Formation mechanism of hafnium oxide nanoparticles by a hydrothermal route

Hafnium oxide nanoparticles (NPs) were synthesized by a hydrothermal route, using hafnium tetrachloride (HfCl4) as the starting material and sodium hydroxide (NaOH) to adjust the pH. Through changing the aging temperature, concentration of NaOH and reaction time, both pure tetragonal hafnium oxide (t-HfO2) and pure monoclinic hafnium oxide (m-HfO2) were obtained. X-ray diffraction (XRD) spectra and transmission electron microscopy (TEM) images indicated that the shapes of t-HfO2 NPs and m-HfO2 NPs were near-spherical and spindle-like, respectively. The formation of t-HfO2 NPs or m-HfO2 NPs is probably related to their crystal cell structure, thermodynamic and kinetic stabilities. Tetragonal HfO2 is produced originally in the process of the formation of monoclinic HfO2. A higher temperature, lower concentration of NaOH, longer reaction time and addition of m-HfO2 seeds are beneficial for the formation of m-HfO2 NPs. By analysis and calculation of the equilibrium constants involving hydrolysis of hafnium ions, the changes in the mole fractions of hafnium hydro-complexes with pH were determined. The Hf(OH)(6)(2-) ion is assigned to the precursory hydro-complex for the formation of HfO2 nanoparticles transformed from Hf(OH)(4) gel according to a comparison between the influences of pH on the equilibrium and the formation of HfO2 particles. Moreover, the formation of HfO2 NPs was obviously promoted and the size was reduced by addition of seeds, suggesting that the formation of HfO2 NPs is controlled by the surface-deposition reaction. The above results are of great importance for studying nano-inorganic solution chemistry.