Purification of carbon nanotubes

MTA SzFKI Carbon Nanotube Learning Seminar

Hernádi KláraUniversity of Szeged

April 18, 2005

What do we have to purify?

• type of carbon nanotubeSWNTMWNT

• synthesis methodarc discharge/laser ablation/HiPCOChemical Catalytic Vapor Deposition

The most common impurities

• various types of carbon (any kind of synthesis)graphitic nanoparticlesamorphous carbonfullerenespolyaromatic hydrocarbons, etc.

• catalyst material (SWNT; CVD)particles containing (transition) metalcatalyst support (CVD)

How to remove contaminants?

Depending on• the reactivity of impurities• the stability of carbon nanotube samples

ChemicalPhysical methodCombined

How to start?

Catalytic particlesSWNT ropes

as-prepared soot: arc-discharge, Fe-Ni catalyst, 500 Torr of He

S. Bandow et al., Appl. Phys. A 67 (1998) 23.

Purification of arc-discharge MWNT

• suspension of 500 ml of distilled water, 2.5 g of SDS (sodium dodecyl sulfate) and 50 mg of MWNT arc powder was sonicated for 15 minutes• sedimentation and centrifugation (at 5000 rpm for 10 minutes) removed all graphitic particles larger than 500 nm• additional surfactant → 12 CMC (critical micelle concentration) → flocculation, formation of aggregates of large objects• smaller objects remain dispersed, and sediment after a few days• after decanting the suspension, procedure is repeated once or twice• material remaining in suspension consisted nearly exclusively of nanoparticles• the sediment contained nanotubes with a content of over 80 % in weight.

J-M. Bonard et al., Adv. Mater 9 (1997) 827.

Purification of arc-discharge MWNT

Purification of arc-discharge (laser) SWNT

a) SWNT soot (20 W/m2 laser)

b) crude material after refluxing in

3 M HNO3 for 16 h (82%)

c) purified sample after oxidation in

air at 550°C for 30 min (20%)

d) after brief annealing at 1500°C in

vacuum

A.C. Dillon et al., Adv. Mater 11 (1999) 1354.A.G. Rinzler et al., Appl. Phys. A 67 (1998) 29.

Purification of arc-discharge (laser) SWNT

combustion of non-nanotube carbon

severe damage of nanotube carbon

~1593 cm-1 and ~1567 cm-1

(1.2 and 1.5 nm diameter SWNT)

Purification of arc-discharge (laser) SWNT

Why annealing can be necessary?

a) raw sampleb-d) „purified” sample

after mixed acid (sulfuric + nitric) treatment

Y. Zhang et al., Carbon 38 (2000) 2055.

Purification of HiPCO SWNT

1st step: to remove carbon coating

100 mLmin-1 air flow at 623K for 30 min.

2nd step: to dissolve Fe nanoparticles

treatment in 35% HCl at roomT for 48 h

pristine

purified

C-M. Yang et al., Nano Letters 2 (2002) 385.

Purification of HiPCO SWNT

Wet oxidation technique• 100 mg of raw HiPco SWNTs• suspended in a mixture of 60 ml of 30% H2O2 and 110 ml of 22% HCl• refluxed under continuous magnetic stirring at 70 °C for 9 h• cooling, filtration and washing with distilled water until pH 7• drying at 120 °C for 30 minthe yield of purified SWNTs was 90%the average diameter of the tubes is 1.5 nm

Cs. Mikó, private communication

Mechanical purification of SWNT

Original sample

1. Suspension of raw material2. Addition of insoluble „powder”3. Strong sonication („Snooker” effect)4. Collection of magnetic particles with a permanent magnet5. Separation of purified carbon nanotubes

L.Thiên-Nga et al., Nano Letters 2 (2002) 1349 .

Mechanical purification of SWNT

empty shells 2-3 graphite layer

N an o tu b e so lu tio n

M ag n e t (0 .1 T )

H ig h p o w er u ltra so n ic fin g er

d ep o sit

Final product:

0 1000 20 00 3000 4000 50 00 6000

Magenic Field [Gauss]

Raw soot: strong ferromagnetic signal

Purified soot

CVD: Silica supported catalysts

Removal of metallic particles(diluted mineral acid)

Sonication

Sedimentation

Decantation

K. Hernadi, et al.: Synth. Metals 77 (1996) 31.

XPS spectra before and after acidic treatment

TEM images before and after mechanical purification

non-destructive

↕low efficiency (~1%)

CVD: Zeolite supported catalysts

catalystparticle

amorphouscarbon

after HF treatmentas-prepared sample

• Purification: dissolving metallic and support particles in HF• Problem: pore structure of zeolite → liberating amorphous carbon• Another purification step is necessary

K. Hernadi, et al.: Zeolites 17 (1996) 416.

CVD: Alumina supported catalysts

• Purification: 1st step: dissolving alumina support in cc. NaOH at boiling point2nd step: removal of metallic particles in acidic treatment

long bundle-like structure

Z. Kónya, NATO-ASI Carbon Filaments and Nanotubes: (Eds.: Biró, L.P., Bernardo, C.A., Tibbetts, G.G., Lambin, Ph.): Carbon Filaments and Nanotubes: Common Origins, Differing Applications? Dordrecht, Kluwer Academic Publishers, 2000, Chapter VI, 85

Removal of amorphous carbon

• Hydrogenation

• Oxidation!

Gaseous ozone at 200°C, 90 min

traces of Co, Zn, Cr

Ozone in acidic solution

traces of Fe, Co

K. Hernadi et al.: Solid State Ionics 141-142 (2001) 205.

Removal of amorphous carbon

H2O2

almost no impur.HClO4

traces of Si, Cl, Cr, Zr

KMnO4 → cc. HCl

no impurities!!!

Result of oxidation

open end after oxidation

Reactivity of different kinds of carbonCompetitive oxidation

↓Damage of carbon nanotubes

(ends, defects, spirals)Raman spectra

CVD: CaCO3 supported catalysts

Non-porous support↓

No amorphous carbonOne-step purification:dissolving metallic particles + support in diluted mineral acid

E. Ljubovic et al.: Chem. Phys. Lett. 378, 9-17 (2003).

SummaryRemoval of impurities

• mechanical separation of different kind

• carbon nanoparticlesoxidation, (dehydrogenation)

• catalyst metalacidic treatment

• catalyst supportsuitable chemical

Thank you for your attention!