Nucleation and growth of phosphate on metal oxide thin films

The kinetics of phosphate uptake by hematite, titania, and alumina were examined by exposing freshly prepared thin films to phosphate solutions at incremental times and subsequently analyzing the surfaces by Auger electron spectroscopy (AES), temperature programmed desorption (TPD), and atomic force microscopy (AFM). Thin film hematite exposed to a sodium phosphate solution demonstrated initially rapid phosphate chemisorption during the first 10 min of solution exposure, followed by island growth. For titania and alumina exposed to a calcium phosphate solution, the initially rapid reaction was completed after 1 and 3 h, respectively. Subsequent rapid three-dimensional growth occurred after 3-4 h for titania and 25-30 h for alumina. The trend in the rate of formation of phosphate film on titania and alumina was comparable to those of in-situ studies using the same solutions. AFM data on the 10-20-mu m scale did not show distinct changes between clean and phosphate-covered alumina and iron oxide surfaces; however, structures 200-500 nm in diameter could be observed on these surfaces after exposure to phosphate solution.