I didn't fully grasp the context of your question. However, from what I understand you could use precipitation as a means to create monodisperse ultrafine particles of ZnO. Dissolve ZnO in HCl or any compatible mineral acid. Dilute the solution in water. Add sodium carbonate solution which will lead to precipitation. Wash and heat the precipitate to get the powders.
Again, this is a very basic approach and I wonder if it qualifies as green chemistry or not. But I hope it helps.
Dear Charles, may I suggest that you adopt the use of a varying percentage of ethanol in water for the dissolution of that salt.....that will still qualify as green chemistry if it works. Best wishes...