Enhanced Gas Response Research Articles

Synthesis, Characterization and Enhanced Sensing Properties of a NiO/ZnO p–n Junctions Sensor for the SF6 Decomposition Byproducts SO2, SO2F2, and SOF2

The detection of partial discharge and analysis of the composition and content of sulfur hexafluoride SF6 gas components are important to evaluate the operating state and insulation level of gas-insulated switchgear (GIS) equipment. This paper reported a novel sensing material made of pure ZnO and NiO-decorated ZnO nanoflowers which were synthesized by a facile and environment friendly hydrothermal process for the detection of SF6 decomposition byproducts. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were used to characterize the structural and morphological properties of the prepared gas-sensitive materials. Planar-type chemical gas sensors were fabricated and their gas sensing performances toward the SF6 decomposition byproducts SO2, SO2F2, and SOF2 were systemically investigated. Interestingly, the sensing behaviors of the fabricated ZnO nanoflowers-based sensor to SO2, SO2F2, and SOF2 gases can be obviously enhanced in terms of lower optimal operating temperature, higher gas response and shorter response-recovery time by introducing NiO. Finally, a possible gas sensing mechanism for the formation of the p–n junctions between NiO and ZnO is proposed to explain the enhanced gas response. All results demonstrate a promising approach to fabricate high-performance gas sensors to detect SF6 decomposition byproducts.

H2 response characteristics for sol–gel-derived WO3-SnO2 dual-layer thin films

This paper describes the deposition of SnO2 and WO3 thin films and WO3-SnO2 dual-layer thin films using the sol-gel process. The microstructure and morphology of these three thin films were analyzed with FE-SEM and X-ray diffraction. The H2 response characteristics, including response magnitude, time and transients of the three samples, were investigated at different operation temperatures and H2 gas concentrations. Although the maximum response magnitude of 29.31 towards 1000ppmH2 gas appeared at 225°C，the WO3-SnO2 dual-layer films still had a response magnitude of 24.23 at 175°C, which is much higher than those of the SnO2 (4.19) and WO3 (6.73) thin films. The linear response magnitude profile of the WO3-SnO2 dual-layer thin films toward H2 gas concentration was obtained. The mechanism of the enhanced gas response characteristics was explained by the band bending theory.

P-Type aliovalent Li(I) or Fe(III)-doped CuO hollow spheres self-organized by cationic complex ink printing: Structural and gas sensing characteristics

The p-type CuO hollow sphere films doped with aliovalent Li(I) or Fe(III) have been prepared directly on the substrate through a self-organization process in the printed films using the formulated cationic complex inks. The Li(I) or Fe(III) ions are substitutionally incorporated into CuO lattice with the variation in lattice parameters and act as a shallow acceptor or donor leading to the increase or decrease of the hole density, respectively. Such an electronic modification shifts the optical band gap and binding energy of Cu. The increased hole density by Li(I) doping reduces the overall sensor resistance and the width of hole-accumulation layer (λd) in CuO grains, resulting in the decrease in gas response toward the reducing ethanol gas to ca. 50% compared to that of undoped CuO. In contrast, the decreased hole density by Fe(III) doping increases the sensor resistance and λd, leading to the enhancement in gas response to ca. 150%, based on the electronic sensitization mechanism. These results show clearly that the substitutional doping of aliovalent cation is effective for the manipulation of electronic structure and gas sensing properties of p-type CuO.

Enhanced gas sensing properties to acetone vapor achieved by α-Fe2O3 particles ameliorated with reduced graphene oxide sheets

A low-cost and environmentally friendly hydrothermal method was utilized to prepare reduced graphene oxide (rGO) and synthesize rGO/α-Fe2O3 composites with different rGO contents. The chemical composition and morphological essence of the as-prepared samples were characterized through multiple techniques. The results indicated that the uniform α-Fe2O3 cubes adhered uniformly on both sides of the crumpled and rippled rGO sheets. In addition, a series of resistive-type gas sensors were fabricated based on the as-prepared rGO/α-Fe2O3 composites as well as pure α-Fe2O3 to compare their gas-sensing properties toward acetone vapor. The composite containing 1.0wt% rGO exhibited an enhanced gas response and its response time was shortened to 2s. We attribute it to the extension of electron depletion layers, the change of charge carrier concentration, which are caused by the formation of local p-n heterojunctions when introducing rGO.

The crystal facet-dependent gas sensing properties of ZnO nanosheets: Experimental and computational study

Herein, we focused on the effects of exposed crystal planes on the gas sensing property of ZnO. For this purpose, we designed and synthesized two porous ZnO nanosheets with different exposed crystal facets (0001) and (101¯0) by a facile hydrothermal routes. The characterization results show that both the porous nanosheets have a near specific surface area about 7.5m2/g, thickness about 100nm, diameter about 5μm and pore size of tens of nanometers. However, their dominating exposed crystal facets are (0001) and (101¯0), respectively. When employed them as sensing materials in gas sensors, porous ZnO nanosheets with dominating exposed (0001) facet exhibit a superior sensitivity than the (101¯0) one. The enhanced gas response is attributed to a large amount of oxygen vacancy defects and unsaturated dangling bonds existing in the ZnO nanosheets with exposed crystal facet (0001), which is favorable for the adsorption of gas molecular on the sensor surface and result in improvement of the gas response. Finally, the calculation of the chemisorption energy of oxygen on ZnO crystal facets also proves the reactive-facet-enhanced gas sensitivity.

Room temperature ferromagnetism and gas sensing in ZnO nanostructures: Influence of intrinsic defects and Mn, Co, Cu doping

Undoped and transition metal (Cu, Co and Mn) doped ZnO nanostructures were successfully prepared via a microwave-assisted hydrothermal method followed by annealing at 500°C. Numerous characterization facilities such as X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), and high-resolution transmission electron microscopy (HRTEM) were employed to acquire the structural and morphological information of the prepared ZnO based products. Combination of defect structure analysis based on photoluminescence (PL) and electron paramagnetic resonance (EPR) indicated that co-existing oxygen vacancies (VO) and zinc interstitials (Zni) defects are responsible for the observed ferromagnetism in undoped and transition metal (TM) doped ZnO systems. PL analysis demonstrated that undoped ZnO has more donor defects (VO and Zni) which are beneficial for gas response enhancement. Undoped ZnO based sensor exhibited a higher sensor response to NH3 gas compared to its counterparts owing to high content of donor defects while transition metal doped sensors showed short response and recovery times compared to undoped ZnO.

Design of α-Fe2O3 nanorods functionalized tubular NiO nanostructure for discriminating toluene molecules.

A novel tubular NiO nanostructure was synthesized by a facile and low-cost hydrothermal strategy and then further functionalized by decorating α-Fe2O3 nanorods. The images of electron microscopy indicated that the α-Fe2O3 nanorods were assembled epitaxially on the surfaces of NiO nanotubes to form α-Fe2O3/NiO nanotubes. As a proof-of-concept demonstration of the function, gas sensing devices were fabricated from as-prepared α-Fe2O3/NiO nanotubes, and showed enhanced gas response and excellent selectivity toward toluene, giving a response of 8.8 to 5 ppm target gas, which was about 7.8 times higher than that of pure NiO nanotubes at 275 °C. The improved gas sensing performance of α-Fe2O3/NiO nanotubes could be attributed to the unique tubular morphology features, p-n heterojunctions and the synergetic behavior of α-Fe2O3 and NiO.

Hydrothermally grown ZnO nanorods arrays for selective NO2 gas sensing: Effect of anion generating agents

Vertically aligned ZnO nanorods (ZNRs) arrays with various aspect ratios were deposited by using a simple and inexpensive hydrothermal route at relatively low temperature of 90°C. The influence of hydroxide anion generating agents in the solution on the growth of ZNRs arrays was studied. Hexamethylenetetramine (HMTA) and ammonia were used as hydroxide anion generating agents while polyethyleneimine (PEI) as structure directing agent. The combined effect of these three agents plays a crucial role in the growth of ZNRs arrays with respect to their rod length and diameter, which controls the aspect ratio. The deposited ZNRs exhibited hexagonal wurtize crystal structure with preferred orientation along (002) plane. The highly crystalline nature and pure phase formation of ZNRs was confirmed from FT-Raman studies. The maximum gas response (Rg/Ra) of 67.5 was observed for high aspect ratio ZNRs, deposited with combination of HMTA, ammonia as well as PEI. The enhancement in gas response can be attributed to high surface area (45 cm2/g) and desirable surface accessibility in high aspect ratio ZNRs. Fast response–recovery characteristics, especially a much quicker gas response time of 32s and recovery time of 530s were observed at 100ppm NO2 gas concentration.

A Volatile Organic Compound Sensor Using Porous Co3O4 Spheres

Porous Co3O4 spheres with bimodal pore distribution (size: 2-3 nm and ~ 30 nm) were prepared by ultrasonic spray pyrolysis of aqueous droplets containing Co-acetate and polyethylene glycol (PEG), while dense Co3O4 secondary particles with monomodal pore distribution (size: 2-3 nm) were prepared from the spray solution without PEG. The formation of mesopores (~ 30 nm) was attributed to the decomposition of PEG. The responses of a porous Co3O4 sensor to various indoor air pollutants such as 5 ppm C2H5OH, xylene, toluene, benzene, and HCHO at 200°C were found to be significantly higher than those of a commercial sensor using Co3O4 and dense Co3O4 secondary particles. Enhanced gas response of porous Co3O4 sensor was attributed to high surface area and the effective diffusion of analyte gas through mesopores (~ 30 nm). Highly sensitive porous Co3O4 sensor can be used to monitor various indoor air pollutants. Key words: Gas sensor, Co3O4, Volatile organic compound, Spray pyrolysis

Fabrication and gas-sensing performance of nanorod-assembled SnO2 nanostructures

The manner how nano building blocks assemble into hierarchical architectures exerts a tremendous influence on gas-sensing property of the metal oxides. Herein, we focus on tuning the 1D SnO2 rectangular nanorods into 3D hierarchical nanoflowers by manipulating the concentration of surfactant (citric acid) as well as the period of hydrothermal process, and then investigate their gas-sensing performance. Nanostructures assembled by short, unordered and well-ordered rectangular nanorods were successfully fabricated, corresponding to infant, abnormal and blooming SnO2 nanoflowers, respectively. In comparison, sensor based on blooming SnO2 nanoflowers exhibited significantly enhanced gas response to ethanol than the other two, not only in terms of the sensitivity but also the response/recovery time, which may be mainly attributed to the sufficient diffusion channel and exposed surface provided by finely dispersed nanorods in the blooming nanoflowers.

Effect of Growth Temperature and Time on Morphology and Gas Sensitivity of Cu2O/Cu Microstructures

A facile hydrothermal synthesis with CuSO4as the copper source was used to prepare micro/nano-Cu2O. The obtained samples have been characterized by X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). With increasing the reaction temperature and time, the final products were successively Cu2O octahedron microcrystals, Cu2O/Cu composite particles, and a wide range of Cu spherical particles. The gas sensitivity of products towards ethanol and acetone gases was studied. The results showed that sensors prepared with Cu2O/Cu composites synthesized at 65°C for 15 min exhibited optimal gas sensitivity. The gas sensing mechanism and the effect of Cu in the enhanced gas response were also elaborated. The excellent gas sensitivity indicates that Cu2O/Cu composites have potential application as gas sensors.

Effects of rare earth element doping on the ethanol gas-sensing performance of three-dimensionally ordered macroporous In2O3

Rare earth (Tm, Er, La, Yb and Ce)-doped In2O3 nanostructures with three-dimensionally ordered macroporous structures (3DOM) were prepared by a colloidal crystal templating method, and their ethanol sensing properties were investigated. Rare earth doping improved the gas response and lowered the optimum operating temperature. Tm-doped 3DOM In2O3 had the best gas-sensing performance, and the gas sensitivity for 100 ppm ethanol was seven times higher than that of pure 3DOM In2O3. The reason for the different sensitivities generated by the five doping elements and the underlying mechanism of the enhanced gas response for the samples were studied.

Fabrication of Cubic p-n Heterojunction-Like NiO/In2O3 Composite Microparticles and Their Enhanced Gas Sensing Characteristics

Oxide semiconductor In2O3 has been extensively used as a gas sensing material for the detection of various toxic gases. However, the pure In2O3 sensor is always suffering from its low sensitivity. In the present study, a dramatic enhancement of sensing characteristic of cubic In2O3 was achieved by deliberately fabricating p-n heterojunction-like NiO/In2O3 composite microparticles as sensor material. The NiO-decorated In2O3 p-n heterojunction-like sensors were prepared through the hydrothermal transformation method. The as-synthesized products were characterized using SEM-EDS, XRD, and FT-IR, and their gas sensing characteristics were investigated by detecting the gas response. The experimental results showed that the response of the NiO/In2O3 sensors to 600 ppm methanal was 85.5 at 260°C, revealing a dramatic enhancement over the pure In2O3 cubes (21.1 at 260°C). Further, a selective detection of methanol with inappreciable cross-response to other gases, like formaldehyde, benzene, methylbenzene, trichloromethane, ethanol, and ammonia, was achieved. The cause for the enhanced gas response was discussed in detailed. In view of the facile method of fabrication of such composite sensors and the superior gas response performance of samples, the cubic p-n heterojunction-like NiO/In2O3 sensors present to be a promising and viable strategy for the detection of indoor air pollution.

Synthesis of CuO microstructures with controlled shape and size and their exposed facets induced enhanced ethanol sensing performance

Ethanol-sensitive sensors were fabricated using various CuO microstructures with controlled size and shape. For the materials preparation, Cu2O microstructures were first synthesized by a facile wet chemical reaction and then were oxidized into CuO under air conditions. Hexahedron-, octahedron-, decahedron- and flower-shaped CuO microstructures were obtained. The particle size of CuO decahedrons can be tailored from 1.27 to 2.23μm by simply adjusting the amount of glucose. Gas sensing properties of various CuO microstructures were also compared. It is interesting that the gas response of the CuO decahedron samples increased with increasing the particle size. The one with the maximum average particle size (2.23μm) has gas responses ≥3 times larger than the one with the minimum average particle size (1.27μm), toward 5 to 100ppm ethanol vapor. It was found that the gas response of the CuO microstructures increased with the increase of the exposure extent of the low index (111¯) facet relative to high index (202¯) facet. This phenomenon was also observed in the gas sensing tests of CuO microstructures with lower crystal quality and different shape. The enhanced gas response may be attributed to the higher exposure extent of the (111¯) facet with highly activity. In addition, the crystal quality, surface defect density and shape also play important roles in the gas sensing performance.

전기방사방법에 의해 합성된 ZnO 중공 나노섬유의 trimethylamine 가스 감응 특성

Abstract Pure and Mo-doped ZnO hollow nanofibers were prepared by single capillary electrospinning and their gas sensing characteristicstoward 5 ppm ethanol, trimethylamine (TMA), CO and H 2 were investigated. The gas responses and responding kinetics were depen-dent upon sensing temperature and Mo doping. Mo-doped ZnO hollow nanofibers showed high response to 5 ppm TMA ( R a /R g = 111.7,R a : resistance in air, R g : resistance in gas) at 400 o C, while the responses of pure ZnO hollow nanofibers was low (R a /R g = 47.1). In addi-tion, the doping of Mo enhanced selectivity toward TMA. The enhancement of gas response and selectivity to TMA by Mo dopingto ZnO nanofibers was discussed in relation to the interaction between basic analyte gas and acidic additive materials. Keywords: ZnO nanofiber, Molybdenum, Gas sensor, Oxide semiconductor, Electrospinning 1. 서론 폭발성가스, 환경가스, 독성가스, 농수산물에서 발생되는 가스등의 미량검지를 위해서 감도 및 선택성이 우수한 가스센서가요구되고 있다. 산화물 반도체형 가스센서는 감응물질의 표면과피검 가스의 반응에 의한 저항 변화를 이용하여 가스를 검출한다[1]. 가스 감응물질로 SnO

Synthesis of fast response, highly sensitive and selective Ni:ZnO based NO2 sensor

The use of metal oxide semiconductor (MOS) gas sensors is limited due to lack of selectivity and high operating temperature. The aim of present investigation is to enhance gas response and sensitivity of the zinc oxide (ZnO) based thin film sensor towards nitrogen dioxide (NO2) by Ni doping. The achieved sensitivity is around 4.2%/ppm at moderate operating temperature of 200°C. The gas response of 108% and 482% is observed at 200°C operating temperature, towards 5ppm and 100ppm NO2, respectively. Ni doping increased the NO2 response and reduced the lower detection limit of NO2 to 5ppm which is much lower than the emergency exposure limit (20ppm). The hybrid structure of nanograined rods and hexagonal flakes are observed which enhanced gas response. The gas response dependence on various physical properties and chemical composition of the sensor is also studied. The response and recovery time of 1.5 atomic percentage (at%) Ni doped ZnO thin film sensor is 11 and 123s, respectively. The response of the sensor is reproducible and it has negligible cross sensitivity for other interfering gases such as CO2, SO2, H2S, NH3, LPG, methanol, ethanol, and acetone.

Highly selective and sensitive xylene sensors using Ni-doped branched ZnO nanowire networks

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 NiCl2 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 5ppm p-xylene (S=42.44) at 400°C. This value is 1.7 and 2.5 times higher than the responses to 5ppm toluene (S=25.73) and C2H5OH (S=16.72), respectively, and significantly higher than the cross-responses to other interfering gases such as benzene, HCHO, trimethylamine, H2, 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.

Effect of solution concentration on physicochemical and gas sensing properties of sprayed WO3 thin films

Sub-stoichiometric tungsten trioxide (WO3) thin films are deposited onto the glass substrates by spray pyrolysis technique using ammonium metatungstate. Effect of solution concentration on structural, morphological, optical, electrical and NO2 sensing properties of WO3 thin films is studied. Films are polycrystalline with monoclinic crystal structure and sub-stoichiometric as observed form the XRD and XPS studies, respectively. The SEM and AFM images show micro grained structure and surface roughness increases with increase in solution concentration. The PL studies revealed that the majority of the defects are the oxygen vacancies. From XPS and PL studies it is observed that, oxygen vacancies decrease with increase in solution concentration. The dielectric constant of the films as a function of frequency is in concurrence with resistivity measurements. Films show reproducible and reversible gas response at various operating temperatures and gas concentrations. Highest sensor response (38%) towards 200 ppm NO2 concentration is observed for the film with 15 mM solution concentration at moderate operating temperature (200 °C). Pd sensitization enhanced gas response to 68% and improved kinetics of the sensor. Films are highly selective towards NO2 as compared with the various gases such as SO2, LPG, NH3 and H2S.

Scalable fabrication of SnO2 thin films sensitized with CuO islands for enhanced H2S gas sensing performance

The detection of H2S, an important gaseous molecule that has been recently marked as a highly toxic environmental pollutant, has attracted increasing attention. We fabricate a wafer-scale SnO2 thin film sensitized with CuO islands using microelectronic technology for the improved detection of the highly toxic H2S gas. The SnO2–CuO island sensor exhibits significantly enhanced H2S gas response and reduced operating temperature. The thickness of CuO islands strongly influences H2S sensing characteristics, and the highest H2S gas response is observed with 20nm-thick CuO islands. The response value (Ra/Rg) of the SnO2–CuO island sensor to 5ppm H2S is as high as 128 at 200°C and increases nearly 55-fold compared with that of the bare SnO2 thin film sensor. Meanwhile, the response of the SnO2–CuO island sensor to H2 (250ppm), NH3 (250ppm), CO (250ppm), and LPG (1000ppm) are low (1.3–2.5). The enhanced gas response and selectivity of the SnO2–CuO island sensor to H2S gas is explained by the sensitizing effect of CuO islands and the extension of electron depletion regions because of the formation of p–n junctions.

3D flower- and 2D sheet-like CuO nanostructures: Microwave-assisted synthesis and application in gas sensors

3D flower- and 2D branching sheet-like CuO nanostructures have been synthesized by a microwave-assisted hydrothermal method. The samples were characterized through X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), infrared spectroscopy (IR), UV–vis diffuse reflectance spectroscopy (UV-DRS), and Brunauer–Emmett–Teller (BET) specific surface area analysis. The phase and morphology observations showed the formation of monoclinic CuO nanostructures with well-defined morphology. BET analysis displayed that the measured surface area was 15.0m2g−1 for CuO flowers, and 20.8m2g−1 for CuO nanosheets. Gas sensing properties of the as-synthesized CuO nanostructures were evaluated by the detection of volatile and toxic gases including ethanol, ethyl-acetate, acetone, xylene, and toluene. It was found that CuO flowers exhibited an enhanced gas response to the five gases at 260°C, compared to CuO nanosheets. Furthermore, at 1000ppm, the CuO flower sensor gave a higher response to ethyl-acetate (Rg/Ra=4.6) and ethanol (Rg/Ra=4.0), in comparison with toluene (Rg/Ra=2.8). In addition, the CuO flowers displayed a rapid response and recovery as well as good reproducibility.