Single-crystalline Te microtubes: Synthesis and NO2 gas sensor application

Tellurium tubular crystals were grown by direct thermal evaporation of tellurium metal in an inert atmosphere on quartz substrates at ambient pressure without employing any catalyst. Tellurium powder was evaporated by heating at 600 °C and was condensed at a substrate temperature of 300–350 °C in the downstream of argon gas at a flow rate of 100 mL/min. The structure and chemical composition of the as-synthesized samples were examined by X-ray diffraction analysis, scanning electron microscopy, energy-dispersive X-rays microanalysis and micro-Raman spectroscopy. Scanning electron microscopy images and X-ray diffraction patterns showed that the as-synthesized Te had a tubular single-crystalline morphology with a hexagonal cross-section. The Te microtubes were typically 0.5–6 mm long, 30–70 μm in external diameter, and 5–20 μm thick. NO₂ gas-sensing properties of the Te microtubes at room temperature were also investigated. They showed a promising sensitivity and response towards tested gas.     

Selectivity of flame-spray-made Nb/ZnO thick films towards NO2 gas

Unloaded ZnO and Nb/ZnO nanoparticles containing 0.25, 0.5 and 1 mol.% Nb were produced in a single step by flame-spray pyrolysis (FSP) technique. The nanoparticles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The BET surface area (SSA_BET) of the nanoparticles was measured by nitrogen adsorption. FSP yielded small Nb particles attached to the surface of the supporting ZnO nanoparticles, indicating a high SSA_BET. The morphology and accurate size of the primary particles were further investigated by TEM. Nb/ZnO nanoparticles paste composed of ethyl cellulose and terpineol as binder and solvent respectively was coated on Al₂O₃ substrate interdigitated with gold electrodes to form thick films by spin coating technique. After the sensing tests, the morphology and the cross-section of sensing film were analyzed by SEM and EDS analyses. The influence on a low dynamic range of Nb concentration on NO₂ response (0.1-4 ppm) of thick film sensor elements was studied at the operating temperatures ranging from 250 to 350 °C in the presence of dry air. The optimum Nb concentration was found be 0.5 mol.% and 0.5 mol.% Nb exhibited an optimum NO₂ response of ∼1640 and a short response time (27 s) for NO₂ concentration of 4 ppm at 300 °C.     

A highly sensitive NO2-selective optode membrane

A new NO₂-sensitive polymeric optode membrane is introduced. It takes advantage of the reactivity of NO₂ with an aquacyanocobalt(III)-cobyrinate derivative. The detection mechanism leads to a protonation of the Nile Blue derivative ETH 5418, which changes its visible spectrum thereby. The polymer layer also contains the aquacyanocobalt(III)-cobyrinate derivative, which significantly reduces the response time and improves the operational lifetime of the sensor compared to a blank membrane. No cross-sensitivity to NO, CO, CO₂ and only a small interference by SO₂ was observed.

A large variety of polymers and plasticizers were tested in order to achieve resistance to high temperature and to improve the lifetime of the sensor. Until now, the plasticized poly(vinylchloride) membranes still remain the material of choice in view of a good compromise between stability, response time and solubility of components.     

声表面波NO_2传感器敏感膜研究进展

由于工业检测、环境监测、医学监测等领域的需求,高性能NO2传感器得到了广泛的研究。声表面波传感器技术的发展为研发高灵敏度、高稳定性、响应快速、小型化的NO2传感器提供了极大的潜能。总结了近30年来声表面波NO2传感器敏感膜的研究现状,并根据现有的研究和传感器的应用需求,深入探讨了声表面波NO2传感器敏感膜面临的挑战和发展趋势。     

Zeolite-modified WO3 gas sensors – Enhanced detection of NO2

Solid-state metal oxide gas sensors with zeolite overlayers have been developed as a means to improve sensor selectivity. Screen printed tungsten oxide (WO₃) sensors were modified by the addition of acidic and catalytic zeolite layers. The sensors were characterised before and after sensing experiments using X-ray diffraction, energy dispersive X-ray analysis and scanning electron microscopy. The sensors were tested against various gases and gas mixtures to assess their discriminatory behaviour. The results show that the sensors response can be tailored to be selective towards specific target gases by changing the zeolite; for example the H-ZSM-5 sensor gave a response 19 times greater to NO₂ than an unmodified control sensor. It was observed that the WO₃ based gas sensors showed a remarkable selectivity towards NO₂ in a gas mixture. The sensors also showed high levels of stability and sensitivity and have potential to be used in electronic nose technology.     

Development of SAW-based multi-gas sensor for simultaneous detection of CO2 and NO2

A 440 MHz wireless and passive surface acoustic wave (SAW)-based multi-gas sensor integrated with a temperature sensor was developed on a 41° YX LiNbO₃ piezoelectric substrate for the simultaneous detection of CO₂, NO₂, and temperature. The developed sensor was composed of a SAW reflective delay lines structured by an interdigital transducer (IDT), ten reflectors, a CO₂ sensitive film (Teflon AF 2400), and a NO₂ sensitive film (indium tin oxide). Teflon AF 2400 was used for the CO₂ sensitive film because it provides a high CO₂ solubility, with good permeability and selectivity. For the NO₂ sensitive film, indium tin oxide (ITO) was used. Coupling of mode (COM) modeling was conducted to determine the optimal device parameters prior to fabrication. Using the parameters determined by the simulation results, the device was fabricated and then wirelessly measured using a network analyzer. The measured reflective coefficient S₁₁ in the time domain showed high signal/noise (S/N) ratio, small signal attenuation, and few spurious peaks. The time positions of the reflection peaks were well matched with the predicted values from the simulation. High sensitivity and selectivity were observed at each target gas testing. The obtained sensitivity was 2.12°/ppm for CO₂ and 51.5°/ppm for NO₂, respectively. With the integrated temperature sensor, temperature compensation was also performed during gas sensitivity evaluation process.     

Acoustic wave-based NO2 sensor: Ink-jet printed active layer

Microsphere-templated BaCO₃ films with well-defined area were deposited onto quartz crystal microbalances by thermal ink-jet printing, and the devices were characterized with respect to their microstructures and NO₂ sensing characteristics. Highly porous three-dimensional BaCO₃ frameworks with promising sensor characteristics were obtained. The printed thin films exhibited reversible frequency shifts following exposure to NO₂ and subsequent recovery under CO/CO₂ at 400 °C. The feasibility of controlled deposition of complex functional films in controlled patterns is discussed in the context of the direct-write features of ink-jet printing.     

Au-doped WO3-based sensor for NO2 detection at low operating temperature

Pure and Au-doped WO₃ powders for NO₂ gas detection were prepared by a colloidal chemical method, and characterized via X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The NO₂ sensing properties of the sensors based on pure and Au-doped WO₃ powders were investigated by HW-30A gas sensing measurement. The results showed that the gas sensing properties of the doped WO₃ sensors were superior to those of the undoped one. Especially, the 1.0 wt% Au-doped WO₃ sensor possessed larger response, better selectivity, faster response/recovery and better longer term stability to NO₂ than the others at relatively low operating temperature (150 °C).     

Selective NH3 gas sensor based on Langmuir-Blodgett polypyrrole film

Polypyrrole thin films have been deposited onto a glass substrate by the Langmuir-Blodgett technique to fabricate a selective ammonia (NH₃) gas sensor. The d.c. electrical resistance of the sensing elements is found to exhibit a specific increase upon exposure to different gases such as NH₃, CO, CH₄, H₂ in N₂ and pure O₂. The polypyrrole thin-film detector shows a considerable increase of resistance when exposed to NH₃ in N₂, and negligible response when exposed to comparable concentrations of interfering gases such as CO, CH₄, H₂ in N₂ and pure O₂. The calibration curve for NH₃ in N₂ at room temperature is measured in the concentration range from 0.01 to 1%. The relative change of the electrical resistance is about 10% for the lower detectable limit of 100 ppm of NH₃ in N₂. The sensitivity of the Langmuir-Blodgett polypyrrole towards ammonia is considerably higher than that of the electrochemical polypyrrole. The fast rise time and the high sensitivity of the detector are reported as a function of number of the polypyrrole layers. Long-term aging tests of the selective NH₃ gas sensor are performed.     

Thin film polypyrrole/SWCNTs nanocomposites-based NH3 sensor operated at room temperature

A PPY/SWCNTs nanocomposite-based sensor with relatively high sensitivity and fast response–recovery was developed for detection of NH₃ gas at room temperature. The gas-sensitive composite thin film was prepared using chemical polymerization and spin-coating techniques, and characterized by Fourier transformed infrared spectra and field-emission scanning electron microscopy. The results reveal that the conjugated structure of the PPY layer was formed and the functionalized SWCNTs were well-embedded. The effects of film thickness, annealing temperature, and SWCNTs content on gas-sensing properties of the composite thin film were investigated to optimize the gas-sensing performance. The as-prepared thin film PPY/SWCNTs composite sensor with optimized process parameters had a response of 26–276% upon exposure to NH₃ gas concentration from 10 to 800 ppm, and their response and recovery times were around 22 and 38 s, respectively.     

Mixed-potential-type zirconia-based NO2 sensor with high-performance three-phase boundary

This paper focuses on the gas sensing properties of the mixed-potential-type NO₂ sensor based on yttria stabilized zirconia (YSZ) and NiO electrode. The sensing performance of the sensor was improved by modifying the three-phase boundary (TPB). Hydrofluoric acid with different concentrations (10%, 20% and 40%) was used to corrode YSZ substrate to obtain large superficial area of TPB. The scanning electron microscope and atomic force microscopic images showed that the 40% HF could form the largest superficial area at the same corroding time (3 h). The sensitivity of the sensor using the YSZ plate corroded with 40% hydrofluoric acid to 20-500 ppm NO₂ was 76 mV/decade at 850 °C, which was the largest among the examined HF concentrations. It was also seen that the sensor showed a good selectivity and speedy response kinetics to NO₂. On the basis of the measurements of anodic and cathodic polarization curves, as well as the complex impedance of the device, the sensing mechanism was confirmed to involve a mixed potential at the oxide sensing electrode.     

Application of m-CNTs/NaClO4/Ppy to a fast response, room working temperature ethanol sensor

A new blended material, m-CNTs/NaClO₄/Ppy, was developed as a gas sensor to detect ethanol concentrations at room temperature. The sensing material polypyrrole (Ppy) was synthesized in situ by UV-photo-polymerization. The multiwall carbon nanotubes (m-CNTs) added enhanced the short-term repeatability of the Ppy-sensing material. The relative resistance variation (R_ethanol/R_air) of m-CNTs/NaClO₄/Ppy was 1.193 when exposed to ethanol of 30,000 ppm. The sensor response and recovery times (both 20 s) were very short to this concentration. An unstable baseline of the sensor was explained by theoretical calculation of molecular dynamics made for ethanol adsorption on polypyrrole, which revealed formation of a new bond, N–HO. Adsorption energy decreased with increasing the number of ethanol molecules adsorbed and was 0.8 kcal/mol at eight adsorbed molecules. Sensor responses of three different Ppy samples were measured to ethanol concentrations of 18,000–40,000 ppm.     

Highly sensitive NO2 sensor based on square-like tungsten oxide prepared with hydrothermal treatment

The square-like WO₃ nanosheets were synthesized by hydrothermal treatment of irregular WO₃ nanosheets prepared through acidification of Na₂WO₄·2H₂O. The obtained square-like and irregular WO₃ nanosheets were characterized with field emission scanning electron microscopy, X-ray powder diffraction and transmission electron microscopy. The gas sensing properties of sensors based on as-prepared samples were investigated. The results indicated that both samples exhibited high response to NO₂. The sensor based on square-like WO₃ nanosheets exhibited remarkably enhanced response and faster response/recovery time for NO₂ compared with that based on irregular nanosheets. Especially, the sensor based on square-like WO₃ nanosheets could detect NO₂ down to 40 ppb, which covered environmental standard. A possible reason for the influence of unique structure on the sensing properties of sensors based on square-like WO₃ was proposed.     

SnO2 thin film sensor with enhanced response for NO2 gas at lower temperatures

Semiconducting SnO₂ thin films having higher value of electrical conductivity have been deposited using RF sputtering technique in the reactive gas environment (30% O₂ + 70% Ar) using a metallic tin (Sn) target for detection of oxidizing NO₂ gas. The effect of growth pressure (12-18 mTorr) on the surface morphology and structural property of SnO₂ film was studied using Atomic force microscopy (AFM), Scanning electron microscopy (SEM) and X-ray Diffraction (XRD) respectively. Film deposited at 16 mTorr sputtering pressure was porous with rough microstructure and exhibits high sensor response (∼2.9 × 10⁴) towards 50 ppm NO₂ gas at a comparatively low operating temperature (∼100 °C). The sensor response was found to increase linearly from 1.31 × 10² to 2.9 × 10⁴ while the response time decrease from 12.4 to 1.6 min with increase in the concentration of NO₂ gas from 1 to 50 ppm. The reaction kinetics of target NO₂ gas on the surface of SnO₂ thin film at the Sn sites play important role in enhancing the response characteristics at lower operating temperature (∼100 °C). The results obtained in the present study are encouraging for realization of SnO₂ thin film based sensor for efficient detection of NO₂ gas with low power consumption.     

A CO2 sensor based upon a continuous-wave thermoelectrically-cooled quantum cascade laser

A CO₂ sensor based upon a continuous-wave thermoelectrically-cooled distributed feedback quantum cascade laser operating between 2305 and 2310 cm⁻¹ and a 54.2 cm long optical cell has been developed. Two approaches for direct absorption spectroscopy have been evaluated and applied for monitoring of the CO₂ concentration in gas lines and ambient laboratory air. In the first approach optical transmittance was derived from the single channel laser intensity, whilst in the second approach a ratio of signal and reference laser intensities (balanced detection) was used. The optimum residual absorption standard deviation was estimated to be 1.9 × 10⁻⁴ for 100 averages of 1 ms duration and 0.1 cm⁻¹ scans over the P(46) CO₂ absorption line of the ν₃ vibrational band at 2306.926 cm⁻¹. A CO₂ detection limit (1 standard deviation) of 36 ppb was estimated for 0.1 s average and balanced detection.     

Tetrakis(alkylthio)-substituted lutetium bisphthalocyanines for sensing NO2 and O3

In this study, the nitrogen dioxide (NO₂) and ozone (O₃) sensing properties of a series bis[tetrakis(alkylthio) phthalocyaninato] lutetium(III) complexes [(C_nH_2n+1S)₄Pc]₂Lu(III) (n = 6, 10, 16) are investigated as a function of concentration in the temperature range between 25 °C and 150 °C. The concentration ranges were 1–10 ppm for NO₂, and 50 ppb–1 ppm for O₃. The response time and the sensor response to NO₂ are measured for approximately 1 min and 100% ppm⁻¹, respectively, for compound 1 at room temperature. At room temperature, all compounds are in the solid phase. The response time decreases to a few seconds with increasing operation temperature to 150 °C. At this temperature, all compounds are in the liquid crystal phase. The fastest response to oxidizing gases is observed at the liquid crystal phase of the Pcs. It has also been observed that the response time and the sensor response depend on the alkyl chain lengths of the Pcs. The doping effect of oxygen has been determined under high purity nitrogen N₂ flow, after exposure to dry air, at a different period of time and after annealing. It has been found that the conductivities of [(C_nH_2n+1S)₄Pc]₂Lu(III) thin films increased after exposure to dry air and the conduction mechanism also changed from ohmic behavior to space-charge-limited conduction.     

On-board phthalocyanine gas sensor microsystem dedicated to the monitoring of oxidizing gases level in passenger compartments

The monitoring of in-vehicle pollutant concentrations by means of an on-board molecular semiconductor gas sensor microsystem is described in this paper. The main objective is to measure in real time with a high level of accuracy the variations of oxidizing gases concentration in bus passenger compartments to inform travelers or commuters and to evaluate the assessment of bus drivers’ exposure. A self-contained gas sensor microsystem of which the sensitive element is constituted by a thin layer of copper phthalocyanine has been developed, validated at laboratory under controlled experimental conditions and then implemented in a bus of the urban network of Clermont-Ferrand, France. Preliminary in-car measurements realized with commercial analyzers show that nitrogen dioxide is the major oxidizing gas present on urban roads and so is considered as the target gas in this study. Tests realized under artificially polluted atmosphere show the high performances obtained with our microsystem, such as high resolution, low threshold, good reproducibility, satisfying concentration range and real time detection. The calibration curve has been determined at laboratory by experiments made under low NO₂ concentrations in the range of those measured in urban atmosphere. The relation between sensor microsystem response and gas concentration is established with accuracy. The validation of our microsystem is illustrated by measurements realized under real conditions, i.e., in an urban bus. It shows that NO₂ concentration variations are mainly correlated with the nature of roads and that rates of pollutant measured in traffic are always greater than those measured by the nearest stations of the air quality control network.     

Fabrication of polypyrrole-phenylalanine nano-films with NH3 gas sensitivity

Polypyrrole (PPy) films complexed with phenylalanine were successfully synthesized via one-step emulsion polymerization. By incorporating phenylalanine into the conductive PPy backbone, highly functionalized PPy films have been produced and found to respond to ammonia gas. The chemical sensors based on the PPy-phenylalanine nanoparticle films displayed enhanced reproducible and reversible responses upon exposure to NH₃ gas. Their response was strongly dependent on the amount of the phenylalanine. The PPy-phenylalanine films were characterized by UV, IR, and XPS spectroscopy and compared to the PPy films. The gas detection and reversible response mechanism are discussed on the basis of these analysis results.     

EPR and DRS evidence for NO2 sensing in Al-doped ZnO

Zinc oxide (ZnO) is a well-known semiconducting multifunctional material wherein properties right from the morphology to gas sensitivity can be tailor-made by doping or surface modification. Aluminum (Al)-incorporated porous zinc oxide (Al:ZnO) exhibits good response towards NO₂ at low-operating temperature. The NO₂ gas concentration as low as 20 ppm exhibits S = 17% for 5 wt.% Al-incorporated ZnO. The NO₂ response increases with operating temperature and concentration and reaches to its maximum at 300 °C without any interference from other gases such as SO₃, HCl, LPG and alcohol. Physico-chemical characterization likes differential thermogravimetric analysis (TG-DTA) electron paramagnetic resonance (EPR) and diffused reflectance spectroscopy (DRS) have been used to understand the sensing behavior for pure and Al-incorporated ZnO. The TG-DTA depicts formation of ZnO phase at 287 °C. The EPR study reveals distinct variation for O⁻ (g = 2.003) and Zn interstitial (g = 1.98) defect sites in pure and Al:ZnO. The DRS studies elucidate signature of adsorbed NO_x species in aluminium-incorporated zinc oxide indicating its tendency to adsorb these species even at low temperatures. This paper is an attempt to correlate the gas sensing behavior with the physico-chemical studies such as EPR and DRS.     

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.