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This review work reports the synthesis, the morphological, structural, spectroscopic, and electrical properties, the sensing performances of TixSn1-xO2 solid solutions, using pristine SnO2 and TiO2 as reference. Each characterization highlighted two behavioral classes for the solid solutions: SnO2-like and TiO2-like, being the oxide with x = 0.25 the borderline sample. Furthermore, two real applications of (Ti, Sn) solid solutions are described: i) in air quality monitoring to detect carbon monoxide (CO), ii) to detect the variation of analytes concentrations in samples of hydraulic fluid headspace collected during the system running.

In this work, a variety of semiconducting oxides were prepared and principally characterized by means of spectroscopic techniques (absorbance FT-IR, diffuse reflectance UV-Vis-NIR) to shed light on the electronic properties and defects involved at the roots of gas sensing capabilities. The thick films were obtained by screen printing technology on which electrical characterization and gas sensing measurements were performed. From the cross analysis of the results, a description of the specific sensing mechanism for each material is proposed.

Titanium dioxide (TiO2) is a functional semiconductor metal oxide that plays a major role in the many applicative fields, mostly including water remediation, surface functionalization and photoanodes for photoelectrochemical reactors. TiO2 is also one of the known chemoresistive materials that can be adopted as sensitive layers in solid state gas sensors. TiO2 has a peculiarly property in the fact that it exhibits two different photoluminescence (PL) spectra, depending on the crystalline polymorph considered (rutile or anatase), and that the PL emission of these two phases reacts differently as exposed to molecular oxygen (O2). We show some representative results of this phenomenon, discussing the possibility to exploit it actual applications.

Introduction Air pollution is a global threat having huge impact on human health and ecosystems [1]. In urban areas, the air quality can be considered as resulting from the overlapping of three different pollution components: regional background, urban background and hot spot events. Therefore, the continuous monitoring of these components is of critical importance. Indeed, the ability to separate the three components is the key to define reduction actions of atmospheric pollution linked to the related sources and for this reason, really effective. This work demonstrates that it is possible to determine, with reliability, sensitivity and selectivity comparable to those of the analytical instruments, the pollutants’ concentration in the atmosphere using specific advanced thick film gas sensors. They are based on nanostructured metal oxides (MOX sensors) which modify their surface properties when interacting with reducing or oxidizing gases [2]. Nowadays, monitoring is usually performed in few fixed stations and through mobile stations (where the conventional equipment is installed on vehicles) for temporary measurement campaigns or occasional environmental impact estimations. The equipment used in these stations is often complex, expensive and difficult to deploy, requiring controlled environmental conditions and frequent maintenance and calibrations [1]. Differently, systems based on MOX gas sensors constitute an advantageous solution because of their low-cost fabrication, maintenance, and installation, low power consumption, small dimensions and of easy deployment for a wide range of situations for example along heavy traffic roads, or in historic centers where fixed monitoring stations cannot be used due to their large size. Experimental The thick film gas sensors were fabricated through screen-printing technology. The sensing layers are based on specific functional materials synthesized in form of crystalline nanometric powders via wet chemistry processes [2]. A careful coordination of all different processes involved in the gas sensors realization, such as functional material preparation, deposition of sensing layers, substrates and electronics were done to optimize the sensors’ precision and accuracy. The arrays of thick film gas sensors were assembled in small remotely controlled units and used for on-site tests. A monitoring unit (see figure A) consists in a small box to lodge the sensors, the electronic circuitry related to each sensor and a main electronic control unit provided with serial connection through which the data are transferred to a GSM module which sends them to an input data server, on request. With the aim to validate the values of gas concentrations obtained through the array of MOX sensors, few monitoring units have been exposed to the real atmosphere beside fixed-site monitoring stations for air quality control. The sensor responses were studied in comparison with the pollutant concentrations as measured by the analytical instruments. In particular, IR and UV spectrophotometers for carbon monoxide and ozone, chemiluminescence analyzer for NOx and gas chromatograph for BTEX. Results and Conclusions The developed monitoring units based on different advanced thick film gas sensors have been successfully prepared with optimal level of reliability, stability, reproducibility and used to detect gaseous pollutants in the atmosphere with the same trend observed by the conventional analytical techniques for long monitoring periods, in different seasons and areas. As example, a laboratory experiment devoted to test the stability and repeatability of sensors is reported in the figure B. In the image, the figure B refers to the alternate signal of two sensors, nominally identical, to 0.1 and 0.8 ppm of benzene. The responses, defined as ratio between the conductance in presence of benzene and the conductance in air, were 1.2 to 0.1 ppm and 2.1 to 0.8 ppm of benzene for both sensors. A second experiment is shown in figure C: the ozone concentration measured in real atmosphere through a WO3 sensor was compared with the measurements carried out by means of a conventional analyzer (UV spectrophotometer). The data are expressed in both cases as hourly average obtaining a quite good agreement. The graph is relating to one month of monitoring, while, in the inset, a zoom on few days is shown. The innovative system, based on a specific MOX sensors array, is able to detect and quantify atmospheric pollutants constituting an advantageous solution for a wide range of situations, as for example along the roads of city centers or along heavy traffic roads, or in historic centers where fixed monitoring stations cannot be used due to their large size. References [1] E. von Schneidemesser, K. Steinmar, E. C. Weatherhead, B. Bonn, H. Gerwig, J. Quedenau, Air pollution at human scales in an urban environment: Impact of local environment and vehicles on particle number concentrations, Sci. Total Environ. 688 (2019) 691-700, doi:10.1016/j.scitotenv.2019.06.309 [2] M.C. Carotta, A. Fioravanti, S. Gherardi, C. Malagù, M. Sacerdoti, G. Ghiotti, S.Morandi, (Ti,Sn) solid solutions as functional materials for gas sensing, Sens.Actuators B: Chem. 194 (2014) 195–205, doi:10.1016/j.snb.2013.12.021. [3] S. Morandi, A. Amodio, A. Fioravanti, A. Giacomino, M. Mazzocchi, M. Sacerdoti, M.C. Carotta, A.N. Skouloudis, Operational functionalities of air-quality W-Sn metal-oxide sensors correlating semiconductor defect levels and surface potential barriers, Sci. Total Environ. 706 (2020) 135731, doi:10.1016/j.scitotenv.2019.135731. Figure 1

The proper operation of a fluid power system in terms of efficiency and reliability is directly related to the fluid state, therefore, the monitoring of fluid ageing in real time is fundamental to prevent machine failures. For this aim, an innovative methodology based on fluid vapor analysis through metal oxide (shortened: MOX) gas sensors has been developed. Two apparatuses were designed and realized: (i) A dedicated test bench to fast-age the fluid under controlled conditions, (ii) a laboratory MOX sensor system to test the headspace of the aged fluid samples. To prepare the set of MOX gas sensors suitable to detect the analytes’ concentrations in the fluid headspace, different functional materials were synthesized in the form of nanopowders, characterizing them by electron microscopy and X-ray diffraction. The powders were deposited through screen-printing technology, realizing thick-film gas sensors on which dynamical responses in the presence of the fluid headspace were obtained. It resulted that gas sensors based on solid solution TixSn1–xO2 with x = 0.9 and 0.5 offered the best responses toward the fluid headspace with lower response and recovery times. Furthermore, a decrease in the responses (for all sensors) with fluid ageing was observed.

ZnO nanofillers, with different morphologies, were synthesized by a facile aqueous sol-gel approach and embedded, at low concentration (i.e. 4 wt.%), into a UV-curable acrylic system. SEM observations showed a homogeneous distribution of the fillers within the cured network. The different morphologies of ZnO nanostructures were found to significantly affect the thermo-oxidative stability and the glass transition temperature of the obtained UV-cured films. Microcantilevers, made of the prepared films with sputtered aluminum contact, were fabricated by using standard microfabrication technology and their piezoelectric response was investigated both at the resonance frequency and at lower frequency values. Despite the low ZnO content, all the UV-cured nanocomposite films showed interesting piezoelectric properties: in particular, the devices containing flower-like nano-structures exhibited the highest root mean square voltage both at 150 Hz and at the resonance frequency (about 0.176 ± 0.001 and 0.914 ± 0.001 mV, respectively). In addition, these devices also showed maximum peak-peak voltage values at both the selected frequencies.

Thick films of tungsten trioxide (WO3), tin dioxide (SnO2), and tungsten-tin (W-Sn) mixed oxides at different nominal Sn molar fraction (0.1 and 0.3) were prepared. The functional materials were synthesized and characterized by SEM, X-ray diffraction, specific surface area measurements (BET), UV-Vis-NIR and IR spectroscopies. The gas sensing measurements highlighted that the mixed oxides based sensors offer a better response towards NO2 with respect to pure WO3 being almost insensitive to CO.

The comprehension of atmospheric pollution levels worldwide is a crucial issue for assessing the health consequences from human exposure to polluted air, and for identifying emission reductions that will be effective for minimizing the potential risks. Advanced interconnected sensors are required that could monitor on real-time toxic gaseous concentration. The success of these instruments depends on the reliability of these devices to quantify and disseminate the pollution levels to nearby citizens. Metal-oxide semiconductors are widely used as functional materials for gas sensing because of their chemo-resistive effect when interacting with gases. Mixed oxides are usually considered for the superior performances shown with respect to the single oxides. In this work, tungsten-tin mixed oxides with different Sn molar fraction (0.0013, 0.12, 0.33 and 0.39 named WS-1, WS-2, WS-3 and WS-4) were synthesized by sol-gel co-precipitation. The powders were characterized by ICP-OES analysis, specific surface area measurements, electron microscopy, X-ray diffraction, UV-Vis-NIR and FT-IR spectroscopies. The powders were also deposited through screen-printing technology, obtaining thick film gas sensors, on which measurements of conductance as a function of temperature, surface potential barrier and dynamical responses in the presence of oxidizing or reducing gases were carried out. Based on the studied properties, the mixed oxides can be divided into two groups: the WO3-like samples (WS-1, WS-2, WS-3) and the SnO2-like sample (WS-4). All samples present pure crystalline structures: this is a new result for the WO3-like samples. There is no literature data reporting about the introduction of so high Sn content in a WO3 structure. The combination of spectroscopic and electrical characterizations allowed the definition of an interpretative model that correlates the deepness of defect levels in the band gap of these materials to the values of the surface potential barrier in air and, as a consequence, to the electrical responses to oxidizing and reducing gases. (C) 2019 Elsevier B.V. All rights reserved.

This study concerns eight samples of ZnO powders, which can be classified in three categories: 3D-structure samples (ellipsoids, flowers), 2D-structure samples (commercial sample, nanosheets), 1D-structure samples (nanorods; short, long and irregular needles). Apart for the commercial sample, the others were synthesized through wet chemistry processes. The morphological and structural features of the ZnO powders were investigated with the purpose to understand how the friction behaviour is affected by the materials intrinsic properties. The tribological analysis was performed using a pin on disc tribometer, studying the influence of the material properties by a multivariate statistical approach. Results show that the abrasive nature of ZnO nanoforms is correlated with the crystallite dimensions. Spherical particles cause a higher damage on surfaces at the interface, with a consequent major change in the roughness. In the case of 1D-structure samples, the friction coefficient is directly proportional to the aspect ratio of the particles.

In this work we elucidate the origin of zinc oxide (ZnO) excitonic photoluminescence modifications induced by adsorption of nitrogen dioxide (NO2). In particular, we investigated the effects of NO2 exposure on excitons radiative recombination efficiency and lifetime in nano-structured vs. micro-structured ZnO films synthesized by wet chemical routes. By combining electrical and optical analyses and focusing on the role played by sample topology, we show that the presence of the oxidant species unambiguously gives rise to a decrease of photoluminescence efficiency and to an increment of the energy barrier at ZnO surfaces, while at the same time it leaves unaffected the probability of exciton radiative recombination. Comparisons between the results obtained for different kind of topologies allow us to infer about the basic fundamental mechanism underlying the effect, namely inhibition of free-excitons formation.

Six different ZnO nanomorphologies were synthesized trough wet chemical routes starting from a water solution of zinc nitrate hexahydrate, obtaining two types of morphologies: bidimensional nanocrystals and nanoparticles aggregates. Powders and films characterizations have been carried out by means of TG–DTA, SEM, and X-ray diffraction analysis. Finally, electrical measurements were performed with the aim to compare conductive properties of the thick films, surface barrier heights and gas sensing features, mainly versus acetone and other VOCs related to the breath gas analysis. Among the different morphologies tested, it turned out that the samples constituted by nanoparticle aggregates exhibited the best performances versus all gases, but especially toward acetone at sub-ppm level.

We investigate the effects exerted by nitrogen dioxide (NO2) exposure on excitonic photoluminescence efficiency and on excitons radiative lifetime in nano-structured versus micro-structured ZnO films synthesized by wet chemical routes. We show that the presence of the oxidant species unambiguously give rise to an increment of the energy barrier at ZnO surfaces, while at the same time it leaves unaffected the decay dynamics of free excitons. By comparing the results obtained for different kind of topologies, we deduce that the fundamental mechanism driving the NO2-induced photoluminescence quenching is the inhibition of free excitons formation.

ZnO samples with different nano-morphologies were synthesized by a facile method in which solutions containing zinc nitrate, ammonium hydroxide or hexamethylenetetramine were heated at different temperatures. Along with morphological and structural analysis, electrical and spectroscopic characterizations have been performed to investigate the role played by the sample morphologies. The spectroscopic results obtained during interaction with acetone well correlate with the electrical measurements: the higher is the intensity of the electronic absorptions in IR spectra, the higher is the electrical response to the tested gas. In particular, samples constituted by particles with well defined and smoothed surfaces are characterized only by the increase of electron population in the conduction band caused by acetone interaction and show peculiar energy barrier features. Samples constituted by particles with indented surfaces give the increase of electron population both in the conduction band and in oxygen vacancies. These last samples show similar energy barrier features.

Household air pollution is ranked the 9thlargest Global Burden of Disease risk (Forouzanfar et al., The Lancet 2015). People, particularly urban dwellers, typically spend over 90% of their daily time indoors, where levels of air pollution often surpass those of outdoor environments. Indoor air quality (IAQ) standards and approaches for assessment and control of indoor air require measurements of pollutant concentrations and thermal comfort using conventional instruments. However, the outcomes of such measurements are usually averages over long integrated time periods, which become available after the exposure has already occurred. Moreover, conventional monitoring is generally incapable of addressing temporal and spatial heterogeneity of indoor air pollution, or providing information on peak exposures that occur when specific indoor sources are in operation. This article provides a review of new air pollution sensing methods to determine IAQ and discusses how real-time sensing could bring a paradigm shift in controlling the concentration of key air pollutants in billions of urban houses worldwide. We also show that besides the opportunities, challenges still remain in terms of maturing technologies, or data mining and their interpretation. Moreover, we discuss further research and essential development needed to close gaps between what is available today and needed tomorrow. In particular, we demonstrate that awareness of IAQ risks and availability of appropriate regulation are lagging behind the technologies.

In this work a novel ozone detection at room temperature (RT) has been investigated. Two functional materials, ZnO and (W 0.9 Sn 0.1 )O 3 − x (WS10) oxides, have been synthesized to prepare thick film gas sensors, both used in conventional heated mode as well as at RT assisted by UV irradiation. As a source of light, a light emitting diode (LED) of 400 nm peak wavelength was used. Under typical operating conditions of the UV-LED, the radiation flux density ϕ over the sensor was of about 5 · 10 17 photons/cm 2 . Powders and films have been characterized by means of TG-DTA, SEM, TEM and XRD. Finally, electrical measurements have been performed on sensing films with the aim to compare conductive properties, surface barrier heights and ozone sensing features with and without UV irradiation. Despite the fact that two types of conventional heated sensors offered quite similar results with respect to ozone sensing, it turned out that, at RT and with the assistance of UV light, ZnO behaved excellently fast detecting ozone at concentrations down to 10 ppb, while for WS10 under the same operating conditions an opposite result was observed, i.e. very low response and long response time.

Resistance and capacitance measurements, as a function of frequency, were carried out to analyze and model the electrical behavior of undoped titanium dioxide (TiO 2 ) films. The sensing measurements were carried out in a chamber with independent temperature, pressure, gas composition, and flow rate control. With the aim of evaluating the electrical response of the sample as a function of the surrounding atmosphere, a frequency range of 100 Hz-4 MHz was used (vacuum or air at atmospheric pressure). The resistance and capacitance responses were obtained with the help of impedance spectroscopy technique. The impedance response versus frequency allows the discrimination between grain-boundary capacitance (Cg b ) and resistance (R gb ) contributions. A model, taking into account the possible effects due to grain size is proposed in order to work out the permittivity of the material.

Standard vapour phase growth process for ZnO tetrapods has been optimized in order to reach a very large yield, a good reproducibility and a single morphology (tetrapods are separated from the other possible ZnO nanostructures). The large yield of the growth and the simple deposition of these nanostructures on an alumina substrate with contacts and heater, allowed us to realize gas sensor prototypes with a relatively low-cost procedure. The obtained ZnO tetrapods-based gas sensors have been tested with different gases (CH 3 CH 2 OH, NO 2 , CO and H 2 S) and, especially, response values S = 25 and S = 100 have been measured towards 1 ppm and 5 ppm of hydrogen sulphide, respectively.

In this work we report the synthesis, microstructure, electric and spectroscopic properties, and sensing performances of Ti x Sn 1 − x O 2 ( x = 0.1, 0.2, 0.3, 0.5, 0.7, 0.9) nano-powders and of SnO 2 and TiO 2 reference samples, prepared via sol-gel route starting from metal-organic precursors working in hydro-alcoholic media. Actually, the attention is particularly focused on properties of the sample with x = 0.2, in comparison with ones of the other solid solutions and of the single oxides. Indeed, this solid solution showed a borderline behaviour between that of the solid solution with x = 0.1 and that of the other solid solutions with x ≥ 0.3. An abrupt change in the structural, electrical and spectroscopic properties has been observed, passing from sample with x = 0.1, showing a behaviour very similar to that of SnO 2 , to one x = 0.3 showing a behaviour very similar to that of TiO 2 . The borderline properties of the mixed oxide with x = 0.2 represent the expected continuous transition among the two behaviours.

Tin–titanium solid solutions at increasing Ti molar fraction, x = 0.1, 0.3, 0.5, 0.7 and 0.9, were prepared by symplectic gel co-precipitation (SGC) method from stoichiometric hydroalcoholic solutions of Ti and Sn metalalkoxides. Structural, spectroscopic and electrical analyses were performed and the results have been correlated. The peaks of the XRD patterns change from the positions of SnO 2 cassiterite to those of TiO 2 rutile, when increasing the Ti content in the solid solution. Photoluminescence (PL), FT-IR and UV–vis-NIR characterizations are in agreement with the formation of solid solutions: PL peak falls either in the visible or IR region, depending on the Ti content in the solid solution; FT-IR spectra reveal a progressive upward shift of the high-frequency absorption edge of the skeletal vibration modes from the value of SnO 2 to one of TiO 2 on increasing the Ti content; UV–vis-NIR spectra reveal a progressive downward shift of the VB–CB absorption edge on increasing Ti content. The changes induced by CO interaction at increasing temperature in the FT-IR and UV–vis-NIR spectra revealed a different behaviour of the materials following the Ti content: for the lowest Ti molar fraction ( x = 0.1) as well as for SnO 2 the reduction gives rise to mono-ionized oxygen vacancies, for the other solid solution the loss of reticular oxygen gives rise to doubly ionized oxygen vacancies and to Ti 3+ ions. From the electrical-conductance point of view, two groups of films can be identified: pure SnO 2 and the solid solution with x = 0.1, and all the other solutions including pure titania, in agreement with the results of FT-IR investigations. The best response to carbon monoxide was achieved by the solutions with x = 0.3 and x = 0.5. Opposite to electrical properties, exhibiting good responses to reducing gases, these agents did not appreciably modify the PL yield of the solid solutions. Conversely, PL emissions were sensitive to NO 2 , particularly for Ti 0.5 Sn 0.5 O 2 .

A study of the sensing properties of chemoresistive metal-oxides vs. light alkanes has been undertaken under wet condition and in presence of ethanol. Screen-printed films of a solid solution of mixed Sn and Ti oxides have been selected for the purpose. We demonstrated that the films are capable of detecting 100 ppm of light alkanes or 500 ppm of methane in wet condition, two concentration levels by far lower than their alarm thresholds, respectively. Ethanol is known to be a harmful interfering gas, though its concentration can be reduced to values lower than 10 ppm by proper filtering. We show that, even in presence of 10 ppm of ethanol, the films steadily responded to alkanes.

Nanostructured ZnO was obtained through two different synthetic processes: a traditional sol–gel (SG) route and a combination of vapor transport process and controlled oxidation methods. In SG synthesis, thermal decomposition of the precursor at 450 °C for 2 h in an oven led to pure zincite nanoparticles. Through the second method, instead, nanocrystalline structures in form of tetrapods (TP) were directly obtained during the synthesis process. Powder and film characterizations have been carried out by means of TG–DTA, SEM, TEM and XRD. Finally, electric measurements have been performed with the aim to compare conductive properties, surface barrier heights and gas sensing features, versus O 3 , NO 2 , CO and H 2 S. Despite the significant difference in morphology, it turned out that both types of sensors offered large responses to oxidizing gases at concentration suitable for environmental monitoring.

New sensors for detecting acetone directly in the exhaled breath have been developed. Several metal oxide materials, ZnO in different nano-morphologies, WO3 pure and Zr-loaded and TixSn1-xO2 solid solutions, have been considered for comparing the functional properties of the corresponding thick films with respect to the target gas. ZnO nano-sheets and bipyramidal nanoaggregates exhibited the large responses towards acetone, however ZnO aggregates of nanocrystals, Zr-WO3 and Ti(x)Sn(1-)xO(2) showed better performances as regards the influence of humidity on the acetone response.

Titania films were obtained through two synthetic processes, a traditional sol-gel method and a hydrothermal route. In SG synthesis, thermal decomposition of the precursor in oven at 400 degrees C for 2 h led to pure anatase TiO2; in HY synthesis, instead, crystalline anatase grains were obtained in autoclave at 200 degrees C for 1 h. To investigate the microstructural evolution of SG and HY titania with temperature, each powder was annealed at 650, 750, 850 degrees C for 1 h and subjected to XRD analysis. Surprisingly, HY titania, contrary to SG, maintained the anatase phase, up to 850 degrees C, without any introduction of foreign elements. The sensing layers, obtained from as grown powders, were fired at 650, 750 or 850 degrees C and tested vs. methane and carbon monoxide. Both types of films fired at 850 degrees C yield insignificant responses to both CO and CH4, demonstrating the lack of influence of the crystalline phase on the gas response. Moreover, as regards the films fired at 650 and 750 degrees C, the gas responses are higher for SG than for HY samples, despite larger particle size. (C) 2007 Elsevier B.V. All rights reserved.

Atomic force and high resolution scanning tunneling analyses were carried out on nanostructured WO3 films. It turned out that the band gap measured by scanning tunneling spectroscopy at surface is lower than the band gap reported in the literature. This effect is attributed to the high density of surface states in this material, which allows tunneling into these states. Such a high density of surface states pins the Fermi level resulting in modest surface activity at room temperature. Photo activation of WO3 results in unpinning of the Fermi level and thereby in higher chemical activity at surface.

A new material based on titanium dioxide modified by tantalum and vanadium has been synthesized. The prepared powders have been deposited to produce gas sensors in the form of thick films through screen-printing technology. The crystalline phase and morphology were examined by XRD and SEM analyses. The results showed that Ta addition inhibited the anatase-to-rutile phase transformation and hindered grain growth during heating, whilst vanadium did the opposite. The sensors have been tested with different gases showing good selectivity; in particular, the samples with greatest content of Ta showed good response to benzene, only slightly influenced by presence of carbon monoxide.

We used SnO2 gas sensors for monitoring ethylene in the range of interest for fruit ripening. The effect of atmospheric humidity on the sensor conductance and response towards ethylene was quantitatively measured. An algorithm for humidity compensation of the sensor’s response was developed and applied to experimental data. The results of our analysis proved useful for the application of chemoresistive sensors for agro-alimentary monitoring.

Nanopowders of pure and of Mn-, Ta- and Zr-loaded (5 wt.%) WO 3 were prepared and printed as thick films. Investigation of the influence of the doping on morphology, structure and gas response versus NO 2 has been performed. Pure nanometric WO 3 was prepared by a modified sol–gel synthesis while loading was carried out by impregnation with Mn(II), Ta(V) and Zr(IV) chlorides. Addition of Ta resulted in grain coalescence and phase transition inhibitions in the layers with respect to pure WO 3 films, being the effect strongly enhanced in the films fired at 850 °C. The Ta-doped films turned out to be the most sensitive films with a response extending down to the sub-ppm domain.

The photoluminescence of rutile and anatase TiO2 has been investigated in the range of 360–890 nm at several temperatures. An unexpected intense near-infrared (1.53 eV) photoluminescence band was recorded for both phases. At low temperatures, the resulting bands form a substructure of equally spaced peaks irrespective of the phase. The spectra were interpreted in the framework of the single-configuration-coordinate model as the phonon-replica effect, originating from ionization of oxygen vacancies.

Sol-gel routes were used to prepare pure and 5 at% and 10 at% Ta- or Nb-dope TiO2 nano-sized powders. The thermal decomposition behaviour of the precursors was studied using simultaneous thermogravimetric and differential thermal analysis (TG/DTA). X-ray diffraction (XRD) analysis showed that the powders heated to 400°C were crystalline in the anatase TiO2 structure. The pure TiO2 powder heated to 850°C showed the rutile structure. The addition of Ta and Nb inhibited the anatase-to-rutile phase transformation up to 950–1050°C. Ta was soluble in the titania lattice up to the concentration of 10 at%, while the solubility of Nb was 5 at%. Thick films were fabricated with these powders by screen printing technology and then fired for 1 h at different temperatures in the 650–1050°C range. Scanning electron microscopy (SEM) observations showed that the anatase-to-rutile phase transformation induces a grain growth of about one order of magnitude for pure TiO2. The addition of Ta and Nb is effective to keep the TiO2 grain size at a nanometric level even at 950°C, though grain growth was observed with increasing temperature. The gas-sensitive electrical response of the thick films were tested in laboratory, in environments with CO in dry and wet air. Conductance measurements showed a good gas response only for the nanostructured titania-based films. For field tests, the prototype sensors were placed beside a conventional station for atmospheric pollutant monitoring. The electrical response of the thick films was compared with the results of the analytical instruments. The same trend was observed for both systems, demonstrating the use of gas sensors for this aim.

Thick films of nanostructured pure TiO 2 , Nb/TiO 2 , Ga/TiO 2 , and Ta/TiO 2 have been fabricated by screen-printing technology. Structural, morphological, and optical studies have been carried out in order to correlate the measurements to electrical performance. We showed that addition of dopants inhibits grain growth due to firing and hinders conversion of anatase to rutile. The films were characterised electrically in terms of conduction properties and grain-to-grain energy barrier. The films proved sensitive to CO and NO 2 . It was also shown that firing temperature and dopant content strongly influence the gas response of the films.

Pure, 5 at%, and 10 at% Ta- or Nb-doped TiO2 nanosized powders were prepared by the sol-gel method. The powders heated to 400°C have the crystalline anatase structure. While the pure TiO2 powder heated to 850°C has the rutile structure, the addition of Ta and Nb inhibited the anatase-to-rutile phase transformation at this temperature. Ta was soluble in the titania lattice up to the concentration of 10 at%, while the solubility of Nb was 5 at%. Thick films were fabricated with these powders by screen printing technology and then fired at 650°C and 850°C for 1 h. SEM observations showed that the anatase-to-rutile phase transformation induces a grain growth of about one order of magnitude for pure TiO2. The addition of Ta and Nb is effective to keep the TiO2 grain size at the nanometric level even at 850°C. Conductance measurements showed that a good gas response is observed only for the nanostructured titania-based films. The CO response of these materials is only slightly affected by humidity.

This work gives results about the characterization of SnO 2 materials, prepared via the sol–gel route, pure and Mo 6+ -added. The materials were characterized as powders or thick films using a variety of techniques. The morphology of the powders was analyzed by XRD, SEM, TEM and HRTEM, their texture by volumetric measurements. The morphology of the thick films was analyzed by SEM. The goal of obtaining powders and films made by regularly shaped and nanosized (30÷50 nm) particles, even after thermal treatments at 850°C is attained. FT-IR spectroscopic and electrical measurements were employed on powders and films, respectively, to obtain information on the electronic effect due to the molybdenum addition. FT-IR results show that Mo lowers the intensity of the light scattered by free electrons and the intensity of a broad absorption, previously assigned to the photoionization of V O + [V O + + hν →V O 2+ +e − (c.b.)]. Accordingly, electrical data show that molybdenum markedly lowers (of about 2 orders of magnitude) the conductance of the films in air. Electrical measurements show that Mo lowers the response of tin oxide towards CO, but leaves almost unaltered or enhances its ability to sense NO 2 , depending on the thermal pretreatments. Both pure and Mo-added materials treated at 650°C show the same response to NO 2 . However, for the pure material treated at 850°C the response to NO 2 is halved, while it is almost unaffected by the thermal treatment on the Mo-added materials. The sensing temperature of maximum response is in any case 150°C. FT-IR spectroscopy was also employed to obtain information on the Mo species present on the surface of the materials after treatments in oxygen and on how they are affected in the presence of the different testing gases. Furthermore surface species formed by NO 2 interaction were carefully investigated.

Ultrafine SmFeO 3 powders have been prepared by the thermal decomposition at 700°C of the corresponding hexacyanocomplex, Sm[Fe(CN) 6 ]·4H 2 O. These powders have been used for the preparation of thick films, screen-printed on alumina substrates with comb-type Au electrodes. The electrical response of the films fired at 750°C, in N 2 and air atmospheres, prepared from pastes with different composition of the organic vehicle (o.v.), showed that they were sensitive to CO and NO 2 in dry and wet air. The composition of the o.v. strongly influenced the electrical conductivity and its activation energy and the gas response of the SmFeO 3 films, which had similar microstructure. The presence in the o.v. of components with hydroxyl groups causes a reaction with adsorbed oxygen species, which results in an increase in the films resistivity, being SmFeO 3 a p -type semiconductor and in their NO 2 response. The thick film processing parameters are thus of primary concern for the NO 2 sensing properties of the SmFeO 3 thick films.

The thermal decomposition of a heteronuclear complex, La[Fe(CN)6] · 5H2O, leads to the preparation of nanosized single-phase perovskite-type LaFeO3 powders. The microstructural evolution of LaFeO3 with the temperature has been studied by x-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The product of the decomposition at 500 °C consists of nanoporous grains which have the morphology of the complex, but diffracting as a monocrystal of LaFeO3. At the higher temperatures, the nanosized particles start to separate from each other, still keeping the shape of the complex grains and forming soft agglomerates. The formation of LaFeO3 from the complex at low temperatures is facilitated by the formation of an orthorhombic transition phase.

The responses of semiconductor gas sensors to the reducing gases are affected by the changes in charge carrier density arising from addition of dopants or catalysts. The aim of our work has been to obtain a better understanding of the influence of Pd on the electrical and electronic properties of thick SnO2 films. We investigated the responses of SnO2 and Pd/SnO2 films to CO in dry and wet (50% RH) air at temperatures increasing from 250°C up to 450°C. The samples were obtained from commercial and laboratory powders. FT-IR spectroscopy has been employed here to provide information on the changes of the electronic properties of the differently prepared samples, and the results have been compared to d.c. electrical measurements. Films prepared with pure tin oxide, both from commercial and laboratory powders, show a maximum of electrical response at 350°C, and show lower responses in wet air than in dry air at temperature lower than 400°C, while the responses is higher in wet air at 400–450°C. Pd always decreases the temperature of the maximum response to CO, both in dry and wet air. Pd addition decreases the response to CO of the films prepared from commercial powders at temperature ≥300°C, both in dry and wet air, while at 250°C there is a moderate increase of the response. In the case of films prepared from laboratory powder, Pd addition does not show a clear trend. Some comparisons with the responses to CH4 analyzed in a previous paper are also reported.

Zinc oxide nano-powders with different morphologies have been synthesized using a wet chemical route, starting from an aqueous solution of Zn(NO3)2·6H2O. The obtained powders shaped like nanoparticles, bipyramidal nanoaggregates, nanoneedles and flower-like nanostructures. The screen-printing technique has been adopted to deposit the functional materials in form of thick films onto miniaturized alumina substrates or onto silicon substrates for PL measurements. Acetone, acetaldehyde, formaldehyde, ethanol and ammonia have been used as testing gases, highlighting that the bipyramidal samples exhibited the best performances versus all gases, but especially toward acetone at sub-ppm level.