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Since 2001, silica microspheres have been reported to be stabilized by highly charged hydrous ZrO2nanoparticles which form halos around the microspheres at pH 1.5. However, the exact mechanisms behind this novel stabilization method in terms of the relevant interaction forces remain unclear. In order to gain a greater insight into this mechanism, the interaction between a silica flat and a silica sphere in different ZrO2nanoparticle suspensions was investigated by the colloid probe technique. The interaction force between a silica flat and a 600 nm silica sphere was first investigated in a ZrO2nanoparticle (D≈ 8 nm) suspension with volume fractions of 10−3, 10−4, 10−5, and 10−6. When the volume fraction of ZrO2is 10−6, only a purely attractive van der Waals force was observed between the silica surfaces. With an increase in the ZrO2nanoparticle volume fraction, a peak was detected on the transition force curve at a ZrO2volume fraction of 10−5while a purely repulsion force was observed for ZrO2volume fractions of 10−4and 10−3. The average distance difference between the peak and the zero distance point on the transition force curve which should define the distance between the halo on the microsphere is approximately 2.3 nm. Additionally, the repulsion increases with the effective zeta potential of the binary composite sphere (BCS, the entity of the silica sphere and the surrounding zirconia particles) on an increase of the nanoparticle volume fraction while the adhesion force decreases, which indicates a denser nanoparticle halo. [ABSTRACT FROM AUTHOR]

In this work, it has been shown that, through a highly controlled process, the chemical etching of the anodic aluminum oxide membrane barrier layer can be performed in such a way as to achieve nanometer-scale control of the pore opening. As the barrier layer is etched away, subtle differences revealed through AFM phase imaging in the alumina composition in the barrier layer give rise to a unique pattern of hexagonal walls surrounding each of the barrier layer domes. These nanostructures observed in both topography and phase images can be understood as differences in the oxalate anion contaminated alumina versus pure alumina. This information bears significant implication for catalysis, template synthesis, and chemical sensing applications. From the pore opening etching studies, the etching rate of the barrier layer (1.3 nm/min) is higher than that of the inner cell wall (0.93 nm/min), both of which are higher than the etching rate of pure alumina layer (0.5−0.17 nm/min). The established etching rates together with the etching temperature allow one to control the pore diameter systematically from 10 to 95 nm. [ABSTRACT FROM AUTHOR]