The role of grain size on the thermal instability of nanostructured metal oxides used in gas sensor applications and approaches for grain-size stabilization

This review examines the influence of grain size on the thermal stability of nanostructured metal oxides by studying their structural parameters. The main consideration was focused on the behavior of SnO2, the most studied and used metal oxide in the field of gas sensor design. It has been shown that a decrease of the grain size was accompanied by a deterioration of the structural stability of the gas sensing material; a change of grain size in the gas sensing material could occur during annealing and exploitation. The study provides an analysis of the reasons for the changes in gas sensor properties during thermal treatment. The effectiveness of different approaches used for stability improvement of metal-oxide structures were considered. In particular, various approaches such as preliminary high-temperature annealing of as-synthesized powders, optimization of the processes of synthesis and deposition, the use of 1-D structures, and metal-oxide doping with various additives were analyzed.