effect of environmental gas on the growth of cnt in catalystically pyrolyzing c 2 h 2

Effect of Environmental Gas on the Growth of CNT in Catalystically Py

rolyzing C2H2

Minjae Jung*, Kwang Yong Eun, Y.-J. Baik, K.-R. Lee, J-K. Shin* and S. T. Kim*

Thin Film Technology Research CenterKorea Institute of Science and Technology

* LG Corporation Institute of Technology

Carbon Nano-Tubes (CNT)

• Unique Structure and Properties

• Suggested Potential Applications– Cold Cathode for FED– Hydrogen Storage Materials– Electrode for Fuel Cell– Nanoscale Transistors12.5㎛12.5㎛

Synthesis of CNTs

• Arc Discharge, Plasma CVD, Laser Ablation, Thermal CVD

• Thermal CVD – Decomposition of hydrocarbon gas with Ni, Co, Fe catalyst– Advantages

• Relatively easy to obtain vertically aligned CNTs. • Can be employed for large scale production system.• Easy to understand the reaction behavior (Near Equilibrium).

Reaction kinetics and the growth mechanism Reaction kinetics and the growth mechanism are not fully understood, yet.are not fully understood, yet.

Reaction kinetics and the growth mechanism Reaction kinetics and the growth mechanism are not fully understood, yet.are not fully understood, yet.

• Analogy to carbon filament growth :

The catalyst surface should not be passivated by any reason.

Passivation : Polymeric encapsulation at low temperature Excess decomposition of hydrocarbon at high temperature

• CNT growth behavior in various environmental gases in thermal CVD

We focused on the passivation behavior We focused on the passivation behavior of the metal catalystof the metal catalyst..

We focused on the passivation behavior We focused on the passivation behavior of the metal catalystof the metal catalyst..

The Present Work

Agglomeration of the film

Si(100)

SiO2

Ni, Co film deposition

Heat treatment @ 800oC H2

3.4nm Ni 6.8nm Ni

300nm300nm 300nm300nm

Formation of Catalyst Particles

Loading system

H2O

Hood

Gas inlet

FurnaceSubstrate holder

Tube type reactor with quartz tube (50800L) at 1 atm.

Procedure: Sample loading after increasing temperature in Ar

Pretreatment for 1hr in H2, N2, H2+N2, H2+Ar, NH3

Total gas flow : 200sccm (NH3 : 100sccm)

Add C2H2 to the environmental gas

Cooling in Ar

300nm300nm

300nm300nm

2.4 vol. % C2H2 at 850℃

In N2 Environment

300nm300nm

1.50㎛1.50㎛ 3.00㎛3.00㎛

H2/(H2+N2) = 0.6

(120sccmH2 / 80sccmN2)

H2/(H2+N2) = 0.85

(170sccmH2 / 30sccmN2)

H2/(H2+N2) = 1

(200sccmH2)

The Same Behavior in H2+Ar Environment

In H2+N2 Environment

2.4 vol. % C2H2 at 850℃

Catalyst Surface after Pretreatment in H2+N2 Environment

0 500 1000 1500 2000 2500

Si

O

NiC

After 0.5min sputtering

dY/d

E (

arb.

uni

t)

Kinetic Energy (eV)

As received

NN22 acts like an inert gas. acts like an inert gas.NN22 acts like an inert gas. acts like an inert gas.

C2H2 2C+H2

Lower Decomposition Rate of C2H2Lower Decomposition Rate of C2H2

Prevent the Catalyst PassivationEnhance the CNT Growth

Role of Hydrogen

at 950℃

1.00㎛1.00㎛

300nm300nm

2.4 vol. % C2H2 in H 2/(H2+N2) = 1

3.00㎛3.00㎛

at 850 ℃

2.4 vol. % C2H2 in H 2/(H2+N2) = 0.35

at 750 ℃

40nm40nm

TEM Microstructure of CNT

Bamboo-like GrowthBamboo-like GrowthBamboo-like GrowthBamboo-like Growth

for 7min at 950℃ with 16.7 vol. % C2H2 in pure NH3

6nm

In NH3 Environment

4.8 vol. % C2H2 for 20min 9.1 vol. % C2H2 for 15min

16.7 vol. % C2H2 for 7min 23.1 vol. % C2H2 for 7min

at 950oC in pure NH3 Environment

300nm300nm

at 950 ℃ with 2.4 vol. % C2H2

in N2+H2 : H/(H+N)=0.75at 950℃ with 16.7 vol. % C2H2

in pure NH3

NH3 Environment Effect

300nm300nm

300nm300nm

In H2+N2

In pure NH3

Catalyst Surface after Pretreatment

Ease of Decomposition of NH3

NH3 N + 3/2 H2

• NHNH33 is much easier to be decomposed than N is much easier to be decomposed than N22

• Increase in activated nitrogen.Increase in activated nitrogen.

• NHNH33 is much easier to be decomposed than N is much easier to be decomposed than N22

• Increase in activated nitrogen.Increase in activated nitrogen.

① EEnhance the graphitic layer formation on the catalyst surface nhance the graphitic layer formation on the catalyst surface

② Enhance the separation of the layer from the catalyst surfaceEnhance the separation of the layer from the catalyst surface

① EEnhance the graphitic layer formation on the catalyst surface nhance the graphitic layer formation on the catalyst surface

② Enhance the separation of the layer from the catalyst surfaceEnhance the separation of the layer from the catalyst surface

Role of Activated Nitrogen

at 950℃ with 16.7 vol. % C2H2 in pure NH3 environment

Ni Co

The Catalyst Effect

0 500 1000 1500 2000

C N

O

NiSi

in NH3

in NH3/(H

2+NH

3)=0.33

in NH3/(H

2+NH

3)=0.05

Kinetic Energy (eV)

dN/d

E (a

rb.u

nit)

0 500 1000 1500 2000

C N

O

Co Si

in NH3/(H

2+NH

3)=0.05

in NH3/(H

2+NH

3)=0.33

in NH3

Kinetic Energy(eV)dN

/dE

(arb

.uni

t)

Ni Co

Catalyst Surface after Pretreatment

at 950℃ with 16.7 vol. % C2H2 in pure NH3 Environment

Ni Co

CNT Growth Without Pretreatment

Nitrogen incorporation to the catalyst Nitrogen incorporation to the catalyst is essential in Niis essential in Ni

Nitrogen incorporation to the catalyst Nitrogen incorporation to the catalyst is essential in Niis essential in Ni

Ni

Ni

Co

Co

9.5 vol.% C2H2

In 33 vol.% NH3+H2

9.5 vol.% C2H2

In 5 vol.% NH3+H2

Conclusions

• CNT growth by balancing the carbon supply with the reaction kinetics at the catalyst surface.– Gas concentration in the environment gas and the reaction

temperature

• Activated nitrogen in NH3 environment play a significant role in the CNT growth kinetics.– Enhancing the graphitic layer formation

– Enhancing the separation of the graphitic layer from the catalyst surface

– Depends on the catalyst materials

12.5㎛12.5㎛

Straight form

Tangled form

Growth of Vertically Aligned CNT

70sec (9.8㎛ /min) 4min (1.1㎛ /min) 7min(0.8㎛ /min )

Intimate Relationship Between Intimate Relationship Between the Growth Rate and the Vertically Aligned CNTthe Growth Rate and the Vertically Aligned CNT

Intimate Relationship Between Intimate Relationship Between the Growth Rate and the Vertically Aligned CNTthe Growth Rate and the Vertically Aligned CNT

Evolution of Vertically Aligned CNT

at 950℃ with 16.7 vol. % C2H2 in pure NH3 Environment