| Nano TiO2-xNx powders were prepared by chemical precipitation process using self-doping. As-prepared TiO2-xNx powders show yellow color and can absorb visible light with the range from 400nm to 800nm. The structure and properties of TiO2-xNx photocatalysts were characterized by UV-vis (UV-visible absorption spectrometer), DTA-TGA, XRD, FE-SEM(Field emission scanning electron microscopy), TEM, BET, FTIR(Fourier transform infrared spectroscopy), XPS(X-ray photoelectron spectroscopy) and ESR(Electron spin resonance). From the results of XPS, ESR and coloration feature, it was inferred that N was embedded into TiO2 powders to form interstitial and substitutional N doping,and N doped in the part of oxygen vacancy was crucial to emerge visible-light activity. Moreover, N doping played a significant role in restraining reoxidation of oxygen vacancy.Doping content of N could be controlled by reactant concentration, pH value and heat treatment temperature. It had also be systemically investigated that the effect of adding dispersant, heat treatment temperature, precipitator species, washing times for hydrate, pH value, preparation method on the absorption intensity in the range of visible light and the visible-light activity. It was shown that addition of dispersant was detrimental for the visible-light activity of TiO2-xNx. At the range from 400nm to 500nm, the absorption intensity was strongest when TiO2-xNx powders were heated at 400℃. The visible-light activity of TiO2-xNx was improved after appropriately increasing washing times of the TiO2·nH2O hydrate or raising pH value. The visible-light activity of TiO2-xNx prepared by chemical precipitation process was better than by mechanochemical method.In this study, the visible-light activity of TiO2-xNx powders was compared with immobilized TiO2-xNx. After immobilization, visible-light activity of TiO2-xNx photocatalysts declined remarkably. For the purpose of overcoming this limitation, the immobilized TiO2-xNx photocatalysts were modified by noble metal. The results showed that the visible-light activity of immobilized TiO2-xNx could be enhanced by Ag,Pt,Pd and Rh depositing on its surface. The immobilized TiO2-xNx for degrading methyl orange solution was in the order of Pd/TiO2-xNx>Pt/TiO2-xNx>Rh/TiO2-xNx>Ag/TiO2-xNx>TiO2-xNx, but the order of photocatalytic activity became Ag/TiO2-xNx>Pd/TiO2-xNx>Pt/TiO2-xNx>TiO2-xNx under UV light irradiation. Thus this can be seen that there is large difference between the photocatalytic activity of M/TiO2-xNx under UV light and under visible lightirradiation, due to the noble metal particles existing in different chemical state on the surface of M/TiO2-xNx, which leading to different photocatalytic mechanism of M/TiO2-xNx photocatalysts. Under UV light irradiation, the noble metal particles can exist as Ag0, Pt0, Pd0, and partial PtCl4, PdO. As a consequence, the photocatalytic behavior of the M/TiO2-xNx can be explained by the metal-semiconductor contacts. That's the lower the Fermi level and the smaller the work function of the noble metal, the higher the photodegradation percentage of methyl orange solution. While under visible light irradiation, Ag,Pt,Pd and Rh exist as Ag0, PtCl4, PdO and Rh0 respectively, which results in that the photocatalytic activity of the M/TiO2-xNx depends on the generation and transfer pathway of photogenerated electron in M/TiO2-xNx photocatalyst. As for Pt/TiO2-xNx and Pd/TiO2-xNx, the absorption intensity in visible light range is enhanced due to the unique electron excitation process. The photogenerated electrons can be transferred from Pt3+ and Pd+ to the Conduction Band (CB) of TiO2-xNx photocatalyst, as a result that Pt/TiO2-xNx and Pd/TiO2-xNx exhibit good photocatalytic activity under visible light irradiation.
|
PDF Full Text Request