Anatase-type TiO2 nanotube arrays (TiO2-NTAs) were grown on Ti foil by anodic oxidation in CH3COOH/NH4F solutions followed by thermal treatment. The surface of TiO2-NTAs was further decorated by palladium and silver metal clusters through a chemical-reduction method and its photocatalytic activity was tested by investigating the degradation of p-nitrophenol (PNP) in aqueous solution under visible-light irradiation and electrical polarization. The effects of preparation variables both on microstructural properties of samples and photocatalytic activity were examined by using the 3D response surface and the 2D contour plots. The experimental investigations carried out by using XRD, SEM, HRTEM, EDS, XRF, ICP-AES, XPS, DRS, and PL, demonstrated a strong relation between the phase structure and the photocatalytic activity of TiO2-NTAs. Titania nanotubes grown in acetic acid solution and thermally post-treated have stable anatase crystal structure, to a point that by performing annealing at 800 degrees C for 3 h, only the 35% of anatase transforms into rutile. Finally, it was shown that the TiO2-NTAs decorated with Pd(0.72 wt%) and Ag(1.26 wt%) particles show higher photocatalytic activity compared with nanotubes modified with single metal particles. It is believed that the high photoactivity of TiO2 nanotubes decorated with Pd-Ag heterostructures is due to the prolonged lifetimes of photogenerated electron-hole pairs. The possible mechanism for the enhanced photocatalytic activity is discussed in detail. (C) 2015 Elsevier B.V. All rights reserved.