A. Tschöpe

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-dependent electrical conductivity of polycrystalline cerium oxide II: Space charge model

Abstract The effect of space charges along grain boundaries on the electrical conductivity of polycrystalline mixed ionic/electronic conductors (MIECs) was investigated. The bulk concentrations of point defects were calculated using the defect chemistry of corresponding single crystalline materials, taking into account the charge neutrality condition. The accumulation or depletion of charged point defects in space charge layers along grain boundaries was described in terms of the Gouy–Chapman theory of liquid electrolytes. Analytical expressions were obtained for the contribution of space charge layers to the electrical conductivity. These expressions were used to calculate the partial conductivities of a polycrystalline material with a simple brick-layer topology. The space charge model was applied to cerium oxide including acceptor ions at typical impurity concentrations. The major objective of the study was the calculation of the grain size dependence of electrical conductivity. In particular, the effect of a positive space charge potential on acceptor segregation, the ionic and electronic partial conductivities and also on the temperature and oxygen partial pressure dependence of electrical conductivity was studied. It was shown that the space charge model yielded a consistent explanation for experimental results on the grain size dependent electrical conductivity of polycrystalline cerium oxide.

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.

Grain size-dependent electrical properties of nanocrystalline ZnO

Undoped and Bi-doped nanocrystalline ZnO was prepared by the inert gas condensation method. Samples of different grain sizes were obtained by annealing treatments at various temperatures. The dc and ac electrical conductivity of undoped and Bi-doped nanocrystalline ZnO was investigated as a function of grain size. In dc measurements, nanocrystalline ZnO of small grain size ( 40 nm) did not exhibit time dependent conduction but exhibited ohmic behavior. This difference in dc conductivity is discussed in terms of the grain size dependence of the average trap density. A small amount of Bi dopant had no measurable effect on the electrical behavior in the small grain size range but resulted in higher specimen resistance in the large grain size rang...

Catalytic properties of nanostructured metal oxides synthesized by inert gas condensation

Abstract The correlation between defects in metal oxides and activity in heterogeneous catalytic oxidation was investigated. Processing of nanocrystalline metal oxides by inert gas condensation was employed to synthesize oxygen-deficient nonstoichiometric cerium oxide. Comparison with stoichiometric nanocrystalline ceria revealed the effect of nonstoichiometry on surface chemical reactivity. Stoichiometric nanocrystalline cerium dioxide was investigated for catalytic activity in two reactions; oxidation of CO by (i) SO2 and (ii) O2. Catalytic activity was found at significantly different temperatures of 560°C in the former and 360°C in the latter reaction. Complementary temperature programmed reduction (TPR) studies were performed and two distinct signals were found, which correlated with the light-off temperatures for each of the catalytic reactions. This demonstrated that the oxygen intermediates, participating in the redox mechanisms were different for both reactions. Nonstoichiometric nanocrystalline CeO2−x exhibited catalytic activity in CO oxidation by O2 and a corresponding TPR-signal at 200°C, which suggested the presence of an even more reactive oxygen species. Fourier-transform infrared spectroscopy (FT-IR) and electrical conductivity were used to study the interaction of chemisorbed CO and O2 with the metal oxides. Surface defects and chemisorbed oxygen were discussed as possible origins of enhanced catalytic activity due to their effects on the nature of surface oxygen species. The combination of these studies yielded detailed insight into the different reaction mechanisms and the chemical interaction between solid surface and gaseous species that are critical towards the engineering of catalytic and gas sensor materials.

Grain-size-dependent thermopower of polycrystalline cerium oxide

The effect of grain boundaries on the defect chemistry of polycrystalline cerium oxide was investigated by measuring the thermopower of cerium oxide samples with grain sizes in the range of 0.13–11.5 Am. The samples were prepared by sintering pellets from a single batch of nanocrystalline 500 ppm Gd-doped cerium oxide powder at different temperatures. A change in the sign of the Seebeck coefficient as function of the grain size was observed, indicating a transition from predominantly ionic conductivity at large grain size to electronic conductivity at small grain size. The experimental results were analyzed using the space charge model for ionic solids, which has already been successfully employed in the analysis of the grain-size-dependent electrical conductivity of cerium oxide. The agreement between the experimental data and the space charge model, regarding both electrical conductivity and thermopower, suggested that the essential effect of the grain boundaries in cerium oxide is the accumulation/depletion of charge carriers in space charge layers, whereas the effect of microstructure on charge carrier mobilities is negligible. From the analysis of experimental results, a value of DU=0.7 V was obtained for the space charge potential at the grain boundaries in cerium oxide. D 2002 Elsevier Science B.V. All rights reserved.

Numerical analysis of space charge layers and electrical conductivity in mesoscopic cerium oxide crystals

The electrical conductivity of mesoscopic cubic cerium oxide crystals with space charge layers along their external boundaries has been calculated as a function of grain size. The equilibrium distribution of electronic and ionic charge carriers in this mixed conductor were obtained by numerical solution of the three-dimensional Poisson–Boltzmann equation in finite-difference approximation. At grain sizes comparable to the screening length λ, space charge layers overlap and penetrate the entire crystallite. Taking into account the inhomogeneous distribution of charge carriers, the partial electronic and ionic conductivities were calculated by numerical solution of the drift-diffusion equations in first-order expansion of the electrical potential under the non-equilibrium conditions imposed by an applied external voltage. Assuming a positive space charge potential, a characteristic grain-size-dependent increase of electronic and decrease of ionic conductivity was found. The obtained numerical results were c...

Nanocrystalline Doped Cerium Oxide as a Catalyst for SO2 Reduction by Co

Nanocrystalline processing by inert gas condensation has the inherent advantages of generating: (1) high surface area nanoclusters, (2) non-stoichiometric oxides, and (3) high dispersions of dopants. This approach is exploited in the synthesis of fluorite-structured catalysts for SO 2 reduction by CO. Nanocrystalline CeO 2-x , La-doped CeO 2-x , and Cu-doped CeO 2-x were produced by magnetron sputtering from a pure or mixed metal target, followed by controlled oxidation of the metallic clusters. The as-prepared doped and undoped nanocrystalline CeO 2-x materials were found to be excellent catalysts for complete SO 2 conversion to elemental sulfur. Undoped nanocrystalline CeO 2-x enabled light-off at 460 °C, a temperature ∼120 °C lower than that over polycrystalline CeO 2 , which is a novel effective catalyst itself. The high catalytic activity of the nanocrystals was associated with their high concentration of oxygen vacancies. Excellent poisoning resistance was also exhibited by the nanocrystalline CeO 2-x samples. These materials have stable activity in the presence of excess CO 2 .

Surface Chemistry of Nanocrystalline Cerium Oxide

Nanocrystalline cerium oxide was synthesized by magnetron sputtering of a metallic target, followed by controlled post-oxidation. The resulting cerium oxide clusters were 2−x with stoichiometric CeO 2 . It further explores the promoting effect of Cu-doping on surface reduction and oxidation. The oxidation states of metal cations were examined with X-ray photoelectron spectroscopy after various oxidizing and reducing heat treatments in a connected reaction chamber. Isothermal pulsed reduction and oxidation of the samples were investigated by thermogravimetric analysis. Reduction properties of the different materials are discussed in terms of their microstructure, oxygen deficiency and chemical composition. These studies will help to understand the importance of bulk defects and synergistic effects in multicomponent and multiphase materials for high surface reactivity.