Controllable bulk growth of few-layer graphene/single-walled carbon nanotube hybrids containing Fe@C nanoparticles in a fluidized bed reactor

A family of layered double hydroxides (LDHs) with varied Fe contents were employed as catalyst precursors for the controllable bulk growth of few-layer graphene/single-walled carbon nanotube (G/SWCNT) hybrids in a fluidized-bed reactor through chemical vapor deposition of methane at 950 °C. All the G/SWCNT hybrids exhibited the morphology of SWCNTs interlinked with graphene layers. The purity, thermal stability, graphitization degree, specific surface area, and total pore volume of the G/SWCNT hybrids decreased with the increasing Fe contents in the LDH precursors. A high yield of 0.97 g_G/SWCNTs/g_cat can be achieved by tuning the Fe content in the FeMgAl LDHs after a 15-min growth. After the removal of the as-calcined FeMgAl layered double oxide flakes, a high carbon purity of ca. 98.3% for G/SWCNT hybrids was achieved when the mole ratio of Fe–Al is 0.05:1. The size and density of Fe nanoparticles decorated in the as-obtained G/SWCNT hybrids depend largely on Fe content in the FeMgAl LDH precursors. Furthermore, the mass ratio of graphene materials to SWCNTs in the as-prepared G/SWCNT hybrids can be well controlled in a range of 0.4–15.1.     

High yield production of semiconducting p-type single-walled carbon nanotube thin-film transistors on a flexible polyimide substrate by tuning the density of ferritin catalysts

We report on a simple method for fabricating pure p-type single-walled carbon nanotube (SWCNT) thin-film transistors (TFTs) on flexible polyimide substrates without selective removal of metallic SWCNTs from the as-grown CNT films. The density of the SWCNTs was controlled by tuning the concentration of ferritin catalyst, resulting in the control of the metallic percolation pathways in the SWCNT TFTs. For a ferritin solution diluted by 1/2000, approximately 60% of the pristine SWCNT TFTs showed p-type behavior with larger on/off current ratios, (I_on/I_off > 10⁴) and a high photosensitivity to the exposure of UV/visible light.     

Raman spectroscopic evidence for interfacial interactions in poly(bithiophene)/single-walled carbon nanotube composites

Electrochemical polymerization of 2,2′-bithiophene (BTh) on single-walled carbon nanotube (SWCNT) films has been studied by Raman scattering and infrared absorption spectroscopy. Covalent functionalization of SWCNTs with poly(bithiophene) (PBTh) in its un-doped and doped states is demonstrated. The occurrence of a charge transfer process at the interface of PBTh and SWCNTs, is shown by: (i) an up-shift of the Raman lines associated with the radial breathing modes of SWCNTs that reveals both a doping process and an additional twisting together as a rope with the conducting polymer as binding agent; (ii) a new Raman band in the range 1430-1450 cm⁻¹ indicating the functionalization of SWCNTs with PBTh in doped and un-doped states; (iii) strong absorption bands situated in the interval 600-800 cm⁻¹ resulting from steric hindrance produced by the nanotube binding to the polymeric chain. Treatment of the PBTh/SWCNT composite with aqueous NH₄OH solution forms un-doped PBTh covalently functionalized SWCNTs. At the resonant excitation of the metallic tubes, an additionally enhanced Raman process is generated by plasmon excitation in the metallic nanotubes. It is evidenced by a particular behavior in the Stokes and anti-Stokes branch of the PBTh Raman line at 1450 cm⁻¹.     

Decrease of carbon nanotube UV light absorption induced by π-π-stacking interaction with nucleotide bases

UV-vis absorption spectra of hybrids formed by carbon single-walled nanotubes (SWCNTs) and single-stranded DNA (ss-DNA), double-stranded DNA (ds-DNA), and synthetic homopolynucleotides (poly(rC) and poly(rG)) have been studied. In the 300-400 nm spectral range a decrease of SWCNT light absorption because of their interaction with polymers was observed. The revealed decrease of nanotube absorption is explained by SWCNT hypochromism that results from π-π-stacking interaction of polymer nitrogen bases with the tube surface. Molecular dynamic simulation demonstrates a spontaneous wrapping of polynucleotides around the nanotube. Simulation showed that pyrimidine oligonucleotide (d(C)₂₅) manifests a higher rate of minimizing the hybrid structure with the wrapped polymer structure around the tube than the purine one (d(G)₂₅). Such polymer behavior can be explained by the purine oligonucleotide property to form a stable ordering self-stacking structure which prevents structural re-orientation, in contrast to pyrimidine polynucleotides.     

Surfactant-free assembling of functionalized single-walled carbon nanotube buckypapers

The electrical and textural properties of single-walled carbon nanotube buckypapers were tunned through chemical functionalization processes. Single-walled carbon nanotubes (SWCNTs) were covalently functionalized with three different chemical groups: Carboxylic acids (-COOH), benzylamine (-Ph-CH₂-NH₂), and perfluorooctylaniline (-Ph-(CF₂)₇-CF₃). Functionalized SWCNTs were dispersed in water or dimethylformamide (DMF) by sonication treatments without the addition of surfactants or polymers. Carbon nanotube sheets (buckypapers) were prepared by vacuum filtration of the functionalized SWCNT dispersions. The electrical conductivity, textural properties, and processability of the functionalized buckypapers were studied in terms of SWCNT purity, functionalization, and assembling conditions. Carboxylated buckypapers demonstrated very low specific surface areas (< 1 m²/g) and roughness factor (R_a = 14 nm), while aminated and fluorinated buckypapers exhibited roughness factors of around 70 nm and specific surface areas of 160-180 m²/g. Electrical conductivity for carboxylated buckypapers was higher than for as-grown SWCNTs, but for aminated and fluorinated SWCNTs it was lower than for as-grown SWCNTs. This could be interpreted as a chemical inhibition of metallic SWCNTs due to the specificity of the diazonium salts reaction used to prepare the aminated and fluorinated SWCNTs. The utilization of high purity as-grown SWCNTs positively influenced the mechanical characteristics and the electrical conductivity of functionalized buckypapers.     

Increasing the thermoelectric power generated by composite films using chemically functionalized single-walled carbon nanotubes

We prepared and characterized flexible thermoelectric (TE) materials based on thin films of single-walled carbon nanotube (SWCNT) composites with polyvinylalcohol. While pristine SWCNTs incorporated in a polymer matrix generated a p-type TE material, chemical functionalization of SWCNTs by using polyethyleneimine produced an n-type TE material. TE modules made of both p- and n-type composite were fabricated to demonstrate TE voltage and power generation. A single p–n junction made of two composite strips containing 20 wt.% of SWCNTs generated a high TE voltage of 92 μV per 1 K temperature gradient (ΔT). By combining five electrically connected p–n junctions an output voltage of 25 mV was obtained upon the applying ΔT = 50 K. Furthermore, this module generated a power of 4.5 nW when a load resistance matched the internal module resistance of 30 kΩ. These promising results show the potential of TE energy conversion provided by the SWCNT composite films connected in scalable modules for applications that require light weight and mechanical flexibility.     

Near-infrared photoresponse in single-walled carbon nanotube/polymer composite films

We present a near-infrared photoresponse study of single-walled carbon nanotube/poly(3-hexylthiophene)-block-polystyrene polymer (SWCNT/P3HT-b-PS) composite films for different loading ratios of SWCNT in the polymer matrix. Compared to the pure SWCNT film, the photoresponse [(light current − dark current)/dark current] is much larger in the SWCNT/polymer composite films. The photoresponse is up to 157% when SWCNTs are embedded in P3HT-b-PS while for a pure SWCNT film it is only 40%. We also show that the photocurrent strongly depends on the position of the laser spot with maximum photocurrent occurring at the metal-film interface. We explain the photoresponse due to exciton dissociations and charge carrier separation caused by a Schottky barrier at the metallic electrode-SWCNT interface.     

Polystyrene composites of single‐walled carbon nanotubes‐graft‐polystyrene

Single‐walled carbon nanotubes (SWCNTs) dispersed in N‐methylpyrrolidone (NMP) were functionalized by addition of polystyryl radicals from 2,2,6,6‐tetramethyl‐1‐piperidinyloxy‐ended polystyrene (SWCNT‐g‐PS). The amount of polystyrene grafted to the nanotubes was in the range 20‐25 wt% irrespective of polystyrene number‐average molecular weight ranging from 2270 to 49 500 g mol^?1. In Raman spectra the ratios of D‐band to G‐band intensity were similar for all of the polystyrene‐grafted samples and for the starting SWCNTs. Numerous near‐infrared electronic transitions of the SWCNTs were retained after polymer grafting. Transmission electron microscopy images showed bundles of SWCNT‐g‐PS of various diameters with some of the polystyrene clumped on the bundle surfaces. Composites of SWCNT‐g‐PS in a commercial‐grade polystyrene were prepared by precipitation of mixtures of the components from NMP into water, i.e. the coagulation method of preparation. Electrical conductivities of the composites were about 10^?15 S cm^?1 and showed no percolation threshold with increasing SWCNT content. The glass transition temperature (T_g) of the composites increased at low filler loadings and remained constant with further nanotube addition irrespective of the length and number of grafted polystyrene chains. The change of heat capacity (ΔC_p) at T_g decreased with increasing amount of SWCNT‐g‐PS of 2850 g mol^?1, but ΔC_p changed very little with the amount of SWCNT‐g‐PS of higher molecular weight. The expected monotonic decrease in ΔC_p coupled with the plateau behavior of T_g suggests there is a limit to the amount that T_g of the matrix polymer can increase with increasing amount of nanotube filler. Copyright © 2012 Society of Chemical Industry     

Fabrication of single-walled carbon nanotube Schottky diode with gold contacts modified by asymmetric thiolate molecules

We have fabricated single-walled carbon nanotube (SWCNT) Schottky diodes by asymmetrically modifying the two Au/SWCNT contacts using different thiolate molecules, methanethiol (CH₃SH) and trifluoroethanethiol (CF₃CH₂SH). Characterization has revealed that highly asymmetrical contacts with Schottky barrier heights of ∼190 and ∼40 meV (increased by over 70% and decreased by over 60%, respectively with respect to that of pristine Au/SWCNT contact of ∼110 meV) were achieved for the Au/SWCNT contacts modified by CH₃SH and CF₃CH₂SH, respectively. The performance of our SWCNT Schottky diodes is as follows: the forward and reverse current ratio (I_forward/I_reverse) higher than 10⁴, a forward current as high as ∼5 μA, a reverse leakage current as low as ∼100 pA, and a current ideality factor as low as ∼1.42. This is at least comparable to, if not better than SWCNT Schottky diodes fabricated with asymmetrical metals, where one contact is a metal with a work function lower than that of SWCNTs to yield a Schottky contact, while the other has a work function higher than that of SWCNTs to achieve an ohmic (more near ohmic) contact.     

The synthesis of covalent bonded single‐walled carbon nanotube/polyvinylimidazole composites by in situ polymerization and their physical characterization

Single‐walled carbon nanotube (SWCNT) polyvinylimidazole (PVI) composites have been prepared by in situ emulsion polymerization. Dispersion of raw SWCNTs in the PVI matrix was improved by surface modification of the SWCNTs using nitric acid treatment and air oxidation. The carbonyl‐terminated SWCNTs were covalently bonded to PVI by in situ polymerization and the SWCNT/PVI composite was thus obtained. The morphological and structural characterizations of the surface‐functionalized SWCNTs and SWCNT/PVI composites were carried out by Fourier transform infrared spectroscopy, X‐ray diffraction, conductivity measurements, scanning, and transmission electron microscopy. Thermograms of the materials were determined by the differential scanning calorimetry technique. The characterization results indicate that PVI was covalently bonded to SWCNTs and a new material was then obtained. The functionalized SWCNTs showed homogenous dispersion in the composites, whereas purified SWCNT resulted in poor dispersion and nanotube agglomeration. SWCNT/PVI composites exhibited chemical stability enhancement in many common solvents. I–V curves of the samples exhibit an ohmic character. Conductivity values for pure SWCNTs, pure PVI and SWCNT/PVI composite were measured to be 3.47, 2.11 × 10⁻⁹, and 2.3 × 10⁻³ S/m, respectively. Because of resonance, a large dielectric constant is obtained for SWCNT/PVI composite, which is not observed for ordinary materials. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Low temperature preparation of a high performance Pt/SWCNT counter electrode for flexible dye-sensitized solar cells

A platinum/single-wall carbon nanotube (Pt/SWCNT) film was sprayed onto a flexible indium-doped tin oxide coated polyethylene naphthalate (ITO/PEN) substrate to form a counter electrode for use in a flexible dye-sensitized solar cell using a vacuum thermal decomposition method at low temperature (120 °C). The obtained Pt/SWCNT electrode showed good chemical stability and light transmittance and had lower charge transfer resistance and higher electrocatalytic activity for the I₃⁻/I⁻ redox reaction compared to the flexible Pt electrode or a commercial Pt/Ti electrode. The light-to-electric energy conversion efficiency of the flexible DSSC based on the Pt/SWCNT/ITO/PEN counter electrode and the TiO₂/Ti photoanode reached 5.96% under irradiation with a simulated solar light intensity of 100 mW cm⁻². The efficiency was increased by 25.74% compared to the flexible DSSC with an unmodified Pt counter electrode.     

Highly enhanced gas sensing in single-walled carbon nanotube-based thin-film transistor sensors by ultraviolet light irradiation

Single-walled carbon nanotube (SWCNT) random networks are easily fabricated on a wafer scale, which provides an attractive path to large-scale SWCNT-based thin-film transistor (TFT) manufacturing. However, the mixture of semiconducting SWCNTs and metallic SWCNTs (m-SWCNTs) in the networks significantly limits the TFT performance due to the m-SWCNTs dominating the charge transport. In this paper, we have achieved a uniform and high-density SWCNT network throughout a complete 3-in. Si/SiO₂ wafer using a solution-based assembly method. We further utilized UV radiation to etch m-SWCNTs from the networks, and a remarkable increase in the channel current on/off ratio (I_on/I_off) from 11 to 5.6 × 10³ was observed. Furthermore, we used the SWCNT-TFTs as gas sensors to detect methyl methylphosphonate, a stimulant of benchmark threats. It was found that the SWCNT-TFT sensors treated with UV radiation show a much higher sensitivity and faster response to the analytes than those without treatment with UV radiation.     

Single-walled carbon nanotube/silicone rubber composites for compliant electrodes

Single-walled carbon nanotube (SWCNT)/silicone rubber composites that can be used in fabricating compliant electrodes are prepared by spraying a mixed solution of ionic-liquid-based SWCNT gel and silicone rubber onto an elastic substrate. Subsequently, the composites are exposed to nitric acid vapor. Scanning electron microscopy and atomic force microscopy images of the composites show that the SWCNTs are finely dispersed in the polymer matrix due to the addition of the ionic liquid. Doping of the SWCNTs by nitric acid can significantly lower the sheet resistance (R_s) of the composites; samples with 4 wt% of SWCNT content exhibit the lowest R_s value (50 Ω sq^?1). This sheet resistance corresponds to a conductivity value of 63 S cm^?1. In addition, the composites retain a high conductivity after several tensile strains are applied. Stretching the composite sample to 300% of the original length increased the R_s value to 320 Ω sq^?1 (19 S cm^?1). Even after 20th stretch/release/stretch cycle, the conductivity remains constant at a value of 18 S cm^?1. These results provide a scalable route for preparing highly stretchable and conductive SWCNT composites with relatively low SWCNT concentrations.     

Influence of lengths of millimeter-scale single-walled carbon nanotube on electrical and mechanical properties of buckypaper

The electrical conductivity and mechanical strength of carbon nanotube (CNT) buckypaper comprised of millimeter-scale long single-walled CNT (SWCNT) was markedly improved by the use of longer SWCNTs. A series of buckypapers, fabricated from SWCNT forests of varying heights (350, 700, 1,500 μm), showed that both the electrical conductivity (19 to 45 S/cm) and tensile strength (27 to 52 MPa) doubled. These improvements were due to improved transfer of electron and load through a reduced number of junctions for longer SWCNTs. Interestingly, no effects of forest height on the thermal diffusivity of SWCNT buckypapers were observed. Further, these findings provide evidence that the actual SWCNT length in forests is similar to the height.     

Solubilization of single-walled carbon nanotubes by entanglements between them and hyperbranched phenolic polymer

Hyperbranched phenolic polymer (HBP) was prepared by Lewis acid-catalyzed polycondensation of bifunctional phenolic monomer with trifunctional phenolic monomer. By choosing an appropriate Lewis acid, HBP was successfully obtained. By using physical adsorption of HBP on a single-walled carbon nanotube (SWCNT) surface, solubilization of SWCNTs was examined. SWCNTs were soluble with extended branches of HBP in N,N-dimethylformamide (DMF) solution, while they were insoluble in a linear phenolic polymer. In the presence of shrinking branches of HBP in tetrahydrofuran, SWCNTs were hardly soluble. Entanglements between extended branches of HBP and SWCNT in DMF solution resulted in high solubility of SWCNTs.     

3D microfabrication of single-wall carbon nanotube/polymer composites by two-photon polymerization lithography

We present a method to develop single-wall carbon nanotube (SWCNT)/polymer composites into arbitrary three-dimensional micro/nano structures. Our approach, based on two-photon polymerization lithography, allows one to fabricate three-dimensional SWCNT/polymer composites with a minimum spatial resolution of a few hundreds nm. A near-infrared femtosecond pulsed laser beam was focused onto a SWCNT-dispersed photo resin, and the laser light solidified a nanometric volume of the resin. The focus spot was three-dimensionally scanned, resulting in the fabrication of arbitrary shapes of SWCNT/polymer composites. SWCNTs were uniformly distributed throughout the whole structures, even in a few hundreds nm thick nanowires. Furthermore, we also found an intriguing phenomenon that SWCNTs were self-aligned in polymer nanostructures, promising improvements in mechanical and electrical properties. Our method has great potential to open up a wide range of applications such as micro- and nanoelectromechanical systems, micro/nano actuators, sensors, and photonics devices based on CNTs.     

Fabrication of Carbon Nanotube/SiO2 and Carbon Nanotube/SiO2/Ag Nanoparticles Hybrids by Using Plasma Treatment

Based on plasma-treated single wall carbon nanotubes (SWCNTs), SWCNT/SiO₂ and thiol groups-functionalized SWCNT/SiO₂ hybrids have been fabricated through a sol–gel process. By means of thiol groups, Ag nanoparticles have been in situ synthesized and bonded onto the SiO₂ shell of SWCNT/SiO₂ in the absence of external reducing agent, resulting in the stable carbon nanotube/SiO₂/Ag nanoparticles hybrids. This strategy provides a facile, low–cost, and green methodology for the creation of carbon nanotube/inorganic oxides-metal nanoparticles hybrids.     

Preparation of single-walled carbon nanotube/TiO2 hybrid atmospheric gas sensor operated at ambient temperature

Single-walled carbon nanotubes (SWCNTs)/TiO₂ hybrid gas sensors operated at a room temperature were fabricated. SWCNTs were stabilized on a Si substrate with interdigitated Pt-electrodes to prepare a gas sensor. Sensing properties of the gas sensor were measured in various concentrations of NO gas. Resistance of the prepared SWCNT based gas sensor decreased with increase of NO gas concentration due to electron transfer from p-type SWCNTs to NO molecules. The SWCNT gas sensor showed high sensitivity and rapid response to the test gas. The hybrid gas sensor using SWCNTs doped with anatase TiO₂ nano-particles was developed, which could work at room temperature under UV-LED (λ = 377 nm) irradiation. It showed rapid recovery to the initial state and higher sensitivity than the SWCNT gas sensor due to TiO₂ photocatalytic effect.     

Fabrication of single-walled carbon nanotube three-dimensional networks inside the pores of a porous silicon structure

Single-walled carbon nanotube (SWCNT) three-dimensional (3-D) networks were first fabricated in the pores of a porous silicon substrate using thermal decomposition of C₂H₂ at 800 °C. Catalyst nanoparticles were uniformly distributed on the inner wall surfaces of the pores using a dipping method combined with ultrasonication. SWCNTs were synthesized along the inner wall surface of the pores, and spanned it. The suspended SWCNTs inside the pores formed 3-D networks in the results of the chaotic overgrowth of SWCNTs in a confined space under thermal vibration, and van der Waals interactions between SWCNTs.     

Fluidized-bed synthesis of sub-millimeter-long single walled carbon nanotube arrays

The rapid growth method for vertically aligned, single walled carbon nanotube (SWCNT) arrays on flat substrates was applied to a fluidized-bed, using ceramic beads as catalyst supports as a means to mass produce sub-millimeter-long SWCNT arrays. Fe/Al₂O_x catalysts were deposited on the surface of Al₂O₃ beads by sputtering and SWCNTs were grown on the beads by chemical vapor deposition (CVD) using C₂H₂ as a feedstock. Scanning electron microscopy and transmission electron microscopy showed that SWCNTs of 2–4 nm in diameter grew and formed vertically aligned arrays of 0.5 mm in height. Thermogravimetric analysis showed that the SWCNTs had a catalyst impurity level below 1 wt.%. Furthermore, they were synthesized at a carbon yield as high as 65 at.% with a gas residence time as short as <0.2 s. Our fluidized-bed CVD, which efficiently utilizes the three-dimensional space of the reactor volume while retaining the characteristics of SWCNTs on substrates, is a promising option for mass-production of high-purity, sub-millimeter-long SWCNT arrays.