Solid-Solution Nanoparticles:  Use of a Nonhydrolytic Sol−Gel Synthesis To Prepare HfO₂ and HfxZr₁₋xO₂ Nanocrystals

A nonhydrolytic sol−gel process closely following that of Hyeon et al. for the synthesis of ZrO 2 nanocrystals ( J. Am. Chem. Soc. 2003, 125, 6553−6557) was used to synthesize highly crystalline and monodisperse HfO 2 nanoparticles. Reactions of hafnium isopropoxide with hafnium halides at high temperature in a strongly coordinating solvent yield nanometer-sized particles of HfO 2 . The size, shape, and crystalline phase of the hafnia particles depend on both the reaction temperature and the halide. The nonhydrolytic cross-condensation method was also extended to the binary metal oxides nanocrystals, i.e., HfO 2 −ZrO 2, ZrO 2 −TiO 2, and HfO 2 −TiO 2 . Efforts to prepare nanocrystals of Hf x Zr 1 - x O 2 over a wide range of x were successful; however, this method could not be used to prepare either Zr x Ti 1 - x O 2 or Hf x Ti 1 - x O 2 . In conjunction with X-ray powder diffraction and high-resolution transmission electron microscopy, Raman spectroscopy identifies the structural phase of the nanocrystals and also proves the formation of the Hf−Zr solid solution nanoparticles via the cross-condensation method. In the case of Hf x Zr 1 - x O 2, for x < 0.5, highly monodisperse, roughly spherical particles of tetragonal Hf x Zr 1 - x O 2 were formed, while for x > 0.5, small nanorods of the monoclinic phase of the binary oxide were obtained.