After pioneering research on the discovery of graphene, the synthesis of various graphene derivatives has begun to be further developed.
Graphene derivatives can be classified according to their size, such as zero-dimensional (graphene quantum dots), one-dimensional (graphene nanoribbons), and three-dimensional (graphene foams). This technical article will focus on the synthesis of graphene quantum dots, a zero-dimensional material.
Graphene has a wide range of application prospects, but due to its zero-bandgap properties, low dispersion in water, and low spectral absorption, it cannot be applied in many fields such as optoelectronics, bioimaging, and semiconductors. Therefore, the preparation of graphene quantum dots (GQDs) is an effective method to modulate the graphene band gap and apply it to nanodevices.
When the transverse size of graphene flakes is reduced to the nanoscale, they become GQDS, zero-dimensional (0D) materials consisting of no more than five layers of graphene flakes. Most GQDs are circular or oval in shape, although there are also triangular and hexagonal dots.
The opening of the band in GQDS in a size-dependent manner due to the quantum confinement effect is one of the significant differences between GQDS and graphene that produce a clear boundary, and the band width increases with the decrease of the size of the quantum dots. Most GQDs have a bandgap between 22~3.1 EV between, with green or blue fluorescence.
It has been found that compared with graphene, GQDS has a very large specific surface area and extremely small size, and the edge can accommodate more active sites (such as functional groups, dopants, etc.), so it is easier to disperse in water. At the same time, it also has other remarkable characteristics such as low toxicity, good biocompatibility, chemical stability, stable photoluminescence and fluorescence emission in a wide spectral range. Due to these unique properties, GQDS is considered an advanced multifunctional material with a wide range of applications, including cancer**, solar cells, biosensors, LEDs, and light detectors, among others.
GQD synthesis can be divided into two categories: top-down and bottom-up preparation techniques.
Bulk graphitized carbon materials (such as MWCNTS, graphene, graphite, graphene oxide, coal, etc.) are used as precursors. Carbon precursors are stripped during the reaction and cleaved into the desired GQDS by chemical, thermal, or physical processes. The top-down synthesis process employs techniques such as redox cutting, pulsed laser ablation (PLA), and electrochemical cutting.
Graphene quantum dots are synthesized by reducing oxidation cutting technology, mainly using strong reducing or oxidizing agent as scissors to cut graphene oxide or graphene sheets. Nonetheless, the process is often described as requiring the use of toxic chemicals and a large number of purification steps;However, there are some exceptions where an environmentally safe oxidant such as H2O2 can be used, which can achieve yields of more than 77% without any purification.
The results show that during electrochemical cutting, the application of an electric potential causes charged ions to enter the graphite layer of the precursor. For example, the researchers reported the synthesis of GQDS with an average size of 2-3 nanometers by using a simple electrochemical stripping device consisting of two graphite rods as electrodes and citric acid and sodium hydroxide in water as the electrolyte. The method also has excellent ability to functionalize and dope GQDS.
Another interesting top-down synthesis method is the PLA method, which uses a focused laser beam to synthesize GQDS from graphite flakes. This technology does not require strong acidic chemicals, providing a viable and environmentally friendly avenue for the study of GQDS. This method can be used to synthesize GQDS of consistent size.
The bottom-up approach, rather than the top-down approach, uses the fusion of smaller precursor molecules (e.g., citric acid, glucose, etc.) to obtain GQDs. Compared to a top-down strategy, a bottom-up approach has the advantage of fewer defects and adjustable size and topography. The most well-known bottom-up synthesis route is microwave-assisted, water bath heating, stepwise organic synthesis and preparation of soft templates.
A typical case is that citric acid and amino acids have been reported to synthesize GQD by hydrothermal method. In this technique, the preparation is done by loading the precursor into an autoclave and subjecting the citric acid to a hydrothermal reaction at a specific time and a defined temperature. This technique simplifies the process of introducing heteroatom doping such as sulfur and nitrogen into the GQD structure. For example, the use of citric acid and ethylenediamine nitrogen-doped GQDS (N-GQDS) has been reported to be 5-10 nm in size.
The hydrothermal process typically takes several hours, which makes it unsuitable for the synthesis of GQD on an industrial scale. Microwave-assisted heating is a well-established remedy. By employing microwave heating, the time required for GQDS growth can be reduced to minutes or even seconds.
Due to the limited precision of the synthesis process of single crystal GQDS with controllable size, the formation process of single crystal GQDS has not been directly observed. In addition, the main limitations of GQDS for industrial and academic research are its low yield and extremely high preparation cost.
Currently, most of the existing top-down or bottom-up GQD synthesis methods have yields of less than 30%, which also require expensive and time-consuming purification operations, which significantly increases the final cost of GQDS. Therefore, future research directions should focus on improving yield and simplifying the purification process, so that the industrial application of GQDS can be more economical.
References
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3. tian, p., et al. (2018). graphene quantum dots from chemistry to applications. materials today chemistry.
4. yan, yibo., et al. (2018). systematic bandgap engineering of graphene quantum dots and applications for photocatalytic water splitting and co2 reduction. acs nano.