A novel ozone detection at room temperature through UV-LED-assisted ZnO thick film sensors

In this work a novel ozone detection at room temperature (RT) has been investigated. Two functional materials, ZnO and (W_0.9Sn_0.1)O_3 − x (WS10) oxides, have been synthesized to prepare thick film gas sensors, both used in conventional heated mode as well as at RT assisted by UV irradiation. As a source of light, a light emitting diode (LED) of 400 nm peak wavelength was used. Under typical operating conditions of the UV-LED, the radiation flux density ? over the sensor was of about 5 · 10¹⁷ photons/cm². Powders and films have been characterized by means of TG-DTA, SEM, TEM and XRD. Finally, electrical measurements have been performed on sensing films with the aim to compare conductive properties, surface barrier heights and ozone sensing features with and without UV irradiation. Despite the fact that two types of conventional heated sensors offered quite similar results with respect to ozone sensing, it turned out that, at RT and with the assistance of UV light, ZnO behaved excellently fast detecting ozone at concentrations down to 10 ppb, while for WS10 under the same operating conditions an opposite result was observed, i.e. very low response and long response time.     

Highly selective ethanol In2O3-based gas sensor

The sensitive composite material was prepared by loading Pt and La₂O₃ into ultrafine In₂O₃ matric material (8 nm) synthesized by microemulsion method. A highly selective ethanol gas sensor was developed based on hot-wire type gas sensor, which was sintered in a bead (0.8 mm in diameter) to cover a platinum wire coil (0.4 mm in diameter). The gas sensor was operated by a bridge electric circuit. The influences of La₂O₃ and Pt additives on C₂H₅OH sensing properties of In₂O₃-based gas sensor were discussed. The addition of La₂O₃ resulted in a prominent selectivity for C₂H₅OH, and the addition of Pt improved the response rate to C₂H₅OH without affecting the sensitivity. The temperature and humidity characteristics of the sensor output were also investigated. The selective sensor had low power consumption, significantly minor humidity and temperature dependence, high selectivity and prominent long-term stability.     

Microwave-assisted synthesis and humidity sensing of nanostructured α-Fe2O3

Nanocrystalline α-Fe₂O₃ has been prepared on a large-scale by a facile microwave-assisted hydrothermal route from a solution of Fe(NO₃)₃·9H₂O and pentaerythritol. A systematic study of the morphology, crystallinity and oxidation state of Fe using different characterization techniques, such as transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy was performed. It reveals that nanostructured α-Fe₂O₃ comprises bundles of nanorods with a rhombohedral crystalline structure. The individual nanorod has 8-10 nm diameter and ∼50 nm length. The as-prepared nanostructured α-Fe₂O₃ (sensor) gives selective response towards humidity. The sensor shows high sensitivity, fast linear response to change in the humidity with almost 100% reproducibility. The sensor works at room temperature and rejuvenates without heat treatment. The as-prepared nanostructured α-Fe₂O₃ appears to be a promising humidity sensing material with the potential for commercialization.     

Structure, properties and gas sensing behavior of Cr2 − xTixO3 films fabricated by electrostatic spray assisted vapour deposition

Single-phase eskolaite crystalline Cr_2 − xTi_xO₃ films (CTO) with a uniform porous microstructure were fabricated via an electrostatic spray assisted vapour deposition (ESAVD) method. The sensing behavior upon exposure to ammonia and ethanol was characterized in a CTO film-based sensor device in terms of response, reproducibility, humidity constraints and sensor stability. The ESAVD process has been shown to be capable of producing CTO films at low temperature (650 °C) and more importantly, it results in a more uniform titanium distribution and better microstructural control than processes based on solid-state chemical reactions. The material with a nominal composition of Cr_1.7Ti_0.3O₃ exhibited the highest sensitivity among the different Cr_2 − xTi_xO₃ compositions examined towards ammonia over the temperature range of 200-500 °C with a peak sensitivity of 2.90 at 200 °C. The CTO materials, when used as sensors, also exhibit excellent responses to ethanol concentration in air. The sensitivity was 0.64 for 10 ppm ethanol, 0.85 for 25 ppm, and 0.92 for 50 ppm, respectively.     

Structural and gas sensing behavior of nanocrystalline BaTiO3 based liquid petroleum gas sensors

In this paper nanosized BaTiO₃ thick films based gas sensor has been fabricated for liquid petroleum gas (LPG). Doping with different concentrations of metal oxides influenced the sensitivity of BaTiO₃ thick films for LPG sensor. We present the characterization of both their structural properties by means of X-ray powder diffraction (XRD) and the electrical characteristics by using gas-sensing properties. X-ray powder diffraction analyses revealed the persistence of cubic phase with grain growth 65 nm. The LPG-sensing properties of BaTiO₃ thick films doped with CuO and CdO are investigated. It was found that 10 wt.% CuO: 10 wt.% CdO doped BaTiO₃ based LPG sensor shows better sensitivity and selectivity at an operating temperature 250 °C which is an important commercial range for LPG alarm development. Incorporation of 0.3 wt.% Pd doped CuO:CdO:BaTiO₃ element shows maximum sensitivity with high cross selectivity to the other gases including CO, H₂ and H₂S at an operating temperature 225 °C for low concentrations of LPG sensor.     

Simultaneous absorption of NOx and SO2 from flue gas with pyrolusite slurry combined with gas-phase oxidation of NO using ozone

NO was oxidized into NO₂ first by injecting ozone into flue gas stream, and then NO₂ was absorbed from flue gas simultaneously with SO₂ by pyrolusite slurry. Reaction mechanism and products during the absorption process were discussed in the followings. Effects of concentrations of injected ozone, inlet NO, pyrolusite and reaction temperature on NO_x/SO₂ removal efficiency and Mn extraction rate were also investigated. The results showed that ozone could oxidize NO to NO₂ with selectivity and high efficiency, furthermore, MnO₂ in pyrolusite slurry could oxidize SO₂ and NO₂ into MnSO₄ and Mn(NO₃)₂ in liquid phase, respectively. Temperature and concentrations of injected ozone and inlet NO had little impact on both SO₂ removal efficiency and Mn extraction rate. Specifically, Mn extraction rate remained steady at around 85% when SO₂ removal efficiency dropped to 90%. NO_x removal efficiency increased with the increasing of ozone concentration, inlet NO concentration and pyrolusite concentration, however, it remained stable when reaction temperature increased from 20 °C to 40 °C and decreased when the flue gas temperature exceeded 40 °C. NO_x removal efficiency reached 82% when inlet NO at 750 ppm, injected ozone at 900 ppm, concentration of pyrolusite at 500 g/L and temperature at 25 °C.     

Synthesis and characterization of CdO-doped nanocrystalline ZnO:TiO2-based H2S gas sensor

This paper reports the preparation and gas-sensing characteristic of ZnO:TiO₂-based hydrogen sulfide (H₂S) gas sensor with different mol% of CdO by polymerized complex method. The structural and gas-sensing properties of ZnO:TiO₂ materials have been characterized using X-ray diffraction and gas-sensing measurement. The electrical resistance response of the sensor based on the materials was investigated at different operating temperatures and different gas concentrations. The sensor with 10 mol% CdO-doped ZnO:TiO₂ shows excellent electrical resistance response toward H₂S gas. The cross sensitivity was also checked for reducing gases like CH₄, CO and H₂ gas. The selectivity and sensitivity of ZnO:TiO₂-based H₂S gas sensor were improved by the addition of 10 mol% of CdO at an operating temperature of 250 °C.     

Neodymium(III)-PVC membrane sensor based on a new four dentate ionophore

A new selective Nd(III) sensor has been made by using N,N′-bis(quinoline-2-carboxamido)-4,5-dimethylbenzene (H₂L₄) as a suitable ionophore. The sensor exhibits Nernstian response to Nd(III) ions in the concentration range of 5.0 × 10^− 6 to 1.0 × 10^− 2 M. It displays a Nernstian slope of 19.5 ± 0.4 mV/decade in the pH range of 2.9-9.2. The proposed sensor also exhibits a fast response time of < l0 s. The detection limit of the proposed sensor is 4.8 × 10^− 6 M, and it can be used over a period of 10 weeks without significant changes in its response. Furthermore, the electrode showed high selectivity toward Nd(III) ion respect to all other lanthanide ions tested. The practical utility of the sensor was demonstrated by using it as an indicator electrode in the potentiometric determination of Nd(III) ions in certified reference material and spiked water samples.     

Influence of annealing on humidity response of RF sputtered nanocrystalline MgFe2O4 thin films

Humidity response of Radio Frequency sputtered MgFe₂O₄ thin films onto alumina substrate, annealed at 400 °C, 600 °C and 800 °C has been investigated. Crystalline phase formation of thin films annealed at different temperature was analyzed by X-ray Diffraction. A particle/grain like microstructure in the grown thin films was observed by Scanning Electron Microscope and Atomic Force Microscope images. Film thickness for different samples was measured in the range 820-830 nm by stylus profiler. Log R (Ω) response measurement was taken for all thin films for 10-90% relative humidity (% RH) change at 25 °C. Resistance of the film increased from 5.9 × 10¹⁰ to 3 × 10¹² at 10% RH with increase in annealing temperature from 400 °C to 800 °C. A three-order magnitude, 10¹² Ω to 10⁹ Ω drop in resistance was observed for the change of 10 to 90% RH for 800 °C annealed thin film. A good linear humidity response, negligible humidity hysteresis and minimum response/recovery time of 4 s/6 s have been measured for 800 °C annealed thin film.     

Preparation and humidity sensing properties of Ba0.8Sr0.2TiO3 nanofibers via electrospinning

Ba_0.8Sr_0.2TiO₃/Poly (vinylpyrrolidone) (BST/PVP) composite fibers were successfully synthesized via electrospinning. The ceramic nanofibers were obtained after calcining the composite at 800 °C for 2 h. The morphology and structure of the BST fibers were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results reveal that the as-synthesized BST nanofibers show a diameter of 50-150 nm with the length over 0.1 mm, and a well-defined perovskite crystal structure. The electrical properties of the as-synthesized BST nanofibers were investigated through an impedance-type humidity sensor. The nanofibers exhibited excellent humidity sensing properties at room temperature. The possible sensing mechanism was proposed.     

Pd-modified TiO2 electrode for electrochemical oxidation of hydrazine, formaldehyde and glucose

The electrocatalysis of the oxidation of hydrazine, formaldehyde and glucose on a nanoporous Pd-modified TiO₂ electrode, prepared by the hydrothermal process, was investigated in 0.1 M NaOH solutions. The electrocatalytic activity of the Pd-modified TiO₂ electrode for the electrochemical oxidation of hydrazine, formaldehyde and glucose is characterized by the low onset potentials of −0.80, −0.70 and −0.85 V (vs Ag,AgCl), respectively. Compared to the oxidation of formaldehyde and glucose, the hydrazine oxidation on the Pd-modified TiO₂ presents the highest anodic oxidation current densities, showing that the Pd-modified TiO₂ electrode is more electro-active for the hydrazine oxidation than for the oxidation of formaldehyde and glucose. Chronoamperograms at different concentrations of hydrazine and formaldehyde showed that the Pd-modified TiO₂ electrode is a promising electrochemical sensor for the detection of hydrazine with a sensitivity of 0.554 mA cm⁻² mM⁻¹ and a detection limit of 0.023 mM, and for the detection of formaldehyde with a sensitivity of 0.20667 mA cm⁻² mM⁻¹ and a detection limit of 0.015 mM. However, it was found from the chronoamperometric responses at various glucose concentrations that a linear plot of the anodic oxidation current density versus glucose concentration developed only in the range of 7-35 mM glucose while an obvious deviation from the linear relationship was observed at both low and large glucose concentrations. Results show that the prepared Pd-modified TiO₂ electrode could be applied to the direct liquid (hydrazine, formaldehyde, and glucose) fuel cells as an effective anodic catalyst, in addition to be a promising electrochemical sensor for the detection of hydrazine and formaldehyde.     

Fabrication of TiO2 nanotubes by anodization of Ti thin films for VOC sensing

Ti thin films were anodized in aqueous HF (0.5 wt.%) and in polar organic (0.5 wt.% NH₄F + ethylene glycol) electrolytes to form TiO₂ nanotube arrays. Ti thin films were deposited on microscope glass substrates and then anodized. Anodization was performed at potentials ranging from 5 V to 20 V for the aqueous HF and from 20 V to 60 V for the polar organic electrolytes over the temperatures range from 0 to 20 °C. The TiO₂ nanotubes were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX). It has been observed that anodization of the deposited Ti thin films with aqueous HF solution at 0 °C resulted in nanotube-type structures with diameters in the range of 30-80 nm for an applied voltage of 10 V. In addition, the nanotube-type structure is observed for polar organic electrolyte at room temperature at the anodization voltage higher than 40 V. The volatile organic compound (VOC) sensing properties of TiO₂ nanotubes fabricated using different electrolytes were investigated at 200 °C. The maximum sensor response is obtained for carbon tetrachloride. The sensor response is dependent on porosity of TiO₂. The highest sensor response is observed for TiO₂ nanotubes which are synthesized using aqueous HF electrolyte and have very high porosity.     

Nanocrystalline chemically modified CdIn2O4 thick films for H2S gas sensor

CdIn₂O₄ sensor with high sensitivity and excellent selectivity for H₂S gas was synthesized by using sol-gel technique. Structural, electrical and gas sensing properties of doped and undoped CdIn₂O₄ thick films were studied. XRD revealed the single-phase polycrystalline nature of the synthesized CdIn₂O₄ nanomaterials. Since the resistance change of a sensing material is the measure of its response, selectivity and sensitivity was found to be enhanced by doping different concentrations of cobalt in CdIn₂O₄ thick films. The sensor exhibits high response and selectivity toward H₂S for 10 wt.% Co doped CdIn₂O₄ thick films. The current-voltage characteristics of 10 wt.% Co doped CdIn₂O₄ calcined at 650 °C shows one order increase in current with change in the bias voltage at an operating temperature of 200 °C for 1000 ppm H₂S gas.     

Influence of the steady background turbulence level on second sound dynamics in He II - II

We report complementary results to our previous publication [Dalban-Canassy M, Hilton DK, Van Sciver SW. Influence of the steady background turbulence level on second sound dynamics in He II. Adv Cryo Eng 2006;51:371-8], both of which are aimed at determining the influence of background turbulence on the breakpoint energy of second sound pulses in He II. The apparatus consists of a channel 175 mm long and 242 mm² in cross section immersed in a saturated bath of He II at 1.7 K. A heater at the bottom end generates both background turbulence, through a low level steady heat flux (up to q_s = 2.6 kW/m²), and high intensity square second sound pulses (q_p = 100 or 200 kW/m²) of variable duration Δt₀ (up to 1 ms). Two superconducting filament sensors, located 25.4 mm and 127 mm above the heater, measure the temperature profiles of the traveling pulses. We present here an analysis of the measurements gathered on the top sensor, and compare them to similar results for the bottom sensor [1]. The strong dependence of the breakpoint energy on the background heat flux previously illustrated is also observed on the top sensor. The present work shows that the ratio of energy received at the top sensor to that at the bottom sensor diminishes with increasing background heat flux.     

Improvement in real time detection and selectivity of phthalocyanine gas sensors dedicated to oxidizing pollutants evaluation

A sensor microsystem prototype, using copper phthalocyanine thin film as sensitive layer, and dedicated to ozone evaluation, was developed. The methodology implemented is based on cyclic sensor recalibrations by thermal cleaning of the sensitive membrane, and on pollutant concentration quantification according to the kinetics of sensor response. Results of laboratory experiments for various NO₂ and O₃ concentrations, in the range of 10-200 ppb, illustrate the selectivity of CuPc sensors towards ozone, obtained by our methodology. We have shown that ozone selectivity is especially improved for short time of exposure (few minutes) and for phthalocyanine layer maintained at low temperature (80 °C). For optimal conditions, our microsystem exhibits a threshold lower than 10 ppb, a resolution lower than 10 ppb, and good reproducibility of measurements. Performances obtained in real urban atmosphere are satisfying to ensure real time evaluation of ozone during several days. Long-term stability and the detection of NO₂ by associating chemical filters to our microsystem will be also discussed.     

Ecotoxicological evaluation of three tertiary wastewater treatment techniques via meta-analysis and feeding bioassays using Gammarus fossarum

Advanced treatment techniques, like ozone, activated carbon and TiO₂ in combination with UV, are proposed to improve removal efficiency of micropollutants during wastewater treatment. In a meta-analysis of peer-reviewed literature, we found significantly reduced overall ecotoxicity of municipal wastewaters treated with either ozone (n = 667) or activated carbon (=113), while TiO₂ and UV was not yet assessed. As comparative investigations regarding the detoxification potential of these advanced treatment techniques in municipal wastewater are scarce, we assessed them in four separate Gammarus-feeding trials with 20 replicates per treatment. These bioassays indicate that ozone concentrations of approximately 0.8 mg ozone/mg DOC may produce toxic transformation products. However, referred effects are removed if higher ozone concentrations are used (1.3 mg ozone/mg DOC). Moreover, the application of 1 g TiO₂/l and ambient UV consistently reduced ecotoxicity. Although activated carbon may remove besides micropollutants also nutrients, which seemed to mask its detoxification potential, this treatment technique reduced the ecotoxicity of the wastewater following its amendment with nutrients. Hence, all three advanced treatment techniques are suitable to reduce the ecotoxicity of municipal wastewater mediated by micropollutants and may hence help to meet the requirements of the European Water Framework Directive.     

H2S sensing properties of La-doped nanocrystalline In2O3

The nanocrystalline powders of pure and La³⁺-doped In₂O₃ with cubic structure were prepared by a simple hydrothermal decomposition route. The structure and crystal phase of the powders were characterized by X-ray diffraction (XRD) and microstructure by transmission electron microscopy (TEM). All the compositions exhibited a single phase, suggesting a formation of solid solution in the concentration of doping investigated. Gas-sensing properties of the sensor elements were tested by mixing a gas in air at static state, as a function of concentration of dopant, operating temperature and concentrations of the test gases. The pure In₂O₃ exhibited high response towards H₂S gas at an operating temperature 150 °C. Doping of In₂O₃ with La³⁺ increases its response towards H₂S and La³⁺ (5.0 wt.% La₂O₃)-doped In₂O₃ showed the maximum response at 125 °C. The selectivity of the sensor elements for H₂S against different reducing gases was studied. The results on response and recovery time were also discussed.     

Polypyrrole thin films for gas sensors prepared by Matrix-Assisted Pulsed Laser Evaporation technology: Effect of deposition parameters on material properties

Thin films of polypyrrole (PPY) were prepared by Matrix-Assisted Pulsed Laser Evaporation (MAPLE) technology from two matrices: water and dimethylsulfoxide (DMSO). The deposition was carried out using a KrF excimer laser (laser fluence F ranged from 0.1 to 0.6 J cm^− 2). This work deals with optimization of two deposition parameters - laser fluence and number of pulses - for both matrices. From the deposition curves, the fluence thresholds, F_th, and maximum growth rates were subsequently determined (water matrix: F_th ~ 0.40-0.45 J cm^− 2, maximum growth rate 0.16 nm pulse^− 1; DMSO matrix: F_th ~ 0.25-0.30 J cm^− 2; maximum growth rate 0.20 nm pulse^− 1). The changes in chemical composition of deposited layers were studied by Attenuated Total Reflection Fourier Transform Infrared spectroscopy. Surface morphology was characterized by Atomic Force Microscopy. A discussion is also presented concerning relationships between laser fluence and chemical composition of deposited layers with respect to their potential application in gas sensors. Finally, the response of a sensor with a MAPLE deposited PPY active layer to air humidity is presented.     

Structure and CO gas sensing properties of electrospun TiO2 nanofibers

Titanium dioxide (TiO₂) nanofibers were fabricated by electrospinning a hybrid solution, which is a mixture of the TiO₂ sol precursor, polymer, and solvent. The structure and gas sensing properties of TiO₂ nanofibers were investigated. By calcining at 600 °C, the polymeric components were decomposed and a multi-layered random network structure of TiO₂ nanofibers was obtained. Polycrystalline TiO₂ nanofibers consist of tetragonal anatase and rutile TiO₂ phases. The diameter ranged from 400 nm to 500 nm and the grain size was about 15 nm. The TiO₂ nanofibers-based sensor exhibited response to CO concentration as low as 1 ppm at 200 °C.     

Structural analysis of aerosol spray pyrolysis ZnO films exhibiting ultra low ozone detection limits at room temperature

ZnO films grown by a home-made Aerosol Spray Pyrolysis (ASP) system at 350 °C have shown a characteristic columnar structure with an inhomogeneous layer structure that extends all the way to the surface. Surface morphology studies revealed grain sizes of the order of 80 nm and a highly porous structure which proved catalytic for the performance as gas sensing element. Conductometric ozone response tests have shown that the films of such a structure may be very effective in detecting ultra low ozone concentrations. The lowest ozone limit detected was 16 ppb at room temperature opening up the road for ultra sensitive inexpensive metal oxide ozone detectors.