Dispersion and Functionalization of Nonaqueous Synthesized Zirconia Nanocrystals via Attachment of Silane Coupling Agents

Zirconia (ZrO2) nanocrystals, synthesized from zirconium(IV) isopropoxide isopropanol complex and benzyl alcohol, were dispersed and functionalized in organic solvents using three kinds of bifunctional silane coupling agents (SCAs), 3-glycidoxypropyltrimethoxy si lane (GPTMS), 3-aminopropyltriethoxysilane (APTES), and 3-isocyanatopropyltriethoxysilane (IPTES). Completely transparent ZrO2 dispersions were achieved in tetrahydrofuran (THF) with all three SCAs, in pyridine and toluene with APTES and IPTES, and in N,N-dimethylformamide with IPTES. Dynamic laser scattering (DLS) measurements and high-resolution transmission electron microscopical (HRTEM) observation indicated that the ZrO2 nanocrystals are dispersed on a primary particle size level. Fourier transform infrared spectroscopy, solid-state C-13- and Si-29 NMR spectroscopy, and thermogravimetric analysis demonstrated that all three SCAs are chemically attached to the surface of the ZrO2 nanoparticles, however, in different bonding modes. Except for GPTMS/ZrO2/THF dispersion and IPTES/ZrO2/pyridine dispersion, all other transparent dispersions have poor long-term stability. The increasing polarity, due to high amount of APTES attached and high hydrolysis and condensation degree of the bonded APTES, and the aggregation, due to interparticle coupling via the bonded triethoxysilyl group, are the causes of the poor long-term stability for the ZrO2 dispersions with APTES and IPTES, respectively. Nevertheless, the APTES-functionalized ZrO2 precipitates can be deagglomerated in water to get a stable and transparent aqueous ZrO2 dispersion via addition of a little hydrochloric acid.