Degradation of Cd-yellow Paints: ab initio Study of the adsorption of oxygen and water on {10.0} CdS surface

The cadmium yellow paints (CdS) used in impressionist and modernist paintings in early 1900s are undergoing several deterioration processes including whitening and discoloration. A relevant effect produced at the surface of the paintings is the growth of discolored crusts formed mainly by white globular hydrated cadmium sulfate (CdSO4*nH2O) and cadmium carbonate (CdCO3). Recent studies [1,2] ascribe to an initial photo-oxidation process of CdS the input for the formation of such whitish compounds. In order to understand the oxidation and hydration mechanisms, at atomic level, of these whitish globules, we present the early stages of a theoretical study on the interaction between the hexagonal (10-10) surface of CdS and O2 and H2O molecules to simulate the combined effects of exposure to air and humidity. More specifically, we determined the favorite adsorption sites and calculated the adsorption energies of the different molecules on top of the surface. The details of the electronic structure of the interactions are given via the bonding charge analysis along with a thorough description of the geometry.