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     

Properties of powders produced by evaporating CeO<Subscript>2</Subscript>/Gd<Subscript>2</Subscript>O<Subscript>3</Subscript> targets exposed to pulsed-periodic radiation of a CO<Subscript>2</Subscript> laser

The characteristics (phase composition, grain shape, grain size distribution, and specific surface area) of Ce_0.78Gd_0.22O_2-δ nanopowders produced by exposing the target to pulsed CO₂ laser radiation are reported. Reasons for a threefold increase in the output of the experimental powder-preparation unit (up to 60 g/h) with the characteristic grain size (≈10 nm) remaining unchanged are discussed.     

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.     

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.     

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.     

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.     

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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Aggregation kinetics of CeO<Subscript>2</Subscript> nanoparticles in KCl and CaCl<Subscript>2</Subscript> solutions: measurements and modeling

To characterize the environmental transport and health risks of CeO₂ nanoparticles (NPs), it is important to understand their aggregation behavior. This study investigates the aggregation kinetics of CeO₂ NPs in KCl and CaCl₂ solutions using time-resolved dynamic light scattering (TR-DLS). The initial hydrodynamic radius of CeO₂ NPs measured by DLS was approximately 95 nm. Attachment efficiencies were derived both from aggregation data and predictions based on the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. The deviations of the DLVO predictions were corrected by employing the extended DLVO (EDLVO) theory. The critical coagulation concentration (CCC) of CeO₂ NPs at pH = 5.6 is approximately 34 mM for KCl and 9.5 mM for CaCl₂. Furthermore, based on the EDLVO theory and the von Smoluchowski’s population balance equation, a model accounting for diffusion-limited aggregation (DLA) kinetics was established. For the reaction-limited aggregation (RLA) kinetics, a model that takes fractal geometry into account was established. The models fitted the experimental data well and proved to be useful for predicting the aggregation kinetics of CeO₂ NPs.     

Synthesis of hierarchical Na<Subscript>2</Subscript>FeP<Subscript>2</Subscript>O<Subscript>7</Subscript> spheres with high electrochemical performance via spray drying

Hierarchical Na₂FeP₂O₇ spheres with nanoparticles were successfully fabricated by a facile spray drying method. A relatively low drying temperature was introduced in order to form a carbon layer on the surface. As a cathode material for sodium-ion batteries, it delivered a reversible capacity of 84.4 mAh g^?1 at 0.1 C and showed excellent cycling and rate performance (64.7 mAh g^?1 at 5 C). Furthermore, a full sodium battery was fabricated using SP-Na₂FeP₂O₇ as the cathode and hard carbon as the anode, suffering almost no capacity loss after 400 cycles at 1 C. Due to its superior electrochemical property and the low materials cost, Na₂FeP₂O₇ is becoming a promising cathode material for large-scale energy storage systems.     

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.     

Novel CeO<Subscript>2</Subscript> nanorod framework prepared by dealloying for supercapacitors applications

The CeO₂ nanorod framework was synthesized via a facile-dealloying method coupled with calcination treatment for supercapacitors. X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) characterizations identified the cubic phase and nanorod morphology of the synthesized sample. Their electrochemical performance was also evaluated by cyclic voltammetry, galvanostatic charge-discharge tests, and cycling performances. The results show that CeO₂ nanorod framework possesses high-specific capacitance and superior charge/discharge stability, which are mainly ascribed to its high-Brunauer-Emmett-Tellar surface area (110.6 m² g^?1). Notably, the CeO₂//AC (Active Carbon) asymmetric supercapacitor device exhibits excellent cycling stability with capacity retention of 133.6% after cycling for 30,000 cycles.     

In situ controlled synthesis of various TiO<Subscript>2</Subscript> nanostructured materials via a facile hydrothermal route

Various TiO₂ nanomaterials, such as nanosheets, nanoflowers, and nanowires were directly self assembled on titanium substrate on a large scale under hydrothermal conditions. The morphology of the formed TiO₂ nanomaterials could be easily tuned by varying the experimental parameters of temperature, reaction time, and the NaOH concentration. A possible formation mechanism was suggested on the basis of the shape evolution of TiO₂ nanostructures by SEM images in combination with XRD patterns of as-grown samples. The optical properties of TiO₂ nanosheets, nanoflowers, and nanowires were characterized by reflectance spectroscopy. The studies revealed that the absorption capability of visible light is obviously different for TiO₂ with different morphologies. Moreover, TiO₂ nanosheets exhibited better light trapping than TiO₂ nanoflowers and TiO₂ nanowires due to their unique nanostructure.     

High frequency impedance spectroscopy study on Gd-doped CeO<Subscript>2</Subscript> thin films

The gadolinia-doped ceria (GDC) thin films were deposited by pulsed laser deposition. Samples with special geometry were prepared which allowed us to characterize GDC film in across-plane direction. The electrical properties of the films were investigated by means of impedance spectroscopy in the frequency range of 10 Hz to 10 GHz and 380–600 K temperature interval. The data analysis was performed by using appropriate equivalent circuit. The equivalent circuit modeled thin GDC film itself, platinum metal connections (traces) in the dielectric medium of sapphire substrate and interfaces between the film and platinum electrodes. Hence, several factors influenced the impedance spectra, namely the properties of substrate, the oxygen-ion transport in the film, ion blocking at the interface between the film and the electrode, and metal traces. The electrical properties of GDC thin films were compared with these of bulk ceramics and showed similar conductivity and dielectric permittivity values. The study also revealed that measurement data at electrical field frequencies of up to 10 GHz were particularly important to correctly estimate electrical properties of GDC thin films, because at high temperatures the electric response of GDC film shifts to high frequencies (higher than 1 MHz at 600 K). The thin film sample preparation for high frequency measurements and fitting of impedance data by using relatively simple equivalent circuit model is presented.     

Synthesis and properties of barium ferrigermanate Ba<Subscript>2</Subscript>Fe<Subscript>2</Subscript>GeO<Subscript>7</Subscript>

The magnetic susceptibility and specific heat of single crystals of the Ba₂Fe₂GeO₇ barium ferrigermanate are investigated. It is revealed that the temperature dependence of the magnetic susceptibility exhibits a kink at a temperature T = 8.5 K. The number of nonequivalent positions of Fe³⁺ ions and their occupancies are determined using Mössbauer spectroscopy. It is shown that the Fe³⁺ ions located in tetrahedral positions T2 are ordered incompletely, which is inconsistent with the results obtained previously. An assumption is made regarding the possible ground magnetic state of the Ba₂Fe₂GeO₇ compound.     

Controlled synthesis of CeO<Subscript>2</Subscript> nanoparticles using novel amphiphilic cerium complex precursors

Maleic anhydride was grafted by long-chain alcohols (1-hexadecanol, 1-octadecanol) to amphiphilic mono-L cis-butene dicarboxylates (L = hexadecyl, octadecyl), i.e., MAH, MAO, respectively. Subsequently, corresponding amphiphilic cerium complexes with these two mono-L cis-butene dicarboxylate ligands (Ce(L')₃, L'= MAH, MAO) were synthesized and behaved as the precursors to prepare CeO₂ nanoparticles for both of which can form nanosized micelle-like aggregates by special self-assembly in the wet chemical process. The nanoparticles were further characterized by Fourier transform-infrared spectroscopy (FTIR), Diffuse reflectance ultraviolet-visible spectra (DRUVS), scanning electron microscope (SEM), transmission electron microscope (TEM), and x-ray diffraction (XRD). Both the CeO₂ nanoparticles are in a cubic fluorite structure and present regular and well-dispersion club-like morphology with average particle size in the range of 40–70 nm. Besides, the strong ultraviolet–visible absorption for these CeO₂ nanoparticles can be found at the long-wavelength ultraviolet to visible region of 200–500 nm.     

Synthesis,sintering, and conductivity of Mn<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>

Studies on the sintering of manganese pyrovanadate depending on the temperature and the crystallite size show that we are prevented from obtaining a bulk ceramic sample by the anisotropic growth of grains. Investigation of the electrical properties of Mn₂V₂O₇ in the temperature range of 250–800°C reveals the activation energy at which bulk conductivity is 0.62 eV.     

Methane sensor based on SnO<Subscript>2</Subscript>/In<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> nanostructure

Two sets of samples of SnO₂/In₂O₃/TiO₂ system have been fabricated with different concentrations of component materials. In the first set TiO₂ with rutile structure was used, while in the second set it has the structure of anatase. Thin films (up to 50 nm) of obtained mixtures were deposited. Their sensitivity and selectivity with respect to methane (CH4) were studied. Nanostructure on the basis of 70%SnO₂ — 10%In₂O₃ — 20%TiO₂(anatase) exhibits sufficient sensitivity to methane.     

Synthesis of ZnAl<Subscript>2</Subscript>O<Subscript>4</Subscript>/α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> complex substrates and growth of GaN films

With the solid phase reaction between pulsed-laser-deposited (PLD) ZnO film and α-Al₂O₃ substrate, ZnAl₂O₄/α-Al₂O₃ complex substrates were synthesized. X-ray diffraction (XRD) spectra show that as the reaction proceeds, ZnAl₂O₄ changes from the initial (111)-oriented single crystal to poly-crystal, and then to inadequate (111) orientation. Corresponding scanning electron microscope (SEM) images indicate that the surface morphology of ZnAl₂O₄ transforms from uniform islands to stick structures, and then to bulgy-line structures. In addition, XRD spectra present that ZnAl₂O₄ prepared at low temperature is unstable at the environment of higher temperature. On the as-obtained ZnAl₂O₄/α-Al₂O₃ substrates, GaN films were grown without any nitride buffer using light-radiation heating low-pressure MOCVD (LRH-LP-MOCVD). XRD spectra indicate that GaN film on this kind of complex substrate changes fromc-axis single crystal to poly-crystal as ZnAl₂O₄ layer is thickened. For the single crystal GaN, its full width at half maximum (FWHM) of X-ray rocking curve is 0.4°. Results indicate that islands on thin ZnAl₂O₄ layer can promote nucleation at initial stage of GaN growth, which leads to the (0001)-oriented GaN film.     

Synthesis and electrochemical properties of P2-Na<Subscript>2/3</Subscript>Ni<Subscript>1/3</Subscript>Mn<Subscript>2/3</Subscript>O<Subscript>2</Subscript>

The pure phase P2-Na_2/3Ni_1/3Mn_2/3O₂ was synthesized by a solid reaction process. The optimum calcination temperature was 850 °C. The as-prepared product delivered a capacity of 158 mAh g^?1 in the voltage range of 2–4.5 V, and there was a phase transition from P2 to O2 at about 4.2 V in the charge process. The P2 phase exhibited excellent intercalation behavior of Na ions. The reversible capacity is about 88.5 mAh g^?1 at 0.1 C in the voltage range of 2–4 V at room temperature. At an elevated temperature of 55 °C, it could remain as an excellent capacity retention at low current rates. The P2-Na_2/3Ni_1/3Mn_2/3O₂ is a potential cathode material for sodium-ion batteries.