BaO-terminated Surface Research Articles

The electronic structure and the oxygen adsorption at BaO terminated surface of GdBaCo2O5.5: A first principles study

The double perovskites GdBaCo2O5.5 (GBCO) is candidate cathode materials for application in intermediate temperature solid oxide fuel cells (IT-SOFCs), attributing to excellent electrical conduction properties and oxidation activity. The electronic structure and the oxygen adsorption at BaO terminated surface of GBCO are investigated using projector augmented-wave (PAW) method based on spin-polarized density functional theory (DFT) with the inclusion of on-site Coulomb interaction. The BaO-terminated surface is adopted according to recent experimental results that this termination is energetically favorable for the double perovskites GBCO. We predict that G-AFM configuration is the most stable structure. The calculated results of oxygen vacancy energy indicate that the lowest energy vacancy positions are in the GdO layers. Charge transfer for the adsorbed oxygen and the surface ions is also examined by Bader charge analysis. The oxygen vacancies can facilitate oxygen adsorption and catalyze the bond cleavage. The existence of oxygen vacancy strengthens surface activity by acting as an active site at the surface. These results at the atomic level can contribute to the detailed understanding of oxygen reduction reaction mechanism.

Ab initio hybrid DFT calculations of BaTiO3 bulk and BaO-terminated (001) surface F-centers

Using a supercell model and a hybrid B3PW exchange-correlation functional, we have performed first principles calculations for the F-center in the BaTiO3 bulk and on the BaO-terminated (001) surface. We find that two Ti atoms nearest to the bulk F-center are repulsed, while nearest eight oxygen and four barium atoms relax toward the oxygen vacancy (by 1.06, 0.71 and 0.08% of the lattice constant [Formula: see text], respectively). The magnitudes of atomic displacements around the F-center located on the BaO-terminated (001) surface in most cases (except for Ti) are larger than those around the bulk F-center (0.1, 1.4 and 1.0% of [Formula: see text], respectively). Our calculated BaTiO3 bulk [Formula: see text]–[Formula: see text] bandgap of 3.55 eV is in an acceptable agreement with the respective experimental bandgap value of 3.2 eV. The pristine BaO-terminated (001) surface [Formula: see text]–[Formula: see text] bandgap (3.49 eV) is reduced with respect to the bulk bandgap value. The bulk and BaO-terminated (001) surface F-center bands in BaTiO3 matrix are located only at 0.23 eV and 0.07 eV under the conduction band (CB) bottom, indicating that the F-center is a shallow donor. The F-center in the BaTiO3 bulk contains charge of 1.103[Formula: see text], whereas slightly less charge, only 1.052[Formula: see text], are localized inside the F-center on the BaO-terminated (001) surface. Our calculations demonstrate considerable increase of the chemical bond covalency between the BaTiO3 bulk F-center and its two nearest Ti atoms equal to 0.320[Formula: see text], and even larger increase for BaO-terminated (001) surface F-center and its nearest Ti atom 0.480[Formula: see text], in comparison to the relevant Ti–O chemical bond covalency in the perfect BaTiO3 bulk 0.100[Formula: see text]. The difference between F-center formation energy in BaTiO3 bulk (10.3 eV) and on the BaO-terminated (001) surface (10.2 eV) trigger the segregation of the F-center from the bulk toward the BaO-terminated (001) surface.

Theoretical Approach to the CH4 Decomposition on BaTiO3(001)

The catalytic activity of BaTiO3(001) to the methane decomposition was analyzed by employing density functional theory (DFT) with an idealized model. As it was expected, the “clean” surfaces of BaTiO3(001), TiO2-terminated and BaO-terminated showed low catalytic activity mainly due to the high energy required to break the C-H bonds of the methane molecule. Our results show that the TiO2-terminated surface requires almost six times less energy to break the first C-H bond and is approximately four times more active for the total decomposition of methane than the BaO-terminated surface. To our knowledge, to date there is no theoretical study focused on the methane decomposition on the BaTiO3(001) surfaces. This study will provide insight of the catalytic behavior of the surfaces of BaTiO3(001).

Theoretical Study of Inorganic Carbonaceous Species Reaction with the Surfaces of BaTiO3(001)

Due to the variety of component elements and their high chemical stability at elevated temperatures, various perovskite oxide materials have been used to replace catalyst containing precious metals as oxidation catalysts or oxygen-activated catalysts (1). Additionally, because of their oxygen mobility through the material the C accumulation is not promoted. Such is the case of the resistance towards C deposition of Ni/BaTiO3-Al2O3 when employed as catalyst for low temperature dry reforming of CH4 (2). During the CH4 reforming, the C deposition is caused mainly due the CH4 decomposition or the CO disproportionation reaction that will lead to the formation of CO2 and C. On the other hand, one of the drawbacks of some perovskite materials such as BaTiO3 is that in the presence of CO2 the formation of a localized stable carbonate phase on the surface is possible (3). This carbonate phase will affect not only the catalytic activity of the material, but also may influence in its electronic properties. In this study the density functional theory (DFT) will be employed to analyze from a theoretical point of view the interaction of CO2 and the CO disproportionation reaction on the surfaces of BaTiO3(001). Our first approach with the BaO-terminated surface revealed that the CO molecule chemisorbs on the surface O atom. Additionally, if another CO molecule from the gas phase will interact with the already adsorbed CO molecule (Eley-Rideal reaction mechanism), the formation of trioxydehydroethene showed high spontaneity and the least stable but still probable to occur is the formation of CO2 and C that chemisorbs on top of an O atom (disproportionation reaction). Moreover, for the CO2 reaction with the BaO-terminated surface, the formation of CO3 anion showed to be quite stable. Furthermore, the CO2 reaction and CO disproportionation reaction will be also analyzed for the TiO2-terminated surface. References J. T. S. Irvine, in Perovskite Oxide for Solid Oxide Fuel Cells, T. Ishihara, Ed., p. 167, Springer, Fukuoka (2008).X. Li, Q. Hu, Y. Yang, Y. Wang, and F. He, Appl. Catal. A, 413– 414, 163, (2012).M. Wegman, L. Watson, A. Hendry, J. Am. Ceram. Soc., 87, 371 (2004).

Theoretical Study of Inorganic Carbonaceous Species Reaction with the Surfaces of BaTiO3(001)

In this study, the density functional theory was employed to analyze from a theoretical point of view the reaction of CO and CO2 with the surfaces of BaTiO3(001). Our results showed that the adsorption of CO is more stable on the TiO2-terminated than on the BaO-terminated slab. Additionally, this interaction led to the stable formation of CO3 on the TiO2-terminated slab. Similarly, the chemisorption of CO2 on both surfaces showed to be high stable, and it led to the CO3 formation on the BaOT slab. Moreover, in order to investigate the possibility of carbon deposition, the CO disproportionation reaction was considered to occur via the Eley-Rideal reaction mechanism. However, it was observed that the carbon deposition is quite energetically demanding, more challenging for the BaO-terminated surface than for the TiO2-terminated, and might not occur at the operating conditions of the solid oxide fuel cells.

Theoretical Approach to the CH4 Decomposition on BaTiO3(001)

Oxide configurations with perovskite structures exhibit some interesting and controllable properties. BaTiO3 compounds have attracted considerable attention due to their potential use as electric and optical applications (1). Additionally, BaTiO3 has been studied for its catalytic properties in the oxidative coupling of methane (2), partial oxidation of methane (3) and for the total oxidation of methane (4) showing some promising results. Moreover, the presence of BaTiO3 on the anode material showed to suppress the C deposition for the low temperature dry methane reforming (5). In a similar way, BaTiO3 has been used as anode for solid oxide fuel cells (SOFCs) exhibiting quite low performance when methane was employed as fuel, but its performance increased in the presence of sulfur containing fuel (6). Although, this material showed some curious properties when used partially or totally as anode for SOFCs to date there are only speculations regarding its potential. In several occasions theoretical studies have been used to elucidate the phenomena behind some exciting properties such as catalytic behaviour. However, this is not any easy task since heterogeneous catalytic processes are complex to model due to the absence of information on the molecule-surface interaction, and the effect that the operating conditions and catalyst preparation have on its performance. In this work an idealized model will be used to describe the methane decomposition on the surfaces of BaTiO3(001) employing density functional theory (DFT) method. Preliminary results show that the TiO2-terminated surface is approximately four times more active for the total decomposition of methane than the BaO-terminated surface. References S. Saha, T. P. Sinha, and A. Mookerjee, Phy. Rev. B, 62, 8828, (2001). M. Teymouri, and E. Bagherzadeh, J. Mat. Sci., 30, 3005, (1995). C. Guo, J. Zhang, W. Li, P. Zhang, and Y. Wang, Cat. Today, 98, 583, (2004). I. Popescu, A. Urda, T. Yuzhakova, I. Marcu, J. Kovacs, and I. Sandulescu, C. R. Chimie, 12, 1072, (2009). X. Li, Q. Hu, Y. Yang, Y. Wang, and F. He, Appl. Catal. A: General, 413– 414, 163, (2012). J. H. Li, X. Z. Fu, J. L. Luo, K. T. Chuang and A. R. Sanger, J. Power Sources, 213, 69 (2012).

Proton conduction at BaO-terminated (001) BaZrO3 surface using density functional theory

We studied proton conduction at a BaO-terminated (001) BaZrO3 surface by using density functional theory. In order to evaluate the proton conductivity at the surface, the space charge layer and structural disorder models were introduced. In the BaO-terminated (001) BaZrO3 surface, a positively-charged proton and an oxygen vacancy were segregated at the surface layer with segregation energies of −0.96 and −0.42eV, respectively; this results in the generation of the Schottky barrier height. The migration energy barrier of the proton at the surface was 1.03eV, and was converted to the proton mobility at the surface. Based on the concentration and mobility of the proton, its conductivity at the surface was evaluated. We find that the surface impedes the proton conduction by six orders of magnitude more than bulk at 900K.

First-principles study of the (001) surface of half-metallic cubic BaCrO3

First-principles calculations have been performed to study the (001) surfaces of half-metallic cubic BaCrO3 with BaO and CrO2 terminations by employing density function theory under the generalized gradient approximation (GGA)+U (Hubbard parameter). Surface relaxations reveal that the CrO2-terminated surface could be rougher than the BaO-terminated surface and an oscillatory (−+−) damping (|Δd12|>|Δd23|>|Δd34|) relaxation phenomenon appears for both the BaO-termination and the CrO2-termination. Meanwhile, the grand thermodynamic potential F indicates that only the BaO-terminated surface can exist steadily in the range of the accessible values of the CrO2 chemical potential. The electronic structure calculations predict that BaO-terminated surface preserves the bulk's half-metallic property. For the CrO2-terminated surface, surface states can be identified which result in the deterioration of its half-metallicity.

Interface-dependent magnetic anisotropy of Fe/BaTiO3: A first principles study

Using first principles calculations, we investigated interface structure effects on the magnetic properties of the Fe/BaTiO3 system. On the BaO-terminated surface, a Fe monolayer is formed as two Fe atoms are adsorbed on the top sites of Ba and O in the (1×1) surface unit and a Fe monolayer (ML) is formed on the TiO2-terminated surface as two Fe atoms are adsorbed on the two O top sites. The magnetic anisotropy energy of Fe was higher on the TiO2–terminated surface (1.5 eV) than on the BaO-terminated surface (0.5 eV). The decomposed electron density of the states showed that the stronger hybridization of Fe with the TiO2 layer than with the BaO layer is the most important reason for the higher magnetic anisotropy energy.

Vacancy-induced magnetism in BaTiO3(001) thin films based on density functional theory

The origin of magnetism induced by vacancies on BaTiO(3)(001) surfaces is investigated systematically by first-principles calculations within density-functional theory. The calculated results show that O vacancy is responsible for the magnetism of the BaO-terminated surface and the magnetism of the TiO(2)-terminated surface is induced by Ti vacancy. For the BaO-terminated surface, the magnetism mainly arises from the unpaired electrons that are localized in the O vacancy basin. In contrast, for the TiO(2)-terminated surface, the magnetism mainly originates from the partially occupied O-2p states of the first nearest neighbor O atoms surrounding the Ti vacancy. These results suggest the possibility of implementing magneto-electric coupling in conventional ferroelectric materials.

Surface properties of the (001) surface of cubic BaMnO3: A density functional theory study

We have theoretically investigated surface properties of the (001) surface in cubic barium manganese (BaMnO3) by the full-potential linear augmented plane wave methods within the local spin-density approximation. We present and discuss the electronic properties of the (001) surface of cubic BaMnO3 with BaO- and MnO2-terminations. Surface structure, Mulliken effective atomic charges, surface energies and stability, band structure, and partial density of states have been obtained. For the BaO-terminated surface, we find that all layer atoms relax inward (toward the central layer), while for the MnO2-terminated surface all layer atoms relax outward (toward the vacuum). The largest relaxations emerge on the first-layer atoms on the two terminations. The surface rumpling of the BaO-terminated is much larger than that of the MnO2-terminated surface. Based on the results of the calculated surface energies and stability, we obtain that only the BaO-terminated surface can exist in the (001) surface of cubic BaMnO3. From the analysis of their band structure, we can see that the BaO-terminated surface has obvious half-metallic character, compared with the bulk materials and the MnO2-terminated surface.

Surface rumples and band gap reductions of cubic BaZrO3 (001) surface studied by means of first-principles calculations

Electronic properties of the (001) surface of cubic BaZrO3 with BaO and ZrO2 terminations have been studied using first-principles calculations. Surface structure, partial density of states, band structure and surface energy have been obtained. We find that the largest relaxation appears in the first layer of atoms, and the relaxation of the BaO-terminated surface is larger than that of the ZrO2-terminated surface. The surface rumpling of the BaO-terminated surface is also larger than that of the ZrO2-terminated surface. Results of surface energy calculations reveal that the BaZrO3 surface is likely to be more stable than the PbZrO3 surface.

Ab initio calculation of Ag monolayer adhesion on BaTiO 3 (100) surfaces

We use ab initio plane waves ultrasoft pseudopotential method to investigate monolayer of Ag deposited on the BaTiO 3 (100) surface. Both BaO- and TiO 2-terminated substrates are considered and the atomic configurations of Ag/BaTiO 3 interface have been fully relaxed. It is found that the O site on TiO 2-terminated surface is more favorable for Ag monolayer adsorption than the BaO site on BaO-terminated surface. The main impetus for the stability of the preferred binding sites is the close proximity of Ag atoms to oxygen atoms.

Density functional simulation of theBaZrO3(011) surface structure

The atomic structure and charge redistribution of different terminations of BaZrO3 (011) surfaces have been studied using density functional simulations. We found that the O-terminated (011) flat surface had the smallest cleavage energy among (011) surfaces, but this value was still twice as large as for the formation of a pair of complimentary (001) surfaces. The density functional calculations allowed us to estimate the excess surface Gibb's free energy and to compare stability of different (011) surfaces as a function of chemical environment. In addition, we compared stability of BaZrO3 (011) surfaces with respect to BaZrO3 (001) surfaces. Within boundaries, where BaZrO3 does not decompose, only the Ba- and O-terminated (011) surfaces appeared to be stable. However, if (001) surfaces are also taken into consideration, the BaO-terminated (001) surface is the only stable surface among all considered (001) and (011) surfaces.

Ab initio study of rumpled relaxation and core-level shift of barium titanate surface

The rumpled relaxation and the core-level shift of full-relaxed BaTiO 3 (0 0 1) surface have been investigated by first-principles calculation. Based on the work function and the electric-field gradient, the right size of vacuum and the slab have been evaluated. The large displacements of ions deviated from their crystalline sites to lead to the formation of the surface rumples have been found. Some fully occupied surface oxygen p states at the top M point of the valance band and the empty surface titanium d states at the edge of the bulk conduction band are observed on the TiO 2-terminated surface. In contrast, on the BaO-terminated surface, two different core levels of the Ba 5p states shifted about 1.29 eV are induced by the bulk perovskite Ba atoms and the relaxation of surface Ba atoms, respectively. Our calculations are consistent with the experimental data.

BaTiO3 thin films grown on SrTiO3 substrates by a molecular-beam-epitaxy method using oxygen radicals

BaTiO3 (BT) thin films were grown by a molecular-beam-epitaxy method using an oxygen radical source. BaO and TiO2 layers were alternately deposited on SrTiO3 (001) (ST) substrates, and the structure of the thin films obtained was evaluated by x-ray diffraction, reflection high-energy electron diffraction, transmission electron microscopy, atomic force microscopy, and x-ray photoelectron spectroscopy. The BT thin films were oriented in the [001] direction and epitaxially grown without misfit dislocations. The lattice constants of the thin films varied with distance from the interface of BT and ST. Near the interface the a value was shorter than that for bulk BT while the c value was longer than that for bulk BT. The surface analysis indicated that adsorbed oxygen was enriched on the BaO-terminated surface in comparison with the TiO2-terminated surface.