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.     

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.     

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.     

Benzene monitoring by micro-machined sensors with SnO2 layer obtained by using micro-droplet deposition technique

SnO₂ thin layers were deposited by the way of the micro-droplet technique. The sensor substrate consisted of a thin membrane developed on oxidised silicon wafer. The sensing layers were deposited by means of the micro-droplet technique into thin layers of about 100 nm. Such devices were tested for benzene detection. The obtained results showed a very high sensitivity for this chemical compound since 500 ppb were detected.The results presented in this paper were not focused on the reactional mechanism of benzene detection but rather on the development of a cheap and sensitive sensor using sol-gel and micro-droplet processes. Since these layers were elaborated using solely tin oxide, the as-obtained sensors are not selective but these one are intended to be used by coupling with additional devices such as chromatographic micro-column and micro-pre-concentrators.     

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.     

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.     

Conduction mechanisms in SnO2 based polycrystalline thick film gas sensors exposed to CO and H2 in different oxygen backgrounds

The conduction mechanism in polycrystalline SnO₂ thick sensing films was modeled and experimentally investigated by means of simultaneous DC electrical resistance and work function changes measurements under CO and H₂ exposure in different oxygen backgrounds. It was shown that, according to the composition of the ambient atmosphere, the conduction changes from the case in which it is controlled by the surface depletion layers to a situation in which the main contribution comes from free charge transport in the surface accumulation layer. This is significant for the interpretation of work function changes measurements results because the relation between the different measured electrical resistance and surface band bending depends on the conduction model. Furthermore, the CO sensing mechanism dependence on the oxygen amount in the ambient was explained.     

Thin film SnO2-based gas sensors: Film thickness influence

The influence of the thickness of SnO₂ films deposited by a spray pyrolysis method on the operating characteristics of gas sensors is analyzed in this paper. It outlines how the thickness of metal oxides is an important parameter for gas sensors in determining the main operating parameters, such as the magnitude and rate of the sensor response and the optimal operating temperature. It is also shown that the optimal film thickness of a gas sensing layer depends on the required sensor parameters.     

On the temperature dependence of the resistive and surface ionisation response of SnO2 gas sensing layers

Gas sensing experiments have been performed on SnO₂ thin films using a wide range of different analyte gases. In these experiments, the SnO₂ layers were specifically configured to observe the familiar resistive (RES) gas response alongside with the novel surface ionisation (SI) response. It is shown that the RES and SI responses, in general, occur in dissimilar temperature ranges and that both follow very different selectivity criteria. Microscopically, both kinds of response proceed through analyte-specific sequences of adsorption, surface reaction, charge transfer and desorption steps. A generalisation of the Ahlers model [1], originally developed to account for the bell-shaped temperature variation of the RES response of thin film metal oxide layers, is shown to quantitatively account for the whole range of SI data. Fits to experimental SI response vs. temperature curves allow the total energy input into the surface ionisation process to be determined and insights into the ionisation mechanism to be gained.     

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.     

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.     

The influence of catalytic activity on the response of Pt/SnO2 gas sensors to carbon monoxide and hydrogen

The article presents the results of research studies on ceramics SnO₂ sensors with Pt catalysts. The role of catalysis in gas sensing mechanisms was investigated. In order to obtain samples with different catalytic activity but with identical Pt loading, the Pt/SnO₂ catalysts were calcined at different temperatures (400-800 °C). Structural analysis of these samples was performed. Among the sensors manufactured with Pt/SnO₂, the highest sensitivity was shown for the sensor obtained with Pt/SnO₂ sample sintered at 800 °C. The correlation between catalytic activity and sensor sensitivity is given.     

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.     

Formaldehyde sensing properties of electrospun NiO-doped SnO2 nanofibers

Formaldehyde is a kind of hazardous gases dangerous to human health. Hence, gas sensor is an essential device to monitor formaldehyde in air, especially in indoor ambient. Semiconductor metal oxides are studied as gas-sensing material to detect most of key gases for decade years. For the purpose of actual application and meeting a variety of conditions, diverse additives added into host material are expected to improve the performance of gas sensors. The formaldehyde gas-sensing characteristics of undoped and NiO-doped SnO₂ (NSO) nanofibers synthesized via a simple electrospinning method were investigated in this study. It is noticed that the addition of NiO causes the distortion at the surface of SnO₂ nanofibers, which is responsible to adjust activation energy, grain sizes and chemical states of host material. The sensors fabricated from NSO nanofibers exhibited good formaldehyde sensing properties at operating temperature 200 °C, and the minimum-detection-limit was down to 0.08 ppm. The response time and recovery time of the sensors were about 50 s and 80 s to 10 ppm formaldehyde, respectively. The sensor shows a good long-term stability in 90 days. The simple preparation and excellent properties significantly advance the viability of electrospun nanofibers as gas sensing materials. The sensing mechanisms of NSO nanofibers to formaldehyde were discussed. The results indicated that NSO nanofibers could be used as a candidate to fabricate formaldehyde sensors in practice.     

Enhancing performance of FSP SnO2-based gas sensors through Sb-doping and Pd-functionalization

Via flame spray pyrolysis (FSP), SnO₂ gas sensing layers have been doped with 0.01-4 wt% Sb as well as 0.01 wt% Pd in combination with 1 wt% Sb. Characterization of these materials through X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) surface analysis, and transmission electron microscopy (TEM) revealed particle grain sizes and crystallinity unchanged by the presence of Sb and/or Pd. The addition of Sb to SnO₂ resulted in the significant decrease in baseline resistance; up to two orders of magnitude in dry air at 300 °C and three orders of magnitude in humid air at 300 °C, which is significant for FSP-prepared gas sensors with high porosity and low particle coordination number since they typically suffer from high baseline resistance. While the baseline resistance was improved with Sb-doping, the sensor signal (R₀/R_gas) remained constant over all concentrations explored. Moreover, regarding the surface functionalization of SnO₂ with Pd in combination with Sb-doping, the reduction of baseline resistance was preserved without influencing sensor signal.     

Growth kinetics of nanograins in SnO2 fibers and size dependent sensing properties

The present study investigates the growth kinetics of SnO₂ nanograins and determines the activation energy and mechanism of the growth in nanofiber form. The activation energy for the growth of the SnO₂ nanograins was estimated to be ∼28.28 kJ/mol, which is an order of magnitude smaller than that of bulk SnO₂. The estimated m value suggests that the growth mechanism of the nanograins is primarily through lattice diffusion in the pore control scheme. Precise control of the calcination temperature and time is necessary to maximize the efficiency of electrospinning-synthesized SnO₂ nanofibers for sensor applications. Importantly, the sensor fabricated with nanofibers of small nanograins showed much better sensing properties to CO and NO₂ comparing with the sensor fabricated with nanofibers of large nanograins. A mechanism to explain this finding is suggested.     

Influence of oxygen backgrounds on hydrogen sensing with SnO2 nanomaterials

The influence of different oxygen backgrounds on the sensing of hydrogen with SnO₂ nanomaterials was investigated and a model was proposed. It is based on two hydrogen reaction mechanisms at the surface of tin oxide that can take place simultaneously; the weight of each mechanism depends on the concentration of oxygen in the ambient atmosphere. In the absence of oxygen the adsorbed hydrogen builds surface donors (rooted hydroxyl groups). In the presence of oxygen the reaction between hydrogen and pre-adsorbed oxygen ions dominates. Due to the fact that this behavior is present for very different nanomaterials, the model probably describes a SnO₂ specific feature.     

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.     

Synthesis and enhanced gas sensing properties of crystalline CeO2/TiO2 core/shell nanorods

Crystalline CeO₂/TiO₂ core/shell nanorods were fabricated by a hydrothermal method and a subsequent annealing process under the hydrogen and air atmosphere. The thickness of the outer shell composed of crystal TiO₂ nanoparticles can be tuned in the range of 5-11 nm. The crystal core/shell nanorods exhibited enhanced gas-sensing properties to ethanol vapor in terms of sensor response and selectivity. The calculated sensor response based on the change of the heterojunction barrier formed at the interface between CeO₂ and TiO₂ is agreed with the experimental results, and thus the change of the heterojunction barrier at different gas atmosphere can be used to explain the enhanced ethanol sensing properties.