STUDIO COMPUTAZIONALE DELLE PROPRIETA' OTTICHE ED ELETTRONICHE DI MATERIALI A BASE DI ZnS

Several reasons have directed the last decades' research towards the study of alternatives energy sources, owing to the unavoidable exhaustion of oil reserves and the need to contain carbon dioxide emissions, i.e. one of the main responsible for green house gas effect and on-going climate change . Among the so-called renewable energy sources, the solar one appears as one of the most promising. Solar radiation can be used in different ways: solar thermal technologies, electricity production, to drive photocatalized reactions, such as the ones aimed at H2 production. H2 is an ideal energy vector: it can be produced from water, it gives, in theory, a pollutant-free combustion and it has a high energy density. These thesis is part of a Research Project of National Interest (PRIN09), aimed at the improvement of the Sulphur-Ammonia cycle. The photocatalyst currently employed in the reaction is Pt/CdS. The requirements a photocatalyst must satisfy are absorbing in the Visible region, in order to exploit the solar energy; having energetic levels properly aligned to the redox couple it must interact with and assure for wide and stable surfaces. ZnS, for instance, could be a good photocatalysts, however, due to its band gap of 3.6 eV, it absorbs in the UV region. A valence and conduction bands engineering is able to tune the electronic structure of semiconductors by doping with metal ions or by preparation of solid solutions with narrow band semiconductors. These goals can be achieved only through a combined experimental and computational approach. These work provided a wide insight into the ZnS sphalerite based materials. The use of different approaches coming from different branches of the computational chemistry and material science allowed for a deep understanding of its electronic and optical properties. The study on the surfaces was aimed at a characterization of the surfaces that the water molecules are free to interact with, without excluding those surfaces that may be present even if saturated. The same kind of studies could be carried out on the cluster optimized in this thesis work in order to use apply also a TD-DFT approach. The aspects studied in this thesis work are the ones of interest for materials that has to be used in photocatalysis applications. To provide the basis for a systematic and real improvement of their properties only a combined experimental and computational effort can be successful. For this reason the computed data have always been compared to the experimental results with the purpose to give a better understanding of the physics of the material and the processes.