Synthesis and optimization of photocatalytic performance of WO3-loaded TiO2 nanotube array layers

TiO₂ nanotube arrays were grown by anodic oxidation of the surface of Ti metal foils in an electrochemical cell with organic electrolyte containing fluorine, using constant bias voltage, 30 V. The average typical diameter and length of the as-prepared tubes were 87 nm and 3.8 μm, respectively. The titania nanotube arrays surface was subsequently loaded with WO₃ non-contiguous thin layers by plasma-assisted RF sputtering. The addition of WO₃ on the nanotube surface resulted in the formation of semiconductor heterojunctions, which lead to supplementary spatial separation of electron-hole pairs and reduced the recombination rate. Macroscopically, this is certified by increased lifetime of the photocatalytic activation and associated increase of surface wettability. In the current experiments, four sample sets with increasing WO₃ coverage were prepared by increasing the sputter-deposition time of the WO₃ material. Nanotube array morphology, structure and elemental composition were investigated by high-resolution scanning electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, and energy-dispersive x-ray spectroscopy. A comparison was made between the characteristics of the WO₃-loaded nanotube layers and a reference bare TiO₂ nanotube arrays layer. Annealing the samples at 420 °C led to the development of predominant anatase. The effects of UV irradiation and the presence of dopant fluorine species were further investigated and discussed as means to modifying the generation of hydroxyl radicals. Optimal conditions for the best photocatalytic performance of titania nanotube arrays were established via monitoring the degradation of methylene blue pollutant.