Control of nanoscale morphology of oxide crystals using aqueous solution systems

We successfully controlled nanoscale morphology of ZnO, SnO₂ and TiO₂ grown in aqueous solution systems. Nanograins, nanorods and nanosheets of the metal oxides were obtained with variation of the preparation conditions including the concentration of the precursor reagents, pH and addition of various organic molecules and inorganic ions. The morphological evolution of wurtzite-type ZnO crystals from hexagonal nanorods to nanoplates and nanosheets was induced by the coexistence of phosphate anions and specific carboxylates. An increase in the concentration of SnF₂ and pH changed the nanoscale morphology of crystalline SnO₂ from grains into rods, flakes and sheets. Nanorods of anatase and rutile were deposited in the solutions of TiF4 and TiOSO₄ by controlling pH and addition of urea. Nanosheets of titanate were prepared with ammonia anions in agar gel containing TiF₄ and easily transformed into nanosheets of anatase.