Vertically Aligned Carbon Nanotube Forest Research Articles

Sensing abilities of embedded vertically aligned carbon nanotube forests in structural composites: From nanoscale properties to mesoscale functionalities

In this paper, Vertically Aligned Carbon Nanotube (VACNT) forests are embedded into two different glass fibre/epoxy composite systems to study their sensing abilities to strain and temperature. Through a bottom-up approach, performing studies of the VACNT forest and its individual carbon nanotubes on the nano-, micro-, and mesoscale, the observed thermoresistive effect is determined to be due to fluctuation-assisted tunnelling, and the linear piezoresistive effect due to the intrinsic piezoresistivity of individual carbon nanotubes. The VACNT forests offer great freedom of placement into the structure and reproducibility of sensing sensitivity in both composite systems, independent of conductivity and volume fraction, producing a robust sensor to strain and temperature.

A simple method to grow millimeters long vertically aligned carbon nanotube forests

Growing long vertically aligned carbon nanotube (VACNT) forests have been achieved so far. However, it requires some additional steps or use of etchants to keep catalyst nanoparticles (NPs) active during VACNT growth in chemical vapor deposition (CVD). Here, we were able to grow a 2.6 mm long VACNT forest with a very simple and cost effective method without using an etchant or other complicated systems. Reducing the catalyst at lower temperature than the growth temperature before VACNT growth provided dense and uniform NPs formation. We observed that NPs, formed after low temperature reduction, did not coarsen at growth temperature at least for 20 min which extends catalyst NP lifetime. Moreover, we find that time varying forest height of VACNT is accurately represented by the Gompertz model of height evolution as a result of low temperature catalyst reduction otherwise it is represented by the monomolecular model. Additionally, use of very low carbon feedstock (10 sccm C2H4) enabled us to prevent carbon coating of active catalyst NPs and ensured a longer NP lifetime. Low carbon use also provided a cost effective and an eco-friendly synthesis of VACNT.

In Situ Measurement of Carbon Nanotube Growth Kinetics in a Rapid Thermal Chemical Vapor Deposition Reactor With Multizone Infrared Heating

Abstract Understanding and controlling the growth of vertically aligned carbon nanotube (VACNT) forests by chemical vapor deposition (CVD) is essential for unlocking their potential as candidate materials for next generation energy and mass transport devices. These advances in CNT manufacturing require developing in situ characterization techniques capable of interrogating how CNTs grow, interact, and self-assemble. Here we present a technique for real-time monitoring of VACNT forest height kinetics applied to a unique custom designed rapid thermal processing (RTP) reactor for CVD of VACNTs. While the integration of multiple infrared heating lamps enables creating designed spatiotemporal temperature profiles inside the reactor, they pose challenges for in situ measurements. Hence, our approach relies on contrast-adjusted videography and image processing, combined with calibration using 3D optical microscopy with large depth-of-field. Our work enables reliably measuring VACNT growth rates and catalytic lifetimes, which are not possible to measure using ex situ methods.

Single-Step Synthesis of Vertically Aligned Carbon Nanotube Forest on Aluminium Foils.

Vertically aligned carbon nanotube (VACNT) forests are promising for supercapacitor electrodes, but their industrialisation requires a large-scale cost-effective synthesis process suitable to commercial aluminium (Al) foils, namely by operating at a low temperature (<660 °C). We show that Aerosol-Assisted Catalytic Chemical Vapour Deposition (CCVD), a single-step roll-to-roll compatible process, can be optimised to meet this industrial requirement. With ferrocene as a catalyst precursor, acetylene as a carbon source and Ar/H2 as a carrier gas, clean and dense forests of VACNTs of about 10 nm in diameter are obtained at 615 °C with a growth rate up to 5 µm/min. Such novel potentiality of this one-step CCVD process is at the state-of-the-art of the multi-step assisted CCVD processes. To produce thick samples, long synthesis durations are required, but growth saturation occurs that is not associated with a diffusion phenomenon of iron in aluminium substrate. Sequential syntheses show that the saturation trend fits a model of catalytic nanoparticle deactivation that can be limited by decreasing acetylene flow, thus obtaining sample thickness up to 200 µm. Cyclic voltammetry measurements on binder-free VACNT/Al electrodes show that the CNT surface is fully accessible to the ionic liquid electrolyte, even in these dense VACNT forests.

Tuning the Wettability of Vertically Aligned CNT–TiO2 Hybrid Electrodes for Enhanced Supercapacitor Performance

AbstractBoosting the energy density of carbon‐based materials is of great importance for developing supercapacitors with high cyclic stability. Chemical vapor deposition (CVD) growth of vertically aligned carbon nanotube (VACNT) forest is reported with millimeter height on Si/SiO2 substrate and engineer its microstructure and wettability for remarkable supercapacitor performance. A simple method to alter the microstructure of VACNT forest by adding small amount of ethanol in KOH electrolyte is demonstrated. The modified electrolyte facilitates the formation of pores and channels in CNT forest and leads to increased active surface area. Microstructure‐engineered CNT forest is further coated conformally with 3 nm of TiO2 using atomic layer deposition. The developed VACNT‐TiO2 hybrid shows 102‐fold increase in energy density, 20‐fold increase in specific capacitance, and 13‐fold increase in power density along with high capacitive retention as compared to bare VACNT in KOH. This improved performance is correlated to formation of micro channels that enable more accessible surface area for electrolytic ions. The demonstrated simple electrolyte engineering approach with increased energy density of aligned CNT‐TiO2 hybrid is relevant for portable energy storage applications.

Phase transformation as the mechanism of mechanical deformation of vertically aligned carbon nanotube arrays: Insights from mesoscopic modeling

Vertically aligned carbon nanotube (VACNT) arrays or “forests” behave mechanically as foams when compressed, exhibiting a characteristic nonlinear stress – strain response. However, the fiber structure of VACNT forests is unlike that of cellular foams, and the microscopic mechanisms of the deformation are quite different. While numerous studies have addressed the mechanical response of VACNT forests undergoing uniaxial compression, the underlying deformation mechanisms are not yet fully established. In this paper, we report the results of large-scale mesoscopic simulations of the uniaxial compression of a VACNT forest composed of 2-μm-long carbon nanotubes (CNTs) as well as three structurally distinct forests composed of 0.6-μm-long CNTs. The simulations reveal that the compressive deformation proceeds as a phase transformation from an original low-density phase composed of vertically aligned CNT bundles to a densified phase with horizontal alignment of CNTs. The two phases are separated by a well-defined interfacial layer, which advances during the compressive deformation through localized bending and folding of nanotubes. For the 2-μm-tall forest, the folding involves correlated displacements of multiple CNT bundles, hinting on the origin of the collective buckling behavior observed in experiments. The characteristic three-stage stress-strain dependence (an initial "elastic" peak followed by an extended plateau region and a sharp rise of stress in the densification regime), commonly observed in experimental probing of the mechanical properties of VACNT forests, is reproduced in all of the simulations, suggesting that the heterogeneous propagation of densification front may be the general mechanism of the mechanical deformation of VACNT forests.

Three-Dimensionally Enlarged Photoelectrodes by a Protogenetic Inclusion of Vertically Aligned Carbon Nanotubes into CH3NH3PbBr3 Single Crystals

We demonstrate that single crystals of methylammonium lead bromide (MAPbBr3) could be grown directly on vertically aligned carbon nanotube (VACNT) forests. The fast-growing MAPbBr3 single crystals engulfed the protogenetic inclusions in the form of individual CNTs, thus resulting in a three-dimensionally enlarged photosensitive interface. Photodetector devices were obtained, detecting low light intensities (~20 nW) from UV range to 550 nm. Moreover, a photocurrent was recorded at zero external bias voltage which points to the plausible formation of a p-n junction resulting from interpenetration of MAPbBr3 single crystals into the VACNT forest. This reveals that vertically aligned CNTs can be used as electrodes in operationally stable perovskite-based optoelectronic devices and can serve as a versatile platform for future selective electrode development.

Dynamic Mechanical Analysis and High Strain-Rate Energy Absorption Characteristics of Vertically Aligned Carbon Nanotube Reinforced Woven Fiber-Glass Composites

The dynamic mechanical behavior and energy absorption characteristics of nano-enhanced functionally graded composites, consisting of 3 layers of vertically aligned carbon nanotube (VACNT) forests grown on woven fiber-glass (FG) layer and embedded within 10 layers of woven FG, with polyester (PE) and polyurethane (PU) resin systems (FG/PE/VACNT and FG/PU/VACNT) are investigated and compared with the baseline materials, FG/PE and FG/PU (i.e., without VACNT). A Dynamic Mechanical Analyzer (DMA) was used for obtaining the mechanical properties. It was found that FG/PE/VACNT exhibited a significantly lower flexural stiffness at ambient temperature along with higher damping loss factor over the investigated temperature range compared to the baseline material FG/PE. For FG/PU/VACNT, a significant increase in flexural stiffness at ambient temperature along with a lower damping loss factor was observed with respect to the baseline material FG/PU. A Split Hopkinson Pressure Bar (SHPB) was used to evaluate the energy absorption and strength of specimens under high strain-rate compression loading. It was found that the specific energy absorption increased with VACNT layers embedded in both FG/PE and FG/PU. The compressive strength also increased with the addition of VACNT forest layers in FG/PU; however, it did not show an improvement for FG/PE.

Electrochemically synthesized and vertically aligned carbon nanotube–polypyrrole nanolayers for high energy storage devices

In this work, we successfully demonstrate the fabrication of vertically aligned carbon nanotube (VACNT)-polypyrrole (PPY) nanocomposites as a “hybrid supercapacitor” material directly integrated on silicon-based electrodes. In contrast to previous works, three distinctive achievements have been accomplished: (1) a “hybrid supercapacitor” using VACNT forest with electroplated PPY and dodecylbenzenesulfonate (DBS) as a dopant in acetonitrile, (2) realizing 500% higher capacitance as compared to the capacitance of electrodes made of VACNT or DBS-doped PPY alone, and (3) highly reversible cycling between −1V and +1V with improved knee frequency at 797Hz. As such this hybrid nanocomposite could become a new class of material for future supercapacitors. We also demonstrate the life-cycle stability of the VACNT-PPY nanocomposite supercapacitor, as well as its rapid charge-discharge capabilities and low leakage current. Electrochemical impedance modeling results suggest that the VACNT-PPY supercapacitor has higher electric double layer capacitance in addition to higher pseudocapacitance compared to uncoated VACNTs.

Elastomer-infiltrated vertically aligned carbon nanotube film-based wavy-configured stretchable conductors.

Elastomer-infiltrated vertically aligned carbon nanotube (VACNT) forests are good candidates for use as stretchable conductors that can retain the electrical properties under relatively large stretching. The electrical performance can be further enhanced in terms of high stretchability and small change in the electrical resistance by using a wavy configuration. In this work, we present a wavy-structured high-performance stretchable conductor prepared by a simple prestraining approach based on polydimethylsiloxane (PDMS)-infiltrated VACNT films. Prior to the infiltration process, the VACNT forests can also be easily micropatterned by a PDMS stamp-assisted contact transfer printing technique. The conductive VACNT forest patterns are fully infiltrated with highly elastic PDMS, and the PDMS/VACNT film is conformally and strongly bonded to the prestrained PDMS substrate with the help of an intermediate thin PDMS layer, resulting in mechanical robustness of the whole device. The fabricated wavy VACNT conductor shows a small resistance change ratio of less than 6% with a tensile strain of up to 100% (prestrained level) and a high reversibility under multiple stretching/releasing cycles with a maximum strain of 100%.

Stretchable and bendable carbon nanotube on PDMS super-lyophobic sheet for liquid metal manipulation

We report a vertically-aligned carbon nanotube (CNT) forest on polydimethylsiloxane (PDMS) sheet as a novel widely stretchable and bendable anti-wetting super-lyophobic surface for naturally oxidized gallium-based liquid metals. The vertically-aligned CNT has inherent chemical inertness and a hierarchical texture combining micro/nanoscale roughness; these two characters render the developed sheet as a super-lyophobic substrate against gallium-based liquid metals. The vertically-aligned CNT forest was first grown on Si substrate and then transferred onto a PDMS sheet by imprinting. It was found that the transferred CNT on the PDMS sheet maintained its vertically-aligned nature as well as hierarchical micro/nano surface morphology. It was found that the static contact angles of the gallium-based liquid metal droplet on the CNT on Si and on the CNT on PDMS were both greater than 155° and the contact angle hysteresis on the CNT on Si was 4° and that on the transferred CNT on PDMS was 19°. These measurement results showed that the surface retains a super-lyophobic property before and after the CNT transfer onto PDMS. We tested the CNT on PDMS sheet for its mechanical flexibility using stretching (50% and 100%) and bending (curvature of 0.1 and 0.4 mm−1). We carried out a bouncing test and a rolling test on the stretched/bent CNT on the PDMS sheet and the results confirmed that the flexible sheet maintains anti-wetting characteristics under bending or stretching conditions.

In-situ observation of carbon nanotube fiber spinning from vertically aligned carbon nanotube forest

Single carbon nanotube (CNT) has exceptional electrical and mechanical properties. And CNT fiber is an interesting application because it can become the wire which is more excellent than exiting one. Some groups reported the fabrication of CNT fiber spun from vertically aligned CNTs forest (VACNF), but mechanism of spinning has not clear yet. In this paper, we present the optimum parameters for spinnable VACNF via the difference between spinnable and unspinnable VACNF.