Strategies And Mechanisms For Improving The Photocatalytic Performance Of Bi₂WO₆ Nanomaterials

The rapid development of technology and the economy has been accompanied by the massive exploitation of fossil fuels since the last century.The accompanying environmental pollution and energy crises have also gradually emerged and become one of the problems that modern human society must face up to.The actual development and application of fossil fuels can cause land collapse,natural resource erosion,greenhouse effect,atmospheric pollution,and other effects,which pose a great challenge to the actual survival of people.For this reason,it is urgent to seek new energy sources to replace fossil fuels.Scientists have been using various methods and means to find and develop new energy sources,new energy materials,and new energy technologies from both theory and practice,since the beginning of the 21st century.Converting inexhaustible solar energy into clean chemical energy,and obtaining new energy with high energy density from nature's abundant water resources are considered to be very strategic and practical approaches.Which is named solar photocatalytic water splitting hydrogen evolution(SPH).In recent years,Aurivillius oxides composed of(Bi₂O₂)^2-layers and(A_n-1B_nO_3n+1)^2-layers have been widely used in the field of photocatalysis due to their unique layered structure.It is also beneficial to charge separation.Among them,the typical sample Bi₂WO₆ can effectively enhance the utilization rate of electric charge due to its unique heterojunction structure(monolayer Bi₂WO₆ is stacked by a sandwich-like structure of[Bi O]⁺-[WO₄]^2--[Bi O]⁺).What's more,the bandgap of Bi₂WO₆ is generally 2.8 e V which is suitable for absorbing sunlight.And the advantages of good light response-ability,good chemical stability,and non-toxic,make Bi₂WO₆ attract the attention of many scientific researchers.However,in the actual application process,the thick sheet layer and uneven dispersion inhibit the catalytic activity in visible light.Because not only the absorption of light is limited but the separation effect of photogenerated electron-hole pairs is also not ideal.Therefore,the modification of Bi₂WO₆ is still considered to be an effective way to improve photocatalytic performance.In this thesis,we take Bi₂WO₆ as the study object,which is one of the most popular samples among Aurivillius oxides.Through three modification means to modify it:thinning effect,nanocomposite,and SPR thermal electron injection.Focusing on solving the poor photogenerated charge separation effect and narrow light absorption range in the photocatalytic process,so that to achieve significant improvement in photocatalytic performance.A series of detailed characterizations were also conducted to explore the internal mechanism of different modification methods to enhance photocatalytic activity.The main studies are as follows:Firstly,due to the strong chemical bond between layer and layer,the two-dimensional Bi₂WO₆ generally appears in the form of blocks or thick sheet layers.It is difficult to prepare and form thin nano-sheet layers not to mention a few layers or a single layer.According to known literature,adding surface-active agents such as CTAB and Na BH₄can achieve the wrapping of surface coordination of unsaturated metals in the bottom-up chemical preparation process,thereby stopping longitudinal growth and thus achieving thinning.Therefore,in this thesis,the Na BH₄-assisted hydrothermal method was first used to obtain stable few-layer Bi₂WO₆nanosheets,which are based on the predecessors.By optimizing the processing time after Na BH₄ addition,obtaining different thickness Bi₂WO₆nanosheets.Series characterization analysis proves that the[BH₄]^-in Na BH₄ can be adsorbed on the Bi³⁺surface of Bi₂WO₆,which has an inhibitory effect on the attraction of chemical bonds between Bi₂WO₆ layers.So the sample is more inclined to grow into a few-layer,even monolayer Bi₂WO₆ nanosheets.The photocatalytic performance is improved due to the thinner thickness and the shorter migration distance.Secondly,in the previous work,we found that the addition of Na BH₄ will cause partial Bi reduction and oxygen vacancies,while achieving thinning.It indicates that the structure of Bi₂WO₆ is partially destroyed.What's more,according to known literature reports,when the concentration of Na BH₄ is high,the material will be further reduced to Bi₂O₃ or even Bi elemental.Moreover,this nanocomposite is achieved by in situ reductions,and the structural force between the components is strong.Besides,there is no lattice imbalance compared to mechanical composite,and the ability of charge separation will be greatly enhanced,either.At the same time,the introduction of Bi can also achieve SPR thermal electron injection,which can further improve the photocatalytic performance of the material.Therefore,this chapter further explores the modification effect of a high concentration of Na BH₄ on Bi₂WO₆.Experimental results show that unlike the low concentration of Na BH₄,the high concentration of Na BH₄ treatment will cause Bi₂WO₆ to undergo a phase transition,and generate Bi and Bi₂O₃.By changing the dosage of Na BH₄ can adjust the relative content of Bi,Bi₂O_3,and Bi₂WO₆.Among them,the Bi/Bi₂O₃/Bi₂WO₆ composite photocatalyst is prepared,and achieves the best photocatalytic activity.Because it can not only form a Bi₂O₃/Bi₂WO₆heterojunction,effectively improve the efficiency of charge separation,but also use the SPR effect of Bi to increase the carrier concentration of the material.Which can effectively improve the photocatalytic performance of the original Bi₂WO₆ sample.Finally,based on the previous chapter,we can confirm that Bi has a precious metal-like SPR effect.But the size of its role in the system and the object of action are not clear due to the complex system in previous work.Therefore,to focus on Bi's SPR electron injection effect,this chapter uses ethylene glycol as the solvent and glucose-assisted solvothermal heat to synthesize the Bi/Bi₂WO₆ composite photocatalyst.A series of experimental analysis results show that the concentration of added glucose has a significant impact on Bi₂WO₆.When the concentration is low,the crystal structure of Bi₂WO₆ will gradually change from crystal to amorphous.And when the concentration is higher,Bi~0 will be reduced on the surface of amorphous Bi₂WO₆.Moreover,the relative content of Bi can also be regulated by changing the concentration of glucose,and the relative content of the reduced Bi is positively correlated with the glucose treatment concentration.The results of the series of experimental analyses show that the modification of the SPR effect of Bi can effectively increase the carrier concentration and improve the charge utilization,thereby improving photocatalytic performance.In this thesis,the Bi₂WO₆catalysts were modified in three different ways:thinning effect,nanocomposites,and SPR thermal electron injection.The intrinsic mechanisms of the three different modifications for photocatalytic performance enhancement were also investigated.This work provides some reference for the preparation of photocatalysts with fast photogenerated charge separation,wide visible light absorption range,and fast interfacial electron transport.It is also expected to extend the application to other Aurivillius oxides and even other types of photocatalytic materials.