In the past several years, research in carbon nanotubes (CNTs) has expanded to applications such as drug delivery, transparent conductive coatings, chemical sensors, electrodes for use in organic light-emitting diodes or lithium ion batteries, as capacitors, field effect transistors, actuators, and for use in filtration applications1–4
. The incorporation of nanotubes into such devices will necessitate processing and handling of relatively large quantities of nanotubes, most likely in powder form. These nanotube powders are difficult to handle and can readily spread through the laboratory and workplace. Reports of potential hazards associated with nanotube inhalation are raising concerns,5–11
and safe handling practices must be considered. Safe handling is particularly important when nanotubes or other nano-powder samples are transported, for example, between preparation and surface modification work-zones.
For most applications, well dispersed nanotubes are desired, but their tendency to agglomerate in solution makes them difficult to handle. To create solution dispersions of nanotubes, surfactants such as sodium dodecyl sulfate (SDS) are often used. Surfactants do not covalently bond with the nanotubes, maintaining the π bonding system while allowing the nanotubes to disperse in solution. Alternative protocols for the attachment polymers and other functional groups can alter the π conjugation system and affect the properties of the nanotubes12–14
Another method to change the surface energy of carbon nanotubes is to use vapor-phase processes to deposit a thin film coating. Atomic layer deposition (ALD) is used because it produces highly conformal coatings and is compatible with mild process conditions suitable for nanotubes. Atomic layer deposition uses a sequence of self-limiting reactions to achieve monolayer control over film thickness. The deposition of thin films alters the surface properties of the material, which can increase hydrophilicity and thus improve dispersion of carbon nanotubes in aqueous solution.
Interest in nanomaterials with modified surface composition is leading to new direct methods to uniformly coat volumes of nanotubes and other small objects (e.g. particles, powders, etc.) using vapor phase processes. Our interest, for example, is to explore ultrathin coatings on carbon nanotubes, where at its ultimate thin limit, the nanoscale coating may not significantly affect the performance of the nanotube for its particular application, but it may be able to diminish or eliminate any possible ill effects of exposure. Because nanotube powders are volatile in flowing gas, caution is required to keep nanotubes from dispersing and spreading throughout the reactor during coating. Most studies of ALD coatings on CNTs are performed using nanotubes that are anchored to a Si substrate, or nanotubes deposited on a TEM grid15,16
. Farmer et al. suspended SWNTs by growing them between two electrodes which helped stabilize them in the reactor and allow direct electrical testing17
. To coat gram quantities of nanotube powders and decrease CNT agglomeration, custom rotary or fluidized bed tool designs have emerged18–20
, but issues related to hazard mitigation during powder loading and unloading are not often discussed.
While researcher safety is always a primary concern, few reports directly discuss practices for safe handling of nano-powders as part of the process design. New procedures describing how to safely handle and contain nanomaterials during vapor phase processing will benefit many research groups in this field. For ALD coating in particular, new procedures and methods will be most attractive if they do not require special deposition equipment designs, but instead are compatible with common commercial or self-fabricated viscous flow ALD reactors. Ideally, any containment method must not interfere substantially with the coating process. The challenge, therefore, is to identify containment methods that secure the nanotubes during transfer in lab air and during vapor phase processing in a viscous flow reactor, while not impeding or otherwise interfering with the ALD reaction sequence.
For this study, two methods were explored to secure and hold carbon nanotubes during laboratory handling and ALD Al2
coating. The reactor used here is a simple self-fabricated viscous flow tubular reactor, and it includes many of the design elements of commercial laboratory-scale tools. One approach we explored used bundles of cotton fibers to absorb nanotubes that were previously dispersed in solvent. A second method used a fiber “basket” (or “wrapping”) to hold the nanotubes, where the nanotubes are loaded as dry powders into a supported fibrous capsule. The fiber “bundle” and “basket” methods both permit the reactant gases to flow through and around the nanotubes, while preventing the tubes from spreading in the reactor during ALD processing. The fiber basket or bundle can hold milligram quantities of nanotubes and improve the handling and transfer of the nanomaterials from a closed glove-box, through the lab and into the ALD reactor without undesired dissemination. The fiber basket method in particular, did not significantly affect the coating process, and we were able to repeatedly and reliably form uniform coatings as thin as 4 nm. We find significant differences in the ALD film coatings and product yields obtained by the bundle and basket methods. Specifically, we show here that the fiber wrapping method has distinct advantages over the fiber bundle approach in terms of handling, yield of coated nanotubes, and quality of the coating process. Using these methods the CNTs were not agitated to prevent aggregation, as would be the case in using fluidized bed or rotary reactors.18
The effectiveness of our fiber containment methods at preventing CNT aggregation was not directly evaluated.