Electronic properties and stabilities of bulk and low-index surfaces of SnO in comparison with SnO2: A first-principles density functional approach with an empirical correction of van der Waals interactions

The electronic properties and stabilities of SnO and SnO2 bulk materials and their low-index surfaces are investigated by density functional theory. An empirical method has been adopted in this study to account for the van der Waals interactions among the Sn-O layers in the bulk and low-index surfaces of SnO. Compared with SnO2, the structural and electronic properties of SnO bulk and its low-index surfaces present some unique features due to the dual valency of Sn. In SnO, the s orbital of Sn has larger contributions than its p and d orbitals in the first valence band (VB) and the p orbital of Sn has a larger contribution than its s and d orbitals in its conduction band (CB). In SnO2, the p and d orbitals of Sn play an important role to form the upper part of the VB and its s orbital dominates in forming the lower parts of the VB and the CB. In both oxides, the s orbital of O forms the second VB with lower energy and its p orbitals are involved in forming the first VB and the CB. The calculated bulk modulus and cohesive energy agree well with the experimental measurements. By constructing all possible symmetrical low-index surfaces of SnO and the (111) surface of SnO2, our results reveal that the calculated surface energies of SnO stoichiometric surfaces are lower than that of the corresponding surfaces of SnO2 due to different bonding between Sn and O in these two oxides. The calculated stabilities of the low-index stoichiometric surfaces of SnO are in the order (001)>(101)/(011)>=(010)/(100)>(110)>(111) while the order in the case of SnO2 is (110)>(010)/(100)>(101)/(011)>(001)>(111). The calculated relationships between surface free energies [gamma(p,T)] and oxygen chemical potentials [mu(O)(p,T)] indicate that the nonstoichiometric O-terminated (110) and (111) surfaces of SnO could be more stable than their corresponding stoichiometric ones when the mu(O)(p,T) reaches its higher O-rich bound, and one Sn-terminated nonstoichiometric (111) surface of SnO2 could be more stable than its stoichiometric ones when the mu(O)(p,T) falls into its lower O-poor region. During surface formation from the bulk, the stable surface usually has small atom displacements. For both SnO and SnO2 the atoms on the (111) surface have larger relaxations than on their other low-index surfaces.