Synergistic oxygen evolving activity of a TiO2-rich reconstructed SrTiO3(001) surface.

In addition to composition, the structure of a catalyst is another fundamental determinant of its catalytic reactivity. Recently, anomalous Ti oxide-rich surface phases of ternary oxides have been stabilized as nonstoichiometric epitaxial overlayers. These structures give rise to different modes of oxygen binding, which may lead to different oxidative chemistry. Through density functional theory investigations and electrochemical measurements, we predict and subsequently show that such a TiO2 double-layer surface reconstruction enhances the oxygen evolving activity of the perovskite-type oxide SrTiO3. Our theoretical work suggests that the improved activity of the restructured TiO2(001) surface toward oxygen formation stems from (i) having two Ti sites with distinct oxidation activity and (ii) being able to form a strong O-O moiety (which reduces overbonding at Ti sites), which is a direct consequence of (iii) having a labile lattice O that is able to directly participate in the reaction. Here, we demonstrate the improvement of the catalytic performance of a well-known and well-studied oxide catalyst through more modern methods of materials processing, predicted through first-principles theoretical modeling.

Atomic and Electronic Structure of the BaTiO(3)(001) (sqrt[5] × sqrt[5])R26.6° Surface Reconstruction.

This contribution presents a study of the atomic and electronic structure of the (sqrt[5] × sqrt[5])R26.6° surface reconstruction on BaTiO(3) (001) formed by annealing in ultrahigh vacuum at 1300 K. Through density functional theory calculations in concert with thermodynamic analysis, we assess the stability of several BaTiO(3) surface reconstructions and construct a phase diagram as a function of the chemical potential of the constituent elements. Using both experimental scanning tunneling microscopy (STM) and scanning tunneling spectroscopy measurements, we were able to further narrow down the candidate structures, and conclude that the surface is either TiO(2)-Ti(3/5), TiO(2)-Ti(4/5), or some combination, where Ti adatoms occupy hollow sites of the TiO(2) surface. Density functional theory indicates that the defect states close to the valence band are from Ti adatom 3d orbitals (≈ 1.4 eV below the conduction band edge) in agreement with scanning tunneling spectroscopy measurements showing defect states 1.56 ± 0.11 eV below the conduction band minimum (1.03 ± 0.09 eV below the Fermi level). STM measurements show electronic contrast between empty and filled states' images. The calculated local density of states at the surface shows that Ti 3d states below and above the Fermi level explain the difference in electronic contrast in the experimental STM images by the presence of electronically distinctive arrangements of Ti adatoms. This work provides an interesting contrast with the related oxide SrTiO(3), for which the (001) surface (sqrt[5] × sqrt[5])R26.6° reconstruction is reported to be the TiO(2) surface with Sr adatoms.

Surface Pyroelectricity in Cubic SrTiO3.

Symmetry-imposed restrictions on the number of available pyroelectric and piezoelectric materials remain a major limitation as 22 out of 32 crystallographic material classes exhibit neither pyroelectricity nor piezoelectricity. Yet, by breaking the lattice symmetry it is possible to circumvent this limitation. Here, using a unique technique for measuring transient currents upon rapid heating, direct experimental evidence is provided that despite the fact that bulk SrTiO3 is not pyroelectric, the (100) surface of TiO2 -terminated SrTiO3 is intrinsically pyroelectric at room temperature. The pyroelectric layer is found to be ≈1 nm thick and, surprisingly, its polarization is comparable with that of strongly polar materials such as BaTiO3 . The pyroelectric effect can be tuned ON/OFF by the formation or removal of a nanometric SiO2 layer. Using density functional theory, the pyroelectricity is found to be a result of polar surface relaxation, which can be suppressed by varying the lattice symmetry breaking using a SiO2 capping layer. The observation of pyroelectricity emerging at the SrTiO3 surface also implies that it is intrinsically piezoelectric. These findings may pave the way for observing and tailoring piezo- and pyroelectricity in any material through appropriate breaking of symmetry at surfaces and artificial nanostructures such as heterointerfaces and superlattices.

Coexisting surface phases and coherent one-dimensional interfaces on BaTiO3(001).

Coexistence of surface reconstructions is important due to the diversity in kinetic and thermodynamic processes involved. We identify the coexistence of kinetically accessible phases that are chemically identical and form coherent interfaces. Here, we establish the coexistence of two phases, c(2 × 2) and c(4 × 4), in BaTiO3(001) with atomically resolved Scanning Tunneling Microscopy (STM). First-principles thermodynamic calculations determine that TiO adunits and clusters compose the surfaces. We show that TiO diffusion results in a kinetically accessible c(2 × 2) phase, while TiO clustering results in a kinetically and thermodynamically stable c(4 × 4) phase. We explain the formation of domains based on the diffusion of TiO units. The diffusion direction determines the observed 1D coherent interfaces between c(2 × 2) and c(4 × 4) reconstructions. We propose atomic models for the c(2 × 2), c(4 × 4), and 1D interfaces.

Influence of nitrogen doping on the defect formation and surface properties of TiO2 rutile and anatase.

Nitrogen doping-induced changes in the electronic properties, defect formation, and surface structure of TiO2 rutile(110) and anatase(101) single crystals were investigated. No band gap narrowing is observed, but N doping induces localized N 2p states within the band gap just above the valence band. N is present in a N(III) valence state, which facilitates the formation of oxygen vacancies and Ti 3d band gap states at elevated temperatures. The increased O vacancy formation triggers the 1 x 2 reconstruction of the rutile (110) surface. This thermal instability may degrade the catalyst during applications.