Nanomaterials -carbon fullerenes and nanotubes

Lecture 3

郭修伯

Carbon fullerenes and nanotubes

• Carbon– graphite form: good metallic conductor– diamond form: wide band gap semiconductor

• Ref:– “Science of Fullerenes and Carbon nanotubes”,

M.S. Dresselhaus, G. Dresselhaus and P.C. Eklund, Academic Press (1996)

Carbon fullerenes

• A molecule with 60 carbon atoms C60

– with an icosahedral symmetry– buckyball or buckmister fullerene– C-C distance 1.44 A (~ graphite 1.42 A)– 20 hexagonal faces + 12 pentagonal faces– each carbon atoms: 2 single bonds (1.46 A)+ 1

double bond (1.40 A)

Carbon fullerenes

• Initially synthesized by Krätschmer et al. 1990

• C60, C70, C76, C78, C80

Fig 6.1

Carbon fullerenes synthesis

– arc discharge between graphite electrodes in 200 torr of He gas

– heat at the contact point between the electrodes evaporates carbon

• form soot and fullerenes

• condense on the water-cooled walls of the reactor

• ~15% fullerenes: C60 (13%) + C70(2%)

– Separation by mass• liquid (toluene) chromatography

Carbon nanotubes

• Ref– M. Terrones, Ann. Rev.Mater. Rev. 33 (2003)

419– K. Tanaka, T. Yambe and K. Fukui, “The

Science and Technology of Carbon Nanotubes” Elsevier, 1999

– R. Saito, G. Dresselhaus and M.S. Dresselhaus, “Physical Properties of Carbon Nanotubes”, Imperial College Press, 1998

Single-wall carbon nanotube (SWCNT)

• diameter and chiral angle =30° : armchair = 0° : zigzag– 0° < < 30° : chiral

Fig 6.2

Fig 6.3

Multi-wall carbon nanotube (MWCNT)

• Several nested coaxial single-wall tubules (chiral tubes)

• typical dimensions:– o.d.: 2-20 nm– i.d.: 1-3 nm– intertubular distance: 0.34 nm– length: 1-100 m

Carbon nanotube synthesis

• Initially synthesized by Iijima (1991) by arc discharge

• Arc evaporation, laser ablation, pyrolysis, PECVD, eletrochemical

• Requires an “open end”:– carbon atoms from the gas phase could land and

incorporate into the structure.

– Open end maintenance: high electric field, entropy opposing, or metal cluster

Carbon nanotube synthesis

• Electric field in the arc-discharge promotes the growth– tubes form only where the current flows on the

larger negative electrode– typical rate: 1 mm/min (100A, 20V, 2000-

3000°C)– the high temperature may cause the tubes to

sinter (defects!!)

Carbon nanotube formation

• Single-wall:– add a small amount of transition metal powder

(e.g. Co, Ni, or Fe)– Thess et al. (1996)

• condensation of laser-vaporized carbon catalyst mixture

• low temp: ~1200°C

• alloy cluster anneals all unfavorable structure into hexagons -> straight nanotubes

Aligned carbon nanotubes

• CVD– on Fe nanoparticles embedded in silica– the catalyst size affects: tube diameter, tube

growth rate, vertical aligned tube density

• Plasma induced well-aligned tubes– on contoured surfaces– with a growth direction perpendicular to the

local substrate surface

Fig 6.5

Fig 6.5

Fig 6.6

Carbon nanotube growth mechanism

• Atomic carbon dissolves into the metal droplet

• diffuses to and deposits at the growth substrate

• mass production– CVD (700~800°C), but poor crystallinity– CVD (2500~3000°C+argon), improved

crystallinity

• base growth? tip growth?

Tip/base growth

• PECVD and pyrolysis:– catalytic particles are found at the tip and

explained by the tip growth model

• thermal CVD using iron as catalyst:– vertical aligned carbon nanotubes– base growth model– both tip and base growth (depend on catalyst)

Carbon nanotubes purification

• Impurities– amorphous carbon and carbon nanoparticles

• gas phase method– remove impurities by an oxidation process

– burn off many of the nanotubes (especially smaller ones)

• liquid phase method– KMnO4 treatment: higher yield than gas phase purification, but

shorter length

• intercalation methods– reacting with CuCl2-KCl, remove impurities

Carbon nanotube properties

• Excellent for stiff and robust structures– carbon-carbon bond in graphite

• flexible and do not break upon bending

• extremely high thermal conductivity

• applications– catalyst, storage of hydrogen and other gases,

biological cell electrodes, electron field emission tips, scanning probe tip, flow sensors