Film Gas Sensors Research Articles

Aluminum-doped TiO 2 nano-powders for gas sensors

Nano-powders of pure and Al-doped TiO 2 ceramics were synthesized using a citrate–nitrate auto combustion method. Powders were then used to make thick film gas sensors to measure selectivity and sensitivity in CO and O 2 environment. Titanyl nitrate solution was prepared using commercial TiO 2 powder, hydrofluoric acid (HF) and concentrated nitric acid (HNO 3). An optimized ratio of citrate to nitrate was used to produce TiO 2 nanoparticles. Powder X-ray diffraction data showed that synthesized TiO 2 nano-powders, pure as well as Al-doped, had stable anatase phase up to 800 °C. X-ray fluorescence confirmed the Al concentrations in synthesized nano-powders. BET surface area analysis showed a decrease at 5 wt.% Al addition, followed by an increase at 7.5 wt.% in specific average surface area. Particle size analysis showed particles below 100 nm for both pure and doped TiO 2 even after calcination at 800 °C. The resistance of Al-doped TiO 2 samples was found to be lower than that of pure TiO 2 in O 2 and CO environment. Al-doped TiO 2 gas sensors were more selective and sensitive to CO and O 2 at an operating temperature of 600 °C than pure TiO 2 powders.

Nanoengineered chemiresistors: the interplay between electrontransport and chemisorption properties of morphologically encodedSnO2 nanowires

The transport properties and gas-sensing performance of chemiresistors based on quasi-1D, single-crystal,SnO2 nanostructures with deliberately synthesized (encoded) segmented morphology areexplored. Such nanostructures were obtained using programmable modulation of the gassupersaturation ratio during their growth. Using hydrogen and oxygen as modelreducing and oxidizing gases, we show that the responsiveness of these structurallymodulated nanowires to gases is improved over that of straight nanowires of the sameaverage diameter. This is due to the presence of small-diameter segments, whichdominate the nanostructure’s transport properties and to the better tolerance of suchnanostructures towards contact effects. The narrow segments approximate theexcellent responsiveness of ‘necks’ between particles in traditional thin film gassensors but with the significant advantage of greater morphological integrity andstability.

TiO 2 thin film gas sensor for monitoring ammonia

Systematic development and mechanistic studies of sensing materials are critical to the design of higher performance gas sensing elements and arrays. Polycrystalline metal-oxide semiconductors such as SnO 2 and TiO 2 are among the most widely used materials for thin film-based conductometric gas sensors. The mechanistic steps responsible for the gas-induced conductance changes of polycrystalline metal-oxide sensors have been investigated. Results are presented for TiO 2 gas sensing films. The TiO 2 films experience an increase in conductance upon exposure to ammonia. Reduction of surface oxygen is proposed as the dominant mechanism for the increase in conductance in TiO 2 sensing films upon exposure to ammonia. Here TiO 2 films of low thickness prepared using DC magnetron sputtering were employed for sensing applications. A suitable operating temperature, sensitivity, response and recovery time of the TiO 2 thin film gas sensor was studied for sensing ammonia.

Preparation and Gas Sensitive Properties of Polyaniline/Titanium Oxide Composite Thin Films

Polyaniline-titanium dioxide (PANI/TiO2) composite thin film and polyaniline (PANI) thin film were prepared by combined techniques of electrostatic self-assembly and in situ chemical oxidation polymerization at room temperature, which were characterized by XPS and SEM. The PANI/TiO2 and PANI thin film gas sensors with planar interdigitated electrodes were fabricated, and the sensitive properties of sensors to toxic gases NH3 and CO were studied at room temperature. The results showed that the sensitivity, response and recovery characteristics of PANI/TiO2 thin film were superior to those of PANI thin film.

Gas sensors: New materials and processing approaches

To satisfy demands for detecting chemicals in the environment, versatile sensors are required to detect a rapidly growing range of chemical species. In this paper, focus is directed on progress being made to develop temperature-independent oxygen sensors based on the perovskite solid solution system SrTi1−x Fe x O3−δ,and on improving the sensitivity of thin film gas sensors integrated on Si-based self heated substrates. A novel strategy to produce macroporous films with high surface area for enhanced chemical activity is described, and how this processing strategy results in markedly improved sensitivity of gas sensors based on a novel material, CaCu3Ti4O12, is demonstrated.

Kinetics of indium oxide-based thin film gas sensor response: The role of “redox” and adsorption/desorption processes in gas sensing effects

The kinetics of In 2O 3-based thin film gas sensor response to reducing and oxidizing gases was analyzed in this article. In 2O 3 films with thickness from 20 to 400 nm were deposited by spray pyrolysis. The influence of operating temperature, air humidity and film thickness on time constant of sensor response was reviewed. Model concepts that allow explanation of the specific character of the In 2O 3 interaction with reducing and oxidizing gases were proposed. It was concluded that water and oxygen adsorption/desorption processes are the main factors controlling the rate of response and recovery of the In 2O 3 gas sensors. Only during response to ozone in dry air the time of intergrain or bulk oxygen diffusion determines the sensor response time. It was established that reduction/reoxidation processes at the surface of In 2O 3 during gas detection are possible at temperatures as low as 120–150 °C.

Gas Sensing Properties of Metal Doped WO3 Thin Film Sensors Prepared by Pulsed Laser Deposition and DC Sputtering Process

Tungsten trioxide (WO3) thin films gas sensors were prepared by the KrF excimer pulsed laser deposition (PLD) method. The films were prepared on the quartz glass, silicon and also on the Al2O3 sensor substrates with platinum interdigitated electrodes. The effect of doping of the platinum (Pt), palladium (Pd) or gold (Au) on the WO3 thin film was also investigated. These metals were doped to the WO3 thin film by the DC sputtering process during the PLD. The substrate temperature and the oxygen pressure were 400 °C and 100 mTorr, respectively, during the deposition. The films were characterized by atomic force microscopy (AFM) and X-ray diffraction (XRD). The sensitivity of the prepared sensors to 60 ppm NO gas was examined using the two terminal resistance method in a chamber at atmospheric pressure and operating temperatures of 25–350 °C. The sensitivity of the WO3 thin films doped with Pt, Pd, or Au was found to be higher than that of the undoped WO3 thin film.

The selective detection of C 2H 5OH using SnO 2–ZnO thin film gas sensors prepared by combinatorial solution deposition

Sensing materials for selective detection of C 2H 5OH were designed using combinatorial solution deposition of SnO 2–ZnO thin films. The SnO 2–ZnO composite sensor prepared by alternate deposition of 10 droplets of SnO 2 and ZnO sols (S50Z50 sensor) showed a high response to 200 ppm C 2H 5OH (S(ethanol) = R a/ R g = 4.69, R a: resistance in air, R g: resistance in gas) at 300 °C, while the gas responses to 100 ppm C 3H 8, 100 ppm CO, 200 ppm H 2, and 5 ppm NO 2 ranged from 1.11 to 1.19. The S(ethanol) value of the S50Z50 sensor was twice that to 200 ppm CH 3COCH 3 (S(acetone)). In contrast, the S(ethanol) and S(acetone) of pure SnO 2 and ZnO thin films were similar to each other. The heterostructure between SnO 2 and ZnO was suggested as one of the probable reasons for the successful discrimination between C 2H 5OH and CH 3COCH 3.

A novel process for fabrication of SnO 2-based thick film gas sensors

A centrifugal coating technique has been developed for SnO 2 thick film gas sensors. The thickness of the films was controlled by changing the concentration of the SnO 2 suspension. For thicknesses above 10 μm, the sensitivity was stabilized. A cold isostatic pressing of the films before sintering resulted in a slight reduction of the sensitivity and a considerable reduction in resistivity, showing the applicability of the process. Centrifugally coated samples showed a much higher sensitivity than screen-printed samples. This result was attributed to the stacking of fine powder at the surface which showed a lower resistivity and a higher sensitivity compared to coarse powder.

Electron beam evaporated carbon nanotube dispersed SnO2 thin film gas sensor

Carbon nanotube (CNT) is a useful material for gas-sensing applications because of its high surface to volume ratio structure. In this work, multi-wall CNTs are incorporated into tin oxide thin film by means of powder mixing and electron beam evaporation and the enhancement of gas-sensing properties is presented. The CNTs were combined with SnO2powder with varying concentration in the range of 0.25–5% by weight and electron beam evaporated onto glass substrates. From AFM and TEM characterization, CNT inclusion in SnO2thin film results in the production of circular cone protrusions of CNT clusters or single tube coated with SnO2layer. Experimental results indicate that the sensitivity to ethanol of SnO2thin film increases by the factors of 3 to 7, and the response time and recovery time were reduced by the factors of 2 or more with CNT inclusion. However, if the CNT concentration is too high, the sensitivity is decreased. Moreover, the CNT doped film can operate with good sensitivity and stability at a relatively low temperature of 250–300∘C. The improved gas-sensing properties should be attributed to the increasing of surface adsorption area of metal oxide produced by CNT protrusion.

Application of ion beam analysis to the selective sublimation processing of thin films for gas sensing

Ion beam analysis was successfully applied to a novel technique, named selective sublimation process (SSP), for deposition of nanostructured gas-sensing films through reactive sputtering. The method consists of the co-deposition of a mixed oxide, one of which has a relatively low sublimation temperature. Annealing at suitable temperature causes the sublimation of the most volatile compound, leaving a layer with adjustable composition. The appropriate choice of thermal treatments and the consequent tailoring of the composition play a crucial role in the determination of the microstructural properties.We developed a model based on diffusion equations that provides a useful guide to control the deposition and processing parameters and we applied the model on the systems TiO2–WO3 and TiO2–MoO3. Rutherford backscattering (RBS) was demonstrated to be effective for the characterization of the diffusion and sublimation processes during SSP. Experimental results fully agree with theoretical prediction, and allowed the calculation of all the parameters involved in SSP.

Dynamic determination of domestic liquefied petroleum gas down to several ppm levels using a Sr-doped SnO2 thick film gas sensor

Sr-doped SnO2 thick film gas sensors were prepared for domestic liquefied petroleum gas (LPG) determination down to several ppm levels using the screen-printing technique. Characterization of Sr-doped SnO2 thick film was investigated by XRD, XPS and DTA-TGA analyses. The sensitivity, selectivity, sintering temperature, and static and dynamic measurement were investigated. The results showed that the Sr-doped SnO2 thick film sensor is suitable for several ppm levels LPG determination because of the high sensitivity (S = 12.7 to 10 ppm LPG). The dynamic measurements showed that the sensor exhibited high sensitivity and selectivity to domestic LPG at 210–300 °C.

Density of occupied and unoccupied states monitored during metal deposition onto phthalocyanine layers

Abstract. Phthalocyanine (Pc) materials find applications in electronic devices such as organic light emitting devices (OLEDs), solar cells, thin film transistors or gas sensors. The electronic properties at interfaces with different materials e.g. inorganic semiconductors or metals, usually dominate the device performance. Therefore the knowledge of the interface electronic properties is required. In the present work valence band photoemission spectroscopy (VB-PES) and inverse photoemission spectroscopy (IPES) were employed to determine the density of occupied and unoccupied states upon silver deposition onto layers of two phthalocyanines (H2Pc and CuPc). The two different Pc molecules give rise to very distinct behaviour already in the initial Stage of silver deposition. While in the CuPc case no shift occurs in the energy levels, the H2Pc highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are shifting simultaneously by 0.3 eV, i.e. the HOMO shifts away from the Fermi level while LUMO shifts towards the Fermi level. As the silver quantity increases the HOMO levels of both Pcs are shifting towards the Fermi level. When the Fermi level is resolved in the VB-PES spectra the H2Pc and CuPc characteristic features are smeared out to some extent. Shifts in HOMO and LUMO energy positions as well as changes in line shapes are discussed in terms of charge transfer and chemical reactions at the interfaces.

Characterization of Chinese vinegars by electronic nose

In this paper, 17 commercial Chinese vinegars, acetic acid and 5% diluted acetic acid were analyzed by an electronic nose containing nine nano ZnO thick film gas sensors, which are doped by 5 wt.% and 10 wt.% TiO 2, 5 and 10 wt.% MnO 2, 1 wt.% V 2O 5, 5 wt.% Bi 2O 3, 0.6 and 2.4 wt.% Ag, and 5 wt.% W, respectively. Principal component analysis (PCA) and cluster analysis (CA) were employed to investigate the presence of classes inside the sample population. It was shown that characterizing the Chinese vinegars by the electronic nose was highly related to their type, raw materials, total acidity, fermentation method and production area and all these influencing factors were not independent. The CA results indicated that the type and fermentation method were more effective than the other influencing factors when the vinegars were analyzed by the electronic nose. Finally, the data colleted by the electronic nose were applied to the learning vector quantization (LVQ) neural network performing the role of recognition and classification of the vinegars. The accuracy in terms of predicting tested vinegar measurements was 72.1%, 76.5%, 77.9%, 94.1% and 82.4% according to their type, raw materials, total acidity, fermentation method and production area, respectively. This work was the first step to establish a gas-sensing fingerprint database of Chinese vinegars and develop a commercial electronic nose on the Chinese vinegars quality control.

Fast response thin film SnO 2 gas sensors operating at room temperature

SnO 2 thin film gas sensors operating at room temperature were prepared with SnCl 4 as the starting material. The gas sensing characteristics of the sensors towards methyl alcohol vapor were investigated. The sensors exhibit ultra-fast and reversible electrical response ( t 90% ∼5 s for response and ∼1 s for recovery) at room temperature. In addition, the electrical responses are reproducible during cycling tests. The particle size of the hydrolyzed SnCl 4 affects the sensitivity of the sensors, but does not have much effect on their response time. The sensors show different electrical response in the detection of the vapors of methyl, ethyl, isopropyl and butyl alcohol at room temperature, which is proposed to be related to the variation in the reducing ability of the alcohols with a different number of methyl groups.

Preparation and characterization of nanocrystalline SnO 2 thin films by PECVD

Nanocrystalline SnO 2 thin films were deposited by RF inductively coupled plasma enhanced chemical vapor deposition (PECVD) using dibutyltin diacetate as precursor. The effects of substrate–nozzle distance ( D sn), RF power and deposition time on the phase and morphologies of the SnO 2 thin films have been studied. The as-deposited SnO 2 thin films were well crystallized even without additional substrate heating and post-annealing. The grain size and film thickness decreased continuously with the increase of D sn and RF power while increased with increasing deposition time. The deposition parameters showed notable effects on the microstructure of the SnO 2 thin films, thus it was possible to optimize the microstructure of the SnO 2 thin films by adjusting deposition parameters to achieve high-performance SnO 2-based thin film gas sensors.

Tin Dioxide Nanodimensional Film Based H2 sensor

AbstractPaper presents results of investigation aimed at the development of tin dioxide nanodimensional film gas sensors (NDFGS) with high sensitivity to hydrogen. Specifics of the performed research consist in the approach based on the combination of utilization nanodimensional metal oxide film, as gas sensitive element of device, and group electron technology methods that allows to arranging from small to large scale manufacturing of given NDFGS.Gas sensitive nanodimensional film (NDF) had been formatted by means of chemical spray pyrolysis method on the both ceramic substrate for study and optimization of electrophysical and gas sensitive characteristics and “chip” of gas sensor (GS) preliminarily produced through group electron technology. Films were deposited at the temperatures in interval 400-500°C from water solutions of SnCl4.5H2O precursor. The thickness of SnO2 films was varied in the range 40-80 nm.Study of electrophysical characteristics of obtained layers has shown that the last one possess resistance on the level 105-106 Ohm/ at the working temperatures of GS. Gas sensitivity was determined as ratio of SnO2 film resistance in the pure air and in the presence of gas impurity in atmosphere (S=Rgas/Rair). Values of R were determined through Van-der-Pauw method.X-ray and SEM investigation has allowed establishing the interconnection between technologic parameters, nanostructure and gas sensitive characteristics of obtained films.Gas sensitivity S of the deposited undoped films has amounted 9-10 relative units to the 100 ppm of hydrogen in air. Modification of gas sensitive properties of tin dioxide NDF through bulk and surface doping with Pd has allowed to increasing hydrogen sensitivity up to almost 104 rel. units, providing the detection of hydrogen at the concentration on the level 1 ppm and shift of the sensitivity maximum from 350°C (in the case of undoped films) to 150°C (Pd doped NDF).Performed sensor chip thermal regimes modeling has allowed developing of the NDFGS topology with reduced power consumption (< 30 mW).

Formation of Highly Porous Gas-sensing Films by In-situ Thermophoretic Deposition of Nanoparticles from Aerosol Phase

AbstractGas sensors based on tin dioxide nanoparticles show high sensitivity to reducing and oxidizing gases. Dry aerosol synthesis applying the flame spray pyrolysis was used for manufacture and directly (in-situ) deposit nanoparticles on sensor substrates. For the first time this technique has been used to synthesize a combination of two stacked porous layers for gas sensor fabrication. Compared to state-of-the-art techniques, aerosol technology provides a direct and versatile method to produce homogeneous nanoparticle films. Two different sensing layers were deposited directly on interdigital ceramic substrates. These porous bottom layers consisted either of pure tin dioxide or palladium doped tin dioxide. The top layer was a palladium doped alumina nanoparticle film which served as a chemical filter. The fabricated gas sensors were tested with methane, CO and ethanol. In case of CH4 detection, the pure tin dioxide sensor with the Pd/Al2O3 filter layer showed higher sensor signals and significantly improved analyte selectivity with respect to water vapor compared to single tin dioxide films. At temperatures up to 250°C the Pd-doping of the tin dioxide strongly increased the sensitivity to all gases. At higher temperatures the sensor signal significantly decreased for the Pd/SnO2 sensor with a Pd/Al2O3 filter on top indicating high catalytic activity.

Multilayered metal oxide thin film gas sensors obtained by conventional and RF plasma-assisted laser ablation

Multilayered thin films of In 2 O 3 and SnO 2 have been deposited by conventional and RF plasma-assisted reactive pulsed laser ablation, with the aim to evaluate their behaviour as toxic gas sensors. The depositions have been carried out by a frequency doubled Nd–YAG laser ( λ = 532 nm, τ = 7 ns) on Si(1 0 0) substrates, in O 2 atmosphere. The thin films have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrical resistance measurements. A comparison of the electrical response of the simple (indium oxide, tin oxide) and multilayered oxides to toxic gas (nitric oxide, NO) has been performed. The influence on the structural and electrical properties of the deposition parameters, such as substrate temperature and RF power is reported.

Nano-crystalline Cu-doped ZnO thin film gas sensor for CO

A Cu-doped ZnO (CZO) film was prepared on a glass substrate by co-sputtering using ZnO and Cu targets. The CZO film possessed a columnar structure consisting of small crystals with an average grain size of around 5nm. The CO-sensing properties of the CZO film were tested at operating temperatures of 150, 300, 350 and 400°C. It was found that CZO-based sensor exhibited the highest sensitivity to CO at 350°C. An obvious change in resistance of the CZO film was also observed when the sensor was exposed to 6ppm CO at 150°C. The sensing mechanism of the nano-structured CZO sensor is discussed.