Effect of lattice strain on the oxygen vacancy formation and hydrogen adsorption at CeO2(111) surface

Using first-principles calculation, the effect of lattice strain on the oxygen vacancy formation at CeO₂(111) surface has been investigated. The tensile strain facilitates the oxygen vacancy formation at the surface and the compressive strain hinders the process. This is in part due to the strengthening or weakening of the surface Ce–O bond under the lattice strain. On the other hand, a more open surface with a larger lattice constant can better accommodate the larger Ce³⁺ and thus facilitate the structural relaxation of the reduced surface. The studies on the strain effect on the atomic hydrogen adsorption at the defect-free CeO₂(111) surface show that the adsorption strength monotonously increases with the increase of the lattice strain, further confirming the tunable surface chemical activity by lattice strain.     

A density functional theory study of formaldehyde adsorption and oxidation on CeO2(1 1 1) surface

The effects of different oxygen species and vacancies on the adsorption and oxidation of formaldehyde over CeO₂(1 1 1) surface were systematically investigated by using density functional theory (DFT) method. On the stoichiometric CeO₂(1 1 1) surface, the C-H bond rupture barriers of chemisorbed formaldehyde are much higher than that of formaldehyde desorption. On the reduced CeO₂(1 1 1) surface, the energy barriers of C-H bond ruptures are less than those on the stoichiometric CeO₂(1 1 1) surface. If the C-H bond rupture occurs, CO and H₂ form quickly with low energy barriers. When O₂ adsorbs on the reduced (1 1 1) surface (O₂/O_v species), the C-H bond rupture barriers of formaldehyde are greatly reduced in comparison with those on the stoichiometric CeO₂(1 1 1) surface. If O₂ adsorbs on oxygen vacancy at sub-layer surface, its oxidative roles on formaldehyde are much similar to that of O₂/O_v species.     

Pd与CeO2(111)面的相互作用的第一性原理研究

采用基于广义梯度近似的投影缀加平面波(projector augmented wave) 赝势和具有三维周期性边界条件的超晶胞模型，用第一性原理计算方法，计算并分析了Pd在CeO₂(111)面上不同覆盖度时的吸附能，价键结构和局域电子结构. 考虑了单层Pd和1/4单层Pd两种覆盖度吸附的情况. 结果表明：1)在单层吸附时，Pd的最佳吸附位置是O的顶位偏向Ce的桥位；在1/4单层吸附时，Pd最易在O的桥位偏向次层O的顶位吸附.2) 单层覆盖度吸附时，吸附原子Pd之间的作用较强；1/4单 关键词： 三元催化剂 Pd 2')" href="#">CeO₂ 吸附 密度泛函理论     

Structural,electronic, stability and reduction properties of perovskite surfaces: The case of rhombohedral BaCeO3

The (100), (110) and (111) surfaces of rhombohedral phase BaCeO₃ perovskite with two kinds of surface terminations are investigated using a periodic DFT + U method. We show that the lowest energy for surface formation via crystal cutting (cleavage energy) corresponds to (100) terminations. Out of all studied terminations, only BaO(100) and BaCeO(110) are stable with respect to precipitation of oxide phases and metals in respective ranges of oxygen chemical potentials, whereas CeO2(100) termination is not stable with respect to CeO₂ precipitation for all temperatures and oxygen partial pressures. Analyzing the electronic properties of the surfaces, we have established that reduction of the cerium oxidation state occurs in response to the local stoichiometry (lack of surface oxygen's, etc.) rather than as a result of breaking of cerium–oxygen bonds and formation of under-coordinated cerium ions. This equally applies to cerium reduction in the case of surface vacancy formation. We have calculated the vacancy formation energies as these can be viewed as a measure of surface activity in the catalytic reaction with various adsorbates. We find that CeO2 termination of the (100) surface and modified O2 termination of the (110) surface (O termination) have the lowest vacancy formation energies.     

A density functional theory study of formaldehyde adsorption on ceria

Molecular adsorption of formaldehyde on the stoichiometric CeO₂(1 1 1) and CeO₂(1 1 0) surfaces was studied using periodic density functional theory. Two adsorption modes (strong chemisorbed and weak physisorbed) were identified on both surfaces. This is consistent with recent experimental observations. On the (1 1 1) surface, formaldehyde strongly chemisorbs with an adsorption energy of 0.86 eV to form a dioxymethylene-like structure, in which a surface O lifts from the surface to bind with the C of formaldehyde. A weak physisorbed state with adsorption energy of 0.28 eV was found with the O of formaldehyde interacting with a surface Ce. On the (1 1 0) surface, dioxymethyelene formation was also observed, with an adsorption energy of 1.31 eV. The weakly adsorbed state of formaldehyde on the (1 1 0) surface was energetically comparable to the weak adsorption state on the (1 1 1) surface. Analysis of the local density of states and charge density differences after adsorption shows that strong covalent bonding occurs between the C of formaldehyde and surface O when dioxymethylene forms. Calculated vibrational frequencies also confirm dioxymethylene formation. Our results show that as the coverage increases, the adsorption of formaldehyde on the (1 1 1) surface becomes weak, but is nearly unaffected on the (1 1 0) surface.     

Topotactical Route to Multiwalled Cerium Oxide Nanotubes from MWCNTs

The development of new strategies for synthesizing 1D cerium oxide (CeO₂) hollow nanostructures has attracted much attention in recent years due to the importance of their superior properties and highly anisotropic geometry. This study reports an unpublished route of fabricating novel multiwalled CeO_2-δ nanotubes (CeO_2-δ NTs) in which the entire volume of functionalized multiwalled carbon nanotubes (f-MWCNTs) is converted into the CeO_2-δ pseudomorph through oxidation and dehydration topotactic reactions. The stable CeO_2-δ (111) planes are topotactically grown on the curved C (002) planes, preferentially exposed along the nanotube axis. In their initial condition, the novel nanotubes consist of Ce oxyhydroxide (CeO_2-x(OH)_x) with residual carbon. When heating the air up to 500 °C, CeO_2-x(OH)_x transforms gradually by dehydration into CeO_2-δ, while the residual carbon is oxidized. Despite compositional changes, nanotubes maintain their multiwalled structural integrity up to ≈550 °C. The CeO_2-δ NTs exhibit an unusually high presence of Ce³⁺ ions and surface O vacancies, contributing to a low direct band gap ranging from 2.67 to 2.32 eV compared to their NPs counterparts (3.2 eV).     

CO<Subscript>2</Subscript> activation on single crystal based ceria and magnesia/ceria model catalysts

Novel multifunctional ceria based materials may show an improved performance in catalytic processes involving CO₂ activation and reforming of hydrocarbons. Towards a more detailed understanding of the underlying surface chemistry, we have investigated CO₂ activation on single crystal based ceria and magnesia/ceria model catalysts. All model systems are prepared starting from well-ordered and fully stoichiometric CeO₂(111) films on a Cu(111) substrate. Samples with different structure, oxidation state and compositions are generated, including CeO_2-x/Cu(111) (reduced), MgO/CeO_2-x/Cu(111) (reduced), mixed MgO-CeO₂/Cu(111) (stoichiometric), and mixed MgO-CeO_2-x/Cu(111) (reduced). The morphology of the model surfaces is characterized by means of scanning tunneling microscopy (STM), whereas the electronic structure and reactivity is probed by X-ray photoelectron spectroscopy (XPS). The experimental approach allows us to compare the reactivity of samples containing different types of Ce³⁺, Ce⁴⁺, and Mg²⁺ ions towards CO₂ at a sample temperature of 300 K. Briefly, we detect the formation of two CO₂-derived species, namely carbonate (CO₃ ^2-) and carboxylate (CO₂ ^-) groups, on the surfaces of all investigated samples after exposure to CO₂ at 300 K. In parallel to formation of the carbonate species, slow partial reoxidation of reduced CeO_2-x/Cu(111) occurs at large doses of CO₂. The reoxidation of the reduced ceria is largely suppressed on MgO-containing samples. The tendency for reoxidation of Ce³⁺ to Ce⁴⁺ by CO₂ decreases with increasing degree of intermixing between MgO and CeO_2-x. Additionally, we have studied the stability of the formed carbonate species as a function of annealing temperature.     

锐钛矿型TiO2(101)面本征点缺陷的理论研究

采用平面波超软赝势方法计算了锐钛矿型TiO₂(101)面存在本征空位和间隙点缺陷的几何结构以及缺陷形成能.首先分析了点缺陷对表面结构的影响,发现不同类型缺陷导致缺陷周围原子有不同的位移趋势：O空位的产生导致空位周围的Ti原子向空位外移动,Ti₁和Ti₂空位的产生均使O₁自发地与周围的O原子团聚,O_i原子易被周围的氧原子吸附而成键,而Ti_i2缺陷几乎 关键词： 第一性原理 2')" href="#">TiO₂ 点缺陷 表面结构     

Intrinsic polyatomic defects in Sc2(WO4)3

The intrinsic formation of polyatomic defects in Sc₂(WO₄)₃-type structures is studied by Mott Littleton calculations and Molecular Dynamics simulations. Defects involving the WO₄²⁻ tetrahedron are found to be energetically favorable when compared to isolated W and O defects. WO₄²⁻ Frenkel and (2Sc³⁺, 3WO₄²⁻) Schottky defects exhibit formation energies of 1.23 eV and 1.97 eV, respectively and therefore may occur as intrinsic defects in Sc₂(WO₄)₃ at elevated temperatures. WO₄²⁻ vacancy and interstitial migration processes have been simulated by classical Molecular Dynamics simulations. The interstitial defect exhibits a nearly 10 times higher mobility (with a migration energy of 0.68 eV), than the vacancy mechanism (with a slightly higher migration energy of 0.74 eV) and thus should dominate the overall ionic conduction. Still both models reproduce the experimental activation energy (0.67 eV) nearly within experimental uncertainty.     

p型K:ZnO导电机理的第一性原理研究

基于密度泛函理论,利用局域密度近似的第一性原理平面波赝势方法,对掺K以及含有氢填隙(H_i)、氧空位(V_O)、锌填隙(Zn_i)和锌空位(V_Zn)的K:ZnO电子结构分别进行了研究.结果表明,1) 单独掺K可引入浅受主,但系统总能增高;2) K与H共掺可降低系统总能,提升稳定性;3) V_O在K+H:ZnO中的形成比Zn_i困难得多,二者都是 关键词： 氧化锌 p型 第一性原理 电子结构     

Adsorption and decomposition of H2O on a K-covered Pt(111) surface

The adsorption of H₂O on clean and K-covered Pt(111) was investigated by utilizing Auger, X-ray and ultra-violet photoemission spectroscopies. The adsorption on Pt(111) at 100–150 K was purely molecular (ice formation) in agreement with previous work. No dissociation of this adsorbed H₂O was noted on heating to higher temperatures. On the other hand, adsorption of H₂O on Pt(111) + K leads to dissociation and to the formation of OH species which were characterized by a work function increase, an O 1s binding energy of 530.9 eV and UPS peaks at 4.7 and 8.7 eV below the Fermi level. The amount of OH formed was proportional to the K coverage for θ_K > 0.06 whereas no OH could be detected for θ? 0.06. Dissociation of H₂O occurred already at T = 100 K, with a sequential appearance of O 1s peaks at 531 and 533 eV representing OH and adsorbed H₂O, respectively. At room temperature and above only the OH species was observed. Annealing of the surface covered with coadsorbed K/OH indicated the high stability of this OH species which could be detected spectroscopically up to 570 K. The adsorption energy of H₂O coadsorbed with K and OH on Pt(111) is increased relative to that of H₂O on Pt. The work function due to this adsorbed H₂O increases whereas it decreases for H₂O on Pt(111). The energy shifts of valence and O1s core levels of H₂O on Pt + K as deduced from a comparison of gas phase and adsorbate spectra are 2.8–4.2 eV compared to ≈ 1.3–2.3 eV for H₂O on Pt (111). This increased relaxation energy shift suggests a charge transfer screening process for H₂O on Pt + K possibly involving the unoccupied 4a₁ orbital of H₂O. The occurrence of this mode of screening would be consistent with the higher adsorption energy of H₂O on Pt + K and with its high propensity to dissociate into OH and H.     

Formation energies and electronic structures of native point defects in potassium dihydrogen phosphate

The formation energies and the equilibrium concentration of vacancies,interstitial H,K,P,O and antisite structural defects with P and K in KH 2 PO 4 (KDP) crystals are investigated by ab initio total-energy calculations.The formation energy of interstitial H is calculated to be about 2.06 eV and we suggest that it may be the dominant defect in KDP crystal.The formation energy of an O vacancy (5.25 eV) is much higher than that of interstitial O (0.60 eV).Optical absorption centres can be induced by defects of O vacancies,interstitial O and interstitial H.We suggest that these defects may be responsible for the lowering of the damage threshold of the KDP.A K vacancy defect may increase the ionic conductivity and therefore the laser-induced damage threshold decreases.     

Oxygen reduction reactions in the SOFC cathode of Ag/CeO2

The interactions between oxygen molecules and a silver surface or a CeO₂(111) supported atomic layer of silver are predicted using first-principles calculations based on spin polarized DFT with PAW method. The juncture between the CeO₂(111), the atomic layer of silver, and O₂ represents a triple-phase boundary (TPB) whereas the interface between silver surfaces and O₂ corresponds to a 2-phase boundary (2PB) in a solid oxide fuel cell (SOFC). Results suggest that the O₂ dissociation process on a monolayer of silver supported by CeO₂(111) surfaces (or TPB) with oxygen vacancies has lower reaction barrier than on silver surfaces (or 2PB), and the dissociated oxygen ions can quickly bond with subsurface Ce atom via a barrierless and highly exothermic reaction. The oxygen vacancies at TPB are found to be responsible for the lower energy barrier and high exothermicity because of the strong interaction between subsurface Ce and adspecies, implying that oxygen molecules prefer being reduced at TPB than on silver surfaces (2PB). The results suggest that, for a silver-based cathode in a SOFC, the adsorption and dissociation of oxygen occur rapidly and the most stable surface oxygen species would be the dissociated oxygen ion with − 0.78|e| Bader charges; the rate of oxygen reduction is most likely limited by subsequent processes such as diffusion or incorporation of the oxygen ions into the electrolyte.     

The structure of the V2O3(0 0 0 1) surface: A scanned-energy mode photoelectron diffraction study

Scanned-energy mode photoelectron diffraction (PhD), using the O 1s and V 2p photoemission signals, together with multiple-scattering simulations, have been used to investigate the structure of the V₂O₃(0 0 0 1) surface. The results support a strongly-relaxed half-metal termination of the bulk, similar to that found in earlier studies of Al₂O₃(0 0 0 1) and Cr₂O₃(0 0 0 1) surfaces based on low energy electron and surface X-ray diffraction methods. However, the PhD investigation fails to provide definitive evidence for the presence or absence of surface vanadyl (VO) species associated with atop O atoms on the surface layer of V atoms. Specifically, the best-fit structure does not include these vanadyl species, although an alternative model with similar relaxations but including vanadyl O atoms yields a reliability-factor within the variance of that of the best-fit structure.     

Study of Ag/Si(111) submonolayer interface: I. Electronic structure by angle-resolved UPS

Angle-resolved ultraviolet photoelectron spectra have been measured for well defined Ag/Si(111) submonolayer interfaces of (1) $\text{Si(111)(}\text{3}\text{×}\text{3}\text{)R30°-Ag}$, (2) “Si(111)(6 × 1)-Ag”, and (3) Ag/Si(111) as deposited at room temperature. Non-dispersive and very narrow (FWHM ～ 0.4–0.5 eV) Ag 4d derived peaks are found at 5.6 and 6.5 eV below the Fermi level for surface (1) and at 5.3 and 6.0 eV for surface (2). Dispersions of sp “binding” states in the energy range between E_F and Ag 4d states have been precisely determined for surface (1). Electronic structures similar to those of the Ag(111) surface, including the surface state near E_F, have been observed for surface (3).     

Scanning tunneling microscopy/spectroscopy study of adsorption of C<Subscript>60</Subscript>F<Subscript>36</Subscript> molecules on the 7 × 7-Si(111) surface

Spatially resolved images of an individual C₆₀F₃₆ fluorofullerene molecules on Si(111)-7 × 7 surface have been obtained by means of scanning tunneling microscopy/spectroscopy (STM/STS). The presence of isomers with different symmetry (T, C ₃, C ₁) has been revealed in STM investigation of initial adsorption stage of C₆₀F₃₆ on silicon surface Si(111)-(7 × 7). The adsorbed fluorofullerene molecule can occupy any adsorption site of silicon surface (corner site, faulted half, unfaulted half) that indicates for strong molecule-substrate interaction. The HOMO-LUMO gap of the adsorbed C₆₀F₃₆ molecules have been estimated from current image tunneling spectroscopy (CITS) and z(V) with engaged feedback measurements. The value of HOMO-LUMO gap observed experimentally was 3 eV. The C₆₀F₃₆ molecules adsorption on Si(111)-(7 × 7) surface was stable and kept equilibrium configuration during several hours.     

Effects of oxygen vacancies on the room-temperature ferromagnetism of Co-doped polycrystalline CeO2

Pure and Co-doped single-phase CeO₂ crystals were synthesized by a solid-state reaction method. Samples of different oxygen vacancy concentration were studied, including (1) as-sintered crystals, (2) powders ground from the same crystal, and (3) a cold-pressed pellet from the ground powder that was unannealed and annealed at 800 °C. By analyzing the magnetic behaviors, surface/volume ratio and O vacancy concentration, the effects of oxygen vacancies on the room-temperature ferromagnetism (RT-FM) of Co-doped CeO₂ were systematically investigated. The results confirm that the RT-FM observed in Co-doped CeO₂ has a direct relationship with the oxygen vacancy concentration, and support the oxygen vacancy mediated FM mechanism.     

First-principles calculations on electronic structures of Fe-vacancy-codoped TiO2 anatase (1 0 1) surface

The electronic structures of Fe-doped TiO₂ anatase (1 0 1) surfaces have been investigated by all spin-polarized density functional theory (DFT) plane-wave pseudopotential method. The general gradient approximation (GGA)+U (Hubbard coefficient) method has been adopted to describe the exchange-correlation effects. Through the density functional calculations for the formation energies of various configurations, the complex of a substitutional Fe plus an O vacancy was found to form easily in the most range of O chemical potential. The calculated density of the states of the system of Fe-doped surface with a surface oxygen vacancy shows a band gap narrowing from 2.8 to 1.9 eV comparing with the pure surface due to the synergistic effects of surface Fe impurities with O vacancies. The system processes high visible light sensitivity and photocatalytic ability by decreasing extrinsic absorption energy. By comparing the partial DOS of some O and Ti atoms lying in the outermost and bottom layers of Fe-doped surfaces, it was found that the influence of Fe impurities on the electronic structure of the system is localized.     

Ionic conductivity and point defects in pure and doped MnF2 and MgF2 single crystals

Conductivity, σ, of MnF₂ and MgF₂ single crystals, pure and doped (with Li⁺, Na⁺, Y³⁺, Gd³⁺), has been measured, from room temperature to 500°C. Further, some crystals were contaminated with O^2? as an additional impurity. These tetragonal (rutile structure) crystals both behave like typical ionic conductors. Of particular interest is the existence of a large anisotropy, σ being largest when measured parallel to the c-axis. Study of the conductivity isotherms and anisotropy as functions of impurity concentration allows identification of the conduction mechanism in terms of the migration of two mobile defects: the fiuorine-ion vacancy, V_F, and interstitial, F_i. A value of 1.44 eV was obtained for the enthalpy of formation of the intrinsic anion Frenkel defect, 0.80 eV for the migration enthalpy of a V_F and 0.88 eV for an F₁ in MnF₂ parallel to the c-axis. Similar values were obtained for MgF₂. This work shows that more information about point defects can be obtained from conductivity measurements in non-cubic cyrstals than in cubic ionic crystals, because of the additional information from conductivity anisotropy.     

Low energy Ar-ion bombardment effects on the CeO2 surface

The structure and electronic properties of epitaxial grown CeO₂(1 1 1) thin films before and after Ar⁺ bombardment have been comprehensively studied with synchrotron radiation photoemission spectroscopy (SRPES). Ar⁺ bombardment of the surface causes a new emission appearing at 1.6 eV above the Fermi edge which is related to the localized Ce 4f¹ orbital in the reduced oxidation state Ce³⁺. Under the condition of the energy of Ar ions being 1 keV and a constant current density of 0.5 μA/cm², the intensity of the reduced state Ce³⁺ increases with increasing time of sputtering and reaches a constant value after 15 min sputtering, which corresponds to the surface being exposed to 2.8 × 10¹⁵ ions/cm². The reduction of CeO₂ is attributed to a preferential sputtering of oxygen from the surface. As a result, Ar⁺ bombardment leads to a gradual buildup of an, approximately 0.69 nm thick, sputtering altered layer. Our studies have demonstrated that Ar⁺ bombardment is an effective method for reducing CeO₂ to CeO_2−x and the degree of the reduction is related to the energy and amount of Ar ions been exposed to the CeO₂ surface.