Propriedades eletrônicas, estruturais e espectroscópicas do Bulk e superfícies do BaSnO3 modelados computacionalmente por teoria de funcional de densidade.

Abstract

Tin-based Perovskites are promising materials due to their physical and chemical properties that are governed by the electronic and structural characteristics of the bulk and their respective surfaces. Such properties allow BaSnO3 (BSO) to be applied as gas sensor, photocatalyst, transparent optical conductor, among other applications. Experimental studies have revealed that the cubic structure of BSO correspond to the thermodynamic ground state. However, tin-based perovskites may adopt polymorphic structures such as tetragonal, rhombohedral and orthorhombic. In this sense, the main aim of this work is the theoretical study of the main properties of BSO in the cubic phase applying the Density Functional Theory, taking into account the influence of the applied methodology in the property’s description. Besides the cubic structure, the polymorphic phases of BSO were evaluated under high pressure (0 to 30 Gpa), aiming to investigate the possibility to obtain other BSO phases. The Murnaghan, Birch-Murnaghan, Poirier-Tarantola and Vinet equations of state (EOS) were used to determine the energy-volume and pressure-volume relationships for the different structures at T = 0 K. Murnaghan's EOS indicated that the transitions of cubic phase → tetragonal → rhombohedral → orthorhombic occur at 8.98, 16.40 and 16.90 GPa, respectively, indicating the possibility of obtaining other phases for BSO. In the second step, the stoichiometric and nonstoichiometric surfaces (0 0 1), (0 1 1) and (1 1 1) of BSO were studied as regard the surface energy and stability. The most stable non-stoichiometric surface is the SnO-terminated (0 0 1), while stoichiometric (0 0 1) and (0 1 1) surfaces may exist simultaneously. This condition had a direct implication in the crystal morphology that was analyzed by the Wullf Construction and allowed the construction of a morphological map for this material. Finally, the stoichiometric surface (0 0 1) with the BaO termination was selected for applications as a water catalyst, taking into account the interaction and possible dissociation of the water molecule on such a surface. From the study of the orientation and site of water adsorption, it was found that water interacts with the surface of BSO through hydrogen. However, topological analysis has shown that the chemical bond has been broken between the surface-interacting hydrogen and the oxygen in the water. These two atoms interact from a hydrogen interaction, but the chemical bond is broken. On the other hand, the adsorbed hydrogen and the superficial oxygen of BSO form a chemical bond and, from the analysis of the band structure and density of states, the interaction and water splitting were confirmed. These results corroborate the proposed ionization of the water molecule on the BSO surface, which represents a hydroxylation process and, from an experimental point of view, becomes an even more relevant aspect concerning dye treatment and discoloration processes.