3.2. Optical properties
Reflectance and transmittance:
Figure 5 shows the reflectance of UV-Vis. Spectrum of Co-ZnFe2O4 and Cu-Zn Fe2O4. The fig. indicates an increase in the reflectance intensity by increasing wavelength.
Figure 6 explains the reflectance intensity of the pure PVDF polymer and prepared nanocomposites. It is obvious that there is clear increase in the PVDF reflectance when nano-fillers were added, the largest values were spotted in case of PVDF/(Co-ZnFe2O4) nanocomposite, this increase reached 5 times the PVDF original value. This observed increase in the nanocomposite reflactance is attributed to light scattering caused by the nanoparticles, where cobalt has higher refractive index [22]. We can conclude that the (PVDF/Co-ZnFe2O4) nanocomposite film had the lower transmittance intensity.
Moreover, Fig. 7, (a) shows a remarkable increase in the PVDF absorption coefficient (α) with the addition of nanofillers. The absorption coefficient (α) of the investigated samples were obtained using the following equation [23].
(2)
where A: absorbance, l: thickness of the specimen.
The high absorbance values in the UV region for the investigated nanocomposite films make them of interest in UV protection applications [24 &25].
The extinction coefficient (K) was calculated using the following equation [23]
K = αλ/4π (3)
where, α and λ are the absorption coefficient, and wavelength respectively.
The relation between the extinction coefficient (K) and wavelength for neat PVDF and (PVDF/Co-ZnFe2O4) nanocomposite is illustrated in Figure. (7 b). It is observed that (K) values increases by increasing the wavelength which attributed to the interaction between the incident photons and electrons. As well as K is increased by the addition of the nanofillers as the density of the PVDF increased by addition of nanofiller.
Optical bandgap:
An elctron can be moves from its normal state to an excited state as it is absorbing energy from an incident photon greater than its band gap energy. In photoexcitation an electron moves from the valence band into the conduction band across the optical band gap. The lower the energy of the band gap (\({E}_{g}\)), the easier for an electron to move from the valance band to the conduction band [26]. In the present work the optical band gap energy can be estimated using Tauc’s relation (T-region):
$${\left(\alpha h\nu \right)}^{n}=A(h\nu - {E}_{g})$$
4
where α is absorption coefficient, A is constant, n indicate the optical transition type (n = 2 indicates direct transition and n = ½ indicates indirect transition) [27–29]. Figure 8 shows the relation between \({\left(\alpha h\nu \right)}^{2}\) verse the incident photon energy (\(h\nu )\). From the linear parts of the obtained curves the direct band gap of the neat PVDF and PVDF / (Co-ZnFe2O4, Cu-Zn Fe2O4,) nanocomposites have been calculated and tabulated in Table (1). It is noticed that the addition of nanoparticles, reduced the PVDF band gap to half its original value, which might be due to incorporation lager number of defects leading to the creation of new levels in the PVDF band gap facilitates the movement of electrons from valance to conduction band.
Table (1): Direct band-gap energy of PVDF and PVDF / (Co-ZnFe2O4, Cu-Zn Fe2O4,) nanocomposites
Composite
|
Direct bandgap (Eg.)(ev)
|
PVDF
|
4.45
|
PVDF / (Co-ZnFe2O4)
|
2.9
|
PVDF / (Cu-ZnFe2O4)
|
2.48
|
Refractive index:
The refractive index is Another mainly important elemental property of material because of its direct relationship with the ion’s electronic polarizability and the local field within the material. The refractive index (n) of composite materials has been calculated via given equation [30],
$$n= \frac{\left(1+R\right)}{\left(1-R\right)}+ \sqrt{\frac{4R}{{\left(R-1\right)}^{2}}-{K}^{2}}$$
5
where R is reflectance and K is the extinction coefficient.
Figure 9 shows the plot of the refractive index (n) versus (λ) of the samples under investigation. A remarkable increase in the refractive index value (n) was observed in the PVDF by the addition of the Co-ZnFe2O4 nanoparticles. This increase can be attributed to the higher intermolecular chemical and physical interaction between the filler and the adjacent PVDF chain segments which lead an improvement of the films densities resulting in higher refractive indices and the higher refractive index [22]. The maximum value for refractive index was related to PVDF/Co-ZnFe2O4 nanocomposite.
Optical conductivity:
The optical conductivity σopt. is an important parameter for studying the electronic states in materials. The optical conductivity was obtained using the following equation, [23]:
$${\sigma }_{opt.}= \frac{\alpha nc}{4\pi }$$
6
α is the absorption coefficient, and (n) is the refractive index of the samples.
Figure10 illustrates the variation of optical conductivity σopt. as a function of photon energy hʋ for the investigated samples. It is observed that the optical conductivity of PVDF increases by the addition of nanoparticles. This increase can be attributed to the creation of new levels in the band gap which facilitates the movement of charges from valance to conduction band [31]. Co-ZnFe2O4 nanoparticles increases the PVDF optical conductivity σopt. four times its original value.
Dielectric constant:
The complex dielectric constant reveals an insight into the behavior of electrical charge carriers in materials. The real part of dielectric constant is representing the amount by which the velocity of light decreases within the material, although imaginary part of dielectric constant represents the amount of energy absorbed in the dielectric material from electric field due to dipole motion. Both the real and imaginary parts of dielectric constant have been calculated by using following expressions [23], [32] [33].
εr = n2 – k2, (7)
εi = 2nk, (8)
where, εr is real part of dielectric constant, εi is imaginary part of dielectric constant. The real part of dielectric constant (εr) and imaginary part (dielectric loss) (εi) as a function of (hν) of the investigated samples shown in Fig. 11 (a, b). It can be noted that the prepared nanoparticles (Co-ZnFe2O4 and Cu-Zn Fe2O4) have improved the dielectric response of the PVDF polymer. The greater dielectric values were observed in case of (Co-ZnFe2O4) nanofiller where it increases the dielectric constant from 0.05 for neat PVDF to 2. These greater dielectric values resulted from the interfacial polarizations at the conductor-insulator interface [34] [35] [36]. As well as the PVDF dielectric loss increases by the addition of the nanoparticles, the higher increase was observed in the PVDF/Cu-Zn Fe2O4 sample this increase is attributed to the increase in n and K with the nanofillers.
Non-linear optical (NLO) properties:
Studying the nonlinear optics of PVDF /(Cu-Zn Fe2O4) nanocomposites is helpful for the usage in several optoelectronic applications [37]. The intensity of incident light causing the occurrence of optical polarizability P in the nanocomposite. The nonlinear electron polarizability PNL can be obtained using the following equations: [38]
$$P= {\chi }^{\left(1\right)}E+ {P}_{NL}$$
9
,
$${P}_{NL}= {\chi }^{\left(2\right)} {E}^{2}+ {\chi }^{\left(3\right)}{E}^{3}$$
10
where E is the electric field of light,
\({\chi }^{\left(1\right)}\) is the linear optical susceptibility,
\({\chi }^{\left(2\right)}\) is the 2nd order nonlinear optical susceptibility and
\({\chi }^{\left(3\right)}\) is the 3rd order nonlinear optical susceptibility [
39], [
40], [
41].
To determine the values of, \({\chi }^{\left(1\right)}\) and \({\chi }^{\left(3\right)}\) these equations can be used
$${\chi }^{\left(1\right)}= \frac{({n}^{2}-1)}{4\pi }$$
11
\({\chi }^{\left(3\right)}= \xi\)(
\({{\chi }^{\left(1\right)})}^{4}\), (12)
where\(\xi\) = 1.7 ×10−10 (esu). Figure 12 ( a, b) indicates the obtained values of \({\chi }^{\left(1\right)}\) and \({\chi }^{\left(3\right)}\) as a function of wavelength for pure PVDF and PVDF /(Co-ZnFe2O4 and Cu-Zn Fe2O4) nanocomposites. For pure PVDF and PVDF /(Cu-Zn Fe2O4) nanocomposite samples, the variations of \({\chi }^{\left(1\right)}\) and \({\chi }^{\left(3\right)}\)are very small. The PVDF and PVDF /(Cu-Zn Fe2O4) values are almost constant and have the same pattern for both \({\chi }^{\left(1\right)}\) and \({\chi }^{\left(3\right)}\). Moreover, it is observed that the Co-ZnFe2O4 nanofiller values of \({\chi }^{\left(3\right)}\) are increased and improved the nonlinear response of the PVDF polymer.
3.3. Energy harvest performance
Piezoelectricity is an electric polarization proportional to an applied mechanical energy or a mechanical deformation proportional to an applied electric field.
3.3.1. Piezoelectric response using a digital storage oscilloscope (DOS)
The working principle of the piezoelectric energy harvester of PVDF is based on the creation of an electric polarization under an applied stress [42].
The prepared nanocomposite films have been tested using a digital storage oscilloscope (DOS), where each film was placed between two copper layers electrodes. With respect to the open circuit (with forward and reverse connection), the generated piezo potential was recorded as a repetitive finger stress has been applied on the upper surface of nanocomposite films. The finger tapping generates a compressive stress on the surface of films, causes the displacement of positive and negative charges in the nanocomposite films.
It is well known that PVDF mainly have a synthetic semi crystalline polymer, and its β-phase and γ-phases are responsible for the piezoelectric power harvesting property [5].
By mechanical deformation and polarization, the structure of alpha phase can be transformed into polar beta phase to achieve piezoelectric characteristics [43].
The filler nanoparticles improved the piezo-potential as shown in Fig. 12 (a-c) where an interaction between the Co-ZnFe2O4 and Cu-Zn Fe2O4 nanoparticles and the dipoles of PVDF (CH2 = CF2).
This enhancement of piezo-potential in PVDF composite films is due to the role of Co-ZnFe2O4 and Cu-Zn Fe2O4 nanofillers in PVDF matrix, which is provide a conducting particle could help charge carriers induced inside the film to move to the surface. Also, the interaction between Co2+, Zn2+, Cu2+ and Fe3+ nanoparticles with CF2- dipoles and O2− nanoparticles of Co-ZnFe2O4 and Cu-Zn Fe2O4 with CH2- dipoles could enhance the piezo response of PVDF polymer.
It can be noted that the nanofillers have enhanced the values of complex dielectric constant and the values of piezo potential of the PVDF polymer as shown in Figure (10, 12). That could be due to the increase in polarization causes a slight increase in the dielectric constant, where the beta phase dipoles rearranged and a transformation of alpha to beta takes place when energy is applied [44].
3.3.2. PFM studies
piezoresponse force microscopy (PFM) was used for measuring piezoelectric response of prepared nano samples. PFM operated in contact-mode by using an alternating voltage Va.c applied to the tip subsequent in an alternating electric field inside the specimen [45] [46]. Herein, piezoelectric samples, a periodic deformation was applied on the samples. The applied deformation causes deflection of cantilever in any directions; torsion or buckling [47]. Plane deformation (a change in the z-axis) was causing the deflection, buckling responds to an in-plane deformation (a diameter changes in the y-axis) as well as torsion is related to another in-plane deformation (a length change in the x-axis) [48]. Resolved lateral mapping of the piezoelectric behavior of the prepared sample can be obtained using PFM. The combination between PFM images of in-plane and out-of-plane which are obtained from PFM are related to domain structure [49]. The area of 2.5 µm ×2.5 µm of the pure PVDF and PVDF/(Co-ZnFe2O4 ,Cu-ZnFe2O4,) nanocomposite films was scanned by using Vac of 510mV applied to the cantilever tip. The response of PFM in the z-axis direction, phase, and amplitude micrographs of the considered samples, respectively is shown in Fig. 13. The out of plane PFM phase pictures Fig. 14 for all tested samples, corresponding to the piezoelectric polarization, both negative (white) and positive (black) zones showed up representing antiparallel ferroelectric nanodomains with 180o domain walls [50] [51]. The white zones related to negative domains with the polarization direction perpendicular to the surface of the PVDF film and Orien situated descending, whereas the dark areas associated with positive domains having the polarization heading upward [52].
In pure PVDF, the well-defined piezoelectric spaces illustrates that the stretched crystallites are the homogeneous β-phase. The PFM amplitude for PVDF/(Co-ZnFe2O4 ,Cu-ZnFe2O4,) nanocomposite films (a piezoelectric contrast due to the defections caused by the applied alternating field has been noted. Therefore, the domains in the PVDF film align along the z-axis direction perpendicular to the sample and the bottom electrode, Fig. 14. The in-plane PFM images Fig. 13 (d-j) were analyzed both for the phase and amplitude dependence of the PFM signal of PVDF/( Co-ZnFe2O4 ,Cu-ZnFe2O4) nanocomposites films. The images Fig. 13, 14 donate the formation of periodic stripe domains acquired at θ = 0o. stripe domains aligned along the last saturating field direction were observed for all the pictures of PVDF/(Co-ZnFe2O4 ,Cu-ZnFe2O4) nanocomposites films.