Nanocrystalline TiO₂/SnO₂ heterostructures for gas sensing

Abstract

The aim of this research is to study the role of nanocrystalline TiO2/SnO2 n–n heterojunctions for hydrogen sensing. Nanopowders of pure SnO2, 90 mol % SnO2/10 mol % TiO2, 10 mol % SnO2/90 mol % TiO2 and pure TiO2 have been obtained using flame spray synthesis (FSS). The samples have been characterized by BET, XRD, SEM, HR-TEM, Mössbauer effect and impedance spectroscopy. Gas-sensing experiments have been performed for H2 concentrations of 1–3000 ppm at 200–400 °C. The nanomaterials are well-crystallized, anatase TiO2, rutile TiO2 and cassiterite SnO2 polymorphic forms are present depending on the chemical composition of the powders. The crystallite sizes from XRD peak analysis are within the range of 3–27 nm. Tin exhibits only the oxidation state 4+. The H2 detection threshold for the studied TiO2/SnO2 heterostructures is lower than 1 ppm especially in the case of SnO2-rich samples. The recovery time of SnO2-based heterostructures, despite their large responses over the whole measuring range, is much longer than that of TiO2-rich samples at higher H2 flows. TiO2/SnO2 heterostructures can be intentionally modified for the improved H2 detection within both the small (1–50 ppm) and the large (50–3000 ppm) concentration range. The temperature T max at which the semiconducting behavior begins to prevail upon water desorption/oxygen adsorption depends on the TiO2/SnO2 composition. The electrical resistance of sensing materials exhibits a power-law dependence on the H2 partial pressure. This allows us to draw a conclusion about the first step in the gas sensing mechanism related to the adsorption of oxygen ions at the surface of nanomaterials.

Nanostructured SmFeO3 Gas Sensors: Investigation of the Gas Sensing Performance Reproducibility for Colorectal Cancer Screening

“…On the other hand, great effort has been devoted to the study and development of advanced nanostructured sensing materials, by considering both MOX and other types of semiconductors [ 7 , 12 , 13 , 14 , 15 ]. The most used techniques were based on the addition of catalysts to the MOX nanostructure surface, on the synthesis of different MOX morphologies and on the preparation of MOX solid solutions [ 6 , 7 , 16 ]. Specifically, the MOX solid solutions showed very good sensing performances due to the high tunability of their physical and chemical properties, which can be controlled during the synthesis process by modifying synthesis parameters and the types and amounts of metals [ 17 ].…”

Nanocrystalline composites in TiO2 and SnO2 system for ammonia resistance sensors

“…The difference of potentials formed as a result of contact between TiO 2 and SnO 2 grains facilitates electron transport from TiO 2 to SnO 2 , which accelerates initial adsorption of oxygen on the surface of SnO 2 grains. Moreover, the smaller the size of grains, the greater the potential difference, and hence, the greater sensor response [8,16,17]. The responses of all sensors with respect to 500, 1000, 2000 and 4000 ppm of ammonia in dry air at 400°C are shown in Fig.…”

“…An increase or a decrease in the oxygen ion concentration results in signal formation [6,7]. Chemisorption of oxygen can be presented as the following equation [8]:…”

“…Thus, if there is a reductive gas in the examined atmosphere, resistance of an n-type semiconductor sensor material is lower [6,7]. Chemisorption of a reductive gas as a reaction of it with the adsorbed oxygen on the sensor surface is described in the example of hydrogen in the following equation [8]:…”

“…It was also observed that as a result of the effect of two above-described types of gas, the electrical resistance (R) of any reductive gas sensor can be expressed as [8]:…”