Co-doped CeO2 Research Articles

Room-temperature ferromagnetism in Co-doped CeO2 nanospheres prepared by the polyvinylpyrrolidone-assisted hydrothermal method

Nanospheres of pure CeO2 and Co-doped CeO2 (Ce1−xCoxO2, 0.01 ≤ x ≤ 0.07) dilute magnetic oxide were prepared by hydrothermal treatment using cerium (III) nitrate, cobalt (III) nitrate, and polyvinylpyrrolidone (PVP) as a surfactant. The prepared samples were studied using x-ray diffraction (XRD), transmission electron microscopy, field-emission scanning electron microscopy (FE-SEM), and UV-Visible spectroscopy. The valence states of Ce and Co ions were determined by using x-ray absorption near edge spectroscopy. The magnetic properties of the samples were studied using vibrating sample magnetometry. The results from XRD indicated that the synthesized samples had a cubic structure without a change in the structure of CeO2 due to Co substitution. FE-SEM micrographs showed that the samples had a spherical morphology. The Co-doped CeO2 showed a red shift of the band gap energy that originates from defects caused by Co substitution. The samples of both CeO2 and Co-doped CeO2 exhibit room temperature ferromagnetism, and the saturated magnetization (Ms) increases with increasing Co content until x = 0.03 and then displays ferromagnetic loops as well as paramagnetic behavior. Oxygen vacancies have been proposed to play an important role in the magnetic properties of Co-doped CeO2.

Investigation of ionic conductivities of CeO2-based electrolytes with controlled oxygen vacancies

Effects of Gd2O3 and SrO co-dopants on microstructural features, ionic conductivities and oxygen vacancy concentrations of two modified CeO2 electrolytes (Ce0.8Gd0.2−xSrxO1.9−0.5x and Ce0.8−xGd0.2SrxO1.9−x) are investigated using X-ray diffractometry (XRD), scanning electron microscopy (SEM) and AC impedance spectroscopy. Lattice parameters of co-doped CeO2 specimens are observed to linearly increase with an increase of co-dopants amount, due to larger radius of Sr2+. Grain size of Ce0.8−xGd0.2SrxO1.9−x is significantly larger than that of Ce0.8Gd0.2−xSrxO1.9−0.5x at the same Sr2+ concentration, because Ce0.8−xGd0.2SrxO1.9−x possesses more oxygen vacancy to migrate oxygen ions, defects and impurities easily, based upon calculation of oxygen vacancy amount. Small addition of SrO into 20 mol% Gd2O3 doped CeO2 (Ce0.8−xGd0.2SrxO1.9−x) is effective in enhancing of ionic conductivity, but a sudden decreasing of conductivity with Sr2+ heavy-doping is found. The relationship among co-dopant concentration, oxygen vacancy radius, average binding energy and ionic conductivity in two different co-doped CeO2 systems are discussed in this work.

Structural and magnetic properties of Ce1−xCoxO2 (0 ⩽ x ⩽ 0.1) nanocrystalline powders

In this work, we report detailed investigations on the structural and magnetic properties of pure and Co-doped CeO2 nanoparticles. All the samples, with different cobalt concentrations, were prepared by the co-precipitation method. To investigate the structural properties and phases present in the samples, systematic, x-ray diffraction measurements were performed in the θ–2θ mode. Phonon modes, from the pure and Co-doped samples, were studied by Raman scattering measurements. Surface morphology of individual nanoparticles was studied by field emission scanning electron microscopy. Interestingly, room temperature ferromagnetism is observed in, both, un-doped and Co-doped samples. The mechanisms of variation in the Raman active mode (F2g) and magnetization, by increasing the cobalt concentration, are briefly discussed.

PVA-assisted synthesis and characterization of nano-crystalline La3+ and Mg2+ co-doped CeO2 electrolyte for intermediate-temperature solid oxide fuel cells

A series of nano-crystalline ceria-based solid solution electrolyte, Ce0.8La0.2−x MgxO2−δ (x = 0.0, 0.05, 0.10, 0.15, and 0.2), were synthesized via the polyvinyl alcohol (PVA) assisted combustion method, and then characterized to the crystalline structure, powder morphology, sintering micro-structure, and electrical properties. Present study showed that Ce0.8La0.2−x Mg x O2−δ was exceedingly stable as a cubic phase in all temperature range and exhibited fine crystals ranging from 15 to 20 nm. After sintering at 1,400 °C, the as-prepared pellets exhibited a dense micro-structure with 96 % of theoretical density. The electrical conductivity was studied using AC impedance spectroscopy and it was observed that the composition Ce0.8La0.1 Mg0.1O2−δ showed higher electrical conductivity of 0.020 S cm−1 at 700 °C. The thermal expansion was measured using dilatometer technique in the temperature range 30–1,000 °C. The average thermal expansion coefficient of Ce0.8La0.1 Mg0.1O2−δ was 12.37 × 10−6 K−1, which was higher than that of the commonly used SOFC electrolyte YSZ (~10.8 × 10−6 K−1).

High-temperature ferromagnetism in Co-doped CeO2 synthesized by the coprecipitation technique

The aim of the present study is to check the influence of annealing under vacuum and a mixture of N(2)-H(2) atmosphere on the magnetic properties of polycrystalline Co-doped CeO(2) diluted magnetic oxides (DMOs) with Co concentrations of 5 at% synthesized using the coprecipitation technique. X-Ray diffraction (XRD) patterns and transmission electron microscopy (TEM) showed for all samples the expected CeO(2) cubic fluorite-type structure and that Co ions are uniformly distributed inside the samples. Room-temperature Raman and photoluminescence (PL) spectroscopies indicate an increase in the concentration of oxygen vacancies upon Co doping and further annealing. Field dependent magnetization measurements revealed a paramagnetic behavior for as-prepared Co-doped CeO(2), while a ferromagnetic behavior appears when the same samples are annealed under vacuum or N(2)-H(2) atmosphere. Temperature dependent magnetization measurements suggest that the observed ferromagnetism is due to the presence of metallic Co clusters with nanometric size and broad size distribution. These results are supported by electron paramagnetic resonance studies.

Up conversion luminescence of Yb 3+–Er 3+ codoped CeO 2 nanocrystals with imaging applications

The effects of Yb 3+ doping on up conversion in Yb 3+–Er 3+ co-doped cerium oxide nanocrystals are reported. Green emission around 545 and 560 nm attributed to the 2H 11/2, 4S 3/2→ 4I 15/2 transitions and red emission around 660 and 680 nm due to 4F 9/2→ 4I 15/2 transitions under 975 nm excitation were studied at room temperature. Both green and red emission intensities increase as the Yb 3+ concentration increases from 0%. Emission strength starts to decrease after the Yb 3+ concentration exceeds a critical amount. The green emission strength peaks around 1% Yb 3+ concentration while the red emission strength peaks around 4%. An explanation of competition between different decay mechanisms is presented to account for the luminescence dependence on Yb 3+ concentration. Also, the application of up converting nanoparticles in biomedical imaging is demonstrated.

Enhanced sulfur and carbon coking tolerance of novel co-doped ceria based anode for solid oxide fuel cells

Doubly doped CeO 2 based anode with Y and Yb is first reported for direct methane solid oxide fuel cells. The power output of the cell with Ni–Ce 0.8Y 0.1Yb 0.1O 1.9 anode and stability at various temperatures are investigated when air is used as oxidant and pure H 2, 5 ppm H 2S containing H 2 and dry CH 4 as fuel, respectively. At 750 °C, the cell displays stable power output for 120 h at 200 mA cm −2 when fueled with dry CH 4, suggesting the carbon deposit is largely absent on the anode, which is confirmed by the SEM observation and EDS results. The results also prove that the rare earth elements Y and Yb affect the sulfur tolerance performance of the anode in a cooperative fashion leading to good anode stability in the contaminated fuel. The SEM and EDS results provide evidence that the cell with Ni–Ce 0.8Y 0.1Yb 0.1O 1.9 anode is tolerant toward the H 2S contamination. The remarkable performances suggest that co-doped CeO 2 anode is an attractive electrode component for direct hydrocarbon solid oxide fuel cells and might also be used as a catalyst for reforming of hydrocarbon fuels and for removal of fuel gas contaminations such as sulfur.

Impact of Ce4+ ion on microstructure and luminescence character of Ho3+/Yb3+ co-doped ZrO2 nanocrystal

By solid-state synthesis method, Ho3+/Yb3+ co-doped CeO2–ZrO2 nano-powders have been prepared. The concentration of Ce4+ ions has greater effect to the oxygen lattice structure. When the concentration of Ce4+ ions is 30mol%, the oxygen lattice is a tetrahedral space group and the luminescence intensity of the sample is strongest. The results show that the lattice structure can be changed by inducting the Ce4+ ions into Ho3+/Yb3+ co-doped ZrO2. And the emission character can be improved.

Fabrication of Sm 3+ and Nd 3+ co-doped CeO 2 thin-film electrolytes by radio frequency magnetron sputtering

Nanocrystalline Sm 3+ and Nd 3+ co-doped CeO 2 thin-film electrolytes with high in-plane electric conductivities have been deposited via radio frequency magnetron sputtering. The results show that with increased RF power or decreased pressure, a reduction in the level of porosity of the film takes place, along with an increased electric conductivity. Maximum electric conductivity of 9 × 10 −3 S cm −1 at 500 °C was achieved with optimal sputtering process. It also indicates that with the change of the substrate from single crystal alumina to polycrystalline alumina, the film structure varies from (1 1 1) preferred orientation to random orientation, accompanied by an enhancement in electric conductivity. Compared with alumina substrates, the quartz substrate shows a negative effect on electric conductivity.

Self assembly of Co doped CeO 2 microspheres from nanocubes by hydrothermal method and their photodegradation activity on AO7

We successfully synthesized self assembled Co doped CeO 2 microspheres from nanocubes via surfactant free hydrothermal technique. The surface shapes of CeO 2 nanoparticles have been controlled and optimized by adjusting the molar concentration and pH value of the precursor. The structural properties of the prepared nanocubes have been investigated using electron diffraction and X-ray diffraction. High resolution transmission electron microscopy studies show that the co doped CeO 2 nanocubes are in polycrystalline nature with highly porous structure. The photo catalytic activity of Co-doped CeO 2 nanocubes under UV–Visible light was examined by degradation of azodyes acid orange 7 (AO7) in aqueous solution. The improved photocatalytic activity was attributed to the better adsorption ability and unique 4f electron configuration of CeO 2.

Residual stress-dependent electric conductivity of sputtered co-doped CeO2 thin-film electrolyte

Sm3+ and Nd3+ co-doped ceria thin-film electrolytes have been deposited on polycrystalline alumina substrates via RF magnetron sputtering. Electric conductivity is evaluated with respect to the residual stress in the film by sin2 ψ-methodology, indicating that an in-plane tensile stress is applied to the as-deposited film. The stress in the film increases as annealing temperature decreases, and there is an enlarged crystal lattice. The results also reveal that the annealed film with a greater stress shows a higher electric conductivity, which might be due to the lower activation energy. The conductivity of the film annealed at 600 °C is as high as 0.009 S cm−1 at 500 °C, and the residual stress is determined to be 542.70 MPa at room temperature.

Solvothermal synthesis of titania/ceria

In this study, CeO2 and a mixture of TiO2 and CeO2 (CeO2-TiO2, Ce: Ti (mol.) = 3: 1) were synthesized by the solvothermal method at different temperatures. The particle size, either CeO2 or CeO2-TiO2 synthesized at 140 °C was less than 10 nm. Nitrogen and carbon co-doped CeO2 was synthesized using hexamethylenetetramine (HMT) as a precipitator at 140 °C. The degradation ratio of acid orange 7 (AO7) could achieve 98.8% with this co-doped CeO2 under an irradiation of a 10 Watt compact fluorescent lamp (CFL). CeO2-TiO2 could degrade 93.3% AO7 by the 10 Watt CFL. In contrast, the degradation ratio of AO7 only reached 43.0% by TiO2. However, a result of CeO2 synthesized using Na2CO3 precipitator gave a degradation ratio of 5.0%, i.e., a 94.9% reduction compared with that of HMT precipitator.

Electrical conductivity of textured Sm 3+ and Nd 3+ Co-doped CeO 2 thin-film electrolyte

Nanocrystalline thin-film electrolytes of Sm 3+ and Nd 3+ co-doped ceria have been deposited on polycrystalline alumina substrates via RF magnetron sputtering. It is found that with the increase of substrate temperature from room temperature to 600 °C, the structure of the film varies from (1 1 1) preferred orientation to random orientation, accompanied by an enhancement in electrical conductivity. It is estimated that the activation energy for oxygen ion migration along (1 1 1) orientation may be higher than other crystal orientations, resulting in a lower conductivity of the (1 1 1) barrier textured co-doped ceria film electrolyte. It also indicates that the electrical conduction is predominantly due to the oxygen ions.

Processing and Characterization of Bi2O3 and Sm2O3 Codoped CeO2 Electrolyte for Intermediate-Temperature Solid Oxide Fuel Cell

A series of ceria-based solid solution electrolyte, Bi2O3 and Sm2O3 codoped CeO2 (Sm0.2−xBixCe0.8O1.9, x=0, 0.05, 0.10, 0.15, 0.20), were synthesized via a Pechini-type gel route. The phase composition was analyzed by the X-ray diffraction. Present study shows that Sm0.2−xBixCe0.8O1.9 is exceedingly stable as a cubic phase in all temperature range. Starting from the Sm0.2−xBixCe0.8O1.9 powder with specific surface area of 23.057 m2/g and using bismuth oxide as sintering aid, the sintering behavior investigation demonstrates that the Sm0.2−xBixCe0.8O1.9 green compacts can be densified to nearly theoretical density at 1300°C for 3 h. The very low sintering temperature prevents excessive grain growth during the heating, which is crucial for obtaining microcrystalline ceramics. In the temperature range of 500°–800°C, AC impedance spectra indicate that the Sm0.2−xBixCe0.8O1.9 has much higher ionic conductivity than SDC (Sm0.2Ce0.8O1.9), and that the highest conductivity value is 3.982 S/m at 750°C. The increased conductivity implies that the effect of cell volume and polarization, associated with Bi3+, played an important role in the anion transport of the materials.

Appearance of Ferromagnetism in Co-Doped CeO2 Diluted Magnetic Semiconductors Prepared by Solid-State Reaction

We report on the magnetic and electronic properties of Co-doped CeO2 oxides prepared by a solid-state reaction. The as-prepared samples exhibit pure paramagnetic behavior, but postannealing at 450 °C for 1 h under H2/N2 atmosphere results in the appearance of ferromagnetism at room temperature. The electronic structure of as-prepared and postannealed Co-doped CeO2 diluted magnetic oxides is investigated using soft X-ray absorption and emission spectroscopy. Co L-edge resonant inelastic X-ray scattering (RIXS) spectra indicate that direct Co−Co bonds form due to the precipitation of Co atoms in CeO2. This is in agreement with zero-field-cooled and field-cooled thermal variations of the magnetization, which indicated the presence of Co clusters with size larger than 8 nm.

Extrinsic ferromagnetism in epitaxial Co-doped CeO2 pulsed laser deposited films

We report on the structural and magnetic properties of 5 at. % Co-doped CeO2 films grown on LaAlO3(001) substrates by pulsed laser deposition. A series of epitaxially grown samples made under different oxygen partial pressures ranging from 10−4 to 10−1 mbar showed a ferromagnetic signal at room temperature. This signal is independent on the oxygen partial pressure during deposition. X-ray photoelectron spectroscopy showed an increasing concentration of defects as the oxygen pressure during deposition decreases. Although x-ray diffraction and transmission electron microscopy observations could not prove the existence of spurious phases, the temperature dependent variation in the magnetization suggests the existence of small magnetic Co clusters with a large distribution of blocking temperatures.

Study of hyperfine interactions in pure and Co-doped CeO2 nanoparticles by PAC spectroscopy using 111Cd

In this work we have used the Perturbed Angular Correlation (PAC) technique to measure the hyperfine interactions in pure nanostructured (crystallite size around 70 nm) as well as doped with 3d transition metal Co (3 at. %) CeO2 samples. The PAC measurements were performed at different temperatures between 23 and 527°C. The results did not reveal any evidence of hyperfine magnetic interactions. Spectra obtained presented three major frequencies related to substitutional sites in CeO2, grain boundary sites and intrinsic defect related sites.

Influence of Thickness on Oxide Ionic Conductivity in Sm<sup>3+</sup> and Nd<sup>3+</sup> Co-Doped CeO<sub>2</sub> Electrolyte

Sm3+ and Nd3+ co-doped CeO2 solid electrolytes with various thicknesses were prepared by citric-nitrate combustion process. The electrical conductivity as a function of electrolyte thickness was determined by ac impedance spectroscopy. The results showed that the ionic conductivity increases with the decrease of the electrolyte thickness approximately and it was estimated that the conductivity enhancement was due to the increased grain boundary conductivity.

Nanowires formation and the origin of ferromagnetism in a diluted magnetic oxide

The ferromagnetic response from Co-doped CeO2 epilayers is shown to be linked to the formation of Co metallic nanowires (diameters in the 3–7 nm range) within the oxide matrix. These results (i) illustrate the key role of the shape anisotropy of metallic inclusions in diluted magnetic oxides (DMO), (ii) stress the importance to monitor the out-of-plane response of DMO, and (iii) suggest that some dilute systems previously thought to be intrinsic ferromagnet may be reexamined.

Effects of oxygen vacancies on the room-temperature ferromagnetism of Co-doped polycrystalline CeO 2

Pure and Co-doped single-phase CeO 2 crystals were synthesized by a solid-state reaction method. Samples of different oxygen vacancy concentration were studied, including (1) as-sintered crystals, (2) powders ground from the same crystal, and (3) a cold-pressed pellet from the ground powder that was unannealed and annealed at 800 °C. By analyzing the magnetic behaviors, surface/volume ratio and O vacancy concentration, the effects of oxygen vacancies on the room-temperature ferromagnetism (RT-FM) of Co-doped CeO 2 were systematically investigated. The results confirm that the RT-FM observed in Co-doped CeO 2 has a direct relationship with the oxygen vacancy concentration, and support the oxygen vacancy mediated FM mechanism.