Study Of Nonmetals And Rare Earths Co-doped TiO₂ With High Visible Light Activity

With the increase in human population, resources are becoming the problem concerned to all the people. Water and energy are the two indispensable resources of the development of human society. As a type of semiconductor photocatalyst, TiO2 can be used in hydrogen production by photosplitting of water, which is a new method to produce energy. Because of the advantages of non-toxic and low cost, TiO2 can be used for water treatment and air purification by advanced oxidization process. But the problem with TiO2 is that, because of wide band gap, TiO2 can only be excited by the UV light of wavelength below 387 nm, and the photogenerated electrons and holes are easy to recombine. So it is important to synthesize high efficient visible-light-responsive TiO2 for the photocatalytic reaction.In order to counter the problems of TiO2 for the practical application, co-doping is used in the present research to modify TiO2 which enhances its photoactivity in visible light region of solar spectrum. The main work is composed of five parts:1. TiO2 co-doped with samarium and nitrogen was synthesized by co-precipitation method with titanium sulfate as titanium precursor and ammonium hydroxide as precipitating agent and nitrogen precursor. The samples were characterized by XRD, TEM, XPS, BET and FTIR. The photocatalytic activities of the samples were evaluated by degradation of salicylic acid under visible light irradiation. UV-Vis spectra showed that N-TiO2 had strong absorption in the visible light region. XPS results showed that nitrogen was incorporated into TiO2 matrix in the form of interstitial doping model existing in the bond of Ti-O-N and O-Ti-N. The doping of samarium inhibited the growth of crystallite size and the transformation from anatase to rutile phase. Meanwhile, the addition of samarium was beneficial for the adsorption of the target contaminant and the transfer of the photogenerated carriers. Therefore, Sm/N-TiO2 presented much higher photocatalytic activity than N-TiO2 and pure TiO2 under visible light irradiation. In our research work, the optimal dosage of samarium was 1.5% for the highest photocatalytic degradation and the sample calcined at 400℃showed the best photoactivity. This could be explained by better absorption in visible region, the appropriate crystallite size, high surface area, stronger pre-adsorption of target contaminant and more efficient separation of electrons and holes.2. TiO2 co-doped with europium and nitrogen was synthesized by precipitation and peptization method and characterized by XRD, XPS, UV-vis spectra and TEM. The results showed the samples were composed mainly of anatase with a little brookite. The great enhancement in photocatalytic activity was attributed to the synergistic effect of nitrogen and europium.1.0Eu/N-TiO2 showed the highest activity and 88% salicylic acid was degraded after 5 h visible light irradiation. The adsorption isotherms illustrated that europium doping was beneficial for the adsorption of salicylic acid and increased the adsorption rate constant and maximum adsorption amount. Meanwhile, for the varied valence, Eu is the shallow trap of electrons and suppresses the recombination of electrons and holes efficiently. The probable degradation mechanism of salicylic acid was investigated by the addition of NaF, Na2S2O3 and K2S2O8. It was verified that salicylic acid was first adsorbed on the surface of the catalysts, followed by the degradation from the hydroxyl radicals generated by the holes (hvb+).3. Terbium and nitrogen co-doped TiO2 was synthesized by in situ esterification and trolamine was used as the nitrogen precursor. The photocatalytic activity was evaluated by the degradation of 20 mg/L AO7. The usage of sulphuric acid as the catalyst for esterification increased the acidity of the surface of the catalyst which enhanced the adsorption of the contaminat. The doping of Nitrogen extended the absorption range to the visible region. The addition of Tb restrained the growth of the crystallite and further enhanced the adsorption of the contaminant. When the dosage of Tb was 1%,1.OTb/N-TiO2 showed the highest photocatalytic activity. After 5 h irradiation under visible light,100% AO7 was decolorized. After 10 h irradiation, the ring of benzene and naphthalin was degraded.4. Mesoporous TiO2 was synthesized by hydrolysis of titanium sulfate for two times free of template. Sm was loaded on the mesoporous TiO2 by impregation in the solution containing certain amount of Sm(NO3)3. Then the samples were hydrothermally treated at 180℃in glucose solution. Sm, C co-doped TiO2 was obtained after washing, filtering and calcined at 250℃for 2 h.0.5Sm/C-TiO2 showed the highest activity under the irradiation of visible light. After 5 h irradiation, the degradation of 20 mg/L phenol is about 95%. The addition of samarium restrained the coaggregation of the TiO2 nanoparticles and ensured the small and homogeneous crystallite size. Carbon is incorporated into TiO2 matrix in the form of interstitial and substitutional and new band was formed in the band of TiO2. The synergistic effects of samarium and carbon enhanced the absorption of visible light, improved the separation of photogenerated holes and electrons and thus increased the visible light photocatalytic activity obviously.5. Well-ordered mesoporous TiO2 co-doped with nitrogen and ytterbium was successfully synthesized by an evaporation-induced self-assembly (EISA) method. TBOT was used as the titanium precursor, n-Butanol was used as solvent and melainine was used as nitrogen precursor. The as-prepared and calcined mesoporous TiO2 materials were characterized by XRD, TEM, UV-vis absorbance spectroscopy, XPS and N2 adsorption/desorption measurements. After calcination at 350℃, the 3.0Yb/N-TiO2 had the highest surface area (219 m2g-1) and retained 2D hexagonal ordered mesoporous structures. In the presence of 3.0Yb/N-TiO2,75% of 20 mg/L phenol solution was degraded after 10 h visible light irradiation and 10 mg/L rhodamine B (RhB) was degraded completely after 4 h. The significant enhancement in the photocatalytic activity of the 3.0Yb/N-TiO2 is attributed to the synergistic effect of N and Yb. The N dopant extended the absorption to the visible region and the Yb dopant was beneficial for stabilizing the mesoporous structure and restraining the recombination of photogenerated holes and electrons.