Effect of synthesis method on the structural, conductive and sensor properties of NiO–In$_2$O$_3$ nanocomposites

The structural, conductive and sensor properties of NiO–In$_2$O$_3$ composites synthesized by hydrothermal and impregnation methods are investigated and compared. The mixed oxide considered consists of nanoparticles with electronic (In$_2$O$_3$) and hole (NiO) conduction bands. The lattice parameters of indium oxide decrease with the introduction of NiO into composites synthesized by the hydrothermal method. The addition of 3% NiO to the hydrothermal composite also increases its specific surface area. The specific surface area and In$_2$O$_3$ lattice parameters in the impregnated samples are essentially independent of the NiO content. The conductivity of impregnated composites is an order of magnitude lower than that of hydrothermal composites. An increase in NiO content leads to a significant enhancement of the sensor response to H$_2$ and CO. In addition, there is a decrease in the optimal operating temperature of hydrothermal and impregnated samples by 60 and 20$^\circ$C, respectively.