Dye‐sensitized TiO2 solar cells based on nanocomposite photoanode containing plasma‐modified multi‐walled carbon nanotubes

A series of anatase TiO₂‐based nanocomposite incorporated with plasma‐modified multi‐walled carbon nanotubes (MWNTs) was prepared by physical blending and shows its capability for efficient electron transport when used as photoanode in dye‐sensitized solar cells (DSSCs). These MWNTs characterized with good dispersal performance were obtained by functionalization technique via in situ plasma treatment and subsequent grafting with maleic anhydride (MA) onto the external walls reported previously. Compared with the conventional DSSCs, the TiO₂ film with 1D carbon nanotubes possesses more outstanding ability to transport electrons injected from the excited dye within the device under illumination. As a result, at an optimum addition of 0.3 wt% MWNTs‐MA in TiO₂ matrix, the photocurrent–voltage (J–V) characteristics showed a significant increase in the short‐circuit photocurrent (J_sc) of 50%, leading to an increase in overall solar conversion efficiency by a factor of 1.5. Electrochemical impedance spectroscopy analyses reveal that the MWNTs‐MA/TiO₂ incur smaller resistances at the photoanode in assembled DSSCs when compared with those in the anatase titania DSSCs. These features suggest that the conducting properties of the MWNTs‐MA within the anodes are crucial for achieving a higher transport rate for photo‐induced electrons in TiO₂ layer by exhibiting lower resistance in the porous network and hence retard charge recombination that could result in poor conversion efficiency. Copyright © 2012 John Wiley & Sons, Ltd.     

Functionalizing Carbon Nanotubes by Grafting on Intumescent Flame Retardant: Nanocomposite Synthesis,Morphology, Rheology,and Flammability

An intumescent flame retardant, poly(diaminodiphenyl methane spirocyclic pentaerythritol bisphosphonate) (PDSPB) has been covalently grafted onto the surfaces of multiwalled carbon nanotubes (MWNTs) to obtain MWNT‐PDSPB and according nanocomposites were prepared via melt blending. After high density PDSPB (65 wt %) were attached to the MWNTs, core‐shell nanostructures with MWNTs as the hard core and PDSPB as the soft shell were formed. The resultant MWNT‐PDSPB was soluble and stable in polar solvents, such as DMF. The optical microscopy and TEM results showed that the functionalized MWNTs can achieve better dispersion in ABS matrix. The linear viscoelastic behavior indicated that MWNT‐PDSPB can form network structure at very low nanotube loading than un‐functionalized MWNTs. The results of flammability showed that better flame retardancy was obtained for ABS/MWNT‐PDSPB nanocomposites due to the better dispersion of MWNT‐PDSPB in ABS matrix. The flammability of the composites is strongly dependent on the network structure of nanotubes which reduces the diffusion of volatile combustible fragments generated by polymer degradation which diffuse towards the surface of the burning polymer to evaporate to feed the flame. The grafting of intumescent flame retardant of PDSPB can improve both the dispersion of nanotubes in polymer matrix and flame retardancy of the nanocomposites.     

Poly(ϵ‐caprolactone)‐Functionalized Carbon Nanotubes and Their Biodegradation Properties

Biodegradable poly(?‐caprolactone) (PCL) has been covalently grafted onto the surfaces of multiwalled carbon nanotubes (MWNTs) by the “grafting from” approach based on in‐situ ring‐opening polymerization of ?‐caprolactone. The grafted PCL content can be controlled easily by adjusting the feed ratio of monomer to MWNT‐supported macroinitiators (MWNT‐OH). The resulting products have been characterized with Fourier‐transform IR (FTIR), NMR, and Raman spectroscopies, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). After PCL was coated onto MWNT surfaces, core/shell structures with nanotubes as the “hard” core and the hairy polymer layer as the “soft” shell are formed, especially for MWNTs coated with a high density of polymer chains. Such a polymer shell promises good solubility/dispersibility of the MWNT–PCL nanohybrids in low‐boiling‐point organic solvents such as chloroform and tetrahydrofuran. Biodegradation experiments have shown that the PCL grafted onto MWNTs can be completely enzymatically degraded within 4 days in a phosphate buffer solution in the presence of pseudomonas (PS) lipase, and the carbon nanotubes retain their tubelike morphologies, as observed by SEM and TEM. The results present possible applications for these biocompatible PCL‐functionalized CNTs in bionanomaterials, biomedicine, and artificial bones.     

Mechanical Properties and Flame‐Retardant Behavior of Ethylene Vinyl Acetate/High‐Density Polyethylene Coated Carbon Nanotube Nanocomposites

High‐density polyethylene coated multiwalled carbon nanotubes (c‐MWNTs) and multiwalled carbon nanotubes (MWNTs) have been dispersed into an ethylene vinyl acetate (EVA) copolymer by mechanical kneading. The effect of c‐MWNTs on tensile properties, thermo‐oxidative degradation, and fire behavior has been studied in comparison with virgin EVA and EVA/MWNTs nanocomposites. Due to the better dispersion of the coated nanotubes, the incorporation of 3 wt % of c‐MWNTs leads to an increase of the Young's modulus, the cohesion of the combustion residues, and a decrease of the peak heat‐release rate.     

Catalytic Twist‐Spun Yarns of Nitrogen‐Doped Carbon Nanotubes

The treatment of free‐standing sheets of multiwalled carbon nanotubes (MWNTs) with a NH₃/He plasma results in self‐supporting sheets of aligned N‐doped MWNTs (CN_x). These CN_x sheets can be easily twist spun in the solid state to provide strong CN_x yarns that are knottable, weavable, and sewable. The CN_x yarns exhibit tunable catalytic activity for electrochemically driven oxygen reduction reactions (ORR), as well as specific capacitances (up to 39 F·g^?1) that are 2.6 times higher than for the parent MWNTs. Due to a high degree of nanotube alignment, the CN_x yarns exhibit specific strengths (451 ± 61 MPa·cm³·g^?1) that are three times larger than observed for hybrid CN_x/MWNT biscrolled yarns containing 70 wt.% CN_x in the form of a powder. This difference in mechanical strength arises from substantial differences in yarn morphology, revealed by electron microscopy imaging of yarn cross‐ sections, as well as the absence of a significant strength contribution from CN_x nanotubes in the biscrolled yarns. Finally, the chemical nature and abundance of the incorporated nitrogen within the CN_x nanotubes is studied as function of plasma exposure and annealing processes using X‐ray photoelectron spectroscopy and correlated with catalytic activity.     

Strong Carbon‐Nanotube–Polymer Bonding by Microwave Irradiation

The vigorous response of multiwalled carbon nanotubes (MWNTs) to microwave irradiation, leading to the release of a large amount of heat, is used to locally melt a plastic matrix adjacent to the nanotubes within a period of seconds. This results in the intercalation of the MWNTs into the polymer matrix at room temperature without any physical damage to the polymer. The so‐called “microwave welding” approach creates a new paradigm for the formation of very strong MWNT–polymer bonds without the use of any adhesive, and represents a significant step forward for the fabrication of functional nanotube composites. Here, we demonstrate the implications of the anisotropic alignment of MWNTs in polymers, patterned conductors/resistors for soft electronics, and high‐strength composites, where the MWNTs are ‘soldered' to flexible polymer substrates.     

Electrostatic Anchoring of Mn4 Single‐Molecule Magnets onto Chemically Modified Multiwalled Carbon Nanotubes

Two different routes that enable the electrostatic grafting of cationic single‐molecule magnets (SMMs) onto the surface of chemically modified anionic multi‐walled carbon nanotubes (MWNTs) are described. The chemical nature and physical properties of the resulting hybrids are discussed on the basis of a complete battery of experimental techniques. The data show that the chemical nature of the SMM unit remains intact, while its magnetic response is significantly affected by the grafting process, which is likely due to surface effects.     

Squaraine‐Doped Functional Nanoprobes: Lipophilically Protected Near‐Infrared Fluorescence for Bioimaging

Hydrophobically stabilized near‐IR fluorescence from self‐assembled nanoprobes composed of amphiphilic poly(maleic anhydride‐alt‐octadec‐1‐ene) (PMAO) and lipophilized squaraine dopants is reported. From comparative studies with varying lipophilicity of squaraine dyes as well as of nanoparticulate polymer matrices, it is found that dual protection by simultaneous lipophilization of the dye‐polymer pair greatly improves the chemical stability of labile squaraine dyes, to produce efficient NIR fluorescence in physiological aqueous milieux. The surface properties of negatively charged PMAO nanoparticles are readily modified by coating with an amine‐rich cationic glycol chitosan with biofunctionality. Efficient cellular imaging and in vivo sentinel lymph node mapping with size and surface‐controlled nanoprobes demonstrate that lipophilic dual protection of NIR fluorescence and the underlying functional nanoprobe approach hold great potential for bioimaging applications.     

Dispersion, hybrid interconnection and heat dissipation properties of functionalized carbon nanotubes in epoxy composites for electrically conductive adhesives (ECAs)

Different types of MWNTs/epoxy composites were prepared with diglycidyl ether of bisphenol F (DGEBF) and bisphenol A (DGEBA) used as epoxy resins. MWNTs were functionalized to enhance the properties of epoxy composites by treatment with strong acids (acid-treated MWNTs, a-MWNTs) followed by m-phenylenediamine grafting (amine grafted MWNTs, m-MWNTs). Raw, a-, and m-MWNTs were dispersed in DGEBF or DGEBA to a concentration of 1 wt.%. X-ray photoelectron spectroscopy and thermogravimetric analysis verified the effectiveness of acid treatment and confirmed the amine-functionalization of the MWNTs. Scanning electron microscopy of the fracture surface of the epoxy matrix showed that chemical functionalization improves compatibility between the epoxy and MWNTs. Good dispersion of MWNTs leads to the improvement in coalescence and pull strength in the quad flat package (QFP) test. Further, the thermal conductivity of MWNTs/epoxy composites was higher than that of pure epoxy resins. In particular, the m-MWNT/epoxy composite has the best heat dissipation properties, due to the formation of an effective network for heat flow.     

Enhancement of Modulus,Strength, and Toughness in Poly(methyl methacrylate)‐Based Composites by the Incorporation of Poly(methyl methacrylate)‐Functionalized Nanotubes

Poly(methyl methacrylate) (PMMA)‐functionalized multiwalled carbon nanotubes are prepared by in situ polymerization. Infrared absorbance studies reveal covalent bonding between polymer strands and the nanotubes. These treated nanotubes are blended with pure PMMA in solution before drop‐casting to form composite films. Increases in Young's modulus, breaking strength, ultimate tensile strength, and toughness of ×1.9, ×4.7, ×4.6, and ×13.7, respectively, are observed on the addition of less than 0.5 wt % of nanotubes. Effective reinforcement is only observed up to a nanotube content of approximately 0.1 vol %. Above this volume fraction, all mechanical parameters tend to fall off, probably due to nanotube aggregation. In addition, scanning electron microscopy (SEM) studies of composite fracture surfaces show a polymer layer coating the nanotubes after film breakage. The fact that the polymer and not the interface fails suggests that functionalization results in an extremely high polymer/nanotube interfacial shear strength.     

Aligning Single‐Walled Carbon Nanotubes By Means Of Langmuir–Blodgett Film Deposition: Optical,Morphological, and Photo‐electrochemical Studies

An alkoxy‐substituted poly(phenylene thiophene) is used in order to suspend single‐walled carbon nanotubes in an organic solvent. The suspension is spread on the air–water interface of a Langmuir trough and the floating film is characterized by means of Brewster angle microscopy and UV‐visible reflection spectroscopy and the compression isotherm is recorded. The polymer/carbon‐nanotube blend is transferred onto different substrates using the Langmuir–Blodgett technique. AFM measurements indicate the formation of globular structures for the samples transferred at low surface‐pressure values and a tubular morphology for high‐pressure‐deposited samples. AFM analysis is repeated on a sample exposed to soft X‐rays for about 5 h and a highly organized structure of bundles of carbon nanotubes rises up. Samples with different numbers of layers are transferred onto ITO substrates by means of the Langmuir–Blodgett method and are tested as photocathodes in a photo‐electrochemical cell. A V_oc of 0.18 V, an I_sc of 85.8 mA, FF of 40.0%, and η of (6.23 × 10^?3)% are obtained.     

Dual‐Functionalized Polymer Nanotubes as Substrates for Molecular‐Probe and DNA‐Carrier Applications

Bifunctionalized polymer nanotubes have been fabricated using vapor‐deposition polymerization in FeCl₃‐adsorbed anodic aluminum oxide membranes followed by attachment of amine‐functionalized silica nanoparticles. The prepared bifunctionalized polymer nanotubes are applied as both a molecular probe and a DNA carrier by conjugating pyreneacetic acid with the amine groups and immobilizing DNA with the carboxylic acid groups on the surface. The number of amine functional groups on the nanotubes' surface can be measured by means of the photoluminescence intensity of pyreneacetic acid conjugated with amine groups, and the number of the residual carboxylic acid groups is calculated by titration with sodium hydroxide. Fourier‐transform infrared spectroscopy, X‐ray photoelectron spectroscopy, transmission electron microscopy, scanning electron microscopy, and confocal laser scanning microscopy have been performed to confirm the complete polymerization of the monomer and the attachment of photoluminescent molecules and single‐stranded DNA.     

Reactive Spinning of Cyanate Ester Fibers Reinforced with Aligned Amino‐Functionalized Single Wall Carbon Nanotubes

We report a new approach of reactive spinning to fabricate thermosetting cyanate ester micro‐scale diameter fibers with aligned single walled carbon nanotubes (SWNTs). The composite fibers were produced by first dispersing the SWNTs (1 wt %) in cyanate ester (CE) via solvent blending, followed by pre‐polymerization, spinning and then multiple‐stage curing. The pre‐polymerization, spinning and post‐spinning cure temperatures were carefully controlled to achieve good spun crosslinked fibers. Both pristine and amino‐functionalized SWNTs were used for the reinforced fiber spinning. Amino‐functionalized SWNTs (f‐SWNTs) were prepared by reacting acid‐treated SWNTs with toluene 2,4‐diisocyanate and then ethylenediamine (EDA). FTIR, optical microscopy and scanning electron microscopy (SEM) showed that the amino‐functionalized SWNTs were covalently and uniformly dispersed into the cyanate ester matrix and aligned along the fiber axis. The alignment was further confirmed using polarized Raman spectroscopy. The composite fibers with aligned amino‐functionalized SWNTs possess improved tensile properties with respect to neat CE fibers, showing 85, 140, and 420% increase in tensile strength, elongation and stress‐strain curve area (i.e., toughness), respectively. NH₂‐functionalization of SWNTs improves their dispersibility, alignment and interfacial strength and hence tensile properties of composite spun fibers. Fiber spinning to align SWNTs using thermosetting resin is novel. Others have reported fiber spinning to align SWNTs in thermoplastics. However, thermosetting CE resins offer the advantages of low and controllable viscosity during spinning and reactivity with amino functional groups to enable f‐SWNT/CE covalent bonding.     

Effect of Nanotube Functionalization on the Coefficient of Thermal Expansion of Nanocomposites

Single‐walled carbon nanotubes (SWNTs) are functionalized through both covalent and noncovalent bonding approaches to enhance dispersion and interfacial bonding. The coefficient of thermal expansion (CTE) of the functionalized‐SWNT‐reinforced epoxy composites are measured with a thermal mechanical analyzer (TMA). Experimental results indicate that changes of the glass‐transition temperature (T_g) in functionalized SWNT–polymer composites are dependent upon the functionalization methods. The CTE below the glass‐transition temperature of nanocomposites with a 1 wt % loading of nanotubes is substantially diminished compared to a neat polymer. A reduction in the CTE of up to 52 % is observed for nanocomposites using functionalized nanotubes. However, the CTE above the T_g significantly increases because of the contribution from phonon mode and Brownian motions of a large number of SWNTs in resin‐crosslinked networks, but the increments are compromised by possible interfacial confinement. A tunable CTE induced through nanotube functionalization has application potentials for high‐performance composites, intelligent materials, and circuit protections.     

High Performance Nanotube‐Reinforced Plastics: Understanding the Mechanism of Strength Increase

Polymer–multiwalled carbon nanotube composite films were fabricated using two types of polymer matrices, namely poly(vinyl alcohol), (PVA) and chlorinated polypropylene. In the first case, the PVA was observed to form a crystalline coating around the nanotubes, maximising interfacial stress transfer. In the second case the interface was engineered by covalently attaching chlorinated polypropylene chains to the nanotubes, again resulting in large stress transfer. Increases in Young's modulus, tensile strength, and toughness of × 3.7, × 4.3, and × 1.7, respectively were observed for the PVA‐based materials at less than 1 wt.‐% nanotubes. Similarily for the polypropylene‐based composites, increases in Young's modulus, tensile strength and toughness of × 3.1, × 3.9, and × 4.4, respectively, were observed at equivalent nanotube loading levels. In addition, a model to describe composite strength was derived. This model shows that the tensile strength increases in proportion to the thickness of the interface region. This suggests that composite strength can be optimized by maximising the thickness of the crystalline coating or the thickness of the interfacial volume partially occupied by functional groups.     

Directed Self‐Assembly of Gradient Concentric Carbon Nanotube Rings

Hundreds of gradient concentric rings of linear conjugated polymer, (poly[2‐methoxy‐5‐(2‐ethylhexyloxy)‐1,4‐ phenylenevinylene], i.e., MEH‐PPV) with remarkable regularity over large areas were produced by controlled “stick‐slip” motions of the contact line in a confined geometry consisting of a sphere on a flat substrate (i.e., sphere‐on‐flat geometry). Subsequently, MEH‐PPV rings were exploited as a template to direct the formation of gradient concentric rings of multiwalled carbon nanotubes (MWNTs) with controlled density. This method is simple, cost effective, and robust, combining two consecutive self‐assembly processes, namely, evaporation‐induced self‐assembly of polymers in a sphere‐on‐flat geometry, followed by subsequent directed self‐assembly of MWNTs on the polymer‐templated surfaces.     

Multifunctional Carbon for Electrochemical Double‐Layer Capacitors

Multifunctional carbon materials are prepared for application as an active electrode material in an electrochemical capacitor displaying both charge storage and binder properties. The synthesis of the materials involves the functionalization of high surface area Black Pearls 2000 carbon black by a covalent attachment of polyacrylic acid. The polyacrylic acid polymer is formed by atom transfer radical polymerization using 1‐(bromoethyl)benzene groups initially bonded to the carbon by spontaneous grafting from the corresponding diazonium ions. The grafting of 1‐(bromoethyl)benzene and polyacrylic acid is confirmed by thermogravimetric analysis, Fourier transform infrared spectroscopy, energy‐dispersive X‐ray spectroscopy, and nitrogen gas adsorption isotherm. The composite electrode films prepared from the modified carbon are more hydrophilic and have better wettability in an aqueous electrolyte than the one prepared with the unmodified carbon. The modified electrodes also show a higher specific capacitance (≈140 F g^?1), a wider working potential window (1.5 V) and excellent specific capacitance retention upon cycling (99.9% after 5000 cycles) in an aqueous 0.65 m K₂SO₄ electrolyte. Moreover, a relatively high specific capacitance (≈90 F g^?1) is maintained at a scan rate of 1000 mV s^?1 with the polyacrylic‐acid‐modified carbon electrode.     

Preparation of High‐Performance Conductive Polymer Fibers through Morphological Control of Networks Formed by Nanofillers

A general method is described to prepare high‐performance conductive polymer fibers or tapes. In this method, bicomponent tapes/fibers containing two layers of conductive polymer composites (CPCs) filled with multiwall carbon nanotubes (MWNT) or carbon black (CB) based on a lower‐melting‐temperature polymer and an unfilled polymer core with higher melting temperature are fabricated by a melt‐based process. Morphological control of the conductive network formed by nanofillers is realized by solid‐state drawing and annealing. Information on the morphological and electrical change of the highly oriented conductive nanofiller network in CPC bicomponent tapes during relaxation, melting, and crystallization of the polymer matrix is reported for the first time. The conductivity of these polypropylene tapes can be as high as 275 S m^?1 with tensile strengths of around 500 MPa. To the best of the authors' knowledge, it is the most conductive, high‐strength polymer fiber produced by melt‐processing reported in literature, despite the fact that only ～5 wt.% of MWNTs are used in the outer layers of the tape and the overall MWNT content in the bicomponent tape can be much lower (typically ～0.5 wt.%). Their applications could include sensing, smart textiles, electrodes for flexible solar cells, and electromagnetic interference (EMI) shielding. Furthermore, a modeling approach was used to study the relaxation process of highly oriented conductive networks formed by carbon nanofillers.     

Effect of multi-walled carbon nanotubes on electron injection and charge generation in AC field-induced polymer electroluminescence

We investigate the use of multi-walled carbon nanotubes (MWNTs) dispersed in an emissive layer of poly (N-vinylcarbazole) (PVK):fac-tris(2-phenylpyri-dine)iridium(III) [Ir(ppy)₃] in alternating current (AC) field-induced polymer electroluminescence (FIPEL) devices. A symmetric device structure, with the polymer/MWNT composite between two dielectric layers, was used to study the effect of MWNTs on charge generation within the active layer. An asymmetric device structure, using one dielectric layer, was used to study band alignment effects of carbon nanotubes in charge injection from a contact. The presence of MWNTs within the emissive layer facilitates effective internal charge generation in the symmetric devices, as would be expected if they acted as a charge source. However, electron injection under AC-driven fields also increases in the asymmetric devices, suggesting a modification to band alignment. Increase in light emission of five times is achieved in composite devices compared to devices with the pure polymer. From the trends in behavior with nanotube loading, we suggest that the nanotubes effectively doped the polymer, modifying energy level alignment in the device and increasing field-induced polarization currents. The combined effects of electron injection and charge generation may pave the way for widespread use of MWNTs in high-performance FIPEL devices.     

Transparent Thin Films of Multiwalled Carbon Nanotubes Self‐Assembled on Polyamide 11 Nanofibers

Electrospun polyamide 11 (PA11) nanofiber films are used as a guide for the deposition of two‐dimensional networks of multi‐walled carbon nanotubes (MWNTs). This method allows for the manufacturing of transparent and electrically conductive thin films. It is demonstrated that the sheet resistance (Rs) and transmittance (T) decrease, as the films become thicker due to longer electrospinning times or larger fibers. The transmittance could be improved by fusing (melting) the fibers at moderate temperatures or impregnating the film with a resin, showing that light scattering rather than absorption by the MWNTs or the polymer was responsible for a low transmittance. As the number of MWNT deposition cycles increases, the Rs decreases with a constant transmittance. A fused 100 nm film obtained after 10 min of electrospinning of the 2 wt % PA11 solution shows Rs = 154 kΩ sq^?1 and T = 83% after ten MWNT deposition cycles. A 95% transmittance was achieved after removing the polymer fibers by heating the glass plate in air (Rs = 440 kΩ sq^?1 after five MWNT deposition cycles).