A critical review on polyvinylidene fluoride (PVDF)/zinc oxide (ZnO)-based piezoelectric and triboelectric nanogenerators

Chirantan Shee; Swagata Banerjee; Satyaranjan Bairagi; Aiswarya Baburaj; Kumar S K Naveen; Akshaya Kumar Aliyana; Daniel M Mulvihill; R Alagirusamy; S Wazed Ali

doi:10.1088/2515-7655/ad405b

1. Introduction

Sensors and smart devices have now become customary in our day to day lives. These gadgets are a gift of the scientific innovation and progress of the current era. As we witness the fruits of such innovations, we cannot deny the enormous energy demand accompanying them. Most of the energy requirements are met by non-renewable fossil-fuel-based energy sources. This puts tremendous pressure on the limited reserves of these energy sources in order to meet the energy demands [1]. As the situation is becoming more alarming over time, there is a need to explore alternative sources of energy. Several non-conventional energy harvesting sources have emerged in this regard. The most common green source of energy, with abundant availability, is based on ambient energy sources. For example, solar energy, wind energy [2, 3], water energy and tidal energy have emerged as promising energy sources. Undeniably, these sources of energy generate appreciable amounts of power; however, their dependence on environmental factors affects their efficient functioning.

The abundant mechanical energy available around us has piqued the interest of the scientific community with regard to scavenging energy from these sources. Triboelectric [4–6] and piezoelectric [7–9] energy harvesting technologies have gained prominence as mechanical energy harvesting techniques. Further, the power generation capability, facile technologies and scope for material development provide immense opportunity for research and innovation in this field. Triboelectricity is driven by contact electrification, while piezoelectricity is a pressure-driven phenomenon. When two surfaces with different electron affinities undergo repeated contact and separation cycles, they generate surface charges of opposite polarity on their respective surfaces. This phenomenon is referred to as contact electrification [10–12]. It is the driving factor behind the triboelectric effect. Piezoelectricity, on the other hand, is a material property. The application of force on piezoelectric materials leads to disturbance in their crystal structure, which causes a potential difference between its surfaces. This potential difference can be harvested in the form of electric energy to power electronic gadgets. This illustrates the direct piezoelectric effect. The dimensional changes in a piezoelectric material as a response to an applied electric field is called the converse piezoelectric effect [13]. Piezoelectric materials can be classified as single crystals, polymers, ceramics and composites [14–18]. Each of these categories of materials have unique characteristics that are explored in a wide variety of applications. Composites are comprised of one or more constituents, where each element has exclusive properties. The properties of a composite are distinct from its constituent elements. They improve upon the advantages of each constituent while overcoming their drawbacks.

Polyvinylidene fluoride (PVDF) is one of the most explored piezoelectric polymers. The appreciable piezoelectric coefficients of this polymer make it suitable for a wide range of applications. The polar nature of this polymer is due to the presence of a CF₂ dipole. PVDF exists in more than one crystalline phase, namely, α, β, γ, etc. All the phases do not show piezoelectric character. This is due to the difference in the polymer chain conformations in each phase. The β-phase of PVDF with all-trans conformation is the desirable electroactive phase of the polymer [13]. There are several ways to achieve and stabilize this phase of PVDF. Techniques like electrospinning result in in situ poling of the polymer chains, resulting in enhanced piezoelectric activity [19, 20]. Piezoelectric composites of PVDF with different incorporated fillers [21–23] have also been explored to produce advanced composites with appreciable properties [14, 24, 25].

Zinc oxide (ZnO) is a popular semiconducting material. This material has multifunctional properties that are explored in areas such as piezoelectricity, antibacterial activity and UV protection. ZnO can be easily synthesized in various morphological structures. Apart from piezoelectricity, ZnO is also used for mechanical energy harvesting with triboelectric effects due to its positive triboelectric charge density.

Extensive research has been carried out to explore the piezoelectric performance of PVDF/ZnO composite. Electrospun mats, solution cast films and several other techniques have been explored with regard to the preparation of PVDF/ZnO piezoelectric composites. The electroactive phase of PVDF can be improved by several means, and piezoelectric ZnO can be synthesized in a relatively easy way to obtain various nanostructures. This promising piezoelectric combination has been used in different forms for a variety of applications. Although there is extensive literature on PVDF/ZnO composites, a conclusive review compiling the different methodologies for the composite preparation, their respective properties and application seems to be scant. This review provides a conclusive report on PVDF/ZnO piezoelectric composites fabricated using different techniques and their properties reported thereof.

2. Mechanical energy harvesting

2.1. Piezoelectricity

Piezoelectricity is a stress-induced phenomenon that helps to convert mechanical energy into electrical energy and vice versa. The word piezoelectricity has been derived from the Greek word 'piezein', which means to squeeze or press. When any piezoelectric material is subjected to stress, it undergoes distortion in its crystalline structure. This causes a local charge imbalance in the unit crystals, leading to the creation of a potential difference between its surfaces (figure 1). This potential difference can be harvested in the form of electrical energy. This phenomenon is called the direct piezoelectric effect. Interestingly, this effect is reversible in nature. When a piezoelectric material is subjected to an electric field, it causes a geometric strain in its structure. This is called the converse piezoelectric effect. This phenomenon can be illustrated mathematically in the form of the following equations [13]:

$\begin{align} P = d \times T \ldots \ldots {\text{for direct piezoelectric effects (i)}}\end{align}$

Figure 1. Refer to the following caption and surrounding text. — **Figure 1.** Schematic representation of crystal structure distortion of piezoelectric materials under stress. Reprinted from [13], Copyright (2021), with permission from © 2021 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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where P is polarization, d is the piezoelectric strain coefficient and T is the applied stress.

$\begin{align} S = d \times E \ldots \ldots {\text{for converse piezoelectric effects (ii)}}\end{align}$

where S is mechanical strain and E is the applied electric field.

These equations can be further reconstructed into linear constitutive equations, as follows:

$\begin{align} {S_p} = s_{pq}^E{T_q} + {d_{pk}}{E_k} \ldots\ldots\ldots\ldots {\text{(iii)}}\end{align}$

$\begin{align} {D_i} = {d_{iq}}{T_q} + \varepsilon _{ik}^T + {E_k} \ldots\ldots\ldots\ldots {\text{(iv)}}\end{align}$

where $s_{pq}^E$ is an elastic compliance tensor at a constant electric field, ${d_{pk}}$ is a piezoelectric constant tensor, ${T_q}$ is mechanical stress in the q direction, $\varepsilon _{ik}^T$ is a dielectric constant tensor under constant stress, ${E_k}$ is the electric field in the k direction, ${D_i}$ is electric displacement in the i direction and ${S_p}$ is the mechanical strain in the p direction.

Piezoelectric materials are broadly classified into single crystals, ceramics, polymers and composites [26]. Each of these classes has its respective advantages and disadvantages. Piezoelectric crystals are among the first materials to show piezoelectricity. They have very good piezoelectric properties, but are expensive for daily use. Piezoelectric ceramics show promising piezoelectric properties. These materials have high piezoelectric coefficients; however, they are fragile and brittle in structure. This fragility restricts their use in areas requiring flexibility. Based on their chemical composition, piezoelectric ceramics can be lead-based or lead-free. Due to the established toxicity issues of lead, the preference for lead-free ceramics is increasing. Numerous lead-free materials have been developed to serve as appropriate alternatives to lead-based piezoelectric ceramics [27]. Piezoelectric polymers are good candidates for sensing purposes. These materials have high-voltage coefficients that render them suitable for functioning as sensors. These are flexible materials and find application in flexible wearables. Piezoelectric composites are generally made up of a matrix and reinforcement. The matrix is usually a piezoelectric polymer, while the filler material is often a piezoelectric ceramic. The composite thus formed has properties that are distinct from its constituent elements. It helps to make a flexible material with appreciable piezoelectric coefficients. Thus, based on target application, the piezoelectric material can be synthesized and modified. This provides a wide area for material development and novel applications [28].

2.2. Triboelectricity

Triboelectric nanogenerators (TENGs) are cutting-edge devices that collect energy from mechanical motion, largely via the triboelectric effect [29]. These devices have attracted considerable interest in recent years because of their ability to transform ambient and human-generated mechanical energy into useful electrical power [30]. TENGs have emerged as a potential technology for a variety of applications, including self-powered electronics, sensor networks and wearable devices, due to their ability to capture energy in a sustainable and efficient manner. The triboelectric effect and electrostatic induction are at the root of a TENG's operation [31]. TENGs are often made up of two materials in contact with each other: one with a high electron affinity, and the other with a low electron affinity [32, 33]. When these materials are rubbed together or mechanically deformed, electrons transfer between them, resulting in a transient charge imbalance [34, 35]. The material with the greater electron affinity is negatively charged, while the other material is positively charged (figure 2). This charge separation generates an electric potential between the two materials. TENGs employ a structured design with electrodes and an external load to harness this electric potential and generate a usable electrical current [36, 37]. The potential difference between the two triboelectric materials causes the flow of electrons, providing an electric current that can be caught by the electrodes and sent to an external circuit as the two triboelectric materials are separated and brought back into contact through mechanical motion. TENGs can create a continuous and renewable source of electrical power due to the continual cycle of separation and contact caused by mechanical motion or vibration [38, 39].

Figure 2. Refer to the following caption and surrounding text. — **Figure 2.** Schematic illustrations of the TENG's structure and materials selection showing the operating principle of the TENGs in contact-separation mode. Reprinted with permission from [40]. Copyright (2012) American Chemical Society.
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The triboelectric series comprises a hierarchical classification of materials based on their proclivity to gain or lose electrons when they come into contact with one another. Materials near the bottom of this series, such as Teflon (polytetrafluoroethylene or PTFE) [41, 42], have a tendency to become negatively charged during contact, whereas metals and other materials at the top tend to acquire a positive charge during contact. With regard to TENGs, this hierarchical classification is critical since it regulates the degree of electron transfer during triboelectric interactions. It is advantageous to use materials with a significant variance in their ranks within the triboelectric series to optimize power generation in a TENG [43, 44]. Materials with a wide range of triboelectric properties are desirable in situations where human motion, such as walking or finger tapping, serves as the energy source. Materials that closely align in the triboelectric series, however, can be selected to assure stable and continuous power output in applications involving continuous mechanical motion or high-frequency vibrations. As a result, rigorous examination of the triboelectric series is required to fine-tune the performance and efficiency of TENGs in a variety of applications [45, 46]. TENGs operate on a set of fundamental equations that govern their energy generation process [47, 48].

They generate an electric charge through the triboelectric effect, where two materials come into contact and then separate. This contact-separation leads to the production of an electric charge, which is represented by the equation, $Q = C \times \Delta V$ . Here, Q denotes the electric charge produced, C signifies the effective capacitance of the TENG and ΔV accounts for the voltage generated due to the contact-separation between the materials.

The voltage generation in TENGs is determined by the difference in work functions (Φ) between the materials involved. This voltage is captured by the equation ${\text{ }}\Delta V = \Phi 1 - \Phi 2$ , where ΔV represents the voltage output. The resulting voltage is a direct consequence of the work function disparity between the two materials, creating an electric potential.

The electric current produced by TENGs follows Ohm's law, which is expressed as ${\text{ }}I = \Delta V/R$ . Here, I represents the current generated, ΔV stands for the voltage produced and R is the external load resistance in the electrical circuit. This equation highlights the dynamic control over the current by adjusting the external load resistance to meet specific power requirements [47, 49, 50].

The choice between these equations depends on whether optimization for high-current or high-voltage output is desired, aligning the TENG's performance with the unique demands of the application at hand. These equations form the core of TENG operation, enabling them to effectively harness mechanical energy for power generation. TENGs can be integrated into a wide range of applications and surroundings that have readily available mechanical energy, including footfall [51, 52], wind [53] and even minor vibrations [51, 52]. Because of their ability to generate power from small intermittent mechanical inputs, they are perfect for powering low-energy devices and sensors, decreasing reliance on traditional batteries and encouraging sustainable energy solutions. TENGs are also low cost and eco-friendly, making them a potential technology for the future of energy harvesting and self-sustaining systems.

3. Piezoelectric and triboelectric materials

A material should have some unique properties to show piezoelectricity. Non-centrosymmetry in a crystal structure is essential for exhibiting piezoelectric properties. Piezoelectric materials can be broadly classified as single crystals, ceramics, polymers and composites. According to the symmetry, single crystals can be divided into 32 groups, amongst which only 21 are non-centrosymmetric in nature. Within these non-centrosymmetric crystal structures, only 20 can show piezoelectric effects. The exceptional group is non-piezoelectric due to other symmetry elements. Although single crystals have higher mechanical quality factors, their costly and complex processing techniques deteriorate their popularity to some extent. Quartz crystal and ammonium dihydrogen phosphate are examples of piezoelectric crystals. With regard to piezoelectric ceramics, they have higher sensitivity, coupling factors, dielectric constants and good chemical stability. By contrast brittleness, high density, costly processing techniques and small strains are well-known limitations of piezoelectric ceramics. Barium titanate (BaTiO₃), ZnO, lithium niobate (LiNbO₃), potassium niobate (KNbO₃) and gallium nitride (GaN) are a few examples of piezoelectric ceramics. Unlike piezoelectric ceramics, where the crystal structure predominantly determines the piezoelectricity, piezoelectric polymers show piezoelectric properties as a result of attraction and repulsion of entangled polymeric chains. Despite inferior electromechanical coupling, piezoelectric polymers are primarily suitable for wearable electronic applications due to their higher flexibility, easy processing technique, lead-free structure, lower cost and lower density. Typical examples of piezoelectric polymers are PVDF, nylon 11 and polyacrylonitrile (PAN). At this point, it is clear that each class of piezoelectric materials has its own advantages and limitations. To realize the benefits of different classes and to suppress their disadvantages, piezoelectric composites are often manufactured by combining filler materials and a polymeric matrix, in which at least one phase is piezoelectric in nature. Synergistic effects of each constituent can be effectively achieved in this class of piezoelectric materials, but neutralization of dipoles is possible due to inconsistent polarization directions. Examples of piezoelectric composites are (BaTiO₃ + PVDF) and (PbTiO₃ + PVDF) [54, 55]. Various forms of ZnO are also being used for addition into the PVDF matrix, which will be thoroughly discussed in the subsequent sections. The triboelectrification process depends on the physical and chemical nature of the two surfaces, environmental factors and the physical mode of the contact–separation cycle. This is why, even after the millennia of its discovery, the involved mechanism has not been made clear until now [56]. The triboelectric series, as discussed earlier, is purely empirical in nature. Contact electrification experiments had been performed followed by measurement of the polarities of the materials as a result of contact–separation cycles. The polarity of the charged surfaces and amount of charge are two independent phenomena. Generally, it is assumed that the further the gap between the two materials, in triboelectric series, the higher the charge produced. Zhang et al stated that triboelectric charge transfer is based on the Lewis basicity of the material [57]. Numerous materials have already been well explored experimentally to assess their triboelectric performance. In this context, PVDF as a polymer and ZnO as a metal oxide perform very well as triboelectric materials. Triboelectric charge density in PVDF is negative, whereas in ZnO it is positive [10, 58]. This is another reason that makes these two multifunctional materials good choices for triboelectric materials.

3.1. PVDF

PVDF, a polymer of difluoroethylene, is known to have maximum piezoelectric properties amongst the common piezoelectric polymers. The CF₂ bond within PVDF exhibits a strong dipole moment of 7 × 10⁻³⁰ C.m. PVDF shows different phases, namely, α, β and γ, depending on their polymeric structure (figure 3(a)). Although the α-phase is thermodynamically stable, it is non-polar in nature due to the alternating trans(T)-gauche(G) conformation, resulting in a zero net dipole moment. The β and γ-phases of PVDF are polar in nature due to the TTTT and TTTGTTTG [59, 60] conformations of polymer chains. There are a few more crystalline forms of PVDF; however, they have not gained much popularity in this aspect [61]. The β-phase of PVDF has high dipole density due to the opposite arrangement of fluorine and hydrogen atoms along the carbon chain. This phase thus produces a high dielectric constant value of 10 [62]. Therefore, this phase is regarded as the desirable electroactive phase of PVDF. It is utilized in piezoelectric energy harvesting. Most of the PVDF polymer-based composites aim to enhance this electroactive phase content to improve the piezoelectric performance of the composite. With regard to the triboelectric effect, the β-phase helps in accumulation of charge at the solid interface, which helps to enhance the performance of TENGs [63]. The presence of fluorine-based functional groups in the polymer chain helps to improve the charge accepting nature of the polymer during the contact electrification phenomenon [62]. Hence, PVDF has been modified to produce PVDF-copolymers with different fluorine-containing groups. The inclusion of bulky fluorine functional groups increases the interchain distance, thus weakening the dipole interaction. This results in relaxor ferroelectric properties of the polymer. Such polymers align their dipole in the direction of the applied electric field. However, due to weak dipole–dipole interaction, the alignment is disturbed once the electric field is removed. This gives good dielectric constant with inferior piezoelectric polarization. Such characteristics affect the piezoelectric and triboelectric properties of the material [64]. The α-phase of PVDF can be transformed to the electroactive β-phase with the help of high-temperature annealing treatment or mechanical stretching. Electrical poling also improves the piezoelectric properties. Piezoelectric polymers require 30–120 kV mm⁻¹ electric field strength for poling, whereas a 3 kV mm⁻¹ field is sufficient for ceramic materials. Apart from the dipole moment, the crystallinity of a polymeric material also determines the piezoelectricity. In a typical polymeric material, the crystallinity can reach as high as 50%. PVDF is often used as a copolymer of trifluoroethylene. This material can offer crystallinity up to 90%, improving the piezoelectric properties as well as the operating temperature of the polymeric system [54].

Figure 3. Refer to the following caption and surrounding text. — **Figure 3.** (a). Crystalline phases of PVDF. Reproduced from [65]. CC BY 4.0. (b) A schematic representation of the electrospinning technique. Reprinted from [66], Copyright (2019), with permission from Elsevier.
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The content of different crystalline phases can be quantified using Fourier transform infrared (FTIR) spectroscopy and x-ray diffraction (XRD) analyses. In FTIR spectra of PVDF, peaks corresponding to 1280 and 840 cm⁻¹ represent presence of the β-crystalline phase, whereas peaks corresponding to 1182, 1073, 878 and 765 cm⁻¹ confirm the presence of α-phase. Using the following equation, the fraction of the β-phase can be calculated. In this equation, A_β and A_α are the absorbance values at the wavenumbers of 841 and 765 cm⁻¹, respectively

$\begin{equation*}F\left(\beta \right){ } = \,\frac{{{A_\beta }}}{{1.26{A_\alpha } + {A_\beta }}}.\end{equation*}$

XRD results also exhibit the signatures of α and β-phases. Peak corresponding to 2θ = 20.8° can be assigned to the β-phase of PVDF. Moreover, three peaks at 2θ = 18.4°, 2θ = 26.6° and 2θ = 35.7° can be attributed to the α-phase of PVDF, and peaks at 2θ = 20.1° can be assigned to the γ-phase [67]. In addition to FTIR and XRD, differential scanning calorimetry (DSC) is another technique for identification of different crystalline phases of PVDF. However, each technique has its own benefits and limitations. In FTIR spectra, the α-phase can be easily distinguishable, whereas peaks for the β-phase and γ-phase are similar. On the other hand, XRD diffraction peaks for the α-phase and γ-phase are superimposed on each other simultaneously, providing an identifiable peak for the β-phase. Additionally, DSC can clearly identify the presence of the γ-phase of PVDF. Therefore, more than one technique can offer vivid identification and quantification of different crystalline phases of PVDF [68].

The electrospinning process has the potential to increase the β-phase content of PVDF due to in situ stretching and poling of polymeric chains. Jiyong et al studied the effect of electrospinning parameters such as applied voltage, flow rate and needle diameter on the content of the β-phase. They observed that with increasing applied voltage, the β-phase content firstly enhances followed by a decreasing trend. An applied voltage of 14–24 kV is found to be beneficial for formation of the electroactive β-phase. An increase in the β-phase with enhancement in the applied voltage can be attributed to the improved dipolar alignment and mechanical stretching during the electrospinning process. However, any further increase in the applied voltage hampers the jet stability and reduces the travelling time of the jet, ultimately causing an increase in α-phase content. Generally, the content of the β-phase decreases with an increase in the flow rate of the polymer solution and an increase in needle diameter [67].

3.2. ZnO

ZnO is a wide band-gap semiconductor which finds extensive use in various industrial sectors. Apart from the obvious properties of ZnO due to its chemical constitutions, the wurtzite structure of ZnO imparts piezoelectric characteristics to it. The structure is devoid of a centre of symmetry and has a polar nature along the [001] crystallographic direction. This influences the piezoelectric nature of ZnO. However, no ferroelectric switching has been observed in this semiconductor material. The structure of ZnO can be illustrated as an alternative arrangement of planes, comprising tetrahedrally coordinated Zn²⁺ and O²⁻ ions along the c-axis. The crystallographic arrangement of anions and cations in a specific manner leads to the generation of polar surfaces. These charge dominated surfaces may result in some topological variations that in turn lead to improvement in the properties of ZnO. Under neutral conditions, the centres of the positive and negative charges coincide with each other. When subjected to stress along the c-axis, these centres deviate away from each other, forming a dipole moment in the crystal [69–71]. ZnO nanostructures of various dimensions can be synthesized through a variety of routes [72]. The numerous synthesis techniques, high piezoelectric coefficients and multifunctional properties of ZnO have escalated the research in the material [73].

4. PVDF/ZnO composite-based nanogenerators (NGs)

4.1. PVDF/ZnO composite-based piezoelectric nanogenerators (PENGs)

4.1.1. Electrospun membrane-based PENGs

Electrospinning is a micro-/nano-fibre manufacturing technique aided by an applied electric field (figure 3(b)). Firstly, the polymer solution is loaded into a syringe. Next, the solution is forced to exit from the needle tip of the syringe to ultimately form a droplet. Thereafter, an external electrical voltage is applied to the needle so that electrical charges accumulate within the polymer solution. When the electrostatic repulsion exceeds the surface tension and viscoelasticity of the solution, the droplet assumes a cone shape and finally stretches to form fibres. The concentration of the polymer solution is a determining factor of charged jet stability. If the concentration is sufficient to maintain the stability of the jet, the elongation of the droplet occurs remarkably, ultimately forming nonwoven meshes, consisting of fibres, on the grounded collector. However, a low concentration of the solution destabilizes the charged jet, which results in small spherical particles after evaporation of the solvent. This is known as electrospraying. Costa et al reported the critical concentration of PVDF solution for electrospraying to electrospinning transition for different solvents [74].

Kim and Fan studied the effects of different structural combinations on the piezoelectric properties of nanofibrous composites. Three combinations were considered for this study as follows: (i) an electrospinning solution was prepared using pre-synthesized ZnO nanorods (NRs) and PVDF followed by electrospinning of the composite solution to obtain a nanocomposite fibrous membrane (ZnO+PVDF); (ii) ZnO NRs were grown on the electrospun PVDF membrane to obtain (ZnO@PVDF); (iii) ZnO NRs were dispersed into water followed by electrospraying of the solution onto the PVDF electrospun membrane to get the (ZnO/PVDF) structure. It was found that for all the cases, the electroactive β-phase of PVDF decreased with the addition of ZnO NRs. The maximum decrement was observed for the (ZnO/PVDF) structure. During electrospray deposition of the ZnO NRs, an electric field was applied to the pristine PVDF membrane in the thickness direction. Initially, after the electrospinning of pristine PVDF, the dipoles were oriented in the fibre axis direction. Afterwards, during the electrospray deposition of the ZnO NRs, the orientation of the dipoles gets disturbed, ultimately resulting in a lower β-phase. However, it can be expected that the resultant dipole orientation will be in the direction of the fibre axis; this is because the electric field during electrospinning of pristine PVDF is more effective compared to the electrical poling during electrospray deposition of ZnO at room temperature. Meanwhile, a decrease in β-phase content for (ZnO + PVDF) can be attributed to less crystallization of the polymer in the presence of filler at higher concentrations. For the ZnO@PVDF structure, the deterioration of the β-fraction can be attributed to the heat relaxation of the PVDF crystalline region during hydrothermal growth of ZnO NRs. A descending order of power output was found for the power outputs arising from ZnO/PVDF, ZnO@PVDF, pristine PVDF and ZnO+PVDF membranes. The reasons for the least power output of the ZnO + PVDF membrane are the decreasing crystallinity of PVDF during electrospinning and the increasing dielectric constant of the nanocomposite structure due to the addition of nanofillers. The higher power outputs for ZnO/PVDF and ZnO@PVDF can be attributed to the triboelectric charge generation. From the FESEM images, it can be visually assessed that, for these two structures, the surface roughness is higher compared to the remaining two structures. Higher surface roughness and specific surface area lead to enhanced surface charge density, thus improving the triboelectricity [75]. Yang et al grew ZnO NRs on an electrospun PVDF surface to obtain a hierarchically interlocked PVDF/ZnO structure with higher pressure and bending sensitivities compared to a pristine PVDF membrane. The mechanical durability and real-time applications of this device have been extensively investigated. The device, fabricated with such a hierarchical structure, can not only monitor physiological movements but is also useful for disease diagnosis (figure 4) [76].

Figure 4. Refer to the following caption and surrounding text. — **Figure 4.** (a) Pressure sensors on different body parts; (b) the electrical current generated from breathing patterns; (c) electrical signals from wrist pulses; (d) three peaks in an expanded pulse; (e) representation of a gait recognition system; (f) detection of gait states during walking in different directions; (g) multiple electrical signals for the left leg during walking in the left direction; (h) the ratio of calf response amplitudes for right and left legs, respectively, in different directions. Reprinted from [76]. Copyright (2020), with permission from Elsevier.
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Feng et al grew ZnO NRs on conductive fabric followed by sandwiching PVDF in between the conductive fabric electrode and the electrode with ZnO NRs. Incorporation of a ZnO layer increases the piezoelectric output voltage noticeably. Apart from piezoelectric properties, this textile-based breathable device exhibits waterproof behaviour. The device, with high sensitivity, durability and output voltage, can be utilized to monitor environmental changes like wind blow and rainfall [77]. Mahanty et al demonstrated the stress concentration behaviour within PVDF/ZnO nanocomposite fibre using the finite element method. PVDF fibre loaded with ZnO nanoparticles (NPs) offers higher stress concentration compared to PVDF loaded with ZnO NRs. The authors depicted the piezoelectric performance of a PVDF/ZnO composite electrospun membrane. The ultra-fast response of the device proves the potential for it to be used in sensing applications. Moreover, the device is not only capable of lighting five LEDs without any electrical storage system but also can charge a capacitor of 2.2 μF after rectification of the output voltage using a bridge rectifier circuit. The device is also capable of sensing several physiological movements, such as pulses and vocal chord vibrations, making the device suitable for monitoring the health condition of patients [78].

Research has been executed on the effects of filler addition on the piezoelectric properties of an electrospun PVDF membrane. Li et al prepared carbon-coated ZnO (ZnO@C) NPs incorporated PVDF electrospun membranes. They observed that with an increase in ZnO@C content within the nanofibrous membrane, the content of the β-phase enhances within the PVDF, which can improve the piezoelectric performance of the device, fabricated using such an electrospun membrane. The carbon layer contains sp² hybridized carbon atoms, which are negatively charged. By contrast, CH₂ dipoles within PVDF are positively charged. Interaction between positively charged dipoles and the negatively charged carbon layer results in regular arrangement of PVDF segments, thus the β-phase is formed. Moreover, the conductive nature of the carbon shell layer accumulates more electrical charges in this region in the presence of an electric field. This further improves the interfacial polarization, ultimately leading to improvement in the electroactive phase of PVDF [79]. Ongun et al incorporated ZnO NPs and Ag-doped ZnO NPs into an electrospun PVDF membrane. They found that the electroactive β-phase of PVDF enhances with the increase in the amount of ionic Ag dopant. With the increase in Ag dopant concentration, the capacitance of the nanocomposite device also enhances [80]. Deng et al prepared a ZnO nanosphere incorporated PVDF electrospun fibrous membrane with a cowpea structure. It was found that the electroactive β-phase of PVDF increases as a result of ZnO addition within the fibrous structure. The sensor was fabricated by spraying Mxene onto the electrospun membrane followed by extraction of the electrode using copper wire. With the increase in the applied force, the open-circuit voltage and short-circuit current have a tendency to rise. The as-prepared piezoelectric sensor can function under both the pressing and bending excitations without any external power source. The sensor not only exhibits high sensitivity and good flexibility but can also help in remote control of a robot hand, proving its application in interactive human–machine interfaces (figure 5) [81]. The addition of a conductive filler and a piezoelectric filler to a polymer matrix has often been carried out to explore their synergistic effects on stabilizing the electroactive phase of the polymer. A carbon nanotube is a widely used conductive filler in piezoelectric composites. The homogeneous dispersion of these fillers has a considerable effect on the piezoelectric performance of the composites. Single-walled carbon nanotubes were decorated with ZnO using a wet chemical route and incorporated into an electrospun PVDF nanofibrous web. The inclusion of this surface with decorated carbon nanotubes promoted the β-phase formation (95%) of PVDF and improved its thermal stability and tensile properties. The NGs produced an output voltage of 15.5 V and a power density of 8.1 μW cm⁻² [82]. The performance of PVDF/ZnO electrospun composite can be influenced by the shape and content of the filler. Nanosticks and NRs have been proven to perform better as piezoelectric reinforcement in an electrospun nanocomposite [83]. Studies on piezoelectric films made from PVDF/ZnO NPs further ascertained the ability of ZnO NPs to promote the β-phase formation of PVDF polymer. ZnO NPs lower the transition energy from the α to the β-phase of PVDF, thus improving its electroactive phase content. This effect is more pronounced at lower levels of ZnO NP addition. At higher levels, however, these NPs halt the polymer chains, adversely affecting the crystallization process. Thus, the overall crystallinity decreases at higher filler loadings. The morphological studies of the composite cross-section further confirm the compatibility of the NPs with the polymer matrix. The optimized composition of the composite yielded an open-circuit voltage of 69 V under an impact force of 1.6 N. It also generated an appreciable surface power density of 250 μW cm⁻² [84]. Simulation studies on PVDF/ZnO piezoelectric composite explained the improved performance of the composite compared to pristine material. It was observed that enhanced distribution of stress at the interface of the filler and matrix increased the Young's modulus and piezoelectric coefficient. These effects led to an enhanced piezoelectric performance of the composite in comparison to pristine polymer [85].

Figure 5. Refer to the following caption and surrounding text. — **Figure 5.** (a) Detecting the power of the gripping of a spring-grip; (b) electrical outputs for various grip strengths; (c) detection of the angle of an opening and closing book; (d) electrical outputs for the closing (red area) and opening (blue area) of a book; (e) a schematic representation of a human–machine remote control system; (f) application of gesture remote control. Reprinted from [81]. Copyright (2018), with permission from Elsevier [81].
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Yi et al synthesized Y-doped ZnO followed by incorporation of the same into PVDF using electrospinning technique. The Y-ZnO incorporated electrospun membrane offered superior piezoelectric output voltage compared to the ZnO incorporated membrane due to the polar face in the former filler. It was also observed that with an increase in the electrospinning time, the piezoelectric output enhances. Corona poling was found to be an efficient technique to improve the piezoelectricity of porous material through dipolar electret formation. The porous electrospun membrane exhibited lower piezoelectricity compared to an aligned electrospun membrane, while the trend reversed after corona poling treatment as a result of higher electret dipole formation within the porous structure. The device fabricated using Y-ZnO incorporated PVDF membrane was able to charge capacitors of various capacitance values. Additionally, it was tested for its ability to produce piezoelectric output voltage from biomechanical motions, such as walking and running [86].

4.1.2. Textile-based materials

The growing field of electronic wearables has led to the expansion of textiles from conventional uses to high-performance applications. The flexibility, conformability, comfort and aesthetic appeal of textiles have made it the appropriate substrate for such applications. Textiles in the form of woven or knitted structures are being widely explored for use as electronic wearable applications. Piezoelectric grade fibres produced through different processes are shaped in the form of textile structures and used to harvest energy from mechanical motions [87].

Melt spinning is one of the widely used processes for the preparation of piezoelectric fibres. There are several studies that illustrate the fabrication and use of melt-spun piezoelectric fibres. In one study, core sheath PVDF-based melt-spun filaments were produced. The core consisted of carbon black/high-density polyethylene acting as an inner electrode, while the sheath was made up of electroactive PVDF. These core sheath piezoelectric PVDF filaments produced a piezoelectric signal with a peak-to-peak voltage of 40 mV when subjected to compression. Such electrode-embedded piezoelectric PVDF filaments can be explored as single fibre sensors [88]. Further, studies have shown that poling of such filaments under appropriate temperature and electric field conditions improves their piezoelectric properties. These filaments when shaped in the form of woven textiles, were suitable for monitoring the heartbeat of humans [89, 90]. Melt extruded PVDF fibres and flexible electrodes were introduced into polyester plain woven fabric to prepare a flexible textile-based sensor. The signals produced by this sensor were different for different force waveforms. The distance between the electrodes was tailored by varying the number of spacer yarns between them. This noticeably influenced the output of the sensor [91]. Sometimes, porous PVDF fibres are also produced through processes like wet spinning to be utilized in filtration application. This porosity can be tuned by varying the drawing ratios and temperatures in the subsequent drawing baths [92].

ZnO has been used extensively in the textile industry due to its properties such as ultraviolet resistance, antifungal activity and piezoelectric characteristics [93]. ZnO NRs were grown on cotton substrate using a low-temperature chemical synthesis method. The cotton substrate was coated with silver to facilitate the growth of nanostructures. Structural and morphological analyses revealed the growth of highly crystalline NRs with a good aspect ratio. The tip deflection in atomic force microscopy revealed a mean output voltage of 9–9.5 mV. The higher flexibility of textile substrates is expected to increase the voltage manifold [94]. A textile-based pressure sensor has also been fabricated using ZnO NRs. For this purpose, three layers were combined to make a sensor. ZnO NRs grown on conductive reduced graphene oxide/polyester fabric formed the top and bottom layer, while a PVDF membrane was sandwiched between them. This sensor had a very low detection limit and high sensitivity of 0.62 V/kPa. Further, it produced an open-circuit voltage around 11.47 V with superior mechanical stability. This sensor could effectively sense different human motions [95]. There are several studies that have employed ZnO in textile substrates and explored its piezoelectric properties [77, 96].

4.1.3. Film-based materials

Piezoelectric films have been prepared by processes such as solvent casting, spin coating and supersonic spraying. These methods are facile and suitable for upscaled production. Poling of piezoelectric films helps to improve their piezoelectricity. The combination of ZnO and PVDF has been explored extensively in the form of films, prepared via different film preparation techniques. ZnO microrods were synthesized chemically using zinc nitrate and hexamethylenetetramine as precursors. The synthesized microrods were mixed with a solution of PVDF in dimethylformamide at different loading levels. PVDF/ZnO composite films were prepared using a supersonic spraying technique. The morphological characterization of the films revealed the successful integration of ZnO microrods into PVDF films, while the structural characterizations of the composite films revealed the stabilization and enhancement of the electroactive β-phase of PVDF. The stretch experienced by the polymer chains during supersonic spraying resulted in the increment of the β phase content of the PVDF polymer. The d₃₃ values obtained for the pristine polymer and composite film showed an increase from 23.3 to 36.3 pm V⁻¹, respectively. The output voltage analysis of the fabricated piezoelectric NG (figure 6) indicated an increase in the output voltage with an increase in ZnO microrod content. However, at higher ZnO loading, the β-phase of PVDF decreased, adversely affecting the output performance of the composite film. The highest output voltage of 15.2 V and maximum power density of 12.5 μW cm⁻² were obtained for the optimized composition of ZnO/PVDF [97]. In another study, ZnO NPs were incorporated into PVDF polymer via a spin-coating technique. The rough surface of the composite film compared to the smooth surface of pristine PVDF film suggested the successful integration of ZnO NPs into the PVDF matrix. Further, elemental analysis carried out using energy-dispersive spectroscopy (EDS) reaffirmed the presence of the expected elements in the nanocomposite film. Structural analyses using XRD and FTIR indicated the β nucleating effect of the ZnO NPs. The negative charge on the surface of the NPs interacts with the positive charge of the polymer chains to cause the effective nucleation of the β-phase. This was further reflected in the enhanced output voltage of 4.2 V for the nanocomposite film compared to 1.2 V for the pristine polymer sample [98]. ZnO nanowires (NWs) were grown hydrothermally on chopped carbon fibre strands. These hierarchical structures were incorporated into spin-coated PVDF films. The incorporation of such hierarchical structures in the nanocomposite film helped to enhance its electrical and mechanical performances. The addition of the hierarchical filler to the polymer matrix helped to enhance its electroactive phase. The interfacial polarization between the semiconductor NWs and the polymer chains deflects the electronegative F atoms to one side of the polymer chain, stabilizing its β-phase conformation. However, too high loading of filler adversely affects the β-phase content of the polymer. A dual interface is formed in this nanocomposite film. The interface between ZnO NWs and the polymer matrix influences the formation of the β-phase of PVDF. Further, the interface between ZnO NWs and carbon fibre helps in the migration of the polarized charges. The filler content has a significant effect on the electrical performances. A higher filler content adversely affected the electrical performance of the nanocomposite films. An optimized amount of filler helped to achieve a trade-off between the piezoelectric and conductive properties, achieving an output voltage as high as 14.91 V, output current of 1.25 μA and output power of 7.9 μW [99]. To improve the piezoelectric properties, ZnO NPs were doped with several metal ions and studied for their electrical performance. A 5% Li-doped ZnO showed striking improvement in its energy harvesting properties compared to pristine ZnO. This optimized level of metal doping helped to create defects in the wurtzite structure of ZnO, increasing its asymmetry and thus improving its piezoelectricity. However, a higher doping level causes screening of piezoelectric charges that adversely affects the piezoelectricity of the doped material. The 5% Li-doped ZnO incorporated PVDF-TrFE spin-coated porous film generated an output voltage of 3.43 V, compared to 0.38 V for pristine ZnO incorporated PVDF-TrFE spin-coated porous film [100]. The size of the loaded filler in the composite films also influences the piezoelectric performance of the material. This was explored for PVDF/ZnO NP incorporated piezoelectric composite. Although the composite performed better compared to the pristine polymer-based film, its performance was significantly affected by the size of the loaded filler. The smaller the filler size, the better was the piezoelectric performance of the composite [101].

Figure 6. Refer to the following caption and surrounding text. — **Figure 6.** (a) Fabrication and application of a PVDF/ZnO piezoelectric NG. Reproduced from [97]. CC BY 4.0. (b) (i) The smooth surface of PVDF, and (ii) the rough surface of PVDF/ZnO film. Reproduced from [98]. CC BY 4.0. (c) Voltage output of metal-doped ZnO NPs. Reprinted from [100]. Copyright (2020), with permission from Elsevier.
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The aspect ratio of fillers, incorporated in a polymer-reinforcement composite, plays a crucial role in governing its electrical properties. To illustrate this, ZnO NRs with varying aspect ratios were synthesized hydrothermally. Varying the reaction time resulted in the synthesis of NRs with different aspect ratios. The highest reaction time yielded NRs with the highest aspect ratio and good crystallinity. The study of the zeta potential reveals the presence of negative charges on the surface of ZnO NRs. The higher the aspect ratio, the greater is the number of negative charges that developed. These negative charges on the surface of ZnO interact with the positive CH₂ dipoles of PVDF polymer chains to enhance and stabilize the β-phase of PVDF polymer. Thus, the incorporation of ZnO NRs of higher aspect ratio into PVDF polymer helps to enhance its electroactive phase content. Further, a higher aspect ratio of fillers helps in homogeneous dispersion of the filler in the polymer matrix [102]. ZnO along with other metal oxide NPs have also been incorporated into PVDF to form advanced polymer composites, suitable for electronic device applications [103].

4.2. PVDF/ZnO composite-based triboelectric energy harvesting

4.2.1. Electrospun membrane-based systems

TENGs are very popular in the field of wasted mechanical energy transduction due to their high electrical power output. Surface decoration with fillers and surface roughening were found to be very effective techniques in the improvement of the energy harvesting performance of electrospun hybrid NGs [104]. Kim and Fan highlighted the influence of surface roughness on electrical power outputs of nanocomposite fibrous membranes. They prepared nanocomposite structures using different combinations of ZnO NRs and PVDF matrix. Electrospinning of ZnO and PVDF composite solution was performed to form nanocomposite fibres. Additionally, ZnO NRs were hydrothermally grown and electrosprayed onto PVDF electrospun fibres to obtain two different structures. The authors highlighted that increasing surface roughness leads to the enhancement of power output due to triboelectrification (figure 7) [75].

Figure 7. Refer to the following caption and surrounding text. — **Figure 7.** FESEM images of (a) pristine PVDF nanofibres, (b) (ZnO + PVDF), (c) (ZnO@PVDF), (d) (ZnO/PVDF). Reproduced from [75], with permission from Springer Nature.
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Pu et al selected two piezoelectric polymers, PVDF and nylon 11, to form different triboelectric layers. The electrospinning technique improves the electroactive phases (β-phase for PVDF and δ'-phase for nylon 11) of the piezoelectric polymers. The addition of ZnO NWs to PVDF and nylon 11 also improves their respective contents of electroactive phases. Incorporation of ZnO NWs improves the thermal stability, tensile strength and Young's modulus of the composite nanofibres and also reduces the breaking elongation. Triboelectric outputs were found to improve in testing with ZnO-loaded PVDF and ZnO-loaded nylon 11 compared to the testing performed using electrospun pristine PVDF and nylon 11 membranes. There are approximately four reasons for this: (i) incorporation of ZnO NWs enhances the electroactive phases of the piezoelectric polymers; (ii) ZnO itself has piezoelectric properties, which are efficient in conversion of mechanical energy to its electrical counterpart; (iii) improvement in tensile strength and Young's modulus of the composite membranes can effectively transfer the applied load between the matrix and ZnO NWs; (iv) such alternation in mechanical properties may improve the friction between the triboelectric layers. The authors demonstrated that the developed TENG can lighten over 100 LEDs and charge capacitors of various capacitance values [105]. Li et al incorporated carbon-coated ZnO (ZnO@C) NPs into PVDF electrospun membranes. An increase in the concentration of ZnO@C NPs results in enhancement in the β-phase of PVDF, as already discussed. Due to improvement in polar phase content, the surface potential of the electrospun membrane was also found to alter. As the concentration of NPs increases from 0% to 5%, the surface potential changes from −130 mV to −740 mV, as obtained from Kelvin probe force microscopy. Due to improvement in the surface potential on the negative side, the triboelectric output voltage also enhances as more potential difference can be created due to the higher possibility of charge transfer [79].

4.2.2. Film-based materials

Film-based ZnO-PVDF materials offer advantages over electrospun fibres due to their simplicity, uniformity, durability and easy integration, making them a cost-effective and customizable choice for various applications, including energy harvesting and sensor technology [106]. This composite excels at converting mechanical energy, such as vibrations and pressure, into electrical energy by combining the intrinsic piezoelectric capabilities of PVDF with the enhanced piezoelectric effect as a result of ZnO NPs [107]. It can be used in a variety of applications, such as wearable electronics and flexible electronics, where it can adapt to diverse surfaces and shapes due to its strength, flexibility and wide compatibility with materials [108, 109]. Singh and Khare achieved a noteworthy advancement in the field of triboelectric energy harvesting through their research. By adding ZnO NRs to a PVDF polymer and combining it with PTFE, they were able to achieve improved triboelectrification (figure 8). When compared to TENGs manufactured from PVDF/PTFE alone, the resultant ZnO-PVDF/PTFE-based TENG demonstrated impressive performance gains, including a 21% rise in output voltage and a significant 60% jump in short-circuit current. Additionally, the remarkable instantaneous output power density of roughly 10.6 mW cm⁻² was attained by this inventive composite. There are several reasons for the increased triboelectrification in this system, one of which is because PVDF has a higher β-phase concentration, which improves polarizability. Furthermore, the incorporation of ZnO NRs resulted in enhanced hydrophobicity, reduced PVDF work function and better surface roughness, all of which enhanced the TENG performance. Interestingly, the ZnO-PVDF/PTFE-based TENG showed a significant 65.6% increase in output power over the PVDF/PTFE-based TENG, and this improvement was ascribed to the changes in PVDF's characteristics as a result of the addition of ZnO [106]. TENGs can have their power density greatly increased by using an interfacial piezoelectric ZnO nanosheet layer, as demonstrated by Narasimulu et al. These TENGs were based on phase inversion membranes made of polyamide-6 (PA6) and ZnSO₃ incorporated PVDF. The ZnO nanosheet electrochemically deposited TENG device demonstrated a remarkable output voltage of around 625 V and a current density of about 40 mA m⁻², which translated into a charge density of roughly 100.6 C m⁻² when subjected to an applied force of 80 N. In comparison to the TENG device with no interfacial layer of ZnO, which produced an output voltage of about 310 V, a current density of about 10 mA m⁻² and a charge density of about 77.45 C m⁻², this represented an improvement. Under compressive stress, the ZnO nanosheets produced a piezoelectric potential that injected charge onto the ZnSnO₃-PVDF membrane's surface, leading to improved charge density and a substantial increase in power density from 0.11 to approximately 1.8 W m⁻². This research underscores the potential for the use of interfacial piezoelectric ZnO nanosheets to enhance energy harvesting and power generation in TENG devices [110]. The efficiency of triboelectric energy harvesting could be revolutionized by modern materials and nanotechnology. Optimizing the combination of PVDF and ZnO for maximum energy conversion, ensuring long-term durability and establishing standardized testing methods are essential steps to realize the full potential of this energy harvesting technology.

Figure 8. Refer to the following caption and surrounding text. — **Figure 8.** (a) A schematic of the fabricated ZnO-PVDF/PTFE-based TENG showing in-plane separation. (b) The respective instantaneous output power density with the different load resistances of PVDF/PTFE and ZnO-PVDF/PTFE-based TENG. Reprinted from [106], Copyright (2019), with permission from Elsevier.
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Hybrid NGs based on PVDF and ZnO constitute a noteworthy development in energy harvesting technology. Through the integration of ZnO's semiconducting characteristics and PVDF's piezoelectric qualities, these NGs effectively transform mechanical energy into electrical power. They provide sustainable and independent energy solutions for a variety of applications, including wearable electronics and self-powered sensors, due to their adaptable and versatile design [111]. By producing a ZnO-PVDF film and combining it with PTFE to create a piezo-tribo-based hybrid NG, Singh and Khare have made a noteworthy advancement. Their investigation showed that adding ZnO to the PVDF matrix improves the material's triboelectric qualities in addition to its piezoelectric qualities. As a result of this breakthrough, a ZnO-PVDF-based piezo-tribo hybrid generator was created that can produce an impressive 2.5 times more power than a bare PVDF-based hybrid NG, with an instantaneous maximum output power of roughly 24.5 μW cm⁻². The noteworthy aspect of this enhancement is that it was achieved without the need for surface treatment or electrical poling: solely by introducing ZnO NRs into the PVDF matrix [112]. This pioneering approach represents a promising direction for simultaneously harnessing the piezoelectric and triboelectric energies from a single device, offering new opportunities for efficient energy harvesting.

5. Challenges and future perspectives

Advancements in nanotechnology and nano-manufacturing techniques are set to revolutionize the production of NGs, enabling large-scale and cost-effective manufacturing. The integration of NGs into everyday objects, such as clothing, sensors and infrastructure, offers a vast array of potential applications, from self-powered wearables to smart cities. Moreover, the ongoing standardization efforts and the establishment of industry guidelines will pave the way for widespread adoption and commercialization. There are particular difficulties with both PENGs and TENGs. With regard to PVDF–ZnO composites, the main challenges are related to material and design, including the development of mechanical structures to maximize energy conversion efficiency and optimization of materials to boost their piezoelectric coefficients. Furthermore, it is critical to guarantee the scalability and endurance for these composites to be appropriate for a variety of uses, such as wearable technologies and large-scale energy harvesting systems. Interestingly, PVDF–ZnO-based TENGs have a unique set of characteristics. The coupling of two complementary materials with differing triboelectric charge densities results in the creation of a large potential difference, leading to the generation of high electrical output. Ensuring TENG durability and long-term performance is a persistent concern, particularly in high-friction situations.

PVDF–ZnO composite advancements are about to take a radical turn because of the development of artificial intelligence (AI) and machine learning (ML). Large-scale material property information can be analysed by AI algorithms to find the best candidates for PENG and TENG applications, maximizing their effectiveness. To ensure that these NGs run as efficiently as possible, ML models can be used to further optimize their configuration and design. Furthermore, PVDF–ZnO composite-based NGs will have a longer lifespan and higher overall efficiency due to AI-driven predictive maintenance algorithms. The creation of novel materials, creative designs and a wide range of applications are expected to accelerate with the integration of AI and ML, increasing the efficiency and adaptability of PVDF–ZnO composites to a variety of scenarios and sectors.

When a piezoelectric material is being used as a triboelectric surface, there is obviously a possibility of getting piezoelectric voltage output along with the triboelectric output. Similarly, during the characterization of piezoelectricity in a piezoelectric material, there is every possibility that triboelectric voltage also arises due to friction between two different materials. Sutka et al stated that often triboelectric output is misinterpreted as piezoelectric output, ultimately reporting irreproducible piezoelectric coefficients. The authors reported that in a piezoelectric generator, contact electrification may arise from static discharge from a rubber glove or a human finger, commonly used to test piezoelectric energy harvesting efficiency. Additionally, shear force within a composite and friction between interfaces also result in triboelectrification within a piezoelectric generator. Device fabrication techniques, testing apparatus, low-resistance electrical measuring instruments and external circuitry can introduce deceptive results within a true piezoelectric output [113–115]. Very few researchers have investigated the path of differentiating the true piezoelectric output from the unwanted triboelectric one. Musa et al confirmed that a triboelectric signal can be bipolar in contact and unipolar in separation in a course of contact–separation cycles [56]. This phenomenon is obviously a unique signature of triboelectrification, distinguishing it from piezoelectric signals. Sutka et al reported that incorporation of conductive tape in between the electrode and polymeric substrate can neutralize the unwanted charges arising from contact electrification. A piezoelectric polymer with sputtered electrodes can be encapsulated with the help of a thin dielectric layer. Thereafter, this device can be coated with a flexible grounded conducting material to dissipate the charges generated due to friction. Additionally, if a piezoelectric material is compressed and released periodically without breaking the contact between the piezoelectric substrate and the force-imparting object, negligible contact electrification can be expected from the contact–separation between these two parts [113]. Suo et al mentioned that the electrical pulses of piezoelectricity and triboelectricity can be distinguished by their natures. The triboelectricity peak is sharp and narrow, whereas the piezoelectricity peak is relatively broad due to strain-induced charge generation. The authors also argued that the total electrical output is the addition of the piezoelectricity and triboelectricity outputs [115]. During piezoelectric energy harvesting, we obviously need another object to exert force onto the piezoelectric material. Contact–separation cycles between this object and a piezoelectric device lead to triboelectric charge generation. Chen et al stated that a triboelectric signal is generated in three stages: namely, contacting, contacted and separating. However, a piezoelectric signal is generated in the contacted stage, which can further be subdivided into compressing, compressed and releasing stages. More specifically, triboelectric signals mainly appear before and after the contact between the object and piezoelectric device, whereas the piezoelectric signal arises during contact between the object and piezoelectric device. By analysing the force signal and electrical signal, both the responses from piezoelectricity and triboelectricity can be identified in a systematic manner. Moreover, the authors also developed a technique to mathematically quantify the piezoelectric output from a hybrid signal [116]. Therefore, there is high chance that the perceived piezoelectric output has a hidden triboelectric component within it. Research should be conducted to separate these two outputs, and the quantification of actual piezoelectric and triboelectric outputs is of vital importance.

6. Conclusion

Due to the present energy crisis concerns, the need for renewable energy sources is amplifying day by day. The sources of mechanical energy are numerous, and this kind of energy is being wasted in various forms. For these reasons, there is growing concern regarding the conversion of these mechanical energies into electrical energy to power small electronics. Piezoelectricity and triboelectricity, two scientific phenomena, have been revealed to be very efficient in this respect. Appropriate material selection and fabrication of an ideal structure out of them are the bases of piezoelectric and triboelectric device manufacturing. Piezoelectric materials can be broadly classified into single crystals, ceramics, polymers and composites. Each class has its own benefits and disadvantages. To partially overcome the limitations of each constituent and to obtain their respective benefits, piezoelectric ceramics are often combined with polymers to obtain piezoelectric composites, with enhanced properties compared to their constituent materials. PVDF is a well-known piezoelectric polymer due to its high piezoelectric properties among the piezoelectric polymers. PVDF offers various phases depending on its chemical conformations. The β-phase, an electroactive phase, of PVDF is known to be primarily responsible for piezoelectric performance of PVDF. Incorporation of a small amount of filler into a PVDF matrix has proved to be effective for generation of the β-phase of PVDF. In this context, ZnO was found to enhance the piezoelectric performance of PVDF. ZnO is known for its functionalities, such as piezoelectricity, pyroelectricity, antibacterial activity and ultraviolet protection. Different nanostructures of ZnO can be easily prepared using various routes. Apart from improving the electroactive phase of PVDF, the addition of ZnO to the PVDF matrix can enhance the mechanical energy harvesting performance due to several other effects. The piezoelectric properties of ZnO itself contribute to the overall electrical performance of PVDF–ZnO composite. Incorporation of ZnO into PVDF results in an increase in surface roughness of the composite structure, thereby improving the triboelectric performance of the resultant material. Although there is good amount of research on mechanical energy harvesting from electrospun PVDF–ZnO composites, the amount of research related to the energy harvesting performance of this particular composite in other structural forms is still lacking. PVDF–ZnO hybrid material has been proven to be very efficient, not only in wasted mechanical energy harvesting but it also performs significantly in sensing and disease-detection areas.

Acknowledgments

The authors are grateful to the Department of Science and Technology and The Government of India for funding the work on piezoelectric device development and, as a part of that, the authors have prepared this review article (Sanction Letter: DST/TDT/DDP-05/2018 (G)) under the Device Development Program). The authors would also like to acknowledge the support of the UK Engineering and Physical Sciences Research Council (EPSRC) through Grant Ref. EP/V003380/1: 'Next Generation Energy Autonomous Textile Fabrics Based on Triboelectric NGs'.

Data availability statement

All data that support the findings of this study are included within the article (and any supplementary files).

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