Diameter and density control of single-walled carbon nanotube forests by modulating Ostwald ripening through decoupling the catalyst formation and growth processes.

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.

Direct wall number control of carbon nanotube forests from engineered iron catalysts.

We present the direct wall number control of carbon nanotube (CNT) forests grown on engineered iron (Fe) catalysts in a catalytic chemical vapor deposition (CVD). Engineered Fe catalysts were fabricated by annealing thickness-tuned (0.8-3 nm) Fe films with small thickness variations prepared by a low-deposition-rate sputter deposition. Extensive scanning electron microcopy (SEM) characterization confirmed that vertically-aligned CNT forests were grown on Fe catalyst films with thickness larger than 1.5 nm. Detailed high-resolution transmission electron microscopy (HRTEM) and micro Raman spectroscopy analysis indicated that 75% of the CNTs grown on an Fe film with 1.5 nm mean thickness were single-walled CNTs while about 67% and 59% of CNTs grown on Fe films with 2.0 and 3.0 nm mean thickness were double-walled and triple-walled CNTs, respectively. The average wall number and outer diameter of CNT forests were found to linearly depend on the mean thickness of Fe catalyst films.

Mechanically durable and highly conductive elastomeric composites from long single-walled carbon nanotubes mimicking the chain structure of polymers.

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.

Mutual exclusivity in the synthesis of high crystallinity and high yield single-walled carbon nanotubes.

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.

Role of subsurface diffusion and Ostwald ripening in catalyst formation for single-walled carbon nanotube forest growth.

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.

A black body absorber from vertically aligned single-walled carbon nanotubes.

Among all known materials, we found that a forest of vertically aligned single-walled carbon nanotubes behaves most similarly to a black body, a theoretical material that absorbs all incident light. A requirement for an object to behave as a black body is to perfectly absorb light of all wavelengths. This important feature has not been observed for real materials because materials intrinsically have specific absorption bands because of their structure and composition. We found a material that can absorb light almost perfectly across a very wide spectral range (0.2-200 mum). We attribute this black body behavior to stem from the sparseness and imperfect alignment of the vertical single-walled carbon nanotubes.

Tailoring temperature invariant viscoelasticity of carbon nanotube material.

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.

Gas dwell time control for rapid and long lifetime growth of single-walled carbon nanotube forests.

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.

Macroscopic wall number analysis of single-walled, double-walled, and few-walled carbon nanotubes by X-ray diffraction.

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.

Confined water inside single-walled carbon nanotubes: global phase diagram and effect of finite length.

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.

Dual porosity single-walled carbon nanotube material.

We present a dual porosity CNT material with a seamless connection between highly porous aligned nanotubes and lowly porous closely packed nanotubes by using capillary action of liquids. Various approaches were developed to fabricate diverse structures using toothpicks, liquid thin films, bubbles, vapors, and superink jet printing. The dual porosity material showed low wear and was useful as a sliding electrical contact.

Crystalline and Electrical Property Improvement of Filtrated, Exfoliated Graphite Sheets by an In-Plane Current and Heating Treatment.

We report an approach to fabricate high conductivity graphite sheets based on a heat-and-current treatment of filtrated, exfoliated graphite flakes. This treatment combines heating (~ 900 °C) and in-plane electrical current flow (550 A·cm-2) to improve electrical conductivity through the reduction of crystalline defects. This process was shown to require only a 1-min treatment time, which resulted in a 2.1-fold increase in electrical conductivity (from 1088 ± 72 to 2275 ± 50 S·cm-1). Structural characterization by Raman spectroscopy and X-ray diffraction indicated that the improvement electrical conductivity originated from a 30-fold improvement in the crystallinity (Raman G/D ratio increase from 2.8 to 85.3) with no other observable structural transformations. Significantly, this treatment was found to act uniformly across a macroscopic (10 mm) sheet surface indicating it is on the development of applications, such as electrodes for energy generation and storage and electromagnetic shielding, as well as on the potential for the development of large-scale treatment technologies.

A background level of oxygen-containing aromatics for synthetic control of carbon nanotube structure.

Growth enhancers, most notably water, have been reported to increase catalyst activity, despite constituting 0.02% of the growth ambient, resulting in dramatic increases in vertically aligned carbon nanotube (CNT) forests. We demonstrate a surprising second function of the growth enhancer, i.e., its ability to control the structure of the CNTs exemplified by highly selective double-walled carbon nanotube synthesis using oxygen-containing aromatics. This approach provides an easy and convenient method to grow various kinds of nanotubes from the same catalyst and same growth conditions.

An efficient carbon precursor for gas phase growth of SWCNTs.

The sp(2) C(2) species, C(2)H(3)/C(2)H(4), has been found as a key precursor for the efficient growth of single-wall carbon nanotubes (SWCNTs) by chemical vapor deposition (CVD) from hydrocarbons.

Selective diameter control of single-walled carbon nanotubes in the gas-phase synthesis.

A novel approach for selective diameter control of single-walled carbon nanotubes (SWNTs) is performed in the gas-phase growth using two kinds of carbon sources with different decomposition properties; the one carbon source (1st carbon source) is the organic solvent which is difficult to decompose in the reactor and the another carbon source (2nd carbon source) is facile to decompose. The diameter distributions of SWNTs synthesized with various conditions of the flow rate of the 2nd carbon source were investigated by resonant Raman scattering, optical absorption, and photoluminescence (PL) mapping measurements. It was found that increasing the flow rate of the ethylene tends to decrease the diameter of synthesized SWNTs, probably due to the earlier nucleation of SWNTs induced by the ethylene addition. The controlling the flow rate of the ethylene used as a 2nd carbon source can selectively tune the diameter distribution of SWNTs in our growth system.

84% catalyst activity of water-assisted growth of single walled carbon nanotube forest characterization by a statistical and macroscopic approach.

We propose a statistical and macroscopic analysis to estimate the catalyst activity of water-assisted growth (super-growth) of single-walled nanotubes (SWNT) and to characterize SWNT forests. The catalyst activity was estimated to be 84% (+/-6%), the highest ever reported. The SWNT forest was found to be a very sparse material where SWNTs represent only 3.6% of the total volume. This structural sparseness is believed to play a critical role in achieving highly efficient growth.

Supramolecular catalysts for the gas-phase synthesis of single-walled carbon nanotubes.

Reversed micelles containing metallic ions have been used as precursors of novel catalysts for the gas-phase synthesis of single-walled carbon nanotubes (SWNTs). This technique possesses the following advantages: (i) excellent solubility in organic solvents, which are used as reactants and (ii) facile preparation of multicomponent catalysts enabling systematic screening of catalyst compositions for the synthesis of SWNTs. In this study, we report the results of the screening study on the catalytic behavior of Fe-Mo binary catalysts during the synthesis of SWNTs. The results suggested that the catalytic ability was closely related to the strain of the crystal structure of Fe-Mo catalysts formed in the reaction and/or the phase transition caused by dissolution of the Mo atoms. The addition of lithium to the Fe-Mo binary catalysts has revealed an increase in the yield of SWNTs.

A sweet spot for highly efficient growth of vertically aligned single-walled carbon nanotube forests enabling their unique structures and properties.

We investigated the correlation between growth efficiency and structural parameters of single-walled carbon nanotube (SWCNT) forests and report the existence of a SWCNT "sweet spot" in the CNT diameter and spacing domain for highly efficient synthesis. Only within this region could SWCNTs be grown efficiently. Through the investigation of the growth rates for ∼340 CNT forests spanning diameters from 1.3 to 8.0 nm and average spacing from 5 to 80 nm, this "sweet spot" was found to exist because highly efficient growth was constrained by several mechanistic boundaries that either hindered the formation or reduced the growth rate of SWCNT forests. Specifically, with increased diameter SWCNTs transitioned to multiwalled CNTs (multiwall border), small diameter SWCNTs could only be grown at low growth rates (low efficiency border), sparse SWCNTs lacked the requirements to vertically align (lateral growth border), and high density catalysts could not be prepared (high catalyst density border). As a result, the SWCNTs synthesized within this "sweet spot" possessed a unique set of characteristics vital for the development applications, such as large diameter, long, aligned, defective, and high specific surface area.

Selective matching of catalyst element and carbon source in single-walled carbon nanotube synthesis on silicon substrates.

We have studied the compatibility of various catalysts for ethylene and ethanol chemical vapor deposition (CVD) syntheses of single-walled carbon nanotubes (SWNTs) on Si substrates. A strong selectivity between the catalyst elemental species and carbon source was found; SWNT yield for Fe (Co) catalysts was much higher for ethylene (ethanol) CVD than for ethanol (ethylene) CVD. This strong and completely opposite selectivity implies significantly different SWNT growth mechanisms for ethanol and ethylene CVD on Si substrates.

CVD growth of single-walled carbon nanotubes with narrow diameter distribution over Fe/MgO catalyst and their fluorescence spectroscopy.

Single-walled carbon nanotubes (SWNTs) with a narrow diameter distribution are synthesized by thermal chemical vapor deposition (CVD) of methane over Fe/MgO catalyst on the basis of parametric study considering Fe loading, reaction temperature and time, methane concentration, and structure of a support material. We found that the porous MgO support gives the SWNTs with a narrow diameter distribution with the mean diameter and standard deviation of 0.93 and 0.06 nm, respectively, only when the Fe loading and reaction temperature are relatively low. The higher Fe loading and/or the higher reaction temperature enlarged the nanotube diameter, forming double-walled carbon nanotubes (DWNTs) in addition to SWNTs. This result indicates that only the diameter of Fe nanoparticles determines the growth of either SWNTs or DWNTs on the MgO support. The fluorescence and absorption spectra of the nanotube dispersion in D(2)O solution with sodium dodecyl sulfate (SDS) were studied to identify their chirality distribution. The fluorescence of the uniform-diameter SWNTs indicates the formation of the near armchair structures. On the other hand, the SWNTs synthesized over the catalyst with a high Fe loading, 3 wt %, showed a wide chirality distribution including the near zigzag structure. The synthesis of the SWNTs with a narrow diameter distribution could be applied to the selection of SWNTs with a specific chirality based on postsynthesis separation.