Synthesis, Modification And Visible-light Photocatalytic Performance Of Nano-microstructured Bi-based Photocatalysts

The development of new visible light-driven photocatalysts is essential to the control of environment pollution and the use of sustainable energies.Bi ₂WO₆ has shown visible-light photocatalytic property in solar energy conversion and the treatment of environment pollution.As an excellent visible-light photocatalyst,it has attracted much attention in recent years.Previous reports demonstrated that some simple nanostructures of Bi₂WO₆ such as nanoparticles,nanorods and nanosheets could be synthesized using soft chemical method.However,three-dimensional(3D) hierarchical structures of Bi₂WO₆ have rarely been reported.Since 3D structures are generally with larger size while their subunits are at the nanosize length scale,they possess the advantages of the subunits during photocatalytic reaction and can be easily recycled.Therefore,the fabrication of 3D Bi₂WO₆ hierarchical structures is highly desirable.On the other hand,carbon has very high conductivity;some reports showed that the carbon coating on UV-driven photocatalysts can significantly improve the photocatalytic performance.But there is no report on the modification of Bi₂WO₆ with carbon and exploitation of the photocatalytic activity of C-Bi₂WO₆ structures.This thesis first discusses the hydrothermal synthesis of Bi₂WO₆ hierarchical structures with different morphologies and investigates their visible light catalytic properties.With respect to the modification of Bi₂WO₆,C-Bi₂WO₆ nanostructures will be fabricated by the hydrothermal carbonization of glucose during the growth of Bi₂WO₆.The photocatalytic activity of C-Bi₂WO₆ nanostructures toward RhB degradation under visible light is investigated,and the effect of carbon content on the photocatalytic performance is discussed.In addition,Bi₃O₄Cl and BiOCl films are fabricated using ZnO nanorod array as template and BiCl₃ as the precursor.The film formation mechanism is primarily investigated.The main contents are as follows:1.The control of Bi₂WO₆ nanostructure morphology by varying the growth temperature,pH value,surfactant concentration,and the molar ratio of starting materials.Bi₂WO₆ dispersed nanosheets, hierarchical microspheres,octahedron- and red cell-like structures can be controllably synthesized.(1) When there are only Bi(NO₃)₃ and Na₂WO₄ in the reaction system,dispersed 2D Bi₂WO₆ square nanosheets can be obtained after 180℃hydrothermal synthesis for 12 h.These nanosheets actually consist of many smaller nanoplates assembled in an edge-by-edge fashion.The formation of 2D nanoplates is attributed to the specific crystal structure of Bi₂WO₆.(2) We discuss the effect of surfactant PVP on the morphology of products and find that PVP plays an important role in the structural formation.When 0.15g PVP is used,the product is Bi₂WO₆ hierarchical microsphere made up of numerous Bi₂WO₆ nanosheets.Without PVP,only dispersed square nanosheets can be attained.With the increase of PVP content,2D round disks and 3D structures emerge and the number of 2D sheets decreases,and finally we can get homogeneous 3D structures.However,with too much PVP added,only 2D round disks can be generated.(3) The morphology of Bi₂WO₆ can also be manipulated by changing the pH value of reaction system.When the pH is 7,the product is hierarchical octahedron-like structure.We propose that the pH can significantly affect the hydrolysis rate of Bi³⁺.At pH=7,the nucleation and growth rates of Bi₂WO₆ are appropriate,the initially formed small plates assemble in an edge-to-edge way with the gradual enlargement of the 2D surfaces.A subsequent layer-by-layer growth of the large nanoplates gives many parallel square plates.Since the formation of parallel plates occurs simultaneously in three dimensions(X,Y,Z),three groups of parallel square plates finally cross-link with(and vertical to) each other,forming octahedron-like structures.(4) When the molar ratio of Na₂WO₄ to Bi₂WO₆ is 1,the structure of nanosheet is compact and the interfaces between primary nanoplates disappear.This is because under this condition the WO₄^2- is excessive,making the nucleation and growth of Bi₂WO₆ very quick.In this regard,primary nanoplates can simultaneously assemble in edge-to-edge and face-to-face fashions,so the interfaces between them have more opportunities to undergo the dissolution-recrystallization process.When Na₂WO₄ and Bi₂WO₆ are both 0.5 mmol,red-cell-like 3D hierarchical structures are attained.2.Based on the different structures obtained above,we study the effect of morphology and structure of Bi₂WO₆ on the visible-light photocatalytic activity.The results show that Bi₂WO₆ hierarchical microspheres and octahedra have the better performance than dispersed 2D nanosheets. The RhB can be thoroughly degraded after 6 h visible light irradiation.All the complex hierarchical structures of Bi₂WO₆ have superior performance over that fabricated by conventional solid-state reaction.In addition,Bi₂WO₆ with the same morphology has different photocatalytic activities in RhB solutions with different pH.When pH=7.5,2D sheets and 3D octahedra have better performance.The sensitivity of photoactivity to the pH value ascribes to the fact that Bi₂WO₆ is not stable under acid environment and can be transformed into H₂WO₄.3.In order to improve the photocatalytic property of Bi₂WO₆,we fabricate C-Bi₂WO₆ structures by the hydrothermal carbonization of glucose during the growth of Bi₂WO₆ followed by high-temperature calcination.The degradation of RhB under visible light using this photocatalyst is further investigated.We find that C-Bi₂WO₆ photocatalyst usually has better performance than pure Bi₂WO₆.The presence of carbon can significantly increase the visible light absorption and dye loading capability,and accelerate the electron transfer rate from photocatalyst to dye RhB.However, too much carbon on the Bi₂WO₆ surface makes the light all absorbed by carbon,the photocatalyst thus can not be utilized,leading to very poor performance,even worse than that of pure calcinated Bi₂WO₆.4.Bismuth oxychlorides are one kind of important semiconductors in Bi-system compounds and have layered Structure and high chemical stability.There are strong inner electric field between Bi-O layer and Cl layer,leading to very fast photo-generated electron-hole separation,so they exhibit good photocatalytic activities.We fabricate for the first time BLOCl film using ZnO nanorod array as the template,BiCl₃ as the precursor and ethanol as the solvent at room temperature.XRD pattern indicates that Bi₃O₄Cl is attained when the immersion time is short,and after long-time reaction, BiOCl is fabricated.SEM results demonstrate both Bi₃O₄Cl and BLOCl films are constructed by very thin nanosheets.TEM result reveals the single-crystal nature of the nanosheet.The bandgaps of Bi₃O₄Cl and BiOCl can be further determined as 2.658eV and 3.15eV respectively by using UV-vis diffuse reflectance spectroscopy.