Nanocrystalline TiO(2)/SnO(2) heterostructures for gas sensing

The aim of this research is to study the role of nanocrystalline TiO(2)/SnO(2) n–n heterojunctions for hydrogen sensing. Nanopowders of pure SnO(2), 90 mol % SnO(2)/10 mol % TiO(2), 10 mol % SnO(2)/90 mol % TiO(2) and pure TiO(2) 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 H(2) concentrations of 1–3000 ppm at 200–400 °C. The nanomaterials are well-crystallized, anatase TiO(2), rutile TiO(2) and cassiterite SnO(2) 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 H(2) detection threshold for the studied TiO(2)/SnO(2) heterostructures is lower than 1 ppm especially in the case of SnO(2)-rich samples. The recovery time of SnO(2)-based heterostructures, despite their large responses over the whole measuring range, is much longer than that of TiO(2)-rich samples at higher H(2) flows. TiO(2)/SnO(2) heterostructures can be intentionally modified for the improved H(2) 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 TiO(2)/SnO(2) composition. The electrical resistance of sensing materials exhibits a power-law dependence on the H(2) 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.