Jingwen Guan

Hydrogen-Catalyzed, Pilot-Scale Production of Small-Diameter Boron Nitride Nanotubes and Their Macroscopic Assemblies

Boron nitride nanotubes (BNNTs) exhibit a range of properties that are as compelling as those of carbon nanotubes (CNTs); however, very low production volumes have prevented the science and technology of BNNTs from evolving at even a fraction of the pace of CNTs. Here we report the high-yield production of small-diameter BNNTs from pure hexagonal boron nitride powder in an induction thermal plasma process. Few-walled, highly crystalline small-diameter BNNTs (∼5 nm) are produced exclusively and at an unprecedentedly high rate approaching 20 g/h, without the need for metal catalysts. An exceptionally high cooling rate (∼10(5) K/s) in the induction plasma provides a strong driving force for the abundant nucleation of small-sized B droplets, which are known as effective precursors for small-diameter BNNTs. It is also found that the addition of hydrogen to the reactant gases is crucial for achieving such high-quality, high-yield growth of BNNTs. In the plasma process, hydrogen inhibits the formation of N2 from N radicals and promotes the creation of B-N-H intermediate species, which provide faster chemical pathways to the re-formation of a h-BN-like phase in comparison to nitridation from N2. We also demonstrate the fabrication of macroscopic BNNT assemblies such as yarns, sheets, buckypapers, and transparent thin films at large scales. These findings represent a seminal milestone toward the exploitation of BNNTs in real-world applications.

Multifunctional single-walled carbon nanotube–cellulose composite paper

Single-walled carbon nanotubes (SWCNTs) have been used as fillers to produce electrically conductive composite papers. While conductive composite papers have been made using other fillers, they either suffer from instability or low conductivity. Using simple papermaking techniques, we have made SWCNT–cellulose composite paper which possesses a conductivity of 3 × 10−2 S cm−1 and is comparable to or exceeds other reports of carbon nanotube–cellulose papers made by layer-by-layer assembly. These composite papers are multifunctional, having both improved electrical conductivity and enhanced flame retardant properties over the control paper.

Toughening of epoxy matrices with reduced single-walled carbon nanotubes

Reduced single-walled carbon nanotubes (r-SWCNT) are shown to react readily at room temperature under inert atmosphere conditions with epoxide moieties, such as those in triglycidyl p-amino phenol (TGAP), to produce a soft covalently bonded interface around the SWCNT. The soft interface is compatible with the SWCNT-free cross-linked cured matrix and acts as a toughener for the composite. Incorporation of 0.2 wt % r-SWCNT enhances the ultimate tensile strength, toughness and fracture toughness by 32, 118, and 40%, respectively, without change in modulus. A toughening rate (dK(IC)/dwt(f)) of 200 MPa m(0.5) is obtained. The toughening mechanism is elucidated through dynamic mechanical analyses, Raman spectroscopy and imaging, and stress-strain curve analyses. The method is scalable and applicable to epoxy resins and systems used commercially.

Cytotoxicity of carbon nanotube variants: a comparative in vitro exposure study with A549 epithelial and J774 macrophage cells.

Abstract While production of engineered carbon nanotubes (CNTs) has escalated in recent years, knowledge of risk associated with exposure to these materials remains unclear. We report on the cytotoxicity of four CNT variants in human lung epithelial cells (A549) and murine macrophages (J774). Morphology, metal content, aggregation/agglomeration state, pore volume, surface area and modifications were determined for the pristine and oxidized single-walled (SW) and multi-walled (MW) CNTs. Cytotoxicity was evaluated by cellular ATP content, BrdU incorporation, lactate dehydrogenase (LDH) release, and CellTiter-Blue (CTB) reduction assays. All CNTs were more cytotoxic than respirable TiO2 and SiO2 reference particles. Oxidation of CNTs removed most metallic impurities but introduced surface polar functionalities. Although slopes of fold changes for cytotoxicity endpoints were steeper with J774 compared to A549 cells, CNT cytotoxicity ranking in both cell types was assay-dependent. Based on CTB reduction and BrdU incorporation, the cytotoxicity of the polar oxidized CNTs was higher compared to the pristine CNTs. In contrast, pristine CNTs were more cytotoxic than oxidized CNTs when assessed for cellular ATP and LDH. Correlation analyses between CNTs’ physico–chemical properties and average relative potency revealed the impact of metal content and surface area on the potency values estimated using ATP and LDH assays, while surface polarity affected the potency values estimated from CTB and BrdU assays. We show that in order to reliably estimate the risk posed by these materials, in vitro toxicity assessment of CNTs should be conducted with well characterized materials, in multiple cellular models using several cytotoxicity assays that report on distinct cellular processes.

Characterization of single-walled carbon nanotubes by scanning transmission X-ray spectromicroscopy: purification, order and dodecyl functionalization.

The C 1s X-ray absorption spectra of several isolated bundles of single-walled carbon nanotubes (SWCNT) have been measured using scanning transmission X-ray microscopy. First the C 1s and O 1s spectra of a purified but unfunctionalized SWCNT were measured. The C 1s --> pi* transition at 285 eV exhibited almost as strong a dichroic effect (spectral dependence on orientation) as that found in multiwalled carbon nanotubes (Najafi; et al. Small 2008, 7, 2279-2285). Second, purified SWCNT were functionalized with dodecyl and then investigated by STXM. Spectral evidence for the dodecyl functionalization is presented and discussed in comparison to the X-ray absorption spectra of aliphatic hydrocarbons. Both orientation and functionalization mapping of an individual SWCNT bundle is demonstrated.

Covalent Functionalization of Boron Nitride Nanotubes via Reduction Chemistry

Boron nitride nanotubes (BNNTs) exhibit a range of properties that hold great potential for many fields of science and technology; however, they have inherently low chemical reactivity, making functionalization for specific applications difficult. Here we propose that covalent functionalization of BNNTs via reduction chemistry could be a highly promising and viable strategy. Through density functional theory calculations of the electron affinity of BNNTs and their binding energies with various radicals, we reveal that their chemical reactivity can be significantly enhanced via reducing the nanotubes (i.e., negatively charging). For example, a 5.5-fold enhancement in reactivity of reduced BNNTs toward NH2 radicals was predicted relative to their neutral counterparts. The localization characteristics of the BNNT π electron system lead the excess electrons to fill the empty p orbitals of boron sites, which promote covalent bond formation with an unpaired electron from a radical molecule. In support of our theoretical findings, we also experimentally investigated the covalent alkylation of BNNTs via reduction chemistry using 1-bromohexane. The thermogravimetric measurements showed a considerable weight loss (12-14%) only for samples alkylated using reduced BNNTs, suggesting their significantly improved reactivity over neutral BNNTs. This finding will provide an insight in developing an effective route to chemical functionalization of BNNTs.

Rapid and controllable covalent functionalization of single-walled carbon nanotubes at room temperature

We report a rapid and efficient procedure to functionalize SWNT where free radicals generated at room temperature by a redox reaction between reduced SWNT and diacyl peroxide derivatives were covalently attached to the SWNT wall.

3D chemically cross-linked single-walled carbon nanotube buckypapers

Single-walled carbon nanotubes (SWCNTs) covalently modified with OH functional groups were assembled into buckypapers through solvent dispersion and vacuum filtration. These SWCNT-OH buckypaper sheets were subsequently crosslinked by wetting with bifunctional linkers followed by hot compression causing reaction between the functional groups of the reagent and OH functional groups on the side-walls of SWCNTs to create three-dimensional (3D) covalently cross-linked buckypapers. Cross-linking also was performed using SWCNTs encapsulated with a functionalized polymer wrapping in a core–shell structure, where OH or/and NH2 groups are available on the surface of the polymeric shell for reaction. The 3D cross-linked SWCNT buckypapers retain the porous character typical of buckypaper, and were characterized for their tensile properties and thermal and electrical conductivities. Several cross-linking approaches dramatically improved the mechanical properties. The strongest and stiffest papers (32 MPa, E = 3.1 GPa), which approach 10× stronger and stiffer than the pristine non-crosslinked buckypaper, were obtained at the expense of a loss of electrical conductivity. In other cases, such as cross-linking using a high-performance epoxy resin monomer, improvements in strength and stiffness of ∼5× were obtained while retaining electrical and thermal conductivity. Therefore, the optimal cross-linking approach would be determined by the desired, multifunctional properties. Additionally, the approach can be used in the preparation buckypaper composites and it is demonstrated that cross-linking using a multifunctional epoxy resin prior to impregnation with the same epoxy resin results in substantially better mechanical properties in comparison to just epoxy-impregnation of pristine buckypaper.

Coupled thermogravimetry, mass spectrometry, and infrared spectroscopy for quantification of surface functionality on single-walled carbon nanotubes

AbstractWe have successfully applied coupled thermogravimetry, mass spectrometry, and infrared spectroscopy to the quantification of surface functional groups on single-walled carbon nanotubes. A high-purity single-walled carbon nanotube sample was subjected to a rapid functionalization reaction that attached butyric acid moieties to the nanotube sidewalls. This sample was then subjected to thermal analysis under inert desorption conditions. Resultant infrared and mass spectrometric data were easily utilized to identify the desorption of the butyric acid groups across a narrow temperature range and we were able to calculate the degree of substitution of the attached acid groups within the nanotube backbone as 1.7 carbon atoms per hundred, in very good agreement with independent analytical measurements made by inductively coupled plasma optical emission spectrometry (ICP-OES). The thermal analysis technique was also able to discern the presence of secondary functional moieties on the nanotube samples that were not accessible by ICP-OES. This work demonstrates the potential of this technique for assessing the presence of multiple and diverse functional addends on the nanotube sidewalls, beyond just the principal groups targeted by the specific functionalization reaction. Figure3D contour map of the FTIR spectra of the species desorbed from the GAP-functionalized SWCNT sample as a function of temperature.

About the solubility of reduced SWCNT in DMSO

Single-walled carbon nanotubes (SWCNT) have been reduced with sodium naphthalide in THF. The reduced SWCNT are not only soluble in dimethylsulfoxide (DMSO) to form a stable solution/suspension, but also react spontaneously at room temperature with DMSO to evolve hydrocarbon gases and are converted into functionalized SWCNT. The degree of functionalization is about 2C% and the addends are mainly methyl and small oxygen-containing hydrocarbons. The functionalized SWCNT are apparently more soluble and stable in DMSO solution. It may open a new era for further processing and applications.