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 met al-oxide quantum dots (QDs) dispersed in alkaline aqueous colloids, at diff erent pH values, is presented. Surface charge density up to about 0.3 C/m(2 ) was obtained for spherical nanoparticles in the diameter range of 8 to 14 run, at pH 10 and 12. The calculated surface charge dens in very good agre ement with the average value reported in the literature, is obtained as lon g as a proton transfer mechanism through the semiconductor-electrolyte inte rface takes place. The data reported here explain the stability found in io nic magnetic fluids at the usual lower/higher pH values and particle sizes.