Some chemical elements have the property of being able to exist in different forms for a given phase state (i.e., solid, liquid or gas). These variations are referred to as allotropes, each of which has a different atomic configuration, physical properties and chemical behavior. Allotropes may have different molecular formulas. Figure 1 illustrates the allotropes of carbon, some of which you may not be familiar with.
Figure 1. Allotropes of carbon: a) Diamond, b) Graphite, c) Lonsdaleite, d) C60 (Buckminsterfullerene or buckyball), e) C540, f) C70, g) Amorphous carbon, and h) single-walled carbon nanotube, or buckytube
We're all familiar with diamonds and their use as gemstones and for industrial cutting tools. With a slight variation in crystal structure we get lonsdaleite, an allotrope with properties similar to that of diamond. This variant is formed from graphite in meteorites when they impact the planet Earth or can be synthesised in a laboratory.
When carbon bonds form a single sheet it is known as graphene. When these sheets are stacked we then form graphite that is used in pencil lead and as a lubricant.
Amorphous carbon, by definition, is carbon that does not have a crystal structure; it is purely random. If observed under a microscope, microscopic segments of graphene and/or graphite may be observed.
Activated carbon (charcoal) is a processed form of carbon with very fine, porous particles. This high surface area powder is extremely adsorbent and will easily bind to molecules, ions and atoms. It is taken orally to treat poisoning, lower flatulence, decrease cholesterol levels, aid hangovers and upset stomach. It can also be applied topically to help clean skin wounds.
Carbon black is a product of incomplete combustion of heavy petroleum products. It is similar to amorphous carbon but is made of fine particles having a partially crystalline character. It also has a high surface to volume ratio similar to that of activated carbon. It is used as a filler in rubber products (tires), plastics, paint and ink.
Soot is an impure carbon particle variant formed from the incomplete combustion of hydrocarbons. It is a carcinogenic airborne contaminant.
A nano material can be defined as a material having one or more dimensions being less than 100 nm. At this scale, quantum mechanical effects become a consideration and the structure (or surface) of the material has unique optical, electrical or mechanical properties. This material can be natural, engineered/manufactured or incidental.
Nano technology basically refers to the engineered manipulation of matter to create a nano material. These materials have novel applications in medicine, electronics, bio-materials, energy production and consumer products.
A nanoparticle can be defined as a nano material with an integral surface layer (ions, organic or inorganic) that fundamentally affects the properties of the particle. The dimensions of these particles are all under 100 nm and have little variation between the longest and shortest dimensions.
An isomer is defined as ionic compounds or molecules which have identical formulas but distinct structures. The different structures may have different properties.
Fullerenes are a more recently discovered allotrope of carbon and these nano-materials can be spherical, elliptical or tubular in shape.
In the center row of Figure 1, we have three spherically shaped fullerene structures: D) C60 E) C540 and F) C70. There are many possible spherical isomers that can form, with a more comprehensive list found at here. C60, C70 and C540 are some of the more stable variants. C60 was the first to be discovered and is referred to as buckminsterfullerene. It is the largest known object to exhibit a wave-particle duality property.
C60 is a hollow structure and metal atoms or gases can be trapped in the middle to form what is known as an endohedral fullerene. These structures are designated as M@C60 with M identifying the trapped metal atom or gas.
Figure 1H shows a carbon nanotube. These tubular structures can be interpreted as one or more rolled up graphene sheets. A single layer forms a Single Wall Carbon Nano Tube (SWCNT) and multiple layers form a Multi Wall Carbon Nano Tube (MWCNT). They can have lengths of up to several millimeters. Properties include high tensile strength, high electrical conductivity, high heat conductivity and relatively low chemical activity.
Spherical fullerenes can also have multiple layers and form what is known as nano-onions. These nano-onions are enclosed in a “Russian doll” manner. In this structure, the chemical representation used by endohedral fullerenes is also used. An example would be C60@C240@C540@C960 where the inner C60 molecule is wrapped by 3 layers having 240, 540 and 960 carbon atoms respectively.
This is only an introduction and there are many more fullerene structures that can be discussed. It is important to understand that each structure has its own set of properties and each must be evaluated separately with respect to health and safety concerns. The properties for C60 do not automatically transfer to other nano materials. The Toxicity page takes a close look at C60 toxicity considerations.