Surface Defect Engineering In Two-dimensional Materials And Their Applications In Photocatalytic Selective Oxidation

Selective oxidation reaction,one of the most effective pathways for synthesis of high value-added organic intermediates,including aldehydes,sulfoxides,imines,etc.,was usually conducted under high temperature and high pressure in industrial production,which brought undesired energy consumption and environmental pollution.As the most economic and cleanest resource,solar energy provides sufficient energy to resolve the above problems if with proper utilization such as photocatalytic selective oxidation.Although the photocatalytic selective oxidation reactions hold great promise for manufacturing these organic intermediates,most of them are still suffering from low conversion efficiency and limited selectivity as compared to conventional thermal-based methods.Bearing these in mind,we proposed that highly selective oxidation reaction can be achieved by modulating the corresponding photocatalytic molecular oxygen activation processes from surface-defect-engineered two-dimensional(2D)photocatalysts.Here,by taking a series of 2D semiconductors as a platform,this paper focuses on achieving selective oxidation of benzyl alcohols,thioethers,and benzylamines with high conversion and selectivity by introducing surface defects including vacancy,functional molecule modification,hybrid structure.With various experimental measurements,characterizations and density functional theory(DFT)calculations on defect structures,the influences of photoabsorption,separation and transport of photoexcited electron-hole pairs,and surface redox chemical reaction processes were revealed from the introduced surface defects.According to clear research of structure-property relationship,this paper provides a new way for designing surface-defect-engineered 2D materials and boosting involved photocatalytic performance.The details of this paper are briefly summarized as follows:1.Photocatalytic selective oxidation of alcohols to aldehydes possess great potential in industrial production,but suffers from low conversion efficiency and selectivity for the uncontrollable oxidation processes.In view of using photogenerated reactive oxygen species(ROS)as the key oxidant in a selective oxidation reaction,we propose that a highly selective oxidation reaction can be achieved by modulating the corresponding photocatalytic molecular oxygen activation processes.Using cubic indium sulfide nanosheet as a model system,we show that charge carriers involved in O2 activation can be optimized with the introduction of surface S vacancies.Benefiting from the enhanced charge separation and transfer processes,the In2S3 nanosheets with S vacancies could simultaneously activate O2 into superoxide radicals via electron transfer under visible-light irradiation to display outstanding activity for the selective oxidation of alcohols to aldehydes with high conversion and selectivity.2.Singlet oxygen is an important ROS,which can react with organic molecules to produce high-value-added organic chemicals.As for 2D semiconductor-based photocatalysts,it is difficult to generate singlet oxygen due to the hardly existent or light-concentration long-lifetime excitonic states.Herein,we built a surface electron conduction suppression(SECS)structure by bonding macromolecule to surface of 2D photocatalysts to facilitate the long-lifetime excitonic states.Using ZnIn2S4 nanosheet as a model system,the surface modified non-conducting polymer will block the electron transfer process and facilitate the dipole-dipole energy transfer process once the electrons are photogenerated in ZnIn2S4 nanosheets.Benefiting from the greatly increased concentration of singlet oxygen,ZnIn2S4 with SECS structure shows greatly enhanced conversion efficiency and selectivity of photocatalytic selective oxidation of sulfides to sulfoxides.3.As one of the most important ROS,superoxide radicals efficiently facilitate the aerobic oxidative coupling of amines to imines.In this work,we propose to build hybrid structure on the surface of 2D materials,where the enhanced photogenerated electrons will increase the O2 activation processes.Using Bi3O4Br nanosheet as a model system,we introduce Bi2O3 onto the surface of Bi3O4Br nanosheets to create a heterojunction.As a result,the separation and transport of photoexcited electron-hole pairs will be facilitated,simultaneously enhancing generation of superoxide radicals.Hence,upon light irradiation,the Bi2O3/Bi3O4Br heterojunction shows a higher 3,3',5,5'-tetramethylbenzidine(TMB)oxidation rates and an enhanced performance of photocatalytic oxidative coupling of amines to imines.