Microstructural characterization of MoO3–TiO2 nanocomposite thin films for gas sensing

This paper focuses on the microstructural characterization of the MoO3–TiO2 nanocomposite thin films prepared by the sol–gel process. The MoO3–TiO2 thin films were prepared by Mo(OC2H5)5 and Ti(OC4H9)4 precursor solutions. Different atomic ratios of the two compounds were investigated. The thin films were deposited on silicon, quartz and sapphire substrates annealed at temperatures of 400, 450, 500 and 600°C for 1 h. The XRD patterns revealed the structure of the molybdenum dominated films consisted mainly of an orthorhombic MoO3 phase with preferential orientation along the (0 1 0) plane. The TEM selected area diffraction patterns revealed the presence of orthorhombic MoO3 and anatase and brookite TiO2 phases. The XPS characterization indicated the films are stoichiometric (MoO3 and TiO2). The SEM analysis showed that the films annealed at 400°C are smooth and uniform with 20–100 nm sized grains. The MoO3 dominated films annealed at high temperatures (T>500°C) have relatively large micrometer particles grown out of the film. The AFM images showed that the MoO3 dominated thin films possess a high surface roughness and the TiO2 dominated films have smooth and uniform nanosized grains. The TEM results showed that the TiO2 dominated films have an average grain size of 6 nm with a narrow distribution. The MoO3 dominated films have an average size of 5 nm but with a broad distribution. The morphological and physical properties of the MoO3–TiO2 nanocomposite can be tailored by altering the ratio of the two compounds, and hence, enhanced thin films for gas-sensing could be achieved.