First‐principles study of the (001) surface of cubic PbTiO3

We present first‐principles calculations on the (001) surfaces of cubic PbTiO₃ with PbO and TiO₂ terminations. The cleavage energy, surface energy, surface grand potential, surface relaxation and surface electronic structure have been investigated by using the projector‐augmented wave method under generalized gradient approximation (GGA). The results show that surface energy of a TiO₂‐terminated surface is a little lower than that of a PbO‐terminated one, thus allowing both terminations to coexist. The PbO‐termination is stable in O‐ and Pb‐rich environments, while on the contrary, the TiO₂‐termination is stable in O‐ and Pb‐poor conditions. In addition, the surface rumpling S of a PbO‐terminated surface is slightly larger than that of a TiO₂‐terminated one. The relaxations dominantly take place on the outermost three layers, and an oscillatory (? + ?) damping (|Δd₁₂ | > | Δd₂₃ | > | Δd₃₄|) relaxation phenomenon appears for both terminations. The band gaps of both PbO‐ and TiO₂‐terminations are slightly lower than that of the bulk. Moreover, the DOS curves of each layer show that for the TiO₂‐termination, the top of the valence band of the first and third TiO₂ layers moves toward Fermi level. Copyright © 2008 John Wiley & Sons, Ltd.     

First-principles Study on the Electronic and Bonding Properties of PbTiO3 (110) Polar Terminations

The electronic structures and bonding properties of the (110) polar terminations of cubic PbTiO₃ were examined by the first-principles calculations at the generalized gradient approximation level. Two stoichiometric (PbTiO and O₂) and three nonstoichiometric(TiO, Pb, and O) terminations were considered in this study. With the aid of the calculated electron density differences, atomic charges, band structures, and densities of states, the charge redistributions and electronic properties were evaluated in detail. Furthermore, based on the calculated results of the cleavage energies, relaxation energies, and surface energies of the investigated terminations, the charge compensation by the modification of the surface stoichiometry and the fillings of surface states were thermodynamically evaluated.     

Pt adsorption on the PbTiO3(110) polar surface: a density functional theory study

The monolayer (ML) and submonolayer Pt on both terminations of PbTiO₃(110) polar surface have been studied by using density functional theory (DFT) with projector‐augmented wave(PAW) potential and a supercell approach. The most favored ML Pt arrangements on PbTiO and O₂ terminations are the hollow site and the short‐bridge site, respectively. By examining the geometries of different ML arrangements, we know that the dominant impetus for stability of the favored adsorption site for PbTiO termination is the Pt–Ti interaction (mainly from covalent bonding), while that for O₂ termination is the Pt–O interaction (mainly from ionic bonding). In addition, the appearance of the gap electronic states in the outermost layers of each termination indicates that a channel for charge transfer between adsorbed layer and substrate is formed. Moreover, the interface hybridization between Pt 5d and O 2p orbitals is also observed, especially for ML Pt on O₂ termination. The stability sequences for various arrangements of 1/2 ML Pt adsorption conform well with those of ML Pt adsorption, and the most stable arrangement is energetically more favorable than the corresponding ML coverage in the view of adsorption energy maximization. The behavior, i.e. the increase in adsorption energy with decrease in coverage, indicates that Pt? Pt interactions weaken those between Pt and the substrate. Copyright © 2009 John Wiley & Sons, Ltd.     

First-principles study of the atomic structures,electronic properties,and surface stability of BaTiO3 (001) and (011) surfaces

The atomic structures, electronic properties, and surface stability of (001) and (011) surfaces of BaTiO₃ are studied by first-principles calculations. Four differently terminated BaTiO₃ surfaces are considered in this study, including (001)-BaO, (001)-TiO₂, (011)-BaTiO, and (011)-O₂ terminations. The relaxations and rumplings are calculated and discussed, finding that the first layer relaxes inwards, while the second layer relaxes outwards for (001) and (110) surfaces. The data obtained for electronic properties show that O_2p states in (001)-BaO/(001)-TiO₂ termination shift to the lower/higher energy region, leading to a wide/narrow band gap. And the new produced surface states are observed in (011) surface terminations, which is mainly attributed to the supplied electrons from outermost surface atoms, even O atoms are oxidized. Furthermore, the (001) surface of BaTiO₃ is found to be more stable than the (011) surface according to the predicted surface energy which is 0.86 and 2.92 J/m² for (001) and (011) surfaces, respectively. Of which, BaO termination is predicted to be more likely to cleavage from the (001) direction than the TiO₂ termination is.     

O deficiency in the rutile TiO2 (110) surface: Ab initio quantum chemical investigation of the electronic properties

The (110) surface of rutile TiO₂ (110) has been modeled using a density functional theory (DFT) plane‐wave pseudo‐potential method (CASTEP). In this study, 6 and 9 atomic‐layer slabs have been examined. The stoichiometric surface converges to a low‐spin solution in both cases with a density of states (DOS) similar to that for the bulk. O deficiencies are introduced by the removal of neutral O atoms thus leaving a neutral model with a surfeit of 2 e^? per vacancy. This results in the partial filling of the Ti t_2g conduction band orbitals and a compensatory shift in the Fermi level. The reduced surface converges to a high‐spin solution in all cases, with the excess spin located within the previously unoccupied Ti t_2g orbitals. Removal of the bridging surface O atoms results in an excess spin of 2 electrons per unit cell with approximately one‐half that for removal of in‐plane surface O atoms and subsurface O atoms. The removal of O atoms from the surface leads to an increase of the band gap, with the largest increase due to the removal of in‐plane 3‐fold coordinated surface O atoms, and the smallest one due to the removal of subsurface O atoms. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Determination of ZnO(0001) surface termination by x‐ray photoelectron spectroscopy at photoemission angles of 0° and 70°

We present an XPS method to determine the termination of the ZnO(0001) surface. By measuring O 1s and Zn 2p_3/2 core‐level x‐ray photoelectron spectra at photoemission angles of 0° and 70° and comparing the intensity ratio (I_O1s/I_Zn2p3)_θ=0/(I_O1s/I_Zn2p3)_θ=70, the Zn and O termination can be distinguished. Calculations show that these two terminations have intensity ratios differing by ～17%. This difference is not affected by a contamination layer provided that the contamination layer thickness is the same for these two differently terminated surfaces. Although this determination method prefers a clean ZnO(0001) surface (in situ measurement), it seems also feasible for surfaces with known contamination layer thickness (ex situ measurement). We have measured ex situ ZnO(0001)‐Zn, ZnO(000&1macr;)‐O single crystals and an epitaxial ZnO film deposited on Al₂O₃(0001). The measured intensity ratios of the first two samples agree with the calculated values for a 0.2 and 0.26 nm contamination layer, respectively. The intensity ratio and the O 1s contamination component intensity of the epitaxial ZnO film are close to those of the ZnO(0001)‐Zn single crystal thus pointing at Zn termination of the film. Copyright © 2004 John Wiley & Sons, Ltd.     

First principles calculations of oxygen reduction reaction at fuel cell cathodes

The efficiency of solid oxide fuel cells (SOFC) depends critically on materials, in particular for the cathode where the oxygen reduction reaction (ORR) occurs. Typically, mixed conducting perovskite ABO₃-type materials are used for this purpose. The dominating surface terminations are (001) AO and BO₂, with the relative fractions depending on materials composition and ambient conditions.Here, results of recent large-scale first principles (ab initio) calculations for the two alternative polar (La,Sr)O and MnO₂ (001) terminations of (La,Sr)MnO₃ cathode materials are discussed. The surface oxygen vacancy concentration for the (La,Sr)O termination is more than 5 orders of magnitude smaller compared to MnO₂, which leads to drastically decreased estimated ORR rates. Thus, it is predicted for prototypical SOFC cathode materials that the BO₂ termination largely determines the ORR kinetics, although with Sr surface segregation (long-term degradation) its fraction of the total surface area decreases, which slows down cathode kinetics.     

Synthesis of Stoichiometric and Bulk CrN through a Solid‐State Ion‐Exchange Reaction

Chromium mononitride (CrN) exhibits interesting magnetic, structural, and electronic properties for technological applications. Experimental reports on these properties are often inconsistent owing to differences in the degree of nonstoichiometry in CrN_x. To date, the preparation of bulk and stoichiometric CrN has been challenging; most products are in the form of a thin film produced by non‐equilibrium processes, and are often nonstoichiometric and poorly crystallized. In this work, we formulated a solid‐state ion‐exchange route for the synthesis of CrN under high pressure. The final CrN product is phase‐pure, stoichiometric, and well‐crystallized in the bulk form. Near‐stoichiometric and well‐crystallized CrN can be synthesized using the same route at atmospheric pressure, making massive and industrial‐scale production technologically feasible. The successful synthesis of stoichiometric and bulk CrN is expected to open new opportunities in diverse areas of fundamental research.     

Combined IR and XPS analysis of the native (1 0 0) surface of single‐crystalline silicon after HFaq etching

A combined analysis, based on angle‐resolved X‐ray photoelectron spectroscopy and multiple‐internal‐reflection infrared spectroscopy, of the (1 0 0) silicon surface after etching in dilute aqueous solution of HF is presented. The analysis shows that the surface is mainly formed by a heterogeneous distribution of SiH, SiH₂ and SiH₃ terminations, but contains (in addition to sub‐stoichiometric oxidized silicon) a form of reduced silicon, not consistent with the currently accepted picture of the native HF_aq‐etched surface. Copyright © 2007 John Wiley & Sons, Ltd.     

Direct Visualisation of the Surface Atomic Active Sites of Carbon-Supported Co3O4 Nanocrystals via High-Resolution Phase Restoration

The atomic arrangement of the terminating facets on spinel Co₃O₄ nanocrystals is strongly linked to their catalytic performance. However, the spinel crystal structure offers multiple possible surface terminations depending on the synthesis. Thus, understanding the terminating surface atomic structure is essential in developing high-performance Co₃O₄ nanocrystals. In this work, we present direct atomic-scale observation of the surface terminations of Co₃O₄ nanoparticles supported on hollow carbon spheres (HCSs) using exit wavefunction reconstruction from aberration-corrected transmission electron microscopy focal-series. The restored high-resolution phases show distinct resolved oxygen and cobalt atomic columns. The data show that the structure of {100}, {110}, and {111} facets of spinel Co₃O₄ exhibit characteristic active sites for carbon monoxide (CO) adsorption, in agreement with density functional theory calculations. Of these facets, the {100} and {110} surface terminations are better suited for CO adsorption than the {111}. However, the presence of oxygen on the {111} surface termination indicates this facet also plays an essential role in CO adsorption. Our results demonstrate direct evidence of the surface termination atomic structure beyond the assumed stoichiometry of the surface.     

A first-principles investigation on interactions between various surface-terminated diamond films

To elucidate the influence of different terminations on diamond surface interaction, the geometry and electronic structures of the diamond films modified by different terminations (H, F, O, NH₂, and OH) are studied by using the first principles method. Strong bonding is formed between the clean diamond surfaces, which suggest an obvious interface interaction. Both H and F terminals have significant effects on the reduction of the interface interactions. Due to the larger difference in electronegativity between C and F, the F termination layer has a higher electron density coverage to give a larger repulsive force. Therefore, the interaction between the F-terminated diamond interfaces is stronger than that between the H-terminated diamond interfaces. The O-terminated diamond surfaces are unstable. The NH₂- and OH-terminals have weak interaction due to the presence of large functional group atoms that leads to an electronic offset.     

第一性原理计算钛酸铅A位掺杂对其负热膨胀性的影响

近年来,实验发现钛酸铅基材料具有负热膨胀性,且其热膨胀程度会受到掺杂元素的影响. 目前所研究的A位掺杂体系中,仅Cd原子掺杂能使钛酸铅负热膨胀性增强. 所以研究A位掺杂钛酸铅,比较Cd原子与其他原子在掺杂钛酸铅时化学键的异同,有助于深刻理解钛酸铅负热膨胀的本质. 本文利用第一性原理,分别优化了Sr、Ba、Cd掺杂钛酸铅的晶格常数,计算了它们的态密度和电荷密度. 结果表明Cd―O键的共价性强于Pb―O键,而Ba―O键和Sr―O键几乎呈离子性,Ba/Sr对Pb的替代削弱了化合物的共价性,降低了自发极化强度. 与实验测量的热膨胀系数对比可以发现,A位原子与氧原子之间的共价性增强,化合物负热膨胀程度升高;若A位原子与氧原子之间的共价性削弱,负热膨胀程度降低. 可见A位原子与氧原子之间的共价性影响了钛酸铅基化合物负热膨胀性.     

The structural,electronic, and magnetic properties of the stoichiometric (001) surface of double perovskite Sr2FeMoO6

The structural, electronic, and magnetic properties of the stoichiometric (001) surface of double perovskite Sr₂FeMoO₆ have been studied by using a 10‐layer FeMoO₄ and SrO terminated (001)‐oriented slab model and the first‐principles projector augmented wave potential within the generalized gradient approximation as well as taking into account the on‐site Coulomb repulsive (U = 2.0 eV for Fe and 1.0 eV for Mo). An outwards relaxation is observed for several layers near surface, and the accompanying layer rumpling has a decrease tend from surface layer to inner layer. Along Fe–O–Mo–O–Fe or Mo–O–Fe–O–Mo chains, the oxygen atom is closer to the adjacent Mo atom than to the adjacent Fe atom. In FeO₆ or MoO₆ octahedra, the two axial TM?O bonds are not equal, and especially, the surface dangling bond makes the remaining one axial TM?O bond slightly shorter than four equally equatorial TM?O bonds. The half‐metallic nature and a complete (100%) spin polarization character ensure the FeMoO₄ and SrO terminated (001)‐oriented slab of double perovskite Sr₂FeMoO₆ a potential application in spintronics devices. The Fe⁺³ and Mo⁺⁵ ions are still in the (3d⁵, S = 5/2) and (4d¹, S = 1/2) states with positive and negative magnetic moments respectively and thus antiferromagnetic coupling via oxygen between them. There is no direct interaction between two nearest Fe–Fe or Mo–Mo pairs, whereas the hybridizations between Fe 3d and 4s, O 2s and 2p, as well as Mo 4d, 5s and 5p orbitals are fairly significant. Copyright © 2016 John Wiley & Sons, Ltd.     

Optical and Electrical Properties of the PLT Films with Various Compositions Deposited by Sol-Gel Process

In order to study optical and electrical properties of (Pb_1-xLa_x)TiO₃ (PLT) films with varying La concentration, the PLT films were deposited by sol-gel process. X-ray diffraction revealed that a pseudocubic phase of the PLT film became dominant with increasing La concentration due to decrease of lattice constant of c-axis. Three-dimensional atomic force microscopy images showed that the grain size and root mean square surface roughness decreased by addition of La. The optical band gap of the PLT films became wider when Pb was substituted with La. The addition of La increased the transparency of the PbTiO₃ film and shifted the threshold for initiation of absorption to shorter wavelength. Hysteresis loops of the PLT films showed that remanent polarization and coercive field decreased with increasing La concentration. In addition, we modified the surface of the PLT film with La concentration of 5% using a keV oxygen ion beam at different doses. The optical band gap of the PLT film was changed by the oxygen ion beam irradiation although the XRD patterns and the transmittance values were not significantly changed. In measuring AFM images of the surface of modified PLT film, significant changes of the grain shape and size were not found. Moreover, polarization and dielectric constant were not changed after oxygen ion irradiation. These results suggested that addition of La could affect the optical and electrical properties of the PbTiO₃ and PLT films and that surface modification by oxygen ion beam modification with 1 keV energy can change the surface property but not bulk property.     

First‐principles study of metal/nitride polar interfaces: Ti/TiN

We have examined the optimal interface structure, ideal work of adhesion and bonding character of polar Ti(110)/TiN(111) interfaces by first‐principles density‐functional plane‐wave pseudopotential calculations. Both Ti‐ and N‐terminated interfaces, including six different interface structures, were calculated. The interface structure for each termination, continuing the TiN crystal structure across the interface, has the largest work of adhesion. Although both terminations yield substantial adhesion energies in the range 3–7 J m^?2, the N‐terminated interface is ～4 J m^?2 stronger than the Ti‐terminated interface. Analysis of the interfacial electronic structure shows that the Ti‐terminated interface is a mixed strong, metallic and weak covalent character, whereas the N‐terminated interface is a polar covalent bond similar to the Ti/TiC interface. Further study of the separation of the optimal interface shows that the cleavages will never fracture at the interface due to the strong bonding, which is consistent with the experimental results. Copyright © 2003 John Wiley & Sons, Ltd.     

High-throughput first-principle calculations of the structural,mechanical, and electronic properties of cubic XTiO3 (X = Ca,Sr, Ba,Pb) ceramics under high pressure

High-throughput first-principle calculations are implemented to study the structural, mechanical, and electronic properties of cubic XTiO₃ (X = Ca, Sr, Ba, Pb) ceramics under high pressure. The effects of applied pressure on physical parameters, such as elastic constants, bulk modulus, Young's modulus, shear modulus, ductile-brittle transition, elastic anisotropy, Poisson's ratio, and band gap, are investigated. Results indicate that high pressure improves the resistance to bulk, elastic, and shear deformation for XTiO₃ ceramics. Pugh's ratios B/G reveal that CaTiO₃ and PbTiO₃ ceramics are ductile, but SrTiO₃ and BaTiO₃ ceramics are brittle under the ground state. The brittle-to-ductile transition pressures are 24.26 GPa for SrTiO₃ and 43.23 GPa for BaTiO₃. Under high pressure, the strong anisotropy promotes the cross-slip process of screw dislocations, and then enhances the plasticity of XTiO₃ ceramics. Meanwhile, XTiO₃ (X = Ca, Sr, Ba) is intrinsically an indirect-gap ceramic, but PbTiO₃ is a direct-gap ceramic. High pressure increases the band gap of XTiO₃ (X = Ca, Sr, Ba) ceramic, but decreases that of PbTiO₃ ceramic. This work is helpful for designing and applying XTiO₃ ceramics under high pressure.     

High Catalytic Efficiency of Nanostructured Molybdenum Trioxide in the Benzylation of Arenes and an Investigation of the Reaction Mechanism

The synthesis and characterization of nanostructured MoO₃ with a thickness of about 30 nm and a width of about 450 nm are reported. The composition formula of the MP (precipitation method) precursor was estimated to be [(NH₄)₂O]_0.169?MoO₃? (H₂O)_0.239. The calcination of the precursor in air afforded nanostructured pellets of the α‐MoO₃ phase. The nanostructured MoO₃ catalyst exhibited high efficiency in catalyzing the benzylation of various arenes with substituted benzyl alcohols, which were strikingly different to common bulk MoO₃. Most reactions offered >99 % conversion and >99 % selectivity to monoalkylated compounds. MoO₃ is a typical acid catalyst. However, the benzylation reaction over nanostructured MoO₃ does not belong to the acid‐catalyzed type or defect site‐catalyzed type, since the catalyst has no acidity and defect site on surface. Characterization with thermal, spectroscopic, and electronic techniques reveal that the catalyst contains fully oxygen‐coordinated MoO₆ octahedrons on the surface but partially reduced species (Mo⁵⁺) within the bulk phase. The terminal oxygen atoms of Mo?O bonds on the (010) basal plane resemble oxygen anion radicals and act as active sites for the adsorption and activation of benzyl alcohols by electrophilic attack. Such sites are indispensable for catalytic reactions since the blocking of these sites by electron acceptors, such as tetracyanoethylene (TCNE), can greatly decrease catalytic activity. This work represents a successful example of combining a heterogeneous catalysis study with nanomaterial synthesis.     

Ambient Pressure X-ray Photoelectron Spectroscopy Study of Oxidation Phase Transitions on Cu(111) and Cu(110)

The surface structure effect on the oxidation of Cu has been investigated by performing ambient-pressure X-ray photoelectron spectroscopy (APXPS) on Cu(111) and Cu(110) surfaces under oxygen pressures ranging from 10⁻⁸ to 1 mbar and temperatures from 300 to 750 K. The APXPS results show a subsequential phase transition from chemisorbed O/Cu overlayer to Cu₂O and then to CuO on both surfaces. For a given temperature, the oxygen pressure needed to induce initial formation of Cu₂O on Cu(110) is about two orders of magnitude greater than that on Cu(111), which is in contrast with the facile formation of O/Cu overlayer on clean Cu(110). The depth profile measurements during the initial stage of Cu₂O formation indicate the distinct growth modes of Cu₂O on the two surface orientations. We attribute these prominent effects of surface structure to the disparities in the kinetic processes, such as the dissociation and surface/bulk diffusion over O/Cu overlayers. Our findings provide new insights into the kinetics-controlled process of Cu oxidation by oxygen.     

Oxygen Atom Adsorption and Diffusion on Pd Low-index Surfaces and （311） Stepped Surface

Introduction Atom adsorption on transition metal surfaces has attracted special attention as a base for understanding the fundamental processes of oxidative catalysis. Particularly interesting is the adsorption and diffusion of oxygen on well-defined metal surfaces. An oxygen covered palladium surface, for example, plays a central role in several important reactions such as oxidation of carbon monoxide and ammonia. In particular, the (100), (111), (110) surfaces and the interactions with oxyge…     

Ab initio calculations of surface electronic states in indium oxide

A slab approach in the framework of ab initio calculations was applied to study surface electronic states in In₂O₃ crystal. Density functional theory (DFT) calculations were carried out employing the WIEN 2k code and using the full potential method with Augmented Plane Waves + local orbitals (APW+lo) formalism. Total and partial DOS (Density of States) were calculated for In and O atoms in two upper (110) surface layers. Comparison of total and partial DOS allowed determining a contribution of electronic states of different In and O surface atoms into formation of surface electronic spectra and corresponding chemical bonds. A dominant ionic character of chemical bonds in In₂O₃ is found. Calculations were performed for three slab models with different geometry parameters. It was shown that an optimal ratio between the whole vertical size of a supercell and the vertical size of atomic cluster has to be chosen. The size of vacuum region in the slab model influences significantly on the reliability of calculated characteristics of the surface electronic structure. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011