Selective Gas Research Articles

Cu‐MOF‐74 as Fluorescent Probe: Selective Optical Sensor for H2S Gas

AbstractIn order to address the air quality issues and industrial gas leakages effectively, gas sensors must be highly sensitive, selective, stable, cost‐effective, and with high life time. We present here a facile solvothermal method to synthesize a Cu based metal‐organic framework (Cu‐MOF‐74) and demonstrated its potential as a selective fluorescent probe for environmental H2S gas detection. Cu‐MOF‐74 based fluorescence sensor shows strong affinity towards H2S with the limit of detection as low as 7 ppm., Though Cu‐MOF‐74 based sensor exhibited high sensitivity and selectivity for H2S gas over H2, SO2 and NH3, Co‐MOF‐74 is inactive towards H2S sensing. The increased fluorescence intensity in presence of H2S gas as well as the decreased fluorescence intensity in presence of NO2 gas was investigated by experimental observation and theoretical calculation. In presence of H2S gas, interaction between Cu and sulphur atoms occur, the bond between Cu and ligand. This reduces the extent of ligand to metal charge transfer, resulting in increase in fluorescence intensity. The present attempt led to the progress of highly selective and sensitive fluorescence‐based environmental gas sensing probe using metal organic framework with a wide operating temperature ranging of 25 °C to 90 °C.

Bilayer nanographene reveals halide permeation through a benzene hole.

Graphene is a single-layered sp2-hybridized carbon allotrope, which is impermeable to all atomic entities other than hydrogen1,2. The introduction of defects allows selective gas permeation3-5; efforts have been made to control the size of these defects for higher selectivity6-9. Permeation of entities other than gases, such as ions10,11, is of fundamental scientific interest because of its potential application in desalination, detection and purification12-16. However, a precise experimental observation of halide permeation hasso far remained unknown11,15-18. Here we show halide permeation through a single benzene-sized defect in a molecular nanographene. Using supramolecular principles of self-aggregation, we created a stable bilayer of the nanographene19-23. As the cavity in the bilayer nanographene could be accessed only by two angstrom-sized windows, any halide that gets trapped inside the cavity has to permeate through the single benzene hole. Our experiments reveal the permeability of fluoride, chloride and bromide through a single benzene hole, whereas iodide is impermeable. Evidence for high permeation of chloride across single-layer nanographene and selective halide binding in a bilayer nanographene provides promise for the use of single benzene defects in graphene for artificial halide receptors24,25, as filtration membranes26 and further to create multilayer artificial chloride channels.

Simultaneously Enhanced Permeability and Selectivity of Pebax-1074-Based Mixed-Matrix Membrane for CO2 Separation

Membrane technology is a promising methodology for carbon dioxide separation due to its benefit of a small carbon footprint. However, the trade-off relationship between gas permeability and selectivity is one obstacle to limiting its application. Herein, branched polyethyleneimine (BPEI) containing a rich amino group was successfully grafted on the surface of the metal–organic framework (MOF) of AIFFIVE-1-Ni (KAUST-8) through coordination between N in BPEI and open metal sites in the MOF and with the resultant maintained BET surface area and pore volume. Both the strengthened CO2 solubility coefficients comig from the additional CO2 adsorption sites of amino groups in BPEI and the reinforced CO2 diffusivity coefficients originating from the fast transport channels created by KAUST-8 led to the promising CO2 separation performance for KAUST-8@BPEI/Pebax-1074 MMM. With 5 wt.% KAUST-8@BPEI loading, the MMM showed the CO2 permeability of 156.5 Barrer and CO2/N2 selectivity of 16.1, while the KAUST-8-incorporated MMM (5 wt.% loading) only exhibited the CO2 permeability of 86.9 Barrer and CO2/N2 selectivity of 13.0. Such enhancement is superior to most of the reported Pebax-1074-based MMMs for CO2 separation indicating a wide application for the coordination method for MOF fillers with open metal sites.

Enhancing CO2 Oversaturation in the Confined Water Enables Superior Gas Selectivity of 2D Membranes.

Due to the global demands on carbon neutralization, CO2 separation membranes, particularly those based on two-dimensional (2D) materials, have attracted increasing attention. However, recent works have focused on the chemical decoration of membranes to realize the selective transport, leading to the compromised stability in the presence of moisture. Herein, we develop a series of 2D capillaries based on layered double hydroxide (LDH), graphene oxide, and vermiculite to enhance the oversaturation of CO2 in the confined water for promoting the membrane permselectivity. By employing the dielectric spectroscopy as a probe to unveil oversaturation, the dissolved CO2 can be enhanced by up to ten times facilitated by water confined in the 2D capillary, particularly constructed by the LDH, endowing the uprise of CO2/N2 separation factor by 43 times. Therefore, our work opens an avenue to the future design of selective membranes by modulating the confined water beyond chemical modification.

Enhancing Selectivity of Two‐Dimensional Materials‐Based Gas Sensors

AbstractTwo‐dimensional (2D) materials have emerged as promising candidates for gas sensing applications due to their exceptional electrical, structural, and chemical properties, which enable high sensitivity and rapid response to gas molecules. However, despite their potential, 2D material‐based gas sensors face a significant challenge in achieving adequate selectivity, as many sensors respond similarly to multiple gases, leading to cross‐sensitivity and inaccurate detection. This review provides a comprehensive overview of the recent advancements for improving the selectivity of 2D gas sensors. It explores material modification strategies, such as functionalizing the sensing components and tuning adsorption dynamics, to enhance selective gas interactions. Engineering approaches, including field‐effect modulation and sensor array design, are also discussed as effective methods to fine‐tune sensor performance. Additionally, the integration of machine learning (ML) algorithms is highlighted for their potential to differentiate among multiple analytes. Prospects for further improving selectivity through material optimization, sensor calibration, and drift compensation are explored, along with the incorporation of smart sensing systems into the Internet of Things (IoT). This review outlines key objectives and strategies that pave the way for next‐generation gas sensors with enhanced selectivity, reliability, and versatility, poised to impact a wide range of applications from environmental monitoring to industrial safety.

Sub-5 Ångstrom Porosity Tuning in Calixarene-Derived Porous Liquids via Supramolecular Complexation Construction.

Precise sub-Ångstrom-level porosity engineering, which is appealing in gas separations, has been demonstrated in solid carbon, polymer, and framework materials but rarely achieved in the liquid phase. In this work, a gas molecular sieving effect in the liquid phase at sub-5 Ångstrom scale is created via sophisticated porosity tuning in calixarene-derived porous liquids (PLs). Type II PLs are constructed via supramolecular complexation between the sodium salts of calixarene derivatives and crown ether solvents. The chemical structure variation and assembly behavior of the porous host upon PL construction are monitored by spectroscopy-, X-ray-, and neutron-scattering techniques. The presence of permanent porosity in calixarene-derived PLs is verified by pressure swing gas uptake, altered CO2 physisorption behavior, and dynamic theoretical simulation. Sub-5 Ångstrom porosity tuning within the PL phase is achieved by introducing bulky substituted groups on the benzene ring of the calixarene host, which then greatly affects the dynamic motion and transport behavior of CO2 molecules and the Xe uptake performance. The approach being demonstrated in this work represents a promising pathway to tune and leverage the porosity effect for enhanced gas uptake capacity and selectivity in liquid sorbents.

Atomically distributed Al-F3 nanoparticles towards precisely modulating pore size of carbon membranes for gas separation

To confront the energy consumption, high performance membrane materials are urgently needed. Carbon molecular sieve (CMS) membranes exhibit superior capability in separating gas mixtures efficiently. However, it remains a grand challenge to precisely tune the pore size and distribution of CMS membranes to further improve their molecular sieving properties. Herein, we report an approach of finely modulating CMS pore structure by using the reactive Al(CH3)3 to in situ defluorinate the polymer precursor to form Al-Fx(CH3)3-x in the polymer matrix, which is further converted to atomic-level Al2O3 and Al-F3 in the polymer matrix. These nanoparticles play the key role in regulating the pore size of CMS membranes by suppressing the formation of unfavorable large pores during pyrolysis, thus enhancing the gas selectivity considerably. The resultant CMS membranes demonstrate a H2/CH4 and CO2/CH4 selectivity of 192.6, and 58.4, respectively, 128% and 93% higher than the untreated samples, residing far above the latest upper bounds.

Selective gas phase pulsed etching of oxides with NbCl5

NbCl5 was found to be a highly selective vapor phase etchant for Ta2O5, TiO2 and ZrO2. The mechanism of Ta2O5 etching was proven with thermodynamic and QMS studies, and selectivity was demonstrated on a patterned Al2O3/Ta2O5 double layer.

Microporous covalent organic frameworks via flexible units and rich heteroatoms for gas uptake and radioactive iodine uptake

Imine-linked FM-COFs, featuring nitrogen-enriched microporous architectures, exhibit superior host-guest interactions and adsorption performance, achieving CO2 uptake of 82.6 cm3 g−1 (273 K) and iodine capacity of 3.98 g g−1 (350 K), excelling in selective gas capture and radionuclide sequestration.

A Scalable Pore-space-partitioned Metal-organic Framework Powered by Polycatenation Strategy for Efficient Acetylene Purification.

Efficient separation of acetylene (C2H2) from carbon dioxide (CO2) and ethylene (C2H4) is a significant challenge in the petrochemical industry due to their similar physicochemical properties. Pore space partition (PSP) has shown promise in enhancing gas adsorption capacity and selectivity by reducing pore size and increasing the density of guest binding sites. Herein, we firstly employ the 2D→3D polycatenation strategy to construct a PSP metal-organic framework (MOF) Ni-dcpp-bpy, incorporating functional N/O sites to enhance C2H2 purification. The polycatenated framework with optimized pore size and regularity, exhibiting significant improvements over traditional PSP MOFs by resolving the critical contradiction of balancing C2H2 uptake (98.5 cm3 g-1 at 298 K, 100 kPa) and selectivity of C2H2/CO2 (3.4), C2H2/C2H4 (5.9), and C2H2/CH4 (96.4) in a MOF. Breakthrough experiments confirm high-purity C2H4 (>99.9 %) and high C2H2 productivity from binary and ternary mixtures. Notably, Ni-dcpp-bpy exhibits excellent water stability, scalability, and regenerability after 20 cycles for separating C2H2/CO2. Theoretical calculations verify that the strong binding of C2H2 is mainly attributed to the C-H⋅⋅⋅O/N interactions between host Ni-dcpp-bpy and guest C2H2 molecules. The polycatenation strategy not only improved industrial C2H2 purification efficiency but also enriched the design diversity of customized MOFs for other gas separation applications.

Pt-Decorated ZnO Nanorods for Light-Assisted Ethanol Sensing and In Situ Analysis of the Sensing Mechanism under Light Irradiation.

ZnO nanorods have attracted much attention owing to their outstanding properties for chemical gas sensors. Although they show greater sensing properties than conventional nanoparticulate ZnO, high operation temperature (>250-350 °C) is required for them to work even if precious metals are deposited on them to sensitize their sensing properties. Light irradiation is one solution for overcoming the high operation temperature and the gas selectivity because it assists the oxidation activity on the surface that affects the sensor response. In this work, the sensing properties of Pt/ZnO nanorods and ZnO nanorods are examined under light irradiation, and the relationship between their sensing properties and surface reaction (ethanol oxidation) is elucidated. Pt/ZnO nanorods show selective sensor responses to ethanol (conditions: 150 °C, 50 ppm ethanol; sensor response, 843; response time, 4.0 min; recovery time, 22 min). In situ spectroscopic observations reveal that the largest amount of oxidation intermediates (acetate species) and oxidation products (CO2 and acetaldehyde) is confirmed during light irradiation. The oxidation reaction of ethanol is facilitated by the deposition of Pt and light irradiation. Thus, the operation temperature of ZnO nanorods decreases, and the selectivity to ethanol is enhanced.

Bi2Sn2O7 Overlayer Assists Bilayer Chemiresistor in Humidity-Independent and Highly Selective Detection of Expiratory Acetone.

Constructing a bilayer structure has not been reported as a method to mitigate the adverse effect of water poisoning on oxide chemiresistors while simultaneously enhancing gas selectivity and sensitivity. To address this challenge, pyrochlore-Bi2Sn2O7 has been first utilized as an overlayer on a ZnO sensing layer for constructing a bilayer acetone chemiresistor, leading to remarkable improvement in the performance for trace-level (500 p-p-b) acetone detection under high humidity (80% relative humidity). In addition, owing to the catalytic predecompositions of ethanol across the overlayer, an outstanding acetone gas selectivity (Sacetone/Sethanol = 2.9) has been achieved, with a more than 4-fold improvement compared with monolayer ZnO chemiresistor (Sacetone/Sethanol = 0.7). More significantly, comprehensive experiments coupled with in situ characterizations have verified the generation of hydroxyl radicals (•OH) on the Bi2Sn2O7 overlayer. These radicals are capable of enhancing the kinetics between •OH and acetone, reducing the activation energy required for the gas sensing reaction, and thus leading to an unexpected phenomenon of enhanced acetone sensitivity under high humid conditions (Sacetoneat80%RH > Sacetoneat5%RH). These demonstrations offer crucial insight into the precise design of highly efficient overlayers for breath sensing.

Highly Selective and Extensive Range Room Temperature Hydrogen Gas Sensor Based on Pd-Mg Alloy Thin Films

Highly Selective and Extensive Range Room Temperature Hydrogen Gas Sensor Based on Pd-Mg Alloy Thin Films

A Fluorine-Functionalized Tb(III)-Organic Framework for Ba2+ Detection.

The development of lanthanide-organic frameworks (Ln-MOFs) using for luminescence sensing and selective gas adsorption applications is of great significance from an energy and environmental perspective. This study reports the solvothermal synthesis of a fluorine-functionalized 3D microporous Tb-MOF with a face-centered cubic (fcu) topology constructed from hexanuclear clusters (Tb6O30) bridged by fdpdc ligands, formulated as {[Tb6(fdpdc)6(μ3-OH)8(H2O)6]·4DMF}n (1), (fdpdc = 3-fluorobiphenyl-4,4'-dicarboxylate). Complex 1 displays a 3D framework with the channel of 7.2 × 7.2 Å2 (measured between opposite atoms) perpendicular to the a-axis. With respect to Ba2+ cation, the framework of activated 1 (1a) exhibits high selectivity and reversibility in luminescence sensing function, with an LOD of 4.34665 mM. According to the results of simulations, compared to other small gas molecules (CO2, N2, H2, CO, and CH4), activated 1 (1a) shows a high adsorption selectivity for C2H2 at 298 K.

Heterogeneity in Diameters of Protein Fibrils and Chitosan for a High CO2/O2 Selectivity and Desired Mechanical Properties of Edible Bioplastic Films.

Edible bioplastics offer a possible approach with great potential to address the challenges of plastic and microplastic crisis and the issue of food waste. In the present study, in situ, induction of fibrillation of legume proteins within chitosan matrix is found to form edible bioplastic films with a high CO2/O2 selectivity coefficient of ≈130. Desirable mechanical properties, including a high elongation at break of ≈230%, are realized to fabricate shopping bags, which can be filled with 5kg of fruits. An atmosphere with a high CO2 content and low O2 concentration is spontaneously achieved after strawberries are packaged with the films and ≈85% of the fruits are edible after 9 days. The contents of CO2 in the equilibrium state and the percent of edible fruits are positively correlated with the CO2/O2 selectivity of the films. By comparison of the edible bioplastic films made from mung bean protein (MP) and pea protein (PP), the fibrillation capability of legume proteins and the heterogeneity in polydisperse diameters of the protein fibrils and chitosan are demonstrated to make great contributions to the dense microstructure, leading to the strong mechanical properties and high gas selectivity of the films. The structure-property relationship is elucidated for high-performance bioplastics films.

A Cone-Like TSV Structured ZnO NO2 Gas Sensor that is Produced Using Room-Temperature Laser Drilling and Radio-Frequency Sputtering

A typical through-silicon via (TSV) has vertical and high aspect ratio holes, making special equipment required to deposit the film on the vertical hole walls. This study used room-temperature laser drilling to fabricate a cone-like TSV structure so that no special process equipment was required. Radio-frequency sputtering was used to deposit SiNx, ITO, and ZnO films on the cone-like TSV structure to form a ZnO NO2 gas sensor. Scanning electron microscopy and focused ion beam results show that the ZnO film connects the front and back sides of the substrate, enabling sensing on both sides. If the concentration of NO2 gas is increased, the sensor response increases and the average sensor response is 34,7%, with an inaccuracy of <± 1% for five 5 consecutive measurements using a NO2 concentration of 3ppm. Reactions with NH3, CO, CO2, and SO2 gases are almost insignificant, giving the cone-like TSV structure ZnO gas sensor good selectivity, stability, and reproducibility for NO2 gas.

Selective Gas Adsorption in Permanently Microporous Coordination Cages with Exposed Metal Sites.

Porous coordination cages (PCCs), molecular analogs of metal-organic frameworks, offer modular platforms for studying the adsorption properties of small molecules, with coordinatively unsaturated metal centers playing a pivotal role in tuning these behaviors. In this work, we present the synthesis, activation, and detailed gas adsorption studies of second-row transition metal-based M24L24 cuboctahedral cages, specifically Mo24(bdc)24, Rh24(bdc)24, and [Ru24(bdc)24]Cl12. These materials represent rare examples of Mo-, Rh-, and Ru-based hybrid porous solids. The synthesis and activation of these cages were optimized to maximize porosity, yielding BET surface areas of up to 832 m2/g. Gas adsorption studies with CO2 and CO reveal distinctive uptake behaviors linked to the metal cations, with Mo24(bdc)24 demonstrating the highest gravimetric CO2 uptake (2.12 mmol/g at 298 K) and [Ru24(bdc)24]Cl12 exhibiting the strongest CO binding (-75 kJ/mol). Additionally, we explore the selective adsorption of unsaturated hydrocarbons, such as ethylene and propylene, revealing strong binding interactions at low pressures as a result of strong metal-hydrocarbon interactions based on pi-backbonding interactions.

Effect of the Chemical Structure of a Self-Assembled Monolayer on the Gas-Sensing Behavior of SnO2 Nanowires.

In this study, detailed investigations of the selective sensing capability of semiconducting metal oxide (SMO)-based gas sensors with self-assembled monolayer (SAM) functionalization were conducted. The selective gas-sensing behavior was improved by employing a simple and straightforward postmodification technique using functional SAM molecules. The chemical structure of the SAM molecules promoted interaction between the gas and SAM molecules, providing a gas selective sensing of SnO2 nanowires (NWs). In addition, a bundle of SnO2 NWs provided a large surface area that could act as a sensing site. SAM functionalization was confirmed by infrared spectroscopy and thermogravimetric analysis, and the selective gas-sensing behaviors were investigated under different sensing conditions. With variations in the chemical structures of the SAM molecules, the selective gas-sensing behaviors also changed as the corresponding intermolecular interaction forces were different. This integration of selective gas sensing and passivation of a sensing surface provides a straightforward approach for the preferred gas sensing of SnO2 NWs. Furthermore, owing to the universal binding characteristics of SAM molecules to the metal oxide surface, this approach can be expanded to other SMO-based gas-sensing platforms.

Tailoring pores of AFN-related zeolites for enhanced separation of CO2/N2 and CO2/CH4

Zeolites with small-pores (8-ring) are widely used in separation of small-molecule gas, however, the precise tuning of aperture sizes to achieve the ideal selectivity still remains challenge due to their limited topology structures. In this work, SAPO-14 zeolite with AFN topology featuring by different 8-ring windows in three-dimension directions has been synthesized and studied in the separation of CO2/N2 or CO2/CH4. Aiming to further adjust pores and channels of the adsorbents, the composite zeolites of SAPO-14 and SAPO-34 (CHA) are prepared by seed-assisted method, and multi-cycle crystallization is adopted to form the interconnected pores of composites and reduce the crystal size. SAPO-14/SAPO-34 zeolites exhibit the comprehensive performance of high CO2 capacity of SAPO-34 and good selectivity of SAPO-14 due to their appropriate pore structure. At 298 K, the CO2 capacity of S-1 composite is 47.24 cm3 g−1, giving a high gas selectivity for CO2/N2 (68.64) and CO2/CH4 (21.61). The highly-efficiency and good reproducibility in gas separation are verified by the binary breakthrough experiments. The strategy of composite zeolite proposed by this work can effectively regulate the pores of adsorbent, thus endow them acceptable selectivity and capacity for target gas.

Fabrication of SnS2/Si Heterostructure for Ultra-High Selective and Rapid Room Temperature NO2 Gas Detection with Enhanced Carrier Mobility

Fabrication of SnS2/Si Heterostructure for Ultra-High Selective and Rapid Room Temperature NO2 Gas Detection with Enhanced Carrier Mobility