Syntheses and applications of tungsten oxide-based nanocrystals
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Abstract
The art of nanocrystal synthesis has been the focus of interest for many years and many researchers. Finding the "right" synthesis for a desired product can be time consuming and very frustrating. Often, syntheses suffer from low reproducibility and comparability when different preparative methods are used.
Therefore, the main objective of this work was to find suitable ways to synthesize nanocrystals of various reduced tungsten oxides, tungsten bronzes and metal tungstates. For tungsten oxides, a synthesis was developed starting from ammonium metatungstate and a combination of oleic acid and oleylamine, which acts both, as solvent and protective surfactant. The simultaneous development of WO3-x nanorods of Magnéli type and hexagonal ammonium tungstate bronzes was observed. The selectivity was analyzed by varying the reaction parameters, e.g., precursor concentration, heating rate and solvent ratio. An unintended quantitative chemical reaction of the solvents towards their condensation product oleyl oleamide was observed and its effect for suppressing ammonium bronze formation was further investigated. A strong absorption of near-infrared light by the anisotropic nanorods and nanocrystals due to localized surface plasmon resonance was observed. It was also found that the maximum of the resonance can be tuned by changing the aspect ratio of the respective nanocrystals synthetically.
WO3-x nanorods catalyze the oxidation of sulfides to sulfoxides with the cheap and green oxidant H2O2. The selectivity in the formation of sulfoxides compared to sulfones is high with reaction times of less than one hour. An advantage of WO3-x nanorods is the low overoxidation to sulfones. The catalytic oxidation of a wide range of sulfides with different chemical structure and the reusability of the catalyst was demonstrated.
As this work progressed, it became apparent that impurities of iron in oleic acid (from the container material) can strongly impact the size and phase selectivity of the nanorod synthesis. A more detailed investigation of this process led to the formation of iron tungstate nanocrystals, which crystallize in the ferberite structure. At the same time, the use of oleyl oleamide led to the formation of a potentially new ferrotungstate phase. The basic structural analysis of this phase was based on a known, related magnesium tungstate phase of still unknown structure, which can be prepared in a similar way.
Synthesis and applications have been investigated for cesium-tungsten bronzes. These nanocrystals have a hexagonal and/or pyrochlore-like cubic crystal structure. The formation of the phases depends on the cesium-tungsten ratio, the heating rate and excess of oleylamine during synthesis. Cubic cesium-tungsten bronzes showed promising haloperoxidase-like properties, which were detected by a phenol red assay. Strong biofilm inhibition of the nosocomial bacterium Pseudomonas aeruginosa up to 35 % and strong suppression of the growth of the fungus Fusarium graminearum up to 54 % were observed
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for both WO3-x nanorods and cubic cesium-tungsten bronzes. In addition, a significant reduction of mycelium was observed with WO3-x nanorods, while cubic cesium-tungsten bronze showed a smaller but comparable effect. Mixed hexagonal/cubic cesium-tungsten bronzes, on the other hand, showed an influence on bacterial growth of Staphylococcus aureus, Pseudomonas aeruginosa and Phaeobacter gallaeciensis under infrared light. Compared to non-irradiated samples, a reduction in bacterial growth of up to 30 % was observed. This effect could be attributed to hyperthermia, which in turn is due to the strong plasmonic absorption of these nanocrystals in the red to near infrared range of light.
In a final project, the influence of transition metals in steel on the biofilm formation of bacteria was investigated. In addition to iron, steel contains other metals such as tungsten, manganese, copper, cobalt and zinc. Steel promotes the proliferation and biofilm formation of the bacterium Phaeobacter gallaeciensis significantly compared to glass and polymer surfaces. The study of metal salts showed that primarily iron and secondarily manganese have a positive effect on bacterial biofilm formation, as they serve as a nutrient for bacteria. Following these results, different (transition-) metal tungstate nanocrystals were tested for their effect on bacterial growth and biofilm formation. In contrast to manganese salts, the manganese tungstate nanocrystals with hübnerite structure significantly reduced biofilm formation by up to 40 % while increasing the number of planktonic cells in the supernatant. The same effect was observed with Pseudomonas aeruginosa and cannot be due to haloperoxidase properties of the nanocrystals. Iron tungstate nanocrystals with ferberite structure showed a similar but less intense influence. The effect could be due to the fact that the surfaces of the iron and manganese tungstate nanocrystals mimic nutrients and cause dispersion of the biofilm by binding relevant siderophores.