Adsorption of metal ions onto hydrophilic silicon surfaces from aqueous solution: Effect of pH

This work examines the pH and time dependence of metal deposition onto hydrophilic silicon surfaces. We find that the surface concentration of metal species is proportional to the pH. Time dependence is seen for only a few metals, notably Cr and Fe in this study, especially at high pH. Si surfaces with wet chemical oxides were exposed to a dilute solution of ten metals: Al, Ba, Ca, Cr, Cu, Fe, K, Ni, Sr, and Zn, in dilute nitric acid solution, from pH 3 to 5.6, for different periods of time, from about 1 to 1000 s. At equilibrium the surface concentration of metals is approximately proportional to the ratio of ionic charge to ionic radius of the metal ions for many of the metal ions which exist as hydrated cations at that particular pH. We observe this proportionality over the pH range under study, although the surface concentration varies over several orders of magnitude. Cations which form amphoteric species, such as Al3+ which forms Al2O3 and Al(OH)(4)(-) at high pH, and several other cations that do not exist as simple cations at some pHs (e.g., Fe3+, Zn2+, and Ni2+ at high pH), do not show this dependence on ionic charge and radius.