The Effect Of Internal Electric Field For Carriers' Separation In Photocatalysis

Since the industrial revolution,the society productivity got huge developed benefited to the using of a large number fossil fuel.People's life has been greatly enriched,but the extensive use of fossil fuels has also brought a series of problems.Now environmental pollution and energy crisis have becoming two major challenges which will influence the sustainable development in the 21st century.Looking for cheap,clean and sustainable energy to replace the fossil fuels is the only solution which can solve to these two problems.Among the several kinds of candidate energy,solar energy has been regarded as the most ideal energy source because of its wide distribution and inexhaustible.The high efficient transformation and utilization of solar energy is not only a focus the field of energy research,but also the key point to solve environmental problems.Since 1972,the Japanese scientists found that H₂O can be broken into H₂ and O₂ with TiO₂ single crystal electrode under the radiation of UV light,semiconductor photocatalytic has attract a lot of attention.For the past decades,a lot photocatalysts have been developed,and popele found that photocatalytic can also be used in the environmental region.Through photocatalytic has got huge development,it is still far from the practical application.The low activity of catalysts caused by the poor light absorption and limited efficiency of carriers is main reason that photocatalytic cannot be used.For example,as the most studied photocatalyst TiO₂,it has a band gap of 3.2 eV and can only use UV light which is less than 5%in the solar energy,besides the separation efficiency of photo generated carriers is also very low.It is crucial to exploit photocatalysts with visible response and high carriers' separation efficiency for the practical application.People have developed several methods to improve the activity of photocatalysts,like element doping,noble metal modification and hetroj unction etc.Besides,preparing novel photocatalysts have been a good method.The study of Ag-based photocatalysts have become a hotspot since AgX?X=Cl?Br?I?,Ag₃PO₄ etc.were prepared and proved having high photocatalytic activity.Most of the current research about Ag-based photocatalysts was focused on localized surface plasmon resonance caused by the nano silver and the constructing of hetrojunction with other materials,little research was carried out on the relationship between the crystal structure and photocatalytic activities.In this paper,we mainly discuss and deeply investigate the influence of structure for the activities through the preparing of several Ag-based photocatalysts.Our results both in experiments and theoretical calculations prove that high separation efficiency of photogenerated carriers caused by the internal electric field is the main reason that these photocatalysts have good performance on photocatalytic region.Our research results provide reference for the designing and preparing of high activity photocatalysts.The content of this thesis is divided into six chapters.In the first chapter,we mainly introduce the development progress,the basic theory of photocatalytic.We also give an investigation that the challenges in photocatalytic region we faced before the practical application.The main method peopled to improve the activity of photocatalysts were discussed.Based on the merits and demerits of the nowadays solutions,we proposed our research contents.In the second chapter,two kind of photoelectrodes were prepared and used for PEC hydrogen production.Firstly,CdS photoanodes were prepared through an electric-deposition method on metal Mo substrate.The CdS photoanodes was composed with uniform nano sheet on the surface.During the reaction in the H₂S gas,some metal Mo was reacted with H₂S,and a MoS₂ formed under the CdS layer.Because of the coating of the CdS,MoS₂ cannot contact the electrolyte solution.CdS and MoS₂ formed a kind of heterojunction with a Janus structure,which promote the separation of the photo generated carriers.Secondary,CuBi₂O₄ photocathodes were prepared through double metal co-depositing on the FTO glasses.The Cu and Bi were transferred into CuBi₂O₄ film at high temperature under the air condition.CuBi₂O₄ is a p-type semiconductor and has very good light absorption properties.CuBi₂O₄ has the potential to be as a photocathode material for PEC hydrogen production.Though CuBi₂O₄ has good visible light response,the poor conductivity of semiconductor materials limits the separation efficiency of carriers.Constructing interface heterojunction has been proven as a good solution to promote the carriers separation.Here,the CuBi₂O₄ photocathodes were modified with a layer of TiO₂,and the CuBi₂O₄ and TiO₂ formed a p-n heterojunction.Owning to internal electric field formed on heterojunction structure,the photo generated electrons will transfer to CB of TiO₂.In other words,the carriers'separation efficiency was enhanced.Thirdly,porous Ag-ZnO microsphere photocatalyst was prepared and used to photocatalytic degrade methane and ethylene gases under room termparature.The Ag-ZnO phtocatalyt shows good stability and high efficiency in oxidizing of methane and ethylene under UV light irradiation.The reaction progresses were investigated through analyzing the degradation products,and we find that both CO₂ and CO were produced in the photocatalystic reaction.In the photocatalytic reactions,Ag nanoparticles play a key role on promoting the separation of photo-generated carriersIn the third chapter,we synthesis a high activity visible light response photocatalyst Ag₉?SiO₄?₂NO₃ through a simple reflux method.The structures and morphologies were characterized by XRD and SEM,respectively.The as prepared Ag₉?SiO₄?₂NO₃ nanoflowers are constructed by some small nanosheets with thickness of about 20 nm.The band gap of Ag₉?SiO₄?₂NO₃ is 1.93 eV,which can absorb visible light up to about 650 nm.Additionally,due to its polar crystal structure,an internal electric field can be formed inside the materials,which can effectively improve the photogenerated charge separation.Owing to the wide visible light absorption and high charge separation efficiency,Ag₉?SiO₄?₂NO₃ exhibit high photocatalytic activity during the degradation of organic dyes and O₂ production under visible light irradiation with comparison to Ag₃PO₄,which make it an ideal material for solar energy absorption and utilization.In the fourth chapter,a novel photocatalyst AgioSi₄0₁3 was synthesized through a facile solid state reaction.The as prepared Ag₁0Si₄Oi₃ was characterized by XRD,SEM,TEM,UV-Vis and XPS.The photocatalytic capability of Ag₁0Si₄O₁3 was estimated through photo-degradation of MB solution and photocatalytic oxygen generation under visible light irradiation.TOC changes of MB solution before and after photocatalytic experiment identified the mineralization of MB molecules.During photocatalytic oxygen generation reaction,Co₃O₄ was used as cocatalyst to boost oxygen evolution.Compared with pure AgioSi₄O₁3,4%Co₃O₄/Ag₁0Si₄O₁3 showed higher efficiency in oxygen evolution.Radical-trapping experiments and results show that holes generated by Ag₁0S₁4O₁3 are main active ingredient-oxidizing organic pollution under light irradiation.In the fifth chapter,A novel high-efficiency visible-light Ag₄?GeO₄?photocatalyst was prepared by a facile hydrothermal method.The photocatalytic activity of as-prepared Ag₄?GeO₄?was evaluated by photodegrading of methylene blue?MB?dye and water splitting experiments.The photodegradation efficiency and oxygen production efficiency of Ag₄?GeO₄?were detected to be 2.9 and 1.9 times higher than that of Ag₂O.The UV-Vis diffuse reflectance spectra?DRS?,photoluminescence experiment and photoelectric effect experiments prove that the good light response and high carrier separation efficiency facilitated by the internal electric field is the main reason for Ag₄?GeO₄?'s excellent catalytic activity.The radical-trapping experiments reveal that photo generated holes are the main active species.First-principles theoretical calculations provide more insight into understanding of the photocatalytic mechanism of Ag₄?GeO₄?catalyst.In the sixth chapter,the summary and prospect were provided.We summarized our research work and the conclusion of our work.We discussed the innovation work and ideas of this work,and we also analyzed the defects in our work.This gives a direction for our future work.In summary,this thesis discussion the relationship between the catalyst structure and photocatalytic properties by designing and preparing photoelectrodes and photocatalysts through solid phase synthesis or electric-deposition methods.Several methods,like X-ray diffraction,scanning electron microscopy,UV-Vis diffuse reflectance spectroscopy and fluorescence spectrum were used to characterize the prepared samples.The activity was evaluated with degrading organic pollution and water splitting.The effect of internal electric field caused by spontaneous polarization and heterojunctions for the carriers' separation was investigated.Our research work provide references for the preparation of high activity photocatalysts,and can be helpful to the fundamental study of photocatalytic field.