Formation and Characterization of an Aqueous Zirconium Hydroxide Colloid

Among the wide variety of routes reported for the chemical synthesis of zirconia, the development of simple, aqueous sol−gel technology is of considerable interest for industrial-scale applications. In this study, zirconium hydroxide nanoparticles were produced by the controlled hydrolysis of zirconium carbonate in nitric acid, followed by gentle heating at 70 °C. Transparent, colorless gels were subsequently produced from the concentrated sols (500 g/L, oxide basis) by drying at ambient temperature. The nanoparticle sols and gels were characterized using a range of techniques, including EXAFS, Raman spectroscopy, dynamic light scattering, and SAXS, which revealed the presence of platelike particles of width 2.8 ± 0.4 nm and thickness 0.5 ± 0.1 nm. The platelets exhibit a surprisingly high degree of short-range ordering, and it is demonstrated that they are composed of stacked layers of two-dimensional “[Zr(OH)4]n” sheets, as proposed (but not established) in earlier studies. The speciation of the nitrate anions in the sols was also investigated by Raman and 14N NMR, which revealed that the majority of anions were closely associated with the nanoparticles (i.e., separated from the surface by several layers of coordinated water molecules), rather than coordinated directly to the surface. The role of such species in maintaining the stability of the nanoparticle sols is discussed.