Piezoelectric Energy Harvesting Nanofibers - Hierarchical Nanostructures for Energy Devices - page 149

of 10 cm and it is observed that they have a relatively uniform diameter in
the range of 315 to 740 nm. On the other hand, the forcespinning process is
conducted under a high speed of 11 000 rpm and the resulting fibers have
diameters that varies greatly ranging from 0.95 to 3.35 mm. The XRD results
in Figure 7.2(d) show a very strong diffraction peak at 2y ¼ 20.61 and a weak
peak at 2y ¼ 36.51 corresponding to 110/200 and 020 reflections of the
orthorhombic b-phase crystal, respectively, in both electrospun and for-
cespun fiber mats, indicating the presence of a ferroelectric b-phase. 58,59
Meanwhile, the characteristic absorption bands of the b-phase at 475, 510,
840, and 1275 cm 1 are also observed in Figure 7.2(e) under FTIR. These
results suggest that pure mechanical stretching can result in a high fraction
of all-trans b-phase at 95% while electrospun fibers from the same material
system can also reach a high fraction of b-phase at approximately 99%. 60 In
summary, the results imply the importance of mechanical stretching in the
formation of the b-phase in PVDF fibers while further studies are needed to
have a full understanding of the piezoelectricity of nanofibers.

d n 3 r 4 n g | 2

7.3 Characterization Method for Piezoelectricity
The applications of piezoelectric nanofibers strongly rely on good piezo-
electric properties. While the formation the b-phase is the key to enhancing
piezoelectricity for PVDF, the PZT nanofibers need to be in the pervoskite
phase. Therefore, piezoelectric analysis techniques and tools will be neces-
sary to characterize crystal/molecular structure and inherent piezoelectric
property of electrospun nanofibers for optimal parameters during the elec-
trospinning processes. Instruments such as X-ray diffraction analysis (XRD),
Fourier transform infrared spectroscopy (FTIR), piezoresponse force micro-
scopy (PFM) and second harmonic generation microscopy (SHG) have all
been utilized for this purpose and they are briefly discussed below.

.

7.3.1 X-Ray Diffraction (X-Ray)
X-Ray diffraction (XRD) has been commonly used for the analysis of the
crystalline structure of materials. Baji et al. 65 have analyzed PVDF fibers
fabricated by far field electrospinning using XRD. It was found that the b-
phase is most abundant in electrospun PVDF nanofibers while other crys-
talline forms also existed. Electrospun nanofibers with smaller diameters
were found to have higher contents of the b-phase probably due to the strong
stretching effect. Other groups have also utilized XRD to analyze the struc-
tures of electrospun polar b-glycine nanofibers 61 and polycrystalline barium
titanate nanofibers. 62 For PZT fibers, XRD is interesting because it can be
used to confirm the presence of the pervoskite phase of the material. This
phase is crucial for piezoelectric PZT. Using a metallo-organics de-
composition (MOD) method for PZT fiber fabrication, the pervoskite phase
is formed during the fiber annealing process between 600 to 850 1C. 63 Wang
et al. used XRD analysis during the annealing process. 63 They observe that