Valence State Of Ce Research Articles

Ultrasensitive resistivity-based ethanol sensor based on the use of CeO2-Fe2O3 core-shell microclusters.

This paper presents a method for synthesis of CeO2-Fe2O3 core-shell nanoparticles (CSNPs). These are shown to display enhanced ethanol sensing properties. Synthesis was done via a two-step process, starting with co-precipitation and followed by applying a sol-gel method. High resolution electron microscopy results revealed the core-shell nature of the particles. Surface morphological studies of the CSNPs showed a microcluster-like structure which is assumed to be responsible for the enhanced sensing response. X-ray photoelectron spectroscopy revealed valence states of Fe(III) and Ce(IV). The material was used in a resisitive sensor for ethanol vapor at room temperature (RT), at a typically applied voltage of 5V. The response of the sensor is higher than that of pristine CeO2 or Fe2O3 sensors towards 100ppm of ethanol at RT. The lower detection limit is 1ppm (with a signal change of 23). The response and recovery times are as short as 3 and 7s, respectively. The sensing mechanism is discussed in detail with respect to n-n heterojunctions formed between n-CeO2 and n-Fe2O3, high catalytic activity of the Fe2O3, and microcluster-like structures of the particles. Graphical abstract Schematic representation of gas sensing mechanism of CeO2-Fe2O3 core-shell nanoparticles (c) along with their morphological images (a&b).

Macrophage polarization by plasma sprayed ceria coatings on titanium-based implants: Cerium valence state matters

With inflammation increasingly recognized as a key factor that influences fracture healing, the immune response is considered as a critical component in determining biomaterial-mediated osteogenesis. Here, the ceria coatings are applied to the titanium substrates via plasma spraying under different process gas compositions, forming A-III (low Ce4+/Ce3+ ratio) and B-IV (high Ce4+/Ce3+ ratio) coatings. Results of protein adsorption and conformation indicated that the exposed cell-binding sites of fibronectin and subsequent cell morphology were associated with the Ce valence states. The A-III and B-IV coatings clearly restricted and promoted the spreading of the adherent RAW264.7 macrophages, respectively. Cell morphology differences between the coatings correlated with a change in the percentage of RAW cells expressing M2 surface markers Cd206/163, together with expression of anti-inflammatory cytokines Il10/1ra, while there was no significant difference between them in the percentage of M1-type macrophages. Owing to its higher catalase-mimetic activity, the B-IV coating exerted stronger inhibitory effects on reactive oxygen species generation and Nfkb activity than either A-III coating or Ti substrate. Consistently, the lowest expression levels of Nfkb-regulated pro-inflammatory cytokines Il6 and Tnfa were found for the B-IV coating. The manipulation of cerium valence states could be a good strategy in the design of orthopedic/dental implant coatings with beneficial immune response.

Optimized composition and improved magnetic properties of Ce-Fe-B alloys

Good magnetic properties of Ce17Fe76.5B6Nb0.5Cu0.5Ga0.5 alloy were obtained in our previous study. In order to increase the Curie temperature of Ce-Fe-B alloy without reducing the magnetic performance, Co element was added to optimize the composition of Ce-Fe-B based alloy. The effect of Nb, Cu, Ga, Co co-addition on phase structures, magnetic characteristics, microstructure and Ce valence state weight of Ce-Fe-B alloy are discussed comprehensively. Magnetic properties of Jr = 0.43 T, Hci = 486 kA/m, (BH)max = 30 kJ/m3 and TC = 437.8 K were obtained in Ce17Fe74.5B6Nb0.5Cu0.5Ga0.5Co2.0 alloy. An obvious remanent magnetization enhancement, decreased recoil permeability and fully closed recoil loops are observed for the optimized alloys. The strengthened short range exchange coupling interaction between adjacent grains contributes to the increased remanence ratio. The synchrotron radiation X-ray absorption near-edge structure fitting result showed that the magnetic Ce3+ valence weight of optimized alloy increased, while the nonmagnetic Ce4+ valence weight decreased compared with the original alloy. Microstructure observations showed that the triple junctions of tetragonal main phase grains are rich in Cu, while Co element prefers to enter into CeFe2 phase. The improved magnetic properties are mainly due to the increased Ce3+ valence weight and strengthened intergranular exchange coupling interactions.

Antibacterial mechanism and activity of cerium oxide nanoparticles

Nanomaterials have been applied as antibacterial agents by virtue of their unique functioning mechanism different from that of conventional antibiotics. Cerium oxide nanoparticles (CeO2 NPs) are important antibacterial agents due to their relatively low toxicity to normal cells and their distinct antibacterial mechanism based on the reversible conversion between two valence states of Ce(III)/Ce(IV). Some studies have been conducted to explore their antibacterial activities; however, systematic research reviews on the related mechanisms and influencing factors are still quite rare. In this review, we discuss the plausible mechanisms of the antibacterial activity of CeO2 NPs, analyze different influencing factors, and summarize various research reports on antibacterial effects on E . coli and S . aureus . We also propose the potential applications and prospects, and hope to provide an in-depth understanding on the antibacterial mechanism and a better guidance to the design and applications of this promising antibacterial material in the future.

Growth and characterization of (La,Ce)OBiS2 single crystals

(La,Ce)OBiS2 single crystals are successfully grown using CsCl flux. The obtained single crystals have a plate-like shape with a size of 0.5–2.0 mm and a well-developed ab-plane. The thickness of the crystals increases with a decrease in the Ce content. The (La,Ce)OBiS2 single crystals exhibit superconducting properties. Moreover, the superconducting transition temperature increases with an increase in the Ce content. The obtained (La,Ce)OBiS2 single crystals exhibit a tetragonal structure and the valence state of Ce in the crystals is determined to be mixed with Ce3+ and Ce4+ based on X-ray absorption fine structure spectroscopy.

Intermediate valence state of Ce in the novel quaternary compound CeRu2Sn2Zn18

A novel quaternary compound CeRu2Sn2Zn18 has been synthesized by a self-flux method. The structural, magnetic, and transport properties have been studied using single crystal X-ray structural analysis and measurements of the electrical resistivity ρ, specific heat C, and magnetic susceptibility χ. The X-ray structural analysis reveals that CeRu2Sn2Zn18 is an ordered variant of the CeCr2Al20- and ZrZn22-types, as found in the isostructural RT2Sn2Zn18 (R = La, Ce, Pr, Nd, T = Fe, Co, Ru). ρ shows no sign of a phase transition down to 2 K and is proportional to T2 below 50 K. χ exhibits a broad peak near 100 K, and the values are on the order of 10−3 emu/mol over a temperature range of 2–300 K. The electronic specific heat coefficient γ is found to be 69 mJ/K2 mol, which is approximately seven-fold larger than that of the La analog. These findings indicate that CeRu2Sn2Zn18 is in the intermediate valence state.

A Versatile, Mild and Selective Reduction of Nitroarenes to Aminoarenes Catalyzed by CeO2 Nanoparticles with Hydrazine Hydrate

AbstractA simple and convenient protocol is reported for the conversion of nitroarenes to aminoarenes by employing CeO2 nanoparticles (n‐CeO2) as heterogeneous solid catalysts using hydrazine hydrate as reducing agent under mild reaction conditions. A quantitative conversion of nitrobenzene is achieved using n‐CeO2 as solid catalyst with aniline selectivity to 93%. Further, this catalyst is also effectively used to transform wide range of nitroarenes to their respective aniline derivatives in higher conversions and selectivities. The reaction is found to be heterogeneous by leaching experiment and n‐CeO2 can be used for three cycles with no drop in its activity. Furthermore, the analyses of the fresh and three times used n‐CeO2 samples by powder X‐ray diffraction (XRD), UV‐Vis diffuse reflectance spectra (DRS), Fourier Transform‐Infrared Spectroscopy (FT‐IR), scanning electron microscope (SEM) and transmission electron microscope (TEM) indicate that the structural integrity, surface morphology and particle size remain unaltered during the course of the reaction. Furthermore, the valence state of Ce in n‐CeO2 is identical in the fresh and three times used samples as evidenced by Fourier Transform Raman spectroscopy. Some of the salient features of this process are operational simplicity, high conversion/selectivity, functional group tolerance, absence of noble metals/additives and reusability of the catalyst.

Growth and physical properties of Ce(O,F)Sb(S,Se)2 single crystals with site-selected chalcogen atoms

Ce(O,F)Sb(S,Se)2 single crystals were successfully grown using a CsCl/KCl flux method. The obtained crystals have a plate-like shape with the typical size of 1–2 mm and well-developed ab-plane, which enables X-ray single crystal structural analysis. The Ce(O,F)Sb(S,Se)2 crystallizes in a monoclinic space group, P21/m, with lattice parameters of a = 4.121(7) Å, b = 4.109(7) Å, c = 13.233(15) Å, β = 97.94(7) °. It is composed of alternate stacking of Ce-(O,F) and Sb-SSe layers, and the Sb-SSe layer includes selective occupation of Se atoms in its in-plane site. The valence state of Ce is estimated to be Ce3+ by X-ray absorption fine spectroscopy analysis. The single crystals show an insulating behavior, and a magnetic ordering around 6 K.

Electronic states of CeT2X20 (T:transition metal, X=Zn and Cd)

The electronic state of single crystals of CeIr2(Zn1−xCdx)20 and CePt2Cd20 has been studied by measuring the electrical resistivity ρ and specific heat C. We found that by substituting Zn with Cd, the lattice parameter expands, and the electronic specific heat coefficient γ and A coefficient, which is T2-term in ρ, are enhanced. Our experimental results reveal that the electronic state of the intermediate valence compound CeIr2Zn20 changes to a moderately heavy fermion state due to the negative chemical pressure. On the other hand, C and ρ of CePt2Cd20 show a clear jump and an abrupt change of slope below 0.3 K, respectively, due to a magnetic transition. The relation between the hybridization strength and Ce valence state has also been investigated by X-ray absorption experiment.

Effect of rare element Ce doping concentration on resistive switching of HfOx film

The switching characteristics of rare element Ce-doped HfOx films were investigated. The effect of Ce doping on oxygen defects was analyzed by X-ray photoelectron spectroscopy (XPS) and first-principle calculations. The variations of valence state of Ce and oxygen vacancies with the increase of Ce doping concentration were demonstrated. Although Ce doping increased dopants-induced oxygen vacancies in HfOx film, the density of oxygen vacancies decreased as the doping concentration increased. Besides, the introduction of Ce dopants enlarged the difference in electronegativity of Hf and O which made it harder for O to escape from HfOx film under voltage. Hence the switching behaviors of Ce-doped HfOx film were affected by multiple factors and can be effectively improved with an appropriate doping concentration (4.3%). A physical model based on the formation and rupture of conductive filaments was proposed to clarify the switching behavior of Ce-doped HfOx samples.

Atomic-Scale Valence State Distribution inside Ultrafine CeO2 Nanocubes and Its Size Dependence.

Atomic-scale analysis of the cation valence state distribution will help to understand intrinsic features of oxygen vacancies (VO ) inside metal oxide nanocrystals, which, however, remains a great challenge. In this work, the distribution of cerium valence states across the ultrafine CeO2 nanocubes (NCs) perpendicular to the {100} exposed facet is investigated layer-by-layer using state-of-the-art scanning transmission electron microscopy-electron energy loss spectroscopy. The effect of size on the distribution of Ce valence states inside CeO2 NCs is demonstrated as the size changed from 11.8 to 5.4 nm, showing that a large number of Ce3+ cations exist not only in the surface layers, but also in the center layers of smaller CeO2 NCs, which is in contrast to those in larger NCs. Combining with the atomic-scale analysis of the local structure inside the CeO2 NCs and theoretical calculation on the VO forming energy, the mechanism of size effect on the Ce valence states distribution and lattice expansion are elaborated: nano-size effect induces the overall lattice expansion as the size decreased to ≈5 nm; the expanded lattice facilitates the formation of VO due to the lower formation energy required for the smaller size, which, in principle, provides a fundamental understanding of the formation and distribution of Ce3+ inside ultrafine CeO2 NCs.

Thermal diffusion of mixed valence Ce in 6Li loaded silicate glass for neutron imaging

The creation and characterization of Ce doped surface regions in 6Li enriched silicate glasses for cold neutron detection is described. A diffusion layer of a dried solution containing cerium III acetylacetonate hydrate was thermally diffused into monolithic glass samples under a reducing atmosphere. The diffusion characteristics of the mixed valence cerium compound were assayed with Rutherford Backscattering Spectrometry. The Ce diffusion activation energy within the glass was found to be 411 ± 53 kJ/mol (4.26 ± 0.55 eV). Radiation induced color centers formed on the diffusion samples as an effect of using alpha bombardment measurements. The ratio of Ce valence states of the diffused cerium was assayed as a function of depth with X-ray Photoelectron Spectroscopy, and the signal from Ce for the color centers was compared to the unirradiated sample surface.

Enhanced photoluminescence response of Ca2+/Ba2+ substituted solid solutions of SrS:Ce phosphors

The quest of improving luminescence efficiency in alkaline earth sulfide phosphors has been of great interest due to their applications in diverse areas. We synthesized SrS:Ce0.05, Sr0.75Ba0.20S:Ce0.05, Sr0.75Ca0.20S:Ce0.05 and Sr0.75Ca0.10Ba0.10S:Ce0.05 phosphors using solid state diffusion method. X-ray diffraction patterns of all the samples were found similar to the cubic SrS phase except minor shifts in angles. To ascertain the valence state of Ce, near edge X-ray absorption fine structure spectroscopy was employed using synchrotron radiations. We simulated Ce M5,4-edges of Ce3+ ions using atomic multiplet calculations, which agrees well with the experimental data, clearly suggesting that Ce enters as Ce3+ in all the samples. The photoluminescence spectra of synthesized phosphors exhibit blue-green emission assigned to the 5d-4f transitions in Ce3+ levels, with minor shifts in wavelength. Interestingly, Ca2+/Ba2+substitution in SrS:Ce phosphors increases the PL intensity considerably. In comparison to SrS:Ce0.05 , PL intensity of Sr0.75Ca0.10Ba0.10S:Ce0.05 phosphors was enhanced by 2.7 times, which may be exploited in many optical applications.

Correlation between emission properties, valence states of Ce and chemical compositions of alkaline earth borate glasses

Although there are many reports investigating emission properties of Ce3+ centers, there is little report on the quantitative Ce3+/(Ce3++Ce4+) ratio, which is one of the most fundamental factors for designing phosphors. In this report, we have investigated the correlation among the optical and emission properties of Ce-doped alkaline earth borate glasses with the Ce3+/(Ce3++Ce4+) ratio determined by X-ray absorption near-edge structure analyses. It is confirmed that the additional carbon increases the population of Ce3+, which is also suggested from the optical absorption edge, Eg opt, due to Ce and the quantum yield (QY). We have revealed that the Ce3+ concentration of the Sr-substituted glass is completely the same as that of the Ca-substituted one that exhibits the highest efficiency, even though the Ce3+ ratio is suggested to depend on the substitution spices from the Eg opt and QY. Therefore, we can conclude that not only the Ce3+ amount but also the local structures of Ce3+ affect the emission properties. In other words, the luminescent efficiency of the Ce3+ centers in amorphous materials strongly depends on the various local coordination states.

Mesoporous Ce-Ti-Zr ternary oxide millispheres for efficient catalytic ozonation in bubble column

Most heterogeneous catalysts available for catalytic ozonation are present as powders and not ideal for column operation in advanced water treatment. In this study, a mesoporous Ce-Ti-Zr ternary oxide (CTZO) millisphere of large size (0.8–1.0 mm in diameter) and high surface area (180 m2 g−1) was synthesized as a highly stable catalyst to meet the hydrodynamic demand in column ozonation. The fraction of Ce(III) is much higher on the surface of CTZO (49.2%) than CeO2 (27.5%), resulting in its high catalytic activity in ozone decomposition and mineralization of oxalic acid (OA), an ozone-refractory probe compound. The most favorable catalytic ozonation of OA was observed at pH 3.0 and the adsorption of OA by CTZO via surface complexation is a requisite step for its ozonation. The EPR analysis and radical scavenging experiment confirmed that OH was the dominating reactive species in catalytic ozonation of OA by CTZO. XPS revealed that the oxidation of OA was mediated by the Ce(III)/Ce(IV) redox cycle, which continously accepts the electron supply from OA ligand and meanwhile donates electron to activate O3 into OH. The catalytic ozonation of OA by CTZO was enhanced in the presence of sulfate due to the generation of sulfate radical. Cyclic runs demonstrated that the CTZO millispheres exhibited high stability for sustainable catalytic ozonation of OA without noticeable release of Ce or change of Ce valence state. This study will provide new insight for the development of efficient ozonation catalysts towards practical application in advanced water treatment.

Effect of Ga addition on the valence state of Ce and magnetic properties of melt-spun Ce17Fe78-xB6Gax (x = 0-1.0) ribbons

The Ce17Fe78-xB6Gax (x=0-1.0) ribbons were fabricated by a melt-spinning technique in order to study the mechanism of the valence variation of Ce and their magnetic properties as well as improve the thermal stability of Ce-based rare earth permanent magnets. The systematic investigations of the Ce17Fe78-xB6Gax (x=0-1.0) alloys show that the room-temperature coercivity increases significantly from 352 kA/m at x = 0 to 492 kA/m at x = 1.0. The Curie temperature (Tc) increases monotonically from 424.5 K to 433.6 K, and the temperature coefficients of remanence (α) and coercivity (β) of the ribbons are better off from -0.56 %/K, -0.75 %/K for x = 0 to -0.45 %/K, -0.65 %/K for x = 0.75 in the temperature range of 300–400 K, respectively. The Ce L3-edge X-ray absorption near edge structure (XANES) spectrums reveal that there is more Ce4+ in ribbons under total electron yield than fluorescence yield as Ce has a high affinity with oxygen. The weight of Ce3+ increases while the weight of Ce4+ decreases in Ga-added alloys. The refined grain size and a more uniform microstructure are mainly attributed to the improved magnetic properties and thermal stability with Ga doping. This paper may serve as a reference for further developing the so-called gap magnets and the effective utilization of the rare earth resources.

Research and development of Ce-containing Nd2Fe14B-type alloys and permanent magnetic materials

Much demanded and overused are the critical rare-earth elements such as Pr, Nd, Dy, and Tb with increasing need of NdFeB-type rare-earth permanent magnets in the enlarging application areas, developing new high-tech industries, and emerging cutting-age frontiers. The balance and efficient use of rare-earth resources comes into being the national strategy, national defense, and border safety for many major countries and regions in the world. (Nd,Ce)FeB-based permanent magnetic materials, which can not only reduce cost but also offer a feasible way for integrated and effective utilization of rare earth resources, have received much attention in recent years. The existence of CeFe2 and the mixed valence state of Ce in CeFeB compound, the different metallurgy behavior and the particular processing as well as potential various magnetic-hardening mechanisms, however, make it quite different from Nd-based alloys. For instance, the coercivity of Ce-containing magnets in some certain composition range, is even higher than that of the counterpart pure Nd-based magnets though the Ce-containing magnets possess inferior intrinsic properties. Consequently, it is very important to design proper composition and structure, optimize processing, and analyze the mechanisms in depth for this kind of magnet. High performance and cost-effective magnets can be fabricated if we can make full use of the composition’s inhomogeneous and abnormal coercivity variation of the Ce-containing permanent magnets. In this paper, we have summarized the phase structures, magnetic properties and microstructures of (Nd,Ce)FeB-based permanent magnetic materials to shed light on further research and development of this type of so-called “gap magnet”.

Evidence of valence state change of Ce^3+ and Cr^3+ during UV charging process in Y_3Al_2Ga_3O_12 persistent phosphors

To elucidate the persistent luminescence mechanism of the Ce3+-Cr3+-codoped Y3Al2Ga3O12 garnet (YAGG), the valence states of Ce and Cr ions before and during UV charging were investigated by X-ray absorption near edge structure (XANES) spectroscopy. In the XANES spectra for Ce LIII and Cr K edges of YAGG:Ce3+-Cr3+ under UV illumination, the valence states of Ce4+ and Cr2+ were detected, but not in the XANES spectra of YAGG:Ce3+ and YAGG:Cr3+. We conclude that the combination of Ce3+ and Cr3+ causes the valence state change into Ce4+ and Cr2+ by UV illumination, which is the mechanism of persistent luminescence.

Increased coercivity for Nd-Fe-B melt spun ribbons with 20 at.% Ce addition: The role of compositional fluctuation and Ce valence state

Abstract It is conventionally accepted that cerium can weaken the magnetic behavior of the Nd-Fe-B magnets. In the present work, however, it is found that the coercivity increases only with the addition of 20 at.% of cerium into the Nd-Fe-B alloy using physical performance measurement system compared to other samples with less or more Ce additions. Unique microstructure and phase composition for the 20 at.% Ce sample have been observed using scanning electron microscope and high resolution transmission electron microscope with energy dispersive spectra. Crystal structure for the alloys was characterized using X-ray diffraction and high resolution transmission electron microscope. Scattering lattice parameters due to compositional fluctuation and Ce valence state have been found to be the direct reason for the abnormal increase of the coercivity for the 20 at.% Ce alloy.

The Effects of Cerium Valence States at Cerium Oxide Coatings on the Responses of Bone Mesenchymal Stem Cells and Macrophages.

Ideal orthopedic coatings should trigger good osteogenic response and limited inflammatory response. The cerium valence states in ceria are associated with their anti-oxidative activity and anti-inflammatory property. In the study, we prepared two kinds of plasma sprayed CeO2 coatings with different Ce4+ concentrations to investigate the effects of Ce valence states on the response of bone mesenchymal stem cells (BMSCs) and macrophage RAW264.7. Both the coatings (CeO2-A and CeO2-B) were characterized via XRD, SEM, and X-ray photoelectron spectroscopy. The CeO2 coatings enhanced osteogenic behaviors of BMSCs in terms of cellular proliferation, alkaline phosphatase (ALP) activity and calcium deposition activity in comparison with the Ti substrate. In particular, the CeO2-B coating (higher Ce4+ concentration) elicited greater effects than the CeO2-A coating (higher Ce3+ concentration). RT-PCR and western blot results suggested that the CeO2-B coating promoted BMSCs osteogenic differentiation through the SMAD-dependent BMP signaling pathway, which activated Runx2 expression and subsequently enhanced the expression of ALP and OCN. With respect to either CeO2-A coating or Ti substrate, the CeO2-B coating exerted greater effects on the macrophages, increasing the anti-inflammatory cytokines (IL-10 and IL-1ra) expression and suppressing the expression of the pro-inflammatory cytokines (TNF-α and IL-6) and ROS production. Furthermore, it also upregulated the expression of osteoinductive molecules (TGF-β1 and BMP2) in the macrophages. The regulation of cerium valence states at plasma sprayed ceria coatings can be a valuable strategy to improve osteogenic properties and alleviate inflammatory response.