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.     

Ultrahigh ethanol response of SnO2 nanorods at low working temperature arising from La2O3 loading

The influences of La₂O₃ loading on the ethanol sensing properties of SnO₂ nanorods were investigated. An obvious enhancement of response was obtained. The response of 5 wt% La₂O₃ loaded SnO₂ nanorods was up to 213 for 100 ppm ethanol at low working temperature of 200 °C, while that of pure SnO₂ nanorods is 45.1. The improvement in response might be attributed to the presence of basic sites, which facilitated the dehydrogenation process. While the working temperature was increased to 300 °C, the sensor response decreased to 16 for 100 ppm ethanol. Additionally, the La₂O₃ loaded SnO₂ nanorods sensors showed good selectivity to ethanol over methane and hydrogen. Our results demonstrated that the La₂O₃ loaded SnO₂ nanorods were promising in fabricating high performance ethanol sensors which could work at low temperature.     

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.     

Improved crystalline structure and H2S sensing performance of CuO-Au-SnO2 thin film using SiO2 additive concentration

In situ SiO₂-doped SnO₂ thin films were successfully prepared by liquid phase deposition. The influence of SiO₂ additive as an inhibitor on the surface morphology and the grain size for the thin film has been investigated. These results show that the morphology of SnO₂ film changes significantly by increasing the concentration of H₂SiF₆ solution which decreases the grain size of SnO₂. The stoichiometric analysis of Si content in the SnO₂ film prepared from various Si/Sn molar ratios has also been estimated. For the sensing performance of H₂S gas, the SiO₂-doped Cu-Au-SnO₂ sensor presents better sensitivity to H₂S gas compared with Cu-Au-SnO₂ sensor due to the fact that the distribution of SiO₂ particles in grain boundaries of nano-crystallines SnO₂ inhibited the grain growth (<6 nm) and formed a porous film. By increasing the Si/Sn molar ratio, the SiO₂-doped Cu-Au-SnO₂ gas sensors (Si/Sn = 0.5) exhibit a good sensitivity (S = 67), a short response time (t_90% < 3 s) and a good gas concentration characteristic (α = 0.6074). Consequently, the improvement of the nano-crystalline structures and high sensitivity for sensing films can be achieved by introducing SiO₂ additive into the SnO₂ film prepared by LPD method.     

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.     

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.     

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.     

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.     

A route to high sensitivity and rapid response Nb2O5-based gas sensors: TiO2 doping, surface embossing, and voltage optimization

We report a novel route for the fabrication of highly sensitive and rapidly responding Nb₂O₅-based thin film gas sensors. TiO₂ doping of Nb₂O₅ films is carried out by co-sputtering without the formation of secondary phases and the surface area of TiO₂-doped Nb₂O₅ films is increased via the use of colloidal templates composed of sacrificial polystyrene beads. The gas sensitivity of Nb₂O₅ films is enhanced through both the TiO₂ doping and the surface embossing. An additional enhancement on the gas sensitivity is obtained by the optimization of the bias voltage applied between interdigitated electrodes beneath Nb₂O₅-based film. More excitingly, such a voltage optimization leads to a substantial decrease in response time. Upon exposure to 50 ppm CO at 350 °C, a gas sensor based on TiO₂-doped Nb₂O₅ film with embossed surface morphology exhibits a very high sensitivity of 475% change in resistance and a rapid response time of 8 s under 3 V, whereas a sensor based on plain Nb₂O₅ film shows a 70% resistance change and a response time of 65 s under 1 V. Thermal stability tests of our Nb₂O₅-based sensor reveal excellent reliability which is of particular importance for application as resistive sensors for a variety gases.     

SnO2 thin films modified by the SnO2–Au nanocomposites: Response to reducing gases

The influence of the SnO₂ surface modification by the SnO₂–Au nanocomposites on conductivity response to such reducing gases as CO and H₂ has been analyzed in the present paper. Both initial SnO₂ films, subjected for surface modification, and SnO₂–Au nanocomposites were deposited by Successive Ionic Layer Deposition (SILD) method. The SnO₂–Au nanocomposites with Au/Sn ratio 1 were synthesized using HAuCl₄ and SnCl₂ precursors. The thickness of the Au-SnO₂ nanolayers varied from 0.7–1.0 nm to 10–15 nm. It was established that the increase in the thickness of the SnO₂–Au nanocomposite layer formed on the surface of the SnO₂ films was accompanied by both the improvement of sensor response and the decrease in response and recovery times. An explanation of the observed effects has been proposed.     

Hydrogen sensing and mechanism of M-doped SnO2 (M = Cr, Cu and Pd) nanocomposite

The microstructure of M-doped SnO₂ (M = Cr³⁺, Cu²⁺ and Pd²⁺) prepared by the sol–gel method and their gas-sensing performance were investigated. In particular, we focus on the effects of metallic ions on the hydrogen sensing behavior of the SnO₂-based sensor. It is found that hydrogen gas response of SnO₂ can be enhanced evidently by adding Pd²⁺, while such effect from Cr³⁺ and Cu²⁺ exhibits somewhat slight. A theoretical study based on first principles calculation shows that SnO₂–Pd (1 1 0) surface enable adsorb more H₂ gas and receive larger electrons from adsorbed H₂ molecule, thereby holding the potential for the improvement of gas response to hydrogen.     

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.     

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.     

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.     

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.     

Enhanced C2H5OH sensing characteristics of nano-porous In2O3 hollow spheres prepared by sucrose-mediated hydrothermal reaction

In₂O₃ hollow spheres with shell thicknesses of ∼150 nm and ∼300 nm were prepared by the one-pot synthesis of indium-precursor-coated carbon spheres via hydrothermal reaction and subsequent removal of core carbon by heat treatment. The gas response (R_a/R_g, R_a: resistance in air, R_g: resistance in gas) of the thin hollow spheres to 100 ppm C₂H₅OH was 137.2 at 400 °C, which was 1.86 and 3.84 times higher than that of the thick hollow spheres and of the nanopowders prepared by precipitation, respectively. The gas sensing characteristics are discussed in relation to the shell configuration of the hollow spheres. The enhanced gas response of the hollow spheres was attributed to the effective diffusion of analyte gas toward the entire sensor surface via very thin and nano-porous shells.     

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.     

SnO2-based R134a gas sensor: Sensing materials preparation, gas response and sensing mechanism

Undoped SnO₂ and porous Al₂O₃ powders were obtained through a simple chemical precipitation process. SnO₂-based gas sensing materials and Al₂O₃ catalytic coating loaded with a noble metal were prepared by impregnation. The SnO₂ and Al₂O₃ powders were characterized by TEM, SEM, nitrogen adsorption-desorption experiment, FT-IR and in situ XRD. Gas responses of the SnO₂-based gas sensors were measured in a static state. The experimental results indicated that the response towards R134a of the SnO₂-based gas sensor can be significantly enhanced by loading noble metal and using catalytic coating. The sensor based on a double layer film SnO₂ (Au)/Al₂O₃ (Au) showed satisfactory results including large response, good selectivity, high long-term stability, fast response and recovery, revealing its potential application in the detection of refrigerants and the maintenance of air condition systems. Finally, a gas sensing mechanism for R134a is suggested and proved by bond energy data, FT-IR spectrum and in situ XRD.     

Nanoplates of α-SnWO4 and SnW3O9 prepared via a facile hydrothermal method and their gas-sensing property

Nanoplates of α-SnWO₄ and SnW₃O₉ were selectively synthesized in large scale via a facile hydrothermal reaction method. The final products obtained were dependent on the reaction pH and the molar ratio of W⁶⁺ to Sn²⁺ in the precursors. The as-prepared nanoplates of α-SnWO₄ and SnW₃O₉ were characterized by X-ray powder diffraction (XRD), N₂-sorption BET surface area, transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). The XPS results showed that Sn exists in divalent form (Sn²⁺) in SnW₃O₉ as well as in α-SnWO₄. The gas-sensing performances of the as-prepared α-SnWO₄ and SnW₃O₉ toward H₂S and H₂ were investigated. The hydrothermal prepared α-SnWO₄ showed higher response toward H₂ than that prepared via a solid-state reaction due to the high specific surface area. The gas-sensing property toward H₂S as well as H₂ over SnW₃O₉ was for the first time reported. As compared to α-SnWO₄, SnW₃O₉ exhibits higher response toward H₂S and its higher response can be well explained by the existence of the multivalent W (W⁶⁺/W⁴⁺) in SnW₃O₉.     

Studies on the gas sensing behaviour of nanosized CuNb2O6 towards ammonia, hydrogen and liquefied petroleum gas

Appreciable changes in resistance of polycrystalline nanosized CuNb₂O₆ upon exposure to reducing gases like hydrogen, liquefied petroleum gas (LPG) and ammonia in ambient atmosphere recognize the material as a gas sensor. Nanosized CuNb₂O₆ synthesized by thermal decomposition of an aqueous precursor solution containing copper nitrate, niobium tartrate and tri-ethanol amine (TEA), followed by calcination at 700 °C for 2 h, has been characterized using X-ray diffraction (XRD) study, transmission electron microscopy (TEM), field-emission scanning electron microscope (FESEM), energy dispersive X-ray (EDX) analysis and Brunauer–Emmett–Teller (BET) surface area measurement. The synthesized CuNb₂O₆ exhibits monoclinic structure with crystallite size of 25 nm, average particle size of 25–40 nm and specific surface area of 55 m² g⁻¹.