Preparation And Properties Of Codoped TiO₂ Visible Light Potocatalytic Materials

In recent years, photocatalysts have attracted considerable attention as they can be used to treat organic pollutants, purify air and produce hydrogen by photocatalytically splitting water. Among the oxide semiconductor photocatalysts, titania is currently a promising photocatalyst because of its low cost, non-toxicity, chemical stability and excelent optical property. However, there have been three obvious deficiencies for practical application of these photocatalysts. The bandgap of TiO2 is wide (≈3.2 eV), so only a small ultraviolet fraction of solar light can be used. The separation of nanosized photocatalysts from wastewater and their recycling are difficult and uneconomical. Moreover, the photocatalytic oxidation process for the pollutants in wastewater is not successive, which results in a low treatment efficiency and an increased loss of photocatalyst. To resolve these problems, in this paper, based TiO2 visible light photocatalytic materials have been synthesized by sol-gel method, vapor hydrolysis-nitridation method and polymer gel template method. The results showed that as-prepared samples revealed excellent visible light responding and reusing properties. Meanwhile, the organic compounds in wastewater can be degraded under continuous-flow with visible light irradiation.Boron and fluorine codoped TiO2 visible-light-driven photocatalysts were prepared by sol-gel method. XRD results showed that the codoping of boron and fluorine could not only efficiently inhibit the grain growth but also suppress the phase transition of anatase to rutile. When the molar ratios of B and F to Ti are 20:100 and 5:100 and calcined at 600℃, UV-vis spectroscopy of the sample showed better visible light responding. XPS results revealed that the Ti-B bonds and Ti-F bonds have formed, which could extend the absorption spectra to visible light region and enhance visible light photocatalytic activity. When the molar ratios of B and F to Ti are 20:100 and 5:100 and calcined at 600℃, the degradation rate of 4-chlorophenol (4-CP) over the sample was about 1.5 to 3.0 times higher than that for other doped samples and undoped TiO2. The total organic carbon (TOC) removal rates of 4-CP showed that 4-CP has been mineralized efficiently with the sample under visible light illumination.Fluorine and carbon codoped TiO2 photocatalysts were synthesized by a simple sol-gel process using tetrabutyl titanate (TBT), ethylenediamine and NH4F as precursors. Results showed that carbon and fluorine codoping could not only efficiently inhibit the grain growth but also suppress the phase transition of anatase to rutile. Moreover, the codoping of carbon and fluorine results in the enhancing of BET surface area for as-prepared samples. Disordered pores have formed by the assembly of TiO2 crystallites during the calcinations process. XPS results showed that the Ti-C and O-Ti-C bonds have formed, which can extend the optical absorption into visible light region and induce new active sites. It also showed that two kinds of fluorine species were formed, which result in higher photocatalytic activity in visible light region. When the volume of ethylenediamine and the molar ratios of F to Ti are 1 mL and 5:100 and calcined at 400℃, the degradation rate of 4-chlorophenol (4-CP) over the sample was the highest than other samples.La2O3/TiO2-xFx photocatalysts were prepared by a simple sol-gel process using tetrabutyl titanate (TBT), La(NO3)3 and NH4F as precursors. XPS results revealed that La2O3 accumulated on the surface of TiO2, which enhanced the surface area of TiO2 and inhibited the recombination of electron-hole pairs. It also showed that two kinds of fluorine species were formed, and which increased the acid active sites and enhanced the oxidation potential of the photogenerated holes in the valance band. UV-vis diffuse reflection spectra of LaO3/TiO2-xFx showed that intraband gap states were present and these are probably responsible for its absorption of visible light while the intrinsic absorption band was shifted slightly to a longer wavelength. At molar ratios of La and F to Ti of 1.5:100 and 5:100 and after calcination at 500℃, the degradation rate of 4-chlorophenol (4-CP) over the sample was about 1.2 to 3.0 times higher than that of the other doped samples and undoped TiO2. The total organic carbon (TOC) removal rates of 4-CP showed that 4-CP was mineralized efficiently in the presence of the sample under visible light illumination.Tetrapod-like core/shell ZnO/TiO2 was prepared by vapor hydrolysis method (VHM) at low temperature. Nitrogen doped tetrapod-like core/shell ZnO/TiO2 visible light driven photocatalysts were synthesized under NH3 flow at different temperatures. FE-SEM and TEM results showed that the as-prepared sample still reserved the T-ZnO shape, suggesting that the photocatalysts can be easy collected from wastewater. XPS results revealed that the N-Ti-0 and Ti-N-0 bonds have formed and the content of nitrogen increasing with temperature enhancing. UV-vis spectra indicated that the optical absorption is stronger, the nitridation temperature is higher. the degradation rate of 4-chlorophenol (4-CP) over the as-sample presented that the temperature of nitridation is a important fact for the photocatalytic activity of as-prepared sample.The TiN nanocrystals were prepared by nitridation of core-shell structured ZnO/TiO2 at 1000℃, which consisted of thin shell of tiny TiO2 nanoparticles deposited on tetrapodlike ZnO by a vapor hydrolysis. The core of ZnO was then removed during nitridation due to the evaporation of Zn species. FE-SEM and TEM results showed that the as-prepared TiN particles with porous and hollow structure were in a crystallite size of 24 nm, and still reserved the shape of tetrapod-like ZnO template. The pore size distribution curves showed that TiN had a narrow pore size distribution with a maximum around 45 nm. The formation of mesopores can be contributed to the assembly of TiN grain during nitridation. The BJH desorption cumulative pore volume and the BET specific surface areas were 0.107 cm3·g-1 as well as 13.8 m2·g-1 for the sample, respectively. The presented structures can improve the diffusion and adsorption of mass and electron transport through its pores. The combination of the hollow and porous structure and novel morphology may be useful for electroanalysis, catalysis and pH sensors.Hierarchical porous, self-supporting La- and F- codoped TiO2 with high durability was prepared using a simple polymer gel template. The TiO2 foams were in the anatase phase, with a specific surface area of up to 88 m2/g and showed a network-like structure containing meso-/macropores. The macropores were with a diameter of ca. 300 nm and a wall thickness of 100-300 nm, while the mesopores were narrow-distributed with a mean diameter of around 5.4 nm. XPS results revealed that La2O3 accumulated on the surface of TiO2, which enhanced the surface area of TiO2 and inhibited the recombination of electron-hole pairs. It also showed that two kinds of fluorine species were formed, and which increased the acid active sites and enhanced the oxidation potential of the photogenerated holes in the valance band. XPS and FTIR results indicated that Ti-S bonds have formed. The UV-Vis spectroscopy of the sample showed that the optical absorption have shift to visible light region, which arised from the Ti-S bonds and oxygen vacancy. The sample showed an excellent visible light photocatalytic activity and a high durability of photocatalytic activity for the degradation of methylene blue (MB) solution. This development could be very useful and widely applicable in wastewater treatment engineering.