Effect of electrostatic interaction on blue shifts of optical absorption edges of SnO2 nanocrystallites

Aqueous sols having SnO₂ crystallites with average diameter less than 2 nm were prepared by ultra-filtration with a flat sheet membrane. Ultraviolet absorption measurements were carried out for the original and ultra-filtrated SnO₂ aqueous sols. Distinct blue shifts are exhibited in the spectra of both the ultra-filtrated acidic and alkaline SnO₂ aqueous sols. Mean electrostatic interaction energies of O and Sn ions in the nanoparticles are obtained from a classical point charge model of the rutile-type structure. The calculated results suggest that the increase of the interaction energy difference between the O and Sn ions as decrease of particle size may be responsible for blue shift of the ultra-filtrated SnO₂ crystallites.