A Comprehensive Review of Strategies toward Efficient Flexible Piezoelectric Polymer Composites Based on BaTiO₃ for Next-Generation Energy Harvesting

Abstract

The increasing need for wearable and portable electronics and the necessity to provide a continuous power supply to these electronics have shifted the focus of scientists toward harvesting energy from ambient sources. Harvesting energy from ambient sources, including solar, wind, and mechanical energies, is a solution to meet rising energy demands. Furthermore, adopting lightweight power source technologies is becoming more decisive in choosing renewable energy technologies to power novel electronic devices. In this regard, piezoelectric nanogenerators (PENGs) based on polymer composites that can convert discrete and low-frequency irregular mechanical energy from their surrounding environment into electricity have attracted keen attention and made considerable progress. This review highlights the latest advancements in this technology. First, the working mechanism of piezoelectricity and the different piezoelectric materials will be detailed. In particular, the focus will be on polymer composites filled with lead-free BaTiO₃ piezoceramics to provide environmentally friendly technology. The next section will discuss the strategies adopted to enhance the performance of BaTiO₃-based polymer composites. Finally, the potential applications of the developed PENGs will be presented, and the novel trends in the direction of the improvement of PENGs will be detailed.

1. Introduction

In the last decade, there has been a sharp increase in wearable and portable electronic devices. Next-generation wearable devices are generally formed with lightweight and flexible structures that meet the requirements of recent advanced applications. Scientists have recently dedicated efforts to providing a continuous power source for uninterrupted operation.

Consequently, the power supply device must offer flexibility, durability, and safety, along with a high energy density. Therefore, energy harvesting has gained extensive attention in recent years. Previously, providing energy for low-power devices was achieved using batteries, which posed a significant threat to the global climate. Therefore, finding more environmentally friendly alternatives is one of scientists’ biggest concerns. Consequently, most recent works have been devoted to renewable energy, as it presents an eco-friendly solution. Various renewable energy sources exist; among them, solar, wind, and mechanical energy are the most sustainable sources currently available in the surrounding environment that is typical during the use of wearable and portable electronic devices.

However, mechanical energy is considered one of the prominent renewable energies due to its abundant availability. It can originate from different sources, including human body movement, wind and liquid flows, and vibration in the surrounding environment [1].

Different technologies have been adopted to harvest mechanical energy and convert it into electricity. The most common technologies are based on piezoelectric [2], triboelectric [3], electromagnetic [4], and electrostatic principles [5]. Due to their numerous advantages, including easy fabrication, lightness, flexibility, and cost-effectiveness, piezoelectric energy harvesters are frequently used to efficiently harvest their surrounding mechanical energy. Piezoelectricity is present in certain materials that can create an electric charge when they are subjected to external mechanical load and vice versa. The piezoelectricity phenomenon dates to 1880 and was discovered by Pierre Curie and Jacques Curie. They investigated the effect of an external load on generating an electrical charge from some natural crystals such as quartz and tourmaline. Up to now, four main classes of piezoelectric materials have been found: single crystals, ceramics, polymers, and composites. Synthetic piezoelectric materials such as ceramics are more efficient than crystals. The most extensively used piezoceramics are lead lanhanum zirconate titanate (PLZT), lead zirconate titanate (PZT), lead magnesium niobate (PMN), and BaTiO₃ [6]. PZT was the most used ceramic owing to its excellent piezoelectric properties (d₃₃ ~300–1000 pC/N) [7] and high electromechanical coupling coefficient (k_p) of 0.69. Many researchers have recently reported that lead-free piezoelectric ceramics have achieved properties similar to PZT-based materials. Because of the environmental concerns associated with lead and to comply the current environmental regulations for new applications, lead ceramics are avoided. Among lead-free ceramics, BaTiO₃ is considered the most promising lead-free material, and it has attracted the attention of many researchers. Numerous investigations have been conducted to synthesize highly performant BaTiO₃-based ceramics with properties comparable to that of PZT. Many strategies have been followed to boost the piezoelectric properties of this material through doping within the ceramics [8,9], or by modification of the synthesis process [10]. Because of the inherent stiffness and brittleness of piezoelectric ceramics, their use in wearable applications is limited. Different from piezoceramics, piezoelectric polymers exhibit lower piezoelectric coefficients but have higher mechanical flexibility, lower stiffness, and higher chemical resistance, resulting in longer device life. The piezoelectric phenomena in a polymer basically come from its molecular structure, which is also called the crystallinity of a polymer. The degree of crystallinity in a polymer is related to the proportions of semicrystalline and amorphous regions present within it. Indeed, the piezoelectricity of a polymer can be ameliorated by increasing the crystalline phase. Nucleation with particles, mechanical orientation, thermal annealing, and high voltage treatment can be applied to induce crystalline phase transformations [11]. PVDF and its copolymers, polyamides and parylene-C, are the most used semicrystalline polymers [12].

The integration of piezoceramic particles with a polymer to form a piezoelectric composite was efficient in strengthening the piezoelectric properties of the material. Therefore, polymer composites based on piezoceramics endow the material with several interesting properties, such as good flexibility with higher piezoelectric properties than pure polymers. This presents a convenient way to overcome the limitations of piezoelectric ceramics as well as bulk polymers.

This review summarizes the recent development of piezoelectric polymer composites based on BaTiO₃ for low-power energy harvesting applications. Therefore, the first part of this review is devoted to understanding the working principle of a piezoelectric energy harvester and the piezoelectric materials classifications. Then, the focus will be on BaTiO₃ lead-free piezoceramic particles, mainly because they are environmentally safe and have good piezoelectric performances. Therefore, an overview of the structure of and the technique used for the synthesis of BaTiO₃ is given while highlighting the main advantages of each technique. Furthermore, this review highlights the importance of embedding BaTiO₃ in polymer to form flexible composites to be used to power wearable and portable electronics. Thus, the most promising strategies for the efficient piezoelectric response of BaTiO_3-based polymer composite energy harvesters are described. In the end, the potential applications of polymer composite nanogenerators as well as a spotlight on the novel trends in this field are given.

2. Piezoelectricity

2.1. Discovery of Piezoelectricity and Materials

Brothers Pierre and Jacques Curie found in 1880 that by applying pressure to crystals (e.g., quartz, Rochelle salts, etc.) generates electrical charges on the surface of these materials. This electromechanical energy conversion is related to the direct piezoelectric effect. Gabriel Lippmann predicted the converse piezoelectric effect (i.e., applying electricity deforms the crystal) mathematically in 1881 and it was demonstrated experimentally by the Curies [13].

The direct piezoelectric effect is due to the electric charge that accumulates in certain solid materials such as crystals (e.g., quartz, etc.), certain ceramics (e.g., lead zirconium titanate (PZT), barium titanate (BaTiO₃), zinc oxide (ZnO), etc.), and certain biological matter (e.g., human bone, tendons, cellulose, etc.) in response to applied mechanical stress [14]. It should be noted that piezoelectric materials exhibit linear behavior when operated under low electric fields or mechanical stress; however, under high electric fields or mechanical stress, non-linear behavior is predicted [15].

2.2. Working Principle and Electromechanical Coupling

A tensorial notation is utilized to identify the coupling between the different units through the mechanical and electrical coefficients. It is common practice to label the directions as shown in Figure 1; i.e., 1, 2, 3 for x, y, z, and 4, 5, 6 for the shear planes perpendicular to their respective axis. This presentation adheres to the IEEE standard for piezoelectricity, which is the most common and good representation of piezoelectric material properties. The IEEE standard assumes that piezoelectric materials exhibit linearity.

The piezoelectric effect represents a remarkable phenomenon exhibited by certain materials and lies at the heart of various technological applications. This effect enables the conversion of mechanical stress or strain into electric charge, and vice versa. In this context, Figure 1 illustrates the direct and converse piezoelectric effects. In the direct effect, by subjection to compression, the material dipole moment decreases, yielding to the generation of a voltage that aligns with the polarity of the poling voltage. Conversely, the application of tension produces a voltage with the inverse polarity to the poling voltage. This inherent property of materials establishes a direct correlation between the mechanical stress applied and the induced electric charge. Subjecting such materials to a periodic tension/compression cycle, an alternating current (AC) voltage can be generated. This characteristic finds extensive use in diverse applications, including sensors, actuators, and energy harvesting devices that necessitate the transformation of mechanical energy into electrical energy. In terms of the converse effect, when a voltage of the same polarity as the poling voltage is applied, the material experiences elongation. This elongation arises from the reorientation of internal dipoles in reaction to the applied electric field. Conversely, reversing the applied voltage leads to the contraction of the material due to the realignment of internal dipoles.

The piezoelectric effect in materials holds great significance in fields such as materials science, engineering, and electronics. Its capacity to transform mechanical stimuli into electrical signals, and vice versa, has facilitated advancements in ultrasound imaging, precision positioning systems, vibration sensors, and acoustic devices.

The piezoelectric behavior is the interaction that occurs when the mechanical properties, stress T and strain S, and the electrical properties, charge density shift D and applied electric field E, interact. The electromechanical equations are as follows for a linear piezoelectric material:

$$S_i= s_{ij}^E{T}_j+d_{mi}{E}_m$$

(1)

$$D_m= d_{mi}^E{T}_i+{\epsilon }_{ik}^T{E}_k$$

(2)

where the index i, j = 1, 2, 3, 4, 5, 6 and m, k = 1, 2, 3 refer to different directions within the material coordinate system, d is the matrix of piezoelectric constants, ε is the dielectric permittivity, and the superscripts T and E indicate a constant electric field and a stress field across the system [16].

2.3. Piezoelectric Coefficients

The “piezoelectric strain constant” d expresses the proportion of developed free strain relative to the applied electric field. The notation $d_{ij}$ means with a displacement or force in the direction j an electric field is applied, or a charge is collected in the direction i. By convention, there is a total of four piezoelectric coefficients; $d_{ij}$, $e_{ij}$, $g_{ij}$, and $h_{ij}$ are defined as presented in the following subsections.

2.3.1. Piezoelectric Charge Coefficient

$$d_{ij}={\left({\displaystyle \frac{\partial D_i}{\partial T_j}}\right)}^E={\left({\displaystyle \frac{\partial S_j}{\partial E_i}}\right)}^T$$

(3)

The piezoelectric charge coefficient ($d_{ij}$) describes the correlation between the induced charge ($D_i$) and mechanical stress applied ($T_j$). This parameter is a crucial indicator of a material’s ability to convert mechanical deformation into electric charge.

2.3.2. Piezoelectric Stress Coefficient

$$e_{ij}={\left({\displaystyle \frac{\partial D_i}{\partial S_j}}\right)}^E={\left({\displaystyle \frac{\partial T_j}{\partial E_i}}\right)}^S$$

(4)

The piezoelectric stress coefficient ($e_{ij}$) describes the complex relationship between the induced stress ($T_j$) and the applied electric field ($E_i$). This coefficient is critical to understanding how a material responds to an electric field by inducing mechanical stress and vice versa.

2.3.3. Piezoelectric Voltage Coefficient

$$g_{ij}={\left({\displaystyle \frac{\partial E_i}{\partial T_j}}\right)}^D={\left({\displaystyle \frac{\partial S_j}{\partial D_i}}\right)}^T$$

(5)

The piezoelectric stress coefficient ($g_{ij}$), also referred to as the voltage output constant, is the relationship between the induced electric field ($E_i$) and the applied mechanical stress ($T_j$). This coefficient is of paramount importance in applications requiring the transformation of mechanical stress to electricity, including the design of sensors and actuators.

2.3.4. The Piezoelectric Stiffness Coefficient

$$h_{ij}={\left({\displaystyle \frac{\partial E_i}{\partial S_j}}\right)}^D={\left({\displaystyle \frac{\partial T_j}{\partial D_i}}\right)}^S$$

(6)

The coefficient ($h_{ij}$) refers to the piezoelectric stiffness coefficient and it clarifies the connection between the induced electric field ($E_i$) and the induced strain ($S_j$). This coefficient provides critical insight into how a material reshapes upon exposure to an electric field and how it responds to mechanical stress by generating an electric field.

All coefficients are related to each other through electrical and mechanical properties.

2.3.5. The Piezoelectric Coupling Coefficient “k”

The effectiveness of a piezoelectric substance as a transducer is gauged by the piezoelectric coupling coefficient. It measures the piezoelectric material’s capacity to transform mechanical stress into electrical energy and the reverse [17].

The piezoelectric coupling coefficient will be represented in terms of other parameters and the direction is given by the following equation:

$$k_{ij}^2={\displaystyle \frac{d_{ij}^2}{S_{ij}^E{\epsilon }_{ij}^T}}$$

(7)

$k_{ij}$ can thus interpreted as follows:

$$k_{ij}= \sqrt{{\displaystyle \frac{mechanical energy stored }{electrical energy applied }}} or, k_{ij}= \sqrt{{\displaystyle \frac{electrical energy applied }{mechanical energy stored }}}$$

(8)

where $k_{ij}$ represents the ratio of stored mechanical energy to applied electrical energy or of stored electrical energy to applied mechanical energy.

2.3.6. Energy Flow of a Piezoelectric Generator

The energy flow in a piezoelectric generator follows a precise process, as shown in the accompanying flow diagram (Figure 2). It begins with the absorption of environmental excitation energy, which is the initial step driven by external sources that lays the foundation for the entire system. Next, the energy is transformed into mechanical vibration energy. Factors such as mismatched mechanical impedance and the damping factor reflection can affect the efficiency of the generator.

The conversion of mechanical vibration into electrical energy involves a phenomenon called Mechanical-Electrical Transduction Loss. This process’s efficiency is influenced by the coupling factor (k) and the piezoelectric coefficients (d, g). These elements are essential for the conversion process but can also contribute to potential losses that must be considered for optimal performance.

The final step of this process is the production of electrical energy output, which can be utilized for various applications. However, this stage has its challenges. Electrical loss, caused by unmatched electrical impedance, adds complexity to the overall efficiency of the generator. The conversion of environmental excitation to electrical output requires minimizing losses at each stage. The flow diagram presents a clear visual representation of the energy conversion process in a piezoelectric generator, identifying areas for optimization.

2.4. Piezoelectric Materials

Piezoelectric materials possess several key characteristics that are crucial for their effective utilization. Firstly, they must exhibit a high piezoelectric coefficient reflecting their capacity to convert mechanical stress into electricity. Secondly, the mechanical properties of piezoelectric ceramics should be optimized to ensure superior strength and stiffness; however, flexible electronic devices require a certain level of flexibility. Thirdly, these materials should have a high voltage breakdown strength to withstand the application of a high electrical potential. Lastly, a relatively high Curie temperature (Tc) is desirable to enable their operation over a broader range of temperatures.

Piezoelectric materials are divided into four main categories: single crystals, ceramics, polymers, and polymer–ceramic composites. Each category possesses distinct properties and processing and application characteristics, leading to diverse applications in various fields. Figure 3 shows common examples from each category [18].

Synthetic materials such as piezoceramics exhibit higher piezoelectric characteristics than natural crystals. Piezoceramics consist of polycrystalline perovskite materials created from numerous single crystal “grains” with similar chemical compositions. However, the orientation in each grain differs, and there is just a slight variation in the space between the ions. These materials have excellent piezoelectric characteristics, and the most widely adopted piezoelectric materials in use today are as follows: lead zirconate titanate (PZT), lead meta niobate (LMN), sodium potassium niobate (SPN), lead titanate (LT), and lead magnesium niobate (PMN). Lead zirconate-titanate (PZT) is a key piezoceramic that is widely employed in energy harvesting and storage applications. Pb (Zr_1−xTi_x) O₃ (PZT) and similar compositions have been the mainstay of high-performance devices, owing to their exceptional dielectric, piezoelectric, and electromechanical coupling coefficients. Compositionally, PZT ceramics lie close to a morphotropic phase boundary (MPB), distinguishing the tetragonal and rhombohedral phases. MPB compositions demonstrate anomalously strong dielectric and piezoelectric properties due to an increased polarization arising from the interaction between two equivalent energy states of the tetragonal and rhombohedral phases, which allows optimal domain reorientation during the polishing process.

However, the presence of lead makes it unsuitable for environmental applications. Lead-free ceramics represent a good alternative to lead ceramics, especially perovskite-type ferroelectrics, since they have relatively high dielectric and piezoelectric properties. Examples of lead-free ceramics are barium titanate (BaTiO₃) and potassium niobate (KNbO₃).

The following section will delve into BaTiO₃-based piezoceramic material which has been deeply explored in recent decades.

3. BaTiO₃ as Lead-Free Piezoelectric Material

3.1. Structure of BaTiO₃

Barium titanate (BaTiO₃) has been extensively utilized over the past 60 years due to its attractive properties. BTO is the most successful ferroelectric material in the world in the production of thin films, ceramics, and composites. It was first discovered in 1944 during the Second World War in Japan, Russia, and the United States [19]. Following the development of a BTO single crystal in 1947, relevant research focused on understanding the ferroelectric region’s phase transitions and structure [20]. At that time, scientists worked to understand the properties of BTO, including the investigation of domain switching and electro-optical and electromechanical properties. In subsequent years, the microstructure, phase transition, ferroelectric properties, dielectric constant, and piezoelectric properties of BTO were increasingly understood [21]. Based on the following properties, BTO was identified as the premier material for ceramic production [22]. First, it is chemically and mechanically very stable; second, it exhibits ferroelectric properties at room temperature and above; and third, it is easy to fabricate and apply in the form of polycrystalline ceramic samples [23]. It is the world’s first ferroelectric ceramic and a suitable candidate for several high-performance applications due to its excellent dielectric [24], ferroelectric [25] and piezoelectric [26] properties. Because of its large dielectric constant and extremely low losses, barium titanate has been used in applications such as capacitors and actuators, transducers, and nanogenerators [27].

Doped barium titanate is widely used in semiconductors and piezoelectric devices and is now among the most extensively utilized ferroelectric ceramics, as illustrated in Figure 4.

The properties of BaTiO₃ have been reported in several publications. Barium titanate is classified within the perovskite group of compounds. Ceramic materials with a perovskite structure are key electronic materials. Barium titanate (BTO) was the first polycrystalline material to combine ferroelectricity and piezoelectricity. Its piezoelectric properties make it very useful in all applications, such as semiconductors, PTC resistors, transducers, ceramics, and energy harvesting systems. Future applications will require materials with good fracture toughness, improved piezoelectric and dielectric properties, and high-temperature stability [28]. BTO also exhibits the pyroelectric property, where spontaneous polarization exists for at least one crystallographic direction, which is used for extremely sensitive control of catalytic effects [29]. Due to their crystal structure, the excellent dielectric and ferroelectric properties of BaTiO₃ ceramics give rise to their high dielectric constants. It is currently one of the best-studied ferroelectric materials and finds applications across various fields due to its simple crystal structure. It belongs to a large family of compounds with the general formula ABO₃, known as perovskites [30]. In the perovskite structure of BaTiO₃, each barium ion is surrounded by 12 oxygen ions, forming a face-centered cubic lattice as illustrated in Figure 4. The titanium atoms occupy octahedral interstitial positions surrounded by six oxygen ions. Due to the large size of the Ba ions, the octahedral interstitial position is relatively large, making it unstable for the smaller Ti ions. To achieve stability, the Ti ions shift to off-center positions concerning the six surrounding oxygen ions, conducting a high degree of polarization with a charge of +4 [31]. By subjection to an electric field, the Ti ions shift from random to aligned positions, leading to a significant overall polarization and dielectric constant. BaTiO₃ has three crystal structures: cubic, tetragonal, and hexagonal, among which the tetragonal form is predominant for its outstanding ferroelectric, piezoelectric, and thermoelectric properties [32]. The crystal structure and polarization properties are strongly temperature dependent. Below 120 °C, BaTiO₃ is cubic with random polarization. Above this temperature, a displacive transformation occurs, changing the structure to tetragonal and inducing ferroelectric behavior, hysteresis, and polarization reversal when an electric field is applied [32].

BTO is a lead-free, environmentally friendly material with excellent and adjustable piezoelectric properties, relatively high Curie temperature, and relatively low sintering temperature [33]. One of the outstanding properties of BTO is that the dielectric constant changes as a function of temperature, density, and grain size. The temperature dependence of the dielectric constant above the Curie point (T > TC) is determined by the Weiss–Curie law. At room temperature, the dielectric constant of BTO is between 3000 and 6000. Due to its high permittivity and dielectric constant, BTO has proven to be useful in many electronic applications [34]. The representative piezoelectric coefficients of various lead and lead-free piezoceramics materials at room temperature are shown below in

Table 1. Typical piezoelectric coefficients of selected materials at room temperature.

Systems	Type of System	Tc (C)	d33 (pC/N)	Kp	Ref.
KNN	Lead ceramic	-	391	-	[35]
PbNb2O6	Lead ceramic	-	83	-	[36]
PZT	Lead ceramic	490	611	-	[37]
PbZr0.54Ti0.46O3	Lead ceramic	-	588	-	[38]
0.67PMN-0.33PT	Lead ceramic	660	-	-	[39]
BaTiO3	Lead-free ceramic	130	140	-	[40]
Nd-doped Bi4Ti3O12	Lead-free ceramic	-	38	-	[41]
BaTiO3 single crystal	Lead-free ceramic	250	80	-	[42]
Li-doped BaTiO3	Lead-free ceramic	130	260	0.4	[43]
Ba1+xTiO3-0.04LiF		95	340	0.5	[44]
Ba (Ti0.96Sn0.04) O3−xY2O3	Lead-free ceramic	84.7	454	0.5	[45]
(1 − x) BaTiO3−xBa (Cu1/3Nb2/3) O3	Lead-free ceramic	96	333	0.4	[46]
BNT	Lead-free ceramic	500	21.5	-	[47]
BNT–BT	Lead-free ceramic	500	21.5		[47]
BNKT	Lead-free ceramic	367.15	198	0.598	[48]
BNT–BZT	Lead-free ceramic	244	168	0.27	[49]
ZnO	Lead-free ceramic	-	44.33	-	[50]
BiFeO3	Lead-free ceramic	870	140	-	[51]

. This table provides a visual representation and comparison of the different properties, which offers an insight into the different characteristics of the various materials in terms of their piezoelectric properties compared to the BTO system.

As shown in Table 1, doping barium titanate (BaTiO₃) is a common strategy for improving its piezoelectric and electrical properties. Doping involves introducing small amounts of elements (dopants) into the material’s crystalline structure. This approach enables modifying and adapting the material’s properties to specific requirements. In the case of barium titanate, doping is often used to improve the piezoelectric and electrical properties.

3.2. Synthesis Methods

Several techniques for synthesizing BaTiO₃ have been discussed in the literature to foster and maximize its piezoelectric performance and to be closer to that of PZT. The chosen method for synthesizing barium titanate is determined by cost, but the final application is even more critical.

In general, powder quality is influenced not only by the route of synthesis but also by the starting materials selected. With the continued miniaturization of electronic devices demanding powders of smaller sizes with carefully controlled morphology, achieving the desired characteristics of the starting powder is becoming a critical question [52]. Successful barium titanate powder production with unique dielectric properties is strongly related to its purity and crystalline structure, which have a major impact on its final properties [53]. There are various methods for obtaining BaTiO₃ ceramics with the required properties, such as the conventional solid-state reaction and chemical methods.

3.2.1. The Conventional Solid-State Reaction

Traditionally, barium titanate has been prepared using a solid-state reaction that involves grinding BaCO₃ or BaO and TiO₂ together as shown in the following Figure 5. Here, the mixture must be calcined at a very high temperature. According to some reports, the required calcination temperature reached 1200 °C [54], and in other works, it was up to 1300 °C [55]. Barium titanate powders made by a solid-state reaction are found to be highly agglomerated involving particles with large sizes (2–5 μm) as well as the presence of numerous impurities. Because of the inherent issues related to the high reaction temperature and non-homogeneous solid-phase reactions, the electrical properties of sintered ceramics are negatively affected [56]. To overcome these issues, several wet chemistry methods are being advanced to produce homogeneous, reactive, and high-purity ultrafine barium titanate powders at low temperatures [57].

3.2.2. Chemical Methods for Barium Titanate Synthesis

The chemical synthesis method consists of using sol-gel, co-precipitation, hydrothermal, and polymeric precursor methods. The advantage of chemical methods is that the quasi-atomic dispersion of the components in a liquid precursor facilitates the synthesis of crystallized powder with nanometer-scale particles and high purity at low temperatures. Powder properties can vary according to the preparation methods used [58,59].

In the next subsections, the different routes used for the chemical synthesis of BaTiO₃ are presented, along with a description of each method’s main advantages for the properties and characteristics of the realized powder and the degree of complexity of the process.

• Sol–Gel Method

The sol-gel process is a method of synthesizing glasses and ceramics based on the formation of metal oxides by hydrolysis from a chemical precursor to form a sol, which is then transformed into a gel as shown in the following Figure 6. Following drying and pyrolysis, this gel yields an amorphous oxide. Further heat treatment can induce crystallization. The sol–gel process offers multiple advantages. Its simplicity and high purity of the product, coupled with low cost and high yield, make it appealing for a wide range of applications. Conducting synthesis at low temperatures ensures energy efficiency and broader compatibility with various materials. Achieving high-scale homogeneity is a notable benefit, allowing for consistent results across various applications.

Furthermore, the versatility of this method is highlighted by its capability to produce materials in various forms, such as porous structures, dense powders, thin fibers, and thin films. Notably, the process excels in obtaining pure, size-controlled, and stable nanoparticles within the 20–200 nm range, contributing to the precise engineering of materials. Additionally, the ease of doping enhances the flexibility and adaptability of this synthesis process, further solidifying its appeal for creating advanced materials with tailored properties. The lone drawback of the sol–gel method is its extended processing time [60]. Wang et al. [56] used the soluble gel method to prepare BaTiO₃ ceramics, the starting reagents being barium stearate. After the process, they obtained a gel that was calcined at different temperatures in the air to obtain high-performance BaTiO₃ nanocrystallites with particle sizes between 25 and 50 nm. Li et al. [61] prepared BaTiO₃ nanopowders by the sol–gel method, where they showed that the BaTiO₃ powders calcined at 700 °C for 2 h were preserved in cubic phase and the average size was 25 nm. These findings indicat that both the grain size and relative density of the ceramics grew with increasing sintering temperature. In 2014, Zhuang et al. [62] fabricated BaTiO₃ (BTO) nanofibers by the sol–gel method combined with the electrospinning method; the fibers calcined at 750 °C for 2 h exhibited good morphology and crystallization. In addition, they conducted a TEM study that demonstrated the BTO nanofibers were polycrystalline, with diameters in the range of hundreds of nanometers, where the existence of domains offered proof of ferroelectric structure and the calculated d₃₃ was 20 pm/V at maximum strain amplitude. Sharma et al. [63] investigated the ferroelectric and dielectric properties of barium titanate (BaTiO₃) bulk ceramics and thin films realized by the sol–gel method. The as-synthesized bulk powder and thin films were initially amorphous but crystallized into a tetragonal phase after heating at 700 °C in the air for one hour. They conducted an electrical study of the obtained powder, where the values of the spontaneous polarization (Ps), remanent polarization (Pr), and coercive field (Ec) of the bulk ceramics were found to be 19.0, 12.6 μC cm⁻², and 30 kV cm⁻¹, respectively. For the film, the values of Ps, Pr, and Ec were, respectively, found to be 14.0, 3.2 μC cm⁻², and 53 kV cm⁻¹. The capacitance–voltage (C–V) characteristics of the film were demonstrated as was polarization hysteresis. The dielectric constant (ε) values of the bulk ceramic and thin film at 1 kHz were found to be 1235 and 370, respectively. Both the films and ceramics exhibited dielectric anomalies at 125 °C, indicating a ferroelectric to paraelectric phase transition. Almeida et al. [59] produced barium titanate powders by the sol–gel method at various calcination temperatures (600 °C, 800 °C, and 1000 °C). They demonstrated an increase in the tetragonal phase and the dielectric constant (15–50 at 20 kHz) with rising temperatures in the samples produced. In addition, the measurement achieved by impedance spectroscopy shows that high temperatures are favorable for dielectric polarization. In fact, the dielectric constant increased with increasing temperature; this relationship illustrates a dependence of the product properties on the synthesis technique, the crystalline phase and the band gap of the material, giving better results for the tetragonal samples calcined at 1000 °C.

• Hydrothermal Method

Hydrothermal synthesis is among the most frequently employed methods for preparing nanomaterials. The hydrothermal route offers a multitude of advantages, presenting a simple, easy, and cost-effective synthesis method. Notably, it excels in producing high-quality 1D nanostructures, particularly nanorods. However, it has drawbacks, as the process is characterized by a prolonged reaction time. Additionally, utilizing a highly concentrated NaOH solution poses another challenge within this otherwise advantageous synthesis approach. It is essentially a solution–reaction approach. Hydrothermal synthesis can form nanomaterials across a broad temperature range, from room to extremely low temperatures, as shown in Figure 7 below. Low- or high-pressure conditions are usually employed to control the shape of the materials, relying on the vapor pressure of the primary reactants. Numerous types of nanomaterials have been successfully synthesized in this way. In hydrothermal synthesis, the nanomaterials’ compositions are well controlled through liquid-phase or multi-phase chemical reactions [64]. Zhu et al. [65] prepared (BTO) using the hydrothermal route; the obtained BTO had a very small and uniform size distribution, exhibiting an average grain size of 65 nm. Kumar et al. [66] prepared BaTiO₃ using the hydrothermal route. They examined it is properties by X-ray diffraction (XRD) which showed a pure tetragonal phase, and the average crystallite size (43.2 nm) was calculated using different methods: Scherrer’s, uniform deformation model (UDM), the uniform deformation energy density model (UDEDM) analysis, and the uniform stress deformation model (USDM). The structure was refined through the Rietveld refinement approach with a well-fitting value (χ₂ = 1.66). The effect of grain size and grain boundary on the material’s electrical properties was also investigated using complex impedance spectroscopy (CIS). Yu et al. [67] investigated a new wet chemical method where the BaTiO₃ nanopowders were prepared by the hydrothermal route at low temperatures for 24 h. They demonstrated that the size of the powders produced under by optimal conditions ranged from 125 nm to 250 nm in diameter.

• Co-Precipitation Method

The process of co-precipitation is an extensively studied method, as presented in the Figure 8 [68]. It provides a simple and practical method of achieving chemical homogeneity by mixing constituent ions at the molecular level under controlled conditions. With co-precipitation by the oxalate route, it is challenging to obtain optimized conditions where the precipitation of the Ba and Ti cations occurs simultaneously. Co-precipitation emerges as a favorable method with distinct advantages, including the benefits of operating at low reaction temperatures and featuring a short reaction time. These attributes contribute to its efficiency and applicability in various contexts.

Despite its advantages, the technique has several notable drawbacks. It may result in uncontrollable shapes and irregular size distribution, creating difficulties in precisely controlling the final product’s characteristics. Additionally, the continuous washing, drying, and calcination required to achieve a pure phosphorus phase are major drawbacks of the co-precipitation method. Zhang et al. [69] prepared tetragonal BaTiO₃ nanopowders using barium chloride and titanium tetrachloride as raw materials and tartaric acid as a precipitating agent with the co-precipitation process. The BaTiO₃ powder obtained had a small average particle size of about 140 nm. Suherman et al. [70] fabricated tetragonal BTO nanopowders which were heated for four hours under different sintering temperatures of 700 °C and 800 °C; they noted that the sintering temperature greatly influences the chemical bonds, the microstructure, and dielectric constant of the ceramics.

• Polymeric Precursor Method

A widely employed polymeric method is the polymeric precursor approach, where a solution containing ethylene glycol, citric acid, and metal ions undergoes polymerization to produce a polyester-type resin, as illustrated in the Figure 9 below. Metal ions were immobilized in a stable polyester network, and no cation segregation was observed during the thermal decomposition of the organic material [71].

Cho et al. [58] have prepared barium titanate by thermally treating polymer precursors containing barium and titanium in the air at 600 °C for 8 h. They obtained a cubic barium titanate powder with particles of around 20 nm. Increasing the heat treatment temperature to 900 °C lead to grain growth, resulting in BaTiO₃ particles several hundred nanometers in size.

Different techniques, such as sol–gel, hydrothermal, and solid-state methods, have their advantages and limitations that affect the structural and electrical properties (

Table 2. Illustration of the different methods used to prepare BaTiO₃ particles.

Synthesis Method	Precursors	Reaction Conditions	Morphologies	Phases	Ref.
Sol–gel assisted solid-state	BaCO3 and TiO2	- Xerogel obtained by heating at 80 °C for 8 h - Ground BaTiO3 was sintered in air at different temperatures (500, 600, 700, 800, 900, 1000, and 1100 °C)	Length: 760 nm–1061 nm Diameter: 73 nm–273 nm	Tetragonal	[72]
Hydrothermal	butyl titanate and barium chloride dihydrate	160 °C for 12 h allowing the autoclave to cool before opening	Average diameter: 30 nm Length: 150 nm	Cubic	[73]
Hydrothermal	BaCl2·2H2O and TiCl4	240 °C for 20 h.	Diameters ranging from 20 to 30 nm and lengths reaching up to >90 nm	Cubic	[74]
Hydrothermal	BaCl2 and titanium (IV) butoxide	200 °C for 24 h	Spherical particles with diameters ranging from 10 to 30 nm	Cubic	[75]
Hydrothermal	titanium dioxide (TiO2) and Ba(OH)2 8H2O	Heated to 200 °C followed by a dwell time of 24 h	Nanoparticles average radius: <25 nm	Cubic	[76]
Hydrothermal	Ba(OH)2·8H2O and TiO2	130 °C for 24 h 130 °C for 48 h 130 °C for 72 h 100 °C for 48 h 150 °C for 48 h 180 °C for 48 h	Nano-cubic barium titanate: 20–500 nm	Cubic	[77]
Sol–gel	Ba(CH3CO2)2 and titanium (IV) isopropoxide	The samples were dried at 200 °C for 12 h and annealed at 700 °C or 800 °C for 1 h	Nanocrystallites with average sizes of 20 nm and 15 nm	Pm-3m cubic	[78]
Sol–gel	barium hydroxide and tartaric acid	calcining treatment at 650 °C	Powders with a size of 50 nm	Tetragonal	[79]
Sol–gel assisted hydrothermal	BaCl2·2 H2O and TiCl4	100 °C–250 °C for 3 h	nanoparticles	Cubic to rhombohedra	[80]
Solid-state	BaCO3 and TiO2	900 °C for 2 h	nanocrystals	-	[81]
Solid-state	BaCO3, TiO2, and ZrO2	1050 °C for 6 h	Powders < 0.5 m	-	[82]

illustrates the different methods used to prepare BaTiO₃ particles).

Optimizing parameters such as temperature, pressure, and precursor concentrations can enhance piezoelectric properties and improve device performance. However, further research is needed to improve the synthesis methods used to produce advanced piezoelectric materials.

Table 2 clearly shows that the selected BaTiO₃ preparation method directly influences the particle size. This size variation in turn plays a crucial role in influencing and improving the piezoelectric, electrical, and dielectric properties. The observed correlation between the preparation method and the particle size highlights the importance of the preparation method for the evolution of the properties of BaTiO₃.

The ability to control the particle size by choosing a specific preparation method provides a strategic means to refine the functionality of BaTiO₃. The particle size can be optimized by fine-tuning the synthesis parameters, contributing to improved properties. This understanding demonstrates the importance of a tailored synthesis approach to achieve specific performance goals and paves the way for progress in developing high-performance piezoelectric materials for various technology areas, particularly energy harvesting.

4. Flexible Piezoelectric Polymer Composite Nanogenerators Based on BaTiO₃ Piezoceramic

Nanogenerators made from flexible piezoelectric polymer composites are increasingly popular for various uses. BaTiO₃ piezoceramics are commonly employed to improve the piezoelectric properties of the nanogenerators. By mixing BaTiO₃ nanoparticles into a polymer, the resulting material can convert mechanical energy into electricity. This process renders the piezoelectric polymer composites suitable for harvesting energy. With the ability to withstand mechanical deformation without sacrificing functionality, the nanogenerators have a prime blend of versatility and strain resistance. Since flexible piezoelectric polymer composite generators have numerous benefits, they ultimately surpass other generator types. Firstly, with the capability to convert up to 80% of mechanical energy into electrical energy—twice as much as traditional generators—the efficiency of these generators is truly unmatched. For those who prioritize convenience, these devices are also lightweight and flexible, making them perfect for wearable tech or other weight-restricted software. And finally, in keeping with a modern eco-friendly mindset, these generators do not require fuel, nor do they discharge any pollutants.

Due to its exceptional piezoelectric response and chemical resistance, PVDF has become the prevailing choice for piezoelectric polymers. Furthermore, its piezoelectric coefficient is relatively higher than other polymer materials. The inclusion of HFP groups in polyvinylidene fluoride–hexafluoropropylene (PVDF–HFP) copolymers has garnered considerable attention due to their advantageous characteristics. These materials exhibit improved mechanical toughness, increased solubility, greater transparency, reduced crystallinity, and enhanced flexibility compared to PVDF [83]. Polylactic acids (PLA) and polyurethane (PU), Polyimide (PI), and cellulose derivatives have several energy-related applications in a wide range of systems [84]. Chemically stable and with a low glass transition temperature, the PDMS polymer is an excellent choice for many nanogenerator designs. Its cured state provides both durability and ease of processing, making it a top contender in the world of polymers [85]. Some performance and applications of piezoelectric nanogenerators based on BaTiO₃, are summarized in

Table 3. Performance and advanced applications of BaTiO₃-based piezoelectric nanogenerators.

Nanoparticles	Polymer	Output Voltage (V)	Applied Force	Application	Ref
0.82Ba(Ti0.89Sn0.11)O3–0.18(Ba0.7Ca0.3)TiO3	PDMS	39	Vertical force of 35 N at 2 Hz	Tactile imitation	[86]
BaTiO3	PDMS	7	-		[87]
BaTiO3	PDMS	10.6	-		[88]
BaTiO3	PVDF	7.99	-		[89]
Ba0.85Ca0.15Ti0.9Zr0.1O3	PVDF-HFP	4.5 1.8	Foot motion, mechanical excitation using an electromechanical shaker		[90]

.

5. Strategies for Efficiency Enhancement of Piezoelectric Polymer Composite Nanogenerators Based on BaTiO₃

Various strategies can be implemented to enhance the applicability and performance of piezoelectric nanogenerators. Nanogenerators’ piezoelectric performance benefit from several factors; the careful choice of the proper selection of piezoelectric materials, along with structural modifications and hybridization, are key factors to consider. Preparation methods for piezoelectric substrates are selected by considering the substrates. Various strategies are discussed in this section to improve the performance of piezoelectric polymer composite nanogenerators using different approaches.

5.1. Modification of the Concentration of BaTiO₃ Particles in Composites

The concentration of BaTiO₃ nanomaterial in composites significantly impacts the dielectric, mechanical, and piezoelectric properties of the material. Higher concentrations of BaTiO₃ lead to improved dielectric, mechanical, and piezoelectric properties, resulting in higher energy conversion efficiency. In addition to its influence on dielectric and piezoelectric properties, the concentration of BaTiO₃ in polymer composites also provides in situ sensing capabilities for probing the mechanical behavior of the resulting material.

The concentration of BaTiO₃ nanomaterials in composites is crucial in defining the mechanical and dielectric properties. At optimized concentrations, BaTiO₃ nanoparticles can enhance the mechanical strength and toughness of composites by acting as reinforcing particles. Therefore, they affect the elasticity of the material, leading to enhanced modulus of elasticity of the composite caused by the addition of BaTiO₃ nanoparticles and contributing to the overall rigidity of the composite. Furthermore, BaTiO₃ possesses high dielectric permittivity which promotes the dielectric constant by the addition of BaTiO_3.

Bouhamed et al. [91] have developed flexible nanogenerators based on BaTiO₃ and a polydimethylsiloxane (PDMS) polymer. Herein, the impact of BaTiO₃ concentration on the performance of the nanogenerator was investigated. The study by Bouhamed et al. shows, as demonstrated in Figure 10, that the concentration of BaTiO₃ greatly influences the output voltage and power of the NGs. The NGs with a high content of piezoceramics particles exhibit high output voltage and power, owing to the enhancement of the composite piezoelectric coefficient. The authors proved that this improvement could also be related to the improved elastic modulus of the composites. In addition, they showed that the elastic modulus was increased by doping BaTiO₃ with PDMS; it decreased at 40 wt.% BaTiO₃, caused by the non-homogeneous arrangement of the particles in the polymer matrix. Ayadi et al. [92] proposed a flexible microgenerator where they demonstrated the correlation between the flexibility and piezoelectric properties of the composites. The proposed flexible microgenerator showed an excellent piezoelectric figure of merit (FoM₃₁ = 13.1 × 10⁻¹² m² N⁻¹), which significantly increased energy harvesting performance (power density of 35 nW/cm²) due to low Young’s modulus values (maximum E = 3.4 MPa) and a large elongation of more than 170%.

5.2. Effect of Processing Methods

The processing method is crucial in determining the microstructure, morphology, and piezoelectric properties of BaTiO₃-based composites. Various sophisticated techniques have been employed to realize polymer composite-based nanogenerators, with notable methods including solution casting, electrospinning, and spin coating as shown in Figure 11. These techniques have emerged as effective means for fabricating nanogenerators, enabling the integration of polymer composite materials to create functional devices with enhanced power generation capabilities. The solution casting method is the simplest and most cost-effective preparation method for realization of a thin nanocomposite layer; it is a straightforward method and involves dissolving a wide range of polymers and nanomaterials in a solvent, then rotating the solution on a substrate surface and letting the solvent evaporate to form a film. It typically requires less specialized equipment and is more cost-effective, making it suitable for large-scale production. Also, this method is scalable; it provides better control over the thickness and uniformity of the film, which is important for applications requiring consistent material properties over a large area. It can produce dense and homogeneous films that often exhibit better mechanical properties, such as tensile strength and flexibility, which are critical for energy harvesting applications [93]. To achieve a higher quality thin film with better uniformity, spin coating is employed, which involves depositing the solution in the middle of a substrate followed by rapid rotation to spread the solution evenly over the surface. Nunes-Pereira et al. [94] spin coated a thin layer of BaTiO₃/poly (vinylidene fluoride–trifluoroethylene) having 12 µm of thickness that generates ∼0.28 μW of power.

In addition, electrospinning is a widely used and promising method for producing functional nanofibers with diameters ranging from tens of nanometers to a few micrometers. It is particularly valued for its ability to produce fibers with controlled orientations and a high surface-to-volume ratio, which is beneficial in energy harvesting applications. The morphology of the nanofibers can be adjusted through changes in electrospinning parameters such as polymer concentration, applied voltage, needle-to-collector distance, and flow rate [95]. Another appealing feature of the use of the electrospinning method over the two other techniques is the ability to introduce in situ poling and stretching of the materials that is not affordable with a solvent-based casting method. Kubin et al. [96] achieved, by using electrospinning method for realization of PVDF/BaTiO₃, a very high rate of the developed piezoactive crystalline phase of up to 99.4% combined with a high level of crystallinity of up to 56.4%. In this field, spin coating is a widely used, effective, versatile, and simple to use technique for creating thin, uniform films of materials, particularly nanocomposites as harvesters. The procedure consists of coating a small amount of a liquid solution to the center of a substrate, which is then rotated rapidly to disperse the liquid by centrifugal force [97]. For many years, the number of articles published on the development of nanocomposites based on the solution casting method has been much greater than for other types of preparation methods. Therefore, among these different methods for producing films used by all researchers in energy harvesting applications, the solution casting method represents the commonly used method because it is easy to produce compared to the spin coating and electrospinning methods.

However, the efficiency of the preparation method always depends on its parameters. On the other hand, the performance of a nanogenerator depends largely on its structure’s ability to convert mechanical energy into electrical energy at the nanoscale, and therefore the selection of a suitable substrate and electrodes to support and collect the generated charge under pressure, plays a critical role in determining the overall performances.

Therefore, the choice of processing parameters, such as temperature, pressure, time, and solvent, also significantly affect the final properties of the composites [98]. In the context of choosing processing parameters, Jeder et al. [99] investigated the effect of the sonication amplitude on the overall properties of BCZT/PVDF–HFP composites. Herein, as shown in Figure 12a, the composite is placed as usual between two electrodes to investigate the piezoelectic properties of the different realized composites. In the study by Jeder et al., three sonication amplitudes were used (20%, 30%, and 40%). The electrical and mechanical analysis shows the highest conductivity and lowest variation of Young’s modulus, respectively, for the composite realized with the highest sonication amplitude, as shown in Figure 12, indicating a good distribution of BCZT in the PVDF–HFP polymer matrix. The increase in sonication amplitude also improves piezoelectric properties that were tested under the application of force from a sewing machine.

Many studies [100,101] investigated the performance of the NGs in relation to the solvent used. For example, Bouhamed et al. [102] studied the impact of using various solvents, such as Aceton/DMF and Aceton/NMP, on the performance BaTiO₃/PVDF–HFP nanogenerators. The results illustrate the improved output performance of the nanogenerator in the case of Aceton/DMF, as depicted in Figure 13a,b.

The presence of dimethyl formamide (DMF) solvent increases the rate of crystalline phases owing to their moderate evaporation rate and significant dipole moment, thereby enhancing the piezoelectric performance. The influence of the solvent evaporation rate and dipole moment on PVDF crystallinity are detailed in Figure 13c.

In summary, the choice of synthesis method and methods’ parameters greatly affect the properties and performance of piezoelectric materials.

5.3. Doping

A promising category of materials can be identified as barium titanate with the typical ABO₃ formula, where A represents a monovalent or bivalent ion and B represents the tri-hexavalent cation.

It has been proven that the ferroelectric and dielectric properties of BaTiO₃ materials can be improved independently or simultaneously by using different dopants at the A-sites Ba²⁺ or B-sites Ti⁴⁺ [103,104,105]. Multiple ion occupancy of A- and/or B-sites in the BaTiO₃ lattice is expected to boost its electrical characteristics. Generally, A-site ions are replaced by metal ions with a larger ionic radius while B-site ions are replaced with smaller metal ions as illustrated in Figure 4. Numerous research investigations have been conducted until the present to investigate the enhancement of the Curie temperature shift and dielectric properties due to dopant impacts [106,107]. However, limited investigations have been conducted on the combined enhancements of dielectric, ferroelectric, and piezoelectric properties because of A- and/or B-site doping [108,109], and even fewer investigations on the effects of excess doping on the ferroelectric and piezoelectric properties of BaTiO₃ piezoceramics have been conducted.

Many researchers have adopted the doping strategy to enhance the performance of polymer composite nanogenerators. For example, Missaoui et al. [110] synthesized doped BaTiO₃ by integrating calcium ions to form Ba_0.8 Ca_0.2 TiO₃ (BCT) in order to enhance the performance of the PDMS polymer composite to harvest body movements. As demonstrated in Figure 14, the NG demonstrated in output voltage of ∼10 V under a gentle tapping finger. Jeder et al. [90] studied the effect of doping in site B as well as in both sites of BaTiO₃. In the work by Jeder et al., Ca²⁺ and Zr⁴⁺ were introduced into the BaTiO₃ crystal network to replace Ba²⁺ and Ti⁴⁺ to realize subsequent flexible composites based on a PVDF–HFP polymer to compare with composites containing BaTiO₃. They found that doping greatly enhances the performance of the nanogenerators to produce a maximum output voltage of ~2 V and output power of 1.9 µW at 40 Hz in the case of BCZT/PVDF–HFP; this is around two times higher than BT/PVDF–HFP. They found that doping in the two sites is more effective.

Further, Ben Ayed et al. [111] illustrate the significance of zinc doping in BCZT to enhance piezoelectric performance as it contributes to minimizing the crystallite particle size and enhancing the β-phase crystallization of the polymer matrix. Under a mechanical shaker, 10 wt.% BCZT/PVDF–HFP exhibited an output voltage of 2.59 V (peak to peak) and a maximum power of ∼2 µW while 10 wt.% Zn-BCZT/PVDF–HFP showed an enhancement of voltage and power of 3.3V and 2.13 µW, respectively (Figure 14c). The addition of zinc to BCZT is an effective way to reduce the crystallite size and enhance the β-phase, which in turn ameliorates the piezoelectric properties of the lead-free piezoelectric material as well as the robustness of a NG. The Zn-BCZT/PVDF–HFP nanogenerator exhibits excellent stability over 1800 cycles of repetitive loading and maintains reliability even after 1 year.

Instead of doping with Zn, Missaoui et al. [112] show that doping with Sn into BCZT has also improved the output voltage and reached around 30 V under finger tapping.

5.4. Surface Modification of BaTiO₃

The compatibility of the fillers with the polymer matrix and their capacity to disperse well within it are always critical requirements for the performance of the nanocomposites. Untreated BaTiO₃ nanoparticles tend to agglomerate due to their high surface charge, making compatibility with the polymer matrix challenging To overcome this problem, surface modification is a very crucial element. Therefore, researchers’ efforts are now concentrated on creating discrete interfaces through surface changes between ceramic particles and a polymer matrix. To ensure that the ceramic filler particles are evenly distributed throughout the polymer matrix, surface modification methods such as hydroxylation, functionalization with an organic modifier (e.g., coupling agent, polymer, etc.), or core-shell formation with an inorganic compound (e.g., gold, cobalt ferrite, etc.) have been used.

Silane is considered the most frequently used organic modifier. Silane coupling agents might decrease agglomerations and promote polymer–particle interactions by lowering the hydrophilic nature and surface energy of the NPs [113,114]. Dalle Vacche et al. [115] investigated the effects of the silane coupling agent (3-aminopropyl)triethoxysilane (APTES) on the characteristics of poly (VDF–TrFE)/BaTiO₃ composites. It decreased aggregate and improved compatibility with the poly (VDF–TrFE) matrix because the modified particles were more evenly spread in the polymer matrix than the unmodified ones. In another study, Li et al. [116] used screen printing to create a PENG out of a surface-modified BTO/PVDF nanocomposite. Triethoxy(octyl)silane (TOs) coating was used to modify the surface; as a result, the TOs coating decreased imperfections, prevented BTO nanoparticle aggregation, and enhanced output performance. After 7500 cycles, a device with evenly scattered BTO nanoparticles showed an output performance of 20 V. A power density of 15.6 W/cm² was attained, which is 150% more powerful than a standard BTO/PVDF-based PENG. According to Fathollahzadeh [117], BaTiO₃ nanoparticles’ surfaces were functionalized with fluoroalkyl silane (FAS) solution to increase their affinity for the PVDF matrix and prevent them from clumping together. Furthermore, adding functionalized BaTiO₃ nanoparticles to the PVDF layer causes the pure PVDF’s piezoelectric response to increase from 1.05 mV to 1.46 mV, which is greater than the sample that has micron-sized BaTiO₃ inserted (1.28 mV).

In different studies, the issue of inorganic fillers’ low compatibility with an organic matrix is effectively resolved by 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA) because its reactive groups (the hydroxyl, carboxyl, and phosphonyl groups) tend to form complexes with the cations of inorganic particles in aqueous media. To tackle this issue, Guo et al. [118] suggested a composite nanogenerator consisting of P(VDF–TrFE) and BaTiO₃-coated PBTCA, offering high voltage and power density output. After 10,000 cycles, the created nanocomposite PENG demonstrated higher output performance than untreated particles with a high output voltage of 45 V and a power density of 0.76 mW/cm².

Developing biocomposites based on a mixture of BaTiO₃ and PVDF is a current, efficient method to combine the benefits of organic and piezoelectric materials. However, the weak interfacial adhesion between the piezoelectric material and flexible polymers allows a direct interaction. To overcome this issue, dopamine was used as a linker to modify the surface of the nanoparticles. Yang et al. [119] discovered that the (PDA) polydopamine-modification method can enhance the dispersion of BTO into the PVDF matrix and minimize the interface hole defects and cracks between components. In contrast to its pure PVDF and BTO/PVDF composites, a PDA@BTO/PVDF sensor demonstrated a quick response of 61 ms and an impressive piezoelectric output voltage of 9.3 V. Moreover, Guan et al. [120] demonstrated that the PENG constructed from polydopamine (Pdop)-modified BaTiO₃ (Pdop-BT@P(VDF–TrFE)) nanocomposite mats showed a notably improved electric output performance compared to electrospun P(VDF–TrFE), BT/P(VDF–TrFE), and Pdop-BT/P(VDF–TrFE)-based PENGs. To be precise, Pdop-BT@P(VDF–TrFE)-based PENGs can collect output voltage and current up to about 6 V and 1.5 μA, which is 4.8 times and 2.5 times higher than P(VDF–TrFE) PENGs.

5.5. Addition of Conductive Particles/Ionic Gel

To improve the polymer matrix’s electrical conductivity and enhance the piezoelectric performance of BaTiO₃-based polymer composites, conductive particles can be incorporated into the composites. Graphene, carbon nanotubes, or metal nanoparticles facilitate charge transport and increase the nanogenerators’ mechanical properties and output performance. For example, with the introduction of carbonaceous particles such as multiwalled carbon nanotubes (MWCNTs), the dielectric constant and dielectric loss rises as MWCNT content increases. M. Iftekhar Uddin et al. [121] studied the impact of MWCNTs on the PVDF–BaTiO₃ matrix on the performance of flexible nanogenerators by varying the concentration of MWCNTs from 0.1 wt.% to 1 wt.%; sample PBC-3 (PVDF–BaTiO₃-0.5 wt.% CNT) demonstrated the optimal dielectric characteristics among all the prepared samples, achieving a maximum output voltage of 4.4 V and a current of 0.66 μA, with an applied force of ~2 N. In another work, Bouhamed et al. [91] investigated the effect of the addition of MWCNTs on the performance of the PDMS–BaTiO₃ composite. The investigation illustrates the important role of MWCNTs; they boosted the performance of the composite two times in comparison to the sample without MWCNTs and with the highest performance at a percolation threshold of around 11.2 V under palm striking. In fact, the network built by the MWCNT fillers enhances the distribution of BaTiO₃ particles, leading to improved electrical and mechanical performances, as shown in Figure 15a. Similarly, Missaoui et al. [110] observed that the integration of MWCNTs within a matrix containing doped BaTiO₃ with Ca ions (BCT) greatly enhanced the performance of the nanogenerator to achieve 2.58 V and a maximum output power of 0.40 μW at 2 MΩ, as demonstrated in Figure 15b. Due to its uniform particle distribution and enhanced electrical and mechanical performance, the nanogenerator has good stability over 13,000 repetitive load cycles.

Furthermore, other investigations report on the synergistic effect of graphene nanosheets and BaTiO₃ nanoparticles, which significantly enhances a nanogenerator’s piezoelectric performance. Shi et al. [122] realized a PENG using an electrospun process which can produce a peak voltage up to 112 V during a finger pressing–releasing process.

Instead of using carbonaceous nanoparticles to foster the performance of piezoelectric nanogenerators based on polymer/BaTiO₃ composite, some researchers used metallic nanoparticles such as silver, gold, and copper nanoparticles. In this context, Bouhamed et al. [101] integrated silver nanoparticles into PVDF–HFP/BaTiO₃ to ameliorate the NG’s performance. The study shows an enhanced output voltage of 2.21 V and output power of 0.22 μW, which are approximately three times and nine times higher, respectively, compared to the composite without Ag NPs.

Not only is the integration of conductive particles interesting in ameliorating the PENGs based on BaTiO₃, but ionic liquids have also gained significant attention in energy harvesting devices such as nanogenerators. When used in combination with BaTiO₃ composite-based nanogenerators, ionic liquids can potentially enhance the performance of the nanogenerators in several ways. Numerous studies have explored the use of ionic liquids to improve the performance of nanogenerators based on BaTiO₃ composites. Researchers have successfully engineered a system that effectively responds to mechanical stimuli by combining piezoionic and piezoelectric activities. This is achieved through the redistribution of mobile ions within the polymer matrix and the presence of electrically polarized ceramic nanoparticles [123].

In conclusion, enhancing the performance of BaTiO₃-based polymer composites for next-generation energy harvesting involves a multifaceted approach. Key strategies include adjusting the concentration of BaTiO₃ particles, optimizing processing conditions, and using polar solvents to improve composite homogeneity and filler–matrix interaction. Surface modification of BaTiO₃ particles ensures better integration and enhances the crystallinity of the composites.

Owing to the increasing in surface area, enhanced interface interaction will be achieved which will help to obtain higher performance at lower filler concentrations. Additionally, incorporating conductive nanofillers enhances self-polarization and improves morphological, electrical, and mechanical properties, particularly at the percolation threshold. However, careful management of the conductive filler content is crucial to maintain a homogeneous and effective composite.

Therefore, adopting a combined approach can offer better performance for the piezoelectric polymer composite nanogenerator.

6. Selected Applications of BaTiO₃-Based Piezoelectric Polymer Composite Nanogenerators

Piezoelectric polymer composite nanogenerators based on BaTiO₃ have become versatile devices with wide applications in various fields. Their ability to generate energy is particularly noteworthy among their countless applications, including energy harvesting, self-powered sensors, wearable electronics, and biomedical devices. Nanogenerators convert vibrations, motions, and pressure collected from various sources into electrical energy with remarkable ability. This makes them ideal for powering compact electronic devices and sensors, eliminating the need for external power sources or batteries. BaTiO₃-based piezoelectric polymer composite nanogenerators have the potential to revolutionize self-powered technologies in common applications.

6.1. Self-Powered Devices for Internet of Things (IoT) Applications

Nanogenerators, such as BaTiO₃-based systems, provide an effective way to collect mechanical energy from various environmental sources, such as vibrations from machinery, human motion, and natural elements like wind or water flow. This accumulated energy is a significant resource that can run small electronic devices or serve as an alternate power source for low-power applications [124]. Integrating BaTiO₃-based nanogenerators into sensors enables the conversion of mechanical stimuli into electrical signals, unlocking transformative capability. This invention has enabled the development of self-powered sensor systems with applications in structural health monitoring, wearable electronics, and biomedical devices [125,126]. BaTiO₃-based nanogenerators are a promising option for powering Internet of Things (IoT) devices due to their efficient power generation. These nanogenerators can effectively harvest energy from human activities, such as finger tapping on touchscreens or body movements, to provide a sustainable power source for wireless sensors and communication modules [127].

6.2. Biomedical Applications

Piezoelectric nanogenerators have gained increasing attention in the field of biomedical applications due to their ability to convert mechanical energy into usable electrical energy. These nanogenerators can be used as power sources for various biomedical devices, eliminating the need for traditional battery systems. Flexible, lightweight devices based on biocompatible materials may be able to harvest energy from the body’s parts or tissues’ contraction and relaxation, offering a sustainable and clean source of energy. In this regard, Ben Ayed et al. [111] investigate the potential of their fabricated piezoelectric nanogenerator in real-life applications, where a 10 wt.% Zn-BCZT/PVDF–HFP-based nanogenerator was connected to different body parts to record the output voltage signals in reaction to the body’s gestures. As illustrated in Figure 16, the generated voltages produced by human movement were 250 mV, 148 mV, 95 mV, and 8 V for finger bending, elbow bending, wrist bending, and walking, respectively. The developed harvester was capable of lighting up an LED by tapping it with a finger, as shown in Figure 16e.

One of the key applications of piezoelectric nanogenerators in the biomedical field is for powering implantable medical devices. These devices, which range from pacemakers to insulin pumps, need a constant source of power to operate. In 2019, Vivekananthan et al. [128] fabricated a lead-free KNaNbO₃/BaTiO₃ NPs nanogenerator impregnated with polydimethylsiloxane (PDMS) flexible composite film. The nanogenerator was designed to monitor sleep and detect sleep disorders in humans. It demonstrated a maximum electrical output of 58 V and 450 nA, indicating its potential as a valuable tool for non-invasive and continuous sleep monitoring. This breakthrough contributes to advancements in health monitoring technologies and opens avenues for the development of biodegradable power sources. The study highlights the promising prospect of using such flexible nanogenerators to power transient implants, which is in line with the pursuit of sustainable and biocompatible energy solutions for medical applications. A central problem was well established: infection caused the failure of orthopedic implants in a clinical setting. Shuai et al. [129] showed that an agar diffusion approach could provide visual evidence of the antibacterial activity of scaffolds by observing their bacterial inhibition zone. Ag nanoparticles would operate as a conductive phase to increase the strength of the polarized electric field on BaTiO₃, causing more domains to align in the direction of the electric field and allowing the piezoelectric effect of BaTiO₃ to be fully realized in the composite scaffold. In another study, Su et al. [130] investigated BaTiO₃ NPs doped PVDF experimentally and through modeling to clarify the fundamental mechanism underlying the interfacial coupling effect in the produced polymer composites to construct high-performance wearable textile bioelectronics.

Developing flexible and stretchable piezoelectric bioelectronic devices for the human body’s soft and malleable tissues presents a significant challenge. Self-powered implantable bioelectronic devices offer real-time monitoring of essential physiological or pathological data, such as body temperature, respiration, arterial pulse, heart rate, blood glucose, pressure, and oxygen levels.

6.3. Automotive Applications

Vehicles are vital in transportation as they carry passengers and cargo. Therefore, vehicle safety remains a primary focus of research. However, certain components of vehicles cannot be monitored in real-time. Combined with nanogenerators, vehicle components can be enhanced to be autonomous. Jiang et al. [131] designed PVDF/BaTiO₃ foams (PBFs) with a hierarchical porous structure using a salt-template method. The study aimed to investigate the unique outcome that arises from the combination of BaTiO₃ and the space-charged electret (SCE) that forms within the pores after poling, resulting in improved piezoelectric and ferroelectric properties. In an innovative application, the PBF is utilized as a wearable sensor to track human body movements and monitor vehicle movements in real-time.

7. Novel Trends of BaTiO₃-Based PENGs

Piezoelectric nanogenerators (PENGs) have recently emerged as cutting-edge, intelligent power sources. Flexible, wearable, and biocompatible BaTiO₃-based PENG devices have garnered extensive research interest due to their potential applications across various fields. To enhance the piezoelectric output performance of polymeric PENGs, researchers have developed nanocomposites that incorporate inorganic piezoelectric nanoparticles into flexible polymers, combining the high piezoelectric response of the nanoparticles with the polymers’ flexibility. However, the significant agglomeration of nanofillers and the unavoidable degradation of flexibility and mechanical properties due to the incorporation of inorganic nanofillers have made this construction suboptimal for PENGs, compromising their stability and durability [132,133,134]. Therefore, a new nanocomposite structure called a “ hierarchical structure” is applied to realize durable and highly performant PENGs. Guan et al. [120] have developed hierarchically architected polydopamine-modified BaTiO₃/P(VDF–TrFE) nanocomposite fiber mats” to meet the demands for high output performance as well as for PENGs’ desirable good flexibility and durable mechanical properties. In [135], researchers found that the alignment of BaTiO₃ particles into a polymer matrix is a convenient solution to enhance the distribution and the material properties. Therefore, alignment of BaTiO₃ particles into a PDMS matrix was conducted by extrusion of the material by direct ink writing technology (DIW). In fact, the shear stress effect during the printing process ameliorated the orientation of BaTiO₃.

Huang et al. [136] introduced a new fabrication route to promote self-polarization of BaTiO₃-based polymer composites by employing dynamic pressure during fabrication. The fabrication process consists of a three-step process including melt-blending, compression molding, and salt leaching. The achieved composites have a d₃₃ value of approximately 51.20 pC/N at a low density of around 0.64 g/cm³.

To promote the crystallization of polymer composite based on BaTiO₃ that induces stronger performance of the composite, novel works are going to enhance the structure though the introduction of a core-shell structure on the external surface of the nanoparticle using several polymers such as ethylene propylene diene monomers, polystyrene, or polymethyl methacrylate (PMMA). For example, Shi et al. [137] successfully prepared a core-shell structured PMMA@BaTiO₃ NWs via surface-initiated in situ atom transfer radical polymerization (ATRP). Then, fibrous piezoelectric nanocomposites of PMMA@BaTiO₃/PVDF–TrFE were created through the electrospinning method. The grafting of PMMA onto the BaTiO₃ particles promotes even dispersion of NWs within the PVDF–TrFE matrix. It improves stress transfer at the interface between the filler and polymer matrix, significantly improving PENG performance. Moreover, Cho et al. [138] create a dense layer of poly(vinylidene fluoride-co-trifluoroethylene) (PVDF–TrFE) on piezoelectric inorganic nanoparticles using the phase separation nano-coating technique. These nanoparticles rely on differences in solubility parameters between good solvents and non-solvents to create porous membranes that maintain stability even in organic solvents or dry phases.

The polymer matrix plays the role of absorbing the nanoscale reinforcements to provide the nanocomposite with the appropriate form, strength, and improved characteristics. The physical characteristics of the nanocomposites, such as their dielectric permittivity and piezoelectric performance, will thus be significantly impacted by these polymer shells.

Thus, various strategies can be utilized to ameliorate the polar phase content and phase transformation of the polymer matrix based on BaTiO₃ and to improve polymer composite crystallinity. Such nanocomposites may be produced using a variety of techniques, such as stretching, hot pressing or annealing, and quenching [139,140]. Xie et al. [141] have developed PVDF–BTO composites using a spin coating process and show that Young’s modulus was enhanced by 190% compared to pure PVDF by using only 7% filler content. Moreover, Ram et al. [142] reported that the Young’s modulus obtained for the PVDF with 5% BTO, 10% BTO, 15% BTO, 20% BTO, and 25% BTO nanofillers had increased by 40%, 205%, 365%, 460%, and 678%, respectively. These improvements in the modulus might be due to the robust molecular interaction between the matrix and BTO. Thus, it follows that the incorporation of nanofillers enhances the PVDF composite film’s capacity to support loads. In order to improve the piezoelectric performance, Khan et al. [143] looked at how phase transitions in BaTiO₃–PVDF composite films were affected by both stretching and the addition of BaTiO₃. The findings suggest that when PVDF composite films are stretched, a different phase change process takes place. Stretching pure PVDF films causes the non-polar α phase to change into the electroactive β phase, whereas stretching BaTiO₃-PVDF composite films mainly causes the weakly electroactive γ phase to change into the more electroactive β phase. In general, it has been demonstrated that stretching BaTiO₃–PVDF films helps to increase the β phase content of PVDF.

8. Conclusions

This comprehensive review focuses on the recent developments in biocompatible, flexible piezoelectric nanogenerators (PENGs) based on lead-free Ba-TiO₃/polymer composites, offering a straightforward overview of the progress in this growing field. The review provides a clear explanation of the working principle of PENGs and emphasizes the connection between mechanical and electrical coupling. An overview of the key materials used for the realization of PENGs is given, with a focus on BaTiO₃/polymer composites to form environmentally friendly energy harvesters suitable for wearable and portable applications. An overview of the structure and the techniques used to synthesize BaTiO₃ is provided, highlighting the main advantages of each technique. This review emphasizes the importance of embedding BaTiO₃ in the polymer to form flexible composites that can be used as low-power sources for wearables and portable electronics. The most promising strategies for achieving an efficient piezoelectric response in BaTiO₃-based polymer composite energy harvesters are described. In this part, the review paper highlighted that modification of concentration of BaTiO₃ particles in composites is one of the key factors influencing the final performance of these composites. Adjusting this concentration is relatively straightforward and significantly impacts the efficiency of the energy harvesters. The processing conditions such as temperature, pressure, time, and solvent, also significantly affect the final properties of the composites. Optimizing these conditions and using polar solvents with moderate dipole moments can improve the homogeneity of the composites and enhance the interaction between the BaTiO₃ filler and the polymer matrix, increasing the composite’s crystallinity. The enhancement of the interaction of BaTiO₃ fillers to the polymer matrix can be guaranteed by surface modification of the particles.

One efficient approach for enhancing the final piezoelectric properties of BaTiO₃ is doping the particle in the two sites to ameliorate the piezoelectricity of the particles and reduce their size, especially when wet chemical methods are adopted as synthesis method. This increase in surface area boosts the interaction at the interface, leading to higher piezoelectric performance at low filler concentrations. Moreover, the addition of conductive nanofillers such as graphene and MWCNTs has been found to enhance the self-polarization of the final composite efficiently. The presence of these conductive fillers positively impacts the overall properties of the composite especially at the percolation threshold, contributing to overall improved performance. However, excessive conductive particles hinder the formation of homogenous and highly performing piezoelectric polymer composites.

Furthermore, the review highlighted the versatile applications of BaTiO₃-based piezoelectric polymer composite nanogenerators, underscoring their potential to revolutionize various fields. These applications include IoT applications, where they can efficiently collect mechanical energy from surrounding sources to power sensors and devices. In biomedical fields, they can power implantable devices and monitor health parameters. In automotive applications, they enhance vehicle components with real-time monitoring capabilities. The review also presents the novel trends shaping the trajectory of this evolving field. Current advancements in this technology indicate a shift from standard paper films to complex composite structures to promote the alignment and orientation of BaTiO₃ particles during processing, primarily through the use of additive manufacturing methods. However, due to the simplicity of creating standard films, this structure remains in use, but with added post-processing steps such as hot pressing or stretching, which can significantly improve the composite’s final properties. Nevertheless, to achieve superior performance, employing core-shell nanostructures is a novel and powerful approach. This method enhances the interaction between inorganic fillers and the organic polymer matrix, resulting in higher dielectric properties with reduced losses. Integrating a shell layer with BaTiO₃ significantly boosts its piezoelectric properties compared to pure BaTiO₃. Combining these recent strategies for enhancing BaTiO₃-based polymer composite energy harvesters can be highly effective in maximizing performance.