Precursor control of crystal structure and stoichiometry in twin metal oxide nanocrystals

Nanocrystals of solid-solution twin metal oxides HfxZr1-xO2 have been prepared by the non-hydrolytic sol-gel condensation of a metal alkoxide with a metal halide. We demonstrate that the alkoxide group can be used to tune the stoichiometry and crystal structure of the obtained nanocrystals. Increased branching of the alkyl group in the metal alkoxide results in the formation of monoclinic rather than tetragonal nanocrystals, whereas longer alkyl chains favor the increased incorporation of Zr in the HfxZr1-xO2 nanocrystals regardless of whether a hafnium or zirconium alkoxide is used as the precursor. Remarkably, the monoclinic phase has been stabilized for a Zr-rich species, pointing to the significant role of the alkoxide precursors in influencing the growth and phase transformation of nanocrystalline materials in hot colloidal synthesis. Considerable control can thus be established over the phase diagram of nanocrystalline Hf/Zr oxides by appropriate choice of precursors.