Numerical calculations of space charge layer effects in nanocrystalline ceria. Part I: comparison with the analytical models and derivation of improved analytical solutions.

Using numerical solutions of the Poisson-equation, one dimensional space charge layer (SCL) concentration profiles in CeO2 are calculated. The SCL conductivity effects of nanocrystalline CeO2 are analyzed as a function of doping content (donor doped, pure and acceptor doped ceria) and SCL potential including not only the standard Gouy-Chapman and Mott-Schottky cases, but also the more complex mixed situations. The results of the numerical approach are compared with the usual analytical approximations. While for the ideal Gouy-Chapman and Mott-Schottky cases for moderate and high potentials the agreement between analytical and numerical solutions is found to be satisfactory, mixed cases and low potential situations cannot be reliably treated by using the standard analytical approaches. Finally, inspired from the numerical solutions, improved analytical equations are proposed which are found to generally yield much more precise results and are accurate even for the mixed situations and low potentials.

Numerical calculations of space charge layer effects in nanocrystalline ceria. Part II: detailed analysis of the space charge layer properties.

The numerical approach presented in Part I is used to investigate in detail some important characteristics of space charge layer (SCL) concentration profiles (steepness, extent, charge contributions and total charge), which determine the resulting SCL effects on the ionic and electronic transport. Here, as a case study the conductivity changes in nanocrystalline ceria are discussed over a broad range of dopant concentration (acceptor and donor-doped) as well as space charge potential values. In addition, the effects of a mobile dopant on the SCL charge carrier profiles are addressed. Finally, using the numerical approach the possibilities of adjusting (under realistic conditions) the SCL effects to improve the conduction properties of nanocrystalline CeO2 are discussed.

Visualizing diastereomeric interactions of chiral amine-chiral copper salen adducts by EPR spectroscopy and DFT.

Single enantiomers of R/S-methylbenzylamine (MBA) were found to selectively form adducts with two chiral Cu-salen complexes, [Cu(II)(1)] (H(2)1 = N,N'-bis(3,5-ditert-butylsalicylidene)-1,2-diaminocyclohexane) and [Cu(II)(2)] (H(2)2 = N,N'-bis-salicylidene-1,2-cyclohexanediamino). The axial g/A spin Hamiltonian parameters of the Cu-MBA adducts were typical of 5-coordinate species. Enantiomer discrimination in the MBA binding was directly evidenced by W-band CW EPR, revealing an 86 ± 5% preference for formation of the R,R-[Cu(1)] + S-MBA adducts compared to R,R-[Cu(1)] + R-MBA; this was reduced to a 57 ± 5% preference for R,R-[Cu(2)] + S-MBA following removal of the tert-butyl groups. The structure of these diastereomeric adducts was further probed by different hyperfine techniques (ENDOR and HYSCORE), although no structural differences were detected between these adducts using these techniques. The diastereomeric adducts were found to possess lower symmetry, as evidenced by rhombic g tensors and inequivalent H(imine) couplings. This was caused by the selective binding mode of MBA onto one side of the chiral Cu(II) complex. DFT calculations were performed on the R,R-[Cu(1)] + S-MBA and R,R-[Cu(1)] + R-MBA adducts. A distinct difference in orientation and binding mode of the MBA was identified in both adducts, confirming the experimental results. The preferred heterochiral R,R-[Cu(1)] + S-MBA adduct was found to be 5 kJ mol(-1) lower in energy compared to the homochiral adduct. A delicate balance of steric repulsion between the α-proton (attached to the asymmetric carbon atom) of MBA and the methine proton (attached to the asymmetric carbon atom) of [Cu(1)] was crucial in the stereoselective binding.

Mixed conductivity in nanocrystalline highly acceptor doped cerium oxide thin films under oxidizing conditions.

We report on the investigation of 10 mol% gadolinium-doped cerium oxide thin films of various microstructures prepared by pulsed laser deposition. Depending on substrate, growth conditions and hence microstructure, the electric conductivity values vary considerably by several orders of magnitude. Remarkably, in the sample with the highest grain boundary density, we even have evidence of substantial electronic conductance under oxidizing conditions despite the large acceptor level. This possibly surprising result can be explained by an increased space charge potential at the grain boundaries in combination with the small grain size of 10 nm that leads to an enrichment of excess electrons while the ion conduction is simultaneously blocked by vacancy-depleted regions.

Boundary effects on the electrical conductivity of pure and doped cerium oxide thin films.

Thin films of CeO(2) (both nominally pure and 10 mol% gadolinium-doped) grown via pulsed-laser deposition were studied. The electrical conductivity of the samples was measured as a function of thickness, temperature and oxygen partial pressure (pO(2)) using impedance spectroscopy. As expected, undoped CeO(2) exhibits electronic conductivity (with activation energy between 1.4 and 1.6 eV) whereas the highly doped samples are oxygen vacancy conductors (activation energy around 0.7 eV for epitaxial films). In order to investigate the influence of the nature of the substrate the thin films were grown on two different substrates, Al(2)O(3) (0001) and SiO(2) (0001), and compared. While the films grown on SiO(2) exhibit a microstructure characterized by columnar grains, the films grown on Al(2)O(3) are epitaxial. Notably, for films on both substrates the conductivity and activation energy vary with film thickness and exhibit remarkable differences when the films on different substrates are compared. In the case of the polycrystalline films (SiO(2) substrate), the space charge layer effects of the grain boundaries dominate over the substrate-film interface effect. In the case of the epitaxial films (Al(2)O(3) substrate), a small interface effect, probably due to a space charge layer or structural strain, is observed.