Grain-size-dependent gas-sensing properties of TiO2 nanomaterials

Abstract

This work deals with TiO2 nanopowders, the grain size of which was intentionally changed from about 10 nm to 30nm by controlling the flow rate of the titanium organic precursor of C16H28O6Ti in flame spray synthesis (FSS). The results of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) compared with the results of Brunauer–Emmett–Teller (BET) nitrogen adsorption isotherms and X-ray diffraction (XRD) revealed non-agglomerated, spherical grains composed mostly of anatase. Annealing at 700°C resulted in a partial phase transformation from anatase to rutile, with rutile crystallites as large as 57 nm. The formation of stoichiometric TiO2 was confirmed by means of X-ray photoelectron spectroscopy. Electrical conductivity determined from dc and ac measurements increased upon reduction in grain size. The dynamic changes in the electrical resistance observed upon interaction with hydrogen over the range of 50–3000ppm at a constant temperature chosen within the interval of 200°C–400°C, were reproducible. Theoretical model, based on the Schottky barriers formation at grain contacts, was applied to the sensor responses obtained in this work. Experimental data of the electrical conductivity in dry air were used to derive the temperature dependence of the band bending energy (EO=0.22–0.38eV over the temperature range of 200–400°C). The difference between the activation energy of water desorption and hydrogen atom adsorption (ΔE) – the only fitting parameter of the adopted model – was found to decrease linearly with the specific surface area (SSA) of the TiO2 nanomaterials.