Characterization of engineered alumina nanofibers and their colloidal properties in water

Aluminum oxide nanofibers having different fiber diameters were characterized to determine if and how the fiber diameter affects their colloidal behavior and solubility in water. The measurements provide initial data on the behavior of alumina nanofibers in aqueous systems. Three different aluminum oxide materials with average fiber diameters ranging from a few nanometers to a few hundred nanometers were acquired from ANF Technology (Nafen™), MemPro Materials, and Pardam Nanotechnology. The morphology, dimensions, chemical composition, and surface properties of each material were measured using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, thermogravimetric analysis, and N2-BET surface area analysis. Properties measured for aqueous suspensions of the fibers included electrophoretic mobility, point of zero charge, apparent hydrodynamic radius via dynamic light scattering, and solubility. TEM images were used to determine the size distribution of the fiber diameter, and the smooth, non-porous, non-tubular structure of the materials. Fiber diameters were 5–10 nm for Nafen™, 90–175 nm for MemPro, and 150–600 nm for Pardam. SEM showed that samples of the dry materials consisted of long (micron sized) fibers having these diameters. Specific surface area ranged from 2.7 m2/g for the largest diameter Pardam fibers to 136 m2/g for the smallest diameter Nafen™ fibers. The pH of the point of zero charge for the three materials ranged from 5.0 for Pardam to 8.5 for Nafen™ in 10 mM NaCl. Dynamic light scattering revealed that the fibers readily aggregate in both deionized water and solutions of high ionic strength. Fiber solubility ranged from a dissolved aluminum concentration of 23–83 μg/L for pH from 6.67 to 8.23. The observed solubilities were similar to those calculated from thermodynamic data for corundum (α-Al2O3) and boehmite (γ-AlO(OH)), phases that were observed by XRD in the aged materials.