Ultra-trace (parts per million-ppm) W6+ dopant ions induced anatase to rutile transition (ART) of phase pure anatase TiO2 nanoparticles for highly efficient visible light-active photocatalytic degradation of organic pollutants

A series of novel photocatalysts were synthesized by incorporating tungsten (W6(+)) ions in to the lattice of pure anatase TiO2 nanoparticles with ultra-trace concentrations (ppm) such as 10 ppm, 50 ppm, 90 ppm and 120 ppm using a simple one-step hydrothermal method under identical conditions. X-ray diffraction (XRD) studies, revealed a spectacular structural phase transformation from anatase to ruffle on doping and associated morphological changes are seen by scanning electron microscopy (SEM). The phase transformation is promoted with the increase of W concentration up to 50 ppm and inhibited thereafter. Microstructural lattice distortions due to the presence of W6(+) ions are monitored by high-resolution transmission electron microscopy (HR-TEM). The valence band X-ray photoelectron spectra (XPS), Ultraviolet photoelectron spectra (UPS) and UV-Vis absorption studies confirm the reduction of band gap for W doped TiO2 mixed phase nanoparticles. The doping causes an enhanced visible light absorption. Room temperature photoluminescence (PL) spectroscopy ensures an oxygen vacancy mediated phase transformation. The photocatalytic performances of the W doped TiO2 catalysts are evaluated for the degradation of Rhodamine B (RhB), Methylene blue (MB) and Methyl orange (MO) aqueous solutions under visible-light irradiation. The efficiency of photocatalytic degradation of the dyes gradually increased with the content of W6+ ions (fill 50 ppm) and then decreased which is consistent with the structural and optical studies. The control experiments, showed possible oxidative species, are hydroxyl (center dot OH-) and oxygen (center dot O-2(-)) radicals while the earlier plays a dominant role. The resuability tests performed for five cycles demonstrate high stability and reusability of the photocatalysts. A possible synergistic mechanism based on the impurity energy levels of W6(+) ions, oxygen vacancies and anatase-rutile heterojunction for the enhancement of the photocatalytic activity under visible-light irradiation is proposed.