The pH dependence of the surface charge density in oxide-based semiconductor nanoparticles immersed in aqueous solution

A new approach to calculate the surface charge density in semiconductor metal-oxide quantum dots (QDs) dispersed in alkaline aqueous colloids, at different pH values, is presented. Surface charge density up to about 0.3 C/m² was obtained for spherical nanoparticles in the diameter range of 8 to 14 nm, at pH 10 and 12. The calculated surface charge density, in very good agreement with the average value reported in the literature, is obtained as long as a proton transfer mechanism through the semiconductor-electrolyte interface takes place. The data reported here explain the stability found in ionic magnetic fluids at the usual lower/higher pH values and particle sizes