Cubic phase stabilization in nanoparticles of hafnia-zirconia oxides: Particle-size and annealing environment effects

Abstract

Amorphous hafnia (HfO2−y), zirconia (ZrO2−y), and hafnia-zirconia (xHfO2−y–(1−x)(ZrO2−y)) nanoparticles were prepared by combining aqueous solutions of hexamethylenetetramine (HMT) with hafnium dichloride oxide (HfOCl2⋅8H2O), zirconium dichloride oxide (ZrOCl2⋅8H2O), or a mixture of these two salts at room temperature. For pure hafnia, transmission electron microscopy showed that the lower cation concentration (0.01M) resulted in the precipitation of smaller amorphous nanoparticles relative to higher concentrations (0.015M–0.04M). Consequently, the lower concentration preparation route coupled with a reducing environment (H2:N2=9:91) during annealing at temperatures between 650 and 850°C allowed for nanoparticles with a cubic structure to be prepared as determined by x-ray diffraction. The structurally cubic hafnia nanoparticles were 6nm or less in diameter and equiaxed. Using the same method (0.01M total metal cation concentration and reducing environment during annealing), nanoparticles of cubic structure were prepared across the entire hafnia-zirconia compositional spectrum, with a critical particle size for the cubic structure of about 6nm. Nanoparticles of tetragonal and monoclinic structure were prepared by increasing the annealing temperature and/or using a less reducing environment. The unique role of HMT in sample preparation is discussed as well.