Abstract：
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 bom-
bardment 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 accel
erated 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.
Key words：mesophase pitch, carbon quantum dots; nitrogen doping
Corresponding Authors： ngq@cup.edu.cn
Cite this article：NING Guoqing, WANG Haibin, YANG Fan, LI Yongfeng. Synthesis of mesophase pitch-based N-doped carbon quantum dots and their fluorescence properties. Petroleum Science Bulletin, 2020, 04: 578-586.
