Page 401 - Organic Electronics in Sensors and Biotechnology

378 Chapter Ten

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and mixtures of distilled water and methanol. Films containing long
77
SWCNT bundles are obtained. Andrade et al. have compared differ-
ent techniques such as dip-coating, filtration, spray coating, and EPD
for CNT deposition. The result suggests that dip-coating and EPD pro-
vide the smoothest CNTs and may be an interesting option for solar
cell applications, among others.
Girishkumar et al. used EPD to deposit a thin film of SWCNT
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modified with tetraoctylammonium bromide (TOAB) in tetrahydro-
furan (THF) on aminopropyltriethoxysilane (APS) coated, optically
transparent electrodes (OTEs) made of conductive glass. The TOAB
binds to the surface of the CNT during sonication by hydrophobic
interactions of its alkyl chain thereby preventing aggregation and set-
tling of nanotubes. The same group has reported the fabrication of a
membrane electrode assembly for hydrogen fuel cells by using EPD
to deposit a SWCNT support and a Pt catalyst on carbon fiber elec-
79
trodes. Both the electrophoretically deposited nanotubes and plati-
num retained their nanostructured morphology on the carbon fiber
surface. Kurnosov et al. have suggested introducing a resistive
80
material on top of the conductive cathode to improve the adhesion of
the CNTs to the substrate and the uniformity of the deposited film. A
suspension of SWCNT in NiCl /isopropyl alcohol was deposited on
2
48
an ITO-coated aluminum cathode. Oh et al. have performed a simi-
lar experiment in which functionalized SWCNTs are stabilized in
MgCl /ethanol and are deposited on ITO-coated glass. In both cases,
2
the nanotubes strongly adhere to the ITO coating. The adhesion has
been attributed to two factors. The first is the interaction between the
hydrophilic CNT and ITO surface. 48, 81 The second is the presence of
the charger salt, MgCl , since Mg ions form hydroxides at the sur-
2+
2
face of the negative electrodes that assist the interfacial bonding. 81
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Girishkumar et al. have introduced APS coating on OTE surface to
obtain more uniform electrophoretically deposited films of CNTs.
Kamat et al. have extensively investigated the assembly of solu-
82
bilized SWCNTs into linear bundles at a high DC voltage (>100 V)
and their deposition on OTE at relatively low DC voltage (∼50 V).
Purified SWCNTs are solubilized by mixing with TOAB in THF. The
SWCNT films of varying thickness are obtained by adjusting the
deposition time. At high DC voltage of >100 V, the CNTs do not
deposit and become aligned perpendicular to the two electrodes
(parallel to the field). The influence of electrode separation has been
80
investigated by Kurnosov et al. They have found that the uniformity
of field emission depended significantly on the electrode separation.
The best uniformity is obtained at the lower end of the separations
tested (0.3 to 1.8 cm). These authors have observed that for larger
electrode separations, the emission sites are concentrated at the edges
of the electrodes due to nonuniformity of the electric field.
Once a porous CNT coating or film is obtained, EPD can be
employed to deposit ceramic or metallic nanoparticles with the aim