The effects of niobium on the structure and properties of VO2 films

Although vanadium dioxide (VO2) is undergoing trials for use in various thermal and optical applications due to its unique characteristics, its phase transition temperature must be decreased significantly for it to be used in smart windows in buildings. Niobium (Nb)-doped vanadium dioxide films were prepared on quartz substrates with sol–gel method, spin coating technique, and subsequent annealing at 550 °C in a pure nitrogen atmosphere. The effects of niobium on the structure, morphology, phase transition temperature and optical transmittance of VO2 films were investigated. The results show that no new phase came into beings after Nb doping, and that all of the films were of high crystalline quality. All of the films were compact from their surface morphologies, but their granules’ shapes evolved from small spheres to larger rods and even blocks as the doping concentration increased. The main valences of the vanadium ions in the films were +4, and Nb5+ ions were well present in the films. Although the phase transition temperature of the thin VO2 film decreased from 67.6 to 28.1 °C, both its optical transmittance and its transmittance mutation decreased significantly as the Nb’s molar concentration increased from 0 to 20 %. The maximum infrared transmittance mutation of the film decreased from 67.02 to 32.52 %.