First-principles study of CaTiO3 oxygen-vacancies (0 0 1) surface

The structural and electronic properties of fully relaxed CaTiO₃ oxygen-vacancies (0 0 1) surface with CaO and TiO₂ terminations are investigated by first-principles plane waves ultrasoft pseudopotential method based on local density approximation. The present results show that the direction and the magnitudes of the atomic relaxations for oxygen-vacancies surface are different from that of the perfect surface. Compared with the TiO₂-terminated oxygen-vacancy surface, the CaO-terminated oxygen-vacancy surface are likely to be observed in oxygen environment conditions. Much different from the perfect surface, the oxygen-vacancy surface becomes metallic caused by some states in the conduction band are lowered and pulled down in the band-gap region.     

Adsorption of atomic oxygen on cubic PbTiO3 and LaMnO3 (001) surfaces: A density functional theory study

We present and discuss density functional theory calculations addressing the electronic structure and energetics of isolated oxygen ad-atoms at the (001) surfaces of PbTiO₃ (PTO) and LaMnO₃ (LMO) cubic perovskites. Both AO- and BO₂-type of surface terminations are considered for each perovskite. Difference electron density analysis has been carried out for each surface to probe local electronic charge redistribution upon oxygen adsorption. Our results show that the (001) surfaces of the two perovskites behave quite differently towards oxygen adsorption. In the case of the PTO (001) surfaces, the adsorbate oxygen atom was found to form a peroxide-type molecular species along with a surface lattice oxygen atom on both PbO- and TiO₂-terminated surface facets. On the other hand, the most stable oxygen adsorption site for the LMO (001) surfaces corresponds to the one expected from a natural continuation of the perovskite lattice. Moreover, the dissociative adsorption of molecular oxygen varies from being only slightly exothermic on the PTO (001) facets to being highly exothermic on the LMO (001) facets. The AO-terminated facets, in general, showed a stronger binding to the adsorbed oxygen.     

First principle study of SrTiO3 (0 0 1) surface and adsorption of NO on SrTiO3 (0 0 1)

The results of first-principles calculations about the two possible terminations of (0 0 1) surface of SrTiO₃ perovskite and adsorption of NO on SrTiO₃ (0 0 1) surface were presented. Both surface parameters (atomic structures and electronic configurations) and adsorption parameters (bond, energy and charge) of NO on SrTiO₃ (0 0 1) surface, which have never been investigated before as far as we know were investigated using density functional theory calculations with the local-density approximation (DFT-LDA). It was found that the two possible terminations of SrTiO₃ (0 0 1) surface have large surface relaxation, which leads to surface polarization and exhibits different reactivity toward the dissociative adsorption of NO. The electronic states of TiO₂-terminated surface have larger difference than that of bulk, so it is more favorable for adsorption of NO than SrO-terminated surface.     

First-principles study for the atomic structures and electronic properties of PbTiO3 oxygen-vacancies (0 0 1) surface

The structural and electronic properties of fully-relaxed PbTiO₃ (0 0 1) oxygen-vacancy surface with PbO and TiO₂ terminations are investigated by first-principles calculations. In contrast to the perfect surface, the smaller surface rumples and interlayer distances have been found. The largest relaxation occurs on the second layer atoms not on the surface layer ones, and some in-gap Ti 3d states at about −1.1 eV below the Fermi-level are observed in the TiO₂-terminated surface caused by oxygen-vacancies. For the PbO-terminated surface, some in-gap Ti 3d states and Pb 6p states also move into the bulk midgap region to become partially occupied, and two different chemical states of the Pb 6s states were found. One is attributed to the bulk perovskite Pb atoms and another one is caused by the relaxation of surface Pb atoms. These theoretical results would give a good reference for the future experimental studies.     

First-principles study of the (0 0 1) surface of cubic SrZrO3

Density functional calculations have been used to investigate the (0 0 1) surface of cubic SrZrO₃ with both SrO and ZrO₂ termination. Surface structure and electronic structure have been obtained. The SrO surface is found to be similar to its counterpart in SrTiO₃, while there are marked differences between the ZrO₂ and TiO₂ terminations in SrZrO₃ and SrTiO₃, respectively, concerning surface relaxation and rumpling. For the ZrO₂-terminated surface of SrZrO₃, the covalency of the interaction between the outmost Zr and the O beneath is enhanced as a result of their bond contraction. The band gap reduction and the presence of the surface states are also discussed in relation with the behavior of the electrostatic potential.     

Hybrid HF–DFT modeling of monolayer water adsorption on (001) surface of cubic BaHfO3 and BaZrO3 crystals

First-principles calculations have been used to study the atomic structure, preferred sites and adsorption energies for water adsorption at different terminations of the cubic phase of perovskite-structured BaHfO₃ and BaZrO₃. By considering different initial positions of water molecules, the possibility of water dissociation has been investigated. It is demonstrated that the site selectivity and the form of adsorbed molecule can be affected by the choice of surface unit cell. Dissociative adsorption was found to be favorable for all surfaces in consideration. Hydroxylation of ZrO₂- and HfO₂-terminated surfaces is accomplished by a noticeable reconstruction of the surface structure of cubic phase towards the orthorhombic phase. Calculated atomic charges in bare and hydroxylated surfaces show that BaHfO₃ crystal is slightly more ionic than BaZrO₃.     

第一性原理研究BaTiO3(001)表面的电子结构

在密度泛函理论的基础上,采用平面波赝势方法计算了立方相BaTiO₃(001)表面的电子结构.结构优化表明最表层原子都向体内弛豫,且金属原子弛豫幅度最大,同时各层层间距变化呈交错分布.对两种表面结构的总能计算发现TiO₂表面稳定性比BaO表面弱,一方面是由于TiO₂表面结构中存在O-2p表面态,使价带和导带中电子态向高能区域偏移.另一方面,TO₂表面附近Ti—O共价键存在强弱差异,有利于发生表面吸附.而在BaO表面结构中,最表层BaO的存在消除了这种差异,因而其表面稳定性较强. 关键词： 第一性原理 钛酸钡 电子结构 表面能     

Quantitative determination of the local structure of H2O on TiO2(1 1 0) using scanned-energy mode photoelectron diffraction

O 1s scanned-energy mode photoelectron diffraction has been used to determine the local structure of molecular water on TiO₂(1 1 0). The adsorption site is found to be atop five-fold coordinated surface Ti atoms, confirming the results of published total energy calculations and STM imaging. The Ti-O_w bondlength is found to be 2.21 ± 0.02 Å, much longer than Ti-O bondlengths in bulk TiO₂ and for the formate (HCOO-) species adsorbed on this surface. This is consistent with relatively weak bonding, and in general agreement with total energy calculations, although all of the published calculations yield bondlengths somewhat longer than the experimental value. Structural optimisation based on the photoelectron diffraction data also provides some information on the associated substrate relaxation. In particular, the bondlength of the five-fold coordinated surface Ti atom to the O atom directly below shows the same contraction (relative to the bulk) as is found for the clean surface, reinforcing the picture of rather weak bonding of the water to this same Ti surface atom.     

Ab initio study of shear strain effects on ferroelectricity at PbTiO3 thin films

Ferroelectric thin film with the perovskite ABO₃ structure have been widely used in technology applications, e.g., actuators in MEMS/NEMS and nonvolatile random access memories (FeRAM). In order to clarify the effect of the shear strain on the ferroelectricity, the PbTiO₃ thin film as a typical one is chosen. The focus of this study is to put on the PbO-terminated (1? × ?1) and c(2? × ?2) surfaces and the TiO₂-terminated (1? × ?1) surface. Based on ab initio density functional theory calculations with the local density approximation, we have found out that in both the PbO and TiO₂-terminated (1? × ?1) models, the ferroelectricity in the PbO layers was enhanced under the positive shear strain while it was suppressed under the negative one. For the TiO₂ layers, the ferroelectricity was slightly enhanced and sharply suppressed under the positive and negative shear strains, respectively. In the PbO-terminated (2? × ?2) model, the AFE phase was suppressed by the FE phase under the positive shear strain while the opposite trend was found under the negative shear strain. For the PbO layers, the ferroelectricity was enhanced under the positive and negative shear strains. For the TiO₂ layers, the influence of the negative shear strain on the ferroelectricity was larger than that of the positive one. In addition, the ideal strength of the PbTiO₃ thin film with the different terminations was investigated as well.     

Periodic density functional theory studies of the VOx/TiO2 (anatase) catalysts: Structure and stability of monomeric species

Periodic density functional theory has been utilized to investigate the structure and stability of monomeric HVO_x species on anatase support. The three most stable surfaces of anatase were investigated, namely the (001), (100) and (101) surfaces. Unlike previous theoretical studies it was found that on the (001) surface vanadia species with five-coordinated vanadium atom are more stable than those with tetrahedrally coordinated vanadium atom. On the other hand, on the (100) and (101) surfaces, the vanadium atom in the vanadia species is still tetrahedrally coordinated. The stability of different VO_x/TiO₂ structures which are not fully dehydrated has been systematically studied and the results show that the vanadia species on the three surfaces follow an order of TiO₂ (001) > TiO₂ (100) > TiO₂ (101). This can be understood from the acidity and basicity of the three anatase surfaces. The results suggest that monomeric VO_x species may be better stabilized if the support exposes more (001) surfaces. Our analyses on electronic structure of the most stable VO_x/TiO₂ structure (D₀₀₁) suggest that its bridging V–O–Ti oxygen atoms may have higher reactivity than the terminal vanadyl oxygen atoms.     

Morphology and growth modes of metal-oxides deposited on SrTiO3

Classical molecular dynamics simulations are used to examine the growth of SrO and TiO₂ thin films on SrTiO₃ (STO). In particular, the simulations consider the deposition of SrO and TiO₂ molecules at incident energies of 0.1, 0.5, and 1.0 eV/atom onto the (0 0 1) surface of STO. The role of surface termination layer (SrO vs. TiO₂) is analyzed. In the case of SrO deposition, smooth, ordered films are produced for all incident energies considered and for both surface terminations. By contrast, in the case of TiO₂ deposition, three-dimensional islands are formed under all conditions. These predictions are in good agreement with experimental data. Importantly, the simulations explain why these differing morphologies are produced for SrO and TiO₂ deposition.     

锐钛矿型TiO2(101)面原子几何及弛豫结构的第一性原理计算

采用平面波超软赝势方法计算了锐钛矿型TiO₂(101)面的表面能和表面原子弛豫结构.首先对TiO₂(101)面的6种不同的表面原子终止结构的体系总能量进行了计算,结果表明终止原子为两配位的O原子、次层为五配位的Ti原子的表面结构最为稳定.针对该表面研究了表面能和原子弛豫与模型中原子层数和真空厚度的关系,当原子层数为12层,真空厚度为0.4nm时,表面能收敛度小于0.01J/m².研究发现:表面上两配位的O原子向里移动约0.0012nm,五配 关键词： 第一性原理 2')" href="#">TiO₂ 表面结构 弛豫     

Ferromagnetism in transition-metal-doped semiconducting oxide thin films

A rather complete work on transition-metal (TM)-doped TiO₂ thin films has been done and room ferromagnetism (FM) is found in the whole series of Sc/V/Cr/Mn/Fe/Co/Ni-doped TiO₂ films. Not only is it remarkable that for the first time, FM at high temperature was achieved in TM-doped TiO₂, but also a very big magnetic moment of 4.2μ_B/atom could be obtained, and direct evidences of real ferromagnets with big domains were shown as well. A similar chemical trend was achieved in TM-doped In₂O₃ films, however, the observed magnetic moment is rather modest, with the maximal value is of only 0.7μ_B/atom for Ni-doped In₂O₃ films. As regards TM-doped SnO₂ films, observed magnetic moments could be very large, with the maximum saturation of 6μ_B per impurity atom for Cr-doped SnO₂ thin films, but it could be influenced very much depending on substrate types. On the other hand, results on TM-doped ZnO films interestingly have revealed that in these systems, the magnetism more likely resulted from defects and/or oxygen vacancies.     

First-principles study of the （001） surface of cubic PbHfO3 and BaHfO3

Using first-principles techniques,we investigate the(001) surfaces of cubic PbHfO3(PHO) and BaHfO3(BHO) terminated with both AO(A=Pb and Ba) and HfO2.Surface structure,partial density of states,band structure,and surface energy are obtained.The BaO surface is found to be similar to its counterpart in BHO.For the HfO2-terminated surface of cubic PHO,the largest relaxation appears on the second-layer atoms but not on the first-layer ones.The analysis of the structure relaxation parameters reveals that the rumpling of the(001) surface for PHO is stronger than that for BHO.The surface thermodynamic stability is explored,and it is found that both the PbO-and the BaO-terminated surfaces are more stable than the HfO2-terminated surfaces for PHO and BHO,respectively.The surface energy calculations show that the(001) surface of PHO is more easily constructed than that of BHO.     

First-principles study of bulk and surface oxygen vacancies in SrTiO<Subscript>3</Subscript> crystal

The structural and electronic properties of the neutral and positively charged oxygen vacancies (F and F⁺ centres) in the bulk and on the (001) surfaces of SrTiO₃ crystal are examined within the hybrid Hartree-Fock and density functional theory (HF-DFT) method based upon the linear combination of atomic orbital (LCAO) approach. A comparison of the formation energy for surface and bulk defects indicates a perceptible propensity for the segregation of neutral and charged vacancies to both SrO and TiO₂ surface terminations with a preference in the latter case which is important for interpretation of space charge effects at ceramic interfaces. It is found that the vacancies reveal more shallow energy levels in the band gap on surfaces rather than in the bulk, in particular, on the TiO₂ surface. The charged F⁺ centre has significantly deeper energy levels both in bulk and on the surfaces, as compared with the neutral F centre.     

Ab initio calculations of the CaTiO3 (111) polar surfaces

The stoichiometric terminations and nonstoichiometric terminations have been constructed for CaTiO₃ (111) surface. The cleavage and surface energies, surface grand potential and surface electronic structure have been calculated for the two main classes of terminations using ab initio plane wave ultrasoft pseudopotential method based on Local Density Approximation (LDA). The results show that the stoichiometric terminations are unstable compared with the nonstoichiometric terminations and the polarity compensation achieved through the modification of the surface stoichiometry is more effective than that by the anomalous filling of the surface states. In the O and Ca chemical environments, only CaO₂ and TiO terminations can be formed; the CaO₃ and Ti terminations cannot be stabilized, even in very O-rich chemical environment.     

Surface effect of natural zeolite (clinoptilolite) on the photocatalytic activity of TiO2

The surface interaction between TiO₂ and natural zeolite, clinoptilolite, has been investigated by means of transmission electron microscope (TEM), atom force microscope (AFM), X-ray diffractometer (XRD), diffuse reflectance infrared Fourier transform (DRIFT) and far Fourier transform infrared ray (FTIR) spectroscopy. And the photocatalytic degradation (PCD) rate of methyl orange (MO), a model of recalcitrant azo dye, in aqueous system has been measured to compare the photocatalytic activities of different photocatalysts. A model has been carried out to explain the incorporation between TiO₂ particles and natural zeolite. The results show that the TiO₂ particles loaded on zeolite are 50 nm or so, smaller than the pure one, and combine with zeolite via chemical force. Since the reserved adsorption ability and the existence of electron trapper, the TiO₂-zeolite performed more efficient at low initial concentration and in the later period of PCD process, as compared with pure TiO₂ nanopowders.     

Spatial and electronic structure of the Ni3P surface

To understand the catalytic effect in the Ni-Ni₃P for the growth of carbon nanostructures, the structural and electronic properties of Ni₃P surface are calculated from first-principles calculations. The calculated surface energies for the (0 0 1)-Ni₄P₄-terminated surface, the (0 0 1)-Ni₈-terminated surface, and the (1 1 0)-Ni₈-terminated surface show that the (0 0 1)-Ni₄P₄-terminated surface is energetically more stable within the allowed range of the chemical potential of P. Through the analysis of the partial density of states of Ni and P atoms in surface and bulk states, respectively, it is further found that due to the bond contractions of the surface layer, the core-level shifts of P atoms in the (0 0 1)-Ni₄P₄-terminated surface make P atoms in the Ni₃P particles act as a catalyst. Finally, the obtained results of the work function show that the (0 0 1)-Ni₄P₄-terminated surface has the largest work function when compared with the other two studied surfaces.     

First-principle calculations on optical properties of C-N-doped and C-N-codoped anatase TiO2

The electronic structures, dipole moment and optical properties of C-N-doped and C-N-codoped anatase titanium dioxide (TiO₂) are studied using the plane-wave ultrasoft pseudopotential method of density functional theory (DFT). The results revealed that the absorption coefficients of pure TiO₂ and N-doped TiO₂ are consistent with experimental values in the visible-light region. The bands originating from C/N-2p states lie in the band gap of doped TiO₂. A visible-light absorption edge red-shift can be observed. The atomic charges have changed, resulting in devation of the center of gravity of the negative electric charge from the positive electric charge in the super-cell, and their dipole moment would not be zero. The dipole moment has large influence on the optical responses in the visible region of TiO₂. Because of the small distance (0.531 nm) between C and N atoms, the covalent bond component was easily enhanced between C atom and adjacent O atom, the covalent bonds making it more difficult for the carrier transfer. Moreover, its optical absorption coefficient is going to reduce in the visible-light region. Under the condition of the larger distance (0.691 nm) between C and N atoms, their interaction can be reduced, which is beneficial to electrons transition; as a result, a significant improvement of the photocatalytic activity of TiO₂ has been found under the visible-light irradiation.