Techn. Universiẗat, Diss., 2002--Chemnitz.

Low dimensional nanostructures have defined the frontier of the research in material science for the last two decades. Presented here are the results of experimental research on growth, device fabrication and application of quasi-one dimensional phthalocyanines and metal oxides to gas-sensing. The possibility of rational tuning of the growth conditions, in order to control composition, morphology, size, orientation and alignment of the grown low-dimensional nanostructures was investigated. Employing custom designed heating stages coupled with optical microscope the in situ approach of monitoring the growth of nanostructures has been realized. Using this method, the growth of VO2 nanowires and nanoplatelets have been investigated and two novel growth mechanisms were discovered and explained. A variety of phthalocyanine and metal-oxide nanowire-based chemical sensors have been proposed, fabricated and tested. The focus of our research was on the development of new sensing principles and the improvement of existing ones. In particular, nanowires of tin and titanium dioxide were proposed to be used as self-heated chemiresistors capable of operating in the absence of an external heater, thus paving the way for ultra-low power consumption sensors. For the first time VO2 nanowires were used to create a nano-Pirani gauge and a gas sensor employing a sharp temperature-driven metal-insulator transition in this material. The sensor is sensitive to both chemically active and inert gases. Its performance is modeled and optimization parameters are presented.

Magister Scientiae - MSc (Biotechnology) / Endophytic bacteria are known to have an endosymbiotic relationship with plants and provide them with many beneficial properties. These bacteria stimulate plant hormones, provide protection from pathogens and increase nutrient availability in the environment. In this study some of these potential growth factors were tested. Endophytic bacteria have the potential to be of great value for the increase of crop production. They offer a variety of processes that aid in plant growth promotion in an ecofriendly manner. The use of endophytic bacteria provides a cheaper and cleaner approach compared to industrial made fertilizers. They also have potential uses in bioremediation to clean the environment polluted by industrial processes. Endophytes were isolated and showed significant growth improvement. Each isolate displayed different morphologies. Isolates were tested for classical growth promotion mechanisms such as the ability to solubilize phosphate, Indole-3-acetic acid and siderophore production. Inductively Coupled Plasma Optical Emission Spectrometry was performed to measure the effect of the isolates on the plants nutrient profile. The isolates were then tested again while the plants were under heavy metal stress to determine if they were still capable of growth promotion. The plants were then assayed for cell death using Evans blue and biomass was measured to determine the effect of vanadium stress. Inductively Coupled Plasma Optical Emission Spectrometry was performed again to assess the change in nutrient profile while under vanadium stress. / 2021-08-31

Exploring the properties of layered materials as a function of thickness has largely been limited to semiconducting materials as thin layers of metallic materials tend to oxidize readily in atmosphere. This makes it challenging to further understand properties such as superconductivity and charge density waves as a function of layer thickness that are unique to metallic compounds. This dissertation discusses a set of materials that use the modulated elemental reactants technique to isolate 1 to 3 layers of VSe2 in a superlattice in order to understand the role of adjacent layers and VSe2 thickness on the charge density wave in VSe2. The modulated elemental reactants technique was performed on a custom built physical vapor deposition to prepare designed precursors that upon annealing will self assemble into the desired heterostructure. First, a series of (PbSe)1+δ(VSe2)n for n = 1 – 3 were synthesized to explore if the charge density wave enhancement in the isovalent (SnSe)1.15VSe2 was unique to this particular heterostructure. Electrical resistivity measurements show a large change in resistivity compared to room temperature resistivity for the n = 1 heterostructure. The overall change in resistivity was larger than what was observed in the analogous SnSe heterostructure. v A second study was conducted on (BiSe)1+δVSe2 to further understand the effect of charge transfer on the charge density wave of VSe2. It was reported that BiSe forms a distorted rocksalt layer with antiphase boundaries. The resulting electrical resistivity showed a severely dampened charge density wave when compared to both analogous SnSe and PbSe containing heterostructures but was similar to bulk. Finally, (SnSe2)1+δVSe2 was prepared to further isolate the VSe2 layers and explore interfacial effects on the charge density wave by switching from a distorted rocksalt structure to 1T-SnSe2. SnSe2 is semiconductor that is used to prevent adjacent VSe2 layers from coupling and thereby enhancing the quasi two-dimensionality of the VSe2 layer. Electrical characterization shows behavior similar to that of SnSe and PbSe containing heterostructures. However, structural characterization shows the presence of a SnSe impurity that is likely influencing the overall temperature dependent resistivity. This dissertation includes previously published and unpublished co-authored materials.

Symmetric nonaqueous redox flow batteries (RFBs) use negative and positive battery solutions of the same solution composition to operate at high cell voltages. This research effort targets these systems since they offer performance improvements derived from using nonaqueous systems and symmetric active species. Nonaqueous solutions permit significantly higher cell voltages than state-of-the-art aqueous RFBs and symmetric active species chemistries reduce the required complexity of cell reactors. Both performance advantages correspond to significant cost improvements beyond already commercially competitive aqueous RFB chemistries. This document focuses on two classes of symmetric nonaqueous RFB chemistries: coordination complexes such as vanadium acetylacetonate [V(acac)₃] or chromium acetylacetonate [Cr(acac)₃], and organic active species such as 9,10-diphenylanthracene (DPA). V(acac)₃ delivers reversible electrochemistry that supports a 2.2 V equilibrium cell potential, but there are some gaps in the understanding of its degradation mechanisms. Cr(acac)₃ supports redox reactions that suggest cell potentials above 4 V, but shows signs of irreversibility in voltammetry experiments and is not yet well understood. Finally, the DPA system could be interesting because it does not use metal active species, and its voltammetry promises cell potentials above 3 V. Yet DPA suffers from low solubility in nonaqueous solvents that limit its practicality. These three systems show promise for symmetric nonaqueous RFBs and offer avenues for further improvement. Voltammetry and spectroelectrochemical electrolysis experiments on the metal coordination complexes clarify the mechanisms behind the voltammetry on these symmetric chemistries. Ligand dissociation causes the irreversible behavior observed in voltammetry on Cr(acac)₃. The same experiments reaffirm the expected cyclability of V(acac)₃. Chemical functionalization of the DPA center is performed to investigate the solubility and reactivity of various derivatives. Functionalizing DPA with ethylene glycol chains to form 'DdPA' significantly increases solubility limits from 0.6 mM and 44 mM for DPA in acetonitrile and 1,2-dimethoxyethane, respectively, to 12 mM and 0.21 M for DdPA in the same solvents. At the same time, DdPA retains redox activity that promises 3 V cell potentials. Ultimately, a custom, nonaqueous-compatible redox flow reactor was designed and used to test the performance of V(acac)₃, DPA, and DdPA under various operating conditions. Contradicting previous reports, V(acac)₃ delivers stable cycling over the 21- cycle experimental protocol. Exploration over a range of flow rates and current densities give energy and power densities up to 1.09 WhL^-1 and 0.16 Wcm^-2, respectively, for the battery solution compositions examined. These experiments further predict values up to 28 WhL^-1 and at least 0.22 Wcm^-2 for optimized V(acac)₃ battery solutions. DPA and DdPA deliver the highest operating potential observed from organic nonaqueous RFBs, discharging at 3 V and 2.9 V, but require further work to understand degradation in the systems.

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Muitos biossorventes naturais têm sido pesquisados por possuírem baixo custo e apresentarem propriedades ligantes, como é o caso da macroalga marinha Sargassum filipendula (S. filipendula) que vem sendo utilizada como material biossorvente no processo de biossorção de metais pesados. No presente trabalho a alga marrom foi utilizada para estudos realizados em batelada, onde se determinou o pH ideal de biossorção de vanádio, a relação sólido/líquido ideal e a importância da velocidade de agitação. O estudo cinético e de equilíbrio dos íons metálicos também foram realizados em bateladas nas seguintes condições de ensaio: (1) 0,10 g de biomassa, 25,0 mL de solução de vanádio igual a 18,0 mg L-1, temperatura 25,0 C e 150 rpm de agitação; (2) 0,10 g de biomassa, 25,0 mL de solução de vanádio igual a 36,0 mg L-1, temperatura 25,0 C e 150 rpm de agitação. A melhor condição para biossorção de vanádio foi encontrada para 36,0 mg L-1 e pH= 2,0. O estudo cinético de biossorção de vanádio mostrou que o modelo de segunda ordem descreve melhor os dados experimentais em 36,0 mg L-1 (R2= 0,9825). O estudo de equilíbrio mostrou um perfil crescente de remoção de vanádio. A melhor eficiência de captação dos íons de vanádio foi de 61,0 % para Co= 40,0 mg L-1 em pH= 2,0. Os dados experimentais da isoterma de vanádio mostraram-se mais adequados ao modelo de Langmuir para pH= 2,0, Os parâmetros de equilíbrio calculados a partir do modelo de Langmuir (b, qmax ) 0,009 e 43,3 mg/g, respectivamente, corroboram melhor para a interpretação dos resultados quando comparados com o modelo de Freundlich (kF, n) 1,56 e 2,41, visto que o coeficiente de correlação é maior para Langmuir / Many natural biosorbents have been investigated for having low cost and submit binding properties, as is the case of marine marcoalgae Sargassum filipendula (S. filipendula) that has been used biosorbent material in the processes of biosorption of heavy metals. In the present study the brown seaweed has been used to study conducted in bach mode, where it was determined the optimum pH for biosorption of vanadium, the solid/liquid ratio ideal and the importance of speed of agitation. The kinetic and equilibrium study of metal ions were also performed in batches in the following test conditions: (1) 0.10 g of biomass, 25.0 mL of vanadium solution equal to 18.0 mg L-1, temperature 303 K and 150 rpm agitation. (2) 0.10 g of biomass, 25.0 mL of vanadium solution equal to 36.0 mg L-1, temperature 303 K and 150 rpm agitation. The optimum conditions for biosorption of vanadium was found to 36.0 mg L-1, pH 2,0. The kinetic study of vanadium biosorption showed that the second order model best describes the experimental data 36.0 mg L-1, (R2 = 0.9825). The balance study showed an increased uptake profile of vanadium. The best collection efficiency of vanadium ion was 46.0 % for Co = 36.0 mg L-1 at pH 2.0. The experimental isotherm data of vanadium were more suitable for the Langmuir model at pH 2.0. The equilibrium parameters calculated from the Langmuir model (b, qmax ) were 0.009 e 43.3 mg/g respectively, to better corroboration to interpretation of results when compared with the Freundlich model (kF, n) 1,56 e 2,41, whereas the correlation coefficient is higher for Langmuir

La diabetes mellitus es un serio desorden metabólicocrónico que se caracteriza por el incremento anormal de laglucosa en la sangre y por complicaciones vasculares yneurológicas. La diabetes mellitus es causada por una falta oun defecto en la acción de la insulina. Si bien actualmenteexisten varios medicamentos orales además de la insulina oanálogos de la insulina, ninguno de estos es el ideal.El vanadio es capaz de imitar los efectos mostrados por lainsulina tanto in vitro como in vivo y se estudia la posibilidadde usar compuestos de vanadio como agentes antidiabéticos. Eneste artículo se revisará la acción insulino-mimética del vanadioy sus posibles mecanismos en comparación con la insulina. / Diabetes mellitus is a serious chronic metabolic disordercharacterized by an increased plasma glucose concentrationand vascular and neurologic complications as well. Diabetesmellitus results from relative or absolute deficiency of insulinsecretion or insulin deficient action. Although there are anumber of oral antidiabetic agents besides insulin or insulinanalogues, none of them is optimal.Vanadium can mimic insulin effects in vitro and in vivoand the possibility of using vanadium compounds asantidiabetic agents is under study. This review will summarizethe insulin mimetic action of vanadium and its possiblemechanisms in comparison with insulin.

>Magister Scientiae - MSc / Vanadium dioxide (VO2) films have been known as the most feasible thermochromic nano-coatings for smart windows which self control the solar radiation and heat transfer for energy saving and comfort in houses and automotives. Such an attractive technological application is due to the fact that VO2 crystals exhibit a fast semiconductor-to-metal phase transition at a transition temperature TM of about 68°C, together with sharp optical changes from high transmitive to high reflective coatings in the IR spectral region. The phase transition has been associated to the nature of the microstructure, stoichiometry and some other surrounding parameters of the oxide. This study reports on the effect of the crystallographic quality controlled by the substrate temperature on the thermochromic properties of VO2 thin films synthesized by inverted cylindrical magnetron sputtering. Vanadium dioxide thin films were deposited on glass substrate, at various temperatures between 350 to 600 0C, deposition time kept constant at 1 hour. Prior the experiment, deposition conditions such as base pressure, oxygen pressure, rf power and target-substrate distance were carefully optimized for the quality of VO2 thin films. The reports results are based on AFM, XRD, RBS, ERDA and UV-VIS. The atomic force microscopy (AFM) was used to study the surface roughness of the thin films. Microstructures and orientation of grain size within the VO2 thin films were investigated by the use of X-ray diffraction technique. The stoichiometry and depth profiles of the films were all confirmed by RBS and ERDA respectively. The optical properties of VO2 were observed using the UV-Vis spectrophotometer.

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