Preparation Of BiFeO₃/TiO₂-Based Nanotube Arrays Composites And Their Photocatalytic Performance

As a typical advanced oxidation process, photocatalytic technology based on semiconductors has become a hot subject to address the elimination of environmental pollutants globally. As the most traditional photocatalyst, TiO2is cheap, non-toxic, stable and reused. However, for the practical application, it cannot meet all the high-demanding fields. Thus, the issue how to improve its photocatalytic performance has aroused widespread concerns. TiO2nanotubes inhenrently show high specific surface area and good electron transfer performance among various morphologies. Nevertheless, the two main defects of TiO2photocatalyst, i.e., low ratio of light energy utilization and easy recombination of photogenerated electron-hole pairs, limit its application in practice. As one of the representatives of perovskite-type materials, BiFeO3possesses unique ferroelectric and ferromagnetic polarization properties. By utilizing the intrinsic electric field of the ferroelectric structure to promote the separation of the photo-induced charges, the local photovoltage intensity and energy conversion efficiency on microscale will be enhanced, which could be utilized to enforce the photo activity of TiO2photocatalyst. Therefore, in this dissertation, BiFeO3and Co doped BiFeo.5Co0.5O3were employed to modify TiO2nanotube array electrode. The morphological, structural, optical and photoelectrochemical properties as well as photodegradation activity were well characterized and analyzed. The main contents are as follows.(1) In this context, firstly BiFeO3/TiO2nanotube arrays were prepared by an anodic oxidation combining electrochemical deposition method. The results showed that after modification of BiFeO3, light absorption of TiO2nabotube arrays exhibited obvious red shift, which improved photoinduced charge separation efficiency of TiO2nanotube arrays. The composite electrode showed an effectively improvement on the photocurrent, wherein the photoelectric conversion efficiency was increased by about7folders compared with TiO2nanotubes, and the modification of BiFeO3effectively restrained the recombination of photogenerated charges.(2) BiFeO3/TiO2nanotube arrays covered Ti electrodes were fabricated by combining anodization of titanium sheet and an ultrasonic immersing method followed by annealing.The photoelectric conversion efficiency with the composite electrode was4.6times of that with nanotube arrays. The photocatalytic degradation performances of rhodamine B was compared over the composite electrode and TiO2nanotube arrays under different experimental conditions. Under visible light irradiation, the degradation efficiency of the composite electrode was10times higher than pure TiO2nanotube arrays.(3) Co-doped BiFeO3, namely, BiFeo.5Coo.5O3, was prepared and loaded onto TiO2nanotube arrays by a vacumm-assisted impregnation technique followed by annealing. Characterization results showed that BiFe0.5Co0.5O3nanoparticles were successfully loaded on the walls of the tubes and above the entrances of tubes. The composite electrode significantly broadened the range of visible-light absorption by pure TiO2nanotube arrays. The separation efficiency of photoinduced electron-hole pairs was effectively enhanced. In addition, the photocatalytic degradation of trifluoroacetic acid (TFA) was investigated by in situ FTIR spectroscopy to evaluate the photocatalytic activity of the composite catalysts. BiFe0.5Co0.5O3/TiO2nanotube arrays exhibited good performance in photocatalytic degradation of gaseous TFA. The degradation ratio reached to75%in6h illumination of simulated sunlight, which was ca.7.5times as high as the degradation ratio by pure TiO2nanotube arrays. CO2was detected as one of the degradation products according to FTIR analysis.