High Gas Response Research Articles

Highly discriminative and sensitive detection of volatile organic compounds for monitoring indoor air quality using pure and Au-loaded 2D In2O3 inverse opal thin films

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 In2O3 and Au-loaded In2O3 2D inverse opal (IO) thin films prepared by heat-treating the precursor-dipped self-assembled polystyrene templates and Au deposition. The Au-loaded In2O3 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.

Humidity-Independent Oxide Semiconductor Chemiresistors Using Terbium-Doped SnO2 Yolk-Shell Spheres for Real-Time Breath Analysis.

The chemiresistive sensing characteristics of metal oxide gas sensors depend closely on ambient humidity. Herein, we report that gas sensors using Tb-doped SnO2 yolk-shell spheres can be used for reliable acetone detection, regardless of the variations in humidity. Pure SnO2 and Tb-doped SnO2 yolk-shell spheres were prepared via ultrasonic spray pyrolysis and their chemiresistive sensing characteristics were studied. The sensor resistance and gas response of the pure SnO2 yolk-shell spheres significantly changed and deteriorated upon exposure to moisture. In stark contrast, the Tb-doped SnO2 yolk-shell spheres exhibited similar gas responses and sensor resistances in both dry and humid [relative humidity (RH) 80%] atmospheres. In addition, the Tb-doped SnO2 yolk-shell sensors showed a high gas response (resistance ratio) of 1.21 to the sub-ppm-levels (50 ppb) of acetone with low responses to the other interference gases. The effects of Tb oxide and the chemical interactions among the Tb oxide, SnO2, and water vapor on this humidity-independent gas sensing behavior of the Tb-doped SnO2 yolk-shell sensors were investigated. This strategy can provide a new road to achieve highly sensitive, selective, and humidity-independent sensing of acetone, which will facilitate miniaturized and real-time exhaled breath analysis for diagnosing diabetes.

Gas sensing properties of p-type CuBi2O4 porous nanoparticulate thin film prepared by solution process based on metal-organic decomposition

Exploring the novel semiconductor-type gas sensors based on complex oxides beyond binary oxides extends the degrees of freedom in chemical sensor research with structural and compositional versatility. In particular, such complex oxides have been reported to exhibit characteristic and promising gas sensing properties, which are mediated by chemical defects. In this work, CuBi2O4 has been prepared in the form of porous nanoparticulate thin film with high surface area-to-volume ratio and small amount of defect (Cu+−oxygen vacancy (VO) complex) by the simple solution process based on metal-organic decomposition (MOD). The film exhibited high gas responses with the specific values of 10.8 toward C2H5OH, 4.2 toward H2, and 2.2 toward CO when measured with 1000 ppm at 400 °C. The particularly high H2S responses (4.7 with 1 ppm, 5.9 with 2 ppm, and 7.4 with 5 ppm) were obtained at 400 °C by the oxidation of H2S on the CuBi2O4 surface. Upon exposure to the oxidizing NO2 gas with low concentrations (≤5 ppm), the resistance of CuBi2O4 thin film sensor was uncommonly increased.

Synthesis and Characterization of Highly Sensitive Hydrogen (H₂) Sensing Device Based on Ag Doped SnO₂ Nanospheres.

In this paper, pure and Ag-doped SnO2 nanospheres were synthesized by hydrothermal method and characterized via X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), and X-ray photoelectron spectra (XPS), respectively. The gas sensing performance of the pure, 1 at.%, 3 at.%, and 5 at.% Ag-doped SnO2 sensing devices toward hydrogen (H2) were systematically evaluated. The results indicated that compared with pure SnO2 nanospheres, Ag-doped SnO2 nanospheres could not only decrease the optimum working temperature but also significantly improve H2 sensing such as higher gas response and faster response-recovery. Among all the samples, the 3 at.% Ag-doped SnO2 showed the highest response 39 to 100 μL/L H2 at 300 °C. Moreover, its gas sensing mechanism was discussed, and the results will provide reference and theoretical guidance for the development of high-performance SnO2-based H2 sensing devices.

Structural Characterization of NiO/Cr2O3Composites and Hydrothermal Synthesis, Properties Gas Sensing

Pure Cr2O3 and NiO/Cr2O3 nanocomposite were luckily synthesized by simple hydrothermal technique. X-ray films diffraction (XRD), Filed Emission scanning electron microscopy (FESEM), Energy dispersive X-ray spectroscopy (EDS), were used to research the crystalline structures, surface morphologies and nanostructures, and element components and their valences of the as-synthesized samples. Moreover, gas sensors based on the synthesized pure Cr2O3 and NiO/ Cr2O3 composites were fabricated and their sensing behavior, to NH3.Gas sensing experiments reference that the NiO/ Cr2O3 composites showed much higher gas response and lower working temperature than those of pure Cr2O3. These results assure that the as-synthesized NiO/ Cr2O3 composites a favorable NH3 sensing material

Enhanced formaldehyde detection based on Ni doping of SnO2 nanoparticles by one-step synthesis

In this work, SnO2 nanoparticles with Ni doping were synthesized via a facile one-step hydrothermal route. The addition of Ni to SnO2 lead to significant enhanced response to formaldehyde (HCHO). The Ni-doped SnO2 materials were used to fabricate HCHO gas sensors and the comprehensive HCHO-sensing properties under diverse working temperatures were tested and studied. With suitable Ni concentration, the SnO2 nanoparticles exhibited a 10 times sensing enhancement with high gas response, excellent selectivity and good stability. Furthermore, the limit of detection (LOD) was experimentally figured out to be as low as 120 ppb, which is much lower than the threshold exposure limit of HCHO proposed by American Conference of Governmental Industrial Hygienists. Importantly, the excellent performances of the as-fabricated gas-sensing devices make Ni-doped SnO2 a promising candidate for HCHO sensing applications. This synthesis strategy here can also give guidance for designing high-performance HCHO gas sensors.

Effect of film thickness on the electrical and ethanol sensing characteristics of LaFeO3 nanoparticle-based thick film sensors

LaFeO3 nanoparticles were prepared by a sol-gel method and chosen as sensing material. A series of thick film sensors were fabricated with one to six layers of LaFeO3 nanoparticles in the thickness of 40–234 µm, so as to study the effect of film thickness on the electrical and ethanol sensing characteristics of LaFeO3 nanoparticle-based thick film sensors. FE-SEM was used to measure the film thickness, and to characterize surface morphology and microstructure inside the sensing matrix. The reduced resistance with increasing film thickness is macroscopically ascribed to the increment of cross-section area for the flowing of charge carriers and is microscopically attributed to the reduction of activation energy for the hopping of small polarons. As for the sensing performance towards ethanol, i.e. prime working temperature, gas response, dynamic response, and gas selectivity, the optimal film thickness depends on the required sensor parameters. Taken together, the sensor with the film thickness of approximately 117 µm showed the best performance, with a fast dynamic response, high gas response and satisfactory selectivity.

On-chip ZnO nanofibers prepared by electrospinning method for NO2 gas detection

In the present study, on-chip ZnO nanofibers were fabricated by means of the electrospinning technique followed by a calcination process at 600 oC towards the gas sensor application. The morphology, composition, and crystalline structure of the as-spun and annealed ZnO nanofibers were investigated by field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX), and X-ray diffraction (XRD), respectively. The findings show that spider-net like ZnO nanofibers with a diameter of 60 – 100 nm were successfully synthesized without any incorporation of impurities into the nanofibers. The FESEM images also reveal that each nanofiber is composed of many nanograins. The combination of experimental and calculated X-ray diffraction data indicate that ZnO nanofibers were crystallized in hexagonal wurtzite structure. For the gas sensing device application, the ZnO nanofibers-based sensors were tested with the nitrogen dioxide gas in the temperature range of 200 oC to 350 oC and concentrations from 2.5 ppm to 10 ppm. The sensing property results indicate that at the optimal working temperature of 300 oC, the ZnO nanofibers-based sensors exhibited a maximum response of 30 and 166 times on exposure of 2.5 and 10 ppm NO2 gas, respectively. The presence of nanograins within nanofibers, which results in further intensification of the resistance modulation, is responsible for such high gas response.

Hierarchically interconnected porosity control of catalyst-loaded WO3 nanofiber scaffold: Superior acetone sensing layers for exhaled breath analysis

Electrospun WO3 nanofibers (NFs) with hierarchically interconnected porosity (HP_WO3 NFs) are designed by using dual sacrificial templates including zero-dimensional (0D) polystyrene (PS) colloids and one-dimensional (1D) multi-walled carbon nanotubes (MWCNTs). Multi-dimensionally interconnected pore-loaded structures offer much enhanced surface area and abundant gas penetration pathway into the inner sensing layers, which are essential requirements for effective gas adsorption-desorption reactions, fast gas diffusion, and high gas response. Moreover, such porous WO3 NFs are homogeneously decorated by Pt nanoparticles (Apo-Pt) encapsulated in a protein nanocage, i.e., apoferritin, which enables uniform catalyst sensitization in interior and exterior of highly gas accessible NFs. The HP_WO3 NFs sensitized by Apo-Pt (Apo-Pt@HP_WO3 NFs) exhibited highly selective and sensitive acetone response (Rair/Rgas = 10.80 ± 0.06 @ 1 ppm) under high humidity atmosphere (90% RH). To investigate the potential suitability as exhaled breath analyzers, we tested three sensor arrays consisting of mesoporous WO3 NFs templated by MWCNTs (MP_WO3 NFs), HP_WO3 NFs, and Apo-Pt@HP_WO3 NFs. The result revealed that targeted acetone molecules were clearly classified against six different interfering molecules (H2S, C7H8, C2H5OH, CO, NH3 and CH4) through principle component analysis (PCA), confirming excellent acetone selectivity of the designed sensor arrays.

Facile Co-precipitation synthesis and ethanol sensing performance of Pd loaded Sr doped SnO2 nanoparticles

In the present communication, we have presented a high performance ethanol sensor based on Pd loaded Sr doped SnO2 nanomaterial. The (0.5, 1.5, 2.5 and 3) wt% Pd loaded 4mol% Sr doped SnO2 nanoparticles were prepared using a co-precipitation method. The samples were characterized by X-ray Diffraction (XRD), High Resolution Transmission Electron Microscope (HR-TEM), Selected Area Electron Diffraction (SAED), Field Emission Gun-Scanning Electron Microscope (FEG-SEM), (SEM-EDAX) Scanning Electron Microscope-Energy Dispersive Analysis by X-ray techniques. The gas response studies such as sensitivity, selectivity and stability towards liquid petroleum gas (LPG), ammonia (NH3), acetone (CH3COCH3) and ethanol (C2H5OH) were measured at 100ppm concentrations. The results show that optimum Pd loading (2.5wt%) results in smaller crystallite size (~3.7nm), lower operating temperature (275°C), higher gas response (93%),better selectivity, faster response (~1s) and quicker recovery (~5s) towards ethanol.

Room temperature acetone-sensing properties of branch-like VO2 (B)@ZnO hierarchical hetero-nanostructures

Hierarchical branch-like VO2 (B)@ZnO hetero-nanostructures with highly crystalline ZnO nanorods grown radially on the surface of the VO2 (B) cores have been prepared via a three-step process. Briefly, the VO2 (B) cores were synthesized by a hydrothermal method and the ZnO shells were formed by two steps: the formation of VO2 (B)@ZnO heteroseeds via a surface adsorption process and the heteroepitaxial growth of ZnO nanorods. The effect of the growth time of ZnO nanorods on the microstructure and acetone-sensing properties was investigated. The crystal structure and morphology of the obtained samples were characterized by X-ray diffractometer, scanning electron microscopy and transmission electron microscope. And the gas-sensing properties of the VO2 (B)@ZnO hetero-nanostructures sensors to acetone were evaluated at room temperature. The results showed that the sensor based on the hetero-nanostructures under an optimal growth time has a high gas response, short response (~ 8 s) and recovery time (~ 18 s), good repeatability and excellent selectivity to 100 ppm acetone at room temperature. The efficient gas-sensing capability of the hetero-nanostructure sensor is associated with the beneficial effect of the hierarchical hetero-nanostructures with novel VO2/ZnO heterojunction and high surface area value.

Sensing performances of pure and hybridized carbon nanotubes-ZnO nanowire networks: A detailed study

In this work, the influence of carbon nanotube (CNT) hybridization on ultraviolet (UV) and gas sensing properties of individual and networked ZnO nanowires (NWs) is investigated in detail. The CNT concentration was varied to achieve optimal conditions for the hybrid with improved sensing properties. In case of CNT decorated ZnO nanonetworks, the influence of relative humidity (RH) and applied bias voltage on the UV sensing properties was thoroughly studied. By rising the CNT content to about 2.0 wt% (with respect to the entire ZnO network) the UV sensing response is considerably increased from 150 to 7300 (about 50 times). With respect to gas sensing, the ZnO-CNT networks demonstrate an excellent selectivity as well as a high gas response to NH3 vapor. A response of 430 to 50 ppm at room temperature was obtained, with an estimated detection limit of about 0.4 ppm. Based on those results, several devices consisting of individual ZnO NWs covered with CNTs were fabricated using a FIB/SEM system. The highest sensing performance was obtained for the finest NW with diameter (D) of 100 nm, with a response of about 4 to 10 ppm NH3 vapor at room temperature.

The gas sensing properties of hafnium oxide thin films depending on the annealing environment

The main focus of this study is to investigate the NO2 gas-sensing properties of HfO2 thin films, taking into the account the importance of the annealing environment and temperature. HfO2 films were produced with 10 nm and 20 nm thicknesses by atomic layer deposition (ALD). They were annealed under N2 and O2 environment with rapid thermal annealing system at two different temperatures. It was found that the NO2 responses of annealed in N2 environment were higher than annealed in O2 environment. The response of 10 nm thickness HfO2 thin film was calculated 22% for N2 environment whereas, it was calculated 18% for O2 environment. The highest gas response is obtained for 10 nm thickness of HfO2 thin film annealed in N2 environment. The performance of the HfO2 thin films reveals that the gas responses rely on annealing temperature as well as annealing atmosphere. The linear dependences of the sensitivity were also observed as the NO2 gas concentrations varied from 10 ppm to 30 ppm in logarithmic forms, indicating that HfO2 thin films offer promising potential for future practical applications in monitoring the NO2 gas.

A fast response & recovery acetone gas sensor based on BiFeO3 nanomaterials with high sensitivity and low detection limit

In present work, BiFeO3 was synthesized at different temperatures (450, 500, 550 °C) by tartaric acid sol–gel method. XRD, SEM and TEM were used to characterize microstructure and morphology of as-prepared BiFeO3. All results indicate that as-prepared BiFeO3 is the nanocrystal with single rhombohedrally distorted perovskite structure with the space group R3c and has irregular shapes and the average grain sizes between 50 and 120 nm. BiFeO3 nanomaterials have a low working temperature of 240 °C and high acetone gas response. Especially for BiFeO3 grown at 500 °C, the sensitivity to 50 ppm acetone is about as high as 30 at 240 °C. The response and recovery time of BiFeO3 sensors is respectively about 5 and 18 s. Moreover, BiFeO3 sensors have a low detection limit and the logarithmic curves of the sensitivity and concentration of BiFeO3 sensors satisfy the linear relationship in the low detection range. These results demonstrate that BiFeO3 can be used as an ideal candidate to fabricate high response acetone sensor. Gas sensing mechanism of BiFeO3 is also discussed on the basis of adsorption and desorption of reducing acetone gas and the reaction with oxygen species on the surface of BiFeO3 nanomaterials. This work can give rise to an interesting choice for researching excellent gas sensing properties of other nonstoichiometric oxides.

Gas-sensing performances of Cd-doped ZnO nanoparticles synthesized by a surfactant-mediated method for n-butanol gas

Zinc oxide nanoparticles with the different Cd doping contents were prepared by with a surfactant-mediated method in this paper. The effects of Cd doping on the gas sensing properties of the ZnO nanoparticles were studied. The morphology and microstructure of as-prepared samples were characterized by X-ray diffraction (XRD); scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM), respectively. The results reveal that all the products are the high crystalline hexagonal wurtzite ZnO crystal structure. The gas-sensing characteristics of the Cd doped ZnO nanoparticles for volatile organic compounds (VOCs) were investigated. At its optimal operation temperature of 300 °C, the sensing properties of the Cd doped ZnO nanoparticles for n-butanol gas exhibit a high-performance gas sensing performances including high gas response, good selectivity, response/recovery time, and repeatability as well as stability. Especially, its response reaches 130 for 100 ppm n-butanol of ZnO nanoparticles with 2.5% Cd doping. Those values demonstrate the potential of using as-prepared Cd doped ZnO nanoparticles for n-butanol gas detection, making them to be promising candidates for practical detectors to n-butanol gas. Apart from these, the mechanism related to the advanced properties was also investigated and presented.

CuO hollow microspheres self-assembled with nanobars: Synthesis and their sensing properties to formaldehyde

CuO hollow microspheres were synthesized via a facile hydrothermal process. Copper nitrate trihydrate (Cu(NO3)2·3H2O) was used as copper source and cetyltrimethylammonium bromide (CTAB) was used as soft template. The morphology and crystal structure of products were systematically investigated by XRD, FESEM, TEM, FT-IR and BET. CuO hollow microspheres were identified as monoclinic structure. The hollow microspheres were assembled by a number of nanobars which were composed of smaller nanograins. The formaldehyde gas sensing properties of CuO products with different morphologies were investigated. The results indicated that the sensor based on CuO hollow microspheres exhibited high gas response, fast reversible and response, good selectivity and stability to formaldehyde gas. This is attributed to hollow structures with the largest specific surface area and an effective gas diffusion path in favor of mass transportation and gas diffusion.

A novel coral rock-like ZnO and its gas sensing

In present work, a novel coral rock-like ZnO with porous structure was successfully synthesized via a solvothermal method and subsequent calcination. We were surprise to find that such ZnO nanostructure exhibited super high gas response to ethanol, which was attributed to the novel coral rock-like structure with porous nature, leading to effective gas diffusion and sufficient surface chemical reaction during sensing measurement.

Individual hollow and mesoporous aero-graphitic microtube based devices for gas sensing applications

In this work, individual hollow and mesoporous graphitic microtubes were integrated into electronic devices using a FIB/SEM system and were investigated as gas and vapor sensors by applying different bias voltages (in the range of 10 mV–1 V). By increasing the bias voltage, a slight current enhancement is observed, which is mainly attributed to the self-heating effect. A different behavior of ammonia NH3 vapor sensing by increasing the applied bias voltage for hollow and mesoporous microtubes with diameters down to 300 nm is reported. In the case of the hollow microtube, an increase in the response was observed, while a reverse effect has been noticed for the mesoporous microtube. It might be explained on the basis of the higher specific surface area (SSA) of the mesoporous microtube compared to the hollow one. Thus, at room temperature when the surface chemical reaction rate (k) prevails on the gas diffusion rate (DK) the structures with a larger SSA possess a higher response. By increasing the bias voltage, i.e., the overall temperature of the structure, DK becomes a limiting step in the gas response. Therefore, at higher bias voltages the larger pores will facilitate an enhanced gas diffusion, i.e., a higher gas response. The present study demonstrates the importance of the material porosity towards gas sensing applications.

Self-template derived CuO nanowires assembled microspheres and its gas sensing properties

Development of high-performance p-type semiconductor gas sensors especially the fast response/recovery speed and high response based on inexpensive non-noble metal oxides without doping or noble metal decoration for the practical applications of gas sensor are highly desirable. In this work, the rapid room-temperature synthesis of innovative and unique parallel copper oxide nanowires assembled microspheres (CuO NMs) with excellent gas sensing property is reported. The morphological characteristics, surface area, porosity, chemical composition and the chemical conversion process are studied carefully. As a result, the CuO NMs exhibit a relatively high gas response, especially the fast response and recovery time to 100ppm n-propanol vapor at relatively low operating temperature of 190°C, respectively. This new rapid self-template derived functional nanomaterials will offer a promising platform for highly sensitive and real-time monitoring p-type gas sensors.

Synthesis, characterization and enhanced acetone sensing performance of Pd loaded Sm doped SnO2 nanoparticles

Abstract In the present communication, we have presented a high performance acetone sensor based on Pd loaded Sm doped SnO 2 nanomaterial. The (0.5, 1, 2 and 3) wt% Pd loaded 6 mol% Sm doped SnO 2 nanoparticles were prepared using a co-precipitation method. The characterization of samples was done by using X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FEG-SEM), Energy Dispersive Analysis by X-rays (EDAX), High Resolution-Transmission Electron Microscopy (HR-TEM) and Selective Area Electron Diffraction (SAED) techniques. The gas response studies such as sensitivity, selectivity and stability towards liquid petroleum gas, ammonia, ethanol and acetone were measured at 100 ppm concentrations. The results show that optimum Pd loading (2 wt%) results in smaller crystallite size (~3.1 nm), lower operating temperature (200 °C), higher gas response (94%),better selectivity, faster response (~3 s) and quicker recovery (~12 s) towards acetone.