CARBON-BASED ALLOTROPES CARBON-BASED ALLOTROPES AND THEIR PROPERTIESAND THEIR PROPERTIES

A.A. 2011-2012A.A. 2011-2012

Fullerenes.

The compounds of pure CarbonThe compounds of pure Carbon

Carbon is a non-metallic element whose most abundant pure allotropes are graphite and diamond, two hypothetically infinite lattices. Only synthetic diamonds are free from impurities.

GraphiteGraphiteThe most abundant natural allotrope of C.• Color and aspect: Black or gray, opaque, non crystalline, non fluorescent• Number of interatomic bonds: 3 (sp2 orbitals, lenght: 1.421 Å)• Atomic density: 1.14 * 1023

• Distance between sheets: 335 nm, no covalent bonds.The arrangement in multiple sheets confers to the graphite high anisotropy, and explains its optical, acoustic and magnetic properties.

DiamondDiamond The second most abundant natural allotrope form of C.

• Color and aspect: colourless if pure, transparent, fluorescent and phosphorescent.• Number of interatomic bonds: 4 (sp3 orbitals, length: 1.54 Å, angle: 109.47°)• Atomic density: 1.77*1023

Diamond crystal have tetrahedral symmetry (A). When hexagonal symmetry is displayed, the allotrope is called lonsdaleite (B).

Properties of Graphite and DiamondProperties of Graphite and DiamondDiamondDiamond GraphiteGraphite

Specific Specific gravitygravity

3.52 gm/cc3.52 gm/cc 2.25 gm/cc.

HardnessHardness 1010 1.5-2.0

OpticsOptics isotropic, transparent;

Abs band at 415.5 nm

(weaker lines at 478,

465, 452, 435, 423 nm)

extreme anisotropy, optically uniaxial (-), high absorption of visible light.

Acoustic insulator.

Electrical Electrical propertiesproperties

Insulator (band gap: 5.47 eV). Semimetal, (band gap -0.04 eV),

high conductivity.

Thermic Thermic propertiesproperties

Expansion Coeff, linear: 1.18 α (10−6 K−1); metastable, no decay at room temperature

Expansion Coeff, linear: 6.5-0.5 α (10−6 K−1), stable

Radiation, Radiation, magnetismmagnetism

phosphorescent and

fluorescent.

non fluorescent,

diamagnetic (caused by the

itinerant electron in semimetal).

Other carbon-based allotropes: Other carbon-based allotropes: nanomaterials.nanomaterials.

Nanodiamonds and buckyballs are discrete nanosized moleculesGraphene, a single layer of graphite with the thickness of a single atomCarbon nanotubes, Single Walled (SWCNT) or MultiWalled (MWCNT). SWCNTs can be considered as the result of the folding of a graphene sheet.With the only exception of graphene, these compounds are found in traces in nature, as in vulcanic rocks and soots.Despite their chemical similarities, the physical chemistry of the nanosized allotropes of the C, changes dramatically in relation to properties like simmetry and size.Fullerenes, graphene and CNTs are compared herein.

NanodiamondsNanodiamondsNanodiamonds generate Nanodiamonds generate when TNT/RDX detonate in when TNT/RDX detonate in the absence of oxygen.the absence of oxygen.Their size if up to 5 nm (1.5 Their size if up to 5 nm (1.5 nm in the exemple here nm in the exemple here shown).shown).Thanks to their properties, Thanks to their properties, and their propensity to be and their propensity to be functionalized at thefunctionalized at thesurface, the nanodiamonds act very well as drug carriers. surface, the nanodiamonds act very well as drug carriers. Their potential toxicity in living systems is under study, and Their potential toxicity in living systems is under study, and seems to be low.seems to be low.

The fullerenesThe fullerenesBuckyballs fullerenes are closed cells of carbon clusters, with icosahedral cubic symmetry. Found in small amounts in sootsThey are odourless, generally soluble at room temperature in organic solvent, not in water.The smallest stable representative is C20, while bucky balls larger than C100 are known. C atoms are arranged in series of hexagonal and pentagonal faces.The hexagonal faces are diamagnetic and aromatic,

the pentagonal ones are paramagnetic and antiaromatics: their properties mutually cancell inside the cluster. The functionalization of the surface is easy.

The smallest buckyball is C20, however the smallest stable fullerene is C36.Bucky balls larger than C100 are known.At room temperature, the fullerenes are soluble in organic solvent, not in water. The colour of the solution varies for different clusters, according to their symmetry. C60 is purple, C70 is reddish brown, C76 and C84 have different colours for their different isomers.Small band gap fullerenes lack solubility, when pure.These allotropes, including C36, C50 and C72, are highly reactive and bind to other fullerenes, to soot particles, or can be functionalized with a lantanide in their core.

Solubility of bucky balls fullerenes.Solubility of bucky balls fullerenes.

SolventSolvent C60C60 C70C70

1,2 dichlorobenzene1,2 dichlorobenzene 2424 3636

carbon disulfidecarbon disulfide 88 1010

xylenexylene 55 44

toluenetoluene 33 11

benzenebenzene 11 11

mesitylenemesitylene 11 11

carbon tetrachloridecarbon tetrachloride 0.50.5 0.10.1

dichloromethanedichloromethane 0.30.3 0.10.1

dodecanedodecane 0.10.1 0.10.1

decanedecane 0.10.1 0.050.05

n-hexanen-hexane 0.050.05 0.010.01

cyclohexanecyclohexane 0.040.04 0.080.08

octaneoctane 0.030.03 0.040.04

pentanepentane <0.01<0.01 <0.01<0.01

Solubility of C60 and C70 related to Solubility of C60 and C70 related to properties of the solvent.properties of the solvent.

The solubility of C60 and C70 in various solvents (see the previous slide) is related to the surface tension of the solvent (graph), not to its octanol/water partition coefficient, nor to its specific gravity/density.

Black solid, odourless.Density: 1.65 g cm-3Standard heat of formation: 9.08 k cal mol-1Index of refraction: 2.2 (600nm)Boiling point: Sublimes at 800KResistivity: 1014 ohms m-1Vapour density: N/AAromatic, Superconductor, Ferromagnetic (polarized at room T°C)

Physical properties of C60Physical properties of C60

Graphene.Graphene.

Graphene is a compound of pure carbon, arranged as an hexagonal lattice in which the C atoms are bound by sp2.

The sheet has the thickness of a single atom; in this respect graphene differentiates from graphite.

Graphene and CNTs as derivatives of Graphene and CNTs as derivatives of graphitegraphite

Diamond and graphite are stable structures, at least at room temperature and atmospheric pressure. To transform them into each other high energy is requiered.

The production of graphene, a single sheet of the graphene allotrope of C, requires instead the separation of only weak, non-covalent bonds.Graphene is an hexagonal lattice of carbon atoms, bound each other as in graphite, whose thickness is that of a single atom.Though a graphene sheet is potentially infinite in the other two dimensions, its thermodynamic stability depends on the number of atoms and on the shape.

Thermodynamic stability of grapheneThermodynamic stability of graphene

To be thermodynamically stable the graphene must reach a minimum size of 6000 atoms, that is a sheet of at least 20 x 20 nm. Most stable structures are larger than 24,000 C.The sheet’s shape is curled, not flat, as shown in the picture, which was obtained “in silico” by using the tool for mimicking the energy minimization, implemented in the NanoRex software.

Graphene is a conductor, even powerful than Cu

Electrical properties of grapheneElectrical properties of graphene

Engineered nanomaterials: Engineered nanomaterials: Single Walled Carbon NanoTubes Single Walled Carbon NanoTubes

(SWCNT).(SWCNT).

Engineered SWCNTs can have different symmetries:

Armchair, m=n; n,n = 5,5Chiral: mn: m,n = 10,5 (in the example, see next slides)Zigzag, m,n = 9,0

The three allotropes can be conceived as generating from a graphene ribbon, rolled up with different axes of symmetry.

SWCNT simmetry….SWCNT simmetry….

A B C

A: Armchair (m,n=5,5), B: Zigzag (m,n=9,0), C: Chiral (m,n=10,5)

and some consequences.and some consequences.

A: Armchair (m,n=5,5), B: Zigzag (m,n=9,0), C: Chiral (m,n=10,5)

Bonding

Anti-Bonding

ACB

Scanning Tunnelling Microscope and Scanning Tunnelling Microscope and conductance measurement.conductance measurement.

Keeping the tip of the STM close to the surface of CNTs, electrons from the tip can jump to the nanotube. A s.c. "tunnelling current“ establishes.The plot of dI/dV by the voltage measures the number of electronic states available for electrons to tunnel at a certain energy (V).

Metallic or semiconducting SWCNTs.Metallic or semiconducting SWCNTs.

The chemicals deals into two main classes for their electrical properties:

1. metals, in which the electric current generally flows freely and there is no energy gap between the valence and the conducting states.

2. semiconductors, in which an energy gap exists and therefore a higher voltage is needed to make electric current flow.

For most materials the metallic or semiconducting nature depends on the chemical composition and 2-D arrangement of atoms and molecules.SWCNTs can show metallic or semiconducting properties in relation to their chirality (n,m) and diameter.

A new exploitation attempt.A new exploitation attempt.

Electrochemical solar cells mimicking

photosynthesis.

Acceptor (porphyrine)

Transmitters (short nucelotides)

SWCNT

Choi et al., 2010, doi:DOI: 10.1117/2.1201007.003130

•

Fullerene-like structures not Fullerene-like structures not Carbon-basedCarbon-based

Boron bucky ballsBoron bucky balls

B16N16

B80

B112, isomers

•De et al., 2011, DOI: 10.1103/PhysRevLett.106.225502•Muya et al., Phys. Chem. Chem. Phys., 2011, 13, 7524–7533

Armchair simmetry (n,m=5,5)

Boron: rosa,Nitride: blu.

Boron-Boron-nitride nitride

nanotubesnanotubes

AknowledgmentsAknowledgments

All the figures, if not otherways indicated, have been constructed with the help of the following softwares, freely distributed:

• Nanoegineering 1, version 1.1.1, by Nanorex• Ninithi 1, by Lanka Software Foundation• UCSF Chimera, by the University of California.

Further readings1. Makarova T. 2004. Magnetism in polymerized fullerenes. In “Frontiers of Multifunctional Integrated Nanosystems” (E. Buzaneva and P. Scharff eds.), Kluwer Academic Publishers, the Netherlands, pag. 331-342.2.Małolepsza E, Witek HA. 2007. Comparison of Geometric, Electronic, and Vibrational Properties for Isomers of Small Fullerenes C20-C36. J. Phys. Chem. A 2007, 111, 6649-6657

Armchair SWCNT (m,n = 5,5):Armchair SWCNT (m,n = 5,5):Electrical propertiesElectrical properties

Chiral SWCNT (m,n = 10,5):Chiral SWCNT (m,n = 10,5):Electrical propertiesElectrical properties

Chiral SWCNT (m,n = 10,5):Chiral SWCNT (m,n = 10,5):Electrical propertiesElectrical properties