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Register a new user3d Transition Metals and Oxides within Carbon Nanotubes by Co-Pyrolysis of Metallocene & Camphor: High Filling Efficiency and Self-Organized Structures
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We demonstrate that a single zone furnace with a modified synthesis chamber design is sufficient to obtain metal (Fe, Co or Ni) filled carbon nanotubes (CNT) with high filling efficiency and controlled morphology. Samples are formed by pyrolysis of metallocenes, a synthesis technique that otherwise requires a dual zone furnace. Respective metallocene in all three cases are sublimed in powder form, a crucial factor for obtaining high filling efficiency. While Fe@CNT is routinely produced using this technique, well-formed Ni@CNT or Co@CNT samples are reported for the first time. This is achieved by sublimation of nickelocene (or cobaltocene) in combination with camphor. These samples exhibit some of the highest saturation magnetization (Ms) values, at least an order of magnitude higher than that reported for Ni or Co filled CNT, by aerosol assisted pyrolysis. The results also elucidate on why Ni or Co@CNT are relatively difficult to obtain by pyrolyzing powder metallocene alone. Overall, a systematic variation of synthesis parameters provides insights for obtaining narrow length and diameter distribution and reduced residue particles outside filled CNT - factors which are important for device related applications. Finally, the utility of this technique is demonstrated by obtaining highly aligned forest of Fe2O3@CNT, wherein Fe2O3 is a functional magnetic oxide relevant to spintronics and battery applications.
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We report a comprehensive study on the electrochemical performance of four different Transition Metal Oxides encapsulated inside carbon nanotubes (CNT). Irrespective of the type of oxide-encapsulate, all these samples exhibit superior cyclic stability as compared to the bare-oxide. Innovative use of camphor during sample synthesis enables precise control over the morphology of these self-organized carbon nanotube structures, which in turn appears to play a crucial role in the magnitude of the specific capacity. A comparative evaluation of the electrochemical data on different samples bring forward interesting inferences pertaining to the morphology, filling fraction of the oxide-encapsulate, and the presence of oxide nano-particles adhering outside the filled CNT. Our results provides useful pointers towards the optimization of critical parameters, thus paving the way for using these synthetically encapsulated and self-organized carbon nanotube structures as anode materials for Li-ion batteries, and possibly other electrochemical applications.
From resonant Raman scattering on isolated nanotubes we obtained the optical transition energies, the radial breathing mode frequency and Raman intensity of both metallic and semiconducting tubes. We unambiguously assigned the chiral index (n_1,n_2) of approximately 50 nanotubes based solely on a third-neighbor tight-binding Kataura plot and find omega_RBM=214.4cm^-1nm/d+18.7cm^-1. In contrast to luminescence experiments we observe all chiralities including zig-zag tubes. The Raman intensities have a systematic chiral-angle dependence confirming recent ab-initio calculations.
We study the low temperature phase behavior of hydrogen within a bundle of carbon nanotubes. Because the carbon environment weakens the attraction between molecules within the same interstitial channel (IC), the ground state of the one-dimensional (1D) system is an uncondensed gas. When the screened attractive interaction between molecules in adjacent ICs is taken into account, the hydrogen ground state is a quasi-1D liquid. The critical temperature of this system is estimated.
This paper assesses the importance of three-body triple dipole interactions for quasi-one dimensional phases of He, Ne, H_2, Ar, Kr and Xe confined within interstitial channels or on the external surfaces of nanotube bundles. We find the substrate-mediated contribution to be substantial: for interstitial H_2 the well depth of the effective pair potential is reduced to approximately one half of its value in free space. We carry out ab initio calculations on linear and equilateral configurations of H_2 trimer and find that overlap interactions do not greatly change the DDD interaction in the linear configuration when the spacing is greater than about 3 A. However, the DDD interaction alone is clearly insufficient for the triangular configurations studied.
The field electron emission from the single-walled carbon nanotubes with their open ends terminated by -BH, -NH, and -O has been simulated. The apex-vacuum barrier and the emission current have been calculated. It has been found that -BH and -NH suppress the apex-vacuum barrier significantly and lead to higher emission current in contrast to the -O terminated structure in the same applied field. The calculated binding energy implies that the carbon nanotubes terminated with -BH and -NH are more stable than those saturated by oxygen atoms or by hydrogen atoms.
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