Synthesis of surface-modified monoclinic ZrO ₂ nanoparticles using supercritical water

We succeeded in simple and rapid synthesis of surface-modified monoclinic ZrO ₂ nanoparticles using a supercritical hydrothermal method. The precursor Zr(OH) ₄ was treated in the presence of various surface modifiers with carboxyl group (-COOH) in a batch-type reactor at 400 °C for 10 min. Oleic, sebacic, dodecanedioic, and 12-aminododecanoic acids were used as surface modifiers. Addition of surface modifiers resulted in smaller particle (crystallite) sizes than the unmodified nanoparticles, suggesting that there is an interaction between the surface modifiers and the nanoparticles. The reduced particle size in the presence of surface modifiers was attributed to the inhibition of the growth of the crystalline surface due to the surface modification. The FT-IR spectra revealed that the surface modifiers were attached to the surface of the nanoparticles through coordination bonds between the carboxylate group (-COO ^-) and the Zr ion. The FT-IR spectra also confirmed the presence of functional groups, such as methyl (-CH ₃), carboxyl (-COOH), and amine (-NH ₂), at the surface. The surface modification was also verified by the thermogravimetric analysis. The number of the surface modifiers attached to the surface of the products was about 2 molecules per nm ². The nanoparticles with carboxyl and amine surface functional groups were water dispersible; the isoelectric point shifted to low pH ranges because of the nature of the groups.