Highly sensitive gas sensors based on hollow SnO2 spheres prepared by carbon sphere template method

Hollow SnO₂ spheres were prepared in dimethylfomamide (DMF) by controlled hydrolysis of SnCl₂ using newly made carbon microspheres as templates. The phase composition and morphology of the material particles were characterized by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The gas sensing properties of sensors based on the hollow SnO₂ spheres were investigated. It was found that the sensor exhibited good performances, characterized by high response, good selectivity and very short response time to dilute (C₂H₅)₃N operating at 150 °C, especially, the response to 1 ppb (C₂H₅)₃N attained 7.1 at 150 °C. It was noteworthy that the response to 0.1 ppm C₂H₅OH of the sensor was 2.7 at 250 °C.     

Selective detection of ethanol vapor and hydrogen using Cd-doped SnO2-based sensors

The effect of CdO doping on microstructure, conductance and gas-sensing properties of SnO₂-based sensors has been presented in this study. Precursor powders with Cd/Sn molar ratios ranging from 0 to 0.5 were prepared by chemical coprecipitation. X-ray diffraction (XRD) analysis indicates that the solid-state reaction in the CdO–SnO₂ system occurs and -CdSnO₃ with pervoskite structure is formed between 600 and 650°C. CdO doping suppresses SnO₂ crystallite growth effectively which has been confirmed by means of XRD, transmission electron microscopy (TEM) and BET method. The 10 mol% Cd-doped SnO₂-based sensor shows an excellent ethanol-sensing performance, such as high sensitivity (275 for 100 ppm C₂H₅OH), rapid response rate (12 s for 90% response time) and high selectivity over CO, H₂ and i-C₄H₁₀. On the other hand, this sensor has good H₂-sensing properties in the absence of ethanol vapor. The sensor operates at 300°C, the sensitivity to 1000 ppm H₂ is up to 98, but only 16 and 7 for 1000 ppm CO and i-C₄H₁₀, respectively.     

Highly sensitive and fast responding CO sensor using SnO2 nanosheets

A highly sensitive and fast responding CO sensor was fabricated from a sheet-like SnO₂. The SnO sheets were prepared by a room temperature reaction between SnCl₂, hydrazine and NaOH, and they were subsequently oxidized into SnO₂ sheets at high temperature (600 °C). The morphology and size of the SnO₂ sheets could be controlled during the formation of SnO, which influence the sensor response (R_a/R_g) and response time to a great extent. The sensor response of SnO nanosheets to 10 ppm CO was enhanced up to 2.34, and the 90% sensor response time could be reduced to 6 s, which are significantly higher and shorter than those of SnO₂ powders (1.57 and 88 s), respectively. The realization of both a high sensitivity and rapid response were explained in terms of rapid gas diffusion onto the entire sensing surface due to the less-agglomerated and very thin structure of SnO₂ nanosheets and the catalytic effect of Pt.     

Effect of surface modification on NO2 sensing properties of SnO2 varistor-type sensors

NO₂ sensing properties of SnO₂-based varistor-type sensors have been investigated in the temperature range of 400-650°C and in the NO₂ concentration range of 15–30 ppm. Pure SnO₂ exhibited a weak nonlinear I–V characteristic in air, but clear nonlinearity in NO₂ at 450°C. The breakdown voltage of SnO₂ shifted to a high electric field upon exposure to NO₂ and the magnitude of the shift was well correlated with NO₂ concentration. Thus, SnO₂ exhibited some sensitivity to NO₂ as a varistor-type sensor. When SnO₂ particles coated with a SiO₂ thin film were used as a raw material for fabricating a varistor, the breakdown voltage in air was approximately the double that of pure SnO₂ and the sensitivity to 15 ppm NO₂ was enhanced slightly. However, the sensitivity to 30 ppm NO₂ decreased. The Cr₂O₃-loading on SnO₂ also led to an increase in the breakdown voltage in air, but the Cr₂O₃ addition was not effective for promoting the NO₂ sensitivity under the present experimental conditions.     

High sensitivity ethanol gas sensor integrated with a solid-state heater and thermal isolation improvement structure for legal drink-drive limit detecting

The paper reports the successful fabrication of ethanol gas sensors with tin-dioxide (SnO₂) thin films integrated with a solid-state heater, which is realized with technologies of micro-electro-mechanical systems (MEMS), and are compatible with VLSI processes. The main sensing part with dimensions of 450×400 μm² in this developed device is composed of a sensing SnO₂ film, which is fabricated by electron-gun evaporation with proper annealing in ambient oxygen gas to yield fine particles and good structure. An integrated solid-state heater with a 4.5 μm-thick cantilever bridge (1000×500 μm²) structure is made of silicon carbide (SiC) material by MEMS technologies. The sensitivity for 1000 ppm ethanol gas reaches as high as 90 with 10 s and 2 min for the response and recovery time, respectively, at an operating temperature of 300°C. Those experimental results also exhibit a much superior performance to that of a popular commercial ethanol gas sensor TGS-822. Therefore, the developed sensor with high performance is a good candidate for some specific application in automobile to detect drink-drive limit and allows an array integration available with various films for controlling each element at separate resistance.     

Characterization of ozone sensors based on WO3 reactively sputtered films: influence of O2 concentration in the sputtering gas, and working temperature

We report on electrical responses of tungsten oxide thin film ozone sensors based on a tungsten trioxide (WO₃)/tin oxide (SiO₂)/Si structure with interdigitated Pt electrodes. The influence of O₂ concentration in the sputtering gas and working temperature of the sensor are investigated. Sensitivity to ozone increases with O₂ content in the sputtering gas. It reaches its highest value for sensors fabricated with 50% O₂. For these sensors, the best ozone sensitivity and shortest response and recovery times are obtained at a working temperature of 523 K. Ozone sensitivity is compared to other ozone sensors.     

Microstructure evolution and gas sensitivities of Pd-doped SnO2-based sensor prepared by three different catalyst-addition processes

We have investigated three ways of impregnating PdO on an SnO₂ gas sensor to achieve a simple and reliable sensor-fabrication process. These impregnating processes are: (1) coprecipitation of SnO₂ and Pd compounds in the solution; (2) addition of PdCl₂ to SnO₂ gel, followed by precipitation; and (3) infiltration of PdCl₂ into calcined SnO₂ powder. Processes (1) and (2) introduce Pd into SnO₂ particles before particle growth is completed. The phase and microstructures of particles have been analysed by X-ray diffraction, scanning and transmission electron microscopes, and an energy-dispering X-ray spectroscope. The presence of Pd in the process of SnO₂ precipitation restrains the growth of SnO₂ particles and enhances a uniform distribution of fine PdO powder on the SnO₂ grains. SnO₂ gas sensors have been fabricated and tested for response to CH₄, C₂H₆ and CO. Processes (1) and (2) show many possibilities of improving SnO₂ gas-sensor sensitivity with a simplified fabrication process.     

SnO2 thin films for gas sensors modified by hexamethyldisilazane after rapid thermal annealing

Simple method of SnO₂ layer modification, using very small quantity of hexamethyldisilazane and rapid thermal annealing in the range 800–1200 °C is proposed. The distribution profile of the dopant elements of C, N, Si in the SnO₂/SiO₂/Si structure is investigated. Penetration of Si in the whole depth of SnO₂ is revealed and formation of SiO₂ regions in the SnO₂ bulk is assumed. Simultaneously, Sn diffusion in the SiO₂ layer is observed. The combination of standard AES and XPS techniques with a hollow cathode discharge method appears to be very useful for detection of traces of dopants in the layers.     

TiO2 thin films from titanium butoxide: Synthesis, Pt addition, structural stability, microelectronic processing and gas-sensing properties

TiO₂ thin films were prepared by spin-coating of a Ti butoxide-derived sol onto oxidized silicon wafers, followed by a heat-treatment at temperatures ranging from 500 to 800 °C. The film thickness after heat-treatment at 500 °C was 50 nm. Pt addition, with a Pt:Ti nominal atomic ratio ranging from 0.01 to 0.1, was achieved by adding solutions of Pt(II) acetylacetonate to the TiO₂ sols. The thin films were investigated by X-ray diffraction, evidencing that Pt promoted the structural transformation of the starting anatase phase of TiO₂ to rutile, with a more enhanced effect with increasing the Pt concentration and/or the heat-treatment temperature. High-resolution transmission electron microscopy evidenced that, when a Pt:Ti atomic ratio of 0.05 and a heat treatment at 500 °C were used, the TiO₂ contained both anatase and rutile phases and interspersed Pt nanocrystals (2–3 nm). This result allowed attributing the structural transformation in TiO₂ to the strain created by the Pt nanocrystals—a conclusion which was further corroborated by the observation that Pd-modified films, prepared under similar conditions, were only composed of anatase TiO₂ and did not contain any Pd nanocrystals. The films heat-treated at 500 °C were able to withstand a full microelectronic processing sequence, including dry etching for gas sensors sensitive area definition, Ti/Pt contact formation, and heater processing on the backside of the sensor substrates. H₂ gas-sensing tests evidenced that the anatase TiO₂ phase was much more sensitive than the rutile one. The presence of Pt further enhanced the gas-sensing properties, lowering the optimum sensor operation temperature to about 330 °C and allowing for the detection of a minimum H₂ concentration of about 1000 ppm.     

Ceria-doped SnO2 sensor highly selective to ethanol in humid air

In our earlier study, we reported that at 300 °C, a 2.0 wt.% CeO₂-doped SnO₂ sensor is highly selective to ethanol in the presence of CO and CH₄ gases [F. Pourfayaz, A. Khodadadi, Y. Mortazavi, S.S. Mohajerzadeh, CeO₂ doped SnO₂ sensor selective to ethanol in presence of CO, LPG and CH₄, Sens. Actuators B 108 (2005) 172–176]. In the present investigation, we report the influence of ambient air humidity on the ethanol selective SnO₂ sensor doped with 2.0 wt.% CeO₂. Maximum response to ethanol occurs at 300 °C which decreases with the relative humidity. The relative humidity was changed from 0 to 80% for different ambient air temperatures of 30, 40 and 50 °C and the response of the sensor was monitored in a 250–450 °C temperature range. As the relative humidity in 50 °C air increased from 0 to 30%, a 15% reduction in the maximum response to ethanol was observed. A further increase in the relative humidity no longer reduced the response significantly. The presence of humidity improved the sensor response to both CO and CH₄ up to 350 °C after which the extent of improvement became smaller and at 450 °C was almost diminished. The sensor is shown to be quite selective to ethanol in the presence of humid air containing CO and CH₄. The selectivity passes a maximum at 300 °C; however it declines at higher operating temperatures.     

Preparation of BST ferroelectric thin film by metal organic decomposition for infrared sensor

Barium strontium titanate (Ba_1−xSr_xTiO₃) ferroelectric thin films have been prepared by metal organic decomposition (MOD) on Pt/Ti/SiO₂/Si and on micromachined wafer with an aim to fabricate dielectric bolometer type infrared (IR) sensor. The XRD pattern and D–V hysteresis curve of the film have been measured in order to investigate the effects of the final annealing temperature and annealing time on the property of the film. The results show that the films annealed at 700 °C or 800 °C all have good perovskite structure, while the film annealed at 800 °C has better ferroelectric loops. Films annealed at 800 °C with different annealing time from 5 to 60 min show a similar perovskite structure, among which films annealed at 30 and 60 min condition have the better ferroelectric loops. Temperature coefficient of dielectric constant (TCD) of the MOD made BST thin film on micromachined substrate is about 1%/K. The uniformity of the BST film on micromachined Si wafer also has been confirmed to be good enough for operation of sensor array. Chopperless operation has been attained and infrared response evaluation of the fabricated sensor also has been carried out with R_v of 0.4 kV/W and D^* of 1.0×10⁸ cm Hz^1/2/W, respectively.     

Titania NOx sensors for exhaust monitoring

Oxide semiconductors have been examined to develop NO_x sensors for exhaust monitoring. Titania doped with trivalent elements, such as Al³⁺, Sc³⁺, Ga³⁺ or In³⁺, has a good sensitivity and selectivity to NO between 450 and 550 °C, and shows rapid response. A sensor probe for monitoring exhaust NO_x has been fabricated. Many kinds of interference gases, such as C₃H₆, CO and SO₂, have been found to have only a slight influence on the sensor response to NO. The influence of O₂ and H₂O is also negligible, except for the cases of 0% H₂O and fuel-rich conditions. In accordance with these results, the sensor probe operates satisfactority in the exhaust gas of various combustion conditions without interference from the various kinds of gas species in the exhaust gases.     

Influence of Pd doping on morphology and LPG response of SnO2

In the present study nanocrystalline pristine and Pd-doped SnO₂ (Pd:SnO₂) with various mol% Pd have been synthesized by a modified Pechini citrate route. Transmission electron microscopy and X-ray powder diffraction studies were used to characterize the morphology, crystallinity, and structure of the SnO₂ and Pd:SnO₂. The response of the pristine SnO₂ and Pd:SnO₂ was studied towards different reducing gases. The 1.5 mol% Pd doping showed an enhanced response of 75 and 95% towards LPG at as low as 50 and 100 °C, respectively, which were quite large high value as compared with pristine SnO₂ (38 and 35% at 50 and 100 °C, respectively). Structural characterization revealed that Pd doping reduced the crystallite size of SnO₂ and helps in the formation of distinct spherical nanospheres at a calcinations temperature of 500 °C. Thus the increase in LPG response can be correlated with the spherical morphology, a decrease in the crystallite size (11 nm) due to doping with Pd as compared with the pristine SnO₂ (26 nm) and main role of Pd as a catalyst.     

CuO/SnO2 thin film heterostructures as chemical sensors to H2S

CuO/SnO₂ heterostructures as well as SnO₂(CuO) polycrystalline films have been studied for H₂S sensing. Gas sensing properties of these materials have been compared in conditions: 25–300 ppm H₂S in N₂ at 100–250°C. A shorter response time of the heterostructures as compared to that of the SnO₂(CuO) films has been found. It is suggested that the improvement of dynamic sensor properties of SnO₂/CuO heterostructures is caused by the localization of electrical barrier between CuO and SnO₂ layers.     

Nanoparticle engineering for gas sensor optimisation: improved sol–gel fabricated nanocrystalline SnO2 thick film gas sensor for NO2 detection by calcination, catalytic metal introduction and grinding treatments

The control of the technological steps such as calcination temperature and introduction of catalytic additives are accepted to be key points in the obtaining of improved sol–gel fabricated SnO₂ thick film gas sensors with different sensitivity to NO₂ and CO. In this work, after proving that the undoped material calcined at 1000°C is optimum for NO₂ detection, grinding is added as third technological step for further modification of particle surface characteristics, allowing to reduce cross-sensitivity to CO. The influence of grinding on the base resistance and on the sensor signals to NO₂ and CO is discussed in detail as a function of the structural differences of the sensing material.     

Characterization and gas sensitivity study of polyaniline/SnO2 hybrid material prepared by hydrothermal route

A polyaniline (PAni)/SnO₂ hybrid material was prepared by a hydrothermal method and characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The XRD pattern suggested that PAni did not modify the crystal structure of SnO₂, but SnO₂ affected the crystallization of PAni to some extent. The gas sensitivity of the PAni/SnO₂ hybrid was also studied to ethanol and acetone at operation temperatures of 30, 60 and 90 °C. It was found that the PAni/SnO₂ hybrid material had gas sensitivity only when operated at 60 and 90 °C, and it showed a linear relationship between the responses and the concentrations of ethanol and acetone at 90 °C. The sensing mechanism was also discussed.     

Analysis of the noble metal catalytic additives introduced by impregnation of as obtained SnO2 sol–gel nanocrystals for gas sensors

In order to clarify the role of the noble metal additives in the gas sensing mechanisms, three of the most common catalytic additives, such as Pd, Pt and Au, have been introduced in a sol–gel obtained tin oxide base material. The additives nominal weight concentrations used were 0.2% and 2%, and they were introduced in the precipitated tin oxide. A posterior calcination treatment was carried out, during 8 h, at the temperatures of 250°C, 400°C, 450°C, 600°C, 800°C and 1000°C. Structural and surface analysis of these nanopowders have been performed. Identification and localisation of metallic, 2+ and 4+ oxidised states of the used noble metals are discussed, and experimental evidences about their effects on the sensor performance are presented. Likewise, effects of their presence on the nanoparticle characteristics, and also on the material sensitivity to CO and CH₄, are analysed and discussed.     

Calorimetric hydrocarbon sensor for automotive exhaust applications

We have developed a calorimetric sensor utilizing a thermoelectric device supported on a planar alumina substrate. By using a highly selective carbon monoxide (CO) oxidation catalyst and a non-selective platinum (Pt) catalyst, the device can be built to detect either CO or hydrocarbons with high selectivity. The CO oxidation catalyst comprises lead-modified platinum and exhibits excellent selectivity over the 200–400 °C temperature range. The thermoelectric device consists of two thick film junctions made of niobium pentoxide (Nb₂O₅)-doped titanium dioxide (TiO₂) and a lithiated nickel (Ni), which are supported on a planar alumina substrate. The thermocouple detects the difference in temperature due to different catalytic reactions over the two junctions and shows a high output signal because of the high Seebeck coefficient of Nb₂O₅-doped TiO₂ (−400 μV/°C). In gas bench tests, the sensor has a linear output of 0–2.75 mV over 0–1000 ppm of propylene and a response time of 2.5 s (at 90% of amplitude) at a gas temperature of 350 °C. An engine dynamometer evaluation shows that the response of the sensor parallels the change in CO and hydrocarbon constituent concentrations when the engine air-to-fuel ratio is varied.     

Ozone sensors on the base of SnO2 films deposited by spray pyrolysis

The gas-sensing properties of SnO₂-based thin films designed for ozone detection are discussed in this paper. The influence of film characteristics on sensor performance is analyzed. SnO₂ films with thickness 30–200 nm were deposited by spray pyrolysis. The SnO₂ films have a response to ozone that is quantitative and rapid and sufficient for use in ozone control and monitoring applications. Sensor performance is compared with similarly prepared sensors fabricated from In₂O₃- and WO₃-based films. The mechanism of the processes controlling the sensor response characteristics is proposed. The data support our conclusion that the reaction with ozone using the SnO₂-film sensors is limited by the adsorption/desorption processes.     

Cerium oxide/SnO2-based semiconductor gas sensors with improved sensitivity to CO

SnO₂-based semiconductor gas sensors have been successfully fabricated and tested for detecting carbon monoxide and methane. The sensitivity and selectivity of the sensors are tailored by incorporation of different additives such as platinum and cerium oxide. While platinum enhances the sensor response to CH₄, ceria suppresses its sensitivity in favor of carbon monoxide. The effect of operating temperature on the performance of sensors is reported. Addition of 10% cerium oxide in the SnO₂ sample leads to an insignificant response to methane even at an elevated temperature of 450°C, while its response to CO remains intact.