A new simple approach to prepare rare-earth metals-modified TiO2 nanotube arrays photoactive under visible light: Surface properties and mechanism investigation

Application of Ti90RE10 alloys (RE = Ho, Er, Nd, Y, Ce, Tm) as a working electrode, instead of Ti pure foil in anodic oxidation in a fluoride-based electrolyte, resulted in formation of well-ordered nanotubes made of TiO2 and RE2O3 mixture, which could be efficiently used for pollutant removal from water and air phase upon UV and visible irradiation and easily separable from the reaction mixture to recycle. The as-prepared NTs were characterized by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDX), diffuse reflectance spectroscopy (DRS), luminescence spectroscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The photocatalytic activity of the synthesized samples was investigated used phenol and toluene as a model compounds to follow degradation kinetics. The influence of the RE metals on the photoelectrochemical performance of modified TiO2 NTs was investigated. Surface morphology analysis showed formation of uniform and vertically oriented NTs structures with an open tube top and smooth walls. The results of EDX, XRD and XPS analysis proved that RE ions exist as surface compounds (RE3+ oxides). Visible light induced photoactivity (both photocatalytic and photoelectrochemical) is mostly pronounced in the presence of nanotubes obtained by anodic oxidation of Ti90Ho10 alloy. These nanotubes are able to degrade toluene in the gas phase even by irradiation of low-powered light source, such as light emitting diodes (λmax = 465 nm). Photocatalytic test of phenol degradation in the presence of scavenger indicates that photogenerated electrons and superoxide radicals play a meaningful role in the photocatalytic degradation of pollutants under visible irradiation. In addition, the photoelectrochemical tests performed under the influence of visible light irradiation confirmed that the RE-modification of TiO2 NTs caused a significant increase of photocurrent (up to 10 times higher). The new and original results on the preparation of RE2O3/TiO2 nanotubes obtained in one step anodic oxidation and photocatalytic activity in aqueous and gas phases represent an important contribution then will benefit photocatalytic surfaces preparation technologies.