Designed single-phase ZrO2 nanocrystals obtained by solvothermal syntheses

Abstract

Design of metal oxide nanoparticles is a key study for obtaining high-functional materials with tunable properties. For that purpose, great progress on chemical methods for controlling the growth of nanoparticles during the synthesis has been made. However, an important challenge faced by this field of study is to completely understand the crystallization process and the chemical pathways during the syntheses that use growth-controlling agents. In this study, the synthesis of ZrO₂ was conducted by a solvothermal methodology using three different organic solvents (oleic acid, benzyl alcohol and octyl alcohol) and adjusting the Zr precursor concentration, aiming to control the ZrO₂ nanocrystal growth and obtain single-phase nanocrystalline particles. X-ray diffraction and Raman spectroscopy experiments indicated the formation of pure monoclinic, cubic and tetragonal ZrO₂ nanocrystals for the syntheses using oleic acid, benzyl alcohol and octyl alcohol as solvents with precursor concentrations of 0.25, 0.50 and 0.50 mol L⁻¹, respectively. Transmission electron microscopy was used to analyze the nanocrystal structure and to assess the crystal size distribution. Infrared and solid-state ¹³C nuclear magnetic resonance spectroscopy confirmed the presence of the organic ligands chemically bonded to the surface of the nanocrystals. Thermogravimetric analysis indicated an increasing ratio of ligand molecules/surface area as the acidity of the solvent increased. With the observations from this study, the solvent pK_a value, the chemical structure and the type of coordination of the capping agents are ascribed as responsible for promoting different growth pathways, allowing pure crystalline phase zirconium oxide nanocrystals to be obtained.