Colloidal Synthesis And Application Of Multinary Semiconductor Nanoparticles With Different Mophorlogies In Field Of Photocatalytic Water Splitting

In order to solve the urgent global energy shortage and environmental pollutionissues,the efficient utilization of solar energy via photocatalytic water splitting based on semiconductor photocatalysts has been paid more and more attentions.As it well known,the morphology of photocatalysts has a derect impact on the performance of photocatalytic water splitting.Therefore,the design and synthesis of photocatalyst with large specific surface area are critical to improve the light conversion efficiency.However,the control of morphology for multinary semiconductor photocatalysts is relatively difficult,because of their complex composition and structure.In this case,we demonstrate the synthsis of multinary semiconductor photocatalysts with different morphologies via colloidal synthesis method,thus further improving the photocatalytic water splitting performance.In addition,we also want to solve some basic problems in colloidal synthesis such as the reaction mechanism,and provide some experience in the control of morphology for other nanoparticles.In order to obtain multinary semiconductor photocatalysts with large specific surface area,we have successively synthesized semiconductor photocatalysts with hollow structure and ultrathin two-dimensional structure.Specifically,the work can be summarized in the following three aspects.1.In chapter II,we demonstrate a facile one-pot synthesis of ternary Cu₂GeS₃hollow nanoparticles based on the Kirkendall effect by using the stable and inexpensive GeO₂ as the Ge source.The as-synthesized Cu₂GeS₃ hollow nanoparticles are demonsteated to be mesoporous with a BET specific surface area of10.5 cm² g^-1.The formation of Cu₂GeS₃ hollow nanoparticles includes the nucleation of solid Cu₇S₄ seed nanoparticles followed by the unequal diffusion of Cu⁺and Ge⁴⁺.Based on the diffusion kinetics and reaction kinetics,we further establish a theory model for the growth of Cu₂GeS₃ hollow nanoparticles.The prerequisite condition for the formation of Cu₂GeS₃ hollow nanoparticles is proposed and the morphology of the product can be controlled.Finally,by using Cu₂GeS₃ hollow nanoparticles as the template,other more complex quaternary Cu₂MGeS₄?M=Zn,Mn,Fe,Co,Ni?hollow nanoparticles are also produced through the subsequent metal ion diffusion.2.In the previous chapter,we successfully prepared ternary Cu₂GeS₃ and quaternary Cu₂MGeS₄?M=Zn,Mn,Fe,Co,Ni?hollow nanoparticles.However their cavity size is small,so the specific surface area is not very large,which limits their application in photocatalytic water splitting.Therefore,in chapter III,we further prepare cubic Cu₂GeS₃ hollow nanoparticles with huge cavities and thin shells by changing the size and crystal phase of Cu_2-xS seed nanoparticles on the basis of the work in chapter II.The size of large Cu₂GeS₃ hollow nanoparticles can reach 60-200nm,but the thickness of the shell is only about 6 nm,leading to a larger BET specific surface area(²2.1 m² g^-1).In addition,the control of the crystal phase of Cu₂GeS₃hollow nanoparticles is disclosed:the crystal phase of Cu_2-xS seed nanoparticles directly determines the crystal phase of the final Cu₂GeS₃ hollow nanoparticles,and the crystal shape of Cu_2-xS seed nanoparticles depends on the sulfur source.Finally,we fabricate a two-layer photoelectrode using both large Cu₂GeS₃ hollow nanoparticles and small Cu₂GeS₃ hollow nanoparticles.Compared with the photoelectrodes prepared by pure large and small Cu₂GeS₃ hollow nanoparticles,the two-layer photoelectrode exhibits higher photocurrents due to the high interface area of the upper layer?large Cu₂GeS₃ hollow nanoparticles?and the ideal compactness of the bottom layer?small Cu₂GeS₃ hollow nanoparticles?.3.In the previous chapter,the photocurrent density has increased to a certain extent via the two-layer photoelectrode,but the photocatalytic performance for water splitting is still not very good in the current field.We think it is ascribed to two aspects:On the one hand,the specific surface area of hollow nanoparticles is still not large enough;On the other hand,the isotropic structure of hollow nanoparticles is not benefical to the separation of photogenerated charges.Therefore,in chapter IV,we prepare a 2D photocatalyst with larger specific surface area,which is more attractive in the field of photocatalytic water splitting.We propose a colloidal two-phase method to synthesize ultrathin BiVO₄ nanosheets.BiVO₄ nanosheets have a monoclinic crystal structure and the active{010}planes are exposed.The thickness is less than 3nm,and the lateral size is as large as 1.2 um.In addition,due to the unique reaction conditions in the two-phase method,the surface of the BiVO₄ nanosheets is free of ligands,and oxygen vacancies are widely distributed.The theoretical calculations show that the oxygen vacancies in the BiVO₄ nanosheets can not only promote the dissociation of water molecules,but also increase the electron density of states on the conduction band edge and valence band edge.Finally,thanks to the unique advantages,BiVO₄ nanosheets exhibit excellent performance in photocatalytic water oxidation.In the presence of AgNO₃ as sacrificial agent,the rate of O₂ evolution for BiVO₄ nanosheets can reach 5.37 mmol h^-1 g^-1 under visible light,which is more than3 times for the BiVO₄ samples prepared by the conventional hydrothermal and co-precipitation method.