2D TiO2 thin films and 3D flower-like TiO2-based nanostructures, also decorated with SnO2, were prepared by chemical and thermal oxidation of Ti substrates, respectively. The crystal structure, morphology and gas sensing properties of the TiO2-based sensing materials were investigated. 2D TiO2 thin films crystallized mainly in the form of rutile, while the flower-like 3D nanostructures as anatase. The sensor based on the 2D TiO2 showed the best performance for H2 detection, while the flower-like 3D nanostructures exhibited enhanced selectivity to CO(CH3)2 after sensitization by SnO2 nanoparticles. The sensor response time was of the order of several seconds. Their fast response, high sensitivity to selected gas species, improved selectivity and stability suggest that the SnO2-decorated flower-like 3D nanostructures are a promising material for application as an acetone sensor.
Vanadium pentoxide thin films were deposited onto insulating support by means of rf reactive sputtering from a metallic vanadium target. Argon-oxygen gas mixtures of different compositions controlled by the flow rates were used for sputtering. X-ray diffraction at glancing incidence (GIXD) and Scanning Electronic Microscopy (SEM) were used for structural and phase characterization. Thickness of the films was determined by the profilometry. It has been confirmed by GIXD that the deposited films are composed of V2O5 phase. The gas sensing properties of V2O5 thin films were investigated at temperatures from range 410–617 K upon NO2 gas of 4–20 ppm. The investigated material exhibited good response and reversibility towards nitrogen dioxide. The effect of metal-insulator transition (MIT) on sensor performance has been observed and discussed for the first time. It was found that a considerable increase of the sensor sensitivity occured above 545 K, which is related to postulated metal-insulator transition.
Reduced graphene oxide and copper oxide multilayer structures were fabricated in a planar configuration by deposition on both ceramic and Si/SiO2 substrates with interdigitated Au electrodes by the spray method. SEM (scanning electron microscopy), TEM (transmission electron microscopy), XRD (X-ray diffraction), and elemental analysis investigations indicated that graphene oxide (GO) was obtained in a form of interconnected flakes consisting of 6–7 graphene layers for GO with the total thickness of ca. 6 nm and 2–3 layers for rGO with the total thickness of 1nm. The lateral size of one flake reached up to 10 micrometers. Copper oxide was obtained by the wet chemical method. The number of sequential layers of the sensing structure was optimized to obtain good sensitivity and acceptable response/recovery times in response to the oxidizing nitrogen dioxide atmosphere. Both semiconductor partners revealed p-type conductivity. Formation of isotype heterojunctions between both semiconductor partners was taken into account and their influence on electrical transport explained. Optimized sensor structures revealed relative sensitivities reaching several tens of percent and acceptable response and recovery times in NO2 concentration ranged from a few to 20 ppm. Possibility of manufacturing sensors working at room temperature was shown, but at the cost of prolonged response/recovery times.
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