified silver nanoparticles against four representative human and foodborne pathogenic bacteria—S. aureus, P. aeruginosa, S. enterica, and E. coli—were 4.7, 2.3, 2.3, and 1.2 μg mL−1, respectively.

A Ni/MH Pouch Cell with High-Capacity Ni(OH)2

Electrochemical performances of a high-capacity and long life β-α core-shell structured Ni0.84Co0.12Al0.04(OH)2 as the positive electrode active material were tested in a pouch design and compared to those of a standard β-Ni0.91Co0.045Zn0.045(OH)2. The core-shell materials were fabricated with a continuous co-precipitation process, which created an Al-poor core and an Al-rich shell during the nucleation and particle growth stages, respectively. The Al-rich shell became α-Ni(OH)2 after electrical activation and remained intact through the cycling. Pouch cells with the high-capacity β-α core-shell positive electrode material show higher charge acceptances and discharge capacities at 0.1C, 0.2C, 0.5C, and 1C, improved self-discharge performances, and reduced internal and surface charge-transfer resistances, at both room temperature and −10 °C when compared to those with the standard positive electrode material. While the high capacity of the core-shell material can be attributed to the α phase with a multi-electron transfer capability, the improvement in high-rate capability (lower resistance) is caused by the unique surface morphology and abundant interface sites at the β-α grain boundaries. Gravimetric energy densities of pouch cells made with the high-capacity and standard positive materials are 127 and 110 Wh·kg−1, respectively. A further improvement in capacity is expected via the continued optimization of pouch design and the use of high-capacity metal hydride alloy.

Ionic Conductivity Measurements-A Powerful Tool for Monitoring Polyol Reduction Reactions.

The reduction of metal precursors during the polyol synthesis of metal nanoparticles was monitored by ex situ ionic conductivity measurements. Using commonly used platinum precursors (K2PtCl6, H2PtCl6, and K2PtCl4) as well as iridium and ruthenium precursors (IrCl3 and RuCl3), we demonstrate that their reduction in ethylene glycol at elevated temperatures is accompanied by a predictable change in ionic conductivity, enabling a precise quantification of the onset temperature for their reduction. This method also allows detecting the onset temperature for the further reaction of ethylene glycol with HCl produced by the reduction of chloride-containing metal precursors (at ≈120 °C). On the basis of these findings, we show that the conversion of the metal precursor to reduced metal atoms/clusters can be precisely quantified, if the reaction occurs below 120 °C, which also enables a distinction between the stages of metal particle nucleation and growth. The latter is demonstrated by the reduction of H2PtCl6 in ethylene glycol, comparing ionic conductivity measurements with transmission electron microscopy analysis. In summary, ionic conductivity measurements are a simple and straightforward tool to quantify the reduction kinetics of commonly used metal precursors in the polyol synthesis.

Modelling TiO2 formation in a stagnation flame using method of moments with interpolative closure

The stagnation flame synthesis of titanium dioxide nanoparticles from titanium tetraisopropoxide (TTIP) is modelled based on a simple one-step decomposition mechanism and one-dimensional stagnation flow. The particle model, which accounts for nucleation, surface growth, and coagulation, is fully-coupled to the flow and the gas phase chemistry and solved using the method of moments with interpolative closure (MoMIC). The model assumes no formation of aggregates considering the high temperature of the flame. In order to account for the free-jet region in the flow, the computational distance, H=1.27 cm, is chosen based on the observed flame location in the experiment (for nozzle-stagnation distance, L=3.4 cm). The model shows a good agreement with experimentally measured mobility particle size for stationary stagnation surface with varying TTIP loading, although the particle geometric standard deviation, GSD, is underpredicted for high TTIP loading. The particle size is predicted to be sensitive to the sampling location near the stagnation surface in the modelled flame. The sensitivity to the sampling location is found to increase with increasing precursor loading and stagnation temperature. Lastly, the effect of surface growth is evaluated by comparing the result with an alternative reaction model. It is found that surface growth plays an important role in the initial stage of particle growth which, if neglected, results in severe underprediction of particle size and overprediction of particle GSD.