Try growing the nanorods without a seed.

Hi you, maybe your sample is not in the crystal state. You can improve the crystalline by annealed  at high temp such as higher 200 degrees Celsius. You can refer this paper:https://www.researchgate.net/publication/278327670_Effect_of_graphene_buffer_layer_on_optical_properties_of_ZnO_nanorods

Conference Paper Effect of graphene buffer layer on optical properties of ZnO nanorods

The most commonly known structure of ZnO is hexagonal in crystalline form. So if perfect conditions for the hexagonal structure growth is provided, you may achieve the desired results. Temperature, time and PH are the key parameters. 

The most crucial thing is nucleation time at which the hexagonal structure is formed, upon which the rods are grown. So add the base slowly to the salt of zinc. Also try to tilt the substrate if you are growing the rods on some substrate.

Here is a useful Article for you. Email me if you could not download that paper

http://link.springer.com/article/10.1007/s10853-010-4294-x

Best Regards

the coalescence of small hexagonal rod into round bar can be seen clear in your picture. there is nothing new, you can add some surfactant agent to passivate the surface and stop the coalescence

Dear Sir

I see that ZnO nanorods you synthesized 

This May be you not control the growth parameters that effect on the growth of ZnO nanorods, try to control the pH of growth solution in acidic rate, your nanorods its come from base level

kind Regards