Size-Resolved Kinetic Measurements of Aluminum Nanoparticle Oxidation with Single Particle Mass Spectrometry

Kinetic analyses for solid-state phase transition of metastable amorphous-AlOx (2.5 < x ≤ 3.0) nanostructures into crystalline alumina polymorphs

Solid-solid phase change materials (SS-PCMs) hold promise for energy storage/dissipation in batteries and energetic materials. Yet, phase change kinetics for SS-PCMs undergoing metastable to semi-stable/stable phase transformations remain relatively ill-studied because trapping metastable phases remain challenging. Recently, we demonstrated the kinetic entrapment and stabilization of a highly disordered and amorphous Al-oxide phase m-AlOx@C (x~2.5-3.0) via laser ablation synthesis in solution (LASiS). We report here, to our knowledge, the first chemical kinetics analysis for S-S phase transition of the m-AlO3@C nanocomposites (< 5-8 nm sizes) into semi-stable equilibrium alumina phases (θ/γ-Al2O3) via disproportionation reaction, while releasing excess trapped gases. Our results indicate the atomic density of the AlO3 structures to be ~5-10 times less than that of the final Al2O3 phases, which led to the hypothesis of a volume shrinkage process during their phase transition. Temperature-dependent X-ray diffraction studies reveal the high-temperature phase transition for m-AlO3 → θ/γ-Al2O3 to follow contracting volume kinetics model, thereby validating our earlier hypothesis. Using the geometric volume contraction model, reaction kinetics analyses from Arrhenius plots reveal the activation energy barrier for the phase transition to be ~270±11 kJ/mol. This makes the activation energy barrier nearly identical to the oxidation of micron-sized Al particles.

Atomistic Insights into Impact-Induced Energy Release and Deformation of Core-Shell-Structured Ni/Al Nanoparticle in an Oxygen Environment

In actual atmospheric environments, Ni/Al composites subjected to high-velocity impact will undergo both intermetallic reaction and oxidative combustion simultaneously, and the coupling of mechanical and multiple chemical processes leads to extremely complex characteristics of energy release. This work employs ReaxFF molecular dynamics simulations to investigate the impact-induced deformation and energy release of a core-shell-structured Ni/Al nanoparticle in an oxygen environment. It was found that Al directly undergoes fragmentation, while Ni experiences plastic deformation, melting, and fragmentation in sequence as the impact velocity increased. This results in the final morphology of the nanoparticles being an ellipsoidal-clad nanoparticle, spherical Ni/Al melt, and debris cloud. Furthermore, these deformation characteristics are strongly related to the material property of the shell, manifested as Ni shell-Al core particle, being more prone to breakage. Interestingly, the dissociation phenomenon of Ni-Al-O clusters during deformation is observed, which is driven by Ni dissociation and Al oxidation. In addition, the energy release is strongly related to the deformation behavior. When the nanoparticle is not completely broken (Ni undergoes plastic deformation and melting), the energy release comes from the oxidative combustion of Al fragments and the intermetallic reaction driven by atomic mixing. When the nanoparticle is completely broken, the energy release mainly comes from the oxidative combustion of the debris cloud. At the same time, the promoting effect of oxygen concentration on the energy release efficiency is examined. These findings can provide atomic insights into the regulation of impact-induced energy release for reactive intermetallic materials.

Changes in the state of matter of KCIO4 to improve thermal and combustion properties of Al/MoO3 nanothermite

To improve the thermal and combustion properties of nanothermites, a design theory of changing the state of matter and structural state of the reactants during reaction was proposed. The Al/MoO3/KClO4 (Kp) nanothermite was prepared and the Al/MoO3 nanothermite was used as a control. SEM and XRD were used to characterize the nanothermites; DSC was used to test thermal properties; and constant volume and open combustion tests were performed to examine their combustion performance. Phase and morphology characterization of the combustion products were performed to reveal the mechanism of the aluminothermic reaction. The results show that the Al/MoO3/Kp nanothermite exhibited excellent thermal properties, with a total heat release of 1976 J·g- 1, increasing by approximately 33% of 1486 J·g- 1 of the Al/MoO3 nanothermite, and activation energy of 269.66 kJ·mol- 1, which demonstrated higher stability than the Al/MoO3 nanothermite (205.64 kJ·mol- 1). During the combustion test, the peak pressure of the Al/MoO3/Kp nanothermite was 0.751 MPa, and the average pressure rise rate was 25.03 MPa·s- 1, much higher than 0.188 MPa and 6.27 MPa·s- 1 of the Al/MoO3 nanothermite. The combustion products of Al/MoO3 nanothermite were Al2O3, MoO, and Mo, indicating insufficient combustion and incomplete reaction, whereas, the combustion products of Al/MoO3/Kp nanothermite were Al2O3, MoO, and KCl, indicating complete reaction. Their "coral-like" morphology was the effect of reactants solidifying after melting during the combustion process. The characterization of reactants and pressure test during combustion reveals the three stages of aluminothermic reaction in thermites. The excellent thermal and combustion performance of Al/MoO3/Kp nanothermite is attributed to the melt and decomposition of Kp into O2 in the third stage. This study provides new ideas and guidance for the design of high-performance nanothermites.

Magnesium-Induced Strain and Immobilized Radical Generation on the Boron Oxide Surface Enhances the Oxidation Rate of Boron Particles: A DFTB-MD Study

Despite their high gravimetric and volumetric energy densities, boron (B) particles suffer from poor oxidative energy release rates as the boron oxide (B2O3) shell impedes the diffusivity of O2 to the particle interior. Recent experiemental studies have shown that the addition of metals with a lower free energy of oxidation, such as Mg, can reduce the oxide shell of B and enhance the energetic performance of B by ∼30-60%. However, the exact underlying mechanism behind the reactivity enhancement is unknown. Here, we performed DFTB-MD simulations to study the reaction of Mg vapor with a B2O3 surface. We found that the Mg becomes oxidized on the B2O3 surface, forming a MgBxOy phase, which induces a tensile strain in the B-O bond at the MgBxOy-B2O3 interface, simultaneously reducing the interfacial B and thereby developing dangling bonds. The interfacial bond straining creates an overall surface expansion, indicating the presence of a net tensile strain. The B with dangling bonds can act as active centers for gas-phase O2 adsorption, thereby increasing the adsorption rate, and the overall tensile strain on the surface will increase the diffusion flux of adsorbed O through the surface to the particle core. As the overall B particle oxidation rate is dependent on both the O adsorption and diffusion rates, the enhancement in both of these rates increases the overall reactivity of B particles.

Spallation of Isolated Aluminum Nanoparticles by Rapid Photothermal Heating

The spallation of isolated aluminum (Al) nanoparticles (NPs) is initiated using rapid photothermal heating. The Al NPs exhibited a nominal diameter of 120 nm, with an average oxide shell thickness of 3.8 nm. Photothermal heating was achieved by coupling a focused laser (446 nm wavelength) to an optical grating substrate and to the plasmonic resonance of the Al NPs themselves. These factors enhanced the absorption cross section by a factor of 8-18 compared to no substrate and generated an Al NP heating rate on the order of 10⁷-10⁸ K/s. Observations indicate that molten Al is ejected from the heated NP, indicating that melting of the Al core is required for spallation. A graphene layer atop the grating substrate encouraged the formation of discrete particles of ejected Al, while irregular elongated filament products were observed without the graphene layer. Numerical simulations indicate that laser-heated Al NPs reach temperatures between approximately 1000 and 1500 K. These observations and experimental conditions are consistent with those anticipated for the melt dispersion mechanism, a thermomechanical reaction mechanism that has not previously been clearly demonstrated. Activating and controlling this mechanism is anticipated to enhance applications ranging from biological phototherapy to energetic materials.

Reaction mechanism of aluminum nanoparticles in explosives under high temperature and high pressure by shock loading

Aluminum nanoparticles (ANPs) can greatly improving the power of explosives. However, the rapid reaction mechanism of ANPs under the simultaneous high temperature and high pressure by shock loading is not...

Application of Single-Particle Mass Spectrometer to Obtain Chemical Signatures of Various Combustion Aerosols

A single-particle mass spectrometer (SPMS) with laser ionization was constructed to determine the chemical composition of single particles in real time. The technique was evaluated using various polystyrene latex particles with different sizes (125 nm, 300 nm, 700 nm, and 1000 nm); NaCl, KCl, MgCO₃, CaCO₃, and Al₂O₃ particles with different chemical compositions; an internal mixture of NaCl and KCl; and an internal mixture of NaCl, KCl, and MgCl₂ with different mixing states. The results show that the SPMS can be useful for the determination of chemical characteristics and mixing states of single particles in real time. The SPMS was then applied to obtain the chemical signatures of various combustion aerosols (diesel engine exhaust, biomass burning (rice straw), coal burning, and cooking (pork)) based on their single-particle mass spectra. Elemental carbon (EC)-rich and EC-organic carbon (OC) particles were the predominant particle types identified in diesel engine exhaust, while K-rich and EC-OC-K particles were observed among rice straw burning emissions. Only one particle type (ash-rich particles) was detected among coal burning emissions. EC-rich and EC-OC particles were observed among pork burning particles. The single-particle mass spectra of the EC or OC types of particles differed among various combustion sources. The observed chemical signatures could be useful for rapidly identifying sources of atmospheric fine particles. In addition, the detected chemical signatures of the fine particles may be used to estimate their toxicity and to better understand their effects on human health.

Review of Experimental Methods for Measuring the Ignition and Combustion Characteristics of Metal Nanoparticles

Investigations in recent decades have shown that the combustion mechanism of metal particles changes dramatically with diminishing size. Consequently, theoretical description of the ignition and combustion of metal nanoparticles requires additional research. At the same time, to substantiate theoretical models, it is necessary to obtain objective experimental information about characteristics of ignition and combustion processes, which is associated with solving serious technical problems. The presented review analyzes specific features of existing experimental methods implied for studying ignition and combustion of metal nanoparticles. This particularly concerns the methods for correct determination of nanoparticles size, correct description of their heat-exchange parameters, and determining the ignition delay and combustion times. It is stressed that the problem exists of adequate comparison of the data obtained with the use of different techniques of particles’ injection into a hot gas zone and the use of different methods of reaction time measurement. Additionally, available in the literature, data are obtained for particles of different material purity and different state of oxide layer. Obviously, it is necessary to characterize in detail all relevant parameters of a particle’s material and measurement techniques. It is also necessary to continue developing advanced approaches for obtaining narrow fractions of nanoparticles and for detailed recording of dynamic particles’ behavior in a hot gas environment.

Aluminium nanoparticle modelling coupled with molecular dynamic simulation method to compare the effect of annealing rates on diethyl ether coating and oxidation behaviours

This study was conducted to examine the influence of annealing rates on coating and oxidation performances of Aluminium (Al) nanoparticle (ANP) by molecular dynamic (MD) simulations. Four levels of cooling rates were utilized on melted ANP to obtain annealed ANP models with different microstructures. Then those nanoparticles were placed into pure diethyl ether or oxygen gas environments to perform coating and oxidation simulations respectively. It was revealed that there was a relatively optimal annealing condition for ANP models to recover the initial microstructure of themselves as much as possible. The ether coating behaviour of annealed ANP model under this condition was better than other models. In contrast, the oxidation of all different models was almost the same. So, the factor of the annealing rate had little effect on the oxidation results. Along with the growth of the oxide layer, the core of ANP still kept its annealed microstructure.

Transformation of engineered nanomaterials through the prism of silver sulfidation

Understanding the structure transformation of engineered nanomaterials (ENMs) is a grand measurement challenge, which impacts many aspects of ENMs applications, such as their efficacy, safety, and environmental consequence. To address the significant knowledge gap regarding the fundamental kinetic rate and extent of ENM transformation in the environment, we present a comprehensive and mechanistic structural investigation of the transformation, aggregation, and dissolution behavior of a polyvinylpyrrolidone-coated silver nanoparticle (AgNP) suspension upon sulfidation in moderately reduced hard water with fulvic acid and dissolved Na2S. This reaction is among the most prevalent and industrially and environmentally relevant ENMs transformation. Using ex situ transmission electron microscopy (TEM) and both in situ and ex situ synchrotron-based small angle X-ray scattering (SAXS) and X-ray diffraction (XRD), we find that sulfidation of faceted AgNPs strongly depends on the crystallographic orientation of the facets, with nanometer-scale passivation layers developed on {111} and {100} facets and continuous nucleation and growth on {110} facets. Nanobeam electron diffraction and atomic resolution imaging show Ag and Ag2S domains both possess a high degree of crystalline order, contradicting amorphous structures as previously reported. In situ SAXS/XRD allowed simultaneous determination of the morphological changes and extent of sulfidation of AgNPs. SAXS/XRD results strongly indicate sulfidation follows first-order reaction kinetics without any aggregation. Aided by their size monodispersity, for the first time, using direct, in situ morphology and atomic-structure probes whose results mutually corroborate, we unequivocally determined the sulfidation rate constant of AgNPs under an environmentally relevant condition (~0.013 min-1 for 68 nm diameter AgNPs). A rigorous analysis of the long-term sulfidation product of the AgNPs under different S/Ag ratios using ex situ SAXS/XRD clearly demonstrates that the silver mass in the original AgNP and transformed Ag/Ag2S NP is preserved. This result has important environmental implications, strongly suggesting that Ag+ ions, a known highly effective antimicrobial agent, are not leached into the solution during sulfidation of AgNPs. The combined nondestructive methodology can be extended to unfold the structure transformation pathway and kinetics in a broad range of ENM systems.

Comparison of Direct and Indirect Laser Ablation of Metallized Paper for Inexpensive Paper-Based Sensors

In this work, we present a systematic study of laser processing of metallized papers (MPs) as a simple and scalable alternative to conventional photolithography-based processes and printing technologies. Two laser-processing methods are examined in terms of selectivity for the removal of the conductive aluminum film (25 nm) of an MP substrate; these processes, namely direct and indirect laser ablation (DLA and ILA), operate at wavelengths of 1.06 μm (neodymium-doped yttrium aluminum garnet) and 10.6 μm (CO₂), respectively. The required threshold energy for each laser processing method was systematically measured using electrical, optical, and mechanical characterization techniques. The results of these investigations show that the removal of the metal coating using ILA is only achieved through partial etching of the paper substrate. The ILA process shows a narrow effective set of laser settings capable of removing the metal film while not completely burning through the paper substrate. By contrast, DLA shows a more defined and selective removal of the aluminum layer without damaging the mechanical and natural fibular structure of the paper substrate. Finally, as a proof of concept, interdigitated capacitive moisture sensors were fabricated by means of DLA and ILA onto the MP substrate, and their performance was assessed in the humidity range of 2-85%. The humidity sensitivity results show that the DLA sensors have a superior humidity sensing performance compared to the ILA sensors. The observed behavior is attributed to the higher water molecule absorption and induced capillary condensation within the intact cellulose network resulting from the DLA process (compared to the damaged one from the ILA process). The DLA process of MP should enable scalable production of low-cost, paper-based physical and chemical sensing systems for potential use in point-of-care diagnostics and food packaging.

Thermoplasmonic Ignition of Metal Nanoparticles

Explosives, propellants, and pyrotechnics are energetic materials that can store and quickly release tremendous amounts of chemical energy. Aluminum (Al) is a particularly important fuel in many applications because of its high energy density, which can be released in a highly exothermic oxidation process. The diffusive oxidation mechanism (DOM) and melt-dispersion mechanism (MDM) explain the ways powders of Al nanoparticles (NPs) can burn, but little is known about the possible use of plasmonic resonances in NPs to manipulate photoignition. This is complicated by the inhomogeneous nature of powders and very fast heating and burning rates. Here, we generate Al NPs with well-defined sizes, shapes, and spacings by electron beam lithography and demonstrate that their plasmonic resonances can be exploited to heat and ignite them with a laser. By combining simulations with thermal-emission, electron-, and optical-microscopy studies, we reveal how an improved control over NP ignition can be attained.

Size Resolved High Temperature Oxidation Kinetics of Nano-Sized Titanium and Zirconium Particles

While ultrafine metal particles offer the possibility of very high energy density fuels, there is considerable uncertainty in the mechanism by which metal nanoparticles burn, and few studies that have examined the size dependence to their kinetics at the nanoscale. In this work we quantify the size dependence to the burning rate of titanium and zirconium nanoparticles. Nanoparticles in the range of 20-150 nm were produced via pulsed laser ablation, and then in-flight size-selected using differential electrical mobility. The size-selected oxide free metal particles were directly injected into the post flame region of a laminar flame to create a high temperature (1700-2500 K) oxidizing environment. The reaction was monitored using high-speed videography by tracking the emission from individual nanoparticles. We find that sintering occurs prior to significant reaction, and that once sintering is accounted for, the rate of combustion follows a near nearly (diameter)(1) power-law dependence. Additionally, Arrhenius parameters for the combustion of these nanoparticles were evaluated by measuring the burn times at different ambient temperatures. The optical emission from combustion was also used to model the oxidation process, which we find can be reasonably described with a kinetically controlled shrinking core model.

A facile route for the synthesis of nanostructured oxides and hydroxides of cobalt using laser ablation synthesis in solution (LASIS)

We used a pulsed laser ablation synthesis in solution (LASIS) to produce cobalt oxide/hydroxide nanoparticles (NPs) with tailored size, morphology and structure at different laser fluences, wavelengths (532 and 1064 nm) and solvent conditions. Specifically, LASIS on bulk Co in the presence and absence of O2 in an aqueous solution initially produces cobalt monoxide (CoO) and single crystal β-cobalt hydroxide (β-Co(OH)2) nanoparticles (NPs) respectively that finally transform into cobaltosic oxide (Co3O4) through oxidation and/or thermal decomposition. Transmission electron microscopy (TEM) and scanning mobility particle sizer (SMPS) measurements on the final products reveal a bimodal size distribution of agglomerated NPs (for the 1064 and 532 nm laser) at low laser fluences, where the ablation mechanism is dominated by vaporization and normal boiling. In contrast, more efficient and predominant explosive boiling at higher laser fluences produces a mono-modal size distribution of spherically shaped primary NPs in agglomerates. Furthermore, higher absorbance of the 532 nm laser by solution-phase colloidal NPs re-ablates them into spherical shapes of larger size (∼13-22 nm) as compared to the ones from using 1064 nm LASIS (∼10-14 nm), while rendering 532 nm LASIS less productive than 1064 nm LASIS over an extended period of time. Finally, Co3O4 nanorods with enhanced localized surface plasmon resonance (LSPR) are synthesized at high pH (pH ≥ 13) and low laser fluence (<5 mJ cm(-2)) conditions. Such nanostructured materials are promising candidates as photocatalysts or additives in nanocomposite materials with enhanced light absorption properties.

Pushing the high-energy limit of plasmonics

The localized surface plasmon resonance of metal nanoparticles allows confining the eletromagnetic field in nanosized volumes, creating high-field "hot spots", most useful for enhanced nonlinear optical spectroscopies. The commonly employed metals, Au and Ag, yield plasmon resonances only spanning the visible/near-infrared range. Stretching upward, the useful energy range of plasmonics requires exploiting different materials. Deep-ultraviolet plasmon resonances happen to be achievable with one of the cheapest and most abundant materials available: aluminum indeed holds the promise of a broadly tunable plasmonic response, theoretically extending far into the deep-ultraviolet. Complex nanofabrication and the unavoidable Al oxidation have so far prevented the achievement of this ultimate high-energy response. A nanofabrication technique producing purely metallic Al nanoparticles has at last allowed to overcome these limits, pushing the plasmon resonance to 6.8 eV photon energy (≈180 nm) and thus significantly broadening the spectral range of plasmonics' numerous applications.

Mechanochemical mechanism for reaction of aluminium nano- and micrometre-scale particles

A recently suggested melt-dispersion mechanism (MDM) for fast reaction of aluminium (Al) nano- and a few micrometre-scale particles during fast heating is reviewed. Volume expansion of 6% during Al melting produces pressure of several GPa in a core and tensile hoop stresses of 10 GPa in an oxide shell. Such stresses cause dynamic fracture and spallation of the shell. After spallation, an unloading wave propagates to the centre of the particle and creates a tensile pressure of 3-8 GPa. Such a tensile pressure exceeds the cavitation strength of liquid Al and disperses the melt into small, bare clusters (fragments) that fly at a high velocity. Reaction of the clusters is not limited by diffusion through a pre-existing oxide shell. Some theoretical and experimental results related to the MDM are presented. Various theoretical predictions based on the MDM are in good qualitative and quantitative agreement with experiments, which resolves some basic puzzles in combustion of Al particles. Methods to control and improve reactivity of Al particles are formulated, which are exactly opposite to the current trends based on diffusion mechanism. Some of these suggestions have experimental confirmation.

Laser-induced plasma chemistry of the explosive RDX with various metallic nanoparticles

The feasibility of exploiting plasma chemistry to study the chemical reactions between metallic nanoparticles and molecular explosives such as cyclotrimethylenetrinitramine (RDX) has been demonstrated. This method, based on laser-induced breakdown spectroscopy, involves the production of nanoparticles in a laser-induced plasma and the simultaneous observation of time-resolved atomic and molecular emission characteristic of the species involved in the intermediate chemical reactions of the nanoenergetic material in the plasma. Using this method, it has been confirmed that the presence of aluminum promotes the ejection process of carbon from the intermediate products of RDX. The time evolution of species formation, the effects of laser pulse energy, and the effects of trace metal content on the chemical reactions were also studied.

Solution phase chemical synthesis of nano aluminium particles stabilized in poly(vinylpyrrolidone) and poly(methylmethacrylate) matrices

The reduction of aluminium trichloride by lithium aluminium hydride in the presence of poly(vinylpyrrolidone) or poly(methylmethacrylate) in mesitylene yielded nano aluminium particles in the matrices of respective polymers. Solution phase synthesis methodology was used successfully to produce composites of various Al/polymer ratios. The composites were characterized by powder XRD patterns and 27Al-NMR with MAS spectroscopic study. The method was useful to produce up to 10 g of nano aluminium that were pure and stable.

The effect of size and size distribution on the oxidation kinetics and plasmonics of nanoscale Ag particles

We employed a simple and effective electroless (EL) plating approach to produce silver nanoparticles (NPs) on bare silicon, on dielectric ZnO nanowires (NWs) and on Si NWs, respectively. The surface stability of the homogeneous Ag NPs formed on the ZnO NW surfaces was investigated by surface enhanced Raman spectroscopy (SERS), which show that the attachment of thiol to the Ag surface can slow down the oxidation process, and the SERS signal remains strong for more than ten days. To further examine the Ag NP oxidation process in air, the oxygen content in the silicon nanowire core/Ag sheath composites was monitored by the energy dispersive x-ray (EDX) method. The amount of oxygen in the system increases with time, indicating the silver NPs were continuously oxidized, and it is not clear if saturation is reached in this time period. To investigate the influence of the Ag NPs size distribution on the oxidation process, the oxygen amount in the NPs formed by EL deposition and e-beam (EB) evaporation on a bare silicon surface was compared. Results indicate a faster oxidation process in the EL formed Ag NPs than those produced by EB evaporation. We attribute this observation to the small diameter of the EL produced silver particles, which results in a higher surface energy.

Ab initio thermodynamics of lithium oxides: from bulk phases to nanoparticles

Lithium ion batteries are nowadays key devices for energy storage, and a great research effort is under way to develop and apply new materials. Recently, new approaches have been proposed that rely on the reversible formation of either Li2O or Li2O2 at the electrodes. The details of their formation and dissolution are, however, still unclear. As a first step towards the understanding of these processes, bulk lithium oxides, their surfaces and their nanoparticles have been here investigated by density functional theory and ab initio thermodynamics. At a pressure of 1 atmosphere of oxygen, Li2O2 is the stable bulk phase below 5 K, where a transition to Li2O takes place. Wulff's construction predicts an octahedral shape for Li2O nanoparticles and the form of a hexagonal prism for Li2O2. By taking into account the effect of the surfaces, a size-dependent phase diagram is calculated. At an oxygen pressure of 1 atmosphere and a temperature of 300 K, Li2O is the stable phase for particles with a diameter larger than approximately 2.5 nm. This size-dependent oxidation behavior of lithium should be taken into account in the design of nanostructured oxygen cathodes.

The algal toxicity of silver engineered nanoparticles and detoxification by exopolymeric substances

In this study, we report that silver ions (Ag(+)) from the oxidative dissolution of silver engineered nanoparticles (Ag-ENs) determined the EN toxicity to the marine diatom Thalassiosira weissflogii. Most of the Ag-ENs formed non-toxic aggregates (>0.22 microm) in seawater. When the free Ag(+) concentration ([Ag(+)](F)) was greatly reduced by diafiltration or thiol complexation, no toxicity was observed, even though the Ag-ENs were better dispersed in the presence of thiols with up to 1.08 x 10(-5) M Ag-ENs found in the <0.22 microm fraction, which are orders of magnitude higher than predicted for the natural aquatic environment. The secretion of polysaccharide-rich algal exopolymeric substances (EPS) significantly increased at increasing [Ag(+)](F). Both dissolved and particulate polysaccharide concentrations were higher for nutrient-limited cells, coinciding with their higher Ag(+) tolerance, suggesting that EPS may be involved in Ag(+) detoxification.

Measurements of hygroscopicity and volatility of atmospheric ultrafine particles during ultrafine particle formation events at urban, industrial, and coastal sites

The tandem differential mobility analyzer (TDMA) technique was applied to determine the hygroscopicity and volatility of atmospheric ultrafine particles in three sites of urban Gwangju, industrial Yeosu, and coastal Taean in South Korea. A database for the hygroscopicity and volatility of the known compositions and sizes of the laboratory-generated particles wasfirst constructed for comparison with the measured properties of atmospheric ultrafine particles. Distinct differences in hygroscopicity and volatility of atmospheric ultrafine particles werefound between a "photochemical event" and a "combustion event" as well as among different sites. At the Gwangju site, ultrafine particles in the "photochemical event" were determined to be more hygroscopic (growth factor (GF) = 1.05-1.33) than those in the "combustion event" (GF = 1.02-1.12), but their hygroscopicity was not as high as pure ammonium sulfate or sulfuric acid particles in the laboratory-generated database, suggesting they were internally mixed with less soluble species. Ultrafine particles in the "photochemical event" at the Yeosu site, having a variety of SO2, CO, and VOC emission sources, were more hygroscopic (GF = 1.34-1.60) and had a higher amount of volatile species (47-75%)than those observed at the Gwangju site. Ultrafine particle concentration at the Taean site increased during daylight hours with low tide, having a higher GF (1.34-1.80) than the Gwangju site and a lower amount of volatile species (17-34%) than the Yeosu site. Occasionally ultrafine particles were externally mixed according to their hygroscopicity and volatility, and TEM/EDS data showed that each type of particle had a distinct morphology and elemental composition.

Generation of Al nanoparticles via ablation of bulk Al in liquids with short laser pulses

Highly stable aluminum nanoparticles (NPs) are generated via ablation of bulk Al in ethanol using either femtosecond (fs) or picosecond (ps) laser sources. The colloidal NPs solutions obtained with fs pulses exhibit a yellow coloration and show an increased optical absorption between 300 and 400 nm, tentatively assigned to the plasmon resonance of nanosized Al. The corresponding solutions after ps ablation are gray colored and opalescent. The average size of the NPs formed ranges from 20 nm for the fs case to 60 nm for the ps case, while a narrower distribution is obtained using the shorter pulses. High Resolution Transmission Electron Microscopy (HRTEM) studies indicate that the NPs are mostly amorphous with single crystalline inclusions. Al NPs generated with short laser pulses slowly react with air oxygen due to the presence of a native oxide cladding, which efficiently passivates their surface against further oxidation.

Sampling nanoparticles for chemical analysis by low resolution electrical mobility classification

The use of electrostatic classification to collect samples of aerosol nanoparticles for chemical analysis is discussed. Our technique exposes the aerosol to negative ions in a unipolar charger with subsequent mobility classification at low resolution and high sampling rate. The negative unipolar charger produces high charged fractions. The low-resolution mobility classifier enables the delivery of high mass concentrations in a well-defined mobility range. The mobility-classified particles are collected by electrostatic precipitation. We summarize experimental and computational work on the performance of the unipolar charger, and we describe the performance of the overall system when used to sample atmospheric particles. For a size distribution measured in Atlanta during a new particle formation event, calculated mass sampling rates of approximately 8 nm particles were about 150 pg/h.

Laser writing of nanostructures on bulk Al via its ablation in liquids

Experimental results are presented on the formation of self-organized nanostructures (NSs) on a bulk Al target under its ablation in liquids--water and ethanol--with short laser pulses from 180 femtoseconds (fs) through 350 picoseconds (ps). NSs are characterized by atomic force microscopy, field emission scanning electron microscopy, optical absorption spectroscopy and x-ray diffraction. The period of NSs does not depend on the laser wavelength used from 248 through 800 nm and is approximately 200 nm. NSs on Al show the characteristic absorption peak in the near UV which has been attributed to plasmon oscillation of electrons. The wings of this peak, extending to the visible, lead to a distinct yellow coloration of the processed Al surface. Ultrafast laser structuring of bulk aluminum in liquids may be potentially a promising technique for efficient production of nanosized aluminum.

T-Jump/time-of-flight mass spectrometry for time-resolved analysis of energetic materials

We describe a new T-Jump/time-of-flight (TOF) mass spectrometer for the time-resolved analysis of rapid pyrolysis chemistry of solids and liquids, with a focus on energetic materials. The instrument employs a thin wire substrate which can be coated with the material of interest, and can be rapidly heated (10(5) K/s). The T-Jump probe is inserted within the extraction region of a linear TOF mass spectrometer, which enables multiple spectra to be obtained during a single reaction event. By monitoring the electrical characteristics of the heated wire, the temperature could also be obtained and correlated to the mass spectra. As examples, we present time-resolved spectra for the ignition of nitrocellulose and RDX. The fidelity of the instrument is demonstrated in the spectra presented which show the temporal formation and decay of several species in both systems. The simultaneous measurement of temperature enables us to extract the ignition temperature and the characteristic reaction time. The time-resolved mass spectra obtained show that these solid energetic material reactions, under a rapid heating rate, can occur on a time scale of milliseconds or less. While the data sampling rate of 10,000 Hz was used in the present experiments, the instrument is capable of a maximum scanning rate of up to approximately 30 kHz. The capability of high-speed time-resolved measurements offers an additional analytical tool for the characterization of the decomposition, ignition, and combustion of energetic materials.

Graphene-stabilized copper nanoparticles as an air-stable substitute for silver and gold in low-cost ink-jet printable electronics

Metallic copper nanoparticles were synthesized by a bottom-up approach, and in situ coated with protective shells of graphene in order to get a metal nanopowder of high air stability and chemical inertness. Using an amphiphilic surfactant, a water-based copper nanocolloid could be prepared and successfully printed onto a polymer substrate by conventional ink-jet printing using household printers. The dried printed patterns exhibited strong metallic gloss and an electrical conductivity of >1 S cm(-1) without the need for a sintering or densification step. This conductivity currently limits use in electronics to low current application or shielding and decorative effects. The high stability of graphene-coated copper nanoparticles makes them economically a most attractive alternative to silver or gold nanocolloids, and will strongly facilitate the industrial use of metal nanocolloids in consumer goods.