10.1117/2.1200902.1492

Electric field assisted fabricationof carbon nanotube-nanocrystalcomposite structuresJames H. Dickerson, Sameer V. Mahajan, Saad A. Hasan, Jo-hann Cho, Milo S. Shaffer, and Aldo R. Boccaccini

Electrophoretic deposition of multilayered structures of carbon nan-otubes and nanocrystals produces nanomaterials containing homoge-neous films with sharp interfaces.

Novel nanomaterial composites comprised of carbon nan-otubes (CNTs) and nanocrystals (NCs) may revolutionizefunctional materials for applications in optical and energy-storage devices.1–5 One major trajectory in the design of thesematerials is the formation of multilayered composite structures.These architectures, with alternating layers of CNTs and NCsseparated by sharp interfaces, are highly relevant to energy-storage devices, such as supercapacitors and fuel cells, inwhich a charge must be reliably separated and transported. Ourability to deposit homogeneous layers of the CNTs and NCssequentially, using an industrially-applicable process, is centralto the development of these composite architectures.

A number of film fabrication techniques, such as dropcasting, spin casting, layer-by-layer, Langmuir-Blodgett, andelectrophoretic deposition (EPD), can be used to fabricate com-posite structures of CNTs and NCs. Of these techniques, EPD isthe most promising in the current context, due to its enhancedsite selectivity, thickness control, and high deposition rate com-pared to other deposition schemes.6 EPD involves the motion ofcharged, polar or dipolar particles that are under the influenceof an applied electric field, and their subsequent deposition onelectrodes of complementary polarity (see Figure 1).

To produce alternating layer heterostructures of CNTsand NCs, we carry out EPD under two different electricfield–current regimes: low field-high current and high field-lowcurrent.7 The different electrical conductivities of the CNT andNC media demand that we use two operating regimes. Wetreat the CNTs so that they form a well-dispersed, stable suspen-sion in polar media (e.g., water),1 while typical NCs disperse in

Figure 1. Schematic of our electrophoretic deposition system.

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non-polar media (e.g., hexane). Large currents flow throughthe CNT suspensions even at low electric fields due to thelarge electrolytic current contribution of the water. In contrast,small electrolytic currents flow through NC suspensions evenat high electric fields. As a result, the low field-high currentregime is preferred for the CNT films, and the high field-low current regime is preferred for the NC films. We combinethese two electric field-current regimes, in sequence, to formalternating layer homogeneous heterostructures of the CNTsand NCs.

We employed multi-walled CNTs and iron oxide (Fe3O4)

NCs with average diameters of 50nm and 20nm, respectively(see Figure 2) to fabricate the heterostructures. Well-dispersed,stable suspensions of the CNTs and NCs are integral to theEPD fabrication of high-quality heterostructures; these are pre-pared in water and hexane, respectively. In one example, theCNT mat-NC film-CNT mat heterostructures were deposited onsteel substrates. First, the CNT mat was deposited onto a steelsubstrate using the low field-high current regime. CNT matswere deposited only on anodes due to the presence ofnegatively-charged acidic groups on the surface of the CNTs.Next, the CNT mats were employed as an electrode to depositthe Fe3O4 NC film using the high field–low current regime. Weobserved that multiple depositions of the NCs onto the CNTmat improved the surface coverage and uniformity of the film.Later, the NC deposited-CNT mat was employed as an anode todeposit a second CNT layer. Figure 3a shows the structure of thesecond CNT mat, which is identical to that of the first CNT mat.The CNTs predominantly lie in-plane with the substrate and arerandomly oriented, producing a porous connected network thatcan be used as an electrode in subsequent device operation. Thesurface roughness of the mat, measured by atomic force mi-croscopy (AFM), is 34.6nm. Figure 3b shows a cross-sectionalimage of the heterostructures. The thicknesses of the threelayers are approximately 6µm, 150nm, and 1µm, for the firstCNT, NC, and second CNT films, respectively. The high mag-nification cross-sectional scanning electron microscopy image(Figure 3c) shows the sharp interfaces between the three layers.

We have successfully integrated low field-high current andhigh field-low current EPD techniques in an alternating se-quence to fabricate CNT-iron oxide NC heterostructures. EPDproduced homogeneous films with sharp interfaces between thelayers. The ability to fabricate such composite architectures ofnanomaterials facilitates a wide range of optical, magnetic, andenergy-storage device applications. The approach can be read-ily extended to combine a range of functional nanocrystals withcarbon nanotube electrodes.

Figure 2. a) Scanning electron microscopy (SEM) image of the multi-walled carbon nanotubes (CNTs) employed. b) Transmission electronmicroscopy (TEM) image of the 20nm iron oxide (Fe3O4) nanocrystals(NCs) employed.

Figure 3. a) SEM image of a CNT mat deposited on a stainless steelelectrode. b) Cross-sectional SEM of the CNT mat–NC heterostructure.c) Magnification of the cross-section.

Author Information

James H. Dickerson, Sameer V. Mahajan, and Saad A. HasanVanderbilt UniversityNashville, TN

Johann Cho, Milo S. Shaffer, and Aldo R. BoccacciniImperial College LondonLondon, UK

References

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6. S. Maenosono, T. Okubo, and Y. Yamaguchi, Overview of nanoparticle array forma-tion by wet coating, J. Nanopart. Res. 5, pp. 5–15, 2003. doi:10.1023/A:10244189317567. S. V. Mahajan, S. A. Hasan, J. Cho, M. S. P. Shaffer, A. R. Boccaccini,and J. H. Dickerson, Carbon nanotube-nanocrystal heterostructures fabricated byelectrophoretic deposition, Nanotechnology 19 (195301), 2008. doi:10.1088/0957-4484/19/19/195301

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