Formation of photocatalytically active TiO₂ layers on TiNb alloys by two-step thermal oxidation

A two-step thermal oxidation process was applied to TixNb binary alloys (x = 0, 1, 10, 15, and 30 at%) to prepare anatase-containing TiO₂ layers, and their photocatalytic activities were evaluated by measuring the water contact angle and decomposition of methylene blue (MB) under UV irradiation. The condition of the first-step treatment was fixed as heating in Ar1%CO atmosphere at 1073 K for 3.6 ks, and the subsequent second-step treatment was conducted in air at 6731073 K for 10.8 ks. The reaction layer formed after the two-step thermal oxidation consisted of TiO₂. The anatase fraction of the TiO₂ layers increased with decreasing second-step temperature and increasing Nb content of the TiNb alloys. In addition, Nb and carbon were introduced into the TiO₂ layers. A water contact angle of around 5° was observed on the TiO₂ layers formed at the second-step temperatures of 673973 K. The rate constant of MB decomposition showed a maximum for an anatase fraction of 0.60.8 at which the recombination of exited electrons and holes are suppressed. The TiO₂ layer formed on the Ti10 at%Nb alloy exhibited a higher rate constant of MB decomposition compared with Ti30 at%Nb, in which the TiNb₂O₇ phase formed. These results indicate that Nb is an effective alloying element for producing a photocatalytically active TiO₂ layer on Ti by the two-step thermal oxidation process. Nevertheless, the presence of an anatase-rich TiO₂ layer and an appropriate Nb content in TiO₂ are required for achieving high photocatalytic activities.