Role of Nitrogen in the Synthesis of Vertically Aligned Carbon Nanotube

Tae-Young Kim*, Minjae Jung, Kwang-Ryeol Lee, Seung-Cheol Lee, Kwang Yong Eun & Kyu-Hwan Oh*

Korea Institute of Science & Technology* Also at Seoul National University

The Korea-US Symposium of Phase Transformations of Nano-Materials, Seoul, Korea (2002. 10. 25)

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 CNT

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

• Thermal CVD – Thermal 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.

40㎚

CNT Growth by Thermal CVD

In H2 , N2 or Ar Environment In NH3 Environment

CNT Growth by Thermal CVD

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

Tangled CNTC2H2+H2600~900

Tangled CNTC2H2+H2, C2H2+N2950

Tangled CNTC2H2+H2, C2H2+N2850

method

ferrocene+xylene

CH4+H2

CH4+N2

CH4+N2

C2H2+Ar

C2H2+NH3

C2H2+NH3

C2H2+NH3

C2H2+NH3

C2H2+NH3

C2H2+NH3

C2H2+NH3

C2H2+NH3

C2H2+NH3

C2H2+NH3

Reaction Gas CatalystTemperatue(oC)

APL 77 3764 (2000)Aligned CNTFe800 Thermal-CVD

APL 76 2367 (2000)Aligned CNTNi700 PE-CVD

JAP 89 5939 (2001)Aligned CNTFe550 PE-CVD

APL 75 3105 (1999)Aligned CNTFe, Ni500 PE-CVD

APL 75 1721 (1999)Tangled CNTNi, Co850~900Thermal-CVD

APL 80 4018 (2002)Aligned CNTNi660< PE-CVD

JAP 91 3847 (2002)Aligned CNT

Ni800~900

Thermal-CVD

DRM 10 1235 (2001)Aligned CNT

Ni950

Thermal-CVD

TSF 398-399 150 (2001)Aligned CNT

Ni, Co950

Thermal-CVD

APL 78 901 (2001)Aligned CNTFe800 Thermal-CVD

APL 77 2767 (2000)Aligned CNTCo825 PE-CVD

APL 77 3397 (2000)Aligned CNTFe750~950Thermal-CVD

APL 77 830 (2000)Aligned CNTCo825 PE-CVD

APL 75 1086 (1999)Aligned CNTNi660 PE-CVD

Science 282, 1105 (1998)Aligned CNTNi666PE-CVD

CitationCNT MorphologySynthesis condition

In H2 , N2 or Ar Environment In NH3 Environment

CNT Growth by Thermal CVD

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

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

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

Activated nitrogen in the environment Activated nitrogen in the environment

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

Activated nitrogen in the environment Activated nitrogen in the environment

Bond Binding Energy (KJ/mol)

N-H < 339

N-N 945

H-H 436

Source : CRC Handbook of Chemistry and Physics 1999-2000

Role of Activated Nitrogen

Two possibilities can be suggested.

1. Catalyst surface modification by nitrogen may enhance the nucleation of graphite layer on the surface and their separation. Importance of pretreatment in NH3 environment

2. Activated nitrogen may play a significant role during CNT growth Importance of NH3 environment during growth

Pretreatment Reaction

NH3 4h H2 + C2H2

Pretreatment Reaction

NH3 1h NH3 + C2H2

C2H2 : 16.7 vol.%

Pretreatment Effect I

Pretreatment Reaction

NH3 + C2H2

Pretreatment Reaction

H2 1h NH3 + C2H2

C2H2 : 16.7 vol.%

0h

Pretreatment Effect II

Deposition

PretreatmentH2+C2H2 NH3 +C2H2

H2 XO

NH3 + H2O

NH3X O

Role of Activated Nitrogen

Two possibilities can be suggested.

1. Catalyst surface modification by nitrogen may enhance the nucleation of graphite layer on the surface and their separation. Importance of pretreatment in NH3 environment

2. Activated nitrogen may play a significant role during CNT growth Importance of NH3 environment during growth

Pretreatment Reaction

H2NH3 + C2H2

0.170.17 0.2310.231

0.050.050.0150.015

X = C2H2 / (NH3+C2H2)

Nitrogen in 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

XPS Analysis of CNTN with sp2 C

N in sp3 environ.

Strain Energy of Tubular Form

Ab initio Pseudopotential Total Energy Calculation

Y. Miyamoto et al, Solid State Comm. 102, 605 (1997)

Nitrogen Incoporation Enhancesthe Pentagon Strucutre

H. Sjostrom et al, Phys. Rev. Lett. 75, 1336 (1995).N. Hellgren et al, Phys. Rev. B 59, 5162 (1999).

• Energy for Pentagon Formation only with carbon atoms= 73.8kcal/mole

• Energy for Pentagon Formation if nitrogen substitute two carbon atoms= 26.2kcal/mole

Role of Activated Nitrogen

strfm

nucleation ErrGV

hrG

222

EELS Analysis of CNT

W.-Q. Han et al, Appl. Phys. Lett. 77, 1807 (2000).

Nanotube Junctions

X. Ma et al, Appl. Phys. Lett. 78, 978 (2001).

PECVD

Nitrogen in CNT

Nitrogen Incorporation

Conclusions

• Enhanced CNT growth in an N2 or NH3 environment is due to nitrogen incorporation into the CNT wall or cap.

• Nitrogen incorporation can reduce the strain energy required for the tubular graphitic layer, which decrease the activation energy for both the nucleation and growth.

• Nitrogen in the CNT would affect their electronic structure, electron transport behavior and chemical activity of the CNTs.

Reactivity of Curved C-N Structures

S. Stafstrom, Appl. Phys. Lett. 77 (24), 3941 (2000)