Synthesis and characterisation of nanosized TiO2–ZrO2 binary system prepared by an aqueous sol–gel process: Physical and sensing properties

Nanostructured TiO2–ZrO2 thin films and powders were prepared by a straightforward aqueous particulate sol–gel route. Titanium (IV) isopropoxide and zirconium (IV) acetate hydrate were used as precursors, and hydroxypropyl cellulose was used as a polymeric fugitive agent in order to increase the specific surface area. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy revealed that the powder were crystallised at the low temperature of 500 °C, containing anatase-TiO2 and tetragonal-ZrO2 phases. Furthermore, it was found that ZrO2 retarded the anatase-to-rutile transformation up to 900 °C. The activation energies for crystallite growth of TiO2 and ZrO2 components in the binary system were calculated 10.16 and 3.12 kJ/mol, respectively. Transmission electron microscope (TEM) image showed that one of the smallest crystallite sizes was obtained for TiO2–ZrO2 binary mixed oxide, being 5 nm at 500 °C. Field emission scanning electron microscope (FESEM) analysis revealed that the deposited thin films had nanostructured morphology with the average grain size of 20 nm at 500 °C and 36 nm at 900 °C. Thin films produced under optimised conditions showed excellent microstructural properties for gas sensing applications. They exhibited a remarkable response towards low concentrations of CO and NO2 gases at low operating temperature of 150 °C, resulted in an increase of thermal stability of sensing films as well as a decrease in the power consumption. Furthermore, calibration curves revealed that TiO2–ZrO2 sensor follows the power law, S = A[gas]B (where S is sensor response, coefficients A and B are constants and [gas] is gas concentration) for the two types of gases, and it has excellent capability for the detection of low gas concentrations.