One dimensional vertically well-aligned ZnO nanorod arrays with uniform thickness and length have attracted the extensive interest of many researchers because they are highly appropriate for the fabrications of different nano-device. ZnO nanorods can be either grown independently or grown on certain substrates. However, a vertical well-aligned ZnO growth on a substrate has more advantages in optoelectronic applications.
For the formation of the one-dimensional ZnO nanostructure, vapor phase synthesis is probably the most extensively explored approach. The typical vapor phase synthesis method includes vapor liquid solid (VLS) growth1, chemical vapor deposition (CVD)2, metal organic chemical vapor deposition (MOCVD)3, physical vapor deposition (PVD)4, molecular beam epitaxy (MBE)5, pulsed laser deposition (PLD)6, and metal organic vapor phase epitaxy (MOVPE)7. Other methods, that do not use any catalyst, such as template-assisted growth8 and electrical field alignment9, have also been employed for the growth of well-aligned ZnO nanorods. Park et al.10,11 have developed a technique for growing vertically aligned ZnO nanorods at a low temperature using catalyst-free MOCVD, which leads to fabrication of vertical Schottky nanorod device arrays. But these techniques demand a specific reaction condition such as high temperatures and high pressures, which go against the large-scale production of this material at low cost.
Solution based method such as hydrothermal or chemical bath deposition (CBD) has many advantages when compared to vapor phase synthesis, such as low temperature, low cost, large area scalable, and ease of handling. Generally, in CBD technique, the reaction occurs at relatively low temperatures (<100°C) compared to vapor phase synthesis methods (>500 C). Thus, solution synthesis methods can be used for a larger choice of substrates including inorganic and organic substrates. Due to the many advantages, in a present day, solution phase synthesis methods have attracted increasing interest. In addition, ZnO nanorods grown using chemical bath deposition methods have more crystalline defects than others mainly due to oxygen vacancies.12 Nanorods with intrinsic defects are capable of exhibiting visible light photocatalysis even without doping with other transition metals13. The general process for vertically aligned ZnO nanorods grown on a substrate by the CBD method is found in the literature.14–17