Atom-Resolved Noncontact Atomic Force Microscopic Observations of CeO2(111) Surfaces with Different Oxidation States: Surface Structure and Behavior of Surface Oxygen Atoms

Oxygen Vacancy Injection on (111) CeO2 Nanocrystal Facets for Efficient H2O2 Detection

Facet and defect engineering have achieved great success in improving the catalytic performance of CeO₂, but the inconsistent reports on the synergistic effect of facet and oxygen vacancy and the lack of investigation on the heavily doped oxygen vacancy keeps it an attractive subject. Inspired by this, CeO₂ nanocrystals with selectively exposed crystalline facets (octahedron, cube, sphere, rod) and abundant oxygen vacancies have been synthesized to investigate the synergistic effect of facet and heavily doped oxygen vacancy. The contrasting electrochemical behavior displayed by diverse reduced CeO₂ nanocrystals verifies that oxygen vacancy acts distinctly on different facets. The thermodynamically most stable CeO₂ octahedron enclosed by heavily doped (111) facets surprisingly exhibited the optimum non-enzymatic H₂O₂ sensing performance, with a high sensitivity (128.83 µA mM^-1 cm^-2), a broad linear range (20 µM~13.61 mM), and a low detection limit (1.63 µM). Meanwhile, the sensor presented satisfying selectivity, repeatability, stability, as well as its feasibility in medical disinfectants. Furthermore, the synergistic effect of facet and oxygen vacancy was clarified by the inclined distribution states of oxygen vacancy and the electronic transmission property. This work enlightens prospective research on the synergistic effect of alternative crystal surface engineering strategies.

Active Pt-Nanocoated Layer with Pt-O-Ce Bonds on a CeO x Nanowire Cathode Formed by Electron Beam Irradiation

A Pt-nanocoated layer (thickness of approx. 10-20 nm) with Pt-O-Ce bonds was created through the water radiolysis reaction on a CeO _x nanowire (NW), which was induced by electron beam irradiation to the mixed suspension of K₂PtCl₄ aqueous solution and the CeO _x NW. In turn, when Pt-nanocoated CeO _x NW/C (Pt/C ratio = 0.2) was used in the cathode layer of a membrane electrode assembly (MEA), both an improved fuel cell performance and stability were achieved. The fuel cell performance observed for the MEA using Pt-nanocoated CeO _x NW/C with Pt-O-Ce bonds, which was prepared using the electron beam irradiation method, improved and maintained its performance (observed cell potential of approximately 0.8 V at 100 mW cm^-2) from 30 to 140 h after the start of operation. In addition, the activation overpotential at 100 mA cm^-2 (0.17 V) obtained for MEA using Pt-nanocoated CeO _x NW/C was approximately half of the value at 100 mA cm^-2 (0.35 V) of MEA using a standard Pt/C cathode. In contrast, the fuel cell performance (0.775 V at 100 mW cm^-2 after 80 h of operation) of MEA using a nanosized Pt-loaded CeO _x NW (Pt/C = 0.2), which was prepared using the conventional chemical reduction method, was lower than that of MEA using a Pt-nanocoated CeO _x /C cathode and showed reduction after 80 h of operation. It is considered why the nanocoated layer having Pt-O-Ce bonds heterogeneously formed on the surface of the CeO _x NW and the bare CeO₂ surface consisting of Ce⁴⁺ cations would become unstable in an acidic atmosphere. Furthermore, when a conventional low-amount Pt/C cathode (Pt/C = 0.04) was used as the cathode layer of the MEA, its stable performance could not be measured after 80 h of operation as a result of flooding caused by a lowering of electrocatalytic activity on the Pt/C cathode in the MEA. In contrast, a low-amount Pt-nanocoated CeO _x NW (Pt/C = 0.04) could maintain a low activation overpotential (0.22 V at 100 mA cm^-2) of MEA at the same operation time. Our surface first-principles modeling indicates that the high quality and stable performance observed for the Pt-nanocoated CeO _x NW cathode of MEA can be attributed to the formation of a homogeneous electric double layer on the sample. Since the MEA performance can be improved by examining a more effective method of electron beam irradiation to all surfaces of the sample, the present work result shows the usefulness of the electron beam irradiation method in preparing active surfaces. In addition, the quantum beam technology such as the electron beam irradiation method was shown to be useful for increasing both performance and stability of fuel cells.

Comparative study of single-atom gold and iridium on CeO2{111}

Oxide-supported single-atom catalysts have shown promise for a variety of heterogeneous processes. In addition to their inherent activity and selectivity, these materials come at much lower financial cost, avoiding the use of full-bodied precious-metal catalysts, but at the conceptual expense that more complex structural and electronic considerations need to be understood if we are to exploit their full potential. Here, we focus on the adsorption of single-atom iridium at both stoichiometric and defective CeO₂{111} surfaces, by means of first-principles density functional theory. Reference calculations for the adsorption of single-atom gold, on the same set of substrates, provide a valuable set of benchmarks against which to interpret our iridium results.

Atomic Scale Characterization of Fluxional Cation Behavior on Nanoparticle Surfaces: Probing Oxygen Vacancy Creation/Annihilation at Surface Sites

Oxygen vacancy creation and annihilation are key processes in nonstoichiometric oxides such as CeO₂. The oxygen vacancy creation and annihilation rates on an oxide's surface partly govern its ability to exchange oxygen with the ambient environment, which is critical for a number of applications including energy technologies, environmental pollutant remediation, and chemical synthesis. Experimental methods to probe and correlate local oxygen vacancy reaction rates with atomic-level structural heterogeneities would provide significant information for the rational design and control of surface functionality; however, such methods have been unavailable to date. Here, we characterize picoscale fluxional behavior in cations using time-resolved in situ aberration-corrected transmission electron microscopy to locate atomic-level variations in oxygen vacancy creation and annihilation rates on oxide nanoparticle surfaces. Low coordination number sites such as steps and edges, as well as locally strained sites, exhibited the greatest number of cation displacements, implying enhanced surface oxygen vacancy activity at these sites. The approach has potential applications to a much wider class of materials and catalysis problems involving surface and interfacial transport functionalities.

Efficient Imine Formation by Oxidative Coupling at Low Temperature Catalyzed by High-Surface-Area Mesoporous CeO2 with Exceptional Redox Property

High-surface-area mesoporous CeO₂ (hsmCeO₂ ) was prepared by a facile organic-template-induced homogeneous precipitation process and showed excellent catalytic activity in imine synthesis in the absence of base from primary alcohols and amines in air atmosphere at low temperature. For comparison, ordinary CeO₂ and hsmCeO₂ after different thermal treatments were also investigated. XRD, N₂ physisorption, UV-Raman, H₂ temperature-programmed reduction, O₂ temperature-programmed desorption, EPR spectroscopy, and X-ray photoelectron spectroscopy were used to unravel the structural and redox properties. The hsmCeO₂ calcined at 400 °C shows the highest specific surface area (158 m²  g^-1 ), the highest fraction of surface coordinatively unsaturated Ce³⁺ ions (18.2 %), and the highest concentration of reactive oxygen vacancies (2.4×10¹⁵  spins g^-1 ). In the model reaction of oxidative coupling of benzyl alcohol and aniline, such an exceptional redox property of the hsmCeO₂ catalyst can boost benzylideneaniline formation (2.75 and 5.55 mmol  g ceria - 1  h^-1 based on >99 % yield at 60 and 80 °C, respectively) in air with no base additives. It can also work effectively at a temperature of 30 °C and in gram-scale synthesis. These are among the best results for all benchmark ceria catalysts in the literature. Moreover, the hsmCeO₂ catalyst shows a wide scope towards primary alcohols and amines with good to excellent yield of imines. The influence of reaction parameters, the reusability of the catalyst, and the reaction mechanism were investigated.

Four-Electron Reduction of Dioxygen on a Metal Surface: Models of Dissociative and Associative Mechanisms in a Homogeneous System

Two different four-electron reductions of dioxygen (O₂) on a metal surface are reproduced in homogeneous systems. The reaction of the highly unsaturated (56-electron) tetraruthenium tetrahydride complex 1 with O₂ readily afforded the bis(μ₃-oxo) complex 3 via a dissociative mechanism that includes large electronic and geometric changes, i.e., a four-electron oxidation of the metal centers and an increase of 8 in the number of valence electrons. In contrast, the tetraruthenium hexahydride complex 2 induces a smooth H-atom transfer to the incorporated O₂ species, and the O-OH bond is cleaved to afford the mono(μ₃-oxo) complex 4 via an associative mechanism. Density functional theory calculations suggest that the higher degree of unsaturation in the tetrahydride system induces a significant interaction between the tetraruthenium core and the O₂ moiety, enabling the large changes required for the dissociative mechanism.

Ce3+-enriched spherical porous ceria with an enhanced oxygen storage capacity

Porous ceria was obtained using a unique solvothermal reaction in acetonitrile, applying high temperature and pressure. The resulting material comprised homogeneous and monodisperse spheres and exhibited an extremely large surface area of 152 m2 g-1. From catalytic performance evaluation by vapor- and liquid-phase reactions, the synthesized porous ceria showed superior and different reaction activity compared with commercial CeO2. To examine the origin of the reaction activity of the present porous ceria, synchrotron hard X-ray photoelectron spectroscopy (HAXPES) measurements were carried out. The systematic study of HAXPES measurements revealed that the obtained porous ceria with the present solvothermal method contained a very high concentration of Ce3+. Moreover, O2-pulse adsorption analyses demonstrated a significant oxygen adsorption capacity exceeding 268 μmol-O g-1 at 400 °C owing to its high contents of Ce3+ species.

Engineered defects in cerium oxides: tuning chemical reactivity for biomedical, environmental, & energy applications

Nanocrystalline cerium oxide (nanoceria) is a rare earth oxide with a complex surface chemistry. This material has seen substantial investigation in recent years in both fundamental and applied studies due largely to more precise characterization of the unique surface structures, which mediate its pronounced redox activity. In particular, oxygen storage/buffering capacities have been thoroughly correlated with synthesis and processing condition effects on other material features such as surface (micro-) faceting, reconstruction, and (extent of) hydration. Key material features such as these modulate nanoceria redox performance by changing the crystal microenvironment. In this review, we present nanoengineering methods, which have produced increased nanoceria performance in biomedical, energy, and catalysis applications. The impact of combined/cooperative theoretical and experimental studies are highlighted throughout.

Visualizing Anisotropic Oxygen Diffusion in Ceria under Activated Conditions

Oxygen reactivity plays a key role in the performance of ceria-based catalysts. Aberration-corrected transmission electron microscopy and molecular dynamics simulations were used to study the oxygen atom diffusion in ceria under activated conditions. Reactive oxygen atom and its real-time diffusion were visualized. The interplay between cerium and oxygen atoms originating from a Coulomb interaction was revealed by the out-of-plane buckling of cerium atoms associated with oxygen transport. Anisotropic oxygen atom diffusion that depends on crystal orientations was discovered, demonstrating a preferential [001] crystallographic diffusion pathway. These findings reveal prospects for applications of anisotropic orientation-relevant fluorite-structured oxides.

Transformations of supported gold nanoparticles observed by in situ electron microscopy

Oxide supported metal nanoparticles play an important role in heterogeneous catalysis. However, understanding the metal/oxide interface and their evolution under reaction conditions remains challenging. Herein, we investigate the interface between Au nanoparticles and a CeO2 substrate by environmental transmission electron microscopy with atomic resolution. We find that the Au nanoparticles have two preferential epitaxial relationships with the substrate, i.e. Type I (111)[-110]CeO2//(111)[-110]Au and Type II (111)[-110]CeO2//(111)[1-10]Au orientation relationships, where Type I is preferred. In situ observations in the presence of O2 show that the gas can stimulate the supported Au nanoparticles to transform between these two orientations even at room temperature. Moreover, when increasing the temperature to 973 K, the transformation of an Au nanoparticle between the two orientation states and a non-crystalline state in the presence of O2 is also observed. DFT calculations of the binding between Au and CeO2 in the two relationships are strongly influenced by the presence of oxygen vacancies. For a given position of a vacancy, there is a significant energy difference between the energy of the two types. However, for some positions, Type I is preferred, and for others, Type II, but the most favourable position of the vacancy for the two types has a very similar energy. This is consistent with the observation of both types of adhesion.

The Structure of Oxygen Vacancies in the Near-Surface of Reduced CeO2 (111) Under Strain

Strain has been widely recognized as important for tuning the behavior of defects in metal oxides since properties such as defect configuration, electronic structure, excess charge localization, and local atomic distortions may be affected by surface strain. In CeO₂, the most widely used promoter in three-way catalysts and solid state electrolyte in fuel cells, the behaviors of oxygen vacancies, and associated Ce³⁺ polarons are crucial in applications. Recent STM and AFM investigations as well as DFT-based calculations have indicated that in the near-surface of CeO₂ (111), at low temperatures and vacancy concentrations, subsurface oxygen vacancies are more stable than surface ones, and the Ce³⁺ ions are next-nearest neighbors to both types of vacancies, which can be explained by the better ability of the system to relax the lattice strain induced by vacancy formation as well as by the excess charge localization. The results also revealed that the interaction between first-neighbor vacancies is repulsive. In this work, the relative stability of surface and subsurface oxygen vacancies at the CeO₂ (111) surface under in-plane strain is investigated by means of DFT+U calculations. The tensile strain favors isolated surface vacancies with next nearest neighbor polarons, whereas isolated subsurface vacancies with nearest neighbor polarons are energetically favored under compressive strain. In addition, the formation of both surface and subsurface dimers is favored over having corresponding isolated species under compressive strain, which implies the possibility of controlling the formation of vacancy clusters using strain. In many applications, ceria is employed as a supported thin film or within a heterostructure in which ceria can be strained, and this study shows that strain can be a useful handle to tune properties of such materials.

Oxygen-Vacancy Dynamics and Entanglement with Polaron Hopping at the Reduced CeO_{2}(111) Surface

The migration of oxygen vacancies (V_{O}) in ceria-based systems is crucial to their functionality in applications. Yet, although the V_{O}'s structure and the distribution of the Ce^{3+} polarons at the CeO_{2}(111) surface has received extensive attention, the dynamic behaviors of V_{O}'s and polarons are not fully understood. By combining density functional theory calculations and ab initio molecular dynamics simulations, we show that the movements of V_{O}'s and polarons exhibit very distinct entanglement characteristics within a temperature range of 300-900 K, and that the positions of the Ce^{3+} polarons play a key role in the V_{O} migration. Long-distance vacancy migration occurs at moderate temperatures when the "suitable" Ce^{3+} distribution remains long enough to promote oxygen migration. This study provides microscopic insight into the interplay between the electronic and ionic charge transport in ceria that will be beneficial for the rational design of conductive ceria-based materials.

Various physicochemical and surface properties controlling the bioactivity of cerium oxide nanoparticles

Amidst numerous emerging nanoparticles, cerium oxide nanoparticles (CNPs) possess fascinating pharmacological potential as they can be used as a therapeutic for various oxidative stress-associated chronic diseases such as cancer, inflammation and neurodegeneration due to unique redox cycling between Ce³⁺ and Ce⁴⁺ oxidation states on their surface. Lattice defects generated by the formation of Ce³⁺ ions and compensation by oxygen vacancies on CNPs surface has led to switching between CeO₂ and CeO_2-x during redox reactions making CNPs a lucrative catalytic nanoparticle capable of mimicking key natural antioxidant enzymes such as superoxide dismutase and catalase. Eventually, most of the reactive oxygen species and nitrogen species in biological system are scavenged by CNPs via an auto-regenerative mechanism in which a minimum dose can exhibit catalytic activity for a longer duration. Due to the controversial outcomes on CNPs toxicity, considerable attention has recently been drawn towards establishing relationships between the physicochemical properties of CNPs obtained by different synthesis methods and biological effects ranging from toxicity to therapeutics. Unlike non-redox active nanoparticles, variations in physicochemical properties and the surface properties of CNPs obtained from different synthesis methods can significantly affect their biological activity (inactive, antioxidant, or pro-oxidant). Moreover, these properties can influence the biological identity, cellular interactions, cellular uptake, biodistribution, and therapeutic efficiency. This review aims to highlight the critical role of various physicochemical and the surface properties of CNPs controlling their biological activity based on 165 cited references.

Oscillations of Cerium Oxidation State Driven by Oxygen Diffusion in Colloidal Nanoceria (CeO2 - x )

The redox performance of CeO2 - x nanocrystals (nanoceria) is always accompanied by the switching of cerium oxidation state between Ce3+ and Ce4+. We monitored Ce3+ → Ce4+ oxidation of nanoceria stimulated by oxidant in aqueous colloidal solutions controlling the luminescence of Ce3+ ions located at different distances from nanoceria surface. The observed Ce3+ luminescence changes indicate that Ce3+ → Ce4+ reaction develops inside nanoceria being triggered by the diffusing oxygen originated from the water splitting on oxidized nanoceria surface. We present the first observation of the pronounced oscillations of Ce3+ luminescence intensity arising from Ce3+ ↔ Ce4+ reversible switching. This threshold effect is to be driven by uptaking and releasing oxygen by nanoceria, when the concentration of oxygen vacancies in nanoceria lattice, oxidant concentration in colloidal solution, and temperature reach certain critical values. So, the ability of nanoceria to uptake and release oxygen depending on the environmental redox conditions really makes it the self-sufficient eternal antioxidant.

IR spectroscopic investigations of chemical and photochemical reactions on metal oxides: bridging the materials gap

In this review, we highlight recent progress (2008–2016) in infrared reflection absorption spectroscopy (IRRAS) studies on oxide powders achieved by using different types of metal oxide single crystals as reference systems.

Co3O4-CeO2/C as a Highly Active Electrocatalyst for Oxygen Reduction Reaction in Al-Air Batteries

Developing high-performance and low-cost electrocatalysts for oxygen reduction reaction (ORR) is still a great challenge for Al-air batteries. Herein, CeO₂, a unique ORR promoter, was incorporated into ketjenblack (KB) supported Co₃O₄ catalyst. We developed a facile two-step hydrothermal approach to fabricate Co₃O₄-CeO₂/KB as a high-performance ORR catalyst for Al-air batteries. The ORR activity of Co₃O₄/KB was significantly increased by mixing with CeO₂ nanoparticles. In addition, the Co₃O₄-CeO₂/KB showed a better electrocatalytic performance and stability than 20 wt % Pt/C in alkaline electrolytes, making it a good candidate for highly active ORR catalysts. Co₃O₄-CeO₂/KB favored a four-electron pathway in ORR due to the synergistic interactions between CeO₂ and Co₃O₄. In full cell tests, the Co₃O₄-CeO₂/KB exhibited a higher discharge voltage plateau than CeO₂/KB and Co₃O₄/KB when used in cathode in Al-air batteries.

Sm doped mesoporous CeO2 nanocrystals: aqueous solution-based surfactant assisted low temperature synthesis, characterization and their improved autocatalytic activity

Mesoporous nanocrystalline high surface area CeO₂ synthesized at 100 °C exhibited improved autocatalytic activities by modifying defect states and reducibility upon Sm³⁺ doping.

A 2D-3D structure transition of gold clusters on CeO2-X(111) surfaces and its influence on CO and O2 adsorption: a comprehensive DFT + U investigation

Detailed knowledge of the structures of gold nanoparticles on ceria surfaces is of fundamental importance to understand their extraordinary activities in catalysis. In this work, we employ density functional theory with the inclusion of the on-site Coulomb interaction (DFT + U) to investigate the structure evolution of small-sized gold (Au, Au4, Au8 and Au12) clusters on four types of reduced CeO2-X(111) surfaces: SSV (single surface oxygen vacancy), LSVT (linear surface oxygen vacancy trimer), dLSVC (double linear surface oxygen vacancy with a surface vacancy dimer and a subsurface vacancy), and TSVT (triangular surface oxygen vacancy trimer). Our results indicate that the relative stabilities of multilayer (3D) structures are strengthened gradually compared with the monolayer (2D) structures with increasing the number of gold atoms. In addition, the 2D-3D structure transition occurs on the size order of Au(2D → 3D)@TSVT > Au(2D → 3D)@dLSVC ∼ Au(2D → 3D)@LSVT > Au(2D → 3D)@SSV, which is determined by the charge transfer magnitude between the CeO2 surfaces and gold clusters. Meanwhile, two competitive nucleation patterns are observed, fcc-like nucleation and hcp-like nucleation, which highly affect the morphology of supported gold clusters. Further site-by-site investigations indicate that the coordination number and the charges of Au atoms are the dominant factors to influence the adsorption strength of CO and O2, and the interface plays a relatively minor role. These findings not only enrich our knowledge of the relationship between surface defects, gold cluster structures and small molecule adsorptions, but also provide a theoretical perspective to help design the optimal Au/CeO2 systems possessing high catalytic efficiency.

Supercharged ceria quantum dots with exceptionally high oxygen buffer action

Supercharged ceria nanoparticles with excellent oxygen storage capacity.

Spectromicroscopic evidence of interstitial and substitutional dopants in association with oxygen vacancies in Sm-doped ceria nanoparticles

Dopant-induced structural differences and defects in Sm doped CeO2 nanoparticles (NPs) exhibiting room temperature ferromagnetism were investigated by complementary spectroscopic analysis, including X-ray Absorption Spectroscopy, Extended X-Ray Absorption Fine Structure analysis, Raman spectroscopy and atomically resolved Scanning Transmission Electron Microscopy-Electron Energy Loss Spectroscopy (STEM-EELS). The CeO2 NPs were prepared by precipitation methods with Sm/Ce ratios ranging from 0 to 0.17 and with typical sizes from 2 to 4 nanometers. These results demonstrated that the nature and the distributions of defects strongly depend on the concentrations of the dopants. Two regimes in the formation of these (Ce1-x, Smx)O2-δ NPs were observed. At lower dopant levels (x < 7%), Sm(3+) atoms mainly replace the Ce atoms in the (Ce(3+)-O(2-) vacancy) complexes which are present in ceria NPs. The dopants are unambiguously observed and localized as diluted by real space STEM-EELS spectromicroscopy done with atomic sensitivity. Nevertheless, this substitution induces a strong structural rearrangement and some Sm dopants are also observed as interstitials in association with Ce vacancies. At higher doping concentrations (x > 7%), a Sm rich phase in association with a high amount of oxygen vacancies is observed at the surface of the particles. It results in the formation of core-shell type nanoparticles with crystallographic continuities where a Sm doped CeO2-δ core is surrounded by a layer of typical (Ce0.7, Sm0.3)2O3 composition.

Oxygen vacancies versus fluorine at CeO2(111): a case of mistaken identity?

We propose a resolution to the puzzle presented by the surface defects observed with STM at the (111) surface facet of CeO 2 single crystals. In the seminal paper of Esch et al. [Science 309, 752 (2005)] they were identified with oxygen vacancies, but the observed behavior of these defects is inconsistent with the results of density functional theory (DFT) studies of oxygen vacancies in the literature. We resolve these inconsistencies via DFT calculations of the properties of both oxygen vacancies and fluorine impurities at CeO2(111), the latter having recently been shown to exist in high concentrations in single crystals from a widely used commercial source of such samples. We find that the simulated filled-state STM images of surface-layer oxygen vacancies and fluorine impurities are essentially identical, which would render problematic their experimental distinction by such images alone. However, we find that our theoretical results for the most stable location, mobility, and tendency to cluster, of fluorine impurities are consistent with experimental observations, in contrast to those for oxygen vacancies. Based on these results, we propose that the surface defects observed in STM experiments on CeO2 single crystals reported heretofore were not oxygen vacancies, but fluorine impurities. Since the similarity of the simulated STM images of the two defects is due primarily to the relative energies of the 2p states of oxygen and fluorine ions, this confusion might also occur for other oxides which have been either doped or contaminated with fluorine.

Imaging the atomic surface structures of CeO2 nanoparticles

Atomic surface structures of CeO2 nanoparticles are under debate owing to the lack of clear experimental determination of the oxygen atom positions. In this study, with oxygen atoms clearly observed using aberration-corrected high-resolution electron microscopy, we determined the atomic structures of the (100), (110), and (111) surfaces of CeO2 nanocubes. The predominantly exposed (100) surface has a mixture of Ce, O, and reduced CeO terminations, underscoring the complex structures of this polar surface that previously was often oversimplified. The (110) surface shows "sawtooth-like" (111) nanofacets and flat CeO2-x terminations with oxygen vacancies. The (111) surface has an O termination. These findings can be extended to the surfaces of differently shaped CeO2 nanoparticles and provide insight about face-selective catalysis.

Oxygen vacancy formation in CeO2 and Ce(1-x)Zr(x)O2 solid solutions: electron localization, electrostatic potential and structural relaxation

Ceria (CeO(2)) and ceria-based composite materials, especially Ce(1-x)Zr(x)O(2) solid solutions, possess a wide range of applications in many important catalytic processes, such as three-way catalysts, owing to their excellent oxygen storage capacity (OSC) through the oxygen vacancy formation and refilling. Much of this activity has focused on the understanding of the electronic and structural properties of defective CeO(2) with and without doping, and comprehending the determining factor for oxygen vacancy formation and the rule to tune the formation energy by doping has constituted a central issue in material chemistry related to ceria. However, the calculation on electronic structures and the corresponding relaxation patterns in defective CeO(2-x) oxides remains at present a challenge in the DFT framework. A pragmatic approach based on density functional theory with the inclusion of on-site Coulomb correction, i.e. the so-called DFT + U technique, has been extensively applied in the majority of recent theoretical investigations. Firstly, we review briefly the latest electronic structure calculations of defective CeO(2)(111), focusing on the phenomenon of multiple configurations of the localized 4f electrons, as well as the discussions of its formation mechanism and the catalytic role in activating the O(2) molecule. Secondly, aiming at shedding light on the doping effect on tuning the oxygen vacancy formation in ceria-based solid solutions, we summarize the recent theoretical results of Ce(1-x)Zr(x)O(2) solid solutions in terms of the effect of dopant concentrations and crystal phases. A general model on O vacancy formation is also discussed; it consists of electrostatic and structural relaxation terms, and the vital role of the later is emphasized. Particularly, we discuss the crucial role of the localized structural relaxation patterns in determining the superb oxygen storage capacity in kappa-phase Ce(1-x)Zr(1-x)O(2). Thirdly, we briefly discuss some interesting findings for the oxygen vacancy formation in pure ceria nanoparticles (NPs) uncovered by DFT calculations and compare those with the bulk or extended surfaces of ceria as well as different particle sizes, emphasizing the role of the electrostatic field in determining the O vacancy formation.

Correlation between evolution of resistive switching and oxygen vacancy configuration in La₀.₅Ca₀.₅MnO₃ based memristive devices

We here report a study of the correlation between the evolution of resistive switching and the oxygen vacancy configuration in La₀.₅Ca₀.₅MnO₃ (LCMO) based memristive devices. By taking advantage of LCMO located at a phase boundary of the metal-to-insulator transition, we observe the development of the high resistive states, depending upon not only the electrical pulse magnitude but also the switching cycles. We discuss the experimental results by an oxygen migration model that involves both single isolated and clustered oxygen vacancies, which are later verified using aberration-corrected scanning transmission electron microscopy.