Study of PVDF/ (Co-ZnFe2O4 and Cu-ZnFe2O4) nanocomposite for the piezo-phototronics applications


 Polyvinylidene fluoride (PVDF) polymer is considered as a promising piezoelectric material whose optical properties need to be improved. Zinc ferrite is an excellent photoelectric material, in the present work it was doped separately by both cobalt and copper. Co-ZnFe2O4 and Cu-ZnFe2O4 nanoparticles were synthesized and characterized to be used as PVDF fillers, aiming to improve its optical properties. The optical properties as well as, the piezoelectric response of the prepared PVDF/ (Co-ZnFe2O4 and Cu-ZnFe2O4) nanocomposites were investigated. A remarkable improvement in the PVDF relative permittivity, optical conductivity, refractive index, non-linear susceptibility, and a great reduction in the band gap energy value is obtained by adding Co-ZnFe2O4 nanoparticles to it. However, Cu-ZnFe2O4 nanoparticles have limited improvement of the PVDF optical properties compared to the Co-ZnFe2O4 nanoparticles. The piezoelectric response of the PVDF polymer is clearly increased by the addition of both Co-ZnFe2O4 and Cu-ZnFe2O4 nanoparticles.


Introduction
In the last decades there is a potential support for using renewable energy, for its economic, environmentally friendly properties [1], [2]. Energy harvesting from mechanical energy (piezo electricity) become very influential lately. Piezoelectricity is produced due to the spontaneous separation of charge within crystal structures in certain conditions. A displacement of the electron relative to their atomic centers, referred to an electric polarization produced by the application of mechanical stress along the appropriate direction of a crystal however, an applied electric field can generate a mechanical distortion in the material. The phenomenon of Piezoelectricity can be used in many applications, including sensors and actuator applications. [3].
Ceramic piezoelectric materials have a great role in the field of sensors [4]. Although these materials have large relative permittivity, facilitate electrical tuning, they are tainted by defects such as low inherent breakdown strength, these materials are brittle which limiting their applications in flexible or wearable devices.
Polyvinylidene fluoride (PVDF) polymer is a good alternative for ceramic materials. It is possessing unique advantages over ceramic such as flexibility, further, PVDF is inert chemically, it has electroactive β-polymorphic phase with great toughness and small fatigue failure can produce remarkable voltage under long-term continuous oscillations. However, PVDF has the disadvantage of low dielectric permittivity. Coupling between polymer and ceramic fillers nanoparticles gives a composite material with new properties. This modification related to insitu growth of β-crystalline phase within the nanocomposites which improve the dielectric and energy harvesting performance of the nanocomposite [5] [6] [7]. The increase in dipole-dipole interaction with the increase of the filler content leads to higher dielectric constant composite material. [8], [6]. [9].
A lot of attention has been paid to the Piezoelectric semiconductor nanomaterials for its excellent properties Combines the excellent properties of nanomaterials and the strength of polymers.
Recently the need for further research in the field of piezotronics and piezo-phototronics increased, where charge-carrier transport is controlled by the application of external mechanical stimuli in flexible devices [10]. The fundamental investigations of piezoelectricity and semiconductor properties utilizing useful nanomaterials prompts the improvement of more intelligent electronics and optoelectronics materials. [11].
In the present study, Co-ZnFe2O4 and Cu-ZnFe2O4 have been investigated as PVDF nanofillers, where Co-ZnFe2O4 and Cu-ZnFe2O4 are reported as semiconductor materials [12], [13]. The investigated nanocomposites were prepared using simple solution casting method [14] The PVDF polymer has dissolved in dimethyl formamide (NMP), Co-ZnFe2O4 and Cu-ZnFe2O4 nanoparticles are synthesized, characterized, and introduced into the solution as the PVDF nanofiller. The effect of Co-ZnFe2O4 compared to Cu-ZnFe2O4 on the PVDF optical properties and piezoelectric response were investigated. The present work aims to study the piezoelectric and optoelectronic properties of PVDF /(Co-ZnFe2O4 and Cu-ZnFe2O4).

Nano powder preparation
Citrate auto-combustion method has been used to synthesize Co-ZnFe2O4 and Cu-Zn Fe2O4 nanoparticles. A stoichiometric ratio of cobalt nitrate, copper nitrate, zinc nitrate, iron nitrate and citric acid were dissolved in a small amount of distilled water and a vigorous stirring was applied to the solution. The pH of the solution was adjusted at 7. The solution temperature was raised up to 250 o C to obtain a fine powder. The obtained powder was calcined for 4 h at 800 o C with rate of 4 o C/min.

(PVDF/ Co-ZnFe2O4 and Cu-Zn Fe2O4) film preparation
A transparent solution of (PVDF/NMP) was obtained by dissolving 3gm. of PVDF powder in 10mL. of (NMP) at room temperature under continuous stirring. To prepare the nanocomposite films 3 milligrams of the Co-ZnFe2O4 and Cu-Zn Fe2O4 nanoparticles were dissolved in appropriate amount of (NMP) and vigorous stirring was applied, the obtained solution were added to the (PVDF/NMP) solution and sonicated for 2 h. Finally, each sample was poured on a clean glass surface on a hotplate kept at 60 o C. The obtained films of about (50×50×0.16mm) washed using distilled water to remove any contaminating particles and for full solidification of the films.

Characterization of nanoparticles
The crystalline phases of Co-ZnFe2O4 and Cu-Zn Fe2O4nanoparticles were identified by Fourier Transform Infrared Spectroscopy instrument (FT-IR) (Perkin Elmer) in the range of 4000-400 cm −1 . The degree of crystallinity was investigated using, X-ray diffraction (XRD) analysis (Bruker, D8 Advance, X-ray diffractometer), operating at 40 kV and current 40 mA with Cu-Ka radiation (l = 1.541 Å). Transmission Electron Microscope (HRTEM) JEM-ARM300F operating at 200 kV was used for topographical investigation of the prepared nanoparticles.

Optical properties investigation
UV-vis spectra of the prepared nanocomposite films obtained using (JASCO Corp., V-570, Rev.

Piezo response Force Microscopy (PFM)
Flex-Axiom AFM was used for measuring the piezo response force. The specifications of the PFM are commercial head type with 115-135 μm length, Co-Cr coated tip with electrical resistivity of 0.01-0.025 Ω·cm with nearly 35nm tip curvature nominal spring constant of 5 N/m and165.08kHz nominal resonance frequency. A.c. voltage was applied to the tip, at a frequency of 165.08kHz for measuring the piezoresponse of samples. Scan rate was 0.5 Hz while the scan area was (250×250) nm 2 . Insulating chamber is used for all measurements at room temperature.

Nanoparticles characterization
Where, D is the crystallite size (nm), K is the particle shape factor (0.9), λ is the target wavelength (nm), β is the corrected full width at half maximum, and θ is the position (angle) at the maximum of the peak at. The estimated particle sizes are 28.8 nm and 35.8 nm. for the Co-ZnFe2O4 and Cu-Zn Fe2O4 nanoparticles, respectively.

FTIR
The FTIR spectra in the range 4000-400 cm-1of Co-ZnFe2O4 and Cu-Zn Fe2O4 nanoparticles are shown in Figure 2. The bands located at 2900-2997 cm −1 attributed to the O-H stretching bond existing in the adsorbed water molecules. The peaks corresponding to 1117 cm -1 band is attributed to the Fe-Co alloy system [16] [17]. Bands present at 535 -533 cm −1 belongs to hematite phases. [18]. 430 cm −1 and 412cm −1 could be ascribed to vibrations of M-O (M denoted to copper or iron) [19]. Also, the band that is observed around 670 cm −1 is assigned to (Fe3O4) [20] [21].

HRTEM
The HRTEM micrographs in Figure 3 (a, b) of Co-ZnFe2O4 and Cu-Zn Fe2O4 respectively showed well-defined cubic shapes. The nano crystallites have cubic spinel structure with average diameter of 55 nm. for the Co-ZnFe2O4 nanoparticles and 75 nm for the Cu-ZnFe2O4. The Cu-ZnFe2O4 particles showed more agglomeration where some particles formed large clusters.

Optical properties
Reflectance and transmittance: Figure 4 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. (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 6, (b). It is observed that (K) values increases by increase 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 which may be explained by the density increase of the PVDF by addition of nanofiller.

Optical bandgap:
An atom 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 ( ), 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 (Tregion): 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], [28], [29]. Figure 7 shows the relation between ( ℎ ) 2 verse the incident photon energy (ℎ ). 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 the creation of new levels in the PVDF band gap facilitates the movement of electrons from valance to conduction band.

Refractive index:
The refractive index is Another mainly important elemental property of material because of its direct relationship with the ions electronic polarizability and the local field within the material.
The refractive index (n) of composite materials has been calculated via given equation [30], where R is reflectance and K is the extinction coefficient. 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].

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]: α is the absorption coefficient, and (n) is the refractive index of the samples. Figure 9 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].
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 Figure 10 (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] = (1) + , where E is the electric field of light, (1) is the linear optical susceptibility, (2) is the 2nd order nonlinear optical susceptibility and (3) is the 3rd order nonlinear optical susceptibility [39], [40], [41].
To determine the values of, (1) and (3) these equations can be used where = 1.7 ×10 -10 (esu). Figure 11( a, b) indicates the obtained values of (1) and ( for both (1) and (3) . Moreover, it is observed that the Co-ZnFe2O4 nanofiller values of (3) are increased and improved the nonlinear response of the PVDF polymer.

Energy harvest performance
Piezoelectricity is an electric polarization proportional to an applied mechanical energy or a mechanical deformation proportional to an applied electric field.

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 Figure 12  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 Co 2+ , Zn 2+ , Cu 2+ and Fe 3+ nanoparticles with CF2-dipoles and O 2-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].

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 Figure 13. 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, Figure 14. The in-plane PFM images Figure 13

Conclusions
Copper doped zinc ferrite and cobalt doped zinc ferrite nanoparticles were successfully synthesized and characterized using different characterization methods. The prepared Co-ZnFe2O4 and Cu-ZnFe2O4 nanoparticles have been used as fillers of the PVDF polymer.
The optical properties as well as piezoelectric response of pure PVDF and PVDF/(Co-ZnFe2O4 and Cu-ZnFe2O4) nanocomposites were studied. The obtained results showed a great development in the relative permittivity, optical conductivity, refractive index, and non-linear susceptibility of the PVDF/(Co-ZnFe2O4) nanocomposite, where PVDF/(Cu-ZnFe2O4) nanocomposite showed a lower improvement in the previous optical properties.
Both PVDF/(Co-ZnFe2O4 and Cu-ZnFe2O4) nanocomposites had lower band gap energy values than that of the pure PVDF, makes them convenient for opto-electronic applications.
The two prepared nanocomposites improved the PVDF piezoelectricity, the PVDF/(Co-ZnFe2O4 and Cu-ZnFe2O4) nanocomposites films show stripe domains aligned along the last saturating field direction when examined using PFM., the domains in the PVDF film align along the z-axis direction perpendicular to the sample and the bottom electrode.
From the obtained results, we can say that PVDF/(Co-ZnFe2O4) nanocomposite is a great candidate for piezo-phototropic applications.

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