High-temperature stable anatase-type TiO2nanotube arrays: A studyof the structure–activity relationship

Anatase-type TiO2 nanotube arrays (TiO2-NTAs) were grown on Ti foil by anodic oxidation inCH3COOH/NH4F solutions followed by thermal treatment. The surface of TiO2-NTAs was further deco-rated by palladium and silver metal clusters through a chemical-reduction method and its photocatalyticactivity was tested by investigating the degradation of p-nitrophenol (PNP) in aqueous solutionunder visible-light irradiation and electrical polarization. The effects of preparation variables bothon microstructural properties of samples and photocatalytic activity were examined by using the 3Dresponse 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 phasestructure and the photocatalytic activity of TiO2-NTAs. Titania nanotubes grown in acetic acid solutionand thermally post-treated have stable anatase crystal structure, to a point that by performing annealingat 800◦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 comparedwith 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.