Preparation And Characterization Of Dy(Ⅲ) Doped Bismuth Molybdate And Bismuth Tungstate Photocatalysts

The ecological security problems brought for the rapid development of industry have become one of the biggest challenges facing the world today,and have aroused great concern from governments and academia in various countries around the world.Water environment security has the greatest impact on human production and life among the known ecological security issues.In recent years,the contradiction between water environment pollution and social industrial progress in China has become increasingly prominent.Due to the large amount of emissions from production factories such as pharmaceutical factories,chemical factories,dye factories,and the wanton use of agricultural fertilizers,more and more refractory,toxic and harmful macromolecular substances have accumulated in the waters,which seriously threatens human health and the safety of the ecosystem.Therefore,it is of great significance to treatment wastewater efficiently,safely and environmentally friendly for the sustainable development of society and healthy survival of human beings.Photocatalytic degradation technology is most popular due to its advantages of high efficiency,stability and safety among the known water pollution treatment methods.Photocatalysts can absorb most solar energy to decompose the harmful substances in wastewater into small inorganic molecules such as water and carbon dioxide through catalytic oxidation,so as to achieve the purpose of safe and environmental degradation.It is a new and efficient green environmental protection treatment technology with good application prospects.Bismuth-based photocatalytic materials have received more and more attention as an important highly catalytically active material.Among them,Bi₂Mo O₆ and Bi₂WO₆ are the two most representative oxides in the Aurivillius family.They have unique[Bi₂O₂]²⁺layered structure,and have important application value in the field of photoelectric conversion and photocatalysis.The activity of the bismuth-based photocatalyst is suppressed because of the disadvantages of low photogenerated carrier separation and migration rate,wide band gap,etc.Therefore,the bismuth-based photocatalyst materials Bi₂Mo O₆ and Bi₂WO₆ were modified by doped with lanthanide rare earth ion dysprosium(Dy³⁺)via solvothermal,roasting and one-step hydrothermal method respectively.Then,through XRD,SEM,TEM,FT-IR,PL,Raman,XPS,UV-Vis DRS,BET and many other characterization methods to analyze the structure and morphology of the samples.In addition,the samples activity was investigated by photocatalysis experiment which used Rhodamine B（Rh B）solution to simulate polluted wastewater and 300W xenon lamp was used to simulate sunlight.The main achievements of the paper are as follows.Dy³⁺doped Bi₂Mo O₆(Dy³⁺/Bi₂Mo O₆)photocatalysts with different molar ratios were prepared by solvothermal method and roasting method.The test results showed that the photocatalytic activity of Bi₂Mo O₆ increased first and then decreased as the amount of Dy³⁺doping increases.The sample with a doping molar ratio of n（Dy）:n（Mo）=3%showed the highest catalytic activity and its degradation rate to Rh B solution was close to 100%under visible light irradiation for 40min.The results of capture experiment showed that the holes（h⁺）,hydroxyl radicals（·OH）and superoxide radicals（·O₂^-）were the main active substances in the photocatalytic reaction of Dy³⁺/Bi₂Mo O₆.The band gap of Bi₂Mo O₆ decreases regularly with the increased of the doping amount of Dy³⁺,which lead to the expansion of the light response range by samples and more visible light can be absorbed and used.Scanning electron microscopy and nitrogen adsorption-desorption test analysis showed that the pure Bi₂Mo O₆ and 3%Dy³⁺/Bi₂Mo O₆ samples were composed of irregular nanosheets with high crystallinity and mesoporous structure.The dysprosium（Dy）was doping into the lattice of Bi₂Mo O₆ in the form of Dy³⁺ions partially replaced Bi³⁺,which reduced the grain size and the recombination rate of photogenerated electron-hole pairs,and increased the specific surface area of Bi₂Mo O₆.Dy³⁺doped Bi₂WO₆(Dy³⁺/Bi₂WO₆)photocatalytic materials with different molar ratios were prepared by a simple one-step hydrothermal method.The degradation rate of pure Bi₂WO₆ to Rh B solution was only 86.7%after 105 min under visible light irradiation.With the increased of Dy³⁺doping amount,the photocatalytic activity of Dy³⁺/Bi₂WO₆showed a trend of increasing first and then decreasing.The sample with the ratio n（Dy）:n（W）=5%had the highest photocatalytic activity which’s degradation rate to Rh B reached 99.8%under the same conditions.The capture experiment results show that superoxide radical（·O₂^-）is the most important active group in the photocatalytic reaction of Dy³⁺/Bi₂WO₆,followed by hole（h⁺）and hydroxyl radical（·OH）.Compared with pure Bi₂WO₆,5%Dy³⁺/Bi₂WO₆ had a lower recombination rate of the photogenerated electron-hole pair,smaller band gap,and wider photo-response range.Analysis of the crystal structure,micromorphology,surface chemical composition,optical properties and specific surface area of pure Bi₂WO₆ and 5%Dy³⁺/Bi₂WO₆ samples showed that the rare earth element dysprosium（Dy）was doped into the lattice of Bi₂WO₆ in the form of Dy³⁺ions.As the amount of doping increased,Dy³⁺partially replaced Bi³⁺and W⁶⁺successively,and had an inhibitory effect on the crystal growth of Bi₂WO₆,resulting in a smaller grain size and larger specific surface area.Samples before and after doping were all in the shape of irregular nanosheets,it indicated Dy³⁺ion doping didn’t change the crystal structure of Bi₂WO₆,but the size and thickness of the nanosheets decreased after doping.Compared Dy³⁺/Bi₂Mo O₆ and Dy³⁺/Bi₂WO₆,it was found that Dy³⁺(R_Dy3+=0.91（?）)doped into the crystal of Bi₂Mo O₆ and partially replaced Bi³⁺(R_Bi3+=1.17（?）)in the crystal lattice,caused cell structure distortion and lattice expansion.As for Bi₂WO₆,Dy³⁺successively replaced Bi³⁺and W⁶⁺in the lattice,resulted in the lattice parameter of Bi₂WO₆ decreased first and then increased and the corresponding interplanar spacing also showed a tendency to smaller first and then bigger.