# Strategic Development of Piezoelectric Nanogenerator and Biomedical Applications
## 3 Strategies
### Effect of Morphology  
The morphology of a piezoelectric material has a substantial impact on piezoelectric performance, and the desired properties can be achieved by tuning the morphology of the material. Different reaction parameters, such as the reaction temperature, concentration, and pH, influence the morphology of the material [58; 59; 60]. Nanostructures formed with different morphologies, such as nanowires, nanoribbons, nanotubes, nanorods, and nanopillars, exhibit different properties. ZnO nanostructures are widely utilized in the fabrication of PENGs because of their chemical stability, biocompatibility, and low cost [61]. ZnO is a potential candidate for PENGs owing to its good piezoelectric response, its availability, and its economical fabrication processes. Nonetheless, ZnO suffers from several difficulties, primarily the screening effect, which hinders its piezoelectric performance. Several strategies have been introduced to minimize these problems and enhance the performance of ZnO-based nanogenerators. Fabricating different morphologies is considered an excellent approach in improving the performance of PENGs. ZnO can be fabricated with different morphologies to achieve desired properties; for example, ZnO nanorods exhibit better performance compared with ZnO nanowires. [PERSON] et al. [14] demonstrated a unique fabrication technique, wherein vertically aligned ZnO nanorods, grown on Si substrate via physical vapor deposition, were transferred to another substrate to create a horizontal nanowire array. They has used a mechanical setup to transfer the nanowire on a resaving substrate shown in Figure 3A(a). Figure 3A(b) shows the SEM image after the transfer process. The receiving substrate was swept across the vertically aligned ZnO nanowire in a counterclockwise direction due to which the nanowire gets detached from the Si substrate and gets horizontally aligned on the receiving substrate. The fabricated device generated an output voltage of 2.03 V with a current of 107 nA. Reportedly, nanogenerators based on high-density, well-aligned ZnO nanorods with decent crystal quality generated an output voltage of 9.5 mV [62]. A nanogenerator based on a laterally grown ZnO nanowire array fabricated via 3D printing generated an output voltage of 2.03 V [11]. Two-dimensional (2D) morphologies, such as ZnO nanodiscs, have been recently developed, and nanogenerators based on ZnO nanodiscs deliver a DC output voltage of 17 V with a current density of 150 nA/cm\({}^{2}\) under vertical compressive strain [19]. Another example of the 2D ZnO nanostructure is the nanosheet, which is an attractive morphology. A high surface-to-volume ratio and good durability under repetitive mechanical damping make ZnO nanosheets a promising candidate for application in PENGs. [PERSON] et al. [18] fabricated a PENG based on ZnO nanosheets, considering the  
Figure 2: Different strategies used to improve the performance of PENGs. drawbacks of 1D nanostructure-based PENGs (the occurrence of fractures when the critical force limit was exceeded). The authors fabricated a 2D ZnO nanosheet-based PENG which was able to produce high output performance without any degradation. The device was fabricated in a stepwise manner. In the first step, the ZnO nanosheets were grown on an Al substrate via an aqueous solution route. Then, in the next step, the Au layer was deposited on a polyethersulphone substrate. The exact structure of the device is shown in Figure 3B(a). The authors demonstrated the effect of the buckling of nanosheets and ionic layer coupling, which were able to provide an efficient DC output. They recorded an output voltage of 0.5 V and a current density of 15 \(\upmu\)A/cm\({}^{2}\). Figure 3B(b,c) shows the switching polarity of the fabricated device under 4.0 kgf. Recently, a PENG based on ZnO nanoflakes with a thickness of 10 nm was synthesized via a novel synthetic route, generating a peak-to-peak open-circuit voltage of 127 V and a short-circuit current of 80 \(\upmu\)A.