Nematic-Like Alignment in SWNT Thin Films from Aqueous Colloidal Suspensions

We present a modification of the vacuum filtration technique for fabricating transparent conductive SWNT thin films with local nematic-like orientational ordering. Dilute SWNT surfactant dispersions are filtered through a vacuum filtration setup in a slow and controlled fashion. The slow filtration creates a region of high SWNT concentration close to the filter membrane. While slowly moving through this region, SWNTs interact and align with each other, resulting in the formation of thin films with local nematic ordering. Scanning electron microscopy and image analysis revealed a local scalar order parameter (S2D) of 0.7–0.8 for slow filtration, three times higher than those produced from “fast filtration” (S2D ≈ 0.24). Orientational ordering is demonstrated with different stabilizing surfactants, as well as with dispersions enriched in metallic SWNTs, produced by density-gradient ultracentrifugation. Simple estimates of relative convective versus diffusive transport highlight the main differences between slow versus fast filtration and the resulting SWNT concentration profiles. Comparisons with previous studies on three stages of liquid-crystal phase transition provide insight into the spontaneous ordering process, indicating the lack of a “healing stage”, which results in a microstructure consisting of staggered domains in our SWNT films

Nematic-Like Alignment in SWNT Thin Films from Aqueous Colloidal Suspensions

We present a modification of the vacuum filtration technique for fabricating transparent conductive SWNT thin films with local nematic-like orientational ordering. Dilute SWNT surfactant dispersions are filtered through a vacuum filtration setup in a slow and controlled fashion. The slow filtration creates a region of high SWNT concentration close to the filter membrane. While slowly moving through this region, SWNTs interact and align with each other, resulting in the formation of thin films with local nematic ordering. Scanning electron microscopy and image analysis revealed a local scalar order parameter (<i>S</i>_2D) of 0.7–0.8 for slow filtration, three times higher than those produced from “fast filtration” (<i>S</i>_2D ≈ 0.24). Orientational ordering is demonstrated with different stabilizing surfactants, as well as with dispersions enriched in metallic SWNTs, produced by density-gradient ultracentrifugation. Simple estimates of relative convective versus diffusive transport highlight the main differences between slow versus fast filtration and the resulting SWNT concentration profiles. Comparisons with previous studies on three stages of liquid-crystal phase transition provide insight into the spontaneous ordering process, indicating the lack of a “healing stage”, which results in a microstructure consisting of staggered domains in our SWNT films

Surface Pressure and Microstructure of Carbon Nanotubes at an Air–Water Interface

This article reports the surface pressure and microstructure of two different types of carbon nanotubes (CNTs) at an air–water interface; namely, as-produced CNTs (nf-CNTs) and CNTs functionalized with carboxyl groups (f-CNTs). Both types of CNTs formed 3D aggregates upon compression using a Langmuir–Pockels trough. However, f-CNTs showed a lower degree of aggregation compared with that of nf-CNTs. This is attributed to the deprotonation of the carboxyl groups within the water subphase, leading to additional electrostatic repulsion between f-CNTs. For the same initial amount of CNTs spread onto the interface, the actual coverage of f-CNTs was higher than that of nf-CNTs at a given trough area. At high compression, f-CNTs formed aligned CNT domains at the interface. These 2D domains resembled 3D liquid–crystalline structures formed by excluded volume interactions. The denser packing and orientational ordering of f-CNTs also contributed to a compressional modulus higher than that of nf-CNTs, as calculated from the surface pressure isotherms. A Volmer equation of state was applied to model the measured surface pressure containing both thermodynamic and mechanical contributions. The Volmer model, however, did not consider the loss of CNTs from the interface due to 3D aggregation and consequently overestimated the surface pressure at high compression. The actual coverage of CNT during compression was back calculated from the model and was in agreement with the value obtained independently from optical micrographs. The findings of this work may have a broader impact on understanding the assembly and collective behavior of rod-like particles with a high aspect ratio at an air–water interface

Facile Synthesis of Co₃O₄@CNT with High Catalytic Activity for CO Oxidation under Moisture-Rich Conditions

The catalytic oxidation reaction of CO has recently attracted much attention because of its potential applications in the treatment of air pollutants. The development of inexpensive transition metal oxide catalysts that exhibit high catalytic activities for CO oxidation is in high demand. However, these metal oxide catalysts are susceptible to moisture, as they can be quickly deactivated in the presence of trace amounts of moisture. This article reports a facile synthesis of highly active Co₃O₄@CNT catalysts for CO oxidation under moisture-rich conditions. Our synthetic routes are based on the in situ growth of ultrafine Co₃O₄ nanoparticles (NPs) (∼2.5 nm) on pristine multiwalled CNTs in the presence of polymer surfactant. Using a 1% CO and 2% O₂ balanced in N₂ (normal) feed gas (3–10 ppm moisture), a 100% CO conversion with Co₃O₄@CNT catalysts was achieved at various temperatures ranging from 25 to 200 °C at a low O₂ concentration. The modulation of surface hydrophobicity of CNT substrates, other than direct surface modification on the Co₃O₄ catalytic centers, is an efficient method to enhance the moisture resistance of metal oxide catalysts for CO oxidation. After introducing fluorinated alkyl chains on CNT surfaces, the superhydrophobic Co₃O₄@CNT exhibited outstanding activity and durability at 150 °C in the presence of moisture-saturated feed gas. These materials may ultimately present new opportunities to improve the moisture resistance of metal oxide catalysts for CO oxidation

High-Performance Carbon Nanotube Transparent Conductive Films by Scalable Dip Coating

Transparent conductive carbon nanotube (CNT) films were fabricated by dip-coating solutions of pristine CNTs dissolved in chlorosulfonic acid (CSA) and then removing the CSA. The film performance and morphology (including alignment) were controlled by the CNT length, solution concentration, coating speed, and level of doping. Using long CNTs (∼10 μm), uniform films were produced with excellent optoelectrical performance (∼100 Ω/sq sheet resistance at ∼90% transmittance in the visible), in the range of applied interest for touch screens and flexible electronics. This technique has potential for commercialization because it preserves the length and quality of the CNTs (leading to enhanced film performance) and operates at high CNT concentration and coating speed without using surfactants (decreasing production costs)

Relationship of Extensional Viscosity and Liquid Crystalline Transition to Length Distribution in Carbon Nanotube Solutions

We demonstrate that the length of carbon nanotubes (CNTs) can be determined simply and accurately from extensional viscosity measurements of semidilute CNT solutions. The method is based on measuring the extensional viscosity of CNT solutions in chlorosulfonic acid with a customized capillary thinning rheometer and determining CNT aspect ratio from the theoretical relation between extensional viscosity and aspect ratio in semidilute solutions of rigid rods. We measure CNT diameter <i>d</i> by transmission electron microscopy (TEM) and arrive at CNT length <i>L</i>. By studying samples grown by different methods, we show that the method works well for CNT lengths ranging from 0.4 to at least 20 μm, a wider range than for previous techniques. Moreover, we measure the isotropic-to-nematic transition concentration (i.e., isotropic cloud point) φ_iso of CNT solutions and show that this transition follows Onsager-like scaling φ_iso ∼ <i>d</i>/<i>L.</i> We characterize the length distributions of CNT samples by combining the measurements of extensional viscosity and transition concentration and show that the resulting length distributions closely match distributions obtained by cryo-TEM measurements. Interestingly, CNTs appear to have relatively low polydispersity compared to polymers and high polydispersity compared to colloidal particles