Gas-sensing properties of In2O3-Au sensors as a function of gold particle size and method of their synthesis

Abstract

Powdered samples of In2O3–Au (0,1 wt%) with Au particle size of 1.5 nm and 3.5 nm have been obtained by introducing either colloidal gold particles of a given size or ionic gold into an indium hydroxide sol followed by its drying and annealing at 600 ◦C. This range of gold particle sizes is of interest for studying adsorption and catalytic properties, since for particles d(Au) ≤3.5 nm quantum size effects are observed that determine the properties of gold particles. The structural features of the samples were studied by X-ray diffraction, TEM, ED, IR spectroscopy, EPR and XPS. The sensitivity of In2O3–Au thick-film sensors with Au particle sizes of 1.5 and 3.5 nm has been measured to CO, acetone and methane. It has been established that the sensitivity of In2O3–Au sensors depends not only on the particle size, but also on the method of introducing gold into the oxide matrix. In2O3–Au thick-film sensors with dAu =3.5 nm were found to be more active towards CO and acetone compared to dAu =1.5 nm. The responses of In2O3 and In2O3–Au sensors to methane at 360 ◦C are nearly equal and do not depend on either dAu or the method of Au introduction. In contrast, the highest sensitivity at temperatures above 400 ◦C is demonstrated by In2O3–Au layers with dAu =1.5 nm when gold was introduced in ionic form rather than in colloidal. It was found that introducing gold ions leads to the most significant increase in the electrical resistance of the sensors over the entire range of operating temperatures (300–470 ◦C) and results in a high response to methane. The fixation of gold on the lateral faces of indium oxide crystals causes the emergence of energy barriers, which impede current transfer in the polycrystalline layer of In2O3–Au. At the same time, introducing colloidal gold particles does not have such a significant effect on the electrical resistance of the sensors. The obtained results also show that the contact area between the oxide and gold nanoparticles is of great importance, along with the size of Au nanoparticles, when the sensor layer interacts with reducing gases.