Easy preparation of self-assembled high-density buckypaper with enhanced mechanical properties.

A controlled assembly and alignment of carbon nanotubes (CNTs) in a high-packing density with a scalable way remains challenging. This paper focuses on the preparation of self-assembled and well-aligned CNTs with a densely packed nanostructure in the form of buckypaper via a simple filtration method. The CNT suspension concentration is strongly reflected in the alignment and assembly behavior of CNT buckypaper. We further demonstrated that the horizontally aligned CNT domain gradually increases in size when increasing the deposited CNT quantity. The resultant aligned buckypaper exhibited notably enhanced packing density, strength, modulus, and hardness compared to previously reported buckypapers.

Experimental consideration of the Hansen solubility parameters of as-produced multi-walled carbon nanotubes by inverse gas chromatography.

The Hansen solubility parameters (HSPs) of as-produced multi-walled carbon nanotubes (APMWCNTs) were determined by means of the inverse gas chromatography (IGC) technique. Due to non-homogeneous surfaces of the APMWCNTs arising from defects and impurities, it was necessary to establish adequate working conditions for determining the HSPs of the CNTs. We then obtained the HSPs of the APMWCNTs and compared these results with earlier reports as determined by using sedimentation and molecular dynamics simulation methods. It was found that the determination of the HSPs of the CNTs by IGC can give an enhanced determination range based on the adsorption thermodynamic parameters, compared to the HSPs determined using sedimentation methods. And the HSPs of the APMWCNTs, determined here, provided good guidelines for the selection of feasible solvents that can improve the dispersion of the APMWCNTs.

Direct-Writable and Thermally One-Step Curable "Water-Stained" Epoxy Composite Inks.

In this study, a simple method for preparing direct-writable and thermally one-step curable epoxy composite inks was proposed. Specifically, colloidal inks containing a mixture of ordinary epoxy resin and anhydride-type hardener with the suspended alumina microplates, as exemplary fillers, are "stained" with small amounts of water. This increases the elasticity of the ink via the interparticle capillary attraction and promotes curing of the epoxy matrix in low-temperature ranges, causing the three-dimensional (3D) printed ink to avoid structural disruption during one-step thermal curing without the tedious pre-curing step. The proposed mechanisms for the shape retention of thermally cured water-stained inks were discussed with thorough analyses using shear rheometry, DSC, FTIR, and SEM. Results of the computer-vision numerical analysis of the SEM images reveal that the particles in water-stained inks are oriented more in the vertical direction than those in water-free samples, corroborating the proposed mechanisms. The suggested concept is extremely simple and does not require any additional cost to the one required for the preparation of the common epoxy-filler composites, which is thus expected to be well-exploited in various applications where 3D printing of epoxy-based formulations is necessary.

Highly dispersible edge-selectively oxidized graphene with improved electrical performance.

We prepared liquid phase exfoliated edge-selectively oxidized graphene (LPEOG) with a high concentration in water (∼14.7 mg ml-1) and a high ratio of a single layer (70%). The edge of graphite was selectively oxidized by step II oxidation of the modified Hummers method, and we subsequently exfoliated the edge-selectively oxidized graphite (EOG) into LPEOG. The edge selective oxidation of the LPEOG was confirmed by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), zeta-potentiometry, Raman spectroscopy, Fourier-transform infrared spectroscopy (FT-IR), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The highly concentrated LPEOG ink can be used in solution processing such as simple drawing or spin casting. Reduced LPEOG showed a higher conductivity (120 000 S m-1) than that of reduced graphene oxide (68 800 S m-1) despite the small lateral size. A transparent conducting film prepared from the LPEOG ink showed a lower surface resistance (∼2.97 kΩ sq-1) at a higher transmittance (>83.0 %T) compared to those of the graphene oxide based film. These results indicate that preservation of π-conjugation of the basal plane of graphene is critical for electrical performance of graphene. Our method facilitates solution processing of graphene for a wide range of applications.

Crucial Role of Oxidation Debris of Carbon Nanotubes in Subsequent End-Use Applications of Carbon Nanotubes.

A facile purification method for oxidized carbon nanotubes (CNTs) is developed to preserve acidic carbon compounds (ACCs) for achieving high-quality dispersion of CNTs. The remaining ACCs, which originated from the surface destruction of CNTs during the oxidation process, are considered to play a crucial role in the dispersion of CNTs in water and various polar protic solvents. To elucidate the concrete role of ACCs, a direct titration method is applied to quantitatively investigate the degree of ionization of both CNTs and ACCs in their aqueous dispersions. While ACCs with strong carboxylic groups (pKa of around 2.9) are easily removed by the neutral or base washing of oxidized CNTs, which is common in the purification process, ACC-selective purification using acid washing preserves the ACCs attached to CNTs, thereby effectively stabilizing CNT dispersions in aqueous solutions. Additionally, the Hansen solubility parameters of ACC-preserved and ACC-removed CNTs were determined by the inverse gas chromatography method to estimate their miscibility in various solvents. The preserved ACCs significantly influenced the dispersibility of CNTs in polar protic solvents, which may widen the possible application of CNTs. Specifically, the ACC-preserved high-quality CNT dispersion produces high-performance CNT buckypaper with densely packed nanostructures. The Young's modulus and tensile strength of these buckypapers reach up to 12.0 and 91.0 MPa, respectively, which exceed those of ACC-removed CNTs in previous reports.

Preparation and Exceptional Mechanical Properties of Bone-Mimicking Size-Tuned Graphene Oxide@Carbon Nanotube Hybrid Paper.

The self-assembled nanostructures of carbon nanomaterials possess a damage-tolerable architecture crucial for the inherent mechanical properties at both micro- and macroscopic levels. Bone, or "natural composite," has been known to have superior energy dissipation and fracture resistance abilities due to its unique load-bearing hybrid structure. However, few approaches have emulated the desirable structure using carbon nanomaterials. In this paper, we present an approach in fabricating a hybrid composite paper based on graphene oxide (GO) and carbon nanotube (CNT) that mimicks the natural bone structure. The size-tuning strategy enables smaller GO sheets to have more cross-linking reactions with CNTs and be homogeneously incorporated into CNT-assembled paper, which is advantageous for effective stress transfer. The resultant hybrid composite film has enhanced mechanical strength, modulus, toughness, and even electrical conductivity compared to previously reported CNT-GO based composites. We further demonstrate the usefulness of the size-tuned GOs as the "stress transfer medium" by performing in situ Raman spectroscopy during the tensile test.

Highly reproducible thermocontrolled electrospun fiber based organic photovoltaic devices.

In this work, we examined the reasons underlying the humidity-induced morphological changes of electrospun fibers and suggest a method of controlling the electrospun fiber morphology under high humidity conditions. We fabricated OPV devices composed of electrospun fibers, and the performance of the OPV devices depends significantly on the fiber morphology. The evaporation rate of a solvent at various relative humidity was measured to investigate the effects of the relative humidity during electrospinning process. The beaded nanofiber morphology of electrospun fibers was originated due to slow solvent evaporation rate under high humidity conditions. To increase the evaporation rate under high humidity conditions, warm air was applied to the electrospinning system. The beads that would have formed on the electrospun fibers were completely avoided, and the power conversion efficiencies of OPV devices fabricated under high humidity conditions could be restored. These results highlight the simplicity and effectiveness of the proposed method for improving the reproducibility of electrospun nanofibers and performances of devices consisting of the electrospun nanofibers, regardless of the relative humidity.