Semiconductor TiO2–Al2O3 thin film gas sensors derived from aqueous particulate sol–gel process

Nanostructured TiO2–Al2O3 films and powders were prepared by a straightforward aqueous particulate sol–gel route. Titanium (IV) isopropoxide and aluminum chloride were used as precursors, and hydroxypropyl cellulose was used as a polymeric fugitive agent in order to increase the porosity. The effect of Al:Ti molar ratio was studied on the crystallization behavior of the products. X-ray diffraction (XRD) revealed that the powders crystallized at 800 °C, containing anatase-TiO2, rutile-TiO2 and cubic-Al2O3 phases. Furthermore, it was found that Al2O3 retarded the anatase to rutile transformation. Transmission electron microscope (TEM) image showed that one of the smallest crystallite sizes was obtained for TiO2–Al2O3 binary mixed oxide, being 3 nm at 600 °C. Field emission scanning electron microscope (FE-SEM) analysis revealed that the deposited thin films had nanostructured morphology with the average grain size in the range 20–70 nm at 800 °C depending on Al:Ti molar ratio. Thin films produced under optimized conditions showed excellent microstructural properties for gas sensing applications. They exhibited a remarkable response towards low concentrations of CO gas at operating temperature of 400 °C.