The sensing performances of gas sensors must be improved and diversified to enhance quality of life by ensuring health, safety, and convenience. Metal-organic frameworks (MOFs), which exhibit an ...extremely high surface area, abundant porosity, and unique surface chemistry, provide a promising framework for facilitating gas-sensor innovations. Enhanced understanding of conduction mechanisms of MOFs has facilitated their use as gas-sensing materials, and various types of MOFs have been developed by examining the compositional and morphological dependences and implementing catalyst incorporation and light activation. Owing to their inherent separation and absorption properties and catalytic activity, MOFs are applied as molecular sieves, absorptive filtering layers, and heterogeneous catalysts. In addition, oxide- or carbon-based sensing materials with complex structures or catalytic composites can be derived by the appropriate post-treatment of MOFs. This review discusses the effective techniques to design optimal MOFs, in terms of computational screening and synthesis methods. Moreover, the mechanisms through which the distinctive functionalities of MOFs as sensing materials, heterostructures, and derivatives can be incorporated in gas-sensor applications are presented.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Abstract
Formaldehyde, a probable carcinogen, is a ubiquitous indoor pollutant, but its highly selective detection has been a long-standing challenge. Herein, a chemiresistive sensor that can detect ...ppb-level formaldehyde in an exclusive manner at room temperature is designed. The TiO
2
sensor exhibits under UV illumination highly selective detection of formaldehyde and ethanol with negligible cross-responses to other indoor pollutants. The coating of a mixed matrix membrane (MMM) composed of zeolitic imidazole framework (ZIF-7) nanoparticles and polymers on TiO
2
sensing films removed ethanol interference completely by molecular sieving, enabling an ultrahigh selectivity (response ratio > 50) and response (resistance ratio > 1,100) to 5 ppm formaldehyde at room temperature. Furthermore, a monolithic and flexible sensor is fabricated successfully using a TiO
2
film sandwiched between a flexible polyethylene terephthalate substrate and MMM overlayer. Our work provides a strategy to achieve exclusive selectivity and high response to formaldehyde, demonstrating the promising potential of flexible gas sensors for indoor air monitoring.
The highly selective detection of trace gases using transparent sensors at room temperature remains challenging. Herein, transparent nanopatterned chemiresistors composed of aligned 1D Au–SnO2 ...nanofibers, which can detect toxic NO2 gas at room temperature under visible light illumination is reported. Ten straight Au–SnO2 nanofibers are patterned on a glass substrate with transparent electrodes assisted by direct‐write, near‐field electrospinning, whose extremely low coverage of sensing materials (≈0.3%) lead to the high transparency (≈93%) of the sensor. The sensor exhibits a highly selective, sensitive, and reproducible response to sub‐ppm levels of NO2, and its detection limit is as low as 6 ppb. The unique room‐temperature NO2 sensing under visible light emanates from the localized surface plasmonic resonance effect of Au nanoparticles, thereby enabling the design of new transparent oxide‐based gas sensors without external heaters or light sources. The patterning of nanofibers with extremely low coverage provides a general strategy to design diverse compositions of gas sensors, which can facilitate the development of a wide range of new applications in transparent electronics and smart windows wirelessly connected to the Internet of Things.
Transparent and visible light‐activated NO2 sensor that can operate at room temperature is presented. The pattern of Au–SnO2 nanofibers with extremely low coverage fabricated by direct‐write near‐field electrospinning exhibits high transparency (≈93%), ultrahigh response to NO2, and reversible sensing behaviors under visible light or natural sunlight, enabling the ppb‐level monitoring of indoor or outdoor NO2.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Volatile aromatic compounds are major air pollutants, and their health impacts should be assessed accurately based on the concentration and composition of gas mixtures. Herein, novel bilayer sensors ...consisting of a SnO2 sensing layer and three different xRh‐TiO2 catalytic overlayers (x = 0.5, 1, and 2 wt%) are designed for the new functionalities such as the selective detection, discrimination, and analysis of benzene, toluene, and p‐xylene. The 2Rh‐TiO2/SnO2 bilayer sensor shows a high selectivity and response toward ppm‐ and sub‐ppm‐levels of benzene over a wide range of sensing temperatures (325–425 °C). An array of 0.5Rh‐, 1Rh‐, and 2Rh‐TiO2/SnO2 sensors exhibits discrimination and composition analyses of aromatic compounds. The conversion of gases into more active species at moderate catalytic activation and the complete oxidation of gases into non‐reactive forms by excessive catalytic promotion are proposed as the reasons behind the enhancement and suppression of analyte gases, respectively. Analysis using proton transfer reaction‐quadrupole mass spectrometer (PTR‐QMS) is performed to verify the above proposals. Although the sensing characteristics exhibit mild moisture interference, bilayer sensors with systematic and tailored control of gas selectivity and response provide new pathways for monitoring aromatic air pollutants and evaluating their health impacts.
Bilayer sensors with tunable Rh‐TiO2 catalytic overlayers have been proposed for highly selective detection of benzene and discrimination of volatile aromatic compounds. The sensors and sensors array in this study will open up a new avenue for accurate indoor air quality monitoring, gas leak detection in the petroleum industry, and assessment of air pollution in congested areas and gas stations.
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Metal‐organic frameworks (MOFs) are promising materials for gas sensing but are often limited to single‐use detection. A hybridization strategy is demonstrated synergistically deploying conductive ...MOFs (cMOFs) and conductive polymers (cPs) as two complementary mixed ionic‐electronic conductors in high‐performing stand‐alone chemiresistors. This work presents significant improvement in i) sensor recovery kinetics, ii) cycling stability, and iii) dynamic range at room temperature. The effect of hybridization across well‐studied cMOFs is demonstrated based on 2,3,6,7,10,11‐hexahydroxytriphenylene (HHTP) and 2,3,6,7,10,11‐hexaiminotriphenylene (HITP) ligands with varied metal nodes (Co, Cu, Ni). A comprehensive mechanistic study is conducted to relate energy band alignments at the heterojunctions between the MOFs and the polymer with sensing thermodynamics and binding kinetics. The findings reveal that hole enrichment of the cMOF component upon hybridization leads to selective enhancement in desorption kinetics, enabling significantly improved sensor recovery at room temperature, and thus long‐term response retention. This mechanism is further supported by density functional theory calculations on sorbate–analyte interactions. It is also found that alloying cPs and cMOFs enables facile thin film co‐processing and device integration, potentially unlocking the use of these hybrid conductors in diverse electronic applications.
Described in this study is a hybridization strategy combining conductive metal‐organic frameworks (MOFs) and polymers for improved gas sensing. Significant enhancements in sensor recovery kinetics, cycling stability, and dynamic range at room temperature are demonstrated. Mechanistic insights into energy band alignments and sorbate–analyte interactions reported here are promising toward future high‐performance chemiresistors as well as novel electronic devices based on polymer/MOF composites.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
•Fabrication of a sensor array using pure and Fe-doped In2O3 nanofibers.•Discriminative detection of benzene, xylene, toluene, ethanol, and formaldehyde.•Distinction between aromatic and non-aromatic ...indoor pollutants using sensor array.•Gas sensing mechanism underlying Fe-induced change in response and selectivity.
Representative indoor volatile organic compounds (VOCs) such as benzene, xylene, toluene, formaldehyde, and ethanol need to be detected in a highly sensitive and discriminative manner because of their different impact on human health. In this study, pure and 0.05, 0.1, 0.3, and 0.5 at% Fe-doped In2O3 nanofibers were prepared by electrospinning and their gas sensing characteristics toward the aforementioned VOCs were investigated. The doping of In2O3 nanofiber sensor with 0.05 and 0.1 at% Fe shifted the temperature to show the maximum responses to benzene, xylene, and toluene, and reduced responses to ethanol and formaldehyde, thus demonstrating changed gas selectivity. The gas sensing characteristics of 0.5 at% Fe-doped In2O3 nanofiber sensor were substantially different from those of the other sensors. Significantly different gas sensing patterns of pure and Fe-doped In2O3 sensors could be used to discriminate between the five different VOCs at 375 °C and to distinguish between the aromatic and non-aromatic gases at all sensing temperatures. The mechanism underlying the Fe-induced change in gas sensing characteristics has been discussed in relation to the variation of catalytic activity, morphology, oxygen adsorption, and charge carrier concentration.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
In manufacturing C−N bond‐containing compounds, it is an important challenge to alternate the conventional methodologies that utilize reactive substrates, toxic reagents, and organic solvents. In ...this study, we developed an electrochemical method to synthesize a C−N bond‐containing molecule avoiding the use of cyanides and amines by harnessing nitrate (NO3−) as a nitrogen source in an aqueous electrolyte. In addition, we utilized oxalic acid as a carbon source, which can be obtained from electrochemical conversion of CO2. Thus, our approach can provide a route for the utilization of anthropogenic CO2 and nitrate wastes, which cause serious environmental problems including global warming and eutrophication. Interestingly, the coreduction of oxalic acid and nitrate generated reactive intermediates, which led to C−N bond formation followed by further reduction to an amino acid, namely, glycine. By carefully controlling this multireduction process with a fabricated Cu–Hg electrode, we demonstrated the efficient production of glycine with a faradaic efficiency (F.E.) of up to 43.1 % at −1.4 V vs. Ag/AgCl (current density≈90 mA cm−2).
By electrochemical method, stable oxalic acid and nitrate substrates were coupled into an amino acid, glycine. Insertion of electrons resulted in their conversion into nucleophile and electrophile, respectively, which led to C=N bond formation between them, as well as the conversion of the reversible C=N double bond to a stable C−N single bond.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Nearly monodisperse hollow hierarchical Co3O4 nanocages of four different sizes (∼0.3, 1.0, 2.0, and 4.0 μm) consisting of nanosheets were prepared by controlled precipitation of zeolitic imidazolate ...framework-67 (ZIF-67) rhombic dodecahedra, followed by solvothermal synthesis of Co3O4 nanocages using ZIF-67 self-sacrificial templates, and subsequent heat treatment for the development of high-performance methylbenzene sensors. The sensor based on hollow hierarchical Co3O4 nanocages with the size of ∼1.0 μm exhibited not only ultrahigh responses (resistance ratios) to 5 ppm p-xylene (78.6) and toluene (43.8) but also a remarkably high selectivity to methylbenzene over the interference of ubiquitous ethanol at 225 °C. The unprecedented and high response and selectivity to methylbenzenes are attributed to the highly gas-accessible hollow hierarchical morphology with thin shells, abundant mesopores, and high surface area per unit volume as well as the high catalytic activity of Co3O4. Moreover, the size, shell thickness, mesopores, and hollow/hierarchical morphology of the nanocages, the key parameters determining the gas response and selectivity, could be well-controlled by tuning the precipitation of ZIF-67 rhombic dodecahedra and solvothermal reaction. This method can pave a new pathway for the design of high-performance methylbenzene sensors for monitoring the quality of indoor air.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
Novel spirobisindane-containing thermally rearranged (TR) benzoxazole(spiroTR-PBO) membranes were prepared by thermal treatment of four newly synthesized spirobisindane-containing hydroxyl-polyimides ...(spiroHPIs). Transport properties of spirobisindane-containing polymers were investigated along with fractional free volume and density measurements, as well as molecular simulation analysis. The resulting spiroTR-PBOs exhibited high free volumes and superior mechanical properties in terms of elongation and tensile strength, and these findings were supported by molecular dynamic simulations. SpiroTR-PBO membranes containing hexafluoroisopropylidene moiety in the polymer backbone showed a six-fold increase in CO2 permeability, compared to that of the spiroHPI precursor. SpiroTR-PBO membranes showed strong potential to be applied in carbon capture and separation of gases because of their high gas permeability coupled with robust mechanical properties.
Display omitted
► Novel spirobisindane-containing thermally rearranged (TR) benzoxazole membranes. ► Robust mechanical properties with large fractional free volume elements. ► Six-fold increase in CO2 permeability of spiroTR-PBO compared with that of spiroHPIs. ► Strong potential to apply in gas separation including carbon capture.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Display omitted
•Co-ZIF-L was used as a new template for co-loading of Co3O4 and PdO nanocatalysts.•Co-ZIF-L is uniformly coated on In2O3 spheres via electrostatic self-assembly.•PdO encapsulated ...Co3O4 catalytic layer was formed by annealing Pd intercalated Co-ZIF-L.•In2O3 co-loaded with PdO/Co3O4 showed highly selective and sensitive detection of acetone.•MOF-derived catalyst loading enables the tailored design of catalyst-oxide heterostructures.
Highly dispersed Co3O4 nanoclusters encapsulating PdO nanoparticles were loaded on In2O3 hollow spheres to design high-performance breath acetone sensors. Nanolayers of two-dimensional (2D) metal-organic frameworks (MOFs), pure and Pd-intercalated leaf-like cobalt zeolitic-imidazolate frameworks (Co-ZIF-L), were uniformly coated (thickness: approximately 10 nm) on the surface of the In2O3 spheres by controlling the growth and self-assembly of 2D Co-ZIF-L on In2O3, which were converted into pure or Co3O4 nanoclusters (size: 10 nm) encapsulating PdO nanoparticles (size: approximately 4 nm) by thermal annealing. The gas response, selectivity, and optimal sensing temperature could be tuned by loading different quantities and configurations of the Co3O4 or Co3O4/PdO nanocatalysts. The In2O3 sensors co-loaded with Co3O4/PdO exhibited ultra-high responses (ratio of resistances in air and gas) to 5 ppm of acetone (145.9) as well as high selectivity over the interference of other biomarker gases at 225 °C, even in high humidity conditions (80% relative humidity), thereby demonstrating the promising potential for monitoring diabetes and the ketogenic diet. This unprecedented acetone sensing performance can be explained by the electronic sensitization due to the formation of p(Co3O4)-n(In2O3) heterojunction and the chemical sensitization due to the synergistic catalytic effect of Co3O4 and PdO. Ultrathin 2D-MOFs incorporating metallic nanoparticles provide a promising template for co-loading two different nanocatalysts in a highly dispersed and well-mixed configuration that can be used to establish diverse catalyst-oxide hetero-nanostructures for various functional applications, including high-performance gas sensors.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
PDF
