General Two-Phase Routes to Synthesize Colloidal Metal Oxide Nanocrystals: Simple Synthesis and Ordered Self-Assembly Structures

Different two-phase approaches have been developed for the synthesis of two classes of monodisperse colloidal metal oxide nanocrystals (NCs): rare earth oxide NCs and transition metal oxide NCs. These routes were simple and inexpensive, using metal salts instead of organometallic compounds, with mild reaction conditions, easily controlled size and shape, and multigram-scale products. The obtained products were characterized by transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), Fourier transform infrared absorption spectroscopy (FTIR), and nitrogen adsorption-desorption isotherms (BET). The possible mechanisms for the formation and growth of nanocrystals were discussed. Accordingly, tert-butylamine (nucleophile agent) and ethanol (reduced agent) are the key factors for the formation of rare earth oxide NCs and transition metal oxide NCs, respectively. Different sizes and shapes of monodisperse nanoparticles such as spherical, cubic, peanut, rod, and hexagonal NCs were obtained, depending on the nature of metal precursors. Furthermore, the effect of monomeric precursor concentration, type of precursors, and reaction time on the size and shape of the products was also studied. The SAED patterns of the obtained NC samples show a set of sharp spots that are characteristic of single crystalline structures indexed accordingly with the structures determined by the XRD spectra. The XPS results revealed the presence of two oxidation states of cerium, samarium, manganese, and cobalt on the nanocrystal Surface, whereas it seems only one oxidation state is present on the surface for Y, Cr, La, Gd, and Er oxide nanocrystals. A large O 1s XPS peak attributed to two oxygen components for these Ce, Sm, Mn, and Co oxide NC samples refer to the defected structure. Our approaches may be also applicable to synthesize other uniform metal oxide NCs as well as doped metal oxide NCs and multicomponent NCs.