NANO LETTERS
Dual Porosity Single-Walled Carbon Nanotube Material
2009 Vol. 9, No. 9 3302-3307
Don N. Futaba,*,† Koji Miyake,‡ Kazuhiro Murata,§ Yuhei Hayamizu,† Takeo Yamada,† Shinya Sasaki,¶ Motoo Yumura,† and Kenji Hata†,| Nanotube Research Center, AdVanced Manufacturing Research Institute Surface InteractiVe Design Group, Nanotechnology Research Institute CollaboratiVe Research Team of Super Inkjet Technology, National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba, 305-8565, Japan, Department of Mechanical Engineering, Tokyo UniVersity of Science, Japan, and Japan Science and Technology Agency, c/o AIST, Kawaguchi, 332-0012, Japan Received May 19, 2009; Revised Manuscript Received July 28, 2009
ABSTRACT We present a dual porosity CNT material with a seamless connection between highly porous aligned nanotubes and lowly porous closely packed nanotubes by using capillary action of liquids. Various approaches were developed to fabricate diverse structures using toothpicks, liquid thin films, bubbles, vapors, and superink jet printing. The dual porosity material showed low wear and was useful as a sliding electrical contact.
Porosity, being the measure of the void spaces of a material, directly reflects on the mass density and surface area and thus critically determines important material properties, such as strength, flexibility, and elasticity. Hence materials of different porosities are utilized for vastly different applications, such as porous platinum for electrodes or nonporous platinum for jewelry. A hybrid material that possesses multiple porosities simultaneously would benefit from multiple functions. An example is a “whisk” broom composed from bundled bristles creating a rigid low porous handle at one end and from unbundled bristles creating a flexible high porous brush at the other. In this letter, we demonstrate the first hybrid dual porosity CNT material composed of a highly porous assembly of aligned nanotubes (forest) and a lowly porous assembly of closely packed nanotubes (solid) seamlessly connected. We fabricated diverse hybrid structures and demonstrated their application as a low wear tribological material, useful as a sliding electrical contact. * To whom correspondence should be addressed. E-mail: d-futaba@ aist.go.jp. † Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST). ‡ Advanced Manufacturing Research Institute Surface Interactive Design Group, National Institute of Advanced Industrial Science and Technology (AIST). § Nanotechnology Research Institute Collaborative Research Team of Super Inkjet Technology, National Institute of Advanced Industrial Science and Technology (AIST). ¶ Department of Mechanical Engineering, Tokyo University of Science, Japan. | Japan Science and Technology Agency. 10.1021/nl901581t CCC: $40.75 Published on Web 08/12/2009
2009 American Chemical Society
When liquids are introduced into an aligned and sparse (highly porous) CNT forest (Figure 1a-c), the capillary forces collapse the pores creating a densely packed (low porous) CNT solid.1 In this work, we used millimeter tall SWNT forests synthesized by water-assisted chemical vapor deposition (“super growth”)2 which had a porosity of ∼97%. We used this phenomenon to locally reduce the porosity, by controlling the delivery point and quantity of the liquid, and made a hybrid material composed of dual porosities. This is different from previous reports where the entire forest was converted into solids or spongelike foams by liquid densification.1,3-5 SWNTs were grown from catalytic thin Al2O3(40 nm)/Fe (1 nm) metal layers sputtered on Si substrates with an oxidized layer of thickness 600 nm. CVD was carried out at with an ethylene carbon source (99.999%) at 50-100 cm3 STP per minute and water (100-200 ppm) as a catalyst preserver and enhancer with a growth time 10-20 min. Water vapor was supplied by passing a portion of the helium carrier gas through a water bubbler. Pure helium (99.9999%) with hydrogen (99.99999%) (total flow 1000 cm3 STP per minute) was used as a carrier gas at one atmosphere with a small and controlled amount of water vapor supplied from the water bubbler. First, to demonstrate this concept, we delivered a limited amount of water/alcohol by a toothpick to the lower end of a millimeter long SWNT forest (Figure 1a). The ratio of water to alcohol was found important to control and limit the wicking. The lower end shrank ∼20-fold, while the upper
Figure 1. Hybrid dual porosity SWNT material. (a,b) Before and after images of a high porosity SWNT forest material (a) and hybrid dual porosity material (b) made by partial collapse of the forest material. White lines indicate the tube alignment direction. (c) Model of the partial collapse process. (d) Mushroom brush structure created by bottom contact using thin liquid layer. Scale bar: 250 µm. Inset, array of mushrooms. (e) Photograph of bubble process. (f) Tepee structures created from top contact using bubble process. Scale bar: 500 µm. (g) Hard-coated SWNT structure created by complete liquid bubble envelopment. Inset, diagram of the hard-coated sparse SWNTs. Scale bar: 250 µm.
region remained unaffected (Figure 1b). Thus, this uniformly porous forest transformed into a hybrid dual porosity material where a forest (high porosity) and a solid (low porosity) were seamlessly and smoothly connected similar to the whisk broom (Figure 1c). In the hybrid material, the forest section possessed a high porosity of ∼97% while the solid section possessed a near ideal (72%) low porosity of 50%. The forest section possesses interesting mechanical properties, such as elastic recovery, dry adhesion, and field emission.6-9 As the solid section resembled fibers, excellent mechanical robustness and strength can be expected.10-13 This “local collapse” concept could be extended by using liquid thin films to control the contact point and to deliver uniform and small amounts to predetermined sections of the forest (or patterned forests). First, we could deliver a thin liquid film along the Nano Lett., Vol. 9, No. 9, 2009
substrate to selectively collapse the base of the as-grown structure to create a “mushroom” type structure (Figure 1d and inset). A bubble process was developed to overcome the reliance on a surface. The use of a self-supported bubble afforded a new degree of control and freedom. In the bubble process, CNT forest structures were brought into contact from above to utilize the inherent of the bubble and then released (Figure 1e). As an example, liquid was delivered from the top of right circular cylinder to create “tepee” structures. The quick delivery of liquid resulted in the abrupt change in porosity along the cylinder length (Figure 1f). Furthermore, by increasing the relative bubble size against the forest size, this process could easily become parallel to create an array of “tepee” structures (Figure 1f). By enveloping the entire 3303
Figure 2. Tribology evaluation. (a) Digital photograph of the SWNT hybrid brush used for testing. (b) Average friction coefficient as a function of reciprocation cycle for the hybrid brush and silicon nitride for all counter surfaces (gold thin film, HOPG, and SWNT solid sheet). (c) Average friction coefficient for each combination. (d) Shear strength per area for each combination. (e) Wear tracks of hybrid brush versus gold thin film. Inset, wear track of silicon nitride versus gold thin film. (Lateral scale (white) 200 nm; vertical scale (black) 2 µm.) (f) Comparison of wear volume for each probe-contact surface combination. (Arrow indicates upper bound as the silicon nitride was stopped prematurely.)
structure into the bubble, we could reduce the porosity on all sides of the structure and create a high density “shell” with a metallic luster surrounding the sparse interior (Figure 1g, and inset, Supporting Information, S1). These examples demonstrate that the contact point and delivered liquid amount are two key factors in determining the structure of the dual porosity material. Moreover, by understanding and controlling these basic parameters as well as more sophisticated methods, such as localized chemical modification, more complex structures, such as depth control could be possible. From its structure, the hybrid material is advantageous as a tribomaterial because the low porous section immobilizes the individual CNTs, and the seamlessly connected high porous section remains flexible. Previous CNT tribological studies carried out on CNTs as the static counter surface revealed the high friction properties of CNTs,14-18 which could be useful for rolling contacts. Distinct from these previous researches, our hybrid material provides the first opportunity to use CNTs as the actuating probe rather than only the static counter surface. 3304
Tribology was evaluated for actuating probes made from the hybrid material in a brush form and silicon nitride ball as a standard tribological material versus three counter surfaces (gold thin film, highly oriented pyrolytic graphite (HOPG), and a high density SWNT solid sheet). Specifically, the low porosity section of the hybrid brush was robustly affixed to the actuating unit of the homemade pin-on-plate tribometer. SWNT hybrid brushes (Figure 2a), which were used as pins for the tribometer could be rigidly fixed to parallel leaf springs, which were used to measure normal load and friction force. This allowed the high porous flexible free end to make contact and reciprocate hundreds of cycles against the counter surfaces at various loads while measuring the friction coefficient (Figure 2b). The opposing contact surfaces were fixed to a voice coil driven two axis scanning stage and linearly reciprocated at a rate of 0.1 Hz and a stroke of 2.5 mm for over 500 cycles at a scanning speed was 0.5 mm/sec. The normal load was controlled by piezoelectric actuator ranging from 20 to 100 mN. The contact area was estimated from the wear scar, and assuming a circular contact area, the width of wear track corresponded to the diameter Nano Lett., Vol. 9, No. 9, 2009
Figure 3. Sliding electrical contacts. (a) Wide hybrid brush connected to wire contact. (b) Side view. Scale bar, 1 cm. (c) Sliding contact made from four wide hybrid brushes. Scale bar, 5 mm. Inset, axial view. (d) Photograph of the DC motor setup. (e) Composite photograph showing the motion of the electrical motor using hybrid brush component.
of the contact area circle. The estimated average friction coefficients for the SWNT-brush and silicon nitride were similar for the solid sheet and HOPG (0.74, 0.08 versus 1.03, 0.09) but differed significantly for Au film (0.77 versus 0.18, respectively (Figure 2c)). These values are similar as previous reports.14-18 Importantly, the friction of the SWNT brushes was more stable than the silicon nitride ball which underwent a number of abrupt changes (Figure 2b). As presented in Supporting Information, Figure S3 the friction force, F, was independent of the normal load, N, meaning that we are measuring the macroscopic coefficient measurements (i.e., µ ) dF/dL). Furthermore, the estimated interfacial shear strength per unit area, τ, which is the ratio of the friction force, F, and the contact area, A (i.e., F ) τ·A), for the SWNT brushes was significantly lower in all cases (Figure 2d). Simply put, this means that the smaller the interfacial shear strength per area, the smaller the friction force which is important for reducing wear. Significantly, the hybrid brush exhibited low wear. In general, wear is the removal of material through surface contact, and the lower the rate of removal translates to longer component lifetime. First, qualitatively, the hybrid brush showed good stability in the friction force that reflects the low wear. Second, the images and cross section (Figure 2e for Au, Supporting Information Figure S2) of the wear scars for the SWNT brushes showed almost undamaged counter surfaces and, significantly, no CNT graphitic debris measurable by Raman spectroscopy. In contrast, for silicon nitride balls, the wear scars showed significant damage to the counter surface. In fact, silicon nitride ball wore through the gold film (Figure 3e (inset)). The wear was quantitatively Nano Lett., Vol. 9, No. 9, 2009
compared by estimating the wear volume, that is, the volume of removed material. The SWNT brushes removed at least 30% less material (Figure 2f) as the test was interrupted when the Au film tore (Figure 2e (inset)). The low wear of CNT hybrid brush stems from two factors: first, all the long SWNTs are immobilized in the low porosity section. Second the high porous section was composed of high density (c.a. 5 × 1011 nanotubes/cm2) millimeter-long SWNTs that had enough flexibility to adjust the shape to increase contact with the target material, a point important to reduce contact pressure and interfacial shear strength directly related to reducing wear. Having low wear is important not only for having long lifetime but also to reduce debris which is the single largest problem for carbonbased brushes. Low wear properties against a metal, for example, gold, holds promise as electrical contact materials. The hybrid material has the potential to be used as sliding electrical contacts from their conductivity and low wear. The high porous section can adjust its shape to increase the electrode contact for better conductivity, and the nanotubes are immobilized at the low porous section resulting in low wear. In addition, we could assemble hybrid materials into device components with controlled structure. To demonstrate this, first we fabricated wide brush-hybrids (1 mm in width). These wide brush-hybrids were made from 1 × 10 mm rectangular forests placed onto a glass slide, and the base was selectively collapsed by a thin liquid film. Second, four wide brush-hybrids were assembled into a “clover” configuration with the low porous section connected to a copper wire and the high porous section forming a uniform, circular exterior (Figure 3c). Third, this component was installed into 3305
Figure 4. Process development. (a) Bed of needles created by vapor deposition. Scale bar: 200 µm. Inset, schematic of the vapor deposition method. (b) Curvilinear patterned SWNT forest using liquid vapor. Scale bar: 1 mm. (c) Honeycomb pattern hybrid created by super inkjet printing. Scale bar: 50 µm. Lines written into the SWNT forest by inkjet printing. Inset, schematic of super inkjet printing. (d) Zoom in of the 15 µm wide lines. Scale bar: 10 µm.
a DC motor as a commutator contact with the high porous circular exterior making contact with the copper electrodes (Figure 3c (inset),d). Upon operation, rotation in excess of 300 rpm (Figure 3e and movie Supporting Information, S1) with a current (∼0.1 A) was achieved. While this CNT brushhybrid may not appropriate to replace conventional carbon brushes in all applications, the low wear and flexibility imply that such commutators might be useful in vibrator motors for cell phones where the lifetime is limited by oxidization of the metal contacts. As demonstrated, the liquid film approach can fabricate useful and functional components, however, the scale and degree of control is limited by the size and thickness of the film. To address these issues, we developed two approaches based on vapor deposition and inkjet printing. Vapor deposition affords uniform deposition over large areas with both precisely controlled amount and rate (Figure 4a (inset)). For example, a bed of needles was made by depositing vapor to a prepatterned rectangular parallel-pipeds cut to 50% depth (Figure 4a). Another example is porcupine-like structure made from vapor deposition to the reverse side of a similarly prepared sample that triggered bowing of the entire sample (Figure 4b). The weakness of this approach is the inability to limit liquid delivery to specific locations that could be overcome with the use of masks. However, superinkjet printing can offer a more direct solution to this issue (Figure 4c (inset)). Superinkjet printing is the current state-of-theart method to deliver small amounts of liquids (minimum droplet size, 0.4 µm) at precise locations (stage resolution, 3306
100 nm). Through this method, increasingly complex hybrid material could be designed over large areas. An example is a honeycomb hybrid material patterned on the as-grown forest (Figure 4c) with line widths of 5 µm (Figure 4d). The honeycomb pattern makes up the low porosity section while the CNTs between and below the drawn pattern remain highly porous. As demonstrated, many different approaches exist addressing issues such as, scalability, resolution, and control to create dual porosity hybrid materials with diverse structures and functions. We have succeeded in creating a dual porosity CNT material of seamlessly connected highly porous aligned nanotubes and lowly porous closely packed nanotubes by using capillary action of liquids. Various approaches were developed to fabricate diverse structures using toothpicks, liquid thin films, bubbles, vapors, and superink jet printing. The dual porosity material showed low wear and was useful as a sliding electrical contact. Acknowledgment. We gratefully acknowledge the helpful contributions by M. Mizuno, K. Matsuno, and S. Yamada. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Futaba, D. N.; et al. Nat. Mater. 2006, 5, 987–994. (2) Hata, K.; et al. Science 2004, 306, 1362–1364. (3) Chakrapani, N.; Wei, B.; Carrillo, A.; Ajayan, P.; Kane, R. S. Proc. Nat. Acad. Sci. U.S.A. 2004, 101, 4009–4013. Nano Lett., Vol. 9, No. 9, 2009
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