Carbon-based nanomaterials are finding increasing applications across electronics, catalysis, energy conversion, storage, and biomedicine. Carbon nano-onions (CMOs), also known as buckyonions or nested fullerenes, are one area of interest.
First described in 1980, carbon nano-onions are nanostructures made of concentric shells of fullerenes, looking like cages within cages. They offer multiple attractive qualities such as a high surface area and large electrical and thermal conductivities.
The conventional methods for producing CNOs have some significant flaws, however. Some call for severe synthesis conditions, such as high temperatures or vacuum, while others require a lot of time and energy.
Techniques do exist to get around such limitations, but they demand complex catalysts, expensive carbon sources, or dangerous acidic or basic conditions. This limits the potential of carbon nano-onions.
Microwave Pyrolysis
Scheme depicting the synthesis of carbon nano-onions via the microwave pyrolysis of fish scales. Credit: Takashi Shirai from NITech, Japan
A simple and convenient way to turn fish waste into extremely high-quality CNOs could change all that, a recent study published in Green Chemistry notes.
A team of scientists from Nagoya Institute of Technology in Japan has developed a construction route that converts fish scales extracted from fish waste after cleaning into carbon nano-onions in just seconds through microwave pyrolysis. Pyrolysis is a process of thermal decomposition of materials at high temperatures in an inert atmosphere.
The team, which included Assistant Professor Yunzi Xin, Master’s student Kai Odachi, and Associate Professor Takashi Shirai, believes the secret to this surprising conversion may have to do with the collagen contained in fish scales, which can absorb enough microwave radiation to produce a fast rise in temperature.
The resulting thermal decomposition or pyrolysis produces certain gases that support the assembly of CNOs. Remarkably, this approach needs no complex catalysts, harsh conditions, or prolonged wait times – the fish scales can be converted into CNOs in less than 10 seconds.
High Crystallinity Carbon Nano-onions
This new synthesis process yields carbon nano-onions with very high crystallinity. This is notably hard to achieve in processes that use biomass waste as a starting material.
Additionally, in the synthesis, the surface of the CNOs is selectively and thoroughly functionalized with (−COOH) and (−OH) groups. This contrasts greatly with the surface of CNOs prepared with conventional methods, which are typically bare and have to be functionalized through additional steps.
This “automatic” functionalization has important implications for the applications of CNOs. When the CNO surface is not functionalized, the nanostructures tend to stick together owing to an attractive interaction known as pi−pi stacking.
This makes it difficult to disperse them in solvents, which is necessary in any application requiring solution-based processes. However, since the proposed synthesis process produces functionalized CNOs, it allows for excellent dispersibility in various solvents.
Another advantage associated with functionalization and high crystallinity is exceptional optical properties.
“The CNOs exhibit ultra-bright visible-light emission with an efficiency – or quantum yield – of 40%. This value, which has never been achieved before, is about 10 times higher than that of previously reported CNOs synthesized via conventional methods,”
Dr. Shirai explains.
Blue-light-emitting Thin Film
To showcase some of the many practical applications of their CNOs, the team demonstrated their use in LEDs and blue-light-emitting thin films. The carbon nano-onions produced a highly stable emission, both inside solid devices and when dispersed in various solvents, including water, ethanol, and isopropanol.
“The stable optical properties could enable us to fabricate large-area emissive flexible films and LED devices. These findings will open up new avenues for the development of next-generation displays and solid-state lighting,”
says Dr. Shirai.
The proposed synthesis technique is environmentally friendly and provides a straightforward way to convert fish waste into infinitely more useful materials. The team believes their work would contribute to the fulfillment of several of UN’s Sustainable Development Goals.
Additionally, if carbon nano-onions make their way into next-generation LED lighting and QLED displays, they could greatly help reduce their manufacturing costs.