Electrical response of Sm2O3-doped SnO2 to C2H2 and effect of humidity interference

Pure and Sm₂O₃-doped SnO₂ are prepared through a sol–gel method and characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The sensor based on 6 wt% Sm₂O₃-doped SnO₂ displays superior response at an operating temperature of 180 °C, and the response magnitude to 1000 ppm C₂H₂ can reach 63.8, which is 16.8 times larger than that of pure SnO₂. This sensor also shows high sensitivity under various humidity conditions. These results make our product be a good candidate in fabricating C₂H₂ sensors.     

Enhanced H2S sensing characteristics of SnO2 nanowires functionalized with CuO

The CuO-functionalized SnO₂ nanowire (NW) sensors were fabricated by depositing a slurry containing SnO₂ NWs on a polydimethylsiloxane (PDMS)-guided substrate and subsequently dropping Cu nitrate aqueous solution. The CuO coating increased the gas responses to 20 ppm H₂S up to 74-fold. The R_a/R_g value of the CuO-doped SnO₂ NWs to 20 ppm H₂S was as high as 809 at 300 °C, while the cross-gas responses to 5 ppm NO₂, 100 ppm CO, 200 ppm C₂H₅OH, and 100 ppm C₃H₈ were negligibly low (1.5–4.0). Moreover, the 90% response times to H₂S were as short as 1–2 s at 300–400 °C. The selective detection of H₂S and enhancement of the gas response were attributed to the uniform distribution of the sensitizer (CuO) on the surface of the less agglomerated network of the SnO₂ NWs.     

Electrostatic sprayed SnO2 and Cu-doped SnO2 films for H2S detection

This paper presents the ability of electrostatic sprayed tin oxide (SnO₂) and tin oxide doped with copper oxide (1, 2, and 4 at.% Cu) films to detect different pollutant gases, i.e., H₂S, SO₂, and NO₂. The influence of a copper oxide dopant on the SnO₂ morphology is studied using scanning electron microscopy (SEM) technique, which reveals a small decrease in the porosity and particle size when the amount of dopant is increased. The sensing properties of the SnO₂ films are greatly improved by doping, i.e., the Cu-doped SnO₂ films have large response to low concentration (10 ppm) of H₂S at low operating temperature (100 °C). Furthermore, no cross-sensitivity to 1 ppm NO₂ and 20 ppm SO₂ is observed. Among the studied films, the 1 at.% Cu-doped SnO₂ layer is the most sensitive in the detection of all the studied gases.     

Controlled synthesis and gas-sensing properties of hollow sea urchin-like α-Fe2O3 nanostructures and α-Fe2O3 nanocubes

Hollow sea urchin-like α-Fe₂O₃ nanostructures were successfully synthesized by a hydrothermal approach using FeCl₃ and Na₂SO₄ as raw materials, and subsequent annealing in air at 600 °C for 2 h. The hollow sea urchin-like α-Fe₂O₃ nanostructures with the diameters of 2–4.5 μm consist of well-aligned α-Fe₂O₃ nanorods with an average length of about 1 μm growing radially from the centers of the nanostructures, have a hollow interior with a diameter of about 2 μm. α-Fe₂O₃ nanocubes with a diameter of 700–900 nm were directly obtained by a hydrothermal reaction of FeCl₃ at 140 °C for 12 h. The response S_r (S_r = R_a/R_g) of the hollow sea urchin-like α-Fe₂O₃ nanostructures reached 2.4, 7.5, 5.9, 14.0 and 7.5 to 56 ppm ammonia, 32 ppm formaldehyde, 18 ppm triethylamine, 34 ppm acetone, and 42 ppm ethanol, respectively, which was excess twice that of the α-Fe₂O₃ nanocubes and the nanoparticle aggregations. Our results demonstrated that the hollow sea urchin-like α-Fe₂O₃ nanostructures were very promising for gas sensors for the detection of flammable and/or toxic gases with good-sensing characteristics.     

Fabrication of N-type Fe2O3 and P-type LaFeO3 nanobelts by electrospinning and determination of gas-sensing properties

N-type Fe₂O₃ nanobelts and P-type LaFeO₃ nanobelts were prepared by electrospinning. The structure and micro-morphology of the materials were characterized by X-ray diffraction (XRD) and scanning of electron microscopy (SEM). The gas sensing properties of the materials were investigated. The results show that the optimum operating temperature of the gas sensors fabricated from Fe₂O₃ nanobelts is 285 °C, whereas that from LaFeO₃ nanobelts is 170 °C. Under optimum operating temperatures at 500 ppm ethanol, the response of the gas sensors based on these two materials is 4.9 and 8.9, respectively. The response of LaFeO₃-based gas sensors behaves linearly with the ethanol concentration at 10-200 ppm. Sensitivities to different gases were examined, and the results show that LaFeO₃ nanobelts exhibit good selectivity to ethanol, making them promising candidates as practical detectors of ethanol.     

Solid-state potentiometric SO2 sensor combining NASICON with V2O5-doped TiO2 electrode

A compact tubular sensor based on NASICON (sodium super ionic conductor) and V₂O₅-doped TiO₂ sensing electrode was designed for the detection of SO₂. In order to reduce the size of the sensor, a thick-film of NASICON was formed on the outer surface of a small Al₂O₃ tube; furthermore, a thin layer of V₂O₅-doped TiO₂ with nanometer size was attached on the NASICON as a sensing electrode. This paper investigated the influence of V₂O₅ doping and sintering temperature on the characteristics of the sensor. The sensor attached with 5 wt% V₂O₅-doped TiO₂ sintered at 600 °C exhibited excellent sensing properties to 1–50 ppm SO₂ in air at 200–400 °C. The EMF value of the sensor was almost proportional to the logarithm of SO₂ concentration and the sensitivity (slope) was −78 mV/decade at 300 °C. It was also seen that the sensor showed a good selectivity to SO₂ against NO, NO₂, CH₄, CO, NH₃ and CO₂. Moreover, the sensor had speedy response kinetics to SO₂ too, the 90% response time to 50 ppm SO₂ was 10 s, and the recovery time was 35 s. On the basis of XPS analysis for the SO₂-adsorbed sensing electrode, a sensing mechanism involving the mixed potential at the sensing electrode was proposed.     

Influence of Cs doping in spray deposited SnO2 thin films for LPG sensors

Detection of low concentrations of petroleum gas was achieved using transparent conducting SnO₂ thin films doped with 0–4 wt.% caesium (Cs), deposited by spray pyrolysis technique. The electrical resistance change of the films was evaluated in the presence of LPG upon doping with different concentrations of Cs at different working temperatures in the range 250–400 °C. The investigations showed that the tin oxide thin film doped with 2% Cs with a mean grain size of 18 nm at a deposition temperature of 325 °C showed the maximum sensor response (93.4%). At a deposition temperature of 285 °C, the film doped with 3% Cs with a mean grain size of 20 nm showed a high response of 90.0% consistently. The structural properties of Cs-doped SnO₂ were studied by means of X-ray diffraction (XRD); the preferential orientation of the thin films was found to be along the (3 0 1) directions. The crystallite sizes of the films determined from XRD are found to vary between 15 and 60 nm. The electrical investigations revealed that Cs-doped SnO₂ thin film conductivity in a petroleum gas ambience and subsequently the sensor response depended on the dopant concentration and the deposition temperature of the film. The sensors showed a rapid response at an operating temperature of 345 °C. The long-term stability of the sensors is also reported.     

Tetrakis(alkylthio)-substituted lutetium bisphthalocyanines for sensing NO2 and O3

In this study, the nitrogen dioxide (NO₂) and ozone (O₃) sensing properties of a series bis[tetrakis(alkylthio) phthalocyaninato] lutetium(III) complexes [(C_nH_2n+1S)₄Pc]₂Lu(III) (n = 6, 10, 16) are investigated as a function of concentration in the temperature range between 25 °C and 150 °C. The concentration ranges were 1–10 ppm for NO₂, and 50 ppb–1 ppm for O₃. The response time and the sensor response to NO₂ are measured for approximately 1 min and 100% ppm⁻¹, respectively, for compound 1 at room temperature. At room temperature, all compounds are in the solid phase. The response time decreases to a few seconds with increasing operation temperature to 150 °C. At this temperature, all compounds are in the liquid crystal phase. The fastest response to oxidizing gases is observed at the liquid crystal phase of the Pcs. It has also been observed that the response time and the sensor response depend on the alkyl chain lengths of the Pcs. The doping effect of oxygen has been determined under high purity nitrogen N₂ flow, after exposure to dry air, at a different period of time and after annealing. It has been found that the conductivities of [(C_nH_2n+1S)₄Pc]₂Lu(III) thin films increased after exposure to dry air and the conduction mechanism also changed from ohmic behavior to space-charge-limited conduction.     

Necked ZnO nanoparticle-based NO2 sensors with high and fast response

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 0.2 ppm of NO₂ at 200 °C. As the concentration of NO₂ increased to 5 ppm, their response was enhanced to approximately 400. During the repeated injection of NO₂ gas with a concentration of 0.4 ppm, the sensors exhibited stable response characteristics. Furthermore, the 90% response and recovery times of the gas sensor were as fast as 13 and 10 s, 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.     

Gas sensing properties of SnO2 nanowires on micro-heater

The SnO₂ nanowires (NWs) network gas sensors were fabricated on a micro-electrode and heater suspended in a cavity. The sensors showed selective detection to C₂H₅OH at a heater power during sensor operation as low as 30-40 mW. The gas response and response speed of the SnO₂ NWs sensor to 100 ppm C₂H₅OH were 4.6- and 4.7-fold greater, respectively, than those of the SnO₂ nanoparticles (NPs) sensor with the same electrode geometry. The reasons for these enhanced gas sensing characteristics are discussed in relation to the sensing materials and sensor structures.     

Fabrication at wafer level of miniaturized gas sensors based on SnO2 nanorods deposited by PECVD and gas sensing characteristics

SnO₂ nanorods were successfully deposited on 3″ Si/SiO₂ wafers by inductively coupled plasma-enhanced chemical vapour deposition (PECVD) and a wafer-level patterning of nanorods layer for miniaturized solid state gas sensor fabrication were performed. Uniform needle-shaped SnO₂ nanorods in situ grown were obtained under catalyst- and high temperature treatment-free growth condition. These nanorods have an average diameter between 5 and 15 nm and a length of 160-300 nm. The SnO₂-nanorods based gas sensors were tested towards NH₃ and CH₃OH and gas sensing tests show remarkable response, showing promising and repeatable results compared with the SnO₂ thin films gas sensors.     

Facile synthesis of hierarchical SnO2 semiconductor microspheres for gas sensor application

Hierarchical SnO₂ microspheres were synthesized by a hydrothermal method at 140 °C using stannic chloride hydrate and sodium hydroxide as starting materials. The individual hierarchical SnO₂ microsphere ranged from 700 to 900 nm in diameter. After these microspheres were heated at 600 °C for 2 h, the spheres were cross-linked into clusters by short SnO₂ nanorods as revealed by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Most importantly, SnO₂ hierarchical microsphere sensor exhibits excellent selectivity and fast response to ethanol. Response and recovery times were 0.6 s and 11 s when the sensor was exposed to 50 ppm ethanol at an operating temperature of 300 °C. Thus, hierarchical structures play a significant role in the field of gas sensing.     

Fe2O3 modified thick films of nanostructured SnO2 powder consisting of hollow microspheres synthesized from pyrolysis of ultrasonically atomized aerosol for LPG sensing

Nanostructured hollow spheres of SnO₂ with fine nanoparticles were synthesized by ultrasonic atomization. Thick film gas sensors were fabricated by screen printing technique. Different surface modified films (Fe₂O₃ modified SnO₂) were obtained by dipping them into an aqueous solution (0.01 M) of ferric chloride for different intervals of time followed by firing at 500 °C. The structural and microstructural studies of the samples were carried out using XRD, SEM, and TEM. The sensing performance of pure and modified films was studied by exposing various gases at different operating temperatures. One of the modified sample exhibited high response (1990) to 1000 ppm of LPG at 350 °C. Optimum amount of Fe₂O₃ dispersed evenly on the surface_, adsorption and spillover of LPG on Fe₂O₃ misfits and high capacity of adsorption of oxygen on nanostructured hollow spheres may be the reasons of high response.     

Synthesis of novel SnO2/ZnSnO3 core-shell microspheres and their gas sensing properities

Large-scale novel core-shell structural SnO₂/ZnSnO₃ microspheres were successfully synthesized by a simple hydrothermal method with the help of the surfactant poly(vinyl pyrrolidone) PVP. The as-synthesized samples were characterized using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). The results indicate that the shell was formed by single crystalline ZnSnO₃ nanorods and the core was formed by aggregated SnO₂ nanoparticles. The effects of PVP and hydrothermal time on the morphology of SnO₂/ZnSnO₃ were investigated. A possible formation mechanism of these hierarchical structures was discussed. Moreover, the sensor performance of the prepared core-shell SnO₂/ZnSnO₃ nanostructures to ethanol was studied. The results indicate that the as-synthesized samples exhibited high response and quick response-recovery to ethanol.     

Characterization of electrospun ZnO–SnO2 nanofibers for ethanol sensor

ZnO–SnO₂ nanofibers have been developed through in situ electrospinning technique and calcination. Poly(vinyl pyrrolidone) (PVP) is selected as fiber template. The composition of products can be controlled concisely by adjusting the compositions in their precursors. Under the optimized experimental conditions, the prepared product shows the desirable sensing characteristics towards ethanol gas at 300 °C, such as high response, excellent linearity in the range of 1–300 ppm, quick response time (5 s) and recovery time (6 s), good reproducibility, stability and selectivity.     

Potentiometric VOC detection in air using 8YSZ-based oxygen sensor modified with SmFeO3 catalytic layer

Potentiometric oxygen sensor was fabricated and applied to detect several volatile organic compounds (VOCs; acetic acid, methylethylketone (MEK), ethanol, benzene, toluene, o- and p-xylene) at sub-ppm levels in the temperatures range of 400–500 °C. The electromotive force (EMF) linearly changed with the logarithm of VOC concentration. Especially for ethanol and MEK, the sensitivity and EMF at 1 ppm were distinctly lowered for the sensor with the SmFeO₃ coated Pt working electrode. It seems that ethanol and MEK were more easily oxidized on the SmFeO₃ surface than the other VOCs. A discriminative detection of ethanol and MEK apart from the others could be achieved with the combination of two types of the sensors, Pt|8YSZ|Pt(ref.) and SmFeO₃/Pt|8YSZ|Pt(ref.).     

Au-doped WO3-based sensor for NO2 detection at low operating temperature

Pure and Au-doped WO₃ powders for NO₂ gas detection were prepared by a colloidal chemical method, and characterized via X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The NO₂ sensing properties of the sensors based on pure and Au-doped WO₃ powders were investigated by HW-30A gas sensing measurement. The results showed that the gas sensing properties of the doped WO₃ sensors were superior to those of the undoped one. Especially, the 1.0 wt% Au-doped WO₃ sensor possessed larger response, better selectivity, faster response/recovery and better longer term stability to NO₂ than the others at relatively low operating temperature (150 °C).     

H2 sensing characteristics of highly textured Pd-doped SnO2 thin films

The effects of the crystallographic orientation on the H₂ gas sensing properties were investigated in highly oriented polycrystalline Pd-doped SnO₂ films, which were obtained using rf magnetron sputtering of a Pd (0.5 wt%)-SnO₂ target on various substrates (a-, m-, r-, and c-cut sapphire and quartz). All the films had a similar thickness (110 nm), root-mean-square (rms) roughness (1.3 nm), surface area, and chemical status (O, Sn, and Pd). However, the orientation of the films was strongly affected by the orientation of the substrates. The (1 0 1), (0 0 2), and (1 0 1) oriented films were grown on (a-cut), (m-cut), and (r-cut) Al₂O₃ substrates, respectively, and rather randomly oriented films were deposited on (0 0 0 1) (c-cut) Al₂O₃ and quartz substrates. In addition, the oriented Pd-doped SnO₂ films were highly textured and had in-plane orientation relationships with the substrates similar to the epitaxial films. The (1 0 1) Pd-doped SnO₂ films on and Al₂O₃ showed a considerably higher H₂ sensitivity, and their gas response decreased with increasing sensing temperature (400–550 °C). The films deposited on and (0 0 0 1) Al₂O₃ showed the maximum sensitivity at 500 °C. The comparison of the H₂ gas response between undoped and Pd-doped SnO₂ films revealed that the Pd-doping shifted the optimum sensing temperature to a lower value instead of improving the gas sensitivity.     

Enhanced H2S sensing characteristics of Pt doped SnO2 nanofibers sensors with micro heater

Gas sensors were designed and fabricated using oxide nanofibers as the sensing materials on micro platforms using micromachining technology. Pure and Pt doped SnO₂ nanofibers were prepared by electrospinning and their H₂S gas sensing characteristics were subsequently investigated. The sensing temperatures of 300 and 500 °C could be attained at the heater powers of 36 and 94 mW, respectively, and the sensors showed high and fast responses to H₂S. The responses of 0.08 wt% Pt doped SnO₂ nanofibers to 4-20 ppm H₂S, were 25.9-40.6 times higher than those of pure SnO₂ nanofibers. The gas sensing characteristics were discussed in relation to the catalytic promotion effect of Pt, nano-scale morphology of electrospun nanofibers, and sensor platform using micro heater.     

Development and application of micromachined Pd/SnO2 gas sensors with zeolite coatings

Zeolite A (LTA)-coated micromachined sensors have been prepared and used in the sensing of individual gases (H₂, CH₄, C₂H₅OH, C₃H₈ and CO, in the 10–1000 ppm range) and gas mixtures. Unlike previous works with conventional sensors, a hydrothermal synthesis was not used to prepare a zeolite film. Instead, a zeolite coating was formed on top of the Pd/SnO₂ surface by microdropping from a zeolite suspension. In spite of this, the response of the sensor with zeolite is significantly different from that of unmodified sensors, and essentially reproduces the performance of zeolite-coated conventional sensors. By avoiding the use of a hydrothermal synthesis the integrity of the sensor is better preserved, and the resulting non-continuous zeolite film has the added advantage of a strong reduction in response times.