The Design And Synthsis Of Semiconductor Photocatalysts And Their Application In Cr(?) Reduction

With the rapid development of modern industry,the burden on the environment is also getting heavier and heavier.Heavy metal ions have become one of the most urgent problems that need to be solved due to the properties of toxicity and stablility in natural environment,which can stay in water for a long time.Among the heavy metal ions,hexavalent chromium ions(Cr(VI))ions are considered as a kind of highly toxic and carcinogenic ions.Using an efficient and economical method to treat Cr(VI)in waste water is of great significance for the protection of the environment and human health.Traditional Cr(VI)treatment methods include chemical precipitation,physical adsorption,membrane filtration and electrolytic method.However,these methods have the disadvantage of low metal ion removal efficiency In recent years,the photocatalytic technology has been used to reduce highly toxic Cr(VI)into low-toxicity trivalent chromium ions(Cr(III))using semiconductor materials as catalysts,which is considered as an effective,low-cost way without producing other harmful substances.However,traditional photocatalytic such as TiO2,CdS,SnS2,Ag2S,WO3 et al.are limited by serious electron-hole recombination,small specific surface area,large band gap width that can only utilize the ultraviolet region in sunlight and low absorption efficiency et al.leading to the low photocatalytic efficiency.Based on the above factors,this paper aims to design and synthesize highly efficient photocatalytic reduction of Cr(VI)semiconductor materials.Specific researches are as follows:A novel hierarchical Bi2WO6 photocatalyst assembled by Bi2WO6 nanosheets with a hollow and rod-shaped appearance has been developed via a facile hydrothermal process.Interestingly,we found that the hydrolysis of Bi(NO3)3 in water can produce solid B16O5(OH)3(NO3)5·342O microrods which can be transformed to hollow-hierarchical Bi2WO6 nanosheets by virtue of the Kirkendall effect.WO42-first react with the surface of the rod-shaped precursor to form smaller Bi2WO6 nanoparticles and then generate Bi2WO6 nanosheets.As the reaction goes on,the material in the precursor gradually moves to the surface to react with the WO42-in the solution and finally forms the hierarchical Bi2WO6.The photocatalytic Cr(VI)reduction test of Bi2WO6 hollow nanorods with super structure showed great promotion of photocatalytic efficiency compare with bulk Bi2WO6.The study found that this is mainly due to its larger specific surface area,higher electron-hole separation efficiency and higher light absorption intensity.A new kind composite material of carbon dots-loaded metal-organic frameworks(MOFs)has been developed.Through impregnating glucose solution inside MIL-53(Fe)by double solvent method,we created numerous carbon dots inside the MOFs under N2 protected calcination at 200 ?,without changing the structure and morphology of the MOF particles.The presence of carbon dots in MOFs have been characterized by photoluminescent spectra,N2 adsorption-desorption isotherm,and HRTEM etc.In contrast to pure MIL-53(Fe),we found that carbon dots-loaded MIL-53(Fe)exhibited an obviously enhanced photocatalytic reduction capability toward Cr(VI)ions.The related mechanism have been revealed by solid UV-Vis spectroscopy,photocurrent,electrochemical impedance spectra and a series of control experiments.It was proposed that the existence of carbon dots in MOFs can increase the absorption of incident light and improve the separation of photogenerated electrons and holes.As such,the developed carbon dots-loaded MIL-53(Fe)composite materials show a significantly enhanced photocatalytic activity for the reduction of Cr(VI)ions.