Why don't you filter off the solid remained in the bottom of the tube and then dry it for a while in an oven?

Filter off using vacuum filtration or centrifuge and dry it in electric oven.

Put your ZnO NP solution in a vacuum filtration system to filter the precipitates followed by a heat treatment (not more than 60 degrees) in a normal hot air oven for 24-36 Hrs.

Take a small sample and dry it e.g. at RT (monitoring the weight loss). How long it takes depends on the use solvent. I also suggest to use this sample for XRD analysis. Additionally I suggest to redisperse the majority if you want to work with a Dispersion. IR analysis might be intersting to monitor the surface modification.

Article Synthesis and testing of ZnO nanoparticles for photo-initiat...

After precipitation, I dry the powder in an oven at 80°C overnight. this is a simple method.

See my paper

DOI:10.1016/j.powtec.2014.04.065

you can also freeze dry it.

Regards

thank you so much for your opinions !! it was great helps!!

The size of ZnO particles are expected to be in which range?

filtering and drying in the oven.

I use Vacuum filtration in general, when I'm in a hurry I put the filtrate in the oven sometimes to accelerate things but only if I'm sure that this will not affect the particles structure and properties.