Film Gas Sensors Research Articles

Gas Sensors Characterization and Multilayer Perceptron (MLP) Hardware Implementation for Gas Identification Using a Field Programmable Gate Array (FPGA)

This paper develops a primitive gas recognition system for discriminating between industrial gas species. The system under investigation consists of an array of eight micro-hotplate-based SnO2 thin film gas sensors with different selectivity patterns. The output signals are processed through a signal conditioning and analyzing system. These signals feed a decision-making classifier, which is obtained via a Field Programmable Gate Array (FPGA) with Very High-Speed Integrated Circuit Hardware Description Language. The classifier relies on a multilayer neural network based on a back propagation algorithm with one hidden layer of four neurons and eight neurons at the input and five neurons at the output. The neural network designed after implementation consists of twenty thousand gates. The achieved experimental results seem to show the effectiveness of the proposed classifier, which can discriminate between five industrial gases.

CO sensitivity of La2O3-doped SnO2 thick film gas sensor

La2O3-doped SnO2 thick films were fabricated via powder mixing of SnO2 and La2O3, and their sensor responses to 10–75ppm CO were characterized. The presence of La2O3 in a SnO2 thick film containing more than 1.3at.% La2O3 was confirmed with X-ray diffraction performed at a slow scan rate of 0.5°/min. The sensor response to 50ppm CO at 350°C was improved from 9.5 to 59.0 with the addition of 2.2at.% La2O3 to SnO2. Compared to the CO sensitivity increase of the undoped SnO2 from 3.2 to 37.6, the 2.2at.% La2O3-doped SnO2 exhibited a remarkable sensitivity improvement from 5.2 to 213.7 with an increase in the CO concentration from 10ppm to 75ppm.

The sensing behaviour of metal oxides (ZnO, CuO and Sm2O3) doped-SnO2 for detection of low concentrations of chlorinated volatile organic compounds

A SnO2-based semiconductor thick film gas sensor was fabricated for detection of chlorinated volatile organic compounds (VOCs) such as dichloromethane (DCM), trichloroethylene (TCE) and 2-chloroethylethylsulfide (CEES) at concentrations of 10, 20 and 10ppm respectively with temperatures ranging from 100 to 400°C. Other semiconductors such as ZnO, CuO in 5.0wt% and Sm2O3 in 1.0, 5.0 and 10.0wt% were added to SnO2 by a co-precipitation method. The samples were characterized by TEM, XRD and BET. The characterization results showed that presence of dopants causes an increase in surface area and a decrease in particle size of the sensing materials. It was found that SnO2–5.0wt% Sm2O3 has the highest response to DCM and CEES, specially at low temperatures. Also the response was quite high for this sensor even at a concentration as low as 0.1ppm. However, presence of the promoters suppressed the sensitivity to TCE.

Acetone vapour sensing characteristics of undoped and Zn, Ce doped SnO2 thick film gas sensor

The nanocrystalline materials of zinc and ceria doped tin oxide [(Zn+Ce)–SnO2] powders were synthesised by coprecipitation method and investigated for their sensing properties towards liquid petroleum gas (LPG), ethanol, ammonia and acetone vapour. The crystal structure and phase of the as synthesised and sintered powders were characterised by X-ray diffractometer and the microstructure by scanning electron microscopy. All the doped and undoped SnO2 compositions revealed single phase solid solution formation. Transmission electron microscope results indicated that well crystallised (Zn+Ce) doped SnO2 particles of size ∼7 nm were obtained at the annealing temperature of 650°C. The reduction in grain size of the metal oxide is a key factor to enhance the gas sensing properties. The doping of zinc, ceria in SnO2 has reduced the grain size and improved the gas response. The results of gas sensing measurements showed that the thick films deposited on alumina substrates using screen printing technique exhibited high response to acetone at an operating temperature of 300°C. Further, the selectivity of the sensor towards acetone with respect to other reducing gases (LPG, ethanol and ammonia) was studied.

Fabrication and Application of Thin Film Semiconductor Sensors for the Detection of Volatile Organic Compounds

Volatile Organic Compounds (VOCs) are highly reactive, often mixed with interfering gases and more importantly, their vapours in the gaseous form respond to the gas sensor devices. Oxide semiconductor based thin film gas sensors play vital role in detecting, monitoring and controlling the presence of hazardous and harmful gases in the environment at very low concentration and hence are tested for the detection of harmful or industrially important VOCs. The demand of sensitive as well stable gas/vapour sensors for direct sensing application has increased the importance of In2O3:SnO2 based semiconductor materials. The paper presents the fabrication aspects of thin film gas sensors based on x1In2O3:x2SnO2 compound and also their application for the detection of some important volatile organic compounds. Thin films of x1In2O3:x2SnO2 were deposited using Thermal Evaporation technique and under the optimized fabrication conditions, the vapour sensors were prepared and mounted on the proper contact jig. Their response to the presence of Volatile Organic Compounds like CCl4 was examined. The stability, the sensitivity and the response time of the sensors were studied for the different concentrations of test vapours.

Nanostructured tungsten oxide gas sensors prepared by electric field assisted aerosol assisted chemical vapour deposition

Nanostructured thin films of tungsten trioxide were deposited on to gas sensor substrates at 600 °C from the aerosol assisted chemical vapour deposition reaction of tungsten hexaphenoxide solutions in toluene under the influence of electric fields. The electric fields were generated by applying a potential difference between the inter-digitated electrodes of the gas sensor substrates during the deposition. The deposited films were characterised using scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The application of an electric field, encouraged formation of enhanced nanostructured morphologies, with an increase in needle length and reduction in needle diameter being observed. The film gas sensor properties were also examined; it was found that the highest response of 110 to 800 ppb NO2 was given by a sensor grown under the influence of a 1.8 × 104 V m−1 electric field and operated at 250 °C, a 2.5 times enhancement compared to a sensor grown in the absence of an electric field under its optimal operating conditions.

Ethanol monitoring by ZnFe2O4 thin film obtained by spray pyrolysis

The results presented in this paper were focused on the easy development of cheap, sensitive and integrable spinel zinc ferrite (ZnFe2O4) thin film gas sensors for the detection of ethanol. ZnFe2O4 layers of about 450 nm thick were deposited by spray pyrolysis technique. The resulting films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Analysis confirmed the formation of monophasic spinel structured thin films with flat surface and dense microstructure consisting of nanosized grains. The sensor detects ethanol concentrations as low as 1 ppm and shows comparably fast behaviour of response and recovery.

WO3 nanoclusters–SnO2 film gas sensor heterostructure with enhanced response for NO2

Abstract The surface of SnO 2 thin film with porous microstructure has been integrated with WO 3 nanoclusters using pulsed laser deposition technique and a novel sensor structure has been realized for detecting low concentration of NO 2 gas (10 ppm) at a low operating temperature (100 °C). The content of WO 3 was found to be very critical and integration of 8 nm thick nanoclusters exhibits a high sensor response of 5.4 × 10 4 towards 10 ppm NO 2 gas with fast response (67 s). The origin of enhanced response characteristics has been analysed in the light of Fermi level and spillover mechanisms and are correlated with the reaction kinetics of NO 2 gas on the sensor surface.

Schottky barriers measurements through Arrhenius plots in gas sensors based on semiconductor films

The oxygen adsorption effects on the Schottky barriers height measurements for thick films gas sensors prepared with undoped nanometric SnO2 particles were studied. From electrical measurements, the characteristics of the intergranular potential barriers developed at intergrains were deduced. It is shown that the determination of effective activation energies from conduction vs. 1/temperature curves is not generally a correct manner to estimate barrier heights. This is due to gas adsorption/desorption during the heating and cooling processes, the assumption of emission over the barrier as the dominant conduction mechanism, and the possible oxygen diffusion into or out of the grains.

온도 가스 감지 다기능성 세라믹 복합 센서

Multifunctional structures with two kinds of materials have been intensively investigated in order to improve their electrical characteristic with two functions simultaneously. However, the research regarding of multifunctional ceramic sensor is still in a preliminary stage and how to integrate them with low-cost and high-yield mass production process remains a challenge issue. In this study, we fabricated the multifunctional ceramic sensor composed of temperature and gas sensors. Moreover, we investigated the CO sensing properties of three dimensional nanostuctured <TEX>$Nb_2O_5$</TEX> thin film gas sensors fabricated with silica (<TEX>$SiO_2$</TEX> nanosphere (<TEX>${\O}$</TEX>= 750 nm). Compared to plain films, the nanostructured films show enhanced gas sensing of greater sensitivity and a faster response. This result reveals that significantly increased sensitivity is an increase in the effective surface area for the adsorption of gas molecules.

Highly sensitive SnO2 porous film gas sensors fabricated by plasma spray physical vapor deposition

Abstract Synergic effect of film structure and sensor geometry on the gas sensor properties was investigated for porous thick film gas sensors fabricated by plasma spray physical vapor deposition (PS-PVD). Sensor model was also developed taking into account of both gas permeation/reaction and spatial current density distributions. It is found that higher responses are fundamentally attained with thinner sensor films, regardless of other parameters. At a fixed thickness, the response is less dependent on the applied voltage or the electrode gap at wider gaps, but it changes significantly when the gap becomes smaller than the critical gap size. This tendency becomes more pronounced for the thicker and denser films. This is explained by the fact that the gas/film reaction is localized near the film surface and the current change upon exposure to the target gas is reduced resulting in the decrease in the response. Furthermore, especially for the sensors with narrow electrode gap and at high applied voltage, a local Joule heating is no longer ignored and increases the temperature at least near the electrode. This affects the response to either increase or decrease, depending on the local temperature that is determined by the sensor design, i.e., gas diffusion speed and reaction, with respect to the optimal sensor operating temperature. As a result of the geometry optimization, the PS-PVD sensors have exhibited the noticeable response of formaldehyde at a concentration as low as 40 ppb.

Manufacturing and investigations of i-butane sensor made of SnO2/multiwall-carbon-nanotube nanocomposite

Nanocomposite SnO2/multiwall-carbon-nanotubes were prepared by the sol-gel method. The morphology of prepared nanocomposite powder was investigated by scanning electron microscopy. Energy-dispersive X-ray and X-ray diffraction analyses were carried out, the element structure of a nanocomposite has been determined. Ceramic and thick film gas sensors made of the nanocomposite multiwall-carbon-nanotube/SnO2/Pd structure were manufactured and investigated. After the sensitization in the Ru(OH)Cl-3 water solution, these sensors have rather high response to isobutane already at 120 degrees C. The response of prepared sensors was measured at different gas concentrations and at different temperatures of the work body. (C) 2012 Elsevier B.V. All rights reserved.

Influences of Al, Pd and Pt additives on the conduction mechanism as well as the surface and bulk properties of SnO2 based polycrystalline thick film gas sensors

Abstract The reasons of the effect of Pd, Pt and Al additives on the sensing and conduction mechanism of SnO 2 based thick film porous gas sensing layers are studied by a combination of DC-resistance, work function changes and catalytic conversion measurements. This is done by analyzing the dependence of the DC resistance on the corresponding band bending changes over a large range and the use of previously reported conduction models. The gained information deals with the surface band bending in the absence of ambient atmosphere oxygen, the position of the Fermi level, the concentration of free charge carriers, the Debye length and the width of the surface charge layer in various ambient conditions. Very interestingly, we found that in all cases the “doping” had an impact on both surface and bulk properties even if the additives and the technology of “doping” were targeted towards surface activation, in the case of Pt and Pd, and bulk compensation of donors, in the case of Al. Besides that, the catalytic conversion experiments indicated that the presence of Pt is associated with the reduction of the material in the absence of ambient atmosphere oxygen.

Solution-Processed Gas Sensors Based on ZnO Nanorods Array with an Exposed (0001) Facet for Enhanced Gas-Sensing Properties

In this paper, thin film gas sensors made from 8-nm-diameter and exposed-{10-10}-facet ZnO nanorods self-aligning along the ceramic tube are fabricated by a simple dip-coating method. On the sensor surface, we successfully synthesize a ZnO nanorods array exposed with (0001) plane in situ by a facile solution-processing technique. Compared with the unprocessed sensor (i.e., dip-coated ZnO film based sensor), the main advantages of the solution-processed sensor are a high sensitivity (3-fold prefactor Ag), fast response (less than 10 s), and low detection limit (1 ppm) to benzene and ethanol. The enhancement in the gas-sensing performance suggests that the effect of exposed facet is dominant rather than the size effect, and the order of gas-sensing properties of ZnO crystal face is (0001) > {10-10}. On the basis of these results, it is found that the surface structure at the atomic level is a key factor in improving the oxygen adsorption and, consequently, the gas-sensing performance of a ZnO nanorods array...

Highly Ordered Large-Area Colloid Templates for Nanostructured TiO&lt;SUB&gt;2&lt;/SUB&gt; Thin Film Gas Sensors

We report a novel process that uses highly ordered colloidal templating to fabricate nanostructured TiO2 thin film gas sensors. An O2 plasma treatment is used to decrease the contact angle of a water droplet on a SiO2/Si substrate from 46 degrees to 3 degrees. The formation of this hydrophilic surface enhances the adhesion of polystyrene microspheres to the substrate during the spin coating of the colloidal solution, leading to a large-area colloid template of closely packed monolayer microspheres on the substrates. Embossed TiO2 thin film gas sensors fabricated through highly ordered colloidal templating using O2 plasma exhibit substantially enhanced gas sensing performance as compared to those without surface treatments prior to colloidal templating.

Properties of SnO2 based gas-sensing thin films prepared by ink-jet printing

SnO2 precursor inks with appropriate viscosity and surface tension were prepared using the sol–gel technique with anhydrous ethanol as the primary solvent. The inks were printed on both alumina ceramic and silicon substrates using an ink printing apparatus produced by modifying a commercial printer. SnO2 based gas-sensing films with different thicknesses and additives were formed, and the morphologies and electrical properties of these films were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and an instrument designed to measure the gas sensing abilities of the films. A linear relationship was observed between the natural logarithm of the electrical resistance of the SnO2 films and the reciprocal of the films’ absolute working temperature in the range between 20°C and 265°C. The electrical and gas-sensing properties of the films changed significantly with the thickness of the films. The film thickness can be easily adjusted by modifying the number of printing events. The SnO2 films selectivity for various gases could be modified by utilizing different additives. This work also showed that ink-jet printing was a convenient and low-cost method to prepare gas-sensing films with controlled film thickness and additive level.

The Role of Grain Size in Response of SnO&lt;sub&gt;2&lt;/sub&gt;- and In&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;-Based Conductometric Gas Sensors

Basing on the experimental results, obtained while studying both structural and gas sensing properties of the SnO2 and In2O3 films deposited by spray pyrolysis method, it was analyzed the influence of the crystallites size on the response of the SnO2-and In2O3-based thin film gas sensors. It was confirmed that that the grain size is one of the most important parameters of metal oxides, controlling almost all operating characteristics of the solid state gas sensors. However, it was shown that the decrease of the grain size is not a universal decision contributing to the improvement of sensor response. The decrease of the grain size could be accompanied either by the improvement, or the worsening of gas sensor operating characteristics.

Effect of top-down nanomachining on electrical conduction properties of TiO2 nanostructure-based chemical sensors

The present research was motivated by the growing interest of the scientific community towards the understanding of basic gas–surface interaction mechanisms in 1D nanostructured metal oxide semiconductors, whose significantly enhanced chemical detection sensitivity is known. In this work, impedance spectroscopy (IS) was used to evaluate how a top-down patterning of the sensitive layer can modulate the electrical properties of a gas sensor based on a fully integrated nanometric array of TiO2 polycrystalline strips. The aim of the study was supported by comparative experimental activity carried out on different thin film gas sensors based on identical TiO2 polycrystalline sensitive thin films. The impedance responses of the investigated devices under dry air (as the reference environment) and ethanol vapors (as the target gas) were fitted by a complex nonlinear least-squares method using LEVM software, in order to find an appropriate equivalent circuit describing the main conduction processes involved in the gas/semiconductor interactions. Two different equivalent circuit models were identified as completely representative of the TiO2 thin film and the TiO2 nanostructure-based gas sensors, respectively. All the circuit parameters were quantified and the related standard deviations were evaluated. The simulated results well approximated the experimental data as indicated by the small mean errors of the fits (in the range of 10−4) and the small standard deviations of the circuit parameters. In addition to the substrate capacitance, three different contributions to the overall conduction mechanism were identified for both equivalent circuits: bulk conductivity, intergrain contact and semiconductor–electrode contact, electrically represented by an ideal resistor Rg, a parallel RgbCgb block and a parallel Rc-CPEc combination, respectively. In terms of equivalent circuit modeling, the sensitive layer patterning introduced an additional parameter in parallel connection with the whole circuit block. Such a circuit element (an ideal inductor, L) has an average value of about 125 μH and exhibits no direct dependence on the analyte gas concentration. Its presence could be due to complex mutual inductance effects occurring both between all the adjacent nanostrips (10 µm spaced) and between the nanostrips and the n-type-doped silicon substrate underneath the thermal oxide (wire/plate effect), where a two order of magnitude higher magnetic permeability of silicon can give L values comparable with those estimated by the fitting procedure. Slightly modified experimental models confirmed that the theoretical background, regulating thin film devices based on metal oxide semiconductors, is also valid for nanopatterned devices.

Study on Gas Sensor of Hybrid Organic Molecule and Sensitive Properties of Toxic Gas

According to the low power and the chemical modifiction properties of organic gas-sensing material, the organic semiconductor material of molecule hybrid CuPc and PANI was used as gas-sensing film, and the gas sensor which has high selectivity to toxic gas was investigated. CuPc was synthesized with Phthalonitrile and p-Cresol as the source material of benzene ring by the template synthesis method of two steps reaction. Through doping with perchloric acid, vitriol and nitric acid, semiconducting doped PANI was obtained. By means of molecular hybrid modification, the intermediate substance of CuPc and PANI was polymerized and formed a kind of new organic semiconductor material CuPcxPANI1-x. Sensor chip was made by virtue of semiconductor technology; the combination of chip and new sensitive material was achieved by virtue of vacuum coating technology. The film surface morphology were characterized through SEM, and it showed that the sensitive material was made of rounded particles whose diameter was about 200nm; the surface of film was deposited by odontoid particals, whose diameter is 200nm or so, and relatively uniform, and its aperture is ordered. The gas sensing properties of thin film were tested by static volumetric method. The results indicate that the new organic semiconductor hybrid material is sensitive to the toxic gases of sulfur dioxide, nitride dioxide, chlorine ect.; the selectivity can be improved by changing the ratio of the distribution; When the heating voltage is 1.5V, the output resistance can be stabilized within 5% and the sensitivity within 10% in 6 months.

Soft-lithographic processed soluble micropatterns of reduced graphene oxide for wafer-scale thin film transistors and gas sensors

PDMS based imprinting is firstly developed for patterning of rGO on a large area. High quality stripe and square shaped rGO patterns are obtained and the electrical properties of the rGO film can be adjusted by the concentration of GO suspension. The arrays of rGO electronics are fabricated from the patterned film by a simple shadow mask method. Gas sensors, which are based on these rGO electronics, show high sensitivity and recyclable usage in sensing NH3.