After groundbreaking research on the discovery of graphene, extensive research was conducted on synthesizing various graphene derivatives.
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Graphene derivatives may be typically classified based on their dimensions, such as zero-dimensional (graphene quantum dots), one-dimensional (graphene nanoribbons), and three-dimensional (graphene foam). This article aims to shed light on the synthesis of graphene quantum dots.
Graphene has a wide range of applications, but due to its zero-band gap property, low dispersibility in water and low spectral absorption, it cannot be used in many areas, including optoelectronics, bioimaging, and semiconductor. Therefore, an effective method for tuning graphene's band gap and using it for nanodevice applications is to produce graphene quantum dots (GQDs).
When the lateral dimensions of graphene sheets are reduced to the nanoscale, they become GQDs, zero-dimensional (0D) materials made up of no more than five layers of graphene sheets. Most GQDs are circular or elliptical in shape, although there are also triangular and hexagonal dots.
Graphene Quantum Dots vs. Graphene
The opening of bandgap in a size-dependent manner due to the quantum confinement effect in GQDs is one of the significant differences that have generated defined borders between GQDs and graphene. The energy band gap widens as the quantum dot size decreases. Most GQDs have a bandgap between 2.2 and 3.1 eV, making them green or blue fluorescent.
GQDs were discovered to accommodate more active sites (e.g., functional groups, dopants) at the edges than graphene due to their very large specific surface area and extremely small size; hence can be dispersed in water more easily.
Together with low toxicity, biocompatibility, chemical stability, stable photoluminescence, and fluorescence emission with a broad spectral range, these features provide distinct advantages over graphene.
Due to these unique properties, GQDs are regarded as an advanced multifunctional material with wide range of applications, including cancer therapy, solar cells, biosensors, LEDs, and photodetectors.
GQD synthesis can be divided into two categories: top-down and bottom-up preparatory techniques.
Top-Down Approach for the Synthesis of Graphene Quantum Dots
Bulk graphitized carbon materials (e.g., MWCNTs, graphene, graphite, graphene oxide, coal etc.) are used as precursors in the top-down process. The carbon precursors are subsequently exfoliated and sliced into required GQDs using chemical, thermal, or physical processes. The top-down synthesis process employs techniques such as oxidative/reductive cutting, pulsed laser ablation (PLA), and electrochemical cutting.
To synthesize GQDs using the reductive/oxidative cutting technique, strong reducing or oxidative agents are used as scissors to cut through graphene oxide or graphene sheets. Nonetheless, this process is typically described as requiring toxic chemicals and extensive purifications; however, there are some exceptions that use environmentally safe oxidative agents such as H2O2 and reach a production yield of more than 77% without any purifications.
It has been shown that applying an electric potential results in charged ions being driven into the precursors' graphitic layers during the electrochemical cutting process. For example, researchers reported the synthesis of GQDs with average sizes of 2-3 nanometers by using a simple electrochemical exfoliation setup consisting of two graphite rods as electrodes and citric acid and sodium hydroxide in water as electrolytes. This method also has outstanding capabilities for functionalizing and doping GQDs.
Another intriguing top-down synthesis route is the PLA method, which uses a focused laser beam to synthesize GQDs from graphite flakes. This technique does not require strong acidic chemicals and thus provides a feasible and environmentally friendly path toward GQDs. This method was reported to synthesize pristine GQDs with uniform sizes.
Bottom-Up Approach for the Synthesis of Graphene Quantum Dots
Bottom-up approaches, instead of top-down methods, employ the fusing of smaller precursor molecules (such as citric acid, glucose etc.) to create GQDs. In contrast to the top-down strategy, the bottom-up approach has fewer flaws and adjustable size and morphology advantages. The most notable bottom-up synthesis routes are microwave-assisted, hydrothermal, stepwise organic synthesis and soft-template.
Typically, citric acid and amino acids, have been reported to synthesize GQDs through a hydrothermal approach. In this technique, precursor is loaded into the autoclave and hydrothermal for a specific time and a defined temperature citric. This technique simplifies introducing heteroatom doping like sulfur and nitrogen into the GQD structure. For example, using citric acid and ethylenediamine nitrogen-doped GQDs (N-GQDs) with a size of 5–10 nanometers has been reported.
The hydrothermal process often takes several hours, making it unsuitable for GQD synthesis on an industrial scale. Utilizing a hydrothermal approach assisted by microwaves is one remedy for this. By using a microwave-based approach, it is feasible to reduce the time needed for GQDs to grow to a few minutes or even seconds.
Challenges Associated with the Synthesis of Graphene Quantum Dots
Synthesis of the size-controlled single-crystalline GQDs has not been directly observed so far due to its limited precision over the synthesis process. Furthermore, the primary limitations of GQDs for industrial and academic operations are their low production yield and extremely high cost.
Currently, most of the existing top-down or bottom-up GQD synthesis methods have a production yield of less than 30%. These approaches also necessitate costly and time-consuming purifying operations, which significantly raise the final cost of GQDs. As a result, future research on GQDs should focus on increasing production yield as well as ease of purification to make GQDs inexpensive for industrial application.
References and Further Reading
Ghaffarkhah, Ahmadreza., et al. (2022). Synthesis, applications, and prospects of graphene quantum dots: a comprehensive review. Small. https://doi.org/10.1002/smll.202102683.
Y. Yan., et al. (2019) Recent Advances on Graphene Quantum Dots: From Chemistry and Physics to Applications. Advanced Materials. https://doi.org/10.1002/adma.201808283.
Tian, P., et al. (2018). Graphene quantum dots from chemistry to applications. Materials today chemistry. https://doi.org/10.1016/j.mtchem.2018.09.007
Yan, Yibo., et al. (2018). Systematic bandgap engineering of graphene quantum dots and applications for photocatalytic water splitting and CO2 reduction. ACS Nano. https://doi.org/10.1021/acsnano.8b00498
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