The Synthesis And Application Research Of Rare Earth Elements Co-doped ZnWO₄-TiO₂ Upconversion Photocatalyst

TiO₂ has been widely used as the photocatalyst material for the dying wastewater treatment,while the extendence of the light spectrum and the improvement of the photocatalyst’s efficiency are the two barriers for the photocatalytic degradation performance:（1）TiO₂ can only use the ultraviolet（UV）light of the solar spectrum,which occupies only 5%of the sunlight energy,leading to the low energy utilization rate of photocatalytic degradation;（2）The photo-generated electronic-hole would recombine quickly,which could also reduce the efficiency of photocatalytic degradation.Due to the two problems above,in this work,the doping of rare earth on conversion material,which can take advantage of the long wavelength,the low frequency light to excite short wavelength,high frequency light,which can not only use the energy of the light source with direct contact,but also can use the light energy through the transformation process of upconversion,thus enhancing the utilizing rate of the light.In order to prevent the recombination of photo-generated electronic-hole pairs effectively,this research use ZnWO₄,a kind of semiconductor material,to combine with TiO₂,which can form n-n heterogeneous structure,making electrons and holes are transferred to different semiconductor conduction band and valence band,respectively.What’s more,ZnWO₄ is also a kind of effective conversion luminescence substrate material,as the carrier of rare earth elements.The main findings were shown below:（1）The Yb³⁺,Tm³⁺rare earth co-doped ZnWO₄（ZYT） materials were prepared by the precipitation method,and the dosage of Tm³⁺and Yb³⁺were optomized.In the process,to determine the best doping rare earth ratio first and eventually conclude that when the content of Tm³⁺ion is 0.5%,Yb³⁺ion content is 20%of ZYT,displays the best upconversion performance.（2）TiO₂/ZnWO₄:Yb³⁺,Tm³⁺（TZYT）were synthesized using the sol-gel method with ZnWO₄:Yb³⁺,Tm³⁺（ZYT）,TTIP and anhydrous ethanol,by the sol-gel method,to synthesis of TiO₂/ZnWO₄:Yb³⁺,Tm³⁺（TZYT）samples.It can be observed that the TZYT sample particle size is smaller than ZYT and TiO₂,the specific surface area of TZYT samples were around 75m²/g,and those of ZYT and TiO₂ are only 13m²/g and37m²/g,respectively.Under the condition of 980 nm laser excitation,the sample of transformation can be seen in the upconversion performance,which exist emission peaks on 364 nm（UV light）,484 nm（green light）and 656 nm（red light）.Photocatalytic degradation results of each sample under a simulated sunlight in full spectrum and near infrared spectra to degrade methyl orange（MO）,the removal rate of 5%TZYT samples are the maximums both under the condition of full spectrum and near infrared spectroscopy,which are 55.6%and 9.0%,respectively,and the removal rate of all the TZYT sample are better than ZYT and TiO₂.（3）To improve the photocatalytic degradation of TZYT,different synthesis conditions of pH（1¹3）,aging time（0¹2h）,calcination temperature（300⁸00℃）were optimized.Rutile phase TiO₂only existed when the pH was 1,while with the increase of pH value there only existed anatase phase TiO₂.When the pH were under acid conditions,the highest removal rate of 5%TZYT samples of MO under the condition of full spectrum in the sun reached to 84.2%,the stronger the alkaline,the removal rate was lower,this was because the greater of alkaline solution,TTIP hydrolysis happened,forming a white precipitation.Anatase phase occurs under the temperature of 300℃ condition,with the increase of temperature,anatase phase TiO₂ increased,but when after 500℃,rutile phase TiO₂ began to appear,as temperatures continued to rise,anatase phase TiO₂ would transfer to the rutile phase TiO₂.At 400℃,the highest removal rate of 5%TZYT sample on MO could reach to 96.5%,while the removal rate under 800℃was only8.7%.Thus,calcination temperature on 5%TZYT sample had impact influence on the removal rate of the MO solution.For the aging time,the TiO₂ phases in all samples seemed to be similar,and no difference appreance was observed.The best MO degradation rate was observed in the TZYT with the aging time of 12 h,with 76.5%,while that of 0 h was 47.7%.（4）The degradation mechanism were discussed with TZYT under the excitation with 980nm laser,and the medium products,the electron-hole pairs or hydroxyl radicals,superoxide free radicals were captured and tested,to track the upconversion photocatalytic performance:Under the light of near infrared（980nm）,the energy of photon was sufficient to make Yb³⁺ion transfer from the ground state ²F_7/2 to excited state ²F_5/2.The energy generated was transferred to Tm³⁺ions through ETU.The Tm³⁺ions transfer from the ground state ¹D₂ to excited state³H₆,from the ground state¹G₄ to excited state ³H₆,as well as from the ground state ¹G₄ to excited state ³F₄.The energy produced by the rare earth ions was transferred to the semiconductor material pure TiO₂ and ZYT by FRET.Electron transferred from TiO₂ to ZYT,and the·OH free radical was generated by the electron hole capturing the water molecule,and the O₂^-·free radical was generated by the electron hole capturing the oxygen molecule,which realizes the photocatalytic degradation.