Nanocrystalline Cerium Oxide Research Articles

Structure-dependent electronic properties of nanocrystalline cerium oxide films

We investigate the electronic properties of nanocrystalline cerium oxide $({\mathrm{CeO}}_{x})$ films, grown by various techniques, and we establish universal relations between them and the film structure, composition, and morphology. The nanocrystalline ${\mathrm{CeO}}_{x}$ films mainly consist of ${\mathrm{CeO}}_{2}$ grains, while a considerable concentration of trivalent ${\mathrm{Ce}}^{3+}$ is distributed at the ${\mathrm{CeO}}_{2}$ grain boundaries forming amorphous ${\mathrm{Ce}}_{2}{\mathrm{O}}_{3}.$ A small portion of ${\mathrm{Ce}}^{3+}$ is also located around O-vacancy sites. The optical properties of the ${\mathrm{CeO}}_{x}$ films are considered, taking into account the reported band-structure calculations. The fundamental gap ${E}_{g}$ of ${\mathrm{CeO}}_{x}$ is due to the indirect $\mathrm{O}2\stackrel{\ensuremath{\rightarrow}}{p}\mathrm{Ce}4f$ electronic transition along the L high-symmetry lines of the Brillouin zone and it is correlated with the $[{\mathrm{Ce}}^{3+}]$ content, explaining the redshift of ${E}_{g}$ in nanostructured ${\mathrm{CeO}}_{x},$ which is due to the ${\mathrm{Ce}}^{3+}$ at the grain boundaries and not due to the quantum-size effect itself. We also correlate the energy position of the $\mathrm{O}2\stackrel{\ensuremath{\rightarrow}}{p}\mathrm{Ce}4f$ electronic transition, which varies up to 160-meV wide, with the lattice constant of the ${\mathrm{CeO}}_{2}$ grains. We also show that the higher-order transitions are more sensitive to film composition. The refractive index, far below ${E}_{g},$ is explicitly correlated with the film density, independently of the ${\mathrm{Ce}}^{3+}/{\mathrm{Ce}}^{4+}$ and O concentrations, grain size, and lattice parameter. The density is also found to be the major factor affecting the absolute value of the ${\ensuremath{\varepsilon}}_{2}$ peak, which corresponds to the $\mathrm{O}2\stackrel{\ensuremath{\rightarrow}}{p}\mathrm{Ce}4f$ electronic transition.

Effect of pH of Medium on Hydrothermal Synthesis of Nanocrystalline Cerium(IV) Oxide Powders

Well‐crystallized cerium(IV) oxide (CeO2) powders with nanosizes without agglomeration have been synthesized by a hydrothermal method in an acidic medium by using cerium hydroxide gel as a precursor. The relationship between the grain size, the morphology of the CeO2 crystallites, and the reaction conditions such as temperature, time, and acidity of the medium was studied. The experiments showed that with increasing reaction temperature and time, the CeO2 crystallites grew larger. The crystallites synthesized in an acidic hydrothermal medium were larger and had a more regular morphology than the ones synthesized in a neutral or alkaline medium when the reaction temperature and time were fixed. The CeO2 crystallites synthesized in an acidic medium were monodispersed; however, there was vigorous agglomeration among the grains synthesized in a neutral or alkaline medium. It was demonstrated that the hydrothermal treatment was an Ostwald ripening process and the acidity (pH) of the used hydrothermal medium played a key role in the dissolution of smaller grains. It is proposed that the dissolution process can control the kinetics of the growth of larger grains.

Low temperature processing of dense nanocrystalline yttrium-doped cerium oxide ceramics

Nanocrystalline cerium oxide ceramics of high homogeneity and nearly full density were prepared. The starting material was synthesized by the direct homogeneous precipitation method using hexamethylenetetramine (HMT). Two alternative routes for consolidation into green bodies were employed: (i) centrifugal casting of an electrostatically stabilized colloidal solution and (ii) cold isostatic pressing of dried ceria powder. The green bodies were sintered at temperatures between 700 and 1000°C. The sinter activities of both nanocrystalline materials were strongly enhanced if compared to microcrystalline ceria. Green bodies, which were generated by colloidal processing exhibited the highest sinter activities, associated with a unique pore structure. The effect of yttrium doping on the grain size after sintering at high temperatures was also investigated. The combination of yttrium doping and colloidal processing allowed for the synthesis of dense nanocrystalline cerium oxide ceramics by pressureless sintering.

Electrical Conductivity and Lattice Defects in Nanocrystalline Cerium Oxide Thin Films

The results of the electrical conductivity and Raman scattering measurements of CeO2 thin films obtained by a polymeric precursor spin‐coating technique are presented. The electrical conductivity has been studied as a function of temperature and oxygen activity and correlated with the grain size. When compared with microcrystalline samples, nanocrystalline materials show enhanced electronic conductivity. The transition from extrinsic to intrinsic type of conductivity has been observed as the grain size decreases to <100 nm, which appears to be related to a decrease in the enthalpy of oxygen vacancy formation in CeO2. Raman spectroscopy has been used to analyze the crystalline quality as a function of grain size. A direct comparison has been made between the defect concentration calculated from coherence length and nonstoichiometry determined from electrical measurements.

Grain size-dependent electrical conductivity of polycrystalline cerium oxide: I. Experiments

The electrical conductivity of polycrystalline cerium oxide was investigated in the nanometer and micrometer size range. Nanocrystalline samples of different grain size were prepared by uniaxial hot-pressing of nanocrystalline powder at various temperatures and pressures. Additional annealing at high temperatures was employed in order to obtain microcrystalline samples. An equivalent-circuit analysis of ac-impedance spectra based on the brick-layer model was performed and the apparent bulk conductivity determined. The effect of a variation in temperature or oxygen partial pressure revealed the rather different nature of the electrical transport properties in the nano- and microcrystalline materials. Nanocrystalline cerium oxide exhibited electronic conductivity under conditions at which microcrystalline samples showed impurity-controlled ionic conductivity. The electronic conductivity of nanocrystalline samples was larger than the intrinsic electronic conductivity of pure single crystalline cerium oxide and was increasing with decreasing grain size. The experimental results were analyzed according to the defect chemistry of cerium oxide and consequences of a space charge effect on the partial electronic and ionic conductivity in polycrystalline cerium oxide will be discussed.

Grain Size Dependence of Electrical Conductivity in Polycrystalline Cerium Oxide

The magnitude and activation energy of electrical conductivity in nanocrystalline cerium oxide exhibit a clear grain size dependence. Experimental results compiled from the literature were analyzed using a space charge model, which takes into account the deviation of point defect concentrations from their bulk values in the vicinity of grain boundaries. The consequences on conductivity arising from such space charge layers were calculated using the brick-layer model (BLM) for grain sizes L large compared to the screening length λ. The obtained results were supplemented by the calculated conductivity in the flat-band limit for L ≪ λ. This combination allowed for a quantitative comparison with experimental values, which were obtained in the mesoscopic regime of grain sizes from 10–40 nm. The analysis yielded a value for the space charge potential in cerium oxide of 0.55 V. This space charge potential is caused by a reduced standard chemical potential of oxygen vacancies in the grain boundary core as compared to the bulk phase.

An X-ray Powder Diffraction Study of the Microstructure and Growth Kinetics of Nanoscale Crystallites Obtained from Hydrated Cerium Oxides

A detailed analysis of the microstructural properties of nanocrystalline cerium(IV) oxide powders produced by thermal dehydration of two hydrated ceria precipitated from the ceric ammonium and sulfate salts in ammonia solutions is described. In addition, it is shown that the diffraction pattern of hydrated ceria can be modeled with the structure data of CeO2 and that the average crystallite size for the precursor CeO2·1.6H2O is 19 Å. The analysis of nanocrystalline CeO2 is based on the modern developments of X-ray powder diffraction line broadening analysis, using 16 Bragg reflections. It is shown that, on average, the crystallites have a spherical shape with volume-weighted and area-weighted diameters in the ranges 264−1349 Å and 178−1079 Å for CeO2 (am-CeO2) obtained from the ammonium-based ceria and 263−1756 Å and 195−927 Å for CeO2 (sul-CeO2) obtained from the sulfate-based ceria. The early stages of the crystallite growth have been studied up to 775 °C and 900 °C, respectively. While sul-CeO2 is strain-free over the temperature range investigated, am-CeO2 contains a small amount of strains decreasing with temperature until 700 °C, where strains are negligible. The kinetics of the crystallite growth has been studied for the two samples, from which activation energies have been calculated. Crystallite sizes are also compared to the values from SEM and BET measurements. Strain-free sul-CeO2 is suggested as a potential candidate as reference material for crystallite size in powder diffraction.

The Study of Nanocrystalline Cerium Oxide by X-Ray Absorption Spectroscopy

X-ray absorption spectroscopy has been used to study the structural and electronic properties of cerium atoms in nano-crystalline cerium oxide. These nanocrystalline cerium oxides were prepared by precipitation followed by the aging process. To increase the particle size, the as-prepared cerium oxides were calcined at various temperatures. The nanocrystalline phase was retained, even when the samples were calcined at 600°C. The analyses of X-ray absorption near edge structures (XANES) for cerium oxides show that the increase in the relative intensity of the transition to 2p4f15d*L final state and its transition energy shifts toward higher energy are due to the increase in covalence between cerium and oxide ligands with increasing particle size. The extended X-ray absorption fine structure (EXAFS) results for cerium oxide show that the third Ce–O shell is degraded and the coordination number around cerium is decreased, resulting in a decrease in the bond distances of RCe–O and RCe–Ce for particle size <15 nm. However, when the particle size is increased, the third Ce–O shell started growing and the coordination number around cerium attained normal coordination. The present results provide evidence for the formation of nanocrystalline cerium oxide prepared by precipitation followed by the aging process.

Hydrothermal Synthesis of Nanocrystalline Cerium(IV) Oxide Powders

Nanocrystalline cerium(IV) oxide (CeO2) powders were prepared by heating solutions of cerium(IV) salts in the presence of urea under hydrothermal conditions at 120° to 180°C. The effects of the concentration of urea and hydrothermal treatment temperature on the morphology and crystallite size of the synthesized particles were investigated. The synthesized particles were angular, ultrafine CeO2, with a cubic fluorite structure. Their crystallite size decreased from 20 to 10 nm with increasing urea concentration from 2 times to 8 times that of the Ce4+ ion. The size only slightly changed by calcining at temperatures below 600°C.

Defect Chemistry and Transport Properties of Nanocrystalline Cerium Oxide

Defect Chemistry and Transport Properties of Nanocrystalline Cerium Oxide

Characterization and Sintering of Reactive Cerium(IV) Oxide Powders Prepared by the Hydrazine Method

Nanocrystalline cerium(IV) oxide (CeO2) powders have been prepared by adding hydrazine monohydrate to an aqueous solution of hydrous cerium nitrate (Ce(NO3)3·6H2O), followed by washing and drying. The lattice parameter of the as‐prepared powder is a= 0.5415 nm. The powder characteristics and sinterability of reactive CeO2 have been studied. The surface areas of powders that have been heated at low temperatures are high, and these surface areas do not decrease to 10 m2/g until the temperature is &gt;1200°C. Crystallite size and particle size are strongly dependent on the heating temperature. Optimum sintered densities are obtained by calcining in the temperature range of 700°–800°C. Ceramics with almost‐full density can be fabricated at a temperature as low as 1150°C.

Catalytic redox activity and electrical conductivity of nanocrystalline non-stoichiometric cerium oxide

Nanocrystalline cerium was synthesized by magnetron sputtering and inert gas condensation. Controlled post-oxidation of the metallic nanoclusters resulted in formation of oxygen-deficient cerium oxide. Catalytic redox reactions such as SO 2 reduction by CO and CO oxidation in air were investigated. A comparison of activity with conventional stoichiometric ceria catalysts was used as a probe to distinguish different surface reactions. Electrical conductivity of nanocrystalline CeO 2− x was investigated by impedance spectroscopy and compared with literature data for single-crystal CeO 2. The results are discussed in terms of the microstructure and defect chemistry of nanocrystalline cerium oxide.

Electrical Conductivity of Pure and Doped Nanocrystalline Cerium Oxide

ABSTRACTWe have previously shown that dense nanocrystalline CeO2−x of approximately 10 nm grain size exhibits enhanced electrical conductivity and an enthalpy of reduction that is more than 2.4 eV lower than that for conventional ceria [1, 2]. These effects were attributed to preferential interface reduction. In this work, we investigated the relationship between interfacial area, heat treatment conditions, and conductivity by varying the grain size of dense samples through annealing at various temperatures. It is shown that the conductivity does not scale in direct proportion to interfacial area. Moderate temperature (700 °C) anneals which change the grain size by only a few nanometers reduce the conductivity by three orders of magnitude. It is suggested that atomistic relaxation occurs at the interfaces, and eliminates many low energy defect sites.

Electrical/Dielectric Properties of Nanocrystalline Cerium Oxide

ABSTRACTElectrical/dielectric properties of nanocrystalline cerium oxide have been studied using impedance spectroscopy, thermopower, and DC 4-point conductivity. The combined techniques identified the effect of poor electroding on impedance spectra. Incomplete contact between the specimen and the electrode induces an additional arc in the impedance spectra. The additional high resistance feature results from the geometric constriction of current flow at the specimen/electrode interface and can be misinterpreted as a grain boundary response. The defect chemistry, nonstoichiometry, and transport properties were investigated in nanoscale ceria and compared with those of microcrystalline material.

The early stages of crystallite growth of CeO2 obtained from a cerium oxide nitrate

An analysis of the microstructure of nanocrystalline cerium oxide produced by thermal decomposition of cerium (IV) oxide nitrate at temperatures in the range 230° to 960 °C is described. Parameters describing the breadths and shape of X-ray diffraction line profiles were obtained by means of pattern decomposition and a method based on the Voigt function was used to obtain information on the temperature dependence of the microstructure. No marked anisotropy in size and strain broadening was observed. It was found that the crystallites are on average spherical, the dimensions increasing significantly with formation temperature, while microstrains decrease with annealing temperature. The results of size determinations were compared with SEM measurements. They suggest a competition between the crystallite growth and “particle” growth processes in the initial sintering of the loosely packed cerium oxide powder.

Electrochemical Synthesis and Sintering of Nanocrystalline Cerium(IV) Oxide Powders

Nanocrystalline CeO2 powders were prepared electrochemically by the cathodic electrogeneration of base, and their sintering behavior was investigated. X‐ray diffraction and transmission electron microscopy revealed that the as‐prepared powders were crystalline cerium(IV) oxide with the cubic fluorite structure. The lattice parameter of the electrogenerated material was 0.5419 nm. The powders consisted of nonaggregated, faceted particles. The average crystallite size was a function of the solution temperature. It increased from 10 nm at 29°C to 14 nm at 80°C. Consolidated powders were sintered in air at both a constant heating rate of 10°C/min and under isothermal conditions. The temperature at which sintering started (750°C) for nanocrystalline CeO2 powders was only about 100°C lower than that of coarser‐grained powders (850°C). However, the sintering rate was enhanced. The temperature at which shrinkage stopped was 200°‐300°C lower with the nanoscale powder than with micrometer‐sized powders. A sintered specimen with 99.8% of theoretical density and a grain size of about 350 nm was obtained by sintering at 1300°C for 2 h.