Ab Initio Studies of Structure, Electronic Properties, and Relative Stability of SnO2 Nanoparticles as a Function of Stoichiometry, Temperature, and Oxygen Partial Pressure

PONCE, C.A.; CARAVACA, M.A.; CASALI, R.A.

JOURNAL OF PHYSICAL CHEMISTRY C

AMER CHEMICAL SOC

Año: 2015 vol. 119 p. 15604 - 15617

1932-7447

Relative stability of nanocrystals of rutile SnO2 with defined sizes and controlled stoichiometry was studied, allowing the full relaxations of nanoparticle (NP) atoms. Once stabilized in energy, the structure was analyzed by a refinement of the pair distribution function (PDF) of the Sn-Sn and Sn-O pairs. The calculated pair distribution functions Gcalc(r) of our NP´s larger than 3 nm indicate a rutile crystalline core of 2.4 nm size, which are well-compared with recent Gexp(r) determined from XRD experiments. Therefore, distorted surface layers are decreased to 0.25-0.30 nm width, for stoichiometric and oxygen in excess NP´s. This finding is confirmed by the inspection of the core electronic density of states (DOS), determined with the projected density of states (PDOS) of atoms included in radii sizes of 0.5, 0.7, 0.9, 1.1, and 1.5 nm. The deviation of the NP total DOS from the bulk total DOS is analyzed inspecting the PDOS contributions of cation and anion sublattices, for atoms placed in the core, in the distorted layer and in the NP surface. In particular, oxygen atoms at the surface can bond forming dimers and trimers whose electronic states can fix the NP Fermi levels and then act as active centers. The dependence of thermodynamic stability of the NP´s with oxygen pressure and temperature is studied. It was found that at normal O2 pressures and T < 860 K, the more stable case is found to be the NP with O in excess while at low O2 pressures and T > 350 K, the stable NP is the deficient in oxygen one. We also studied the changes of Mulliken populations of Sn and O atoms with respect to the bulk. We found more important variations in those atoms located close or at the surface. These changes strongly depend on NP stoichiometry. This study therefore helps to improve the understanding of the electronic and thermodynamic properties of SnO2 nanocrystals in actual environmental conditions of operation in gas sensor devices.