Preparation And Photocatalytic Performances Of Monolayer Bi₂MoO₆ Nanosheets For The Selective Oxidation Of Benzylic Alcohols

As the global energy crisis and environment problem increase prominently,the photocatalytic technology that applies to organic synthesis has attracted widespread attention.The core of photocatalysis is the development of photocatalyst.But there are some drawbacks that suffer from low quantum efficiency and the utilization of solar energy exist in traditional photocatalyst.So it is urgent for us to explore visible light catalyst.Bi-based semiconductors with visible light response have gained immense research interests.But there are massive shortcomings over bulk bismuth semiconductor,such as low specific surface area,the long transportation path of photogenerated carriers and few surface active sites,which call for us to reduce the dimensionality from 3D bulk to the monolayer.Recently,monolayer nanosheets have drawn immense attention owing to their distinctive characteristics in the field of photocatalytic.Firstly,the thickness of monolayer nanosheets with several nanometers makes the catalytic active center site fully expose and the distance of photoinduced carrier migration to the surface become shorter,which restrains the recombination of the carriers in bulk effectively so as to accomplish the effective separation of the carriers.At the same time,the monolayer Bi2MoO6 nanosheets with oxygen vacancies decoration could in-situ assemble to hierarchical sphere,further achieving the enhancement of light adsorption and surface area,improving the photocatalytic efficiency simultaneously.In this thesis,the bottom-up strategy was used to synthesize the monolayer nanosheets.Additionally,we explore the reasons for the difference of photocatalytic performance by preparing hierarchical sphere in-situ assembled by monolayer nanosheets with different oxygen vacancies concentration decoration,providing a platform for designing highly efficient and robust single layer nanosheet catalysts for transforming organic contaminants.The main research contents and results are as follows:(1)Monolayer Bi2MoO6 nanosheets were synthesized successfully via cetyltrimethylammonium bromide(CTAB)-assisted bottom-up method for the first time.The morphology,structure and thickness of the monolayer nanosheets were characterized well by XRD,TEM,HRTEM,SEM and AFM.These results suggested that the pure and high crystallinity monolayer Bi2MoO6 nanosheets were prepared successfully.Taking this sample as a photocatalyst for selective oxidation of benzyl alcohol,monolayer Bi2Mo06 nanosheets show 8 times higher activity(26.5%conversion and 99%selectivity)than its bulk counterpart(3.2%conversion and 99%selectivity)under visible light irradiation.We investigate the reasons for the enhancement of the photocatalytic performance by a series of characterizations.The results show that the monolayer nanosheets have larger surface areas,ultrafast charge separation,higher capacity of oxygen activation,and the fact that they have more Lewis acid-base active sites and can form abundant surface complexes,further boosting the photocatalytic performance.Moreover,a feasible reaction mechanism was proposed to elucidate the whole process of photocatalytic reaction.(2)The monolayer Bi2Mo06 nanosheets are composed of dispersed nanosheets,which suffer from limited surface area and/or light absorption.We prepare hierarchical sphere in-situ assembled by monolayer Bi2MoO6 nanosheets with different oxygen vacancies decoration.Utilizing XRD,TEM,HRTEM,AFM,SEM to characterize its structure,morphology and thickness,these results reveal that the sample is fabricated successfully.Taking this sample as a photocatalyst for selective oxidation of benzyl alcohol,the monolayer Bi2MoO6 nanosheets with rich oxygen vacancies exhibit 5.5 times higher activity(40.2%conversion and 99%selectivity)than the monolayer Bi2MoO6 nanosheets under visible light irradiation.Meanwhile,the reasons why photocatalytic activity is enhanced are explained by a series of characterizations.These results reveal that oxygen vacancies serve as the centre of electrons capturing and the adsorbed sites of oxygen,which may increase both the separation of photoinduced carriers and the capacity of oxygen activation.Moreover,a feasible reaction mechanism was proposed to elucidate the whole process of photocatalytic reaction.