mechanisms of single-walled carbon nanotube growth and deactivation from in situ raman measurements...
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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
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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
Matthieu Picher (Univ. Montpellier): PhD student (looking for a postdoc position in 2010…)
Raul Arenal (CNRS-ONERA): HR TEM
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