A facile and patternable method for the surface modification of carbon nanotube forests using perfluoroarylazides

Article abstract of DOI:10.1021/ja8003446

A facile patterning method for the functionalization of vertically aligned carbon nanotubes is described. Modification of the surface of nanotube forests with hydrophilic, hydrophobic, or polymerizable small molecules was achieved via UV-triggered attachment of perfluoroarylazides. Multiple functionalizations of the tube surface can be achieved. Macro- and micropatterning of forest substrates were demonstrated. Superhydrophobic surfaces containing superhydrophilic regions were prepared. Copyright

Full text of DOI:10.1021/ja8003446

Published on Web 03/11/2008
A Facile and Patternable Method for the Surface Modification of Carbon
Nanotube Forests Using Perfluoroarylazides
Stefan J. Pastine,† David Okawa,†,§ Brian Kessler,§ Marco Rolandi,†,‡ Mark Llorente,§
Alex Zettl,§,‡ and Jean M. J. Fre´chet*,†,‡
College of Chemistry and Department of Physics, UniVersity of California, Berkeley, California 94720-1460, and
Materials Sciences DiVision, Lawrence Berkeley National Laboratory, Berkeley, California 94720
Received January 15, 2008; E-mail: frechet@berkeley.edu
In nature, a specific function is often provided by a unique
combination of surface roughness and chemical composition.
Preeminent examples include the Lotus leaf, the Water Strider leg,
and the Gecko foot. The chemical vapor deposition (CVD) of arrays
of vertically aligned carbon nanotubes (CNTs) provides a direct
means to confer microscale structure with inherent nanoscale
roughness to a surface (Figure 1). Hierarchal roughness and the
possibility for chemical functionalization make nanotube forests
attractive scaffolds for the fabrication of new materials. However,
in order to realize the potential of this biomimetic approach, two
issues need to be addressed: (1) the durability of nanotube forests
Figure 1. Patterned arrays of CNT forests provide both micro- and
nanoscale structure to a substrate.
is typically low; and (2) a facile method for chemical patterning of
forests is lacking. Herein we describe a simple scheme for patterning
robust CNT forests.
The structural integrity of vertical nanotube surfaces has been
limited by weak adhesion of the film to the substrate and the
propensity for film deformation upon solvent exposure due to strong
van der Waals forces between tubes.1 We addressed this issue by
synthesizing multiwalled carbon nanotube (MWCNT) forests via
a modified CVD technique.2 To increase forest adhesion to the
substrate, 10 nm thick iron catalyst films and a high ethylene
concentration were used during growth. Specifically, flowing pure
ethylene at 200 sccm for 10 min at a growth temperature of
750 °C resulted in forests with an average nanotube diameter of
approximately 40 nm. Importantly, the as-grown forests were
resistant to deformation by strong solvent streams and significant
mechanical pressure/scratching. We believe that the observed
durability stems from a cementing effect caused by amorphous
carbon deposited on the nanotube surface during growth.
To develop a chemical patterning method, we investigated nitrene
insertion into CNTs. Nitrenes have been used for the covalent
modification of single-wall CNTs (SWCNTs) and MWCNTs.3,4
Generation of nitrenes by photolytic decomposition of azides
provided an opportunity for modifying forests in a spatially
controlled manner. With this in mind, generic perfluoroarylazide 1
was selected as a scaffold for displaying molecular information on
the surface of the nanotubes (Figure 2). The perfluoroarylazide
moiety was selected as the nitrene precursor because of its high
intermolecular insertion efficiency relative to other aryl- or alky-
lazides.5 Furthermore, the specific azide chosen has a carboxyl
group through which a variety of functionality can be attached via
standard coupling reactions. It was envisioned that the R group in
1 could be used to tailor the surface properties, and that patterning
could be achieved via exposure through a photomask.
Figure 2. Perfluoroazides employed to modify CNT forests.
surface energy of the forests. The wetting behavior of a surface is
dictated by a combination of surface roughness and chemical
composition. Both superhydrophobic surfaces (contact angle (CA)
> 150°; sliding angle < 5°) and superhydrophilic surfaces (CA
near 0°) have aroused great interest for potential practical applica-
tions such as self-cleaning windows and stain resistant textiles.6
Due to the nanoscale roughness of the template, attachment of 2
was expected to confer superhydrophilic properties to the substrate,
while 3 was expected to confer superhydrophobic properties. As a
basic procedure, substrates were dipped in a solution of azide in
acetone (10 mg/mL), allowed to dry, exposed to UV radiation (λ
) 254 nm) for 5 min, and then rinsed by an acetone stream. Forests
treated with 2 exhibited “sponge-like” behavior after UV exposure,
with a water contact angle diminishing to nearly 0° after a few
seconds. In stark contrast, forests treated with 3 exhibited super-
hydrophobic behavior; water droplets had nearly a spherical shape
and were unstable on the surface due to the small hysteresis between
the advancing and receding CAs (θA/θR ) 162°/161°).7
Having demonstrated that azides 2 and 3 can transform the
properties of nanotube forests, we next turned our attention to azide
4. The 2-bromoisobutyrate moiety in 4 serves as an initiator for
atom transfer radical polymerization (ATRP)8 and enables the
grafting of a variety of polymers. UV-triggered attachment of 4
followed by polymerization of N-isopropylacrylamide (NIPAAm)
under ATRP conditions resulted in modification of the forest surface
with poly-NIPAAm.9 Fourier transform infrared spectroscopy
(FTIR) confirmed the presence of poly-NIPAAm on the substrate.
To test if the azide side chain can modify surface properties,
model compounds 2 and 3 were prepared. The hydroxyl-containing
2 and the fluoroalkyl-containing 3 were designed to control the
† College of Chemistry, University of California, Berkeley.
§ Department of Physics, University of California, Berkeley.
‡ Lawrence Berkeley National Laboratory.
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10.1021/ja8003446 CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
Figure 3. Scanning electron micrographs of poly-NIPAAm patterned on
MWNT forests via polymerization initiator azide 4. Dark areas represent
polymer region. (a) Polymerized background with isolated unmodified
regions. (b) Unmodified background with patterned regions of polymer.
Figure 4. Superhydrophilic patterns on superhydrophobic backgrounds:
(a) a macrochannel. (b) Dark field image of selective condensation on
micropatterned hydrophilic regions.
To determine if modification was a result of grafting from the
nanotube surface, a forest was coated in 4 and half of the substrate
was masked during UV exposure. In contrast to the exposed region,
no distinctive signals were observed in the FTIR spectrum of the
masked region. Furthermore, no polymer decomposition was
observed by thermogravimetric analysis (see Supporting Informa-
tion). These results indicated that a surface-initiated polymerization
was enabled by the phototriggered attachment of azide initiator 4.
Using appropriate photomasks, we prepared substrates with isolated
polymerized features as small as 5 µm, as well as a polymerized
background with isolated pristine regions as small as 60 µm (Figure
3).
4b). This specialized surface effectively serves as a synthetic mimic
to the back of the Stenocara beetle of the Namib desert.12 To the
best of our knowledge, this is the first demonstration of a stable
superhydrophobic surface with hydrophilic features of less than 100
µm.
In conclusion, we have developed a simple patterning method
for the functionalization of CNT forests. The nanoscale roughness
of the forest template was utilized to make superhydrophilic patterns
on a superhydrophobic background. Ongoing work in our labora-
tories is directed toward fabricating advanced materials by coupling
the optical and/or electrical properties of aligned CNTs to chemical
modification.
The exact site of functionalization had to be clarified because
of the likely presence of amorphous carbon on the forest substrates.
To ensure that CNTs can be functionalized under our treatment
conditions, we prepared bucky paper from HiPCO SWCNTs. The
static CA of the paper increased from 68 to 110° after treatment
with 3. Further evidence for covalent attachment was provided by
Raman spectroscopy. After modification, an increase in the D band
intensity (ID) occurred with a concomitant decrease in the G band
intensity (IG) (see Supporting Information). The observed increase
in the ID/IG ratio is indicative of an incorporation of defect sites in
the nanotube lattice.3A second treatment with 3 resulted in a further
increase in the ID/IG ratio, indicating that additional modification
of the substrate occurred.
The ability to increase the extent of functionalization by a second
treatment suggested that different azides could be used to further
tailor or alter the surface properties of forest substrates. This was
demonstrated by reversal of the surface energy of a forest template.
Superhydrophilic samples produced after modification with 2
became superhydrophobic by a subsequent treatment with 3. This
overriding of hydrophilicity is attributed to the significantly longer
length of the fluoroalkyl chain in 3, relative to the hydroxyl chain
in 2. Considerable attention has been paid to developing surfaces
with differential wettability.10 However, only a few examples of
stable patterned superhydrophilic-superhydrophobic surfaces have
been reported.11 The ability of 3 to override 2 allowed the fabrication
of superhydrophobic substrates with hydrophilic regions in two
simple steps: (i) blanket modification with 2; and (ii) treatment
with 3 followed by irradiation through an appropriate photomask.
Figure 4a demonstrates the wetting characteristics of a prepared
macrochannel. Water was effectively confined to the hydrophilic
region of the substrate by the superhydrophobic background.
Droplets that landed on the superhydrophobic region rolled either
off the substrate or into the hydrophilic channel. The macropatterned
films remained stable for at least 1 month. A micropatterned
substrate was also prepared using a quartz mask with 80 µm squares.
A microdroplet array was produced from selective condensation
of water vapor in the hydrophilic regions of the surface (Figure
Acknowledgment. The authors acknowledge financial support
from the Office of Basic Energy Sciences, Division of Materials
Sciences and Engineering, of the U.S. Department of Energy
contract DE-AC02-05CH11231. We thank NIH (fellowship to
S.J.P.), Intel (fellowship to M.R.), DOD (NDSEG fellowship to
B.K.) and the Miller Foundation for a Miller Professorship to A.Z.
Supporting Information Available: Experimental procedures and
spectroscopic data for starting materials and products. This material is
available free of charge via the Internet at http://pubs.acs.org.
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