Influence of substrate temperature and silver-doping on the structural and optical properties of TiO2 films

Evaporation of titanium together with activated oxygen is used to grow TiO2 films and simultaneously with silver to grow Ag-TiO2 films (5 at.% Ag) onto sapphire substrates at three different substrate temperatures: − 190, 30, and 200 °C. The obtained films were characterized by X-ray powder diffraction, Raman, X-ray photoelectron, ultraviolet-visible spectroscopy, and transmission electron microscope investigations. The properties of TiO2 films varied with the substrate temperature. Amorphous, transparent TiO2 films were grown at − 190 °C and opaque, polycrystalline films at 200 °C, respectively. Surprisingly, at room temperature black, amorphous TiO2 films are obtained which transform at 350 °C into a mixture of the anatase and brookite polymorph. In the amorphous state of the TiO2 films a predefined rutile arrangement is suggested by Raman investigations, and the contraction of the lattice constant c of anatase phases (tetragonal, space group I 41/amd) depending on the substrate temperature is experimentally observed. The silver-doped TiO2 films deposited at − 190 and 30 °C contain Ag-particles with 2 nm in size inside the TiO2 matrix, which after annealing segregate under increasing particle sizes. The silver-doping stabilizes the anatase polymorph and yields to reduced titanium species in the films especially during deposition at 30 °C. The Ag-TiO2 films deposited at − 190 °C are transparent up to 350 °C. In the undoped as well as silver-doped TiO2 films the rutile polymorph is directly formed at 200 °C as main phase.