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 ZrO2 nanocrystals (J. Am. Chem. Soc. 2003, 125, 6553-6557) was used to synthesize highly crystalline and monodisperse HfO2 nanoparticles. Reactions of hafnium isopropoxide with hafnium halides at high temperature in a strongly coordinating solvent yield nanometer-sized particles of HfO2. 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., HfO2-ZrO2, ZrO2-TiO2, and HfO2-TiO2. Efforts to prepare nanocrystals of HfxZr1-xO2 over a wide range of x were successful; however, this method could not be used to prepare either ZrxTi1-xO2 or HfxTi1-xO2. 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 HfxZr1-xO2, for x < 0.5, highly monodisperse, roughly spherical particles of tetragonal HfxZr1-xO2 were formed, while for x > 0.5, small nanorods of the monoclinic phase of the binary oxide were obtained.