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 TiO ₂-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.