Based on the information you've provided, it sounds like your method for removing the alumina left a residue. If it's residue, it is probably an aluminium salt or hydroxide, depending on the method you used to remove the alumina. most residues can be removed by washing the solids before centrifugation, though the choice of solvent for washing will depend on the composition of the residue. 

It's also possible that your removal method was incomplete. If that's the case, you'll need to adjust the time, strength, and/or volume of the removal agent.

Dear Mark R. Haase.

Thanks for your kind answer. Can I ask more?  maybe the residue (I indicated it the white precipitate) is aluminum salt or hydroxide formed during dissolution of Al2O3.  you said it can be removed by washing the solids.  solid you mean is Al plate ?  I don't understand why you washed Al plate after dissolution of Al2O3 because all residue is already in dissolution solution (which is NaOH)

Could you explain more detail about washing process of solid.

Thanks

Sorry for the ambiguity. What I meant by solids was the nanorods. After you've removed the alumina, the nanorods may have some salt residue - even with good filtering, some of the dissolution liquid will remain on them. Washing the nanorod solids should help remove any residual salts or incompletely dissolved alumina.