Preparation And Catalytic Performance Research Of CO2-Responsive Cabon Quantum Dots And CQDs/BiOBr Nanosheets

As a novel species of luminescent carbon nanomaterials,carbon quantum dots（CQDs）have attracted extensively research interest due to their fascinating properties,including excellent dispersibility,vis ible light absorption,photo-induced electron transfer ability,et.al.In photocatalysis,CQDs have excellent photocatalytic activity when used as catalysts.However,it is still a great challange to seperate and recycle CQDs from solution because of its s uperior solubility.Therefore,it is of great significance to design a emerging class of multi-functional GQDs with both high photocatalytic activity and the feature of rapid separation.In addition,as one of the promising Bi-based semiconducter materials,the two-dimensional layered bismuth oxyhalides（BiOX,X=Cl,Br or I）have a promising application perspect in photocatalysis due to their appropriate band structure,stable light absorption et.al.However,the photocatalytic performance of BiOBr was stil l limitted by the low efficiency of light absorption and rapid recombination of photogenerated electron-hole pairs.In order to improve the photocatalytic activity of BiOBr materials,it is a reliabel method to load CQDs on the surface of BiOBr nanosheets because of the superior light absorption,electron-accepting and transport properties of CQDs.In view of this,we have prepared GQDs with CO ₂-triggered reversible phase transfer perfomance and CQDs/BiOBr compound nanosheets.Concurrently,the photocatalytic performance and mechanism of photocatalytic oxidative coupling of amines and degradation of ligin model of both two catalysts have been investigated.The main contents and results of this work are as follows:In the first part of this work,the carboxyl-coated graphene quantum dots（GQDs-COOH）are prepared by using concentrated acid to oxidate commecial graphite.Then,theGQDs-COOHwerefunctionalizedby3-dimethylaminopropylamine to obtain GQDs-DMA.The physical and chemical properties of both GQDs were investigated by PL,TEM,XPS measurements.When bubbling CO₂ to this system,the GQDs-DMA could fastly transfer to aqueoues phase from CH₂Cl₂.In addition,the GQDs-DMA could return to CH₂Cl₂phase upon bubbling Ar at 60℃.The GQDs-DMA also exhibit a good stability and this unique reversible and responsive phase transfer behavior can be realized in several cycles.The ¹³C NMR results indicate that this phase transfer phenomenon related to the protonation-deprotonation process of GQDs-DMA and CO₂ in aqueoues phase.Furthermore,the reversibility of GQDs-DMA was quantitatively calculated to be90%by PL test.Meanwhile,the photocatalytic experiment shows that GQDs-DMA have a higher selectivity and conversion for visible-light promoted oxidative coupling of amines in CH₂Cl₂and it can be easily separated and recycled from solution by bubbling CO₂.In the second part,CQDs-BiOBr nanosheets were used to degradate ligin model in visible light irradiation.The co-doped B,N,S-CQDs were fabricated by one-step solvothermal method at 180℃and 6h.Subsequently,the CQDs-BiOBr compound nanosheets were successfully synthesized by using Bi（NO₃）₃·5H₂O and Na Br as precursor and adding a certain amounts of CQDs.The structure information,o ptical properties and chemical composition of pristine BiOBr and CQDs-BiOBr nanosheets are obtained by XRD,TEM,FT-IT and XPS et.al.In addition,the 5%CQDs-BiOBr nanosheets demonstrate a highest photocurrent intensity and a lowest interfacial charge transfer resistance by electrochemical measurements,indicating the light absorption ability and charge separation efficiency of BiOBr nanosheets could be effectively enhanced by introducing appropriate amounts of CQDs.Undoubtedly,compare to other BiOBr nanosheets,the 5%CQDs-BiOBr nanosheets displayed the highest photocatalytic performance for ligin model degradation.