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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 TiO2 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 TiO2 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 TiO2 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. Formaldehyde, a probable carcinogen, is a ubiquitous indoor pollutant, but its ultraselective detection has been a long-standing challenge. Here, the authors develop a chemiresistive sensor that can detect ppb-level formaldehyde in an exclusive manner at room temperature. [ABSTRACT FROM AUTHOR]

Water poisoning, the dependence of gas‐sensing characteristics on moisture, in oxide chemiresistors remains a long‐standing challenge. Various approaches are explored to mitigate water poisoning but they are often accompanied by significant deterioration of sensing capabilities such as gas response deterioration, gas selectivity alteration, and sensor resistance increase up to unmeasurable levels. Herein, a novel sensor design with a moisture‐blocking Tb4O7 overlayer is suggested as a facile and universal strategy to remove moisture poisoning without sacrificing intrinsic sensing properties. A submicrometer‐thick coating of Tb4O7 overlayer on In2O3 sensors effectively eliminates the humidity dependence of the gas‐sensing characteristics without significantly altering the gas response, selectivity, and sensor resistance. Furthermore, the general validity of the water‐blocking effect using the Tb4O7 overlayer is confirmed in diverse gas sensors using SnO2, ZnO, and Pd/SnO2. The negligible moisture interference of the bilayer sensor is explained in terms of the hydrophobic nature of the Tb4O7 overlayer and the prevention of formation of the OH radical by the interaction between Tb4O7 and In2O3. A universal solution to design diverse humidity‐independent gas sensors with different gas selectivities can open up new pathways toward building accurate and robust gas sensors with new functionalities and high‐performance artificial olfaction. [ABSTRACT FROM AUTHOR]

Monolayers of periodic porous Co3O4 inverse opal (IO) thin films for gas-sensor applications were prepared by transferring cobalt-solution-dipped polystyrene (PS) monolayers onto sensor substrates and subsequent removal of the PS template by heat treatment. Monolayer Co3O4 IO thin films having periodic pores ( d≈500 nm) showed a high response of 112.9 to 5 ppm C2H5OH at 200 °C with low cross-responses to other interfering gases. Moreover, the selective detection of xylene and methyl benzenes (xylene+toluene) could be achieved simply by tuning the sensor temperature to 250 and 275 °C, respectively, so that multiple gases can be detected with a single chemiresistor. Unprecedentedly high ethanol response and temperature-modulated control of selectivity with respect to ethanol, xylene, and methyl benzenes were attributed to the highly chemiresistive IO nanoarchitecture and to the tuned catalytic promotion of different gas-sensing reactions, respectively. These well-ordered porous nanostructures could have potential in the field of high-performance gas sensors based on p-type oxide semiconductors. [ABSTRACT FROM AUTHOR]

An amperometric CO2 sensor was fabricated using a Pt|NASICON (Na+ conductor, Na3Zr2Si2PO12)|Pt cell and a porous Na2CO3–BaCO3 (1:1.7 in molar ratio) auxiliary layer. The sensor with the Na2CO3–BaCO3 layer showed a faster response than that with the Na2CO3 layer, and the sensor signal was barely influenced by the humidity. When operated in an amperometric mode with an applied external electric field, the sensor could detect the CO2 concentration stably and precisely without the need for a reference gas or a reference electrode material, and the sensor signal was constant for extended test periods (60 days). The sensing mechanism and the long-term stability is discussed in terms of the amperometric operation of the sensor. [Copyright &y& Elsevier]

Bilayer Sensors: A General Solution to Mitigate Water Poisoning of Oxide Chemiresistors: Bilayer Sensors with Tb4O7 Overlayer (Adv. Funct. Keywords: bilayer sensors; gas sensors; oxide semiconductors; Tb 4O 7 water poisoning EN bilayer sensors gas sensors oxide semiconductors Tb 4O 7 water poisoning 1 1 1 02/05/21 20210203 NES 210203 In article number 2007895, Jong-Heun Lee and co-workers show that the coating of a moisture-blocking Tb SB 4 sb O SB 7 sb overlayer provides a general and facile solution to mitigate the water poisoning effect of oxide semiconductor gas sensors without significant alteration of the gas-sensing characteristics such as gas response, selectivity, and sensor resistance, enabling the design of reliable gas sensors and artificial olfaction with the strong endurance against the variation in the ambient moisture. Bilayer sensors, gas sensors, oxide semiconductors, Tb 4O 7 water poisoning. [Extracted from the article]

The selective detection of two different gases, NO2and C2H5OH, has been achieved using a p-type Co3O4-decorated n-type ZnO nanowire (NW) network sensor. The gas selectivity was explained by the catalytic effect of nanocrystalline Co3O4and the extension of the electron depletion layer viathe formation of p–n junctions. [ABSTRACT FROM AUTHOR]

Pristine SnO 2 nanowires (NWs), Cr 2 O 3 -decorated SnO 2 NWs, and SnO 2 –Cr 2 O 3 core-shell nanocables (NCs) were prepared by thermal evaporation, and their gas-sensing characteristics were investigated. The decoration of discrete p-type Cr 2 O 3 nanoclusters on SnO 2 NWs increased their response to trimethylamine (TMA). Highly sensitive and selective detection of TMA was attributed to the chemical affinity and catalytic activity of Cr 2 O 3 toward TMA. In contrast, the SnO 2 –Cr 2 O 3 NCs, formed by the coating of a continuous p-type Cr 2 O 3 overlayer on SnO 2 NWs, showed negligibly low responses to all analyte gases used. The variations in gas-sensing characteristics due to configuration changes in one-dimensional SnO 2 –Cr 2 O 3 hetero-nanostructures are explained and discussed in relation to the gas-sensing mechanisms of n- and p-type oxide semiconductors. [ABSTRACT FROM AUTHOR]

Abstract: A highly sensitive H2 gas sensor was prepared by the thermal oxidation of a Ti plate at 600–1000°C. The H2 sensitivity (the ratio of the resistances between N2 and 1.0% H2 balanced with N2) of the oxidized TiO2 increased exponentially with increasing oxidation temperature up to 900°C but decreased drastically at 1000°C. The maximum sensitivity to 1.0% H2 was 1.2×106, which is the highest value reported in the literature. The mechanism for the enhanced H2 sensing was examined by investigating the phase, thickness and morphology of the oxidized TiO2 layer and the sensor response time. The thermally oxidized specimens exhibited an approximately linear dependence of the sensitivity on the H2 concentrations from 50 to 10,000ppm, and excellent sensitivity (∼103) was still obtained in the sensor operated at temperatures as low as 150°C. [Copyright &y& Elsevier]

Cr 2 O 3 /ZnCr 2 O 4 nanocomposite powders and phase-pure ZnCr 2 O 4 powders were prepared by the galvanic replacement of Zn in ZnO hollow spheres by Cr and subsequent heat treatment. The Cr 2 O 3 /ZnCr 2 O 4 nanocomposite powders prepared by galvanic replacement consisted of nanocrystalline Cr 2 O 3 and ZnCr 2 O 4 particles, which showed a high response (resistance ratio) of 69.2 to 5 ppm xylene at 275 °C and excellent xylene selectivity. In contrast, both Cr 2 O 3 and ZnO powders showed selectivity to ethanol, and no significant selectivity to a specific gas was found for the ZnCr 2 O 4 powders. The Cr 2 O 3 /ZnCr 2 O 4 composite powders consisting of coarse particles prepared by solid-state reaction showed relatively low response and selectivity to xylene. The high selectivity and response to xylene of the Cr 2 O 3 /ZnCr 2 O 4 nanocomposite powders were attributed to the synergistic promotion of the methylbenzene-sensing reaction by two sensing materials with different catalytic activities and the high chemiresistive variation of the small particles, respectively, both of which result from the intimate and uniform mixing of nanocrystalline Cr 2 O 3 and ZnCr 2 O 4 particles. [ABSTRACT FROM AUTHOR]

Urchin-like Mg-doped ZnO nanowire networks were prepared by MgO-seeded vapor-phase growth of ZnO nanowires, and their potential as gas-sensing materials was investigated. The response (resistance ratio) of the urchin-like Mg-doped ZnO nanowire networks to 5 ppm C 2 H 5 OH at 350 °C was as high as 343, which is significantly higher than that of pure ZnO nanowire networks (7.0). In addition, the Mg-doped ZnO nanowire network sensors showed excellent selectivity to C 2 H 5 OH and an unprecedentedly high response (28.8) even to 0.25 ppm C 2 H 5 OH. The enhancement of the gas response and selectivity to C 2 H 5 OH was attributed to Mg-doping-induced decrease of the charge carrier concentration, the change of nanowire thickness/morphology, and the catalytic promotion of the C 2 H 5 OH sensing reaction. [ABSTRACT FROM AUTHOR]

Abstract: A SnO2 nanowires (NWs) sensor was coated with a functional La2O3 layer by solution deposition and its gas sensing characteristics were compared with those of an uncoated SnO2 NWs sensor. The responses (R a/R g, R a: resistance in air, R g: resistance in gas) of an undoped SnO2 NWs sensor to 100ppm C2H5OH and CH3COCH3 were 10.5 and 9.6, respectively, while those to 100ppm C3H8, CO, and H2 ranged from 3.1 to 3.3. The R a/R g values of the La2O3-coated SnO2 NWs to 100ppm C2H5OH and CH3COCH3 increased to 57.3 and 34.9, respectively, but there was no significant change in the responses to C3H8, CO, and H2. The ∼5.5-fold enhancement in R a/R g to C2H5OH as well as the selective detection of C2H5OH in the interference gases such as C3H8, CO, and H2 was accomplished simultaneously by coating with La2O3. In particular, the selective discrimination of C2H5OH over CH3COCH3, which is a difficult issue on account their similar chemical nature, was possible. [Copyright &y& Elsevier]

Abstract: A highly sensitive and fast responding CO sensor was fabricated from a sheet-like SnO2. The SnO sheets were prepared by a room temperature reaction between SnCl2, hydrazine and NaOH, and they were subsequently oxidized into SnO2 sheets at high temperature (600°C). The morphology and size of the SnO2 sheets could be controlled during the formation of SnO, which influence the sensor response (R a/R g) and response time to a great extent. The sensor response of SnO nanosheets to 10ppm CO was enhanced up to 2.34, and the 90% sensor response time could be reduced to 6s, which are significantly higher and shorter than those of SnO2 powders (1.57 and 88s), respectively. The realization of both a high sensitivity and rapid response were explained in terms of rapid gas diffusion onto the entire sensing surface due to the less-agglomerated and very thin structure of SnO2 nanosheets and the catalytic effect of Pt. [Copyright &y& Elsevier]

Abstract: Sensing materials for selective detection of C2H5OH were designed using combinatorial solution deposition of SnO2–ZnO thin films. The SnO2–ZnO composite sensor prepared by alternate deposition of 10 droplets of SnO2 and ZnO sols (S50Z50 sensor) showed a high response to 200ppm C2H5OH (S(ethanol)= R a/R g =4.69, R a: resistance in air, R g: resistance in gas) at 300°C, while the gas responses to 100ppm C3H8, 100ppm CO, 200ppm H2, and 5ppm NO2 ranged from 1.11 to 1.19. The S(ethanol) value of the S50Z50 sensor was twice that to 200ppm CH3COCH3 (S(acetone)). In contrast, the S(ethanol) and S(acetone) of pure SnO2 and ZnO thin films were similar to each other. The heterostructure between SnO2 and ZnO was suggested as one of the probable reasons for the successful discrimination between C2H5OH and CH3COCH3. [Copyright &y& Elsevier]

[Display omitted] • WO 3 spheres doped with Nb were prepared by ultrasonic spray pyrolysis. • Doping of Nb into WO 3 spheres leads to the formation of ferroelectric phase. • Nb-doped WO 3 sensor showed ultrahigh response and selectivity to acetone. • Nb-doped WO 3 sensor can be used for monitoring the efficiency of ketogenic diets. • Enhanced reaction between ferroelectric materials and polar acetone was verified. A ketogenic diet (KD) can enable more effective weight loss when performed with a portable breath acetone sensor capable of monitoring KD efficiency. However, current portable sensors—such as oxide semiconductor gas sensors—cannot selectively detect low concentrations of acetone in human breath, hindering their practical applications. Herein, we report an ultraselective and highly sensitive breath acetone sensor, established using WO₃ doped with 5–20 atom% Nb. The pure WO₃ sensor showed low gas responses (S = resistance ratio – 1) to all analytic gases, including acetone, ethanol, carbon monoxide, ammonia, hydrogen, and toluene (S = 0.1–2.1). In contrast, the 10 atom% Nb-doped WO₃ sensor exhibited a high gas response (S = 15.0 at 1 ppm) and ultrahigh selectivity to acetone over ethanol interference (response ratio = 22.3), even in highly humid conditions (relative humidity: 80 %). Moreover, the response time was as short as 4 s, and the detection limit was as low as 8.9 ppb, enabling breath acetone to be monitored in a rapid and precise fashion. The excellent acetone sensing performance of Nb-doped WO₃ sensors was then examined from diverse perspectives, including changes in porosity, surface area, oxygen adsorption, and charge-carrier concentration, and with respect to the formation of ferroelectric ε-WO₃ domains caused by Nb doping. This work will trigger development of a new class of portable breath sensors that can be used to monitor KD efficiency. [ABSTRACT FROM AUTHOR]

• Co-ZIF-L was used as a new template for co-loading of Co 3 O 4 and PdO nanocatalysts. • Co-ZIF-L is uniformly coated on In 2 O 3 spheres via electrostatic self-assembly. • PdO encapsulated Co 3 O 4 catalytic layer was formed by annealing Pd intercalated Co-ZIF-L. • In 2 O 3 co-loaded with PdO/Co 3 O 4 showed highly selective and sensitive detection of acetone. • MOF-derived catalyst loading enables the tailored design of catalyst-oxide heterostructures. Highly dispersed Co 3 O 4 nanoclusters encapsulating PdO nanoparticles were loaded on In 2 O 3 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 In 2 O 3 spheres by controlling the growth and self-assembly of 2D Co-ZIF-L on In 2 O 3 , which were converted into pure or Co 3 O 4 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 Co 3 O 4 or Co 3 O 4 /PdO nanocatalysts. The In 2 O 3 sensors co-loaded with Co 3 O 4 /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(Co 3 O 4)-n(In 2 O 3) heterojunction and the chemical sensitization due to the synergistic catalytic effect of Co 3 O 4 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. [ABSTRACT FROM AUTHOR]

• Highly selective and sensitive trimethylamine sensor. • Two-dimensional SnO2 inverse opal structures decorated with Cr2O3 nanoclusters. • Unprecedentedly high response (resistance ratio) of 301.3–5 ppm trimethylamine. • High gas response due to oxide p-n junction and highly chemiresistive neck region. • Superior trimethylamine selectivity enhanced by catalytic Cr 2 O 3 nanoclusters. Two-dimensional SnO 2 inverse opal (2D IO) structures with uniform surface decoration of Cr 2 O 3 nanoclusters were prepared using a Sn-solution-dipped sacrificial polystyrene monolayer template method and were employed for chemiresistive gas-sensor applications. The responses (resistance ratio) of the pure SnO 2 2D IO sensor to 5 ppm trimethylamine (TMA) and ethanol were as high as 209.5 and 242.3, representing improvements of two orders of magnitude compared with that of a dense SnO 2 thin film sensor. The high gas response of the porous SnO 2 IO sensor is attributed to the superior gas accessibility as well as the large chemiresistive variation at the thinnest neck region of the 2D IO structures. Further enhancements of the TMA selectivity and response were achieved by decorating the surface of the SnO 2 IO structure with Cr 2 O 3 nanoclusters. The control of the TMA-sensing characteristics of the SnO 2 2D IO sensor through the decoration with Cr 2 O 3 nanoclusters was explained by the change of the electron-depletion layer in the SnO 2 particles due to the formation of the p(Cr 2 O 3)-n(SnO 2) junction, as well as the catalytic promotion of the gas-sensing reaction by Cr 2 O 3. The oxide 2D IO films with high gas accessibility and chemiresistive neck structures are promising nanoarchitectures for gas-sensor applications, and the formation of hetero-nanostructures via decoration with different oxide semiconductor nanoclusters having dissimilar work functions and catalytic activity allows the enhancement of the selectivity toward a specific gas, as well as the gas response. [ABSTRACT FROM AUTHOR]

• Ultrasensitive and selective methylbenzene sensor using Ti-NiO multiroom micro-reactors. • Versatile tunability on gas response, selectivity, sensitivity, and detection limit. • Unprecedentedly high response (resistance ratio) of 337.8 to 1 ppm p -xylene. • Elucidation of gas reforming/oxidation mechanism for selective gas detection. Pure and Ti-doped NiO multiroom spheres were prepared via ultrasonic spray pyrolysis, and their gas sensing characteristics were investigated. The sensor using 10 at% Ti-doped NiO multiroom spheres exhibited an unprecedented high response (resistance ratio = 337.8) to 1 ppm p -xylene at 350 ℃, whereas the sensor using pure NiO multiroom spheres exhibited a negligibly low response (1.3). Moreover, the control of the Ti doping and film thickness provided intriguing strategies for tuning the xylene and methylbenzene sensing characteristics, such as the selectivity, response, sensitivity (slope between response and gas concentration), and detection limit. The versatile tunability on gas sensing characteristics was explained by the Ti-doping-induced variation of the oxygen adsorption, mesoporosity, specific surface area, and charge-carrier concentration, as well as the control over the reforming and oxidation of the analyte gases using the multiroom-structured micro-reactors with high catalytic activity. [ABSTRACT FROM AUTHOR]