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
literature1,2 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.
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 oxide3-5 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
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
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
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
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
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
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,
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).
R. V. Pulikollu and S. M. Mukhopadhyay, "Nanoscale coatings for control of
interfacial bonds and nanotube growth", Appl. Surf. Sci. 253, 7342-7352, (2007).
Copyright AZoNano.com, Professor Sharmila M. Mukhopadhyay