Eng. Ovidiu FLOREA

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.

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 similar to 6 nm and mesoporosity for CP and a non-uniform morphology with larger nanoparticles of similar to 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 mu m and 100 mu m. 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.

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.

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 mu m and 30 mu m. 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 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.

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 cor-responding 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 degrees C and, alternatively, at 500 degrees C. Structural, morphological, and spectroscopic investigations were involved. As such, the XRD patterns showed a higher crystallinity degree for the NiO calcined at 500 degrees C. Such an aspect is in line with the XPS data indicating a lower surface hydroxylation relative to NiO calcined at 400 degrees 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.

The paper presents the possibility of detecting low H2S concentrations using CuWO4. The applicative challenge was to obtain sensitivity, selectivity, short response time, and full recovery at a low operating temperature under in-field atmosphere, which means variable relative humidity (%RH). Three different chemical synthesis routes were used for obtaining the samples labeled as: CuW1, CuW2, and CuW3. The materials have been fully characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). While CuWO4 is the common main phase with triclinic symmetry, different native layers of CuO and Cu(OH)(2) have been identified on top of the surfaces. The differences induced into their structural, morphological, and surface chemistry revealed different degrees of surface hydroxylation. Knowing the poisonous effect of H2S, the sensing properties evaluation allowed the CuW2 selection based on its specific surface recovery upon gas exposure. Simultaneous electrical resistance and work function measurements confirmed the weak influence of moisture over the sensing properties of CuW2, due to the pronounced Cu(OH)(2) native surface layer, as shown by XPS investigations. Moreover, the experimental results obtained at 150 degrees C highlight the linear sensor signal for CuW2 in the range of 1 to 10 ppm H2S concentrations and a pronounced selectivity towards CO, CH4, NH3, SO2, and NO2. Therefore, the applicative potential deserves to be noted. The study has been completed by a theoretical approach aiming to link the experimental findings with the CuW2 intrinsic properties.

Polycrystalline NiO thick film-based gas sensors have been exposed to different test gas atmospheres at 250 degrees 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.

In this work, (Ba0.75Sr0.25) (Ti0.95Co0.05) O-3 perovskite nanostructured material, denoted subsequently as Co-doped BaSrTiO3, was synthesized in a one-step process in hydrothermal conditions. The obtained powder was heat-treated at 800 degrees C and 1000 degrees C, respectively, in order to study nanostructured powder behavior during thermal treatment. The Co-doped BaSrTiO3 powder was pressed into pellets of 5.08 cm (2 inches) then used for thin film deposition onto commercial Al2O3 substrates by RF sputtering method. The microstructural, thermal, and gas sensing properties were investigated. The electrical and thermodynamic characterization allowed the evaluation of thermodynamic stability and the correlation of structural features with the sensing properties revealed under real operating conditions. The sensing behavior with respect to the temperature range between 23 and 400 degrees C, for a fixed CO2 concentration of 3000 ppm, highlighted specific differences between Co-doped BaSrTiO3 treated at 800 degrees C compared to that treated at 1000 degrees C. The influence of the relative humidity level on the CO2 concentrations and the other potential interfering gases was also analyzed. Two possible mechanisms for CO2 interaction were then proposed. The simple and low-cost technology, together with the high sensitivity when operating at room temperature corresponding to low power consumption, suggests that Co-doped BaSrTiO3 has a good potential for use in developing portable CO2 detectors.

A one-step method assisted by hydrothermal treatment was approached to obtain nanocrystalline 1 and 10 mol. % In doped 2 mol.% Pd-SnO2 powders using a non-ionic surfactant Brij52 and Polyethylene glycol 6000 (PEG) as templates. Depending on In content, the samples were labeled as Pd1InSn and Pd10InSn. The obtained materials consist of nanosized crystallites packed into micrometric grains with a high porosity, as revealed by the morphological and structural investigations (SEM, TEM). A dependence of the grain size with respect to the In content has been revealed i.e. the sample Pd1InSn was showing an average grain size of around 10 nm, whilst for the sample Pd10InSn the average grain size was found to be around 5 nm. The XPS investigations highlighted the differences occurred in the surface chemistry in terms of surface hydroxylation as well as the chemical states of Pd. The sensing properties towards different CO concentrations have been examined under infield background conditions, at low operating temperature of 50 degrees C. The sensing mechanism model for CO was discussed in detail according to the possible interplay between oxygen and water related species based on the experimentally results acquired through simultaneous electrical resistance and work function measurements.

Mesoporous CeO2:Mn3O4 materials (3:7 and 7:3 molar ratio) were prepared by co-precipitation and deposited as porous thick films over alumina (Al2O3) planar substrate provided with Pt meander. The aim was oriented towards detecting low levels methane (CH4) at moderate operating temperatures. Herein we demonstrated that the sensitivity of catalytic micro-converters (CMCs) towards a given peak of CH4 concentration corresponds to specific gas-surface interaction phenomena. More precisely, a transition from thermal conductivity to combustion rate is likely to occur when CMCs are operated under real atmospheric conditions (normal pressure, presence of relative humidity, and constant operating temperature). The response to CH4 was analyzed over different gas flows and different gas concentrations under the same operating regime. The materials were fully characterized by adsorption-desorption isotherms, H-2-Temperature Programmed Reduction (H-2-TPR), X-ray Diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning Electron Microscopy (SEM), and Raman spectroscopies. Thus, the applicative aspect of using CeO2:Mn3O4 as moderate temperature CMC for CH4 detection is brought to the fore.

Pd deposited on CeOx-MnOx/La-Al2O3 has been prepared as a sensitive material for methane (CH4) detection. The effect of different amounts (1.25%, 2.5% and 5%) of Pd loading has been investigated. The as prepared materials were deposited on Pt microcoils using a drop-coating method, as a way of developing pellistors operated using a Wheatstone bridge configuration. By spanning the operating temperature range between 300 degrees C and 550 degrees C, we established the linearity region as well as the maximum sensitivity towards 4900 ppm of CH4. By making use of the sigmoid dependence of the output voltage signal from the Wheatstone bridge, the gas surface reaction and diffusion phenomena have been decoupled. The pellistor with 5% Pd deposited on CeOx-MnOx/La-Al2O3 exhibited the highest selective-sensitivity in the benefit of CH4 detection against threshold limits of carbon monoxide (CO), sulfur dioxide (SO2) and hydrogen sulfide (H2S). Accordingly, adjusting the percent of Pd makes the preparation strategies of pellistors good candidates towards CH4 detection.

This study presents the synthesis and characterization of lanthanum-modified alumina supported cerium-manganese mixed oxides, which were prepared by three different methods (coprecipitation, impregnation and citrate-based sol-gel method) followed by calcination at 500 degrees C. The physicochemical properties of the synthesized materials were investigated by various characterization techniques, namely: nitrogen adsorption-desorption isotherms, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and H-2-temperature programmed reduction (TPR). This experimental study demonstrated that the role of the catalytic surface is much more important than the bulk one. Indeed, the incipient impregnation of CeO2-MnOx catalyst, supported on an optimized amount of 4 wt.% La2O3-Al2O3, provided the best results of the catalytic combustion of methane on our catalytic micro-convertors. This is mainly due to: (i) the highest pore size dimensions according to the Brunauer-Emmett-Teller (BET) investigations, (ii) the highest amount of Mn4+ or/and Ce4+ on the surface as revealed by XPS, (iii) the presence of a mixed phase (Ce2MnO6) as shown by X-ray diffraction; and (iv) a higher reducibility of Mn4+ or/and Ce4+ species as displayed by H-2-TPR and therefore more reactive oxygen species.

Nanoclustered Pd (2 mol%) was used to decorate Zn doped SnO2 (10 mol% Zn) in order to increase its sensing performances. Zn doped SnO2 built from nanoparticles was prepared by a hydrothermal method using a nonionic surfactant -Brij52 and Tripropylamine (TPA) as co-templates. The presence of well-dispersed Zn2+ ions in the SnO2 matrix leads to a nonstoichiometric surface. Pd was deposited by subsequent wet impregnation using hydrazine as reducing agent. The as obtained powders were deposited as thick layers onto commercial substrates, in order to obtain the sensitive structures. The coexistence of a mixture of valence states (Pd-0, Pd2+ and Pd4+) was highlighted on the surface of the as prepared layers. Several aspects have been followed regarding the Zn and Pd dispersion into the SnO2 matrix: the large scale and low scale morphology (SEM and TEM/HRTEM) in relation with the synthesis route, the obtained crystallographic phases (XRD, SAED) and the way in which the Zn2+ ions are inserted into the SnO2 structure (XRD, XPS, EPR), the spatial distribution of the added chemical elements, Zn and Pd (SEM, STEM, EDS). All these morphological and structural aspects, as well as the Pd surface chemistry, have been correlated with the sensing properties of the nanostructured materials under controlled gas atmosphere. Through this study, we could harvest the specific role of the aforementioned loadings towards selective detection of low NO2 concentrations, between 350 ppb to 5 ppm, at low operating temperature of 100 degrees C, for infield conditions.

Sensors based networked mesoporous SnO2 nanostructures templated by non-ionic surfactant - Brij (R) 35 were prepared via hydrothermal chemistry route. Specific patterns of the structure, morphology, surface chemistry and sensing properties were obtained by pH and autogenous pressure tuning. Consequently, the as-obtained SnO2 powders were subjected to extensive BET, Raman, TEM, HRTEM and XPS complementary investigations. The sensitive films were obtained by screen printing deposition of the powders onto commercial Al2O3 substrates. The gas sensing properties were assessed towards different hazardous gas species: CO, CH4, NH3, NO2, SO2 and H2S over a wide range of operating temperatures. Our particular SnO2 HP sensor synthesized at high autogenous pressure showed the highest selective-sensitivity to H2S (< 5 ppm) under 50% relative humidity (RH). The enhancement in the H2S sensitivity at low operating temperature under infield conditions was found to be closely connected to the morphological aspects and surface chemistry, peculiarities that can be assessed as consequences of the chemical tuning.

Nanostructured Cu mol. 5% doped perovskite material Ba0.75Sr0.25TiO3 was synthesized under hydrothermal conditions and further on deposited via RF sputtering technique onto commercial Al2O3 substrates provided with Au interdigital electrodes and Pt heater. The obtained thin films have been analyzed by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM, TEM), X-ray photoelectron spectroscopy (XPS), Differential Scanning Calorimetry-Thermogravimetry (DSC-TG) and electrical investigations. The H2S (5-90 ppm) gas-surface interaction at moderate operating temperature (T-op = 250 degrees C) associated with the interplay between the pre-adsorbed oxygen species and surface dipolar hydroxyl groups, has been highlighted by simultaneous work function and electrical resistance measurements. By correlation with the XPS spectra, the dominant role of surface hydroxylation was revealed and the subsequent gas sensing mechanism under operando conditions has been addressed. (C) 2018 Elsevier B.V. All rights reserved.

SnO2 loaded with 1 wt.% NiO based gas sensors, have been tested under humid background towards different H2S concentrations when operated at 350 degrees C. Knowing that water dissociation takes place, leading to the formation of strongly bonded surface hydroxyl groups, usually hindering the subsequent gas detection, it is of highly importance to understand the role of NiO as "OH groups trapper" element. By using complementary phenomenological investigations (simultaneous electrical resistance and work function measurements) we could emphasize the role of loading element in mitigating the water effects, further reflected through the different H2S interaction mechanisms. As such, the exposure of the SnO2 - 1 wt.% NiO sensors to different relative humidity levels was mirrored through a decrease in surface band bending due to the water donor effect and to a minor increase in the electronic affinity explained by the low adsorption of surface OH groups.

NiO/mesoporous SnO2 was deposited by incipient wetness impregnation of the SnO2 powder prepared by hydrothermal synthesis route templated by Brij (R) 35. The sensing properties were acquired and the dependence on the operating temperature of the sensitive material was pointed out. Sensitive selectivity towards H2S detection was highlighted at 350 degrees C.

Mesoporous SnO2 prepared by a hydrothermal synthesis route assisted by the ionic surfactant Cetyltrimethylammonium bromide, has rutile-type tetragonal symmetry, small homogeneous nanocrystallite size of similar to 4 nm and good thermal stability. Porosity analysis revealed high surface area similar to 127 m(2)/g and a narrow pore size distribution, with an average pore diameter similar to 4 nm. The mesoporous structure is likewise advantageous towards enhancing the surface reactivity and subsequent gas sensing performances. The role played by the surface hydroxylation on the NO2 sensing mechanism was discussed with respect to the associated photoelectron spectral components. Under humid air, associated with the in-field conditions, the highest sensitivity was attained at 150 degrees C, were the sensor signal towards NO2 is 4 times higher than the one recorded in dry air. This feature has been experimentally demonstrated by simultaneous electrical resistance and work function changes measurements conducted in the range of 400-5000 ppb NO2. (C) 2016 Elsevier B.V. All rights reserved.