The main focus of this thesis is to study solid-state reaction pathways by controlling structures during the synthesis of scandium vanadium oxides. Besides the synthesis of structures involving different oxidation states, the stable phases were identified by means of ex-situ powder X-ray diffraction as well as in-situ high temperature oxidation. For the synthesis of Sc4/3V2/3O3 (Sc2VO4.5) and Sc2VO5 optimized methods are presented for the formation of a cubic bixbyite-type and tetragonal phase, respectively. In an ex-situ study Sc2VO5 was obtained by oxidation of Sc4/3V2/3O3 (Sc2VO4.5). A deeper understanding of the structural changes can be obtained by focussing on the sub-lattices throughout the reaction pathway. The phase change from cation-disordered bixbyite Sc2VO4.5 to cation-ordered tetragonal Sc2VO5 was studied through oxidation in trace amounts of oxygen. Moreover, the oxidation of bi-phasic samples of Sc4/3V2/3O3 (Sc2VO4.5) and Sc2VO5 clearly indicates the competition of thermodynamic and kinetic routes during heating. Thermodynamic and kinetic oxidation products were compared based on the high temperature in-situ oxidation as well as ex-situ experiments. The product of topotactic oxidation of bixbyite Sc4/3V2/3O3 (Sc2VO4.5) crystallizes in a defect fluorite structure Sc4/3V2/3O3+ε, (= Sc2VO5+δ). The oxidation of the Sc4/3V2/3O3 (Sc2VO4.5) bixbyite phase results in the formation of the reconstructed tetragonal Sc2VO5 structure at high. This finding is contrasted with the low temperature topotactic oxidation resulting in a disordered defect fluorite phase. Moreover, the conventional solid state method was used to explore the possibility of obtaining tetragonal Sc2VO5. The results showed that solid state methods might not be suitable for the preparation of Sc2VO5. / February 2017

Studies of semiconducting glasses have become of great interest in recent years because of their importance in the theory of solid state physics and in their applicability to electronic devices. Of these classes of materials the transition metal oxide glasses are much the most thoroughly studied. In these glasses the transition metal oxide e. g. V205-TiO2-MoO39 WO3, Cu0 is a major constituent i.e > 50 mol%. We start this work with a discussion about a critical review of the history, formation and modern theory of non-crystalline semiconductors in general and glassy state in particular. The aim of the experimental work in present study is to check the validity of the theories and models proposed so far to explain the origin and the nature of the charge carriers, structure, electrical and optical properties of some transition metal oxide glasses based on vanadium pentoxide. For this purpose series of binary V205-P205 glass samples containing 50 to 90 mol% V205 as well as ternary V2015-P205-Te02 glasses containing 60 mol% V2051 (40-x) mol% P205 and x mol% Te02 in which x varies from 5 to 35 were prepared by normal cooling from the melt. It was found that the glass forming region of the system under consideration is fairly large and in binary V205-P205 systems, glass with up to 95 moll V205 could be prepared. Density measurements indicate that in binary V205 - P205 glasses the density increases linearly with increasing V205 content and in ternary V205 - P205 - TeO2 systems density increases with increasing Te02 content. It was also found that the density of both systems are affected by annealing temperature. Electron spin resonance (E.S.R.) studies show that in vanadate glasses, the vanadium exists in more than one valency state mainly V5+ and V4+ of which V4+ is paramagnetic and detectable by E. S. R. This could be taken as some evidence of a hopping conduction mechanism in vanadate glasses and conduction is due to transfer of charge form V4+ to V5+ ions and this is discussed in the terms of small polaron. It is found that the conduction in these glasses is Ohmic up to a field of the order of 4x 105 Vcm-1 with an activation energy range from 0.31 to 0.48 e.V depending on composition and independent of temperatures in our range of temperature (above room temperature). Above this field conduction becomes non-Ohmic which is found to be due to lowering the potential barrier of the carriers at high electric field as was predicted by Poole and Frenkel. Memory switching is observed in thin blown film samples of both binary and ternary glass systems, which is associated with field-induced crystallization of a localized region and formation of conductive channel in the switched area due to self heating effect. In other words the conducting zone consists of VO2 crystals which possess more metal-like conductivity. Infra-red absorption spectra of these glasses revealed that some of the absorption bands of glasses and crystalline V205 are similar which is some evidence that the vanadium ions exists in six-fold co-ordination in disordered glassy systems as well as ordered crystalline V205. The fundamental absorption edge of these glasses occurs in the short wave length region of the visible and is dependent on composition, and the fundamental absorption arises form direct forbidden transitions and occurs at a photon energy of about 1.9-2.6 e.V depending on composition. The absorption edge in these glasses is found to be of the same order of magnitude as that in crystalline V205.

No description available.

Platelets permeabilized by high voltage electric discharges have provided a valuable model system in which to analyse the roles of Ca²⁺ ions and guanine nucleotides in the regulation of secretion by exocytosis. In the present study, the effects of fluoride or fluoroaluminate and of vanadate or pervanadate on secretion of platelet dense granule constituents, and the roles of activation of phospholipase D (PLD), phospholipase C (PLC) and protein kinase C (PKC) in secretion, have been investigated. Electropermeabilized human platelets containing [¹⁴C]5-HT in their dense granules were suspended in a glutamate medium containing ATP and incubated for 10 min at 25°C with, variously, Ca²⁺ buffers, KF/AlCl₃, vanadate/H₂O₂, guanine nucleotides and phorbol 12-myristate 13-acetate (PMA). KF/AlCl₃, which activates heterotrimeric G proteins but not low-M, GTP-binding proteins, caused a Ca²⁺ -dependent [¹⁴C]5-HT secretion; maximal effects were obtained with 10 mM KF plus 10 μM AlCl₃ at a pCa of 6, when 53% of [¹⁴C]5-HT was released. Secretion induced by KF/AlCl₃ in the presence of Ca²⁺ correlated with the stimulation of [3H]diacylglycerol accumulation in permeabilized platelets containing [³H]arachidonate-labelled phospholipids. KF/AlCl₃ also stimulated the phosphorylation of pleckstrin (P47) in permeabilized platelets incubated with [γ-³²P]ATP, indicating the activation of PKC. In the absence of Ca²⁺ (pCa > 9), KF/AlCl₃ caused none of the above effects. These actions of KF/AlCl₃ were attnbutable to the activation of PLC, since KF/AlCl₃ also stimulated the formation of [³H]inositol phosphates in [³H]inositol-labelled permeabilized platelets in the presence of Ca²⁺. PLD activity, measured as the formation of[³H]phosphatidylethanol (PEt) from [³H]arachidonate-labelled phospholipids in the presence of ethanol, could not be detected after stimulation of platelets by KF/AlCl₃ in the absence or presence of Ca²⁺. However, KF/AlCl₃ inhibited the [³H]PEt formation (PLD activity) induced by GTPγS. In the absence of Ca²⁺ (pCa >9), the inhibitory effects of KF/AlCl₃ on [¹⁴C]5-HT secretion induced by GTPγS alone or GTPγS plus PMA correlated well with their inhibitory effects on [³H]PEt formation. At pCa 6, KF/AlCl₃ had only a small inhibitory effect on GTPγS-induced secretion and inhibited GTPγS-induced PLD activity more strongly than GTPγS-induced PLC activity. These results suggest that PLD is important for Ca²⁺ -independent secretion, and that, although both PLD and PLC may play roles in Ca²⁺ -dependent secretion, PLC is likely to be the more important. In the presence of Ca²⁺, either vanadate or H₂O₂ caused concentration-dependent stimulations of [¹⁴C]5-HT secretion, [³H]DAG formation and [³H]PEt formation. At pCa 6, low concentrations of vanadate and H₂O₂, which would be expected to form pervanadate, acted synergistically to stimulate [¹⁴C]5-HT secretion, which correlated with [³H]DAG formation. However, vanadate with H₂O₂ had a biphasic effect on PLD activity that did not correlate with secretion. In addition, at pCa 6, GTPγS-induced PLD activity was abolished by vanadate with H₂O₂, whereas GTPγS-induced secretion and PLC activity were only partially inhibited. These results support the idea that both PLC and PLD are involved in the regulation of secretion but have different contributions to Ca²⁺ dependent and Ca²⁺ -independent secretion. The results are consistent with activation of platelet PLC by a heterotrimeric G protein, but suggest that different mechanisms, possibly involving a low-M, GTP-binding protein, may be involved in the regulation of PLD activity. / Thesis / Master of Science (MSc)

Zugl.: Hamburg, Univ., Diss., 2008

Universiẗat, Diss., 2003--Münster (Westfalen).

Thesis (Ph. D.)--Ohio State University, 2008. / Title from first page of PDF file. Includes bibliographical references (p. 228-234).

Photoelectrochemical studies were performed on bismuth vanadate (BiVO₄) to understand chemical and physical properties of the photocatalysts, and to improve the photoactivity for water oxidation. Scanning electrochemical microscopy (SECM) was used to screen various dopants for BiVO₄, to calculate the photoconversion efficiencies to chemical energy at BiVO₄ electrodes, and to study the water oxidation intermediate radicals at the surface of BiVO₄. Tungsten and molybdenum doped BiVO₄ (W/Mo-BiVO₄) shows a photocurrent for water oxidation that is more than 10 times higher than undoped BiVO₄. Photoelectrochemical measurements and material analysis were done to discuss the factors that affect performance of BiVO₄. Finite elements analysis was also performed to explain the electron-hole transport and electrochemical reactions at W/Mo-BiVO₄ electrodes in solutions. Addition of conductive or electron accepting materials, e.g. reduced graphene oxide, into BiVO₄ was tried to study the electron-hole transport phenomena in the metal oxide electrodes. Surface adsorbed radicals produced during the water oxidation at W/Mo-BiVO₄ were interrogated by using SECM that the surface coverage and decay kinetics of adsorbed hydroxyl radicals at W/Mo-BiVO₄ were measured. The quantum efficiencies of the injected photon conversion to chemical energy were obtained from the photoelectrochemical measurements by using SECM. SECM techniques and finite elements analysis were also used to measure the faradaic efficiency of water oxidation at W/Mo-BiVO₄ under irradiation. Finally, unbiased water splitting to generate hydrogen and oxygen from water splitting only using photon energy at W/Mo-BiVO₄ electrodes was demonstrated in a dual n-type semiconductor or Z-scheme device. / text

Techn. Universiẗat, Diss., 2001--Dresden.

Techn. Hochsch., Diss., 2002--Aachen.