The alkalinity of the precursor solution is an important parameter for controlling the morphology of the formed products.
At higher concentration of the alkali, it was assumed that a large number of [Zn(OH)4]
2 units which act as the growing nuclei for the rods are formed.
At low and medium concentration of the potassium hydroxide, it was considered that both Zn(OH)2 and [Zn (OH)4]2 species are present in the solution.
Although the A3 and A2 rods form with the same mechanism, the A1 rods have the lowest aspect ratio with smaller diameter and shorter length.
As the OH concentration increases, the nucleation rate is increased.
But the growth rate is inhibited.
Hence the aspect ratio of the rods formed is as follows: A1oA3oA2.
Also, when the OH concentration exceeds a saturation limit, the growth units are shielded by the excess OH ions before they are incorporated into the crystal unit and hence the growth rate of the rods is highly inhibited.
For this reason, the crystallite size of A1 is smaller and has the lowest aspect ratio compared to the other two products.
The same effect was well explained by QiXiao et al.
Fig. 4. Variation of aspect ratio and crystallinity of the samples with pH of the
reacting solution.