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

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

  • Slide 1
  • Mechanisms of single-walled carbon nanotube growth and deactivation from in situ Raman measurements Laboratoire des Collodes, Verres et Nanomatriaux Universit des Sciences et Techniques du Languedoc - CNRS Montpellier, France Vincent JOURDAIN
  • Slide 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
  • Slide 3
  • CVD micro-reactor Setup for in situ Raman measurements Catalyst: - 5 layer of Ni or Co on SiO 2 /Si - NO underlayer (e.g. Al 2 O 3 ) Growth conditions: - ethanol (6 Pa - 5 kPa) diluted in argon or pure methane - 450C - 900C Raman measurements: - l = 532 nm - P = 12 mW (on substrate)
  • Slide 4
  • Ex situ characterization Room 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)
  • Slide 5
  • Catalyst activation methane, 650C Argon purge Introduction of the carbon precursor Pretreatment: oxygen from RT to 700C ethanol, 700C In the growth conditions, methane and ethanol reduce cobalt oxides. The catalyst reduction occurs quickly. The nanotube growth starts after the catalyst is reduced.
  • Slide 6
  • Catalyst activation Reducing 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=850C Possible origin: the catalyst particle has to reach carbon supersaturation to initiate the growth. T carbon solubility precursor pressure for supersatutarion
  • Slide 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 (850C, P EtOH =10Pa) Possible origins? Ostwald ripening and/or diffusion in the substrate at high temperature
  • Slide 8
  • Growth kinetics - initial rate - lifetime - final yield Normalize Integrate G(t) = . . (1 e -t/ ) T = 800C 1s acquisition time Fit Acquire
  • Slide 9
  • Growth kinetics Low temperatureHigh temperature Yield vs. Temperature vs. Temperature LTMTHTLTMTHT
  • Slide 10
  • 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 P EtOH : both growth and deactivation are influenced by the availability of the surface products of ethanol decomposition. lifetime initial growth rate Apparent reaction order n = 1.2
  • Slide 11
  • 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 lifetimeinitial growth rate LTMT
  • Slide 12
  • E a LT = -1.9 eV E a HT = 1.0 eV E a ,LT = 2.8 eV E a , HT ~ 0 eV E a ,HT + E a ,HT = 1.0eV E a ,LT + E a ,LT = 0.9eV lifetimeinitial 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
  • Slide 13
  • Density of defects vs. growth parameters G/D ratio from ex situ Raman measurements
  • Slide 14
  • G/D ratio vs. temperature Apparent activation energy for the healing of defects at the nanotube-catalyst interface (~1 eV for Ni and Co) E a G/D = 0.9 eV E a G/D = 1.0 eV
  • Slide 15
  • E a G/D = 0.9 eV E a G/D ~ E a HT Is defect healing by the catalyst the life-prolonging step? Apparent activation energy for the healing of defects at the nanotube-catalyst interface (~1 eV for Ni and Co) E a G/D = 1.0 eV G/D ratio vs. temperature
  • Slide 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?)
  • Slide 17
  • Acknowledgements Eric Anglaret (Univ. Montpellier): Raman spectroscopy [email protected] Matthieu Picher (Univ. Montpellier): PhD student (looking for a postdoc position in 2010) [email protected] Raul Arenal (CNRS-ONERA): HR TEM [email protected]
  • Slide 18
  • Yield vs. Temperature vs. Temperature LTMTHT LTMTHT Summary Surface reactions Defect healing Ostwald ripening Our results support that the yield is limited by:
  • Slide 19
  • Possible growth mechanism
  • Slide 20
  • Theoretical interpretation? (1) Puretzky et al., Applied physics A, 2005 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
  • Slide 21
  • Density of defects: influence of the precursor pressure
  • Slide 22
  • E a LT = -1.9 eV E a HT = 1.0 eV E a ,LT = 2.8 eV E a , 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) E a ,HT + E a ,HT = 1.0eV E a ,LT + E a ,LT = 0.9eV life-prolonging Theoretical interpretation?
  • Slide 23
  • What is a Single Wall Carbon Nanotube? Unidimensional structure. Excellent mechanical properties. Physical properties remarkably dependent on the molecular structure. C h = n a 1 + m a 2 : chiral vector Tube circumference
  • Slide 24
  • General growth mechanism for CCVD synthesis
  • Slide 25
  • Temperature calibration Hipco SWCNTs 532 nm
  • Slide 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.
  • Slide 27
  • Slide 28
  • High temperature deposition of amorphous carbon 900C