Aqueous Suspensions Of Carbon Nanotubes Research Articles

A Review of Spectral Methods for Dispersion Characterization of Carbon Nanotubes in Aqueous Suspensions

Characterization is a crucial step in the study of properties of nanomaterials to evaluate their full potential in applications. Carbon nanotube-based materials have properties that are sensitive to size, shape, concentration, and agglomeration state. It is therefore critical to quantitatively characterize these factors in situ, while the processing takes place. Traditional characterization techniques that rely on microscopy are often time consuming and in most cases provide qualitative results. Spectroscopy has been studied as an alternative tool for identifying, characterizing, and studying these materials in situ and in a quantitative way. In this paper, we provide a critical review of the spectroscopy techniques used to explore the surface properties (e.g., dispersion) characteristics of carbon nanotubes in aqueous suspensions during the sonication process.

The photochemistry of carbon nanotubes and its impact on the photo-degradation of dye pollutants in aqueous solutions

It is reported that carbon nanotubes (CNTs) could either generate reactive oxygen species (ROS) under light irradiation or serve as high-efficient scavenger for ROS. However, it is unclear which role predominates as CNTs enter into aquatic environment. To answer this question, a systematic study of the photochemistry of a pristine and a surface-functionalized CNTs in aqueous suspensions was investigated with both time-resolved and steady state analytical approaches. The transient absorption spectra demonstrate that CNTs could be photo-ionized and trap hydrated electrons upon high energy irradiation. In steady state UV irradiation, CNTs could promote the generation of ROS, such as 1O2 and OH. However, in the presence of H2O2, the OH scavenging effect predominated in the aqueous suspensions of CNTs. The presence of CNTs suppressed the photo-degradation of dye pollutants, as an integrated result of inner filter effect, adsorption effect, and ROS generation and scavenging effect. The results provide useful information for the understanding of the environmental implications of CNTs.

Direct growth of layered double hydroxides in an aqueous suspension of carbon nanotubes and their application as fillers in polypropylene

Abstract Interesting composites with three-dimensional (3D) structure that combine one-dimensional (1D) carbon nanotubes (CNTs) with two-dimensional (2D) lamellar flakes show excellent potential properties for application in polymer matrices. Herein, we describe the construction of a composite of CNTs and 2D flakes by in situ growth of layered double hydroxides (LDH) in CNTs aqueous suspensions. It has been found that the morphology of the LDH/CNTs composites can be easily tuned by changing the CNTs concentration ( C CNTs ) and the LDH growth time. The introduction of CNTs did not severely perturb the main crystalline structure of LDH. A plausible model for the in situ growth mechanism is proposed to describe the temporal evolution of the LDH morphology as well as the influence of C CNTs on the morphology of the LDH/CNTs composites. The as-obtained LDH/CNTs 3D composites with interlayer dodecyl-benzenesulfonate anions have proved to be efficient fillers, significantly improving the mechanical properties and thermal stability of polypropylene (PP).

Separation of dispersed carbon nanotubes from water: Effect of pH and surfactants on the aggregation at oil/water interface

With the extensive use of carbon nanotubes (CNTs), the environmental health risk raised by this nanomaterial is concerned. Dispersed CNTs in aqueous suspension greatly increase the possibility of human and ecological exposures, but little attention has been paid to the decontamination/remediation of CNT pollution. In this study, an oil/water interface aggregation (OWIA) method was proposed for the decontamination of CNTs aqueous suspension. Tuning the water pH to suitable ranges (⩽1.8 or ⩾13.2) or adding cationic surfactant CTAB (0.075–1mM) can cause the aggregation of CNTs at the oil/water interface, which allows the subsequent separation of CNTs from the liquids. Through the OWIA process, more than 99% of the CNTs were removed from water, and the remnant CNTs in water were less than 1mg/L. Adding surfactant or extreme pHs may change the charged status of CNTs’ surface, and induced the aggregation of CNTs when the repulsion force between individual CNTs diminished. The cationic surfactant CTAB molecules have different interaction models with dispersed CNTs, depending on the concentrations of CTAB. This study provides a feasible approach for future decontamination of CNTs or other nanoparticles from aqueous environments.

Controlled Aggregation of Biopolymer‐Wrapped Carbon Nanotubes in Aqueous Suspension, Induced by Cationic Porphyrin

SummaryUnusual aggregation of biopolymer‐wrapped single‐walled carbon nanotubes (SWCNTs) induced by cationic porphyrin meso‐tetrakis(4‐N‐methyl‐pyridyl)porphine (TMPyP4) in aqueous suspension has been studied by absorption spectroscopy and molecular dynamics simulation. After the addition of a small dose of TMPyP4 (10−6 M) into poly(rC):SWCNTs aqueous suspension, nanotubes begin to aggregate, but the aggregates are stable without precipitation during a few days. Spectral observations indicate that the porphyrin core stacked with π‐systems of the biopolymer:SWCNT hybrid and all nanotube species take part in this aggregate formation without any selection. Molecular dynamic simulation demonstrates that the stable complex consisting of two nanotubes can be formed when TMPyP4 molecules bound nanotubes both owing to π‐π stacking of the porphyrin core with the surface of one nanotube and with the oligonucleotide adsorbed onto another nanotube. TMPyP4 molecules not only couple the biopolymer:nanotube hybrids, but also they stitch polymer's strands wrapping around each nanotube resulting in continuous floatation of aggregates in water. It was shown that the aggregate is stable even at 373 K. At this temperature additional contacts between two neighbor's polymers appear which strengthen this aggregate too. The controlled aggregation of biopolymer‐wrapped carbon nanotubes can be employed at the fabrication of multifunctional supramolecular assemblies, photovoltaic cells and in biosensing.

Enhanced Solubilization of Carbon Nanotubes in Aqueous Suspensions of Anionic–Nonionic Surfactant Mixtures

Single-walled carbon nanotubes (SWCNTs) suspensions in aqueous solutions of sodium dodecyl sulfate (SDS) and saturated fatty acids (Cn) are studied. The quality of the dispersions is analyzed by photoluminescence spectroscopy (PL) as a function of the Cn chain length. Resonant Raman scattering (RRS) measurements and molecular dynamics (MD) simulations were also carried out in order to study the effect of the surrounding medium on SWCNTs properties in suspensions. Both PL and RRS data indicate an increased individualization of SWCNTs in the dispersions for Cn’s having an alkyl chain longer than SDS. MD simulations showed the formation of mixed Cn-SDS aggregates around a nanotube in water and a Cn binding energy to the nanotube wall that increases linearly with chain length. The enhanced solubilization of SWCNTs is thus interpreted in terms of the reduced electrostatic repulsion within the surfactant aggregates and the increased binding energy to the nanotube wall. Powders prepared by the evaporation of dis...

Synthesis of Porous Materials and Their Microstructural Control through Ice Templating

Ice-templating is a simple, practical kind of template synthesis which consists of sol-gel polycondensation, unidirectional freezing of hydrosols or hydrogels, and pore-preserving drying method, such as freeze drying, thereby resulting in desirable porous materials. The unidirectional freezing may be achieved by either immersion freezing or contact freezing. Freeze-dried materials contain porous microstructures that are replicas of the ice crystals formed during the unidirectional freezing process. Two similar but slightly different applications of ice-templating will be investigated here. Fabrication of macroporous foam materials by unidirectional freezing of an aqueous suspension of carbon nanotubes (CNTs) dispersed by chitosan is a concrete example of functionalization of an electroconductive foam, which is a composite material made from a polymer and CNTs. As expected, their electroconductive properties can be controlled by the fabrication method and condition. Additional examples of porous materials synthesized by ice-templating are porous microfibers and microhoneycombs of silica, titania, silica-alumina, titania-silica, and even carbon. In the preparation, the morphology and porous structure can be controlled by the freezing conditions and synthesis conditions of the corresponding hydrogels, respectively.

Experimental Investigation of Rheological Behavior and Pressure Drop of Aqueous Suspensions of Carbon Nanotubes in a Horizontal Tube

A new engineering medium, called nanofluid, have attracted a wide range of researches on many processes in engineering applications. Many experimental and theoretical studies on viscosity of nanofluids are controversial and the shear rate dependent viscosity is frequently neglected. The aim of the present work is to investigate the effect of the rheological behavior of aqueous suspensions of carbon nanotubes on the friction factor and the pressure drop in a horizontal tube under laminar and turbulent flow. The particles weight concentration used here is 0.01%. The rheological behavior was measured for a range of temperature from 0 °C to 40 °C. The results showed that the nanofluids exhibit a shear thinning behavior at very low shear stress. The analysis results of the flow characteristics showed a non-Newtonian behavior under laminar flow and a Newtonian behavior under turbulent flow.

Effects of antifreezes and bundled material on the stability and optical limiting in aqueous suspensions of carbon nanotubes

AbstractThis work gives data on the stability of dispersions of single wall carbon nanotubes stabilized by sodium dodecylbenzenesulfonate in binary polar solvents “water + antifreeze” (glycerol, polyethyleneglycole) with eutectic compositions. The absorption spectra of the suspensions demonstrate no changes during 1‐year storage with temperature spanning from −40 to +40 °C. The systems provide relevant optical power limiting properties, the one with glycerol showing good resistance to optical bleaching effects. We also demonstrate that aqueous dispersions of nanotubes exhibit considerable enhancement of optical limiting parameters alongside an increase of the bundled material populace.

Flexible, transparent electrodes using carbon nanotubes

We prepare thin single-walled carbon nanotube networks on a transparent and flexible substrate with different densities, using a very simple spray method. We measure the electric impedance at different frequencies Z(f) in the frequency range of 40 Hz to 20 GHz using two different methods: a two-probe method in the range up to 110 MHz and a coaxial (Corbino) method in the range of 10 MHz to 20 GHz. We measure the optical absorption and electrical conductivity in order to optimize the conditions for obtaining optimum performance films with both high electrical conductivity and transparency. We observe a square resistance of 1 to 8.5 kΩ for samples showing 65% to 85% optical transmittance, respectively. For some applications, we need flexibility and not transparency: for this purpose, we deposit a thick film of single-walled carbon nanotubes on a flexible silicone substrate by spray method from an aqueous suspension of carbon nanotubes in a surfactant (sodium dodecyl sulphate), thereby obtaining a flexible conducting electrode showing an electrical resistance as low as 200 Ω/sq. When stretching up to 10% and 20%, the electrical resistance increases slightly, recovering the initial value for small elongations up to 10%. We analyze the stretched and unstretched samples by Raman spectroscopy and observe that the breathing mode on the Raman spectra is highly sensitive to stretching. The high-energy Raman modes do not change, which indicates that no defects are introduced when stretching. Using this method, flexible conducting films that may be transparent are obtained just by employing a very simple spray method and can be deposited on any type or shape of surface.

Dispersion and aspect ratio of carbon nanotubes in aqueous suspension and their relationship with electrical resistivity of carbon nanotube filled polymer composites

Multiwalled carbon nanotube (MWCNT)/acrylonitrile butadiene styrene (ABS) composites were prepared by a processing method using solvent–nonsolvent precipitation. Size distributions of MWCNT agglomerates in aqueous suspension were investigated in order to predict aspect ratio of nanotubes by evaluating the effects of sonication time, MWCNT content, and surfactant. Aspect ratios of MWCNTs were predicted on the basis of the size distribution measurements for MWCNT agglomerates. Sonication time or applied sonic energy has a strong effect on the size distribution of MWCNT agglomerates. Compared with simple shear mixing method, it was shown that this processing method is more suitable for the MWCNT/ABS composites. An electrical percolation threshold was observed for the weight fraction of MWCNTs in the range of about 0.5–1.0wt.%. Shorter MWCNTs are more suitable to induce fine dispersion, but lead to higher percolation threshold weight fraction. It was illustrated that fine dispersion can overcome the handicap of short length or low aspect ratio of MWCNTs.

Unusual aggregation of poly(rC)-wrapped carbon nanotubes in aqueous suspension induced by cationic porphyrin

Unusual aggregation of biopolymer-wrapped single-walled carbon nanotubes (SWCNTs) induced by cationic porphyrin meso-tetrakis(4-N-methyl-pyridyl)porphine (TMPyP4) in aqueous suspension has been studied by absorption spectroscopy and molecular dynamics simulation. After the addition of a small dose of TMPyP4 (10−6 M) into a poly(rC)–SWCNTs suspension, the absorption intensity of the nanotubes decreases, SWCNT bands demonstrate a red-shift and visible aggregation of SWCNTs is observed at concentrations above 10−5 M. However, formed aggregates are stable in suspension without precipitation for a few days. The Soret band of TMPyP4 in the absorption spectra exhibits hypochromism which is indicative of the π–π stacking interaction of the porphyrin core with π-systems of the biopolymer–SWCNT hybrid. The aggregation process of poly(rC)–SWCNT affects the spectral position of the Soret band, which is red-shifted. The spectral shift depends on TMPyP4 concentration: 18 nm at low porphyrin concentrations (10−7–10−6 M) and 14 nm at concentrations higher than 10−6 M. Such behavior of the Soret band is interpreted in terms of different types of porphyrin binding with poly(rC)–SWCNT resulting in effective nanotube aggregation. The molecular dynamics simulation is in good agreement with the experiment and demonstrates that the most stable complex, consisting of two nanotubes, can be formed when TMPyP4 molecules bind nanotubes, both owing to π–π stacking of the porphyrin core with the surface of one nanotube and with the oligonucleotide adsorbed onto another nanotube. The electrostatic interaction between porphyrin positively charged groups and the negative phosphate backbone of the oligonucleotide increases the stability of this complex as well.

Biodistribution of Aqueous Suspensions of Carbon Nanotubes in Mice and Their Biocompatibility

In this study, we prepared two-types of water-dispersible carbon nanotubes (CNTs) and investigated their biodistribution in mice as well as bio-/cyto-compatibility. After administration, their organs were excised at various post-injection times, then observed using both optical and transmission electron microscopy (TEM). The color of the liver and lung markedly darkened, suggesting that administered CNTs reached these organs. By TEM observation, the CNTs were found in the liver and lung. They were observed even in the kidney and spleen, though their distributions in those organs were very low compared with that in liver and lung. Therefore, most of the administered CNTs would be accumulated in the liver or lung. However, the time profile of the body weight of CNT-administered mice was close to that of control mice. In addition, we estimated the cytocompatibility of the water-dispersible CNTs for hepatocytes. According to a TNF-alpha assay of the cells cultured with CNTs, the expression level was almost the same as that of the control. These results suggested that the water-dispersible CNTs have good bio-/cyto-compatibility under this condition.

Stability and thermal conductivity enhancement of carbon nanotube nanofluid using gum arabic

This experimental study reports on the stability and thermal conductivity enhancement of carbon nanotubes (CNTs) nanofluids with and without gum arabic (GA). The stability of CNT in the presence of GA dispersant in water is systematically investigated by taking into account the combined effect of various parameters, such as sonication time, temperature, dispersant and particle concentration. The concentrations of CNT and GA have been varied from 0.01 to 0.1 wt% and from 0.25 to 5 wt%, respectively, and the sonication time has been varied in between 1 and 24 h. The stability of nanofluid is measured in terms of CNT concentration as a function of sediment time using UV-Vis spectrophotometer. Thermal conductivity of CNT nanofluids is measured using KD-2 prothermal conductivity meter from 25 to 60°C. Optimum GA concentration is obtained for the entire range of CNT concentration and 1–2.5 wt% of GA is found to be sufficient to stabilise all CNT range in water. Rapid sedimentation of CNTs is observed at higher GA concentration and sonication time. CNT in aqueous suspensions show strong tendency to aggregation and networking into clusters. Stability and thermal conductivity enhancement of CNT nanofluids have been presented to provide a heat transport medium capable of achieving high heat conductivity. Increase in CNT concentrations resulted in the non-linear thermal conductivity enhancement. More than 100–250% enhancement in thermal conductivity is observed for the range of CNT concentration and temperature.

Adjustment of the nanosecond laser pulse duration by using a carbon nanotube suspension

It is shown that the scanning of an optical cell with an aqueous suspension of carbon nanotubes along the optical axis of a focused laser beam leads to a smooth change in the duration of nanosecond laser pulses transmitted through the suspension. Pulses shorten owing to the pulse trailing edge cutoff, when the cell approaches the beam waist. In experiments, pulses of a YAG:Nd3+ laser with passive Q-switching and duration of 16 ns were continuously controlled in the range 16–10 ns.

Sonication Power for Length Control of Single-Walled Carbon Nanotubes in Aqueous Suspensions Used for 2-Dimensional Network Formation

Control over the average length of single-walled carbon nanotubes (SWNTs) in suspension is of critical importance to characterizing and developing flexible, transparent thin films that use percolative transport to achieve reproducibility in electronic properties. This paper demonstrates how the average length of SWNTs in aqueous suspensions can be controlled by the conditions used to form (sonication) and purify (low-G centrifugation) the dispersions. The effect of ultrasonic probe sonication, which was used to disperse SWNT bundles into suspension, on the length and extent of defects on the nanotubes was investigated via atomic force microscopy (AFM) and confocal Raman spectroscopy, respectively. Quantitative information about the suspension concentration and the effect of sonication power on unbundling the SWNTs was obtained via UV−vis and near-IR spectroscopy, respectively. To obtain a clear understanding of the effect of sonication power on SWNT suspensions, repeated low-G centrifugation cycles were u...

Shapes of laser radiation pulses modified by nonlinear scattering in aqueous suspension of carbon nanotubes

An improved scheme of z-scanning was used to study the parameters of nanosecond 1064-nm laser radiation pulses scattered at right angle under the conditions of optical limiting in an aqueous suspension of purified carbon nanotubes (CNTs). CNTs were synthesized by the electric-arc evaporation of graphite. It is established that the amplitude, shape, duration, and temporal position of the peak of scattered light pulses significantly depend on the laser radiation power density. The results agree with the mechanism of thermoinduced nonlinear scattering that is operative during the optical limiting of laser pulses in CNT suspensions.

Length Distribution of Single‐Walled Carbon Nanotubes in Aqueous Suspension Measured by Electrospray Differential Mobility Analysis

The first characterization of the length distribution of single-walled carbon nanotubes (SWCNT) dispersed in a liquid by electrospray differential mobility analysis (ES-DMA) is presented. Although an understanding of geometric properties of SWCNTs, including length, diameter, aspect ratio, and chirality, is essential for commercial applications, rapid characterization of nanotube length distributions remains challenging. Here the use of ES-DMA to obtain length distributions of DNA-wrapped SWCNTs dispersed in aqueous solutions is demonstrated. Lengths measured by ES-DMA compare favorably with those obtained from multiangle light scattering, dynamic light scattering, field flow fractionation with UV/vis detection, and atomic force microscopy, validating ES-DMA as a technique to measure SWCNTs of <250 nm in length. The nanotubes are previously purified and dispersed by wrapping with oligomeric DNA in aqueous solution and centrifuging to remove bundles and amorphous carbon. These dispersions are particularly attractive due to their amenability to bulk processing, ease of storage, high concentration, compatibility with biological and high-throughput manufacturing environments, and for their potential applications ranging from electronics and hydrogen-storage vessels to anticancer agents.

Broadband light emission from optically-trapped carbon nanotubes

Optical micromanipulation of carbon nanotubes (CNT), that has hitherto been elusive, has been achieved in an aqueous suspension of CNTs in an optical tweezers setup, via the mediation of microbubbles. These microbubbles are formed when CNT bundles are brought to the focal volume. With the bubbles themselves being easily trappable in the tweezers, the CNTs adhered to the surface of the microbubbles can now be held immobilised. CNTs held in the optical trap focus are found to emit bright light in the visible, which may be a sustained glow, or an uncontrolled flare. These observations and the underlying mechanisms will be discussed in the talk.

Thermal Conductivity Measurement for Carbon-Nanotube Suspensions with 3ω Method

The stable and homogeneneous aqueous suspension of carbon nanotubes was prepared in this study. The stability of the nanofluids was improved greatly due to the use of a new dispersant, humic acid. The thermal conductivity of the aqueous suspension was measured with the 3ω method. The experimental results showed that the thermal conductivity of the suspensions increases with the temperature and also is nearly proportional to the loading of the nanoparticles. The thermal conductivity enhancement of single-walled carbon nanotubes (SWNTs) suspensions is better than that of the multi-walled carbon nanotubes (MWNTs) suspensions. Especially for a volume fraction of 0.3846% SWNTs, the thermal conductivity is enhanced by 40.5%. Furthermore, the results at 30°C match well with Jang and Choi’s model.