# Strategic Development of Piezoelectric Nanogenerator and Biomedical Applications
## 2 Piezoelectric Materials
### Perovskite
#### 2.2.2 Lead-Free Perovskite Materials  
Owing to the toxicity and hazardous nature of lead, researchers are now focusing on the application of lead-free perovskite materials for high-performance PENGs. Piezoelectric lead-free perovskite ceramics are generally polycrystalline materials consisting of irregular collective small grains and are typically prepared through solid-state reactions and sintering processes. The piezoelectric coefficient is governed by the phase transition temperature; the polarization vectors change their direction during the phase transition (tetragonal-orthorhombic/monoclinic, orthorhombic/monoclinic-rhombohedral), thus resulting in a higher piezoelectric coefficient [27]. Lead-free perovskite materials have the general molecular formula ABO3, where cations A and B form a family of oxides represented as A\({}^{\rm II}\) B\({}^{\rm IV}\) O\({}_{3}\), A\({}^{\rm III}\) B\({}^{\rm III}\) O\({}_{3}\), and A\({}^{\rm I}\) B\({}^{\rm V}\) O\({}_{3}\). (I) A\({}^{\rm II}\) B\({}^{\rm IV}\) O\({}_{3}\)  
One of the most extensively used lead-free perovskite materials is barium titanate (BaTiO\({}_{3}\) BTO). Studies have been conducted on BTO as a model material to investigate its ferroelectric and piezoelectric properties at the nanoscale [28; 29]. In BTO, a phase transition occurs as the temperature decreases, and the phase transforms from tetragonal to orthorhombic, and eventually to rhombohedral [30; 31]. In 2009, a BTO-based composite was reported as a high-performance piezoelectric material with a piezoelectric coefficient of d\({}_{33}\approx\) 620 pc/N [32]; additionally, a composite of barium zirconate titanate-barium calcium titanate 0.5 Ba (Ti\({}_{0.8}\) Zr\({}_{0.2}\))-(Ba\({}_{0.7}\) Ca\({}_{0.3}\)) TiO\({}_{3}\) (BZT-BCT) exhibited a drastic enhancement in performance. The BZT-BCT nanowire synthesized via a two-step hydrothermal process exhibited an open-circuit voltage of 6.25 V and an overall power density of 2.25 \(\upmu\)W/cm\({}^{3}\)[33]. To further enhance the performance, different strategies, such as morphological alterations, have been adopted. Numerous techniques, such as solvothermal synthesis [34], a molten salt method [35], and supercritical fluid technology [36], have been reported for the synthesis of BZT-BCT. Note that the piezoelectric properties of the bulk and nanomaterials differ significantly owing to the difference in their surface area-to-volume ratios [37]. Another promising material is BNT (also known as NBT: (Na\({}_{1/2}\) Bi\({}_{1/2}\)) TiO\({}_{3}\)), which exhibits a perovskite structure at room temperature. Herein, the phase transition from cubic to tetragonal and then to rhombohedral occurs as the temperature decreases from approximately 540 \({}^{\circ}\)C to 300 \({}^{\circ}\)C [38]. NBTs have strong ferroelectric properties, with high remnant polarization (Pr = 38 \(\upmu\)c/cm), and several relaxer ferroelectric (ferroelectric materials that exhibit high electrostriction) NBT-based compositions have high piezoelectric coefficients; these have been used to fabricate high-performance lead-free PENGs. NBT materials exhibit shallow depolarization temperatures and significant leakage current values, which are major drawbacks of these materials [39]. (II) A\({}^{\text{III}}\) B\({}^{\text{III}}\) O\({}_{3}\)  
A well-known example of an A\({}^{\text{III}}\) B\({}^{\text{III}}\) O\({}_{3}\)-type lead-free perovskite material is bismuth ferrite (BiFeO\({}_{3}\), BFO) [40]. BFO is a piezoelectric material that exhibits a high Curie temperature (approximately 809 \({}^{\circ}\)C-830 \({}^{\circ}\)C); thus, it is a suitable candidate for high-temperature applications [41; 42]. In particular, BTO and PbTiO3 were alloyed with BFO to achieve rhombohedral and tetragonal phase boundaries, which improved the piezoelectric response [43]. BFO has the unique property of being magnetic and strongly ferroelectric at room temperature [44; 45].