The investigation of nitrogen effect on Carbon nanotube

growth by ab-initio calculation

The investigation of nitrogen effect on Carbon nanotube

growth by ab-initio calculation

2003.10.11Hyo-Shin Ahn*,**, T.Y. Kim*,**, S.-C. Lee*, K.-R. Lee* and D.-Y.

Kim**

*Future Technology Research Division, KIST **School of Materials Science and Engineering, Seoul Nat’l

Univ.

Effect of Nitrogen on CNT growth

Nitrogen incorporation enhances CNT growth drastically that vertically aligned CNT can be fabricated16.7 vol. % C2H2 in NH3, CVD process

Vertically aligned multi-wall CNT of 30~40nm in diameter Very high growth rateChemical Physics Letters, Vol. 372, 603(2003)

What is the role of Nitrogen in CNT growth?

Experimental result

Nitrogen effect: possibility 1Nitrogen effect: possibility 1

Nitrogen effect:Reduction in the strain energy of CNT

Due to the strain energy, growth rate can be retarded.

When nitrogen atoms locate on defect or strained site of carbon network, system energy lowers.

Illustration of the defect stabilization by nitrogenPRB. 59, No. 7, 5162(1999)

Nitrogen effect: Change of growth kinetics

• During the growth, nitrogen will change the growth behavior• No remarkable experimental results of nitrogen effect on CNT growth• We do not even know the exact mechanism of the CNT growth in atomistic scale

Nitrogen effect: possibility 2Nitrogen effect: possibility 2

Method :Computational calculation

Dmol3: commercial package of DFT (density functional theory) Ab-initio calculation

Very accurate calculation results

Strong in energy calculation - energetics

Transition State calculation – growth kinetics

Calculating activation energy for chemical reaction

Dmol3: commercial package of DFT (density functional theory) Ab-initio calculation

Very accurate calculation results

Strong in energy calculation - energetics

Transition State calculation – growth kinetics

Calculating activation energy for chemical reaction

Energetics on CNT wallStrain energy due to curved wall

Conventional design for the calculation: CNT unit cell - Larger radius CNT needs more atoms

Due to the computing power, ab-initio calculation cannot describe real size system.

40nm

Real CNTCalculated strain energy by ab-initio : up to 6Å

Strain energy 1/R2

Cluster design / Curved clusters

By Calculating the energies of curved pieces of graphite (cluster), the energy of CNT with corresponding radius can be calculated

Journal of Computer-Aided Materials Design, 5, 279 (1998)

Cut out Attach Hydrogen

Radius(Å)

E(e

V/a

tom

)

Expanding the scale by cluster design

Cluster design

• Introducing cluster design calculation, the energies of large size CNT can be calculated

Strain energy of CNT 1/R2

~10Å

Bulk design

~35Å

• Over the radius of 35Å strain energy disappears

Energy of flat graphite plate

Nitrogen effect on Strain energy

• Strain energy becomes negligible when radius of CNT is larger than 3~4nm.• Radius of the vertically aligned CNTs is typically 15~20 nm, thus CNT has no strain energy. Strain energy reduction by the Nitrogen incorporation would be negligible.

Growth kineticsTwo kinds of edges

zigzag

Calculation of transition state (activation barrier of reaction) on each step of atom attachment

armchair

Growth on zigzag edge:Rate determining step!

176meV

No barrier

No barrier

160meV

64meV

Armchair and zigzag edge

armchair

zigza

g

As the growth proceeds, the proportion of zigzag edge will increase

Nitrogen incorporation:154meVNitrogen incorporation:152meV

152meV

88meV No barrier

No barrier

154meV

Nitrogen incorporation into zigzag edge

No barrier

No barrier

No barrier

176meV

Pure Carbon: 176meV

No barrier

538meV

Nitrogen incorporation: 176meVNitrogen incorporation: ~153meVSmaller than armchair edge growth

Required energy for reaction

152meV

87meV

179meV

96meV

160meV

64meV

Growth on nitrogen doped armchair

Required energy for reaction Pure Carbon: 160meV / Nitrogen incorporated C: 152meV

No remarkable effect

Growth on nitrogen doped zigzag

333meV

No barrier

No reaction!

When nitrogen locates at the top of the hexagon ring, energy barrier for the growth vanishes

No barrier

No barrier

No barrier

176meV

No barrier

Near the nitrogen incorporated Near the nitrogen incorporated region (top site), region (top site), the activation the activation energy for carbon network energy for carbon network growth disappearsgrowth disappears

No barrier

No barrier

No barrier

No barrier In pure C system:176meV

zigzagzigzag

Growth on nitrogen doped carbon system

Summary – growth kinetics

In pure carbon systemArmchair edge can grow faster, then growth on zigzag edge is rate determining step.

Nitrogen incorporation into zigzag edge-lowers energy barrier-makes the growth rates of zigzag edge similar to that of armchair.

Incorporated nitrogen effect on carbon attachment

Activation energy becomes lower.nitrogen in top site of zigzag edge, makes all energy barriers for the growth disappear.

Nitrogen enhances the growth of zigzag edge.

ConclusionConclusion

Multi-wall CNT with tens of nanometer size has no excess strain energy.

Reducing the strain energy is not a major reason for the enhanced CNT growth by nitrogen incorporation in large CNT.

Nitrogen incorporation significantly affect the growth kinetics by lowering the activation barriers for the growth.

Nitrogen incorporation into armchair edge

160meV 303meV

5455meV137meV

Energy required for the growth : 160meVNo remarkable effect of nitrogen incorporation

No Reaction

64meV

<pure carbon>

C24H12

C54H18

C96H24

C150H30

← Cluster size increases(graphite)

Linear relationshipRelatively small size graphite cluster can reflect whole graphite sheet/CNT

Expanding the scale of calculationExpanding the scale of calculationCalculation by ClusterCalculation by Cluster

Expanding the scale of calculationExpanding the scale of calculationCalculation by ClusterCalculation by Cluster

Journal of Computer-Aided Materials Design, 5, 279 (1998)

C24H12 C54H18

C96H24 C150H30