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 <i>T</i>_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.