Low-temperature synthesis
Low-temperature synthesis of carbon onions by chemical vapor deposition using a nickel catalyst supported on aluminum
A mass of carbon onions have been successfully synthesized via catalytic decomposition of methane over an Ni/Al catalyst at a low-temperature (600 °C). The carbon onions as-obtained have diameters ranging from 5 to 50 nm and consist of several concentric carbon layers surrounding a hollow core.
A practical method for the production of hollow carbon onion nanoparticles
Researchers at Tianjin University have developed a method for obtaining pure carbon onion nanoparticles in large quantities. They use nitric acid to dissolve the nickel from the carbon-coated nickel nanoparticles. In this method, the decomposition of methane in the presence of Ni/Al catalyst particles produces carbon nano-onions as the primary product. And the nanoparticles with nickel nanoparticle encapsulated are very easily purified to be hollow onions by the subsequent nitric acid treatment. The method requires a relatively low energy input with the correct choice of catalyst precursor and purification time, thus making it industrially attractive.
Carbon implantation
To make carbon onion with carbon implantation, 120 KeV carbon ions implant into crystalline copper substrate at 700º to 1000ºC. During the process, 10-5 Pa of vacuum is maintained. Carbon onions are formed on the surface of substrate.
Formation of carbon onions with Pd clusters in a high-resolution electron microscope
Carbon onions have been produced in a transmission electron microscope by electron irradiation of amorphous carbon in the presence of Pd clusters and form intercalated onions. High-resolution electron microscopy revealed structural changes of the onion surface, and atom clouds were observed at the pentagonal vertices. In some onions, Pd atoms were intercalated between the graphite onion sheets, and a structural model for the intercalation has been proposed.
Anode Materials for Rechargeable Lithium-Ion Batteries
Researchers of China prepared Carbon nano-onions (CNOs) at 600 °C by a simple reaction between copper dichloride hydrate (CuCl2•2H2O) and calcium carbide (CaC2). The morphology and structure revealed that large quantities of CNOs consisting of quasi-spherically concentric graphitic shells with high purity and uniform size distribution (about 30 nm) were obtained. The crystal water in CuCl2•2H2O plays an important role in the formation of CNOs. The CNOs as-obtained exhibit high capacity and excellent cycling performance as anode materials for lithium-ion batteries, which can deliver a reversible capacity of 391 mAh g–1 up to 60 cycles.
Carbon onions give diamond
Researchers of Germany have reported that spherical particles of carbon consisting of concentric graphite-like shells ('carbon onions') can be formed by electron irradiation of graphitic carbon materials. When such particles are heated to nearly 700 °C and irradiated with electrons, their cores can be transformed to diamond. Under these conditions the spacing between layers in the carbon onions decreases from 0.31 in the outer shells to about 0.22 nm in the core, indicating considerable compression towards the particle centres which allows diamond to nucleate—in effect the carbon onions act as nanoscopic pressure cells for diamond formation.
Researchers of University of Edinburgh in the UK explain the reason further that the radiation bombardment knocks entire atoms out of the structure of nanotubes, causing the resulting defects to ricochet through the structure. This makes the structure bend and buckle, eventually forming into carbon spheres.
In fact, under intense radiation bombardment, this process turns multiwall nanotubes into carbon onions, ie concentric spheres. As more atoms are knocked out of the structure, the spheres shrink, placing enormous streeses on the layers beneath. It is this stress and resulting pressures that eventually causes diamond to nucleate and form at the centre of the onion.
Carbon-onions for ultra capacitors
Ultra capacitors or super capacitors are electrochemical systems that store energy within their double-layered structure consisting of opposite charged materials.. Electrodes made from activated or porous carbon are used in the production of the highest rated super capacitors. Although this provides a high storage capacity, it slows down the rate at which charging and discharging occurs.
The integration of carbon onions has opened new horizons in the use of micro-scale energy storage for applications for which conventional electrolytic capacitors are not sufficient. The use of onion-like carbons (OLC) in the development of micro-super capacitors now seems to be a promising venture.
A team of researchers made a capacitor using onion-like shells of graphene for electrodes to get enhanced energy and power densities.
Although their surface area is rather low compared to the surface of the activated carbons, it has qualitative value since it is fully accessible to the electric charges. The team began with creating an exposed electrode out of OLC. The OLC (at 6-7 nm diameters) could adhere onto the electrode without any binding agent or polymer separator making the process easier.
The rate of charge and discharge and power density was very high compared to the activated carbon capacitor and thin film lithium battery.
Applications
Applications include portable electronics, ultra capacitor technology biomedical implants, micro-sensors, etc. Their various properties enable them to serve as nanocapsules for drug delivery. In the nanocapsule drug delivery systems the external graphite layers providing protection to substances contained within and can serving as a template for the attachment of desirable functional groups. It can be used in other applications such as components of magnetic recording systems, magnetic fluids, electromagnetic shielding materials, reinforcement of composite materials, magnetic storage media, wear-resistant materials etc. They are also a potential solid lubricant similar to Tungstenite (WS2) nanoparticles having an onion-like structure. Carbon onions can even serve as
Bucky diamond (onion) coexists with nanodiamond
The transformation of nanodiamonds into carbon onions, and vice versa, has lead to the introduction of a new intermediate phase of carbon, coined “bucky diamond,” with a diamond core encased in an onion-like shell. Using a model based on the atomic heat of formation to describe the phase stability of carbon nanoparticles Australian researchers showed that bucky diamond occupies a coexistence region, spanning the calculated upper limit of fullerene stability and the lower limit of nanodiamond stability.
Electromagnetic Wave-Absorbing Coatings
A team of investigators from Belarus, Belgium, Russia and the United States have developed coatings that can efficiently absorb wide-band electromagnetic waves. This could be used as a countermeasure against terrorists who try to use electromagnetic radiation to lock-on to airplanes with surface-to-air missiles or to disrupt their avionics. Electromagnetic wave-absorbing technology can be used to reduce radar signatures. The basic absorbing component is onion-like carbon (OLC), which is produced by the transformation of nanodiamonds. These carbon nanostructures have specific properties that make them ideal materials for electromagnetic wave absorption.
The OLC is embedded in a polymer layer, which would then be deposited on the surface of the device to be protected. Nanodiamond aggregates of defined size and surface group composition are purified and OLC and OLC-based nanocomposites are incorporated into polymer matrixes and films. Nanodiamond fractions have now been developed and a series of OLC-polymer composites has been fabricated. Films with nanodiamonds have also been fabricated. Test results confirmed that OLC is an efficient shielding material for
Lubricating nanoparticle
The carbon nano-onion can be considered as a new kind of interesting lubricating nanoparticle. Used as lubricant additives, carbon nano-onions lead to a strong reduction of both friction and wear, even at low temperature.
It is found that lubricious iron oxide nanoparticles are generated in the core of the steel contact through mechanisms that are not yet known. The molecular dynamics simulation indicates that the lubrication mechanism of the onions is based on a coupled process of rolling and sliding inside the contact area. most of carbon onions seem to remain intact under friction processes and do not generate graphitic planes, which is in contrast to the previously determined behavior of MoS2 fullerenes that are mainly exfoliated inside the contact area and liberate lubricating lamellar sheets of h-MoS2.
Solid lubricant layer
Japanese researchers formed carbon onion layer compounded with gold by dispersing carbon onions on a silicon wafer coated with gold. The carbon onion layer compounded with gold has kept lower-friction coefficient for a longer time than gold layer in a certain range of gold film thicknesses and normal forces. in addition, carbon onion layer on a self-assembled monolayer exhibited the low-friction property under a wide range of normal forces.