How to Prepare Graphene Quantum Dots
After pioneering research into the discovery of graphene, the synthesis of various graphene derivatives has been explored in greater depth.
Graphene derivatives can be categorized according to their dimensions, such as zero-dimensional (graphene quantum dots), one-dimensional (graphene nanoribbons) and three-dimensional (graphene foams). This technical article will focus on elucidating the synthesis of graphene quantum dots, a zero-dimensional material.
What is a Graphene Quantum Dot？
Graphene has a wide range of applications, but due to its zero bandgap property, low dispersibility in water, and low spectral absorption, it cannot be applied in many fields such as optoelectronics, bio-imaging, and semiconductors. Therefore, the preparation of graphene quantum dots (GQDs) is an effective method to regulate the bandgap of graphene and apply it to nanodevices.
When the lateral dimensions of graphene flakes are reduced to the nanometer scale, they become GQDs, zero-dimensional (0D) materials consisting of no more than five layers of graphene flakes. Most GQDs are circular or elliptical in shape, although there are also triangular and hexagonal points.
Graphene Quantum Dots (GQDs) vs Graphene
The energy bands in GQDs open in a size-dependent manner due to quantum confinement effects, which is one of the significant differences between GQDs and graphene that produce well-defined boundaries, and the energy band widths increase with decreasing quantum dot size. Most GQDs have a band gap between 2.2 and 3.1 eV and have green or blue fluorescence.
It has been found that GQDs are more easily dispersed in water due to their very large specific surface area and extremely small size compared to graphene, and the edges can accommodate more active sites (e.g., functional groups, dopants, etc.). It also has other remarkable features such as low toxicity, good biocompatibility, chemical stability, stable photoluminescence and fluorescence emission in a wide spectral range. Due to these unique properties, GQDs are considered to be an advanced multifunctional material with a wide range of applications, including cancer therapy, solar cells, biosensors, LEDs, and photodetectors.
The synthesis of GQDs can be categorized into two types: top-down and bottom-up preparation techniques.
Top-down synthesis of graphene quantum dots
Bulk graphitized carbon materials (e.g., MWCNTs, graphene, graphite, graphene oxide, coal, etc.) were used as precursors. The carbon precursors are stripped during the reaction process and cut into desired GQDs by chemical, thermal, or physical processes.The top-down synthesis process employs techniques such as oxidation/reduction cutting, pulsed laser ablation (PLA), and electrochemical cutting.
The synthesis of graphene quantum dots using reductive/oxidative cutting techniques mainly involves the use of strong reducing or oxidizing agents as scissors to cut graphene oxide or graphene sheets. Nonetheless, this process is often described as requiring the use of toxic chemicals and a large number of purification steps; however, there are some exceptions where environmentally safe oxidizing agents such as H2O2 can be used, and yields of over 77% can be achieved without any purification.
The results show that the application of an electric potential during electrochemical cutting leads to the entry of charged ions into the graphite layer of the precursor. For example, the researchers report the synthesis of GQDs with an average size of 2-3 nm by using a simple electrochemical stripping device consisting of two graphite rods as electrodes and citric acid and sodium hydroxide in water as electrolytes. The method also offers excellent functionalization and doping of GQDs.
Another interesting top-down synthesis method 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 provides a viable and environmentally friendly route to the study of GQDs. The method can be used to synthesize GQDs of uniform size.
Bottom-up synthesis of graphene quantum dots
Bottom-up approaches, rather than top-down approaches, employ the fusion of smaller precursor molecules (e.g., citric acid, glucose, etc.) to obtain GQDs.Bottom-up approaches have the advantage of fewer defects and tunable size and morphology compared to top-down strategies. The most well-known bottom-up synthetic route is the stepwise organic synthesis and preparation of soft templates by microwave-assisted, water-bath heating.
Typically, citric acid and amino acids have been reported for the synthesis of GQDs by hydrothermal method.In this technique, the preparation is accomplished by loading the precursor into an autoclave and subjecting the citric acid to a hydrothermal reaction at a specific time and specified temperature. This technique simplifies the process of introducing heteroatom doping such as sulfur and nitrogen into the GQD structure. For example, nitrogen-doped GQDs (N-GQDs) using citric acid and ethylenediamine have been reported to be 5-10 nm in size.
The hydrothermal process usually takes several hours, which makes it unsuitable for synthesizing GQDs on an industrial scale.The method of utilizing microwave-assisted heating is a more complete remedy. By employing the microwave heating method, the time required for the growth of GQDs can be reduced to minutes or even seconds.
Challenges associated with synthesizing graphene quantum dots
Size-controllable single-crystal GQDs have not yet been directly observed in their generation due to the limited precision of their synthesis process. In addition, the main limitation of GQDs for industrial and academic research is their low yield and extremely high preparation cost.
Currently, most of the existing top-down or bottom-up GQD synthesis methods have yields below 30%, and these methods also require expensive and time-consuming purification operations, which greatly increase the final cost of GQDs. Therefore, future research should focus on improving the yield and simplifying the purification process to make the industrial application of GQDs more economically efficient.
References
1. Ghaffarkhah, Ahmadreza., et al. (2022). Synthesis, applications, and prospects of graphene quantum dots: a comprehensive review. Small. https://doi.org/10.1002/smll.202102683.
2. 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.
3. 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
4. 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