中间相沥青基掺氮碳量子点的制备及其荧光特性

Synthesis of mesophase pitch-based N-doped carbon quantum dots and their fluorescence properties

由中间相沥青中的纳米尺度碳质微晶,直接碳化制备碳量子点(CQDs),是制备碳量子点的新方法。本文在此方法的基础上,对中间相沥青基碳量子点的掺氮工艺及其荧光特性进行了系统考察。采用两种方法制备掺氮碳量子点(NCQDs),一种是用氮等离子体轰击中间相沥青基CQDs,另一种方法是在原料油中混入三聚氰胺,制备掺氮中间相沥青,进而将掺氮的碳质微晶分散在硅片上,碳化得到NCQDs。对所得NCQDs,我们采用扫描电子显微镜、透射电子显微镜和原子力显微镜进行表征,并进行了X射线光电子谱和荧光发射光谱分析。氮等离子体轰击制备的NCQDs(NCQDs-P)中,氮元素主要以吡啶氮和石墨氮的形式存在,等离子体轰击并未明显改变碳量子点的尺寸。而三聚氰胺法制备的NCQDs(NCQDs-C)中氮元素主要为吡咯氮和石墨氮,随着三聚氰胺添加量的增加,NCQDs-C的粒径由2.2 nm增长至4.5 nm。这可能是由于含氮基团具有较高反应活性,加快了沥青大分子的聚合过程,使得在相同煅烧温度和时间下得到了更大尺寸的碳质微晶。NCQDs-C与未掺杂N的CQDs的荧光特性相似,荧光峰位置不随激发波长变化而变化。与之不同,NCQDs-P荧光峰位置随激发波长位置移动而变化,且等离子体轰击时间越长,NCQDs-P荧光峰位置移动越明显,这是因为吡啶氮的引入增加了碳材料在费米能级附近的态密度,导致激发态能隙减小,内转化和振动弛豫变得容易。本研究表明,两种掺氮方法均可以实现氮掺杂碳量子点的高效制备,获得的掺杂碳量子点具有均一的粒径分布,N/C原子比可达8.3%,但是两种掺氮方法对碳量子点的尺寸和掺氮的类型会产生不同的影响,并进一步影响到掺氮碳量子点的荧光特性。

Direct carbonization of dispersed carbonaceous microcrystals in mesophase pitch is a new approach to the synthesis of carbon quantum dots(CQDs).Based on this new method,this work presents a systematic investigation of the synthesis and fluorescence characterization of N-doped CQDs(NCQDs)derived from mesophase pitch.Two different methods are adopted to produce NCQDs:1)Nitrogen plasma bombardment of the CQDs prepared from mesophase pitch,and 2)Carbonization of N-doped carbonaceous microcrystals,which are prepared by introducing melamine into raw oil.In the nitrogen plasma bombardment synthesis,a nitrogen plasma was produced at 220-230 V bias voltage with a discharge current of 0.128 A.The plasma treatment time was 5,20 and 30 min.In the synthesis using melamine,the oil slurry to melamine mass ratios of 4:1 and 1:1 were tested.The as-prepared NCQDs were characterized by scanning and transmission electron microscopy,atomic force microscopy,X-ray photoelectron spectroscopy and photoluminescence emission spectroscopy.Our results show that both methods are capable of producing NCQDs,with an N to C molar ratio of up to 8.3%.On the other hand,the two different synthesis methods affect the particle size and the N atom configuration of NCQDs,consequently resulting in different fluorescence characteristics.In the NCQDs prepared with plasma(NCQDs-P),N atoms are mainly in the configurations of pyridinic and graphitic N,while the N binding modes of NCQDs prepared with melamine(NCQDs-C)are pyrrolic and graphitic N.The nitrogen plasma bombardment has not significantly changed the particle size of CQDs.However,the particle size of NCQDs-C increases from 2.2 nm to 4.5 nm as the added amount of melamine increases.It is likely due to the higher reactivity of N-doped groups,which might have accelerated the aggregation of pitch molecules and resulted in larger carbonaceous microcrystals at the same calcination temperature and time.In the fluorescence curves of NCQDs-C and the undoped CQDs,the fluorescence peak positions do not change with the excitation wavelength,while the peaks for NCQDs-P change as the excitation wavelength varies.Larger peak movement in the fluorescence curve for NCQDs-P is observed at longer plasma etching time.It is likely due to the introduction of pyridinic N,which has increased state density near the Fermi level and consequently reduced the energy gap between different excited states,thus leading to easier internal conversion and vibration relaxation.Our work provides a new approach to efficient synthesis of NCQDs,and gives a better understanding of the effect of N atom configuration on the fluorescence properties of CQDs.