Metal–organic frameworks (MOFs) with high surface area, tunable porosity, and diverse structures are promising platforms for chemiresistors; however, they often exhibit low sensitivity, poor ...selectivity, and irreversibility in gas sensing, hindering their practical applications. Herein, we report that hybrids of Cu3(HHTP)2 (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) nanoflakes and Fe2O3 nanoparticles exhibit highly sensitive, selective, and reversible detection of NO2 at 20 °C. The key parameters to determine their response, selectivity, and recovery are discussed in terms of the size of the Cu3(HHTP)2 nanoflakes, the interaction between the MOFs and NO2, and an increase in the concentration and lifetime of holes facilitated by visible-light photoactivation and charge-separating energy band alignment of the hybrids. These photoactivated MOF–oxide hybrids suggest a new strategy for designing high-performance MOF-based gas sensors.
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IJS, KILJ, NUK, PNG, 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.
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IJS, KILJ, NUK, PNG, UL, UM
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•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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
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•CdS NO2 gas sensor operated under both fluorescent lamp and natural solar light.•2D nanoflake array CdS films for enhanced light absorption and charge transport.•CdS gas sensor with ...high response to NO2, little influence by humidity.•Elucidation of visible-light-enhanced NO2 sensing mechanism.
A Highly ordered CdS nanoflake array was fabricated by CVD, and its gas sensing characteristics were investigated. The sensor exhibited high response (resistance ratio) of 89% to 5 part per million (ppm) nitrogen dioxide (NO2) under green LED illumination (wavelength 500–540nm, irradiance 21W/m2) with excellent selectivity and little interference by humidity. Moreover, the sensor showed promising potential for operating under fluorescent lamp and natural solar light, which can be used for medical diagnosis and indoor/outdoor environment monitoring. This performance is attributed to the low band gap energy (2.4eV) of CdS and the unique morphology of nanoflake array which can enhance both the light absorption and conductivity.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Remarkable efforts have been devoted for enhancing gas selectivity of chemiresistors via tuning the sensing materials. However, the selective detection of low reactive gases remains challenging. ...Herein, we report a new strategy for selective detection of low reactive gases by patterning nanofibers and tuning the catalytic property of electrode. In this approach, straight single In2O3 nanofibers are patterned onto Au, Pt, and ITO interdigitated electrodes (IDEs) via direct-write near-field electrospinning; the resulting low coverage of the sensing materials (∼0.12%) exposes the electrode to analyte gases. The gas sensing characteristics of the sensors are determined by the catalytic activity of each electrode. Furthermore, the functionalization of Pt IDE with Au nanoparticles could achieve extremely high selectivity and response toward xylene. The sensing properties and mechanisms of the nanopatterned sensors are investigated regarding electrode composition, degree of electrode exposure, and catalyst location on the electrode and/or sensing materials. The key strategies for achieving high selectivity are the conversion of low reactive xylene gas into more reactive intermediate species while highly reactive interference gases are completely oxidized at open catalytic electrodes. Catalyst functionalization and exposure of electrodes can provide new guidelines for designing high performance gas sensors for new applications.
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•1D metal oxide single nanofiber patterns with exposed electrodes are prepared via near-field electrospinning methods.•The effect of the catalytic activity of the electrode on gas sensitivity and selectivity are described.•Au functionalized Pt electrodes improved responses and selectivity of In2O3 single nanofibers to xylene gas.•A potential of patterned sensors as indoor air quality monitoring application is presented.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Water poisoning has been a long-standing problem in oxide semiconductor gas sensors. Herein, for the first time, we report that pure and Pr-doped Ce4W9O33 provide humidity-independent gas sensing ...characteristics. Ce4W9O33 and Pr-doped Ce4W9O33 powders with a porous structure have been successfully prepared by ultrasonic spray pyrolysis and subsequent annealing at low temperature (600 °C). Interestingly, these p-type oxide semiconductors exhibited nearly the same gas sensing characteristics at 300 °C regardless of humidity variation, whereas pure WO3 showed a significant decrease of sensor resistance and gas response when the atmosphere is changed from dry to relative humidity 80%. Furthermore, Ce4W9O33-based sensors showed highly selective and sensitive detection of ppm-level trimethylamine (TMA). Moisture-endurant gas sensing characteristics were discussed in relation to surface regeneration through the hydroxyl scavenging reaction assisted by abundant Ln3+ (Ln = Ce, Pr) in (Ce1−xPrx)4W9O33 (x = 0–0.3) and TMA selectivity was explained by the acid–base interaction between the analyte gas and sensing material. Phase-pure ternary or quaternary oxides with a decreased oxidation state of lanthanide components provide a new and general strategy to design humidity-independent gas sensors with new functionality.
Water poisoning has been a long-standing problem in oxide semiconductor gas sensors. Herein, for the first time, we report that pure and Pr-doped Ce 4 W 9 O 33 provide humidity-independent gas ...sensing characteristics. Ce 4 W 9 O 33 and Pr-doped Ce 4 W 9 O 33 powders with a porous structure have been successfully prepared by ultrasonic spray pyrolysis and subsequent annealing at low temperature (600 °C). Interestingly, these p-type oxide semiconductors exhibited nearly the same gas sensing characteristics at 300 °C regardless of humidity variation, whereas pure WO 3 showed a significant decrease of sensor resistance and gas response when the atmosphere is changed from dry to relative humidity 80%. Furthermore, Ce 4 W 9 O 33 -based sensors showed highly selective and sensitive detection of ppm-level trimethylamine (TMA). Moisture-endurant gas sensing characteristics were discussed in relation to surface regeneration through the hydroxyl scavenging reaction assisted by abundant Ln 3+ (Ln = Ce, Pr) in (Ce 1−x Pr x ) 4 W 9 O 33 ( x = 0–0.3) and TMA selectivity was explained by the acid–base interaction between the analyte gas and sensing material. Phase-pure ternary or quaternary oxides with a decreased oxidation state of lanthanide components provide a new and general strategy to design humidity-independent gas sensors with new functionality.
Volatile sulfur compounds (VSCs), such as H2S and methyl mercaptan (MM), are malodorous and harmful gases. H2S and MM share analogical origins. However, they should be discriminated because of their ...different odor thresholds and health impacts. Herein, a nanopattern chemiresistor composed of aligned CuO-loaded CuFe2O4 nanofibers was prepared via direct-write near-field electrospinning. The CuO/CuFe2O4 sensor with the atomic ratio of Cu:Fe = 3:4 exhibited extremely high responses to ppm-level H2S and MM compared with other nanopattern sensors with different compositions (Cu/Fe = 2:4, 4:4, and 0:1). This can be attributed to the gas-accessible morphology of the nanopattern sensor, the formation of a nanoscale p(CuO)-n(CuFe2O4) junction, and the intimate reaction between the discretely loaded CuO nanoparticles and the VSCs. Furthermore, the CuO/CuFe2O4 sensor exhibited high selectivities to H2S and MM at 200 ℃ and 400 ℃, respectively. The selective dual detection of H2S and MM using a single CuO/CuFe2O4 sensor via the simple modulation of the sensing temperature opens a new route for indoor air quality monitoring, ventilation control, halitosis diagnosis, and wine quality monitoring.
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Nanopattern sensor using 1D CuO/CuFe2O4 nanofibers fabricated by direct-writing near-field electrospinning exhibits high selectivity and response to volatile sulfuric compounds and the modulation of sensing temperature enables the discrimination between H2S and methyl mercaptan.
•CuO/CuFe2O4 nanopattern sensor was fabricated using near-field electrospinning.•Sensor temperature modulation enabled selective dual detection of H2S and methyl mercaptan.•PCA using the array of four Cu-Fe-O nanofiber sensors enabled the discrimination between volatile sulfuric compounds and interference gases.•Mechanism underlying gas selectivity control was elucidated by proton transfer reaction quadruple mass spectrometry.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
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.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Water poisoning has been a long-standing problem in oxide semiconductor gas sensors. Herein, for the first time, we report that pure and Pr-doped Ce
4
W
9
O
33
provide humidity-independent gas ...sensing characteristics. Ce
4
W
9
O
33
and Pr-doped Ce
4
W
9
O
33
powders with a porous structure have been successfully prepared by ultrasonic spray pyrolysis and subsequent annealing at low temperature (600 °C). Interestingly, these p-type oxide semiconductors exhibited nearly the same gas sensing characteristics at 300 °C regardless of humidity variation, whereas pure WO
3
showed a significant decrease of sensor resistance and gas response when the atmosphere is changed from dry to relative humidity 80%. Furthermore, Ce
4
W
9
O
33
-based sensors showed highly selective and sensitive detection of ppm-level trimethylamine (TMA). Moisture-endurant gas sensing characteristics were discussed in relation to surface regeneration through the hydroxyl scavenging reaction assisted by abundant Ln
3+
(Ln = Ce, Pr) in (Ce
1−
x
Pr
x
)
4
W
9
O
33
(
x
= 0-0.3) and TMA selectivity was explained by the acid-base interaction between the analyte gas and sensing material. Phase-pure ternary or quaternary oxides with a decreased oxidation state of lanthanide components provide a new and general strategy to design humidity-independent gas sensors with new functionality.
Highly sensitive, selective and humidity-independent detection of trimethylamine was achieved using p-type semiconducting pure and Pr-doped Ce
4
W
9
O
33
.
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