Optical properties of gold/multilayer-graphene/carbon nanotube hybrid materials

Discrete dipole approximation (DDA) method was utilized to simulate optical properties (extinction spectra and normalized electric field distribution) of multilayer graphene shell encapsulated gold nanoparticles (GNPs) and their heterostructures with carbon nanotube (CNT). The results were also compared with those for gold (Au) nanoparticles and CNT–Au nanoparticle heterostructures. Encapsulation of Au nanoparticle with a multilayer graphene shell in GNPs did not suppress optical properties and surface plasmons of the former. Tunable optical characteristics of GNPs were calculated that showed resonance peak wavelengths, corresponding to encapsulated Au nanoparticle, between ∼527 and ∼663 nm as a function of size and multilayer graphene shell thickness. Enhanced optical/plasmonic behavior and intense ‘hot spots’ were estimated for CNT coated with tightly-packed GNPs as compared to CNT coated with bare Au nanoparticles.     

Sonochemical hybridization of carbon nanotubes with gold nanoparticles for the production of flexible transparent conducing films

We have demonstrated the fabrication of flexible, transparent, conducting multiwalled carbon nanotube (MWCNT)/gold nanoparticle hybrid films with improved optoelectronic properties by combining the ionic liquid-assisted sonochemical method (ILASM) for hybrid synthesis with the vacuum filtration (VF) method for thin film preparation. Au nanoparticles (NPs) with diameters of 10.3 ± 1.5 nm were uniformly distributed onto the sidewalls of MWCNTs through ILASM, and flexible, transparent, conducting films of Au/MWCNT hybrids (HBs) were reproducibly fabricated by the VF method. In particular, the sheet resistance of Au-MWCNT-HB films was more than 2-fold lower than the sheet resistance of pristine MWCNT films due to the well-interconnected three-dimensional nanotube network structure and the synergistic effect of hybridization of MWCNTs with Au-NPs.     

Effect of carbon nanoparticle type,content, and stress on piezoresistive polyethylene nanocomposites

This study examines the piezoresistive behavior of polyethylene (PE) composites containing different types of carbon nanoparticle fillers. The fillers investigated are single‐wall carbon nanotube (SWCNT), multi‐wall carbon nanotube (MWCNT), and graphene nanoplatelets (GNP), which were dispersed in PE through melt blending in concentrations ranging between 0.5 and 10 wt%. The dispersion and nanocomposite morphology were investigated using scanning electron microscopy and X‐ray diffraction with strong evidence found for shear‐induced orientation of GNP nanoparticles during the compression molding process. The conductivity and permittivity of the composite materials was investigated using impedance spectroscopy and the lowest percolation threshold and highest electrical conductivity was observed for SWCNT composites, followed by MWCNT and GNP. The compressive piezoresistance of the nanocomposites was measured and the initial, elastic, and plastic deformation regions were all identifiable by the resistance measurements. The main finding of this study is that the piezoresistance of MWCNT nanocomposites is more sensitive to the effects of varying stress and composition than SWCNT nanocomposites. This indicates an evolving filler network in the case for MWCNT, while a static network for SWCNT, which is explained by the higher aspect ratio and surface area of the latter. POLYM. ENG. SCI., 55:1643–1651, 2015. © 2014 Society of Plastics Engineers     

Electronic transport in composites of graphite oxide with carbon nanotubes

We show that the presence of electrically insulating graphite oxide (GO) within a single wall carbon nanotube (SWCNT) network strongly enhances electrical conductivity, whereas reduced graphite oxide, even though electrically conductive, suppresses electrical conductivity within a composite network with SWCNTs. Measurements of Young’s modulus and of Raman spectra strongly support our interpretation of the “indirect” role of the oxide groups, present in GO within the SWCNT-GO composite, through electronic doping of metallic SWCNTs.     

The effect of electro-degradation processing on microstructure of polyaniline/single-wall carbon nanotube composite films

Mesostructured polyaniline/single-wall carbon nanotube (PAni/SWCNT) composite film has been prepared through electrochemical polymerization/degradation processing. The microstructures of the films are observed before and after electro-degradation. Initial twisty SWCNT bundles are broken down and linked by the polymerization of PAni. Both crystalline and disordered PAni regions coexist in the PAni/SWCNT composite nanowires. The disordered regions are gradually dissolved while the crystalline regions are basically preserved after electro-degradation. The formation mechanism of the composite with special construction has been proposed. In addition, cyclic voltammetry measurements demonstrate that the electroactive performance of PAni/SWCNT composite is enhanced after electro-degradation. It is found that the specific capacitance of electro-degraded composite reaches up to 848.7 F/g, more than twice over the untreated film, which is ascribed to its profitable charge accessible interface and increased available crystalline PAni regions.     

Reduced graphene oxide–gold nanoparticle membrane for water purification

The reduced graphene oxide–gold nanoparticle (rGO–Au NP) membranes are prepared by vacuum filtration method. The sizes of the Au NPs on the surface of the rGO are about 8–10 nm, and the lattice spacing of Au NPs is 0.0241 nm, which is relative to the cubic lattice of the gold crystal. The layer-by-layer stacking structure of rGO–Au NP membrane can be observed clearly by field emission scanning electron microscopy. The water flux of the rGO–Au NP membrane is as high as 204.1 L m^?2 h^?1 bar^?1, and its retention for Rhodamine B (RhB) is as high as 99.79%.     

Improved electrical conductivity of a non-covalently dispersed graphene–carbon nanotube film by chemical p-type doping

Using a high-pressure air spray we developed a method to deposit electrically-conducting thin films consisting of non-covalently dispersed graphene and carbon nanotubes. The graphene–carbon nanotube film was immersed in a nitric acid and followed by exposure to fuming nitric acid. The acid treatment induced an increased concentration of atomic nitrogen on the graphene basal plane and carbon nanotube sidewall. This result indicates chemical p-type doping of the graphene oxide–carbon nanotube film. After the two acid treatments, the spray coated graphene oxide–carbon nanotube films on a glass substrate exhibit a low sheet resistance of 171 Ω/sq, and a high transmittance of 84% at a wavelength of 550 nm.     

Morphology of composite particles of single wall carbon nanotubes/biodegradable polyhydroxyalkanoates prepared by spray drying

Spray drying was investigated as a strategy for producing single wall carbon nanotube (SWCNT)/polymer composites. The spray-drying method produced SWCNT/poly(3-hydroxybutyrate) and SWCNT/poly(3-hydroxyoctanoate) composite particles in which the SWCNTs have been trapped in a well-dispersed state throughout the polymer matrix. Increasing SWCNT content in the composite led to a change in particle morphology from spherical and smooth to rosette shape with angular distortions. The technique shows potential for bulk carbon composite fabrication.     

Comparative studies on single-layer reduced graphene oxide films obtained by electrochemical reduction and hydrazine vapor reduction

The comparison between two kinds of single-layer reduced graphene oxide (rGO) sheets, obtained by reduction of graphene oxide (GO) with the electrochemical method and hydrazine vapor reduction, referred to as E-rGO and C-rGO, respectively, is systematically studied. Although there is no morphology difference between the E-rGO and C-rGO films adsorbed on solid substrates observed by AFM, the reduction process to obtain the E-rGO and C-rGO films is quite different. In the hydrazine vapor reduction, the nitrogen element is incorporated into the obtained C-rGO film, while no additional element is introduced to the E-rGO film during the electrochemical reduction. Moreover, Raman spectra show that the electrochemical method is more effective than the hydrazine vapor reduction method to reduce the GO films. In addition, E-rGO shows better electrocatalysis towards dopamine than does C-rGO. This study is helpful for researchers to understand these two different reduction methods and choose a suitable one to reduce GO based on their experimental requirements.     

Controlling the thickness of carbon nanotube random network films by the estimation of the absorption coefficient

Here, we investigate the thickness of single-walled (SWCNT) and multi-walled carbon nanotube (MWCNT) random network films by angle-resolved X-ray photoemission spectroscopy. Furthermore, we estimate the absorption coefficient of carbon nanotube (CNT) films through the Lambert–Beer law, by measuring film optical spectra. Moreover, the knowledge of the absorption coefficient provides an easier, reliable, and faster method of investigation for generic CNT film thickness. In addition, the absorption coefficient leads to the information of the absorption length for SWCNT and MWCNT films, which is a physical quantity of fundamental interest for optoelectronic applications, such as light emitting diodes, photovoltaics, and in general light absorbers.     

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.     

Synthesis and failure behavior of super-aligned carbon nanotube film wrapped graphene fibers

Significant progress has been made in recent years in research on wet spinning and hydrothermal synthesis of graphene fibers. In this paper, we report the relationship between the mechanical performance of graphene oxide (GO) fibers and their processing parameters in wet spinning. With super-aligned carbon nanotube (CNT) film wrapping, the specific strength and electrical conductivity of reduced GO (rGO) fibers were simultaneously enhanced by 22% and 49%, respectively. Thicker CNT film wrapping of the rGO fiber induces a core–sheath structure; the resulting interfacial debonding and slippage under fiber axial loading were examined. The findings of this study provide guidelines for optimization of high-performance graphene/CNT hybrid fibers.     

Graphene induced carbonization of polyimide films to prepared flexible carbon films with improving-thermal conductivity

Carbon films prepared by polyimide (PI) films treated under 1500°C exhibit favorable thermal conductivity. However, the bonds of carbon films will fracture and recombine which will cause shrinking and forming defects. The flexibility of the carbon films will be greatly reduced, and then affect the application of the carbon films in the field of thermal conduction. When the films prepared by the graphene oxide/polyimide (GO/PI) composite films and the reduced graphene oxide/polyimide (rGO/PI) composite films, respectively, rGO and GO can fill the defects, then increasing the flexibility of the carbon films and inducing the carbonation process. Because of the high thermal conductivity and the six-membered ring structure of rGO and GO, the carbonization temperature will decrease and save costs. When the composite films treated under 1500°C, the thermal conductivity increases with the content of rGO and GO. There are connections between PI and graphene. As the amount of rGO and GO increases, the strong interactions between the rGO or GO and PI lead to contact that enhances its thermal conductivity. However, the rGO and GO have different effects on the films flexibility and thermal conductivity and the differences will be described in the article.     

Non-covalent modification of graphene sheets in PEDOT composite materials by ionic liquids

An ionic liquid (IL) supported composite of poly(3,4-ethylene dioxythiophene) (PEDOT) and graphene oxide (GO) is presented. GO was dispersed in ILs and electropolymerization carried out after loading of EDOT to the dried dispersion. The content of GO was optimized to obtain high electrical conductivity of the composite material. The IL acts as the dispersant for GO and as dopant in the synthesis of PEDOT leading to films with a highly porous structure indicated from the scanning electron microscopy (SEM) images. Subsequently, GO was reduced electrochemically by cyclic voltammetry to obtain PEDOT/rGO composite films. The successful formation of composite materials was confirmed using Raman and X-ray photoelectron spectroscopy (XPS) techniques. XPS was also used to verify removal of oxygen-containing functional groups upon electrochemical reduction of the composite films. The electrochemical properties of PEDOT, PEDOT/GO and PEDOT/rGO were studied using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). The results show that electrochemical reduction clearly increases the capacitance of the composite and furthermore the cycling stability. Such an increase could be obtained if >20 cycles, extending to highly negative potentials (−2.0 V), was used during the electroreduction of incorporated GO. Owing to the high porosity, favorable electrochemical properties and cycling stability these hybrid materials shows great potential towards supercapacitor applications.     

Synergy effects between single-walled carbon nanotubes and polypyrrole on the electrocatalysis of their composites for the oxygen reduction reaction

To develop the high-efficiency carbon alloy catalysts with significant interaction of C–N, the synergy effects of the polypyrrole/single-walled carbon nanotube (PPy/SWCNT) composites have been studied for the oxygen reduction reaction (ORR) in an alkaline solution. The synergy effects between PPy and SWCNT have been found in the synthesized PPy/SWCNT composites by the characterization using FE-SEM, Raman spectroscopy, XRD, TGA, electrical conductivity, and the electrochemical measurements for the surface area and the electrochemical activities of sites: electron transfer rate, differential capacitance and chemisorbed H-stripping voltammetry. The synergy effects have significantly affected the electrochemical properties of the PPy/SWCNT composites and the electrocatalytic potential at the active sites. The PPy/SWCNT composites synthesized using the electrochemical polymerization method with an identified PPy:SWCNT weight ratio of 1:2 show the best performance of the ORR. For the electrode process on the PPy/SWCNT composites, a mixture of two-electron and four-electron processes occurred. It was found that the synergy effects in the PPy/SWCNT composite electrodes synthesized using the electrochemical polymerization method with a thin net-like PPy film were stronger than those using the chemical polymerization method with a thick shell-like PPy film and played an important role in the ORR electrode kinetics.     

Controlled assembly of graphene shells encapsulated gold nanoparticles and their integration with carbon nanotubes

Controlled growth and uniform patterning of graphene/carbon shells encapsulated gold nanoparticles (GNPs) on silicon wafer or on high curvature carbon nanotubes (CNTs) is reported here. This was achieved by utilizing patterned gold nanoparticles with controlled sizes (∼30–600 nm) via gold film dewetting process. Surface-oxidized and patterned nanoparticles were used as sacrificial catalysts for the chemical vapor deposition (CVD) growth of graphene/carbon shells. The shell morphological evolution and thickness as well as surface migration of nanoparticles during the CVD process were studied as a function of the gold nanoparticles size. Reduced surface migration and coalescence was observed for gold nanoparticles after the CVD growth and this was attributed to the initial formation of graphene/carbon shells as well as stable dispersion of the dewetted gold nanoparticles. It is proposed that graphene/carbon shell growth was controlled by Ostwald’s ripening, surface gold oxide, and reducing CVD growth environment. Furthermore, complex heterostructures based on CNTs coated with GNPs were fabricated by dewetting Au films on CNTs and followed by surface oxidation and CVD growth steps. CNTs successfully survived multiple processing steps and selective growth of graphene shells around Au nanoparticles was achieved and studied using microscopic and spectroscopic methods.     

Solid-phase electrochemical reduction of graphene oxide films in alkaline solution

Graphene oxide (GO) film was evaporated onto graphite and used as an electrode to produce electrochemically reduced graphene oxide (ERGO) films by electrochemical reduction in 6 M KOH solution through voltammetric cycling. Fourier transformed infrared and Raman spectroscopy confirmed the presence of ERGO. Electrochemical impedance spectroscopy characterization of ERGO and GO films in ferrocyanide/ferricyanide redox couple with 0.1 M KCl supporting electrolyte gave results that are in accordance with previous reports. Based on the EIS results, ERGO shows higher capacitance and lower charge transfer resistance compared to GO.     

Microwave autoclave synthesized multi-layer graphene/single-walled carbon nanotube composites for free-standing lithium-ion battery anodes

Multi-layer graphene sheets have been synthesized by a time-efficient microwave autoclave method and used to form composites in situ with single-walled carbon nanotubes. The application of these composites as flexible free-standing film electrodes was then investigated. According to the transmission electron microscopy and X-ray diffraction characterizations, the average d-spacing of the graphene–single-walled carbon nanotube composites was 0.41 nm, which was obviously larger than that of the as-prepared pure graphene (0.36 nm). The reversible Li-cycling properties of the free-standing films have been evaluated by galvanostatic discharge–charge cycling and electrochemical impedance spectroscopy. Results showed that the free-standing composite film with 70 wt% graphene exhibited the lowest charge transfer resistance and the highest charge capacity of about 303 mAh g⁻¹ after 50 cycles, without any noticeable fading.     

Enhanced optical output power of blue light-emitting diodes with quasi-aligned gold nanoparticles

The output power of the light from GaN-based light-emitting diodes (LEDs) was enhanced by fabricating gold (Au) nanoparticles on the surface of p-GaN. Quasi-aligned Au nanoparticle arrays were prepared by depositing Au thin film on an aligned suspended carbon nanotube thin film surface and then putting the Au-CNT system on the surface of p-GaN and thermally annealing the sample. The size and position of the Au nanoparticles were confined by the carbon nanotube framework, and no other additional residual Au was distributed on the surface of the p-GaN substrate. The output power of the light from the LEDs with Au nanoparticles was enhanced by 55.3% for an injected current of 100 mA with the electrical property unchanged compared with the conventional planar LEDs. The enhancement may originate from the surface plasmon effect and scattering effect of the Au nanoparticles.     

Pinecone shape hydroxypropyl-β-cyclodextrin on a film of multi-walled carbon nanotubes coated with gold particles for the simultaneous determination of tyrosine, guanine, adenine and thymine

A conductive composite film containing functionalized multi-walled carbon nanotubes (fMWCNTs), gold nanoparticles (Au) with hydroxypropyl-β-cyclodextrin (HPβCD) as catalysts have been synthesized on glassy carbon, gold and on indium tin oxide electrodes by potentiostatic methods. The presence of fMWCNTs and HPβCD in the composite film enhances the active surface coverage concentration of Au by 397.0%. The presence of nanoparticles of gold catalyst in the film enhances the functional properties and produces an overall increase in the sensitivity of the modified electrodes. These modified electrodes exhibit promising electrocatalytic activity towards the oxidation of tyrosine (TYR), guanine (GU), adenine (AD) and thymine (THY) present in pH 7.4 aqueous solutions. Well separated voltammetric peaks are obtained between TYR and GU (80 mV), GU and AD (290 mV), AD and THY (185 mV) present in the analyte mixture. The sensitivity values of the composite films from cyclic voltammetry (CV) and semi derivative differential pulse voltammetry (DPV) show that, the composite film modified electrodes are efficient and they could be applied in biosensor devices. However, a detailed comparison between the sensitivities obtained using CV and semi derivative DPV shows that, the sensitivity obtained in semi derivative DPV technique is higher than CV. Electrochemical quartz crystal microbalance, scanning electrochemical microscope and scanning electron microscope techniques have been used for the electrochemical characterizations and surface morphology studies.