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ABSTRACT: A continuous and wide range control of the diameter (1.9-3.2 nm) and density (0.03-0.11 g cm(-3) ) of single-walled carbon nanotube (SWNT) forests is demonstrated by decoupling the catalyst formation and SWNT growth processes. Specifically, by managing the catalyst formation temperature and H2 exposure, the redistribution of the Fe catalyst thin film into nanoparticles is controlled while a fixed growth condition preserved the growth yield. The diameter and density are inversely correlated, where low/high density forests would consist of large/small diameter SWNTs, which is proposed as a general rule for the structural control of SWNT forests. The catalyst formation process is modeled by considering the competing processes, Ostwald ripening, and subsurface diffusion, where the dominant mechanism is found to be Ostwald ripening. Specifically, H2 exposure increases catalyst surface energy and decreases diameter, while increased temperature leads to increased diffusion on the surface and an increase in diameter.
Small 04/2013; · 8.35 Impact Factor
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ABSTRACT: We report a new direction for highly efficient carbon nanotube (CNT) synthesis where in place of conventional highly reactive carbon feedstocks at low concentrations, highly stable carbon feedstocks at high concentrations were shown to produce superior yields. We found that a saturated hydrocarbon that is considered to possess a low reactivity is delivered at high concentrations could achieve an extremely high growth yield, (2.5 times that when using ethylene). This result stems from the unique behavior where the CNT yield linearly increased with carbon concentration, in contrast to more reactive carbon feedstocks where the yield peaks. We propose that the mechanisms for the growth kinetics for high and low reactivity carbon feedstocks are fundamentally different where the latter benefits from a longer catalyst lifetime because of a relatively low production rate of carbon impurities.
ACS Nano 03/2013; · 10.77 Impact Factor
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ABSTRACT: Alignment represents an important structural parameter of carbon nanotubes (CNTs) owing to their exceptionally high aspect ratio, one-dimensional property. In this paper, we demonstrate a general approach to control the alignment of few-walled CNT forests from nearly random to nearly ideally aligned by tailoring the density of active catalysts at the catalyst formation stage, which can be experimentally achieved by controlling the CNT forest mass density. Experimentally, we found that the catalyst density and the degree of alignment were inseparably linked because of a crowding effect from neighboring CNTs, that is, the increasing confinement of CNTs with increased density. Therefore, the CNT density governed the degree of alignment, which increased monotonically with the density. This relationship, in turn, allowed the precise control of the alignment through control of the mass density. To understand this behavior further, we developed a simple, first-order model based on the flexural modulus of the CNTs that could quantitatively describe the relationship between the degree of alignment (HOF) and carbon nanotube spacing (crowding effect) of any type of CNTs.
ACS Nano 06/2012; 6(7):5837-44. · 10.77 Impact Factor
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ABSTRACT: We report the mutually exclusive relationship between carbon nanotube (CNT) yield and crystallinity. Growth conditions were optimized for CNT growth yield and crystallinity through sequential tuning of three input variables: growth enhancer level, growth temperature, and carbon feedstock level. This optimization revealed that, regardless of the variety of carbon feedstock and growth enhancer, the optimum conditions for yield and crystallinity differed significantly with yield/crystallinity, preferring lower/higher growth temperatures and higher/lower carbon feedstock levels. This mutual exclusivity stemmed from the inherent limiting mechanisms for each property.
Journal of the American Chemical Society 05/2012; 134(22):9219-24. · 9.91 Impact Factor
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ABSTRACT: By using long single-walled carbon nanotubes (SWNTs) as a filler possessing the highest aspect ratio and small diameter, we mimicked the chain structure of polymers in the matrix and realized a highly conductive elastomeric composite (30 S/cm) with an excellent mechanical durability (4500 strain cycles until failure), far superior to any other reported conductive elastomers. This exceptional mechanical durability was explained by the ability of long and traversing SWNTs to deform in concert with the elastomer with minimum stress concentration at their interfaces. The conductivity was sufficient to operate many active electronics components, and thus this material would be useful for practical stretchable electronic devices.
Nano Letters 04/2012; 12(6):2710-6. · 13.20 Impact Factor
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ABSTRACT: Here we show that essentially any Fe compounds spanning Fe salts, nanoparticles, and buckyferrocene could serve as catalysts for single-walled carbon nanotube (SWNT) forest growth when supported on AlO(x) and annealed in hydrogen. This observation was explained by subsurface diffusion of Fe atoms into the AlO(x) support induced by hydrogen annealing where most of the deposited Fe left the surface and the remaining Fe atoms reconfigured into small nanoparticles suitable for SWNT growth. Interestingly, the average diameters of the SWNTs grown from all iron compounds studied were nearly identical (2.8-3.1 nm). We interpret that the offsetting effects of Ostwald ripening and subsurface diffusion resulted in the ability to grow SWNT forests with similar average diameters regardless of the initial Fe catalyst.
Journal of the American Chemical Society 02/2012; 134(4):2148-53. · 9.91 Impact Factor
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Advanced Materials 08/2011; 23(32):3686-91. · 13.88 Impact Factor
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ABSTRACT: Using carbon nanotubes (CNTs) as building blocks, we fabricated a viscoelastic material. In contrast to existing conventional materials where the stiffness (storage modulus) increases when the viscosity (damping ratio) decreases, both of these two aspects could be simultaneously improved for the viscoelastic CNT material. This allows fabricating both strong and highly viscous materials. This unique phenomenon was explained by a zipping and unzipping of carbon nanotubes at contacts as the origin of viscoelasticity.
Nano Letters 08/2011; 11(8):3279-84. · 13.20 Impact Factor
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ABSTRACT: The heat history (i.e., "dwell time") of the carbon source gas was demonstrated as a vital parameter for very rapid single-walled carbon nanotube (SWNT) forest growth with long lifetime. When the dwell time was raised to 7 s from the 4 s used for standard growth, the growth rate increased to 620 μm/min: a benchmark for SWNT forest growth on substrates. Importantly, the increase in growth rate was achieved without decreasing either the growth lifetime or the quality of the SWNTs. We interpret that the conversion rate of the carbon feedstock into CNTs was selectively increased (versus catalyst deactivation) by delivering a thermally decomposed carbon source with the optimum thermal history to the catalyst site.
Nano Letters 08/2011; 11(9):3617-23. · 13.20 Impact Factor
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Haruka Kyakuno,
Kazuyuki Matsuda,
Hitomi Yahiro,
Yu Inami,
Tomoko Fukuoka,
Yasumitsu Miyata,
Kazuhiro Yanagi,
Yutaka Maniwa,
Hiromichi Kataura,
Takeshi Saito, Motoo Yumura,
Sumio Iijima
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ABSTRACT: Studies on confined water are important not only from the viewpoint of scientific interest but also for the development of new nanoscale devices. In this work, we aimed to clarify the properties of confined water in the cylindrical pores of single-walled carbon nanotubes (SWCNTs) that had diameters in the range of 1.46 to 2.40 nm. A combination of x-ray diffraction (XRD), nuclear magnetic resonance, and electrical resistance measurements revealed that water inside SWCNTs with diameters between 1.68 and 2.40 nm undergoes a wet-dry type transition with the lowering of temperature; below the transition temperature T(wd), water was ejected from the SWCNTs. T(wd) increased with increasing SWCNT diameter D. For the SWCNTs with D = 1.68, 2.00, 2.18, and 2.40 nm, T(wd) obtained by the XRD measurements were 218, 225, 236, and 237 K, respectively. We performed a systematic study on finite length SWCNT systems using classical molecular dynamics calculations to clarify the effect of open ends of the SWCNTs and water content on the water structure. It was found that ice structures that were formed at low temperatures were strongly affected by the bore diameter, a = D - σ(OC), where σ(OC) is gap distance between the SWCNT and oxygen atom in water, and the number of water molecules in the system. In small pores (a < 1.02 nm), tubule ices or the so-called ice nanotubes (ice NTs) were formed irrespective of the water content. On the other hand, in larger pores (a > 1.10 nm) with small water content, filled water clusters were formed leaving some empty space in the SWCNT pore, which grew to fill the pore with increasing water content. For pores with sizes in between these two regimes (1.02 < a < 1.10 nm), tubule ice also appeared with small water content and grew with increasing water content. However, once the tubule ice filled the entire SWCNT pore, further increase in the water content resulted in encapsulation of the additional water molecules inside the tubule ice. Corresponding XRD measurements on SWCNTs with a mean diameter of 1.46 nm strongly suggested the presence of such a filled structure.
The Journal of chemical physics 06/2011; 134(24):244501. · 3.09 Impact Factor
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ABSTRACT: The layer number is of great importance for nanocarbon materials, such as carbon nanotubes (CNTs) and graphene. While simple optical methods exist to evaluate few-layer graphene, equivalent analysis for CNTs is limited to transmission electron microscopy. We present a simple macroscopic method based on the (002) X-ray diffraction peak to evaluate the average wall number of CNTs in the range from single- to few-walled. The key was the finding that the (002) peak could be decomposed into two basic components: the intertube structure (outer-wall contacts) and the intratube structure (concentric shells). Decomposition of the peaks revealed a linear relationship between the average wall number and the ratio of the intertube and intratube contributions to the (002) peak. Good agreement with CNTs having average wall numbers ranging from 1 to ∼5 demonstrated this as a macroscopic method for average wall number analysis.
Journal of the American Chemical Society 03/2011; 133(15):5716-9. · 9.91 Impact Factor
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ABSTRACT: We present the wall number control of carbon nanotube (CNT) forests grown on metal cat-alyst films in a water-assisted chemical vapor deposition (CVD) by measuring the sheet resistances of metal catalyst films. Catalyst film thicknesses and thickness variations are monitored using a 2-point-based electrical characterization methodology. The electrical characterization and high-resolution transmission electron microscopy analysis showed that single-, double-, and triple-walled CNT forests were grown on iron (Fe) catalyst films with mean sheet resistances of 646.63, 75.40, and 27.84 MX/sq, respectively. The average wall number and outer diameter of CNT forests were found to linearly depend on the log-arithm of the mean sheet resistances of Fe catalyst films.
Carbon 01/2011; 49. · 5.38 Impact Factor
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ABSTRACT: Viscoelasticity describes the ability of a material to possess both elasticity and viscosity. Viscoelastic materials, such as rubbers, possess a limited operational temperature range (for example, for silicone rubber it is -55° to 300°C), above which the material breaks down and below which the material undergoes a glass transition and hardens. We created a viscoelastic material composed from a random network of long interconnected carbon nanotubes that exhibited an operational temperature range from -196° to 1000°C. The storage and loss moduli, frequency stability, reversible deformation level, and fatigue resistance were invariant from -140° to 600°C. We interpret that the thermal stability stems from energy dissipation through the zipping and unzipping of carbon nanotubes at contacts.
Science 12/2010; 330(6009):1364-8. · 31.20 Impact Factor
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ABSTRACT: The main gas phase intermediates, leading the efficient chemical vapor deposition (CVD) growth of single wall carbon nanotubes (SWCNTs), have been explored through systematic experimental and theoretical studies. The growth efficiencies of the basic radical/neutral species (sp2 C2: C2H3/C2H4, sp3 C2: C2H5/C2H6, and sp3 C1: CH3/CH4) have been compared by supplying C2H4, C2H6, and CH4, as carbon sources to grow SWCNTs. The gaseous composition of the exhaust was analyzed by an in situ direct sampling mass spectrometric technique using vacuum ultraviolet (VUV)-single photon ionization (SPI)-time-of-flight mass spectrometry (TOFMS). For verification via theoretical prediction, a CHEMKIN calculation was performed. A kinetic analysis of the experimental and theoretical results was compared with thermal decomposition phenomena of the used hydrocarbons, and hence, it was concluded that the key gas phase intermediates produced from the complex gas phase reactions as a final and efficient species capable of initiating the CVD growth of SWCNTs are sp2 C2 species, C2H3/C2H4. Additional significance is the production of highly pure SWCNTs from C2H6 since to date production of either only multiwall carbon nanotubes (MWCNTs) or a mixture of MWCNTs and SWCNTs have been reported. This result will be beneficial for selecting suitable hydrocarbons for the efficient growth of SWCNTs.
10/2010;
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Advanced Materials 09/2010; 22(35):E235-41. · 13.88 Impact Factor
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ABSTRACT: A novel system for fractionating single wall carbon nanotubes (SWCNTs) by length via a three-step cross-flow filtration has been developed in which three membrane filters of different pore sizes, 1.0, 0.45, and 0.2 microm, were used. SWCNTs dispersed in water with the help of sodium carboxymethylcellulose (CMC) detergents were successfully sorted into four samples, and the atomic force microscopy (AFM) observation of those samples confirmed that their length distribution peaks are within the expected ranges from pore sizes of used filters. However, the result of the similar filtration process using a different detergent, sodium dodecylbenzenesulfonate (SDBS), showed no pronounced correlation between the length distribution of SWCNTs and the pore size. The observed difference in the sorting phenomena caused by the detergent type suggests that the permeation property depends on the complex structure resulting from the dispersed SWCNTs and detergent molecules.
ACS Nano 07/2010; 4(7):3606-10. · 10.77 Impact Factor
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ABSTRACT: For precise characterizations of single-walled carbon nanotubes (SWCNTs) by optical absorption spectroscopy, the background extinction that originated from precipitable impurities and SWCNT bundles has been experimentally determined by the interval centrifugation and difference spectrum (IC-DS) technique. The baseline correction using the line shape of the obtained background extinction revealed the actual absorption spectrum of absolutely debundled SWCNTs with detailed features. The chirality distribution, including both semiconductive and metallic SWCNTs, was evaluated by deconvoluting the corrected absorption spectrum into multiple Lorentzian lines, which was well consistent with the result of photoluminescence (PL) mapping measurements. Another peculiarity is that the UV absorption characteristics of SWCNTs hidden in the original observed spectra by overlapping the background extinction also appeared. The baseline correction by the IC-DS technique provides a useful analysis method for characterizing SWCNTs, a complement/alternative to the similar analysis done by PL spectroscopy.
05/2010;
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Advanced Functional Materials 12/2009; 20(3):422 - 428. · 10.18 Impact Factor
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Advanced Materials 12/2009; 21(47):4811-5. · 13.88 Impact Factor
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ABSTRACT: A gas shower system was introduced to improve the growth of single-walled carbon nanotube (SWNT) forests by controlling the gas flow direction. Delivery of gases from the top of the forest enabled direct and precise supply of ethylene and water vapor to the Fe catalysts. As such, this approach solved one of the limiting factors of water-assisted chemical vapor deposition method (CVD), that is, delivery of the very small optimum water level to the catalysts. Consequently, this approach improved SWNT forests growth stability, uniformity, reproducibility, carbon efficiency (32%), and catalyst lifetime. With this improved growth, we could synthesize a 1 cm tall forest with 1 x 1 cm size. Also we employed this approach to grow an A4 size SWNT forest to highlight the scalability of water-assisted CVD.
ACS Nano 11/2009; 3(12):4164-70. · 10.77 Impact Factor
