Selective growth of monoclinic and tetragonal zirconia nanocrystals.

The growth of metal oxide nanocrystals with a well-organized crystalline phase is of fundamental and technological interest because in this way it is possible to tune their size-dependent unique properties. In this communication, we demonstrate the selective growth of monoclinic and tetragonal ZrO(2) nanocrystals of <10 nm diameter driven by controlling the surface energy, with and without capping by N(CH(3))(4)(+). The nanocrystals were grown in aqueous solution at 150 degrees C. It was revealed, by transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and optical absorption studies, that the nanocrystals capped by N(CH(3))(4)(+) have a less defective pure monoclinic phase, while those without capping have a pure tetragonal phase with highly disordered oxygen vacancies. The N(CH(3))(4)(+) capping on the surface oxygen site can reduce the surface energy low enough to stabilize the monoclinic phase. By contrast, the bare surface oxygen site has higher energy; thereby, the lower surface energy tetragonal phase is formed spontaneously. The present concept is a promising universal approach to control the crystal phases of technologically important oxide nanocrystals.