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Abstract: High-performance gas sensors prepared using p-type oxide semiconductors such as NiO, CuO, Cr2O3, Co3O4, and Mn3O4 were reviewed. The ionized adsorption of oxygen on p-type oxide semiconductors leads to the formation of hole-accumulation layers (HALs), and conduction occurs mainly along the near-surface HAL. Thus, the chemoresistive variations of undoped p-type oxide semiconductors are lower than those induced at the electron-depletion layers of n-type oxide semiconductors. However, highly sensitive and selective p-type oxide-semiconductor-based gas sensors can be designed either by controlling the carrier concentration through aliovalent doping or by promoting the sensing reaction of a specific gas through doping/loading the sensor material with oxide or noble metal catalysts. The junction between p- and n-type oxide semiconductors fabricated with different contact configurations can provide new strategies for designing gas sensors. p-Type oxide semiconductors with distinctive surface reactivity and oxygen adsorption are also advantageous for enhancing gas selectivity, decreasing the humidity dependence of sensor signals to negligible levels, and improving recovery speed. Accordingly, p-type oxide semiconductors are excellent materials not only for fabricating highly sensitive and selective gas sensors but also valuable additives that provide new functionality in gas sensors, which will enable the development of high-performance gas sensors. [Copyright &y& Elsevier]

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: The Co3O4 nanofibers were prepared by electrospinning and their gas sensing characteristics were investigated. The Co3O4 sensors prepared by heat treatment of as-spun precursor fibers at 500 and 600°C showed well-developed one-dimensional morphologies and exhibited high responses to 100ppm C2H5OH (R g/R a =51.2 and 45.3; R g, resistance in gas; R a, resistance in air) at 301°C with negligible cross-responses to 100ppm CO, C3H8, and H2 (R g/R a =1.02–2.7). In contrast, the most of one-dimensional morphology of the Co3O4 specimen was lost and the response to 100ppm C2H5OH became significantly lower when the heat treatment temperature was increased to 700°C or when the nanofibers were ultrasonically disintegrated into primary particles. The significant decrease of the gas response was explained and discussed in relation to the gas sensing mechanism of a p-type semiconductor, the morphology of specimens, and the connecting configuration between nanoparticles and nanofibers. [Copyright &y& Elsevier]

Highlights • Fabrication of a sensor array using pure and Fe-doped In 2 O 3 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. Abstract 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 In 2 O 3 nanofibers were prepared by electrospinning and their gas sensing characteristics toward the aforementioned VOCs were investigated. The doping of In 2 O 3 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 In 2 O 3 nanofiber sensor were substantially different from those of the other sensors. Significantly different gas sensing patterns of pure and Fe-doped In 2 O 3 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. [ABSTRACT FROM AUTHOR]

The lack of gas selectivity in oxide semiconductor chemiresistors has long been an obstacle to realizing discriminative detection of indoor volatile organic compounds (VOCs) with different health impacts. A simple and reliable algorithm to discriminate between critically harmful aromatic VOCs (benzene, xylene, and toluene) and less harmful ethanol is suggested by the simple combination of sensor signals from pure In 2 O 3 and Au-loaded In 2 O 3 2D inverse opal (IO) thin films prepared by heat-treating the precursor-dipped self-assembled polystyrene templates and Au deposition. The Au-loaded In 2 O 3 IO sensor showed unprecedentedly high responses to 5 ppm ethanol (resistance ratio = 1640.2) and comparably high responses to 5 ppm benzene, p- xylene, and toluene (resistance ratio range of 674.5–1012.9). Such high gas responses were attributed to the periodically porous and thus highly gas-accessible structures, while the clear discrimination between aromatic VOCs and ethanol was achieved by tuning gas selectivity through systematic control of the size, morphology, and loading concentration of Au nano-catalysts. The results of this study can be used for reliable and precise monitoring of indoor air pollutants. [ABSTRACT FROM AUTHOR]

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 (NO 2 ) under green LED illumination (wavelength 500–540 nm, irradiance 21 W/m 2 ) 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 (2.4 eV) of CdS and the unique morphology of nanoflake array which can enhance both the light absorption and conductivity. [ABSTRACT FROM AUTHOR]

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 In 2 O 3 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. [Display omitted] • 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 In 2 O 3 single nanofibers to xylene gas. • A potential of patterned sensors as indoor air quality monitoring application is presented. [ABSTRACT FROM AUTHOR]

An ultrasensitive and selective trimethylamine (TMA) sensor was fabricated by the urea-assisted uniform loading of Au nanoparticles (∼5 nm) on Cr 2 O 3 yolk-shell spheres prepared by ultrasonic spray pyrolysis. The response (resistance ratio) of the Cr 2 O 3 yolk-shell spheres uniformly loaded with fine Au nanoparticles to 5 ppm TMA was 200.9 at 225 °C, which was significantly higher than those of pure Cr 2 O 3 yolk-shell spheres (24.6) and Cr 2 O 3 yolk-shell spheres loaded with heavily agglomerated Au particles (9.2). The detection limit of Cr 2 O 3 yolk-shell spheres uniformly loaded with Au nanoparticles was as low as 4.3 ppb. The unprecedentedly high TMA response is explained and discussed in relation to the gas-accessible morphology of the yolk-shell spheres, and the catalytic and electronic promotion of the gas sensing reaction by both Cr 2 O 3 and well-dispersed Au nanoparticles. [ABSTRACT FROM AUTHOR]

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]

The pure and 0.3 wt% Pt-doped SnO 2 hollow nanospheres were prepared by the oxidation of pure and Pt-doped Sn nanoscrystals embedded in carbon matrix and their gas sensing characteristics were investigated. The formation of hollow morphology was attributed to the nanoscale Kirkendall effect due to rapid outward diffusion of Sn ions and relatively slow inward diffusion of oxygen. Pure SnO 2 hollow nanospheres showed a high response (resistance ratio) of 93.3 when exposed to 5 ppm ethanol. The response to 5 ppm ethanol was significantly increased to 1399.9 with doping 0.3 wt% Pt. In addition, selectivity to ethanol was also enhanced by Pt doping. Ultrasensitive and selective detection of ethanol in pure and Pt-doped SnO 2 nanospheres is explained by the effective electron depletion in hollow structures and catalytic promotion of gas sensing reaction. [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]

Branched ZnO nanowires (NWs) doped with Ni were grown by a three-step vapor phase method for the sensitive and selective detection of p -xylene. ZnO NWs were directly grown on sensor substrates with Au electrodes, which were transformed into NiO NWs by the thermal evaporation of NiCl 2 powder at 700 °C. ZnO branches doped with Ni were grown from NiO NWs by the thermal evaporation of Zn metal powder at 500 °C. The stem NiO NWs played the role of catalyst for the growth of ZnO branches through vapor–liquid–solid mechanism. The Ni-doped branched ZnO NWs showed enhanced gas response ( S = resistance ratio) to methyl benzenes, especially to 5 ppm p -xylene ( S = 42.44) at 400 °C. This value is 1.7 and 2.5 times higher than the responses to 5 ppm toluene ( S = 25.73) and C 2 H 5 OH ( S = 16.72), respectively, and significantly higher than the cross-responses to other interfering gases such as benzene, HCHO, trimethylamine, H 2 , and CO. The selective detection of xylene was attributed to the catalytic role of the Ni component. This novel method to form catalyst-doped hierarchical ZnO NWs provides a promising approach to accomplish superior gas sensing characteristics by the synergetic combination of enhanced chemiresistive variation due to the increased number of branch-to-branch Schottky barrier contacts and the catalytic function of the Ni dopant. [ABSTRACT FROM AUTHOR]

Highlight: [•] Methyl benzene sensor using Cr-doped Co3O4 nanorods was suggested. [•] Responses to xylene and toluene of pure Co3O4 nanorods were high. [•] Doping of Cr to Co3O4 nanorods significantly enhanced selectivity to methyl benzenes. [Copyright &y& Elsevier]

Abstract: The CuO-loaded SnO2 hollow spheres were prepared by ultrasonic spray pyrolysis and their H2S sensing characteristics in dry and humid atmospheres for the diagnosis of halitosis were investigated. The loading of CuO to SnO2 hollow spheres decreased the humidity dependence of H2S sensing characteristics down to negligible level, increased the gas response (ratio of resistance to air and gas) in highly humid atmosphere (relative humidity 80%) to 1ppm H2S from 3.13 to 22.4, and enhanced the selectivity to H2S over CO, NH3, CH3COCH3, C6H6, CH3C6H5, (CH3)2C6H4, and NO at 300°C. Ultraselective and ultrasensitive detection of H2S under highly humid atmosphere with negligible interferences from other biomarker gases provides a promising and reliable analysis tool for real-time diagnosis of halitosis from exhaled breath. [Copyright &y& Elsevier]

Abstract: This work presents a simple and versatile route to produce macroporous p-type metal oxide thin films. Two-dimensional arrays of p-type NiO films with a hollow hemisphere structure were fabricated by colloidal templating and RF-sputtering followed by a subsequent heat treatment. The diameter and shell thickness of the NiO hemisphere were 800nm and 20nm, respectively. X-ray diffraction and high-resolution transmission electron microscopy analysis indicate that the pure NiO phase with grain size of 10nm was obtained at calcination temperatures that exceeded 450°C. Close-packed arrays of hollow NiO hemispheres were found to exhibit p-type gas sensing properties against (CO, H2, C3H8, CH4, NO2, and C2H5OH), leading to significantly enhanced responses to C2H5OH (R gas /R air =5.0 at 200ppm). [Copyright &y& Elsevier]

Abstract: WO3⋅H2O nanoplates were prepared by the acidification of Na2WO4∙2H2O and converted into monoclinic WO3 nanoplates by heat treatment. The sizes, morphologies and preferred orientation of the WO3 nanoplates could be controlled by manipulating the acidity of the solution used for the acidification reaction. All of the WO3 nanoplates showed the selective detection of NO2 in the presence of other reducing gases, such as C2H5OH, CH3COCH3, CO, C3H8, and H2. The gas response, selectivity, and response speed were optimized by varying the morphology of the sensing materials and operation temperature. The WO3 nanoplates with a mean edge size of 192nm showed the most rapid gas response along with a high response and selectivity to NO2 when operated at 300°C. [ABSTRACT FROM AUTHOR]

Abstract: The NO2 gas sensing characteristics of semiconductor type gas sensors with channels composed of necked ZnO nanoparticles (NPs) were investigated in this study. The heat treatment of the NPs at 400°C led to their necking and coarsening. The response of the necked-NP-based sensors was as high as 100 when exposed to 0.2ppm of NO2 at 200°C. As the concentration of NO2 increased to 5ppm, their response was enhanced to approximately 400. During the repeated injection of NO2 gas with a concentration of 0.4ppm, the sensors exhibited stable response characteristics. Furthermore, the 90% response and recovery times of the gas sensor were as fast as 13 and 10s, respectively. These observations indicate that the non-agglomerated necking of the NPs induced by the heat treatment significantly enhances the gas sensing characteristics of the NP-based gas sensors. [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]

• 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]

• Ultrasensitive and selective xylene and toluene sensor using Nb-doped NiO hollow spheres. • Unique dual-mode sensor to detect xylene and toluene using a single sensing material. • Unprecedentedly high response (resistance ratio) of 1752 to 5 ppm p -xylene. • Elucidation of sensing mechanism underlying excellent gas selectivity and response. A single gas sensor with dual functionality for ultrasensitive and highly selective detection of p -xylene and toluene was designed using NiO hollow spheres doped with Nb. The pure and Nb-doped NiO hollow spheres were prepared by one-pot ultrasonic spray pyrolysis and subsequent heat treatment at 500 °C for 2 h. The Nb-doped NiO hollow spheres ([Nb]/[Ni] = 0.1) showed an ultrahigh response to 5 ppm of p -xylene (resistance ratio = 1752) and toluene (resistance ratio = 607), with negligible cross-responses to 5 ppm ethanol, benzene, carbon monoxide, and formaldehyde. In contrast, pure NiO hollow spheres showed negligibly low responses to 5 ppm of all analyte gases. In addition, the Nb-doped NiO hollow spheres exhibited dual sensing characteristics for selectively detecting p -xylene and toluene at 350 °C and 400 °C, respectively. The significant improvement of the response and selectivity for p -xylene and toluene can be explained by the high gas accessibility of hollow spheres, the Nb-doping-induced decrease in the charge carrier concentration, and the catalytic promotion of gas reforming reaction of less reactive xylene and toluene into more active species. The dual function of selectively detecting p -xylene and toluene in Nb-doped NiO hollow spheres is explained by the competition between oxidative filtering and gas reforming reaction depending on the operation temperature and sensing film thickness. The Nb-doped NiO hollow spheres can be used to design a single gas sensor with dual selectivity of xylene and toluene for reliable monitoring of the indoor air quality. [ABSTRACT FROM AUTHOR]