Degradation of Cd-yellow pigment: an ab initio study of defects and adsorption of oxygen and water on CdS

The cadmium yellow paints used in impressionist and modernist paintings in early 1900s are undergoing several deterioration processes, including whitening and discoloration. Relevant e↵ects produced at the surface of modern paintings include the growth of discolored crusts, formed mainly by white globular hydrated cadmium sulfate CdSO4*nH2O and cadmium carbonate (CdCO3 ). In view of the fact that the pigment, cadmium sulfide, was historically synthesized by means of dry and wet processes and that CdCO3 and CdSO4 are reagents for this procedure, their identification alone does not constitute conclusive proof of photo-oxidation. The origins of such chemical and physical alterations are still under debate. Structural defects in CdS, among other possible causes like photo-oxidation processes, may play a role in the degradation process. Their presence in the pigment surface alters the electronic structure of cadmium sulfide by forming acceptor levels in the gap of the semiconductor. Such levels make the surface more reactive in the interaction with external agents (oxygen, water ...). To this end, we present a theoretical study of points defects, namely Cd- and S- vacancies, in the structural wurtzite structure (bulk) and [10¯10] CdS surface. In order to understand, at atomic level, the oxidation and hydration mechanisms of these whitish globules, we present the early stages of the interaction between the hexagonal clean and defective [10¯10] surface of CdS and O2 and H2O molecules to simulate the combined e↵ects of exposure to air and humidity. The geometrical and electronic structures as well as the vacancy formation and adsorption energies are determined with the use of a first principles method. All the calculations are performed within the framework of the Density Functional Theory (DFT) in the Generalized Gradient Approximation (GGA-PBE) with the use of ultrasoft pseudopotentials. Despite the standard DFT-GGA ensures a correct order of electronic states, the G0W0 calculations are strongly required to attribute an accurate position of the trap level. Considering the possibility to investigate in a broader spectral range, such theoretical method would be helpful in the interpretation of experimental evidences on fluorescence emissions produced from by yellow altered. This thesis highlights the key role that first-principles methods can play in the application of materials science to art conservation.