Beginning with the discovery of fullerenes (1985) , moving to carbon nanotubes in (1991) , and then to graphene in (2004) , carbon nanomaterials have been widely studied and have shown to have a multitude of potential applications. However, another unique allotropic form of carbon, Carbon Nano Onions (CNOs) [4-5-8-11], which was actually discovered before these other materials, has long been overshadowed by these more popular and much more thoroughly investigated carbon nanomaterials.
Public interest in these carbon nanomaterials is great, research funding is flowing, with the number of scientific projects skyrocketing: between 1980 and 2010, a total of 5,000 projects were published, followed by another 5,000 in just the next three years (2011 and 2014). However, the overall amount of knowledge has only increased marginally, especially in the toxicological field .
In the last several years, Carbon Nano Onion (CNOs) have attracted a great deal of attention because of their unique chemical and physical properties. These properties are different from those of other nano-sized carbon materials. CNOs consist of multilayered quasi-spherical and polyhedral shaped shells that represent another new nanophase of carbon material that owes its name to its concentric layered structures resembling that of an onion .
Over the years, CNOs have been extensively studied to understand their underlying nature and to explore their potentiality for a wide variety of technological applications . Extensive and promising research has been published in the following areas:
- Electromagnetic shielding capabilities of CNOs at microwave, infrared and terahertz frequencies
- Gas and energy storage
- Ultrahigh power micrometer-sized supercapacitors accruing from their accessible external surface area for ion absorption
- Solid lubricants
- Electrode material for Li-ion batteries
- Fuel cells
- Heterogeneous catalysis
- Biocompatible nanocapsules for drug delivery systems.
Although CNOs has been synthesized by many different methods in the last 30 years, up and until now large scale production had yet to be fully achieved.
Sumio Iijima first discovered CNOs while looking at a sample of carbon black in a transmission electron microscope (TEM) in 1980 . The small amount of CNOs that were created in vacuum was observed as a byproduct of the synthesis of carbon black. From then until today the methods that have been developed to synthesize CNOs still remain in their infancy and suffer from issues relating to quality, quantity and purity. Various synthesis pathways have led to discoveries of CNOs with unique physical properties including varying shell numbers, graphitic structures, core types, and precursors.
Twelve years after the first discovery of the CNOs, Daniel Ugarte (1992) is credited with developing a formation mechanism for the creation of spherical graphitic structures by focusing an electron beam on a sample of amorphous carbon. This process caused the carbon to graphitize and began to curl. After sufficient time, it closed on itself forming an onion like structure. It is surmised that the curving and closure occurred in order to minimize the surface energy of the newly formed edge planes of graphite which are about 30x that of the basal plane.
Other synthesis pathways have led to CNOs being formed by other means, such as the following:
- Kuznetsov et al.  obtained OLC (Onion Like Carbon) by high-temperature annealing of diamond nanoparticles under vacuum
- Cabioc’h et al. reported CNOs by high dose carbon ion implantation into copper and silver
- Sano et al. fabricated CNOs by arc discharge between two graphite rods immersed in water
- Chemical vapor deposition has also been considered as a viable method to synthesize CNOs by many chemists.
Over the years many other ways have been utilized to synthesize CNOs. These methods include:
- RF plasma
- Shock compression
- High energy ball milling
- Laser irradiation
Most of the methods mentioned above, however, require high energy input yet have a very low yield of CNOs. This results in the material collected being a secondary byproduct. For applications such as fuel-cell electrodes, large quantities of the material are desired. Unfortunately, CNOS can only be produced in minute quantities by most of these methods. Moreover, it is not practical to use CNOs obtained by some methods for ordinary applications due to the high investment and running costs.
Because of its multitude of applications, it is of paramount importance to be able to synthesize this material economically in consistent size, high purity and large volume. According to the literature available, to date, the method that is the most utilized for research on the characterization and functionalization of carbon nano onions is Kuznetsov et als  published method of creating a subclass of carbon nano onions (carbon like onions) by thermal transformation of nanodiamonds in a vacuum. According to published claims, the material can be made in sizeable volume using this method, as opposed to the very limited volume production of the other above mentioned processes.
(Nov. 2014), Juergen Bartelmess and Silvia Giordani, published an extensive review on the development of carbon nano onions (CNOs) and the many pathways (over the years) that have been used to synthesize this material. (Carbon nano-onions (multi-layer fullerenes): chemistry and applications: Beilstein J. Nanotechnol. 2014, 5, 1980–1998). First, it briefly summarizes the most important synthetic pathways for their preparation and it gives the reader an update over new developments in recent years. It also gives an overview of the fields of applications, in which CNO materials were successfully implemented. Some specific successes highlighted were in a number of different electronic applications such as, “electrode materials in capacitors, as anode materials in lithium-ion batteries, and as catalyst support in fuel cells. They have even attracted the interest of NASA researchers for their tri-biological properties as additives for aerospace applications. Despite much interest in different carbon-based nanomaterials, CNOs as functional constructs for intracellular transport have not been widely explored. However, given their size, homogeneity and purity (compared with carbon nanotubes) they could in principle add an important new avenue for the transport of imaging and therapeutic agents. These carbon particles have demonstrated a higher cellular uptake, low cytotoxicity and lower inflammatory potential, than CNTs, thus creating a very promising future for their biomedical applications.”
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