Characterization of TiO2 films obtained by cathodic arc

ARIEL KLEIMAN; ADRIANA MÁRQUEZ; DIEGO G LAMAS

Puerto Madryn

Congreso; 2ª Reunión de la Asociación Argentina de Cristalografía; 2006

Titanium dioxide has been widely investigated due to its interesting physical and chemical properties. The use of TiO2 in photo-assisted degradation of organic molecules by UV radiation is one of the subjects more investigated on this material. It is well known that TiO2 exhibit three polymorphs: rutile (tetragonal, space group P42/mmm), anatase (tetragonal, space group I41/amd) and brookite (orthorhombic, space Pcab). Rutile is the only stable phase, whereas anatase and brookite are metastable at all temperatures. It has been shown that these metastable phases can be retained in nanocrystalline materials. The retention of the anatase phase has been widely investigated since it exhibits the highest photocatalytic efficiency during chemical reactions. Several techniques have been employed to prepare TiO2 thin films such as sol-gel, CVD, reactive sputtering, laser ablation and cathodic arc deposition. Cathodic arcs consist in a high current discharge running between two electrodes immersed in a vacuum chamber. These devices are characterized by the production of metallic plasma with high degree of ionization and high kinetic energy of the ions. As it is well known, the particle energy and the substrate temperature influence straightforwardly on the film microstructure. In this work TiO2 thin films were prepared on glass substrates at different temperatures, between room temperature and 400 ºC, employing a cathodic arc device. The crystalline structure of the films was determined by X-ray diffraction (XRD). The surface morphology was studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Transmittance in UV-visible region was also measured. All films deposited at temperatures lower than 300 ºC were amorphous, whereas films obtained at higher temperatures grew in crystalline anatase phase. This threshold is in agreement with the experimental conditions for the synthesis of amorphous or anatase TiO2 proposed in Löbl diagram [1] considering the kinetic energy of the ions (~ 0,5 eV). Amorphous films were post-annealed at 400 ºC. Phase transition amorphous-to-anatase was observed after the treatment. Optical measurements showed an almost constant transmittance for l between 400 and 800 nm with an average value above 80 % for all the films. For l < 400 nm an important fall in the transmittance value is observed, this behavior is due to the characteristic TiO2 absorption in the U-V region. The film surfaces are composed of columnar grains, pretty dense and without the presence of voids. The films crystallized during post-annealing exhibit grains between 50 and 80 nm in size and a surface roughness of 5 nm, whereas films crystallized in-situ, present grains between 15 and 30 nm in size and a surface roughness of 2 nm. Although the temperatures employed in-situ to grow the crystallized films were similar to that used in the post- annealing of the amorphous films, the grain sizes obtained in each case were very different between them. This fact has been already remarked by Löbl, the size of the anatase grains becomes smaller at a higher deposition temperature, because an increasing of the substrate temperature increases the nucleation rate of the anatase phase which influences on the grain growth. As the photocatalysis is a surface process, a smaller grain size, that increases the surface-to-volume ratio, improves the photocatalytic activity. This fact suggest that as-deposited crystalline films should have a better performance for photocatalitic applications than the others films.