CO Gas Sensor Research Articles

Development of a CO2 gas analyzer for monitoring soil CO2 concentrations

Measurement of soil CO2 concentrations is important for investigating the dynamics and diffusion of CO2 in soil. In this study, we developed a small CO2 analyzer for measuring in situ-soil CO2 concentrations. The CO2 analyzer consists of a module containing an infrared CO2 gas sensor, a temperature sensor, and a relative humidity sensor. These sensors are installed in a protective box with an air vent, which is suitable for burying in the soil. The output response time of the CO2 analyzer was 349 s, as evaluated from the phase lag after input of known CO2 concentrations. This response time is short enough to measure soil CO2 concentrations, because variations in concentration are slower than the response time of the analyzer. In a field test, we used the CO2 analyzer to measure soil CO2 concentrations at five depths (0–50 cm) over 2.5 months. While the CO2 concentration generally increased with depth, the amplitude of the variation in CO2 concentration decreased with depth. The phase lag of the variations in soil CO2 concentration also increased with depth, as did soil temperature. The tests confirm that the CO2 analyzer is applicable to continuous monitoring of soil CO2 concentrations.

Silicon Carbide Nanotubes As Potential Gas Sensors for CO and HCN Detection

Semiconducting carbon nanotubes (CNTs) have demonstrated extreme sensitivity to molecules such as NH3, NO, NO2, ans so forth. Yet, intrinsic CNTs cannot be used to detect some highly toxic molecules such as CO and HCN. In this article, we examine the possibility of silicon carbide nanotubes (SiCNTs) as a potential gas sensor for CO and HCN detection by first-principles calculations based on density functional theory (DFT). It is found that CO and HCN molecules can be absorbed to Si atoms on the wall of SiCNTs with binding energies as high as 0.70 eV and can attract finite charge from SiCNTs. By comparison to oxygen absorption on CNTs, we infer that molecular CO and HCN absorbed on SiCNTs can induce significant change in the conductivity of SiCNTs. In view of the high portion of the reactive area, SiCNTs can be potential efficient gas sensors for CO and HCN detection.

Thickness dependence of sensor response for CO gas sensing by tin oxide films grown using atomic layer deposition

Ultrathin tin oxide films were deposited on flat hotplate templates using atomic layer deposition (ALD) techniques with SnCl4 and H2O2 as the reactants. The resistance of the SnOx ALD films across an electrode gap on the hotplate template was observed to oscillate and decrease versus the number of sequential SnCl4 and H2O2 reactions at 250 ◦ C. The resistance also varied with exposure to O2 and CO pressure at 300 ◦ C and 325 ◦ C. A wide range of SnOx ALD film thicknesses between 15.9 A and 58.7 A was prepared by varying the number of sequential, self-limiting SnCl4 and H2O2 reactions. The CO gas sensor response was then measured for these SnOx ALD film thicknesses at 300 ◦C. The CO gas sensor response increased for increasing thicknesses between 15.9 A and 26.2 A and decreased for increasing thicknesses between 26.2 A and 58.7 A. The results were interpreted in terms of the Debye length and resistance for the SnOx ALD films. The Debye length is comparable with the SnOx ALD film thickness of 26.2 A corresponding to the maximum responsivity for CO gas sensing. For film thicknesses >26.2 A, the responsivity decrease was explained by a larger fraction of the film with thickness greater than the Debye length that is not affected by the O2 and CO chemisorption. For film thicknesses <26.2 A, the responsivity decrease was attributed to the increasing resistance of the SnOx ALD film. The gas sensor response was temperature dependent and displayed its highest responsivity at temperatures between 250 ◦ C and 325

CO Gas Sensing by Ultrathin Tin Oxide Films Grown by Atomic Layer Deposition Using Transmission FTIR Spectroscopy

Ultrathin tin oxide films were deposited on SiO2 nanoparticles using atomic layer deposition (ALD) techniques with SnCl4 and H2O2 as the reactants. These SnO(x) films were then exposed to O2 and CO gas pressure at 300 degrees C to measure and understand their ability to serve as CO gas sensors. In situ transmission Fourier transform infrared (FTIR) spectroscopy was used to monitor both the charge conduction in the SnO(x) films and the gas-phase species. The background infrared absorbance measured the electrical conductivity of the SnO(x) films based on Drude-Zener theory. O2 pressure was observed to decrease the SnO(x) film conductivity. Addition of CO pressure then increased the SnO(x) film conductivity. Static experiments also monitored the buildup of gas-phase CO2 reaction products as the CO reacted with oxygen species. These results were consistent with both ionosorption and oxygen-vacancy models for chemiresistant semiconductor gas sensors. Additional experiments demonstrated that O2 pressure was not necessary for the SnO(x) films to detect CO pressure. The background infrared absorbance increased with CO pressure in the absence of O2 pressure. These results indicate that CO can produce oxygen vacancies on the SnO(x) surface that ionize and release electrons that increase the SnO(x) film conductivity, as suggested by the oxygen-vacancy model. The time scale of the response of the SnO(x) films to O2 and CO pressure was also measured by using transient experiments. The ultrathin SnO(x) ALD films with a thickness of approximately 10 A were able to respond to O2 within approximately 100 s and to CO within approximately 10 s. These in situ transmission FTIR spectroscopy help confirm the mechanisms for chemiresistant semiconductor gas sensors.

Effect of CO on Characteristics of AlGaN/GaN Schottky Diode

Pt Schottky diode gas sensors for CO are fabricated using AlGaN/GaN high electron mobility transistor (HEMTs) structure. The diodes show a remarkable sensor signal (3 mA, in N2; 2mA in air ambient) biased 2 V after 1% CO is introduced at 50° C. The Schottky barrier heights decrease for 36meV and 27meV in the two cases respectively The devices exhibit a slow recovery characteristic in air ambient but almost none in the background of pure N2, which reveals that oxygen molecules could accelerate the desorption of CO and offer restrictions to CO detection.

A review of electrolyte and electrode materials for high temperature electrochemical CO2 and SO2 gas sensors

Abstract Carbon dioxide and sulfur dioxide are two common combustion products, so monitoring their contents in exhaust gases is important to reduce harmful emissions. Solid electrolyte based sensors are well suited to the high temperature aggressive environments in combustion environments. However, the design of effective high temperature electrochemical sensors for CO2 and SO2 requires strategic combinations of electrolyte and electrode materials to provide satisfactory response and stability. The approaches and materials used in the development of CO2 and SO2 sensors, many of which are common to both types of sensors, are reviewed in this paper.

Fabrication of resistive CO gas sensor based on SnO2 nanopowders via low frequency AC electrophoretic deposition

A resistive CO gas sensor has been fabricated using AC electrophoretic deposition (ACEPD) technique. SnO2 thick films are deposited by applying low frequency (0.01–1,000 Hz) AC electric field to a stable suspension of SnO2 nanoparticles in acetyl acetone. A carbon film base electrode is used as deposit substrate. Effect of CO gas exposure on conductivity of the SnO2 film at 300 °C is investigated. Results show that the sensor is sensitive and its response is repeatable. This work shows that ACEPD can be used as an easy and cheap technique for fabrication of electronic devices such as ceramic gas sensors.

Influence of magnesia modification on the properties of copper oxide supported on γ-alumina

XRD, BET, TPR, UV–vis DRS and in situ FT-IR were employed to investigate the dispersion, reduction and CO 2-adsorption behaviors of copper oxide supported on magnesia modified γ-Al 2O 3 (Mg–Al) samples. The results indicate that magnesia could be highly dispersed on the surface of γ-Al 2O 3 to form a monolayer and the dispersion capacity is about 1.55 mmol/(100 m 2 γ-Al 2O 3). For copper oxide supported on Mg–Al samples, both the dispersion capacity and the reduction temperature of surface CuO decrease with the MgO loading. CO 2-adsorption IR results show that the surface strong basic amount for the catalysts increases with the dispersed MgO loading. In addition, the activity of CO oxidation suggests that the main active species in this system should be small CuO cluster and the existence of dispersed MgO enhances the activity of CO oxidation. The catalysts might be applied in pollution control devices for vehicle exhaust, CO gas sensors, catalytic combustion and gas purification of CO 2 lasers. All the results have been discussed by the consideration of the variation of γ-Al 2O 3 surface structure before and after magnesia modification.

Resistive CO gas sensors based on In2O3 and InSnOx nanopowders synthesized via starch-aided sol–gel process for automotive applications

Abstract Pure and Sn-doped In 2 O 3 nanopowders have been synthesized by a starch-aided sol–gel process. A detailed characterization by means of TEM and HRTEM, TG–MS, XRD and 119 Sn solid-state NMR analysis has been carried out. It has shown that the grains of the samples as-synthesized and dried at 120 °C are in the nanometer range. Moreover, on the Sn-doped In 2 O 3 sample, the homogeneous distribution of the dopant with no segregation phase effects has been demonstrated. The thermal treatment at 550 °C induced an increase of grain size up to about 30 nm and of crystallinity. The behaviour of the resistive gas sensors based on the synthesized nanopowders in the monitoring of carbon monoxide for automotive applications has been evaluated. Electrical and sensing tests have been performed on the sensor devices in a thick film configuration depositing pure and Sn-doped In 2 O 3 nanopowders by screen-printing over a ceramic substrate. The results have been discussed in relation to the chemical and microstructural properties of the synthesized nanopowders. The good sensing behaviour of these samples has been associated with their special features such as very small grains and high oxygen vacancies due to the peculiar reductive character of starch pyrolysis.

Homogeneous coating of Au and SnO2 nanocrystals on carbon nanotubes via layer-by-layer assembly: a new ternary hybrid for a room-temperature CO gas sensor

CNT/Au/SnO(2) nanotubes have been synthesized through homogeneous coating of Au and SnO(2) nanocrystals on carbon nanotubes (CNTs) and applied in a room-temperature CO gas sensor.

Carbon Dioxide Emissions and Carbonation Sensors

The gases with higher heat capacities than those of O2 and N2 cause greenhouse effects. Carbon dioxide (CO2) is the main greenhouse gas associated with global climate change. At the present time, coal is responsible for 30–40% of world CO2 emissions from fossil fuels. There was a higher correlation between the amount of carbon dioxide emission and percentage of carbon in the fuel for all equations. The squares of correlation coefficients were 0.9999, 0.9978, and 0.9995. The gas sensing characteristics of MgO and CaO as the CO2 gas sensors and CO2 emission capacities selected carbonaceous fuels have been investigated. It was found that increasing the percentage of carbon in carbonaceous fuel caused CO2 emission increase. Carbonation is a stabilization of CO2 by solidification process. The availability of a CO2 fixation technology would serve as insurance in case global warming causes severe restrictions on CO2 emissions. In order to prevent rapid climate change, it will be necessary to stabilize CO2 as carbonate by the carbonation process. The carbonation was carried out using MgO and CaO as CO2 sensors. The yield of carbonation increased with increasing temperature. The rate of carbonation conversion sharply increased in the initial 20 min and then reduced and reached a plateau value after about 40 min. The carbonation conversion with MgO is higher than that of CaO.

Investigation of miniature CO2 gas sensor based on NASICON

A miniature CO2 gas sensor based on NASICON (sodium super ionic conductor) thick film was fabricated. The solid-electrolyte NASICON material was synthesized through an inorganic-reagent-based sol-gel method. The resulting materials were characterized by means of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). NASICON paste was coated on a piece of alumina substrate attached to a platinum heater. Li2CO3-BaCO3 binary carbonate in molar ratio 1 : 1.5 was utilized as the sensing electrode. Within a wide range of CO2 volume ratio concentration from 500 to 5000 ppm, the output electromotive force (EMF) of the sensor followed Nernst equation well at high working temperature. The response and recovery times were 20 and 58 s, respectively. This miniature CO2 gas sensor possessed extra merits such as low power consumption, miniaturized framework, and easy fabrication.

ZnO Nanowire-Based CO Sensors Prepared at Various Temperatures

ZnO nanowire-based CO gas sensors were fabricated by growing single crystal ZnO nanowires on patterned templates at various temperatures. It was found that the average length of the nanowires increased while the average diameter of the nanowires decreased as we increased the growth temperature from . It was also found that the nanowires became significantly shorter as the growth temperature was increased. By measuring the resistivity change of the samples at , it was found that the sensor responses were of 3, 17, 57, 7.5, and 5% for the ZnO nanowires grown at 600, 650, 700, 750, and , respectively. Furthermore, it was found that the sensitivity of the sensor was relatively increased when the concentration of injected CO gas increased.

The effect of multiwalled carbon nanotube doping on the CO gas sensitivity ofSnO2-based nanomaterials

A simple in situ method is applied for the synthesis ofSnO2-coatedmultiwalled nanotubes (MWNTs), in which MWNT bundles are uniformly coated with a monolayer ofSnO2 nanoparticles(∼4 nm).A hybrid SnO2 material combined with 0.1% wt MWNTs is fabricated and its CO gas-sensing properties are evaluated.SnO2-coated MWNTs are homogeneously distributed in theSnO2 matrix and the specific surface area of the hybrid material is nearly two times higher compared topure SnO2 material. The electrical resistance is found to be dramaticallydecreased by three orders of magnitude compared to a conventionalSnO2 sensor. The CO gas sensitivity is found to be three times higher than that of aSnO2 sensor, with good stability, indicating that the addition of 0.1% wtMWNTs is effective in improving the sensitivity towards CO. A model isproposed to explain the improvement of CO gas sensitivity for our hybridMWNT/SnO2 material.

Anatase TiO 2 films based CO gas sensor: Film thickness, substrate and temperature effects

Electrical response of anatase TiO 2 films is discussed for different concentrations of CO gas (20–100 ppm) in a nitrogen gas ambient. We investigated temperature (100–300 °C) and film thickness (100–1000 nm) effects for films deposited on glass, sapphire (0 0 0 1) and, Si(1 0 0) substrates. In general, there is a drop in resistance of the device when exposed to CO gas. Films deposited on sapphire showed a larger decrease in the resistance at 300 °C compared to those deposited on glass and silicon substrates. However, films grown on glass and silicon substrates showed a larger decrease in the resistance values for temperature around 200 °C when CO (ppm) values are greater than 40. The change in resistance of the films varies as square root of the CO gas concentration at 200 °C for films deposited on all the three substrates. In general, the decrease in resistance for thicker films is large compared to thinner ones, indicating participation of the bulk in the detection of CO gas. In the presence of O 2, film loses its sensitivity to CO gas when the concentration level of O 2 approaches 1800 ppm.

Preferential Growth of SnO2 Triangular Nanoparticles on ZnO Nanobelts

ZnO−SnO2 binary nanostructure was synthesized by a simple vapor-phase transport method. Pyramid-like single-crystalline SnO2 nanoparticles were found grown on the surface of the ZnO nanobelts with definite orientations, aligning into a line along two preferential directions [0110] and [0111]. The ZnO−SnO2 binary nanostructures were studied and their growth mechanism was discussed. The ZnO−SnO2 binary nanostructures were used to fabricate CO gas sensor with sensitive response. The photoluminescence of the binary oxide nanostructure was also measured.

Electrospun Nanofibrous Polyacrylonitrile(PAN)/ Fe2O3Membrane as Co2Gas Sensor

Polyacrylonitrile (PAN)/Fe₂O₃ nanocomposite membranes with a thickness of 0.02 ㎜ were electrospun by adding 0 to 5 wt% of Fe₂O₃ into PAN. The surface tension, density, kinematic viscosity and dynamic viscosity of the PAN solution were determined to be 33.8±1 mN/m, 0.9794 g/㎖, 1548.6 ㎟/sec and 1516.7 cP, respectively. The average diameters of PAN fibers containing 0, 1, 2, 3, and 4 wt% Fe₂O₃ particles were 300, 260, 210, 130, and 90 nm, respectively. Fourier-transform infrared spectroscopy results showed that the addition of Fe₂O₃ nanoparticles to the PAN mat reduced the absorption peak intensity at 2242 ㎝ -1 (C≡N bond) while it caused a sharp increase in the peak intensity at 2356 ㎝ -1 (C=O bond). Thus, it appears that an appropriate amount of Fe₂O₃ nanoparticles in the PAN backbone leads to an improvement of the performance of the CO₂ gas sensor, most likely due to the change of functional groups in the membrane.

CO2 gas sensitivity of sputtered zinc oxide thin films

For the first time, sputtered zinc oxide (ZnO) thin films have been used as a CO2 gas sensor. Zinc oxide thin films have been synthesized using reactive d.c. sputtering method for gas sensor applications, in the deposition temperature range from 130–153°C at a chamber pressure of 8·5 mbar for 18 h. Argon and oxygen gases were used as sputtering and reactive gases, respectively. ZnO phase could be crystallized using a pure metal target of zinc. The structure of the films determined by means of X-ray diffraction method indicates that the zinc oxide single phase can be fabricated in this substrate temperature range. The sensitivity of the film synthesized at substrate temperature of 130°C is 2·17 in the presence of CO2 gas at a measuring temperature of 100°C.

ZnO nanowire-based CO sensors prepared on patterned ZnO:Ga/SiO2/Si templates

This investigation discusses the growth of high-density single crystalline ZnO nanowires on patterned ZnO:Ga/SiO2/Si templates and the fabrication of ZnO nanowire-based CO gas sensors. The ZnO nanowires grown on a sputtered ZnO:Ga layer were vertically aligned while those grown directly on a SiO2 layer were randomly oriented. Additionally, the average length of the nanowires increased and the average diameter of the nanowires decreased as the amount of zinc metal powder in the quartz tube was increased from 0.1 to 0.25g. As the amount of zinc metal power increased to 0.3g, the nanowires became markedly shorter. Measuring the resistivity change of the samples at 320°C indicated that the sensor responses (((Ra−Rb)/Ra)×100%, Ra is the resistivity in air and Rb in CO gas) of the ZnO nanowire CO sensors prepared with 0.1, 0.15, 0.2, 0.25 and 0.3g zinc metal powder were 5%, 8%, 35%, 57% and 29%, respectively.

Impedance Analysis Study on the Sensing Process of BaTiO&lt;sub&gt;3&lt;/sub&gt; Based PTC Ceramics in CO Gas

Impedance spectroscopy is suitable to evaluate the electric structure and behavior by probing the relaxational processes during the charge transport within this kind of electrically inhomogeneous PTCR ceramics. This technique has been used to characterize the change of structure and behavior in sensing process of La-doped PTC-type BaTiO3 as a CO gas sensor. The kinetic behavior of PTCR-CO ceramics in sensing process was detected. The sensitivity of the PTCR-CO comes from the change in the grain-boundary resistance of the ceramics with different temperatures in surrounding atmosphere. An important indirect evidence for the creation of new electron traps in grain boundaries of the semiconducting PTCR ceramics have been found using by impedance spectrum. And the creation of the new defects and electron traps in grain boundaries, [VO •], [e¢], and/or [Vo ×], still plays a dominate role.