## 3 Structures
### Nanords  
Recently, ZnO-nanorods (NRs) comprised of PENG (ZnO-T-PENG) have been successfully developed [73]. Figure 7 shows the fabrication process of a vertically aligned ZnO NRs array. They are synthesized by using a novel hydrothermal method. In the first step, a precursor solution was formulated by mixing hexamethylenetetramine (HMTA), and zinc acetate dehydrate in deionized water. The nylon fabric piece was washed using alcohol and acetone and deionized with water by means of the ultrasonic method. Then the surface of the fabric is coated with silver (Ag) paste as an electrode by a facile screen-printing  
Figure 6: (**a**) SEM image of the CdS NWs produced by the hydrothermal method; (**b**) XRD pattern of the CdS NWs produced by the hydrothermal method; (**c**) schematic diagram of the AFM measurement system; (**d**) line profiles scanned across CdS NWs; (**e**) (i**) SEM image of the CdS NWs prepared by PVD process (**ii**) TEM image of the CdS NWs prepared by the PVD process (**iii**) HRTEM image of the CdS NWs; (**f,g**) topography and subsequent output voltage image of CdS NW respectively reproduced with permission from [72], AIP Publishing, 2008.  
method, as shown in Figure 7a,b. Figure 7c shows the arrangement of ZnO NRs synthesized vertically on the surface of an Ag-coated fabric. After being submerged in a precursor solution for 5 min with the fabric attached to a PTFE holder, a consistent ZnO Seed layer was created on the fabric. The PTFE holder with fabric was then once more submerged in a reaction mixture in a sealed beaker and maintained in a water bath for 4 h. Subsequently, the fabric was removed, cleaned, and repeatedly annealed. A vertical ZnO nanotube array with hexagonal cross-sections was then created on the face of the Ag-coated fabric, as seen in Figure 7e. Figure 7f shows the structure of ZnO NRs patterned textile-based-PENG (ZnO-T-PENG), consisting of a vertically arranged ZnO NRs array inserted between the Ag-coated fabric. Figure 8a shows the process of the piezoelectric charge generation of the PENG. When a vertical force is exerted over the surface of the ZnO-T-PENG on the top and bottom sides, respectively, an equal quantity of positive and negative charges are produced, as shown in Figure 8a(i). As a current form, the electrons will start to flow from the top electrode to the bottom electrode (Figure 8a(ii)). The strain on the ZnO NRs increases as more force is applied. Electric charges start to flow backward from the bottom electrode to the top electrode as the applied force is withdrawn (Figure 8a(iii)). The electrons stop flowing when all the force is removed from the ZnO nanorods, as seen in Figure 8a(iv). As illustrated in Figure 8b,c, this PENG can produce an output power of 20 nA and an output voltage of 4 V and 0.8 V, respectively, for palm clapping and finger bending. The development of smart wearable electronics can be aided by this kind of PENG [74]. Similar to this, another method uses piezoelectric ZnO NRs on a standard paper substrate to transform mechanical energy into electrical energy [75]. The standard packing paper with excellent flexibility was employed as the paper substrate in the synthesis of ZnO paper. Radiofrequency (RF) magnetron sputtering was used to coat these pieces of paper with a thick ZnO layer after they had been cleaned with acetone, ethanol, and deionized water. The ZnO NRs were grown by a simple hydrothermal process. The solution that was used was composed of acetate dehydrate and HMTA. After that, the solution was poured into the Teflon reaction kettle, inside of which paper substrates covered in ZnO seeds were hanging vertically. The ZnO paper was then removed and washed. It was washed with deionized water and then baked at 100 C. To act as an electrode, silver (Ag) paste was applied to the two ZnO paper's ends. A flexible ZnO paper nanogenerator (ZPNG) was created after drying.