Formation of continuous tungsten oxide coatings on carbon nanotubes using two different methodsScience of Advanced Materials 5, 1467 (2013)
S. Coskun, K.K. Koziol

Carbon nanotube/tungsten dioxide (CNT/WO2) hybrids were prepared by in-situ formation and coating of WO2 nanoparticles on CNTs using two alternative routes, both based on an aqueous solution processing technique and carried out at room temperature. In the first route, functionalized CNTs were used, whereas the other utilized pristine CNTs with benzyl alcohol as a surfactant to enhance the adhesion of nanoparticles. Microstructural and phase characterizations of the samples were carried out using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. For both types of CNTs, a 30% ethanol–70% water solution led to continuous coatings, however, weaker interaction between the CNTs and oxide particles was obtained when pristine CNTs were used for coating. Moreover, it was seen that the amount of ethanol in the solution had a crucial role on forming the coating when pristine CNTs were used.

doi: 10.1166/sam.2013.1642
The production of aligned MWCNT/polypyrrole composite filmsCarbon 60, 229 (2013)
T. Schnoor, G. Smith, D. Eder, K.K. Koziol, G. Burstein, A. Windle, K. Schulte

We synthesized highly porous composite films of vertically aligned carbon nanotubes (CNTs) and polypyrrole (PPy) by employing a simple in situ electropolymerisation process from aqueous pyrrole solutions. By contacting the entire CNT array, each CNT acts as a highly active anode that facilitates uniform deposition and immediate polymerization of pyrrole directly on the CNT surface. As a result, each individual CNT was coated with a thin layer of PPy with a tunable thickness between 4 and 38 nm. At the same time the vertical array structure of the CNTs was preserved. A major advantage of this route is that the resulting composite film is highly porous and can be post-filled with other polymers, metals or ceramics to create new multifunctional composites and hybrid structures.

doi: 10.1016/j.carbon.2013.04.016
A facile method to tune electronic properties of carbon nanotube filmsMaterials Letters 106, 137 (2013)
D. Janas, S. Boncel, A.A. Marek, K.K. Koziol

We report a simple approach of tailoring electronic properties of carbon nanotube (CNT) films. The process was carried out in the gas phase using fuming nitric acid vapors, gases generated from piranha solution or ozone in high and low concentration in O2 as reactants. The results showed a significant alteration in terms of electrical resistance, wherein it was changed by –50% up to +1,300% depending on the employed conditions. Furthermore, as a result of chemical functionalization of the CNT backbone, we witnessed changes to the sample morphology both on the micro- and nanoscale, what could be an asset. Most interestingly, fuming HNO3 treatment introduced carboxylic and phenolic groups to the structure and as a result of hydrogen bonding, we observed bundling up of the material. This strategy could be a tool to yield CNT macroassemblies of predetermined electronic and structural properties without the need of problematic multi-step solution processing.

doi: 10.1016/j.matlet.2013.04.111
Rapid electrothermal response of high-temperature carbon nanotube film heatersCarbon 59, 457 (2013)
D. Janas, K.K. Koziol

Electrothermal materials, which convert electric energy into heat have been known for many years now. They are the heart of plethora of devices surrounding us, but their isotropic structure does not allow for tailoring of operating conditions. The carbon nanotube film heaters developed in house were built for the first time on a counter-intuitive concept to pass electric current against the alignment axis. They render superior and time-independent performance reaching terminal temperature without lag. Just 1 mg of these ultra-light free-standing films outperforms 120 g of nichrome, most commonly used resistive material. They are envisioned to become the next generation heating materials particularly in the applications, in which weight (aviation and aerospace industry), size (microreactor technology) or speed (kinetic systems) is of importance.

doi: 10.1016/j.carbon.2013.03.039
Directing chondrogenesis of stem cells with specific blends of cellulose and silkBiomacromolecules 14, 1287 (2013)
N. Singh, S. Rahatekar, K.K. Koziol, H.S. Tien, A.J. Patil, S. Mann, A.P. Hollander, W. Kafienah

Biomaterials that can stimulate stem cell differentiation without growth factor supplementation provide potent and cost-effective scaffolds for regenerative medicine. We hypothesize that a scaffold prepared from cellulose and silk blends can direct stem cell chondrogenic fate. We systematically prepared cellulose blends with silk at different compositions using an environmentally benign processing method based on ionic liquids as a common solvent. We tested the effect of blend compositions on the physical properties of the materials as well as on their ability to support mesenchymal stem cell (MSC) growth and chondrogenic differentiation. The stiffness and tensile strength of cellulose was significantly reduced by blending with silk. The characterized materials were tested using MSCs derived from four different patients. Growing MSCs on a specific blend combination of cellulose and silk in a 75:25 ratio significantly upregulated the chondrogenic marker genes SOX9, aggrecan, and type II collagen in the absence of specific growth factors. This chondrogenic effect was neither found with neat cellulose nor the cellulose/silk 50:50 blend composition. No adipogenic or osteogenic differentiation was detected on the blends, suggesting that the cellulose/silk 75:25 blend induced specific stem cell differentiation into the chondrogenic lineage without addition of the soluble growth factor TGF-β. The cellulose/silk blend we identified can be used both for in vitro tissue engineering and as an implantable device for stimulating endogenous stem cells to initiate cartilage repair.

doi: 10.1021/bm301762p
Ionic liquids-based processing of electrically conducting chitin nanocomposite scaffolds for stem cell growthGreen Chemistry 15, 1192 (2013)
N. Singh, K.K. Koziol, J. Chen, A.J. Patil, J.W. Gilman, P.C. Trulove, W. Kafienah, S.S. Rahatekar

In the present study, we have successfully combined the biocompatible properties of chitin with the high electrical conductivity of carbon nanotubes (CNTs) by mixing them using an imidazolium-based ionic liquid as a common solvent/dispersion medium. The resulting nanocomposites demonstrated uniform distribution of CNTs, as shown by scanning electron microscopy (SEM) and optical microscopy. Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction confirmed the α-crystal structure of chitin in the regenerated chitin nanocomposite scaffolds. Increased CNT concentration in the chitin matrix resulted in higher conductivity of the scaffolds. Human mesenchymal stem cells adhered to, and proliferated on, chitin–CNT nanocomposites with different ratios. Cell growth in the first 3 days was similar on all composites at a range of (0.01 to 0.07) weight fraction of CNT. However, composites at a 0.1 weight fraction of CNTs showed reduced cell attachment. There was a significant increase in cell proliferation using 0.07 weight fraction CNT composites, suggesting a stem cell enhancing function for CNTs at this concentration. In conclusion, the ionic liquid allowed the uniform dispersion of CNTs and dissolution of chitin to create a biocompatible, electrically conducting scaffold permissive for mesenchymal stem cell function. This method will enable the fabrication of chitin-based advanced multifunctional biocompatible scaffolds where electrical conduction is critical for tissue function.

doi: 10.1039/c3gc37087a
Length-dependent electrical and thermal properties of carbon nanotube-loaded epoxy nanocompositesComposites Science and Technology 81, 42-47 (2013)
M. Russ, S.S. Rahatekar, K.K. Koziol, B. Farmer, H.-X. Peng

Sonication-induced scission was used to obtain carbon nanotubes (CNTs) with aspect ratios averaging 50 (short CNTs) and 500 (long CNTs). A series of carbon nanotube/epoxy nanocomposite samples with CNT content up to 1.0 wt.% were prepared using these tubes. Electrical and thermal characterisation was conducted to identify changes in such properties and the extent of these changes as a function of aspect ratio and CNT content. Results show significantly enhanced electrical conductivities and a modest increase in the thermal conductivities in both sets of samples. Further analysis of electrical conductivity of the long CNT and short CNT-based epoxy nanocomposites was carried out in order to understand their viability for EMI shielding and electrostatic dissipation applications. The processing behaviour of the long and short CNT-based composites and the corresponding electrical properties is also discussed which will help the researchers to select the right type of carbon nanotube aspect ratio achieve specific electrical properties as well as good processing behaviour.

doi: 10.1016/j.compscitech.2013.03.011
Facile processing of zeolite based catalyst support for carbon nanotube synthesisMaterials Letters 93, 404 (2013)
S.W. Pattinson, A.H. Windle, K.K.K. Koziol

Before many of the most promising applications of carbon nanotubes can be realized, nanotube characteristics including chirality, length and diameter must be optimised. One of the most important aspects of supported carbon nanotube synthesis is the interaction of the catalyst with the substrate. The highly uniform and dense pore structures of zeolites make them ideally suited for improving carbon nanotube synthesis, but their powder form renders them incompatible with many commonly used and highly developed synthesis techniques. Existing methods for making planar substrates from zeolites are complex and unsuitable for incorporation into carbon nanotube synthesis. We present an investigation into facile methods for forming zeolites into substrates, demonstrating that these are compatible with many different CNT synthesis techniques and even able to support the growth of aligned arrays of CNTs that are hundreds of microns long. These substrates can be easily produced and incorporated into existing CNT synthesis processes and should therefore aid in the use of zeolites in CNT synthesis generally. This method is also compatible with any powder based porous material such as activated alumina or silica gel, allowing the production of substrates with a wide variety of pore size distributions and surface properties.

doi: 10.1016/j.matlet.2012.10.119