Semiconductor metal oxides (SMOxs) are widely used in gas sensors due to their excellent sensing properties, abundance, and ease of manufacture. The best examples of these sensing materials are SnO2 and TiO2 that have wide band gap and offer unique set of functional properties; the most important of which are electrical conductivity and high surface reactivity. There has been a constant development of SMOx sensor materials in the literature that has been accompanied by the improvement of their gas-sensitive properties for the gas detection. This review is dedicated to compiling of these efforts in order to mark the achievements in this area. The main material-specific aspects that strongly affect the gas sensing properties and can be controlled by the synthesis method are morphology/nanostructuring and dopants to vary crystallographic structure of MOx sensing material.

Low level copper-doped CoFe2O4 has been synthesized and shows an increasing of saturation magnetization up to 6% Cu, subsequently decreasing above 6%.

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Energy shortage and environmental pollution issues are two of the biggest challenges of our century. Therefore, research for innovative new materials to sort out such issues are urgent. In this...

Thin films are being used more and more in gas sensing applications, relying on their high surface area to volume ratio. In this study, ZnO thin film was produced through a thermal aerosol spraying and chemical vapor deposition (spray-CVD) process at 500 °C using zinc acetate as a precursor. The phase identification and the morphologies of the film were investigated by XRD and SEM, respectively. Gas-sensing properties of the ZnO thin film were evaluated toward NO2, CO, and NO at a moderate temperature range (400–500 °C) in dry and humid air (relative humidity = 2.5, 5, 7.5, and 10% RH). The obtained results show good sensor signal for both NO2 (R/R0 = 94%) and CO (92%) and poor sensor signal to NO (52%) at an optimum temperature of 450 °C in dry air. The response and recovery times decrease with the increase of NO2 concentration. In the presence of humidity (10% of RH), the sensor is more than twice as sensitive to NO2 (70%) as CO (29%), and accordingly, exhibits good selectivity toward NO2. As the amount of humidity increases from 2.5 to 10% RH, the selectivity ratio of ZnO thin film to NO2 against CO increases from 1 to 2.4. It was also observed that the response and the recovery rates decrease with the increase of relative humidity. The significant enhancement of the selectivity of ZnO thin film toward NO2 in the presence of humidity was attributed to the strong affinity of OH species with NO2.

This work deals with the substantially high-temperature hydrogen sensors required by combustion and processing technologies. It reports the synthesis of undoped and Ni-doped TiO2 (with 0, 0.5, 1 and 2 mol.% of Ni) nanoparticles by a co-precipitation method and the obtained characteristics applicable for this purpose. The effect of nickel doping on the morphological variation, as well as on the phase transition from anatase to rutile, of TiO2 was investigated by scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The resistive sensors prepared with these powders were tested toward H2 at 600 °C. The results indicate that 0.5% Ni-doped TiO2 with almost equal amounts of anatase and rutile shows the best H2 sensor response (ΔR/R0 = 72%), response rate and selectivity. The significant improvement of the sensing performance of 0.5% Ni-doped TiO2 is mainly attributed to the formation of the highest number of n-n junctions present between anatase and rutile, which influence the quantity of adsorbed oxygen (i.e., the active reaction site) on the surface and the conductivity of the material.

Undoped and Co-doped TiO 2 nanoparticles were synthesized by a facile co-precipitation method and calcined at 700 • C. The phase identification carried out by XRD measurements and Raman spectroscopy analysis of calcined powders reveals the formation of mainly anatase phase for undoped TiO 2 , and 0.5 mol.% Co-doped TiO 2 whereas rutile phase for 1 mol.% Co-doped TiO 2 . The sensors prepared with these powders deposited on interdigital (IDE) sensor platforms were tested toward NO 2 and H 2 sensing properties at 600 • C. As the undoped and 0.5% Co-doped TiO 2 reveal n-type behavior, 1% Co-doped TiO 2 shows p-type semi-conductive behavior. One percentage Co-doped TiO 2 exhibits good sensing performance toward NO 2 while the undoped TiO 2 powder yields the best sensor performance toward H 2 at 600 • C. This indicates that the crystal structure of TiO 2 sensing material must be adjusted depending on the nature of target gas. The results indicate that the main factor influencing high temperature gas sensor performance of nanoparticulate TiO 2 is either the alteration of its electronic structure or the type of polymorphs.

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