Bias-heating recovery of MWCNT gas sensor

A sensor was made using Au/Cr electrodes on a glass substrate and a thin carbon multiwall nanotube film printed between them. A bias-heating method was used completely to desorb gas molecules and restore its initial conductance. The temperature of the thin carbon nanotube film varied depending on the magnitude of the voltage used, and this relationship was investigated. After being used to detect NO₂, the sensor returned to its initial conductance. This method enables complete recovery without additional processing steps, such as the fabrication of heat structure and ambient heating.     

Solid-state sensors for in-line monitoring of NO2 in automobile exhaust emission

Three types of planar solid-state sensors for measuring NO₂ in a gas mixture has been designed and tested in the laboratory under controlled atmosphere between 573–723 K. The concentration of NO₂ in the gas mixture was in the range of 0–500 ppm with the balance gas consisting of air. The three types of NO₂ gas sensors that have been tested in this investigation can be schematically represented as follows:Pt, NO₂ + air, NaNO₃ + Ba(NO₃)₂ | NASICON disk | Porous YSZ disk | NO₂ + air, Pt (I)Pt, NO₂ + air, NaNO₃ + Ba(NO₃)₂ | NASICON disk | YSZ thin film | NO₂ + air, Pt (II)Pt, NO₂ + air, Pt | YSZ disk | Au – Pd, NO₂ + air, Pt (III)In sensor (I) the two solid electrolyte disks were attached by diffusion bonding at elevated temperature whereas in sensor (II) the (8 mol% Y₂O₃–ZrO₂) YSZ thin film was deposited on (Na₃Zr₂Si₂PO₁₂) NASICON disk by radio frequency (RF) magnetron sputtering technique. The measured open circuit electromotive force (Emf) of each sensor was found to attain stable value at all the concentrations of NO₂ in the gas mixture and also varied linearly as a function of the logarithm of the partial pressure of NO₂ in the gas mixture. The time required to reach 90% of the stable emf at a fixed concentration of NO₂ and at a constant temperature was found to be 30–40 min for sensor (I) and approximately 2–3 min for sensor (II) and (III).     

Highly sensitive free-base-porphyrin-based thin-film optical waveguide sensor for detection of low concentration NO<Subscript>2</Subscript> gas at ambient temperature

Meso-5,10,15,20-tetrakis-(4-tertbutyl phenyl) porphyrin was synthesized using Adler–Longo method and was served as sensing material. Electronic absorption spectra of the porphyrin chloroform solution and its thin film were studied comparatively. An optical waveguide sensor based on free-base porphyrin was fabricated by spin coating method. Absorption variation of porphyrin film was studied before and after exposure to NO₂, H₂S, SO₂, and volatile organic gases. XRD patterns of porphyrin film before and after exposure to analytes (NO₂, SO₂, and H₂S) were provided, and light source of the OWG testing system was selected. This facile-prepared sensor exhibited high sensitivity and selectivity to NO₂ with fast response time of 3 s and slow recovery time of 10 min or so and was capable of measuring NO₂ down to 10 ppb at ambient temperature. Scanning electron microscopy was employed to characterize film morphology before and after contact with NO₂. Film thickness was measured before (71.3 nm) and after (76.8 nm) exposure to NO₂, and film thickness variation value (5.20 nm) was calculated. The sensing behavior of the studied sensing device was tested through mixture of H₂S, SO₂, and VOC gases with NO₂ and without NO₂ for determination of the selectivity of the device. Film stability was probed by UV–Vis spectra, and response values of sensing element to NO₂ gas were detected after several days of film preparation, and its RSD value was 1.66%.     

Effects of N2-annealing conditions on the sensing properties of Pt/HfO2/SiC Schottky-diode hydrogen sensor

Hafnium oxide (HfO₂) used as the gate insulator of metal-insulator-SiC Schottky-diode hydrogen sensors is annealed in nitrogen at different temperatures and durations for achieving a better performance. The hydrogen-sensing properties of these samples are compared with each other by taking measurements under various temperatures and hydrogen concentrations using a computer-controlled measurement system. The sensor response of the device is found to increase with the annealing temperature and time because higher annealing temperature and longer annealing time can enhance the densification of the HfO₂ film; improve the oxide stoichiometry and facilitate the growth of an interfacial layer to give better interface quality, thus causing a significant reduction of the current of the sensor under air ambient. The effects of hydrogen adsorption on the barrier height and conduction mechanism of the devices are also investigated.     

Characterization of a nanosized TiO2 gas sensor

Thin films were obtained by r.f. reactive sputtering from a Ti_.1W_.9 target onto a Si substrate followed by annealing in air at 800 °C. The thermal treatment results in a nanosized TiO₂ thin film with high surface-to-volume ratio. The nanosized structure, its stability, together with the ease of preparation, make this material suitable as a gas sensor. The sensing layer proved capable to detect 20 ppm of NO₂ at a temperature suitable for monitoring of exhaust gases of engines. Its high sensitivity suggests use of this sensor for environmental purposes.     

Microsensor based on low temperature cofired ceramics and gas-sensitive thin film

A novel design of gas sensor using low temperature cofired ceramics (LTCC) and thin film technologies is presented. The LTCC structure is composed essentially of two ceramic layers with interlayer thick film Pt heater, interdigitated electrodes on top, contact pads and metallic connections realised by vias. The thin films of both SnO₂ and In₂O₃, intentionally doped and activated, were deposited on top of the structure. With some modifications of the lamination process and heat treatment parameters, the authors obtained the upper ceramic layer with the roughness not exceeding 250 nm, what was suitable for thin film technology. The films deposited onto such LTCC structure revealed the sensing properties very similar to the reference films deposited onto glass. The gas-sensitive films were tested with changing concentrations of reducing and oxidising gases in air. The necessary sensor working temperature was obtained and stabilised using a custom-built digital controller. The low heat capacity of the sensor structure enabled also a sinusoidal temperature control. The satisfactory results obtained by the authors indicate that the connection of LTCC and thin film technologies can lead to the fabrication of good quality gas sensors.     

Improvement in lifetime of pseudo-Schottky diode sensor: Towards selective detection of O3 in a gaseous mixture (O3, NO2)

This article concerns a pseudo-Schottky diode Palladium/Indium-Phosphide (Pd-InP) gas sensor. The catalytic activity of such a palladium layer coupled with a pseudo-Schottky structure enables the measurement of very low concentrations of two highly oxidant gases: nitrogen dioxide (NO₂) and ozone (O₃). The submission of the sensor to long O₃ exposures leads to a degradation of its sensor characteristics (response time, recovery time and sensitivity) due to oxidation of the palladium metallization by O₃. Therefore, to improve sensor lifetime and reduce drift, a methodology based on cyclic regeneration of the sensor's palladium surface (carbon monoxide (CO) reduction associated with thermal treatment) has been developed. The pseudo-Schottky gas sensor associated with this methodology exhibits reproducible responses, significant resolution and real time detection in the range of 20-100 ppb for NO₂ and O₃. Moreover, a sensor exposed to 20 ppb of O₃ presented twice the response of the same sensor exposed to 100 ppb of NO₂ (10.5 nA for 6.5 nA). Selectivity towards O₃, with this methodology, is demonstrated in the case of atmospheric pollution monitoring.     

Oxidizing gas sensing properties of mesogenic copper octakisalkylthiophthalocyanine chemoresistive sensors

In this study, the effect of oxidizing gases, such as oxygen (O₂), nitrogen dioxide (NO₂), and ozone (O₃), on a liquid-crystalline copper octakisalkylthiophthalocyanine[(C₆S)₈PcCu] thin film was investigated in the temperature range of 25-150 °C. Starting from a chloroform solution of (C₆S)₈PcCu, a jet-spray technique in an inert ambient atmosphere was used to coat the thin film of the compound on to an Interdigital Transducer (IDT) with gold electrodes. The concentration ranges for NO₂ and O₃ exposed to the (C₆S)₈PcCu thin film were 1-10 ppm and 50 ppb-50 ppm, respectively. The response time in NO₂ measurements was observed to be approximately 1 min at room temperature, and it decreased to a few seconds with increasing temperature. A good sensor response of 2000% ppm^− 1 was observed when the sensor was exposed to 1 ppm NO₂ at room temperature. The oxidizing gases were found to be desorbed by annealing the thin film.     

Gas sensing characteristics of MEMS gas sensor arrays in binary mixed-gas system

MOS gas sensor arrays based on MEMS gas sensor platforms were developed for the detection of carbon monoxide (CO), nitrogen oxides (NO_x) and ammonia (NH₃), and their gas sensing characteristics in binary mixed-gas system were investigated. Three gas sensing materials with nano-sized particles for these target gases, Pd–SnO₂ for CO, In₂O₃ for NO_x and Ru–WO₃ for NH₃ were synthesized using a sol–gel method. All the sensors showed good properties for their target gases at the optimum points for micro-heater operation. From the experimental data in MEMS gas sensor arrays in a binary mixed system, the gas sensing behavior and sensor response in mixed gas systems were scrutinized. The gas sensing behaviors to the mixed gas systems suggested that specific adsorption and selective activation of adsorption sites might occur in gas mixtures and offer the priority for the adsorption of specific gas. Thorough analysis of the sensing performance of the sensor arrays will make it possible to discriminate the components in gas mixtures as well as their concentrations.     

On structural and high temperature electrochemical properties of ZrO2 thin film coating on Zr metal produced by carbonate melt

Melt of NaCO₃ can favor oxidation of Zr to form ZrO₂ thin film on Zr surface, which is used to make Zr/ZrO₂ oxidation/reduction electrode of pH sensor for testing elevated temperature aqueous solutions. Using SEM, EPMA, XPS, EXAFS and HRTEM, we found that ZrO₂ film is tightness and solid with 20 μm thickness composed by nanometer-sized monoclinic crystals. Zr/ZrO₂ interface is characterized of zoning structure according to topography and chemical composition in five zones: oxygen-rich ZrO₂, ZrO₂, oxygen-rich Zr metal, oxygen-bearing Zr and Zr from outmost to center. Melt oxidation process of Zr involved oxidation time, air and temperature. The air is important effect on structural and electrochemical properties of ZrO₂ thin film for making elevate temperature electrochemical sensor. If oxygen air largely presented in carbonate melting process, ZrO₂ thin film is not tightness and not for oxidation/reduction electrode.     

Novel method for fabrication of NiO sensor for NO2 monitoring

Nickel oxide (NiO) sensor films were prepared on glass substrate by a sol–gel spin coating technique. These films were characterized for their structural and morphological properties by means of X-ray diffraction, field emission scanning microscopy and atomic force microscopy. The NiO films are oriented along (200) plane with the cubic crystal structure. These films were utilized in nitrogen dioxide gas (NO₂) sensor. The dependence of the NO₂ response on operating temperature, NO₂ concentration was investigated. The NiO film showed selectivity for NO₂ over Cl₂ compared to H₂S $ \left( {{\text{S}}_{{{\text{NO}}_{ 2} }} /{\text{S}}_{{{\text{Cl}}_{ 2} }} = 3 7. 5,{\text{ S}}_{{{\text{NO}}_{ 2} }} /{\text{S}}_{{{\text{H}}_{ 2} {\text{S}}}} = 3. 4} \right) $ . The maximum NO₂ response of 23.3 % with 85 % stability at gas concentration of 200 ppm at 200 °C was achieved. The response time of 20 s and recovery time of 498 s was also recorded with same operating parameters.     

Fabrication of a room-temperature NO2 gas sensor based on WO3 films and WO3/MWCNT nanocomposite films by combining polyol process with metal organic decomposition method

Polyol process was combined with metal organic decomposition (MOD) method to fabricate a room-temperature NO₂ gas sensor based on a tungsten oxide (WO₃) film and another a nanocomposite film of WO₃/multi-walled carbon nanotubes (WO₃/MWCNTs). X-ray diffractometry (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to analyze the structure and morphology of the fabricated films. Comparative gas sensing results indicated that the sensor that was based on the WO₃/MWCNT nanocomposite film exhibited a much higher sensitivity than that based on a WO₃ film in detecting NO₂ gas at room temperature. Microstructural observations revealed that MWCNTs were embedded in the WO₃ matrix. Therefore, a model of potential barriers to electronic conduction in the composite material was used to suggest that the high sensitivity is associated with the stretching of the two depletion layers at the surface of the WO₃ film and at the interface of the WO₃ film and the MWCNTs when detected gases are adsorbed at room temperature. The sensor that is based on a nanocomposite film of WO₃/MWCNT exhibited a strong response in detecting very low concentrations of NO₂ gas at room temperature and is practical because of the ease of its fabrication.     

Transition metal oxides covered Pd film for optical H2 gas detection

We have developed a simple fiber optic Fabry-Perot interferometer (FPI) sensor that is used to detection and measure concentration of hydrogen gas in the air. The operating principle of the sensor is discussed in this paper, and it was noticed that the wavelength positions of the FPI reflectance peaks change with the concentration of hydrogen gas. The sensor has been successfully used to monitor concentration of H₂ in the air below Lower Explosion Limit (LEL). The sensor utilizes a layered sensing structure. This structure includes gasochromic titanium dioxide (TiO₂) and nickel oxide (NiO_x) sensing film. The optical H₂ gas sensor has a very short response time and a fast regeneration time at room temperature.     

Room-temperature H2S gas sensing properties of carbonized silicon nanoporous pillar array

Through thermally treating silicon nanoporous pillar array (Si-NPA) in a graphite crucible in a vacuum furnace at 1100 °C, a continuous thin film composed of cubic SiC nanoparticles was prepared and its room-temperature resistive sensing properties were measured. The sensor was found to be with high sensitivity and an upper limit concentration of 1200 ppm for H₂S detection. Through carrying out the experiments of adsorption-desorption dynamic cycles and long-time air-ambient storage, the sensor was demonstrated to be with high repeatability and long-term stability. The response and recovery times were determined to be ~ 123 and ~ 114 s, respectively. The sensing mechanism was put forward through analyzing the possible adsorption modes of H₂S molecules on SiC/Si-NPA. The existence of the detecting limit concentration was attributed to the single-layer adsorption of H₂S molecules, whose quantity was restricted by the effective adsorption sites formed on SiC/Si-NPA. Our results show that SiC/Si-NPA might be an ideal sensing material for fabricating low-concentration H₂S gas sensors.     

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.     

Nanomaterial-based opto-electrical oxygen sensor for detecting air leakage in packed items and storage plants

A novel nanomaterial-based sensor has been fabricated for detecting air leakage in packaged foods, medical devices and inert gas storage plants. The sensor involved ion-pairing of methylene blue with dodecyl sulphate to produce a water-insoluble form of dye, which along with nano TiO₂ as a sensitiser, is used to create an UV-activated oxygen-sensitive indicator that can be printed on a variety of hydrophobic polymers. The developed sensor not only gives colourimetric indication of air leakage but also provides measurable change in electrical behaviour on air exposure. It bleached quite rapidly and became white in colour (with in a minute) under UV light and recovered its original blue colour when exposed to air. The rate of recovery of the original colour of the sensor is proportional to the ambient oxygen level. The sensor has also been connected to an electrical circuit to generate signals and it is useful to produce an alarm on entering the air into the evacuated or inert gas storage plants. Such a combination of materials and processing offers the potential of producing a low-cost, sensitive and reusable oxygen sensor.     

Gas sensors based on doped-CNT/SnO2 composites for NO2 detection at room temperature

For the first time nitrogen or boron doped carbon nanotubes were added into a SnO₂ matrix to develop a new hybrid CNTs/SnO₂ gas sensors. The hybrid sensor is utilised to detect low ppb concentrations of NO₂ in air, by measuring resistance changes of thin CNTs/SnO₂ films. The tests are performed at room temperature. For comparison, pure SnO₂ and N or B-substituted CNT sensors are also examined. Comparative gas sensing results reveal that the CNTs/SnO₂ hybrid sensors exhibit much higher response towards NO₂, at least by a factor of 10, and good baseline recovery properties at room temperature than the blank SnO₂ and the N or B-substituted CNT sensors. This finding shows that doping SnO₂ with low quantity of CNTs doped with heteroatoms can dramatically improve sensitivity.     

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.     

Effect of NO2 in air on the electrical conductance of In2O3 films with and without added ZnO prepared by R.F. Sputtering

The conductance of indium oxide films with and without added zinc oxide consisting of ultrafine particles with a mean particle size of 30 nm, decreased by a factor of about 500 upon changing from air to 10 p.p.m. NO₂ in air at 150° C. The dramatic decrease in the conduction caused by NO₂ adsorption is mainly attributed to the increase in the activation energy,E, in the Arrhenius relationshipG=G _o exp (-E/kT), while the pre-exponential factor,G _o , is almost constant. The conductance measured in air and NO₂ /air for the films with and without added zinc oxide can be fitted to this relationship in the temperature range below 250° C. The decrease in the carrier concentration is considered as the origin of the decrease in conductance caused by NO₂ adsorption. The alternative explanation in terms of an increase in the grain-boundary potential, i.e. the mobility gap, can be ruled out because the depletion layer width is larger than the mean particle size.     

Optical properties and aging of gasochromic WO3

The limits of WO₃ as an optical gas sensor were derived by establishing the change in optical constants induced by 2% H₂ in Ar. Using Langmuir's adsorption equation, it was found that at low H₂ concentrations a high sensitivity is predicted, but the coloration could saturate at 57.9% of the material's maximum ion adsorption. The air poisoning problem observed in these devices was shown to be remedied by coating with a permeable polydimethylsiloxane layer, thus eliminating common atmospheric gases as the possible poisoning agents.