Ordered structures of defect clusters in gadolinium-doped ceria

The nano-domain, with short-range ordered structure, has been widely observed in rare-earth-doped ceria. Atomistic simulation has been employed to investigate the ordering structure of the nano-domain, as a result of aggregation and segregation of dopant cations and the associated oxygen vacancies in gadolinium-doped ceria. It is found that the binding energy of defect cluster increases as a function of cluster size, which provides the intrinsic driving force for the defect cluster growth. However, the ordered structures of the defect clusters are different from the chain model as previously reported. Adjacent oxygen vacancies prefer to locate along <110>/2 lattice vector, which results in a unique stable structure (isosceles triangle) formation. Such isosceles triangle structure can act as the smallest unit of cluster growth to form a symmetric dumbbell structure. This unique dumbbell structure is hence considered as a building block for the development of larger defect clusters, leading to nano-domain formation in rare-earth-doped ceria.     

Microstructural evolution in a CeO2-Gd2O3 system

Microstructural evolution in a CeO2-Gd2O3 system at atomic and nanoscale levels with increasing Gd concentration has been comprehensively investigated by transmission electron microscopy. When the Gd concentration was increased from 10 to 80 at.%, the phase transformation from ceria with fluorite structure to solid solution with C-type structure was not a sudden change but an evolution in the sequence of clusters, domains, and precipitates with C-type structure in the fluorite-structured matrix. Moreover, the ordering of aggregated Gd cations and oxygen vacancies in these microstructural inhomogeneities developed continuously with increasing Gd concentration. This microstructural evolution can be further described based on the development of defect clusters containing Gd cations and oxygen vacancies.     

Optimization of ionic conductivity in solid electrolytes through dopant-dependent defect cluster analysis

Atomistic simulation based on an energy minimization technique has been carried out to investigate defect clusters of R(2)O(3) (R = La, Pr, Nd, Sm, Gd, Dy, Y, Yb) solid solutions in fluorite CeO(2). Defect clusters composed of up to six oxygen vacancies and twelve accompanied dopant cations have been simulated and compared. The binding energy of defect clusters increases as a function of the cluster size. A highly symmetric dumbbell structure can be formed by six oxygen vacancies, which is considered as a basic building block for larger defect clusters. This is also believed to be a universal vacancy structure in an oxygen-deficient fluorite lattice. Nevertheless, the accurate positions of associated dopants depend on the dopant radius. As a consequence, the correlation between dopant size and oxygen-ion conductivity has been elucidated based on the ordered defect cluster model. This study sheds light on the choice of dopants from a physical perspective, and suggests the possibility of searching for optimal solid electrolyte materials through atomistic simulations.     

Atomically dispersed copper species on ceria for the low-temperature water-gas shift reaction

The structure of copper species, dispersed on nanostructured ceria(particles, rods and cubes), was analyzed by scanning transmission electron microscopy(STEM) and X-ray photoelectron spectroscopy(XPS). It was interestingly found that the density of surface oxygen vacancies(or defect sites), induced by the shape of ceria, determined the geometrical structure and the chemical state of copper species. Atomically dispersed species and monolayers containing few to tens of atoms were formed on ceria particles and rods owing to the enriched anchoring sites, but copper clusters/particles co-existed, together with the highly dispersed atoms and monolayers, on cubic ceria. The atomically dispersed copper sites and monolayers interacted strongly with ceria, involving a remarkable charge transfer from copper to ceria at their interfaces. The activity for the low-temperature watergas shift reaction of the Cu/CeO_2 catalysts was associated with the fraction of the positively-charged copper atoms, demonstrating that the active sites could be tuned by dispersing Cu species on shape-controlled ceria particles.     

Ce0.8Gd0.2-xPrxO1.9固溶体的合成与表征

利用X射线衍射(XRD)、拉曼光谱(Raman)、X射线光电子能谱(XPS)和交流阻抗谱对溶胶-凝胶法制备的稀土双掺杂固溶体Ce0.8Cd0.2-xPrxO1.9(x=0,0.02,0.10)的结构和导电性进行了研究.XRD结果表明,经800℃焙烧所得样品都形成了单相立方萤石结构,平均晶粒尺寸在23～30 nm之间;X...     

Special quasirandom structures for gadolinia-doped ceria and related materials

Gadolinia doped ceria in its doped or strained form is considered to be an electrolyte for solid oxide fuel cell applications. The simulation of the defect processes in these materials is complicated by the random distribution of the constituent atoms. We propose the use of the special quasirandom structure (SQS) approach as a computationally efficient way to describe the random nature of the local cation environment and the distribution of the oxygen vacancies. We have generated two 96-atom SQS cells describing 9% and 12% gadolinia doped ceria. These SQS cells are transferable and can be used to model related materials such as yttria stabilized zirconia. To demonstrate the applicability of the method we use density functional theory to investigate the influence of the local environment around a Y dopant in Y-codoped gadolinia doped ceria. It is energetically favourable if Y is not close to Gd or an oxygen vacancy. Moreover, Y-O bonds are found to be weaker than Gd-O bonds so that the conductivity of O ions is improved.     

An insight into the structure,conductivity and ion dynamics of Sr-Sm co-doped ceria oxygen ion conductors: Effect of defect interaction

In this work, we investigate the structure, conductivity and ion dynamics of mixed di and tri-valent doped Ce_0.8Sm_0.2-xSr_xO_2-δ (x = 0–0.2) oxygen ion conductors. The lattice parameter and root mean square strain are significantly affected by the ionic radius of dopants and their solubility into ceria lattice. Due to the solubility limit of Sr²⁺ ions, SrCeO₃ phase increases with the doping concentration of Sr²⁺. The increase of Sr²⁺ ions into ceria lattice promotes the formation of large defect clusters by expense of formed oxygen vacancies. The coulombic interaction between oxygen vacancies with substituted dopant cations enhances with Sr²⁺ ions due to decrease of the value of dielectric constant of the compositions. The defect interaction significantly affects the conductivity values by means of increase of SrCeO₃ phase and defect clusters. The conductivity values are found to be consistent with the migration and association energy. The scaled spectra of dielectric tangent loss and real part of complex conductivity confirm the temperature and defect interaction independent nature of hoping mechanism in the compositions.     

铜-氧化铈界面:几何及电子结构

纳米催化材料的性能主要由粒子尺寸、形貌和界面决定,即活性位点的电子及几何结构.尺寸、形貌可控的纳米催化材料的合成及其反应性能的研究,即催化剂的构效关系,一直是催化领域的研究热点.氧化物负载的金属催化剂广泛应用于多相催化反应过程.基于氧化铈优异的氧化还原性能, Cu/CeO2催化剂在CO氧化、N2O消除、水气变换、甲醇合成等反应中表现出优异性能.其中,通过铜物种与氧化铈表面化学键合形成的金属-载体界面通常被认为是催化活性中心.铜物种和氧化铈的相互作用主要体现在氧化铈固定铜物种,而铜物种促进氧化铈的氧化还原能力,涉及Cu^2+/Cu^+/Cu^0和Ce^3+/Ce^4+之间电子的传输和转移.Cu/CeO2催化剂活性位的原子结构与金属-载体相互作用程度密切相关.氧化铈形貌和铜负载量是决定界面电子和几何结构的重要因素.常见的纳米氧化铈形貌包括纳米粒子(多面体)、纳米棒和纳米立方体,可分别选择性暴露(111)、(110)和(100)晶面;这些晶面上原子配位环境和化学性能决定了铜-氧化铈的键合方式和界面结构.与暴露{100}晶面的纳米立方体相比,主要暴露{100}/{110}镜面的氧化铈纳米棒、暴露{111}/{100}晶面的纳米粒子与铜物种具有更强的金属-载体相互作用程度,也更有利于铜物种的分散.铜的负载量也显著影响铜物种在特定氧化铈表面的分散度和化学状态;随着铜负载量的增加,可在氧化铈表面形成层状铜、铜团簇和铜纳米粒子.通常情况下,低负载量有利于单层、双层铜物种的形成,高负载量时则出现多层铜和铜纳米粒子.催化活性位通常是由铜原子与氧化铈上的氧空穴相互作用产生,与氧化铈表面氧空穴的数量和密度密切相关,即氧化铈形貌.本文总结了Cu/CeO2催化剂的研究进展,讨论了氧化铈形貌和铜负载量对铜物种分散度和化学状态的影响规律,总结了铜氧化铈界面结构的多维度表征结果,比较了Cu/CeO2催化剂在CO氧化、水气变换及甲醇合成中的活性位结构和反应机制.     

掺杂F-,Sr2+的氧化铈紫外屏蔽和氧化催化性研究

采用沉淀法合成F-,Sr2+单掺杂和共掺杂的氧化铈,探讨了不同掺杂离子对氧化铈紫外屏蔽性能和氧化催化性能的影响.XRD结果表明,F-,Sr2+单掺杂或共掺杂的CeO2均为立方萤石结构,但衍射峰因掺杂离子的不同而稍有偏移.UV-Vis光谱表明,F-,Sr2+单掺杂和共掺杂的氧化铈的紫外屏蔽性能均优于纯氧化铈.共掺杂氟锶后的氧化铈的氧化催化性明显比纯氧化铈的弱,但是,单掺杂氟离子或锶离子的氧化铈的氧化催化性比纯氧化铈的强.     

La2O3对CuO/CeO2水煤气变换反应催化剂微观结构及催化性能的影响（英文）

以La2O3为助剂,采用共沉淀法制备了具有良好催化活性和热稳定性的CuO/CeO2-La2O3水煤气变换反应催化剂,其中,当La2O3含量为2wt%时,催化剂的催化性能最为优异.同时运用X射线衍射、N2吸附-脱附、Raman光谱、程序升温还原等手段,研究了不同含量的La2O3对CuO/CeO2催化剂微观结构及催化性能的影响.结果表明,La2O3助剂进入了载体CeO2的晶格并对CuO/CeO2催化剂的微观结构和催化性能产生了直接影响,适量La2O3的添加可以抑制CuO和CeO2晶格的长大、增强CuO与CeO2间的相互作用、提高催化剂的比表面积、促进CeO2载体中生成更多的氧空位,CuO/CeO2催化剂的催化活性和热稳定性也明显改善.     

Ce1-xGdxO2-δ固溶体中缺陷物种的拉曼光谱研究

采用溶胶-凝胶法制备一系列Ce1-xGdxO2-δ固溶体。利用紫外(325 nm)和可见(514 nm)Raman光谱,X射线粉末衍射(XRD),透射电子显微镜(TEM)和紫外可见漫反射光谱(UV-Vis DRS),考察了Ce1-xGdxO2-δ固溶体的缺陷物种的分布以及Gd含量对缺陷浓度的影响。结果表明:Ce1-xGdxO2-δ固溶体中存在氧缺位(～560 cm-1)和GdO8型缺陷结构(～600 cm-1)。根据样品对Raman激发光的吸收,紫外Raman光谱反映样品的表层信息,可见Raman光谱反映样品的整体信息。Ce1-xGdxO2-δ固溶体表层氧缺位(να)和GdO8型缺陷物种的浓度(νβ)均高于固溶体体相,这归因于缺陷物种的表面富集。然而,相比于GdO8型缺陷物种,体相中氧缺位浓度增加较表层中的更显著。     

TiC、TiC1-x、(Ti1-xNbx)C电子结构的计算

采用离散变分x_α法（DV-X_α法）对TiC理想晶体、空位和掺杂缺陷结构中的电子结构进行了计算．通过选取分子簇模型,模拟了理想晶体、空位和掺杂缺陷情况．采用多重散射离散变分X_α法,通过自治迭代来求解局域密度泛函方程,得到了各个分子簇模型的电子结构．分析计算结果发现,在理想TiC结构的态密度图中,费米能级位于两峰之间．但在费米能级处的电子态密度不为零,这提供了TiC导电性的来源．在空位模型中,发现电子态密度在费米能级处有较大的值,说明空位的存在有利于提高TiC的导电能力．对于Nb掺杂后的电子结构,在费米能级处存在一个电子态密度峰,因而也有利于提高其导电性．在计算的过程中考虑到了分子簇模型边界条件带来的电行转移效应对电子结构的影响,通过提供适当的环境势,得到了较精确的计算结果．与已有的计算结果进行了对比,有较好的一致性．     

Oxygen Nonstoichiometry, Conductivity, and Seebeck Coefficient of La0.3Sr0.7Fe1−xGaxO2.65+δ Perovskites

The total electrical conductivity and the Seebeck coefficient of perovskite phases La_0.3Sr_0.7Fe_1−xGa_xO_2.65+δ (x=0-0.4) were determined as functions of oxygen nonstoichiometry in the temperature range 650-950°C at oxygen partial pressures varying from 10⁻⁴ to 0.5 atm. Doping with gallium was found to decrease oxygen content, p-type electronic conduction and mobility of electron holes. The results on the oxygen nonstoichiometry and electrical properties clearly show that the role of gallium cations in the lattice is not passive, as it could be expected from the constant oxidation state of Ga³⁺. The nonstoichiometry dependencies of the partial molar enthalpy and entropy of oxygen in La_0.3Sr_0.7(Fe,Ga)O_2.65+δ are indicative of local inhomogeneities, such as local lattice distortions or defect clusters, induced by gallium incorporation. Due to B-site cation disorder, this effect may be responsible for suppressing long-range ordering of oxygen vacancies and for enhanced stability of the perovskite phases at low oxygen pressures, confirmed by high-temperature X-ray diffraction and Seebeck coefficient data. The values of the electron-hole mobility in La_0.3Sr_0.7(Fe,Ga)O_2.65+δ, which increases with temperature, suggest a small-polaron conduction mechanism.     

Co3O4/Ce0.8Pr0.2O2催化剂用于乙醇水蒸气重整反应的研究

采用共沉淀法制备了Co3O4/Ce0.8Pr0.2O2催化剂,并将其用于乙醇水蒸气重整制氢反应,考察了Co3O4负载量以及Pr掺杂对催化剂性能的影响.采用X射线衍射、程序升温还原、热重分析和透射电子显微镜对催化剂的结构和表面性质进行表征.结果表明,催化剂中部分Co进入到载体的晶格中,使载体发生畸变产生更多的氧空位;载体中Pr掺杂有利于生成更多的氧空位,提高了催化剂的抗积碳性能,同时Pr掺杂可以增强Co3O4与载体之间的相互作用,提高金属Co的抗烧结性能;15%Co3O4/Ce0.8Pr0.2O2催化剂具有最好的催化活性,在反应温度为400℃,空速80 000 mL/(g.h),n(H2O)∶n(EtOH)=3的条件下可将乙醇完全转化;10 h稳定性测试结果表明该催化剂具有较好的稳定性.     

Influence of intrinsic defects on the properties of zinc oxide

The effects of oxygen vacancies and zinc interstitials on the structure and energy of zinc oxide were studied with the semiempirical MO method MSINDO. Cyclic clusters were chosen as model systems. Single and multiple removal of oxygen atoms and zinc interstitials in zinc oxide served to determine the defect formation energy and the band gap. The interaction between two and three oxygen vacancies was investigated. The vacancies cause a decrease of the band gap, which originates from an occupied defect level. This is also found for zinc interstitials under zinc rich conditions. The defect formation energy of such zinc interstitials is found to be lower than that of oxygen vacancies at 0 K but decreases for oxygen vacancies and increases for zinc interstitials with increasing temperature.     

Auger and electron energy loss spectroscopies of small palladium particles supported on (111) oriented MgO films

The most common tool used to characterize supported metal clusters is the transmission electron microscope. The main advantage of TEM is the combination of high (lateral) resolution imaging with electron diffraction. However the TEM observations are usually made ex-situ i.e. UHV deposited clusters have to be exposed to the atmosphere during transfer to the TEM. This could be a severe limitation for very small reactive clusters. This paper demonstrates that electron spectroscopies can provide in-situ information on the cluster growth (AES), on the electronic structure (ELS) and on the local atomic order (SEELFS) of the clusters. These techniques were applied to Pd clusters of varying size (～10–200 Å) vapor-deposited on thin (111) MgO support films under UHV conditions. An expansion of the lattice and a shift of the loss peaks towards higher energies are observed with decreasing particle size.     

Structural and Magnetic Property of Ion Irradiated TiO2 Single Crystals

Ferromagnetism is induced in pure TiO₂ single crystals by oxygen ion irradiation. The ferro-magnetism is observed up to room temperature and is with weak temperature dependence. By combining X-ray diffraction, Rutherford backscattering/channelling, Raman scattering, and electron-spin resonance spectroscopy, supperconducting quantum interference device, displacement per atom, we measured the lattice damage accumulation with increasing flu-ences. A defect complex, i.e., Ti³⁺ on the substitutional accompanied by oxygen vacancies, has been identified in the irradiated TiO₂. This kind of defect complex results in a local (TiO_6-x) stretching Raman mode. We elucidate that Ti³⁺ with one unpaired 3d electron provide the local magnetic moments.     

Preparation and optical properties of high-quality oriented of Al and Er co-doped ZnO thin films

High-quality c-axis oriented Al and Er co-doped ZnO films were prepared on the quartz glasses by sol?Cgel method. In order to obtain the optimal processing parameters for the growth of the oriented film, an L₁₆ (4⁵) orthogonal experimental design was chosen. The experimental results show the rank of 5-factors as follows: Er at.%?>?the number of coating layer?>?annealing temperature?>?Al at.%?>?the concentration of the sol. The Al and Er co-doped film prepared using the optimal parameters exhibits the preferential orientation along the c-axis perpendicular to the substrate surface. In addition, the structural, morphological and optical properties of the films were studied by X-ray diffraction, scanning electron microscopy, and UV?Cvisible spectrophotometer, respectively. The photoluminescence spectra were also used to characterize the luminescence properties of the samples. It is found that when ZnO was co-doped with 7?% Al and 1.5?% Er, the blue emission centered at 465?nm disappears and the green emission centered at 547?nm increases with a blue shift, resulted from the rapid reducing of the interstitial Zn defect, and increasing of the oxygen defects and vacancies caused by Al³⁺ and Er³⁺ dopants.     

Zinc Oxide Defect Microstructure and Surface Chemistry Derived from Oxidation of Metallic Zinc: Thin-Film Transistor and Sensor Behavior of ZnO Films and Rods

Zinc oxide thin films are fabricated by controlled oxidation of sputtered zinc metal films on a hotplate in air at temperatures between 250 and 450 °C. The nanocrystalline films possess high relative densities and show preferential growth in (100) orientation. Integration in thin-film transistors reveals moderate charge carrier mobilities as high as 0.2 cm² V⁻¹s⁻¹. The semiconducting properties depend on the calcination temperature, whereby the best performance is achieved at 450 °C. The defect structure of the thin ZnO film can be tracked by Doppler-broadening positron annihilation spectroscopy as well as positron lifetime studies. Comparably long positron lifetimes suggest interaction of zinc vacancies (V_Zn) with one or more oxygen vacancies (V_O) in larger structural entities. Such V_O-V_Zn defect clusters act as shallow acceptors, and thus, reduce the overall electron conductivity of the film. The concentration of these defect clusters decreases at higher calcination temperatures as indicated by changes in the S and W parameters. Such zinc oxide films obtained by conversion of metallic zinc can also be used as seed layers for solution deposition of zinc oxide nanowires employing a mild microwave-assisted process. The functionality of the obtained nanowire arrays is tested in a UV sensor device. The best results with respect to sensor sensitivity are achieved with thinner seed layers for device construction.     

Growth and Electronic Properties of Ag Nanoparticles on Reduced CeO2-x(111) Films

Ag nanoparticles grown on reduced CeO_2-x thin films have been studied by X-ray photoelec-tron spectroscopy and resonant photoelectron spectroscopy of the valence band to understand the effect of oxygen vacancies in the CeO_2-x thin films on the growth and interfacial elec-tronic properties of Ag. Ag grows as three-dimensional particles on the CeO_2-x(111) surface at 300 K. Compared to the fully oxidized ceria substrate surface, Ag favors the growth of smaller particles with a larger particle density on the reduced ceria substrate surface, which can be attributed to the nucleation of Ag on oxygen vacancies. The binding energy of Ag3d increases when the Ag particle size decreases, which is mainly attributed to the final-state screening. The interfacial interaction between Ag and CeO_2-x(111) is weak. The resonant enhancement of the 4f level of Ce³⁺ species in RPES indicates a partial Ce⁴⁺→Ce³⁺ re-duction after Ag deposited on reduced ceria surface. The sintering temperature of Ag on CeO_1.85(111) surface during annealing is a little higher than that of Ag on CeO₂(111) surface, indicating that Ag nanoparticles are more stable on the reduced ceria surface.