Reactive Gas Research Articles

Reversible and Ultrasensitive Detection of Nitric Oxide Using a Conductive Two-Dimensional Metal-Organic Framework.

This paper describes the use of a highly crystalline conductive 2D copper3(hexaiminobenzene)2 (Cu3(HIB)2) as an ultrasensitive (limit of detection of 1.8 part-per-billion), highly selective, reversible, and low power chemiresistive sensor for nitric oxide (NO) at room temperature. The Cu3(HIB)2-based sensors retain their sensing performance in the presence of humidity, and exhibit strong signal enhancement towards NO over other highly toxic reactive gases, such as NO2, H2S, SO2, NH3, CO, as well as CO2. Mechanistic investigations of the Cu3(HIB)2-NO interaction through spectroscopic analyses and density functional theory revealed that the Cu-bis(iminobenzosemiquinoid) moieties serve as the binding sites for NO sensing, while the Ni-bis(iminobenzosemiquinoid) MOF analog shows no noticeable response to NO. Overall, these findings provide a significant leap in the development of crystalline metal-bis(iminobenzosemiquinoid) based conductive 2D MOFs as highly sensitive, selective, and reversible sensing materials for the low-power detection of highly toxic gases.

Reversible and Ultrasensitive Detection of Nitric Oxide Using a Conductive Two‐Dimensional Metal–Organic Framework

This paper describes the use of a highly crystalline conductive 2D copper3(hexaiminobenzene)2 (Cu3(HIB)2) as an ultrasensitive (limit of detection of 1.8 part‐per‐billion), highly selective, reversible, and low power chemiresistive sensor for nitric oxide (NO) at room temperature. The Cu3(HIB)2‐based sensors retain their sensing performance in the presence of humidity, and exhibit strong signal enhancement towards NO over other highly toxic reactive gases, such as NO2, H2S, SO2, NH3, CO, as well as CO2. Mechanistic investigations of the Cu3(HIB)2‐NO interaction through spectroscopic analyses and density functional theory revealed that the Cu‐bis(iminobenzosemiquinoid) moieties serve as the binding sites for NO sensing, while the Ni‐bis(iminobenzosemiquinoid) MOF analog shows no noticeable response to NO. Overall, these findings provide a significant leap in the development of crystalline metal‐bis(iminobenzosemiquinoid) based conductive 2D MOFs as highly sensitive, selective, and reversible sensing materials for the low‐power detection of highly toxic gases.

Surface restructuring and predictive design of heterogeneous catalysts.

Heterogeneous catalysts, which often consist of metal nanoparticles loaded on supports such as metal oxides, can undergo several types of restructuring under reaction and catalytic conditions. Advanced characterizations now allow the surface structure of a catalyst in a gas phase to be determined to some extent. Metal nanoparticles may experience changes in shape, surface structure and composition, and atomic packing in response to the pressure of a reactant or product gas, temperature, and reaction between the catalyst surface and gas. Metal oxide supports can partially or fully encapsulate metal nanoparticles, altering their electronic structure and reactivity. Restructuring is a main approach to creating active catalytic sites. The rational design of catalysts must anticipate that such restructurings can occur under reaction or catalytic conditions and gain knowledge of how they occur. It is expected that computational studies can predict such restructurings and that advanced synthesis may be used to prepare pristine catalysts with enhanced resistance to restructurings.

Residual Stress Analysis in High‐Speed Physical Vapor Deposition Coatings

Several studies focus on impact of residual stress in coatings, predominantly those synthesized by conventional physical vapor deposition (PVD) techniques like arc PVD and magnetron sputtering. High‐speed PVD (HS‐PVD) is based on hollow cathode gas flow sputtering, enabling the deposition of thick coatings s > 20 μm in contrast to the mentioned processes, where coating thickness is limited due to compressive residual stresses. Therefore, the effect of residual stresses on HS‐PVD coatings and adhesion was analyzed for the first time. The aim is to evaluate the influence of diverse substrate materials, different coating systems, and process parameters on the residual stress states in HS‐PVD coatings. Different coating systems like AlCrN and AlCrO are deposited at different reactive gas flows, coating times, and bias voltages. The residual stress of oxide coatings, deposited on cemented carbide and steel X40CrMoV5–1, is analyzed using X‐ray diffraction (XRD) and the sin2ψ method. For AlCrN coatings, in addition to the XRD method, the residual stresses are measured by focused ion‐beam‐digital image correlation ring‐core method to investigate different measuring methods. Both coating systems show higher adhesion strength with increasing thickness. Lower residual compressive stresses are unexpectedly observed at higher coating thickness using both analysis methods.

Influence of the RF-power on the etching yields and surface morphology in reactive ion beam etching of Si and photoresist

The change in ion energy distribution and composition of a reactive ion beam produced by an RF-excited ion beam source and operation with a mixture of CHF3 and O2 was investigated and correlated with the etching behavior. To this end, measurements were performed with an energy-selective mass spectrometer to determine ion energy distributions, current density measurements for the measurement of current density distributions of the ion beam, and tactile measurements to determine the etching rates of Si and photoresist. The morphology of the photoresist was measured with a scanning force microscope. In particular, alterations in the etching yield and surface morphology of the photoresist can be observed in response to changes in the applied RF-power. An increase in plasma density leads to an increase in fragmentation processes of the injected reactive gases, resulting in the formation of smaller fragments. These smaller fragments have a chemical impact on the substrate surface, which affects the etching performance. These effects can have significant consequences in the context of long-time reactive ion beam processing for patterning applications.

Towards atomic-scale smooth surface manufacturing of β-Ga2O3 via highly efficient atmospheric plasma etching

Abstract The highly efficient manufacturing of atomic-scale smooth β-Ga2O3 surface is fairly challenging because β-Ga2O3 is a typical difficult-to-machine material. In this study, a novel plasma dry etching method named plasma-based atom-selective etching (PASE) is proposed to achieve the highly efficient, atomic-scale, and damage-free polishing of β-Ga2O3. The plasma is excited through the inductive coupling principle and carbon tetrafluoride is utilized as the main reaction gas to etch β-Ga2O3. The core of PASE polishing of β-Ga2O3 is the remarkable lateral etching effect, which is ensured by both the intrinsic property of the surface and the extrinsic temperature condition. As revealed by density functional theory-based calculations, the intrinsic difference in the etching energy barrier of atoms at the step edge (2.36 eV) and in the terrace plane (4.37 eV) determines their difference in the etching rate, and their etching rate difference can be greatly enlarged by increasing the extrinsic temperature. The polishing of β-Ga2O3 based on the lateral etching effect is further verified in the etching experiments. The Sa roughness of β-Ga2O3 (001) substrate is decreased from 14.8 to 0.057 nm within 120 s, and the corresponding material removal rate reaches up to 20.96 μm/min. The polished β-Ga2O3 displays significantly improved crystalline quality and photoluminescence intensity, and the polishing effect of PASE is independent of the crystal faces of β-Ga2O3. In addition, the competition between chemical etching and physical reconstruction, which is determined by temperature and greatly affects the surface state of β-Ga2O3, is deeply studied for the first time. These findings not only demonstrate the high-efficiency and high-quality polishing of β-Ga2O3 via atmospheric plasma etching but also hold significant implications for guiding future plasma-based surface manufacturing of β-Ga2O3.

Exploration on structure-activity relationship over substituted Co-doped spinel AlFe2O4 for enhanced CO2 hydrogenation into C2-C4 hydrocarbons

As the main greenhouse gas, CO2 has caused some serious environmental problems. Hence the abatement of CO2 allows of no delay. Among all strategies for CO2 reduction, CO2 hydrogenation reveals the great potential for industrial application, and becomes the efficient path to achieve target of “Carbon Neutral”. In this work, the Co-doped AlFe2O4 catalysts were firstly reported for CO2 hydrogenation test, and the enhancement of Co doping on activity was explored systematically via a series of conventional characterization methods. The Fischer-Tropsch synthesis-based CO2 hydrogenation was conducted over Co-doped AlFe2O4, thus the dissociation of CO2 and the carbon chain growth were both important. The observation of CoFe2O4 in the composites indicated the strong electronic interaction between Co and Fe species. This could strengthen the electrons transfer to generate more metallic Co and Fe species during H2 reduction, and then increased the Fe3C percentage during CO2 hydrogenation process. Moreover, the generated Co0 species could provide the additional adsorption sites for reactant gas. Therefore, the strategy of Co doping could strengthen the CO2 hydrogenation performance, reflected in the increase of CO2 conversion, the selectivity and space time yield of C2-C4 products. However, the formed “too stable” structure in the condition of the excessive Co doping prohibited the electrons transfer and the generation of Fe3C active species, thus existing an optimum Co doping amount.

Synthesis and Characterization of Boron Nitride Thin Films Deposited by High-Power Impulse Reactive Magnetron Sputtering.

In the present research, hexagonal boron nitride (h-BN) films were deposited by reactive high-power impulse magnetron sputtering (HiPIMS) of the pure boron target. Nitrogen was used as both a sputtering gas and a reactive gas. It was shown that, using only nitrogen gas, hexagonal-boron-phase thin films were synthesized successfully. The deposition temperature, time, and nitrogen gas flow effects were studied. It was found that an increase in deposition temperature resulted in hydrogen desorption, less intensive hydrogen-bond-related luminescence features in the Raman spectra of the films, and increased h-BN crystallite size. Increases in deposition time affect crystallites, which form larger conglomerates, with size decreases. The conglomerates' size and surface roughness increase with increases in both time and temperature. An increase in the nitrogen flow was beneficial for a significant reduction in the carbon amount in the h-BN films and the appearance of the h-BN-related features in the lateral force microscopy images.

Rapid determination of 90Sr in seawater using a novel porous crown-based resin and tandem quadrupole ICP-MS/MS in cool plasma and O2-He mode

There has been an increasing need for the rapid and automated analysis of 90Sr in seawater for the purpose of radiological assessment and emergency response after the FDNPP accident. In this work, a rapid method for the determination of 90Sr in seawater was established using ICP-MS/MS with cool plasma and collision-reaction gases combined with a new porous crown-based resin separation. A nanofiltration membrane was applied to rapidly pre-treat seawater samples within 3 min and remove most of the matrix components from 500 mL to 250 mL for the first time. A comprehensive investigation was performed to study the extraction behaviors of Sr, Zr, Ge, Y and matrix elements on the new resin, showing an excellent separation efficiency (> 103) using a U-shaped resin column. Based on their different first ionization potentials, the 90Zr+ and 89Y+ interferences were completely eliminated under cool plasma conditions. The interferences of 74Ge16O+ and 88Sr were effectively suppressed by using 2 mL/min He-0.3 mL/min O2 as the collision and reaction gases in the dynamic collision/reaction cell. Combined with chemical separation, the overall decontamination factors of Zr, GeO and Y were 1.9 × 1010, 6.9 × 107 and 6.9 × 107 respectively, and 88Sr1H2+ tailing was reduced by more than 200 times compared to a conventional ICP-MS. A detection limit of 4.6 pg/L (24.0 Bq/L) for 90Sr was achieved with a sample analysis turn-around time of 6 h for a set of 12 samples. The method was validated by analyzing spiked seawater samples.

Solution-Processed TiO2/ZnFe2O4 Heterostructure for Stable Multilevel Memristor with Room-Temperature Reactive Gas Selectivity.

Solution-processed oxide-based heterojunctions that work in diverse directions will be ideal alternatives for cost-effective, stable, and multifunctional devices. Here, we have reported a stable multilevel resistive switching (RS) at the solution-processed TiO2/ZnFe2O4 heterointerface with endurance stability over 104 cycles and retention over 105 s. It can maintain the switching after dripping water onto the device, followed by drying at 100 °C and at an operating temperature of up to 200 °C. As the switching mechanism is governed by filamentary and interface-dominated charge conduction, our device shows additional tunability in the low resistance state (LRS) by changing environmental conditions. The inability to form filaments results in almost negligible switching under a vacuum or inert environment with an LRS loss. Meanwhile, the presence of reducing gas leads to a depletion layer lowering at the TiO2/ZnFe2O4 heterointerface by removing the surface-adsorbed oxygen molecules that help filament conduction through the interface and, hence, a change in LRS. Furthermore, different reaction capacities of different reactive gas environments with the surface-adsorbed oxygen molecule lead to discrete ON-OFF ratios, presenting a pathway to identify several reactive vapors like ammonia, formaldehyde, and acetone at room temperature and presenting a new approach for integrating RS and room temperature gas sensing in the multifunctional device technology.

Molecular Transformations for Direct Synthesis of Thorium Dioxide Films

AbstractNew Th(IV) complexes with high volatility were synthesized and applied as molecular single‐source precursors for chemical vapor deposition (CVD) of ThO2 thin films. The change in the steric profile of alkoxide ligands and concomitant reduction in the molecular weight of [Th(L)2(OR)2] (1‐OR, R=iso‐propyl (1‐OiPr) or tert‐butyl (1‐OtBu), L=N‐(4,4,4‐trifluorobut‐1‐ene‐3‐one)‐methoxyethylamide)) had a profound effect on the vapor pressure (at 10−5 mbar) evident in the drop in sublimation temperature to 100 °C for the iso‐propoxide derivative against 130 °C observed for the tert‐(1‐OtBu). Hirshfeld surface analysis of both complexes showed different degrees of intermolecular H⋅⋅⋅F interactions responsible for the observed differences in the volatilities of the two complexes. Both mixed‐ligand compounds (1‐OiPr, 1‐OtBu) were applied in the CVD process to deposit thorium oxide thin films without any carrier or reactive gas that verified their suitability as single‐source precursors to ThO2 coatings. Thin films of ThO2 were grown at 500 °C and 600 °C on Si/SiO2 substrates, which showed presence of C, N, and F presumably originating from the incorporation of ligand fragments in the growing CVD deposits. Subsequent calcination of CVD‐grown ThO2 films at 800 °C in air led to phase pure ThO2 coatings with uniform morphology.

Effects of Tube Diameter and Gas Flow Rate on the Discharge Dynamics Characteristics and Reactive Species of Nanosecond Pulsed Discharge Plasma in Contact With Water

ABSTRACTThe use of millimeter quartz tubes to generate non‐thermal plasma in contact with liquid is widely applied in medicine, such as the effective disinfection of catheter tubes and tooth cavities. Here, the effects of tube diameter and gas flow rate on discharge dynamics and reactive species characteristics of nanosecond pulse needle‐water discharge are studied using an ICCD camera and optical emission spectra. The discharge diffusion is increased significantly by an appropriate combination of tube diameter and gas flow rate. Besides, the discharge intensity, emission spectra intensities of reactive species, gas temperature, and electron density increase with the increase of quartz tube diameter and gas flow rate, which contributes to enhance the production of OH and H2O2 species in aqueous samples.

Development of soft x-ray absorption spectroscopy technique with simultaneous measurement in electron- and fluorescence-yield modes from high vacuum to ambient pressure for observation of surface adsorbed species.

To understand the reactions of heterogeneous catalysts at the solid-gas interface under actual reaction conditions, it is important to develop a method to observe the surface-adsorbed species during the reaction, including the changes before and after the adsorption of light elements involved in the surface reaction. We developed a soft x-ray absorption spectroscopy (XAS) technique that allows simultaneous measurements in the electron- and fluorescence-yield modes in the pressure range of 10-4-1 × 105Pa. In the developed system, the reaction gas near the sample surface is separated from the beamline vacuum by a Si3N4 window and confined to a small area to suppress x-ray absorption by the gas. The electron-yield spectra were obtained by measuring the sample current while applying a bias potential to the Si3N4 window. XAS measurements were performed from high vacuum to ambient pressure by setting the bias potential to 600 and 39V below and above 100Pa, respectively. An anatase TiO2 nanoparticle-deposited film was prepared by spin coating, and soft XAS was performed to observe the photocatalytic oxidative decomposition reactions of isopropanol in the presence of water and oxygen. The obtained O K-edge spectra showed that it is possible to observe adsorbed oxygen on solid oxides even under ambient pressure conditions containing 0.1% of oxygen gas.

Enhanced and prolonged adsorption of ammonia gas by zeolites derived from coal fly ash.

In this study, zeolites were synthesized from coal fly ash (CFA) using NaOH (Na-X type) and KOH (chabazite-K type) and used for the adsorption of ammonia gas in a fixed-bed reactor. Magnetic separation was used to decrease the Fe impurity content from 6.84 to 2.37 wt.%, and to increase the Si and Al content in non-magnetic fly ash (NMFA). BET surface areas of the zeolites increased significantly to 425.28 m2/g with an increase in micropore volume (ca. 0.12 cm3/g). FTIR and NH3-TPD analyses revealed that the increased surface area provided more acidic sites for enhanced ammonia gas adsorption. In particular, the zeolite (i.e., ZNF-X) synthesized from NMFA and NaOH showed the highest ammonia gas adsorption capacity (64.9 mg/g), owing to its high BET surface area and micropore volume. Although the initial adsorption capacity of commercial 13X zeolite (74.5 mg/g) was higher than that of ZNF-X, a much higher recycling efficiency for ZNF-X (92.0%) was achieved than for 13X (31.5%) during the five recycling tests because of the lower ammonia desorption temperature of ZNF-X. The results provide new insights into the synthesis of zeolites by upcycling of CFA and demonstrate the potential use of the generated materials for enhanced and prolonged removal of ammonia gas.

Experimental analysis of a hybrid thermochemical cycle driven by intermediate grade heat sources for energy storage and combined cold and work productions

This paper presents an experimental study of a hybrid thermochemical cycle for the simultaneous cogeneration of cold and mechanical power, and thermal storage based on medium temperature sources. This concept is based on the integration of an expander machine on the reactive gas flow between the evaporator and the reactor to allow mechanical work production. The paper shows the proof of concept of this hybridization by continuous mechanical production and cold during the whole production phase: 68.33 kJ mechanical energy and 15.18 MJ cold, stable pressure ratio at the expander (around 1.3) and stable rotational speed (around 400 rpm). The prototype conception and design are detailed. The integration of the expander creates a pressure difference between the evaporator and the reactor (contrary to a classical thermochemical cycle), and thus an experimental sensitivity study investigating the effect of the cold production temperature at the evaporator (which defines the inlet pressure of the expander) on the mechanical production of the prototype and its dynamics is done. This allowed to analyze the strong coupling between the reactor and the expander in different operating conditions, experimentally and from a theoretical point of view. Indeed, the identified limitations of these two main components, it is worth mentioning that this is the first hybrid thermochemical prototype with stable output productions. The prototype works properly by providing continuous mechanical production during the whole production phase under different cold temperatures and constant electrical load, in comparison with what was found in the literature.

Optimization of LPCVD Deposition Conditions of Silicon-Rich Silicon Nitride to Obtain Suitable Optical Properties for Photoluminescent Coating

Silicon nitride is a commonly used material for ceramic applications and in the fabrication processes of integrated circuits (ICs). It has also increased in interest from the scientific community for use as a functional coating due to its physical, mechanical, electrical, and optoelectronic properties. In particular, silicon-rich silicon nitride (SRSN) has been considered in the photovoltaic industry as a down-conversion film for solar cells. In this work, SRSN films have been obtained by the Low-Pressure Chemical Vapor Deposition (LPCVD) technique at low to moderate deposition temperatures with a variation in the precursor gas pressure ratio. The SRSN films showed a wide photoluminescence (PL) in the visible region (without a high-deposition temperature or annealing process) and suitable optical properties (refractive index and absorption in the UV) to be used as photoluminescent coating on silicon solar cells. The absence of high-deposition temperatures could preserve the original structure of silicon solar cells, once the SRSN layer was applied. In addition, control of the reactive gas pressure ratio and deposition temperature showed an influence on the refractive index, the surface roughness, and the PL emission.

Selective Synthesis of 3D Aligned VO2 and V2O5 Carbon Nanotube Hybrid Materials by Chemical Vapor Deposition.

3D carbon nanotube hybrid materials containing VO2 and V2O5 evenly distributed onto vertically aligned carbon nanotubes (VACNTs) is reported. Adjustable loading of particles in controllable sizes and shapes onto the VACNTs was developed via a stepwise chemical vapour deposition (CVD) approach. Solid VO(acac)2 is chosen as vanadium source. CO2 acts as reactive gas for the pre-functionalisation of the VACNTs. The process temperature was identified as key parameter to control the deposited vanadium oxide phase. A temperature of 550 °C results in monoclinic VO2, while 600 °C results in the deposition of V2O5 onto the VACNT support. The morphology and the amount of deposited material was found to be dependent on the reactor dimensions and the degree of functionalisation of the carbon support. An increase of the D/G ratio of the VACNT from 0.75 to 1.08 caused by a CO2 treatment step within the process led to an increase of the particle coverage from a scarce coverage without prior CO2 treatment to a dense coverage of the VACNT support after 15 min of CO2 exposure time. Size and crystallinity of the as deposited particles can be further adjusted by a controlled heat treatment after VO(acac)2 precursor deposition.

Investigation of the Frying Fume Composition During Deep Frying of Tempeh Using GC-MS and PTR-MS.

This study employed proton transfer reaction mass spectrometry (PTR-MS) and gas chromatography-mass spectrometry (GC-MS) to identify and monitor volatile organic compounds (VOCs) in frying fumes generated during the deep frying of tempeh. The research aimed to assess the impact of frying conditions, including frying temperature, oil type, and repeated use cycles, on the formation of thermal decomposition products. A total of 78 VOCs were identified, with 42 common to both rapeseed and palm oil. An algorithm based on cosine similarity was proposed to group variables, resulting in six distinct emission clusters. The findings highlighted the prominence of saturated and unsaturated aldehydes, underscoring the role of fatty acid oxidation in shaping the frying fume composition. This study not only corroborates previous research but also provides new insights into VOC emissions during deep frying, particularly regarding the specific emission profiles of certain compound groups and the influence of frying conditions on these profiles.

Experimental study of voltage uniformity and stability of H2/O2 PEM fuel cell stack with dead-end anode and recirculation cathode

Dead-end anode and recirculation cathode is an operating mode effectively improving the gas reactant utilization of hydrogen–oxygen proton exchange membrane fuel cell (H2/O2 PEMFC) stack. However, the difference of single cells in the stack, will be further expanded due to the flooding and impurities and necessary purge for removing them. In this work, the voltage uniformity and stability of H2/O2 PEMFC under different operating conditions are investigated based on a dead-end anode and recirculation cathode PEMFC stack. Research indicates that enhancing the current density and temperature, lowering the inlet gas pressure will diminish the uniformity in stack voltage. The analysis of the experimental pattern reveals a positive correlation between the voltage uniformity and stability. This is due to the water flooding or membrane dehydration phenomenon of the single-cell leading to a reduction of the single-cell voltage and the voltage uniformity of the stack, while the water flooding or membrane drying phenomenon makes the stack more unstable and the purge interval shorter. Based on these experimental results, the best operating scheme for the designed stack is proposed, which is of some guidance and reference value for the practical application of air-cooled PEMFC in the hundred-watt class.

Unraveling the Oxygen Vacancy-Performance Relationship in Perovskite Oxides at Atomic Precision via Precise Synthesis.

Understanding the fundamental effect of the oxygen vacancy atomic structure in perovskite oxides on catalytic properties remains challenging due to diverse facets, surface sites, defects, etc. in traditional powder catalysts and the inherent structural complexity. Through quantitative synthesis of tetrahedral (LaCoO2.5-T), pyramidal (LaCoO2.5-P), and octahedral (LaCoO3) epitaxial thin films as model catalysts, we demonstrate the reactivity orders of active-site geometrical configurations in oxygen-deficient perovskites during the CO oxidation model reaction: CoO4 tetrahedron > CoO6 octahedron > CoO5 pyramid. Ambient-pressure Co L-edge and O K-edge XAS spectra clarify the dynamic evolutions of active-site electronic structures during realistic catalytic processes and highlight the important roles of defect geometrical structures. In addition, in situ XAS and resonant inelastic X-ray scattering spectra and density functional theory calculations directly reveal the nature of high reactivity for CoO4 sites and that the derived shallow-acceptor defect levels in the band structure facilitate the adsorption and activation of reactive gases, resulting in more than 23-fold enhancement for catalytic reaction rates than CoO5 sites.