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)
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