Production of carbon nanotubes

Lecture 2

Methods:

1. Pyrolysis of hydrocarbons in the presence of metal catalysts (1950 - present)

Main products: multi-walled carbon nanotubes.Catalysts: Fe, Co, Ni Temperature: 800 -1200 CPrecursors: C2H2, CH4 (gases), benzene (solution), camphour (solid)

Experimental set up

a. One-step process (gas precursors)

Furnace 800-1200 C

H2/

Ar

C2H

2, o

r C

H4

catalysts

CNTs grown in substrateQuartz tube or ceramic tube

Furnace 800-1200 C

catalysts

H2/

Ar

benzene

One-step process (solution precursors)

b. Two step-process (solid precursors)

150-300 C 800-1200 C

catalystscamphour

Ar/

H2

150-300 C 800-1200 C

ferrocene

Ar/

H2

Two step-processMetallic-organics as precursors: Ferrocene (五環鐵 )

Fe

CNTs deposited on tube walls

Random arrangement of multi-walled CNTs

He or (Ar) atmosphere

Anode (+) Cathode (-)

20- 35 V, 70-120 A

Arc temperature 2500-3500 C

2. DC Arc discharge of graphite (1990-present)

Products: Fullerenes and multi-walled CNTs

See website at http://tw.youtube.com/watch?v=8N79nlhwcgM&feature=related

正極消耗

負極長出銀灰色碳積物稱為 (carbon deposit)

真空腔壁層積碳灰 , 稱為 carbon soot: fullerenes 藏于 soot 中

分離

Side view Front view Cut open

Multi-walled CNTs

Carbon deposit at cathode

Hollow anode

cathode

Filling with Fe, Co, Ni or others to produce single-walled CNTs

陽極 陰極腔體

原始煤灰5cm

正極消耗

負極長出銀灰色碳積物稱為 (carbon deposit)

真空腔壁層積碳灰 , 稱為 carbon soot: fullerenes 及 SWNTs 藏于 soot 中

Filling with B, B2O3 or boron compounds to produce B-doped multi-walled CNTs

anode

B

Filling with Ta, Ta2O5 or Tatanlum compounds to produce encapsulated TaC multi-walled CNTs

anode

3. Laser ablation of Co-Ni-graphite (1995-present)

Products: single-walled carbon nanotubes

Co-Ni-graphite

1200 C

furnace

Laser beam cooler

SWNTs found on cooler

He

4. Electrolysis of graphite in molten ionic salts (1995-present)

Product: multi-walled CNTs, low melting points metals (Sn, Pb, Bi) encapsulated CNTs

furnace

Graphite crucible

Graphite rod

LiCl, KCl, NaCl

5. others, derivatives of pyrolysis, e.g. plasma CVD, MOCVD,… etc

Products: variety of CNTs, including SWNTs, MWNTs

Improved techniques on nanotube growth

Substrate and catalyst modification

改變金屬催化劑形態 (簡單矩陣 )

Science, 283,512,1999

Thin film of Fe

比較複雜碳管矩陣

Chem.Phys.Lett, 371, 433, 2003 APL. 79, 3155, 2001

APL, 79, 1534, 2001

Nature, 416, 495, 2002.

碳管整齊排列之意義為何 ?1.提昇場發射性能 (field emission) ? (not true, because threshold field and turn-on-voltage is even higher than random arranged CNTs)

2. 陣列成長時 , 碳管之結晶性 , 結構 或 長度 , 直徑比雜亂成長 較為均勻一致 (possibly true, at least tube length and diameter are rather uniform than random growth of CNTs on substrate).

Growth mechanisms of CNTs

Characteristics of CNT growth in arc discharge process1. No catalyst --- multi-walled CNTs (fact)

2. Electric field driven growth (not certain ?) E

+ -

5-10V/m

3. High temperature (fact)

4. Less defects on CNTs and high degree of graphitization (high crystallinity) (Fact).

Arc-CNTs

Pyrolysis-CNTs

5. CNT length is limited to 5-15 m, tube diameter is similar to pyrolysis grown CNTs (fact)

6. open-ended growth (reasonable)

wrongPossibly right

Open-endBoth-end growth

7. Carbon ions exchange in arc zone (C+ > C-)

C+ C++ -C-

C-

8. CNT growth in arc discharge requires buffer gas (He, Ar, 300-500 torr). In a vacuum tube cannot grow (Fact).

9. Probability of hexagon formation is higher than that of pentagons in arc process, so CNT growth continues, otherwise pentagons lead to tube closure. When tube closure occurs tube growth stops. (fact)

Incoming ions

2

3

1

Growth mechanisms of CNTs in pyrolysis process

1. Metal catalysts are needed, metal particles are always at tube tips (fact).

2. H2 is always needed (not true), because pyrolysis of C60 can also produce CNTs in Ar or N2 flow.

3. Lower production temperature compared with arc discharge, so CNTs are defective structures (fact).

4. Two types of growths have been argued since 1970 by M. Endo and RTK Baker

5. Lift-off growth and rooting growth

Modification of Baker’s model

Fe

Room temp800-1200C

C2H2 C2H2

Atoms strongly vibrate in lattice

C2H2 C2H2

dropletH

Cold Fe

Hot Fe

C

Migration of C through Fe droplet

When C diffuses through Fe, carbidic species formCarbidic species: Intermediate phase of carbide

Baker’s model cannot explain why CNTs are hollow structure

Migration of C through Fe droplet

solid structure

Lift-off growth

Rooting growth

None-sense mechanisms on CNT growth 極其無聊之碳管成長機制

1. Scooter mechanism by rice group

CCo

growth

2. Co-C60 catalyzed growth in arc process

CoC

Why they proposed these none sense mechanisms?

Because at a time they did not see metal articles at tube tips!

Particularly, the Co-C60 catalyzed growth model was proposed based on two wrong images.

Image 1

Image 2

1.1-1.2Å7Å1.1-1.2Å

True is finally found by Rice group itself Chem.Phys.Lett, 260, 471, 1996

Endo and Baker’s models