Größenselektive Synthese von nanokristallinem Cobaltoxidhydroxid - Partikelwachstum, strukturelle Fehlordnung, magnetische Eigenschaften

The aim of this work was the systematic investigation of the physical and chemical properties of nc-CoOOH depending on the average crystallite size and the composition. The synthesis is carried out in aqueous basic solution by oxidation of freshly precipitated Co(OH)₂ with atmospheric oxygen or bromine or by oxidation of an aqueous cobalt (II) solution with in-situ bromate. Depending on the synthesis route, nc-cobalt oxide hydroxide of variable composition is obtained. The prepared samples also vary in particle diameter, degree of disorder, specific surface area and particle morphology and magnetic behavior. It was shown that smaller particles grow more than larger ones. At higher temperatures larger particles are obtained by CoOOH-Br₂ and CoOOH-BrO₃⁻ and a faster particle growth is observed. In contrast to that CoOOH-O₂ reveals a similar particle diameter after 24 h regardless of the temperature. The application of a general growth law is not possible. The oxidation with O₂ and the subsequent hydrothermal annealing provides formanisotropic particles with an aspect ratio of 2-4 and average particle diameter between 12 nm and 31 nm. In addition to the enlarged c-lattice parameter, these samples are characterised by the symmetry-forbidden reflex which occurs as a shoulder on the (101)-reflection at a smaller diffraction angle. The low paramagnetic moment can be explained by partially oxidised Co²⁺. The charge neutrality can be achieved by the substitution of oxide ions by hydroxide ions in the structure. The specific surface area decreases with increasing particle diameter from approximately 100 m²/g to 37-60 m²/g. Oxidation with bromine and subsequent hydrothermal treatment leads to form-isotropic particles with an average particle diameter between 6 nm and 26 nm. The calculated amount of Co²⁺ in CoOOH-Br₂ (6-8%) is higher than in CoOOH-O₂ (1-2%). XP and NEXAFS spectra provide another evidence of partially oxidized Co²⁺ as the cause of the magnetic moment. Divalent cobalt XP spectra have intense satellites compared to Co³⁺. Comparison of the XPS measurement with reference spectra of pure Co (III) and Co (II) compounds confirms the presence of Co²⁺. NEXAFS show no surface enrichment of Co²⁺. With increasing particle diameter, the specific surface area of 77-90 m²/g decreases to 30-42 m²/g. Oxidation with bromate and subsequent hydrothermal treatment yields form-anisotropic particles with an average particle diameter between 3 and 21 nm. In general, this synthesis route at room temperature yields the smallest particles (BrO₃⁻: 3-5 nm, Br₂: 6-9 nm, O₂: 12-18 nm) and at the same time the most disordered material. Untreated samples typically do not show (104)- and (107) but have an anomal reflex width at half maximum of (015) and an overlap of (012) and (101). In comparison with annealed CoOOH-O₂ or -Br₂ the structural disorder is higher in the corresponding CoOOH-BrO₃⁻, caused by the shift of (101) or (006) to a smaller angle or of (110) to a larger diffraction angle, respectively. In addition, the absence of (104) and (107) and the anomalous reflex width at half maximum of (015) and (012) are typical for structural disorders as well. The paramagnetic contribution and the resulting amount of Co²⁺ of untreated CoOOH BrO₃⁻ (6-10%) is comparable to CoOOH Br₂ (6-8%) and significantly larger than CoOOH-O₂ (1-2%). The specific surface area decreases with increasing particle diameter of 190-213 m²/g to 74-158 m²/g. The examined samples have micropores with a particle diameter up to 6 nm. Based on the different synthesis routes, nc-CoOOH can be produced with a different degree of disorder. DIFFaX simulations indicate AB stacking faults due to the found reflection broadening and shifts. The influence of possible stacking faults on the reflection position and width can be investigated with the help of DIFFaX simulations. The method for global optimisation and refinement of microstructures investigates refinement and the resulting microstructure model as the effects of expanding the unit cell. This model shows the disruptions of the ideal stacking sequence of CoOOH-O₂ by AB and ABC stacking faults, with AB stacking faults occurring most frequently. For CoOOH Br₂, less regular stacking faults and more random shifts are observed. The largest amount of stacking faults exists in CoOOH-BrO₃⁻. Among random shifts and few ABC layer sequences, the ideal layer sequence is interrupted by many small AB layers. The consistency of the developed model was verified by PDF analysis. For CoOOH-O₂ with few stacking faults, the PDF provides a good fit to the reported data. The structure model leads to a better adaptation to the measured data at larger distances and therefore better describes the real structure of the examined sample. The numerous stacking faults in the structure of CoOOH-BrO₃⁻ cause a moderate adjustment using the reported data. The microstructure model significantly improves the fit, as evidenced by the near perfect match between measured and calculated curves at distances greater than 10 Å.