Sampling and dilution of nanoparticles at high temperature

<p>Sampling and dilution of flame-generated, fractal-like ZrO₂ aerosols is investigated by aerosol mass/mobility measurements and microscopy. Two broadly used sampler configurations, a straight-tube (ST) and a hole-in-a-tube (HiaT), at three different in-flow orientations and hole diameters are evaluated. The mobility size distributions, mass-mobility exponent, <i>D_fm</i>, prefactor, <i>k_fm</i>, and average primary particle diameter are obtained at 10–60 cm height above the burner (HAB) of fuel-rich (hot) and fuel-lean (cold) spray flames by differential mobility analyzer (DMA) and aerosol particle mass (APM) measurements using a recent power law for fractal-like particles. The primary particle diameter, agglomerate size distributions, and corresponding standard deviations from aerosol measurements are compared to those by counting images of particles collected by thermophoretic sampling along the flame centerline. Once new particle formation is completed in the flame, both sampler configurations result in nearly identical particle size distributions. Furthermore, all HiaT samplers result in similar mobility size distributions at all orientations, regardless of hole size. Sampling using a downstream in-flow hole orientation results in slightly larger Sauter mean diameters than those obtained by upstream or sidestream ones, especially for the cold flame. Additionally, a correlation is developed by Discrete Element Modeling (DEM) for the agglomerate <i>D_fm</i> evolution to its asymptotic value of 2.2 as function of the average number of primary particles per agglomerate, <i>n_va</i>, or the relative particle density with pre-exponential constant <i>k_fm</i> = 1.18, regardless of primary particle size. This is in good agreement with an experimentally obtained correlation in terms of relative particle density as well as with experimental data for ZrO₂, Ag, and Cu nanoparticles.</p> <p>© 2016 American Association for Aerosol Research</p

Pd Subnano-Clusters on TiO₂ for Solar-Light Removal of NO

Palladium subnano-clusters (<1 nm) on TiO₂ nanoparticles have been prepared in one step by flame aerosol technology. Under solar light irradiation, these materials remove NO_<i>x</i> 3 or 7 times faster than commercial TiO₂ (P25, Evonik) with or without photodeposited Pd on it. X-ray photoelectron spectroscopy (XPS) reveals that such photodeposited Pd consists of metallic Pd along with several Pd oxidation states. In contrast, flame-made Pd subnano-clusters on TiO₂ dominantly consist of an intermediate Pd oxidation state between metallic Pd and PdO. In that intermediate state, the Pd subnano-clusters are stable up to, at least, 600 °C for 2 h in air. However, a fraction of them are reduced into relatively large (>1 nm) metallic Pd nano-particles by annealing in N₂ at 400 °C for 2 h, as elucidated by XPS and scanning transmission electron microscopy. The Pd subnano-clusters interact with oxygen defects on the TiO₂ surface, as shown by Raman spectroscopy. This interaction suppresses CO adsorption on Pd, as observed by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), analogous to strong metal–support interactions (SMSI) of nano-sized noble metals on TiO₂

In Situ Monitoring of the Deposition of Flame-Made Chemoresistive Gas-Sensing Films

Flame-deposited semiconducting nanomaterials on microelectronic circuitry exhibit exceptional performance as chemoresistive gas sensors. Current manufacturing technology, however, does not monitor in situ the formation of such nanostructured films, even though this can facilitate the controlled and economic synthesis of these sensors. Here, the resistance of such growing films is measured in situ during fabrication to monitor the creation of a semiconducting nanoparticle network for gas sensors. Upon formation of that network, the film resistance drops drastically to an asymptotic value that depends largely on the film structure or morphology rather than on its thickness and size of nanoparticle building blocks. Precursor solutions of various concentrations enable the flame deposition of Sb-doped SnO₂ sensing films of different morphologies, each of which exhibit a characteristic in situ resistance pattern. Low precursor concentrations (1 mM) lead to thin (ca. 0.16 μm) films with slender columnar structures of increasing diameter (up to 25 nm) after prolonged deposition (up to 6 min) and show an oscillating in situ resistance during their fabrication. On the other extreme, high precursor concentrations (100 mM) lead to thick (up to 80 μm) dendritic and porous films consisting of nanoparticles with relatively small primary particle diameter (around 7 nm) that remain invariant of deposition duration, which is in agreement with the stable in situ resistance. Such dendritic films exhibit a sensor recovery time that is an order of magnitude longer than that of those made at lower concentrations. The above understanding enables the rapid and economic flame synthesis of thin gas sensors consisting of minimal semiconducting nanomaterial mass possessing a tuned baseline resistance and exhibiting excellent response to ethanol vapor

Coagulation–Agglomeration of Fractal-like Particles: Structure and Self-Preserving Size Distribution

Agglomeration occurs in environmental and industrial processes, especially at low temperatures where particle sintering or coalescence is rather slow. Here, the growth and structure of particles undergoing agglomeration (coagulation in the absence of coalescence, condensation, or surface growth) are investigated from the free molecular to the continuum regime by discrete element modeling (DEM). Particles coagulating in the free molecular regime follow ballistic trajectories described by an event-driven method, whereas in the near-continuum (gas-slip) and continuum regimes, Langevin dynamics describe their diffusive motion. Agglomerates containing about 10–30 primary particles, on the average, attain their asymptotic fractal dimension, <i>D</i>_f, of 1.91 or 1.78 by ballistic or diffusion-limited cluster–cluster agglomeration, corresponding to coagulation in the free molecular or continuum regimes, respectively. A correlation is proposed for the asymptotic evolution of agglomerate <i>D</i>_f as a function of the average number of constituent primary particles, <i>n̅</i>_p. Agglomerates exhibit considerably broader self-preserving size distribution (SPSD) by coagulation than spherical particles: the number-based geometric standard deviations of the SPSD agglomerate radius of gyration in the free molecular and continuum regimes are 2.27 and 1.95, respectively, compared to ∼1.45 for spheres. In the transition regime, agglomerates exhibit a quasi-SPSD whose geometric standard deviation passes through a minimum at Knudsen number <i>Kn</i> ≈ 0.2. In contrast, the asymptotic <i>D</i>_f shifts linearly from 1.91 in the free molecular regime to 1.78 in the continuum regime. Population balance models using the radius of gyration as collision radius underestimate (up to about 80%) the small tail of the SPSD and slightly overpredict the overall agglomerate coagulation rate, as they do not account for cluster interpenetration during coagulation. In the continuum regime, when a recently developed agglomeration rate is used in population balance equations, the resulting SPSD is in excellent agreement with that obtained by DEM

Antioxidant and Antiradical SiO₂ Nanoparticles Covalently Functionalized with Gallic Acid

Gallic acid (GA) and its derivatives are natural polyphenolic substances widely used as antioxidants in nutrients, medicine and polymers. Here, nanoantioxidant materials are engineered by covalently grafting GA on SiO₂ nanoparticles (NPs). A proof-of-concept is provided herein, using four types of well-characterized SiO₂ NPs of specific surface area (SSA) 96–352 m²/g. All such hybrid SiO₂-GA NPs had the same surface density of GA molecules (∼1 GA per nm²). The radical-scavenging capacity (RSC) of the SiO₂-GA NPs was quantified in comparison with pure GA based on the 2,2-diphenyl-1-picrylhydrazyl (DPPH^•) radical method, using electron paramagnetic resonance (EPR) and UV–vis spectroscopy. The scavenging of DPPH radicals by these nanoantioxidant SiO₂-GA NPs showed mixed-phase kinetics: An initial fast-phase (<i>t</i>_1/2 <1 min) corresponding to a H-Atom Transfer (HAT) mechanism, followed by a slow-phase attributed to secondary radical–radical reactions. The slow-reactions resulted in radical-induced NP agglomeration, that was more prominent for high-SSA NPs. After their interaction with DPPH radicals, the nanoantioxidant particles can be reused by simple washing with no impairment of their RSC

Pressure- and Temperature-Induced Monoclinic-to-Orthorhombic Phase Transition in Silicalite‑1

The thermal, mechanical, and volumetric behavior of silicalite-1, an all-silica Mobil Five (MFI) zeolite, is elucidated by atomistic simulations. A flexible force field was selected and validated from a set of force fields to capture the intramolecular interactions of the crystal lattice. This force field accounts for realistic bond, angle, and torsional interactions among atoms of the framework alongside with conventional Lennard-Jones and Coulomb interactions. By monitoring the behavior of silicalite-1 as a function of pressure and temperature, a fully reversible monoclinic-to-orthorhombic phase transition (polymorphism) was revealed in accordance with experimental data. Thermodynamic considerations dictate that this is a second-order phase transition in the Ehrenfest classification. Additionally, reversible pressure-induced amorphization was captured by our model and was associated with the formation of linear zones of increased distortion running parallel to the straight and sinusoidal channels of this zeolite. Remarkably high isothermal compressibility (small bulk modulus) was calculated for orthorhombic silicalite-1, in excellent agreement with experimental data, rendering silicalite-1 as the most compressible zeolite known to date. The rigid unit mode model was identified as the dominant structural mechanism for negative thermal expansion (NTE), typically observed over a wide temperature range in MFI zeolites. Better understanding of the monoclinic-to-orthorhombic phase transition and molecular mechanisms associated with energy dissipation and NTE in zeolites provides control over the framework microstructure, allowing for enhanced molecular sieving, tunable selectivity in separation processes, mechanical stability, and substantially amplified catalytic efficiency in petrochemical applications

Impact of Humidity on Silica Nanoparticle Agglomerate Morphology and Size Distribution

The effect of humidity on flame-made metal oxide agglomerate morphology and size distribution is investigated, for the first time to our knowledge, and compared to that on soot, which has been widely studied. Understanding the impact of humidity on such characteristics is essential for storage, handling, processing, and eventual performance of nanomaterials. More specifically, broadly used agglomerates of flame-made silica nanoparticles are humidified at various saturation ratios, <i>S</i> = 0.2–1.5, and dried before characterization with a differential mobility analyzer (DMA), an aerosol particle mass (APM) analyzer, and transmission electron microscopy. At high humidity, the constituent single and/or aggregated (chemically bonded) primary particles (PPs) rearrange to balance the capillary forces induced by condensation–evaporation of liquid bridges between PPs. Larger agglomerates restructure more than smaller ones, narrowing their mobility size distribution. After humidification at <i>S</i> = 1.5 and drying, agglomerates collapse into compact structures that follow a fractal scaling law with mass–mobility exponent <i>D</i>_fm = 3.02 ± 0.11 and prefactor <i>k</i>_m = 0.27 ± 0.07. This critical <i>S</i> = 1.5 for silica agglomerates is larger than the 1.26 obtained for soot because of the hydrophilic surface of silica that delays water evaporation. The relative effective density, ρ_eff/ρ, of collapsed agglomerates becomes invariant of mobility diameter, <i>d</i>_m, similar to that of fluidized and spray-dried granules. The average silica ρ_eff/ρ = 0.28 ± 0.02 is smaller than the 0.36 ± 0.04 measured for the humidified-dried soot because of the larger size of silica aggregates, <i>d</i>_m/<i>d</i>_p, and number of constituent primary particles, <i>n</i>_p, of diameter <i>d</i>_p. This is verified by tandem-DMA (TDMA) measurements, yielding maximum <i>d</i>_m = 3<i>d</i>_p or 5<i>d</i>_p and <i>n</i>_p = 13 or 36 for the soot or silica aggregates studied here, in good agreement with those reported from microscopy and high-pressure agglomerate dispersion. A scaling law relating the initial <i>d</i>_m,o to <i>d</i>_m, <i>D</i>_fm, and <i>k</i>_m after condensation-drying is developed. The mass–mobility relationship of collapsed silica and soot agglomerates obtained by combining this law with fast TDMA measurements is in excellent agreement with that measured by the direct, but tedious, DMA-APM analysis

Deep Tissue Imaging with Highly Fluorescent Near-Infrared Nanocrystals after Systematic Host Screening

Photoluminescent inorganic nanoparticles are attractive as bioimaging contrast agents because they do not degrade by photobleaching and do not suffer from concentration quenching as clinically applied organic dyes. Here, for the first time, a large variety of oxide, phosphate, and vanadate nanocrystals doped with Nd³⁺ are systematically examined and compared as down-converting photoluminescent contrast agents to understand underlying physical properties and to identify the brightest composition. These inorganic crystals are particularly attractive for bioimaging in the near-infrared (NIR) window, where absorption and scattering by human tissue are reduced drastically. Through close control of their crystal size, the resulting fluorescence properties are quantitatively compared under NIR excitation. Most interestingly, BiVO₄ doped with Nd³⁺ is shown to be the most efficient composition. Its application as a photoluminescent NIR imaging contrast agent is demonstrated <i>ex vivo</i> with chicken skeletal muscle and bovine liver tissues. Under a harmless laser power density (0.2 W/cm²), fluorescent BiVO₄ particles could be clearly detected at an injection depth of 20 mm by a simple commercial camera

Quantifying the Origin of Released Ag⁺ Ions from Nanosilver

Nanosilver is most attractive for its bactericidal properties in modern textiles, food packaging, and biomedical applications. Concerns, however, about released Ag⁺ ions during dispersion of nanosilver in liquids have limited its broad use. Here, nanosilver supported on nanostructured silica is made with closely controlled Ag size both by dry (flame aerosol) and by wet chemistry (impregnation) processes without any surface functionalization that could interfere with its ion release. It is characterized by electron microscopy, atomic absorption spectroscopy, and X-ray diffraction, and its Ag⁺ ion release in deionized water is monitored electrochemically. The dispersion method of nanosilver in solutions affects its dissolution rate but not the final Ag⁺ ion concentration. By systematically comparing nanosilver size distributions to their equilibrium Ag⁺ ion concentrations, it is revealed that the latter correspond precisely to dissolution of one to two surface silver oxide monolayers, depending on particle diameter. When, however, the nanosilver is selectively conditioned by either washing or H₂ reduction, the oxide layers are removed, drastically minimizing Ag⁺ ion leaching and its antibacterial activity against E. coli. That way the bactericidal activity of nanosilver is confined to contact with its surface rather than to rampant ions. This leads to silver nanoparticles with antibacterial properties that are essential for medical tools and hospital applications

Scale-up of Nanoparticle Synthesis by Flame Spray Pyrolysis: The High-Temperature Particle Residence Time

The scale-up of nanoparticle synthesis by a versatile flame aerosol technology (flame spray pyrolysis) is investigated numerically and experimentally for production of ZrO₂. A three-dimensional computational fluid dynamics model is developed accounting for combustion and particle dynamics by an Eulerian continuum approach coupled with Lagrangian description of multicomponent spray droplet atomization, transport, and evaporation. The model allows the extraction of the high-temperature particle residence time (HTPRT) that is governed by the dispersion gas to precursor liquid mass flow ratio as well as the flame enthalpy content. The HTPRT is shown to control the primary particle and agglomerate size, morphology, and even ZrO₂ crystallinity in agreement with experimental data. When the HTPRT is kept constant, the production rate for ZrO₂ nanoparticles could be scaled up from ∼100 to 500 g/h without significantly affecting product particle properties, revealing the HTPRT as a key design parameter for flame aerosol processes