CeO<Subscript>2</Subscript> and Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> modified Ni-YSZ anode for solid oxide fuel cell

Ni sintering at high temperature (～ 800 °C) operation drastically degrades the performance of Ni-yttria-stabilized zirconia (YSZ) anode in solid oxide fuel cell (SOFC). Mixed ionic and electronic conductive oxides such as CeO₂ and Nb₂O₅ enhance the dispersion of Ni, CeO₂ enhances the redox behavior and promotes charge transfer reactions, and Nb₂O₅ increases the triple phase boundary. In the present work, anode-supported SOFC is fabricated and tested in H₂ fuel at 800 °C. YSZ and lanthanum strontium manganite (LSM)-YSZ are used as the electrolyte and composite cathode with NiO-YSZ, CeO₂-NiO-YSZ, and Nb₂O₅-NiO-YSZ as an anode. The peak power density obtained for the cell with 10% CeO₂–30% NiO-YSZ anode at the 5 and 25 h of operation is 330 and 290 mW cm^?2 which is higher than that for 40% NiO-YSZ anode (275 mW cm^?2 at 5 h). The peak power density obtained for the cell with 10% Nb₂O₅–30% NiO-YSZ anode at the 5 and 25 h of operation is 301 and 285 mW cm^?2 which is higher than that for 40% NiO-YSZ anode (275 mW cm^?2 at 5 h). Physical characterization has been carried to study morphology, elemental analysis, particle size, and phase formation of the fabricated anode before and after cell operation to correlate the cell performance.     

Effect of PVA addition on formation of spray-deposited Sm<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>CoO<Subscript>3</Subscript> thin films on CeO<Subscript>2</Subscript> substrates

Nano-crystalline films of Sm_0.5Sr_0.5CoO₃ (SSC) have been formed on CeO₂ substrates by spraying stoichiometric aqueous solution containing Sm, Sr, and Co ions. Effect of polyvinyl alcohol (PVA) addition as a complexing agent in spray solution on stoichiometry, crystallite size, morphology, and transport properties of film are studied. The results showed that the SSC cathode had maximum crystallite size for 40% PVA addition. Electrical performance of film decreases with decrease in the particle size, while the electronic to ionic predominance transition temperature decreases with decreasing particle size. These films are studied for their potential application as a cathodic material in developing intermediate temperature solid oxide fuel cells.     

Facile synthesis of uniform hierarchical composites CuO-CeO<Subscript>2</Subscript> for enhanced dye removal

The hierarchically shaped CuO-CeO₂ composites were prepared through a facile solvothermal method without using any template. The as-prepared products were characterized by X-ray powder diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, and N₂ adsorption–desorption analysis. In the characterization, we found that CuO-CeO₂ composites were showed uniform size and morphology which were consisted of the secondary nanoflakes interconnected with each other. Most interestingly, the composites showed efficient performance to remove methyl blue and Congo red dyes from water with maximum adsorption capacities of 2131.24 and 1072.09 mg g^?1, respectively. In addition, because of their larger surface area and the unique hierarchical structures, the adsorption performance of the CuO-CeO₂ composites is much better than the materials of CuO and CeO₂.     

Low-frequency Raman scattering from CeO<Subscript>2</Subscript> nanoparticles

Raman scattering measurements were performed on CeO₂ nanoparticles at room temperature. Low-frequency modes are assigned to confined acoustic vibrations of spherical CeO₂ nanoparticles. Frequencies of these vibrational modes have been calculated in the elastic continuum approximation, which considers a nanoparticle as a homogeneous elastic sphere. We assumed stress-free boundary conditions. The specific dependence of the vibrational frequency on the particle diameter enables the determination of the particle size from the experimental Raman frequency. The particle size value calculated in this way agrees well with the value acquired from the phonon confinement model. PACS 61.46.Df; 73.63.Bd; 63.22.+m     

Synthesis,characterization, and ecotoxicity of CeO<Subscript>2</Subscript> nanoparticles with differing properties

CeO₂ nanoparticles with various characteristics find an increasing number of applications in the electronic, medical, and other industries and are therefore likely released in the environment. This calls for investigations linking the physicochemical properties of these particles with their potential environmental impacts. In this study, CeO₂ nanoparticle powders were prepared using three different precursors [Ce(NO₃)₃, CeCl₃, and Ce(CH₃COO)₃] and annealing temperatures (300, 500, and 700 °C). This procedure resulted in nine different types of nanoparticles with differing size (5–90 nm), morphology, surface Ce³⁺/Ce⁴⁺ ratio, and slightly different crystal structures as characterized using transmission electron microscopy, dynamic light scattering, X-ray photoelectron spectroscopy, and X-ray diffraction measurements with Rietveld refinement. These CeO₂ nanoparticles underwent toxicity testing at concentrations up to 64 mg L^?1 using Daphnia magna. Toxic effects were observed for three particle types with EC50 values between 5 and 64 mg L^?1. No clear correlation was observed between the physicochemical properties (size, shape, oxygen occupancy, Ce³⁺/Ce⁴⁺ ratio) of the nanoparticles and their toxicity. However, toxicity was correlated with the amount of Ce remaining suspended in the test medium after 24 h. This indicated that toxic effects may depend on the colloidal stability of CeO₂ nanoparticles during the first day of exposure. Therefore, being readily suspended and remaining stable for several days in the aquatic media increases the likelihood that CeO₂ nanoparticles will cause unwanted adverse effects.     

Hydrothermal synthesis of CeF<Subscript>3</Subscript> nanocrystals and characterization

CeF₃ nanocrystals with plate-like and perforated morphologies were successfully synthesized via a facile hydrothermal route. The nanocrystals of CeF₃@silica can dispersed in aqueous solution were also prepared. The effects of fluoride sources on the morphology and microstructure of the nanocrystals were investigated by means of transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and powder X-ray diffraction (XRD). Results indicate that the morphology of the rare earth compound nanocrystals can be well tuned by selecting proper fluoride sources. The ultraviolet (UV) absorption peak of the CeF₃ nanocrystals is slightly blue shifted along with the decrease of size. And the photoluminescence (PL) intensity of the CeF₃ nanocrystals is closely related to size and microstructure as well.     

Structural transformations in the Al<Subscript>85</Subscript>Ni<Subscript>6.1</Subscript>Co<Subscript>2</Subscript>Gd<Subscript>6</Subscript>Si<Subscript>0.9</Subscript> amorphous alloy during multiple rolling

The effect of multiple rolling at room temperature on the structure and crystallization of the Al₈₅Ni_6.1Co₂Gd₆Si_0.9 amorphous alloy has been studied using transmission electron microscopy, differential scanning calorimetry, and X-ray diffraction. The total plastic strain is 33%. It has been shown that the deformation results in the formation of aluminum nanocrystals with the average size that does not exceed 10–15 nm. The nanocrystals are formed in regions of localization of plastic deformation. The deformation decreases the thermal effect of nanocrystallization (∼15%) as compared to the heat release at the first stage of crystallization of the unstrained sample. The morphology, structure, and distribution of precipitates have been investigated. Possible mechanisms of the formation of nanocrystals during the deformation have been discussed.     

Flame-synthesized Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:Tb<Superscript>3+</Superscript> nanocrystals as spectral converting materials

In flame spray pyrolysis (FSP), the generation of uniform nanoparticles can be quite challenging due to difficulties in controlling droplet sizes during liquid spraying and uneven flame temperature. Here, we report a method to produce relatively uniform nanocrystals of a Tb³⁺ doped Y₂O₃ phosphor. In ethanol, metal nitrate precursors were simply mixed with organic surfactants to form a homogeneous solution which was then subjected to FSP. Depending on relative concentrations of the surfactant (oleic acid) to the metal precursors (yttrium and terbium nitrates), different sizes and morphologies of Y₂O₃:Tb³⁺ particles were obtained. By adjusting the surfactant concentration, Y₂O₃:Tb³⁺ crystals as small as 20~25 nm were acquired. X-ray diffraction and transmittance electron microscopy were used to prove that as-synthesized nanoparticles were highly crystalline due to the high temperature of FSP. X-ray photoelectron spectroscopy revealed that terbium dopants were well distributed throughout Y₂O₃ particles and a small portion of carbonate impurities were remained on the surface of particles, presumably originated from incomplete combustion of the organic surfactants. Photoluminescence (PL) spectra of Y₂O₃:Tb³⁺ nanocrystals exhibited a green light emission ensuring that the terbium doping was successfully occurred. However, when post-annealing was performed on the nanocrystals, their PL was dramatically enhanced indicating that quenching centers such as carbonate impurities and surface defects may have been removed by the annealing process. Owing to the continuous processability of FSP, this current method can be a practical way to produce nanoparticles in a large quantity. The obtained Y₂O₃:Tb³⁺ nanocrystals were used to fabricate a transparent film with poly-ethylene-co-vinyl acetate (poly-EVA) polymer, which was suitable for a spectral converting layer for a solar cell.

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Nanocrystallization of an amorphous Fe<Subscript>80</Subscript>B<Subscript>20</Subscript> alloy during severe plastic deformation

The structural evolution of an amorphous Fe₈₀B₂₀ alloy subjected to severe plastic deformation at room temperature or at 200°C was studied. Deformation leads to the formation of α-Fe nanocrystals in an amorphous phase. After room-temperature deformation, nanocrystals are localized in shear bands. After deformation at 200°C, the nanocrystal distribution over the alloy is more uniform. Possible causes of the crystallization of the amorphous phase during severe plastic deformation are discussed.     

EPR of Gd<Superscript>3+</Superscript> ion in mixed CeO<Subscript>2</Subscript>-Y<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocrystals

This paper reports on the results of EPR studies of mixed CeO₂-Y₂O₃ crystals (including nanosized crystals) doped by gadolinium ions. It is revealed that the width of the line corresponding to the allowed transition 1/2 ↔ −1/2 between the Kramers-conjugated states |±1/2〉 of the Gd³⁺ ion decreases with a decrease in the powder size from macrosizes to nanosizes. The observed dependence can be due to the increase in the unit cell size during grinding of the samples.     

Multiband tight-binding theory of disordered A<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>B<Subscript>1-</Subscript><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>C semiconductor quantum dots

Zero-dimensional nanocrystals, as obtained by chemical synthesis, offer a broad range of applications, as their spectrum and thus their excitation gap can be tailored by variation of their size. Additionally, nanocrystals of the type A _x B_{1- x} C can be realized by alloying of two pure compound semiconductor materials AC and BC, which allows for a continuous tuning of their absorption and emission spectrum with the concentration x. We use the single-particle energies and wave functions calculated from a multiband sp ³ empirical tight-binding model in combination with the configuration interaction scheme to calculate the optical properties of Cd _x Zn_{1- x} Se nanocrystals with a spherical shape. In contrast to common mean-field approaches like the virtual crystal approximation (VCA), we treat the disorder on a microscopic level by taking into account a finite number of realizations for each size and concentration. We then compare the results for the optical properties with recent experimental data and calculate the optical bowing coefficient for further sizes.     

CO<Subscript>2</Subscript> activation on single crystal based ceria and magnesia/ceria model catalysts

Novel multifunctional ceria based materials may show an improved performance in catalytic processes involving CO₂ activation and reforming of hydrocarbons. Towards a more detailed understanding of the underlying surface chemistry, we have investigated CO₂ activation on single crystal based ceria and magnesia/ceria model catalysts. All model systems are prepared starting from well-ordered and fully stoichiometric CeO₂(111) films on a Cu(111) substrate. Samples with different structure, oxidation state and compositions are generated, including CeO_2-x/Cu(111) (reduced), MgO/CeO_2-x/Cu(111) (reduced), mixed MgO-CeO₂/Cu(111) (stoichiometric), and mixed MgO-CeO_2-x/Cu(111) (reduced). The morphology of the model surfaces is characterized by means of scanning tunneling microscopy (STM), whereas the electronic structure and reactivity is probed by X-ray photoelectron spectroscopy (XPS). The experimental approach allows us to compare the reactivity of samples containing different types of Ce³⁺, Ce⁴⁺, and Mg²⁺ ions towards CO₂ at a sample temperature of 300 K. Briefly, we detect the formation of two CO₂-derived species, namely carbonate (CO₃ ^2-) and carboxylate (CO₂ ^-) groups, on the surfaces of all investigated samples after exposure to CO₂ at 300 K. In parallel to formation of the carbonate species, slow partial reoxidation of reduced CeO_2-x/Cu(111) occurs at large doses of CO₂. The reoxidation of the reduced ceria is largely suppressed on MgO-containing samples. The tendency for reoxidation of Ce³⁺ to Ce⁴⁺ by CO₂ decreases with increasing degree of intermixing between MgO and CeO_2-x. Additionally, we have studied the stability of the formed carbonate species as a function of annealing temperature.     

CeO<Subscript>2</Subscript> coating to improve the performance of Li[Li<Subscript>0.2</Subscript>Mn<Subscript>0.54</Subscript>Ni<Subscript>0.13</Subscript>Co<Subscript>0.13</Subscript>]O<Subscript>2</Subscript>

The Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ coated with CeO₂ has been fabricated by an ionic interfusion method. Both the bare and the CeO₂-coated samples have a typical layered structure with R-3m and C2/m space group. The results of XRD and TEM images display that the CeO₂ coating layer on the precursor could enhance the growth of electrochemically active surface planes ((010), (110), and (100) planes) in the following ionic interfusion process. The results of galvanostatic cycling tests demonstrate that the CeO₂-coated sample has a discharge capacity of 261.81 mAh g^?1 with an increased initial Coulombic efficiency from 62.4 to 69.1% at 0.05 °C compared with that of bare sample and delivers an improved capacity retention from 71.7 to 83.4% after 100 cycles at 1 °C (1 °C?=?250 mA g^?1). The results of electrochemical performances confirm that the surface modification sample exhibits less capacity fading, lower voltage decay, and less polarization.     

Room-temperature synthesis of pure perovskite-related Cs<Subscript>4</Subscript>PbBr<Subscript>6</Subscript> nanocrystals and their ligand-mediated evolution into highly luminescent CsPbBr<Subscript>3</Subscript> nanosheets

Currently, all-inorganic cesium lead-halide perovskite nanocrystals have attracted enormous attentions owing to their excellent optical performances. While great efforts have been devoted to CsPbBr₃ nanocrystals, the perovskite-related Cs₄PbBr₆ nanocrystals, which were newly reported, still remained poorly understood. Here, we reported a novel room-temperature reaction strategy to synthesize pure perovskite-related Cs₄PbBr₆ nanocrystals. Size of the products could be adjusted through altering the amount of ligands, simply. A mixture of two good solvents with different polarity was innovatively used as precursor solvent, being one key to the high-yield Cs₄PbBr₆ nanocrystals synthesis. Other two keys were Cs⁺ precursor concentration and surface ligands. Ingenious experiments were designed to reveal the underlying reaction mechanism. No excitonic emission was observed from the prepared Cs₄PbBr₆ nanocrystals in our work. We considered the green emission which was observed in other reports originated from the avoidless transformation of Cs₄PbBr₆ into CsPbBr₃ nanocrystals. Indeed, the new-prepared Cs₄PbBr₆ nanocrystals could transform into CsPbBr₃ nanosheets with surface ligands mediated. The new-transformed two-dimensional CsPbBr₃ nanosheets could evolve into large-size nanosheets. The influences of surface ligand density on the fluorescent intensity and stability of transformed CsPbBr₃ nanosheets were also explained. Notably, the photoluminescence quantum yield of the as-transformed CsPbBr₃ nanosheets could reach as high as 61.6% in the form of thin film. The fast large-scale synthesis of Cs₄PbBr₆ nanocrystals and their ligand-mediated transformation into high-fluorescent CsPbBr₃ nanosheets will be beneficial to the future optoelectronic applications. Our work provides new approaches to understand the structural evolution and light-emitting principle of perovskite nanocrystals.

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Synergistic effect of Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> and CeO<Subscript>2</Subscript> co-modified titanate nanosheet heterojunction on enhanced photocatalytic degradation

CeO₂ and Fe₂O₃ co-modified titanate nanosheet (Fe₂O₃/CeO₂@TNS) was prepared by one-pot hydrothermal method; the photocatalyst exhibited large surface area with CeO₂ and Fe₂O₃ particles well dispersed on the surface. The results of XRD, BET, and Raman proved that the CeO₂ and Fe₂O₃ introduced in the TNS influenced its structure evolution from 3D to 2D. The modification resulted in a shift of the absorption edge toward a longer wavelength and the band gap reduced to 2.87 eV. The three-component systems performed excellent photocatalytic activity and cycle stability on phenol and methyl blue (MB) solution under sunlight; nearly total phenol and MB were degraded in dozens of minutes. And the reaction rate constant (K) of Fe₂O₃/CeO₂@TNS on phenol degradation was 1.77, 3.25, 4.88, and 13-fold of Fe₂O₃@TNS, CeO₂@TNS, bare TNS, and P25, respectively. The enhanced photocatalytic activity could be ascribed to the efficient separation of photogenerated pairs through the formation of tandem n-n-n heterojunction among the three-component systems. This work will be useful for the design of other tandem n-n-n heterojunction photocatalytic systems for application in energy conversion and environmental remediation.

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Nanoparticles size effects in thermoluminescence of oxyfluoride glass-ceramics containing Sm<Superscript>3+</Superscript>-doped CaF<Subscript>2</Subscript> nanocrystals

Oxyfluoride glass-ceramic in the system SiO₂–Al₂O₃–CaF₂–SmF₃ containing Sm³⁺-doped CaF₂ nanocrystals in the range from 15 to 150 nm size were produced by using the controlled ceramization of the precursor glass. The incorporation of the Sm³⁺-dopant ion in the glass ceramic creates new electron-trapping centers and thermoluminescence (TL) method has been used in order to trace their evolution during glass ceramization. The 370 °C TL peak observed in precursor glass has been assigned to the recombination of the electrons released from the Sm²⁺-traps in the amorphous glass network. In the glass-ceramic sample containing nanocrystals with about 15 nm size the new weak TL peaks at 270, 290, and 310 °C were attributed to the recombination of the electrons released from the Sm²⁺-traps located mainly at the surface of the CaF₂ nanocrystals. In the glass-ceramic sample containing nanocrystals with about 150 nm size, the new TL peaks at 232, 270, and 302 °C size have been assigned to the recombination of the electrons released from the Sm²⁺-traps located inside the CaF₂ nanocrystals.     

Highly efficient solid-state synthesis of carbon-encapsulated ultrafine MoO<Subscript>2</Subscript> nanocrystals as high rate lithium-ion battery anode

A simple and highly efficient method is developed for the one-step in situ preparation of carbon-encapsulated MoO₂ nanocrystals (MoO₂@C) with core-shell structure for high-performance lithium-ion battery anode. The synthesis is depending on the solid-state reaction of cyclopentadienylmolybdenum tricarbonyl dimer with ammonium persulfate in an autoclave at 200 °C for 30 min. The large amount of heat generated during the explosive reaction cleaves the cyclopentadiene ligands into small carbon fragments, which form carbon shell after oxidative dehydrogenation coating on the MoO₂ nanocrystals, resulting in the formation of core-shell structure. The MoO₂ nanocrystals have an equiaxial morphology with an ultrafine diameter of 2–8 nm, and the median size is 4.9 nm. Hundreds of MoO₂ nanocrystals are encapsulated together by the worm-like carbon shell, which is amorphous and about 3–5 nm in thickness. The content of MoO₂ nanocrystals in the nanocomposite is about 69.3 wt.%. The MoO₂@C anode shows stable cyclability and retains a high reversible capacity of 443 mAh g^?1 after 50 cycles at a current density of 3 A g^?1, owing to the effective protection of carbon shell.     

Photoluminescence of GeO<Subscript>2</Subscript> films containing germanium nanocrystals

Germanium nanocrystals were formed in a GeO₂ film during the process of germanium monoxide gas-phase deposition onto a sapphire substrate and studied by photoluminescence (PL) and Raman scattering spectroscopy. A PL peak in this heterosystem was observed in the visible region at room temperature. The sizes of Ge nanocrystals were estimated from the position of a Raman peak corresponding to scattering by localized optical phonons in germanium. The PL peak position calculated with allowance for the electron and hole size quantization in Ge nanocrystals coincides well with the experimentally observed position of this peak.     

Dependence of electronic structure on the grain size in Mn<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocrystals

. The dependence of electron structure on the grain size in Mn₂O₃ nanocrystals has been investigated using X-ray photoelectron spectroscopy. It is found that the electron structure is sensitively dependent on the average grain size. The experimental results indicate that the O–1s core-level spectra are hardly shifted with decreasing average grain size; however, for a sample with a small grain size (e.g. a 9-nm sample), the shoulder peak evidently appears on the higher-binding-energy side. The main peak width of the Mn–2p core-level spectra becomes wide for samples with decreasing average grain size. The remarkable aspect of the Mn–3s core-level spectra is that the peak is multiply split, with a complex peak structure. The exchange interaction of the 3s and 3d electrons in the Mn³⁺ ion can be used to explain this behavior. PACS 71.23.-k; 73.22.-f     

Facet‐Dependent Effect of Well‐Defined CeO2 Nanocrystals on the Adsorption and Dephosphorylation of Phosphorylated Molecules

CeO₂ nanocrystals (CN) with different morphologies (i.e., cube, octahedron, and rod) are prepared and the facet‐dependent effect of these CeO₂ nanocrystals on the adsorption and dephosphorylation of phosphorylated molecules is investigated for the first time using the model p‐nitrophenyl disodium orthophorphate (p‐NPP). Due to their different surface atomic configurations, the {111} and {110} facets have much higher adsorption capacity and kinetic catalytic activities than {100} facets. All the CeO₂ nanocrystals can intensely promote the dephosphorylation reaction owing to the strong interaction between Ce cations and phosphoryl oxygens resulting in the cleavage of phosphoester bonds. As was expected, the above facet‐dependent effect can be verified by the practical application results of the CeO₂ nanocrystals on the capture and dephosphorylation of phosphopeptides. Thus, surface engineering could be a useful and feasible strategy for not only fundamentally understanding the interaction between crystal facets and molecules but also effectively developing high‐performance functional materials.