Electrowetting of aligned carbon nanotube films

Electrowetting is one approach to reducing the interfacial tension between a solid and a liquid. In this method, an electrical potential is applied across the solid/liquid interface which modifies the wetting properties of the liquid on the solid without changing the composition of the solid and liquid phases. Electrowetting of aligned carbon nanotube (CNT) films is investigated by the sessile drop method by dispensing deionized (DI) water or 0.03 M NaCl droplets (contacted by Au wire) onto aligned CNT films assembled on a copper substrate. The results demonstrate that electrowetting can greatly reduce the hydrophobicity of the aligned CNTs; the contact angle saturation for DI water and 0.03 M NaCl droplets occurs at 98 and 50 degrees , respectively. The combined effects of the geometrical roughness and the electrical potential on the contact angle are briefly discussed and modeled. Such a strategy may be invoked to controllably reduce the interfacial tension between carbon nanotubes (CNTs) and polymer precursors when infiltrating the monomers into the prealigned nanotube films.     

Control over the wettability of an aligned carbon nanotube film

Three-dimensional anisotropic aligned carbon nanotube microstructures were constructed by the chemical vapor deposition method on silicon templates with well-defined structure. It brought about new properties of wettability. Superhydrophobic (contact angle > 150 degrees ) and very hydrophilic (contact angle < 30 degrees ) properties can both be achieved by a simple change of structural parameter.     

Large-scale fabrication of aligned single-walled carbon nanotube array and hierarchical single-walled carbon nanotube assembly

Aligned single-walled carbon nanotubes (SWNTs) and hierarchical SWNT assembly were fabricated by electrospinning. The high fiber elongation and high DC electric field applied during the electrospinning process result in the orientation of the SWNTs along the axial direction of the fiber. The alignment of the electropsun composite fiber transfers this local SWNT orientation to macroscopically aligned SWNTs. After removing the polymer component from the aligned composite fiber, we produced large area aligned SWNTs. The results show that the directional control of SWNT alignment and debundling of SWNTs into individual tubes can be simultaneously realized.     

Tailoring the wettability of TiO2 nanotube layers

Self-organized TiO₂ nanotube layers were grown on Ti by electrochemical anodization. As prepared, these layers showed a super-hydrophilic wetting behaviour. When modified with organic molecules, octadecylsilane (C₁₈H₃₇SiH₃) or octadecylphosphonic acid (C₁₈H₃₇PO(OH)₂), the layers show a super-hydrophobic behaviour. We demonstrate how the tubular geometry of the TiO₂ layers combined with UV induced decomposition of the organic monolayers (SAM) can be used to adjust the surface wetting properties to any desired degree from super-hydrophobic to super-hydrophilic.     

Covalent attachment and hybridization of DNA oligonucleotides on patterned single-walled carbon nanotube films

DNA oligonucleotides were covalently immobilized to prepatterned single-walled carbon nanotube (SWNT) multilayer films by amidation. SWNT multilayer films were constructed via consecutive condensation reactions creating stacks of functionalized SWNT layers linked together by 4,4'-oxydianiline. Aminated- or carboxylated-DNA oligonucleotides were covalently immobilized to the respective carboxylated or aminated SWNT multilayer films through amide bond formation using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride. UV-vis-NIR spectroscopic analysis indicated that the SWNT film surface density increased uniformly according to the number of reaction cycles. Scanning electron microscopy and contact angle measurements of the SWNT multilayer film revealed a uniform coverage over the substrate surface. The covalent attachment of DNA oligonucleotides to the SWNT multilayer films and their subsequent hybridization with complementary oligonucleotides were verified using X-ray photoelectron spectroscopy and fluorescence-based measurements. This is the first report demonstrating that DNA oligonucleotides can be covalently attached to immobilized SWNT multilayer films. The anchored DNA oligonucleotides were shown to exhibit excellent specificity, realizing their potential in future biosensor applications.     

Single-walled carbon nanotube combing during layer-by-layer assembly: from random adsorption to aligned composites

Oriented SWNTs in polymer composites have shown dramatic improvements in the physical properties of a composite because of the anisotropic shape and properties of SWNTs. Controlled alignment of SWNTs during composite fabrication implies better material function performance. This letter reports a new fabrication technique whereby aligned SWNTs and robust SWNT-polymer composites can be made using a fusion method of SWNT combing and layer-by-layer (LBL) assembly. As we previously reported, LBL assembly demonstrated exceptional processing ability in constructing the uniform distribution of a SWNT-polymer composite. Combined with this uniformity, this SWNT combing technique endows controlled alignment of single-stranded SWNTs in a SWNT-polymer composite system. SWNT combing employs air-water interfacial forces to change the molecular topography from the random adsorption state to the stretched alignment of SWNTs. More specifically, air-water interfacial forces are associated with an excess viscous drag force and an intrinsic dewetting rate along SWNTs. Moreover, the alignment efficiency of SWNTs is high enough to construct a multilayered LBL film with horizontal-linear weaving structures. This simple method also can be applied for aligning other nanowire materials because it utilizes simple geometric features of SWNTs.     

Solution-assisted assembly of organic semiconducting single crystals on surfaces with patterned wettability

Two efficient approaches to assembling organic semiconducting single crystals are described. The methods rely on solvent wetting and dewetting on substrates with patterned wettability to selectively direct the deposition or removal of organic crystals. Substrates were functionalized with different self-assembled monolayers (SAMs) to achieve the desired wettabilities. The assembly of different organic crystals over centimeter-squared areas on Au, SiO2, and flexible plastic substrates was demonstrated. By designing line features on the substrate, the alignment of crystals, such as CuPc needles, was also achieved. As a demonstration of the potential application of this assembly approach, arrays of single-crystal organic field-effect transistors were fabricated by patterning organic single crystals directly onto and between transistor source and drain electrodes.     

Protein electrochemistry using aligned carbon nanotube arrays

The remarkable electrocatalytic properties and small size of carbon nanotubes make them ideal for achieving direct electron transfer to proteins, important in understanding their redox properties and in the development of biosensors. Here, we report shortened SWNTs can be aligned normal to an electrode by self-assembly and act as molecular wires to allow electrical communication between the underlying electrode and redox proteins covalently attached to the ends of the SWNTs, in this case, microperoxidase MP-11. The efficiency of the electron transfer through the SWNTs is demonstrated by electrodes modified with tubes cut to different lengths having the same electron-transfer rate constant.     

Creating patterned carbon nanotube catalysts through the microcontact printing of block copolymer micellar thin films

We report a route for synthesizing patterned carbon nanotube (CNT) catalysts through the microcontact printing of iron-loaded poly(styrene-block-acrylic acid) (PS-b-PAA) micellar solutions onto silicon wafers coated with thin aluminum oxide (Al(2)O(3)) layers. The amphiphilic block copolymer, PS-b-PAA, forms spherical micelles in toluene that can form quasi-hexagonal arrays of spherical PAA domains within a PS matrix when deposited onto a substrate. In this report, we dip a poly(dimethylsiloxane) (PDMS) molded stamp into an iron-loaded micellar solution to create a thin film on the PDMS features. The PDMS stamp is then put in contact with a substrate, and uniaxial compressive stress is applied to transfer the micellar thin film from the PDMS stamp onto the substrate in a defined pattern. The polymer is then removed by oxygen plasma etching to leave a patterned iron oxide nanocluster array on the substrate. Using these catalysts, we achieve patterned vertical growth of multiwalled CNTs, where the CNTs maintain the fidelity of the patterned catalyst, forming high-aspect-ratio standing structures.     

Electrochemical modification of vertically aligned carbon nanotube arrays

Electrochemical oxidation and reduction were utilized to modify vertically aligned carbon nanotube (CNT) arrays grown on a porous network of conductive carbon microfibers. Ultrafast and complete CNT opening and purification were achieved through electrochemical oxidation. Highly dispersed platinum nanoparticles were then uniformly and densely deposited as electrocatalysts onto the surface of these CNTs through electrochemical reduction. Using supercritical drying techniques, we demonstrate that the unidirectionally aligned and laterally spaced geometry of the CNT arrays can be fully retained after being subjected to each step of electrochemical modification. The open-tipped CNTs can also be electrochemically detached in full lengths from the supporting substrates and harvested if needed.     

Gold nanoparticle mediated formation of aligned nanotube composite films

We report on the formation of highly anisotropic nanotube composite materials, made by the attachment of gold nanoparticles to the surface of the single-walled carbon nanotubes, followed by preparation of an aligned composite film by compression in a Langmuir-Blodgett trough. The gold is attached in a one-step sonication procedure. The gold-modified nanotube material forms a stable suspension in toluene and has been characterized by atomic force and scanning force microscopy, energy-dispersive X-ray spectroscopy, and Raman spectroscopy. The aligned films have highly anisotropic electrical properties, with a factor of approximately 3000 difference in the conductivity between the aligned and perpendicular directions.     

Room-temperature assembly of directional carbon nanotube strings

Micromold with microchannels was employed in assembly of directional free-standing single-walled carbon nanotube (SWNT) strings at room temperature. The new postgrowth assembly approach could, in principle, apply not only to a wide range of SWNTs in their soluble or dispersible forms, including small diameter (0.7-0.8 nm) SWNTs, covalent- and noncovalent-functionalized SWNTs, monodispersed SWNTs with identical diameter and chirality, and fullerenes@SWNTs, which either cannot survive the high-temperature treatment or cannot be synthesized by current CVD method, but also to other soluble or dispersible one-dimensional nanostructures.     

Multifunctional chemical vapor sensors of aligned carbon nanotube and polymer composites

Partially coating perpendicularly aligned carbon nanotube arrays with an appropriate polymer thin film along their tube length provides a novel concept for developing new sensors of high sensitivity, good selectivity, and excellent environmental stability for the detection of a broad class of chemical vapors with low power consumption. The absorption and desorption of chemical vapors by the polymer matrix cause changes in the inter-tube distance and, hence, the surface resistance across the nanotube film. Simple measurements of the resistance change, therefore, constitute the nanotube-polymer chemical vapor sensors. These rationally designed, aligned carbon nanotube-polymer composite films are flexible and can be effectively integrated into many systems for a wide range of potential applications, including their use as multifunctional sensors for sensing chemical vapors, mechanical deformations, thermal and optical exposures.     

Construction of various nanostructures on carbon nanotube films

Construction of nanostructures on surfaces has appealed intensive attention due to its significant applications in diverse fields. Especially, engineering surface properties via surficial nanostructures is actually the creation of functional interface-based materials and slated to be the key aspect for the future of materials science. Although many efforts have been made, there are only a few reports about the construction of nanostructures on carbon nanotube film surfaces. The big challenge for constructing on carbon films is that these carbon assemblies are easy to be dispersed by immersion in a chemical solution. Here, in this paper, we have shown for the first time the fabrication of different kinds of nanostructures, i.e. nanoneedles, nanoparticles, nanospirals, on carbon nanotube films by using facile and cheap electrodeposition method and precise physical deposition method. We pretreat the films by an electrical method to strengthen the films to avoid dispersion during the electrodeposition process. These composite films are still very flexible after coating with nanostructures. Compared with those precise physical deposition methods, the facile electrodeposition method is more suitable for constructing nanostructures on carbon nanotube films, due to the low requirement for planeness of films. It is interesting to find that these nanostructures can endow superhydrophobicity or higher conductivity for these flexible composite films, which greatly broaden the potential applications for carbon nanotube films in the fields of battery, moisture self-cleaning, electrostatic energy harvesting, and enhancing condensation heat transfer for more efficiency of energy utilization, environmental, and thermal management.     

High capacity vertical aligned carbon nanotube/sulfur composite cathodes for lithium-sulfur batteries

Binder free vertical aligned (VA) CNT/sulfur composite electrodes with high sulfur loadings up to 70 wt% were synthesized delivering discharge capacities higher than 800 mAh g(-1) of the total composite electrode mass.     

Charging and discharging of the electrochemically swelled,aligned carbon nanotube fibers

Aligned carbon nanotube fibers are macroscopic materials with remarkable properties, such as high specific strength, stiffness, extreme flexibility as well as electrical and thermal conductivity. It is demonstrated that when subjected to negative potentials, these structures undergo the process of swelling in which the increase of their external dimension is observed. Swelling is believed to be caused by cation insertion in the process similar to intercalation. The efficiency of swelling was determined both in organic and aqueous solutions of different pH. Chronocoulometry was used as the technique to monitor the charging-discharging processes of swollen ACNT fibers in a presence of different electrolytes, i.e. LiCl, NaCl and KCl. The possibility of performing the charging-discharging cycles multiple times indicates that the swollen ACNT fibers can be considered as an advantageous material for electrodes in ion batteries and supercapacitors.     

Vertically aligned carbon nanotube based electrodes: Fabrication, characterisation and prospects

We present a methodology to fabricate carbon nanotube based electrodes using plasma enhanced chemical vapour deposition. The metal catalyst nanoparticles used to promote nanotube growth are removed using a water plasma treatment in combination with an acid attack. The final integrated microelectrode-based devices present excellent electrocatalytic properties that make them suitable for electrochemical applications. The presented methodology enables the construction of highly regular and dense vertically aligned carbon nanotube (VACNT) forests that can be confined within the patterned bounds of a desired surface. These VACNT electrodes display very low capacitive currents and are amenable to further chemical modifications.     

Molecular beam-controlled nucleation and growth of vertically aligned single-wall carbon nanotube arrays

The main obstacle to widespread application of single-wall carbon nanotubes is the lack of reproducible synthesis methods of pure material. We describe a new growth method for single-wall carbon nanotubes that uses molecular beams of precursor gases that impinge on a heated substrate coated with a catalyst thin film. In this growth environment the gas and the substrate temperature are decoupled and carbon nanotube growth occurs by surface reactions without contribution from homogeneous gas-phase reactions. This controlled reaction environment revealed that SWCNT growth is a complex multicomponent reaction in which not just C, but also H, and O play a critical role. These experiments identified acetylene as a prolific direct building block for carbon network formation that is an order of magnitude more efficient than other small-molecule precursors. The molecular jet experiments show that with optimal catalyst particle size the incidence rate of acetylene molecules plays a critical role in the formation of single-wall carbon nanotubes and dense vertically aligned arrays in which they are the dominant component. The threshold for vertically aligned growth, the growth rate, the diameter, and the number of walls of the carbon nanotubes are systematically correlated with the acetylene incidence rate and the substrate temperature.