Elena Mora

Honda Research Institute USA, Inc., Mountain View, California, United States

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Publications (18)44.03 Total impact

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    ABSTRACT: In this manuscript we present the thermodynamics of iron-carbon nano particles at low temperature. By combining classical molecular dynamics simulations, ab initio calculations, finite temperature thermodynamics modeling, and the “size/pressure approximation”, we address carbon-induced fluidization, size-induced eutectic point shift, and reduced solubility at the nanoscale. The results are used to describe, as functions of particle size, three scenarios in the catalytic chemical vapor deposition growth of single single-walled carbon nanotubes, corresponding to steady state-, limitedand no-growth.
    Physics Procedia. 01/2010; 6:16-26.
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    ABSTRACT: Carbon filaments can be grown using hydrocarbons with either exothermic or endothermic catalytic decomposition enthalpies. By in situ monitoring the evolution of the reaction enthalpy during nanotube synthesis via methane gas, we found that although the decomposition reaction of methane is endothermic an exothermic process is superimposed which accompanies the nanotube growth. Analysis shows that the main contributor in this liberated heat is the radiative heat transfer from the surroundings, along with dehydrogenation reaction of in situ formed secondary hydrocarbons on the catalyst surface and the carbon hydrogenation/oxidation processes. This finding implies that nanotube growth process enthalpy is exothermic, and particularly, it extends the commonly accepted temperature gradient driven growth mechanism to the growth via hydrocarbons with endothermic decomposition enthalpy.
    ACS Nano 03/2009; 3(2):379-85. · 12.03 Impact Factor
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    ABSTRACT: We report on the lowest temperature of SWCNT growth using endothermic decomposition of CH4 gas on a specially activated alumina-supported Fe:Mo catalyst. However, the observed lowest growth temperature (560 degrees C) is higher than that reported previously for exothermic feedstock type. Our observation indicates that the decomposition threshold temperature of the feedstock limits the SWCNT growth. This study also suggests that, by using more active carbon feedstock or somehow facilitating its decomposition, one could enable the synthesis of SWCNT at an even lower temperature.
    Journal of the American Chemical Society 10/2008; 130(36):11840-1. · 10.68 Impact Factor
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    ABSTRACT: Fe nanoclusters are becoming the standard catalysts for growing single-walled carbon nanotubes via chemical vapor decomposition. Contrary to the Gibbs-Thompson model, we find that the reduction of the catalyst size requires an increase of the minimum temperature necessary for the growth. We address this phenomenon in terms of solubility of C in Fe nanoclusters and, by using first-principles calculations, we devise a simple model to predict the behavior of the phases competing for stability in Fe-C nanoclusters at low temperature. We show that, as a function of particle size, there are three scenarios compatible with steady state growth, limited growth, and no growth of single-walled carbon nanotubes, corresponding to unaffected, reduced, and no solubility of C in the particles.
    Physical Review Letters 05/2008; 100(19):195502. · 7.73 Impact Factor
  • Elena Mora, Avetik R. Harutyunyan
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    ABSTRACT: The effect of catalyst composition, hydrocarbon flow rate, and synthesis temperature on the single-walled carbon nanotubes (SWCNTs) grown was studied through the evolution of the corresponding catalyst lifetime. Catalyst lifetime was estimated based on the analysis of Raman spectra of SWCNTs obtained by sequential introduction of 12CH4 and 13CH4 gases at different stages of the catalyst activity during bulk nanotube growth. We found that proper modification of the Fe catalyst composition with Mo increases the lifetime around 3 times, increasing the catalytic activity and yield of tubes grown. A 9 times increase in the hydrocarbon flow rate (from 6.4 × 10-4 to 3.8 × 10-2 CH4 mol/s g Fe) results in a decrease in the catalyst lifetime; however, the formed carbon atoms rate increases around 2 times (from 1.8 × 1021 to 3.5 × 1021 C atoms/s g Fe) increasing the nanotube growth rate (from 1.2 to 2.4 nm/s), along with improving the nanotube quality. An increase in the synthesis temperature of ΔT 140 °C shortened the lifetime around 9 times (from 90 to 10 min), increasing, however, the growth rate and resulting in a similar nanotube yield. The kinetics of nanotube formation in terms of carbon atoms diffusion ability and solubility is discussed.
    03/2008;
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    ABSTRACT: Various diameters of alumina-supported Fe catalysts are used to grow single-walled carbon nanotubes (SWCNTs) with chemical vapor decomposition. We find that the reduction of the catalyst size requires an increase of the minimum temperature necessary for the growth. We address this phenomenon in terms of solubility of C in Fe nanoclusters and, by using first principles calculations, we devise a simple model to predict the behavior of the phases competing for stability in Fe-C nanoclusters at low temperature. We show that, as a function particles size, there are three scenarios compatible with steady state-, limited- and no-growth of SWCNTs, corresponding to unaffected, reduced and no solubility of C in the particles. The result raises previously unknown concerns about the growth feasibility of small and very-long SWCNTs within the current Fe CVD technology, and suggests new strategies in the search of better catalysts. Research supported by Honda R.I. and NSF.
    03/2008;
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    ABSTRACT: Single-walled carbon nanotubes (SWCNTs) were grown inside of a differential scanning calorimetry (DSC) apparatus with an attached mass spectrometer (MS), using different hydrocarbons (CH4 and C2H4) and alumina supported (Fe, Fe/Mo, and Ni) catalysts. This set-up allowed to in situ follow the evolution of calorimetric, thermogravimetric and MS data during the synthesis. A Raman spectrometer (with laser excitations wavelengths 532 and 785 nm) was used for verification of the growth of SWCNTs. DSC studies at temperatures ˜650-900 C of the interaction between the hydrocarbons and the preliminary reduced alumina supported catalysts showed a release (C2H4) or absorption (CH4) of heat depending on the type of hydrocarbon used. The effect of this energy on the growth of SWCNTs was studied. We found that the incubation time for nanotube nucleation depends on the hydrocarbon type and flow rate, as well as on the synthesis temperature. The origin of the initial endothermic peak observed during nanotube growth with both hydrocarbon sources will be discussed. Furthermore, the kinetics and thermodynamic of hydrocarbon decomposition, carbon atoms diffusion and solid carbon structure formation dependence on the catalyst and synthesis parameters will also be presented.
    03/2008;
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    ABSTRACT: Combining in situ studies of the catalyst activity during single-walled carbon nanotube (SWCNT) growth by mass spectrometry with differential scanning calorimetry and Raman spectroscopy results, the authors expose the favorable features of small catalyst for SWCNT growth and their relationship with synthesis parameters. The sequential introduction of 12C and 13C labeled hydrocarbon reveals the influence of catalyst composition on its lifetime and the growth termination path. Ab initio and molecular dynamics simulations corroborate “V”-shape liquidus line of metal-carbon nanoparticle binary phase diagram, which explains observed carbon-induced solid-liquid-solid phase transitions during nanotube growth.
    Applied Physics Letters 04/2007; 90(16):163120-163120-3. · 3.79 Impact Factor
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    ABSTRACT: Using catalytic decomposition, a technique for the production of singe-walled carbon nanotubes (SWCNTs) is reported with a production rate up to 6 g h−1 after purification, and scaling capability up to 220 g h−1. This is achieved by injection of pre-prepared alumina supported catalyst powder into a modified vertical floating reactor. The product is collected in several cyclones connected in series. Wide range Raman studies (laser excitations from λ = 1064 to 488 nm) and temperature programmed oxidation measurements of the samples collected from the different cyclones show that SWCNTs were separated in situ by tube diameter. This is attributed to the different residual times of the catalyst in the reaction zone depending on particle diameter. A series of computational fluid dynamics calculations of the flow and heat transfer in the reactor, as well as modeling of catalyst particle transport reveals the parametrical dependence of the process.
    Carbon. 04/2007; 45(5):971–977.
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    ABSTRACT: We investigate the viability of formation of very small diameter (< 0.5nm) freestanding SWNTs by CVD based on concept of carbon diffusion through the catalyst particle, originated from the vapor-liquid-solid growth mechanism. We found that the decrease of particle size required for nucleation of small diameter tubes results in a significant increase of catalytic decomposition temperature of hydrocarbon and, accordingly, the temperature required for nucleation and growth of nanotubes. However, high temperature increases the mobility of particles and endorses their agglomeration with formation of bigger particles, as well as leads to deactivation of catalyst by formation of intermetallic compounds with support material. The results of Raman spectroscopy, (n,m) assignments of the grown tubes and TEM studies for the smallest diameter tubes are presented. Performed ab-inito and molecular dynamics simulations qualitatively explain the experimental finding based on size dependent carbon solubility of catalyst, by analyzing supported nanocatalyst-carbon binary phase diagram.
    03/2007;
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    ABSTRACT: A series of Fe catalysts, with different mean diameters, supported on alumina with different molar ratios, was studied before and after carbon single walled nanotubes growth using magnetic measurements and Raman scattering techniques (laser excitation wavelengths from 1.17 to 2.54 eV) to follow changes on catalyst particle size and composition, as well as the relationship between particle size and diameter of nanotubes grown. In all cases, an increase and redistribution of the particle size after the growth was concluded based on the blocking temperature values and Langevin function analysis. This is explained in terms of agglomeration of particles due to carbon-induced liquefaction accompanied with an increase in the catalyst mobility. For large particles no direct correlation between the catalyst size and the nanotube diameters was observed.
    Journal of Applied Physics 08/2006; 100(4):044321-044321-8. · 2.21 Impact Factor
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    ABSTRACT: Fe nanoparticles are widely used as catalysts for carbon nanotube growth. In order to better understand the melting and phase transition properties of these nanoparticles, molecular dynamics (MD) simulations are performed to determine the melting point of Fe-C nanoparticles (˜ 1- 4 nm size) as a function of size and carbon concentration. The temperature dependence of the total energy and the Lindemann index characterize the melting of nanoparticles. For free (unsupported) Fe-C clusters, it is observed that the eutectic point (in the phase diagram) shifts with nanoparticle size. We have investigated how the presence of a substrate affects the melting process of the Fe-C nanoparticles.
    03/2006;
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    ABSTRACT: Interaction of iron catalyst nanoparticles with a substrate may influence nucleation and growth mechanism of carbon nantobutes (CNT) by shifting the melting temperature of the supported iron particles. To account for this effect we have used ab initio calculations to develop empirical potentials between iron nanoparticles and oxide substrates. Simulations have been performed on Fe/Al2O3 system with Al2O3 substrate fully relaxed. We have demonstrated that the surface rearrangement effects are significant but can be naturally incorporated into a simple Morse potential, which describes the total nanoparticle-substrate binding. The influence of different substrate surface terminations and positions of Fe layers on the strength of binding are discussed. Potentials for Fe and other oxide substrates are also being evaluated.
    01/2006;
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    ABSTRACT: A series of Fe catalysts with different mean diameter supported on alumina with different molar ratios were studied before and after SWNTs growth by using magnetic and Raman measurements to follow changes on catalyst particle size and its relationship with diameter of grown tubes. After the growth, based on blocking temperature values and Langevin function analysis, it was determined that for all catalysts, an increase and redistribution of particle size occurred. This is explained in terms of particle agglomeration, due to carbon-induced liquefaction accompanied with an increase in catalyst mobility. The free path of supported Fe particles was estimated to be >2.1nm. For big particles no correlation between catalyst size and nanotube diameter was observed. Analysis of the intensity of Breit-Wigner-Fano line contribution in the Raman G-band revealed that big catalyst particles are more selective to tubes chiralities, and more favorable to the growth of particular metallic tubes.
    01/2006;
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    ABSTRACT: The evolution of the iron catalyst during carbon single-walled nanotube growth is studied using calorimetry, temperature-programmed oxidation and Raman measurements. Carbon-induced solid-liquid, and solid-liquid-solid phase transitions of the nanocatalyst during the synthesis were observed. We found that liquid phase is favored for the growth of nanotubes, while the solidification of the catalyst nearly terminates the growth. No growth was observed below the eutectic point, when the catalyst is in solid phase. Our results support a growth mechanism of single-walled carbon nanotubes on liquid catalyst particles.
    Applied Physics Letters 07/2005; 87(5):051919-051919-3. · 3.79 Impact Factor
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    ABSTRACT: Two groups of cobalt nanoparticles with spherical and disk shape were used to grow carbon single-walled nanotubes by chemical vapor deposition. To elucidate the synthesis conditions and for comparison purpose, a third group of cobalt catalyst prepared by common wet chemistry was used. After the synthesis, electron-microscopic studies revealed that the disk-shape particles with size less than 20 nm were transformed into spheres. Meanwhile, calorimetric measurements showed that the given synthesis temperature was lower than the melting point of the catalyst nanoparticles. Our result supports a growth concept based on the formation of nanotubes on carbon-induced liquefied metal nanoparticles.
    Applied Physics Letters 04/2005; 86(15):153113-153113-3. · 3.79 Impact Factor
  • Elena Mora, Toshio Tokune
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    ABSTRACT: Despite intense studies, the growth mechanism of carbon single- walled nanotubes (SWNTs) is still debated and current synthesis methods do not allow for full control over the growth. There has been much discussion of whether the active catalytic species are in the liquid or solid phase during SWNTs formation, which is a key to understand and to control the growth of these materials. However, the actual phase of the catalyst and its evolution during carbon SWNTs growth still has to be experimentally verified. We report the observation of carbon induced solid-liquid and solid--liquid-solid phase transitions of the iron nanocatalyst during the synthesis, using differential scanning calorimetry and Raman scattering measurements. We found that as long as the nanocatalyst is in a liquid state, SWNTs growth occurs and continues until its solidification. Moreover, no growth was observed below the eutectic point, when the catalyst is always in solid phase.
    01/2005;
  • Elena Mora, Toshio Tokune
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    ABSTRACT: Despite the discovery of carbon single walled nanotubes (SWNTs) over a decade ago, the growth mechanism is still not fully understood. The question of whether the catalyst remains solid or not during the growth is still the subject of intense research. In the study reported here, two groups of catalysts with spherical and disk shape cobalt nanoparticles were used to grow SWNTs by chemical vapor deposition. The Raman spectra of the carbon deposits confirmed the formation of SWNTs. Additionally, electron microscopy images revealed that the disk shaped particles with diameter less than 20 nm were transformed into spheres during the synthesis. This implies that SWNTs were formed on the liquefied metal nanoparticles. However, calorimetric studies conducted on the pristine catalyst showed that the synthesis temperature was lower than the melting point of the catalyst nanoparticles. According to the cobalt-carbon binary phase diagram, the saturation of the metal with carbon atoms causes a decrease in the melting point. Based upon our results, we believe that SWNTs grow on the liquefied nanoparticles and that the liquefaction is caused by the diffusion of carbon atoms in the nanoparticles.
    01/2005;

Publication Stats

140 Citations
44.03 Total Impact Points

Institutions

  • 2007–2008
    • Honda Research Institute USA, Inc.
      Mountain View, California, United States
  • 2005–2006
    • The Ohio State University
      • Department of Physics
      Columbus, Ohio, United States