S Polarz

Publications of S Polarz

• Lithium related deep and shallow acceptors in Li-doped ZnO nanocrystals

  Authors: C. Rauch, W. Gehlhoff, M. R. Wagner, E. Malguth, G. Callsen, R. Kirste, B. Salameh, A. Hoffmann, S. Polarz, Y. Aksu, M. Driess

  Journal of Applied Physics. 02/2010;

  We study the existence of Li-related shallow and deep acceptor levels in Li-doped ZnO nanocrystals using electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopy. ZnOWe study the existence of Li-related shallow and deep acceptor levels in Li-doped ZnO nanocrystals using electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopy. ZnO nanocrystals with adjustable Li concentrations between 0% and 12% have been prepared using organometallic precursors and show a significant lowering of the Fermi energy upon doping. The deep Li acceptor with an acceptor energy of 800 meV could be identified in both EPR and PL measurements and is responsible for the yellow luminescence at 2.2 eV. Additionally, a shallow acceptor state at 150 meV above the valence band maximum is made responsible for the observed donor-acceptor pair and free electron-acceptor transitions at 3.235 and 3.301 eV, possibly stemming from the formation of Li-related defect complexes acting as acceptors.

• Organometallics meet colloid chemistry: a case study in three phases based on molecular carbonyl precursors containing zinc and manganese.

  Authors: A. Orlov, A Roy, M. Lehmann, M. Driess, S Polarz

  Journal of the American Chemical Society. 02/2007; 129(2):371-5.

  Two organometallic compounds containing zinc and manganese in different ratios are used as single-source precursors for the preparation of various new, bimetallic oxide materials with nanoscaledTwo organometallic compounds containing zinc and manganese in different ratios are used as single-source precursors for the preparation of various new, bimetallic oxide materials with nanoscaled dimensions. It is shown that the materials synthesis can be performed in the solid-state, the liquid-phase, and even in the gas-phase. The molecular composition of the precursors determines the composition of the resulting materials. In addition, two novel methods for the preparation of highly crystalline metal oxide colloids are presented: The coupling between a gas-phase process and a colloidal approach, and the application of ozone as an oxidant for the transformation of metal carbonyls into oxides in the liquid phase.

• Mesosynthesis of ZnO-silica composites for methanol nanocatalysis.

  Authors: S Polarz, F Neues, M.W.E. van den Berg, W. Grünert, L. Khodeir

  Journal of the American Chemical Society. 09/2005; 127(34):12028-34.

  Methanol catalysis meets chemistry under confined conditions. Methanol is regarded as one of the most important future energy sources. ZnO/Cu composite materials are very effective in heterogeneousMethanol catalysis meets chemistry under confined conditions. Methanol is regarded as one of the most important future energy sources. ZnO/Cu composite materials are very effective in heterogeneous catalysis for methanol production due to the so-called strong metal-support interaction effect (SMSI). Therefore, materials of superior structural design potentially representing model systems for heterogeneous catalysis are highly desired. Ultimately, such materials could help to understand the interaction between copper and zinc oxide in more detail than currently possible. We report the preparation of nanocrystalline, size-selected ZnO inside the pore system of ordered mesoporous silica materials. A new, liquid precursor for ZnO is introduced. It is seen that the spatial confinement significantly influences the chemical properties of the precursor as well as determines a hierarchical architecture of the final ZnO/SiO(2) nanocomposites. Finally, the ability of the materials to act as model systems in methanol preparation is investigated. The materials are characterized by a variety of techniques including electron microscopy, X-ray scattering, solid-state NMR, EPR, EXAFS, and Raman spectroscopy, and physisorption analysis.

• A Systematic Study on Zinc Oxide Materials Containing Group I Metals (Li, Na, K)-Synthesis from Organometallic Precursors, Characterization, and Properties

  Authors: S. Polarz, A. Orlov, A. Hoffmann, M. R. Wagner, C. Rauch, R. Kirste, W. Gehlhoff, Y. Aksu, M. Driess, M. W. E. van den Berg, M. Lehmann

  Chemistry of Materials. 21(16):3889-3897.

• Dynamical Changes in the Cu−ZnO_x interaction observed in a model methanol synthesis catalysts

  Authors: M.W.E. van den Berg, S Polarz, O.P. Tkachenko, K. Kaehler, M. Muhler, W. Gruenert

  Catal. Lett. 128(2009),49-56.

  Journal article

• Cu/ZnO aggregates in siliceous mesoporous matrices: Development of a new model methanol synthesis catalyst

  Authors: M.W.E. van den Berg, S. Polarz, O.P. Tkachenko, K.V. Klementiev, M. Bandyopadhyay, L. Khodeir, H. Gies, M. Muhler, W. Grünert

  Journal of Catalysis.

  Copper and zinc were introduced into mesoporous siliceous matrices with the goal of obtaining model methanol synthesis catalysts with intense interaction between copper and the ZnO promoter. TheCopper and zinc were introduced into mesoporous siliceous matrices with the goal of obtaining model methanol synthesis catalysts with intense interaction between copper and the ZnO promoter. The preparation methods included various aqueous routes starting from acetate solutions (into MCM-48) and a route involving an organometallic step—thermolysis of a liquid heterocubane of Zn4O4 type ([CH3ZnOCH2CH2OCH3]4) in a wormhole-type silica of 5 nm average pore size—followed by aqueous Cu (nitrate) impregnation. The materials were characterized by XRD, nitrogen physisorption, N2O frontal chromatography, TPR, and EXAFS, and their methanol synthesis activity was measured at 493 K and normal pressure. In the aqueous preparations with acetate solutions, excessive formation of silicates (particularly zinc silicate) led to damage of the pore system. A significant delay in Cu reduction was assigned to the influence of micropores formed, together with some copper silicate formation. These samples exhibited poorly accessible Cu surface areas despite small Cu particle sizes indicated by EXAFS and disappointing methanol synthesis activity. In contrast to this, a highly active catalyst was obtained via the heterocubane route that meets industrial standards in terms of reaction rate per Cu surface area. Orientation studies (EXAFS at the CuK and ZnK edges) reflecting a redox behavior of the ZnOx component illustrate the potential of this catalyst type for use in basic studies of the Cu–ZnOx interaction in methanol synthesis catalysts.

• Dynamical Changes in the Cu-ZnOx interaction observed in a model methanol synthesis catalysts

  Authors: M.W.E. van den Berg, S Polarz, O.P. Tkachenko, K. Kähler, M. Muhler, W. Grünert

  Catal. Lett. 128(2009),49-56.

  Journal article

• A Systematic Study on Zinc Oxide Materials Containing Group I Metals (Li, Na, K)-Synthesis from Organometallic Precursors, Characterization, and Properties

  Authors: S Polarz, A. Orlov, A. Hoffmann, M.R. Wagner, C. Rauch, R. Kirste, W. Gehlhoff, Y. Aksu, M. Driess, M.W.E. van den Berg, M. Lehmann

  Chem. Mater. 21(2009)16,3889.

  Journal article

• Cu/ZnO aggregates in siliceous mesoporous matrices - Development of a new model methanol synthesis catalyst

  Authors: M.W.E. van den Berg, S Polarz, O.P. Tkachenko, K.V. Klementiev, M. Bandyopadhyay, L. Khodeir, H. Gies, M. Muhler, W. Gruenert

  Journal of Catalysis 241(2006),446.

  Journal article