Hygroscopic behavior of individual submicrometer particles studied by X-ray spectromicroscopy

A novel application of single particle scanning transmission X-ray microscopy (STXM) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy is presented for quantitative analysis of hygroscopic properties and phase transitions of individual submicrometer particles. The approach utilizes the exposure of substrate-deposited individual particles to water vapor at different relative humidity followed by STXM/NEXAFS spectromicroscopy analysis. The hygroscopic properties of atmospherically relevant NaCl, NaBr, NaI, and NaNO(3) submicrometer particles were measured to evaluate the utility of the approach. An analytical approach for quantification of a water-to-solute ratio within an individual submicrometer particle during hydration and dehydration cycles is presented. The results for the deliquescence and efflorescence phase transitions and quantitative measurements of water-to-solute ratios are found in excellent agreement with available literature data. Oxygen K-edge NEXAFS spectra of submicrometer sodium halide droplets are reported along with a unique experimental observation of the formation of the halide-water anionic complex in NaBr and NaI microdimensional droplets. The analytical approach provides a unique opportunity for spectromicroscopy studies of water uptake on environmental particles collected in both laboratory and field studies.     

Attenuated total reflection fourier transform infrared spectroscopy to investigate water uptake and phase transitions in atmospherically relevant particles

In this study, attenuated total reflection Fourier transform infrared (ATR-FT-IR) spectroscopy is used to investigate water uptake and phase transitions for atmospherically relevant particles. Changes in the ATR-FT-IR spectra of NaCl, NH(4) NO(3), (NH(4))(2)SO(4), Ca(NO(3))(2), and SiO(2) as a function of relative humidity (RH) are presented and discussed. For these various particles, water can (1) become adsorbed on the particle surface; and/or (2) become absorbed in the particle structure to form a hydrate salt; and/or (3) become absorbed by the particle to form a liquid solution. Spectral features and analyses that distinguish these various processes are discussed. For the salts that do undergo a solid to liquid phase transition (deliquescence), excellent agreement is found between the measurements made here with ATR-FT-IR spectroscopy, a relatively simple, inexpensive, and readily available analytical tool, compared to more expensive, elaborate aerosol flow reactor systems using tandem differential mobility analyzers. In addition, for particles that adsorb water, we show here the utility of coupling ATR-FT-IR measurements with simultaneous quartz crystal microbalance (QCM) measurements. This coupling allows for the quantification of the amount of water associated with the particle as a function of relative humidity (f(RH)) along with the spectroscopic data.     

Evaporation of water from particles in the aerodynamic lens inlet: an experimental study

The extremely high particle transmission efficiency of aerodynamic lens inlets resulted in their wide use in aerosol mass spectrometers. One of the consequences of transporting particles from high ambient pressure into the vacuum is that it is accompanied by a rapid drop in relative humidity (RH). Since many atmospheric particles exist in the form of hygroscopic water droplets, a drop in RH may result in a significant loss of water and even a change in phase. How much water is lost in these inlets is presently unknown. Since water loss can affect particle size, transmission efficiency, ionization probability, and mass spectrum, it is imperative to provide definitive experimental data that can serve to guide the field to a reasonable and uniform sampling approach. In this study, we present the results of a number of highly resolved measurements, conducted under well-defined conditions, of water evaporation from a range of particles, during their transport through an aerodynamic lens inlet. We conclude that the only sure way to avoid ambiguities during measurements of aerodynamic diameter in instruments that utilize low-pressure aerodynamic lens inlets is to dry the particles prior to sampling.     

Temperature-controlled confocal Raman microscopy to detect phase transitions in phospholipid vesicles

Optical-trapping confocal Raman microscopy enhances the capabilities of traditional Raman spectroscopy for the analysis of small particles by significantly reducing the sampling volume and minimizing background signal from the particle surroundings. Chemical composition and structural information can be obtained from optically trapped particles in aqueous solution without the need for labeling or extensive sample preparation. In this work, the challenges of measuring temperature dependent changes in suspended particles are addressed with the development of a small-volume, thermally conductive sample cell attached to a temperature-controlled microscope stage. To demonstrate its function, the gel to liquid-crystalline phase transitions of optically trapped lipid vesicles, composed of pure 1,2-ditridecanoyl-sn-glycero-3-phosphocholine (DTPC), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), were detected by changes in Raman spectra of the lipid bilayer. The Raman scattering data were found to correlate well with differential scanning calorimetry (DSC) results.     

Ion etching methods for depth profiling of complex three-dimensional samples in combination with scanning Auger electron microscopy

Ion etching of surfaces combined with detection of secondary events (particles or radiation emitted) are used for depth profiling of samples with interesting features at-, near-, or somewhat below the surface. These methods are destructive and relatively slow, and compete with non-destructive methods like Rutherford backscattering spectroscopy, energy-dispersive X-ray spectroscopy in the scanning electron microscope or angle-resolved photoemission spectroscopy, which are non-destructive and relatively faster methods. In this work we have concentrated on the use of noble gas ion sputtering with low-energy beams in combination with electron excited Auger electron spectroscopy and imaging for analysis of nanostructured and microstructured samples. No attempt will be made here to justify this method over the other methods, as their relative merits depend on the nature of the sample and the problem at hand. We have thus chosen to study samples and problems for which this technique would be obvious to use. This work is also aimed at providing practical standards and guidelines (“metrology”) for the use of the technique in the context of industrial nanotechnology. The use of Auger electron spectroscopy instead of photoemission spectroscopy is preferred for laterally non-uniform samples due to the presently better resolution capabilities of electron beams and narrower information depths of typical Auger electron transitions. The use of Auger electrons for concentration sampling, and low-energy beams of noble gas ions for sputtering, reduces the adverse influence of atomic mixing. In this report two systems are intensively studied with sputter profiling in combination with Auger electron spectroscopy and scanning electron imaging: a hard disk and a surface of a stainless-steel sample.     

Ettringite and calcium sulfoaluminate cement: investigation of water content by near-infrared spectroscopy

Calcium sulfoaluminate (CSA) cement is a sulfate-based binder whose high-performance hydraulic behavior depends on the rapid formation of ettringite, when grinded clinker is hydrated in presence of gypsum. Ettringite is a calcium aluminum sulfate mineral characterized by high water content, estimated as 32 water molecules per formula unit. Three examples of utilization of near-infrared (NIR) spectroscopy are here shown. First of all, information on water distribution in pure ettringite was deduced and compared with infrared analyses. Then its thermal behavior has been followed up to 400 °C, allowing to improve the knowledge about water loss and thermal decomposition of this hydrated phase. Finally, the obtained results have been employed in order to follow hydration of CSA cement sample, demonstrating thus that NIR spectroscopy, being highly sensitive to water amount and distribution, can be an extremely useful tool for hydration studies.     

Spatially correlated fluorescence/AFM of individual nanosized particles and biomolecules

Individual fluorescent polystyrene nanospheres (<10-100-nm diameter) and individual fluorescently labeled DNA molecules were dispersed on mica and analyzed using time-resolved fluorescence spectroscopy and atomic force microscopy (AFM). Spatial correlation of the fluorescence and AFM measurements was accomplished by (1) positioning a single fluorescent particle into the near diffraction-limited confocal excitation region of the optical microscope, (2) recording the time-resolved fluorescence emission, and (3) measuring the intensity of the excitation laser light scattered from the apex of an AFM probe tip and the AFM topography as a function of the lateral position of the tip relative to the sample substrate. The latter measurements resulted in concurrent high-resolution (approximately 10-20 nm laterally) images of the laser excitation profile of the confocal microscope and the topography of the sample. Superposition of these optical and topographical images enabled unambiguous identification of the sample topography residing within the excitation region of the optical microscope, facilitating the identification and structural characterization of the nanoparticle(s) or biomolecule(s) responsible for the fluorescence signal observed in step 2. These measurements also provided the lateral position of the particles relative to the laser excitation profile and the surrounding topography with nanometer-scale precision and the relationship between the spectroscopic and structural properties of the particles. Extension of these methods to the study of other types of nanostructured materials is discussed.     

Hydration phase diagram for sodium cobalt oxide Na0.3CoO2·yH2O

The hydration phase diagram for sodium cobalt oxhydrate, Na_0.3CoO₂·yH₂O (y = 0, 0.6 and 1.3), was determined as a function of relative humidity at 298 K It is found that greater than 75% relative humidity is needed for complete hydration of anhydrous Na_0.3CoO₂ to the superconducting phase Na_0.3CoO₂·1.3H₂O. Dehydration studies show that a minimum of 43% relative humidity is needed to maintain the stability of the fully hydrated superconducting phase. The intermediate hydrate, Na_0.3CoO₂·0.6H₂O, is stable between 10 and 50% relative humidity on hydration, and 35 and 0% relative humidity on dehydration.     

Determination of mean diameter and particle size distribution of acrylate latex using flow field-flow fractionation, photon correlation spectroscopy, and electron microscopy

Flow field-flow fractionation (flow FFF) was employed to determine the mean diameter and the size distribution of acrylate latex materials having diameters ranging from 0.05 to 1 μm. Mean diameters of the samples determined by flow FFF are in good agreement with those obtained from photon correlation spectroscopy (PCS). Scanning electron microscopy (SEM) yielded a mean diameter that is about 20% lower than those obtained from flow FFF or PCS, probably due to the shrinkage of particles during sample drying and high-vacuum measurements. It was found that flow FFF is particularly useful for the determination of particle size distributions of latex materials having broad size distributions. Flow FFF separates particles according to their sizes and yields an elution curve that directly represents the particle size distribution of the sample. In PCS, measurements had to be repeated at more than one scattering angle to obtain an accurate mean diameter for the latex having a broad size distribution. Flow FFF was fast (less than 12 min of run time) and showed an excellent repeatability in measuring the mean diameter with ±5% relative error.     

Fluidized Bed Medium Separation (FBMS) using the particles with different hydrophilic and hydrophobic properties

Three kinds of particles with different hydrophilic and hydrophobic properties were used as fluidized particles of Fluidized Bed Medium Separation (FBMS). A minimum fluidization velocity, an apparent specific gravity of fluidized bed and floating-sinking behaviors of dry and wet coals were measured in the range of relative humidity from 50% to 80%. In a hydrophilic particle, the fluidization became unstable with increasing relative humidity because particle aggregation took place at a high humidity, and hence floating-sinking behaviors depend on changes in a relative humidity. On the other hand, in a highly hydrophobic particle, the fluidization was stable and floating-sinking behaviors based on the specific gravity difference were obtained even for wet coals and at a high relative humidity. Therefore, the FBMS using a highly hydrophobic particle is applicable at a high relative humidity without a control device of relative humidity.     

Simultaneous measurements of individual ambient particle size, composition, effective density, and hygroscopicity

The interaction between atmospheric particles and water vapor impacts directly and significantly the effect that these particles exert on the atmosphere. The hygroscopicity of individual particles, which is a quantitative measure of their response to changes in relative humidity, is related to their internal compositions. To properly include atmospheric aerosols in any model requires knowledge of the relationship between particle size, composition, and hygroscopicity. Here we demonstrate the capability to conduct in real time the simultaneous measurements of individual ambient particle hygroscopic growth factors, densities, and compositions using a humidified tandem differential mobility analyzer that is coupled to an ultrasensitive single-particle mass spectrometer. We use as an example the class of particles that are composed of sulfate mixed with oxygenated organics to illustrate how multidimensional single-particle characterization can be extended to yield in addition quantitative information about the composition of individual particles. We show that the data provide the relative concentrations of organics and sulfates, the density of the two fractions, and particle hygroscopicity.     

Synthesis of carbon films containing diamond particles by electrolysis of methanol

Carbon films containing diamond particles were deposited onto a Si (100) substrate by electrolysis of methanol under a direct current potential of 1200 V, with a current density of about 52 mA/cm², at atmospheric pressure and in the temperature range of 50-55 °C. The surface morphology, microstructure and crystalline structure of the deposited films were characterized by scanning electron microscopy (SEM), Fourier transformation infrared (FTIR) spectroscopy, Raman spectroscopy and transmission electron microscopy (TEM) respectively. The SEM images show that the films are formed by particle clusters and a surrounding glassy phase. The Raman spectra of the films indicate that the particle clusters are composed of diamond and that the glassy phase is composed of amorphous carbon. The FTIR measurements suggest the existence of hydrogen which is mainly bonded to the sp³ carbon in the films. The transmission electron diffraction patterns further indicate that the particles in the films consist of single-crystalline diamond. Both TEM and Raman measurements have confirmed unambiguously the formation of diamond crystals in the deposit, although the particles are not uniformly distributed on the entire surface.     

Spectroscopic measurement of the vapour pressure of ice

We present a laser absorption technique to measure the saturation vapour pressure of hexagonal ice. This method is referenced to the triple-point state of water and uses frequency-stabilized cavity ring-down spectroscopy to probe four rotation-vibration transitions of at wavenumbers near 7180?cm(-1). Laser measurements are made at the output of a temperature-regulated standard humidity generator, which contains ice. The dynamic range of the technique is extended by measuring the relative intensities of three weak/strong transition pairs at fixed ice temperature and humidity concentration. Our results agree with a widely used thermodynamically derived ice vapour pressure correlation over the temperature range 0°C to -70°C to within 0.35 per cent.     

Development of an intelligent polymerized crystalline colloidal array colorimetric reagent.

We have developed a novel colorimetric reagent for the determination of Pb2+, pH, and temperature. This colorimetric reagent consists of a dispersion of approximately 100-microm particles composed of an intelligent polymerized crystalline colloidal array (IPCCA). The IPCCA particles are composed of a hydrogel polymerized around a face-centered cubic (fcc) array of monodisperse, highly charged polystyrene colloidal particles. These IPCCA particles diffract visible light because the (111) planes of the fcc polystyrene colloidal particle array have an approximately 200-nm lattice constant. The IPCCA particles also contain a molecular recognition agent that actuates array volume changes as a result of changes in analyte concentration or temperature. This results in changes in the IPCCA lattice constants, which shifts the wavelength of light diffracted. We report here the use of these sensing materials in a liquid dispersion that can be poured into a sample solution. This diffraction measurement method is analogous to X-ray powder diffraction measurements. The diffraction wavelength is monitored at a defined angle relative to the incident light.     

Synthesis of Mn-doped CeO<Subscript>2</Subscript> nanorods and their application as humidity sensors

Mn-doped CeO₂ nanorods have been prepared from CeO₂ particles through a facile composite-hydroxide-mediated (CHM) approach. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The analysis from the X-ray photoelectron spectroscopy indicates that the manganese doped in CeO₂ exists as Mn^4 +. The responses to humidity for static and dynamic testing proved doping Mn into CeO₂ can improve the humidity sensitivity. For the sample with Mn% about 1·22, the resistance changes from 375·3 to 2·7MΩ as the relative humidity (RH) increases from 25 to 90%, indicating promising applications of the Mn-doped CeO₂ nanorods in environmental monitoring.     

Mssbauer Effect in Ultrafine Particles with Fe-C Solid Solution, γ-Fe and Fe_3C Phases

Ultrafine particles prepared by evaporating pure Fe in CH4 atmosphere using arc-dischargeheating method, were found to consist of Fe-C solid solution, γ-Fe and Fe3C phases. EfFect of annealing temperature on phase transformation and hyperfine interactions has been investigated by Mossbauer spectroscopy, X-ray diffraction (XRD), differential thermal analysis and thermogravimetry (DTA-TG), transmission electron microscopy (TEM), oxygen determination and vibrating sample magnetometer (VSM) measurements. It was observed that phase transformation of γ-Fe to α-Fe occurs during annealing in vacuum. The mechanism causing the change of hyperfine interactions with annealing temperature differs for Fe-C solution and interstitial compounds. DifFerence of hyperfine interactions of Fe-C solid solution in the starting sample and its annealed samples is ascribed to the improvement of activation of interstitial carbon atoms. Stress-relieving in structure of annealed Fe3C particle can result in a weak influence on hyperfine interactions. Parameters fitted to the Mossbauer spectra show the existence of superparamagnetism in all the samples. Absorbed and combined oxygen on particle surface of the starting sample were determined.     

Simulation of the hydration kinetics and elastic moduli of cement mortars by microstructural modelling

The ability of the VCCTL microstructural model to predict the hydration kinetics and elastic moduli of cement materials was tested by coupling a series of computer simulations and laboratory experiments, using different cements. The novel aspects of this study included the fact that the simulated hydration kinetics were benchmarked using real-time measurements of the early-age phase composition during hydration by in situ X-ray diffraction. Elastic moduli are measured both by strain gauges (static approach) and by P-wave propagation (dynamic approach). Compressive strengths were measured by loading mortar prisms until rupture. Virtual samples were generated by VCCTL, using particle size distribution and phase composition as input. The hydration kinetics and elastic moduli were simulated and the numerical results were compared with the experimental observations. The compressive strength of the virtual mortars were obtained from the elastic moduli, using a power-law relation. Experimentally measured and simulated time-dependence of the major cement clinker phases and hydration product phases typically agreed to within 5%. Also, refinement of the input values of the intrinsic elastic moduli of the various phases enabled predictions of effective moduli, at different ages and different water-to-cement mass ratios, that are within the 10% uncertainty in the measured values. These results suggest that the VCCTL model can be successfully used as a predictive tool, which can reproduce the early age hydration kinetics, elastic moduli and mechanical strength of cement-based materials, using different mix designs.     

TiO2 hydrosols with high activity for photocatalytic degradation of formaldehyde in a gaseous phase

Two types of TiO2 hydrosols (TOSO and HTO) were prepared from titanium sulfate (TiOSO4) and metatitanic acid (H2TiO3) by a chemical precipitation-peptization method, respectively. The prepared hydrosols were characterized by means of X-ray diffraction, particle size distribution, scanning electron microscopy, UV-vis spectroscopy, Fourier transform infrared spectroscopy, Brunauer-Emmett-Teller and Barret-Joyner-Halender methods. The results showed that the TiO2 hydrosols with an anatase crystal structure had smaller particle sizes, higher surface areas, larger pore volume, and higher transparence than Degussa P-25 suspension. The photocatalytic activity of the TiO2 hydrosols was evaluated for formaldehyde degradation under UVA illumination in a gaseous phase. The results demonstrated that the photocatalytic activity with the catalyst loading of 2mgcm(-2) was ranked as an order of HTO>TOSO>P-25. The photocatalytic activity was further studied using the HTO catalyst under different experimental conditions. The results showed that catalyst loading, relative humidity, and initial concentration could influence the efficiency of HCHO photocatalytic degradation. It was found that a catalyst loading of more than 2mgcm(-2) and a relative humidity of 55% were two essential conditions for achieving the best performance under these experimental conditions. The repeated experiments indicated that the HTO catalyst was reasonably stable and could be repeatedly used for the HCHO oxidation under UVA irradiation. This investigation would be helpful to promote the application of TiO2 photocatalytic technique for indoor air purification.     

Mechanistic interpretation of the slow bronchial clearance phase

A slow bronchial clearance phase, describing the removal of insoluble particles from the sol layer, has recently been incorporated into the ICRP human respiratory tract model. A striking feature of that clearance phase is the size dependence of the slowly cleared fraction. In the theoretical approach presented here, it is suggested that the slow bronchial clearance phase includes three main mechanisms: (1) re-transfer of particles onto the gel layer after a certain time delay, (2) intra- or intercellular transfer of particles through the airway epithelium and subsequent transport to the blood or lymph nodes, and (3) uptake of particles by airway macrophages. The experimentally observed inverse relationship between the slow clearance fraction and geometric particle size is interpreted as an increasing capture probability in the sol layer and transepithelial transport with decreasing particle diameter.     

Quantitative Evaluation of SERS-Active Ag Film Nanostructure by Atomic Force Microscopy

The reliability of an image analysis algorithm for atomic force microscopy (AFM) of thin metal films was evaluated by comparison with manual analysis of images and transmission electron micrographs of Ag films deposited on Formvar-coated Cu grids. In order to extract quantitative nanostructural information using the algorithm discussed herein, the optimal fitting parameters were found to be low-pass filtering to reject high-frequency noise, a 5 × 5 point grid for identification of particle maxima, and a linear least-squares fit to a hemispheroidal model of particle shape. Metal particle dimensions were defined from the height and radius of the hemispheroid fit. Due to the close spacing of particles in these Ag films, tip geometry causes the greatest error in the height measurements, rather than width measurements. In addition, the effect of scanning parameters such as scan rate and size, applied load, and humidity on particle count and dimensions was examined. Increasing the scan rate reduced the number of resolvable Ag particles, decreased the apparent particle height, and increased the apparent particle radius. Under conditions of low capillary force, a net repulsive force of ～19 nN resulted in subtle tip-induced changes in the Ag surface morphology. The Ag film surface was damaged at a net repulsive force of ～23 nN. At slow scan rates, the moisture layer did not significantly affect the quality of the AFM images obtained over a broad relative humidity range. Finally, the Ag surface structure was found to be very homogeneous over a relatively large area.