Preparation and characterization of vanadium-titanium oxide thin films via the evaporation technique followed by the post-annealing treatment

This study emphasized the influence of different post-annealing temperatures and film thicknesses on the materials and CO gas sensing properties of vanadium titanium oxide (VTO) thin films. The VTO samples were prepared using the evaporation technique followed by post-annealing treatment. The results of orthorhombic V₂O₅, tetragonal TiO₂, and monoclinic V₂Ti₃O₉ were observed from the XRD analysis and Raman spectroscopy. The crystallinity of the VTO samples was enhanced with an increase in thicknesses and post-annealing temperatures. From SEM and AFM results, the different shapes and sizes (i.e., 50–1000 nm) of grains as well as smooth surfaces are noticed, however, the larger grains are formed at higher annealing temperatures. Higher post-annealing temperature promotes the formation of oxygen vacancies, as a result, the oxygen composition in VTO films is improved. The VTO thin films can transmit light up to 80 % in the visible region (higher transmittance for a thickness of 50 nm) and estimated bandgaps were noticed in the range of bandgaps of vanadium oxide and titanium oxide. From electrical properties, n-type semiconductors with carrier concentrations ranging from 10¹⁰ to 10¹² cm^-3 were found. The interplanar distances for VTO samples seen from TEM analysis resembled the results obtained from XRD measurements. Finally, the CO gas sensing response of VTO samples was studied by analyzing the change in the electrical resistances at different operating temperatures and CO gas concentrations. By analyzing the structural, morphological, optical, electrical and gas sensing properties, the post-annealing temperature of 550 °C showed comparatively the best condition for sensing CO gas.