Single Crystalline CeO2 Research Articles

Cerium coordination-dependent surface intermediates regulate activity in dimethyl carbonate synthesis from CO2 and methanol

The direct synthesis of dimethyl carbonate (DMC) from CO2 and methanol is a potential green alternative to the current industrial methods using toxic chemicals. Although many CeO2-based catalysts have been explored, the key structural factor affecting their DMC activity is still unclear. Herein, single-crystalline CeO2 catalysts bearing distinct but well-defined Ce coordination structures were examined. We found that methanol is the key species being activated by surface Ce sites (cf. CO2) to produce methyl carbonate (MC) and later DMC. The reactivity of surface methoxy species towards CO2 varies considerably with its configuration determined by Ce coordination structures. The head-to-head terminal methoxy species converts CO2 to MC faster than its atilt counterparts, resulting in a higher DMC activity. The bridging methoxy species is, however, too stable to react with CO2. The established structure-activity relation here should also guide the design of CeO2-based catalysts in other reactions involving methanol activation.

Crystallinity-modulated hollow CeO2−x nanorods as free radical scavengers for long-term photostability in organic photovoltaics

Single-crystalline CeO2−x hollow nanorods possess more Ce3+ states and wider surface area than other types of cerium oxide nanostructures and those enable the resultant organic photovoltaic cells to have improved photostability.

Selective Oxidative Coupling of Methane to Ethylene in a Solid Oxide Electrolyser Based on Porous Single-Crystalline CeO2 Monoliths.

Catalytic conversion of CH4 to C2 H4 plays an important role in the light olefin industry. Here, we report the electrochemical conversion of CH4 to C2 H4 /C2 H6 at the anode with the electrolysis of CO2 to CO at the cathode in a solid oxide electrolyser. We constructed well-defined interfaces that function as three-phase boundaries by exsolving single-crystalline Ni nanoparticles in porous single-crystalline CeO2 monoliths. We engineered the chemical states and flux of active oxygen species for the oxidation of CH4 at the anode by controlling voltage and temperature. We show the unprecedented C2 selectivity (C2 H4 and C2 H6 ) of ≥99.5 % at a CH4 conversion of ≈7 %. The electrolyser exhibits excellent durability without performance degradation being observed in a continuous operation of 100 hours. Our work enables a novel path for the selective conversion of CH4 /CO2 into useful chemicals, and the technique of building well-defined interfaces may find potential applications in other fields.

Single crystalline CeO2 nanotubes

Here, we report a facile synthetic methodology to prepare uniform single crystalline CeO2 nanotubes through a hydrothermal transformation of CeO2 nanorods in aqueous Ce(NO3)3 solution. A chemically driven etching-dissolution-deposition mechanism is proposed, involving the surface Ce3+ hydrolysis and dissolution at tips of nanorods and subsequent redeposition and crystallization on the outer sides of nanorods. Compared to CeO2 nanorods, CeO2 nanotubes exhibited the richer structural defects, higher reducibility and larger surface area, leading to a higher haloperoxidase-like activity.

Catalytic performance of Ni catalysts supported on CeO2 with different morphologies for low-temperature CO2 methanation

In this study, a series of Ni catalysts supported on CeO2 with different morphologies including nanopolyhedrons (PH), nanorods (NR), nanoparticles (NP) and nanocubes (NC) was prepared via hydrothermal / wet impregnation method. The catalytic performance of as-prepared catalysts was evaluated for low-temperature CO2 methanation. The Ni/CeO2 catalysts exhibited a superior CO2 conversion and CH4 selectivity over METH®134, a commercial methanation catalyst. The following order of activity was experimentally found : Ni/CeO2-PH > Ni/CeO2-NR > Ni/CeO2-NP > Ni/CeO2-NC > METH®134. Among different CeO2 morphologies, the Ni/CeO2-NR catalyst exhibited the largest surface area and the highest reducibility, providing the high oxygen vacancies/oxygen storage capacity (OSC). Nevertheless, the strong metal-support interaction (SMSI) between Ni and Ce of the Ni/CeO2-NR catalyst determined by H2-TPR posed a negative impact on the CO2 conversion at low temperature. Unexpectedly, the Ni/CeO2-PH catalyst possessed a single crystalline CeO2 nanostructure of ca. 7.4 nm with relatively high surface area and high reducibility especially at low reduction temperature. Therefore, the Ni/CeO2-PH catalyst was found to be the optimum catalyst for low-temperature CO2 methanation.

Single and polycrystalline CeO2 nanorods as oxygen-electrode materials for lithium-oxygen batteries.

Single and polycrystalline CeO2 nanorods (NRs) were prepared for application as oxygen-electrode electrocatalysts for lithium-oxygen batteries. The CeO2 NRs were prepared via a time-controlled hydrothermal process. At a high current rate of 1000 mA g-1, the single crystalline CeO2 NRs exhibited a higher reversibility and a lower voltage gap than polycrystalline CeO2 NRs. We compared the oxygen reduction and evolution kinetics of single and polycrystalline CeO2 NRs using electrochemical impedance spectroscopy.

Shape Effect of MnOx-Decorated CeO2 Catalyst in Diesel Soot Oxidation

Abstract A series of MnOx-decorated CeO2 (MnCeO) catalysts with different morphologies have been prepared by the shape-controlled synthesis of single-crystalline CeO2, such as nanorod, nanoparticle, and nanocube, by a hydrothermal method followed by the deposition of MnOx phase by a simple impregnation method. Their shape-dependent physicochemical properties were investigated by means of XRD, BET, UV–vis, HRTEM, XPS, TPR and oxygen storage/release properties measurements. The MnOx/CeO2 nanorod (MnCeO-NR) showed prominent carbon soot combustion activity at lower temperature under tight contact mode with a T50 temperature of 317 °C, which is attributed to larger surface area and higher oxygen release rate, as compared with particle and cubic catalysts. MnCeO-NR also exhibited higher catalytic activity (a shift of T50 by 127 °C) in NO-assisted soot combustion under a loose contact mode. Not only the higher surface area but also the superior NO conversion ability at low temperature as well as greater NO absorption contributes to the improved catalytic activities. The MnCeO-NR further displayed high stability and satisfactory recyclability without significant loss of catalytic activity. The use of cheaper combinations without noble metals makes the MnOx/CeO2 catalyst a promising candidate to satisfy future requirements in industry.

Ultrasmooth Ru(0001) Films as Templates for Ceria Nanoarchitectures

Single crystalline magnetron sputter-deposited Ru(0001) epitaxial thin films on c-plane sapphire were prepared and used as a template for reactive CeO2 growth. Low-energy electron microscopy and diffraction, as well as transmission electron microscopy and atomic force microscopy, experiments were performed to investigate the crystallinity and morphology of the prepared films. Multiple cycles of Ar+ sputtering and high-temperature annealing produces films of exceptional surface quality. High-temperature reactive ceria growth leads to perfectly aligned triangular single-crystalline CeO2(111) islands of extraordinary morphological and structural homogeneity. At the chosen growth conditions, ceria nucleation takes place only at V-shaped surface defects on the otherwise atomically flat Ru terraces, opening up the possibility to influence the nucleation by introducing artificial surface defects using standard etching techniques. Due to their high crystallinity and extraordinary surface quality, these substrates...

Enhanced Hydroxyl Radical Scavenging Activity by Doping Lanthanum in Ceria Nanocubes

Ceria nanoparticles have been reported to possess special antioxidant and catalytic properties due to their unique redox characteristics. In this study, single-crystalline CeO2 nanocubes were synthesized which demonstrated hydroxyl radical scavenging properties. In order to further enhance such activity, 10% lanthanum (La) was doped into the nanocubes by a hydrothermal method. The as-synthesized, La-modified ceria nanocubes presented improved radical scavenging activity spanning a range of concentrations and durations. The structure and redox behavior of the nanocubes were characterized using X-ray diffraction (XRD), inductively coupled plasma (ICP), X-ray photoelectron spectroscopy (XPS), high resolution electron microscopy (HREM), temperature-programmed reduction (TPR), oxygen storage capacity (OSC), etc. A mechanism was proposed on how the incorporation of La could affect the redox as well as scavenging properties of CeO2 nanocubes. This new nanomaterial may be potentially used as protective agent in b...

Organization of cubic CeO2 nanoparticles on the edges of self assembled tapered ZnO nanorods via a template free one-pot synthesis: significant cathodoluminescence and field emission properties

The present investigation explores the controlled architecture of a CeO2–ZnO nanocomposite via a template-free, low temperature, facile single step solvothermal approach. This complex architecture depicts cubic single crystalline CeO2 nanoparticles (size ∼15 nm) grown on the edges of tapered ZnO nanorods with definite orientations and alignments. The formation of wurtzite ZnO, cubic CeO2 and the coexistence of Ce3+ and Ce4+ on the surface of the CeO2–ZnO nanocomposites are confirmed using various characterization tools. The finding of such unique nanostructures by a facile method is exemplified by a plausible growth mechanism. Surprisingly, the aqueous mediated ultrasonication reaction conferred the formation of crystalline ZnO nanotubes of diameter ∼50 nm. Spatially resolved cathodoluminescence spectra are obtained by linearly scanning an individual CeO2–ZnO nanorod along its length, which reveals the size-dependent surface effects. Interestingly, such hybrid CeO2–ZnO nanoarchitecture is observed to exhibit enhanced field emission properties, demonstrating better current stability as compared to other ZnO nanostructures. This is attributed mainly to strong surface interactions between the Ce-ionic species and the ZnO nanorods. Herein, a soft-chemical approach is used for the first time to architect a binary oxide nanostructure, which is otherwise accomplished using high temperature techniques, as reported elsewhere. Also, the present work not only gives insight into understanding the hierarchical growth behaviour of the CeO2–ZnO nanocomposite in a solution phase synthetic system, but also provides an efficient route to enhance the field emission performance of ZnO nanostructures, which could be extended to other potential applications, such as chemical sensors, optoelectronic devices and photocatalysts.

Hydrothermal synthesis of CeO2nanorods using a strong base–weak acid salt as the precipitant

Single-crystalline CeO2 nanorods of the size of about 30 nm diameter and 300 nm length were synthesised by using a strong base–weak acid salt as the precipitant, by a simple hydrothermal method without any surfactant. The structure and properties of the CeO2 nanorods were characterised by X-ray diffraction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, selected-area electron diffraction, UV–visible absorption and scanning electron microscopy field emission. Reaction parameters, such as the reaction temperature, the reaction time, the molar ratio of the reactants and the pH value, were investigated by a series of control experiments. The effect of the strong base–weak acid salt in the synthesis of the CeO2 nanorods was discussed.

Controllable Electrochemical Synthesis and Photocatalytic Activity of CeO2 Octahedra and Nanotubes

Single-crystalline CeO2 octahedra were successfully grown on Cu substrates via a facile electrochemical approach, and free-standing CeO2 nanotubes with high crystallinity were also obtained by adjusting the reaction temperature. A ferromagnetic behavior at 5 K arising from their intrinsic defects is observed for the CeO2 octahedra and nanotubes. Furthermore, the CeO2 octahedra and nanotubes exhibit a good photocatalytic performance in the degradation of an organic pollutant in waste water.

Hydrothermal Synthesis of Single Crystalline Nano CeO2 and Its Structural, Optical, and Electronic Characterization

Single crystalline CeO2 powder was prepared by precipitation-hydrothermal technique. The hydrothermal treatment temperature and time were the two parameters chosen to observe their effect on nature of ceria. The synthesized samples were characterized for structural, optical, and electrical properties using X-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electron microscope (SEM), transmission electron microscope (TEM), ultraviolet-visual (UV-Vis), and photoluminescence (PL) spectroscopy. Single crystalline CeO2 nanoparticles of about 5 to 9 nm were formed when the precipitated precursor was heated at 200°C for 3 h. The mean crystalline diameter (MCD) obtained from XRD using Scherrer's formula matched with the particle size as observed from TEM confirming the single crystalline nature of ceria. The crystallinity of sample increased with increase in reaction temperature and time while the lattice parameters decreased, as observed from XRD data. Observation from the micrographs obtained from SEM indicated the decrease in particle size as well agglomeration with increase in reaction temperature which was also confirmed by blue shift in absorption spectra of the sample obtained at 200°C. Room temperature PL spectra of CeO2 nano crystals, excited at wavelength 300 nm, showed emission at 400 and 360 nm corresponding to CeO2 synthesized at temperatures 160°C, and 200°C with corresponding E g values of 2.94 eV and 3.60 eV, respectively.

Template-free hydrothermal synthesis and characterisation of single crystalline Ce(OH)CO 3 and CeO 2 with spindle-like structures

Single crystalline Ce(OH)CO3 and CeO2 with spindle-like structures were synthesised by a facile hydrothermal synthesis method. Then the effect of reaction time and concentration of cerium precursor on the formation and morphologies of Ce(OH)CO3 and CeO2 were further investigated. The products were characterised by X-ray powder diffraction, transmission electron microscopy and scanning electron microscopy. The results show that the single crystalline Ce(OH)CO3 with spindle-like structures could be obtained at 120°C for 8 h. The present method shows an easy process without high reaction temperature, long reaction time, surfactant and template to attain final Ce(OH)CO3 products. Furthermore, single crystalline CeO2 with spindle-like structures had been fabricated by a oxidation process at 600°C for 2 h using Ce(OH)CO3, with spindle-like structures as the precursor. Meanwhile, the single crystalline spindle-like CeO2 shows a high catalytic activity on CO conversion.

Template-Free Hydrothermal Synthesis of CeO2 Nano-octahedrons and Nanorods: Investigation of the Morphology Evolution

Uniform single-crystalline CeO2 nano-octahedrons and nanorods were synthesized by a facile hydrothermal synthesis process only using Ce(NO3)3·6H2O as cerium resource and Na3PO4·6H2O as mineralizer, into which no surfactant or template was introduced. By tuning the hydrothermal treatment time, the morphology evolution between the nano-octahedron and nanorod was observed. Furthermore, the synthesizing mechanism and the morphological evolution of different shapes were investigated. Unlike traditional hydrothermal synthesis of CeO2 nanostructures using strong base as precipitant, Na3PO4 does not leave any impurity in the hydrothermal reaction system and makes the process very simple to obtain and separate the octahedral and rodlike morphology.

Single-crystalline ceria nanocubes: size-controlled synthesis, characterization and redoxproperty

Single-crystalline CeO2 nanocubes were synthesized through a hydrothermal treatment. By varyingreaction temperature and the NaOH concentration, the size control ofCeO2 nanocubes has been achieved, which produces the nanocubes with a controllableedge length in the regime of 20–360 nm. HRTEM studies reveal that theCeO2 nanocubes expose their high energy {001} planes. Consequently, it is demonstrated that theCeO2 nanocubes exhibit excellent reducibility and high oxygen storage capacity, indicating theyare potential novel catalytic materials.

Origin of crystalline quality deterioration in epitaxial growth of CeO2 layers on Si substrates

CeO 2 layers with various degrees of crystallinity are characterized to get insight into the origin of the deterioration of epitaxial layers on Si substrates. The surface morphology of CeO2 layers is studied using atomic force microscopy and is correlated with the crystallinity of the layer determined by reflection high-energy electron diffraction. It is clearly observed that the surface morphology changes according to the crystallinity of the CeO2 layers. Single crystalline CeO2(110) layers with good crystallinity have a nanometer-scale periodically corrugated surface structure, which consists of (111) facets. As the crystalline quality of the CeO2 layers becomes worse, the number of irregularly shaped hillocks and tetrahedral hillocks increases and corresponds to distorted (110) grains and (111)-oriented polycrystalline grains, respectively. It is found that the crystalline quality is not uniform; different regions of crystallinity are distributed from place to place and the ratio of their population changes according to the degree of crystalline quality. Direct observation of irregularly shaped hillocks is carried out using cross-sectional lattice images from high resolution transmission electron microscopy. These features are discussed to understand the origin of the irregularly shaped hillocks in terms of the surface properties of the substrates at the early stage of growth.

Interfacial properties of single-crystalline CeO2 high-k gate dielectrics directly grown on Si (111)

Interfacial properties of single-crystalline CeO2 high-k dielectrics directly grown on Si (111) were investigated by comparing metal–insulator–semiconductor field-effect transistors (MISFETs) without any interfacial layer [(w/o-IL); direct growth of CeO2 on Si] and those with an interfacial layer (w-IL). FET characteristics, such as the drain current and the S factor, for the w/o-IL MISFET were much worse than those for the w-IL MISFET. The in-gap states attributed to the oxygen defects were detected in the CeO2 directly grown on Si by x-ray photoelectron spectroscopy measurements. The large interface state density induced by the oxygen defects at the CeO2/Si interface may deteriorate the w/o-IL MISFET performances.

Electrical Properties of Single Crystalline CeO2 High-k Gate Dielectrics Directly Grown on Si (111)

We have grown epitaxial single crystalline CeO2 directly on Si (111) by molecular beam epitaxy. Metal-insulator-semiconductor (MIS) capacitors were fabricated to evaluate electrical properties of CeO2 films. By eliminating an interfacial layer, equivalent oxide thickness (EOT) as small as 0.38 nm was obtained for a CeO2 physical thickness of 5 nm. The dielectric constant (ε) was calculated to be ε∼52, which is two times larger than the value reported for bulk (polycrystalline) CeO2 (ε∼26). Enhancement of ε is probably attributable to the anisotropy of the single crystalline CeO2 and/or the lattice distortion of CeO2 directly grown on Si. A leakage current density of ∼ 1 A/cm2 was obtained at an equivalent electrical field of 5 MV/cm for the CeO2 MIS capacitors, which is much smaller than the expected value for SiO2 capacitors with the same EOT. It is revealed that single crystalline CeO2 is promising as an alternative gate dielectric.

Electrical Characteristics of Metal/Cerium Dioxide/Silicon Structures

Electrical characteristics of a Au/CeO2/Si structure have been measured by means of capacitance-voltage (C–V) and conductance-voltage (G–V) methods. Single crystalline and polycrystalline CeO2 films are grown on silicon substrates using electron-beam evaporation. MOS characteristics of the samples are classified by the crystallinity of the CeO2 films rather than by the orientation or conduction type of the Si substrate. Polycrystalline CeO2 film samples show a fine structure around the flat-band voltage in the C–V curves and injection-type hysteresis. It is clarified that the characteristics of the single crystalline CeO2 film sample are free from such undesirable properties caused by negative charge and carrier injection. Dielectric constant εr of the CeO2 films is estimated to be around 20, which is close to the bulk value (∼26).