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The current pandemic situation gives us the opportunity of the following research, which is a plea to use the principle of proportionality as a rational argument in substantiating the restrictive measures adopted by the authorities in order to prevent the spread of Sars-CoV-2 virus. Thus, after (1) we found it appropriate to make general considerations related to governmental measures for public health and safety in the introductory part, we continued with (2) the presentation of the proportionality standard, as a jurisprudential model used both at national and international level, in order to develop (3) proportionality in a narrow sense afterwards, using the formula of professor Robert Alexy, which we consider representative for the structure of balancing competing rights, using a quantifiable model, in which respect we have exemplified the applicability of this formula (4). Finally, we concluded that the principle of proportionality can be a convincing structural approach in the legal reasoning designated to promote the balance of principles (6), although we acknowledge some of the limitations of the proposed method, which we consider that can be overcome (5).

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.

Insufficient hand hygiene compliance of Health care workers is a widespread problem in medicine leading to several preventable infections each year. Hence, World Health Organization (WHO) presumes, that up to 1 in 10 patients acquires a Health care associated infection (HCAI) while receiving treatment in health care facilities. Apart from hand hygiene adherence and compliance, a correct hand disinfection technique regarding the amount of hand rub and exposure time is crucial for hand hygiene efficacy. Since alcohol-based hand rub is prevalent in the medical sector, the present study presents a new method for anonymous, electronic hand hygiene monitoring with commercial Ethanol gas sensors. In contrast to previous attempts of electronic monitoring, which mostly merely count the amount of hand hygiene actions, the present approach exploits different algorithms of Machine Learning to assess and categorize the hand disinfection technique into correct and incorrect. To demonstrate functionality of the mentioned approach, labeled hand disinfection data was collected in a pilot study over the period of one year with two different sensor setups. In detail, on the one hand a commercial portable gas sensor tag is used, which can be directly attached to the surgical gown. On the other hand, the performance of several commercial gas sensors is investigated, which are stationarily mounted next to the hand rub dispenser station. The present study follows three subgoals: Firstly, suitability of the stationary vs. portable setup shall be assessed. Secondly, the adequacy of different commercial gas sensor types is evaluated and thirdly the performance of possible algorithms for time series classification of the given sensor response is investigated, i.e. Support Vector Machines (SVM) as well as different Deep Learning algorithms, such as Multilayer Perceptron (MLP), Fully Convolutional Neural Networks (FCN) and a Bidirectional Long-Short-Term Memory network (Bi-LSTM). It could be found that the stationary setup is significantly superior compared to the portable setup. For most of the sensors within this stationary setup an accuracy score between 85% and 87% could be obtained for classification of correctly and incorrectly performed hand disinfections.

Our previous studies demonstrated that rare-earth oxycarbonates Ln

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.

A “new” gas sensing material; Co 3 O 4 , which is mainly known as a good catalyst for CO combustion, was investigated by means of resistance and catalytic conversion measurements. A systematic screen of sensors with sensitive layers of undoped, 0.2 wt% Pd- and Pt-doped Co 3 O 4 was done by variation in temperature, humidity, test gas, and test gas concentration. It was found that all three kinds of materials show best response to EtOH (sensor signals up to 28,000 @ 100 °C, 0% r.h., 90 ppm), good response to CO (sensor signals up to 100 @ 100 °C, 0% r.h., 50 ppm), but only a marginal response to CH 4 exposure, in particular at 50% r.h. The occurrence of maxima in sensor signals at 50% r.h. indicates a change in the surface reaction mechanism for both EtOH (175–200 °C) and CO exposure (125–150 °C). The 0.2 wt% Pt-doped sensors display superior gas sensing properties concerning recovery times and sensor signal height as well as a higher overall catalytic conversion. Additionally, at 0% r.h. and under exposure to CO a comparison with the benchmark material SnO 2 @ 300 °C reveals even higher sensor signals for Co 3 O 4 @ 100 °C.

Metal (TiO and Zn) phthalocyanines that are not currently investigated for gas sensing applications have been deposited by thermal vacuum evaporation as thin layers and evaluated in operando Kelvin Probe setup as NO2 sensing materials. From the raw data their calibration curves have been inferred and some sensing parameters determined. Comparative conductometric evaluations have also been performed. On the basis of the experimental information the main peculiarities of the gas responses have been identified and discussed.

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.

The paper reviews the main issues concerning the work function changes in gas sensitive materials, giving few representative examples. Starting from an historical perspective, the most relevant field effect gas sensing approaches are presented and discussed in connection with their proven practical or possible applications. The specific features of both, conducting and insulating gas sensing materials, are summarily analysed and compared with the goal of advancing towards a better understanding of the mechanisms responsible for the field effect gas sensing.

The problem of a fair and effective voting system in the EU deserves particular emphasis, due to the fact that several issues to be agreed upon concern matters of crucial national interest. The aim of this paper is to assess, in the enlargement context, the advantages and disadvantages of different voting systems within the EU, studying the statistical distributions of the rate of winning and blocking coalitions on the basis of a simulation model. The different scenarios of the voting procedures’ results confirm the positive effects of the Penrose square root rule, which remains the only valid voting system to be adopted in the future for the main EU decision-making bodies.

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.

The sorption of water vapour and gaseous ammonia into polyacrylic acid (PAA) induces a mass uptake and changes the electrochemical properties of the material. In order to review and confirm phenomenological models describing these processes, systematic infrared (IR) studies have been performed. The IR spectra are taken in the 400–4000 cm−1 wavenumber range and the IR spectrum of the polymer in dry air is taken as the reference for calculation of the absorbance spectra. The interpretation of increasing and decreasing bands in the IR spectra draws a consistent picture of the processes in the polymer, so that the models can be confirmed and, additionally, more detailed information is gained.

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.

The paper provides a critical assessment of the progress made in the field of gas sensing metal oxide semiconductors (MOX) based on the Hall effect measurement as the main experimental data source. At the beginning, the focus is on the morphologic and the functional peculiarities of this type of material that differentiate them among the larger class of polycrystalline, granular and porous semiconductors. On this basis the complications in defining and extracting the electrical parameters of MOX, especially under operating conditions, are discussed. Different experimental solutions addressing the Hall effect measurements on gas sensing MOX reported in the literature are then briefly presented as the foundation of the discussions about the most utilized modelling procedures. Two main trends are identified: the solid-state and the chemo-physical ones. The solid-state methods focus on the crystallite energy structure, introducing the external influences as model parameters describing the interface trap distribution. The chemo-physical models firstly give a physical picture of the chemical interaction of the ambient with the solid-state surface by connecting the target gases partial pressures with the surface trap concentration and energy and, afterwards, follow the formalism used by the solid-state models. For each type of approach, representative examples and applicative illustrations are considered. An extended overview of the accessible data is included at the end of the paper in a tabulated form.

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.

Semiconducting In2O3 gas sensors have been fabricated by two different deposition techniques, i.e., spin-coating and screen-printing. In both cases the same starting material – sol-gel-prepared cubic In2O3 – was used for the deposition in order to ensure a better comparability of the different sensing layers. The morphology of the layers has been characterized using scanning electron microscopy (SEM) technique. The layers deposited by different methods show similar grain size and porosity. Furthermore, Dc electrical tests have been performed to analyze the sensing properties of the different gas sensors. Reducing gases (CO and propanal) as well as oxidizing gases (NO2 and ozone) were used as test gases in the background of dry and humidified synthetic air. All measurements were performed at several temperatures. It was found that the spin-coated and screen-printed layers show different sensing properties, i.e., screen-printed sensors showed higher sensor signals than spin-coated sensors for CO, propanal, ...

The interaction of oxygen and nanocrystalline SnO 2 has been investigated by means of simultaneous work function change and resistance measurements. Various oxygen concentrations between 50 ppm and 6000 ppm have been dosed at two different temperatures. At 400 °C, only ionic species have been identified. However, at 200 °C dipolar species play the major role reflected by a massive decrease in electron affinity.

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.

Simultaneous work function change and conduction measurements have been performed on undoped SnO 2 thick film sensors in order to investigate the surface reactions of CO under dry and humid conditions. On their basis, changes in the electron affinity have been observed for the reaction with water and the reaction with CO in the background of water. Additionally, by comparing the results for the changes in band bending, one can conclude that both water and CO compete for the same reaction sites, i.e. preadsorbed oxygen species. The kinetics of work function and band bending changes show that in the case of humid air more than one dipole species is involved in the reaction with CO.

Sensing of propane with SnO2-based thick film gas sensors was studied by using a combination of experimental techniques. In order to assign reaction pathways, direct IR surface spectroscopic measurements (DRIFT) were performed. For a better understanding of the full picture, the spectroscopic information was completed with catalytic conversion and electrical measurements. We observed that during propane interaction with SnO2 the surface ionic species play a crucial role. The results indicate that: the C3H8 dehydrogenation is initiated by hydrogen abstraction over a surface acid–base lattice pair site (Sn4+O2−) while its further transformation is caused by reaction with adsorbed oxygen ions and with surface hydroxyl groups. In parallel with the rupture of the CH and/or CC bond, the ionic carboxylate species are formed and through further oxidative dehydrogenation, forms CO2− and CO32− surface species. The carbonates and carboxylates are finally desorbed as CO2(g).

The conversion of CO and different hydrocarbons by palladium-doped tin oxide sensors was studied in conditions with low oxygen and low water concentration for temperatures between 200 and 400 °C. Although, under normal conditions, oxygen plays an important role in the detection of these reducing analytes, relatively large sensor signals could be observed. For most conditions, carbon dioxide and water could be observed as reaction products even at low oxygen content.

The paper investigates the gas response of nanocrystalline SnO2 based thick film sensors upon exposure to carbon monoxide (CO) in changing water vapour (H2O) and oxygen (O2) backgrounds. The sensing materials were undoped, Pt- and Pd-doped SnO2. We found that in the absence of oxygen, the sensor signal (defined as the ratio between the resistance in the background gas, R0 and the resistance in the presence of the target gases, R, namely R0/R) have the highest values. These values are higher for doped materials than for the undoped ones. The presence of humidity increases dramatically the sensor signal of the doped materials. In the presence of oxygen, the sensor signal decreases significantly for all sensor materials. The results indicate that there is a competitive adsorption between O2 and H2O related surface species and, as a result, different sensing mechanisms can be observed for CO.

This paper investigates the effect of water vapour in CO sensing by using Pd doped SnO2 sensors realized in thick film technology as an example of the basic understanding of sensing mechanisms applied to sensors. The results of phenomenological and spectroscopic measurement techniques, all of them obtained under working conditions for sensors, were combined with modelling in order to derive conclusions able to be generalized to the field of metal oxide based gas sensors. The techniques employed were: dc conductance, ac impedance spectroscopy, work function (by using the Kelvin probe method), catalytic conversion and diffuse reflectance infrared Fourier transform measurements. The most important conclusion is that the different parts of the sensor (sensing layer, electrodes, substrate) are all influencing the gas detection and their role has to be taken into consideration when one attempts to understand how a sensor works.

The catalytic conversion of methane, propane and toluene by palladium-doped tin oxide sensors was measured in dry and humid conditions for several temperatures between 200 and 400 °C. To distinguish between the consumption caused by the sensitive material and the consumption caused by the transducer, additional measurements were performed with substrates equipped with electrodes and heater only. For all gases, the presence of humidity decreases both consumption and sensor signal. For methane and propane, no consumption by the substrate and electrodes/heater was found and the sensor signals are, in comparison with toluene, low. In humid conditions, there is a direct correlation between the consumption by the sensitive material and the sensor signal towards toluene. In dry conditions, this correlation is more complex and indicates a more complex interaction path.

An optimized fuzzy inference system for carbon monoxide and methane concentration estimation is presented and compared to the three most common linear methods: PLS, PCR and MLR, and also to nonlinear extensions of PLS. The system optimization includes: rule pruning, membership function optimization by Solis–Wett algorithm, rule consequents optimization and sensor selection by sequential floating feature selection (SFFS) algorithm. An extensive data set obtained from a sensor array composed of five metal oxide gas sensors operated at two working temperatures in different humidity conditions is used for the method evaluation. Advantages and drawbacks of both linear methods and fuzzy systems are discussed and compared.

It is shown that a single thermally-modulated tin oxide based resistive microsensor can discriminate between two different pollutant gases (CO and NO2) and their mixtures. The method employs a novel feature-extraction and pattern classification method, which is based on a 1-D discrete Wavelet transform and a Fuzzy ARTMAP neural network. The wavelet technique is more effective than FFT in terms of both data compression and drift rejection. Furthermore, Fuzzy ARTMAP networks lead to a 100% success rate in gas recognition in just 2 training epochs, which is significantly lower than the number of epochs required to train the back-propagation network.

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.

Arrays consisting of thick-film SnO2 sensors (based on three different materials and operated at two different temperatures) were exposed to mixtures of carbon monoxide (0, 3, 7 and 14 ppm), methane (0, 400, 1500 and 5000 ppm) and water (0, 30 and 50% RH at 25°C). It was possible to discriminate between different mixtures of CO and CH4 in the presence of different relative humidities in air. In addition, it was possible to quantify both CH4 and CO in the background of the other by using multivariate data analysis.

The surface interaction between differently prepared tin oxide samples and CO was studied by Diffuse Reflectance Infrared Fourier Transform (DRIFT)-spectroscopy at room temperature. As samples, commercial powders (Merck) and home-made powders obtained by a sol–gel preparation were used. Several bands corresponding to isolated and rooted OH groups, coordinated water and condensed water in two forms: ordered layers of water molecules and water in the liquid form, were found. Their appearance and intensity depended on the sample origin and storing conditions. Moreover, the spectra of all samples show bands corresponding to different hydrated protons (H3O+, H5O2+) and carbonate ions of different intensities. To get information about the interaction between SnO2 powders and CO, in-situ measurements were conducted. We found that differently bound water and specific surface OH-groups react with CO and that surface carbonate ions dissociate because of acidic intermediate products. As a result, different bands corresponding to physisorbed CO appear at approximately 2200 cm−1, together with changes in the region corresponding to covalent bound chelating and bridging bidentate carbonates and carboxylate (between 1700 and 1000 cm−1).

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.

Nanocrystalline thick-film SnO2 sensors with different dopants were fabricated by an optimized screen printing process and subsequent annealing. Powders were used as starting materials which were prepared by a wet chemical process from SnCl4. Microanalysis was performed of both, the precursors and the final sensor materials with their different annealing conditions. Gas sensing tests with CO, CH4 and NO2 in air with controlled humidity were correlated with results from X-ray photoemission spectroscopy (XPS), Raman spectroscopy and transmission electron microscopy (TEM). As an interesting result, the distribution of the transition metal dopants Pd and Pt (as deduced from TEM and XPS data) rules out the existence of metallic clusters or even atoms in the metallic state at the surface. This finding does not allow to explain the sensor effects on SnO2 based materials as usually done by means of spill-over effects or Fermi energy control.