Tip Shaping for ZnO Nanorods via Hydrothermal Growth of ZnO Nanostructures in a Stirred Aqueous Solution

To enhance the field emission (FE) properties of emitters based on ZnO nanostructures, the growth of ZnO nanostructures on ZnO nanorods (NRs) (1–1.2 μm in length and ∼200 nm in diameter) in a disturbed hydrothermal growth (HTG) solution is demonstrated. Experimental results reveal that the degree of disturbance of the aqueous solution determines both the shape and location of the synthesized ZnO nanostructures. For stirring speeds of 300 and 600 rpm (rpm), NR-like ZnO nanostructures with a reduced uniform diameter (70–120 nm) and a tapered shape but a rough surface are grown on the basal plane of ZnO NRs, respectively. For stirring speeds of 900 and 1150 rpm, ZnO needles (40–70 nm and 15–20 nm in diameter, respectively) were synthesized along the {101̅0} planes of the ZnO NRs with coherent c-planes. FE characteristics of ZnO-NRs emitters with and without the second stage growth of ZnO nanostructures are reported and compared. Possible growth mechanisms which govern the physical characteristics of the ZnO nanostructures synthesized in the HTG process are proposed and discussed