## 3 Structures
### Nanotubes  
Nanotube-based PENGs are very well recognized for powering miniature autonomous, independent systems. Recently, simple, inexpensive, and scalable piezoelectric nanogenerators were fabricated using stretchable and flexible BaTiO\({}_{3}\) nanotubes [76]. Figure 10a shows a schematic of a nanogenerator composed of lead-free BaTiO\({}_{3}\) nanotubes synthesized hydrothermally by forming composites with PDMS polymers. A direct poling procedure was used to create flexible and transparent nanogenerators. According to Figure 10a, there are five layers in the nanogenerator. Two Au/Cr films act as two upper and bottom electrodes,  
Figure 9: (**a**) Higher magnification SEM image of the ZnO NRs; (**b**) structural diagram of an indi–vidual paper fiber grown with ZnO NRs; (**c**) output voltage and (**d**) output current of the ZPNG subjected to repeated cycles of FS and FR; measured (**e**) output voltages and (**f**) output currents at different frequencies and a constant applied strain reproduced with permission from [75], RSC Pub, 2012. BaTiO\({}_{3}\) nanotubes and PDMS composites act as piezoelectric potential generators under applied voltage, and a PS substrate. The PS substrate and PDMS serve as supporting and shielding layers to keep the nanogenerators' conformation. The BaTiO\({}_{3}\) nanotube/PDMS composite's SEM picture is shown in Figure 10b, which also illustrates how adaptable the manufactured nanogenerators are. The output voltage and current of the constructed nanogenerators are shown in Figure 10c,d. The nanogenerator generates an open circuit voltage of 5.5 V and a short circuit current of 350 nA with a load of 1 MPa. Because nanotubes are so small, nanogenerators have tremendous performance. To determine whether the generated output signal is the result of the piezoelectric effect, a switching polarity test is carried out. Nanogenerators are connected in two modes, forward and reverse. A positive output signal is generated during the forward link, as shown in Figure 10c, and an opposite output signal is recorded during the reverse link, as shown in Figure 10d. The COMSOL simulation model used to determine the distribution of piezoelectric potential inside the nanogenerator is depicted in Figure 10e. An upper PDMS layer, a BaTiO\({}_{3}\) layer, and a bottom PS substrate make up the simulation's three layers. In a similar vein, a hydrophobic piezoelectric nanogenerator based on poly (vinylidene fluoride)-carbon nanotube (PVDF-CNT) foam that is moisture-tolerant was created [77]. Figure 11 shows the fabrication process of nanogenerators based on hydrophobic piezoelectric PVDF-CNT foams fabricated by the solution route. PVDF-CNT foam fabrication involved mixing PVDF pellets, table salt (NaCl), and multi-walled carbon nanotubes (MWCNTs) in a solvent. The solution was held for 1 h at 60 \({}^{\circ}\)C with magnetic stirring. After that, the solution was put into a crucible made of silica gel and heated to 90 \({}^{\circ}\)C for 5 h. A black polymer composite was produced after cooling. The composite foam was submerged in boiling water to wash away the salt after the outermost layer of the composite sample was removed with sandpaper. Pictures of PVDF-CNT demonstrate how flexible it is. The chain structure of PVDF-CNT foam is crystallized and clearly defined. In order to create piezoelectric nanogenerators, conductive silver (Ag) paint was employed as an electrode. Figure 12a,b illustrates how a compressive force increase of 0.02 kgf resulted in a high output voltage of roughly 12 V and a current density of 30 nA/cm\({}^{2}\) from a flexible piezoelectric PVDF-CNT foam-based nanogenerator. The identical settings as in Figure 12c,e were used to test the piezoelectric output current. To confirm that the generated output signal is the result of the piezoelectric effect, a shifting polarity test is carried out. For this cause, forward as well as reverse linkages are used to connect nanogenerators. The operation of the piezoelectric PVDF-CNT foam-based nanogenerator is depicted in Figure 12g.