Selectively Tuning the Substrate Adhesion Strength of Aligned Carbon Nanotube Arrays via Thermal Postgrowth Processing.

The excellent intrinsic properties of aligned nanofibers, such as carbon nanotubes (CNTs), and their ability to be easily formed into multifunctional 3D architectures motivate their use for a variety of commercial applications, such as batteries, chemical sensors for environmental monitoring, and energy harvesting devices. While controlling nanofiber adhesion to the growth substrate is essential for bulk-scale manufacturing and device performance, experimental approaches and models to date have not addressed tuning the CNT array-substrate adhesion strength with thermal processing conditions. In this work, facile "one-pot" thermal postgrowth processing (at temperatures Tp = 700-950 °C) is used to study CNT-substrate pull-off strength for millimeter-tall aligned CNT arrays. CNT array pull-off from the flat growth substrate (Fe/Al2O3/SiO2/Si wafers) via tensile testing shows that the array fails progressively, similar to the response of brittle microfiber bundles in tension. The pull-off strength evolves nonmonotonically with Tp in three regimes, first increasing by 10 times through Tp = 800 °C due to graphitization of disordered carbon at the CNT-catalyst interface, and then decreasing back to a weak interface through Tp = 950 °C due to diffusion of the Fe catalyst into the substrate, Al2O3 crystallization, and substrate cracking. Failure is observed to occur at the CNT-catalyst interface below 750 °C, and the CNTs themselves break during pull-off after higher Tp processing, leaving residual CNTs on the substrate. Morphological and chemical analyses indicate that the Fe catalyst remains on the substrate after pull-off in all regimes. This work provides new insights into the interfacial interactions responsible for nanofiber-substrate adhesion and allows tuning to increase or decrease array strength for applications such as advanced sensors, energy devices, and nanoelectromechanical systems (NEMS).

Micro- and Macrostructures of Aligned Boron Nitride Nanotube Arrays.

Boron nitride nanotubes (BNNTs) possess a broad range of applications because of several engineering-relevant properties, including high specific strength and stiffness, thermal stability, and transparency to visible light. The morphology of these nanoscale fibers must be controlled to maximize such properties, which can be achieved by synthesizing long aligned arrays of crystalline hexagonal boron nitride (hBN) nanotubes. Herein, we synthesize high-quality millimeter length, vertically aligned (VA-) BNNTs using free-standing carbon nanotube (CNT) arrays as scaffolds. In addition to high optical transparency of the VA-BNNTs, we also demonstrate several micro- and macrostructures of BNNTs via patterning and/or postprocessing of the arrays, including engineering of either disconnected or interconnected tubes in VA-, horizontally aligned (HA-), or coherently buckled BNNTs. The internanotube spacings and interconnections between aligned BNNT can thus be tailored to create BN macrostructures with complex shapes and advantaged morphologies for hierarchical materials and devices.

Substrate adhesion evolves non-monotonically with processing time in millimeter-scale aligned carbon nanotube arrays.

The advantageous intrinsic and scale-dependent properties of aligned nanofibers (NFs) and their assembly into 3D architectures motivate their use as dry adhesives and shape-engineerable materials. While controlling NF-substrate adhesion is critical for scaled manufacturing and application-specific performance, current understanding of how this property evolves with processing conditions is limited. In this report, we introduce substrate adhesion predictive capabilities by using an exemplary array of NFs, aligned carbon nanotubes (CNTs), studied as a function of their processing. Substrate adhesion is found to scale non-monotonically with process time in a hydrocarbon environment and is investigated via the tensile pull-off of mm-scale CNT arrays from their growth substrate. CNT synthesis follows two regimes: Mode I ('Growth') and Mode II ('Post-Growth'), separated by growth termination. Within 10 minutes of post-growth, experiments and modeling indicate an order-of-magnitude increase in CNT array-substrate adhesion strength (∼40 to 285 kPa) and effective elastic array modulus (∼6 to 47 MPa), and a two-orders-of-magnitude increase in the single CNT-substrate adhesion force (∼0.190 to 12.3 nN) and work of adhesion (∼0.07 to 1.5 J m-2), where the iron catalyst is found to remain on the substrate. Growth number decay in Mode I and carbon accumulation in Mode II contribute to the mechanical response, which may imply a change in the deformation mechanism. Predictive capabilities of the model are assessed for previously studied NF arrays, suggesting that the current framework can enable the future design and manufacture of high-value NF array applications.

Multifunctional nanocomposite structural separators for energy storage.

Separators in energy storage devices such as batteries and supercapacitors are critical elements between the much-researched anodes and cathodes. Here we present a new "structural separator" comprised of electrically-insulating aligned alumina nanotubes, which realizes a structural, or mechanically robust, function in addition to allowing charge transfer. The polymer nanocomposite structural separator is demonstrated in a supercapacitor cell and also as an interface reinforcement in an aerospace-grade structural carbon fiber composite. Relative to a polymeric commercial separator, the structural separator shows advantages both electrically and structurally: ionic conductivity in the supercapacitor cell is doubled due to the nanotubes disrupting the semi-crystallinity in the polymer electrolyte, and the structural separator creates an interface that is 50% stronger in the advanced composite. In addition to providing direct benefits to existing energy storage devices, the structural separator is best suited to multifunctional structural energy storage applications.

Ultrahigh-Areal-Capacitance Flexible Supercapacitor Electrodes Enabled by Conformal P3MT on Horizontally Aligned Carbon-Nanotube Arrays.

Nanocarbon electronic conductors combined with pseudocapacitive materials, such as conducting polymers, display outstanding electrochemical properties and mechanical flexibility. These characteristics enable the fabrication of flexible electrodes for energy-storage devices; that is, supercapacitors that are wearable or can be formed into shapes that are easily integrated into vehicle parts. To date, most nanocarbon materials such as nanofibers are randomly dispersed as a network in a flexible matrix. This morphology inhibits ion transport, particularly under the high current density necessary for devices requiring high power density. Novel flexible densified horizontally aligned carbon nanotube arrays (HACNTs) with controlled nanomorphology for improved ion transport are introduced and combined with conformally coated poly(3-methylthiophene) (P3MT) conducting polymer to impart pseudocapacitance. The resulting P3MT/HACNT nanocomposite electrodes exhibit high areal capacitance of 3.1 F cm-2 at 5 mA cm-2 , with areal capacitance remaining at 1.8 F cm-2 even at a current density of 200 mA cm-2 . The asymmetric supercapacitor cell also delivers more than 1-2 orders of magnitude improvement in both areal energy and power density over state-of-the-art cells. Furthermore, little change in cell performance is observed under high strain, demonstrating the mechanical and electrochemical stability of the electrodes.

Low-Temperature Growth of Carbon Nanotubes Catalyzed by Sodium-Based Ingredients.

Synthesis of low-dimensional carbon nanomaterials such as carbon nanotubes (CNTs) is a key driver for achieving advances in energy storage, computing, and multifunctional composites, among other applications. Here, we report high-yield thermal chemical vapor deposition (CVD) synthesis of CNTs catalyzed by reagent-grade common sodium-containing compounds, including NaCl, NaHCO3 , Na2 CO3 , and NaOH, found in table salt, baking soda, and detergents, respectively. Coupled with an oxidative dehydrogenation reaction to crack acetylene at reduced temperatures, Na-based nanoparticles have been observed to catalyze CNT growth at temperatures below 400 °C. Ex situ and in situ transmission electron microscopy (TEM) reveal unique CNT morphologies and growth characteristics, including a vaporizing Na catalyst phenomenon that we leverage to create CNTs without residual catalyst particles for applications that require metal-free CNTs. Na is shown to synthesize CNTs on numerous substrates, and as the first alkali group metal catalyst demonstrated for CNT growth, holds great promise for expanding the understanding of nanocarbon synthesis.

Influence of Different Defects in Vertically Aligned Carbon Nanotubes on TiO2 Nanoparticle Formation through Atomic Layer Deposition.

The chemical inertness of carbon nanotubes (CNT) requires some degree of "defect engineering" for controlled deposition of metal oxides through atomic layer deposition (ALD). The type, quantity, and distribution of such defects rules the deposition rate and defines the growth behavior. In this work, we employed ALD to grow titanium oxide (TiO2) on vertically aligned carbon nanotubes (VACNT). The effects of nitrogen doping and oxygen plasma pretreatment of the CNT on the morphology and total amount of TiO2 were systematically studied using transmission electron microscopy, Raman spectroscopy, and thermogravimetric analysis. The induced chemical changes for each functionalization route were identified by X-ray photoelectron and Raman spectroscopies. The TiO2 mass fraction deposited with the same number of cycles for the pristine CNT, nitrogen-doped CNT, and plasma-treated CNT were 8, 47, and 80%, respectively. We demonstrate that TiO2 nucleation is dependent mainly on surface incorporation of heteroatoms and their distribution rather than structural defects that govern the growth behavior. Therefore, selecting the best way to functionalize CNT will allow us to tailor TiO2 distribution and hence fabricate complex heterostructures.

Effect of surface functionality of silica nanoparticles on cellular uptake and cytotoxicity.

Mesoporous silica nanoparticles (MCM-41) with different surface chemistry were used as carrier system to study its influence on drug delivery and anticancer activity of curcumin (CUR). CUR was encapsulated in pristine MCM-41 (hydrophilic and negatively charged), amino functionalized MCM-41 (MCM-41-NH2 which is hydrophilic and positively charged), and methyl functionalized MCM-41 (MCM-41-CH3 which is hydrophobic and negatively charged) and evaluated for in vitro release and cell cytotoxicity in human squamous cell carcinoma cell line (SCC25). Various techniques were employed to evaluate the performance of these materials on cellular uptake and anticancer activity in the SCC25 cell line. Both positively and negatively charged surfaces demonstrated enhanced drug release and anticancer activity compared to pure CUR. Positively charged nanoparticles showed higher cell uptake compared to negatively charged nanoparticles owing to its electrostatic interaction with cells. However, hydrophobic surface modified nanoparticles (MCM-41-CH3) showed no improvement in drug release and anticancer activity due to its poor wetting effect. Cell cycle analysis and cell apoptosis studies revealed different pathway mechanisms followed by the positively and negatively charged nanoparticles but exhibiting similar anticancer activity in SCC25 cells.