The
atomic and electronic structures of SnO2 surfaces
are closely related to their catalysis and gas detection properties.
In this study, the atomic structure of SnO2 (100) and (101)
surfaces has been directly imaged by aberration-corrected transmission
electron microscopy. Combined with density functional theory calculations
and image simulations, we show that the (100) and (101) surfaces are
terminated with Sn, with large outward relaxations. Bond valence analysis
indicates that the surface stability can be attributed to the dual
oxidation states (+4 and + 2) that are stable for Sn. For both surfaces,
the oxidation state of Sn at the surface is reduced to +2 to compensate
the surface polarity. The (301) facets that form steps at the (100)
surface are also reduced, with SnO stoichiometry and + 2 oxidation
state for Sn.
Atomic-scale structures and energetics of the Co 3 O 4 (001) surface are studied combining high-resolution transmission electron microscopy observations and density functional theory calculations. The results show that the surface remains perfectly bulk-truncated in a wide range of oxygen chemical potentials. This is in sharp contrast to Fe 3 O 4 , for which the (001) surface is stabilized by the formation of cationic point defects. Besides the well-known "subsurface cation vacancy" structure, which is stable for Fe 3 O 4 (001) at high chemical potentials of oxygen, a new structure of Fe 3 O 4 (001) involving surface interstitials and subsurface vacancies is shown here to be stable at low chemical potentials of oxygen. The different surface behaviors between Co 3 O 4 and Fe 3 O 4 can be attributed to the different formation energies of point defects.
The Mn(III) ions at Mn 3 O 4 surface are hypothesized to contribute to catalytic activity in oxygen reduction reaction. However, the surface structure and stability of Mn 3 O 4 are far less understood. Here, the atomic structures of the widespread ( 101) and (001) surfaces of Mn 3 O 4 are determined by combining aberration-corrected transmission electron microscopy and DFT calculations. The surface stabilization mechanisms and the oxidation states of Mn are revealed and correlated to the catalytic activity of the surfaces. The results show that the (101) surface undergoes a subsurface reconstruction, forming a rock-salt-type surface layer. The Mn(III) ions are in the outermost layer of the (001) surface but in the subsurface of the (101) surface. The surface partition of the Mn(III) ions provides a microscopic understanding to the observed higher catalytic activity of the (001) surface relative to the (101) surface and would contribute to further development of novel catalysts based on Mn 3 O 4 .
The determination of the atomic structure of oxide surfaces has been a challenge because of their structural complexity. In this study, the coexistence of two types of reconstructions of the spinel (100) surface and their derivatives has been revealed by aberration-corrected transmission electron microscopy experiments and density functional theory calculations. Both reconstructions have the same oxygen sublattice as the bulk, but different cation distributions. The first type involves a full substitution of subsurface Mg atoms by Al. The second type involves a surface interstitial atom and a subsurface vacancy. The two types of surfaces avoid metallization and have significantly lower energies than the structural models known to date. The flexibility in cation distribution accounts for the sensitivity of the surface structure of spinel-type oxides on preparation routes, providing insights into the understanding and controlling of surface behavior and properties.
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