Optical and ferromagnetic properties of hydrothermally synthesized CeO2/CuO nanocomposites

Nanostructured CeO₂/CuO composites are synthesized using a facile hydrothermal reaction. Results signify that Cu ions prefer to enter into CeO₂ lattice forming solid solution at low concentration, and would be transformed into CuO phase at moderate concentration. Moreover, the addition of CuO species into CeO₂ promotes the reduction of Ce⁴⁺ and the creation of oxygen vacancy (V_O) defects. Raman analyses confirm V_O concentration initially increases and then decreases with the increasing CuO phase and the sample Ce1Cu2 exhibits the highest defect concentration. The room temperature ferromagnetic behavior is observed firstly in CeO₂/CuO nonmagnetic system and the maximal saturation magnetization appears in Ce1Cu2. The emergent ferromagnetism appears to be relevant to the extensive V_O defects, which can be interpreted by the indirect double-exchange model. The synthetic interaction between CeO₂ and CuO results in the redshift of the bandgap in prepared CeO₂/CuO nanocomposites.     

Tunable Ferromagnetic Behaviors Observed in Highly Orientated Co‐Doped ZnO Thin Films by the Bandgap Engineering

Highly orientated Co‐doped Zn(Mg, Cd)O thin films have been prepared on Si (001) substrate using pulsed laser deposition (PLD). Our results indicate that Mg and Cd‐doping can tune the bandgap of ZnO thin film. Apparent room‐temperature ferromagnetism is observed in these films, which can be modified by the bandgap of ZnO. A narrow bandgap enhances the ferromagnetism of the films through the Cd‐doping, and wide bandgap does the opposite through the Mg‐doping, which may be ascribed to the ferromagnetic (FM) exchange interaction between the 3d states of magnetic Co ions and the impurity band. These experimental results provide some new evidence that the ferromagnetism in Co:ZnO is closely related to crystal defects and improve our knowledge about ferromagnetism in dilute magnetic oxides.     

A new insight into the role of transition metals doping with CeO2–ZrO2 and its application in Pd-only three-way catalysts for automotive emission control

CeO₂–ZrO₂ (CZ) doped with different transition metals was prepared through a co-precipitation and supercritical drying method and the corresponding Pd-only TWCs were characterized. Pd/CZFe and Pd/CZCo exhibit the best catalytic activity and the widest operation window, in accordance with the decrease of the light-off and full-conversion temperature. However, Pd/CZCr restrains the catalytic property. It may be due to the different effect of transition metals on the property of CZ. The introduction of Fe and Co promotes the surface atom ratios of Ce/Zr, makes part of Ce⁴⁺ transferred into Ce³⁺ in order to maintain the electrical neutrality and seems to facilitate the reduction of Ce⁴⁺ → Ce³⁺ or the formation of oxygen vacancies of CZ. The increasing concentration of oxygen vacancies for CZFe leads to the enhancement of DOSC. Moreover, the introduction of Fe favors the transfer of oxygen in different atmospheres.     

Oxygen vacancy luminescence and band gap narrowing driven by Ce ion doping with variable valence in SnO2 nanocrystals

Although considerable research works have witnessed the important modulations of oxygen vacancies on the optical, electrical, and magnetic properties of SnO₂ nanostructures, it is not easy to control oxygen vacancy defects in such systems.The difficulty stems from that oxygen vacancy is a kind of atomic defect, and its distribution is sensitive to process conditions and external factors, which makes direct characterization and purposeful control difficult. The purpose of this work on Ce-doped SnO₂ nanocrystals is to investigate the tolerance of the host lattice to Ce ions, the population and evolution of Ce³⁺/Ce⁴⁺ ions, and the possibility to adjust oxygen vacancies by Ce³⁺ ions, and then focus on the influence of oxygen vacancy defects on the band gap and luminescence performance. As Ce doping concentration increases from 0 to 12 at.%, the doped system changes from Ce³⁺ dominated at low doping amount (≤3 at.%) to Ce³⁺/Ce⁴⁺ coexistence at medium doping concentration (3 at.% ∼ 9 at.%), to occurrence of CeO₂ impurity phase at over doping (∼12 at.%). The optimum doping occurs at 6 at.%, which corresponds to the saturated critical point of Ce³⁺ content and the maximum oxygen vacancy concentration. Importantly, the oxygen vacancies in the current Ce-doped SnO₂ nanocrystals is directly regulated by the Ce³⁺ ion concentration on the Sn sites, which plays an important role in the band gap tuning and visible light emission. With Ce concentration increasing from 0 to 12 at.%, the band gap monotonicity decreases from 3.36 eV to 3.12 eV, while the intensity of the oxygen vacancy luminescence band first increases and then decreases, with the turning point at 6 at.%. Both band gap narrowing effect and enhanced emission indicate that Ce-doped SnO₂ should be a promising method to design and manufacture visible light responsive SnO₂ based optoelectronic materials by manipulating oxygen vacancy defects.     

Experimental evidence and mechanism of the oxygen storage capacity in MnO2-Ce(1−x)ZrxO2/TiO2 catalyst for low-temperature SCR

The oxygen storage capacity of amorphous CeO₂ and its mechanism were investigated in a Zr-doped MnO_x-CeO₂/TiO₂ catalyst at low temperatures. The oxygen storage capacity of several catalysts was determined by the release of lattice oxygen upon reduction of Ce⁴⁺ to Ce³⁺. We designed a temperature programmed reduction analysis using ammonia gas to measure the amount of lattice oxygen release and identify a decrease in reaction-onset temperature from 136 °C to 75 °C upon doping the catalyst with Zr. Additional reduction was observed in Zr-doped MnO_x-CeO₂/TiO₂ and this was attributed to an increase in temperature sensitivity of thermal vibrations of the first Ce–O coordination shell. The temperature dependence of the thermal vibrations was identified by examining the behavior of the Debye–Waller factor as a function of temperature with fitting the extended X-ray absorption fine structure.     

Preparation of biaxially textured Ce1-x(Y0.2Zr0.8)xOδ buffer layers on RABiTS NiW tapes by chemical solution deposition

Highly (100)-oriented Ce_1-x(Y_0.2Zr_0.8)_xO_δ (CYZO) films were prepared on biaxially textured NiW substrates by a chemical solution deposition approach using metal inorganic salts as starting materials. It has been found that both the preferential orientation and surface roughness of CYZO films decrease gradually with increasing of the doping percentage of Y³⁺ and Zr⁴⁺ ions. The epitaxial growth relationship of (220)_CYZO//(200)_NiW and [00?l]_CYZO//[001]_NiW was demonstrated by XRD texture measurement as well as atomic resolution STEM observation. XRD, Raman and XPS spectra results indicate that Y³⁺ and Zr⁴⁺ ions were indeed introduced into CeO₂ lattice to substitute Ce⁴⁺ ions and form cubic fluorite CYZO solid solution. Moreover, CeO₂ buffer layer can be endowed a strong enough capability to prevent element diffusion through co-doping of yttrium and zirconium, provided that an optimal doping ratio of them is adopted. This will provide a new approach to fabricating strong-barrier single buffer layer for coated conductor.     

Catalytic Efficiency of Ceria–Zirconia and Ceria–Hafnia Nanocomposite Oxides for Soot Oxidation

The catalytic oxidation of soot particulates has been investigated over CeO₂, CeO₂–ZrO₂ and CeO₂–HfO₂ nanocomposite oxides. These oxides were synthesized by a modified precipitation method employing dilute aqueous ammonia solution. The prepared catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) and BET surface area methods. The soot oxidation has been evaluated by a thermogravimetric method under ‘tight contact’ conditions. The XRD results revealed formation of cubic CeO₂, Ce_0.75Zr_0.25O₂ and Ce_0.8Hf_0.2O₂ phases in case of CeO₂, CeO₂–ZrO₂ and CeO₂–HfO₂ samples, respectively. TEM studies confirm the nanosized nature of the catalysts. Raman measurements suggest the presence of oxygen vacancies, lattice defects and oxide ion displacement from normal ceria lattice positions. UV-Vis DRS studies show presence of charge transfer transitions Ce³⁺←O^2? and Ce⁴⁺←O^2? respectively. The catalytic activity studies suggest that the oxidation of soot could be enhanced by incorporation of Zr⁴⁺ and Hf⁴⁺ into the CeO₂ lattice. The CeO₂–HfO₂ combination catalyst exhibited better activity than the CeO₂–ZrO₂. The observed high activity has been related to the nanosized nature of the composite oxides and the oxygen vacancy created in the crystal lattice.     

Preparation of oxygen vacancy-controllable CeO2 by electrotransformation of a CeCl3 solution and its oxidation mechanism

CeO₂ was successfully prepared by electrotransformation of CeCl₃ solution in previous work. In this work, the oxidation mechanism of Ce³⁺ was determined by cyclic voltammetry and a potential-pH diagram, which indicated that Ce³⁺ was oxidized by oxygen during the electrotransformation, and the important role of the oxygen content in solution was confirmed. Thus, the influence of atmosphere (air/argon/oxygen) on CeO₂ preparation was investigated by adjusting the gas type and flow rate. XRD patterns demonstrated the cubic fluorite structure and grain size of CeO₂. With an increase in the gas flow rate, the grain size decreased, but the particle size increased. This result indicated that the smaller grains contained more lattice defects and higher surface energy, so the grains preferred to agglomerate together, forming larger particles. XPS spectra proved that Ce³⁺ and Ce⁴⁺ both existed in the CeO₂ samples. Raman scattering revealed that more oxygen vacancies could be produced with an argon atmosphere and high gas flow.     

Effect of yttrium and praseodymium on properties of Ce0.75Zr0.25O2 solid solution for CH4–CO2 reforming

Ce_0.75Zr_0.25O₂ solid solutions doped with Y³⁺ or Pr⁴⁺/Pr³⁺ were prepared by the co-precipitation method, and their physicochemical properties were characterized by means of N₂ adsorption, X-ray diffraction, X-ray photoelectron spectroscopy, FT-Raman, and H₂ temperature-programmed reduction and thermogravimetric analysis. Their performance in CH₄–CO₂ reforming was also tested in an atmospheric fixed-bed reactor. Ce_0.75Zr_0.25O₂ and Y³⁺ or Pr⁴⁺/Pr³⁺ doped Ce_0.75Zr_0.25O₂ solid solutions are of CaF₂ structure, and the thermal stability of Ce_0.75Zr_0.25O₂ is enhanced by doping Y³⁺ or Pr⁴⁺/Pr³⁺. Comparing with Ce_0.75Zr_0.25O₂, the migration of bulk lattice oxygen species become easier and the content of surface oxygen species is higher in the doped Ce_0.75Zr_0.25O₂, which is due to either oxygen vacancies or/and structural distortion resulted from the doping. The activity of the solid solutions in CH₄–CO₂ reforming is closely related to the surface oxygen species. Y³⁺ or Pr⁴⁺/Pr³⁺ doped Ce_0.75Zr_0.25O₂, especially the former, show higher activity than Ce_0.75Zr_0.25O₂, and Y³⁺ doped Ce_0.75Zr_0.25O₂ possesses better stability. All of the catalysts have good coke resistance. The catalyst deactivation is mainly due to the catalyst sintering.     

High magneto-optical performance of GdFeO3 thin film with high orientation and heavy Ce3+ doping

In this paper, GdFeO₃ thin films with high orientation and heavily Ce³⁺ doping were deposited by radio frequency magnetron sputtering with a matching substrate. The effects of substrates and Ce³⁺ doping on the structure, magnetic and magneto-optical properties of thin films were investigated. As a result, Ce³⁺ doping can not only increase the saturation magnetization but also greatly enhance the magnetic circular dichroism signals of Ce:GdFeO₃ thin films. Based on the density functional theory calculation, it can be found that the probability of electron transition between Ce³⁺ 4f and Fe³⁺ 3d and the difference in the absorption of right and left circularly polarized light increase, which results in the strong magneto-optical effect of Ce:GdFeO₃/STO thin films.     

Entropy-stabilized metal-CeOx solid solutions for catalytic combustion of volatile organic compounds

Doped Ceria with abundant oxygen vacancies exhibits enhanced performance in heterogeneous oxidation. In principle, doping 50 mol% divalent cations (such as: Cu²⁺, Zn²⁺, and Mg²⁺) into CeO₂ lattice would produce an exceptional catalyst with maximum active oxygen species. However, the huge size gap between Ce (IV) and divalent metal cations obstructs its synthesis. Here, we utilize the theory of increasing configurational entropy with five metal dopants to lower the Gibbs-free energy, and successfully incorporate 50 mol% divalent metal cations into CeO₂ lattice. This unique doping environment endows Ce_0.5Zn_0.1Co_0.1Mg_0.1Ni_0.1Cu_0.1O_x two features: (a) Abundant active oxygen species for excellent performance in volatile organic compounds catalytic oxidation; (b) Bring multi reactive sites, which enable the simultaneous combustion of carbon monoxide, propylene and toluene. Moreover, the increased entropy value makes Ce_0.5Zn_0.1Co_0.1Mg_0.1Ni_0.1Cu_0.1O_x an ultra-stable catalyst in both thermal and hydrothermal conditions (e.g., Working >200 hr in water-resistance experiment).     

Enhanced oxygen mobility and reactivity for ethanol steam reforming

This article describes a strategy for increasing oxygen storage capacity (OSC) of ethanol steam reforming (ESR) catalysts. Sintering and carbon deposition are major defects of nickel‐based catalysts for ESR; tuning oxygen mobility (OM) of CeO₂‐based supports can overcome these drawbacks and promote H₂ production. We have successfully increased OSC and OM by adding Mg into the lattice of Ni/CeO₂ to promote H₂ production in ESR. The insertion of Mg into the CeO₂ lattice efficiently promotes the reduction of Ce⁴⁺ according to X‐ray powder diffraction (XRD) and temperature‐programmed reduction (TPR) analysis. Mg‐modified Ni/CeO₂ catalysts have larger OSC and smaller nickel crystallite size compared with bare Ni/CeO₂. The optimal Mg addition is 7 mol % (Ni/7MgCe) with the best OM. We also present evidence indicating that Mg addition significantly promotes ethanol conversion and H₂ production in ESR, and that Ni/7MgCe yields the best performance due to the high OM of the support. These Mg‐modified catalysts also produce less carbon deposition compared with Ni/CeO₂, and the amount of deposited carbon decreases with increasing Mg addition. Ni/7MgCe has the best resistance to carbon deposition owing to the excellent OM. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Structural modifications of nanostructured ceria CeO2,xH2O during dehydration process

Hydrated nanostructured cerium dioxide CeO₂, xH₂O (hydrated nanoceria) has been synthesized in room conditions via a precipitation route. This hydrated nanoceria phase has been subjected to thermal decomposition in the temperature range from 25 °C to 800 °C. At least three decomposition steps have been observed in thermal and thermogravimetric analyses. Three different samples of cubic nanoceria respectively obtained at room temperature (RT-nanoceria), 80 °C (80-nanoceria) and 600 °C (600-nanoceria) have been studied by X-Ray diffraction, Raman spectroscopy and scanning electron microscopy analyses. The analyses of X-ray diffraction profiles and Raman vibrational bands have clearly shown that dehydration is accompanied by increasing crystallite size, lattice parameter contraction. The cubic structure of hydrated RT-nanoceria might be associated with a complex chemical formula unit involving Ce⁴⁺, Ce³⁺ mixed valences, oxygen vacancies, lattice and surface water and OH^? proton species.     

Mg doping effects on the microstructure and piezoelectric characteristics of ZnO:Li films deposited at room temperature using an RF sputtering deposition method

In this work, Density functional theory (DFT + U) was used to simulate Mg doped LZO (ZnO:Li 3 mol%) based on an atomic substitution concept. The calculation results indicate the piezoelectric constants e₃₃, e₃₁ and lattice constant a have a tendency to increase significantly due to Mg doping effects. Also, an RF magnetron sputtering system was used to prepare Mg-doped LZO films (ZnO: Li 3 mol%) with Mg doping concentrations of 0, 3, 6, 9 mol% at room temperature. A further analysis shows that when the proposed films are deposited at room temperature, Li atoms will bond with oxygen atoms to form Li₂O₂ to help to enhance the piezoelectric properties of the films. This is also found to be one of the reasons for the error in the simulated lattice constants.In addition, the doping of Mg induces lateral tensile stress in the ZnO structure. This phenomenon can produce lattice distortion and change the asymmetry of the ZnO atomic center at room temperature, thus improving the piezoelectric properties (piezoelectric coefficient d₃₃ is increased from 4.5 to 26.3 pm/V). The reported d₃₃ value is the highest compared to other reports on ZnO-based films deposited at room temperature.     

Enhancing piezoelectric properties of Ba0.88Ca0.12Zr0.12Ti0.88O3 lead-free ceramics by doping Co ions

The piezoelectric properties of lead-free Ba_0.88Ca_0.12Zr_0.12Ti_0.88O₃ (BCZT) ceramics were greatly optimized by doping Co ions using a CoO powder. The role of Co²⁺ and Co³⁺ in enhancing the piezoelectric properties and the relationship between the content ratio Co³⁺/Co²⁺ and piezoelectric performance were studied. The X-ray diffraction patterns of all samples indicated that crystalline phases were a BCZT-based single perovskite structure regardless of the Co ion content. The phase transition temperature and lattice distortion degree were related to the Co ion content and the content ratio Co³⁺/Co²⁺ because Co²⁺ resulted in higher oxygen vacancy generation, whereas Co³⁺ induced larger lattice shrinkage. The ceramic containing 0.10 wt% of Co ion showed the best piezoelectric and dielectric performance with the highest piezoelectric constant d₃₃ ~ 490 p.m./V at room temperature and the highest Curie temperature T_c of 110 °C, which increased by 29% and 16%, respectively. In this case, the content ratio Co³⁺/Co²⁺ reached the maximum value of 0.86. The high piezoelectric properties and phase stability of BCZT ceramics by doping Co ions make these ceramics promising piezoelectric materials for practical applications.     

Microstructural and electrical changes in Ca0.9R0.1CeNbMoO8 (R = Y,Sm, Nd,La) ceramics induced by rare-earth ion doping

Rare-earth ions doped Ca_0.9R_0.1CeNbMoO₈ (R = Y, Sm, Nd, La) ceramics have been successfully prepared by solid-state method, and their modifications to the microstructure and electrical properties are also investigated. The rare-earth ions doped ceramics exhibit the scheelite structure. With the increase in the radius of rare-earth ions, the lattice distortion and bond interaction will be enhanced, and the consistency of grain size will be reduced. The ceramics exhibit negative temperature coefficient (NTC) thermistor characteristics in the temperature range of 473 K-1273 K, and the activation energy decreases with the increase of the radius of rare-earth ions. Rare-earth ions doping can increase the content of Ce³⁺ ions and promote the conductivity of ceramics. Except for Sm³⁺-doped ceramics, the high-temperature aging rate of other ceramics is less than 2%. The existence of some metastable Sm²⁺ ions in Sm³⁺-doped ceramics not only increases the activation energy, but also reduces the high-temperature stability of the ceramics.     

Effects of dopants with various valences on densification behavior and phase composition of a ZrO2–SiO2 nanocrystalline glass-ceramic

Effects of dopants with different valences on the densification behavior and phase composition of a ZrO₂–SiO₂ nanocrystalline glass-ceramic (NCGC) during pressureless sintering were investigated in this study. The raw powder of Ca²⁺, La³⁺, Ce⁴⁺ and Ta⁵⁺ ions doped ZrO₂–SiO₂ (referred to as Ca-ZS, La-ZS, Ce-ZS, Ta-ZS, respectively) and pure ZrO₂–SiO₂ (PZS) sample were synthesized by sol-gel method, followed by pressureless sintering. Compared with the PZS sample, doping of Ca²⁺ and La³⁺ ions significantly promoted the densification of the NCGCs. The “densification promotion” effect was attributed to the formation of oxygen vacancies and the decrease of SiO₂ viscosity due to doping of aliovalent cations. The dopants with various valences showed significant effects on the phase compositions of the NCGCs during sintering. Doping of Ca²⁺ ion accelerated the reaction kinetics between ZrO₂ nanocrystallites and amorphous SiO₂ to yield ZrSiO₄. The La³⁺ ion acted as destabilizer of t-ZrO₂, which resulted in a rapid tetragonal (t) to monoclinic (m) ZrO₂ phase transformation during sintering, while in the Ta⁵⁺ and Ce⁴⁺ ions doped sample, the phase transformation occurred gradually. All the doping ions increased the lattice parameters and the volume of t-ZrO₂ unit cell, while the effects of the doping ions on the lattice parameters of m-ZrO₂ unit cell were more complex.     

The influence of chelating agents on cerium oxide decorated on graphite synthesized by the hydrothermal route

Morphology features of cerium oxide nanoparticles, such as size and agglomeration, are important as a coating that improves corrosion resistance and as reinforcement in mechanical applications. In this work, the influence of two heat treatments (160° and 190 °C) in combination with three different chelating agents in the preparation of CeO₂ and CeO₂ decorated on graphite (CeO₂_Gr) nanoparticles is studied. The novelty of this work is that CeO₂_Gr was successfully prepared using the hydrothermal method. All the samples evaluated by X-ray diffraction exhibit a single fluorite-type structure in the cubic phase and Fm3m space group. The spherical harmonics method using the Fullprof Suite program was used to determine the average crystallite sizes, which were 9 nm for CeO₂ and 7 nm for CeO₂_Gr. Transmission electron micrographs for the prepared samples with citric acid showed non-agglomerate particles with homogeneous particle sizes and a quasi-spherical shape distribution. Raman spectra show a band centre at 600 cm^-1 associated with the presence of Frenkel-type oxygen vacancies that induced the reduction of Ce⁴⁺ to Ce³⁺. The analysis of X-ray photoelectron spectra corroborates the coexistence of Ce³⁺ and Ce⁴⁺ species for CeO₂ and CeO₂_Gr nanoparticles. This work forms new perspectives in the development of CeO₂ decorated on graphite prepared by the hydrothermal method to obtain composites not only for sensing applications and wastewater treatment but also for corrosion resistance and reinforcement materials.     

Effect of Zr-doping on Pd/CexZr1−xO2 catalysts for oxidative carbonylation of phenol

Ce–Zr solid solution (Ce_xZr_1−xO₂, CZO) was prepared by the citric acid sol–gel method. The CZO was then used as a support for Pd/CZO catalysts for the oxidative carbonylation of phenol to diphenyl carbonate. The Pd/CZO catalyst showed enhanced activity and diphenyl carbonate selectivity compared with the Pd/CeO₂ catalyst. The catalytic performance of Pd/CZO was influenced by the calcination temperature of the CZO support. X-ray diffraction, scanning electron microscopy, N₂ adsorption–desorption measurements, X-ray photoelectron spectroscopy and H₂ temperature-programmed reduction measurements were used to investigate the effects of Zr doping and calcination temperature. The catalytic performance of Pd/CZO and Pd/CeO₂ for the oxidative carbonylation of phenol was affected by several factors, including the specific surface area, Ce³⁺ and/or oxygen vacancy content, oxygen species type and Pd(II) content of the catalyst. All these properties were influenced by Zr doping and the calcination temperature of the CZO support.     

Defect generation and morphology transformation mechanism of CeO2 particles prepared by molten salt method

Ceria is widely used in industrial fields due to its unique chemical properties. In this work, a series of CeO₂ particles with controllable morphology, size, and defect concentration were obtained by a simple molten salt method. The adjustment of temperature and molten salt concentration has a considerable effect on the morphology and particle size of the final CeO₂ particles while prolonging the holding time has little effect. Ion doping and reducing atmosphere calcination were used to regulate the defect concentration to improve the chemical activity of CeO₂ particles. SEM results show that the morphology of CeO₂ particles transforms from sphere to octahedron under the two treatments. The Rietveld refinement results and the XPS spectra indicate that increasing calcination temperature, reducing atmosphere calcination and ion doping are beneficial to improving the oxygen vacancies and Ce³⁺ concentration of CeO₂ samples, which are the reason for enhancing the photocatalytic activity of the samples. Moreover, the oriented attachment, agglomeration and merging of crystals formed by the decomposition of cerium precursors are the key to the growth of CeO₂ particles. Aggregates with exposed low-energy planes merge directly to form particles of various morphologies to maintain their own low energy.