3.1. Morphological and Structural elucidation of IZO nanocomposite
Detailed morphologic and structural characterization of IZO nanocomposite was investigated by transmission electron micrographs (TEM). The synthesized composite was observed to have a quasi-spherical shape and some elliptical geometries, as shown in Fig. 2a. All the images reveal an accumulation of tightly bonded nanoparticles. The average size of the particles is determined from TEM images using the software ImageJ to be around 76 nm. The estimated distance between the two adjacent planes from the lattice fringes is 0.28 nm for the (010) plane of ZnO and 0.17 nm for the (044) plane of In2O3, as shown in Fig. 2b. The interface's selected area electron diffraction (SAED) pattern, which comprises of zone diffraction spots, is depicted in Fig. 2c. These mixed diffraction patterns of the ZnO quasi-spherical interface also suggest the presence of the In2O3 crystalline nucleus on the surface.
Figure 2(d) shows the powder XRD pattern of the IZO nanocomposite synthesized by hydrothermal technique and calcined at 900°C for 6 h. The strong and intense peaks correlating to the wurtzite phase of ZnO (JCPDS card 36-1451) and the cubic phase of In2O3 (JCPDS 6-0416) indicate that the samples are highly crystalline. The composite sample shows ZnO and In2O3 peaks without any other peaks, demonstrating successful composite formation between ZnO and In2O3. The crystallite size of the synthesized composite was calculated using the Debye-Scherrer equation (Eq. 1) as stated below:
$$D=\frac{k\lambda }{\beta \text{cos}\theta } \left(1\right)$$
Where, D is crystallite size, k is Scherrer constant having value 0.94, \(\lambda\) is the wavelength of X-ray (0.154 nm) and \(\beta\)is full width at half maximum (FWHM) of the diffraction peak. D values of all the peaks were calculated separately and finally their average was taken as crystallite size (D) of the nanocomposite and it was found to be ~ 33.19 nm.
FT-IR spectrometric analysis was done in the range of 500\({ \text{c}\text{m}}^{-1}\) to 4000 \({\text{c}\text{m}}^{-1}\) to detect the functional groups in the IZO nanocomposite shown in Fig. 2(e). The vibration peak at 590 \({\text{c}\text{m}}^{-1}\) wavenumber corresponds to the characteristic stretching vibration in the structure of ZnO. In literature, depending upon synthesis and experimental conditions, various Zn-O band positions have been reported at 423\({\text{c}\text{m}}^{-1}\) [13], 490 \({\text{c}\text{m}}^{-1}\) [14], 580 \({\text{c}\text{m}}^{-1}\) [15], 677 \({\text{c}\text{m}}^{-1}\) [16], supporting our result. Moreover, multiple peaks overlapped in the region near 420–600 \({\text{c}\text{m}}^{-1}\) are assigned to the stretching of ZnO and In2O3 [17, 18]. The peak at 1390 \({\text{c}\text{m}}^{-1}\) is related to the symmetric and asymmetric stretching vibrations of adsorbed carbonate anion [19].Whereas the band at 1646 \({\text{c}\text{m}}^{-1}\) is attributed to the bending vibrations of adsorbed water on the surface [20]. The band located at 2325 \({\text{c}\text{m}}^{-1}\) is due to the stretching mode of CO2 present in the air [21]. The band at 3446 \({\text{c}\text{m}}^{-1}\) arises due to the stretching and bending vibration mode of the -OH group [22].
3.2. Morphological and Structural elucidation of PVDF/IZO polymer nanocomposite
The atomic force microscopy was employed to understand the changes in the PVDF matrix upon doping with IZO. Figure 3(a-d) demonstrates the topographic images of neat PVDF and PVDF/IZO composite films. Compared to the neat PVDF film, the composite film with 1% IZO has increased surface roughness, as shown in Fig. 3b. This increase is attributed to the homogenous distribution of nanofillers on the PVDF surface. The increase in the surface roughness was directly related to the enhancement of surface charge density of the PVDF films, which further results in better piezoelectric properties compared to the bare PVDF films. The 2% IZO-doped PVDF film AFM image demonstrates the distribution of nanofiller homogeneously throughout the PVDF matrix.
The distribution of inorganic nanofillers and their effect on the shape and size of the crystalline PVDF film was investigated using the SEM technique. The SEM image of the neat PVDF film (Fig. 4a) consists of well-defined spherulite structures corresponding to the \(\alpha\)-phase of the PVDF film. Upon introduction of nanofillers of various weight percentages, substantial change in the size and shape of the spherical structure of the PVDF film was observed, as shown in Fig. 4b and Fig. 4c. There is a considerable reduction in the granular size of the PVDF as the weight percentage of the IZO is increased, suggesting the uniform distribution of nanofillers. A similar reduction in the size of crystalline PVDF spherulites was observed by Lopes et al. [23]. The inorganic nanofillers act as nucleating agents, producing smaller spherulites than neat PVDF [24]. Further increase in the weight percentage of IZO to 3% agglomeration of nanofillers was observed as shown in Fig. 4d, whereas no agglomeration was observed with 1% PVDF/IZO.
The EDX mapping was also performed to confirm the distribution of nanofiller inside the PVDF matrix. The EDX images of the nanocomposite films indicate the successful incorporation of Indium oxide and ZnO nanoparticles on the surface of the PVDF, as shown in Fig. 5b and Fig. 5c. The 2% PVDF/IZO nanocomposite film EDX image Fig. 5b illustrates the smaller granular size PVDF particles than the neat PVDF (Fig. 5a) without any nanofiller. These results agree with the SEM analysis and XRD data. As observed in SEM analysis, the further increase in weight percent resulted in the agglomeration of nanoparticles on the surface of the PVDF. It is also clearly visible in the EDX images, as shown in Fig. 5c.
The crystallinity and phase purity of the synthesized polymer nanocomposites were analyzed with the help of XRD and FT-IR spectroscopy. Different XRD peak patterns are expected based on the phase of the PVDF film. Figure 6a depicts the XRD peak pattern of neat PVDF exhibited in the characteristic XRD peak at 2θ = 18.5° corresponding to the \(\alpha\)-PVDF and a strong peak at 2θ = 20.2° arising from\(\beta\)-phase of the PVDF. The 1% PVDF/IZO, XRD pattern exhibits a sharper peak at 2θ = 20.1°, indicating an increase in the \(\beta\)-phase of PVDF upon adding nanofiller IZO. Similarly, with 2% and 3% PVDF/IZO nanocomposites, enhanced content of \(\beta\)-PVDF is visible, as shown in Fig. 6a. The intensity of diffraction peak at 2θ = 18.59° (110) is reduced. In contrast, the peak from the electroactive \(\beta\) -phase appears at 2θ = 20.6° (200/110) increased with increase in the weight percentage of IZO nanofiller [25–29]. The enhancement of active \(\beta\)-phase is desirable for piezoelectric applications. The characteristic XRD peaks of In2O3 were observed at 2θ = 30.6° (222), 51.0° (044) and 60.74° (226). The XRD pattern also consists of weak peaks at 2θ = 31.81° (010) and 68.10° (112) characteristic of ZnO. However, their intensity is very low as minimal amount of ZnO was used.
FT-IR spectral analysis was performed to identify various functional groups present in the nanocomposite films and to calculate the ratio of \(\alpha\) -PVDF and \(\beta\)-PVDF phase in all the composite films. Figure 6b illustrates the FTIR spectra of neat PVDF and IZO-doped PVDF film in the 700–4000 cm -1 range. According to Gregorio et al. [30], the relative fraction of the \(\beta\)-phase, F (\(\beta\)), (Table 1) in a sample containing only \(\alpha\) and \(\beta\)-PVDF is calculated in Eq. 2:
$${ F\left(\beta \right)=\frac{{X}_{\beta }}{{X}_{\alpha }+{X}_{\beta }}=\frac{{A}_{\beta }}{1.26{A}_{\alpha }+{A}_{\beta }}}_{} \left(2\right)$$
where \({X}_{\alpha }\) and \({X}_{\beta }\) are the crystalline mass fractions of the α and \(\beta\) phases. It was predicted that FTIR absorption followed the Beer-Lambert law, and the absorption A (\(\alpha\)) and A (\(\beta\)) were determined at wavenumbers of 763 cm − 1 and 840 cm − 1, respectively [31].
Table 1
Refractive fraction of calculated β-phase content.
Sample name | F (\(\varvec{\beta }\)) (%) |
Neat PVDF | 47.34% |
1% PVDE/IZO | 58.07% |
2% PVDE/IZO | 60.27% |
3% PVDF/IZO | 57.83% |
The relative composition of \(\alpha\) and \(\beta\) phase in each composite film is calculated using Eq. 2, and results are summarized in Table 1. It is evident from the FTIR spectra that the addition of nanofiller IZO caused the enhancement of the intensity of peaks corresponding to \(\beta\)-phase and a diminishing of the intensity of peak corresponding to \(\alpha\)-phase. The maximum percentage of \(\beta\)-phase (F (\(\beta\)) = 60.27%) was obtained for 2% PVDF/IZO, whereas neat PVDF has \(\beta\)-phase content of 47.34%. This confirms that the addition of IZO nanofiller results in the enhancement of \(\beta\)-phase content, which further aids in improvement in the piezoelectric properties.
3.3. Dielectric properties of PVDF/IZO polymer nanocomposites
The impact of varying IZO nanofiller concentrations in PVDF films on their electrical characteristics was investigated to create polymer composite materials with outstanding dielectric properties. Figure 7 (a-b) shows the dependence of the dielectric constant (ε′) and dielectric loss (ε′′) on the frequency of neat PVDF and PVDF/IZO nanocomposites with different weight percentages. Because of the dipole polarisation occurring within the composite's films, the dielectric constant rises with temperature (Fig. 7a). The dielectric constant of the PVDF/IZO composite films was higher than that of the neat PVDF this is due to the cross-facial polarisation as well as high \(\beta\)-phase content in the nanocomposites induced by the IZO filler [32]. The PVDF/IZO with 3% of IZO filler shows the highest dielectric constant of 50 among all the samples, which is approximately 2 times greater than the neat PVDF. This high polarization enhances piezoelectric properties [33, 34]. The dielectric constant (ε′) and dielectric loss (ε′′) can be expressed using impedance data with the following Equations (3) and (4) [35].
$$\epsilon {\prime }\left(\omega \right)=\frac{-{Z}^{{\prime }{\prime }}}{\omega {C}_{0}\left({Z}^{{\prime }2}+{Z}^{{\prime }{\prime }2}\right)}$$
3
$${\epsilon }^{{\prime }{\prime }}\left(\omega \right)=\frac{{Z}^{{\prime }}}{\omega {C}_{0}\left({Z}^{{\prime }2}+{Z}^{{\prime }{\prime }2}\right)}$$
4
Whereas \({Z}^{{\prime }}\) and \({Z}^{{\prime }{\prime }}\) real and imaginary impedance,\(\omega\) is the angular frequency, \({\epsilon }^{{\prime }}\) and \({\epsilon }^{{\prime }{\prime }}\) are real and imaginary permittivity and \({C}_{0}\) represents vacuum capacity.
The conductivity behaviour of the PVDF/IZO composite films are shown in the Fig. 7c. The values of conductivity increased with the amount of charge carriers that are present in the system, resulting in the greatest value for uniformly distributed with IZO nanofiller, whereas 2% and 3% PVDF/IZO shows much slighter deviation in increment compared to the 1% PVDF/IZO as shown in Fig. 7c. The frequency verses tanδ of neat PVDF and PVDF nanocomposites filled with different weight percentages of IZO nanofiller are shown in Fig. 7d. The loss tangents of the PVDF polymer composites films rise with increasing in frequency. The relaxation behaviour of polymer composite films at low frequencies is due to the conductive character of composites, while high-frequency dielectric behaviour is related to polarization relaxation loss. As shown below, the relaxation (R) is related to the minimum frequency (fmin), which is the lowest value from the frequency of tanδ [36].
$$R=\frac{1}{{f}_{\text{m}\text{i}\text{n}}}$$
4
For 2% PVDF/IZO, maximum relaxation was observed at 67760.4 Hz frequency with the value 1.48x 10− 6, whereas 3% PVDF/IZO maximum relaxation was observed at 64280.5 Hz frequency with the value is 1.556 X 10− 5.
3.4. Thermal stability of PVDF/IZO polymer nanocomposite
Thermogravimetric analysis of polymer nanocomposites was done to understand nanofiller concentration's influence on the composite materials' thermal stability. As expected, the addition of 1% IZO nanofiller to the PVDF film resulted in improved thermal stability, as shown in Fig. 8a. Similar observation of enhanced thermal degradation temperature was also reported by Jaleh et al., with the ZnO doped PVDF nanocomposites [37]. Surprisingly, further increasing nanofiller concentration doesn't lead to more thermal stability. This may be due to the interference of nanofillers on the polymeric chains at higher concentrations. A similar trend was also observed for the polymer composite PVDF-HFP/Ni-doped ZnO nanocomposites [27]. Compared to the neat PVDF, the degradation temperature values for 1% PVDF/IZO raised by 36°C. The interaction between PVDF and IZO nanofiller may contribute to the resistance of nanocomposites to thermal degradation. The FTIR spectra and XRD patterns show an extremely strong interaction between nanoparticles and the PVDF matrix [37]. The DTA curves for all composites with varying concentration of IZO filler are shown in Fig. 8b. The decomposition temperature from the DTGA curves for 1% PVDF/IZO, 2% PVDF/IZO and 3% PVDF/IZO are 481.1°C, 475.1°C and 476.1°C respectively. They further claim that the moving of the degradation temperature is due to the homogenous distribution and inorganic character of the IZO nanofiller, which improves the thermal stability of the PVDF matrix.
Figure 8
TGA thermograms of phase separation (a) TGA (b) Derivative TGA of PVDF films with different percentages of IZO filler.
3.5. Melting and Crystallization Behavior of PVDF/ IZO polymer nanocomposite
Figure 9 depicts neat PVDF and PVDF crystallization behaviours with IZO nanofiller. In Fig. 9b, the crystallization temperature Tc of neat PVDF was observed at 130.4°C while cooling down from the melt. A slight increase in the Tc for the polymer nanocomposites of PVDF with IZO to 132.0°C indicates the nucleation effects of inorganic nanofillers. The melting curves for the polymer nanocomposites also show a similar trend as shown in Fig. 9a. Among all the polymer PVDF with IZO nanofiller, the 2%PVDF/IZO composite film has a considerable shift in the melting temperature Tm=161.9°C to that of neat PVDF which has Tm=160.1°C. This shifting Tm to higher temperatures in the polymeric composites indicates the transformation of PVDF into different phases to that of the neat of PVDF. The DSC results reveal that the IZO no composites have enhanced \(\beta\)-phase, consistent with the XRD and FTIR data. These results are also consistent with the observed piezoelectric performance of the 2% PVDF/IZO polymer composite, which exhibited the highest output voltage compared to the other PVDF/IZO composites.
3.6. The effect of IZO nanofiller on Piezoelectric output voltage of PVDF and its composite
To fabricate piezoelectric nanogenerators (PENG), phase-separated films were sliced into circular pieces of diameter 3cm and thickness of ~ 150 µm. Both the sides of circular films were polished with silver paste and followed by the adhesion of Cu tape. The circuit was built by connecting copper wires with electrodes at room temperature, as shown in Fig. 10a, and the working mechanism is provided in Fig. 10b. Four different PENGs with a high β-phase fraction were created by varying the weight percentage of nanofiller IZO. Figure 11(a-d). depicts the direct piezoelectric effect output voltage of piezoelectric nanogenerators (PENG) with working parameters of 1.2 kHz and resistance 3 MΩ for generating a typical AC-type signal.
The phase-separated nanocomposite films were exposed to mechanical pressure generated by finger tapping. The resulting output voltage obtained was linearly dependent on the weight percentage of the IZO nanofiller incorporated into the PVDF matrix. The highest peak-to-peak output voltage of 1.32 V was observed for the composite film containing 2% PVDF/IZO, which is almost 26 times higher than the output voltage observed in the case of neat PVDF film as shown in Fig. 11c. However, it was observed that at higher loading of nanofiller (3% PVDF/IZO) the output voltage was found to be 0.65V (Fig. 11d), which is lower than that is obtained in the case of 2% PVDF/IZO nanocomposite film. This is attributed to the uniform distribution of nanofiller IZO in the PVDF matrix at lower loading, as illustrated by SEM micrographs in Fig. 5. Similar findings were also reported by Parangusan et al., for Ni-ZnO doped PVDF-HFP matrix [27].
The addition of IZO nanofillers to PVDF affects the functional as well as structural characteristics of the final phase-separated polymer composite films by inducing nucleation, interfacial contact, and supercooling [38]. The higher output voltage obtained for 2% PVDF/IZO can be attributed to enhanced \(\beta\)-phase active PVDF as evidenced by FTIR and XRD. Kumar et al. also reported similar observations for the electrospun nanofiber P(VDF-TrFE)/ZnO [39]. Applying mechanical force generates a potential in the nanoparticles, which aids in the alignment of the PVDF dipoles via stress-induced polarization. The increased piezoelectric voltage generation in the PVDF/IZO polymer composite film is due to this self-polarization [40].