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 fo r only a few metals, notably Cr and Fe in this study, especially at hi gh pH. Si surfaces with wet chemical oxides were exposed to a dilute s olution of ten metals: Al, Ba, Ca, Cr, Cu, Fe, K, Ni, Sr, and Zn, in d ilute nitric acid solution, from pH 3 to 5.6, for different periods of time, from about 1 to 1000 s. At equilibrium the surface concentratio n 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 ex ist as hydrated cations at that particular pH. We observe this proport ionality over the pH range under study, although the surface concentra tion varies over several orders of magnitude. Cations which form ampho teric 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 dep endence on ionic charge and radius.