Ultrathin CeO2 Research Articles

Visible-light driven nitrogen-doped petal-morphological ceria nanosheets for water splitting

Water splitting is a promising sustainable technology for solar-to-chemical energy conversion. Herein, we successfully fabricated nitrogen-doped ultrathin CeO2 nanosheets by using field poppy petals as templates, which exhibit an efficiently catalytic activity for water splitting. Abundant oxygen vacancies and substitutional N atoms were experimentally observed in the film due to its unique biomorphic texture.In view of high efficiency and long durability of the as-prepared photocatalyst, this biotemplate method may provide an alternative technique for using biomolecules to assemble 2D nanomaterials.

Three-dimensional graphene network supported ultrathin CeO2 nanoflakes for oxygen reduction reaction and rechargeable metal-air batteries

An efficient oxygen reduction reaction (ORR) on the air cathode is a prerequisite for high-performance metal-air batteries (MABs), as it decreases the excessive charge-discharge overpotential to reduce side-reactions. Here, we develop a composite catalyst for MABs by loading ultrathin CeO2 nanoflakes (UCNFs) on a three-dimensional graphene (3DG) network to form the UCNFs@3DG. The UCNFs@3DG composite features a high activity approaching the commercial Pt/C catalyst in the ORR, probably due to synergic effects of the catalytically active UCNFs and the conductive 3DG network. The UCNFs@3DG can serve as an efficient air cathode in a series of MABs, delivering a capacity of 613 mAh h gZn−1 in the Zn-air battery (ZAB) and 839 mAh h gAl−1 in the Al-air battery (AAB), respectively. Finally, the UCNFs@3DG cathode in lithium-oxygen batteries (LOBs) achieves a high discharge capacity of 21,166 mAh h gc−1 at 0.2 A gc−1 and over 15,000 mAh h gc−1 under elevated current densities up to 2 A gc−1, validating the UCNFs@3DG as a general air cathode in MABs.

Designed synthesis of thin CeO2 nanowires-supported Pt electrocatalysts with pore-interconnected structure and its high catalytic activity for methanol oxidation

A strong local coordination structure of Pt nanoparticles (NPs) over ultra-thin CeO2 nanowires (NWS) has been synthesized to investigate the role of CeO2 NWS on the enhancement of methanol oxidation activity. The Pt/CeO2 NWS catalyst was characterized by X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy and electrochemical tests. The results showed that Pt/CeO2 NWS exhibited 1.23 times higher activity and 1.22 times longer-time durability over a commercial 20 wt% Pt/XC-72 catalyst. To further enhance the activity of Pt/CeO2 NWS catalyst and mass transfer ability, the in situ carbonization of urea had been adopted to provide N-doped carbon doping layer and porous interconnection between CeO2 NWS network, and the resulted catalyst showed 1.38 times greater activity than undoped Pt/CeO2 NWS. It was ascribed to the pyridine-like and pyrrole-like nitrogen in N-doped carbon layers, which greatly increased the electrical conductivity of CeO2 NWS and decreased the loss of electron transfer energy, as well as the strong interaction between Pt NPs and CeO2 NWS with their anisotropy and unique pore-interconnected structure.

Equilibrium oxygen storage capacity of ultrathin CeO2-\u03b4 depends non-monotonically on large biaxial strain

Elastic strain is being increasingly employed to enhance the catalytic properties of mixed ion–electron conducting oxides. However, its effect on oxygen storage capacity is not well established. Here, we fabricate ultrathin, coherently strained films of CeO2-δ between 5.6% biaxial compression and 2.1% tension. In situ ambient pressure X-ray photoelectron spectroscopy reveals up to a fourfold enhancement in equilibrium oxygen storage capacity under both compression and tension. This non-monotonic variation with strain departs from the conventional wisdom based on a chemical expansion dominated behaviour. Through depth profiling, film thickness variations and a coupled photoemission–thermodynamic analysis of space-charge effects, we show that the enhanced reducibility is not dominated by interfacial effects. On the basis of ab initio calculations of oxygen vacancy formation incorporating defect interactions and vibrational contributions, we suggest that the non-monotonicity arises from the tetragonal distortion under large biaxial strain. These results may guide the rational engineering of multilayer and core–shell oxide nanomaterials.

Ultrathin Conductive CeO2 Coating for Significant Improvement in Electrochemical Performance of LiMn1.5Ni0.5O4 Cathode Materials

LiMn1.5Ni0.5O4 (LMNO) has a huge potential for use as a cathode material in electric vehicular applications. However, it could face discharge capacity degradation with cycling at elevated temperatures due to attacks by hydrofluoric acid (HF) from the electrolyte, which could cause cationic dissolution. To overcome this barrier, we coated 3–5 micron sized LMNO particles with a ∼3 nm optimally thick and conductive CeO2 film prepared by atomic layer deposition (ALD). This provided optimal thickness for mass transfer resistance, species protection, and mitigation of cationic dissolution at elevated temperatures. After 1,000 cycles of charge-discharge between 3.5 V–5 V (vs. Li+/Li) at 55°C, the optimally coated sample, 50Ce (50 cycles of CeO2 ALD coated) had a capacity retention of ∼97.4%, when tested at a 1C rate, and a capacity retention of ∼83% at a 2C rate. This was compared to uncoated LMNO particles that had a capacity retention of only ∼82.7% at a 1C rate, and a capacity retention of ∼40.8% at a 2C rate.

Spillover Reoxidation of Ceria Nanoparticles

Interest in resolving the mechanisms behind ceria’s activity has been intense due to the numerous industrial applications including those in heterogeneous catalysis. In this work, we study the reduction and reoxidation of ultrathin CeO2(111) nanoislands on Rh(111) and Pt(111) substrates, so-called inverse model catalysts, with a combination of real and reciprocal space techniques based on X-ray photoemission electron microscopy (XPEEM) and low energy electron microscopy. Soft X-ray microfocused illumination was employed to reduce the ceria islands, which we are able to control by varying the oxygen partial pressure within the measurement chamber. Low energy electron diffraction measurements of the irradiated ceria films demonstrate the formation of an ordered array of oxygen vacancies leading to a (√7 × √7)R19.1° superstructure attributed to the ι-phase (Ce7O12)(111). Resonant photoelectron spectroscopy provides the required high sensitivity to detect small changes in Ce3+ concentration. The high spatial ...

Thermally Stable Hierarchical Nanostructures of Ultrathin MoS2 Nanosheet-Coated CeO2 Hollow Spheres as Catalyst for Ammonia Decomposition.

MoS2 ultrathin nanosheet-coated CeO2 hollow sphere (CeO2@MoS2) hybrid nanostructures with a 3D hierarchical configuration were successfully constructed from a facile two-step wet chemistry strategy: first, CeO2 formed on a silica core which served as a template and was subsequently removed by NaOH solution to attain hollow spheres, and then few-layered ultrathin MoS2 nanosheets were deposited on the CeO2 hollow spheres through a hydrothermal process. As a proof of concept application, the as-prepared CeO2@MoS2 hybrid nanostructures were used as catalytic material, which exhibited enhanced catalytic activity in ammonia decomposition for H2 production at high temperature. It was demonstrated that, even with a structural transformation from MoS2 to MoNx under harsh conditions of ammonia decomposition at high temperature (700 °C), the 3D hierarchical nanostructures of the CeO2@MoNx were well kept, indicating the important role of the CeO2 support.

Lattice Strain Defects in a Ceria Nanolayer.

An ultrathin two-dimensional CeO2 (ceria) phase on a Cu(110) surface has been fabricated and fully characterized by high-resolution scanning tunneling microscopy, photoelectron spectroscopy, and density functional theory. The atomic lattice structure of the ceria/Cu(110) system is revealed as a hexagonal CeO2(111)-type monolayer separated from the Cu(110) surface by a partly disordered Cu–O intercalated buffer layer. The epitaxial coupling of the two-dimensional ceria overlayer to the Cu(110)-O surface leads to a nanoscopic stripe pattern, which creates defect regions of quasi-periodic lattice distortions. The symmetry and lattice mismatch at the interface is clarified to be responsible for the topographic stripe geometry and the related anisotropic strain defect regions at the ceria surface. This ceria monolayer is in a fully oxidized and thermodynamically stable state.

Core–shell structured CeO2@MoS2nanocomposites for high performance symmetric supercapacitors

MoS2 ultrathin nanosheet-coated CeO2 hollow sphere (CeO2@MoS2) nanocomposites with a 3D core–shell architecture were successfully prepared via a facile wet chemistry strategy. The core–shell structured CeO2@MoS2 nanocomposites were systematically characterized using XRD, SEM, TEM and XPS analysis techniques. As a proof of concept application, symmetric supercapacitors were assembled using the CeO2@MoS2 nanocomposites, and aqueous 1 M Na2SO4 or 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid as the electrolyte. The as-obtained CeO2@MoS2 nanocomposites exhibited superior performance in symmetric supercapacitors.

The Interaction of Cobalt with CeO2(111) Prepared on Cu(111)

The interaction of cobalt with an ultrathin CeO2(111) film on Cu(111) was studied by low energy ion scattering spectroscopy and X-ray photoelectron spectroscopy. At room temperature, Co grows three-dimensionally on the ceria film. For small Co coverages (0.2 ML), oxidation to Co2+ occurs together with the formation of Ce3+ in the ceria. At higher Co coverages (0.7 and 2 ML), an increasing fraction of Co remained metallic after deposition. Annealing to higher temperatures induced diffusion of Co2+ ions into the ceria film, and for metallic Co, presumably agglomeration occurs. At T ≥ 800 K, oxidation of significant amounts of Co0 to Co2+ is found, along with diffusion into the ceria film, and partly also in the copper crystal underneath. On a reduced CeOx surface, the redox reaction between cobalt and cerium was hindered, which, in turn, slowed down cobalt migration into the ceria film.

Template- and surfactant-free synthesis of ultrathin CeO2 nanowires in a mixed solvent and their superior adsorption capability for water treatment

Ultrathin CeO2 nanowires with a diameter of 5 nm and an aspect ratio of more than 100 can be prepared by a one-step refluxing approach in a mixed solvent composed of water and ethanol without using any templates or surfactants. The formation mechanism of the as-synthesized ultrathin nanowires has been investigated. The as-synthesized CeO2 nanowires with a high surface area of 125.31 m2 g-1 exhibited excellent wastewater treatment performance with high removal capacities towards organic dyes and heavy metal ions. In addition, the as-synthesized CeO2 nanowires can adsorb Congo red selectively from a mixed solution composed of several dyes. Successful access to high quality ultrathin nanowires will make it possible for their potential application in catalysis and other fields.

Spectromicroscopy of a Model Water–Gas Shift Catalyst: Gold Nanoparticles Supported on Ceria

Nanometer-sized gold particles supported on ceria are an important catalyst for the low-temperature water–gas shift reaction. In this work, we prepared a model system of epitaxial, ultrathin (1–2 nm thick) CeO2–x(111) crystallites on a Rh(111) substrate. Low-energy electron microscopy (LEEM) and X-ray photoemission electron microscopy (XPEEM) were employed to characterize the in situ growth and morphology of these films, employing Ce 4f resonant photoemission to probe the oxidation state of the ceria. The deposition of submonolayer amounts of gold at room temperature was studied with scanning tunneling microscopy (STM) and XPEEM. Spatially resolved, energy-selected XPEEM at the Au 4f core level after gold adsorption indicated small shifts to higher binding energy for the nanoparticles, with the magnitude of the shift inversely related to the particle size. Slight reduction of the ceria support was also observed upon increasing Au coverage. The initial oxidation state of the ceria film was shown to influen...

Integration of atomic layer deposition CeO2 thin films with functional complex oxides and 3D patterns

We present a low-temperature, <300°C, ex-situ integration of atomic layer deposition (ALD) ultrathin CeO2 layers (3 to 5 unit cells) with chemical solution deposited La0.7Sr0.3MnO3 (LSMO) functional complex oxides for multilayer growth without jeopardizing the morphology, microstructure and physical properties of the functional oxide layer. We have also extended this procedure to pulsed laser deposited YBa2Cu3O7 (YBCO) thin films. Scanning force microscopy, X-ray diffraction, aberration corrected scanning transmission electron microscopy and macroscopic magnetic measurements were used to evaluate the quality of the perovskite films before and after the ALD process. By means of microcontact printing and ALD we have prepared CeO2 patterns using an ozone-robust photoresist that will avoid the use of hazardous lithography processes directly on the device components. These bilayers, CeO2/LSMO and CeO2/YBCO, are foreseen to have special interest for resistive switching phenomena in resistive random-access memory.

Preparation, formation mechanism and photocatalysis of ultrathin mesoporous single-crystal-like CeO2 nanosheets

For the first time we synthesized mesoporous CeO2 nanosheets with single-crystal-like, ultrathin and uniform-sized structures through the thermal decomposition of specially prepared intermediates of Ce(OH)CO3 nanosheets. The resulting single-crystal-like CeO2 porous nanosheets are only 2.4 nm thick, as measured by AFM. BET analysis indicated that the pores of the nanosheets were centered at 3.7 nm. A novel structure transformation mechanism from 1D to 0D, then to a 2D assembly was clearly revealed. Moreover, such ultrathin CeO2 porous nanosheets exhibited much higher photocatalytic activity for dye degradation than that of polycrystalline CeO2 nanoparticles.

Microstructure modification of La2Zr2O7 buffer films for coated conductors by metal organic decomposition

La2Zr2O7 (LZO) film directly deposited on Ni-5 at.%W by a chemical solution technique, metal organic decomposition (MOD), indicated a poor texture characteristic, which would result in high-angle grain boundaries in subsequent YBa2Cu3O7−δ (YBCO) associated with weak-link behavior. Different ultrathin MOD-LZO, Y2O3, and CeO2 seed layers (~several nanometers) with various annealing temperature were inserted to improve the crystallographic alignment. The relation between the texture and annealing temperature was systematically investigated. A CeO2 seed layer allows us to grow high quality LZO epitaxial films with values of full width at half-maximum around 5.61° and 5.13° for the Φ-scan of (222) and rocking curve of (400) LZO, respectively, which is comparable to the microstructure of films grown using physical vapor deposited Y2O3 as a seed layer. This buffer template, serving for YBCO coated conductors, could potentially decrease the overall fabrication cost.

Ultra-thin cerium oxide nanostructures through a facile aqueous synthetic strategy

In this work, synthesis of ultra-thin 2D CeO2 nanoribbons is reported for the first time along with rod-shaped nanostructures via a surfactant free aqueous method. CeO2 nanoribbons of length >100nm have been synthesised through a mild reaction route involving ammonia precipitation and subsequent heating and ageing at 0°C. 2D nanostructures are proposed to be formed as a result of self-assembly of CeO2 molecules. The ribbons exhibited amorphous X-ray diffraction pattern and TEM showed a ribbon like morphology with crystal facets of ∼0.33nm spacing corresponding to (111) plane of CeO2. HR-TEM confirmed that the nanorods are enclosed by {111} planes and have preferentially grown in {110} direction. The surface area of the powder containing majority nanorods is 82m2/g and their average pore size is ∼3nm. The nanostructures exhibited optical band gaps dominated by the regular quantum confinement effect as well as presence of defects.

Structure of Ultrathin Epitaxial CeO2 Films Grown on Si(111)

The structure of ultrathin epitaxial CeO2 films grown on Si(111) by molecular beam epitaxy was investigated using high-resolution Rutherford backscattering spectroscopy. The observed elemental depth profiles showed that there were oxygen defects in the CeO2 films and the amount of oxygen defects was about 10%. The strain distribution in the CeO2 film was measured using a channeling technique. It was found that the [100] axis of CeO2 was tilted toward the [211] direction by ∼0.7°. The observed tilt angle was more than twice larger than the expected tilt angle calculated from the lattice mismatch by assuming a pseudomorphic growth of CeO2. The present results suggest that the crystal structure of the CeO2 film was changed from a cubic structure due to the oxygen defects.

Orientation selective epitaxial growth of CeO[formula omitted] layers on Si(1 0 0) substrates using reactive DC magnetron sputtering with substrate bias

Epitaxial growth of ultra-thin CeO2 layers on Si(100) substrates is studied using reactive DC magnetron sputtering enhanced with an inductively coupled RF plasma. Orientation-selective epitaxial growth of CeO2(100) and CeO2(110) is realized by controlling substrate bias and the growth rate, adopting a two-step growth method: ultrathin metallic Ce layer deposition at room temperature followed by silicidation process at several hundreds degree C, and subsequent reactive sputtering in an Ar/O2 mixture environment using a Ce metal target at elevated temperature. It is important that epitaxial growth of both directions is realized on practical H-terminated surfaces obtained by the usual wet process. Details on the growth parameter maps for the orientation selection are reported in terms of substrate bias, plasma power and the growth rate.

Improved Characteristics of Ultrathin CeO2 by Using Postnitridation Annealing

This work demonstrates the improved characteristics of an ultrathin dielectric by using the plasma treatment with additional rapid thermal annealing. The after the treatment exhibits superior characteristics such as a small effective oxide thickness (∼2.25 nm), a low leakage current a high breakdown electric field (−24 MV/cm), a long projected 10 yr lifetime (−12 MV/cm), a small capacitance-voltage hysteresis (25 mV), and a high barrier height for Frenkel-Poole emission (0.55 eV). These good properties are attributed to the nitrogen incorporation into the dielectric to eliminate the traps after annealing. The postnitridation annealing appears to be a very useful treatment for future ultrathin metal-oxide gate dielectrics. © 2004 The Electrochemical Society. All rights reserved.