Conductometric sensor for ammonia and ethanol using gold nanoparticle-doped mesoporous TiO2

Abstract

We describe uniform and high-temperature-stable mesoporous TiO2 beads functionalized with gold nanoparticles (AuNPs-TiO2) for use in conductometric sensing of gases and organic vapors. The size of the interconnected main mesopores of the TiO2 beads ranges from 8 to 15 nm, and the AuNPs have diameters between 8 and 10 nm. The mesoporous TiO2 beads are formed during calcination while the structure-directing template agent is removed. Monodispersed AuNPs are formed by reduction in-situ and are placed inside the mesoporous TiO2 framework. This prevents aggregation of the AuNPs even at 500 °C. The materials were characterized by UV–vis spectroscopy, scanning and transmission electron microscopy, nitrogen adsorption-desorption, and X-ray diffraction. Comb-type gold electrodes were then fabricated on an alumina substrate and are shown to display excellent properties in terms of sensing ammonia, ethanol, methanol or acetone. The sensitivity (defined as the ratio of resistivities under vapor and air) of a typical AuNPs(0.5 %)-TiO2 gas sensor for ethanol reached up to 5.65 at above 600 ppm at 75 °C. Response time and recovery times (t90 ≤ 20 s) are faster than (or comparable to) other metal-doped TiO2 sensors, and working temperatures are much lower. An interesting observation was made in that the changes in the conductivity of the sensor change with temperature. The sensor prepared with AuNPs(0.5 %)-TiO2 is of the p-type (in its response to ammonia gas) at 45 °C, but becomes n-type at 20 °C. Obviously, rather slight changes in temperature lead to a complete change in the direction of the conductometric signal change. This may provide a new aspect in terms of selective and highly sensitive detection of ammonia at ambient and slightly elevated temperatures.