Detection of organic chemical vapors with a MWNTs-polymer array chemiresistive sensor

Organic chemical hazardous gases pose a significant threat to human life and the environment. An urgent need exists for the development of reliable chemical sensors that would be able to identify these hazardous gases. In a recent study, conductive carbon nanotubes were mixed with six polymers with various chemical adsorption properties to produce a composite thin film for the fabrication of a chemical sensor array. A silicon wafer was used as a microelectrode substrate for a resistance sensor fabricated using a typical semiconductor manufacturing process. This sensor array was then used to identify hazardous chemical gases at various temperatures. Results for two hazardous gases, ammonia (NH₃) and chloroform (CHCl₃), tested with the six polymers at different temperatures, indicated that the variation in sensitivity/resistance increased when the temperature increased. It was found that the MWNTs-PVP and MWNTs-PMVEMA sensing films had high sensitivity, excellent selectivity, and favorable reproducibility in detecting the two chemical agent vapors. In addition, we derived the solubility parameter (Δδ) to demonstrate the sensitivity of the polymers to ammonia (NH₃). The results showed that smaller solubility parameter corresponds to a stronger interaction between NH₃ gas and polymers, and increased sensitivity. Additionally, we used the statistical methods of principal component analysis to identify the interaction of hazardous gases with the MWNTs-polymer sensor.     

Gas sensor using single-wall carbon nanohorns

We fabricated a gas sensor using single-walled carbon nanohorns (SWNHs) produced by the gas-injected arc-in-water method. This gas sensor consisted of agglomerated SWNHs as a coating film between Al electrodes on a glass substrate and the shift of the electric resistance of this coating film caused by gas adsorption was monitored. Its sensing property was examined for the detection of NH₃ and O₃ at room temperature. It was confirmed that the electrical resistance of the SWNHs film increases with adsorption of NH₃, whereas the adsorption of O₃ induced the decrease of the resistance. A model to correlate the gas concentration and the sensing property was proposed focusing on the detection of NH₃ based on mono-layer adsorption and a second-order interaction of adsorbed gas molecules for charge transfer.     

Properties of Indium Oxide Semiconducting Sensors Deposited by Different Techniques

Semiconducting In₂O₃ gas sensors have been fabricated by two different deposition techniques, i.e., spin-coating and screen-printing. In both cases the same starting material – sol-gel-prepared cubic In₂O₃ – was used for the deposition in order to ensure a better comparability of the different sensing layers. The morphology of the layers has been characterized using scanning electron microscopy (SEM) technique. The layers deposited by different methods show similar grain size and porosity. Furthermore, Dc electrical tests have been performed to analyze the sensing properties of the different gas sensors. Reducing gases (CO and propanal) as well as oxidizing gases (NO₂ and ozone) were used as test gases in the background of dry and humidified synthetic air. All measurements were performed at several temperatures. It was found that the spin-coated and screen-printed layers show different sensing properties, i.e., screen-printed sensors showed higher sensor signals than spin-coated sensors for CO, propanal, and NO₂. The most striking differences appeared in the detection of ozone. In this case, spin-coated sensors showed a higher performance than screen-printed sensors. Higher ozone concentrations led to saturation effects for the latter.     

The potential of porous silicon gas sensors

Recent developments in porous silicon gas sensors have been reviewed. Monitored species detection levels, and the mechanisms of sensing for different sensor designs are also discussed. Porous silicon surface modification methods have been employed for detecting different gas molecules; H₂O, ethanol, methanol, isopropanol, CO_x, NO_x, NH₃, O₂, H₂, HCl, SO₂, H₂S and PH₃.     

CO sensing with SnO2 thick film sensors: role of oxygen and water vapour

The paper investigates the gas response of nanocrystalline SnO₂ based thick film sensors upon exposure to carbon monoxide (CO) in changing water vapour (H₂O) and oxygen (O₂) backgrounds. The sensing materials were undoped, Pt- and Pd-doped SnO₂. We found that in the absence of oxygen, the sensor signal (defined as the ratio between the resistance in the background gas, R₀ and the resistance in the presence of the target gases, R, namely R₀/R) have the highest values. These values are higher for doped materials than for the undoped ones. The presence of humidity increases dramatically the sensor signal of the doped materials. In the presence of oxygen, the sensor signal decreases significantly for all sensor materials. The results indicate that there is a competitive adsorption between O₂ and H₂O related surface species and, as a result, different sensing mechanisms can be observed for CO.     

Hierarchical pseudo-cubic hematite nanoparticle as formaldehyde sensor

To develop a low cost and scalable gas, sensor for the detection of toxic and flammable gases with fast response and high sensitivity is extremely important for monitoring environmental pollution. This work reports a facile method for preparing pseudo-cubic hierarchical α-Fe₂O₃ nanostructured materials as well as their implementation in gas sensor application. The α-Fe₂O₃ is developed using Fe(NO₃)₃ and ethylene glycol followed by a facile and one-step solvo-thermal reaction without subsequent heat treatment. The pseudo-cubic nanostructures were having an average edge length of 5–10 nm. The solvent played the role of ligand and synergistically affected olation and oxolation process along with dehydration to form final product. The sensor performance of α-Fe₂O₃ in the detection of toxic and flammable gases such as formaldehyde (HCHO), ethanol (C₂H₅OH), and carbon monoxide (CO) was evaluated. As-synthesized nanostructured hematite showed better sensing performance towards formaldehyde. The fabricated gas sensor showed temperature sensitivity sensing performance for the same gas. In addition, ethanol, formaldehyde vapours, and carbon monoxide gas-sensing properties were tested and the sensing performance of the synthesized material was found to be in the order of HCHO > C₂H₅OH > CO. This sensing performance is attributed to the large specific surface area of the pseudo-cubic nanoparticles.     

Unidentified H2 gas sensing characteristics of BaTiO3:Cu composition

A systematic evaluation of the gas sensing properties of Cu-modified BaTiO₃ pellets for CO, H₂ and LPG gases was carried out in the present investigation. The concentration of Cu in BaTiO₃ was varied systematically from 1 to 11 wt.%. The highest value of sensitivity factor (SF) of 1388 for H₂ and 557 for CO gases was obtained at considerably lower optimum operating temperatures of 180 and 250 °C respectively for 9 wt.% of Cu concentration. The sensor tends to saturate at 5.0 and 3.5% for H₂ and CO gases respectively. The XRD and SEM characterization techniques were employed to understand the possible reasons for the high performance of the sensor material with 9 wt.% of Cu. The study revealed that this sensor material shows extraordinarily promising properties for H₂ gas sensing application.     

CO2 and H2 detection with a CHx/porous silicon-based sensor

There has been great interest in the last years in gas sensors based on porous silicon (PS). Recently, a gas sensing device based on a hydrocarbon CH_x/porous silicon structure has been fabricated. The porous samples were coated with hydrocarbon groups deposited in a methane argon plasma. We have experimentally demonstrated that the structure can be used for detecting a low concentration of ethylene, ethane and propane gases [Gabouze N, Belhousse S, Cheraga H. Phy State Solidi (C), in press].In this paper, the CH_x/PS/Si structure has been used as a sensing material to detect CO₂ and H₂ gases. The sensitivity of the devices, response time and impedance response to different gas exposures (CO₂, H₂) have been investigated.The results show that current-voltage and impedance-voltage characteristics are modified by the gas reactivity on the PS/CH_x surface and the sensor shows a rapid and reversible response to low concentrations of the gases studied at room temperature.     

Evaluation of Constants of Electron–Phonon Coupling between Gas Molecules and Graphene

The adsorption of CO, NO, NO₂, Н₂О, and NH₃ molecules on ideal graphene and graphene doped with aluminum is analyzed using simple models. The constants of electron–phonon coupling are evaluated with the Lennard-Jones 6–12 potential for ideal graphene and the 2–4 potential for doped graphene. It is demonstrated that the dimensionless electron–phonon-coupling constant for ideal graphene is ζ ? 1, while ζ ~ 1 corresponds to graphene doped with aluminum. Ways to use both types of graphene as a resistive gas sensor are discussed.     

Trace-Level,Multi-Gas Detection for Food Quality Assessment Based on Decorated Silicon Transistor Arrays

Multiplexed gas detection at room temperature is critical for practical applications, such as for tracking the complex chemical environments associated with food decomposition and spoilage. An integrated array of multiple silicon-based, chemical-sensitive field effect transistors (CSFETs) is presented to realize selective, sensitive, and simultaneous measurement of gases typically associated with food spoilage. CSFETs decorated with sensing materials based on ruthenium, silver, and silicon oxide are used to obtain stable room-temperature responses to ammonia (NH₃), hydrogen sulfide (H₂S), and humidity, respectively. For example, one multi-CSFET sensor signal changes from its baseline by 13.34 in response to 1 ppm of NH₃, 724.45 under 1 ppm H₂S, and 23.46 under 80% relative humidity, with sensitive detection down to 10 ppb of NH₃ and H₂S. To demonstrate this sensor for practical applications, the CSFET sensor array is combined with a custom-printed circuit board into a compact, fully integrated, and portable system to conduct real-time monitoring of gases generated by decomposing food. By using existing silicon-based manufacturing methodologies, this room-temperature gas sensing array can be fabricated reproducibly and at low cost, making it an attractive platform for ambient gas measurement needed in food safety applications.     

Effect of hydrogen on SiNx films deposited by Cat-CVD method

This study is aimed at improving the characteristics of silicon nitride (SiNx) film deposited by catalytic chemical vapor deposition (Cat-CVD) method. Cat-CVD method can deposit SiNx films that have low hydrogen content and high density at low temperature without any plasma damage to substrates. Usually silane (SiH₄) and ammonia (NH₃) are used for source gases. Then adding hydrogen (H₂) gas to source gases makes characteristics of Cat-CVD SiNx improved. When using H₂ gas, hydrogen content in SiNx film becomes lower and electronic reliability becomes higher.     

Chemiluminescence analyzer of NOx as a high-throughput screening tool in selective catalytic reduction of NO

A chemiluminescence-based analyzer of NO_x gas species has been applied for high-throughput screening of a library of catalytic materials. The applicability of the commercial NO_x analyzer as a rapid screening tool was evaluated using selective catalytic reduction of NO gas. A library of 60 binary alloys composed of Pt and Co, Zr, La, Ce, Fe or W on Al₂O₃ substrate was tested for the efficiency of NO_x removal using a home-built 64-channel parallel and sequential tubular reactor. The NO_x concentrations measured by the NO_x analyzer agreed well with the results obtained using micro gas chromatography for a reference catalyst consisting of 1 wt% Pt on γ-Al₂O₃. Most alloys showed high efficiency at 275 °C, which is typical of Pt-based catalysts for selective catalytic reduction of NO. The screening with NO_x analyzer allowed to select Pt-Ce_(X) (X=1–3) and Pt–Fe₍₂₎ as the optimal catalysts for NO_x removal: 73% NO_x conversion was achieved with the Pt–Fe₍₂₎ alloy, which was much better than the results for the reference catalyst and the other library alloys. This study demonstrates a sequential high-throughput method of practical evaluation of catalysts for the selective reduction of NO.     

A hybrid gas-sensing material based on porous carbon fibers and a TiO2 photocatalyst

The electrode material for a gas sensor was prepared from electrospun carbon fibers. The electrode material was chemically activated to enlarge the gas adsorption sites, and carbon nanotubes (CNTs) were embedded into the polyacrylonitrile-based carbon fibers to enhance their electrical conductivity. TiO₂ was used as an additive to promote NO gas degradation and to improve their response in NO gas sensing. The chemical activation process increased the specific surface area and pore volume of the carbon fibers to values in excess of 2000 m²/g and 1.0 ml/g, respectively. To investigate the photocatalytic effects of the TiO₂ additive, NO gas sensing was conducted in the presence and absence of ultraviolet irradiation. The subsequent results indicate that the response of the sensor was improved due to the TiO₂-photocatalyzed decomposition of NO gas and the subsequent adsorption of HNO₂, NO₂, and HNO₃. The electrical resistance of the sensor was significantly reduced during NO gas sensing due to the electron hopping effect, and highly efficient gas adsorption was observed. In conclusion, a sensitive gas sensor electrode was realized by fabricating a porous material to increase the efficiency of gas adsorption, adding CNTs to improve its electrical conductivity and adding TiO₂ photocatalysts to promote NO decomposition.     

Influence of effective surface area on gas sensing properties of WO3 sputtered thin films

WO₃ thin films having different effective surface areas were deposited under various discharge gas pressures at room temperature by using reactive magnetron sputtering. The microstructure of WO₃ thin films was investigated by X-ray diffraction, scanning electron microscopy, and by the measurement of physical adsorption isotherms. The effective surface area and pore volume of WO₃ thin films increase with increasing discharge gas pressure from 0.4 to 12 Pa. Gas sensors based on WO₃ thin films show reversible response to NO₂ gas and H₂ gas at an operating temperature of 50-300 °C. The peak sensitivity is found at 200 °C for NO₂ gas and the peak sensitivity appears at 300 °C for H₂ gas. For both kinds of detected gases, the sensor sensitivity increases linearly with an increase of effective surface area of WO₃ thin films. The results demonstrate the importance of achieving high effective surface area on improving the gas sensing performance.     

Study on Ambient Air Quality of Megacity Delhi,India During Odd–Even Strategy

State Government of Delhi had adopted odd–even scheme on vehicles plying in megacity Delhi to understand and improve the air quality of Delhi. To understand the effect of odd–even scheme on the concentration of pollutants, we have analysed the concentrations of chemical constituents [organic carbon, elemental carbon, water soluble inorganic components, trace elements and stable carbon and nitrogen isotopic composition (δ¹³C_TC) and N (δ¹⁵N_TN)] of PM_2.5 and PM₁₀ along with mixing ratios of trace gases (NO _x , CO, SO₂ and NH₃) data collected at an urban site of megacity Delhi during first phase (Phase-I: winter 2016) and second phase (Phase-II: summer 2016). During the Phase-I of the scheme, mass concentrations of PM_2.5 and PM₁₀ were changed by ?13 and ?5%, respectively, whereas, concentrations of PM_2.5 and PM₁₀ were changed by +18 and +16%, respectively during the Phase-II as compared to before the implementation of the scheme. The analysis of chemical constituents of PM_2.5 and PM₁₀ reveals that the odd–even strategy marginally changed the concentrations (markers) of vehicular emission. During both the phases, mixing ratios of trace gases (NO _x , CO, SO₂ and NH₃) were reduced non-significantly during the odd–even scheme as compared to before the implementation of the scheme.     

Development of a selective gas sensor utilizing a perm-selective zeolite membrane

Reported here is a novel sensor that utilizes a zeolite film to selectively limit gas exposure of the sensing surface. A unique amperometric sensor design based on a non-porous mixed conducting sensing electrode enables the formation of a continuous zeolite film covering the entire sensor surface. The sensor was tested in a variety of oxygen containing gases. The sensor without a zeolite film responded strongly to both oxygen and carbon dioxide at a bias of 1.8 V. In contrast, the sensor coated with a zeolite film showed a discernable, but diminished response to oxygen, and a more marked drop in response to CO₂ indicating that the diffusion of oxygen through the zeolite film is preferential to that of CO₂. The response of the zeolite coated sensor to a mixture of oxygen and carbon dioxide gases is attributed primarily to the oxygen content. Expanding this concept using a variety of different zeolite structures covering an array of sensors, complete analyses of complex gaseous mixtures could be performed in a very small device.     

Pulsed laser deposition of (WO3)1 − x(Nb2O5)x thin films: Characterization and gasochromic studies

A series of (WO₃)_{1 − x}(Nb₂O₅)_x (x = 0, 0.05, 0.1 and 0.15) mixed oxide films were fabricated by pulsed laser deposition (PLD) at 27 Pa oxygen partial pressure on ITO glass substrates. The thickness of the (WO₃)_{1 − x}(Nb₂O₅)_x thin films is about 350 ± 30 nm and their surface has a uniform morphology. A layer of platinum (Pt) was then sputtered onto the surface of the film. The hydrogen gas sensing performance of Pt catalyst activated (WO₃)_{1 − x}(Nb₂O₅)_x thin films were investigated. The cycling of the coloration was obtained from UV–vis spectra. Gasochromic coloration of (WO₃)_{1 − x}(Nb₂O₅)_x thin films were investigated at room temperature in H₂–N₂ mixtures containing 1–100 mol% of H₂. The results show that the shortest response time of (WO₃)_{1 − x}(Nb₂O₅)_x/Pt hydrogen sensor is within 30 s and the highest transmittance change (ΔT) varies from 20% to 30%.     

An evanescent field absorption gas sensor at mid-IR 3.39 μm wavelength

Trace gases such as H₂O, CO, CO₂, NO, N₂O, NO₂ and CH₄ strongly absorb in the mid-IR (>2.5 μm) spectral region due to their fundamental rotational and vibrational transitions. CH₄ gas is relatively non-toxic, however, it is extremely explosive when mixed with other chemicals in levels as low as 5% and it can cause death by asphyxiation. In this work, we propose a silicon strip waveguide at 3.39 μm for CH₄ gas sensing based on the evanescent field absorption. These waveguides can provide the highest evanescent field ratio (EFR)>55% with adequate dimensions. Moreover, EFR and sensitivity of the sensor are highly dependent on the length of the waveguide up to a certain limit. Therefore, it is always a compromise between the length of the waveguide and EFR in order to obtain greater sensitivity.     

Interface Oxygen Vacancy-Enhanced Co3O4/WO3 Nanorod Heterojunction for Sub-ppm Level Detection of NOx

Herein, a cobalt oxide/tungsten oxide (Co₃O₄/WO₃) p–n heterojunction for NO_x detection is developed and optimized. Field-emission scanning electron microscopy shows that the WO₃ nanorods are embellished with Co₃O₄ nanostructure. X-ray photoelectron spectroscopy reveals the presence of oxygen vacancy on the Co₃O₄ coupled with WO₃ heterojunction. Compared to bare WO₃ and pure Co₃O₄, the Co₃O₄/WO₃ heterojunction sensor shows significant sensitivity to NO_x at 200 °C. The sensor exhibits higher linearity from 0.4 to 10 ppm of NO_x, with a sensing response of 4.4–93%. The NO_x sensitivity of the Co₃O₄/WO₃ heterojunction sensor is ninefold higher than that of the pure WO₃ sensor. Even in a high humidity (84%) environment, the Co₃O₄/WO₃ heterojunction sensor demonstrates high NO_x sensitivity. The sensor maintains remarkable stability measured for up to 4 weeks. The possible NO_x sensing mechanism of the Co₃O₄/WO₃ heterojunction is additionally discussed.     

Composite hydroxyapatite/TiO2 materials for photocatalytic oxidation of NOx

Hydroxyapatite/TiO₂ composite photocatalysts were obtained from sol–gel prepared TiO₂ and commercial hydroxyapatite (HA) powders. Composites with different HA/TiO₂ ratio were studied to assess the influence of HA on the morphology and the photocatalytic behavior of the materials. Morphological SEM analysis revealed that the presence of HA diminishes the aggregation of TiO₂ particles and leads to their higher dispersion in the composites that was confirmed by the N₂ adsorption–desorption isotherms and Barret–Joyner–Halenda analysis. The photocatalytic activity of the prepared catalysts was examined by monitoring photocatalytic oxidation of NO_x model gases over catalysts under UV illumination. The NO_x oxidation over the composite catalysts was improved in comparison with pure TiO₂ powder. Moreover, the decrease of the TiO₂ content, which is the photocatalytically active component in the composites, resulted in enhanced NO_x removal. Maximum activity was recorded for composites with HA/TiO₂ ratios 1 and 2 that was related to improved TiO₂ dispersion and NO₂ trapping by the composite materials.