Silica coatings on α-alumina particles: Analysis and deposition mechanism

Coatings are applied to particulate materials to enhance surface properties while retaining desired bulk properties. Characterization of such coatings presents a challenge. In this paper we present an example of a particle surface analysis and assess how factors such as roughness and coating thickness distributions affect quantitative conclusions. Silica coatings were deposited from aqueous solution on to α-alumina particles of 150 nm average size. Transmission electron microscopy, x-ray photoemission spectroscopy and secondary ion mass spectrometry/depth profiling were used to analyze coatings of thicknesses of tens of nanometers to submonolayer. The three techniques were in excellent agreement in the thickness ranges where they overlapped. XPS does not allow assessment of the uniformity or thickness distribution of the coatings. However, depth profiles are quantitatively consistent with a model of random attachment of the silica units to the particle surface during growth, with no preference for the uncoated alumina surface over the previously deposited silica.