Imaging the atomic surface structures of CeO2 nanoparticles

in situ observation of reversible phase transitions in Gd-doped ceria driven by electron beam irradiation

Ceria-based oxides are widely utilized in diverse energy-related applications, with attractive functionalities arising from a defective structure due to the formation of mobile oxygen vacancies (V O ⋅ ⋅ ). Notwithstanding its significance, behaviors of the defective structure andV O ⋅ ⋅ in response to external stimuli remain incompletely explored. Taking the Gd-doped ceria (Ce0.88Gd0.12O2-δ) as a model system and leveraging state-of-the-art transmission electron microscopy techniques, reversible phase transitions associated with massiveV O ⋅ ⋅ rearrangement are stimulated and visualized in situ with sub-Å resolution. Electron dose rate is identified as a pivotal factor in modulating the phase transition, and both theV O ⋅ ⋅ concentration and the orientation of the newly formed phase can be altered via electron beam. Our results provide indispensable insights for understanding and refining the microscopic pathways of phase transition as well as defect engineering, and could be applied to other similar functional oxides.

A CeO2 (100) surface reconstruction unveiled by in situ STEM and particle swarm optimization techniques

The reconstruction of the polar CeO2 (100) surface has been a subject of long-standing debates due to its complexity and the limited availability of experimental data. Herein, we successfully reveal a CeO2 (100)-(4 × 6) surface reconstruction by combining in situ spherical aberration-corrected scanning transmission electron microscopy, density functional theory calculations, and a particle swarm optimization-based algorithm for structure searching. We have further elucidated the stabilizing mechanism of the reconstructed structure, which involves the splitting of the filled Ce(4f) states and the mixing of the lower-lying ones with the O(2p) orbitals, as evidenced by the projected density of states. We also reveal that the surface chemisorption properties toward water molecules, an important step in numerous heterogeneous catalytic reactions, are enhanced. These insights into the distinct properties of ceria surface pave the way for performance improvements of ceria in a wide range of applications.

Composition-dependent morphologies of CeO2 nanoparticles in the presence of Co-adsorbed H2O and CO2: a density functional theory study

Catalytic activity is affected by surface morphology, and specific surfaces display greater activity than others. A key challenge is to define synthetic strategies to enhance the expression of more active surfaces and to maintain their stability during the lifespan of the catalyst. In this work, we outline an ab initio approach, based on density functional theory, to predict surface composition and particle morphology as a function of environmental conditions, and we apply this to CeO2 nanoparticles in the presence of co-adsorbed H2O and CO2 as an industrially relevant test case. We find that dissociative adsorption of both molecules is generally the most favourable, and that the presence of H2O can stabilise co-adsorbed CO2. We show that changes in adsorption strength with temperature and adsorbate partial pressure lead to significant changes in surface stability, and in particular that co-adsorption of H2O and CO2 stabilizes the {100} and {110} surfaces over the {111} surface. Based on the changes in surface free energy induced by the adsorbed species, we predict that cuboidal nanoparticles are favoured in the presence of co-adsorbed H2O and CO2, suggesting that cuboidal particles should experience a lower thermodynamic driving force to reconstruct and thus be more stable as catalysts for processes involving these species.

Unveiling the Structure of Oxygen Vacancies in Bulk Ceria and the Physical Mechanisms behind Their Formation

Understanding the structures of oxygen vacancies in bulk ceria is crucial as they significantly impact the material's catalytic and electronic properties. The complex interaction between oxygen vacancies and Ce3+ ions presents challenges in characterizing ceria's defect chemistry. We introduced a machine learning-assisted cluster-expansion model to predict the energetics of defective configurations accurately within bulk ceria. This model effectively samples configurational spaces, detailing oxygen vacancy structures across different temperatures and concentrations. At lower temperatures, vacancies tend to cluster, mediated by Ce3+ ions and electrostatic repulsion, while at higher temperatures, they distribute uniformly due to configurational entropy. Our analysis also reveals a correlation between thermodynamic stability and the band gap between occupied O 2p and unoccupied Ce 4f orbitals, with wider band gaps indicating higher stability. This work enhances our understanding of defect chemistry in oxide materials and lays the groundwork for further research into how these structural properties affect ceria's performance.

Advanced Biological Applications of Cerium Oxide Nanozymes in Disease Related to Oxidative Damage

Due to their excellent catalytic activities, cerium oxide nanoparticles have promise as biological nanoenzymes. A redox reaction occurs between Ce3+ ions and Ce4+ ions during which they undergo conversion by acquiring or losing electrons as well as forming oxygen vacancies (or defects) in the lattice structure, which can act as antioxidant enzymes and simulate various enzyme activities. A number of cerium oxide nanoparticles have been engineered with multienzyme activities, including catalase, superoxide oxidase, peroxidase, and oxidase mimetic properties. Cerium oxide nanoparticles have nitric oxide radical clearing and radical scavenging properties and have been widely used in a number of fields of biology, including biomedicine, disease diagnosis, and treatment. This review provides a comprehensive introduction to the catalytic mechanisms and multiple enzyme activities of cerium oxide nanoparticles, along with their potential applications in the treatment of diseases of the brain, bones, nerves, and blood vessels.

Direct in-situ insights into the asymmetric surface reconstruction of rutile TiO2 (110)

The reconstruction of rutile TiO2 (110) holds significant importance as it profoundly influences the surface chemistry and catalytic properties of this widely used material in various applications, from photocatalysis to solar energy conversion. Here, we directly observe the asymmetric surface reconstruction of rutile TiO2 (110)-(1×2) with atomic-resolution using in situ spherical aberration-corrected scanning transmission electron microscopy. Density functional theory calculations were employed to complement the experimental observations. Our findings highlight the pivotal role played by repulsive electrostatic interaction among the small polarons -formed by excess electrons following the removal of neutral oxygen atoms- and the subsequent surface relaxations induced by these polarons. The emergence and disappearance of these asymmetric structures can be controlled by adjusting the oxygen partial pressure. This research provides a deeper understanding, prediction, and manipulation of the surface reconstructions of rutile TiO2 (110), holding implications for a diverse range of applications and technological advancements involving rutile-based materials.

Metal-Organic Framework Membrane Constructor: A Tool for High-Throughput Construction of Metal-Organic Framework Membrane Models

With the rapid development of metal-organic framework (MOF) membranes for separation applications, computational screening of their separation performance has attracted increasing interest in the design and fabrication of such materials. Although bulk crystal models in MOF databases are often used to represent MOF membrane structures, membrane models in slab geometries are still essential for researchers to simulate the separation performance, particularly to understand the effects of the surface/interface structure, pore sieving, and exposed lattice plane on guest permeability. However, to date, no database or method has been established to provide researchers with numerous membrane models, restricting the further development of related theoretical studies. Herein, we propose an algorithm and develop a tool called the "MOF-membrane constructor" to realize the high-throughput construction of membrane models based on the MOF crystal structures. Using this tool, membrane models can be generated with desired sizes, reasonable surface terminations, and assigned exposed crystal planes. The tool can also deduce the most prominent surface in the Bravais-Friedel-Donnay-Harker morphology or identify the pores in MOF crystals and automatically determine an exposed plane for each membrane model. Thus, an MOF-membrane database can be established rapidly according to user simulation requirements. This study can considerably improve the efficiency of building MOF membrane models and may be beneficial for the future development of simulation studies on MOF membranes.

Photocatalytic Co-Reduction of N2 and CO2 with CeO2 Catalyst for Urea Synthesis

The effective conversion of carbon dioxide (CO2 ) and nitrogen (N2 ) into urea by photocatalytic reaction under mild conditions is considered to be a more environmentally friendly and promising alternative strategies. However, the weak adsorption and activation ability of inert gas on photocatalysts has become the main challenge that hinder the advancement of this technique. Herein, we have successfully established mesoporous CeO2-x nanorods with adjustable oxygen vacancy concentration by heat treatment in Ar/H2 (90 % : 10 %) atmosphere, enhancing the targeted adsorption and activation of N2 and CO2 by introducing oxygen vacancies. Particularly, CeO2 -500 (CeO2 nanorods heated treatment at 500 °C) revealed high photocatalytic activity toward the C-N coupling reaction for urea synthesis with a remarkable urea yield rate of 15.5 μg/h. Besides, both aberration corrected transmission electron microscopy (AC-TEM) and Fourier transform infrared (FT-IR) spectroscopy were used to research the atomic surface structure of CeO2 -500 at high resolution and to monitor the key intermediate precursors generated. The reaction mechanism of photocatalytic C-N coupling was studied in detail by combining Density Functional Theory (DFT) with specific experiments. We hope this work provides important inspiration and guiding significance towards highly efficient photocatalytic synthesis of urea.

Bulk and Surface Contributions to Ionisation Potentials of Metal Oxides

Determining the absolute band edge positions in solid materials is crucial for optimising their performance in wide-ranging applications including photocatalysis and electronic devices. However, obtaining absolute energies is challenging, as seen in CeO2 , where experimental measurements show substantial discrepancies in the ionisation potential (IP). Here, we have combined several theoretical approaches, from classical electrostatics to quantum mechanics, to elucidate the bulk and surface contributions to the IP of metal oxides. We have determined a theoretical bulk contribution to the IP of stoichiometric CeO2 of only 5.38 eV, while surface orientation results in intrinsic IP variations ranging from 4.2 eV to 8.2 eV. Highly tuneable IPs were also found in TiO2 , ZrO2 , and HfO2 , in which surface polarisation plays a pivotal role in long-range energy level shifting. Our analysis, in addition to rationalising the observed range of experimental results, provides a firm basis for future interpretations of experimental and computational studies of oxide band structures.

Recent advances and perspectives of CeO2-based catalysts: Electronic properties and applications for energy storage and conversion

Cerium dioxide (CeO₂, ceria) has long been regarded as one of the key materials in modern catalysis, both as a support and as a catalyst itself. Apart from its well-established use (three-way catalysts and diesel engines), CeO₂ has been widely used as a cocatalyst/catalyst in energy conversion and storage applications. The importance stems from the oxygen storage capacity of ceria, which allows it to release oxygen under reducing conditions and to store oxygen by filling oxygen vacancies under oxidizing conditions. However, the nature of the Ce active site remains not well understood because the degree of participation of f electrons in catalytic reactions is not clear in the case of the heavy dependence of catalysis theory on localized d orbitals at the Fermi energy E _F . This review focuses on the catalytic applications in energy conversion and storage of CeO₂-based nanostructures and discusses the mechanisms for several typical catalytic reactions from the perspectives of electronic properties of CeO₂-based nanostructures. Defect engineering is also summarized to better understand the relationship between catalytic performance and electronic properties. Finally, the challenges and prospects of designing high efficiency CeO₂-based catalysts in energy storage and conversion have been emphasized.

Surface potentials of acceptor- and donor-doped CeO2 thin films and their relation to oxygen surface exchange

Surface Fermi level positions, ionisation potentials, and work functions of acceptor-, donor-, and nominally undoped CeO₂ have been determined by means of in situ photoelectron spectroscopy on films grown with different surface orientation and preparation conditions. The Fermi energy varies in accordance with the doping and oxygen activity. The ionisation potentials are largely unaffected by the preparation conditions and surface orientation. They are comparable for nominally undoped, 1% donor-doped, and 1% acceptor-doped films. The majority of the 10% Gd-doped films exhibit a 0.5 eV lower ionisation potential, which might be related to the presence of a surface space-charge region. The lower ionisation potential of the 10% Gd-doped films compensates for their lower Fermi energies and eventually results in work functions being largely independent on doping. Oxygen surface exchange coefficients determined using secondary ion mass spectrometry and conductivity relaxation experiments reveal similar magnitudes and are not strongly affected by doping type, concentration, and surface orientation. The results indicate that surface space-charge regions are crucial for oxygen surface exchange but do not allow to finally identify the rate determining step for oxygen incorporation into CeO₂-based materials.

Investigations of the Effect of H2 in CO Oxidation over Ceria Catalysts

The preferential CO oxidation (so-called CO-PROX) is the selective CO oxidation amid H2-rich atmospheres, a process where ceria-based materials are consolidated catalysts. This article aims to disentangle the potential CO–H2 synergism under CO-PROX conditions on the low-index ceria surfaces (111), (110) and (100). Polycrystalline ceria, nanorods and ceria nanocubes were prepared to assess the physicochemical features of the targeted surfaces. Diffuse reflectance infrared Fourier-transformed spectroscopy (DRIFTS) shows that ceria surfaces are strongly carbonated even at room temperature by the effect of CO, with their depletion related to the CO oxidation onset. Conversely, formate species formed upon OH + CO interaction appear at temperatures around 60 °C and remain adsorbed regardless the reaction degree, indicating that these species do not take part in the CO oxidation. Density functional theory calculations (DFT) reveal that ceria facets exhibit high OH coverages all along the CO-PROX reaction, whilst CO is only chemisorbed on the (110) termination. A CO oxidation mechanism that explains the early formation of carbonates on ceria and the effect of the OH coverage in the overall catalytic cycle is proposed. In short, hydroxyl groups induce surface defects on ceria that increase the COx–catalyst interaction, revealed by the CO adsorption energies and the stabilization of intermediates and readsorbed products. In addition, high OH coverages are shown to facilitate the hydrogen transfer to form less stable HCOx products, which, in the case of the (110) and (100), is key to prevent surface poisoning. Altogether, this work sheds light on the yet unclear CO–H2 interactions on ceria surfaces during CO-PROX reaction, providing valuable insights to guide the design of more efficient reactors and catalysts for this process.

In Situ Resolving the Atomic Reconstruction of SnO2 (110) Surface

Understanding surface reconstruction of nanocrystals is of great importance to their applications, however it is still challenging due to lack of atomic-level structural information under reconstruction conditions. Herein, through in situ spherical aberration corrected scanning transmission electron microscopy (STEM), the reconstruction of nanocrystalline SnO₂ (110) surface was studied. By identifying the precise arrangements of surface/subsurface Sn and O columns through both in situ bright-field and high-angle annular dark-field STEM images, an unexpected added Sn₂O model was determined for SnO₂ (110)-(1 × 2) surface. The protruded Sn^δ+ of this surface could act as the active sites for activating O₂ molecules according to our density functional theory (DFT) calculations. On the basis of in situ observation of atomic-level reconstruction behaviors and DFT calculations, an energy-driven reconstruction process was also revealed. We anticipate this work would help to clarify the long-standing debate regarding the reconstruction of SnO₂ (110) surface and its intrinsic property.

Enhanced Artificial Enzyme Activities on the Reconstructed Sawtoothlike Nanofacets of Pure and Pr-Doped Ceria Nanocubes

In this work, a simple one-step thermal oxidation process was established to achieve a significant surface increase in {110} and {111} nanofacets on well-defined, pure and Pr-doped, ceria nanocubes. More importantly, without changing most of the bulk properties, this treatment leads to a remarkable boost of their enzymatic activities: from the oxidant (oxidase-like) to antioxidant (hydroxyl radical scavenging) as well as the paraoxon degradation (phosphatase-like) activities. Such performance improvement might be due to the thermally generated sawtoothlike {111} nanofacets and defects, which facilitate the oxygen mobility and the formation of oxygen vacancies on the surface. Finally, possible mechanisms of nanoceria as artificial enzymes have been proposed in this manuscript. Considering the potential application of ceria as artificial enzymes, this thermal treatment may enable the future design of highly efficient nanozymes without changing the bulk composition.

Crystal plane effect of ceria on supported copper catalyst for liquid-phase hydrogenation of unsaturated aldehyde

Ceria has been widely used as catalyst support displaying a size- or shape-dependent catalytic performance due to the strong metal-support interaction (SMSI) effect with active metal. Almost all the studies on the SMSI effect of ceria-supported metal catalysts are involved generally in gas-phase reaction, but rarely in the liquid-phase reaction system. In this work, Cu/CeO₂-P (copper loaded on nano-polyhedral CeO₂ with (111) terminated surface) was investigated its catalytic performance on liquid-phase hydrogenation and studied the SMSI effect by comparing with the catalysts supported on nano-rod and nano-cube CeO₂. It was found that Cu was highly dispersed on the external surface of ceria in the Cu/CeO₂-P catalyst via a moderate SMSI effect. Furthermore, the degree of the interaction showed great influence on the chemical state of Cu species, and the ratio of (Cu⁺+Cu⁰)/Cu²⁺ in Cu/CeO₂-P was higher than Cu/CeO₂-R (Cu loaded on nano-rod CeO₂ with (110) plane) and Cu/CeO₂-C (Cu loaded on nano-cube CeO₂ with (100) facet). As a result, the Cu/CeO₂-P catalyst showed the best catalytic performance among three types of catalysts. Based on series of catalytic investigations, the catalytic performance in liquid-phase hydrogenation was intrinsically relevant to the crystal plane effect and reduced Cu proportion induced by an appropriate SMSI effect, which was completely different from gas-phase hydrogenation.

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.

Design strategies for ceria nanomaterials: untangling key mechanistic concepts

The morphologies of ceria nanocrystals play an essential role in determining their redox and catalytic performances in many applications, yet the effects of synthesis variables on the formation of ceria nanoparticles of different morphologies and their related growth mechanisms have not been systematised. The design of these morphologies is underpinned by a range of fundamental parameters, including crystallography, optical mineralogy, the stabilities of exposed crystallographic planes, CeO_2-x stoichiometry, phase equilibria, thermodynamics, defect equilibria, and the crystal growth mechanisms. These features are formalised and the key analytical methods used for analysing defects, particularly the critical oxygen vacancies, are surveyed, with the aim of providing a source of design parameters for the synthesis of nanocrystals, specifically CeO_2-x. However, the most important aspect in the design of CeO_2-x nanocrystals is an understanding of the roles of the main variables used for synthesis. While there is a substantial body of data on CeO_2-x morphologies fabricated using low cerium concentrations ([Ce]) under different experimental conditions, the present work fully maps the effects of the relevant variables on the resultant CeO_2-x morphologies in terms of the commonly used raw materials [Ce] (and [NO₃^-] in Ce(NO₃)₃·6H₂O) as feedstock, [NaOH] as precipitating agent, temperature, and time (as well as the complementary vapour pressure). Through the combination of consideration of the published literature and the generation of key experimental data to fill in the gaps, a complete mechanistic description of the development of the main CeO_2-x morphologies is illustrated. Further, the mechanisms of the conversion of nanochains into the two variants of nanorods, square and hexagonal, have been elucidated through crystallographic reasoning. Other key conclusions for the crystal growth process are the critical roles of (1) the formation of Ce(OH)₄ crystallite nanochains as the precursors of nanorods and (2) the disassembly of the nanorods into Ce(OH)₄ crystallites and NO₃^--assisted reassembly into nanocubes (and nanospheres) as an unrecognised intermediate stage of crystal growth.

Oxygen Vacancy-Related Cathodoluminescence Quenching and Polarons in CeO2

We used cathodoluminescence (CL) spectroscopy to characterize the oxygen vacancies (V_O) in ceria (CeO₂). The effects of the processing atmosphere and thermal quenching temperature on the nature and distribution of the intrinsic defects and on the spectroscopic behavior were investigated. The presence of polarons and associates of the polarons with the oxygen vacancies such as (V_O ^••-Ce_Ce ^')^• is demonstrated. CL intensity quenching above a critical concentration of V_O has been shown. Even though the emission centers in all samples are the same, their concentration changes with the oxygen partial pressure of the processing atmosphere. Deconvolution of the observed CL spectra shows that the emissions originating from the F⁰ centers prevail over those of F⁺ centers of V_O when the defect concentration is high.

Surface Refaceting Mechanism on Cubic Ceria

Polar surfaces of solid oxides are intrinsically unstable and tend to reconstruct due to the diverging electrostatic energy and thus often exhibit unique physical and chemical properties. However, a quantitative description of the restructuring mechanism of these polar surfaces remains challenging. Here we provide an atomic-level picture of the refaceting process that governs the surface polarity compensation of cubic ceria nanoparticles based on the accurate reference data acquired from the well-defined model systems. The combined results from advanced infrared spectroscopy, atomic-resolved transmission electron microscopy, and density functional theory calculations identify a two-step scenario where an initial O-terminated (2 × 2) reconstruction is followed by a severe refaceting via massive mass transport at elevated temperatures to yield {111}-dominated nanopyramids. This significant surface restructuring promotes the redox properties of ceria nanocubes, which account for the enhanced catalytic activity for CO oxidation.

Surface termination and stoichiometry of LaAlO3(001) surface studied by HRTEM

As an important topic of condensed matter physics, metal oxide surfaces often exhibit exotic properties such as high catalytic activity, enhanced ferroelectricity and electronic phase transition, originating from the different local symmetry with respect to the bulk. As the structure determination of oxide surfaces presents challenges to conventional surface science techniques like scanning tunneling microscopy, aberration-corrected transmission electron microscopy (TEM) has been increasingly used to solve structures of oxide surfaces. In this work, the (001) surface of LaAlO₃, one of the most used components of oxide heterostructures, has been investigated. Our TEM experiments and extensive image simulations show that the La-O terminated LaAlO₃(001) surface undergoes significant reconstructions, forming La vacancies on the surface layer. Energetically, the LaAlO₃(001) surface is stable with the reconstructed La-O termination in a wide range of oxygen chemical potentials. Polarity compensation, reduced density of states at the Fermi level and bond enhancement of subsurface oxygen anions all contribute to the stabilization of the reconstructed surface.

Controllable design, synthesis and characterization of nanostructured rare earth metal oxides

Rare earth metal oxide nanomaterials have drawn much attention in recent decades due to their unique properties and promising applications in catalysis, chemical and biological sensing, separation, and optical devices. Because of the strong structure–property correlation, controllable synthesis of nanomaterials with desired properties has long been the most important topic in nanoscience and nanotechnology and still maintains a grand challenge. A variety of methods, involving chemical, physical, and hybrid method, have been developed to precisely control nanomaterials, including size, shape, dimensionality, crystal structure, composition, and homogeneity. These nanostructural parameters play essential roles in determining the final properties of functional nanomaterials. Full understanding of nanomaterial properties through characterization is vital in elucidating the fundamental principles in synthesis and applications. It allows researchers to discover the correlations between the reaction parameters and nanomaterial properties, offers valuable insights in improving synthetic routes, and provokes new design strategies for nanostructures. In application systems, it extrapolates the structure–activity relationship and reaction mechanism and helps to establish quality model for similar reaction processes. The purpose of this chapter is to provide a comprehensive overview and a practical guide of rare earth oxide nanomaterial design and characterization, with special focus on the well-established synthetic methods and the conventional and advanced analytical techniques. This chapter addresses each synthetic method with its advantages and certain disadvantages, and specifically provides synthetic strategies, typical procedures and features of resulting nanomaterials for the widely-used chemical methods, such as hydrothermal, solvothermal, sol–gel, co-precipitation, thermal decomposition, etc. For the nanomaterial characterization, a practical guide for each technique is addressed, including working principle, applications, materials requirements, experimental design and data analysis. In particular, electron and force microscopy are illuminated for their powerful functions in determining size, shape, and crystal structure, while X-ray based techniques are discussed for crystalline, electronic, and atomic structural determination for oxide nanomaterials. Additionally, the advanced characterization methodologies of synchrotron-based techniques and in situ methods are included. These non-traditional methods become more and more popular because of their capabilities of offering unusual nanostructural information, short experiment time, and in-depth problem solution.

Graphical Abstract:

Dispersion of copper oxide species on nanostructured ceria

Copper oxides species deposited on ceria rods, particles, and cubes were examined for low-temperature oxidation of CO. It was found that the shape of ceria altered the dispersion and chemical state of copper species considerably. CuO_x monolayers and bilayers were formed on ceria rods and particles, while multilayers and faceted particles co-existed on ceria cubes. The formation of Cu⁺ species at the copper-ceria interface involved a significant charge transfer from copper oxides to the ceria surface via a strong electronic interaction, which was more pronounced on ceria rods. The concentrations of surface Cu⁺ and oxygen vacancies followed the order rods > particles > cubes, in line with their catalytic activity for CO oxidation at 343 K.

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.

Formation mechanism and size control of ceria nanocubes

Ceria nanocubes of 10–20 nm were hydrothermally synthesized, involving Ce(OH)₃ rods and small-sized CeO₂ polyhedra/cubes as building blocks.

Clarifying the impacts of surface hydroxyls on CO oxidation on CeO2(100) surfaces: a DFT+U study

In heterogeneous catalysis, surface hydroxylation is well recognized as a common phenomenon under realistic reaction conditions.

Atomically dispersed platinum on low index and stepped ceria surfaces: phase diagrams and stability analysis

Through the combination of density functional theory calculations and ab initio atomistic thermodynamics modeling, we demonstrate that atomically dispersed platinum species on ceria can adopt a range of local coordination configurations and oxidation states that depend on the surface structure and environmental conditions. Unsaturated oxygen atoms on ceria surfaces play the leading role in stabilization of PtOx species. Any mono-dispersed Pt0 species are thermodynamically unstable compared to bulk platinum, and oxidation of Pt0 to Pt2+ or Pt4+ is necessary to stabilize mono-dispersed platinum atoms. Reduction to Pt0 leads to sintering. Both Pt2+ and Pt4+ prefer to form the square-planar [PtO4] configuration. The two most stable Pt2+ species on the (223) and (112) surfaces are thermodynamically favorable between 300 and 1200 K. The most stable Pt4+ species on the (100) surface tends to desorb from the surface as gas phase above 950 K. The resulting phase diagrams of the atomically dispersed platinum in PtOx clusters on various ceria surfaces under a range of experimentally relevant conditions can be used to predict dynamic restructuring of atomically dispersed platinum catalysts and design new catalysts with engineered properties.

Coordination Polymer to Atomically Thin, Holey, Metal-Oxide Nanosheets for Tuning Band Alignment

Holey 2D metal oxides have shown great promise as functional materials for energy storage and catalysts. Despite impressive performance, their processing is challenged by the requirement of templates plus capping agents or high temperatures; these materials also exhibit excessive thicknesses and low yields. The present work reports a metal-based coordination polymer (MCP) strategy to synthesize polycrystalline, holey, metal oxide (MO) nanosheets with thicknesses as low as two-unit cells. The process involves rapid exfoliation of bulk-layered, MCPs (Ce-, Ti-, Zr-based) into atomically thin MCPs at room temperature, followed by transformation into holey 2D MOs upon the removal of organic linkers in aqueous solution. Further, this work represents an extra step for decorating the holey nanosheets using precursors of transition metals to engineer their band alignments, establishing a route to optimize their photocatalysis. The work introduces a simple, high-yield, room-temperature, and template-free approach to synthesize ultrathin holey nanosheets with high-level functionalities.

Reverse Monte Carlo modeling for low-dimensional systems

Reverse Monte Carlo (RMC) is one of the commonly used approaches for modeling total scattering data. However, to extend the capability of the RMC method for refining the structure of nanomaterials, the dimensionality and finite size need to be considered when calculating the pair distribution function (PDF). To achieve this, the simulation box must be set up to remove the periodic boundary condition in one, two or three of the dimensions. This then requires a correction to be applied for the difference in number density between the real system and the simulation box. In certain circumstances an analytical correction for the uncorrelated pairings of atoms is also applied. The validity and applicability of our methodology is demonstrated by applying the algorithms to simulate the PDF patterns of carbon systems with various dimensions, and also by using them to fit experimental data of CuO nanoparticles. This alternative approach for characterizing the local structure of nano-systems with the total scattering technique will be made available via the RMCProfile package. The theoretical formulation and detailed explanation of the analytical corrections for low-dimensional systems – 2D nanosheets, 1D nanowires and 0D nanoparticles – is also given.

A DFT+U revisit of reconstructed CeO2(100) surfaces: structures, thermostabilities and reactivities

Cerium dioxide (CeO2) shows wide catalytic applications by virtue of its excellent oxygen storage capacity. The CeO2(100) surface has aroused particular interest because of its intrinsic polarity; however, it suffers from structural reconstruction, which consequently hinders experimental and theoretical studies. In this work, we performed density functional theory calculations with on-site Coulomb interaction correction to investigate and further correlate the geometric and catalytic properties of reconstructed CeO2(100) surfaces. By introducing CeO2 units on a previous O-terminal model, the surface exposed CeO4 pyramids with gradual increase in coverage and eventually transformed into a Ce-terminal structure. The corresponding thermostabilities were evaluated by calculating the surface energy and oxygen vacancy formation energy. We also showed that the CO oxidation on the reconstructed CeO2(100) surfaces favored the Mars-van-Krevelen mechanism. The most stable CeO4-terminal type of reconstruction, covered with a half overlayer of CeO4 pyramids on the surface, was capable of directly producing CO2 without forming bent CO2 intermediates and carbonate byproducts. Moreover, coordinatively unsaturated Ce ions at the pyramid apex provided extra accommodation to the reacting CO, thus lowering the reaction barrier of the key CO coupling step relative to that of the O-terminal surface. We finally generalized a unified picture of the dynamic changes in the thermostability and catalytic activity along with the structural reconstruction of the CeO2(100) surface. The CeO4-terminal type of reconstruction was theoretically predicted to be highly efficient for catalyzing CO oxidation.

Atomic Mechanism in Layer-by-Layer Growth via Surface Reconstruction

Layer-by-layer growth played a critical role in the fine design of novel materials and devices. Although it has been widely studied during materials synthesis, the atomic mechanism of the growth remains unclear due to the lack of direct observation at the atomic scale. Here, we report a new mode in layer-by-layer growth via surface reconstruction on MoO₂ (011) by environmental transmission electron microscopy and density functional theory calculations. Our in situ environmental transmission electron microscopy results demonstrate that the layer-by-layer growth of MoO₂ experiences two steps that occur in an oscillatory manner: (1) the formation of an atomic ledge by transforming a section of the reconstructed layer to the intrinsic surface layer and then (2) the spontaneous reconstruction of the newly formed intrinsic surface section. Thus, the surface reconstruction can be considered as an intermediated phase during the layer-by-layer growth of MoO₂. A similar phenomenon was also observed in the MoO₂ dissolution procedure.

Multiscale Modeling of Agglomerated Ceria Nanoparticles: Interface Stability and Oxygen Vacancy Formation

The interface formation and its effect on redox processes in agglomerated ceria nanoparticles (NPs) have been investigated using a multiscale simulation approach with standard density functional theory (DFT), the self-consistent-charge density functional tight binding (SCC-DFTB) method, and a DFT-parameterized reactive force-field (ReaxFF). In particular, we have modeled Ce₄₀O₈₀ NP pairs, using SCC-DFTB and DFT, and longer chains and networks formed by Ce₄₀O₈₀ or Ce₁₃₂O₂₆₄ NPs, using ReaxFF molecular dynamics simulations. We find that the most stable {111}/{111} interface structure is coherent whereas the stable {100}/{100} structures can be either coherent or incoherent. The formation of {111}/{111} interfaces is found to have only a very small effect on the oxygen vacancy formation energy, E_vac. The opposite holds true for {100}/{100} interfaces, which exhibit significantly lower E_vac values than the bare surfaces, despite the fact that the interface formation eliminates reactive {100} facets. Our results pave the way for an increased understanding of ceria NP agglomeration.

Controlling the {111}/{110} Surface Ratio of Cuboidal Ceria Nanoparticles

The ability to control the size and morphology is crucial in optimizing nanoceria catalytic activity as this is governed by the atomistic arrangement of species and structural features at the surfaces. Here, we show that cuboidal cerium oxide nanoparticles can be obtained via microwave-assisted hydrothermal synthesis in highly alkaline media. High-resolution transmission electron microscopy (HRTEM) revealed that the cube edges were truncated by CeO₂{110} surfaces and the cube corners were truncated by CeO₂{111} surfaces. When adjusting synthesis conditions by increasing NaOH concentration, the average particle size increased. Although this was accompanied by an increase of the cube faces, CeO₂{100}, the cube edges, CeO₂{110}, and cube corners, CeO₂{111}, remained of constant size. Molecular dynamics (MD) was used to rationalize this behavior and revealed that energetically, the corners and edges cannot be atomically sharp, rather they are truncated by {111} and {110} surfaces, respectively, to stabilize the nanocube; both the experiment and simulation showed agreement regarding the minimum size of ∼1.6 nm associated with this truncation. Moreover, HRTEM and MD revealed {111}/{110} faceting of the {110} edges, which balances the surface energy associated with the exposed surfaces, which follows {111} > {110} > {100}, although only the {110} surface facets because of the ease of extracting oxygen from its surface and follows {111} > {100} > {110}. Finally, MD revealed that the {100} surfaces are "liquid-like" with a surface oxygen mobility 5 orders of magnitude higher than that on the {111} surfaces; this arises from the flexibility of the surface species network that can access many different surface arrangements because of very small energy differences. This finding has implications for understanding the surface chemistry of nanoceria and provides avenues to rationalize the design of catalytically active materials at the nanoscale.

Subsurface reconstruction and saturation of surface bonds

Surface reconstructions and stabilization mechanisms have been great challenges for insulators. Based on accurate determination of the long-sought atomic structure of the spinel (1 1 1) surface, here we show that the surface is stabilized by an unconventional mechanism. In general, solid surfaces have unsaturated chemical bonds and are prone to atomic reconstruction to saturate them. The spinel (1 1 1) surface, however, has the surface bonds fully saturated, while the unsaturated bonds remain only in the subsurface. It undergoes a reconstruction that keeps the topmost atomic layer unchanged, but has the subsurface atoms completely rearranged. Such a reconstruction results in a perfect compensation of the surface polarity and a large reduction in the surface energy. This work provides surprising insights into the surface stability and physical and chemical behaviors of complex oxides and insulators.

In situ oxidation and reduction of cerium dioxide nanoparticles studied by scanning transmission electron microscopy

Cerium dioxide nanocubes and truncated octahedra were reduced and oxidized in the scanning transmission electron microscope. The reduction process was stimulated by the electron beam and oxidation was supported by background gases in the microscope environment. High-angle annular dark field imaging is sensitive to local lattice distortions that arise as oxygen vacancies are created and cerium cations reduce enabling high spatial resolution characterization of this process with temporal resolution on the order of seconds. Such measurements enable us to differentiate and infer that the observed behavior between the nanocubes and truncated octahedra may be due to the difference in crystallographic termination of surfaces. In situ measurements taken with different partial pressures of oxygen reveal the cerium oxidation state and the dose rate threshold for the onset of beam reduction are influenced by the environment. Increasing oxygen partial pressure reduces the Ce3+ content and decreases susceptibility to electron beam driven reduction.

Atomic-Scale Valence State Distribution inside Ultrafine CeO2 Nanocubes and Its Size Dependence

Atomic-scale analysis of the cation valence state distribution will help to understand intrinsic features of oxygen vacancies (V_O ) inside metal oxide nanocrystals, which, however, remains a great challenge. In this work, the distribution of cerium valence states across the ultrafine CeO₂ nanocubes (NCs) perpendicular to the {100} exposed facet is investigated layer-by-layer using state-of-the-art scanning transmission electron microscopy-electron energy loss spectroscopy. The effect of size on the distribution of Ce valence states inside CeO₂ NCs is demonstrated as the size changed from 11.8 to 5.4 nm, showing that a large number of Ce³⁺ cations exist not only in the surface layers, but also in the center layers of smaller CeO₂ NCs, which is in contrast to those in larger NCs. Combining with the atomic-scale analysis of the local structure inside the CeO₂ NCs and theoretical calculation on the V_O forming energy, the mechanism of size effect on the Ce valence states distribution and lattice expansion are elaborated: nano-size effect induces the overall lattice expansion as the size decreased to ≈5 nm; the expanded lattice facilitates the formation of V_O due to the lower formation energy required for the smaller size, which, in principle, provides a fundamental understanding of the formation and distribution of Ce³⁺ inside ultrafine CeO₂ NCs.

Protecting Ceria Nanocatalysts-The Role of Sacrificial Barriers

Forces acting on a functional nanomaterial during operation can cause plastic deformation and extinguish desirable catalytic activities. Here, we show that sacrificial materials, introduced into the catalytic composite device, can absorb some of the imposed stress and protect the structural integrity and hence the activity of the functional component. Specifically, we use molecular dynamics to simulate uniaxial stress on a ceria (CeO₂) nanocube, an important functional material with respect to oxidative catalysis, such as the conversion of CO to CO₂. We predict that the nanocube, protected by a "soft" BaO or "hard" MgO sacrificial barrier, is able to withstand 40.1 or 26.5 GPa, respectively, before plastic deformation destroys the structure irreversibly; the sacrificial materials, BaO and MgO, capture 71 and 54% of the stress, respectively. In comparison, the unprotected nanoceria catalyst deforms plastically at only 2.5 GPa. Furthermore, modeling reveals the deformation mechanisms and the importance of microstructural features, insights that are difficult to measure experimentally.