Study On Synthesis And Photocatalytic Performance Of Several Micro/Nano Semiconductor Materials

Environment pollution has been becoming a big problem in our developing society.In particular,organic wastes due to various types,large quantity,and harmfulness are a severe challenge for water resource and environment of our country.However,the commonly applied physical and chemical methods are difficult to remove these organic wastes completely.The photocatalytic oxidation technology has attracted extensive attention because of its friendliness to environment,low cost,and no secondary pollution.The traditional photocatalysts such as TiO₂ have wide band gaps and are suffering from easy recombination of photoelectrons and holes.This leads to low photocatalytic activity,and hence their applications are restricted.Therefore,exploring highly efficient photocatalysts is of great significance for the treatment of organic wastes.This thesis is aiming to improve the catalytic activity of photocatalysts to degrade organic wastes.Three micro/nanostructured semiconductor photocatalysts were synthesized via sol-gel method,solvothermal thesis,and electrospinning.Their structures,size,morphologies and photocatalytic performance were characterized in details.The as-synthesized photocatalysts exhibited good photocatalytic activities toward the degradation of bisphenol A?a typical environmental endocrine disruptor?,methylene blue,and rhodamine B in wastewater.Firstly,we applied sol-gel method to prepare a Gd-SiO₂-TiO₂ composite photocatalyst doped with rare earth element Gd.The syntheses of Si-TiO₂,Gd-TiO₂,and Gd-SiO₂-TiO₂were optimized to have tetraethyl orthosilicate and gadolinium oxide as the respective Si and Gd source and calcination temperature of 550oC with duration of 2h.When the doping amount of Si and Gd was 9.1%and 0.2%,respectively,the photocatalysts exhibited the best photocatalytic activity.They were further characterized by FTIR,XRD,SEM and UV-vis DRS.The calcination temperature affected the crystalline phases of samples.When the calcination temperature is above 600oC,a mixture of anatase and rutile phases was obtained.The Si and Gd co-doped samples can improve the catalytic activity of photocatalysts.When doped with only Gd,the sample has a minor rutile phase in addition to the major anatase phase;while doped with only Si,it is a pure anatase phase.This indicates that incorporation of Si can effectively inhibit phase transformation and benefit stability of photocatalysts.Gd atoms introduced into crystals can induce lattice distortion of TiO₂,which will delay the recombination between photoelectrons and holes.The light absorption threshold and band gap of the Si and Gd co-doped sample are about 407 nm and 3.05 eV,respectively.By comparing with TiO₂?387nm,3.2 eV?,the co-doped sample has red-shifted absorption threshold and narrower band gap.This suggests that energy for exciting photoelectrons is smaller,favoring photocatalytic reactions.When the Gd-SiO₂-TiO₂ composite photocatalyst was used to degrade BPA,the reaction conditions were optimized:the optimal dosage of catalyst is0.400g/L for BPA of 20mg/L in an organic wastewater;pH of 5⁷ leads to the best photocatalytic activity;introduction of H₂O₂ or O₂ significantly improves the photocatalytic degradation of BPA.It is speculated that BPA is decomposed to phenol and acetone,and then decomposed to carbon dioxide and water.The combination of ultrasonic and photocatalysis techniques can improve the degradation efficiency of MB.Secondly,solvothermal synthesis was applied to fabricate semiconductor photocatalysts,BiOCl_xI_1-x microspheres with three-dimensional petal-like structures.The synthetic temperature,reaction time,and ratio of Cl and I govern the photocatalytic activity of samples.But,high temperature and long dwelling time can induce agglomeration of nanoparticles,resulting in reduced active sites and photocatalytic activity.Appropriate ratio of Cl and I can enhance the visible light response of BiOCl and oxidation capability of the holes generated in BiOI.The optimized synthesis conditions are:molar ratio of Cl/I is 2:8 and the reaction temperature is 120oC for 9h.The samples were characterized by FTIR,XRD,SEM,and UV-vis DRS.The unique microspheres with three-dimensional petal-like structure benefit transfer of the photo-generated charge carriers.The XRD patterns reveal that BiOI_0.8Cl_0.2 is composed of both BiOCl and BiOI.The lattice of BiOI_0.8Cl_0.2 is distorted when doped by Gd.The average size of the crystalline is 8.14nm.The band gap of BiOI_0.8Cl_0.2 is 2.068eV,and its light absorption threshold is about 600nm.Comparing with the pure BiOCl,the visible light absorption edge shifts positively.The light absorption performance of 5%Gd/BiOI_0.8Cl_0.2is best while 7.5%Gd/BiOI_0.8Cl_0.2 is worst.BiOI_0.8Cl_0.2 exhibits higher photocatalytic activity than pure BiOCl and BiOI in degradation of methylene blue.The different doping amounts of Gd change the photocatalytic efficiency:5%>2.5%>0%>10%>7.5%.Excessive amounts of Gd form a complex center of electrons and holes and reduce photocatalytic efficiency.BiOI_0.8Cl_0.2 possesses both high catalyti activity and chemical stability.It shows little decrease in the photocatalytic activity after five cycles.This degradation of methylene blue follows the first-order kinetic model.When pH of solution is11 and photocatalyst amount is 0.25g/L,the photocatalytic efficiency of 5%Gd/BiOI_0.8Cl_0.2 irradiated by 350W xenon lamp for 90min reaches 95.7%.In the photocatalytic system of 5%Gd/BiOI_0.8Cl_0.2,h⁺is major active group,then is·O₂^-,the impact of·OH is minimum.Finally,ZnWO₄ nanofibers were fabricated by electrospinning technology,as well as ZnWO₄ particles with irregular shapes prepared by solid-phase recation.They were characterized by using FTIR,XRD,SEM,TEM,UV-vis DRS,XPS,ESR and EIS.It was revealed that the photocatalytic activity of ZnWO₄ is better than that of ZnWO₄ particles.The main reasons are:?1?different structures of ZnWO₄ fibers and particles,leading to various surface areas of 110m²/g vs.75m²/g;?2?the band gap of ZnWO₄ fibers is about 3.58eV,which is lower than that of ZnWO₄ particles?3.67eV?;?3?the emission intensity of ZnWO₄fibers is lower than that of ZnWO₄ particles,due to the abundant lattice defects for photogenerated holes and electrons;?4?ZnWO₄ fibers can separate photogenerated electrons from holes more effectively and promote interfacial charge transfer;?5?the active group of·OH on ZnWO₄ fibers is more than that of ZnWO₄ particles.In the photocatalytic degradation of RhB,the ZnWO₄ nanofibers exhibited better performance than the irregular particles.It took about 45 min for ZnWO₄ fibers to reach a degradation efficiency of 70%,and the degradation efficiency of more than 90%is achieved after 90 min under the simulated sunlight irradiation.After five cycles,no obvious change in the photocatalytic activity for degradation of methylene blue.