Adsorption Of Atmospheric Gases Research Articles

First-principles and machine learning modeling on adsorption of atmospheric gases on two-dimensional Ruddlesden–Popper halide perovskite surface

The interactions between the atmospheric gases and the halide perovskite materials are receiving attention in these years before the extensive industrial deployment of halide perovskite materials. In this manuscript, we combine first-principles calculation and machine learning techniques to evaluate the interactions between the atmospheric gas molecules and a two-dimensional Ruddlesden–Popper halide perovskite Cs2PbBr4 surface based on the adsorption energies and automatically design advanced molecular descriptors for the target output. The impacts of density functionals are considered while an accurate machine learning model (r = 0.954 and R 2 = 0.951) is obtained based on the XGBRF ensemble algorithm. Importantly, the symbolic regression automatically finds an effective hybrid descriptor that exhibits high correlation with the target output that is comparable with the machine learning model; the symbolic regression-derived descriptor is mathematically simple and chemistry-aware, which complements the debatable ‘black-box’ machine learning model. Both feature importance ranking and symbolic regression indicate the importance of the functional-dependent energy levels of the perovskite systems and the amide/hydroxyl functional groups of the molecules. The present study highlights the viability of combining density functional theory and machine learning techniques to model the low-dimensional perovskite structures under the atmospheric conditions.

Ferromagnetism in Defected TMD (MoX2, X = S, Se) Monolayer and Its Sustainability under O2, O3, and H2O Gas Exposure: DFT Study.

Spin-polarized density-functional theory (DFT) has been employed to study the effects of atmospheric gases on the electronic and magnetic properties of a defective transition-metal dichalcogenide (TMD) monolayer, MoX2 with X = S or Se. This study focuses on three single vacancies: (i) molybdenum "VMo"; (ii) chalcogenide "VX"; and (iii) di-chalcogenide "VX2". Five different samples of sizes ranging from 4 × 4 to 8 × 8 primitive cells (PCs) were considered in order to assess the effect of vacancy-vacancy interaction. The results showed that all defected samples were paramagnetic semiconductors, except in the case of VMo in MoSe2, which yielded a magnetic moment of 3.99 μB that was independent of the sample size. Moreover, the samples of MoSe2 with VMo and sizes of 4 × 4 and 5 × 5 PCs exhibited half-metallicity, where the spin-up state becomes conductive and is predominantly composed of dxy and dz2 orbital mixing attributed to Mo atoms located in the neighborhood of VMo. The requirement for the establishment of half-metallicity is confirmed to be the provision of ferromagnetic-coupling (FMC) interactions between localized magnetic moments (such as VMo). The critical distance for the existence of FMC is estimated to be dc≅ 16 Å, which allows small sample sizes in MoSe2 to exhibit half-metallicity while the FMC represents the ground state. The adsorption of atmospheric gases (H2O, O2, O3) can drastically change the electronic and magnetic properties, for instance, it can demolish the half-metallicity characteristics. Hence, the maintenance of half-metallicity requires keeping the samples isolated from the atmosphere. We benchmarked our theoretical results with the available data in the literature throughout our study. The conditions that govern the appearance/disappearance of half-metallicity are of great relevance for spintronic device applications.

The modulation of terahertz photoconductivity in CVD grown n-doped monolayer MoS2 with gas adsorption

The as-grown atomically thin transition metal dichalcogenides (TMDs) usually have extremely large surface-to-volume ratios. Hence, the electrical and optical properties are always affected by the adsorption of atmospheric gases. By using optical pump terahertz (THz) probe spectroscopy, THz photoconductivity dynamics of chemical vapor deposition grown monolayer MoS2 has been investigated in nitrogen, air and oxygen atmospheric environments. Our study reveals that the photoconductivity of MoS2 at THz frequencies is dramatically altered by the adsorption of oxygen. The relaxation dynamics of photoconductivity strongly depends on atmospheres, which is attributed to photo-excited different quasi-particles, such as free charge carriers, excitons and trions. This study highlights the role of oxygen passivation of the photoconductivity in monolayer MoS2, which may have important applications for the design of future MoS2-based electronic devices.

Effect of Pre-Irradiation Annealing and Laser Modification on the Formation of Radiation-Induced Surface Color Centers in Lithium Fluoride

It is shown that surface color centers of the same type are formed in the surface layer and in regions with damaged crystal structure inside crystalline lithium fluoride after γ-irradiation. Results are presented from a study of the effect of pre-irradiation annealing on the efficiency with which surface centers are formed in lithium fluoride nanocrystals. Raising the temperature for pre-irradiation annealing from room temperature to 250°C leads to a substantial reduction in the efficiency with which these centers are created. Surface color centers are not detected after γ-irradiation for pre-irradiation annealing temperatures of 300°C and above. Adsorption of atmospheric gases on the crystal surface cannot be regarded as a necessary condition for the formation of radiation-induced surface centers.

Adsorption of Atmospheric Gases on Pu Surfaces

ABSTRACTSurface adsorption represents a competition between collision and scattering processes that depend on surface energy, surface structure and temperature. The surface reactivity of the actinides can add additional complexity due to radiological dissociation of the gas and electronic structure. Here we elucidate the chemical bonding of gas molecules adsorbed on Pu metal and oxide surfaces. Atmospheric gas reactions were studied at 190 and 300 K using x-ray photoelectron spectroscopy. Evolution of the Pu 4f and O 1s core-level states were studied as a function of gas dose rates to generate a set of Langmuir isotherms. Results show that the initial gas dose forms Pu2O3 on the Pu metal surface followed by the formation of PuO2 resulting in a layered oxide structure. This work represents the first steps in determining the activation energy for adsorption of various atmospheric gases on Pu.

Extreme sensitivity of graphene photoconductivity to environmental gases

Graphene is a single layer of covalently bonded carbon atoms, which was discovered only 8 years ago and yet has already attracted intense research and commercial interest. Initial research focused on its remarkable electronic properties, such as the observation of massless Dirac fermions and the half-integer quantum Hall effect. Now graphene is finding application in touch-screen displays, as channels in high-frequency transistors and in graphene-based integrated circuits. The potential for using the unique properties of graphene in terahertz-frequency electronics is particularly exciting; however, initial experiments probing the terahertz-frequency response of graphene are only just emerging. Here we show that the photoconductivity of graphene at terahertz frequencies is dramatically altered by the adsorption of atmospheric gases, such as nitrogen and oxygen. Furthermore, we observe the signature of terahertz stimulated emission from gas-adsorbed graphene. Our findings highlight the importance of environmental conditions on the design and fabrication of high-speed, graphene-based devices.

Interpretation of Transient Photocurrents in Coplanar and Sandwich PIN Microcrystalline Silicon Structures

ABSTRACTWe report on the use of coplanar transient photoconductivity and post-transit time-of-flight spectroscopy techniques in the study of carrier transport in microcrystalline silicon films prepared over a range of crystallinities. Coplanar samples are susceptible to post-deposition oxidation and reversible adsorption of atmospheric gases, which may alter the apparent density of states. Coplanar measurements suggest lower deep defect densities in more highly crystalline films, but this is due at least in part to an increased occupancy of these states. A comparison of results obtained using both techniques suggests anisotropic transport, with reduced band tailing (greater structural order) along the direction of film growth, a larger defect concentration around the column boundaries, and a higher defect density within the amorphous tissue than in optimised single-component amorphous silicon films.

Adsorption of Atmospheric Gases at the Air−Water Interface. 4: The Influence of Salts

We have measured the standard free energies and standard enthalpies of adsorption as well as saturated surface coverages for hexanoic acid and 1-propanol adsorbed at the air−aqueous interface of sodium chloride (NaCl) and ammonium sulfate ((NH4)2SO4) solutions of concentrations between 0 and 4 mol L-1. Temperature-dependent surface tension measurements are used to obtain ΔG0aq→σ and ΔH0aq→σ; using known Henry's law constants and estimated values of the salting-out coefficient we calculate the gas−interface adsorption parameters ΔG0g→σ and ΔH0g→σ. A small (∼1−2 kJ mol-1) decrease in ΔG0aq→σ with increasing salt concentration is accompanied by an increase (∼10−30%) in saturated surface coverage, consistent with the expected “salting-out” behavior. Adsorption from the gas phase becomes somewhat less favorable with increasing salt concentration for both organic species. The standard enthalpy of adsorption from the gas phase decreases with increasing salt concentration for hexanoic acid, but not for 1-propanol. This may be a consequence of acid dimer formation at the interface becoming important at higher coverages.

Structural effects on adsorption of atmospheric gases in mixed Li,Ag–X‐zeolite

AbstractSilver is known to strongly affect the adsorptive properties of zeolites. In the synthesis of mixed Li,Ag low‐silica X‐type (LSX) zeolite, adding very small amounts of silver and specific dehydration conditions results in a sorbent with enhanced adsorptive characteristics for air separation. The location of the extraframework silver in relation to the aluminosilicate framework is of primary importance for elucidating the effect of silver cations on the adsorptive characteristics of the zeolite. In this work, mixed Li,Ag ion‐exchanged zeolites were synthesized and treated to promote the formation of intracrystalline silver clusters. These samples were structurally characterized using Rietveld refinement of neutron powder diffraction data. Structural characterization revealed the presence of cations in a novel site II* in mixed Li,Ag–LSX zeolites that were vacuum‐dehydrated at 450°C. Cations in this site II* are more interactive with the atmospheric sorbates of interest than silver at the conventional site II location. Vacuum dehydration at 450°C induced thermal migration of Ag+ from site II to site II* and gives rise to the superior properties for air separation.

Adsorption of Atmospheric Gases at the Air−Water Interface. 3: Methylamines

The surface-bound state of methylamines adsorbed at the air−water boundary has been studied using a combination of equilibrium surface tension measurements and ab initio quantum calculations. Methylamine, dimethylamine, and trimethylamine all adsorb at the water surface, with saturated coverages roughly corresponding to the average concentration of “free” surface hydrogens. The standard enthalpies of adsorption from the gas phase show a correlation with the standard enthalpies of solvation, implying that partial solvation takes place at the air−water boundary. Comparison of experimental adsorption enthalpies and ab initio amine−water binding enthalpies suggests that the binding at the surface consists primarily of a single hydrogen bond to a surface water molecule possessing a “free” hydrogen.

Influence of Residual Water on the Adsorption of Atmospheric Gases in Li−X Zeolite: Experiment and Simulation

Li−LSX zeolite (Si/Al = 1.0) is currently the best sorbent in use in the separation of air by adsorption processes. In this present work we have synthesized fully exchanged Li−LSX zeolite and measu...

817. The adsorption of atmospheric gases on molecular sieves at low pressures and temperatures. The effect of preadsorbed water: S A Stern and F S Di Paolo, J vac sci technol, 4 (6), Nov/Dec 1967, 347–355.

817. The adsorption of atmospheric gases on molecular sieves at low pressures and temperatures. The effect of preadsorbed water: S A Stern and F S Di Paolo, J vac sci technol, 4 (6), Nov/Dec 1967, 347–355.

The Adsorption of Atmospheric Gases on Molecular Sieves at Low Pressures and Temperatures. The Effect of Preadsorbed Water

The adsorption of nitrogen, helium, neon, and air on Molecular Sieves 5A, 10X, and 13X was studied at 77.3 °K and in the pressure range from 102 to 10−6 Torr. The capacities of freshly activated adsorbent samples for nitrogen were found to increase markedly below 10−3 Torr after consecutive adsorption-desorption cycles. This “conditioning effect” may explain the large discrepancies in the data of previous investigators. The nitrogen isotherms can be represented above 10−2 to 10−4 Torr by either the Freundlich or the Dubinin–Radushkevich equations, while the helium and neon isotherms obey Henry’s law. The effect of preadsorbed water is to decrease the adsorptive capacity of Molecular Sieve 5A for nitrogen; when the amount of water exceeds 7 wt%, the capacity for nitrogen is almost completely suppressed. It is suggested that this behavior is due to the migration of hydrated cations, which block the apertures to the cube octahedral cavities of the adsorbent.

Effect of Temperature on Photo-Exoemission

Measurements have been made with a Geiger counter of photo-exoemission from aluminum. The effects of time, temperature, and various adsorbates on the electron emission rate were measured. Adsorption reduces the electron emission rate; ion bombardment within the counter is shown to remove the effects of adsorbed gases. Adsorption of atmospheric gases can explain previously reported decay in emission and temperature effects.