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

Abstract

Evaporation of titanium together with activated oxygen is used to grow TiO2 films and simultaneously with silver to grow Ag–TiO2 films (5at.% 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 2nm 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.