Solar-responsive photocatalytic activity of amorphous TiO2 nanotube-array films

We prepared amorphous and anatase TiO2 nanotube-array (NA) films by anodic oxidation followed by annealing. We characterized the as-prepared samples with X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), photoluminescence spectrum (PL), electron spin resonance (ESR), and so on. The XPS and EPR spectra show that the surface of amorphous TiO2 NA films absorbs chemically dissociated water much easier than that of anatase TiO2 NA films. The amorphous TiO2 NA films show higher photocatalytic activity than anatase TiO2 NA films in RhB degradation under the irradiation of simulated solar light. Furthermore, the photocatalytic activity of amorphous TiO2 NA films ascribes to visible-infrared light rather than UV light. On the contrary, that of anatase TiO2 NA films mainly attributes to UV light. The EPR spectra, photocurrent density and photocatalytic activity evaluation reveal that totally different mechanisms play the key role in photocatalytic degradation of RhB dye for amorphous and anatase TiO2 NA films. For amorphous one, visible light excites electrons from RhB dye molecules and they quickly transfer to the conduction band of amorphous TiO2 NA films, which acted as electron trapping centers. The electron loss of RhB molecules results in their oxidation. In a word, the dye self-sensitization is responsible to the high photocatalytic degradation of RhB dye. On the other hand, the classical theory of photocatalysis could perfectly explain the photocatalytic activity of anatase one. Therefore, this work confirmed on the fact that amorphous TiO2 is inactive. This work completely clarified the origin of high photocatalytic activity of amorphous TiO2.