Microstructure characterization of sol-gel prepared MoO3–TiO2 thin films for oxygen gas sensors

Abstract

Binary metal–oxide MoO3–TiO2 films have been prepared using the sol-gel technique. The thin films were annealed at several temperatures including 400, 450, 500, 550, and 600 °C for 1 h. The morphology, crystalline structure, and chemical composition of the films have been analyzed using scanning electron microscopy (SEM) and atomic force microscopy, x-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), and x-ray photoelectron spectroscopy (XPS) techniques. The SEM analysis showed that there are two different sizes of grains in the films annealed at temperatures of 400, 450, 500, and 550 °C. One grain type is small with 20–100 nm; the other is a large grain type several micrometers in length. The XRD analysis revealed that the films annealed at 400 °C were a mixture of orthorhombic and hexagonal MoO3 phases. The films annealed at 450 °C showed an increase in the hexagonal phase. A preferential orientation growth along the (100) plane of the hexagonal phase and the (010) plane of the orthorhombic phase has been found in both samples. RBS and XPS analysis showed that the films were stoichiometric. When the annealing temperature was increased beyond 500 °C, the concentration ratio of MoO3–TiO2 decreased due to the evaporation of MoO3. For the study of the electrical and gas sensing properties, the films were deposited on sapphire substrates with interdigital electrodes on the frontside and a Pt heater on the backside. The MoO3–TiO2 thin films are sensitive to oxygen gas. The film has exhibited the O2 response (S=Rg/Rb) of 2.1, 8.1, and 80 for 120, 1000, and 10 000 ppm concentration of O2, respectively.