Search

Our previous studies demonstrated that rare-earth oxycarbonates Ln2O2CO3 (Ln = La, Nd, and Sm) and rare-earth oxides Ln2O3 (Ln = Nd, Sm, Gd, Dy, Er, and Yb) are sensitive to CO2 and that hexagonal ...

We recently demonstrated acetylene and ethylene selectivity for LaFeO3 (LFO)-based gas sensors. To understand the origin of sensing, we combined catalytic conversion measurements with simultaneousl...

LaFeO3 (LFO) thick film gas sensors were obtained by depositing the same sensitive material on alumina substrates provided with electrodes made out of different noble metals, namely platinum and gold. Their acetylene and ethylene sensing properties were investigated in dry and humid conditions at different operating temperatures. The main finding is that the nature of the electrode materials has an impact on the sensor performance: in the case of Pt electrodes good responses to both ethylene and acetylene were recorded at 150 °C; in the same conditions, in the case of Au electrodes a good response was recorded only for acetylene. In order to understand the underlying reasons, we investigated the surface chemistry changes associated with the nature of electrodes by operando Diffuse Reflectance Infrared Fourier Transformed (DRIFT). We found that regardless of the electrode material, the surface reaction responsible for the sensor signals takes place for acetylene. However, in the case of ethylene, platinum is required for the reaction to occur.

Monitoring the gas composition around power cables using gas sensors is a promising method for detecting their overheating. In this paper we screened the sensing performance in dry and humid conditions of a variety of commercial and homemade sensors. The target gases were 2-Ethylhexanol (2-EH), Dioctyl terephthalate (DOTP), and Benzene; they are the main gases evaporated from overheated polyvinyl chloride (PVC), which is the material of choice for the external insulation of power cables. We found that the tested sensors show different sensing performances to the three target gases, generally with a fast response and slow recovery, the latter especially for 2-EH and DOTP. We also found that the responses to DOTP and 2-EH are correlated and, generally, larger than the ones to benzene. We also showed that by combining the sensors into a sensor array and applying PCA it is possible to distinguish between 2-EH and DOTP, on the one hand, and benzene, on the other hand. However, a more refined pattern recognition method is needed to distinguish between 2-EH and DOTP.

Noble metal/metal oxide nanocomposites are very important in various fields of catalysis and play an evenly important role in gas sensing. Although there are many similarities regarding the choice of materials, the synthesis and the fundamental mechanisms, both research fields have been mostly treated inde¬pendently up to now. In both fields, open questions regarding the elementary steps in the interaction of the gases with the active species and the role of the noble metal, the semiconducting metal oxide support and the interface remain. In this concept article, we first outline the importance of such composites in catalysis and gas sensing focussing on Pt/Pd as well as CeO2 and SnO2 as transition metal oxides. Next, the state of art of both fundamental and relevant surface reactions and electronic mechanisms are described. Finally, we highlight the synergy of jointly exploring catalysis and gas sensing of noble metal/metal oxide nanocomposite materials and the benefit for both research fields if they are dealt with simultaneously using advanced characterization and operando methods, sophisticated preparation techniques, testing of the performance, and predictive theoretical modelling.

Gas sensor based on perovskites, such as LaFeO3 (LFO), have been used successfully to detect various target gases [1,2]. […]

Rare-earth oxycarbonates Ln2O2CO3 (Ln = rare-earth element) have been identified as materials for chemoresistive CO2 gas sensors. Among them, previous studies identified monoclinic La2O2CO3 as the best performing one. However, not all rare-earth elements have been investigated and, moreover, La2O2CO3 monoclinic phase is metastable and this can influence the long term performance. In this work, we have synthesized rare-earth oxycarbonates Ln2O2CO3 (Ln = La, Nd, and Sm) including monoclinic and hexagonal La2O2CO3, rare-earth oxides Ln2O3 (Ln = Nd, Sm, Gd, Dy, Er, and Yb) and LnO2 (Ln = Ce) by calcination of oxalate hydrate or the acetate hydrate precursors in air. All the materials, except for CeO2 and Nd2O3, were sensitive to CO2. All CO2 sensitive materials, except for monoclinic La2O2CO3 and Nd2O2CO3, were stable and their performance is sufficient for practical use. Hexagonal La2O2CO3 shows the best overall performance. The results of operando investigations indicate that the origin of CO2 sensing is the competitive adsorption between carbonates and hydroxyl groups.

Gas sensing using metal oxides can be a highly cost effective and reliable technology in a variety of medical and industrial applications. However, selectively sensing a specific gas in a complex gas mixture continues to be a significant challenge. Here, we focus on acetylene and ethylene sensing with LaFeO3 (LFO) based sensors with novel applications, such as online monitoring and maintenance of electrical power transformers, in mind. We prepared LFO’s via sol gel method at five different calcined temperatures (500–900 °C). X-ray diffraction (XRD) patterns showed that all five materials were single phased with a perovskite crystal structure. We then used these materials as active sensing layers during exposure to various test gases, including C2H2, C2H4, CH4, C2H6, CO, CO2 and H2 at different operating temperatures (150 °C, 200 °C, 250 °C and 300 °C). All of our sensors showed a significant response to unsaturated hydrocarbons, namely acetylene and ethylene, but not to the other gases. We further improved this high selectivity of our sensors, to only detect acetylene and not ethylene, by controlling the operating temperature. We then tested the effects of different backgrounds, such as humidity, and CO2 levels, on our LFO sensors. The sensors were saturated faster in humid than dry conditions, in particular at lower operating temperatures, and there was no influence of CO2. Therefore, our results demonstrate novel oxide-based sensors capable of distinguishing between acetylene and ethylene, enabling interesting new industrial applications.

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.

Flame spray pyrolysis (FSP) allowed quenching the high temperature phase In4Sn3O12 in the form of highly single crystalline particles of about 6 nm. These nanoparticles were in situ deposited to form stable porous films on interdigitated electrodes. The resulting gas sensors were tested for formaldehyde sensing in the low ppb range. Comparing systematic composition of the In–Sn–oxide system ranging from pure In2O3, In1.9Sn0.1O3 (ITO), In4Sn3O12 and SnO2 with their corresponding mixtures showed by far the best sensor performance at 250 °C for 43% of (Sn/(Sn + In), corresponding to the In4Sn3O12 phase. The sensors tested using this phase outperformed state of the art metal oxide devices. The In4Sn3O12 phase was stable beyond the operation time and temperature used here, demonstrating its enormous but largely undiscovered potential in the future.

The conduction mechanism in polycrystalline SnO2 thick sensing films was modeled and experimentally investigated by means of simultaneous DC electrical resistance and work function changes measurements under CO and H2 exposure in different oxygen backgrounds. It was shown that, according to the composition of the ambient atmosphere, the conduction changes from the case in which it is controlled by the surface depletion layers to a situation in which the main contribution comes from free charge transport in the surface accumulation layer. This is significant for the interpretation of work function changes measurements results because the relation between the different measured electrical resistance and surface band bending depends on the conduction model. Furthermore, the CO sensing mechanism dependence on the oxygen amount in the ambient was explained.

The driving forces for organic and printed electronics are the display and lightning, solar cell, battery and electronics (e.g., RFID) industries1. The complete technology chain is being established in the fields of materials preparation, processing and characterization equipment, and production. Over the last decade, there has been a significant increase in the efforts dedicated to the development and implementation of electronic components on flexible and stretchable substrates for other types of application, such assensing, and notable results have been obtained by different research groups2-8. This technology could result in sensors being introduced to new settings, by significantly reducing their production cost and by adding new functionalities.

A model for the detection of CO in the presence of humidity is proposed for thick porous film gas sensors based on p-type CuO. The sensing mechanism is investigated by means of simultaneous DC electrical resistance and work function changes measurements combined with appropriate modeling of the conduction in the polycrystalline sensing film. The experiments were performed at 150 °C in dry and humid air backgrounds. The conclusion is that, very similarly to the case of undoped SnO2, the explanation of the cross-interference of water in the CO detection is the fact that both react with pre-adsorbed oxygen ions.

Undoped nanosized In2O3 with n-type conduction was produced in both polymorphic forms (cubic and rhombohedral) and deposited by screen-printing as thick films. These films show high sensitivity to low O3 concentration levels. They have been investigated by four point conductance and Hall effect measurements under sensor operating conditions (elevated temperature and ozone exposure). The effective values of the charge carrier concentration and mobility have been calculated from the experimental records using the recipe for the single crystals. The response to O3 is discussed in the frame of the standard models for gas sensors. The observed deviations from the model are explained in connection with the film crystalline structure and microscopic parameters spread.

Temperature, gas and humidity resistive/capacitive sensors on plastic substrates, suitable for mobile applications, like smart RFID tags, have been produced and investigated. The sensor concept is providing simple, versatile and low power solutions for temperature, humidity and gas detection. The device structure was devised together with the data evaluation strategies based on the latest generation ΣΔ analog (resistance and capacitance) to digital converters. The possibility of developing gas sensors on humidity sensitive substrates, having temperature corrected responses is demonstrated. The proposed sensor is aimed to evolve towards “flexible and full plastic” implementations.

This paper investigates the conduction mechanism in Cr2O3 gas sensing thick films. A citrate combustion method was used for the preparation of the metal oxide and ethanol vapour as a test gas. The manner in which surface reactions induced electrical changes are affecting the sensor signals inputs was explored by simultaneous dc and work function changes (Kelvin probe method). The identification of the contributions to conduction of the different sensing layer elements was made possible by ac impedance spectroscopy measurements. A conduction model, which qualitatively explains the experimental findings, was elaborated on the basis of the acquired experimental data and the information provided in literature. The model validity should apply, besides Cr2O3, to all p-type metal oxides used as gas sensitive materials.

Dry aerosol synthesis applying the flame spray pyrolysis was used to manufacture and directly (in situ) deposit tin dioxide nanoparticles on sensor substrates. For the first time this technique was used to synthesize a combination of two porous layers for gas-sensor fabrication. Two different sensing layers were deposited on ceramic substrates, i.e., pure tin dioxide and palladium-doped tin dioxide. The top layer was a palladium-doped alumina as a filter. The fabricated sensors were tested with methane, CO, and ethanol. In the case of CH4, the pure tin dioxide sensor with the Pd/Al2O3 filter showed higher sensor signals and improved 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, indicating high catalytic activity.

Highly reactive gallium oxynitride powders have been prepared by thermal nitridation under ammonia of a gallium oxide precursor synthesized via a soft chemistry process (citrate method). For the first time, thick film sensors were prepared from the oxynitride powders and subsequently compared to GaN thick film sensors prepared from nitride powders obtained by the conventional nitridation of commercial gallium oxide β-Ga 2 O 3 . The former showed large ethanol response in comparison to that of the GaN sensors in the 220–320 °C temperature range. The effects of water vapour and sensitive layer thickness on ethanol sensing were also examined. The sensing behaviour of the two materials and the influences of the two parameters – relative humidity and layer thickness – on the latter appeared to be quite different.

The role and activity of catalytic additives on solid-state gas sensors are determined by the additive chemical state, aggregation form and interaction with the semiconductor oxide. All these parameters depend on the technological steps involved in the element introduction and the treatments applied to the sensor material. The aim of this work is to analyse the influence of the additive introduction procedure on the gas sensor performance. In order to achieve this objective, two sets of different palladium, platinum or gold modified tin oxide materials have been prepared. In a first set of samples, additives were introduced by impregnation of the previously thermally stabilised oxide. In the second set, catalyst addition was carried out before any thermal treatment was applied. The study of the catalytically modified materials, calcined at different treatment temperatures between 250 and 1000°C, has been performed by means of HRTEM, XRD, XPS, and Raman spectroscopy. The influence of both processes on additive surface concentration, chemical state, nanoparticle growth and resistivity values are presented and discussed. Moreover, electrical characterisation of the sensors prepared from these materials has been carried out.

This review deals with gas sensors combining a metal oxide based sensing layer and a substrate realized by using micromachining. It starts by giving an overview of the design principles and technology involved in the fabrication of micromachined substrates examining thermal and mechanical aspects. Both kinds of micromachined substrates, closed-membrane-type and the suspended-membrane-type, are discussed. The deposition of the sensing layer is complicated by the mechanical fragility of the micromachined substrates. Different approaches used for the formation of the sensing layer such as thin film and thick film deposition techniques are reviewed. Finally, the gas sensing function of the sensitive layer is analyzed and various ways for extracting the information are presented with respect to the improvement of sensor performance brought by this new approach.

In order to clarify the role of the noble metal additives in the gas sensing mechanisms, three of the most common catalytic additives, such as Pd, Pt and Au, have been introduced in a sol–gel obtained tin oxide base material. The additives nominal weight concentrations used were 0.2% and 2%, and they were introduced in the precipitated tin oxide. A posterior calcination treatment was carried out, during 8 h, at the temperatures of 250°C, 400°C, 450°C, 600°C, 800°C and 1000°C. Structural and surface analysis of these nanopowders have been performed. Identification and localisation of metallic, 2+ and 4+ oxidised states of the used noble metals are discussed, and experimental evidences about their effects on the sensor performance are presented. Likewise, effects of their presence on the nanoparticle characteristics, and also on the material sensitivity to CO and CH4, are analysed and discussed.

The effects of the introduction of Pt and Pd by impregnation in sol–gel fabricated SnO 2 nanoparticles after calcination are reported in this paper. The differences in base resistance and sensitivity of sensors prepared using these powders are presented and explained — taking into account the chemical states of the metal additives and the generated surface states in the band gap of the SnO 2 .

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.

In2O3 thin films prepared by sol–gel method make it possible to detect low levels (several hundreds ppb) of nitrogen dioxide in air. The possibility of grain size control in indium oxide-sensing layers has been established by using of two preparation methods—electron beam evaporation (EB) and sol–gel technique (SG). SG-prepared samples show smaller particles sizes (down to 5 nm), higher state of agglomeration, higher sensor resistance in air and higher response to NO2 in comparison to EB samples. Sol–gel technique leads to the preparation of polycrystalline indium oxide with particle sizes of about 5–6 nm after calcination at 400°C and 20 nm after calcination at 700°C. The initial state of particle agglomeration in initial indium hydroxide sol (IHS), stabilized with nitric acid, influences the structure and surface morphology of the resulting indium oxide. While the In2O3 layer prepared by using low agglomerated IHS is smooth and porous, In2O3 layers prepared from highly agglomerated IHS consist of two regions—thin layer and crystallite agglomerates in cubic and rectangular parallelepiped form. The last shows the best results in terms of NO2 sensitivity. Sensor resistance and NO2 sensitivity increase with the decrease of the grain sizes in In2O3.

This chapter gives an overview of the conduction mechanisms in semiconducting metal oxide (SMOX) sensing films and their impact on the transduction of surface chemistry into a measurable sensor signal (relative change of resistance). Following general discussion on the functioning of sensors based on SMOX porous, sensing films, modeling of the conduction in p- and n-type materials is performed. The theoretical results explain why the sensor signals are lower for p-type oxides when compared with n-type oxides. The modeling concepts are verified by applying them to experimental results obtained with sensors based on both n- and p-type SMOXs with good results. In combination with the modeling results, further experiments in more realistic conditions (exposure to CO in humid air) demonstrate that also in these conditions the switch between conduction mechanism controlled by surface depletion layer to one controlled by surface accumulation layer may occur.

A comparison of the stability and sensitivity for two different sensor types (semiconductor SnO 2 devices and amperometric electrochemical sensors) has been performed. Sensitivities and drifts in the signal and in the background for various concentrations of CO have been studied for thick film SnO 2 sensors (undoped, Pt and Pd doped) over a period in excess of 8 months. Similar performance data have been recorded for amperometric sensors over a period in excess of 3 years. The two sensor types investigated here were also compared to well known commercial SnO 2 -based sensors at similar concentrations.

The influence of the calcination temperature and doping on the structural and electrical properties of SnO 2 nanopowders for gas sensors is investigated. The effectivity of grinding before and/or after calcination of the oxide will be discussed in terms of both structural modifications and electrical response of gas sensors.

Journal of Nanoparticle Research, 8 (6), ISSN:1388-0764, ISSN:1572-896X

A nondestructive methodology is reported to obtain the grain size distribution of SnO2 nanoparticles, with grain sizes below 10 nm, using low-frequency Raman scattering measurements. The position of the main low-frequency Raman bands depends on the grain size and can be correlated with the spheroidal vibration modes of a spherical elastic body with “rigid” boundaries. The grain size distributions deduced from Raman scattering are in agreement with the direct measurements from transmission electron microscopy, especially for the smaller grains.

An array of highly sensitive and mechanically stable gas sensors based on different sol–gel fabricated Pd-doped SnO 2 nanocrystalline thick films has been developed for the analysis of ternary mixtures in the concentration ranges of 0–100 ppm CO, 0–4000 ppm CH 4 and 0–50% relative humidity. The selectivity of the sensors has been modulated by varying the percentage of Pd content and the contacts geometry, while the use of micromachined hotplates as substrates for the sensors allowed a reduction of heater power consumption and a fast and accurate temperature control. Principal component analysis (PCA) as pattern recognition and principal component regression (PCR) as multicomponent analysis method have been used to analyze these mixtures qualitatively and quantitatively obtaining good results.

Thin and porous SnO2 films (70 nm thick with grain size between 10 and 30 nm) have been prepared by e-beam evaporation onto alumina substrate provided with platinum electrodes. The Ohmic character of the contacts was preserved in all measurement conditions utilized for investigations. The dependence of electrical conduction on the composition of the ambient atmosphere has been studied by means of Hall and four point conductance measurements. The experiments were performed in different gas atmospheres containing N2, O2, and CO and at different operation temperatures (between room temperature and 420 °C). A relatively low effective mobility (530 cm2 V1 s1) and a high charge carrier effective concentration (10181019 cm3) were deduced when using the single crystals recipe, as required by the established models for granular materials. The analysis of these experimental data showed the inadequacy of the geometrical models and effective medium theories to correctly extract the electrokinetic parameters from conductance and Hall measurements in the case of gas sensitive layers and to predict their temperature and gas composition dependences. The conventional approach fails because it considers the samples at different temperatures as one physical system while, in fact, the surface chemistry in oxygen atmosphere leads to new trap generation, which is equivalent to the doping level modification. The use of a nonconventional approach, taking into account the film interaction with the ambient through quasichemical equations, and associated mass action laws together with the surface scattering influence on the carrier mobility allowed for the understanding of the involved mechanisms and good fits for the experimental data.

Semiconductor sensors based on nanocrystalline In2O3 and MoO3–In2O3 thin films are found to be very sensitive to detecting low concentrations (100–200 ppb) of ozone and nitrogen dioxide. In this work, the sensitive layers were prepared by a sol–gel method. Mo-loading (MoO3–In2O3 samples) was performed by coprecipitation of In–Mo mixed hydroxides and subsequent drying and annealing (700°C, air). A simple adsorption model for target gases (NO2, O3) is proposed. According to this model O2− and O− are the predominant species at the In2O3 surface during the ozone interaction. NO2 interaction with In2O3 is dissociative and leads to the formation of atomic oxygen species at the surface.

We report on multi-sensor platforms on plastic foils for environmental monitoring. Polymer-based capacitive sensors for humidity and volatile organic compounds (VOC)s, semiconducting metal oxides (MOX) based chemoresistive sensors for reducing/oxidizing gases and a Pt thermometer have been integrated together on a polyimide sheet and their performances characterized. The MOX gas sensors exhibited good sensitivity to CO and ethanol. The differential operation of the capacitive humidity sensors resulted in increased signals and reduced response/recovery times.

A multisensor platform on plastic foil for environmental monitoring has been produced and its gas sensing performance, investigated. It is an array of conductometric metal-oxide (MOX) and capacitive polymer gas sensors integrated with a resistive platinum thermometer on a polyimide sheet substrate. The feasibility of simultaneous measurement of oxidizing and reducing gases, volatile organic compounds (VOCs), humidity and temperature has been demonstrated. MOX signals comparable with those of the devices realized on ceramic substrates have been obtained. Due to its structure, the platform is very versatile and, by using different sensor configurations and sensing materials, it allows the detection of a broad spectrum of gaseous analytes over wide concentration ranges. From the raw signals, temperature and humidity-corrected gas responses have been inferred which have been used for the calibration of the platform sensors. All the integrated devices were stable and gave reproducible signals for more than two months of operation, even when the MOXs ran continuously at 300 degrees C. The performed investigation proved the device concept viability and the reliability of its practical implementation. (C) 2012 Elsevier B.V. All rights reserved.

Interdigitated capacitive transducers have been inkjet printed onto flexible substrates and optimized for gas sensing applications. Their characteristics have been improved by tuning the annealing/sintering conditions and making use of additional passivation procedures, such as Ag electroplating with Ni or Parylene-C coating of the whole device surface. The as-prepared transducers printed with Ag ink could be utilized in capacitive gas sensing structures up to 70% relative humidity only. Higher humidity levels irreversibly altered the nominal capacitance and conductance of the devices, limiting their practical application. This drawback could be eliminated through the upgrading routes specified above and stable operation over wide temperature ranges has been achieved. From the acquired data a quite accurate description of the sorption processes, involving the printed conducting layers and the supporting material, has been inferred. Furthermore the parasitic sensitivity to test vapours from the class of Volatile Organic Compounds in the concentration range of Threshold Limit Value - Time-Weighted Average as possible interfering analytes has been estimated. Test gas sensors obtained from the optimized transducers by additional coating with gas sensing films poly(ether urethane) successfully passed the laboratory evaluations and seem to be appropriate for use in real devices. (C) 2012 Elsevier B. V. All rights reserved.

Starting with a hydrated tin dioxide powder of 3nm size obtained by the sol-gel method, the effects of thermal treatments and grinding on the final structural properties of the nanoparticles have been investigated. It is observed that two regions with different structural properties can be distinguished in the range of temperatures between 250 and 1000°C. In spite of their large grain size (>15nm), above 350–450°C, these nanocrystals can be used as precursor powders for gas sensors because of their higher stability. Grinding of the starting material before calcination will be proved as a suitable method to obtain the sarne type of stability with lower grain size.