The Study On Synthesis And Photocatalytic Activity Of Nano-TiO₂Doped With Bi-RE-element

Recently, TiO₂-based heterogeneous photocatalysis has attracted more and moreattention, since it can degrade organic pollutants efficiently. However, there are stillsome problems in TiO₂-based photocatalysis, such as low quantum efficiency andlittle utilization of solar energy and so on. So it is a great challenge to solve theseproblems for many scientists. It is found that doping is considered to be an efficientway in respect of enhancing the photocatalytic activity. Rare earth elements havespecial electronic structure, which can serve as electron trap and suppress therecombination of electron-hole pairs. Single element doping has been subject ofextensive research in TiO₂doping with rare element. To the best of our knowledge,the photocatalytic activity of RE metals codoped TiO₂has seldom been reported. Inaddition, traditional sol-gel method will result in the aggregation of the nano-TiO₂particles in the calcination process. It is easy to form larger particles. However,improved sol-gel method （ultrasonic assisted sol-gel） can be helpful to reduceagglomeration.In this paper, nano-TiO₂photocatalysts doped with La, Sm, Y, Zr weresuccessfully synthesized by ultrasonic assisted sol-gel method. The photocatalyticactivities were evaluated using methyl orange （MO） as target pollutant under UV lightirradiation. The effect factors, such as the species and the amount of dopant, thecalcinated temperature and so on, had been investigated in the experimental. Furthermore, the difference between single-element doping and bi-element doping had beenstudied well. The influences of doping on the structure, morphology, opticalabsorption capacity, chemical configuration were systematically studied by usingX-ray diffraction （XRD）, scanning electron microscopy （SEM）, UV-visible diffusereflectance spectra （UV-vis DRS） and X-ray photoelectron spectroscopy （XPS）,respectively. The conclusions can be drawn as the follows:（1） The photocatalytic activities of nano-TiO₂obtained from ultrasonic assistedsol-gel were significantly higher that prepared by traditional sol-gel method.Dispersion of nano-TiO₂prepared by ultrasonic assisted sol-gel method had beensignificantly improved. It could improve the higher photocatalytic activities due toincrease the contact area with the substrate.（2） The photocatalyst series were prepared by ultrasonic assisted sol-gel method,using La（NO₃）₃·6H₂O, Y（NO₃）₃·6H₂O, Sm（NO₃）₃·6H₂O, Zr（NO₃）₄·5H₂O as dopant resources. The effect of concentration of dopant, annealing temperature and thespecies of doping element were investigated. The experimental results showed thatphotocatalytic activities of impure samples were improved largely compared to thebare TiO₂. It indicated that doping can improve the photocatalytic activity ofnano-TiO₂. The optimum percentages of dopant for La, Y, Sm, Zr doped photocatalystcalcined at500oC were0.5%,0.5%,0.5%,0.3%respectively. There were certainrelationships between separation of election and hole and photocatalytic activity.Probably the dopant would surpress photoinduced electron-hole pair separation athigher or lower doping content. Obviously, the optimum annealing temperature of allsamples was500oC.（3） There always exist unfavorable factors, such as oxygen vacancy and surfacedefect, brought by charge imbalance for single element doping. However, thebi-element doping is likely to maintain charge balance via charge compensation in thecrystals. At the optimum doping percentage and annealing temperature, La/Y-TiO₂,Sm/Y-TiO₂and Sm/Zr-TiO₂were synthesized successfully by ultrasonic-assistedsol-gel method respectively. Compared to single element doping, co-doping samplesshowed the higher photocalytic activity. It indicted that dopants of co-doping played asynergistic role in the improvement of the photocatalytic properties of titania. Inaddition, co-doping system accelerated the formation of·OH, which would helpimprove the photocalytic activity. However, Compared with the Sm or Y-dopedsamples, the codoped sample presented poorer UV-light absorption behavior. Thephotocalytic activity was lower due to due to the synergistic reaction of two dopingelements leading to recombination of electron and hole.（4） First-principles density functional theory （DFT） based calculations of theelectronic and optical properties of pure, La, Y, La-Y, Zr doped anatase rutile TiO₂had been performed using the CASTEP module of Materials Studio packagerespectively. It confirmed that the electron on the valence band can transit to theimpurity states because of doping. So the electronic excitation in doping systembecame easier compared to pure TiO₂, and the absorption edge shift to the visible lightregion. The result of computation and theory were comparable with those obtainedfrom experiment.