This study presents the development and analysis of a highly sensitive SnO2-based gas sensor for acetone detection, enhanced through the wet impregnation with WO3. Structural and morphological investigations confirm the tetragonal structure of the SnO2 crystalline phase and reveal irregular grain boundaries that become less pronounced with increasing WO3 content. Surface chemistry analyses confirm the presence of well-dispersed WO3 on the SnO2 surface. Thick and porous sensing layers were fabricated by screen printing onto commercial alumina substrates. The influence of operating temperature on surface interactions was evaluated by measuring the sensor response at a fixed acetone concentration. This enabled the selection of SnO2:WO3 5 % for sensing measurements, at an operating temperature of 280 degrees C, over a wide range of acetone concentrations, dosed in dynamic airflow with variable relative humidity. Simultaneous DC electrical resistance and Contact Potential Difference measurements allow decoupling of ionosorption from dipolar effects on the SnO2:WO3 5 % surface and explain the rise in the sensor response to acetone as relative humidity increases. The chosen concentration range for acetone, between 0.25 and 5 ppm, and for relative humidity, between 10 and 90 %, creates the premises for the further development of applications for non-invasive monitoring of diabetic exhalation.

The morpho-structural and defect properties of SnO2 nanoparticles, obtained by hydrothermal synthesis at 120 degrees C, 140 degrees C and 160 degrees C, using a SnCl2 precursor, were comparatively investigated and correlated with their NO2 sensing performance for in-field conditions. The constructive contributions of the nanoparticle size, faceting and oxygen vacancy concentrations had a positive effect on the sensor performances for the two samples synthesized at lower temperatures. These samples had almost similar, smaller size and the proportion of the more active, higher-index facets over the {110} facets was significantly larger than for the sample prepared at 160 degrees C. The concentration of paramagnetic defects, associated to complexes of oxygen vacancies in the (101) planes at the SnO2 surface, increased with the synthesis temperature decrease. A sensor signal of 74 for the NO2 detection limit of 3 ppm, at the operating temperature of 100 degrees C, under dynamic air flow with in-field-like relative humidity of 50 %, was obtained for the sample grown at 120 degrees C. The sensor signal was about four times higher compared to the 140 degrees C sample with similar size and morphology and about nine times higher than in the case of the 160 degrees C sample. In addition to its high NO2 sensitivity, the 120 degrees C sample had a low sensor response for potential interfering gases as CH4 and CO2 and was relatively stable over a period of 20 months. Our results evidence the direct correlation between the sensing properties and the surface oxygen vacancy complexes and highlight the importance of an in-depth atomic-level investigation approach for the controlled synthesis of an application-oriented material.

The tuning sensitivity towards CO2 detection under in-field-like conditions was investigated using SnO2-sensitive material deposited onto Al2O3 substrates provided with platinum electrodes with interdigital gaps of 100 µm and 30 µm. X-ray diffraction, low-magnification and high-resolution transmission electron microscopy, and electrical and contact potential difference investigations were employed to understand the sensing mechanism involved in CO2 detection. The morpho-structural analysis revealed that the SnO2 nanoparticles exhibit well-defined facets along the (110) and (101) crystallographic planes. Complex phenomenological investigations showed that moisture significantly affects the gas sensing performance. The experimental results corroborated the literature evidence, highlighting the importance of Pt within the interdigital electrodes subsequently reflected in the increase in the CO2 sensing performance with the decrease in the interdigital gap. The catalytic efficiency is explained by the distribution of platinum at the gas-Pt-SnO2 three-phase boundary, which is critical for enhancing the sensor performance.

The outstanding properties exhibited by the p-type NiO nanostructures can be greatly affected by morpho-structural and defect characteristics with constructive or competing effects. We have conducted an in-depth study on NiO nanoparticles obtained by hydrothermal synthesis and submitted to various thermal treatments, to monitor the evolution of their structural properties and the effect of the thermal history on their CO sensing. Correlated electron paramagnetic resonance and analytical transmission electron microscopy investigations evidenced an amount of up to 1 % metallic nickel clusters close to surface in the NiO nanoparticles calcined at 400 degrees C and 500 degrees C for 8 h. Subsequent annealing in vacuum and in air of the sample calcined at 400 degrees C resulted in different size distributions and morphology of the NiO nanoparticles and an increase/decrease of the nickel phase, respectively. Comparative CO sensing tests on the two pristine samples and on the sample calcined at 400 degrees C and further annealed in air at 500 degrees C for 2 h showed an increase in the baseline resistance of the later due to the decrease of free charge carriers induced by the dissolution of the nickel clusters. The overall CO sensing results show a strong dependence on the samples thermal history.

This work investigates the conduction mechanism of hydrothermally grown Gd2O3-sensitive material in order to explain its electrical resistance behaviour when exposed to increasing concentrations of CO2 under in-field conditions. To achieve this, the experimental investigation began with X-ray photoelectron spectroscopy of the Gd2O3 microstructure to verify the oxidation states of the surface. Subsequently, the impact of constant atmospheric factors such as oxygen and relative humidity on the electrical resistance of the Gd2O3 layer was examined. Finally, a progressive dosing of CO2 concentrations ranging from 400 to 3000 ppm was conducted. The DC electrical resistance measurements were performed using a computer-controlled Gas Mixing System operated under a dynamic gas flow regime. Experimental data was validated using the Boltzmann distribution statistics and the grain-to-grain Schottky barrier model. The results highlight the preservation of the n-type semiconductor behaviour of Gd2O3 irrespective of the background relative humidity and bring the oxidising character of CO2 to the fore.

This work presents the ability of WO3-based sensors to detect low traces of acetone, specifically within the range of 0.25-5 ppm, specific to the in -field atmosphere. The WO3 powder was synthesised through the hydrothermal method. Morpho-structural investigations showed a monoclinic structure and a good crystallization of the WO3 powder, containing well -grown and faceted grains along low -index crystallographic planes. The paste obtained by mixing the powder with propanediol was screen -printed as a thick layer onto commercial alumina substrates, obtaining the chemical sensors. A dynamic computer -controlled Gas Mixing System was utilized to ensure controlled airflow with variable relative humidity and acetone concentrations. The sensor response was explained based on physico-chemical equations, taking into consideration pre -adsorbed species of oxygen and water, both of which are relevant constituents of atmospheric conditions. The results highlight the applicative potential of WO3, having a good signal-to-noise ratio in relative humidity conditions up to 90% and a pronounced sensitive selectivity to acetone.

This study investigates the influence of synthesis methods and electrode geometry on the physico-chemical properties of 5%Gd-doped SnO2. Two distinct synthesis routes, co-precipitation and hydrothermal growth, were employed, resulting in powders denoted as SnO2: Gd 5%-CP and SnO2: Gd 5%-HT. Morpho-structural and textural analyses reveal a uniform morphology consisting of quasi-spherical nanoparticles with dimensions of ~6 nm and mesoporosity for CP and a non-uniform morphology with larger nanoparticles of ~42 nm, with irregular shapes and macroporosity for the HT sample, respectively. The powders were deposited onto alumina substrates equipped with platinum interdigital electrodes with alternative gaps of 200 μm and 100 μm. The back-side heater allows for variation in the temperature of the layer. Sensing properties assessed under in-field-like atmospheres simulated by a computer-controlled Gas Mixing System reveal higher sensitivity to methane compared to carbon dioxide. Although the sensor signals did not differ quantitatively, they exhibited distinct saturation tendencies with an increasing methane concentration, attributed to the morpho-structure and porosity induced by the synthesis method. Differentiation was achieved by varying the interdigital gap of the electrodes, highlighting different sensor signals and conduction mechanisms, determined by the specific size of the crystallites.

The response of nickel oxide gas sensors towards CO and H2 and the underlying gas sensing mechanisms were investigated with special focus on the influence of ambient humidity interference. Surface reactions were tracked by using diffuse reflectance infrared Fourier transformation spectroscopy with simultaneous resistance measurements. The sensor response to both gases is barely influenced by the background humidity. Spectroscopic results reveal that the underlying processes at the surface are almost identical for CO and H2 reception and similar to the effect of the removal of oxygen. Accordingly, the detection of the analytes is based purely on the reduction and oxidation of the oxide material instead of the formation of analyte specific surface species.

In this study, we report the implications of the synthesis method on Gd2O3 sensitivity to CO2. The rare-earth oxide was prepared by wet chemical co-precipitation and by hydrothermal method. The obtained powders labelled Gd2O3-CoP and Gd2O3-HT were deposited as thick films over commercial Al2O3 substrates provided with Pt electrodes and a back-side heater. Both powders consist of the same crystallographic phase, with a significant difference appearing in selected area electron diffraction patterns, transmission electron microscopy images at higher magnification and X-ray diffraction spectra, with respect to the crystallization degree. The associated role in sensing properties is revealed via electrical resistance variations determined by CO2 concen-trations in the range between 400 and 3000 ppm and variable relative humidity between 0 and 50%RH, similar to the in-field atmosphere. The proposed CO2 interaction mechanism is based on phenomenological in-vestigations which highlight the electronic affinity variation as the effect of dipoles induced by the in-field conditions on the Gd2O3 surface.

The effect of interdigital gap distance on the sensing performance was evaluated towards carbon monoxide target gas with p-type NiO-based gas sensors. Both base electrical resistance and sensor signal increase with the interdigital gap distance increase. While the former is a direct consequence of the high resistive bulk component, the latter was attributed to the decrease in the gas-insensitive metal-semiconductor contacts. A numerical simulation of the sensor signal concerning the interdigital gap and grain size revealed the “dead” role played by the metal-semiconductor contacts in the overall gas sensing performance.

Chemical sensors translate charge exchange chemical interactions into electrical resistance variations. The interactions are modulated by the operating temperature and facilitated by doping the base material, which is generally a metal oxide semiconductor. In the present study, we doped SnO 2 with Gd to increase the vacancies number and make it more reactive. The detection potential of greenhouse gases is investigated by modulating the operating temperature detection of sensors based on $\mathrm{S_{1-x} G d_{x} O_{(4-x) / 2}}$ thick layers.

In this study, two alternative synthesis routes have been used in obtaining gas-sensitive NiO materials. The structural and morphological aspects were systematically investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM), revealing significant differences further mirrored in their sensing performances. Simultaneous electrical resistance and contact potential differences have been involved aiming to decouple the energetic contributions: work function (ΔΦ), surface band bending (qΔVs) and electron affinity (Δχ). Two sensing mechanism scenarios explained the enhancement and downgrading in the sensor response to carbon monoxide (CO) concerning the synthesis strategies. The role of relative humidity (RH) was considered throughout the electrical operando (in-field) investigations.

NiO sensitive material was prepared by using a hydrothermal synthesis route using ethylenediaminetetraacetic as structural agent. The sensing properties towards NH3 detection were acquired under real operating conditions by means of variable relative humidity. The gas sensing interaction mechanism was brought to the fore by means of simultaneous electrical resistance and work function measurements.

The paper aims to identify the CO2 interaction mechanism for chemical sensors based on Gd-doped SnO2, SnO2 and Gd2O3 powders deposited as thick sensitive layers. The low reactivity of CO2 conferred by the thermodynamic stability and chemical inertia can be offset by the presence of relative humidity. The sensitive powders were prepared by wet chemical co-precipitation method. The Gd concentration was varied from 1% to 20 at% in order to determine the limit for Gd integration as a doping ion prior to chemical segregation as a secondary phase. Analytical transmission electron microscopy points to a homogeneous Gd doping of the nanostructured SnO2 powders for low doping concentrations and the formation of a nanocomposite based on SnO2 as main phase and cubic Gd2O3 as secondary phase for the highly doped samples. The electrical resistance is either influenced by the density of oxygen vacancies, or is the result of compensation for two opposite behaviours into the SnO2-Gd2O3 nanocomposite structures. The CO2 exposure to humid atmosphere determines distinct behaviours corresponding to SnO2 and Gd2O3 as constitutive elements. The associated CO2 interaction mechanism is based on simultaneous DC electrical resistance and Contact Potential Difference measurements, which allow decoupling the ionosorption from the dipolar processes, thus highlighting specific chemical interactions on the SnO2 and Gd2O3 surfaces.

NiO-sensitive materials have been synthesized via the hydrothermal synthesis route and calcined in air at 400 °C and, alternatively, at 500 °C. Structural, morphological, and spectroscopic investigations were involved. As such, the XRD patterns showed a higher crystallinity degree for the NiO calcined at 500 °C. Such an aspect is in line with the XPS data indicating a lower surface hydroxylation relative to NiO calcined at 400 °C. An HRTEM microstructural investigation revealed that the two samples differ essentially at the morphological level, having different sizes of the crystalline nanoparticles, different density of the surface defects, and preferential faceting according to the main crystallographic planes. In order to identify their specific gas-sensing mechanism towards CO exposure under the in-field atmosphere, the simultaneous evaluation of the electrical resistance and contact potential difference was carried out. The results allowed the decoupling of the water physisorption from the chemisorption of the ambient oxygen species. Thus, the specific CO interaction mechanism induced by the calcination temperature of NiO has been highlighted.

This letter highlights the role of synthesis temperature over the morpho-structural properties of SnO2. Specific crystalline nanoparticles with quasi-tetragonal and quasi-hexagonal morphologies are faceted, suggesting a high reactivity to atmospheric oxygen. This is a premise for the sensing ability of SnO2 in detecting CO2. The in-field conditions are ensured by dynamic synthetic air flow with variable relative humidity, a wide range of CO2 concentrations and potential interfering gases at their specific detection limits.

Today's view on gas sensors end-users is more directed toward miniaturization, low power consumption, and intelligent device integration aiming to reply to several hot issues such as high sensitivity, optimum selectivity, fast response/recovery transients, and good long time stability [...].

Polycrystalline NiO thick film-based gas sensors have been exposed to different test gas atmospheres at 250 °C and measured via simultaneous electrical resistance and work function investigations. Accordingly, we decoupled different features manifested toward the potential changes, i.e., work function, band-bending, and electron affinity. The experimental results have shown that the presence of moisture induces an unusual behavior toward carbon monoxide (CO) detection by considering different surface adsorption sites. On this basis, we derived an appropriate detection mechanism capable of explaining the lack of moisture influence over the CO detection with NiO-sensitive materials. As such, CO might have both chemical and dipolar interactions with pre-adsorbed or lattice oxygen species, thus canceling out the effect of moisture. Additionally, morphology, structure, and surface chemistry were addressed, and the results have been linked to the sensing properties envisaging the role played by the porous quasispherical–hollow structures and surface hydration.

NiO-loaded SnO2 powders were prepared involving two chemical procedures. The mesoporous SnO2 support was synthesized by a hydrothermal route using Brij 35 non-ionic surfactant as a template. The nickel loadings of 1 and 10 wt.%. NiO were deposited by the wet impregnation method. The H2S sensing properties of xNiO-(1-x)SnO2 (x = 0, 1, 10%) thick layers deposited onto commercial substrates have been investigated with respect to different potential interfering gases (NO2, CO, CO2, CH4, NH3 and SO2) over a wide range of operating temperatures and relative humidity specific for in-field conditions. Following the correlation of the sensing results with the morphological ones, 1wt.% NiO/SnO2 was selected for simultaneous electrical resistance and work function investigations. The purpose was to depict the sensing mechanism by splitting between specific changes over the electron affinity induced by the surface coverage with hydroxyl dipoles and over the band bending induced by the variable surface charge under H2S exposure. Thus, it was found that different gas-interaction partners are dependent upon the amount of H2S, mirrored through the threshold value of 5 ppm H2S, which from an applicative point of view, represents the lower limit of health effects, an eight-hour TWA.

Sn1-xGdxO(4-x)/2 is prepared by a wet chemical co-precipitation method. Doping concentrations range from 1% to 20% to determine the solubility limit for Gd integration as a doping ion prior to chemical segregation as a secondary phase. Rietveld analysis highlights the presence of Gd2O3 as weak crystallization secondary phase even for 5 at. % Gd and analytical transmission electron microscopy points to a homogeneous Gd doping of the nanostructured SnO2 powders for low doping concentrations and the formation of a nanocomposite based on SnO2 as main phase and cubic Gd2O3 as secondary phase for the highly doped samples. The associated chemical maps reveal compositional inhomogeneities. The electrical measurements show the resistance behaviour when Gd doping level and CO2 concentration vary, highlighting the opposite sensing behaviour of SnO2 and Gd2O3 in the presence of moisture and their compensating effect at the heterojunction interface between SnO2 and Gd2O3 nanocomposite for 5 to 20%Gd doped SnO2 samples. The proposed CO2 detection mechanism is based on simultaneous DC electrical resistance and Contact Potential Difference measurements which allow decoupling the ionosorption processes from the dipolar interaction under in-field conditions, for SnO2 and Gd2O3 samples. The applicative aspects are completed by the selective sensitivity evaluation against potentially interfering gases.