Graphene quantum dots with nitrogen and oxygen derived from simultaneous reaction of solvent as exfoliant and dopant
Graphical abstract
Introduction
Graphene quantum dots (GQDs) are composed of two-dimensional (2D) few-layer graphene with nanoscale dimensions and have attracted considerable attention because of their exceptional properties of band-gap opening and fluorescent tuning in contrast to graphene sheets [1], [2]. Precisely controlled GQDs have a finite band-gap because of quantum confinement and edge effects; thus, they are promising materials for various photonic and optoelectronic applications such as light emitting-diodes, biological imaging, and photovoltaics [3], [4], [5], [6], [7].
Numerous efforts to control the unique properties of GQDs, such as tunable band-gap and photoluminescence yield, have been reported. It is basically known that the properties of the GQDs can be tuned by size control and heteroatom doping [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. In other words, the quantum confinement effect is highly related to the size or surface/edge state of GQDs. However, the size effect of GQDs is strictly restricted because of the infinite exciton Bohr radius of graphene compared to that of the metal-based quantum dots [19], [20]. On the other hand, heteroatom doping can easily tune the band-gap and spectrum regions of GQDs from blue to red. Therefore, to control the properties of GQDs, the size and surface chemistry should be thoroughly considered from initial step to synthesize the GQDs.
Until now, various methods have been reported to form small-sized GQDs and incorporate heteroatoms into GQDs. These can be briefly summarized by two methods: top-down and bottom-up approaches. The top-down approach is a common method involving cutting bulk carbon materials, including graphite, coal, and carbon fibers, by using a strong acid [2], [21], [22]. The bottom-up approach is a chemical synthesis of hexagonal nanocarbon fragments from several organic precursors [11], [23], [24], [25], [26], [27], [28]. However, both approaches mostly produce low-quality GQDs that unavoidably contain structural defects, and this issue arose from following rationale. The top-down approach generally involves harsh oxidation steps to reduce the size, inherently generating permanent in-plane defects in GQDs. In addition, the synthesis of GQDs by bottom-up and some top-down approaches mostly involve the use of non-graphitic and amorphous carbon sources (e.g. coal, citric acid, glucose, and urea) as raw materials and an incomplete dehydration process for converting to CC, leading that CC bonds could not assemble the perfect hexagonal structure [29]. Therefore, the products resulting from these approaches are not GQDs but carbon nanodots (CNDs) or carbon dots (CDs), inevitably degrading their intrinsic optical and electrical properties.
Heteroatom doping effectively modulates the optical and electrical states of GQDs. Many researchers have reported on nitrogen-containing GQDs (N-GQDs) (or nitrogen-containing carbon materials) and their properties [30], [31]. Yeh et al. reported on the band-gap configuration changes of nitrogen-doped graphene oxide quantum dots (N-GOQDs) compared to pristine GQDs [32]. Replacing the oxygen functional groups on the edge of graphene oxide (GO) sheets with nitrogen-containing groups transforms the orbitals and confines π-electrons in GQDs. Recently, Tetsuka et al. reported the tunable energy levels and optical properties of GQDs as a result of nitrogen functionalization on the edge, inducing an effective sp2 orbital resonance within the carbon atoms [33]. They also revealed that nitrogen doping of GQDs led to enhanced charge transport ability because of the long carrier lifetime of N-GQDs. However, the incorporation of N atoms into the graphene lattice through the above strategy is likely to disrupt the sp2 hybridization of C atoms and even induce structural defects in GQDs, which possibly hinders their chemical stability as well as electrical and/or optical properties [29], [34]. However, doping in both hexagonal and defect sites cannot be avoided because the starting material, GO, contains defective structures. Furthermore, chemical reduction processes, which are generally used for heteroatom doping in oxygenated carbon materials, give rise to a number of permanent defects in GQDs, where oxygen functional groups are detached. Therefore, a systematic study on the development of edge N-functionalized GQDs with high crystallinity and outstanding intrinsic properties is necessary.
Here, we proposed a novel one-pot reaction for producing edge-N-/O-functionalized GQDs (NO-GQDs) with high crystallinity and electrical properties using graphite as the raw material. A simple solvothermal reaction using N-methyl-2-pyrrolidone (NMP), which has surface energy similar to graphite, can realize NO-GQDs through a simultaneous process of exfoliation, scission and finally transform the graphite into the GQDs with heteroatoms derived from the decomposed NMP solution. The synthesized NO-GQDs have a less defective and more selectively edge-functionalized structure compared to other reported GQDs. To investigate the electrical properties of NO-GQDs, we applied them to the additive of hole-transporting materials (HTMs) in an optoelectronic device, i.e., a perovskite solar cell (PeSC). We confirmed that NO-GQDs facilitated hole-extraction from the photoactive layer and guaranteed a more stable operation of PeSCs.
Section snippets
Preparation of NO-GQDs
Raw graphite (0.1 g) was dispersed in NMP (N-methyl-2-pyrrolidone, 70 ml) via ultrasonic treatment for 1 h. For the hydrothermal process, the mixture was put in a 150 mL Teflon-lined autoclave and elevated temperature to 300 °C and maintained for 24 h [35]. The resultant solution was filtered to remove the non-exfoliated graphite by using a 200-nm mesh filter. The NO-GQDs powder was obtained by heating the solution at 170 °C for 5 h; the powder was then dried in a N2 furnace (400 °C for 6 h) to
Results and discussion
The schematic procedure for the one-step synthesis of heteroatom doped-GQDs is shown in Fig. 1(a). Recent theoretical works demonstrated that the surface tension of a solvent for graphite exfoliation should be between 40 and 50 mJ m−2 (Table S1) [39], [40]. Among various solvents, NMP was reported to have the most appropriate surface energy for the effective exfoliation of graphite. It is evident that NMP droplets exhibit high affinity with graphite with a low contact angle of 28.2°, as shown
Conclusion
In conclusion, we developed a novel method to produce high-quality heteroatomic NO-GQDs using graphite and NMP as source materials. Through a hydrothermal process, the intercalation and decomposition of NMP molecules into graphite enabled NMP to function as the exfoliator and the dopant. The resulting NO-GQDs exhibited high quantum yield due to their well-confined structures and the presence of N-/O- functional groups. Furthermore, by incorporating NO-GQDs as the additive of HTMs into PeSCs, a
Acknowledgements
This work was supported by a grant from Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2018M1A2A2061989 and NRF-2018R1D1A1B07045368) and “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20174010201540).
Conflicts of interest
The authors declare no conflict of interest.
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