60 s; this spin-coating process was repeated ten times. Between each coating cycle,the substrate was placed on a hot plate at 150 ◦C for 2min. After spin coating, the substrate was placed on a hot plate at 210 ◦C for 1 h to form ZnO and to decompose the PVA chain and create carbon backbone grids [36]. Five set of substrates were annealed at different temperatures ranging from 250 ◦C to 550 ◦C, after which the substrates were ready for use.The second step was the growth of ZnO nanorods. First, 0.1 mol/L of zinc nitrate hexahydrate Zn(NO3)2·6H2O was dissolved in DI water at 70 ◦C, and an equal molar concentration of hexamethylenetetramine (C6H12N4) was dissolved in DI water at 70 ◦C. The two solutions were then mixed in a beaker containing the ZnO–PVA seed layer coated on silicon substrates. The substrates were inclined at 80◦ in the beaker and then placed inside a microwave oven for 2 h at 90 ◦C. After complete growth of the ZnO nanorods, the substrates were first washed with hot DI water and then with hot ethanol to remove the remaining salt. They were then dried with nitrogen gas. Finally, the substrates were heated at 550 ◦C for 1 h in an oxygen flow (5cm3/min).