Preparation And Performance Research Of Tm, N Doped TiO₂ Nano-composite Photocatalysts

To improve the solar utilization ratio of nano-TiO₂, inhibit the recombination of photogenerated e-/h+ and enhance the quantum efficiency. TiO₂ nano-composite photocatalyts with high performance have been exploited, which could offer novel photocatalytic materials used in the treatment of environmental pollutants with photocatalytic technique.Tm-doped TiO₂ nano-composite photocatalysts with different doping contents or calcined at different temperatures were prepared by a sol-gel method. The effects of Tm-doping contents and calcination temperatures on their phase structures, crystallite sizes, light absorption performances and photoluminescence properties were investigated by the techniques such as XRD, DRS and PL. Their photocatalytic activities were evaluated by the photocatalytic degradation of methylene blue (MB) in aqueous solution. The results indicated that the presence of low amount Tm in TiO₂ could strongly inhibit the phase transformation from anatase to rutile. However, the effect of inhibition could gradually weaken with the increase of Tm-doping content. Tm-doping could result in both a slight decrease of their light absorption performance in ultraviolet region and a blue shift of their optical absorption edge. It can be verified that the decrease sequence of PL spectrum intensity for the samples doesn't accord with the increase sequence of their photoactivity completely. Tm-doped sample with doping contents of 0.075wt% calcined at 550℃show the highest photocatalytic activity, which present two phase structure and include the anatase phase of 91%. Its crystallite size is 24.48 nm. The main origin of the higher UV photoactivity of the Tm-doped sample is that the separation of the photogenerated electron-hole pairs can be promoted by Tm-doping, leading to increasing quantum efficiency.On this basis, preparation process research of Tm-doped, N-doped and (Tm, N)-codoped TiO₂ nano-composite photocatalysts synthesized via a sol-hydrothermal method was systematically carried out. The structures and performance characterizations of prepared samples were investigated by the techniques such as XRD, BET, XPS, FT-IR, DRS and PL. In the meantime, the influencing mechanism of Tm and N doping on TiO₂ photoactivity was discussed. The results indicated that Tm-doped sample with doping contents of 0.075wt% calcined at 550℃showed excellent UV photoactivity. Its UV photoactivity is 63.19%, its specific photoactivity is 5.92×10^-5 mol·g^-1·h^-1. Compared with Tm-doped sample (its specific photoactivity is 8.64×10^-6 mol·g^-1·h^-1) prepared by sol-gel, its specific photoactivity markedly improved 6.85 times. N-doped sample with ammonia water adding amount of 2 mL calcined at 440℃showed excellent visible light photocatalytic activity. Its visible light photoactivity is 62.63%, its specific photoactivity is 5.87×10^-5 mol·g^-1·h^-1. Compared with N-doped sample (its specific photoactivity is 6.59×10^-6 mol·g^-1·h^-1) prepared by sol-gel, its specific photoactivities markedly improved 8.91 times. Obviously, samples prepared by the sol-hydrothermal process could lead to excellent microstuctural properties, higher crystallinities, smaller grain sizes, higher photoactivies. These indicated that the sol-hydrothermal process were significantly better than sol-gel process. The Tm-doping can bring about higher photogenerated electron-hole separation efficiency, effectively inhibit the phase transformation, improve crystallinity degree. These factors facilitate its improvement of UV photoactivity. The N-doped sample exhibits outstanding visible light photoactivity. The N-doping can bring out its absorption edge red-shift, broaden light response range to visible light region, produce large numbers of surface states and defects, as well as increase the contents of surface hydroxyl groups, which result in the enhancement in visible light photoactivity. Visible light photoactivity of (Tm, N)-codoped sample was between that of TiO₂ and that of N-doped TiO₂, UV photoactivity lower than TiO₂, indicating no expectative synergetic effects produced from Tm and N codoping.