Morphology of ZnO nanostructures can be altered by varying pH of the precursor’s solution during the growth process. It was clearly demonstrated by Baruah et al.51 that the pH plays a major part in the morphology and dimensions of the nanorods grown through the hydrothermal process. They found that the growth of the nanorods was observed to be faster in the basic medium rather than in acidic medium. However, slightly acidic conditions during the initial growth from the nucleation sites resulted in significant variations in the lateral and longitudinal dimensions, opening up possibilities of tailoring the length and width of the nanorods to match different applications. This may be an attractive option for manufacturing ZnO nanorods based devices at a commercial level as the dimension of the nanorods can be controlled simply by the variation in a single parameter i.e. the pH of the chemical bath, keeping all other parameters like concentration of the bath, growth time and temperature constant.
Wahab et al.52 varied the pH of the precursor solution from 6 to 12 by the addition of sodium hydroxide (NaOH). They found that morphology of ZnO nanostructures significantly varies from sheet-like (pH 6) to the rod-like structure of ZnO (pH 10–12). Farmer et al.17 showed that ZnO nanorods were formed at pH 5-7 of the precursor’s solution, while nanoflowers form at pH 9-10. Rana et al.53 using microwave method showed that when the pH of the solution was at 6.8, the ZnO nanorods were formed which persisted in the pH range of 6.8–9. However, an instantaneous change in the shapes and the size of the nanostructures was observed when the pH was increased beyond 10. A morphology transition from nanorods to nanoflowers was observed at pH 11. They also found that the solution gave birth to nanostars and nanotetrapods when the pH of the precursors was further raised to 12 and 13, respectively.