Environmental Engineering Reference
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Fig. 10 SEM images of TiO
2
nanotube arrays grown around a Ti wire: Top view (a), inset is
cross-sectional view, magnified top view (b) and magnified cross-sectional view (c). The current-
voltage
characteristics
(d)
of
DSSCs
using
TiO
2
nanotube
arrays
with
different
lengths.
(Reprinted with permission from Ref. [
205
]. Copyright American Chemical Society)
2.1.4 Electrospinning and Electrospray Methods
Electrospinning is a simple and versatile nanofabrication technique for preparing
several continuous 1D nanofibers, including polymers, ceramics, composites and
metals, with controllable diameters ranging from a few nanometers to several
micrometers [
215
,
216
]. Electrospinning works using the principle of asymmetric
bending of a charged liquid jet accelerated by a longitudinal electric field (Fig.
11
)
[
217
]. A diversity of soluble and fusible polymers (e.g., polyvinylpyrrolidone (PVP),
polyacrylonitrile (PAN), polyurethane (PU) and polyvinyl acetate (PVAc)) can be
electrospun to form nanofibers from their precursor solutions [
218
,
219
]. If the
polymeric solution contains the inorganic precursors (e.g., TiO
2
, SnO
2
and ZnO),
organic/inorganic composite nanofibers are obtained. These are subsequently cal-
cinated at high temperature to thermally decompose organic components, and pro-
duce inorganic nanofibers with minimal morphological change [
135
,
220
]. The
diameter, alignment, and morphology of these nanofibers can be tailored by con-
trolling the liquid injection rate, intensity of the electric field, and shape of the
collector surface, respectively. The diameter of nanofibers also depends on the
intrinsic properties of the polymeric solution such as the viscosity and surface charge.
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