Ultrathin CeO2 Research Articles

Ultra-thin order–disorder CeO2 nanobelts as the non-carbon support of the PtCu catalyst towards methanol oxidation and oxygen reduction reactions

By using the ultra-thin CeO2 nanobelts (2D-CeO2) with order–disordered structure as the non-carbon support, the PtCu/2D-CeO2 catalyst exhibits outstanding MOR and ORR activities.

A universal multifunctional rare earth oxide coating strategy to stabilize high-nickel lithium layered oxide cathode

LiNi0.8Co0.1Mn0.1O2 (NCM811) high-nickel lithium layered oxide material has gained recognition as a promising cathode material for lithium-ion batteries (LIBs) due to its relatively high output capacity. However, the presence of high nickel content leads to capacity fading resulting from surface residue lithium and cathode-electrolyte interfacial side reactions, posing a significant threat to the cycle life for battery. In this study, we have developed an ultrathin and uniform CeO2&LiCeO2 nanoscale rare earth composite functional coating in-situ for NCM811 cathode, utilizing residual lithium and rare earth oxide. Ex-situ scanning electron microscope (SEM), electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectrometer (XPS) analyses confirm the reduction in interfacial side reactions, leading to improved stability at the cathode-electrolyte interface. This improvement can be attributed to the removal of surface residual lithium and the formation of stable CeO2&LiCeO2 functional coatings. As a result, the NCM811 modified with the rare earth composite functional coating exhibits a higher capacity retention rate after 100 cycles (90.9% vs. 86.1%) and enhanced rate capacity (136 mAh g−1 vs. 117 mAh g−1) at 5 C than the NCM811. Even the designed full battery achieves an energy density as high as 253 Wh kg−1. This work presents an innovative approach to utilize residual lithium and rare earth oxide for in-situ functional coating formation, ultimately enhancing the lithium storage performance of high-nickel NCM811 cathodes.

Engineering of Micro-mesoporous two-dimensional CeO2-based heterojunction oxides for energy storage applications

With growing demands on energy supply and storage, there is a need for advanced devices that can meet the high power and energy requirements. One such device is a supercapacitor, which is classified into two types, namely the electrical double-layer capacitor (EDLC) and the pseudocapacitor. Pseudocapacitors show energy storage properties that are between EDLCs and batteries and generally, transition metal oxides are used for these applications. Recent work has demonstrated that CeO2 is a potential material for such applications owing to its reversible charge-switching capabilities and good stability in different environments. In the present work, novel ultrathin nanosheets of CeO2-x and a composite of ruthenium (Ru)-CeO2-x, with the Ru in the form of mixed heteroatoms and dopants were synthesised using a Ce-based coordination polymer (Ce-CP) as the precursor using modified anodic chronoamperometric electrodeposition (MACE) technique. The ready exfoliation of Ce-CP in triethanolamine, which was the exfoliation medium, followed by solvent removal and annealing at 300 °C resulted in the formation of ultrathin 2D CeO2 nanosheets that are partially stacked, resulting in the formation of flower-like 3D nanostructures. The ultrathin 2D CeO2 formed 3D flower-like nanostructures with high concentrations of defects, active sites, and mesopores, which increased with the concentration of Ru in the CeO2-x nanostructure. Additionally, the specific surface area of the Ru-CeO2-x samples was higher, increasing to 354 m2·g−1 compared to the value of 146 m2·g−1 for pristine CeO2-x. The cyclic voltammetry analysis revealed that the introduction of Ru increased the capacitance of pristine CeO2-x from 665 F·g−1 to 923 F·g−1 at a scan rate of 5 mV·s−1. These findings suggest that Ru-CeO2-x nanosheets have potential for use in supercapacitor applications.

Nanostructured CeO2 ultrathin film deposited by the Langmuir Blodgett technique for highly sensitive and specific detection of sub ppm level NO2 gas at room temperature

Room temperature NO2 sensor based on ultrathin CeO2 deposited by LB technique was fabricated. High sensing and selectivity to ppb level NO2 gas was observed due to large number of surface adsorbed oxygen species on nanostructure CeO2 surface.

Interaction of Hydrogen with Cu-Modified Cerium Oxide Surfaces

We investigate the interaction between molecular hydrogen and ultrathin epitaxial CeO2 films modified with a 2% concentration of Cu atoms using X-ray photoemission spectroscopy (XPS) during thermal reduction cycles in H2. The XPS measurements are combined with density functional theory calculations to obtain further insight into the observed modifications of the film surface. Our results show that the presence of Cu atoms decreases the barrier for H2 dissociation in comparison to that on pure ceria surfaces, leading to the formation of surface OH groups after exposure to H2. Moreover, surface oxygen vacancies are generated already at mild temperatures (470 K), most likely via water formation and desorption. The presence of surface oxygen vacancies and hydroxyls contributes to the observed large increase in surface Ce3+ concentration with increasing reduction temperature. In spite of these atomic scale modifications, the surface morphology observed by scanning tunneling microscopy remains substantially unchanged on the length scale of tens of nm.

Two-dimensional heterostructures built from ultrathin CeO2 nanosheet surface-coordinated and confined metal-organic frameworks with enhanced stability and catalytic performance.

Two-dimensional (2D) metal–organic framework (MOF) based heterostructures will be greatly advantageous to enhance catalytic performance because they increase the contact surface and charge transfer. Herein, a novel 2D heterostructure named CeO2@NiFe-MOFs, in which monolayer NiFe-MOFs is coordinated with ceria (CeO2) to improve catalytic and stability performance, is successfully constructed by the strategy of in situ growth on the surface of ultrathin CeO2 nanosheets being functionalized with monolayer carboxylic acid groups. The 2D heterostructure possesses a sandwich structure, where monolayer NiFe-MOFs are coordinated to both the top and bottom surface of CeO2 nanosheets via joining carboxylic acid groups. In particular, CeO2 with robust coordination plays a significant role in the anchoring of carboxylic acid groups and binding strength of heterostructures. The 2D CeO2@NiFe-MOF heterostructure with a joint effect of metal–ligand coordination not only presents good structural stability but also significantly enhances the oxygen evolution reaction (OER) efficiencies in comparison to bare NiFe-MOFs, achieving a current density of 20 mA cm−2 at a low overpotential of 248 mV as well as durability for at least 40 h. Meanwhile, the electronics, optics, band gap energy and local strains of CeO2 decorated with 2D NiFe-MOFs are different to the properties of bare CeO2. Our study on the construction of an ultrathin CeO2 surface-coordinated and confined MOF layer may pave a new way for novel 2D MOF composites/heterostructures or multi-functional 2D CeO2 materials to be used in energy conversion or other fields.

Construction of Z–scheme 1D CdS nanorods/2D ultrathin CeO2 nanosheets toward enhanced photodegradation and hydrogen evolution

The synthesis of semiconductor-based photocatalytic materials has been developed rapidly because of the wide application potential in solving environmental pollution and energy crises. In this study, a noval Z–scheme heterojunction by in situ assembly of two-dimensional (2D) ceria nanosheets and one-dimensional (1D) cadmium sulfide nanorods (CdS/CeO2) was constructed and employed for effective photocatalytic degradation of contaminant and hydrogen production under irradiation. The morphologies and physicochemical properties of as-synthesized nanostructures were analyzed by a series of characterizations. The optimized CdS/CeO2 showed more superior degradation efficiency of tetracycline than the most CdS-based materials as reported, and exhibited about 1.8 folds higher hydrogen evolution efficiency than pristine CdS. A direct Z–scheme charge transfer mechanism was proposed by combining radicals trapping experiments and electron paramagnetic resonance spectra, which accelerated the efficient transport of photo-induced charge carriers. The present work demonstrates that CdS/CeO2 nanomaterial with high photocatalytic performance has a promising application in the environmental treatment and green energy production.

Thermal behaviour of Cu and Au nanoparticles grown on CeO2 thin films

Bimetallic catalysts are often more active and/or selective than their monometallic counterparts. The behaviour of such catalysts is frequently strongly dependent on the molar ratio of the two elements as well as nanoparticle size and the interaction with the support material. X-ray photoelectron spectroscopy (XPS) is an excellent surface analytical technique for probing the electronic properties of catalytic systems. When a mixture of pure and alloyed particles is present, it is more difficult to extract information from XPS given that it is a spatial averaging technique. Recently, the technique of medium energy ion scattering (MEIS) has been exploited to investigate the depth-dependent composition of nanoparticles on planar surfaces. Herein, we combine the two techniques to investigate the nature of Cu and Au nanoparticles deposited onto ultrathin CeO2 films on Si(111) examining their morphology and chemical composition as a function of annealing temperature for samples that have been maintained in an ultrahigh vacuum environment and exposed to air. The Cu/Au/CeO2/Si(111) is chosen as a model system in order to provide insight into how the catalytic properties of Cu/Au/CeO2 depend on the presence of discrete Cu and Au particles versus fully intermixed Cu/Au systems.

Electronic and optical properties of ultrathin cerium dioxide: A many-body GW-BSE investigation

Ultrathin CeO2 shows enormous potential as promising optical, catalytic, and electronic materials. However, accurate calculations of the basic structural, electronic, and optical properties of ceria in the ultrathin limit have been lacking by far. In this work, structural, electronic, and optical properties of ultrathin CeO2 are investigated. Phonon dispersion calculations are performed for ultrathin CeO2 to justify its dynamic stability. Band structures and density of states of ultrathin CeO2 are calculated by the density functional theory. After the GW correction, the quasiparticle band energies of ultrathin CeO2 from monolayer to tetralayer are predicted. Band edge positions of ultrathin CeO2 are also determined, and their thickness dependence is displayed for tunable photocatalytic applications. The optical properties of monolayer CeO2 have been calculated by solving the Bethe-Salpeter equation and eliminating the ambiguity introduced by the vacuum layer added in the supercell. Our calculations predict that monolayer CeO2 has great in-plane optical performance in the ultra-violet region and can be tuned for further photocatalytic and optoelectronic applications.

Ultrathin CeO2 nanosheets as bifunctional sensing materials for humidity and formaldehyde detection

Issues like morphology control and further multifunctional applications are of significant importance for rare earth nano-oxides, e.g., cerium dioxide (CeO2) nanostructures, however, relevant results in this respect are rather limited up to now. In the present work, ultrathin CeO2 nanosheets were synthesized through a facile low-temperature hydrothermal method. The structure, morphology and specific surface area of these CeO2 nanosheets were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and N2 adsorption–desorption. Significantly, CeO2 nanosheets have the potential as bifunctional sensing materials to detect both humidity and formaldehyde vapor. The CeO2 nanosheet humidity sensor exhibited excellent sensing characteristics in the relative humidity range of 11%–97% with the response value as high as 3.1 × 104. Meanwhile, the CeO2 nanosheet gas sensor showed superior sensitivity and repeatability with fast response/recovery speed toward formaldehyde vapor at 300 °C. Finally, the humidity and formaldehyde sensing mechanism were discussed as well.

Catalytic Stability Studies Employing Dedicated Model Catalysts.

Long-term stability of heterogeneous catalysts is an omnipresent and pressing concern in industrial processes. Catalysts with high activity and selectivity can be searched for by high-throughput screening methods based maybe on educated guesses provided by ab initio thermodynamics or scaling relations. However, high-throughput screening is not feasible and is hardly able to identify long-term stable catalyst so that a rational and knowledge-driven approach is called for to identify potentially stable and active catalysts. Unfortunately, our current microscopic understanding on stability issues is quite poor. We propose that this gap in knowledge can be at least partly closed by investigating dedicated model catalyst materials with well-defined morphology that allow for a tight link to theory and the application of standard characterization methods. This topic is highly interdisciplinary, combining sophisticated inorganic synthesis with catalysis research, surface chemistry, and powerful theoretical modeling. In this Account, we focus on the stability issues of Deacon catalysts (RuO2 and CeO2-based materials) for recovering Cl2 from HCl by aerobic oxidation and how to deepen our microscopic insight into the underlying processes. The main stability problems under harsh Deacon reaction conditions concomitant with a substantial loss in activity arise from deep chlorination of the catalyst, leaching of volatile chlorides and oxychlorides, and decrease in active surface area by particle sintering. In general, powder materials with undefined particle shape are not well suited for examining catalyst stability, because changes in the morphology are difficult to recognize, for instance, by electron microscopy. Rather, we focus here on model materials with well-defined starting morphologies, including electrospun nanofibers, shape-controlled nanoparticles, and well-defined ultrathin crystalline layers. CeO2 is able to stabilize shape-controlled particles, exposing a single facet orientation so that comparing activity and stability studies can reveal structure sensitive properties. We develop a quasi-steady-state kinetic approach that allows us to model the catalyst chlorination as a function of temperature and gas feed composition. For the case of pure CeO2 nanocubes, this simple approach predicts chlorination to be efficiently suppressed by addition of little amounts of water in the reaction feed or by keeping the catalyst at higher temperature. Both process parameters have great impact on the actual reactor design. Thermal stabilization of CeO2 by intermixing Zr has been known in automotive exhaust catalysis for decades, but this does not necessarily imply also chemical stabilization of CeO2 against bulk-chlorination since Zr can readily form volatile ZrCl4 and may quickly lose its stabilizing effect. Nevertheless, with model experiments the stabilizing effect of Zr in the Deacon process over mixed CexZr1-xO2 nanorods is clearly evidenced. Even higher stability can be accomplished with ultrathin CeO2 coatings on preformed ZrO2 particles, demonstrating the great promise of atomic layer deposition (ALD) in catalysis synthesis.

Core-shell-like structured α-MnO2@CeO2 catalyst for selective catalytic reduction of NO: Promoted activity and SO2 tolerance

MnO2 is effective to selectively catalytically reduce NO (SCR) at low-temperature as an environmental catalyst, but suffers seriously from SO2 poisoning. The α-MnO2@CeO2 catalyst with a core-shell-like structure was synthesized by a two-step hydrothermal method. The CeO2 shell with a depth of 10 nm was deposited outside α-MnO2, consisting of CeO2 nanoparticles with a diameter of less than 5 nm. The ultra-thin CeO2 shell was porous and did not affect the gaseous molecules diffusing. Furthermore, the valence of Mn decreased and the redox property improved due to the CeO2 shell, thus increasing the NO removal efficiency. Moreover, CeO2 shell could bind SO2 strongly in priority to the poisoning of MnO2, thus functioning as the sacrifice to protect the α-MnO2 core from SO2 poisoning. The preferential adsorption of SO2 on the CeO2 shell was because of its strong covalent binding with SO2. SO2 was oxidized and solidified by lattice oxygen, leaving the gas phase and avoiding further attack on active center Mn. This work offers a new route for the design of SO2-resistant Mn-based catalysts for SCR of NOx at low temperatures.

Ultrathin Atomic Layer-Deposited CeO2 Overlayer for High-Performance Fuel Cell Electrodes.

Obtaining a catalyst with high activity and thermal stability is essential for high-performance energy conversion devices operating at an elevated temperature. Herein, the design and fabrication of a heterogeneous catalyst with an ultrathin CeO2 overlayer via atomic layer deposition (ALD) on Pt electrodes for low-temperature solid oxide fuel cells (LT-SOFCs) is reported. The cell with a CeO2-overcoated (five ALD cycles) Pt cathode shows lower activation resistance by 50% after a 10 h operation and higher thermal stability by a factor of 2 compared with the cell with a Pt-only cathode, which is known to be the best single catalyst at 450 °C. Eventually, a thin-film SOFC with a highly active and stable CeO2-overcoated cathode based on an anodized aluminum oxide (AAO) substrate demonstrates a high peak power density of 800 mW cm-2 at 500 °C, which is the highest performance ever reported for an AAO-based SOFC at this temperature.

Proton-assisted creation of controllable volumetric oxygen vacancies in ultrathin CeO2\u2212x for pseudocapacitive energy storage applications

Two-dimensional metal oxide pseudocapacitors are promising candidates for size-sensitive applications. However, they exhibit limited energy densities and inferior power densities. Here, we present an electrodeposition technique by which ultrathin CeO2−x films with controllable volumetric oxygen vacancy concentrations can be produced. This technique offers a layer-by-layer fabrication route for ultrathin CeO2−x films that render Ce3+ concentrations as high as ~60 at% and a volumetric capacitance of 1873 F cm−3, which is among the highest reported to the best of our knowledge. This exceptional behaviour originates from both volumetric oxygen vacancies, which enhance electron conduction, and intercrystallite water, which promotes proton conduction. Consequently, simultaneous charging on the surface and in the bulk occur, leading to the observation of redox pseudocapacitive behaviour in CeO2−x. Thermodynamic investigations reveal that the energy required for oxygen vacancy formation can be reduced significantly by proton-assisted reactions. This cyclic deposition technique represents an efficient method to fabricate metal oxides of precisely controlled defect concentrations and thicknesses.

Active and stable Pt-Ceria nanowires@silica shell catalyst: Design, formation mechanism and total oxidation of CO and toluene

Cerium oxide is one of the most important rare earth metal oxides in catalysis, however, the sintering problem of noble metals and CeO2 at higher temperature (e.g., >700 °C) is still unresolved. Herein, Pt nanoparticles self-assembled on ultra-thin CeO2 nanowires (NWs) and then confined inside a thermally robust porous silica shell (Pt-CeO2NW@SiO2) were introduced. The thickness of CeO2 NWs was just ˜2.0 nm. Moreover, Pt-CeO2 NW@SiO2 showed significantly enhanced catalytic performances for total oxidation of CO and toluene. The increased catalytic properties are attributed to the strong metal-support interaction effect between Pt and CeO2 NWs at sub-nanoscale. Most importantly, the special core-shell structure also affords excellent sintering resistance retention up to 700 °C for 100 h, due to the guarding effect of porous silica shell. Finally, the formation mechanism of Pt-CeO2 NW@SiO2 was investigated in detail. Current strategy should inspire many rational designs of rare-earth metal-based nanocatalysts for real-world catalysts.

A Discovery of an Unexpected Metal Dissolution of Thin-Coated Cathode Particles: Its Theoretical and Experimental Explanations

The degree of metal dissolution of cathode materials is a critical parameter in determining Li-ion battery performance. Ultra-thin coated LiMn2O4 (LMO) cathode particles via Atomic Layer Deposition (ALD) exhibit superior battery performance over bare LMO particles. However, we have observed that ultra-thin CeO2 coating intensifies Mn dissolution of LMO during cycling of lithium-ion battery, whereas ultra-thin Al2O3 coating tends to inhibit Mn dissolution. A detailed study of Density Functional Theory (DFT) has been carried out to illustrate the experimental observations. First, the manganese vacancy formation energy is calculated, along with the bonding strengths of Mn-O of uncoated, Al2O3 coated, and CeO2 coated particles via Crystal Orbital Overlap Population (COOP) calculations. Further, the projected density of state calculation of Mn is used to confirm the electronic occupancy of Mn atom for each case. Surprisingly, the atomic analysis is consistent with the experimental observations. This is the first report that coatings can accelerate metal dissolution, which can show new insights into ALD coatings and their impact on metal dissolution in cathode materials.

Atomic-layer-deposited ZrO2-doped CeO2 thin film for facilitating oxygen reduction reaction in solid oxide fuel cell

Ultra-thin ZrO2-doped CeO2 (ZDC) interlayers (20 nm thick) with varying doping ratios of 0, 20, and 60 mol% were prepared using atomic layer deposition (ALD), and were investigated as cathodic interlayers for low-temperature solid oxide fuel cells (LT-SOFCs). The inclusion of ZrO2 in CeO2 film induced the reduction of Ce4+ to Ce3+ with higher concentration of oxygen vacancies, and also enhanced the resistance of the film to the coarsening at elevated temperature (800 °C), well preserving the nanoscale fine grain structure. As a result, the maximum power density of the cell with 20 mol%-doped ZDC interlayer improved by 57% compared to the cell without the interlayer due to enhanced activation process at the cathode, which seems to be due to higher oxygen vacancy population as well as higher grain boundary density at the electrolyte-cathode interface.

Ultrathin CeO2 coating for improved cycling and rate performance of Ni-rich layered LiNi0.7Co0.2Mn0.1O2 cathode materials

In this study, we have successfully coated the CeO2 nanoparticles (CeONPs) layer onto the surface of the Ni-rich layered LiNi0.7Co0.2Mn0.1O2 cathode materials by a wet chemical method, which can effectively improve the structural stability of electrode. The X-ray powder diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS) are used to determine the structure, morphology, elemental composition and electronic state of pristine and surface modified LiNi0.7Co0.2Mn0.1O2. The electrochemical testing indicates that the 0.3 mol% CeO2-coated LiNi0.7Co0.2Mn0.1O2 demonstrates excellent cycling capability and rate performance, the discharge specific capacity is 161.7 mA h g−1 with the capacity retention of 86.42% after 100 cycles at a current rate of 0.5 C, compared to 135.7 mA h g−1 and 70.64% for bare LiNi0.7Co0.2Mn0.1O2, respectively. Even at 5 C, the discharge specific capacity is still up to 137.1 mA h g−1 with the capacity retention of 69.0%, while the NCM only delivers 95.5 mA h g−1 with the capacity retention of 46.6%. The outstanding electrochemical performance is assigned to the excellent oxidation capacity of CeO2 which can oxidize Ni2+ to Ni3+ and Mn3+ to Mn4+ with the result that suppress the occurrence of Li+/Ni2+ mixing and phase transmission. Furthermore, CeO2 coating layer can protect the structure to avoid the occurrence of side reaction. The CeO2-coated composite with enhanced structural stability, cycling capability and rate performance is a promising cathode material candidate for lithium-ion battery.

Optical Analysis of Oxygen Self‐Diffusion in Ultrathin CeO2 Layers at Low Temperatures

AbstractAn optical in situ strategy for the analysis of oxygen diffusion in ultrathin ceria layers with a thickness of 2–10 nm at temperatures between 50 and 200 °C is presented, which allows for the determination of diffusion coefficients. This method is based on the sensitivity of the photoluminescence (PL) intensity of InGaN nanowires to adsorbed oxygen. The oxygen diffusion through an ultrathin CeO2 coating deposited on the InGaN nanowires is monitored by analyzing the transient PL behavior of the InGaN nanowires, which responds to changes of the oxygen concentration in the environment when the corresponding oxygen concentration is established at the CeO2/InGaN interface due to diffusion through the coating. Quantitative evaluation of the oxygen diffusion in CeO2 based on a model considering Langmuir Adsorption and recombination yields a diffusion coefficient D of (2.55 ± 0.05) × 10−16 cm2 s−1 at a temperature of 100 °C. Temperature‐dependent measurements reveal an Arrhenius type behavior of D with an activation energy of (0.28 ± 0.04) eV. In contrast, no oxygen diffusion is detected for an ultrathin layer (≥5 nm) of Al2O3, which is known as a poor oxygen ion conductor within the analyzed temperature regime.

Interaction of HCl with a CeO2(111) Layer Supported on Ru(0001): A Theory and Experiment Combined Study

Ultrathin crystalline CeO2(111) films were grown on Ru(0001) in order to study the interaction of HCl with this surface as a first step of the Deacon reaction with experimental techniques, including low energy electron diffraction (LEED), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and thermal desorption spectroscopy (TDS) in combination with density functional theory calculations (DFT+U). HCl molecules adsorb dissociatively on CeO2(111), forming a well-ordered (3x3)R30° overlayer structure with one Cl and one H per surface unit cell. DFT calculations indicate that HCl adsorption is exothermic by 1.15 eV and proceeds via an acid–base reaction. The mixed overlayer structure is stabilized by Lewis acid–base pairing (∼0.4 eV). Stoichiometric CeO2(111) films are likely to be not very active in the Deacon process since at 800 K the recombination of adsorbed H* and Cl* to form HCl is far more preferred over Cl* + Cl* recombination to form the desired product Cl2.