Laboratoire des Colloïdes, Verres et Nanomatériaux

1. Mechanisms of single-walled carbon nanotube growth and deactivation from in situ Raman measurements Vincent JOURDAIN Laboratoire des Colloïdes, Verres et Nanomatériaux Université des Sciences et Techniques du Languedoc - CNRS Montpellier, France

2. Motivations The 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

3. Setup for in situ Raman measurements CVD micro-reactor • 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)

4. Ex situ characterization SEM Raman Room temperature l = 532nm G band RBM D band • Dense entanglement of SWNTs (less than 10 nm thick) • Low amount of disordered carbon TEM (Raul Arenal, ONERA)

5. Catalyst activation Introduction of the carbon precursor methane, 650°C Pretreatment: oxygen from RT to 700°C Argon purge 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.

6. Catalyst activation Reducing the catalyst is not enough to initiate the growth. The precursor pressure must also exceed a threshold value. : no growth T=850°C The threshold pressure increases with increasing temperature. Possible origin: the catalyst particle has to reach carbon supersaturation to initiate the growth. T   carbon solubility   precursor pressure for supersatutarion  At high temperature and low ethanol pressure, the catalyst is reduced but still unactive

7. 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

8. Growth kinetics • Acquire T = 800°C 1s acquisition time • Normalize • Integrate G(t) = .. (1 – e -t/ ) • Fit - initial raten - lifetime - n = final yield

9. Growth kinetics Low temperature High temperature n, vs. Temperature Yield vs. Temperature LT MT HT LT MT HT

10. Initial growth rate and lifetime vs. ethanol pressure initial growth rate lifetime Apparent reaction order n = 1.2 • 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 • t and n are anticorrelated when increasing PEtOH: both growth and deactivation are influenced by the availability of the surface products of ethanol decomposition.

11. Initial growth rate and lifetime vs. temperature MT LT lifetime initial growth rate • 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

12. Initial growth rate and lifetime vs. temperature MT LT Eat,LT= -1.9 eV Ea,HT ~ 0 eV lifetime initial growth rate Eat,HT = 1.0 eV Ea,LT= 2.8 eV Ea,HT+ Ea,HT = 1.0eV Ea,LT+ Ea,LT = 0.9eV • At LT and MT, t and n are also anticorrelated when increasing temperature: confirms ethanol decomposition is a common step for growth and deactivation. • The constant difference of activation energies between t and n (~1eV) suggests the existence of an additional life-prolonging step of Ea ~1 eV.

13. Density of defects vs. growth parameters G/D ratio from ex situ Raman measurements

14. G/D ratio vs. temperature EaG/D = 1.0 eV EaG/D = 0.9 eV Apparent activation energy for the healing of defects at the nanotube-catalyst interface (~1 eV for Ni and Co)

15. G/D ratio vs. temperature EaG/D = 1.0 eV EaG/D = 0.9 eV Apparent activation energy for the healing of defects at the nanotube-catalyst interface (~1 eV for Ni and Co) EaG/D ~ Eat,HT Is defect healing by the catalyst the life-prolonging step?

16. 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?)

17. Acknowledgements Matthieu Picher (Univ. Montpellier): PhD student (looking for a postdoc position in 2010…) picher@lcvn.univ-montp2.fr Eric Anglaret (Univ. Montpellier): Raman spectroscopy eric@lcvn.univ-montp2.fr Raul Arenal (CNRS-ONERA): HR TEM raul.arenal@onera.fr

18. Summary Our results support that the yield is limited by: Surface reactions Defect healing Ostwald ripening LT MT HT n, vs. Temperature LT MT HT Yield vs. Temperature

19. Possible growth mechanism

20. Theoretical interpretation? THE MODEL Competitionbetween the formation of a carbonaceous layer (deactivation) & the formation of a SWNT. G(t) = ..(1 – e (-t/) ) 3 elementarysteps  3 kinetic constants (1) Puretzky et al., Appliedphysics A, 2005

21. Density of defects: influence of the precursor pressure

22. Theoretical interpretation? Eat,LT= -1.9 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 n& unfavorable to t(activation energy 2.8 eV) - There is an additional process involved in the lifetime (Ea of 1 eV) Eat,HT = 1.0 eV “life-prolonging “ Ea,LT= 2.8 eV Ea,HT+ Ea,HT = 1.0eV Ea,LT+ Ea,LT = 0.9eV

23. What is a Single Wall Carbon Nanotube? Ch = na1 + ma2 : chiral vector Tube circumference • Unidimensional structure. • Excellent mechanical properties. • Physical properties remarkably dependent on the molecular structure.

24. General growth mechanism for CCVD synthesis

25. Temperature calibration Hipco SWCNTs 532 nm

26. 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.

28. High temperature deposition of amorphous carbon 900°C