THz radiation generation via the interaction of two‐colour ultra‐short laser pulses with SO2 and NH3 gases

In this work, using a two‐dimensional particle‐in‐cell Monte Carlo collision computation method, terahertz (THz) radiation generation via the interaction of two‐colour, ultra‐short, high‐power laser pulses with the polyatomic molecular gases sulphur dioxide (SO₂) and ammonia (NH₃) is examined. The influence of SO₂ and NH₃ pressures and two‐colour laser pulse parameters, i.e., pulse shape, pulse duration, and beam waist, on the THz radiation generation is studied. It is shown that the THz signal generation from SO₂ and NH₃ increases with the background gas pressure. It is seen that the THz emission intensity for both gases at higher laser pulse durations is higher. Moreover, for these polyatomic gases, the plasma current density increases with increase in the laser pulse beam waist. A more powerful THz radiation intensity with a larger time to peak of the plasma current density is observed for SO₂ compared to NH₃. In addition, many THz signals with small intensities are observed for both polyatomic gases. It is seen that for both SO₂ and NH₃ the generated THz spectral intensity is higher at higher gas pressures.     

Controlled Formation of Patchy Anisometric Fumed Silica Supraparticles in Droplets on Bent Superhydrophobic Surfaces

This article presents a study on the formation of anisometric, ellipsoidal supraparticles by evaporation‐induced self‐assembly from multicomponent colloidal dispersion droplets deposited on a superhydrophobic surface. Performing the formation process on bent surface substrates grants precise control on the shape and spatial orientation of the final dried supraparticles. Due to the V‐shaped surfaces providing interfacial blockage, anisotropic evaporation rates occur with respect to the direction of the bending channel. This proportionally leads to inhomogeneous accumulation of fumed silica (FS), used as structure guiding component. Thus, upon the increase of FS‐particle interaction via ionic strength (NaCl), this so‐formed shell provides enough anisotropic stiffness resulting in predictable droplet deformation with the elongation orientation being perpendicular to the bending axis. The anisotropic evaporation rates were monitored and quantified using an established, empiric kinetic model and taking into account surface geometry. Employing this reliable control of elongation direction and using additional Fe₃O₄@SiO₂ core–shell nanoparticles, anisometric magnetic Janus supraparticles with defined patch position were prepared, which are not accessible on flat surfaces. The results can find application in the controlled, easy to scale up, nanofabrication process of patchy anisometric supraparticles.     

Oxide Materials for Development of Integrated Gas Sensors—A Comprehensive Review

In the recent past a great deal of research efforts were directed toward the development of miniaturized gas-sensing devices, particularly for toxic gas detection and for pollution monitoring. Though various techniques are available for gas detection, solid state metal oxides offer a wide spectrum of materials and their sensitivities for different gaseous species, making it a better choice over other options. In this article a critical parameter analysis of different metal oxides that are known to be sensitive to various gaseous species are thoroughly examined. This includes phase of the oxide, sensing gaseous species, operating temperature range, and physical form of the material for the development of integrated gas sensors. The oxides that are covered in this study include oxides of aluminum, bismuth, cadmium, cerium, chromium, cobalt, copper, gallium, indium, iron, manganese, molybdenum, nickel, niobium, ruthenium, tantalum, tin, titanium, tungsten, vanadium, zinc, zirconium, and the mixed or multi-component metal oxides. They cover gases such as CO, CO₂, CH₄, C₂H₅OH, C₃H₈, H₂, H₂S, NH₃, NO, NO₂, O₂, O₃, SO₂, acetone, dimethylamine (DMA), humidity, liquid petroleum gas (LPG), petrol, trimethylamine (TMA), smoke, and many others. Both doped and undoped oxides are analyzed for the compatibility with silicon processing conditions and hybrid microcircuit fabrication techniques. In silicon processing conditions, they are further analyzed for the suitability for simple silicon surfaces, silicon-on-insulator (SOI) surfaces, and micromachined silicon geometries for different operating temperatures. Discussion on gas-sensing properties of each material and its applications are described in the text in alphabetical order of the elemental oxides. Further, the gas-sensing properties like sensitivity, detection limits, operating temperature, and so on are summarized in tables al ong with relevant references. The figures incorporated in the present review are primarily based on discussions and data in tables. However, these figures provide a qualitative comparison and present a pictorial view to examine suitability of a material for a particular application. From the known parameters, the present study clearly indicates the suitability of certain materials and the gases that they cover for the development of integrated micro gas sensors. A clear picture has been brought out for the development of silicon-based processing technology. Various parameters are discussed for the selection of these materials, to examine their suitability and practical problems that are being associated. Etching of these metal oxides and the reliability of devices are also discussed.     

Preparation of Reinforced Anisometric Patchy Supraparticles for Self-Propulsion

The preparation of fumed silica-based anisometric supraparticles with well-defined catalytically active patches suitable for self-propulsion is presented here. These sub-millimeter-sized particles can self-propel as they contain Pt-covered magnetite (Fe₃O₄) nanoparticles, where the Pt can decompose catalytically a “fuel” like H₂O₂ and thereby propel the supraparticles. By their magnetic properties, the catalytically active nanoparticles can be concentrated in patches on the supraparticle surface. The goal is to obtain robust supraparticles with well-defined patchiness and long-time stability during self-propulsion through evaporation-induced self-assembly (EISA) on a superhydrophobic surface. The latter is a major issue as oxygen evolution can lead to the disintegration of the supraparticles. Therefore, enhanced mechanical stability is sought using a number of different additives, where the best results are obtained by incorporating polystyrene microspheres followed by heat treatment or reinforcement with microfibrillated cellulose (MFC) and sodium trisilicate (Na₂SiO₃). The detailed internal structure of the different types of particles is investigated by confocal micro-X-ray fluorescence spectroscopy (CMXRF), which allows for precisely locating the catalytic Fe₃O₄@Pt nanoparticles within the supraparticles with a resolution in the µm range. The insights on the supraparticle structure, together with their long-time stability, allow fabricating optimized patchy supraparticles for potential applications in propulsion-enhanced catalysis.     

Simulation of operation of multiwave remote gas-analyzer based on NH3-laser

Numerical analysis of a multiwave path gas-analyzer, based on a NH₃-laser pumped by CO₂-laser radiation, is performed for model detection of concentrations of a series of molecular species such as NH₃, HCN, phosgene, NHO₃, CO₂, and H₂O. The potentialities of the gas analyzer and uncertainty of the gas concentration detection were estimated for a 4 km horizontal atmospheric path. The estimation took into account the absorption of laser radiation by the atmospheric aerosol and molecular gases under study and distortion of the laser beam due to atmospheric turbulence.     

Capture of pure toxic gases through porous materials from molecular simulations

ABSTRACT

In the last three decades, the air pollution is the main problem to affect human health and the environment in China and its contaminants include SO_2, NH_3, H₂S, NO₂, NO and CO. In this work, we employed grand canonical Monte Carlo simulations to investigate the adsorption capability of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) for these toxic gases. Eighty-nine MOFs and COFs were studied, and top-10 adsorption materials were screened for each toxic gas at room temperature. Dependence of the adsorption performance on the geometry and constructed element of MOFs/COFs was determined and the adsorption conditions were optimised. The open metal sites have mainly influenced the adsorption of NH₃, H₂S, NO₂ and NO. Especially, the X-DOBDC and XMOF-74 (X = Mg, Co, Ni, Zn) series of materials containing open metal sites are all best performance for adsorption of NH₃ to illustrate the importance of electrostatic interaction. Our simulation results also showed that ZnBDC and IRMOF-13 are good candidates to capture the toxic gases NH_3, H₂S, NO₂, NO and CO. This work provides important insights in screening MOF and COF materials with satisfactory performance for toxic gas removal.     

Thiol-Functionalized Palladium Nanoparticles Networks: Synthesis,Characterization, and Room Temperature (Toxic) Vapor Detection

The preparation of three different functionalized palladium nanoparticles (PdNPs) systems for room temperature BTX (benzene, toluene, p-xylene) sensing detection and their morphostructural characterization is described. PdNPs are prepared through a two-phase water/toluene wet chemical reduction method in the presence of bifunctional organic thiols as stabilizing agents suitable for the formation of covalently linked PdNPs networks: p-terphenyl-4,4″-dithiol (PdNPs-TR), biphenyl-4,4′-dithiol (PdNPs-BP), or with 9,9-didodecyl-2,7-bis(acetylthio)fluorene (PdNPs-FL). Comparing the hydrodynamic diameter values, TR and BP ligands help to obtain networks consisting of spherical NPs of about 2 nm, in which each bifunctional ligand act as a bridge between PdNPs. In contrast, PdNPs-FL show a population centered at <2R_H> = 45 ± 5 nm. To perform preliminary gas sensing measurements, PdNPs networks are cast deposited on interdigitated electrodes to study their resistive response toward volatile organic compounds (VOCs) such as benzene (0–5%), toluene (0–1.7%), and p-xylene (0–0.4%) (BTX) and common interfering gases (H₂S, NH₃, SO₂, and relative humidity, RH). PdNPs-FL show enhanced response to BTX with an appreciable response also toward H₂S and RH. PdNPs-TR exhibit a better ability to discriminate benzene gas with a negligible response after H₂S exposure. Moreover, all the PdNPs systems show little to no response to NH₃ and SO₂ gases, offering an interesting perspective in practical sensing applications.     

Effect of drying technique on the physicochemical properties of sodium silicate-based mesoporous precipitated silica

The conventional drying (oven drying) method used for the preparation of precipitated mesoporous silica with low surface area (>300 m²/g) and small pore volume is often associated with a high production cost and a time consuming process. Therefore, the main goal of this study was to develop a cost-effective and fast drying process for the production of precipitated mesoporous silica using inexpensive industrial grade sodium silicate and spray drying of the precipitated wet-gel silica slurry. The precipitated wet-gel silica slurry was prepared from an aqueous sodium silicate solution through the drop-wise addition of sulfuric acid. Mesoporous precipitated silica powder was prepared by drying the wet-gel slurry with different drying techniques. The effects of the oven drying (OD), microwave drying (MD), and spray drying (SD) techniques on the physical (oil, water absorption, and tapping density), and textural properties (specific BET surface area, pore volume, pore size, and % porosity) of the precipitated mesoporous silica powder were studied. The dried precipitated mesoporous silica powders were characterized with field-emission scanning electron microscopy; Brunauer, Emmett and Teller and BJH nitrogen gas adsorption/desorption methods; Fourier-transform infrared spectroscopy; thermogravimetric and differential analysis; N₂ physisorption isotherm; pore size distribution and particle size analysis. There was a significant effect of drying technique on the textural properties, such as specific surface area, pore size distribution and cumulative pore volume of the mesoporous silica powder. Additionally, the effect of the microwave-drying period on the physicochemical properties of the precipitated mesoporous silica powder was investigated and discussed.     

In situ auger electron spectroscopy applied to the study of chemisorption and diffusion during reactive implantation of titanium into iron

Auger electron spectroscopy (AES) has been used to monitor the composition of Fe and FeTi surfaces, in situ, during Ti, Fe and Ar ion implantation in the presence of CO, C₂H₂, CH₄, N₂ and NH₃ gases at pressures of approximately 10^-6 Torr. Implantation-induced subsurface carburization was observed for Ti⁺ implantation into Fe in CO and C₂H₂ and for Ar⁺ implantation into an FeTi alloy in CO. In these cases, Ti carbide-like bonding of carbon at the surface preceded the subsurface carburization. Carburization was not observed when CH₄ gas was present during Ti implantation of Fe nor was it observed when CO gas was present during Fe self-implantation. Subsurface nitridation, similar to but not exactly analogous to the carburization, occurred during Ti⁺ implantation of Fe with NH₃ gas present. Nitridation did not occur with N₂ gas present even though some nitrogen was adsorbed. The influences of surface chemical reactions are considered, along with other chemical and physical arguments, in proposing an interpretation of these results.     

Adsorption and diffusion behaviors of H2, H2S,NH3, CO and H2O gases molecules on MoO3 monolayer: A DFT study

Molybdenum trioxide (MoO₃) with α-phase is a promising material for gas sensing because of its high sensitivity, fast response and thermodynamic stability. To probe the mechanism of superior gas detection ability of MoO₃ monolayer, the adsorption and diffusion of H₂, H₂S, NH₃, CO and H₂O molecules on two-dimensional (2D) MoO₃ layer are studied via density functional theory (DFT) calculations. Based on calculated adsorption energies, density of states, charge transfer, diffusion barriers and diffusion coefficient, MoO₃ shows a superior sensitive and fast response to H₂ and H₂S than CO, NH₃, H₂O, which is consistent with experimental conclusions. Moreover, the response of MoO₃ to H₂S and H₂ will be obviously enhanced at high gas concentration, and the incorporation of H₂ and H₂S results in an obvious increasing in DOS near Fermi level. Our analysis provides a conceptual foundation for future design of MoO₃-based gas sensing materials.     

Separation of electrode reactions in multi electrode amperometric sensors

Multi electrode amperometric sensors based on stabilized zirconia have been studied with respect to the simultaneous detection of oxygen and NO. Both gases are of particular interest in oxygen rich exhaust gases. With a setup consisting of two subsequent electrodes it was possible to separate the reduction of oxygen and the reduction of NO spatially. Hence the currents of the two electrodes are directly correlated with the oxygen and the NO concentrations. Combustible gases like CO are oxidized at the oxygen electrode, thus lowering the effective oxygen concentration in the cell. H₂O and CO₂ are partly reduced at the NO electrode. This effect strongly depends on the applied potential and may be eliminated by an appropriate selection of the working parameters. The results demonstrate the potential of such multi electrode cells to the simultaneous detection of several gas components. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

Redox reactions and inward cationic diffusion in glasses caused by CO and H2 gases

We find that inward diffusion of network-modifying cations can occur in an iron-containing silicate glass when it is heat-treated in CO/CO₂ (98/2 v/v) or H₂/N₂ (1/99 v/v) gases at temperatures around the glass transition temperature. The inward diffusion is caused by the reduction of ferric to ferrous ions and this diffusion leads to formation of a silica-rich surface layer with a thickness of 200–600 nm. The diffusion coefficients of the network-modifying divalent cations are calculated and they are different for the glasses treated in the CO and H₂ gases. At the applied partial pressures of CO and H₂, the H₂-bearing gas creates the silica-rich layer more effectively than the CO-bearing gas. The layer increases the hardness and chemical durability of the glass due to the silica network structure in the surface layer.     

Sensing low concentrations of CO using flame-spray-made Pt/SnO2 nanoparticles

Tin dioxide nanoparticles of different sizes and platinum doping contents were synthesized in one step using the flame spray pyrolysis (FSP) technique. The particles were used to fabricate semiconducting gas sensors for low level CO detection, i.e. with a CO gas concentration as low as 5 ppm in the absence and presence of water. Post treatment of the SnO₂ nanoparticles was not needed enabling the investigation of the metal oxide particle size effect. Gas sensors based on tin dioxide with a primary particle size of 10 nm showed signals one order of magnitude higher than the ones corresponding to the primary particle size of 330 nm. In situ platinum functionalization of the SnO₂ during FSP synthesis resulted in higher sensor responses for the 0.2 wt% Pt-content than for the 2.0 wt% Pt. The effect is mainly attributed to catalytic consumption of CO and to the associated reduced sensor response. Pure and functionalized tin dioxide nanoparticles have been characterized by Brunauer, Emmett and Teller (BET) surface area determination, X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) while the platinum oxidation state and dispersion have been investigated by X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS). The sensors showed high stability (up to 20 days) and are suitable for low level CO detection: <10 ppm according to European and 50 ppm according to US legislation, respectively.     

Raman and fluorescence lidar measurements of aircraft engine emissions

Laser induced Raman and fluorescent measurements were made in the exhaust of a gas turbine engine with a new field portable instrument devised specifically for gas turbine exhaust measurements. The gas turbine exhaust was analyzed by conventional instruments for CO, CO₂, NO, NO_x, total hydrocarbons, smoke and temperature, and these data were used as a ‘calibration’ standard for the evaluation of the laser Raman instrument. Results thus far indicate good correlations for CO₂, O₂, smoke, hydrocarbons and temperature. The instrument was not sensitive enough for NO detection but the data analysis indicates that 100 ppm may be detectable with instrument improvements. CO analysis was not attempted, but it is expected that CO could be detected with further research. NO₂ (or NO_x) was not attempted because theoretical and experimental laboratory analysis indicated severe interference with CO₂. The conclusion was that laser Raman shows a good potential for aircraft gas turbine emission analysis.     

Disodium terephthalate/multiwall-carbon nanotube nanocomposite as advanced anode material for Li-ion batteries

Organic small molecule materials have attracted extensive attention due to their environmentally friendly, sustainability, and low cost which can be obtained from biomass and recyclable resources for Li/Na-ion batteries. However, the intrinsic poor electronic conductivities and the dissolution in organic liquid electrolyte lead to poor electrochemical performance, thus preventing them from practical application. To tackle these issues, herein, we take disodium terephthalate (Na₂TP) as an example and report an organic/multiwall-carbon nanotube nanocomposite via a simple spray drying methodology as an anode material for Li-ion battery. It delivers improved electrochemical performance compared to the pristine Na₂TP microspheres produced by the same spray drying method and the bulk microsized Na₂TP prepared by a conventional water-crystallization method. This is mainly due to the as-prepared nanocomposite can shorten the Li-ion diffusion distance, form highly conductive network and slow the dissolution rate. Our simple methodology could be of interest designing newly organic composites.     

Waveguide co-laser with an intracavity optothermal cell for gas analysis systems

We investigate the possibilities for intracavity optothermal detection of microimpurities of gases. The generation regimes of a frequency-tunable wavequide CO-laser with an intracavity absorbing cell that contains NH₃ and NO₂ gases as absorbers are investigated experimentally. The conditions for stationary generation of such a laser are discussed. It is shown that the intracavity siting of the optothermal cell in a waveguide CO-laser makes it possible to increase the detecting ability of a gas analyzer by more than an order of magnitude as compared to the case of its external position. For NO₂ and NH₃ gases the magnitude of the minimum detected concentrations is ≈10 and 30 ppb, respectively. Moscow State Engineering Physics Institute, 31, Kashirskoe shosse, Moscow, 115409, Russia. Translated from Zhurnal Prikladni Spektroskopii, Vol. 64, No. 5, pp. 686–690, September–October, 1997.     

Colloidal Core–Satellite Supraparticles via Preprogramed Burst of Nanostructured Micro‐Raspberry Particles

Colloidal molecules, or more general supraparticles, i.e., particles which are themselves assembled of smaller nanoparticles in a defined way, are known to be synthesizable via bottom‐up assembly techniques in colloidal dispersion. The amount of synthesizable particles is mostly limited to milligrams. Herein, a bottom‐up‐programed, triggerable top‐down process is reported to obtain core–satellite supraparticles, i.e., particles composed of a larger core particle surrounded by smaller satellite particles. The key is to prepare a nanostructured, microparticulate powder into which defined burst behavior is preprogramed. Once the system is mechanically triggered, it bursts into well‐defined nanosized core–satellite supraparticles. Scale‐up is easily feasible and several hundred grams per batch can be demonstrated. The product is a ready‐to‐use and flexibly processible powder. Upon simple mixing with a polymer, it disintegrates into the preprogramed core–satellite supraparticles, thus forming a highly sophisticated nanocomposite with the polymer matrix. A pure silica nanoparticle system and a silica–iron oxide nanoparticle hybrid system are presented to demonstrate the versatility of the approach. Enhanced mechanical and unexpected magneto‐optical properties with the particle system are found. The disintegration of the microparticles into individual core–satellite colloidal supraparticles is confirmed via in situ liquid cell transmission electron microscopy.     

The response of yttria stabilised zirconia oxygen sensors to carbon monoxide gas

Pressures to reduce pollution and conserve fuel have created a growing interest in emission control, highlighting the need for reliable chemical sensors to detect pollutant gases. With particular regard to automotive emission control, this study examines chemical sensors based on yttria stabilised zirconia (YSZ) solid electrolytes for detection and control of carbon monoxide. It was found that combustible gases in the exhaust interfere with O₂ measurement when a YSZ oxygen conducting solid electrolyte is used. The detected CO interference on the measurement of O₂ could be utilised in two ways. Firstly, to upgrade the performance of the oxygen sensor in an atmosphere containing combustibles by minimising the effect of the combustibles. Alternatively, the interference can be controlled and quantified for CO detection. Sensors with Pt and Au electrodes were tested in O₂ and CO atmospheres. The measured response was evaluated in relation to theoretical expectations. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Uptake of 15NH3 by Picea abies in Closed Chamber Experiments

Abstract

Above-ground deposition of anthropogenic trace gases like NH₃ and NO_x is considered as a main factor for nitrogen (N) loading of Picea abies ecosystems. In order to quantify NH₃ deposition, tracer experiments with ¹⁵N labelled NH₃ were carried out in fumigation chambers (GSF München).

NH₃ uptake is linearly related to the gas concentration in the air, but the relation differs between organs and depends on N-nutrition of the organs. Plants well supplied with N have a lower NH₃ uptake per g dry weight then plants deprived of N. Only a small amount of the offered gas deposits to the external plant surfaces. The NH₃ uptake rates of spruce indicate that NH₃ may be regarded as being just as or even more important as environmental pollutant than NO_x with respect to N loading of spruce ecosystems.