Piezoelectric Energy Harvesting Nanofibers - Hierarchical Nanostructures for Energy Devices - page 143

nanogenerators. Film-based nanogenerators are often made by the spin-on
or thin-film deposition methods. 2,3 Mechanical strains due to bending,
vibration or compression of the thin-film structure can be the sources of
energy generation. Nanowire-based nanogenerators 4 are typically made of
semiconducting materials such as ZnO, 1,5,6 ZnS, 7 GaN 8,9 or CdS. 10,11 These
piezoelectric nanowires have been demonstrated to build up an electrical
potential when mechanically strained by AFM tips, 1 zig-zag electrodes 12 or
compliant substrates 13 to convert mechanical strains into electricity. The
third group of nanogenerators is based on nanofibers often constructed by
the electrospinning process to be discussed in detail in this chapter.
In terms of piezoelectric materials, lead zirconate titanate (PZT), which is a
ceramic material that exhibits exceptionally good piezoelectric properties,
has been studied as fiber-based energy harvester recently. 14-16 Even though
PZT has very high piezoelectric property compared to other polymer based
materials, it has been mainly studied as a film-based structure and typically
integrated with microelectromechanical systems (MEMS) cantilevers with
proof masses. 17-19 These energy harvesters utilize the mechanical reson-
ances of cantilevers to induce larger mechanical strain for energy generation.
However, PZT fibers generally require high temperature annealing (4600 1C)
to enhance the piezoelectric property. 20 Furthermore, PZT fibers made by the
electrospinning process require mixing of PZT with solvents that lower the
density of the PZT and lead to lower overall power eciency. On the other
hand, organic nanofibers made of polymeric polyvinylidene fluoride
(PVDF) have been studied as nanogenerators. 21-23 Compared with the
aforementioned nanomaterials, PVDF nanofibers have a unique good com-
bination of material properties: flexibility, biocompatibility and availability
in ultra-long lengths, various thicknesses and shapes, and they are light-
weight, making them an interesting candidate for energy harvesting appli-
cations in wearable and/or implantable devices.
This chapter will describe piezoelectric nanofiber nanogenerators while
nanowire and nanofiber-based nanogenerators and their fabrication processes
have been extensively discussed in several review papers. 4,24,25 Furthermore,
electrospun piezoelectric PVDF nanofibers are the main materials to be re-
viewed as they are the most commonly studied piezoelectric nanofiber
nanogenerators. The key fabrication methodology to make PVDF nanofibers is
the electrospinning process such that a key section in this chapter discusses
the details of electrospinning processes to make piezoelectric nanofibers for
high yield, scalable, and cost effective nanofiber nanogenerators.
Ceramic PZT and polymeric PVDF are two key piezoelectric materials
which have been successfully demonstrated in the making of nanofiber
nanogenerators. In these demonstrations, either near-field electrospinning
(NFES) 26 or the conventional far-field electrospinning (FFES) process has
been the key manufacturing tool to produce nanofibers. 27 For the NFES
process, a continuous single nanofiber can be deposited in a controllable
manner for nanogenerators while the FFES process can make dense nano-
fiber networks on large areas for nanogenerator demonstrations. In general,

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