V.J. Rico, A. Borrás, F. Yubero, J.P. Espinós, F. Frutos, A.R. González-Elipe
Journal of Physical Chemistry C, 113 (2009) 3775-3784
doi: 10.1021/jp805708w

Ta₂O₅ thin films with different nanostructure and surface roughness have been prepared by electron evaporation at different angles between the evaporation source and the substrates. Large variation of refraction indexes (n) from 1.40 to 1.80 were obtained by changing the geometry of evaporation and/or by annealing the evaporated films at increasing temperatures up to 1000 °C to make them crystalline. Very flat and compact thin films (n = 2.02) were also obtained by assisting the growth by bombardment with O₂⁺ ions of 800 eV kinetic energy. A similar correlation has been found between the wetting contact angle of water and the roughness of the films for the evaporated and evaporated + annealed samples, irrespective of their procedure of preparation and other microstructural characteristics. When the films were illuminated with UV light of h > E_g = 4.2 eV (E_g, band gap energy of Ta₂O₅), their surface became superhydrophilic (contact angle < 10°) in a way quite similar to those reported for illuminated TiO₂thin films. The rate of transformation into the superhydrophilic state was smaller for the crystalline than for the amorphous films, suggesting that in Ta₂O₅ the size of crystal domains at the surface is an important parameter for the control of this kinetics. Changes in the water contact angle on films illuminated with visible light were also found when they were subjected to implantation with N₂⁺ ions of 800 eV kinetic energy. The origin of this photoactivity is discussed in terms of the electronic band gap states associated with the nitrogen-implanted atoms. The possibility of preparing antireflective and self-cleaning coatings of Ta₂O₅ is discussed.