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 ZrO2 nanocrystals of <10 nm diameter driven by controlling the surface energy, with and without capping by N(CH3)(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(CH3)(4)(+) have a less defective pure monoclinic phase, while those Without capping have a pure tetragonal phase with highly disordered oxygen vacancies. The N(CH3)(4)(+) capping oil the surface oxygen site call 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