Polymer-wrapped Carbon Nanotubes Research Articles

M-N-C-Supported Catalysts for Carbon Dioxide Reduction Reaction

Electrochemical carbon dioxide reduction (CO2RR) is a promising approach to converting CO2 into value-added chemicals using renewable electricity and to ultimately reducing the dependence on fossil resources. However, achieving sufficient activity and selectivity in economically viable CO2 electrolyzers presents a great challenge for CO2RR catalysts.1 Carbons are an important and particularly suitable component of a majority of CO2RR catalysts due to their excellent electronic conductivity, relatively easily achievable high porosity and hierarchical pore structure.2, 3 Thanks to these benefits, the metal-nitrogen-carbon (M-N-C) materials, containing at least 95 at% of carbon, have attracted special interest due to their promising selectivity for CO in CO2RR.4 In particular, the Ni-N-C support has been used to improve selectivity of Cu-based CO2RR catalysts for ethylene, attributed to the enhancement of CO generation during CO2RR.5 However, a comprehensive study is still needed to understand the effect of composition and morphology of M-N-C materials as supports for CO2RR.In this presentation, we will summarize the results of our recent study that has focused on the effect of composition (e.g., different metal centers) and morphology (e.g., porosity) of M-N-C supports on the activity and selectivity of metal (e.g., Cu) nanoparticles. We will specifically concentrate on possible advantages/disadvantages of using M-N-C materials as performance enhancing supports rather than autonomous CO2RR electrocatalysts. Acknowledgement Research presented in this work was supported by the Laboratory Directed Research and Development program of Los Alamos National Laboratory under project number 20230065DR. References (1) Masel, R. I.; Liu, Z.; Yang, H.; Kaczur, J. J.; Carrillo, D.; Ren, S.; Salvatore, D.; Berlinguette, C. P. An industrial perspective on catalysts for low-temperature CO2 electrolysis. Nature Nanotechnology 2021, 16 (2), 118-128.(2) Jhong, H.-R. M.; Tornow, C. E.; Kim, C.; Verma, S.; Oberst, J. L.; Anderson, P. S.; Gewirth, A. A.; Fujigaya, T.; Nakashima, N.; Kenis, P. J. A. Gold Nanoparticles on Polymer-Wrapped Carbon Nanotubes: An Efficient and Selective Catalyst for the Electroreduction of CO2. ChemPhysChem 2017, 18 (22), 3274-3279.(3) Baturina, O. A.; Lu, Q.; Padilla, M. A.; Xin, L.; Li, W.; Serov, A.; Artyushkova, K.; Atanassov, P.; Xu, F.; Epshteyn, A.; et al. CO2 Electroreduction to Hydrocarbons on Carbon-Supported Cu Nanoparticles. ACS Catalysis 2014, 4 (10), 3682-3695.(4) Liang, S.; Huang, L.; Gao, Y.; Wang, Q.; Liu, B. Electrochemical Reduction of CO2 to CO over Transition Metal/N-Doped Carbon Catalysts: The Active Sites and Reaction Mechanism. Advanced Science 2021, 8 (24), 2102886.(5) Wang, X.; de Araújo, J. F.; Ju, W.; Bagger, A.; Schmies, H.; Kühl, S.; Rossmeisl, J.; Strasser, P. Mechanistic reaction pathways of enhanced ethylene yields during electroreduction of CO2–CO co-feeds on Cu and Cu-tandem electrocatalysts. Nature Nanotechnology 2019, 14 (11), 1063-1070.

Click-Chemistry-Mediated Synthesis of Silver Nanoparticle-Supported Polymer-Wrapped Carbon Nanotubes: Glucose Sensor and Antibacterial Material.

We report a novel approach for the synthesis of silver nanoparticles (NPs) stabilized on polymer-wrapped carbon nanotubes (Ag@polymer/CNTs) for the non-enzymatic glucose sensing and antibacterial activity applications. Poly(styrene-alt-maleic anhydride) (PSM) was functionalized with amino furan to obtain furan-modified poly(styrene-alt-maleic anhydride) (PSMF), which was later grafted onto the surface of CNTs by Diels–Alder “click” reaction to afford a polymer/CNTs hybrid material. The photo-deposition technique was applied to immobilized small-sized (∼10 nm) AgNPs on the surface of the polymer/CNTs hybrid material using visible light irradiation. The resulting material, Ag@polymer/CNTs, showed promising electrocatalytic activity for the non-enzymatic glucose sensing and antibacterial activity in vitro assays toward Escherichia coli, Staphylococcus aureus, and Bacillus cereus bacteria strains. Covalent-bonded polymer layer-bearing carboxylic pendent groups to the CNTs might be playing a pivot role in not only stabilizing AgNPs but also facile electron-transfer reaction, thus demonstrating better activity.

High transconductance and current density in field effect transistors using arrays of bundled semiconducting carbon nanotubes

We examine if the bundling of semiconducting carbon nanotubes (CNTs) can increase the transconductance and on-state current density of field effect transistors (FETs) made from arrays of aligned, polymer-wrapped CNTs. Arrays with packing density ranging from 20 to 50 bundles μm−1 are created via tangential flow interfacial self-assembly, and the transconductance and saturated on-state current density of FETs with either (i) strong ionic gel gates or (ii) weak 15 nm SiO2 back gates are measured vs the degree of bundling. Both transconductance and on-state current significantly increase as median bundle height increases from 2 to 4 nm, but only when the strongly coupled ionic gel gate is used. Such devices tested at −0.6 V drain voltage achieve transconductance as high as 50 μS per bundle and 2 mS μm−1 and on-state current as high as 1.7 mA μm−1. At low drain voltages, the off-current also increases with bundling, but on/off ratios of ∼105 are still possible if the largest (95th percentile) bundles in an array are limited to ∼5 nm in size. Radio frequency devices with strong, wraparound dielectric gates may benefit from increased device performance by using moderately bundled as opposed to individualized CNTs in arrays.

A simple simulation-derived descriptor for the deposition of polymer-wrapped carbon nanotubes on functionalized substrates.

Controlling the deposition of polymer-wrapped single-walled carbon nanotubes (s-CNTs) onto functionalized substrates can enable the fabrication of s-CNT arrays for semiconductor devices. In this work, we utilize classical atomistic molecular dynamics (MD) simulations to show that a simple descriptor of solvent structure near silica substrates functionalized by a wide variety of self-assembled monolayers (SAMs) can predict trends in the deposition of s-CNTs from toluene. Free energy calculations and experiments indicate that those SAMs that lead to maximum disruption of solvent structure promote deposition to the greatest extent. These findings are consistent with deposition being driven by solvent-mediated interactions that arise from SAM-solvent interactions, rather than direct s-CNT-SAM interactions, and will permit the rapid computational exploration of potential substrate designs for controlling s-CNT deposition and alignment.

Brightening of Long, Polymer-Wrapped Carbon Nanotubes by sp3 Functionalization in Organic Solvents.

The functionalization of semiconducting single-walled carbon nanotubes (SWNTs) with sp3 defects that act as luminescent exciton traps is a powerful means to enhance their photoluminescence quantum yield (PLQY) and to add optical properties. However, the synthetic methods employed to introduce these defects are currently limited to aqueous dispersions of surfactant-coated SWNTs, often with short tube lengths, residual metallic nanotubes, and poor film-formation properties. In contrast to that, dispersions of polymer-wrapped SWNTs in organic solvents feature unrivaled purity, higher PLQY, and are easily processed into thin films for device applications. Here, we introduce a simple and scalable phase-transfer method to solubilize diazonium salts in organic nonhalogenated solvents for the controlled reaction with polymer-wrapped SWNTs to create luminescent aryl defects. Absolute PLQY measurements are applied to reliably quantify the defect-induced brightening. The optimization of defect density and trap depth results in PLQYs of up to 4% with 90% of photons emitted through the defect channel. We further reveal the strong impact of initial SWNT quality and length on the relative brightening by sp3 defects. The efficient and simple production of large quantities of defect-tailored polymer-sorted SWNTs enables aerosol-jet printing and spin-coating of thin films with bright and nearly reabsorption-free defect emission, which are desired for carbon nanotube-based near-infrared light-emitting devices.

Brightening of Long, Polymer-Wrapped Carbon Nanotubes By Large Scale sp3 Functionalization

Networks of long, polymer-sorted semiconducting single-walled carbon nanotubes (SWNTs) show excellent transport of both electrons and holes and are thus perfectly suited for electrically-driven light emission in the near-infrared (NIR) [1,2]. So far, their application in near-infrared light-emitting devices has been hampered by their low photoluminescence quantum yield (PLQY) and strong self-absorption owing to their small Stokes shift. In recent years, the controlled introduction of a low density of sp3 defects as exciton trapping sites was shown to lead to red-shifted emission with longer exciton lifetimes and improved PLQYs. However, due to the polarity of the employed diazonium salt reagent, this approach was limited to aqueous dispersions of SWNTs with often short tube lengths and the undesired presence of stabilizers. Here, we present a novel route to sp3 functionalization with diazonium salts in organic, non-halogenated solvents, thereby making it compatible with polymer-wrapped long SWNTs produced by shear force mixing. As a result of the low density of exciton quenching sites on shear mixed SWNTs [3], the excitons are efficiently channeled to the emissive defect sites, leading to absolute PLQYs of 4 % for functionalized (6,5) SWNT dispersions in toluene. This new method paves the way toward the large scale production of high-quality sp3 functionalized SWNTs and hence also thin films for optoelectronic devices with narrowband near-infrared emission. [1] Rother et al., ACS Appl. Mater. Interfaces 2016, 8, 5571-5579. [2] Brohmann et al., J. Phys. Chem. C 2018, 122, 19886-19896. [3] Graf et al., Carbon 2016, 105, 593-599. Figure 1

Rheological and electrical properties of polystyrene nanocomposites via incorporation of polymer-wrapped carbon nanotubes

Carbon nanotubes (CNTs) are used as nanofillers in polymer nanocomposites to improve the properties of a matrix polymer because of their excellent properties. However, CNTs tend to aggregate due to the strong van der Waals force between CNTs. To solve the problem, surface-modified CNTs with hydrophilic polymers, such as polyvinyl pyrrolidone (PVP) and polystyrene sulfonate (PSS), were employed. Polystyrene (PS)/CNT nanocomposites were fabricated by latex technology, which is a suitable method for dispersing nanofillers in aqueous particle suspension. The effect of the incorporation of the modified CNT was significant, thereby resulting in increased modulus at lower frequencies when compared to that of neat PS. Electrical percolation thresholds of PS/PSS-wrapped CNT and PS/PVP-wrapped CNT nanocomposites corresponded to 0.39 and 0.52 wt.%, respectively. The PS/PSS-CNT nanocomposites exhibited higher rheological and electrical properties than the PS/PVP-CNT counterparts due to the hydrophilic nature of PSS with strong negative charge groups.

Improved electrical stability of silver NWs based hybrid transparent electrode interconnected with polymer functionalized CNTs

Silver nanowires have high potential to replace the conventional transparent conductors in many applications. However, the electrical instability of silver nanowires especially when heated as a result of electrical current passage causes the failure of this type of conductors and thus may limit their uses in many applications. In this study, a novel approach based on interconnecting the individual nanowires with conductive polymer wrapped carbon nanotubes is examined to improve the stability of silver nanowires network. The hybrid electrode with modified carbon nanotubes shows highly stable electrical performance in comparison to silver nanowires electrode. Under continuous electrical stress, slight variation in the sheet resistance of the hybrid electrode was noticed and was about two order of magnitude less than that of silver nanowires electrode. Moreover, the thermal studies reveal that the hybrid electrode stabilizes at much lower temperature than that of silver nanowires electrode. The findings show that the proposed approach is very promising to overcome the failure issue of silver nanowires and to increase the feasibility of their use for device applications.

Excited-State Interaction of Semiconducting Single-Walled Carbon Nanotubes with Their Wrapping Polymers.

We employ photoluminescence and pump–probe spectroscopy on films of semiconducting single-walled carbon nanotubes (CNTs) of different chirality wrapped with either a wide band gap polyfluorene derivative (PF12) or a polythiophene with narrower gap (P3DDT) to elucidate the excited states’ interplay between the two materials. Excitation above the polymer band gap gives way to an ultrafast electron transfer from both polymers toward the CNTs. By monitoring the hole polaron on the polymer via its mid infrared signature, we show that also illumination below the polymer band gap leads to the formation of this fingerprint and infer that holes are also transferred toward the polymer. As this contradicts the standard way of discussing the involved energy levels, we propose that polymer-wrapped CNTs should be considered as a single hybrid system, exhibiting states shared between the two components. This proposition is validated through quantum chemical calculations that show hybridization of the first excited states, especially for the thiophene–CNT sample.

Gold Nanoparticles on Polymer-Wrapped Carbon Nanotubes: An Efficient and Selective Catalyst for the Electroreduction of CO2.

Multiple approaches will be needed to reduce the atmospheric CO2 levels, which have been linked to the undesirable effects of global climate change. The electroreduction of CO2 driven by renewable energy is one approach to reduce CO2 emissions while producing chemical building blocks, but current electrocatalysts exhibit low activity and selectivity. Here, we report the structural and electrochemical characterization of a promising catalyst for the electroreduction of CO2 to CO: Au nanoparticles supported on polymer-wrapped multiwall carbon nanotubes. This catalyst exhibits high selectivity for CO over H2 : 80-92 % CO, as well as high activity: partial current density for CO as high as 160 mA cm-2 . The observed high activity, originating from a high electrochemically active surface area (23 m2 g-1 Au), in combination with the low loading (0.17 mg cm-2 ) of the highly dispersed Au nanoparticles underscores the promise of this catalyst for efficient electroreduction of CO2 .

Wireless Oxygen Sensors Enabled by Fe(II)-Polymer Wrapped Carbon Nanotubes.

Oxygen causes food spoilage and drug degradation, which is addressed commercially by modified atmosphere packaging. We report herein a wireless oxygen sensor, O2-p-CARD, from solution processed FeII-poly(4-vinylpyridine)-single-walled carbon nanotube composites on commercial passive near-field communication tags. A large irreversible attenuation in the reflection signal of an O2-p-CARD was observed in response to oxygen at relevant concentrations, enabling non-line-of-sight monitoring of modified atmosphere packaging. These devices allow for cumulative oxygen exposure inside a package to be read with a conventional smartphone. We have demonstrated that an O2-p-CARD can detect air ingress into a nitrogen-filled vegetable package at ambient conditions. This technology provides an inexpensive, heavy-metal-free, and smartphone-readable method for in situ non-line-of-sight quality monitoring of oxygen-sensitive packaged products.

(Invited) Polymer Wrapped Carbon Nanotubes As Highly Effective Hole Transporting Layers for New Perovskite and Quantum Dot Photovoltaic Devices

Our recent work (1) has shown that polymer wrapped carbon nanotubes can be used to transform the properties of the hole transporting layer in a number of different perovskite based solar cell configurations. This presentation will report on two examples where the CNT hole transporter produces enhanced device performance, firstly for tin oxide/perovskite alloy/CNT devices and secondly in PbS colloidal quantum dots (CQD) where improved charge extraction by the CNT is particularly important. Halide perovskite solar cells remain to generate a lot of interest and enthusiasm among researchers, largely due to the prospect of readily transitioning from a purely lab based technology to real-world scales. This expectation is largely based on the rapid increase in efficiency over the past few years to values above 22%. An essential component of this device architecture are the two charge selective contacts. The n-type contact in the majority of all devices is TiO2, either as a thin compact layer, or a mesostructured configuration, but recently, SnO2 has moved into the spotlight as n-type contact with recent studies reporting excellent steady-state performances. The p-type contact on the other side of the absorber has been investigated even more intensively. The best-performing devices still use the “original” hole-transport material, spiro-OMeTAD, which provided the crucial breakthrough for the perovskite solar cell in 2012, but which suffers from the fact that it is not sufficiently conductive for efficient charge-transport, and requires extrinsic doping, typically with Li-TFSI, which has been shown to detrimentally impact the device stability. This presentation will demonstrate that for FA0.83MA0.117Pb(I0.87Br0.17)3 based solar cells we can achieve steady-state efficiencies of up to 18.8%, by using polymer-wrapped single-walled carbon nanotubes (SWNTs) as an inert conductive element in undoped spiro-OMeTAD, exceeding the performance of their fully doped counterparts. In a completely different example we can use the same polymer wrapped SWNTs as a hole collection electrode for PbS based CQD photodetectors which are capable of operating over a spectral range extending beyond one micron. We find that the use of a polymer wrapped CQD combined with PMMA filler more than doubles the current collection in such devices and enables efficiencies of over 7%. In addition the SWNT layer produces a substantial improvement in long term device stability in normal ambient conditions. (1) Habisreutinger, S. N.; Leijtens, T.; Eperon, G. E.; Stranks, S. D.; Nicholas, R. J.; Snaith, H. J. Nano Lett. 2014, 14, 5561–5568.

Improving the transesterification and electrical conductivity of vitrimers by doping with conductive polymer wrapped carbon nanotubes

Vitrimers, thermosets with exchangeable covalent bonds, have recently attracted increasing attention in the field of functional polymer materials. However, their transesterification rates and electrical properties are inadequate for many practical applications. In this study, we showed that doping vitrimers with conductive polymer wrapped carbon nanotubes could effectively facilitate the transeterification for stress relaxation and endow vritrimers with enhanced electrical conductivity. The vritrimer network was formed by curing epoxy with citric acid, in the presence of polypyrrole wrapped carbon nanotubes (CNT/PPy) as dopant. The transesterification performance, evaluated by stress relaxation analysis, showed 3.6 times faster relaxation rate, reduced transesterification activation energy and 15°C lower Tv, after doping with only 3wt% of CNT/PPy. The improved transesterification in stress relaxation, benefited from the higher thermal conductivity of carbon nanotubes and the interfacial interaction between CNT/PPy and vitrimer matrix. In contrast, pure CNT as dopant results in little enhancement suffering from strong agglomeration in the matrix. Tensile fracture analysis suggested the major role of π-π and p-π conjugation in the doping enhancement. In addition, CNT/PPy doping improved the conductivity for several orders of magnitude. This work provides a promising method for lowering temperature of transesterification and fabricating vitrimers with improved performance and extended applications.

Facile construction of mitochondria-targeting nanoparticles for enhanced phototherapeutic effects.

Phototherapy, as a noninvasive therapeutic procedure, has been applied to treat tumors. However, the application of phototherapy is often compromised by its low efficiency. Herein, we developed a novel nanoplatform based on cationic amphiphilic polymer-wrapped carbon nanotubes (rPAA@SWCNTs) with a photosensitizer, indocyanine green (ICG), for phototherapy. The as-prepared nanoparticles exhibited excellent mitochondria targeting due to the synergistic properties of highly positive charges from the polycations on the corona and the high hydrophobicity from the carbon nanotubes in the core. Moreover, the high buffer capacity of the polycations facilitated the endosomal escape of nanoparticles via a proton-sponge effect. When irradiated with an 808 nm NIR laser, ICG/rPAA@SWCNTs could precisely damage mitochondria with high efficiency and produce reactive oxygen species (ROS) and hyperthermia, which further induced the ROS burst from damaged mitochondria. The overproduced ROS accumulated in mitochondria ultimately resulted in mitochondrial damage and cell death. Therefore ICG/rPAA@SWCNTs may be able to achieve an amplifying phototherapeutic effect.

High Surface Area Electrodes Derived from Polymer Wrapped Carbon Nanotubes for Enhanced Energy Storage Devices.

Electrical double layer capacitors store energy on two adjacent layers, resulting in fast charging and discharging, but their energy density is limited by the available surface area. In this study, using poly(methyl methacrylate) assisted sonication, carbon nanotube buckypapers with specific surface area as high as 950 m(2)/g have been processed. Performance of these high surface area buckypapers have been evaluated as supercapacitor electrodes. The energy density of these high surface area electrodes at low power density of 0.68 kW/kg was 22.3 Wh/kg, and at high power density of 84 kW/kg was 3.13 Wh/kg using the ionic liquid electrolyte.

Phosphorus-nitrogen containing polymer wrapped carbon nanotubes and their flame-retardant effect on epoxy resin

A novel phosphorus-nitrogen containing polymer wrapped carbon nanotubes (CNT-PD-x, x denoted the feed ratio) were facilely prepared via strong π-π stacking interactions between the poly(phenylphosphonic-4,4’-diaminodiphenyl-methane) (PD) and the walls of carbon nanotubes (CNTs). The content of polymer PD of CNT-PD-x can be controlled by adjusting the feed ratio of polymer monomers to CNTs. The structure and properties of CNT-PD-x were characterized by Fourier transformed infrared (FT-IR) spectroscopy, 1H nuclear magnetic resonance (1H NMR), transmission electron microscopy (TEM) and thermo gravimetric analysis (TGA) measurements. The CNT-PD-x was incorporated into epoxy resin for improving the flame retardancy. The LOI value reached to 33.6% when the mass fraction of CNT-PD-x (x = 20) was 4 wt%. Compared with CNTs, the same addition of CNT-PD-x (x = 10) reduced the PHRR and THR of epoxy resin more effectively. The results implied the gas-condensed phase flame-retardant effect of CNT-PD-x, which is ascribed to the combined action of the polymer PD and CNTs on the flame retardancy of epoxy resin.

Unambiguous Diagnosis of Photoinduced Charge Carrier Signatures in a Stoichiometrically Controlled Semiconducting Polymer-Wrapped Carbon Nanotube Assembly.

Single-walled carbon nanotube (SWNT)-based nanohybrid compositions based on (6,5) chirality-enriched SWNTs ([(6,5) SWNTs]) and a chiral n-type polymer (S-PBN(b)-Ph4 PDI) that exploits a perylenediimide (PDI)-containing repeat unit are reported; S-PBN(b)-Ph4 PDI-[(6,5) SWNT] superstructures feature a PDI electron acceptor unit positioned at 3 nm intervals along the nanotube surface, thus controlling rigorously SWNT-electron acceptor stoichiometry and organization. Potentiometric studies and redox-titration experiments determine driving forces for photoinduced charge separation (CS) and thermal charge recombination (CR) reactions, as well as spectroscopic signatures of SWNT hole polaron and PDI radical anion (PDI(-.) ) states. Time-resolved pump-probe spectroscopic studies demonstrate that S-PBN(b)-Ph4 PDI-[(6,5) SWNT] electronic excitation generates PDI(-.) via a photoinduced CS reaction (τCS ≈0.4 ps, ΦCS ≈0.97). These experiments highlight the concomitant rise and decay of transient absorption spectroscopic signatures characteristic of the SWNT hole polaron and PDI(-.) states. Multiwavelength global analysis of these data provide two charge-recombination time constants (τCR ≈31.8 and 250 ps) that likely reflect CR dynamics involving both an intimately associated SWNT hole polaron and PDI(-.) charge-separated state, and a related charge-separated state involving PDI(-.) and a hole polaron site produced via hole migration along the SWNT backbone that occurs over this timescale.

Simultaneous Improvement of Hole and Electron Injection in Organic Field-effect Transistors by Conjugated Polymer-wrapped Carbon Nanotube Interlayers.

Efficient charge injection is critical for flexible organic electronic devices such as organic light-emitting diodes (OLEDs) and field-effect transistors (OFETs). Here, we investigated conjugated polymer-wrapped semiconducting single-walled carbon nanotubes (s-SWNTs) as solution-processable charge-injection layers in ambipolar organic field-effect transistors with poly(thienylenevinylene-co-phthalimide)s. The interlayers were prepared using poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) or poly(9,9-dioctylfluorene) (PFO) to wrap s-SWNTs. In the contact-limited ambipolar OFETs, the interlayer led to significantly lower contact resistance (Rc) and increased mobilities for both holes and electrons. The resulting PTVPhI-Eh OFETs with PFO-wrapped s-SWNT interlayers showed very well-balanced ambipolar transport properties with a hole mobility of 0.5 cm2V-1S-1 and an electron mobility of 0.5 cm2V-1S-1 in linear regime. In addition, the chirality of s-SWNTs and kind of wrapping of conjugated polymers are not critical to improving charge-injection properties. We found that the improvements caused by the interlayer were due to the better charge injection at the metal/organic semiconductor contact interface and the increase in the charge concentration through a detailed examination of charge transport with low-temperature measurements. Finally, we successfully demonstrated complementary ambipolar inverters incorporating the interlayers without excessive patterning.

Origin of Open Circuit Voltage in Solar Cells Based on Polymer Wrapped Carbon Nanotubes and Fullerenes

Thin films of semiconducting p-type single-walled carbon nanotubes (SWCNTs) have been recently used both for photo-absorption and for hole transport in the quest for all-carbon solar cells [1-6]. Quantum efficiency as high as 43% at the peak absorption wavelength of the nanotubes has been reported using single-chirality tubes [3]. These devices were of bilayer type, with efficiencies around 1%. More recently, bulk heterojunction (BHJ) type solar cells were also reported with a certified efficiency of 3.1% [2]. With the major focus on the interface between SWCNTs and fullerenes where the excitons generated by photo-absorption get dissociated, varying values of open circuit voltage (Voc ) have been obtained. Common to all these devices is the use of semiconducting polymers such as polyfluorene and polythiophene along with the nanotubes. During the separation process for single chirality tubes, the excess polymer is usually removed to obtain a polymer-nanotube mass ratio of 1:1. However, along with the SWCNT interfaces, there will still exist polymer-fullerene and polymer-electrode interfaces, which can influence the device performance. For example, S. Na et al [7] have shown that a polyfluorene derivative interfacial layer on the cathode side can in a BHJ solar cell with poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61(PCBM) can increase its efficiency. We report the effect of polymers on the performance of two single chirality SWCNT-C60 bilayer solar cells using: (a) poly(9,9-dioctylfluorene-2,7-diyl) (PFO) separated (7,5) tubes and (b) 9,9-di-octylfluorenyl-2,7-diyl and bipyridine copolymer (PFO-BPy) separated (6,5) tubes. We show that along with polymer-SWCNT weight ratio, the absolute weight of the polymer is also a critical parameter affecting the device operation. Performance of devices with and without SWCNTs both having the same polymer content is compared. In devices without SWCNTs, the polymer alone acts as interfacial layer between C60 and the anode. Polymer concentration dependence is also studied in these devices. It is found that the polymer layer, even at very low concentrations, increases the device efficiency by nearly an order of magnitude. While the reference cell with only C60 gave an efficiency of 0.07%, the devices with PFO interfacial layer had efficiencies up to 0.6% with an increase in all three efficiency determining parameters. The Voc of devices with PFO increased in the range of 0.2 -0.5 V over that of the C60 reference device, increasing with PFO layer thickness. In addition, while the PFO-SWCNT-C60 device gave a Voc of 0.40 V, PFO-C60 device gave a Voc of 0.56 V even though both devices contain equal concentration of PFO. These results indicate that the origin of Voc may be the PFO-C60 interface and that the presence of SWCNTs has a negative effect on achievable Voc . With PFO-SWCNT-C60 device working like a ternary system with PFO concentration dependent efficiency, optimization of this system should take into account the PFO-C60 interface as well.

Enhancement of Platinum Mass Activity at Polymer-Wrapped Carbon Nanotube-Based Fuel Cell Electrocatalysts

Polymer electrolyte fuel cells (PEFCs) are one of the most promising power sources for cars and houses due to their high-energy conversion efficiency [1-7]. Cost reduction and improved durability are the two major targets for accelerating the commercialization of PEFCs. To achieve these goals, the development of a novel method to fabricate platinum (Pt)-based electrocatalysts with a high mass activity, deposited on durable conductive support materials, is necessary. In this study, we report a facile approach to grow homogeneously dispersed Pt nanoparticles (Pt) with a narrow diameter distribution in a highly controllable fashion on polymer-wrapped carbon nanotubes (CNTs)[8]. A PEFC cell employing a composite with the smallest Pt nanoparticle size (2.3 nm diameter) exhibited a ~8 times higher mass activity compared to a cell containing Pt with a 3.7 nm diameter. This is the first example of the diameter control of Pt on polymer-wrapped carbon supporting materials. using a minimal amount of Pt.