Mechanisms of single-walled carbon nanotube growth and deactivation from in situ Raman measurements

Laboratoire des Colloïdes, Verres et Nanomatériaux

Université des Sciences et Techniques du Languedoc - CNRS

Montpellier, France

Vincent JOURDAIN

MotivationsThe nanotube yield in catalytic CVD is limited by:

- Activation processes

- Growth kinetics

- Deactivation processes

Why Raman spectroscopy?

Advantages- structural information (SWNTs vs. MWNTs, disordered C, …)- resonance effect: intense and specific signal- micron-large probed area: statistical information

A few disadvantages:- the information is averaged on a large number of nanotubes- resonance effect: too specific information?

In situ measurements

CVD micro-reactor

Setup for in situ Raman measurements

Catalyst: - 5Å layer of Ni or Co on SiO2/Si- NO underlayer (e.g. Al2O3)

Growth conditions:- ethanol (6 Pa - 5 kPa) diluted in argon or pure methane- 450°C - 900°C

Raman measurements:- l = 532 nm- P = 12 mW (on substrate)

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10000

20000

30000

40000

Ra

ma

n in

ten

sity

(a

.u.)

Raman shift (cm-1)

(x3)

Ex situ characterizationRoom temperature

= 532nm

SEM

RBM

D band

G band

• Dense entanglement of SWNTs (less than 10 nm thick)

• Low amount of disordered carbon

Raman

TEM

(Raul Arenal, ONERA)

Catalyst activationmethane, 650°C

Argon purge

Introduction of the carbon precursor

Pretreatment:

oxygen from RT to 700°C

ethanol, 700°C

• In the growth conditions, methane and ethanol reduce cobalt oxides.• The catalyst reduction occurs quickly.• The nanotube growth starts after the catalyst is reduced.

Catalyst activationReducing the catalyst is not

enough to initiate the growth.

At high temperature and low ethanol pressure, the catalyst is

reduced but still unactive

: no growth

The precursor pressure must also exceed a threshold value.

The threshold pressure increases with increasing temperature.

T=850°C

Possible origin: the catalyst particle has to reach carbon supersaturation to initiate the growth.

T carbon solubility precursor pressure for supersatutarion

Catalyst deactivation at high temperature

Once reduced, the catalyst layer rapidly restructures at high temperature as revealed by:

- a decreased activity

- increased nanotube diameters

Nanotubes grown in standard conditions

Nanotubes grown in the same conditions after 14 min in the high-temperature non-activated region (850°C, PEtOH=10Pa)

Possible origins? Ostwald ripening and/or diffusion in the substrate at high temperature

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18T = 700°CP = 59 Pa

T = 700°CP = 20 Pa

G b

an

d A

rea

(a

.u.)

Time (s)

T = 575°CP = 20 Pa

Growth kinetics

- initial rate

- lifetime

- final yield

• Normalize

• Integrate

G(t) = .. (1 – e -t/ ) G(t) = .. (1 – e -t/ )

T = 800°C

1s acquisition time

• Fit

• Acquire

Growth kineticsLow temperature High temperature

Yield vs. Temperature vs. Temperature

LT MT HT LT MT HT

Initial growth rate and lifetime vs. ethanol pressure

• The initial growth rate displays two regimes as a function of ethanol pressure: limited by the gas-phase precursor supply at low ethanol pressure limited by surface reactions at high ethanol pressure

• and are anticorrelated when increasing PEtOH: both growth and deactivation are influenced by the availability of the surface products of ethanol decomposition.

lifetimeinitial

growth rate

Apparent reaction order n = 1.2

Initial growth rate and lifetime vs. temperature

• At LT and MT, the initial growth rate also displays two regimes as a function of temperature:

limited by surface reactions at low temperature limited by the gas-phase precursor supply at medium temperature

lifetime initial growth

rate

LTMT

EaLT = -1.9 eV

EaHT = 1.0 eV

Ea,LT = 2.8 eV

Ea,HT ~ 0 eV

Ea,HT + Ea

,HT = 1.0eV Ea,LT + Ea

,LT = 0.9eV

lifetime initial growth

rate

LTMT

• At LT and MT, and are also anticorrelated when increasing temperature: confirms ethanol decomposition is a common step for growth and deactivation.

• The constant difference of activation energies between and (~1eV) suggests the existence of an additional life-prolonging step of Ea ~1 eV.

Initial growth rate and lifetime vs. temperature

Density of defects vs. growth parametersG/D ratio from ex situ

Raman measurements

G/D ratio vs. temperature

Apparent activation energyfor the healing of defects at the nanotube-catalyst interface

(~1 eV for Ni and Co)

EaG/D = 0.9 eV

EaG/D = 1.0 eV

EaG/D = 0.9 eV

EaG/D ~ Ea

HT

Is defect healing by the catalyst the life-prolonging step?

Apparent activation energyfor the healing of defects at the nanotube-catalyst interface

(~1 eV for Ni and Co)

EaG/D = 1.0 eV

G/D ratio vs. temperature

Conclusion A threshold precursor pressure to initiate the growth

Two regimes for the initial growth rate Surface-limited regime: precursor decomposition and carbon diffusion Gas-phase diffusion-limited regime

Growth rate & lifetime are anticorrelated A common step for the growth and the deactivation (supply of the surface by carbon atoms?)

Constant difference of activation energies between Growth rate & lifetime at LT and MT A life-prolonging step of Ea~1 eV

Measured activation energy for the annealing of defects at the nanotube-catalyst interface of ~1eV (for Ni and Co) Is the annealing of defects the life-prolonging step? Is an accumulation of defects responsible for the deactivation?

Change of behavior at HT: Suggests the appearance of an additional deactivation mechanism at high temperature (Ostwald ripening?)

Acknowledgements

Eric Anglaret (Univ. Montpellier): Raman spectroscopy

eric@lcvn.univ-montp2.fr

Matthieu Picher (Univ. Montpellier): PhD student (looking for a postdoc position in 2010…)

picher@lcvn.univ-montp2.fr

Raul Arenal (CNRS-ONERA): HR TEM

raul.arenal@onera.fr

Yield vs. Temperature

vs. Temperature

LT MT HT

LT MT HT

SummarySurface

reactionsDefect healing

Ostwald ripening

Our results support that the yield is limited by:

Possible growth mechanism

Theoretical interpretation?

(1) Puretzky et al., Applied physics A, 2005

G(t) = .. (1 – e (-t/) ) G(t) = .. (1 – e (-t/) )

Competition between the formation of a carbonaceous layer (deactivation) & the formation of a SWNT.

THE MODEL

3 elementary steps

3 kinetic constants

Density of defects: influence of the precursor pressure

EaLT = -1.9 eV

EaHT = 1.0 eV

Ea,LT = 2.8 eV

Ea,HT ~ 0 eV

- Measured Ea = sums of the activation energies of elementary

steps

-There is a common step (carbon flux at the surface) : favorable to & unfavorable to (activation energy 2.8 eV)

- There is an additional process involved in the lifetime (Ea of 1 eV)

Ea,HT + Ea

,HT = 1.0eV Ea,LT + Ea

,LT = 0.9eV

“life-prolonging “

Theoretical interpretation?

What is a Single Wall Carbon Nanotube?

• Unidimensional structure.

• Excellent mechanical properties.

• Physical properties remarkably

dependent on the molecular structure.

Ch = na1 + ma2 : chiral vector Tube circumference

General growth mechanism for CCVD synthesis

Temperature calibration

Hipco SWCNTs

532 nm

Evolution of final G band Area:

An optimum partial pressure is observed for each temperature.

This optimum pressure shifts to higher pressures with increasing temperature.

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56 mW

8 mW

48 mW

16 mW

24 mW

30 mW

Ra

ma

n In

ten

sity

(a

.u.)

Raman shift (cm-1)

1592 cm-1

40 mW

12mW (Power used for In Situ Raman measurements)

High temperature deposition of amorphous carbon

900°C