yes previous researchers gave some real data about annealing temperature. But we can not ignore the effect of precursor. In our case ZnO obtained from ZnNO3 at 550 deg C, we got best crystallinity with 20 nm size, but above it the particle size gradually increases. if the precursor do have lots of organic framework, u should limit yourself within 350 deq C.. Best u should check within this range at different temp