Scheme depicting the synthesis of carbon nano-onions via the microwave pyrolysis of fish scales. The top inset shows the rise in temperature of the fish scales due to microwave absorption over a period of 10 seconds, as well as a proposed formation mechanism for the carbon nano-onions. (Right) transmission electron microscopy images showing the morphology of the synthesized carbon nano-onions and photographs of CNO dispersion in ethanol, an emissive flexible film, and an LED containing CNO © Takashi Shirai from NITech, Japan
In a recent study published in Green Chemistry, a team of scientists from Nagoya Institute of Technology in Japan found a simple and convenient way to turn fish waste into extremely high-quality CNOs. CNOs are a type of carbon-based nanomaterial that are widely used in the catalysis, energy conversion and storage, biomedicine and electronics sectors thanks to their low toxicity and chemical stability. After they were first identified in 1980, researchers noted that the nanostructures were composed of concentric shells of fullerenes, resembling cages within cages. This gives CNOs a high surface area and large electrical and thermal conductivities – making them highly useful across multiple industries.
Unfortunately, the conventional methods for producing CNOs have serious drawbacks. Some require harsh synthesis conditions, like high temperatures or a vacuum, while others are time and energy intensive. Some techniques can circumvent these limitations, but instead call for complex catalysts, expensive carbon sources, or dangerous acidic or basic conditions. This greatly limits the potential of CNOs.
However, researchers from Nagoya Institute of Technology found a breakthrough. The team, which included assistant professor Yunzi Xin, master’s student Kai Odachi, and associate professor Takashi Shirai, developed a synthesis route in which fish scales extracted from fish waste after cleaning are converted into CNOs in mere seconds through microwave pyrolysis.
The researchers weren’t able to pinpoint the exact reason why fish scales could be easily converted into CNOs, but they believe that it is related to the collagen in the fish scales. The collagen can absorb enough microwave radiation to produce a fast temperature spike. This leads to thermal decomposition or “pyrolysis,” which produces certain gases that support the assembly of CNOs. What is remarkable about this approach is that it needs no complex catalysts, harsh conditions or prolonged wait times; the fish scales can be converted into CNOs in less than 10 seconds.
Moreover, this synthesis process yields CNOs with very high crystallinity. This is remarkably difficult to achieve in processes that use biomass waste as a starting material. Additionally, during synthesis, the surface of the CNOs is selectively and thoroughly functionalised with (−COOH) and (−OH) groups. This is in stark contrast to the surface of CNOs prepared with conventional methods, which are typically bare and have to be functionalised through additional steps.
This “automatic” functionalisation has important implications for applications of CNOs. When the CNO surface is not functionalised, 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 new synthesis process generates functionalised CNOs, it allows for an excellent dispersibility in various solvents.
Yet another advantage associated with functionalisation and the high crystallinity are the exceptional optical properties. Dr Shirai explains: “The CNOs exhibit ultra-bright visible-light emission with an efficiency (or quantum yield) of 40 percent. This value, which has never been achieved before, is about 10 times higher than that of previously reported CNOs synthesised via conventional methods.”
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 CNOs 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,” speculates Dr Shirai. “These findings will open up new avenues for the development of next-generation displays and solid-state lighting.”
Furthermore, 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 CNOs make their way into next-generation LED lighting and QLED displays, they could greatly help reduce their manufacturing costs.
