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Graphene quantum dots (GQDs) have drawn significant attention as a result of their exceptional qualities and prospective applications in a variety of biological and technical fields.
There are several prospective applications for quantum dots in electronic devices, such as solar cells, LEDs, imaging, electronic displays, as a result of their distinctive size-dependent electro-optical qualities. Therefore, these materials have been the subject of considerable study.
However, as a result of the high costs of quantum dots, their use and research into them have been limited. The high toxicity of inorganic quantum dots has also been a major limiting factor. GQDs have recently emerged as an appealing cost-effective alternative. GQDs are also non-toxic, have high solubility, steady photoluminescence, and superior surface grafting, thus making them promising replacements for standard (inorganic) quantum dots.
The fabrication procedures for the GQDs can be categorized into two classes: top-down and bottom-up. The first involves taking carbon materials, such as graphene or carbon nanotubes, and breaking them into bits through chemical or physical processes. The second fabrication class involves GQDs being created through synthesis or carbonization from organic source materials.
Top-Down
The most common top-down method to create GQDs is known as oxidative cleavage. The method involves breaking of carbon-carbon bonds in a source material by oxidizers such as sulfuric acid or nitric acid. Oxidative cleavage is a straightforward and efficient preparation method, but there are some issues, such as an oxidizer that is too strong causing incineration or explosion and complex post-processing requirements.
The hydrothermal (or solvothermal) synthesis technique is a basic and rapid process that involves cutting carbon materials under high pressure and high temperature. Typically, the carbon materials have to be exposed to a strong oxidizer before the main phase of the process.
One of the typical issues with oxidative cleavage or hydrothermal is a relatively long reaction time. Commonly used in the preparation of nanomaterials, microwave heating can shorten reaction times and boost production yield.
One technique involves the use of ultrasound to create tens of thousands of tiny bubbles in a liquid, which generate high pressure and energy when expanding, contracting and rupturing. These forces effectively break apart carbon-carbon bonds to create GQDs.
In the electrochemical oxidation process, a carbon source material functions as a working electrode that is oxidatively split into the GQDs under high voltage. In one type of electrochemical oxidation process, the carbon-carbon bonds fractured directly. In the other type of process, water is oxidized into a hydroxyl free radical or an oxygen free radical that oxidatively cleaves source material into GQDs. The GQD solutions produced by electrochemical oxidation show substantial amounts of stability, but the process requires lengthy pre-treatment and purification processes. Furthermore, it is challenging to scale up production with these processes due to low product yield.
A recent study has shown promise for the top-down preparation of GQD from relatively inexpensive carbon sources like coal. As a result of their low cost, coal-derived GQDs have significant potential for industrial applications and might be a cost-effective and green alternative to standard quantum dots.
In a standard process, coal is agitated in strong nitric acid and heated to between 100 120 degrees Celsius for several hours. The temperature of the solution is then reduced, and the nitric acid is evaporated away for reuse. The GQDs that are produced must be filtered and purified.
Bottom-Up
Bottom-up fabrication class involves GQDs being created through synthesis or carbonization from organic source materials.
In a controllable synthesis process, GQDs are produced from phenyl-containing source materials by controllable, step-by-step synthesis in an organic solvent. GQDs produced by this method have a proper quantity of carbon atoms, as well as consistent size and shape. However, the preparation method includes intricate multi-step reactions, which take quite a while and have a relatively low yield.
The production of GQDs through molecular carbonization is a fairly green and easy process. It processes organic molecules or polymers via dehydration and carbonization measures. Size and structure of GQDs are challenging to control with this method.
Sources and Further Reading
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