Ppm In Synthetic Air Research Articles

Porous and Nanostructured CuBr Networks for Selective Ammonia Sensing at Room-Temperature

Introduction Ammonia (NH3), a major industrial commodity (142 Mt in 2017 [1]), is toxic as well as a tracer for food spoilage detection and important breath marker for impaired kidney function [2]. As a result, there is a strong interest in selective sensors that detect NH3 concentrations over a large range: from 250 to 2900 ppb in mouth-exhaled breath down to few ppb in indoor air at high relative humidity. Also, it occurs usually in gas mixtures containing a myriad of compounds requiring also high NH3 selectivity. Most promising as a solid-state and low-cost sensor material is CuBr featuring high selectivity and sensitivity to low NH3 concentrations (e.g. 20 ppb [3]) even at room temperature. However, current fabrication methods usually result in micro-sized CuBr particles [3] and rather dense film morphologies [4] that could impede efficient NH3 diffusion in the film and interaction with the CuBr surface. Here, we introduce a novel fabrication route yielding highly porous and nanostructured CuBr films for improved NH3 sensitivity and fast response dynamics at room temperature [5]. Methods Porous and nanostructured CuBr films are obtained by a flame-aerosol based method on interdigitated electrodes. Their sensing performance was evaluated at room temperature in gas mixtures prepared with high-resolution mass flow controllers, as described in detail elsewhere [6]. In brief, analyte gases (i.e. NH3, isoprene, ethanol, methane, acetone, hydrogen, acetic acid, methanol, formaldehyde and CO) were supplied from calibrated cylinders (10 or 50 ppm in synthetic air, PanGas) and dosed to dry synthetic air (Pangas 5.0, CnHm and NOx ≤ 100 ppb) with calibrated mass flow controllers. Humidity was added by bubbling dry synthetic air through distilled water and admixing it to the analyte flow. All transfer lines were made of inert Teflon to minimized analyte gas adsorption and heated to 55 °C to avoid water condensation. The sensor film resistance was continuously monitored with a multimeter (Keithley, Integra Series 2700, USA). Results and Conclusions The porous CuBr films were tested for sensing of 5 - 5000 ppb NH3 at 90% RH (Figure 1a). When exposed to 5000 ppb of NH3, the resistance rapidly increases from 47 kΩ to 13 MΩ corresponding to a response (S) of 276. When exposed to NH3, the Cu+ as charge carriers are immobilized by forming that results in the observed resistance increase. Remarkably, this interaction is rapid and reversible even at room temperature, as indicated by the full recovery of the initial resistance baseline (dashed line, Figure 1a) and in line with literature [3]. Most impressively, NH3 concentrations down to 5 ppb (see also inset of Figure 1a for higher magnification) are detected and can be distinguished clearly from 10 and 20 ppb. The signal-to-noise-ratio (SNR, > 70) is remarkable and should enable the detection of even lower concentrations with an extrapolated lower limit of detection (LOD) of 210 parts-per-trillion (ppt) considering a typical SNR of 3. This is superior to state-of-the-art room temperature NH3 sensors: For instance, polyaniline sensors detect 40 ppb NH3 at ≥ 90% RH [7], but these are usually for single-use only as they recover too slowly.The porous CuBr sensor was tested with isoprene, ethanol, methane, acetone, hydrogen, acetic acid, methanol, formaldehyde and carbon monoxide (CO), all at a concentration of 1 ppm and 90% RH (Figure 1b). Most remarkably, the sensor responds strongest to NH3 with selectivity > 30 with CO being the highest one (> 290). This is similar to thermally deposited CuBr films with a CeO2 overlayer [3] for some of these analytes. Other NH3 sensors feature lower selectivities to these analytes. In fact, polymer-based PEDOT:PSS nanowires [8] are quite sensitive to ethanol (NH3 selectivity ~15 vs. CuBr ~40).In conclusion, such nanostructured and porous CuBr films are quite attractive for wearable breath analyzers [9] and detectors for distributed air and food quality monitoring networks with stringent power requirements.

Mesoporous Tyrosine Functionalized BTC-ZnO Composite for Highly Selective Capacitive CO Sensor

Drop-casting is a simple deposition technique where a slurry-like solution of nanomaterial is coated on small substrates, which are then allowed to dry under controlled temperature and pressure ambience. The present work focuses on the tyrosine functionalized benzene-tri-carboxyamide (Tyr-BTC) and ZnO composite based sensor deposited using drop-cast method for sensing carbon monoxide (CO) at room temperature. The composition of the composite surface was varied by altering the molar concentration of the constituents. The performance of the mesoporous structures in room temperature gas sensing was studied for the CO concentration range of 5–300 ppm in synthetic air. The highly amplified gas sensing response of approximately 94% at 300 ppm of CO by Tyr-BTC-ZnO in molar concentration 2:3 of Tyr-BTC-ZnO was observed. The improvement in the capacitance change, quicker response time, good linearity and reversibility was attributed to the synergistic effect between ZnO and tyrosine functionalized BTC.

Porous and Nanostructured CuBr Networks for Selective Ammonia Sensing at Room-Temperature

Introduction Ammonia (NH3), a major industrial commodity (142 Mt in 2017 [1]), is toxic as well as a tracer for food spoilage detection and important breath marker for impaired kidney function [2]. As a result, there is a strong interest in selective sensors that detect NH3 concentrations over a large range: from 250 to 2900 ppb in mouth-exhaled breath down to few ppb in indoor air at high relative humidity. Also, it occurs usually in gas mixtures containing a myriad of compounds requiring also high NH3 selectivity. Most promising as a solid-state and low-cost sensor material is CuBr featuring high selectivity and sensitivity to low NH3 concentrations (e.g. 20 ppb [3]) even at room temperature. However, current fabrication methods usually result in micro-sized CuBr particles [3] and rather dense film morphologies [4] that could impede efficient NH3 diffusion in the film and interaction with the CuBr surface. Here, we introduce a novel fabrication route yielding highly porous and nanostructured CuBr films for improved NH3 sensitivity and fast response dynamics at room temperature. Methods Porous and nanostructured CuBr films are obtained by a flame-aerosol based method on interdigitated electrodes. Their sensing performance was evaluated at room temperature in gas mixtures prepared with high-resolution mass flow controllers, as described in detail elsewhere [5]. In brief, analyte gases (i.e. NH3, isoprene, ethanol, methane, acetone, hydrogen, acetic acid, methanol, formaldehyde and CO) were supplied from calibrated cylinders (10 or 50 ppm in synthetic air, PanGas) and dosed to dry synthetic air (Pangas 5.0, CnHm and NOx ≤ 100 ppb) with calibrated mass flow controllers. Humidity was added by bubbling dry synthetic air through distilled water and admixing it to the analyte flow. All transfer lines were made of inert Teflon to minimized analyte gas adsorption and heated to 55 °C to avoid water condensation. The sensor film resistance was continuously monitored with a multimeter (Keithley, Integra Series 2700, USA). Results and Conclusions The porous CuBr films were tested for sensing of 5 - 5000 ppb NH3 at 90% RH (Figure 1a). When exposed to 5000 ppb of NH3, the resistance rapidly increases from 47 kΩ to 13 MΩ corresponding to a response (S) of 276. When exposed to NH3, the Cu+ as charge carriers are immobilized by forming that results in the observed resistance increase. Remarkably, this interaction is rapid and reversible even at room temperature, as indicated by the full recovery of the initial resistance baseline (dashed line, Figure 1a) and in line with literature [3]. Most impressively, NH3 concentrations down to 5 ppb (see also inset of Figure 1a for higher magnification) are detected and can be distinguished clearly from 10 and 20 ppb. The signal-to-noise-ratio (SNR, > 70) is remarkable and should enable the detection of even lower concentrations with an extrapolated lower limit of detection (LOD) of 210 parts-per-trillion (ppt) considering a typical SNR of 3. This is superior to state-of-the-art room temperature NH3 sensors: For instance, polyaniline sensors detect 40 ppb NH3 at ≥ 90% RH [6], but these are usually for single-use only as they recover too slowly.The porous CuBr sensor was tested with isoprene, ethanol, methane, acetone, hydrogen, acetic acid, methanol, formaldehyde and carbon monoxide (CO), all at a concentration of 1 ppm and 90% RH (Figure 1b). Most remarkably, the sensor responds strongest to NH3 with selectivity > 30 with CO being the highest one (> 290). This is similar to thermally deposited CuBr films with a CeO2 overlayer [3] for some of these analytes. Other NH3 sensors feature lower selectivities to these analytes. In fact, polymer-based PEDOT:PSS nanowires [7] are quite sensitive to ethanol (NH3 selectivity ~15 vs. CuBr ~40).In conclusion, such nanostructured and porous CuBr films are quite attractive for wearable breath analyzers [8] and detectors for distributed air and food quality monitoring networks with stringent power requirements.

A Semiconductor Gas System of Healthcare for Liver Disease Detection Using Ultrathin InN-Based Sensor

The gas sensor based on platinum-catalyzed ultrathin indium nitride (Pt-InN) was used to detect liver disease by sub-ppm ammonia (NH3) concentration variation in an ambient environment simulating exhaled human breath. Because the InN with strong surface electron accumulation extensively enhanced sensitivity, the NH3 gas sensor minimum detection limit is 0.2 ppm. Meanwhile, the sensor enables to distinguish slight deviation in NH3 concentration between 0.2 and 0.8 ppm that brings simple diagnosis of liver disease for portable and homecare clinical applications. Liver cancer is first of the top ten causes of death in Asia. Abnormal concentrations of the breath volatile organic compounds (VOCs) are reported to correlate with unhealthy/injurious body/organ conditions; for instance, ammonia (NH3) gas for renal and liver disease. Therefore, the detection of ammonia concentration in exhaled breath more important. In this study, an indium nitride (InN) gas sensor of 10 nm in thickness has achieved detection limit of 0.2 ppm ammonia. The sensor has a size of 1 mm by 2.5 mm, while its sensing area is 0.25 mm by 2 mm and deposited with a 10 nm thick Pt film on the sensing window to investigate the effect of catalyst on ammonia gas detection, shown in Fig 1. The ultrathin InN epilayer extensively enhances sensing sensitivity due to its strong electron accumulation on roughly 5–10 nm deep layers from the surface. Sensitivity is expected to be much better due to the natural electronic characteristics of as-grown InN films, including a narrow band gap, excellent electron transport characteristics (mobility > 1,000 cm2·V·s), a background high electron density (typically in excess of 1 × 1018 cm-3), and the unusual phenomenon of strong surface electron (charge) accumulation (1.57 × 1013 cm- 2) for the InN films grown on AlN film on a buffer layer . We proposing the miniaturized prototype, and packaging the ammonia gas sensor system (Including: Pt-InN sensor chip, heater, K-type thermocouple) on the 8 pin dual in-line package (DIP) for the portable human breath liver disease physiological detection application, shown in Fig 2. The current variation (DI) of a Pt-InN ammonia sensor under exposure to various ammonia concentrations from 0.2 ppm to 0.8 ppm in synthetic air at 200 °C is depicted in Fig 3. The experiments were repeatedly performed while the ammonia gas was turned on for 5 minute and off (i.e., in pure synthetic air) for another 5 minute at various ammonia concentrations. The current went up as ammonia concentration increased; such current increment was induced by the reduction of pre-adsorbed oxygen atoms. When ammonia molecules are introduced to the sensing system, the hydrogen atoms on dissociated ammonia molecules react with pre-adsorbed oxygen atoms, reducing the effects of surface depletion. Platinum as catalyst can increase output current signals as well as reduce response in comparison with Pt-InN, indicating that such an ammonia concentration can be analyzed in air. The dynamic responses of the Pt-InN sensor indeed demonstrate the great ability to sense ammonia gas at the sub-ppm level, and promising device for the realization of non-invasive ammonia sensing to detect ammonia in exhaled breath as a marker for liver disease. Figure 1

Hydrogen sensing by sol–gel grown NiO and NiO:Li thin films

Hydrogen sensors have been prepared using nickel oxide (NiO) and lithium-doped nickel oxide (NiO:Li) thin films, deposited on glass substrates by the sol–gel spin coating technique. The surface morphology, structure, optical and electrical properties of the obtained films were studied. Hydrogen sensing results are presented for three operating temperatures (140, 160, and 180°C) and for hydrogen concentrations ranging from 1000 to 15,000ppm in synthetic air. The NiO and NiO:Li (2% and 8% doping concentrations) sensors show maximum responses for the operating temperature of 180°C. When tested at different hydrogen concentrations in air, the lithium-doped NiO sensors showed a higher response than the undoped NiO films.

Effect of Post-Deposition Annealing Treatment on the Structural, Optical and Gas Sensing Properties of TiO<sub>2</sub> Thin Films

One of the potential applications of TiO2 is its use in gas sensor technology. The aim of this work was to study the gas sensing properties of TiO2 thin films in combination with the effect of post-deposition annealing treatment. Titanium dioxide thin films with thickness 100 nm were prepared by the reactive dc magnetron sputtering. The thin films were deposited on sapphire substrate from a titanium target in an oxygen atmosphere. The samples were then post-annealed in air in the temperature range 600 °C 1000 °C. Crystal structure, surface topography and absorption edge of the thin films have been studied by X-ray Diffraction technique, Atomic Force Microscopy and UV-VIS Spectroscopy. It was found that the phase gradually changed from anatase to rutile, the grain size and roughness tended to increase with increasing post-annealing temperature. The effect of these factors on gas sensing properties was discussed. For electrical measurements comb-like Pt electrodes were prepared by standard photolithography and the films were exposed to different concentrations of H2 gas up to 10000 ppm in synthetic air at various operating temperatures from 200 °C to 350 °C.

Vertically Aligned ZnO Nanorod Sensor on Flexible Substrate for Ethanol Gas Monitoring

Vertically aligned ZnO nanorods were grown on flexible polyimide films by a thermolysis assisted chemical solution method to fabricate an ethanol sensing material. Flexible ZnO nanorod sensors were examined to monitor ethanol gas by varying the working temperature from 300 to 125°C and by changing the ethanol concentration in a range from 100 to 10 ppm in synthetic air. A flexible ZnO nanorod sensor can detect 100 ppm of ethanol gas with a sensitivity of 3.11 at 300°C. When compared with ZnO nanorod sensors fabricated on a silicon dioxide hard substrate, its sensing performance exhibited competitive sensitivity.

Nitric oxide gas sensors with functionalized carbon nanotubes

AbstractNitric oxide (NO) gas sensing with carboxyl functionalized single‐ and multi‐walled carbon nanotubes (SWCNTs and MWCNTs) is demonstrated in this work. Stable solutions of the carboxylated nanotubes were made and then drop‐cast on alumina substrates equipped with gold electrodes and also with platinum heater elements. The obtained resistive nanotube sensors were tested in a 2‐point setup at different temperatures and NO gas concentrations up to 100 ppm in synthetic air and also in argon buffer. When exposed to NO, the conductivity of the sensors changed up to ∼40% for SWCNTs and ∼12% for MWCNTs; however, in the investigated regime, the response was found to be fairly independent on NO concentration. Though all sensors showed practically instantaneous gas response, the time needed for saturation (few tens of minutes) was found to be concentration dependent. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of the nanostructural characteristics on the gas sensing properties of pulsed laser deposited tin oxide thin films

Tin oxide (SnO 2) thin films doped with ∼2 at.% Pt have been deposited onto alumina substrates by reactive pulsed laser ablation of a pure SnO 2 target in an oxygen atmosphere. At a KrF laser intensity of 5×10 8 W/cm 2 and a substrate deposition temperature ( T d) of 300 °C, the pulsed laser deposition (PLD) of SnO 2 thin films was investigated under various oxygen background pressures ( P(O 2)) ranging from 1 to 200 mTorr. While all the deposited films consist of pure polycrystalline SnO 2 phase, their mean grain size and nanoporosity, as deduced from TEM observations, were found to be highly influenced by P(O 2). Indeed, the size of the nanograins that compose the SnO 2 columns was found to increase from ∼4 nm for P(O 2)≤100 mTorr to about 10 nm for P(O 2)≥150 mTorr. The SnO 2 films deposited under P(O 2)≥100 mTorr are shown to exhibit an open inter-columnar porosity, while those grown under P(O 2)=50 mTorr are characterized by randomly distributed spherical nanopores (∼1 nm). The deposited PLD SnO 2 films were integrated into gas-sensing devices and their sensing performance was evaluated for CO gas concentrations ranging from 10 to 250 ppm in synthetic air. It is pointed out that the gas sensitivity of PLD SnO 2 films, having grain size in the (3–10 nm) range, is predominantly influenced by the porosity of the nanostructure.

High-Temperature Potentiometric NO2 and CO Sensors Based on Stabilized Zirconia with Oxide Sensing Electrodes

ABSTRACTThis paper reports the efforts made in our laboratory to develop electrochemical sensors that might detect NO2 and CO at high temperatures for On Board Diagnostic (OBD) application. The non-Nernstian behaviour of zirconia-based electrochemical NO2 sensors with various oxides as sensing electrodes was studied in the temperature range 450–700°C. Both pellets and tape-casted layers (150 μm of thickness) of yttria-stabilized zirconia (YSZ) were used for fabrication of the sensors. Pt electrodes were painted on both sides of the pellets or as two parallel fingers on one face of the layers. One of the Pt electrodes was covered with a thick-film oxide electrode. Various oxides were tested as sensing electrodes, either p- or n-type semiconductors, including WO3 and LaFeO3. The role of ionic conductivity of the oxide electrodes was investigated using Sr-doped perovskite-type oxides, such as LaxSr1-xFeO3, a mixed ionic-electronic conductor. The sensors were tested as potentiometric and amperometric devices. The performance of these devices was promising: fast and stable responses to different NO2 concentrations (20–1000 ppm in synthetic air) were observed at high temperatures. The role of the metallic electrodes is also studied. The sensing mechanism of the sensors is discussed.

Elaboration and characterization of tin oxide–lanthanum oxide mixed layers prepared by the electrostatic spray pyrolysis technique

Lanthanum oxide–tin oxide mixed layer was found to show promising sensing properties to CO 2 in the air. In this paper, we describe the characterization of such layers fabricated by the electrostatic spray pyrolysis technique. Analysis by scanning electron microscopy (SEM) and atomic force microscopy (AFM) show a structure of grain aggregates, while energy dispersion analysis reveal that the original and final compositions are very close with an uniformly distribution in the layer. X-ray diffraction gives the nature of the deposited oxide, which is LaOCl and SnO 2. Electrical characterization versus temperature and concentration reveal a sensitivity peak for the 2% lanthanum oxide mixed layer at about 425°C. A variable frequency (13 MHz–500 Hz) ac voltage was applied to all the sensor structures to perform the complex impedance spectrum measurements, at a temperature range 350–550°C, and a CO 2 concentration range 400–2000 ppm in synthetic air. For all these measurements, we obtain the same equivalent circuit, which fits very well the experimental data, and allows us to propose a conduction model.

Measurements of VOCs in soils through a tin oxide multisensor system

A tin oxide multisensor with 15 sensor elements has been prepared using the r.f. reactive sputtering technique. Some components of this multisensor have been doped with Pt, Pd and Al by sputtering. Each sensor has been characterized by electrical measurements for various volatile organic compounds (VOCs) in a concentration range from 50 to 500 ppm in synthetic air at 300 °C. The best response has been obtained by Pt and Pd doped sensors for the oxygenated gaseous compounds, but for all gases tested a good recognition is shown. For the concentration range used it has been possible to detect the threshold limit values (TLV) for airborne contaminants of toluene, octane and methyl-ethyl-ketone. The influence of the oxygen species (O − and O 2 −), ionosorbed on the sensing surface, on the VOCs detection is also discussed.

Response of Some SnO2 Gas Sensors to H 2 S after Quick Cooling

The hydrogen sulfide response of two commercial gas sensors and a thick‐film sensor containing 10 weight percent (w/o) of was studied in a concentration range of 0.1–50 ppm in synthetic air. The experiments were carried out at a temperature of 150°C both by allowing the sensors to stabilize at this temperature, and by quick cooling from an initial temperature of 400°C. Some continuous temperature cycling experiments were also performed in order to study the reproducibility of the readings and any drift with time. The primary aim was to study the feasibility of using temperature cycling to increase the performance of sensors for . A major increase in the sensitivity of the sensors to was achieved by means of the cooling process. The response times to exposure in synthetic air were also shorter after cooling to 150°C than at the same temperature without any cooling process. Continuous temperature cycling also improved both the response and recovery times. Mixing of porous sensors with powder may allow the selectivity of sensors to to be increased if the cooling process is used for detection (as mixing increases sensitivity to but drastically reduces that to , for instance). Water vapor increased the sensitivity of the sensors to and reduced the response and recovery times.