Immobilization of carbon nanotubes on functionalized graphene film grown by chemical vapor deposition and characterization of the hybrid material

We report the surface functionalization of graphene films grown by chemical vapor deposition and fabrication of a hybrid material combining multi-walled carbon nanotubes and graphene (CNT–G). Amine-terminated self-assembled monolayers were prepared on graphene by the UV-modification of oxidized groups introduced onto the film surface. Amine-termination led to effective interaction with functionalized CNTs to assemble a CNT–G hybrid through covalent bonding. Characterization clearly showed no defects of the graphene film after the immobilization reaction with CNT. In addition, the hybrid graphene material revealed a distinctive CNT–G structure and p–n type electrical properties. The introduction of functional groups on the graphene film surface and fabrication of CNT–G hybrids with the present technique could provide an efficient, novel route to device fabrication.     

Influence of Cu nanoparticle size on the photo-electrochemical response from Cu-multiwall carbon nanotube composites

We show that Cu metal nanoparticle-multiwall carbon nanotube (MWCNT) assemblies can act as a new hybrid photoactive layer in photo-electrochemical devices. The carbon nanotube (CNT) composites were formed by a controlled thermal deposition of copper which produced crystalline metal nanoparticles localized on the carbon tube outer walls. The photoresponse evaluated in terms of IPCE (incident photon-to-charge carrier generation efficiency) varied for different sized-Cu-MWCNT samples across all the visible and near ultraviolet photon energy range with respect to the response of bare MWCNTs. In the case of 0.2 nm Cu nominal thickness, the IPCE increased, reaching 15%, a value 2.5 times higher than that measured for bare MWCNTs. As the Cu nominal coverage thickened, the IPCE started to decrease and become totally ineffective after 1 nm deposited Cu. The IPCE increase found was interpreted as being the result of a remarkable charge transfer between the Cu metal nanoparticles and the CNTs due to the formation of a strong ionic bond at their interface. The results obtained prove that the metal nanoparticle-CNT composites have optical, electrical and structural properties that can be applied in a variety of nanoscale architectures for novel photo-electrochemical devices.     

Two-dimensional distribution of carbon nanotubes in copper flake powders

We report an approach of flake powder metallurgy to the uniform, two-dimensional (2D) distribution of carbon nanotubes (CNTs) in Cu flake powders. It consists of the preparation of Cu flakes by ball milling in an imidazoline derivative (IMD) aqueous solution, surface modification of Cu flakes with polyvinyl alcohol (PVA) hydrosol and adsorption of CNTs from a CNT aqueous suspension. During ball milling, a hydrophobic monolayer of IMD is adsorbed on the surface of the Cu flakes, on top of which a hydrophilic PVA film is adsorbed subsequently. This PVA film could further interact with the carboxyl-group functionalized CNTs and act to lock the CNTs onto the surfaces of the Cu flakes. The CNT volume fraction is controlled easily by adjusting the concentration/volume of CNT aqueous suspension and Cu flake thickness. The as-prepared CNT/Cu composite flakes will serve as suitable building blocks for the self-assembly of CNT/Cu laminated composites that enable the full potential of 2D distributed CNTs to achieve high thermal conductivity.     

Aligned graphene nanoribbons and crossbars from unzipped carbon nanotubes

Aligned graphene nanoribbon (GNR) arrays have been made by unzipping of aligned single-walled and few-walled carbon nanotube (CNT) arrays. Nanotube unzipping was achieved by a polymer-protected Ar plasma etching method, and the resulting nanoribbon array can be transferred onto any chosen substrate. Atomic force microscope (AFM) imaging and Raman mapping on the same CNTs before and after unzipping confirmed that ˜80% of CNTs were opened up to form single layer sub-10 nm GNRs. Electrical devices made from the GNRs (after annealing in H₂ at high temperature) showed on/off current (I _on/I _off) ratios up to 10³ at room temperature, suggesting the semiconducting nature of the narrow GNRs. Novel GNR-GNR and GNR-CNT crossbars were fabricated by transferring GNR arrays across GNR and CNT arrays, respectively. The production of such ordered graphene nanoribbon architectures may allow for large scale integration of GNRs into nanoelectronics or optoelectronics.     

Microstructure and mechanical properties of Al2O3 composites with surface-treated carbon nanotubes (CNTs): dispersibility of modified carbon nanotubes (CNTs) on Al2O3 matrix

Aluminum oxide (Al2O3) matrix have been reinforced by the multi-walled carbon nanotubes (MWCNTs) to overcome the inherent brittleness of Al2O3 matrix. In order to increase mechanical properties of MWCNTs-Al2O3 composites, MWCNTs need to be well dispersed and individually incorporated in Al2O3 matrix. In this work, aluminum hydroxide (Al(OH)3) used as a Al2O3 precursor and MWCNTs were mixed in an aqueous solution for the homogeneous mixing of hetero-particles, as functions of the content of MWCNTs and the potential hydrogen (pH) of Al(OH)3 suspension. Firstly, MWCNTs were purified and modified by an acid reagent, inducing that the dispersibility of MWCNTs is increased in an aqueous solution by carboxylic group given on the surface of MWCNTs. The modified MWCNTs were added in the Al(OH)3 suspension, and then the mixture was filtered at room temperature. The filtered powders were formed using an uniaxial pressing and then densified by a pressureless heat treatment. As the pH is decreased the Al(OH)3 particles are well dispersed in an aqueous solution, due to the increment of repulsive force between particles with a same surface charge. MWCNTs are individually incorporated into Al2O3 matrix up to 1 vol.% MWCNTs, whereas MWCNTs are aggregated at the composite with 3 vol.% MWCNTs. Therefore, control of the pH and the MWCNTs content are key factors to be considered for the fabrication of MWCNTs-Al2O3 composites with high functional properties.     

Hybrid Films of Graphene and Carbon Nanotubes for High Performance Chemical and Temperature Sensing Applications

A hybrid composite material of graphene and carbon nanotube (CNT) for high performance chemical and temperature sensors is reported. Integration of 1D and 2D carbon materials into hybrid carbon composites is achieved by coupling graphene and CNT through poly(ionic liquid) (PIL) mediated‐hybridization. The resulting CNT/PIL/graphene hybrid materials are explored as active materials in chemical and temperature sensors. For chemical sensing application, the hybrid composite is integrated into a chemo‐resistive sensor to detect a general class of volatile organic compounds. Compared with the graphene‐only devices, the hybrid film device showed an improved performance with high sensitivity at ppm level, low detection limit, and fast signal response/recovery. To further demonstrate the potential of the hybrid films, a temperature sensor is fabricated. The CNT/PIL/graphene hybrid materials are highly responsive to small temperature gradient with fast response, high sensitivity, and stability, which may offer a new platform for the thermoelectric temperature sensors.     

Functionalization of multi-walled carbon nanotubes by radiation-induced graft polymerization in aqueous solution

A facile strategy for the preparation of water-soluble multi-walled carbon nanotubes (MWCNTs) in aqueous solution is proposed, where MWCNTs were treated by the concentrated mix acid and then functionalized by simultaneous radiation-induced graft polymerization of acrylic acid under the γ-ray. The structure of the grafted MWCNTs (denoted as MWCNT-g-PAAc) is characterized by means of FT-IR, ^lH NMR, Raman spectroscopies, TGA analysis, and TEM imaging. The degree of grafting was dependent on the reaction conditions such as the monomer concentration and the inhibitor concentration. The stability of the aqueous solution of MWCNT-g-PAAc is highly improved, where the solution is stable for more than 30 days at the concentration 1.5 g/L.     

Enhanced crack detection sensitivity of carbon fiber composites by carbon nanotubes directly grown on carbon fibers

The ability of highly conductive hybrid carbon–fiber/carbon nanotube loaded epoxy composites to sense matrix cracking damage in situ is demonstrated. Multi-walled carbon-nanotubes (MWCNTs) are grown perpendicular to and on the surface of a woven carbon–fiber fabric using a chemical vapor deposition process. An increase in sensitivity of resistance change under interlaminar fracture is shown through a series of double cantilever beam (DCB) tests on samples prepared with MWCNTs grown on both sides of carbon–fiber fabric lamina placed at the top and bottom surfaces of an 8-layer test panel whereas samples with MWCNTs inside the samples did not show much increase in sensitivity of resistance change compared with the baseline samples without MWCNTs. The results suggest that the addition of surface positioned hierarchical carbon-nanotube lamina on composite structures has the potential for autonomic sensing of internal matrix damage.     

Growth of horizontally aligned dense carbon nanotubes from trench sidewalls

Horizontally aligned, dense carbon nanotubes (HADCNTs) in the form of CNT cantilevers/bridges were grown from selected trench sidewalls in silicon substrate by chemical vapor deposition (CVD). The as-grown CNT cantilevers/bridges are packed with multiwalled carbon nanotubes (MWCNTs) with a linear density of about 10 CNTs μm(-1). The excellent horizontal alignment of these CNTs is mainly ascribed to the van der Waals interactions within the dense CNT bundles. What is more, the Raman intensity ratio I(G)/I(D) shows a gradual increase from the CNT roots to tips, indicating a defect gradient along CNTs generated during their growth. These results will inspire further efforts to explore the fundamentals and applications of HADCNTs.     

Fabrication of polyacrylamide–carbon nanotubes by One-Step Radiation-Induced Graft Polymerization

A facile strategy for the preparation of organic nitrogen functional multi-walled carbon nanotubes (MWCNTs) in aqueous solution is proposed, where MWCNTs were functionalized by simultaneous radiation-induced graft polymerization of acrylamide under the γ-ray. The structure of the poly(acryl amide) grafted MWCNTs (denoted as MWCNT-g-PAM) is characterized by means of FT-IR, XPS spectroscopies, TGA analysis and TEM imaging. The degree of grafting dependent on the reaction conditions such as the monomer concentration was also discussed in this paper.     

Adsorption of ciprofloxacin and its role for stabilizing multi-walled carbon nanotubes and characterization

In this study, multi-walled carbon nanotubes (MWCNTs) were dispersed in an aqueous solution using ciprofloxacin (CF) without chemical modification. We found that CF is a useful stabilizer for MWCNTs and the dispersions were stable for more than one month. Scanning electron microscopy, Uv-visible spectroscopy and cyclic voltammetry (CV) showed MWCNTs coated with CF molecules. Dry film of MWCNTs/CF was prepared and characterized by SEM, Uv-vis and CV. MWCNTs/CF dry film can be used as a biocompatible platform for other applications including protein and enzyme immobilization.     

Composite films prepared by immersion deposition of manganese oxide in carbon nanotubes grown on graphite for supercapacitors

Manganese oxide/carbon nanotube (CNT) composite films on graphite were prepared by growing CNTs on the substrate using chemical vapor deposition (CVD), followed by immersion in an aqueous solution of potassium permanganate. The CVD growth created favorable conditions for deposition of the oxide on the electrode, and an aligned porous structure of the composite films, which originated from the CNT growth, could be managed. Electrochemical behaviors of the CNT and the composite films for supercapacitors were studied in 1 M Na₂SO₄ solution. While the oxide deposition in the CNT films was identified as contributing to capacitance enhancement, it was also found that a mild heat treatment could improve performance of the composite films.     

Three dimensional single-walled carbon nanotubes

We report a simple fabrication method of creating a three-dimensional single-walled carbon nanotube (CNT) architecture in which suspended CNTs are aligned parallel to each other along the conventionally unused third dimension at lithographically defined locations. Combining top-down lithography with the bottom-up block copolymer self-assembly technique and utilizing the excellent film forming capability of polymeric materials, highly uniform catalyst nanoparticles with an average size of 2.0 nm have been deposited on sidewalls for generating CNTs with 1 nm diameter. This three-dimensional platform is useful for fundamental studies as well as technological exploration. The fabrication method described herein is applicable for the synthesis of other very small 1D nanomaterials using the catalytic vapor deposition technique.     

Hierarchical carbon nanostructure design: ultra-long carbon nanofibers decorated with carbon nanotubes

Hierarchical carbon nanostructures based on ultra-long carbon nanofibers (CNF) decorated with carbon nanotubes (CNT) have been prepared using plasma processes. The nickel/carbon composite nanofibers, used as a support for the growth of CNT, were deposited on nanopatterned silicon substrate by a hybrid plasma process, combining magnetron sputtering and plasma-enhanced chemical vapor deposition (PECVD). Transmission electron microscopy revealed the presence of spherical nanoparticles randomly dispersed within the carbon nanofibers. The nickel nanoparticles have been used as a catalyst to initiate the growth of CNT by PECVD at 600°C. After the growth of CNT onto the ultra-long CNF, SEM imaging revealed the formation of hierarchical carbon nanostructures which consist of CNF sheathed with CNTs. Furthermore, we demonstrate that reducing the growth temperature of CNT to less than 500°C leads to the formation of carbon nanowalls on the CNF instead of CNT. This simple fabrication method allows an easy preparation of hierarchical carbon nanostructures over a large surface area, as well as a simple manipulation of such material in order to integrate it into nanodevices.     

Macrodispersion of multi-walled carbon nanotubes for conductive films

Understanding of the effect of the multi-walled carbon nanotube (MWCNT) dispersion process on physical properties of MWCNT film is crucial in process optimization of MWCNT film-based products. In the present work, the electrical conduction property of MWCNT films according to various conditions in MWCNT dispersion is investigated. Spectroscopic analysis of dispersed MWCNTs show that the electrical resistance of the MWCNT conductive film is affected by an increase in the electrical contacts between adjacent CNTs due to CNT debundling and physical damage caused by ultrasonic processing. Based on the two conflicting parameters, dispersion guidelines for highly conductive MWCNT film are presented.     

Enhanced electrochemical detection performance of multiwall carbon nanotubes functionalized by aspartame

Inexpensive, non-toxic, and biocompatible materials that can disperse multiwall carbon nanotubes (MWCNTs) in aqueous solutions through a non-covalent approach while retaining their unique electronic and photonic properties are highly preferred. In this article, we introduce the use of an amphiphilic dipeptide derivative, aspartame, as an effective dispersing agent in preparing highly stable suspensions under ultrasonication. The results demonstrate that aspartame was absorbed by the nanotube surface possibly because of non-covalent π–π stacking between the aromatic group of aspartame and the CNT backbone. In addition, the resulting MWCNT/aspartame composites remained stably dispersed over a wide range of pH values. The chronoamperometric measurements of MWCNT/aspartame composite-coated electrodes for hydrogen peroxide demonstrated better electrochemical detection performance, as characterized by significantly enhanced step current, higher sensitivity, and reduced potential compared with bare electrodes.     

Solubilization of multi walled carbon nanotubes under a facile and mild condition

Multi-walled carbon nanotubes (MWCNTs) functionalized with amino groups was prepared via chemical modification of carboxyl groups introduced on the carbon nanotube surfaces. Oxidation of MWCNTs was performed with ozone in aqueous phase and amidation of generated carboxylic groups, was occurred with amines in the presence of HATU as a coupling agent. Obtained functionalized MWCNTs are soluble in many common organic solvents. The functionalized MWCNTs were characterized in detail using FTIR-ATR, Raman CHN and SEM methods.     

A convenient route to disperse pristine carbon nanotubes in polystyrene with no damage of nanotube electronic structure

A convenient route to prepare polystyrene (PS)/pristine carbon nanotube (CNT) nanocomposite was developed; its electronic structure and electrical conductivity were also studied. Using their well-dispersed system in N-methyl-2-pyrrolodone, pristine multi-walled CNTs (MWCNTs), without any surface organo-modification, were greatly debundled and uniformly dispersed in PS matrix via a simple solution blending procedure. The dispersion structure of the prepared PS/pristine MWCNT nanocomposite was confirmed by scanning electron microscope and laser scanning confocal microscopic images. Moreover, Raman spectroscopy indicated that this method could effectively preserve the electronic structure of the pristine CNTs in the prepared nanocomposite, and the PS/pristine MWCNT nanocomposite showed greatly enhanced electrical conductivity.     

Ultrasound-aided formation of gold nanoparticles on multi-walled carbon nanotubes functionalized with mercaptobenzene moieties

A hybrid of multi-walled carbon nanotube (MWCNT) and gold nanoparticle (Au NP) was prepared under ultrasound irradiation. The approach starts with the functionalization of the walls of MWCNTs with mercaptobenzene moieties for the subsequent immobilization of Au NPs. From the Raman spectra, mercaptobenzene was proven to exist on the MWCNTs. Gold ions were added to the aqueous dispersion of functionalized MWCNTs (f-MWCNTs), and were reduced with the aid of ultrasound and ammonium hydroxide. The reduced gold nanoparticles were examined from the TEM images. Au NPs adhered specifically on the thiol groups of mercaptobenzene to be deposited uniformly on the outer walls of the f-MWCNTs. The application of ultrasound led to a high yield of MWCNT-Au nanocomposites and to the dense distribution of the Au NPs. Moreover, the synthesis reaction rate of the hybrid was considerably enhanced relative to synthesis with mechanical agitation. Through an adsorption test using gold-binding-peptide-(GBP)-modified biomolecules, the hybrid's potential for biological diagnosis was verified.     

Flexible electroluminescent device with inkjet-printed carbon nanotube electrodes

Carbon nanotube (CNTs) inks may provide an effective route for producing flexible electronic devices by digital printing. In this paper we report on the formulation of highly concentrated aqueous CNT inks and demonstrate the fabrication of flexible electroluminescent (EL) devices by inkjet printing combined with wet coating. We also report, for the first time, on the formation of flexible EL devices in which all the electrodes are formed by inkjet printing of low-cost multi-walled carbon nanotubes (MWCNTs). Several flexible EL devices were fabricated by using different materials for the production of back and counter electrodes: ITO/MWCNT and MWCNT/MWCNT. Transparent electrodes were obtained either by coating a thin layer of the CNTs or by inkjet printing a grid which is composed of empty cells surrounded by MWCNTs. It was found that the conductivity and transparency of the electrodes are mainly controlled by the MWCNT film thickness, and that the dominant factor in the luminance intensity is the transparency of the electrode.