Determination of the aerosol particle size distribution by means of the diffusion battery: Analytical inversion

The algorithm of the analytical inversion of aerosol size distribution is proposed in this work. As the diffusion battery separates particles into several fractions according to their diffusivity, the total spectrum can be represented as the sum of spectra of fractions. Analytical formulas are derived to calculate mean diameters for particles in different fractions using diffusion battery penetrations as input parameters. The spectra of fractions are approximated by lognormal functions. Two analytical solutions for the aerosol size distribution inversion problem are discussed. The sizing accuracy of analytical solutions is investigated, comparing them with the measurements through transmission electron microscopy using the laboratory-generated NaCl aerosol. The agreement is demonstrated to be within 10% accuracy. It is shown that in case of two-mode size distribution, the spectrum components are well resolved for rather distant peaks (modal diameters of 10 and 300?nm) and poorly resolved for nearby modes (50 and 300?nm). To improve the peak resolution, the procedure of spectrum correction is applied demonstrating an excellent peak separation. Finally, the peak resolution is experimentally verified for the laboratory-generated two-mode spectra of tungsten oxide–NaCl aerosol with the modal diameters of 10 and 60?nm, respectively. Both analytical solutions demonstrated good peak resolution.

Copyright © 2018 American Association for Aerosol Research     

High resolution Auger electron imaging of supported metal particles

High spatial resolution Auger electron spectra and images of supported metal particles have been obtained in a UHV scanning transmission electron microscope. An edge resolution < 3 nm has been achieved. The number of atoms in a small particle can be estimated from the integrated intensity of the Auger electrons. This method is very useful for detecting and measuring particles with sizes smaller than the incident probe size. Ag clusters containing less than 20 atoms have been detected when supported on a thin carbon film.     

Effects of Mixing State on Black Carbon Measurements by Laser-Induced Incandescence

Laboratory experiments and theoretical calculations were made to characterize the performance of a Single Particle Soot Photometer (SP2) manufactured by Droplet Measurement Technologies (DMT), which was designed to measure the mass and the mixing state of individual black carbon (BC) or elemental carbon (EC) particles, based on the laser-induced incandescence (LII) technique. In this study, graphite was used as a surrogate of EC. Graphite particles with mass equivalent diameters of 110–200 nm were layered with organic liquids (glycerol and oleic acid) to produce coated graphite with diameters up to 650–800 nm. These were sampled by the SP2 to measure the waveforms (i.e., time development) of the LII and scattering signals. The peak temperature and the peak LII signal of graphite particles were independent of the coating thickness or the coating material to within experimental errors. These results indicate that the mass of EC can be measured by using peak LII signal without interference by the coating conditions. It was also shown that the difference between the times of the scattering and LII peaks can be used as an indicator of coating on EC with thicknesses larger than about 100–200 nm. LII and scattering waveforms were calculated using a newly developed theoretical model that takes into account the physical processes controlling the temperature and evaporation rate of the coated graphite particle in the laser beam. The calculations reproduced the general features of observed waveforms of LII and scattering signals, providing a firm theoretical basis for the interpretation of the SP2 data.     

Characterization of dispersed hydroprocessing catalysts prepared in reversed micelles

Fe, Co and Ni particles were prepared in water/polyoxyethylene-4-laurylether/n-hexane and water/polyoxyethylene-4-laurylether/decahydronaphthalene microemulsions by reduction of metal nitrates dissolved in the water pools of the reversed micelles. The particles were mostly monodispersed with average diameters in the range 8 to 23 nm, as determined by dynamic light scattering (DLS). However, significantly smaller size estimates were obtained using transmission electron microscopy (TEM). The average DLS particle diameters increased with increased average diameter of the reversed micelles, and the diameter of the reversed micelles increased with an increase in the microemulsion water: surfactant ratio and metal ion concentration. The diameter of the reversed micelles was also dependent upon the metal dissolved in the water pool, increasing in the order Ni < Co < Fe. These trends are explained in terms of changes that occur to the microemulsion hydrophilic-lipophilic balance. Preparation of nickel and cobalt sulfides by sulfidation of the metal salt with H₂S at low temperature, yielded much larger diameter particles (average diameter 75 nm). Measurement of the activity of the sulfided catalysts showed that the Co catalyst was more active than the Ni catalyst for the hydrocracking of diphenylmethane, and Co was more effective than Fe in reducing coke yield during Cold Lake residue hydroprocessing.     

Carbon Nanotube Penetration Through Fiberglass and Electret Respirator Filter and Nuclepore Filter Media: Experiments and Models

Three different respirator filter media (two electrets and one fiberglass) were challenged with monodisperse multi-walled carbon nanotubes (MWCNTs) of mobility diameters 20–500 nm at 5.3 and 10.6 cm s^?1 face velocities. The penetration data were compared with that of sphere-like NaCl particles. The MWCNT penetrations were generally lower than those of NaCl at both face velocities in all three filters. However, the MWCNTs had a slightly higher penetration than the NaCl in the fiberglass filter at 10.6 cm s^?1 face velocity when their mobility diameters were lower than 50 nm and the alignment effect was expected to occur. Results from the scanning electron microscopic (SEM) analysis supported the hypothesis of the alignment effect, which showed that the MWCNTs tend to be straighter or with higher aspect-ratios at the mobility sizes less than 100 nm, leading them more readily to align with the flow. Therefore, caution should be exercised when respirators are used against the MWCNTs with the mobility diameters less than 100 nm. The single fiber theory predicted the penetration of both particles in the fiberglass filters well for the particles with below 100 nm mobility diameters but discrepancies occurred beyond 100 nm. The theory still predicted the NaCl penetration through the electret filters well for the sizes below 100 nm but only predicted the MWCNT penetration well for ～20–30 nm. The Nuclepore filter and the corresponding capillary tube model were adopted to study the mechanical deposition mechanisms of MWCNTs. The model was found to predict MWCNT penetration very well when the effective length of the MWCNT was taken into account.

Copyright 2014 American Association for Aerosol Research     

Properties of gold nanostructures sputtered on glass

We studied the electrical and optical properties, density, and crystalline structure of Au nanostructures prepared by direct current sputtering on glass. We measured temperature dependence of sheet resistance and current-voltage characteristics and also performed scanning electron microscopy [SEM] analysis of gold nanolayers. It was shown that within the wide range of temperatures, gold nanolayers (<10 nm) exhibit both metal and semiconducting-like type of conductivity. UV/Vis analysis proved the semiconducting characteristic of intrinsic Au clusters. SEM analysis showed the initiatory stadium of gold layer formation to be running over isolated islands. Gold density calculated from the weight and effective thickness of the layers is an increasing function of the layer thickness up to approximately 100 nm. In thin layers deposited on solid surface, a lattice expansion is observed, which is manifested in the increase of the lattice parameter and the decrease of metal density. With increasing layer thickness, the lattice parameter and the density approach the bulk values.     

Structure of Co and Co oxide clusters in MCM-41

The structural properties of Co/MCM-41 with pore diameters between 2.9 and 3.6 nm prepared by direct synthesis and impregnation were investigated. For both preparation methods, the size of the metal particles decreased with the pore diameter. For Co/MCM-41 with the same pore diameter we observed that the direct synthesis method led to significantly smaller metal clusters compared to the impregnation method. For all Co/MCM-41 samples constraints of the metal cluster sizes were observed, which are speculated to result from influences of the micro structure during the formation of the catalyst precursor.     

In-situ transmission electron microscopy observation of particle size effect of nanoscale platinum catalysts on the formation of fullerenelike carbon shells

Drastically different catalytic behaviors of nanometer-sized platinum particles which have diverse sizes are observed using in-situ transmission electron microscopy. For small platinum catalyst particles (with diameters less than 5 nm), carbon shells form on the surfaces of the platinum catalyst particles. The formation of the carbon shells starts from the nucleation of amorphous carbon on the preferred (1 1 1) planes of platinum nanoparticles. For these small platinum catalyst particles encapsulated in graphitic shells, they are passivated by the carbon shells and their coalescence is hindered by the surrounding shells. After the platinum catalyst particles ultimately coalesce, they interact to form a compact platinum particle after breaking the encapsulating shells. For larger platinum nanoparticles (with diameters larger than 5 nm), no encapsulation of platinum nanoparticles is observed and there occurs only the coalescence of platinum nanoparticles.     

The Development of a New Thermophoretic Precipitator for Scanning Transmission Electron Microscope Analysis of Ultrafine Aerosol Particles

A thermophoretic precipitator specifically for collecting ultrafine aerosol samples onto electron microscope support grids has been designed, and built. The precipitator has been designed to deposit particles of diameter less than 100 nm discretely onto support grids, with uniform deposition velocity across the size range. In addition, it has been designed to be compact, and portable. Preliminary investigations indicate it to give discrete deposits suitable for single particle analysis. Qualitatively, particle deposition velocity appears uniform between 4 and 30 nm, with a slight decrease towards higher diameters, although this is yet to be confirmed by comparison with reference particle size distribution analysis methods. Particle distribution on the microscope grid was shown to be uneven on a millimeter scale, but relatively even on a micrometer scale, enabling good characterization of the deposit.     

Quasi-radial growth of metal tube on si nanowires template

It is reported in this article that Si nanowires can be employed as a positive template for the controllable electrochemical deposition of noble metal tube. The deposited tube exhibits good crystallinity. Scanning electron microscope and transmission electron microscope characterizations are conducted to reveal the growth process of metal tube, showing that the metal tube grows quasi-radially on the wall of Si nanowire. The quasi-radial growth of metal enables the fabrication of thickness-defined metal tube via changing deposition time. Inner-diameter-defined metal tube is achieved by choosing Si nanowires with desired diameter as a template. Metal tubes with inner diameters ranging from 1 μm to sub-50 nm are fabricated.     

Modelling stiffness of polymer/clay nanocomposites

Aligned nanoclay particles can be distributed randomly in a polymer matrix even at high volume fractions, but randomly oriented particles cannot be randomly distributed at high volume fractions. Instead a nanocomposite where there are clusters of nearly aligned particles is obtained. The clusters of nearly aligned particles form an effective particle with lower aspect ratio. This phenomenon which produces a nanocomposite of less stiffness than might have been expected has implications for the processing of nanoclay polymer composites.It is shown by comparing two-dimensional to three-dimensional finite element studies that the two-dimensional model, often used because it is simpler, does not accurately predict the stiffness. The Mori-Tanaka model is shown to give a reasonably accurate prediction of the stiffness of clay nanocomposites whose volume fraction is less than about 5% for aligned particles but underestimates the stiffness at higher volume fractions. On the other hand for randomly oriented particles the Mori-Tanaka model overestimates the stiffness of clay nanocomposites.     

Sol–Gel Route to Synthesis of Microporous Ceramic Membranes: Preparation and Characterization of Microporous TiO2 and ZrO2 Xerogels

This paper focuses on the preparation and characterization of pure TiO₂ and ZrO₂ xerogels. The preparation method is based on a sol–gel technique using metal tert -amyloxides as precursors to produce nano-sized metal oxide particles which are subsequently packed in a gelation process, eventually resulting in microporous xerogele. The unsupported TiO₂ and ZrO₂ xerogele produced in this manner have a mean pore diameter less than 2 nm and more than 50% microporosity. However, these gels, in their pure form, are thermally stable only to 350°C. Improved thermal stabilities of mixed metal oxide xerogels will be reported elsewhere.     

Growth of large-diameter (∼4 nm) single-wall carbon nanotubes in the nanospace of mesoporous material SBA-15

Single-wall carbon nanotubes (SWCNTs) have been synthesized by supported-catalyst chemical vapor deposition (CCVD) using one-dimensional (1D) channels of mesoporous silica (SBA-15; mean channel diameter, 6.0 nm) functionalized with carboxyl groups where Co and Fe complexes are encapsulated. The synthesized SWCNTs have much larger diameters than the SWCNTs synthesized by conventional CCVD. Transmission electron microscope observations reveal that large-diameter SWCNTs (<4.2 nm) are grown in 1D channels of SBA-15. Large metal particles formed in the channels should play an important role in the growth of the SWCNTs with larger diameters.     

Coordination,atom reorganization,and catalysis of palladium in zeolite cages

Zeolite encaged palladium clusters undergo thorough atomic reorganizations when exposed to an adsorptive such as carbon monoxide or when used as catalysts e.g. in CO hydrogenation. Exposure to carbon monoxide of metal particles, which are initially anchored to zeolite walls via proton bridges, transforms the metal clusters into small, highly mobile carbonyl clusters. They coalesce and form larger clusters. At low temperature, this process is limited by the geometric constraints of the cage windows. At higher temperatures, further growth of metal particles occurs, conceivably via partial destruction of the zeolite matrix. The interaction of the metal particles with zeolite protons gives rise to electrondeficient metal clusters, which catalyze neopentane at a much higher rate than neutral metal particles. Such clusters might also act as collapsed bifunctional sites in bifunctional catalysis.     

Contributions of TMAH Surfactant on Hierarchical Structures of PVA/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>–TMAH Ferrogels by Using SAXS Instrument

The magnetic hydrogels combining polyvinyl-alcohol (PVA) and Fe₃O₄ (magnetite)–TMAH (tetra-methyl ammonium hydroxide) have been successfully fabricated via a Freezing-thawing route. The magnetite nanoparticles were prepared from iron sands by using coprecipitation method. The transmission electron microscopy image revealed that the magnetite nanoparticles with a reaction temperature of 30 °C had the average particle size of 12 nm in clusters of aggregation. The result was similar to the particle size obtained from X-ray diffraction data analyzed by Scherer equation. Furthermore, synchrotron small angle X-ray scattering data were analyzed by using two lognormal distributions to calculate the distribution of the individual magnetite particles. Meanwhile, Teubner-Strey and Beaucage models were employed to observe the distribution of magnetite particles coated by TMAH as a surfactant. The data analysis showed that the magnetite particles within the magnetic hydrogels formed aggregations with diameters of cluster particles in the range from 13.1 to 31.8 nm. Interestingly, the diameter of clusters particle increased from 13.1 to 31.8 nm along with the increasing concentration of ferrofluids from 1 to 15 wt%. This phenomenon was predicted to result from the effect of TMAH as a surface reactant agent that prevented the aggregation by coating the surface of the magnetite nanoparticles.     

The pore structure of chrominophosphate catalysts

A series of chrominophosphates (CrPs) with various P/Cr ratios were prepared by the precipitation method. The pore structures of these catalysts were characterized by nitrogen adsorption and mercury-penetration porosimetry. The results indicated that the micro-pores with diameters less than 20 nm were due to the dehydration process and had a slit-shaped geometry. There were two types of large pores (meso- and macro-) with diameters greater than 20 nm. These pores had a cylindrical pore-shape. The meso-pores can be attributed to the packing of particles and the macro-pores are essentially due to the packing of the aggregates of the particles.     

Densification of Alkoxide-Derived Fine Silica Powder Compact by Ultra-High-Pressure Cold Isostatic Pressing

Powder compacts of alkoxide-derived fine silica powders were consolidated into a highly dense and uniform structure by ultra-high-pressure cold isostatic pressing of granules with controlled structure. The diameters of spherical and nearly monosized amorphous silica particles, prepared from metal alkoxide, were successfully controlled in the range of 9 to 760 nm by varying the concentration of ammonia. Close-packed granules of these powders were produced by spray drying. These powders were isostatically pressed up to 1 GPa at room temperature. Although the average particle diameter was less than 100 nm, the maximum relative density of the compacts was more than 78% of theoretical density. The optimum particle size to obtain highly dense compacts was in the range of 30 to 300 nm at 1 GPa. Furthermore, the ratio of mode pore diameter in these compacts to particle diameter was less than 0.155, which corresponded to the minimum ratio of calculated three-particle pore channel radii for hexagonal close packing. Viscous deformation of particles under ultra-high isostatic pressure played an important role in the densification of the compacts.     

Characterization of a Modified Expansion Condensation Particle Counter for Detection of Nanometer-Sized Particles

A newly developed condensation particle counter provides measurements of aerosol particle number densities for size diameters as low as 3 nm. This Expansion Condensation Particle Counter (ECPC) operates based on fast adiabatic expansion with specialized detection and evaluation of the temporal development of light scattered by the ensemble of growing droplets. In its new configuration the ECPC has been modified such that a previously needed calibration factor became obsolete. In this article the new design is described which now includes a fast pressure sensor for monitoring the pressure drop inside the measurement chamber. Extensive laboratory experiments for characterizing the ECPC are described where sulfuric acid droplets with diameters between ～2.5 nm and 23 nm have been utilized. Water as well as butanol are demonstrated to be suitable working fluids. One experiment using tungsten oxide (WOx) particles shows that a 50% cut-off size diameter as low as 2.5 nm can be reached for this ECPC with a detection efficiency of several percent for particles as small as 1.4 nm. High and low supersaturations are experimentally examined and the corresponding different cut-off sizes are obtained. Measurements of ambient urban air in Mainz (Germany) obtained by this ECPC are juxtaposed to those from a TSI UCPC 3025A with satisfactory agreement. Similarly, in-situ data recorded with two ECPC units in the city of Isfahan (Iran) are shown to demonstrate the suitability of the technique for traffic related pollution measurements. Also, in future applications coarse information on the chemical nature of nucleated particles can be obtained by simultaneously using various condensing liquids in different channels of the ECPC setup.     

Examining Elemental Surface Enrichment in Ultrafine Aerosol Particles Using Analytical Scanning Transmission Electron Microscopy

The surface structure and chemistry of ultrafine aerosol particles (typically particles smaller than 100 nm in diameter) play key roles in determining physical and chemical behavior, and is relevant to fields as diverse as nanotechnology and aerosol toxicity. Analytical scanning transmission electron microscopy (STEM) is one of the few analytical methods available that is potentially capable of characterizing ultrafine particles at subnanometer resolution. We propose a method that enables STEM to characterize and quantify elemental surface enrichment within radially symmetrical particles at a spatial resolution of less than 1 nm when used in conjunction with electron energy loss spectroscopy (EELS) and X-ray energy dispersive spectroscopy (EDS). Although the method relies on a number of assumptions for complete particle characterization, estimation of the depth of an outer layer of elemental enrichment should be possible with relatively few assumptions. A preliminary investigation of the method has been carried out using particles from gas metal arc welding on mild steel. Using the analysis method, we were able to characterize Si and O enrichment in a number of particles. Two particles were investigated extensively using EELS and EDS analysis. Both techniques allowed surface enrichment of Si to be identified and quantified in the particles, although the relatively poor sensitivity of EDS was a limiting factor in the analysis. EELS allowed rapid data collection and enabled surface enrichment of Si and O to be characterized. Using a simple model to describe elemental composition with radial position, it was estimated that Si and O were enriched in an outer layer around the particle approximately 1 nm deep.     

Supra‐monolayer coverages on small metal clusters and their effects on H2 chemisorption particle size estimates

H₂ chemisorption measurements are used to estimate the size of supported metal particles, often using a hydrogen‐to‐surface‐metal stoichiometry of unity. This technique is most useful for small particles whose sizes are difficult to estimate through electron microscopy or X‐ray diffraction. Undercoordinated metal atoms at the edges and corners of particles, however, make up large fractions of small metal clusters, and can accommodate multiple hydrogen atoms leading to coverages which exceed 1 ML (supra‐monolayer). Density functional theory was used to calculate hydrogen adsorption energies on Pt and Ir particles (38–586 atoms, 0.8–2.4 nm) at high coverages (≤3.63 ML). Calculated differential binding energies confirm that Pt and Ir (111) single‐crystal surfaces saturate at 1 ML; however, Pt and Ir clusters saturate at supra‐monolayer coverages as large as 2.9 ML. Correlations between particle size and saturation coverage are provided that improve particle size estimates from H₂ chemisorption for Pt‐group metals. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3109–3120, 2018