Preparation & Characterization Of Novel N-doped TiO₂ And The Investigation On Its Visible Light Photocatalytic Behavior

Many researchers have attempted to extend the light responsible range of TiO₂ from the ultraviolet (UV) to the visible light region. One approach is to dope metal-ion so that it is sensitive to visible light. However, the metal-doped materials suffer from a thermal instability , an increase of carrier-recombination centers, or the requirement of an expensive ion-implantation facility . Another method is to modify TiO₂ with nonmetal-ion. Recently,the N-doped TiO₂ has attracted wide attention for its high photocatalytic activity in the visible light region.In this study, novel N-doped TiO₂ was obtained by thermal treatment of nanotube titanic aci(denoted as NTA) in NH3 flow, the products were denoted as N-NTA. P25-TiO₂ was also treated under the same conditions as that of NTA, and the products obtained were denoted as N-P25. The photocatalytic activity of N-NTA and N-P25 samples were evaluated by the oxidation of propylene. The results show that with the increase of NH3 treating temperature, both series of samples have a max propylene removal situated at T=600°C, 24.9% for N-NTA(600) and 7.3% for N-P25(600), respectively. The former is ca. 3.4 times of the latter. The XRD and DRS results show that this novel N-doped TiO₂ belongs to anatase and can absorb visible light. The chemical state of the doped-N atoms incorporated into the TiO₂ particles is determined in the form of NO by X-ray photoelectron spectra. ESR results demonstrate that this novel N-doped TiO₂ have a large number single-electron-trapped oxygen vacancy (denoted as Vo?), and modified by chemisorbed NO. Asymmetrical ESR peak centered at 3484G, g=2.004, , which represent the characteristics of Vo?. Accompanying with the NH3 treating temperature increased to 500°C and 600°C, the intensity of g=2.004 peak(h) enhanced, at the same time, two weak satellite peaks (3454G, g=2.023; 3516G, g=1.987) appear at its both sides. The distance from satellite peak to g=2.004 peak center is 32±1 G.Synthetically considering the characterization results of N-NTA, three conclusions were obtained as follows:1, Novel N-doped TiO₂ with single-electron-trapped oxygen vacancy were prepared by the dehydration of nanotube titanic acid under NH3 atmosphere. The products have a continuous absorption in the visible light region.2, The chemical state of the N atoms incorporated into the TiO₂ particles is determined to be NO from X-ray photoelectron spectra, which can effectively suppress the photoluminescence of V_o^·defects so as to facilitate the photogenerated charges transfer to the surface reactive centers to conduct redox reaction.3, This novel N-doped TiO₂ belongs to anatase and its visible-light photocatalytic activity is originated from the photoactive centers, i.e. single-electron-trapped oxygen vacancy (denoted as V_o^·) modified by chemisorbed NO in well-crystallized anatase TiO₂ (denoted as V_o^·-NO-Ti). The higher V_o^·-NO-Ti concentration, the better visible light photocatalytic activity.The photocatalytic activity of N-NTA and N-P25 samples were also evaluated by decoloration of methylene blue irradiating under visible irradiation. The results show that the visible light photocatalytic activity of novel N-doped TiO₂ is higher than N-P25 samples.Based on the discussion, we inferred that the formula for the NH3 treatment of NTA at T=400-600°C may be written as follows: Reaction takes two steps: i, Accompanying with the dehydration, NTA converts from orthorhombic form to anatase TiO₂-x(V_o^·)x; ii, NH3 reacts with oxygen atom of TiO₂-x(V_o^·)x to form NO with additional V_o^·, NO will be chemisorbed in the vicinity of V_o^·. At T=700°C, a transition temperature for NH3 treating of NTA, TiN forms (formula 3). Though TiN can strongly absorb visible light, it is inert for photocatalytic oxidation of C₃H₆:To confirm it correct, NTA was calcinated at 600°C for 2h in air atmosphere to prepare novel anatase, and then in NH3 atmosphere for 4h to prepare N-doped TiO₂, which denoted as N-NTA624. The investigation on the visible-light photocatalytic behavior of N-NTA624 and N-P25(600) correlated with their physicochemical properties was conducted.