Preparation Of Doped Graphene Quantum Dots And Their Applications

Fluorescent carbon dots?CDs?is another kind of zero-dimensional carbon nanomaterials,which was discovered after the two-dimensional graphene,the one-dimensional carbon nanotubes and the zero-dimensional fullerene.As one type of novel fluorescent materials,CDs have shown extraordinary talents in the fluorescent world because they have a lot of advantages and the widespread applications.Along with the time passage and intensive investigation,CDs can all be grouped into three main categories: graphene quantum dots?GQDs?,carbon nanodots?CNDs?and polymer dots?PDs?.In order to make study more targeted,our research only focused on GQDs to survey,what is called "showing fundamental reasons from detail description ",we have conducted in-depth research into these fields: the formation mechanism of GQDs;the physical and chemical properties,optical properties and new applications.In addition,we have taken the ideas from the concept of doping in the semiconductor field,and tried to explain the phenomenon of GQDs modification from the heteroatom doping perspective.Then,we have extened the modified GQDs into the broader fields such as photocatalysis.The whole dissertation is composed of the following four parts:1.Citric acid?CA?is chosen as the carbon source,because it is common chemical with define structure in our daily life.We well-designed a set of experiments by changing the kind of amine to understand the formation of GQDs.Based on the tracking the reaction process,the formation mechanism of GQDs was proposed for the first time.At high temperature and basic media,the carboxyl groups?-COOH?in the CA starts dehydration-condensation reaction first,and then the –COOH will react with the amine group?-NH2?to form amide group,forming the pyrrole N in the graphite,then the pyrrole N will change into graphitic N.Next,we found that the quantum yield?QY?of the N doped GQDs,which were obtained from heating CA and ethylenediamine?EDA?in the distilled water,could reach as high as 94% for blue light emission by optimizing the conditions.After understanding the formation mechanism of the N doped GQDs,we changed the above-mentioned passivant and N source from EDA to diethylenetriamine?DETA?,and used different kinds of solvent: distilled water?H2O?,Dimethyl Formamide?DMF?and even solvent free.Then we obtained the N doped GQDs with different kinds of emission light: blue?N-GQDs-B?,green?N-GQDs-G?and yellow?N-GQDs-Y?.We found that the effective conjugate existing in these system N doped GQDs continuously increased and the amount of –COOH decreased relatively.The phenomenon is caused by two primary factors.One is that the H2 O is a kind of polar protic solvent,it will provide more protons and inhibit the dehydration reaction.The other is that the DMF is a kind of aprotic solvent,it cannot provide the proton and facilitate the dehydration reaction,and then the amount of N will be introduced into the graphite structure.And with the increase in the amount of the effective conjugate and N,the emission fluorescence of N doped GQDs can be red shift from blue?450 nm?to green?550 nm?and yellow?580 nm?.Then we applied the prepared GQDs in cell imaging fields.2.Taking the idea from the doping concept of semiconductor,we took CA as carbon source,thiourea as passivant and N/S source and H2 O as the solvent,the S,N co-doped GQDs?S,N-GQD?were prepared firstly.They emit blue light,the QY can reach 71%.A broad absorption band in the visible region appeared in S,N co-doped GQDs,because of the sulfur doped,which alters the surface state of GQDs.The excellent photocatalytic performance of the S,N-GQD/TiO2 composites had been demonstrated by degradation of rhodamine B?RhB?under visible light.The apparent rate of S,N-GQD/TiO2 is higher than that of N-GQD/TiO2 and P25 TiO2 under visible light irradiation,respectively.And we changed the above-mentioned solvent from H2 O to DMF.A facile solvothermal route to synthesize S,N co-doped graphene quantum dots?S,N-GQDs?with unique optical properties is demonstrated.Three absorption bands are observed at 338,467,and 557 nm,which is different from any previous reports.The photoluminescent spectra display emissions in three primary colors that are independent of the excitation wavelength.The as-prepared S,N-GQDs provide an attractive means of effectively tuning their optical properties for the purpose of exploiting new applications in visible-light photocatalytic and bioimaging.3.The optical properties of GQDs has been significantly changed after doping N and S,so we continued to choose Se as another dopant because it has larger atomic radius than N and S.After the Se doping,Se&N-CDs have a narrow band gap 2.5 eV.Fluorescent Se&N-CDs expand the absorption into visible light region?500 nm?and show relative high PL QY 52% in green emission region.It provides the possibility of making use of the fluorescent Se&N-CDs as an important tool of noninvasive imaging due to the better penetration and relative harmlessness of the longer wavelength.Moreover,we demonstrate the Se&N-CDs can take the place of the expensive fluorescent dye like indocyanine green?ICG?to assist the process of diagnosis and providential treatment for ophthalmic disease,because these GQDs had low toxity and good biocompatibility.4.Recently,g-C₃N₄-carbon dots?g-C₃N₄-C?composite has been demonstrated an efficient photocatalyst for overall water splitting via H2O2 as intermediate.We developed a unique one-step homogeneous synthesis route to prepare g-C₃N₄-GQD composites?C₃N₄-GQDs catalyst?by heating CA and urea.The band gap of g-C₃N₄-GQDs can be tuned from 2.84 to 2.08 eV by simply changing the content of CA.We also confirm that GQDs in the g-C₃N₄ has two functions: cocatalyst for H2 production and catalyst for H2O2 decomposition.Our findings give a detailed understanding on the working mechanism of g-C₃N₄-Cx composites,which will beneficial to rational design highly efficient metal-free photocatalyst.