Preparation And Visible Light Catalytic Properties Of C,n,f-doped Tio₂ Nanoparticles

Due to excellent photocatalytic properties, titanium dioxide has been widely used in many fields, such as degradation of pollutants, self-cleanning coating, and so on. However, more practical applications have been limited by its wide band gap, which require ultraviolet light as the optical excitation source. Shifting the optical response region of TiO₂ to visible spectral range means that the sun light can be used as the optical excitation source. It is helpful for saving energy and environmental protection. So many researchers are focused on enlarging the optical response region of TiO₂ to degrade organics under the sun radiation.It is an important way to prepare TiO₂ with visible light response by doping nonmental. In this paper, a series of TiO₂ photocatalysts were prepared using titanium tetrachloride as raw material. C-TiO₂ was synthesized using citric acid as carbon source by a simple hydrolysis method. N-TiO₂ and F-TiO₂ with visible light response were prepared using NH4NO3 and NaF as doped sources by hydrothermal method. The structure, morphology and light adsorption of such photocatalysts were characterized by XRD, TEM, SEM, UV-Vis, XPS, X-ray absorption fine structure(XAFS), etc. The photocatalytic degradation of methyl-orange was used as model reaction to evaluate the visible light activity of TiO₂ samples. It suggested that a best doping concentration existed for every dopant and the mechanism of doped TiO₂ for the visible-light activity was discussed.The research results were suggested as following:(1)The content of anatase phase of the sample rised with increasing amount of citric acid. The photocatalysts were well dispersed and their particle sizes increase with increasing amount of citric acid. The experimental result showed that the surface area of pure TiO₂ was 92.232m2·g^-1. After adding citric acid, the surface area of sample increased to 187.747m2·g^-1. The increase of surface area was helpful to improve their photocatalytic properties.(2)With the addition of NH4NO3, the samples were rutile phase composed of nanorods. The surface area of undoped TiO₂ was 41.479m2·g^-1. When the addition amount of NH4NO3 was 0.4%, its surface area was 50.645 m2·g^-1.(3)The TiO₂ samples prepared with small amount of NaF were rutile phase with the morphology of the microspheres composed of nanorods. When the mole percentage of NaF to titanium ions was 2.0%, the anatase phase could be observed in the TiO₂ powder. The content of anatase phase was 20.7% and the rutile phase was 79.3%, with the surface area of 40.355 m2·g-1.(4)The UV–Vis adsorption spectroscopy showed that C,N,F-doped TiO₂ appeared a certain visible light adsorption and the optical band edge exhibited a certain red-shift compared with that of undoped TiO₂ and Degussa P25. The photocatalytic activity of prepared samples were tested under 12W LED light with a major emission wavelength at 460nm as visible light source and methyl-orange as model reaction. The research suggested that the final degradation rate of the optimum samples were as following: 2.0%C-TiO₂ was found to be nearly 90% within 3h. 0.5%N-TiO₂, 0.4%F-TiO₂ were about 62% and 72% within 5h.(5)The XAFS technique was employed to analyse the change of the local structure around Ti atom. It showed the structure information of doping atoms, such as coordination number, bond distance, etc. With the increasing of N and F dopant, the coordination number and Ti-O bond become bigger. XPS and XAFS analysis indicated that C and F atoms had been doped in the lattice of crystalline TiO₂ by substituting the oxygen or Ti atom. Non-metal doping changed the energy band of TiO₂ and improved photocatalytic activity under visible light irradiation.