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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.

Gas sensors are essential in many areas, from environmental monitoring to quality and safety control of industrial processes, food and healthcare. Those based on metal oxides semiconductor (MOX) provide a versatile solution to detect many different gases with low costs, small dimensions, easy to install and use. On the other hand, sensor performances depend on many factors, first of all the selection and the design of the nanostructured functional material. The most used oxides are SnO2, TiO2, ZnO and WO3, although many strategies could be introduced to increase sensing performances of the functional materials such as the use of solid solutions characterized by higher performances with respect to the single oxides. The system TixSn1-xO2, with Ti molar ratio x = 0.1, 0.2, 0.25, 0.3, 0.5, 0.7, 0.9), is very attractive because the two pristine oxides, SnO2 and TiO2, show several similarities in crystalline structure as well as in electronic properties and thus can easily form a solid solution. On the other hand, they exhibit some differences, such as electrical transport properties and gas sensing behavior. This work represents a complete study in the field of MOX gas sensing. It starts from the design of the sensitive material, then the study of their properties from a fundamental point of view, concluding with two real successful real applications (CO detection in air quality monitoring and analytes concentrations monitoring in hydraulic fluid headspace).

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.