The Many Faces of Carbon

Carbon has “a lean and hungry look,” as Shakespeare has Julius Caesar saying about Cassius.  A relatively small atom with 6 protons, it exhibits tremendous flexibility in its dealings with other carbons and other species, and is the basis for all organic chemistry.  While all organic compounds contain carbon, not all carbon-containing compounds are organic.  However, there are approximately 20 million known organic compounds and about ½ million known inorganic compounds, making carbon the hands-down winner of the most promiscuous element contest. 

Carbon is capable of joining covalent bonds in three ways, at three levels of quantum state: sp, sp2 and sp3.  It is uniquely capable of catenation, joining with other carbons to form long chain molecules.  Carbon’s “hunger” comes from the fact that it has an outer shell with 4 electrons, while the full shell wants to have eight by virtue of the octet rule.  It is therefore capable of 4 single-bond covalent connections and is eager to share them.

As a testament to carbon’s ability to form bonds with itself, over 500 allotropes have been described in the literature.  It is well beyond the scope of this blog to enumerate and describe all of them, but some of the more common and useful ones have made our list of favorites.

Graphene-Related:  Graphene is defined as a hexagonal, 2-dimensional crystal derived only of carbon atoms.  The bonding angle of 120° at the sp2 shell assures the formation of a hexagonal lattice and creates the stability that allows for properties that include superior electron mobility, superior phonon mobility, virtually inert chemistry, extreme tensile strength and flexibility.

• Flake – Can be derived by chemical or physical exfoliation of graphene oxide, then reduced. Can also be produced by re-formation of high carbon feedstock through plasma or flash joule heating.  Can also be created by exfoliating graphene oxide into flaks, then reducing the resulting material chemically.
• Few-layer graphene Nanoplatelets – Similar to the production of flake.
• Film – High purity films are essentially macro-scale flakes and are typically produced by Chemical Vapor Deposition (CVD). Methane is introduced into an autoclave at 1000°C with copper foil inside acting as a catalyst.  The carbon atom bonds in the methane are released and the carbon self-assembles onto the copper foil.
• Nanotubes – Single wall or multi-wall nanotubes are produced in a variety of ways. The nanotube is shaped as a tiny sheet of graphene curled around and attached to itself to create a tubular structure on the nano-scale.  In multi-wall NTs, the outermost tube defines the diameter and inside the tube are some number of other coaxially expressed tubes.  The nanotubes themselves can show lattices arranged as Zigzag or Chair morphologies.
• Fullerenes – One of the earliest known morphologies became known as Buckminsterfullerenes, Fullerenes or BuckyBalls.  Named for the architect responsible for geodesic domes, these are round hollow structures made up of six- and five-sided polygonal graphene rings.  Typical manufacturing methods use either the arc-discharge method for smaller quantities, or the combustion method to convert organic compounds into fullerenes in low-oxygen environment, similar to pyrolysis.  The main feature of the process is the separation of the fullerenes from the predominant soot (disorganized carbons and other species) in the resultant mixture.

Carbon additives:  Because of carbon’s chemical flexibility, the addition of “foreign” atoms into the graphene lattice is quite possible, and can change the behavior of the underlying lattice structure.  Depending on the desired characteristic, modifications can include functionalization, enhancement of electron storage, increased electron mobility, increased strength in specific environments, or other desirable traits. 

• Oxygen - By adding “oxygen contaminants” to the hexagonal lattice of graphene, the resulting structure can change from hydrophobic (graphene) to hydrophilic (graphene oxide) with oxygen breaking up the regularity of the lattice. This change also turns the lattice from electrically conductive to an electrical insulator, a radical change of behavior.
• Carboxyl Groups - The –COOH group takes advantage of the proclivity of carbon to form multiple bonds. The carbon in the group is double bonded to the first oxygen atom and is single bonded to the hydroxyl group (-OH).  In its entirety the carboxyl becomes single bonded to the edges of graphene flake, thus easing the graphene into polar solvents.
• Fluorine - The addition of a fluorine “contaminant” into the carbon lattice greatly increases the electron storage capability of graphene, thus becoming a desirable material for battery cathode/anode sets.

Graphite-Related:  Graphite is defined as an amorphous form of carbon at the macro level, but made up of very small sheets of graphene that easily slide over each other.  This gives graphite some of its characteristics, softness, lubricity, and electrical conductivity.  As a naturally occurring mineral, graphite is mined in several specific areas of the world where deposits of relatively pure forms exist.  It has been known for thousands of years for its ability to leave marks on other surfaces (the word derives from the Greek word γράφειν or graphein, which means writing stone).

Branched Spheres:  This form is a recent addition to the carbon catalog.  Produced in a plasma process, the form appears as a central solid sphere with extended branches, all of pure carbon.  This form is finding its way into the market as an additive to cementitious substances and to polymers used for tires.  The unique morphology strengthens underlying crystalline structures at very low loading levels and is likely to become an important factor in reducing the carbon footprint of the concrete industry by reducing the volume of pours without reducing the strength of the concrete.

Graphite Oxide:  Similar to graphene oxide, used in many situations where its solubility is desired along with its slipperiness.  It is used in car waxes, water purification and as a graphene precursor.

Diamond:  Each carbon is attached to four others in a strainless chair configuration through sp3 bonds.  As a naturally occurring 3-dimensional crystal, formed under heat and pressure, this mineral is highly valued for both commercial characteristics and for its intrinsic beauty.

Carbon Black:  An amorphous, paracrystalline form of carbon derived from the incomplete combustion of petrochemicals.  Used as a filler in tires, for inks and as a UV stabilizer.