Diffusion Of Gas Molecules Research Articles

Realizing the Control of Electronic Energy Level Structure and Gas-Sensing Selectivity over Heteroatom-Doped In2O3 Spheres with an Inverse Opal Microstructure.

Understanding the effect of substitutional doping on gas-sensing performances is essential for designing high-activity sensing nanomaterials. Herein, formaldehyde sensors based on gallium-doped In2O3 inverse opal (IO-(Ga xIn1- x)2O3) microspheres were purposefully prepared by a simple ultrasonic spray pyrolysis method combined with self-assembled sulfonated polystyrene sphere templates. The well-aligned inverse opal structure, with three different-sized pores, plays the dual role of accelerating the diffusion of gas molecules and providing more active sites. The Ga substitutional doping can alter the electronic energy level structure of (Ga xIn1- x)2O3, leading to the elevation of the Fermi level and the modulation of the band gap close to a suitable value (3.90 eV), hence, effectively optimizing the oxidative catalytic activity for preferential CH2O oxidation and increasing the amount of adsorbed oxygen. More importantly, the gas selectivity could be controlled by varying the energy level of adsorbed oxygen. Accordingly, the IO-(Ga0.2In0.8)2O3 microsphere sensor showed a high response toward formaldehyde with fast response and recovery speeds, and ultralow detection limit (50 ppb). Our findings finally offer implications for designing Fermi level-tailorable semiconductor nanomaterials for the control of selectivity and monitoring indoor air pollutants.

Unusual adsorption behaviours and responsive structural dynamics via selective gate effects of an hourglass porous metal-organic framework.

An hourglass porous metal–organic framework, LIFM-12, constructed on a T-shaped flexible ligand with Cu2+ paddle-wheel clusters, shows temperature and gas adsorption responsive structural dynamics upon reversible molecular guest binding. Temperature-dependent single crystal and powder X-ray diffraction experiments show that the open gate status of the framework with adaptive behaviours facilitates kinetic diffusion of gas molecules resulting in the sequential filling of pores of different sizes, thus creating a breathing behaviour reminiscent of the observation of several steps in adsorption isotherms. In addition, adsorption studies revealed that LIFM-12 performs exceptional adsorption selectivity of 10–25 for CO2versus light gases N2, CH4, and CO and up to 200 for C3H6versus CH4.

Efficient and Robust Fabrication of Microscale Graphene Drums

Few-layer graphene drums have been extensively studied as bulging devices to analyze their mechanical and optical properties. A controllable and efficient fabrication method of such drums is of great significance for related studies. We propose a one-step approach to fabricate graphene drums through modified mechanical exfoliation. Graphene sheets are annealed on the prepatterned SiO2 substrate with micrometer-sized holes. With such an approach, one can fabricate graphene drums with larger height using higher annealing temperature or larger holes. Larger strain within the graphene drums could also be achieved using a higher annealing temperature. The strain of the graphene drums gradually decreases over time within 24 h. The underlying mechanism is attributed to the diffusion of gas molecules initially trapped at the graphite/substrate interface. This self-consistent mechanism prevents the drums from being destroyed by the pressure difference across graphene layers in principle. Compared with the existing...

Nanoporous Organic Field‐Effect Transistors Employing a Calixarene Dielectric for Sub‐ppb Gas Sensing

AbstractThe increase of exposure to volatile organic compounds in general, and endocrine disruptors in particular, through daily healthcare consumer goods urgently calls for the development and commercialization of cheap and easy‐to‐use sensors to detect these molecules in air. Organic semiconductors (OSCs) promise the mass fabrication of cheap and flexible sensors, under the condition that the fabrication of functional thin films having the right morphology can be fully harnessed. Nanoporous morphologies are interesting in the field of gas sensing because they facilitate the diffusion of gas molecules to the active volume of organic field‐effect transistors (OFETs), at the vicinity of the gate dielectric. Nanoporous films of OSCs are, however, challenging to produce and to transfer into working sensors. Usage of nanoporous OSC films deposited on a sensitive 4‐tert‐butylcalix[6]arene gate dielectric as the active layer of an OFET‐based gas sensor is proposed here. The semiconducting molecules are self‐assembled into nanoporous films at the interface between air and water, allowing their transfer to practically any gas‐sensitive dielectric substrate. The OFET built on this film stack is used here to sense the presence of methyl 4‐hydroxybenzoate (methylparaben) in air at concentrations below 1 ppb.

Nonlinear Thermodiffusion in Gases at Moderate Temperatures

Abstract The paper discusses the properties of the nonlinear thermodiffusion equation corresponding to the diffusion processes, which occur with a finite velocity. In the previous papers, A. J. Janavičius proposed the nonlinear diffusion equation with the diffusion coefficient directly proportional to the concentration of impurities. This equation provides a more exact description of the profiles of impurities in Si crystals. The heat transfer in gases carries a greater average kinetic energy based on nonlinear diffusion of gas molecules from hot regions to the coldest ones with a finite velocity by random Brownian motions. In this case, the heat transfer in gases can be described by using nonlinear thermodiffusion equation with heat transfer and thermodiffusion coefficients directly proportional to temperature T. The obtained approximate analytical solutions are successfully applied in defining temperature profiles and heat transfer coefficients in gases as well as providing opportunities for practical applications. It has been concluded that heat spreading in gases depends on temperature differences and pressure in gases.

Effects of Grain Boundary Density on the Gas Sensing Properties of Triethylsilylethynyl‐Anthradithiophene Field‐Effect Transistors

AbstractIn this study, triethylsilylethynyl‐anthradithiophene (TES‐ADT) films with different density of grain boundaries are used for systematic investigation of effects of grain boundary density on gas sensing properties of TES‐ADT field‐effect transistors (FETs). Grain boundary density is simply controlled by changing mixing time of TES‐ADT solution. Higher mixing time leads to higher grain boundary density, and field‐effect mobility decreases with increased grain boundary density. However, gas sensing properties exhibit the opposite behavior. Drain current and field‐effect mobility of FETs based on TES‐ADT film with higher grain boundary density increase much more upon exposure of NO2 with electron withdrawing character. In addition, dynamic gas sensing tests reveal that response rate and sensitivity of a TES‐ADT sensor are enhanced remarkably by an increase of grain boundary density in TES‐ADT films. Grain boundaries provide a pathway for diffusion of gas molecules into channel regions and thus increase of grain boundary density is beneficial for development of highly sensitive OFET gas sensors.

Ionic Liquid Microstrips Impregnated with Magnetic Nanostirrers for Sensitive Gas Sensors.

Ionic liquids (IL) have been regarded as promising electrolytes as substitutes for volatile aqueous or organic solvents for electrochemical gas sensors. However, ILs are viscous, and the slow diffusion of gas molecules leads to poor sensitivity and sluggish response times. Herein, we describe a strategy using an array of microstrips of IL containing magnetic nanoparticles as nanostirrers for enhanced mass transport and gas sensing. Magnetic CoFe2O4 nanoparticles are synthesized and dispersed in a hydrophobic IL [BMP][Ntf2]. First, the convection effect of the IL dispersion was studied using the reversible redox couple ferrocene/ferrocenium ion. In a rotating magnetic field, steady-state currents for oxidation of dissolved ferrocene are three to five times greater than that in an unstirred solution. Then, the IL dispersion is micropatterned onto a gold electrode using microcontact printing. A self-assembled monolayer was printed onto a gold surface creating 70 μm wide hydrophobic lines with a 30 μm gap between them. Upon applying the IL dispersion into the gap, a 30 μm wide array of microstrips was successfully fabricated. The system is demonstrated as an oxygen sensor in the range of volume fraction of O2 of 50-500 ppm giving a linear calibration with a sensitivity of 1.94 nA cm-2 ppm-1.

Solvent Exchange Leading to Nanobubble Nucleation: A Molecular Dynamics Study.

The solvent exchange procedure has become the most-used protocol to produce surface nanobubbles, while the molecular mechanisms behind the solvent exchange are far from being fully understood. In this paper, we build a simple model and use molecular dynamics simulations to investigate the dynamic characteristics of solvent exchange for producing nanobubbles. We find that at the first stage of solvent exchange, there exists an interface between interchanging solvents of different gas solubility. This interface moves toward the substrate gradually as the exchange process proceeds. Our simulations reveal directed diffusion of gas molecules against the gas concentration gradient, driven by the solubility gradient of the liquid composition across the moving solvent–solvent interface. It is this directed diffusion that causes gas retention and produces a local gas oversaturation much higher near the substrate than far from it. At the second stage of solvent exchange, the high local gas oversaturation leads to bubble nucleation either on the solid surface or in the bulk solution, which is found to depend on the substrate hydrophobicity and the degree of local gas oversaturation. Our findings suggest that solvent exchange could be developed into a standard procedure to produce oversaturation and used to a variety of nucleation applications other than generating nanobubbles.

Transport of dissolved gases through unsaturated porous media

The natural porous media (e.g. soil, sand, peat etc.) usually are partially saturated by groundwater. The saturation of soil depends on hydrostatic pressure which is linearly increased with depth. Often some gases (e.g. nitrogen, oxygen, carbon dioxide, methane etc.) are dissolved into the groundwater. The solubility of gases is very small because of that two assumptions is applied: I. The concentration of gas is equal to solubility, II. Solubility depends only on pressure (for isothermal systems). In this way some part of dissolved gas transfers from the solution to the bubble phase. The gas bubbles are immovably trapped in a porous matrix by surface-tension forces and the dominant mechanism of transport of gas mass becomes the diffusion of gas molecules through the liquid. If the value of water content is small then the transport of gas becomes slow and gas accumulates into bubble phase. The presence of bubble phase additionally decreases the water content and slows down the transport. As result the significant mass of gas should be accumulated into the massif of porous media. We derive the transport equations and find the solution which is demonstrated the accumulation of gases. The influence of saturation, porosity and filtration velocity to accumulation process is investigated and discussed.

Beyond Equilibrium: Metal–Organic Frameworks for Molecular Sieving and Kinetic Gas Separation

Metal–organic frameworks (MOFs) are a class of crystalline inorganic–organic hybrid materials that have demonstrated huge potential in gas separation due to their ultrahigh porosity, boundless chemical tunability, as well as surface functionality. Most gas separations realized in MOFs are under an equilibrium state and are dependent on the difference in thermodynamic affinities of gases to MOFs, whereas nonequilibrium separation such as kinetic and molecular sieving separation attracting growing attention in the past decade is achieved based on the difference in the size and diffusivity of gas molecules. In this perspective, we first discuss the pore size, temperature, and pressure effect on gas diffusion as well as nonequilibrium gas separation in MOFs. Second, we introduce current techniques reported to measure intracrystalline gas diffusivity. Third, we review recent progress in MOF-based nonequilibrium N2/O2 separation, CO2 capture, and hydrocarbon separation. In addition, we describe the hydrogen iso...

High selectivity to ppb-level HCHO sensor based on mesoporous tubular SnO2 at low temperature

Distinctive mesoporous SnO2 microtubes were synthesized via a facile and environmentally friendly solvothermal method. The gas sensor based on such SnO2 microtubes exhibited not only the lower detection limit to detect formaldehyde and remarkable selectivity comparison with other various gases when operating within 92°C, but also good linear relationship under concentration ranges of ppm level (1–100ppm) and ppb level (10–100ppb). The sensing properties of such SnO2 microtubes have been greatly improved compared to previously reported hollow structures. This might be attributed to their large specific surface area and porous structure, which can promote the diffusion of gas molecules and their adsorption on the surface of the sensing material.

Diffusion of gas molecules on multilayer graphene surfaces: Dependence on the number of graphene layers

The diffusion of gas molecules on multilayer graphene surfaces is of great importance for a wide range of applications in gas-related industries. This study calculates diffusion coefficients for gas diffusion on single layer or multilayer graphene surfaces based on molecular dynamics simulations with a major emphasis on the effect of the number of graphene layers. The results show that the gas diffusion on these graphene surfaces is mainly controlled by molecular collisions in the adsorption layer; because the contributions of the gas adsorption energy and the gas collision energy are always comparable with the gas adsorption energy becoming slightly stronger with increasing number of graphene layers. Therefore, the surface diffusion coefficient decreases gradually with increasing number of graphene layers owing to the larger number of adsorbed molecules on graphene surfaces with more layers. Notably, the diffusion coefficients do not depend strongly on the number of graphene layers when there are a large number of graphene layers due to the limited interaction distance between the gas molecules and the graphene atoms. Furthermore, the variations of the surface diffusion coefficient with the number of graphene layers and the gas species are confirmed from the probability distributions of the molecular jump length on the graphene surface in a given time period.

Nonlinear thermal conductivity in gases

The nonlinear diffusion equation corresponds to the diffusion processes which can occur with a finite velocity. A.J. Janavičius proposed nonlinear equation which describes more exactly the diffusion of impurities in Si crystals in many interesting practical applications. The heat transfer in gases is also based on diffusion of gas molecules from hot regions to the coldest ones with a finite velocity by random Brownian motions. In this case the heat transfer can be considered using similar nonlinear thermal diffusivity equation. The approximate analytical solution of this nonlinear equation can be used for the experimental analysis of thermal conductivity coefficients using temperature profiles dependence on different temperatures and pressures in gases.

Pressure sensor based on MEMS nano-cantilever beam structure as a heterodielectric gate electrode of dopingless TFET

Micro-electromechanical systems (MEMS) technology has enticed numerous scientists since recent decades particularly in the field of miniaturized-sensors and actuators. Pressure sensor is pivotal component in both of the forerunning fields. The pursuance of a pressure sensor is exigently relying upon its different physical properties i.e. Piezo-resistive, Piezoelectric, Capacitive, Magnetic and Electrostatic. This article presents an outline and scrutiny of the Doping-less Cantilever Based Pressure Sensor using tunnel field effect transistor technology. The propounded pressure sensor based on the principle of capacitive gate coupling, due to which the tunneling current is modified. Additionally, to enhance the affectability of pressure sensor, the work function of metal gate electrode is amended using gas molecule diffusion. Simulation uncovers a phenomenal relationship amongst hypothetical and practical accepts of configuration. The pressure sensor is composed at Silvaco Atlas tool utilizing 40 nm technologies. The performance results exhibit that the proposed model consumes ≤1 mW power and 250 μA tunneling current per nm bending of cantilever beam structure. The inclusive length of the proposed device is 100 nm.

Hydrothermal Synthesis of Lanthanide Stannates Pyrochlore Nanocrystals for Catalytic Combustion of Soot Particulates.

Nanocrystalline La2Sn2O7 and La2Sn1.8Co0.2O7 with a phase-pure pyrochlore structure were synthesized by a hydrothermal method, and their catalytic activity was investigated for soot combustion. The as-synthesized catalysts presented relatively larger surface area, and pore volume, which was benefit to the gas molecule diffusion in the reaction. A uniform spherical structure with particle size of 200–500 nm was found in SEM. The samples via hydrothermal route are more active for catalytic soot combustion, ascribing to the spherical morphology, high surface area and improved oxygen mobility. After Co, the reducibility was improved and surface oxygen vacancy was produced, resulting in the enhanced activity and selectivity to CO2 formation.

HCl gas adsorption/desorption properties of poly(N-isopropylacrylamide) brushes grafted onto quartz resonator for gas-sensing applications

Poly(N-isopropylacrylamide) (PNIPAM) chains were grafted onto a quartz resonator using surface-initiated atom transfer radical polymerization (SI-ATRP). The PNIPAM loading was varied by changing the grafting time and the monomer concentration. AFM analysis revealed that the increase in PNIPAM loading was attributable to increases in the chain length and in the corresponding brush height. The HCl gas adsorption/desorption properties of the resulting PNIPAM brushes were measured by a quartz crystal microbalance (QCM) technique. The PNIPAM brushes adsorbed large amounts of HCl gas, and the adsorption capacity was linearly related to the PNIPAM loading, indicating the homogeneous adsorption of HCl gas inside the PNIPAM brushes. On the other hand, the rate of desorption of the HCl molecules was lower than that of the adsorption process, and slowed with increases in chain length. Reversibility was insufficient, especially at longer chain lengths due to the difficulty of the diffusion of gas molecules from inside the brushes. The reversibility was improved by raising the measuring temperature from 30°C to 50°C, but it was still up to 80%, suggesting the irreversibility of the adsorption.

Study of transitions between wetting states on microcavity arrays by optical transmission microscopy.

In this article, we present a simple and fast optical method based on transmission microscopy to study the stochastic wetting transitions on micro- and nanostructured polymer surfaces immersed in water. We analyze the influence of immersion time and the liquid pressure on the degree of water intrusion in individual microcavities on these surfaces as well as the lifespan of their superhydrophobicity. We show that transitions among the three wetting states (Cassie, Cassie-impregnating, and Wenzel) occur with a certain pressure threshold (300 mbar for a microcavity diameter of 7.5 μm). Below this threshold, the transitions between the Cassie and the Cassie-impregnating states are reversible, whereas above this threshold, irreversible transitions to the Wenzel state start to occur. The transitions between the different wetting states can be explained by taking into account both the Young-Laplace equation for the water menisci in the cavities and the diffusion of dissolved gas molecules in the water. In addition, the wetting transitions had a stochastic nature, which resulted from the short diffusion distance for dissolved gas molecules in the water between neighboring cavities. Furthermore, we compared the contact angle properties of two polymeric materials (COC and PP) with moderate hydrophobicity. We attributed the difference in the water repellency of the two materials to a difference in the wetting of their nanostructures. Our experimental observations thus indicate that both the diffusion of gas molecules in water and the wetting properties of nanostructures are important for understanding the sustainability of superhydrophobicity of surfaces under water and for improving the structural design of superhydrophobic surfaces.

Compilation and evaluation of gas phase diffusion coefficients of reactive trace gases in the atmosphere: volume 1. Inorganic compounds

Abstract. Diffusion of gas molecules to the surface is the first step for all gas–surface reactions. Gas phase diffusion can influence and sometimes even limit the overall rates of these reactions; however, there is no database of the gas phase diffusion coefficients of atmospheric reactive trace gases. Here we compile and evaluate, for the first time, the diffusivities (pressure-independent diffusion coefficients) of atmospheric inorganic reactive trace gases reported in the literature. The measured diffusivities are then compared with estimated values using a semi-empirical method developed by Fuller et al. (1966). The diffusivities estimated using Fuller's method are typically found to be in good agreement with the measured values within ±30%, and therefore Fuller's method can be used to estimate the diffusivities of trace gases for which experimental data are not available. The two experimental methods used in the atmospheric chemistry community to measure the gas phase diffusion coefficients are also discussed. A different version of this compilation/evaluation, which will be updated when new data become available, is uploaded online (https://sites.google.com/site/mingjintang/home/diffusion).

Rapid and selective detection of acetone using hierarchical ZnO gas sensor for hazardous odor markers application

Hierarchical nanostructured ZnO dandelion-like spheres were synthesized via solvothermal reaction at 200°C for 4h. The products were pure hexagonal ZnO with large exposure of (002) polar facet. Side-heating gas sensor based on hierarchical ZnO spheres was prepared to evaluate the acetone gas sensing properties. The detection limit to acetone for the ZnO sensor is 0.25ppm. The response (Ra/Rg) toward 100ppm acetone was 33 operated at 230°C and the response time was as short as 3s. The sensor exhibited remarkable acetone selectivity with negligible response toward other hazardous gases and water vapor. The high proportion of electron depletion region and oxygen vacancies contributed to high gas response sensitivity. The hollow and porous structure of dandelion-like ZnO spheres facilitated the diffusion of gas molecules, leading to a rapid response speed. The largely exposed (002) polar facets could adsorb acetone gas molecules easily and efficiently, resulting in a rapid response speed and good selectivity of hierarchical ZnO spheres gas sensor at low operating temperature.

Non-Fickian diffusion and the accumulation of methane bubbles in deep-water sediments.

In the absence of fractures, methane bubbles in deep-water sediments can be immovably trapped within a porous matrix by surface tension. The dominant mechanism of transfer of gas mass therefore becomes the diffusion of gas molecules through porewater. The accurate description of this process requires non-Fickian diffusion to be accounted for, including both thermal diffusion and gravitational action. We evaluate the diffusive flux of aqueous methane considering non-Fickian diffusion and predict the existence of extensive bubble mass accumulation zones within deep-water sediments. The limitation on the hydrate deposit capacity is revealed; too weak deposits cannot reach the base of the hydrate stability zone and form any bubbly horizon.