Once it's in powder form, it's no longer nano - that is, the powder does not contain independent, discrete particles < 100 nm. Rather a powder is a collection of fused sub- and post-micro aggregates and agglomerates. 'Nanopowder' is an oxymoron. Reduce the size of these agglomerates by a long time in a colloid mill. Best route to preserve the nano nature is to keep the material in colloidal suspension or in atomized form.