| The increasing consumption of fossil energy has resulted in a large amount of CO2 being discharged into the atmosphere,exacerbating the problem of global warming.Using solar energy to convert CO2 into valuable chemical fuels(eg:CH4,CH3OH,CO,etc.)can not only alleviate the greenhouse effect,but also meet part of the energy demand.Among them,catalysts play a key role in the photocatalytic CO2 reduction process.However,the low separation efficiency of photogenerated carriers(electronhole pairs)and poor CO2 adsorption/activation capacity on catalysts result in low CO2 reduction activity.Among many catalysts,we mainly study Bi2O2(OH)(NO3).Bi2O2(OH)(NO3)has a typical layered structure and an internal spontaneous polarization electric field,which has a natural advantage in the separation of electronhole pairs.Even so,the low separation efficiency of photogenerated carriers in bulk Bi2O2(OH)(NO3)materials remains high due to the long carrier transport distance.In this study,we enhanced the separation and migration efficiency of photogenerated carriers and improved the adsorption/activation efficiency of CO2 molecules by constructing ultrathin structures,modulating the surface composition,and building heterojunctions as strategies to enhance the photocatalytic CO2 reduction performance of Bi2O2(OH)(NO3).The main research results are as follows:1.Ultrasonic-assisted exfoliation was used to prepare ultrathin Bi2O2(OH)(NO3)nanosheets(BON-N3).TEM results showed that 10-20 μm bulk Bi2O2(OH)(NO3)material was exfoliated into nanosheets of 1.7-3.4 nm atomic level thickness.Moreover,the specific surface area of ultrathin Bi2O2(OH)(NO3)(4.70 m2 g-1)was 8.5 times larger than that of bulk Bi2O2(OH)(NO3)(0.49 m2 g-1).The effect of material thickness on performance was investigated by photocatalytic CO2 reduction performance test.When illuminated for 3 h,the CO yield of the ultrathin nanosheet catalyst(8.4 μmol g-1)was 5.6 times higher than that of the bulk catalyst(1.5 μmol g-1).The enhanced photocatalytic activity can be attributed to the efficient separation of photogenerated carriers by the ultrathin structure.2.A series of ultrathin Bi2O2(OH)1+x(NO3)1-x nanosheets with adjustable surface composition were successfully realized by adjusting the OH-concentration on the surface of ultrathin BON-N3 catalysts by means of simple acid treatment method,which led to partial substitution of OH-by NO3-on the nanosheet surface.XPS results demonstrate the successful regulation of surface composition.The influence of the material surface composition on the performance was investigated by photocatalytic CO2 reduction performance test.After 3 h of illumination,the CO yield of the optimal catalyst was increased to 16.7 μmol g-1,which was about 10 times higher than that of bulk Bi2O2(OH)(NO3).Theoretical calculations revealed that the surface NO3-has stronger charge accumulation/depletion behavior with adjacent Bi atom than surface OH-conducive to the transfer of photogenerated charge from bulk phase to catalyst surface.Therefore,the photocatalytic activity is improved.This work may provide a powerful strategy for the design of surface controlled 2D ultrathin photocatalyst for efficient CO2 reduction.3.N-GQDs/Bi2O2(OH)(NO3)heterojunction catalysts(BON-NGx)in 0D/2D configuration were constructed by hydrothermal method with N-doped graphene quantum dots(N-GQDs)loaded on ultrathin Bi2O2(OH)(NO3)nanosheets.TEM results showed that N-GQDs with particle size of 1～3 nm and ultrathin Bi2O2(OH)(NO3)nanosheets were successfully compounded together.The effects of synthesis conditions and loading of N-GQDs on CO production were investigated by photocatalytic CO2 reduction performance tests.The yield of BON-NG0.5 was 13.7 μmol g-1or CO2 reduction after 3 h illumination,which was twice as high as that of BON-N3.Nanosecond-level time-resolved fluorescence spectra and other results confirmed that the enhanced photogenerated charge transfer and separation efficiency were the main reasons for the enhanced photocatalytic CO2 reduction activity of the composites.This work will provide more ideas for the design of novel high-efficiency photocatalysts.4.Bi2WO6/Bi2O2(OH)(NO3)heterostructure catalyst(BON-BWx)was constructed by combining ultrathin Bi2O2(OH)(NO3)nanosheets and Bi2WO6 nanosheets with 2D/2D configuration.TEM results showed that 2D Bi2WO6 and ultrathin Bi2O2(OH)(NO3)nanosheets were successfully compounded together.Through the photocatalytic CO2 reduction performance test,the influence of Bi2WO6 loading on CH4 production was investigated.BON-BW2 composite exhibited the best photocatalytic CO2 reduction activity,and the yield of the main product CH4 was 11.0μmol g-1 after 3 h illumination,which was three times that of Bi2WO6.The enhanced photocatalytic CO2 reduction activity should be attributed to the formation of a 2D/2D heterojunction with a larger heterojunction interface that facilitates the separation and migration of photogenerated carriers.This study can provide a reference for the rational design of heterojunction photocatalyst structures. |
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