Preparation And Characterization Of TiO₂Nanocomposite Photocatalyst With Visible Light Response

To improve the solar utilization ratio of nano-TiO₂and the quantum efficiency,TiO₂nano-composite photocatalyts with high-performance and visible light responsewere exploited, which could offer novel high-performance photocatalytic materials forthe treatment of environmental pollutants with a photocatalytic technique.Firstly, un-doped, N-doped, Gd-doped and Gd-N-codoped TiO₂nanophoto-catalysts were successfully prepared by a sol-hydrothermal method. Their photocatalyticactivities were evaluated by photocatalytic decoloration of methyl orange （MO） dye inaqueous solution under UV and visible light irradiation, respectively. Thephysicochemical properties of as-prepared samples were characterized by XRD, XPS,BET, UV-vis and PL analytic techniques. The effect mechanisms of Gd-N-codoping onthe photoactivity of TiO₂were also discussed. The results showed that Gd-doping couldeffectively inhibit the recombination of photogenerated electron-hole pairs, thenenhancing photo quantum efficiency. Gd-doping could inhibit the growth ofcrystalline size and improve the surface textural properties. Therefore, the UVphotoactivity was obviously enhanced by Gd-doping. N-doping could make theabsorption edge red-shift to visible light range by narrowing the electronic bandstructure, which enhanced the visible light photoactivity. Gd-N-codoping could furtherincrease the absorption to visible light by narrowing the electronic band structure, andfurther decrease the crystalline size and improve the surface textural properties,therefore increasing the thermal stability. Compared to the single-doped TiO₂,Gd-N-TiO₂significantly improved the photoactivity for decoloring methyl orange inaqueous solution under visible light irradiation, which was ascribed to the synergeticeffect of Gd-N-codoping.To further enhance the visible light photoactivities of nano-TiO₂, on the basis of theresearch on the Gd-N-doping, Gd-N-P-tridoped anatase-TiO₂nano-compositephotocatalyst was successfully prepared by a one-pot two-phase separatedsol-solvothermal process using phosphoric acid as phosphorus source. Thephysicochemical properties of as-prepared samples were characterized X-ray diffraction,N₂physical adsorption-desorption, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, UV-vis absorbance spectroscopy and photoluminescencespectroscopy. The photocatalytic activities of the sampleswere evaluated by degradationof4-chlorophenol （4-CP） under simulated sunlight irradiation. Results indicated that theGd-N-P-tridoped TiO₂with P-doping amount of5wt.%exhibited the highestphotoactivity under simulated sunlight light irradiation among as-prepared undoped,single-doped and codoped samples, which should ascribed to a synergetic effect ofactions played by Gd, N and P. The tridoped sample achieves an excellentphotocatalytic performance with Kappof0.0322min^-1, which is2.75times more thanthat of the commercial P25TiO₂(Kapp=0.0117min^-1). Based on the effect of radicalscavengers and N₂purging on the photocatalytic process, we deduced that OH radicalis the most important active species for4-CP degradation. The enhanced photoactivityof Gd-N-P-tridoped TiO₂is attributed to the synergetic effect of enhanced visible lightabsorption, reduced charge carriers recombination, increased surface hydroxyls, andimproved surface textural properties.In addition, a novel high-performance Sm-N-P-TiO₂（note as SNPTO） wassuccessfully prepared by a sol-solvothermal process. The physicochemical properties ofas-prepared samples were characterized by X-ray diffraction, transmission electronmicroscopy, N₂adsorption-desorption isotherm, X-ray photoelectron spectroscopy,Fourier transform infrared spectroscopy, UV-vis absorbance spectroscopy,photoluminescence spectroscopy and zeta potential measurements. Results show that,among as-prepared undoped, single-doped and codoped samples, SNPTO presents thehighest photoactivity for degrading4-CP under simulated sunlight irradiation with theapparent rate constant of2.83×10^-2min^-1, which is3.98times more than that ofcommercial P25TiO₂(Kapp=7.11×10-3min^-1). The TOC removal rate for4-CP solution（20mg/L） in the presence of SNPTO （0.4g/L） reaches87.13%after simulated sunlightirradiation for120min, indicating that4-CP can bemineralized efficiently. Experimentsconcerning radical scavengers indicate that OH in the solution is the main reactivespecies. The enhanced photoactivity of SNPTO is attributed to a synergistic effect of theincrease in the visible light absorption ability, surface hydroxyl and specific surface area,the improvement of the surface textural properties, as well as more efficient inhibitionof the recombination of photogenerated electrons and holes. SNPTO exhibits goodphotochemical stability and can be reused for5times with less than1.6%decrease in the4-CP removal efficiency. This research provides a very effective and stable visiblelight active photocatalyst for environmental application.Study on degradation of industrial wastewater simulated by tetra-chlorophenol（4-CP） in SNPTO suspension under irradiation of a xenon lamp was carried out toinvestigate the influence of different conditions on the4-CP degradation rate. Theresults showed that4-CP is completely degraded and the apparent rate constant Kappachieves4.95×10^-2min^-1, under the conditions: a initial concentration of4-CP is40mg/L （adjust the pH value to5.23）, a light intensity is500W, a SNPTO dosage is1.5g/L, a solution temperature is309K, which is3.26times of P25TiO₂. The degradingreaction fits the first-order kinetics well within the range of experimental concentrationsand the degradation rate decreases with the increase of solution concentration.