Surface characterization of coated powders : Al2O3-SiO2-coated TiO2

The aim of this study has been to find experimental strategies to characterize surface of coated powders. We have studied hydrous alumina + silica-coated TiO2 pigments using different XPS methods, isoelectric points (ieps) and x-ray fluorescence (XRF). We tested especially the applicability of a simple XPS analysis method introduced by Tourgaard et al. Coated TiO2 powders were analysed qualitatively with ieps, XPS surface concentrations, XPS depth profiling, and Ti 2p photoemission peak-to-background ratio D, The ratio D together with surface concentrations, was found to characterize well the quality of the coating. Differences in surface layers were affirmed also by ieps and XPS profiling. The asymptotic value of D for uncoated, uncontaminated TiO2 powder, interpolated from qualitative results, was ∼25 eV. This is within 25% of the predicted universal constant (25.2 eV) for transition metals and alloys. Coating layer thicknesses were calculated from XRF and XPS data, the latter assuming an exponential attenuation model with superficial carbon. Based on the value of D, Ti 2p photoelectron mean path lengths (R) also were calculated from the experimental equation by Tougaard. We found good correlation between the results from conventional XPS layer calculations and the Tougaard equation. The methods are independent of each other. Together, they seemed to give more reliable information on the non-ideal powder surfaces studied than either of the methods alone.