Nanoscale Multifunctional Materials: Nature Inspired Hierarchical Architectures

The largest variety of efficient and elegant multifunctional materials is seen in natural biological systems, which very seldom occur in the simple geometrical shapes of traditional man-made materials. For bio-materials involved in surface-interface related processes, common geometries involve capillaries, dendrites, hair, or fin-like attachments supported on larger substrates. It may be beneficial to incorporate similar hierarchical structures in the design and fabrication of multifunctional synthetic materials that involve surface sensitive functions such as sensing, reactivity, charge storage, thermal/electrical transport or stress transfer.

If one were to select a base material for creating such structures, graphitic carbon will perhaps be the most versatile. Hexagonal sheets of sp2 carbon can have unprecedented mechanical strength, electrical and thermal conductivity within the plane, but weaker bond-strength and conductivities normal to the planes. Therefore, properties of graphene based solids can often be dictated by relative orientation of the hexagonal planes in the overall solid.

Among various grapheme-based structures, carbon nanotubes (CNT) can be suitable building blocks for the biomimetic hierarchical structures, due to their geometry and dimensions. Moreover, there is reasonable evidence in the literature^1,2 that many of their electrical, thermal, mechanical and magnetic properties can be tailored though control of radius, chirality, helicity, and stacking that can, in-turn, be controlled through process parameters.

Recent Advances

Significant effort is being directed in Dr. Mukhopadhyay’s laboratory to fabricate and understand materials involving multiple length scales and functionalities. This review will focus on carbon nanotubes attached on larger graphitic solids, which can range from simple flat graphite to complex cellular foams having open-interconnected porosity.

Porous cellular structures can behave like lightweight solids providing significantly higher surface area compared to compact ones. Depending on what is attached on their surfaces, or what matrix is infiltrated in them, these core structures can be envisioned in a wide variety of surface active components or net-shape composites. If nanotubes can be attached in the pores, the surface area within the given space can be increased by several orders of magnitude, thereby increasing the potency of any desired surface functionality³.

This concept may sound straightforward, but until very recently, there were no established procedures for creating strongly attached nanotubes on uneven porous materials. Recent developments in this group have made this possible, thanks to a precursor nano-layer of reactive oxide^3-5 that can be created in microwave plasma. This opens up the possibility of taking a functional material of any shape and size, and attaching nanotubes on them for added surface functionality. Figure 1 shows images of CNT attached on porous graphitic foam obtained by this process.

Figure 1. Hierarchical porous carbon created by attaching nanotubes on microcellular foam. Images at various magnifications: (a) 50X (b) 500X (c) 20,000 X and (d) 150,000 X.

When this type of foam core is infiltrated with a matrix material such as epoxy, the excess interfacial area causes significant increase in interlaminar strength between the two phases. Figure 2 shows mechanical test results on foam-epoxy composites created with and without CNT attachment. The regular foam forms a brittle composite that shatters in compression, but the CNT-attached foam forms a ductile composite that allows extensive plastic deformation. These foam materials are now being tested as possible scaffolding for biomedical composites.

Figure 2. Compression testing of foam-epoxy composite specimens: comparison of foams with and without CNT attachment. (a) stress-strain plots, (b) photograph of untreated foam-epoxy composite after testing (brittle composite is easily crushed), (b) photograph of composite made with CNT-attached foam after testing (significantly tougher composite that deforms without fracturing). All test samples had starting dimensions of 6X6X6 mm cube.

Figure 3 shows bone cells cultured on them. Image analyses and biological assays indicate that CNT attachment results in higher density of bone cells having improved biological function. Since graphite is very biocompatible, these types of hierarchical cellular composites may be promising candidates for future biomedical implants.

Figure 3. Bone cells cultured on foam: (a) Electron Microscope images showing cells grow well on carbon foam (b) Cell staining images showing details of the nuclei (blue) and Cytoplasm (pink).

In addition to composite formation with matrix materials, the surfaces of these structures can be functionalized as needed for electrochemical and other surface-sensitive applications. Figure 4 shows nanoparticles of Pd attached on CNT-attached structures resulting in a miniature solid with exceptionally high electrochemical activity. These structures are currently being tested for hydrogen storage and water purification.

Figure 4. Palladium nano-particles attached on the CNT-foam material of Figure 1. This structure shows exceptionally high catalytic activity, and has many potential applications.

In summary, Mother Nature has always used hierarchical structures such as capillaries and dendrites to increase surface area and related functionality of living devices. Material scientists are just beginning to use this concept and create structures where nanotubes can be attached to larger surfaces and subsequently functionalized. This article mentions only a small sampling of materials and devices that can be enhanced by this technique. In principle, many more applications can be envisioned and created. As new architectures develop, a new wave of surface-sensitive devices related to sensing, catalysis, photo-voltaics, cell scaffolding, and gas storage applications is bound to follow.

References

1. Peter J. F. Harris, "Carbon Nanotube Science: Synthesis, Properties and Applications", Cambridge University Press, (2009).
2. M. S. Dresselhaus, G. Dresselhaus, Phaedon Avouris, "Carbon Nanotubes: Synthesis, Structure, Properties and Applications", Springer, (2001).
3. S. M. Mukhopadhyay, A. Karumuri and I. T. Barney, "Hierarchical nanostructures by nanotube grafting on porous cellular surfaces", J. Phys. D: Appl. Phys. 42, 195503, (2009).
4. R. V. Pulikollu, S. R. Higgins, S. M. Mukhopadhyay, "Model nucleation and growth studies of nanoscale oxide coatings suitable for modification of microcellular and nano-structured carbon." Surf. Coat. Technol., 203, 65-72, (2008).
5. R. V. Pulikollu and S. M. Mukhopadhyay, "Nanoscale coatings for control of interfacial bonds and nanotube growth", Appl. Surf. Sci. 253, 7342-7352, (2007).