Effects of substrate microstructure on the formation of oriented oxide nanotube arrays on Ti and Ti alloys

The formation of nanotubular oxide layers on Ti and Ti alloys has been widely investigated for the photocatalytic degradation of organic compounds due to their excellent catalytic efficiency, chemical stability, and low cost and toxicity. Aiming to improve the photocatalytic efficiency of this nanostructured oxide, this work investigated the influence of substrate grain size on the growth of nanotubular oxide layers. Ti and Ti alloys (Ti-6Al, Ti-6Al-7Nb) were produced by arc melting with non-consumable tungsten electrode and water-cooled copper hearth under argon atmosphere. Some of the ingots were heat-treated at 1000 °C for 12 and 24 h in argon atmosphere, followed by slow cooling rates to reduce crystalline defects and increase the grain size of their microstructures. Three types of samples were anodized: commercial substrate, as-prepared and heat-treated samples. The anodization was performed using fluoride solution and a cell potential of 20 V. The samples were characterized by optical microscopy, field-emission scanning electron microscopy and X-ray diffraction. The heat treatment preceding the anodization process increased the grain size of pure Ti and Ti alloys and promoted the formation of Widmanstätten structures in Ti6Al7Nb. The nanotubes layers grown on smaller grain and thermally untreated samples were more regular and homogeneous. In the case of Ti-6Al-7Nb alloy, which presents a α + β phase microstructure, the morphology of nanotubes nucleated on α matrix was more regular than those of nanotubes nucleated on β phase. After the annealing process, the Ti-6Al-7Nb alloy presented full diffusion process and the growth of equilibrium phases resulting in the appearance of regions containing higher concentrations of Nb, i.e. beta phase. In those regions the dissolution rate of Nb2O5 is lower than that of TiO2, resulting in a nanoporous layer. In general, heat treating reduces crystalline defects and promotes the increasing of the grain sizes, not favoring the process of nanotube nucleation and growth on the metallic surface.